PLC单片机 外文翻译 外文文献 英文文献 可编程逻辑控制器PLC.docx

PLC单片机 外文翻译 外文文献 英文文献 可编程逻辑控制器PLC.docx

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PLC单片机外文翻译外文文献英文文献可编程逻辑控制器PLC

ProgrammableLogicControllers（PLC）

1.AboutProgrammableLogicControllers（PLC）

Everyoneknowsthere’sonlyoneconstantinthetechnologyworld,andthat’schange.ThisisespeciallyevidentintheevolutionofProgrammableLogicControllers（PLC）andtheirvariedapplications.Fromtheirintroductionmorethan30yearsago,PLChasbecomethecomerstoneofhundredsofthousandsofcontrolsystemsinawiderangeofindustries.

Atheart,thePLCisanindustrializedcomputerprogrammedwithhighlyspecializedlanguages,anditcontinuestobenefitfromtechnologicaladvancesinthecomputerandinformationtechnologyworlds.Themostprominentofwhichisminiaturizationandcommunications.

ProgrammablelogiccontrollersI/Ochannelspecificationsincludetotalnumberofpoints,numberofinputsandoutputs,abilitytoexpand,andmaximumnumberofchannels. Numberofpointsisthesumoftheinputsandtheoutputs.PLCmaybespecifiedbyanypossiblecombinationofthesevalues. Expandableunitsmaybestackedorlinkedtogethertoincreasetotalcontrolcapacity. Maximumnumberofchannelsreferstothemaximumtotalnumberofinputandoutputchannelsinanexpandedsystem. PLCsystemspecificationstoconsiderincludescantime,numberofinstructions,datamemory,andprogrammemory. ScantimeisthetimerequiredbythePLCtocheckthestatesofitsinputsandoutputs. Instructionsarestandardoperations（suchasmathfunctions）availabletoPLCsoftware. Datamemoryisthecapacityfordatastorage. Programmemoryisthecapacityforcontrolsoftware.

AvailableinputsforprogrammablelogiccontrollersincludeDC,AC,analog,thermocouple,RTD,frequencyorpulse,transistor,andinterruptinputs. OutputsforPLCincludeDC,AC,relay,analog,frequencyorpulse,transistor,triac. ProgrammingoptionsforPLCincludefrontpanel,handheld,andcomputer.

Programmablelogiccontrollersuseavarietyofsoftwareprogramminglanguagesforcontrol. TheseincludeIEC61131-3,sequentialfunctionchart（SFC）,functionblockdiagram（FBD）,ladderdiagram（LD）,structuredtext（ST）,instructionlist（IL）,relayladderlogic（RLL）,flowchart,C,andBasic. TheIEC61131-3programmingenvironmentprovidessupportforfivelanguagesspecifiedbytheglobalstandard:

SequentialFunctionChart,FunctionBlockDiagram,LadderDiagram,StructuredText,andInstructionList.Thisallowsformulti-vendorcompatibilityandmulti-languageprogramming. SFCisagraphicallanguagethatprovidescoordinationofprogramsequences,supportingalternativesequenceselectionsandparallelsequences. FBDusesabroadfunctionlibrarytobuildcomplexproceduresinagraphicalformat.Standardmathandlogicfunctionsmaybecoordinatedwithcustomizablecommunicationandinterfacefunctions. LDisagraphiclanguagefordiscretecontrolandinterlockinglogic.ItiscompletelycompatiblewithFBDfordiscretefunctioncontrol. STisatextlanguageusedforcomplexmathematicalproceduresandcalculationslesswellsuitedtographicallanguages. ILisalow-levellanguagesimilartoassembly.code.Itisusedinrelativelysimplelogicinstructions. RelayLadderLogic（RLL）,orladderdiagrams,istheprimaryprogramminglanguageforprogrammablelogiccontrollers（PLC）.Ladderlogicprogrammingisagraphicalrepresentationoftheprogramdesignedtolooklikerelaylogic. FlowChartisagraphicallanguagethatdescribessequentialoperationsinacontrollersequenceorapplication.Itisusedtobuildmodular,reusablefunctionlibraries. Cisahighlevelprogramminglanguagesuitedtohandlethemostcomplexcomputation,sequential,anddataloggingtasks.ItistypicallydevelopedanddebuggedonaPC. BASICisahighlevellanguageusedtohandlemathematical,sequential,datacapturingandinterfacefunctions.

Programmablelogiccontrollerscanalsobespecifiedwithanumberofcomputerinterfaceoptions,networkspecificationsandfeatures. PLCpoweroptions,mountingoptionsandenvironmentaloperatingconditionsareallalsoimportanttoconsider.

2.INTRODUCTION

ForsimpleprogrammingtherelaymodelofthePLCissufficient.AsmorecomplexfunctionsareusedthemorecomplexVonNeumanmodelofthePLCmustbeused.AVonNeumancomputerprocessesoneinstructionatatime.Mostcomputersoperatethisway,althoughtheyappeartobedoingmanythingsatonce.ConsiderthecomputercomponentsshowninFigure1.

Figure1SimplifiedPersonalComputerArchitecture

Inputisobtainedfromthekeyboardandmouse,outputissenttothescreen,andthediskandmemoryareusedforbothinputandoutputforstorage.（Note:

thedirectionsofthesearrowsareveryimportanttoengineers,alwayspayattentiontoindicatewhereinformationisflowing.）ThisfigurecanberedrawnasinFigure2toclarifytheroleofinputsandoutputs.

Figure2AnInput-OutputOrientedArchitecture

Inthisfigurethedataenterstheleftsidethroughtheinputs.（Note:

mostengineeringdiagramshaveinputsontheleftandoutputsontheright.）IttravelsthroughbufferingcircuitsbeforeitenterstheCPU.TheCPUoutputsdatathroughothercircuits.Memoryanddisksareusedforstorageofdatathatisnotdestinedforoutput.Ifwelookatapersonalcomputerasacontroller,itiscontrollingtheuserbyoutputtingstimulionthescreen,andinputtingresponsesfromthemouseandthekeyboard.

APLCisalsoacomputercontrollingaprocess.Whenfullyintegratedintoanapplicationtheanalogiesbecome;

Inputs-thekeyboardisanalogoustoaproximityswitch.

Input-circuits-theserialinputchipislikea24Vdcinputcard.

Computer-the686CPUislikeaPLCCPUunit.

Output-circuits-agraphicscardislikeatriacoutputcard.

Outputs-amonitorislikealight.

Storage-memoryinPLCissimilartomemoriesinpersonalcomputers.

ItisalsopossibletoimplementaPLCusinganormalPersonalComputer,althoughthisisnotadvisable.InthecaseofaPLCtheinputsandoutputsaredesignedtobemorereliableandruggedforharshproductionenvironments.

3.OPERATIONSEQUENCE

AllPLChavefourbasicstagesofoperationsthatarerepeatedmanytimespersecond.Initiallywhenturnedonthefirsttimeitwillcheckitsownhardwareandsoftwareforfaults.Iftherearenoproblemsitwillcopyalltheinputandcopytheirvaluesintomemory,thisiscalledtheinputscan.Usingonlythememorycopyoftheinputstheladderlogicprogramwillbesolvedonce,thisiscalledthelogicscan.Whilesolvingtheladderlogictheoutputvaluesareonlychangedintemporarymemory.Whentheladderscanisdonetheoutputswillbeupdatedusingthetemporaryvaluesinmemory,thisiscalledtheoutputscan.ThePLCnowrestartstheprocessbystartingaselfcheckforfaults.Thisprocesstypicallyrepeats10to100timespersecondasisshowninFigure3.

Figure3PLCScanCycle

SELFTEST-Checkstoseeifallcardserrorfree,resetwatch-dogtimer,etc.（Awatchdogtimerwillcauseanerror,andshutdownthePLCifnotresetwithinashortperiodoftime-thiswouldindicatethattheladderlogicisnotbeingscannednormally）.

INPUTSCAN-Readsinputvaluesfromthechipsintheinputcards,andcopiestheirvaluestomemory.ThismakesthePLCoperationfaster,andavoidscaseswhereaninputchangesfromthestarttotheendoftheprogram（e.g.,anemergencystop）.TherearespecialPLCfunctionsthatreadtheinputsdirectly,andavoidtheinputtables.

LOGICSOLVE/SCAN-Basedontheinputtableinmemory,theprogramisexecuted1stepatatime,andoutputsareupdated.Thisisthefocusofthelatersections.

OUTPUTSCAN-Theoutputtableiscopiedfrommemorytotheoutputchips.Thesechipsthendrivetheoutputdevices.

Theinputandoutputscansoftenconfusethebeginner,buttheyareimportant.Theinputscantakesasnapshotoftheinputs,andsolvesthelogic.Thispreventspotentialproblemsthatmightoccurifaninputthatisusedinmultipleplacesintheladderlogicprogramchangedwhilehalfwaythroughaladderscans.Thisproblemcouldhavesevereeffectsoncomplexprogramsthataredevelopedlaterinthebook.Onesideeffectoftheinputscanisthatifachangeininputistooshortinduration,itmightfallbetweeninputscansandbemissed.

WhenthePLCisinitiallyturnedonthenormaloutputswillbeturnedoff.Thisdoesnotaffectthevaluesoftheinputs.

4．TheInputandOutputScans

WhentheinputstothePLCarescannedthephysicalinputvaluesarecopiedintomemory.WhentheoutputstoaPLCarescannedtheyarecopiedfrommemorytothephysicaloutputs.Whentheladderlogicisscanneditusesthevaluesinmemory,nottheactualinputoroutputvalues.Theprimaryreasonfordoingthisissothatifaprogramusesaninputvalueinmultipleplaces,achangeintheinputvaluewillnotinvalidatethelogic.Also,ifoutputbitswerechangedaseachbitwaschanged,insteadofallatonceattheendofthescanthePLCwouldoperatemuchslower.

5．TheLogicScan

Ladderlogicprogramsaremodelledafterrelaylogic.Inrelaylogiceachelementintheladderwillswitchasquicklyaspossible.Butinaprogramelementscanonlybeexaminesoneatatimeinafixedsequence.ConsidertheladderlogicinFigure4,theladderlogicwillbeinterpretedleft-to-right,top-to-bottom.Inthefiguretheladderlogicscanbeginsatthetoprung.Attheendoftherungitinterpretsthetopoutputfirst,thentheoutputbranchedbelowit.Onthesecondrungitsolvesbranches,beforemovingalongtheladderlogicrung.

Figure4LadderLogicExecutionSequence

Italsobecomesimportantwhenconsideringoutputusage.ConsiderFigure5,thefirstlineofladderlogicwillexamineinputA