Composite performance of variable stiffness actuator for exoskeleton administrated via impedance control and disturbance observer

This paper presents an integration of two significant targets, i.e., reducing the output impedance and the vibration at the trajectory tracking, into one variable stiffness actuator (VSA) system. The VSA is operated within the cascade impedance control framework, in which the inner torque control loop is designed with the unknown input observer (UIO). The gain-scheduled UIO cooperates with the low and high physical stiffness of the VSA to achieve the low-output impedance and steady trajectory tracking, respectively. The rationality of these implementations is presented considering the stiffness constant and the performance of the control system. The VSA and the gain scheduling-based variable impedance control follow the paradigm of the continuous variable impedance task. In accordance with the knee joint torque, the low and high impedance task with the enhanced performance are applied to a knee exoskeleton with the VSA joint. The effectiveness of the controllers is experimentally verified on a VSA prototype and the exoskeleton with the test person. The robust stability and passivity of the whole control system with the bounded parameter variation rate are investigated.