Page - 72 - in Proceedings of the OAGM&ARW Joint Workshop - Vision, Automation and Robotics

connector plug was made out of more reflective plas- tic, resulting in a few cases when reflections caused the accuracy issues regarding the rotation. However, these issues were observed very rarely under specific viewing angles, and matching accuracy dropped below 90%, so these cases could be easily identified. Fig. 7. Results of the template matching. A high variety of angles and lighting conditions were tested. Viewing angles up to 45◦ resulted in successful detection with accuracy dropping beyond that. Row 1: Type 2 connector plug. Row 2: Type 1 socket. Row 3: Type 2 socket. C. Eye-to-HandCalibration In the given configuration, the structure of the connector plug was used as a marker for eye-to- hand calibration. During the calibration process it was turned to face the vision sensor, while during the normal operation it faces away from the camera. Furthermore, the outer ring of the plug is angled, so the pins of the plug had to be used as reference points to get the accurate calibration. The end point of the connector plug was rotated around each of the axis as well as moved to different locations within the field-of-view of the vision sensor. In total, 26 poses were recorded and used until the calibration converged. Additionally, 3 instances were discarded because of the incorrect template matching result. The average translation error within the work- ing space was reduced to 1.5mm, which was sufficient for our application at this stage. Possibly, having more poses would reduce the positional error even further. With the eye-to-hand calibration completed, coordinate frames for the camera position and the end point of the connector plug can be added to the model, as shown in Figure 8. D. FindingChargingPortPoseandRobotMovements As the final evaluation, we used the whole process pipeline and analysed whether the plug-in motion was successful or not. There were 10 runs executed in total using Type 2 connectors. For the first 5 runs the charging port was Fig. 8. Eye-to-hand calibration results. Visualisation of the assigned coordinate frames to the vision sensor, the end-effector of the robot and the end point of the connector plug. Resulting point cloud is overlayed onto the visualisation of the robot model. angled at 10◦ in relation to the vision sensor, and for the remaining 5 runs, the angle was increased to 30◦. The robot successfully connected the plug 8 out of 10 times. Both failures occurred by missing the rotation of the plug, which were determined by the misalignment of the guidance slot on the charging port. However, the safety stop automatically initialised in both of the cases ensuring that the robot stopped before causing any damage. TABLE I SUMMARY OF THE PLUG-IN MOTION EXPERIMENTS WITH CHARGING PORT PLACED AT TWO DIFFERENT ANGLES Exp ChargingPortAngle10◦ ChargingPortAngle30◦ 1 Success Success: Misalignment 2 Success: Misalignment Failed:Missed rotation 3 Success Success 4 Failed:Missed rotation Success: Misalignment 5 Success: Misalignment Success: Misalignment However, even when the plug was successfully in- serted in the charging port, there were some alignment issues. In 5 out of 8 successful runs, the plug was not fully inserted into the charging port. It was caused by a small angular offset varying between 2◦ and 5◦. The contact was still made, so the charging process would be successful, however, there was additional strain due to imperfect alignment. The misalignment occurred more frequently during the experiments, where the charging port was placed at 30◦ angle. The results are summarised in Table I. As expected, the unplugging process was successful during all the runs. It simply follows already executed trajectory in the inverse order, meaning that as long as the position of the charging port did not change during the time it was plugged in, there should be no issues with the unplugging process. IV. CONCLUSIONS AND FUTURE WORK We have presented a vision-guided and robot-based automatic EV and PHEV charging station. The goal is toallowautomatedconductive fast chargingofelectric and hybrid vehicles and avoid the issue of a charged car taking up the space when it is not necessary. 72