Development Of The Vacuum Interferometric Comparator For Calibrating The Fine Linear Encorders And Scales

A new one dimensional laser interferometric comparator has been developed for the calibration of the fine linear encorders and scales up to 1600mm. In the comparator, the interferometer is fully arranged in vacuum and the calibration objects are mounted under atmospheric conditions. The Abbe's principle in the alignment of workpiece with the measuring beam is satisfied in the structure of a long measuring range. A traveling slide table, on which the calibration objects are mounted, is supported on guide rails by the air bearings and is driven through a recirculating ballscrew. The exhaust of the air bearings is guided to the exterior of the booth in which the comparator is placed. The travel of the table is measured by a reference interferometer with a beam path in vacuum shielded by an evacuated metal bellow, so that the effect of refractive index is eliminated. The laser beam is led by a polarization plane maintaining glass fiber from a self-designed stabilized HeNe laser, which is placed in an adjacency room, to the beam inlet of the main unit. The measurement system can input the interferometer signal by the encorder signal or the scale signal, and input the encorder or scale data by the interferometer signal. The system resolution is approximately 0.8nm and maximum traveling measurement speed is 20mm/s at continuous measurement. The uncertainly (K=2) of measurement is approximately 30nm in linear encorders of 500mm length and, approximately 40nm in scales of 500mm, although it depends on the length and the characteristics of encorders and scales. It is successful such a high accuracy that the uncertainty of measurement is smaller than 40nm in encorders of I m length.