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MotorsandNumericalControlMotorschangeelectricalenergyintomechanicalenergy.Thisissimplythemechanicalforcesbetweenmagnets.whentwomagnetsorelectromagnetsapproacheachother,onewillbeeitherpulledtowardorpushedawayfromtheother.1.CONSTRUCTIONEachtypeofmotorisbuilttodoitsownspecialjob.Allofthemhavetwomainparts.Onepartiscalledthestator.Thestatorisstationaryitdoesnotmove.Theotherpart,therotor,ismountedonbearingssoitcanrotate.Therotorshaftsticksoutbeyondthehousing.Foramotor,therotorshaftiscoupledtoamechanicalload.2.MOTORSThepowerinputtoamotoriselectrical.Voltageisappliedtoamotorsterminalsresultinginacurrent.Thepoweroutputofamotorismechanical.Thispoweristransmittedbytherotorshaftasatorque.Thistorquetendstorotateaload,suchasafanorpump.Asamotordrivesaload,mechanicalpowerisdrawnfromthemotor.Themotor,inturn,drawselectricalpowerfromthesourceatthesamerate3.NUMERICALCONTROLThemajordisadvantageofmachinetoolautomationthusfardescribedliesintheeconomicsoftheprocess.Itisexpensivetotoolainstallationsforautomaticproduction.Therefore,unlessthepartistobemadeinverylargenumbers,thecostbecomesprohibitive.Greatneedexistsforamethodthatpermitsrapidautomaticproduction,economicalinjoblotamounts.Theanswerhasbeenfoundinthenumericalcontrolofmachinetools.ComputerNumericalControl.NumericalcontrolNCisaformofprogrammableautomationinwhichtheprocessingequipmentiscontrolledbymeansofnumbers,letters,andothersymbols.Thenumbers,letters,andsymbolsarecodedinanappropriateformattodefineaprogramofinstructionsforaparticularworkpieceorjob.Whenthejobchanges,theprogramoftheparticularworkpiecechanges.ThecapabilitytochangetheprogramiswhatmakesNCsuitableforlowvolumeandmediumvolumeproduction,anditismucheasiertowritenewprogramsthantomakemajoralterationstotheprocessingequipment.Thereisawideareaofperformanceduplicationbetweennumericalcontrolandautomatics.Numericalcontrol,however,offersmoreflexibility,lowertoolingcost,quickerchanges,andlessmachinedowntime.Inmachiningcontours,numericalcontrolcanmathematicallytranslatethedefinedcurveintoafinishedproduct,savingtimeandeliminatingtemplates.Thiscaninturnimproveaccuracy.Anotheradvantageappearstobegreatsavingofmachinetime,theequivalentogincreasingproductivecapacitywithnoincreaseinfacilities.NumericalcontrolN/Cisaformofprogrammableautomationinwhichtheprocessingequipmentiscontrolledbymeansofnumbers,letters,andothersymbols.Thenumbers,letters,andsymbolsarecodedinanqppropriateformattodefineaprogramofinstructionsforaparticularworkpartorjob.Whenthejobchanges,theprogramofinstructionsischanged.ThecapabilitytochangetheprogramiswhatmakesN/Csuitableforlowandmediumvolumeproduction.Itismucheasiertowriteprogramsthantomakemajoralterationsoftheprocessingequipment.Therearetwobasictypesofnumericallycontrolledmachinetoolspointtopointandcontinuouspathalsocalledcontouring.Pointtopointmachinesuseunsynchronizedmotors,whittheresultthatthepositionofthemachiningheadcanbeassuredonlyuponcompletionofamovement,orwhileonlyonemotorisrunning.TheN/Csystemconsistsofthefollowingcomponentsdatainput,thetapereaderwiththecontrolunit,feedbackdevices,andthemetalcuttingmachinetollorothertypeofN/Cequipment.Datainput,alsocalledmantocontrollink,maybeprovidedtothemachinetoolmanually,orentirelybyautomaticmeans.Manualmethodswhenusedasthesolesourceofinputdataarerestrictedtoarelatibelysmallnumberofinputs.Examplesofmanuallyoperateddevicesarekryboarddials,pushbuttons,switches,orthumbwheelselectors.Thesearelocatedonaconsolenearthemachine.Dialsareanalogdevicesusuallyconnectedtoasynchrotyperesolverorpotentiometer.Inmostcases,pushbuttons,switches,andothersimilartypesofselectorsaredigitalinputdevices.Manualinputrequiresthattheoperatorsetthecontrolsforeachoperation.Itisaslowandtediousprocessandisseldomjustifiedexceptinelementarymachiningapplicationsorinspecialcases.Inpracticallyallcases,informationisautomaticallysuppliedtothecontrolunitandthemachinetoobycards,punchedtapes,orbymagnetictape.EightchannelpunchedpapertapeisthemostcommonlyusedformofdatainputforconventionalN/Csystems.Thecodedinstructionsonthetapeconsistofsectionsofpunchedholescalledblocks.Eachblockrepresentsamachinefunction,amachiningoperation,oracombinationofthetwo.TheentireN/Cprogramonatapeismadeupofanaccumulationofthesesuccessivedatablocks.Programsresultinginlongtapesarewoundonreelslikemotionpicturefilm.Programsonrelativelyshorttapesmaybecontinuouslyrepeatedbyjoiningthetwoendsofthetapetoformaloop.Onceinstalled,thetapeisusedagainandagainwithoutfurtherhandling.Inthiscase,theoperatorsimplyloadsandunloadstheparts.Punchedtapesarepreparedontypewriterswithspecialtapepunchingattachmentsorintapepunchingunitsconnecteddirectlytoacomputersystem.Tapeproductionisrarelyerrorfree.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.Highreadingratesarenecessarytomaintaincontinuousinstallationsmotionotherwise,dwellmarksmaybegeneratedbythecutteronthepartduringcontouringoperations.Thereadingdevicemustbecapableofreadingdatablocksataratefasterthanthecontrolsystemcanprocessthedata.AfeedbackdeviceisasafeguardusedonsomeN/Cinstallationstoconstantlycompensateforerrorsbetweenthecommandedpositionandtheactuallocationofthemovingslidesofthemachinetool.AnN/Cmachineequippedwiththiskindofadirectfeedbackcheckingdevicehaswhatisknownasacloseloopsystem.Positioningcontrolisaccomplishedbyasensorwhich,duringtheactualoperation,recordsthepositionoftheslidesandrelaysthisinformationbacktothecontrolunit.Signalsthusreceivedarecomparedtoinputsignalsonthetape,andanydiscrepancybetweenthemisautomaticallyrectified.Inanalternativesystem,calledanopenloopsystem,theisinstallationspositionedsolelybysteppingmotordrivesinresponsetocommandsbyacontroller.Thedegreeofworkprecisiondependsalmostentirelyupontheaccuracyoftheleadscrewandtherigidityoftheinstallationsstructure.Withthissystem,thereisnoselfcorrectingactionorfeedbackofinformationtothecontrolunit.Intheeventofanunexpectedmalfunction,thecontrolunitcontinuestoputoutpulsesofelectricalcurrent.If,forexample,thetableonaN/Cmillingmachineweresuddenlytobecomeoverloaded,noresponsewouldbesentbacktothecontroller.Becausesteppingmotorsarenot
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