Bi-resonant structure with piezoelectric PVDF films for energy harvesting from random vibration sources at low frequency
Introduction
With the fast development of the low power wireless sensor networks and the internet of things (IoT), energy harvesting technology has recently attracted a great deal of research interest as a promising technique to replace the traditional batteries [1]. The traditional batteries not only require their costly replacement, especially for sensors at inaccessible locations, but also cause pollution of the environment. Many energy sources from the environment such as light [2], RF radiation [3], thermal gradient [4] and mechanical motions [5], [6] can be harvested to provide sustainable power supply to the wireless electronics. The kinetic energy of mechanical vibration is generally the most versatile and ubiquitous ambient energy source [5], and three types of vibrations energy harvesters, electrostatic [7], [8], [9], [10], [11], electromagnetic [12], [13] and piezoelectric [14], [15], [16], [17] have been studied a lot. Piezoelectric energy harvesting devices have been most intensively studied because of their simple configuration, high conversion efficiency and compatible manufacturing process.
To maximize the harvested power output, most of the piezoelectric energy harvesters (PEH) utilize a linear vibrating structure of mass-cantilever system [18], which provides optimal power output at a high resonant frequency (typically larger than 200 Hz), as shown in Fig. 1. However, the environmental vibrational frequencies are spectrally distributed and usually below 100 Hz (especially abundant below 50 Hz). Therefore, frequency up-conversion structures are designed to match the ambient excitation [19], [20]. Tang et al. [19] demonstrated that by using magnetic repulsion forces to achieve non-contact frequency up-conversion, an average power generation of over 10 μW can be achieved within a broad frequency range of 10–22 Hz under 1 g (9.8 m/s2) acceleration. On the other hand, the traditional PEH has a very limited operating bandwidth nearby its resonant frequency. The performance of the energy harvester will decrease to a large extent when the external excitation frequency shifts away from the resonant frequency of the device. Many efforts have been made to improve the bandwidth of the energy harvesters. Lin et al. [21] implemented a multi-cantilever piezoelectric generator with current standard MEMS fabrication techniques, where the resonant frequencies of the device are between 237 Hz and 244.5 Hz.
In this paper, we have developed a polyvinylidene fluoride (PVDF) films based piezoelectric energy harvester (PPEH) with a bi-resonant structure shown in Fig. 2(a), which consists of two cantilever-mass systems to achieve two different resonant frequencies. On each of the stainless steel cantilevers, PVDF film is attached to generate electric energy from the stress caused by external vibration sources. The PVDF based polymeric piezoelectric films are used instead of the PZT materials in this demonstration because PVDF is a lead-free polymer material, which is more compatible to the CMOS/MEMS technology. In addition, the PVDF polymer material has lower Young’s modulus which can result in lower resonant frequency of the structure [22]. As shown in Fig. 2(c), there is one specific resonant frequency for each beam-mass system. When one of the masses is oscillating at resonance, the vibration amplitude may be large enough to make the mass collide with the other mass and drive the latter into forced vibration mode. Therefore, the latter mass also oscillates to a significant level even though the frequency is off its resonant frequency. By series connection of the circuits of the PPEH-top (PPEH-T) and PPEH-bottom (PPEH-B), the device bandwidth can be widened. In an optimal design, the PPEH with the bi-resonant structure can outperform the sum of the two subsystems in terms of the energy harvested from random vibration sources. It should be noted that this dual resonant structure can also be applied to the other vibrational energy harvesters such as electrostatic or electromagnetic transduction methods [23].
Section snippets
Mechanical model
Fig. 2(b) shows the mechanical model of the device. The mechanical performance of the device can be analyzed under a few basic assumptions:
- (1)
the magnitude of the mass displacement is small compared to the beam length, so that the “stiffening effect” and the nonlinearity of the beam can be neglected;
- (2)
the two beam-masses are perfectly aligned, and the collision between them is one-dimensional and elastic with no energy loss;
- (3)
the electromechanical coupling force may be neglected in this device as it
Simulation
For the energy harvester with a single resonant structure, a theoretical investigation has previously been made to study the device performance under a random vibration source [24]. The energy harvester with bi-resonant structure in this paper is more complicated. A numerical calculation with Matlab/Simulink modeling is used with the parameters listed in Table 1. Fig. 3 shows the Simulink model to study the performance of the bi-resonant structure based on the mechanical equations mentioned
Experiment
An experimental PPEH device with bi-resonant structure was fabricated to validate the numerical modeling. As shown in Fig. 2(a), the proposed bi-resonant structure for PPEH comprises of PPEH-T and PPEH-B, both of which consist of a stainless steel beam and a proof mass at the free end. The gap distance g0 between the two cantilevers is set to 0.5 mm, which is the minimum gap we can control during the experiment. The length and width of the two cantilevers are 55 mm and 18 mm, respectively. The
Conclusion
This paper presents the design and experimental characterization of a piezoelectric PVDF films energy harvesting device with bi-resonant structure for wider bandwidth response. Thanks to the two separate resonant frequencies, the vibration amplitudes of two cantilever-mass structures produces strong coupling when colliding at sufficiently large acceleration. With an optimal design of the resonant frequencies of the two subsystems, the energy harvester with bi-resonant structure can provide a
Acknowledgements
This work is supported by National Natural Science Foundation of China (Project No.: 51505209) and Guangdong Natural Science Foundation (Project No.: 2015A030313812 and 2016A030306042). The Shenzhen Key Laboratory of 3rd Generation Semiconductor Devices is supported by Project No.: ZDSYS20140509142721434.
Shanshan Li received the M.Sc. degree in materials engineering from Shenzhen University, Shenzhen, China, in 2013. Since 2014, she has been a research assistant in the department of Electronic and Electrical Engineering, Southern University of Science and Technology, China. Her current research interests include piezoelectric sensors and vibration-based micro energy harvesters.
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Shanshan Li received the M.Sc. degree in materials engineering from Shenzhen University, Shenzhen, China, in 2013. Since 2014, she has been a research assistant in the department of Electronic and Electrical Engineering, Southern University of Science and Technology, China. Her current research interests include piezoelectric sensors and vibration-based micro energy harvesters.
Andrea Crovetto received the M.Sc. degree in physics engineering and nanotechnology from Politecnico di Milano, Italy, in 2012. His studies included a project on electret-based energy harvesting at the Technical University of Denmark. He is currently pursuing the Ph.D. degree in thin-film solar cell technology at the Technical University of Denmark. His research interests include electromechanical transducers, photovoltaics, microfabrication, and modeling of semiconductor devices.
Ole Hansen received the M.Sc. degree in micro-technology from the Technical University of Denmark (DTU), Lyngby, Denmark, in 1977. Since 1977 he has worked with micro-and nano-technology and applications of the technology within electronics, metrology, sensing, catalysis and energy harvesting. He is Professor at DTU Nanotech, the Technical University of Denmark, where he is heading the Silicon Microtechnology group, with activities within lithography based micro- and nanotechnology. Current research interests include sustainable energy, photocatalysis and tools for characterizing catalytic processes. Since 2005 he has been part of the Danish National Research Foundation Center CINF, Center for Individual Nanoparticle Functionality.
Xinxin Li received the B.S. degree from Tsinghua University and the Ph.D. degree from Fudan University. For a long period of time, his research interests have been in the fields of micro/nano sensors and MEMS/NEMS. He was a Research Engineer with Shenyang Institute of Instrumentation Technology, Shenyang, China, for five years. He was also with The Hong Kong University of Science and Technology, Kowloon, Hong Kong, as a Research Associate and with Nanyang Technological University, Singapore, as a Research Fellow. He then joined Tohoku University, Sendai, Japan, as a Lecturer (Center of Excellence Research Fellowship). Since 2001, he has been a Professor with the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai. From 2007 he has been serving as the Director of the State Key Laboratory of Transducer Technology, China. He has invented more than 100 patents and published more than 300 papers in refereed journals and conference proceedings (including about 170 SCI journal papers). He is on the Editorial Board of the Journal of Micromechanics and Microengineering. Prof. Xinxin Li is now serving as a Technical Program Committee member for IEEE MEMS, and as an International Steering Committee member for Transducers.
Fei Wang received the B.S. degree in mechanical engineering from the University of Science and Technology of China, Hefei, China, in 2003, and the Ph.D. degree in microelectronics from Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai, China, in 2008. He was a Postdoctoral Researcher with the Department of Microtechnology and Nanotechnology, Technical University of Denmark, where he had been promoted to assistant professor since 2010. Since 2013, he has been an associate professor in the department of Electronic and Electrical Engineering, Southern University of Science and Technology, China. His current research interests include micro energy harvesting, MEMS and NEMS devices, IC and semiconductor testing.