Real and Complex Exciter Forces for Experimental Modal Analysis - A Simulation Study -.

Kurzfassung

Most modal analysis testing of aerospace structures is performed by applying the phase resonance test technique, and the experiences gained with this procedure are based on longtime applications. Most -fforts to improve this test technique were focused on the determination of the real exciter force distribution to isolate the individual normal modes. Nevertheless, in the absence of efficient force tuning and force placement procedures, many normal modes have been identified as complex normal modes only. Strong damping coupling effects were assumed to explain this dynamic behavior. Therefore, some publications treat the relationship between the real and complex normal modes and offer numerical algorithms for the calculation of the real normal modes. In the recent past the exclusive isolation of the complex normal modes was proposed, assuming that the excitation of these normal modes is simpler and more reliable. Based on these publications, the real and complex exciter force formulations were applied on an eleven-degree-of-freedom model. The mean error of the normal mode shapes is presented as a comparison of the test results. These results document advantages and shortcomings of the individual real and complex exciter force tuning formulations. Also taking into consideration the neighbouring normal modes, the discrepancies and the accuracy of most test results can be improved significantly. This parameter variation documents the requirement of consistent control of the dynamic responses of the test structure and shows that "complex normal modes" may also be a result of incomplete -xcitation. Taking into account the test philosophies of some popular phase separation techniques, most published "complex normal modes" merely seem to be the result of incomplete excitation, simultaneously regarding the small damping factors of aerospace structures.