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Alex Lucas

Metrology

Which Are Better: Cross Scanners or Single-Line Scanners?

Finally, a single scanner solution to measure free-form surfaces and critical features.

Published: Thursday, July 30, 2009 - 10:02

For decades, traditional touch probes on coordinate measuring machines (CMMs) have been the gold standard by which parts have been inspected and verified. However, this time-consuming process becomes an even bigger drain on a quality department’s valuable resources as the parts it is charged with inspecting contain increasingly complex free-form surfaces that take an exponentially longer time to thoroughly inspect.

Quickly and accurately digitizing free-form surfaces was the first niche application for laser-scanning probes, a technology that uses a single laser, sweeping back and forth (this appears as a "line" on the surface of an object). As the laser line is moved across the surface of the object the reflected laser is received by optics in the scan probe and, depending on the type of scanner, a variety of techniques are used to determine the location of the laser beam as it scans an object, and a 3-D point cloud is created. Laser scanners are fast, accurate, and can collect thousands of points a second making them ideal for free-form shapes. A variety of single-line laser probes have long been available for retrofit on existing CMMs and do a wonderful job at defining this complex geometry. Unfortunately, these probes struggle when trying to measure critical features such as circles and slots, and ultimately require a tool change from a laser probe back to a conventional touch probe. With a multistripe cross-scanner probe, the time-consuming issue of switching back and forth between probes (and often, between software packages) is eliminated. A cross scanner offers a one-scanner-fits-all solution to quickly digitize complex surfaces and to accurately measure critical features.

Figure 1 illustrates the typical point distribution across a circular feature with a single-stripe scanner and a multiple-stripe cross scanner. The point distribution collected by the single-stripe scanner would lead to only six points being used to determine the feature’s size and location, whereas the point distribution collected by the cross scanner leads to 18 points being used to calculate the same values. This results in greater accuracy and reliability for this and any other given measurement.

Figure 1: Single-stripe vs.cross scanner on a circular feature

Another dramatic advantage of a cross scanner is that the assembly of the internal optics of the probe has been optimized for scanning deep into holes and pockets. This translates to impressive reductions in cycle time and set-up time, and extraordinary improvements in accuracy. Features such as circles and slots can be measured in a single scan path, thereby eliminating the need for multiple scanner positions, and thus cutting the cycle time to inspect a part. Multiple scanner positions are not necessary to measure these features so the set-up procedure for a cross scanner is greatly simplified because only a single probe articulation must be qualified prior to measurement, rather than three to four different positions. Finally, and most remarkable, is the inherent accuracy improvement the cross scanner offers on feature measurements. By scanning much deeper into pockets than standard single-line scanners, the cross scanner offers a much stronger accuracy correlation to conventional touch-probe measurements than any single-line scanner on the market today. Figure 2 offers a 3-D view of a scan across two features and the depth that the scanner is capable of collecting data. By scanning into the depth of a feature, the resulting calculation becomes more accurate because fillets and chamfers can be ignored as part of the calculation. A typical “aerial view” of a feature would include all such entities and lead to skewing the accuracy of the size and position by including extraneous data in the calculation.

Figure 2: A cross scanner is capable of scanning into holes.

Cross scanners offer a host of benefits in addition to eliminating the need to swap between a laser probe and a touch probe. For large surfaces that lack critical features, such as automotive fenders and jet engine nacelles, a cross scanner can run in “single-stripe” mode, essentially tripling the coverage area of a single scan. The next generation of digital cross scanners offers more than four times greater scan coverage when running in single-stripe mode vs. old, analog single-line scanners, thanks to greatly improved data processing capabilities and a larger field of view. Figure 3 illustrates the expected scan coverage area of a single-stripe, analog laser scanner that typically collects stripes of data at a rate of 25–30 lines per second. Compared with a digital cross scanner that collects stripes at a rate of 75–80 lines per second, the speed improvement is magnified by running the cross scanner in single-stripe mode and the 15 mm larger stripe width offered by the next generation scanner.

Figure 3: Single-stripe vs. cross scanner in single-stripe mode

With government-mandated fuel efficiency standards constantly increasing, the way cars are assembled takes on a greater level of significance. Gap and flush inspection between assembled automotive doors and bodies is a critical factor in allowing engineers to model a car’s aerodynamic performance and, in turn, determine its fuel efficiency. Thanks to ideal distribution of the laser stripes, a cross scanner is the best-suited probe for automotive gap and flush inspection. With multiple laser stripes, no reorientation of the probe is required to measure this complex and crucial geometry. A single scan path can be defined to follow the 3-D curvature of the gap, and the cross scanner will automatically collect the data. A single laser scanner would require post-processing multiple smaller data sets. These data sets from a single laser scanner take a longer amount of time to collect and a longer amount of time to analyze. See figure 4 below.

Figure 4: Single-stripe vs. cross scanner making a gap measurement

A variant of the cross scanner is a cross scanner with a long standoff. A scanner’s standoff is defined by the distance between the scanning probe itself and the scanner’s field of view. Most scanners in the market today offer standoff distances that vary 3–5 in. (75–125 mm). Scanners with standoff distances of 7–10 in. (175–250 mm) are classified as long-standoff scanners. These scanners offer greater versatility when measuring parts that have features that would otherwise obstruct a scanner with a shorter standoff distance or when maneuvering around clamps and other fixtures required to hold the part in a constrained position. A long-standoff cross scanner offers the best of both worlds. It combines the maneuverability of a larger standoff distance, and the dense and thorough data collection ability that is offered by using multiple scanning stripes.

Figure 5: Cross scanner with long standoff.

 

Industry leaders such as Boeing Co., Honda, Lockheed Martin, and Volvo are already realizing the improvements offered by cross scanners. The laser scanning market has come a long way from the old single-stripe, standard offset days when scanning anything beyond curved surfaces required switching to a touch probe. The fact that cross scanners offer the advantage of accurately measuring prismatic features, thus eliminating the need to swap between probes is only enhanced by being able to choose the type of cross scanner to meet your standoff distance needs. While the laser scanner market continues to evolve, the new breed of cross scanners proves that the future is now.

 

Figure 6: Cross scanner measuring a curved part.

 

 

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About The Author

Alex Lucas’s picture

Alex Lucas

Alex Lucas is the sales development manager for scanning products at Nikon Metrology. Lucas’s responsibilities include meeting with potential customers to better understand their inspection processes, and advising how incorporating laser scanning technology can increase inspection throughput and improve their bottom line. With a degree in mechanical engineering from the University of Michigan, Ann Arbor. Lucas possesses the knowledge to offer practical hardware and software to meet any inspection challenge.