From human hand joints to continuum robot: how articular surface morphology shapes flexibility and stability in template-based designs

The design of continuum robots often involves a dilemma between flexibility and stiffness, where increased flexibility may reduce stiffness and control precision. The human hand achieves both power grasp and precision grasp by leveraging different joint structures, particularly in the thumb, which plays a key role in balancing dexterity and stability. Inspired by the three distinct joints of the human thumb, we designed three types continuum manipulators featuring uniaxial, ball-and-socket, and saddle joints (SJ). A templated surface design was employed to control all other variables, ensuring that the only difference among the joint contact surfaces was their Gaussian curvature. The analysis covers aspects such as kinematic modeling, finite element simulations, workspace measurement, and stiffness experiments. Experimental results show that the workspace of the SJ manipulator is 0.73 times that of the ball-and-socket joint (BSJ) and 1.69 times that of the uniaxial joint (UJ). In terms of stability performance, the SJ achieves a maximum increase of 5.51 times in torsional stiffness and 2.68 times in bending stiffness compared to the BSJ. Compared to the UJ, the maximum improvements are 3.73 times in torsional stiffness and 2.44 times in bending stiffness. This suggests that the SJ continuum structure design can enhance stiffness while maintaining flexibility. This work provides a new approach for achieving a balanced flexibility and stability in continuum robot design.