Issue 23

D. Castagnetti, Frattura ed Integrità Strutturale, 23 (2013) 87-93; DOI: 10.3221/IGF-ESIS.23.09

From Fig. 6 it appears that, at the first eigenfrequency, the tip deflection is exactly the same for all the laminas (as discussed above), and is many times higher than that of subsequent eigenmodes. The resistive load seems to have no effect on the tip deflection. By contrast, despite not showed here for brevity, it was observed that the tip deflection linearly depends on the base acceleration. The RMS voltage in Fig. 7 is quite similar for lamina #1 and #2, and, as expected, the higher values are obtained by increasing the resistive load up to an open circuit condition (solid black bars). A small difference is observed at the third eigenfrequency, where the higher RMS voltage is obtained for a resistive load of 100 k  ( empty bars). Probably, a more accurate investigation on this eigenfrequency would be needed. Moreover, the RMS voltage is noticeably low at the first and second eigenfrequency when the 10 k  resistive load (grey bars) is applied to the piezoelectric patches. Similarly to the tip displacement in Fig. 6, also the RMS voltage is much more high at the first eigenfrequency and is proportional to the base acceleration. Fig. 8 shows that the overall output power of the converter at the fundamental eigenfrequency is an order of magnitude higher than at the second and third eigenfrequencies. The output power increases more than linearly with the base acceleration. In addition, both for the first and third eigenfrequency the resistive load significantly affects power generation. Despite the highest power generation is obtained at the first eigenfrequency, also at subsequent eigenfrequencies the converter is able to provide a useful power output, which can be maximized by choosing the optimal resistive load for that given frequency. On the whole, a multi-frequency converter allows to harvest ambient energy in a given frequency range and can be particularly efficient in applications dealing with excitations which are not controllable or are intrinsically frequency variant over a wide range. Further experimental tests will be performed to compare this fractal-inspired multi-frequency converter with a traditional multi-cantilever solution.

C ONCLUSIONS

A

fractal-inspired, multi-frequency, piezoelectric energy converter, which is a square thin sheet structure with inner cuts, is designed and experimentally investigated. The converter exhibits three eigenfrequencies in the range between 0 and 120 Hz: the first eigenfrequency corresponds to that of an equivalent cantilever, the second and third eigenfrequency comes from the inner cantilevers. The electric power generation under different levels of resistive loads (from open circuit up to a low electric resistance) is good, in particular at the first eigenfrequency, and increases almost linearly with the base acceleration. A multi-frequency converter, as presented here, can be particularly efficient for energy harvesting involving ambient vibrations whose frequency is uncertain or varying over a wide range.

R EFERENCES

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