  This paper presents a study of the characterization of the singular manifold
of the six-degree-of-freedom parallel manipulator commonly known as the Stewart
platform. We consider a platform with base vertices in a circle and for which
the bottom and top plates are related by a rotation and a contraction. It is
shown that in this case the platform is always in a singular configuration and
that the singular manifold can be parameterized by a scalar parameter.
