16 P.S. Varoto Fig. 2.7 Experimental results from sine sweep tests: (a) input acceleration frequency spectra; (b) FRF a b Fig. 2.8 Nonlinear measurements: (a) repulsive magnetic force; (b) attractive magnetic force observed when sine sweep is used to drive the system under study. In this case, care should be taken in choosing appropriate excitation levels such that the system’s response falls within linear limits. On the other hand, the use of sine sweep signal is suitable for studying nonlinear behavior, and should be preferred in this case in relation to random or pseudorandom excitation signals, since the later tends to smooth nonlinear effects mostly due to the nature of the signal (white noise) and the linearization effects of the Fourier transform used to compute the FRF. Finally, the results shown in Fig. 2.8 were obtained with the test setup shown in Fig. 2.4. In this case the system was driven by a sinusoidal excitation signal that covered a reduced frequency range. The linear distance between the tip magnet attached to the free end of the harvester and the magnet fixed on the vertical tower shown in Fig. 2.4 was varied in order to generate different magnetic forces, thus inducing different nonlinear effects on the dynamics of the harvester. Figure 2.8a show results for a repulsive magnetic force while Fig. 2.8b show similar results but in this case the magnets were inverted such that an attractive magnetic restoring force could be achieved. Results clearly show differences in amplitude and on the location of the resonant peak, indicating that the use of the magnets significantly altered the dynamic behavior of the harvester.
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