Foundation of PLC外文翻译PLC发展史Word文件下载.docx
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外文翻译
专业:
班级学号:
学生姓名:
指导教师:
二〇一一年六月
本科生毕业设计
原文1:
Programmablelogiccontrollers
译文1:
可编程逻辑控制器
原文2:
FoundationofPLC
译文2:
PLC基础
专业班级:
学院:
2011年6月
Programmablelogiccontroller(PLC)iseight10-Yearonbehalfnewgenerationindustrythatdevelopthecontrolequip,andisanautomaticcontrol,calculatorwiththethingthatthecorrespondencetechniquecombinetogether,andisathespotequipmentsforexclusivelyusedforingtheindustryproductionlinecontroling.MakethePLCthereischaracteristicsofobviousoneselfonthedesignwiththelong-termandcontinuousthatcirculatebecauseofthespecialofthecomplexity,usageenvironmentofthecontrolobject:
Thedependableishigh,andtheadaptabilityiswide,andhavetocorrespondbyletterthefunction,andweavethetheconvenience,constructionmoldpiece.Gatherthethecontrolinthemoderninthesystem,thePLChavealreadybecomeakindofimportanceofbasiccontrolunit,controltherealmtheinsideintheindustryappliedtheforegroundisveryandextensive.
Aprogrammablelogiccontroller(PLC)isasolid-statedevideusedtocontrolmachinemotionorprocessoperationbymeansofastoredprogram.ThePLCsendsoutputcontrolsignalsandreceivesinputsignalsthroughinput/output(I/O)devices.APLCcontrolsoutputsinresponsetostimuliattheinputsaccordingtothelogicprescribedbythestoredprogram.Theinputsaremadeupoflimitswitches,pushbuttons,thumbwheels,switches,pulses,analogsignals,ASCIIserialdata,andbinaryorBCDdatafromabsolutepositionencoders.Theoutputsarevoltageorcurrentleverstodriveenddevicessuchaslolenids,motorstaters,relays,lights,andsoon.Otheroutputdevicessuchincludeanalogdevices,digitalBCDdisplays,ASCIIcompatibledevices,servovariable-speeddrives,andevencomputers.
Programmablecontrollersweredeveloped(circain1968)whenGeneralMotorsCorp,andotherautomobilemanufacturerswereexperimentingtoseeiftheremightbeanalterantivetoscrappingalltheirhardwiredcontrolpanelsofmachinetoolsandotherproductionequipmentduringamodelchangeover.Thisannualtraditionwasnecessarybecauserewiringofthepanelswasmoreexpensivethanbuyingnewoens.
Theautomotivecompaniesapproachedanumberofcontrolequipmentmanufacturersandaskedthemtodevelopacontrolsystemthatwouldhavealongerproductivelifewithoutmajorrewiring,butwouldstillbeunderstandabletoandrepairablebyplantpersonnel.Thenewproductwanamda“programmablecontroller”.
TheprocessorpartofthePLCcontainsacentralprocessingunitandmemory.Thecentralprocessingunit(CPU)isthe“trafficdirector”oftheprocessor,thememorystoresinformation.Comingintotheprocessoraretheelectricalsignalsfromtheinputdevices,asconditionedbytheinputmoduletovoltagelevelsacceptabletoprocessorlogic.TheprocessorscansthestateofI/OandupdatesoutputsbasedoninstructionsstoredinthememoryofthePLC.Forexample,theprocessormaybeprogrammedsothatifaninputconnectedtoalimitswitchistrue(1imitswitchclosed),thenacorrespondingoutputwiredtoanoutputmoduleistobeenergized.Thisoutputmightbeasolenoidforexample.Theprocessorremembersthiscommandthroughitsmemoryandcomparesoneachscantoseeifthatlimitis,infact,closed.Ifitisclosed,theprocessorenergizesthesolenoidbyturningontheoutputmodule.
Theoutputdevice,suchasasolenoidormotorstater,iswiredtoanoutputmofule’sterminal,anditreceivesitsshiftsignalfromtheprocessor,ineffectthepeocessorisperformingalongandcomplicatedseriesoflogicdecisions.ThePLCperformssuchdecisionssequentiallyandinaccordancewiththestoredprogram.similarly,analogI/Oallowstheprocessortomakedecisionsbasedonthemagnitudeofasignal,ratherthanjustifisonoroff.Forexample,theprocessormaybeprogrammestoencreaseordecreasethesteamflowtoaboiler(analogoutput)basedonacomparisonoftheactualtemperatureintheboiler(analoginput)tothedesiredtemperature.thisisoftenperformedbyutilizingthebuilt-inPID(proportional,integral,derivative)capabilitiesoftheprocessor.
Properpowertotheprogrammablecontrolleriscritical.Today’ssystemsareavailableinawidevarietyofelectricalconfigurations.Virtuallyallaredesignedforuseinsingle-phasepowersystems,andmostarenowbeginningtobeofferedwiththeoptionalabilitytooperateinaDCsupplyenvironment.ACdesignsareofferedineithersinglevoltagesupplies,suchas115or230VAC;
whilesomecanbeconfiguredaseitherthroughaselectionmadeonthepowersupply.Propergroundingofthepowersupplyconnectionisrequiredforasafeinstallation.Someprogrammablecontrollerdesignshaveindividualgroundingconnectionsfromracktoface-platesandothersystemcomponents,socaremustbetakentofollowwellelectricalpracticeinsystemgroundingduringelectricalinstallation.Incertainapplications,a24or120VDCpowersupplyisrequired.ThisiscommonforinstallationsthataxemadewherenoACpowerisavailable,suchasremoteelectricalgenerationstations.ItisalsofoundwhereACpowerisunreliableandwherelossofcontrolisconsideredanunacceptablesituation
Eventhebestoftoday'
swell-designedandmanufacturedprogrammablecontrollersrequireoccasionalpreventativemaintenanceandrepair.Thissectionlooksatsomeofthetoolsprovidedbythemanufacturerandtechniquesforgeneralmaintenance.Mostofthemedium-andlarge-sizedprogrammablecontrollersystemsavailabletodayaredesignedtobemaintainedbyindividualswithawidevarietyofskills,withoutthebenefitofin-depthformaltrainingofthispieceofequipment.Thisisaccomplishedinthedesignbyprovidingindividualmodulesoffunctionalityinstalledinachassisservicedfromthefront(allmoduletypesincludingpowersupplies).Frontaccessiscriticaltopropermaintenance.Thisallowseasyinspectionandreplacementofthesuspectedbadmodule.ModulehealthisdeterminedbyinspectingtheLEDindicatorsnormallyprovidedonthefrontofeachmodule.Typicalindicatorswillbeonoroffdependingonthedesignandindividualconditionofthemoduleinquestion.VariousCPUandI/OmoduleswillhaveindicatorsshowingI/Ocontrolcommunicationsstatus,memoryintegrity,powersupplytolerancecheck,scanintegrity,andothers.Onfuturecontrollerdesigns,andeventodayonafewsystems,itislikelythatEnglishlanguagemessageswillbedisplayedonthecontrolleradvisingtheuserormaintenancepersonnelthataparticularfailurehasoccurredandrecommendedactionstotake.
Themodulardesignanddiagnosticindicatorsare,ofcourse,important,butwouldbequiteuselesswithoutwelldesigneddocumentationprovidedbythemanufacturerfortheprogrammablecontrollersysteminquestion.ProperdocumentationwillhavesectionsdedicatedtoeachmajorsubsystemincludingCPU,I/O,andprogrammingdevice.Eachshouldexplainindepththestop-by-stopinspectionofthesystem.Allpossiblecombinationsoffailuremodeshouldbelisted,alongwithsuggestedactionsforrepair.Thiswillmostofteninvolveonlythesubstitutionofare-placementboardforthesuspectedfailedunit.Theuserisurgedtopurchaseasetofsparemodulesforthesysteminquestionasrecommendedbythemanufacturer.Thisisnormally,ataminimum,asinglereplacementmoduleforeachCPUandprogrammingdeviceserviceablemodule,andspareI/Omodulesequalto10%ofthenumberinthesystem.
BecauseaPLCis“softwarebased”,itscontrollogicfunctionscanbechangedbyreprogrammingitsmemory.Keyboardprogrammingdevicesfacilitateentryoftherevisedprogram,whichcanbedesignedtocauseanexistingmachineorprocesstooperateinadifferentsequenceortorespondtodifferentlevelsof,orcombinationsofstimuli.Hardwaremodificationsareneededonlyifadditional,changed,orrelocatedinput/outputdevicesareinvolved.
Programmablecontrollermemoryisformattedintobits,bytes,andwordsofmemory.Abitisasinglestorageelementforeitherazerooraone.Abyteconsistsofeightbits,andaword(normally)consistsof16bits,ortwobytes.Somesystemsstilluseawordlengthofeightbits,butmosthaveadopteda16bitword,eventhoughtheymayusean8bitmicroprocessor.
Dependingonthespecificdesignoftheprogrammablecontroller,itwillhaveastatedmemorycapacity.Thisisanindication,althoughnottheonlyone,ofthecapabilityandpowerofthesystem.Mediumandlargecontrollersarenormallyexpandablefromonememorysizetotheirmaximumsize.Smallcontrollersarenormallyfixedintheirmemorysize.Sizeofthememorycapacitymustbeexaminedrelativetothewordsize(8bitor16bit)andutilization.Whileitisclearthattwicetheinformationcanbestoredina16bitwordthaninan8bitword,itmaynotbeimmediatelyclearthatsomecontrollersutilizememorymoreefficientlythanothers.Forexample,anormallyopencontactanditsassociatedreferenceaddress(e.g.Input1),mayusein8bitbyteeachforstorage.Combined,theyconsumeone16bitword.Somecontrollersmayusemorememorythanthisfortheseinstructionsorothers.Inalargeprogram,theseinefficienciescanbuildoneachothertocauseapoorutilizationofthesystemmemory.Acarefulanalysisofthevariousprogrammablecontrollermodelsisrequiredtoassessutilizationefficiency.Normalpracticecallsforanadditional20%-40%ofmemorysizetobespecifiedtoallowformodificationsandlaterexpansion.Thisanalysis,combinedwithknowledgeoftheapplicationneeds,willallowforanintelligen