A great many manufactured sheet materials are produced or modified in one or more continuous operations. In order to control the manufacturing operations, various properties of the sheet material such as its thickness, composition, macrostructure, surface finish, color and the like must be sensed continuously while the sheet is moving rapidly past one or more sensing elements placed alongside the path of the sheet.
The thickness or caliper of moving sheets is commonly measured by passing the sheet over a base member or reference plate on one side of the sheet while a pneumatic surface follower automatically positions itself at a constant distance from the surface of the sheet on the other side. The base member may contain one sensing element such as a magnetic reluctance or eddy current type of proximeter element, while the surface follower may contain another sensing element such as a proximeter target element. The proximeter responds to its distance from the target, and the sheet caliper is derived in effect by subtracting the constant distance of the sheet follower from the surface. A typical arrangement is described in U.S. Pat. No. 3,818,327 to Alexander.
In my copending U.S. Pat. application Ser. No 6/255,424 filed Apr. 20, 1981 and now U.S. Pat. No. 4,434,649 for Improvements in Measuring Apparatus there is described a gauge that is particularly useful for measuring the caliper of paper sheets, utilizing a new and improved surface follower that is capable of normally maintaining its mean distance the one sheet surface constant within a fraction of a micron despite changes in sheet surface roughness or line speed. Up to the present time, however, to my knowledge it has not been possible to maintain the other surface of the sheet in a constant distance relation to the base member with a comparable degree of accuracy and precision, due to the effects of minute wrinkles in the sheet and variations in the thickness of the residual boundary layer of air between the base member and the traveling sheet.