5. Pretest Analysis for Modal Survey Tests Using Fixed Base Correction Method

A fixed base correction method that uses acceleration constraint shapes as references to transform flexible or dynamically active boundary conditions into fixed boundaries has been recently implemented for modal tests. The method uses test data directly to generate constraint shapes associated with accelerometer measurements at the test article and test fixture interface that are then used as references when calculating corrected fixed base frequency response functions (FRFs). The main challenge with the method is that at least one disturbance source, such as a modal shaker, must be applied to the boundary structure for each constraint shape used, so it is advantageous to understand how many constraint shapes may be required to fix a boundary for test planning purposes. This paper outlines a procedure that uses multipoint constraint equations in an analysis model of an integrated test article and its test fixture to determine the number of exciters necessary to apply the fixed base correction method. The method is verified by comparing mode shapes of the fixed base test article to the system model with a number of multipoint degrees of freedom constrained.