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Questions and Answers about Combat Robotics from Team Run Amok.

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6802 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.

Flashback Post from 2018: What's an OPTO?
Q: I see several different brands of brushless ESCs labeled 'OPTO'. What does that mean? When would I want 'OPTO'? Would it ever be a bad thing? [Pomona, California]

A: [Mark J.] A complete answer requires a bit of history.

Back in the day...

The small, high-power brushed motors powering early electric model aircraft created a huge amount of high-frequency electrical 'noise' and voltage spikes from the arcing of the motor brushes at high speed. This 'noise' traveled along the power wires and found its way to the sensitive and vulnerable electronics in the R/C receiver, wreaking havoc with reception.

To protect the receiver from this electrical noise, 'OPTO' speed controllers included a device called an optical isolator in the line between the receiver and the power circuits. This 'opto-isolator' converted the receiver output signal to a modulated light beam, then back to an electrical signal for the ESC. Electrical noise couldn't jump back across the optical gap to get to the receiver. Genius!

But there's a catch...

If you use a battery eliminator circuit (BEC) to power the receiver from the main battery pack the electrical noise can use it as a path to the receiver, nullifying the benefit of the opto-isolator. Fully isolating the receiver from motor 'noise' requires that the receiver have its own battery, so OPTO ESCs had no BEC. Inconvenient, but it worked.

Jump to present day

Brushless motors have replaced the 'noisy' brushed motors in model aircraft and the need for opto-isolation no longer exists in most applications. Few brushless 'hobby' ESCs have an optical barrier to electrical noise, yet many still carry the OPTO label. In current usage it's simply an indication that the ESC has no battery eliminator circuit. Eliminating the BEC shaves off a couple grams of weight and eliminates the need to 'clip' the red power line in the receiver cable when multiple BEC equipped ESCs are on the 'bot.

You need something in your power system to provide power to your receiver, and a convenient way to do that is to make sure that one of the ESCS for the weapon or drive motors has an BEC -- but you only need one.


Q: Hi there. I’m looking for information on competitors who competed at San Fransisco Robot Wars event in 1997 and I’m also looking for pictures of them. Thank you. [Apparently from New Zealand, via two proxy servers in the UK]

A: [Mark J.] Info from the 1997 San Francisco Robot Wars is more difficult to obtain because the entire US Robot Wars organization was shut down immediately following the event. They didn't even have time to update their website with photos and results. There are a few good sources of info and photos:


Q: What are the materials that are used to make weapons in combat robots? [Digital Ocean]

A: [Mark J.] Your question comes from a web server associated with web proxy services that mask user location. Please remove your VPN service and resubmit. If you have nothing to hide, don't try to hide it. No Proxies! I'm very serious about safety in combat robotics. I no longer answer 'Ask Aaron' questions from geographic regions that ask only about building more powerful robot combat weapons with little to no regard for building safer robot combat arenas. Builders from these regions often attempt to sneak questions thru by disguising their true locations with VPN and proxy services. Questions that arrive via known proxy servers are generally ignored because of the time it takes for me to determine the true origin of the question.


Q: Hi Mark. Thank you for sharing your snail cam design. I work in a different field and am interested in using it to generate impact. We have a tight space, the minimal cam radius is 5mm and the maximal cam radius is 8mm. We want to use disk spring to generate 1000N impact force. Do you see if it is possible? Any comments will be appreciated. Thank you! [Weston, Florida]

A: [Mark J.] You're not giving me much to go on, Florida. The snail cam is a reasonable mechanism to compress and release spring force, and it is fairly simple to calculate the energy stored in the compressed spring, but translating stored energy into 'impact force' is complex, requires many assumptions, and varies with the characteristics of the object being impacted.

Here's a link to a good article at wired.com: How Do You Estimate Impact Force?


Q: Given that μ = 52, σ = 35 and n = 35, what is P(X ≤ 73) and P(X ≥ 73)? [Singapore]

A: [Mark J.] Not a robot question, but it's a slow Saturday morning and I do have a minor in math...

Calculate the Z-score:

Z = (x - μ) / σ = (73 - 52) / 35 = 0.6

Consult a Z-score table:

P(x ≤ 73) = P(z = 0.6) = 0.7257

P(x ≥ 73) = P(z = -0.6) = 0.2743


Q: hi ive been enjoying reading you site for a few years finally decided to ask a questions hopefully this doesn't qualify for a bad hamburger or cheerleader question

anyway im currently designing/building a 30lb ring spinner (not a good idea but i like a challenge) and im looking at ways to power the ring so currently my plan is to use two brushless motors on a 3 to 1 ratio on the inside of the ring via a large hdpe ring gear. my main concern with this is that the impacts would damage the weapon motors or break teeth off the inside of the ring so my plan is to use a ridged tpu [Thermoplastic Polyurethane] my thinking is that the tpu teeth would flex and "skip" on a big hit protecting the motor would this work or am i just going to rip apart the rubber if this wouldn't work what would you suggest to protect the motors. i unfortunately cannot use belts due to a lack of internal space (mostly rubbish design on my part) i could run the motors with wheels on to drive the ring but my fear was that i wouldn't be able to get the torque required to spin the weapon

heres some info that might be useful

  • the ring weighs just short of 5kg

  • the brushless motors are N5065 320KV 1820W from banggood because i could afford to buy lots of spares due to the cost admittedly this will probably bite me in the butt

  • the module of the gears is 4.2 with 100 on the ring gear and 30 on the motors

again apologies if this is a rubbish questions and for the shocking grammar [Abingdon, England]

A: [Mark J.] Well, you already know that a ring spinner is a bad idea -- that saves me some time.

The combination of "flex" and "gears" is a bad one. The teeth are large, but I doubt you'd have enough clearance for them to flex enough to 'skip'. I worry that the flexi TPU teeth would distort under drive torque loading and jam at the gear interface, causing huge drag. That would bog down the motors and create enough heat to soften the plastic, creating more trouble.

I'm not a fan of friction drive for just the reason you mention. Stick with HDPE and make the ring and pinions as tall as possible for strength. I assume the pinion gears are mounted on the motor cans? Support both ends of the weapon motors: extend or reverse the motor shafts and add bearing blocks to the far end. If the ring support is strong enough you should be OK.


Q: Hello Mark
I come back to you for our robot flipper machine. We followed your advice and installed a RC Polulu 2802 switch, but as regards the solenoid valve the best is a 5/2 model. Should the pilot be internal or external, and naturally open or closed?
Thank you in advance for your advice. [Provence-Alpes-Côte d'Azur, France]

A: [Mark J.] Hello, Jean Pierre...

For your purposes you'll want a solenoid operated valve with an internal pilot. Valves with an external pilot are for systems that must be able to actuate even when there is no pressure for them to control -- never the case in a robot flipper.

All 5/2 (five port, two position) pneumatic valves have two output ports. In the non-energized state:

  • One output port is connected to exhaust: normally closed (N.C.)
  • The other port is connected to pressure: normally open (N.O.)
You will connect the N.C port to the extension side of your actuator, and the N.O. port to the retract side. In that configuration the actuator will not move from its retracted state when the system is pressurized with the valve non-energized.
Q: I stumbled onto some crazy butane jet lighters that are intended for cigars but seem pretty dang impressive. Would it be practical to put one of these in an ant or beetle? [The Aether]

A: [Mark J.] I fought against BattleBots champion Donald Hutson's jet lighter weaponed ant 'Incinerator' in a melee at 'Bot Bash' in 2002. Somebody ejected it thru a pushout quickly, but it continued to wander around the 'moat' attempting to fry the other 'bots that fell in. Donald's like that.

All flame bots are just for showing off -- they do not win tournaments. You need a method to turn on the gas and light the flame remotely because no organizer will let you 'flame on' before the arena door is locked and the match has started. Don't write back and ask me how to make that ignitor; if you don't already know how you're not ready to build a flame-bot. The event also has to allow flame weapons. Most organizers aren't too keen on 'bots that could melt holes in their expensive Lexan.


Q: I'm building a robot for a competition which was cancelled due to the COVID-19 obvious. Before everything was shut down, I was able to record the actual free-running RPM of the output shaft on one of the gearmotors. The RPM translates to about 14 feet per second. I'd like to have at least 8 feet per second in actual performance. I was curious what kind of decrease in speed I might expect when the robot is completely assembled versus this theoretical speed. Each motor would be driving two sets of wheels and I figured we would be close, but are there any methods I can use to calculate this that isn’t crazy complicated? Thanks! [Reddit/battlebots]

A: [Mark J.] Brushed Permanent Magnet Direct Current (PMDC) motors produce their maximum torque at stall, with torque falling away linearly with increasing RPM. At no-load free-running RPM the torque usable torque has dropped to zero -- if there was any available torque the RPM would continue to climb. Brushed DC motor performance curves From a standing start your robot will accelerate more and more slowly as the speed climbs and torque diminishes. With greater speed comes greater friction and aero drag. When the total drag climbs to the point that it matches the available torque from your drivetrain, acceleration stops: you're at top speed.

Actual top speed should be 85% or more of calculated speed. Getting to only 57% of calculated top speed would indicate that you either have a great deal of drag in your drivetrain or you have insufficient gear reduction and are 'bogging' the motors. Check for free-running in your drivetrain before adjusting gear ratio.

I've written a guide to help determine optimum gear ratios for combat robots. It goes thru the math and then points you to my version of the Tentacle Torque-Amp calculator to let you compare the effects of gearing changes on acceleration and top speed within an arena of given size: Combat Robot Drive Train - Optimum Grear Ratio Selection.

I've also written up a walk-thru of a drivetrain optimization for a combat robot: Example Drivetrain Analysis using the Tentacle Torque Calculator.


Q: Hey there, i am purchased a leopard 58110 motor for my 30kg bot for weapon and I am using a ZTW 200amps ESC but I don't know why my motor isn't able to pull the weapon. I am unable to figure out that did I made a mistake in selection of each or i am putting some wrong programming values while programming my ESC.. please help me (nepal)

A: [Mark J.] You claim to be from Nepal, but your IP trace says your message came from Mumbai. Ask Aaron no longer answers questions from robot builders competing in India (Why not?).

Assuming you are from Nepal, you have provided far too little information for me to even take a guess at a solution to your problem. All you've identified are the weapon motor, weapon ESC, and robot weight. So many things that could be wrong -- so little information provided. About all I can tell you is that the motor and ESC are compatible and that the default ESC programming should be an adequate starting point. Something else is wrong. The hamburger is bad.

Q: Hey Mark, thanks for answering my previous question. It seems that you haven't visited india in past 2 years.. the arena quality is much improved since 2018 as the Indian robotics community boycott all those events having unsafe arena...

May be you should start answering all indian roboteers from today. Thank-you. [Mumbai, India]

A: Maybe you should develop a sense of honor and stop lying about where you're from.

Maybe you should stop kidding yourself about robot combat safety on the Indian subcontinent.

Here are a few video highlights from regional 2019 tournaments with horrible safety conditions:


Q: Hi there, I dunno if you'll know the identities of [these three] UK entries during the 1995 UK Robot Wars pilot. I know they were all built by Derek Foxwell and I've checked the archived website for images and names but I'm struggling to discover who they were. Any ideas? [Bodmin, England]

A: [Mark J.] The best info I have is that the three robots in the photos were small machines weighing perhaps 5 Kg that were built by Derek Foxwell for 'proof of concept' trials staged months prior to the Robot Wars pilot event that came to be known as the 1995 UK Open Competition. Short film clips from a compilation of the UK Open were released years later as part of the 'Robot Wars - The first Great Wars' video. That compilation shows the three small robots in what may have been a 'Robot Soccer' event.

The featured fights at the UK Open had early versions of U.K. robots 'Eubank the Mouse' and a multibot version of 'Grunt' called 'Grunts' versus well-known US Robot Wars competitors 'The Master', 'La Machine', and 'Thor'. Both Eubank and Grunt appeared in later seasons of Robot Wars, but the names and full story of the three 'bots you ask about are undocumented.


Q: My son is making a 3lb battle bot. He wants to run the drive motors off of a 2s 300 mAh battery and run the weapon on a 3s 1100 mAh battery. I sent a sketch up of how he wants to wire it up. Somebody on [social media] said we would have a "ground loop" and I'd blow something up. What's a "ground loop", and do I really have a problem with this circuit? [Orlando, Florida]

A: [Mark J.] I've re-drawn your sketch for clarity.

  1. A ground loop may occur when you have two or more batteries wired in series to obtain a higher voltage and some component taps into the wire connecting the two batteries in order to operate at reduced voltage. This can create a 'floating ground' at a higher voltage potential than the ground used by other components that will 'dead short' thru the receiver ground leads and melt receiver leads and/or destroy one of the batteries. You can see an animated diagram and a more complete discussion of ground loops in this archived Ask Aaron post.
  2. Your circuit has no ground loop. It does use two batteries, but they independently power two separate components that each share a common ground. No floating ground -- no ground loop. It will work, but there is a simpler circuit that will accomplish the same power distribution.
The simpler solution is to wire all ESCs to a single 11.1 volt battery and restrict the output of the transmitter sticks controlling the drive motors by transmitter programming. The function used to do this has different names on different makes of radio; check your transmitter manual for 'Adjustable Travel Volume' (ATV), 'Travel Adjust', or 'End Points' for instructions. Setting the ATV to 66% on forward, reverse, left, and right stick commands will reduce the voltage the drive motors 'see' from 11.1 volts to 7.4 volts. One battery, one power switch, and simpler pit procedure.
Q: How exactly does the 'Greedy Snake' lifter mechanism work? I’ve seen it used on bots like 'SlamMow', 'Claw Viper', and 'Barróg Doom' and kinda have an idea as to how the system works, but I feel like I don’t get it entirely despite being a seemingly simple setup. Are there any diagrams, pictures, or videos that best describe how it works?

As always, thanks! [Social Media]

A: [Mark J.] The mechanism in question is often referred to as a single motor clamp lifter. Powered rotation of a spur gear (shown with a red dot) drives a clamping arm attached to a second spur gear downward until on object stops it's motion and prevents further rotation of its gear. Continued rotation of the powered spur gear will then rotate the entire gearbox and attached lifting platform around the axis of the powered gear. Reversing the rotation of the powered gear will lower the platform and release the clamp. In practice, there may be additional gears involved to provide additional gear reduction in order to gain the substantial torque needed for the lift. The animation has been reduced to only the critical components.

Note The clamping force is created by weight on the lifting platform. As the lift angle increases there is less weight pressing down on the platform: clamping force becomes essentially zero when the lifter is vertical.


Q: Hello Mark,
First of all I want to thank you for your previous answer [five posts down the page].

For our flipper type robot we are considering a Burkett 5404 2 port 24v type valve associated with a Wasp motor controller. What do you think of this choice? And what connection for a double acting cylinder?

Thank you for your reply. Best regards, Jean Pierre [Provence-Alpes-Côte d'Azur, France]

A: [Mark J.] The Burkert 5404 is the solenoid valve of choice for pneumatic flipper systems because of its ability to flow a large volume of gas very quickly, but I don't think it's suited to your use. As I recall, you are building a 20 KG robot. A single Burket 5404 weighs more than a kilo and you'll need at least two valves of this type for your flipper.

The 5404 is a simple 2-port open/close valve. It can open to allow gas to flow into your actuator but when it closes there is no place for the gas in the actuator to go -- you need a second 2-port valve to open and exhaust the gas from the actuator to allow it to retract -- see the diagram. If controlling both valves from a single controller you will want the pressure valve to be the 'normally closed' type and the exhaust valve to be 'normally open' so that an actuation signal will open the pressure valve and close the exhaust.

Two 2-port valves will suffice for a flipper if you can live with a gravity or spring-powered retract. If you want a powered retract you'd need more valves to operate the retract side of the actuator. By this point a quarter of the weight of your 'bot could be valves and hoses!

Take a look at our Tips and Tricks for Pneumatic Weapon Systems page for some alternative designs.


Q: What kind of robot would be most effective/useful in this era of COVID-19? (Preferably in the form of a haiku) [Pittsburgh, Pennsylvania]

A: [Mark J.] Who wants dibs on the name "Antisocial Distancing"?

Social distancing:
Ship bot, but no transmitter.
Control via Skype.

Q: My FS2A 4 channel receiver has the failsafe only on the third set of pins. What small receiver can I use with my FSi6x transmitter that will failsafe correctly? [Social Media]

A: [Mark J.] The FS2A receiver will failsafe correctly with your FSi6x on all channels, but the set-up process is not well described in the receiver documentation. Chinese radio gear is like that. Here's what the docs say:

Failsafe settings:
The receiver can normally receive the transmitter signal, push the rocker to the the failsafe set, long press the receiver bind button, the blue light flashes quickly several times and then always bright , failsafe settings is successful.
Allow me to translate Banggood English to [Actual English]:
  1. "The receiver can normally receive the transmitter signal,"

     [Turn on the transmitter and receiver]

  2. "push the rocker to the the failsafe set,"

     [Move the transmitter sticks and switches to the failsafe positions and hold them there for the next step]

  3. "long press the receiver bind button, the blue light flashes quickly several times and then always bright,"

     [Hold down the receiver bind button 'til the LED flashes, then stays steady]

  4. "failsafe settings is successful."

     [You're done]

This is a separate process from binding the receiver, and is done after binding is completed.

Motor Spec Converter
Speed: RPM
Voltage: volts
Output: watts
Kv: RPM
Kt: oz-in
Stall: amps
Q: I'd like to see if a pair of 36 Volt 1000 Watt 3000 RPM Electric Scooter Motors will work for my 'bot, but the motors have no specs. Is there any way I can use the Tentacle Torque Calculator to see what the performance of my 'bot would be with these motors? [Social Media]

A: [Mark J.] You have more motor information than you may think.

I've written a JavaScript 'Motor Spec Converter' to quickly transform motor RPM, voltage, and output watts into the inputs needed for the Tentacle calculator. Fill in the blue input cells with your values and click the 'Calculate' button.

Are you interested in the math behind the calculator? Take a look at Converting Motor Specifications.


Q: Hi there. I purchased an Endbots DESC lemon to use in a Beetleweight project, but am finding acquiring a DSM2 transmitter in Europe troublesome. Is it possible to add a different receiver to the project to allow me to use an alternate transmitter? Failing that, are there any other dual channel ESCa that work well for beetles?

Thanks, James [Smithton, Scotland]

A: [Mark J.] Hi James. I don't have an Endbots DESC lemon here in the shop, but I believe that it is just a Endbots DESC board with modified RX input pin locations that allow a Lemon-RX to be flipped over and soldered to the back of the board. De-soldering the Lemon-RX should allow you to wire any receiver onto the exposed channel pins. I would suggest writing to Endbots to verify this; they accept support requests at Sales@endbots.com

Failing that, the popular Beetle DESC at the moment is the Scorpion Mini from Robot Power. It is a fair bit larger and heavier than the Endbots DESC, but has a much higher current rating (6.5 amp continuous) and will handle up to 28 volts.

Some builders prefer using two of the single-channel Wasp ESCs from Robot Power. A pair of them are a little lighter than a single Scorpion Mini, and it's easier to find space in a cramped beetle for two small boards than one larger one.


Q: Hello Mark. First of all congratulations for your website which is very enriching for novice builders like me (we are French).

My question: We are working on the design of a pneumatic combat robot (20 kgs), we have tested the chassis part and made the choice of HPA to actuate the jack. The best to control the valve is an ESC or a relay?

Thank you for your reply. [Provence-Alpes-Côte d'Azur, France]

A: [Mark J.] I'm glad that you are enjoying Ask Aaron! You have a choice of three types of R/C interfaces to control your pneumatic valve:

  • A mechanical relay with an R/C interface like the Polulu 2804. Mechanical relays are subject to damage and failure under heavy shock loading and for that reason are not recommended for combat robot use.
  • A solid-state R/C switch like the Polulu 2802. Solid-state switches have no moving parts to fail under shock loading and are a better choice for combat robots. Some solid-state switches require adding a 'flyback diode' across the power output terminals when used to operate an inductive load like your solenoid valve -- be sure to check the manual. The Polulu 2802 requires no diode.
  • A small ESC like the Fingertech tinyESC would provide a forward/reverse current rather than on/off if plugged into a switched receiver output. You could get around that with transmitter programming or you could find a single-direction ESC, but an R/C switch would be easier.

goBILDA transmitter Q: Is the goBILDA Element-6 Radio Control System any good for an ant or beetle? [Glendora, California]

A: [Mark J.] I'm gonna say no. Take a look at the Team Run Amok guide to what radio functions you actually need. The goBILDA does not score well:

  • The manual is only two pages long, and the first page is a picture of the transmitter.
  • The manual makes no mention of if or how the system failsafes -- a proper failsafe is mandatory for robot combat.
  • The feature list is quite basic: servo reversing on 4 of the 6 channels, standard trims, two non-standard mixes, and a very limited dual-rate switch.
  • The included receiver is large for an insect robot, and the uncommon S-FHSS protocol limits the range of replacement receiver choice.
For the same price you can buy a full-featured FlySky FS-i6 radio with much greater capability.


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.

Aaron's Greatest Hits! More of Aaron's Poems Aaron's Minecraft High Dive Video Aaron's World of Warcraft Player Guide


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