可编程序控制器毕设外文翻译

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1、TheProgrammableLogicControllerEarlymachineswerecontrolledbymechanicalmeansusingcams,gears,leversandotherbasicmechanicaldevices.Asthecomplexitygrew,sodidtheneedforamoresophisticatedcontrolsystem.Thissystemcontainedwiredrelayandswitchcontrolelements.Theseelement

2、swerewiredasrequiredtoprovidethecontrollogicnecessaryfortheparticulartypeofmachineoperation.Thiswasacceptableforamachinethatneverneededtobechangedormodified,butasmanufacturingtechniquesimprovedandplantchangeovertonewproductsbecamemoredesirableandnecessary,amor

3、eversatilemeansofcontrollingthisequipmenthadtobedeveloped.Hardwiredrelayandswitchlogicwascumbersomeandtimeconsumingtomodify.Wiringhadtoberemovedandreplacedtoprovideforthenewcontrolschemerequired.Thismodificationwasdifficultandtimeconsumingtodesignandinstalland

4、anysmall"bug"inthedesigncouldbeamajorproblemtocorrectsincethatalsorequiredrewiringofthesystem.Anewmeanstomodifycontrolcircuitrywasneeded.ThedevelopmentandtestinggroundforthisnewmeanswastheU.S.autoindustry.Thetimeperiodwasthelate1960'sandearly1970'sandtheresult

5、wastheprogrammablelogiccontroller,orPLC.Automotiveplantswereconfrontedwithachangeinmanufacturingtechniqueseverytimeamodelchangedand,insomecases,forchangesonthesamemodelifimprovementshadtobemadeduringthemodelyear.ThePLCprovidedaneasywaytoreprogramthewiringrathe

6、rthanactuallyrewiringthecontrolsystem.ThePLCthatwasdevelopedduringthistimewasnotveryeasytoprogram.Thelanguagewascumbersometowriteandrequiredhighlytrainedprogrammers.Theseearlydevicesweremerelyrelayreplacementsandcoulddoverylittleelse.ThePLChasatfirstgradually,

7、andinrecentyearsrapidlydevelopedintoasophisticatedandhighlyversatilecontrolsystemcomponent.Unitstodayarecapableofperformingcomplexmathfunctionsincludingnumericalintegrationanddifferentiationandoperateatthefastmicroprocessorspeedsnowavailable.OlderPLCswerecapab

8、leofonlyhandlingdiscreteinputsandoutputs(thatis,on-offtypesignals),whiletoday'ssystemscanacceptandgenerateanalogvoltagesandcurrentsaswellasawiderangeofvoltagelevelsandpulsedsignals