Patent GB-1185181-A (GB)
Title: Method and Apparatus for Improving Backscatter Gauge Response
Inventor: HETENHOUSER LELAND SHERMAN
IPC: G01N23/16
Filed: 1967-03-17
Abstract: 1,185,181. Photo-electric measurement of coating thickness. INDUSTRIAL NUCLEONICS CORP. March 17, 1967 [March 18, 1966], No. 12782/67. Heading G1A. In the measurement of the weight per unit area of coating applied to a surface of a base material, the thickness of the base material itself is measured before coating takes place, and this measurement used in maintaining the spacing between the backscatter gauge, which measures the coating weight, and the surface of the base material, constant, so as to obtain maximum sensitivity from the gauge. This is done because there is an optimum value for the air-gap between gauge and coated material, at which variations in the detector output due to passline (air-gap) changes are at a minimum. In one arrangement, described with reference to Fig. 1, the measurement on the base material 14 is carried out and the thickness value obtained used by an operator to set a control knob 46 ganged to the movable tap of a potentiometer 44. The latter produces a corresponding voltage output which is used by a servo-unit to raise the backscatter gauge 20 by the thickness of the base material. The servo-unit operates through a pinion and rack gear system 34, 36 which is associated with a position transducer 38 producing a feedback signal representative of the position of the gauge above the base surface which signal is compared with the pre-set base thickness signal to adjust the input to the servo. The backscatter gauge employs a #-particle source 22 and detection chamber 24 producing an output which is measured at 28 and read-out at 50. The apparatus is initially set up with no material passing through and the thickness setting on zero, when the backscatter gauge is raised to a position above the reference plane determined by roller 18, where maximum detector response is obtained. At this position, the feedback generator is set to zero. Other arrangements are described in which the thickness of the uncoated base material is measured continuously instead of using the manually pre-set thickness signal generator. This permits changes in the thickness of the base to be allowed for, the gauge being raised or lowered automatically in accordance with these changes. The thickness may be sensed by contact transducers of the differential transformer or caliper transducer type or, preferably, by a non-contact, photoelectric transmission gauge. In one of these arrangements, Fig. 3 (not shown), error sensing and averaging means are provided to prevent movement of the gauge in response to short term changes in the base thickness and set a tolerance level. A delay circuit is incorporated to allow for the distance of the base thickness gauge from the coating thickness gauge. Fig. 4 (not shown) illustrates apparatus in which steel strip is coated with zinc by a dipping process and the coating weight measured. The coatings on both sides of the strip (64) are measured by separate backscatter gauges which are each associated with a bridge circuit for comparing the outputs of base thickness measuring servo and the gauge position signal generator. An initial manual set-up adjustment means are provided. Fig. 5 (not shown) illustrates a gauge which can be traversed back and forth across the coated material. In order to prevent damage to the gauge, vertical displacement or transverse movement of the gauge away from the material is carried out at the appropriate time after a weld has been detected by the base thickness gauge. This arrangement is described and claimed in Specification 1, 185, 182. The invention is also applicable to monitoring the plastic coating on fibrous base material.