Advanced Imaging

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Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

Geometry Class

Solving tough applications with lighting angles
© Epic Vision Solutions
Following a number of tests, Epic Vision Solutions found a lighting solution to inspect cans for dents at the speed of production.
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By William Biederman, Biederman & Associates

Machine vision technology continues to improve yearly with better lenses, better software tools, and more robust cabling technologies, as well as smaller and more sophisticated cameras. There also has been a big improvement in the availability of robust LED lighting with appropriate triggering and strobing capabilities.

What continues to ensure success with difficult applications is the understanding and adaptation of unique geometries of lighting, filtering and lenses to illuminate and quantify only the features that need to be seen in an application. Without the ability to properly illuminate these difficult applications, the other advances in technology, while helpful, will not lead to a solution.

Epic Vision Solutions (St. Louis, Mo.) is a machine vision integration firm that typically gets the tougher applications. For example, a customer in the consumer packaging industry uses aluminum cans for its products. Because of the nature of the products it is important to have cans that are not dented. Specifically, they need cans that do not have dents or deformations greater than 0.020 inches (0.50 mm).

The initial engineering study showed that it was possible to silhouette the cans and to detect dents provided the dents were at the edge of the silhouette. After days of testing they determined that the dents would only be "detectable" on a narrow four-degree arc of the can's surface. Since they would inspect both sides at once, they would only be able to detect dents on eight degrees of the surface per image. This meant that they would need to position the can and take images at 45 unique locations. In essence they would spin the can and take 45 images to completely inspect the surface. Since these cans were on a production line that had an average speed of 220 cans per minute, they needed to find another way to illuminate this application so that the line could run at speed.

Further testing showed that if they could control the rays of light and the angle of incidence to the surface of the can they could cause dents to show up as shadows on the surface. They engineered a new light source that could be placed below the perimeter surface of a turntable that would "carry" the cans around the perimeter and inspect the cans at the same, but opposite, angle as the incident light. This yielded good results but the cans were decorated and this decoration caused some false shadows and false dark spots that were interpreted as shadows and showed as false dents.

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