PLASTIC OPTICAL FIBER BASED SENSING SYSTEM FOR CRACK MONITORING

In this paper we present a low-cost system for crack evolution monitoring based on plastic optical fiber displacement sensors. The system is devised to perform continuous monitoring of civil engineering structures with performance similar to commercial devices based glass optical fiber sensors, but with a remarkably lower cost. Several sensors have been fabricated and tested, showing good sensing capabilities i.e. working range up to 3.5cm, resolution 10μm and repeatability 5μm. A closed-loop light modulation/detection scheme is implemented to reduce the environmental noise sensitivity. The developed system includes two types of control units, where up to eight transducers can be connected to provide multi-point distributed sensing and that can be interfaced to a PC either via USB port or via GSM/GPRS modem. The later allows remote monitoring and automatically sends periodical reports of the measurements and is able to issue warnings in case of displacements above a given threshold. Extensive measurements on a test-bench and in an environmental chamber have been carried out to extract calibration curves and to find suitable compensation methods for temperature fluctuations. Preliminary tests of an 8-sensors prototype have been carried out in a heritage building in Venice.