Patent GB-1495516-A (GB)
Title: Measuring the thicknesses of a first material on opposite sides of reinforcing cords of a second material
IPC: G01N23/16
Filed: 1974-12-09
Abstract: 1495516 Thickness measuring by x-ray backscatter INDUSTRIAL NUCLEONICS CORP 9 Dec 1974 [12 Dec 1973] 53136/74 Heading G1A The thickness of a ply material such as rubber applied to both sides of reinforcing cords, e.g. of steel, is measured by making x-ray backscatter measurements on both sides of the ply. In the case of a vehicle tire, the total ply thickness and the position of the cords within the ply can be found, as can the total amount of rubber. In Fig.1 rubber is formed on to opposite sides of cords 10 by rolls 14-17, and the ply is examined by two x-ray backscatter gauges 20, 21 which can be driven simultaneously across the ply. Each gauge produces a signal which depends on the amount of rubber on both sides of the cords; the two gauge signals are combined at 30 to produce two signals representing the amount of rubber on each side of the cords, and these signals can be displayed at 33, 34. Alternatively by summing the signals a measure of the total rubber is obtained, and this may be combined with a pre-set indication of cord size to give the overall ply thickness; a ratio of the signals gives the cord position within the thickness of the ply. The measuring heads 20, 21, each have an x-ray source 23, Fig.2 whose beam passes to the ply via a circular window 25 of an ionization chamber 24. This window shapes the beam so that the cords of the ply do not cause too much fluctuation in the backscatted rays received by an annular ionization chamber 26 as the heads scan across the ply. The chamber 24 monitors the beam intensity, but is not required if the source is an isotope since the intensity would then be effectively constant. Each head carries a plate 29 opposite the source of the other head, for calibration of the other head when no ply is present. The x-rays preferably have an energy between 25-60 keV, this being a compromise between providing insensitivity to changes in rubber compositions and providing sufficient absorption by the cords.  Fig.3 shows the circuitry for the upper head 20, that for head 21 being similar as indicated at 36.  The signals from chambers 24, 26 are fed via high impedance circuits 37, 38 to a ratio circuit 40 to cancel out source-intensity variations, then via a calibration amplifier 41 to a subtractor 50 and variable gain device 51. The calibration amplifier is adjusted when no ply is present and also when a reference body is present so as to set the extremes of the response of the apparatus, Fig.4 (not shown); the circuits 50 responds to the position of switch 54, itself indicative of the type of ply to be measured, so that for a given ply the measurement output signifies the departure from a desired value rather than an absolute measurement; similarly circuit 51 responds to switch 54 to control the sensitivity of the measurement. The network 55 combines the processed signals from the two heads so as to solve an equation which removes from each head signal the effects of the rubber on the far side of the cords. The signals W T , W B  produced represent the rubber thickness on each side of the  cords, and can be displayed or can be combined as at 65, 66, 68 or 125 to measure the balance, the cord position, the rubber thickness (or weight) or the total thickness (or weight). The equation to be solved, and the manner in which a known ply or an equivalent absorber is used to determine sensitivity constants of the apparatus for use in network 25, is given.