We have developed a micro-electro-mechanical systems (MEMS) electrostatic vibratory power generator with over 100 μWRMS of (root-mean-square) output electric power under 0.03 GRMS (G: the acceleration of gravity) accelerations. The device is made of a silicon-on-insulator (SOI) wafer and is fabricated by silicon micromachining technology. An electret built-in potential is given to the device by electrothermal polarization in silicon oxide using potassium ions. The force factor, which is defined by a proportional coefficient of the output current with respect to the vibration velocity, is 2.34 × 10−4 C/m; this large value allows the developed vibration power generator to have a very high power efficiency of 80.7%. We have also demonstrated a charging experiment by using an environmental acceleration waveform with an average amplitude of about 0.03 GRMS taken at a viaduct of a highway, and we obtained 4.8 mJ of electric energy stored in a 44 μF capacitor in 90 min.

We have developed a metal-cantilever electrostatic vibration power generator that has comb-drive electrodes made of a SOI wafer at the tip of the cantilever. The comb-drive electrodes in which built-in electric field is given by potassium ion electret technique acts as an energy transducer from mechanical vibration power to electrical one. The metal cantilever part is formed separately, and assembled with the comb-electrodes on a bakelite plastic. This device showed strong nonlinear frequency responses, in which spring hardening was observed when applied acceleration was small while spring softening when high acceleration. About 30μW output power was observed at 0.03GRMS acceleration, and the maximum 90% conversion efficiency achieved at 0.01GRMS acceleration.

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