Development of a 6-DoF motion system for realizing a linear datum for geometric measurements

In order to further improve the linear datum based geometric measurement accuracy and expand the measurement range, a 6-DoF motion system is developed for realizing a linear datum in the form of motion trajectory of the contact point (CP) of an absolute displacement measurement probe. This linear datum is established based on the concept of coordinate measurement and it does not contain straightness error in theory. The 6-DoF motion system consists of a 6-DoF fine stage and a 1-DoF coarse stage. The probe is moved by the 6-DoF fine stage which is magnetically noncontact supported and parallelly noncontact actuated. A CP-centred 6-DoF metrology model and a CP-centred 6-DoF motion model are established for elimination of Abbe error and on-line compensation of motion error of CP, respectively. 1-DoF coarse stage is controlled with relative position between two stages to extend the limited motion range of 6-DoF fine stage along the linear datum. Effectiveness of the metrology and motion models is verified through experiment. Straightness error of a 91.5 mm long line of an optical flat is measured by the proposed system and a commercial Fizeau interferometer. Comparison shows a consistency with standard deviation of 11 nm. Another experiment indicates that the proposed system could be used to realize a linear datum within a range of 220 mm with a repeatability of standard deviation of 7 nm.