外文翻译-DCS与PLC之间的关系：讨论新兴的工厂控制系统的发展趋势

英文原文CementingtherelationshipbetweenDCSandPLC:Areviewofemergingtrendsinplantcontrolsystems.Author:MoinShaikhCompany:SiemensEnergy&Automation,Inc.AbstractThispaperexploresthechangingrolesoftraditionaldistributedcontrolsystems(DCS)andprogrammablelogiccontrollers(PLC)usedtoautomatecementmanufacturingprocesses.Thetwotechnologiesinitiallyservedtwodifferentcontrolrequirements.However,improvementsinmicroprocessor-basedcontrollerscreatedconditionsfortwotechnologiestomerge.Theshifttowardcommercial,off-the-shelfautomationtechnology,software-basedcontrolverseshardcontrolanduseofnon-proprietarynetworkshascreatedanewclassofsystemscalledhybridprocessautomationsystems.Inaddition,theroleoftheplantcontrolsystemhasbeenexpandedfromjustprocessvisualizationandcontroltoincludeprocessoptimization,plantassetmanagement,energymanagement,andinventorycontrol.Cementplantscannowusetheprocesscontrolsystemstoimproveoperationalandenergyefficiency.ControlsystemvendorshaveadopteduniquetermstodifferentiatetheirsystemsfromthetraditionaldefinitionofaDCSorPLC.SomehaveexpandedtheirprocessontrolsystemstoofferverticalintegrationwithITsystemsandhorizontalintegrationwithintelligentdrives,motors,processinstrumentationanddiscretecontroltechnologies.Thispaperaddressesanumberofrequirementsplacedonthecementautomationsystemsasaresultofthesedevelopmentsandidentifiestechnologiesthatofferthebestsolutiontomeetthechallengesfacedbycementproducers.IntroductionTheprocessofmanufacturingcementhasgonethroughmanychangesovertheyears.Thewideacceptanceofthedryprocessofcementmakingratherthanthewetandpre-calcinationsprocessesareamongthesechanges.Inanycase,themanufacturingprocesscontinuestohavethemostsignificant,quantifiableinfluenceonoverallcementqualityandefficiency,sustainability,andenvironmentalimpactoftheplant.Theautomationrequirementsofthecementindustryareunique,eventhoughonthesurfacetheymayseemsimilartootherindustrialprocesses.Legacyautomationsystemsinstalledinexistingcementplantshavenotkeptpacewithnewtechnologiesnowavailabletotheprocessindustry.Thegrowingsizeandcomplexityofthecementmanufacturingprocesshaveplacednewdemandsonautomationsystemvendors.Byunderstandingtheserequirementsandthecontrolandautomationtechnologiesavailabletoday,wecaneffectivelyselectthebestplatformfortheautomationofapplicationsinthecementindustry.CementPlantAutomationRequirementsWhatisuniqueaboutautomatingthecementmanufacturingprocess?Cementproductioninvolvesthetransformationofmaterialthroughcrushingandgrinding.Limestoneistheprimaryingredient.Therawmealundergoesphysicalchangethroughthecalcinationsprocessinthekilnwhereitisheatedupto1600oC.Theautomationsystemsconveymaterialthroughdifferentstagesoftheprocessandroutes.Theautomationsystemmanagesthestartingandstoppingsequencesoftheconveyingandmaterialhandlingsystems.Cementproductioncanbeconsideredasacombinationofindividualprocessessuchasblendingandpreparationofrawmeal,temperatureandspeedcontrolofthekilnandkilnfeedcontrol.Clinkercoolingprocessrecoverstheheatandlowersthetemperatureoftheclinkersoitcanbestored.Speedcontrolofpre-heaterfanandtheexhaustfanmaintainnegativepressureinsidethekilnhoodandensurethecorrectflowofhotgases.FeedcontroloftheFinishMillstomaintainthroughputandblendingofthecementwithadditivesintheFinishMillstoproducedifferenttypesofcement.Basictemperature,airflowandpressurecontrolareverycriticaltotheproductionprocessandareappliedatvariousplacesintheprocess.Thecementindustryalsouseslargemotorsanddrivesthatcontrollargepiecesofmechanicalequipment.Interlockrequirementsforthistypeofequipmentareuniquewhencomparedtotheequipmentusedintypicalprocessindustries,suchaschemical.Themotorsandmechanicalequipmentneedstobeprotectedfromoperatormistakesandincorrectoperation.Forthispurposetwotypesofinterlocksareprogrammedinthecontrolsystems.Thefirsttypeiscalledsoftinterlocks,implementedusinglogicinthecontrolsystem,andsecondtypeofinterlocksareimplementedusinghardwiringandarecalledhardinterlocks.Therearealsodifferenttypesofinterlockseffectiveindifferentmodesofoperationofthemechanicalequipment.Improvingenergyefficiencyinthecementproductionisabigchallengeforcementindustrysinceitisabigconsumerofelectricity,oil,coal,andnaturalgas.Thecementindustryisalsothethirdlargestemitterofgreenhousegasesglobally.DivisionoftheCementPlantfromControlsPerspectiveAcementplantscontrolsystemcanbedividedintothreesectionsorprocesses-front-endwherematerialisprepared,mainsection,andbackendforpackingstorageanddispatching.Therearealsoauxiliarysystemsandutilitiesprovidingessentialmaterialsandservicestothemainprocess.Thefrontendprocessescanalsobecalledthepreparationphase.Inthepreparationphasetherawmaterial,whichismainlylimestone,isgrindedandcombinedwithothermaterials,suchasshaleandirontogiveitspecificchemicalcomposition.Afterthefinalmixisprepared,itisstoredinarawmealsilountilitcanbefedintothepre-heater.Incoal-firedkilnsthereisacoalmill,anditpulverizesthecoalthatisfedintothekilnusingtheburner.Rawmealisfedintothepre-heaterwhereitgraduallygainstemperatureasitentersthekiln.Thefinepowderofrawmealduetotherotationofthekiln,andtheheattransformsintocirculargreystonescalledclinker.Clinkerispassedintoaclinkercoolerwhereitiscooledtoatemperaturewhereyoucanactuallyholditintoyourhand.Insidethecooler,heatfromthehotclinkerisrecuperatedandfedbackintotheprocess.Theclinkerisstoredintheclinkersilo.Theback-endprocessesmainlyinvolvethefinishmillsareas.Inthefinishmillsection,therawclinkerisgroundedfinelyandismixedwithdifferentadditivestoproducedifferentkindsofcementbasedoncustomerrequirements.Thismaterialisfinallystoredindifferentsilos.Thematerialcanbebaggedorloadedontotrucks,railscarsorbargesforbulkdistribution.OperationsandPlantControlThejobofthecontrolsystemoperatoristoensurethattheplantkeepsrunningatmaximumthroughputandfuelefficiency.Operatorscontrolthesequenceofbeltsandselecttheroutesformaterialtransport.Theseconveyorbeltsmovematerialfromoneprocesssectiontothenext.Operatorstakeactionsonalarmsrelatedtotheequipmenttheycontrolsuchaslargedrivesandmotors.Interlocksprotecttheequipmentfromdamageandarevisualizedasalarmsfortheoperators.Operatorsalsotakeactiononalarmsthatdisruptthemainprocessandusetrendingdatatoensurethatkeyprocessvariablesarewithintheiroperationallimits.Operatorsusemainplantcontrolsystemtocontrolkilnfeed,aswellasfuelinput.Afuelinjectionsystemmayusecoal,oil,oracombinationofcombustiblematerials.Sometimesadvancedprocesscontrol(APC)overseesalltheprocessvariables.APCalsoprovidesoptimizedcontrolthroughschemesbasedonMPC,fuzzylogic,orneuralnetworks.Similarcontrolsschemesareusedforrawmillandcoalmilloperations.Cementmillareasincludehydraulicsystemsforballmills,millfeedingcircuits,blendingsystemstoproducedifferenttypeofcement,materialseparatorsystems,andsilodischargesystems.Cementplantsalsohavespecializedsystemsforwaterinjectioninballmills,coolerhydraulicgratecontrolsystems,circularStackerandReclaimersystems,longitudinalStackerandReclaimer,andlabautomationsystems.Additionally,moreandmoreplantsareusingnonconventionalfuelssuchastiresandindustrialandresidentialwasteproductsinadditiontooil,gas,andcoal.OverallPlantControlStrategiesTherearetwostrategiestoimplementoverallcontrolincementplants.ThedistributedapproachusesindividualPLCstocontrolauxiliarytasks,includinglabautomation,coolergratecontrol,hydraulicpowerpacks,ReclaimerandStackers,watertreatment,compressedair,andbaghouses.TheseauxiliarysystemsareusuallycontrolledthroughdedicatedPLCssuppliedwithmechanicalequipment.Theseauxiliarysystemsare,inturn,controlledbytheplantsmaincontrolsystemthatistypicallyaDCStypesystem.ThemainplantcontrolsysteminterfacestotheseindividualPLCsvianetworkinterfacesorhardwiringandprovidestheoverallcommandandprocesscontrolenvironment.Thiscontrolapproachisusedwhentheplantcontrolsystemcannotmeetspecificauxiliaryapplicationsrequirements.Thisapproachisalsopartlydrivenbytheneedofequipmentsupplierstoguaranteeperformance.Theymustprovidecontrolsystemswiththeirequipmentsothatthecontrollogicfunctionsareproperlyimplementedtoensurecorrectoperationandcontroloftheequipment.Thesuppliersusuallydeveloptheirsolutionsbasedontheproductcapabilitiesofasingleautomationvendor.ItiscosteffectivefortheprocessOEMtouseasinglePLCvendor,becauseitdoesnotrequirethemtodevelop,maintain,andsupporttheirprogramsonmultiplePLCplatforms.Themaindisadvantagewiththisapproachisthatthereisalwaysalevelofambiguitybetweentheauxiliarysystemsandthemainplantcontrol.Onedisadvantageisthatmaintenanceandcontrolpersonneltreatthesesystemsasblackboxes,sinceonlytheequipmentvendorknowsthewhyandhowofthelogicinsidethePLC,orifthecontrolisbasedonadedicatedcontroller.Inmostcasesthetoolsandsoftwareusedtoimplementthesesystemsareverydifferentfrommainplantcontrolsoftware,requiringthemaintenancepeopletolearnseveraldifferentprogrammingpackagesfromdifferentvendors.Managingthedifferentprogramsandversionsofsoftwarepackagesandkeepingthemupdatedoverthelifecycleoftheplantisverycostly.Inthecentralizedcontrolapproach,thecontrolsystemmustmeettheoverallplantcontrolrequirements,suchasplantassetmanagement,energymanagementandadvancedprocesscontrol,andinterfacewithCMMSandERPsystems.Thesystemmustalsobeabletomeettheprocesscontrolrequirementsoftheanalogcontrolandbeabletoexecuteportionsofthelogicinafastercycletomeetthespeedcontrolrequirementsofthesubcontrols.Thesystemmustbeabletosupportvarioustypesofprogramminglanguagesincludingfunctionblocks,structuredprogramminglanguages,andladderlogic.ThesystemsthathavethesecapabilitiescombinethefeaturesofbothDCSandPLC,sometimescalledhybridcontrolsystems.Inthecentralapproach,themainplantcontrolsystemprovidestheoveralllogicforthemainprocessandtheauxiliarysystems.TheauxiliarysystemshaveremoteI/Osthatareconnectedtothemainplantcontrolsystem;thelogicforthecontroloftheauxiliarysystemresidesinthecentralcontroller.PLC-ProgrammableLogicControllersProgrammablelogiccontrollersinitiallyemergedasareplacementoftherelaycontrolsystems.BeforePLC,control,sequencing,andsafetyinterlocklogicformanufacturingprocesseswasaccomplishedusinghundredsofrelays,camtimers,drumsequencers,anddedicatedclosed-loopcontrollers.Theprocessforupdatingsuchfacilitiestomakechangestotheprocesswasverytimeconsumingandexpensive,astherelaysystemsneededtoberewiredbyskilledelectricians.PLCseliminatedtheneedtophysicallyrewiretherelaysystemsasitcannowbedonebymodifyingthesoftware.HencesoftwareinthePLCbecamethebrainofthemanufacturingprocess.ThebasicconceptsofPLCprogrammingarecommontoallmanufacturers.DifferencesinI/Oaddressing,memoryorganization,andinstructionsetsmeanthatPLCprogramsareneverperfectlyinterchangeablebetweendifferentvendors.Evenwithinthesameproductlineofasinglemanufacturer,differentmodelsofferdifferentlevelsofprogrammingcapability,resultinginprogramsbeingnon-interchangeable.TheIEC61131-3specificationisanefforttostandardizethePLCprogramminglanguagesanddefinecommoninstructionsetsandprogramminglanguages.IEC61131-3currentlydefinesfiveprogramminglanguagesforprogrammablecontrolsystems:FBD(functionblockdiagram),LAD(ladderdiagram),SCL(structuredtext,similartothePascalprogramminglanguage),STL(instructionlist,similartoassemblylanguage),andSFC(sequentialfunctionchart).Thesetechniquesemphasizelogicalorganizationofoperationswithaprogram.InPLC-basedsystems,logiccontrolandprogrammingworksinthesamewaysasrelaylogic.PLCsareprogrammedinladderlogic,whichstronglyresemblesaschematicdiagramofrelaylogic.ModernPLCscanbeprogrammedinavarietyofways,fromladderlogic,tomoretraditionalstructuredprogramminglanguages,Candfunctionblocks.Itisnowonderthatladderlogicisthepreferableformofprogramminglanguageforelectriciansandtechniciansresponsibleforthemaintenanceofmanufacturingfacilities.Sincenotmuchchangeinthethinkingprocessisrequiredtoprogramladderlogic,itwasthenaturalfirststepfortheelectricianswhowerefamiliarandhadexperienceworkingwithrelays.TheelementsusedinPLCprogramming,suchasinputs,arecalledcontacts.Outputsarecalledcoils,asinrelaycoil.ThiskindoflanguageisfoundinalmostallthePLCprogrammingsoftware.Programsaredividedintomultiplerungsofladder.Eachrungofladderlanguagetypicallyhasonecoilatthefarright.Somemanufacturersmayallowmorethanoneoutputcoilonarung.-()-aregularcoil,truewhenitsrungistrueornormallyopenrelaycontactclose-()-anotcoil,falsewhenitsrungistrueornormallycloserelaycontactopens-Aregularcontact,truewhenitscoilistrue(normallyfalse)-Anotcontact,falsewhenitscoilistrue(normallytrue)Thecoilmayrepresentaphysicaloutputthatoperatesadevice,suchasamotorcontactor,connectedtothePLCoutput.Itmayalsorepresentaninternalstoragebittouseelsewhereintheprogram.ThelimitationwiththePLCwayofthinkingisthatitgivesaprogrammeraverylimitedviewoftheoverallplantsequencecontrol.Italsomakesthelogicverycomplicatedtotroubleshootifalargenumberofcontactsandcoilsareused.Thisisalsoknownasthebottomsupapproachtoprogramming.Itisveryeasytolookattheindividualpiecesoftheequipmentandtheirinterlocks.However,itisverydifficulttounderstandthecompleteplantcontrollogic,becausetheladderlogicquicklybecomesverycomplicatedasthenumberofdevicesandtheirinteractionsincrease.Ladderlogicdoesnotprovidethekindofinterfacethataprocesscontrolengineercaneasilyunderstand,becausehisviewoftheplantisintheformofgroups,routes,andindividualobjectssuchasmotors,PID,anddiverters.Drumsequencersaresometimesprogrammedinladderlogictoimplementprocesssequences.Drumlogiccanbeimplementedinmanydifferentways,dependingonhowaparticularprogrammerapproachesaproblem.Keepingthedrumlogicseparatefromtherestoftheinterlockingandcontrolschemeisverylaterpointbecomesverydifficult,especiallyforapersonwhoisnotfamiliarwiththefunctionaldetailsoftheparticularequipmentandprotectioninterlocks.Mosttechniciansandmaintenancepeopledonotunderstandwhyaparticularpieceoflogicwasimplemented.Theywill,however,usesoftwareandhardwarejumperstobypassthelogic.Thiscandamagetheequipmentandposeserioussafetyriskstoworkers.Someequipmentcontrolprogramsforequipment(circularstackersandgratehydrauliccontrol)areverycomplicatedandrequiremathematicalcomputations.Thesecalculationsmustbeperformedinthespecificamountoftime(10msorless).Suchstringentexecutionrequirescontrollerswithhighspeedcomputationcapability.Oncethesesub-controlsareprogrammed,theirlogicrarelyneedstobemodifiedorchanged;onlymodificationintheprocessparametersisrequired.DCS-DistributedControlSystemsDCSevolvedwithintheprocesscontrolindustry.DCSareknownforregulatorycontrolcapabilityandredundancy.TheDCSarchitectureswerecentrallyorientatedwithmaincontrolroomasameetingpointforlongcablesfortheinputandoutput.Theywerebasedonproprietarycomponents-operatingsystems,networks,hardware,andconfigurationtoolswithallthehardwarelocatedcentrallyinacontrolledenvironment.TraditionalDCSwerenotdesignedwithopeninterfacesthusrestrictingthecommunicationwithinthesystemboundaries.Communicationinterfaceswithothervendorsequipmentrequiresexpensiveandtimeconsuminghardwareandsoftwaredevelopmentprocess.TheearlierdaysofprocesscontrolwasmainlyisolatedtoacertainnumberofloopsorPIDcontrollers(ProportionalIntegralandDerivative)tocontroltemperature,pressure,steam,andflow.AstheelectronicstechnologydevelopedandthePLC-basedsystemsbecamepervasive,itbecomespossibletoprogramanumberofPIDloopsinasinglecontrollerinarackawayfromthefield.ThePIDcontrollersthatwereinitiallymadeofdedicatedelectronichardwareintheearlierDCSsystemscannowbeimplementedinacost-effectivewayinasingle,microprocessor-basedcontroller.Someofthefieldbusbasedtechnologies,suchasFoundationFieldbus,stillsupporttheconceptof“ControlintheField,”whereasingle,independentcontrolloopcanbeimplementedinthefieldawayfrommaincontroller.TheprogrammingenvironmentforthetraditionalDCSwasbasedonfunctionblockscommonlyunderstoodintheprocesscontrolindustry.Thereisnotabigneedtoprogramthecomplicatedlogicoperationsthatrequiredhighspeedprogramexecution,sincemostofthetemperatureandpressureloopsresponsetimeisin100msorhigher.Inprocessplants,motorsanddriveswereprogrammedandcontrolledviathePLC.Asthemicroprocessorbecamefasterandcheaper,itbecamepossibletoimplementsomeofthePIDloopsinthesoftware,sincesoftware-basedcontrollerswereabletomeettheinputtooutputcycletimerequirementsoftheoverallprocesscontrolloop.Highspeedcontrolloops,suchasthoseusedinmetalprocessing,stillusededicatedcontrolhardwareduetoperformancereasons,butmostoftheprocessloopsinacementplantcanbeprogrammedinasinglecontroller/PLCusingsoftware.UnderstandingthePLCvs.DCSdivideNowletuslookinmoredetailatthekeyattributesofthePLCvs.theDCStypeofcontrol.Manufacturingapplicationsinvolveassemblyofspecificitems-thingsaPLCwasoriginallyusedinthiscase.Atypicalcharacteristicofthistypeofprocessisthattheoperatorcanusuallymonitorthethingsvisuallyastheyprogressthroughthemanufacturingline.Themanufacturingprocessisverylogiccontrolintensive,oftenwithhigh-speedrequirements.Higherspeedmeansmorethingscanbeproducedpershiftwhichtranslatesintomoreproduction.ThistypeofprocessisoftencontrolledbyaPLCandHumanMachineInterface(HMI)combination.Theprimarygoalistomaketheequipmentperformcertainsteps,andthecontrolisonlyneededtomanagetheexceptiontothesetasks;insuchcases,PLCistherightchoice.Processautomationapplicationstypicallyinvolvethetransformationofrawmaterialsthroughthereactionofcomponentchemicalsortheintroductionofphysicalchangestoproduceanew,differentproduct-stuff.Theplantiscomposedofoneormoreprocessunitoperationsconnectedtogether.Onekeycharacteristicisthattheoperatorcantseetheproduct.Thereisusuallyalargeamountofsimpletocomplexanalogcontrol(i.e.PIDorloopcontrol),andtheresponsetimeisnotthatfast(100msorgreater).ThistypeofprocessisoftencontrolledbyaDCS,althoughtheanalogcontrolcapabilityofaPLCmaybemorethanadequateinsomeapplications.Adeterminingfactorintheselectionprocessisoftenhowlargeinscopethecontrolapplicationis(i.e.plant-wideversussingleunitandnumberofI/Opoints).SomeoftheexamplesofPLCtypecontrolincementarecoolergratehydrauliccontrol,hydraulicsystemforballmills,circularStacker,andRecalimer.Thevalueoftheproductandthecostofdowntimeisanotherwayoflookingattheautomationrequirements.Thecostofdowntimeisthelossofproduction,butitcanalsoleadtoothercosts.Forexample,inthePyroprocessingsection,therestartofkilnrequiresmorefueltobri