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6586 Questions and Answers about Combat Robotics from Team Run Amok

Team Run Amok receives a lot of email asking about the design and operation of 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

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.
 


There are two weapon sprockets, so which one gets the clutch?
Q: When designing a heavyweight [electric] hammer (potentially applies to other weight classes) I've seen builders use a slip clutch at both the motor output or at end of the reduction the hammer shaft/sprocket connection. Is this just preference or is there a practical advantage to one vs the other? [Seattle, Washington]

A: [Mark J.] The design choice usually comes down to the availability of suitable torque limiters. If you're planning an off-the-shelf solution you'll find that affordable sprocket slip clutches are most commonly available is sizes better suited to the hammer end of the system.

There is a small mechanical advantage to placing the clutch on the motor end: it allows the inertia of the full chain and sprocket to contribute to the hammer impact rather than dissipating that energy into the slip clutch. That's typically not a lot of energy, but if you're after every last available erg of impact you might justify a custom motor-side clutch. I wouldn't bother.


Forgotten secrets of master hammer designers revealed!
Q: So I'm about to take the leap into the 30 lb sportsman's class. I've had a few designs bouncing around in my head and came up with on that I've (prematurely) gotten excited about. I thought of it last night so I don't have a lot of details but I'll pitch the basic concept.

It's a pneumatically driven rack and pinion hammer bot. As I'm sure you can imagine, the piston drives the rack, which rotates the pinion gear, which is attached to a devastating hammer that will smash the opponent into tiny little pieces (as is the case with all hammer bots).

I have a full machine shop to work with, CAD and combat experience, and a decent budget. Is this a viable design that I should tackle, or should I hit the cheerleader button? [Albany, Oregon]

A: [Mark J.] Viable? The most feared and successful combat robots to ever swing a hammer/axe have been pneumatic rack and pinion designs.

  • Pioneering hammerbot 'The Judge' is a member of the Combat Robot Hall of Fame with a record of 24 wins / 12 losses. The 'rack and pinion' assembly in 'The Judge' was actually created from chain belts and sprockets, but was functionally identical to a conventional rack/pinion. When the giant overhead hammer came down, the whole building shook and the crowd shouted "GUILTY!"
  • Overhead axebot TerrorHurtz used a true pneumatic rack and pinion to power its way to multiple UK tournament championships. Yes, they are also in the Hall of Fame.
Start with this archived post on pneumatic rack/pinion design, and follow the links therein to more info. Electric hammers are well and good, but if you want to rattle the box you need to go pneumatic!
Four genius ways to use bungee cords you'll wish you'd discovered.
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.


Friends told him to swap his drive motor to his weapon. Do you think he should?
Q: We are getting suggestions that we should use Ampflow A28-400 for weapon and Ampflow A28-150 for drive. But I think we should use A28-400 for drive and A28-150 for weapon. Please tell us the reason for this [ITT Kanpur, India]

A: [Mark J.] You've told me absolutely nothing about your robot or your weapon, but you want me to recommend motors. It this the type of engineering they teach at ITT Kanpur?

That's all the assistance I'm willing to provide. I've seen the combat arena at ITT Kanpur: very thin plastic poorly mounted to the outside of a weak wood frame. It can't safely contain the robots that currently compete in it -- I'm not going to help improve Indian robots until the arenas are much better.
Everything was fine until he turned the transmitter on. You won't believe what happened next!
Q: Hi Mark,

I'm having some trouble with TinyESC V2s and an FRSky X4R receiver on 4S Lipo (fully charged). When I power up with the remote turned off, everything works fine. Red flashing lights on both Tinys, red flashing light on the X4R. As soon as I turn the remote on, the Tinys get really hot and power browns out on the reciever within seconds, a few seconds later it shuts off entirely. At the same time, the Tinys get really hot.

I've tried using only one Tiny at a time, to see if the trouble is with one of them and to prevent conflicts with the BECs. Same thing happens.

The motors are Silver spark clones, but I doubt that's important since they aren't even running when this happens (they do run fine for the few seconds before power browns out though).

Thanks for any help. [British Columbia, Canada]

A: [Mark J.] You've been caught in a trap caused by your particular combination of components, Vancouver:

  • Your X4R receiver has a telemetry function. As soon as a transmitter link is established the X4R starts transmitting a telemetry signal and its power consumption goes up. FrSky says the X4R pulls up to 100 mA in full operation -- that's a lot for a receiver.
  • Your tinyESCs have BECs that de-rate as the supply voltage rises. On 4 cells (14.8 volts) each tiny can only supply about 28 mA -- 56 mA total.
Here's the sequence of the failure:
  1. When the transmitter is turned on it links to the receiver;
  2. The receiver starts transmitting telemetry back to the transmitter and its power consumption spikes;
  3. The BECs in the tinyESCs can't keep up with the increased current demand from the receiver and they overheat;
  4. As the voltage drops, the receiver browns out then quits.
There doesn't seem to be a way to turn off the telemetry function of this receiver. Add a stand-alone BEC that can provide 100 mA of receiver power from a 14.8 volt supply and you'll be fine. Alternately, switch to a receiver without telemetry.

Is your transmitter suddenly twitchy? Consider this radical solution.
Q: Hey I'm back! Another one of the problems I've been procrastinating about is that my Flysky FS-i6 transmitter has lately been maxing out the input one way or another when I barely twitch the stick. Since you seem to have quite a bit of knowledge about this transmitter, do you have any ideas of what I might do to fix this? Also, thank you so much for answering all of my questions. [Oak Harbor, Washington]

A: [Mark J.] You're welcome, Oak Harbor.

I suspect your problem is mechanical. The FlySky FS-i6 is a great bargain, but to sell such a full-featured transmitter for under $40 they had to cut a few quality corners. In particular the stick gimbal assemblies are not great, and it sounds like one of your gimbal potentiometers has gone flaky.

The good news is that you can buy a replacement stick assembly for less than $10 - shipping included. Select the 'Direction' model to replace the right gimbal that self-centers both axis, or the 'Throttle' model to replace the left gimbal that centers only one axis.

There are multiple YouTube videos that will guide you thru the simple process of replacing the stick assembly. Try this one: How To Replace A FlySky Defective Gimbal. If you need to correct the stick centering after replacing the gimbal there is a trick the maker of the video didn't know: the 'Sticks Adjust' function of the Secret Menu will take care of it. Best luck.

Q: As a clarification to my earlier question, I forgot to mention that both sticks are doing this, and it was not a problem when I first received it. I'm not running a servo off the BECs on the tinyESCs, just the receiver. Does this affect your advice?

A: Gimbals wear out over time -- cheap ones wear out fast. Replacing two gimbals will run $20 plus your time and effort. Replacing the whole transmitter will cost $40 and solve any possible transmitter issues. Let's make sure its the transmitter:

Go into the 'Functions' menu and select 'Display'. Move your sticks around. Are you seeing the same glitchy behavior on this display that you are having with your robot?

  • If yes - go into the 'System' menu, scroll all the way to the bottom and perform the 'Factory Reset. This will wipe out your programming, so make some notes before you proceed. Try the 'Display' test again. If it's still bad you need new gimbals or a full new transmitter.
  • If no - the transmitter is fine and the receiver becomes suspect.
Let me know what you find.

Q: There has been no change in the behavior of the sticks. This transmitter also had had problems with binding, so I think I'll just get a new one and use this one for spare parts.

A: That makes good sense to me.


Seven things hipster builders wish they'd known before they went brushless.
Q: Hey it's me form the Oak Harbor/Anacortes area again. My robot design has been going through a CAD redesign, but now I'm ready to get back to actually getting the thing to move. A long while back I asked about brushless drive for beetleweights, and you reccomended the Afro 20A ESC. But, they are out of stock at Hobbyking again. Would you recommend any other similar ESC's? [Oak Harbor, Washington]

A: [Mark J.] The Afro ESCs were a stable 'known quantity' produced over a fairly long period of time. Unfortunately, the Atmel chips used in the Afro ESCs have started to dry up and the model aircraft industry that drives brushless ESCs has largely moved away from the robot-friendly SimonK firmware and toward BLHeli firmware. There aren't likely to be any more Afros made.

It's difficult to recommend a replacement ESC for entry-level brushless drive. The main problem is that most small brushless ESCs are produced in relatively low numbers and disappear from the market as fast as robot builders can figure out their quirks and decide if they're useful. Once you've picked out a brushless ESC you'll still need to go in and modify the firmware settings to allow for forward/reverse operation and make a few tweeks to motor brake settings to make them suitable for a robot drivetrain. See if this page makes any sense to you; if it reads like Chinese algebra you're in for long and painful learning experience.

I'm not a good source for answers to questions on brushless ESC firmware settings or for the current 'hot flavor' of ESC. A reasonable place to start would be the Facebook Combat Robotics group. My suggestion is to avoid going 'brushless hipster' and stick to brushed drive motors until you've run out of other aspects of robot design to master.

Q: OK, I've gathered up the hardware needed to set the Simonk firmware on my Afro ESC, identified the leads on the circuit board I need to connect, figured out my release version, and identified the correct 'target' for the SimonK firmware settings tool. Now I have no idea what firmware settings to modify! What settings do I use for robot drive? [Late night phone call from a Midwest area code. Who gave you my number?]

A: [Mark J.] Congratulations on making it that far! As a starting point, use the default values - with these exceptions:

[COMP_PWM]   ON *
[MOTOR_BRAKE] ON
[RC_PULS_REVERSE] ON
[RC_CALIBRATION] OFF
[STOP_RC_PULSE] 1050
[FULL_RC_PULSE] 1950
[RCP_DEADBAND] 25
[BRAKE_POWER] MAX_POWER
[BRAKE_SPEED] 8
* For ESCs rated above 60 amps leave [COMP_PWM] 'OFF'.
Three ways receivers failsafe: number three can leave you crying.
Q: I'm having issues getting my HobbyKing Transmitter to failsafe. When I cut power to the transmitter, my servo which I modified for continuous rotation keeps going in its last direction. However, that same exact transmitter channel will properly failsafe drive motors, a brushless, and even a store bought continuous rotation servo I have. I have tried the solution in the archive using the bind plug and setting the sticks but to no success.

Are there any other options or should I just go get a real radio? [Roseville, California]

A: [Mark J.] The Ask Aaron archives have a few different binding sequences to set failsafe for different transmitter/receiver systems, including a specific sequence for the HK T6A-V2 transmitter paired with the TR6A-V2 receiver. Other HK transmitter/receiver pairs may not repond to that sequence. Worse, that sequence is not officially documented and is subject to firmware changes, which are frequent and often unannounced with HobbyKing products.

A receiver with failsafe capability may offer one or more failsafe modes:

  1. Hold last instruction - not suitable for combat robots;
  2. Send a specific programmed position instruction - a good robot response; or
  3. Shut off the output signal and hope that the connected device knows what to do. Quite a few radios do this. When it works it works well, but when it doesn't...
It sounds like your particular transmitter/receiver is using that third mode and that most of your ESCs and servos are responding with a correct shut-down, but that your modified servo does not have the correct response.

HobbyKing radio systems are poorly documented, subject to change without notice, and generally user hostile. My opinion is that they are not worth the frustration they cause for anything beyond very simple applications. I'd suggest an upgrade. A 'real radio' will not only solve your current problem, but the added features will -- if properly applied -- improve your ability to better control your 'bot.


You'll be shocked by this simple spinner blade hack.
Q: Would a fingertech chipper blade work on a direct drive overhead 1lb spinner, or would a blade that has a "bow tie" design (think bow ties made of AR400) similar to Cobalt's large impactor, except there's one on each side work? I think it will work better due to the outward weight distribution.

The blade diameter of the theoretical blade is similar to the aforementioned chipper. If its important, here is the motor: Fingertech D2822/17 1100kV Outrunner. [Charleston, West Virginia]

A: [Mark J.] You're correct in thinking that more mass out toward the outer edge is better for energy storage. A simple bar is poor, a 'bow tie' bar with flared ends would be better, and a disk would be better still. However, there are other considerations: strength, ease of construction, and 'bite' for a few.

Consider also the benefit to a 'single toothed' weapon. A weapon with a single impactor can spin twice as fast while retaining the same 'bite'. Since kinetic energy increases with the square of speed, doubling the speed increases energy storage by a factor of four! Alternately, you might spin the single-toothed weapon at the same speed and benefit from twice the bite.

The illustration shows a relatively simple modification to the FingerTech chipper blade: trimming off one impactor and drilling a balance hole near the other end, turning it into a single-toothed weapon. I think that might make for a simple and very effective overhead spinner. I should warn you that a long direct-drive spinner design is very hard on the motor. An unmodified outrunner really isn't designed to absorb the large side-loading it will take from weapon impacts. Pack a few spare weapon motors if you go that route.


This bot won't self-right -- can you spot the solution in the photo?
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.


A crazy walker with too much gyro force? The reason left me spinning.
Q: Hello, Mark! Today I come to you with an unusual idea. Please let me know what you think!

The idea behind the robot is essentially to make a fusion between 'Clean Sweeper' and 'Wrecks'. The robot has two weapons (either a pair of 6" Diablo Sawblades or a pair of Fingertech Bars) connected by a singular axle. The weapons would rest a distance off the ground to prevent them from hitting the floor when the robot tips too far over, and would use an angle limiter to keep the servo from tipping too far (at most, I'd say 22.5 degrees left or right is the aim for the maximum angle). A weapon motor will be connected by a 1/8" urethane belt, and the robot will move by using a servo. The servo has a stick connected to it, and will pull up or push down against the ground to make it move. In theory, the servo goes up, the directional stick goes up, tipping the robot to the right and forcing the robot to turn right. The servo goes down, the directional stick goes down, tipping the robot to the left and forcing the robot to go left. My problem is this: Would the two spinners be one too many? Would the gyroscopic forces be too much for it to be able to stay upright, causing it to fall on its weapons? Are there any pieces of advice you could give on as to how this design could be improved on? I mean, I know this is a robot built to be a showboat more than anything, but it's like an antique car: it's fun to look at them, but it's even more fun to watch somebody drive it!

Thanks, and have a good one! [Newton, Illinois]

A: [Mark J.] Is that you, Champaign? Your server address has drifted about 70 miles to the south.

The 6" sawblades weigh about 5 oz each, so I'm guessing that this is a beetleweight. As long as you realize that this isn't going to be a competitive robot, you're generally on the right track. Take a look at this video of a small non-combat gyro walker that uses a servo to actuate walking same way your design does.

  • All gyroscopic walkers I've seen have their weapons centered to avoid having them 'tilt' into the arena surface. The body of the robot will slowly tilt one way or the other, and having a rotating blades on the outside edge of the 'bot requires a lot of clearance -- you'll be completely pwned by wedges.
  • Don't worry about 'too much' gyroscopic force. The gyro force creates the turning force in a gyro walker, and it is what keeps the 'bot from tipping sideways. More gyro force will not cause one side to lift as it does for a gyro-dancing vertical spinner; you can't have 'too much' gyro in a walker, but you can certinly have too little. Spin the blades fast to get more gyro force.
  • The gyroscopic forces will stabilize the robot from tilting left/right, but they will not offer any stability in tilting to the front/back. You'll need some failrly long 'feet' on the ends of your 'leg' to prevent toppling. The gyro forces will also not help to counter reaction from a weapon 'hit'. You're going to quickly find yourself inverted with your saw blades dragging along the arena floor and no way to self-right.
There are good reasons for the layout used by 'Wrecks': center-placed weapon, rear-placed legs, a weapon guard under the blade for the front of the robot to pivot on... Watch some videos of combat gyro-walkers and learn from their designs.
Comment: There's a small error you made in answering the gyroscopic precession question. There is such a thing as too much gyroscopic force for a precession walker. The maximum pivot speed is expressed as (force of gravity) * (distance between centre of gravity and the point of contact with the ground) / (angular momentum of the disk). Which means if you spin the disk twice as fast you (very counter-intuitively) actually cause the robot to pivot half as fast. You can see this in spinning tops because as they slow down, the wobble speed increases.

I've spent some (too much) time trying to figure out gyroscopes myself for future bots and I'm not an expert but I figured I can still help with the info I somewhat understand.

The 'Hyperphyics' site explanation on spinning top physics is probably one of the most complete sources for this info that isn't completely indecipherable.

Thanks for the excellent resource! [Ontario, Canada]

A: [Mark J.] I've always believed that the best way to learn about something is to try to explain it to someone else. The other half of that process is to have someone tell you when you've made a mistake. Thank you for the correction!

The Hyperphysics website is my go-to source for understandable explantions of physics concepts -- you'll find it referenced multiple times in the Ask Aaron Archives. I'm a bit embarassed that I didn't take the time to check there for the math. In my defense, I believe it preferable to have 'too much' of something and be able to throttle back than to have too little with no option to recover.

Thanks again for the correction and the hyperphysics reference.


These new combat wheels look amazing, but they hide a serious flaw.
Q: So, recently BaneBots released their new set of compliant wheels and I'm very curious as to what they do differently than other wheels and am fairly confused about the concept. Could you give any insight to the pros and cons of the compliant wheels vs their normal wheels? [Altoona, Pennsylvania]

A: [Mark J.] You may have noticed the popularity of thick, foam rubber tires in small combat robots. The thick foam absorbs a great deal of a spinner weapon impact before it can be passed on to damage the drive train. Their down-side is relatively poor traction -- a layer of latex or silicone rubber applied to the surface of the foam rubber improves traction greatly.

The new 'compliant' wheels from BaneBots are constructed with a small wheel hub and a deep tire made of a high-traction compound in a 'cut-out' pattern that allows it to deform to absorb impacts. In theory, this construction will combine the best qualities of the regular BaneBots wheels and the 'Lite Flite' style foam wheels. In practice... well, we'll need to see how well they do in combat. Note A builder on the on-line forums reports that there is a lot of variation in the 'squishiness' of the compliant wheels, even within the same size and compound. There may be some production consistency problems. Stay tuned.


Stunning photo: you won't believe what fights in this arena.
Q: Planning for a almost featherweight(12KG). I got these gearmotors 555 sized from a Chinese supplier for 10$ each. They do about 800 RPM in 12V, but I plan to go with slight over-volting, 14.4v. It will be a 4 wheeled robot, powered by 4 of these motors.

Question is, Drum. I want to make a drum spinner, and taking the suggestion from riobotz tutorial book,it should weigh 3KG, just the drum alone. And drum will be powered by a 775 with a high current relay, will convert to brushless later. Does this plan have a big flaws.??

Thank you [Dhaka, Bangladesh]

A: [Mark J.] Yes, the weapon has a very large flaw -- it's a dangerous spinner weapon that will fight in an arena like the one pictured below. I answered some questions about a wedge/lifter 'bot from Bangladesh a couple months ago, but I won't offer any help with spinner weapons that will fight in open arenas. The risk of serious injury is far too great for 'Ask Aaron' to support.


Q: No question, just wanted to say have a very merry Christmas :D [Altoona, Pennsylvania]

A: [Mark J.] Thank you, Altoona -- you as well.


Q: Hi! I was wondering if there were any advantages of 'Kraken' having a Pneumatic Crusher over a hydraulic or battery powered one. Ty and Robots ftw! [Providence, Rhode Island]

A: [Mark J.] 'Kraken' has a non-conventional weapon system that uses a heavy lift airbag instead of the usual pneumatic cylinder to power the jaw. The large area of the lifting bag can produce a great deal of force from relatively low air pressure, but requires a lot of air to pass thru a small port to inflate. That small port is a serious problem that will slow down weapon actuation. It also requires a pair of stiff plates to sandwich the bag and some custom links carry the force to the weapon jaw.

Pneumatics are comparatively simple and robust compared to other options:

  • Their solution is simpler and more reliable than a hydraulic system that requires a motor, motor controller, fluid reservoir, high-pressure pump, 5-port valve, and a bunch of pressure hoses.
  • An electric crusher requires a very heavy and expensive gearbox to survive the massive torque, and electric motors don't survive long when bogged down near stall to provide their maximum torque.
Based on performance I don't think I can recommend the airbag approach. The team claimed 10,000 pounds of force out at the piercing end of the jaws, but I don't think the weapon performed up to expectations.
I've been hard at work on the whole 'runamok.tech' website for several weeks. Toward the end of October the number of 'clicks' I was getting from Google searches abruptly dropped by nearly 2/3rds. Google has been placing great emphasis on 'responsive' web pages that adapt to display on screens of both large and small devices. I've been keeping up with reponsive web techniques, but apparently not at the pace Google desired.

Most of the runamok.tech pages have now been re-structured. You may notice some format differences in specific pages -- particularly on phones and tablets -- but the page contents have not been modified. I can only hope that my efforts will please the Google deity.


Q: What is Run Amok's favorite hobby, and alcoholic beverage now that she is retired? [Richmond, Virginia]

A: [Mark J.] 'Run Amok' enjoys painting abstract road art (see photo). Her favorite beverage is a 'Northern Thwackbot' -- a shot of WD-40 with a squirt of blue Loctite.

Q: Hey this is not really a question but more of a correction. There are events other than RoboGames that fight larger 'bots. AVC Sparkfun in Colorado runs 60lbers, and Maker Faire Orlando runs 250lbers.

A: I didn't mention those events in response to this earlier question because:

  • 'AVC Sparkfun' had exactly two 60-pound robots at their 2018 event. That's not worth dragging a robot to Colorado; and
  • 'Battles at Maker Faire Orlando' had a good number of 220-pound heavys in 2017, but had no event in 2018 and shows no scheduled big 'bot event for 2019 [see below].
That's not enough activity for me to encourage a builder to start construction on anything above 30-pounds.

Q: The buildersdb.com shows that 13 heavyweights registered for Battles at Maker Faire Orlando 2018. [Douglasville, Georgia]

A: Thanks for the correction. BotRank has no results from the event and the 'Registered Robots' page at the makerfaireorlando.com website has an expired security certificate. If you want to fight 220-pound heavys it looks like Maker Faire is your only current US option.


Q: Hello! I just had a quick question about lifting mechanisms. I've noticed that some lifting mechanisms, like the ones on the featherweights Banana Bender (AUS) and Mad Rush (UK), as well as a number of lifting mechanisms on UK ants that use an angled piece that connects to the lifting arm rather than having the motor connected to the weapon itself.

Other than protecting the motor or servo, are there any other advantages to using this over a conventional lifting mechanism of the same size? What are the drawbacks? How does one calculate what kind of gearing ratio is needed in order to lift opponents?

((I know the last question sounds silly, but I didn't know if calculating the ratio needed to lift an opponent with a particular motor is different with these designs than with normal lifting arms)) [Jacksonville, Illinois]

A: [Mark J.] What you've got there is a '4-bar Mechanism'. The design is widely used in combat flippers and axes: 'BioHazard', 'Ziggy'. 'Pad Thai Doodle Ninja', 'Shunt' and a great many more. A search of the Ask Aaron Robot Weapons archive yields more than 100 hits for '4-bar'.

The calculations required to optimize a 4-bar mechanism and to determine the motor torque requirements are quite complex: take a look. There is a 4-Bar Simulator written by Adam Wrigley of T.i. Combat Robotics that will give the motor torque requirement for the common layouts of lifters using a 4-bar mechanism, but I will point out that the design used by 'Banana Bender' is a non-standard use of the 4-bar and would not be covered by the T.i. simulator.

Take a look thru the archives for discussions of the advantages of the standard 4-bar design. Briefly: the 4-bar allows greater choice in the 'arc' of the lifter, may reduce the peak torque requirements of the gearmotor, and can increase lifter speed by spreading the power input more evenly over the full sweep of the lift. A custom 4-bar design like 'Banana Bender' is something that third-year mechanical engineering students might undertake just to show off.


Q: Hi Mark,
The german here with some updates on the omnidrive bot. After searching the keywords I was indeed excited but sobered out pretty quick after I realized I couldn´t implement existing concepts into a combat capable bot. The usuak transmission in FIRST bots is very fragile indeed. Way to many connections that can slip while receiving a punch or two. So either I learn how to drive like god, skip the idea or cut down the concpet to the rugedness of a old russian Tank.

The only way to avoid the transmission desaster, (which in my opinion turned roboteers away from this concept in the first place) is to implement a solution where the motor is inside the Hub and get´s turned around with it. 360° of course would be the optimum. So no wires attached or a 180° solution which I had in mind at first. Then I found some wireless charging station plans, using coils, but I skiped that because of weight.

In the end it really struck me when I took apart a Drill motor. Brushes! I drew a quick sketch outlining the concept:

On the Topplate of the enclosure, there are two copper rings. One is connected to the positive terminal and the second one to the negative terminal. Up too four brushes share each copper trace (splitting the amps and working as a backup) and join together on the respective motor connection. I presume something between 15-20mm² in diameter on the brushes and Wire. Maybe i can find some carbon brushes used in Makita motors or else.

The rest stays the same as before. A ballbearing on the top enclosure and a aluminim ring in the lower part with (nylon?) V-pulley rolls. Which will function as an absorber on the lower part of the axis. Encoders on the Top and on the wheel will do the position control for the ESC´s to do the adjustments.

This became a hell of a journey and sadly not many people are excited to join in. Luckily I found some in my hometown who even have the lathe so machining parts might start next year.

Any suggestions or recommendations? I know that open traces for 140A (stall torque) and upwards are a bit messy though. As soon as we have the first prototype we will check radio interference and counter measures.

With kind regards [Niedersachsen, Germany]

A: [Mark J.] I'm pleased to find that you're continuing development of your swerve drive, Niedersachsen. It's more than a bit too complex for my personal taste, but it will certainly be an accomplishment to get it operational.

I strongly recommend that you use carbon brushes. A web search for "benefits of carbon motor brushes" should convince you of their merits at high current levels: self-lubricating, self-cleaning, reduced radio interference, and they don't weld themselves to the copper slip rings.

Replacement 'carbon brush assemblies' for electric drills are widely available; they include brushes, brush holders, and tension springs in a handy little package. Should do nicely.


Q: So, over the past few years of browsing the archives trying to better myself as a builder I have always noticed that you have a very strong opinion against ring spinners. In most cases I have held the same opinion rather strongly up until recently I have been seeing people be more and more successful with them over time. One of the best examples I have seen of this is the 1lb bot 'Mr. Roomba'.

Can you give any insight on what they do for that bot that makes it so much more successful than others? And if someone were to look into making one what your best advice for them would be? [Grove City, Pennsylvania]

A: [Mark J.] The primary audiences for 'Ask Aaron' are and always have been 1) new builders, and 2) builders frustrated by their lack of success. I'm going to paraphrase 'Ask Aaron FAQ' #28 and paste in 'ring spinner' everywhere it originally said 'flame weapon':

Q: Would it be a good idea to make a 'bot with a ring spinner that would destroy the competition?

How does insert ring spinner name here's weapon work?

A: You can search the archive for ring spinner to find our many previous posts on ring spinners. Briefly:

  • Ring spinners are generally ineffective in any weight class.
  • Ring spinners are built by builders who just want to show off for other builders.
  • We will not encourage ring spinner construction here because we don't want novice builders drawn to a design with which they will fail.
  • By the time you are experienced enough to build a successful ring spinner you won't need to ask us how to do it.
  • That ring spinner that you're about to tell me is successful was built by a talented and determined builder who improved the design thru multiple versions. That same commitment would produce a successful robot with nearly any weapon.
The difference between a successful robot and a poor one is seldom a single factor -- it's a dozen elements plus scrupulous attention to detail. In the case of 'Mr. Roomba' (and many successful robots) a great deal of the success comes not from the weapon but from the attention given to the chassis, radio sorting, and drivetrain -- 'Mr Roomba' drives beautifully. Here's an equation I haven't published previously:
(Talent + Knowledge + Experience + Commitment) * Money0.5 = Combat Success
'Ask Aaron' can help with knowledge and we can broadly speak from experience, but we can't do much for the talent, commitment, and money elements. If you're not winning matches it isn't because you don't have a ring spinner, and if you want a ring spinner to impress other builders you're going to need more than 'Ask Aaron' can provide.
Q: I loved BotsIQ in high school and wanted to continue with the weight class after I graduated. I look in the Builders Database but cannot seem to find an event. I'm looking for a 15lb competition or something very close. [Monroeville, Pennsylvania]

A: [Mark J.] The 15 pound class is almost exclusively limited to school-based competitions. The only open competitions I know of for this 'dogeweight' class are associated with the 'Battle Beach' tournaments in Florida.

You'll have more luck shaving off some weight and finding a tournament with a 12-pound class. The big annual NERC Motorama event in Harrisberg Pennsylvania has 12-pound 'hobbyweight' and 30-pound 'featherweight' classes. The event is coming up in February -- registration closes soon.

Q: I also want to make a truly big bot as I am starting my own machine shop. I was thinking a 60lb or larger, but also am having trouble finding an event. Do you know of any for that as well?

A: The larger the 'bot the harder it is to find a competition. Robogames in northern California is the only open US tournament fighting 'bots larger than 30 pounds, and they have cancelled their 2019 tournament. Keep an eye on Robot Combat Events as well as The Builders Database.


Comment: Hey, no question, just writing in on the JMT speed controllers [see archived post] (was suggested them by Melanistic Leopard's builder as well). 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.


Q: I've just read your new FlySky FS-i6 Transmitter Combat Guide. Well done on making this excellent resource, I learnt heaps, I have one question. Is there any way to use one of the transmitter switches to quickly reconfigure elevon control settings when the bot is running inverted? [New Zealand]

A: [Mark J.] I'm glad to hear you're enjoying the FlySky combat guide, New Zealand.

The bad news is that you'd need to spend a fair bit more than the cost of the FlySky FS-i6 to get a transmitter you can program to perform an invert mix fix with a switch. The programing you want requires at least four custom mixes that can be assigned to a transmitter switch: instructions are given on the Team Cosmos website. Your FS-i6 has only three custom mixes and none of them can be turned 'on/off' with a transmitter switch.

The good news is that you can add a bit of hardware that will give you a switchable invert solution. Several on-board mixers like the FingerTech tinyMixer have two inputs for the channels to me mixed plus a third input cable that plugs into an auxilliary channel (5 or 6) on your receiver. Assign that auxilliary channel to one of the transmitter switches on the FS-i6 with the 'Aux Channels' function and a flip of that switch will change the mixer output to inverted operation.

Q: Following with interest. I've been trying to figure out an 'invert' switch on my Taranis Q X7 and I can't seem to do it properly. Either left/right inputs is inverted or forward/back is. Am I simply trying to do something beyond the transmitter's capabilities? [Reddit comment]

A: Taranis transmitters use the highly configurable 'OpenTX' firmware. There is very little that an OpenTX transmitter cannot do, but figuring out how to implement even simple functions can be a challenge.

Here's a video showing how to setup an inverted airplane 'flight mode' on a Taranis transmitter. The same method works for inverted robot control -- don't reverse the rudder or aileron channels, just the elevator.



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   

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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