An Underactuated Variable Stiffness Continuum Robot with Multi-Layer Jamming Spherical Joints

Continuum robots(CRs) are featured with high compliance and adaptability. However, existing CRs commonly face issues of insufficient stiffness and actuation redundancy. This paper proposes a novel design of multi-layer jamming spherical joint (MLJSJ) and develops a pneumatically actuated variable-stiffness continuum robot. A segmented spherical expansion actuator airbag, resembling Tanghulu, is fabricated via the lost-wax method to meet the positive-pressure actuation requirements. The proposed robot consists of four serially connected continuum segments, each containing four spherical joints for smooth bending motion. By leveraging multi-layer frictional interactions within jamming structures, the robot achieves rapid and wide-range stiffness modulation under relatively low air pressure. Each continuum segment can be locked via a dedicated actuation airbag, with its pressure regulated by a proportional valve to enable rapid switching between rigid and flexible states. By switching the lock-unlock states of segments, the variable-stiffness mechanism decouples multi-segment motions into single-segment bending, enabling sequential actuation of all segments by using only four embedded alloy rods. A dedicated experimental setup was developed to characterize performance, demonstrating that a single segment reaches a maximum stiffness of 1403.03 N/m, with a stiffness modulation ratio of 35.85 and switching times ranging from 0.07 s to 0.43 s. Furthermore, by controlling the stiffness states and bending motions of different segments, the single segment of robot demonstrates a maximum payload capacity of 2 kg and versatile motion flexibility verified through obstacle avoidance and grasping tasks.