GB1366369A - Detection of faults on surfaces - Google Patents

Abstract

1366369 Counting faults in moving webs FERRANTI Ltd 30 Nov 1972 [30 Nov 1971] 55626/71 Heading G4D [Also in Division G3] In an arrangement for counting surface faults in webs of paper, plastics, steel, &c., a beam of light is repetitively scanned across the surface of the web 11 and light emanating from the surface is collected, e.g. by a photomultiplier 16, and applied to means 25 whereby a fault signal is produced each time there is a change in the collected light level. The invention provides a means whereby the fault signals are counted only while the scanning beam is directed on to the web. Near the beginning 17 of each scan the beam falls on a detector 19 shown as a photodiode thus producing a pulse; this pulse C, which can be adjustable to allow for various sized webs, triggers a generator 24 which produces a pulse D which extends from just before the earliest time that the beam could encounter the edge of the sheet to just after the latest. The collector 16 output is differentiated at 25 to provide the fault pulses which are applied to a counter 31. Only large fault pulses (i.e. due to the beam passing the edge of the web) pass a comparator 26 to trigger a circuit 27 arranged to deliver a signal E continuously before a trigger pulse and until 80 m.secs. after the arrival of a pulse. Signals D, E and the large fault pulse are applied to the three inputs of an AND gate 28 which thus delivers an 80 m.sec. pulse as the beam passes the edge of the web only if the edge occurs in an acceptable position. After a delay 29 to avoid a spurious count from the rest of the large fault pulse, the 80 m.sec. pulse enables the counter 31 which then counts subsequent fault pulses for a period dependent upon the known width of the web. In a modification (Fig. 3, not shown) further timing pulses are generated before and after pulse D and are employed to open further gates which control servo units to correct the position of the web if its edge does not occur during pulse D; a count can then be made during the next scan. Where the web is likely to vary in width a similar arrangement for detecting its trailing edge and stopping the counter may be provided. Or, alternatively, (Fig. 5, not shown) two separate scanning arrangments, each of which employs a multifacetted mirror 15, may be arranged so that each detects one edge as a leading edge.