Multi-frequency array of nonlinear piezoelectric converters for vibration energy harvesting

Energy harvesting from wideband and random vibrations demands for techniques and solutions to overcome limitations of linear converters, which best operate when the frequency of vibrations matches the resonant frequency of the converter. Viable approaches comprise, among others, the use of array of linear harvesters with different resonant frequencies, the adoption of nonlinear techniques or the exploitation of mechanically coupled converters. This work proposes a piezoelectric energy harvester where the combination of such techniques is explored in a unique device. The harvester is composed of an array of four piezoelectric cantilevers fabricated by screen printing lead zirconate titanate (PZT) films on a stainless steel substrate. Nonlinear behavior of the cantilevers is achieved by exploiting the interaction between a magnet and their ferromagnetic steel substrate. Two configurations have been analyzed where the magnet is either fixed on the harvester base, or it is elastically suspended through a spring on the harvester base. The latter configuration introduces a mechanical coupling among the cantilevers. For the experimental characterization, the prototype has been excited by band-pass filtered white-noise mechanical vibrations in the range 10-100 Hz with amplitude in the range 0.1-2.2 g. The obtained experimental results show that the rms values of the open-circuit output voltage for each cantilever increase in the nonlinear configuration and it can be up to 215% higher for the case with the elastically suspended magnet compared to the case of the same array operated in linear regime.