Effect of elastic element on self-excited electrostatic actuator

Self-excited electrostatic actuators, which are also called Franklin bells or Gordon bells, are one of the simplest oscillators among various types of actuators. The actuators oscillate with DC voltage and simple structures mainly composed of only two electrodes and a conductive armature. These features seem suitable for micro mechatronic devices, including micro robots; however, these actuators only have a small number of practical applications. The high voltage for actuation, damage by electrical discharge, and mechanical loss with collisions between the armature and the electrodes keep the actuators away from practical usages. This research focuses on the mechanical loss with collisions between the armature and the electrodes and proposes a method for decreasing this loss by energy recovery with elastic elements. This study first conducts a model-based analysis of the behavior of the proposed system that combines the electrostatic self-excitation and the elastic elements. The results are then compared with the experimental results of a prototype actuator. The simple analysis predicted the increase of the oscillating frequency with the elastic elements, limiting the condition of the self-excitation caused by the energy loss of the damping factor. The results of the experimental verification showed the same tendency as the analytical predictions: increase of the oscillating frequency and existence of a critical value for the oscillation limit. Although comparatively large errors were confirmed between the simulation and the experiment under a lower applied voltage, experimentally obtained frequencies have errors less than 6.4% compared with the analytical result under the applied voltage of over 1.3 kV.