Experimental and theoretical studies on MEMS piezoelectric vibrational energy harvesters with mass loading

Abstract

Experimental and theoretical investigations on micro-scale multi-morph cantilever piezoelectric vibrational energy harvesters (PZEHs) of the MicroElectroMechanical Systems (MEMS) are presented. The core body of a PZEH is a “multi-morph” cantilever, where one end is clamped to a base and the other end is free. This “fixed-free” cantilever system including a proof-mass (also called the end-mass) on the free-end that can oscillate with the multi-layer cantilever under continuous sinusoidal excitations of the base motion. A partial differential equation (PDE) describing the flexural wave propagating in the multi-morph cantilever is reviewed. The resonance frequencies of the lowest mode of a multi-morph cantilever PZEH for some ratios of the proof-mass to cantilever mass are calculated by either solving the PDE numerically or using a lumped-element model as a damped simple harmonic oscillator; their results are in good agreement (disparity≤0.5%). Experimentally, MEMS PZEHs were constructed using the standard micro-fabrication technique. Calculated fundamental resonance frequencies, output electric voltage amplitude V and output power amplitude P with an optimum load compared favorably with their corresponding measured values; the differences are all less than 4%. Furthermore, a MEMS PZEH prototype was shown resonating at 58.0±2.0Hz under 0.7g (g=9.81m/s2) external excitations, corresponding peak power reaches 63μW with an output load impedance Z of 85kΩ. This micro-power generator enabled successfully a wireless sensor node with the integrated sensor, radio frequency (RF) radio, power management electronics, and an advanced thin-film lithium-ion rechargeable battery for power storage at the 2011 Sensors Expo and Conference held in Chicago, IL. In addition, at 58Hz and 0.5, 1.0g excitations power levels of 32, and 128μW were also obtained, and all these three power levels demonstrated to be proportional to the square of the acceleration amplitude as predicted by the theory. The reported P at the fundamental resonance frequency f1 and acceleration G-level, reached the highest “Figure of Merit” [power density×(bandwidth/resonant frequency)] achieved amongst those reported in the up-to-date literature for high quality factor Qf MEMS PZEH devices.