PLC相关的外文英语文献与翻译.docx
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PLC相关的外文英语文献与翻译.docx
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PLC相关的外文英语文献与翻译
PLC相关的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/Omodule
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