Low-Cost Adaptive Array Demodulation of Bragg-Grating and Interferometric Sensors for Health Monitoring of Marine Structures

摘要

Fiber Bragg Orating (FBG) sensors are an increasingly important emerging technology in the area of intelligent structural health monitoring (SHM) of civil, mechanical, naval and aerospace structures. A large number of FBG sensors can be easily written in a single fiber. For SHM applications, FBG sensors are typically used to monitor static or dynamic strains. The strains cause spectral shifts in the FBG sensor transmittivity (equivalently reflectivity). The spectral shifts can be monitored in several ways with an appropriate demodulator. For spectrally-encoded FBG sensors, extant demodulation schemes can be classified into three categories, namely: scanning type, spectrometry- based and interferometry-based. Scanning type techniques include Fabry-Perot scanning filters, acousto-optic tunable filters, and tunable laser sources. All these scanning type techniques suffer from the fact that at any instant only one FBG sensor can be interrogated. Such approaches are not applicable if all the sensors have to be interrogated simultaneously for the purpose of monitoring impact signals and acoustic emissions. Spectrometric methods suffer from low sensitivity and are not suitable for dynamic measurements if several sensors have to be active at all times. Interferometric methods such as the Mach-Zehnder interferometer are ideally- suited to monitor dynamic strains; however they require electronic feedback to actively compensate for any quasistatic drift to maintain the inter ferrometer at quadrature. This makes the cost of multiplexing high since each sensor requires its own feedback system. A cost-effective and parallel demodulation scheme for arrays of FBG sensors is therefore necessary. In work over the past two years, we have demonstrated a novel two-wave mixing (TWM) wavelength demodulation scheme for FBG sensors that has the ability to compensate for quasistatic drifts without the need for active stabilization.