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外文翻译--可编程逻辑控制器.doc

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外文翻译--可编程逻辑控制器.doc

ProgrammablelogiccontrollerAprogrammablelogiccontrollerPLCorprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,amusementrides,orlightingfixtures.PLCsareusedinmanyindustriesandmachines.Unlikegeneralpurposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbatterybackedornonvolatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.1.HistoryThePLCwasinventedinresponsetotheneedsoftheAmericanautomotivemanufacturingindustry.Programmablelogiccontrollerswereinitiallyadoptedbytheautomotiveindustrywheresoftwarerevisionreplacedtherewiringofhardwiredcontrolpanelswhenproductionmodelschanged.BeforethePLC,control,sequencing,andsafetyinterlocklogicformanufacturingautomobileswasaccomplishedusinghundredsorthousandsofrelays,camtimers,anddrumsequencersanddedicatedclosedloopcontrollers.Theprocessforupdatingsuchfacilitiesfortheyearlymodelchangeoverwasverytimeconsumingandexpensive,aselectriciansneededtoindividuallyrewireeachandeveryrelay.In1968GMHydramatictheautomatictransmissiondivisionofGeneralMotorsissuedarequestforproposalforanelectronicreplacementforhardwiredrelaysystems.ThewinningproposalcamefromBedfordAssociatesofBedford,Massachusetts.ThefirstPLC,designatedthe084becauseitwasBedfordAssociateseightyfourthproject,wastheresult.BedfordAssociatesstartedanewcompanydedicatedtodeveloping,manufacturing,selling,andservicingthisnewproductModicon,whichstoodforMOdularDIgitalCONtroller.OneofthepeoplewhoworkedonthatprojectwasDickMorley,whoisconsideredtobethefatherofthePLC.TheModiconbrandwassoldin1977toGouldElectronics,andlateracquiredbyGermanCompanyAEGandthenbyFrenchSchneiderElectric,thecurrentowner.Oneoftheveryfirst084modelsbuiltisnowondisplayatModiconsheadquartersinNorthAndover,Massachusetts.ItwaspresentedtoModiconbyGM,whentheunitwasretiredafternearlytwentyyearsofuninterruptedservice.Modiconusedthe84monikerattheendofitsproductrangeuntilthe984madeitsappearance.TheautomotiveindustryisstilloneofthelargestusersofPLCs.2.DevelopmentEarlyPLCsweredesignedtoreplacerelaylogicsystems.ThesePLCswereprogrammedinladderlogic,whichstronglyresemblesaschematicdiagramofrelaylogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicians.OtherearlyPLCsusedaformofinstructionlistprogramming,basedonastackbasedlogicsolver.ModernPLCscanbeprogrammedinavarietyofways,fromladderlogictomoretraditionalprogramminglanguagessuchasBASICandC.AnothermethodisStateLogic,averyhighlevelprogramminglanguagedesignedtoprogramPLCsbasedonstatetransitiondiagrams.ManyearlyPLCsdidnothaveaccompanyingprogrammingterminalsthatwerecapableofgraphicalrepresentationofthelogic,andsothelogicwasinsteadrepresentedasaseriesoflogicexpressionsinsomeversionofBooleanformat,similartoBooleanalgebra.Asprogrammingterminalsevolved,itbecamemorecommonforladderlogictobeused,fortheaforementionedreasons.NewerformatssuchasStateLogicandFunctionBlockwhichissimilartothewaylogicisdepictedwhenusingdigitalintegratedlogiccircuitsexist,buttheyarestillnotaspopularasladderlogic.AprimaryreasonforthisisthatPLCssolvethelogicinapredictableandrepeatingsequence,andladderlogicallowstheprogrammerthepersonwritingthelogictoseeanyissueswiththetimingofthelogicsequencemoreeasilythanwouldbepossibleinotherformats.2.1ProgrammingEarlyPLCs,uptothemid1980s,wereprogrammedusingproprietaryprogrammingpanelsorspecialpurposeprogrammingterminals,whichoftenhaddedicatedfunctionkeysrepresentingthevariouslogicalelementsofPLCprograms.Programswerestoredoncassettetapecartridges.Facilitiesforprintinganddocumentationwereveryminimalduetolackofmemorycapacity.TheveryoldestPLCsusednonvolatilemagneticcorememory.Morerecently,PLCsareprogrammedusingapplicationsoftwareonpersonalcomputers.ThecomputerisconnectedtothePLCthroughEthernet,RS232,RS485orRS422cabling.Theprogrammingsoftwareallowsentryandeditingoftheladderstylelogic.GenerallythesoftwareprovidesfunctionsfordebuggingandtroubleshootingthePLCsoftware,forexample,byhighlightingportionsofthelogictoshowcurrentstatusduringoperationorviasimulation.ThesoftwarewilluploadanddownloadthePLCprogram,forbackupandrestorationpurposes.Insomemodelsofprogrammablecontroller,theprogramistransferredfromapersonalcomputertothePLCthoughaprogrammingboardwhichwritestheprogramintoaremovablechipsuchasanEEPROMorEPROM.3.FunctionalityThefunctionalityofthePLChasevolvedovertheyearstoincludesequentialrelaycontrol,motioncontrol,processcontrol,distributedcontrolsystemsandnetworking.Thedatahandling,storage,processingpowerandcommunicationcapabilitiesofsomemodernPLCsareapproximatelyequivalenttodesktopcomputers.PLClikeprogrammingcombinedwithremoteI/Ohardware,allowageneralpurposedesktopcomputertooverlapsomePLCsincertainapplications.Regardingthepracticalityofthesedesktopcomputerbasedlogiccontrollers,itisimportanttonotethattheyhavenotbeengenerallyacceptedinheavyindustrybecausethedesktopcomputersrunonlessstableoperatingsystemsthandoPLCs,andbecausethedesktopcomputerhardwareistypicallynotdesignedtothesamelevelsoftolerancetotemperature,humidity,vibration,andlongevityastheprocessorsusedinPLCs.Inadditiontothehardwarelimitationsofdesktopbasedlogic,operatingsystemssuchasWindowsdonotlendthemselvestodeterministiclogicexecution,withtheresultthatthelogicmaynotalwaysrespondtochangesinlogicstateorinputstatuswiththeextremeconsistencyintimingasisexpectedfromPLCs.Still,suchdesktoplogicapplicationsfinduseinlesscriticalsituations,suchaslaboratoryautomationanduseinsmallfacilitieswheretheapplicationislessdemandingandcritical,becausetheyaregenerallymuchlessexpensivethanPLCs.Inmorerecentyears,smallproductscalledPLRsprogrammablelogicrelays,andalsobysimilarnames,havebecomemorecommonandaccepted.TheseareverymuchlikePLCs,andareusedinlightindustrywhereonlyafewpointsofI/Oi.e.afewsignalscominginfromtherealworldandafewgoingoutareinvolved,andlowcostisdesired.Thesesmalldevicesaretypicallymadeinacommonphysicalsizeandshapebyseveralmanufacturers,andbrandedbythemakersoflargerPLCstofillouttheirlowendproductrange.PopularnamesincludePICOController,NANOPLC,andothernamesimplyingverysmallcontrollers.Mostofthesehavebetween8and12digitalinputs,4and8digitaloutputs,andupto2analoginputs.Sizeisusuallyabout4wide,3high,and3deep.MostsuchdevicesincludeatinypostagestampsizedLCDscreenforviewingsimplifiedladderlogiconlyaverysmallportionoftheprogrambeingvisibleatagiventimeandstatusofI/Opoints,andtypicallythesescreensareaccompaniedbya4wayrockerpushbuttonplusfourmoreseparatepushbuttons,similartothekeybuttonsonaVCRremotecontrol,andusedtonavigateandeditthelogic.MosthaveasmallplugforconnectingviaRS232orRS485toapersonalcomputersothatprogrammerscanusesimpleWindowsapplicationsforprogramminginsteadofbeingforcedtousethetinyLCDandpushbuttonsetforthispurpose.UnlikeregularPLCsthatareusuallymodularandgreatlyexpandable,thePLRsareusuallynotmodularorexpandable,buttheirpricecanbetwoordersofmagnitudelessthanaPLCandtheystillofferrobustdesignanddeterministicexecutionofthelogic.4.PLCTopics4.1FeaturesThemaindifferencefromothercomputersisthatPLCsarearmoredforsevereconditionssuchasdust,moisture,heat,coldandhavethefacilityforextensiveinput/outputI/Oarrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,analogprocessvariablessuchastemperatureandpressure,andthepositionsofcomplexpositioningsystems.Someusemachinevision.Ontheactuatorside,PLCsoperateelectricmotors,pneumaticorhydrauliccylinders,magneticrelays,solenoids,oranalogoutputs.Theinput/outputarrangementsmaybebuiltintoasimplePLC,orthePLCmayhaveexternalI/OmodulesattachedtoacomputernetworkthatplugsintothePLC.4.2SystemscaleAsmallPLCwillhaveafixednumberofconnectionsbuiltinforinputsandoutputs.Typically,expansionsareavailableifthebasemodelhasinsufficientI/O.ModularPLCshaveachassisalsocalledarackintowhichareplacedmoduleswithdifferentfunctions.TheprocessorandselectionofI/Omodulesiscustomisedfortheparticularapplication.Severalrackscanbeadministeredbyasingleprocessor,andmayhavethousandsofinputsandoutputs.AspecialhighspeedserialI/Olinkisusedsothatrackscanbedistributedawayfromtheprocessor,reducingthewiringcostsforlargeplants.4.3Userinterface

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