Small R/C servo mechanism
1. electric motor
2. position feedback potentiometer
3. reduction gear
4. actuator arm
A servomechanism, usually shortened to just servo, is a device used to provide mechanical control at a distance. For example, a servo can be used at a remote location to proportionally follow the angular position of a control knob. The connection between the two is not mechanical, but electrical or wireless, for example.
The most common type of servo is that mentioned, which gives positional control. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force, though other types that use hydraulics, pneumatics or magnetic principles are available. 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. A whole science of this type of system has been developed, known as control theory.
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 airplanes use servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move the focus.
Typical servos give a rotary (angular) output, though linear types are common too, using a screw thread to give linear motion, or using a linear motor.
Another device commonly referred to as a servo is used in automobiles to amplify the steering or braking force applied by the driver. In this form this device is not a true servo, but rather a mechanical amplifier.
Also in the industrial machines servos are used to perform complex motion.