## 3 Results  
Figure 6 shows the autonomous driving robot recognizing road signs in the underground mine, driving straight and taking left and right turns; in addition, the matching results of road signs are shown. In the straight section, we confirmed that the robot drove along the centerline of the tunnel, measuring both the distance to the wall in the left and right directions without recognizing the road sign (Figure 6a). In the left and right turn sections, we confirmed that the robot drove along the left and right walls by recognizing the road sign and switching the autonomous driving mode (Figure 6b,c). In the matching result in Figure 6, it can be seen that the x and y coordinates of the detected road sign were output. In addition, the rotation angle of the mark, the scale for the template image, and the matching score were calculated. The autonomous driving robot drove safely in an underground mine tunnel of approximately 60 m for 128 s without a global map, and we confirmed that, after recognizing road signs, it returned stably while following the left and right walls.  
Figure 5: Conceptual diagram of the (**a**) field experiment area and (**b**) road signs installed in the underground mine.  
Figure 7 illustrates the process of changing the autonomous driving mode when the autonomous robot drove through the tunnel. While driving through the underground mine experiment area, it drove for 49 s in the centerline autonomous driving mode, 25 s in the right wall-following mode, and 45 s in the left wall-following mode. We confirmed that the robot's driving mode switched when six road signs were recognized; furthermore, when the wall-following mode lasted for more than 15 s, it switched back to centerline tracking autonomous driving mode.  
Figure 6: Field experimental scenes, view of vision camera and matching results of the autonomous driving robot in (**a**) the straight section, (**b**) the right turn section, and (**c**) the left turn section.  
Figure 7: Graph showing the change in autonomous driving and wall-following mode in the field experiment.  
Figure 8 presents the data of the LiDAR sensor obtained from three road types (two-way intersection, narrow-to-wide section, and wide-to-narrow section) and the robot's driving direction. At the two-way intersection, after recognizing the road sign in the right direction, the robot drove along the right side at a constant distance. In the narrow-to-wide section where the width of the road widened rapidly, after recognizing the road signs in the left direction, the robot drove along the left side. In addition, in the wide-to-narrow section, after recognizing the sign in the left direction, it was possible for the robot to safely enter the narrow path without colliding with the right wall.  
Figure 9 shows the results of recognizing road signs when the robot was driving in an underground mine. The autonomous driving robot recognized all road signs at a total of six points and correctly classified the road signs in the left and right directions. In Figure 9, two road signs were captured together. However, the road signs recognition system calculated the distance by comparing the size of the matched image with the template image and recognized the relatively close road marker first. In addition, we confirmed that the autonomous robot sequentially recognized the road signs in the back. The matching score for a total of six points was calculated as 979.14 points on average, the scale was 80-120%, and the rotation was measured to be \(\pm 10^{\circ}\) (Table 2).  
Figure 8: Tunnel shape obtained from LiDAR sensor in two-way intersection, narrow-to-wide, and wide-to-narrow sections.  
Figure 9: View of recognizing road signs in the experimental area using vision system developed in this study. (**a**) Site 2. (**b**) Site 3. (**c**) Site 4. (**d**) Site 5. (**e**) Site 6. (**f**) Site 7.