外文翻译--电动机和数控技术.doc

MotorsandNumericalControlMotorschangeelectricalenergyintomechanicalenergy.Thisissimplythemechanicalforcesbetweenmagnets.whentwomagnets(orelectromagnets)approacheachother,onewillbeeitherpulledtowardorpushedawayfromtheother.1.CONSTRUCTIONEachtypeofmotorisbuilttodoitsownspecialjob.Allofthemhavetwomainparts.Onepartiscalledthestator.Thestatorisstationary；itdoesnotmove.Theotherpart,therotor,ismountedonbearingssoitcanrotate.Therotorshaftsticksoutbeyondthehousing.Foramotor,therotorshaftiscoupledtoamechanicalload.2.MOTORSThepowerinputtoamotoriselectrical.Voltageisappliedtoamotorsterminalsresultinginacurrent.Thepoweroutputofamotorismechanical.Thispoweristransmittedbytherotorshaftasatorque.Thistorquetendstorotateaload,suchasafanorpump.Asamotordrivesaload,mechanicalpowerisdrawnfromthemotor.Themotor,inturn,drawselectricalpowerfromthesourceatthesamerate3.NUMERICALCONTROLThemajordisadvantageofmachinetoolautomationthusfardescribedliesintheeconomicsoftheprocess.Itisexpensivetotoolainstallationsforautomaticproduction.Therefore,unlessthepartistobemadeinverylargenumbers,thecostbecomesprohibitive.Greatneedexistsforamethodthatpermitsrapidautomaticproduction,economicalinjob-lotamounts.Theanswerhasbeenfoundinthenumericalcontrolofmachinetools.ComputerNumericalControl.Numericalcontrol(NC)isaformofprogrammableautomationinwhichtheprocessingequipmentiscontrolledbymeansofnumbers,letters,andothersymbols.Thenumbers,letters,andsymbolsarecodedinanappropriateformattodefineaprogramofinstructionsforaparticularworkpieceorjob.Whenthejobchanges,theprogramoftheparticularworkpiecechanges.ThecapabilitytochangetheprogramiswhatmakesNCsuitableforlow-volumeandmedium-volumeproduction,anditismucheasiertowritenewprogramsthantomakemajoralterationstotheprocessingequipment.Thereisawideareaofperformanceduplicationbetweennumericalcontrolandautomatics.Numericalcontrol,however,offersmoreflexibility,lowertoolingcost,quickerchanges,andlessmachinedown-time.Inmachiningcontours,numericalcontrolcanmathematicallytranslatethedefinedcurveintoafinishedproduct,savingtimeandeliminatingtemplates.Thiscaninturnimproveaccuracy.Anotheradvantageappearstobegreatsavingofmachinetime,theequivalentogincreasingproductivecapacitywithnoincreaseinfacilities.Numericalcontrol(N/C)isaformofprogrammableautomationinwhichtheprocessingequipmentiscontrolledbymeansofnumbers,letters,andothersymbols.Thenumbers,letters,andsymbolsarecodedinanqppropriateformattodefineaprogramofinstructionsforaparticularworkpartorjob.Whenthejobchanges,theprogramofinstructionsischanged.ThecapabilitytochangetheprogramiswhatmakesN/Csuitableforlow-andmedium-volumeproduction.Itismucheasiertowriteprogramsthantomakemajoralterationsoftheprocessingequipment.Therearetwobasictypesofnumericallycontrolledmachinetools:point-to-pointandcontinuous-path(alsocalledcontouring).Point-to-pointmachinesuseunsynchronizedmotors,whittheresultthatthepositionofthemachiningheadcanbeassuredonlyuponcompletionofamovement,orwhileonlyonemotorisrunning.TheN/Csystemconsistsofthefollowingcomponents:datainput,thetapereaderwiththecontrolunit,feedbackdevices,andthemetal-cuttingmachinetollorothertypeofN/Cequipment.Datainput,alsocalled“man-to-controllink”,maybeprovidedtothemachinetoolmanually,orentirelybyautomaticmeans.Manualmethodswhenusedasthesolesourceofinputdataarerestrictedtoarelatibelysmallnumberofinputs.Examplesofmanuallyoperateddevicesarekryboarddials,pushbuttons,switches,orthumbwheelselectors.Thesearelocatedonaconsolenearthemachine.Dialsareanalogdevicesusuallyconnectedtoasynchro-typeresolverorpotentiometer.Inmostcases,pushbuttons,switches,andothersimilartypesofselectorsaredigitalinputdevices.Manualinputrequiresthattheoperatorsetthecontrolsforeachoperation.Itisaslowandtediousprocessandisseldomjustifiedexceptinelementarymachiningapplicationsorinspecialcases.Inpracticallyallcases,informationisautomaticallysuppliedtothecontrolunitandthemachinetoobycards,punchedtapes,orbymagnetictape.Eight-channelpunchedpapertapeisthemostcommonlyusedformofdatainputforconventionalN/Csystems.Thecodedinstructionsonthetapeconsistofsectionsofpunchedholescalledblocks.Eachblockrepresentsamachinefunction,amachiningoperation,oracombinationofthetwo.TheentireN/Cprogramonatapeismadeupofanaccumulationofthesesuccessivedatablocks.Programsresultinginlongtapesarewoundonreelslikemotion-picturefilm.Programsonrelativelyshorttapesmaybecontinuouslyrepeatedbyjoiningthetwoendsofthetapetoformaloop.Onceinstalled,thetapeisusedagainandagainwithoutfurtherhandling.Inthiscase,theoperatorsimplyloadsandunloadstheparts.Punchedtapesarepreparedontypewriterswithspecialtape-punchingattachmentsorintapepunchingunitsconnecteddirectlytoacomputersystem.Tapeproductionisrarelyerror-free.Errorsmaybeinitiallycausedbythepartprogrammer,incardpunchingorcompilation,orasaresultofphysicaldamagetothetapeduringhandling,etc.Severaltrialrunsareoftennecessarytoremoveallerrorsandproduceanacceptableworkingtape.Whilethedataonthetapeisfedautomatically,theactualprogrammingstepsaredonemanually.Beforethecodedtapemaybeprepared,theprogrammer,oftenworkingwithaplanneroraprocessengineer,mustselecttheappropriateN/Cmachinetool,determinethekindofmaterialtobemachined,calculatethespeedsandfeeds,anddecideuponthetypeoftoolingneeded.Thedimensionsonthepartprintarecloselyexaminedtodetermineasuitablezeroreferencepointfromwhichtostarttheprogram.Aprogrammanuscriptisthenwrittenwhichgivescodednumericalinstructionsdescribingthesequenceofoperationsthatthemachinetoolisrequiredtofollowtocuttheparttothedrawingspecifications.Thefunctionofthecontrolunitmaybebetterunderstoodbycomparingittotheactionofadialtelephone,where,aseachdigitisdialed,itisstored.Whentheentirenumberhasbeendialed,theequipmentbecomesactivatedandthecalliscompleted.Siliconphotodiodes,locatedinthetapereaderheadonthecontrolunit,detectlightasitpassesthroughtheholesinthemovingtape.Thelightbeamsareconvertedtoelectricalenergy,whichisamplifiedtofurtherstrengthenthesignal.Thesignalsarethensenttoregistersinthecontrolunit,whereactuationsignalsarerelayedtothemachinetooldrives.Somephotoelectricdevicesarecapableofreadingatratesupto1000characterspersecond.Highreadingratesarenecessarytomaintaincontinuousinstallationsmotion；other-wise,dwellmarksmaybegeneratedbythecutteronthepartduringcontouringoperations.Thereadingdevicemustbecapableofreadingdatablocksataratefasterthanthecontrolsystemcanprocessthedata.AfeedbackdeviceisasafeguardusedonsomeN/Cinstallationstoconstantlycompen-sateforerrorsbetweenthecommandedpositionandtheactuallocationofthemovingslidesofthemachinetool.AnN/Cmachineequippedwiththiskindofadirectfeedbackcheckingdevicehaswhatisknownasaclose-loopsystem.Positioningcontrolisaccomplishedbyasensorwhich,duringtheactualoperation,recordsthepo