Development and Evaluation of a Friction Model for Tendon-Driven Soft Robotic Devices

The Capstan formula is a common theoretical model that has been widely used to characterize friction between tendons and sheaths in tendon-driven transmission systems. Although several factors affect the friction in these systems, only two factors, the friction coefficient and the curvature angle of the sheaths, are taken into account in this theoretical model. Thus, understanding friction behavior still remains a significant limitation of control system performance for robotic systems that use tendon-driven mechanisms. This study aims to develop an improved friction model to more accurately determine the friction in tendon-driven systems. It considers the physical properties of the tendons and the sheaths by calculating the contact area and the adhesion force between them. The proposed friction model was verified by simulation and benchtop experiments, and compared with the Capstan formula. The results demonstrate that the error is reduced between 45% and 95% depending on the tendon angle and the sheath curvature. Thus, the proposed friction model can be used to characterize the friction between the tendons and sheaths in tendon-driven wearable devices, which could result in improved accuracy and better control of these devices.