Design and Analysis of a Cable-Driven Lower Limb Rehabilitation Robot With Variable Stiffness Joints

Abstract The cable-driven mechanism combines the advantages of rigid linkage mechanisms and flexible cable systems, featuring a compact structure, large workspace, low mass and inertia, and flexible motion capabilities. These characteristics hold significant theoretical and practical values in the field of lower limb rehabilitation robots. This article proposes a cable-driven lower limb rehabilitation robot with variable stiffness properties, consisting of a seat mechanism and a rehabilitation mechanism. Additionally, variable stiffness joints are introduced, enabling both active and passive stiffness adjustment. First, based on Lie group theory, the dynamic equations of the lower limb rehabilitation mechanism were derived, providing an efficient modeling method for multi-body systems with flexible joints. Second, this model was used to calculate the stiffness of robot's joints, which was then set as a constraint to construct a quadratic optimization function for the cable tension. This function is used to evaluate the changes in robot's joint stiffness and cable tension during rehabilitation training. Finally, simulation examples and experimental tests were conducted to verify the accuracy of the model and the feasibility of the rehabilitation training program.