AVS 54th International Symposium
    MEMS and NEMS Tuesday Sessions
       Session MN-TuP

Paper MN-TuP13
Capacitive Displacement Sensing for Comb Drive Actuators Operating in Aqueous Media

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Session: MEMS and NEMS Poster Session
Presenter: P. Ponce, Stanford University
Authors: P. Ponce, Stanford University
V. Mukundan, Stanford University
B. Murmann, Stanford University
B.L. Pruitt, Stanford University
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We present a system that is capable of measuring displacements in a comb drive actuator operating in aqueous electrolytes. Underwater electrostatic actuators are promising tools for manipulation of biological samples in media.1,2 Optical techniques have been reported for measurement of these actuator displacements.3 Apart from accuracy and portability, electrical measurements are beneficial in setting up feedback systems for controlled actuation. Relative changes between the capacitances of the comb drive are measured by connecting them to parallel oscillator circuits. The operational frequencies of each oscillator depend directly on its corresponding capacitance value. In order to avoid electrolysis and electrostatic shielding effects in ionic media, the oscillators resonate at high frequencies (around 2-10 MHz) and the voltages across the comb drive electrodes are limited to approximately 100 mV, peak-to-peak. A major advantage of the described system is its ability to operate in ionic media without common adverse effects, such as electrolytic breakdown and electrode corrosion. The resultant oscillating signals are multiplied with each other and filtered to obtain a sinusoidal signal whose frequency is determined by the capacitance offset between the parallel oscillators. The sine wave is then applied to a frequency-to-voltage converter that yields a DC voltage signal. The system exhibits a change of approximately 16 kHz for each pF offset in capacitance. Based on the performance of the circuitry used, these results translate into a capacitive offset measurement accuracy on the order of 10 fF. The signal that contains information of the measured comb drive displacement is a DC voltage. This purely electrical signal allows the current device to be considered as an abstract "black box" for the purposes of creating a controllable feedback system. One plausible use for this system topology is the development of a method for applying specific forces onto cells adhered to the comb drive actuator.

1 T. L. Sounart, T. A. Michalske, and K. R. Zavadil, "Frequency-Dependent Electrostatic Actuation in Microfluidic MEMS," Journal of Microelectromechanical Systems, vol. 14, pp. 125-133, 2005.
2 V. Mukundan and B. L. Pruitt, "Experimental Characterization of Frequency Dependent Electrostatic Actuator for Aqueous Media," presented at Solid State Sensors and Actuators, Hilton Head Island, 2006.
3 D. J. Burns and H. F. Helbig, "A System for Automatic Electrical and Optical Characterization of Microelectromechanical Devices," Journal of Microelectromechanical Systems, vol. 8, pp. 473-482, 1999.