Measuring with Light and Sight
With the array of choices available to measure critical dimensions, noncontact measurement techniques from vision to high-tech lasers are increasingly offered for measurement and inspection. Will they replace contact probes, or complement them?
By Bruce Morey
Noncontact metrology solutions are increasingly offered on precise CMMs. From shadow lighting techniques to high-technology lasers, engineers have a number of options for measuring manufactured parts.
"Our customers demand flexibility," says Keith Vince, optical systems specialist, of Hommel-Etamic (Rochester Hills, MI), describing a key driver for developing their high-precision optical shaft systems. "The customer base is changing, whether it is automotive, aerospace, military, or medical," Vince explains. "Where before we dealt with the major automakers directly, we are now selling much more to Tier 1 and Tier 2 suppliers who have global operations. They need a measuring system that is easy to train, can switch from one part to the next quickly, and can measure ever smaller tolerances."
One such system, the Opticline system for measuring shafts, uses the shadow principle. A green laser illuminates a rotating shaft placed between it and a line detector made of a charged-couple device (CCD) sensor. Although it uses optical principles, no image is ever formed—think of it as noncontact gaging. Measurements include both static and dynamic length measurements, evaluations of geometry, and evaluation of form and position tolerances. New test plans are created within minutes, with up to 100 characteristic measurements, according to the company. The shadow optical principle provides for fast collection of measurements.
Another Hommel-Etamic optical metrology system, the IPS-10, inspects cylindrical bores with a diameter of 14–100 mm and depths to 450 mm. Designed for production environments, the system achieves cycle times that are only seconds per part, producing complete optical inspection of a bore's inside surface. The system detects surface flaws such as cavities, scratches, porosity, valleys, edge flaws, and unmachined surfaces automatically. The system is well-suited to flexible production because adaptation to a new part type is said to be easy.
Constraining the measurement system makes it easier to program, paradoxically providing the flexibility Hommel-Etamic's customers require. Opticline measures only shafts, and the IPS-10 measures bores and measures them only optically. "Opticline is a gage that concentrates on diametrical measurements only," says Vince, "we take one diameter at a time. Instead of three axes we have two degrees of freedom, the diameter and length."
"One reason to choose a laser is that writing a measurement program for a touch probe is often involved and expensive in terms of manpower," explains Jim Clark of Metris USA (Brighton, MI). The company offers bridge, gantry, and horizontal-arm CMMs. It offers two varieties of lasers for use on their CMM machines as well as both point and analog touch-trigger probes. "There is a whole science and process behind writing an accurate measuring program for a touch-trigger probe," Clark explains. "They are best when that single program is going to be used to measure many parts, say in a production environment. However, if you need to precisely measure a single part or just a few, the choice of a laser probe may be superior. They are much easier to write measurement programs for."
Another ideal situation for a laser probe is to measure free-form surfaces, such as artificial knee implants or turbine blades, according to Matt Mulherin, applications engineer for Metris USA. "On the other hand, prismatic objects.objects with flat, regular surfaces and holes, like engine blocks.are better measured with touch-trigger probes," Mulherin says. "Precisely detecting edges is more difficult with a laser scanner, whereas the probe can touch it from a variety of angles and position the edge more accurately."
Most laser techniques scan a single line across a surface, making it difficult to pick up abrupt changes caused by sharp edges, holes, and slots (think of Mulherin's engine block example.) However, the speed at which a laser scanner can collect data spurred Metris to develop a solution to overcome these limitations. By putting three laser line-scanners into a single head, each offset by 120°, the Metris XC-50 provides a high-accuracy, high-density cloud of points that picks up many of the features a single line-scanner cannot. The XC-50 provides 15 µm of accuracy (to a 1-sigma sphere) with a resolution of 0.1 mm. Each of the three lines scans a stripe 50-mm wide at 6400 points/second.
A noncontact alternative to lasers is to use simple white light and passive CCD cameras. One way to get the 3-D position of any point in an image is using the equations of photogrammetry. By measuring known points on the object by imaging from different angles, an algorithm calculates any position of the object in space. Another technique pioneered by Opton Co. Ltd. (Aichi, Japan) is to project a white light with a grating "shadow" onto the workpiece. A single CCD vision camera, mounted on a CNC-controlled multiaxis positioning platform with three autofocusing laser lights, captures the image. Using proprietary mathematical algorithms, a computer reconstructs the shape of the object. It does this by comparing the distortion of the grating on the surface to the known position of the grating lens housed in the xenon-strobe lighting unit.
"Our grating technique eliminates the need for having known control points in the view of the camera," says Tobby Li, manager of 3-D measurements for MiiC America Inc. (Canton, MI), Opton's USA branch. "The key element is that we know the position of the 3-D sensor [camera and light unit] using the CNC-controlled positioner." The Surftizer series of multiaxis 3-D measuring systems MiiC offers comes in four sizes, measuring a volume of 300 x 300 x 300 mm for the Surftizer 300S system up to a large 5400 x 1400 x 2400-mm volume for the Surftizer 6000L. As an example, the 300S offers point spacing of 0.06 mm in X-Y with 0.015 mm accuracy. It collects a "patch" of 75 x 50 mm in less than 10 sec. Although it collects in patches, the software that comes with the Surftizers automatically merges patches of points. The systems are programmed through a teach joystick, and program files are saved for automatic measurement, such as in production environments.
"Our technique, which grew out of automotive design centers, is excellent for free-form parts like turbine blades, propellers, or car body sides, as well as more regular shapes such as engine blocks," says Li. The 3-D sensor method does have restrictions, including looking into deep holes or through transparent objects like glass. "A touch probe has its place; therefore, we offer as an option an integrated sensor package of touch probe and co-axial laser point sensor," Li explains.
Rather than make a choice, contact or noncontact, multisensor measurement options continue to grow in popularity. "People want more out of every machine they get," says Mark Arenal managing director, L. S. Starrett Kinemetrics (Athol, MA). "People want a single machine that will do a lot of things. Multiple sensors give them that flexibility."
Starrett extended the size of its Galileo systems by introducing the Galileo AV1824 video measurement system in February. The system includes vision, touch probe, and laser-scanning options. The gantry-type metrology solution with CNC is targeted for QC labs, research, engineering, and manufacturing environments. It offers a measurement volume of 24 x 18 x 6" (610 x 457 x 152 mm) on an X-Y-Z measuring stage. Maximum workload (evenly distributed) is 220 lb (100 kg) without translight or 44 lb (20 kg) with the translight. Accuracy is ensured by a stable granite base design with an encoder resolution of 0.5 µm.
The AV1824's color optical system provides zoom magnification of 12:1 with a programmable magnification range from 15 to 550x. A dual-output LED illuminator, ring light, and coaxial illumination provides lighting options. Four-quadrant high-incidenceangle LED transmitted illumination is also available. Other options include a Renishaw PH 6 contact probe with TP20 module, Optimet Mark III Laser Probe and a CNC rotary positioning device.
"While multisensor systems are flexible and enable one to do a variety of measurement work, they do require a higher level of training," cautions Arenal. Although all of Starrett's Galileo series offer multiple sensors, it is rare that all three are used. Arenal explains: "In my experience, an application would use either a trigger touchprobe and an optical system, or a laser probe and an optical system. It is not common that all three would be used."
One of the biggest challenges that Arenal sees is an easy interface between CAD software and the measurement machine. "People don't want skilled engineers running a measurement machine," explains Arenal, which is why the company's approach includes the Metronics Quadra-Chek QC-5000 3-D Metrology Software, providing video edge detection and full CNC control. It uses the MS Windows operating system, importing data in .dxf or IGES formats to create measurement programs, and collects and stores the generated data.
Recognizing that multisensor CMMs are becoming the standard in metrology today, Carl Zeiss IMT Corp. (Maple Grove, MN) announced in January 2008 the introduction of its multiapplication sensor system (MASS). Intended to extend the capabilities of its Prismo and Accura bridge-type CMM machines, the MASS platform permits the fast change of contact and optical sensors during CNC-guided measuring operations.
Carl Zeiss offers two optical sensors, both compatible with the MASS system. The ViScan 2-D camera with structured lighting is particularly well-suited for a part with small or 2-D geometries and soft parts such as sheetmetal, rubber, or plastic parts and low-contrast test pieces. While optical sensors are useful for measuring soft parts, the Zeiss ViScan camera sensor is also suitable for geometric features smaller than a contact probe's minimum stylus tip radius, typically 0.15 mm. The LineScan sensor uses a scanning laser to measure up to 250,000 points per second, and is good for car bodies, mold/tool making, model construction, and design, as well as for touch-sensitive or finely structured surfaces. Contact probes that are available with MASS include Zeiss Vast XT, Vast gold contact probe, and the Vast XXT.
Carl Zeiss also offers the O-Inspect workstation equipped with a Vast XXT contact probe and a ViScan optical sensor. O-Inspect is targeted for applications in the plastics industry, medicine, and automotive technology, and in precision mechanics.
Another major provider of metrology solutions, Brown & Sharpe (North Kingstown, RI), a brand of Hexagon Metrology, offers the new Optiv line of multisensor measurement systems. Recognizing the value of multiple sensors, the system features optical, camera, laser, and tactile probing options, packaged in a variety of applications. High-resolution video cameras feature fixed, two-step fixed, or CNC zoom optics, with halogen or LED illumination for noncontact measurement. Bestfit routines provide automatic contour tracing and analysis. A through-the-lens laser provides autofocus and surface-profile scanning. Finally, an array of touch-trigger probes and analog touch probes are available to round out the options. The Optiv comes in three basic lines: the Optiv Performance, Advantage and Reference lines. The Performance is an entry-level, "benchtop" design for inspection of small to medium size parts. The Advantage bridge-style features CNC controlled precision drives. The Optiv Reference, the high-end model, features the highest precision measurement accuracy with air bearings and vibration dampers. The Optiv dual-Z axis, available on some models, allows individual sensors to ride on its own axis, allowing what the company calls true multisensor performance.
Optiv is controlled by Hexagon's PC-DMIS software. In addition, PC-DMIS is used for off-line programming of the CMM. A special PC-DMIS Vision edition provides CAD programming functionality to vision-based software. Offline programming capabilities using CAD, including simulated lighting, magnification, and camera image, are features said to be especially useful for a vision-based measurement system. CAD translators and direct CAD interfaces to all major CAD formats provide for interfacing with part data.
Machine Vision for Go/No-Go Inspections
One often thinks of machine vision as a high-technology tool, which is to say expensive. A machinevision system for inspection often involves extensive hardware and software purchases, integrated by a company with specialized expertise. Rockwell Automation (Milwaukee) wanted to change all that with its recent introduction of the MultiSight vision sensor.
"We wanted to make machine vision easy by creating a low-end vision sensor instead of a high-end machine vision tool. Five years ago, what would have taken a systems integrator about three days to set up we now can do with a MultiSight sensor that takes about an hour to set up," says Brian Schriver, product manager for error-proofing sensors for Rockwell. "Vision sensors like MultiSight are intended to fill the gap between standard sensors and complex vision systems.
The image recorder of the MultiSight is a CCD chip with a resolution of 640 x 480 pixels. Designed for inspection, it cannot provide measurement information. Its purpose is to provide go/no-go information. A region of interest on the part to be inspected is designated and tolerance levels are defined. This virtual detector configuration—a pattern that the vision sensor is looking for—is stored in the sensor's nonvolatile flash memory for reference when the sensor is in stand-alone operation mode. As parts move through the sensor's field-of-view, it performs a pixel-by-pixel normalized gray-scale correlation to the stored pattern. If it matches within tolerance, the part is 'go'; if not; the part is 'no-go'. Signals are sent to PLCs via a simple PNP-type output for processing.
"MultiSight is especially good for detecting presence or absence, completeness, position, markings, and labeling," says Schriver.
This article was first published in the May 2008 edition of Manufacturing Engineering magazine.