Design and experiments of the binocular visual obstacle perception system for agricultural vehicles

Machine learning was efficiently incorporated to design a visual perception system for obstacle-free path planning in agricultural vehicles. The present system aims to ensure the safety and reliability of intelligent agricultural vehicles in the process of autonomous navigation. Hardware and software were mainly included in the system. The hardware consisted of visual perception and navigation control module. Since the visual perception task needed real-time image processing, the embedded AI computer Jetson TX2 was taken especially as the core of computing to operate. A deep Convolutional Neural Network (CNN) was used to identify agricultural obstacles. The complex structure and uneven illumination were considered in the agricultural environment, thereby enhancing stability in object detection. The CNN performance of environmental features was much better, compared with the traditional detection using artificially designed features. Moreover, better detection was achieved under continuous learning features in the current task from the large-scale dataset. The improved YOLOv3 was utilized to integrate object detection for the simultaneous output of all information, including category, location, and depth estimation. A binocular camera was used to capture the left and right images, all of which were firstly input into the improved YOLOv3 model for object detection. The output of the improved YOLOv3 model was used for object matching to complete obstacle recognition, where the relationship of obstacles was determined in the left and right images. The location of matching objects was then used to calculate the parallax of the obstacle between left and right images. Finally, the parallax of the obstacle was input into the binocular imaging model for depth estimation. The accuracy of depth estimation was improved, with the increase of model sensitivity to the X-axis of images. The mean error, mean error ratio, and mean square error of depth estimation were greatly improved, compared with the original YOLOv3 and HOG+SVM model. The experimental results showed that the embedded AI computer-processed images in real-time, ensuring the detection accuracy of the improved YOLOv3 model. In object detection, a highly accurate identification was achieved in the agricultural obstacles with an average accuracy rate of 89.54%, and a recall rate of 90.18%. In the first kind of obstacle, the mean error and mean error ratio of the improved YOLOV3 model were 38.92% and 37.23% lower than those of the original one, while 53.44% and 53.14% lower than those of the HOG+SVM model, respectively. In the second kind of obstacle, the mean error and mean error ratio of the improved YOLOV3 model were 26.47% and 26.12% lower than those of the original one, while 41.9% and 41.73% lower than those of the HOG+SVM model, respectively. In the third kind of obstacle, the mean error and mean error ratio of the improved YOLOV3 model were 25.69% and 25.65% lower than those of the original one, while 43.14% and 43.01% lower than those of the HOG+SVM model, respectively. In addition, there was no obvious change in the mean error, mean error ratio, and mean square error of the three models, when changing the distance between obstacle and vehicle. The average error ratio was 4.66% in the depth estimation of obstacles under the dynamic scenario, and the average time was 0.573 s. An electrically controlled hydraulic steering was also used in time for obstacle avoidance during depth warning. The findings can provide an effective basis for environment perception for agricultural vehicles in autonomous navigation. In the following research, the more lightweight YOLOv3-tiny model and the terminal processor Xavier with higher computing power can be selected to conduct the depth estimation, aiming to increase the real-time inference speed of visual perception system in modern agriculture. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.