Control loop: Difference between revisions

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A '''control loop''' is the fundamental building block of [[control system]]s in general 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 (SP).

There are two common classes of control loop: open loop and closed loop.

There are two common classes of control loop: open loop and closed loop. In an open-loop control system, the control action from the controller is independent of the process variable. An example of this is a central heating boiler controlled only by a [[timer]]. The control action is the switching on or off of the boiler. The process variable is the building temperature. This controller operates the heating system for a constant time regardless of the temperature of the building.

* In aan closedopen-loop control system, the control action from the controller is dependentindependent onof the desired and actual process variable. InAn the caseexample of thethis is a central heating boiler analogy,controlled thisonly would utilizeby a [[thermostattimer]]. toThe monitorcontrol theaction buildingis temperature,the andswitching feedon backor aoff signalof tothe ensureboiler. theThe controllerprocess outputvariable maintainsis the building temperature close to that set on the thermostat. A closed-loopThis controller hasoperates athe feedbackheating loop which ensures the controllersystem exertsfor a controlconstant actiontime toregardless control a process variable atof the sametemperature value asof the setpointbuilding. For this reason, closed-loop controllers are also called feedback controllers.<ref name=":0" />

* In closeda closed-loop control system, the control action from the controller is dependent on the desired and actual process outputvariable. In the case of the boiler analogy, this would includeutilize a [[thermostat]] to monitor the building temperature, and thereby feed back a signal to ensure the controller output maintains the building attemperature theclose temperatureto that set on the thermostat. A closed -loop controller therefore has a feedback loop which ensures the controller exerts a control action to givecontrol a process outputvariable at the same value as the "reference input" or "set point"setpoint. For this reason, closed -loop controllers are also called feedback controllers.<ref name="auto:0">"Feedback and control systems" - JJ Di Steffano, AR Stubberud, IJ Williams. Schaums outline series, McGraw-Hill 1967</ref>

==Open-loop and closed-loop ==

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Fundamentally, there are two types of control loop: ''[[open-loop control]]'' (feedforward), and ''[[closed-loop control]]'' (feedback).

There are two common classes of control loop: open loop and closed loop.* In an open-loop control system, the control action from the controller is independent of the "process output" (or "controlled process variable"). AnA 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 or /off of the boiler., Thebut processthe controlled variable isshould be the building temperature., Thisbut controlleris operatesnot thebecause heatingthis systemis foropen-loop acontrol constantof timethe regardlessboiler, ofwhich thedoes temperaturenot give closed-loop control of the buildingtemperature.

[[File:Electromechanicaltimer.JPG|thumb|upright|An electromechanical timer, normally used for open-loop control based purely on a timing sequence, with no feedback from the process]]

* In open-closed loop control, the control action from the controller is independentdependent ofon the "process output". (orIn "controlledthe processcase variable").of Athe goodboiler example ofanalogy, this iswould include a centralthermostat heatingto boilermonitor controlledthe onlybuilding bytemperature, aand timer,thereby sofeed thatback heata issignal appliedto forensure athe constantcontroller time,maintains regardlessthe ofbuilding at the temperature ofset on the buildingthermostat. TheA closed loop controller therefore has a feedback loop which ensures the controller exerts a control action isto thegive switchinga on/offprocess ofoutput the boiler,same butas the controlled"reference variableinput" shouldor be the"set buildingpoint". temperatureFor this reason, butclosed isloop notcontrollers becauseare thisalso iscalled open-loopfeedback controllers.<ref name="auto">"Feedback and control ofsystems" the- boilerJJ Di Steffano, whichAR doesStubberud, notIJ giveWilliams. closed-loopSchaums controloutline ofseries, theMcGraw-Hill temperature.1967</ref>

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">"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 Standards 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

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The controller function can be a discrete controller or a function block in a computerised control system such as a distributed control system or a [[programmable logic controller]]. In all cases, a control loop diagram is a very convenient and useful way of representing the control function and its interaction with plant. In practice at a process control level, control loops are normally abbreviated using standard symbols in a [[Piping and instrumentation diagram]], which shows all elements of the process measurement and control based on a [[process flow diagram]].<ref>{{cite web|title=Piping and instrumentation diagram P&ID|url=http://processflowsystems.com/piping-instrumentation-diagram-pid/|website=Process Flow Systems|accessdate=16 September 2017|archive-date=20 July 2020|archive-url=https://web.archive.org/web/20200720181608/http://processflowsystems.com/piping-instrumentation-diagram-pid/|url-status=dead}}</ref>

At a detailed level, the control loop connection diagram is created to show the electrical and pneumatic connections. This greatly aids diagnostics and repair, as all the connections for a single control function are on one diagram.

==Loop and control equipment tagging==