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.