A piezoelectric power harvester with adjustable frequency through axial preloads

We study the technique to adjust the performance of a piezoelectric bimorph vibrating in the flexural mode through axial preloads, which is useful for a power harvester to effectively scavenge energy from ambient mechanical vibrations/noise with varying-frequency spectra. The external circuit connected to the bimorph is simplified as an impedance in the analysis. Analytical solutions are derived. The analyses show that resonance happens when the natural frequency of the bimorph is adjusted adjacent to the external driving frequency by preloading, and the output power density can be raised many more times in that case. The mechanism for an axial preload to improve the bimorph performance at varying-frequency vibrations is examined in detail.