## 4 Discussion  
We implemented a method to test the performance of a genetic algorithm in terms of optimizing a manipulator with various mechanical links. Four different types of mechanical links located between the manipulator joints were implemented. The performance of the genetic algorithm was tested with various tasks and two cost functions.  
We observed that optimizing a manipulator with a simple (basic, linear, or rounded) link type led to better results in all of our tests. The Hermite spline links underperformed even after a 10-fold increase in evaluations. It is possible that the increased search space caused by more parameters creates a combinatorial explosion and that increasing the generation count 10 times is not enough. There also could be certain fundamental disadvantages of spline-shaped links between joints. Further investigations with different types of splines could confirm this phenomenon.  
When comparing the differences between the simpler types of joints, we observed that both rounded and linear links performed better than the basic links in task 1. However, in task 3, the linear links performed significantly better than linear or rounded links, depending on the used cost function. This could mean that the right choice of link type is task-specific or that basic links perform better on tasks with unchanging orientation of the end-effector. During task 2, the base, linear, and rounded links showed no statistically significant differences.  
Including torque in the cost function did not have a significant effect on the results. This could be caused by the combined cost function, which was defined as a weighted sum of all of its parts. A multiobjective optimization method might be more effective in showing  
Figure 19: Best results for task 3: (**a**) basic links; (**b**) linear links; (**c**) rounded links; (**d**) Hermite spline links.  
Figure 17: Best results for task 1: (**a**) basic links; (**b**) linear links; (**c**) rounded links; (**d**) Hermite spline links.  
Figure 18: Best results for task 2: (**a**) basic links; (**b**) linear links; (**c**) rounded links; (**d**) Hermite spline links.  
how much the torque measurements affect the results. This was considered outside the scope of this article, since we wanted to compare different types of links.  
When examining the resulting kinematic structures for each task, we noticed that the solutions for task 1 with basic links resulted in more degrees of freedom. This could be caused by the distance between the trajectory and the robot base in combination with the length of the predefined links. Optimization produced manipulators with more DOF, and consequently a greater number of smaller links instead of longer links. The other types of links that were length-adjustable resulted in 3 or 4 DOF with longer links.  
Overall, the simplicity of the genotype seems to be a significant factor when optimizing a manipulator for a specific task. Assembling the manipulator with curved links does not seem to provide any significant advantage, although we think that there still might be certain advantages in using curved links if a different optimization algorithm is used and there are obstacles in the environment.