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Force control on antagonistic Twist-drive Actuator robot

In recent researches relating to robotic fingers, a novel power transmission mechanism had been designed, and developed as Twist-drive Actuators or Twisted String Actuator systems (TSA). These actuation structures, state of the art, include compact designs, light-weight mechanical structures, inherent compliance, and variable gearing, resulting in the fabrication of anthropomorphic robotic hands. However, those studies mentioned position control of the joint, and focused on the control performance of the pulling forces of a twisted string. This paper, therefore, introduces a novel joint mechanism composed of an Antagonistically-twisted Round-belt Actuator (ARA), which is able to make rotating motion by means of contraction forces induced by twisting small-diameter elastic round-belts. A noteworthy point in this paper is that the ARA robot is extremely well-suited for contact force control, which is exerted on the tip of the single joint robot. First, we demonstrate the existence of hysteresis characteristics of static contraction forces induced by the round-belt twisting, in which three belts are simultaneously twisted at constant low-speed. In addition, there exists a stress relaxation phenomenon when strongly twisting the elastic round-belts. We reveal that such sorts of discontinuous and nonlinear properties do not influence control performance in either the contraction force on the belts or contact force on the tip of the robot. Finally, this paper clearly shows stable and accurate tracking performance of the contact force of the ARA robot. In these experiments, we newly develop a twin/triple-twisted round-belt structure for the agonist side actuator, thus enhancing the contact forces effectively.