Multibody kinematic dimensional synthesis of a novel passive ankle exoskeleton with biomimetic kinematic response containment

Abstract To address uncertainty-induced kinematic response fluctuations in passive ankle exoskeletons, a bioinspired passive compliant ankle-exoskeleton structural concept is proposed. The design integrates four biomechanical subsystems: (I) a soleus muscle-inspired elastic-damping energy storage assistance mechanism, (II) an ankle soft tissue-mimetic elastic load-bearing structure, (III) a foot–ankle complex motion-inspired buffering-propulsion unit, and (IV) an ankle-joint motion-inspired passive compliant support, collectively forming an integrated shank-ankle–foot composite bioinspired system. Based on prescribed sagittal-plane human input and simplified foot–ground boundary constraints, a human–exoskeleton–environment equivalence kinematic model is established. The Chebyshev polynomial response-interval method is applied to quantify the destabilizing effects of wearing-position uncertainty on human–exoskeleton coordination. A bioinspired kinematic response containment-smoothness (BKRCS) index is proposed for human–exoskeleton optimization. Structural parameters are optimized via an enhanced particle swarm optimization-genetic algorithm (PSO-GA) hybrid strategy using multi-objective formulation. Simulation results show that the optimized configuration reduces the selected comprehensive kinematic metric, while different key response variables exhibit mixed trends: the objective-function value converges to 0.2276, the BKRCS decreases to 0.1902, the fluctuations in displacement, velocity, and acceleration of upper L Superscript 45 e L 45e $L^{\text{45e}}$ and theta Superscript 13 re θ 13re $\theta ^{\text{13re}}$ all exhibit a downward trend; although the displacement and velocity responses of upper L Superscript 78 e L 78e $L^{\text{78e}}$ increase slightly, its acceleration fluctuations are effectively suppressed. Numerical results indicate that the optimized mechanism exhibits smoother kinematic responses and reduced sensitivity to wearing-position uncertainty. Thus, this study provides a theoretical framework and methodological basis for the biomimetic structural design, uncertainty kinematic evaluation, and multi-objective kinematic dimensional synthesis of passive ankle exoskeletons.