Design and Development of a Polycentric Knee Exoskeleton

Abstract

This paper presents the design, development, and preliminary proof-of-concept validation of a compact robotic knee exoskeleton for lower-limb assistive application. The proposed system addresses the trade-off between biomechanical compatibility, structural compactness, and reduced mechanical complexity through a lightweight single-degree-of-freedom architecture incorporating a polycentric knee mechanism. The exoskeleton integrates a compact brushless motor unit directly into the wearable structure, combined with a high-ratio transmission system based on a Capstan-driven architecture and a gear-reduction stage, enabling compact actuation. In addition, an embedded sensing system based on magnetic encoders and inertial measurement units was integrated to enable onboard monitoring of joint kinematics during movement execution. Experimental validation was conducted during representative lower-limb tasks, including walking and sit-to-stand movements. Joint kinematics were analyzed through gait-cycle and task-cycle normalization procedures to evaluate the consistency and repeatability of the measured motion. In addition, a comparative gait analysis was performed to assess the biomechanical impact of the proposed polycentric mechanism during walking by comparing natural walking, polycentric-joint walking, and walking with a locked single-axis configuration. The experimental results demonstrate the functional feasibility of the proposed design and provide preliminary evidence of repeatable kinematic behavior during representative lower-limb tasks. Furthermore, the comparative analysis showed that the polycentric configuration better preserved gait symmetry and reduced compensatory adaptations compared with the locked single-axis configuration. These results highlight the potential of reduced-complexity polycentric architectures for developing compact, wearable knee exoskeletons with improved biomechanical compatibility.