本科毕业生外文文献翻译.docx

本科毕业生外文文献翻译.docx

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本科毕业生外文文献翻译

本科毕业生外文文献翻译

学生姓名：

史衍彬

指导教师：

荣丽红

所在学院：

信息技术学院

专业：

农业电气化与自动化

中国·大庆

2010年4月

SCMprofile

IntroductionofProgrammablecontrollers

Fromasimpleheritage,theseremarkablesystemshaveevolvedtonotonlyreplaceelectromechanicaldevices,buttosolveanever-increasingarrayofcontrolproblemsinbothprocessandnonprocessindustries.Byallindications,thesemicroprocessorpoweredgiantswillcontinuetobreaknewgroundintheautomatedfactoryintothe1990s.

HISTORY

Inthe1960s,electromechanicaldevicesweretheorderofthedayassfarascontrolwasconcerned.Thesedevices,commonlyknownasrelays,werebeingusedbythethousandstocontrolmanysequential-typemanufacturingprocessesandstand-alongmachines.Manyoftheserelayswereinuseinthetransportationindustry,morespecifically,theautomotiveindustry.Theserelaysusedhundredsofwiresandtheirinterconnectionstoeffectacontrolsolution.Theperformanceofarelaywasbasicallyreliable-atleastasasingledevice.Butthecommonapplicationsforrelaypanelscalledfor300to500ormorerelays,andthereliabilityandmaintenanceissuesassociatedwithsupportingthesepanelsbecameaverygreatchallenge.Costbecameanotherissue,forinspiteofthelowcostoftherelayitself,theinstalledcostofthepanelcouldbequitehigh.Thetotalcostincludingpurchasedparts,wiring,andinstallationlabor,couldrangefrom$30~$50perrelay.Tomakemattersworse,theconstantlychangingneedsofaprocesscalledforrecurringmodificationsofacontrolpanel.Withrelays,thiswasacostlyprospect,asitwasaccomplishedbyamajorrewiringeffortonthepanel.Inadditionthesechangesweresometimespoorlydocumented,causingasecond-shiftmaintenancenightmaremonthslater.Inlightofthis,itwasnotuncommontodiscardanentirecontrolpanelinfavorofanewonewiththeappropriatecomponentswiredinamannersuitedforthenewprocess.Addtothistheunpredictable,andpotentiallyhigh,costofmaintainingthesesystemsasonhigh-volumemotorvehicleproductionlines,anditbecameclearthatsomethingwasneededtoimprovethecontrolprocess–tomakeitmorereliable,easiertotroubleshoot,andmoreadaptabletochangingcontrolneeds.

Thatsomething,inthelate1960s,wasthefirstprogrammablecontroller.Thisfirst‘revolutionary’systemwandevelopedasaspecificresponsetotheneedsofthemajorautomotivemanufacturersintheUnitedStates.Theseearlycontrollers,orprogrammablelogiccontrollers（PLC）,representedthefirstsystemsthat1couldbeusedonthefactoryfloor,2couldhavethere‘logic’changedwithoutextensiverewiringorcomponentchanges,and3wereeasytodiagnoseandrepairwhenproblemsoccurred.

Itisinterestingtoobservetheprogressthathasbeenmadeinthepast15yearsintheprogrammablecontrollerarea.Thepioneerproductsofthelate1960smusthavebeenconfusingandfrighteningtoagreatnumberofpeople.Forexample,whathappenedtothehardwiredandelectromechanicaldevicesthatmaintenancepersonnelwereusedtorepairingwithhandtools?

Theywerereplacedwith‘computers’disguisedaselectronicsdesignedtoreplacerelays.Eventheprogrammingtoolsweredesignedtoappearasrelayequivalentpresentations.Wehavetheopportunitynowtoexaminethepromise,inretrospect,thattheprogrammablecontrollerbroughttomanufacturing.

AllprogrammablecontrollersconsistofthebasicfunctionalblocksshowninFig.10.1.We’llexamineeachblocktounderstandtherelationshiptothecontrolsystem.Firstwelookatthecenter,asitistheheart（oratleastthebrain）ofthesystem.Itconsistsofamicroprocessor,logicmemoryforthestorageoftheactualcontrollogic,storageorvariablememoryforusewithdatathatwillordinarilychangeasafunctionpowerfortheprocessorandmemory.NextcomestheI/Oblock.ThisfunctiontakesthecontrollevelsignalsfortheCPUandconvertsthemtovoltageandcurrentlevelssuitableforconnectionwithfactorygradesensorsandactuators.TheI/Otypecanrangefromdigital（discreteoron/off）,analog（continuouslyvariable）,oravarietyofspecialpurpose‘smart’I/Owhicharededicatedtoacertainapplicationtask.Theprogrammerisshownhere,butitisnormallyusedonlytoinitiallyconfigureandprogramasystemandisnotrequiredforthesystemtooperate.Itisalsousedintroubleshootingasystem,andcanprovetobeavaluabletoolinpinpointingtheexactcauseofaproblem.ThefielddevicesshownhererepresentthevarioussensorsandactuatorsconnectedtotheI/O.Thesearethearms,legs,eyes,andearsofthesystem,includingpushbuttons,limitswitches,proximityswitches,photosensors,thermocouples,RTDS,positionsensingdevices,andbarcodereaderasinput;andpilotlights,displaydevices,motorstarters,DCandACdrives,solenoids,andprintersasoutputs.

Nosingleattemptcouldcoveritsrapidlychangingscope,butthreebasiccharacteristicscanbeexaminedtogiveclassifyanindustrialcontroldeviceasaprogrammablecontroller.

（1）Itsbasicinternaloperationistosolvelogicfromthebeginningofmemorytosomespecifiedpoint,suchasendofmemoryorendofprogram.Oncetheendisreached,theoperationbeginsagainatthebeginningofmemory.Thisscanningprocesscontinuesfromthetimepowerissuppliedtothetimeititremoved.

（2）Theprogramminglogicisaformofarelayladderdiagram.Normallyopen,normallyclosedcontacts,andrelaycoilsareusedwithinaformatutilizingaleftandarightverticalrail.Powerflow（symbolicpositiveelectronflow）isusedtodeterminewhichcoiloroutputsareenergizedordeenergized.

（3）Themachineisdesignedfortheindustrialenvironmentfromitsbasicconcept;thisprotectionisnotaddedatalaterdate.TheindustrialenvironmentincludesunreliableACpower,hightemperatures（0to60degreeCelsius）,extremesofhumidity,vibrations,RFnoise,andothersimilarparameters.

Generalapplicationareas

Theprogrammablecontrollerisusedinawidevarietyofcontrolapplicationstoday,manyofwhichwerenoteconomicallypossiblejustafewyearsago.Thisistruefortwogeneralreasons:

1therecosteffectiveness（thatis,thecostperI/Opoint）hasimproveddramaticallywiththefallingpricesofmicroprocessorsandrelatedcomponents,and2theabilityofthecontrollertosolvecomplexcomputationandcommunicationtaskshasmadeitpossibletouseitwhereadedicatedcomputerwaspreviouslyused.

Applicationsforprogrammablecontrollerscanbecategorizedinanumberofdifferentways,includinggeneralandindustrialapplicationcategories.Butitisimportanttounderstandtheframeworkinwhichcontrollersarepresentlyunderstoodandusedsothatthefullscopeofpresentandfutureevolutioncanbeexamined.Itisthroughthepowerofapplicationsthatcontrollerscanbeseenintheirfulllight.Industrialapplicationsincludemanyinbothdiscretemanufacturingandprocessindustries.Automotiveindustryapplications,thegenesisoftheprogrammablecontroller,continuetoprovidethelargestbaseofopportunity.Otherindustries,suchasfoodprocessingandutilities,providecurrentdevelopmentopportunities.

Therearefivegeneralapplicationareasinwhichprogrammablecontrollersareused.Atypicalinstallationwilluseoneormoreoftheseintegratedtothecontrolsystemproblem.Thefivegeneralareasareexplainedbrieflybelow.

Description

TheAT89C51isalow-power,high-performanceCMOS8-bitmicrocomputerwith4KbytesofFlashprogrammableanderasablereadonlymemory（PEROM）.ThedeviceismanufacturedusingAtmel’shigh-densitynonvolatilememorytechnologyandiscompatiblewiththeindustry-standardMCS-51instructionsetandpinout.Theon-chipFlashallowstheprogrammemorytobereprogrammedin-systemorbyaconventionalnonvolatilememoryprogrammer.Bycombiningaversatile8-bitCPUwithFlashonamonolithicchip,theAtmelAT89C51isapowerfulmicrocomputerwhichprovidesahighly-flexibleandcost-effectivesolutiontomanyembeddedcontrolapplications.

Functioncharacteristic

TheAT89C51providesthefollowingstandardfeatures:

4KbytesofFlash,128bytesofRAM,32I/Olines,two16-bittimer/counters,afivevectortwo-levelinterruptarchitecture,afullduplexserialport,on-chiposcillatorandclockcircuitry.Inaddition,theAT89C51isdesignedwithstaticlogicforoperationdowntozerofrequencyandsupportstwosoftwareselectablepowersavingmodes.TheIdleModestopstheCPUwhileallowingtheRAM,timer/counters,serialportandinterruptsystemtocontinuefunctioning.ThePower-downModesavestheRAMcontentsbutfreezestheoscillatordisablingallotherchipfunctionsuntilthenexthardwarereset.

PinDescription

VCC：

Supplyvoltage.

GND：

Ground.

Port0：

Port0isan8-bitopen-drainbi-directionalI/Oport.Asanoutputport,eachpincansinkeightTTLinputs.When1sarewrittentoport0pins,thepinscanbeusedashighimpedanceinputs.Port0mayalsobeconfiguredtobethemultiplexedloworderaddress/databusduringaccessestoexternalprogramanddatamemory.InthismodeP0hasinternalpullups.Port0alsoreceivesthecodebytesduringFlashprogramming,andoutputsthecodebytesduringprogramverification.Externalpullupsarerequiredduringprogramverification.

Port1

Port1isan8-bitbi-directionalI/Oportwithinternalpullups.ThePort1outputbufferscansink/sourcefourTTLinputs.When1sarewrittentoPort1pinstheyarepulledhighbytheinternalpullupsandcanbeusedasinputs.Asinputs,Port1pinsthatareexternallybeingpulledlowwillsourcecurrent（IIL）becauseoftheinternalpullups.Port1alsoreceives