Abstract: This paper describes the design and control of a portable and lightweight knee exoskeleton for people with knee dysfunction. The knee exoskeleton is designed based on our custom quasi-direct drive actuation composed of a DC motor unit and a transmission mechanism encompassing three gears. An online gait generation method based on gait step calculation and direct measurement using Inertial Measurement Unit (IMU) sensors is proposed to provide continuous assistance during walking. Based on the generated gait trajectory online, Active Disturbance Rejection Control (ADRC) incorporating the feedforward compensation approach is proposed to help the human leg move in consideration of external disturbances. The developed knee exoskeleton has been employed successfully to assist impaired users with knee dysfunction to walk in a health recovery center. The experimental results indicate that the online gait generation method and the proposed control method are suitable for the knee exoskeleton. The maximum value of the target knee angular position is approximately 50° and the mean of the control torque for the knee joint is located in the interval of [? 2 Nm, 4 Nm]. The developed knee exoskeleton has the potential to regain normal gait to improve strength and endurance during walking in their activities of daily life.

Key words: Exoskeleton, Knee dysfunction, Quasi-direct drive, Active disturbance rejection control, Gait generation