A servomechanism, sometimes shortened to servo, is an automatic device that uses error-sensing negative feedback to correct the performance of a mechanism and is defined by its function.[1] It usually includes a built-in encoder.[2] A servomechanism is sometimes called a heterostat since it controls a system's behavior by means of heterostasis.
The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position, speed or other parameters.[3] For example, an automotive power window control is not a servomechanism, as there is no automatic feedback that controls position—the operator does this by observation. By contrast a car's cruise control uses closed loop feedback, which classifies it as a servomechanism.