On-line compensation for deflection in instruments using focused beams

Abstract

A focused light beam instrument measures characteristics of webs of sheet material. The instrument and a reference surface, a backing tile in a carousel, are on opposite sides of the web which is maintained a fixed distance from the reference surface by a Bernoulli hold down device such that variations in the gap affect the beam focus. The instrument and reference surface scan the web via a scanning frame. The instrument is calibrated-standardized in an off sheet position where a first surface, a carousel backing tile, is moved opposite to the instrument and in the plane of the web. The instrument measures the first surface and a second surface, another carousel backing tile, which is moved opposite to the instrument but spaced farther from or closer thereto than the first surface. The distance of the instrument from the web is calculated from the difference between the measurements of the first and second surfaces. The calculated distance is used to determine an operating position on an instrument response curve and the instrument is calibrated accordingly. A second or third order polynomial is fitted to the instrument response curve and differentiated at the operating point on the curve to determine the rate of change of the measurement characteristic of the instrument at that point. Gap changes and the determined rate of change of the measurement characteristics are used to compensate gap change errors in measurements made by the instrument.