A machine-vision framework for electric vehicle charging hole localization based on contour-driven detection and geometric verification

The accurate localization of charging holes is an indispensable perception step for the automatic charging system of electric vehicles, since the success of subsequent charging-gun insertion depends directly on the precise estimation of the geometric centers of the holes on the vehicle charging interface. This paper presents a lightweight machine-vision framework for charging hole localization based on contour-driven detection and geometric verification. The proposed framework integrates image filtering, RGB-to-HSL color-space conversion, parameterized HSL adjustment, two-stage contour detection, minimum enclosing circle fitting, and Hough circle verification into a deterministic pipeline. The framework is interpretable, does not require a learning-based training process, and can be deployed on conventional CPU-based computing resources. The image data used in this study were acquired by a monocular RGB camera mounted on the charging robot end-effector, in the same sensing configuration in which the system is intended to operate. A total of 80 images were collected from six vehicles equipped with the same five-hole charging interface, under realistic variations of illumination, viewing angle, reflective interference, and background complexity. Detailed parameter selection rationale, a parameter perturbation analysis, and a condition-specific ablation analysis are reported. On an Intel Core i7-12700 CPU with 16 GB RAM and no GPU acceleration, the proposed framework achieves an overall mean localization error of 4.2 pixels and a 96.25% detection success rate, with an average per-image processing time of approximately 60 ms. The results demonstrate that the proposed framework provides accurate, robust, and engineering-deployable charging-hole localization for the target charging interface.