4.3 Acoustic “Truth” Test The acoustic test setup is shown in Fig. 4.3. This serves as our “truth” test, or the actual environment for this research. The test was performed at the Sandia National Laboratories acoustic test facility with the direct-field acoustic test (DFAT) approach. Twelve VT-99 model speaker cabinets and six low frequency model VS-Q speakers surround the test article in eight speaker stacks. Multiple Input Multiple Output (MIMO) control was accurately maintained at the six control microphones. All six microphones had the same flat topped drive specification with 0.1 coherence between microphones. The drive levels were at the limits of speaker/amplifier capability. Dozens of accelerometers were mounted on the skin, rack and yoke and dozens more were on internal components. Twelve other response microphones were also placed around the unit. The industrial test structure was supported on its rack with a large yoke made of steel bars, which allowed the structure to be suspended and rotated into a vertical position to facilitate the placement of speakers radially around it. (The rack is not visible in this view, but the steel bars of the yoke are visible). 4.4 MIMO Simulation Test The MIMO shaker simulation test was set up in a modal testing laboratory at Sandia National Laboratories. The rack and industrial structure were suspended as shown in Fig. 4.1. Several different shaker arrangements were executed for various purposes not germane to this paper. The MIMO simulation configuration was chosen based on information gained from these configurations. 4.4.1 MIMO Simulation Test Setup The MIMO simulation configuration had two floor mounted shakers attached to the belly of the industrial structure, and four attached to the rack. Two lateral shakers were attached near the forward and aft ends of the rack and two more vertically oriented shakers were attached at the forward and aft ends of the rack. The vertical shakers had a 21.5 tilt from vertical simply to keep the shaker from interfering with the bungee cord support. (See the vertical shaker in Fig. 4.2). Flat random voltage inputs which produced about 45 N RMS force from 25 to 4000 Hz were utilized to calculate accelerometer response to input voltage FRFs for the six shaker drives. One hundred averages were used with a frequency resolution of 2.5 Hz. These FRFs were used to back calculate input voltages to drive the shakers to attempt to produce the same accelerometer power spectral densities (PSDs) as were measured in the acoustic “truth” test. The simulation was adjusted in steps with three iterations to the final physical simulation. 37 internal accelerometers were chosen as accelerometers of interest for which the shaker simulation attempted to match the acoustic test PSDs. 4.4.2 MIMO Simulation Test Results A metric which provides a single plot comparison of the overall match of the simulation was the sum of the 37 PSDs. This sum of PSDs for the acoustic test (blue) and the vibration test (red) is shown in Fig. 4.4. For most of the band, the vibration test closely envelopes the acoustic test. The band from 3500 to 4000 Hz does not match near as well as the rest of the frequency range. In Fig. 4.5 is shown a couple of the PSDs from individual accelerometers that gives an indication of the “best” and “worst” gage responses. In the worst gage, there are several bands where the vibration test overshoots the desired PSD. These results are very encouraging, and the vibration simulation with six shakers is certainly much better than could be obtained by setting the unit on a shaker table and attempting to control all 37 gages. There was a portion of the structure that had some intermittent nonlinearity. In Fig. 4.6 one can see the PSD of an accelerometer in this region. The plot shows the truth test result that we desire to match in blue. The red plot shows one physical vibration simulation measurement and the magenta plot shows another physical vibration simulation where everything in the vibration test was repeated exactly as before. The nonlinearity is intermittent. 32 R.L. Mayes and D.P. Rohe
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