Note: High-efficiency energy harvester using double-clamped piezoelectric beams Yingmei Zheng, Xuan Wu, Mitesh Parmar, and Dong-weon Lee Citation: Review of Scientific Instruments 85, 026101 (2014); doi: 10.1063/1.4862820 View online: http://dx.doi.org/10.1063/1.4862820 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing

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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 026101 (2014)

Note: High-efficiency energy harvester using double-clamped piezoelectric beams Yingmei Zheng, Xuan Wu, Mitesh Parmar, and Dong-weon Leea) MEMS and Nanotechnology Laboratory, School of Mechanical Systems Engineering, Chonnam National University, Gwangju, South Korea

(Received 15 September 2013; accepted 7 January 2014; published online 3 February 2014) In this study, an improvement in energy conversion efficiency has been reported, which is realized by using a double-clamped piezoelectric beam, based on uniaxial stretching strain. The buckling mechanism is applied to maximize axial stress in the double-clamped beam. The voltage generated by using the double-clamped piezoelectric beam is higher than that generated by using other conventional structures, such as bending cantilevers coated/sandwiched with piezoelectric film, which is proven both theoretically and experimentally. The power generation efficiency is enhanced by further optimizing the double-clamped structure. The optimized high-efficiency energy harvester utilizing double-clamped piezoelectric beams generates a peak output power of 80 μW, under an acceleration of 0.1g. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4862820] In recent years, the wireless sensors network (WSN) has been considered to be one of the most important technologies.1, 2 Numbers of sensors in the WSN can gather and wirelessly transmit/receive real-time information of the environment,3 which can realize various functions, such as military surveillance systems,4 car accident prediction,5 intelligent buildings,6 and so forth. The battery is the most common choice to power the widely spread wireless sensors. Unfortunately, owing to their limited energy, a lot of manpower and maintenance cost are required to replace the batteries for thousands of individual wireless sensors. Moreover, it is impossible to replace batteries for built-in wireless sensors.7 Thus, worldwide research on energy harvesters utilizing ambient energy, like solar, wind, and vibration, is being carried out to find a substitute for batteries in WSN.8, 9 Among the many options, piezoelectric vibration energy harvesting has been regarded as a suitable energy resource for WSN, due to its ability of miniaturization.10 However, for a micro-sized piezoelectric energy harvester, the power generation efficiency is limited, due to its high resonant frequency, caused by its small size. A great deal of research has been engaged in the efficiency improvement for piezoelectric energy harvesting devices.11 Tang et al. proposed a bi-stable frequency up-conversion piezoelectric energy harvester to enhance the energy scavenging bandwidth.12 Galchev et al. presented a piezoelectric parametric frequency increased generator for harvesting low-frequency vibrations.13 In this research, aiming at improving the energy conversion efficiency, the design, fabrication, and characterization of a novel energy harvester using double-clamped polyvinylidene difluoride (PVDF) beam is reported. The cantilever structure is widely used in piezoelectric energy harvesters. Piezoelectric materials are directly bonded at the fixed end of the cantilever for energy harvester apa) Author to whom correspondence should be addressed. Electronic mail:

[email protected]

0034-6748/2014/85(2)/026101/3/$30.00

plications. When external force is applied to the cantilever, the cantilever achieves maximum stress on the fixed end, and the stress will decrease with the distance of the position from the fixed end. This leads to the reduction of stress generations for piezoelectric material. For a double-clamped beam structure, since both fixed ends of the supported beam endure stress from the axial direction, uniform stress distribution is expected, and the stress effect received by the cantilever itself would be smaller. However, the instability of the beam is enhanced, when it receives maximum stress from both ends supported with the piezoelectric beam. When the stress exceeds the allowable limits, the buckling phenomenon occurs. Therefore, the increase of power generation is expected with the introduction of large axial stress.14 This occurrence of the pre-buckling phenomenon phase can be applied to piezoelectric material.15 The present study replaces the existing bending mode piezoelectric material at both ends of the supported (doubleclamped) beams, and maximized the efficiency of the doubleclamped beam mode. To compare the energy conversion efficiency of the two different structures, as Fig. 1(a) shows, a conventional bending mode sandwich beam is designed with the same position of the double-clamped beam, which is located at the bottom of the cantilever. After designing it, the stress analysis is conducted by the finite element simulation program ABAQUS. As shown in Figs. 1(b) and 1(c), the simulation resulted in average Von Mises stress of 135.2 kPa in the double-clamped beam, and 56.75 kPa in the sandwich beam. In order to characterize the vibrating energy harvester, a measurement system is configured. Through a function generator and a shaker, the energy harvester carrying both doubleclamped and sandwich structures can be vibrated at various frequencies. In the experiment, the maximum output is achieved when the vibration frequency of the shaker matches the resonance frequency of the cantilever. Figure 2 describes the output voltage according to the frequency changes of the

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FIG. 1. (a) A schematic of the double-clamped beam and sandwich beam structure. [(b) and (c)] Stress FEM analysis of the double-clamped piezoelectric beam, using ABAQUS.

FIG. 3. The output power and voltage versus load resistance for the doubleclamped beam.

fabricated PVDF-based energy harvester. The peak output voltage with open circuit occurred in both structures at the resonance point. The experimental results have a favorable agreement with that of the simulation results, and the output voltage value of the double-clamped beam is almost twice as large as that of the sandwich beam, in a vibration frequency range from 1 to 30 Hz at an acceleration of 0.1g. When the piezoelectric device is used as power source in the circuit, a suitable impedance matching is required to increase the efficiency of power transmission. Figure 3 identifies a graph of output voltage and power variations, with different load resistance values. The results indicate that the output voltage tends to increase sharply until a load resistance of 5 M, and after reaching this saturation point there are no significant changes. When the load resistance is taken as 12 M, the peak output voltage is 1.1 V at 0.1g acceleration. Similarly, the maximum output voltage increases with increase in acceleration (0.1g, 0.2g, 0.3g). However, considering the output power at 0.1g acceleration, the maximum

output power is found to be 0.41 μW at 0.5 M of load resistance. In this study, additional optimization has been conducted to further improve the output and energy conversion efficiency. The inset in Fig. 4 describes the experimental concept for optimizing the double-clamped energy harvester. Many variables affect the output voltage; however, the height of the PDVF junction (supporting gap) has the most prominent effect. Therefore, in order to optimize the output characteristics, stress analysis for various junction thicknesses (0.5–16.5 mm) is performed using a finite element model. It can be observed in Fig. 4 that the maximum simulated stress value can be obtained with a junction height of around 6.5–10.5 mm. Based on the simulation results, we have analyzed the output characteristics of PVDF energy harvesters by fabricating the energy harvester with a junction height in the range of 0.5–16.5 mm, as shown in Fig. 4. As the junction height increases, the peak output voltage also increases. However, the energy density decreases after a certain height is reached due to rapid waveform changes. The experimental results indicate that the highest output voltage is achieved with a junction height of 6.5 mm at 0.1g acceleration. Therefore,

FIG. 2. The output voltage versus the input frequency for the doubleclamped beam and sandwich beam.

FIG. 4. The experimental and simulation results of changing thickness in the supporting gap of the double-clamped beam.

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FIG. 5. The resistance matching for different double-clamped beams at an applied acceleration of 0.1g. The inset shows average stress variation with the length of the PVDF beam.

the optimized junction height is fixed to 6.5 mm for further experiments. While fixing the cantilever’s length to 200 mm and varying the PVDF’s length, the characteristic evaluation of the double-clamped piezoelectric beam is conducted. As shown in Fig. 5 inset, finite element analysis is conducted by modeling the length ratio of the cantilever and the piezoelectric beam to 5:1, 4:1, 3:1, 2:1, and 1:1; as a result, the highest stress value occurs at a length ratio of approximately 3:1. Consequently, the energy harvester is fabricated at ratios of 3:1 and 5:1 for comparison. Figure 5 shows the output power of different double-clamped cantilevers (cantilever:PVDF ratios – 3:1 and 5:1). In the experiment, a peak output power of 45 μW is achieved, with an optimized length ratio of 3:1 at 0.1g acceleration. The junction height is 6.5 mm as for former optimization. Based on the above two optimization experiments for the double-clamped piezoelectric beam, an optimized energy

FIG. 6. The output power and peak voltage of the optimized energy harvester versus load resistance for the double-clamped beam.

Rev. Sci. Instrum. 85, 026101 (2014)

harvester is fabricated and experimentally characterized. The configuration is similar to Fig. 4 inset, but the steel cantilever (200 mm length) carries an optimized double-clamped PVDF beam on both upper and bottom sides. As shown in Fig. 6, at an acceleration of 0.1g, this optimized energy harvester generates a peak voltage of 15 V, with load resistance up to 10 M. A maximum output power of 80 μW is obtained with a 0.2 M load resistance for the optimized double-clamped PVDF energy harvester, which is much higher than the output before optimization. In summary, this study performed the design, fabrication, characteristic evaluations, and structural optimizations of an energy harvester, based on a double-clamped PVDF beam. FEM is utilized to analyze the stress distribution. Through the double-clamped structure, the power efficiency has been enhanced, which is further improved by optimizing the thickness of the double-clamped junction, as well as the length ratio of the cantilever to the double-clamped piezoelectric beam. The structurally optimized double-clamped piezoelectric energy harvester generates a maximum power of 80 μW at 0.1g acceleration. In future work of this design, further structural minimization will be conducted, such as size reduction and utilizing a folded beam. Additionally, for mass production of the minimized energy harvester, an integrated manufacturing technique will be utilized to fabricate the supporting gap and double-clamped beam. Moreover, array structures can be employed to increase the total power generation. This work was supported by the National Research Foundation of Korea (NRF) grant, funded by the Korea government (MEST) (Grant No. 2012K1A3A1A20031500).

1 I.

F. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Cayirci, IEEE Commun. Mag. 40, 102 (2002). 2 T. A. Phan, V. Krizhanovskii, and S. G. Lee, in Proceeding of the IEEE Custom Integrated Circuits Conference (IEEE, 2007), p. 675. 3 M. Gastpar and M. Vetterli, in Proceeding of the IEEE INFOCOM, New York, USA (IEEE, 2002), pp. 1577. 4 M. Winkler, K. D. Tuchs, K. Hughes, and G. Barclay, J. Telecommun. Inform. Technol. 2, 37 (2008). 5 X. Wu, M. Parmar, and D. W. Lee, “A seesaw-structured energy harvester with superwide bandwidth for TPMS application,” IEEE/ASME Trans. Mechatron. (in press). 6 Y. Ammar, A. Buhrig, M. Marzencki, B. Charlot, S. Basrour, K. Matou, and M. Renaudin, in Proceedings of Joint sOc-EUSAI Conference, Grenoble, France (2005), pp. 287. 7 C. Alippi and C. Galperti, IEEE Trans. Circ. Syst. 55, 1742 (2008). 8 A. Hajati and S. G. Kim, Appl. Phys. Lett. 99, 083105 (2011). 9 P. J. Cornwell, J. Goethal, J. Kowko, and M. Damianakis, J. Intell. Mater. Syst. Struct. 16, 825 (2005). 10 H. A. Sodano, G. Park, and D. J. Inman, J. Strain 40, 49 (2004). 11 K. A. Cook-Chennault, N. Thambi, and A. M. Sastry, Smart Mater. Struct. 17, 043001 (2008). 12 Q. C. Tang, Y. L. Yang, and X. X. Li, Smart Mater. Struct. 20, 125011 (2011). 13 T. Galchev, E. E. Aktakka, and K. Najafi, J. Microelectromech. Syst. 21, 1311 (2012). 14 X. Chen, L. S. Ma, Y. M. Zheng, and D. W. Lee, Appl. Phys. Lett. 98, 073107 (2011). 15 X. Chen, L. S. Ma, Y. M. Zheng, X. X. Li, and D. W. Lee, J. Micromech. Microeng. 22, 015011 (2012).

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Note: high-efficiency energy harvester using double-clamped piezoelectric beams.

In this study, an improvement in energy conversion efficiency has been reported, which is realized by using a double-clamped piezoelectric beam, based...
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