Electrostatic vibration energy harvester with 2.4-GHz Cockcroft–Walton rectenna start-up

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Electrostatic vibration energy harvester with 2.4-GHz Cockcroft–Walton rectenna start-up

Hakim Takhedmit, Zied Saddi, Armine Karami, Philippe Basset, Laurent Cirio

To cite this version:

Hakim Takhedmit, Zied Saddi, Armine Karami, Philippe Basset, Laurent Cirio. Electrostatic vibra-

tion energy harvester with 2.4-GHz Cockcroft–Walton rectenna start-up. Comptes Rendus. Physique,

Académie des sciences (Paris), 2017, 18 (2), pp.98-106. �10.1016/j.crhy.2016.12.001�. �hal-01434988�

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Energy and radiosciences / Énergie et radiosciences

Electrostatic vibration energy harvester with 2.4-GHz Cockcroft–Walton rectenna start-up

Dispositif de récupération d’énergie vibratoire par transduction

électrostatique, pré-chargé par une rectenna Cockcroft–Walton à 2,4 GHz Hakim Takhedmit

^a^,^∗

, Zied Saddi

^a

, Armine Karami

^b

, Philippe Basset

^a

,

Laurent Cirio

^a

aUniversitéParis-Est,ESYCOM(EA2552),UPEM,ESIEE-Paris,CNAM,77454Marne-la-Vallée,France bLaboratoired’informatiquedeParis6(LIP6),UniversitéParis6,Paris 75005,France

a r t i c l e i n f o a b s t r a c t

Articlehistory:

Availableonlinexxxx

Keywords:

Rectenna

Cockcroft–Waltonrectiﬁer Energyharvesting Electrostatictransduction Bennet’sdoubler

Mots-clés : Rectenna

CircuitderectiﬁcationCockcroft–Walton Récupérationd’énergie

Transductionélectrostatique DoubleurdetensiondeBennet

In this paper, we propose the design, fabrication and experiments of a macro-scale electrostaticvibrationenergyharvester(e-VEH),pre-chargedwirelesslyforthe ﬁrsttime with a2.4-GHz Cockcroft–Walton rectenna.The rectenna is designed and optimized to operateat low power densities and providehigh voltage levels:0.5 V at0.76 μW/cm² and 1 V at1.53 μW/cm².The e-VEH uses aBennet doubler as aconditioning circuit.

Experimentsshow a23-Vvoltageacrossthetransducer terminal,whenthe harvesteris excitedat25 Hzby1.5gofexternalacceleration.Anaccumulatedenergyof275 μJanda maximumavailablepowerof0.4 μWareachieved.

©²⁰¹⁶Âcadémie^des^sciences.^Published^byÊlsevier^Masson^SAS.^Thisîsânôpenâccess articleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

r é s um é

Cetarticleproposelaconception,laréalisationetlesmesuresd’untransducteurélectrosta- tique, à base d’une capacité macroscopique, pré-chargé par une rectenna de type Cockcroft–Waltonà2,4GHz.Larectennaest conçueetoptimiséepour fonctionneràdes niveauxdepuissancefaiblesetfournirdestensionsélevées :0,5Và0,76 μW/cm²et1 V à1,53 μW/cm².Le transducteurélectrostatique utiliselecircuit deconditionnement de Bennet. Lesmesuresdusystèmecompletmontrent destensionssupérieuresà23 Vaux bornesdutransducteur, lorsqu’ilest excité à25 Hzetavecuneaccélération externede 1,5g.Uneénergiecumuléede275 μJetunepuissancedisponiblede0,4 μWontpuêtre obtenues.

©²⁰¹⁶Âcadémie^des^sciences.^Published^byÊlsevier^Masson^SAS.^Thisîsânôpenâccess articleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

*

Correspondingauthor.

E-mailaddress:[email protected](H. Takhedmit).

http://dx.doi.org/10.1016/j.crhy.2016.12.001

1631-0705/©²⁰¹⁶Âcadémie^des^sciences.^Published^byÊlsevier^Masson^SAS.^Thisîsânôpenâccessârticleûnder^the^CC^BY-NC-ND^license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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JID:COMREN AID:3363 /SSU [m3G; v1.194; Prn:23/12/2016; 12:30] P.2 (1-9)

2 H. Takhedmit et al. / C. R. Physique•••(••••)•••–•••

Fig. 1.Block diagram of an RF energy harvester.

1. Introduction

Advancesinwirelesscommunicationsandlow-consumptionelectronicsinrecentdecadeshave contributedtotheemer- gence of sensors andconnected objects in differentfields.An exponential growth of thenumber of devicesis expected withtheadventoftheMachine-to-Machine(M2M)andtheInternetofThings(IoT).Theenergyautonomyofsuchdevices constitutesoneofthemainobstaclesbeforereachingfullmobility.Insteadoftraditionalbatteriesthatrequireperiodicre- placementorrechargingandraiserecyclingissues,energyharvesting,consistinginconvertingtheenergyofambientsources such aselectromagneticwaves,vibrations, thermal, solarandwindintoelectrical energy,becamea potentially promising solution.From theseambientsources, electromagneticwavesandmechanicalvibrationsareofparticularrelevance dueto theiravailability.Vibrationharvestersarebasedonthetransductionmechanism,andtheyaretypicallyofthreekinds:elec- tromagnetic,piezoelectricandelectrostatic[1–5].Inelectrostatic-vibrationenergyharvesters(e-VEHs),mechanicalenergyis convertedintoelectricity byamechanicalattractionforceduetochargedvariablecapacitorplates.Thisforce opposesthe motionofthemobileplate. Thegeneratedpowerfrommechanicaltoelectricalconversionisproportionaltothesquareof theaccumulatedquantityofchargeinthecapacitor.Therefore,anexternalsourceprovidingsufficientvoltageisnecessary to convertvibrationsintoelectricity inasufficient manner.One solutionconsistsinusingan electretlayer[6,7].Another solutionconsistsinusingatransducerpre-chargecontaining apowersourceandaconditioningcircuitthatgeneratesthe biasvoltageitselfandthencreatesaforcebetweenthetwoplatesofthevariablecapacitor[8–14].Inthispaper,wepropose theuseofanRFenergyharvester,commonlycalledrectenna[15–23],topre-chargeane-VEHdevice.Severalconfigurations ofrectennae arereportedintheliterature,the singleseries[16–18]andshunt[16–19] arethemostused.However,such topologiesdeliversmallDCvoltages.Indeed,forhigheroutput,thevoltagedoubler[16,20,21],theGreinachertopology[22]

andthevoltagemultiplier[23]arefurtherappropriate.

Usually,whendesigning rectennas,themainissueconsistsinmaximizing theelectricalpowerdeliveredtotheloador theRF-to-dcconversionefficiency[24,25].Anewandefficientdual-dioderectennawasreportedin[24].Aglobalefficiency higherthan 80%andadcoutputvoltageof2.6Vovera1050- resistiveloadhavebeenachievedatapowerdensityof 0.22 mW/cm²(E∼²⁹^V/m).^{A compact}ândêfficient^2.45-GHz^rectenna^was^presentedⁱⁿ^[25],^whereâ^circularly^polarized shortedring-slotantennawasused.Thereportedrectennaexhibitsamaximumefficiencyof69%andanoutputdcvoltage of1.1 Vatalowpowerdensityof20 μW/cm² (E=⁸.7V/m).

This paperdescribesthe design, fabrication andexperimentsof an e-VEH, witha Bennetdoublerasthe conditioning circuit,pre-chargedbyarectennacircuitat2.4GHz.Theoutlineisasfollows.Section2presentsthedesignandexperiments oftheCockcroft–Waltonrectenna.Thefabricationandoperationofthemechanicaltransducer,includingtheBennetdoubler, isreportedinSection3.Further,inSection4,theexperimentalresultsofthefullcircuitarepresentedanddiscussed.Finally, section5concludesthepaper.

2. Cockcroft–Waltonrectennaat2.4GHz

TheblocdiagramofanRFpowerharvesterisillustratedinFig. 1.ItcontainsareceivingantennafollowedbyanRF-to-dc rectiﬁerandoptionallyanenergystoragedevice.

ArectifierisoftenmadeupofacombinationofSchottkydiodes,aninputRFfilter,andanoutputbypasscapacitor.The input filter,localizedbetweenthereceiving antennaandthediodes,is alow-pass filterthat rejects unwantedhigh-order harmonicscreatedbythenon-linearbehaviorofthediodes.Italsoprovidesimpedancematchingbetweentheantennaand therectifier[15,16].

Topre-chargethee-VEH,highoutputvoltageissuitable.Thesingleseries,thesingleshunt oreventhevoltagedoubler wereshowntobeinsuﬃcient.Othertopologies,lessconventional,shouldbeused.TheCockcroft–Waltonvoltagemultiplier withseveralcascaded stagesproves tobe an eﬃcientsolution[26,27]. Thissection describesthestudyofsuch a circuit, fromdesigntoexperiments.

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Fig. 2.Voltage multiplier scheme (a); one stage multiplier (b).

Fig. 3.Geometryofthe rectiﬁer:L1=45, L2=4, L3=8.4,L4=4, L5=7.3, L6=12.5, W1=46, W2=3.5, W3=0.8,θ=45^◦ (dimensionsarein millimeters).

2.1. Voltagemultiplierdesign

ThevoltagemultiplierschemeisshowninFig. 2a.Itcontains severalidenticalcascadedstages;eachstagecontainstwo SkyworksSMS7630Schottkydiodes(D₁andD₂)[28]andtwoequalcapacitors(C₁andC₂)asillustratedinFig. 2b.

TheoutputDCvoltagedependsonthecapacitorsC1 andC2 andevenmoreonthenumberofstages.Itincreaseswhen the number ofstages increases. However, for an RF powerlevel, there is a tradeoff betweenthe number of stages and losses. Indeed,whenlosses(substrate,parasiticcomponents,electromagneticcouplings. . . ), becomeimportantthevoltage gainbecomesinsigniﬁcant.

AmicrostripCockcroft–Waltonvoltagemultiplier,operatingat2.4GHz,isproposed(Fig. 3).Itisdesignedandoptimized, underADS(Advanced Design System)software,by usinga globalanalysistechnique [29].The simulations were achieved by couplingamomentum electromagneticsimulatorandHarmonicBalance (HB).The circuitcontains sixcascadedstages andisfed byamicrostripline(Zc=⁵⁰).The circuitisetchedonArlon25Nsubstrate(

ε

r=³.4,thickness=¹.524mm, tanδ=⁰.0025). Seriescapacitors (C=⁶⁸^pF) ^were ^properly ^chosen ând^modeled, ^by ^taking înto âccount ^their ^parasitic effects.Theoutputload R_Lisvery large:itissetat100M.Aquarterwavelengthradial stub(L₆,θ) isolatestheoutput loadfrom RF incident power. Toachieve an impedancematching betweenthe RF source (or receiving antenna) andthe rectifier,an openstub(L5, W2) associatedwithaquarterwavelengthtransformer(L2+^L3+^L4, W3) was calculatedand used.Thecirculartopologyofthedesignandthearrangementofthedifferentcomponentsarecarefullychosentodecrease thedimensionsofthecircuit.

2.2. Experimentalcharacterization

Thecircuitwasfabricatedandmeasured.Fig. 4showsthesimulatedandmeasuredoutputdcvoltageagainstfrequency, from0.5to 4 GHz,at −^{15 dB m.} ^Good^agreement ^can ^be ^observed^betweentheoretical andexperimental results. At2.4 GHz,themeasureddcvoltageis0.85V,andthatachievedbyADSsimulationis1.05V.

Fig. 5shows a comparison betweenthe simulatedand measured curves ofdc voltage,as a function of theinput RF power from−²⁵^to −^{5 dB m,}ât ^{2.4 GHz.} ^Maximum ôutput^dc ^voltages ôf^3.16 ând^3.36^V ^have^been ôbtained^by ÂDS simulationandexperiment,respectively.

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Fig. 4.Output dc voltage vs. frequency.

Fig. 5.Output dc voltage vs. input RF power.

A microstrippatchantenna, operatingat2.4GHz, was designedandassociated withthe rectifier.Itpresentsan input returnlossof19dBandagainof4dBi.Thefullrectennawascharacterizedinsideananechoicchamber.Themeasurement setup contains anRF sourceanda12-dBigaintransmittinghorn antenna.The deviceundertestis placedinthefarfield region atadistanceof1.5m fromthe hornantenna.Fig. 6 showsthedcoutput voltageagainstpowerdensityfrom0to 10 μW/cm².Ourresultsshowthatwhenpowerdensityincreases,dcvoltageincreases.However,forhigherpowerdensities, the output voltageis limitedby the reversebreakdown voltageof thediodes. Measured dcvoltages of1, 2and3 V are obtainedat1.53 μW/cm²(E=².4V/m),3.5 μW/cm²(E=³.63V/m)and8 μW/cm²(E=⁵.5V/m)powerdensities(electric fieldstrength),respectively.

Fig. 7representsthemeasured voltageevolution,overacapacitiveloadof1mF,fordifferentpowerdensities:1,3and 10 μW/cm².Theresultsshowthatthecapacitortakeslessthan5mintochargeandthenstoresenergyofabout281 μJat 1 μW/cm²,1620 μJat3 μW/cm²and5445 μJat10 μW/cm².

3. Electrostaticvibrationenergyharvester 3.1. Conditioningcircuit

Mostconditioningcircuitsreportedintheliteraturerequireinductiveelementsandswitches[8–12]togenerateahigh biasvoltage.However,inductiveelementsarenotcompatiblewithabatchmanufacturingprocessandswitchesrequiread- ditionalpower-consumption-controlcircuits.Recentlyissued,aconditioningcircuitbasedontheBennetdoublergenerated high biasvoltage withoutusing anyswitch orinductor[13,14].Its operation hasbeen demonstratedwitha macro-scale variable differential capacitor, whosevariation was induced by an external motor. Moreover, authors in[30] presentthe studyofamonophase(singlecapacitor)MEMSe-VEHusingsuchakindofconditioningcircuit.Fig. 8showsBennet’sdou-

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Fig. 6.Measured dc voltage vs. power density (RL=^{100 M}).

Fig. 7.Voltage waveform over capacitive load (C=1 mF).

Fig. 8.Bennet’s doubler conditioning circuit.

blercircuit,itcontainsthreecapacitors,thevariablecapacitorCvar,Cres=¹^μF,^and^Cstore=⁴⁷^nF,^and^three^diodes ^D1,D2 andD3 (JPAD5).Theinitialpre-chargeV0 appliedtoCres issuppliedbytherectennapresentedintheprevioussection.The operationofBennet’sdoublerisdescribedin[30].

3.2. Descriptionofthestructure

Theschematicdrawingofthee-VEHprototypeisshowninFig. 9.

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Fig. 9.Geometry of the e-VEH:L1=W1=150,L2=119,W2=100 (dimensions are in mm). Top view (a) and proﬁle view (b).

Fig. 10.Photograph of the prototype showing the Cockcroft–Walton rectenna, the conditioning circuit and the macro-scale variable capacitor.

The device includesa macro-scale variablecapacitorandaBennetdoublerconditioningcircuit. Thetop view (Fig. 9a) showstherectennaandtheconditioningcircuitonthesamesubstrate,whichislinkedtothemobileplateofthevariable capacitorwithfourTeflonbolts(Fig. 9b).Themacro-scale variablecapacitorismadebytwo circulardopedsiliconwafers 100mmindiameterand0.5mminthickness,pastedonasquareepoxyboard1.5mminthickness.Thewaferofthemobile plateisprovidedwitha50-μm-thickSiO2 insulatinglayertopreventshortcircuitswiththesecondwafer.Thesecondplate of thevariablecapacitorisfixed toa shaker.Fourflatmetal springs,60 mm inlengthand15mm inwidth, areused to linkthetwoplatesofthevariablecapacitor,oneoneachsideoftheepoxylayer.Thesespringsweredimensionedsothat themobileplateofthecapacitorremains paralleltothefixedonewhenitmoves.EachendofaspringisfixedtothePCB layersusingfournylonbolts.

4. Experimentsofthee-VEHsystem

ThephotographoftheprototypeisdepictedinFig. 10.Theexperimentswerecarriedoutusingthemacro-scaleresonant variablecapacitordescribedintheprevioussection.Theresonancefrequencywas25 Hz.Themeasuredunbiasedtransducer capacitancevariationof1.5gamplitudeat25 Hzfrequencywas:Cmax/C_min=²⁵⁰^{pF/40 pF (}

η

₌6.25).

Fig. 11showstheschematic diagramoftheexperimentalsetup.Thee-VEH prototypeismountedonashaker (Bruel&

Kjaertype7541)andplacedinananechoicchamberatadistanceR=1.5m fromatransmittinghornantenna,wherethe far-ﬁeldconditionissatisﬁed.Anaccelerometer,adheredtotheshaker,isusedtocontrolandthenregulatetheacceleration.

Theexperimentwascarriedusingexternalvibrationsat25Hzwithanaccelerationamplitudeof1.5g.

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Fig. 11.Schematic diagram of the experimental setup.

Fig. 12.MeasuredevolutionofvoltageacrossCres(a)andaccumulatedenergyonCres(b)forseveralbiasvoltagesof0.5,1and2 V,with25 Hz/1.5g externalvibrations.

The measured voltage evolution across C_res at several pre-charge voltages is shown in Fig. 12a. The output voltage progressivelyincreasesupto23 V,wheresaturationoccurs.ThisvoltageincreaseacrossCres correspondstotheaccumulated energy.ThesaturationofthevoltageacrossCres isduetothespring-softeningeffectinducedbyelectromechanicalcoupling [30]. Indeed, thefrequency corresponding to the maximumdisplacement of theharvester’s movable electrode decreases progressively whenthe self-biasing provided by theconditioning circuit increases[31]. At some point, thisfrequency of maximum displacementis too far fromthe frequencyof the mechanicalexcitation, andthe power gain dueto the bias increase is balanced by a smaller transducer capacitance variation. Fordifferent pre-charge voltages, the output voltage across Cres tendstowards the samevalue. However, thetime requiredto achieve saturationdecreases when biasvoltage increases.The optimal voltage,which is deﬁnedas themaximum gradient of voltageV/t, iscalculated for different valuesofthepre-chargevoltage.ThevaluesofV_optandtherequiredtimetoreachitaresummarizedinTable 1.

The energyaccumulatedinCres,fordifferentvaluesofthe pre-chargevoltage(0.5,1, and2 V),isplottedinFig. 12b.

Energy increases against time and reaches more than 275 μJ ina few tens of minutes. The maximum available power, deﬁnedas the gradientof energy E/t, is calculatedfor differentcurves. When thebias voltage increases, the e-VEH needslesstimetoreachthemaximumavailablepowerof0.4 μW.TheresultsaresummarizedinTable 1.

Theaccumulatedenergyismainlyprovidedbythemechanicaltransducer.Indeed,therectennaprovidesonly0.125,0.5 and2 μJforpre-chargevoltagesof0.5,1and2V,respectively.

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Table 1

Optimalvoltageandmaximumavailablepowerfordifferentpre-chargevoltages, with25 Hz/1.5gexternalvibrations.

Biasvoltage (V)

Optimalvoltage (V)

Maximumavailable power(μW)

Requiredtime (min)

0.5 14.5 0.4 43

1 14 0.4 24

2 14.3 0.4 19

5. Conclusion

Thispaperpresentstheﬁrstexperimentsrelatingtoanelectrostatic-vibrationenergy-harvesterstart-upusingRFwaves.

The RFharvesterconsistsofaCockcroft–Waltonrectennaat2.4GHz. Thecircuitwas fabricatedandvalidated.Itprovides 0.5Vat0.76 μW/cm² and2Vat3.5 μW/cm².TheBennettdoublerisusedastheconditioningcircuit.Itdoesnotneedany inductiveelementorswitch.Experimentsshowa23-Vvoltageacrossthetransducerterminal,whentheharvesterisexcited at25Hzby1.5g ofexternalacceleration.Anaccumulatedenergyof275 μJandamaximumpowerof0.4 μWareavailable fortheload.

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