Improved Capacitance Model Involving Fringing Effects for Electret-Based Rotational Energy Harvesting Devices

Electret-based rotational energy harvesting (EBREH) technology is promising to overcome the electrostatically converted power limitation of velocity-damped resonance generators. Its structural parameter-dominated capacitance variation and parasitic capacitance should be correctly evaluated for the optimal design of EBREH devices regarding the high power output implement. Basically, finite-element analysis (FEA) seems the only access to predict capacitances across the complicated interdigitated electrodes in 3-D space because of partially or completely neglected fringing effects in currently available theoretical models. In this paper, we proposed a more efficient model for the capacitance of EBREH devices based on four-positioned capacitors, i.e., coplanar, parallel-plate, flat-plate nonparallel, and sector coplanar capacitors. Distinctively differing from other models, our model takes 3-D fringing effects associated with fringing fields into account properly in each type of the capacitor. In addition, the accuracy of the proposed model was much improved by combining parallel-wire capacitance theory and Schwarz-Christoffel mapping. Our model was verified through both the FEA and practical measurements with a fabricated EBREH structure. The measurement indicated that this novel capacitance model can be readily applicable to rotary and linear motion-driven EBREH devices and electrostatic sensors for further design optimizations with device parameters.