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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>"Feedback and control systems" - JJ Di Steffano, AR Stubberud, IJ Williams. Schaums outline series, McGraw-Hill 1967</ref>
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|isbn= |pages=}}</ref>
Likewise, a "'Feedback Control System' is a system which tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control."<ref>{{cite book|title= The Origins of Feedback Control|last=Mayr|first= Otto| author-link= Otto Mayr| year= 1969|publisher =The Colonial Press, Inc.|___location= Clinton, MA USA|isbn= |pages=}}</ref>
The advanced type of automation that revolutionized manufacturing, aircraft, communications and other industries, is feedback control, which is usually ''continuous'' and involves taking measurements using a [[sensor]] and making calculated adjustments to keep the measured variable within a set range.<ref>Bennett, Stuart (1992). A history of control engineering, 1930-1955. IET. p. p. 48. {{ISBN|978-0-86341-299-8}}.</ref> The theoretical basis of closed loop automation is the discipline of [[control theory]].
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