Development of a novel pull-cutting end-effector for ex-situ robotic harvesting of white asparagus based on MBD-DEM coupling simulation

The existing selective harvest of white asparagus originally imitates manual operation, belongs to in-situ harvesting mode, and improves harvesting speed by reverse motion compensation resulting in complicated mechanical and control. Asparagus spear grows underground with limited harvesting space, brittle and easily damaged. The design of robotic end-effector for efficient selective harvest is still a challenge. Inspired by ‘fishing with nets in moving boat’, this paper firstly proposed a novel ex-situ harvesting mode with sequential subprocess of move-penetrating, pull-cutting, lifting-out, and eject-throwing and the rigid-flexible coupling pull-cutting end-effector (PCEE) was designed. Secondly, the model of PCEE-soil-spear was established, and the two-way coupled MBD-DEM (Multi-Body Dynamics and Discrete Element Method) analysis was utilized to the harvesting process to clarify the interactive forces and element motions. Finally, the harvesting test bench with designed PCEE was built and tested harvesting performance. The experimental results showed that the maximum soil resistances in the moving and penetrating directions during the move-penetrating subprocess and in the moving direction during the pull-cutting subprocess were 445 N, 530 N and 624 N, and the relative errors between the test and simulation were 5.3 %, 11.7 % and 15.2 %, respectively, which validated the MBD-DEM coupling model. The harvest success rate of proposed PCEE was 90 % and the average single cycle time was 3.2 s, which demonstrated its effectiveness and feasibility for the selective harvesting robots of white asparagus.