13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- In our experiments, at first, the relationship between the cantilever vibration frequency and excitation frequency was investigated. Then different load resistances were connected to the prototype to determine the optimum load resistances for the electromagnetic and piezoelectric modules. After that, the experimentally determined optimum load resistances were used to obtain the quantitative effects of the excitation amplitude and frequency on the power output and the total power density. Finally, the optimum excitation frequency for maximum power output and the total output power at the optimum excitation frequency vs. the excitation amplitude were measured. Since it was found that the power generation of the directly impacted piezoelectric plates (PZT-T-IMP and PZT-B-IMP) was much lower than that of the other four piezoelectric modules under the same excitation condition, this part of power extraction is neglected in the following study. All the following experiments were conducted only for the electromagnetic and piezoelectric modules at the excitation frequency around the resonance frequency of the middle cantilever beam (frc ≈ 11 Hz). 4 Results and Discussion Applying an excitation to the prototype and making the cantilever tip impact with the frames, the relationship between the vibration frequency of the cantilever beam fc and the excitation frequency fs has been measured firstly, and the result is shown in Fig. 4. It is manifested that either the cantilever tip vibrates freely or there is impact between the cantilever tip and frames, the cantilever vibration still has the same frequency as the external excitation. Vibration frequency of the cantilever beam (Hz) Figure 4. The vibration frequency of the cantilever beam fc vs. the excitation frequency fs. According to Eqs. (9) and (10), the theoretical optimum load resistances can be obtained. In our VEH prototype, Ro =3.9 kΩ, Lo = 0.3 H, Cp = 33 0 / T p p p LW H ε ε = 0.92925 nF, fc=11 Hz and frf is measured to be 169.5 Hz using Polytec Scanning Vibrometer PSV-300, such that RLEO and RLPO are calculated to be 3.9 kΩ and 202 kΩ approximately. To acquire these values experimentally, the output power Po vs. load resistance RL was measured for the electromagnetic and piezoelectric modules under the excitation amplitude of 2 mm (rms) and excitation frequency of 11 Hz. As shown in Fig. 5(a), the optimum resistance for electromagnetic module RLEO is found to be about 4 kΩ; from Fig. 5(b), it is found that the optimum resistance for each piezoelectric module RLPO is almost the same and equals 200 kΩ approximately, which well agree with the theoretical values.
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