Wide dynamic range direct accelerometer

Abstract

The acceleration input-digital-output sigma-delta modulator accelerometer and the finite-element analysis of the mechanical structure are described. The performance obtained from prototype units is also presented. Silicon micromachining techniques were used to fabricate a high-precision, micro-gee accelerometer. Operating in a closed-loop configuration, the accelerometer utilizes electrostatic field sensing and electrostatic force feedback. The sensor assembly consists of three silicon chips, bonded together at the wafer level. The micromachined sensor is operated in a vacuum to eliminate nonlinear viscous damping and to provide a high-Q second-order mechanical resonant circuit. Near-critical damping is provided by the closed-loop control system. The control system is a highly oversampled sigma-delta modulator that produces a wide dynamic range and a direct digital output. These accelerometers can be built to serve widely diverse markets at a reasonable cost. Frequency ranges from DC to thousands of hertz, accelerations from nano-gees to hundreds of gees, and dynamic ranges in excess of 120 dB can all be accommodated in small rugged silicon accelerometers.