servo motor

http://en.wikipedia.org/wiki/Servomechanism

servo, is a device used to provide control of a desired operation through the use of feedback. 

servomechanism |ˈsərvōˌmekəˌnizəm| |ˈsərvoʊˌmɛkəˌnɪzəm|
noun
a powered mechanism producing motion or forces at a higher level of energy than the input level, e.g., in the brakes and steering of large motor vehicles, esp. where feedback is employed to make the control automatic.

http://www.seattlerobotics.org/guide/servos.html

A Servo is a small device that has an output shaft. This shaft can be positioned to specific angular positions by sending the servo a coded signal. As long as the coded signal exists on the input line, the servo will maintain the angular position of the shaft. As the coded signal changes, the angular position of the shaft changes.

http://www.openservo.com

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http://www.societyofrobots.com/actuators_servos.shtml

http://en.wikipedia.org/wiki/Electric_motor

http://en.wikipedia.org/wiki/Control_theory

PWM = Pulse-width modulation

RC servos

: The servo is controlled by three wires: ground (black/orange), power (red) and control (brown/other colour) and will move based on the pulses sent over the control wire. The servo expects a pulse every 20 ms, or 0.02 s in order to gain correct information about the angle. The width of the servo pulse dictates the range of the servo's angular motion.

http://en.wikipedia.org/wiki/Motion_control

http://www.rane.com/par-s.html#servo_loopservo-loop; servo-locked loop; servo-mechanism A self-regulating feedback system or mechanism. Typically a feedback system consisting of a sensing element, an amplifier, and a (servo)motor, used in the automatic control of a mechanical device (such as a loudspeaker). In audio, usually the name applies to a class of electronic control circuits comprised of an amplifier and a feedback path from the output signal that is compared with a reference signal. This topology creates an error signal that is the difference between the reference and the output signal. The error signal causes the output to do whatever is necessary to reduce the error to zero. A loudspeaker system with motional feedback is such a system. A sensor is attached to the speaker cone and provides a feedback signal that is compared against the driving signal to create more accurate control of the loudspeaker. Another example is Rane's servo-locked limiter™ which is an audio peak limiter circuit where the output is compared against a reference signal (the threshold setting) creating an error signal that reduces the gain of the circuit until the error is zero.