Tube Mechanism for Positively Pressurized Variable Stiffness by Lever and Wire

In the field of soft robotics, research on variable stiffness mechanisms that combine flexible structures for deformation with a solid structure for transmitting external forces is underway. This investigation is focused on a mechanism for achieving a high adjustable locking force with minimal actuators, enabling arbitrary shape locking. The paper introduces a novel shape-locking mechanism that utilizes the expansion force of a single rubber tube, which is positively pressurized with air, to generate frictional force. Positive pressurization facilitates clamping of wires and contact between joints, producing a joint holding torque. The holding torque in the mechanism is theoretically modeled and compared with its measured value in the bending directions of the prototype under an applied pressure. The results confirm that the holding torque increases in the presence of wires at each internal pressure value. Moreover, the holding torque, both with and without wires, is evaluated through a combination of theoretical models and actual measurements. These results demonstrate the viability of the proposed mechanism for the development of soft manipulators.