Competing optical instruments for the estimation of Full Field FRFs

Thanks to the progress of optical measurement systems, known to use non-contact transducers (with their own peculiarities, application domains and drawbacks), nowadays addressing complex testing problems, especially direct dynamic measurements and modal analyses on lightweight structural components, is becoming even more common than in the recent past. Furthermore, obtaining vibration patterns with optical gears, in broad frequency bands with high spatial resolution, is easier than before, for the advantage of many advanced applications in vibration engineering, testing and product development. The characteristics and uniqueness of three different measurement technologies, based on distinguished physics and processing grounds, are here compared on a shared test rig with multiple excitations. SLDV, Hi-Speed DIC and Dynamic ESPI are here challenged in a complex and unique matching test for the estimation of FRFs with high spatial resolution, exploiting, as benchmark of the methodologies, a vibrating lightweight rectangular plate. The plate was selected for its exhibiting a broad band dynamics with high modal density and potentially repeated and closely spaced roots in the common frequency domain, where the techniques can find an operative intersection. The comparison of estimated FRFs and related Coherences, here as quality features of the measurements, is detailed with specific notes on acquisition and processing techniques, in order to draw extended conclusions on the full field measurement approaches, specific gears and the multiple excitation sources. The peculiar pointwise procedure, here proposed in details as the minimal paradigm to make a meaningful comparison, is addressed by means of discrete geometry transforms to put all the three technologies on an effective and consistent trial at each physical location of the real surface. The obtained results are a significant step forward in understanding the cross comparison of what these optical technologies, which belong to different domains and developing paths, can offer nowadays to further extend the experimental characterisation of vibrating components.