Abstract
A MEMS electrostatic kinetic energy harvester (e-KEH) of about 1 cm(2), working at ultralow frequency (1-20 Hz), without any supported additional mass on its mobile electrode, and working even without a vacuum environment is reported. The prototype is especially suitable for environments with abundant low frequency motions such as wearable electronics. The proposed e-KEH consists of a capacitor with a finger-teeth interdigited comb structure. This greatly reduces the air damping effect, and thus the capacitance variation remains important regardless of the presence of air. With the new design, the energy transduced per cycle of excitation is no less than 33 times higher than the classic design within 10-40 Hz/2 g (peak), while is 85 times higher at 15 Hz/2 g (peak). An enclosed miniature ball combined with non-linear stoppers enables the oscillation of the movable electrode through impact-based frequency up-conversion mechanism, which is also improved by the low air damping. Thanks to this new design, a higher efficiency than the classic gap-closing comb structure is obtained, as a larger range of working frequency (1-180 Hz) in air. A maximum energy conversion of 450 nJ/cycle is obtained with a bias voltage of 45 V and an acceleration of 11 Hz, 3 g (peak). Working with a diode AC-DC rectifier, the proposed KEH is able to support up to 3 RFID communications within 16 s while operated at 11 Hz, 3 g (peak).