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Manual inspection of a four-inch wide, eight-inch circumference elastomeric band is difficult because there is no objective way for a human inspector to determine what is or is not a defect. In general, defects are areas that are darker or lighter than the surrounding material. Invotec Engineering, Miamisburg, OH, was asked by the company that produces the bands if machine vision could be used to define an objective specification and produce repeatable inspection results. Working with the customer, Invotec engineers developed a specification that considers both the size of a light or dark area and the degree to which it contrasts with the surrounding material.
During the prototype phase, Invotec engineers worked with customer engineers to mathematically define each type of defect and develop the lighting and vision tools needed to consistently identify defects. It was determined that multiple inspection processes were needed to characterize the product. “When looking for field defects such as inclusions or thin spots, the system applies two histograms to look for areas of high contrast,” Invotec Controls Engineer Mike Updike said. “One histogram looks for dark areas and one histogram looks for light areas. If no such areas are found, then we assume that the region is free of defects. If an area of high contrast is found, either light or dark, a regional blob tool is applied to find the potential defect. The blob tool uses information from the histogram to better classify the size of the defect. Once the suspect area has been localized with the regional blob tool, another histogram tool and a defect specific blob tool are applied directly to the potential defect. The output of these last two tools is a contrast level, defect size and defect location. These three values are compared to the preset thresholds that define a reject.”
Invotec developed a manually loaded and actuated prototype fixture as a first step towards moving the application into the production environment. This fixture holds the band with a pair of one-inch diameter rods. These rods are spaced to place the part under a small amount of tension. The front rod is manually rotated to present the membrane to the camera. The rear roller has a rather high rotating friction. This applies some additional stretch to the membrane as it stretches slightly before the rear roller begins to rotate…
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Click here to see the June 2010 full edition (article, page 20).
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