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Abstract
Fibre Bragg Grating based strain sensing systems have found important applications in structural health monitoring of civil infrastructures such as bridges, tunnels and buildings. In my thesis research I systematically investigated on various factors that affect the short-term and long-term performance of fibre Bragg grating strain sensing systems using the Fibre Fabry-Perot Interferometer (FFPI) Tunable Filter (TF) as interrogator.
A number of issues concerning the short-term system performance, such as system tuning frequency, signal spectral position shift, electrical nonlinear effect of TF, noise and algorithm efficiency, are identified and investigated. A number of other issues concerning the long-term system performance, such as thermal and electrical polarization effects of TF, and data acquisition (DAQ) card calibration, are identified and investigated as well. Based on the investigation on these factors, related techniques have been developed to minimise the effects of these factors and enhance system performance.
Several techniques are developed to improve the short-term system performance. To eliminate signal position shift and increase system tuning frequency on the short-term performance, a parallelly synchronised operation mode of DAQ card and CPU is proposed. Compared with the traditional serial mode, the parallel mode is more efficient and stable. The electrical nonlinearity of TF is compensated using a correction curve obtained from DC-AC test which determines the TF electrical response. The system induced noise are suppressed by a carefully designed FIR low pass filter which makes fully use of CPU signal processing capability and does not affect system tuning frequency. In order to achieve the best system performance, a proper algorithm to determine the FBG wavelength shift from data points has been selected and optimised.
Several techniques are developed to improve the long-term system performance. To suppress the thermal effect of TF on the long-term performance, the relationship between signal wavelength shift and temperature of TF is investigated and determined experimentally. Also the polarisation effect of TF on system performance is tested and a specific duration is required to ensure the polarisation of TF saturated and its effect on system performance minimised.