Résumé
This paper introduces a lower mobility parallel kinematic crane able to generate Scara motions (three translations and one rotation about a vertical axis). A crane is an underconstrained cable robot: it requires gravity acting on the traveling plate in order to tense the cables. The proposed crane can resist, to a certain extent, against outside forces and torques in all directions of the 6-dimensional task space. This feature results from the use of pairs of cables linking the actuators and the traveling plate. The proposed crane is derived from the I4 parallel robot. Thus, its traveling plate is articulated which provides a wide range of orientation. It is hyperstatic in the sense that one of the eight cables can be removed while keeping the same kinematic relationships. However, for symmetry reasons all the eight cables are kept (this feature is interesting in case of a cable breakdown). The input/output geometrical and kinematic models required for control are derived. Then, the cables tensions are obtained enabling the determination of the static workspace defined as the domain of reachable space where the cables remain taut under the action of gravity.