The Vibrofiber sensor is a Fabry-Perot cavity formed between two broadband fiber gratings creating interference fringes. It was introduced three yearsago to monitor the vibration and temperature rise of the stator end winding in apower generator. [1] This paper will discuss the use of Vibrofiber to monitor thedeflection of the bridge under adverse conditions: wide temperature swings,excess load, strong winds, earth quakes, etc. The fringes in these cavity sensorshave features like peaks and valleys, which are sensitive to temperature andstrain. When the bridge becomes over loaded, we are interested in knowing theextent of the deflections; in addition, we might want to locate the cause of theoverload. A simple Sagnac FBG interferometer has been invented to providesuch diagnostics. A pair of long fiber with such cavity sensors can be installedon the underside of the target bridge segment between two supporting columns.The objective is to monitor the deflection together with any torsion in the bridgedeck. Each of these 2 long fiber segments has a pair of cavity sensors, one tomeasure the deflection as a result of the excess strain, another to measuretemperature and provide compensation for the deflection data. An array ofcavity sensors with different center wavelengths will be used to support thetypical multi-segment bridge structure. The interrogation unit is based on atunable laser that can be programmed to hop to different ITU grids. A separateDFB laser will run a grating based Sagnac interferometer, measuring weight inmotion, identifying the speed and the maker of vehicle in traffic, and providingdeflection diagnostics. Overloaded trucks and speeding vehicles can be capturedand tagged for corrective actions. The interrogation unit is equipped withwireless Ethernet communication enabling the monitoring of many bridges froma central location; similarly, warning can be initiated to alert the central trafficcontrol ahead of any problems.
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