Identification of Principal Rigid Body Modes Under Free-Free Boundary Condition

This paper focuses on the problem of identifying all individual principal rigid body modes and the associated mass or principal inertia of moment, which can be called modal mass, of flexible structures under the free-free boundary condition with fewer multi-location excitations than the number of those modes. The rigid body mass matrix of the structure can be identified by using both the parameters of inertia, which are determined previously by a modal parameter estimation, and the coordinates of measurement points. As all rigid body properties can be obtained from the mass matrix, it becomes possible to simulate the FRFs between any two measurement points with inclusion of the contribution of rigid body motions even by only experimental modal analysis technique. First, the theory is explained. Then, a numerical simulation and two actual identifications for a plate structure and an automotive body component are carried out to demonstrate the validity and the usefulness of the theory.