## 3 Results and Discussion
### Results of the Optimisation of the CPM Mechanism Links
#### 3.3.1 Results of Optimising Link 3 of the CPM Main Positioner  
The input parameters for the optimisation were the moment values previously determined using the Rigid Dynamics toolbox of ANSYS Workbench. The analysis looked at each of the positioner links of the CPM separately. It is not possible to create a new structure for the entire kinematic chain. The Generative Design synthesis required the additional definition of boundary conditions in the form of surfaces that had to be excluded from the optimisation, as their shape and purpose were necessary to maintain the full functionality of the original solution. In the case of link 3, these were the hinge surfaces by which the link was attached to the other moving and fixed links of the mechanism. In addition, for the purposes of the research, an additional area of 5 \(\times\) 4 \(\times\) 1 cm was defined in the structure of the link, which in a real solution would replace the area used to fix the support of the femoral part of the limb. An important assumption in the research was to define the plane of symmetry for the optimised solution and to identify max stiffness as a complementary parameter. Most structures cannot be generated correctly without this parameter. Results were generated assuming a volume limit of between 20% and 50%. Values below and above these assumptions did not satisfy the mass and displacement conditions of link 3 and were therefore not included in the results. For the AL 6061 material, the mass of link 3 before optimisation was 0.81712 kg. Preliminary strength tests in Creo Parametric showed that the von Mises stress for link 3 was 121.4 MPa, and displacement was 5.43 mm. Such a high value of the displacement parameter is due to the fact that the simulations have neglected the influence of the stiffness of the shaft connections 3 and 4, which in the real solution ensures sufficient stiffness of the connection in the kinematic pair 3-4. The initial results, to which we will relate the obtained results of the generated structures, will allow us to select the optimum solution in terms of minimum mass and least deformation. For each of the newly generated structures, additional numerical analyses are carried out to verify the result obtained, consisting of the determination of the strength parameters: von Mises stress and displacement. The results of the optimisation of link 3 of the positioner mechanism are summarised in Tables 4-6. The number of finite elements used in the numerical study was 23,012. A single finite element had a size of 0.003 m.  
A prominent feature of the Generative Design numerical study is the mass of the optimised object and the associated output geometry that the object adopts for optimisation in Creo Parametric. This is clearly higher for the same material, as can be seen in Table 4. For the AL 6061 material, the initial mass of the link is approx. 2.64 times higher (was 2.153 kg) than the original link (0.81712 kg) in the same material. This is because when analysing and generating the new object structure, the software takes the entire volume of the object as the initial mass. The main problem identified when attempting to use other digital engineering tools that offered the ability to perform optimisation using generative design techniques (Fusion 360 and Autodesk Inventor, among others, were used) was that it was not possible to confirm by additional strength testing that the structure under test actually met the assumed strength constraints. This was only possible by applying CAD modelling techniques that used the generated geometry to manually model a new solid object with a spatial shape similar to the optimisation result. This is a very labour-intensive and time-consuming activity when it comes to achieving quite complex spatial forms, and the end result is nevertheless an approximation of the result achieved. The Creo Parametric programme allowed such studies to be carried out without modifying the optimised shape, which is a major advantage of the tool and hence its selection.