可编程控制器外文翻译中英文翻译外文文献翻译.docx

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可编程控制器外文翻译中英文翻译外文文献翻译

毕业设计中英文翻译

院系

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班级

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指导教师

20**年4月

ProgrammableLogicControllers（PLC）

1、Motivation

ProgrammableLogicControllers（PLC）,acomputingdeviceinventedbyRichardE.Morleyin1968,havebeenwidelyusedinindustryincludingmanufacturingsystems,transportationsystems,chemicalprocessfacilities,andmanyothers.Atthattime,thePLCreplacedthehardwiredlogicwithsoft-wiredlogicorso-calledrelayladderlogic（RLL）,aprogramminglanguagevisuallyresemblingthehardwiredlogic,andreducedtherebytheconfigurationtimefrom6monthsdownto6days[MoodyandMorley,1999].

AlthoughPCbasedcontrolhasstartedtocomeintoplace,PLCbasedcontrolwillremainthetechniquetowhichthemajorityofindustrialapplicationswilladhereduetoitshigherperformance,lowerprice,andsuperiorreliabilityinharshenvironments.Moreover,accordingtoastudyonthePLCmarketofFrostandSullivan[1995],anincreaseoftheannualsalesvolumeto15millionPLCsperyearwiththehardwarevalueofmorethan8billionUSdollarshasbeenpredicted,thoughthepricesofcomputinghardwareissteadilydropping.TheinventorofthePLC,RichardEMorley,fairlyconsidersthePLCmarketasa5-billionindustryatthepresenttime.

ThoughPLCsarewidelyusedinindustrialpractice,theprogrammingofPLCbasedcontrolsystemsisstillverymuchrelyingontrial-and-error.Alikesoftwareengineering,PLCsoftwaredesignisfacingthesoftwaredilemmaorcrisisinasimilarway.Morleyhimselfemphasizedthisaspectmostforcefullybyindicating[MoodyandMorley,1999,p.110]:

`Ifhouseswerebuiltlikesoftwareprojects,asinglewoodpeckercoulddestroycivilization.”

Particularly,practicalproblemsinPLCprogrammingaretoeliminatesoftwarebugsandtoreducethemaintenancecostsofoldladderlogicprograms.ThoughthehardwarecostsofPLCsaredroppingcontinuously,reducingthescantimeoftheladderlogicisstillanissueinindustrysothatlow-costPLCscanbeused.

Ingeneral,theproductivityingeneratingPLCisfarbehindcomparedtootherdomains,forinstance,VLSIdesign,whereefficientcomputeraideddesigntoolsareinpractice.ExistentsoftwareengineeringmethodologiesarenotnecessarilyapplicabletothePLCbasedsoftwaredesignbecausePLC-programmingrequiresasimultaneousconsiderationofhardwareandsoftware.Thesoftwaredesignbecomes,thereby,moreandmorethemajorcostdriver.Inmanyindustrialdesignprojects,morethanSO0/aofthemanpowerallocatedforthecontrolsystemdesignandinstallationisscheduledfortestinganddebuggingPLCprograms[Rockwell,1999].

Inaddition,currentPLCbasedcontrolsystemsarenotproperlydesignedtosupportthegrowingdemandforflexibilityandreconfigurabilityofmanufacturingsystems.Afurtherproblem,impellingtheneedforasystematicdesignmethodology,istheincreasingsoftwarecomplexityinlarge-scaleprojects.

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、Resume

APLC（programmableLogicController）isadevicethatwasinventedtoreplacethenecessarysequentialrelaycircuitsforcontrol.ThePLCworksbylookingatitsinputanddependingupontheirstate,turningon/offitsoutputs.Theuserentersaprogram,usuallyviasoftwareorprogrammer,whichgivesthedesiredresults.

PLCisusedinmany"realworld"applications.Ifthereisindustrypresent,chancearegoodthatthereisaPLCpresent.Ifyouareinvolvedinmachining,packing,materialhandling,automatedassemblyorcountlessotherindustries,youareprobablyalreadyusingthem.Ifyouarenot,youarewastingmoneyandtime.AlmostanyapplicationthatneedssometypeofelectricalcontrolhasaneedforaPLC.

Forexample,let'sassumethatwhenaswitchturnsonwewanttoturnasolenoidonfor5secondandthenturnitoffregardlessofhowlongtheswitchisonfor.Wecandothiswithasimpleexternaltimer.Butwhatiftheprocessincluded10switchesandsolenoids?

Weshouldneed10externaltimes.Whatiftheprocessalsoneededtocounthowmanytimestheswitchindividuallyturnedon?

Weneedalotofexternalcounters.

AsyoucanseethebiggertheprocessthemoreofaneedwehaveforaPLC.WecansimplyprogramthePLCtocountitsinputandturnthesolenoidsonforthespecifiedtime.

Wewilltakealookatwhatisconsideredtobethe"top20"PLCinstructions.Itcanbesafelyestimatedthatwithafirmunderstandingoftheseinstructionsonecansolvemorethan80%oftheapplicationsinexistence.

OfcoursewewilllearnmorethanjusttheseinstructiontohelpyousolvealmostALLpotentialPLCapplications.

ThePLCmainlyconsistsofaCPU,memoryareas,andappropriatecircuitstoreceiveinput/outputdata.WecanactuallyconsiderthePLCtobeaboxfullofhundredsorthousandsofseparaterelay,counters,timesanddatastoragelocations,Dothesecounters,timers,etc.reallyexist?

No,theydon't"physically"existbutrathertheysimulatedandbeconsideredsoftwarecounters,timers,etc..Theseinternalrelaysaresimulatedthroughbitlocationsinregisters.

Whatdoeseachpartdo?

Letmetellyou.

InputRelays

Theseareconnectedtotheoutsideworld.Theyphysicallyexsitandreceivesignalsfromswitches,sensors,ect..Typicallytheyarenotrelaysbutrathertheyaretransistors.

InternalUtilityRelays

Thesedonotreceivesignalsfromtheoutsideworldnordotheyphysicallyexist.theyaresimulatedrelaysandarewhatenablesaPLCtoeliminateexternalrelays.Therearealsosomespecialrelaysthatarededicatedtoperformingonlyonetask.Somearealwaysonwhilesomearealwaysoff.Someareononlyonceduringpower-onandaretypicallyusedforinitializingdatathatwasstored.

Counters

Theseagaindonotphysicallyexist.Theyaresimulatedcountersandtheycanbeprogrammedtocountpulses.Typicallythesecounterscancountup,downorbothupanddown.Sincetheyaresimulated,theyarelimitedintheircountingspeed.Somemanufacturersalsoincludehigh-speedcounters