Radial-Layer Jamming Mechanism for String Configuration

In the soft robotics field, soft structures are required for safe human interactions, adaptive shape grippers, and haptic interfaces. Conversely, rigid structures are necessary for high force transmission devices and precise position control applications. The tunable stiffness mechanism is a crucial technology in such areas, and various methods were proposed. Extant studies examine fixing the segments of the string by the tension of a tendon to instantly stiffen the structure and they can be constituted with rigid materials. However, the conventional wire tensioning mechanism exhibits a limitation in the holding torque, and the optimum segment morphology and stiffening performance are not clear. In the study, we demonstrate a new tendon-driven jamming mechanism referred to as a 'radial-layer jamming' with multiple layered joints that can realize high holding torque. Three basic segment structures are presented, and the design method is demonstrated. We measured the holding torque of the joints and compared the torque between three different designs. The results indicate that all three structures can control the holding torque by the pulling force of the wire. Furthermore, the findings suggest that the radial layer jamming can maintain the joints as fixed even if it has an extended structure. The holding force of the radial layer jamming did not significantly decrease when compared to other structures even if the number of joints increased. These layered approaches can contribute to the performance improvement of various variable stiffness mechanisms that utilize friction.