自动化英语论文——温度控制简介和pid控制器

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IntroductionstotemperaturecontrolandPIDcontrollersProcesscontrolsystem.Automaticprocesscontrolisconcernedwithmaintainingprocessvariablestemperaturespressuresflowscompositions,andthelikeatsomedesiredoperationvalue.Processesaredynamicinnature.Changesarealwaysoccurring,andifactionsarenottaken,theimportantprocessvariables-thoserelatedtosafety,productquality,andproductionrates-willnotachievedesignconditions.Inordertofixideas,letusconsideraheatexchangerinwhichaprocessstreamisheatedbycondensingsteam.TheprocessissketchedinFig.1Fig.1HeatexchangerThepurposeofthisunitistoheattheprocessfluidfromsomeinlettemperature,Ti(t),uptoacertaindesiredoutlettemperature,T(t).Asmentioned,theheatingmediumiscondensingsteam.Theenergygainedbytheprocessfluidisequaltotheheatreleasedbythesteam,providedtherearenoheatlossestosurroundings,iiithatis,theheatexchangerandpipingarewellinsulated.Inthisprocesstherearemanyvariablesthatcanchange,causingtheoutlettemperaturetodeviatefromitsdesiredvalue.[21Ifthishappens,someactionmustbetakentocorrectforthisdeviation.Thatis,theobjectiveistocontroltheoutletprocesstemperaturetomaintainitsdesiredvalue.OnewaytoaccomplishthisobjectiveisbyfirstmeasuringthetemperatureT(t),thencomparingittoitsdesiredvalue,and,basedonthiscomparison,decidingwhattodotocorrectforanydeviation.Theflowofsteamcanbeusedtocorrectforthedeviation.Thisis,ifthetemperatureisaboveitsdesiredvalue,thenthesteamvalvecanbethrottledbacktocutthestearrflow(energy)totheheatexchanger.Ifthetemperatureisbelowitsdesiredvalue,thenthesteamvalvecouldbeopenedsomemoretoincreasethesteamflow(energy)totheexchanger.Allofthesecanbedonemanuallybytheoperator,andsincetheprocedureisfairlystraightforward,itshouldpresentnoproblem.However,sinceinmostprocessplantstherearehundredsofvariablesthatmustbe maintainedatsomedesiredvalue,thiscorrectionprocedurewouldrequiredatremendousnumberofoperators.Consequently,wewouldliketoaccomplishthiscontrolautomatically.Thatis,wewanttohaveinstnnnentsthatcontrolthevariableswJtbomrequ)ringinterventionfromtheoperator.(siThisiswhatwemeanbyautomaticprocesscontrol.Toaccomplish~hisobjectiveacontrolsystemmustbedesignedandimplemented.ApossiblecontrolsystemanditsbasiccomponentsareshowninFig.2.Fig.2HeatexchangercontrolloopThefirstthingtodoistomeasuretheoutlettemperaVareoftheprocessstream.Asensor(thermocouple,thermistors,etc)doesthis.Thissensorisconnectedphysicallytoatransmitter,whichtakestheoutputfromthesensorandconvertsittoasignalstrongenoughtobetransmittertoacontroller.Thecontrollerthenreceivesthesignal,whichisrelatedtothetemperature,andcomparesitwithdesiredvalue.Dependingonthiscomparison,thecontrollerdecideswhattodotomaintainthetemperatureatitsdesiredvalue.Baseonthisdecision,thecontrollerthensendsanothersignaltofinalcontrolelement,whichinturnmanipulatesthesteamflow.Theprecedingparagraphpresentsthefourbasiccomponentsofallcontrolsystems.Theyare(1)sensor,alsooftencalledtheprimaryelement.(2)transmitter,alsocalledthesecondaryelement.(3)controller,the"brain"ofthecontrolsystem.(4)finalcontrolsystem,oftenacontrolvalvebutnotalways.Othercommonfinalcontrolelementsarevariablespeedpumps,conveyors,andelectricmotors.Theimportanceofthesecomponentsisthattheyperformthethreebasicoperationsthatmustbepresentineverycontrolsystem.Theseoperationsare(1)Measurement(M):Measuringthevariabletobecontrolledisusuallydonebythecombinationofsensorandtransmitter.(2)Decision(D):Basedonthemeasurement,thecontrollermustthendecidewhattodotomaintainthevariableatitsdesiredvalue.(3)Action(A):Asaresultofthecontroller'sdecision,thesystemmustthentakeanaction.Thisisusuallyaccomplishedbythefinalcontrol element.Asmentioned,thesethreeoperations,M,D,andA,mustbepresentineverycontrolsystem.PIDcontrollerscanbestand-alonecontrollers(alsocalledsingleloopcontrollers),controllersinPLCs,embeddedcontrollers,orsoftwareinVisualBasicorC#computerprograms.PIDcontrollersareprocesscontrollerswiththefollowingcharacteristics:ContinuousprocesscontrolAnaloginput(alsoknownas"measuremem"or"ProcessVariable"or"PV")Analogoutput(referredtosimplyas"output")Setpoint(SP)Proportional(P),Integral(I),and/orDerivative(D)constantsExamplesof"continuousprocesscontrol"aretemperature,pressure,flow,andlevelcontrol.Forexample,controllingtheheatingofatank.Forsimplecontrol,youhavetwotemperaturelimitsensors(onelowandonehigh)andthenswitchtheheateronwhenthelowtemperaturelimitsensortumsonandthenmmtheheateroffwhenthetemperaturerisestothehightemperaturelimitsensor.Thisissimilartomosthomeairconditioning&heatingthermostats.Incontrast,thePIDcontrollerwouldreceiveinputastheactualtemperatureandcontrolavalvethatregulatestheflowofgastotheheater.ThePIDcontrollerautomaticallyfindsthecorrect(constant)flowofgastotheheaterthatkeepsthetemperaturesteadyatthesetpoint.Insteadofthetemperaturebouncingbackandforthbetweentwopoints,thetemperatureisheldsteady.Ifthesetpointislowered,thenthePIDcontrollerautomaticallyreducestheamountofgasflowingtotheheater.Ifthesetpointisraised,thenthePIDcontrollerautomaticallyincreasestheamountofgasflowingtotheheater.LikewisethePIDcontrollerwouldautomaticallyforhot,sunnydays(whenitishotteroutsidetheheater)andforcold,cloudydays.Theanaloginput(measurement)iscalledthe"processvariable"or"PV".YouwantthePVtobeahighlyaccurateindicationoftheprocessparameteryouaretryingtocontrol.Forexample,ifyouwanttomaintainatemperatureof+or--onedegreethenwetypicallystriveforatleasttentimesthatorone-tenthofadegree.Iftheanaloginputisa12bitanaloginputandthetemperaturerangeforthesensoris0to400degreesthenour"theoretical"accuracyiscalculatedtobe400degreesdividedby4,096(12bits)=0.09765625degrees.[~]Wesay"theoretical"becauseitwouldassumetherewasnonoiseanderrorinourtemperaturesensor,wiring,andanalogconverter.Thereareotherassumptionssuchaslinearity,etc..Thepointbeing--with1/10ofadegree"theoretical"accuracy--evenwiththeusualamountofnoiseandotherproblems--onedegreeofaccuracyshouldeasilybeattainable.Theanalogoutputisoftensimplyreferredtoas"output".Oftenthisisgivenas0~100percent.Inthisheatingexample,itwouldmeanthevalve istotallyclosed(0%)ortotallyopen(100%).Thesetpoint(SP)issimply--whatprocessvaluedoyouwant.Inthisexample--whattemperaturedoyouwanttheprocessat?ThePIDcontroller'sjobistomaintaintheoutputatalevelsothatthereisnodifference(error)betweentheprocessvariable(PV)andthesetpoint(SP).InFig.3,thevalvecouldbecontrollingthegasgoingtoaheater,thechillingofacooler,thepressureinapipe,theflowthroughapipe,thelevelinatank,oranyotherprocesscontrolsystem.WhatthePIDcontrollerislookingatisthedifference(or"error")betweenthePVandtheSP.SETPOINTP，I，&DCONSTANTSDifferenceerrorPIDcontrolalgorithmprocessoutputvariableFig.3PIDcontrol 　　Itlooksattheabsoluteerrorandtherateofchangeoferror.Absoluteerrormeans--isthereabigdifferenceinthePVandSPoralittledifference?Rateofchangeoferrormeans--isthedifferencebetweenthePVorSPgettingsmallerorlargerastimegoeson.Whenthereisa"processupset",meaning,whentheprocessvariableorthesetpointquicklychanges--thePIDcontrollerhastoquicklychangetheoutputtogettheprocessvariablebackequaltothesetpoint.Ifyouhaveawalk-incoolerwithaPIDcontrollerandsomeoneopensthedoorandwalksin,thetemperature(processvariable)couldriseveryquickly.ThereforethePIDcontrollerhastoincreasethecooling(output)tocompensateforthisriseintemperature.OncethePIDcontrollerhastheprocessvariableequaltothesetpoint,agoodPIDcontrollerwillnotvarytheoutput.Youwanttheoutputtobeverysteady(notchanging).Ifthevalve(motor,orothercontrolelement)isconstantlychanging,insteadofmaintainingaconstantvalue,thiscouldcausemorewearonthecontrolelement.Sotherearethesetwocontradictorygoals.Fastresponse(fastchangeinoutput)whenthereisa"processupset",butslowresponse(steadyoutput)whenthePVisclosetothesetpoint.Notethattheoutputoftengoespast(overshoots)thesteady-stateoutputtogettheprocessbacktothesetpoint.Forexample,acoolermaynormallyhaveitscoolingvalveopen34%tomaintainzerodegrees(after thecoolerhasbeenclosedupandthetemperaturesettleddown).Ifsomeoneopensthecooler,walksin,walksaroundtofindsomething,thenwalksbackout,andthenclosesthecoolerdoor--thePIDcontrollerisfreakingoutbecausethetemperaturemayhaveraised20degrees!Soitmaycrankthecoolingvalveopento50,75,oreven100percent--tohurryupandcoolthecoolerbackdown--beforeslowlyclosingthecoolingvalvebackdownto34percent.