Servo motors are similar to DC motors; however, there is an important difference. While DC motors are generally designed to move in a continuous way—rotating 360 degrees at a given speed—servos are generally designed to move in a limited set of angles. In other words, in the DC motor world, you generally want your motors to spin with a continuous rotation speed that you control. In the servo world, you want your motor to move to a specific position that you control. This is done by sending a Pulse-Width-Modulated (PWM) signal on the control connector of the servo. PWM simply means that you are going to change the length of each pulse of electrical energy in order to control something. In this case, the length of this pulse will control the angle of the servo, as shown:
These pulses are sent out with a repetition rate of 60 Hz. You can position the servo at any angle by setting the correct control pulse.
