外文翻译--可编程逻辑控制器（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可编程逻辑控制器（PLC）可编程逻辑控制器（PLC）是一种固态电子装置，它利用已存入的程序来控制机器的运行或工艺的工序。PLC通过输入/输出(I/O)装置发出控制信号和接受输入信号。PLC依据已存入程序所规定的逻辑控制输出装置响应输入装置的激励。输入装置由限位开关、按钮、手轮、开关、脉冲、模拟信号、ASCII系列数据和来自于绝对位置解码器的二进制或BCD数据组成。输出的是驱动电磁线圈、电动起动器、继电器、指示灯等终端设备的电压或电流电平。其他输出装置包括模拟装置、数字BCD显示、ASCII兼容装置、伺服变速驱动器、甚至计算机。当通用汽车公司和其他制造商们正在试验看能否有另一种方法来销毁型号转变过程中机床的所有布线控制面板各其他生产设备时，PLC被研制成了（大约在1968年）。这种年度例行工作是必要的，因为控制面板的重新布线比购买新的控制面板要贵得多。汽车公司与许多控制设