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5816 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 Aaron, could you tell me, for a spinning drum that stores 10,000J [30 Lbs class] what is the difference between a 10mm bite and a 20mm bite? Thanks. [Valle del Cauca, Colombia]

A: [Mark J.] Bite (what's bite?) is calculated as a maximum depth of opponent insertion into the arc of a spinning weapon at a given weapon RPM and forward velocity. You'll get that maximum bite rarely, just like 13 black only comes around rarely on a roulette wheel. Sometimes your luck will be very poor and you'll hit your opponent just as an impactor is facing them and get no bite at all! On average, you'll get half the max bite -- less as your attack speed drops.

  • Consider an opponent who was wise enough to avoid exposed sharp edges in their robot design. Given a nice sharp angle to attack you don't need a lot of bite, but if you're forced to attack a flat or gently curved surface you need all the bite you can get.
  • Greater bite also is a bonus when your attack velocity drops. A weapon with a lot of bite can still be effective in close quarters when you don't have a chance to back away and take a high-speed run. Watch some combat videos and pay attention to the speed at which most weapon hits are made.
What's the diffference between 10mm and 20mm bite? A few more match wins. Bite is good -- more bite is better.
I've been writing quite a bit of JavaScript lately, so what's a little more? Here's a quick 'Bite Calculator' -- fill in your values in the blue input cells and click 'Calculate':

Rotary Weapon Bite Calculator
Weapon: rpm Insert Time: sec
Impactors: # Max Bite: mm
Attack: mph

Q: When did the first insect class robots first appear? Where was the first insect class tournament? [a question I asked myself, since nobody else had]

A: [Mark J.] The first antweights were something of an accident. At the time of the earliest combat robot events, R/C transmitter / receiver sets came with a full set of servos, servo mounting hardware, servo 'horns', and a receiver battery pack. The builders each had a drawer full of servos and no particular use for them.

Someone figured out how to modify servos for continuous rotation which turns them into tiny high-torque gearmotors with built-in ESCs! Attach a jar lid to a one of those servo horns, screw it onto the servo output shaft, plug two of them into a receiver along with one of those unused receiver batteries and tape the whole mess onto a stiff piece of cardboard -- instant mini robot!

The first 'Antweight World Series' tournament for 150 gram 'bots was held in September of 1999 in Reading, England (video). The event appears to have been held on a scrap piece of wallboard propped up in someone's living room. I'm not sure it should count.

The first organized U.S. insect tournament I can document was the SOZBOTS antweight event in Sun Valley, California on February 23, 2002 (video).

SOZBOTS welcome

Q: Rank these designs from a to f .
  1. A complete control type of bot with a vertical saw blade
  2. A clamp bot with a drill on it's clamp
  3. Number 1 with the saw replaced with a horizontal bar [Herndon, Virginia -- suspiciously close to Ashburn and Reston]

A: [Mark J.] If you're asking someone for a favor, the correct form is: Please rank these designs from a to f.

You didn't mention what weight class you're considering, but there are some serious problems with these designs:

  • An effective clamp and lift system like 'Complete Control' takes up a lot of weight, which leaves VERY little for a workable saw, drill, or spinner weapon;
  • You have very little control over what part of your opponent happens to be beneath your drill or saw, which makes the chances of something vulnerable being exposed to your weapon very slim;
  • Lifting a clamped opponent up into a spinning weapon offers very little advantage over simply running the weapon into them, plus there's now a very good chance that the impact of your spinner will rip your clamp apart.
If you're fond of this type of weaponry, think about a simpler design. Consider a 'dustpan' with a saw on a light moving arm like 'Gloomy'.
Q: i've read lots about overvolting motors, but haven't found anything in the archives about overvolting the ESC. if my ESC is rated at 50 volts and i'm using two 8S lipo batteries in series to produce a nominal voltage of 59 volts, will i instantly fry my ESC or will it just get very hot until it dies? [Utah]

A: [Mark J.] DO NOT OVERVOLT MOTOR CONTROLLERS!

  • Flowing too much CURRENT thru an ESC will cause heat to build up faster than it can dissipate and lead to component failure, either slow or fast depending on the degree of overload.
  • Putting too much VOLTAGE into an ESC will exceed the dielectric properties of the components. What's that smell? Instant failure.
Manufacturers do allow for a bit of extra voltage. A hobby ESC rated 12 volts will be fine with a 12 volt 'nominal' battery that shows a 13+ volt 'resting' voltage, but you don't want to push it farther than that. There are a few exceptions -- usually toy-level controllers that carry an artificially low voltage cap for safety (like the Vex Motor Controller 29. However, bumping up the nominal voltage nearly 20% on a seriously rated 50 volt ESC is a quick ticket to ozone city.
Q: Why doesn't witch doctor have gyroscopic forces acting on it, one side doesn't lift [Reston, Virginia]

A: [Mark J.] See this post on gyroscopic forces farther down the page.


Q: Hey, Aaron! I was thinking about building a beetleweight drum spinner for a competition in Chicago. The design I want to build will have a 1-pound drum. There's only two problems:
  1. What type of design should I use that would support a heavy drum like that?
  2. How do I cut out a drum from a piece of steel?
If you could answer one (or both) of my questions, that would be excellent! [Kollier W. , Illinois]

A: [Mark J.] I can offer some help:

  1. Take a look at the photos of the Weta 1 and Weta 2 beetle drum spinners for some general guidance on drum support. They use strong, thick-section extensions of the main frame rails to support their drums.
  2. I can't teach you metal shop fabrication in a short answer. I can tell you that most builders do not start with a solid block of metal! Typically a builder will find a suitable length of thick-wall aluminum tube and will fabricate end caps and add impactors.
There are many posts on drum design and fabrication in the Ask Aaron Weapon Archive. Search there for 'drum'.
Q: what types of particles are represented by the crosses (x) [Ovenden, England]

A: [Mark J.] You might have included a link to the diagram. It took me a couple minutes to find it.

The section heading is "simple covalent bonding" and a covalent bond is defined as "a chemical bond that involves the sharing of electron pairs between atoms". It's not a great leap to infer that the crosses are electrons. I don't think you have a future in chemistry -- why don't you go build a robot?


Q: Hey Mark,

How does the flipper on Lock jaw in Battlebots Season 2 harness the power of the springs? From what I could tell it was winched back but how was it able to fire then reset again? Additionally, could this method of flipping be utilized in all other weight classes as an alternative to pneumatics? Thanks in advance! [Straight Outta Facebook]

A: [Mark J.] Donald H. doesn't divulge much about his robot designs, and I can't see enough detail in the photos of 'Lock-Jaw' to understand the clutch mechanism. Fortunately there are builders who do share their spring-powered flipper designs:

  • Dale Hetherington has built pretty much every exotic robot design there is. Take a look at his very detailed build log for 'Dead Air'.
  • I'm personally very fond of the snail-cam spring loader for it's simplicity and adaptability to small weight classes.
That will get you a start.
Q: Does the RPM of motor matters while building battlebot? [Kathmandu, Nepal]

A: [Mark J.] Build a lot of BattleBots in Nepal, do they? The power of the motor matters, and power is the product of torque and RPM:

Power [watts] = Torque [Nm] * RPM / 9.549

American builders aim for about 100 watts of power per pound of robot weight, although sucessful robots can be built with as little as 10 watts per pound. When you have enough power you can adjust gear reduction between the motor and the wheels to get the required speed and acceleration for your design.

Team Run Amok offers tools to help with robot design and motor selection: here is an example drive motor analysis using the Tentacle Drivetrain Calculator.


Kitbots beetle spinner kit Q: I have a saifu v2 kit and I want to make it like Klazo from near chaos Robotics with the wedgelets. The wedgelets need to be used to get under DDT. Ok here is the question. Will the wedgelets work? [Ashburn, Virginia]

A: [Mark J.] The titanium wedgelets or 'horns' used by 'Klazo' and recently adopted for the Kitbots 'Weta 2' beetleweight kit (photo at left) are not added to assist in 'getting under' a specific opponent type -- the UHMW wedgelets formed in the side rails of the 'Saifu 2' are entirely adequate for that. The problem is that the thin slicing blades on horizontal spinners like 'DDT' cut thru the UHMW plastic like butter. The titanium holds up much better to that type of attack.

If you're going to add a design feature, make sure you know what it's supposed to do.


Q: Hello,
Do you know a site where I can buy pinions? I am trying to mount a motor with a 5mm shaft to a 72.3 gearbox. (I'm the dude with a ton of grammar mistakes, I'm from Ohio.) (This will be my third robot.) Thanks! [IP address: Orlando, Florida]

A: [Mark J.] You'd think that something small and apparently simple would be easy to find in the 'net - wouldn't you? Unfortunately, pinion gears are a problem.

Advice: don't try to order an internet pinion to match up with that gearbox. All you know is the motor shaft size you need -- the PDF doesn't give the required gear pitch, tooth count, or length. The number of 'wrong' gears you might find, buy, and throw away is mind boggling.

I suggest that you, the gearbox, and the motor visit your local R/C hobby shop. They likely have a box of assorted pinions under the counter that you can go thru to find something that works or that can be modified to work.

It may be easier (and possibly less expensive) to search eBay for complete gearmotors with a 5mm motor input shaft. Pull the motor, switch the pinion to the motor you want to use. and bolt the new motor to the gearbox. This is common practice when adapting a specific motor to a combat robot application. It saves a lot of trouble.


Q: Hello Mark,

I've heard it said that Wrecks' vertical disk (30-35 lbs) has a much higher Moment of Inertia than Electric Boogaloo's vertical spinning weapon (~70 lbs). With the assumption that Wrecks is using a similar motor (big assumption), how is this possible?

-David R. [Livermore, CA]

A: [Mark J.] A little clarification:

The Moment of Inertia (MOI) is NOT a direct measure of how much energy a spinning weapon stores; the motors or speed of rotation have no bearing on the moment. MOI is a measure of the energy needed to change the rate at which the weapon is spinning. Its value depends on the mass of the weapon and (most importantly) on how that mass is distributed relative to the rotational axis.

Consider:

  • A small chunk of matter is rotating around an axis at a given RPM at a distance of six inches. With each rotation the chunk of matter travels a distance of ( 2 * 6" * pi = ) approximately 37.7".
  • That same small chunk of matter is now rotating around an axis at the same RPM at a distance of 12 inches. With each rotation the cunk of matter travels a distance of ( 2 * 12" * pi = ) approximately 75.4".
  • The chunk of matter 12" from the axis must be travelling twice as fast as it did when only 6" from the axis in order to complete a revolution in the same length of time.
  • The kinetic energy of a moving object increases with the square of its velocity (K = mv2), the mass 12" away from the axis has four times as much kinetic energy as an equal mass 6" from the axis when rotating at the same RPM.
  • Doubling the distance of the mass from the rotational axis doubled the speed of the mass, which raised the energy stored by the spinner at any given speed by a factor of four (velocity2). This also increases the MOI of the spinner by a factor of four, even though the mass of the spinner has not increased.
Now, let's take a look at the spinner weapons on 'Electric Boogaloo" and "Wrecks":

'Electric Boogaloo' has a bar spinner weapon. A bar has a lot of its mass close to the rotation axis and relatively little mass far from the axis. The formula for the MOI of a rectangular bar spinner is:

MOI Rectangular Bar = (mass/12) * (length2 + width2)

'Wrecks' has a large diameter spinner with most of the mass concentrated in a ring at the outer edge -- far away from the axis of rotation. The formula for a the MOI of a thick ring (discounting the supporting spokes) is:

MOI Thick Ring = (mass/2) * (inner radius2 + outer radius2)

Let's use the new Run Amok JavaScript Spinner Weapon Calculator to compare the MOI of the two designs based on rough estimates of their sizes:

  • A steel bar similar to the spinner weapon used by 'Electric Boogaloo': 24 inches long by 4 inches wide by 2.5 inches thick weighs 68.6 pounds and has an MOI of 0.99 kgm2.
  • A steel ring similar to the spinner weapon used by 'Wrecks': 15 inch outer radius, 12 inch inner radius, and 0.5 inch thick weighs 36.2 pounds and has an MOI of 1.95 kgm2.
The ring weapon is a bit more than half the weight of the bar weapon, yet has nearly twice the Mass Moment of Inertia. That's a much more efficient use of weapon mass for energy storage.

Q: i am the new builder who recently asked about brushless motors, im on a tight budget main supplies were bought by my school club, so i cant go and buy brushed. our club is building 4 bots for a competition, but in my team none of us have built, and teams cant help each other. any tips for gearing down my brushless motors, they are 4900 kv 19 turn [South Brooksville, Florida]

A: [Mark J.] So... tight budget, no experience, unspecified components, the other teams can't help, you're 3000 miles away, and you want to build a combat robot.

I cannot even guess at the power available from your motors from the information you have provided -- '4900 kv' motors come in all sizes and power outputs. The Hamburger is Bad. Useful information would include the brand / model of the motor and how many volts your battery provides.

As I said in the earlier post, if you want to use conventionally sized wheels for your beetleweight robot (say about 2" diameter) you're going to need a gear reduction of around 40:1. From a practical standpoint that will require a multi-stage reduction that would be complex, fragile, and (I'm guessing) beyond your capacity to produce. It's time to get creative!

Here are two high-reduction combat robot drivetrains that require no gears, pulleys, or sprockets:

  • Friction drive The shaft of the motor presses directly against the surface of the tire and relys on friction to transmit power to the wheel. You will get some slippage, but if your motor has a 2mm to 3mm shaft the reduction ratio works out about right for a fast spinning motor like yours.
  • Tiny wheels If direct-drive for a normal sized wheel isn't practical, how about a really tiny wheel? Mount a very small wheel to the motor shaft and mount the motor at an angle to put the wheel in contact with the floor. Unconventional, but a successful antweight robot used this method as a weight-saving measure.
In your situation, I'd go for the tiny wheels. The 'wheels' could be something as simple as a small plumbing washer, a short length of small diameter rubber tubing, or even a few layers of electrical tape wound around the shaft. They won't last long, but they're easy to replace.
Q: If you attach 2 motors to one spinner blade will the blade go 2 times as fast as one motor? P.S don't say anything about weight. [Ashburn, Virginia]

A: [Mark J.] If you tie two people together, are they twice as fast? Power is additive, speed is not:

  • two motors = same speed
  • two motors = twice the power
The weapon will be no faster but it will spin up to that same speed in half the time -- it will be twice as quick.

You can gear up to get greater speed, but you will get a proportional reduction in torque. Ain't no such thing as a free lunch.


Q: How do electric hammers not burn out [...just down the road from Ashburn, Virginia]

A: [Mark J.] If you aren't careful, they do burn out.

The motor for an electrically powered hammer weapon needs to be powered off at either end of the weapon swing to avoid an extended 'stall' condition where the motor would consume damaging current levels. This can be done a couple of ways:

  • Most builders control the hammer with one of the spring-centered joysicks on the transmitter and release the stick at either end of the weapon swing. Just don't forget to release the stick!
  • Some R/C relay boards and a few ESCs (like the Talon SRX) have provisions for 'limit switches' to sense when the controlled device has reached the end of its travel and automatically shut off power in that direction. This allows an electric hammer to be actuated by a simple single-throw switch on the transmitter -- typically channel 5.

Q: Show me a picture of killerhurtz and deadpan side by side [Ashburn, Virginia]

A: [Mark J.] You've confused 'Ask Aaron' with Siri. Show me a little respect.


Q: hello, so I'm building my first battle bot, i was going for ant weight but my college club is doing a beetle weight so i am looking to do that, my motors are brushless 4900kv, and they are cogging on the low end when i try to use them, also I'm having issues finding a proper wheel that gets enough traction due to my lack of torque, my question is i am trying to gear down the motors to help with the cogging, i have all the parts but i have no idea how to attach an axle to a battle bot. I just get confused on how to add a shaft with the ability to rotate. [South Brooksville, Florida]

A: [Mark J.] [Mark J.] It sounds like you jumped into the deep end of the pool before you even watched someone else swim.

  • 'First combat robot' and 'brushless drive motors' are a very poor combination.
  • You've attempted to simply attach a wheel directly to the shaft of a 4900kv brushless motor? You're lucky it's cogging and didn't just melt.
  • You've told me nothing about your motors except the 'kv', but as a guess you're going to need to gear them down about 40:1 to get adequate torque and controllable speed. That's not something you can do with a simple pinion and spur gear. The most common approach is to attach a brushless motor to a multi-stage planetary gearbox -- but I'm getting ahead of myself.
Advice: use the brushless motors for another project. The 'kv' is too large for a combat robot drivetrain, and you shouldn't be using a brushless drivetrain in your first 'bot anyhow -- they're a real pain to get working well.

More advice: get out on the 'net and find some build logs for beetleweights to get an idea of what works and get some design ideas before you go any farther. Here's one to get you started: Naked Singularity beetleweight.

Still more advice: browse the shop at BotKits.com to get an idea of the type of components used in competitive beetleweight robots.


Q: What is the best motor to use for a horizontal spinner [Los Angeles, California]

A: [Mark J.] The Hamburger is Bad.

See Ask Aaron Spinner Weapon FAQ.


Q: [product name expunged] are unreliable [Ashburn, Virginia]

A: [Mark J.] Sez you. If you want to express unsubstantiated opinions get your own website.


Q: Is there any way to find archive for all questions in order? I am struggling to find my old questions. [Bristol, England]

A: [Mark J.] There is no single time-ordered archive of all 5793 questions at 'Ask Aaron'. When posts 'roll off' the bottom of the main page they are moved to the top of one of the topic archives.

There are search boxes both at the bottom of the main 'Ask Aaron' page and at the top of the Ask Aaron FAQ that allow you to search the archives. A search there for 'Bristol' shows recent archive entries by you in the Robot Design and Construction, Ants, Beetles, and Fairies, and Robot Weapons archives.

Once in one of the archives you can use your browser to search the page for 'Bristol' or any other phrase you like.


Q: If you have [insert spinner name here] then should you destroy [insert knocked out bot here] if it is getting counted out? [Ashburn, Virginia]

A: [Mark J.] Look over at the other team and tilt your head to one side.

  • If they wave you off or tap out -- back away.
  • If they flip you off or moon you -- trash 'em.

Q: When you looked over at the other team and tilted your head, have any builders mooned you? [Elkridge, Maryland]

A: [Mark J.] Only metaphorically.

At our second trip to Robot Wars we had pit space next to a tracked robot decorated like a military tank that had the noisiest drive train imagineable. I was badly jet-lagged and was trying to catch a nap under my pit table, but every five minutes that damned tracked robot would test their drive.

We drew the tank and a BattleBots veteran in the first-round three-way elimination match, and I wanted a good shot at that noisy bugger. The treaded noisemaker was quickly disabled but I wanted a trophy to make up for my interrupted nap. I lined up to take a run up the rear wedge of the tank to shear off their decorative turret. I got the turret, but also high-centered my 'bot on top of the tank. We were both immobilized and were being counted out -- but the BattleBots veteran had just finished his fearsome E-Tek powered disk spinner a few hours earlier and was looking for a target to test it on...

Moral of the story: don't tick off anyone in the pits, and don't turn your back on a spinner.


Q: Hello! I am a high school student that I building a new robot. I am building a drum spinner, and that spinner will be operated by a Brushless motor with the specs of:
  • Turns: 10T
  • Voltage: 12S Lipoly
  • RPM/V: 560kv
  • Motor Poles: 10
  • Internal resistance: 0.017 Ohm
  • Max Loading: 100A
  • Max Power: 4200W
I currently do not have the dimensions of weapon itself, but I am interested in knowing the "equation" in finding the speed of the weapon at full speed. I have the Wh of the battery, Volts, and and most of the other specs. Also, where can I find the equations to calculate torque, and stall torque. [Dublin, Ohio?]

A: [Mark J.] What's wrong with this group of questions?

Given the syntax, grammar, and language structure, I have trouble believing that the author is from Ohio. Further, the motor and weapon design are unusual for a combat robot that might be constructed by an American high school student in the mid-west.

If the author is a high school student in Ohio and they are building a combat robot this size, they should have a local mentor to guide them in design, construction, and safety. The mentor should be providing the answers to questions like these.

Either way, I'm not comfortable answering your questions. The best I'm willing to do is to point you to this Wikipedia article on 'Motor Constants' and warn you that stall torque on brushless motors is much less than the calculated value due to the software in the motor controller limiting current at low motor speed.


Q: How do you lose weight for a modded D2 kit 3 oz overweight it hat two dual aluminum wedges or 1 titanium wedge but how do you lose weight for the dual aluminum wedges [Massachusetts]

A: [Mark J.] The D2 Beetleweigth Robot Kit is designed to run with one wedge, and I'm not sure why you'd want to add a second wedge in the rear. I've seen a lot of fight videos of D2s and in the fights they lost I don't think a rear wedge would have helped.

The base kit plus battery, receiver, and ESC weighs in at about 42 ounces, leaving six ounces to add your own modifications. Adding a rear-mounted D2 aluminum wedge plus mount and brackets comes to about 8 ounces and will put you overweight without adding much functionality to the robot.

Three ounces is a lot of weight to shave off a beetle, and I'm guessing that you're restricted to part replacement and simple modification:

  • You can shave 1.2 ounces by replacing the aluminum bottom panel of the robot with one of the polycarbonate top panels - plus you save the weight of four mounting screws.
  • Replacing the 1.5" wide wheel/tires with the lighter 1" wide wheel/tires from the Ant1 kit will save you another 0.8 ounce.
  • Replacing the recommended Scorpion Mini ESC with a pair of FingerTech tinyESCs will save a little better than 0.5 ounce.
  • Trimming off the bent-down ends of the added aluminum wedge will save at least another ounce.
I don't think that a rear wedge is worth all this effort and weakening of the robot structure. I'd consider some other type of modification, or just run the very effective D2 as it comes.
Q: I've been taking classes for welding, and as I've been practicing, I've been thinking about translating some of those welding techniques into my future builds. I have a few questions regarding some design choices about welding:

1) I've learned that construction sites will lay a crisscross surfacing weld on their heavy machinery scoops, such as an excavator bucket. They do this to increase the surface hardness and to lengthen the life span of the component. I was wondering how well an anti spinner scoop could hold up if I were to put on a layer of weld and grind the surface flush. Would this make for a stronger anti spinner scoop, or would it not do anything?

A: [Mark J.] That's an imprecise, labor intensive, and unpredictable method of hardening a steel surface. Suggest you read up on the wide array of more standard hardening techniques. I think you'll get better results.

2) I've noticed that many builders when they weld will only do one pass. How come builders don't do multipass welds? [Oregon]

A: Multipass welds are only needed in very specific situations. There's a good discussion on when to use multipass at the Miller Welding Forum.


Q: What thickness should a titanium disc for a 1 lb bot be? I am designing it to look like DDT except a rectangle [Ashburn, Virginia]

A: [Mark J.] How long is a piece of string?

As discussed in the Spinner FAQ a spinner is a flywheel that stores rotational energy. Its size and shape is primarilly selected for the ability to store and deliver the maximum amount of destructive energy for the allowable weight.

  • Titanium alloy is not the preferred material for small spinner weapons. Robots like 'DDT' use specialized hard and tough steel alloys.
  • A weapon disk, bar, or drum is not primarilly a structural element, but it must be designed to withstand the forces of impact. Those impact forces are determined by the dimensions of the weapon and the speed at which it rotates, neither of which you have given me.
  • If it's a rectangle, it isn't a disk -- and a rectangle is not nearly as effective at energy storage as the carefully engineered single-toothed cutout disk on 'DDT'.
  • See Frequently Asked Questions #17.
  • See The Hamburger is Bad.
Q: I am the guy that asked the disc question and I need to know if a park 250 motor will do good on a 0.465 or so pound disc.

A: The performance of a given motor with a spinner weapon depends on more than the weight of the disk, bar, or drum. Suggest you read the Ask Aaron Spinner Weapon FAQ for info on weapon motor selection and spinner design. In general I can say that the Park 250 is considerably smaller than a typical ant spinner weapon motor.

Q: The CHASSIS is a rectangle not the disc it is a 4 toothed CIRCLE. The dimensions WILL BE 4x4 inches spinning preferred speed is 4,000 rpm maximum is 18,000 rpm. Is S7 tool steel ok for it?

A: 18K RPM? It sounds like you're planning to direct-drive a heavy 4" diameter 1/8" thick steel disk with a very small brushless motor. Your spin-up time will be awful, the 2mm Park 250 motor shaft and bearings are not suited to handle large impacts, and the heavy loading during the slow spin-up will cook the motor very quickly. You can direct-drive a small weapon, but not a heavy disk with a large moment of inertia. Run a belt-drive reduction to eliminate these issues.

You don't see many 4-toothed disk weapons. If you had read the Spinner FAQ you'd know that more weapon teeth = poorer weapon bite. I strongly suggest reducing your tooth count to no more than two teeth, and please do read the Spinner FAQ.

S7 tool steel typically comes from the supplier in a soft 'annealed' state so that it may be cut and machined. The final product must be heat-hardened in order to develop its hard and tough properties. Heat treating a small and thin piece like your disk is likely to warp it. The current 'DDT' disk is 6 inches in diameter and waterjet cut from pre-hardened 1074/1075 spring steel to avoid this warping issue.

Q: The size doesn't matter, is the park 250 good for an ant class robot

A: Size does matter. When I say that the Park 250 is 'considerably smaller' than a typical ant spinner weapon motor, I refer not only to its physical size but to its power output as well. It is both considerably smaller and less powerful than a typical ant weapon motor like the Turnigy D2822/17 1100KV Outrunner. As noted above, its 2mm output shaft is also too small to use with an otherwise unsupported direct-drive antweight weapon.

Q: We are never going to make a fairy so even if we will make a belt drive we have to waste a perfectly good motor? We are gonna use the can of the park 250 motor for the belt drive. A lot of questions.

A: You can spend your time and money to build a whole robot around an underpowered weapon motor just because you have it, or you can sell that motor on eBay and buy a proper motor that will give you a chance at success. I know what I'd do.

If you want to go ahead, a belt reduction of 3:1 would be about right. Using the can of the motor to drive a belt will require a large pulley on the disk to get that 3:1 ratio -- about 2.5 inches in diameter.

Q: Will a power 25 outrunner with a 3S battery work for the disc I am the disc guy. PS I asked a lot of questions in 1 day. I will maybe suck it up and build a fairy.

A: You want about 100 - 150 watts output power for your ant weapon. Your Park 250 has about 50 watts, and the E-flight Power 25 has up to 600 watts. Worse, the Power 25 weighs almost seven ounces - which combined with your seven ounce disk leaves about two ounces for the rest of your 'bot.

Perhaps you could take a couple days off to think thru your options before sending in more questions? You've used up your quota for today.

Q: Disc guy here maybe the park 270 bc on the robot marketplace the power 25 is back ordered. Has robot combat changed since 5 years ago? P.S I found a park 370 maybe it could work. What is a quota?

A: Maybe, yes, maybe, definition of 'quota'.


Q: Are there any R/C transmitters made like game controllers, or can I hack a game controller to control an R/C transmitter? [direct email]

A: [Mark J.] I don't know of any currently available game controller style standard R/C transmitters, but you can hack an existing controller for R/C. This seems like a good idea to gamers who are very familiar with the controller-style layout -- but the thing is, you aren't going to like it.

All those games you play on your favorite console were designed and tuned to respond correctly to the inputs of the game controller, but real-world stuff like combat robots have their own real-world control requirements that are much better delt with by the control range and layout of conventional transmitter inputs.

Everybody I know who went thru all the trouble to hack a game controller for robot combat quickly gave it up and went to a conventional controller. They simply work better. Suck it up and learn to use a real transmitter.

Q: In response to the guy who asked about gaming-like transmitters, Team Bronco use the Vexnet Joystick, which is designed very much like the controller of a gaming console. Wouldn't this work for them? [Fremont, California]

A: [Mark J.] Sometimes I know more about the builder than shows up in the question and answer. The question came from a novice builder of insect-class 'bots.

  • The VEXnet System Bundle is $399.99;
  • The required VEX ARM Cortex-based Microcontroller is way too large for an insect-class 'bot;
  • The VEXnet System Bundle is $399.99;
  • The VEXnet system is not plug-in compatable with standard combat robot components, like motor controllers;
  • The VEXnet System Bundle is $399.99;
  • Have you tried to read the manual?
  • The VEXnet System Bundle is $399.99;
  • Adjusting system settings requires knowledge of the 'easyC V4' or 'ROBOTC for VEX Robotics v4.x' languages;
  • Did I mention that the bloody thing costs four-hundred bucks?
Team Bronco extensively modified the VEXnet system to get it to operate with their combat robot. Unless you have the experience and specialized resources available to 'Team Bronco' and fully understand what you're getting into I cannot recommend that you invest in this entirely non-standard control system for your combat robot.
Q: Aaron, how good do the anti wedge rubber pad things work? How many do you need to achieve ultimate friction? Thanks, Anonomous

A: [Mark J.] I wasn't sure what you were talking about, but I asked around and was pointed to a rubber strip used by some UK antweights that rubs the arena surface when the nose of the 'bot rides up an opponent's wedge and has its wheels/treads lifted off the arena floor (thanks, Ian). You can see in this video that it works pretty well (the blue 'bot has the rubber strip). A single strip running the width of the chassis will give you all the grip you're going to get.

Personally, I think that the problem this solves is better addressed by designing your 'bot so that the drive wheels aren't lifted off the floor if the nose of the robot is raised. That keeps your drive wheels operational and gives you some ability to push back - and steer!


Q: How to make 2 [pneumatic] cylinders work in sync? Use 2 buff tank and 2 valve or 1 buff tank with 1 valve [to] supply 2 cylinders? Thank you (just like Subzero) [Yunnan, China]

A: [Mark J.] I see that you've just asked the same question of Subzero's builders on their Facebook page. The better question might be, "Why use two pneumatic cylinders instead of just one of larger diameter?"

If I had to use two cylinders I'd want both to fill from a single gas supply thru a single valve to avoid any pressure imbalance. Why don't we wait a few hours and see what the builders say?


Subzero's builders did reply a few hours later:

Team Hammertime / Teamxd: That version was one supply tank with 2 rams, one valve and no buffer tank.

Twin cylinder 'Subzero' - 2009

Q: What's the best & safest way for someone who's only done non-weaponed bots to do their first weaponed one? [New Jersey]

A: [Mark J.] I'm not sure how to respond to 'best' but I can offer some safety guidance. You didn't mention how large a robot you are interested in building or the type of active weapon you are considering, so I'll have to keep this general.

  1. Review your basic workshop safety practices:
    • Wear eye protection when there is danger of flying chips, abrasive dust, or irritants.
    • Remove rings and other jewelry before operating machinery.
    • Keep your workspace and the floor around it clean and un-cluttered.
    • Securely anchor materials being drilled, ground, or machined.
    • Cover and secure sharp edges and points in the work area when not in use. Use gloves where appropriate.
    • Do not wear loose clothing around power tools -- a work apron may be appropriate.
    • Monitor your lithium battery charging, and use a suitable charging container if needed.
    • If it's gonna be loud, wear hearing protection.
  2. Keep safety restraints on weapons that prevent them from operating unexpectedly until you are ready to test/use the weapon.
  3. Do not test or operate your weapon without containment suited to your weapon and robot.
  4. Be cautious of mechanism pinch-points: articulated levers, chains, hinged surfaces.
  5. Keep your design and expectations at a level appropriate for your building skills and resources.
  6. If you aren't sure you understand the safe operation of specialized equipment, fittings, or assemblies -- ask!

[Mark J.]: Welcome back! 'Ask Aaron' has been off-line for a week for our annual break, but I've been busy working on a new combat robot design tool. You can take a peek here: JavaScript Spinner Weapon Calculator

Q: Dear Aaron, which horizantal spinner has more effect on the other bot? And to you? Thx, [Google Fiber ISP]

A: [Mark J.] I'm not sure I understand your question. The base physics of a horizontal spinner are the same if it's a bar or disk -- or a top/mid/undercutter. The effectiveness depends on other factors such as energy storage, 'bite', and chassis stability. Suggest you read the Ask Aaron Spinner Weapon FAQ and then send in a more focused question.


Q: Is there a way to calculate a spinning weapon's gyro effect? I've seen bots that were similar to each other yet one had HUGE problems with gyro and one didn't. How can I make sure ours isn't like the first one? [Kansas City, MO]

A: [Mark J.] There are multiple posts about designing to minimize gyroscopic forces on your 'bot in the Ask Aaron Combat Robot Design Archive -- search there for 'gyroscopic'.

Many of those posts refer to the Total Insanity Gyroscopic Effect Calculator as a tool useful in adjusting robot design to better cope with the weapon gyro forces. The T.i. gyro calculator requires the 'Mass Moment of Inertia of Weapon' as an input, which can be calculated with the Run Amok Spinner Weapon Calculator


Comment: Hey Mark, I've made a video of the Fight My Bot Championship 2016 in China [link fixed]. There are 8 fights including Semi Final, Grand Final, 3rd Playoff and a 6-Player Rumble! I hope you enjoy it : ) [Zhejiang, China]

Reply: [Mark J.] Sharp robots, nice arena, good crowd, and a great video. Thanks!

Update: An expanded video with more fights is now up -- FMB Championship Video Clips 2.


Q: Hi again, I've stumbled upon a problem, the snail cam spring reloader [needs to stop] after one full rotation, I cannot seem to find a suitable solution to do so with a gear motor. Do you think a stepper motor is better for this or is there a way to make motor start and stop after one rotation and a push of one button if you will. [Bristol, UK]

A: [Mark J.] When you turn off the windshield wipers on an automobile have you noticed how they continue for the rest of the wipe stroke and then stop in the park position? That's what you're looking for and here's how you do it:

  • The circuit shown at right allows the gearmotor to run until the flipper arm (not shown) reaches the fully loaded position and presses down to open the 'interrupter' micro switch -- stopping the gearmotor.
  • A momentary closure of the normally open R/C switch re-starts the gearmotor long enough to fire the flipper, which closes the interrupter switch.
  • The gearmotor then continues to run thru the rest of the reload cycle to an automatic stop.
The positioning of the micro switch is simplified if you use a lever-style switch that can be bent to fine-tune the switch point. The micro and R/C switches must have enough current capacity to handle the gearmotor, and the micro switch must be wired 'normally closed' (NC). For larger robots the micro switch can trigger a relay with the capacity to handle the motor load.

Micro switch

Q: I was actually thinking to have snail cam consist of 2 shapes, 1) the main cam and 2) a smaller cam with sudden increase in radius where reloading needs to stop that way there is no need for finest tuning. [Bristol, UK]

A: You have lots of options on the interrupter. A micro switch is simple, but you can certainly use other sensor types: infrared emitter/detector, inductive proximity, magnetic... whatever you're comfortable with.


Q: Hey Mark. Care to be a hero today?

I recently purchased a new 'Weta 2' from Kitbots. At some point the drive system started to get a little bit funky. I'm not sure if I just dug around the insides too much and maybe did something to it, but I don't believe that it was like this upon delivery. I've asked countless people about this gearmotor/esc issue and has eluded them all, including both Pete Smith and Kurtis Wanner of Fingertech fame.

Here is the video

[Mark J.]: The video shows the robot initially responding pretty well to transmitter commands, but when it enters a left spin on hard left transmitter stick it slows rapidly to a stop. A hard right stick yields the same result on a right spin.

I have tried to rectify this issue by swapping out the drive ESCs with new ones. Same issue. I even bought a pair of botkits 22mm gearmotors and tried those out instead. Same issue. I even suspected that perhaps the weapon ESC was taking too much power away from the drive system. But upon disconnecting power to the Weapon ESC and trying again...well. You know. Same thing. And yes, both the batteries I used to test this were at full charge.

Before I go out and spend all of my money replacing everything imaginable to no longer experience this, I thought I'd come to you. Do you have any idea what could be going on?

Thank you sir, David R. [Livermore, CA]

A: [Mark J.] So, you 'asked countless people' before finally becoming so desperate as to ask me... It's good to know where I rank. 

  • It isn't the battery - you have two, fully charged
  • It isn't the drive ESCs - you swapped them out
  • It isn't the drive motors - you tried another set
  • It isn't the weapon ESC - you disconnected it

"How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?

- Sherlock Holmes, "The Sign of the Four"

If it isn't any of those components, what's left? Improbably: it's your radio. I recognize the radio you're using in the video: a $25 Chinese unit known for awful quality control. Try running the 'bot with the receiver unpacked from that tight Weta electronics bay to make sure the circuit board isn't getting warped. If that doesn't help, borrow another radio for a test. I know it's unlikely -- but what else can it be?

Update: I see on the forums that David R. has switched radio gear and is no longer asking about his response problem. I guess I was right.



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

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