Patent Number: 061335789
Section: summary

The present invention is directed to a nuclear gauge for measuring a characteristic of moving sheet material and alignment compensation and more specifically to where the moving sheet material is paper being produced by a paper making machine. BACKGROUND OF THE INVENTION In measuring the basis weight of moving sheet material a radiation source is mounted on one side of the sheet material and on the other side is a receiving transducer. The pairs of transducers are then scanned in a cross direction (perpendicular to the movement or machine direction of the sheet material) to provide a cross direction measurement of that particular characteristic. When a nuclear gauge is used, such as with radioactive krypton gas, such nuclear source must be appropriately shielded when not in use. U.S. Pat. No. 5,432,353 illustrates one technique of shielding where the encapsulated nuclear source is mounted for rotation about a horizontal axis (parallel to the moving web) and when not in use merely rotated to face a side wall of its enclosure. See also U.S. Pat. No. 5,233,195. Since it is desirable to decrease the air gap between the radiation source and the moving web, this source should be as close to the sheet as possible. Thus, normally there is no room for a shutter to shield the source and this was the reason for the mounting in the above patents. Such shielding of the source by rotating it is space-saving. However, where a source other than krypton, such as promethium, is used, which has a much larger area of radiation and thus the encapsulated cylinder is larger, it would be too space consuming to mount such a source as illustrated in the above patents, and still provide proper shielding and also space for a standardization flag. Also as discussed above, in regard to the alignment between the radiation source and the detector, increasing demands for accuracy of measurement requires some type of error compensation for mechanical imperfections in the transfer mechanisms causing the radiation source and detector to scan across the paper in a cross direction. One form of correction is illustrated in U.S. Pat. No. 5,010,766, in the name of the present inventor and assignee. In that technique a separate unit senses the misalignment and then makes appropriate compensation. OBJECT AND SUMMARY OF INVENTION It is therefore the general object of the present invention to provide an improved nuclear gauge for measuring a characteristic of moving sheet material and alignment compensation. In accordance with the above object, there is provided apparatus for measuring a characteristic of a moving web of sheet material having a pair of transducers on opposite sides of the web the pair of transducers being scanned across the web. One of the pair of transducers has an aperture with a center axis perpendicular to the web and in close proximity to the web and includes an encapsulated radiation source which in a first position emits radiation along a radiation axis substantially coincident with the center axis through the aperture and through said web to the other of said pair of transducers. The other transducer includes detector means for sensing radiation received through the web from the radiation source. The encapsulated radiation source is mounted for movement in a plane perpendicular to said radiation axis and parallel to said web from an axis of movement, offset from the center axis, between a stowed position where the radiation from said nuclear source is shielded to the first position. In addition from a detector standpoint there is provided a multi-segmented planar detector having three or more segments for use in measuring a characteristic of a moving web in conjunction with a radiation source on the opposite side of the web. The source has a radiation axis and a relatively large area of radiation emission. Each of the segments of the detector is symmetrically arranged around a center detector axis which is nominally coincident and aligned with the radiation axis whereby in this coincident condition electrical signals representative of received radiation having passed through the moving web to each detector segment are equal. Processing means eliminating any error due to misalignment between the axes by processing the signals (one from each said segment) to eliminate dependence on misalignment.