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Line-Scan Camera 'Peels' Spark Plugs

Photonics Spectra
Jun 1998
Ruth A. Mendonsa

In mass production, keeping defective products to a minimum and cutting waste are primary concerns. A major manufacturer of auto parts and electronics set a goal to ship products with zero defects and to reduce scrap by better controlling its production process. The company chose an imaging system with a line-scan camera to solve its quality control inspection problem.

A Dalsa line-scan camera "peels" the spark plug to acquire a 2-D image of the model number.

The manufacturer needed a system that could inspect the gaps on spark plugs, making sure they are within tolerance; to check that the posts are correctly seated and straight; and -- most difficult -- to ensure that the printed type and model numbers are legible and not missing any characters. The company turned to Acuity Imaging, a division of Robotic Vision Systems Inc. (RVSI), to find a solution.

Peeling the model

The first two requirements were straightforward, but the third requirement, print verification, presented a problem. Because the print runs around the circumference of the spark plug insulator, several standard RS-170 cameras would have been needed to provide complete coverage.

The job required a camera that could image the spark plugs as if they had been peeled and flattened. RVSI Acuity chose Dalsa's CL-CB-0512A single-output line-scan camera, which runs at 20 MHz with 8-bit digital output, as part of its system. The camera views the characters on the spark plugs by taking a vertical scan of the circumference of the barrels as the plugs are rotated. It reads the lines one at a time, and the frame grabber compiles the lines to create a 2-D image, which is then compared with a reference image. This technique enables the camera to capture 100 percent of the circumference of the spark plug. The system identifies missing and partial characters as well as print smears, and notifies operators of any discrepancies.

The resulting system, based on the Powervision platform, is integrated with Acuity's Image Analyst software, which includes a menu-driven interface and user-defined display capabilities. Another Acuity vision system works in conjunction with the print verification system to address the other two requirements.

According to RVSI Acuity, the spark plug manufacturer has realized its goal of zero defects shipped and significantly reduced scrap. In addition, using two different systems to create one solution was very cost-effective, because the gap and post inspection didn't require the advanced capabilities needed for print verification.

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