Identification of modal parameters from transmissibility measurements

It is a well-known fact that linear dynamic behavior of structures can be studied by modeling the relation between force(s) [input(s)], acting on a structure, and their resulting structural vibration response(s) [output(s)]. For industrial structures, in their real in-operation conditions, it often becomes hard (or impossible) to experimentally measure the excitation. For this reason system identification techniques have been developed that work on a basis of response data only. However, current output only techniques have serious limitations when applied to some practical cases. One limiting constraint of the current OMA techniques is that the non-measured excitations to the system in operation must be white-noise sequences. In practice however, structural vibrations observed in operation cannot always be considered as pure white-noise excitation. Current techniques may encounter difficulties to correctly identify the modal parameters. In this paper a new OMA approach to identify modal parameters from output-only transmissibility measurements is introduced. This method does not make any assumption about the nature of the excitations to the system. In general, the poles that are identified from transmissibility measurements do not correspond with the system's poles. However, by combining transmissibility measurements under different loading conditions, it is shown in this paper that modal parameters can be identified. In this contribution a numerical experiment on a cantilever beam was conducted. A comparison was made between the results of a classic input–output and the new output-only modal analysis based on transmissibility measurements.