  The subject of this paper is a special class of parallel manipulators. First,
we analyze a family of three-degree-of-freedom manipulators. Two Jacobian
matrices appear in the kinematic relations between the joint-rate and the
Cartesian-velocity vectors, which are called the "inverse kinematics" and the
"direct kinematics" matrices. The singular configurations of these matrices are
studied. The isotropic configurations are then studied based on the
characteristic length of this manipulator. The isoconditioning loci of all
Jacobian matrices are computed to define a global performance index to compare
the different working modes.
