Toward Damage-Less Robotic Fragile Fruit Grasping: A Closed-Loop Force Control Method for Pneumatic-Driven Soft Gripper

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.