Evaluation of parameters for electrostatic energy harvesters

Electrostatic micro generators based on variable capacitors driven by proof mass motion constitute an important class of motion energy harvesters. We present an analytical approach for calculation of the variable capacitance in non-trivial harvester structures where fringing fields can not be neglected. We restrict attention to transducers consisting of metal strip electrodes on opposing dielectrics, i.e. in-plane overlap plate energy harvesters. A small gap-based transducer structure consisting of two dielectric wafers with a metal strip pattern on each wafer is optimized with respect to the ratio of electrode width a to electrode pitch p. For charge constrained operation, we find that the optimum is a=0.4p and that the capacitance variation is considerably reduced with respect to the parallel plate formula. The formulation is further developed with an additional ground plane and inclusion of end effects. A compact model is developed for this modified structure and is used to simulate the device in continuous mode with constant voltage bias. The acceleration level and bias voltage are evaluated in optimization of output power.