外文翻譯--可編程邏輯控制器.doc

ProgrammablelogiccontrollerAprogrammablelogiccontroller(PLC)orprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,amusementrides,orlightingfixtures.PLCsareusedinmanyindustriesandmachines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.1.HistoryThePLCwasinventedinresponsetotheneedsoftheAmericanautomotivemanufacturingindustry.Programmablelogiccontrollerswereinitiallyadoptedbytheautomotiveindustrywheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.BeforethePLC,control,sequencing,andsafetyinterlocklogicformanufacturingautomobileswasaccomplishedusinghundredsorthousandsofrelays,camtimers,anddrumsequencersanddedicatedclosed-loopcontrollers.Theprocessforupdatingsuchfacilitiesfortheyearlymodelchange-overwasverytimeconsumingandexpensive,aselectriciansneededtoindividuallyrewireeachandeveryrelay.In1968GMHydramatic(theautomatictransmissiondivisionofGeneralMotors)issuedarequestforproposalforanelectronicreplacementforhard-wiredrelaysystems.ThewinningproposalcamefromBedfordAssociatesofBedford,Massachusetts.ThefirstPLC,designatedthe084becauseitwasBedfordAssociateseighty-fourthproject,wastheresult.BedfordAssociatesstartedanewcompanydedicatedtodeveloping,manufacturing,selling,andservicingthisnewproduct:Modicon,whichstoodforMOdularDIgitalCONtroller.OneofthepeoplewhoworkedonthatprojectwasDickMorley,whoisconsideredtobethefatherofthePLC.TheModiconbrandwassoldin1977toGouldElectronics,andlateracquiredbyGermanCompanyAEGandthenbyFrenchSchneiderElectric,thecurrentowner.Oneoftheveryfirst084modelsbuiltisnowondisplayatModiconsheadquartersinNorthAndover,Massachusetts.ItwaspresentedtoModiconbyGM,whentheunitwasretiredafternearlytwentyyearsofuninterruptedservice.Modiconusedthe84monikerattheendofitsproductrangeuntilthe984madeitsappearance.TheautomotiveindustryisstilloneofthelargestusersofPLCs.2.DevelopmentEarlyPLCsweredesignedtoreplacerelaylogicsystems.ThesePLCswereprogrammedinladderlogic,whichstronglyresemblesaschematicdiagramofrelaylogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicians.OtherearlyPLCsusedaformofinstructionlistprogramming,basedonastack-basedlogicsolver.ModernPLCscanbeprogrammedinavarietyofways,fromladderlogictomoretraditionalprogramminglanguagessuchasBASICandC.AnothermethodisStateLogic,averyhigh-levelprogramminglanguagedesignedtoprogramPLCsbasedonstatetransitiondiagrams.ManyearlyPLCsdidnothaveaccompanyingprogrammingterminalsthatwerecapableofgraphicalrepresentationofthelogic,andsothelogicwasinsteadrepresentedasaseriesoflogicexpressionsinsomeversionofBooleanformat,similartoBooleanalgebra.Asprogrammingterminalsevolved,itbecamemorecommonforladderlogictobeused,fortheaforementionedreasons.NewerformatssuchasStateLogicandFunctionBlock(whichissimilartothewaylogicisdepictedwhenusingdigitalintegratedlogiccircuits)exist,buttheyarestillnotaspopularasladderlogic.AprimaryreasonforthisisthatPLCssolvethelogicinapredictableandrepeatingsequence,andladderlogicallowstheprogrammer(thepersonwritingthelogic)toseeanyissueswiththetimingofthelogicsequencemoreeasilythanwouldbepossibleinotherformats.2.1ProgrammingEarlyPLCs,uptothemid-1980s,wereprogrammedusingproprietaryprogrammingpanelsorspecial-purposeprogrammingterminals,whichoftenhaddedicatedfunctionkeysrepresentingthevariouslogicalelementsofPLCprograms.Programswerestoredoncassettetapecartridges.Facilitiesforprintinganddocumentationwereveryminimalduetolackofmemorycapacity.TheveryoldestPLCsusednon-volatilemagneticcorememory.Morerecently,PLCsareprogrammedusingapplicationsoftwareonpersonalcomputers.ThecomputerisconnectedtothePLCthroughEthernet,RS-232,RS-485orRS-422cabling.Theprogrammingsoftwareallowsentryandeditingoftheladder-stylelogic.GenerallythesoftwareprovidesfunctionsfordebuggingandtroubleshootingthePLCsoftware,forexample,byhighlightingportionsofthelogictoshowcurrentstatusduringoperationorviasimulation.ThesoftwarewilluploadanddownloadthePLCprogram,forbackupandrestorationpurposes.Insomemodelsofprogrammablecontroller,theprogramistransferredfromapersonalcomputertothePLCthoughaprogrammingboardwhichwritestheprogramintoaremovablechipsuchasanEEPROMorEPROM.3.FunctionalityThefunctionalityofthePLChasevolvedovertheyearstoincludesequentialrelaycontrol,motioncontrol,processcontrol,distributedcontrolsystemsandnetworking.Thedatahandling,storage,processingpowerandcommunicationcapabilitiesofsomemodernPLCsareapproximatelyequivalenttodesktopcomputers.PLC-likeprogrammingcombinedwithremoteI/Ohardware,allowageneral-purposedesktopcomputertooverlapsomePLCsincertainapplications.Regardingthepracticalityofthesedesktopcomputerbasedlogiccontrollers,itisimportanttonotethattheyhavenotbeengenerallyacceptedinheavyindustrybecausethedesktopcomputersrunonlessstableoperatingsystemsthandoPLCs,andbecausethedesktopcomputerhardwareistypicallynotdesignedtothesamelevelsoftolerancetotemperature,humidity,vibration,andlongevityastheprocessorsusedinPLCs.Inadditiontothehardwarelimitationsofdesktopbasedlogic,operatingsystemssuchasWindowsdonotlendthemselvestodeterministiclogicexecution,withtheresultthatthelogicmaynotalwaysrespondtochangesinlogicstateorinputstatuswiththeextremeconsistencyintimingasisexpectedfromPLCs.Still,suchdesktoplogicapplicationsfinduseinlesscriticalsituations,suchaslaboratoryautomationanduseinsmallfacilitieswheretheapplicationislessdemandingandcritical,becausetheyaregenerallymuchlessexpensivethanPLCs.Inmorerecentyears,smallproductscalledPLRs(programmablelogicrelays),andalsobysimilarnames,havebecomemorecommonandaccepted.TheseareverymuchlikePLCs,andareusedinlightindustrywhereonlyafewpointsofI/O(i.e.afewsignalscominginfromtherealworldandafewgoingout)areinvolved,andlowcostisdesired.Thesesmalldevicesaretypicallymadeinacommonphysicalsizeandshapebyseveralmanufacturers,andbrandedbythemakersoflargerPLCstofillouttheirlowendproductrange.PopularnamesincludePICOController,NANOPLC,andothernamesimplyingverysmallcontrollers.Mostofthesehavebetween8and12digitalinputs,4and8digitaloutputs,andupto2analoginputs.Sizeisusuallyabout4wide,3high,and3deep.MostsuchdevicesincludeatinypostagestampsizedLCDscreenforviewingsimplifiedladderlogic(onlyaverysmallportionoftheprogrambeingvisibleatagiventime)andstatusofI/Opoints,andtypicallythesescreensareaccompaniedbya4-wayrockerpush-buttonplusfourmoreseparatepush-buttons,similartothekeybuttonsonaVCRremotecontrol,andusedtonavigateandeditthelogic.MosthaveasmallplugforconnectingviaRS-232orRS-485toapersonalcomputersothatprogrammerscanusesimpleWindowsapplicationsforprogramminginsteadofbeingforcedtousethetinyLCDandpush-buttonsetforthispurpose.UnlikeregularPLCsthatareusuallymodularandgreatlyexpandable,thePLRsareusuallynotmodularorexpandable,buttheirpricecanbetwoordersofmagnitudelessthanaPLCandtheystillofferrobustdesignanddeterministicexecutionofthelogic.4.PLCTopics4.1FeaturesThemaindifferencefromothercomputersisthatPLCsarearmoredforsevereconditions(suchasdust,moisture,heat,cold)andhavethefacilityforextensiveinput/output(I/O)arrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,analogprocessvariables(suchastemperatureandpressure),andthepositio