Low-cost intensity-based plastic optical fibre sensing system for permanent structural health monitoring

Fibre optic sensors are attracting an increasing interest in structural monitoring applications because of their sensitivity combined with other unique properties such as immunity to electrostatic discharges, intrinsic fire safety and minimum invasiveness. However, their wide diffusion in permanent monitoring systems is still hampered by the costs of the interrogators. This limitation can be overcome using plastic optical fibres to fabricate displacement transducer based on the variation of the fibre link attenuation. The intensity-based measurement technique and the use of plastic optical fibres allow keeping the advantages of optical fibre sensors, while reducing the overall costs to that of conventional electromechanical sensors. In this paper the characterization of the developed sensing system, including the long term performances in a two-year in-situ test in a prestigious monument in Venice, is presented. Several transducers with different embodiments have been fabricated and tested achieving resolution better than 0.02 mm in a 3.5 cm range. An experimental evaluation of the temperature effects and of the temperature compensation methods is also considered.