VarWrist: An Anthropomorphic Soft Wrist With Variable Stiffness

Robotic wrists play a crucial role in enhancing the dexterity and stability of robotic end-effectors. Existing rigid robotic wrists tend to be complex and lack flexibility, while soft robotic wrists often struggle with limited load-bearing capacity and lower accuracy. Human wrists feature multi-degrees of freedom and variable stiffness, which help human hands to accomplish daily tasks. This study presents an innovative anthropomorphic soft robotic wrist, VarWrist, equipped with a fiber jamming variable stiffness module, enabling stiffness adjustment through vacuuming. VarWrist consists of three parallel bellows, utilizing a positive-negative pneumatic actuation strategy to mimic human wrist motion. In addition, the trajectory equation of the rotation center was fitted through modeling. We developed a prototype of VarWrist and assessed its performance. Results indicate that the soft wrist surpasses the motion range of human wrists, achieving flexion (81.9°), extension (78.5°), ulnar deviation (70.5°), and radial deviation (70.5°). The bending motion trajectory showed a 73% increase in similarity to human motion compared to fixed-axis rotation, with VarWrist exhibiting a significant range of variable stiffness (resting state: 206%, working state: 155%). Demonstration experiments confirm that this wrist facilitates a dexterous hand in completing grasping tasks that would be unattainable by the hand alone.