New Frontiers in Measurement
Integrating different types of sensors into common platforms has created flexible measuring stations and CMMs
Using multiple sensors for measurement and inspection is a trend that seems to be accelerating. Sensors used for today's metrology include touch (tactile) probes, probes that use color or proximity at their tips, camera vision systems, and various flavors of lasers. Newer X-Ray Computed Tomography (CT) sensors look deep into parts. While each sensor has its purpose for particular applications, combining them is becoming easier.
For Stephen Flynn, president of Optical Gaging Products (OGP) (Rochester, NY) putting multiple sensors together is a trend that is not so new, though the company has added some twists lately. "We developed our first multisensor system in 1986—we were actually ahead of the technology at that time," says Flynn. "Today it's very common to have multiple sensors on any system we ship." OGP specializes in high-precision systems configured around a video sensor. Making them multisensor systems means integrating additional probes and sensors.
In Flynn's opinion, multisensor metrology is driven by customers who need speed, accuracy, and the ability to measure small features, coupled with the ease of a single setup. "Capability is really what it's all about," observes Flynn. Each sensor has its own strengths and limitations—combining them allows each to contribute its individual strengths. For instance, according to OGP, high-magnification video detects edges directly, while touch probes measure surfaces that intersect to form edges. Lasers and scanning white-light sensors are superior for measuring surface contours. Flynn points to the increasing speed of low-cost computing platforms coupled with easy-to-use software as key enabling technologies for multisensor metrology.
OGP's SmartScope line of multisensor CMMs ranges from compact benchtop units to floor models with large measuring volumes. Their latest is the SmartScope Quest 300, a largecapacity, benchtop model. It has a designed-for-multisensor mechanical design with machined-in axial straightness and perpendicularity. A number of optional sensors are available. Parts with important features on different surfaces can be mounted on an MTR rotary indexer, and maneuvered into optimal positions for measurement by any available sensor.
Werth Inc. (Old Saybrook, CT) is another company that has migrated from optical-only metrology to a multisensor approach that uses a full range of multiple sensor technologies. Many of their new machines also include rotary axes to satisfy setup requirements, and combine optical sensors, lasers, contact probes, and their own high-precision tactile-optical fiber probe. An example of a new multisensor offering is the Werth ScopeCheck V for measuring turned and milled parts in shop-floor environments. Available sensors include a Werth-supplied video sensor, an optical sensor with full image processing, a laser sensor, Renishaw point-topoint touch trigger, and dynamic scanning sensors, and Werth's Fiber and Contour sensors.
The Optiv line of multisensor metrology systems is offered by Brown and Sharpe, a division of Hexagon Metrology (North Kingstown, RI). Using video cameras as primary sensors, Optiv systems are augmented by optional touch probes, laser sensors, or white light sensors (WLS). Introduced in September, the Optiv Classic is an entry-level model in the existing Optiv Reference, Advantage, and Performance line. Each of these three existing systems is intended as a building-block system that can be outfitted with different sensors and machine capabilities, providing flexibility for future growth, according to Zvonimir Kotnik, Hexagon Metrology product manager for vision and multisensor products. With more than 25 possible size and accuracy combinations, plus a wide variety of accessories, Optiv systems offer a lot of choice.
The multisensor trend is migrating to CMMs as well. Where vision-based systems were well-suited for measurement and inspection of smaller, more-precise parts, CMMs are designed to handle larger parts, according to Kotnik. Where CMMs once used only touch probes, options for augmenting them with other sensors are increasing in number. An example of this trend is Hexagon's release in September of the CMM-V high-resolution camera. Compatible with Brown & Sharpe or Sheffield CMMs, it also fits on Tesastar-m and Renishaw PH10M/MQ wrists that are designed for measuring large components. "A fixed-optic sensor with a small field of view and only one light source, it's a low-cost multisensor upgrade for CMMs targeted for small features, cut-outs in sheetmetal, and easily deformed parts," explains Kotnik.
Another example is the ScanShark laser contour probe, which is intended for inspection, reverse engineering, or combined full inspection and reverse engineering. Offered with a common plug-and-play interface, the device is compatible with Romer portable arms as well as with Brown & Sharpe or Sheffield CMMs and Tesastar-m wrists. Kotnik also points to software designed to be relatively sensor-independent as a key enabler for the expanding use of multisensor technologies. "Using our PC-DMIS software, users familiar with touch probes would not find it overly difficult to develop a measuring program for other sensors," he asserts.
White-light sensors are a sensing technology whose development excites Kotnik. The WLS is a Through-the-Lens (TTL) sensor. Unlike the TTL laser, WLS does not care about the amplitude or signal strength of the return signal, it just measures light in excess of 30,000 color levels. From these, a single color is chosen for measuring. Kotnik lists several advantages for the WLS, among them:
- Improved accuracy due to a light spot as small as 1.5 µm;
- Improved access, because it measures angles in excess of 88° to the probe, unlike a laser;
- Insensitivity to surroundings, because it's not affected by material conditions, color or texture; and
- Easy installation, because it is lightweight.
Applications that can benefit from the use of WLS measurement systems include measurement of precision molded parts, small machined parts, small ceramic or stamped parts, bone screws, or any other small, precision part. And, of course, white-light technology works well as part of a multisensor system.
An example of a WLS that could be included in a multisensor system is Werth's Chromatic Focus Probe. It distinguishes the distances between the various wavelengths of colors from a white light source after they are refracted by a lens to different focal lengths. Another is OGP's noncontact white-light-scanning Rainbow Probe, with a spot size as small as 2 µm, which also analyzes the optical spectrum of reflected light to determine changes in surface heights.
A new frontier in sensing metrology lies in adapting Xray CT to precise measurement of parts. While combining multiple sensors will advance the state of the art, most such systems today combine sensors that only measure a surface. Complicated geometries, especially those with recesses and undercuts hidden deep in parts, require writing complex measurement programs, and require complicated movement of the part, sensors, or both. In some cases, parts must be cut open to measure interiors adequately, making 100% inspection impossible. To look deep into a part and measure it precisely, some companies are pursuing CT. While CT has been used for industrial purposes for some time to identify the presence of flaws and cracks, the new adaptations turn CT systems into accurate measurement devices that deliver accuracy in microns. After capturing hundreds or thousands of X-ray slices of an object, CT reconstructs them into a 3-D volumetric picture of the part. Powerful computers and computer clusters that can handle the tremendous data load from CT are enabling this advance.
"Our X-ray CT systems measure completely and accurately in one setup," says Jeff Bibee, a vice president at Werth. He notes that X-rays are typically not accurate, with focal point sizes in the 50–100-µm range. But the company has developed a compensation system that marries the multisensor approach to X-rays. The result: A system that provides an accurate CT metrology for the precision-part industry. "Using a multisensor approach on the first article only, we gather accurate data with sensors such as optical or tactile probes to establish a calibration point cloud. We then best-fit the X-ray CT data to the calibration point cloud to recalibrate the X-ray data. This step compensates for beam hardening, and the distortion effects typical with X-rays. Now the system measures accurately—with X-rays—within a few microns."
The phenomenon called beam hardening results when an X-ray tube produces various frequencies, or polychromatic beams. Because materials absorb radiation differently for different frequencies (low frequencies tend to be absorbed more than high frequencies), this behavior causes an artifact in measurement. The 3-D reconstruction of the X-rays of the workpiece is based on measuring all frequencies.if they don't all get through at the same time, the measurement is distorted. Because the X-ray penetrates the material faster as the lower frequencies are absorbed, the X-ray is said to be "harder", so this distortion is called "beam hardening." The broader the spectrum of the X-ray emitter, the bigger the piece, and the more "absorbant" the material, the more the beam will harden and the more the distortion must be accounted for.
The Werth Tomoscope can measure a maximum volume of 500 x 350 x 350-mm with a best maximum permissible error (MPE) of (2.5 + L/120) µm. Because it's a multisensor system, the Tomoscope also includes optional optical sensors, image processing, and trigger and dynamic touch probes. The smaller, more-precise Tomocheck device offers a best MPE of (0.25 + L/900) µm. Using this unit, first-article inspection, according to Werth, can be completed in 20 min rather than requiring days.
"I like to think of X-ray CT as the CMM of the future," says Kevin Legacy, manager of computed tomography and engineering for Carl Zeiss IMT Corp., Industrial Measuring Technology (Minneapolis). Zeiss' Metrotom, which was developed for measuring precision parts, offers a sensing volume of 300 x 300 x 300 mm. The company also recognized the inherent inaccuracies caused by beam hardening and radiation scattering. Another concern for Zeiss, as Legacy explains it, is the volumetric variation in X-ray-derived measurements. A distortion of 0.001" (0.03 mm) on the surface may grow to a 0.006" (0.15-mm) distortion deep inside the part as the X-rays interact with holes, inclusions, and the material.
"There are three elements to our strategy to compensate for possible distortion," explains Legacy. "First we build Metrotom around a high-quality measuring machine. Second, we precalibrate the machine with artifacts that are traceable to known standards, such as NIST and PTB. We measure them exactly as we do on a CMM today. Third, we do some internal processing that results in our not having to do any kind of comparison to a [sensing] device of a higher standard, such as a touch probe." He describes achieving an accuracy specification as high as MPEE= 9+L/50 µm over the entire volume of the machine. For the Metrotom, Zeiss recommends a yearly calibration similar to a CMM calibration.
Materials suitable for X-Ray tomography are currently limited to relatively low-density materials (less than 4 gm/cm3), according to Legacy. He cites aluminum, composites, plastics, and ceramics as appropriate materials. "Measuring denser materials is directly related to power and better beam focusing. Currently, we are limited to producing X-rays at about 225 kV," explains Legacy. He points to future improvements in focus ability and better detection of X-rays, rather than simply more power, as the key to measuring denser materials. Devices such as X-ray collimators may provide some of this enhanced capability.
What Sensors Do
Even while multisensor technology is expanding, the classes of typical sensors used to date fit into one of three categories (not counting X-rays).
Touch probes physically contact the surface of the measured part. The length of the styli and styli diameter used can affect accuracy. For certain materials, such as rubber, touch probes deform the surface and may provide inaccurate readings.
Vision probes are noncontact and are the fastest and most accurate for 2-D measurement in the plane perpendicular to the optical axis of the camera (X-Y). Much modern video measurement equipment uses zoom lenses, autofocus, and large stage travels in the Z axis to perform accurate, noncontact measurements anywhere within the system's measurement volume.
Lasers used in multisensor systems can employ triangulation or direct their beams through a visionsystem camera (this is termed a through-the-lens laser or TTL). Triangulation lasers typically mount away from the video optical axis, while TTLs typically are on-axis and mount through the lens. Either type measures single focus points, or is scanned across surfaces to derive profiles. Some off-axis triangulation-type lasers are limited by the angle of light striking the surface. An advantage of TTL is that the laser shares the optical path with the video system. This characteristic allows rapid switching between the two sensor types, and makes it possible to monitor the position of the laser in the video image.
More specialized high-accuracy sensors offered today include Werth's Fiber probe, which has a 10µm spherical-diam measuring head that uses fiber technology. Optical Gaging Products provides a new TTL laser for its SmartScope Quest multisensor systems. Using interferometry, the TTL laser measures features with a resolution down to 0.5 µm at working distances of 200 mm—a shorter working distance offers finer resolution.
This article was first published in the November 2008 edition of Manufacturing Engineering magazine.