Capacitive Position-Sensing Interface for Micromachined Inertial Sensors


A monolithic MEMS/circuits technology enables the implementation of multiple  sensors and more functions, such as sensing, signal processing and actuation, on a  single chip. In this work, issues related to monolithic integration of micromachined  inertial sensors, including accelerometers and gyroscopes, are investigated. Mechanical sensitivity, noise, and experimental performance verification of the open-loop accelerometer are discussed. The resolution of a position-sensing  accelerometer can be improved by increasing the mechanical sensitivity and the accuracy of position measurement. 1¹G=pHz (1G = 9:8m=s2) acceleration sensitivity and 0:1pm=pHz (1pm = 10¡12m) displacement resolution can be achieved using capacitive position-sensing techniques.

 Issues in designing a ΣΔ closed-loop sensor, such as system modelling,  noise interaction, and verification of an analytical ΣΔ loop model with simulation and  measurement results, are also described. A 2nd-order electromechanical ΣΔ force- feedback loop is transformed into a discrete-time ΣΔ modulator. The 1-bit  quantizer is linearized using the least mean square (LMS) method that takes its nonlinear behavior into account. In the linearized model, the effective quantizer gain is  determined statistically based upon the RMS value of the signal at its input. Reducing loop gain of a ΣΔ closed-loop sensor increases the output quantization noise.  Interaction between electronic noise and quantization noise in the 2nd-order ΣΔ closed- loop sensor is revealed.

 Vibratory rate gyroscopes for all three-axis have been implemented using  capacitive position-sensing and ΣΔ force-feedback techniques. Unlike many  published solutions that require a vacuum or high operating voltage, gyroscopes developed in this work operate at atmospheric pressure using a single 5V supply voltage. In  addition to the demonstration of functional prototype devices, this work has advanced the understanding of capacitive position measurement and provided measurement  results to verify many of the design trade-offs.

 The specific contributions of this work include: 1. The accuracy of an open-loop acceleration sensor is improved by increasing  both the mechanical sensitivity and the accuracy of position measurement. The  accuracy of position measurement is improved by optimizing the electromechanical interface, where the major concerns are damping and wiring parasitics. 2. By understanding noise interaction and its effect on ΣΔ loop  operation, noise floor of the ΣΔ closed-loop sensor is improved. Results from the  analytical model and simulation are in good agreement with measurement results. 3. The position-sensing interface can be adapted easily to demonstrate new MEMS concepts that rely on position measurement, making it a modular approach. For instance, a novel vertically-driven X/Y-axis gyroscope has been implemented using a position-sensing interface that has been designed for a Z-axis  gyroscope.

Xuesong Jiang
Roberto Horowitz
Publication date: 
May 31, 2003
Publication type: 
Ph.D. Dissertation
Jiang, X. (2003). Capacitive Position-sensing Interface for Micromachined Inertial Sensors. United States: University of California, Berkeley.

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