Patent GB-921362-A (GB)
Title: Improvements in or relating to the checking of sheet material for optically detectable irregularities
IPC: G01N33/34
Filed: 1959-04-10
Abstract: 921,362. Photo-electric inspection of sheet material. FELDMUHLE PAPIER - UND ZELLSTOFFWERKE A. G. April 10, 1959 [April 11, 1958], No. 12307/59. Class 40 (3). [Also in Group XXX] In an apparatus for the inspection of sheet material for blemishes such as spots, pin-holes, tears and surface irregularities, a spot of light having uniform intensity traverses the material as it moves through a scanning zone, and the transmitted or reflected light is detected by one or more photo-cells whose output feeds a limiter circuit which gives a signal only when the photo-cell output deviates from a given value by a certain amount. The sheet 10, for example paper, is passed through the scanning zone by endless belts 11 and by a belt 15 which is air permeable and of different colour from that of the web. The scanning zone and associated equipment are enclosed in a casing 13 at super-atmospheric pressure to exclude dust and the sheet 10 is held flat against belt 15 by suction, a box 16 being connected to a vacuum pump. The scanning device comprises a lamp 21, e.g. a gas-discharge lamp, and optical system (not shown) comprising an adjustable diaphragm to project a small sharply-focused light spot on to a multimirror drum 22 which is rotated at high speed so that the spot rapidly traverses the sheet in a line at right-angles to the direction of motion of the sheet. Light reflected from the sheet is received by a photo-multiplier 24 whose output feeds a limiter 37 as stated above. The limiter output may operate a visible or audible alarm, or may produce a record on magnetic or punched tape. If the invention is used for the inspection of separate sheets, it is arranged that there is no fault signal from the limiter when the gap between two sheets is being scanned. In one arrangement, feelers 30 detect the passage of a gap and operate a timedelay switch 32, which lights a lamp 33 when the gap reaches the scanning zone. Light from lamp 33 is reflected to the photo-cell 24 to mask the effect of the gap. Preferably, feelers are provided at both sides of the sheets so that tears at one edge only will not actuate the switch 32. An alternative method uses a second light source 38, e.g. a fluorescent lamp, continuously energized so that light passes through the perforated top of the suction box 16 and the perforated belt 15 to excite the photo-cell 24 when a gap passes the scanning zone. The latter method does not allow the detection of tears and holes in the sheet and in a second embodiment, Fig. 2 (not shown), to obviate this difficulty, two lamps are provided beneath and at either side of the sheet and there are two photo-cells, these corresponding to photo-cell 24 in Fig. 1, but above either side of the sheet. Only if both the photo-cells are energized is a gap present. Plane mirrors may be positioned above the web so that as the scanning spot enters and leaves the scanning zone the output of the photo-cells does not fall. The scanning light source is preferably fed with a stabilized voltage. Colour filters may be used in the light beam. When the invention is used for sorting, relays and time delay switches are associated with one or more deflectors which sort the sheets into two or more categories, e.g. according to the position or number of their blemishes. More than one scanning section may be required. By using two successive endless belts and two associated suction boxes, sheet material may be inspected by transmitted light, the light source being positioned beneath the gap between the belts.