## 5 Experimental Process and Results Analysis
### Detection Experiment of Flange Port of Tank Car  
In this experiment, the tank car flange was detected using an on-site image of the tank car. Figure 12 shows a rear-view image of a typical in-service tank car. In the automatic docking system, the preset initial point of the small crane tube was 1.65-m high and 2-m away from the target. After adjusting the screening parameters of target circle size, flange port target detection was carried out in the specific range of the rear image of the tank car.  
Figure 11: Flange end face physical detection experiment. (**a**) Represents the detection condition that is closest to the real working condition. (**b**) Represents the detection condition when the flange deviation angle is large. (**c**) Represents the detection condition with similar chromatic aberrations between the target flange and the background. (**d**) Represents the optimal working conditions. (**e**) Represents the detection of interference from ambient light to the flange surface.  
Figure 12: LNG tank car rear.  
In order to verify the effectiveness of the method, this paper conducted detection experiments on multiple groups of image data. Figures 13-15 show the original images of three groups of experimental images and the detection results. Taking Figure 13a as an example, with the lower left corner of the image as the origin point, the measured pixel coordinates of the flange port center are [210.74, 370.41], [621.00, 365.71], and [1048.78, 388.04], respectively. After coordinate transformation, \(\mathbf{P}_{uv}=\mathbf{KTP}_{uv}\), in which \(\mathbf{P}_{uv}\) is the pixel coordinate, \(\mathbf{K}\) is the camera internal parameter, \(\mathbf{T}\) is the camera external parameter, and \(\mathbf{P}_{uv}\) is the camera coordinate. The internal XOY coordinates of the world-scale camera coordinate system are [32.20, 53.61], [91.63, 55.87], and [151.80, 57.17]. The \(z\)-axis coordinates, namely the distance, are detected by other sensors.  
The results show that the center of the flange face can still be located accurately in field working conditions, and the lowest running time is 0.694 s. This method detects the center and diameter of multiple images, and the outer diameter of the tested flange is actually 165 mm. It can be seen from Table 2 that the diameter accuracy errors detected by the three groups of experiments are all less than 3 mm and the accuracy errors are affected by differences in the approximation ratio. The higher the average approximation ratio is, the lower the accuracy error is. The results show that the proposed method can effectively locate the target flange position in actual field conditions and meet the real-time measurement requirements of project engineering, providing technical support for practical engineering applications.  
Figure 13: On-site tank car A. (**a**) Flange practicality picture. (**b**) Detection results of (**a**).  
Figure 14: On-site tank car B. (**a**) Flange practicality picture. (**b**) Detection results of (**a**).  
Figure 15: On-site tank car C. (**a**) Flange practicality picture. (**b**) Detection results of (**a**).  
Using the flange picture data set, real item images, and the field conditions simulation, the flange end face pose in the process of the automatic butt joint of a small crane tube was detected.