Control loop: Difference between revisions

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{{short description|Fundamental building block of control systems}}

A '''control loop''' is the fundamental building block of [[control systemssystem]]s in general and [[industrial control systemssystem]]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.

* 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 a closed-loop control system, the control action from the controller is dependent on the desired and actual process variable. In the case of the boiler analogy, this would utilize a [[thermostat]] to monitor the building temperature, and feed back a signal to ensure the controller output maintains the building temperature close to that set on the thermostat. A closed-loop controller has a feedback loop which ensures the controller exerts a control action to control a process variable at the same value as the setpoint. For this reason, closed-loop controllers are also called feedback controllers.<ref name=":0" />

==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).

* 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.

[[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>

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

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>

|publisher =The Colonial Press, Inc.|___location= Clinton, MA USAUS}}</ref>

TheLikewise; definition"A of''Feedback aControl closed loop control system according to the British Standard InstitutionSystem'' is "a control system possessingwhich monitoringtends feedback,to themaintain deviationa signalprescribed formed as a resultrelationship of thisone feedback beingsystem usedvariable to controlanother theby actioncomparing functions of athese finalvariables controland elementusing inthe such a waydifference as toa tendmeans toof reduce the deviation to zerocontrol."<ref>{{cite book|title= The Origins of Feedback Control|last=Mayr|first= Otto| author-link= Otto Mayr| |year= 19701969|publisher =The Colonial Press, Inc.|___location= Clinton, MA US}}</ref>

|publisher =The Colonial Press, Inc.|___location= Clinton, MA USA}}</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}}</ref>

===Other examples===

[[File:Feedback loop with descriptions.svg|thumb|right|upright=1.3|A [[block diagram]] of a [[negative feedback]] [[control system]] using a [[feedback loop]] to control the process variable by comparing it with a desired value, and applying the difference as an error signal to generate a control output to reduce or eliminate the error]]

[[File:Industrial control loop.jpg|thumb|upright=1.3|Example of a single industrial control loop; showing continuously modulated control of process flow.]]

An example of a control system is a car's [[cruise control]], which is a device designed to maintain vehicle speed at a constant ''desired'' or ''reference'' speed provided by the driver. The ''controller'' is the cruise control, the ''plant'' is the car, and the ''system'' is the car and the cruise control. The system output is the car's speed, and the control itself is the engine's [[throttle]] position which determines how much power the engine delivers.

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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==