Wireless Energy Transmission System For Low-Power Device

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WirelessEnergyTransmissionSystemForLow-PowerDevicesPeterSpies,PhilipBabelDepartmentPowerEfficientSystemsFraunhoferIISNuremberg,GermanyPeter.spies@iis.fraunhofer.deAbstract—Thispaperdescribesanelectronicsystemforwavelengthinductivecouplingisemployed.Well-knownwirelessenergytransmissiontosupplylow-powerelectronicexampleforthiseffectisthetransformerinformoftwodeviceslikewirelesssensors.Itpresentsthefirstreceivercoupledinductors.Thebasicrelationswillbepresentedhere.ASICsforthiskindofapplication.TwoprototypeswithsupplyThemagneticinductanceBofonecoilofthetransformeriscurrentsof4and100milliAmperesserveasproof-of-concept.Furthermore,severalimplementationsoftransmittersandN⋅r2receiverswithoff-the-shelfcomponentsarediscussed,showing1B=μ⋅⋅I,031differentsystemoptions.2⋅()r2+x22whereµ0isthemagneticfieldconstant,N1isthenumberofwindingsontheprimaryinductorofthetransformer,ristheradiusofthiscoil,xisthedistancefromthecentertheI.MOTIVATIONcoilandI1isthecurrentthroughthisinductor.ThemutualLow-powerdevicescommonlyhaveanenergystorageinductanceorcouplingcoefficientMdefinesthefluxfromelementlikeabatteryoracapacitortoprovideenergyfortheprimarywinding,whichisenclosedbythesecondarytheirdifferentelectroniccircuits.Inordertoberecharged,winding.Itiscalculatedasfollowstheenergystoragemustbecable-connectedtoavoltageregulatorandthemainspowersupply.Regulator,wiringandN2⋅B⋅A2M=connectorsaswellastheenergystorageitself,especiallyinIcaseofabattery,areoftenasourceoferror.Moreover,1,regulatorsandconnectorsarenotalwayscompatiblewithwhereN2andA2arethenumberofwindingsandtheareadifferentapplicationdevices,sothateachdevicetypeneedsofthesecondarycoil,respectively.TheinducedvoltageU2itsowncustom-tailoredchargingsolution.Theneedforinthesecondarycoiliswiring,connectorsorevenbatteriesinelectronicequipmentcanbeeliminatedbywirelessenergytransmission.Itistheidealsolutionwhenlittleboardspaceisavailableandlowcostsareofparamountimportance.Besidesmobileterminalsω⋅MU=⋅I.andwirelesssensorsapplicationareascovertotally21122encapsulatedorisolateddevicesanddevicesinremoteor⊬⊬2·2·∆⊬ω·⊬ω⋅L·¸inaccessiblelocations.Dependingontheapplicationandthe∆1−∆¸¸+∆2¸employedcomponents,i.e.antennasandcapacitors,different∆∆∆∆ω¸¸∆R¸¸¸««R¹¹«2¹frequenciescanbeusedforpowertransfer.Furthermore,it«¹canbecombinedwithdatacommunicationforsoftwareupdates,read-outofwirelesssensorsandthelike.Theentirefunctionalitycanbeimplementedonasingleintegratedωisthefrequency,ωRtheresonancefrequencyandL2circuit,withonlyafewexternalcomponents[1][2].andR2aretheinductanceandtheresistanceofthesecondarycoil,respectively.Inawirelessenergytransmissionsystem,II.OVERVIEWtheprimarycoilwillbetheantennaofthetransmitterandDifferentphysicalprinciplescanbeusedforwirelessthesecondarycoilwillbetheantennaofthereceiverattheenergytransmission.Fordistancessmallerthantheapplicationdeviceside[3]. Forlargerdistances,electro-magneticcouplingisthetogetherwithacapacitortobuildaresonancecircuitatitseffecttobeusedforenergytransmission.Thereceiveenergyinput.Duetothisresonance,themaximumpossibleenergyiscalculatedasfollowsisusedforthesupplyofthedevice.AccordingtothedesiredfrequencyωtheinductanceLoftheantennaandthe2Pt⋅λcapacitanceCmustchosenwiththefollowingequation:P=,r24⋅π⋅R1ω=wherePtisthetransmitpower,RthedistancebetweenLCtransmitterandreceiverandλisthewavelength[4].TheinducedvoltageisstronglydependedonthedistanceThesystemarchitectureisthesameforbothprinciples,betweentransmitterandreceiver.Varyingdistancesresultinwhereastheelectro-magneticcomponentsliketheantennafluctuatingvoltages.Integratedcircuits(IC)workonlyinahavetobeadaptedandoptimizedforthevariousphysicallimitedvoltagerange,sospecialprotectionorlimitingprinciples.circuitsareused.ForcommercialICsthisrangeise.g.20V,forASIC-designsthisvoltagerangeissmaller.ToavoidIII.IMPLEMENTEDSYSTEMovervoltageandapossibledestructionofthechip,alimitationcircuitnecessary.InFigure2thevoltageisAwirelessenergytransmissionsystemconsistsoftheregulatedbyanNMOStransistor,controlledwithantransmitterandthereceiver.Thetransmittergeneratestheamplifier.Thistransistorregulatesthemaximumvoltageelectro-magneticfieldandcomprisesanoscillator,apoweraftertherectifierindirectlybyshorteningthecurrent.Aamplifier(PA),amatchingcircuitandanantenna.Thefeedbackloopdividesthevoltageandcomparesitwithaoscillatorproducesaperiodicsignalwiththechosencarrierfixedreference.Accordingtotheresultsofthiscomparisonfrequency.BecauseofcombinationwithstandardRFIDtheNMOStransistoriscontrolledtoreducethevoltage.protocolsfordatatransfer,the13.56MHzfrequencywasusedinthepresentsystem.Thisrangebetween13.553and13.567MHzisemployedbyawidevarietyofradioserviceslikepressagenciesandtelecommunications.InadditiontoR1RFIDsystems,remotecontrolsystems,remotecontrolledT1Loopmodels,demonstrationradioequipmentandpagerscanbeAmpVreffoundhere.R2ThegainofthepoweramplifierdeterminestheinductorVoltageorantennacurrentandthusthetransmissiondistanceandtheReferencereceivepoweratthereceiver(seeII).Thispowerislimitedaccordingtothefrequencyrange,definedfromthenationalregulatoryauthority.Inthe13.56MHzband,thepowermustnotexceed42dBµA/mat10mdistance(EN300220,ENFigure2:Schematicofthelimiter300330,EN300440)[4].ToprovidetheappropriatesupplyvoltagefortheThematchingcircuitimplementsthematchingbetweenapplicationdeviceoranenergystorageelementlikeabatterythePAoutputandtheantennatoguaranteemaximumpoweroracapacitor,avoltageregulatorisused.Iflowboardspacetransfertotheantenna.Dependingonthefrequencyusedandisimportantandonlysmallcurrentsareprocesses,alineartherequireddistance,thesizeoftheantennahastoberegulatorshouldbechosen.Incomparisonswitchingchosen.Theoptimumdistanceforenergytransmissionwithregulatorswillmaintainbetterefficiencybutrequirealargeinductivecouplingishalfthediameteroftheantenna.inductor,whichcouldnotbeimplementedonachip.Figure3showstheschematicofthelinearregulator,alsocalledseriesregulator,becausethecontrolelementinformofthePMOStransistorT1isinserieswiththeload.Atfirst,theoutputvoltageVoutmustbedividedbyavoltagedividerwhichisbuildoftworesistors.ThisvoltageisthencomparedwithaconstantvoltageVrefandthedifferenceisamplifiedbyaloopamplifier.TheoutputofthisamplifierFigure1:BlockdiagramofthetransmittercontrolsthepasselementT1oftheregulator.Ifthedesiredvoltageisreached,thecurrentisreducedbythepasselement.Iftheoutputvoltageisbelowthedesiredlevel,theThereceiveroftheWETisdirectlyconnectedtothepasselementisopenedtoincreasetheoutputvoltage.applicationdevice.Itusestheelectro-magneticfluxoftheTheoutputvoltageisrelatedtothecircuitparameteristransmittertoproduceaconstantvoltageforthesupplyofthefollowingway:thisapplicationdevice.Thereceivercomprisesanantenna ⊬R·customizetodifferentsupplyvoltagesandtransmissionV=V⋅∆1+1¸rates.Thedemonstratorboardincludesamicrocontrolleroutref∆¸«R2¹MSP430andseveralLEDs,whicharepoweredbythewirelessenergytransmission.TheantennaisalsobuildbyAsthisequationshows,theoutputvoltageVoutisnotseveralcopperwindingsonthePCB.dependentontheinputvoltageVin,thusfluctuationattheinputarenotseenattheoutput.Figure3:SchematicofthevoltageregulationFigure4:ImplementationofreceiveronaflexiblefoilPCBThereceivepowerwhichnearlyequaltotheoutputIV.IMPLEMENTATIONALTERNATIVESpowerfordifferentsupplyvoltagesasafunctionoftheThetransmitterofawirelessenergytransmissionsystemdistanceisshowninFigure5.Themaximumpowerisisveryoftendesignedwithoff-the-shelfcomponents,between80und140mW,dependingontheoutputvoltage.Itbecauseitisanexternaldevicewhichisnotpartofthefallsoffto5mWatadistanceof6cm.applicationunit.Thussmallvolumeisnotmandatory.Therearetwoalternativestorealizethereceiverofa160,00wirelessenergytransmissionsystemwithelectronicdevices140,00andcomponents.120,00Ifsmalldevicesizeisrequestedorsmallcomponent100,002,60Vcountisimportant,allfunctionalblockscanbeimplemented80,003,30V60,00onanintegratedcircuit.TheonlycomponentwhichisPower/mW5,00V40,00difficulttoimplementonchipistheantenna.Uptonow,20,00therearenoexamplesforsuchadeviceon-chip[3][4].0,00020406080100120TheadvantagesofanintegratedcircuitwithminimumDistance/mmexternalcomponents(e.g.foilantenna)aresmallboardspace,smalldevicecountandsmallsystemcosts.Duetotheusuallylargedevelopmenttime,hugedevelopmentcostsareFigure5:OutputpowerofflexiblefoilPCBnecessary.Furthermore,thecustomizationisdifficult,becausechangingdeviceparametersofanintegratedcircuitFurthermore,tworeceiverASICswereimplemented,whichisonlypossiblewithexternalcomponentsfortuning.providepowertorechargeLithium-Ionbatteries.ThechargeDiscretesolutionswithstate-of-theartdevicesofferthecurrentsof4and120mAweredictatedbythemaximumbenefitofshortdevelopmenttime,smalldevelopmentcostschargecurrentofthebatteriesused.FortheseICsa0.35µmandfastcustomization.Downsidesarelargeboardspace,CMOStechnologywasused.largedevicecountandlargesystemcosts.The4mAreceiverisusedinaflexiblechipcardandneedsV.DEMONSTRATORSANDRESULTSonlyanexternalantenna.ThisantennawasrealisedwithseveralcopperwindingsonaflexiblefoiltoachievetheAccordingtothesystemalternativesdiscussedinIII,severalmaximumthicknessof0.5mmofthesechip-cards.Thedemonstratorsandprototypeswererealised.Theydifferinsmart-cardisshowninFigure6.theprovidedsupplycurrentandthedepthofintegration.The120mAusesalsoaflexibleantenna,butrequiresAnimplementationonaflexibleprintedcircuitboard(PCB)additionalexternalcomponentstotunetotherightservesasarapidprototypesystem,whichiseasytofrequency.ThelayoutofthisASICisshowninFigure7. VI.SUMMARYANDCONCLUSIONSThispaperdiscussesseveralimplementationalternativesforwirelessenergytransmissionsystems.Itpresentsrealizationexamplesforthesealternatives.Wirelessenergytransmissionisinvestigatedinnumerousapplicationsduetoitssimplicityandadvantages.Maingoalinafurtheroptimizationistoincreasethetransmissionpowerandnottoexcesstheallowedpowerlevelsinthefrequencybandsused.Inconnectionwithenergystorageelementslikebatteriesorsupercapssmallamountofenergycanbecollectedduringappropriatetimestopowershorttransmissionbursts.Figure6:Implementationinasmart-cardACKNOWLEDGMENTTheauthorwouldliketothankFranz-XaverArbinger,MichaelKirchner,SantiagoUrquijo,JavierGutierrezandJosefBernhardfortheircontribution.REFERENCES[1]G.Wiesspeiner:DrahtlosAkkusaufladen,www.bticcs.com[2]F.Arbinger,P.Spies,G.Rohmer:WirelessBatteryChargerChipforSmart-CardApplications,www.smart-power.fraunhofer.de[3]K.Finkenzeller:RFIDHandbook,SecondEdition,Wiley,2003[4]S.Roundy,P.wright,J.Rabaey:EnergyScavengingforWirelessSensorsNetworkswithspecialFocusonVibrations,KluwerAcademicPublishers,NewYork,2004.PeterSpiesstudiedElectricalEngineeringattheUniversityofErlangen/GermanyandgraduatedwithaDipl.-Ing.degreein1997.Since1998,heiswiththeFraunhoferIIS,powerefficientsystemsdepartment.Hewasworkingonthefieldofmulti-standardfront-endsandsystemsimulationsforcommunicationapplications.Since2001heisgroupmanagerofthetechnologiesforterminaldevicesgroupwhereheisdoingresearchanddesignonthefieldofpowerandbatterymanagement,energytransmissionandenergyharvesting.Focusofhisgroupisintegratedcircuitandsystemdesignaswellassoftwaredevelopment.OtherareasofhisworkaresensorsignalprocessingandinterfaceFigure7:LayoutoftheASICfor120mAchargecurrenttechnologiesliketransceiversortransponders.