This paper presents a novel disturbance function method to avoid turning point singularities for the semi-regular hexagons 6-SPS Gough-Stewart manipulator. Through analysis of the configuration bifurcation characteristics of the manipulator at the type-II singular points, it is found that the type-II singularities under signal input parameter belong to turning point bifurcation. The configuration patterns for the manipulator to pass through the turning points are divided into three types: persistent, non-persistent and path configuration. Utilizing the universal unfolding approach, the configuration bifurcation characteristics under the perturbation pa- rameters applied to the extendable legs are analyzed. The investigation reveals that all configuration branches converged in the same singular point in the unperturbed system will be separated in the disturbed system. Based on this discovery, a novel method for the parallel manipulator to pass through the singular points with the desired configuration is presented. Then the disturbance functions for the manipulator to pass through the turning points with the persistent configuration and the non-persistent configuration are constructed. The method presented in this paper can be applied to avoiding the singularities in such cases where the path and orientation of the manipulator are strictly programmed.
The singular points of a 6-SPS Stewart platform are distributed on the multi-dimensional singularity hypersurface in the task-space, which divides the workspace of the manipulator into several singularity-free regions. Because of the motion un- certainty at singular points, while the manipulator traverses this kind of hypersurface from one singularity-free region to another, its motion cannot be predetermined. In this paper, a detailed approach for the manipulator to traverse the singularity hypersurface with its non-persistent configuration is presented. First, the singular point transfer disturbance and the pose disturbance, which make the perturbed singular point transfer horizontally and vertically, respectively, are constructed. Through applying these dis- turbances into the input parameters within the maximum loss control domain, the perturbed persistent configuration is transformed into its corresponding non-persistent one. Under the action of the disturbances, the manipulator can traverse the singularity hy- persurface from one singularity-free region to another with a desired configuration.
