  This paper focuses on the kinematic properties of a new
three-degree-of-freedom hybrid manipulator. This manipulator is obtained by
adding in series to a five-bar planar mechanism (similar to the one studied by
Bajpai and Roth) a third revolute passing through the line of centers of the
two actuated revolute joints of the above linkage. The resulting architecture
is hybrid in that it has both serial and parallel links. Fully-parallel
manipulators are known for the existence of particularly undesirable
singularities (referred to as parallel singularities) where control is lost [4]
and [6]. On the other hand, due to their cantilever type of kinematic
arrangement, fully serial manipulators suffer from a lack of stiffness and from
relatively large positioning errors. The hybrid manipulator studied is
intrinsically stiffer and more accurate. Furthermore, since all actuators are
located on the first axis, the inertial effects are considerably reduced. In
addition, it is shown that the special kinematic structure of our manipulator
has the potential of avoiding parallel singularities by a suitable choice of
the "working mode", thus leading to larger workspaces. The influence of the
different structural dimensions (e.g. the link lengths) on the kinematic and
mechanical properties are analysed in view of the optimal design of such hybrid
manipulators.
