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Questions and Answers about Combat Robotics from Team Run Amok.

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Fifteen Years of Ask Aaron [Click Me] Ask Aaron banner image
6736 Questions and Answers about Combat Robotics from Team Run Amok

Team Run Amok receives a lot of email about designing and building combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) asked for a question and answer page to document our responses.

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click the blue button.

The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ

Caution Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot. Do not operate combat robots without proper safeguards.
 

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...

Electric Lifter FAQ

Q: How well do clamping-style shaft collars resist axial shock? Cheers. [Sydney, Australia]

A: [Mark J.] Many designs qualify as a 'clamping' collar, and some are better than others for both torque transmission and axial shock. Holding 'well enough' depends on the specific application, information you have elected to withhold. I can comment that clamping collars are generally preferable to set screw collars.

Q: If I were to machine my own clamping-style shaft collar and shaft, would there be any benefit to including a Morse taper on the mating surfaces?

A: Adding a matching shaft and collar taper will greatly improve axial shock resistance in one direction -- and entirely screw up resistance in the other direction. Unless the shock loading direction is completely predicatable (and nothing in combat robotics is completely predictable) I would strongly avoid any taper.

I'm assuming that you're asking about shaft collars because your application requires axial adjustment along the shaft. If not, a simple machined groove and snap-in circlip are the standard and preferred solution.


Q: Hi Mark, I've had a robot weapon idea bouncing around in my head for a little while and I wanted to bounce it off someone else to figure out if it was conceptually sound or not.
The idea is to build an overhead bar spinner, but make the bar out of a material like styrene butadiene rubber, then make teeth out of steel that would go on the ends. The theory being that when the bar contacts another robot the teeth would hit like normal but the contact time is then increased due to the deforming of the rubber.
This would lead to a greater transfer of momentum so that opposing robot would get thrown harder and the shock load on the “floppy bar” robot would be reduced, right? Is that a valid strategy, or is there something I'm missing?

Thank you for your insight, Joe [Glendale, California]

A: [Mark J.] I believe you're severely underestimating the energy level of an effective combat spinner weapon, Joe. The impacts are so violent that a solid steel bar undergoes elastic deformation of the type and magnitude you propose for a rubber bar, and a rubber bar would flex so much as to qualify as a type of 'flail'. Scroll down eight posts to find an earlier discussion on flail weapons. Go read that and come back. I'll wait...

In addition to being ineffective, flails require that they be positioned such that the extreme 'wobbly' deformations they experience on even a glancing impact don't send the impactors into contact with the robot itself. A robot with a flail in an overhead bar position is going to be at least as dangerous to itself as to its opponent. I'm gonna have to give this concept a strong 'nope'.

Second opinion:


Q: Hey Its me from Anacortes again. I've been having traction issues lately with my Beetle weight wedge. I seem to have three options: use magnets, make my tires more sticky, or use wider tires. Magnets obviously have disadvantages, such as getting stuck to the floor or other robots. I was thinking of using carpet tape around my wheels like Robert Cowan's "Psychotic Break," but I don't know if the stickiness would last all three minutes, especially in a dirty arena. Wider tires is also an option, but they would be much heavier and more expensive than the Fingertech wheels and hubs I'm using now. Not to mention the width of the tires would make them hard to protect, and more vulnerable to spinners.

So in your opinion, what would be the most viable option? [Anacortes, Washington]

A: [Mark J.] Great traction is something best designed into a 'bot from the start. I don't know the layout of your wedge, but you may be able to shift some components or change wheel placement to get optimum weight on the drive wheels. See section 2.7.7 of the RioBotz Combat Tutorial. This isn't usually a problem for 4-wheel drive 'bots.

If it's too late to consider radical design revisions, we can look at some 'fixes'.

  • Magnets The usual problem with magnetic downforce is that it's overdone. A couple of small chassis-mounted magnets positioned very close to the wheels can give a significant traction boost without turning the whole 'bot into a magnetic hazard. Just a pound of downforce on a beetle can make a big difference -- if you want to experiment.
  • Sticky Tires The carpet tape on 'Psychotic Break' was a desperation attempt to adjust for the poor weight distribution on a big horizontal spinner. In a dirty arena the crud would build up VERY quickly and render the traction worse than it was to start with. A more conventional approach is to coat foam tires like yours with liquid latex or silicone rubber. This doesn't actually make them 'sticky', but does provide a surface with much greater 'grip' than the foam. See this post in the Ask Aaron Archives.
  • Wide Tires switching to wider tires does not generally provide better traction. See this post in the Ask Aaron Archives for an explanation.
My suggestion is to try a latex or silicone coating on your tires and clean them with alcohol and a rag before every match. If you decide to try magnets, keep them small and close to the wheels.
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: I'm having some difficulty making a brushless saw weapon motor run off the Channel 5 switch of my FS-T6a radio. When I first built my 'bot I simply plugged the weapon ESC (Great Planes GPMM2130) into Channel 5 and when I flipped the switch the motor would reach full throttle. This has been working fine, but at the last event it stopped working. Flipping the Channel 5 switch now just makes the weapon ESC beep and twitch the motor. Plugging into Channel 6 gives a similar response on its switch.

What's going on here, and how can I bring back on/off switch control? [Whereami, Idunno]

A: [Mark J.] Your weapon ESC has a 'Safe Start' arming sequence that requires a 'full-off/full-on/full-off' sequence from the radio to arm itself before operation. The FlySky T6a has an unusual interaction between the transmitter switches and knobs; the switches just activate and deactivate their associated knobs. Those knobs got bumped out of 'full on' position and now the ESC never gets a 'full-on' command -- it's failing the arming sequence.

Here's your correct arming sequence everytime you power on the 'bot:

  1. With everything off, set both transmitter switches (SW.A and SW.B) off and rotate both knobs (VR.A and VR.B) full right.
  2. Turn transmitter 'on'.
  3. Turn robot power 'on' (one or two beeps).
  4. Flip weapon switch 'on' (one or two beeps).
  5. Flip weapon switch 'off' (four beeps) - ESC is now ready.

Comment: The ESC made the beeps and started to work. I turned it off and on a couple of times and it still works. Thanks!


Q: I've heard of an insect class robot that was powered entirely by its spinner weapon making contact with the arena floor. What can you tell me about that robot? Would it be worth building something similar? [North End of Southern California]

A: [Mark J.] That would be Gene Burbeck's beetleweight 'One Fierce Low Ryda' (build log). The 'bot fought at the 2008 Motorama event where it did quite well, winning 6 of 8 fights.

The huge friction-driven horizontal weapon blade ran on a live spinning shaft that stuck out the bottom of the robot, where it was fitted with a 'cupped drive nub' of traction material. When the weapon was parallel to the arena floor the shaft rested on the floor like the point of a spinning top. Tilting the robot chassis with servos attached to the un-powered rear wheels changed the contact point of the nub and caused it to drag the robot in a corresponding direction. Greater tilt equals greater speed.

'One Fierce Low Ryda combat robot

Video One Fierce Low Ryda vs. Ron Video One Fierce Low Ryda vs. D12

Gene's 'WackerDrive' is pretty much the only possibly competitive design that hasn't shown up on the BattleBots reboot. If you're looking for an unusual design to impress the competitor selection committee I'd say it's definitely worth a shot.


Q: Hey Mark, I want to try building a sensored brushless motor powered lifter. I'm looking at a worm-drive gearbox with an additional belt-drive stage so I can just "park" the lifter with the motor unpowered and not worry about it slipping back down.

The motor I'm considering gives a "30-second Max Current" rating but does not provide a torque constant (Kt). Is there a way to calculate how much torque I can get from the motor at that 30-second current level? [The Panhandle]

A: [Mark J.] You're very brave to try a brushless lifter, but your plan sounds workable. You don't have the motor Kt, but I'm sure you have the speed constant Kv, so there is an equation that will give you an approximate torque level at a given current draw. I'd dial back the number it gives by about 20% to adjust for 'real world' conditions:

Torque (oz-in) = (1352 * Amps) / Kv
Example Your motor has a Kv of 1800 RPM/volt and you want to hold your current draw to 20 amps. The equation is:
(1352 * 20 Amps) / 1800 = 15 oz-in torque
Gear for 80% of that number (12 oz-in) at maximum lifter load and you should be OK.

Q: I've upgraded my R/C system to a Spektrum DX6i and I swear that my bot is slower now. I'm using tinyESCs and Elevon transmitter mixing. The mix rates are set to 100%. Is there some setting I've overlooked? How do I get my speed back? [Anaheim, Azusa, and Cucamonga]

A: [Mark J.] This is a known issue with Spektrum Elevon mixing. Due to an error in transmitter firmware the two mixed channels each only provide about half the full travel response. You can bump the mix rates up to the maximum (125%), but that still doesn't get you all the way to full throttle. Here's the complete fix:

The Programmable Mix Fix - Double the response of the elevator channel by mixing it with itself in a programmable mix:
  1. From the main screen, press and release the roller to access the "ADJUST LIST".
  2. Scroll to "MIX 1" and select.
  3. Set both master and slave channels to "ELEV"
  4. Set the "RATE" for both "D" and "U" to 100%.
  5. Set "SW" to "ON".
  6. Set "TRIM" to "ACT".
  7. Exit to the Main Screen.
You can repeat the above steps on "MIX 2" with "AIL" for master and slave channels to bring the turn rate up to full speed if you like, but most 'bots have their turn rate set too high already. I'd leave the turn rate alone.

Alternate Solution - If you'd rather not fiddle with the transmitter, you can change the response of your motor controllers to provide full throttle at the lower signal level. Follow the calibration procedure on the setup sheet that came with your tinyESCs.

Q: That's got it! New question. My DX6i transmitter is currently powered by four AA alkaline batteries, but I have a spare 2s lipo battery that will fit inside the battery compartment. Can I patch in a matching connector and power my DX6i transmitter with the rechargeable 2s lipo or would it be too much voltage?

A: The Spektrum transmitters have a wide range of acceptable voltage input. I'm told that they will operate without issue from 2s, 3s, and even 4s LiPo packs. So, you can do it, but the capacity of a LiPo small enough to stuff into that battery compartment is going to be pretty small compared to a set of high-quality, low self-discharge NiMH AA cells.


Q: I know this is an odd question, but do blades with curved edges tend to get better bite or any other benefits, or does it just sacrifice a lot? For context, I'm referring to a blade with a slight curved indent into one side, somewhat like a scimitar or other related weapon. [Former Flail Idiot, Missouri]

A: [Mark J.] 'Bite' is a very specific technical term when referring to spinner weapons. As defined in the Ask Aaron Spinner Weapon FAQ:

Bite is the maximum depth of opponent insertion into the arc of a spinning weapon at a specific combination of weapon RPM and robot forward velocity.
The shape of the weapon impact surface has no place in that calculation -- however, there is a related property of spinner weapons commonly called 'grab' that does depend on the angle of the impact surface. Consider the two spinner impactors below: Two spinning impact weapons with different impact angles
  • The 'straight' impactor on the left is going to tap the forward edge of the green target, which may send the target spinning away, avoiding a more forceful impact.
  • The 'forward raked' impactor on the right is going to reach deeper into the body of the target and -- at least in this example -- produce a more forceful impact.
Such angled impactors are common on sub-lightweight robots. The only real down-side is more rapid wear on the impact surface -- which some designs counter by having a dual-faced impactor that can be removed when worn and flipped over to present an un-worn, sharp-edged face. Two-faced reversible spinner weapon impactor design

Q: I could not find any mentions of a weapon type my team is considering using in the 15 lb weight class. Our design is utilizing a spinning shell around a stationary core, with 3 portions of chain from a chainsaw with weights on the end. The chains are going to be short enough to not be considered an entrapment device.

The purpose of the weapon would be to create a nice barrier between their weaponry and our shell, while still either bludgeoning them with a weighted end on each chain, or potentially cutting into them with the chains. Do you think this would be a valid primary weapon? [Kansas City, Missouri]

A: [Mark J.] Weighted chain weapons are a type of 'flail'. The BattleBots Wiki has a category page devoted to robots armed with spinning flails with photos and links. Chain flails were fairly common weapons in the early days of combat robotics, but builders quickly learned that rigid weapons were much better than floppy weapons.

There are several 'flail' posts in the Ask Aaron Robot Weapons archive. In the box below I've re-printed an archived post that sums up their performance issues.

Q: My question this time involves flails. Simply put, what chain length to flail weight ratio is the best? Also, why has nobody ever mounted flails vertically, like on a drum or a flywheel? Finally, would a horizontal or vertical mounted set of flails do best? [Urbana, Illinois]

A: [Mark J.] If you perform a numeric analysis on flail weapon performance it's quite clear that the ideal chain length is 'zero'. An effective spinner weapon stores kinetic energy and delivers that energy in a single massive impact on your opponent. Flails swing out of the way after striking and transfer only a fraction of the energy stored in the spinning mass of the weapon -- it's like trying to hit a home run with a rubber baseball bat. Solidly mounted hard 'impactors' have proven to be much more effective weaponry.

You can't spell 'FLAIL' without 'FAIL'.

There has never been a successful flail robot -- either vertical like 'Morrigan' from Near Chaos Robotics, or horizontal like 'Chains Addiction' -- and I don't expect to ever see one. The only thing they do well is make noise. Avoid.

If you're really set on building a flail spinner, perhaps you should talk with our creative support department:

Q: I believe you just saved my team a LOT of time, I appreciate it! -Flail idiot

A: You're welcome. Don't be hard on yourselves for finding this design appealing; many teams have gone down this same blind alley.


Q: What is it about [insert robot name here] that makes it so dominant? Is it the weapon? Is it the armor? Is it an ultra-sharp wedge? Is [insert driver name here] a super awesome driver? Is it some exotic radio control system? Are they cheating? Tell me! [Suddenly asked everywhere]

A: [Mark J.] Let's say that you're going to jump out of an airplane at 20,000 feet. Which of the following features do you want your parachute to have?

  • A backup system;
  • An easy to locate ripcord;
  • Human safety rated materials;
  • A secure harness buckle system;
  • Inspected and packed by a qualified technician;
  • Upgrades based on the experience of previous uses;
  • A canopy large enough to slow your descent to a safe speed;
  • Extensive modeling covering a wide range of uncommon conditions.
You'd better have them all; it will do you little good to have a really huge canopy if your harness buckle keeps popping open. A good parachute does everything it needs to do under all foreseeable conditions.

A robot like [insert robot name here] is built and maintained by a team with the talent, experience, dedication, and funding needed to cover all the bases. And no, they don't cheat.


Q: I want to run the weapon in my new beetleweight at 6s [22.2 volts] but that's crazy high for my drive motors. What's the best way to reduce the voltage to the drive?

A) Resistors?
B) A 12V step-down voltage regulator?

Can I do this without creating a ton of heat? [Houston, Texas]

A: [Mark J.] The correct answer is C) None of the above.

  • Resistors do not limit voltage -- they limit current. Because your drive motor impedence varies with the load, a fixed power resistor in series would not provide a constant voltage reduction, but would limit current and reduce drive motor torque. Unsatisfactory.
  • The linear voltage regulator you found has a maximum current flow of 600 mA, and that current rating drops with increasing input voltage. Certainly not enough current to operate your drive motors. Bogus.
If your drive ESC can handle 6s voltage you can effectively limit the voltage your drive motors will 'see' by limiting the throttle response at the transmitter. Check your radio manual for 'Adjustable Travel Volume (ATV)', 'End Point Adjust', or 'Travel Adjust' for set-up options.

Alternately, if you are using transmitter Elevon mixing you may limit throttle response by setting the Elevator (channel 2) response to less than 100%. Start with 50% and adjust as needed to tune the drive motor performance to a suitable level.


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.

Here's your shortcut:

  1. Goto: Combat Robotics Design Tools;
  2. Scroll to first entry: Team Tentacle Torque / Amp-Hour Calculator;
  3. Follow link to 'Example Drivetrain Analysis using the Tentacle Torque Calculator'.
  4. Read it.
  5. 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.
  6. Check performance figures as described in the example drivetrain analysis.
  7. 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're going with 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: 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 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: I kind of need some help here. I have a drum weapon beetle and my scorpion mini esc kind of turns off sometimes when it gets hit. We use two 12v polulu motors for the locomotion and a propdrive 1200kv with a flycolor 36A esc for the weapon. The battery is 3s 1000 mAh. We are sure that the problem is not the connections and its not shortening anything.

Even when the locomotion is not working, the weapon still runs nice. When I turn the weapon off and on during the combat, sometimes the locomotion comes back. Does it have anything to do with the electrical noise or some kind of interference in the receiver? [Rio de Janeiro, Brazil]

A: [Mark J.] I'm gonna bet that you have the 'Lipo' jumper in place on your Scorpion Mini ESC. The Lipo jumper activates a circuit that will shut down the Scorpion Mini output when your supply voltage drops below 3 volts per cell -- even for an instant. Here's the event sequence:

  • You take a good weapon-to weapon hit;
  • Your weapon stalls and pulls a ton of current;
  • The large current drain drops Lipo voltage below 9 volts; and
  • Your drive ESC shuts down and takes a few seconds to restart.
There should be nothing on a combat robot that lets the machine decide to shut itself down -- except the radio fail-safe. It's fine to have sensors trigger audio or visual warnings about overloads, but let the driver decide if it's a good time to ease up or shut down. Pull that Lipo jumper off and throw it away.
Q: Is there a difference between the D-Pack gearbox and the Magmotor gearbox? [Fairfax, Virginia]

A: [Mark J.] The 'D-Pack' was an industrial hydraulic pump motor similar in size to the S28-400 Magmotor. In the early 2000's the D-pack was available from surplus dealers for a very reasonable price and found use as both drive and weapon power in a number of combat robots. The mounting face and hole spacing are different for the two motors, so gearboxes for them reflect those differences.

D-pack motor dimensioned drawing


Q: This is something I’d like to tack onto the previous question, but I’ve heard people online saying “shell spinners are obsolete”. What do you think? Do you think that these designs have no place in the future? What other designs might one consider obsolete (besides hammers, but don’t let Al Kindle know I said that). [Joliet, Illinois]

A: [Mark J.] I'm gonna pass on this discussion. I already get enough hate mail for discouraging builders with new ideas. If I start talking builders out of old ideas there won't be anything left.

"Prediction is very difficult, especially if it's about the future."

- Nils Bohr 


Chinese combat robot 'Flaming Wheel Q: So here’s a question: there was a Chinese competitor called... Flame Wheel? Hot Wheel? Not for sure, it’s one of those two names. Anyways, I was wondering if there’s any advantage to having the [impactor] mass placed outwards like seen in this picture in comparison to having small, sharp teeth like Gigabyte or Captain Shrederator. [South Bend, Indiana]

Spinner impactor clear path A: [Mark J.] The robot is King of Bots season 2 competitor 'Fēng huǒ lún' or 'Flaming Wheel'. An unrelated robot fought in King of Bots season 1 under the name 'Hot Wheel'.

'Flaming Wheel' had two interchangeable spinning shells:

  1. A 'tall impactor' shell - pictured; and
  2. A slope-sided 'undercutter' shell similar to 'Megabyte'.
The choice of shell depends on the perceived vulnerabilities of their opponent. The undercutter is used to attack targets with soft surfaces or exposed edges, like tires or simple wedges. The tall impactor is reserved for hard and/or smooth surfaces where it can deliver a blunt-force 'body blow'.

The unusual design of the tall impactors does provide strong support that also keeps impact forces away from the clylindrical shell itself, but the impactors are placed in a position that is shrouded by the gradual taper of the top and bottom mounting plates. The impactor itself needs a clear shot at the opponent without any material in the rotation path that might tap the target out of the way. I don't know what the "bite" calculations for 'Flaming Wheel' look like, but there is nothing to gain by adding material that can get in the way of your impactor path.

Energy storage advantage? Minimal.


Q: I have a choice of two versions of the same brushless motor to power my weapon: a 750 Kv and an 1100 Kv. The 750 Kv version will give me more torque, right? [Everywhere]

A: [Mark J.] There are a lot of 'half-truths' about brushless motors floating around out on the 'net that are confusing robot builders and leading to poor design choices. You can find this one all over the 'net:

"A lower Kv motor has more torque"

There is a kernal of truth in that statement, but it is incomplete and misleading.

The true part -- the motor speed constant (Kv) and torque constant (Kt) are inversely proportional -- if one goes down the other goes up. Compared to a higher Kv version, a lower Kv motor will produce more torque per amp of current.

The incomplete part -- lowering the motor Kv increases the electrical resistance (Ri), which reduces the current the motor will draw.

The misleading part -- if allowed to pull unrestricted current, the lower Kv motor version will produce both less torque and less power.

Here's a real-world example comparing two versions of the AXI 2208 motor:

AXI 2208/34 Gold Line V2 motor
  • Kv: 1100 RPM/volt
  • Kt: 1.23 in-oz/amp
  • Ri: 0.260 ohm
  • Voltage: 3s
  • Max Current (60 seconds): 10 amps
  • Torque @ Max Current: 12.3 in-oz
  • RPM @ Max Current: 9350
  • Max Rated Power: 100 watts
AXI 2208/20 Gold Line V2 motor
  • Kv: 1820 RPM/volt
  • Kt: 0.74 in-oz/amp
  • Ri: 0.089 ohm
  • Voltage: 3s
  • Max Current (60 seconds): 18 amps
  • Torque @ Max Current: 13.3 in-oz
  • RPM @ Max Current: 17,280
  • Max Rated Power: 182 watts

The 1820 Kv version of this motor generates greater torque than the 1100 Kv version throughout the RPM range. When bogged down to the '60 second' current maximum it produces 8% more torque, and it does so while spinning 85% faster than the lower Kv motor -- a power increase of: (1.08 * 1.85) - 1 = 99.8%.

There are a number of reasons why you might choose a lower Kv version of a motor for a specific application, but "because it has more torque" is not one of them.


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.

My question stems from this previous post:

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.

  1. 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(?)
  2. Would the same MOI advantage be true for the carbon-fiber disk with steel ring options?
  3. Would shaving the spokes into an airfoil provide a useful difference?
  4. Would the minor density increase of copper over steel decrease the ring volume by a more than negligible amount aerodynamically?
  5. If not, at what density would it matter; hypothetically tungsten for example?
  6. 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.
For our purposes there are three categories of aerodynamic drag:
  • 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.
  1. 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.
  2. 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.
  3. Not as useful as answer #1 above.
  4. The thickness of the disk is relatively unimportant as it adds only a smidge to the surface area.
  5. See answer #4.
  6. 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...

Beetleweight Meltybrain spinner 'Halo' 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. Twin AX-2810Qs qualify as huge overkill and are used quite successfully by pictured beetle melty spinner 'Halo'.

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.


Proper set screw usage for combat robot 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.
Scorpion ESC detail 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?

Viking funeral 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.

VEX 29 motor controller Q: Hey its me from Anacortes again. First I want to thank you for diagnosing the BEC problem with my ESC [two posts down the page]. 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 post in the Ask Aaron 'Ants, Beetles, and Fairies' archive for a list of available options and a discussion of the cheap Chinese solution.

  • My choice would be a pair of the Robot Power Wasps.
  • 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:

Estimated Kitbot 1000 RPM gear motor specs

  • 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.
Note - I've updated our Tentacle Torque-Amp Calculator with the 'Kitbots 1000 RPM (estimated)' motor specs as shown above.

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:

  1. Is it possible to use them for 1-stick drive?

  2. 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?

  3. Precisely how do I connect two VEX 29 ESCs to a single battery & receiver system?

A: Three answers...

  1. 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.
  2. 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.

  3. 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.
Wiring diagram for VEX 29 ESCs running four motors at 14.8 volts

Wolverine wedge killer forks Q: When two wedge/ramps meet, what design choices dictate which gets under who? Is it the length of the wedge? The high? How heavy it is? I mean other properties that help clearance with the ground but still don’t get stuck to the uneven floor? [Port Angeles, Washington]

A: [Mark J.] The saying goes, "You can't have your cake and eat it too." You can't have a razor-edged paint-scraping wedge and still not get stuck on a raised floor seam. The best design for getting under wedges is actually something other than a 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 have a good chance of riding down in a lower section of the arena floor and slipping under a wide wedge.

Get to know the arena you fight in. Learn where the bad floor spots are and make a habit of running across the floor seams at an angle where you can. If you're still getting stuck too often, feather that sharp leading edge just a little. Uneven arena floor causes wedge gaps



Two photos of Aaron Joerger Remembering Aaron Joerger, 1991 - 2013

The 'Ask Aaron' project was important to Aaron, and I continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me. Aaron's obituary

- Mark Joerger   
Killer Robot drawing by Garrett Shikuma

Q: how can robots help us deal better with hurricanes and why? [Ontario, California]

A: [Aaron] Few people in Nebraska are threatened by hurricanes, so send a swarm of killer robots into low Atlantic and gulf coastal areas to drive the puny human inhabitants toward Nebraska. Problem solved.

Robot haiku:

That's obviously
A question from your homework.
Do your own research.

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