## 4 Experimental Evaluation
### Application to Testbed  
A real-scale boring machine testbed was constructed to evaluate the practicality of the proposed pushing-based control method, as illustrated in Figure 11. The overall setup of the boring machine testbed and the bushing placement process are presented. The setup specifically integrates a robot manipulator and external elements, such as a worktable and slots, enabling pushing actions that effectively leverage the surrounding environment. Detailed sequences of the experiments conducted on the testbed, showcasing the pushing process and the insertion of the bushing into the slot, are illustrated in Figure 12. This also demonstrates the robot's ability to successfully place the bushing within the slot with a clearance of approximately 5 mm. Figure 13 highlights the consistent placement of the bushing into the slot, irrespective of its initial position, further validating the feasibility of the proposed method. The entire scenario is available at [[https://www.youtube.com/watch?v=TSW7](https://www.youtube.com/watch?v=TSW7) lbNnbRQ]([https://www.youtube.com/watch?v=TSW7](https://www.youtube.com/watch?v=TSW7) lbNnbRQ) (accessed on 2 December 2024), where three consecutive operations were captured in a single continuous take. The proposed method was compared to two alternative approaches. The first approach attempts to place the bushing into the machine slot without considering the initial position, while the second approach implements pushing primitives that use motion planning instead of a control-based method. The method that does not consider the initial position only succeeded when the gripper precisely grasped the bushing at a specific point. Our experiments conducted with three initial positions, short, middle, and long, resulted in a success rate of 33%. In practice, the slot's clearance is limited to approximately 5 mm, allowing successful placement only when the grasping position falls within this range. The motion planning approach also showed a low success rate. This method requires predefined slip distances between the gripper and the bushing. Without additional vision sensors for real-time feedback of the initial grasp position, it can only handle one of the  
Figure 11: A testbed configuration describing real-world bushing machine as illustrated in Figure 2.  
Figure 12: Detailed experimental results from the testbed, illustrating the pushing process and the insertion of the bushing into the slot. three initial positions. Its success rate was limited to 27%, including failures caused by inconsistencies in the grasp friction state, which prevented accurate implementation of the desired slip distance. Moreover, attempting to create more slip than necessary to reach the edge increased the contact force between the gripper and the external pusher, causing the manipulator to diverge. This instability makes the motion planning approach unreliable. In contrast, the proposed control-based method maintained a consistent force during slip, allowing the gripper to position the bushing at the edge accurately, regardless of its initial position. The method achieved a 97% success rate, with the only failure case occurring when the bushing slipped out of the gripper during slip control. Importantly, the proposed method does not cause divergence or other issues even in cases of failure, making it a stable option for process automation. The results of the comparison among the three methods are summarized in Table 2. \begin{table}
\begin{tabular}{l c c} \hline \hline
**Method** & **Success Rate (\%)** & **Success/Attempts** \\ \hline No-Pushing Approach & 33 & 10/30 \\ \hline Motion Planning Pushing Approach & 27 & 9/30 \\ \hline Proposed Controlled Pushing Approach & 97 & 29/30 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of success rates. Figure 13: Experimental results from the testbed showing successful insertions into the slot by the precise placement of the bushing when the gripper is positioned at **(a)** short, **(b)** middle, and (c)** long initial grasping positions.