Ask Aaron: questions and answers about combat robots

6828 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.

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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.

Taranis Q-X7 Combat Guide
I've answered a wide range of individual questions about using the Taranis Q-X7 transmitter for combat robots. The OpenTX firmware used in the Q-X7 offers features previously unheard of in a transmitter priced below $100, but the user interface is radically different and intimidating for even experienced radio users.
I decided to author a combat robot guide for the Q-X7 based on the format of my FlySky FS-i6 transmitter combat guide, but exploring the OpenTX firmware with step-by-step examples setting up functions commonly found in combat robots. Work through these examples and you'll have a good working knowledge of the OpenTX firmware.

Taranis Q-X7 Combat Guide

Q: According to the Kinetic Energy calculator, my beetleweight weapon is spinning [up to] 1000 joules by 2 seconds. The Spinning Weapon FAQ said that a weapon should have the weight class of the bot in kilos x 60 x 2. That adds up to about only 163 joules. The motor in the calcs is the Sunny Sky Outrunner in the Vector Kit and the weapon is a 10 x 2 inch steel bar with a thickness of about 1/10 of an inch. Does my weapon really have that KE? [Potomac Mills, Virginia]
A: [Mark J.] No, your weapon does not spin up to 1000 joules of kinetic energy in two seconds. It would have helped if you had included the numbers you entered into the Run Amok Spinner Weapon Kinetic Energy Calculator so I could see where you went wrong. Here are my numbers...
According to the EndBots product website your weapon bar has the following specs:

Thickness: 4.75 mm
Length: 224 mm
Material: Steel - 7800 kg/m³ density
Weight: ~300 grams
Width: 40 mm (back-calculated from weight)
The Brushless Motor Stall Torque Estimator on the Kinetic Energy Calcualtor Help Page gets these input values for the Sunny Sky outrunner:

Voltage: 11.1 volts
Kv: 980
Ri: 133 milliohms
The estimated output values are 10,800 RPM (rounded down) and 0.85 Newton meters stall torque. Your pulley drive reduction is 2 to 1. Here's the generated spin-up graph: The performance numbers are likely a bit optimistic. Trying to spin a big bar that fast will generate enough aerodynamic drag to limit top speed and lengthen spin-up time. I'd guess that you meet the 'typical' spin-up speed and energy storage formula, and not much more.
Q: I am making a beetleweight 'Tombclone' horizontal spinner. About how much of the total robot weight should be allocated to the spinner? [Winchester, Virginia]
A: [Mark J.] Grant Imahara's "Kickin' Bot: An Illustrated Guide to Building Combat Robots" was the first to promote the well-acepted '30-30-25-15 rule' for robot weight allocation:

30% to the drive system - motors, gearboxes, and wheels;
30% to the weapon system - weapon, motor, and belts/pulleys;
25% to the structure and armor; and
15% to the batteries and electronics.
For a weapon-centered design like a big bar spinner you might steal an extra ounce or two for your bar -- about one pound devoted to your weapon is close to right.
Q: I am the Tombclone person and I am not experienced with beetle spinners, only antweight ones, so I am asking for a brushless motor recommendation for the beetleweight HS.
A: I recommend that you search the Ask Aaron Archives before asking a new question. With more than 6800 questions and answers, there is a good chance that your answer is waiting there for you. Here's a Q&A from the Ants, Beetles, and Fairies Archive:
Q: About how large a brushless motor do I need for a beetle spinner weapon? [Cincinnati, Ohio]
A: [Mark J.] Specifics will depend on your design, but popular beetleweight spinner weapon motors run about 150 watts per pound of robot and about 6% of the total robot weight. For a beetle, that typically translates to outrunner motors in 28mm or 36mm diameters.
As noted, the details of motor selection will depend on the specific elements of your weapon design -- belt reduction ratio, battery voltage, etc. I suggest searching for successful beetleweights with similar weapon layouts to your design to see what they use. For example:

Combat Robot Hall of Fame beetleweight 'Silent Spring' belt drives it's horizontal weapon with a powerful AXI 2814 outrunner;
The Vector Beetleweight Bar Spinner Kit uses greater belt reduction to spin its weapon with the more budget-friendly Sunny Sky 2212 outrunner.
Don't try to direct-drive a beetleweight spinner with an un-modified motor. The increased impact levels of the heavier weight class will very quickly destroy stock motors.
Q: I saw the graph of the number of robot fights and tournaments for each year on the Team Run Amok webpage. You say there that the data for the graph comes from SPARC Botrank. Is that data available to everyone? [Sacramento, California] A: [Mark J.] It is! You can go to their events list to find the events listed by date and the number of individual fights for that event. Clicking on the event name will take you to a list of all the fights at the event listed by weight class.
You might also be interested in this graph of similar data from the combat robot tournament registration site BuildersDB for comparison. Mouse-over the graph above, or click this link: Builders Database Events Chart. The BuildersDB chart shows 'Registered Bots' instead of 'Fights' because that's what the site tracks. Not all events report results to BotRank, and not all events use BuildersDB for registration -- but both charts show similar trends.

Q: I'm having trouble setting up my new FS-i6 transmitter with my endbots desc. I set up the FS-i6 with elevon mixing according to the instructions in your FlySky FS-i6 Transmitter for Combat Robotics guide but I can’t get the channels to behave. Moving the stick up with neither channel reversed, the right side moves forward and the left backwards. Reversing channel one has no effect, and having channel two or both reversed results in the left moving forward and the right backwards.
What am I missing? The wiring is the same as I had working on my last transmitter and I’m pretty sure I had just reversed one of the channels. Thanks! [Boulder, Colorado]
A: [Mark J.] The odd responses to stick input that you describe are expected if you have both transmitter 'elevon' mixing and Endbots DESC on-board mixing turned on. The transmitter mixing is much more configurable, so leave it on and turn off the Endbots mixing.
The Endbots DESC has an absurd an unusual method of selecting on-board mixing: it toggles on/off each time you calibrate the DESC. Recalibrating your DESC will turn off the on-board mixing and should fix your problem.
Reply: Desc mixing did the trick, thanks!

Q: A builder on [social media] recently mentioned using a 2205 2300kv brushless weapon motor in their combat antweight. Was that a typo? Does anyone make an outrunner only 5mm thick? [Akron, Ohio]
A: [Mark J.] I understand your confusion. There are two ways to measure a motor for classification:

Some manufacturers classify their brushless outrunners by the diameter and length of the motor can and base. A '2205' motor under this system would be 22 millimeters in diameter and only 5 millimeters long. No, nobody makes a hobby motor that size.
Other manufacturers - particularly of quadcopter motors - classify their brushless outrunners by the diameter and length of only the motor stator; the wire-wound source of the electromagnetic force that drives the motor.
A '2205' quad motor like the pictured 'Emax RS2205-2300' would be a '2817' if classified by its can dimensions.
Q: So I recently got a FrSky QX7 Transmitter to replace my dying HobbyKing entry transmitter, but frankly, I'm a little overwhelmed - seems a lot of the OpenTX and Taranis tutorials are geared towards planes, so filtering out and applying the information I want is proving tricky. I've successfully bound the receiver to the tx, but I need to do the following:

Setup an invert switch

Mix for drive and steering on the right stick

Turn down the output on the steering mix to make it more controllable

Trim the throw of the weapon servo on channel one, as the linkage only allows for around 40 degrees of rotation, while the servo is trying to output 45.
All simple enough on the old Tx! Can you point me in the direction of some good guides, or give me some pointers? Thanks!! [Glasgow, Scotland]
A: [Mark J.] I'm not so sure that all of those functions are simple, ore even possible on your old HoobyKing transmitter - but they are certainly functions that the Q-X7 can handle. There is very little that an OpenTX transmitter cannot be set-up to do, but all that capability comes with a large and complex system of menus and adjustments. Figuring out how to implement even simple functions can be a challenge.
Before you buy ANY radio, download and read the manual.
If it doesn't make sense to you, buy a different radio.

Here's a video for the invert mix. Don't reverse the rudder or aileron channels, just the elevator for an inverted robot with an elevon mix.
Here's a video for single stick channel mixing. Ignore his percentage settings: set the throttle inputs to 100, and try the steering inputs at 50.
To adjust the steering sensitivity, increase or decrease the steering mix inputs you learned how to do in the video above. You can also add fancy response curves to decrease response near the center -- but you can look that up later.
For your purpose, just limit the travel 'throw' of your weapon servo channel from the 'Outputs' screen. The default for a channel is -100 +100 -- something like -90 +90 should be close to what you need.

UPDATE - There are many guides for the Taranis Q-X7 out on the 'net, but they all address model aircraft applications. I've written a guide aimed at setting up functions common in combat robots. Plenty of examples, and plenty of screen shots.
Taranis Q-X7 Combat Guide

Q: Brushless drive motors have me really confused. My beetle has brushed motors but I'd like to build a feather and it seems that everybody is using brushless for their feathers. I don't know where to start. Is there a beginner guide someplace? [Pittsburgh, Pennsylvania]
A: [Mark J.] My usual advice for beginners in any aspect of combat robotics is to look at what successful builders are doing and follow their example. That doesn't work very well with brushless drive where the difference between poor and excellent performance relies so much on the unseen ESC firmware and user-modified settings. Unfortunately, both firmware and currently favored settings change quite frequently.
I've pulled together a collection of posts from the Ask Aaron archives that may be of some use in selecting suitable brushless motors for both weapon and drive use. It also contains a complete 'off-the-shelf' solution for featherwight brusless drive that requires no modification:
Brushless Motor Selection

Q: I've been watching videos about battle hardening brushless outrunner weapon motors. Doesn't globbing a lot of epoxy on the outside of the stator increase heat build-up? How much epoxy do I really need? [Ames, Iowa]
A: [Mark J.] I don't know where the practice of building a thick epoxy 'shell' around stator windings became popular. It does reduce stator cooling, and it does not prevent the lower layers of wire from shifting under impact load. A better method:

Drop your epoxy dispenser (long-cure, not 5-minute epoxy) into a glass of hot water to get warm and 'runny'.
Pop the stator into a 150-degree oven and let it warm up all the way thru.
Mix up a batch of the warm epoxy and brush/pour/drip it onto the stator windings.
The warm epoxy will 'wick' into the windings. Keep adding epoxy as it continues to soak in. Rotate the stator to better distribute the epoxy and check for drips. When satisfied, pop the stator back into the warm oven -- the heat speeds curing.
The epoxy that soaked in between the windings now aids cooling via improved conduction, and the wires are bonded together to prevent shifting.
Q: Seth Schaffer gave you a shout out on [social media site] for solving the weapon instability problem he had with his antweight "Mini Mulcher". What did you tell him? [Tacoma, Washington]
A: [Mark J.] Seth is just showing his appreciation for a very small pointer. Both Justin Marple and I remarked that the long, narrow, and heavy weapon blade on antweight 'Mini Mulcher' was a prime candidate for a bad case of 'polhode instability' that would explain Mini Mulcher's wild gyrations. Seth did the hard work of chasing down the problem with an extensive testing program and created a new 'axe head' weapon design that eliminated the polhode problem. A word to the wise was sufficient.
Q: I don't understand what caused Mini Mulcher's instability. The weapon was spinning on it's "maximal inertial axis" which should be stable. What went wrong?
A: Yes, according to the intermediate axis theorem it was spinning on a stable axis and everything was fine... until there was a change in the stored energy level. A change in stored energy will induce a small polhode motion wobble.
If the energy storage capacity of the intermediate axis is significantly different from that of the axis of maximum inertia, the wobble will self-damp and pass without notice. Unfortunately the weapon blade's intermdiate axis momentum was quite close to that of the maximum inertia axis and the wobble was able to flip the rotation axis 90 degrees -- throwing the entire robot perpendicular to the arena floor. The weapon striking the floor then sent the 'bot skyward.
The new weapon blade has the inertia of the intermediate axis well separated from that of the major principal axis and the polhode wobble damps out. Problem solved.

Flashback Post from 2018: Gyro Effect Calculator
Q: Is there a calculator to determine if a bot will flip over based on the MOI of the weapon, the distance from the wheels to the weapon shaft, and the diameter of the wheels? Can you calculate how fast you would be able to turn based off the information from the spinner spreadsheet? Does overall width matter in stability of the weapon and in the turning ability of the bot - are wider bots more stable?
Many Thanks! [New Castle, Pennsylvania]
A: [Mark J.] Yes there is a calculator, but the variables used by the calculator are a little different. It will tell you how fast you can rotate without wheel lift, and if you know the weapon MOI you don't need the spinner spreadsheet. All the design factors are explained on the calculator page. So... where is this calculator? If your question starts with 'Is there a calculator...' your first stop should be the 'Combat Robot Design Tools from Team Run Amok' page:
...I realized that there was another tool that should be salvaged from the wreckage of the T.i. Combat Robotics site. Their 'Designing Around the Gyroscopic Effect' page has a full explanation of the forces at play when a vertical spinning weapon exerts a lifting force on a turning robot. That page is now safe here at RunAmok.tech.
After walking you thru the math, the page offers a simple javascript calculator that will model the stability of specific robot designs to let you know how serious the gyro effect will be on your planned robot. If you're designing a big drum or vertical disk/bar spinner you'll want to make use of this calculator to avoid unpleasant surprises in the behavior of the finished robot.
Follow the link above to our design tools page and scroll most of the way down the page to find the link to the 'Total Insanity Gyroscopic Effect Calculator'. I'd give you the direct link here, but you'll benefit from looking over the other tools on that page.
Q: Which weapon type is good for beginners? I know wedge is the best but because my local event has active weapon rule, I can't build wedge. So could you tell me which one is recommended and which one is not? [Seoul, Korea]
A: [Mark J.] A combat robot is a complex assembly of components and systems that all have to work together to be successful. Forcing new builders to add an active weapon (lifter, flipper, spinner, crusher...) is a disservice to both new builders and the combat robot community. It creates a barrier to participation that practically guarantees frustration and failure for novice builders.
If your first robot must have an active weapon, concentrate on the basics and build a robot that does not rely on its active weapon to be effective. Start with a strong, capable chassis with a wedge and enough weight allowance left over to add a small active weapon system. The weapon can be whatever you're comfortable bulding. A simple electric lifter would be a good choice, but don't allow the active weapon to compromise your strong drive train and wedge. Your active system is secondary and entirely expendable.
Aaron's Wisdom I've said this often but builders don't want to believe me:

The weapon may be the least important system on a combat robot.
If you're not winning matches it isn't because you have a poor weapon.

Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.

Q: Somebody made a comment about the "hamburger is bad" rule on [a combat robot social media site]. Can you tell me what that rule is? [Henderson, Nevada]
A: [Mark J.] You came to the right place.
'Ask Aaron' has a long history of receiving requests for very specific recommendations on materials or components that provide far too little information for us to give a useful answer. We needed a name for this type of question and a way to explain our problem in answering. In 2010 we published a webpage titled "The Hamburger is Bad" that uses a fictional exchange of questions and answers about whether a 1/4 pound hamburger is good to depict the problem. The name caught on in the combat robot community.
So, "the hamburger is bad" is not so much a rule as a shorthand request to provide adequate information when asking a question.
Q: I have a 3S LiPo and I want to run the drive on the full 11.1 volts and tap the balance plug to get 7.4 volts for the receiver and servo. I'm worried about screwing something up and getting a short thru a 'ground loop'. How do I do this safely? [Social Media]
A: [Mark J.] You're wise to worry. If you tap the balance plug incorrectly you will have the receiver 'ground' at a different voltage potential than the drive 'ground' and bad things will happen: wires melting, LiPo cells exploding, the works. When tapping the balance plug for a voltage less than the full battery voltage, always use the full-pack ground (pin 1) on the plug. You could, for example, get 7.4 volts from the balance plug by tapping pin 2 and pin 4, but failing to include pin 1 creates the dreaded ground loop that shorts out one or more cells thru the receiver ground bus.
Make sure everything uses the same ground and you'll be fine.
Q: Maybe it's just my imagination, but Hypno-Disc was always interesting to me in that it when it turned, it would turn about a point that was closer to the front of the robot rather than the centre and sling the back out. How did it manage this? Was it just weight distribution that caused this effect or was it something about the wheels? [Ashford, England]
A: [Mark J.] It's not your imagination. I won't comment on 'Hypno-Disc' directly (see FAQ #37) but I will direct you to the Hypno-Disc website where you may notice some differences between their front and rear tires starting in Series 5.
You may also find this post in the AskAaron Archives interesting.

Q: "Mounting plates" sold by Fingertech have served me well on ant & beetle motors, is there an equivalently easy solution for fairyweight motors? [Woburn, Massachusetts]
A: [Mark J.] The popular 'N20' style motors used in 150 gram 'bots are constructed differently than commonly used ant and beetle motors. They require greater support for their open gearboxes than 'face mount' style plates like the FingerTech Flat Motor Mount provide.
A search for "N20 mounts" will find many clamp-style mounts, like the Pololu Micro Metal Gearmotor Bracket shown. That's your fairyweight equivalent easy solution.
Q: How do I charge a 36v Li-ion battery like those found in hoverboards? [Roseville, California]
A: [Mark J.] Call me crazy, but have you considered a36v lithium charger? Lithium batteries need 'smart' chargers that carefully monitor the charging process. Hoverboards and scooters have on-board charger electronics and just need a power supply, but you'll need a complete charger. Do not use any charger not intended to charge the specific type of lithium battery you're using. Improper charging can very easily result in a fiery and explosive failure of the battery.
Note The battery you reference (link removed) does not have the high discharge rate commonly needed in a combat robot. A typical combat 4400 mAh LiPo battery would have a max continuous discharge rate of more than 250 amps, while the battery you reference has a discharge rate of only 30 amps. If you can live with that low discharge rate, shop around a bit; I've seen that same battery from other sources for half that price.
If you're interested in the technical details of lithium ion battery charging, check this article at Battery University.
Q: I'm using a 20A Afro esc flashed with blheli to power a Turnigy 2209-980 brushless outrunner. I'm also using a Turnigy 850mAh 3 cell battery to power all the electronics. 30c-60c discharge. At low and high rpms, the weapon, when it gets a good hit on something, shuts off and re calibrates. I have installed a large capacitor between the battery leads that helps with this problem quite a bit. Anything above 50% power on the throttle shuts down the weapon when it gets a good hit. Rapidly throttling the power all the way up to 100% also achieves this.
While watching the electronics during the incident, they all seem to shut down for a quarter of a second before coming back on and re calibrating. On my previous robot with a drum spinner, I also had this issue with a 750kv motor. During the competition I was able to wait for the electronics to come back online and drive away. If you need any other information about the electronics or the robot, just tell me. [Arlington, Virginia]
A: [Mark J.] The clue to the cause of the problem is that your installation of a 'large capacitor' across the battery leads helped. You're getting a serious voltage drop when the weapon motor is pulling a lot of power and your electronics are 'browning out'. Two possible solutions -- I'd do both:

Since ALL your electronics are shutting down I'm suspicious that it's the receiver that's causing the system reset when the voltage dips. Remove the 'large capacitor' from the battery leads and connect it across the power and ground pins on an unused receiver port. This will sustain voltage on the 5-volt receiver bus.
How old is your battery? The math says that it should provide (60 * 0.85 =) 51 amps of peak surge power, but it sounds like it isn't doing it. The current batch of Turnigy 850 mAh 3S LiPos are rated 60c-120c and should have no trouble keeping up with your weapon motor.
Q:'UnMakerBot' builder Zach Goff says his lightweight champion has a reprogrammed quadcopter brain that keeps it from flipping over. Is this real? How does it work? [Bay Area, California]
A: [Mark J.] Many combat robots use a single-axis 'heading hold' gyro to improve straight-line tracking and provide steady turn response. The solid-state gyro accomplishes this by adjusting drive motor speeds to make the actual turn rate (yaw) matches the turn commands coming from the R/C transmitter.
Zach has expanded on this by repurposing a multirotor flight controller with three solid-state gyros aligned at right angles to each other to detect rotation in any of the three axis of motion: pitch, roll, and yaw. Unlike an aircraft, a combat robot has no direct control over motion in the pitch and roll axes -- but information about movement on those axes can be used to modify robot motion in a way that may keep it out of trouble:

A sudden up-pitch to the front or rear of the robot may indicate a weapon strike that threatens to flip the 'bot over. If you still have wheels on the ground, a command by the pitch gyro for hard accelleration away from the detected strike may keep the robot upright and on its wheels.
A 'gyro dance' caused by too high a turn rate on a robot with a vertical spinner weapon can be detected by the roll gyro, which can then command a reduction in the turn rate or weapon speed to keep the 'bot on its wheels and avoid a vulnerable position.
The hardware found in a stock flight controller can perform these functions, but the firmware must be modified to direct robot-correct responses. This isn't a project for the timid, but you can gain some advanced help in keeping your 'bot on the ground.

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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Questions and Answers about Combat Robotics from Team Run Amok.