A Soft Gripper Design for Apple Harvesting with Force Feedback and Fruit Slip Detection

Abstract

This research presents a soft gripper for apple harvesting to provide constant-pressure clamping and avoid fruit damage during slippage, to reduce the potential danger of damage to the apple pericarp during robotic harvesting. First, a three-finger gripper based on the Fin Ray structure is developed, and the influence of varied structure parameters during gripping is discussed accordingly. Second, we develop a mechanical model of the suggested servo-driven soft gripper based on the mappings of gripping force, pulling force, and servo torque. Third, a real-time control strategy for the servo is proposed, to monitor the relative position relationship between the gripper and the fruit by an ultrasonic sensor to avoid damage from the slip between the fruit and fingers. The experimental results show that the proposed soft gripper can non-destructively grasp and separate apples. In outdoor orchard experiments, the damage rate for the grasping experiments of the gripper with the force feedback system turned on was 0%; while the force feedback system was turned off, the damage rate was 20%, averaged for slight and severe damage. The three cases of rigid fingers and soft fingers with or without slip detection under the gripper structure of this study were tested by picking 25 apple samples for each set of experiments. The picking success rate for the rigid fingers was 100% but with a damage rate of 16%; the picking success rate for soft fingers with slip detection was 80%, with no fruit skin damage; in contrast, the picking success rate for soft fingers with slip detection off increased to 96%, and the damage rate was up to 8%. The experimental results demonstrated the effectiveness of the proposed control method.

Perceptual Soft End-Effectors for Future Unmanned Agriculture

“…Of course, traditional end-effectors in harvesting processes may cause damage to agricultural products due to rigid contact [ 5 ]. Such damage directly affects the quality of agricultural products and increases production costs.…”

“…In the process of apple picking, researchers employed sensors to establish a relationship between gripping force, tension, and servo torque. This approach effectively eliminated potential peel damage that may arise during the picking operation [ 5 ]. Additionally, Jintao et al leveraged data gathered from sensors to achieve fruit classification [ 109 ].…”

“…In the future of unmanned agriculture, end-effectors, as an integral component, will primarily fulfill the tasks of harvesting, sorting, and detecting certain agricultural products [ 4 ]. However, these end-effectors present unique challenges compared to other sectors due to the irregularity of product positioning during harvesting, variations in sizes and shapes, and the fragile nature of agricultural products [ 5 ]. Additionally, end-effectors for grasping and manipulation applications in the agri-food domain must be robust and precise.…”

Robots and Autonomous Machines for Sustainable Agriculture Production

“…The third category has seven papers under the following sub-heading: Designs related to intelligent machinery execution [3,13,15,25]. Zhang et al [25] designed a pruning manipulator with five degrees of freedom for jujube trees.…”

“…It is of reference value to solve the problems of poor working conditions and the labor intensity of manually pruning jujube trees. Chen et al [3] developed a fin ray structure-based soft gripper mechanical model and its real-time servo-driven control strategy to reduce the potential danger of damage to the apple pericarps during robotic harvesting. Yu et al [15] designed a cotton seeder duckbill welding robot to improve the automation, welding efficiency, and welding quality of duckbill welding of cotton seeds.…”