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How to build a 3-pound combat robot

Introduction

Why did I write this?  I didn't see a good, or even a bad, how-to for someone just getting started in combat robots and who wants to target the 3-pound Beetleweight class.  A conversation with a friend -- who's interested in building his own -- made me think I should collect my experiences and what little I know.  (There is, however, an excellent how-to for 12-pound bots.)

What are my credentials for writing it?  Honestly, I don't really have any.  I have built three robots in this weight class; the first two were lucky to make it onto the arena floor, the most-recent one hasn't been tested in battle yet.  As a Computer Science prof, I sponsor a university-level combat robotics club, so I've seen students struggle with these same design problems.  I know next to jack about mechanical engineering and only slightly more about fabrication.  So my credentials are "I'm a beginner and here's what I've learned."  (However, I *do* have a PhD in Computer Engineering, which helps a little with the electronics...)

Materials

In this section, I'll talk about parts and materials that I've actually used, or at least seen in action.  This isn't to say these are the best, but it gives you something with which to start.

Wheels and Hubs

If you're a beginner like me, you're probably going the route of attaching wheels directly to drive motors.  The upside is that it's easy; the downside is that it makes you more vulnerable to motor damage.

Most of the time, you're dealing with a wheel that slides onto a motor shaft and is secured with a set screw that presses against the shaft.  Often the shaft will have a "D profile" (meaning it kind of looks like a capital D when you look at the shaft's cross-section); in that case the set-screw presses against the flattened part of the shaft for better fit.

For both types of wheel I've used, the wheel is actually two pieces: a hub that slides onto the motor shaft and is secured by a set screw and the actual wheel that then is attached onto the hub:
  • Banebots Wheels and hubs: I used a pair of their 2-7/8" green wheels with the hex hubs in my first two bots.  They're easy to install, inexpensive and light.  Cons: 
    • the wheels can wobble on the hubs if the fit isn't tight
    • less traction than I would have liked
  • LiteFlite wheels: inexpensive foam-rubber wheels.  I initially wrote them off as cheap, until I noticed a LOT of other builders using them.  There are several hub options here:
    • stock hubs that come with them: not recommended for robot combat as there isn't a good way to secure them to a motor shaft
    • LiteFlite "hubs": I'd call these a shaft adapter, but whatever.  I've used these with some success on 1-pound robots.  They are limited to motors with 3mm or (they claim) 1/8" shafts.  I tried to drill one to 1/8" and pretty much destroyed it.
    • Aluminum hubs: I've not used these yet but they have to be better than using the stock hubs by themselves.  These seem to be the cheapest option for using LiteFlite wheels with a 4mm motor shaft.
    • Dave's Hubs: Pricey but NICE.  I'm using these on Naked Singularity and, at least from a building point-of-view, they rock.

Drive Motors

Technically, you are looking for gear motors.  These are brushed DC electric motors that have a gearbox attached to them.  The gearbox is necessary because the motors generally spin too fast to be useful on its own.  I mention "brushed" motors because you'll also hear about "brushless" motors, often as weapon motors; the main thing here is remember to use brushed motors for your drive system.

I have probably stressed more over motor selection than anything else.  There are several criteria for evaluating what you want:
  • torque: higher torque mean more ability to push your opponent around
  • speed: higher speed means more ability to out-maneuver your opponent.  Usually measured in RPM (revolutions per minute).
  • stall current: this is how much amperage your motor draws when it is trying to spin, but can't.  I'll talk more about why this one is important later.
  • gear ratio: related to speed and torque, this is how many times the motor spins for one revolution of the output shaft.
The gear ratio is a decent proxy for torque and speed:
  • lower gear ratio: more speed, less torque
  • higher gear ratio: less speed, more torque
Gear ratios are written, for example, as 50:1 (read "50 to 1"), which in this case means the motor spins 50 times for every one time the output shaft spins.

Here are the ones I've tried and what I've learned:
  • Beetle B62 / B16: these are both the same motor but with different gear ratios (62:1 and 16:1, respectively).  The B62 works well with larger wheels (e.g., 2-7/8"), as the larger wheel compensates for the slower motor, speed-wise.  The B16 is faster but less torque-y.  I'm using it on Naked Singularity due to its smaller wheels.  These motors fall apart easily, however, especially in cases where an overpowered weapon motor vibrates so bad the bot loses traction.  Not that I'd know about that....  They must be "battle hardened" to protect against coming apart and leaving microscopic gearbox parts all over the arena.  Wrap the gearbox in a layer of electrical tape and use Lock Tite on the gearbox screws.
  • FingerTech motors: I've used these mainly on 1-pound 'bots, but I did try them out with my second 3-pounder.  While they're powerful enough (especially at the 20:1 gear ratio), the shaft is more fragile than the 3-pound weight class suggests.  If you can't come up with a design that supports the shaft on both ends (which is harder to build than it souns) stay away.  These work well with the Lite Flite wheels and Lite Flite hubs.

Electronic Speed Controller

The electronic speed controller (ESC) receives signals from your radio and translates them into the proper signals to control the speed of your motors.  For oddball reasons, the radio input is in the form of Pulse Position Modulation (PPM), which is actually the control signal for a servo.  In other words, anything (like, say, an Arduino) that can control a servo can also control an ESC.

The output of an ESC is Pulse Width Modulation (PWM, not to be confused with PPM).  This is what controls a motors speed.  PWM literally turns the motor off and on very fast.  It varies the speed by varying the amount of time the motor is "on" versus "off".  "On" for longer equal faster speed and vice versa.  (Note that an Arduino can output PWM, too, but can not supply the current for even the small motors combat 'bots use -- so don't try!)

Things to look for in an ESC:
  • number of channels: usually either 1-channel (can only control 1 motor) or 2-channels (can control two motors)
  • current rating
  • reversible: some ESCs, especially the brushless ones, can only spin one direction.  Make sure your drive-motor ESCs are reversible!
  • Battery Elimination Circuit (BEC): this is a major convenience -- it allows the radio receiver (see below) to get its power directly from the ESC instead of having its own dedicated battery cable.  The power is supply through the servo-style cables that connect the receiver and ESC.
I have tried two different ESCs:
  • Scorpion XL: 2-channels, can handle a boatload of current, and has lots of other cool features I've never really used.  Cons: pricey.  Size can also be an issue.
  • Banebots BB-3-9.  single-channel, small, light.  You can buy two for about 1/2 the price of a Scorpion XL.  Cons: I've had a little trouble with one sporadically not recognizing the signal from the radio system.  Usually requires a power on/off to fix.
Naked Singularity is using my Scorpion XL right now.  The XL's current rating is high enough that each channel can control two motors in lockstep, which is nice since this is a 4-wheel-drive bot.

Radio System

The radio system is 2 parts: a transmitter (Tx) aka your remote control and the receiver (Rx).  Most of the time they are packaged together.  The Rx is installed in the bot and connects to the ESC(s).  Things to look for in a radio system:
  • 2.4 GHz signal.  This is the "newer" type of radio system and it is much more robust than the old crystal-based systems.  And they're cheap enough now (you can get knockoffs from Hobby King for as little as $30) that you have no excuse NOT to go this route.
  • Enough channels to do what you want.  If you're just starting, a 4-channel system is plenty.
I have budget Futaba system that ran about $100 and it does everything I need.  As I mentioned before, Hobby King now sells equivalent systems for much, much less.

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