Broadband energy harvester for low-frequency rotations utilizing centrifugal softening piezoelectric beam array

Harvesting the rotational energy has drawn great attention in the past decade with its abundance in our living environment and its great potential to power the wireless transmitter. However, issues exist that small-scale and resonant-frequency-based linear harvesters typically have high natural frequencies and narrow operating frequency bandwidth. Therefore, to design a broadband energy harvester for low-frequency rotations, this paper proposes the centrifugal softening beam array harvester. The broad bandwidth is achieved by arranging groups of inverted piezoelectric beams at different rotational radii in a straight array, which results in different centrifugal softening coefficients. This eliminates the tedious work of pre-designing and manufacturing beams with different geometries in a conventional beam array harvester. Furthermore, the centrifugal softening effect can decrease the resonant frequency of the harvester, leading to the higher energy output under low-frequency rotations. In addition, the analytical model of such centrifugal softening beam array with different interface circuits is firstly proposed based on the equivalent impedance method. Results show that for both parallel and series connections of piezoelectric beams, the output power is highest and the bandwidth is broadest with the parallel-SSHI circuit compared with those with the standard AC/DC and series-SSHI circuits. Despite that both the parallel-SSHI and series-SSHI circuits can significantly improve the output power compared with the standard circuit, the series-SSHI circuit has a degradation on the frequency bandwidth compared with the other two circuits. Specifically, when attached to the parallel-SSHI circuit, for ten identical parallel-connected beams with decreasing centrifugal softening coefficients, the 3 dB bandwidth can be broadened by up to 1240.7% within the frequencies lower than 20 Hz compared with that of a single beam.