Parallel robots have proved they can have better performances than serial ones in term of rigidity and payload-to-weight ratio. Nevertheless their workspace is largely reduced by the presence of singularities. In particular, the Type 2 singularities (parallel singularities) separate the workspace in different aspects, corresponding to one (or more) robot assembly modes. In order to enlarge the workspace size, it has been proved that a mechanism can cross the singularity loci by using an optimal motion planning. However, if the trajectory is not robust to modeling errors, the robot can stop in the singularity and stay blocked.
Therefore, the objective of this paper is to show new general procedure that allows the exit of a parallel manipulator from a Type 2 singularity. Two strategies are presented. The first one proposes the computation of an optimal trajectory that makes it possible for the robot to exit the singularity. This trajectory must respect a criterion that ensures the consistency of the robot dynamic model all along the singularity loci. The second trajectory consists in declutching one of the robot actuator in order to change the kinematic and dynamic behavior of the mechanism so that no singularity exists anymore. Theoretical works are illustrated, discussed and analysed through simulations achieved on a planar five-bar mechanism.
Online optimal motion generation with guaranteed safety in shared workspace