Architectures for vibration-driven micropower generators

JOURNALOFMICROELECTROMECHANICALSYSTEMS,VOL.13,NO.3,JUNE2004429
ArchitecturesforVibration-DrivenMicropower
Generators
PaulD.Mitcheson,StudentMember,IEEE,TimC.Green,SeniorMember,IEEE,EricM.Yeatman,Member,IEEE,
andAndrewS.Holmes,Member,IEEE

Abstract—Severalformsofvibration-drivenMEMSmicro-
generatorarepossibleandarereportedintheliterature,with
potentialapplicationareasincludingdistributedsensingand
ubiquitouscomputing.Thispapersetsoutananalyticalbasis
fortheirdesignandcomparison,verifiedagainstfulltime-do-
mainsimulations.Mostreportedmicrogeneratorsareclassified
aseithervelocity-dampedresonantgenerators(VDRGs)or
Coulomb-dampedresonantgenerators(CDRGs)andaunifiedan-
alyticalstructureisprovidedforthesegeneratortypes.Reported
generatorsareshowntohaveoperatedatwellbelowachievable
powerdensitiesanddesignguidesaregivenforoptimisingfuture
devices.Thepaperalsodescribesanewclass—theCoulomb-force
parametricgenerator(CFPG)—whichdoesnotoperateina
resonantmanner.Forallthreegenerators,expressionsandgraphs
areprovidedshowingthedependenceofoutputpoweronkey
operatingparameters.Theoptimizationalsoconsidersphysical
generatorconstraintssuchasvoltagelimitationormaximum
orminimumdampingratios.Thesensitivityofeachgenerator
architecturetothesourcevibrationfrequencyisanalyzedand
thisshowsthattheCFPGcanbebettersuitedthantheresonant
generatorstoapplicationswherethesourcefrequencyislikelyto
vary.Itisdemonstratedthatmechanicalresonanceisparticularly
usefulwhenthevibrationsourceamplitudeissmallcomparedto
theallowablemass-to-framedisplacement.TheCDRGandthe
VDRGgeneratethesamepoweratresonancebutgivebetterper-
formancebelowandaboveresonancerespectively.Bothresonant
generatortypesareunabletooperatewhentheallowablemass
framedisplacementissmallcomparedtothevibrationsource
amplitude,asislikelytobethecaseinsomeMEMSapplications.
TheCFPGis,therefore,requiredforsuchapplications.[944]

IndexTerms—Inertial-generators,microelectromechanicalde-
vices,micropowergenerators,self-poweredsystems,vibration-to-
electricenergyconversion.

I.I
NTRODUCTION

P
ROSPECTSforwearableandubiquitouscomputing

willbegreatlyenhancedifminiaturecomputingnodes
requiringalmostnomaintenancecanbeprovided[1].In
particular,thesenodesshouldbeself-poweredinorderto
avoidthereplacementoffinitepowersources[2],forexample,
byscavengingenergyfromtheenvironment.Withtheeverreducingpowerrequirementsofbothanaloganddigitalcircuits[3],powerscavengingapproachesarebecomingincreasinglyrealistic.Onesuchapproachistodriveanelectromechanicalconverterfromambientmotionorvibration.FabricationofManuscriptreceivedOctober14,2002;revisedMay9,2003.ThisworkwassupportedbytheEPSRCandtheDCInitiative(EUFrameworkV)undertheORESTEIAproject.SubjectEditorS.D.Senturia.TheauthorsarewiththeDepartmentofElectricalandElectronicEngineering,ImperialCollege,LondonSW72BTU.K.(e-mail:pmitcheson@iee.org).DigitalObjectIdentifier10.1109/JMEMS.2004.830151suchpowersourcesusingMEMStechnologyisattractiveinordertoachievesmallsizeandhighprecision.Vibration-drivengeneratorsbasedonelectromagnetic[4]–[7],electrostatic[8]–[10]orpiezoelectrictechnologies[11],[12]havebeendemonstrated.Whilesomeofthere-portedgeneratorshavealreadybeenfabricatedusingMEMStechniques[7],[8],[10],[11],othershavebeenmadeonamesoscalewiththeintentionoflaterminiaturizingthedevicesusingMEMS[6],[13].Ageneralandunifiedanalyticalframeworkforsuchdeviceshasnotpreviouslybeenestablished.Wepresentinthispapersuchaunifiedframework,basedonthreefundamentalgeneratorclasses.ThisanalysisshowsthatpreviouslyreportedMEMSgeneratorshaveoperatedwellbelowtheoreticallyachievablepowerdensities,andprovidesamethodologyfordesigningop-timizedgeneratorsforparticularapplications.WeconsiderthatanyMEMSvibration-drivengeneratorcon-sistsofaproofmass,,movingwithinaframe.Theoperatingprincipleisthattheinertiaofthemasscausesittomoverela-tivetotheframewhentheframeexperiencesacceleration.Thisrelativedisplacementcanthenbeusedtogenerateenergybycausingworktobedoneagainstadampingforce,,realizedbyanelectricormagneticfield,orbystrainingapiezoelectricma-terial.Themassisattachedtotheframebyasuspensionwhichmaybedesignedsolelytoconstrainthemotionofthemass,ortoalsocreatearesonantmass-springsystem.Thedisplacementofthemassfromitsrestpositionrelativetotheframeisdenotedas.Theabsolutemotionoftheframeisandthatoftheproofmassis.Weconsiderharmonicsourcemotion,sothat,thusbeingthesourcemotionamplitude.Equivalently,istheamplitudeofthemass-to-framedisplacement.Inaparticularoperatingcase,willbemechanicallyconstrainedbytheconstructionofthedevice.Wedefineasamaximumpossibleforapartic-ulardevice.Agenericmodelofavibration-drivengeneratorisshowninFig.1.Theupperlimitonpowergenerationforagivensizeof
generatorisultimatelydependentuponthenatureofthe
dampingforcebywhichenergyisextracted.Thispaper
considerstworesonantgenerators,onedampedbyaforce
whichisproportionaltovelocity,thevelocity-dampedreso-
nant-generator(VDRG),andonedampedbyaconstantforce,
theCoulomb-dampedresonant-generator(CDRG),andalso
onenonlineargenerator,theCoulomb-forceparametric-gener-
ator(CFPG).AllthreecanbeimplementedinMEMS,using
electromagnetics(fortheVDRG)orelectrostatics(fortheother
two).

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