Fragile fruit uploading and packaging are labor-intensive and time-consuming steps in postharvest industry. With the aging of the global population, it is supposed to develop robotic grasping systems to replace manual labor. However, damage-less grasping of fragile fruit is the key problem in robotization. Inappropriate grasping force will result in damage, early-stage bruise, or slip. Benefits from the advantages of softness and compliance of a pneumatic-driven soft gripper have been widely adopted for agricultural product and food manipulation. Nevertheless, pneumatic gripper is a complex, multivariable, nonlinear, and long time-delay control system, which is difficult to achieve robust closed-loop grasping force control. In this study, we aim to solve this problem and developed a robotic grasping force control system with pneumatic gripper and matrix force sensor. The force distribution condition was explored to tackle the problem in changing of the main contact point. A double closed-loop control method was proposed based on Kalman filter (KF) and proportion integration differentiation controller with dead band. The external and internal control loops were force controller and air pressure of the pump controller, respectively. The double closed-loop controller with dead band achieved robust grasping force control through air pressure. The experimental results validated the effectiveness of the KF method for denoising and the matrix force visualization method for exploring grasping mechanism. Ablation studies were carried out to demonstrate the effectiveness of the multiple grasping force sensing units in matrix form and the dead band in the controller. The maximum steady-state error was 0.07 N. In addition, the generalization performance and the antidisturbance ability of the grasping force control system was also validated. In summary, the problem in closed-loop control of the grasping force for pneumatic gripper has been solved in our study, and the method in this research is potential to be deployed in fruit postharvest industry.

中文翻译：

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迈向无损伤的机器人易碎水果抓取：一种气动软夹持器的闭环力控制方法。


易碎水果的上传和包装是采后行业中劳动密集型且耗时的步骤。随着全球人口老龄化，它应该开发机器人抓取系统来取代体力劳动。然而，无损伤地抓取易碎的水果是机器人化的关键问题。不适当的抓握力会导致损坏、早期瘀伤或滑倒。气动软抓手的柔软性和柔顺性优势已广泛应用于农产品和食品操作。然而，气动夹爪是一个复杂、多变量、非线性和长延时的控制系统，很难实现鲁棒的闭环抓取力控制。在本研究中，我们旨在解决这个问题，并开发了一种带有气动夹爪和矩阵力传感器的机器人抓取力控制系统。探索力分布条件以解决主接触点变化的问题。提出了一种基于卡尔曼滤波 （KF） 和带死区比例积分微分控制器的双闭环控制方法。外部和内部控制回路分别为力控制器和泵控制器的气压。带死区的双闭环控制器通过气压实现了强大的抓取力控制。实验结果验证了 KF 方法去噪和矩阵力可视化方法探索抓取机制的有效性。进行了消融研究，以证明矩阵形式的多个抓握力传感单元和控制器中的死区的有效性。最大稳态误差为 0.07 N。 此外，还验证了抓取力控制系统的泛化性能和抗干扰能力。综上所述，本研究解决了气动夹爪抓取力的闭环控制问题，本研究的方法有望应用于水果采后行业。