A High Stability Optical Shadow Sensor with Applications for Precision Accelerometers

Abstract

Gravimeters are devices which measure changes in the value of the gravitational acceleration, \textit{g}. This information is used to infer changes in density under the ground allowing the detection of subsurface voids; mineral, oil and gas reserves; and even the detection of the precursors of volcanic eruptions. A micro-electro mechanical system (MEMS) gravimeter has been fabricated completely in silicon allowing the possibility of cost e-effective, lightweight and small gravimeters. To obtain a measurement of gravity, a highly stable displacement measurement of the MEMS is required. This requires the development of a portable electronics system that has a displacement sensitivity of $\le 2.5$ nm over a period of a day or more. The portable electronics system presented here has a displacement sensitivity $\le 10$ nm$/\sqrt{\text{Hz}}$ ($\le 0.6$ nm at $1000$ s). The battery power system used a modulated LED for measurements and required temperature control of the system to $\pm$ 2 mK, monitoring of the tilt to $\pm$ 2 $\mu$radians, the storage of measured data and the transmission of the data to an external server.

8 Pages, 12 figures, 5 equations, currently submitted and under review at IEEE Sensors SIEL