MEMS IMU for AHRS applications

Northrop Grumman, LITEF is developing MEMS (micro-electro-mechanical systems) based Inertial Measurement Units (IMU) for future attitude and heading reference systems (AHRS) with a target accuracy of 5 deg/h for the gyroscopes and 2.5 mg for the accelerometers. Within the technology development phase, prototype single axis gyroscopes have been realized and extensively tested for effects including temperature, acoustic and vibration sensitivities. These devices employ micro-machined all-silicon gyroscope sensor chips processed with deep reactive ion etching (DRIE). Silicon fusion bonding ensures pressures smaller than 3middot10^-2 mbar. Sophisticated analog electronics and digital signal processing condition the capacitive pick-off signals and realize full closed loop operation. The current results with overall bias error smaller than 2 deg/h to 5 deg/h, scale factor error <1200 ppm, measurement range >1000 deg/s and angular random walk <0.4 radic/vh indicate that stable production of 5 deg/h gyroscopes is realistic. The fabrication technology for capacitive, pendulous accelerometer chips is based on that used for the gyros with only an increase in the enclosed pressure to obtain overcritical damping. Pulse width modulation (PWM) within a digital control loop is used to realize closed loop operation. Accelerometer chips have been tested over temperature with a residual bias error <2.0 mg and a scale factor error <1400 ppm. These sensor chips have been integrated into an IMU whereby the power budget and size of the sensor electronics have been optimized. In this paper the salient features of the gyro and accelerometer designs are presented together with an overview of the IMU system architecture. Measurement results, with a focus on environmental characteristics and robustness, are included.