Investigation on a New Approach for Designing Articulated Soft Robots With Discrete Variable Stiffness

Soft robots made of elastic and compliant materials offer various advantages, such as safe interaction, adaptiveness, and easy fabrication. However, the highly dissipative nature of elastic materials intrinsically limits certain functions, such as force transmission, positioning accuracy, and load capability. Herein, in this article, we present a design approach for developing articulated soft robots with variable stiffness. We propose a bioinspired bistable articulated joint that consists of a rigid joint and bistable structures. We realize stiffness change through forming a locking function by triggering the bistable structure to switch between two stable states. The trigger force is generated by heating shape memory alloy springs connected to the bistable structures. After the locking function is triggered, the bending stiffness of the articulated joint is highly amplified. Moreover, the joint can be locked at different angles through mechanically preprograming bistable structures fabricated with a single three-dimensional print. With this approach, we developed an articulated soft robot as a case study. The robot demonstrated variable stiffness, changeable configurations, and a large workspace. Moreover, the approach proposed in this article provides valuable insights into the utilization of bistability for developing variable-stiffness robotic systems.