Bionic design of universal gripper for nursing robot with hybrid joints and variable Equivalent Link Length

Currently, most rehabilitation/nursing robots attach the human body to the end of the robot by ‘binding’ solution, which makes the operation complicated and greatly limits their applications. Therefore, it is necessary to study a universal gripper that can directly grasp human limbs as the end-effector of the robot. Inspired by the human hand, this paper proposes a bionic under-actuated gripper that imitates the structure of human hand. The concept of Equivalent Link Length (ELL) is proposed to optimize the envelope effect. And the structure with hybrid rotational and translational joints is proposed to increase gripping stiffness and to avoid the harmful component in the grip force. Theoretical analyses and experiments on envelope effect, force distribution and load capacity show that the propose gripper can grasp cylindrical (limb-shaped) objects with wide applicable size range, and also has a high load capacity. Furthermore, the gripper has the characteristic of almost constant force transfer ratio, which reduces the number of sensors required by the system. These results show that the gripper has the potential to be used in the nursing robot system.