Program control

Example

Forexample,usingaheatertoadjustthetemperatureofaroomcanberegardedasaprocess,becauseitspurposeistomakeanoutput(temperatureinthisexample)reachanidealvalueWithin(forexample,20degrees),andthisoutputdoesnotchangewithtime.Inthisexample,thetemperatureisacontrolvariable,butifyouuseatemperaturemetertomeasurethetemperaturetodeterminewhethertoheat,thetemperatureisalsoinputvariable.Theidealtemperature(20degrees)isthetargetvalue.Thestateoftheheater(suchasthevalvethattheheatercontrolstheflowofhotwater)willchangewiththecontrol,whichisacontrolledvariable>.ThecontrolvariablesofprocesscontrolareusuallyprocessvariablessuchastemperatureT,pressureP,flowF,liquidlevelL,componentA,andPHvalue.

Acommoncontroldeviceisaprogrammablelogiccontroller,orPLCforshort.ThePLCreadsmanyanalogordigitalinputs,andtheinternalprogramgeneratesanalogordigitaloutputbasedontheinput.Iftheexampleinthepreviousparagraphisextended,theroomtemperatureistheinputofthePLC,andtheinternalprogramwillcomparethedifferencebetweentheinputvariablesandthetargetvalue,andthendeterminewhethertheheatingvolumeshouldbeincreasedordecreased.TheoutputofthePLCwillbeusedtochangethesizeoftheheatervalveopening.Larger(ormorecomplex)systemsmaybecontrolledbyDistributedControlSystem(DCS)orSCADA.

Classificationinpractice

Inpractice,processcontrolcanbeclassifiedinthefollowingways(however,someapplicationsusethefollowingtypesofprocesscontrolatthesametime):

• Discrete:Discreteprocesscontrolisusedinmanymanufacturing,sportsorpackagingapplications.Forexample,theautomaticassemblyofroboticarms,whichiscommoninautomobileproduction,isanexampleofdiscreteprocesscontrol.Mostoftheseapplicationsaretheproductionofindividualpartsinaproduct,suchasmetalstampingandforming.

• Batch:Someapplicationsneedtomixtherawmaterialsinaspecificamount,inaspecificwayandtime.Likemakingglue,itusuallytakesaperiodoftimetomixtheingredientsinaheatedcontainer.Similarexamplesincludefood,drinkormedicine.Thebatchprocedureisapproximatelyusedinindustrieswhoseannualoutputrangesfromafewkilogramstoseveralthousandtons.

• Continuous:Generally,aphysicalsystemcanberepresentedbysomecontinuousvariablesthatdonotchangedrasticallyovertime.Forexample,thewatertemperaturecontrolintheheaterisanexampleofcontinuousprocesscontrol.Forexample,themanufactureofpetroleum,chemicalsorplasticsbelongstocontinuousprocesscontrol.Continuousprocesscontrolisgenerallyusedinindustrieswithannualoutputrangingfromseveralthousandtonstoseveralmilliontons.

StatisticalProcessControl

StatisticalProcessControl(SPC)isaneffectivewaytomonitoraprogramwithacontrolchart.Itsspecialfeatureisthatitcanmonitorthemainoutputofaprogramanditschangesatthesametime.Collectdataatdifferentpointsintheprogramtofindtheprogramvariationthatwillaffectthequality,andtrytodetectandcorrectittoreducetherateofdefectiveproductstotheclient.Statisticalprocesscontrolemphasizesearlydetectionofproblemsandpreventionofproblems.

Relateditems

• Automaticcontrol

• Controller

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• Actuator

• Automation

• PIDController

• Feedback

• ProgrammableLogicController