# Structural Optimization of MicroMED Dust Analyzer
## 4 Experimental Assessment  
As mentioned before, mass minimization plays a key role in aerospace components design. In this light, among the proposals probed within this research work, Model A was considered the most promising one for MicroMED mechanical hardware future development. Despite the optimal OB robustness being positively proven numerically via FEAs, for completing the circle an experimental activity to further freeze the adopted workflow was carried out. Indeed, a mockup of the Model A was manufactured by milling from an AI7075-T6 slab (Figure 16) and was tested to measure the free natural frequencies. Before the experimental testing activities, a dimensional and geometrical check was performed to state the actual mockup footprint, to update the FEM. The mass was measured with a Gibertini balance having a resolution of 0.01 g, obtaining 63.88 g. The mass mismatch with the Model A Computer-Aided Design (CAD) was attributed to a slight difference between the real and theoretical component dimensions. Indeed, a measure of the ribs grid and the overall OB geometrical features--carried out with a Mitutoyo fiftieths gauge and a Micro-Epsilon laser scanner having a resolution of 2 \(\upmu\)m--confirmed the presence of a dimensional unevenness. The measurement outcome is reported in Appendix A, providing a comparison between the theoretical and real dimensions of Model A. The activity was propoedeutic to the actual free vibrations test because it allowed updating the Model A CAD model with the real mockup dimensions and preserving its representativeness. This was mandatory in view of a comparison between the numerical free natural frequencies (i.e., found running a free modal analysis on PTC Creo Parametric/Simulate) with the experimental one. Moreover, as highlighted in Figure 16 during the manufacturing process the 0.3 mm skin was not able to withstand the milling head-cutting force and was plastically deformed by the tool action. However, since the damage was localized to a restricted area of the component, the mockup was considered eligible to be investigated in this experimental activity. Indeed, its structural integrity was not compromised and the vibrating modes not involving the damaged region were expected to be unaffected by the manufacturing defect. Finally, before the actual resonances research experimental activity, the Model A updated CAD was subject to a free modal analysis to estimate the free natural frequencies and discover which vibrating modes were eligible to be observed during the resonances research. Indeed, considering a non-contact measurement method including a Laser Doppler Vibrometer (LDV--Polytec OFV 505 sensor head) was used for the experimental activity, the free modal analysis was necessary to plan where to point the laser head to properly catch the vibrating modes and measure their frequencies. Figure 16: Manufactured Model A mockup highlighting the damaged region (in the red box) (**a**) and the laser scanner measurement setup (**b**). The FEA outcomes are reported in Figure 17 and the numerical results are summarized in Table 4. As can be seen, with only three LDV laser spot positionings (i.e., at one of the peripheral mockup holes, at the skin between the LG and the PG, and near the component damaged area) it was theoretically possible to catch all the investigated vibrating modes. An overview of the adopted experimental setup is depicted in Figure 18. For the free vibrations test, the Model A mockup was suspended on elastic ropes and excited with a series of sweep sines (i.e., five consecutive linear sweeps lasting 30 s each, in the 80 to 5120 Hz frequency range and 200 mV peak-to-peak wide) produced by the Keysight Technologies 33220A function generator and delivered to a loudspeaker placed below the OB. The real input delivered at the mockup was measured with the PCB Piezotronics 130E20 ICP microphone (i.e., having a sensitivity of 48.1 mV/Pa).