Machine vision reveals the defects on automotive glassThe use of safety glass is increasing Strengthened glass, either laminated or tempered, is widely used in the construction of buildings and vehicles. Its manufacture involves a complex process comprising several stages characterized by large temperature fluctuations. During this process many types of defects can occur. Because of the nature of the final product, quality control needs to be very stringent. Large cost savings can be made by rejecting defective components as soon as possible, as value is continually added throughout the production process. Architecture glass is observed by critical human eyes from day to day. The same applies to laminated glass which is used in automobile windscreens. The defects should be removed in factory, they cannot be washed away later. During the lengthy manufacturing process, automobile windscreens are susceptible to scratches, abrasions, rubbing, chipping, contamination, dirt, adhesive glass particles and bubble or stone inclusions. Some defects, for instance tin marks, can be found on raw glass as it enters the process and many others can suddenly appear at later stages. The cutting and grinding of flat glass and also the drilling of holes can be the cause of edge chipping or breaking and such damaged glass elements should be removed from the conveyor before they go to following manufacturing stages. The quality control work is often done by manual means; looking at finished glasses against dark/bright background as is shown in Picture 1.
Picture 1. The visual inspection of high- quality glass products is a tough task. Development project has been supported by EU CCD Photonics Ltd (Espoo, Finland) and Splintex Glaverbel Italy S.r.l. (Roccasecca, Italy) have developed together with Glassrobots Oy (Pirkkala, Finland) a family of SAFEGLASS cameras which can be used for controlling the optical quality of flat glass, windscreens, tempered glass and curved architecture glass in transmission, dark field and polarization modes. The development work has been funded by the European Cluster Integrated Machine Vision (EUTIST-IMV). The cluster is a part of the Information Society Technologies Programme (IST) of the Fifth Framework Programme of the European Commission.Satakunta Polytechnic (Pori, Finland) is the Principal Contractor of this activity. Cameras are sensitive to many defect types The general design of SAFEGLASS- cameras is shown in Pictures 2 and 3. In these examples, the cameras are looking at laminated windscreens (WS) and flat glass after machining and washing. The WS application is based on two camera units: the first one is used in a transmission mode (T) and the second one is used in darkfield mode (D). The installation of two units requires a free space of 80 millimeters between two rolls of the conveyor system. The illumination parts are located behind the glass. The various types of defects are observed either in transmission (T) or darkfield (D) mode. These types are indicated in Picture 3. The transmission camera acts as a master unit by mapping the windscreen on the basis of fade band printing or along machined edges in case of side lites. The dark field camera, on the other hand, ”sees” nothing from a perfect windscreen. However, it is sensitive to light scattering from scratches etc. The quality of flat glass edges is also controlled in the dark field mode. The organisation of separate defect maps is done by the master unit and resulting data is then transmitted to a PC computer, where the defect map is shown immediately after the windscreen has passed the inspection gate. The operator is able to define various quality zones having their own alarm limits according to defect magnitudes. In case of windscreens, the chauffeur’s side is more important than the side of a passenger. A zone can be also of ”ignore defect” type or of ”defect requested” type. The latter one is used to control the presence of drilled holes. Several small defects can be added together by using the ”combine defects” feature. The image can be rotated and mirrored to be more understandable for the operator. Practical experience The windscreen cameras have been positioned immediately before the packaging site of finished products at Splintex Glaverbel plant in Roccasecca, Italy. The SAFEGLASS system indicates the points of occasional defects to the human operator who is then able to quickly confirm the seriousness of the findings. In some cases the operator is capable of removing some loose contamination from the glass surface and he gives the final acceptance of product quality. The defect data from successive windscreens is stored in the computer memory. The statistical analysis of data has in some cases shown the accumulation of defects in certain areas of glass. The maintenance personnel has been able to trace the origin of faults to a damaged machine part in the conveyor mechanism.
Picture 2. SAFEGLASS cameras occupy only a narrow slot in the glass machinery.
 Picture 3. SAFEGLASS cameras are capable of catching a wide variety of annoying defects in glass. Splintex Glaverbel Italy S.r.l. produces every year about one million windscreens, one million backlites and two million sidelites for automobile industries. The SAFEGLASS camera has been used for windscreen quality control since 2001. The following Table gives an idea of the resolving power of the system in practical industrial conditions.
Flat glass camera keeps an eye on tin marks In addition to windscreen quality control, SAFEGLASS cameras have been used also in flat glass lines. In this application the cameras have been installed immediately after edge grinding and the washing machine. The mapping of sidelite glass elements is based on edge grinding, which enables the detection of bad edge chips and also the area control in case of broken glass. The early detection of ”tin mark waves” is also important. One example of a sidelite image is shown in Picture 4. Our picture does not present an industrial product, but a worn secondhand sidelite which is used in laboratory conditions as a calibration example. Defects are only rarely seen at Splintex factory lines. This means that a typical glass image consists only of edge points. Instead of fadeband printing, the position and shape of a glass are governed on the basis of edge grinding. Mr. Miguel Antinarelli is responsible for continuous improvement engineering at the Roccasecca plant of Splintex Glaverbel. He says proudly that more than 12 000 pieces of machined glass have been inspected using the flat glass version of SAFEGLASS during the two months period from December 2002 to February 2003. Roughly five percent of all incoming glass has been rejected by the control system and low- quality glasses have been returned to float glass factories for recycling. Most defects have been bubbles, more than half of all defects. Tin marks have been found on every tenth glass. Hits, chips, glue, scratches, abrasion and contamination are less frequent and disturbs, optical distortion, edge chips, stones, breakings, yellow marks and attached glass particles are rather rare appearances.
 Picture 4. Defect "candidates" are displayed in the computer screen. This secondhand sidelite has many scratches and contamination. Spin-off company CCD Photonics Ltd is a spin-off company from Technical Research Centre of Finland (VTT), founded in 1990 by Dr. Kimmo Simomaa, one of the founding members of the Vision Club of Finland.
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