High level simulation of an electrostatic micromotor

To date, electrostatic microactuators have mostly been simulated using tools that involve accurate but complex finite element analysis methods. When such an analysis forms part of a full electro-mechanical simulation, the quantity of computations necessary is excessively demanding whenever rapid results are required. High-level simulation of electrostatic actuation that includes closed-form expressions of the static and dynamic behaviours of the device, seems a best alternative for rapid prototyping. The work presented in this article is focused on the high-level simulation of a particular class of actuators, the wobble electrostatic micromotor. The high-level simulation of the motor and its surrounding electronics (control loop, power supply, sensory circuitry) shows aspects of its performance that cannot be seen by any other means. As in conventional electronic systems, this approach also offers a faster and cheaper way of designing and debugging system models, by exchanging Intellectual Property (IP) blocks across different designer teams. The double-rotor double-stator wobble motor designed and tested at Heriot Watt University [1] is simulated using the high-level language VHDL-AMS (VHSIC [Very High Speed Integrated Circuits] Hardware Description Language - Analogue Mixed Signal). The micromotor is to be included into a minimally invasive catheter system for removing arterial plaque. Conformal mapping techniques are employed to get a novel, accurate and simple analytical model for the torque exerted by the micromotor. A two-dimensional approximation has been chosen for the motor geometry. By means of two different conformal transformations the complex geometry induced by the electrostatic air gap region is transformed into a simple model. The analytical torque function can then be calculated through the resulting expression of the capacitance. The mathematical model obtained is suitable for the VHDL-AMS description language. Torque results obtained by Finite Element Analysis [1] and the proposed simulation model show good agreement. Having simulated the dynamic behaviour of the motor, closed-loop control of the excitation of the motor is now being designed with the same high level language. Monolithic integration on the same die of the microsystem and the electronic circuitry is underway for full validation of the methodology.