This study sought to construct and empirically validate a discrete element method (DEM) particle model representing post-tillage soil blocks. This model was developed to facilitate a detailed examination of granular movement and contact mechanics during the shaping process of planting chambers for rape plants. The research specifically targeted the sticky, cohesive soil prevalent in rice paddy fields of the middle and lower Yangtze River region. Simulations were conducted using EDEM software to improve the accuracy with which soil-tool interactions are predicted for the design and optimization of mechanical transplanters. The physical and bonding parameters of the sticky soil were calibrated using the Hertz-Mindlin with Johnson-Kendall-Roberts (JKR) contact model and Hertz-Mindlin with Bonding contact model. A particle replacement method was adopted to create a discrete element model of cohesive soil aggregates with different shapes and sizes after rotary tillage. The accumulation angle of soil aggregates was used as the evaluation index in both the simulation and physical experiments. Design-Expert software was used to design a four-factor, three-level simulation experiment to identify the optimal parameter combinations for the physical and mechanical properties of the sticky soil and the JKR contact model, which comprised a soil-soil static friction coefficient of 0.32, soil-soil rolling friction coefficient of 0.10, soil-steel static friction coefficient of 0.51, and surface energy of soil for the JKR model of 5.50 J/m². Next, the steepest climbing test and Box-Behnken orthogonal combination test were then used to narrow down the range of values for the significant factors and identify the optimal parameter combinations for the bonding contact model parameters, which included a bonding bond normal contact stiffness of 2.1×10⁶ N/m, a bonding bond tangential contact stiffness of 2.2×10⁶ N/m, a normal ultimate stress of 0.55 MPa, a tangential ultimate stress of 0.55 MPa, and a bonding radius of 12 mm. Field experiments were conducted using a flat box device to measure the soil evenness and firmness after ridge formation and compaction by a rotary tiller. The results of these experiments were compared with the discrete element simulation optimization results. The relative errors between the field test results and the simulation test results for soil flatness and compaction were 10.7% and 9.8%, respectively, which indicated good accuracy of the parameters calibrated and optimized by EDEM discrete element simulation software. Overall, this research can provide a reference for understanding the working mechanism and optimizing the parameters of soil touching components in rape transplanting equipment.

Seedling transplanting is one of the most important steps during oilseed rape cultivation. However, critical challenges still remained in seedling transplanting, such as the high pot damage rates. Some difficulties are also in separating the seedling plugs from their trays. In this study, an innovative rotary feeding mechanism was proposed to reduce the adhesive forces between seedling plugs and tray walls, thereby improving the efficiency and reliability of the transplanting. A rotary tray conveyer was also designed and tested in the seedling transplanting machine for oilseed rape. Specifically, the damage to the seedling structure was minimized to maintain the high retrieval success rates. The mechanical behavior of the seedling plugs was investigated to determine the influencing forces on the integrity during separation. Internal forces, including the cohesion between substrate particles and root-soil binding, were analyzed alongside the external forces such as the friction and adhesion between the seedling plug and tray walls. A rotary bending approach was introduced to verify the device, wherein the trays were bent along the specific trajectories to expand the openings at the top while compressing the bottom. After that, the contact area and adhesion forces were reduced to easily separate the plugs without structural failure. The critical parameters were optimized using a theoretical model, such as the bending radius and the angle. The key components were included in the rotary tray conveyer: a seedling tray frame, a chain conveyor system, transmission mechanisms, and seedling protection rails. A servo motor of the conveyor system was used to provide intermittent motion, thus driving the trays along a curved path. The controlled deformation was applied at the specific angles and radii, as the trays moved through the predefined bending segments. As such, the adhesive forces were significantly reduced after deformation. Extensive experiments were conducted to evaluate and optimize the performance of the device. Three critical factors were determined, including the substrate moisture content, rotary radius, and bending angle. A systematic investigation was also made to determine the effects of these parameters on the pot damage rates and detachment force reduction. Orthogonal experiments were performed, where the 40 day oilseed rape seedlings were taken as the substrate moisture contents of 50%±1% %, 55%±1%, and 60%±1%; the rotational radius of 58, 113, and 168 mm; and the bending angles of 35°, 55°, and 75°. The results indicated that better performance was achieved in a pot damage rate of 2.01% and a detachment force reduction rate of 21.08%. The optimal configuration was also obtained in a substrate moisture content of 50%±1 %, a rotary radius of 113 mm, and a bending angle of 55°. Field trials also validated the effectiveness of the device in practice. The critical metrics were measured, such as the seedling retrieval success rates during operation at a working speed of 0.4 m/s. An average retrieval success rate of 96.75% was improved from 3.70% to 6.01%, compared with the existing transplanters. The field trials also identified promising potential improvements. The vibration-induced errors were mitigated by the occasional interference caused by overlapping seedling stems. Significant advancements were obtained to solve the limitations of the traditional seedling transplanting machines. The rotary bending was employed to reduce the adhesion forces. The precise and efficient plug separation was realized for the structural integrity. The reliability and efficiency of the automated transplanting were enhanced to reduce the labor dependency for agricultural productivity. Furthermore, the finding can also provide valuable guidelines for developing transplanting technologies for crops with similar requirements of plug separation.