Observer-based active impedance control of a knee-joint assistive orthosis

In this study, a new active impedance control method of a knee joint orthosis is proposed by using a nonlinear observer to estimate the human joint torque. The proposed method is an alternative to the sensor-based impedance controller in which the human joint torque is estimated by using the electromyography (EMG) sensor, force/torque sensors, etc. The use of the nonlinear observer can efficiently overcome the shortcomings of the EMG and force/torque sensors, such as complexity of use, encumbrance, modeling, sensitivity to noise, high cost, etc. Additionally, a main goal of the proposed active impedance control is to decrease the impedance of the human-orthosis system to a desired level that the wearer can easily achieve. To achieve this goal, a virtual desired impedance model is designed to estimate human intention movements; a nonlinear disturbance observer based sliding mode controller is proposed to track the human estimated movements. Two experiments were carried out to verify the performance of the proposed nonlinear observer and sliding mode controller, as well as the effectiveness of the active impedance assistance strategy. The results show that the proposed method can effectively decrease the impedance of the human-orthosis system according to the desired impedance model. At the same time, the required human muscle torque for ensuring the knee joint flexion/extension movement can be also significantly decreased with the active impedance assistance.