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Sensors and Actuators A 142 (2008) 329–335

Piezoelectric multifrequency energy converter for powerharvesting in autonomous microsystems

Marco Ferrari ∗, Vittorio Ferrari, Michele Guizzetti, Daniele Marioli, Andrea TaroniDipartimento di Elettronica per l’Automazione and Istituto Nazionale Fisica della Materia INFM-CNR,

Universita di Brescia, Via Branze 38, 25123 Brescia, Italy

Received 6 October 2006; received in revised form 3 July 2007; accepted 3 July 2007Available online 17 August 2007

bstract

A multifrequency mechanoelectrical piezoelectric converter intended for powering autonomous sensors from background vibrations is presented.he converter is composed of multiple bimorph cantilevers with different natural frequencies, whose rectified outputs are fed to a single storageapacitor. The structure of the converter, description of the operation, and measurement data on the performances are reported. Experimental resultshow the possibility of using the converter with input vibrations across a wideband frequency spectrum, improving the effectiveness of the overall

nergy conversion over the case of a single converter. The converter was used to supply power to a battery-less sensor module that intermittentlyeads the signal from a passive sensor and sends the measurement information via RF transmission, in this way forming an autonomous sensorystem with improved measure-and-transmit rate.

2007 Elsevier B.V. All rights reserved.

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eywords: Piezoelectric converter; Autonomous sensor systems; Power harves

. Introduction

In battery-powered autonomous sensing nodes and measure-ent microsystems, one of the main concerns is the lifetime

f the system, particularly when it must work for a long timen unattended operation. For this reason, the retrieval of electri-al energy by conversion from freely-available ambient sources,uch as mechanical vibrations, is a very promising alternative1–8]. Vibration-powered converters provide the maximum out-ut when operated at resonance [9], but this condition is difficulto guarantee when the excitation is not controllable or intrinsi-ally frequency-variant over a broad bandwidth. The converteramping can be increased to widen its response bandwidth andake fine tuning less important, but this can negatively affect

he peak output level. Therefore, a trade-off in general resultsetween the bandwidth span and the output level achievable with

given single converter.

This work proposes an approach where a multifrequencyonverter array (MFCA) is created by combining multiple con-

∗ Corresponding author. Tel.: +39 030 3715899; fax: +39 030 380014.E-mail address: marco.ferrari@ing.unibs.it (M. Ferrari).

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nergy scavenging; Wideband spectrum vibrations

erters with different frequency responses. This allows obtainingwider equivalent bandwidth while, at the same time, granting

n improvement in overall conversion effectiveness.The proposed approach is tested with a converter array made

f three piezoelectric bimorphs, but it can be generalized to dif-erent converter numbers and other conversion principles. Theesulting increase in size is modest, especially with possible

EMS implementations of the concept.

. Piezoelectric multifrequency energy converter

The structure of the multifrequency piezoelectric converterrray is shown in Fig. 1. It is made of three piezoelectricimorph cantilevers (produced by Low Power Radio Solutionnd available at RS Components, code 285–784) with the sameimensions, equal to 15 mm × 1.5 mm × 0.6 mm, and differentasses (m1 = 1.4 g, m2 = 0.7 g, m3 = 0.6 g) at the free end. As a

onsequence, each cantilever has a different fundamental reso-ant frequency, also due to residual differences in the clamping

oints of the cantilevers to the base of the MFCA structure.

When excited by mechanical vibrations at the base, the can-ilevers operate in flexure, mainly working in the 31 mode.

echanical energy is converted into electrical energy, and

330 M. Ferrari et al. / Sensors and Actuators A 142 (2008) 329–335

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Fig. 1. The structure of the piezoelectric energy converter array.

ach cantilever exhibits a voltage (V1, V2, V3) between itslectrodes.

At low frequency, a piezoelectric converter can be conve-iently represented by the Thevenin equivalent circuit formedy a voltage source VP in series with the parallel of a resistor RPnd a capacitor CP derived from the classical model composedf a charge generator Q with a capacitor CP and a resistor RPonnected in parallel [10]. In the Thevenin model, the voltageource VP is given by ((Q/CP)(jωRPCP)/(1 + jωRPCP)) and it isherefore dependent on the angular frequency ω becoming nullor ω = 0 [11]. This is consistent with the fact that a piezoelectriclement cannot generate a DC voltage. Typical values of RP andP for the used devices, measured with an impedance analyzer atbout 100 Hz under no mechanical excitation, are about 30 M�

nd 750 pF, respectively.The AC outputs produced by the converters are rectified and

ed to the storage capacitor Cb through a doubler circuit as shownn Fig. 2.

In principle, using in this case a piezoelectric converter, theapacitors C are not strictly necessary and they can be omitted,ecause the internal capacitances of the converters are alreadyresent. The value of the capacitor C is 47 nF, which is larger thanP. Therefore, the resulting series capacitance can be assumed

qual to CP. The diodes D are 1N4148 with typical reverse cur-ent as low as 25 nA, chosen to minimize the capacitor dischargeue to leakage.

Fig. 2. Electrical circuit of the multifrequency energy converter.

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ig. 3. Spice simulation results relative to the circuit of Fig. 2 with only oneonverter active (VPM1 = 10 V, VPM2 = VPM3 = 0 V, Cb = 1.5 nF).

To analyse the operation of the circuit, Spice simulations wereone under sinusoidal regime by initially assuming to have onective cantilever only and leaving the remaining two connecteds dummy elements, i.e. with their equivalent voltage generatorset to 0 V.

Fig. 3 shows the obtained results, namely the voltage pro-uced by the generator representing the active cantilever, theoltage across the storage capacitor Cb = 1.5 nF and the currenthat flows into the capacitor.

Assuming ideal diodes, neglecting the resistor RP due to itsigh value, and considering that (C·CP)/(C + CP) ≈ CP, the fol-owing expressions can be derived that give the increment of theoltage �VCb(k) across the storage capacitor and the chargingime �tc(k), where k is the period considered, and VPM is theeak amplitude of the signal VP assumed to be sinusoidal witheriod T:

VCb (k) = VCb(k) − VCb(k − 1)

= CP

CP + Cb

[1 − cos

(2π

T�tc(k)

)]VPM (1a)

tc(k) = T

2πarccos

(VCb (k − 1) − VPM

VPM

)(1b)

y solving the system, the following formula for �VCb(k) cane obtained:

VCb (k) = CP

CP + Cb(2VPM − VCb(k − 1)) (2)

q. (2) shows that the voltage increment �VCb(k) at the currenteriod k is proportional to a ratio of capacitances, and to theifference between the peak-to-peak voltage 2VPM generatedy the converter and the voltage VCb(k − 1) reached across theapacitor in the previous period. Asymptotically, VCb reachesVPM and this represents a typical behaviour of a voltage-doublerircuit.

Defining the charging angle ϑ as 2π times the ratio between

he charging time �tc and the period T, it follows from Eq. (1b)hat:

(k) = 2π�tc(k)

T= arccos

(VCb (k − 1) − VPM

VPM

)(3)

M. Ferrari et al. / Sensors and Actuators A 142 (2008) 329–335 331

Fig. 4. Spice simulation results relative to the circuit of Fig. 2 with all of thec

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onverters active (VPM1 = VPM2 = VPM3 = 10 V, Cb = 4.7 �F).

q. (3) shows that ϑ only depends on the period index k and its independent from both the frequency 1/T and the amplitudePM of the input signal because VCb(k−1) is proportional to VPM

or any k > 1. Therefore, higher frequencies give faster chargeowards 2VPM, while the number of periods for VCb to reach aiven fraction of 2VPM is constant.

Extending the analysis to the case where all of the cantileversre active and driven at different frequencies and amplitudes isot straightforward, and a closed formula equivalent to Eq. (2)s difficult to derive. Spice simulations were therefore run andhe obtained results are shown in Fig. 4. All of the cantileversre active and assumed to generate similar output voltageslbeit at different frequencies. It can be observed that the stor-ge capacitor is now charged by all the converters, increasinghe amount of the converted energy and reducing the chargingime. On the other hand, if the output amplitudes of the con-erters are markedly different or change over time due to therequency-variant excitation, the storage capacitor is chargedtep-by-step prevalently by the converter with the highest instan-aneous output level. The converter that at a given time is in suchcondition behaves as the dominant device, while the remainingonverters do not supply a significant net charge to the storageapacitor.

Under stationary conditions, the highest amplitude generatedy any of the converters in the array determines the asymp-otic value of VCb. During the charging, all of the convertersnitially contribute, although with different weights related tohe respective amplitudes. As VCb rises, the converters witheak-to-peak amplitudes surpassed by VCb become unproduc-ive to charge Cb. It can be observed that in the ideal conditionhen leakage in the rectifiers is suitably small, such unproduc-

ive converters are simply inactive. On the other hand, wheneakage is significant each unproductive converter is a potentialource of losses that has to be taken into account in the practicalases.

Summarizing, for a given input excitation, the converter array

s expected to shorten the charging time of the storage capacitor

b to a given voltage level compared to the use of any subset ofonverters in the same array, in particular compared to the usef a single converter.

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ig. 5. Typical measured signals produced by the piezoelectric cantilevers underechanical excitation.

. Experimental results

.1. Wideband excitation

Preliminary tests were carried out under stationary widebandxcitation by using a vibration exciter realized by a DC motorith an unbalanced flywheel. The rotation speed was first set

o generate a dominant excitation at the base of the cantileverrray at a fundamental frequency fa of about 15 Hz, plus higherrequency harmonics and wideband random components. Thepen-circuit output voltages V1, V2, and V3 produced by thehree cantilevers, measured with high-input-impedance buffers,re shown in Fig. 5 where a complex frequency content is visi-le. The voltage on the capacitor Cb was then measured eitherxciting all of the cantilevers, or exciting one cantilever onlynd leaving the remaining two connected as dummy elementsot to alter the electrical impedance. The value of Cb of courseffects the charging time and the energy that can be stored in theapacitor. Using a low capacitance value reduces the chargingime but a small amount of energy can be stored, and vice versa.he value of the used storage capacitor Cb was set to 4.7 �F,

n order to have enough stored energy to drive a load made bylow-power circuit for some milliseconds, having at the same

ime a reasonably short charging time.Fig. 6 shows the typical measured voltages with Cb = 4.7 �F,

emonstrating that the use of three cantilevers combined in theFCA as shown in Fig. 2 increases the amount of converted

nergy.The same tests were repeated with the excitation fundamental

requency set to fb of around 30 Hz. In this case, the cantilevershows a higher response than the cantilever 1, changing the

rder in the ranking of the cantilevers with respect to the case ofhe excitation frequency set to fa. This is consistent with the facthat the input vibrations in the second case were more distributedt higher frequency superimposing to the response bandwidth ofhe cantilever 3.

The top-left inset of Fig. 6 shows the responses of each can-ilever working alone normalized to those obtained with thehree of them working together for the two different excitationrequencies.

332 M. Ferrari et al. / Sensors and Actuators A 142 (2008) 329–335

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ig. 6. Typical measured voltages on the capacitor Cb with an excitation fun-amental frequency fa of about 15 Hz; in the top-left inset: normalized responsef the cantilevers with different excitation frequencies.

.2. Sinusoidal excitation

Subsequent tests were carried out under stationary sinusoidalxcitation by mounting the MFCA on an electrodynamic shakerBruel & Kjaer 4290). When driven with an electrical sig-al, the shaker produces a vertical vibration. Fig. 7 shows theiezoelectric MFCA realized, mounted on the electrodynamichaker.

The frequency responses of the three cantilevers were mea-ured using a gain-phase analyzer (HP 4194A) to drive the shakerith a sinusoidal signal Vi and detect the voltage VO = V1, V2,3 from each cantilever. The peak amplitude of Vi was 0.5 Vorresponding to a peak vertical acceleration of about 0.02 g.he frequency was varied over the range from 10 to 400 Hz.ig. 8 shows the resulting experimental graphs peaked aroundifferent resonant frequencies, according to the corresponding

antilever masses. In particular the respective three fundamen-al resonant frequencies are about f1 = 113 Hz, f2 = 183 Hz, and3 = 281 Hz.

ig. 7. Piezoelectric multifrequency energy converter mounted on the electro-ynamic shaker.

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ig. 8. Typical frequency responses of the piezoelectric cantilevers forming theFCA.

The complete MFCA was then tested under stationary exci-ation by driving the shaker with sinusoidal signals. The drivingrequency was sequentially set to the different values corre-ponding to the resonant frequencies of each single cantilever,amely f1, f2, and f3.

The vertical acceleration produced by the electrodynamichaker was set to a peak value of around 1 g.

The cantilever open-circuit output voltages V1, V2, and3 were simultaneously measured using high-input-impedanceuffers. Fig. 9 reports the typical results obtained with the exci-ation frequency set to f3. It can be observed that the outputoltage V3 is larger than the other outputs. Similar results werebtained for V1 and V2 with excitations at f1 and f2.

Considering each cantilever working alone, excited at itsesonance frequency with a vertical peak acceleration seto 1 g and on matched load condition, power levels equalo P1 = 89 �W, P2 = 57 �W, P3 = 57 �W, and power densi-ies equal to w1 = 0.23 �W/mm3, w2 = 0.30 �W/mm3, w3 =.76 �W/mm3 are, respectively, obtained.

Such figures are difficult to combine into an estimation of theutput power level of the whole MFCA under the general casef wideband excitation, because of the nonlinear behaviour of

he voltage-doubler circuits.

ig. 9. Typical measured signals produced by the piezoelectric cantilevers at thexcitation frequency f3.

M. Ferrari et al. / Sensors and Actuators A 142 (2008) 329–335 333

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ig. 10. Block diagram of the MFCA connected to a battery-less sensor modulehat interfaces with a passive sensor and transmits the measurement informationia a RF link.

.3. Autonomous sensor system

The MFCA was then tested to supply power to a battery-ess sensor module that interfaces with a passive sensor (NTChermistor Epcos K164/10K) and transmits the measurementnformation via a RF link as described in [12,13]. The blockiagram of the MFCA connected to the sensor module is shownn Fig. 10. The sensor module plus the MFCA used as the powerupply form an autonomous sensor system.

When excited by mechanical vibrations, the MFCA chargeshe storing capacitor like in Fig. 6. The voltage VCb on the stor-ge capacitor is monitored by a custom designed circuit basedn discrete MOS transistors that acts like an ultra-low-leakageoltage-controlled switch. The circuit connects and disconnectshe load from the storage capacitor when the voltage VCb reachesn upper or a lower threshold level, respectively. In the presentmplementation, the upper and the lower threshold levels wereet to 7.7 and 2 V, respectively, but they can be varied depend-ng on the MOS transistors. When the voltage on Cb reaches thepper threshold level, the stored energy is used to supply the sen-or module including the RF transmitter (Maxim MAX1472) toend the sensor reading. Therefore, the voltage across Cb sud-enly drops until the lower threshold level is reached. At thisoment, the load is automatically disconnected from the stor-

ge capacitor by means of the voltage-controlled switch circuit,nd the charging cycle of the storage capacitor restarts. The

ensor module therefore operates under intermittent power sup-ly, and the time interval between two consecutive ON cycles,hich produce measure-and-transmit operations, will be called

witching time in the following.

ftaT

able 1witching times and cantilever open-circuit output peak amplitudes with different fre

Cantilever 1 Cantilever 2 Cantilev

1 = 113 Hz 17.8 s X X

2 = 183 Hz X 25.8 s X

3 = 281 Hz X X 12.2 s

4 = 145 Hz X X X

ig. 11. Typical measured voltages on the capacitor Cb when different can-ilevers are excited at the frequency f2.

Measurements were taken both connecting the sensor moduleo all of the cantilevers in the MFCA at the same time, andonnecting it to one cantilever at the time for comparison.

The frequency of the sinusoidal excitation was sequentiallyet to different values, including the three resonant frequencies1, f2, and f3, plus the additional frequency f4 = 145 Hz purposelyhosen to be off resonance for all of the cantilevers.

The voltage across the storing capacitor Cb and the switchingime were measured under the different conditions of frequencyxcitation and different cantilevers active in the array, alwaysaintaining the vertical peak acceleration set to 1 g.Fig. 11 shows the voltage on the storing capacitor Cb when

he excitation frequency was set at f2. The switching time whennly the cantilever 2 was active is about 25 s. When all of theantilevers were made active, the switching time decreased tobout 14 s. The plots corresponding to the cantilevers 1 and 3perated alone at f2 are not shown in Fig. 11 because none of theiresponses was high enough to ever trigger the RF transmission.he decrease of the switching time shown in Fig. 11 represents aarked improvement. That is also in qualitative accordance with

he predictions of Section 2, because it provides clear evidencehat converters 1 and 3, that alone cannot supply enough energyo reach the switching threshold, indeed produce a significantffect when combined with converter 2.

The results obtained for different combinations of excitationrequency and active cantilevers are summarized in Table 1, thateports the measured switching times and open-circuit voltagesor each cantilever.

It can be observed that under resonant excitation, i.e. at either

1, f2, or f3, the corresponding cantilever in the array could alonerigger the transmission, as expected. On the other hand, therere frequency-cantilever pairs, denoted by the “X” symbol inable 1, which did not provide enough converted energy to

quency-cantilever pairs

er 3 All V1 V2 V3

14 s 25 V 5 V 10 V14.3 s 5 V 16 V 7 V6.6 s 5 V 2 V 13 V

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34 M. Ferrari et al. / Sensors and

rigger the transmission. Moreover, under off-resonance exci-ation, i.e. at f4, none of the cantilevers alone was able to triggerhe transmission.

Conversely, it is remarkable that with the complete MFCAhe converted energy was high enough to trigger the transmis-ion at all the tested frequencies, including at f4. In this latterase, considering that the three converters provide similar outputoltages, the advantage offered by the MFCA over a single con-erter is the enhanced current supply which enables to overcomeosses.

In all the cases, the improvement in switching time was sig-ificant, thereby demonstrating the effectiveness of the proposedpproach that obtains a converter with a wider equivalent band-idth by combining multiple cantilevers working in different

requency ranges.

. Conclusions

A piezoelectric multifrequency converter array has beeneveloped and tested. The results show the possibility of man-facturing a miniaturized system made of a set of cantileveronverters designed to work in different frequency ranges,o widen the overall equivalent bandwidth of the converterrray.

For a single converter followed by a voltage-doubler recti-er, a theoretical treatment has been developed showing that,nder sinusoidal regime and when losses are neglected, theharging angle of the storage capacitor is only dependent onhe period index k and independent from the excitation fre-uency and amplitude. When different converters work togethern the MFCA under the most general case of wideband exci-ation, the situation is much more complicated to allow for

direct extension of the theoretical treatment. As qualitativeonsiderations, it was observed that the storing capacitor isharged prevalently by the converter with the highest instan-aneous output level which depends on the excitation frequency.nder wideband stationary excitation, all of the converters

ontribute to shorten the charging time as long as their peak-o-peak output voltage is larger than the voltage across thetorage capacitor. The results were confirmed by Spice simula-ions.

The MFCA was tested alone and then used to supply power tobattery-less sensor module that intermittently reads the signal

rom a passive sensor and sends the measurement informationia RF transmission, in this way realizing an autonomous sensorystem.

Using the complete MFCA under sinusoidal mechanical exci-ation at different frequencies, a marked improvement in thewitching time between measure-and-transmit operations of theutonomous sensor module was demonstrated at all the testedrequencies.

Furthermore, it was verified that the sensor module coulde correctly powered, thereby triggering the transmission, even

or an excitation frequency that was off resonance for all ofhe cantilevers in the MFCA. On the other hand, at that samerequency none of the cantilevers used alone was able to providenough energy to operate the sensor module.

B

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ators A 142 (2008) 329–335

Though in the present study the piezoelectric effect was takennto consideration, the concept of the MFCA can be extended tother energy-conversion principles.

With possible MEMS implementations of the concept,btaining compact structures with converters of different res-nant frequencies is comparatively simple due to the flexibilityn the control of dimensions [14,15]. It is therefore expected thathe modest increase in size over the use of a single converter cane outweighed by the increase in performance.

eferences

[1] S. Roundy, P.K. Wright, J. Rabaey, A study of low level vibrations asa power source for wireless sensor nodes, Comput. Commun. 26 (2003)1131–1144.

[2] J.A. Paradiso, M. Feldmeier, A compact, wireless, self-powered pushbuttoncontroller, in: Proceedings of ACM UBICOMP Conference, Atlanta GA,September, 2001, pp. 299–304.

[3] M. Ferrari, V. Ferrari, D. Marioli, A. Taroni, Modeling, fabrication andperformance measurements of a piezoelectric energy converter for powerharvesting in autonomous microsystems, in: Proceedings of IMTC 2005,vol. III, Ottawa, ON, Canada, 17–19 May, 2005, pp. 1862–1866.

[4] C.B. Williams, R.B. Yates, Analysis of a micro-electric generator formicrosystems, Sens. Actuators A 52 (1996) 8–11.

[5] E. Lefeuvre, A. Badel, C. Richard, L. Petit, D. Guyomar, A comparisonbetween several vibration-powered piezoelectric generators for standalonesystems, Sens. Actuators A 126 (2) (2006) 405–415.

[6] T. Starner, Human-powered wearable computing, IBM Syst. J. 35 (3/4)(1996) 618–629.

[7] N.M. White, P. Glynne-Jones, S.P. Beeby, A novel thick-film piezoelectricmicro-generator, Smart Mater. Struct. 10 (4) (2001) 850–852.

[8] G. Poulin, E. Sarraute, F. Costa, Generation of electrical energy for portabledevices comparative study of an electromagnetic and a piezoelectric sys-tem, Sens. Actuators A 116 (3) (2004) 461–471.

[9] P.D. Mitcheson, T.C. Green, E.M. Yeatman, H.S. Holmes, Architecturesfor vibration-driven micropower generators, J. Microelectromech. Syst. 13(3) (2004) 429–440.

10] J.G. Webster, The Measurement, Instrumentation and Sensors Handbook,CRC Press, Boca Raton, FL, 1999.

11] M. Ferrari, V. Ferrari, D. Marioli, A. Taroni, Modeling, fabrication andperformance measurements of a piezoelectric energy converter for powerharvesting in autonomous microsystems, IEEE Trans. Instrum. Meas. 55(6) (2006) 828–834.

12] M. Ferrari, V. Ferrari, D. Marioli, A. Taroni, Thick-film piezoelectric energyconverter for power harvesting in autonomous microsystems, in: Proceed-ings of XIX Eurosensors Conference, vol. 1, Barcellona, Spain, 11–14September, 2005, p. TP46.

13] M. Ferrari, V. Ferrari, D. Marioli, A. Taroni, Autonomous sensor modulewith piezoelectric power harvesting and RF transmission of measurementsignals, in: Proceedings of IMTC 2006, Sorrento, Italy, 24–27 April, 2006,pp. 1663–1667.

14] I. Sari, T. Balkan, H. Kulah, A wideband electromagnetic micro powergenerator for wireless microsystems, in: Proceedings of Transducers’07 &Eurosensors XXI Conference, vol. 1, Lyon, France, 10–14 June, 2007, pp.275–278.

15] M. Renaud, T. Sterken, A. Schmitz, P. Fiorini, C. Van Hoof, R. Puers,Piezoelectric harvesters and MEMS technology: fabrication, modeling andmeasurements, in: Proceedings of Transducers’07 & Eurosensors XXIConference, vol. 1, Lyon, France, 10–14 June, 2007, pp. 891–894.

iographies

arco Ferrari was born in Brescia, Italy, in 1974. In 2002, he obtained the elec-ronics engineering degree at the University of Brescia. In 2006 he received theesearch doctorate degree in Electronic Instrumentation at the same university.

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Asassociate professor at the University of Modena and since 1986 he has been full

M. Ferrari et al. / Sensors and

is research activity deals with the energy conversion via the piezoelectric effector powering autonomous microsystems and sensors for physical and chemi-al quantities with the related signal-conditioning electronics. In particular hes involved with piezoelectric acoustic-wave sensors in thick-film technology,esign of oscillator circuits and frequency-output signal conditioners.

ittorio Ferrari was born in Milan, Italy, in 1962. In 1988, he obtained theaurea degree in physics cum laude at the University of Milan. In 1993 he

eceived the research doctorate degree in electronic instrumentation at the Uni-ersity of Brescia. He has been an assistant professor and an associate professort the Faculty of Engineering of the University of Brescia until 2001 and 2006,espectively. Since 2006 he has been a full professor of Electronics. His researchctivity is in the field of sensors and the related signal-conditioning electronics.articular topics of interest are acoustic-wave piezoelectric sensors, microreso-ant sensors and MEMS, autonomous sensors and power scavenging, oscillatorsor resonant sensors and frequency-output interface circuits. He is involved in

ational and international research programmes, and in projects in cooperationith industries.

ichele Guizzetti was born in Brescia, Italy, in 1981. In 2006, he obtained thelectronics engineering degree at the University of Brescia. Since 2006 he is

peii

ators A 142 (2008) 329–335 335

hD student in electronic instrumentation at the same university. His researchctivity deals with power harvesting and autonomous systems. In particular hes involved with piezoelectric and thermoelectric energy conversion.

aniele Marioli was born in Brescia, Italy, in 1946. In 1969, he obtained thelectrical engineering degree at the University of Pavia, Italy. From 1984 to989 he was an associate professor in applied electronics and since 1989 he haseen a full professor of electronics at the University of Brescia. His researcheld is the design, realization and test of sensors, electronic instrumentation,nd signal processing electronic circuits. He is author and coauthor of morehan 200 scientific papers published in international and national journals andonference proceedings.

ndrea Taroni was born in 1942. In 1966, he received the degree in physicalcience from the University of Bologna, Italy. From 1971 to 1986, he was an

rofessor of electrical measurements at the University of Brescia. He has donextensive research in the field of sensors for physical quantities and electronicnstrumentation, both developing original devices and practical applications. Hes author of more than 100 scientific papers.