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A characteristic of the open-loop controller is that it does not use [[feedback]] to determine if its output has achieved the desired goal of the input. This means that the system does not observe the output of the processes that it is controlling. Consequently, a true open-loop system can not engage in [[machine learning]] and also cannot correct any errors that it could make. It also may not compensate for disturbances in the system.
==Open-loop and closed-loop (feedback) control==
Fundamentally, there are two types of control loop; open loop 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. The process output is the building 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>"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|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|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 name="Bennett 1993"/> The theoretical basis of closed loop automation is [[control theory]].
== Examples ==
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