This research investigates the design of capacitive position sensing circuits for electrostatic micropositioners. The goals of this investigation are to understand the interaction among the micropositioners, the driving circuits, and the position sensing circuits, to explore fundamental limitations of capacitive position sensing, and to utilize position sensing circuits to implement feedback loops to improve the dynamics of micropositioners. A dual-stage servo system for magnetic disk drives is used as the framework of this research.
At the circuit level, capacitive position sensing schemes for monolithic implementations are discussed. A technique of interfacing three-terminal sense elements to fully-differential sensing electronics is presented. The fundamental resolution imposed by the amplifier thermal noise is studied. A circuit technique that enables the sensing circuit to potentially achieve the fundamental resolution is proposed.
At the integration level, methods of sharing a single set of electrodes between the high voltage drive of the micropositioner and the precision low-voltage sensing signal are presented. Issues related to the interface between capacitive position sensing and electrostatic actuation are investigated and their solutions are presented.
To demonstrate the feasibility of the design techniques developed in this research, a prototype capacitive position sensing circuit for micropositioners has been designed. An experimental electrostatic positioning system utilizing this sensing circuit has a position sensitivity of 35mV/um, an offset of approximately I0mV, a thermal noise floor of 0.19 angstrom/Hz or approximately 3.4nm in 25kHz bandwidth, and a feedthrough-limited resolution of l0nm for maximum driving voltages +/-10V. A simple feedback loop utilizing the position sensing circuit and a lead filter reduces the micropositioner settling time from over l00msec to 0.7msec.
July 31, 1998
Wongkomet, N. (1998). Position Sensing for Electrostatic Micropositioners. United States: Electronics Research Laboratory, College of Engineering, University of California.