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5655 Questions and Answers about Combat Robotics
from Team Run Amok


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

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click 'Got Question?'
The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ
In Memorium: 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   
 
Caution
Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot.

Q: Hi, I have a hammer robot and I choose to directly attach the ram 3 inches below the fulcrum. The pivot point is also just a pin though an aluminum bar. The ram has a 6 inch throw so the hammer has a roughly 100 to 120 degree angle cocked. My question is, would it be worth it to add gearing to get a 180 degree swing and bearings for better efficiency or keep the rugged, less component design. Thank you for you help - Team Humphrey [West Virginia]

A: [Mark J.] The amount of 'work energy' available from your actuator is not increased by adding gearing to expand the range of motion, but a rack and pinion can increase efficiency in converting the linear motion of the actuator into the rotary motion of the hammer. However, careful attention to the hammer linkage geometry can keep the thrust vector favorable without the added weight and complexity of a gear system -- see example at right. I recommend keeping it simple.

Your pivot is a weak point in the structure of your weapon arm. You didn't share any details of your arm design, but you should be very cautious about enlarging the hole in the arm at this highly stressed point. Without knowing more about the design I can't make a recommendation on the benefits. If you've got a big, meaty chunk of aluminum around that pivot I'd recommend boring the hole just enough to press in an oilite bushing to avoid steel-on-aluminum purely for reliability. If well lubricated your simple pivot has minimal frictional loss, but the bushing will prevent spalling and wear that can lead to failure.

Pneumatic hammer range of motion

Q: Hello! Thank you so much for spending time answering these questions.

I noticed that 'Complete Control' uses a large worm gear to drive its lifting forks. This gives them significant speed reduction in a compact package. This got me thinking about how almost all the combat robots I have seen use some combination of chains, pinion gears, and planetary gearboxes for all their speed reduction needs. Do you happen to know of any other combat robots that use bevel gears, worm gears, or other less-typical speed reduction methods to drive a mechanism? [Madison Heights, Michigan]

A: [Mark J.] There are generally good reasons for the selection of the common drive mechanisms found in combat robots. One of the primary design considerations in drivetrain selection is:

If I hit this with a really big hammer will it still work?

  • Chains and timing belts tollerate a good amount of twisting or misalignment in any axis. They are robust, efficient, easy to implement, and widely available.
  • Planetary gearboxes are efficient, compact, and available from a variety of sources in a very broad range of reduction ratios. They provide a simple bolt-up drivetrain solution, and simple is good.
  • If a true geartrain is needed, spur gears are simple to deal with as they require precision only in axis orientation and in/out mesh clearance.
  • Bevel gears require precision in three dimensionss: up/down, left/right, in/out, plus axial alignment. That makes their mountings difficult to machine and sensitive to failure if (when) the housing takes a good hit. Some builders do find use for bevel gears in low-profile drives for overhead horizontal spinning weapons.
  • Worm gears are notoriously inefficient in power transmission. A lifting or clamping mechanism can afford this inefficiency, but the high frictional losses generally render them unsuitable for high-speed weapons or propulsion. In the early days of combat robotics, a fair number of robots used worm-drive gearmotors scavenged from electric wheelchairs, but better options are available now.
  • Ball screw linear actuators show up from time to time in combat robots. Heavyweight champion 'BioHazard' used linear actuators to power its four-bar lifter mechanism, and BattleBots ABC Season 1 competitor 'Overhaul' used ball screw actuators in its lifting and clamping mechanisms.
  • BioHazard 1996

Q: Dear Mark,in the new season of Battlebots lots of vertical spinning weaponed-bots are seemingly getting smaller to give weight allowances for better armour,like Poison Arrow and Witch Doctor.But from Witch Doctor's rather shocking loss to Red Devil in the round of 32 I think being small is not a really good idea,which makes them become perfect targets for clampbots to get a hold of,do you agree? [Chinese Forum]

A: [Mark J.] I've known Red Devil's builder Jerome Miles for many years. He is a fine young man, a great builder, and a talented driver. He also got very, very lucky in his fight against 'Witch Doctor'.

Improvements in battery and brushless motor technology have made it possible to shrink the mass and size of effective spinner weapons. Robots with these more advanced weapons are quick, maneuverable, and deadly efficient in deploying their weaponry. Any weapon system has weaknesses against specific counter attacks but the high-speed single tooth disks can certainly hold their own in a tournament. Don't form your opinion on the outcome of a single battle.

Q: And another question:Lock-Jaw and Yeti had a very tough match that went for full 3 minutes in the qualifying round,if Battlebots is using the rules of last year or the old days of TV era,would Lock-Jaw ever have a chance to be judged as the winner?

A: I make a point of not judging robot combat matches based on edited video. I've seen matches in person that looked very different from the same match when shown on TV.

That said, I believe that the revised judging guidelines for BattleBots ABC season 2 greatly favor more destructive weapons such as the large drum on 'Yeti'. Under the old rules I believe that 'Lock-Jaw' would have won both the aggression and control categories and would have had a good chance with the judges overall.


Q: How is T-minus's flipping device so effective, since the ram is nearly horizontal when actuated. Wouldn't this initially direct the majority of the force horizontally instead of vertically? I would have thought a flipper would become more powerful the closer to vertical the ram is oriented; how does the T-minus design allow such force upon actuation? I'm trying to see it in terms of the statics behind the design. Thanks! [Grand Rapids, Michigan]

A: [Mark J.] You're entirely correct; from a statics point of view the design of the 'T-Minus' flipper is terribly inefficient. To efficiently convert the linear action of the actuator to rotational motion of the single-pivot lifter, the actuator should pivot to remain perpendicular to the lifter arm motion. This was a primary design consideration for my heavyweight lifter 'The Gap'.

The complication is that all of the robot systems must work together, and concentrating on the efficiency of any single system leads to design compromises in other systems. Inertia Labs elected to concentrate on a well-armored compact and maneuverable low-profile design. That design required a 'lay-down' initial position for the actuator. The actuator never gets close to perpendicular alignment with the flipper arm motion and the force vectors are horribly inefficient.

So, back to your original question: how is T-Minus's flipper so effective? BRUTE FORCE! Inertia Labs made up for inefficiency with a big actuator, huge valves, large ports, and scary high gas pressure. If you have enough power you can get away with inefficiency, and the overall robot design is brilliant.

Combat Robot 'T-Minus' flipper design

Q: I had an idea and I wanna know if it could work, i dont think I saw this design anywhere. Imagine a hammerbot a little like terrorhurtz or the one i send you videos a in aquestion below. But, the rack and pinion isnt connected directly to the shaft of the hammer. The pinion is on a dead shaft, bolted or weld on a sprocket. Above, we have a smaller sprocket, wich is bolted on the hammer. In theory, i could get more speed out of the same actuator, by gearing it with a ratio of 4:1, for example. Do you think it could work? It's not for any weight class in particular, just a design i had in my head that i thouh was worth sharing to you.

Thanks a lot for all you do for the combat robot community, you inspired me to build robot, you showed me it wasnt only reserved to pros :) [Quebec, Canada]

A: [Mark J.] I'm glad to see you're enjoying combat robotics and spending some time thinking about design improvements. In the gear train you describe, the 'pinion' on the dead shaft is called an idler gear. An idler gear has no effect on the gear ratio -- you would get the same gearing if your 'smaller sprocket' rode directly on the rack without the added complexity and weight.

About Gearing: Your pneumatic actuator can produce only a certain amount of power as defined by the cylinder bore, the gas pressure, and the rate at which the gas can flow thru the valves and ports into the cylinder. Power is a function of time and is described by the formula:

Power(t) = Force * Velocity

Gearing changes the ratio of force to velocity, but does not change power. You can 'gear down' to get greater force (torque in this case) and reduced speed, or you can 'gear up' to increase speed with reduced torque.

To be effective your hammer weapon must accelerate to as great a speed as possible in only half a revolution -- it is torque that creates that acceleration.

  • If you reduce the torque by gearing up too much the hammer will accelerate slowly and not achieve its best speed in the distance available.
  • If you reduce the speed by gearing down too much the hammer will accelerate quickly to a peak speed that is much lower than it might have if it were geared to use the full distance available.
What you're looking for is the gearing that provides the torque needed to accelerate the hammer to peak speed just as it impacts your opponent, maximizing the power from the actuator.

Q: ok, but if i use a chain instead of a gear? would it still nt affect the gearing of the hammer?

A: I don't see how the system you decribed could be implemented with a chain, but regardless...

...no number of intermediate idler gears, idler wheels, or idler sprockets in a drivetrain will impact the overall gear reduction. The reduction ratio is calculated from only the sizes of the first and last elements in the sequence.

Q: Can you explain what "brushless" means in a way that someone who hasn't studied engineering since high school can understand? [Logan, Ohio]

A: [Mark J.] I just did a re-write on Frequently Asked Questions #23 -- see if that helps.

Brushless motor animation

Q: Hi Aaron, I read through your your archive as requested, I had a followup question.

My idea for drive in a 30 pound robot has been to drive 6 wheels with timing belts from one gearbox. I've seen a couple times in the past you have suggested around 3/8" steel shafting for this weight class. My idea was to use vex 1/2" 7075 shafting, where it would go:

endcap/pulley/inner bearing/outer bearing/shaft collar/wheel/endcap

...with basically no space between the bearing, 1/4" shaft collar, and wheel.

I would file a small weak point just after the shaft collar, so that if struck by a weapon strong enough to break the axle it would tend to snap off after the shaft collar and not bind up the rest of the drive, the shaft collar would stop the shaft from working out of its bearing and falling into the robot.

Is this a terrible idea? I could put a cheap 5mm titanium rod or something through the hex to reinforce it. I just figured it would be best for the wheels to tear off and sacrifice themselves rather than take other things with them. [Dublin, Ohio]

A: [Mark J.] I understand how nice it is to build with VEX components. They all fit together and assemble without trouble, they all come from a single source, and they are reasonably priced -- but they really aren't designed to withstand full-on featherweight robot combat.

  • A hollow 1/2" hexagonal aluminum shaft is not equivallent to a solid 3/8" hardened steel shaft, but might work.
  • The aluminum shaft is more likely to bend than snap off; ditch the shaft collar and don't add a weak point.
  • If the wheels are exposed, 'squishy' tires help absorb weapon impacts; sacrifice wheels - not axles.

Q: Mark, why can't I post question on Ask Aaron these days? Is there any problem in system? I'm not from India. [Jiangsu, China]

A: [Mark J.] No problems on this end, no other reports of problems from outside, and this post got thru. I don't have an explanation for your difficulty.

Q: That's amazing, I can send questions now! Mark, why does 'Lucky' and 'Son of Ziggy' take a lot of time to make their weapons ready before they can use weapons again? That's a deadly drawback!

A: I haven't noticed any particular delay on weapon reset for 'Son of Ziggy' (video). I believe SOZ uses a spring powered return on the flipper and it takes just a moment for the high pressure gas vent from the pneumatic actuator so the spring can pull the weapon back down.

'Lucky' was rushed into battle before the flipper could be fully sorted and a lot of problems surfaced at BattleBots. Scroll down three posts to find a report.


Q: where should i buy the materials for a combat robot [Helotes, Texas]

A: [Mark J.] See Frequently Asked Questions #16. Read the rest of the FAQ while you're there.


Q: does some kind of rack attachment exist to put on a pneumatic ram? I would want something like the one terrorhurtz use. Thanks a lot :) [Quebec, Canada]

A: [Mark J.] The 'Terrorhurtz' weapon rack is all custom machine work. There are industrial pneumatic actuators that perform a similar function, but they're expensive and heavy [example]. Google: 'pneumatic rotary actuator'.

Q: how did john reid make the rack and pinion system? did he just put grooves on a longer shaft he then put in the pneumatic ram? it seems like it in the picture. or what about the robot in this video? or this video?

Pneumatic rack and pinion assembly

A: John Reid cut precision gear teeth into an extended length shaft on the pneumatic actuator. He has also provided a roller to support that shaft from the underside. I would think that the gear teeth would interfere with the front seal on the actuator, so there's more going on than is explained in the photos.

The robots in the other videos have their hammers driven by rack gears that have been attached to the pneumatic system. The attachment and support of the racks is critical, and the method used by the builders in the videos is not clearly shown.

British builders are famous for scrounging parts from scrapyards and repurposing them, so I suspect that those racks were scavenged from a discarded mechanical assembly. I would point out that neither of the bots in the videos are delivering what I would consider 'damaging' blows.

There are other designs for pneumatic axes: see this post in the Ask Aaron Robot Weapons Archive for discussion of the pneumatic weapons on 'SlamJob' and 'The Judge'.


Q: how does Ziggy's flipper seems so powerful compared to Lucky's? Aren't they built by the same guys? [Quebec, Canada]

A: [Mark J.] My original answer to this question doesn't hold water. I attributed the reduced performance to BattleBots rules prohibiting the use of custom pneumatic components. It seems that I may be misreading the BattleBots Design Rules. I thought section 10 was quite clear on pneumatic components:

"There are no specific restrictions on the system design; however, the pneumatic system must use best practices and commercially available components that are rated for the operating pressures used."

I received a note from a reader in Massachusetts who was on-site at BattleBots 2016 and who offers a better explanation:

I have to disagree with you on the Lucky vs. Ziggy question. If Battlebots rules prohibited custom pneumatic components, how do you explain Bronco's black-box variable pressure system or Chomp's custom everything-except-the-tank? Also, having been at the event I know that there were custom components inside of Lucky, although not to the same degree as the other pneumatic bots.

You can feel free to ask Mark or Rob for the specific details, but the story is that the short time period they had for building prevented testing until the last minute. At that point, it was shown that the spring-retract and release valve system simply wouldn't do, and firing the system on full pressure (which was significantly greater than Ziggy's) would guarantee the arm being jammed or unable to cycle. Even operating at a greatly reduced pressure, the arm still got jammed repeatedly, and was only occasionally able to cycle back down.

There were other fundamental design issues in Lucky's flipper that kept it from being as potent as Ziggy's, but they're of a more mathematical nature and since I wasn't privy to the whole design process I can't really answer in good faith. Really, all of Lucky's issues came down to their status as a last-minute addition and the extremely short time that they actually had to build in.

Thanks, Massachusetts. I look forward to seeing what 'Lucky' can do with the bugs sorted.


Q: what kind of ICE engine people use to power spinner? i know that Icewave uses a fireman saw engine, but i dont seem to be able to find one anywhere (ebay, mcmaster carr). Is there other type can use? i know it might not be the most efficient way or the most simple way to power a spinner.. I just want to see if i can make one.. like i always love hammerbot, even if they are not really that efficient. [Quebec, Canada]

A: [Mark J.] Chainsaw engines are popular choices for ICE spinners -- light, powerful, and easy to obtain. Outputs around one horsepower for every 20 pounds of robot weight are about right. Check carefully with event organizers for rules specific to internal combustion engines at their event. Many events simply do not allow ICE. Current BattleBots rules (Rev. 2016.2):

Internal combustion engines are allowed, but with the following requirements:
  • The engine must use a self-starter that is activated by remote control.
  • Any electric fuel pumps must be able to be shut off by remote control.
  • If the engine uses a separate fuel tank, the tank and fuel line must be well protected.
  • The fuel tank must be vented (no pressurized tanks) with a vent system that will not continuously leak fuel if the bot is upside-down.

ICE weapons are tempermental, unreliable, and have a poor record in combat. They are most certainly not a sane choice for novice builders.


Q: Hello again Mark, sorry to bother you again.

We've come to a few decisions on the material for our drivetrain. We're going for 22mm diameter Silver Steel axles, and duplex sprockets, along a similar drive train to yours, somewhat.

Now here's the question.

The 24v 750w DC motors that we're going to use has a much smaller shaft in length and diameter than that of the silver steel. We need to make it longer. Do we just hook up a sprocket to the motor then run that to an axle which in turn drives the other axles? Or is there a way to connect the thicker axle to the thinner one?

Any help would be much appreciated. [Sheffield, England]

A: [Mark J.] I strongly advise against attempting to extend/enlarge the motor shaft -- all sorts of problems and weaknesses result. Run a sprocket on the motor shaft to an idler axle, then duplex from there to the drive wheels. Take a look at this prior post in the Ask Aaron Design and Construction Archive.

Note: if I was bothered by combat robot questions I wouldn't carry on answering them. Keeps me sane. Kinda.


Q: Hi Mark,
I am wanting to mount a pulley to a 5mm motor shaft. After scouring the internet, there doesn't seem to be any pulleys of the size that I need (2" OD) with a 5mm bore. I saw a previous post about shimming the shaft and was hoping you could go a little more in depth on the best way to do that. Could I just take a piece of shaft that matches the inside diameter of the pulley and drill a 5mm hole through it? Then drill a hole through the side to secure the set screw from the pulley to the 5mm shaft? [Corvallis, Oregon]

A: [Mark J.] There are good reasons why you can't find so large a pulley for so small a shaft-- it's poor engineering practice. You haven't told me what you're building, but I can't think of an application where this would be a good idea. I'd feel much better about this if you'd share your design with me; I might be able to head off a problem.

If you'd rather learn for yourself why this is probably a bad idea, your solution is on the right track. Rather than starting with something the right outside diameter and trying to bore a precise concentric hole thru the center, it's much easier to start with a piece of thick-wall tubing with an inner diameter matching your 5mm shaft. You can turn down the outer diameter on a lathe if needed to match the inner diameter of the pulley. Drill a hole for the set screw to pass thru and you're set.

Thick wall bress tubing

Q: Just a note on pulleys and small-diameter motor shafts, I have actually seen pulleys that large for even smaller shafts (in Tamiya kits) but they're made more like spoked wheels. I don't know if that changes anything about it, though. [Illinois]

A: Such pulleys are made for low-power low-speed applications -- certainly not to attach to the shaft of a high-speed motor in a combat robot. I don't know what the builder from Corvallis is trying to make, but I can't think of a combat application where a 2" pulley on a 5mm motor shaft is a good idea.

Q: I am building a hobbyweight with a small (~2 lb.) vertical spinning bar sticking out of the front wedge. I recently attended an event in which a couple of the other competitors were running Turnigy brushless motors for their belt-driven weapons (both of them did well). I am basically just trying to replace my heavy brushed motor with a lighter brushless motor while keeping the belt drive. What should I attach to a brushless motor (5mm shaft) to spin that 2 pound bar on my hobbyweight? [Albany, Oregon]

A: Your questions are streaming in from a couple different IP servers -- one reads Corvallis, one Albany. Confusion reigns.

I need more info:

  • Dimensions of your spinning bar (length, width, thickness);
  • Diameter of the pulley on the bar;
  • What brushed motor you are replacing, and at what voltage; and
  • Which Turnigy brushless motor you want to swap in.
I suspect you'll need a larger reduction ratio than you have with your brushed motor, and so will need a smaller motor pulley. Send me the info and I'll run the numbers.

Q: The steel bar is 5" x 3" x 1/2" (roughly, a couple of the corners are taken off a little bit). It is attached to a 3" pulley. I was running a Kawasaki 21.6V circular saw motor with a 7s lipo. I haven't yet selected which Turningy motor but was thinking something along the lines of the Turnigy XK3665-1200KV and running it with a 3s or 4s.

A: Hmmm... I have no clue about the power output of your circular saw motor, and I suspect you don't either. How did you decide on a 2" to 3" pulley ratio for the weapon?

The Turnigy XK3665-1200KV is an inrunner motor that would spin at close to 18,000 RPM on a 4-cell battery, but running it on 4 cells rather than its rated 7 cells reduces the output power by almost 70% [1 / (7 / 4)^2 = 33% of max power]. Pick a motor rated for the number of cells you want to use. For 4-cells something like the Turnigy Aerodrive SK3-3548-1050kv would be about right.

Your weapon bar is puny. At 8000 RPM (too fast) it stores less than 570 joules of energy. Consider adding thickness, increasing the diameter, or going to a full disk. Changing out the bar for a 6" diameter steel disk 1/2" thick bumps the 8000 RPM energy storage to nearly 1700 joules -- no longer puny.

Running a 1" diameter motor pulley to a 2" pulley on the weapon could work nicely for this set-up. Make sure the belt width is adequate to carry this amount of power.

Q: Thanks a lot for the advice. Looks like I'll be making some changes. Your answers to my previous questions have been spot on, by the way. One more thing; where can I find smooth pulleys for a 5mm shaft? I've looked all over and can only find timing belt pulleys that are that small.

A: Go ahead and use the timing pulleys and belt. A couple of tricks to get the needed slippage:

  • Use the stock pulleys and turn the timing belt inside-out; or
  • File down the teeth on the larger pulley -- leave just a bit to retain some belt grip, and run the belt a little loose.
I've seen both of these cheats used with success.
Q: Mark,

Long time reader/worshiper of the website, first time contacting.

So, first and foremost, my condolences about Aaron. I can't imagine what it must be like to have to go through such a tragedy.

Secondly, on a lighter note, I was wondering if there was a way to rig two controllers to the same receiver. We have a Dx6i controller already, as well as an AR600 receiver already. We were planning to get a pistol grip controller for the drive, and use the dx6i for the weapon. Any advice about which controller/how to rig it up will be exceptionally useful. Thank you so much for all of the information that you have provided already through this website!

-Camden, Captain of Team Xenos [Ohio]

A: [Mark J.] Thank you for your condolences, Camden.

A digital 'spread spectrum' radio receiver like the Spektrum AR600 'binds' to a single R/C transmitter and ignores signals from any other source. A single transmitter can be bound to multiple receivers, but a receiver can bind to only one receiver at a time -- you cannot use a single spread spectrum receiver to process signals from multiple transmitters.

My recommendation: use two receivers with your two transmitters. Bind each receiver to its respective transmitter, then plug your weapon into one receiver and your drive into the other.

Q: Hey thanks for the advice.
I had one follow-up question
I've seen some 12/15 pound robots that have much, much more acceleration and agility than they seem like they should have, particularly some 2 wheel drive wedges with heavy weapons that aren't even over the wheels, while there are some 4 (or more) wheel drive wedges using the same sort of wheels but they spin like crazy without much traction and don't move as well even though they have 100% of their weight overtop of wheels.
Why is that? Is it just too much power and the wheels break traction? is it because there isn't enough weight on each wheel to compress the rubber a little? [Ohio]

A: [Mark J.] Traction is simple, but traction 'myths' confuse many builders and create the poor performance you have noticed.

Max Pushing Thrust = Weight on Powered Wheels * Coefficient of Friction

There are many posts in the Ask Aaron Design and Construction Archive about optimizing traction. Search there for "coefficient of friction".

Quick summary: you already understand 'weight on powered wheels', and the only other factor in the equation is the coefficient of friction between the tire material and the arena surface.

  • Not all 'rubber' is equally 'grippy' and some very grippy rubber quickly gathers up dust, oil and crud from the arena surface that coats the tire surface and drasticaly reduces traction.
  • Clean your tires with a solvent that leaves no residue BEFORE EVERY MATCH. I like 'lighter fluid' on a clean rag -- but be cautious with flammible solvents!
  • Foam rubber tires like 'Lite Flights' have poor grip. Their grip can be improved by coating their surface with high-grip latex or silicone rubber.
  • Tire width has only an indirect influence on traction, and wide tires decrease turning agility. Keep tire width reasonable.
  • A tire that has 'broken traction' and spinning is still providing good thrust, although directional control may suffer. Bonus: spinning helps clean the tire surface and can warm the rubber which (within reason) improves traction.
Another consideration: four-wheel robots drag their wheels sideways when turning. This makes them a little sluggish and induces wheel spin. Two-wheel and six-wheel 'bots (with lowered center wheels) don't have this problem and are more nimble in turns.

Winner: Strangest Question in the Last Year

Q: Would you consider [name redacted] to be an obscure robot that only entered one event and disappeared, or is it too soon for that since its first event was less than a month ago? [Illinois]

A: [Mark J.] The rule [see FAQ #32] applies to an "obscure old robot that fought only once and lost". The robot in question is a FingerTech 'Viper' kit with the spinner weapon add-on. Combat record: 1 win, 1 loss. So far pretty 'obscure' and mostly 'uninteresting', but not 'old'. Certainly failing to fight in the last couple weeks doesn't qualify as 'disappeared'.


Q: Hello Mark, I was having some ridiculous thoughts in my head (as usual) and I was curious, has anybody ever tried to make a water combat class? Like stick two R/C boats/submarines in a small pool or large fish tank and then put weapons on them? I would love to build an arena myself but, I don't have any funds to undertake a project of that magnitude. Thanks-Luke [Alabama]

A: [Mark J.] Check R/C Warship Combat. It's been going on for a long time. Lots of videos.

Q: Hello Mark, I have watched some of the videos about R/C ship combat but that isn't what I meant. I meant a small combat class with more traditional robotic weaponry like (spinner, flipper, wedge, axe, etc.)

A: Water spinners? Sounds slower, less dynamic, and more expensive than dry combat. Combat arenas cost enough to build without making them hold water. Interesting to think about, but I don't see this gathering a following.


Q: Do you know of a source of right angle 3 pin PWM connectors? I have a tight squeeze where I don't have room for the wires coming straight up from the PWM connector. By PWM connector I mean the interface from the RC receiver to the ESC. I could bend the wires over at a 90 degree angle, but I am worried about inducing a break in the wire from the sharp bend. [Texas]

A: [Mark J.] I've never seen right-angle servo plugs -- they'd interfere with each other in the tight quarters, wouldn't they? There are receivers with plug connectors in the end rather than the top which helps in tight spaces, but the plugs themselves are all alike.

Suggestion: it's possible to remove the metal connectors from the servo plug (video), shave away some plastic from one side of the plug to allow a gentle wire bend, then re-assemble the plug. I've done this in tight spaces and never had a wire breakage issue.


Q: I am looking into using brushless drive on a hobbyweight with p80 gearboxes (since 2 p60's look too fragile for a pusher), but my research has been having issues.

I found that Kt can be calculated by 1352.4/kv, using this I enter the information from hobbyking into the robot drive calculator, but the numbers always seem very, very poor, I must be missing something. I realize that "max amps" isn't stall amps on hk's motor stats for the calculator but it is the best I have, how do I make a decision on what motor to use, or is there any easy math to choose a brushless as opposed to brushed motor? [Dublin, Ohio]

A: [Mark J.] The formulas that apply to torque for brushed motors do not directly apply to hobby brushless motors, and there are good reasons why such motors don't give figures for 'stall torque' and 'stall amps'.

Brushed motors are easy. Hook one up to a battery and it runs. Their torque curve is linear and their power curve is entirely predicatable. That makes selection of a brushed motor for a robot drive simple.

Brushless motors are not easy. Hook one up to a battery and it will twitch and then quickly melt. They require an external motor controller to supply to each of their three input leads with power of the correct polarity at just the right instant and then change everything an instant later as the motor rotates. The timing and amount of current supplied to the motor is controlled by software in the motor controller that monitors the speed of the motor and adjusts everything thousands of times a second.

Now here's the problem: hobby brushless motors and controllers are made for model aircraft and light little model cars that don't rely on low-speed pushing torque. The controllers aren't designed to provide the current needed to produce a lot of motor torque and their software severely limits current flow when the motor is operating at low RPM, protecting the controller but killing low speed torque. The motors are also designed to run in their upper RPM range and cannot handle the heat generated by high current levels.

This all explains why brushless motors don't list values for 'stall torque' -- low speed torque depends on the combination of motor and controller and is limited by the controller software. The torque and power curves for a brushless motor are not predictable from the motor specs, they don't look much like the brushed curves at all, and the curves for the same motor will be different with different controllers. It is possible to hack into the controller software and modify the parameters governing low speed torque, but you're about as likely to blow up the controller and/or motor as you are to improve performance. It's all a black art.

So, how do you select a brushless drive motor? Overkill. Builders are selecting drive motors based on rated output. The favored formula at the moment is about 100 watts per pound of 'bot weight. That puts you in the market for a pair of 600 watt power rated motors. Pick a gear reduction that gives you some reasonable speed for the arena you'll be driving and hope for the best. You'll still likely need to hack into your selected controllers to set a few things. Like I said, it's a black art.

Torque and power curves for brushed PMDC motors
Possible torque and power curves for a brushless motor & controller

As with all component selection, I advise looking at competitors with similar designs to see what motors and controllers are being used with success. At least you might have someone to ask for help if you can't get your drive to work! Personally, I'd stick with brushed motors for a pushybot.


Now that I've explained why stall amperage means nothing in a hobby brushless motor, let me tell you how to calculate it -- just to be complete:

Theoretical Stall Amps = Operating Voltage / Motor Internal Resistance

From that formula, a brushless motor running on 4 LiPo cells (14.8v) with an internal resistance of 0.077 ohm has a theoretical stall current of 192 amps -- but you won't find a controller capable of providing that current at stall, and if you were able to provide that current the motor would melt. The number is meaningless.



Remembering Aaron... 

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.
Killer Robot drawing by Garrett Shikuma

Not all the questions we receive at Ask Aaron are serious. Some are odd, some misdirected, and a few are incomprehensible. Aaron enjoyed dealing with these questions, and I've collected some of his best responses in a special page:
Aaron's Greatest Hits
Aaron's Greatest Hits


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