Editing Servomechanism (section)

===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.