Extraction of free-free modes from constrained vibration data for flexiblemultibody models

This paper presents a method for identifying the free free modes of a struc ture by utilizing the vibration data of the same structure with boundary co nditions. In modal formulations for flexible body dynamics, modal data are primary known quantities that are obtained either experimentally or analyti cally. The vibration measurements may be obtained for a flexible body that is constrained differently than its boundary conditions in a multibody syst em. For a flexible body model in a multibody system, depending upon the for mulation used, we may need a set of free-free modal data or a set of constr ained modal data. If a finite element model of the flexible body is availab le, its vibration data can be obtained analytically under any desired bound ary conditions. However, if a finite element model is not available, the vi bration data may be determined experimentally. Since experimentally measure d vibration data are obtained for a flexible body supported by some form of boundary conditions, we may need to determine its free free vibration data . The aim of this study is to extract, based on experimentally obtained vib ration data, the necessary free free frequencies and the associated modes f or flexible bodies to be used iii multibody formulations. The available vib ration data may be obtained for a structure supported either by springs or by fixed boundary conditions. Furthermore, the available modes may be eithe r a complete set, having as many modes as the number of degrees of freedom of the associated FE model, or an incomplete set.