A Variable Stiffness Supernumerary Robotic Limb with Pneumatic-Tendon Coupled Actuation

Supernumerary robotic limbs (SRLs) can assist humans in achieving efficient and comfortable work in daily life or industrial assembly scenarios, requiring SRLs to switch between rigidity and flexibility to perform compliant movements while also providing stable support for humans to reduce fatigue from prolonged standing, existing SRLs struggle to achieve this transition. In this study, a variable stiffness supernumerary robotic limb (VSSRL) is implemented, capable of adjusting its position and stiffness through pneumatic-tendon coupled actuation. The position of the VSSRL is accurately modulated by tendons, while its stiffness is controlled by pneumatic-tendon coupled actuation, tendons significantly increase the overall stiffness of the VSSRL, and the fiber-reinforced actuators (FRAs) can dynamically adjust its stiffness in response to changes in dynamic loads. Furthermore, a kinematic model of the VSSRL and a stiffness model under the coupling of FRAs and tendons are developed. Then, the trajectory and stiffness of the VSSRL in task execution are assigned based on human motion, and a multi-objective control system for both position and stiffness of the VSSRL is designed based on reinforcement learning (RL) algorithm, achieving collaborative control of position and stiffness for the VSSRL. The accuracy of the control system is validated through experiments, which demonstrate that the load capacity of the VSSRL is significantly enhanced under the action of tendons and FRAs, and that the VSSRL is able to provide various modes of assistance for daily life activities.