Dual Variable-Radius Drum for Transmission Ratio Linearization and Stroke Enhancement in Twisted String Actuators

Twisted string actuators (TSAs) have emerged as promising actuators in robotics owing to their compliant, lightweight nature, and high transmission ratios. However, practical utilization of TSAs remains limited due to their restricted stroke length and intrinsically nonlinear transmission ratio (TR). While variable-radius pulleys (VRPs) can mitigate these issues, they exhibit poor scalability as their volume grows disproportionately with the required stroke or compensation TR range. This paper proposes the dual variable-radius drum TSA (DVRD-TSA) and mathematically proves that its architecture offers superior scalability and compactness compared to existing mechanisms as performance demands increase. For a baseline comparison to validate our model, we fabricated a prototype and compared it to a conventional TSA under constant load conditions with identical initial length. The experiment confirmed that our DVRD-TSA delivers a substantially larger linear stroke (210.3 mm, 72.5%) compared to conventional TSA (85.3 mm, 29.4%), while maintaining a comparable peak torque (25.82 Nmm vs. 24.21 Nmm), and successfully tracks its target near-constant transmission ratio (1.475 rd/mm) with low error. This work presents a compact, passive, and scalable solution that overcomes two major drawbacks of TSAs, nonlinear TR and limited stroke, thereby making them a more compelling option for robotic applications.