Impedance Reduction Control of a Knee Joint Human-Exoskeleton System

Reducing human lower limb effort during walking is still a challenge for wearable exoskeletons. From the wearer's viewpoint, joint impedance increases while wearing the exoskeleton due to the fact that the exoskeleton's mass, friction, and gravity are added to those of the human lower limbs. Active impedance is required from the exoskeleton to compensate not only the exoskeleton's mechanical impedance but also that of the human limb. This paper presents a nonlinear impedance reduction control (IRC) approach applied to a knee joint exoskeleton during flexion/extension movements. The IRC approach ensures impedance adaptation of the human-exoskeleton system toward a desired level. The proposed method is an alternative to sensor-based impedance controllers where the human joint torque is estimated using electromyography (EMG) sensors, force/torque sensors, and so on. The performance of the proposed approach and its robustness with respect to modeling uncertainties are theoretically analyzed and discussed. Experiments were conducted with four healthy subjects to verify the effectiveness of human effort reduction. The root mean square of the EMG signals of the muscles involved in the flexion/extension was used as a metric. The experimental results show that EMG signal levels can be efficiently reduced by inertia, damping, and gravity compensation compared with those obtained without wearing the exoskeleton.

I. Introduction

In the past few decades, an increasing number of exoskeleton devices have been developed to provide human performance augmentation either for daily living activities or for rehabilitation purposes [1]–[6]. In terms of powered assistive exoskeletons, one of the main goals is to reduce the wearer’s effort during a movement initiated by the wearer without sacrificing his/her control priority, particularly for the wearer with partial movement capabilities, such as the elderly and patients affected by muscular weaknesses and/or movements disorders. However, interaction between the wearer and an assistive device is greatly limited by the inherent mechanical impedance of the coupled human-exoskeleton system. Important human efforts are required for achieving a desired movement if the exoskeleton interacting with the human limbs has a relatively high impedance.