Metrology for medical device manufacturing is in a class all its own. How are providers adapting their equipment for faster, more accurate measurements?
Metrology for medical devices needs to become more capable as those devices get more varied and complex. Manufacturers must inspect dental implants, coronary stents, orthopedic joints, and implanted electronic devices. Surgeons are increasingly using intricate, sometimes one-off, surgical tools. There is also a growing number of consumable items, such as hypodermic needles, made on rapid production lines. They all need ever more precise quality control.
Vision systems that leverage image analysis are often a good match for medical devices. Why vision? Many reasons. “First, for this industry, noncontact is important,” explained Mike Creney, vice president of sales and services for Mitutoyo America Corp. (Aurora, IL). “Many devices cannot be touched after they are manufactured.” Noncontact is important for delicate devices frequently made of myriad technologies. “It is not uncommon to see a device with an injection-molded plastic housing containing electronic and metal parts, like a needle,” he explained.
They also need more accuracy and resolution than most touch probes can provide. “A typical touch probe provides 5–10 µm of accuracy, depending on the machine. But a vision machine can provide down to 2 µm [of accuracy].” Some Mitutoyo systems provide down to 0.25 µm of accuracy. Vision systems also tend to be faster. “A touch probe might provide a point per second; our vision systems measure whole features per second. For the right application, it provides a lot of accuracy for an affordable price,” stated Creney.
Larger Images, Faster Throughput
New challenges and opportunities are emerging. “The medical manufacturing industry is increasingly wanting to measure to higher accuracies within a larger field of view,” said Mark Arenal, general manager for Starrett Kinemetric Engineering (Laguna Hills, CA). A critical function is determining where edges and features are. The finer the spatial resolution of each pixel, the more accurate. Previously, they would increase magnification and focus on smaller fields of view (FOV) to increase resolution.
The critical technology enabling a move to larger FOV systems is enhanced image processing in the software that can provide the same accuracy from a larger image, according to Arenal. That, in turn, is enabled by today’s powerful, yet inexpensive computing systems. “We use robust edge detection and sub-pixeling algorithms to resolve the details on these bigger images,” said Arenal. “Traditionally you might have used a FOV of 1–2 mm; now FOVs of 50–75 mm are common.” The result is faster measurements and greater flexibility. Orthopedic knee implants, as an example, can be measured in one image, capturing a complete profile at once, according to Arenal.
Vision systems and cameras have also improved. Like others in the industry, Starrett now offers telecentric lenses as an option. Though heavier and bulkier than a conventional lens, telecentric lenses capture light with minimal optical distortion. This translates to a greater depth of field. With them, it is easier to put more or larger parts in the FOV. “We also offer systems with interchangeable telecentric and zoom lenses for greater flexibility,” said Arenal. Starrett also offers vision systems equipped with touch and laser probes and their measurement software can incorporate them into a single program.
The company’s newest offerings, the Horizontal Digital Video (HDV) series, are horizontal systems that digitally emulate many capabilities of the venerable optical comparator using DXF/CAD files instead of Mylar overlays as a measurement reference. Their next product is a variation of the HDV. “In October we are going to announce the FLIP-HVR vision measurement system,” said Arenal. “It has a FOV of 175 mm and can be used in either a vertical or horizontal format to accommodate a wide variety of parts and applications, from threaded shafts to nonprismatic contoured geometries.”
Carl Zeiss Industrial Metrology Technology LLC (Maple Grove, MN) is a relatively new entrant in the field of vision metrology systems, according to Jay Elepano, product manager-optical CMMs for the company. He thinks this is to their advantage. As one of the leading providers of CMMs, Zeiss entered into developing vision systems with CMM thinking. It is all about thinking in 3D: “Medical parts are 3D and they need to be measured on a system calibrated [and operated] like a 3D CMM,” he said, noting that image-based vision systems inherently measure in 2D. “Knee joints, bone screws, or surgical units are complex, 3D shapes.”
Vision systems can provide some degree of depth measurement using scales attached to the Z-axis combined with focus and contrast measurements. “These are often referred to as 2.5D systems, in contrast to true 3D,” he said. They require an additional sensor to truly measure 3D, to touch, scan or use a noncontact device, sometimes with articulation. For example, the Zeiss multisensor O-Inspect line comes standard with a telecentric lens, adaptive illumination system, and a Zeiss VAST XXT scanning analog probe, with an option for a confocal white light system. The Zeiss systems also use large FOVs, important to boost speed and throughput.
Recognizing this, throughout the industry practically every vision provider offers various additional devices to measure in 3D, including touch probes, scanning probes, laser point and line scanners, and white light or chromatic sensors.
The key to truly accurate 3D measurement is in the calibration, according to Elepano. “Since we had no legacy, we saw vision systems equipped with touch or scanning probes as CMMs that were also equipped with a vision probe,” he explained. “They are multisensor 3D CMMs, not 2D vision systems with a probe attached for optional 3D.” Towards that end, Zeiss vision systems are fully calibrated to the international ISO10360 standard first developed for CMMs. Its newest addendum, part 7, introduced in 2011, is applicable to Cartesian CMMs using imaging probing systems, according to the ISO website. “We calibrate our systems volumetrically, in all points in 3D, assuming the probe is the most accurate device, just like a CMM, and provide offsets to the vision system,” he explained.
Creney from Mitutoyo, a company also with a pedigree in CMMs, agreed that there is a need to comply with international accuracy specifications. “We adhere to the ISO 10360-7 standard for vision systems,” he said, noting that Mitutoyo contributed to defining the specification.
New Sensors, New Frontiers
Creney also points to the growing number of optical, noncontact sensing probes entering the market, such as lasers or white light sensors. Such noncontact probes not only do not mar parts, which can be the case with contact probes, but they also measure features smaller than the radius of touch probe end ball. “These multisensor vision systems are appealing in medical because they can do 2D measurements, some contouring in 2D and 2.5D, and true scanning of 3D in one system instead of three,” he said. He also pointed out that Mitutoyo recently purchased TAG optics, a company specializing in a unique liquid tunable acoustic lens that provides ultra-fast varifocal focusing. With sub-microsecond speeds, the TAG Lens is fast enough to scan 3D volumes in real-time, creating virtual large depths-of-focus.
Mitutoyo’s Points From Focus (PFF) shows how an operator can extract 2.5D using only the vision camera. Using contrast detection, the objective lens of the vision system is moved in the Z-direction as the PFF algorithm detects the peak of each contrast in each pixel, determining its Z coordinate. Available on the QuickVision line of vision-based multisensor systems, the QV Apex system is Mitutoyo’s workhorse for medical customers, according to Creney.
Systems, Interfaces and the Future
Tom Groff, vice president of North American sales, OGP Division of QVI (Rochester, NY) agreed that multisensor vision systems help solve a growing class of metrology problems in medical devices. While new sensors are always exciting, his company’s current focus mirrors the industry trend of creating more capable systems by combining existing sensors with more capable software.
“Our most impressive new offering is the Fusion 400, a combination of everything we do well in a single machine,” he said. It offers a telecentric large field-of-view (LFOV) camera with selectable optical configurations—a 100-mm FOV with long depth of focus and a 20-mm FOV with high magnification and autofocus. “One benefit of large FOV is that you don’t need to create a parts program since you can now see the whole part. Just put it on the machine and measure it in one snapshot,” he said.
Other sensors available include a touch trigger probe, an SP25 analog scanning probe, and OGP’s own on-axis TeleStar Plus interferometric through-the-lens laser. OGP also offers its Feather and Rainbow micro probes for measuring fragile parts or fine features often found in medical applications. While video can quickly measure most part features, having other sensors available on the same machine means the operator can potentially measure all features without having to move the part. This versatility reduces measurement time and improves productivity, according to Groff.
Another interesting device developed by OGP—the FlexPoint line of CMMs—illustrates the continuing convergence of CMMs with video. Both a smaller bench-top version and a bridge-style floor model CMM can be equipped with the same probes as the Fusion, and with what OGP calls the VersaFlex multisensor head. This offers up to three simultaneously available sensors on an articulating probe head. The beauty of this is that there is no downtime to swap out sensors from a rack, or recalibrate them, according to Groff.
It can also be equipped with a unique video sensor mounted like a probe.
This is a key point—one of the limitations of traditional video systems is they were fixed while a moving stage moved a part under them. To get different views required fixturing to present the part to the camera at different angles. The new QVI sensor assembly uses an articulating probe head to move the camera and other sensors in 3D around a fixed part. So, while vision systems may well evolve into full CMMs, CMMs may just as well evolve into flexible video systems. It is worth paying attention to.