Abstract: A novel 3-PCRNS spherical 3-DOF parallel mechanism is proposed. The limb kinematic chain consists of a circular prismatic pair PC, a revolute pair RN ,where the subscript N denotes the revolute axes in 3 limbs intersect at a common point, and a spherical joint S. Mobility, singularity and input selection of this mechanism is analyzed via screw theory. In the general non-singular configuration, the 3 PCRNS limb kinematic chain exerts 3 constraining forces on the moving platform, which intersect at a common point, the mechanism center. The 3 constraining forces constrains the 3 translational DOFs of the moving platform, thus the mechanism has 3 rotational DOFs. In the singular configuration, the 3 PCRNS limb kinematic chain exerts 6 constraining forces on the moving platform, which are linearly dependent. The maximum linear independent number of the 6 constraining forces is 5, thus 5 DOFs of the moving platform are restricted. The mechanism only has one instanta-neous rotation DOF about the z axis. The circular prismatic pair can be actuated, thus the rotational DOF of the moving platform about the z axis is decoupled from the other 2 rotational DOFs.

Key words: Degree of freedom, Parallel mechanism, Screw theory