A flex-compressive-mode piezoelectric transducer for ... - IEEE Xplore

IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,

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A Flex-Compressive-Mode Piezoelectric Transducer for Mechanical Vibration/Strain Energy Harvesting Xiaotian Li, Mingsen Guo, and Shuxiang Dong Abstract—A piezoelectric transducer for harvesting energy from ambient mechanical vibrations/strains under pressure condition was developed. The proposed transducer was made of two ring-type piezoelectric stacks, one pair of bow-shaped elastic plates, and one shaft that pre-compresses them. This transducer works in flex-compressive (F-C) mode, which is different from a conventional flex-tensional (F-T) one, to transfer a transversely applied force F into an amplified longitudinal force N pressing against the two piezo-stacks via the two bowshaped elastic plates, generating a large electric voltage output via piezoelectric effect. Our experimental results show that without an electric load, an F-C mode piezo-transducer could generate a maximum electric voltage output of up to 110 Vpp, and with an electric load of 40 kΩ, it a maximum power output of 14.6 mW under an acceleration excitation of 1 g peak-peak at the resonance frequency of 87 Hz.

I. Introduction

T

oday, the energy harvesting from light, thermal, magnetic, or mechanical energy in the ambient environment is an important research topic [1]–[5]. With recent progresses in wireless and MEMS technology, sensor systems are being popularly used in various areas, including human body care, bridge or engine early health monitoring, etc. [6]–[8]. However, replacement of small power supplies and batteries in sensor systems would be a tedious task. Therefore, it is quite interesting to supply a small amount of power for sensor systems from environmental energy [9], [10]. In addition, because of the shortage in energy sources, people are also seeking environmental energy to replace part of the electric energy used in daily life. Therefore, another interesting application is to harvest the mechanical energy from highway or railway for generating electric energy, which may supply a small to medium amount of power for powering road lights or even electric motors if there are enough vehicles/trains running [11]. One of the most effective methods for power harvesting systems is to use piezoelectric materials to convert mechanical vibration or strain energy to electric energy based on the piezoelectric effect. During the past ten years, there has been an explosion of research in the area of harvesting

Manuscript received July 23, 2010; accepted October 28, 2010. This work was supported by the National Natural Science Foundation of China (Grant No. 50872002) and National Basic Research Program of China (973 Program, Grant No. 2009CB623303). The authors are with the Department of Advanced Materials & Nanotechnology, College of Engineering, Peking University, Beijing, China (e-mail: [email protected]). Digital Object Identifier 10.1109/TUFFC.2011.1862 0885–3010/$25.00

energy from ambient vibrations by using the direct piezoelectric effect [12]–[15]. Piezoelectric materials are very good prospects for mechanical energy conversion because they have a good electromechanical coupling effect. Piezoelectric energy harvesting devices are also much simpler than, for example electromagnetic or electrostatic devices. For these reasons, piezoelectric energy harvesting devices have attracted much attention. Conventional piezoelectric harvesting devices are based on a piezoelectric unimorph or bimorph cantilever configuration, i.e., one or two piezoelectric elements laminated with one long elastic plate, and they are operated in bending mode. In general, piezoelectric cantilever type harvesters generate only a very small power output, and they cannot work under pressure. In 2004, Uchino’s group at Pennsylvania State University developed a piezoelectric cymbal transducer which operated in flex-tensional mode for vibration energy harvesting, which could work well under a small force load. At the resonance frequency of ~100 Hz, the power generated by the cymbal-type transducer under forced vibrations with a force load of 7.8 N could reach 39 mW across an electrical load of 400 kΩ [16]. This group reported their cymbal transducer operating under a larger force load of 70 N in 2006, and an increased power output of ~100 mW was achieved at the resonance frequency of 200 Hz across a 200 kΩ resistor [17]. Later, a cymbal transducer based on 0.71Pb(Mg1/3Nb2/3) O3-0.29PbTiO3single crystal was reported by the Shanghai Institute of Ceramics. This modified cymbal transducer can produce a maximum power of 14 mW at 500 Hz with a proof mass of 17.0 g [18]. Currently, researchers mainly focus on cantilever and cymbal-type piezoelectric transducers as energy harvesting devices. The cantilever piezoelectric device is easily broken under only a small pressure [see Fig. 1(a)]. Although the cymbal transducers have a larger force load capability, the piezoelectric elements inside the transducers work in tensional stress status under an applied force load because of the cymbal’s flex-tensional mode [see Fig. 1(b)], which may result in a crack occurring in piezoelectric elements after working for some time. Although a simple piezoelectric stack can bear a large amount of pressure, as a mechanical vibration energy harvester, its resonance frequency is too high because of its high stiffness, which may result in a poor electromechanical coupling in low-frequency ranges. Therefore, it is necessary to develop a different mode piezoelectric transducer that can work well under a force load. In this paper, we report a flex-compressive (F-C) mode piezoelec-

© 2011 IEEE

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Fig. 2. Simplified flex-compressive (F-C) mode of piezoelectric transducer.

transducer. Under one pair of applied load F, the axial pressure force against to piezoelectric stacks is N. The force amplification factor β is given as Fig. 1. Conventional piezoelectric energy harvesters: (a) cantilever type and (b) cymbal type.

tric transducer for energy harvesting, suited for working under pressure condition. II. Work Principle and Analysis It is well known that piezoelectric ceramic materials can bear a large pressure, but not a large tensional stress. Therefore, conventional flex-tensional mode piezoelectric transducers have a serious problem when working under a large force loads. To avoid working in the tensional stress state, we proposed a flex-compressive (F-C) mode piezoelectric transducer in which the piezoelectric elements always work in a compressive state under a force load. Fig. 2 shows a simplified model of an F-C piezoelectric transducer. The proposed transducer was made of two ring-type piezoelectric stacks, one pair of long, bow-shaped elastic plates and one pre-press set. The bow-shaped elastic plates in the transducer provide flex-compressive (F-C) mode conversion, transferring a transversely applied force F into an amplified longitudinal force N and pressing against the two piezo-stacks. Note that they may be called cymbal elements if they are disc-type elastic elements. As a result, the F-C transducer is permitted to work in a much larger pressure state without worrying about cracks occurring in the piezoelectric ceramic elements. Clearly, this F-C mode is significantly different from a conventional F-T one. See the simplified model of the F-C piezoelectric transducer shown in Fig. 2. Assume that the height of the arc is dc, the arc length rc, and the arc angle α. The dc and rc are important design parameters, which determine the force amplification factor β of the F-C mode piezoelectric

β = N/F = 1/tan α,

(1)

where tan α = dc/rc. Clearly, a large force amplification factor β may be obtained by choosing a suitable arc angle α. From the piezoelectric constitutive equation, the induced electric field E under a stress N at open-circuit and at low-frequency conditions is

E = g33N/S,

(2)

where g33 is the piezoelectric voltage constant, and S is the cross-sectional area of the piezo-stack. If the applied force N is caused by acceleration force of a mass m attached to the top of one arc-shaped element in the transducer, the N is

N =

mg . tan a

(3)

Correspondingly, the generated voltage VOC at open condition is

VOC = g 33Nt/S =

g 33mgt , S tan a

(4)

where t is the thickness of one piezoelectric element. Under resonance driving conditions, the generated voltage becomes the maximum:

VOC,Res »

g 33mgt Q m, S tan a

(5)

where Qm is the mechanical quality factor of the transducer. When an external resistor RLoad exists, we should use the following equivalent circuit to estimate the output voltage VLoad and power P [16], see Fig. 3.


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Fig. 3. Equivalent circuit of piezoelectric transducer.

V Load = VOC

R Load R Load +

P =

1 j wC

+ RS

,

2 V Load , 2R Load

(6)

(7)

where RS = (tan δ)/(2πfC) is the internal resistance related to the mechanical and dielectric losses of the piezoelectric materials. Clearly, we can obtain the maximum output voltage VLoad,max = VOC/ 2 and the maximum 2 power Pmax = V Load,max /(2R Load) when RLoad = |RS + 1/(jωC)|. III. Experimental Results and Discussion

Fig. 4. Flex-compressive (F-C) mode of piezoelectric transducer, (a) modified configuration design, (b) piezoelectric stack, and (c) photograph of the assembled energy harvester.

A. Structure Design and Fabrication A flex-compressive mode piezoelectric transducer was designed and assembled according to the simplified F-C model as shown in Fig. 2, which was made of two ringtype piezoelectric stacks, each of them containing 10 lead zirconate titanate (PZT) plates, one pair of long bowshaped elastic rectangular plates and one shaft that holds them together with a pre-compression force, see Fig. 4 (a). In the transducer, piezoelectric stacks were made of multiple PZT-4 rings with 15 mm outer diameter, 5 mm inner diameter, and 1.0 mm thickness. These PZT rings were bonded together with electrodes using epoxy resin in an electrical parallel connection, and then cured at 60°C for several hours to form a multilayer piezoelectric stack for generating an enhanced mechanical-to-electric coupling effect, see Fig. 4(b). A photograph of the harvester we fabricated is shown in Fig. 4(c) The bow-shaped F-C elastic elements in the transducer were made of spring steel plates, 40 mm in length, 10 mm in width, and 0.5 mm in thickness. The force amplification factor β of the F-C elastic elements was designed to be 5 to 15, depending on the applied load. In our experiment, the amplification factor was designed as 15. The F-C elastic elements in the transducer transfer a transversely applied force F into an amplified longitudinal force N pressing against the two piezo-stacks to generate electric output.

B. Measurement Setup and Procedure The frequency spectrum of the assembled F-C mode piezoelectric transducer was first characterized using an Agilent 4294A precision impedance analyzer (Agilent Technologies, Santa Clara, CA). Then the mechanical-toelectrical energy coupling performances were measured using the experimental setup shown in Fig. 5(a). A mechanical shaker, Dynamic Systems HEV-20 (Nanjing University of Aeronautics and Astronautics, Institute of Vibration Engineering Research, Nanjing, China), which can supply a maximum force of 200 N within a frequency range of 5 to 2000 Hz, was used to excite mechanical vibrations. One cap of one F-C elastic element of the transducer was glued to the shaker with epoxy resin. The output voltage from the transducer was monitored using an Agilent digital oscilloscope (model DSO6014A). The whole experimental setup was put on a spongy cushion on a table to avoid any interference from the surroundings. Various load resistances were used to characterize the performance of the transducer, including the output voltage V and power P. The power generated under different load resistances RL was calculated using (7).

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Fig. 5. Measurement setup.

IV. Results and Discussion First, we characterized the resonance performance of the piezoelectric transducer. Fig. 6(a) shows the measured impedance spectrum of a single multilayer PZT stack under free conditions within a wide frequency range from 40 Hz to 200 kHz, and a resonance frequency point f0 = 91.56 kHz was found, which corresponds to the firstorder longitudinal resonance frequency of the piezoelectric stack. Fig. 6(b) shows the measured impedance spectrum of the assembled F-C mode transducer, in which the two F-C elastic elements were included. It is clear to see that the resonance peak was dramatically lowered from f0 = 91.56 kHz to f0 = 2.3 kHz; the low stiffness of F-C elastic elements decreases the transducer system’s resonance frequency significantly. For mechanical energy harvesting, we hope the resonance frequency of the transducer is very low, close to the ambient vibration frequency. Obviously, a single piezoelectric multilayer stack can’t be used directly as an energy harvester effectively. Subsequently, we measured the generated electric output under a mechanical vibration excitation. The F-C transducer was fixed to the shaker by its lower cap, as shown in Fig. 5, and it was found that the resonance frequency of the F-C transducer was further lowered to 196 Hz and a peak voltage output of 28.9 V at open condition was observed under a mechanical vibration excitation (with 1 g peak-to-peak acceleration), see Fig. 7. The lower resonance frequency can be attributed to the effect of its own weight and the boundary condition change of the F-C transducer, which may result in vibration mode change of the F-C transducer. Fig. 7 also shows the generated voltage output of the F-C transducer as a function of vibration frequency under a force load. Under an applied load of 1.9 N to the upper cap, the resonance frequency of the F-C transducer decreased again from f0 = 196 Hz to f0 = 87 Hz, whereas the generated voltage output increased dramatically from 28.9 V to 111 V at open condition. The increased voltage output can be attributed to the larger

Fig. 6. Impedance spectra of the piezoelectric stack and the transducer, (a) spectrum of one multilayer PZT stack under the free condition, (b) spectrum of the F-C mode transducer under the free condition.

Fig. 7. The generated voltage as a function of mechanical vibration frequency.

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and P = 1.2m W. With a force load of 1.9 N, the calculated voltage and power outputs are VLoad = 64.7 V, and P = 21 mW, respectively. Clearly, these calculated values are close to the measured data. V. Conclusion

Fig. 8. Output power as a function of vibration frequency under different electric loads, with and without a load of 1.9 N.

stress applied to the piezoelectric stacks of the transducer under a force load. The generated power output of the F-C transducer as a function of vibration frequency under different electrical or force loads were also measured, as shown in Fig. 8. Without a force load, the generated peak power output from the F-C transducer was ~1 mW at the resonance frequency point of 196 Hz under 1 g peak-peak acceleration excitation, which corresponded to an optimum electrical matching load of 25 kΩ. Under a force load of 1.9 N, the generated peak power output increased to ~15 mW at the lower resonance frequency point of 87 Hz under 1 g peak-peak acceleration excitation, which corresponded to an optimum electric matching load of 40 kΩ. Clearly, the F-C transducer has a better performance under a large force load. Note that the two piezoelectric stacks in F-C transducer have a much higher power density compared with those in bimorph or cymbal type transducers, and they also are strong enough to bear a much higher pressure without worrying about breakdown. [15] Therefore, F-C transducers have potential to generate higher power output under a larger force load. Also, it is possible to integrate F-C transducers into footwear or pavements for harvesting large vibration or strain energy from human walking. Finally, we compared the measured data with calculated values using the derived equations in Section II. The piezoelectric ceramic material used in our experiments is PZT-4, and its piezoelectric voltage constant and material loss are g33 = 26.1 × 10−3 Vm/N, and tan δ = 0.0038, respectively. The measured static capacitance of the assembled piezoelectric stack is CS = 36.75 nF, and the mechanical quality factor Qm ~ 25. Without a load, the resonance frequency of the F-C transducer is 196 Hz. By using (4)–(9), the calculated RLoad is found to be 22.5 kΩ, correspondingly, the calculated voltage and power outputs across the RLoad at the resonance are VLoad = 15.13 V,

In conclusion, an F-C mode piezoelectric transducer for harvesting energy from ambient mechanical vibration or strain under pressure was developed. The proposed F-C mode transducer contains one pair of piezoelectric stacks that can bear a large force load, and one pair of bowshaped elastic plates that work in F-C mode to transfer a transversely applied force load into an amplified longitudinal force pressing against the two piezo-stacks. The experimental results show that the F-C transducer could generate a maximum electric voltage output of up to 110 Vpp and a maximum power output of 14.6 mW under an acceleration excitation of 1 g peak-peak at the resonance frequency point of 87 Hz. The F-C transducer showed other advantages over the conventional bimorph or cymbal-type piezoelectric transducers, such as its ability to withstand a larger force load and its lower resonance frequency because it operates in the F-C mode, higher power density in the piezoelectric stacks, and higher voltage output. The F-C transducer has the potential to be integrated into footwear or pavements which can convert vibration or strain into electricity from human walking, based on the piezoelectric effect. References [1] B. Demmig-Adams and W. W. Adams, “Photosynthesis: Harvesting sunlight safely,” Nature, vol. 403, pp. 371–374, Jan. 2000. [2] R. Hu, B. A. Cola, N. Haram, J. N. Barisi, S. Lee, S. Stoughton, G. Wallace, C. Too, M. Thomas, A. Gestos, M. E. D. Cruz, J. P. Ferraris, A. A. Zakhidov, and R. H. Baughman, “Harvesting waste thermal energy using a carbon-nanotube-based thermo-electrochemical cell,” Nano Lett., vol. 10, pp. 838–846, Feb. 2010. [3] S. Dong, J. Zhai, J. F. Li, D. Vieland, and S. Priya, “Multimodal system for harvesting magnetic and mechanical energy,” Appl. Phys. Lett., vol. 93, art. no. 103511, Sep. 2008. [4] A. Erturk, J. Hoffman, and D. J. Inman, “A piezomagnetoelastic structure for broadband vibration energy harvesting,” Appl. Phys. Lett., vol. 94, art. no. 254102, Jun. 2009. [5] S. Priya, J. Ryu, C. S. Park, J. Oliver, J. J. Choi, and D. S. Park, “Piezoelectric and magnetoelectric thick films for fabricating power sources in wireless sensor nodes,” Sensors, vol. 9, pp. 6362–6384, Aug. 2009. [6] H. Kudo, T. Sawada, E. Kazawa, H. Yoshida, Y. Iwasaki, and K. Mitsubayashi, “A flexible and wearable glucose sensor based on functional polymers with soft-MEMS techniques,” Biosens. Bioelectron., vol. 22, no. 4, pp. 558–562, Oct. 2006. [7] D. Satake, H. Ebi, N. Oku, K. Matsuda, H. Takao, and M. Ishida, “A sensor for blood cell counter using MEMS technology,” Sens. Actuators B, vol. 83, no. 1–3, pp. 77–81, Mar. 2005. [8] S. Kim, S. Pakzad, D. Culler, J. Demmel, G. Fenves, S. Glaser, and M. Turon, “Health monitoring of civil infrastructures using wireless sensor networks,” in Proc. 6th Int. Symp. Information Processing in Sensor Networks, 2007, pp. 254–263. [9] J. A. Paradiso and T. Starner, “Energy scavenging for mobile and wireless electronics,” IEEE Pervasive Comput., vol. 4, no. 1, pp. 18–27, Jan.-Mar. 2005. [10] P. D. Mitchson, E. M. Yeatman, G. K. Rao, A. S. Holmes, and T. C. Green, “Energy harvesting from human and machine mo-

li et al.: flex-compressive-mode transducer for energy harvesting tion for wireless electronic devices,” Proc. IEEE, vol. 96, no. 9, pp. 1457–1486, Sep. 2008. [11] S. Priya, “Advances in energy harvesting using low profile piezoelectric transducers,” J. Electroceram., vol. 19, no. 1, pp. 165–182, 2007. [12] S. Roundy, P. K. Wright, and J. M. Rabaey, Energy Scavenging for Wireless Sensor Networks: With Special Focus on Vibrations. New York, NY: Springer, 2004. [13] Z. L. Wang and J. Song, “Piezoelectric nanogenerators based on zinc oxide nanowire arrays,” Science, vol. 312, no. 5771, pp. 242–246, Apr. 2006. [14] R. Guigon, J. J. Chaillout, T. Jager, and G. Despesse, “Harvesting raindrop energy: Experimental study,” Smart Mater. Struct., vol. 17, art. no. 015039, Jan. 2008. [15] S. R. Anton and H. A. Sodano, “A review of power harvesting using piezoelectric materials,” Smart Mater. Struct., vol. 16, pp. R1–R21, May 2007. [16] H. W. Kim, A. Batra, S. Priya, K. Uchino, D. Markley, R. E. Newnham, and H. F. Hofmann, “Energy harvesting using a piezoelectric ‘cymbal’ transducer in dynamic environment,” Jpn. J. Appl. Phys., vol. 43, pp. 6178–6183, Sep. 2004. [17] H. W. Kim, S. Priya, and K. Uchino, “Modeling of piezoelectric energy harvesting using cymbal transducers,” Jpn. J. Appl. Phys., vol. 45, pp. 5836–5840, Jul. 2006. [18] B. Ren, S. W. Or, X. Zhao, and H. Luo, “Energy harvesting using a modified rectangular cymbal transducer based on 0.71Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 single crystal,” J. Appl. Phys., vol. 107, art. no. 034501, Feb. 2010.

Xiaotian Li was born in 1989 in Hubei, China. She received the B.Sc. degree in mechanics and aerospace technology from Peking University, Beijing, China in 2009. She is currently pursuing her Ph.D. degree in advanced materials and nanotechnology at Peking University. Her research interests include piezoelectric materials, sensors, and actuators.

703 Mingsen Guo was born in 1980 at Hubei, China. He received the B.Sc. degree in physics, the M.Sc. degree in physics of condensed matter, and the Ph.D. degree in physics and chemistry of materials from Wuhan University, Wuhan, China, in 2002, 2005, and 2008, respectively. He is currently a postdoctoral fellow at Peking University, Beijing, China. From December 2005 to October 2007, he was a research assistant at Hong Kong Polytechnic University, HK SAR, China. His main research interests include piezoelectric materials, sensors, and actuators.

Shuxiang Dong (M’03) is a professor of Advanced Materials & Nanotechnology of the College of Engineering at Peking University. He received his B.S. degree in semiconductor physics from Wuhan University, Wuhan, China, in 1982, and his M.S. degree in acoustics physics and Ph.D. degree in electronic materials and devices from Tsinghua University, Beijing, China, in 1989 and 1993, respectively. Dr. Dong was a research associate at Materials Research Laboratory, The Pennsylvania State University, State College, PA, from Jan., 2000 to Dec., 2001, and a research scientist at Materials Science & Engineering, Virginia Tech, Blacksburg, VA, from Jan., 2002 to May, 2008. He has authored more than 90 peer-reviewed papers and 15 patents. His research interests include piezoelectric and magnetoelectric transducers, sensors, actuators, ultrasonic motors, and other functional devices.