Patent GB-1401288-A (GB)
Title: Electronic defect detecting apparatus
IPC: G01N21/90
Filed: 1972-08-02
Abstract: 1401288 Photo-electric flaw detectors BALL CORP 2 Aug 1972 [16 Aug 1971] 36086/72 Heading G1A [Also in Division H3] In a system for detecting defects in an object, video signals produced by a camera are processed to remove those portions resulting from the edges of the object. Optical system (Fig. 1). Objects, e.g. bottles, pass along a chute (12) between a semi-diffuse light source (22) provided with a collimating lens (24) and a TV camera (26) connected to the processing circuit which operates a deflector gate (18), or a reject arm, to remove defective objects from the main path. The presence of an object in the measuring zone may be detected by means of a light source and photo-cell arrangement which strobes the main light source. The defects and edges of the object show as dark areas against a light background due to the refractive characteristics at regions of changing thickness but detection sensitivity may be increased by covering the central part of the light source with a black mask in which case the defects and edges appear as bright spots against a dark background. Signal Processing. (Figs. 4 & 5). The horizontal and vertical sync pulses produced by the camera trigger four monostable multi-vibrators which are arranged to produce horizontal and vertical lead and trail signals HLEAD, HTRAIL, VLEAD and VTRAIL. The inverses of these signals are used to inhibit a comparator (172) producing negative going output pulses from dark spots of the video signal which is passed through a high pass filter (142) and a buffering circuit to the comparator. The video signal pulses are therefore blanked at the edges of the picture area to define a reference window within which the object is located. Edge removal, Fig. 6. In each line scan the pulse produced by the leading edge of the object is removed by means of a circuit including monostable 210 which is triggered by the first pulse to produce a signal EDGE 1 which inhibits a NAND gate 200 to which the video pulses are applied. The monostable 210 also sets a bi-stable 250 which inhibits NAND gate 200 before the occurrence of the first pulse and also inhibits the input to monostable 210 so that the latter can not be triggered a second time during the line. Bi-stable 250 is reset at the start of each line by the HLEAD signal. Each video pulse also triggers transistor 260 into conduction to charge capacitor 268. At the beginning of HLEAD of the next line the comparator 272 is triggered into the low state and capacitor 284 charges until the voltages at points A and B are equal. The comparator then switches to the high state. The voltage on capacitor 284 which was inversely related to the time between the last pulse and the start of HLEAD is therefore reproduced on capacitor 284 which discharges slowly until the voltage is equal to a reference applied to comparator 312 which switches producing an EDGE 2 signal commencing just before the time at which the last pulse of the previous line occurred. This signal also inhibits NAND gate 200 to suppress the pulse due to the second edge of the object. The first pulse of each line also triggers a monostable 342 which resets after a time sufficient for the scanning of the width of the object to provide a WIDE signal which is used in the classifying circuitry. Detection and Classifying circuits, (Fig. 7). The defect pulses are used to set a bi-stable (402) which forms the first stage of a shift register (400) clocked by HTRAIL. The outputs of the stages of the register are gated to provide an output if a defect is present in any three of four consecutive lines. Circuitry is also provided for determining. whether or not an object is being scanned and whether this is a single object, half an object or one and a half objects. The presence of half or one and a half objects is recorded by a bi-stable 468 and the output of this is combined with the classification and defect presence signals of the next line to control the movement of a deflector gate to the reject or accept positions as appropiate via a time delay produced by a monostable 504. Strobe system, (Fig. 8). If a strobed light source is used the output of the photo-cell prepares the strobe circuit which is triggered at the start of the next frame and the classification circuitry is omitted. The detector circuit may incorporate switching so that a defect is detected in response to defect pulses in three or four consecutive lines or in two of three or three of four consecutive lines. The defect output is applied to the first of a series of four monostables which trigger in cascade and the last monostable operates a reject arm after a suitable delay which is adjustable. An alternative circuit for generating the window defining signals is given (Fig. 9).