Patent GB-1437951-A (GB)
Title: Apparatus for detercting blemishes in sheet material
IPC: H03K5/02
Filed: 1972-05-12
Abstract: 1437951 Thresholding circuits P NASH 14 May 1973 22393/72 Heading H3T [Also in Division G1] A device for detecting flaws in moving webs of sheet material 1, Fig. 1, comprising a row of photo-cells 16 arranged transversely to the direction of movement of the web and lamps 4, 5, 6, illuminating the web from above and below, is characterized in that the outputs of the cells, shown at 20,..., 27, Fig. 2, are combined in pairs in amplifiers 51, whose outputs are in turn combined over diodes 54, ... , 57 to feed a single threshold circuit 62, thereby feeding to the threshold circuit a signal in which the noise component is of smaller peak-to-peak amplitude than that of any of the signals from the individual amplifiers. This reduction in the noise variance-but not in its absolute level-results because the arrangement is such that only the largest of the signals applied to diodes 54, ... , 57 is applied to the threshold circuit 62, and is illustrated in Fig. 17A, which shows how the range of variation 281 of the largest 278 of three random noise signals 275, 276 and 277 is less than the range of variation 279 of either of the individual noise signals. Rather than using a differential amplifier, as in Fig. 2, having inverse outputs, an operational amplifier 79, Fig. 3, can be used, in which case phototransistor pairs 64, 65 have their outputs subtracted and applied to one input 73 of the amplifier and its output is rectified positively by a diode 83, and negatively by a diode 86. As in Fig. 2, the output of many such amplifiers are combined over diodes 83, 84, 85 and 86, 87, 88, to provide the output signals, one 90 being applied to a threshold circuit constituted by a saturable amplifier 93, and the other (not referenced) being applied to a similar threshold circuit, not shown, with a threshold level of E 1 , Fig. 12A, surface defects causing signals such as 202 are detected, but small but sharply defined defects that cause spikes such as 198 are not. In a modification, Fig. 13, the signal Fig. 12A from amplifier 79, is differentiated 205, 206, to give a signal, Fig. 12B, in which the presence of the small but sharply defined defect 200 is readily detectable, and this signal is mixed over diodes 215 (together with other similar signals), with the output from amplifier 79 to form the signal 90 that is applied to the threshold circuit. In a further modification designed to recognize surface defects such as a pencil line in the direction of movement of the web, which defects whilst producing only a small increase 219, Fig. 14A, in the background noise level 218, do produce a signal of considerable duration, the signal 90 of Fig. 3 is integrated Fig. 15 so as to produce a signal Fig. 14B, which does, at its peak, exceed the threshold level E 1 . The integrator is fed via a circuit 224, 226 which is arranged to cut out the background noise 218, Fig. 14A, and is reset by pulses 221, Fig. 14B, applied to a terminal 240, Fig. 15, these pulses being produced by a uni-junction transistor oscillator (Fig. 16, not shown). By selecting an appropriate integrating capacitor 229, and the frequency of the reset pulses 221, the integration can be made to extend over varying lengths of the web being examined.