Abstract: A 4-xPxRxRxRyRN parallel mechanism with variable mobility is proposed. The four revolute pairs linked to the moving platform are set symmetrically with two groups of collinear axes (denoted by u and v axis). Mobility analysis and actuation selection are performed by using screw theory. In the initial configuration, the moving platform of the 4-xPxRxRxRyRN parallel mechanism is parallel to the base and the moving platform has three finite translational DOFs and two instantaneous rotational DOFs with rotation axes being any two lines parallel to the base. The moving platform can rotate finitely about the u or v axis from the initial configuration. After the finite rotation, the moving platform has three finite translational DOFs and one finite rotational DOF about the u or v axis. The rotational DOF about the u axis is exclusive to that about the v axis, namely, if the moving platform rotates finitely about the u axis, no finite rotation about the v axis can happen unless the moving platform is parallel to the base again and vice versa. To obtain the total control of the change of mobility, five actuators are required to perform redundant actuation for the 4-xPxRxRxRyRN parallel mechanism.

Key words: Mobility, Parallel mechanism, Screw theory