Mass design method considering force control errors for two-redundant cable-suspended parallel robots

For redundant cable-suspended parallel robots, gravity imposes a predictable minimum or maximum limit on the tension in all cables. The hybrid joint-space control strategy is adopted in this study. In this case, the chosen redundant cables are force-controlled, whereas the remaining ones are length-controlled in the joint space. This strategy aims at maintaining all cable tensions within the pre-defined force boundaries under the influence of force control errors. This is by designing the end-effector mass while simultaneously ensuring that the cable force control errors can be realized by the controllers in the physical world. Three design indexes are proposed for the basic mathematical framework and a comprehensive methodology is devised to design the end -effector mass (gravity) of cable-suspended parallel robots with two degrees of redundancy. A trajectory planning, based on the French CoGiRo layout, is adapted to validate the analysis and design process by using multibody dynamics simulations. Further, we found that the choice of the optimal cable combination is essential for performing force control, and control failures may occur when inappropriate combinations are selected.