A capacitive energy harvester based on in-plane rotary combs is proposed and studied. It is capable of collecting kinetic energy from planar ambient vibrations for low frequency operation. The design and simulation of capacitance among rotary combs, ladder spring and resonant frequency of the whole rotary comb energy harvester are presented in this paper. This device is fabricated in SOI (silicon-on-insulator) wafers by deep silicon etching technology. The dimensions of the prototype are about 7.5 mm × 7.5 mm × 0.7 mm. A maximum measured output power in air for vibrations of 0.5 g, 1 g, 1.5 g, 2 g and 2.5 g is 0.11 μW, 0.17 μW, 0.24 μW, 0.3 μW and 0.35 μW, respectively, when the loading resistance matches the parasitic resistance of 80 MΩ at the resonant frequency of 110 Hz. In order to reduce the air damping effect, the prototype is packaged by having a metal cap to form the vacuum level of 3 Torr. The testing results in vacuum level of 3 Torr show that the resonant frequency decreases from 110 Hz in air to 63 Hz, and the maximum electrical output power at 0.25 g is 0.39 μW.
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