A new vacuum interferometric comparator for calibrating the fine linear encoders and scales

A new one-dimensional laser interferometric comparator has been developed for the calibration of the fine linear encoders and scales up to 1600 mm. In the comparator, the interferometer is fully arranged in vacuum and the calibration objects are mounted under atmospheric conditions. The Abbe’s principle on the alignment of workpiece with the measuring beam is satisfied in the structure of a long measuring range. A travelling slide table, on which the calibration objects are mounted, is supported on guide rails by the air bearing and is driven through a recirculating ballscrew. The exhaust of the air bearing 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 stabilised He–Ne 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 encoder signal or the scale signal, and input the encoder or scale data by the interferometer signal. The system resolution is approximately 0.8 nm and maximum travelling measurement speed is 20 mm/s at continuous measurement. The uncertainly (k=2) of measurement is approximately 30 nm in linear encoders of 500 mm length and, approximately 40 nm in scales of 500 mm, although it depends on the length and the characteristics of encoders and scales. It is successful such a high accuracy that the uncertainty of measurement system is smaller than 40 nm in encoders of 1 m length.