On the identification of rigid body properties of an elastic system

This paper presents a procedure for identification of the full set of rigid body properties (overall mass, center of gravity location and moments of inertia) of a given elastomechanical system which is based on experimental modal analysis. The approach uses measured inertance frequency response functions (FRF) up to the first elastic natural frequency of the free/free system which are obtained either by testing the system suspended in soft springs or by resting the fixed/free system with an additional measurement of the interface forces. In a first step the underlying rigid body response is extracted from the FRFs by taking the influences of the elastic modes into account. A second step then uses these data to estimate the rigid body properties. The theoretical foundation of the identification procedure is presented as well as the requirements for setting up an appropriate vibration test. Furthermore the procedure will be classified with respect to existing methods and advantages and disadvantages are compared. A fan/motor unit which is used in air conditioning systems of cars has been tested and the identification procedure was applied in order to estimate the rigid body properties. The test set-up will be described and the identification results will be presented, In addition a comparison to results coming from standard pendulum testing will show a very good correlation and thus emphasizes the efficiency of the method.