ANoise-FreeServo-SpindleforHigh-speedNCGearGrindingMachinesTakashiEmura,LeiWang,HisashiNakamura,MasashiYamanakaandYasushiTeshigawaraDepartmentofMechatronicsandPrecisionEngineering,TohokuUniversityAoba,Aramaki,Aobaks,Sendai,980-77JapanAbstract-TheauthorsdevelopedaproductivetypeNC(NumericallyControlled)geargrindingmachineforautomobileplants,ThisNCma-chinerequireshigh-accuracysynchronizationbetweenthegrindingspin-dleandworkspindle.Wemustusehigh-powerservomotorsandservo-amplifierstosatisfythespecificationrequiredforhigh-speedgrinding.However,sincethehigh-powerservo-amplifiercausesverylargerippleofmotorcurrentatPWM(Pulse-WidthModulation)frequencyandin-ducesstrongnoise,itbecomesdifficulttogetthestablesynchronousrota-tionamongspindles.Therefore,theauthorstriedtousetwo-phasetypePUtoachievenoise-freehigh-speedsynchronousspindlesandtoreducethenoisecausedbythecurrentripplewithakindofcurrentfilter.Inthispaper,anapproachforsuppressingthenoiseandexperimentalresultsaredescrlbed.I.INTRODUCTIONGeargrindingtakesextremelylonggrindingtimeanditpre-ventsincreasingtheproductivityinmassproductionofgears.Thus,theauthorsdevelopedaproductivetypeNCgeargrind-ingmachineforautomobileplants,whicharethemostproduc-tiveplantsofgears.Thismachinerequirestwolarge-powerservo-spindlesforitsgrinding-spindleandworkspindle.heseservo-spindlesalsohavetobesynchronouslycontrolledwithhighprecisionevenrotatingatahighspeed.Theauthorsusedhigh-powerandhigh-speedservomotorstodrivethespindles.Forexample,theservomotorforgrinding-spindlehasaratedpowerof22kW,ratedoutputtorqueof22N-mandmaximumrotaryspeedof10000r/min.Theservomotorforworkspindlehasaratedpowerof16kW,ratedoutputtorqueof82N.mandmaximumrotaryspeedof2000r/min.TheseservomotorsaredrivenbyPWM(Pulse-WidthMod-ulation)typeservo-amplifiers,andthePWMtypeamplifieroflargepowercausesaverylargerippleinmotorcurrentwhichappearsatPWMfrequency.Sincethislargecurrentripplein-ducesastrongnoisewhichcausestroubletocontrolsystems,itisdifficulttorealizesynchronouscontrolofhighaccuracy.Therefore,theauthorstriedtousetwo-phasetypePLL(Phase-LockedLoop)togetnoise-freehigh-speedsynchronousspin-dlesandtriedtouseakindofcurrentfiltertoreducetherippleofmotorcurrent.Experimentswerecarriedouttoconfirmtheef-fectivenessofthenoise-freesystems.Inthispaper,anapproachforsuppressingthenoiseandgettingthehigh-speedandhigh-precisionsynchronousspindlesisdescribedwithexperimentalresults.11.NOISECAUSEDBYPWMProblemsofnoiseofthegrinding-spindleismoreseverethanthatoftheworkspindlebecausetheratedpowerofthegrinding-spindleislargerthanthatoftheworkspindle.Thuswemainly0-7803-3026-9195$4.0001995IEEEdescribeaboutthecaseofgrinding-spindleinthefollowings.Thegrinding-spindleisdrivenbyahigh-powerbrushlessser-vomotor.Sincethemaximumoutputvoltageoftheservo-amplifierislimitedto200Vandtheratedpoweroftheser-vomotoris22kW,thecurrentoffieldcoilhastobemorethan200Aatapeakload.Forthislargecurrent,awirediameteroffieldcoilbecamelarge,andthenumberofturnsbecameafewbecauseofthelimitationofwindingspace.Thusinductanceofthefieldcoilwasconstrainedtoaverysmallvalue.Themea-suredinductanceofonephaseisabout30pHatlkHzanditisreducedtoverysmallvalueoflessthan1.5pHat25kHzofPWMfrequency.Sincethecoilinductanceisextremelysmalllikethis,itselectricaltimeconstantisverysmall.Thismeansthatwecangetagoodcontrolcharacteristics.However,theextremelyshorttimeconstantcausesverylargecurrentrippleatPWMfrequency,andthiscurrentrippleinducesthestrongnoise.Thestrongnoisegivesseriousproblemstocontrolsys-tems.Inordertoanalyzetheripplecurrent,numericalsimulationofmotorcurrentwascarriedout.Fig.l(a)issimplifiedmodelofsynchronousmotorforsimulation,wherecoilinductanceLis1.44pH,coilresistanceRis0.15SZandvoltageofDClineEis200V.PWMoutputvoltageEu,EvandEwofservo-amplifieraresuppliedonterminalofthreephasesU,V,W.PWMpatternshowninFig.l(b)aregeneratedbycompar-ingreferencevoltageeu,eVandewwithtrianglewaveegofwhichfrequencyis25kHz.Fig.2(a)isanexampleofsimulatedresults,wheretheout-putfrequencyoftheservo-amplifieris300Hz.AsshowninFig.2(a),motorcurrentisoverlappedbylargenoise.Fig.2(b)givesthedetailsituationbyconiparingsimulatedandmeasuredresultsofcoilcurrent,wheresimulatedresultsagreedwiththemeasuredresults.Thusthesimulatedresultsshowsthatthemo-torcurrenthasverylargecurrentrippleatPWMfrequency.Thissimulationshowsclearlythattheservomotorandpoweramplifiergenerateverystrongnoisesforthesakeofsmallnessofcoilinductance.Thecoilinductancedecreasessharplyathighswitchingfrequencyinsuchlargepowermotorbecauseofthemagneticcharacteristicoffieldcore.ThenoisescausedbyswitchingisaverysevereprobleminthisNCmachineandhastobeovercomeinordertoachievethehighprecisionsyn-chronouscontrol.Therefore,thefollowingsectionsdescribetwodifferentap-proachesforovercomingswitchingnoiseproblem.SectionHIpresentsasynchronouscontrolmethodoftwo-phasetypePLLwhichenablesthesystemengageahigh-precisionandhigh-692UEvWEUEVEW(b)Fig.1.Vu,V,V,arebackEMF(electromotiveforce)Motormodelfornumericalsimulation.I,I,I,arelinecurrentandn3EsUEV.I,I-1001I.I.I.I.II0123Time(ms)(a)Simulatedmotorcurrent.InITime(2Opddiv)(b)Comparisonofsimulatedandmeasuredresults.Fig.2.Rippleofmotorcurrent.speedsynchronouscontrolunderstrongnoisesituation.Sec-tionIVgivesdiscussionofusinglargepowerfiltertosuppressswitchingnoiseandtheexperimentalresults.111.SYNCHRONOUSSYSTEMBYTWO-PHASETYPEPLLThedevelopedgeargrindingmachinerequireshigh-speedsyn-chronouscontrolwithhighaccuracy.Thehigh-speedandhigh-precisionsynchronouscontrolsystemsarerealizedusuallybyusingahigh-resolutionandhigh-speedencoderwhoseoutputistwo-phasesinusoidalwave.However,thevoltageofthesinusoidalwavesisusuallyverysmall,anditisdifficulttodetectthissmallvoltagewithoutproblemsofthenoiseunderthestrongnoisesurroundings.Thismeansthatwecannotusetheconventionalcontrolmethodofhigh-precisionsynchroniza-tion.Therefore,theauthorsusedtwo-phasetypePLLinthissynchronoussystem.Thetwo-phasetypePLLwasfirstpro-posedbyEmuralanditsnoiserejectionabilitywasalreadyverified2-4.Fig.3isblockdiagramofthehigh-speedsynchronoussys-tembetweengrindingspindleandworkspindleoftheNCgeargrindingmachine.Bothoftheservo-spindleswerecontrolledbytwo-phasetypePLL.Acommandpulsegeneratorgeneratestwopulsetrainswhosefrequencyratioisthevelocityratioofgrinding-spindletoworkspindle.Positioncontrollersoftwo-phasetypePLLcontrolthespindlessoasforthemtofolloweachcommandpulseaccurately.Byusingtwo-phasetypePLL,high-speedandhigh-precisionsynchronouscontrolwasreal-ized.Twointerpolatorsoftwo-phasetypePLLinterpolateen-codersignalwithhighresolutionandtheinterpolatedoutputpulsesofrotationalanglearesenttoservo-amplifiers.A.High-resolutiondetectionofrotaryanglewithinterpolatoroftwo-phasetypePLLTheservo-amplifierswhichweusedthistimerequireatwo-phaserectangularsignalforgeneratingthethree-phasefieldcurrentthatsynchronizeswithrotorangle.However,wecouldnotusetheconventionalinterpolatorsofcomparatortypebe-causeofthenoiseinducedbytheripplecurrent.Therefore,theauthorsdesignedspecialinterpolatorswhichusetwo-phasetypePLLasshowninFig.4todetectrotaryanglesofspindles.Circuitsofthisinterpolatorwasdesignedmainlyaccordingtothedescriptionin3,andsotheyareonlybrieflydescribedinthefollowing.Inputsignalisatwo-phasesinusoidalwave(sin&,cosBi)obtainedfromencoder.Atrackingsignaloftwo-phasesinu-soidalwave(sine,cos0,)isgeneratedbyavoltagecontrolledoscillator(VCO).ThisVCOiscontrolledsoastotrackthein-putsignal.Phasedifferenceofinputsignalandtrackingsig-naliscalculatedbyphasedetector(PD).Loopfilter(LF)re-moveshigh-frequencycomponentsofoutputofPDandoutputtoVCO.V/FconvertercontainedinVCOgeneratespulsetrain693Commandpulsegenerator&pulsedistributorr1IcontrollerWorkoieceOpticalencoder/.Fig.3.Ahigh-spxdsynchoussystemforNCgeagrindingmachines.InputFig.4.Blockdiagramofinterpolatorusingtw-phasetype.PLL.withthefrequencyproportionaltoinputvoltage.Thepulsetrainsiscountedwithabinarycounter,andtheoutputofcounterisfedtoaROM.Two-phasesinusoidalwaveindigitalvalueiswrittenintheROManditisoutputwhenaddressed.D/Acon-vertersconverttheROMoutputtoanalogvoltage.Thesinu-soidalwaveiscontrolledsoastotrackinputoftwophasesinu-soidalsignalbycontrollingthephasedifferenceto0,thismeansthatPLLisinlockedstate.Underthelockedstate,pulsefromV/Fbecomestotheinterpolatedpulseofinputsignalwhichistherotaryangleofencoder.Themostsignificantdifferenceoftwo-phasetypePLLfromconventionalPLListhatwecanalwaysobtainthetrue2-phasesinusoidalwavewhichtracksinputsignal.Thismeansthatithasexcellentnoisesuppressioncharacteristicsandwiderangeofinputfrequency.Thiscircuitcanalsolockwhenmotorchangesitsrotarydirection.Opticalencodersofveryhighprecisionareusedtodetectro-taryangleofshaft.Numberofopticalslitsis1800forgrinding-spindleand9000forworkspindle.Wecanget2-phasesinu-soidalwavesfromtheencoders.Thedevelopedinterpolatordividesthemby128timesandoutput2-phaserectangularpulsesignal.Hencetheresolutionis5.6and1.1respectively,andthemaximumfrequencybecomesupto38.4MHz.Thepulsesarefedtopoweramplifiersandaswellusedtomonitorsyn-chronouscontrol.In3,amagneticscalewithinductiontypeheadwasusedastherotarysensor.Becauseinductiontypeheadcannotworkatlowspeed,theinterpolatorcouldnotworkatrotaryspeedbelow300r/min.However,thenewlydevelopedsystemcanworkatanyspeedbelow10000r/mforgrindingspindleandbelow2000r/minforworkspindle.Ofcourse,wecangivereversiblerotationstothespindles.Ingeneral,detectionwithsohighaccuracyandhighfre-quencyiscriticallyaffectedbynoisesanditisverydifficulttorealizestablecontrol.However,thenewinterpolatorscande-tecttherotorangleswithoutmisoperation.Fig.5illustratesanexampleofexperimentalresults.InFig.5,inputsignalmeanssignalobtainedfromencoderandtrackingsignalmeansthesinusoidalwaveofVCO.AsshowninFig.5,signalfromencoderisdistortedbystrongnoise.However,thesinusoidalwavewhichispreciselytrackingtheinputsignalhasnodistor-tion,becauseitisgeneratedbyVCOwhoseoutputwaveformistruesinusoidalwave.WecangettheinterpolatedpulsesfromtheoutputpulsesofV/Fconverter,bywhichthetrackingsinu-soidalwaveswereconstructed.B.Servo-ControllerwithTwo-PhaseTypePLLServo-controllerusesthetwo-phasetypePLLmethodaswell.TheschematicisshowninFig.6.Aservo-errorde-694Time)Fig.5.Experimentalresultsofinterpolatoroftwo-phasetypePLL.(Workspindle,500rhnin)sine,Asine,ServoServo+f1amplifierIMotor6sine,Fig.6.Schematicofservomechanismoftwo-phasetypePLL.tectoroftwo-phasetypePLLisusedtodetectservo-errorwithveryhighprecision.PIDcontrollercontrolsthemotortorquetomaketheservo-errorto0.Referencesignalofthisloopisapairoftwo-phasesinusoidalwaves(sin8；,cose；),andtheirphaseiscomparedwiththetwo-phasesinusoidalwavesobtainedfromencoderbythefollowingvectorphaseoperation.cos8icos8,-sin8；sin0,=cos8d-cos8；sin8,+sin8；cos8,=sin8d(1)*Hencewecangetapairoftwo-phasesinusoidalwavesofwhichargumentisphasedifferenceBd=8；-8,.Sincewecandetectthephasedifference8dwiththesameprincipleastheinterpola-tormentionedpreviously,theservo-errordetectoroftwo-phasetypePLLdetectstheservo-errorwithhighresolutionandhighaccuracy.Anotherfeatureofthisloopisthattheservo-errordetectorisabletodetectangularvelocityerrorwithhighaccuracy.ThisisbasedontheexcellentcapabilityofPLLinfrequencydetectionasknown.Byfeedbackoftheangularvelocityerro