Inside a Servo Motor
A servo motor mainly consists of a DC motor, gear system, a position sensor which is mostly a potentiometer, and control electronics.
The DC motor is connected with a gear mechanism which provides feedback to a position sensor which is mostly a potentiometer. From the gear box, the output of the motor is delivered via servo spline to the servo arm. The potentiometer changes position corresponding to the current position of the motor. So the change in resistance produces an equivalent change in voltage from the potentiometer. A pulse width modulated signal is fed through the control wire. The pulse width is converted into an equivalent voltage that is compared with that of signal from the potentiometer in an error amplifier.
The difference signal is amplified and provided to the DC motor. So the signal applied to the DC servo motor is a damping wave which diminishes as the desired position is attained by the motor.
When the difference between the desired position as indicated by the pulse train and current position is large, motor moves fast. When the same difference is less, the motor moves slow.