Design and fabrication of a new vibrationbased electromechanical power generator.docx

Design and fabrication of a new vibrationbased electromechanical power generator.docx

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Designandfabricationofanewvibrationbasedelectromechanicalpowergenerator

Designandfabricationofanewvibration-based

electromechanicalpowergenerator

M.El-hami1,P.Glynne-Jones1,N.M.White1,M.Hill2,S.Beeby1,

E.James1,A.D.Brown1,J.N.Ross1

1DepartmentofElectronicsandComputerScience,2SchoolofEngineeringSciences

UniversityofSouthampton,

Highfield,SouthamptonSO171BJ,UK.

ABSTRACT

Adeviceisdescribedforgeneratingelectricalpowerfrommechanicalenergyinavibratingenvironment.Thedesignutilisesanelectromagnetictransduceranditsoperatingprincipleisbasedontherelativemovementofamagnetpolewithrespecttoacoil.Theapproachissuitableforembeddedremotemicrosystemsstructureswithnophysicallinkstotheoutsideworld.Simulation,modellingandtestresultsfollowingfabricationofafirstprototypehavedemonstratedthatgenerationofpracticalamountsofpowerwithinareasonablespaceispossible.Powergenerationofmorethan1mWwithinavolumeof240mm3atavibrationfrequencyof320Hzhasbeenobtained.

Keywords:

Self-powered,renewablepowergeneration,Vibration.

1INTRODUCTION

Overrecentyears,aninteresthasdevelopedinmicroelectromechanicalsystems（MEMS）andthesubjecthasmaturedtothepointwhereitsapplicationstoawiderangeofareasarenowclearlyfeasible.Applicationssuchasmedicalimplantsandembeddedsensorsinbuildingsandsimilarstructures,arejustafewofmanyexamples.Thesupplyofpowertosuchsystemshassofarbeenthroughbatteries.However,inlong-livedsystemswherebatteryreplacementisdifficultandinapplicationsconsistingofcompletelyembeddedstructureswithnophysicallinkstotheoutsideworld,generatingpowerfromambientsourcesbecomesimperative.Systemsthatdependonbatterieshavealimitedoperatinglife,whilesystemshavingtheirownself-poweredsupplyunithaveapotentiallymuchlongerlife.Apotentialandpromisingalternativesolutiontobatteriesistheuseofminiaturerenewablepowersupplyunits.Suchdevicesconvertenergyfromexistingsourcesenergywithintheirenvironmentintoelectricalenergy.

Ambientenergymaybeavailablewithintheenvironmentofasystemandisnotstoredexplicitly.Thesourceofsuchenergies,however,dependsontheapplication.Themostfamiliarambientenergysourceissolarpower（lightenergyfromambientlightsuchassunlight）.Thermalenergyisanotherambientenergysource（thermoelectricgeneratorsgenerateelectricitywhenplacedacrossatemperaturegradient）[1].Flowofliquidsorgases,energyproducedbythehumanbody[2]andtheactionofgravitationalfields[3]areotherambientenergysourcepossibilities.OtherexampleswhichdependoninjectedenergyratherthannaturallyoccurringambientenergyfieldsincludeelectromagneticfieldsusedinRFpoweredtags[4],inductivelypoweredsmartcards[5]andnon-invasivepacemakerbatteryrecharging[6].Ourapproachusesmechanicalvibrationastheambientenergysourceforgenerationofelectricalpower[7,8].Therefore,inthispaperavibration-basedmagnet-coilpowergeneratorisdescribed.

Themostimportantparametersinfluencingthedesignofsuchasystemareitsphysicalsizeandconversionefficiencies.Thesizeisdependentontheenergyrequirementandmustbeassmallaspossible,tobecompatiblewiththegeneraldesignobjectivesofMEMS.Howeverasthesizeofthedeviceisreduced,mechanicalresonancestendtoincreaseinfrequencyanditisthechallengeofgeneratingpowerfromcomparativelylowvibrationalfrequencies（hundredsofHzratherthankHz）thatisaddressedinthiswork.Theambientenergymaybeatapremiuminaparticularenvironmentsotheconversionefficiencymustbeashighaspossible.Toanalysethetransformationefficiencyandtoassesstheinput-outputrelationshipofsuchagenerator,fullelectromechanicalandmagneticanalyseshavebeencarriedout.Finiteelement（FE）techniquesforthemagneticfielddistributionsolutionhavebeenemployed.Fabricationandtestresultsofafirstprototypebasedonsimulationandmodellingresultsarefullydiscussed.Practicalamountsofpowerwithinreasonablespace（quarterofacubiccentimetre）havebeenachieved.

2THEGENERATOR

Aschematicdiagramoftheproposedelectromechanicalpowergenerator,illustratingdimensionedandlabelleddrawingsofbeam/magnetassemblyisshowninfigure1.Thesystemconsistsofacantileverbeamsupportedbythehousing.Themassonthebeamismadeupoftwomagnets（onepole）mountedonac-shapedcore.Arrangingthemagnetsinthiswayprovidesauniformmagneticfieldintheair-gap.Themainpurposeofthecoreistoprovideapathandguidethemagneticfluxthroughitwithaminimumoffluxleakage.Thecoilismadeupofanumberofsinglesolidcoreenamelledcopperwires.Itisplacedintheair-gapbetweenthemagnetsatrightanglestothedirectionofthemovementofthemass.

Theoperatingprincipleofthedeviceisasfollows.Asthehousingisvibrated,amechanicalinputforcefeedsintoasecondordermechanicalsystem,themassmovesrelativetothehousingandenergyisstoredinthemass-beamsystem.Thisrelativedisplacement,whichissinusoidalinamplitude,causesthemagneticfluxtocutthecoil.ThisinturninducesamotionalelectromotiveforceonthecoilduetoFaraday’slaw.Themagnitudeofthisvoltageisproportionaltotherateofchangeofthecoilposition.Theelectricalsysteminvolvedissimplyafirst-orderLRcircuitwiththeinductanceofthecoilinserieswiththeloadresistanceandtheparasiticresistanceofthecoil.

2.1Designanalysis

Inordertodetermineandpredictthepracticalperformanceofthedeviceelectromechanicalandmagneticanalyseshavebeenundertaken.

Considerthesystemshowninfigure1toconsistofapointmass（m）mountedontheendofabeamprovidingaspringstiffnessk（thevalidityofthisassumptionwillbediscussedlater）.Variablesxandyarethedisplacementsoftheeffectivemassandthevibrationhousingrespectively.Itisassumedthatthemassofthehousingismuchgreaterthanmandmovementofthehousingisunaffectedbythemovementofthegenerator.Forasinusoidalexcitation

whereYistheamplitudeofvibrationandtheangularfrequencyofvibration,thefollowingdifferentialequationofmotionisobtained[9].

（1）

wherezistherelativedisplacementofthemasswithrespecttothevibratingbody,

kisthebeamstiffnessandcisadampingcoefficient.Thesolutiontoequation

（1）isgivenby:

（2）

Theinstantaneouselectricalpower,Pi,generatedbythesystemis

（3）

Whereceistheportionofthedampingattributabletoelectricalpowergeneration.

Hencethemagnitudeofthegeneratedpower,

is

（4）

andthegeneratedpowermaybewrittenas:

（5）

where

isthenaturalfrequencyofthesystemand

istheelectromagnetictransducerdampingfactor.Theoveralldampingfactorofthesystem,

includeslossesduetofriction,airresistance,etc.,

andisgivenby:

（6）

Thevoltage,e,andcurrent,i,generatedwithinthesystemcanbedescribedbythefollowingequations:

（7）

（8）

whereFeistheforcegeneratedbytheelectromechanicalcoupling,RcandLcaretheresistanceandinductanceofthecoilrespectivelyandthetransformationfactoris:

（9）

Here,Nisthenumberofturns,Bistheaveragefluxdensityintheair-gapandNlistheeffectivelengthofcoil.

IfthecurrentisdrivingaloadofresistanceRL,theelectricallygeneratedforcewillbe

（10）

Hencetheelectricallygenerateddamping,ce,willbe

（11）

andatfrequencieswheretheinductiveimpedanceismuchlowerthantheresistiveimpedances,theelectricallygenerateddampingratiowillbe

（12）

Operatingthedeviceatresonance,when

（aswe’reconsideringrelativelyhighQsystems）,andsubstitutingfromequation（12）intoequation（5）givesthetotalelectricalpowergeneratedas:

（13）

2.2Determinationofdesignparameters

Fordeterminationofthedesignparameters,bothanalyticalandnumericaltechniqueshavebeenemployed.AnalyticaldesignprogrammesimplementedinMatLabenvironmentsarespecificallytailoredtocalculatethephysicalparameters,dimensionsandotherfunctionalrequirementsofthedevice.Ageneral-purposeelectromagneticfieldanalysisCADsoftwarepackage（VF-OPERA）thatusesfiniteelementmethodshasbeenusedtomodelandsolvethemagneticfielddistributioninarangeofdesignvariantsofthedevice[10].Thedesignparametersareiterativelyrefined,ultimatelyresultingintheoptimumdesign.

Forconvenienceandeaseoffabricationanarbitrarysize（thicknessandcross-sectionarea）forthemagnetandarelativelylargeair-gapof3mmhasbeenchosen.Thecorethicknessisdeterminedbytheneedtocarrythemagneticfluxinthecircuitwithoutexceedinganominalsaturationfluxdensityof1.6Tesla.ThecoreandmagnetsaremodeledinX-YsymmetryinOPERA-2dbydrawingtheirexactgeometryasasetofnon-overlappingregions.Figure2showsthemesheddiagramofthemagnetsandcoregeometry.TheirB-Hcharacteristicsaredefinedbycurvefittingandtocalculatethefluxdensityintheair-gapnon-linearstaticsolutionofthesystemisperformed.Figure3showsthevariationinfluxdensityalongthemagnetheightthroughthecentreoftheair-gap.Thenon-linearityoffluxdensityisduetofringingeffect.Theaveragefluxdensityintheair-gapiscalculatedbycomputingthefluxdensityalonganumberofpathsandtakingtheaverage.Figure4clearlyillustratesthatthevariationoffluxinthecorehavingathicknessof1mmiswellmaintainedwithinthepredefinedlevelof