  A classification of a family of 3-revolute (3R) positining manipulators is
established. This classification is based on the topology of their workspace.
The workspace is characterized in a half-cross section by the singular curves.
The workspace topology is defined by the number of cusps and nodes that appear
on these singular curves. The design parameters space is shown to be divided
into nine domains of distinct workspace topologies, in which all manipulators
have similar global kinematic properties. Each separating surface is given as
an explicit expression in the DH-parameters.
