A Method for Response Replication at Component-Level in MIMO Random Testing 41 ence SDM can significantly affect the response intensity on the DUT. Therefore, not adjusting the reference PSDs may cause significant deviation from the target vibration environment. These results highlight how the MPMis capable of replicating the target vibration environment. Furthermore, Figure 4 presents the comparison between the drives required to conduct each test. The plots on the left show the drive PSDs, the bar plot on the right illustrates the difference in voltage RMS value indB with respect to the drives required by the target measurement. It can be noticed that the MPMachieved the greatest drive reduction for each one of the three drives. In particular, theMPMachieveda1.46dB, 0.89dBand1.02dBreduction for drive 1, 2 and 3, respectively. On the other hand, the MDMtab, achieved a reduction of 0.23dB, 0.11dBand 0.25dBfor drive 1, 2 and 3, respectively. It should also be noticed that the MDMtiptest determined a significant increase in the energy required to conduct the test. This is caused by the physical constraints of the system. In fact, when controlling the response of the specimen, the control target must abide to the dynamic behaviour of the specimen. In particular, at the resonance frequencies the response of the DUT is driven by its natural mode shape and the phase and coherence of the response are independent of the phase and coherence of the multi-axis excitation. Figure 5 shows the comparison between phase of the CSDs obtained during the MDM tip test (red crosses) and the phase required to achieve the minimum drives according to the MDMwhen controlling the tip accelerometer (green circles) at each natural frequency. The phase obtained during the test is determined by the dynamic behaviour of the specimen and, as a consequence, does not always match the phase required by the MDM. This phase mismatch between the control and target phase determines an increase in the required drives. Moreover, at the 55 117 350 436 Hz 10-8 10-6 10-4 10-2 V2/Hz PSD drive 1 Target MPM MDM tab MDM tip 55 117 350 436 Hz 10-8 10-6 10-4 10-2 V2/Hz PSD drive 2 Target MPM MDM tab MDM tip 55 117 350 436 Hz 10-6 10-4 10-2 V2/Hz PSD drive 3 Target MPM MDM tab MDM tip Drive RMS difference from target Drive 1 -2 -1.5 -1 -0.5 0 0.5 1 dB MPM MDM tab MDM tip Drive 2 -1 0 1 2 3 4 dB MPM MDM tab MDM tip Drive 3 -2 -1 0 1 2 3 4 dB MPM MDM tab MDM tip Fig. 4 Comparison of the drives required by the target measurement, the MPMand the MDMwhen controlling the taband tab accelerometer (dB reference: drive 1: 0.23 V, drive 2: 0.22 V, drive 3: 0.16 V).
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