A Rigid-Soft Pneumatic Wrist with Fixed Rotation Axes and Active Jamming Variable Stiffness Mechanisms.

The wrist can greatly enhance the manipulative performance of dexterous hands. In addition, due to the flexibility of soft materials, the soft wrist has obvious advantages in environmental adaptability and safety. However, existing soft wrists often have small motion curvatures and cannot modulate stiffness. Inspired by the human wrist, we proposed a rigid-soft pneumatic wrist design with fixed axes, realizing variable stiffness through jamming mechanisms. The wrist is composed of two rotatory joints in series to achieve pronation/supination and flexion/extension motions. Each joint is driven by two opposite-mounted actuators, resulting in active bidirectional movement. The pre-stretched mounting of pneumatic actuators eliminates the stiffness torque effects. Both joints have independent jamming stiffening mechanisms controlled by air pressure and require no additional power source, which makes the stiffness control more flexible. Experimental results demonstrated that the maximum output torques of the two joints reached 1.18 Nm and 1.40 Nm, and the jamming mechanism increased the stiffness of the joints by 55.3% and 67.2% in the practical test, respectively. The proposed rigid-soft wrist provides a feasible solution for enhancing the manipulative ability of soft dexterous hands.