Linear scale calibration machine

Abstract

A Zeiss length measuring machine was retrofitted for the calibration of linear scales, with a pitch distance in the order of a few micrometers. High resolution optics, equipped with a CCD camera with a built in computer was added, for capturing the line sizes and distances. An appropriate LED illumination system is responsible for through the lens illumination and backlighting. The carriage on which the scale rests provides linear positioning motion. The paper discusses various definitions of the pitch distance. From these definitions simple algorithms are derived to determine the pitch distance. Important issue is the elimination of the influence of scale contamination. The main point is to minimize the effect of non-linearity and diffraction, by using appropriate optics and illumination, hereby improving measurement accuracy. The carriage carrying the scale is driven by ultrasonic piezomotor providing nanometer resolution. The displacement is measured by an HP laser interferometer. For higher resolution the CCD camera system can be replaced by a near field microscope.