(This article was originally presented as a handout at a TCRoboWars competitor meeting.)
Gearing - Most motors cannot drive wheels directly. They run too fast and have too little torque. They must be geared down. In general the more you gear the motors down the slower they go but the more torque they have. You can gear them down with gears, chains or belts. Gears are more reliable, belts are the cheapest and easiest but can slip.
Reversible - It is possible to have a robot with no reverse but it will be harder to control. Make sure your motors can run in reverse. Many car starter motors cannot run in reverse unless modified.
Voltage - The voltage and current of your motor will determine the ratings for the rest of the electronics in your bot. Bigger, higher voltage motors require bigger and more expensive batteries and controllers. 12 volt motors are by far the most popular.
Current - The current draw of your motor will determine how much current your battery must supply and how big your speed controller must be. Higher current motors also need heavier wiring. Test your motors current draw under a typical load but also check it when the motor is stalled. The stall condition is the maximum current your electronics must be able to handle. Unless you have a lot of slippage in your drive train (for example if you use belts) you will stall the motor during battle and pull full current, so plan the electronics to handle it.
Number of Channels - The biggest decision with radio selection is how many channels. You need at least two, one for each drive wheel. You then need channels for weapons and other controls. A good 4 channel FM radio will cost about $170. Some people go with two two-channel radios. This way one person can drive the bot and another person can control the weapons.
Frequency Selection - You usually will have a limited selection of frequencies based on what the store has in stock. Make sure you check the TCRobowars web page to see what frequencies are already taken. Two robots cannot use the same frequency. If you get two two-channel radios for your robot, they will have to be on two different channels.
Ground vs Air - The FCC has designated one set of radio control frequencies for Aircraft use and one set for ground vehicle use. For a robot you should get a ground radio. In past competitions we have used both air and ground radios, but there is a chance that the FCC will crack down and not allow air radios at competitions. Most radios for ground use a pistol grip instead of joysticks because they are designed for driving RC cars. Finding a radio for ground use that uses joysticks and has more than 3 channels can be difficult. Hub Hobby in Richfield has them at times. Many places can order them for you. You can also find them on the Internet.
Full forward / full reverse - The quick and dirty way to control your motors with your radio is to use the servos that come with the radio to mechanically operate switches. The servo is mechanically linked to the switches so that when it is full one direction it pushes switches that apply power to run the motor forward, full the other way it pushes switches to run the motor backward. In the center position the motor is off. You need one of these setups for each motor. A drawback of this approach is that you don't have much control over the robots speed. It is either full speed forward, stopped or full speed backward. This can make the bot difficult to control.
Mechanical speed Control - Some RC cars have rheostat speed controllers that are mechanically linked to a servo to adjust the speed of the motor. Most of these kind of controller do not have reverse.
Electronic Speed Controller (ESC) - Electronic speed controllers connect to your radio receiver the same way a servo would. They have outputs to control a motor. There is a wide range of ESC's available depending on the amount of current they can handle. On the lower end of the price scale, TEKIN speed controllers handle around 20 AMPs and cost around $100. You need one for each motor. Many of the bots you see on TV use VANTEC speed controllers. VANTEC makes a dual speed controller that can handle about 20 amps and controls two motors for around $280. A nice feature of this dual controller is that you can mix the left and right motor signals, allowing you to drive the robot with 1 joystick more like a car than 2 joysticks like a tank.
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