## 2 Target Recognition and Pose Estimation of the Fuel Tank Inlet
### Recognition Method and Results Based on RGB-D Camera  
To achieve the positioning of the female refueling connector, this study first conducts object detection on the female connector. Then, using the origin of the center box of the identified target as the docking target point for the female end of the refueling connector, the paper provides position and posture information to the robotic arm. When the robotic arm is in its current position, the image of the female connector is located at the exact center of the camera's field of view, and the end effector of the robotic arm is parallel to the plane of the female connector. This position is used as the origin point for the robotic arm. To simulate a real refueling scenario, a six-degree-of-freedom simulation motion platform is utilized. By adjusting the various degrees of freedom of the motion platform, data can be obtained in different positions and postures. Since the camera's field of view is limited, excessive movement may cause the target connector to move out of the camera's view. To ensure the accuracy and controllability of the experiment, the range of motion of the six-degree-of-freedom platform is restricted, ensuring that the movement does not exceed 10 cm and the tilt angle does not exceed 5 degrees. Such limitations ensure that the target connector always remains within the camera's field of view, effectively avoiding data loss or inaccuracies due to the target moving out of sight. A partial visualization of the dataset  
Figure 1: Fuel tank and connector. (**a**) Schematic diagram of the special fuel tank; (**b**) simulated fuel tank; (**c**) dry-type self-sealing connector; (**d**) male and female ends of the connector. and the distribution of Center coordinates and normalized values of width and height are shown in Figure 2. In Figure 2, the left panel visualizes six typical orientations of the target connector and displays their recognition results. In the right panel, we statistically analyze the widths, heights, and center points of the detected bounding boxes to illustrate the distribution and sizes of the recognition boxes as well as the positions of the targets in the images. Together, these two panels provide an overview of the distribution of the recognized target connector boxes, the positions of their center points, and the sizes of the recognition boxes. In this study, the YOLOv8 [5] algorithm is employed for object detection. YOLOv8 abandons the C3 module used in the previous YOLOv5 [6] and instead utilizes the more advanced C2f module for feature extraction. The incorporation of more branches in the C2f module allows for a richer gradient feedback to the sub-branches, enhancing the extraction of features from images. It is noteworthy that the center point of the annotated bounding box is taken as the central point of the circular connector. This approach effectively addresses the challenge of determining the docking point for circular connectors in a tilted position. In the process of automatic docking, obtaining the pose information of the fuel inlet is essential to ensure its alignment. Perspective-n-Point (PnP) is a method used to solve the correspondence between 3D points and their 2D projections. It describes how to estimate the pose of a camera when the positions of n 3D space points are known, as illustrated in Figure 3.  
Figure 3: Schematic diagram of the PnP algorithm. Figure 2: Visualization of part of the dataset. After calibration, the intrinsic matrix \(K\) of the camera is known. The homogeneous coordinates of the 3D coordinate \([X_{w}\ Y_{w}\ Z_{w}]^{T}\) in space can be represented a \([X_{w}\ Y_{w}\ Z_{w}\) 1] \({}^{T}\). The coordinates of the projection point \([u,v]^{T}\) in homogeneous coordinates can be expressed as \([u,v]^{T}\). The intrinsic matrix is denoted by \(K\), and the aim is to solve for the rotation matrix \(R\) and the translation matrix \(T\). The formula is as follows:  
\[z_{c} \begin{bmatrix}u\\ v\\ 1\end{bmatrix} =K