A novel variable impedance compact compliant ankle robot for overground gait rehabilitation and assistance

This paper presents the modular design and control of a novel variable impedance compact compliant wearable ankle robot (powered Ankle-Foot Orthosis, AFO) for overground gait rehabilitation and assistance mainly for stroke patients. Design of AFO, its construction, kinematics, working principle, actuation, control etc. are described. A novel variable impedance compact compliant series elastic actuator (SEA) is designed to actuate the AFO with variable impedance and compliance. The actuator design consists of a servomotor, a ball screw, a torsional spring connecting the motor and the ball screw via a pair of spur gear, and a set of translational springs connecting the ball screw nut to the output link. Two types of springs of the actuator with different stiffness provide variable impedance and compliance to the AFO based on the range of operational force. The translational springs with low stiffness are used to handle the low force operation that reduces friction, impedance and impact. The torsional spring, being in the high speed range, has high effective stiffness and improves bandwidth in large force operation when the translational springs are fully compressed. Application of the variable impedance compact compliant actuator makes the AFO compact, and it may enhance user-friendliness. Simulation results confirm the effectiveness of the AFO design. Competitive advantages of AFO over its existing counterparts are described and its future extensions are discussed.