A Variable-Stiffness Robotic Link Based on Rotating-Rectangle Auxetic Structures for Safe Human-Robot Interaction

Industrial robotics emphasizes the development of collaborative robots (co-bots) designed to work safely alongside human operators. To minimize the risk of injury during physical human-robot interactions (pHRI), it is common to incorporate compliance into the joints or links of robotic manipulators. This work introduces a variable-stiffness robot link with an auxetic tubular design capable of achieving stiffening through a constantlength shape-morphing process under vacuum. The constant length is maintained in compliant and stiffened states, allowing the same robot arm kinematic model to be used regardless of the stiffness mode. Based on the structure's geometric design and shape-morphing behavior, we develop a kinematic model and a design optimization algorithm. We construct a physical robot arm prototype consisting of two auxetic stiffening links to validate our approach. Experiments are conducted to evaluate the bending stiffness of the links and demonstrate how stiffening enhances the arm's robustness and payload capacity. Safety during interactions is ensured by maintaining compliance when the robot links are not stiffened, highlighting the adaptability of the proposed design.