Nonlinear disturbance observer based sliding mode control of a human-driven knee joint orthosis

• This paper deals with the control of a knee joint exoskeleton for rehabilitation purposes.
• An observer is proposed to estimate the muscular torque developed by the wearer.
• A robust terminal sliding mode control method combined with the observer is presented.
• Asymptotic stability is proved by means of a Lyapunov analysis.
• Experiment tests were conducted with three subjects.

The present paper deals with the control of a knee joint orthosis intended to be used for rehabilitation and assistive purposes. A model, integrating human shank and orthosis, is presented. To reduce the influence of the uncertainties in muscular torque modeling on the system control, a nonlinear observer is proposed to estimate the muscular torque developed by the wearer. Additionally, a robust terminal sliding mode control approach combined with the nonlinear observer is presented. To illustrate the effectiveness of the proposed control method, a comparison with two control methods, basic sliding mode and sliding mode with nonlinear observer, are also given. The asymptotic stability of the presented approaches and observer convergence are proved by means of a Lyapunov analysis. Furthermore, the proof of advantage of the robust terminal sliding mode control method with the nonlinear observer (improving the tracking precision and reducing the required time for eliminating external disturbances) is proposed as well. The experiment results show that the robust terminal sliding mode control approach combined with the nonlinear observer has a significant advantage with respect to the position tracking and robustness regarding the modeling identification errors and external disturbances.