Design and Experimental Verification of a Quasi-Passive Variable Stiffness Ankle Exoskeleton for Human Walking Assistance

Abstract

Exoskeleton robots are an effective method for enhancing human walking ability. This letter introduces a quasi-passive variable stiffness ankle exoskeleton, which absorbs negative work produced during ankle dorsiflexion in the stance phase of the gait cycle and releases energy to assist plantar flexion during push-off. Compared to powered exoskeletons, this design does not require high-power actuators but instead relies on a clutch and elastic component to mimic the interaction between muscles and tendons for assistance. Compared to passive exoskeletons, the designed clutch can adapt to different users. Compared to fixed-stiffness exoskeletons, the novel variable stiffness energy storage mechanism passively adjusts stiffness to mimic the biomechanical properties of the ankle joint. The proposed exoskeleton identifies gait phases based on a control strategy using foot force sensors. This strategy controls the exoskeleton's energy recovery and release by determining the gait cycle phase and changing the clutch state. Finally, a level-ground walking experiment was conducted with six healthy participants. Results showed that wearing the exoskeleton reduced the root mean square (RMS) change rate of soleus EMG activity by 7.25% and decreased the net metabolic rate during walking by 3.6%.