Vision & Sensors

Multisensor Metrology

April 29, 2008
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Probing the world of vision and multisensor metrology.



With so many changes in the industry, it is useful to consider the rapidly evolving nature of vision and multisensor technology and review some of the probing technologies.

A short time ago there would be no need for this discussion because vision only inhabited the flatlands of 2-D measurement, where it was used effectively for high-volume validation of such objects as printed circuit boards. Recently, more sophisticated vision measurement systems became available, but only a few people who understood the various tradeoffs involving lighting choices, focus and depth of field, and who had mastered the esoteric skill of vision measurement system coding could make use of them. This is no longer the case.

Noncontact and multisensor system manufacturers and metrology software developers have joined forces to create measurement equipment, sensors and software that bring vision and multisensor metrology into the mainstream of manufacturing metrology. New hardware directly addresses the needs of manufacturers of 3-D parts. Various noncontact and tactile sensors can be cross-calibrated to yield highly comparable results, permitting the use of various data collection instruments without compromising accuracy.

On the software side, new vision and multisensor products now use computer-aided design (CAD) models, imported or directly linked, as the basis for programming and evaluation. In some instances, this is cross-platform software that is comparable or identical to the software used on popular coordinate measuring machines (CMMs), portable measurement systems and NC spindle probes. It is powerful, easy to program and provides the operator with automated utilities for managing the requirements unique to vision and other types of noncontact probes.

So, which type of sensor is best for particular applications? Here are some rough guidelines:

• Vision, of course, is the fastest method for traditional measurements on an XY plane. It also is good, but not always great, for making depth measurements, as long as the camera is normal to the surface or the software has true vector compensated 3-D capabilities. This is an excellent tool when measuring features that are either inaccessible to a tactile probe or would be deformed by contact with it. However, since the camera must stop and refocus every time it moves along the Z-axis, it is slower than a tactile probe because, unlike a probe stopping at the focal point, the vision probe has to pass the surface to see it.

• Tactile probes have long been the standard for CMMs because they can approach the part from any angle. On multisensor systems, they make good partners with vision cameras because they can be cross-calibrated to provide accurate, repeatable results regardless of which sensor collected what set of data points. Couple this with an ability to normalize the probing vector and projection to CAD and you have a winner.

• Laser sensors are very fast. They can capture thousands of data points per second, making them an excellent choice for scanning under the right circumstances. However, because the laser works by streamed feedback of light requiring a continuous and relatively constant power and amplification return to the array, its correct operation depends on the angle of incidence of the beam and the relative reflectivity of the part’s surface. So, operators must take care in using them. In extreme cases, an increase in the angle of incidence will result in a loss of signal. Even worse, a variation in the surface finish can distort the feedback information enough to make results misleading.

• White Light Sensing also is very fast, but it is dependent only on the evidence of a signal returning from the part as opposed to the power of the signal. As such, it is much more resistant to the geometric and reflective changes in a part’s surface than the laser.

The manufacturers of probes, sensors, software and measurement equipment are excited about the advanced capabilities these emerging technologies offer. They are providing lots of choices, so operators need to explore their options carefully to find out what will work best for them. This is meaty stuff and conventional wisdom frequently does not apply. So ask the vendor your toughest questions and be prepared to have your eyes opened.

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