A study on configuration synthesis and kinematic performance of a high-flexibility humanoid thumb base joint

• A novel pneumatic flexible spatial bending joint is proposed for the base joint of the humanoid dexterous thumb, integrating a flexible actuator with a rigid constraint layer and adopting a multi-muscle collaborative drive mode. • The joint realizes 3-degree-of-freedom motion (axial elongation + spatial bending), with an axial elongation up to 40% of its own length, 360° omnidirectional bending capacity, and a maximum bending angle of 60° in any direction. • When the base joint cooperates with proximal and distal joints, the thumb achieves a maximum forward bending angle of 180°, a reverse bending angle of 37°, and an in-plane motion range of 220°, realizing human-like flexible movements. • The joint solves the defects of poor human-machine interaction safety of traditional rigid-driven robots and weak load-bearing capacity of flexible manipulators, with a clamping force up to 2N while ensuring flexibility. • The humanoid dexterous hand based on this joint exhibits excellent adaptability to objects of different shapes/sizes, high compliance, and strong safety, providing a new solution for high-flexibility bionic manipulator design. For the design of humanoid dexterous mechanical hands, we have always been committed to achieving flexibility and adaptability comparable to human hands. For this purpose, in this paper we proposed a design with a pneumatic flexible spatial bending joint as the base joint of the thumb, which combines a flexible actuator and a rigid constraint layer. This design not only ensures the flexibility of the hand but also takes into account its stiffness. By applying a multi-muscle collaborative drive method, we can precisely control the finger bending angle and clamping force, thereby achieving a more natural bending grip and fine manipulation. A static model of the joint was established and verified through experiments. The elongation of the joint can reach 40% of its own length, and the joint can bend in any direction within a 360° range, with a maximum bending angle of 60° in any direction (0-60°). The kinematic equation of the flexible finger was also established and verified through experiments. When the base joint, proximal joint, and distal joint work simultaneously, the maximum forward bending angle of the thumb is 180°, with a maximum reverse bending angle of 37°, and an in-plane motion range of 220°. Finally, a clamping force test was conducted on the fingers. When different combinations of air pressure were applied, the clamping force of the fingers can reach 2N.