Patent GB-1486918-A (GB)
Title: Process and apparatus for detecting longitudinal faults on moving webs of material
IPC: G01N21/892
Filed: 1974-12-18
Abstract: 1486918 Photo-electric flaw detection in webs AGFA-GEVAERT A G 18 Dec 1974 [20 Dec  1973] 54602/74 Heading G1A A method of detecting and localising longitudinal faults by light reflected from a moving web comprises: scanning at least part of the width of the web in transverse lines; reciprocating the position of the scanning line transversely over the whole width of the web at constant velocity; summing, during a scanning cycle comprising a predetermined number of scans, the signals representing reflected light received at instants corresponding to two fixed points in the scanning line; separately storing the summed signals received from each point; and indicating a fault only if a summation deviating from a summed noise signal is detected at both points in succession. Apparatus for detecting and localizing longitudinal faults by light reflected from a moving web 8, Fig. 1, includes: an auto-collimation scanner 20, Fig. 3, mounted on a transversely mobile carriage 21 and comprising a light source 1, Fig. 1, mirror wheel 5, focusing device 11, 7 and photo-cell 13 for scanning in transverse lines 14; and an electronic processing device 37, Fig. 3 for interrogating the scanner at two fixed time marks within the scanning line during a number of scannings, and for averaging the signals received at the instants of interrogation..  Scanner, Fig. 1. For detecting e.g. longitudinal casting stripes or scratches in a moving photographic film 8, a mirror wheel 5 angularly displaces a pencil beam directed to a parabolic mirror 7, which reflects a transversely-moving beam to form a spot 3a scanning a succession of lines 14 on the film 8. Reflected light returning on a reflex path is deflected by a beam-splitter 2 on to a measurement photodiode or transistor 13. A starting photo-cell 16 to one side of the mirror 17 supplies a starting pulse just before the beginning of each  scan. A typical scan comprises a spot extending 2 mm in the direction of web motion and sweeping a line 70 mm long, 800 times a second. Traverse arrangement, Fig. 3. The scanner, mounted on a wheeled carriage 20, is reciprocally traversed over the whole width of the web 8 at e.g. 100 mm/s by a motor 25 controlled by limit switches 35, 36. An analogue on digital transducer 38 supplies a co-ordinate signal to an electronic processor 37. Successive scan lines are thus transversely displaced by 0À125 mm; and the web speed (e.g. 1 m/s) is chosen in relation to the spot size to ensure that the scanned lines overlap. Signal processor, Fig. 4a. Just before each scan a starting pulse provided by the photo-cell 16, amplified at 41 and shaped in a Schmitt trigger 42, is applied to four monostables 43-46 in cascade, the odd monostables 43, 45 defining, via an OR gate 49, two "sample" periods of a sample-andhold unit 40 receiving amplified signals from the measurement photo-cell 13. The ends of the "sample" periods correspond to two fixed points on the scan line spaced by 40 mm; at these instants the sample-and-hold unit 40 maintains the lastreceived signal, which is converted to a frequency at the rate of 10 KHz/volt in converters 50, 51 dealing with positive and negative signals respectively. The monostable 44 gates pulses from the converters 50, 51 for a period of 500 Ás in response to the signal held after sampling at the first scanning point, into an up-down counter 56 which has been pre-set to a count of 32. The monostable 46 similarly gates a digitized signal held after sampling at the second scanning point into a counter 62. Because of the motions of the web and of the traversing carriage, measurement signals sampled at the scanning points normally fluctuate about a mean level, so that the pulses accumulated in the counters 56, 62 tend to sum to zero, leaving the stored count substantially unchanged at 32. Any consistent positive or negative trend in the signal due to a longitudiual flaw affecting either scanning point in a number of successive scans will, even if below the noise level, cause a significant change in the stored count. If, within either successive scans, the count in either counter 56 on 62 has arrived at either zero or 64, that counter emits a fault recognition signal. One of a pair of monostables 73, 74 is put in a storage state by a signal 78 or 79 received from the traverse control (34) Fig. 3 and representing the direction of traverse. If one counter 56 or 62 responds to the first scanning point to traverse a suspected fault, the output from that counter is stored until the second scanning point traverses a corresponding region longitudinally displaced along the web; if the counter corresponding to that second point also detects a fault, one of the AND gates 75, 76 sends a fault recognition signal to an indicator and recorder 80. Each counter 56, 62 is reset to a count of 32 after eight consecutive scans by a ¸ 8 unit 68. Lamp Control. For stabilizing the reflected light level against normal average film reflectance variations, a peak detector (88) Fig. 6a (not shown) gated by the starting pulse to respond to an initial pulse from the measuring detector 13 at the beginning of each scan line, provides an output which is smoothed and compared with a reference signal to control a transistor (96) supplying current to the lamp 1.