Design and evolution of a triad twisted string actuator for controlling a two degrees of freedom joint: improving performance and simulating active transmission adjustment

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m1"> <mml:mrow> <mml:mo>±</mml:mo> </mml:mrow> </mml:math> 14.5° to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m2"> <mml:mrow> <mml:mo>±</mml:mo> </mml:mrow> </mml:math> 26° for a single axis, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m3"> <mml:mrow> <mml:mo>±</mml:mo> </mml:mrow> </mml:math> 6° to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m4"> <mml:mrow> <mml:mo>±</mml:mo> </mml:mrow> </mml:math> 20° for a dual axis movement. Angular feedback sensor accuracy increased from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m5"> <mml:mrow> <mml:mo>±</mml:mo> </mml:mrow> </mml:math> 0.21° to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m6"> <mml:mrow> <mml:mo>±</mml:mo> </mml:mrow> </mml:math> 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.