A Normal Mode Identification Test Using Multiple Inputs

Dynamic load analysis for qualifying aerospace structures often requires that the analytical based modal models have modes which have been verified or derived directly from a normal mode identification test. Normal modes are the characteristic solution of a lumped parameter eigenvalue problem, and including measured normal modes in loads analysis assures the correct contribution of each mode in forming the linear combinations of the modal response. The difficulty in testing for normal modes is that most structures are not truly linear and have physical limitations which prevent placing the exciters in the ideal locations needed to isolate and define normal modes. A normal mode test approach utilizing multiple exciter inputs was successfully used on a TRW spacecraft. Several testing and analysis techniques were utilized and compared. The approach demonstrates the need for multiple indicators to assure confidence in testing.