Velocity Based Hybrid Position-Force Control of Cable Robots and Experimental Workspace Analysis

One important design criteria of a cable-driven parallel robot is its workspace. Disturbance effects on drive systems, pulley mechanisms and cables have a major impact on the workspace due to their influence on cable forces. Thus, the computed wrench-feasible workspace with ideal kinematic constraints may not be valid while operating the platform of a cable robot. To ensure that the platform moves within the workspace, cable forces can be controlled, whereby kinematic inaccuracies such as calibration errors affecting the platform’s positional accuracy. Hence, in this paper a concept of the velocity based hybrid position-force controller is presented in order to pretension cable forces on one hand, and ensure positioning accuracy on the other hand. The controller stability is investigated on extreme poses outside the wrench-feasible workspace hull of a dynamic model of a planar cable robot in simulation environment. Thereupon, the hybrid controller was implemented in a real-time control system and analyzed on the cable robot setup COPacabana with experiments inside and outside the computed workspace border. The analysis of measured signals is based on statistical investigation by performing multiple measurements of the same platform trajectory. Both, simulation and experiments show that the proposed velocity based hybrid position-force controller is feasible and can be applied to move the platform outside the workspace.