A cable-driven human assistive system has been developed to separate actuators from a human body. In the system, the assistive torque is transmitted via cables from the actuators to the end-effector which is to be attached on a human joint. The use of cables in flexible tubes allows for users to move freely without carrying the heavy actuators. However, the varying cable friction according to the curvature of the flexible tubes sets a challenge on the precise generation of the desired torque. To generate the desired torque precisely, a hierarchical control scheme is applied to the system. In this paper, the algorithms for determining the desired assistive joint torque and corresponding cable tensions are proposed. To determine the desired assistive torque, a rehabilitation strategy inspired by a potential field is discussed. For corresponding cable tensions, the algorithms for the cable tension controller which considers the varying cable friction as well as a bias for maintaining appropriate cable tensions are proposed. The performance of the proposed controller is verified by experiments.

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