Capacitive Sensor Interface for an Electrostatically Levitated Micromotor

This paper presents the design and performance of a capacitive sensor interface dedicated to a microelectromechanical systems (MEMS) micromotor electrically levitated in five DOFs. The position and orientation of the rotor are detected by measuring differential rotor-electrode capacitances with a set of capacitance-to-voltage converters (CVCs). The sensor contains multiplexed electrodes for both capacitive sensing and force feedback, and a set of common electrodes for carrier exciting with an aim to eliminate ohmic connection with the levitated rotor. The proposed interface circuit is based on a symmetrical structure containing two half ac bridges, more robust against parasitic capacitances, capable of detecting capacitance changes with frequency higher than 10 kHz, and able to decouple multiaxis position signals of a levitated rotor. An electronic equivalent model of the sensing circuit has been developed and used to analyze the sensor performance. The major nonidealities and their effects on the accuracy of the position sensing are discussed. The performance of the sensing circuit was experimentally investigated on a prototype interface circuit. The experimental results confirm the principles of operation and the performance of the interface for the multiaxis levitated devices using capacitive position sensors.