PLCProgrammableLogicControllers(PLCs),alsoreferredtoasprogrammablecontrollers,areinthecomputerfamily.Theyareusedincommercialandindustrialapplications.APLCmonitorsinputs,makedecisionsbasedonitsprogram,andcontrolsoutputstoautomateaprocessormachine.ThiscourseismeanttosupplyyouwithbasicinformationonthefunctionsandconfigurationsofPLCs.Inthe1960s,electromechanicaldevicesweretheorderofthedayasfarascontrolwasconcerned.Thesedevicescommonlyknownasrelays,werebeingusedbythethousandstocontrolmanySequential-typemanufacturingprocessesandStand-alonemachines.Manyoftheserelayswereinuseinthetransportationindustry,morespecifically,theautomothveindustry.Theserelaysusedhundredsofwiresandtheirinterconnectionstoaffectacontrolsolution.Theperformanceofarelaywasbasicallyreliableatleastasasingledevice.Butthecommonapplicationsforrelaypanelscalledfor300to500ormorerelays,andthereliabilityandmaintenanceissuesassociatedwithsuppostingthesepenelsbecameaverygreatchallenge.Costbecameantherissue,forinspiteofthelowcostoftherelayitself,theinstalledcostofthepanelcouldbequitehigh.Thetotalcostinduelingpurchasedparts,wiring,andinstallationlabor,couldrangefrom$30-$50perrelay.Tomakemattersworse,theconstantlychangingneedsofaprocesscalledforrecurringmodificationsofaconstantlychangingneedsofaprocesscalledforrecurringmodificationsofacontrolpanel.Withrelays,thiswasacostlyprospect,asitwasaccomplishedbyamajorrewiringeffortonthepanel.Inaddition,thesechangesweresometimespoorlydocumented,causingasecondshiftmaintencancenightmaremonthslater.Inlightofthis,itwasnotuncommontodiscardanentirecontrolpanelinfavorofanewonewiththeappropriatecomponentswiredinamannersuitedforthenewprocess.Addtothistheunpredictable,andpotentiallyhight,costofmaintainingthesesystemsasonhigh-volumemotorvehideproductionlines,anditbecameclearthatsomethingwasneededtoimpronethecontrolprocesstomakeitmorereliable,easiertotroubleshoot,andmoreadaptabletochangingcontrolneeds.Thatsomething,inthelate1960s,wasthefirstprogrammablecontroller.Thisfirst“revollutionary”systemwasdevelopedasaspecificresponsetotheneedsofthemajorautomotinemanufacturersintheUnitedStates.theseearlycontrollers,orprogrammablelogiccontrollers(PLC),representedthefirstsystemsthat(1)couldbeusedonthefactoryfloor,(2)couldhavethere“l(fā)ogic”changewithoutextensiverewiringorcomponentchanges,and(3)wereeasytodiagnostandrepairwhenproblemsoccurred.Itisinterestingtoobservetheprogressthathasbeenmadeinthepast15yearsinthelate1960smusthavebeenconfusingandfrighteningtoagreatnumberofpeople.Forexample,whathappenedtothehardwiredandeletromechanicaldevicesthatmaintenancepersonnelwereusedtorepairingwithhardtools?Theywerereplacedwith“computers”disquisedaselectromicsdesignedtoreplacerelays.Eventheprogrammingtoolsweredesignedtoappearasrelayequivalentpresentations.Wehavetheopportunitynowtoexaminethepromise,inretrospect,whattheprogrammablecontrollerbroughtmanufacturing?Allprogrammalecontrollersconsistofthebasicfunctionalblocks.Wewillexamineeachblocktounderstandtherelathonshiptothecontrolsystem.Firstwelookedatthecenter,asitistheheardofthesystem.Itconsistsofamicroprocessor,logicmemoryforthestorageoftheactualcontrollogic,storageorvariablememoryforusewithdatathatwillordimarilychangeasafunctionofthecontrolpropramexecution,andapowersupplytoprovideelectricalpowerfortheprocessorandmemory.NextcomestheI/Oblock.ThisfunctiontakestheconrtollevelsignalsfortheCPUandconvertsthemtovoltageandcurrentlevelssuitable.TheI/Otypecanrangefromdigital,analog,oravarietyofspecialpurpose“smart”I/Owhicharededicatedtoacertainapplicationtask.Theprogrammingisnormallyusedonlytoinitiallyconfigureandprogramasystemandisnotrequiredforthesystemtooperate.Itisalsousedintroubleshootingasystem,andcanprovetobeavaluabletoolinpinpointingtheexactcauseofaproblem.ThefielddevicesshownhererepresentthevarioussensorsandactuatorsconnectedtotheI/O.Thesearethearms,legs,eyes,andearsofthesystem,includingpushbuttons,limitswitches,proximitysuitches,photosensors,thermocouple,positionsensingdevices,andbarcodereaderasinput；andpildtlight,displaydevices,motorstarters,DCandACdrivers,solenlids,andprintersasoutputs.1.BasicPLCOperationPLCsconsistofinputmodules,aCentralProcessingUnit(CPU),andoutputmodulesorpoints.Aninputacceptsavarietyofdigitaloranalogsignalsfromvariousfielddevices(sensors)andconvertsthemintoalogicsignalthatcanbeusedbytheCPU.TheCPUmakesdecisionsandexecutescontrolinstructionsbasedonprograminstructionsinmemory.OutputmodulesconvertcontrolinstructionsfromtheCPUintoadigitaloranalogsignalthatcanbeusedtocontrolvariousfielddevices(actuators).Aprogrammingdeviceisusedtoinputthedesiredinstructions.TheseinstructionsdeterminewhatthePLCwilldoforaspecificinput.Anoperatorinterfacedeviceallowsprocessinformationtobedisplayedandnewcontrolparameterstobeentered.Pushbuttons(sensors),inthissimpleexample,connectedtoPLCinputs,canbeusedtostartandstopamotorconnectedtoaPLCthroughamotorstarter(actuator).2.Hard-WiredControlPriortoPLCs,manyofthesecontroltasksweresolvedbycontactorsorrelaycontrols.Thisisoftenreferredtoashardwiredcontrol.Circuitdiagramshadtobedesigned,electricalcomponentsspecifiedandinstalled,andwiringlistscreated.Electricalcomponentsnecessarytoperformaspecifictask.Ifanerrorwasmadethewireshadtobereconnectedcorrectly.Achangeinfunctionorsystemexpansionrequiredextensivecomponentchangesandrewiring.3.AdvantagesofPLCsThesame,aswellasmorecomplextasks,canbedonewithaPLC.WiringbetweendevicesandrelaycontactsisdoneinthePLCprogram.Hard-wiring,thoughstillrequiredtoconnectfielddevices,islessintensive.Modifyingtheapplicationandcorrectingerrorsareeasiertohandle.ItiseasiertocreateandchangeaprograminaPLCthanitistowireandrewireacircuit.FollowingarejustafewoftheadvantagesofPLCs:(1)Smallerphysicalsizethanhard-wiresolutions.(2)Easierandfastertomakechanges.(3)PLCshaveintegrateddiagnosticsandoverridefunctions.(4)Applicationscanbeimmediatelydocumented.(5)Applicationscanbeduplicatedfasterandlessexpensively.4.SiemensPLCsSiemensmakesseveralPLCproductlinesintheSIMATICS7family.Theyare:S7-200,S7-300,andS7-400.(1)S7-200TheS7-200isreferredtoasamicro-PLCbecauseofitssmallsize.TheS7-200hasabrickdesignwhichmeansthatthepowersupplyandI/Oterminalareon-board.TheS7-200canbeusedonsmaller,stand-aloneapplicationssuchaselevators,carwashes,ormixingmachines.Itcanalsobeusedonmorecomplexindustrialapplicationssuchasbottlingandpackagingmachines.(2)S7-300andS7-400TheS7-300andS7-400PLCsareusedinmorecomplexapplicationsthatsupportagreaterofI/Opoints.BothPLCsaremodularandexpandable.ThepowersupplyandI/OconsistofseparatemodulesconnectedtotheCPU.ChoosingeithertheS7-300orS7-400dependsonthecomplexityofthetaskandpossiblefutureexpansion.YourSiemenssalesrepresentativecanprovideyouwithadditionalinformationonanyoftheSiemensPLCs.5.CPUCPUisamicroprocessorsystemthatcontainsthesystemmemoryandisthePLCdecision-makingunit.TheCPUmonitorstheinputsandmakesdecisionsbasedoninstructionsheldintheprogrammemory.TheCPUperformsrelay,countingtiming,datacomparison,andsequentialoperations.6.ProgrammingDevicesTheprogramiscreatedinaprogrammingdevice(PG)andthentransferredtothe