Low frequency piezoelectric energy harvesting at multi vibration mode shapes

A multi-degree of freedom micro-energy harvester has been designed, fabricated, and tested and sub 100 Hz natural frequencies have been achieved. This design's resonant frequencies at its first three mode shapes are within the low ambient vibration frequency range. The structure is fabricated from a silicon substrate with Aluminum Nitride (AlN) energy harvesting elements on thin silicon beams and uses a chip as a proof mass. The nonlinear stiffness due to stretching strain in the structure provides a wider harvestable frequency bandwidth in each mode. The nonlinear load-deflection equation for the second mode shape of the device, which corresponds to vertical oscillation and maximum harvester deflection, has been modeled using finite element simulation. The first three natural frequencies of 71.8, 84.5, and 188.4 Hz were measured experimentally for the presented harvester. A frequency bandwidth of 10 Hz has been obtained for the second mode shape under a base excitation of 0.2 g. A maximum open circuit voltage of 1 V and power output of 136 nW with a load resistance of 2 MΩ have been measured using this harvester. Using a synchronized switch harvesting on inductor (SSHI) electrical interface and Lead Zirconate Titanate (PZT), simulations estimate that the power output could be improved to ∼3.1 μW.