Author：Ming Ma, Quansheng Jiang, Haochen Wang, Yehu Shen & Fengyu Xu

Abstract

The pneumatic gripper in industrial applications has the advantages of structure simplicity and great adaptability, but its gripping power is usually limited due to the low modulus of soft materials. To address this problem, a novel bionic pneumatic gripper inspired by spider legs is proposed. The design has two pairs of symmetrical fingers, each finger consists of two pneumatic actuated joints, two rigid links and one pneumatic soft pad. The rigid link connects the pneumatic chamber which is enclosed in a retractable shell to increase the actuation pressure and the gripping force. The compressibility and elasticity of the soft joint and pad enable the gripper to grasp fragile objects without damage. The modeling of the bionic gripper is developed, and the parameters of the joint actuators are optimized accordingly. The prototype is manufactured and tested with the developed experimental platform, where the gripping force, flexibility and adaptability are evaluated. The results indicate that the designed gripper can grasp irregular and fragile items in sizes from 40 to 140 mm without damage, and the lifting weight is up to 15 N.

 Introduction

Gripping and manipulation are the most significant ways to interact with the environment in industrial logistics transport. Although traditional robotic grippers possess a high load-bearing capacity and control precision, they are not appropriate for grasping irregular, soft, or delicate objects. On the other hand, a universal grasping device with great adaptability is required in industrial production, agriculture, supermarkets, and disaster relief . The ideal gripper should adapt to the shape of the target object, increase the area of the contact patches between the object and end-effectors, and automatically adjust its grip force. Soft grippers can passively deform to meet the contour of a variety of target items because of their compliance . Elastomer actuators have the propensity to deform and distribute loads uniformly due to their soft materials and hollow pressure structure, which normally keeps the maximum applied force of an actuator within a safe range. This property makes many elastomer actuators suitable for interaction with humans and fragile objects.

Soft pneumatic grippers have become the focus of current research due to their high flexibility . Mutlu et al.  developed a gripper with a calabash chamber that could capture objects weighing up to 180 g. The adaptive palm and surface patterned feature are combined, the soft gripper elevates the grasping force . To increase the rigidity of the soft pneumatic gripper, Miron et al.  developed a silicone soft gripper wrapped in a woven fabric, which demonstrated a significant improvement in gripping capabilities. Yang et al. achieved a soft gripper with variable stiffness by adding particles to the soft gripper’s joints. Sun et al.  designed a soft pneumatic gripper inspired by pangolin scales with great rigidity and could hold 0.5 kg objects. The soft grippers proposed by these communities can successfully execute a variety of grasping tasks under objects pose uncertainties when compared to traditional robotic grippers. However, the grabbing forces of soft grippers are generally insufficient because of the low Young’s modulus of soft materials .

When one examines the anatomical makeup of our hands, they reveal a combination of soft muscles and hard bone skeletons. According to the biological evidence, which points to a similar strategy in soft robotics, a multi-material gripper will improve its performance in terms of bearing its weight and exerting heavier payloads. Some research institutions have invented mixed-material grippers , but these grippers have flaws such as rigid materials not being used as support structures, a difficult production procedure, or the inability to grasp the object like a finger.

This paper merged the rigid materials with the soft chamber, inspired by the structure of spider legs , which are thin and lengthy, with the ability to hold multiple times the body's weight and a quick reaction time, and proposed a bionic pneumatic four-finger soft gripper. This hybrid gripper with four fingers combines the intense pressures imposed by rigid fingers with the adaptable behavior of soft pneumatic actuators mechanism that increases gripping quality and the range of applications. The proposed grippers’ performance is evaluated using three types of experiments: (i) force exertion experiments to determine the maximum forces that can be applied by the fingers, (ii) finger bending angle test, and (iii) grasping tests with various everyday objects.

The rest of the paper is organized as follows: Sect.  designs the Structure of the soft gripper, Sect.  presents the mathematical model of the soft finger, and Sect. details the grasping experiments and the results. Finally, the findings and future directions are discussed in Sect.