Camera Smooths Flatness Measurements

Daniel C. McCarthy

Flatness is an important geometric tolerance for many engineering designs. For years, automotive designers have specified flatness per inch for manufactured parts, but few techniques could provide adequate measurements in that format. A flatness measurement system developed by OG Technology Inc. uses large-format progressive-scan cameras from Dalsa Inc. to deliver a new solution.

OG Technology has already sold several of its flatness gauges, called the OG 3000, to customers such as Ford Motor Co. The automaker helped to evolve the technology into a production-quality instrument. The systems can deliver up to 1 million data points in about 30 seconds with submicron-range resolution and can display flatness topographically.

The system comprises a camera, a frame grabber, fluorescent lights, an optical flat, a dye and a computer. Although many applications require little more than standard cameras, many of the parts measured by Ford require large-format imagers. For these applications, OG Technology relies on Dalsa's CA-D7 area-scan camera, which allows the OG 3000 to measure rough surfaces up to 24 x 24 in. (600 x 600 mm). In contrast, interferometry-based instruments are limited to measuring envelopes less than 12 in. in diameter, and they cannot handle rough or discontinuous surfaces.

"We use a CCD chip as a group of independent and well-aligned light meters -- the more the better," said Tzyy-Shuh Chang, vice president of OG Technology.

Parts to be measured are placed into a glass tray filled with water. The water carries a specific concentration of dye, and the bottom of the tray is an ultrasmooth optical flat that acts as a reference plane.

Two fluorescent lights beneath the glass illuminate the part's surface, and a 12-bit camera captures the image. The computer measures the span between the optical flat and the part's surface by evaluating the attenuation of light through the dye.

After rigorous testing of the CA-D7 camera, OG selected it for its 12-bit pixel depth and low noise. "We also prefer to have a camera with 100 percent fill factor so that our instruments can provide better repeatability," said Chang.

He credited Dalsa's camera for the recent record-setting flatness measurement achieved with the OG 3000. In a gauge repeatability and reproducibility test conducted by Ford, three operators measured 10 different parts -- each operator measuring the same part three times. Each of the 90 measurements captured more than 250,000 data points. The results of the test showed that OG's instrument delivered 12 percent repeatability with 25-µm tolerance on a surface with many isolated islands and thin trails.