Fiber Bragg grating sensors based system for strain measurements

Abstract

This paper is aimed at describing the realization of an electronic system for interrogation of fiber optic strain sensors for several application fields. One of the most important industrial sector where fiber sensors are becoming more popular is the aerospace industry, where the main applications are oriented to the strain monitoring inside metal or composite structures. These analyses are aimed mainly at obtaining higher safety level, higher structural reliability and lower maintenance and operative costs of aerospace structures. In particular, FBG sensors can be employed in the nondestructive control sector applications as structural test, structural health monitoring and loads monitoring. The work is centered on fiber optic Bragg grating (FBG) sensors, due to the possibility to detect strains in specific points of complex structures getting useful information for design and stress analysis. This application is of particular interest in the case of composite structures, fiber metal laminates and bonded structures. In fact, in these cases fiber optic sensors, thanks to their reduced dimensions, can be embedded in the part at critical points during the lamination or bonding process. In fact, fiber optic Bragg sensors, protected with a suitable coating like poly amide, can afford the autoclave cure cycle, without any damage, up to 9 bar, 180 /spl deg/C typically used for consolidation of composite parts. FBG sensors can be interrogated with different type of opto-electronic instrumentation and the resulting information, typically the electrical output signal produced by a photodetector, has to be acquired, sampled and elaborated by means of a dedicated electronic system. From a practical point of view, two different solutions could be adopted to implement the acquisition/processing of the data: a specialized hardware or a general purpose system. The first solution should lead to high effective, low cost systems, but the leak of flexibility, joined to high design cost and time, make the application-specific-system solution not a feasible one, except in the case of application with a large market. As counterpart, a solution based on a standard platform represents a valid answer to the flexibility needs. From this point of view the Pcbus extension for instrumentation (PXI) represents the best trade off between low cost PC platform and high performances devices as the VMEbus extensions for instrumentation (VXI) onces. Moreover, the advanced timing and synchronization available in the PXI devices allow precise time-based measurements to be performed. In the work the authors present an advanced interrogation system characterized by a high grade of portability and upgrading realized by using standard technology. The internal structure of the system and the user friendly graphical interface are also deeply described.