Modal analysis of vibrating structure using a Fiber Bragg Grating system

In this paper, modal analysis is carried out by using a Frenet frame-based shape measurement algorithm. This algorithm is validated through experiments conducted with a long slender, aluminum cantilever structure (65.625′′×2.0′′×0.125′′), which is instrumented with eight serially multiplexed fiber Bragg grating (FBG) sensors. The multiplexed FBG sensors measure the bending strain distribution along the cantilever structure, and this distribution is used to calculate the dynamic shape of the structure forced by a base excitation. When excited close to a natural frequency, the experimentally obtained shape deviates from the corresponding mode shape of the cantilever beam. This deviation is related to the influence of other modes of the cantilever beam structure. The structural shape data was processed by using modal analysis techniques to determine the modal coefficients and the associated spatial modes that best represent the structure’s vibration. The results obtained for the modal coefficients are found to compare well with results of Fourier transform analysis (FFT) of signals recorded over time. This type of real-time analysis may be useful for applications in structural health monitoring where changes in the modal coefficients are used as indicators of the damage to the structure.