On the Role of Modal Coupling in Model Updating

In this paper a new technique is proposed for updating parameters in a Finite Element (FE) model based upon modal coupling. Modal coupling exists between vibration modes with respect to parametric variations and is often associated with curve veering and vibration localisation. Examination of the modal coupling leads to the development of a new set of system properties, providing a different emphasis for updating schemes. In particular, the properties convey information about the system which cannot be deduced from the eigenvalues alone, yet can be measured experimentally more accurately than the eigenvectors. This information is found to be especially useful in nominally symmetric or periodic systems where parameters may be indeterminable using traditional techniques. It is thought that the new methods may find particular application in the tuning of bladed rotor assemblies. The presentation begins with a discussion of veering and localisation, and its manifestation in symmetric structures. The modal coupling and related properties are then derived, in tandem with methods of extracting these quantities from experimental data. Next, the FE model and updating scheme are outlined, and to conclude an experimental example is given, demonstrating the arrival at a unique solution to an otherwise ambiguous updating problem. Nomenclature β angle between current eigenvector set and datum eigenvector set in the mass-normalised basi