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A '''closed-loop controller''' or '''feedback controller''' is a [[control loop]] which incorporates [[feedback]], in contrast to an ''[[open-loop controller]]'' or ''non-feedback controller''.
A closed-loop controller uses feedback to control [[state (controls)|states]] or [[Negative feedback#Overview|outputs]] of a [[dynamical system]]. Its name comes from the information path in the system: process inputs (e.g., [[voltage]] applied to an [[electric motor]]) have an effect on the process outputs (e.g., speed or torque of the motor), which is measured with [[sensor]]s and processed by the controller; the result (the control signal) is "fed back" as input to the process, closing the loop.<ref>{{Cite journal |last=Bechhoefer |first=John |date=2005-08-31 |title=Feedback for physicists: A tutorial essay on control |url=https://link.aps.org/doi/10.1103/RevModPhys.77.783 |journal=Reviews of Modern Physics |volume=77 |issue=3 |pages=783–836 |doi=10.1103/RevModPhys.77.783|url-access=subscription }}</ref>
In the case of linear [[feedback]] systems, a [[control loop]] including [[sensor]]s, control algorithms, and actuators is arranged in an attempt to regulate a variable at a [[Setpoint (control system)|setpoint]] (SP). An everyday example is the [[cruise control]] on a road vehicle; where external influences such as hills would cause speed changes, and the driver has the ability to alter the desired set speed. The [[PID algorithm]] in the controller restores the actual speed to the desired speed in an optimum way, with minimal delay or [[Overshoot (signal)|overshoot]], by controlling the power output of the vehicle's engine.
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