Twisted string actuators (TSAs) are gaining popularity in modern engineering and robotic applications. However, in conventional actuators of this type, the twisted part of the string should not be in contact with any surfaces or objects because this may interfere with the propagation of twisting. This imposes significant constraint on potential applications of TSAs. In this paper, we investigated the feasibility of using TSAs inside conduit and demonstrated that the twists of the string can be fully propagated through the sheath and that consistent periodic behavior of the twisted string can be achieved. In addition, we investigated input-output position and force characteristics of TSAs for various deflection angles of the conduit, effect of lubrication on transmission efficiency, and compared it with conventional cable sliding transmission. We found that TSA has higher transmission efficiency than sliding due to decreased friction between the string and conduit, which is further improved by lubrication. We have managed to achieve 85% of force transmission efficiency for the case of lubricated twisting, as opposed to the 71.74% for lubricated sliding.
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