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1自动化081Y10208110王嘉浩PLCs--Past,PresentandFutureEveryoneknowsthere'sonlyoneconstantinthetechnologyworld,andthat'schange.ThisisespeciallyevidentintheevolutionofProgrammableLogicControllers(PLC)andtheirvariedapplications.Fromtheirintroductionmorethan30yearsago,PLCshavebecomethecornerstoneofhundredsofthousandsofcontrolsystemsinawiderangeofindustries.Atheart,thePLCisanindustrializedcomputerprogrammedwithhighlyspecializedlanguages,anditcontinuestobenefitfromtechnologicaladvancesinthecomputerandinformationtechnologyworlds.Themostprominentofwhichisminiaturizationandcommunications.TheShrinkingPLCWhenthePLCwasfirstintroduced,itssizewasamajorimprovement-relativetothehundredsofhard-wiredrelaysandtimersitreplaced.AtypicalunithousingaCPUandI/Owasroughlythesizeofa19televisionset.Throughthe1980sandearly1990s,modularPLCscontinuedtoshrinkinfootprintwhileincreasingincapabilitiesandperformance(seeDiagram1fortypicalmodularPLCconfiguration).Inrecentyears,smallerPLCshavebeenintroducedinthenanoandmicroclassesthatofferfeaturespreviouslyfoundonlyinlargerPLCs.ThishasmadespecifyingalargerPLCjustforadditionalfeaturesorperformance,andnotincreasedI/Ocount,unnecessary,aseventhoseinthenanoclassarecapableofEthernetcommunication,motioncontrol,on-boardPIDwithautotune,remoteconnectivityandmore.PLCsarealsonowwell-equippedtoreplacestand-aloneprocesscontrollersinmanyapplications,duetotheirabilitytoperformfunctionsofmotioncontrol,dataacquisition,RTU(remotetelemetryunit)andevensomeintegratedHMI(humanmachineinterface)functions.Previously,thesefunctionsoftenrequiredtheirownpurpose-builtcontrollersandsoftware,plusaseparatePLCforthediscretecontrolandinterlocking.TheGreatCommunicatorPossiblythemostsignificantchangeinrecentyearsliesinthecommunicationsarena.Inthe1970sModiconintroductionofModbuscommunicationsprotocolallowedPLCstocommunicateoverstandardcabling.ThistranslatestoanabilitytoplacePLCsincloserproximitytorealworlddevicesandcommunicatebacktoothersystemcontrolsinamainpanel.2Inthepast30yearswehaveseenliterallyhundredsofproprietaryandstandardprotocolsdeveloped,eachwiththeirownuniqueadvantages.Today'sPLCshavetobedatacompilersandinformationgateways.Theyhavetointerfacewithbarcodescannersandprinters,aswellastemperatureandanalogsensors.Theyneedmultipleprotocolsupporttobeabletoconnectwithotherdevicesintheprocess.Andfurthermore,theyneedallthesecapabilitieswhileremainingcost-effectiveandsimpletoprogram.AnotherprimarydevelopmentthathasliterallyrevolutionizedthewayPLCsareprogrammed,communicatewitheachotherandinterfacewithPCsforHMI,SCADAorDCSapplications,camefromthecomputingworld.UseofEthernetcommunicationsontheplantfloorhasdoubledinthepastfiveyears.Whileserialcommunicationsremainpopularandreliable,Ethernetisfastbecomingthecommunicationsmediaofchoicewithadvantagesthatsimplycan'tbeignored,suchas:*Networkspeed.*Easeofusewhenitcomestothesetupandwiring.*Availabilityofoff-the-shelfnetworkingcomponents.*Built-incommunicationssetups.IntegratedMotionControlAnotherresponsibilitythePLChasbeentaskedwithismotioncontrol.Fromsimpleopen-looptomulti-axisapplications,thetrendhasbeentointegratethisfeatureintoPLChardwareandsoftware.Therearemanyapplicationsthatrequireaccuratecontrolatafastpace,butnotexactprecisionatblazingspeeds.Theseareapplicationswherethestand-alonePLCworkswell.ManynanoandmicroPLCsareavailablewithhigh-speedcountingcapabilitiesandhigh-frequencypulseoutputsbuiltintothecontroller,makingthemaviablesolutionforopen-loopcontrol.Theonecaveatisthatthecontrollerdoesnotknowthepositionoftheoutputdeviceduringthecontrolsequence.Ontheotherhand,itsmainadvantageiscost.Evensimplemotioncontrolhadpreviouslyrequiredanexpensiveoptionmodule,andattimeswasrestrictedtomoresophisticatedcontrolplatformsinordertomeetsystemrequirements.Moresophisticatedmotionapplicationsrequirehigher-precisionpositioninghardwareandsoftware,andmanyPLCsofferhigh-speedoptionmodulesthatinterfacewithservodrives.Mostdrivestodaycanaccepttraditionalcommandsfromhost(PLCorPC)controls,orprovidetheirowninternalmotioncontrol.Thetrendhereistointegratethemotioncontrolconfigurationintothelogiccontrollerprogrammingsoftwarepackage.ProgrammingLanguages3AfacetofthePLCthatreflectsboththepastandthefutureisprogramminglanguage.TheIEC61131-3standarddealswithprogramminglanguagesanddefinestwographicalandtwotextualPLCprogramminglanguagestandards:*Ladderlogic(graphical).*Functionblockdiagram(graphical).*Structuredtext(textual).Instructionlist(textual).Thisstandardalsodefinesgraphicalandtextualsequentialfunctionchartelementstoorganizeprogramsforsequentialandparallelcontrolprocessing.Basedonthestandard,manymanufacturersofferatleasttwooftheselanguagesasoptionsforprogrammingtheirPLCs.Ironically,approximately96percentofPLCusersrecentlystilluseladderdiagramstoconstructtheirPLCcode.Itseemsthatladderlogiccontinuestobeatopchoicegivenit'sperformedsowellforsolong.HardwarePlatformsThemodernPLChasincorporatedmanytypesofCommercialofftheShelf(COTS)technologyinitsCPU.ThislatesttechnologygivesthePLCafaster,morepowerfulprocessorwithmorememoryatlesscost.The