Beeby, SP

Elliott, ADT

Mitcheson, PD

Zhu, D

Spiral - Imperial College Digital Repository

MULTILAYER PIEZOELECTRIC ENERGY HARVESTING USING SINGLESUPPLY PRE-BIASING FOR MAXIMUM POWER GENERATIONA.D.T. Elliott1*, D. Zhu2, S.P. Beeby2 and P.D. Mitcheson11Electrical and Electronic Engineering, Imperial College, London, UK2Electronics and Computer Science, University of Southampton, Southampton, UKAbstract: This paper studies the method of increasing output power of multilayer piezoelectric energyharvesters using Single Supply Pre-biasing (SSPB). In SSPB, it is important that the pre-biasing voltage beapplied at the right moment. Multilayer piezoelectric energy harvesters are well suited SSPB as there is no phaselag between piezoelectric charge displacement on each layer and one layer can therefore be used for switch timingcontrol. Compared to previously reported SSPB circuits, the power consumption of the circuit has beensignificantly reduced. Experimentally, the output power of a bimorph single-layer energy harvester using SSPBhas been increased by a factor of three compared to that using a bridge rectifier. Furthermore, the possibility ofimplementing SSPB in bimorph double-layer energy harvesters has also been investigated.Keywords: single supply pre-biasing, multi-layer piezoelectricINTRODUCTIONWireless devices such as biomedical sensors arelimited in their application by the finite energy storedby the battery powering them. Thus energy harvestersare being designed to eliminate this constraint byextracting power from the local environment.However it is not simply a case of finding atransduction method and powering a device. Acomplete system must be designed taking into accountthe transduction method and the power electronicsattached to the output.Piezoelectric vibrational energy harvesters generatea sinusoidal voltage when a mechanically oscillatingsinusoidal stress is applied to the device. The simplestform is a cantilever beam with a single-layer of piezoattached to one side of a substrate. However theoutput power can be increased by attaching a secondlayer on the bottom of the beam [1] to form a bimorphstructure. The benefit of this structure is that the twopiezo layers are moved further away from the neutralline to experience more stress and thus potentiallygenerate more energy.Output power can be further increased bymanufacturing bimorph double-layer energyharvesters. They have approximately 40% higheroutput power than single-layer energy harvesterswhen the total thicknesses of piezoelectric layers inthe two cases are the same [2] when connected to anoptimized resistive load.Rectification of the piezo’s output must beperformed in order to store the energy on a capacitoror battery. The obvious solution is a bridge rectifier,however more power can be extracted by optimizingthe electrical damping applied to the piezo [3]. Themost successful method is Single Supply Pre-biasing(SSPB), which can theoretically generate twice asmuch power as Synchronous Switched Harvester onInductor (SSHI), [3] and has been shownexperimentally to generate 14% more power [4] thantheoretically possible by SSHI [5]. This paperpresents the combining of these two powermaximization strategies.THEORY OF OPERATIONFigure 1 shows cross sections of the bimorphsingle-layer and bimorph double-layer energyharvesters. In bimorph energy harvesters there is nophase lag between piezoelectric layers. This isadvantageous as a layer can be used to provide a sensesignal for switch control timing.SubstrateElectrodePZTPolarization directionVout VsenVoutVsen(a) Bimorph single-layer energy harvester(b) Bimorph double-layer energy harvester123Figure 1: Multilayer piezo construction.978-0-9743611-9-2/PMEMS2012/$20©2012TRF 141 PowerMEMS 2012, Atlanta, GA, USA, December 2-5, 2012For the single-layer energy harvester, one piezolayer is used for energy harvesting and the other isused for sensing. Therefore, only half of the totalavailable capacitance can be used to harvest. Bycontrast, only one of the four piezoelectric layers isused to provide the sensing signal in the double-layerenergy harvester and thus three quarters of the totalavailable piezo can be used for harvesting.Figure 2: SSPB H-bridge circuit for discharging andpre-biasing the piezo [4].SSPB operates by placing charge on thepiezoelectric beam at its maxima points of travel byclosing switch pairs (Figure 2). The charge generatesa piezoelectric force which opposes the motion of thebeam as it travels in the opposite direction whilst allswitches are open. This increases the work done bythe beam and hence more power can be extracted.When the beam reaches the opposite point ofextreme travel, the pre-bias charge and the generatedcharge are both discharged into the energy storagecomponent through the same set of switches, beforethe beam is pre-biased with opposite polarity by thesecond pair of switches and the operational cyclerepeats. Figure 3 shows a typical waveform of theoutput voltage using SSPB.Figure 3: Voltage waveform across piezo when pre-biasing is applied to increase power extraction [6].IMPLEMENTATIONImplementation of the system can be considered infour main blocks: transduction, peak detection, piezopre-biasing, and switch timing control (Figure 4).Figure 4: Multilayer piezo connected to SSPB circuitimplementation.The chosen transduction method was a multilayerpiezoelectric energy harvester (as shown in Figure 5)fabricated using screen printing techniques. Eachlayer was printed via its respective pre-meshed screenand was then dried and cured at specific temperatures.After the fabrication processes, the piezo layers werepolarized under constant electric field at 200°C.Details in fabrication can be found in [1].Figure 5: Photo of the multilayer piezo energyharvester.The peak detection circuit (Figure 6) operates bymeasuring the differential voltage across the sensepiezo using a low power AD8500 operationalamplifier [4]. The instantaneous voltage is comparedto a lossy peak-hold version held by a capacitor togenerate the triggers for the switch timing control.Power consumption of the peak detector wasreduced over previous implementations [4] by 91% to8.64 µW. This improvement was achieved byanalyzing the current paths in the circuit andsubsequently increasing the resistor values of thelossy peak-hold circuits.142Figure 6: Low power peak detection circuit [7].The switches need to be operated in pairs (P1-N4, P2-N3) and held on for a half electrical resonance cycleformed by the piezo and inductor. A low powerIGLOO nano FPGA was selected to implement thecontrol algorithm (Figure 7). When a peak isdetected, the FPGA pulls the input to the dischargetiming resistor and fires the required switch pair. Thesignal is held high until the voltage on the capacitorreaches the FPGA’s input high threshold. A 1 µs timerthen begins to prevent shoot-through, before the pre-bias switch pair are fired and timed in the same way.Figure 7: Switch timing control circuit [4].The voltage across the piezo increases above theVcc voltage rail as well as below the ground rail, hencethe switches must block in both directions. Currentmust also flow in both directions in order to pre-biasand discharge the piezo. Previous implementationsused optically isolated TRIACs to provide thenecessary switching characteristics [6], however theyare power intensive to control and have a slow turn-onand turn off times.A better solution is shown in Figure 8 along withtheir gate drives [4]. The series connected p-type(BSH201) and n-type (BSS138) MOSFETs ensure thepiezo is electrically isolated. The n-type is drivendirectly from the FPGA (IGLOO AGLN250),however the p-type MOSFET required the voltage tobe level shifted so a capacitor and voltage clampingdiode were used. The high side p-type MOSFET isdriven using a DC decoupled potential divider.Figure 8: H-bridge circuit implementation [4].The inductor chosen was designed to maximizepower generation with minimal volume [7], thus a5.354 mH with volume 0.152 cm3 from Coilcraft wasselected.EXPERIMENTAL RESULTSThe single-layer bimorph was tested first using onelayer to generate and one layer to sense. Thegeneration layer had a piezo capacitance of 28.5 nFand when combined with the inductor, the Q-factorwas measured to be 4.9. A mechanical excitationforce of 200 mG rms at 70 Hz was applied inducing a2.2 V open circuit voltage across the piezo.The power generated was measured using aYokogawa WT210 Digital Power Meter for both abridge rectifier and the SSPB circuit. The theoreticalmaximum power extraction possible using SSHI wasalso calculated from [3].Table 1 presents the results from the testing. Undera relative small excitation the output the SSPB circuitgenerates over three times as much power as thesimple bridge rectifier. However it performs onlyequally well compared to the theoretical limit of SSHIdue to the peak detection circuit being slightly late indetecting when the cantilever is at its extreme point oftravel as can be seen in Figure 9.Table 1: Average charging power at optimal voltagefor single-layer bi-morph energy harvester usingdifferent electrical interfaces.Electrical interfaceto beamAverage charging power(total charged energydivided by charging time)Bridge rectifier 18 µWSSHI (theoretical) 60 µWSSPB (experimental) 60 µW143The energy lost is due to the early firing of theswitches. The expression for the theoretical maximumpower extracted by SSPB [3] can be modified to allowfor this by multiplying by a correction factor,(1-sin( t)) where t is the timing inaccuracy and theangular excitation frequency. The timinginaccuracy can be found by measuring the timebetween the switch firing and the actual peak. In thiscase the correction factor is (1-sin(0.16 )), thus thepower loss is approximately 58 µW.Power loss = Vpo2f0Cp (1-sin( t)) (1)The testing on the bimorph double-layer energyharvester was less successful than the single-layerenergy harvester. This is suspected to be due to thesensing layer and the three harvesting layers sharing acommon connection (see the dotted line in Figure 1b).Therefore, the sensing signal is not completelydecoupled from the generated power signal. When thepower signal is pre-biased, the sensing signal is alsoaffected, which caused the system to fire at the wrongtime.CONCLUSIONA complete design for piezoelectric energyharvesters has been presented. Improvements incontrol circuitry power consumption have been made,however a better peak detection implementation isstill required to reach the theoretical limits of singlesupply pre-biasing.Preliminary results using a single-layer bi-morphwith SSPB are encouraging however future work isrequired in redesigning the double-layer energyharvesters so that the sensing layer and generatinglayers can be isolated.Furthermore, a simulation will be run to study thedeformation of the three parallel connected generatinglayers under the same voltage and possible solutionswill be proposed based on the simulation results.ACKNOWLEDGEMENTSThis work was supported by EPSRC under grantnumber EP/G070180/1, “Next Generation Energy-Harvesting Electronics: Holistic Approach”.REFERENCE[1] D. Zhu, S. P. Beeby, M. J. Tudor and N. R. Harris,“A credit card sized self powered smart sensornode,” Sensors and Actuators A: Physical, vol.169, no. 2, pp. 317-325, October 2011.[2] D. Zhu, A. Almusallam, S. Beeby, J. Tudor and N.Harris, “A Bimorph Multi-layer PiezoelectricVibration Energy Harvester,” in PowerMEMS2010, Leuven, pp. 335-338, 2010.[3] J. Dicken, P. D. Mitcheson, I. Stoianov and E. M.Yeatman, “Power-Extraction Circuits forPiezoelectric Energy Harvesters in Miniature andLow-Power Applications,” IEEE Transactions onPower Electronics, in press 2012.[4] A. D. T. Elliott and P. D. Mitcheson, “Powerdensity Improvement of a Piezoelectric EnergyHarvester through use of a Micropower Switch-Mode Interface,” in IEEE Sensors 2012, Taipei,2012.[5] M. Lallart, C. Richard, L. Garbuio, L. Petit and D.Guyomar, “High efficiency, wide load bandwidthpiezoelectric energy scavenging by a hybridnonlinear approach,” Sensors and Actuators, vol.165, no. 2, pp. 294-302, February 2011.[6] J. Dicken, P. D. Mitcheson, A. Elliott and E. M.Yeatman, “Single-Supply Pre-biasing Circuit forlow-amplitude energy harvesting applications,” inPowerMEMS, Seoul, South Korea, pp. 46-49,2011.[7] A. D. T. Elliott and P. D. Mitcheson,“Implementation of single supply pre-biasing withsub-energy harvesting,” in PowerMEMS, Atlanta, p. InPress, 2012.Figure 9: Generation piezo voltage (yellow), sensepiezo voltage (pink) and inductor current (green)waveforms with respect to time for single-layerbimorph energy harvester with SSPB output.144

Multilayer Piezoelectric Energy Harvesting Using Single Supply Pre-biasing for Maximum Power Generation

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