Design of variable impedance actuator for knee joint of a portable human gait rehabilitation exoskeleton

This paper presents the design of a variable impedance actuator, called BAFSA, for the knee joint of a portable human gait rehabilitation exoskeleton. Such an actuator is specifically tailored to this joint, aimed at the application of robotic-assisted gait training therapies to restore the normal function of the impaired knee, taking into account kinematics, kinetics and anthropometric requirements along the gait cycle. Mechanical design and functioning of the actuator are thoroughly shown, with particular emphasis upon the variable stiffness mechanism, which consists of an axial floating spring, bidirectionally actuated in an antagonistic way. This is combined with a complimentary system, named BLAPS, that allows to vary automatically the preload stiffness, and sustain it before external loads with no further energy cost to the actuator motors. Simulations reproducing a normal gait cycle on level terrain are carried out to demonstrate the feasibility of the design, considering both power and energy consumption to validate the actuator performance. Thus, the actuator here developed is suitable to implement different rehabilitation strategies on the impaired knee joint. Finally, complete disclosure is achieved by presenting the technical specifications of the BAFSA, which fulfills all the requirements that were initially established as design criteria.