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.

The twisted string actuator is an actuator that is gaining popularity in various engineering and robotics and applications. However, the fundamental limitation of actuators of this type is the uni-directional action, meaning that the actuator can contract but requires external power to return to its initial state. This paper proposes 2 novel passive extension mechanisms based on buckling effect to solve the uni-directional issue of the twisted string actuator. The proposed mechanisms are mechanically simple and compact and provide a nearly-constant extension force throughout the operation range. The constant force can fully extend the twisted string actuator with minimal loss of force during contraction. The designed mechanisms are evaluated in a series of practical tests, and their performances are compared and discussed.

Twisted string actuators are widely used in different areas of robotics, however, their lifetime remains one of the main concerns for robotics. There are no comprehensive studies that relate the lifetime of a twisted string to its material properties and operation conditions, which can be important to improve the life cycle. This paper provides preliminary results of an extensive experimental study on twisted string actuators, using strings made of different materials and operating in various working conditions. We experimentally investigated the life time of two different strings and provided empiric mathematical model which relates operating conditions of TSAs and string material and properties to their endurance. Based on experimental results, the observations are made that the life cycle of the strings decreases exponentially with the increase in load forces and stroke and increases linearly with the increase in the number of strings.