plc外文文献译文.docx

plc外文文献译文.docx

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plc外文文献译文

Programmablelogiccontroller

Aprogrammablelogiccontroller（PLC）orprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,amusementrides,orlightingfixtures.PLCsareusedinmanyindustriesandmachines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.

1.History

ThePLCwasinventedinresponsetotheneedsoftheAmericanautomotivemanufacturingindustry.Programmablelogiccontrollerswereinitiallyadoptedbytheautomotiveindustrywheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.

BeforethePLC,control,sequencing,andsafetyinterlocklogicformanufacturingautomobileswasaccomplishedusinghundredsorthousandsofrelays,camtimers,anddrumsequencersanddedicatedclosed-loopcontrollers.Theprocessforupdatingsuchfacilitiesfortheyearlymodelchange-overwasverytimeconsumingandexpensive,aselectriciansneededtoindividuallyrewireeachandeveryrelay.

In1968GMHydramatic（theautomatictransmissiondivisionofGeneralMotors）issuedarequestforproposalforanelectronicreplacementforhard-wiredrelaysystems.ThewinningproposalcamefromBedfordAssociatesofBedford,Massachusetts.ThefirstPLC,designatedthe084becauseitwasBedfordAssociates'eighty-fourthproject,wastheresult.BedfordAssociatesstartedanewcompanydedicatedtodeveloping,manufacturing,selling,andservicingthisnewproduct:

Modicon,whichstoodforMOdularDIgitalCONtroller.OneofthepeoplewhoworkedonthatprojectwasDickMorley,whoisconsideredtobethe"father"ofthePLC.TheModiconbrandwassoldin1977toGouldElectronics,andlateracquiredbyGermanCompanyAEGandthenbyFrenchSchneiderElectric,thecurrentowner.

Oneoftheveryfirst084modelsbuiltisnowondisplayatModicon'sheadquartersinNorthAndover,Massachusetts.ItwaspresentedtoModiconbyGM,whentheunitwasretiredafternearlytwentyyearsofuninterruptedservice.Modiconusedthe84monikerattheendofitsproductrangeuntilthe984madeitsappearance.

TheautomotiveindustryisstilloneofthelargestusersofPLCs.

2.Development

EarlyPLCsweredesignedtoreplacerelaylogicsystems.ThesePLCswereprogrammedin"ladderlogic",whichstronglyresemblesaschematicdiagramofrelaylogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicians.OtherearlyPLCsusedaformofinstructionlistprogramming,basedonastack-basedlogicsolver.

ModernPLCscanbeprogrammedinavarietyofways,fromladderlogictomoretraditionalprogramminglanguagessuchasBASICandC.AnothermethodisStateLogic,averyhigh-levelprogramminglanguagedesignedtoprogramPLCsbasedonstatetransitiondiagrams.

ManyearlyPLCsdidnothaveaccompanyingprogrammingterminalsthatwerecapableofgraphicalrepresentationofthelogic,andsothelogicwasinsteadrepresentedasaseriesoflogicexpressionsinsomeversionofBooleanformat,similartoBooleanalgebra.Asprogrammingterminalsevolved,itbecamemorecommonforladderlogictobeused,fortheaforementionedreasons.NewerformatssuchasStateLogicandFunctionBlock（whichissimilartothewaylogicisdepictedwhenusingdigitalintegratedlogiccircuits）exist,buttheyarestillnotaspopularasladderlogic.AprimaryreasonforthisisthatPLCssolvethelogicinapredictableandrepeatingsequence,andladderlogicallowstheprogrammer（thepersonwritingthelogic）toseeanyissueswiththetimingofthelogicsequencemoreeasilythanwouldbepossibleinotherformats.

2.1Programming

EarlyPLCs,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.Functionality

ThefunctionalityofthePLChasevolvedovertheyearstoincludesequentialrelaycontrol,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.Sizeisusuallyabout4"wide,3"high,and3"deep.MostsuchdevicesincludeatinypostagestampsizedLCDscreenforviewingsimplifiedladderlogic（onlyaverysmallportionoftheprogrambeingvisibleatagiventime）andstatusofI/Opoints,andtypicallythesescreensareaccompaniedbya4-wayrockerpush-buttonplusfourmoreseparatepush-buttons,similartothekeybuttonsonaVCRremotecontrol,andusedtonavigateandeditthelogic.MosthaveasmallplugforconnectingviaRS-232orRS-485toapersonalcomputersothatprogrammerscanusesimpleWindowsapplicationsforprogramminginsteadofbeingforcedtousethetinyLCDandpush-buttonsetforthispurpose.UnlikeregularPLCsthatareusuallymodularandgreatlyexpandable,thePLRsareusuallynotmodularorexpandable,buttheirpricecanbetwoordersofmagnitudelessthanaPLCandtheystillofferrobustdesignanddeterministicexecutionofthelogic.

4.PLCTopics

4.1.Features

ThemaindifferencefromothercomputersisthatPLCsarearmoredforsevereconditions（suchasdust,moisture,heat,cold）andhavethefacilityforextensiveinput/output（I/O）arrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,analogprocessvariables（suchastemperatureandpressure）,andthepositionsofcomplexpositioningsystems.Someusemachinevision.Ontheactuatorside,PLCsoperateelectricmotors,pneumaticorhydrauliccylinders,magneticrelays,solenoids,oranalogoutputs.Theinput/outputarrangementsmaybebuiltintoasimplePLC,orthePLCmayhaveexternalI/OmodulesattachedtoacomputernetworkthatplugsintothePLC.

4.2Systemscale

AsmallPLCwillhaveafixednumberofconnectionsbuiltinforinputsandoutputs.Typically,expansionsareavailableifthebasemodelhasinsufficientI/O.

ModularPLCshaveachassis（alsocalledarack）intowhichareplacedmoduleswithdifferentfunctions.TheprocessorandselectionofI/Omodulesiscustomisedfortheparticularapplication.Severalrackscanbeadministeredbyasingleprocessor,andmayhavethousandsofinputsandoutputs.AspecialhighspeedserialI/Olinkisusedsothatrackscanbedistributedawayfromtheprocessor,reducingthewiringcostsforlargeplants.

4.3Userinterface

PLCsmayneedtointeractwithpeopleforthepurposeofconfiguration,alarmre