New Self-Motions of Parallel Manipulators

In this paper we present two results on geometry of parallel manipulators with planar platform and base (Stewart–Gough type platforms). In the first one we show that for this type of manipulators there is always one quadratical equation on Euler parameters between equations describing the geometry of the platform. This equation has a strong geometrical meaning – it is identically zero iff the platform is architecturally singular. This yields a very simple system of equations for architectural singularity and proves automatically the self-mobility. In the second result we describe a large class of manipulators with self-motions, a special case of which was already discussed in Borel (1908) and Bricard (1906).