Patent GB-1234425-A (GB)
IPC: G01N21/89
Filed: 1968-10-17
Abstract: 1,234,425. Detecting defects photoelectrically. C. BRICHARD, Oct. 17, 1968 [Nov.31, 1967], No.49299/68. Heading G1A. At least one of the co-ordinates of a defect in a sheet or strip is determined by successively scanning adjacent bands of the sheet or strip by a radiation beam displaced directly or indirectly by at least one electrical factor, the magnitude of which at the instant when the beam encounters a defect being used as a measure of the co-ordinate of such defect. Defects may be detected by the transmission or reflection of a light or electron beam and the results may inform a computer controlling the cutting of metal, glass, plastics or paper strips. A glass sheet 30<SP>1</SP>, Fig.4, is scanned rasterwise by a television type camera tube 33<SP>1</SP> fed by X and Y sweep generators 40<SP>1</SP>, 40<SP>11</SP>. A line of light is located near the start of the line scans so that the video signal from electrode 38<SP>1</SP> includes reference pulses in addition to pulses due to defects. The video pulses trigger a digital volmeter 43<SP>1</SP> to sample the X deflection signal, values corresponding to reference and defect pulses being transmitted to lines 45<SP>1</SP> and 58<SP>1</SP> respectively. A differentiator 48<SP>1</SP> delects the flyback of the camera spot causing gate 47<SP>1</SP> to blank the video signal and bi-stable circuit 51<SP>1</SP> to set, informing voltmeter 43<SP>1</SP> that the next video pulse will be a reference pulse. A further digital voltmeter 43<SP>11</SP> samples the Y deflection signal in response to defect pulses, but ignores reference pulses. A discriminator circuit 57<SP>1</SP> determines whether or not each defect signal (a) is of more than a predetermined amplitude, (b) is of more than a predetermined duration, or (c) has more than a predetermined rate of change and if so opens respective pairs of gates 64<SP>1</SP>, 64<SP>11</SP>; 65<SP>1</SP>, 65<SP>11</SP>; 63<SP>1</SP> ,  63<SP>11</SP>; to apply the co-ordinate values from voltmeters 43<SP>1</SP>, 43<SP>11</SP> to lines 61<SP>1</SP>, 61<SP>11</SP>; 62<SP>1</SP>, 62<SP>11</SP>; 60<SP>1</SP> ,  60<SP>11</SP>; Discriminator 57<SP>1</SP> includes means for distinguishing a single defect giving rise to a video signal which changes sign once or more from a plurality of separate smaller defects. If the glass sheet moves rapidly, the camera tube may make a linear scan, Fig. 3 (not shown). Readout of the digital voltmeter is synchronized with presentation of information thereon by an arrangement of bi-stable circuits and gates, Fig. 5 (not shown). If a slow acting digital voltmeter is used the deflection signal samples are held on a capacitor which is discharged in response to the scan flyback, Fig. 6 (not shown). In a circuit giving directly the difference between the co-ordinates of a defect and the reference line, Fig. 7, the deflection signal applied at 704 charges capacitors 702 and 703 equally until a reference pulse at 709 sets a bi-stable 709 to cut off transistor 706. Capacitor 702 continues to charge, and voltmeter 701 which monitors the voltage difference between capacitors 702 and 703 is read out by a defect pulse at 717 opening a gate 719 via a synchronizing circuit 718. In another embodiment, Figs. 1 and 2 (not shown) scanning is performed by a mirror galvanometer which, energized by a signal of the form A arc tan Bt, deflects a light beam through a glass sheet on to a linearly extensive photo-sensitive device. For wide strips, a plurality of scanners may be used, Fig.8 (not shown) or a single scanner may be traversed across the strip, Fig.9 (not shown).