AU702965B2 - A method of bending glass sheets - Google Patents

Description

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AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant: Actual Inventor: Address for Service: PERFECT GLASS WINDSCREEN INDUSTRIES PTY LTD PATRICE GEORGE ROBERT

DOHERTY-BIGARA

CULLEN CO., Patent Trade Mark Attorneys, 240 Queen Street, Brisbane, QId. 4000, Australia. o r r Invention Title: A METHOD

SHEETS

OF BENDING GLASS Details of Associated Provisional Application: No. PN8315 The following statement is a full description of this invention, including the best method of performing it known to us.

This invention relates to the bending of glass sheets and more particularly to the bending cf glass of glass sheets by gravitational deformation at elevated temperatures. The invention finds particular application in the bending of glass sheet for the preparation of vehicle windscreens and the following description will primarily be concerned with glazings for vehicles. However, the invention is applicable to the bending of glass sheets for any purpose.

An initial step in the manufacture of vehicle windscreens is the bending of glass sheets cut to the desired shape. A common method utilises the effect of gravity on sheets which have been plasticised by heating. In this method, a glass sheet is mounted on an outline mould which is conveyed through a bending oven.

The oven includes a number of preheating cells which raise the temperature of the glass from about 250°C to about 560°C. The glass sheet on the outline mould is then conveyed into a bending cell which comprises an array of heating elements about the glass. The glass is heated to temperatures of up to about 720'C at this stage which plasticises the glass. The glass then sags under the influence of gravity and tends towards a curvature dictated by the outline mould. Following bending, the glass and mould is conveyed back through the oven to anneal the glass.

For manufacture of laminated windscreens, a glass/laminate/glass sandwich 20 is subjected to the bending process followed by a laminating process.

Curvature of windscreens is typically more pronounced at the ends thereof.

SDifferential plasticisation, with the glass being more plastic in some zones such as :i near the ends then in other zones, is required to form windscreens of compound •curvature. This differential plasticisation is effected by controlling the number and 25 disposition of heaters that are energised during the bending process.

ooe• The heaters in the bending cell are manually controlled. Viewing apertures are provided in a wall of the oven, typically an end wall, so that an operator can inspect the windscreen being bent. Each heater of the array has a switch by which the operator can control which heaters are energised.

The reproducability and efficiency of windscreen production using a process such as that described above is dependant on the skill of the operator. Excessive .i heating, or too little heating, through inaccurate assessment of the curvature of a screen may lead t, an article which falls outside of tolerance specifications and will therefore need to be rejected. A conservative approach by the operator, with excessively cautious energisation of heaters, may lead to higher reproducability of L _I screen curvature but will slow up production.

There is thus a need for a glass bending oven of the gravity bending type which can be used to efficiently produce sheets having a correct bend while eliminating human error from the operation of the oven. That is, there is a need for electronic or mechanical control over the bending cell stage of the process so that a fully automatic procedure is provided.

It is an object of the present invention to provide a method of controlling the bending of glass sheets such as for windscreen production, during conveyance of the sheets through a bending oven.

It is also an object of the invention to provide a system for controlling bending of glass sheets in a bending oven.

In a first aspect, the invention provides a method of controlling bending of a glass sheet in a bending oven, the method comprising the steps of: i) obtaining an image of the profile of said glass sheet; ii) digitising said image and comparing said image with a reference profile; iii) energising specific heaters of a heater array in said oven in response to differences between said image and said reference profile; and repeating steps to (iii) until said glass sheet has a curvature substantially identical to said reference profile.

20 In a second aspect, there is provided a system for controlling bending of a sheet of glass in a bending oven, the system comprising: i) an optical system for transmitting an image of a glass sheet profile to the exterior of said oven; ii) a video camera for receiving said transmitted image; iii) a frame grabber for digitising said image; iv) a central processing unit; and v) a programmable logic control for controlling said oven heaters.

The invention also includes within its scope a bending oven which includes the above described control system.

The following abbreviations are used hereafter: CPU central processing unit .PLC programmable logic control.

In the method according to the invention, the deformation at various positions along a glass sheet are measured and compared with a reference profile which contains information on the desired deformation. If the measured data 4 differs in any way from the ideal data then the heaters are adjusted accordingly via a PLC to bring the actual curvature as close as possible to the ideal curvature of the reference profile.

The cycle time of the analysis as described above is about two seconds.

The optical system of the second aspect of the invention consists of various mirrors and lenses placed outside of the oven. An image of the glass sheet is reflected to the external mirrors for delivery to the camera. Transmittal is conveniently via inspection ports in the end wall of the oven adjacent the bending cell. The system typically comprises at least two mirrors and associated focussing lenses.

The image focussed on the camera can than be processed by the frame grabber for delivery to the CPU. Appropriate software is provided, the preferred software being C" for Windows using the Global Lab Image Processing library.

Successive images digitised by the frame grabber are subtracted from one another and the resultant image used for image processing purposes. The resultant image is a crescent, which represents the amount of bending that has occurred between the capture of the two images. By measuring the distance between the leading and trailing edge of this crescent at various places along the crescent, it is possible to determine the curvature of the glass sheet. This data is 20 then compared to an ideal data set and any differences are noted. Based on the *differences between the actual curvature and the ideal curvature the appropriate configuration of the heating array can be inferred.

Control of the oven heaters is mediated by a PLC connected to the CPU S•and hard wired into the oven. Once the appropriate heating array configuration has been determined by the vision system it is output to the PLC via the serial port of the CPU. The PLC then switches on or off the different heating elements in the oven based on the corrective data it has received from the CPU. Switching is typically via a relay.

This cycle is repeated a number of times until the glass sheet has achieved its final curvature.

~The invention will now be exemplified by describing a bending oven for S. windscreen manufacture which includes a control system according to the invention. Reference will be made to the accompanying drawings in which: Figure 1 is a schematic representation of a control system according to the invention; Figure 2 is an elevational view of a bending oven which has a bending control system associated therewith; Figure 3 is an elevational view of the oven shown in Figure 2 showing the bending cell end of the oven; and Figures 4 to 8 are flow charts of functions from a program for use with the control system of the invention.

Referring now to Figure 1, there is shown a control system having an optical system 1 which receives images from oven 2. An image from oven 2 is focussed on video camera 3 for subsequent digitising by frame grabber 4. CPU receives the digitised image which is delivered to the software system 6. Software system 6 also receives input from a database system 7, a communication system 8, and an image processing module 9. Software system 6 is controlled by a control system Database 7 stores the switching configurations for the various model windscreens and the corresponding images. It also stores initialisation data for each windscreen. The database engine provides quick and easy access using the dBase format.

Communication in system 8 is via an OMRON LK201-V1 host link unit to an OMRON C200H CPU 5 which provides switching of the heating elements as well 20 as informing the CPU of the location of the windscreen/outline mould combination within the bending cell. The communication through the system also performs the act of conveying the windscreen/outline mould combination to the next cell once the bending operation is complete.

Processed data is then inputed to PLC 11 which ultimately controls the on- 25 off cycles of heaters (not shown) in oven 2.

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A bending oven including the control system described above is depicted in Figures 2 and 3. Referring first to Figure 2, there is shown oven 21 which includes a conveyer the path of which is generally indicated by arrow 22. The oven includes a loading station 23 at which outline moulds carrying glass sheets can be loaded onto the conveyor. The oven also includes a plurality of preheating cells, 24 to 29, a prebending annealing cell 30, a bending cell 31 and recycling zones 32 to 34. The actual bending step occurs in bending cell 31 in the area generally indicated by asterisk 35. Electrical supply cables to heating elements in preheating cells the annealing cell, and the bending cell, plus other oven components are routed from an electrical switch cabinet not shown in the drawing

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via duct 36.

The bending-cell end of oven 21 is shown in Figure 3. The figure shows wall 37 of the bending cell, which wall includes three inspection ports 38 to Wall 37 also has a control panel 41 for manual control of oven functions and a video monitor 42 for monitoring loading operations.

When in position for bending, a windscreen on its outline mould is positioned in the bending cell roughly level with the inspection ports. Two video cameras 43 and 44 are mounted adjacent inspection ports 38 and 40 for receiving images of the profile of the windscreen via appropriately positioned mirrors. The mirrors, and associated focussing lenses, constitute optical system 1 of Figure 1.

Cabling 45 is provided to route the video images to the frame grabber, item 4 of Figure 1.

The operation of the bending oven including the control system of Figure 1 will now be described with reference to all figures. An outline mould carrying a windscreen to be bent is placed on the conveyer at loading station 23 (see Figure The conveyer is then activated to carry the windscreen, and any subsequently loaded windscreen/outline mould combinations, through the preheating cells, 24 to 29 of Figure 2, the prebend annealing cell, 30 of Figure 2, and finally to the bending cell 31. During passage through the preheating cells and the prebend 20 annealing cell, the windscreen is heated from an initial temperature of about 250°C to a prebending temperature of about 560*C.

Once the windscreen is in position in the bending cell, this position being generally indicated by asterisk 35 of Figure 2, the control system comes in to play.

It will be appreciated that conveyor movement is arrested during the bending 25 process.

SAn identification code for the particular windscreen to be bent is entered into the software system via the communication system, This initiates the following steps: 1) particular heaters within the bending cell heater array are energised the heaters energised will be determined by the desired ~curvature of the windscreen; and 2) a reference profile is called up from the database system.

Energisation of heaters in the heater array gives regional plasticisation of the windscreen achieved at a temperature of about 720'C which allows bending of the same. Continued monitoring of the windscreen profile then commences with c~ i I the image obtained from the bending cell compared with the reference profile.

Differences between the received image and the reference profile give rise to onoff cycling of appropriate heaters in the heater array to achieve a bend identical to the reference profile. Once the correct bend is achieved, all heaters in the bending cell are de-energised and the conveyor transports the windscreen back through the oven, allowing cooling of the windscreen, then to the loading station where it can be demounted.

Because the database system can hold a plurality of reference profiles, a corresponding plurality of windscreens can be bent in a bending oven including the control system according to the invention. Indeed, as many as about 800 different windscreens can be processed with a cycle time per windscreen of about 2 seconds.

Functions (subroutines) of a computer program suitable for use with the control system of the invention will now be briefly described with reference to Figures 4 to 8. In these figures, the term "bucket" is used in place of "outline mould" and the abbreviation "DB" for "database". All times or elapsed times referred to in the flow charts are measured from the time when the bucket (outline mould) is fully raised into the bending cell.

CmRecord() (Figure 4) 20 This function is used to record data for a single windscreen. The function is given a windscreen name through an input dialog box and then searches the FRAME database for the frame name. The bending sequence will then be .o.

recorded provided that it has not already been recorded or that it has not been locked.

25 Record() Figures 5a to This function performs the recording of data for a given windscreen. The frame name is passed to the function from either the CmRecordo function or the 9Run() function when the Run() function is used to bend the screen and it does not exist. The details of this function are provided in the flow chart (Figures 5a to which outlines the basic program flow for the Record() function. The function S..treats the left and right hand sides of the image separately which is not indicated

C..

in the flow chart. From the point where the program grabs an image to where it calculates the distance of the glass from the mould, the left and right hand sides are operated on independently. The program simply performs the procedures firstly for the left hand side and then for the right hand side, e.g. Grablmage(Left), La I I I II Grablmage(Right), ScanDown(Left), ScanDown(Right), ScanUp(Left), ScanUp(Right), CalculateDistance(Left), CalculateDistance(Right) and so on.

CmRunO Figure 6 This function is used to bend a single windscreen. The function gets a screen name through an input dialog box and then passes it to the Run() routine.

Run( Figures 7a to 7d This function performs the bending of a given windscreen. The screen name is passed to the function from either the CmRun( function or the CmBendRun( function. The details of the Run() function are provided in the flow chart (Figure 7) which outlines the basic program flow for the function. Again, the function treats the left and right hand sides of the image separately. From the point where the program grabs an image to where it matches the data with the database (either on time or on vision), the left and right hand sides are operated on independently. As in the Record() function, the program performs the 15 procedures firstly for the left hand side and then for the right hand side, e.g.

Grablmage(Left), Grablmage(Right), ScanDown(Left), ScanDown(Right), ScanUp(Left), ScanUp(Right), CalculateDistance(Left), CalculateDistance(Right), MatchData(Left), MatchData(Right), MatchTimer(Left), MatchTimer(Right) and so on. The left hand side may finish at different times in which case the program will switch off all of the heating elements for the finished side whilst continuing to bend the unfinished side.

CmBendRun() Figure 8 The is the main bending routine. The operator must type in the windscreens in the run against their respective bucket numbers. This is performed via a dialog box. The program then searches the FRAME databases to ascertain which screens are stored and which need to be recorded. The operator is alerted 0 if there are screens in the run that need recording. The schedule is stored in the SCHEDULE database so that if during the day one of the screens is changed the operator only needs to type in the screen name for the new screen and does not need to retype the entire run. The program then drops into a loop and cycles through the schedule bending the screens that have been recorded and recording any new screens. After one or two cycles all screens in the schedule should liave been recorded. It will continue operating in this way until the ESCAPE key is pressed. If the run is stopped during the day for any reason the sequence can be regained by typing into schedule dialog box the bucket number of the next bucket 9 due to enter the furnace.

It will be appreciated that many modifications can be made to the control system and method of use as exemplified above without departing from the broad ambit and scope of the invention.

Claims (13)

1. A method of controlling bending of a glass sheet in a bending oven, the method comprising the steps of: i) obtaining an image of the profile of said glass sheet; ii) digitising said image and comparing said image with a reference profile; iii) energising specific heaters of a heater array in said oven in response to differences between said image and said reference profile; and repeating steps to (iii) until said glass sheet has a curvature substantially identical to said reference profile.

2. The method according to claim 1, wherein in step said image of the profile of said glass sheet is obtained via: an optical system comprising at least two mirrors and associated focussing lenses; and, at least two video cameras.

3. The method according to claim 1 or claim 2, wherein in step (ii) said image i 15 is digitised by a frame grabber and said reference profile comprises a computer file.

4. The method according to any one of claims 1 to 3, wherein in step (iii) said energising of specific heaters of said heater array is via a programmable logic S" control. V 20

5. The method according to any one of claims 1 to 4, wherein said curved glass sheet produced in step (iii) is a windscreen.

6. A curved glass sheet when produced by the method according to any one of claims 1 to 4.

7. The curved glass sheet of claim 6 which is a windscreen.

8. A system for controlling bending of a sheet of glass in a bending oven, the system comprising: S, i) an optical system for transmitting an image of a glass sheet profile to the exterior of said oven; ii) a video camera for receiving said transmitted image; iii) a frame grabber for digitising said image; iv) a central processing unit; and v) a programmable logic control for controlling said oven heaters.

9. The system according to claim 8, wherein said optical system comprises at least two mirrors and associated focussing lenses.

10. A glass bending oven which includes the system according to claim 8 or claim 9.

11. A system for controlling bending of a glass sheet in a bending oven, which system is substantially as hereinbefore described with reference to Figure 1.

12. A glass bending oven as defined in claim 10 and substantially as hereinbefore described with reference to Figures 2 and 3.

13. A method of controlling bending of a glass sheet in a bending oven, which method is substantially as hereinbefore described with reference to the example. DATED THIS 27TH DAY OF FEBRUARY 1997 PERFECT GLASS WINDSCREEN INDUSTRIES PTY LTD By their Patent Attorneys CULLEN CO a,. •2.22 c e *0 -I Abstract The invention provides a method of controlling bending of a glass sheet in a bending oven. In the method, an image of the profile of the glass sheet is obtained, the image is digitised and compared with a reference profile, specific heaters of a heater array in the oven are energised in response to differences between the image and the reference profile, and the foregoing steps are repeated until the glass sheet has a curvature substantially identical to the reference profile. The invention also provides a system for controlling bending of a sheet of glass in a bending oven and a bending oven which includes the system. ee ee