A High Stability Optical Shadow Sensor with Applications for Precision Accelerometers

TL;DR

This paper presents a highly stable optical shadow sensor integrated into a portable electronics system for precise displacement measurements, enabling improved MEMS gravimeters for geophysical applications.

Contribution

The development of a portable, highly sensitive displacement measurement system with stability over a day, suitable for MEMS gravimeters and other precision sensors.

Findings

Displacement sensitivity of ≤10 nm/√Hz achieved

System maintains stability over 24 hours

Temperature and tilt control enhance measurement accuracy

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 $\leq 2.5$ nm over a period of a day or more. The portable electronics system presented here has a displacement sensitivity $\leq 10$ nm$/\sqrt{\textrm{Hz}}$ ($\leq 0.6$ 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.