422 Y. Soucy and F. Brassard Table 35.1 Test-based modal parameters of the rover Mode no Frequency (Hz) Damping ratio (%) Main axis ofmotion Description 1 4.5 2.1 Y Wheel mode: rigid body translation of rover in Y axis 2 5.1 8.1 X, Y Wheel mode: most likely rigid body translation mode of rover in X axis, coupled with mode at 4.5 Hz 3 6.2 0.7 X, Y, Z Mode not well defined and corresponding to small peaks appearing in FRF’s in Z axis. Strongly coupled with mode at 5.4 Hz 4 8.8 4.3 X, Y, Z Wheel mode: rigid body translation of rover in Z axis, strongly coupled with lower frequency modes 5 13.0 4.0 Z Wheel mode: rigid body rocking mode of rover about X axis 6 19.0 1.3 X, Y Bending mode of avionics box in X axis 7 22.0 1.7 X, Z Torsion mode of avionics box, strongly coupled with mode at 19 Hz 8 24.7 3.0 Y, Z Bending mode of gear box in Y axis 9 26.5 2.9 Z Bending mode of avionics box in Z axis 10 27.8 2.2 X, Y, Z Likely to be bending mode of battery box in Y axis, highly coupled with some other modes 11 29.2 2.4 X, Y, Z Likely to be bending mode of shaft attaching the gear box to main frame of rover, highly coupled mode 12 30.2 1.7 Y Bending mode of shaft 13 36.1 2.6 X, Y Not clearly identified, but related to dynamics in gear box, or in the shaft 14 41.2 2.7 X, Y Likely to be torsion mode of battery box 15 48.6 2.3 X, Y, Z Second mode of battery box in Y axis or related to dynamics in the shaft 16 53.9 2.4 X, Y, Z Appear to be a combination of bending and torsion modes of the shaft After these preliminary runs, it was originally planned to perform other runs with different exciter configurations in order to eventually excite the rover in all three orthogonal directions. For each exciter configuration, two complementary runs were to be performed in order to get data for all 60 accelerometers; each run was to be done with two exciters to generate more uniform vibration within this nonlinear test article. Finally, for logistic reasons, no other test was performed after the preliminary runs discussed earlier. It was decided from analysing the acquired data that complete mode shapes of the rover could be extrapolated from symmetry or, if required, from comparison with data already obtained by testing the rover and mast assembly. It turned out that the derivation of mode shapes for the rover was achieved for the first 16 modes using these two sources of information. This modal data base of the rover was more than what was required for this activity. However, with hindsight, considering the effort required for defining some of these modes, performing some complementary runs in the first place might have been a better decision. 35.5.2 Estimation of Modal Parameters By curve fitting the FRF’s for the left half of the rover obtained from the last preliminary run, the modal parameters of the rover for 16 modes were derived and are presented in Table 35.1. The mode description is as considered while looking at the rover data only, i.e. without comparing with the modes of the rover and mast assembly. The LMS PolyMAX curve fitting algorithm was used to perform these derivations. 35.6 Modal Testing of Rover and Mast Assembly with Portable Exciter This section presents some details and results of modal testing performed on the rover and mast assembly. As mentioned earlier, one of the main reasons for testing the assembly at that time was to investigate possible resonance in the mast that could cause detrimental motion to the payloads supported on top of the mast.
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