Camera control of windscreen production
pays back in one year.
Quality is also improved

Laser- based scanners have been used in the quality control of float glass production for years, but the quality assessment of end products is generally based on human visual inspection before shipping them to customers. CCD Photonics Ltd (Espoo, Finland) and a large glass company, have together developed a family of SAFEGLASS cameras which can be used for controlling the optical quality of flat glass, windscreens, tempered glass and architecture glass. The development work was partly funded by the European Cluster Integrated Machine Vision (EUTIST-IMV) and by the National Technology Agency TEKES, Finland.. Several cameras are now running in plants of this glass company

The long road of windscreen glass from flat to bent

The initial progress of glass through the manufacturing process of laminated windscreens is illustrated in Picture 2. The shapes and sizes of inner and outer glass panes are first cut from flat glass which is followed by the grinding of glass edges. After machining glass panes are washed and dried by blowing hot air before separating their paths on their way to printing, bending and laminating.

The raw glass may have defects in it: some bubbles or tiny lenses might have avoided the eye of a laser scanner in the float glass production line, tin marks and many dark inclusions are usually not detected there. Many scratches on glass have been created during packaging and while moving on conveyor rolls. Edge chips, major and minor, an also cracks or breakages are sometimes generated during grinding, not to mention the critical act of hole drilling.

Most defects are easily detected by means of a machine vision device located in the position of the camera gate. Picture 3 shows that the SAFEGLASS camera system needs only a free space of 90 mm between conveyor rolls or belts. This space is often available immediately after a washing machine. The optical inspection of glass defects is based on transmission and dark field imaging. The use of two independent methods increases the reliability of the system. Scratches, abrasions, finger prints, adhesive glass particles and edge chips are normally visible only due light scattering in dark field conditions. Tin marks, inclusions, contamination and dirt are visible only in a transmission mode against illuminated background, whereas bubbles are detected in both modes.

Operator can check recent defects on a visual display

Glass images with a defect map are shown immediately on a visual display of a PC computer. Picture 4 is an illustration of the user interface. Every glass is shown in a full size for a short period and the decision of glass quality is made on the basis of quality zones and alarm limits. The operator is free to handle important issues of the machine line while the camera system makes the initial quality decision. He can keep an eye on cutting, grinding and drilling problems and he can control the operation of a washing machine. The camera system provides an alarm relay which gives a command to a vacuum gripper in case of a bad glass. Bad glasses are moved onto a side conveyor/desk, where the operator can check them at a convenient time. Severely defected glass parts are returned to recycling, but some dirt particles can be removed manually and these parts can be guided back to the production line.

In addition to normal zones, the operator can define specific hole zones around drilling areas. A hole zone generates a quality circle around the center of hole edge readings. If any crack extends outside the quality circle, the alarm state is asserted. Such a hole is shown in Picture 5. The operator can also distinguish between missing, non-completed and good holes. Ignore zones are used to prevent alarms from un-dried water droplets at the rear end of glass, normal zones can be adjusted to provide feasible alarm limits for dark field and transmission defects. The sensitivity of every detection channel can be adjusted according to the actual haze condition of glass material.

In addition to windscreen production, the SAFEGLASS system can be used to control the quality of other automotive windows: roofs, sidelites and backlites. These glass panes are usually darker, down to 12 percent transmissivity. The brightness of illumination lamps is set according to glass darkness. The nominal measurement width of the basic SAFEGLASS camera is 1250 mm, which is limited by the length of low cost tube lamps. The standard camera type has two CCD ( Charge Coupled Device ) sensors, both having the total of 8000 picture elements giving the nominal resolution of 0.16 mm across the camera gate.

Operator can check recent defects on a visual display

Why should we install a SAFEGLASS camera gate across the glass conveyor in a windscreen (WS) production line? We may make the following educated guess: the manufacturing cost of one WS could be for instance 30 EUR, we work at least 300 days in a year producing more than one thousand units in one day.

If we have a defect in glass which is not observed after washing and we proceed with this part to the final inspection stage, we lose time, electricity, screen printing efforts, pvb plastic and the second glass part used as a companion in lamination. It is also important to send glass panes to printing in a clean condition. Having loose or adhesive glass particles or chips on glass surface would mean problems in silk work. Simply stated - we lose money. The final loss would be about 20 EUR using this imaginative cost calculation. If there are five bad glass units, we have a loss of 100 EUR in one day, which adds up to 30 000 EUR in one year. This happens to be the approximate cost of one basic camera system without installations at the plant.

In the previous example, we have used a rather arbitrary unit cost of 30 EUR. Of course, the plant could be more efficient producing one windscreen for not more than 15 EUR. In order to have a pay back time of one year, the camera system should be able to reject ten bad glass units instead of five in one day. Even this amount of rejects would be only one percent of the daily production, not more.

After listening carefully and patiently to these explanations, a responsible in the glass company smiles at all this reasoning: ”Having a short pay back time is an important issue, but having a good quality is maybe even more important. We have frequent visitors from car manufacturing industries and they will certainly know about our resources available for maintaining the quality level required. We are happy to show our customers that the high quality is not only beautiful words in brochures or in internet pages. We want to have a partnership with them. Customer Satisfaction can be achieved only, if we look forward together!”

Contact details: Dr. Kimmo Simomaa CCD Photonics Ltd Tapiontie 20 FIN-02720 Espoo Finland Phone: +358-9-5093417 Mobile: +358-50-5260122 email: kimmo.simomaa@kolumbus.fi www.kolumbus.fi/ccdphotonics MACHINE VISION CAN SOLVE YOUR PROBLEMS