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...)
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.
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:
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:
The gear ratio is a decent proxy for torque and speed:
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:
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:
I have tried two different ESCs:
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.
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:
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.