NumericalControl机械英语文章翻译.docx
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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
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