$In Situ$ Measurement of Charging Process in Electret-Based Comb-Drive Actuator and High-Voltage Charging

We report in situ measurements of the charging process for an electret-based comb-drive actuator and an experiment on high-voltage charging. The ac current flowing between the comb-drive electrodes, which is proportional to the displacement, is monitored during the application of a dc bias voltage at an elevated temperature. As the charge voltage increases, it opposes the applied voltage, causing the current to decrease and become a minimum, which indicates the completion of the charging process. The proposed electret-based method allows voltage-controlled charging in silicon microelectromechanical systems (MEMSs) devices, which offers advantages for commercial MEMS device fabrication. This paper also discusses the importance of high-voltage charging for improving the efficiency of the actuator as an electrostatic vibration-energy harvesting device. A charging mechanism is proposed to explain the experimental results, and the stability of the movable comb-drive electrode against the pull-in effect is discussed. Using the proposed method, a large force factor is obtained, and charging to a voltage of up to 380 V is demonstrated.