A Capacitive Detection Scheme with Inherent Self-calibration for Resonant MEMS

Background: SEVERAL important classes of MEMS devices, such as resonators, gyroscopes, and chemical sensors, rely on resonance phenomenon in their operation. In these devices, resonant motion needs to be actuated, sensed, and controlled. Capacitive phenomena are commonly used for transduction in vibratory MEMS devices due to the ease of fabrication, low sensitivity to temperature changes, and other practical advantages. However, conventional capacitive detection schemes produce a signal proportional to such system parameters as nominal sense capacitance, carrier voltage, and gain of the current amplifier. These dependencies constitute a need to calibrate individual MEMS devices to address fabrication imperfections, and fluctuation of the parameters due to changing environment and aging. A detection technique independent of these system parameters can be of great advantage. Technology: University researchers have developed a novel capacitive detection method for resonant devices which is robust to parameters variation. The approach constructively utilizes inherent nonlinearity of parallel plate sense capacitors in order to measure amplitude of motion. In the case of parallel plate detection signal, multiple harmonics exist and carry redundant information on the amplitude of mechanical motion. The amplitude of motion can be extracted from the ratio of two simultaneously measured harmonics. Unlike conventional methods, the proposed measurement algorithm does not depend on such system parameters, as nominal sense capacitance, probing voltage, and trans-impedance gain of the motional current amplifier. Feasibility of the developed approach has been demonstrated experimentally, with a real-time measurement algorithm having been simulated. Application: The technique is especially valuable for robust capacitive detection and self calibration in resonant structures, such as gyroscopes, resonant microbalances, and chemical sensors.

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