Measurement of rigid body modes for dynamic design.

For experimental building block approach or substructuring in dynamic design, the measurement of rigid body modes of each component of the structure in free-free state is required. The rigid body modes influence much the level of total response after assembling the components. This paper first describes the necessity of such rigid body modes in building block approach by using a numerical simulation. By taking into account the rigid body motion of a component in free-free state analytically, the method is developed to determine the location of the center of gravity, the moment of inertia around each principal axis based on measured level of frequency response function in low frequency range. The computer program developed is applied to a simple beam structure to confirm that the correct rigid body modes can be obtained. For a complex structure, the rigid body modes of a engine whose geometry is very complex are appropriately determined by the method proposed.