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Non-contact metrology.

A look at what's new in vision-based devices

Machine vision, laser scanning, optical comparators, and microscopes are generally considered the non-contact methods of choice for gaging parts and quality in today's metrology world, supplementing and in many cases supplanting traditional tactile methods that require physical contact with the part being measured.

Ten years ago, contact and non-contact methods had very clearly defined applications with little crossover. If an inspection task involved parts too fragile or too small to touch, the job went to vision. Otherwise, the job went to more traditional contact methods. Recent advances in vision optics, computers, and software have blurred the dividing line. And the emerging multi-sensor vision systems, which add laser and/or touch probes, virtually eliminate the dividing line.

"Traditionally, touch-probe and vision system technologies have been complementary rather than competitive," says Charles Seifert, director of metrology, Pacific Precision Laboratories, Chatsworth, CA. "For example, take a company that makes high-speed flextures, contacts, and terminals for the mass storage industry. No one would attempt to use a CMM to inspect these parts, but vision is a natural for these small and flexible components. Yet the same company needs a touch-probe CMM to inspect the molds and dies used to produce these stamped parts.

"Another example of how vision and touch-probe CMMs complement one another can be found in the aerospace industry, where you have many parts such as panels that use through holes, diameters, o-ring grooves, and other simple features that can be seen with direct line-of-sight 3D machine vision systems. These features can be digitized and dimensioned much more quickly on a vision system. Then the part can be taken to a traditional touch-probe CMM to record complex features--such as intersecting diameters and holes, point-to-plane, and line-to-plane measurements--and dimensions that are out of a vision system's field of view, such as side walls, bore dimensions, and cylindricity."

William R Gilman, marketing communications manager, Optical Gaging Products Inc, Rochester, NY, agrees: "No one would attempt to use a vision system to measure an engine block; it's a perfect application for a touch-probe CMM. On the other hand, no one would attempt to use a touch-probe CMM to measure small, fragile, delicate parts, or two-dimensional parts with maybe hundreds or thousands of features. These can clearly be handled better by vision."

Yet, in recent years, several advancements have been responsible for enhancing vision's allure as an alternate data gathering device for dimensional measurements. Vision system speed and power have been improved by higher resolution cameras, more robust image processing methods, less expensive computational power, and more user-friendly software. Vision system capabilities have been improved through the addition of lasers, transforming vision from a 2D technology to a 3D technology.

"Vision is no longer a technology going in search of niche markets. Today, we sell to virtually every industry you can think of. There is no one industry that accounts for the majority of business," said Mr Gilman.

"Other types of sensors, particularly laser scanners and triangulation devices, have shown an ability to excel in those applications where vast amounts of 3D data are required. In many cases, lasers have been combined with either contact or vision systems to augment their performance," continues Mr Gilman.

Enter the laser

The addition of a laser transforms vision from a 2D to a 3D technology. For example, laser triangulation can provide the real-time feedback necessary to maintain image focus on a part whose third axis geometry cannot be predicted--a warped or bowed circuit board, for instance. With the laser constantly monitoring the Z-axis, the vision system can focus faster and more accurately.

Another use of laser triangulation is for scanning a contour along a section. In this mode, the laser can digitize thousands of points along a curved surface in the time it would take a tactile (contact) probe to move from one point to the next.

"The use of lasers makes machine vision much faster, more accurate, and more repeatable," says Mr Seifert. "For example, suppose you wanted to take a step dimension from the bottom of an o-ring grove to the top surface. With vision alone, it would take the system anywhere from three to five seconds to autofocus on each of the planes. In contrast, it takes the laser only a fraction of a second to perform the Z-axis measurement."

A recent advance in laser technology can be found in the WEGU MMC 800 Multi Sensor CMM and exemplifies what the addition of a laser can mean to a vision system. The patented laser is coaxial to the center of the CCD video camera and shares the same optical path. "The benefits of linear projection are numerous," says Ed Bresnan, engineer/applications manager, WEGU, Brewster, NY:

* Line of sight is vertical, thus beam obstructions are not so often encountered.

* The laser assists in focusing the CCD camera, thus eliminating gray-scale focusing errors associated with optical systems and increasing accuracies while reducing focusing time to less than 0.2 seconds.

* Single point measurements are obtained in less than 0.2 seconds.

* High-speed laser scanning/digitizing at up to 5000 points-per-second is achieved.

"No matter how smart a video processor is with all of its subpixeling capability, it can't 'rationalize' a focus accurately on surfaces, edges, etc, with changing contrast, tooling marks, textures, etc. With the coaxial laser, we focus in less than 0.2 seconds on surfaces ranging from polished mirrors to flat black fiber plastics," says Mr Bresnan.

WEGU adds a second Z-axis which houses a touch probe for use in the same manner as on a conventional CMM, to inspect features that are either out of sight of the optical/laser sensor or are best suited to be inspected in a contact manner.

Software advances

The combination of low-cost computational power and knowledge-based software has done much to enhance the capabilities of today's vision-based systems and lower cost.

Years ago, you needed the computational power of a $50,000 workstation to manipulate all the data that a typical vision system could provide--a PC/XT was just not up to the task. But today's powerful yet low cost Intel 80486 and Motorola 68040-based PCs are more than adequate to handle all but the most demanding data processing requirements of today's vision system.

"Ten years ago, an entry-level vision system with an 8" x 6" staging platform cost $100,000. Today, vision systems that are probably ten times faster with triple the measurement-function capability can be purchased for less than $50,000," says Mr Gilman.

One of the advantages of vision-based systems is that all of the information from a scene is available at one time. Knowledge-based software such as the IQ system from Optical Gaging Products can break each image down into primitive geometries (lines, circles, etc) with no prior knowledge of the content of the image.

The traditional role of the operator programming every phase of the measurement has been reduced to one of guiding the programming by configuring the system's output and supplying information it can't deduce from the raw data.

"For instance, without knowledge-based software you have to tell the vision system that it will be measuring a circle, select a measurement tool, place the tool over the image, size it, etc, before even acquiring the data. Then, the data it acquires will fit into a circle. With knowledge-based software, you merely image the part, and the system will draw a graphic representation on the screen. Then, with a mouse, you simply point to those features for which you want measurement," says Mr Gilman.

Today's vision systems are less expensive, faster, more accurate, and more easily programmed than ever before. Augmented with additional sensors such as laser and touch probe, there aren't many measurement applications that vision-based systems can't handle.
COPYRIGHT 1993 Nelson Publishing
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Copyright 1993 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Stovicek, Donald R.
Publication:Tooling & Production
Date:Sep 1, 1993
Words:1295
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