Accuracy Analysis of Non-Overconstrained Spherical Parallel Manipulators

This paper presents a new approach to compare several non-overconstrained solutions of the 3RRR Spherical Parallel Manipulator (SPM). The 3 − RRR SPM is known to be highly overconstrained, which causes several problems of mounting the mechanism, but has the advantage of having high rigidity thus a good accuracy. Several works in the literature proposed non-overconstrained versions of this mechanism. However, very few dealt with the problem of the consequence of modifying an overconstrained mechanism into a non-overconstrained one, mainly from an accuracy point of view. In this work, we present an analysis of the accuracy of five different non-overconstrained SPMs, i.e., 3 − RCC, 3 − RRS, 3 − RUU, 2 − RSC − RRR and RSC − RRU − RRC. These five SPMs are then compared in pose accuracy due to manufacturing errors of their links. The error on the position and orientation of the platform, due to manufacturing errors, are computed in 1000 different configurations within their workspace. The obtained results showed that the lowest orientation errors, not exceeding 2.4 °, are obtained for the 3-RCC. However, the smallest position errors, not exceeding 0.4 mm, were obtained for the case of the 2-RSC-RRR manipulator. These results show that different non-overconstrained versions of the SPM can exhibit different pose accuracies.