Vibratory threshing mechanism and multi-species threshing parameter testing of wine grapes based on discrete element method

This study aimed to achieve precision harvesting of wine grapes by investigating their vibratory threshing mechanism and measuring key harvesting parameters across eight varieties at different maturity stages. This study measured the geometric and physical parameters as well as the mechanical properties of Marselan bunches. The average values of the fruit detachment force (FDF) and fruit bursting force (FBF) at the optimal ripeness stage were found to be 3.22 N and 8.52 N, respectively. Based on the measurement results, a discrete element flexible model of grape bunches was established using the Bonding V2 Model. A vibration-based threshing parameter testing system for wine grapes was developed, featuring adjustable vibration frequency, angular amplitude, and movement speed, as well as functionality for detecting the vibration intensity of grape bunches. The accuracy of the simulation model was verified by comparing results with bench tests. A multifactor simulation test was conducted on Marselan grapes at the harvestable stage, using a Box-Behnken Design (BBD). The experimental factors included frequency, angular amplitude, movement speed, and sugar content, with threshing rate and breakage rate as response variables. The relative errors between simulation and bench tests were 2.36 % for threshing rate, 7.86 % for breakage rate, and 10.3 % for maximum acceleration amplitude. Analysis revealed two primary forms of berry threshing: impact threshing form, which predominantly affects the breakage rate, and vibratory threshing form, which primarily influences the threshing rate. It was identified that operational parameters (frequency, angular amplitude, and movement speed) and inherent varietal characteristics (FDF, FBF, and berry mass) were key factors affecting threshing quality. Furthermore, the relationships between these parameters and the threshing rate and breakage rate were clarified. The coefficient of breakage (CB) was introduced as an indicator of grape suitability for vibratory threshing for the first time. Grapes with a CB below 0.1 were considered unsuitable for mechanical threshing. Finally, orthogonal tests were conducted to identify the optimal combination of threshing parameters for several varieties at different maturity levels. The results provide a reference for optimizing the mechanized harvesting of wine grapes.