外文翻譯--可編程邏輯控制器（PLC）.doc

1ProgrammableLogicControllersProgrammablelogiccontroller(plc)isasolid-statedeviceusedtocontrolmachinemotionorprocessoperationbymeansofastoredprogram.ThePLCsendsoutputcontrolsignalsandreceivesinputsignalsthroughinput/output(I/O)devices.APLCcontrolsoutputsinresponsetostimuliattheinputsaccordingtothelogicprescribedbythestoredprogram.Theinputsaremadeupoflimitswitches,pushbuttons,thunbwheels.Switches,pulses,analogsignals,ASCLLserialdata,andbinaryorBCDdatafromabsolutepositionencoders.Theoutputsarevoltageorcurrentlevelstodriveenddevicessuchassolenoids,motorstarters,relays,lights,andsoon.Otheroutputdevicesincludeanalogdevices,digitalBCDdisplays,ASCIIcompatibledevices,servovariable-speeddrives,andevencomputers.Programmablecontrollersweredeveloped(circain1968)whenGeneralMotorsCorp,andotherautomobilemanufacturerswereexperimentingtoseeiftheremightbeanalternativetoscrappingalltheirhardwiredcontrolpanelsofmachinetoolsandotherproductionequipmentduringamodelchangeover.Thisannualtraditionwasnecessarybecauserewiringofthepanelswasmoreexpensivethanbuyingnewones.Theautomotivecompaniesapproachedanumberofcontrolequipmentmanufacturersandaskedthemtodevelopacontrolsystemthatwouldhavealongerproductivelifewithoutmajorrewiring,butwouldstillbeunderstandabletoandrepairablebyplantpersonnel.Thenewproductwasnameda“programmablecontroller”.TheprocessorpartofthePLCcontainsacentralprocessingunitandmemory。Thecentralprocessingunit(cpu)isthe“trafficdirector”oftheprocessor,thememorystoresinformation.Comingintotheprocessoraretheelectricalsignalsfromtheinputdevices,asconditionedbytheinputmoduletovoltagelevelsacceptabletoprocessorlogic.TheprocessorscansthestateofI/oandupdatesoutputsbasedoninstructionsstoredinthememoryoftheplc.Forexample,theprocessormaybeprogrammedsothatifaninputconnectedtoalimitswitchistrue(limitswitchclosed),thenacorrespondingoutputwiredtoanoutputmoduleistobeenergized.Thisoutputmightbeasolenoid,forexampleoneachscantoseeifthatlimitswitchis,infact,closed.Ifitisclosed,theprocessorenergizesthesolenoidbyturningontheoutputmodule.Theoutputdevice,suchasasolenoidormotorstarter,iswiredtooutputmodulesterminal,anditreceivesitsshiftsignalfromtheprocessor,ineffect,theprocessorisperformingalongandcomplicatedseriesoflogicdecisions.ThePLC2performssuchdecisionssequentiallyandinaccordancewiththestoredprogram.Similarly,analogI/Oallowstheprocessortomakedecisionsbasedonthemagnitudelfasignal.Ratherthanjustifitisonoroff.Forexample,Theprocessormaybeprogrammedtoincreaseordecreasethesteamflowtoaboiler(analogoutput)basedonacomparisonoftheactualtemperatureintheboiler(analoginput)tothedesiredtemperature.Thisisoftenperformedbyutilizingthebuilt-inPID(proportional,integral,derivative)capabilitieslftheprocessor.BecauseaPLCis“softwarebased”,itscontrollogicfunctionscanechangedbyreprogrammingitsmemory.Keyboardprogrammingdevicesfacilitateentryoftherevisedprogram,whichcanbedesignedtocauseanexistingmachineorprocesstooperateinadifferentsequenceortorespondtodifferentlevelsof,orcombinationsofstimuli.Hardwaremodificationsareneededonlyifadditional,changed,orrelocatedinput/outputdevicesareinvolved.AllprogrammablecontrollersconsistofthebasicfunctionalblocksshowninFigure10.1.Wewillexamineeachblocktounderstandtherelationshiptothecontrolsystem.Firstwelookedatthecenter,asitistheheartofthesystem.Itconsistsofamicroprocessor,logicmemoryforthestorageoftheactualcontrollogic,storageorvariablememoryforusewithdatathatwillordinarilychangeasafunctionofthecontrolprogramexecution,andapowersupplytoprovideelectricalpowerfortheprocessorandmemory.NextcomestheI/Obloke.ThisfunctiontakesthecontrollevelsignalsfortheCPUandconvertsthemtovoltageandcurrentlevelssuitableforconnectionwithfactorygradesensorsandactuators.TheI/Otypecanrangefromdigital,analog,oravarietyofspecialpurpose“smart”I/Owhicharededicatedtoacertainapplicationtask.Theprogrammerisnormallyusedonlytoinitiallyconfigureandprogramasystemandisnotrequiredforthesystemtooperate.Itisalsousedintroubleshootingasystem,andcanprovetobeavaluabletoolinpinpointingtheexactcauseofaproblem.ThefielddevicesshownhererepresentthevarioussensorsandactuatorsconnectedtotheI/O.Thesearethearms,legs,eyes,andearsofthesystem,includingpushbuttons,limitswitches,proximityswitches,photosensors,thermocouples,positionsensingdevices,andbarcodereaderasinput；andpilotlight,displaydevices,motorstarters,DCandACdrivers,solenoids,andprintersasoutputs.Anadaptivecontrolsystemisonewhoseparametersareautomaticallyadjustedtocompensateforcorrespondingvariationsinthepropertiesoftheprocess.Thesystemis,inaword,“adapted”totheneedsoftheprocess.Naturallytheremustbe3somecriteriaonwhichtobaseanadaptiveprogram.Tospecifyavalueforthecontrolledvariableisnotenough-adaptionisnotrequiredtomeetthisspecification.Some“objectivefunction”ofthecontrolledvariablemustbespecifiedinaddition.Itisthisfunctionthatdeterminestheparticularformofadaptionrequired.Theobjectivefunctionforagivenprocessmaybethedampingofthecontrolledvariable.Inessence,therearethentwoloops,oneoperatingonthecontrolledvariable,theotheronitsdamping.Becausedampingidentifiesthedynamicloopgain,thissystemisdesignatedasadynamicadaptivesystem.Itisalsopossibletostipulateanobjectivefunctionofthesteady-stategainoftheprocess.Acontrolsystemdesignedtothisspecificationisthensteady-stateadaptive.Thereis,inpractice,solittleresemblancebetweenthesetwosystemsthattheirclassificationunderasingletitle“adaptive”hasledto,muchconfusion.Aseconddistinctionistobemade,thisnotontheobjectivefunction,butratheronthemechanismthroughwhichadaptionisintroduced,ifenoughisknownonaprocessthatparameteradjustmentscanberelatedtothevariableswhichcauseitspropertiestochange,adoptionmaybeprogrammed.However,ifitisnecessarytobaseparametermeansofafeedbackloop.Thisisknownasaselfadaptivesystem.4可編程邏輯控制器（PLC）可編程邏輯控制器（PLC）是一種固態(tài)電子裝置，它利用已存入的程序來控制機器的運行或工藝的工序。PLC通過輸入/輸出(I/O)裝置發(fā)出控制信號和接受輸入信號。PLC依據(jù)已存入程序所規(guī)定的邏輯控制輸出裝置響應輸入裝置的激勵。輸入裝置由限位開關、按鈕、手輪、開關、脈沖、模擬信號、ASCII系列數(shù)據(jù)和來自于絕對位置解碼器的二進制或BCD數(shù)據(jù)組成。輸出的是驅動電磁線圈、電動起動器、繼電器、指示燈等終端設備的電壓或電流電平。其他輸出裝置包括模擬裝置、數(shù)字BCD顯示、ASCII兼容裝置、伺服變速驅動器、甚至計算機。當通用汽車公司和其他制造商們正在試驗看能否有另一種方法來銷毀型號轉變過程中機床的所有布線控制面板各其他生產(chǎn)設備時，PLC被研制成了（大約在1968年）。這種年度例行工作是必要的，因為控制面板的重新布線比購買新的控制面板要貴得多。汽車公司與許多控制設