PLC相关外文英语文献与翻译.docx

PLC相关外文英语文献与翻译.docx

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PLC相关外文英语文献与翻译

Relays

TheProgrammableLogicController

Earlymachineswerecontrolledbymechanicalmeansusingcams,gears,leversandotherbasicmechanicaldevices.Asthecomplexitygrew,sodidtheneedforamoresophisticatedcontrolsystem.Thissystemcontainedwiredrelayandswitchcontrolelements.Theseelementswerewiredasrequiredtoprovidethecontrollogicnecessaryfortheparticulartypeofmachineoperation.Thiswasacceptableforamachinethatneverneededtobechangedormodified,butasmanufacturingtechniquesimprovedandplantchangeovertonewproductsbecamemoredesirableandnecessary,amoreversatilemeansofcontrollingthisequipmenthadtobedeveloped.Hardwiredrelayandswitchlogicwascumbersomeandtimeconsumingtomodify.Wiringhadtoberemovedandreplacedtoprovideforthenewcontrolschemerequired.Thismodificationwasdifficultandtimeconsumingtodesignandinstallandanysmall"bug"inthedesigncouldbeamajorproblemtocorrectsincethatalsorequiredrewiringofthesystem.Anewmeanstomodifycontrolcircuitrywasneeded.ThedevelopmentandtestinggroundforthisnewmeanswastheU.S.autoindustry.Thetimeperiodwasthelate1960'sandearly1970'sandtheresultwastheprogrammablelogiccontroller,orPLC.Automotiveplantswereconfrontedwithachangeinmanufacturingtechniqueseverytimeamodelchangedand,insomecases,forchangesonthesamemodelifimprovementshadtobemadeduringthemodelyear.ThePLCprovidedaneasywaytoreprogramthewiringratherthanactuallyrewiringthecontrolsystem.

ThePLCthatwasdevelopedduringthistimewasnotveryeasytoprogram.Thelanguagewascumbersometowriteandrequiredhighlytrainedprogrammers.Theseearlydevicesweremerelyrelayreplacementsandcoulddoverylittleelse.ThePLChasatfirstgradually,andinrecentyearsrapidlydevelopedintoasophisticatedandhighlyversatilecontrolsystemcomponent.Unitstodayarecapableofperformingcomplexmathfunctionsincludingnumericalintegrationanddifferentiationandoperateatthefastmicroprocessorspeedsnowavailable.OlderPLCswerecapableofonlyhandlingdiscreteinputsandoutputs（thatis,on-offtypesignals）,whiletoday'ssystemscanacceptandgenerateanalogvoltagesandcurrentsaswellasawiderangeofvoltagelevelsandpulsedsignals.PLCsarealsodesignedtoberugged.Unliketheirpersonalcomputercousin,theycantypicallywithstandvibration,shock,elevatedtemperatures,andelectricalnoisetowhichmanufacturingequipmentisexposed.

AsmoremanufacturersbecomeinvolvedinPLCproductionanddevelopment,andPLCcapabilitiesexpand,theprogramminglanguageisalsoexpanding.Thisisnecessarytoallowtheprogrammingoftheseadvancedcapabilities.Also,manufacturerstendtodeveloptheirownversionsofladderlogiclanguage（thelanguageusedtoprogramPLCs）.ThiscomplicateslearningtoprogramPLC'singeneralsinceonelanguagecannotbelearnedthatisapplicabletoalltypes.However,aswithothercomputerlanguages,oncethebasicsofPLCoperationandprogramminginladderlogicarelearned,adaptingtothevariousmanufacturers’devicesisnotacomplicatedprocess.MostsystemdesignerseventuallysettleononeparticularmanufacturerthatproducesaPLCthatispersonallycomfortabletoprogramandhasthecapabilitiessuitedtohisorherareaofapplications.

Itshouldbenotedthatinusage,aprogrammablelogiccontrollerisgenerallyreferredtoasa“PLC”or“programmablecontroller”.Althoughtheterm“programmablecontroller”isgenerallyaccepted,itisnotabbreviated“PC”becausetheabbreviation“PC”isusuallyusedinreferencetoapersonalcomputer.Aswewillseeinthischapter,aPLCisbynomeansapersonalcomputer.

Programmablecontrollers（theshortenednameusedforprogrammablelogiccontrollers）aremuchlikepersonalcomputersinthattheusercanbeoverwhelmedbythevastarrayofoptionsandconfigurationsavailable.Also,likepersonalcomputers,thebestteacherofwhichonetoselectisexperience.Asonegainsexperiencewiththevariousoptionsandconfigurationsavailable,itbecomeslessconfusingtobeabletoselecttheunitthatwillbestperforminaparticularapplication.

ThetypicalsystemcomponentsforamodularizedPLCare:

1.Processor.

Theprocessor（sometimescallaCPU）,asintheselfcontainedunits,isgenerallyspecifiedaccordingtomemoryrequiredfortheprogramtobeimplemented.Inthemodularizedversions,capabilitycanalsobeafactor.Thisincludesfeaturessuchashighermathfunctions,PIDcontrolloopsandoptionalprogrammingcommands.Theprocessorconsistsofthemicroprocessor,systemmemory,serialcommunicationportsforprinter,PLCLANlinkandexternalprogrammingdeviceand,insomecases,thesystempowersupplytopowertheprocessorandI/Omodules.

2.Mountingrack.

3.Inputandoutputmodules.

Inputandoutput（I/O）modulesarespecifiedaccordingtotheinputandoutputsignalsassociatedwiththeparticularapplication.Thesemodulesfallintothecategoriesofdiscrete,analog,highspeedcounterorregistertypes.

DiscreteI/Omodulesaregenerallycapableofhandling8or16and,insomecases32,on-offtypeinputsoroutputspermodule.Modulesarespecifiedasinputoroutputbutgenerallynotbothalthoughsomemanufacturersnowoffermodulesthatcanbeconfiguredwithbothinputandoutputpointsinthesameunit.ThemodulecanbespecifiedasAConly,DConlyorAC/DCalongwiththevoltagevaluesforwhichitisdesigned.

Analoginputandoutputmodulesareavailableandarespecifiedaccordingtothedesiredresolutionandvoltageorcurrentrange.Aswithdiscretemodules,thesearegenerallyinputoroutput;howeversomemanufacturersprovideanaloginputandoutputinthesamemodule.AnalogmodulesarealsoavailablewhichcandirectlyacceptthermocoupleinputsfortemperaturemeasurementandmonitoringbythePLC.

PulsedinputstothePLCcanbeacceptedusingahighspeedcountermodule.Thismodulecanbecapableofmeasuringthefrequencyofaninputsignalfromatachometerorotherfrequencygeneratingdevice.Thesemodulescanalsocounttheincomingpulsesifdesired.Generally,bothfrequencyandcountareavailablefromthesamemoduleatthesametimeifbotharerequiredintheapplication.

Registerinputandoutputmodulestransfer8or16bitwordsofinformationtoandfromthePLC.Thesewordsaregenerallynumbers（BCDorBinary）whicharegeneratedfromthumbwheelswitchesorencodersystemsforinputordatatobeoutputtoadisplaydevicebythePLC.

OthertypesofmodulesmaybeavailabledependinguponthemanufacturerofthePLCandit'scapabilities.Theseincludespecializedcommunicationmodulestoallowforthetransferofinformationfromonecontrollertoanother.OnenewdevelopmentisanI/OModulewhichallowstheserialtransferofinformationtoremoteI/Ounitsthatcanbeasfaras12,000feetaway.

4.Powersupply.

Thepowersupplyspecifieddependsuponthemanufacturer'sPLCbeingutilizedintheapplication.Asstatedabove,insomecasesapowersupplycapableofdeliveringallrequiredpowerforthesystemisfurnishedaspartoftheprocessormodule.Ifthepowersupplyisaseparatemodule,itmustbecapableofdeliveringacurrentgreaterthanthesumofallthecurrentsneededbytheothermodules.ForsystemswiththepowersupplyinsidetheCPUmodule,theremaybesomemodulesinthesystemwhichrequireexcessivepowernotavailablefromtheprocessoreitherbecauseofvoltageorcurrentrequirementsthatcanonlybeachievedthroughtheadditionofasecondpowersource.Thisisgenerallytrueifanalogorexternalcommunicationmodulesarepresentsincetheserequire±DCsupplieswhich,inthecaseofanalogmodules,mustbewellregulated.

5.Programmingunit.

Theprogrammingunitallowstheengineerortechniciantoenterandedittheprogramtobeexecuted.Init'ssimplestformitcanbeahandhelddevicewithakeypadforprogramentryandadisplaydevice（LEDorLCD）forviewingprogramstepsorfunctions,asshown.Moreadvancedsystemsemployaseparatepersonalcomputerwhichallowstheprogrammertowrite,view,editanddownloadtheprogramtothePLC.ThisisaccomplishedwithproprietarysoftwareavailablefromthePLCmanufacturer.ThissoftwarealsoallowstheprogrammerorengineertomonitorthePLCasitisrunningtheprogram.Withthismonitoringsystem,suchthingsasinternalcoils,registers,timersandotheritemsnotvisibleexternallycanbemonitoredtodetermineproperoperation.Also,internalregisterdatacanbealteredifrequiredtofinetuneprogramoperation.Thiscanbeadvantageouswhendebuggingtheprogram.Communicationwiththeprogrammablecontrollerwiththissystemisviaacableconnectedtoaspecialprogrammingportonthecontroller.Connectiontothepersonalcomputercanbethroughaserialportorfromadedicatedcardinstalledinthecomputer.

AProgrammableControllerisaspecializedcomputer.Sinceitisacomputer,ithasallthebasiccomponentpartsthatanyothercomputerhas;aCentralProcessingUnit,Memory,InputInterfacingandOutputInterfacing.

TheCentralProcessingUnit（CPU）isthecontrolportionofthePLC.Itinterpretstheprogramcommandsretrievedfrommemoryandactsonthosecommands.InpresentdayPLC'sthisunitisamicroprocessorbasedsystem.TheCPUishousedintheprocessormoduleofmodularizedsystems.

Memoryinthesystemisgenerallyoftwotypes;ROMandRAM.TheROMmemorycontainstheprograminformationthatallowstheCPUtointerpretandactontheLadderLogicprogramstoredintheRAMmemory.RAMmemoryisgenerallykeptalivewithanon-boardbatterysothatladderprogrammingi