Design and Control of a Knee Exoskeleton with Multiple Actuators

Abstract

This thesis presents the design, modelling and control of a knee exoskeleton aimed at assisting individuals with knee disorders during stand-to-sit-to-stand (STS) motions. The knee exoskeleton is developed by a four-bar mechanism actuated by a linear actuator and is controlled using electromyography (EMG) sensors to detect the user's muscle signals. The system is designed to provide the required knee joint torque and range of motion necessary for smooth transitions between standing and sitting postures. Through detailed simulations, it is demonstrated that the exoskeleton achieves the desired angle of thigh rotation and reduces user effort during STS motions. A comparative study between linear and rotary actuators reveals that both can provide adequate assistance at the knee joint, with the rotary actuator delivering a higher torque output. This feature allows the exoskeleton to be powered by either or by the actuator depending on the user's needs. To address reliability concerns, the exoskeleton incorporates a fault-tolerant control system using a fault detection, isolation and reconfiguration (FDI) technique. This system enables the exoskeleton to continue functioning even if one of the actuators experiences a fault, ensuring user safety and continuous operation. The design parameters of the knee exoskeleton are further optimized to enhance user comfort by maximizing the angle of thigh rotation during STS movements. The optimization is performed using the interior point method, implemented in MATLAB environment, to ensure optimal kinematic performance while maintaining mechanical feasibility. The bond graph technique is employed to model and simulate the system dynamics, offering an efficient framework for multi-domain system analysis and control. Experimental validations confirm the effectiveness of the knee exoskeleton in reducing user effort during STS transitions. The exoskeleton provides up to 60% of the external assistance required, significantly easing the burden on the user. This work contributes to the advancement of knee exoskeletons by providing a robust, fault-tolerant design that can be used in rehabilitation settings or by individuals with mobility impairments. The results of this research pave the way for future developments in assistive devices designed to enhance mobility and independence for users with knee disorders.