Design and experiment of the whole-skin peeling system of Channa argus based on three-dimensional reconstruction

Channa argus is one type of predatory fish in the family Channidae native to fresh waters in eastern China. The skin of Channa argus has been an ideal substitute for the shortage of wild high-grade leather raw materials, due to the beautiful pattern, strong toughness, excellent breathability, waterproof, and strong anti-expansion. Annual production of Channa argus skin has been more than 500 000 tons in the broad market of China. However, the manual peeling of Channa argus skin has been limited to the deep processing industry in large-scale production at present, leading to low efficiency, low integrity of fish skin, and security risks. Therefore, this study aims to design the automatic whole-skin peeling system of Channa argus using the 3D (Three-Dimensional) reconstruction. The morphological parameters were also collected, where the head, body and tail sections were changed dramatically. The sticky skin was then fixed for the peeling operation on the hard hip fin. The whole-skin peeling machine was designed, including clamping, cutting, skin and flesh separation, as well as peeling devices. The vision system was selected with the industrial camera, LED (light-emitting diode) lamp band and upper computer. The industrial camera was used to capture the images of Channa argus. The LED lamp band was to provide an ideal optical environment for image acquisition. The upper computer was also used to preprocess the images. 3D reconstruction of images was performed to identify and locate the pectoral fin position using YOLOv5s. The cloacal aperture position was then identified and located by ROI (region of interest). The operation path of each mechanism was finally realized, according to the 3D point cloud model. The control system was designed to accurately control the travel mechanisms and the actuators in the whole-skin peeling device. The position feedback PID (Proportion Integral Derivative) with magnetic encoder was used for the closed-loop control of the motion module, in order to avoid the out-of-step phenomenon during operation. In the actuators, the air path was designed to realize the precise control of the cylinder gripper, horizontal slide cylinder, longitudinal slide cylinder, and double rod double shaft cylinder, according to the control requirements of pneumatic components. The duty ratio of the PWM (pulse width modulation) signal was set to realize the control of a DC (direct current) motor speed, in order to calibrate the relationship between PWM duty ratio and rotational speed. The performance of the prototype was evaluated by the total system operation duration, fish skin removal rate, skin and flesh separation index and breakage rate of fish skin of the whole-skin peeling device. The test results showed that a high peeling speed was achieved in the machine. The total operation duration of the system varied with the size of the Channa argus, ranging from 172 to 195 s, with an average of 183 s. After peeling, the fish skin removal rate ranged from 93.10% to 95.70%, with an average value of 94.60%. There was a large actual area of Channa argus skin after stripping, which effectively reduced the waste of raw materials. The separation index of skin and meat ranged from 9.91 to 9.98, with an average value of 9.93. There was less fish left on the skin after stripping, indicating a higher stripping efficiency than before. The breakage rate of fish skin ranged from 5.00% to 10.00%, with an average of only 6.25%. The small number of damaged fish skin in each group fully met the requirements of whole-skin peeling technology. This finding can provide a strong reference for the development and design of the whole-skin peeling system of Channa argus.