ORIGINAL RESEARCH article
Front. Robot. AI
Sec. Robot Design
Design and Evolution of a Triad Twisted String Actuator for Controlling a Two Degrees of Freedom Joint: Improving Performance and Simulating Active Transmission Adjustment
Damian John Crosby 1
Joaquin Carrasco 2
William Heath 3
Lutong Li 4
Andrew Weightman 4
1. Department of Mechanical and Aerospace Engineering, The University of Manchester, Manchester, United Kingdom, England, M13 9PL
2. Department of Electrical and Electronic Engineering, The University of Manchester, Manchester, United Kingdom
3. Bangor University School of Computer Science and Electronic Engineering, Bangor, United Kingdom
4. Department of Mechanical and Aerospace Engineering, The University of Manchester, Manchester, United Kingdom
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Abstract
Actuated universal joints are used in a wide range of robotic applications, including mobile snake robots, snake-arm robots and robotic tails. They are employed in applications such as search and rescue and confined space inspection. These can use remote cables, fluid driven systems, or inline motors. To realise the benefits of inline actuation while keeping the system compact with a high power to weight ratio, an actuated universal joint (AUJ) was developed using an "antagonistic triad" of three twisted string actuators in our previous work. However, the design had numerous drawbacks in its prototype form, namely a limited angle range, poor accuracy due to the angular feedback sensors used, and issues with string failure due to mechanical design choices. In this publication, we performed a root-cause analysis of these issues, and partially or fully mitigated some of them by reducing the distance between the twisted string actuator (TSA), removing geometry which caused premature string failure, and exchanging the angular feedback sensors for more accurate ones. As a result, angle range was increased from ±14.50° to ±26.00° for a single axis, and ±6.00° to ±20.00° for a dual axis movement. Angular feedback sensor accuracy increased from ±0.21° to ±0.11°, and no string failures occurred within load limits. The performance of the mechanism was further characterised with additional experiments for increased follower load and angular velocity. A novel method to adjust the transmission ratio during operation (active transmission adjustment) was proposed and simulated, and its advantages over existing mechanisms for a snake robot in a multi-segment configuration were theoretically evaluated.
Summary
Keywords
Force control actuators, Inverse dynamic control, Robotjoint module, snake robots, Twisted String Actuation System
Received
05 December 2025
Accepted
20 February 2026
Copyright
© 2026 Crosby, Carrasco, Heath, Li and Weightman. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Damian John Crosby
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.