2 F. Trainotti et al. In particular, the use of standard measurement devices like piezo accelerometers in the FRFs acquisition process may not be suitable for applications requiring a high level of accuracy. Therefore, a valid alternative for motion detection can be identified in the use of laser interferometry, thanks to its non-invasive and high quality measurement technique. In this contribution, the potential of laser technology in the acquisition of FRFs within FBS is investigated. The reliability of laser measurements compared to those taken with standard accelerometers is evaluated through the analysis of the final coupling results. In Sect. 1.2, the theoretical background on Experimental Dynamic Substructuring is recalled, with particular focus on frequency-based approaches and interface modelling techniques. The laser vibrometry is shortly discussed in Sect. 1.3 along with its advantages and drawbacks over traditional measurement techniques. In Sect. 1.4, two measurement campaigns are carried out, one with accelerometers and the other with a laser Doppler vibrometer. A DS coupling is performed and the results of both case studies are compared. A brief summary of findings and conclusions is given in Sect. 1.5. 1.2 Experimental Dynamic Substructuring This section briefly reviews the theoretical concepts underlying the Experimental Dynamic Substructuring. In particular, a general overview of Frequency Based Substructuring and Virtual Point Transformation is provided in Sects. 1.2.1 and 1.2.2 respectively. 1.2.1 Frequency Based Substructuring The assembly procedure in the frequency domain according to a dual approach is named Lagrange Multiplier—Frequency Based Substructuring (LM-FBS) [1, 8, 9]. This method, which operates with admittance notation, evaluates locally a set of interface DoFs for each single substructure in the system and considers the interface forces as unknown variables. The aim of LM-FBS is to derive the admittance of the assembled systemYAB from the separate admittances of the two subsystems YA and YB. Consider the system depicted in Fig. 1.1. The subsystems’ admittances are known and the substructures’ DoFs are grouped in internal DoFs ((∗)A 1 and (∗) B 3 ) and interface DoFs ((∗) A 2 and (∗) B 2 ). The vectors of displacements, applied forces and reaction forces are denoted by u, f and g respectively. The governing equation of motion for the uncoupled system in the frequency domain is written in a compact form: u=Y(f +g) ⇒ ⎡ ⎢⎢ ⎣ uA 1 uA 2 uB 2 uB 3 ⎤ ⎥⎥ ⎦ = ⎡ ⎢⎢ ⎣ YA 11 Y A 12 0 0 YA 21 Y A 22 0 0 0 0 YB 22 Y B 23 0 0 YB 32 Y B 33 ⎤ ⎥⎥ ⎦ ⎛ ⎜⎜ ⎝ ⎡ ⎢⎢ ⎣ fA 1 fA 2 fB 2 fB 3 ⎤ ⎥⎥ ⎦ + ⎡ ⎢⎢ ⎣ 0 gA 2 gB 2 0 ⎤ ⎥⎥ ⎦ ⎞ ⎟⎟ ⎠ (1.1) Thematrix Y represents the admittance of the uncoupled system, built in a block diagonal form by the admittances of the subsystems YA and YB. Fig. 1.1 Assembly of subsystems A and B at the interface DoFs u2
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