Patent GB-978572-A (GB)
Title: Improvements in or relating to inspection systems
Inventor: MCCREANOR RICHARD ALBIN
IPC: G01N21/89
Filed: 1961-11-17
Abstract: 978,572. Photo-electric detection of surface flaws. JONES & LAUGHLIN STEEL CORPORATION. Nov. 17, 1961, No. 41334/61. Heading G1A. [Also in Division H3] Apparatus for detecting visible flaws on the surface of moving material comprises an electron optics device arranged to scan an image of the surface of the material with an electron beam to produce a video signal including pulses each having a width proportional to the width of the flow image scanned by the beam, and means coupled to the device for integrating the widths of at least a portion of the pulses in the signal to produce an indicating voltage when the sum of the widths so integrated reaches a predetermined amount. In the apparatus shown in block diagram form in Fig. 2, a luminous ingot 16, passes on rollers 18 beneath a vidicon tube 20 scanning in a line perpendicular to the direction of travel of the ingot and giving a video output signal B, Fig. 3, a and d marking the edges of the ingot and b and c indicating bright and dark flows respectively, the signal being blanked out during the flyback scan from e to f. Waveform B now passes through clipping circuit 50, 52, 54, 56 to produce waveform C which is then differentated in 60 to give waveform D. The negative pulses are separated from waveform D, inserted and recombined with the positive pulses in 70 to give waveform G. Waveform C is also passed to two Schmitt trigger circuits 102 and 112, the former being supplied via a ¢ ms delay line 98, to give waveforms I and L respectively. These waveforms are then used in AND gates 76 and 110 respectively to eliminate from waveform G the pulses due to steps a and d (waveform B) to give waveform M which in turn gives waveform N from flip/flop 118. Tachometer 28, connected to rollers 28 produces a sinusoidal output O which produces waveform S by means of circuits 122, 124---140 and by way of waveform P, Q and R. Waveform S is then applied to gate 120 together with waveform N so that only pulses 146 and 148 (waveform T) reach gate 152 to pass a number of oscillations from 154 (waveform U) depending in number on the width of the flow pulses 146 and 148, the number of oscillations being counted by decode counters 160-164. The last counter 164 is set to give an output to memory 168 when the total number of recorded oscillations has reached a predetermined value. Waveform S is also applied to the binary counters 172-176, such that, if for example, there are 12 binaries in series, requiring 4096 pulses for the last counter to give an output and the tachometer is set to give 4096 pulses for every foot run of the ingot, a pulse will be given to multivibrator 178 every foot so travelled. The resulting marking pulses from 178 periodically reset the decode counters 160-164 to zero, so that the counters only give an output signal to actuate scarfing head 22 when the total width of faults per foot of the ingot reaches a predetermined limit. An override circuit 48 is included so that the scarfing head will be actuated to cut away a very prominent localised dark flow producing a large negative pulse on waveform C going below the triggering voltage of Schmitt triggering or multivibrator circuit 186 and producing pulses at the input of the integratory circuit 196 in addition to those produced by the leading and trailing edges a and d of waveform B or C. The integrating circuit 196 passes on a pulse via multivibrators 198, 200 to the memory 168 to actuate the scarfing head when the frequency of these additional pulses becomes too high. If in the circuit of Fig. 2 very narrow flaws are scanned, the differentiation of the resulting pulses, e.g. those shown at 636, 648 and 656 in waveform C of Fig. 10, will produce single pulses such as 694 in waveform N of Fig. 10, causing flip/flop 118 to produce waveform N<SP>1</SP>, Fig. 10 resulting in incorrect pulse lengths being recorded by the integrating circuit 46. This objection is overcome by inserting flip/ flop 718, Fig. 9 and comparator 764 between amplifier 116 and flip/flop 118, to give the embodiment of Fig. 9. In this modification the sweep pulses of waveform A are inverted and passed through a delay line 750 (waveform P, Fig. 10) and applied to the grid of triode 722, forming half of the flip/flop 718. The pulses of waveform N are applied, negative going, to both of the flip/flop grids giving an output from the anode of triode 722 as shown in waveform Q, which is applied to one input of a comparator circuit 764) the undelayed inverted sweep pulses (waveform O) being applied to the second input. The arrangement of the comparator is such that the cathode potential of diode 776 only rises enough to trigger the one shot multivibrator 780 when both diodes 766, 768 are cut off by pulses from the flip/flop 718 and waveform O respectively, resulting in the diode 776 taking all the current. The resulting output from 780 is as in waveform S and is applied, together with waveform N, delayed by one cycle (waveform T), to gate 782, resulting in the elimination of all those error inducing cycles having a single pulse (waveform U). Detailed descriptions of the individual circuit elements are given. Specification 978,573 is referred to.