Abstract: A novel 3-RRUR parallel manipulator is studied. The mechanism having high stiffness and symmetric simple structure can be applied to high precision assembly or used to constitute force sensor or parallel machine tool and micromotion parallel manipulator. It is easy to check and repair when using the 3-RRUR parallel manipulator for there is no moving pair in its limbs. Firstly, the screw system expressing the movements of each limbs is set up by the use of screw theory. The constraining screw system is built up. Then, its degree of freedom and moving characteristics are solved by researching the constraining screw system. The degree of freedom is 4, there are 3-dimensional motions and 1-dimensional rotation around axes vertical to the plane where the axes of kinematic pairs joining every limbs and the fixed platform lie in. In succession, the rationality of the input joints chosen is discussed with the aid of reciprocal screw theory and the method of analyzing the characteristics and the relativity of the constraints to the moving platform after adopting stiffened driving of the moving pairs. Then the constraint equations are obtained considering its moving characteristics and the forward and inverse position kinematics solutions of the mechanism and corresponding numerical examples are given. The research result will play an important role in pushing forward the application of this kind of manipulators.

Key words: Kinematics, Parallel manipulator, Spatial mechanism