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6688 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

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.
 

Q: Hey, its me from Anacortes again. I've run into a couple more hitches in the electronics of my beetleweight wedge. A while back you basically had to walk me through how to install the fingertech tinymixer properly, and at the time it seemed to work fine.

But now that I've put the chassis of the robot together, the right side drive motors suddenly reverse direction for a split second when I use more than 75% throttle. This happens every 1-3 seconds of drive, making it largely uncontrollable. I suspected that the tinymixer was the cause, and sure enough, when I removed it the motors responded to power normally. Is this some sort of defect in my tinymixer, or somthing wrong with my setup? (running four kitbots 1000 rpm gearmotors with Scorpion mini esc with a 4s lipo)

My second problem is (I think) my radio. What is "RX battery" and what can I do to stop the FS-i6 beeping? [Anacortes, Washington]

A: [Mark J.] Second problem first. No... wait... both problems at once.

You've got three different components in your system that can provide R/C channel mixing -- that's two too many. Your FS-i6 transmitter is the best choice to provide mixing because it has adjustments that can tailor the robot response to your driving preferences. Instead of sorting out which components are currently doing what, let's turn off all the excess mixing options and set the transmitter up correctly.

  1. Remove the tinyMixer from your 'bot and plug the left/right Scorpion Mini ESC inputs directly into channels 1 and 2 of your receiver.
  2. Remove the 'MIX' jumper from the Scorpion Mini board. **
  3. Read my FS-i6 Combat Set-Up Guide. Then read it gain.
  4. If you've done any mix programming on the transmitter, reset it to factory defaults.
  5. Install the Quick and Dirty Combat Set-up from the combat set-up guide onto your transmitter.
Now all mixing is handled right in the transmitter and if something doesn't feel right in the way the robot responds you can adjust the transmitter settings 'til you like it.

RX Battery - Your FS-i6 radio is bi-directional: the transmitter 'talks' to the receiver, and the receiver 'talks back' to the transmitter. The 'RX Battery' display on your transmitter display screen reports on the voltage your receiver reports that is being fed. This is important if you're flying a gas-powered airplane that has a separate battery for the radio receiver that might be running low on charge. You don't have a separate receiver battery -- your receiver gets power from the 'Battery Eliminator Circuit' (BEC) in your Scorpion ESC. You can ignore the RX Battery display.

Beeping - Turning on your FS-i6 transmitter without making sure that all the control sticks and switches are in 'safe' start-up positions will trigger alarms that will continue until you set everything to the 'safe' condition. This is likely the source of your 'beeping'. See the 'Turning On the Transmitter' section in the 'FS-i6 Combat Guide'.

Note - I'd also remove the 'LIPO' jumper on the Scorpion Mini board to keep the ESC from shutting down the robot in the middle of a combat match if your 4s LiPo voltage drops below 12 volts -- but that's your call.

Q: Thanks for the quick response! Now, for some clarification: I installed the tinymixer only for the invert switch, not because I need another way to mix. It was the only active mixer when I experienced problems with it. For the RX battery, it showed no bars, and was beeping because of this. Also the robot did not respond in this state. So does this mean the BEC is not working properly?

A: The RX Battery Display Shows No Bars! - That's the missing piece. I looked up the specs on the BEC output for your Scorpion Mini and its functioning correctly, but there's a catch...

  • Your receiver sends telemetry back to the transmitter, and that draws a lot more power than a receiver without telemetry.
  • On a 2s LiPo (7.4 volts) the BEC on your Scorpion can supply 100 mA of current; thats enough for your receiver.
  • On a 4s LiPo (14.8 volts) the BEC on your Scorpion can only manage about 30 mA of current. That's enough for a standard receiver, but not enough for yours.
  • Your BEC can't keep up with the current demand, the voltage drops, and the receiver gets glitchy.
Your receiver is happy anywhere between 4.0 and 6.5 volts but your BEC can't hold that voltage at the 90 mA the receiver is trying to draw. If you're gonna run a 4s battery you're gonna need a higher output BEC. Here's a clip from a post in the Ask Aaron Archives from a builder who had similar issues when he added a servo powered lifter to his 'bot:
Q: It turns out the radio gear does work fine with just the Sabertooth ESC, so the added current drain from the servo was the problem. Based on your recommendation, I have placed an order for a BEC rated at 3A. I don't know whether or not you have already posted a wiring diagram of how this should be fitted together? I really don't want to mess this up.

A: [Mark J.] This diagram should get you thru the process:

  • Most BECs come with a pair of power input wires that connect to the main battery supply and a three-wire cable that plugs into any free receiver port. If your new BEC is different, write back and send a picture.
  • It's best to have only one power source powering the receiver. The center red leads on the cables from the Sabertooth to the receiver carry power from the on-board BEC to the receiver. If you examine the connectors on these cables you will find the little barbs that hold the connector pins into the plastic holder. With a small probe you can unclip that barb. Pull the center pin backward out of the holder and tape it back out of the way (video). Do that for each of the Sabertooth cables.
  • Alternately, you can simply clip the red center wire on each of the Sabertooth receiver cables.

Q: Ok so it just so happens that I have a BEC laying around... I jerry-rigged it to the wiring, and now the beeping has stopped and the RX bars are full, but the robot still won't move. Any thoughts or ideas?

A: Wait... 'still won't move'? This is the first you've said about not moving. You had a problem with the right side motors reversing unexpectedly near full throttle when we started -- when did it stop moving?

Q: I said "also the robot did not respond in this state" referring to when the RX bars were empty and transmitter started beeping.

A: I'm confused by the sequence of events...

  • You had the unexpected reversing issue with the right-side motors.
  • Then you removed the tinyMixer and motor response was normal.
  • Then (maybe) you reinstalled the tinyMixer?
  • Then (maybe) the RX bars went to empty and the motors no longer responded??
  • Then you patched in a BEC and the RX bars filled up but the motors still do not respond???
I'm kinda lost. What components are in the 'bot right now? Are the speed/direction LED lights on the Scorpion Mini responding to R/C inputs? If you pull out the tinyMixer does anything change? What are the specs for your battery?
Q: So, I've got a question...

I'm working on the designs for an antweight (US) lifter, and was running the calculations of using a Silverspark motor to power the weapon. However, the math seems to be a bit... off. Here's what I have:

Lifter Arm Length: .33 feet

Maximum Lifting Weight: 2 lbs

Maximum Torque at Gearbox (ft-lb): (.33 feet x 2 lbs) = .66 lb-ft

Max Load At Gearbox (in-oz): (.66 lb-ft x 192) = 126.72 oz-in

Motor Stall Torque: 0.64 oz-in (Fingertech Silverspark @ 6v)

Torque Overage Factor: 2

Gear Ratio Required: ((126.72 / 166) x 2) = ~1.5:1

Is that really it in terms of a gear ratio? All I need is about a 1.5:1 reduction to lift another ant with a Silverspark motor at 6v? I feel like I've done something wrong.

Also, I plan on running the motors at 12v - 14v rather than the recommended 6v. How does this affect the stall torque, or does it remain constant regardless of the amount of voltage applied to the motors? [Newton, Illinois]

A: [Mark J.] You were doing fine right up to the end. That last line should be:

Gear Ratio Required: ((126.72 / 0.64) x 2) = 396:1
It seems that you inadvertantly inserted the 166 oz-in stall torque of the Banebots RS-775 18v @ 18v that you used in your earlier calculation for a featherweight lifter instead of the 0.64 oz-in for the Silver Spark @ 6v.

Stall torque, stall current, and no-load RPM of a brushed PMDC motor increase proportionally with voltage. Increasing voltage from 6 volts to 12 volts will double both the speed and the torque of the motor, so your new calculation will be:

Gear Ratio Required: ((126.72 / 1.28) x 2) = 198:1
At 12 volts, the 200:1 Silver Spark gearmotor should do.

Q: I saw that a D2 kit was mentioned in robots appearing on at least 10% of the ballots [in 2019 balloting for the Combat Robot Hall of Fame]. I didn't see a lot of recognition for that bot on forums. Did people vote for that bot specifically or did they just say "D2 Kit" on their ballots and you picked the one that is the best in your opinion, and/or having good combat records? [Winchester, Virginia]

A: [Mark J.] I do not take such liberties with ballots for the Hall. I simply count and report on all the verified ballots as submitted.

I've learned that forum chatter is a very poor predictor of Hall balloting. The 'D2 Kit' in question is 'Captain Doom', a 'bot popular in both UK and US balloting because of its success in the UK Bugglebots web-based mini-series and multiple US competitions. Several Bugglebots competitors showed up in the 2019 balloting.

As I dutifully reported, 'Captain Doom' appeared by name on more than 10% of the ballots received for 2019.


Q: Hey there, Mark! This might be a dumb question, but I'll ask it anyways. Over on Facebook people mentioned the Fingertech vertical spinner which is apparently [expletive deleted]. I noticed, however, that somebody mentioned if it was crafted from AR400 instead of Titanium, it would hit harder. Why? Is it because of the mass increase? Is that really enough to make this spinning weapon more effective? [Kankakee, Illinois]

A: [Mark J.] Changing the material of the FingerTech vertical spinner blade from Grade 5 Titanium to AR400 Steel would result in a heavier, stiffer weapon that would in fact 'hit harder' -- but the improvement would be too small to help this specific weapon.

  • The FingerTech titanium vertical blade is a VERY small weapon blade. With a 1" spin radius and a weight of 10 grams, I calculate that it stores about 1.5 joules of energy at 11,000 RPM. A blade the same size and shape made from steel would weigh about 17 grams and store about 2.5 joules of energy. A typical antweight spinner weapon will store more than ten times that much energy.
  • Titanium is only about 60% as stiff as steel. That means that a titanium weapon of a given design will flex more on impact and will be less effective at transmitting the impact energy to your opponent -- like a fist in a boxing glove versus a bare fist. A steel version of the weapon blade would 'hit harder' than the titanium version, but there is so little energy involved here that impact flexing would be negligable for either material.
Read thru the Ask Aaron Spinner Weapon FAQ for more info on effective spinner weapon design.
Q: is it a good idea to put a titanium original sin style attachment on a d2 kit to stop wheels being shredded? [Fairfax, Virginia]

A: [Mark J.] I'm not sure what 'attachment' you mean. The D2 robot kit comes with a big titanium plow similar to the one commonly used by 'Original Sin', and I don't recall OS ever using wheel protection of any sort.

The D2 uses big foam tires that can absorb a lot of abuse and are inexpensive to replace. I think you're best advised to leave the wheels open as designed.

Q: i am the d2 guy and i mean that spinners keep getting on the wedge, to the top of my robot and shredding my wheels. this original sin attachment has a blocker to keep horizontals off the top.

A: Ahh... you're talking about Original Sin's "bar spinner trap" (pictured). Team 'Late Night Racing' built the trap specifically to counter Last Rites' huge bar spinner by forcing it to repeatedly impact the hard steel trap and possibly break. It has had mixed success in that role.

A similar design could certainly keep horizontal spinners from climbing your plow, but your driving has to be spot-on to keep that spinner away from the hard edges on the exposed ends of the trap. Don't use it against vert spinners.


Q: Hey Mark! This is probably a "cheerleader" question, but I wanted to at least try and give it a chance.

My team and I want to build a heavyweight. We know we want a strong chassis, now it's just time to figure out the weapon! We want to try a front-hinged flipper, but we aren't quite sure of where to start. Is there a "good" number to shoot for in terms of joules per pound, like spinners? We aren't necessarily trying to get the same power as bronco, as it's going more forward versus upward; we just want enough to get them over the wall.

Sorry if the hamburger is bad, we're mostly theorizing at this stage. [Akron, Ohio]

A: [Mark J.] Yes, it's a question. First I'll explain why your team shouldn't build a heavyweight flipper, then I'll point you to answers to your 'starting point' question in case you choose to make the mistake of going ahead and building it.

Why not? If you decided to take up mountain climbing, it would be a poor idea to catch a plane to Nepal and start walking up the southwest face of Mt. Everest; you should start with a smaller mountain. A heavyweight flipper is Mt. Everest -- expensive, dangerous, and not for the inexperienced. The questions you're asking make it clear that you have no experience with pneumatics and no experience with big combat robots. It's failrly certain that the selection committee for the only heavyweight robot tournament currently operating in North America will not look kindly upon your application to compete. Build some smaller combat robots, work out the finer points of pneumatic flippers on a featherweight, and put together a combat résumé that will get you to your goal.

Ignore my advice? A pneumatic flipper is not a 'one number' system (and neither are spinners). You need a system with enough theoretical force to accelerate your opponent into an arc that will achieve your purpose, but you also need to flow gas from the storage tank into your actuator as quickly as possible to get the speed 'pop' that separates a flipper from a lifter. That last part isn't easy. I have a couple of tutorials and a spreadsheet tool that you should find useful.

Q: I seemed to have forgotten one REALLY IMPORTANT word when writing my flipper question: E L E C T R I C. My team has built several robots in the past (a 4 bar lifter and some spinners) with decent success in smaller weight classes. It's true it'll be our first heavyweight, which is why we don't want to build something we aren't familiar with, as you said.

A: Heavyweight... ELECTRIC... Flipper. Woof. The cheerleader had one look at this and took the day off, so I guess it's up to me.

There are a few insect-class flippers using exotic electric servos, but the square-cube law pretty much rules out heavier flippers directly driven from an electric motor. Sub-lightweight electric-kinetic flippers store up energy in a flywheel to get enough 'pop' to launch opponents. The only remotely successful heavy electric flipper has been Team Whyachi's 'Warrior SKF' (aka 'Warrior Clan' and 'Warrior Dragon'), although many of their victories have been via direct attacks with their flywheel/spinning disk rather than attacks from their anemic kinetic energy flipper. Here are the primary problems:

  1. A flywheel flipper system requires a custom machined 'dog clutch' as explained by Dale Hetherington at his Flip-o-Matic page. Dale developed the Flip-o-Matic for use in hobby and featherweight 'tabletop' competitions that don't allow exposed high-speed spinner weapons. Dale has had good success under those conditions, but in open heavyweight competition you're going to be badly outclassed by more powerful weaponry.
  2. All electric/kinetic flippers share an unusual attribute: they have very short actuation ranges. For example, the flipper on 'Warrior SKF' moves only a few inches. This allows the flipper to expend all of the limited stored energy an a short but explosive burst. Extending this stored energy over a longer 'throw' -- like the large angular throw of a front hinged flipper -- means less energy per unit of distance travel and would require a HUGE flywheel to store enough energy to be effective. A pneumatic system can simply tap into more pressurized gas to continue the motion, but the kinetic energy stored in a flywheel depletes quickly and there won't be any more 'til it spins up again. It really isn't the right choice to power a long-throw, front-hinge flipper.
Build a nice featherweight to test your exotic design. The cost to build and perfect a feather will be less than the cost of a single redesign on a heavy. Besides, BattleBots reboot season 4 isn't guaranteed...


Q: This may be a dumb and frequently asked question, but how does one vote for the 2019 Hall of Fame opening? [Sunbury-on-Thames, England]

Q: Maybe Im just overlooking it, but Im not seeing the link to submit my Hall of Fame ballot anywhere? [Bergenfield, New Jersey]

A: [Mark J.] The link is deliberately not posted on 'Ask Aaron'. As noted at The Combat Robot Hall of Fame:

"Membership in the Hall is by ballot of the global combat robot community. Notification and balloting instructions appear in combat robot forums in the US, UK, and Australia in August of odd-numbered years."
Specifically, notices have been posted on: This approach is intended to keep the voting largely within the builder community and avoid hoards of casual fans from flooding the ballot box with every 'bot they saw on last week's episode of 'Battling Robots' -- we get a lot of those anyhow. If you'd like to cast a ballot, drop into one of the listed forums and do a search for 'Hall of Fame'. You've got 'til August 16th.


Q: Voting for the Combat Robot Hall of Fame opens Thursday, but when does it close?

A: [Mark J.] The voting site will open at 12:01 AM (PDT) on Thursday, August 1st and will remain open for a full 16 minutes. Please be ready with your ballot and keep the line moving.

WARNING This website is provided "as is" without warranty of any kind, either expressed or implied. In no event unless required by law will the author of this website be liable for general, special, incidental, or consequential damages arising from its use. This website is intended for educational purposes only. Avoid prolonged exposure to this website. Side effects of reading this website are not common but may include itching, redness and occasional fainting. Do not read this website if you are allergic to this website or any of its words. If condition persists, consult your physician. Do not read this website while operating a vehicle or heavy equipment. Any resemblance between this website and others, for better or for worse, is purely coincidental. Reading the website does not constitute legal advice. Should you require legal advice, seek a legal advisor. Contents of this website are under pressure. Shake well before using. Void where prohibited. Use only as directed. For external application only. Batteries not included. Objects in mirror are closer than they appear. May contain peanuts. Your mileage may vary.

Q: Sixteen minutes?

A: Did I say 16 minutes? Sorry... meant to say 16 days. Voting will close on Friday, August 16th at 11:59 PM (PDT). I plan to announce the voting results on Sunday, August 18th.


Q: My 1lb antweight is exhibiting some odd behavior. The weapon blade is the fingertech titanium blade. I have a V-spec 2205 motor, a 450mah 4s 70c battery, a 20a brushless ESC and two fingertech ESCs with accompanying silversparks. When I spin my weapon up to max or near max throttle one or more of these behaviors *sometimes* exhibit themselves: The power LED starts flickering, the robot starts doing donuts, power to the weapon motor gets cut and it spins down. As soon as I turn the throttle down it fixes itself and its free to spin back up again. Is this a problem with the weapon ESC not being able to handle the weapon? [St. Louis, Missouri]

A: [Mark J.] Your primary problem is not your weapon ESC; the symptom of an overstressed ESC is a puff of smoke and unrecoverable loss of motor power. You didn't mention your weapon layout, but assuming that you're running the weapon direct-drive your problem is the weapon motor.

The FingerTech weapon blade as used as an add-on weapon for the Viper robot kit is designed to spin at 8000 to 12000 RPM under the power of an 1100 Kv direct drive motor. You're attempting to spin that blade at about 34,000 RPM with a 2350 Kv motor that has a lower torque constant than the design motor. The motor simply isn't capable of spinning the weapon to that speed given the maximum current output (about 30 amps) of the battery pack you've chosen. The motor 'bogs', you get serious 'voltage sag' from the battery, and your electronics 'brown out' -- which gives you the symptoms you're reporting.

Your battery won't put up with this level of abuse for long, and may have been damaged already. If the plastic coating is at all 'puffy' and no longer tight, the battery shoud be safely disposed of immediately -- under no circumstances either charged or used.

A larger battery pack capable of supplying the 60 or 80 amps the motor might require operating bogged down to perhaps 15,000 RPM in this application would simply transfer the failure point to the weapon ESC or the the motor itself, which is rated for only around 25 amps continuous current.

Scrap your weapon motor and pick a lower Kv unit that will be able to deliver enough torque to spin the blade at a more reasonable terminal speed. Don't over-do it. You'll likely want a battery with a bit more peak current output and - depending on the selected motor - a larger capacity ESC as well.

Q: The battery seems fine and that sounds like a big overhaul that I don't really have time for, as a short term "hot fix" could I simply off-set the throttle in my transmitter settings to prevent the motor from revving up to full speed?

A: The problem is that your weapon motor can't rev up to full speed because of the aerodynamic drag of the weapon at such unrealistic speeds. Applying less throttle will reduce the current to the motor, but it also proportionally cuts torque. You might well need to cut throttle to around half to keep all the components in their safe zones. Spin-up times will greatly suffer. Try a new throttle setting in a practice session and check temperatures carefully. I'm also concerned that your battery may not have the capacity needed for a full match with a bogged weapon motor sucking down big amps. Best luck.

Q: How come I never had this problem with a 3s 65c battery and my emax 2204 motor? Similar KV to current motor.

A: Increasing voltage has rather large consequences.

Wattage increases by the square of voltage; the increased voltage also increases the current. Bumping up from 3 cells to 4 cells (a 33.3% increase) results in 1.333 * 1.333 = 1.78 a 78% increase in wattage, motor output power, and peak battery drain.

A similar effect is seen on weapon aero drag. Increasing the weapon speed by 33% means that it has to move 33% more air out of the way per unit time and it has to move each unit of air out of the way 33% quicker. That's the same formula highlighted above. The motor now has to work 78% harder to maintain the faster weapon speed -- if it has enough torque to maintan that speed at all.

Q: Do you think just stepping back down to the old 3s lipo could solve this?

A: Quite likely, yes.

Q: Just a quick update for anyone in the future reading who has a similar issue, I tried the 3s and it didn't fix the problem, it just made my robot weaker. I'm going to try offsetting the throttle so its max signal is like 75% throttle. I can't see any reason this wouldn't work, as the robot works fine if I manually keep the throttle under 80% or so, so if I do not give any future updates assume that this fix worked... or alternatively that the blade flew off and lodged itself in my trachea.

A: The failure of the three-cell battery to solve the problem casts suspicion on a new component in addition to the high current draw issue. You haven't mentioned the specifics of your weapon ESC except to say that it has a 20 amp rating, but it's possible that it's faulty or has been damaged by the current load.

  • If your weapon ESC supports calibration to your radio signal range, run that calibration sequence and try the 3s battery again.
  • If calibration doesn't help, consider replacing the weapon ESC.
From 1,734 miles away that's about all I can suggest for a true 'fix'.

Q: This is the ESC: DYS XSD 20A Brushless Speed Controller. Are you suggesting that upgrading the current rating to say 30a could help or just that it may be faulty and in need of replacement.

A: The conversation has wandered around a bit -- lets recap:

  • I'm confident that your 'brown out' problem stems from attempting to spin a 6-inch weapon bar to 34,000 RPM with a low-torque/high-RPM V-Spec 2205 motor powered by a 450mah 4s 70c battery via a 20a brushless ESC. This set-up is pulling more current than any of these components can handle.
  • Based on your prior 'bot that ran a similar set-up on a 3s pack, replacing the 4s pack with a 3s pack should eliminate the problem -- but it does not. This leads me to believe that the weapon ESC may be faulty or damaged.
  • Running the weapon motor on the 4s pack with a reduced max-throttle weapon setting on your transmitter won't not work, but it doesn't address the actual fault.
  • I'm suggesting that you replace the weapon ESC and try it again on a 3s pack largely because I don't like leaving a mystery unsolved, but also because a faulty/damaged ESC may completely fail in combat without additional warning. If it passes this test you can go back to running it on 4s at reduced throttle with increased confidence.
  • If you want to run a 4s pack with full weapon throttle, upgrading just the ESC won't get you there. Neither your motor or battery pack are likely to survive on a 4s pack with your weapon load. If you plan on upgrading the motor and battery at some point you can certainly move to a higher capacity ESC now while running at reduced throttle -- it won't hurt.

Q: It's also been suggested to me that the BEC on fingertech ESC's isn't the best and this could also be part of the problem.

A: That was my first thought when I read your 'flickering power light' symptom, but overstressed BECs on tinyESCs behave like this. BEC output and receiver current demands don't increase with raising weapon motor speed, so why would the problem only manifest above a certain throttle level? Try an alternate source of receiver power if you like, but I'm not optimistic.

Q: THE STANDALONE BEC FIXED IT! I CAN GO UP TO 100% LUDICROUS SPEED WITH NO POWER ISSUES!!!!!!!!!!!!! (at least on the 3s battery, I'll update with the 4s later hopefully I don't run into an Icarus situation)

90 minutes later...

On further consideration I've decided to leave well enough alone and run on 3s, the 4s battery is heavier and it does more to improve top speed than spin up times. Top speed is something that I don't need (It's already stupid fast at 3s, 26000 RPM if the calculations hold true) and would decrease bite, spin up is good enough already and way better than last years set up. I'll use the extra weight for wheel guards. I do feel bad for having spent the money on the bigger batteries but I think I may just need to accept that this hobby is a money sinkhole by nature.

A: Super awesome! I think that's a solid decision -- but there is a troubling question remaining: what's drawing so much 5 volt power?


Q: Hey Mark. It's been almost 25 years since the first major combat robotics tournament (thanks for putting in so much work to recreate the brackets and everything from it and so many other tournaments) so I'm curious, what do you think of the journey combat robotics has had so far, and where do you see it going in the future? [Hackensack, New Jersey]

A: [Mark J.] To quote the Grateful Dead, "Lately it occurs to me what a long, strange trip it's been."

I could write a book on this topic, and anything less would not do the topic justice. Until my book comes out you should find and read a copy of "Gearheads -- The Tubulent Rise of Robotic Sports". Here's what I say about it in my book review page:

Gearheads -- The other 'must read' on robotic combat is a very different type of book: Gearheads, by Brad Stone. On the surface, this book is a history of the rise of combat robots from their early underground roots to international media phenomenon. It's also a story about what happens to people when large chunks of money become involved.

It's all here -- the early Robot Wars competitions in San Francisco, the clash between the creative and business aspects of robot combat, the legal turmoil, the personalities, the excitement... I locked myself in a quiet room and read the book cover-to-cover. Food crumbs on the pages mark where I grabbed a quick snack while continuing to read!

Anyone with an interest in the story behind combat robots needs to read this book. It is a fascinating read, and does a fine job of keeping all the parties involved very human. Highly recommended!


Q: I was looking through the Combat Robot Hall of Fame and I noticed that some of the robots have more than one name and that each of those names is tracked separately on Botrank. Doesn't changing the name make it a new robot? [Reddit comment]

A: [Mark J.] Botrank is a service to the robotics community that tracks the competition records of combat robots and attempts to rank them within their weight class. BotRank simply tracks robots by the name they register for at a tournament. Given the number of robots and fights that they have to track this is a reasonable approach for them to take; a new name starts a new combat tracking record.

The Combat Robot Hall of Fame has a greater interest in the true heritage of a robot nominated for recognition. An increase or decrease in the weight of a robot or a name change (on a whim or to meet television production requirements) does not by itself create a new robotic entity. The Hall believes that the identity of a robot lies in its design concept. That can be a challenge to sort out.

I've put together a fairly lengthy explanation of how The Hall sorts this out - with examples: When two robots are really one, and when one robot is really two.


Q: Hi Mark, question to unrelated to building a robot but more about the culture of the sport. I was wondering why there has never been a successful combat robotics game. I searched the archives and saw one series attempt but with mostly negative reviews. I can't imagine it would be difficult to create once the physics engine is sorted out. "Brand Name" robots like the ones in the HoF would be nice, but knock offs can be done for free. Use a similar set up to a need for speed game: start with a small budget and build something from preset configurations, win matches to get better materials, decals, etc. Do you think fear of lawsuits is the issue? I imagine there's enough interest but am wondering if it is a difficulty issue, a monetary issue, a legal issue or something else in the community. At least I think it would be a good way to draw in new fans. Look forward to hearing your insights. Thanks again for all you do for the community. [Pittsburgh, Pennsylvania]

A: [Mark J.] You mean something like this: Robot Rumble 2?


Q: Hey, I'm not to sure if this has been asked, but how I should go about heat treating my s7 weopaon? [McAllen, Texas]

A: [Mark J.] It's not practical to heat treat S7 tool steel in your home workshop. Heat Treating S7 is a multi-step process requiring precise temperature control up to 1750° fahrenheit with carefully monitored heating and cooling rates. You need a specialized, fully instrumented furnace plus experience in the art. Backyard 'hillbilly' metalurgy will simply turn your weapon into a warped, brittle, useless piece of scrap.

Find a commercial heat treating company nearby (like Texas Heat Treating in Round Rock) and explain what you're doing. They just might be BattleBots fans and be willing to slip your little piece in with a larger commercial order.


Q: I know Hellachopper isnt the best bot to be asking questions about, but I was reading about two of its apparent abilities on their Facebook Page and they have me thinking more than I should:

EXTENDABLE FLAILS - They apparently have the ability to extend and retract their flails at will. First off, what would happen if they extended or retracted at full speed? How would that effect the bot, forces applied to the robot, spin speed, etc? Second, what would happen if they simply let go all of a sudden, letting the flails pop out quickly and violently?

SELF-BALANCING ARMS - How does that work? How can the arms simply balance themselves with a hammer missing? [Naperville, Illinois]

A: [Mark J.] I generally won't discuss combat robots that have never even fought a match. 'Hellachopper' is best known for catching fire during its 2016 BattleBots safety check - twice. BattleBots leads builders to try some crazy things.

The physics involved are a little beyond the usual short answers I give here. I've spent quite some time writing and re-writing the answers to keep them as brief as possible...

Extendable Flails - Extending or retracting the flails by itself will not change the total rotational energy of the weapon -- but since the Moment of Inertia of the weapon changes as the flails move in or out, the weapon speed will change.

  • Spinning at constant speed with the flails 'in': extending the flails will slow the weapon RPM. Tip speed remains nearly constant, but the the impactors now travel in a larger circle. If the weapon motor has additional torque available at the lower weapon speed, it can apply that torque to spin the weapon to a higher speed. Calculations for 'Hellachopper' presented on the Reddit forum claim that aerodynamic drag with the flails extended would prevent the weapon from attaining full target speed in that configuration.
  • Spinning at best speed with the flails 'out': retracting the flails (not easy because there's a lot of cetrifugal force acting on them) would increase the weapon RPM. Tip speed remains nearly constant, but the the impactors now travel in a smaller circle. If the weapon motor cannot maintain the weapon at that increased speed, the speed will fall to the level the motor can sustain.
Watch this video on Conservation of Angular Momentum to see this effect in action. The rate at which the flails extend will have no impact on the above changes -- unless the flail cables snap from the sudden load.

Self-Balancing Arms - Self balancing systems are fairly common. Some washing machines have a 'donut' shaped tube partially filled with liquid wrapped around the tub that will compensate for an imbalanced load of clothes during the spin cycle. Somewhat counter-intuitively, the liquid will shift position in the tube to position itself at the location of lowest energy, which will be opposite the 'heavy' side of the washer load as it spins.

Several combat robots with hollow drum weapons have used a product made for commercial truck tires that works the same way. Tiny ceramic beads act like the fluid in the washing machine system to redistribute mass to maintain balance, even if the weapon is damaged and balance changes.

Hellachopper's balance system is based on this same principle, but it relys on flail anchor points in sliding grooves moving to change the distribution of mass. Clever, but I'm not entirely convinced that it will work; the balancing 'fluid' needs to be free to move independent of the rotation of the weapon, and the flail anchors are not entirely 'fluid'.


Q: Hey Mark,
I've been doing a little more research on combat robot weapons (specifically for beetleweights) and a popular choice for beater bars seem to be the Fingertech Beater Bar kit. I'm wondering if the 6061-T6 aluminum used in the Fingertech beater bar is a good choice in general for a beetleweight beater bar, as the RioBotz tutorial doesn't seem to show aluminum as a favorable weapon material for sharp one piece spinners. If I had a more expansive budget, I'd probably use some S7 or some other impact resistant steel and heat treat it but as I'm sure you're aware the costs for this can build up quickly. It looks like if I'm going to go for an eggbeater I'm pretty much relegated to the use of aluminum. So how strong of a choice is that kind of aluminum for a beater bar, or even aluminum in general, in the beetleweight class?

As a side note, I may end up ordering some aluminum from mcmaster-carr and machining it to have a higher MOI than the fingertech bar, while retaining a similar shape.

Thanks, Jack from Lake Charles, LA

A: [Mark J.] I don't think you understand how the FingerTech beater bar is used, Jack. Aluminum is used for the 'body' of the beater, but sharp-edged steel machine screws are threaded in as the 'impactors' that acually strike the opponent. The design is popular in part because of the ease with which the impactors can be replaced as they are blunted. It's much cheaper than replacing the whole weapon -- and the sharp edge of your spinner weapon will be blunted quickly.

The RioBotz Guide is a valuable resource, but it was written ten years ago -- before the availability of materials and components that are in widespread use in current robot combat. Small 'bot one-piece spinner weapons are commonly waterjet cut from pre-hardened abrasion-resistant steel (AR-400, AR-500...) which reduces cost by eliminating heat-treating and problems with warping that accompany S7 tool steel.

Insect-class robots are also now less about weapon MOI than they were ten years ago due to the overkill power available from hobby-grade brushless weapon motors. Once you can bounce your opponent off the ceiling of the arena at will the importance of high MOI spinners is greatly reduced.

Two suggestions

1) Pay attention to the warning that appears at the top of the Ask Aaron Robot Weapons Archive:

Aaron's Wisdom  I've said this often, but builders don't want to believe me:

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

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

2) Build yourself a robot and enter a tournament. You'll learn more in your first combat match than you'll learn from studying design for a year.



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
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Do your own research.

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