In-Line Mobile Measurement

May 19, 2003
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Performing in-line test, measurement and inspection in an automated, production environment requires a fast, accurate and robust system that can withstand the environmental challenges found on the factory floor. Optical measuring instruments are one way to measure in this environment because they are less vibration sensitive than contact-based alternatives. But optics have to see something to measure it--just as tactile needs to touch--which requires a part fixtured in a position that allows the measurement tool to "see" the part and make the optical probe.

One way to establish a line of sight is by making the optical system mobile. Some companies move optical probes on rails; one Israeli-based company puts them on wheels or mounts them on robots.

The Optigo 3D vision measurement system from CogniTens 3D Vision Systems Ltd., which has U.S. offices in Wixom, MI, uses a mapping device that allows it to be moved around a part and still take an accurate picture. The Optigo comes in two general configurations: The 100 E model is mounted on a wheeled tripod for mobility around measured objects on the shop floor. The Optigo on Robot is mounted on an industrial robot arm and can be used in measurement cells.

The Optigo's yellow optical measurement head encloses three high-resolution, 1.5 mega pixel charge-coupled device (CCD) cameras. The cameras takes 2-D images of up to 450 millimeters by 670 millimeters, and then uses algorithms to turn the 2-D images into 3-D images, says Meny Benady, a research scientist at CogniTens.

Larger areas such as on car doors, hoods and side panels are mapped with Optigo photogrametric mapping devices. A sequence of 2-D images are taken of the entire measured object. Texture markings, in the form of small, reflective targets, are placed on different areas of the measured surface or the part environment such as on jigs. The Optigo maps the targets so that it knows from where the individual tiles were taken.

"The camera captures multiple images, which can be stitched together to create an accurate mosaic of the entire object," Benady explains. Because of the Optigo's mobility and ability to stitch together many images, any free form object of any size can theoretically be measured.

Point clouds
The Optigo can capture up to 400,000 points per view and it is accurate down to 30 microns. It produces clouds of points that can be exported to standard computer-aided design (CAD) and analysis software systems. From this dense surface data, the Optigo can extract both cross-sections from any orientation or spacing and coordinate measuring machine-style single point measurements. It does surface analysis and generates a color-coded map of the surface. And, it takes feature measurements such as holes, slots and edges that are detected and measured for analysis and deviation detection.

The technology requires a non-reflective surface. However, paint, surface markings and stains do not affect the measurement data, CogniTens says.

For the features or measurements that are described by their surface values, such as surface points, cross sections or holes in thick materials, the Optigo projects dense surface texture, allowing it to create the point clouds.

Features that are described by surface discontinuities such as a sheet metal hole, or optic discontinuities such as trim lines that are engraved and highlighted on production tools, can be computed from a basic "clear" image, meaning that the system does it without the projection of texture, says Benady.

Tough going
A key to measuring in a production environment is a robust tool. According to Benady, the Optigo system can operate in tough industrial conditions, including:

  • Temperatures that can range from 50 to 95 F.
  • Vibration, which might affect contact measurement, does not hurt data integrity because of the fast acquisition time.
  • Typical industrial illumination such as florescent and sodium mercury lighting.

The ability to make fast measurements is another key to in-line production. The Optigo captures each tile in about 0.2 second and processes the data in 15 seconds for a total throughput per tile of about 16 seconds. The Optigo on Robot can be used in measurement cells incorporated into the production line in a go-no go operation or at the end of the line for post-processing inspection. The automated measurement cell includes one or more robots that are run by a dual processor Pentium III, 1GHz PC or better, and operates in a Windows NT OS environment.

"The advantage of using flexible robots for the measurement cell is that a number of different measurement programs can be created for each measurement cell," says Benady. "These programs may cover different vehicle models, or simply measure different areas or features for a certain model."

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