# Structural Optimization of MicroMED Dust Analyzer
## 2 Materials and Methods  
The MicroMED OB is the support plate on which the instrument subassemblies and functional components are firmly fixed. The OB is a ribbed aluminum component (i.e., manufactured in Al7075-T6 aluminum alloy) characterized by high mechanical stiffness, provided by the grid of ribs lying on its interface plate. In this research work, the main investigation dealt with the topology and sizing of the OB ribs, pursuing the optimal configuration towards mass minimization achievement (i.e., hereafter referred to as Model A) maintaining comparable dynamic performance with respect to the current OB design. Furthermore, a different layout was probed, aiming to optimize the OB dynamic performance (i.e., hereafter referred to as Model B). The latter proposal was obtained by maintaining the unchanged mass budget allocated to the ribs grid while maximizing the OB resonance frequencies. The TO method was preferred to the other structural optimization approaches, i.e., the size and the shape optimization methods, because of its design freedom. Both Model A and Model B redesign studies exploited two commercial software programs, i.e., PTC Creo Parametric/Simulate and Altair Inspire. The former was adopted for computer-aided modelling and FEAs, while the latter was employed for the TO. A schematic of the workflow exploited for the redesign is depicted in Figure 2.  
Within the optimization process, Altair Inspire relies on a computational method based on the material distribution technique. This process adopts the material density as the design variable and returns an optimal material distribution in a discretized reference domain [24; 25]. A binary solid model is produced, and for each reference domain finite element either a unitary or null value is assigned to the design variable. Consequently, a finite element will be finally part of the optimal structure only if its associated design variable has a non-null value. In the latter case, the material density is assigned to the finite element. The software engine, to cope with the occurrence of the intermediate values of the design value (i.e., between 0 and 1) during the optimization iterations, adopts a penalization approach based on the Solid Isotropic Material with Penalization (SIMP) algorithm [26; 27]. Therefore, the outcome will be composed of either voids or solids. In this research work, the optimization cycles imposed the finite element structure mechanical stiffness maximization as the design target. At the beginning of each iteration, the overall mass of the reference domain was progressively reduced by a percentage within the total reference domain volume. For the OB optimization, at the beginning of each TO step, 70% of the design volume removed was imposed as an output condition. Before running the optimizations, a filled OB solid model was developed, as depicted in Figure 3. Solid model defeaturing was carried out as the first task to reduce the computational complexity of the FEAs, while maintaining the representativeness of the numerical outcomes. The Finite Element Model (FEM) held three lumped masses connected via rigid links to the OB areas intended to support the instrument subassemblies. Such areas were excluded from the design domain and thus were not involved in the optimization cycles. These features--92 g for the OH + Inlet/Outlet (IO), 51 g for the Pump Group (PG), and 29 g for the Laser Group (LG)--were introduced to consider the main instrument's subassemblies  
Figure 3: Altair Inspire TO model of the OB. The brown region represents the design domain subject to the SIMP approach and the grey one was left unaltered during the optimization iterations. Figure 2: Block chart of the phases included within the OB redesign and optimization workflow. but in the meantime to run a streamlined optimization process. The lumped masses were estimated by assigning the aluminum density (i.e., 2819 kg/m\({}^{3}\)) and assuming a 10% margin as the worst-case scenario.