Uncertainty in vision based modal analysis: probabilistic studies and experimental validation

This paper analyses the issues related with the application of photogrammetric methods in the context of modal analysis. In this field of mechanical testing, the vibrations of the measurand are monitored. The possibility to measure displacement of several monitored points without directly instrumenting those, pushes the interest toward the application of an image based measurement system. However, the process of acquiring a moving target with a camera is complex due to the motion blur phenomenon. The effects of this particular phenomenon on uncertainty generation and propagation are discussed in parallel with Monte Carlo simulation and experimental approach. The case study here proposed focuses on the estimation of the main modal parameters (first two resonant frequencies and amplitudes) of a structure with the help of 2D Blob Analysis, with particular focus in comparing the performances of photogrammetry with the ones of classic motion transducers. The results of both the approaches confirm that the choice of using photogrammetry as an input for modal analysis should be made carefully, since the actual behavior of a structure can be distorted. Nonetheless, the results show that particular care should be made in the choice of structural excitation when using vision based techniques.