5 Effective Use of Scanning Laser Doppler Vibrometers for Modal Testing 57 Fig. 5.17 UB100X model after update using global subsets, then local parameters on the reduced test geometry. Change in: (a, b) density; (c, d) Young’s modulus 0 0 50 100 150 200 250 50 100 150 FEA EMA 200 250 Before Updating (20 Pairs). Before Updating (Reduced EMA, 16 Pairs) Local Updating (Reduced EMA, 18 Pairs) Subset Updating (Reduced EMA, 18 Pairs) Subset, Local Updating (Reduced EMA, 6 Pairs) Local Updating (20 Pairs) Subset Updating (20 Pairs) Subset , then Local Updating (16 Pairs) Fig. 5.18 Frequency-frequency plot for paired mode shapes (MAC 50 %). No restriction on frequency difference was set for the complete dataset, whilst a maximum frequency difference of 50 % was set for the reduced dataset to minimise pairing of aliased modes 5.4 Conclusions In this paper, various means of performing modal testing using SLDVs were discussed, with particular attention to the challenges encountered in importing measured data into existing modal analysis software packages. A case study of the application of a hybrid SLDV and accelerometer test geometry was demonstrated, and the correlation with a basic FE model through to high order modes was shown to be extremely good. The SLDV was shown to be a useful tool for modal analysis, although the line-of-sight limitation of such a device when trying to characterise the structural dynamics of a threedimensional structure must be considered. Development of virtual testing tools to optimise use of SLDV is highly desirable and would significantly increase the benefit in the use of SLDVs for modal analysis. Various model updating trials were detailed including use of a reduced number of degrees of freedom. It was shown that the use of many degrees of freedom increased the number of model and test mode shapes which could be paired, and reduced
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