Variable Length-Angle and Stiffness Joint Mechanism that Enables Extension, Contraction and Rotation Elements by S-shape Folded Flat Tube

In recent years, the need for a variable stiffness mechanism to control the stiffness of a robot structure has been observed in soft robotics. There are various stiffness methods, and various linear mechanisms have been proposed to achieve extension, contraction, bending, and joint rotation of each method. However, to the best of our knowledge, no linear mechanism with variable stiffness in the two axes of extension, contraction, and joint rotation. This is because, in the conventional variable stiffness method, the air tube of the pneumatic actuator encased in the structure cannot maintain the desired shape under pressure due to wrinkling and buckling that occur when the air tube is deformed in response to the extension, contraction, and joint rotation of the structure. Therefore, it was necessary to develop a new method of encasing the air tube. The mechanism proposed in this study is to bend the flat tube that serves as the flow path into an S-shape to achieve extension, contraction and joint rotation, and to apply internal pressure to make the stiffness variable. Using a prototype based on this original principle, we confirmed the performance of switching stiffness in conjunction with extension, contraction and joint rotation. Experiments measuring the holding force during extension, contraction and the holding torque during joint rotation revealed that the holding force and holding torque were highly dependent on the pressure-receiving area between the mechanism and the S-shaped folded flat tube. In the future, we aim to apply this mechanism to a posture holding assist.