Analysis, design, and optimization of a multifunctional lower-limb rehabilitation mechanism

Based on screw theory, this study analyzes the rehabilitation motions of the hip, knee, and ankle joints in a seated posture, determines the required degrees of freedom (DOFs) for each motion, and experimentally measures their respective motion ranges. Utilizing the application of screw theory in statics, a 2RPS-UPU reconfigurable parallel mechanism is designed, featuring three motion modes: 2R1T, 2T1R, and 2R2T. The corresponding rehabilitation motions for each mode, along with the overall rehabilitation workflow, are described in detail. Inverse kinematic equations for each motion mode are derived using the closed-loop vector method, and the corresponding workspaces are analyzed to confirm their alignment with rehabilitation requirements. Furthermore, Jacobian matrices for the three modes are obtained, and performance evaluations are conducted with respect to dexterity and stiffness. An improved multi-objective genetic algorithm is then employed to optimize key structural parameters, aiming to enhance global dexterity, stiffness, and workspace coverage. The optimization results indicate a 30.7% increase in workspace, a 12.3% improvement in global dexterity, the global maximum deformation index is reduced by about 40%. By enabling multi-mode switching and structural optimization, the proposed reconfigurable mechanism offers a highly adaptable and efficient solution for lower-limb rehabilitation, demonstrating strong potential for clinical application.