WO1999064845A1

WO1999064845A1 - Defect detecting unit

Info

Publication number: WO1999064845A1
Application number: PCT/BE1999/000068
Authority: WO
Inventors: Hugues Lefevre; Guy Renard
Original assignee: Glaverbel
Priority date: 1998-06-05
Filing date: 1999-06-01
Publication date: 1999-12-16
Also published as: AU4124599A; GB9812091D0

Abstract

A defect checking unit for transparent sheet material (4) comprises one or more laser light sources (1), a light configuring device (2) associated with each light source (1) to configure the light into a planar beam, and a light detection device (7), in which the light source (1) and configuring device (2) are disposed to transmit the planar beam through the sheet (4) to be checked, characterised in that the transmitted light forms an image band on a screen (5) for inspection by the light detection device (7).

Description

Defect Detecting Unit

The invention relates to a device for detecting in transparent sheet material defects that can lead to distortion or attenuation of light passing through the material. It is particularly concerned with detecting in sheet glass produced in a float glass process the presence of micro-bubbles, of inclusions such as stones and of zones in the sheet where the density or thickness of the glass differs from those of the sheet as a whole.

Many proposals have been made to examine the quality of transparent sheet material by directing a light beam towards it and inspecting the reflected or transmitted light for signs of defects. GB patent specification 880135 detects flaws in sheets of glass using apparatus which projects a thin light curtain on to the surface of a sheet and has a photosensitive detector which receives light reflected from the surface and creates an output signal when the reflected light differs from a flaw-free reference condition. EP-A- 060160 relates to detecting defects leading to surface deformations in a moving ribbon of glass by using under the glass a linear light source and over the glass a network of photodetectors. The photodetectors are disposed so that a defect in the glass deflects light in the thin light curtain passing through the said glass to the photodetectors, increasing the light quantity they receive in order to identify the presence of a defect and its location in the glass. WO 93/6467 teaches the use of a thin light curtain formed from a laser source to detect defects in glass sheets. A CCD (charge couple device) camera detects a light image reflected as dots corresponding to the defects. According to the luminosity of the image of a defect detected by the camera, it is possible to determine the approximate size of the defect. According to the above-described units, the defect detection device is directed to the inspected material at its intersection with a thin light curtain obtained from the light source. However, because of the small size of the defects, the image recorded by the detection device does not offer a sufficient resolution to allow a precise identification of the type of the said defects.

It has also been suggested to place a screen remote from the examined substrate in order that large size images of defects can be displayed. WO 94/9358 describes a system for detecting defects on a moving glass ribbon in which a laser source placed under the ribbon emits a beam shaped into a thin light curtain directed transversally across the ribbon and obliquely with respect to the surface. A skimming light further illuminates surface deposits. A light background is disposed under the ribbon, on the same side as the laser source, to serve as a contrast reference for the detection system. A CCD camera placed on the other side of the ribbon observes the background and registers dark spots indicative of the presence of defects in the ribbon and extra brightness indicative of deposits on the ribbon surface. The recorded image of a defect is situated between two bright lines corresponding to the intersections of the laser planar beam with the faces of the ribbon. This permits the location of the defect in the ribbon depth to be determined. DE 4444165 teaches spreading a linear laser beam into a three-dimensional beam that passes through a planar transparent substrate. A large planar image of the substrate as well as of the defects it comprises is displayed on a ground-glass screen. The images of the defects can then be visually analysed by an operator or automatically treated by use of cameras. Another defect detection unit using projection of a two dimensional image of the substrate for inspection on a screen is disclosed by EP 0484237. Such devices allow a better resolution of the image of the defects since their size is enlarged on a screen. Furthermore, since the detection device is not focused on the examined material, surface deposits on this substrate are not identified as defects. Nevertheless, since the projection is performed by use of light beams giving large images on the screen, the contrast of these images may be insufficient for a precise analysis of the images displayed, and thus for an accurate identification of the types and shapes of the defects.

The present invention overcomes these problems by providing a defect detection unit for displaying images of defects present in a transparent sheet material with an increased resolution compared with the prior art proposals, thereby enabling precise identification of the types, sizes and shapes of the detected defects.

According to the invention there is provided a defect checking unit for transparent sheet material passing through the unit, which unit comprises one or more laser light sources, a light configuring device associated with each light source to configure the light into a planar beam, and one or more light detection devices, the light source(s) and configuring device(s) being disposed to transmit the planar . beam through the sheet to be checked, characterised in that the transmitted light forms an image band on a screen for inspection by the light detection device(s).

Compared with prior devices using a linear scanning laser beam to detect defects in a moving sheet of material, the unit of the invention permits simultaneous examination of a whole width of the sheet, thereby permitting more rapid detection of the defects and avoiding the use of complex components such as a spinning or oscillating mirror to impose a to-and-fro scanning movement throughout the width of the sheet.

The laser light source, which may for example be a laser tube or a laser-emitting diode, provides a point source of light. The light emitted by the said source is configured into a beam having a wide but shallow cross sectional shape, referred to herein as a "planar beam". The device to configure the planar beam is conveniently a suitable lens assembly and can be regarded as a "line maker".

In the absence of defects in the sheet, the image formed on the screen by the said planar beam is a generally parallel-sided band of light with a shallow depth relative to its width across the screen. This image band has a gaussian light intensity profile across its depth, i.e. its light intensity varies across its depth from a maximum at the centre line parallel to the parallel sides to a minimum at each of the parallel sides. The gaussian variation of light intensity across the band depth can be represented by a bell-shaped curve with its crest at the centre line.

Each light detection device may be focused along a line within the said band parallel to, and to one side of, the centre line of the said profile. The light intensity recorded by each light detection device thus remains constant provided that the incident planar beam meets no defect in the transparent sheet material.

If the planar beam does meet a defect within the sheet, it is locally deflected and the image band formed on the screen is distorted by a peak corresponding to the defect. According to the orientation of the said peak, which may project in either direction peφendicular to the centre line of the image band, the detection device(s) will record either local increases and decreases, or only decreases, in the light intensity as the peak traverses or does not traverse the focusing line of the said device. If the focusing line of the detection unit were not located within the image band, peaks created within the band would not be recorded.

Two or more detection devices can be used, at least one being focused on each side of the centre line of the image band on the screen. In such a configuration of the invention, the focusing line of each detection device does not need to be located within the depth of the said profile. Indeed, whatever is the orientation of a local peak in the image band, the peak will be detected by at least one detection unit when the crest of the light intensity profile of this peak traverses the focusing line of this unit. As the defect moves forward with the sheet through the unit, successive images of peaks appear on the screen until the defect passes beyond the planar beam. The succession of peaks is recorded by at least one light detection device, which permits the construction of a complete two dimensional image of the defect. A processing unit analyses these data in order to identify and measure the defects.

In focusing the detection devices outside the light intensity profile of the image band, one can determine the sensitivity of detection of the unit. Indeed, the focusing may be adjusted so that low peaks in the said image are not observed, in order that only defects of a certain size are detected. The planar laser beam is focused on the transparent sheet material. Typically, the depth (i.e. thickness) of the light band appearing at the intersection of the planar beam and the transparent sheet material is 300 μm or less. Consequently, very small defects in the inspected sheet can be detected and identified. For safety reasons and because high power lasers cannot be continuously activated, relatively low power lasers are used according to the invention. Particularly suitable are lasers emitting in the green portion of the luminous spectrum, especially when infrared cutting transparent material is to be checked by a detecting unit according to the invention. Indeed, this type of lasers doesn't emit light in the red and infrared portion of the luminous spectrum. Therefore, loss of light power of the laser incident beam due to absoφtion by infrared cutting transparent material is reduced compared with traditional red light emitting lasers. Moreover, most of the detection devices that can be used according to the invention are more sensitive in the green that in the red portion of the luminous spectrum.

The use of a planar beam has the advantage of spreading the available power of the laser source over a shallow band across the material to be tested, so that the light intensity is sufficient to obtain an accurate level of contrast of the images formed on the screen. This is not the case if for the same power of the laser source, a three dimensional beam is used in order to project a planar image on the screen. The choice of a type of laser to be used in a detecting unit according to the invention is therefore influenced by the capability of the laser beam to be focused into a band as shallow as possible. For example, YAG type lasers are appropriate for this puφose.

The light intensity in the laser planar beam depends on its angular spread. The value of this spread has to be adjusted according to the power of the laser source in order to achieve a contrast of the image on the screen that matches the sensitivity of the detection device(s).

The angular deviation of a linear beam comprised in the planar beam, relative to its axis, influences the length of the optical path this beam follows through the glass ribbon. This length determines the amplification factor of the image of the beam on the screen. Therefore, according to their locations in the glass ribbon width, identical defects may create images of different sizes. To avoid this phenomenon, which could lead to mistakes in determining the size of these defects, it is preferred to reduce the angular spread of the planar beam.

Besides, because of the optical characteristics of the lenses used to turn light emitted by the laser point source into a planar beam, the light intensity at the extremities of the width of an image formed on the screen differs from that of the rest of the image. This difference could also lead to mistakes in the identification of the defects. Therefore, it is preferred that the detection device does not record the extremities of the image formed on the screen.

Where the planar beam strikes the material to be tested, it forms a light band transverse to the direction of movement of the sheet material. The width of this band across the sheet depends on the angular spread of the planar beam. Typically, a width of sheet of between 0.5 and 3 m can be analysed by a unit according to the invention. In embodiments of the invention for testing greater widths of material, for example a 3 m or more wide ribbon of glass from a float line, two or more laser point sources, associated line-making devices and light detection devices may be employed.

The distance separating the screen image from the intersection line between the planar beam and the sheet under test affects the extent of amplification of the size of the defects in the said screen image. One can thus modify this amplification in adjusting the relative position of the screen and the sheet. For a given relative position, the amplification also depends on the position of the image on the screen. This position is determined by the length of the optical path followed by the planar beam through the sheet in such a way that the longer the path, the greater the amplification. The length of the said path depends itself in part on the incident angle of the planar beam on the sheet surface and in part on the thickness and refractive index of the sheet material.

Typically, the preferred angle of incidence of the planar beam upon the surface of the sheet under examination according to the invention is generally in the range 10 to 90°. The precise determination of the value of this angle depends namely on the types of defects that one wishes to detect. Indeed, defects arising from a difference in density or thickness of the glass compared with the sheet as a whole require a small value of the angle of incidence to achieve the screen display. Whether such defects are to be displayed can thus be determined by appropriate choice of the said angle. According to one preferred configuration of the unit of the invention, the or each light detection device is disposed at the same side of the sheet of material as the light source(s) and configuring device(s) and thus the screen image is observed by the light detection device(s) through the said sheet. In this way, the relative position of the image and of the line of detection of the light detection device on the screen remains constant, whatever the thickness, the shape or the refractive index of the sheet material analysed. Curved sheets of material can thus be inspected according to the invention.

In another preferred embodiment of the invention the unit includes a semi-mirror in the path of the planar beam from the or each configuring device to the sheet material. This permits adjustment of the angle at which the planar beam strikes the sheet material and thus allows a given position of image on the screen to be achieved for any thickness or refractive index of the sheet material under examination. Furthermore, such an adjustment allows to focus the detection device on the intersection line of the planar beam and the sheet material. This enables to properly inspect discrete sheet material pieces. Indeed, one needs to achieve this focusing in order that the marginal portions of the said pieces can be inspected, since otherwise, the focusing line of the detection device might not traverse the sheet material at the said portions so that it wouldn't be accurately located on the screen relatively with the image of the planar beam. The semi-mirror also permits the light source(s) and configuring device (s) to be conveniently distanced from the light detection device. Most preferably the light detection device has a direct line of sight (its "sight axis") through the semi-mirror and through the sheet material to the image on the screen while the or each light source and configuring device are offset from the said axis such that the planar beam is reflected by the semi- mirror towards the sheet material and screen. Alternatively, a small mirror being sized according to the cross section of the said planar beam can be used instead of a semi-mirror. In a further preferred embodiment of the invention a tilting glass plate is located across the sight axis of the or each light detection device between the light detection device and the semi-mirror or mirror here above referred. Adjusting the angle of the plate to the sight axis changes the optical path of the sight axis through the plate and changes the band on the screen observed by the light detection device. The plate thus provides a means of adjusting the position of the focusing line of the light detection device on the screen. This can be used to adjust the detection sensitivity of the unit.

The preferred type of light detection device is a CCD camera. Such a camera offers the advantage of generating image signals in digital form which can be readily processed to provide a complete analysis of the type and size of defects present. A further advantage is that the processing can be conducted automatically. A particular benefit of the invention thus arises when the sheet material being tested is a ribbon of glass from a float glass production line, since the analytical results allow the line operator to modify the operational parameters of the glass furnace to reduce the number and the size of the defects and at best to avoid them altogether.

Other types of detection devices can nevertheless be used, such as photo-detectors ensembles or CMOS type captors. The latter offer a particularly good light sensitivity level as well as the possibility to adjust the sensitivity of each individual pixel they comprise.

Because of the shallow angle of incidence of the planar beam upon the sheet to be tested, some of the incident light upon the sheet is not transmitted but is reflected. In a further embodiment of the invention the unit includes a second screen, in this case disposed to the side of the face of the sheet on which the planar beam is incident. This additional screen is aimed at displaying the image of the reflected part of the incident beam, so that it is also recorded by the detection device. This configuration increases the luminous intensity that reaches the detection device and thereby improves the definition of the recorded images. Furthermore, an internal defect will lead to a peak in the image of the transmitted part of the beam and not in the image of its reflected part. On the other hand, a surface defect will lead to a peak in both of these images. The light intensities recorded by the detection device will thus be different in each of these two cases. This enables defects associated with surface irregularities to be distinguished from those that are not.

Preferably, the screens on which the images are displayed are coated with a reflecting material having such optical properties that it reflects light in its direction of incidence. Consequently, no specular reflection occurs, so that substantially the whole quantity of light reflected by such a screen reaches the detection device. This increases the resolution of the images recorded compared with traditional screens.

In another configuration of the invention, at least one screen made of a translucent material can be used. At least one detection device is then located on the side of the screen opposite to the side on which the images are displayed.

Preferably, the light detection device and the other elements of the unit, except the screen, are mounted on the same chassis or in the same chamber as a monobloc. This configuration facilitates disposition of several such monoblocs above the sheet material in order to inspect its whole width simultaneously and thereby allow the light detection devices to give a simultaneous reading of defects across the said whole width.

The invention is further illustrated with reference to the accompanying figure 1, which is a diagrammatic side view of a device according to the invention. A ribbon of glass to be tested, of which a part is indicated in the figure by reference number 4, is horizontally disposed and moved through the device in the direction shown by the arrow D.

The device comprises a laser light source 1, a line-maker lens 2 and a pivoting semi-mirror 3. The source 1 is in the form of a diode and generates a point source of laser light. The lens 2 forms the laser light into a planar beam . The semi-mirror 3 deflects the planar beam at an angle of 20° towards the ribbon 4, to form a transverse band of laser light across its upper surface. Given this angle of incidence of the laser light to the glass surface, most of the light passes through the ribbon 4 but part of the light is reflected.

Two screens, 5 and 6, are disposed vertically to receive light from the source 1. Screen 5 is located below the level of the ribbon 4 and receives an image band from the light passing through, and refracted by, the ribbon 4. Screen 6 is located above the level of the ribbon 4 and receives an image band from the light reflected by the upper surface of the ribbon 4.

A camera 7 of the linear charge couple device (CCD) type is located, relative to the ribbon 4, beyond the semi-mirror 3 and serves as a defect detection unit. The laser source 1, lens 2, semi-mirror 3 and camera 7 are mounted on a common frame (not illustrated). The camera 7 is disposed such that its sight axis 9 is aligned with the planar beam passing through the ribbon 4 and is focused on a line across the width of the image band formed by the said beam on the screen 5. A rotatable plate 8 of dense glass is also mounted on the camera support frame and placed across the sight axis 9 between the camera 7 and semi-mirror 3.

Rotation of the plate 8 around an axis peφendicular to that of the camera sight axis 9 modifies the optical path of the light in the said glass and thus in turn modifies the position of the focusing line of the camera 7 on the screen 5. The said rotation thus allows adjustment of the distance which separates the said focusing line from the image on the screen 5.

Light reflected by the upper surface of the ribbon 4 to strike the screen 6 is in turn reflected back by the screen 6 to the said surface and thus adds to the light transmitted by the glass to the camera 7. This configuration permits the loss of luminous intensity to be limited and thus improves the definition of the images obtained on the screen 5 and recorded by the camera 7.

When light passing through the ribbon 4 meets a defect it is deflected to form a corresponding peak in the image band formed on the screen 5. The peak is observed and recorded by the camera 7. As the ribbon 4 moves forward the defect continues to be illuminated, thereby creating a succession of peaks in the successive images recorded by the camera 7, until the defect passes beyond the transverse band of light striking the glass surface. The collectively recorded succession of peaks thus constitutes a series of electrical signals giving a complete image of the defect. Data processing of the said signals permits an automatic survey of the type and of the size of the defects, which permits in turn modification of the parameters of the furnace in order to reduce the number and the size of these defects.

Figures 2 to 4 illustrate typical images displayed on the screen 5. Figure 4 shows a portion of the image band 11 formed on the screen in the absence of any defects. The image band 11 has parallel upper and lower sides 12 and 13 respectively and a centre line 14 which represents the line of maximum brightness of the image. The plate 8 is adjusted to position the camera's focusing line 15 parallel to and within the image band 11 between the upper side 12 and the centre line 14. In this no-defect state, the intensity of image is the same at all points along the focusing line 15.

Figure 3 illustrates a typical distortion of the image band 11 caused by a defect in the glass ribbon 4. The distorted image includes an upwardly-directed peak 16, which creates changes in the light intensity along the focusing line 15. For example at points 18 and 19 the camera 7 detects local increases in light intensity, relative to the no-defect state, whereas at points 20 and 21 it detects local decreases in light intensity.

Figure 4 illustrates a similar distortion to that of Figure 3 but with a defect which causes a downwardly-directed peak 17. In this case the camera 7 detects at points 22 and 23 local decreases in light intensity relative to the no- defect state. Over the distance between points 22 and 23, where there is no illumination of the screen 5 by the image band 2, the camera 7 detects the absence of illumination.

Claims

1. A defect checking unit for transparent sheet material (4) passing through the unit, which unit comprises one or more laser light sources (1), a light configuring device (2) associated with each light source (1) to configure the light into a planar beam , and one or more light detection devices (7), the light source(s) (1) and configuring device(s) (2) being disposed to transmit the planar beam through the sheet (4) to be checked, characterised in that the transmitted light forms an image band on a screen (5) for inspection by the light detection device(s) (7).

2. A defect checking unit as claimed in claim 1, in which each light detection devices is a CCD camera.

3. A defect checking unit as claimed in claim 1 or claim 2 , in which the laser light source is a laser tube or a laser-emitting diode.

4. A defect checking unit as claimed in any preceding claim, in which the light configuring device is a lens assembly.

5. A defect checking unit as claimed in any preceding claim, which includes two or more laser point sources and associated light configuring devices employed in alignment.

6. A defect checking unit as claimed in any preceding claim, in which the angle of incidence of the planar beam upon the surface of the sheet under examination is in the range 10 to 90┬░.

7. A defect checking unit as claimed in any preceding claim, in which the light detection device is disposed at the same side of the sheet of material as the light source(s) and configuring device(s).

8. A defect checking unit as claimed in any preceding claim, which includes a semi-mirror in the path of the planar beam from the or each configuring device to the sheet material.

9. A defect checking unit as claimed in claim 8, in which the light detection device has a "sight axis" giving a direct line of sight through the semi-mirror and through the sheet material to the screen while the or each light source and configuring device are offset from the said axis such that the planar beam is reflected by the semi-mirror towards the sheet material and screen.

10. A defect checking unit as claimed in claim 8 or claim 9, which includes a tilting glass plate located across the sight axis of the light detection device between the light detection device and the semi-mirror.

11. A defect checking unit as claimed in any preceding claim, in which the light detection device and the other elements of the unit, except the screen, are mounted on the same chassis or in the same chamber.

12. A defect checking unit as claimed in any preceding claim, which includes a second screen, in this case disposed above the sheet being tested, to the side of the face of the sheet on which the planar beam is incident.