It is known that a parallel manipulator at a singular configuration can gain one or more degrees of freedom and become uncontrollable, that is, it might not reproduce a stable motion along a prescribed trajectory. However, it is proved experimentally that there is possible passing through the singular zones. This was simulated and shown through numerical examples and illustrated on several parallel structures. In this paper, we determine the optimal dynamic conditions generating a stable motion inside the singular zones. The obtained results show that the general condition for passing through a singularity can be defined as follows: the end-effector of the parallel manipulator can pass through the singular positions without perturbation of motion if the wrench applied on the end-effector by the legs and external efforts of the manipulator are orthogonal to the twist along the direction of the uncontrollable motion. This condition is obtained from the tion of the uncontrollable motion. This condition is obtained from the inverse dynamics and analytically demonstrated by the study of the Lagrangian of a general parallel manipulator. Numerical simulations are carried out using the software ADAMS and validated through experimental tests.

Three-degree-of-freedom planar parallel robots are increasingly being used in applications where precision is of the utmost importance. Clearly, methods for evaluating the accuracy of these robots are therefore needed. The accuracy of well designed, manufactured, and calibrated parallel robots depends mostly on the input errors (sensor and control errors). Dexterity and other similar performance indices have often been used to evaluate indirectly the influence of input errors. However, industry needs a precise knowledge of the maximum orientation and position output errors at a given nominal configuration. An interval analysis method that can be adapted for this purpose has been proposed in the literature, but gives no kinematic insight into the problem of optimal design. In this paper, a simpler method is proposed based on a detailed error analysis of 3-DOF planar parallel robots that brings valuable understanding of the problem of error amplification.

This paper is focused on the study of singularity of planar parallel manipulators taking into account the force transmission, i.e. study of singularity of planar manipulator by introducing the force transmission factor. Thus the singularity zones in the workspace of the manipulator are defined not only by kinematic criterions from the theoretical perfect model of the manipulator but also by the quality of force transmission. For this purpose, the pressure angle is used as an indicator of force transmission. The optimal control of the pressure angle for a given trajectory of the manipulator is realized by means of legs with variable structure. The suggested procedure to determination of the optimal structure of the planar parallel manipulator 3-RPR is illustrated by two numerical simulations.

It is widely claimed that parallel robots are intrinsically more accurate than serial robots because their errors are averaged instead of added cumulatively, an assertion which has not been properly addressed in the literature. This paper addresses this void by comparing the kinematic accuracy of two pairs of serialparallel 2-DOF planar robots. Only input errors are considered and all robots are optimized for accuracy, the only constraint being that they cover a given desired workspace. The results of this comparison seem to confirm that parallel robots are less sensitive to input errors than serial robots. However, this comparison is too limited to draw any general conclusions. Besides, it is virtually impossible to make a meaningful comparison between other pairs of serial and parallel robot. Therefore, there is no simple answer to this question of superiority. _____________________________________________________________________________ EST-CE QUE LES ROBOTS PARALLÈLES SONT PLUS PRÉCIS QUE LES ROBOTS SÉRIELS ? RÉSUMÉIl est généralement dit que les robots parallèles sont intrinsèquement plus précis que les robots sériels parce que leurs erreurs sont moyennées au lieu d'être ajoutées. Cependant, cette hypothèse n'a jamais été vérifiée dans la littérature. Cet article cherche à répondre à cette question en comparant la précision ciné-matique de deux paires de robots sériels et parallèles à 2 degrés de liberté. Seules les erreurs sur la commande sont prises en compte et tous les robots sont optimisés afin de les rendre les plus précis possible, la seule contrainte étant qu'ils couvrent tous le même espace de travail. Les résultats de cette comparaison semblent confirmer le fait que les robots parallèles sont moins sensibles aux erreurs sur la commande que les robots sériels. Cependant, cette comparaison est trop simpliste pour permettre à tirer des conclusions générales. De plus, il n'est pas possible de comparer d'autres paires de robots sériels et parallèles. Ainsi, il n'y a pas de réponse simple à cette question de supériorité.