Force-Distribution Sensitivity to Cable-Tension Errors: A Preliminary Investigation

In order to control cable-driven parallel robots (CDPRs), it is necessary to keep all cable tensions within (positive) known limits during motion. For CDPRs having more cables than end-effector degrees of freedom, a feasible force distribution within cables should be computed beforehand. This paper aims at evaluating how a tension error in one cable affects the overall distribution of tensions in the other cables, by focusing on planar overconstrained CDPRs with four cables. The cable whose tension error more limitedly impact the force distribution is analytically determined by computing the right nullspace of the manipulator structure matrix. It is then shown how the cable least influencing the force distribution changes throughout the wrench-feasible workspace. Lastly, the results of the proposed analysis are used to perform a motion-control experiment on a prototype, where, for any configuration of the end-effector, the cable least influencing the force distribution is tension-controlled, while the remaining ones are length-controlled.