Nonlinear Observer-Based Sliding Mode Control for Robot-Aided Bilateral Human-Compliant Rehabilitation Training of Upper Limb

Robotic-assisted rehabilitation therapy has been a promising way in improving upper limb motor function. This paper proposes a multi-mode training control method for a bilateral upper limb rehabilitation robotic system, with which human-compliant rehabilitation training can be provided. Firstly, an admittance controller is built to transform the human-robot interaction force to compliant desired trajectory. Then, by integrating with super-twisting algorithm, a nonlinear observer is designed to estimate the lumped disturbance exerted on the driving revolute joint, including the active force applied by human subject, the force of friction, the model uncertainty, et al. To guarantee that the state of position converges to the desired value in real time, a high-order sliding mode controller combined with the disturbance compensation from the observer is proposed. Additionally, based on the aforementioned several methods, multiple bilateral training modes are constructed for patients in different rehabilitation stages. The overall system including the constructed bilateral rehabilitation robotic system and the proposed control method is verified in several experiments, demonstrating the advantage of the controller on interaction compliance with respect to normal method in addition to the capability of multiple rehabilitation training modes.Note to Practitioners—This work is motivated by the patients’ needs of the compliance and comfort during the human-robot interaction in the robot-aided rehabilitation training process. Thus, a nonlinear observer-based sliding mode controller combined with admittance model is proposed in this paper. The developed control method has the following functionalities: <xref ref-type="disp-formula" rid="deqn1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(1)</xref> Ensuring the compliance of the desired trajectory via the constructed admittance model. <xref ref-type="disp-formula" rid="deqn2" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</xref> Solving the estimation of lumped disturbance exerted on the robotic system based on nonlinear force observer. <xref ref-type="disp-formula" rid="deqn3" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(3)</xref> Ensuring the trajectory tracking with high accuracy under unknown disturbances via sliding mode controller combined with the observer compensation. The controller can be potentially applied in lots of areas: 1) Human-robot compliant collaboration or interaction, e.g., human-robot cooperative manipulation, robotic surgery; 2) Precise motion of robotic arm under unknown disturbance.