Closed-loop electromechanical sigma-delta microgravity accelerometers.

Abstract

Micromachined inertial sensors are one of the most important groups of silicon based sensors. High precision accelerometers with micro-gravity (mug) resolution have a number of applications including navigation and guidance, space microgravity measurements, and automotive industry. Recently, MEMS-based capacitive accelerometers have become very attractive due to their high sensitivity, low-temperature sensitivity, simple structure, low cost, drastically reduced size and weight, and low power dissipation. The objective of this thesis is to investigate the limitations of microaccelerometer systems and develop <italic>a micro-g resolution accelerometer system with its interface electronics</italic> for inertial navigation applications. The focus of this research is on the interface electronics and the system design. The interface electronics forms a 2<super>nd</super> order Sigma-Delta modulator together with the sensor and operates it in an oversampled electromechanical Sigma-Delta loop to read the sensor capacitance variation, force-rebalance the proof mass, and obtain a direct digital output. Closed-loop operation increases the dynamic range and reduces the sensitivity to variations in mechanical characteristics. The 1<super>st</super> generation interface circuit has a 95dB dynamic range and can resolve better than 75aF. The complete module has a measured acceleration sensitivity of 430 mV/g with 3.5mug/&radic;Hz noise floor in open-loop. Closed loop operation of the system has been achieved for the first time and provides a resolution of 25mug/&radic;Hz. Noise analysis of the 1<super>st</super> generation system shows that the interface electronics limits system performance. Therefore, a 2<super> nd</super> generation interface circuit was been developed to achieve mug resolution. This chip operates from a 1MHz clock and provides an adjustable sensitivity between 0.2 and 1.2V/pF with a resolution better than 20aF and a dynamic range up to 140dB. It has been shown that this new circuit can resolve 1mug/&radic;Hz in open-loop when it is combined with high-sensitivity out of plane (z-axis) accelerometers. By using this chip a complete 3-axis mug-resolution accelerometer system has been realized. In addition to the 2nd-order Sigma-Delta technique, a novel interface electronics design has been introduced for sub-mug resolution accelerometers. This new technique employs two accelerometers in a multi-step Sigma-Delta modulator architecture and provides high SNR while improving the dynamic range. It has been shown that this new architecture improves the system resolution by a factor of more than two compared to the 2<super>nd</super>-order Sigma-Delta modulator.