13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- 0 20 40 60 80 100 120 0 50 100 150 200 250 300 Load resistance (kΩ) EM Excitation frequency: 11 Hz Excitation amplitude: 2 mm (rms) Figure 5. Output power Po as a function of the load resistance RL for (a) the electromagnetic module and (b) the piezoelectric modules when the excitation amplitude and frequency are 2 mm (rms) and 11 Hz, respectively. Connecting resistors with the above experimentally determined optimum resistances to the VEH prototype individually, power consumed by the loads has been measured as a function of the excitation amplitude at the excitation frequency of 11 Hz. As shown in Fig. 6(a), the output power of the electromagnetic module and the total output power of the piezoelectric modules are at the same order. It is also seen that as the excitation amplitude increases, the change of the output power of electromagnetic and piezoelectric modules manifests in a similar way. At first, the output power of each energy harvesting module increases quickly as the excitation amplitude increases; with further increase of the excitation amplitude, the increasing speed of the output power slows down. With the increase of the excitation amplitude, the impact speed increases, resulting in the increase of harvested energy. However, when the excitation amplitude is larger than 1 mm (rms), the cantilever tip impacts the top and bottom frames, which limits the further increase of vibration velocity of the cantilever tip and the further increase of the output power. The total power density is calculated by the ratio of the total output power to the volume of VEH. As shown in Fig. 6(b), for the proposed VEH at the excitation frequency of 11 Hz, the total power density increases with the increase of the excitation amplitude. Output power (μW) Total power density (μW per cubic centimeter) Figure 6. (a) Output power and (b) the total power density vs. the excitation amplitude at the excitation frequency of 11Hz. The output power Po vs. the excitation frequency fs at the excitation amplitude of 1 mm (rms) has also been measured, and the result is shown in Fig. 7. For the electromagnetic module, with the (a) (b)
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