Evaluation of a pure moment exciter for measurement of rotational DOF frequency response functions

Experimental data in the form of a mobility matrix is needed for a variety of purposes including substructure synthesis, Structural Dynamics Modification (SDM) and updating of Finite Element Models (FEM). A computational based mobility matrix contains responses from both translational and rotational components. Experimentally, the rotational components are usually ignored because of the inherent difficulty in their measurement. A review of previous direct and indirect efforts to estimate rotationally excited frequency response functions is presented. Next a novel direct experimental method to measure translation and rotation responses from a pure moment excitation is discussed. The method uses a set of geared eccentric masses to create a pure moment. The theory of operation is discussed first. A set of laboratory tests was performed to evaluate the performance of a prototype device. Analytical predictions were nominally within 5% of the experimental values for the desired moment excitation. However, unwanted forces and cross axis moment excitations were also measured. Simulations indicate that non ideal performance results from manufacturing and assembly variations. The investigation concludes with an assessment of the method's potential for direct experimental measurement of rotational components in the mobility matrix.