Design, Modeling, and Testing—A Compact Variable-Stiffness Actuator for Knee Joint Dimensions

Stroke rehabilitation exoskeletons require joint mechanisms capable of replicating physiological stiffness modulation to adapt to varying gait phases. This paper presents a novel compact variable-stiffness mechanism (VSM) for knee exoskeletons, based on a simplified three-bar linkage topology. The proposed design achieves a pre-configurable quasi-stiffness range of 0.15-2.0 NM/deg. Static characterization under a 2 kg load demonstrated up to 23.0 N of collision force attenuation in softening regimes (λ < 2.3) through passive viscoelastic dissipation, whereas hardening behavior (λ ≥ 2.3) preserved precise torque-angle characteristics scalable to physiological loading. A parametric analysis showed an 89% correlation between the theoretical and scaled experimental stiffness profiles for values from 0.5 to 2.5. The proposed architecture enables decoupled optimization of impact safety and positional precision, offering a clinically adaptable solution for hemiparetic gait assistance.