Machine vision controls the quality of windscreens and architecture glassThe optical quality of glass products is an important issue: their appearance is judged by human eyes in daily use. Laminated safety glass is widely used in automobiles and in other transport vehicles and the use of tempered architecture glass is rapidly increasing. The manufacturing process of both types of products has many complex stages involving large temperature variations and the final products are always visible to the eyes of critical users. People insist upon having a good visibility through glass, but even small flaws are immediately apparent. Machine vision methods can be used to control the visual quality of glass during the manufacturing process.The manufacturing process of automobile and architecture glasses is a long way from float glass cutting and washing through heat treatment, bending and tempering and further to final inspection. The problems appearing at glass production are often visible as optical deviations. Automobile windscreens are susceptible to scratches, abrasions, rubbing, chipping and contamination dirtyness as well as to bubble and stone inclusions during the lengthy manufacturing process. Some defects, for instance tin marks, can be found already on raw glass as it enters the process and many others can suddenly appear at later stages. The manufacturing process of architecture glass is normally based on the use of special furnaces for bending and tempering. Each glass piece coming out from a furnace should be checked for eventual contact points in order to control the thermal and timing parameters of the bending process. Low- cost camera has many operation modes This article describes a low- cost system, SAFEGLASS, to inspect glass components automatically at the different stages of their production. The new camera operates in transmission, dark field and distortion modes. The polarization mode was discussed in the 2001 issue of this publication. Each imaging method has special application areas in glass inspection.The system improves the overall quality of the manufactured components and allows significant savings to be made by reducing the number of defective items produced. The development of the new camera system has been carried out in co-operation between three European companies as a part of European Take-up of essential Information Society Technologies, Integrated Machine Vision (EUTIST- IMV). Glaverbel Italy S.r.l., Splintex (Roccasecca, Italy) is an industrial manufacturer of automobile glass products, Glassrobots Oy (Pirkkala, Finland) is a developer and manufacturer of glass production machinery for automotive and architectural uses. CCD Photonics Ltd (Espoo, Finland) applies machine vision methods which are combined with machinery robots to maintain the correct quality level at industrial glass production. Satakunta Polytechnic (Pori, Finland) is the Principal Contractor of this activity funded by the European Union. The project partners have developed the SAFEGLASS method to handle the problems at industrial glass production. The basic unit of the system is an electronic camera with two detectors looking at glass in transmission and dark field modes. The actual shape and dimensions of a glass are determined using the first mode which also reveals most defects, whereas the second mode is used to complete the image with data showing bubbles and scratches. The camera can also be equipped with polarisation filters which are capable of telling the zones of excessive residual stresses in tempered glass. The SAFEGLASS camera adapts to changes in primary illumination. The presence of eventual defects is determined using glass colour information obtained from the first item off a production run. In the case of windscreens, the SAFEGLASS camera maps the contour of glass and then the system divides the scene into zones A and B which correspond to chauffeur and passenger areas having different quality tolerances. If any excessive defect is found, an alarm relay will be activated. Our Picture 1. shows an actual case of defects captured from the industrial production line of windscreens: the image shows lines of multiple defects in a broken glass. The example is a real case acquired in industrial conditions. The camera unit has been designed with ease of use in mind. The essential idea of the SAFEGLASS method is to put an investigation device in a place, where it is truly needed. One camera unit can be installed at almost any node on a manufacturing line. A typical inspection site comprises a narrow gap measuring less than 80 millimetres between two consecutive conveyor mechanisms. A standard fluorescent tube is positioned below the moving glass parts and the camera is located above them. The camera has two separate imaging sensors which are capable of revealing most defects using the two modes. Low cost has been an important design consideration. Initial experience is promising The major benefit of this system is the possibility of removing bad parts from the production line as soon as they have been damaged and revealed. This means huge cost savings as potential problem sources can be located early and bad parts are eliminated before further processing. The installation of the prototype camera at Splintex Glaverbel is shown in the photo, Picture 2. A long time collection of rejected windscreens is shown as a heap in the right side, the illumination assembly in the middle and the camera itself is located above the conveyor, where it is capable of controlling the whole width of the windscreen. The images of windscreens can be displayed using a personal computer located in the front. The use of an external computer is not necessary in the SAFEGLASS use: all data processing is done by means of a signal processor inside the camera enclosure. Says Mr.
Miguel Antinarelli (Continuous Improvement Eng.) from Splintex:
"We produce millions of glass items for automobile industry in every year,
windshields, backlites, sidelites and encapsulated parts. We want to be the
best in this industry and we control the quality of every part before shipping.
We have been testing the SAFEGLASS camera in the transmission mode for several
months and we have found that the system is capable of revealing
almost any type of contamination at the conveyor speed of 300 mm/s. The system
is also very sensitive to tin marks, bubbles, digs, scratches, stones and
even to occasional hair appearance between laminated glass sheets."
Distortion defects are catched from architecture glass Bent and tempered architectural glass is the construction and facade material of the future and the demand is rapidly growing. New construction applications are springing up around the world and architectural applications of glass are becoming more sophisticated. Similarly, the size of curved glass surfaces continues to grow, which means that tempered safety glass is needed in larger constructions than before. The applications of cylindrically bent tempered glass are many: wavelike and curved facades, cylindrical curtainwall panels, partition walls inside offices, skylights, staircases, showcases and so on. The bending and tempering process creates the properties desired for glass, such as strength, using controlled heat processing. Heating the glass to 615 … 650 degrees Centigrade, followed by rapid cooling, creates residual compressive stress on the surface of the glass and a corresponding residual tensile stress in the interior. The resulting stress balance gives glass the desired properties. Tempered, bent glass withstands mechanical strain 4 – 5 times better than annealed glass. This load-bearing ability makes tempered glass suitable for many places that require mechanical strength. Tempered bent glass can also withstand much greater variations in temperature than normal glass. This property makes bent tempered glass more suitable for building facades than laminated glass, for example. Another feature is the increased safety. Tempered glass shatters rather into small pebble-like pieces than larger sharp pieces. It is evident that the furnace process is very sensitive in producing architecture glass. Timing and heating parameters are critical factors in bending the glass. The SAFEGLASS camera has been used in distortion mode to reveal any point defects in architecture glass in co-operation with Glassrobots Oy. The imaging system is used to develope the parameter setting properties of the bending/tempering process of their Rainbowmaker TSF Combi furnace system. Mr. Kari Vähä-Antila (Techn. Dir.) regards the initial testing of SAFEGLASS as very promising: "We look forward of having also the magnitude values of distortion points in the defect map. These values would give a nice basis for quality assurance."
CCD Photonics
Ltd is a spin-off company from Technical Research Centre of Finland
Contact details: Dr. Kimmo
Simomaa
|
