PLC英文翻译.docx

PLC英文翻译.docx

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PLC英文翻译

ProgrammableLogicControllers（PLCs）

1。

AboutProgrammableLogicControllers（PLCs）

PLCs（programmablelogiccontrollers）arethecontrolhubsforawidevarietyofautomatedsystemsandprocesses.Theycontainmultipleinputsandoutputsthatusetransistorsandothercircuitrytosimulateswitchesandrelaystocontrolequipment.Theyareprogrammableviasoftwareinterfacedviastandardcomputerinterfacesandproprietarylanguagesandnetworkoptions.

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

AvailableinputsforprogrammablelogiccontrollersincludeDC,AC,analog,thermocouple,RTD,frequencyorpulse,transistor,andinterruptinputs. OutputsforPLCsincludeDC,AC,relay,analog,frequencyorpulse,transistor,andtriac. ProgrammingoptionsforPLCsincludefrontpanel,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（PLCs）.Ladderlogicprogrammingisagraphicalrepresentationoftheprogramdesignedtolooklikerelaylogic. FlowChartisagraphicallanguagethatdescribessequentialoperationsinacontrollersequenceorapplication.Itisusedtobuildmodular,reusablefunctionlibraries. Cisahighlevelprogramminglanguagesuitedtohandlethemostcomplexcomputation,sequential,anddataloggingtasks.ItistypicallydevelopedanddebuggedonaPC. BASICisahighlevellanguageusedtohandlemathematical,sequential,datacapturingandinterfacefunctions.

Programmablelogiccontrollerscanalsobespecifiedwithanumberofcomputerinterfaceoptions,networkspecificationsandfeatures. PLCpoweroptions,mountingoptionsandenvironmentaloperatingconditionsareallalsoimportanttoconsider.

2。

INTRODUCTION

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

Figure11SimplifiedPersonalComputerArchitecture

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-memoryinPLCsissimilartomemoriesinpersonalcomputers

ItisalsopossibletoimplementaPLCusinganormalPersonalComputer,althoughthisisnotadvisable.InthecaseofaPLCtheinputsandoutputsaredesignedtobemorereliableandruggedforharshproductionenvironments.

3。

OPERATIONSEQUENCE

AllPLCshavefourbasicstagesofoperationsthatarerepeatedmanytimespersecond.Initiallywhenturnedonthefirsttimeitwillcheckit’sownhardwareandsoftwareforfaults.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.Thispreventspotentialproblemsthatmightoccurifaninputthatisusedinmultipleplacesintheladderlogicprogramchangedwhilehalfwaythroughaladderscan.Thuschangingthebehaviorsofhalfoftheladderlogicprogram.Thisproblemcouldhavesevereeffectsoncomplexprogramsthataredevelopedlaterinthebook.Onesideeffectoftheinputscanisthatifachangeininputistooshortinduration,itmightfallbetweeninputscansandbemissed.

WhenthePLCisinitiallyturnedonthenormaloutputswillbeturnedoff.Thisdoesnotaffectthevaluesoftheinputs.

3。

1TheInputandOutputScans

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

3。

2TheLogicScan

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

Figure4LadderLogicExecutionSequence

Thelogicscansequencebecomeimportantwhensolvingladderlogicprogramswhichuseoutputsasinputs.Italsobecomesimportantwhenconsideringoutputusage.ConsiderFigure5,thefirstlineofladderlogicwillexamineinputAandsetoutputXtohavethesamevalue.Thesecondlinewill