A novel cable configuration method for fully-actuated parallel cable-driven systems: Application in a shoulder rehabilitation exoskeleton

Cable-driven exoskeletons play a significant role in poststroke rehabilitation. Since cables can only pull but not push, conventional parallel cable-driven exoskeletons are usually over-actuated, leading to uncertain dynamics. This paper proposes a cable configuration method utilizing the antagonistic cable pair to achieve full-actuation. The antagonistic cable pair is formed by assigning a passive impedance cable corresponding to an active driving cable. This method is illustrated as feasible through force closure analysis based on screw theory, and is implemented in a shoulder rehabilitation exoskeleton. Subsequently, the dynamics model of the exoskeleton is formulated, and the isomorphic mapping between cable tension space and joint space is established. Additionally, the elastic fluctuation absorption mechanism is presented to absorb the length fluctuation produced by the antagonistic cable pair, through the deformation of elastic elements. The continuity of cable tension is evidenced through its finite and smooth partial derivatives. Finally, the experimental results of prototype validate that the fully-actuated parallel cable-driven exoskeleton can be achieved with the proposed method.