Patent GB-906947-A (GB)
Title: Detection of defects in sheet glass
IPC: G01N21/89
Filed: 1960-10-27
Abstract: 906,947. Photo-electric flaw evaluation. PITTSBURG PLATE GLASS CO. Oct. 27, 1960 [Nov. 2, 1959 (3)], No. 36899/60. Class 40 (3). The invention relates to a photo-electric inspection system for sheet glass, especially polished plate glass, to enable glass inspected to be subdivided into top quality areas suitable for mirrors, areas suitable for commercial plate glass, and reject areas. For this purpose the prevelance and seriousness of four types of defect are determined and may then be fed together with positional information about the defects to a storage device and computer which calculates the best way of cutting the glass into smaller sized sheets. Alternatively, markings may be made directly on the glass to indicate defects and the best way to sub-divide the sheet determined by a cutter. The four types of defects considered are as follows: Type A defects, which are microscopic. surface defects and, if present, frequently extend over relatively wide areas. Type B defects, which occur within the body of the glass and result from incomplete blending of the ingredients: the lengthwise dimension of these defects is usually aligned with direction of drawn of the glass. Type C defects which are point type defects in or at the surface of the glass due to solid or gaseous inclusions or small surface fractures. Finally, Type D defects, which are linear surface indentations such as scratches. In the detection and assessment of Type A defects, a source of light S (Fig. 1) is associated with a lens system L1 which forms an image of the source at the plane of aperture al in plate PL. This small aperture serves as a point source from which light reaches and passes through glass sheet or ribbon G moving in the direction indicated by the arrow. A rotary scanning disc D disposed in the light path beyond the glass has a small aperture a2 offset from its axis of rotation, which is preferably coincident with the axis of the system. The light passing through aperture a2 is directed by lens system L2 to photo-multiplier T. Constructional details of units containing the elements on the two sides of the glass are described with reference to Figs. 2 to 7 (all not shown). These units are mounted on the top and bottom longer sides of a vertically mounted elongated rectilinear framework through the central space in which the glass is passed on a conveyer. The framework is arranged for oscillation in a direction at right angles to the glass movement and a double-acting hydraulic system causes it to move at a constant linear velocity except for short periods when a change in the direction of travel is being made. Oscillation of the framework together with rotation of the disc D causes a scan on the surface of the moving glass sheet as indicated in Fig. 12. Normally, the glass will be wider than can be adequately scanned by one optical system in which case two or more independent systems are mounted on a single oscillatory framework. Fig. 12 shows how the scans of two systems are arranged to overlap sufficiently for outputs when the velocity changes, as the direction of framework travel reverses, to be ignored. The signals from the (or each) photo-multiplier are amplified and then fed to a full-wave rectifier and a smoothing circuit discriminating against brief large amplitude signals. The amplitude of the resultant signal is a measure of the severity of type A defects and may be compared with pre-set potentials to determine the grading and/or cutting of the glass in the computer unit. For the determination of Type B defects, the optical system used is that indicated in Fig. 14. A similar source unit to that referred to in connection with Type A defects directs light at an angle on to the moving glass. On the other side of the glass and on a continuation of the optical axis of the source unit is provided a disc D<SP>1</SP> with a narrow elongated slit s in it aligned with the direction of draw of the glass. Light passing through the slit falls on the photo-multiplier T placed immediately behind it. The pair of units, or a plurality of pairs of units, for the detection of Type B defects are mounted on a similar oscillatory framework to that previously described so that each system covers a zig-zag path relative to the glass. The output of the photo-multiplier(s) is amplified, rectified and fed to the computer. A combined inspection device for Type C and Type D defects is shown schematically in Fig. 20. Light from a system giving an approximately point source at aperture a in plate P1 is collimated by lens system L2 and directed through the glass at an angle of approximately 78 degrees to the surface thereof. The transmitted light is received by objective lens system L3 which is followed by a beam separator BS. This is a transparent glass plate set at an angle to the optical axis and having a small central mirrored portion. The mirror directs the substantially parallel light from the glass to a fixed narrow slit s1 backed by a rotating disc D1 provided with four equispaced transparent arcuate portions as indicated in Fig. 26. Light passing through the small aperture formed by the intersection of the slit and one of the arcuate apertures is gathered by lens system L4 and directed to photo-multiplier T1. Scattered light falling on the transparent part of separator BS falls on a similar system comprising slit s2, disc D2 with arcuate slots, lens system L5 and photo-multiplier T2. In the practical construction described, two optically separate systems are formed into twin units using slots s1, s2, aligned along radii of discs D1, D2 on opposite sides of the axes of rotation so that disc D1 and disc D2 are common to the two systems and perform the same function in each (Figs. 21 to 25, all not shown). A plurality of such " twinned " inspection devices are mounted on a rectangular framework arranged round the glass as previously described. The framework is mounted for lateral movement and controlled so that the inspection devices are always in the same position relative to one edge of the glass sheet. Rotation of the scanning discs causes each inspection device to scan repeatedly across a portion of the width of the glass and the plurality of such devices to scan the whole width. Type C defects show as dark spots on a light field and information as to the size of these spots and reduction of light intensity caused by them is derived from the output of photo-multiplier T1 and fed to the computing or indicating arrangement. Type D defects scatter the light and appear to photomultiplier T2 as light spots on a dark ground. The amplitude and duration of the output signal give an indication as to the seriousness of the defect and this signal, together with all the other defect-assessment signals is fed to the computer or other utilization device. The Type C and Type D inspection devices may be constructed separately if desired and any of the devices may operate by reflected rather than transmitted light if this is appropriate.