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Distinguishing physical modes from mathematical modes in the modal analysis of complex systems, such as full vehicle structures, is a difficult and time-consuming process. The major tools frequently used are stabilization diagrams, mode indicator functions, or modal participation factors. When closely spaced modes are to be identified, the stabilization diagrams and mode indicator functions are no longer effective. Even the reciprocities of mode shapes and modal participation factors cannot be well satisfied to indicate whether a mode is a physical one, when measurement errors are large. To overcome these difficulties, an optimization procedure is developed, whereby physical modes can be sorted out in a given frequency range while the error between measured and synthesized frequency responses is minimized. An optimal subset selection algorithm is used in this procedure.

A testing procedure has been developed for measuring rotation/moment impedance functions. At connection points between components, both rotations and moments have important contributions in describing the dynamic characteristics of a coupled system. Analytically, both rotations and moments are included at connection points and are necessary for achieving a high fidelity model of a system. Experimentally, these effects have been historically neglected since no acceptable rotational transducers exist. If high fidelity impedance models are to be developed from experimental data, it is important to measure rotational impedance functions. In this paper a testing method is developed which uses the motion of a rigid body attached at a point of interest to determine displacements, forces, rotations and moments at the point of interest.