Design of a variable ratio capstan drive for robotic applications

This paper presents the design and validation of a passive position-based variable transmission (PVT) using the capstan drive technology for robotic applications. The proposed mechanism uses variable-radius pulleys and helical cable grooves to passively adjust its transmission ratio based on its output angle. A mathematical model is developed to fully define the pulley geometries according to a variable ratio curve that is optimized for the chosen application. The proposed concept is implemented in a physical prototype and experimentally validated through kinematic and dynamic tests, confirming a variable ratio across its motion range and reduced mechanical impedance (higher backdrivability) in mid-range positions. These results are a first proof of concept towards integrating passive variable transmissions into robotic joints, offering a lightweight and reliable alternative to active solutions.