Design of a Compact Variable Stiffness Joint for Modular Lower Limb Assistive Exoskeleton

In response to the deficiency of traditional rigid-driven exoskeletons, this paper presents the mechanical design and control strategy of a compact variable stiffness joint mechanism for modular lower limb assistive exoskeletons. The stiffness variation is achieved by modifying the lever arm principle, while allowing for adjustable configurations to cater to different stiffness requirements. The range of stiffness variation of the proposed joint is first mathematically modeled, and its stiffness characteristics, as well as the effects of different configurations, are then analyzed. In accordance to the mechanism design of the joint, a control strategy for a modular compliant lower limb assistive exoskeleton is proposed. The stiffness adjustment and output torque requirements are analyzed and modeled and a solution to control variables are derived for the 40% assistance case. The case study results indicate a relatively low coupling between the stiffness and torque of the mechanism.