Design of an Active and Passive Control System for a Knee Exoskeleton with Variable Stiffness Based on a Shape Memory Alloy

Exoskeletons have been widely used in the field of human body function enhancement, and the application scenarios of exoskeletons have gradually diversified. The current application environment is characterized by high mobility, long periodicity, and uncertainty. To adapt to these characteristics, in the present study, an active and passive control system for a variable-stiffness knee exoskeleton based on a shape memory alloy (SMA) is developed. To enhance the mobility of the knee exoskeleton, the stiffness adjustment function is realized by SMA wires. To adapt to the long periodicity of application scenarios, the passive control mode is constructed, during which the exoskeleton drives human legs. Moreover, to address the uncertainty of the application scenarios, the active control mode is constructed, during which the exoskeleton follows the human leg movement when the wearer is in an irregular movement state. A gait recognition system is used to switch between the active and the passive control modes. Based on the prototype, a comprehensive experimental verification of the control system was carried out. The experimental results show that the constructed control system can realize the active and passive control function. In addition, the variable-stiffness function of the knee exoskeleton has the effect of reducing the driving error in scenarios with high mobility requirements.