1. RWAform broadband signal. A JR3 load transducer measures interface loads, and the six Kistler 8630B5 Piezobeam accelerometers shown in Figure 7 measure accelerations. Figure 8 shows the six different loading scenarios used to obtain the RWA dynamic mass matrix. In (a), the stinger attached to the small plate below the load transducer applies a white noise in the RWA's +x direction. While this is mostly an fxforce, it is not applied perfectly along the x-axis and is not applied exactly at the interface. Therefore it includes elements of force and moment in six axes, all of which are measured by the load transducer. Configuration (b) is similar but rotated by 90degreessothat fydominates. In(c),(d),and(e),aleverarmisusedtoapplymoments at the interface. The lever arm was designed to be stiff, yet light, so it would not alter the dynamics of the assembly. In (c), a +x force is applied with a moment arm in z, creating a load dominated by fxand myat the interface. In (d), the same force is applied with an additional moment arm along y, creating a load dominated by fx, my, and mz. (e)is similar to(c),butrotated by 90 degrees so that fyandmxdominate.Finally,(f)provides anfzdisturbance.Withthesesixconfigurations,allforce and moment axes are excited. In each of the six tests, a Tektronix Fourier Analyzer DAQ records the white noise disturbance, the six accelerometer voltages, and the six transducer voltages. It then computes in real time the transfer functions
2. FIG. 8: Six"Free-Free"RWA LoadingConfigurations
3. Error [%] 1 21.6 21.7 -0.6 2 28.3 28.4 -0.5 3 68.9 63.8 +8.0 4 87.1 88.6 -1.7 twomodesisthuscriticalinvalidatingtheFEM,andthe error in the higher modes is acceptable. Note that the dominant modes lie at 21.7 Hz and 28.4 Hz, and since the RWA used in this study is spun up to 2590 RPM, or 43 Hz, it excites these modes. Hence this structure and RWA are well-suited for acoupling study. In the coupled validation test, the wheel, mounted as showninFigure11,isspunandaKistler8630B5Piezo-beam accelerometer is used to measure the structure's performance. The accelerometer signal is passed through the same signal conditioning unit used in the dynamic mass tests and is recorded by the dSpace DAQ as a voltage time history. The same conversion factors used in the dynamic mass tests are used to convert the accelerationfromvoltstometric units,m/s2.Finally,the time history is processed to yield the acceleration PSD, which is converted to a displacement PSD using Equation(29).