A High Stability Optical Shadow Sensor with Applications for Precision Accelerometers

ABSTRACT

Displacement sensors are found in a variety of applications including gravitational wave detectors, precision metrology, tissue imaging, gravimeters, microscopy and environmental monitoring. Most of these applications benefit from the use of displacement sensors that offer both high precision and stability. This is particularly the case for gravimetry where measurements are often taken over multi-day timescales. In this paper we describe a custom-built microcontroller-based displacement sensor that has been utilised in a micro-electromechanicalsystem (MEMS) gravimeter. The system runs off battery power and is low-cost, portable and lightweight. Using an optical shadow sensor technique, and by designing a digital lock-in amplier based around a dsPIC33 microcontroller, we demonstrate a displacement sensitivity of 10 nm/Hz down to 300 s, and an rms sensitivity of 1 nm over timescales of one day. The system also provides real time monitoring/control of temperature,using an AD7195 ratiometric bridge to provide mK control of three separate PT100 sensors. Furthermore, a tilt sensor conditioning circuit is incorporated to drive a pair of electrolytic tilt sensors, resulting in the ability to monitor 2 axis tilt at the level of 1 microradian over approximately 1 day. The sensor system described is thus multifunctional and capable of being incorporated into precision accelerometers/gravimeters, or indeed other applications where long term displacement/temperature monitoring is necessary.

EXISTING SYSTEM :

Displacement sensors have many applications, one such application in research is the use of a shadow sensor in Advanced LIGO (aLIGO), the ground-based interferometric gravitational wave detector. Displacement sensors also have industrial applications such as environmental monitoring, biomedical sensing , microscopy, the manufacturing industry , hydraulic device position monitoring  and gravimeters. Recently a Micro-electro Mechanical System (MEMS) gravimeter has been fabricated and tested, demonstrating the possibility for small, low-cost and lightweight gravimeters. The MEMS chip is manufactured from a single piece of silicon, and is comprised of a proof mass suspended from microscopic springs (or flexures). The gravimeter requires a displacement sensor that is stable over timescales of days with sensitivities of nanometers over this period. The MEMS also requires the use of temperature control, tilt monitoring and conditioning electronics, whilst also fitting the criteria of being low-cost, lightweight and portable. For example, to measure a  useful accelerations of 40 µGal 1, a displacement sensitivity of ±2:5 nm is required for a 2 Hz resonator. The displacement of the MEMS proof mass is measured using the lock-in technique  with a shadow sensor such as the one used in aLIGO. Although the shadow sensor presented by N. A. Lockerbie et al has very good performance,     69 ± 13 pmrms/p Hz, this performance is at high frequency (500 Hz) in order to monitor violin modes of the suspension fibres. Our application requires stability over day timescales in order to sense earth tides. There is no available system which provides nanometer displacement stability over this time period, that can also monitor temperatures at the mK level, and provide tilt readout better than 1 µrad. The system also needs to be portable and run off battery power.

PROPOSED SYSTEM :

In this paper we describe a custom-built microcontroller-based displacement sensor that has been utilised in a micro-electromechanical nsystem (MEMS) gravimeter. The system runs off battery power and is low-cost, portable and lightweight. Using an optical shadow sensor technique, and by designing a digital lock-in amplier based around a dsPIC33 microcontroller, we demonstrate a displacement sensitivity of 10 nm/Hz down to 300 s, and an rms sensitivity of 1 nm over timescales of one day. The system also provides real time monitoring/control of temperature, using an AD7195 ratiometric bridge to provide mK control of three separate PT100 sensors. Furthermore, a tilt sensor conditioning circuit is incorporated to drive a pair of electrolytic tilt sensors, resulting in the ability to monitor 2 axis tilt at the level of 1 microradian over approximately 1 day. The sensor system described is thus multifunctional and capable of being incorporated into precision accelerometers/gravimeters, or indeed other applications where long term displacement/temperature monitoring is necessary.

 

CONCLUSION :

A highly stable optical shadow sensor has been demonstrated along with its digital readout and control. The system is shown to be able to measure displacements with a sensitivity of 0.6 nm over an integration time of 1000 s, whilst maintaining this sensitivity over periods of a day or longer. All functionality is obtained from a micro-controller-based, custom electronics board. This electronics board can measure and control several temperatures to ±2 mK; monitor changes in tilt to ±1 µrad; power and modulate an LED; convert µA of current to measurable voltages; demodulate a digitised signal; compute digital filters; and decimate the output signals. Whilst the shadow sensor has been designed for us in a low-cost portable MEMS gravimeter, the sensor could be re-purposed to serve in many precision sensing applications.