Tension Maintenance Mechanism for Control Consistency of Twisted String Actuation-Based Hyper-Redundant Manipulator

Abstract

Hyper-redundant manipulators have been developed for hazardous environment exploration due to their flexibility and high agility in workplace. In this research, we designed a hyper-redundant manipulator by integrating Twisted String Actuators (TSAs) and Rolling Contact Joints (RCJs) to overcome the limitations of traditional cable-driven system, such as difficulties with long-distance power transmission, and to achieve high payload capability with a compact design. To prevent instantaneous tension loss by the slack and to enhance control consistency of the manipulator by preserving the relationship between contraction ratio of TSA and motor rotations, we proposed a tension maintenance mechanism using compression springs at the distal end of the manipulator. Additionally, to reduce losses from string contact friction, spring sheaths were inserted along the joint holes. Our approaches enhance the repeatability and position controllability of the manipulator. We noted a 33.5% reduction of error in repeatability test along with 35.9% and 38.8% improvements in piecewise position control accuracy and precision compared to a conventional manipulator, respectively, leading to enhanced controllability. We also experimentally verified that the proposed manipulator can maintain its trajectory with a variance of less than 2.83% up to 1600 g. Overall, our manipulator has the potential to expand the exploration environments in which robots can be used by simultaneously demonstrating large payload and controllability.