Multisensor metrology systems have existed for years, but the technology has become more popular recently as manufacturers seek more versatile measurement tools in the latest systems combining vision, laser, and touch-probe scanning technology.

As precision parts require ever-tighter tolerances and increase demands on quality, some benchtop metrology systems formerly dedicated to video inspection can now handle touch probes and lasers to provide multisensor measurement in a compact package. In some cases, features that were formerly found only on larger multisensor metrology equipment are now migrating to smaller benchtop-style multisensor measurement systems, partly due to the increased computer power available with advanced microprocessors.

Teaming optical measurement with touch probe or laser scanning offers manufacturers several advantages, including lowering capital equipment costs by combining two or three types of metrology systems into a single multisensing system. "Our NEXIV vision system combines both optical measuring and through-the-lens [TTL] laser, so we carry two of the popular probes--laser and vision, or video--that qualify as a multisensor tool," says Michael W. Metzger, department manager, Measuring Instruments, Nikon Instruments Inc. (Melville, NY). "The combination of the vision with the TTL laser really maximizes accuracies, speed of throughput, and the ability to measure complex contours."

Multisensor system from Optical Gaging Products uses a touch probe to measure a metal part.

Nikon's lineup includes the NEXIV VMR 3020 tabletop multisensing system and the 6555 bridge-design models. Manufacturers use benchtop systems for measuring smaller parts, such as electronics and smaller precision-machined parts, while the bridge design measures larger, heavier parts. Multisensing systems come in two basic platforms, one based on a standard CMM-type platform, and others based on standard optical or vision-based systems, Metzger adds. "With the CMM, the part is stationary and the probe moves. If you're going to put an engine block on a measuring device, you do not want to move that engine block around; you want to move around the probe. And the probe head on a CMM is feather-light, so that it can maintain that sensitivity of the touch-trigger probe. On a vision or optical-based system, the part moves along X and Y on a stage, and the optics move up and down in the Z-axis direction, giving the height measurements. They're two completely different workstation designs."

Standard CMM-type platforms can handle very heavy parts due to their robust construction, while most vision-based platforms can only be used with lighter parts. "With an optical-based platform, you really should use a less-heavy part, because it is often cost-prohibitive to make a stage good enough to move a heavy part around without that stage sagging when you go to one side or the other," Metzger says. "If you took that engine block and put it on a stage that moves, when you move all the way over to the right, that stage is likely to bend, at least to some extent."

Multisensing measuring technology has slowly evolved as manufacturers sorted out the technical issues involved in effectively integrating multiple sensing capabilities. "People experimented with cameras on CMMs for about 20 years," notes Tom Charlton, director of advanced technology, Mahr Federal Inc. (Providence, RI). "It started very early, with PCB manufacturers building vision systems, and I believe the first machine designed from the beginning to be an integrated [multisensing] system was a machine made by a German company called Wegu, which was acquired by Mahr in 1999."

Early systems had mixed success for a couple of reasons, Charlton adds. "One was the importance of lighting, illumination, to using the video cameras, and the software for traditional CMMs wasn't really the right software. It needed special characteristics for vision." With Wegu's patented technology, Mahr's system evolved into an integrated positioning machine with a CCD camera supplying vision capability, plus laser scanning, and touch-probe functionality. Mahr recently introduce the new compact benchtop system, the Marvision Multiscope MS 222 3D CMM, which offers features found on larger multisensor systems, including integrated automatic touch trigger, automatic probe changer, motor zoom, fast video focus, and automatic feature recognition. With a measuring envelope of 9.8 X 7.9 X 7.9" (250 X 200 X 200 mm), the unit's maximum load is 22 lb (10 kg) and stage resolution is rated at 0.5µm.

Benchtop multisensor systems have some limitations compared to larger systems, Charlton concedes. "One is that they tend to provide lower accuracy, for a number of reasons, partly because they are low-end entry systems," he notes, "and also because they tend not to be used in as good an environment. They don't have a big stable platform base, and they usually don't go into temperature-controlled labs, so they tend to be a little less accurate.

"In terms of sheer volume sold, the smaller benchtop systems probably account for 40% of the sales," he says. "Because they're so much lower in price, they serve as an entry-level system for a lot of people. They're all full-blown multisensor machines, the difference is the size of the part--something like 75% of all the parts made will fit in a breadbox."

Benchtop systems with higher-end features began appearing recently, as increasing microprocessor power made it easier to incorporate more capabilities into the smaller packages. "In the early years, the first vision machines were larger, floor-style machines, and the benchtop system really came into its own maybe eight or nine years ago," notes Frank Demski, product manager for Mitutoyo America Corp. (Aurora, IL).

"What's considered new in that segment is the fact that the capabilities that were once only available in large, expensive floor-style machines have been miniaturized, and are becoming widely available now on the smaller-stage, benchtop-style machines," adds Demski.

With the downsizing of PC boards, the entire CNC machine controller has shrunk, Demski notes. "We've got fully automated, CNC-controlled benchtop-style machines now that are self-contained units. All of the drive systems, servos, and controls are integrated into the benchtop units, instead of the freestanding controllers that were coupled to them in the past."

Among Mitutoyo's offerings, the Quick Vision family of CNC multisensors offers higher levels of accuracy and speed (up to 3X faster) than previous models, and the company's UMAP (Ultrasonic Micro and Accurate Probe) multisensor system uses vision and ultrasonic probing to probe tiny holes in fuel-injection nozzles and other miniature part features.

Originally developed for Toyota Motor Corp. (Tokyo) for measuring the ports on the fuel injector nozzles for the automaker's engines, the UMAP system has been sold in Japan for about a year, and Mitutoyo introduced the units to the US market in April, notes Demski. "It was originally custom-built for Toyota," Demski says of the UMAP. "We've had such a demand here in the US in the fiber-optics industry for precise measurement of these microholes in the fiber-optic connectors, fiberoptic ferrules, that we've decided that particular machine is a very good fit for that market. We have interest from several other engine manufacturers to use this type of system for the same fuel-injection application."

Benchtop Marvision Multiscope 222 3D CMM offers vision, touch probe, and laser sensors in a compact scanning system.

Cost-effectiveness of multisensing equipment also can enable manufacturers to cut costs by replacing older metrology systems with new multisensor units. "A multisensing environment allows you to attack a lot more applications with one system, as opposed to having three pieces of capital equipment sitting in your lab," says Tom Groff, product manager for video systems, Optical Gaging Products Inc. (OGP, Rochester, NY).

OGP's SmartScope Quest line of multisensor metrology equipment includes both benchtop and larger floor-standing systems, adds Groff, with the company supplying the machines predominantly to the medical, automotive, aerospace, plastics, and electronics industries. OGP, which has sold multisensing systems for about 20 years, designed the Quest system from the ground up to support multisensing, as opposed to simply adding sensors to an existing system. OGP recently introduced its new Quest 650, a floor-standing multisensor system with video, laser, touch probe, articulating probe heads,and continuous contact probing that features a rigid granite base with bridge construction and a measurement volume of 24 X 26 X 12" (600 X 660 X 300 mm) in X-Y-Z and an option of 16" (400 mm) in Z axis.

As parts become more complex and tolerance requirements increase, the demand for enhanced capabilities from measurement equipment has driven interest in multisensor technologies, notes OGP's Fred Mason. "If you can do it all on one machine, you can get the advantages of fixturing the part once, and getting all your results at the same time," he adds.

Multisensor equipment theoretically offers manufacturers the best of all worlds. Optical technology excels in edge detection, with fast, noncontact data acquisition; tactile or touch-probe abilities can measure surfaces and access internal part features that are inaccessible by other sensors; and lasers can do surface profiling and offer speedy data acquisition in noncontact measurement.

Integrating sensor technologies also can pose a challenge, requiring cross-correlation of each sensor to the others, notes Nikon's Metzger. "On a vision or optical type of platform, you have the choice of video measuring, touch probe, and laser as the three primary probes people are using," he says. "When you work with a vision system or an optical system, the touch probe is offset from the optical axis of the machine. The lens is typically in the center of the system and the touch probe is over to the side, because you can't have the touch probe directly in line with the optics or the touch probe will block the image, and the same is true with lasers. On optical or vision-based systems, you can have a laser that goes through the lens, or you can have a laser that is bolted onto the side of the optical head, or is offset from the optical axis.

Vision and ultrasonic probing allows Mitutoyo's UMAP multisensor system to probe tiny holes in fuel-injection nozzles and other miniature part features.

"At Nikon, we're always TTL, but many other manufacturers are not. The technical advantage is that the on-axis vision and the on-axis TTL laser have a strong correlation to one another, because they're both using the same optical system to provide the probe point, which is the focusing of the lens and the focusing of the laser. If you have two probes on the same tool and they are offset, you must cross-correlate them to each other. In a multisensor tool, sometimes you measure with the optics, sometimes you measure with the touch probe or the laser, and if all three of those are offset from another, then you have to cross-correlate or cross-calibrate. All probes must be considered a potential source of error or bias to determine the uncertainty of the system."

Vision or optical systems' biggest challenge has been in measuring shades of gray, or defining contrast when measuring parts. "Contrast is the opposition of black and white, that's high contrast," Metzger says, "and low contrast would be gray and gray. When we use a vision system to measure something, most of the time it's gray and gray, so there's low contrast in the things that we measure. Our computers and the software algorithms used to determine the change in a gray-to-gray edge are improving all the time, because computers are getting faster, and computers are getting smarter. In our computers, we use the Cognex Corp. [Natick, MA] vision system, which is a leader in low-contrast edge detection."

In most cases, lack of industry standards in software means most metrology vendors supply their own versions for customers. "There are several aspects to the software issue," Mahr's Charlton notes. "The critical enabling piece is being able to treat data that comes from different kinds of sensors, all in the same context. You take a point, an X-Y-Z point with a touch probe, a camera, or with a laser--you'd like to be able to handle all those points as all just data, and integrate them into a single geometric feature, or compute the distance between a surface measured with a touch probe and a surface measured with a camera. You'd like the system to make it easy for you to do so. With the earlier systems, you really couldn't do that. You could take the touch probe points, and you could take the camera points, but it wasn't easy to get them into a common coordinate system."

Industry standard metrology software consists mainly of the Dimensional Measurement Interface Standard (DMIS), an ANSI standard since 1990, notes Charlton, although several related standardization efforts--including work by the Advanced Metrology Group at DaimlerChrysler Group (Auburn Hills, MI) and the Metrology Interoperability Consortium at the Automotive Industry Action Group (AIAG, Southfield, MI), plus the STEP (Standard for the Exchange of Product Model Data) effort--have been underway for years (see the article "Metrology for Manufacturing Means Business," in the June 2002 issue of Manufacturing Engineering).

"In multisensor systems, there are virtually no standards and there's no common software," Charlton adds. "Every big vendor has come up with their own--they've solved these technical problems in their own way with their own software."