CMOS systems and circuits for sub-degree per hour MEMS gyroscopes
Micromachined gyroscopes constitute one of the fastest growing segments of the microsensor market. The application domain of these devices is quickly expanding from automotive to consumer and personal navigation systems. Examples include anti-skid and safety systems in cars and image stabilization in digital cameras. However, MEMS angular rate sensors today do not meet the sub-degree-per-hour resolution and bias drift requirements needed in high precision applications such as inertial measurement units (IMU) for GPS augmented navigation, robotics, unmanned surveillance vehicles, aircraft and personal heading references.
The objective of our research is to develop system architectures and CMOS circuits that interface with high-Q silicon micromachined vibratory gyroscopes to implement navigation-grade angular rate sensors. The MEMS sensor used in this work is an in-plane bulk-micromachined mode-matched tuning fork gyroscope (M2 - TFG), fabricated on silicon-on-insulator substrate. The use of CMOS transimpedance amplifiers (TIA) as front-ends in high-Q MEMS resonant sensors is explored. A T-network TIA is proposed as the front-end for resonant capacitive detection. The implemented T-network TIA provides on-chip transimpedance gains of up to 25MΩ, has a measured capacitive resolution of 0.02aF/√Hz at 15kHz, a wide dynamic range of 104dB in a bandwidth of 10Hz and consumes 400μW of power.
Another important contribution of this work was developing a scheme to substantially improve the noise and drift of micromachined gyroscopes by adaptively biasing the mechanical structure, such that the sensor is operated in so-called mode-matched condition. Mode-matching leverages the inherently high quality factors of the microgyroscope and results in significant improvement in the Brownian noise floor, electronic noise, sensitivity and bias drift of a microgyroscope. We developed a novel architecture that utilizes the often ignored residual quadrature error in a gyroscope to achieve and maintain perfect mode-matching (i.e. 0Hz split between the drive and sense mode frequencies), as well as electronically control the sensor bandwidth.
A CMOS implementation was developed that allowed mode-matching of the drive and sense frequencies of a gyroscope at a fraction of the time taken by current state-of-the-art techniques. Further, this mode-matching technique allows for maintaining a controlled separation between the drive and sense resonant frequencies, providing a means to increasing sensor bandwidth as well as dynamic range. The mode-matching CMOS IC, implemented in a 0.5μm 2P3M process, and control algorithm have been interfaced with a 60μm thick mode-matched tuning fork gyroscope (M2 - TFG) to implement an angular rate sensor with bias drift as low as 0.3°/hr—two orders of magnitude lower than commercially available gyroscopes and the lowest recorded to date for a silicon MEMS gyro.