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This paper presents a comparison study of two unique micro-motion parallel kinematic manipulators (PKM): a three degrees of freedom (DOF) micro-motion PKM and a 3-DOF micro-motion PKM with actuation redundancy. The 3-DOF micro-motion PKM has three linear-motion driving units, while the 3-DOF micro-motion PKM with actuator redundancy has four of these units. For both designs, the linear motion driving units are identical and both machines have the same passive link in the middle of the structure. The manipulator with actuator redundancy is designed to prevent singularity and to improve stiffness. The inverse kinematics and Jacobian of these two PKMs are analyzed, the stiffness properties are determined and global stiffness is optimized using genetic algorithm. A comparison of the global stiffness indicates that the 3-DOF micro motion PKM with actuator redundancy outperforms the one without actuator redundancy.