Impedance-based Backdrivability Recovery of a Lower-limb Exoskeleton for Knee Rehabilitation

This paper features the performance of the AGoRA lower-limb exoskeleton in knee rehabilitation therapy. Given that stroke rehabilitation focuses on functionality recovery, robotic devices for such purpose need to be able to move the limbs safely and naturally. Thus, rigid orthoses have lately implemented impedance-based controllers to ensure compliance of non-backdrivable mechanical structures and guarantee easy operation. Following such compliance recovery concepts, the exoskeleton presented here utilizes an admittance controller to deploy a learning mode (LM) in which a trajectory pattern defined by a therapist is recorded for future use. Once the trajectory has been saved, an assistance mode (AM) based on a gravity compensation module passively replays such behavior on the user's limb while the motor position and velocity profiles are monitored. A preliminary test in a healthy subject was conducted to assess the performance of this control architecture. Results drawn from this trial suggest that the proposed therapeutic approach might be useful for early stages of knee rehabilitation where continuous repetitive movements are recommended, since position and velocity errors were not greater than ±1.5°, and ±17% of maximum velocity value, respectively.