Monopterus albus (rice field eel) is one of the commercially important fish species in Asian areas. It is often required to efficiently and accurately grade the different body sizes in aquaculture production and standardized processing. In this study, a roller-type machine was proposed for the rice field eel grading. A vibration mechanism and a spraying device were integrated to fully meet the demand for the high-throughput and high-precision size classification. The numerical simulation and experimental validation were conducted to evaluate the grading performance of the equipment. A coupled dynamic model of the grading machine was then established to optimize the operating parameters using RecurDyn–EDEM co-simulation technology. Specifically, the rigid–flexible multibody dynamics of the grading system was modeled in RecurDyn platform, including the vibration exciter, grading sieve, and supporting structure. While the discrete element method was implemented to characterize the motion, collision behavior, and contact interactions of individual rice field eels in EDEM software. The optimal contact models and mechanical parameters were defined to represent the eel–eel and eel–machine interactions. Spraying-assisted lubrication was evaluated to improve material dispersion and then reduce excessive friction during grading. The performance of classification was quantitatively assessed to select grading efficiency and accuracy as the primary indices. A single-factor experiment was conducted to clarify the independent effects of vibration frequency, sieve inclination angle, and feeding quantity on the grading performance. The results showed that the grading efficiency first increased and then decrease, with an increase in the number of rice field eels, while the accuracy decreased gradually. The grading efficiency increased, as the inclination angle of the grading sieve increased, whereas the grading accuracy decreased. Furthermore, the grading efficiency increased continuously, with increasing vibration frequency, while the grading accuracy first increased and then decreased. Subsequently, a multi-factor orthogonal experiment was applied to identify the relative importance of each parameter and their combined influence on grading efficiency and accuracy. Physical experiments were further verified under optimal parameters using a prototype grading machine. A comparison was made on the numerical predictions and measurement. The results showed that the greatest influencing factors on both grading accuracy and efficiency were the grading sieve inclination angle, followed by feeding quantity and vibration frequency. The grading performance was reduced, due to insufficient dispersion, material accumulation, or excessive sliding velocity on the sieve surface. Parameters was then optimized to improve grading performance using Design-Expert software, with the grading efficiency and grading accuracy as the objective variables. The optimal combination of parameters was determined as a vibration frequency of 30 Hz, a grading sieve inclination angle of 2.6°, and a feeding quantity of 27 individuals. The better performance was achieved in the grading efficiency of 0.637 individuals per second, and the average grading accuracy of 0.867. The relative errors between experiment and simulation were 7.378% for the grading efficiency and 3.46% for the grading accuracy, indicating better consistency and prediction of the RecurDyn–EDEM coupled simulation. The optimal grading parameters were obtained from the complex interaction between vibration-driven machinery and elongated aquatic organisms. A reliable basis can provide for the structure and performance of eel vibratory grading equipment. This finding can offer practical technical support for the intelligent, efficient, and scalable grading of aquatic products with similar morphologies.

The scale of Monopterus albus farming in Hubei Province is large, but there are few pre-treatment processing devices. The current Monopterus albus pretreatment device is mainly dissected and killed. Aiming at the problems of poor stability, low efficiency, and low meat yield of existing Monopterus albus deboning devices, an adaptive Monopterus albus deboning device was designed, which was mainly composed of laparotomy components, transitional components, and deboning components. The theoretical analysis of the process of fish laparotomy, transition transportation, and deboning was carried out, and the key factors affecting the performance of the deboning device were obtained. Taking the meat yield rate and sensory quality scores as evaluation indexes, the single-factor test of the distance between the pressure wheel and the V-block, the initial clearance of the transition wheel, the rotational speed of the transition wheel, the shape of the deboning blade disc and the rotational speed of deboning blade disc were carried out to determine the reasonable range of each parameter. The test results showed that when the distance between the pressure wheel and the V-block was 15.50-16.75 mm, the meat yield rate was higher; when the initial clearance of the transition wheel was 9 mm, the sensory quality scores of the fish body was the highest; the increase of the rotational speed of the transition wheel and the rotational speed of the deboning blade disc is beneficial to improve the meat yield rate and sensory quality scores; and the shape of the boning blade disc had little influence on the meat yield rate and the sensory quality scores. The orthogonal test of four factors and three levels was carried out with the distance between the pressure wheel and the V-block, the initial clearance of the transition wheel, the rotational speed of the transition wheel, and the rotational speed of the deboning blade disc as the test factors. The results showed that the order of significance of each test factor on the meat yield rate was the distance between the pressure wheel and the V-block, the initial clearance of the transition wheel, the rotational speed of the transition wheel, and the rotational speed of the deboning blade disc; and the order of significance on the sensory quality scores was the initial clearance of the transition wheel, the distance between the pressure wheel and the V-block, the rotational speed of the transition wheel and the rotational speed of the deboning blade disc. When the distance between the pressure wheel and the V-block was 16.75 mm, the initial clearance of the transition wheel was 9 mm, the rotational speed of the transition wheel was 648 r/min, and the rotational speed of the deboning blade disc was 3 200 r/min, the deboning effect was the best. The meat yield rate of the deboning device was 84%, and the total sensory quality scores of the fish after processing reached 17.3. The device can process Monopterus albus with a body mass of 75-150 g, and the production efficiency reaches 16.8 strips/min. Compared with the current commercially available deboning device, the feeding is more convenient and the production efficiency is higher. It can provide a reference for the research of Monopterus albus pretreatment device.

Monopterus albus is one of the popular aquatic fish in Asia. Manual processing of eel cannot fully meet the large-scale production at present, due to the time-consuming, laborious, and low safety. In view of its slender and slimy body, an eel-cutting machine is required for the consumption of Monopterus albus. In this study, a miniature machine of automatic cutting was designed for the living Monopterus albus with a sticky surface. The machine consisted of conveying, clamping, and cutting parts. The double-clamped wheel to roller was used to capture the eel for transportation during cutting. The switch between laparotomy and back dissection was realized to change the feeding channel. The adaptive mechanism was equipped to meet the requirements of different sizes of eel cutting. Firstly, the geometry and weight parameters of Monopterus albus were measured using a straight ruler and precision electronic balance. The tribological parameters on the surface of Monopterus albus were measured by the inclined plane. The mechanical parameters of Monopterus albus meat were verified by compression test. Secondly, according to the rigid-flexible coupling principle, the rigid-damping model was equivalent to the contact between the Monopterus albus body and the clamping wheel and slide. The rigid-flexible coupling model was established by Abaqus and Adams simulation software. The analysis model was optimized to take the conveying time of the fish body, output speed, clamping force, and whole machine vibration as the test indexes in single-factor tests, such as the spring stiffness, slope Angle of the slide, and the speed of the clamping wheel. The results show that the clamping force first increased and then decreased with the increase of spring stiffness and the vibration amplitude of the whole machine decreased. But there was little influence on conveying time and output speed. The output time of the fish body decreased with the increase of slope angle, while the output speed, clamping force, and vibration of the whole machine increased. There was an increase in the conveying time of the fish body, the output speed, the clamping force, and the vibration of the whole machine, with the increase of the speed of the clamping wheel. The single- and multi-factor analyses were carried out to obtain the optimal working conditions. Specifically, the clamping wheel speed was 600 r/min, the spring stiffness was 1.2×10³ N/m, and the slope angle was 16°. Finally, the processing experiment was performed on the automatic cutting machine for Monopterus albus. There was consistency in the vibration amplitude range and the relationship between the three directions of the machine measuring points with the theoretical analysis. At the same time, the output speed of the fish body output machine was also measured to better agree with the theoretical analysis. The cutting of the machine under optimal working conditions has fully met the requirements. The correctness of the theoretical analysis was also verified by the test. The rigid-flexible coupling model can provide a strong reference to research and develop the subsequent equipment.