Dimensions in Medical Metrology
From optical comparators to computed tomography, metrology equipment is adapting to aid advances in medical manufacturing
By Bruce Morey
Medical devices extend life or make living more comfortable. Driven by advances in machining, new devices are getting smaller and more complicated. Other devices, while not getting smaller, are increasing in demand, meaning production volumes are increasing. From looking into parts more deeply, combining sensors for more complete coverage, or speeding up processes in time, metrology equipment is improving in many dimensions for medical devices. This includes optical comparators, video metrology systems, CMMs, touch probes, and laser scanners. Even metrologygrade X-ray Computed Tomography (CT) is now offered. Each sensor has limitations and strengths.
There are large classes of medical parts that are ideal for three-axis vision-based metrology notes Allen Cius, product specialist for vision systems for Mitutoyo (Aurora, IL). This is because the feature sets are typically small—tens of microns—and require a high degree of measurement certainty. These include multilumen tubing, medical packaging, dental components, and cardiac components. The fact that vision systems measure without contact is another plus. "In many cases, touching the part in any way can introduce unwanted contamination." Mitutoyo offers a range of three-axis vision-based metrology systems. They offer systems with accuracy of 1.5 µm as well as higheraccuracy systems such as their QuickVision Ultra system, which delivers 250 nm of accuracy in the X and Y axes and 1.5 µm in Z.
Cius also points out that capabilities have grown by combining multiple sensors in one system. Why is this trend so prevalent on systems designed originally for vision? "You can guide an additional sensor—laser or touch probe—to the feature you want to measure using the vision camera. When a feature is very small—a situation that's often encountered in medical—you can use the camera to position the other sensor and engage the feature of interest properly." Vision can be good for edges and sharp contrasts. Touch probes typically measure surfaces and complex 3-D profiles that vision simply cannot reach. Scanning analog probes are good for collecting many data points quickly. Lasers collect even more without touching the surface (noncontact), critical in some medical applications.
Cius predicts automation will become just as important in medical as it is in other industries. "The trend is always going to be towards eliminating human intervention in metrology systems," says Cius. As an example, Mitutoyo offers its QV Stream vision-based system for high throughput applications. While it offers 1.5-µm accuracy, what is unique is that it uses stroboscopic illumination and a progressive scan camera to boost speed, measuring in 45 min up to 30,000 diameters that once took 8 hr.
Advances in optical comparators offer their own solutions. "There is demand from medical manufacturers for optical comparators," says Mark Arenal general manager for Starret Kinemetric Engineering Div. (Athol, MA). "For example, while we still offer static overlays, new technology for comparators includes automatic edge detection and CNC controls. Even as they are still used as a go/no-go gage, enabled by edge detection and CNC, people now use comparators like a CMM," he explains. Optical edge detection remains common, but with the addition of an OV2 optical-video adapter, Starrett converts their optical comparators into low-cost video measuring systems. "With that video lens, you can now view the workpiece in two ways: on a video monitor or as a typical projector-type image. Yet, you can still take optical edge-detection points. These are more accurate and precise than a traditional overlay." He notes that more than half of the optical comparators shipped by Starrett record discrete measurements rather than being used as go/no-go gages, a percentage he expects to continue to grow.
"Medical-device manufacturing is investing in technology, especially domestically in the US," explains Arenal. "Much of the technology manufacturing is here in the US, driven by the need to meet FDA regulations and the need to be close to the end-user. People can justify the additional expense for health care. Increasing efficiency, eliminating operator interaction, and more data comparisons to CAD models are all trends we are seeing in medical device manufacturing."
Tom Groff, metrology product manager for Optical Gaging Products (OGP; Rochester NY), agrees that advances in comparator technology are important. "Medical devices in some cases might have thousands of overlay charts. Eliminating overlay charts means you do not have to create new ones if you make a change." In response, in September 2009 OGP introduced its electronic overlay eCAD software. Available on specially configured OGP Contour Projectors, it projects a profile tolerance band derived from a CAD model onto the comparator viewing screen.
This example points to the three key ingredients of metrology for medical devices Groff sees:
- Sensors, especially multisensory,
- Software that enables throughput and sensor data fusion, and
- Procedures to ensure regulatory compliance.
Beyond comparators, Groff also agrees that multisensor capability is important, as reflected in OGP's Smart-Scope Specialist 300, which is designed specifically for medical device manufacturing. It's built around a vision system as the base sensor technology, and additional sensor options include a touch probe, a telecentric interferometric through-the-lens (TTL) laser, microprobes, or a continuous-contact scanning probe. Groff emphasizes the importance of software that ties the sensors together. "That is the crux of it all, relating different sensors and orientations to maintain good, accurate metrology. One part set up, one datum exploited by multiple sensors improves throughput and accuracy."
The final element is supporting FDA regulatory compliance, including the Code of Federal Regulation Title 21 for Food and Drugs (CFR 21). "We offer three basic items to help companies with their compliance. First, we offer a documentation package that takes them through a validation procedure. Second, we provide a service for Installation Qualification and Operational Qualification (IQ/OQ). Third, our Smart Feature software tracks the user through a log-on procedure, and provides an electronic signature for that user's measurements. It satisfies the CFR 21 Part 11 requirement that the manufacturer know who performed every measurement, providing the needed traceability for each individual part so vital in medical manufacturing," explains Groff.
Other companies recognize the need to bring a third dimension to the 2-D world of vision. "The most important element of Hexagon's multisensor strategy is bringing full 3-D capabilities to a traditionally 2-D class of products," states Gary Hobart of Hexagon Metrology Inc. (Elgin, IL). "[Our] PC-DMIS Vision software makes full CAD integration with vision and multiple sensor machines a reality. All the sensors work seamlessly with the same interface. You can program from and compare results to the CAD model, so you always have that absolute reference. This capability is especially important with medical parts, which are growing ever more complex. There can be 3-D surfaces to inspect where comparison with the CAD is the only realistic way to ensure that the part matches the design."
While vision systems now often include CMM-like tactile probes, CMMs also now offer full suites of sensors, including lasers and vision. CMMs equipped with indexing heads offer five-axis measuring capability. "We can even add a rotary table to get a sixth axis," explains Dan Jeanloz applications engineering manager for Hexagon Metrology Inc. (North Kingstown, RI). "For medical, typically we use five axes, and for complex contours we supply an analog scanning probe." A typical CMM intended for medical applications is a Brown & Sharpe Global equipped with a scanning probe with 5 µm best accuracy in a 700 x 1000 x 700-mm measuring volume.
Applications with small 2-D features can drive a need for vision on a CMM, which Hexagon satisfies with its CMM-V vision probe mounted directly on the i5-axis indexing head. "With a scanning probe or touch-trigger probe, we can measure a 0.020 or 0.030" [0.51 or 0.76-mm] diam hole. A CMM-V camera measures down to a 0.005" [0.127-mm] hole," notes Jeanloz.
Depth is a dimension with special characteristics. Measuring a surface is one thing, penetrating into the part with X-ray Computed Tomography (CT) provides something quite different. A few companies are now offering these systems as metrology systems, though they were first developed for Nondestructive Test (NDT.) CT captures several thousand X-ray 'slices' of an object, and reconstructs them into a volumetric picture of the part. This 'voxel' dataset can be measured or compared to a CAD model. The ultimate advantages include access to undercuts, internal cooling channels, and voids in one setup even while fixturing is much simpler. It also measures the density in each voxel, which can be important, depending on the application.
While providing a new dimension to metrology, CT has a few technical limitations notes Kevin Legacy, manager computed tomography and engineering for the Metrotom CT CMM from Zeiss IMT (Brighton, MI), such as measuring materials typically no denser than 4 gm/cc. He cites aluminum, composites, plastics, and ceramics as ideal materials. These are materials frequently found in medical devices. "Because of the small size of medical devices, Metrotom is more applicable to medical than other industries, especially since many medical devices are going to lightweight supermaterials." It should be noted that in some cases even denser materials could be measured, as long as they are thin enough. "Penetration depth is the key parameter. We can measure goldplated pins because the gold, though very dense, is thin." Offering two sizes, their Metrotom 1500 has a cylindrical measuring volume of 300 mm in diam x 350-mm high with accuracy specification up to MPE = (9+L/50) microns. The smaller Metrotom 800 has a measuring volume of 125 mm in diam x 150-mm high with an MPE of (4.5 + L/100) microns.
Legacy points out that the Metrotom CT, while useful in manufacturing quality control and process control, also aids in the product development process. Since its introduction, says Legacy, CT has proven especially useful in understanding how assemblies of parts mate to one another, an interface often hidden from more-conventional metrology sensors. "Recently we measured a biopsy instrument, an assembly of small gears and racks. It was a clever little device that unfortunately did not work. We scanned the assembly, then individual parts to help the designer understand why."
Another company that offers CT Metrology capability is Nikon Metrology (Brighton, MI) with its XT H 225 X-ray and CT inspection system. Its maximum scan-area is 250 x 330 mm for a single scan, with an accuracy that is approximately 1/3 of a volume element (voxel), according to David Bate, manager, X-ray Centre of Excellence for Nikon Metrology. "Voxel sizes are material and partsize dependent, and can range from 1 to 5 µm or larger. Depending on the accuracy you want to achieve, you can trade accuracy for time. The longer the collection time, the more accurate the data set. A high-accuracy data set might take as long as 1 hr."
Bate also notes that CT metrology builds on the foundations of Xray NDT. Typical customers use the XT H 225 for both inspection and metrology. He agrees that an important use is seeing how assemblies of parts fit together. Their longest-term application is related to drug delivery systems such as asthma inhalers and tablet dispensers. "Strictly speaking, these applications are not metrology, but new applications built on that experience including measuring inside heart pacemakers, where metrology is important. Dimensions are important because they have to fit in a specific place in the chest cavity." Other applications include inhaler-type devices and pacemakers. An application of note is looking at titanium screws used in implants. In this application, checking both the pitch and density across the profile is important for the customer. Although titanium would normally be too dense to use X-ray, the small depth of penetration (10–20 mm) allows the 225-kV system to capture the data.
Sometimes new capability builds its own demand. That's the observation of Mike Knicker, president of QPLUS Laboratories (Irvine, CA), who remarks, "as medical manufacturers have more confidence in metrology equipment to deliver accurate measurements, they increase their expectations even further by tightening tolerances." Q-PLUS is a development partner with Starret Kinemetric. The company provides dimensional inspection services, reverse engineering, calibration, and consulting to end users, as well as metrology equipment. He notes that with the advent of noncontact laser scanners on his Starret Galileo systems, complicated shapes medical manufacturers would once omit from measurement are now included in their protocols.
Are tolerances getting tighter in medical devices? "The tightest print specification I might see is maybe 0.0002" [0.005 mm]," replies Knicker. "I might have seen that ten years ago, but it's more commonplace today. Tight specs are specified more frequently because engineers have more confidence that they can be measured." However, he cautions that a tighter tolerance does not always mean advancement. Robust engineering means delivering functionality that does not require more tolerance than needed, or that increases manufacturing cost.
"Customers always want faster, better, and cheaper. Advances in both sensors and software will deliver that," he remarks. "One of the ways new sensors go mainstream is that people stop seeing them as novelties and more as necessities. I watched that with laser sensors. Once, they were something to be marveled at and feared. Now they are an integral device to be employed regularly, where appropriate."
This article was first published in the May 2010 edition of Manufacturing Engineering magazine.