Stiffness control of a coupled tendon-driven robot hand

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Abstract
This paper presents the methods of design and control for a coupled tendon-driven robot hand with stiffness control capability. By using the tendon as a force transmission mechanism, compact design of the robot hand is achieved with the developed controller. Problems specific to the tendon characteristics are considered, such as the slacking problem of tendons when the joint is disturbed, and the tendon elongation problem when the collocated position sensing method is used for compact design. To cope with these problems, two fundamental algorithms are developed and implemented on a laboratory apparatus, the POSTECH Hand II. First, a position estimation algorithm is developed to evaluate the accurate position of the hand, leading to an antagonistic tendon controller. Secondly, an active stiffness control algorithm is developed to control the fingertip force. It is shown that the finger produces excellent linear stiffness characteristics which justifies the effectiveness of the proposed algorithm. The object stiffness control is also implemented to exert desired force to the environment when the hand grasps an object, and is evaluated via experiments.

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