Three-finger grasp planning and experimental analysis of picking patterns for robotic apple harvesting

Abstract

Operational efficiency is an important bottleneck in the industrialization of apple picking robots. The action planning of grasping and picking is key to improving the operational efficiency of picking robots. In this study, first, from the perspective of grip stability, the grasping positions of a three-finger end-effector were planned in two cases to determine an optimal three-finger grip posture: one that is parallel to the apple stem–calyx axis and the other being centripetal to the apple equatorial plane. Second, based on the gripping posture parallel to the apple stem-calyx axis, four picking patterns were designed: vertical pull, horizontal pull, vertical rotation, and rotation–horizontal pull. With the horizontal displacement, vertical displacement, and rotation angle as the main factors influencing the picking process, it was found through ABAQUS simulation software and the response surface analysis method that the rotation–horizontal pull pattern with the combination of horizontal displacement and rotation angle yields the minimum separation force for apple piking. With the three-fingered hand in the optimal grasping posture parallel to the stem–calyx, the four picking patterns were tested on a dedicated apple picking experimental device equipped with a grip force acquisition sensor, and the test results showed that the average three-finger grip force required for rotary tug-pull picking was the lowest (10.33 N). The proposed rotation–horizontal pull pattern with the three-finger grip posture was proven to be the best pattern for apple picking. The results of this study provide a theoretical basis for further research on apple picking robots.