A Servo Motor can be either a DC, AC or other type of motor and includes a device to know it’s position (ex.: potentiometer, digital encoder…). Inside most Servo Motors you will find: A motor, gears, some type of limit stops that will limit the movement of the shaft, a potentiometer of some kind for position feedback and some integrated circuit to move the servo to a specific position.
A Standard Servo has around 180 degrees of motion. These can be modified to make them rotate 360 degrees or you can buy them already made this way.
The modification involves opening the Servo case, removing the limiting device and disconnect the potentiometer from the shaft. Once done the Servo will rotate in either direction endlessly since it has no way of knowing it’s position anymore and there are no limit switches to stop it.
Most Servo Motors have three wires:
White or Yellow: Control Wire.
To move a Servo you send a pulse to the control wire. This process is referred to Pulse Coded Modulation.
Standard Servos expects to see a pulse every 20ms. Depending on the length of this pulse the Servo will move to a specific angle.
For example a 1.5ms pulse will make the Servo move to the 90 degree position (which normally is the neutral or middle position).
A pulse shorter than 1.5ms will be move the Servo closer to 0 degrees and a longer one will move closer to 180 degrees.
Servos can have a lot of torque for their size and they also draw power in proportion to how hard they are working. So if you project is not moving much weight than the Servo will not consume much energy.