The servo motor is an engine, that can drive to different set point positions and then maintains these in the sense of a control drive. The position is thereby in case of a rotatory engine a angle position and in the case of a linear motor a distance position. Starting and maintaining the given position are made by a position control.
The term servo motor does not describe a physical action principle like for instance designations such as DC motor or Induction Machines, but the name makes from its earlier operational task as auxiliary drive (servus lat. slave). Generally however brushless direct current motors or vector-regular asynchronous/synchronous motors are meant with servo actuator.
For exact position adjustment the servo motor and his controller (servo amplifier) have a Feedback system, that 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 into 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 also digitally and be compared by means of a suitable computer circuit with a set point.