Eelectric motor converts electrical energy into mechanical energy. The reverse process, that of converting mechanical energy into electrical energy, is accomplished by a generator or dynamo. Traction motors used on locomotives often perform both tasks if the locomotive is equipped with dynamic brakes. Electric motors are found in household appliances such as fans, refrigerators, washing machines, pool pumps and fan-forced ovens.
Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force. Other types of servos use hydraulics, pneumatics, or magnetic principles. Usually, servos operate on the principle of negative feedback, where the control input is compared to the actual position of the mechanical system as measured by some sort of transducer at the output. Any difference between the actual and wanted values (an “error signal”) is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. An entire science known as control theory has been developed on this type of system.
Servos are found in many applications. They operate the throttle of engines that use a cruise control. CNC machines use servos to make the motion axes of a machine tool follow the desired tool path. Fly-by-wire systems in aircraft use servos to actuate the control surfaces that control the aircraft. Radio-controlled models use RC servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus.
