A Method to Determine the Electret Charge Potential of MEMS Vibrational Energy Harvester Using Pure-White Noise

A high-speed measurement method has been developed to electrically determine the potential voltage of the permanent charge or electret formed in a MEMS (microelectromechanical systems) vibrational energy harvester. While the conventional admittance method requires long time to observe the waveforms of the output power in time domain that diminishes when the built-in electret potential is compensated by an external voltage, this work proposes a new method to uniquely and promptly determine the electret potential by using the FFT (fast Fourier transform) analysis on the vibrational energy harvester excited by the white-noise voltage; the resonant peak in the frequency domain diminishes when the electret potential is compensated by the applied external bias voltage. White noise has been used in conventional method to measure the frequency response of devices of linear characteristics. However, due to the randomness of the input signal, random noise is inherently introduced to the measurement result, which urges us to repeat the measurement and to perform time-averaging to smooth out the noise. Therefore we used a newly developed white noise so-called "pure" white noise, which has a totally flat power spectrum, as it is synthesized from waveforms of a constant magnitude and random phases. We compute the inverse Fourier transform of such pure-white noises to instantly obtain the power spectrum without using mathematical averaging, and thus the throughput of measurement is enhanced ten-times faster.