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The term servo or servo-mechanism refers to a feedback control system in which the controlled variable is:

-        Mechanical position, or

-        Time derivatives e.g. velocity and acceleration.

Following characteristics are usually required for a feedback control system:

i.            High accuracy;
ii.           Remote operation;
iii.          Fast-response;
iv.          Unattended control.

Following are the essentials of a feedback control system:

  1. An error detecting device. It determines when the regulated quantity is different from the reference quantity and sends out the error signal to the amplifier.
  2. An amplifier. The amplifier receives the error signal and then supplies power to the error-correcting device, which in turn changes the regulated quantity so that it matches the reference input.

A servo-motor should entail the following characteristics:

  1. The output torque of the motor should be proportional to the voltage applied (i.e. the control voltage which is developed by the amplifier in response to an error signal).
  2. The direction of the torque developed by the servo-motor should depend upon the instantaneous polarity of the control voltage.