Fiber Jamming Mechanism for Back-Stretchable McKibben Muscles

McKibben muscles struggle to perform passive elongation movements from their natural length by an external force when air pressure is not applied. This problem occurs in systems with mutual interference between artificial muscles, such as antagonistic drive. To solve this problem, we developed a new type of McKibben muscle called a back-stretchable McKibben muscle (BSM), comprising a thin McKibben muscle and a pneumatically driven variable stiffness mechanism for the tensile direction. However, in principle, the developed variable stiffness mechanism requires the use of parts with high bending stiffness. Consequently, unlike the thin McKibben muscle, the BSM cannot operate in a bent state, maintaining high flexibility even when air pressure is applied. This study proposes a new mechanism with low bending stiffness when air pressure is applied and not applied to improve the variable stiffness mechanism of the BSM. The proposed mechanism presents low tensile and bending stiffnesses when air pressure is not applied, and the bending stiffness remains unchanged when air pressure is applied, increasing only the tensile stiffness. We have devised a pneumatically driven fiber jamming mechanism (HJM) with these characteristics. In this study, a prototype was fabricated, and the experiments confirmed that the tensile stiffness can be increased by 4.3 by applying pressure while the bending stiffness remains almost unchanged.