Variable Stiffness Mechanism for Single-Joint Lower Limb Wearable Exoskeleton: A Review

Lower limb wearable exoskeleton can support the rehabilitation of the patient and offer assistance of the healthy human. But the traditional rigid exoskeleton mechanism is not inherently compliant, resulting in human-machine collision, lacking safety and environmental adaptability. The variable stiffness mechanism (VSM) is proved can improve inherent compliance of the exoskeleton, with the advantages of wide range of stiffness adjustment, considerable control bandwidth, high flexibility and security. However, the application of VSMs in wearable exoskeletons is still in the laboratory stage with limited commercial application cases, as designing a comfortable, compact VSM that takes into account compliance, safety, and stiffness adjustment capability is a major challenge. A detailed review of existing VSMs is provided for different lower extremity joints including hip, knee, and ankle joints to accelerate the application of VSM in exoskeleton. The VSMs were classified, summarized and compared according to active and passive assistance. The review focused on the structures and interface attachment components for the mechanical compliance of the VSMs. For passive VSM, more attention should be paid to the bio-inspired compact design, strictly following the load pattern of the lower limb muscles, and flexible adjustment of the stiffness. For active VSM, it tends to combine flexible force transmission systems with novel stiffness adjustment methods. In the future, developing active-passive combined exoskeletons is bounded to be a new trend.