KINEMATIC AND FORCE ANALYSIS OF THE PELVIS-FOOT SECTION OF AN EXOSKELETON FOR THE REHABILITATION OF INDIVIDUALS WITH MUSCULOSKELETAL DISORDERS

Abstract

This study presents a kinematic and force analysis of the pelvis–foot section of a passive lower-limb exoskeleton aimed at assisting the rehabilitation of individuals with musculoskeletal disorders. The architecture features a two-segment foot linked by a cylindrical hinge with a rubber outsole, shank and thigh links, a pelvic plate, and energy-storing spring-damper modules positioned at the ankle, knee, and hip. In the unloaded configuration, the segments are set to a quasi-physiological arrangement (e.g., a 120–140° foot-shank angle); during the sit-to-stand cycle the elastic modules accumulate and release energy to reduce user effort. The kinematic model employs a closed-vector-loop formulation for a planar three-link chain with three generalized coordinates; numerical trajectories are generated in MathCad software using a two-stage timing profile (total cycle duration of 2 s). A MapleSim multibody model is developed to validate the analytical motion laws, confirming characteristicpaths: a circular knee locus about the ankle, an elliptic hip trajectory, and a controlled trunk lean-and-recovery. The load analysis considers vertical static loading and the transition to bending during sitting; the thigh link, as a critical member, is evaluated by finite-element analysis in SolidWorks Simulation software. The maximum equivalent stress is approximately 154 MPa with a tip deflection not exceeding 0.3 mm under design load up to 1.5 kN, satisfying the strength criterion for steel St. 3 (with allowable stress of about160 MPa). Overall, the proposed design and the parameterization of the spring-damper units are sufficient to assist sit-to-stand motions and provide a sound basis for subsequent optimization to user anthropometrics and rehabilitation protocols.