Design and experiment of tomato picking end-effector based on non-destructive pneumatic clamping control

Abstract: Harvesting of fresh fruit remains heavily reliant on manual labor. It is the most time-consuming and laborious part of the whole fruit production process, which accounts for about 40% of the total workload. Therefore, automatic fruit harvesting technology and agricultural picking robots become a focus of research, and one of the key questions is how to design an end-effector of harvester suitable for specific fruits without damaging fruit during the picking process. A pneumatic sucking-gripping integrated non-destructive end-effector for tomato was developed and a non-destructive control method was proposed in this study. Firstly, an innovative multi-link 3-finger mechanism with spatial envelope was designed, which was arranged symmetrically on a 3-finger type slide guide air gripper and driven by it. The vacuum cup was fixed in the center of three claws and was hinged to claws through multi-linkage. Based on this structure, the degrees of freedom (DOF) of vacuum cup were limited to the axial direction of the air gripper, when three claws opened or closed. Thus, the two motions of pulling and gripping the fruit which was sucked by a vacuum cup were driven by a single active component in order. Secondly, the structural parameters of end effector with a maximum envelope range of 156 mm and the maximum moving distance of sucker 38.7 mm are determined by establishing of kinematic model. Meanwhile, the relative relation between the motion velocity and acceleration of each component was obtained based on ADAMS kinematics and dynamics simulation analysis, as well as the relative relation between the holding force and pneumatic system pressure and fruit size. Thirdly, an electro-pneumatic system of end-effector with pressure feedback and pressure continuous regulation function was built. The Arduino Due programmable controller was used as the controller to control the proportional pressure regulating valve, so that the clamping force can be adjusted continuously in real time. In addition, FSR-402 pressure sensors were placed inside the three gripper fingers to detect the clamping force of each finger in the picking process. By analyzing the time domain signal of the digital quantity of the pressure sensors on the gripper fingers under the two slip conditions of the linear movement and rotation of the fruit relative to the gripper fingers, the dual-threshold slip judgment algorithm based on dynamic standard deviation rising fluctuation node and non-destructive picking control strategy based on slippage criterion and threshold of damage limit pressure were presented. Finally, the actuator performance test was carried out in the vegetable planting base of Bishan District, Chongqing, China. 204 ripe tomatoes of Hongyun 712 cultivar with an outer diameter from 50 to 100 mm were selected for field trials. The indexes of picking success rate were defined to evaluate the performance of the end-effector. The direct damage rate and a 72 h browning rate were used to measure the performance of non-destructive clamping control method. The experimental results showed that the picking success rate was 96.03% of this actuator, direct damage rate was 1.58%, 72 h browning rate was 1.76%, and time consuming of one picking process was 5 s, which demonstrated the picking end-effector could sufficiently meet the usage demand of fruit picking robot.