X-ray or gamma ray systems or devices – Specific application – Absorption
Patent
1993-03-05
1995-03-21
Church, Craig E.
X-ray or gamma ray systems or devices
Specific application
Absorption
378 90, G01B 1502
Patent
active
054003806
DESCRIPTION:
BRIEF SUMMARY
For several decades measurement of sheets of metal in production plants where the sheets are continuously moving, such as steel mills, beverage can or automobile factories, has been accomplished by measuring the absorption of a beam of radiation. This method of measurement is described in particular in the paper "Steel Measurement by Radiation Gauges" by D. Gignoux, published in "Steel Technology International" (1990) on pages 257 to 260.
When the radiation beam consists of electrons, i.e. beta radiation, the measurement of the absorption by the material can be translated readily into a measure of mass per unit area of the material. For some applications the mass per unit area is the quantity that is used to represent thickness. In other cases, it suffices to divide the mass per unit area by the density to obtain the physical thickness expressed in units of length. Beta radiation cannot be obtained in very intense beams as a practical matter, so that the measurements are too slow to be suitable for modern mill control computers. Another problem is that beta radiation is suitable to measure relatively thin material only.
X-ray beams are used the most often. They can be obtained with enough intensity and their energy can be varied so that it is possible to measure aluminium, steel, other metals and some plastics materials. However, the figure obtained for the absorption by a sheet of a certain metal is very much dependent on the composition of the metal.
For instance, when gauging aluminium, if the sheet contains a small percentage of some metal higher than aluminium in the periodic table of elements such as zinc, iron or copper, measurement may be inaccurate if the composition is not taken into account. If the composition is known, it is quite possible to relate the thickness to the absorption measurement. In most cases, the relation consists essentially of using the basic function relating absorption to thickness of pure metal and multiplying the result by an alloy factor. The requirement of modern mills for an accuracy of measurement of 0.1% requires, for certain aluminium alloys, that the composition be known very accurately, this is not really possible. Furthermore, sometimes the variation in composition between one end of the coil and the other is sufficiently large to create errors. Several schemes have been proposed for solving this problem. One consists of coupling the X-ray gauge to a gauge using a beam of beta particles generally produced by a radioisotope. This has several disadvantages in practice as it requires 2 different gauges measuring at 2 different points. Another scheme is the possibility of using 2 X-ray gauges producing beams of two different energies so that they do not have the same absorption characteristic. This method is good only when the concentration of only one alloying element varies. For instance, zinc in aluminium is uncertain, also the two measurements need to be made with an extreme degree of accuracy for the ultimate result to be itself of sufficient accuracy.
According to the invention, there is provided a method of calculating the thickness of a sheet material comprising directing a beam of photons against the sheet material from a photon source located on one side of said sheet material, detecting reflected or "backscattered" radiation from said sheet material in a first detector located on the same side of said sheet material and detecting transmitted radiation in a second detector located on the opposite side of the said sheet material, combining the signals of transmitted and reflected or "backscattered" photon beams, measuring and processing the data from both detectors for a number of calibrating samples, obtaining the thickness as a function of the detected data and applying this function to determine the unknown thickness of a sample.
The invention also provides an apparatus for calculating the thickness of a sheet material comprising a source of photons located on one side of said sheet material, adapted to direct a beam of photons from said photon source agai
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Gignoux Dominque
Gouel Roland M.
Church Craig E.
Data Measurement Corporation
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