Among the several ways adopted for the characterization of manipulator performances is identification of the typologies of maneuvers allowed by the manipulator kinematic arrangement. The multiplicity of configurations corresponding to a given hand position for serial manipulators, or to a given set of input values for fully-parallel manipulators, makes one wonder whether a manipulator can perform the maneuver of changing its configuration without meeting singularities. The paper shows that singularity-free configuration change is possible both for general-geometry serial and fully-parallel architectures. Results are reported that shed light on potential performances of manipulators and allow, in perspective, a deeper exploitation of their structural peculiarities.

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Parallel mechanisms show desirable characteristics such as a large payload to robot weight ratio, considerable stiffness, low inertia and high dynamic performances. In particular, parallel manipulators with fewer than six degrees of freedom have recently attracted researchers' attention, as their employ may prove valuable in those applications in which a higher mobility is uncalled-for. The attention of this paper is focused on translational parallel mechanisms (TPMs), that is on parallel mechanisms whose output link (platform) is provided with a pure translational motion with respect to the frame. It deals with the general problem of the topological synthesis and classification of TPMs and it investigates both their constraint and direct singularities. In particular, it identifies for the first time special families of fully-isotropic mechanisms. Such manipulators exhibit outstanding properties, as they are free from singularities and show a constant orthogonal Jacobian matrix throughout their workspace. As a consequence, both the direct and the inverse position problems are linear and the kinematic analysis proves straightforward.

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