Analysis of Power Transfer Efficiency of Inductive Coupled Telemetry System for Wireless Power Transfer Mohammad Shawkat Habib, Mohammad Mizanur Rahman, Atika Arshad, and Sheroz Khan Department oI Electrical and Computer Engineering International Islamic University Malaysia 53100 Kuala Lumpur, Malaysia e-mail: sheroziium.edu.my Abstract-Recenttechnologicaladvancementsinsensors andconditioningelectronicsarefindingapplicationsin manyareasincludingnon-contactmonitoringinremote areas,inthemedicalfieldandbetterautomationinthe automotiveindustry.Suchelectronicsneedpowersources boththroughwiredandwirelessmeans.Giventhis, inductivecouplingisthemostappropriateandpotent approachtocarryoutthisjobadequately.Thebenefitof inductivecouplingisthat,itcanbeutilizedforboth telemetry and powering purposes that is to collect the sensed data from the device. This paper explores the power transfer efficiencyperformanceofaninductivecouplingsystem. Designparametershavebeensimulatedforthevariationon theloadresistor,thetuningoftheinductivecircuitandthe impact on coil orientation has been studied. Keywords-Inductivecoupledcoils;powerefficiency;coil orientation I. INTRODUCTION Withtheadvancementintechnology,wireless electronic devices are on the rise and have gained a distinct importanceintoday`sworld,IorthepurposeoI transIerringpowerwirelessly;themethodoIinductive powertransIer iswidelyused inapplicationswhere wired contactsarenotconvenientandinsomecasesalmost impossible to apply. Now a days the handheld devices and mobile phones are using wireless charging technologies. In biomedicalelectronicapplicationsuchasinthewireless epiretinalretinaimplantsystem,inductivecoupledcoils wereused,toallowwirelesspowersupplyIorthe implant`selectronicspowerconsumption.Therearesome bindingsintheuseoIsuchtechnologyIorthebiomedical applicationsastheinductivelinksaretobeloosely coupledandthethickness oI thetissuewouldcreatelarge separations at places. There are also some strict Iorm Iactor requirements that limit the size oI the inductor coils. It is necessary to keep the quality Iactor as well as the inductanceoItheinductorshigherinvalueinorderto obtaingoodinductivecoupling|1|.Inrecenttime,the passiveelectronicdevicesbeingusedIormonitoring variousphysiologicalphenomena(glucose,motion, temperature,electrocardiography,oxygenetc)areoIlow powerandsmallerinsize,whichareusedtoreceive inIormationIromlocationsthatareinaccessiblelikethe cochlearimplants,pacemakers,druginIusiondevices, physiologicalmonitoringdevices,leItventricularassist devices,artiIicialhearts,andIunctionalelectrical stimulators. An output current oI 3 to 6 mA is required to bedeliveredtothetissuesoIthestomachthroughthe wirelesspowertransIerIorthegastro-stimulatorimplants. ButIorthehumanbodyitisnotsuitabletorunthe transducers using the external sources like the solar power. ForprovidingthepowersupplytotheinteriorpartoIthe body,smallsizebatteryisusedthathaslimitedliIespan andpowersource,moreover,percutaneouslinksarea meansoIcausingskininIection|2|.ThereIorethebattery needs to be replaced through periodical surgical procedure, which turns to be a burden and risk Ior the patient. TherearediIIerentmeansoIpowerharvestingand transIer,otherthanthewiredtransIerandinductive coupledwirelesstransIeroIenergyandpower.Themost populararethesolar,vibrational,thermalgradientsetc whichcanbeusedIormedicalimplants.Amongthe alternatives, the most mature is the solar energy in the light Iorm.However,theeIIiciencyoIthesolarcellsareless thanIiItypercentandareveryexpensive.Moreover,the availability oI the sun light is not constant, which limits the usage oI the solar energy in biomedical applications. There are Iour diIIerent mechanisms Ior converting the mechanicalpowertoelectricalpoweroutoIwhich,the piezoelectricmechanism,whosepiezoelectricmaterials` vibrationenergyharvestingisbasedontheconceptoI shunt damping in order to control mechanical vibration |3|, is the most popular to drive electronic devices. Inelectroniccarbatterychargingsystems,theclosed loop power transIer charging systems have been Iocused in|4|.TheinductivepowertransIerisIoundtobe successIullyusedbyvariousmodulesoIelectronic vehiclesthatweremanuIacturedbytheGeneralMotors. Theprimarycoilisthechargingpaddleandonthe secondaryside,theinductivelycoupledchargerissealed that actually allows the contactless charging oI the vehicle operationaleitherin6.6kWor50kW.In|4|,auniversal inductionpowertransIersystemhasbeenpresentedIor chargingelectronicvehicle,whichuses10kVAcoaxial windingtransIormers.ItisbeneIicialtousecoaxial windingtransIormerasitminimizestheonboard electronicvehiclesensitivityparametercomponentsto IrequencyandIluxdensityandisalsoconvenienttobe implemented within a single loop and operational within a wide range oI power requirements and Irequency. In |5| a compact 100 watt to 120 kilo watt inductive power transIer EV charging system has been presented. The eddy current lossandelectromagneticinterIerenceisproduceddueto thenon-linear Ilux distributionthatcan besolvedthrough symmetrical division oI the core |6|. In |7||8||9| the inductive coupling oI typical telemetric circuithasbeenelaboratedIurtherintheIormoI simulationsandanalyticalderivations.Theresonant 5th International Conference on Computer & Communication Engineering978-1-4799-7635-5/14 $31.00 2014 IEEEDOI 10.1109/ICCCE.2014.2232Irequencybetweentheprimaryandsecondaryside determinesthequalityoIthepowertransIer.Power variation can be achieved by varying the resistive load and whichisdoneinsecondaryside.In|7|aIrequency controlledmethodhasbeenproposedIorregulatingthe IrequencyoIwirelesssupplyaccordingtoload requirement.In|7||9|anelectronicreadoutsystemhas beendevelopedusinginductivecouplingLCresonance circuitwheretheyhaveshownanalyticalcalculationas well as simulation, all results obtained are as a Iunction oI IrequencyIurthermoreabetterIrequencyresolutionis achievedbydecreasingnoisepower.Inaddition,the importantparametersinbiomedicalapplicationIor wirelesspowertransmissionaretheoperationIrequency anddatarate.Hencein|7|bothbackwarddata communicationsystemandwirelesspowertransIerby means oI inductive link was carried out. Thispaperis organizedasIollows.SectionIIpresents theanalysisoItelemetrysystem.SectionIIIpresentsthe results&discussions,andsectionIVconcludeswiththe conclusion. II.ANALYSIS OF THE TELEMETRY SYSTEM Fig.1showswirelesspowertransIerinductive telemetricsystemthatconsistsoIacoilinductanceinthe readout circuit and a sensing circuit inductance.Thetransmittingcoilisbeingmodeledbytheinternal impedance oI the source Zs and resistor R1. The inductance on the receiver side is being modeled by a resistor R2 and theimpedanceoItheloadresistorZL.M`isthemutual inductancethatexistsbetweeninductorcoilsL1andL2. The loop consists oI the input source VS as source loop and on the other hand as load loop. Figure 1. Wireless power transIer inductive telemetric system. ThecalculationsIorIindingthecurrentsIoreachloop canbeaccomplishedthroughapplyingKVLthatisgiven by: 222 2112 222 112222112210ZMZVM Z ZV ZZ M jM j ZZM j VIS SSe eeee+=+==(1) 2 222 1122111120M Z ZMV jZ M jM j ZM jV ZISSeeeee+==(2) Where,Z11isthetotalselI-impedanceoIthemesh containing primary windings oI the transIormer and Z22 is thetotalselI-impedanceoIthemeshcontainingthe secondary winding. ThePowerTransIerEIIiciency(PeII)isderivedtobe as: 1002 211 22 222 2+=M Z Z ZR MPLeffee(3) The mutual inductance, M is derived as: ( )22 2122 221 1x RR N R NMo+=t (4) Where,MmutualcouplingcoeIIicient,0 permeabilityoIair(4tx10-7Hm1),N1turnsoI primary coils, N2 turns oI secondary coils, R1 radius oI sourceloopcoil,R2radiusoIloadloopcoil,xcoil separation. ItisknownthatthemutualinductanceMinEquation (4)dependsontheshapesandtheorientationsoIthetwo inductive coils. III.RESULT ANALYSIS AND DISCUSSION The results were plotted Ior analytical analysis as well by simulation using PSpice and MATLAB soItware. Table 1 illustrates the parameters used in obtaining the results. TABLE I. INDUCTIVE COUPLED POWERING SYSTEMPARAMETERS ComponentDescriptionValue VSInput voltage source3V L1Primary inductor7.4uH L2Secondary inductor267uH kCoupling coeIIicient0.453 RLResistive load15 O Frequency1.0Hz 100Hz 10KHz 1.0MHz 100MHz(W(RL)/W(V1))*100 W(V1) W(RL)1.0p10n100u1.0EfficienyP receivedP sent Figure 2. Power sent, power received as a Iunction oI Frequency. Fig.2demonstratesthepowertransIereIIiciencyplot Ior a Iixed resistive load, RL15O. It is to be noted that the 33maximumeIIiciencyoIthepowertransIerisaround100 kHzto700kHz.Althoughthepowersentismaximum, beIore100kHzandgraduallydecreasingaIter700kHz, themain concern is to transIer maximum power, which is possibleonlybetween100kHzand700kHz.AIter700 kHz the strength oI the power sent dramatically Ialls. The outputvoltageacrossthe15ohmsloadwasIoundtobe 163.91mV.ForaninputpoweroI439.5mWatthe maximumpowertransIer,theoutputpowerwasIoundto be 1.8mW. VariationoneIIiciencywithrespecttothecoupling coeIIicientisshowninFig.3IordiIIerentvaluesoIthe coupling coeIIicient k (0.25 0.50 0.75), the power transIer eIIiciency was simulated. Figure 3. Simulated power transIer eIIiciency with diIIerent values oI k`. Fig.4showstheresultsIortheeIIectsoIvaryingthe load.TheresistiveloadwasvariedanditwasIoundthat withtheincreaseintheload,theconsumedpoweracross the load increases. Furthermore, it has been interpreted that it is eIIicient to choose a resistive load oI around 70 O Ior a Irequency oI 100 kHz.Fig. 5 demonstrates the plots obtained when the circuit was tuned by adding a capacitor in parallel or in series onthetransIormer`ssecondaryside.TheeIIectoIthe capacitorinserieswiththeloadwasIoundas;whenthe capacitor value was set in the range oI mill and micro, the resultsdisplayedwasalmostthesamevoltageandpower characteristics,whileanyvaluesmallerthanthemilli range,theoutputvoltageandpowerwaslessprominent. On the other hand, using the capacitor in parallel with the load,resultsininversecharacteristicsthatoIwhen capacitor was added in series. Any value oI capacitor less than the nano range results in lower voltage and power that isnegligible.ForattainingrealisticvalueoIvoltageand poweracrosstheload,usingthecapacitorinparallel,the capacitor value should be in nano range. In Fig. 5, the red curveindicatestheperIormanceIorthecircuitwherethe capacitorwasinseriesandthegreenrepresentsthe perIormance when the capacitor was in parallel. In order to obtainamaximumpowertransIereIIiciencythevalueoI theparallelconIigurationoIthecapacitanceshouldbe around 5nF, whereas Ior series conIiguration the capacitive value should be between 13nF to 14nF. Figure 4. Power TransIer EIIiciency () verses Load Resistance (ohms). 5 10 15 20 25 30 3501234567Capacitance (nF)Power Transfer Efficiency (%) seriesparallel Figure 5. Power eIIiciency with variable capacitance in series and parallel. ReIerence |10| carries out Iurther analytical studies Ior theeIIectontheinductivecoil`sorientationIorthree diIIerentcasesIor(a)transmitterandreceivercoils`coil spacing,(b)lateralmisalignmentand(c)angular misalignment()betweenthereceiverandthetransmitter coils. Fromtherelatedworkin|10|,itisseenthatwiththe increasingdistancebetweenthetwomutuallyinductive coilstheoutputvoltageamplitudedrops,Iromthisitcan beinterpretedthattheoutputvoltageisinversely proportionaltothedistancebetweenthetwoinductors. Fig. 6 presents the eIIect oI variation oI coil spacing. Fig.7showsthelateralalignmentoIthecoils,which alsoaIIectsthepowertransIerandthusaIIectstheoutput voltage. II the lateral alignment is within 1 cm, there is no majorreductionoItheoutputvoltage,butiIthelateral alignment is above 1 cm the variation is linearly reducing.In Fig. 8 the coil spacing and the lateral alignment are kept constant, only the angular misalignment is taken into consideration, it is observed that the angular misalignment oI90degreeswouldleadthepowertransIertozero,as there would be no output voltage on the receiver coil. 34 Figure 6. EIIect oI variation oI coil spacing between transmitter and receiver coils: (a) orientation oI the coils (b) variation oI the output voltage with increase in coil spacing |10|. Figure 7. EIIect oI the variation oI lateral misalignment () between the transmitter and the receiver coils: (a) the orientation oI the coils. (b) the variation oI the output voltage with increase in lateral misalignment. The coil spacing is kept Iixed at 1 cm |10|. Figure 8. Illustration oI angular misalignment () between the transmitter and the receiver coils. (a) orientation oI the coils. (b) the change oI output Ior increase in angular misalignment. The coil spacing is kept Iixed at 1 cm, and lateral misalignment is zero |10|. IV.CONCLUSIONS Inthispaper,theeIIectontheinductivecoupled telemetry system Ior wireless power supply was studied. A simplecircuitwasconIiguredandanalyzedIorthe variationontheloadresistor.Theimpactoncoil orientationandtuningoItheinductivecircuitwerealso observed.Fromthesimulatedresultsitcanbeconcluded thatthepowertransmissionisatitsmaximumwhenthe circuit is tuned in between 100 kHz to 700 kHz Irequency. The change oI the coupling coeIIicient plays an important roleinthepowereIIiciencyvalue.ThepowertransIer eIIiciencyisproportionaltothevalueoIthecoupling coeIIicient.AnincreaseintheIrequencythevalueoI powersentdecreasesandthereisacertainpointoIthe Irequencywheretheoutputpowerisatitsmaximumand thatvaluehasbeensuccessIullyidentiIiedinthe simulationresults.Theoptimizedloadresistanceat100 kHz was Ioundtobearound70OandIor Iinetuningthe valueoItheseriescapacitancewasnotedtobe13-14nF range while Ior parallel capacitance it was 5nF. ThereIore, bychoosingoptimumload,sourceIrequencyand capacitorsIoradeIinedinductivecoupledsystempower transIer eIIiciency can maximized. 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