Impedance modulation control of a lower limb exoskeleton to assist sit-to-stand movements

As an important movement of the daily living activities, sit-to-stand (STS) movement is usually a difficult task facing elderly and dependent people. In this article, a novel impedance modulation strategy of a lower limb exoskeleton is proposed to provide appropriate power and balance assistance during STS movements while preserving the wearer’s control priority. The impedance modulation control strategy ensures adaptation of the mechanical impedance of the human-exoskeleton system towards a desired one requiring less wearer’s effect while reinforcing the wearer’s balance control ability during STS movements. A human joint torque observer is designed to estimate the joint torques developed by the wearer using joint position kinematics instead of electromyography (EMG) or force sensors; a time-varying desired impedance model is proposed according to the wearer’s lower limb motion ability. A virtual environmental force is designed for the balance reinforcement control. Stability and robustness of the proposed method are theoretically analyzed. Simulations were implemented to illustrate the characteristics and performance of the proposed approach. Experiments with four healthy subjects were carried out to evaluate the effectiveness of the proposed method and show satisfactory results in terms of appropriate power assist and balance reinforcement