Tuning of Normal Modes by Multi-Axial Base Excitation

Normal mode testing is commonly performed on large aerospace structures in order to identify their eigenfrequencies, mode shapes, modal damping values, and generalized masses. Normally, multiple-point sinusoidal excitation is applied. For each normal mode, the exciter configuration is adapted in location and magnitude in order to compensate for the internal damping forces of the test structure. An alternative to multiple-point excitation is a driven-base test using a multi-axial shaker table. Multi-axis shaker facilities have been developed for structural dynamic qualification. However, they can be used for normal mode testing as well, whereby the overall structure is accelerated in space in order to compensate for the internal damping forces. This requires a suitable combination of all six spatial dof of the base excitation. In the case of resonance, the structure responds in quadrature related to the base excitation. If this criterion is fulfilled, the structure vibrates in an eigenmode of vibration in the sense of a sdof system. This article gives the theoretical background of the test method with particular emphasis on the damping estimation, on the determination of the generalized mass, and on finding a suitable base axis combination. A suitable test rig for the normal mode testing by base excitation, DLR's Multi-Axis Vibration Simulator (MAVIS) in Goettingen(FRG), is presented. The test procedure and test data evaluation as applied to a test structure characterized by closely spaced eigenfrequencies is presented. The advantages and disadvantages of normal mode testing by means of base excitation are pointed out