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Vision Keeps Pace with Growing Metrology Demand
by Winn Hardin, Contributing Editor - AIA Posted 01/08/2013Most, if not all, machine vision systems make measurements, but only metrology systems make a measurement to a specific, usually widely accepted standard. Standards can come from anywhere—the National Institute of Standards and Technology, the U.S. Food and Drug Administration (F.D.A.), or even internal documents that specify the alignment of plastic mesh over a cell phone speaker. In each of these cases, only machine vision systems offer a cost-effective solution for automatically comparing production to objective standards.
The challenge for machine vision technologists is how to bridge the real world of the production line to the mathematical world of the “standard” while cost-effectively accommodating constantly changing system requirements from a growing number of industries in need of metrology solutions.
Metrology’s Growing Customer Base
The automotive, semiconductor, electronic and display industries have supported the development of machine vision solutions for metrology applications. In recent years, newcomers in a number of industries, from consumer electronics to paper and plastics, have joined these traditional markets.
“Pharmaceutical and medical device manufacturing are not necessarily newcomers to metrology, but as new regulations emerge, such as the U.S. Pharmacopeail Convention on glass lamellae, or fragments caused by delamination in injectable glass vials, you see the need for advanced machine vision systems like metrology spread throughout the manufacturing process,” says Jason Baechler, Lighting and Imaging North America Business Manager, Industrial MV/Industrial/Microscopy/Cosmetics/POF at SCHOTT North America (San Jose, California).
Other growth areas for machine vision metrology solutions include electrostatic printers, paper, plastics and consumer electronics. “You see it in cell phones where manufacturers push for these devices to be visually perfect,” explains Dr. Ben Dawson, Teledyne DALSA Industrial Products Division (Billerica, Massachusetts), Director of Strategic Development. “When you open up a phone, people act like you’re opening a jewel, and manufacturers want to reinforce that belief. OEM specifications dictate everything, down to the number of microns that a speaker grill has to be aligned to the face of the phone.”
Are You Calibrated Both Ways?
Because the whole point of a metrology system is to make a measurement to an independent standard, nothing is as important as calibrating the vision system. Many end users realize they need to calibrate the vision system to the standard’s coordinate system, but they may overlook an important second step.
“There are two types of calibration that are often conflated here,” says Teledyne DALSA’s Dawson. “One is absolute calibration, where your software says 26.3 pixels equals 1 millimeter or some unit of measurement. That’s what most people think of when they think of calibration. The second step that can be overlooked is correcting for optical distortion. Even the best lens introduces some fraction of distortion to the image. Telecentric and double Gaussian lenses are excellent at limiting distortion, but around the edges of the image, you still might need to lay down a grid and figure out how to correct for optical distortion in your image.
“Now, if your part is always the same size, such as a piston, you can set up the system to use the sweet spot in the lens and not be overly concerned about optical distortion,” continues Dawson. “But if your system is inspecting a variety of parts that may fall at different locations in the field of view, and you need absolute measurements, you’ll need to correct for optical distortion in your software. You can’t underestimate the importance of software when it comes to metrology systems because it’s what helps you to calibrate and correct for distortions.”
Faster, Better Metrology
“There’s a war that’s been going on for a long time,” says SCHOTT’s Baechler. “Ask a software designer and they’ll tell you that you don’t need a better lens, just better software. Ask an optics manufacturer and they’ll tell you how important optics are for software. What is driving both are the new cameras with larger sensors and smaller pixel pitches. Today, customers are realizing that the optics sales pitch we’ve made for years—that the quality of the camera needs to match the quality of the optics—is finally finding acceptance.”
The common customer mistake used to be pairing a high-quality camera with a CCTV-quality zoom lens and expecting a high-quality image for metrology. Today, the surprise is that customers are looking for high quality fixed magnification solutions rather than high-quality zoom lenses, which can make the optical system more complicated if you require multiple lenses for your machine or application. “You would think that a high-end telecentric or low-distortion zoom lens would have been the next progression, but while customers now understand their lenses have to support the optical requirements of the sensor, they’re still trying to save money and improve imaging speed wherever they can through fixed magnification lenses,” Baechler says. “For us, the upside is customers now ask us to design, develop and supply a complete sensor, optical, lighting solution because designing complex optical systems is what we do extremely well. And not necessarily what they do.”
Bigger sensors with more pixels also increase the demands on the backend of the machine vision system, namely data acquisition and processing.
“Our Karbon-CXP and new Cyton-CXP frame grabbers are designed specifically for high data transfers at high speeds,” says Donal Waide, Director of Sales at BitFlow, Inc. (Woburn, Massachusetts). “We’ve always focused on being the fastest boards for getting your data into the PC, and the CoaXPress train has really sped that up. In addition to the high speed of CoaXPress, the longer cable length availability means that you don't have to put your PC in your cleanroom environment for semiconductor, electronics and medical manufacturing. With more CXP cameras coming out this year at Stuttgart’s Vision 2012, we expect CXP will drive metrology demand for our highest-speed boards.”
With the growth in demand of 5-megapixel (MP) cameras with 3.4-micron pitch, standard optics cannot match the quality of the sensor. Combine this physical fact with the growing number of industries seeking machine vision metrology solutions, and it is safe to say customers will continue to demand higher quality vision components for metrology systems that return the favor by driving quality in the customers’ products.
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