Multiple fiber optic twin-sensor-array based on Michelson optical low-coherence reflectometer

Abstract

The implementation of a large sensing network based on fiber optic sensors for the structural health monitoring of a new five arch stone bridge with a total span of 60 m over Vizela River (Felgueiras, Portugal) is presented. In addition to the implemented instrumentation with conventional sensors, four different types of fiber Bragg grating based sensors were specially developed for measuring temperature, strain and displacement in several critical points of the structure. In total, 85 Bragg sensors were installed and more than 800 m of fiber cables were deployed along the bridge. The fiber optic sensing network installed on the bridge was entirely monitored by using a single measurement unit designed for collecting and archiving the signals from all the fiber Bragg grating sensors, and also to allow remote access to the important data through a standard GPRS connection. 1.2 Why fiber optics? In general, modeling structures like the Vila Fria bridge brings several issues on how to define parameters to calibrate the mathematical models of the structure and on how to be certain about the results predicted by those models. Therefore, the possibility of monitoring an arch stone bridge from the beginning of its construction is of particular interest because it allows the installation of a large number of sensors according to a pre-established plan and accessing points impossible to achieve in structures that are already built. With the large amount of information that can be generated, improved calibration and models validation can be obtained. The fact that the bridge can then be monitored for a long period also allows the evaluation of its structural behavior and its relation to the usually observed pathologies in this kind of structures. Nevertheless, the establishment of a large sensing network for monitoring multiple parameters that are structurally relevant is not always easy when using conventional electric technology. In fact, conventional sensors suffer from high EMI/RFI sensitivity, environment induced drift and require individual electrical stimulus that makes large scale structural health monitoring highly complex, particularly in hazard-environments common in Civil Engineering monitoring applications. Fiber optic sensors, and in particular, fiber Bragg gratings, constitute a particularly appropriate and competitive alternative for structural health monitoring applications. In fact, fiber Bragg grating sensors add to the long recognized advantages of fiber optic sensors (e.g., immunity to EMI/RFI, remote monitoring, small size and weight, electrical isolation, intrinsically safe operation, high sensitivity, long-term reliability) the inherent multiplexing capability and the ability to provide absolute measurements without the need for referencing. Fiber Bragg grating technology is therefore becoming the natural substitute for the conventional sensing technologies by easing health monitoring of large structures during construction, load tests and long-term service. 1.3 Fiber optic monitoring project The monitoring of the Vila Fria stone masonry bridge pursued two main objectives: the establishment and calibration of analytical modeling techniques suitable for the numerical simulation of similar constructions; the evaluation and systematization of the main features of this type of structures in terms of their structural behavior and their relation with the most frequent structural pathologies. To achieve these goals, a vast instrumentation project was put forward to transform the bridge into a “live laboratory”, which included, in addition to the installation of several conventional sensors described in reference 6, the deployment of a large fiber sensing network for the measurement of temperature (28 sensors), displacement (48 sensors) and strain (9 sensors). Some of the sensors used in the bridge were specifically developed by FiberSensing for this project, as it is the case of the LPDS – Linear Position and Displacement Sensor, and the Long Gauge. Most temperature sensors were used for referentiation of both fiber optic and conventional sensors. The sensors were deployed in a tree network configuration with 15 branches. To interrogate this sensing network, a BraggMeter measurement unit containing an optical switch with 16 channels was adopted. In total, more than 800 m of fiber optic cables were installed in the bridge. Figure 2 shows a simplified scheme of the fiber optic sensing network architecture. Technical Cabinet Measurement Unit BraggMeter Fibre Optic Switch 1x16 (Built-in) Remote Transmission Module Optical cables to sensing network branches 1 16 Typical sensing branch (up to 10 sensors) LPDS Temp. sensor Strain sensor