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A '''control loop''' is the fundamental building block of [[control system]]s in general [[industrial control system]]s and [[industrial control system]]s in particular. It consists of the process sensor, the controller function, and the final control element (FCE) which controls the process necessary to automatically adjust the value of a measured [[process variable]] (PV) to equal the value of a desired [[set-point (control system)|set-point]] (SP).
==Open-loop and closed-loop ==
[[File:Electromechanicaltimer.JPG|thumb|right|An electromechanical timer, normally used for open-loop control based purely on a timing sequence, with no feedback from the process.]]
Fundamentally, there are two types of control loop: open-loop (feedforward) control, and closed loop (feedback) control.
In open-loop control, the control action from the controller is independent of the "process output" (or "controlled process variable"). A good example of this is a central heating boiler controlled only by a timer, so that heat is applied for a constant time, regardless of the temperature of the building. The control action is the switching on/off of the boiler, but the controlled variable should be the building temperature, but is not because this is open-loop control of the boiler, which does not give closed-loop control of the temperature.
In closed loop control, the control action from the controller is dependent on the process output. In the case of the boiler analogy this would include a thermostat to monitor the building temperature, and thereby feed back a signal to ensure the controller maintains the building at the temperature set on the thermostat. A closed loop controller therefore has a feedback loop which ensures the controller exerts a control action to give a process output the same as the "reference input" or "set point". For this reason, closed loop controllers are also called feedback controllers.<ref name="auto"/>
The definition of a closed loop control system according to the British Standard Institution is "a control system possessing monitoring feedback, the deviation signal formed as a result of this feedback being used to control the action of a final control element in such a way as to tend to reduce the deviation to zero."<ref>{{cite book|title= The Origins of Feedback Control|last=Mayr|first= Otto| author-link= Otto Mayr| year= 1970
|publisher =The Colonial Press, Inc.|___location= Clinton, MA USA}}</ref>
==Application==
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