外文翻譯--數(shù)字控制.doc

1附錄:外文文獻NumericalControlOneofthemostfundamentalconceptsintheareaofadvancedmanufacturingtechnologiesisnumericalcontrol.Controllingamachinetoolusingapunchedtapeorstoredprogramisknownasnumericalcontrol(NC).NChasbeendefinedbytheElectronicIndustriesAssociation(EIA)as“asysteminwhichactionsarecontrolledbythedirectinsertionofnumericaldadaatsomepoint.thesystemmustautomaticallyinterpretatleastsomeportionofthisdata.”thenumericaldatarequiredtoproduceapartisknownasapartprogram.Anumericalcontrolmachinetoolsystemcontainsamachinecontrolunit(MCU)andthemachinetoolitself.TheMCUisfurtherdividedintotwoelements:thedataprocessingunit(DPU)andthecontrolloopsunit(CLU).TheDPUprocessesthecodeddatafromthetapeorothermediaandpassesinformationonthepotionsofeachaxis,requireddirectionofmotion,feedrate,andauxiliaryfunctioncontrolsignalstotheCLU.TheCLUoperatesthedrivemechanismsofthemachine,receivesfeedbacksignalsconcerningtheactualpositionandvelocityofeachoftheaxes,andsignalsthecompletionofoperation.TheDPUsequentiallyreadsthedata.WheneachlinehascompletedexecutionasnotedbytheCLU,antherlineofdataisread.Adataprocessingunitsconsistsofsomeorallofthefollowingparts:1)Datainputdevicesuchasapapertapereader,magnetictapereader,RS232-Cport,etc2)Data-readingcircuitsandparity-checkinglogic3)Decodingcircuitsfordistributingdataamongthecontrolledaxes4)Aninterpolator,whichsuppliesmachine-motioncommandsbetweendatapointsfortoolmotionAcontrolloopsunit,ontheotherhandconsistsofthefollowing:1)Positioncontrolloopsforalltheaxesofmotion,whereeachaxishasaseparatecontrolloop2)Velocitycontrolloops,wherefeedcontrolisrequired3)Decelerationandbacklashtakeupcircuits4)Auxiliaryfunctionscontrol,suchascoolanton/off,gearchange,spindleon/offcontrol2GeometricandkinematicdataaretypicallyfedfromtheDPUtotheCLU.TheCLUthengovernsthephysicalsystembasedonthedatafromtheDPU.Numericalcontrolwasdevelopedtoovercomethelimitationofhumanoperators,andithasdoneso.Numericalcontrolmachinesaremoreaccuratethanmanuallyoperatedmachines,theycanproducepartsmoreuniformly,theyarefaster,andthelong-runtoolingcostsarelower.ThedevelopmentofNCledtothedevelopmentofseveralotherinnovationsinmanufacturingtechnology:ElectricdischargemachiningLaser-cuttingElectronbeamweldingNumericalcontrolhasalsomademachinetoolsmoreversatilethantheirmanuallyoperatedpredecessors.AnNCmachinetoolcanautomaticallyproduceawidevarietyofparts,eachinvolvinganassortmentofwidelyvariedandcomplexmachiningprocesses.Numericalcontrolhasallowedmanufacturerstoundertaketheproductionofproductsthatwouldnothavebeenfeasiblefromaneconomicperspectiveusingmanuallycontrolledmachinetoolsandprocesses.HistoricalDevelopmentofNCLikesomanyadvancedtechnologies,NCwasborninthelaboratoriesoftheMassachusettsInstituteofTechnology.TheconceptofNCwasdevelopedintheearly1950swithfundingprovidedbytheU.S.AirForce.Initsearlieststages,NCmachineswereabletomakestraightcutsefficientlyandeffectively.However,curvedpathswereaproblembecausethemachinetoolhadtobeprogrammedtoundertakeaseriesofhorizontalandverticalstepstoproduceacurve.Theshorterthestraightlinesmakingupthesteps,thesmootheristhecurve.EachlinesegmentinthestepsshowninthecloseupinFig.2.17hadtobecalculated.ThiswasacumbersomeapproachthathadtobeovercomeifNCwastodevelopfurther.Thisproblemledtothedevelopmentin1959oftheAutomaticallyProgrammedTools(APT)language.ThisisaspecialprogramminglanguageforNCthatusesstatementssimilartoEnglishlanguagetodefinethepartgeometry,describethecuttingtoolconfiguration,andspecifythenecessarymotions.ThedevelopmentoftheAPTlanguagewasamajorstepforwardinthedevelopmentofNCtechnology.TheoriginalNCsystemswerevastlydifferentfromthoseusedtoday.Themachineshadhardwiredlogiccircuits.Theinstructionalprogramswerewrittenonpunchedpaper,whichwaslatertobereplacedbymagneticplastic3tape.Atapereaderwasusedtointerprettheinstructionswrittenonthetapeforthemachine.Together,allofthisrepresentedagiantstepforwardinthecontrolofmachinetools.However,therewereanumberofproblemswithNCatthispointinitsdevelopment.Amajorproblemwasthefragilityofthepunchedpapertapemedium.Itwascommonforthepapercontainingtheprogrammedinstructionstobreakortearduringamachiningprocess.Thisproblemwasexacerbatedbythefactthateachsuccessivetimeapartwasproducedonamachinetool,thepapertapecarryingtheprogrammedinstructionshadtobererunthroughthereader.Ifitwasnecessarytoproduce100copiesofagivenpart,itwasalsonecessarytorunthepapertapethroughthereader100separatetimes.Fragilepapertapessimplycouldnotwithstandtherigorsofashotfloorenvironmentandthiskindofrepeateduse.Thisledtothedevelopmentofaspecialmagneticplastictape.Whereasthepapertapecarriedtheprogrammedinstructionsasaseriesofholespunchedinthetape,theplastictapecarriedtheinstructionsasaseriesofmagneticdots.Theplastictapewasmuchstrongerthanthepapertape,whichsolvedtheproblemoffrequenttearingandbreakage.However,itstilllefttwootherproblems.Themostimportantofthosewasthatitwasdifficultorimpossibletochangetheinstructionsenteredonthetape.Tomakeeventhemostminoradjustmentsinaprogramofinstructions,itwasnecessarytointerruptmachiningoperationsandmakeanewtape.Itwasalsostillnecessarytorunthetapethroughthereaderasmanytimesastherewerepartstobeproduced.Fortunately,computertechnologybecamearealityandsoonsolvedtheproblemofNCassociatedwithpunchedpaperandplastictape.1)AdventofDirectNumericalControlThedevelopmentofaconceptknownasdirectnumericalcontrol(DNC)solvedthepaperandplastictapeproblemsassociatedwithnumericalcontrolbysimplyeliminatingtapeasthemediumforcarryingtheprogrammedinstructions.Indirectnumericalcontrol,machinetoolsaretied,viaadatatransmissionlink,toahostcomputer(Fig2.18).Programsforoperatingthemachinetoolsarestoredinthehostcomputerandfedtothemachinetoolasneededviathedatatransmissionlinkage.Directnumericalcontrolrepresentedamajorstepforwardoverpunchedtapeandplastictape.However,itissubjecttothesamelimitationsasalltechnologiesthatdependonahostcomputer,themachinetoolsalsoexperiencedowntime.Thisproblemledtothedevelopmentofcomputernumericalcontrol.42)AdventofComputerNumericalControlThedevelopmentofthemicroprocessorallowedforthedevelopmentofprogrammablelogiccontrollers(PLCs)andmicrocomputer.Thesetwotechnologiesallowedforthedevelopmentofcomputernumericalcontrol(CNC).WithCNC,eachmachinetoolhasaPLCoramicrocomputerthatservesthepurpose.Thisallowsprogramstobeinputandstoredateachindividualmachinetool.Italsoallowsprogramstobedevelopedoff-lineanddownloadedattheindividualmachinetool.CNCsolvedtheproblemsassociatedwithdowntimeofthehostcomputer,butitintroducedanotherproblemknownasdatamanagement.Thisisaproblemallworksettingsdependentonmicrocomputershave.Thesameprogrammightbeloadedontendifferentmicrocomputerswithnocommunicationamongthem.Thisproblemistheprocessofbeingsolvedbylocalareanetworksthatconnectmicrocomputersforbetterdatamanagement.Theproblemofdatamanagementledtothedevelopmentofdistributednumericalcontrol.3)AdventofDistributedNumericalControlDistributednumericalcontrol(alsocalledDNC)takesadvantageofthebestaspectsofdirectnumericalcontrolandcomputernumericalcontrol.Withdistributednumericalcontroltherearebothhostcomputersandlocalcomputersattheindividualmachinetools(Fig2.19).Thisa