Development of two-layer multiple transmitter fibre optic bundle displacement sensor and application in structural health monitoring

Abstract

This paper presents a study of fibre optic bundle displacement sensor with multiple layers of transmitters. By arranging the transmitter of two-layers in certain ratio, an enhancement of the linear range as well as sensitivity can be achieved compared to conventional bifurcation arrangement. A theoretical model using the Gaussian beam assumption has been developed to characterise the response curve of the various two-layer configurations. The designed two-layer multiple transmitter fibre optic sensor is fabricated to validate the results. The predicted displacement response shows an enhancement of the linear range and sensitivity of four and two times, respectively, better than the corresponding ranges for the conventional bifurcation configuration and is validated by experimental static calibration. Harmonic excitation test is employed to deduce its dynamic displacement response behaviour. Results show constant sensitivity of about 0.363mV/μm (absolute deviation less than 5%) over a bandwidth up to 2.2kHz with minimum measurable displacement amplitude of about 50nm at 2.2kHz and signal-to-noise ratio of around 26dB. The minimum measurable displacement is found to be 18nm at 3.4kHz with a signal-to-noise ratio of around 6dB. Finally, application of the proposed sensor in structural health monitoring is demonstrated experimentally with comparison of conventional contact and non-contact sensors.