Closed-loop electromechanical sigma-delta microgravity accelerometers

Micromachined inertial sensors are one of the most important groups of silicon based sensors. High precision accelerometers with micro-gravity (μg) 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 a micro-g resolution accelerometer system with its interface electronics for inertial navigation applications. The focus of this research is on the interface electronics and the system design.; The interface electronics forms a 2nd order Σ-Δ modulator together with the sensor and operates it in an oversampled electromechanical Σ-Δ 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 1st 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 3.5μg/√Hz noise floor in open-loop. Closed loop operation of the system has been achieved for the first time and provides a resolution of 25μg/√Hz.; Noise analysis of the 1st generation system shows that the interface electronics limits system performance. Therefore, a 2 nd generation interface circuit was been developed to achieve μg 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 1μg/√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 μg-resolution accelerometer system has been realized.; In addition to the 2nd-order Σ-Δ technique, a novel interface electronics design has been introduced for sub-μg resolution accelerometers. This new technique employs two accelerometers in a multi-step Σ-Δ 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 2nd-order Σ-Δ modulator.