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{{Short description|Feedback controller}}
A '''closed-loop controller''' or '''feedback controller''' is a [[control loop]] which incorporates [[feedback]], in contrast to a ''[[open-loop controller]]'' or ''[[non-feedback controller]]''.▼
[[File:Industrial control loop.jpg|thumb|300px|Example of a single industrial control loop; showing continuously modulated control of process flow.]]
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.▼
[[File:Closed Control Loop.svg|thumb|300px|Illustration of a Closed Loop Control consisting of [[Setpoint (control system)|Set Point]] <math>w(t)</math>, [[Feedback|Measured Output]] <math>y_m(t)</math>, Measured Error <math>e(t)</math>, Controller Output <math>u(t)</math>, System Input <math>u_s(t)</math>, Disturbance <math>d(t)</math>, and System Output <math>y(t)</math>]]
▲A '''closed-loop controller''' or '''feedback controller''' is a [[control loop]] which incorporates [[feedback]], in contrast to
▲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.
Control systems that include some sensing of the results they are trying to achieve are making use of feedback and can adapt to varying circumstances to some extent. [[Open-loop controller|Open-loop control systems]] do not make use of feedback, and run only in pre-arranged ways.
Closed-loop controllers have the following advantages over open-loop controllers:
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* reduced sensitivity to parameter variations
* improved reference tracking performance
* improved rectification of random fluctuations<ref>{{Cite journal |last=Cao |first=F. J. |last2=Feito |first2=M. |date=2009-04-10 |title=Thermodynamics of feedback controlled systems |url=https://link.aps.org/doi/10.1103/PhysRevE.79.041118 |journal=Physical Review E |volume=79 |issue=4 |pages=041118 |doi=10.1103/PhysRevE.79.041118|arxiv=0805.4824 }}</ref>
In some systems, closed-loop and open-loop control are used simultaneously. In such systems, the open-loop control is termed ''[[feed forward (control)|feedforward]]'' and serves to further improve reference tracking performance.
A common closed-loop controller architecture is the [[PID controller]].
[[File:Ideal feedback model.svg|thumb|right | A basic feedback loop]]
== Open-loop and closed-loop ==
{{excerpt|Control loop#Open-loop and closed-loop}}
==Closed-loop transfer function==
{{
The output of the system ''y''(''t'') is fed back through a sensor measurement ''F'' to a comparison with the reference value ''r''(''t''). The controller ''C'' then takes the error ''e'' (difference) between the reference and the output to change the inputs ''u'' to the system under control ''P''. This is shown in the figure. This kind of controller is a closed-loop controller or feedback controller.
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==PID feedback control==
{{main|PID controller}}
[[File:PID en.svg|right|thumb|400x400px|A [[block diagram]] of a PID controller in a feedback loop
A proportional–integral–derivative controller (PID controller) is a [[control loop]] [[feedback mechanism]] control technique widely used in control systems.
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