Optimization and validation of root-cutting device for Chinese cabbage harvester based on discrete element method

Optimization of the structural and working parameters for a cutting system are essential to improve the working performance and reduce the cutting energy consumption and mechanical damage of the vegetable harvester machinery. An asymmetric root-cutting system was developed to efficiently solve the problems of high cutting loss and energy consumption of mechanized harvesting of Chinese cabbage. Discrete element method (DEM) and microscopic observation were utilized to establish the simulation model of Chinese cabbage, and a delamination particle model was proposed in terms of contact interactions. The experimental methods of Plackett-Burman design (PBD), steepest ascent (SA), and Box Behnken design (BBD) were applied to determine significant factors from eight major influential factors and to optimize the structural and working parameters of the cutting system, and further validated experiments were performed according to the practical feasibility of the optimal combinations of parameters. Finally, the cutting mechanisms were revealed both at a macro and micro level by comparative analyses of the cutting process using EDEM simulation. On this basis, the designed root-cutting device was configured on the self-propelled Chinese cabbage harvester for experimental verification. Results showed that the four most impactful factors on maximum reaction force (MRF) of root-cutting were determined in the order of cutting pitch angle (D), cutter speed (A), walking speed (E), and cutter overlap amount (B), and most interactions between interactive factors had consequences for the cutting force change, except A and cutting position (C). The minimum value of MRF was 88.62 N when the cutter speed, cutter overlap amount, cutting pitch angle, and walking speed were 186.91 r min -1, 17.87 mm, 12.26 degrees, and 0.30 m s- 1, respectively. Results of the validated experiments indicated that the measured MRF was 93.41 N under the specified parameters of 187 r min -1, 18 mm, 12 degrees, and 0.3 m s- 1. This value was found to be comparable to the original value, within an acceptable percentage change of 5.41 %. These findings demonstrate the feasibility of optimizing the cutting system based on DEM. In comparison to the three most prevalent combinations of cutting disks, the cutting system utilizing one concave disk and one sawtooth disk could efficiently sever roots over shorter periods during the cutting process. The combined disks were arranged at an angle of 12 degrees in the longitudinal direction. The forces of each layer of the particle structure inside the root were inconsistent during the root-cutting process. The internal bonding forces of the two-layer particle model played a role as a structural support of the parenchyma in the root epidermis, and the connecting epidermis between the inner and outer epidermis stand greater forces. The result of the field experiment showed that the average productivity of the Chinese cabbage harvester was 0.11 hm2 h-1, and the average qualified rate of root-cutting was 93.40 %, which demonstrated that the optimized root-cutting device efficiently fulfills the harvest requirements of high efficiency, and low cutting energy consumption and damage.