Experimental Modal Substructuring to Extract Fixed-Base Modes from a Substructure Attached to a Flexible Fixture

It is well known that fixed boundary conditions are often difficult if not impossible to simulate experimentally, but they are important to consider in many applications. Mayes and Bridgers ["Extracting Fixed Base Modal Models from Vibration Tests on Flexible Tables," IMAC XXVII, Orlando, Florda, 2009.] recently presented a method whereby one can determine the fixed base modes of a structure from measurements on the system and the fixture to which it is attached. They used modal constraints in conjunction with an admittance approach to perform the substructuring computation, demonstrating that the first fixed-base mode could be estimated accurately using the coupling procedure, even though the free fixture had a mode at almost the same frequency. This work builds on that by Mayes and Bridgers, but employs modal substructuring instead of frequency-response based methods to estimate the fixed-interface modes. The method is validated by applying it to a experimental measurements from a simple test system meant to mimic a flexible satellite on a shaker table. A finite element model of the subcomponents was also created and the method is applied to its modes in order to separate the effects of measurement errors and modal truncation. Excellent predictions are obtained for many modes of the fixed-base structure, so long as modal truncation is minimized, verifying that this modal substructuring approach can be used to estimate fixed-base modes of a structure without having to measure the connection point displacements and rotations and even with the limitations inherent to real measurements.