FPGA-implementation-of-a-signal-synthesizer-for-dr

FPGAImplementationofaSignalSynthesizerforDrivingaHigh-powerElectrostaticMotorFumitakaKimura,AkioYamamotoandToshiroHiguchiDepartmentofPrecisionEngineeringTheUniversityofTokyo7-3-1Hongo,Bunkyo-ku,Tokyo113-8656,JAPANEmail:{k-bunbun,akio,higuchi}@aml.t.u-tokyo.ac.jpAbstract—Thispaperdescribesamotordrivercircuitforahigh-powerelectrostaticmotorusingFieldProgrammableGateArray(FPGA).Electrostaticactuatorsarepopularinthefieldofmicronornanotechnologiesbutsomehigh-powerelectrostaticmotorshavebeendevelopedfornormal-sizedapplications.DualExcitationMultiphaseElectrostaticDrive(DEMED)isoneofthestrongestactuatorsamongthem.Therearevariousdrivingmethodsforthemotor(DEMED),however,regardlessofdrivingmethods,developingdrivingcircuitsisoneofthedifficultiesforpracticalapplications.Tooperatethemotor,oneortwothree-phasesignalsarerequiredandamplitudesofthemmustbeafewkV.Hencemulti-channelsignalsynthesizersandhigh-poweramplifiersareneededfordrivercircuits.Inaddition,portabilityisrequiredforsomeapplicationsaswell.Inthisresearch,therefore,quadDirectDigitalSynthesizers(DDS)arebuiltinsidetheoneFPGA.Intheperformanceevaluation,thisdrivercircuitgeneratesmotordrivingsignalsbasedonthepositioncommandthatisgivenbyahigher-levelcontroller.Itisconfirmedthattheelectrostaticmotoroperatesfollowingreferencepositiondata,byusingthisFPGA-basedmotordriver.I.INTRODUCTIONElectrostaticactuatorsarewidelyusedinthefieldofMicroElectroMechanicalSystems(MEMS)becausetheelectrostaticforcehassomeadvantagesinthatfieldcomparedwiththeelectromagneticforce[1–9].Ontheotherhand,itisconsideredthattheelectrostaticforceisgenerallyunfittedtoactuatenormal-sizedequipments.However,somegroupshavedevel-opedelectrostaticactuatorswhichcanbeappliedtonormal-sizedequipmentsbyaccumulatingmicroelectrostaticactuatorelements[10–12].Amongthem,dualexcitationmultiphaseelectrostaticdrive(DEMED)proposedbyNiinoetal.[10]isoneofthestrongestelectrostaticactuators.Itconsistsofapairofthinflexiblefilms,onwhichthree-phaseelectrodes,alignedinamicro-scalepitch,areaccumulated.Thismotorcangeneratesufficientthrustforceortorquetoactuatenormalmechatronicequipments.Hencethismotoriscalledhigh-powerelectrostaticmotor.Inaddition,themotorcontainsnoferromagneticmaterials,soitisexpectedtobeusedforspecialapplicationsinstrongorprecisemagneticfields[13],[14],e.g.amotiongeneratorinanMRenvironment.OneofthedifficultiesindevelopingapracticalsystemusingtheelectrostaticmotoristhedrivercircuitbecausethemotorrequiresafewkVacvoltages.Somedrivercircuitshavebeendevelopedthatcanbecategorizedintothreegroupsbasedonoperationmethods;singlefrequencyoperation,modulatedsinglefrequencyoperation,anddualfrequencyoperation.Singlefrequencyoperationisthemostsimplemethod.Asinglethree-phasehigh-voltagesignalisappliedtobothelectrodesofthetwofilmsandthenfilmsmoverelativelytoeachother.Arelativevelocityofthefilmsisproportionaltotheappliedsignalfrequency.Oneofdisadvantagesofthismethodisthatitrequireshigh-voltageamplifiersthathavefrequencyrangestartingfromDC.Suchanamplifiercostsalotandthesizeishugecomparedwiththemotor.Anotherproblemisthatthemotorperformancebecomesunstableatalowspeed.Modulatedsinglefrequencyoperationisanextendedmethodofthepreviousmethod.Anadvantageofthisprincipleisthatsmall-sizedtransformerscanbeusedtoamplifysignals,insteadofthehigh-voltageamplifiers,thatleadstoreductionofcostandsizeofthedrivingcircuits.Inaddition,theproblemoftheunstableperformanceatalowspeedissolved.Butthereisoneprobleminitsperformance;ageneratedthrustforceisreducedbyhalfofnormalsinglefrequencyoperation.Thethirdprincipleisdualfrequencyoperation.Inthisprinciple,small-sizedtransformerscanbeusedasamplifiers,aswellasmodulatedsinglefrequencyoperation.Inaddition,bothperformanceproblemsdescribedabovedonothappeninthismethod,sothismethodshowsthebestperformanceamongthethreemethods.However,itrequirestwothree-phasehigh-voltagesignalsthatmakesthedrivingsystemcomplex;doubleamountofsignalgeneratorsandamplifiersareneeded.Somestudieshavebeendonetosolvethisproblemregard-ingcomplexityofdualfrequencyoperation.Yamamotoetal.[15]developedamotordrivercircuitusingamodifiedanalogPhase-LockedLoop(PLL)circuitandtransformers.ButtheanalogbasedsignalgeneratorcontrolledbyPLLislessflexibleinitsconfiguration,andtherefore,itcannotcoverawiderangeofthemotoroperation.Nishijimaetal.[16]andZhangetal.[17]developedadigitaldrivercircuitusingDirectDigitalSynthesizers(DDS).DDShasmoreflexibilitythananalogbasedsignalgenerators.Itiseasiertogeneratesignalswithwiderangeoffrequencies.Todrivetheelectrostaticmotorindualfrequencyoperation,sixDDSsarerequiredandtheyhavetobeoperatedinsynchronizationtogeneratetwothree-phasesignals.Inpreviousdevelopments,sixcommerciallyavailableDDSchipsarecontrolledbymicrocontrollers.Insuchconfigurations,communicationspeedbetweenamicrocontrollerandDDSchipsrestrictscontrolratesofDDSsand,978-1-4244-9312-8/11/$26.00©2011IEEE1295inturn,themotorcontrolrate.Inthispaper,multiDDSsareimplementedontheFPGA(FieldProgrammableGateArray)tosolvetheaboveproblem.Inthenextsection,thepaperreviewsdrivingmethodsofthemotorandpreviousdevelopme