Abstract:A novel 3-UPU symmetrical parallel mechanisms is proposed, whose base and moving platform are always symmetrical about a middle plane. It has two rotational and one translational (2R1T) degrees of freedom (DOFs). The mobility and motion characteristics of this mechanism are studied by using the screw theory. And it has been approved that the moving platform can rotate continuously about a fixed axis or fixed point, which can be chosen freely on the middle plane. Therefore this

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... has no parasitic motion, which will bring much convenience to the kinematics analysis and controlling. Meanwhile, it also has a translational DOF along the normal of the middle plane, and the moving platform can realize continuous translation along this direction. Based on the structure characteristics of this mechanism, the orientation of moving platform can be represented by two angle parameters, which is very convenient and visual. The forward and inverse solutions are derived and the analytical solutions are obtained which is very simple and make it easy for further study. The workspace is analyzed, which is large enough for ordinary requirement. Key words:3-DOFs;2R1T;parasitic motion;symmetrical parallel mechanism;orientation workspace