Editing Servomechanism (section)

==Applications==

===Position control===
[[Image:Pl control valve.jpg|thumb|right|Globe control [[Valve actuator|valve with pneumatic actuator]] and "positioner". This is a servo  which ensures the valve opens to the desired position regardless of friction]]
A common type of servo provides ''position control''. Commonly, servos are [[electric current|electric]], [[hydraulic]], or [[pneumatic]]. They 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 type of [[transducer]] at the output. Any difference between the actual and wanted values (an "error signal") is amplified (and converted) and used to drive the system in the direction necessary to reduce or eliminate the error.  This procedure is one widely used application of [[control theory]]. Typical servos can give a rotary (angular) or linear output.

===Speed control===
Speed control via a [[Governor (device)|governor]] is another type of servomechanism. The [[steam engine]] uses mechanical governors; another early application was to govern the speed of [[water wheels]]. Prior to World War II the [[constant speed propeller]] was developed to control engine speed for maneuvering aircraft. Fuel controls for [[gas turbine]] engines employ either hydromechanical or electronic governing.

===Others===
Positioning servomechanisms were first used in military [[fire-control]] and [[marine navigation]] equipment. Today servomechanisms are used in [[CNC|automatic machine tools]], satellite-tracking antennas, remote control airplanes, automatic navigation systems on boats and planes, and [[antiaircraft]]-gun control systems. Other examples are [[fly-by-wire]] systems in [[aircraft]] which use servos to actuate the aircraft's control surfaces, and [[radio-controlled model]]s which use RC servos for the same purpose. Many [[autofocus]] cameras also use a servomechanism to accurately move the lens. A [[hard disk drive]] has a magnetic servo system with sub-micrometer positioning accuracy. In industrial machines, servos are used to perform complex motion, in many applications.