Comparative analysis of the repeatability performance of a serial and parallel robot

The paper proposes a new procedure to compare the repeatability of serial and parallel robots based on the stochastic ellipsoid theory. The ISO9283 position repeatability index is estimated but also other performance criteria built upon the stochastic ellipsoid geometrical characteristics. The choice of the best comparison criterion is investigated and different solutions are proposed, associated with the task specificity. For each criterion, maps are built to determine the set of workspace points where the serial robot is better than the parallel robot. The ratio of the workspace surface where one robot is better than the other is computed and the results are analysed. Contrary to the common opinion that parallel robots are more accurate than serial robots, we prove here that the repeatability performance depends mainly on the chosen performance criterion. Another result found is that the considered RRRRR parallel robot keeps the same repeatability in all its workspace.