Micro-Machined Variable Capacitors for Power Generation

Variable capacitors are key elements in electrostatic micro-power generators. In such devices inertial forces are used to do work against the electric field of the capacitor, thereby converting mechanical energy to electrical potential energy that can be extracted by a suitable circuit. Applications are envisaged in portable, wearable or implantable electronic devices where body motion could provide the mechanical energy source. This paper describes the fabrication and initial testing of a micromachined variable capacitor for power generation, with overall dimensions in the meso-scale. The measured capacitance of the device varies from 100 pF to around 1pF as the mass moves from initial to final position, corresponding to a hundred-fold increase in voltage if the device is operated in constant charge mode. Initial tests of the capacitor on a vibration system (10Hz) have shown that a periodic high voltage output of 2.3 kV can be generated if the capacitor is charged by a voltage source of 26 V. This corresponds to an energy conversion rate of 2.4 μJ per cycle, or 24 μW at a vibration frequency of 10Hz.