Study of Environmental Survivability and Stability of Folded MEMS IMU

This paper presents a study on the structural rigidity of an Inertial Measurement Unit (IMU) implemented using the Folded MEMS approach. For such folded structures, environmental survivability and stability is a natural concern. We investigated IMUs of 35 mm(3) (small) and 130 mm(3) (medium) in volume, in the shape of a cube and a pyramid, and considered the reinforcement methods, including epoxy, eutectic solder, and silicon welding. Experimental characterization of a medium cube with eutectic reinforcement showed the maximum tilt of 0.2 mrad on the cube's sidewalls under temperature variation 25 degrees C90 degrees C and the tilt of 0.2mrad under 10Hz to 20kHz vibration. An IMU error model was implemented, with the target sensors performance of tactical-grade, using in- run bias stability and scale factor (SF) stability for gyroscope (1 degrees/hr, 100ppm) and for accelerometer (100 mu g, 100ppm), and a pre- defined trajectory with 122s flight time. The 0.2 mrad misalignment for the medium eutectic-soldered cube corresponded to a Circular Error Probable (CEP) rate of 1.25 nmi/hr, and a typical tactical grade IMU corresponded to CEP rate <= 1nmi/hr. We also concluded that the 35mm(3) eutectic-bonding reinforced cubes will survive the mechanical shock under 10,000g, while silicon welding reinforced cubes of 35 mm(3) would survive the shocks of 20,000g.