Semi-supervised surface defect detection of wind turbine blades with YOLOv4

Timely inspection of defects on the surfaces of wind turbine blades can effectively prevent unpredictable accidents. To this end, this study proposes a semi-supervised object-detection network based on You Only Looking Once version 4 (YOLOv4). A semi-supervised structure comprising a generative adversarial network (GAN) was designed to overcome the difficulty in obtaining sufficient samples and sample labeling. In a GAN, the generator is realized by an encoder- decoder network, where the backbone of the encoder is YOLOv4 and the decoder comprises inverse convolutional layers. Partial features from the generator are passed to the defect detection network. Deploying several unlabeled images can significantly improve the generalization and recognition capabilities of defect-detection models. The small-scale object detection capacity of the network can be improved by enhancing essential features in the feature map by adding the concurrent spatial and channel squeeze and excitation (scSE) attention module to the three parts of the YOLOv4 network. A balancing improvement was made to the loss function of YOLOv4 to overcome the imbalance problem of the defective species. The results for both the single- and multi-category defect datasets show that the improved model can make good use of the features of the unlabeled images. The accuracy of wind turbine blade defect detection also has a significant advantage over classical object detection algorithms, including faster R-CNN and DETR.