GB1303521A - - Google Patents

Abstract

1303521 Detecting cracks in steel strip BETHLEHEM STEEL CORP 29 Jan 1970 [31 Jan 1969] 4326/70 Heading G1A In an apparatus for detecting edge cracks in a moving strip 16, including a light source 72 and two photodetectors, e.g. photomultipliers, 86, 88, wherein the direct light path between one detector and the source is displaced both laterally and longitudinally of the strip from the direct light path between the other detector and the source, means 164 are provided for producing an output signal indicative of a crack only when an output signal from one detector precedes an output signal from the second detector by the time taken by the strip to travel from the first light path to the second. The light paths are defined by the apertures 104, 106 in a light shield 92, and each path contains a U.V. transmitting filter 78, 80, a window 82, 84 and a lens 90, 91. When an edge crack passes between apertures 104 the resulting signal pulse from photomultiplier 86 is supplied to a gate 156, which also receives pulses from a strip driven tachometer 150. The resulting output pulse from gate 156 is delayed in a pre-set counter 158 for a time interval corresponding to the strip travel time from apertures 104 to 106. The pulse is then fed to gate 162 to coincide with a pulse produced by photomultiplier 88 in response to the same crack. The resulting signal from gate 162 may be used to mark a data record, sound a horn or illuminate a signal light. The output signal from bistable switch 160 is also supplied to a further pre-set counter 168 which introduces a time delay before resetting the various bistable switches, thus allowing for errors in calibrating the tachometer. Only edge cracks of sufficient lateral extent to cross both optical paths will produce a warning signal, pinholes passing through undetected. The position of the shield 92 with respect to the strip edge is maintained constant by both a coarse and a fine control. The fine control is provided by a system including a light source 100 and detector 102. The detector output feeds a rack and pinion drive Figs. 5, 6 (not shown). The coarse position control comprises a U-shaped housing surrounding the light shield 92 Figs. 1-3 (not shown), this housing containing three filament lamps (58, 60, 62) located above the strip cooperating with three photo-detectors (66, 68, 70) located below the strip, there being a lens (64) in each light path. The photo-detector outputs are fed to a motor which controls a screw drive (50).