Active-Impedance Control of a Lower-Limb Assistive Exoskeleton

We propose a novel control method for lower-limb assist that produces a virtual modification of the mechanical impedance of the human limbs. This effect is accomplished by making the exoskeleton display active impedance properties. Active impedance control emphasizes control of the exoskeleton´s dynamics and regulation of the transfer of energy between the exoskeleton and the user. Its goal is improving the dynamic response of the human limbs without sacrificing the user´s control authority. The proposed method is an alternative to myoelectrical exoskeleton control, which is based on estimating muscle torques from electromyographical (EMG) activity. Implementation of an EMG-based controller is a complex task that involves modeling the user´s musculoskeletal system and requires recalibration. In contrast, active impedance control is less dependent on estimation of the user´s attempted motion, thereby avoiding conflicts resulting from inaccurate estimation. In this paper we also introduce a new form of human assist based on improving the kinematic response of the limbs. Reduction of average muscle torques is a common goal of research in human assist. However, less emphasis has been placed so far on improving the user´s agility of motion. We aim to use active impedance control to attain such effects as increasing the user´s average speed of motion, and improving their acceleration capabilities in order to compensate for perturbations from the environment.