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第1页共7页RelaysTheProgrammableLogicControllerEarlymachineswerecontrolledbymechanicalmeansusingcams,gears,leversandotherbasicmechanicaldevices.Asthecomplexitygrew,sodidtheneedforamoresophisticatedcontrolsystem.Thissystemcontainedwiredrelayandswitchcontrolelements.Theseelementswerewiredasrequiredtoprovidethecontrollogicnecessaryfortheparticulartypeofmachineoperation.Thiswasacceptableforamachinethatneverneededtobechangedormodified,butasmanufacturingtechniquesimprovedandplantchangeovertonewproductsbecamemoredesirableandnecessary,amoreversatilemeansofcontrollingthisequipmenthadtobedeveloped.Hardwiredrelayandswitchlogicwascumbersomeandtimeconsumingtomodify.Wiringhadtoberemovedandreplacedtoprovideforthenewcontrolschemerequired.Thismodificationwasdifficultandtimeconsumingtodesignandinstallandanysmallbuginthedesigncouldbeamajorproblemtocorrectsincethatalsorequiredrewiringofthesystem.Anewmeanstomodifycontrolcircuitrywasneeded.ThedevelopmentandtestinggroundforthisnewmeanswastheU.S.autoindustry.Thetimeperiodwasthelate1960'sandearly1970'sandtheresultwastheprogrammablelogiccontroller,orPLC.Automotiveplantswereconfrontedwithachangeinmanufacturingtechniqueseverytimeamodelchangedand,insomecases,forchangesonthesamemodelifimprovementshadtobemadeduringthemodelyear.ThePLCprovidedaneasywaytoreprogramthewiringratherthanactuallyrewiringthecontrolsystem.ThePLCthatwasdevelopedduringthistimewasnotveryeasytoprogram.Thelanguagewascumbersometowriteandrequiredhighlytrainedprogrammers.Theseearlydevicesweremerelyrelayreplacementsandcoulddoverylittleelse.ThePLChasatfirstgradually,andinrecentyearsrapidlydevelopedintoasophisticatedandhighlyversatilecontrolsystemcomponent.Unitstodayarecapableofperformingcomplexmathfunctionsincludingnumericalintegrationanddifferentiationandoperateatthefastmicroprocessorspeedsnowavailable.OlderPLCswerecapableofonlyhandlingdiscreteinputsandoutputs(thatis,on-offtypesignals),whiletoday'ssystemscanacceptandgenerateanalogvoltages第2页共7页andcurrentsaswellasawiderangeofvoltagelevelsandpulsedsignals.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.Inthe第3页共7页modularizedversions,capabilitycanalsobeafactor.Thisincludesfeaturessuchashighermathfunctions,PIDcontrolloopsandoptionalprogrammingcommands.Theprocessorconsistsofthemicroprocessor,systemmemory,serialcommunicationportsforprinter,PLCLANlinkandexternalprogrammingdeviceand,insomecases,thesystempowersupplytopowertheprocessorandI/Omodules.2.Mountingrack.ThisisusuallyametalframeworkwithaprintedcircuitboardbackplanewhichprovidesmeansformountingthePLCinput/output(I/O)modulesandprocessor.Mountingracksarespecifiedaccordingtothenumberofmodulesrequiredtoimplementthesystem.Themountingrackprovidesdataandpowerconnectionstotheprocessorandmodulesviathebackplane.ForCPUsthatdonotcontainapowersupply,therackalsoholdsthemodularpowersupply.Therearesystemsinwhichtheprocessorismountedseparatelyandconnectedbycabletotherack.Themountingrackcanbeavailabletomountdirectlytoapanelorcanbeinstalledinastandard19wideequipmentcabinet.MountingracksarecascadablesoseveralmaybeinterconnectedtoallowasystemtoaccommodatealargenumberofI/Omodules.3.Inputandoutputmodules.Inputandoutput(I/O)modulesarespecifiedaccordingtotheinputandoutputsignalsassociatedwiththeparti