Parameter optimization of stroke-adjustable and monodirectional pulling fruit harvester

Abstract: The fruit harvester is one of the most common machines in orchards. However, current mechanical harvesters are not suitable for orchard harvesting due to fixed operating parameters, especially the frequency and amplitude. Therefore, a monodirectional pulling fruit harvester with adjustable stroke was proposed based on slider-crank mechanism to improve the harvesting efficiency. The harvester consisted mainly of the case, a steel cable, and a front actuator. In particular, the shaking mechanism in the case was composed of one crank-rocker-slider mechanism and one crank-slider mechanism. The former was used to generate linear reciprocating vibration and the latter to adjust stroke. The stroke changed with the angle of the stroke-adjusting crank. Furthermore，a kinematic model of the shaking mechanism was established to describe its stroke and range dynamics. In order to define the influence of linkage length on the stroke and range, a program was developed based on the displacement equation of the shaking mechanism. The calculation result indicated that the shaking crank had more influence on the stroke than the rocker and stroke-adjusting crank. In addition, the stroke increased with the length of the shaking crank, but decreased with the length of the rocker and stroke-adjusting crank. Range was significantly affected by those two cranks and increased significantly with them. To make the fruit harvester more compact and avoid interferences among all components, the dimension of the shaking mechanism was optimized based on a genetic algorithm. The acceleration curve of the actuator slider at various rotation angles of the stroke-adjusting crank was acquired by further calculations. Moreover, the three-dimensional model of the shaking mechanism was built according to the optimal parameters. The dynamics of the shaking mechanism were simulated in ADAMS software when the shaking crank and stroke-adjusting crank rotated. The rotation speeds of the shaking crank and the stroke-adjusting crank were set to 840 r/min and 7 r/min, respectively. Then the driving torque of those two cranks, the displacement and the acceleration of actuating slider were obtained. The simulation results showed that the driving torque of those two cranks fluctuated wildly while the mechanism generated a two-frequency, reciprocating movement. Finally, based on the kinematic analysis and dynamics simulations, a physical prototype was developed. After that, five Chinese hickory trees with similar age and height in the orchard were measured by digital dynamometer to obtain the average separating force of the fruit, and then the fruit was harvested by the stroke-adjustable fruit harvester to evaluate its performance. The results revealed that the average separating force of fruit was 7.3 N, and the average harvesting percentage was 63.9%, which demonstrated that this mechanical harvester can be used to harvest fruit. By further analysis of problems and causes in the experiment, the cantilever structure in this mechanism needs to be regulated; otherwise it would easily cause vibration when the machine runs at ordinary high speed. In addition, the impact of small branches on energy transmission should be fully considered because part of the fruit grew at the end of small branches.