Variable-capacity electrostatic micromotors

The desire to miniaturize actuators is not new, and the progress made in this field, notably by the watchmaking industry, proves it. But now that conventional machining methods have reached their limits, new manufacturing techniques, mostly derived from processes used in microelectronics, are making it possible to produce micrometer-scale mechanical parts. The first micromotors machined on silicon appeared at the end of the 1980s. In fields of application where dimensions play a very important role, such as medical and biological engineering, space and instrumentation, the stakes are high.

Like "classical" motors, i.e. those of macroscopic size, several operating principles can be demonstrated. In this article, however, we shall confine ourselves to the study of variable-capacity electrostatic micromotors. After presenting some manufacturing processes and realizations, we will study the effects of miniaturization on the static and dynamic performance of electromechanical converters. As the benefits of electrostatic systems in very small dimensions become apparent, we will discuss the principles of operation and then present a systematic sizing method which, based on simple topological rules and a given specification, enables us to define an optimum structure in terms of maximum mean torque. Finally, to better appreciate the theoretical performance of this type of micromotor, we will study its dynamic behavior and present simulation results corresponding to several operating configurations.