Method for measuring the degree of bending in a bent glass sheet

Abstract: The invention relates to a method for measuring the degree of bending in a bent glass sheet. The measuring is effected by means of cameras (1) having a common imaging field within the boundaries of a surface being measured. The cameras are set at an arbitrary but stationary angle relative to each other. This prior known principle is useful also in the application of the invention, as the glass surface along a profile to be measured is made visible to the cameras by means of an illuminating line (7) whose light has a high intensity over the wavelength range of 250-330 nm. Hence, an extremely thin film of tin, left on the glass surface in the manufacture thereof, becomes visible.

Description: The present invention relates to a method for measuring the degree of bending in a bent glass sheet. For example, in the gravitational bending of automotive windshields and backlights, the degree of bending is difficult to control with a sufficient accuracy. After a bending operation, it is necessary to check the degree of bending in order to discard the incorrectly bent pieces. This has been done by using mechanical equipment, wherein the glass is secured to a checking bench and mechanical tracers are used for checking the degree of bending at certain points. This is a slow and tedious process and the number of checking points is limited. The necessity for measuring the degree of bending also involves glasses that have been bent with a "pressbend" method or by means of an arching conveyor. Thus, the question may be about measuring the final bending of bent tempered glass produced by means of a roller-equipped furnace. Patent publication US-4,928,175 discloses a method for the three-dimensional monitoring of a target space with two or more video cameras, which are set at an arbitrary but stationary angle relative to each other and which have a common imaging field in which a target to be monitored is located. However, this prior known method is not as such suitable for measuring the degree of bending in a glass sheet, since glass is transparent over the band of wavelengths detected by the cameras. An object of the invention is to provide a method, whereby said camera-imaging based technique can be applied to measuring the degree of bending in a glass sheet. This object is achieved by means of the invention on the basis of the characterizing features set forth in the annexed claim 1. The non-independent claims disclose preferred embodiments of the invention. The invention will now be described in more detail with reference made to the accompanying drawings, in which fig. 1 shows a schematic view of an apparatus for carrying out a method of the invention; fig. 2 shows an end section of a bent glass sheet, whose curvature profile is the object of measuring; fig. 3 shows video signals for a pair of cameras used for measuring; and fig. 4 depicts schematically a preferred illumination apparatus for illuminating the surface of a glass sheet so as to produce at a desired point on the glass-sheet surface an illuminating line visible to the cameras. The structural design of an apparatus used in the method is as follows. At least two video cameras 1 are mounted on a common frame 3, such that the cameras 1 remain at a stationary angle relative to each other. This angle can be arbitrarily selected, yet in such a manner that the cameras 1 have a common imaging field which covers the object of measuring. The video signals produced by the cameras 1 are processed in a computer 2 as described in Patent publication US-4,928,175. The same applies also to the calibration of a measuring system, i.e. determination of the bearing factors of the cameras by means of known points. A glass sheet 5, whose bending profile is being measured, is placed within an imaging field 6 common to the cameras. During the course of imaging, the glass sheet is not required to have any precise disposition or attitude. Thus, the measuring can be effected directly from the conveying track. Above the measuring point is set a light source 4, whose light has a high intensity over the wavelength range of 250-330 nm. Thus, the extremely thin layer of tin formed on one surface of the glass during its manufacturing becomes visible to the cameras 1. An illuminator having a powerful intensity at the wavelength of 253,7 nm has been used earlier for illuminating a glass surface after visually inspecting on which surface of the glass a layer of tin is formed. In the invention, this per se known phenomenon is exploited in a new type of measuring method. Between the light source 4 and the glass 5 is provided a slit screen 10, admitting therethrough one or more light beams for producing an illuminating line 7 on the surface of glass 5. Thus, a desired number of illuminating lines are produced at locations whose curvature profile is to be measured. Optionally, the screen 10 can be manipulated in such a manner that the illuminating line 7 shifts to various locations on the surface of glass 5, the curvature profiles of the corresponding points being measured sequentially. With the present assembly, the measuring is efected as follows. The image fields of both video cameras 1 are scanning criss-crossing lines 9. Video signals V of both cameras 1 receive an intensity pulse whenever the cameras detect the illuminating line 7 on the glass surface. Thus, during each field line of both cameras 1 there occurs a pulse 8, whose moment of occurrence is determined by which pixel of a CCD-image field the pulse originates from. Since, after calibrating the measuring system, the relative disposition of cameras is known, the relative disposition of pulses 8 occurring during each pair of lines can be used for determining the 3-D coordinates of a point of the illuminating line 7 falling on the crossing point of lines 9. Depending on the number of image points included in the CCD-image fields of cameras 1, it is possible to determine the 3-D coordinates of even more than 200 points for indicating the surface-curvature profile along the illuminating line 7. In addition to this, it is possible to determine the curvature profile of glass-sheet edges by placing at least two cameras 1 in such a manner that both have the same edge surface of a glass sheet within their field of vision. It has been unexpectedly discovered that illumination of a surface of the glass sheet 5 with the light source 4 sheds light also on a glass-sheet edge surface. This "illumination" of a glass edge can also be used for measuring the glass size as both cameras are able to see the opposite glass-edge surfaces or the entire glass outline. The invention can be applied in a second bending examination effected downstream of a bending furnace. Thus, the measured curvature profile is compared to a predetermined profile and, if an anomaly exceeding a certain limit is detected as a result of the comparison, the glass sheet is rejected. The invention can also be applied to the regulation of a bending process in such a manner that, if anomalies exceed a certain limit, a regulation or adjustment of the bending process is effected in order to compensate for the anomalies. In this application, it is of course necessary to monitor both the distribution and average of the bending. In another application of the invention, the measuring result can be compared to the points of the real CAD-file of glass or to a surface formed thereby. The minimum number of cameras required for carrying out the measuring is two. In practice, however, the number of cameras may be more than that, typically e.g. four cameras. The illumination system shown in fig. 4 comprises a high-pressure mercury vapour lamp 11, having a relatively high intensity also over the UV-range. A diffraction mirror 12 and a collimation lens 13 are used for focusing the beam of light on an x-y-mirror 15, which is included in a scanner 14 and capable of maneuvering in x-y-directions the light beam directed at the glass surface. The line scanning occurs e.g. in y-direction and the interline field deflection in x-direction. The line may have a width of up to 20-30 mm. Filters positioned in the path of light as well as selections of mirrors and reflective surfaces can be used for limiting the wavelength band, such that the beam of light mainly comprises radiation having the desired wavelength range of 250-330 nm. The glass sheet 5 carries a small amount of tin also on that surface which has been the top surface during the manufacturing process. The illumination system shown in fig. 4 is sufficiently effective for making the illuminating line 7 visible also on this surface which carries a considerably lesser amount of tin. For example, the helium-cadmium laser emits laser light of 325 nm, which is capable of producing fluorescence points on a glass surface. Thus, the invention is also operable by means of laser light but the illumination system of fig. 4 is more effective in terms of its illuminating qualities as well as more economic in terms of its price when compared to laser-light sources commercially available on the filing date of this application. The invention is not limited to the above embodiment. For example, the illuminating line 7 on the glass surface need not be continuous but it may consist of a number of points or a single moving point. The source of light 4 may comprise a fluorescent tube or a laser capable of producing a required wavelength. The light can also be brought from a light source to a target by means of a photoconducting cable. The term "light" as used in this context must be understood comprehensively so as to comprise electromagnetic radiation wavelengths outside visible light, particularly the wavelength band of 250-330 nm within the UV-range. Many other details may also vary within the scope of the annexed claims.

Claims: A method for measuring the degree of bending in a bent glass sheet, characterized in that the measuring is effected in a per se known manner by means of cameras (1), which are set at an arbitrary but stationary angle relative to each other and which have a common imaging field within the boundaries of a surface being measured, and that a film of tin on the glass surface is made visible to the cameras by focusing light of a suitable wavelength on the glass surface. A method as set forth in claim 1, characterized in that the glass surface is provided with a focused illuminating line (7) or a number of luminous points along a profile being measured. A method as set forth in claim 1 or 2, characterized in that the light has a high intensity over the wavelength range of 250-330 nm. A method as set forth in any of claims 1-3, characterized in that the illuminating line (7) on the glass surface is used for determining therealong the 3-D coordinates for more than 200 points, said coordinates corresponding to the surface curvature profile along the illuminating line (7). A method as set forth in any of claims 1-4, characterized in that the glass surface is provided with a number of focused illuminating lines (7) or the illuminating line (7) is translated along the surface, such that the curvature profile can be measured at a number of different points. A method as set forth in any of claims 1-5, characterized in that the measured curvature profile is compared to a predetermined profile and, when an anomaly exceeding a certain limit is detected as a result of the comparison, the glass sheet is rejected. A method as set forth in any of claims 1-6, characterized in that the measured curvature profile is compared to a predetermined profile and, when anomalies exceeding a certain limit are detected as a result of the comparison, the bending process is adjusted to compensate for the anomalies. A method as set forth in any of claims 1-7, characterized in that at least two cameras (1) are positioned in such a manner that both have visibility on the same edge surface and/or two opposite edge surfaces of a glass sheet. A method as set forth in any of claims 1-3, characterized in that the source of light comprises a high-pressure mercury vapour lamp (11), at least some of the wavelengths outside the wavelength range of 250-330 nm being eliminated from its radiation, and that a thus produced beam of light is swept by means of a scanner mirror (14, 15) along the surface of a glass sheet (5). A method as set forth in claim 8, characterized in that illumination of the glass edge is used for measuring the glass size. A method as set forth in any of claims 1-11, characterized in that the measuring result is compared to the points of the real CAD-file of glass or to a surface formed thereby.

Inventors: YLI-VAKKURI ERKKI, YLI-VAKKURI, ERKKI
Applicants: TAMGLASS ENG OY, TAMGLASS ENGINEERING OY
Application year: 1996
Main classification: G01B11
IPC codes: G01B11