Knee exoskeletons are devices that can enhance users’ mobility and strength. Their compatibility with the user’s joint motion is crucial for proper functioning. Typically, knee hinges are designed to replicate the instantaneous center of rotation (ICR) of an average knee. This study represents a knee exoskeleton worn by a user as a one DoF closed-kinematic chain, which enables calculating the displacement between the exoskeleton and the user’s thigh. The problem of optimizing the exoskeleton hinges to yield low relative movements during flexion and extension motions is formulated based on the proposed analysis. Also, the 4-bar knee model was simulated with different exoskeleton hinge designs to demonstrate the ability of the proposed analysis to predict the relative motions under different alignment conditions. The performance of an optimized polycentric geared knee hinge is then compared experimentally with that of a typical fixed center hinge. Experimental results showed that the optimized polycentric hinge yields less relative motion and a lower chance of exoskeleton migration. The proposed method emphasizes the importance of minimizing relative motion rather than replicating the knee’s ICR and provides a systematic way to design and evaluate various knee hinge designs.
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