Wanru Liu

Lateritic soils in tropical regions feature cohesive textures and high specific resistance, driving up energy demands for tillage and harvesting machinery. However, current equipment designs lack discrete element models that account for soil moisture variations, and the microscopic effects of water content on lateritic soil deformation remain poorly understood. This study aims to calibrate and validate discrete element method (DEM) models of lateritic soil at varying moisture contents of 20.51%, 22.39%, 24.53%, 26.28%, and 28.04% by integrating the Hertz–Mindlin contact mechanics with bonding and JKR cohesion models. Key parameters in the simulations were calibrated through systematic experimentation. Using Plackett–Burman design, critical factors significantly affecting axial compressive force—including surface energy, normal bond stiffness, and tangential bond stiffness—were identified. Subsequently, Box–Behnken response surface methodology was employed to optimize these parameters by minimizing deviations between simulated and experimental maximum axial compressive forces under each moisture condition. The calibrated models demonstrated high fidelity, with average relative errors of 4.53%, 3.36%, 3.05%, 3.32%, and 7.60% for uniaxial compression simulations across the five moisture levels. Stress–strain analysis under axial loading revealed that at a given surface displacement, both fracture dimensions and stress transfer rates decreased progressively with increasing moisture content. These findings elucidate the moisture-dependent micromechanical behavior of lateritic soil and provide critical data support for DEM-based design optimization of soil-engaging agricultural implements in tropical environments.

Design and performance test of a combined and adjustable precise rice seed metering device

With the aim to solve the problems of plug seedlings being damaged and the low success rate in the process of picking up seedlings, an open-and-close seedling picking manipulator with four fingers was designed. The clamping scheme with quadrangle inserting while clamping was designed in order to reduce the disturbance and injury of pot matrix soil. The working principle of the manipulator was expounded, and a mathematical model of the mechanism movement was established. The force transmission of the mechanism and force between finger and pot was analyzed, and the constraint condition of optimum force transmission efficiency and low damage when picking up seedlings was analyzed. Based on theoretical calculation and analysis, a set of optimal parameters and the trace curve of the finger end point were obtained. Based on the above theoretical calculation, kinematics parameters were analyzed and verified by Adams software. IF and STEP velocity functions were used to define the motion form of the driving source to simulate manipulator opening away from the seedling, straight down near the seedling, inserting into pot while clamping, and lift-off after the insertion to depth 45 mm. The simulation result proved the end point trajectory obtained by the motion simulation was basically consistent with that obtained by theoretical calculation. Velocity and acceleration curves of each mechanism component were obtained, and the result proved the velocity and acceleration of the tip of the finger changed greatly, and the inertia impact was large; the inertia force helped to clamp the pot. The manipulator was installed at the end of the transplanting platform. A plug seedling of “Zhongnong Luheng line pepper 363” was taken as the object, and the pot moisture content, seedling pick-up frequency, and finger material were used as experimental factors for the seedling pick-up test. The results showed that the above three factors had significant effects on the rate of pot damage and the rate of successful seedling pick-up. The optimum level was that when the moisture content was 45%, the frequency of picking seedlings was 20 plants·min⁻¹, and the clamping finger material was carbon steel; the damage rate of the pot body was 1.98%, and the success rate of picking seedlings was 98%. This manipulator has the advantages of stable seedling picking and a low damage rate and can be used for transplanting plug seedlings of plants such as pepper and tomato.