13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- increase of fs, Po first increases and then reaches a summit at 15 Hz, passing which it begins to decrease. For the piezoelectric module, Po is almost zero when fs is less than 9 Hz; it increases as fs increases from 9 Hz to 15 Hz; it reaches the maximum at 15 Hz and decreases as fs increases further. From equations (7) and (8), the impact frequency range at the excitation amplitude of 1 mm (rms) is calculated to be from 9.6 to 13.3 Hz, which means a too small or too large excitation frequency may decrease the vibration of the cantilever tip and suppress the impact from happening, which qualitatively explains the phenomenon shown in Fig. 7. The maximum total output power and optimum excitation frequency fs for the excitation amplitude of 1 mm (rms) are 0.8 mW and 15 Hz, respectively. In addition, comparing Figs. 6(a) and 7, it can be found that the energy harvesting of the proposed VEH is more sensitive to the excitation frequency than the excitation amplitude. To further understand the energy harvesting characteristics of the VEH prototype, the optimum excitation frequency fso for maximum total output power and the total output power at fso (= the maximum total output power Pm) under different excitation amplitude have been measured, and the results are shown in Fig. 8. It is found that the maximum total output power Pm and optimum excitation frequency fso increase with the increase of the excitation amplitude. From equation (8), it is seen that the upper limit of the impact frequency range increases as the excitation amplitude increases. It seems that the impact frequency range may increase the optimum excitation frequency. 0.6 0.7 0.8 0.9 1.0 100 200 300 400 500 600 700 800 900 The total output power Optimum excitation frequency Excitation Amplitude (mm) (rms) 11 12 13 14 15 Figure 7. Output power Po vs. the excitation frequency fs at the excitation amplitude of 1 mm (rms). Figure 8. The optimum operating frequency fso and the total output power at fso vs. the excitation amplitude. 5. Conclusions In this paper, a hybrid internal impact type vibration energy harvester that uses the piezoelectric effect and electromagnetic induction both has been investigated experimentally. It is found that, for a given excitation frequency, when the excitation amplitude increases, the total output power increases fast first and the increasing speed slows down when the excitation amplitude increases further; for a given excitation amplitude, there is a frequency range in which the total output power increases with the increase of the excitation frequency; the optimum excitation frequency and its corresponding total output power increases with the increase of the excitation amplitude. The total output power and power density are found to be 0.8 mW and 15 µW/cm3, respectively, when the excitation amplitude and frequency are 1 mm (rms) and 15 Hz, respectively. The proposed vibration energy harvester may be used to harvest the vibration energy of low frequency vibration sources, which exist universally in nature, artificial structures, machines, vehicles, etc. Acknowledgements
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