Parallel mechanisms frequently contain an unstable type of singularity that has no counterpart in serial mechanisms. When the mechanism is at or near this type of singularity, it loses the ability to counteract external forces in certain directions. The determination of unstable singular configurations in parallel robots is not trivial, and is usually attempted via exhaustive search of the workspace using an accurate analytical model of the mechanism kinematics. This paper investigates the determination of unstable singular poses for the platform type of parallel mechanisms using a coordinate-independent approach. We also suggest a joint braking method for the trajectory control in the presence of such singularities.
Electronic Journal of Computational Kinematics, 1(1), February, 2002.