SELECTION OF COMPONENT MODES FOR CRAIG-BAMPTON SUBSTRUCTURE REPRESENTATIONS

Three measures of modal dynamic importance are studied for the purpose of ranking Craig-Bampton substructure fixed interface mode shapes bas ed upon their contribution. to forces at the substructure interface, m odal velocity, or modal displacement. These measures can be employed t o identify, mode shapes which are dynamically Research Assistant. impo rtant and thus should be retained in a reduced analytical representati on, or identified during a modal survey of the substructure. The first method considered, Effective Interface Mass, has been studied previou sly. However, new results are presented showing the relation between E ffective Interface Mass and a commonly used control dynamics measure o f modal importance called approximate balanced singular values. In con trast to the general case of approximate balanced singular values, Eff ective Interface Mass always gives an absolute measure of the dynamic importance of mode shapes. The EIM method is extended to consider moda l velocity and modal displacement outputs. All three measures are appl ied to a simple substructure called the General Purpose Spacecraft. It is shown that each of these measures provides an efficient method for ranking the dynamic importance of Craig-Bampton fixed interface modes such that a reduced representation will accurately reproduce the subs tructure's response in the frequency range of interest.