Closed-loop MEMS accelerometer: From design to production

Abstract

The paper reports on a navigation grade 30g MEMS accelerometer based on digital ΔΣ modulation and capacitive sensing. The closed-loop accelerometer is a fully integrated system comprising a uniquely designed MEMS device enclosed in a specially built LCC package with a proprietary ASIC. Like Physical Logic's high-end open-loop MEMS accelerometers, now known as the MAXL-OL-2000 series, the closed-loop design benefits from the in-plane architecture using SOI wafer. Key features of the design are discussed, from the MEMS transducer to system wide considerations of low noise, high linearity, and robust stability of the control design. System level simulation results are presented and compared to the test results from the most recently fabricated MAXL-CL-3030 closed-loop ΔΣ accelerometer with 30g range. Physical Logic has developed a closed-loop MEMS accelerometer with an objective to reach inertial navigation grade performance. Both the high-end open-loop and closed-loop MEMS accelerometers employ a similar transducer design. In a different manner from the commonly used out-of-plane technique for bulk micromachining, an in-plane design using SOI wafer was adopted. The advantages of the approach are full bridge capacitive sensing for parasitic rejection, a highly symmetric mechanical structure for better temperature stability and elimination of the need for vacuum packaging for better reliability. A large proof mass, which is realized in SOI wafer handle layer, contributes to enhanced sensitivity. The closed-loop system architecture operates as a 4th order fabricated MAXL-CL-3030 closed-loop modulator used to convert external acceleration into a high frequency single bit digital signal. The design challenges and considerations are described with emphasis on noise, linearity, and stability. The test results of the MAXL-CL-3030 closed-loop ΔΣ accelerometer confirm the navigation grade design. Measurements are presented demonstrating results of <20 μg bias stability, 0.01 % typical non-linearity, and less than 10 μg/g2rms Vibration Rectification Error (VRE) up to a 2 kHz frequency range.