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SUMMARY This paper deals with a comparative study of two planar two degrees of freedom parallel mechanisms: one with 2-RRR and the other with 3-RRR structures. For a fair comparison, both 3-RRR and 2-RRR mechanisms are symmetrical. The kinematics and Jacobian matrix of the two mechanisms are studied. Conditioning index, stiffness index and payload index are proposed to compare the condition, stiffness and payload performance of the two mechanisms. The results show that the 2-RRR and 3-RRR mechanisms have the similar condition performance, and the planar 3-RRR mechanism has better payload and stiffness performance than the 2-RRR one.
Wu et al. (Fri,) studied this question.