Light Touch with Noncontact Metrology
Accuracy, data rates, and usability
are all improved
By Bruce Morey
The breadth of noncontact technologies is expanding. Suppliers are improving vision and lasers. X-ray computed tomography for precision measurement is a growing presence. The benefits are many. For example, metrology using machine vision–like most noncontact systems—is generally faster than a touch probe, explains Gary Hobart of Hexagon Metrology (North Kingstown, RI). “With it, you measure on the fly, collecting more data quickly and easily.” He also notes that it is especially good for measuring flexible components or micromachined parts too small for touch probing. “Medical is another application because those components—knee and hip replacements, for example—need to be manufactured in a sterile environment,” he says. Touch probes can even leave small marks on highly engineered surfaces.
However, like other noncontact technologies, Hobart points out that vision started out as a bit of a “black art.” “With vision, you have to understand three qualities: lighting, magnification, and edge quality,” he explains. In addition, to keep vision metrology systems competitive, providers need to use commercial off-the-shelf lighting and camera systems, both of which exhibit wide variance in illumination and calibration. “Controlling those removes the two biggest variables in the use of vision for metrology,” he says.
In September, Hexagon Metrology introduced its OptivScan vision-based CMM, part of the Optiv line under the company’s Brown and Sharpe brand, to the North American market. Coupling a touch probe with the vision camera comes in handy. There are places a camera can’t see, and taking a datum is always useful. The OptivScan combines a high-resolution CCD camera with a Leitz LSP-X1 probe. Previous Optiv models provided their own brand of touch probes. Finally, programming vision systems off-line through importing a CAD model is just as important for vision-based systems as touch-probe systems. The Optiv line uses the PC-DMIS Vision product from Hexagon.
Why use the LSP-X1? “Our customers wanted to use the same probe they had on their other CMMs,” Hobart answers. Accuracy is about 0.5 to 1 mm within a measuring volume of 650 × 600 × 300 mm (XYZ). Like other Optiv systems, OptivScan is equipped with PC-DMIS Vision software. Using this software enables users to call on CAD model data for programming inspections. Base models are three-axis with two additional axes optional.
Hobart explains that the OptivScan is the first product designed with expertise from across the many brands and companies brought together in the last few years under the Hexagon name. It features a common distributed controller, which the company plans to use on all future metrology products and CMMs. As Hobart describes it, this common platform enables common service calls. No longer will a customer need to call a vision service engineer or touch-probe service engineer—one call, one visit will only be needed because of the distributed controller.
Careful engineering and continuous improvement will also improve noncontact technologies, such as vision. As an example of the steady improvements that are behind the scenes, but delivering impressive results are the new high resolution (HR) objective lenses from Mitutoyo America Corp. (Aurora, IL). “These new lenses improve image quality about 50% over our standard lenses,” explains Allen Cius, product specialist for vision systems. He details the specific improvements in their 1× lens using the new HR series. “The numerical aperture increased from 0.5 to 0.8,” he states, noting that numerical aperture helps to increase resolving power. “The working distance increased from 34 to 40.6 mm,” he relates, noting this increases flexibility in taking measurements. The HR lens also doubles the effective brightness and reduces flaring by 60%. “The resolving power increased from 5 to 3.3 µm all because of this improved lens,” he remarks. He notes similar improvements in their 2.5× power objective lenses: Numerical Aperture improved from 0.14 to 0.21; 2× improvement in effective brightness; 80% improvement in flaring; resolving power increased from 2 µm to just 1.3 µm. Currently fitted to new models of their QV 363 line, Cius expects the HR lenses to become standard on all vision systems from Mitutoyo.
Multiple sensors, each providing its own strength, are becoming common among vision and non-contact sensors. The best example of that trend is the newly introduced HN6060 from Nikon Metrology Inc. (Brighton, MI) that features four separate noncontact sensors along with a touch probe. The HN6060 is equipped with touch probes and optical heads with built-in (through the lens) TTL laser with automatic focus (AF) common with Nikon’s NEXIV VMR series video-measuring system. To these it adds two more sensors: a high-precision laser scanner and shape-from-focus (SFF) inspection sensor. “The laser is a special telecentric lens designed by our factory engineers in Japan,” explains Mike Provenzano, applications engineer for Nikon. “While other lasers deliver on accuracy on the order of 25 µm, this laser measures about 5 µm,” he states, quoted against the Japanese JIS B 7441 standard for accuracy. Using an improved CCD collector, Nikon also claims it now measures low-reflectivity or glossy surfaces without spraying. The SFF sensor uses active texture pattern based on Ronchi grid techniques to collect data on shiny parts, according to Provenzano. The company claims this measures mirror surfaces, also to 5 µm accuracy. Provenzano also points out that Nikon spent considerable effort building a solid CMM-style base for these sensors, with a honeycomb bridge and carbon-fiber Z-Ram with linear scales graduated to 20 nm in all three axes.
“This system was designed to measure hypoid gears,” explains Hajime Kosawa, managing director-the Americas for Nikon. “These are important for automotive fuel economy and were very difficult to measure accurately until we built the HN6060. Now, we are looking for new applications.” Such applications could include precision gears, turbine blades, or orthopedic implants. Provenzano notes that 5 µm of accuracy opens new applications in 3-D wear analysis, for example, once the domain of profilometers.
Noncontact technologies are also finding their place and improving accuracies in portable applications as well. Romer, a brand of Hexagon Metrology (North Kingstown, RI) announced in July its latest laser scanner for use on portable CMM arms. The CMS108 by Romer boasts an accuracy of 20 µm. The company claims this is a 16% gain in accuracy over previous scanning solutions. Using “flying dot” technology means the scanner detects changes in color and surfaces through their reflectivity, according to Romer. The laser scanner also transitions from matte to shiny features without additional calibration.
Nikon Metrology has also been improving its offerings in portable laser scanning. The MMDx 200 is an addition to the existing MMDx 50 and 100 devices model maker line. The MMDx collects up to 175 stripes/second, which translates to over 80,000 points/sec over its 200-mm stripe with an accuracy published at 22.5 µm to 1 s. “We see this device measuring larger pieces such as large casting, sheetmetal pieces, or heavy industrial machinery,” says Matthew Gibbons, applications engineer for Nikon.
Nikon has also adapted all of this MMDx line to its K-Scan optical CMM, replacing the less capable MMD line of hand-held scanners, according to Gibbons. The K-Scan provides wide area “walk around” measuring capability, using triangulation to LED targets to lock-in the position of the part. An operator then uses a probe or laser scanner to measure data along the part’s surface. “The MMDx in the optical CMM gives us added capability to scan surfaces with multiple colors, high gloss, or that are highly reflective,” explains Gibbons “Previous versions of laser scanners would have a difficult time going from, for example, black to white to orange all on a glossy surface. You would get noise, reflection issues, and you would have to adjust the intensity of the laser based on that color. Now, the laser will update automatically as we scan along. Of the 1000 points we will collect on that line, we can adjust—on the fly—the laser intensity for each of those points.” The MMDx package also includes Nikon’s ESP 3 (Enhanced Sensor Performance-version 3) software package. Accuracy specification for the combined MMDx with the K-Series.
“With vision, you have to understand three qualities: lighting, magnification,
and edge quality.”
Another approach to wide area, walk around metrology is using passive optical systems that measure known points from photoreflector targets coupled with sophisticated photogrammetry algorithms. Creaform (Levis, Quebec, CA) offers its line of such optics based scanners—HandyPROBE, metraSCAN, and Handyscan. It also saw the need to improve their accuracy by introducing its MaxSHOT 3D device. MaxSHOT 3D uses a ring-light flash and digital camera to collect and establish the position of a series of photoreflectors placed on the part, including a calibrated bar they call a scale bar. The targets used for the MaxSHOT are coded targets, with specific numbers of reflectors used only by the MaxSHOT system.
“The MaxSHOT 3D provides a highly accurate positioning model, much like others who use photogrammetry do,” explains Dan Brown, product manager for vision products for Creaform. “What is different is we made it accessible to the user with real-time validation of the model as they are collecting it.” The MaxSHOT feeds data to the company’s VXelements software that displays the points collected. Points are color-coded to indicate if they are recognized by the system. As the user takes data, the system displays points on a screen. Green points are valid; points marked yellow are those where the user needs to take more data. It produces only 3-D coordinates of the photoreflective targets, displayed as a point cloud in XYZ. Once a model establishes these control points, their other scanning systems will incorporate the more precise location data of the retro-reflectors in their scans of the objects.
How much more accuracy does MaxSHOT 3D give to a scanning device? “With parts that are about 1–2 m in size, MaxSHOT will improve accuracy by [a factor of] 2,”explains Brown. “However, with larger parts, say 15' [4.57 m] or more—about the size of a car—you might get five times or more accuracy than using a scanning device by itself.” This translates to about 150-µm accuracy over an object the size of a car, according to Brown.
Another form of wide-area, noncontact metrology is the class of systems known as structured white light. In April, Hexagon Metrology introduced its newest CogniTens systems, the WLS400M and WLS400A. Blue light from LED sources replaces its previous white-light sources. “Blue Light allows us to filter out ambient light using wavelength as a discriminator,” explains Cliff Bliss of CogniTens. The blue light wavelength also limits reflections. This means measuring reflective parts without using a developer spray common for shiny surfaces. In addition, the LEDs have a much longer life cycle that lowers operating cost, according to Bliss.
The Cognitens WLS400 systems uses two different modes, stereo correlation and structured ‘white light’ (really now blue.) “Although both are common, we provide both in a single system,” remarks Bliss. The stereo vision mode projects a known texture and, through correspondence matching, finds the same point in two images to calculate points on a 3-D surface. Using spatial correlation from neighboring pixels means the algorithm delivers subpixel accuracy. “We acquire data in less than 10 ms in stereo vision correlation mode,” explains Bliss. Why is speed important? Vibrations in the environment don’t affect the measurements. This means collecting data on manufacturing plant floors and other challenging environments. Vibrations up to hundreds of Hz do not affect the results, according to Bliss, who reports the accuracy for this mode of the system as about 25 µm (2 s) for surfaces and about 100 µm (2 s) for hole features.
For reverse-engineering applications, the system offers its structured light mode that projects stripes onto the measured part and shifts the pattern, known as a phase-shift technique. It uses five projections to take measurements. It also takes longer to acquire data, 1–2 sec. While it produces a smoother .stl file, this technique is more susceptible to reflections and illumination changes, usually requiring a surface coating to get optimal results. “This mode is best for dimensional inspection applications and reverse engineering,” explains Bliss.
Noncontact metrology has gone “inside” in recent years with the introduction of computed tomography (CT) X-ray machines. Wenzel America Ltd. (Wixom, MI) is introducing a new line of smaller CT machines for metrology that promises to deliver as much accuracy as their larger brethren. “Wenzel acquired the technology when it purchased the Volumetrik company,” explains Giles Gaskell, applications manager for Wenzel. “Previously, all CT machines used for metrology were based on medical technology. The founder of Volumetrik, Martin Simon, produced a new generation of CT machines that broke away from that concept. These machines are desktop or workstation sized.” Wenzel’s exaCT series comes in two models: the space required for the larger evaluation station measures 2300 × 1290 × 1460 mm; the smaller measurement station requires only 1600 × 960 × 1810 mm. Wenzel’s exaCT provide cylindrical measuring volumes that
Multiple sensors, each
providing its own strength, are becoming common among vision and
range from 100 to 250 mm in diameter by 300-mm tall, with detector pixel sizes ranging from 20 to 100 µm. “By housing the sources and detectors in a smaller machine, accuracy, and resolution are actually improved because the source is closer to the part and the detectors,” explains Gaskell. He emphasizes Wenzel customizes the machines for the needs of individual customers, hence the range of measuring volumes and pixel resolutions. Like other CT machines, he also points out that there are some material limitations. “We recommend light metals such as aluminum, some steels within a range of thicknesses, as well as plastics,” he says. “It is really useful for assemblies, especially where bits of metal are molded into plastic and you need to know if those are in there correctly.” He describes parts and assemblies up to the size of a football as ideal, including medical device implants, automotive and aerospace electrical connectors, consumer packaging, high-precision plastics, and metal mold-ins. ME
This article was first published in the November 2011 edition of Manufacturing Engineering magazine. Click here for PDF.