Pulley friction compensation for winch-integrated cable force measurement and verification on a cable-driven parallel robot

In a cable-driven parallel robot, elastic cables are used to manipulate the end effector within the workspace. Cable force measurement is necessary for several control algorithms like cable force control, contact control, or load identification. The cable force sensor can be placed directly at the connection point on the platform or somewhere along the cable using pulleys. The pulleys between the force sensor and the platform disturb the force measurement accuracy due to friction. This paper deals with modeling and compensation of the friction. The friction behavior in the drive train with focus on the effects of the pulleys is non-trivial, as the cable movement consists of microscopic and macroscopic movements and standstills. Friction models from Coulomb and Dahl are adapted to deal with the pulley friction. The experimental evaluation showed an improvement of 70% with respect to the uncompensated case.