Design and implementation of an underactuated gripper with enhanced shape adaptability and lateral stiffness through semi-active multi-degree-of-freedom endoskeletons

Grasping is a key task for robots to interact with humans and the environment. Soft grippers have been widely studied and some have been applied in industry and daily life. Typical soft grippers face two challenges: lack of stiffness and insufficient adaptability to various objects. Inspired by the human hand, this paper proposes a soft-rigid hybrid pneumatic gripper composed of fingers with soft skin and rigid endoskeletons, and an active palm. Through different combinations of the four joints' locking states within the rigid endoskeleton, each finger obtains 9 different postures in its inflating state and 13 different postures in its deflating state, endowing the gripper with the capability of adapting to a wider variety of objects. Simultaneously, due to the endoskeletons, the lateral stiffness of the gripper is significantly enhanced (load-to-weight ratio similar to 7.5 for lateral grasping). We also propose a series of grasping strategies for grasping objects with different sizes and shapes to utilize the versatile configurations of the gripper. Experiments demonstrated that the gripper conformed well to the surfaces of cylindrical and prismatic objects and successfully grasped all tool items and shape items in the Yale-CMU-Berkeley object set.