  The subject of this paper is a special class of three-degree-of-freedom
parallel manipulators. The singular configurations of the two Jacobian matrices
are first studied. The isotropic configurations are then found based on the
characteristic length of this manipulator. The isoconditioning loci for the
Jacobian matrices are plotted to define a global performance index allowing the
comparison of the different working modes. The index thus resulting is compared
with the Cartesian workspace surface and the average of the condition number.
