Re: STEPPER MOTOR
A stepper motor typically has 4 coils. Inside the motor, these coils relate to metal parts on the shaft in such a way that each step is only so many degrees. One stepper motor I use has 20 steps per rotation, or 18 degrees per step.
By activating the coils in sequence, the motor only rotates to the next step. Coil one is 0 degrees, turn on coil 2 while turning off coil 1, 18 degrees. Turn on coil 3 while turning off coil 2, 27 degrees. You have a very precise control of how fast the motor turns in a single rotation. You can hold one coil on and not step, thereby locking the motor. To turn the other way, reverse steps.
Another kind of stepper motor only has 2 coils. You reverse the voltage alternately on each coil to get the next step.
I've seen stepper motors that had 400 steps per revolution that I've pulled off old hard drives.