Questions and Answersabout Combat Robotics from Team Run AmokPrivacy Policy
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Plant Filament
Q: Is there a benefit to using PETG over PLA+/Pro for Plastic Ants? [Rocklin, California]
A: Mark J. The short answer is "no". PETG is heavier for equivalent strength, more difficult to print, and no tougher than a good quality PLA+. Avoid.
Be sure to check tournament rules for the specific filament types they allow.
It Rubs a Little
Q: Just a question about beater bar kits. How does the weapon manage keep its axial position along the shaft? Would the weapon be prone to sliding since the needle roller bearings won't resist axial movement? Why don't they use thrust bearings on the sides? [Mandaluyong City, Phillipines]
A: Mark J. I assume you're referring to the Fingertech 1-pound and 3-pound beater bar assemblies. These are not "drop-in" weapons -- the design of the weapons supports and fitments are left to the builder. There is typically only a small clearance between the beater and the weapon supports.
Adjustment of the weapon pulley lateral position will allow the beater to spin centered in its mounting with minimal rubbing friction against the weapon mount.
On impact the beater will slide to place either smooth aluminum or the Delrin pully into contact with a weapon support that is typically very slick UHMW polyethylene -- both good thrust interfaces.
You may certainly add thrust bearings and washers, but there would be little if any performance increase.
For larger robots I absolutely recommend better axial control for any style of spinner, but at the insect-scale it isn't required.
Not the Whole Saw!
Comment: Hi. Julian here, the builder of Mako.
You previously answered a question about Mako inaccurately and I have been getting a lot of messages asking me about it. I don't use an off the shelf saw-motor or complete setup. I use an off-the-shelf saw *blade*. I specifically use a drone motor with a TPU hub.
I am a big fan by the way, I don't want this message to come off as negative. [Norwich, Connecticut]
Reply:Mark J. I greatly appreciate corrections and clarifications, Julian. The quote from the video I reference in my answer saying that the weapon was "a literal off-the-shelf circular saw that you could walk into Home Depot and buy" combined with the design of your blade retainer led me to an incorrect conclusion -- thanks for writing in to set things straight.
The structure of your weapon hub is now a mystery. If you would care to send in a photo I would be pleased to publish it to provide a complete answer to the question from Zanesville.
Date marker: January 2026
Walk into Home Depot
Q: Hi Mark,
I was watching an NHRL stream and was interested in a beetleweight (3 lb) overhead saw robot called "Mako". I was having trouble figuring out how exactly the circular saw was mounted to the weapon motor, as it appears to be held on only by a TPU hub and lock nut (no screws or fasteners threaded directly into the motor. Any feedback is greatly appreciated. [Zanesville, Ohio]
A: Mark J. My original answer to this question appears in the box below, but Mako's builder wrote in to clarify the information given in the cited video. Julian Papasian's correction appears below the box.
Mako's weapon is not the usual hobby brushless outrunner with a printed hub like 'Cheesecake'. As disclosed in this video Mako's saw is "...a literal off-the-shelf circular saw that you could walk into Home Depot and buy." It's likely a 4.5" cordless framing saw torn down to just the motor and hub.
Circular saws have blade mounts that look just like what I see in the photos. What appears to be a TPU hub in one photo is likely a polished version of the metal retainer that is catching yellow reflection from the printed chassis.
Comment: Hi. Julian here, the builder of Mako.
You answered a question previously about Mako that was inaccurate and I have been getting a lot of messages asking me about it. I don't use an off the shelf saw-motor or complete setup. I use an off-the-shelf saw *blade*. I specifically use a drone motor with a TPU hub.
I am a big fan by the way, I don't want this message to come off as negative.
Reply: Mark J. I greatly appreciate corrections and clarifications, Julian. The quote from the video combined with the retaining structure on your blade led me to an incorrect conclusion -- thanks for writing in to set the record straight.
The structure of your weapon hub is now a mystery. If you would care to send in a photo I would be pleased to publish it to provide a complete answer to the question from Zanesville.
Jumbo Can-o-Worms
Q: One of the more common lifter setups in insect classes is a rear hinged lifter arm with some sort of 2 part linkage further down the arm providing the lift (Compared to the servo horn providing torque right at where it's hinged). This allows for a greater range of movement for a longer arm.
However my maths is failing me (See bad drawing attached for definitions of symbols). For calculating the force on the lifter arm from the servo in the setup is it as simple as x*sin(theta)? However isn't that just the force transmitted between the servo arm and the arm of the linkage? What about transmission of force between the arm linkage and the lifter arm itself? Are there further losses there?
And then to calculate whether you can lift something or not, is it (F * y) - (W * z) (Where F is the force from the lifter motor on the arm, W is the weight of the robot being lifted, y is the distance between pivot and lifting point and z is the distance between pivot and robot being lifted)?
Hope this makes sense
Thanks in advance. [Somewhere around Manchester, England]
A: Mark J. You've opted to open up the jumbo can-o-worms, eh? What you have is known as a 4-bar mechanism, and the calcs are especially nasty because the lever advantages keep changing as the lift progresses. In your sketch the initial advantage is poor and a good deal of torque is needed, but as theta increases the advantage improves and less torque is required. The general approach is to calculate the actual rise in the tip of the lifter for each change of say 1 degree of servo arm motion and convert that into a torque requirement for each progressive angle.
I know this because I wrote a pair of 4-bar Excel spreadsheets that perform these calculations for standard "Biohazard" style 4-bar lifters (movement up and forward) and for the servo linkage you describe (single pivot hinge motion). That is the good news.
The spreadsheet I wrote for the servo-powered single-pivot lifter has the correct geometry for your purpose, but the example layout has a hinge point much farther forward than your design. You will need to adjust bar lengths and angles to morph into your longer rear-hinge design. This may well take some time. Here are the spreadsheet link assignment letters as they apply to your sketch:
Give it a shot. If the calculations balk, you can wade thru the "Equations" tab on the spreadsheet and make adjustments as needed. Download the Team Run Amok Servo Lifter Spreadsheet from our Combat Robot Design Tools page.
About Eighty Grams
Q: what motor to use for a beetleweight eggbeater robot [Barnet, United Kingdom]
Q: Hi - I'm building my first UK antweight robot, which is a giant 50g bar overhead spinner, with inspiration from robots icewave, moros and bloodsport. It's 2WD, with the brushless motor in between the two drive motors, and a screw in the back to put the blade at an angle. My problem is so: When testing the drive, it's completely fine, when testing the weapon, it's also fine. When I drive and spin the weapon, the robot is incredibly unbalanced, and pings around my test box like an air hockey game.
Could you give me some tips on how to make it balance better? (Also the weapon bar itself is balanced.)
Cheers! [Eaton, England]
A: Mark J. That's a very pretty 'bot with a lovely spinner bar, but you've changed a critical design element from the robots you credit as inspirations. That change is causing your problem.
The spinner bars for 'Icewave', 'Moros', and 'Bloodsport' are all precisely horizontal -- the rotational axis of the weapon remains pointing straight up when the robot turns. You have elected to place "...a screw in the back to put the blade at an angle." That angle causes the direction the weapon axis points to change when the robot turns. Combined with the huge rotational inertia of your weapon this causes a strong gyroscopic reaction, which raises one side of the 'bot, which then causes additional changes in the weapon axis, which then... you get the idea. This is why you don't see angled horizontal spinners.
There is no fix other than tilting the spinner axis back upright and making sure it stays like that. 'Bloodsport' and 'Icewave' are four-wheeled chassis to help keep their blades 'flat' while 'Moros' is two-wheeled but has the center of mass well back toward the trailing rear skid for stability.
Yes, setting your bar fully horizontal raises the weapon up too high to be terribly useful. Call it a rookie mistake. Do not be tempted to 'droop' the impactor end of the bar -- that causes a different type of instability.
Resetting the Rat
Q: Has anyone tried to make a robot like 'Rat Amok' with a 'Snail Cam' (second of four designs on our Spring Flipper Designs page)? A rat trap with a near infinite number of resets sounds very effective.
- sincerely, Iceywave [West of San Antionio]
A: Mark J. I'm quite proud of 'Rat Amok' -- she is a one-of-a-kind antweight that won her first tournament and was later victorious at the "King of Robotica" match between myself and season two champion Mike Konshak with a clean OOTA ejection. Her one-shot weapon struck fear into her opponents, and the suspense of "when will it fire?" added much to the drama of her matches.
A 'snail cam' mechanism is quite bulky due to the large spiral cam. It would be difficult to translate the linear motion of the cam follower into a 180 degree rat trap reset. I suspect this is why no one has attempted a snail cam version of 'Rat Amok' -- but your question got me thinking...
I just now took Rat Amok off the shelf and measured the torque required for a reset against the torsion springs. A full 180 degree reset requires an initial torque at the axle of 12 kg-cm that rises to 29 kg-cm at the end. That is well within the capacity of high-torque R/C servos that weigh about 2 ounces and have up to 270 degrees of motion.
Rather than a snail cam, I believe that the 'Servo Latch' (fourth of four designs on our Spring Flipper Designs page) would be more easily modified to reset the trap, latch it, and trigger the release. Might be fun!
Too Tight for a Gearbox
Q: My design for an antweight full body spinner is very tight on space. I don't have room for regular gearmotors, even the little N10 size, and I can't find any right angle gearboxes small enough and fast enough to work.
Are there any alternative drive methods that might work in such a small space? [Social Media]
A: Mark J. I dug thru the Ask Aaron Archives and found a post from 2016 that described a couple unconventional combat robot drivetrains that require no gears, pulleys, or sprockets:
Friction drive The shaft of the motor presses directly against the surface of the tire and relies on friction to transmit power to the wheel. You will get some slippage, but if your motor has a 2mm to 3mm shaft the reduction ratio works out about right.
Tiny wheels If direct-drive for a normal sized wheel isn't practical, how about a really tiny wheel? Mount a very small wheel to the motor shaft and mount the motor at an angle to put the wheel in contact with the floor. Unconventional, but a successful antweight full-body spinner used this method as a space and weight-saving measure. The 'wheels' could be something as simple as a small plumbing washer, a short length of small diameter rubber tubing, or even a few layers of electrical tape wound around the shaft. They won't last long, but they're easy to replace.
About So Big
Q: What is the usual size of an 'insect sized' arena? [Lagos, Nigeria]
A: Mark J. A common size for beetleweight competition is 8' by 6'. One-pound antweight arenas are often 4' by 4'.
Here is a YouTube video covering the design and building of a 4' by 4' antweight arena with a link to a parts list and design drawings.
Malenki Weapon Voltage Boost
Q: I would like to get more power from my fairyweight spinner, but I use a Malenki-Nano dual ESC/receiver that limits me to a 2S lipo battery. I don't need more drive speed but my spinner waepon would be much better at the higher voltage. I know there is a high voltage version of the Malenki but is there anything I can do that will allow me to run my Malenki-Nano on a 3S battery? [Social Media]
A: Mark J. There is a similar situation that involves operating a lifter servo weapon at 2S when it is plugged into a receiver that can take no more than the voltage from a Battery Eliminator Circuit (BEC). The two situations have similar solutions.
With your Malenki, it is possible to use a battery with more than 2S to provide power to your brushless ESC by tapping into the balance plug of the same battery to obtain 2S power for the ESC/receiver and your drive motors. A diagram for a 3S battery is provided below.
There are a few drawbacks to this solution:
You will need to switch
S
the positive leads to both the weapon ESC and to the Malenki. DO NOT ATTEMPT to use a single power switch on the ground wire! Removing the battery ground causes a 1S reverse voltage to both the Malenki and weapon ESC.
Make certain to use the same ground potential for both the Malenki and the weapon ESC. See this archived post for details.
The three battery cells will not drain at the same rate. Balance charge the battery every time.
More Than You Need
Q: Could a spring from a typical rat trap be repurposed into an antweight flipper?
- sincerely, Iceywave [West of San Antonio ✪]
A: Mark J. Sincerely? I suspect you're just prompting me to post a pic of Team Run Amok's infamous antweight snapper/crusher/flipper 'Rat Amok'.
In truth a rat trap can store far more energy than is required for a properly designed antweight spring powered flipper -- a mouse trap spring should be adequate. I found an analysis on the energy stored in a typical mouse trap at the Physics Stack Exchange:
I calculated the torsion constant in a Victor Original Mousetrap with a spring arm length of 4.3cm) to be approximately 0.09088 N×m/Rad by using Hooke's Law and comparing the angle between the spring arm and the wood base when hanging different weights from the arm with the trap upside down.
I then used τ = -kθ (torque applied to the spring arm by the weight is equal to torsional constant times the angle rotated) along with the values from one of the weights to calculate how far the spring is twisted by default [θ = (τweight / k) - θweight ]. This came out to about 73.63°.
I used this 'starting angle' to calculate how much potential energy (U = 1/2kθ2) the spring would have when totally open (an additional 180° from that last angle we found). The maximum energy that you could get out of this "standard" mousetrap is approximately 0.815 J.
Using the potential energy calculator at OMNI Calculator reveals that 0.815 joules -- if perfectly converted to vertical speed -- could loft a 1-pound object just over 17 feet straight up.
Think Wider
Q: Do people still use the FingerTech S3M belt at Beetleweight? I am working on a horizontal spinner but I don't know if the belt would be able to survive. [Sacramento-ish]
A: Mark J. The 4mm wide S3M FingerTech belts are uncommon in beetleweight spinners -- particularly for a belt long enough to reach out to a horizontal weapon. The S3M tooth profile is OK but the magnitude of the power transfer calls for a wider belt:
The EndBots 'Vector' horizontal bar spinner beetleweight kit used a 0.25" wide (6.35 mm) XL profile timing belt.
For their 3lb Beater Bar assembly FingerTech uses a wide S3M pulley that accepts up to a 8.25 mm belt.
The vertical Peter Bar Weapon Kit from Repeat Robotics uses a 9mm wide 3M profile belt.
The Spec Sheet is Wrong
Q: Why is my drivetrain supposedly so close to stalling? If I drop the voltage to 11.1V then it's apparently going to be unable to drive even though I've run these motors on a Fingertech beater before at 3S. Motors in question are the generic 22mm gearmotors available on the Fingertech site. [Arlington, Virginia]
A: Mark J. You have correctly entered the motor specs given by FingerTech (and other sources) for this motor -- but those specs are incorrect . Per the spec sheet, the 12 volt stall torque for the gearmotor is 27.8 oz-in @ 4.9 amps (Kt = 5.67) but actual test results from multiple builders yield an average stall torque almost three times greater.
For comparision: At 12 volts, the very much smaller FingerTech Silver Spark 22:1 has a similar no-load speed and stalls at 22.7 oz-in @ 2.1 amps for a Kt of 10.8 -- the spec for the 22mm stall torque is obviously too low. See: Converting Motor Specs. Based on the real world numbers I estimate the torque constant for these 22mm gearmotors at 16.3 oz-in per amp. Plug that into the drivetrain calculator and your output will make sense.
Also, the "Torque (per motor) to spin wheels" calculation in the drivetrain calculator does not refer to the torque required to simply drive the robot across the arena. It is the torque required to provide the full pushing force and "beak traction" to spin the wheels and prevent motor stall under heavy pushing. This value should ideally be attained at no more than half the stall amperage of the drive motor. See the Optimizing Drivetrains page for a full explanation.
PS: How much leftover battery life should I pack? The spinner weapon spreadsheet tells me that I'll be using 1.03 amp hours per match, so would a 1500 maH battery work? How do I know if the margin is too small, and if the battery will start to die before the end?
A: The Spinner Weapon Spreadsheet provides a gross estimate of the battery capacity required to spin up your weapon a specified number of times and maintain top speed for the given duration of the match. Take that number with a grain of salt. It does not factor in things like excessive aerodynamic drag from spinning a weapon at stupid-fast speed. It also does not include the battery capacity required by the robot drivetrain.
Generally, rounding up capacity in the 25% to 50% range is about right. You will find out if the margin is too small by testing. Most battery chargers will tell you the mAh required to restore the battery to full charge after a match so that you may determine if the battery has too much or too little capacity.
Literal Top Post
Q: In a beetleweight bar spinner design driven by a timing belt, is it good to have one pulley be smooth? This robot seems to do that, and it would definitely make CAD work easier if I can just use a fingertech toothed pulley for the weapon and fit a smooth 3D-printed one (more appropriate for my CAD skills than a full-on timing pulley) around the motor can.
Three Minutes Later...
Just realized that the literal top post [on the Ask Aaron page] answered my initial question, oops. Do I need to worry about tensioning with one smooth side using a fingertech belt? [Ashburn, Virginia]
A: Mark J. If you run two toothed pulleys you can run a calculated fixed pulley spacing and be fine. If you have one smooth pulley you will need some method of tension adjustment -- one or two tensioning idlers as in your example photo or perhaps adjustable weapon motor mounts.
Beetleweights are still small enough to consider running two toothed pulleys with a timing belt -- you can do that and avoid the tensioning issue. Take a look at the STL files for the EndbotsVector Beetle kit. You might find it easy to modify the motor can 0.25" XL pulley from the Vector to fit your weapon motor. Consider battle hardening your weapon motor if you choose to do this.
Increases with the Cube of Speed
Q: These weapon performance numbers [at right] from the Run Amok Spinner Weapon Energy Calculator are looking a bit optimistic. I used ChatGPT to calculate the internal resistance of the SunnySky 2450 Kv 2212 motor since they only have the specs up to 1400 Kv version on the site.
I'm guessing aero forces will be the limiting factor here and overload the motor, but can you weigh in on how this weapon would perform? Would it be better to go with a lower kv? [An iCloud Server in New York]
A: Mark J. I recognize those weapon numbers. You either have or are duplicating a Vector beetleweight kit. The Vector had the 980 Kv SunnySky 2212 weapon motor that would draw a continuous 8 amps @ 14.8 volts to overcome the aerodynamic drag at 6500 RPM (370 joules). That's about 90 watts and the motor is rated for a continuous power output of 385 watts -- it could do that all day.
You are correct in worrying about aero forces at high RPM. Increasing weapon speed increases aerodynamic drag with the cube of speed:
Doubling the speed requires 23 = 2 × 2 × 2 = 8 times the energy; so...
Doubling the speed of that 6500 RPM weapon to 13,000 RPM (5000 RPM below your target speed) would require 90 watts × 8 =720 continuous watts to overcome aerodynamic drag; but...
The 2450 Kv SunnySky 2212 has a continuous power rating of only 450 watts; which means...
The motor will not hit those performance numbers and it will melt trying.
The output for the same weapon from the full Run Amok Spinner Weapon Spreadsheet (below) shows that you can probably get away with using the 1250 Kv version of the SunnySky 2212 at 14.8 volts to push the weapon speed toward 8000 RPM (520 joules) at a continuous 15 amps @ 14.8 volts. That's about 220 watts with the motor rated at 518 watts. If you want more than that you'll need a larger motor with a higher continuous output limit.
Press Fit
Q: How do robots like Sauron and Vector design their pulleys to fit around the outrunner can? Do they just build it with equal diameter and press fit it on? [Ashburn, Virginia]
A: Mark J. Typically, yes. Actually the pulley inner diameter is a few thousandths of an inch smaller than the can diameter for a tight press fit. See this post in the Ants, Beetles, and Fairies archive for a photo and a link.
Too Squishy
Q: Hi Mark,
I have been pondering the usefulness of tangential drive for antweight (1 lb) applications, but have concerns regarding its compatibility with foam wheels. In particular, the Repeat Robotics Repeat Tangent motors seem to be effective on the likes of robots like "Super Space Turtle", but I have only seen them used with tires made via custom-molded polyurethane/rubber. Could it be that foam wheels are too "squishy" to create any meaningful contact points with the shaft or simply have a poor friction coefficient (if the latter, could this be mitigated using liquid latex/some other traction coating)? This seemed like a "too good to be true" lightweight 4WD solution. Any feedback is greatly appreciated. [Zanesville, Ohio]
A: Mark J. You are entirely correct to worry about this, Zanesville. Tangential drive (YouTube video) and squishy foam tires do not mix. The transfer of power from the drive shaft to the tire is the product of the coefficient of friction and the force by which the shaft is pressed against the tire. Power in excess of this power limit results in slippage, and getting enough force against a foam tire will depress the shaft so deeply into the tire that drive force vectors would no longer be 'tangential' and power transfer efficiency plummets -- with or without a traction coating on the tire.
Note that pressing hard against a tire may place considerable side-loading on the motor shaft. This is a type of load that most hobby brushless outrunners are not designed to accommodate.
Note also that the tangential drive video linked above has the test bed operating at low power and not doing any high-resistance pushing. Gears don't slip under high loading, but friction drive can. I like gears.
Extra Credit: A tangent-drive antweight with 2.0" diameter drive wheels has a calculated top speed of 12 MPH. If the drive train is modified to accept 1.5" wheels, what will the calculated top speed of the robot be?
16 MPH
12 MPH
9 MPH
8 MPH
The calculated speed is still 12 MPH. With a tangent-drive one motor revolution moves the robot forward by one shaft diameter -- wheel diameter is irrelevant.
Stubby Shaft Solutions
Q: I'm a new bot builder trying to build an ant for the first time entirely on my own. I'm planning on doing a servo four-bar lifter and I'm planning to use four 30.4mm diameter 14mm wide LEGO tires. I'm curious about what you'd recommend for the drive motors to go with those wheels, given that the robot is a control bot. I've only build heavily aggressive horizontals before so it's a little different.
I was thinking n20s/30s (but I could totally be wrong) but all the ones I can find that actually have specs have a mere 8.6mm long shaft. Is there a good way to use those short shafts in a wheel that is nearly double the width? I see people all over using them with little to no issues but every time I have tried to it has not gone well because the shaft was wobbly because it was just not long enough. [Purdue, University]
A: Mark J.Good quality N20 gearmotors would have ample power for your control ant, but the gearboxes are potato chip fragile. In the 150 gram class they're fine, but even with wheel protection they don't survive well in full combat one-pound ants. For full combat I can't recommend anything without a stout planetary gearbox -- something like the Repeat Robotics Brushed Mk2. They are larger and heavier than N20s, but two would be enough.
If you're building for the plastic ant class you could certainly give N20s a shot. If you do a web search for N20 long shaft you'll find multiple choices for both threaded and smooth shafts in assorted lengths. The pictured example (fingers for scale) from eBay has a 23mm long shaft. Cheap eBay N20s typically have low-power motors, but you can swap in the high-power carbon brush robot motors if needed.
I must note that it is unusual to first pick your wheels and then design the drivetrain around them.
Date marker: January 2025
Raising the Roof
Q: How effective would adding spacers to shock mount the lexan top panel be to counter the new, devastating hammer saws like Strikepoint in the beetleweight class? (On a 4WD wedge) Although having the armor resting above the chassis would lead to a less than ideal outcome against horizontals that manage to rip into the gap, I figure that it would buy me a few hits against overhead disc spinners and saws by giving it some distance away from the electronics. [Behind a Cloudflare Curtain]
A: Mark J. Any specific guidance I might give would depend on design details you have not provided. Beetle, 4WD wedge, Lexan top panel -- are we talking about a BotKits D2? General comments:
Lexan (polycarbonate) gets much of its strength from its ability to flex and stretch. Think of it as stiffened transparent rubber.
Lexan cracks and fails when it is fastened in a way that concentrates the stress of flexing to a small area. I typically recommend oversized mounting holes with rubber grommet inserts to allow some 'give' around the fasteners.
Raising a top panel on small spacers removes the support of the chassis edges and increases stress around the fastening points. The added height also increases stress on the fastening screws. You haven't mentioned how much additional space you are considering.
Is damage from overhead weapons something your 'bot has actually experienced, or is this speculation on your part? If this is an actual problem I might suggest simply replacing the top panel with something that does not flex enough to endanger your internal components: carbon fiber, garolite, or perhaps thicker Lexan.
You Already Have Gearboxes
Q: I want to get my D2 beetle running again, but I want to use something stronger than the generic Botkits 22mm gear motors to stay competitive and I can't get the Botkits motors to last on 4S. The Just 'Cuz Dragon 22mm Dartbox Gearmotors look great, but the weird clamp drive motor mounting system the D2 uses won't work with a square Just 'Cuz gearbox.
Any alternate recs for good 22 mm brushed gearmotors? [Social Media]
A: Mark J. Let me save you some money. The motor bolt pattern on the BotKits 22mm gearbox is correct for 130-size Dartbox motors, so just remove the stock motors and bolt up any of the brushed Nerf/Dartbox motors you like. You may need to shorten the shaft just a bit and/or transfer the pinion gear from the old motor.
If you want to stick with the 4S lipo pack I'd suggest the Just 'Cuz Gecko replacement motors for the Dartbox Pro. They're dirt cheap, will run all day on 4S, and at 16 volts pull only 7 amps each at stall. The Dragon motors can pull two-and-a-half times that many amps at their 12 volt max, require extra heavy duty ESCs, and are complete overkill for a four-motor beetle.
Reply: Nice, thank you
Heat Kills Motors
Q: Are there any brushless outrunner motors in the 2822 size class that can handle 14.8 volts? (can't find any online) If not, what will happen if I try to spin one only rated for 11.1 volts with a 4S? I'm doing an antweight based on the Fingertech mini beater bar, and I want to use a 4S for more drive and weapon speed, but although the page for the beater bar electronics bundle claims that it can handle 14.8 volts, the page for the motor itself (2822 1100 kv) says it can only accommodate a 2S-3S lipo. [Close to D.C.]
A: Mark J. The maximum voltage a motor can handle depends in part on how much load is placed on it. Too much load will 'bog down' the motor during spinup, causing it to draw extra current over an extended period of time. Extra current means more heat, and heat kills motors.
The FingerTech antweight beater bar kit has a relatively small weapon spun via a 1.7:1 reduction timing pulley set. This reduction reduces the torque load on the motor, shortens spin-up time, and prevents overheating on a 4S battery. If the same motor was used with a 1:1 direct drive it would not be prudent to run it at 4S.
If you want more weapon speed I would suggest sticking with a 3S battery and selecting a higher Kv 2822 weapon motor. Greater drive motor speed can make a 'bot difficult to control and is rarely usable in an insect-size arena. See: Traction and Reflex Limited.
Q: 1lb beater bot guy here. I found out that Fingertech sells 2600 kv 2822 motors, which should do nicely for increasing the weapon speed. However, the page for the motor says that the motor draws 24A, and "over double the power output [of the 1100 kv motor] requires a larger ESC." The 40A ESC is a full 6 grams heavier than the 20A ESC, which I also may or may not have already ordered, so I decided to seek a second opinion.
Palm Beach Bots appears to be reselling this motor with the claim that it is compatible with the 20A ESC, so I'm at an impasse here. Should I bite the bullet and grab the 40A, especially considering that I'm still planning to use the 14.8V battery which would probably draw more current on spinup? (since more weapon speed on a vert = good, and I drove a D2 kitbot at 4S for a couple years, so I'm reasonably confident that I can handle whatever speed two Silver Sparks can output) Or will the max current being above the ESC's rating be irrelevant since the motor will stop drawing a lot of current after spinup?
A: You're operating under a number of bad assumptions. Where to start...
I don't know where you got: more weapon speed on a vert = good. More speed equals more stored energy, but more speed also equals less 'bite' -- and bite wins matches. I suggest you read the Ask Aaron Spinning Weapon FAQ with particular attention to the 'Rotational Speed' section. Running a 2600 Kv motor with the FingerTech antweight beater at 4S will have the two-impactor weapon spinning at a theoretical 22,600 RPM. The weapon will have almost no 'bite' and - unless you get very lucky - will just skitter across the surface of your opponent.
The 2600 Kv 2822 motor Palm Beach Bots sells is "Viper compatible" but the specs and measurements do not match the motor sold by FingerTech. It is similar, but not the same.
When a specification sheet for a brushless motor gives a "Max Current" it is NOT the greatest current the motor can draw; it is the most current it can continuously draw without failing from thermal overload. A typical 2600 Kv 2822 outrunner might have an internal resistance of less than 0.05 ohm and can draw more than 120 amps if bogged down into its mid-RPM range. This is why it is important to load the motor lightly and let it pass thru the lower RPM range quickly.
Likewise, a "20 amp" hobby ESC can provide 20 amps of current for some (usually unspecified) period of time, and will provide much more for short periods.
Yes, increasing voltage does result in a proportional increase in current draw, both during spinup and perhaps disproportionately at max weapon RPM with a weapon that has a lot of aerodynamic drag.
Beater bar weapons have notoriously high aerodynamic drag. Attempting to spin one at well over 20,000 RPM will bog down your weapon motor enough to draw well more than 25 continuous amps of current and it will melt -- unless of course the brushless ESC fails first.
Driving a weaponless four-wheel-drive beetle on 4S is a whole lot different from driving a two-wheel-drive ant on 4S with a very fast vert weapon adding very large gyro forces into the mix. You'll spend a lot of time sideways on one wheel -- see Designing Around the Gyroscopic Effect.
I gave my best advice in answering your first set of questions. I have now given the background on that advice. More is not necessarily better, and more may bring disasterous consequences. Proceed as you please.
Less Cogging
Q: Is there a reason to choose a lower Kv motor for a given weapon versus a higher one if the higher one is capable of a higher speed/torque? I understand the argument of choosing a lower Kv to get better bite, but couldn't the same be accomplished by limiting the throttle from the radio? [Probably Sacramento]
A: Mark J. Yes, there is good reason to consider a lower Kv version of a given motor for an unsensored direct-drive spinner weapon. Even though a high Kv motor produces greater PEAK torque and power, a comparable low Kv motor has a smoother start-up with less "cogging" and better low-speed torque. A low Kv motor has a higher number of wire coils in the stator which provides better position feedback to the ESC, allowing the delivery of greater current at low speeds to boost initial spin response.
Limiting throttle to the weapon motor will reduce top speed, but it also reduces the current supplied to the motor which reduces both torque and power. If you can run a belt drive reduction to the weapon to take the torque load off the high Kv motor and let it spin up into its power band it can be a better choice, but for a direct drive weapon pick a Kv that keeps speed in the usable range for the weapon.
Stator Size Matters
Q: Do you know if the Sunnysky 2212 is roughly comparable in power output to a Repeat 2822? They both physically look to be the same size, but I cannot find any power information or a datasheet for the Repeat Motor. [Probably Sacramento]
A: Mark J. The two motors are difficult to compare because their motor designations refer to differing measurements.
The '2212' designation for the SunnySky motor refers to the stator dimensions: 22mm diameter and 12mm thick;
The '2822' designation for the Repeat Robotics motor refers to the external dimensions of the motor can: 28mm diameter and 22mm overall length.
A 2205 Stator
The can diameter of the two motors is the same but the overall length of the SunnySky at 30mm is quite a bit greater than the Repeat Robotics motor at 22mm, and it weighs a fair bit more as well. This represents a considerably thicker stator. For brushless motors with similar voltage constants (Kv) a larger stator generally means more power, so the 1250 Kv version of the SunnySky should be a good bit more powerful than the 1100 Kv version of the Repeat Robotics motor. This also holds for the higher Kv versions of the two motors.
I will point out that the Repeat Robotics weapon motors are quite popular, and they are marketed as being designed for durability in direct-drive applications. Motor selection is not all about power.
I emailed Peter Garnache at Repeat Robotics to ask about performance specs for his direct-drive 2822 weapon motors -- like their internal resistance or continuous output power. He was kind enough to respond quickly:
Mark,
I don't have either of those numbers on hand. I find that the current draw of the motors varies heavily on the weapon system that they're spinning and the speed they spin at. In general the higher KV motors will pull more current. I'd choose a kv to get your tip speed between 150-250mph, and then do some testing with a power meter to figure out what your actual current draw is.
Q: Hello,
If I want to make a competitive plastic ant with oversize drive motors, would you recommend brushed or brushless? Palm Beach Bots recently added a couple of brushed and brushless beetle drive options that both seem light enough to put in an ant, as well as the new dual brushless drive esc that was just released. This robot would most likely be a 4wd vert or 2wd drum with a hub motor powered weapon.
Do you think this would be practical overall? It would gain a lot of pushing power and speed but I feel like I would see more people oversizing drive components if it was worth doing. I'm sure I can make everything fit, but figured I should ask before I buy any components. Thanks! [Logan, Utah]
A: Mark J. You are wise to note that oversize drive motors are not commonly encountered. If big motors gain an edge in pushing power, why isn't everyone using them? If speed is a sure path to victory, why are chunky brushless drive motors so rare?
Pushing power is limited by traction. Once the wheels meet their traction limit additional drivetrain torque does not increase pushing power. The maximum pushing force a robot can generate is dependent on the weight bearing down on the driven wheels and the traction of the tire/arena pairing:
Maximum pushing force = Weight Supported by the Drive Wheels × Coefficient of Friction
An oversized drivetrain will "break traction" and spin the drive wheels at a small fraction of its power, but will not by itself generate additional pushing force from the tires. If you are fighting in a steel-floored arena and event rules allow it you may use chassis magnets to increase the apparent weight on the drive wheels and make use of increased motor power -- but there are significant problems with magnetic downforce, as described by multiple posts in the Ask Aaron Archives.
There is only so much speed that can us effectively used in an insect-class arena. Like pushing force, acceleration force is also traction limited -- excess torque will break the tires free and hamper directional control of the robot. Even small ant brushless drive motors like the 24 gram Repeat Mini Mk3 deliver more speed and power than most drivers can use. Oversized drive motors will simply increase your frequency of running into the arena walls.
To answer your questions:
For the reasons above, I do not consider oversized beetle drive motors in a plastic ant to be practical.
Unless you have significant experience driving insect-class 'bots and reflexes like an over-caffeinated cat, I would recommend a nice pair of Repeat Drive Brushed Mk2 drive motors.
Q: Hey, Oversized ant drive guy here again, good call on the traction, I hadn't considered it. However, I have thought of one more point.
Since spinning weapons get more bite the faster they charge at something, would it be practical in that way to increase drive speed? Most of my successful robots have essentially been fast glass cannons that rely on outmaneuvering and ending the fight without taking damage. In this way, would it be wise to have an oversize drive to allow for faster retreats and a hit and run style strategy? Or perhaps just the antweight size brushless drive options?
I've mostly used the repeat brushed mk2's, but do have a set of mk3 mini brushless motors on hand. The brushless seem to not perform as well, but it might be due to them being paired to some of the very first brushless drive esc's that were released.
Thanks as always for the knowledgeable perspectives!
A: Brushless motors are only as good as the controller firmware and setup. Trying to control them with ESCs not perfectly matched to their requirements will yield very poor results. I like brushed drive motors because they're stupid simple to set up.
As mentioned in the post above, acceleration force is also traction limited. All torque above the physical traction limit will simply set the wheels free-spinning without adding to acceleration. More power won't give you as much improved performance as you think, and the effect on robot control can be catastrophic. The Tentacle Drivetrain Calculator takes this traction limit into account when modeling robot performance, so let's use it to compare the performance of two ants powered by normal and oversized motors.
The robots are two-wheel drive with 2" diameter wheels and 3S LiPo batteries. The only difference is that one is powered by the familiar Repeat Drive Brushed Mk2 ant motors while the other is powered by the more-than-twice as powerful Repeat Compact Brushed motors. Both are modeled for a 6-foot sprint across the arena floor:
For the six-foot sprint, the 115% power increase results in:
A speed increase of 5% from 5.96 MPH to 6.25 MPH.
An elapsed time decrease of 14% from 1.08 sec to 0.95 sec.
Do you think that's worth the effort? Even if the additional power did not adversely affect robot control, I would say not.
Turn Two Into Four
Q: How can I convert an antweight into a 4 wheel drive? What gears or pulleys are to be used for that? [London, England]
A: Mark J. Given your location I'll assume we're discussing 150 gram UK antweights. You haven't told me anything about the design you would like to 'convert' so my comments must be very general.
The simplest and by far most common 150g 4WD solution is to add another pair of motors to directly drive the new pair of wheels. A pair of N20 motors weigh 18 grams; a pair of N10 gearmotors weigh 16.8 grams.
I do see a few 150g gear drive solutions that use custom printed plastic gears and hubs: Team Panic video.
I don't know of off-the-shelf components for the hubs, pulleys, and stub axles needed for a generic belt-drive conversion for a 150g robot. You would need to fabricate many if not all of these components.
Gear or belt drive 4WD antweight designs are generally used only where there is no space for extra motors due to weapon layout. Note that these solutions might very well weigh more than an added pair of motors and would likely be less reliable.
Hypothetical Thinking
Q: Hi. I am thinking about building a hypothetical 1 lb cam lifter akin to NHRL's "Supreme Ruler" or "Needle/Ace" from BattleBots. So far, I am in the stage of part selection and I have some questions.
I have decided to use two KingMax KM1203MD servos to directly rotate the lifting forks. I also plan on using a 3S battery to power the robot's electrical system. Based on the operating voltage of the servo, it can operate between 5.0 and 8.4 volts (fully-charged 2S battery). Since I would be running it off a battery of 11.1V, I worry about damaging the servos by overvolting them. As such, I found a FingerTech Robotics 9.6V 4A Switching UBEC Regulator that is very lightweight. Is this still insufficient for reducing the voltage? Other UBECs I have seen that drop the voltage to 8.4 V weigh approximately 5 times more than the proposed Fingertech UBEC. It might also help to know that I plan on running FingerTech TinyESCs (as they have integrated 5V BECs). Recommendations for parts or solutions (especially simple ones that I am likely overlooking) are very welcome.
If Fingertech's UBEC is viable, then I am struggling to understand how the wiring diagram on their website works (located below product description and/or specifications). There are lots of instructions about IN+/- & OUT +/- wires and splitting connectors that I don't understand. Help with a possible wiring diagram or anything to make the layout clearer is greatly appreciated.
Any other advice you may have about cam lifter design is also welcome and appreciated.
Thank you very much. [Hidden in an iCloud]
A: Mark J. I'm wondering how you decided on two KingMax servos.
A cam lifter slides a pair of thin but wide blades under the opponent then axially rotates the blades 90 degrees to high-center their 'bot. The maximum torque required for this action would be the weight of your opponent times the width of the blade. For a 0.45 kg antweight with 2.5 cm wide blades that comes to 0.45 × 2.5 = 1.125 kg-cm. Even with a 100% torque safety margin to keep the servo cool and happy, a single 2.25 kg-cm servo with linkages to both blades would be fine for your purpose.
A pair of the KingMax servos @ 8.4 volts deliver a combined 21 kg-cm of torque -- about ten times what you require. They are massive overkill but if you have weight, space, and budget not really a flaw.
Overvolting There is no 'standard' used by servo manufacturers to determine a maximum allowable voltage, and there is no agreement on a safety margin. Some servos are limited by the voltage ratings of their electronics, while others may establish a voltage limit to remain under the current capacity of the electronics or motor when stalled. Running a servo at a voltage above the stated limit may:
Instantly destroy the electronics before the servo even moves;
Fry the electronics with the servo operating under load;
Fry the servo motor with the servo operating under load; or
Cause no harm at all.
There's no way to tell for a specific servo without trying, but you don't need to try and you don't need a UBEC regulator. Keep reading...
Battery Tap You can tap into the balance connector on your 3S LiPo to feed 7.4 volts direct to your servo while still supplying 11.1 volts to your motors. The amount of current used by the servos is small relative to the drive motors -- just balance charge the LiPos every time to keep the cells happy. If your receiver can run at 7.4 volts, clip the red receiver wires on the tinyESCs and the wiring diagram (which shows a single dual-channel ESC) looks like this:
If the receiver needs a lower voltage, leave the tinyESC leads alone and run the servo power and ground direct to the 7.4 volts:
If you still want to use a UBEC, the voltage regulator, battery, and servo are wired in like this, with power and ground (red and black) wires running to the ESCs and the signal wire (white) going to the receiver:
Reply: Hi, cam lifter person again.
I decided upon using a KingMax KM1203MD servo because I used the desired stall torque formula from the Electric Lifter FAQ, multiplying the theoretical blade length, opponent weight, and the 1.67 factor together. I was unaware that the formula is different for cam lifters. Having extra torque is good, but my initial choice was absolutely overkill.
Length of Lifter Arm (inches) × Weight Class (ounces) × 1.67 = Desired Stall Torque (oz-in)
Response: Mark J. Aha! Cam lifters were not a thing when the lifter FAQ was written. "Length" in the above formula refers to the maximum distance perpendicular to the axis of rotation at which your opponent might be lifted -- which in the case of a cam lifter will generally refer to the width of the blade rather than the length that gets shoved under them. I've added a section on cam lifters to the lifter FAQ.
Q: Additionally, I didn't consider that the blades could be run off a single servo. What kind of linkage system would you recommend for this application? Upon digging through Wikipedia and remembering an old Youtube video, my best guess would be some sort of modified "Watt linkage", but I would like to hear your thoughts.
A: A Watt's linkage is tricky to get to work for this application, and it isn't needed. Although 'Supreme Ruler' and 'Needle/Ace' rotate their blades in opposing directions it's more efficient to rotate them in the same direction to avoid scraping them across the underside of the opponent as the edges converge. A simple linkage setting a tie rod between two "steering" arms and running a drag link to the servo will work nicely. As the blades draw closer to vertical the torque requirement is reduced, so you don't need to get fancy. I leave trigonometry behind this as an exercise for the reader. Crude animation:
There are some refinements available, but it's late and it's been a long day.
Addendum: Your wiring diagrams were extremely helpful, and your suggestions will certainly help me lower the amount of weight I need for electronics. [Hidden in an iCloud]
Q: I've heard of ants making a simple hub motor mount for weapons out of TPU, but I'm not sure how they do it. Is it a press fit? Do they use set screws? [Sacramento, California]
A: Mark J. I'm going to interpret "ants making a simple hub motor mount for weapons" as an antweight direct-drive weapon hub rather than a mount for a weapon hub motor made by ants. Tell me if I'm wrong.
I don't believe I've seen a good match for what you describe. I've seen soft and squishy TPU sandwiched between motor/hub and around the mounting screws to cushion the motor from impact shock, but I can't recall seeing a single-piece TPU weapon hub. I can tell you:
I would not trust a compliant weapon hub for a US antweight held in place solely by a press-fit; and
TPU is far too soft to hold set screws.
If you could provide a 'bot name I might be more helpful.
Q: Specifically I'm talking about this one horizontal spinner named Cheesecake. I know he has a lot of videos posted on his channel but I haven't found one yet explaining the hub. I was just hoping to see if you had any insight thanks.
A: That gives me something to work with. I wrote to the builder of 'Cheesecake' and asked for some details on the weapon hub. Alex F. was kind enough to write back quickly and confirm my assumption that the hub bolts to the top of the weapon motor, but the design has a very clever method of locking the weapon blade in place:
Yep! It's TPU, and it's bolted through to the rotor. Let me see if I can get a good illustration...
A very few minutes later...
Here's a cross section of the hub. It pushes down onto the motor, is held down by three screws, and the act of pushing it down onto the motor causes the collet to flare out, clamping onto the inner diameter of the weapon bar.
Thanks Alex!
I also found this side view of the weapon hub that shows the slots in the side of the hub that allow it to flex enough to 'click' down into the hole in the weapon blade before the assembly slides down over the rotor and is secured by screws into the top of the rotor:
I think that gives a good overview of the weapon hub design.
Can You Trust the Gerbil?
Q: Hello, I am wondering what thickness of TPU I should use for my beetleweight minibot? I started off with quarter inch, but it turned out to be to heavy. What should I do? [South of Hartford, Connecticut]
Pssst! Down here... It's me, Roger the Web Gerbil.
Mark is kinda touchy about questions that don't provide the silly little details that might give him a chance to come up with a quantified answer. This time though, I think he missed something. I have access to all of his notes and the exotic software that he doesn't share with the robot community, and I found this equation filed under "If All Else Fails":
Plugging your values into the equation I get: "Make it thinner 'cause making it thicker won't make it lighter."
Two Control Pins
Q: Hi, I need help finding a dual motor esc that is brushless and has 2 control pins going to the receiver. [Bay Area, California]
A: Mark J. I'll guess this is for an insect class robot? I'm not entirely sure what you mean by "2 control pins going to the receiver". A typical dual ESC will have either:
Two 3-wire cables: each with (black) ground, (red) 5 volt+, and (white) signal wires; or
One 3-wire cable as above plus a second (often yellow) signal wire with its own single-pin plug.
If you are looking for an brushless dual motor ESC for a receiver with a serial protocol (IBus, SBUS...) I know of no such beast.
Left Weapon Spins Clockwise
Q: Hello. I was examining a previous question about the successful 3 lb robot Droopy regarding how it produces motion through torque reaction and gyroscopic precession. I have some questions:
1. Let's say my layout uses a two-blade system akin to Droopy's that spins the left blade counterclockwise and the right blade clockwise. Would the body spin clockwise (right) due to the conservation of angular momentum when the left blade is given more power than the right, and the body counterclockwise if the right is given more power? I want to make sure my understanding of the post is correct before continuing.
A: Mark J. I think I may be responsible for your confusion. My original description of Droopy's locomotion had an error in the direction of weapon spin. I promptly corrected that description in the Ask Aaron Design and Construction archive, but I suspect you were reading a copy of the original version that I had overlooked in the 'Ants, Beetles, and Fairys' archive. I corrected that version today.
Here's the correct description as given to me by Droopy's builder Tommy Wong:
Acceleration of the blade on the left side (clockwise rotation) causes the right side to lift up and pivot due to the simultaneous effect of two variables:
Torque Reaction - The conservation of angular momentum causing the bot to rotate in the opposite direction;
Gyro Effect - Rotating the gyro-causing vertical component CCW off axis, lifting the right side, analogous to a vertical spinner being rotated CCW.
2. If I were to program this robot such that I could flip a switch or push the throttle to make the weapons spin in the intended directions (at say, a constant 50% power), and use the aileron to shift the power from (50% left, 100% right) to (100% left, 50% right), how would I set it up/go about doing so? Is this even remotely possible?
A: Most transmitters are capable of this type of multi-channel mixing. It is essentially a simple(?) modification to the standard robot Elevon mix. A transmitter using OpenTX or EdgeTX firmware would have a very different setup than say a FlySky FS-i6, but such a control scheme is entirely do-able. If you tell me what transmitter you have and where you want the controls located, I can assist.
Sorry if the hamburger is bad. Thank you for your time. [Hidden in the iCloud]
The hamburger is fine and you are welcome. My apologies for causing your confusion.
Q: Hi, gyro walker person from yesterday here.
1. I originally had the spin direction opposite that of Droopy's (left CCW & right CW rather than left CW & right CCW), but in hindsight there must be some merit to spinning the blades in the latter directions given Droopy's success, so let's use that as a reference.
A: There is considerable merit to spinning the blades in the directions used by Droopy. You can try the reverse directions to see the result but I don't think you'll like it.
2. While I don't have the transmitter on hand, I plan on using the FlySky FS-i6. This is a rough idea of my proposed control layout:
Throttle UP: both weapons spin up in the intended directions at 50% power
Throttle DOWN: both weapons are stationary/slowed down to a halt
Aileron LEFT: robot turns left (100% power left blade, 50% power right blade?)
Aileron RIGHT: robot turns right (50% power right blade, 100% power right blade?)
If there is a more optimal control layout that you think would work better, I am open to input. Hope this helps.
A: I feared you would want to do this with a FlySky FS-i6. The available pre-programmed mixes assume reversing ESCs with "off" in the center, but you'll be using single-direction ESCs that interpret the output from a spring-centered joystick as 50% throttle. For safety purposes we can't have your 'bot snap to 50% throttle on two spinner weapons as soon as the transmitter is turned on. To safely attain the precise controls you've specified we'll need to use custom mixes. The FS-i6 has only three custom mixes -- that may not be enough...
Three Hours Later...
OK, I think I've got it. I don't have a FS-i6 in my workshop at the moment so I'm unable to test, but it works on paper. Suggest you read thru my FlySky FS-i6 Combat Programming Guide to become familiar with navigating the FS-i6 menu system and the processes for entering new values into the function fields.
This is where I warn you that the project you have chosen is not an easy build. The machine itself will be difficult to construct, will likely require multiple revisions, and you will find the process of entering and troubleshooting the required transmitter programming frustrating and daunting.
Here we go...
Receiver ports:
The ESC controlling the left motor will plug into receiver port 3
The ESC controlling the right motor will plug into receiver port 4
In the FS-i6 "Functions" menu:
Set Endpoints for CH3 to low side: 100%, high side: 0%.
This limits the "throttle" stick input to 50%.
Set Endpoints for CH4 to low side: 0%, high side: 0%.
This prevents unintentional "rudder" input from impacting the motor speeds.
Set Custom Mix#1
Mix is: ON
Master: CH3
Slave: CH4
Pos Mix: 100%
Neg Mix: 100%
Offset: 0% May need to be 50%?
This mirrors the "throttle" stick output to receiver ports 3 and 4.
Set Custom Mix#2
Mix is: ON
Master: CH1
Slave: CH3
Pos Mix: 0%
Neg Mix: -100% May be better at -50%
Offset: 0%
This adds throttle to the left motor when the "aileron" stick is moved left of center.
Set Custom Mix#3
Mix is: ON
Master: CH1
Slave: CH4
Pos Mix: 100% May be better at 50%
Neg Mix: 0%
Offset: 0%
This adds throttle to the right motor when the "aileron" stick is moved right of center.
I might hope that a kind reader with an FS-i6 might enter the above settings and report the channel outputs from the "Display" function.
At 'power on' channels 3 and 4 should be at "low" output - all the way left in the display.
As the left vertical "throttle" stick is raised from bottom to top, channels 3 and 4 should increase to 50% power - centered in the display.
As the right horizontal "aileron" stick is moved to the right, channel 4 should increase to maximum - all the way right in the display.
As the right horizontal "aileron" stick is moved to the left, channel 3 should increase to maximum - all the way right in the display.
This is all too 'fiddly' to be right on the first try...
Reply: Hi. Thank you so much for answering my torque reaction walker inquiries. I know it was probably a hassle, but I greatly appreciate your help and commitment to this website.
Response: You are very welcome. I enjoy a good challenge, and I have a need to pay back the combat robot community for the help given to me when I needed assistance.
I've received a bit of feedback on my proposed mixes that confirms the approach is correct. I've annotated the mixes above with possible improvements. Still waiting on actual confirmation from someone with an FS-i6 in hand -- stay tuned.
Puttin' On a Gear
Q: How would I go about attaching a printed gear to the output of a servo? Right now I can only think of gluing the gear to a servo horn but I presume there's a better way. [South of Sacramento]
A: Mark J. For components that will disable a critical system if they fail, the general rule is No Glue - No Tape - No Zip Ties -- but there are exceptions. It would greatly help to know how large your gear is and what you will drive with it.
It is possible to print an internal spline to match your servo output shaft -- but it takes a higher-resolution printer than you likely have. 3D printing RC servo splines.
If your gear is large enough, you may attach it to the top of a servo horn with small screws. This is not ideal for highly stressed components.
You can purchase pre-made gears with splines that will directly attach to servo splined shafts from sources like goBILDA and Servo City.
You can file/cut away the splined servo output into a square output shaft and print a gear to match. You will lose the fine position adjustment of the splines, but your transmitter may be able to adjust well enough for your application.
I forgot one:
Print the gear with a 'press fit' onto the servo shaft. You can then drill an axial hole along the junction of the shaft and hub and drive in a self-tapping screw to locate the hub and lock rotation. This is known as a 'Dutch Key'. Machinists consider this sloppy work but it can be useful.
May or May Not
Q: What shaft to use for a beetleweight robot? [Indianapolis, Indiana]
A: Mark J. 'Ask Aaron' is not a free engineering service. Even if we were, no competent engineer would spec material or thickness before knowing a great deal more about the design than you have told us. The Hamburger is Bad.
If you're asking about a weapon shaft I can tell you that the Repeat Robotics Peter Bar Beetleweight Weapon Kit uses a 5/16-18×5" Grade 8 bolt, well supported on both ends. Something similar may or may not be suitable for your unspecified design.
I Have Pages for That
Q: I should really stop saying "I'm not going to be back until (insert date here)" because that's the signal for my brain to start thinking of some crazy combat robot ideas : /
Anyway, I'm going to attempt my first ever bot with an active weapon! It's a 150g drum spinner, and I've gotten most of the design figured out. The only part I need your help with is the weapon.
What's the best motor to use for the spinner? I tried searching the archives for "fairyweight weapon motor" but couldn't find any recommendations. [West of San Antonio ✪]
A: Mark J.
Inputs of "150 gram robot" and "drum spinner" do not give me enough information to supply a "best" output. Weapon motors should ideally be matched to the Moment of Inertia of the specific weapon which varies with the mass, shape, and specific dimensions of the weapon.
Your drum might be very large for a fairyweight and have a large MoI;
Your drum might be very small for a fairyweight and have a small MoI;
You may be spinning the weapon with a belt drive with some amount of speed reduction from the pulleys;
You may be driving the weapon directly.
I can provide two bits of general guidance:
The weapon motor size formula in the Ask Aaron Brushless Motor Selection Guide pumps out a motor weight of about 12 grams for a fairyweight spinner. That is a VERY GENERAL number to make sure you're in the right ballpark.
'BE 1806 2300Kv' outrunners (18 grams) are popular weapon motors in both kit and scratch-built 150 gram spinners. They are a little 'overkill' but should be adequate for a direct-drive drum about which I have been given no other information.
Q: I was also gonna make it a single-toothed spinner. Is there a method to calculate the center of gravity of an asymmetrical spinner? If needed, I can send a picture of the weapon.
A: A good CAD program will give you the CG for an object like an asymMetrical disk or a single-toothed drum. It isn't something reasonably done by hand calculation.
I'll mention that single-toothed drums are a pain to machine and balance. A common and very effective substitute is a symmetrical drum or beater bar with offset teeth: two teeth set outboard on one side, and two teeth (or one wide tooth) centered inboard on the other side (see images below). For insect-class robots the 'teeth' are often large screws inserted into threaded holes for ease of replacement. Several beater bar kits are made this way.
Q: FACE PALM - I never considered making the drum with offset teeth! That idea is much better than what I was thinking. Thanks!
The spinner will be belt driven, so there's much less strain on the motor that way. I take your building advice seriously. : )
Lastly, are there any kits for fairyweight spinners? Robot Combat Wiki doesn't list any. If there are, then I could use all the electronics and make a custom chassis for it.
-sincerely,Iceywave
A: Glad to hear you've been paying attention about direct-drive spinner motor strain -- but you'll find that almost all 150 gram class spinners are direct drive. The Square-Cube Law tells us that you can get away with higher stress loadings on small structures (like a 150 gram robot) than you can on larger structures (like a heavyweight robot). The smaller the robot, the less sense the added complexity of weapon belts and pulleys makes. For a fairyweight drum supported at both ends, direct-drive is worth considering.
I know of no full kits for fairy spinners -- but Bristol Bot Builders offers component kits for this weight class:
A 150g Drive Kit: wheels, motors, motor mounts, receiver, dual ESC and assorted hardware.
You may not want to order your parts from the UK, but you can certainly use the kits as a parts list and grab components from US sources. I would likely build around a Malenki Nano integrated receiver/ESC.
What About the Blue Wire?
Q: so hi i just recently got into this amazing hobby and I am making an antweight for the first time. I want to have tank drive so i can move two motors sepretley so I bout an esc made to control 2 motors. There is a small issue though i cant find anywhere how to control the two motors with one esc separately so i can turn. I can do this because there's only one output wires so i can either have all motors on one channel or another.
For reference I'm using the flip sky dual drive bidirectional esc (brushed) with two brushed motors hooked up to the basic fly sky i6x receiver controller by the fly sky i6x please ask questions if you need more details to help. [Dillsburg, Pennsylvania]
A: Mark J. One of the problems with buying an inexpensive product for your combat robot is the lack of an understandable user manual. The instructions available on the FlipSky product website are a poorly translated jumble of incomplete and contradictory information, and their wiring diagram is entirely wrong. Before purchasing a component for your robot I recommend that you download the manual to see if it makes sense to you. If it does not, find another product.
Let's see if I can sort this out for you. In your problem description you refer to "only one output" wire:
I'm guessing that you're talking about the 3-wire (white/red/black) cable that plugs into the CH2 port on your FS-iA6 receiver.
There is also a single blue wire with a small connector. That wire plugs into the top pin of the CH1 port on your FS-iA6 receiver
I've made a sketch of an 'end view' of your receiver showing the receiver ports. I've color-coded the pins that the two receiver cables plug into:
There is a tiny three-position switch near the blue wire attachment to the circuit board. This switch is used to select one of three ESC modes. One of these three modes will give you single-stick "mixed" control of the two motors on CH1 and CH2, but the FlipSky instructions for the switch make no sense:
This ESC has three modes, choose it before before power on (toggle switch):
Mixed control (differential speed) control mode by toggling switch inward
Two-way synchronous control mode (switch inward)
Two independent control modes (switch extension)
From your description you have the ESC in synchronous control mode -- both motors controlled by a single R/C channel. Move the toggle to another position and see what control result that gives you. If that isn't right just power down, move the switch to the third position, and power back on.
If I did not correctly understand your problem, please write back with more information -- and next time spend an extra couple dollars to buy an ESC with readable instructions. It will save us both a lot of time and trouble.
A Novelty Entertainer
Q: It's me, Iceywave. Here to ask an actual question about building a robot!
I'm planning on making an antweight torque-reaction thwackbot - and before you ask, NO, I don't plan on winning any tournaments with it. It's a "novelty bot" intended for entertainment purposes and I was wondering about which 'SilverSpark Motors' to use for the drive.
Its design will be similar to the 2020 version of 'Axe Backwards' and (if my calculations are right) I should have around 6 to 4 ounces left to dedicate to the weapon, which will be sticking out about 3.5 inches from the circumference of the wheels. So, should I use high torque/low speed or low torque/high speed 'SilverSparks' for the bot? Even though it's a "novelty entry" I still want it to get in some good shots during a fight.
I hope I gave enough information. If I haven't, just let me know and I'll try to provide more details.
- sincerely, Iceywave [West of San Antonio ☆]
A: Mark J. 'Axe Backwards' drew power from its full-body vert spinner to add power to its axes, but a true torque reaction thwack get its weapon power from the Newtonian reaction of a drivetrain attempting to accelerate/decelerate the mass of the robot chassis.
To directly answer your motor question:
The greater the torque that is applied toward robot acceleration, the greater the counter-torque that is applied to flip the chassis/weapon over in the other direction. This might lead you to believe that the better gear motor for the purpose would be the one with the greater torque, but torque is traction limited. This means that as soon as the drive train generates enough torque to break traction and spin the drive wheels no additional counter-torque is available to power your weapon strike.
Pull up the
Tentacle Drivetrain Calculator and enter the specs for your motors and chassis. You will likely find that you will not require a low-speed Silver Spark to get enough torque to max-out the thwack performance -- particularly if your traction is reduced by having a big lump of mass out on the end of a weapon arm that leaves only 10 or 12 ounces of weight on the drive wheels.
Ideally your design should have large enough wheels to allow some counter-balancing mass to be placed behind the axle. This makes it possible to have a heavy weapon tip and still generate enough reaction force to achieve good weapon acceleration.
Q: I was wanting to know what 'Silver Spark' motors would work best for my design. I figured high speed would cause the bot to-while providing a much quicker attack—thrust backward, meaning that it wouldn't hit the opponent unless the opposing bot was moving towards it. Or worst case scenario, the motors wouldn't have enough torque to turn it over, rendering it useless. I was wondering what your thoughts are.
-sincerely,Iceywave
A: Let me expand on my answer a bit.
Once you have enough torque to break traction and spin the wheels, adding additional torque will not accelerate the robot faster and it will not improve the reaction hammer action.
I wrote my original response before I had your design sketch. It looks like the wheels are about five inches in diameter. For purposes of your weapon actuation that means you've got about 3/4ths of a revolution to make things happen before you're out of range -- half a revolution for the weapon and a quarter of a revolution of the wheel.
Let's run some numbers thru the Tentacle Drivetrain Calculator, shrink the arena size to 0.8 foot, and compare the acceleration of a couple of the Silver Spark gear ratios thru that short distance:
The 50:1 Silver Sparks at 7.4 volts in an ant with 5" wheels have a top speed of 4.22 MPH and will spin from a dead stop thru 3/4ths of a revolution in 0.41 second.
The 100:1 Silver Sparks at 7.4 volts in an ant with 5" wheels have a top speed of 2.11 MPH and will spin from a dead stop thru 3/4ths of a revolution in 0.38 second.
Torque has been limited by traction such that acceleration and weapon actuation for the two motors is essentially identical. I'd pick the 50:1 for their greater speed. Note that attacks made by decelerating as you approach your opponent are also traction limited, but breaking from a higher top speed may impart greater energy into your weapon.
If your weapon won't 'turn over' because it's too heavy you'll need to swap some weight from the weapon tip to out behind the axle to partially counter-balance the mass -- or maybe boost voltage to a 3S battery. A lot of reaction hammer design is trial and error.
Reply: Thanks for all the very helpful info! You just managed to clear up so much confusion that I couldn't figure out on my own.
-sincerely, Iceywave
Q: I'm back with another question about my thwackbot, which has now been moved to the 'Plastic Antweight' class. Not sure if you can consider that a promonition or a demotion?
Anyway, the wheels are going to be 5 inches in diameter. Only problem? FingerTech doesn't make wheels that size. I was going to print them out using PLA (the same plastic that the chassis is made of) and wrap electrical tape around them to improve traction. Would you happen to know a better way to make custom wheels?
- sincerely, Iceywave
A: Mark J. You are not the first builder to include a component in your design only to discover that said component does not exist. However, having a 3-D printer on your workbench does not automatically make it the best option to solve your problem -- see: The Law of the Instrument.
There are multiple interpretations of "better way". Do you want something simpler? More durable? Providing greater traction? More impressive in appearance? Casting polyurethane tires on custom hubs is popular with builders looking for impressive appearance with optimal traction, but it is certainly not simple. Since your thwackbot is "for entertainment purposes" I think something a little wacky might be appropriate.
In the very early antweight days the popular drive motors were R/C servos hacked for continuous rotation. The rotation was slow, so large diameter wheels were needed to get reasonable speed. It was common to use plastic lids from peanut butter jars with a rubber band stretched around the circumference for wheels - electrical tape does not provide great traction. How about an update of that wheel style?
The screw-on lids for large jars of assorted snack items sold at Costco (and elsewhere) are 4.5" in diameter, made of polypropylene, and weigh 20 grams each.
Drill a bunch of holes around the rim and insert rubber grommets for traction. The grommet thickness will take the diameter out close to 5 inches.
Bolt a 3mm hub like the FingerTech Sumo Hub to the center of the lid and you've got a unique, entertaining, and functional thwackbot wheel.
If you want something off-the-shelf, they do make 5" diameter foam wheels for R/C airplanes. Bodge a hub and maybe coat them with latex.
Q: One more question. I'm currently designing the 3D files for the chassis but I ran into some problems with how I'll attach the top and bottom panels together. I was thinking of using 6-32 'Flat Head' screws (length doesn't really matter) and directly screwing them into place, but I don't know if that will work without the strips inside the holes. If it doesn't, the other option would be a nut-strip, but I don't know how I would attach it to the inside of the chassis.
Anyways, I love that idea for the wheels. : )
-sincerely,Iceywave
A: For un-threaded holes in firm plastic you'll want to use self tapping screws rather than "machine screws" made to thread into pre-tapped threads or nuts. Drill the hole in the piece the screw will bite into just a little smaller than the major diameter of the screw. A 6-32 self tapping screw typically requires a 0.012 diameter hole (#31 drill bit) and makes its own threads as it is driven in place.
There are special "thread forming" screws for low-density plastics like UHMW, but a generic self tapper will be fine for PLA.
Resistance is Not Futile
Q: I've built robots with printed hubs and O-ring tires that worked pretty well, but the O-rings that were mounted on the hubs cracked and deteriorated in a few weeks. The O-rings still in the bag were fine.
I've also used small rubber bands to tension my forks. I again found that the mounted bands became brittle and snapped while the bands remaining in the bag were still good.
The O-rings were from eBay and the bands were from a local discount store. Can I get better longevity with rings and bands from another source? Do I need a different type of rubber? [Social Media]
A: Mark J. Exposure to atmospheric ozone is the downfall of many types of rubber. Inexpensive O-rings made from Nitrile and 'Buna-N' rubber degrade quickly when exposed to air, and natural latex rubber bands have a similar problem.
It's easy to find both O-rings and rubber bands made of synthetic EPDM (ethylene propylene diene monomer) rubber that strongly resists degradation from ozone, sunlight, and common cleaning chemicals. Just keep them away from petroleum oils.
Q: I just finished my antweight after a redesign to give it more interior space: the issue was that the wires took up much more space that I anticipated, even after wire shortening. I put the sizes of the objects in my CAD and I saw they fit with tons of space to spare, and I had most of the electronics just in the middle, no specific parts to hold them. How do I properly plan around the amount of space I need for wires inside my bot? [Cambridge, Massachusetts]
A: Mark J. The amount of space the assorted wires take up in a small 'bot often comes as a shock to new builders. I suggest using both types of CAD to avoid the need for a redesign:
Computer Aided Design: Draw up your 'bot using your new realization of wire space requirements.
Cardboard Aided Design: Build a cardboard box the size of your designed component bay and physically see if you can get everything to fit. Adjust dimensions as needed and transfer back to your drawing.
A few minutes with carboard and tape can save frustration, time, and expense.
Also related to your antweight: shortly before your question arrived a builder in Virginia wrote in and asked that I pass on their experience with a problem similar to your Repeat Robotics drivetrain issues:
Some advice for the person with the RR brushless motor, I recently had the same issue, and it turns out one of the wires had disconnected inside of the motor [Rocky Mount, Virginia]
I'm curious about the resolution of your drivetrain problem.
Reply: It turns out the phase wires disconnected on both my Repeat Brushless motors. I sent the video to Peter and he told me what the issue was, and sent me two replacement motors for free. Great guy.
Slippery Dark Forces
Q: I am a relatively new builder, and I'm looking to build a beetleweight. My idea is that it will be a large, potentially curved wedge, with a small spinner at the top, perhaps similar to the attached picture. It will be 2.W.D and have a very low clearance to the ground, perhaps aided by some magnets on the underside. How would you recommend I accurately bend metal in this way to form a wedge, and do you have any other suggestions and things to consider? I am rather new to this and have never made a spinner before! (I will build a prototype out of HDPE/Polycarbonate first, and then make a proper shell out of aluminium.)
Your photo shows version 8 of Russ Barrow's antweight 'Dark Pounder'. You can see vertical lines along the length of the curved body indicating that the piece was hand formed. Thin metal may be formed to a curve by simply pressing it around a curved surface, moving it a bit, and repeating the process as the curve takes shape. Thicker metal will require a substantial curved form (tree trunk?) and a mallet. It takes time, patience, and a good deal of 'fiddling'.
Team Dark Forces moved on from the large curved structure for the next version of this robot. The photo below shows version 9 of 'Dark Pounder' in front of version 8. Russ found that the fully curved structure of version 8 made it very difficult to slide an opponent up into the small weapon -- they kept slipping off to the sides. Version 9 concentrated on "delivering the opponent to the weapon".
Aluminum is in general far too soft to make suitable armor for the current beetleweight level of competition. When they were made eight years ago, both versions of antweight 'Dark Pounder' shown used formed titanium for their bodies, and other 'bots from the team used 'spring steel'. Current wedge material tends toward thick flat panels of abrasion resistant steel. I don't think that "hammered around a tree trunk sheet metal" is a good bet in 2024.
Too Puny by Half
Q: Can I use a simple 300 rpm torque N20 motor for a flipper on a US antweight instead of a servo? My flipper arm is about 3" long. Will this work well? [Social Media]
A: Mark J. Nope. A Pololu 100:1 N20 @ 7.4 volts puts out about 2 watts of power and stalls at 27 oz·in torque. With a 3" arm it will stall at a lifting force of 27 ounce·inch / 3 inch = 9 ounces. Not a flipper -- not even a lifter.
Note also that the N20 gearboxes are weak. Pololu recommends an absolute upper torque limit of 25 oz·in, so going to a higher gear ratio won't help. N20 motors are used as lifter/flippers in 150 gram UK ants, but they are too puny for this use in 16 ounce US antweights.
Q: My motor seems to be "twitching" and can barely spin up. When it stalls at a high power, the ESC beeps and beeps again when I return the stick to normal. I checked that all the solder connections are good and that everything is wired correctly. I have done no modification to the default ESC settings, and only cut the shaft on the motor. Any idea what could be causing the twitching, or how to fix it? Also, why is it beeping?
Setup:
Repeat Mini Mk. 3
Repeat Brushless Drive ESC
FS2A Receiver
Flysky FS-I6 Transmitter
[Cambridge, Massachusetts]
A: Mark J. If only the one gearmotor behaves this way:
Repeat Robotics warns about using long mounting screws:
These gearboxes can only accept ~2mm of face screw engagement. Using screws that are too long will result in damage.
If the gearmotor shaft rotates smoothly or you've never mounted the motor, forward your question to Peter Garnache: repeatrobotics@gmail.com
If both gearmotors motors behave the same way and their shafts spin freely by hand:
Why is the ESC beeping?
It is common for an ESC to beep on startup if the transmitter stick is not at zero throttle.
The ESC will not operate until the stick is centered.
The second beep comes when the stick is centered as a notice that the ESC is armed and ready.
Why is the ESC re-starting?
Attempting to operate the motor is briefly dropping system voltage below the minimum needed for the ESC to operate.
Why is the voltage dropping so low?
LiPo batteries are shipped with a minimal charge for safety and long term storage.
My guess is that you didn't give your LiPo a full charge after you received it.
Charge the battery and try again.
What if my LiPo is fully charged?
Forward your question to Peter Garnache: repeatrobotics@gmail.com
Tossing the Caber
Q: I am designing an antweight (1lb) brushless lifter that I would like to be able to throw opponents (not into the ceiling or anything). I am planning to use a Flash Hobby 1204 2500Kv motor, connected to an 135:1 planetary gearbox. The lifting arm is 6.22in long, and the robot will be powered by an 11.4V LiHV battery. At 211.11 rpm, do you think that this would be too fast? Would it provide enough torque to not be too slow under load? (The manufacturer does not give stall torque.) Also, I know that brushless motor torque depends on the ESC, so I am planning to use AM32. Would you say that this is necessary? Thanks! [Outskirts of Madison, Wisconsin]
A: Mark J. The unsensored brushless motors and ESCs we use to build small robots are borrowed from the model aircraft hobby industry. They are designed to spin-up lightweight propellers to about 80% of their free RPM and run at that speed for several minutes. That task is far different from being bogged down against a heavy load right at zero RPM and being asked to develop enough torque to accelerate that load upward against gravity fast enough to toss it into the air. It's like asking a chicken to toss the caber -- they aren't made for it.
Add to this handicap your choice of a brushless motor that weighs 6 grams 0.21 ounce and puts out about 50 peak watts of power maybe 10 watts at low RPM on a 4S lipo and you're running 3S. A typical antweight brushless spinner weapon motor that has a couple seconds to come up to speed against its load weighs perhaps 30 grams and pumps out more than 200 watts of power (see Brushless Motor Selection). With that tiny motor you're not going to be tossing opponents into the air, and the strain of moderately quick lifts may drive the motor into thermal failure.
As noted in FAQ #17 Ask Aaron is not a free engineering service - I'm not going to model the performance of your weapon for you. However, we do supply design tools like the Team Run Amok Electric Hammer Spreadsheet that can be used to model flipper designs. A lifter/flipper is effectively an overhead hammer with a very heavy hammer head (your opponent's 'bot) that traverses a small arc.
Two Days Later... My curiosity got the better of me, so I broke down and modeled your flipper performance.
I brought up the Electric Hammer Spreadsheet and plugged in the numbers for your flipper. Rather than make guesses about the torque curve of that tiny brushless motor I put in the specs for a brushed RF-370 motor that has comparable output wattage. The brushed motor has low-speed torque better suited to lifter duty than the brushless motor, so calculated performance will be a 'best case' model. I ran multiple simulations to determine the optimum gear reduction for the RF-370 motor:
Performance:
Energy = 1.00 joule
Time to 45° = 0.11 second
Terminal Speed = 1.04 m/sec
An object launched at a 45 degree angle with a velocity of 1.04 m/sec will reach a calculated height of... 2.76 centimeters.
Plastic Traction
Q: dont know if you have any experience in the plastic ant category, but two things
first, what would be the best infill for high-impact hits with PLA plus
secondly, what would be your thoughts on our current design, and issues that you can spot (yes I know it needs a top-plate, and there are no screws) Ive had problems in the past with traction, so that's mainly what I'm looking for. [Close to Raleigh, NC]
A: Mark J. It's a very pretty design, isn't it.
First question: I claim no specific knowledge in 3D printing PLA, but I do know that more infill equals more strength. How much strength you need depends on your design and expected impact forces. There are two groups on Facebook that might provide more useful responses:
Second question: I don't know what your previous designs looked like, but if you build the current design I foresee continuing traction problems in your future. You've placed your wheels as far back in the chassis as possible to get chassis clearance in both upright and inverted modes. Doing so leaves a lot of the robot's weight supported by those long forks.
Take a look at successful two-wheeled insect spinnerbots. They have the motors as far forward in the chassis as reasonably possible, with battery and electronics hanging out behind to shift the 'bot center of gravity closer to the wheels. Unfortunately, unless you switch to much larger wheels you don't have room for much rear overhang. This is a serious flaw.
The rule-of thumb for two-wheel 'bots is to put ~65% of the robot weight on the wheels. More than that may let the 'bot raise the nose off the floor under acceleration -- see Section 2.2.7 of the RioBotz Combot Tutorial. Given the wheel placement on your design I'm guessing that less than 50% of the weight is on the wheels, which will give both poor traction and poor turning response. No amount of latex or silicon rubber coating on the tires is gonna help.
Q: this was our earlier design, and it had the most abysmal center of gravity possible, which was over the forks, we are currently using 2.25 inch wheels, what do you thing would be the best size option?
A: You're not trying to tell me that the little circle marker on the current design render is the calculated center of gravity, are you? I'm entirely not buying that. Nope. No way.
From a practical standpoint, I'm really not sure your design is salvageable. That very large and heavy spinner hangs so far out in front that it's going to take a lot of mass out behind the wheels to make the drive work:
Flip the contents of the chassis box to move the drive motors to the front;
Find the new location of the center of gravity (CG);
Extend the chassis box rearward and shift enough mass aft to move the CG to a point about 1.5" ahead of the axle line;
See how big the wheels need to be to get the new lump sticking out behind the 'bot off the floor.
Taking a fresh look at the design has me worrying about another problem. When (not if) the 'bot lands on its side it's going to be very difficult to get it back on its wheels. The upper and lower forks will provide a wide and stable base, and the weapon is going to sit very close to horizontal. You're going to be pretty well stuck.
As I said at the start, it's a pretty design -- I just don't think it's practical.
Q: for the forks, would having what basically is a plastic popsicle stick going perpendicular to the forks help us with that issue [of being stuck on its side]?
A: I'm sure you have an image in your mind as to where that stick would be placed and how long it might be, but my psychic abilities are quite limited these days.
You could construct a cardboard model of your design and experiment with adding various lengths of lateral 'spinner bait' to get an idea of how much help they might provide in righting.
You could decide that the design has too many flaws to fix by tacking on doodads.
Q: I was desoldering a FingerTech Silver Spark and the little metal tab that I solder to the wire to broke off. I tried adding some solder to the remaining visible metal but its not sticking well. Is there anything I can do to save it or is the motor just screwed? [Random Comcast Server]
A: Mark J. The photo shows what the inside of the Silver Spark motor 'endbell' looks like. The nylon that the metal tab disappears into goes down a fair distance and becomes the support for a motor brush. If you're REALLY determined to save the motor, you can carve away some of the nylon to reveal more of the brass tab, then use a good quality soldering flux (I use Rubyfluid Soldering Paste) to tin the exposed brass. It will then be easy to attach the power wire.
If your soldering skills are not good, I'd suggest buying a new Silver Spark and saving the gearbox as a spare.
Gotta Be Tight
Q: Hello, I am building a 1lb undercutter bot based on Jameson Go's 'Silent Spring/DDT' build, but am not sure how to securely mount the weapon to the bottom. I am currently planning on using a pulley driven disc on a dead shaft, with bearings in a printed hub/pulley attached to the disc. Do you think a long nut/coupling hex rod in the chassis above the blade with a nylock nut on the end would support the weapon shaft? or what would be a more robust way to make sure the weapon shaft does not go anywhere? I don't want the blade to bend upwards and chop my weapon belt. Attached is a mock up of the basic structure. Thanks! [Redmond, Washington]
A: Mark J. That's how 'Silent Spring' does it: a socket head shoulder bolt with a nylock nut (photos below). Note that you're going to torque this down, so you're going to need a substantial spacer block around the shoulder bolt between the two chassis plates to take the pressure.
I wouldn't trust a printed hub holding both bearings to take the loads this style of weapon will put on it. 'Silent Spring' has the top bearing in an aluminum pulley and the lower bearing in the weapon disk itself.
How are you mounting the weapon motor? Your mock-up appears to have the spinning can pressed into the chassis plates and the motor base sticking out unsupported. Can I assume that's just a rendering error?
Q: On the topic of the latest post of a Silent Spring inspired bot, I can say that it can work out well even with a printed pulley since that is exactly what I did for my 1lb Shock! kits [GrabCad]. The big thing I did was adding a pair of concentric washers between the bearings to take the compressive loads and help axially retain the pulley and disk in place. [Newark, Delaware]
A: Mark J. Thank you, Ryan. Your 'Shock!' weapon design is well thought out with the lower bearing in the weapon disk itself and the upper bearing in the printed pulley. In their question, Redmond spoke of a design with "bearings in a printed hub/pulley attached to the disc" which led to my comment that I wouldn't trust a printed hub holding both bearings. I think you might agree that placing both bearings in the printed pulley/hub would be risky.
Q: Hello, Redmond undercutter guy again. Good call on the bearing spacing, I will implement one into the disc and one into the pulley rather than both in the pulley. The weapon motor will be mounted similar to 'Silent spring', but I have not designed a housing for it yet. However, a few more questions about the shaft mounting:
1. Do you know how he mounts the shaft so securely to the frame? It looks like just a hole in the print with a nut on top, but wouldn't the print deform/break in big hits? Ive been looking around at various undercutter designs but can't seem to find exactly how they keep the shaft from moving or bending at all.
A: The mount is exactly as it appears. Take a look at the assembly instructions for the Shock! kit. Steps 9 and 10 cover weapon assembly. When assembled and torqued down the shoulder bolt turns into a structural element that locks the two chassis plates together. The Shock! chassis plates are cut from high-strength carbon fiber composite material.
'Silent Spring' has a thick, single-plate chassis printed from NylonG (early versions) or NinjaTek Cheetah TPU. Printed chassis plates do flex - it helps to absorb the energy of the hit - but they snap back into place (you hope). Design note: wider spaced chassis plates make a stronger base for an undercutter.
2. How thick of a bolt do you think is safe enough to assume won't break? Thanks!
A: The Shock! kit used this 1/4" shoulder bolt. Please note that a shoulder bolt (about $8) is hardened alloy steel and is MUCH STRONGER than a nasty hardware department bolt made to mount the horn on your bicycle handlebars. Do not go cheap on this critical part.
It's a Zen Thing
Q: If the first design you should build is a wedge, what's the second? [Saratoga Springs, Utah]
A: Mark J. When you have built and competed with your wedge you will better understand:
The magnitude of the challenges presented by differing types of weapons; and
How well your knowledge and skill level suit those challenges.
Your next step will be obvious.
Zen teaches that enlightenment is achieved through the profound realization that one is already an enlightened being.
Mark J. Peter Garnache of Repeat Robotics recently gave comments on the current beetleweight drive motor solutions in answer to a question in the Facebook Combat Robotics group. Peter was kind enough to allow me to repost his comments here as a guest commentary. Thanks, Peter!
Q: What's a good motor & gearbox for beetleweight robots?
A: [Peter Garnache] My recommendations for drive motors from my 6 years of building 3lbers are as follows: Prices rounded - February 2024
Off-The-Shelf Gearmotors
below $1025GA-370 1000 RPM Spur Gearmotor - Super cheap but not very powerful or durable. Various flavors available from eBay or Amazon.
$15Absolute Chaos Double Spur Gearmotor - An inexpensive option with good power and torque. While not as durable as an all metal planetary gearbox, they are a big improvement over the generic spur gearmotors. Added by Mark J.
$26Repeat Compact Brushed - Super light (45 gram) 22mm dual bearing gearmotor with billet 4mm hardened steel shaft. Added by Mark J.
$30Servocity Gearmotor - Tried and true, used on plenty of bots. Kinda expensive tho.
$32Botkits Gearmotor - Planetary 22mm gearmotor built for combat. Hardened steel output shafts and reasonable amounts of power.
$36 to $49Just 'Cuz Dartbox - Super high power brushed planetary with low weight. ESCs are expensive and large to be able to handle the power of the motors. 6mm shaft upgrade is very helpful for direct drive applications.
$45Rectified Brushless - Very light but the 4mm output shaft is suboptimal for direct drive applications. Also available as a $25Brushed Drive.
$51Fingertech Mega Spark Brushless - Plenty of power with the durability of a 24mm planetary. Great option for any drive system.
$55Repeat Drive Max Brushless - Highest power beetle brushless commercial off-the-shelf option. Super chonky 24mm planetary gearbox is unlikely to break in combat. Hardened 4130 steel 6mm output shafts. 6S ready out of the box. $35Same gearbox with a 3S brushed motor.
Assemble-It-Yourself Gearmotors
$42 w/1806Robotmatter Mercurybox - Add a 1806 motor & pinion yourself. Good balance of durability vs weight. Has a 6mm shaft option and the 22mm gearbox is smaller than Repeat Max and Mega Spark while still having larger gears than other 22mm planetaries.
$29 w/1806Hennkwell Conversion - Cheapest 6mm shaft brushless drive around, but requires elbow grease to make and keep working. All you need is:
The 2 metal stages of Hennkwell gearbox transplant perfectly into aluminum ring gear and mate to the BotKits pinion for 2mm shafts. You can re-use the faceplate and motor adapter plate as well to create a 2 stage, ~17:1 gearbox with a 6mm output shaft.
Direct Spur Gear Drive
CustomJust 'Cuz Single Stage Brushless CAD - Requires more engineering work to make happen but with a 1000kv motor and single stage gear reduction it can be the most compact and lightest drive option. It's a 1000 Kv drone motor with an R/C car pinion gear driving a spur gear on the wheel.
Q: For my horizontal spinner, the motor I want to use is too tall for the robot with the pulley attached as well. There aren't any shorter motors available that can handle the current I need to run, and making the chassis taller would be a waste of weight and harm the rest of the design. I've heard that some robots put the pulley directly on the outside of the brushless outrunner. How would I go about attaching this to, say, a BadAss 2305? [I-95, West of Boston]
A: Mark J. The "belt-around-weapon-motor" technique generally uses a custom 3D-printed pulley that presses tightly onto the weapon motor can. The pictured can-pulley for a timing belt was used for the horizontal weapon on the EndBots Vector beetleweight kit (discontinued) that used a Sunny Sky X2212 weapon motor. You can see the pulley being installed on the motor in the Vector assembly video.
I have seen robots that simply wrap a timing belt directly around the can without a pulley of any type on the motor. This 'pulleyless' design requires precise alignment, accurate belt tensioning, and a fair amount of 'fiddling'. I'll note that the narrow flat space on the BadAss 2305 can makes it a questionable candidate for a pulleyless design -- you might consider using a round belt with its easier to design pulleys.
The large diameter of the motor can means that you will not be able to get much speed reduction between the motor and the weapon pulley.
Testing, Testing, 1 - 2 - 3
Q: I already have experience building simple wedge and lifter bots and am looking into building my first spinner. This bot would be a 1 pound antweight. How would I go about building a safe and functional test box where I can practice and test my bot? [Camarillo, California]
A: Mark J. You've made a good decision to build a test box for your antweight spinner. You may want to consider building a test box capable of handling slightly larger 'bots to be ready for your next steps. I know of several guides on the construction of insect-class test boxes:
FingerTech Robotics has put together a very nice set of step-by-step plans for an inexpensive antweight test box, including a parts list you can fill from your local home center.
Absolute Chaos Robotics has an outline of the construction of their new beetleweight test box that includes a bill of materials. The photo they include of their earlier antweight test box made from a steamer trunk may give you some ideas as well.
Just 'Cuz Robotics offers a 12 minute 'How to Build a Test Box' video about their sliding-top beetleweight test box. Includes a bill of materials in the comments section.
Not Designed For This
Q: Hi Mark. Thank you for continuing to run such a wonderful site with so much information for builders of all levels! If this question is already in the archives, forgive me as I could not locate it after quite a bit of searching.
I'm running an antweight and bettleweight lifter using a commercial servo. I power the servo by removing the red wire, and directly running it into the battery power circuit to bypass any voltage limitations in powering from the receiver. I'm also using a directly mounted lifter arm - so torque requirements are pretty easy to calculate.
Many servos I'm interested to try are only rated for up to 8.4 V, and not the ~11V or ~14V I'm running for 3S or 4S. In a combat robot application, what is the risk I would have to over-voltage these servos? How would I know if I'm causing damage? Any idea on how to estimate the limit of over-voltaging?
These servos are designed to be constantly moving as a steering servo on cars and boats or adjusting various parts of a plane, helicopter, or drone. In combat robots, a lifter servo very rarely moves. It doesn't see the same continuous load (which I assume the rating is based on), and only needs to handle very infrequent use, so heat won't build up to the same extent.
I can deal with a little shorter life on my servo since they break on some frequency anyway due to the hits they take. But instant or very rapid failure would be a problem.
Thanks! Matt [Milford, Ohio]
A: Mark J. The components of a servo mirror the components of a robot drivetrain: a PMDC motor, a gear train, and an electronic speed controller. The servo also has a positional feedback circuit that enables it to attain and hold a specific position.
As you point out, hobby servos are designed for very different conditions than they see in a combat robot lifter. The frequent and precise position adjustments they perform in R/C cars and aircraft generally require only brief bursts of torque to achieve the commanded position, and the torque needed to hold that position is commonly a small fraction of their output capability.
The heavy lift and hold operation in your application requires big gobs of torque applied over an extended time period. A PMDC motor has a direct linear relationship between torque production and current consumption, and more current equals more heat in both the motor and the speed controller. Increasing voltage will increase both the speed and stall torque of your servo motor which means still more heat... But although current/heat can lead to servo failure it is not the primary factor that determines the servo voltage rating.
Common R/C vehicles that use servos share design concerns with combat robots in terms of space and weight; a smaller and lighter device is preferred over one that is larger and heavier. The individual components that go into servo control boards (capacitors, power FETs...) can be made with thinner internal insulation and tighter tolerances if they run at lower voltages. This reduces the weight and bulk of the entire assembly. Note that some components see voltage spikes far above the input voltage to the board. If the voltage rating of these components is exceeded for even a moment the component may be destroyed even though current and heat remain within allowable limits.
To directly answer your questions:
The primary risk of over-volting a servo is an immediate and complete failure of the servo control electronics. The secondary risk is longer term heat-related failure of either the electronics or the motor.
The common failure modes do not provide appreciable warning of failure. Servos have no external heat sinks to monitor, and telltale odors will be largely contained by the servo case. I suggest extensive testing of all over-volted components under combat conditions to better understand their limits and avoid failure in competition. Be prepared to destroy a few.
How much (if any) over-volting a specific servo can take varies wildly. Different manufacturers design with differing safety margins. Cheap Chinese servos are known to vary the specs and quality of their components based on availability, so one batch may take over-volting well and the next batch may immediately fail. It's a crap shoot.
Date marker: January 2024
1/10th Scale = 1/1000th Weight
Q: Now I did create an antweight of Run Amok however i might have created the wrong kind of antweight what I meant was actually a fairyweight a uk antweight and the rc you send me a link to was convenient and it would be nice to try and find an rc to create a fairyweight version of Run Amok something a little bit shorter not too short for a fairyweight Run Amok scaled R/C per say? [Erskine, Scotland ☆]
A: Mark J. As I recall, you were searching for an R/C toy to convert into an operational 1/10th scale model of Run Amok, Erskine. I don't remember any specific weight class being mentioned and I don't remember the specific toy for which I sent you a link.
From a scale standpoint, if you shrank 'Run Amok' to 1/10th its size (44 inch length => 4.4" length) it would weigh:
1/10 × 1/10 × 1/10 × 162 lbs = 0.162 pounds
That would make a 1/10th scale Run Amok only half as heavy as a UK Ant. I don't think you want it any smaller! I think it would fit in the required 4" cube for a UK ant -- at an angle. A correct 1/10th Run Amok would have a 2.35" wheelbase and a 3.0" width. R/C toy cars do not advertise their dimensions in such detail. Best luck.
Requires Stupid Torque
Q: I'm designing a beetle with a spring powered overhead axe. A spring is cocked and then released, rotating the axe into another robot. I'm going to be using a 21 in-lb torsional spring. I'm aiming for at least 80 joules of energy storage, and according to my calculations that 21 in-lb torsional spring should hold 105 joules:
Spring constant (k) = 21 in-lb / 90 degrees= 0.233
Does that look right to you? [A Cry in the Darkness]
A: Mark J. No, it doesn't. Converted to kinetic energy, 105 joules is enough to send a 1 kilogram mass better than 10 meters straight up. Springs the size you're considering don't store anywhere near that much energy.
Your potential energy storage equation is correct, but it requires inputs in SI units: 'k' in Nm per radian, 'Θ' is displacement in radians, and the output comes out in joules.
Q: I've been planning a 'Son of Whyachi' style beetleweight and am planning on using a super powerful motor such as a propdrive v2 5050 from hobbyking but was wondering is it too overkill for the weight class? [Social Media]
A: Mark J. The impact power of a spinner weapon comes from the kinetic energy stored in the rotating mass of the weapon rotor. That stored energy is MUCH greater than the instantaneous power available from the motor itself. A more powerful motor will spin a given weapon up to a given speed faster, but your weapon's speed is limited by decreasing weapon 'bite' and the mass allowance for the weapon rotor is decreased by the larger motor's increased mass. There is a "sweet spot" for motor size.
Analysis of current successful spinner robots shows a trend toward weapon motors of a weight that falls along a logarithmic function of the robot's weight [chart above]. A typical successful beetleweight robot has a weapon motor that weighs about 6% of the robot's weight -- a PropDrive v2 5050 motor would make up about 25% of a beetleweight's weight. More information on this weight relationship is available here: Combat Robot Brushless Motor Selection.
Keep it Snug
Q: How much slack is acceptable for a FingerTech timing belt? I use a 22 tooth and 42 tooth pulley for the 4mm wide belt. The center distance I designed is 60.85 mm, which gives me 2.7 mm of slack. Is that too much? [Cambridge, Massachusetts]
A: Mark J. FingerTech's recommendation is to keep slack on their S3M timing belts under 1mm:
"A perfectly tensioned belt will have no slack. If you have a set Center Distance you will probably have some slack (but not much thanks to our many belt sizes!). Under 1mm is good for a robot. Above starts getting sloppy so you will want a tensioner, or maybe go up in pulley size."
A 62mm center distance would give 0.5mm slack with a 74 tooth belt.
How Bad Is It?
Q: Do you know the dimensions for the 33.3:1 SilverSparks? If I have to build a new one, I'll probably 3D an entirely different chassis.
A: Your Viper kit has a pair of 33.3:1 SilverSpark motors, do you not have a way to measure them?
Gearbox Length: 12.7mm (0.50in)
Motor Length: 28.7mm (1.13in)
Gearbox Diameter: 16mm (0.63in)
Shaft Diameter: 3mm (0.12in) with flat along length
Q: Hi Mark, sorry about not measuring my motors myself. I... I don't know what went through my head but I clearly didn't think I could measure it myself for some reason. Probably teenage anxiety.
P.S. - Is this hamburger bad?
- Sincerely, Iceywave.
A: I've seen worse -- but yes, it's bad. I give it four pickles on the bad hamburger scale.
You Need to Add Something
Q: how do I connect power to my servo? [Vestal, New York]
A: Mark J. That depends on your servo, receiver, and battery, Vestal.
A standard servo operates in the 5 volt to 6 volt range and is designed to be powered from the receiver power buss. Its flat 3-wire cable plugs into your receiver, with the three wires providing it with voltage, ground, and a position signal from the receiver.
The LiPo battery connects to your Electronic Speed Controller (ESC);
The Battery Eliminator Circuit (BEC) in the ESC reduces the battery voltage to a level suitable for your receiver; and
The flat 3-wire cable from the ESC carries power to the receiver.
Plugging you servo into the receiver passes on the BEC power to your servo -- but there is a problem. The tiny BEC in your small ESC produces enough current to operate you receiver, but not enough to operate a servo. You will need a more powerful BEC. See this post in the Ants, Beetles, and Fairies archive for more details and a circuit diagram.
Some servos and receivers can operate from higher voltages and may be wired to bypass the BEC entirely and be powered directly from the battery. I happen to know that your receiver cannot operate at LiPo voltage, but maybe your servo can -- check the servo spec sheet. If the servo can handle your full battery voltage it may be wired to as shown in the diagram below.
One of Many
Q: Can I run brushed nerf motors on Vex 29's or would that be too much for them? [Sacramento, California]
A 'NERF motor' is a general class of 130 or 180 size brushed motors that have been modified for greater performance in NERF dart guns. There are many varieties that range in performance from fairly mild amp-sippers to wild current-chugging monsters.
For any motor the current draw is proportional to the load placed on it. A given motor running a high gear reduction with a small diameter wheel in an antweight will pull much less current than the same motor running a low gear reduction with a large diameter wheel in a beetle with magnetic downforce.
So, a NERF gearmotor like the DartBox Viper in a beetle on 11.1 volts with 1.5" wheels is likely within the capacity of a rewired Vex 29 ESC. However, a DartBox Dragon on 14.8 volts with 3" wheels and 6 pounds of downforce is likely to convert your Vex 29 into smoldering embers.
Public Service Announcement - Insect class 'bots REALLY do not need the stupid levels of power and speed available from either brushless or Nerf motors. In a typical insect arena it just can't be put to effective use -- even with a heap of magnet downforce. The stock BotKits motors are plenty. What we're discussing here is just for showing off.
Is That a Typo?
Q: I cannot find this in the archives - how do you connect the malenki nano to the KST DS215MG servo? The servo has a black, brown and red wire. Thanks in advance. [Franklin, North Carolina]
A: Mark J. Black, brown, and red wires are not standard -- is that a typo? The KSTs I've seen have yellow, brown, and red wires:
Voltage + is always the center wire in the servo wire ribbon (red).
Voltage - is the darker of the two outside wires (brown).
Signal is the remaining outside wire (yellow).
The diagram uses Futaba wire colors:
Voltage + is red;
Voltage - is black;
Signal is white.
Too Many Eliminators
Q: I am making a beetleweight (3lb) combat robot, but I have an odd problem. The weapon esc has a switch to turn the weapon motor and esc on and off. When turned off the drive motors work perfectly. When turned on the weapon works as expected but the drive motors stop responding and just twitch.
I tried fixing it using seperate batteries for the weapon and drive, but the issue still persists. I've attached a parts list and a photo of the wiring. What can I do to fix this? [Social Media]
A: Mark J. You have a rather odd selection of R/C car components and electronics. I'm not sure they are all 'battle ready' for use in a beetle, but I think I can help you at least get them all running together. I've converted your wiring photo into a diagram for clarity:
First: The drive ESCs are dual-channel units made to independently control two motors with up to five amps of current to each motor. You have both outputs wired together to power a single motor, presumably an attempt to get ten amp capacity from the ESC. This Does Not Work! Each of the two independent channels has its own has its own 'clock driver' to time the output power pulses, so the power pulses will be out of synch with each other. This can cause both control and power consumption issues.
You haven't had these problems because only one half of each drive ESC is operable. The 'white' lead wires that control one half of each ESC are connected to the receiver, but the other half of each ESC is controlled by the yellow lead wires which are not connected. We need to correct this wiring issue, but it is not the source of your failing drive ESC problem.
Second: A voltage mismatch is the reason your drive motors stop when the weapon is turned on. The BEC in the weapon ESC has a higher voltage output than do the BECs in the drive ESCs. When the weapon is turned on the increased voltage on the receiver power buss to which they are all connected causes the drive BECs to shut down, which removes internal power to the drive ESCs. Poof, no drive motor response.
Remove one of the drive ESCs entirely.
Rewire the remaining ESC with one set of red/black output wires to one drive motor and the other set of red/black output wires to the second drive motor.
Connect the currently unused yellow lead from the drive ESC to the receiver on the channel previously used by the removed ESC -- plug it onto the same pin previously used by the white lead of the removed ESC.
Cut and tie back or remove the center red wire in the three-wire cable from the drive ESC to the receiver. This will isolate the drive BEC from the receiver buss voltage and prevent it from shutting down.
Here's the new wiring diagram:
Inserts Don't Hold
Q: Over the past few events I've had screw inserts ripped out of hdpe parts causing the whole wedge to ripoff. Is there an optimal way to connect two plates at a 90 degree angle? Or does it vary by what material is being used? What approaches are commonly used? Could you please respond to this in relation to us beetles? [Redmond, Washington]
A: Mark J. Threaded inserts in hard plastics are OK but in soft plastics like HDPE or UHMW polyethylene they are - as you have discovered - sub-optimal. The grip obtained in a soft plastic by heat-set, self-tapping, or press-in inserts just won't handle high-load impacts -- the plastic has too much flex.
I have three options for you:
Many insect-class builders use simple wood screws for this type of connection into soft plastic with good results -- but there are special 'thread forming' screws made with widely spaced and deep threads that are designed to hold in thermoplastics; search for "Plastite" screws. Pick a suitably long screw, drill the correct diameter pilot hole, and sink 'em in.
If the part will be removed frequently you can cross-drill the chassis and install barrel nuts with machine screws. This type of fastening is commonly used in assemble-it-yourself furniture. They're more work than simple screws and you may have trouble finding small barrel nuts, but the screws will break before they pull thru.
Although it has fallen a bit out of favor you can certainly use FingerTech Nutstrip. It adds a bit of weight and takes up a little room, but it holds well.
Low / Hard / Both / Neither
Q: Hi & thanks for sharing all these questions and answers!
I'm a new builder & I've been having fun trying to create a design for an two wheel antweight undercutter. (I am planning to make a wedge bot first when it comes to constructing something). I'm having a really hard time understanding a few of the details of how to mount the weapon and the weapon shaft. Some more important context: I've been thinking of making almost everything out of laser cut metal with some sheet metal bending & tying it together with bolts. Also, I'm aiming to use a pulley to separate the weapon shaft from the weapon motor. Here I have two questions.
First for the weapon shaft, I've seen the horizontal spinner diagrams you've posted but these seem to have a flanged bearing / bearing housing with bolt holes to attach to the frame, to handle the axial loads (eg from being hit by a vertical spinner). (Those diagrams were really helpful, BTW, in helping me understand that it's important to think about the inner ring and outer ring of the bearing separately!) Most bearing housings I have found are very large but it does look like some small ones can be purchased (although they seem expensive to me and might be heavy). Perhaps I am searching for them in the wrong way. Anyway, would you think such a bearing housing is reasonable in antweight robots or would it be better to use some sort of washer with holes in it to hold a flanged bearing in place? I keep thinking that there must be something simple here that I am missing.
Second I have it in my head that it's better for an undercutter to be as low to the ground as possible (without hitting the floor), partly due to reading some of the answers here talking about how drum spinners are more vulnerable on the bottom of their rotation. But having a whole hub and bolts (and sliding foot, for a 2 wheel design) below the weapon seems to leave a lot of space. I can imagine putting the hub above, but then there are still the bolts. They could be countersunk / counterbore, but that presents some issues as well: I'm not expecting the weapon to be very thick anyway; I don't have a machine shop; and if I have my parts made at a laser cutting service, it doesn't look like I can get countersunk / counterbore holes unless I use a softer material (e.g. chromoly instead of ar500). I have a feeling that one or both of these goals (hardest weapon material or lowest ground clearance) is not so important for antweight. Can you help me understand to what extent these are reasonable things to chase after for an antweight? Or do you know of a more one-sided mounting method that I could try? Thanks! [Silver Spring, Maryland]
A: Mark J.
You did a fine job of pouring thru the Ask Aaron archives in search of examples and diagrams, Silver Spring. The drawings you found are somewhat generic and include elements necessary for larger robots than you are planning. I think you may have missed one post down at the very bottom of the Ask Aaron Robot Weapons archive that is more relevant to an insect-class robot:
Q: do you have a drawing on how an undercutter blade is fixed to a bot? i know u did for a fbs and a horizontal spinner. [Quebec, Canada]
A: Mark J. The mounting is pretty much like an FBS turned upside-down. The weapon shaft is rounded on the ends and usually sticks out both below the blade and above the chassis to allow the 'bot to invert.
For an antweight robot you may simply insert flanged ball bearings into the chassis from the pulley side and secure them with a drop of Loctite/epoxy/prayer. Assuming the pulley and any needed spacers take up the full gap and the top/bottom chassis sections are fastened together nearby, the bearings will remain in place. Note that some builders prefer to use flanged oilite bushings instead of ball bearings as the oilites are less likely to fail from a heavy impact. The bearing housings in the McMaster link you included are neither required nor suitable as they are roller bearings which are not designed for axial loading.
There is such a thing as too low for an undercutter blade. You want enough clearance to keep the blade clear of arena floor imperfections and to avoid floor contact if the 'bot gets 'jostled' from a hit. Having a weapon that is more dangerous to your 'bot than to your opponent is embarrassing.
The clamp-style shaft mount is superior to other styles for small-diameter shafts.
The hub is made to accommodate a 1/8th inch shaft but is easy to drill out to larger sizes.
The squeeze clamp blade mount is both secure and simple for an antweight with no chance of shearing off thru-bolt mounting hardware.
Make the blade hard and don't sweat the low.
Time Out to Pump!
Q: I have thought about converting a bb gun that you pump to store up pressure with into a pneumatic flipper is this possible? I'm guessing that you'll give me a cheerleader button. [Vestal, New York]
A: Mark J. No, Vestal - the Cheerleader doesn't get this one. You haven't told me much about your conversion plan, so I'll have to guess:
Before the match you pump up a very small air gun chamber for your flipper.
At an opportune moment in the match you pull the converted trigger mechanism with a servo.
With a bit of luck you manage to flip your opponent and exhaust the air chamber.
Then you call a "time out" while you pump up the flipper again?
Q: One pump fires one bb. I will have a strong geared up servo pump it once and then I will have another servo pull the trigger.
A: Combat robot weapons typically measure their power in joules of kinetic energy, and there isn't much energy output from a single pump on an air pump gun. The kinetic energy of a moving object is equal to half of the object's mass (1/2*m) multiplied by the velocity squared. A 'BB' pellet weighs about 0.35 gram and a small pump air gun might propel a 'BB' at 200 feet-per-second from a single pump. This works out to 0.65 joule of kinetic energy.
That level of kinetic energy does not compare well to other weaponry available: the 5-inch plastic bar spinner weapon you have written in about previously stores about 56 joules. Given the weight, complexity, and construction challenges of a converted air gun flipper I think it is a poor choice.
Q: how do you convert a servos lifting power to a servos crushing power? [Vestal, New York]
A: Mark J. Hobby servos do not come with a rated 'lifting power'. The specs they come with are:
Speed: The time needed to traverse 60 degrees of rotation under no load; and
Stall Torque: The maximum (stall) torque the servo can produce.
When coupled to a simple lifting/clamping arm, the 'crushing' power of this servo is the stall torque of the servo divided by the length of the arm as measured from the center of rotation to the point of clamping contact.
Example: The Zoskay DS3235 35KG Servo operating on 7.4 volts has the following specifications:
Speed: 0.11 sec (at no load)
Stall torque: 35 kg-cm
Attached to a 14 centimeter clamping arm, the 'crushing' power of the servo is: 35 kg-cm ÷ 14 cm = 2.5kg.
Calculating the lifting power of the same servo and arm requires the inclusion of a de-rating factor: actual lifting requires the arm to move rather than just hold the weight at stall. Reducing the maximum lift load rating by about 40% allows for good lifting speed and less servo stress-- as discussed in the Ask Aaron Electric Lifter FAQ.
Two additional notes:
Servos don't like to spend a lot of time stalled at full load. They don't live long under these conditions.
High torque servos require more current than the small battery eliminator circuit in an ESC can provide. Either wire them directly to a battery of suitable voltage, or provide a stand-alone high-current BEC. This is also covered in the Electric Lifter FAQ.
One of These is Theoretical
Q: Hi Mark. Thank you very much for your advice (and Aaron's) on this site. I had my first antweight competition last month and duly bought a wedge. I didn't do that well but had fun and learnt a lot about design. I am now looking to make a beetleweight wedge flipper (based off Eruption because it was a pretty successful design and would allow the spring to be mounted at the back of the robot) based of the spring mechanisms you've specified, which leads me to my question. Would it be possible to use a slip-geared winch design, rather than a choo-choo disc?
The reach of a choo-choo mechanism is theoretically equal to the radius (r) of the disc it is mounted to. If I were to use a slip-geared winch then the reach is equal to slightly less than the circumference of the gear (2πr - x, let's say) no? This would allow the cord to be mounted further along the flipper plate from the axis, giving it more mechanical advantage which should allow a more powerful spring. This would also increase the bulk of the mechanism, but it could be laid flat out on the chassis, rather than a general increase in choo-choo disc size which would increase height.
Am I barking up the wrong tree here? I haven't been able to find any examples of this online, even in the FRC forums.
Kind regards, [Brisbane, Australia]
A: Mark J. I'm happy to hear that you had a good time at your first event, Brisbane. It's a bit of a step from wedge to spring flipper -- you might want to take a look at the build report for this antweight spring flipper to get an idea of what you're in for.
I'm not following your contention that a 'Choo-Choo' linkage has a reach equal to the radius of the driven disk. I make the reach equal to the diameter of the disk plus the length of the "fling" arm attached to said disk (the dark blue arm in the animation). In the animated example the reach is almost three diameters, and the design could be modified to make it more.
The reach of a slip-geared winch is dependent on the circumference of the pinion gear that drives the spur sector. A small pinion will require a larger portion of its diameter to be 'toothless' in order to provide adequate slip clearance for the spur sector. In the animation, the reach is about 2/3rds the circumference of the pinion.
Large Block of Text Removed.
I started to write an extensive comparison of the two spring flipper mechanisms you're considering, but thought better of it. Let me summarize:
You're having trouble finding references to combat robots using the slip-gear mechanism on a spring flipper because I don't think there are any. The slip-gear is compact and inexpensive to make with simple modifications to off-the-shelf parts. In its simplest form it requires linearly increasing torque from the driving gearmotor as the mechanism approaches the set point, which slows the speed of the reset. The re-engagement of gear mesh can jam if anything gets tweeked.
The actual work done to 'cock' the spring in your flipper is the same regardless of the mechanism used to accomplish the work or the attachment point on the flipper plate, although some mechanisms place highly variable torque requirements on the motor powering them while doing that work.
Response: Hi Mark, it's Brisbane again. I don't know why I got it in my head that Choo Choo mechanisms were limited to the diameter of the disc (radius was a mistype, oops). Thanks for the quick response, I think the choo choo mechanism will suit my needs just fine. It's true that it's a bit of a step up from a traditional wedge, but I figured that if it didn't work in time for the competition in May that I could still run it as a passive wedge.
High Performance
Q: Looking at a Nerf motor for weapon drive and something doesn't add up...
120 watts for a nerf motor is nuts considering standard gearmotors are in the 1 watt range and maxxon motors of this size are barely pushing 3W. I seem to be off by a couple of magnitudes, so what is the actual power of these things?? [Bristol, England]
A: Mark J. Your calculations are correct; your error is comparing these motors to low-bidder consumer motors intended to power toys, office products, or kitchen gizmos. The proper comparison is with high-performance motors of similar weight:
A 180-size motor like The Neo Hellcat weighs about 32 grams and pumps out around 120 Watts of power;
A 2822-size Turnigy Aerodrive SK3 outrunner weighs 32 grams and is rated 115 Watts max continuous power.
A: Big wattage output requires big wattage input. Hook up a motor with a 40 amp stall to a LiPo with an 18 amp surge rating and bog it down: battery go poof. If your gear reduction is high enough to keep the current draw down where that itty bitty battery can cope you can get away with it, but the motor is overkill for your purpose. Maybe the smaller and lighter OOD Valkyrie would be a better option to wind the spring on your axe-ant?
Q: I don't follow the three formulas that "Bristol" used to calculate the power of the nerf motor. Might you explain them? [Syracuse, N.Y.]
A: 'Bristol' took a few shortcuts, but left an ample trail of crumbs to follow if you know the base equations.
Background: A brushed permanent magnet direct current electric motor develops peak power output when loaded to half its maximum RPM. When loaded to half of its maximum RPM the motor consumes half of its stall current. Here are the three sparse equations:
12V × 0.5 × 40A = 240W
Calculates electrical input power with motor at peak power output:
Voltage × 1/2 Stall Current
= 12 volts × 1/2 × 40 amps = 240 watts
Put all together, calculates peak mechanical power:
1916 rad/sec × 0.62 Nm
= ~120 watts
Gonna Need a Bigger Spring
Q: I'm designing an antweight (150g) robot with a hammer for a weapon. I want to power it with 2 torsion springs however I am having a hard time calculating the energy that a spring can hold, the springs will wind up 180 degrees and I'm aiming for 50 joules per spring.
Here are the specifications stats for a random spring I've found:
Series: Torsion Springs Outside Diameter (mm): 6.15 Wire Diameter (mm): 0.51
Max Torque (N-mm): 22.597 Deflection to Max Torque: 180.00
Radius (mm): 12.7 To Work Over Rod/Mandrel (mm): 4.06 Free Position of Ends: 180
Length of Leg (mm): 25.4 Body Length (mm): 3.18 Direction of Wind: Right Hand
Total Coils: 5.00 Material: Music Wire
I can sort the springs by "Max Torque", "rate" and other dimensional descriptors but no spring constant afaik.
Could you walk me through on how to calculate the energy stored?
P.S. Is 100 joules an appropriate amount for a stabbing weapon? I was originally aiming for much less inspired by real-world mouse traps, however, I saw you say that an average antweight spinner holds about 63 joules of energy.
Cheers! [Bristol, England]
A: Mark J. A few points:
A few years ago a spinner storing 63 joules of kinetic energy was average -- for a 454 gram antweight.
Using a spring with a hundred joules of stored energy to power your weapon does not necessarily translate into a weapon carrying a hundred joules of kinetic energy.
It isn't realistic to use spinner weapon energy standards for an overhead hammer weapon. A spinner weapon takes a second or two to accelerate the weapon up to full energy storage. Trying to instantaneously accelerate a hammer up to 100 joules of kinetic energy in 180 degrees of arc will just flip your antweight over backwards before the weapon touches anything.
I'd suggest dialing the energy back by an order of magnitude... or two.
You do need the 'Spring Constant' to calculate stored energy, so let's get that first. For easy conversion to joules we want units in Newton-meters and radians. 180 degrees = pi radians, so the equation for your example spring is:
Torsion Spring Torque Constant
= Torque [N-m] ÷ Deflection [radians]
= 0.0226 N-m ÷ 3.142 radians
= 0.00719 N-m / radian
Now we can plug that into the equation for torsion spring energy storage:
So... if your example spring with 22.597 N-mm of torque at 180 degrees stores 0.0355 joule, you'll need a spring with 636.71 N-mm of torque to store one joule.
Response: Apologies for not being clear enough, the motor will wind up the springs over 3-5 seconds with a release mechanism. I didn't realize you referred to 450g antweights so 100 joules seems too high indeed. I am looking at a 2-watt motor so I'll be aiming for 3-4 joules per spring.
Something like:
Editor's Note:Bristol is now going to demonstrate mastery of algebra skills by combining and simplifying the 'Tension Spring Torque Constant' and 'Torsion Spring Energy' equations above. Pay attention - this will be on the unit test.
Max Torque (N-mm): 2372.68
Deflection to Max Torque: 180.00
Energy = (1/2) × π2 × 2372.68 ÷ 1000 ÷ π
= 2372.68 × π ÷ 2000 = 3.73 J
Reply: No, my apologies! Your question was clear, but the poor wording on one of my concerns misled you.
I wasn't worried about winding your springs. I was worried that the Newtonian action of abruptly releasing 100 joules of stored energy to accelerate your hammer would cause a Newtonian reaction that would flip your robot in the other direction. You may find that the rotational release of even 6 or 8 joules lifts the nose of your robot. Let's hope it won't be a problem.
Response: P.S. Your response was so quick and cheap that I'm starting to wonder if it's any good
Reply: Now that we have identified an error attributable to the speed of my reply, I hope that you might gain confidence in the rest of my response. Perhaps I should raise my billing rate.
Q: I'm 3d printing a 1-lb grappler bot. I have a servo that I have been testing with but when I plug it into my receiver it just steals the power from my motors. What am I doing wrong? [Social Media]
A: Mark J. A servo can draw quite a bit of current -- often more current than the small battery eliminator circuit (BEC) in the drive motor ESC can provide. If your receiver is powered simply by plugging your ESC into it that's likely your problem. When your servo motor moves it draws current, the BEC can't provide enough current, and the voltage drops below what the receiver needs to control your robot correctly. You'll need a separate high-current BEC to power your servo: something like this.
As shown in the diagram, you'll need to snip away the center red wire in the three-wire cables from your ESCs to the receiver. This prevents the ESC and the new stand-alone BEC from both trying to power the receiver.
Gotta Get Down to Go Up
Q: I have been seriously thinking about getting into combat robots for a few months now so obviously I have been reading this website for a few months. I just became a father this year and I am so sorry to hear about Aaron! I wanted my first bot to be a vertical spinner but reading your website (FAQ #8) and some other forums (NO SPINNER) I settled on a 4 bar lifter. If it fails I still essentially have a wedge bot but before that I can try to do stuff actively. Anyway I have it all drawn up in CADD and I ran the 4-bar Calculator but ...
The graph showing the start position is very different from the drawing. It seems the negative on the "rear bar base rise up" causes the big issues.
I can extend D a little...but no way could I get to 26mm.
Do you have any ideas how I could improve the lifters performance?
"New Dad" [Slidell, Louisiana]
A: Mark J. You stopped reading FAQ #8 on 'first bot' a little early. After it says no spinner it continues with no lifter, no flamethrower, no crusher. Keep it simple....
Before I get to your questions, I spotted a couple errors in your spreadsheet inputs:
Base [Frame]: A is measured in a straight line from the Base of the Rear Bar (Crank) to the Base of the Front Bar (Rocker). As your Crank Base has been dropped below the Rocker Base, the measurement is just a bit longer than their separation along the X-axis. I make the correct measurement to be about 33.9 mm.
As shown in 'Figure 1 - Elements' on the 4-bar spreadsheet, Extension: R includes the length of Top Bar [Connector]: C. That makes the correct value for your design as drwan 79.0 mm rather than 57.48 mm.
To your questions:
1) The problem you've encountered is not directly due to setting a negative value for the Rear Bar Base Rise. The geometry of your design causes the rear end of the Top Bar [Connector] to start its rise faster than the front end, which causes the front tip of Extension: R to briefly dip down below the base of the Front Bar [Rocker]. This negative relative height of the Extension tip breaks the calculation engine formulas and crashes the spreadsheet. The giveaway is the #NUM! Error that appears below the 'Front Bar Torque Chart':
Shortening Rear Bar [Crank] to 12 mm will prevent the initial 'dip' and allow the spreadsheet to run correctly, but the performance of the system is not very good: there is a very limited range of servo motion combined with a high torque requirement.
2) The recommended lengths for Front Bar [Rocker]: D are critical for getting best lift height from lifters powering the Rear Bar [Crank] over a full range of motion. Lifters powered by the Front Bar [Rocker] may ignore the recommended Rocker length range, but should note that Rocker motion may be limited by very short Rear Bar [Crank] lengths.
3) Four-bar design is as much art as science. Keep playing with values 'til you get a workable design. A couple suggestions:
It is desirable to place the Front Bar [Rocker] much closer to the lift point than you have it. This will reduce the torque loading on the servo and place less stress on the entire system. Flipping the servo end-for-end will place the servo output shaft about 12 mm farther forward in the 'bot and change the geometry of the mechanism for the better.
Take a look a successful small 4-bar lifters like 'Pad Thai Doodle Ninja' to get a general idea of bar positions and ratios. This is much easier than earning a degree in mechanical engineering from MIT. Are you sure you don't want to build a wedge?
One last note: You're currently entering measurement values in grams and millimeters so the units for torque are expressed in g-mm. If you enter the lengths as centimeters the torque output will be in the standard g-cm. Yes it's easy to convert, but I suggest staying in standard units to avoid confusion.
Update - After working with the Run Amok 4-bar Calculator to answer the above questions from "New Dad", I noticed a few things that needed improvement or clarification. It all made sense to me three years ago when I wrote it, but the passage of time now allows me to see it with new eyes. I'm pleased to announce the release of Version 1.3:
Pop-up reporting of errors caused by trigonometric violations;
Rocker and crank end point angle display for both front and rear bar powered lifters;
Instant warning and display of impending 'retract lock' condition for powered front bar designs;
A new selectable option to reduce distortion in the linkage display plot; and
Re-written documentation.
A lot of late-night coffee and head scratching went into this version. I think you'll find it to be a big upgrade!
Date marker: January 2022
Play a Different Game
Q: I have a 3 lb bot and I can't seem to win the low ground in fights. My bot's wedge struggles to get underneath opponents and I can't hit them with my spinner as a result. What improvements can I make to the wedge to make it more effective? [Palo Alto, California]
A: Mark J. Since you didn't tell me anything about your current wedge I'm unable to suggest specific improvements. In general, playing the 'lowest wedge' game can be frustrating.
Search the Ask Aaron: Design and Construction archive for 'best angle', 'which gets under', and 'lowest wedge' to find detailed information in multiple prior posts on this topic.
Sometimes it's not the 'bot, it's the driver. Getting around to attack from an angle instead of trying to rush head-on can produce superior results.
A wedge is often not the best option for fighting another wedge. A wide single piece dragging wedge will ride along the highest part of the floor it contacts, which leaves the rest of the wedge spaced above the arena surface. Multiple hinged 'wedge killer' forks' like those developed by BotKits for their D2 beetle kit have a better chance of finding a lower spot in the arena floor to get under full-width wedges.
BotKits recommends swapping in drop forks against opponents with wedges or drum/egg-beater weapons, then going back to a wedge against other weaponry. I think that's good advice.
Thwack It Good
Q: Hello mark, first I just want to say that this website is and I only wish that I had found It sooner!
Any way, I am working on designing an ant weight Overhead Thwack bot as my fourth combat robot. I am looking into the Repeat Drive Mini – Mk2 motors but I have run into a problem. Section 6.8 of the RioBotz Combat Robot Tutorial explains how to calculate the needs of an Overhead Thwack bot drive train but that only works for a brushed motor. Is there any way short of "trial and error" to figure out how to design my drive train?
By the way, I am aware that a thwack bot is not the most viable design but I am feeling experimental. Thanks! [Amarillo, or maybe Chicago?]
A: Mark J. Glad you're enjoying the site, Amarillo. Take your time; there's a lot to dig thru.
The RioBotz tutorial recommends a current limiting motor controller set to keep the maximum drive torque provided by a brushed gearmotor close to a theoretical traction-limited 'sweet spot'. They weren't thinking about a YOLO brushless ant thwacker.
Repeat Robotics has been kind enough to provide a chart comparing the "throttle response" of various ant gearmotors to their 'Mini Mk2' gearmotors, presumably paired with their Repeat Brushless Drive ESC:
Activation of the thwack takes place in the first half a revolution of the drive, so we're interested in torque very low in the RPM range. If we assume that 'throttle response' is a function of torque, we can see from the chart that the Repeat Brushless Drive mirrors the low RPM response of the BotKits gearmotor (presumably their 16MM DC gearmotor). So, a reasonable starting point for your thwack drivetrain might be to input the torque specs of the BotKits gearmotor into the RioBotz design formula. But there's a potential problem...
The unknown quantity is how well the Repeat Reprogrammed BLHeli_S ESC handles abrupt throttle reversal. For an unsensored brushless ESC starting from a standstill is very different from suddenly slamming the throttle the other direction, and there may well be a significant delay while the ESC sorts out commutation timing to reverse the motor direction. This might seriously screw up a brushless overhead thwackbot. It's a good thing that you're feeling 'experimental'.
Since it's all about torque I'd go with good-old predictable brushed drive -- and I wouldn't worry too much about theoretical wheelspin.
Afterthought: The RioBotz guide formula is intended to squeeze every last joule of performance out of the overhead reaction hammer weapon, but what's best for the weapon does not provide an optimum drivetrain. Given that a maximum performance overhead thwack hammer is still ineffective I can't recommend hobbling the drivetrain to make the weapon just a little bit better.
Q: Hey Mark, its the thwack bot guy again. Just thought I would let you know that it is Amarillo. You were right the first time. Anyway, I appreciate the afterthought - it was quite helpful. Do you have any other tips for building overhead thwack bots?
A: You likely have enough weight allowance to add a wedge mounted to bearings on the gearmotor output shafts. Either a simple 'Toe Crusher' style wedge or a full 'Sworded History' style plow will add aggression potential. Yes, when inverted the full plow would be goofy but thwacks are all about goofy.
Tweakeroo
I'm having an issue with my beetleweight. I am running 4 silverspark motors with fingertech ESCs on 4s. I know silversparks aren't great in a beetle, but it's an overhead spinner and I couldn't find another option that was flat enough to work.
When I try to turn while at a complete stop only two motors, one on each side diagonally, will spin. All 4 motors spin just fine going forward and backward, and I can turn just fine when I am already moving forward or backward. My guess is that it is an issue with the load on the drive motors, but I'm hoping it's not cause there's no way to change those with this design.
Have you come across this or do you have any suggestions on what to try to improve it? [Social Media]
A: Mark J. Your chassis is tweeked. It's warped enough to put most of the weight on two diagonal wheels and very little weight on the other two wheels. All the wheels are actually spinning, but one pair is spinning so much slower that it looks like they're just dragging.
The lightly loaded wheels are breaking traction and spinning freely in a stationary turn while contributing little to the turning motion.
The heavily weighed wheels are getting much better traction, doing all the real work, and cannot break free to spin as rapidly as the other pair.
Un-tweek the chassis to get equal weight on all wheels and you'll be fine.
If the chassis itself can't be straightened you can shave down the tire diameter a bit on the wheels that bear too much weight. This will shift weight onto the other wheels and equilize traction.
It's All About Torque
Q: How powerful are the brushed Nerf motors for beetleweights? Are they as strong as the brushless outrunners? Are there any tricks to getting them to work well in a beetle? [Reno, Nevada]
A: Mark J. It's difficult to directly compare brushed and brushless motors, but brushed has certain advantages:
The first nice thing about brushed motors is that they produce much greater torque in the lowest RPM range than do comparable brushless motors.
The second nice thing about brushed motors is that there is no need to experiment with the motor controller firmware to get reliable start-up performance.
I've mentioned before that you don't need either brushless or Nerf brushed motor performance to be competitive in the beetlewight class. Stock Kitbots motors are plenty. I've recently added a few Nerf motors to the Tentacle Drivetrain Calculator so that you might compare the performance of these high-performance motors to the Kitbots motors:
DartBox Viper Gearmotor
DartBox Dragon Gearmotor
Nerf: Fang ReVAMPed
Nerf: MTB Neo Rhino
The drivetrain calculator does not include brushless motors because their performance is so dependent on the set-up of the brushless ESC used with them, but if I were building a beetle and wanted stupid performance levels I would design the 'bot around Nerf brushed motors for their ease of use.
Like any brushed motor, it is important to 'run them in' at low-speed/no-load to allow the brushes to wear a bit and match the curve of the commutator. Failure to do this may result in electrical arcing that can damage the brushes and commutator.
Put a small drop of lubricating oil on the motor bushings/bearings.
Secure the motor and apply about 1/4 to 1/3 its rated voltage.
Allow the motor to run unloaded for 10 to 15 minutes. Monitor for heat build-up and pause for cooling if needed.
The motor speed should be constant and stable at the end of this period - if not, continue until it is.
Reverse the polarity and run in the opposite direction for 10 to 15 minutes, as above.
A properly run-in motor will run cooler, produce more power, and be more reliable. It's well worth the time and effort.
As Much Art as Science
Q: Hello, I'm the guy that had the 1lb vert that wouldn't spin. The ESC you recommended cleared up a lot of headaches, so many thanks.
I recently competed in my first competition with my plastic 1lb Drum spinner, 'Rupture'. While getting some good hits and damage with the weapon, the drum had a tendency to chip off large chunks of the drum after a few hits, rendering the drum useless due to unbalance. It seemed to consistently shear off in roughly the same spots on each end of the drum, but I'm not 100% sure why. Each drum was 80% infill PETG, but consistently shattered on impact against robots even without active weapons, even when spinning at lower speeds. Any ideas on how to fix this? [Redmond, Washington]
A: Mark J. 3D printing is sometimes as much an art as a science. PTEG has some printing requirements that - if not met - can result in a brittle print that fractures in just the way your drum is failing. There are print tweaks to correct the problem but it may take some experimentation to find the right combination. Some of the possible actions likely to improve your print:
Dry your PETG filament in an oven at 60°c for a minimum of 6 hours prior to printing;
Decrease cooling fan speed or turn it off entirely;
Comment: Hey, I saw you mention my use of JMT R/C Car motor controllers in 'Melanistic Leopard'. I just wanted to note that I used them for a short time in 'Shadow Leopard' after that but have since stopped, because their lack of drag brake became too annoying to put up with. I will note that since the post I made on Reddit back in 2018, I encountered a weird issue with them where they only actually work on 2S or 4S, and attempting to run them on 3S actually causes them to catch fire. They also tend to lose internal power to the logic side of the controller and the BEC when run on 4S, so the limiting factor to their voltage seems to be the voltage regulator.
I'm still willing to recommend them for people who are just messing around with fun builds, but I wouldn't put them in anything intended to be competitive. [InquisitorWarth]
Reply: Mark J. Thanks for the update. If you've only got $7 and you need a pair of brushed motor controllers for your beetle, JMTs may be your best answer. I know there are builders who love to save a few bucks but as your notes point out: cheap comes with weird -- particularly if you pay little attention to voltage ratings. I'm glad you were able to get some use from them and I think your current advice is sound.
Can't Stop Shakin'
Q: Hi, I'm 11 years old and new to combat robot building. I just built a fairy weight bot with a hammer that's operated by a servo and everything is operated by a Malenki Nano ESC/receiver. I have a mode 2 Flysky i6 transmitter.
The transmitter's right stick is operating the drive and that all works ok. I have the left stick operating the hammer and am having issues with this. When I try to use the hammer it doesn't just move forward and backwards but also shakes a lot when I'm not touching the stick. Do you know why this might be happening and is there a way to fix it? [Westlake Village, California]
A: Mark J. That shaking is called 'servo jitter' and is often caused by electrical interference or voltage fluctuation. It would help to know what servo you are using, but I can offer some general guidance:
If your servo is wired as shown in the diagram above it MUST be rated for operation at 7.4 volts or more. Most servos are rated for lower voltages and will behave erratically or fail if powered directly from a 2S LiPo battery.
If your servo is not rated for 7.4 volts you may use a Battery Eliminator Circuit (BEC) between the battery and your servo. The BEC must supply a voltage within the operating range of your servo with enough current to support full stall torque -- at least one amp for a micro servo.
The servo signal wire (white or orange/yellow) may be too close to specific components on the Malenki ESC or the drive motors. Try moving the signal wire away from sources of possible interference.
Some inexpensive servos are very sensitive to jitter. If everything appears to be in order and you are still getting uncontroled servo motion you may want to upgrade your servo.
Write back and tell me more about your servo if you need help selecting and installing a BEC.
Q: First off, thanks so much for answering my question! I have a TIAN KONG RC MG90 micro-servo. It looks like it's only rated for 4.8 volts. Is there a certain BEC I would use with this? This is a picture of my bot and what the inside looks like so I don't have much room. For the amount of space I have, would it be better to get a good higher rated servo and are there any you would suggest that are not too expensive?
A: Nice little 'bot! I'm always happy to support new builders.
You need a BEC that supplies enough current for full-power operation of your micro-servo (about one amp) at a voltage it can handle. I checked the manufacturer's specs and your Tian Kong servo is rated up to 6 volts, so a small BEC like the ShareGoo 5 volt 3 amp BEC would do nicely. It weighs about 1/4 ounce (7 grams), costs about $8, provides more than enough current for your servo, and it looks like you have room for it. You can cut the wires shorter to save space. It installs like so:
If you'd rather swap out your servo I have a couple of low-price options:
The distributor for the Malenki ESC in England recommends the ALZRC DS452PM Micro Servo for direct use with 2S LiPo voltage -- but I can find no US sources for this product. You can buy one on eBay for $12 but it takes about a month to ship direct from China.
The EcoPower 827 Metal Gear Micro Servo is available from multiple US sources for about $15. It is rated for 8.4 volts and provides good performance for its price class.
Using a BEC to drop the voltage to your servo will reduce the speed and power of your hammer by a bit, while switching to a high-voltage servo will keep your weapon performance close to its current level.
Comment: Thank you so much for all your help! Your website is really cool. Can't wait to get my bot working
Q: Hello, I am constructing a 1-lb vert running off of an Endbots Lemon-RX receiver hooked to a spektrum transmitter, a D2830 Brushless Motor(850kv), a 40a turnigy esc to run it, and a 2s or a 3s LiPo battery to run everything. my biggest issue right now is that whenever I power up my weapon (pulley driven) the motor is slow to start up and very often refuses to go above ~10% power regardless of stick position on the transmitter.
The weapon power to stick position is consistent until reaching a certain point on the controller, where it will not spin any faster above that, even while under no load and just spinning freely. Sometimes the motor will only stutter, and on a rare occasion it will spin to normal speed and the motor speed to stick position will be consistent like it should be. I've determined that the transmitter is not the issue here. Is there any way to tell what could be the problem? Thanks [Redmond, Washington]
A: Mark J. Your symptoms point to a problem with the firmware settings in your Turnigy ESC. You didn't mention which Turnigy ESC series you have, but I'm going to guess that you have the Turnigy MultiStar that's commonly used in drones. Low kv motors like your D2830-850 require different timing-advance and soft-start settings than do higher kv drone motors.
You can break out the Turnigy manual and figure out how to change the programming parameters:
Commutation Timing - start with the lowest setting and move upward as needed.
Startup Power - start low and increase until you have best spin-up.
Alternately, you can just swap out the ESC for one intended for use with a fixed-wing outrunner. Ask around to get an ESC recommendation from a builder running a similar weapon motor and design; the Turnigy Plush series might do.
It Still Won't Spin
Q: Hey, it's me again with the 1-lb vert with spin up issues, I've tried multiple KV motors of different brands, multiple different batteries, a different ESC (all of the ones I have used are Turnigy Multistar ones found on HobbyKing), and I've checked all my connections and joints, but the same scenario keeps popping up: The drivetrain works just fine, but the weapon motor continues to refuse to go past 10% power with any load on the motor. Do you think the problem is the ESCs or the Lemon-RX board? I'm really not too sure what it is at this point. One more thing to note is sometimes when I have the weapon motor trying to spin the weapon and I give it a little boost, it will speed up to full speed like it's supposed to and will have consistent speed with the stick on the controller. It seems like it's biggest problem is getting past the 10% throttle mark and then it works fine.
Thanks in advance, what you do for the community is greatly appreciated. [Redmond, Washington]
A: Mark J. My answer remains the same as it was the first time you asked this question -- see post immediately above. This type of brushless startup problem is typically traced to ESC firmware settings that are incompatible with the motor. MultiStar ESCs have their firmware settings optimized for drone applications that use a different size and style of motor than the weapon motor you have chosen to use.
User changes to the MultiStar ESC firmware without the MultiStar programming card (no longer available) are very limited, so I think it would be best to switch to an ESC designed for use with fixed-wing aircraft - like the AeroStar or Turnigy Plush. These would be likely to come out of the box with firmware settings that match the needs of your largish ant-sized brushless outrunner weapon motor.
Asking the Wrong Question
Q: I'd like to get more lifting power for my FingerTech Viper lifter add-on. The robot currently has a 300 mAh 2S Turnigy Nano-Tech battery. What battery can I use to increase the power? [Reddit]
A: Mark J. The servo included with the Viper lifter can provide about two pounds of lift out at the tip of the lifter arm. Why do you want more lifting power in the one-pound antweight class?
The Viper lifter servo electronics have a maximum voltage rating of 10 volts, so bumping up to a 3-cell 11.1 volt LiPo battery would be risky and would provide only a small performance increase.
You could upgrade the servo to a more powerful unit that could lift faster under load, but if you're trying for enough speed to 'flip' your opponent you'll be disappointed.
If you want a true flipper you will need to replace the entire weapon with a new design. Perhaps something like a spring powered flipper.
A Tricky Balance
Q: Hello,
I bought the FingerTech's "Viper Vertical Spinner Add-On" as a first weapon to use for an ant (rest of bot won't be viper kit though). My first thought is - what if I ordered custom teeth from a site like sendcutsend that were a little longer or thicker to add weight and weapon diameter, and possibly at different sizes (but same weight) that could provide an asymmetrical design? Is there anything I should keep in mind before playing around with this idea, or any way you would approach the exercise?
Best, Neil [Tukwila. Washington]
A: Mark J. Thicker or (reasonably) longer teeth will not significantly increase the moment of inertia of this weapon system -- but it will significantly increase the stress on the tooth and the tooth mounting site. Increasing weapon diameter only adds energy storage to the extent the you move mass away from the center outward to occupy the increased span. Don't expect to gain much weapon performance from small changes to the teeth.
Going asymmetric has the potential to increase weapon "bite", which is a good thing -- but the process is a bit trickier than you might think...
You're starting with a balanced spinning mass and removing two identical teeth that are equidistant from the center of rotation -- so far so good.
If the new teeth are not identical in shape they may not balance even if they are equal in weight -- you need to calculate not only the weight but also the center-of-mass for the new teeth.
Example: You add two new teeth to opposite sides of an otherwise balanced rotating mass. The new teeth weigh the same, but one tooth is longer and thinner which places it's center of mass 10% farther from the axis of rotation than the shorter and thicker tooth.
This is like placing two kids on a teeter-totter but seating one kid 10% farther from the fulcrum point: they don't balance. To get the system back in balance you need the shorter/thicker tooth to weigh 10% more than the longer/thinner tooth to make up for being closer to the center of rotation.
In this case the calculation of tooth mass-centers is complicated because the teeth are not mounted radially with respect to rotation. A decent CAD program can sort this type of balance calculation out for you, or you could attempt to balance the assembly with the new teeth by hand or trial-and-error. Given that this is your first weapon I'd recommend that you run the FingerTech drum "as is" and save asymmetric weaponry for a future robot.
Q: I recently completed my first active weapon bot. It is an antweight lifter. How would I go about building a weapon lock for it? All the resources and videos I've seen focus on spinner safety which is understandable but I don't want to take chances with a lifter either. Any safety tips? Can I just wrap a bungee cord around it? [San Francisco]
A: Mark J. Section 3.4 of the SPARC Robot Construction Specifications provides expectations for weapon locking devices:
Moving weapons that can cause damage or injury must have a clearly visible locking device in place at all times when not in the arena. Locking devices must be painted in neon orange or another high visibility color. Locking devices must be clearly capable to stopping, arresting or otherwise preventing harmful motion of the weapon. Weapon locking pins must be in place when weapon power is applied during a robot's power-on procedure. This includes all powered weapons regardless of the power source or weight class.
I don't believe a bungee cord qualifies as "clearly capable" in the above capacity. Robots will typically use some form of spring lock pin that passes thru a hole in the moving part of the weapon and thru a second hole in the chassis or armor to prevent motion. A spring wire retainer holds the pin in place to prevent it from falling out.
Small robots may use other locking methods depending on weapon design. FingerTech sells a small but strong Safety Clamp for ant/beetle robots that you may find suitable.
Quicker Than Gaston
Q: Salutations!
I am planning on building my first combat robot in the plastic antweight category. Since it is my first time I am just building a simple wedge. My design involves only two wheels, and I'm wondering if you need a four wheel drive system in order to be competitive as a wedge.
Another concern of mine is that, according to my calculations, my 'bot's top speed will be just over 1mph, which seems a bit slow. Is it possible to be competitive with this speed?
Finally, my last question is regarding batteries. If two different brands of lipo batteries have the same specs, is there any difference in performance (e.g. Galaxy vs. NanoTech)?
Thanks. [Isolated from Competition in New England]
A: Mark J. Building a wedge for your first 'bot is a fine plan. Your questions, by the numbers:
1) Two-wheeled antweight wedges can be quite competitive. Wedgebots based on the FingerTech Viper kit have done quite well. A few notes:
It may well be that "No one's slick as Gaston" but Gaston's wheels look too slick. Are they just placeholders in your render? A plastic disk with a single screw threaded thru it into a driveshaft will be neither reliable nor capable of providing adequate traction. A proper wheel hub that will not fail when (not if) a screw loostens is a must. So are tires with reasonable traction.
If knocked on its side (and it will be) Gaston will be stuck unable to set itself back on its wheels. Losing a match that way is really embarrassing. Have something sticking out the side to prevent the 'bot being stable in that position and help it roll back onto its wheels.
The extreme rearward placement of your wheels places much of the weight of the 'bot on the sliding wedge. That leaves the wheels without much weight bearing down on them, and weight is traction. A two-wheeled wedge 'bot works well with 65% or 70% of the weight of the robot on the wheels. Consider moving the gearmotors forward and placing some components behind the drive axles to shift weight onto the drive wheels.
If you move the motors and wheels forward, Gaston will likely be able to get stuck on the back panel with wedge in the air and wheels off the ground. Add something that will prevent this from being a stable position, just as recommended for the side-stuck problem above.
2) At 1 MPH everyone's going to be quicker than Gaston. A whole lot quicker. An adequate ant speed is around 4 MPH, reaching that speed in about 4 feet. Many ants are MUCH quicker.
With wheels exposed to impact you'll also be well advised to have gearboxes with all metal gears and stout steel axle shafts at least 3mm in diameter. The FingerTech Silver Spark motors (or similar) are popular in the one-pound antweight class.
If you switch to gearmotors with long axle shafts, do not use those shafts as the 'something sticking out' to help Gaston back onto its wheels. A weapon hit to the end of that exposed shaft will send destructive shock straight into the gearbox. Cut the shaft flush with the wheel hub.
3) For the purposes of a simple ant wedge you need not worry about differences between LiPoly brands or flavors. Select something with a connector compatible with your charger and robot wiring harness. A 300 mAh pack should be adequate for Gaston.
Gaston Returns!
Q: Hello, it's the builder of Gaston again [see six posts down the page]. I took your advice, and created a completely new design for Gaston. I moved the center of mass closer to the wheels, and switched the motors to these: ServoCity 970 RPM Economy Gear Motor
According to the team tentacle calculator, my 'bot's top speed will now be about 4mph. However, according to the results from the calculator, I don't even need a 0.1 amp hours for a 3 minute match! Did I do something wrong, or are these motors just that powerful? Also, I attached a new rendering of Gaston (don't worry, the wheels and shafts are a work in progress, the body is the main focus). What are your thoughts on the new chassis shape and the motors? Thanks!
A: Mark J. The Servo City motors you have selected are sized correctly for 3-pound 'beetleweight' robots. They are very heavy for an antweight (over 3-ounces each) and offer no performance advantages over commonly used ant motors.
The FingerTech motors I recommended are commonly used in antweight robots for good reasons:
They weigh in at a hair over 1-ounce each and offer more than enough power to exceed the traction limits of your wheels; no additional pushing power would come from greater motor power.
They come with a robust planetary gearbox that is more resistant to shock failure than is the spur gearbox on the Servo City Economy Gear Motor.
Speed for the 22:1 FingerTech motors with a 3-cell LiPo battery is a bit better than for the oversized Servo City motor with the same battery.
Current consumption is a factor of load placed on the motors. As I had no specifications for your motors or the diameter of Gaston's wheels I estimated required battery capacity conservatively at 300 mAh. Now that I can derive Gaston's wheel diameter from your reported speed with the 970 RPM motor (1.5") and I know the motor specs I can verify that you could use a smaller battery, but suitable 3-cell lipos smaller than 450 mAh or 2-cell lipos smaller than 300 mAh are hard to find.
Your new design presents new problems. The smaller wheels mean that Gaston will be completely stuck when (not if) flipped over onto its back. The forward placement of the wheels also creates the possibility of Gaston being stuck upright balanced on the rear panel (this WILL happen). Switching to the smaller gearmotors will save you 4 ounces of weight -- put that to use.
Four Small or Two Large?
Q: I don't know if this sounds like a silly question, but I really would like to know.
Everyone knows 1-pounders use those 16mm gear motors (like those from Fingertech) for 2WD systems. Am I able to use four of those motors to make a 4WD system for a 3-pounder, or is it too much weight that could bend the shafts? If this is the case, what would you recommend for a 4WD system that could squeeze into a 3-pound vert?
Big fan of your site BTW - Chris. [Armadale, Western Australia]
A: Mark J. Four of the Fingertech-style 16mm gearmotors can deliver enough drive power for a spinner-weaponed beetleweight robot, but their 3mm shafts and relatively small gears are simply not strong enough to survive the stresses of the larger weight class. The go-to motors in beetleweights are the BotKits-style 22mm gearmotors. Four of them will weigh in at 10 to 12 ounces, but two will provide enough power for a spinner weaponed beetle. Take a look at Jamison Go's build log for his beetleweight spinner 'Cake' for an example using a weight-saving belt drive to get 4-wheel drive from two 22mm gearmotors. It's more complex than mounting four motors, but it saves a bunch of weight!
Save Weight Elsewhere
Q: Hey Mark! Happy to report that work on my 1lb robot is close to done… but currently, I have a few issues and want your advice. [South Bend, Indiana]
1) I'm overweight, and looking to trim some fat around the edges where I can. Currently I have six Shoulder Bolts between two AR500 disks and a TPU pulley, the whole system weighing about 6oz. I had added that many as I was worried about the weapon flexing too much on impact due to the TPU, ultimately warping or bending the weapon. In hindsight however, I could be overengineering a piece I don't have to. Would you recommend removing some of these shoulder bolts?
A: Mark J. I'm going to hide behind FAQ #17. No competent engineer would spec a material or layout without knowing a great deal more about the design than you have made available. I can offer some general notes:
Your design has shoulder screws separated by a fairly 'squishy' spacer. When one of the disks gets a good hit and the other misses, a LARGE shear force will be applied to the screws. You've designed an effective bolt cutter. Ideally there should be hardened steel pins thru the disks and into a non-squishy spacer to take the shear load and leave the screws with only the tension loading for which they are designed. Given that it's an ant you might get away without hardened pins and a non-yielding spacer if you can find shoulder screws made of hardened alloy steel rather than the weak 303/316 stainless steel commonly found in the off-the-shelf variety. How many screws? I'd save weight elsewhere.
2) I'm trying to figure out a good way to build the body. The entire frame currently is a unibody frame from Taulman 910 with armor panels that are replaceable. In order to install the armor, I have counterbored holes to hide the bolts… which is fine for the farther sides of the frame, but not for the front as I worry they'll be prone to impacts from spinners. I wanted to print it all as a unibody but can't as there is at least no way in this design to mount the drive motors and wheels as the space is way too small to fit my tools in. How do you think this could be improved upon? What's some advice you could give for designing and mounting angled panels like these?
A: I'm a fan of a flush countersink rather than a counterbore for areas exposed to likely spinner impacts. The flush surface offers little opportunity for an impactor to catch an edge. The countersunk fasteners will also spread the retaining load over a larger area for better survivability.
Uncertain what your top panel is made of. If it's something nice and stiff (carbon fiber?) a few more fasteners holding it in place will add considerably to the stiffness of the chassis structure. Particularly at the rear corners behind the motor mounting. That would help make up for the lack of a full unibody.
I'm concerned about the mounting of the weapon axle. The top-view render you sent (not reprinted here) appears to show the axle free-floating in a simple hole in the plastic. The oilite weapon bushings do not allow you to tighten the dead axle down tight to make the axle a structural part of the chassis. Consider switching to ball bearings with an internal tube spacer against the inner races that will allow the axle to be snugged down. See the cross-section drawing of the vertical weapon on 'Algos'. Add metal washers on both sides of the plastic between the plastic mount sites and the fastener heads and bearing races.
I realize that my design suggestions don't help with your overweight issue, but without them your opponents will start removing weight for you.
Q: Hey, thanks for the advice. Here's a bit more info on the materials of the weapon:
(2x) 2in x .25in AR500 flywheels
(2x) 95A TPU pulley sides (the thin portions between the discs and the pulley)
(1x) 95A TPU pulley (for two Fingertech belts)
(2x) .25in ID x .25in wide needle roller bearings
(6x) Oilite Thrust Bearings
(6x) Alloy Steel Shoulder Bolts (comparable to Grade 8 steel, according to McMaster-Carr)
(6x) Nylon Locking Nuts (18-8 Stainless Steel)
I have considered making the weapon pulley toothless and either from aluminum or TPU, but I'm not sure how large I'd make the diameter: I'd assume 1.007in? Would that alongside possibly keying the weapon help alleviate some of the shear load off the shoulder bolts?
A: I realize that the diameter of a man's shoulder screws is a very personal matter, but that information is somewhat critical in my calculation of their survivability. Based on the holes in your weapon pulley drawing I'm gonna guess at 1/8". Estimating shear strength at 60% of tensile strength is reasonable. It would also be generous of you to share just how fast you plan to spin this weapon. I can approximate the weapon's moment of inertia but I need the RPM to get a rough number for the stored energy that might be applied to the shoulder screws in a worst-case scenario. I'll also need an address to which I can send my bill for two hours of engineering consulting time. Off-the-top-of-my-head guesses are free, but if you're gonna make me run the numbers it can get expensive.
You can always have a go at the calculations yourself: Bolt in Single Shear Equation and Calculator. Note that placing a squishy spacer in between the two plates makes the situation worse: it adds lever arm length and requires bending analysis as well as shear.
I would vote for an aluminum pulley over TPU. A stiffer material will be more effective in tempering the shear forces on the shoulder screws if keyed into the disks.
The pulley size you are seeking is the Pitch Diameter and finding that is not a simple matter. The calculation depends on both the pulley and the construction of the belt. From www.gizmology.net:
The pitch diameter of a pulley is not the outside diameter. Or the inside diameter. In fact, the pitch diameter is very difficult to measure directly. If you cut a belt and look at the end, you'll see a row of fibers near the outside surface. This is the tension carrying part of the belt; the rest of the belt exists only to carry the forces from the pulley to and from these fibers. The pitch diameter of any pulley is measured at these fibers. If you think about this for a moment, you'll see that the pitch diameter of a pulley depends not just on the pulley itself, but on the [thickness] of the belt.
I'm Pulling For You
Q: I have an antweight flipper built on a FingerTech Viper chassis. Recently attended my first event, and while the robot survived every match it spent a significant amount of time getting flipped over. This event allowed unlimited magnet downforce. Many events specify a limit on downforce (usually 1 lb for an antweight) and at some point too much downforce over-stresses the drive motors. I am planning to mount magnets on the bottom of the chassis in front of the wheels, but they won't be contacting the arena floor.
How do you go about designing a magnet system to achieve a desired downforce? Seems like it should be simple, but I'm having a hard time understanding how "magnet strength" is specified for a particular magnet, figuring out how to calculate the size of the magnets needed, and the impact of distance from the attracting surface. There are also several permanent magnet materials to choose from. Which type is preferred for combat robotics? [Milford, Ohio]
A: Mark J. "Rare Earth' Neodymium magnets are the most powerful permanent magnets by weight and are thereby the preferred magnets for combat robot downforce applications. Neodymium magnets have a grade designation: an 'N' followed by two numbers (N35, N42,...). Larger numbers mean greater strength, and higher cost. Any letters following the numbers do not matter for our purpose.
K&J Magnetics has an On-Line Magnetic Attraction Force Calculator that will give an estimate of downforce for a neodymium magnet of specific grade, size, and distance from a generic 'steel plate'. They also provide links to their inventory for magnets of the type entered into the calculator.
Example: A round N38 grade magnet 0.75" in diameter, 0.125" thick, and spaced 0.25" above a steel floor will provide a 0.59 pound downforce.
The K&J calculator also generates a chart you can mouse-over to see how distance to the steel plate effects downforce. Play around with magnet sizes, grades, and spacing distance to get the results desired. K&J prices are good and their selection is excellent.
Date marker: January 2022
A Kinda Lumpy Disk
Hello, Mark! I've been working on and off with my ant as of late and have two design-related questions: [Naperville, Illinois]
Q1: How would I calculate the kinetic energy for this design using your calculator?
A: Mark J. The kinetic energy storage capacity of a given spinner weapon design depends on the speed of rotation and the Mass Moment of Inertia (MOI) of the rotor. The online Team Run Amok Spinner Weapon Kinetic Energy Calculator can determine the MOI of a rotor made up of simple geometric shapes: disks, bars, and tubes. Calculating the precise MOI of more complex shapes like your weapon is more difficult. Options:
Many Computer Aided Design (CAD) programs are able to calculate the MOI of an object sketched up using the program. Check to see if your CAD program has that ability. If it does, obtain the MOI from the program, enter it directly into the "Asymetric: MOI" field of the Kinetic Energy Calculator, and set all the size parameters to zero.
Since your rotor shape is a kinda lumpy disk, we can get a fairly good estimate of its MOI by thinking of it as a simple disk sized half way between the radius of the round hub portion and the larger radius at the outer edge of the impactor. The estimate will be a little high, but pretty close.
Let's try that last option. I'm guessing that the weapon pulley in your render is a 30 tooth Fingertech (32mm diameter) and estimate the diameter at the outer edge of the impactor to be 44mm. Splitting the difference gives a diameter of 38mm (a 19mm radius). Each diskette looks to be about 8mm thick, so double that to account for two steel diskettes. I'll assume the contribution of the aluminum pulley and fasteners to be negligible.
Pumping those numbers into the kinetic energy calculator with an assumed speed of 8,000 RPM gives kinetic energy storage of... nine joules. That's pitiful. A typical antweight spinner weapon has about seven times that much stored energy. Go bigger!
Q2: How would I go about fitting the bearings into the weapon? I don't have access to any fancy or expensive tools and figured it wouldn't be very easy to just press-fit two metal parts together.
A: An interference press fit requires a high precision drilled hole -- much tighter tolerances than you might get from a waterjet cut hole. If you have a correctly sized hole a it's not all that tough to tap or vise-press a needle roller bearing into place.
I know of builders that fix bearings into slide-fit holes with Loctite 640 retaining compound. This is not the same stuff as the common blue Loctite threadlocker. The Loctite is effective -- but don't get it in the bearing!
You might consider flanged oilite bushings instead of needle roller bearings. The flange keeps the bushing in place and the bronze bushing can absorb enormous impact loading. Correctly oiled, the frictional difference is negligible.
The Guide Says...
Q: Hi, Back again, but this time building a beetle! What brushless motor would you recommend for vertical or drum spinners of the 1.5KG weight class?
Thanks! [York, England]
A: Mark J. The Ask Aaron Combat Robot Brushless Motor Selection Guide will give you general parameters for weapon and drive motors for a given robot weight. For reasons provided in the guide, a brushless motor massing about 90 grams with a power output around 450 watts will do nicely for a typically sized beetle weapon. Examples:
Selection of motor diameter/length will depend on available space in your design, and selection of motor Kv rating will depend on the weapon design and drive method. Since all you've told me is "beetle vert or drum" I can't narrow it down more than that for you.
The Miracle is...
Q: Hi again, this time having a problem with Lipos…
I bought a 2s LiPo charger from the BBB shop and when I attempt to charge any of my Turnigy 300mah LiPo batteries, the light is red for about for seconds, then flashes briefly and repeats this. I have plugged these batteries into my robot, but it's not turning on?
Thanks for your help! [Eton, England]
A: Mark J. This same charger is sold on Amazon.com as the Blomiky H102. Several reviews of the Blomiky report problems similar to what you describe. The miracle is that most of these little £3 ($5) chargers actually do kinda work. Your's doesn't. Send it back to the Bristol Bot Builders shop and ask for a new one.
Note: The Blomiky advert on Amazon estimates that the H102 will take about 5 hours to charge a 1000 mAh battery -- that's about 90 minutes to charge your 300 mAh pack. You might want to consider spending a bit more for a faster charger.
Q: About my recent question concerning chargers, I do have a fingertech, proper lipo charger, which came with the fingertech viper kit, but I don't know how to use it, and also my £3 BBB charged batteries much faster than 90 minutes when it worked. If my fingertech charger isn't suitable either, could you link me with a decent, working one with correct accessories and a video on how to use it?
Many thanks again!
A: You have options:
If you're happy with the charge rate and simplicity of your little £3 USB charger, you could buy a couple more of them (one to use and one as a spare) and live with the fact that a £3 charger isn't likely to last very long.
The GT Power C6D Mini that came with your Viper kit is a very capable charger, but I'm not surprised to hear that you can't figure out how to use it. It has a horribly complex and poorly translated Chinese manual. Sorting thru all the available menu options for battery type, diagnostics, and charge rates is quite a challenge. I've found a video demonstrating charging with the C6D Mini, but the demonstrator already knows what he's doing and doesn't take the time to explain his actions.
For small capacity LiPo batteries you do not need a complex or expensive charger, but you do need something reliable. The Turnigy E3 Compact 2S/3S Lipo Charger is a very simple and inexpensive (£9) balance charger that plugs into a standard wall outlet (available in US, EU, and UK plug styles) and will charge small 2-cell or 3-cell Lipo batteries in a reasonable length of time. Just plug the battery into the correct port and charging starts automatically.
The Turnigy E3 with the UK plug is currently out-of-stock at HobbyKing, but there are other sources for this charger. Look around -- I think it's the right charger for you.
Electron Maze
Q: Hi, I'm still working on my antweight spinner and space doesn't seem to be an issue anymore which is great! I am still struggling with the wiring though so could you create a wiring diagram utilising 2 red bbb ESCs, the BBB brushless motor and Brushless speed controller for a basic antweight spinner bot?
Thank you very much! [Oxford, England]
A: Mark J. This previous post has a wiring diagram and notes for the Bristol Bot Builders Red ESCs with N-20 motors. Adding the wiring for the brushless weapon ESC and motor looks like this:
You'll need to add a switch in the battery line, as covered in the assembly notes on the Bristol Bot Builders website. If the weapon motor spins the wrong direction, reverse any two of the three weapon motor connections to the weapon ESC.
Note that the BBB Red ESCs are being phased out in favor of the BBB Edition Antweight Dual ESC. The wiring diagram for the Dual ESC is the standard combat robot wiring diagram as seen in FAQ #19.
Nerf or Nothing
Q: Do you know if I can buy 22mm gearboxes (like the one's used in D2 kits) without the motors? I want to mess around with some high power 130 size motors for my next build and I'm just going to toss the stock ones anyway. [Roseville, California]
A: Mark J. When you buy the 22mm gearmotors from ServoCity or BotKits the gearboxes are 95% of the expense; the brushed motors only add about $1.50. You'll need the pinion gears from the stock motors for the conversion, so just buy the whole package so you can back-convert if you don't like those fancy Nerf motors.
Fitting It All In
Q: Hi, Still working on this second antweight and after will definitely move up to a beetle, but still after 4 design revamps am struggling to fit all of my electronics inside my robot - I have looked over multiple designs on the internet but am still having trouble trying to fit everything into a 4' cube - bearing in mind this is a spinner. I was planning to make something not too dissimilar from the pictured robot but was wondering how all of these electronics were fitted in. Probably not a very answerable question but any tips you can give me would be much appreciated.
Thanks! [Oxford, England]
A: Mark J. Note for unseasoned American builders: antweights in the United Kingdom are restricted in both weight (150 grams) and size (must fit in a 4" cube). Some general tips:
Standard combat robot components are too large. Specialty UK Ant components are available from multiple suppliers, like Bristol Bot Builders. Go tiny.
Wires and connectors can take up a lot of room. Cut wires to length and solder connections to eliminate connectors whenever possible.
UK ants can get by with surprisingly small capacity batteries. See what similar 'bots use.
Stack components upward as needed; battery/motors/ESC/receiver.
Squish.
Q: Hi, about my recent question involving a quest for space, you mentioned batteries - I am currently running a turnigy nanotech 2S 300mah battery and I know it is extremely overkill. Where could I find a (much) smaller battery and what mAh rating would be nescessary? I think the BBB site recommends a 180mah battery but browsed the entirety of google and could not find any of those at all. Thanks!
A: The battery capacity requirement depends on details of your drive train and weapon. The BBB battery recommendation of 180mAh is for their drivetrain kit without weapon: adding a spinner weapon may put you back close to 300mAh. Run some simulated matches with your 300mAh battery - strapped to the outside of the 'bot if necessary - to see how long it provides power and downsize as appropriate. If your charger tracks amp-hours needed to recharge your battery after a standard length match you can use that as a guide.
Small capacity LiPo batteries have become scarce. A search for "E-Flite 180mAh 2S" turns up a good number of responses. They are not cheap, but they are half the size of your 300mAh Turnigy.
Looking back at some of my older questions from a few years ago, I now know how kind of “stupid” they are. Firstly, I wanted to apologize for the pain I forced you through having to answer about those crazy designs.
HOWEVER, I have recently started the process of converting an Elegoo Smart Car 3.0 robotics kit Into a small beetle lifter, my first combat robot.
I hope this isn't one of the questions you don't like to answer but I was wondering if there is a good way to buy a (preferable smaller) beetle weight chassis? I have only been able to find kits and the chassis on the robot kit is way to heavy (it gives me 0.7 pounds for armor, weapon, and radio stuff), but it would be fairly easy to fit everything into a smaller chassis. If so, thanks! As always, go Run Amok! [New York, New York]
A: Mark J. I'm puzzled about what components you plan to transfer from your 'Smart Car' to a beetleweight combat robot, New York.
The yellow plastic gearmotors are slow, underpowered, and way too fragile for beetleweight combat;
The Uno R3 motor controller board does not appear to interface to combat legal 'radio stuff';
The lithium battery is current limited for 'safety' and has no balance charging connector;
The tiny kit servo doesn't have enough torque to be an effective lifter; and
The press-on yellow plastic wheels will not survive in combat.
What else is there?
There are a few generic beetleweight designs available to download and send to a 3D printing service (example) but they are made to fit standard combat components and would not be suited to work with your salvaged smart car pieces. You might consider dropping down a notch to build an antweight. Your components are still sub-standard, but there is a wider selection of antweight printable chassis from which to pick.
Very Hot, Very Fast
Q: Hi Mark.
You might remember me from a few months ago when I asked you to diagnose some of the gremlins we were having with the Scorpion Mini Esc in my beetle. We ended up sending it back to Robot Power and they supposedly repaired it, but now we have a different issue. None of the lights on the esc will come on when we have it connected to a battery, but when we connect it to the receiver one of the chips gets very hot, very fast.
Is this just a bad board? Or is there something else we might be missing that could help to fix it? I really like the scorpion mini and I'd like to think this is just a bad board among a field of otherwise very robust ESC's.
Thanks! [Baton Rouge, Louisiana]
A: Mark J. I remember you, Baton Rouge. Sorry to hear that you're continuing to have trouble with the usually-reliable Scorpion ESC. Maybe you have the exception that proves the rule. It's difficult enough to troubleshoot an electronics problem with the board on the bench in front of me. Diagnosing a problem based on limited info and the board in an unknown circuit is effectively impossible, but I'll take a stab at it.
One weakness of the Scorpion Mini is its battery eliminator circuit (BEC). Does the hot chip look like the photo at right, and is it next to the 'BEC' label on the ESC board? That's the voltage regulator that reduces battery voltage to 5 volts to power the ESC and the receiver. Either the voltage regulator has gone bad or your receiver bus is drawing too much power. Take a look at this post in the Ask Aaron Radio and Electronics Archive that covers some of the symptoms that can result from overloading the Scorpion BEC. It sounds like you've cooked the voltage regulator.
Are you trying to power a servo via a receiver port?
Are you using a receiver with telemetry?
Are you running more than a 3s LiPo battery?
If you're convinced that you didn't overload the Scorpion's BEC it could just be a bad one, but if you're drawing too much 5-volt power you'll need to add a stand-alone BEC that can provide more current before you replace the Scorpion or you'll just cook it too. See my advice in the post I linked above.
Q: Hi I have never built a bot before and I'm trying to build a 3lb bot. what do you think the material should be and what type of weapon should it have? [Homewood, Illinois]
A: Mark J. Those are questions I can't answer. Your choice of materials depends on your skills, your experience, and your resources:
What experience do you have in material fabrication?
What tools do you have available in your workshop?
How much money are you willing to spend on your 'bot?
For a start you can read thru the first eight posts in the Ask Aaron FAQ -- pay special attention to number 8.
Consider a one-pound antweight robot for your first 'bot. The 3-pound beetleweight class is highly competitive and difficult for a new builder to break into.
There are a number of small combat robot kits available that will make your first 'bot a simpler build. Several kits are mentioned in the Ask Aaron Toy Hacks and Kits archive.
Browse this Ants, Beetles, and Fairies archive to get ideas from designs by other builders.
Q: I'm building a 3lb (beetleweight) overhead horizontal spinner [render attached]. Everything is enclosed within a box made of nylonx (carbon fiber infused nylon) and I'm looking at different materials for the top and bottom plates of the bot. These top and bottom plates aren't meant to be armor, but more structural components to avoid flex in the bot. I've looked into 2mm carbon fiber (about $20-30 per plate from cncmadness) and 1mm titanium ($50 per plate from sendcutsend).
I was wondering which one you think would work best and if there are other places I could get titanium CNCed or laser-cut. Thank you! [Arlington, Virginia]
A: Mark J. You're considering two materials with very different properties.
Grade 5 Titanium has properties that are useful for armor: high tensile strength, impact resistance, and springy toughness. But the strutures you're fabricating are not intended to function as armor.
Generic Carbon Fiber (there are different types) has properties useful for structures that are not directly exposed to hard impact: a high strength to weight ratio, and very high stiffness. The structures you're fabricating are intended to be strong and stiff to limit chassis flex.
A 2mm panel of carbon fiber will weigh about 15% less than a 1mm titanium panel, but will be about five times as stiff and have nearly six times the yield strength. This makes it sound like an easy choice and I think it is -- but mounting carbon fiber panels requires special attention. Pan-head screws will work but do not over-torque them. The screws are tightened just enough to hold the panel in place without squashing the soft CF under the screw head. Oversize metal washers are an option. Do not countersink CF for flat-head screws!
I don't generally recommend specific shops for parts fabrication. I will say that SendCutSend has a good reputation among combat robot builders. If you'd like to take your chances there are many on-line metal fabrication shops you could try.
Note: You didn't ask, but there are good reasons why you don't see three-blade spinner weapons. Some of those reasons can be found in the Spinner Weapon FAQ. You might consider a weapon shape similar to the stable and effective Mini Mulcher 'Axehead' weapon.
Q: This is another question regarding the overhead horizontal spinner. I've redesigned my tri-blade weapon to match Mini Mulcher's weapon, and I was wondering why the tri-blade design wouldn't work. My guess is that it'll act like a vertical disc spinner where any rotational force not along its axis of rotation will make it gyro. Thanks for the quick response!
A: The problem with the tri-blade isn't stability, it's lack of bite. You need to move two of the impactor tips in far enough to make room for a deep impact by the remaining impactor. I only mentioned 'stable' because 'Mini Mulcher' is well known for the serious stability problems it had with an earlier spinner design. See: Polhode Motion for a full explanation.
In addition to increasing the dimensions of the 'Axehead' to move from ant to beetle size, you will likely want to increase the cross-sectional area a bit more to account for the 'square-cube law' -- see Ask Aaron FAQ #17.
Q: What brushed drive motors should I use for a 3lb beetle weight with 2 inch wheel [Renton, Washington]
A: Mark J. I'm going to make a few assumptions:
This is your first combat robot;
The robot will have only two motors;
You will be competing in a 'standard' 8-foot arena.
Basic: The commonly used basic beetle brushed drive motor is the Kitbots 1000 RPM gearmotor. These motors are quite inexpensive, but require a bit of reinforcing to survive well in combat. Kitbots will 'battle harden' the motors for an additional fee, or you can follow the battle hardening instructions yourself.
You can find these same motors for a little less money on EBay (search there for "25GA motor") but builders have reported receiving motors that don't match the correct specs. I recommend buying from Kitbots.
Upgrade: The commonly used upgrade beetle brushed drive motor is the BotKits aluminum 22mm gear motor. These motors are more expensive, but are lighter and much more durable. As a bonus the gearboxes are well suited for conversion to more powerful motor options when you are ready.
Q: The Bristol Bots Builders shop sells an N-20 motor with a built-in ESC. They say it gets rid of a lot of wiring clutter in a 150-gram 'bot, but I don't understand how it wires up. What does the wiring diagram look like for two of these motors? [Reddit]
A: Mark J. The BBB N-20 wiring diagram is very simple. The battery connects to the receiver power and ground leads on an unused port, and the three-wire motor leads plug into the appropriate receiver ports for signal and power. Very neat and tidy, but drawing power thru the receiver ports creates a few limitations:
Your receiver must operate at the same voltage as the motors, which limits your choice of receivers. For their 150 gram kits BBB uses the Flysky FS2A AFHDS receiver, rated for 10 volts.
You'll need to construct a cable with a switch to run from the receiver power + and - pins to your battery connector. Make sure you get the polarity correct!
Don't try to power a weapon thru the receiver ports -- the thin circuit board traces cannot carry much current. If you need weapon power split off separate battery leads to the weapon ESC to power it directly.
See the BBB website for additional notes.
NOTE: Yes, you can use the same trick robot builders use to run the VEX 29 ESC at higher voltage than the receiver -- but that would ruin the tidy wiring layout and defeat the whole purpose of the compact motor/ESC package.
Q: I saw a youtube video of a fight with a beetle named "Droopy" that has two horizontal disks but no wheels. It kinda skates forward on skids, but I dont think its a bristlebot. Can you explain how it moves? [Atlanta, Georgia]
A: Mark J. Builder Tommy Wong calls 'Droopy' a gyro walker, but it doesn't work on the same principle as vertical gyro walkers like 'Wrecks'. It's more accurate to call it a torque reaction walker that gets help from a small gyro effect.
When you apply power to a horizontal spinner weapon, the weapon motor apples torque to the spin the weapon in one direction, but it also applies torque in the opposite direction that attempts to spin the robot chassis in the other direction. By increasing and decreasing weapon motor torque you can get the robot to rotate back and forth, but it won't move forward.
Droopy's name gives you a clue about how it turns that torque rotation into forward motion. A perfectly horizontal spinner does not lift the leading wheel when turning like a vertical spinner does, but Droopy's counter-rotating weapon disks each angle down just a bit toward the outer edges of the 'bot. That little bit of 'droop' creates enough gyroscopic force when the 'bot turns to transfer weight off the leading skid and force the 'bot to pivot around the skid more distant from the advancing
side.
My original description had the weapon spin direction backward. Here's the correction Tommy gave me:
Acceleration of the blade on the left side (clockwise rotation) causes the right side to lift up and pivot due to the simultaneous effect of two variables:
Torque Reaction - The conservation of angular momentum causing the bot to rotate in the opposite direction;
Gyro Effect - Rotating the gyro-causing vertical component CCW off axis, lifting the right side, analogous to a vertical spinner being rotated CCW.
Keep alternating weapon power from side to side and the robot 'walks' forward. It's slow, it can't back up, and it may not get you a weight bonus -- but it's kinda cool.
Q: I am making a beetleweight 'Tombclone' horizontal spinner. About how much of the total robot weight should be allocated to the spinner? [Winchester, Virginia]
A: Mark J. Grant Imahara's "Kickin' Bot: An Illustrated Guide to Building Combat Robots" was the first to promote the well-accepted '30-30-25-15 rule' for robot weight allocation:
30% to the drive system - motors, gearboxes, and wheels;
30% to the weapon system - weapon, motor, and belts/pulleys;
25% to the structure and armor; and
15% to the batteries and electronics.
For a weapon-centered design like a big bar spinner you might steal an extra ounce or two for your bar -- about one pound devoted to your weapon is close to right.
Q: I am the Tombclone person and I am not experienced with beetle spinners, only antweight ones, so I am asking for a brushless motor recommendation for the beetleweight HS.
A: I recommend that you search the Ask Aaron Archives before asking a new question. With more than 6800 questions and answers, there is a good chance that your answer is waiting there for you. Here's a Q&A from elsewhere in this archive:
Q: About how large a brushless motor do I need for a beetle spinner weapon? [Cincinnati, Ohio]
A: Mark J. Specifics will depend on your design, but popular beetleweight spinner weapon motors run about 150 watts per pound of robot and about 6% of the total robot weight. For a beetle, that typically translates to outrunner motors in 28mm or 36mm diameters.
As noted, the details of motor selection will depend on the specific elements of your weapon design -- belt reduction ratio, battery voltage, etc. I suggest searching for successful beetleweights with similar weapon layouts to your design to see what they use. For example:
The Vector Beetleweight Bar Spinner Kit [discontinued] uses greater belt reduction to spin its weapon with the more budget-friendly Sunny Sky 2212 outrunner.
Don't try to direct-drive a beetleweight spinner with an un-modified motor. The increased impact levels of the heavier weight class will very quickly destroy stock motors.
Q: A builder on [social media] recently mentioned using a 2205 2300kv brushless weapon motor in their combat antweight. Was that a typo? Does anyone make an outrunner only 5mm thick? [Akron, Ohio]
A: Mark J. I understand your confusion. There are two ways to measure a motor for classification:
Some manufacturers classify their brushless outrunners by the diameter and length of the motor can and base. A '2205' motor under this system would be 22 millimeters in diameter and only 5 millimeters long. No, nobody makes a hobby motor that size.
Other manufacturers - particularly of quadcopter motors - classify their brushless outrunners by the diameter and length of only the motor stator; the wire-wound source of the electromagnetic force that drives the motor.
A '2205' quad motor like the pictured 'Emax RS2205-2300' would be a '2817' if classified by its can dimensions.
Q: "Mounting plates" sold by Fingertech have served me well on ant & beetle motors, is there an equivalently easy solution for fairyweight motors? [Woburn, Massachusetts]
A: Mark J. The popular 'N20' style motors used in 150 gram 'bots are constructed differently than commonly used ant and beetle motors. They require greater support for their open gearboxes than 'face mount' style plates like the FingerTech Flat Motor Mount provide.
A search for "N20 mounts" will find many clamp-style mounts, like the Pololu Micro Metal Gearmotor Bracket shown. That's your fairyweight equivalent easy solution.
Q: I'm using a 20A Afro esc flashed with blheli to power a Turnigy 2209-980 brushless outrunner. I'm also using a Turnigy 850mAh 3 cell battery to power all the electronics. 30c-60c discharge. At low and high rpms, the weapon, when it gets a good hit on something, shuts off and re calibrates. I have installed a large capacitor between the battery leads that helps with this problem quite a bit. Anything above 50% power on the throttle shuts down the weapon when it gets a good hit. Rapidly throttling the power all the way up to 100% also achieves this.
While watching the electronics during the incident, they all seem to shut down for a quarter of a second before coming back on and re calibrating. On my previous robot with a drum spinner, I also had this issue with a 750kv motor. During the competition I was able to wait for the electronics to come back online and drive away. If you need any other information about the electronics or the robot, just tell me. [Arlington, Virginia]
A: Mark J. The clue to the cause of the problem is that your installation of a 'large capacitor' across the battery leads helped. You're getting a serious voltage drop when the weapon motor is pulling a lot of power and your electronics are 'browning out'. Two possible solutions -- I'd do both:
Since ALL your electronics are shutting down I'm suspicious that it's the receiver that's causing the system reset when the voltage dips. Remove the 'large capacitor' from the battery leads and connect it across the power and ground pins on an unused receiver port. This will sustain voltage on the 5-volt receiver bus.
How old is your battery? The math says that it should provide (60 × 0.85 =) 51 amps of peak surge power, but it sounds like it isn't doing it. The current batch of Turnigy 850 mAh 3S LiPos are rated 60c-120c and should have no trouble keeping up with your weapon motor.
Q: I stumbled onto some crazy butane jet lighters that are intended for cigars but seem pretty dang impressive. Would it be practical to put one of these in an ant or beetle? [The Aether]
A: Mark J. I fought against BattleBots champion Donald Hutson's jet lighter weaponed ant 'Incinerator' in a melee at 'Bot Bash' in 2002. Somebody ejected it thru a pushout quickly, but it continued to wander around the 'moat' attempting to fry the other 'bots that fell in. Donald's like that.
All flame bots are just for showing off -- they do not win tournaments. You need a method to turn on the gas and light the flame remotely because no organizer will let you 'flame on' before the arena door is locked and the match has started. Don't write back and ask me how to make that ignitor; if you don't already know how you're not ready to build a flame-bot. The event also has to allow flame weapons. Most organizers aren't too keen on 'bots that could melt holes in their expensive Lexan.
Q: Hi there. I purchased an Endbots DESC lemon to use in a Beetleweight project, but am finding acquiring a DSM2 transmitter in Europe troublesome. Is it possible to add a different receiver to the project to allow me to use an alternate transmitter? Failing that, are there any other dual channel ESCa that work well for beetles?
Thanks, James [Smithton, Scotland]
A: Mark J. Hi James. I don't have an Endbots DESC lemon here in the shop, but I believe that it is just a Endbots DESC board with modified RX input pin locations that allow a Lemon-RX to be flipped over and soldered to the back of the board. De-soldering the Lemon-RX should allow you to wire any receiver onto the exposed channel pins. I would suggest writing to Endbots to verify this; they accept support requests at Sales@endbots.com
Failing that, the popular Beetle DESC at the moment is the Scorpion Mini from Robot Power. It is a fair bit larger and heavier than the Endbots DESC, but has a much higher current rating (6.5 amp continuous) and will handle up to 28 volts.
Some builders prefer using two of the single-channel Wasp ESCs from Robot Power. A pair of them are a little lighter than a single Scorpion Mini, and it's easier to find space in a cramped beetle for two small boards than one larger one.
Q: I'm looking to make a Beetle weight hammer/pick ax bot and saw you mention a pneumatic hammer system is the way to go. Would it be possible to fit into the US Beetle class weight limit if not what is the best substitute? [Spartanburg, South Carolina]
A: Mark J. Did I really say that? A pneumatic system can deliver a great deal more speed and power to a hammer or lifter weapon than a direct-acting electric system -- but they are also a great deal more complex and dangerous. Whether they are 'the way to go' depends on your experience in building combat robots, expertise in metalworking, access to a machine shop, and expectations of performance.
Take a look at the Team DaVinci Guide to Understanding Pneumatics for an overview of the combat robot pneumatic systems. Also check with the organizers of any events where you wish to compete for special tournament rules and restrictions on pneumatic systems.
Search the Ask Aaron Ants, Beetles, and Fairies archive for 'pneumatic' to find dozens of earlier posts on insect class pneumatic systems. You'll find some alternatives with that same search.
There have been many pneumatic beetles, ants, and even fairies, but there is no 'off-the shelf' set of components currently available that will work for insect-class combat robots. You'll need to carefully modify existing pieces or make your own -- not a simple task and not for the inexperienced.
You can download the Team Run Amok Electric Hammer Spreadsheet and model the performance of an electric hammer sized for your 'bot -- but you'll find that the destructive energy available from such systems is disappointingly small compared to other weapon choices. A pneumatic hammer would be more powerful, but would still fall well short of other weapons.
No type of beetleweight hammer is likely to be effective at winning matches against spinners or powerful wedges, so 'the way to go' here might be to make it easy and just mount a hammer on a quick R/C servo. It won't do much damage, but neither will a more complex hammer system.
Date marker: January 2020
Q: So, I'm a bit befuddled... what exactly is Torque Overage Factor? I've read the Lifter FAQ and the archives, but there's not really a description as to what it is or what exactly it represents, unless I'm missing something. [Mattoon, Illinois]
A: Mark J.
Take another look at the first two questions in the Electric Lifter FAQ:
Q: How do I calculate the torque needed from a gearmotor to operate a simple beetleweight lifter?
and...
Q: Is the 1.67 multiplication to provide safety or is the number produced by the equation the actual minimum oz-in?
I've gone in and highlighted the reference there to the torque overage factor. Write back if it still doesn't make sense.
Q: Hey guys!
Since the last time I posted my beetleweight has gone from having a hammer to a hammersaw. I had a major issue with the hammer this entire season where the pinion gear thats pressfit onto the hammer motor keeps breaking free of the shaft, killing the hammer.
Version 1 My hammer system used a 370W 3.17mm shaft brushless motor that went directly into a 32mm planetary gearbox with and then to a gear driven 3:1. Total ratio 42:1. The issue happened when the hammer had no where else to go. If the hammer is back and we tried to push the hammer back further it just broke the pinion loose immediately. The pinion also broke free over general use, we struggled to get through a 3 min match with the hammer flailing 24/7. Our strategy of "just dont mess up controlling it" didn't work well. We used green loctite and superglue to try to mend this issue, it didn't work, it just popped loose.
Version 2 When rebuilding the hammersaw I downsized the hammer arm power massively and swapped in a toothed belt drive to curb this issue. Unfortunately, the motor shaft was even smaller at only 2mm diameter. We run a 2440 brushless into a 22mm gearbox. The arm """worked""" at one comp - was only swung maybe 6 times, and only took one full impact hit. During a post comp weapon test however, the arm started slipping again and I just know it's the same stupid pinion issue.
I found a setscrewed 3.17mm gear and dremeled a 'D' shape in the motor shaft for the V1 hammer, but the entire gearbox locks up so I haven't been able to durability test it yet. I think I need to clean and re-grease the 32mm gearbox.
Mini question: how important is grease in planetary gearboxes?
Mini answer: Grease is critical, but that setscrew pinion is probably interfering with rotation and causing the lock-up.
Unfortunately there is no way I can find a set screwed 2mm gear pinion. I've seen videos of people soldering pinions, does this work?
Other than soldering the gears and incorporating a slip clutch into the arm for V3 I have no idea what else to do to fix this. Do you have any tips? [Ontario, Canada]
A: Mark J. Your problem isn't the pinion gear or the press-fit attachment method -- the problem is poor quality motor shafts.
Hobby-grade brushless motors are designed to be used with clamp-style propeller hubs on model aircraft or with large diameter setscrew pinions on R/C cars. The shaft tolerances in those applications are not critical, so there is no need to add to manufacturing expense by using high-precision shafts.
A press-fit pinion gear requires precise tolerance on shaft diameter. Without a correct interference fit the pinion will not hold. Attempting to bodge the fit with Loctite or super glue is futile, and the gear fit is too tight to allow a proper solder joint -- there has to be a little room for the solder to wick in between the gear and shaft.
It is possible to obtain and install precision shafts on small brushless motors, but it's a difficult job with an inrunner and an unsensored brushless motor is a poor choice for the job of operating a hammer in the first place.
I'd replace the brushless motor with a brushed Nerf motor. These motors have precision shafts designed for press-fit gears, and their low-end torque is much superior to your 2440 brushless. Peak power output is lower, but the added stall torque will make up for it.
Press-Fit Pinion Tip Don't re-use a press-fit pinion. If you pull one off a shaft, replace it with a new pinion. Press-fitting a second time is not quite as secure as it was the first time.
New! Electric Lifter FAQ
There's been increasing interest in electric lifter weapons in recent months. I think the interest may stem from designs that have appeared at BattleBots and the various Chinese events. I had previously edited together posts from 'Ask Aaron' into topic area FAQs for LiPoly Batteries and Spinner Weapons, so I decided to try that same approach with electric lifters. Let me know what you think...
Q: I keep hearing builders mentioning the crazy powerful NERF drive motors in their beetles, but I have no clue. Are they some kinda inrunner? Maybe an outrunner? I can't find anything called NERF at Hobbyking, Robot Marketplace, or Servo City. What does NERF stand for? Where can I find them? How do I use them? [A Guy I Met in Line at Starbucks]
A: Mark J. It's not easy being a 'brushless hipster', particularly down in the insect weight classes. The commonly used motors and controllers suitable for the classes are sourced from hobby model aircraft suppliers. They are light, powerful, and inexpensive - but poorly designed for use as robot drive motors:
Motor shafts are made of poor quality metal;
Mounting 'outrunner' style motors for drive is challenging;
The sensorless controllers offer very poor low-speed torque;
Selecting controller firmware and proper settings is a black art; and
Shaft diameter tollerences are inadequate to assure proper gear retention.
All these problems have created a kind of 'brushless backlash' in the insect classes, and somebody came up with an alternative from an unlikely source.
Do you remember 'Nerf guns'? In the early 2000's they were hugely popular toys that fired small plastic foam darts or balls. They're still out there, and a small industry has developed around modifying these toys for extreme performance. We're talking about firing rates of 12 rounds per second! This performance upgrade requires lots of modified parts, but at the heart are high-performance versions of the lowly Mabuchi FA-130 and FF-180 brushed motors. These are the same size as the motors used in the original toys, but they pump out up to fifteen times the power! They bolt right up to the Servo City 22mm gearboxes or to the upgraded BotKits 22mm gearboxes to create a beetleweight drive solution comparable in weight and performance to brushless alternatives with most of the heartache removed.
Public Service Announcement - Insect class 'bots REALLY do not need the stupid levels of power and speed available from either brushless or Nerf motors. In a typical insect arena it just can't be put to effective use -- even with a heap of magnet downforce. The stock BotKits motors are plenty. What we're discussing here is just for showing off.
Where was I? Oh yea... you'll want to use high quality brushed ESCs with Nerf motors because they pull a lot of amps. It would be reasonable to run one RobotPower Wasp for each motor. Most Nerf motors are rated for 3s (11.1 volt) Lipo packs, but do yourself a favor and run them on 2s (7.4 volt). A web search for "nerf mod motors" will turn up sources for the motors -- you might start at Out of Darts.
Q: Hey Mark, I've taken up the helm of my university's Beetleweight team and we are building a new robot this year. We built one last year but it was a wedgebot and essentially a crash course on how not to build a combat robot: we went way over the weight limit, 3d printed a chassis, and haphazardly threw some silver spark motors in there. Surprisingly enough it ended up working but thankfully I had the foresight to never enter it into a competition.
Now that I'm at the helm, we're currently working on selecting drive motors. Of course, we want something that will supply sufficient torque and speed, but isn't too heavy. But what exactly should we be looking for in a motor? I've seen websites give information on motors such as their size, weight, KV, stall and no-load torque, and max. current limitations, but how do we use all of these statistics to compare motors to each other to determine which ones fit our needs best? What kind of calculations can we do to determine the needs for our purposes? I've seen calculators on this website relating to the max. current draw and gearbox selections, but that assumes that you've chosen a motor and as of right now that seems to be our first roadblock.
I guess another question is, what are some good vendors for the beetleweight class motors? I've seen Fingertech, BotKits, KitBots, and then there's also RobotShop. Do you have any other recommendations for that?
Just to avoid a bad hamburger, I'd like to mention the following:
We are planning on using BaneBots wheels, haven't quite determined the size yet;
Going to have a UHMW chassis; and finally,
Planning on using a beater bar setup, likely the Fingertech stock kit.
I'm sure this has been answered already in the archives, but as a busy college student I simply do not have the time to go through every submission under "drive motors."
Thanks, Bengal Reauxbotics [Baton Rouge, LA]
A: Mark J. You've taken on a leadership roll for a complex project but can't find an hour to scan thru our website? That time would be well spent in finding not only the answers to your current questions but multiple nuggets of information you haven't yet realized you need to know. Asking for a shortcut handed to you on a platter does not bode well for your new robot, but maybe you'll learn something from a second failed robot that you didn't learn from the first.
Scroll to first entry: Team Tentacle Torque / Amp-Hour Calculator;
Follow link to 'Example Drivetrain Analysis using the Tentacle Torque Calculator'.
Read it.
Go to the calculator, select 'Kitbot 1000 RPM (Estimated)' from the 'Motor' pull-down menu and select '3 lb' from the 'Weight Class' pull-down menu.
Check performance figures as described in the example drivetrain analysis.
Repeat analysis with other motor/voltage/wheel/gearing options until you're happy.
The motors sources you named cover the popular brushed beetle motor sources, and they are popular for good reason. Don't even think about some poorly documented, unknown eBay special in an effort to save a couple bucks.
This all assumes that you've elected to use brushed drive motors -- which I strongly recommend given your level of experience. If you're set on being cool brushless drive hipsters I'm afraid that you'll have to carve a good chunk of time out of your busy schedule to read up on that option -- there's way too much on that topic to summarize here.
Q: I'm building an antweight with a lifter plow, kinda like 'Duck!'. The lifter is powered by twin servos that need to run in opposite directions to work together on the lift, but my transmitter only has elevon mixing and I'm already using that for the drive motors. I have a Y-harness to connect both servos to a single receiver port. Can I re-wire one servo to respond in the opposite direction? [Ontario, Canada]
A: Mark J. I can save you a lot of time and frustration. R/C airplanes have a similar problem getting the twin servos controlling wing flaps to respond in opposite directions, and they have a quick and simple solution to the problem. A web search for "servo signal reverser" will reveal multiple sources for a tiny, inexpensive device that plugs into a servo lead and reverses its response. Do that!
Q: Hi Mark, I've been working on a flywheel flipper design for a while now (I know, I know, get your cheerleader button ready but hear me out first...) I want a high-power flipper that can manage proper ceiling-hitting throws not just a lifter in the 3lb class. I don't have enough experience with pneumatics to be comfortable with a DIY solenoid setup and in the 3lb class it seems like there aren't many part options off-the-shelf for big power done safely at low weight. 'Kelpie' and 'Launchpad' are two options in that class along with 'Anticide' even smaller so while it isn't impossible, it's not the challenge I'm stuck on.
There are a couple small flywheel flippers that seem to work well, though not many THAT small, likely for good reason, but I've convinced myself it's not too far-fetched. All the flywheel flippers I've found seem to approach the task in a different way. The lack of a tried and true method yet multiple successful solutions really peaks my interest. I also think this question might directly relate to spinner weapons so the answer may be useful for people who aren't directly in need of a cheerleader button.
Q: Hello, I was curious as to how friction and RPM relate to each other when talking about a spinning weapon? If I increase my RPM to increase energy storage will I also loose more energy to friction in the process? Also, is this increase linear or exponential?
In the answer you mention Aerodynamic drag:
If your weapon is a thin disk that displaces little air as it rotates this isn't too much of a problem...
I'm trying to quantify 'too much' and see at what point i'm doing extra math and making the design more difficult/less reliable to add a fraction of a Joule to a multi-hundred Joule system. For my flywheel I could see about making it in a variety of different ways. Assume all options weigh the same:
solid steel disk
spoked steel disk
solid carbon-fiber disk with steel outer ring attached
spoked carbon-fiber disk with steel outer ring attached
solid/spoked carbon disk with higher density metal ring like copper or tungsten (if machining it or paying for it were really even an option)
This question is more hypothetical out of curiosity of the math, but to avoid being a bad hamburger my intentions are a 6-7" wheel weighing just under 1lb. I have not yet selected a motor option because I'm not far enough along in the design process but Silent Spring uses an AXi 2814 (1390kv I think) on a 4s lipo to power it's similarly sized disk so lets assume that ballpark.
I assume the moment of inertia gains of the spoked vs solid steel disk would far outweigh the aerodynamic drag as far as peak Joule storage given the same motor(?)
Would the same MOI advantage be true for the carbon-fiber disk with steel ring options?
Would shaving the spokes into an airfoil provide a useful difference?
Would the minor density increase of copper over steel decrease the ring volume by a more than negligible amount aerodynamically?
If not, at what density would it matter; hypothetically tungsten for example?
Where would I find the math to calculate these differences or might you know of some epic aero spreadsheet?
I'm sure the answer directly relates to rotational speed and given that I'm spinning a flywheel I will probably spin much faster than a normal spinner as bite isn't a concern anymore. Maybe this question is indeed only useful for cheerleader-needers after all... [Waltham, Massachusetts]
A: Mark J. Let's have a look at the rest of the sentence you reference from my reply to that earlier post:
If your weapon is a thin disk that displaces little air as it rotates this isn't too much of a problem, but if the weapon is a big and aerodynamically ugly beater bar you're going to run into a great big wall of resistance.
Lift-Induced Drag -- caused by redirecting airflow to produce lift;
Form Drag -- caused by an object displacing air as it moves; and
Skin–Friction Drag -- caused by the viscous friction of air against the surface of the moving object.
I typically address 'form drag' when talking about robot weapons as it is by far the greatest contributor in a typical design. Your case is different. A spinning solid and impactorless disk is not generating lift and neither is it directly displacing a volume of air by moving thru it. That leaves only skin-friction drag, which is small.
Form drag concerns around 'spokes vs. spokeless' can be eliminated by covering the open areas of a spoked disk with a thin cover -- even something as simple as packing tape.
An ideal flywheel places as much mass as far away from the rotational axis as possible to obtain maximum energy storage from the mass, so the CF disk will work for that purpose at least as well as a spoked steel center. Note that CF is strong but brittle.
Not as useful as answer #1 above.
The thickness of the disk is relatively unimportant as it adds only a smidge to the surface area.
See answer #4.
You're chasing decimal places. The calculation of skin-friction drag is nasty and the complication added by a rotating object is boggling. Design your energy storage disk for the maximum joules you're willing to pay for, keep it strong, clean up surface voids with tape, and launch somebody thru the ceiling.
Let's hope your dog clutch can stand the abuse...
Q: Hi Mark, 3lb Flywheel flipper guy again.
For a section of my loading mechanism I'd really like to use a motor in such a way that it could end up taking a pretty serious shock load (the residual energy left over after a flip, or worse, a missed flip's energy). The motor will need some decent torque and very low speed. To give a perspective on torque requirements, I'm essentially using this 2nd motor attached to the flipper arm mechanism in such a way that it can be used to drive the arm almost as a lifter bot. This would normally would lead me to a gearbox, but given the shock situation I'm left looking for a direct-drive system where the motor can be back-driven.
Any steppers with enough torque are crazy heavy, and I know of no brushless motors in a small package and super low KV other than gimbal motors and I'd think they couldn't handle the current requirements of staying under load.
I know some planetary gearboxes can be back-driven. Is killing power to a motor on a planetary gearbox and letting it be back-driven a common safety practice for gearboxes or is that asking for trouble like it seems?
I know it is common to use a timing belt upside down with toothless pulleys or a stretchy round urethane belt to eat up spinner shock but I worry that if my belt has enough slip to save the gearbox it will also have too much slip such that I won't be able to drive the loader mechanism. A spinner would still spin up eventually with tons of slip vs my lifter just won't ever lift as I burn the belt. Is the belt better than I'm picturing because of the magic of slipping vs static friction?
It seems my quest is not without a solution at least. On Battlebots the main arm of Jameson Go's SawBlaze has had more than a couple instances of aggressively snapping back after the heavier vertical spinner style 'saw' blade takes a bite and seems to work just fine. That's also chain driven, though I can't find any info on the motor and gearbox combo used other than his 30lb version megatRON uses Banebots P60 planetary gearboxes on RS550 motors to do the same job (can't tell if there is some kind of slipper clutch in there though too?). Also, whatever DUCK! is using this season for the flipper arm drive motors are quite clearly just fine with being back-driven with some serious shock. Any guess as to what Hal is using in there?
Hopefully one of my many assumptions is just blatantly wrong and there is a simple solution, if not, it's back to the drawing board for my design. Thank you very much for your time! [Cambridge, Massachusetts]
A: Mark J. I'm not getting a good picture of your design from your description. I'm guessing that you're trying to raise the flipper arm up just a bit with the lifter motor to engage some type of dog clutch on the flywheel and initiate a 'flip' -- but I don't understand how residual flywheel energy would backdrive such a system. Have you read thru Dale Hetherington's Flip-o-Matic page? Regardless, if that's what you need let's see what we have.
A direct-drive electric motor solution for the system you describe does not exist in this size range. If it only needs to lift a small distance a cam drive might suffice.
Planetary gearboxes can be back-driven, but a high-reduction planetary has enough inertia that a heavy shock load will destroy it even if the motor is unpowered.
Don't look to larger robots for a solution. The energy levels and material strengths don't translate well over the size difference.
Belts are a very popular and practical solution in insect classes. It's entirely possible to have a belt system that is slip-free up to a specific torque level and then breaks free to absorb shock load.
I'd suggest a timing belt and two toothed pulleys. Shave down the face of one pulley (the larger one if they are different sizes) to reduce the depth of the notches. Leave just enough depth to still give the toothed belt a little purchase, and run the belt a little loose. It will take some testing to find the right combination of notch depth and tension to hold just well enough to lift, but the system will meet your needs.
Q: I'm investigating using brushless drive on a 3-lb Meltybrain spinner and I was wondering if you had some advice on picking motors. I've only used brushed in the past, which is a pretty straightforward process with a torque calculator. However, I can't seem to find any analogous tools or equations for brushless motor torque.
It seems like people have had success with the DYS BE1806 and AX-2810Q, but simply based on the motor characteristics on HobbyKing I don't know what makes them better than others. I know that a lower Kv means higher torque, but how do you know that the Kv is low enough to drive your bot? [Social Media]
A: Mark J. You won't find drive train tools analogous to the
Tentacle Torque Calculator for brushless motors because brushless performance depends as much on the controller firmware as on the motor. All the possible combinations are staggering in number and largely theoretical in practice. The way brushless drive is done is to go for overkill and not worry all that much about the details. See this previous discussion in the Ask Aaron archives: Brushed vs. Brushless Drive.
There's a "rule of thumb" for brushless motors circulating out on the builder forums that appears to work well for drive motor selection. The rule states that a set of brushless drive motors together weighing between 2% and 3% of the total weight of the robot will have more than enough power and adequate 'thermal mass' to survive. By that rule a pair of DYS-BE1806s is about right for a beetle and twin AX-2810Qs is overkill (which might be just right for a Melty).
What makes a motor better than others? Somebody used it successfully on their robot and word got out. Brushless meltybrain spinner? You're on your own. Best luck.
Q: Set screws on my insect-class robot hubs are driving me crazy. They won't hold!!! All I've got is hand tools. Is there anything I can do to make my wheel/gear/pulley hubs more reliable? [Social Media]
A: Mark J. The tiny set screws as supplied with small hubs are generally inadequate to reliably hold against the forces imparted by robot combat. Exotic clamping/splining solutions that can eliminate the problem require custom machining. But don't despair -- it is entirely possible to massively improve the reliability of set screw hubs using only hand tools. Follow along:
Enlarge and tap the screw hole. Go big. A screw even a bit larger in diameter than the shaft is appropriate.
Grind or file a WIDE flat on the shaft. Not a divot, not a hole -- a flat. Grinding away as much as half the shaft is not too much if your screw is that wide.
Install a flat-bottom set screw -- not a 'cup' or 'cone' tip. Grind one flat if you must.
Buy a good quality hex key (Allen wrench) to properly torque down the screw. A good tool here does make a difference.
Loctite! If you've done the steps above, the standard blue Loctite thread locker is fine. A drop or two on the set screw threads before assembly will do. If you're trying to short-cut the list with just a standard size flat-bottom screw and a narrow flat on the shaft, blobbing nasty red threadlocker on the shaft and screw before assembly may help... a little.
Note - I've edited this post to remove a lot of back-and-forth questioning and general confusion about what was actually happening with the robot. The names have been changed to protect the innocent, but the outcome remains the same.
Q: Hey, its me from Anacortes again. I've run into a couple more hitches in the electronics of my beetleweight wedge. A while back you basically had to walk me through how to install the fingertech tinymixer properly, and at the time it seemed to work fine.
But now that I've put the chassis of the robot together, the right side drive motors suddenly reverse direction for a split second when I use more than 75% throttle. This happens every 1-3 seconds of drive, making it largely uncontrollable. I suspected that the tinymixer was the cause, and sure enough, when I removed it the motors responded to power normally.
I wanted to get some driving practice in, so I was just driving it around when the RX battery alarm went off on my FS-i6 transmitter and the robot became unresponsive. The transmitter RX battery display shows no bars and the speed/direction lights on the Scorpion are not responding to transmitter inputs. Is this some sort of hardware problem, or is somthing wrong with my setup? I'm running four kitbots 1000 rpm gearmotors with Scorpion mini esc and a 1300mah 95C 4s lipo. [Anacortes, Washington]
A: Mark J. Your FS-i6 radio is bi-directional: the transmitter 'talks' to the receiver, and the receiver sends telemetry (like the voltage level at receiver) back to the transmitter. That telemetry function causes the receiver to draw a lot more power than a passive receiver.
I looked up the specs on the BEC output for your Scorpion Mini. Its max output decreases with increasing input voltage to the Scorpion:
On a 2s LiPo (7.4 volts) the BEC on your Scorpion can supply 100 mA of current -- that is enough for your bi-directional receiver.
On a 4s LiPo (14.8 volts) the BEC on your Scorpion can only manage about 30 mA of current. That's enough for a standard receiver, but not enough for yours.
Your BEC couldn't keep up with the current demand. It overheated, the voltage dropped, and you ran into a series of worsening failures:
The tinyMixer has an operating input range from 5-12 volts and would be the first component to suffer from dropping voltage. The odd reversing of the right-side motors occurred when the voltage from the overheating BEC fell below 5 volts. Removing the tinyMixer temprorarily resolved the problem, but the current draw from the receiver was still too great for the BEC.
Your FS-iA6B receiver would be happy anywhere between 4.0 and 6.5 volts. After you removed the tinyMixer and and tried to get in some driving practice your BEC continued to overheat and the voltage continued to drop. Your receiver sent one last telemetry signal as the voltage plunged below 4 volts, triggering the RX low-voltage alarm on the transmitter.
Q: Ok so it just so happens that I have a BEC laying around. I jerry-rigged it to the wiring to power the receiver directly and removed the red wire power pins from the Scorpion receiver leads. The low-voltage alarm has stopped and the RX telemetry bars are full, but the robot still won't move. Any thoughts or ideas?
A: Patching in the stand-alone BEC got power back to the receiver and restored operation of the radio system, but the Scorpion ESC is internally powered by the output of its own BEC and that has failed. You can try re-connecting one of the red power wires from the Scorpion to the receiver to see if you can back-power the ESC circuits. I don't think it'll work, but it won't take much effort to try.
Q: OK, I plugged the red power back into the receiver, and now the left drive motors are responding properly but the right side is still dead. Also, the ESC heats up considerably whenever the robot is turned on now. I don't remember it heating up that quickly before the RX alarm went off. What can I do next?
A: Yea, the ESC is toast. All that heat is coming from the charred remains of the former BEC still trying to do something useful and mostly just turning current into heat. Here's what you do:
Make a little paper boat;
Drop in the Scorpion Mini;
Take it to the beach;
Set fire to the boat; and
Sail it off into the sunset.
Q: Hey its me from Anacortes again. First I want to thank you for diagnosing the BEC problem with my ESC [above]. I never would have figured it out on my own. Anyway, now that I've had some time to reflect on the untimely demise on my ESC, and the fact that I'm way over budget, I'm ready to look for a new ESC. Now, I've seen that some builders seem to be able to save space by using two basic ESCs rather than a single dual ESC. Knowing that I will be running four Kitbots 1000 rpm gearmotors on a 4s lipo, and using a BEC, would you recommend this? [Anacortes, Washington]
A: Mark J. You aren't using the on-board mixing or BEC on the Scorpion Mini ESC so your replacement can certainly be more 'basic', but your beetle wedge doesn't seem to have any space problems. Some of the commonly used single channel options won't handle a 4s LiPo battery so you may have limited choices. What ESCs are you considering?
Q: Well that's the problem, I actually do need more space in my robot. (I will be getting a smaller 4s battery, 1500 mah is just to practice.) But as far as ESCs, I don't really know where to start. I know this isn't a free engineering service, but could you please try to point me in the right direction?
A: There aren't a lot of choices. Take a look at this earlier post for a list of available options and a discussion of the cheap Chinese solution.
If you want to save a little money and are willing to do a bit of work, a pair of the Vex 29 ESCs might do the trick.
Given that you're running a stand-alone ESC you might just be able to get away with a pair of the Chinese JMT controllers (search eBay for 'JMT motor controller'), but you'll likely want to buy a couple of spares...
Q: I like the price and simplicity of the VEX 29 ESCs, but will running two motors apiece draw too much current? I don't know much about how current draw works, and I won't be saving any money if they get destroyed.
A: Current draw is dependent on the load placed on the motors, not the number of motors:
Pushing around a 3-pound robot with one Kitbots 1000 RPM motor per side takes a certain level of power that is loaded on those two motors.
Pushing around a 3-pound robot with two Kitbots 1000 RPM motors per side takes just about that same level of power, but now that load is distributed over four motors.
The total motor load and thereby the total current draw is close to the same for either case.
You can run this thru the
Tentacle Torque-Amp Calculator for verification. The distributed version of the calculator does not include the Kitbots motor in its pull-down selections, but I use the following motor settings in my personal copy:
Enter those settings, bump the voltage to 14.8, set weight @ 3 pounds, specify 3" wheels and one motor per side: the total current draw for the robot pushing full throttle against a wall comes to 2.81 amps spread across the two ESCs.
Keep all those settings and increase to two motors per side: the total current draw for the robot pushing full throttle against a wall stays at 2.81 amps spread across the two ESCs.
Now, 'real world' you've added some extra gearbox friction and bearing loss with the extra motors that will add a pinch to the current draw, but you're so far under the capacity of the ESCs that it's insignificant. If you like the VEX 29s, use 'em.
Q: I'm reading thorough The Variable Constant's Guide to Vextrollers and I think I get the gist of how to follow his instructions to hack each individual one. Now I have 3 questions about next steps:
Is it possible to use them for 1-stick drive?
There's a part at the end of the guide about a BEC making the second GND wire redundant that I don't understand. Can you explain?
Precisely how do I connect two VEX 29 ESCs to a single battery & receiver system?
A: Three answers...
Yes. Any ESC that accepts a standard PWM R/C signal can be used with your tinyMixer or (if you learn how to drive inverted) with the superior on-board mixing available in your FS-i6 transmitter.
The white (sometimes orange) 'signal' wire from the R/C receiver carries a pulsed electric signal to control the ESC. That signal pulse requires a path for the electrons to return to the power source to complete the circuit or the electrons just pile up and the signal goes nowhere. The standard 3-wire PWM cable includes a black (sometimes brown) 'ground' wire that provides this return path, but as long as the receiver has at least one ground wire any additional grounds are superfluous. Your stand-alone Battery Eliminator Circuit (BEC) provides a 'ground' wire to the receiver, eliminating the need for a ground from the ESC to the receiver. You don't need to remove the VEX case, you don't need to solder on any new wires, and all you connect from the VEX to the mixer or receiver is the white signal wire.
If you examine the receiver connector on the VEX you will find the little barbs that hold the connector pins into the plastic holder. With a small probe you can unclip the barbs for the red and black wire pins (video), pull them backwards out of the holder, clip the pins off, and they're ready to connect to the battery. Leave the white wire and pin in the holder and plug it directly into your tinyMixer.
See wiring diagram below.
If a mixer isn't used the white signal wires from the VEX 29s would plug directly into the receiver.
The VEX 29 does not have a BEC, so you will require either a stand-alone BEC or a weapon ESC with an internal BEC.
Q: So, I've got a question...
I'm working on the designs for an antweight (US) lifter, and was running the calculations of using a Silverspark motor to power the weapon. However, the math seems to be a bit... off. Here's what I have:
Lifter Arm Length: .33 feet
Maximum Lifting Weight: 2 lbs
Maximum Torque at Gearbox (ft-lb): (.33 feet x 2 lbs)= .66 lb-ft
Max Load At Gearbox (in-oz): (.66 lb-ft x 192) = 126.72 oz-in
Motor Stall Torque: 0.64 oz-in (Fingertech Silverspark @ 6v)
Torque Overage Factor: 2
Gear Ratio Required: ((126.72 ÷ 166) x 2) = ~1.5:1
Is that really it in terms of a gear ratio? All I need is about a 1.5:1 reduction to lift another ant with a Silverspark motor at 6v? I feel like I've done something wrong.
Also, I plan on running the motors at 12v - 14v rather than the recommended 6v. How does this affect the stall torque, or does it remain constant regardless of the amount of voltage applied to the motors? [Newton, Illinois]
A: Mark J. You were doing fine right up to the end. That last line should be:
Gear Ratio Required: ((126.72 ÷ 0.64) x 2) = 396:1
It seems that you inadvertently inserted the 166 oz-in stall torque of the Banebots RS-775 18v @ 18v that you used in your earlier calculation for a featherweight lifter instead of the 0.64 oz-in for the Silver Spark @ 6v.
Stall torque, stall current, and no-load RPM of a brushed PMDC motor increase proportionally with voltage. Increasing voltage from 6 volts to 12 volts will double both the speed and the torque of the motor, so your new calculation will be:
Gear Ratio Required: ((126.72 ÷ 1.28) x 2) = 198:1
At 12 volts, the 200:1 Silver Spark gearmotor should do.
Q: My 1lb antweight is exhibiting some odd behavior. The weapon blade is the fingertech titanium blade. I have a V-spec 2205 motor, a 450mah 4s 70c battery, a 20a brushless ESC and two fingertech ESCs with accompanying silversparks. When I spin my weapon up to max or near max throttle one or more of these behaviors *sometimes* exhibit themselves: The power LED starts flickering, the robot starts doing donuts, power to the weapon motor gets cut and it spins down. As soon as I turn the throttle down it fixes itself and its free to spin back up again. Is this a problem with the weapon ESC not being able to handle the weapon? [St. Louis, Missouri]
A: Mark J. Your primary problem is not your weapon ESC; the symptom of an overstressed ESC is a puff of smoke and unrecoverable loss of motor power. You didn't mention your weapon layout, but assuming that you're running the weapon direct-drive your problem is the weapon motor.
The FingerTech weapon blade as used as an add-on weapon for the Viper robot kit is designed to spin at 8000 to 12000 RPM under the power of an 1100 Kv direct drive motor. You're attempting to spin that blade at about 34,000 RPM with a 2350 Kv motor that has a lower torque constant than the design motor. The motor simply isn't capable of spinning the weapon to that speed given the maximum current output (about 30 amps) of the battery pack you've chosen. The motor 'bogs', you get serious 'voltage sag' from the battery, and your electronics 'brown out' -- which gives you the symptoms you're reporting.
Your battery won't put up with this level of abuse for long, and may have been damaged already. If the plastic coating is at all 'puffy' and no longer tight, the battery shoud be safely disposed of immediately -- under no circumstances either charged or used.
A larger battery pack capable of supplying the 60 or 80 amps the motor might require operating bogged down to perhaps 15,000 RPM in this application would simply transfer the failure point to the weapon ESC or the the motor itself, which is rated for only around 25 amps continuous current.
Scrap your weapon motor and pick a lower Kv unit that will be able to deliver enough torque to spin the blade at a more reasonable terminal speed. Don't over-do it. You'll likely want a battery with a bit more peak current output and - depending on the selected motor - a larger capacity ESC as well.
Q: The battery seems fine and that sounds like a big overhaul that I don't really have time for, as a short term "hot fix" could I simply off-set the throttle in my transmitter settings to prevent the motor from revving up to full speed?
A: The problem is that your weapon motor can't rev up to full speed because of the aerodynamic drag of the weapon at such unrealistic speeds. Applying less throttle will reduce the current to the motor, but it also proportionally cuts torque. You might well need to cut throttle to around half to keep all the components in their safe zones. Spin-up times will greatly suffer. Try a new throttle setting in a practice session and check temperatures carefully. I'm also concerned that your battery may not have the capacity needed for a full match with a bogged weapon motor sucking down big amps. Best luck.
Q: How come I never had this problem with a 3s 65c battery and my emax 2204 motor? Similar KV to current motor.
A: Increasing voltage has rather large consequences.
Wattage increases by the square of voltage; the increased voltage also increases the current. Bumping up from 3 cells to 4 cells (a 33.3% increase) results in 1.333 × 1.333 = 1.78 a 78% increase in wattage, motor output power, and peak battery drain.
A similar effect is seen on weapon aero drag. Increasing the weapon speed by 33% means that it has to move 33% more air out of the way per unit time and it has to move each unit of air out of the way 33% quicker. That's the same formula highlighted above. The motor now has to work 78% harder to maintain the faster weapon speed -- if it has enough torque to maintan that speed at all.
Q: Do you think just stepping back down to the old 3s lipo could solve this?
A: Quite likely, yes.
Q: Just a quick update for anyone in the future reading who has a similar issue, I tried the 3s and it didn't fix the problem, it just made my robot weaker. I'm going to try offsetting the throttle so its max signal is like 75% throttle. I can't see any reason this wouldn't work, as the robot works fine if I manually keep the throttle under 80% or so, so if I do not give any future updates assume that this fix worked... or alternatively that the blade flew off and lodged itself in my trachea.
A: The failure of the three-cell battery to solve the problem casts suspicion on a new component in addition to the high current draw issue. You haven't mentioned the specifics of your weapon ESC except to say that it has a 20 amp rating, but it's possible that it's faulty or has been damaged by the current load.
If your weapon ESC supports calibration to your radio signal range, run that calibration sequence and try the 3s battery again.
If calibration doesn't help, consider replacing the weapon ESC.
From 1,734 miles away that's about all I can suggest for a true 'fix'.
Q: This is the ESC: DYS XSD 20A Brushless Speed Controller. Are you suggesting that upgrading the current rating to say 30a could help or just that it may be faulty and in need of replacement.
A: The conversation has wandered around a bit -- lets recap:
I'm confident that your 'brown out' problem stems from attempting to spin a 6-inch weapon bar to 34,000 RPM with a low-torque/high-RPM V-Spec 2205 motor powered by a 450mah 4s 70c battery via a 20a brushless ESC. This set-up is pulling more current than any of these components can handle.
Based on your prior 'bot that ran a similar set-up on a 3s pack, replacing the 4s pack with a 3s pack should eliminate the problem -- but it does not. This leads me to believe that the weapon ESC may be faulty or damaged.
Running the weapon motor on the 4s pack with a reduced max-throttle weapon setting on your transmitter won't not work, but it doesn't address the actual fault.
I'm suggesting that you replace the weapon ESC and try it again on a 3s pack largely because I don't like leaving a mystery unsolved, but also because a faulty/damaged ESC may completely fail in combat without additional warning. If it passes this test you can go back to running it on 4s at reduced throttle with increased confidence.
If you want to run a 4s pack with full weapon throttle, upgrading just the ESC won't get you there. Neither your motor or battery pack are likely to survive on a 4s pack with your weapon load. If you plan on upgrading the motor and battery at some point you can certainly move to a higher capacity ESC now while running at reduced throttle -- it won't hurt.
Q: It's also been suggested to me that the BEC on fingertech ESC's isn't the best and this could also be part of the problem.
A: That was my first thought when I read your 'flickering power light' symptom, but overstressed BECs on tinyESCs behave like this. BEC output and receiver current demands don't increase with raising weapon motor speed, so why would the problem only manifest above a certain throttle level? Try an alternate source of receiver power if you like, but I'm not optimistic.
Q: THE STANDALONE BEC FIXED IT! I CAN GO UP TO 100% LUDICROUS SPEED WITH NO POWER ISSUES!!!!!!!!!!!!! (at least on the 3s battery, I'll update with the 4s later hopefully I don't run into an Icarus situation)
90 minutes later...
On further consideration I've decided to leave well enough alone and run on 3s, the 4s battery is heavier and it does more to improve top speed than spin up times. Top speed is something that I don't need (It's already stupid fast at 3s, 26000 RPM if the calculations hold true) and would decrease bite, spin up is good enough already and way better than last years set up. I'll use the extra weight for wheel guards. I do feel bad for having spent the money on the bigger batteries but I think I may just need to accept that this hobby is a money sinkhole by nature.
A: Super awesome! I think that's a solid decision -- but there is a troubling question remaining: what's drawing so much 5 volt power?
Q: Hey Mark,
I've been doing a little more research on combat robot weapons (specifically for beetleweights) and a popular choice for beater bars seem to be the Fingertech Beater Bar kit. I'm wondering if the 6061-T6 aluminum used in the Fingertech beater bar is a good choice in general for a beetleweight beater bar, as the RioBotz tutorial doesn't seem to show aluminum as a favorable weapon material for sharp one piece spinners. If I had a more expansive budget, I'd probably use some S7 or some other impact resistant steel and heat treat it but as I'm sure you're aware the costs for this can build up quickly. It looks like if I'm going to go for an eggbeater I'm pretty much relegated to the use of aluminum. So how strong of a choice is that kind of aluminum for a beater bar, or even aluminum in general, in the beetleweight class?
As a side note, I may end up ordering some aluminum from mcmaster-carr and machining it to have a higher MOI than the fingertech bar, while retaining a similar shape.
Thanks, Jack from Lake Charles, LA
A: Mark J. I don't think you understand how the FingerTech beater bar is used, Jack. Aluminum is used for the 'body' of the beater, but sharp-edged steel machine screws are threaded in as the 'impactors' that acually strike the opponent. The design is popular in part because of the ease with which the impactors can be replaced as they are blunted. It's much cheaper than replacing the whole weapon -- and the sharp edge of your spinner weapon will be blunted quickly.
The RioBotz Guide is a valuable resource, but it was written ten years ago -- before the availability of materials and components that are in widespread use in current robot combat. Small 'bot one-piece spinner weapons are commonly waterjet cut from pre-hardened abrasion-resistant steel (AR-400, AR-500...) which reduces cost by eliminating heat-treating and problems with warping that accompany S7 tool steel.
Insect-class robots are also now less about weapon MOI than they were ten years ago due to the overkill power available from hobby-grade brushless weapon motors. Once you can bounce your opponent off the ceiling of the arena at will the importance of high MOI spinners is greatly reduced.
Two suggestions
1) Pay attention to the warning that appears at the top of the Ask Aaron Robot Weapons Archive:
Aaron's Wisdom I've said this often, but builders don't want to believe me:
The weapon may be the least important system on a combat robot.
If you're not winning matches it isn't because you have a poor weapon.
Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.
2) Build yourself a robot and enter a tournament. You'll learn more in your first combat match than you'll learn from studying design for a year.
Date marker: June 2019
Q: What was the first "typical beetledrum" (2wd beetleweight, outboard wheels with wrap-around guards, centerline mounted drum directly mounted to the [longitudinal] frame rails) design? At the moment the earliest ones I can find are 'Weta' and 'Grande Tambor', but I get the feeling that the design goes back even further... [Asheville, North Carolina]
A: Mark J. Trying to determine 'who was first' gets very sticky very fast. The answer really depends on how far from 'typical beetledrum' you're willing to stray.
'Grande Tambor' made its first combat appearance at the Schiele Museum Clash of the Bots event in July of 2010. That first version of the 'bot had no wheel guards, did not use UHMW polyethylene frame rails, and had significant rear overhang not seen in a current 'typical' design.
'Weta - God of Ugly Things' made its first appearance at that same event. That first version of the 'bot had a belt-driven drum, lacked 'anti-wedge' fingers, and used an atypical 'inrunner' weapon motor.
The two builders learned from the strengths of each other's 'bots and both evolved toward the pervasive current design. If you are looking for earlier roots of current design you might need to drop down to the antweight class. 'Poco Tambor' made its first combat appearance in December of 2007 and had all the key design elements later upscaled for 'Grande Tambor'.
Q: Hi it's me from Anacortes again. It has been a week since my first competition with the beetleweight wedge I've been working on. 'Firecracker', as it was called, had its first battle against 'Dark Pummeler', and the short fight ended with my robot in five pieces. This being said, thanks to the kind and encouraging attitude of the other builders, it not a discouraging experience. I've begun a redesign of Firecracker, and along with the new ideas, I have some new questions.
I am looking into the prospect of making the front and back wall of my robot out of aluminum (6061) but I don't really have a solid idea of how strong it is in the world of combat robots. I know this is not a lot of info to work off of, but do you think that a 1/4 inch aluminum plate reinforced by nutstrip would hold up somewhat well as a front wall/armor? Ideally most of the hits would be taken by the wedge, but I've quickly learned that things don't always go exactly as planned... [Anacortes, Washington]
A: Mark J. A first-round draw against any of Russ Barrow's 'bots is a tough break -- initiation by fire. Glad to hear you weren't discouraged.
I'd be fine with 1/4" 6061 alloy for front and rear panels in a beetleweight, but I'm not a fan of relying entirely on nutstrip to hold the corners together. Machine screws are made of material and temper to optimize tensile strength -- not resistance to shear forces. Take a look at this video from Robert Cowan for details. You can use Robert's hardened pin technique, or you can 'slot and tab' (see illustration) the panels to take some of the shear load off the screws. That will greatly reduce the number of pieces you'll need to pick up after a match.
Q: I've got a 3D printed plastic antweight with a direct-drive drum weapon. The brushless outrunner weapon motor is pressed into the drum. The motor gets so hot after a couple minutes of operation that it melts the plastic and the drum starts to slip out of position. Do I need to put something between the motor and the drum or is there some way to cool the motor? [On-line Forum]
A: Mark J. Small hobby brushless outrunner motors are made for model aircraft and are designed to operate with a strong airflow to dissipate the considerable heat that they generate. If you cut off all airflow thru the motor by stuffing it into a blind plastic tube you're going to rapidly generate a lot of trapped heat -- more than enough to melt a PLA printed drum.
Drill some 1/4" holes from the outer surface of the drum into the central void of the drum. As the drum spins the holes will pull air out of the drum. Keep the vent holes in the motor base uncovered and you'll generate cooling airflow thru the motor. If you're lucky you can get it to whistle like a flute.
Q: Okay, so I've noticed that the teeth of the Saifu Antweight kit can be moved around. Normally, the teeth are placed with one side having the teeth in the far left and far right holes, whereas the other side puts the teeth in the middle left and middle right holes (the way I'm describing it makes sense, right?). I was wondering if there would be any advantage against any robots to place all four of the teeth either in the farthest holes or in the center holes. I know that this would fiddle with the way energy is distributed in the drum, but I didn't know if it would be worthwhile, or just an unnecessary thing to play around with.
AS ALWAYS, THANKS! [Chester, Illinois]
A: Mark J.Don't play with it! The 'normal' tooth arrangement as you describe it is the correct and most effective arrangement of the moveable teeth for all situations. Each of the four teeth has a clear path in its rotation which assures that the 'bite' of each tooth is maximized. It also maintains both static and dynamic drum balance.
I suggest that you read sections 6.3.1 and 6.3.2 of the RioBotz Combat Tutorial for a complete description of 'bite' for spinner weapons. Read the rest of the tutorial while you're there.
Q: This is me again from Anacortes! The Beetleweight wedge I've been building finally has it's electronics! I took your advice and am not going brushless right away, but I have plans to do so in the future. Upon the arrival of the final part, I excitedly botched together a test chassis using some spare 3d printed parts, cardboard, and a lot of hot glue. Everything works after testing using at 2s lipo, but I have run into two problems.
First, the green wire ports on the Scorpion Mini [ESC] I am using are nothing but trouble. I had a lot of trouble getting the wires to stay in as I positioned the motors and drove around, no matter how tight I screwed in the retaining screws. I really don't want to use any solder, as I do not plan to keep this esc in my robot, and want it to be easy to sell or reuse for another project once I switch to brushless. Is there a way to better secure the wires to the esc in a less permanent fashion?
My second problem is that I forgot to solder the capacitors onto the Kitbots 1000 rpm gearmotors that I bought. How exactly should they be soldered on? Is there a wrong and right way? I'll admit that I'm not exactly sure how they work, only that you've said that they "make thing easier for your escs." Anyway thank you so much for your time! [Anacortes, Washington]
A: Mark J. I'm always happy to get a progress report and I really love the cardboard test chassis! I think I can help you with your problems.
The Scorpion Mini ESC has screw terminals to accept power input and twin motor output wires. A wire slides into the terminal and tightening a screw clamps the wire in place. The problem you're having comes from the stranded wire shifting and loosening after it is clamped in place. You can prevent the wires from loosening by twisting the wire ends, applying a dab of soldering flux, and 'tinning' the wire with a touch of hot solder. Where you have two wires in a single port, solder them together and insert them as a single wire. This will prevent the wires from compressing and shifting when clamped.
The motor capacitors are important! Solder one leg of the capacitor to each of the motor terminals, as shown in the photo. Polarity does not matter - either leg to either terminal. You may want to slide a length of wire insulation over the bare capacitor leads to prevent accidental shorts. An explanation of the need for the capacitors plus some tips on properly 'breaking in' your motors are found in this archived post.
Q: Hey guys! Its a little specific and I'm really sorry if its not the kind of question you want to receive :/
I run a 3lbs Beetleweight hammer bot and for some reason my brushless drive weapon system always has like a quarter to a half second of delay on it! For the first version of the hammer I used basically the same weapon setup as an earlier hammer bot "Dain" did. I ran a Fingertech 2838 22 max amp, 300W 22 max amp, 300W 2970kv motor hooked up to the next generation R/C car Trackstar ESC that Dain had. I powered it all by a 850mah 3S battery, a Fingertech receiver and 20AWG wires (whoops). The entire time the hammer had this bad input lag. Despite being hooked up to the same drive receiver as a lag-less drive system
Since the first iteration I've swapped to a bigger motor (650W, 50 something-ish max amps) gotten and programmed a bigger 60A Trackstar ESC, have bought a higher 80C rating battery and I have even changed the radio and receiver and upped the wires to 16AWG and yet I still swing with 0.25 to 0.5 seconds of lag when compared to Dain throughout the entire match! It's maddening!
I've never been great with electronics and I'm honestly out of ideas as to what causes this. It was the exact same problem with the 30A ESC as the 60A, the same with 2 entirely different radio and the same with a bigger C rating and lower gauge wiring!! I really feel like I've cycled out all of the electrical components and still have the same issue. I would love your opinion on this.
Again genuinely sorry if this isn't the kinda question you're looking for on this forum. I've tried to include as much info to avoid the bad hamburger. Have a good one! [Ontario, Canada]
A:
Mark J. Your question is exactly the type of question we do like to see, Ontario. You've got a problem that has you stumped and it's a problem that other builders might run into as well.
The bad news is that you've put a lot of time and effort trying to fix the electronics when the electronics aren't causing the delayed response. The good news is that there's a little secret about brushless motors that I'm willing to tip. Follow along...
Brushed and brushless motors are very different:
Brushed motors are simple. Hook a brushed motor directly to a battery and it spins right up, producing huge chunks of start-up torque.
Brushless motors are not simple. Hook one directly to a battery and it will twitch and quickly melt; they require an intelligent external motor controller to supply each of their three input leads with power of the correct polarity at just the right instant and then change everything an instant later as the motor rotates.
Getting a 'sensorless' style brushless motor (like yours) to start rotating is a tricky problem. The correct current polarity to apply to each of the leads to spin the motor in the right direction depends on the relative positions of the magnets and coils in the motor, but with the motor at rest the sensorless controller does not know what that position is. The controller has to 'guess' and send a small current pulse thru one pair of the motor leads, then it waits to detect and analyse the electrical feedback from the motor when the motor starts to rotate in response to that current pulse.
This start sequence works well if the motor has a light load like an airplane propeller, but if the motor has a heavy load (like an overhead hammer) the small current pulse may not cause the motor to move enough to provide feedback to the controller. The controller will wait a few milliseconds and try again... and again... and again until the motor eventually twitches enough to allow the controller to figure out the correct polarity sequence and provide more current to properly spin the motor.
If there is enough free play ('slop') in the gearbox and/or belt drive between the motor and the hammer so that the motor can spin freely for about one full revolution before it takes up the load of lifting the hammer the whole start-up process takes only an instant. However, if the drive train beween the motor and load is very 'tight' and there is not enough free play to allow the motor to start its spin-up without load you're going to get the type of delay you describe.
You've given me a complete description of your hammer electronics but you have provided no details about the mechanics of the hammer drive train. Twisted Sick Robotics' beetleweight 'Dain' has the weapon motor mated to a 14:1 gearbox that then goes to a 3:1 belt drive for a combined 42:1 reduction. It's a good bet that the combined play in the two-stage gearbox along with a fairly loose timing belt allow the weapon motor enough no-load slop to get thru the startup sequence muy pronto. Given your description of the problem it's also a good bet that your unspecified weapon drive train does not have enough slop. Add some.
Reply: Hi Mark! I appreciate the in-depth tips. I run the exact same gearbox and additional reduction as 'Dain' and - credit where credit is due - WOW it works well! The only difference is that I run the additional reduction using specially cut gears, not belts. I'll be looking into a sensored weapon motor setup or swapping to a belt drive system now. Thanks for all your help! Cheers!
A: Very pretty, Ontario! It would be a pity to remove those bespoke gears; a sensored motor and controller will cure your response delay. May your opponents all have fragile top armor.
Q: Do you know of any good power switches for antweights? FingerTech seems to be out of stock for their mini power switches at the moment [Marysville, California]
A: Mark J. 'Good' depends on you design and current requirements.
If you really need 40 amps continuous current the FingerTech Mini Switch is a fine choice -- if it was 'in stock'.
If your design has room and allows access, my preferred method for a high current switch in a small and light package is to make a 'removable link' from a suitable wire connector -- see the diagram. I use Deans connectors for this purpose.
If you need a direct substitute for the Mini Switch and your 'bot pulls no more than 6 amps continuous (15 amp burst) you can use the FingerTech 3.5mm Switch/Charge Jack. Insert the plug and the robot is off; remove the plug and the robot is on.
The 3.5mm switches are used by the FingerTech Viper antweight kits, and they are both reliable and 'in stock'.
Q: Have there ever been any 1-lb robots that used bungee cords to power a spring-loaded weapon (eg axe or flipper)? And were they any good? [Newton Center, Massachusetts]
A: Mark J. Take a look at this post about successful UK antweight 'Militant' in the Ask Aaron archives. It uses rubber bands to power the flipper with twin servos providing the reset and triggering.
Elastic power is uncommon as a flipper or axe power source. Metal springs can provide more power for their weight and may be formed in compact torsion coils to save space and apply force directly in a rotational orientation. Take for example Team Run Amok's champion antweight 'Rat Amok'. Several of the weapon designs featured on our Spring Flipper Designs page could use elastic power, and could be modified to function as axe weapons as well.
Back to your question, I do not know of any 1-pound robots with bungee-powered weapons. Go ahead and make one.
Q: I was hoping I could get some feedback on my antweight. I'm trying to clean up this design a little. It has a reverse lifter like 'Firestorm' that uses nutstrip attached to a servo to push the plastic lifter forward. That works for the most part but it doesn't go forward enough to self right. Also the nutstrip sticks out the back of the wedge, making it susceptible to snagging.
I was wondering if you had a way to fix these two design problems? [Gaithersburg, Maryland]
A: Mark J. You have enough 'nutstrip' there to re-arrange into a compact '4-Bar' mechanism that will solve both of your issues. See the diagram below. Play with the lengths of the blue and green bars and the hinged attachment point of the blue bar to your lifter until you get enough extension to 'self-right'. It should all tuck neatly within the length of your 'bot.
Q: I'd like get into robot combat and I thought I'd start with an antweight. I've got a drawer full of parts from flying drones and I thought I'd modify some servos to power my first 'bot. What are the advantages and disadvantages of servo drive? [The Aether]
A: Mark J. The 1-pound class robots got their start back in the late 1990's when R/C gear came with servos included. Combat robot builders all had a bunch of unused servos in their parts bins, and when they figured out how to modify those analog servos for continuous rotation tiny robots started wandering around the pits. It didn't take long for the first fight to break out.
Twenty years later there are much better options for insect class propulsion.
A 'standard' R/C servo modified for use as a gearmotor is neither fast nor powerful. Servos are designed to respond to standard R/C receiver output signals by moving the output shaft to a specific angle relative to the servo body. They are not designed to operate as gearmotors and they are certainly not made to survive the abusive loads suffered by combat robot drivetrains. However, most analog servos can be modified to function as a gearmotor.
There are very few advantages to using a modified servo:
They have a small speed controller built in which simplifies wiring and reduces cost.
They just plug into a receiver output port to draw power and a control signal.
Their rectangular shape is generally simple to mount to the chassis.
The disadvantages are many:
Standard servos use fragile plastic gears and cases.
Tougher metal gear/case servos cost more than real gearmotors/controllers.
Servos require modification to convert them to continuous rotation gearmotors.
The power output of standard servos is very low compared to common gearmotors.
Servos require further modification to operate above receiver voltage level.
Even with modification, standard servos should not be run above 9 volts.
The table below compares the performance of 'standard' Futaba S148 servos modified for antweight drive to the FingerTech 'Silver Spark' gearmotor commonly used in antweights. You can see that the performance of the modified servos fall far below the true gearmotors. Overall, servo power drive just isn't worth the trouble in an antweight.
Antweight: 1" wheels
Futaba S148 Servo @ 6.0V
Silver Spark 11:1 @ 11.1V
Full Rotation
Speed Mod
Dimensions:
1.6 x 0.8 x 1.4"
0.63 x 0.63 x 1.5"
Weight:
1.6 oz
1.0 oz
Output Power:
1.1 watts
6.7 watts
Stall Current:
0.75 Amp
2.4 Amps
Stall torque:
38 oz·in
2.2 oz·in
10.8 oz·in
Shaft speed:
50 RPM
875 RPM
1573 RPM
Robot top speed:
0.14 MPH
2.6 MPH
4.7 MPH
8-foot sprint:
27.6 sec
3.2 sec
1.4 sec
Torque note:
Ants with 1" wheels may stall below 3 oz·in
Q: The performance table mentions a servo 'Speed Mod'. What's that and how is it done?
A: The basic servo gear layout is a four-stage reduction that includes two gears that rotate independently on a central shaft: the blue and red gears in the diagram below. In the 'standard' Futaba servos typically hacked in the early days of combat the total gear reduction adds up to 278:1, which gives an output speed around 50 RPM. To get any type of speed at all out of that slow shaft rotation requires very large diameter wheels -- awkward and bulky.
The 'speed mod' involves removing one of the four compound gears (the yellow 'Top Hat' gear in the diagram) and mechanically fusing the two gears (red and blue) that share the central gear shaft so that they will spin together as a single compound gear. This eliminates two of the reduction stages, leaving a total reduction of about 16:1 which spins the output shaft 17.5 times as fast.
Unfortunately, the speed mod also reduces the output torque by 17.5 times. Even with the smallest possible wheels the servos will stall under heavy pushing loads -- a highly undesirable outcome. The tiny servo motor simply does not provide enough power to make the 'speed mod' more than a novelty.
Q: What are your thoughts on cheapo Chinese ESC's like this one? [eBay link redacted] It's been reported to work up to 4s by others, and I plan on using a 3s battery with a 1000rpm kitbots 25mm gearmotor. [Middletown, New York]
A: Mark J. So, you want to trust your success at a tournament on a pair of $4.65 ESCs (shipped free from Hong Kong) pushed more than 30% beyond their rated voltage 'cause somebody says that they've gotten away with even greater abuse? Allow me to reprint an excerpt from Frequently Asked Questions #16:
Q: Where can I get really cheap combat robot parts?
A: You're going to spend a lot of time and effort building your 'bot and going to a tournament. When some component fails in combat and puts you out of the competition, you're gonna wish you hadn't gone cheap. In particular, don't scrimp on electronics! With experience you'll learn where you can save money, but it's not gonna be on key components like speed controllers and wheel hubs.
'Cheap' and 'Combat Robot' don't go together!
That said, some teams are willing to use the really cheap off-brand electronics, motors, and batteries available from China. Documentation is awful, customer service non-existent, quality control unheard of, and delivery uncertain -- use them at your own risk.
When 'bots with these ESCs start winning tournaments feel free to use them. Paying more for a component doesn't guarantee success in combat, but it does buy some peace of mind. If you pay $30 for an ESC and it fails you can blame the manufacturer, but if you pay $5 and it fails you can only blame yourself.
Comment: I believe the questioner was referring to a Reddit thread from a while back, the suggestion of those ESCs was made by Sean McCartin because he's used something like them successfully on 'Melanistic Leopard'.
I'm still shelling out for the more expensive ones, because it's only $30-40 difference in the end on a $300-500 bot, but you can't deny he's had tournament success with them. [Vancouver, British Columbia]
Reply: I'm not sure how long Sean used JMT controllers in 'Melanistic Leopard' or if he still uses them. He had been having drive controller problems a few months earlier and may have gone to them in desperation. Here's the unenthusiastic recommendation he gave in a Reddit post from March, 2018:
"As for drive ESCs, I know people would probably question my judgement for suggesting them, but I actually have a set of JMT 10A 1/16th scale RC Car ESCs in Melanistic Leopard (the no-brakes version, of course). The manufacturer rates them for 2S (technically 7-cell NiMH), but they run fine on up to 4S. You'll have to cut the switches off and short the switch leads, though. If you don't feel comfortable running something THAT cheap or at voltages beyond manufacturer specifications, Coboxite's suggestions [VEX29, Robot Power Wasp, Fingertech Tiny ESC, and Botbitz 30amp] are pretty good as well, though do note that the Fingertech TinyESC is going to be running really close to its current limits in the Beetleweight class."
Sean may have apologetically used JMT controllers in a tournament, but the JMTs are certainly not mainstream. I'm not going to recommend their use until they are.
Comment: Hey, no question, just writing in on the JMT speed controllers [#JMT]. My experience with them has been frankly less than excellent. I switched to them after having a poor experience with the VEX29s, but I've blown around six of the JMT controllers doing little more than just wheels-up tests and I've had several come in that had a "tap brake" despite specifically ordering them without it.
The BEC seems to be the main source of problems (even had one catch fire on me), and a controller being used to power the receiver seems to get alarmingly hot while just idling. They also don't seem to have much if any protection from motor noise. I personally use them in my beetleweight only to try and figure out just how many different ways I can get them to fail, and in some starter robots for young builders (where low cost is really the only important element).
I agree on your stance that you're better off paying for a more trusted and reliable speed controller. [Alberta, Canada]
A: Mark J. Everyone likes a bargain, and once in a while you can find a batch of Chinese ESCs that have some 'better than adequate' chips soldered to the board just because they were the parts available that day. The next batch of Chinese ESCs that looks kinda the same ain't gonna be as awesome. The JMT is one of those 'next batch' items.
The JMT controller's 8.4 volt max rating is likely based on BEC voltage regulator limits. Cliping the red power wires on the JMT receiver cables and powering the receiver from the weapon BEC (if you have one) will take all the load off the voltage regulator and cut down on flames. I suspect that's how 'Melanistic Leopard' was set up.
I understand the temptation; you can find the JMT 10 amp controller priced under $4. Just don't do it.
Four Years Later...
Comment: Hey, I saw you mention my use of JMT R/C Car motor controllers in 'Melanistic Leopard'. I just wanted to note that I used them for a short time in 'Shadow Leopard' after that but have since stopped, because their lack of drag brake became too annoying to put up with. I will note that since the post I made on Reddit back in 2018, I encountered a weird issue with them where they only actually work on 2S or 4S, and attempting to run them on 3S actually causes them to catch fire. They also tend to lose internal power to the logic side of the controller and the BEC when run on 4S, so the limiting factor to their voltage seems to be the voltage regulator.
I'm still willing to recommend them for people who are just messing around with fun builds, but I wouldn't put them in anything intended to be competitive. [InquisitorWarth]
Reply: Mark J. Thanks for the update. If you've only got $7 and you need a pair of brushed motor controllers for your beetle, JMTs may be your best answer. I know there are builders who love to save a few bucks but as your notes point out: cheap comes with weird -- particularly if you pay little attention to voltage ratings. I'm glad you were able to get some use from them and I think your current advice is sound.
Q: Hello, Mark! Quick question: I've seen people build full-body insect spinners from mixing bowls a couple of times now and was wondering: are they structurally capable of withstanding the impacts a full-body spinner in the insect classes would normally face? I don't think they'd be structurally sound enough to withstand constant abuse.
Also, how would one mount teeth to them? Can you weld teeth onto them, or would you have to fasten them using bolts? [Jacksonville, Illinois]
A: Mark J. Small mixing bowls are typically made from 304 grade stainless steel about 0.6mm thick. This steel is designed to be corrosion resistant and is fairly tough, but is neither hard nor resistant to deformation. If your idea is to simply add impactors and spin it up, it will likely not last thru a single fight -- but that's not how you convert a bowl into an insect spinner shell:
Bolt a thick steel ring to the rim of the inverted bowl to strengthen the structure and increase the rotational inertia. A couple of 'undercutter' impactors incorporated into the ring make for a nice bonus.
Welding a small impactor to the thin and malleable steel bowl is unlikely to work -- the localized impact force will buckle the bowl. Bolting impactors thru the bowl and a large 'fender washer' or reinforcing plate will spread the impact force over a larger area of the bowl wall and offer a better chance of survival. Large bolt heads will themselves make pretty good impactors.
The area surrounding the shell mounting hub should also have a reinforcing plate to prevent distortion in this critical area.
Even with these modifications the bowl isn't going to last long, but since it's all bolted together you can simply transfer the reinforcements to a new bowl. Have a couple drilled and ready.
Q: I am considering making bottom-mounted wedgelets for my 1lb antweight drumspinner. I would like to know if you have any tips for making and mounting them. [Cleveland, Tennessee]
A: Mark J. All the elements of a combat robot need to work together to be successful. I hesitate to discuss the design and construction of an element when knowing only that it will be attached to an antweight drum spinner. It would also be useful to know what metal working tools and skills the builder has available.
Q: About how much ground clearance do I need for a four wheel drive beetleweight wedge running Lite Flite or similar foam wheels on what appears to be a fairly smooth arena? Would 2.5 mm be too little? How about 5? I am trying to make my robot have a low center of gravity. I hope this is enough info. [Oak Harbor, Washington]
A: Mark J. Given your location I assume we're talking about the Western Allied Robotics arena? It's been a while since I last saw that particular arena, and 'appears to be fairly smooth' can leave you embarrassingly high-centered. I dropped a note to someone who knows the arena well asking for their opinion. Here's what WAR's Rob Farrow says about the arena:
Hey Mark,
Beetles fight in the big arena built-up from 4'x4' square panels made from steel sheet. The assembled floor is pretty flat but not perfect. 1/4" clearance is plenty and 1/8" is probably fine for a 4-wheel bot.
Hope this helps, Rob
Thanks Rob!
Q: I'm down to the last stages on my beetleweight, but I've run into a snag. My receiver is working, the signal goes just fine to my brushless weapon motor when tested on all three channels I'm using. However, my brushed drive motors won't respond on any of the channels. The motors run fine when voltage is applied directly, so I've narrowed the problem down to the ESC.
I'm using a Dual TB6612FNG from RobotShop, and I've triple checked all my wiring. Is there something I'm missing? Is the ESC compatible with an RC signal? It says it's PWM. Any help would be greatly appreciated. [Mark from Vancouver]
A: Mark J. So, you were out shopping for a beetle ESC and all the solutions were running about $50. Then you came across the Dual TB6612FNG for $4.95. What a savings! Did you at any point wonder why the Dual TB6612FNG was available at a 90% discount over the stuff other builders use for their beetles?
The reason it's so cheap is that is isn't a brushed motor controller -- it's a brushed motor driver, and there's a big difference.
Brushed DC motor drivers only provide power amplification based on a Pulse Width Modulation (PWM) signal and a separate 'forward/backward' signal. Your R/C receiver puts out a Pulse Position Modulation (PPM) signal intended to instruct a servo to move to a specific position. A motor driver does not have the capability to decipher that PPM signal.
Brushed DC motor controllers are DC motor drivers PLUS an on-board microcontroller that translates a specific input signal protocol (like PPM) into the PWM and direction signals that the driver requires. That microcontroller interface is the expensive part of a low-power controller.
Scrap the TB6612FNG. Its 1 amp continuous output is very light for a beetle and the expense/complexity of adding a pair of signal converters make it unworkable. Without full details of your drivetrain I hesitate to recommend a specific controller, but the Scorpion Mini ESC is pretty much bulletproof in beetles, has a good reputation in combat, and is very user-friendly.
Reply: Thanks once again! There's more of a learning curve on this stuff than I thought when I started out. I think that's the last hurdle for me to get over before Seattle. I decided to go with a pair of Wasps instead of the Scorpion because I simply don't have the space in my chassis for the Scorpion, but as I understand they're battle tested as well.
Q: What do you think of brush wheels for antweights? Too flimsy? [Syracuse, New York]
A: Mark J. Are we talking 'bristlebots'? Calling them 'brush wheels' is misleading -- wheels spin, bristle brushes just hop up and down.
Combat bristlebots like 'Vibrant' are novelties: poor speed, awful pushing power, no reverse. What part of that do you like? Some builders enjoy making weird 'bots just to be different. Avoid if you're trying to win.
Q: Would an HDPE unibody work for a beetle wedge if it was protected by grade 5 titanium plates? I was hoping the unibody would work as a sort of shock absorber to protect against high energy spinners. [Oak Harbor, Washington]
A: Mark J. Polyethylene unibodies are common and successful in insect class combat robots, but UHMW polyethylene is preferable to HD. An image search for 'UHMW beetle robot' will provide many examples. No metal cladding is needed -- save the grade 5 titanium for your wedge.
Q: Hey Mark, question about insect-class pneumatics:
I understand that a larger cylinder bore equals more useable force, however, how do you calculate the limit of this, when the slower piston speed due to larger bore becomes a greater limitation than the additional force?
For example, say my valve has a flow rate (scfm) of 2.1469; I am thinking of switching from a cylinder with 5/8" bore to 3/4". Could you point me in the right direction, even external links on the math behind calculating this?
Thanks! [Utrecht, Netherlands]
A: Mark J. It's very difficult to model the speed of pneumatic systems, in part because of the interaction of multiple system elements on gas flow and in part because of the compressible nature of gasses. Ultimate forc
Q: Just a question about beater bar kits. How does the weapon manage keep its axial position along the shaft? Would the weapon be prone to sliding since the needle roller bearings won't resist axial movement? Why don't they use thrust bearings on the sides? 
Comment: Hi. Julian here, the builder of Mako.
Q: My design for an antweight full body spinner is very tight on space. I don't have room for regular gearmotors, even the little N10 size, and I can't find any right angle gearboxes small enough and fast enough to work.
Q: What is the usual size of an 'insect sized' arena? [Lagos, Nigeria]
Q: These weapon performance numbers [at right] from the Run Amok Spinner Weapon Energy Calculator are looking a bit optimistic. I used ChatGPT to calculate the internal resistance of the SunnySky 2450 Kv 2212 motor since they only have the specs up to 1400 Kv version on the site.
Q: How do robots like Sauron and Vector design their pulleys to fit around the outrunner can? Do they just build it with equal diameter and press fit it on? [Ashburn, Virginia]
I have been pondering the usefulness of tangential drive for antweight (1 lb) applications, but have concerns regarding its compatibility with foam wheels. In particular, the Repeat Robotics Repeat Tangent motors seem to be effective on the likes of robots like "Super Space Turtle", but I have only seen them used with tires made via custom-molded polyurethane/rubber. Could it be that foam wheels are too "squishy" to create any meaningful contact points with the shaft or simply have a poor friction coefficient (if the latter, could this be mitigated using liquid latex/some other traction coating)? This seemed like a "too good to be true" lightweight 4WD solution. Any feedback is greatly appreciated. [Zanesville, Ohio]
Q: I'm a new bot builder trying to build an ant for the first time entirely on my own. I'm planning on doing a servo four-bar lifter and I'm planning to use four 30.4mm diameter 14mm wide LEGO tires. I'm curious about what you'd recommend for the drive motors to go with those wheels, given that the robot is a control bot. I've only build heavily aggressive horizontals before so it's a little different.
A: Mark J. Any specific guidance I might give would depend on design details you have not provided. Beetle, 4WD wedge, Lexan top panel -- are we talking about a BotKits D2? General comments:
Q: I want to get my D2 beetle running again, but I want to use something stronger than the generic Botkits 22mm gear motors to stay competitive and I can't get the Botkits motors to last on 4S. The Just 'Cuz Dragon 22mm Dartbox Gearmotors look great, but the weird clamp drive motor mounting system the D2 uses won't work with a square Just 'Cuz gearbox.
Q: 1lb beater bot guy here. I found out that Fingertech sells 2600 kv 2822 motors, which should do nicely for increasing the weapon speed. However, the page for the motor says that the motor draws 24A, and "over double the power output [of the 1100 kv motor] requires a larger ESC." The 40A ESC is a full 6 grams heavier than the 20A ESC, which I also may or may not have already ordered, so I decided to seek a second opinion.
If the receiver needs a lower voltage, leave the tinyESC leads alone and run the servo power and ground direct to the 7.4 volts:
If you still want to use a UBEC, the voltage regulator, battery, and servo are wired in like this, with power and ground (red and black) wires running to the ESCs and the signal wire (white) going to the receiver:
There are some refinements available, but it's late and it's been a long day.
Thanks Alex!
I think that gives a good overview of the weapon hub design.
1. Let's say my layout uses a two-blade system akin to Droopy's that spins the left blade counterclockwise and the right blade clockwise. Would the body spin clockwise (right) due to the conservation of angular momentum when the left blade is given more power than the right, and the body counterclockwise if the right is given more power? I want to make sure my understanding of the post is correct before continuing.
Q: What shaft to use for a beetleweight robot? [Indianapolis, Indiana]
Let's see if I can sort this out for you. In your problem description you refer to "only one output" wire:
There is a tiny three-position switch near the blue wire attachment to the circuit board. This switch is used to select one of three ESC modes. One of these three modes will give you single-stick "mixed" control of the two motors on CH1 and CH2, but the FlipSky instructions for the switch make no sense:
Q: It's me, Iceywave. Here to ask an actual question about building a robot!
Aluminum is in general far too soft to make suitable armor for the current beetleweight level of competition. When they were made eight years ago, both versions of antweight 'Dark Pounder' shown used formed titanium for their bodies, and other 'bots from the team used 'spring steel'. Current wedge material tends toward thick flat panels of abrasion resistant steel. I don't think that "hammered around a tree trunk sheet metal" is a good bet in 2024.
Second question: I don't know what your previous designs looked like, but if you build the current design I foresee continuing traction problems in your future. You've placed your wheels as far back in the chassis as possible to get chassis clearance in both upright and inverted modes. Doing so leaves a lot of the robot's weight supported by those long forks.
Q: this was our earlier design, and it had the most abysmal center of gravity possible, which was over the forks, we are currently using 2.25 inch wheels, what do you thing would be the best size option?
Q: I was desoldering a FingerTech Silver Spark and the little metal tab that I solder to the wire to broke off. I tried adding some solder to the remaining visible metal but its not sticking well. Is there anything I can do to save it or is the motor just screwed? 
A: Mark J. That's how 'Silent Spring' does it: a socket head shoulder bolt with a nylock nut (photos below). Note that you're going to torque this down, so you're going to need a substantial spacer block around the shoulder bolt between the two chassis plates to take the pressure.
A: Mark J. Thank you, Ryan. Your 'Shock!' weapon design is well thought out with the lower bearing in the weapon disk itself and the upper bearing in the printed pulley. In their question, Redmond spoke of a design with "bearings in a printed hub/pulley attached to the disc" which led to my comment that I wouldn't trust a printed hub holding both bearings. I think you might agree that placing both bearings in the printed pulley/hub would be risky.
Q: For my horizontal spinner, the motor I want to use is too tall for the robot with the pulley attached as well. There aren't any shorter motors available that can handle the current I need to run, and making the chassis taller would be a waste of weight and harm the rest of the design. I've heard that some robots put the pulley directly on the outside of the brushless outrunner. How would I go about attaching this to, say, a BadAss 2305? 
A: Mark J. The components of a servo mirror the components of a robot drivetrain: a PMDC motor, a gear train, and an electronic speed controller. The servo also has a positional feedback circuit that enables it to attain and hold a specific position.
Q: Now I did create an antweight of Run Amok however i might have created the wrong kind of antweight what I meant was actually a fairyweight a uk antweight and the rc you send me a link to was convenient and it would be nice to try and find an rc to create a fairyweight version of Run Amok something a little bit shorter not too short for a fairyweight Run Amok scaled R/C per say? 
Q: I'm designing a beetle with a spring powered overhead axe. A spring is cocked and then released, rotating the axe into another robot. I'm going to be using a 21 in-lb torsional spring. I'm aiming for at least 80 joules of energy storage, and according to my calculations that 21 in-lb torsional spring should hold 105 joules:
First: The drive ESCs are dual-channel units made to independently control two motors with up to five amps of current to each motor. You have both outputs wired together to power a single motor, presumably an attempt to get ten amp capacity from the ESC. This Does Not Work! Each of the two independent channels has its own has its own 'clock driver' to time the output power pulses, so the power pulses will be out of synch with each other. This can cause both control and power consumption issues.
Many insect-class builders use simple wood screws for this type of connection into soft plastic with good results -- but there are special 'thread forming' screws made with widely spaced and deep threads that are designed to hold in thermoplastics; search for "Plastite" screws. Pick a suitably long screw, drill the correct diameter pilot hole, and sink 'em in.
If the part will be removed frequently you can cross-drill the chassis and install barrel nuts with machine screws. This type of fastening is commonly used in assemble-it-yourself furniture. They're more work than simple screws and you may have trouble finding small barrel nuts, but the screws will break before they pull thru.
Although it has fallen a bit out of favor you can certainly use FingerTech Nutstrip. It adds a bit of weight and takes up a little room, but it holds well.
Q: how do you convert a servos lifting power to a servos crushing power? [Vestal, New York]
I'm not following your contention that a 'Choo-Choo' linkage has a reach equal to the radius of the driven disk. I make the reach equal to the diameter of the disk plus the length of the "fling" arm attached to said disk (the dark blue arm in the animation). In the animated example the reach is almost three diameters, and the design could be modified to make it more.
The reach of a slip-geared winch is dependent on the circumference of the pinion gear that drives the spur sector. A small pinion will require a larger portion of its diameter to be 'toothless' in order to provide adequate slip clearance for the spur sector. In the animation, the reach is about 2/3rds the circumference of the pinion.
Response: P.S. Your response was so quick and cheap that I'm starting to wonder if it's any good 
Q: I'm 3d printing a 1-lb grappler bot. I have a servo that I have been testing with but when I plug it into my receiver it just steals the power from my motors. What am I doing wrong? [Social Media]
To your questions:
Shortening Rear Bar [Crank] to 12 mm will prevent the initial 'dip' and allow the spreadsheet to run correctly, but the performance of the system is not very good: there is a very limited range of servo motion combined with a high torque requirement.
2) The recommended lengths for Front Bar [Rocker]: D are critical for getting best lift height from lifters powering the Rear Bar [Crank] over a full range of motion. Lifters powered by the Front Bar [Rocker] may ignore the recommended Rocker length range, but should note that Rocker motion may be limited by very short Rear Bar [Crank] lengths.
One last note: You're currently entering measurement values in grams and millimeters so the units for torque are expressed in g-mm. If you enter the lengths as centimeters the torque output will be in the standard g-cm. Yes it's easy to convert, but I suggest staying in standard units to avoid confusion.
A: Mark J. Since you didn't tell me anything about your current wedge I'm unable to suggest specific improvements. In general, playing the 'lowest wedge' game can be frustrating.
Q: Hello mark, first I just want to say that this website is 
and I only wish that I had found It sooner!
Thanks! [Amarillo, or maybe Chicago?]
Activation of the thwack takes place in the first half a revolution of the drive, so we're interested in torque very low in the RPM range. If we assume that 'throttle response' is a function of torque, we can see from the chart that the Repeat Brushless Drive mirrors the low RPM response of the BotKits gearmotor (presumably their 16MM DC gearmotor). So, a reasonable starting point for your thwack drivetrain might be to input the torque specs of the BotKits gearmotor into the RioBotz design formula. But there's a potential problem...
If you'd rather swap out your servo I have a couple of low-price options:
Q: Hello,
Q: Salutations!
Q: Hello, it's the builder of Gaston again [see six posts down the page]. I took your advice, and created a completely new design for Gaston. I moved the center of mass closer to the wheels, and switched the motors to these: ServoCity 970 RPM Economy Gear Motor
According to the team tentacle calculator, my 'bot's top speed will now be about 4mph. However, according to the results from the calculator, I don't even need a 0.1 amp hours for a 3 minute match! Did I do something wrong, or are these motors just that powerful? Also, I attached a new rendering of Gaston (don't worry, the wheels and shafts are a work in progress, the body is the main focus). What are your thoughts on the new chassis shape and the motors? Thanks!
Everyone knows 1-pounders use those 16mm gear motors (like those from Fingertech) for 2WD systems. Am I able to use four of those motors to make a 4WD system for a 3-pounder, or is it too much weight that could bend the shafts? If this is the case, what would you recommend for a 4WD system that could squeeze into a 3-pound vert?
1) I'm overweight, and looking to trim some fat around the edges where I can. Currently I have six Shoulder Bolts between two AR500 disks and a TPU pulley, the whole system weighing about 6oz. I had added that many as I was worried about the weapon flexing too much on impact due to the TPU, ultimately warping or bending the weapon. In hindsight however, I could be overengineering a piece I don't have to. Would you recommend removing some of these shoulder bolts?
Q: I have an antweight flipper built on a FingerTech Viper chassis. Recently attended my first event, and while the robot survived every match it spent a significant amount of time getting flipped over. This event allowed unlimited magnet downforce. Many events specify a limit on downforce (usually 1 lb for an antweight) and at some point too much downforce over-stresses the drive motors. I am planning to mount magnets on the bottom of the chassis in front of the wheels, but they won't be contacting the arena floor.
Hello, Mark! I've been working on and off with my ant as of late and have two design-related questions: [Naperville, Illinois]
Q2: How would I go about fitting the bearings into the weapon? I don't have access to any fancy or expensive tools and figured it wouldn't be very easy to just press-fit two metal parts together.
Q: Hi again, this time having a problem with Lipos…
Q: About my recent question concerning chargers, I do have a fingertech, proper lipo charger, which came with the fingertech viper kit, but I don't know how to use it, and also my £3 BBB charged batteries much faster than 90 minutes when it worked. If my fingertech charger isn't suitable either, could you link me with a decent, working one with correct accessories and a video on how to use it?
Many thanks again!
You'll need to add a switch in the battery line, as covered in the assembly notes on the Bristol Bot Builders website. If the weapon motor spins the wrong direction, reverse any two of the three weapon motor connections to the weapon ESC.
Q: Hi, Still working on this second antweight and after will definitely move up to a beetle, but still after 4 design revamps am struggling to fit all of my electronics inside my robot - I have looked over multiple designs on the internet but am still having trouble trying to fit everything into a 4' cube - bearing in mind this is a spinner. I was planning to make something not too dissimilar from the pictured robot but was wondering how all of these electronics were fitted in. Probably not a very answerable question but any tips you can give me would be much appreciated.
Thanks! [Oxford, England]
Q: Hello!
One weakness of the Scorpion Mini is its battery eliminator circuit (BEC). Does the hot chip look like the photo at right, and is it next to the 'BEC' label on the ESC board? That's the voltage regulator that reduces battery voltage to 5 volts to power the ESC and the receiver. Either the voltage regulator has gone bad or your receiver bus is drawing too much power. Take a look at this post in the Ask Aaron Radio and Electronics Archive that covers some of the symptoms that can result from overloading the Scorpion BEC. It sounds like you've cooked the voltage regulator.
A: Mark J. You're considering two materials with very different properties.
A: The problem with the tri-blade isn't stability, it's lack of bite. You need to move two of the impactor tips in far enough to make room for a deep impact by the remaining impactor. I only mentioned 'stable' because 'Mini Mulcher' is well known for the serious stability problems it had with an earlier spinner design. See: Polhode Motion for a full explanation.
Q: "Mounting plates" sold by Fingertech have served me well on ant & beetle motors, is there an equivalently easy solution for fairyweight motors? [Woburn, Massachusetts]
Q: I stumbled onto some crazy butane jet lighters that are intended for cigars but seem pretty dang impressive. Would it be practical to put one of these in an ant or beetle? [The Aether]
Q: Hi there. I purchased an Endbots DESC lemon to use in a Beetleweight project, but am finding acquiring a DSM2 transmitter in Europe troublesome. Is it possible to add a different receiver to the project to allow me to use an alternate transmitter? Failing that, are there any other dual channel ESCa that work well for beetles?
A: Mark J. Did I really say that? A pneumatic system can deliver a great deal more speed and power to a hammer or lifter weapon than a direct-acting electric system -- but they are also a great deal more complex and dangerous. Whether they are 'the way to go' depends on your experience in building combat robots, expertise in metalworking, access to a machine shop, and expectations of performance.
Since the last time I posted my beetleweight has gone from having a hammer to a hammersaw. I had a major issue with the hammer this entire season where the pinion gear thats pressfit onto the hammer motor keeps breaking free of the shaft, killing the hammer.
Q: Hey Mark, I've taken up the helm of my university's Beetleweight team and we are building a new robot this year. We built one last year but it was a wedgebot and essentially a crash course on how not to build a combat robot: we went way over the weight limit, 3d printed a chassis, and haphazardly threw some silver spark motors in there. Surprisingly enough it ended up working but thankfully I had the foresight to never enter it into a competition.
Q: I'm building an antweight with a lifter plow, kinda like 'Duck!'. The lifter is powered by twin servos that need to run in opposite directions to work together on the lift, but my transmitter only has elevon mixing and I'm already using that for the drive motors. I have a Y-harness to connect both servos to a single receiver port. Can I re-wire one servo to respond in the opposite direction? 
Q: Set screws on my insect-class robot hubs are driving me crazy. They won't hold!!! All I've got is hand tools. Is there anything I can do to make my wheel/gear/pulley hubs more reliable? 
Q: Hey, its me from Anacortes again. I've run into a couple more hitches in the electronics of my beetleweight wedge. A while back you basically had to walk me through how to install the fingertech tinymixer properly, and at the time it seemed to work fine.
A: Yea, the ESC is toast. All that heat is coming from the charred remains of the former BEC still trying to do something useful and mostly just turning current into heat. Here's what you do:
Q: Hey its me from Anacortes again. First I want to thank you for diagnosing the BEC problem with my ESC [above]. I never would have figured it out on my own. Anyway, now that I've had some time to reflect on the untimely demise on my ESC, and the fact that I'm way over budget, I'm ready to look for a new ESC. Now, I've seen that some builders seem to be able to save space by using two basic ESCs rather than a single dual ESC. Knowing that I will be running four Kitbots 1000 rpm gearmotors on a 4s lipo, and using a BEC, would you recommend this? [Anacortes, Washington]
Q: Hi it's me from Anacortes again. It has been a week since my first competition with the beetleweight wedge I've been working on. 'Firecracker', as it was called, had its first battle against 'Dark Pummeler', and the short fight ended with my robot in five pieces. This being said, thanks to the kind and encouraging attitude of the other builders, it not a discouraging experience. I've begun a redesign of Firecracker, and along with the new ideas, I have some new questions.
Q: I've got a 3D printed plastic antweight with a direct-drive drum weapon. The brushless outrunner weapon motor is pressed into the drum. The motor gets so hot after a couple minutes of operation that it melts the plastic and the drum starts to slip out of position. Do I need to put something between the motor and the drum or is there some way to cool the motor? [On-line Forum]
Q: Okay, so I've noticed that the teeth of the Saifu Antweight kit can be moved around. Normally, the teeth are placed with one side having the teeth in the far left and far right holes, whereas the other side puts the teeth in the middle left and middle right holes (the way I'm describing it makes sense, right?). I was wondering if there would be any advantage against any robots to place all four of the teeth either in the farthest holes or in the center holes. I know that this would fiddle with the way energy is distributed in the drum, but I didn't know if it would be worthwhile, or just an unnecessary thing to play around with.
Q: This is me again from Anacortes! The Beetleweight wedge I've been building finally has it's electronics! I took your advice and am not going brushless right away, but I have plans to do so in the future. Upon the arrival of the final part, I excitedly botched together a test chassis using some spare 3d printed parts, cardboard, and a lot of hot glue. Everything works after testing using at 2s lipo, but I have run into two problems.
Mark J. Your question is exactly the type of question we do like to see, Ontario. You've got a problem that has you stumped and it's a problem that other builders might run into as well.
A: Mark J. 'Good' depends on you design and current requirements.
Q: Have there ever been any 1-lb robots that used bungee cords to power a spring-loaded weapon (eg axe or flipper)? And were they any good? [Newton Center, Massachusetts]
The 'speed mod' involves removing one of the four compound gears (the yellow 'Top Hat' gear in the diagram) and mechanically fusing the two gears (red and blue) that share the central gear shaft so that they will spin together as a single compound gear. This eliminates two of the reduction stages, leaving a total reduction of about 16:1 which spins the output shaft 17.5 times as fast.
Q: I am considering making bottom-mounted wedgelets for my 1lb antweight drumspinner. I would like to know if you have any tips for making and mounting them. [Cleveland, Tennessee]
Q: About how much ground clearance do I need for a four wheel drive beetleweight wedge running Lite Flite or similar foam wheels on what appears to be a fairly smooth arena? Would 2.5 mm be too little? How about 5? I am trying to make my robot have a low center of gravity. I hope this is enough info. [Oak Harbor, Washington]
Q: What do you think of brush wheels for antweights? Too flimsy? [Syracuse, New York]
Q: Would an HDPE unibody work for a beetle wedge if it was protected by grade 5 titanium plates? I was hoping the unibody would work as a sort of shock absorber to protect against high energy spinners. [Oak Harbor, Washington]