NumericalControl机械英语文章翻译.docx

NumericalControl机械英语文章翻译.docx

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NumericalControl机械英语文章翻译

NumericalControl

　　Oneofthemostfundamentalconceptsintheareaofadvancedmanufacturingtechnologiesisnumericalcontrol.

　　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,etc

　　2）Data-readingcircuitsandparity-checkinglogic

　　3）Decodingcircuitsfordistributingdataamongthecontrolledaxes、

　　4）Aninterpolator,whichsuppliesmachine-motioncommandsbetweendatapointsfortoolmotion

　　Acontrolloopsunit,ontheotherhandconsistsofthefollowing:

　　1）Positioncontrolloopsforalltheaxesofmotion,whereeachaxishasaseparatecontrolloop

　　2）Velocitycontrolloops,wherefeedcontrolisrequired

　　3）Decelerationandbacklashtakeupcircuits

　　4）Auxiliaryfunctionscontrol,suchascoolanton/off,gearchange,spindleon/offcontrol

　　GeometricandkinematicdataaretypicallyfedfromtheDPUtotheCLU.

　　TheCLUthengovernsthephysicalsystembasedonthedatafromtheDPU.

　　Numericalcontrolwasdevelopedtoovercomethelimitationofhumanoperators,andithasdoneso.Numericalcontrolmachinesaremoreaccuratethanmanuallyoperatedmachines,theycanproducepartsmoreuniformly,theyarefaster,andthelong-runtoolingcostsarelower.ThedevelopmentofNCledtothedevelopmentofseveralotherinnovationsinmanufacturingtechnology:

　　lElectricdischargemachining

　　2Laser-cutting

　　3Electronbeamwelding

　　Numericalcontrolhasalsomademachinetoolsmoreversatilethantheirmanuallyoperatedpredecessors.AnNCmachinetoolcanautomaticallyproduceawidevarietyofparts,eachinvolvinganassortmentofwidelyvariedandcomplexmachiningprocesses.Numericalcontrolhasallowedmanufacturerstoundertaketheproductionofproductsthatwouldnothavebeenfeasiblefromaneconomicperspectiveusingmanuallycontrolledmachinetoolsandprocesses.

　　HistoricalDevelopmentofNC

　　Likesomanyadvancedtechnologies,NCwasborninthelaboratoriesoftheMassachusettsInstituteofTechnology.TheconceptofNCwasdevelopedintheearly1950swithfundingprovidedbytheU.S.AirForce.Initsearlieststages,NCmachineswereabletomakestraightcutsefficientlyandeffectively.

　　However,curvedpathswereaproblembecausethemachinetoolhadtobeprogrammedtoundertakeaseriesofhorizontalandverticalstepstoproduceacurve.Theshorterthestraightlinesmakingupthesteps,thesmootheristhecurve.EachlinesegmentinthestepsshowninthecloseupinFig.2.17hadtobecalculated.ThiswasacumbersomeapproachthathadtobeovercomeifNCwastodevelopfurther.

　　Thisproblemledtothedevelopmentin1959oftheAutomaticallyProgrammedTools（APT）language.ThisisaspecialprogramminglanguageforNCthatusesstatementssimilartoEnglishlanguagetodefinethepartgeometry,describethecuttingtoolconfiguration,andspecifythenecessarymotions.ThedevelopmentoftheAPTlanguagewasamajorstepforwardinthedevelopmentofNCtechnology.TheoriginalNCsystemswerevastlydifferentfromthoseusedtoday.Themachineshadhardwiredlogiccircuits.Theinstructionalprogramswerewrittenonpunchedpaper,whichwaslatertobereplacedbymagneticplastictape.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）AdventofDirectNumericalControl

　　Thedevelopmentofaconceptknownasdirectnumericalcontrol（DNC）solvedthepaperandplastictapeproblemsassociatedwithnumericalcontrolbysimplyeliminatingtapeasthemediumforcarryingtheprogrammedinstructions.Indirectnumericalcontrol,machinetoolsaretied,viaadatatransmissionlink,toahostcomputer（Fig2.18）.Programsforoperatingthemachinetoolsarestoredinthehostcomputerandfedtothemachinetoolasneededviathedatatransmissionlinkage.Directnumericalcontrolrepresentedamajorstepforwardoverpunchedtapeandplastictape.However,itissubjecttothesamelimitationsasalltechnologiesthatdependonahostcomputer,themachinetoolsalsoexperiencedowntime.Thisproblemledtothedevelopmentofcomputernumericalcontrol.

　　2）AdventofComputerNumericalControl

　　Thedevelopmentofthemicroprocessorallowedforthedevelopmentofprogrammablelogiccontrollers（PLCs）andmicrocomputer.Thesetwotechnologiesallowedforthedevelopmentofcomputernumericalcontrol（CNC）.WithCNC,eachmachinetoolhasaPLCoramicrocomputerthatservesthepurpose.Thisallowsprogramstobeinputandstoredateachindividualmachinetool.Italsoallowsprogramstobedevelopedoff-lineanddownloadedattheindividualmachinetool.CNCsolvedtheproblemsassociatedwithdowntimeofthehostcomputer,butitintroducedanotherproblemknownasdatamanagement.Thisisaproblemallworksettingsdependentonmicrocomputershave.Thesameprogrammightbeloadedontendifferentmicrocomputerswithnocommunicationamongthem.Thisproblemistheprocessofbeingsolvedbylocalareanetworksthatconnectmicrocomputersforbetterdatamanagement.Theproblemofdatamanagementledtothedevelopmentofdistributednumericalcontrol.

　　3）AdventofDistributedNumericalControl

　　Distributednumericalcontrol（alsocalledDNC）takesadvantageofthebestaspectsofdirectnumericalcontrolandcomputernumericalcontrol.Withdistributednumericalcontroltherearebothhostcomputersandlocalcomputersattheindividualmachinetools（Fig2.19）.Thisallowstheprogramstobestoredinthehostcomputersand,thereby,bettermanaged.However,italsoallowsthemtobedownloadedtolocalmicrocomputersorPLCs.ItalsoallowsforlocalinputandinteractionthroughmicrocomputersorPLCsatthemachinelevels.

　　NCMachineComponents

　　TherearefourcomponentsinanyNCmachine:

　　lTheactualNCtool

　　2Themachinecontrolunit（MCU）

　　3ThecommunicationinterfacebetweentheNCmachineandtheMCU

　　4AvarietyofaccessoriesforperformingspecificjobsontheNCmachine

　　TheactualNCmachinemaybeamillingmachine,lathe,drill,oranyothertypeofmachinetool..TheMCUisthecontrolunitthatholdstheprogramsthatinstructtheNCmachine.TheMCUalsohasvariousdevicesavailableforoperatorinput.InformationcontainedintheMCUiscarriedtotheactivatorsontheNCmachinethroughthecommunicationinterface.TheseactivatorsreceivetheelectronicsignalsfromtheMCUandcausethemechanicalapparatusoftheNCmachinetooperate.

　　LesssophisticatedNCmachineshaveopen-loopactivators.Anopen-loopactivatorcanreceiveasignalandcarryouttheinstructionscontainedinthatsignal,butcannotfeedbacktotheMCUtoshowthatinstructionscarriedinthesignalhavebeenproperlycompleted.MoresophisticatedNCmachineuseclosed-loopactivators.Aclosed-loopactivatorcanreceiveandcarryoutasignalandf