A servo motor is a rotary actuator that allows for precise control of angular position. It consists of a motor coupled to a sensor for position feedback. It also requires a servo drive to complete the system. The drive uses the feedback sensor to precisely control the rotary position of the motor.
The term "servo motor" does not describe a physical principle such as the terms DC Motor or Induction Machines. The term comes from the earlier task as an auxiliary drive, "servus" being Latin for "slave". The terms "servo motor" and "servo actuator" do not refer to the same thing as servo actuator generally refers to brushless direct drive motors.
For exact positioning, the servo motor and its controller (servo amplifier) use a Feedback system which monitors the actual position (e.g. the rotating angle referring to a start position) of the motor axis. This measurement is done by a rotating encoder e.g. Resolver, incremental encoder or absolute encoder.
The electronic regulation compares the signal of this encoder with a given position set point. If a deviation is present, then the motor is turned in that direction, which guarantees a smaller distance to the set point. This leads to the fact that the deviation is reduced. The procedure repeats itself until the current value lies incremental or via approximation within the tolerance limits of the set point..
Alternatively the motor position can be seized digitally and be compared by means of a suitable computer circuit with a set point.