Voluntary Assist-as-Needed Controller for an Ankle Power-Assist Rehabilitation Robot

Although existing assist-as-needed (AAN) controllers have been designed to adapt the robotic assistance to patients' movement performance, they ignore patient's active participation. This study proposed a voluntary AAN (VAAN) controller considering both movement performance and active participation for an ankle rehabilitation robot.According to the trajectory tracking error of the human-robot cooperation movement, the controller can switch among four working modes, including robot-resist, free, robot-assist, and robot-dominant mode. In order to reflect patients' active participation, the voluntary torque of the ankle joint was estimated by an EMG-driven musculoskeletal model. The control torque in robot-resist, free, and robot-assist mode was determined by the voluntary torque of ankle joint multiplied by an assistance ratio to encourage subjects' active participation, and a stiff torque was provided in robot-dominant mode. The controller was evaluated with 2 healthy subjects and 5 stroke patients on an ankle rehabilitation robot to investigate the clinical impact on the stroke patients.The experiment results showed that as patients' disability level increased, the trajectory tracking error increased and the proportion of human-dominant time and the voluntary torque of ankle joint decreased. Moreover, the results showed that the proposed VAAN controller achieved higher human contribution ratio than that of previous studies.The proposed VAAN controller can adapt the working mode to the movement performance and promote the subjects to participate actively.Based on its performance, the proposed VAAN controller has potential for use in robot-assisted rehabilitation.