Robots and Servos
The robot used in this article is a model of simplicity. The frame is made from Radio Shack perf boards and a small piece of bass wood (available from almost any hobby shop). The large wheels are also from a hobby shop and are made for model airplanes. A few angle braces and a small caster from Home Depot complete the mechanical construction.
The two drive wheels are inexpensive servo motors made for radio-control vehicles. These make motor drive very simple. Each motor shaft connects internally to a potentiometer that controls a pulse-generating circuit. To move the motor, you send a pulse to it approximately every 20 ms. The motor generates its own pulse and compares the two pulses. Suppose the potentiometer in the motor (which is connected to the shaft) is set so that the internal circuit generates a 1-ms pulse. If you send a 1-ms pulse, the motor will not move. However, if you send, for example, a 1.2-ms pulse, the motor will move until the internal circuit is also generating a 1.2-ms pulse. Or, if you sent a 0.8-ms pulse, the motor would move in the opposite direction to make the internal pulse match the pulse you supply.
Normally, these servos don't rotate. They simply move in an arc (useful for an airplane's control surface or a car's steering wheel, for example). However, it is possible to modify the servo so that its potentiometer is not connected to the shaft (you also have to remove the stops that prevent it from rotating through 360 degrees). If you adjust the potentiometer so that the servo generates a 1.5-ms pulse, you can easily control the motor. A 1.5-ms pulse makes the servo hold position. A shorter pulse makes the motor rotate in one direction and a longer pulse makes the motor rotate in the opposite direction.
You can even control the speed of the motor by controlling the length of the input pulse. A 1.6-ms pulse makes the motor move more slowly than a 2-ms pulse because the motor perceives more error with a 2-ms pulse. Of course, with the potentiometer disconnected from the shaft, the motor can never correct the perceived error, so the motor just continues to turn as long as you keep supplying pulses.
This is perfect for controlling a small robot. Since the two servos are mounted back to back, if you feed the same pulse to both motors, the robot will spin in place. That's because if you rotate both motors clockwise (for example), they will spin in opposite directions since they are back to back.
The solution is to turn one wheel clockwise and the other one counterclockwise. This requires two separate signals, one to drive each wheel. Of course, if you want the robot to turn, you can send identical pulses to each wheel.
—A.W.
