Optimization of continuous sensor placement for modal analysis: Application to an optical backscatter reflectometry strain sensor

The methods for experimental modal analysis are consolidated, as are the sensors used to perform this analysis. The characteristics of the most commonly used sensors are to be “discrete” and “independent”, that is, each sensor can measure the magnitude of interest in a single point and can be placed independently from the others on the structure. This kind of layout is widely used, for example, with accelerometers, but it presents a strong complexity if the structure is extended or characterized by high modal density in the frequency range of interest, or if the number of modes to be identified is high. Often the “discrete” sensors also have nonnegligible masses compared to the mass of the structure to be analyzed, thus introducing the well-known “mass effect”. Recently, new dynamic sensors have become available based on optical fibers such as the well-known Fiber Bragg grating sensors or the more recent Optical Backscatter Reflectometry (OBR) sensors. Both are sensors of the “continuous” type, or rather, they are a succession of sensors connected in series inside a single optical fiber. The purpose of this paper is to explore the use of OBR for modal analysis and introduce an original algorithm that allows the OBR sensor to be arranged in an optimized manner, to identify up to a certain number of modes in the most effective way and to respect the constraints imposed by the optical fiber itself, such as fiber length or maximum sensor curvature. The results are illustrated by means of the experimental results obtained on a structure characterized by a simple geometry.