Micro Accelerometer Design with Digital Feedback Control

In this work, design of capacitive accelerometers with CMOS electronics is investigated. The goal of this investigation is to form accurate behavioral models at three different levels of abstraction: system, electrical, and mechanical. Analysis of these models permits a deeper understanding of both accelerometer design and design trade-offs. The results of this work are used to obtain an accurate prediction of accelerometer performance.
At the system level, simultaneous force-balancing and analog-to-digital conversion of the analog input acceleration to digital output is achieved using a sigma-deltaforce-feedback loop. Analysis of the feedback loop shows that the system is unstable without compensation. Compensation of the feedback loop is accomplished with a forward path, discrete-time lead filter. Modeling and simulations are used to determine optimal compensator characteristics and system robustness.
Design of CMOS-compatible interface electronics is also investigated. Time-multiplexing of the sigma-delta feedback loop enables errors introduced by the electronic interface to be measured and canceled. A method of interfacing the sense-element to a fully-differential electrical interface is described. To obtain accurate predictions of resolution, a detailed noise analysis is presented. Fabricated devices invariably exhibit an acceleration offset due to mismatches from the manufacturing process. A technique for trimming this offset after packaging is demonstrated.
Mechanical design of the sense-element is found to play a fundamental role in the determination of both dynamic range and noise floor. Understanding of factors determining mechanical sensitivity, noise, and robustness is obtained through detailed analyses.
Efficacy of both the design techniques and the analytical models developed through this research has been experimentally verified. The experimental test-bed used for verification is a monolithic three-axis surface micromachined accelerometer. The 0.2 micro-gram proof-mass is formed from a 2.3 micron-thick layer of polysilicon. A measured dynamic range of 84 dB, 81 dB and 70 dB is achieved in the x-, y-, and z-axes respectively. While the 4mm x 4mm chip operates from a 5 Volt power supply, the proposed design techniques are applicable to low voltage technologies. Comparisons of calculated, simulated, and measured parameters show excellent agreement between modeled and observed behaviors.
Publication date: 
May 31, 1997
Publication type: 
Ph.D. Dissertation
Lemkin, M. A. (1997). Micro Accelerometer Design with Digital Feedback Control. United States: University of California, Berkeley.

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