6164 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.

    In Memoriam: 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   
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
Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot.

'Ask Aaron' has recently received criticism in the combat robot forums for being unfriendly, downbeat, and overly discouraging toward builders with 'new ideas'. Our use of statistical analysis, mathematical modeling, and historical perspective to provide the best available guidance to new builders wishing to build successful robots is apparently ruining the sport by reducing the number of wacky and marginally competitive designs that appear at tournaments. It was directly and seriously suggested that I encourage rookie competitors of all ages to build 'melty-brain flail spinners with homemade Arduino-based control systems' because they're so awesome.

In response to this criticism I am pleased to unveil the new 'Ask Aaron' Cheerleader Mode:
  1. DO NOT search the archives for previous posts on your design idea -- those old posts might discourage you.
  2. Type your combat robot design idea into the usual question entry box at the bottom of this page.
  3. Ignore the depressing button and click the perky new button instead.
You will receive an immediate encouraging response. Hooray! Combat robotics is saved! Give it a try.

Q: What is the difference between a crusher and a hammer [Arlington, Virginia]

A: [Mark J.] Same as the difference between a hammer and a vise:

  • A hammer hits things - quickly;
  • A vise squeezes things from two sides - slowly.

Q: Just out of curiosity, is there any combat robotics related application for the NPC-A0045512 Gearmotor? If not, what is this thing even used for? [Charlotte, North Carolina]

A: [Mark J.] Nope, it's a military item (the color gives that away). This $9600 gearmotor is used to rotate gun turrets on armored vehicles.

Q: What are the physics of average wedges beating most average spinners? [Fairfax, Virginia]

A: [Mark J.] Physics... Do you mean 'probability'?

The findings were essentially the same for both studies -- you may be surprised by the results...
Q: Thanks for the new brushless spreadsheet, it helps immensely in planning. Your calculations are based on the SimonK firmware, but how different is that from the default firmware most chipsets come with? Or is flashing to SimonK such an essential part of getting a bot ready for the arena that I shouldn't even think about using off the shelf firmware? Still learning the process, I'm in the midst of designing two beetleweight bots and I just learned about SimonK a couple of days ago (just when I thought I had all the requisite bases covered).

Is there a good SimonK tutorial you can recommend that explains the settings for weapon motors? All I've found for robotics is that you can set the ESC up with a reverse mode for drive motors (the Robert Cowan video you linked to in the FAQ), but I don't know what settings to tweak for a weapon motor. [Vancouver, Canada]

A: [Mark J.] I'm glad to hear that you're finding the new Brushless Spinner Weapon Spreadsheet useful. As noted in the release info, the current spreadsheet is a beta version and I will be adding information about other firmware as I am able to dig thru their code and decipher the details of their startup sequence. For now I can tell you that all the un-sensored brushless ESC firmware flavors have a common approach to motor start-up and low-speed operation:

  1. There is a brief period where the ESC responds to R/C input and applies a little power while it sorts out the electrical responses it receives from the motor to make sure the motor has started rotating and is spinning in the right direction.
  2. Next, the firmware increases the power available a bit more and monitors the rising motor speed. This 'current restricted' period protects both the motor and the ESC from overcurrent until the Back EMF (video) rises high enough to restrict current on its own.
  3. Once this 'safe' speed is achieved, the firmware ends the current restriction and as much power as the R/C commands can flow to the motor.
Note The firmware 'restricts' current flow by reducing the current pulse width to a set percentage of full power. This is not the same as true 'current limiting' which cuts off current above a set amperage limit. You can still exceed the current capacity of the controller.

The nice thing about SimonK firmware is the large number of parameters that are user adjustable. If you know what you're doing you can match the firmware to the performance needs of your application. If you don't know what you're doing you can smoke both the motor and the ESC. Very, very few people know what they're doing.

Fortunately, brushless ESCs are widely available that have SimonK or BLHeli firmware already installed -- no flashing required. A brushless weapon is MUCH less demanding on firmware than is a brushless drive system, and the default settings for either of those firmware packages will work well with a reasonably designed spinner weapon. I highly recommend this approach. If you really want to tinker with setup, you can play with the parameters listed in this earlier post that determine when the current restriction is lifted -- at your own risk.

If you do want to dig deeper into setting SimonK parameters, get a fresh cup of coffee and read thru 'How to Be a Brushless Hipster'. Charles knows what he's doing.

Q: Okay, I read through Charles Guan's treatise on brushless drives. It made my head hurt, but I think I've absorbed most of the important information.

A: Charles excels at making heads hurt.

Q: The brushless motors I'm looking at as weapon motors (ACK-3510CP-630KV) list a max current of 22 amps and recommend a 40 amp ESC. I am intending to direct-drive the weapons. I know that's a risk and has downsides, but I see so many belts and chains come off on insect weights, and recently there have been several highly-successful insect weight bots with direct drive (Sgt. Cuddles, Weta2 kits, Margin of Safety).

A: 'Sgt. Cuddles' and 'Weta' kits have low moment of inertia weapons supported on both ends, and 'Margin of Safety' has a custom motor with a HUGE bearing mounted in a large aluminum plate to absorb impact. A little bird tells me that your design has a large moment of inertia weapon at one end of the stock motor and the single point of motor support at the other end. I'm gonna guess... three hits before the motor comes apart and that 4mm shaft bends.

1) Does using a larger ESC make any sense? Will it, for instance, let me push the drives to slog through that low-rpm phase faster without exploding the capacitors?

2) Is it equally practical (or, perhaps even better) to do as Charles does and purchase some really good capacitors to replace the ones on the ESC? I have pretty good experience with soldering and electronics, so I figure I can probably handle that. As far as I can tell, this is one of the big differences between say a 40 and a 60 amp ESC, and if I'm reading his blog right it's the capacitors that balance the surge of energy required as they ramp up to speed.

A: Better caps and more/better MOSFETs are seldom a bad idea, but I suspect that the weak link in your direct-drive high MOI weapon will be the extended time your motor will be pulling amps beyond rated capacity during spinup. With your current motor design you may need to extend the ESC current restriction to a higher RPM range to keep the motor from smoking.

Q: I know I'm probably getting way over my head as a newb, but for me the learning and experimenting aspect is one of the most exciting aspects of the sport.

A: I understand perfectly -- but efficient experimentation requires a baseline condition against which the results may be compared. I recommend starting with the stock set-up and finding your weak points. Team Juggerbot had a saying that should be better known: 'Damage is weakness leaving the robot'. Break it, then make it stronger.

Q: Also, I know it's early, but are you thinking about attending the Seattle Bot Bash this spring? I'm hoping to get there with at least one working bot. I'll be the guy watching as his 10 year-old daughter drives (see, this is why I want two bots).

Thanks again for all your help and for maintaining this fantastic resource!

A: You're very welcome. No promises about SBB -- best luck to you and your daughter.

Q: So I'm straining the motor too much, plus its 4mm shaft and bearings can't stand the rigors of direct-drive to the weapon for long. Clearly some kind of pulley system is necessary unless I'm going to build my own motor (some day, maybe, but not on my first bot). I think something with a slim motor up front with a very short belt between weapon and motor pulleys would probably be best (similar to Silent Spring). Then the question becomes, what kind of ratio do I need?

I hope I'm correct in assuming that the delay where the ESC is restricting current to the motor can be used as a proxy to guesstimate "motor strain". If so, what do you think would be a safe(ish) level of motor strain, measured by the amount of time it takes before the motor kicks into full power?

Thanks again, this is all immensely helpful!

A: You're on the wrong track, Vancouver. The proxy for strain on a permanent magnet DC motor is current consumption. While the ESC 'soft-start' function is active, the stress on the motor is greatly reduced. I've been working on a new feature for the Brushless Spinner Spreadsheet -- let me show you a few current consumption graphs:

Vancouver sent me some specs and drawings for his weapon that he would prefer not to share. These graphs are based on that info.

1) Here's your direct-drive weapon as it stands:

Your weapon motor is highly stressed, pulling WAY above its 22 amp max rating for almost two seconds. It won't last long and neither will your ESC.

2) Here's your weapon with a 2:1 drive reduction:

Better. There's still a big power/stress spike when the soft-start function stops, but the total time your motor spends frying is greatly reduced.

We can reduce the peak current consumption by tinkering with the SimonK parameters to extend the soft-start RPM range.

3) Here's your weapon with a 2:1 reduction and the TIMING_RANGE3 parameter in the SimonK firmware reduced from '1024' to '256':

Now we're getting into the survivable range. Your spin-up time is down around 1 second, the peak current has been cut by half, and the blade speed is at a reasonable level to get some 'bite'. Your energy storage drops from 900 joules to about 225 joules, but that's entirely adequate for a beetleweight.

The need to tinker with SimonK firmware settings indicates to me that the selected motor is simply too small for so large a spinner bar. My recommendation is to either shrink the weapon or grow the motor. I'd shrink.

Concept: Antbotica-esque competition, but A) it's in a Plastic Class, and B) it's also a rapid build-a-thon: instead of a 'Bring Your Own Bot' model, all competitors arrive maybe 2-5 days before the competition and are handed kits holding motors, batteries, controllers, fasteners, etc. all the non-plastic essentials for a bot, and a spool of thermoplastic filament (with a different color in each kit perhaps). Then, they are pointed to a workshop with many 3d printers, and they spend the next couple-to-a-few days designing and constructing their bot.

Sure, it's obviously very costly especially being a multiday event requiring a workshop with plenty of printers. You can charge the competitors more than a typical competition of course; you're paying for their bots and hosting the build experience in a proper shop. Also, maybe it could be geared towards teens with a STEM education focus and STEM education funding. Instructors/mentors guiding & teaching kids on how to accomplish the design they want, from the CAD to the controls.

My region has a few events like that: coding camps hosted by schools in summer, dancing hexapods at the library, FIRST is big here, etc.. How feasible do you see this concept ever being, considering it could be refined a lot? What do you think the biggest issue(s) would be? [Buffalo, New York]

A: [Mark J.] I don't think I'm the person to ask about this. You need to parade the idea in front of a group of builders that might be interested in attending this type of event, and that's not me.

The event doesn't match my design/build methods: I'm a grumpy old man who doesn't CAD, doesn't print, would be frustrated by the lack of hardware choice, and who doesn't see the need to enforce 'plastic' for an 'Antbotica' style event that eliminates the advantages of exotic materials and construction by its design. With identical hardware I suspect that you'd end up with a field of very similar plastic 'bots.

Perhaps the Combat Robotics group on Facebook would like to comment on your event? If not, you can always click the button.

Comment: Cheerleader mode is funny. [Wilmington, Delaware]

Reply: [Mark J.] Thanks. There's a team in England that isn't amused.

Q: Hey hey! I was wondering if you'd like to take a look at my articles about rejected BattleBot designs on the BattleBots Wiki. What do you think of them? Are there any of these rejects that stand out to you? Which one do you, personally, think should've been in Battlebots? Thanks! [Decatur, Illinois]

A: [Mark J.] I fondly remember the earlier series of BattleBots where no design was rejected. Anyone could build a combat robot and simply show up for the tournament, and hundreds of teams did just that. If your 'bot passed tech inspection you were in. That spawned great diversity and interest in combat robotics. It was first a robot tournament and second a television show.

In the ABC series the creative purity of the builders was overridden by a need to present a design that might 'audition' well and catch the eye of the producers. The fighting ability of the robot takes a back seat to audience appeal, and your design can be rejected simply because if might not film well. Is your team well financed enough to build a duplicate robot to have standing by so that repairs won't slow down the production? That would never fly in a true sporting event, but it was a plus at ABC BattleBots.

I'm not going to second guess the BattleBots producers over their selections and rejections, but I will say that the most capable reject in your collection is 'Team Velocity'. Their record is outstanding, their design was appealing, and they certainly would have been in contention for the championship.

Q: Hi, I have a few questions.

First, I have a disk for my fairyweight which has two different geometries that I can reverse for different armor types. The disk is 108mm diameter, weighs 35g and is 3mm thick grade 5 titanium. I was wondering if the sharp or flat side would be better for 3d printed armor? [Great Falls, Montana]

A: [Mark J.] Clever. Your design in general looks good. General rules:

  • Flat for hard metal surfaces;
  • Sharp for hard plastic / soft metal;
  • Cutting knife edge perpendicular to the axis if your only targets are spongy tires or ablative foam armor.
See the recent post from Jamison Go near the top in the Ask Aaron Ants, Beetles and Fairies archive where he discusses his strategy.

Q: Also, have people had success using fans to create more down-force for a pusher, when magnets are not an option? And if not, what is the secret to making a pusher work well?

A: The experts in fan-downforce are the builders of line-follower and maze-runner robots. The photo at right shows the underside of a line-following 'bot with fan downforce. There is a post in the Ask Aaron Robot Design & Construction archive with some links to useful info. Search that archive for 'suckbot'.

General pusher tips

  • Put as much of the robot's weight on powered wheels as possible. Section 2.7.7 of the RioBotz Combat Tutorial provides guidance on wheel placement for two-wheel drive 'bots.
  • Foam tires can be coated with silicone rubber or latex to provide a surface with more grip. Multiple posts on this in the archives.
  • Clean your tires before every match with solvent -- I use lighter fluid on a cotton rag.

Q: Hey there,
Just wondering if there is a good baseline for flipper force to weight ratio? I'm shopping for pneumatic cylinders for a 30 pounder I've designed (think Wheely Big Cheese with guarded wheels) and the rams im coming across have a force rating of like 450-500 pounds. This number seems to be too low to really throw an opponent with much authority. Is there a force number I should say "Ok it can't be less than this" or am I over thinking it? [Adrian, Michigan]

A: [Mark J.] Ram force is only one of the elements needed for an effective flipper. There are four primary factors that must work together:

  1. Ram force: calculated as gas pressure times the piston area;
  2. Ram extension: over how great a distance can the force be applied;
  3. Lifter geometry: the physical lifter mechanism usually decreases force and increases speed; and
  4. Gas flow efficiency: how quickly the gas can get into the cylinder to provide effective force.
That last factor is the critical factor in flipper effectiveness. Pressure regulators, hoses, ports, and valves must be selected to flow the gas as quickly as possible from the pressure tank thru the system and into the cylinder in order to extend the ram at maximum speed under the load of your opponent's mass.
  • A featherweight ram weapon that provides a maximum 500 pound force but which extends slowly because of poor gas flow will only be a 'lifter', but...
  • A smaller diameter ram delivering only 200 pounds of force may fill quickly enough from the same valving system to produce an effective 'flip'.
The calculations involved in modeling flipper performance are not trivial. Typically, flipper builders will simply obtain the pneumatic components with the greatest flow capacity available and hope for the best. If you wish to model your flipper design, take a look down at the bottom of our Combat Robot Design Tools page for links to the 'Team Da Vinci: Understanding Pneumatics' page and the 'Hassocks Hog: Pneumatic Flipper Spreadsheet'.

Q: With a certain televised competition allowing limited forms of entanglement as a "counter" to the ever-increasing number of powerful spinners, do you think more competitions in smaller weight classes will consider adding similar rules? Would you like to see entanglement in any form return to robot combat? [Alberta, Canada]

A: [Mark J.] Every time an entanglement post appears on 'Ask Aaron' I get hate mail from builders in the US who take this as an attempt to emasculate their robots and ruin the sport. No other topic causes so great a reaction -- it makes me wonder a bit about builders. Outside the 'sportsman' class, I don't see any possibility for measures that might reduce destruction.

Q: i have an ev warrior . i need to locate part of friction drive train. i need the drive shaft than links the two motors together that turns the knurled roller which engages rear tire . do you know where i can find [El Monte, California]

A: [Mark J.] There are multiple EV Warrior bike parts suppliers on-line, but none of them list the drive shaft. Try asking in the Motorized Bicycle Forum.

Q: Hey, it's me again! I've been keeping quiet and busy here at home with my bot. But alas, I still have questions that desire to be answered. So, here they are! As with every time, your response is greatly appreciated! [Decatur, Illinois -- close to Champaign]

1) What exact purpose (other than serving as a directional tool) does the arm of Megabyte serve?

A: [Mark J.] The tube sticking up thru the center of 'Megbyte' serves as the dead shaft for the rotating shell, is bent to provide a directional cue, and aids self-righting by preventing the 'bot from flopping all the way onto its back.

2) How does one go about adding safety switches and locks to full body spinners?

A: Use a hollow tube for the weapon dead shaft, run the power wires up the inside of the tube, and mount the kill-switch at the top. The lock can be a caribiner run thru matching holes in the shell and chassis.

3) IF the only choice of a material to use for a combat robot was wood, which kind should one use and why?

A: Just as you use different types of metal for different parts of your robot, so also would you use different types of wood in different places. Marine grade plywood would be useful for support structure in place of magnesium or UHMW polyethylene due to the uniform strength of the multiple cross-grain layers.

Trivia All of Team Run Amok's robots have at least one component made of wood as a tribute to our first 'bot, Robotica champion 'Run Amok'.

4) Do you know anything about Chinkilla? I've heard rumors of Jay Leno having the robot, but not having it appear on BB due to copyrights and legal things of the like. Do you think Chinkilla would ever make a return, even if it's for shits and giggles?

A: 'Chin-Killa' was a demo 'bot supposedly made by 'The Tonite Show' tech crew for Jay Leno. It was built for a publicity tie-in to BattleBots and was never intended to compete, being overweight (more than 400 pounds) and generally unsuited to combat. In BattleBots season 2 it did fight a televised exhibition match against BattleBots co-founder Trey Roski's ultra-heavyweight 'Ginsu'. It ain't comin' back.

5) Do you think putting a motor inside a drum for a heavyweight would be a good idea or a bad idea? '841' is a modified weta kit, and the weapon inside the drum is really good, especially for making the robot more compact! However, I have a feeling that it wouldn't work so well with bigger bots...

A: The high energy storage levels of modern robots combined with the 'scale factor' (FAQ #17) have rendered hollow drums generally unsuitable for large combat robots.

6) What are your thoughts on Expulsion's ability to self-right? Plain genius or plain silly?

6.5) What are your thoughts on Expulsion's camouflage during the 10-way rumble?

A: 'Ask Aaron' stopped answering questions about Robot Wars competitors many years ago. See FAQ #37.

7) If I were to use a shovel head as armor for a robot, what kind(s) of shovel heads should I look for?

A: Try one of these.

8) What's the weirdest location a robot has come from, in your opinion? I did some research and I found robots from Iran, Nepal, and Pakistan!

A: My workshop. Ya oughta see it.

Q: I've seen many insect weight builders using the Fingertech pulleys and belts to power weapons and drive systems. How do those builders attach said pulley onto a weapon. I'm a bit confused about the documentation of those pulleys on how they are mounted. Could you show me examples or tell me the best way to mount them. Thanks! [Columbia, Tennessee]

A: [Mark J.] The FingerTech pulleys and wheel hubs have threaded holes with set-screws that tighten down onto the support shaft to lock them in place. The belt turns the pulley, the pulley turns the shaft, the shaft turns the wheel hub. FingerTech blade weapon hubs have a similar design with screws that pull two sections of the hub together and clamp the shaft in place.

If your design has the weapon rotating on a non-moving 'dead shaft' you can align the pulley/shaft/weapon assembly and drill crosswise thru the pulley into the weapon, fixing the pulley in place with screws into or thru the weapon rotor. FingerTech offers a pre-bored 30 tooth timing pulley designed for this purpose and used on their beetleweight beater-bar weapon.

Major Update to the Run Amok Spinner Spreadsheet

I've known for some time that the brushless motor torque estimates used to model spinner weapon performance in the Run Amok Spinner Excel Spreadsheet were very conservative, but the complex non-linear torque curve imposed on sensorless BLDC motors by their controller firmware was difficult to model correctly.

Over the last week I 'bit the bullet' and dug into the popular 'SimonK' firmware deep enough to find the key. I've modified my spinner spreadsheet to emulate the default soft-start sequence of typical sensorless controllers. This greatly improves brushless spinner weapon modeling by the spreadsheet.

The new 'Brushless Spinner Weapon Spreadsheet' is a beta release while I work to improve the documentation and pull in some real-world feedback. You can download and test the beta version from a temporary web page I've set-up for the purpose:

Brushless Spinner Spreadsheet Beta

I've used test results from the new emulation to update the on-line brushless torque estimators in the Spinner Weapon FAQ and the JavaScript Spinner Weapon Calculator help file. The new values they provide are much improved, but I recommend using the full spreadsheet to get the best estimate of your brushless spinner performance.

Q: Hi Mark - feedback on your beta brushless weapon calculator.

7800 kg/m3
I think that there is an error in the kinetic energy output, although I'm not sure how to correct it. I've attached a couple of screenshots showing the differences between the beta spreadsheet and the Team Cosmos KE calculator using a drum with the following specifications:

Your spreadsheet gives the correct weight of 35.38Kg, whereas the Team Cosmos calculator gives the weight as 31.48Kg. To eliminate the weight as the variable, I have changed the Team Cosmos calculator to show the correct weight.

In terms of kinetic energy at 50% (4271RPM), the Team Cosmos calculator shows 25,243 joules. The beta spreadsheet shows 132,045 joules. I'm not quite how much this is off by, but the numbers don't stack up.

Regards, Rob K [Trumpington, England]

A: [Mark J.] You've been caught in a dimension input difference between the calculators, Rob.

  • The Team Cosmos calculator asks for the diameter of the tube;
  • The Run Amok spreadsheet asks for the outer radius of the tube.

Plugging the input values from the Team Cosmos calculator screenshot you sent (a steel tube 250mm long and 100mm in radius with a 35mm wall) into the Run Amok spreadsheet gives essentially identical output: 35.38 kilos storing 25,306 joules @ 4271 RPM.

Thanks for checking with me and for testing the beta Brushless Spinner Spreadsheet.

Q: Hey Mark! The new brushless spreadsheet looks good. How exactly do you figure out the speed where a SimonK ESC switches from startup into full power mode? [Philadelphia, Pennsylvania]

A: [Mark J.] Thanks. The power level and switchover process was a deep riddle. Configuration variables and their explanations on the SimonK Home Page turned out to be outdated or just plain wrong. With some help from brushless gurus Cosmin Gorgovan (who confirmed my suspicions in the firmware) and Charles Guan (who did his best to confuse me) I worked my way to the bottom:

  1. In spite of what the documentation says, the default controller firmware settings will limit the power pulse length to 1/4th [PWR_MAX_RPM1] of the full power pulse length [MAX_POWER] until the motor speeds up enough to complete one 'commutation' in under 1024 -sec [TIMING_RANGE3]. That's [60 seconds per minute / 0.001024 seconds per commutation] = 58,594 commutations per minute. Great... but what's a commutation?
  2. A hobby brushless motor (three phase) requires six electrical power switchings (commutations) to complete one 'electrical revolution'. That makes [58,594 commutations per minute / 6 commutations per e-revolution] = 9766 eRPM at switchover to full power. Interesting... now what's an eRPM?
  3. An electrical revolution is the mechanical rotation of the motor required to move thru one pair of permanent magnet 'poles' and return to the electical starting point in the commutation sequence. Switchover to full power happens at [9766 eRPM * 2 magnet poles per pair / number of magnet permanent magnet poles]. Awesome... how many poles does my motor have?
    Commutation Speed to RPM Conversion
    Commutation: -sec Speed: RPM
    Magnets: poles
  4. The motor specs will tell you how many poles a motor has; it's always an even number. A typical outrunner might have 14 poles, where a typical inrunner might have 4.
    • For a 14-pole outrunner motor: cutover to full power comes at [19532 / 14] = 1395 RPM.
    • For a 4-pole inrunner motor: cutover to full power comes at [19532 / 4] = 4883 RPM.
In general form: 20,000,000 / (commutation in -sec * magnetic poles) = RPM.
Q: Hi, I have seen somewhere that a robot used a banebot wheel for a spinning disk. What are the advantages and disadvantages of using a wheel for a spinner? [Great Falls, Montana]

A: [Mark J.] There have been a few 'toothless' spinner 'bots. Pictured is 15-pound 'Mad Cow Disease' that uses multiple Colson wheels as a spinning drum weapon. The robot has a 4-4 record at BotRank, so it can hold its own.

Since toothless spinners rely on contact friction rather than 'bite' they can be spun very fast to store big energy without penalty and they are effective against smooth surfaces that standard spinners just skitter across. They are also durable, inexpensive, and easy to replace.

Primary disadvantage: their hits are friction-limited, so they aren't effective at delivering really huge impacts. I suspect a high-speed toothless disk could be a very effective weapon in a fairyweight 'bot...

Q: I've seen several types of game controller style R/C transmitters. Do you recommend an XBee interface for an Xbox wireless controller or a compact controller look-alike transmitter such as the Blade MLP4DSM? [Fort Collins, Colorado]

A: [Mark J.] Game controllers work wonderfully well for controlling software designed to be controlled by game controllers. Nothing in the real world has that same relationship with game controllers.

Get a conventional R/C transmitter. Their features and design have been refined over the last five or six decades to be an effective and transparent interface between your hands and real-world objects like combat robots. It really won't take you long to adapt.

Q: This is regarding the selection of motors for weapon system.

I've noticed that selection of a brushed motor is simpler since the RPM and torque characteristics are linear. However, brushless motors do not follow this linear nature. I've also noticed that selection of a brushless weapon motor is primarily based on experience, brand and the stator diameter for a specific kv rating. Also the ESC plays a major role.

How do I select a BLDC for my drum with 0.01 kgm2 moment of inertia, weighing 4.8 kg?

Can you recommend some BLDC motors with a kv rating in the range of 300-400. I've been looking at Alien Power System and Scorpion motors. I have smoked a Propdrive 5060 with 380kv running on 6s earlier. The ESC accompanying this was a Redbrick 200amp model. [Braunschweig, Germany]

A: [Mark J.] I'm told that there aren't many combat robot builders in Germany and that the German Roboteers Association doesn't know of anyone building a drum robot such as you describe. They'd like to get in touch with you.

While you've noticed quite a bit about brushless motors you may not have noticed that these very compact and lightweight motors do not like to be bogged-down in their lower RPM range. Hobby brushless motors are designed to spin very light propellers quickly into their upper RPM range before encountering significant loading.

I've tried modeling the performance of your weapon powered by the Propdrive 5060 380kv. I suspect that you're using the low-RPM outrunner to direct-drive the weapon drum -- a very poor idea for a weapon this large. The Propdrive has a maximum current rating of 90 amps, but my model shows the motor drawing nearly three times that amperage for the first three-quarters of the fairly long spin-up. I'm not surprised that you smoked it.

Featherweght drum weapons are typically powered by a belt reduction from a brushless inrunner motor. This design allows the motor to build RPM quickly for greater efficiency. Read thru the build report for featherweight 'Big Ripto' for the general layout. When in doubt, copy a successful example. 'Big Ripto' uses a KB 44-74 1500KV brushless inrunner weapon motor rated 2600 watts at a maximum 120 amps.

Q: Why do you take so long to answer stuff? [Wilmington, Delaware]

A: [Mark J.] I've got a life, but I almost always 'answer stuff' in less than a day. If that's not fast enough for you, just attach a $20 bill to your question.

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.

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