Variable-stiffness actuator based on coupled fiber jamming: design and application

Soft robots show fantastic adaptability, flexibility and safety, which are widely used in unstructured environments and human-robot interactive scenarios. However, soft robots also exhibit low load-bearing capability due to the low stiffness limited by the intrinsic property of soft materials. Therefore, adjusting stiffness of soft robots according to the corresponding application is important to expand the scope of soft robotic applications. Since the majority of variable stiffness mechanisms adjust stiffness by compressing discrete rigid components, which leads to the reduction of stretchability and structural flexibility. This paper proposes a new coupled fiber jamming mechanism. The mechanism is featured with coupled fibers, which consist of fibers and sliding tubes in a coupled arrangement. The coupled fibers can adapt to the stretching state of the actuator as the sliding tubes slides along the corresponding fibers. Based on the coupled fiber jamming mechanism, a variable stiffness soft actuator is developed, featuring a two-chamber configuration where both chambers are embedded with coupled fibers. If two chambers are inflated with different pressures, the actuator will generate bending motion, while the differential pressure of the two chambers would also force the coupled fibers inside the chambers to jam each other, which substantially enhances the actuator's stiffness. Theoretical models of stiffness and bending angle were developed and the experiments demonstrate the effectiveness of the proposed fiber jamming mechanisms.