Retrofitting a length measuring machine for linear scale calibration

The aim of the project, described in this paper, is to retrofit a Zeiss length measuring machine for the calibration of linear scales, with a scale division of a few micrometers. The retrofitting consists of optics, equipped with a CCD camera with a builtin computer, for capturing the line width and the scale division, an appropriate through the lens and backlight illumination system, a motion system carrying the scale and providing linear motion at constant speed. The paper starts with discussing the various definitions of the scale division. From these definitions simple algorithms, to determine the pitch, are derived. 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 a pair of ultrasonic piezomotors 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.