A Bioinspired Gear-Rolling Knee Exoskeleton for Enhanced Human-Exoskeleton Kinematic Compatibility

OBJECTIVE: This paper aims to reduce human-exoskeleton knee joint misalignment arising from the nonuniform and subject-specific motion of the tibiofemoral joint, thereby improving kinematic compatibility between the user and the exoskeleton. METHODS: A bioinspired gear-based knee exoskeleton is proposed, featuring a planetary gear mechanism that approximates the physiological rolling behavior of the human knee and a three-stage gear compensatory transmission that accommodates residual sliding-induced mismatch. A virtual human-exoskeleton interaction model based on sliding misalignment metric was developed to quantify kinematic misalignment and guide gear-parameter optimization. The proposed design was evaluated through numerical simulations, prototype implementation, human-subject experiments, and bench tests to characterize kinematic consistency and drive performance. RESULTS: Relative to a single-axis knee joint, the proposed mechanism reduced knee joint misalignment by approximately 70%. Torque transmission tests conducted on a mechanically decoupled platform showed close agreement between commanded and measured output torques, with peak deviations of approximately 8%-15% during extension and 5%-13% during flexion. Back-drivability characterization further indicated low passive resistance, with back-driving torque accounting for less than 5% of the rated assistive torque. CONCLUSION: The proposed gear-based knee joint enables anatomical motion approximation and internal misalignment accommodation while maintaining reliable torque transmission within a deterministic kinematic structure. SIGNIFICANCE: This work provides a generalizable mechanical and modeling framework for addressing knee joint misalignment in wearable exoskeletons and supports the development of systems with improved human-robot kinematic compatibility.