Human-in-the-Loop Modeling and Control of an Upper Limb Exosuit with Tendon-Sheath Actuation

Cable-driven exosuits have the advantages of being lightweight, compact, and well-adapted due to avoiding extensive usage of rigid structures to transmit power. However, applying too many flexible materials into the system will cause some problems such as insufficient stiffness, easy deformation, and non-negligible friction, thus increasing the difficulty of synergistic control between the human and cable-driven exosuit. In this letter, we attempt to control the exosuit effectively and reliably to provide assistance for the elbow, by analyzing the force and position relationship of connecting parts between the power source and the joint. The fundamental laws of Tendon-Sheath Mechanism (TSM) are deeply analyzed, which has obtained the corresponding mathematical description of force transmission and cable elongation. A spring-damping model is introduced between the tendon end and forearm to describe the character of textile-based interface, whose parameters could be identified by the least square method with the forgetting factor. Then, these theoretical analyses are incorporated into the human-in-the-loop modeling that can achieve appropriate motion generation from the desired joint torque to the actuator output. In the experiments, we have evaluated the output performance of the fabricated prototype and its efficacy on muscle activity based on the identification of spring coefficient and damping coefficient. The results demonstrate that the exosuit is able to respond appropriately to the changes of elbow position while realizing accurate tracking of the torque profile (the RMSE is 0.092 ± 0.011 Nm). In addition, it can effectively reduce the magnitude of muscle activation (29.52% reduction in biceps activation), thereby alleviating physical fatigue.