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外文翻译--可编程逻辑控制器(PLC).doc

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外文翻译--可编程逻辑控制器(PLC).doc

1ProgrammableLogicControllersProgrammablelogiccontrollerplcisasolidstatedeviceusedtocontrolmachinemotionorprocessoperationbymeansofastoredprogram.ThePLCsendsoutputcontrolsignalsandreceivesinputsignalsthroughinput/outputI/Odevices.APLCcontrolsoutputsinresponsetostimuliattheinputsaccordingtothelogicprescribedbythestoredprogram.Theinputsaremadeupoflimitswitches,pushbuttons,thunbwheels.Switches,pulses,analogsignals,ASCLLserialdata,andbinaryorBCDdatafromabsolutepositionencoders.Theoutputsarevoltageorcurrentlevelstodriveenddevicessuchassolenoids,motorstarters,relays,lights,andsoon.Otheroutputdevicesincludeanalogdevices,digitalBCDdisplays,ASCIIcompatibledevices,servovariablespeeddrives,andevencomputers.Programmablecontrollersweredevelopedcircain1968whenGeneralMotorsCorp,andotherautomobilemanufacturerswereexperimentingtoseeiftheremightbeanalternativetoscrappingalltheirhardwiredcontrolpanelsofmachinetoolsandotherproductionequipmentduringamodelchangeover.Thisannualtraditionwasnecessarybecauserewiringofthepanelswasmoreexpensivethanbuyingnewones.Theautomotivecompaniesapproachedanumberofcontrolequipmentmanufacturersandaskedthemtodevelopacontrolsystemthatwouldhavealongerproductivelifewithoutmajorrewiring,butwouldstillbeunderstandabletoandrepairablebyplantpersonnel.Thenewproductwasnamedaprogrammablecontroller.TheprocessorpartofthePLCcontainsacentralprocessingunitandmemory。Thecentralprocessingunitcpuisthetrafficdirectoroftheprocessor,thememorystoresinformation.Comingintotheprocessoraretheelectricalsignalsfromtheinputdevices,asconditionedbytheinputmoduletovoltagelevelsacceptabletoprocessorlogic.TheprocessorscansthestateofI/oandupdatesoutputsbasedoninstructionsstoredinthememoryoftheplc.Forexample,theprocessormaybeprogrammedsothatifaninputconnectedtoalimitswitchistruelimitswitchclosed,thenacorrespondingoutputwiredtoanoutputmoduleistobeenergized.Thisoutputmightbeasolenoid,forexampleoneachscantoseeifthatlimitswitchis,infact,closed.Ifitisclosed,theprocessorenergizesthesolenoidbyturningontheoutputmodule.Theoutputdevice,suchasasolenoidormotorstarter,iswiredtooutputmodulesterminal,anditreceivesitsshiftsignalfromtheprocessor,ineffect,theprocessorisperformingalongandcomplicatedseriesoflogicdecisions.ThePLC2performssuchdecisionssequentiallyandinaccordancewiththestoredprogram.Similarly,analogI/Oallowstheprocessortomakedecisionsbasedonthemagnitudelfasignal.Ratherthanjustifitisonoroff.Forexample,Theprocessormaybeprogrammedtoincreaseordecreasethesteamflowtoaboileranalogoutputbasedonacomparisonoftheactualtemperatureintheboileranaloginputtothedesiredtemperature.ThisisoftenperformedbyutilizingthebuiltinPIDproportional,integral,derivativecapabilitieslftheprocessor.BecauseaPLCissoftwarebased,itscontrollogicfunctionscanechangedbyreprogrammingitsmemory.Keyboardprogrammingdevicesfacilitateentryoftherevisedprogram,whichcanbedesignedtocauseanexistingmachineorprocesstooperateinadifferentsequenceortorespondtodifferentlevelsof,orcombinationsofstimuli.Hardwaremodificationsareneededonlyifadditional,changed,orrelocatedinput/outputdevicesareinvolved.AllprogrammablecontrollersconsistofthebasicfunctionalblocksshowninFigure10.1.Wewillexamineeachblocktounderstandtherelationshiptothecontrolsystem.Firstwelookedatthecenter,asitistheheartofthesystem.Itconsistsofamicroprocessor,logicmemoryforthestorageoftheactualcontrollogic,storageorvariablememoryforusewithdatathatwillordinarilychangeasafunctionofthecontrolprogramexecution,andapowersupplytoprovideelectricalpowerfortheprocessorandmemory.NextcomestheI/Obloke.ThisfunctiontakesthecontrollevelsignalsfortheCPUandconvertsthemtovoltageandcurrentlevelssuitableforconnectionwithfactorygradesensorsandactuators.TheI/Otypecanrangefromdigital,analog,oravarietyofspecialpurposesmartI/Owhicharededicatedtoacertainapplicationtask.Theprogrammerisnormallyusedonlytoinitiallyconfigureandprogramasystemandisnotrequiredforthesystemtooperate.Itisalsousedintroubleshootingasystem,andcanprovetobeavaluabletoolinpinpointingtheexactcauseofaproblem.ThefielddevicesshownhererepresentthevarioussensorsandactuatorsconnectedtotheI/O.Thesearethearms,legs,eyes,andearsofthesystem,includingpushbuttons,limitswitches,proximityswitches,photosensors,thermocouples,positionsensingdevices,andbarcodereaderasinputandpilotlight,displaydevices,motorstarters,DCandACdrivers,solenoids,andprintersasoutputs.Anadaptivecontrolsystemisonewhoseparametersareautomaticallyadjustedtocompensateforcorrespondingvariationsinthepropertiesoftheprocess.Thesystemis,inaword,adaptedtotheneedsoftheprocess.Naturallytheremustbe3somecriteriaonwhichtobaseanadaptiveprogram.Tospecifyavalueforthecontrolledvariableisnotenoughadaptionisnotrequiredtomeetthisspecification.Someobjectivefunctionofthecontrolledvariablemustbespecifiedinaddition.Itisthisfunctionthatdeterminestheparticularformofadaptionrequired.Theobjectivefunctionforagivenprocessmaybethedampingofthecontrolledvariable.Inessence,therearethentwoloops,oneoperatingonthecontrolledvariable,theotheronitsdamping.Becausedampingidentifiesthedynamicloopgain,thissystemisdesignatedasadynamicadaptivesystem.Itisalsopossibletostipulateanobjectivefunctionofthesteadystategainoftheprocess.Acontrolsystemdesignedtothisspecificationisthensteadystateadaptive.Thereis,inpractice,solittleresemblancebetweenthesetwosystemsthattheirclassificationunderasingletitleadaptivehasledto,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备制造商打交道,请他们开发一个控制系统,这个系统要有较长的生产寿命而无须主要线路重新布线,并且能被工厂人员所理解和维修。这个新系统被称作可编程控制器。PLC的处理器部分由中央处理器和存储器组成。中央处理器(CPU)是处理器的交通控制器存储器存信息。从输入装置来的电信号进入处理器后,经输入模块整理成处理器逻辑单元接受的电压电平。处理器监测I/O的状态,然后依据储存在PLC存储器中指令更新输出。例如,处理器可被编程以便当连接限位开关的输入位置时(限位开关闭合),连在输出模块上的输出装置被接通,例如,这个输出装置可以是电磁线圈。处理器通过存储器记录下这个指令并与每次检测相比较以确定限位开关是否真的闭合。如果闭合,处理器通过接通输出模块接通电磁线圈。诸如电磁线圈或电动起动机之类的输出装置被连接到输出模块的接线上,并从处理器接受它的位移信号。实际上,处理器在完成一系列长而复杂的逻辑判断。PLC按顺序并根据存储的程序来执行这样的判断。同样地,模拟I/O装置允许处理器依据信号的大小而不是其接通或关闭来做判断。例如,处理可被编程为根据锅炉实际温度(模拟输出)。这通常是用处理器的内置PID(比例,积分、微分)能力来实现的。因为PLC是基于软件的其控制逻辑功能可通过对存储器再编程而改变。键盘编程装置使修改的程序的输入更方便,该程序可以被设计成使现有机器或工序以不同顺序运行,或响应不同水平的激励或激励组合。只有当涉及到附加的、更改的或重新定位的输入/输出装置时,才需要个性硬件。可编程控制起都包含了基本的功能模块,为了理解控制系统的关系,我们将

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