## 4 Discussion  
From the experimental results presented in Figure 5, it was confirmed that the needle handling accuracy could be improved when the needle-hooking task was carried out using the optimized conditions (Conditions (A), (B), and (C)). Thus, the optimization of the mechanism with the invisible area rate of the tip of the forceps manipulator and the moving volume as variables was suggested. Among the conditions compared in this experiment, no significant difference was found when Conditions (A) and (E) were compared, as shown in Figure 6. From the results in Figures 7 and 8, a significant difference in the invisible area rate was not observed when comparing Conditions (A) and (D). Moreover, a significant difference in the moving volume was confirmed. In contrast, a significant difference was confirmed in the invisible area rate when compared with Condition (E). However, no significant difference was confirmed in the moving volume. Condition (D) was the mechanism with the largest moving volume under all the conditions. Although the invisible area rate was similar between the compared conditions, it was considered that the operation accuracy was improved by the ease of operation, given that the moving volume of Condition (A) was reduced by optimization. In contrast, Condition (E) was the mechanism with the largest invisible area rate. The invisible area rate under Condition (A) was improved by optimization; however, it was larger than that under Condition (E). Moreover, no significant difference was confirmed when comparing the moving volumes of Conditions (A) and (E). Based on the results, it was considered that the improvement of the needle handling accuracy was not confirmed because the difficulty of the operation was not resolved, given that the improvement of the moving volume was not sufficiently achieved. Condition (A) was the mechanism in which the invisible area rate was the smallest among the Pareto optimal solutions. Hence, it was considered that the visual field information of the tip of the forceps manipulator could be obtained clearly when performing the task under Condition (A). In the case where such visual field information can be clearly obtained, the linear trajectory of the tip of needle from the initial position to the insertion point was confirmed. When the needle was moved linearly and hooked at the target point, as shown in Figure 9, the visual field information of the needle was blocked by the near side of the surgical tool. If the visual field information of the needle was blocked, the operator could not recognize the motion of the needle. As a result, the accuracy of the needle hooking at the exit point decreased. In addition, the variations occurred easily. Therefore, it was considered that the error distance Err under Condition (A) was not significantly shortened when compared with the mechanism that did not satisfy the Pareto optimal solution. Based on the above, it is necessary to consider the blocking ratio of the tip of the surgical tool and the visual field information of the needle. However, this study has potential limitations. The effectiveness of the optimization we described in this paper is based on the difference in the length of the tip joint of forceps manipulator and the motion of the needle-hooking task. For designing, the other DH parameters should be verified. For developing, the physical size of the instruments should be  
Figure 9: Hiding the tip of the needle during the suturing motion. optimized. The surgical robot is also required to apply other surgical tasks for congenital esophageal atresia in pediatric surgery. For example, the surgical technique involves the cooperation of the left and right forceps manipulator. Given that the required motions differ between the left and right forceps manipulator, it is necessary to carry out the optimization for each mechanism of the manipulator based on the task required by each manipulator. Thus, in the future, we will optimize and evaluate under those conditions.