Centering grabbing with variable orientation envelope for hydroponic leafy vegetables cultivated in pipeline

Hydroponic leafy vegetables can be enveloped and then clamped into the center of the planting hole in the cultivated pipeline, in order to avoid the root residues for the accurate root cutting of stems. However, the centering envelope grabbing mechanism with the fixed orientation can vary greatly with the different initial inclination angle and the position of hydroponic leafy vegetables. In this study, a variable orientation grabbing mechanism was proposed to adjust the orientation relative to the center of the planting hole during grabbing. Taking Lvbao hydroponic Chinese kale as the research material, the related properties were measured to determine the structural parameters. The initial opening angle of 15°, the closing angle of 30°, the rotating angle of 45°, and two times rotating speed of the two fingers were determined for the gripper during envelope grasping. A comparison test was performed on the grasping performance and success rate of the fixed and variable orientation grabbing mechanism with different initial inclination angles and positions using multi-body dynamics simulation with ADMAS software and laboratory grabbing test. The simulation results showed that the clamping inclination angle increased significantly with the increase of the initial inclination angle. At the initial inclination angles of 50°, 65°, and 80°, the mean clamping inclination angles of 84.25°, 85.86° and 87.76° for the variable orientation centering and clamping state, were better than those of 82.17°, 83.73° and 86.41° for the fixed orientation approach. At the position of P₁-P₈, the mean clamping inclination angles of 84.89°, 85.93°, 84.81°, 86.86°, 85.16°, 86.47°, 86.70°, and 86.78° for the variable orientation centering and clamping state, were better than those of 82.67°, 85.09°, 84.64°, 85.09°, 84.13°, 83.17°, 83.94°, and 83.53° for the fixed orientation. The grabbing test also show that the grasping inclination angle increased, as the initial inclination angle increased. In the inclination angles of 45°-60°, >60°-75°, and >75°-90°, the mean grasping inclination angles of the variable orientation grabbing at different positions were 85.78°, 87.91°, and 87.57°, respectively, which were better than those of 83.08°, 85.00° and 86.76° for the fixed orientation. At the position of P₁-P₈, the mean grasping inclination angles of 87.18°, 87.10°, 84.83°, 88.13°, 87.63°, 86.90°, 87.92°, and 88.21° for the variable orientation grabbing, were better than those of 80.89°, 86.98°, 88.07°, 85.88°, 81.56°, 83.99°, 85.51°, and 86.68° for the fixed orientation. In the hydroponic Chinese kale with different initial inclination angles, the mean values of grabbing success rate were 91.67%, 95.83% and 100.00% for the variable orientation, which were better than those of 87.50%, 91.67% and 100.00% for the fixed orientation. Except for the P₂ position, the mean values of grabbing success rate of variable orientation grabbing at different position were higher than those of the fixed orientation. It infers that the simulation was consistent with the test. The variable orientation grabbing was more applicable for the better performance of the centering envelope grabbing mechanism. High-speed photographic images presented that the main reason for the grabbing failure at P₂ was that the double-layer finger was directly contacted and pushed with the extended leaves of the hydroponic Chinese kale during entering and positioning. Therefore, it can be expected to avoid the collision of the fingers and the extended leaves in future. The finding can provide a strong reference for the harvesting equipment of hydroponic leaf vegetables.