Comparing Fixed-Base and Shaker Table Model Correlation Methods Using Jim Beam

Abstract

The key to any dynamic model correlation is an understanding of how the boundary conditions of the test article interact with the test data. Due to budget and schedule constraints, some spacecraft programs opt to correlate the spacecraft dynamic model during the Environmental Qualification Test, conducted on a large shaker table. While this saves cost to the spacecraft program, the base-drive analysis of the dynamic model incorrectly assumes the boundary condition between the shaker and the spacecraft to be completely fixed, except for the prescribed force input.

This paper follows-up research published in IMAC 36, “Comparing Free-Free and Shaker Table Model Correlation Methods using Jim Beam.” In that study a free-free impact modal test, a “fixed” base impact modal test on top of the shaker, and a base-drive vibration test were used to assess Finite Element Model (FEM) correlation using different boundary conditions. The NAVCON Jim Beam, a simple and well characterized structure featured in the round robin tests of IMAC 27, was chosen as the test article. Conclusions showed that due to the non-linear compliance of the shaker table, most time would be spent accounting for the boundary condition in the correlation, rather than correlating the test article itself.

Previous testing was conducted with the Jim Beam flush mounted to the shaker table, which restricted the motion of the bending and shear modes. To mitigate this constraint, this paper included the use of “donut” force gauges inserted between the shaker table and the Jim Beam. Not only was the direct force input of the vibration test measured, but the gauges acted as spacers which relieved the constraint on mode shapes caused by contact with the shaker table. This constraint was a source of error in the previous modal data.

The premise of this follow-on study is the same as before; to compare test data of the same structure with identical instrumentation across different boundary conditions. First, a fixed base modal impact test of the Jim Beam was conducted on a slip table to approximate a modal plate. During this test the Jim Beam was mounted on four disconnected force gauges to simulate the same bolted interface as the shaker table. Second, the Jim Beam was transferred to a large shaker table and a vibration test was conducted. Results of the two tests were compared to investigate the validity of using environmental test data alone to correlate a dynamic model.