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Inspection perfection

Nov. 1, 2008
The process of melting sand and forming it into glass is a manufacturing method that dates back thousands of years. Though contemporary production methods

The process of melting sand and forming it into glass is a manufacturing method that dates back thousands of years. Though contemporary production methods benefit from greater scientific knowledge of additives that improve the durability of glass, today's manufacturing process is still far from perfect. Finished glass often contains bubbles or cracks, so each piece must be inspected — either by a machine or a trained worker.

Some plants rely on mechanical inspection systems that make direct contact with finished glass products, but they often require additional steps: Bottles, for example, may have to be rotated, filled with compressed air, or plugged with a gauge. High-end camera-based inspection systems are available, but may be cost prohibitive depending on the size of the plant. In one case, where cost was a factor, engineers came up with an alternative, a retrofit, which was simpler and more reliable than mechanical inspection and less expensive than a full-fledged camera-based solution.

“The original system had an optical component to locate defects, but no processor,” recalls Thorsten Gonschior, president of Spectral Process, a system integrator headquartered in Erkelenz, Germany. “Since the inspection system was no longer on the market, we decided to replace it part by part with a scalable subsystem.”

Hot stuff

Glass is formed when a mixture of silicon oxide, sodium carbonate, calcium oxide, magnesium oxide, and other minerals are heated to a temperature well above 1,000° C. If it's intended for containers, the molten glass is pressed into molds and then 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, such as bottles, for cracks, enclosures (bubbles), and pressed artifacts.

“Keeping defective bottles off a production line is essential. Anything but a smooth surface can cause injuries. And carbonated beverages might go flat if the bottles have defects and can't be sealed properly,” he adds. “The container must be free of burrs, chips, or sharp edges to pass inspection.”

Keeping it simple

Opening Inspector can be retrofitted for a wide variety of glass inspection machinery. The subsystem consists of an Iris P-Series smart camera from Matrox Imaging of Quebec, Canada, plus a power supply, and a custom-designed illumination device. As the core of the system, the smart camera not only performs visual inspection, but also reads the sensors and updates actuators through digital I/O. The associated software employs a number of modules, including blob analysis, edge finder, and metrology, to measure the container's inner and outer diameters and locate enclosures, cracks, and over-pressed structures.

In theory, Opening Inspector works with glass of any color; clear, brown, and green glass are considered standard, though clear is the most challenging to inspect. Adjusting the intensity of the illumination device or the camera's amplifier creates the right conditions for acquiring a useable image. A number of complex subsystems can be integrated into the Opening Inspector with Ethernet connections Gonschior is planning to add a 2-D actuator with high resolution to control a labeling arm.

Reflections on illumination

“Glass has a bad reputation when it comes to illumination,” notes Gonschior. Indeed, both the material itself and its shape create challenges. On a microscopic level, glass is non-uniform and has a moonlike surface with craters. These irregularities affect the way light reflects and refracts at the surface, producing highly contrasted images. The rounded form of a bottle and deviations in glass wall thickness only compound the problem. However, Gonschior was able to resolve many of these illumination issues with his custom solution. The light source uses diffuse light that reflects into the camera wherever it encounters damaged regions.

Gonschior believes the Opening Inspector has an edge over other inspection solutions. By developing his setup with the smart camera, he can offer the system at a fraction of the cost of many traditional glass-checking systems. What's more, manufacturers often need to inspect more than 400 containers per minute. With his scalable design, Gonschior simply adds more cameras to the existing production line's inspection process when a higher throughput it needed.

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Special thanks to Thorsten Gonschior, president of Spectral Process, for this month's vision lesson.

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