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The Bright Spot

Smart camera and MIL shine a light on glass inspection

The melting of sand and forming it into glass is a manufacturing process that dates back to ancient days. Though contemporary production methods benefit from greater scientific knowledge of the properties of various additives to improve the durability of the glass itself, the manufacturing process is far from perfect. The finished glass can contain bubbles or cracks, so each piece must be inspected. Thorsten Gonschior, President and founder of Spectral Process (Erkelenz, Germany) says that formal inspection of glass is as old as glass production itself. "And there are still some regions today where inspection is performed manually, by the human eye."

Some plants rely on mechanical inspection systems that have some kind of direct contact with the container. The container might be rotated, be filled with compressed air, or plugged with a gauge. There are also many high-end camera-based inspection systems, but Gonschior explains that even though manufacturers are under pressure to maintain strict quality control (especially for food and beverage containers), there are plant owners worldwide who are not capable or willing to spend 250 000 to 500 000 for an inspection system. Cost is clearly an issue, but manufacturers understand they have little choice but to keep up with high quality standards. They must also improve the production process if they want to maintain a competitive edge.

Gonschior was confident that he could provide a lower-cost alternative when one of his customers asked him to retrofit an existing machine. "That original system had an optical component to locate the defects, but no processor," he says. "The original machine was no longer available on the market, so we had the idea of replacing one part of the full inspection process with a scalable subsystem."

Hot stuff

Glass is made when a mixture of silicon oxide, sodium carbonate, calcium oxide, magnesium oxide and other minerals are heated to a temperature well over 1000C. If the glass is intended for containers, the molten glass is pressed into molds and then annealed, that is, slowly cooled in a temperature-controlled kiln. At this point, cold end inspection is performed on the glass. Gonschior calls his system the Opening Inspector, as it checks the opening of hollow glass containers (i.e., bottles) for cracks, enclosures (bubbles), and pressed artifacts. "Keeping defective bottles off a production line is essential for the beverage industry. Anything but a smooth surface can cause injuries. Carbonated beverages might go flat if the bottles have defects and can't be sealed properly," he says. The container, then, must be free of burrs, chips, or sharp edges if it is to pass inspection.

Keep it simple

The Opening Inspector can be retrofitted for a wide variety of glass inspection machinery. The "sub-system" consists of a Matrox Iris P-Series smart camera, a power supply and a custom-designed illumination device. Gonschior opted for the Matrox Iris P-Series camera for a number of reasons. "It's cost-efficient, small, yet powerful for its range," he says. A smart camera also eliminates a lot of what he calls "additional engineering": housing, computer, electrical connections, etc. More importantly, "After testing the Iris [camera], I realized Matrox had done a good job… I found no flaws or non-functioning components in the hardware or software like I usually do with [off-the-shelf] components."

As the core of the system, the Matrox Iris P-Series camera not only performs the visual inspection, but also reads the sensors and updates actuators through digital I/O. The software application uses a number of modules in the Matrox Imaging Library (MIL), in particular Blob Analysis, Edge Finder, and Metrology, to measure the container's inner and outer diameters and locate the enclosures, cracks, and over-pressed structures. The eye can easily distinguish over-pressed structures, but the software requires clear definitions to detect them. In theory the Opening Inspector works with glass of any color; clear, brown and green glass are considered standard. Adjusting the intensity of the illumination device or the camera's amplifier creates the right conditions to acquire a useable image. Clear glass is the most challenging of all to inspect.

good.jpg overpressed.jpg overpressed_binary.jpg

Left to right: A bottle that passes inspection; an over-pressed bottle opening; the binary image of the over-pressed bottle with the defect highlighted. All images courtesy of Spectral Process.

A number of complex sub systems can be integrated into the Opening Inspector with Ethernet connections. For example, Gonschior is planning to use a 2D actuator with high way resolution to control a labeling arm. "Integrating network-capable third party devices into the Matrox Iris network is easy and straightforward," notes Gonschior.

Reflections on illumination

"Glass has a bad reputation when it comes to illumination," notes Gonschior. Indeed, both the material (the glass) and its shape create illumination challenges. On a microscopic level, glass is non-uniform, has a moon-like surface, with craters. These irregularities affect the way the light reflects and refracts through the surface; the resulting images can show strong contrasts in the background micro structure. The roundish bottle shape and deviations in the bottle's wall thickness only compounds the problem. Gonschior was able to resolve many illumination issues with his custom solution. The light source uses diffuse light and makes the glass reflect light into the camera at those spots that contain the damage, (the inspection vector). "It sounds easy but the development was anything but," recalls Gonschior.

A bright idea

"Gonschior believes the Opening Inspector has an edge over the competition. By developing his system with Matrox Imaging's Iris P-Series, he can offer the system at a fraction of the cost of many "established" glass inspection systems. Moreover, manufacturers want to inspect over 400 containers per minute. With his scalable design, Gonschior simply adds more cameras to the existing inspection line when a higher throughput is needed.

What's next?

Once Gonschior is satisfied with the Opening Inspector's robustness and stability in the industrial environment, he plans to offer additional inspection stations. These will inspect side walls and bottoms for defects, and read dot codes and human readable text. Developing these new inspection stations should be straightforward for Gonschior, who already has experience with laser-based code readers. As Gonschior sells and installs more systems, he will develop remote software tools for central control, statistics, supervision, and set up.

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