基于气动无损夹持控制的番茄采摘末端执行器设计与试验

为实现类球形果实采收过程中稳定夹持和无损采摘,该研究以番茄为研究对象,设计了一款全气动吸-夹一体式无损采摘末端执行器。首先设计空间多连杆三爪机构,采用3个夹持爪单元空间轴向均布的方式构成空间多连杆末端执行器主体机构,实现中心吸盘回拉果实和夹持爪夹持果实两个动作由单一主动气缸驱动并实现顺序动作;其次,建立末端执行器夹持爪单元的数学模型,并确定满足夹持爪尖端张开最大范围156 mm和吸盘回拉移动最大距离38.7 mm条件下的末端执行器结构参数,通过ADAMS软件对其进行运动学和动力学仿真分析,获得各部件间运动速度和加速度的相对关系,以及夹持力与气动系统压力和果实尺寸的关系。最后,设计并搭建具有压力负反馈和气压连续调节功能的电气伺服控制系统,通过分析果实在拉动和转动两种情况的滑移试验,提出基于动态标准差波动上升节点的双阈值滑移判别算法和基于滑移判据及损伤极限压力的无损采摘控制策略。204个不同尺寸番茄果实的实地采摘试验表明,末端执行器采摘成功率为96.03%,采摘过程耗时5 s,采摘过程中的直接损伤率为1.58%,72 h褐变率为1.76%。结果表明该采摘末端执行器具有较好的采摘效果,可满足实际工作需求。

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

Abstract: Harvesting of the fresh fruit is 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 robot 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 envelop was designed, which was arranged symmetrically on 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 was limited to the axial direction of air gripper, when three claws opened or closed. Thus, the two motions of pulling and gripping the fruit which was sucked by vacuum cup were driven by single active component in order. Secondly, the structural parameters of end effector with 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 outer diameter from 50 to 100 mm were selected for field trials. The indexes of picking success rate was defined to evaluate the performance of end-effector. The direct damage rate and 72 h browning rate was 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.