196 J. Baqersad et al. Table 19.2 Correlation between extracted modal parameters for MIMO and impact hammer tests MIMOtest Impact hammer test Correlation Mode # Freq. (Hz) Damp. (%) Freq. (Hz) Damp. (%) Freq. diff. (%) MAC (%) 1 5.3 0.9 5.3 0.7 0.2 98.7 2 5.4 0.7 5.4 0.6 0.0 99.5 3 5.6 0.5 5.6 0.5 0.0 97.9 4 7.7 5.4 – – N/A N/A Fixture mode 5 7.7 3.6 – – N/A N/A Fixture mode 6 8.9 4.8 8.9 4.9 0.8 98.6 Fixture mode 7 14.9 1.3 14.9 1.5 0.0 99.9 8 15.4 0.7 15.4 0.7 0.0 99.9 9 16.1 1.3 16.1 1.3 0.0 99.9 10 18.3 5.8 18.9 2.8 3.2 22.0 Fixture mode 11 21.1 3.3 21.1 2.9 0.0 94.6 12 23.2 4.5 23.3 4.1 0.2 98.4 Fixture mode 13 25.5 2.8 25.5 2.6 0.1 99.1 Fixture mode 14 29.7 1.5 29.7 1.5 0.1 99.5 15 30.2 3.1 30.2 3.1 0.0 97.2 16 36.3 0.6 36.2 0.6 0.1 99.9 17 37.3 2.4 37.3 2.4 0.0 97.9 18 37.9 1.8 37.9 1.8 0.2 98.9 19 51.3 0.7 51.3 0.8 0.0 99.8 20 58.3 0.7 58.3 0.7 0.0 99.5 21 58.9 0.7 58.9 0.7 0.1 99.6 22 62.1 0.5 62.1 0.5 0.1 99.9 23 67.6 0.6 67.6 0.6 0.0 99.4 24 69.1 0.6 69.1 0.3 0.1 99.9 effects of the blades. After obtaining the modes of the assembly (Fig. 19.6), a considerable change in some modes of the fixture (Fig. 19.5) was seen. This change is more significant in the modes that are dependent to the rotational inertia of the blades. The results of this paper show the significant effects of rotational inertia on the structures where mass is distributed far from the rotation axis. Another important lesson that can be learned from this measurement was the effects of impact locations on the extracted modes. For the impact test, if the measured FRFs for the case when impact forces are only made on a single blade were used, not all the modes could be extracted; this was observed in the results but was not presented in this paper due to space limitation. For instance, if the measured FRFs of the test when only Blade 1 was excited were used, mode 2 of the turbine could not be identified. Therefore, if the input force is distributed on all the components of a structure in a modal test, a better representation of the modes (or even more modes of the structure) can be extracted. This fact also clarifies why SIMO test results do not compare very well with the MIMO and impact tests. For SIMO test, the force was imparted to the turbine only through a single point and on a single blade; therefore, not all the modes might be effectively excited. 19.7 Conclusion The results of the study revealed there are very closely space modes in the three-bladed turbine attached to the steel block. The modes of the assembled turbine were categorized into: (1) collective flapwise modes, (2) differential flapwise modes, (3) collective edgewise modes, (4) differential edgewise modes, and (5) fixture modes. In this paper, the technique for installing shakers in oblique orientations was implemented for a wind turbine blade; the modes of the structure could be found without the need for finding the installed angles of the shakers. Comparing the MIMO and SIMO results showed that a complicated structure such as a wind turbine that has several connections among the subcomponents needs to be excited by several shakers. Using multiple shakers leads to a uniform distribution of the energy over the entire structure and a better coherence in the measurement. On the other hand, a single shaker could not effectively excite all the modes of the structure. The results of the study also showed that impact hammer modal test is still one of the powerful techniques to excite structures for an experimental modal test. The results revealed that for a complicated structure with several substructures, impact hammer
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