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.
Highlights
G RAVIMETERS are devices used to measure the local acceleration of gravity, g
We demonstrate a micro-controller based electronics board capable of not just the displacement sensiti√vities required in measuring changes in g of ≤ 40 μGal/ Hz, which, was successfully taken into the field[CITEFIELD] for gravimetry measurements, but a high stability optical displacement sensor complete with electronic readout and control that could be used for other precision sensing applications
Whilst the shadow sensor has been presented to be used in a small, battery-powered, cost efficient and lightweight Micro-electro Mechanical System (MEMS) gravimeter, the shadow sensor could be re-purposed to serve in many precision sensing applications
Summary
G RAVIMETERS are devices used to measure the local acceleration of gravity, g. To obtain the necessary displacement sensitivity of the MEMS proof mass, a large suite of electronics is required to: modulate and demodulate the LED/signal, measure and control the temperatures via use of digital to analogue converters (DACs), convert the photocurrent to a voltage (IV converter), convert the analogue signals to digital signals and to perform filtering. It was noted that as the system was miniaturised, a small and cost effective lock-in amplifier would have to be designed and created It was observed from [11] [12] [13] [14] that digital lock in amplifiers could have the necessary performance for the gravimeter system and could even be implemented in low. Further thought behind designing a custom board is that many of the individual pieces of electronic equipment required all cost in excess of £1000 each This would not be in keeping with the main objective of portability and affordability of the complete system. We demonstrate a micro-controller based electronics board capable of not just the displacement sensiti√vities required in measuring changes in g of ≤ 40 μGal/ Hz, which, was successfully taken into the field[CITEFIELD] for gravimetry measurements, but a high stability optical displacement sensor complete with electronic readout and control that could be used for other precision sensing applications
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