Here’s a look at the development path of the world’s first direct scanning laser tracker. Article by Joel Martin, Hexagon Geosystems.
Manufacturing innovations have often been the driving force behind new developments in the field of metrology—the science of measurement. New combinations of hardware and software are allowing engineers to solve problems in new ways that simply weren’t possible before.
In the late 1990s, technological advancements delivered a new device known as the laser tracker, which has gone on to establish itself as a worldwide standard for large-scale alignment and verification tasks. A laser tracker is a portable coordinate measurement machine (PCMM) that uses a laser beam to accurately measure and inspect the features of an object in 3D space. This beam is sent to a spherically mounted retro-reflector touching the object to measure two angles and a distance, thus calculating its position and defining it with an X, Y, Z coordinate.
Laser trackers were quick to find their home in large-scale manufacturing, largely because no other measurement solution could accomplish such tasks. They allowed engineers to perform wing-to-body alignments or even tooling verification faster and more accurately than ever before. But the first generation of laser trackers had their own special issues, such as when line of sight between the laser tracker and the reflector was interrupted and the operator would have to walk the steel sphere back to a home position to pick up the laser beam from the tracker.
This limitation reduced operator efficiency, and consequently cost money, especially if the reflector was being tracked from a distance of some 20m away. While workarounds were available, it was not uncommon to see the connection interrupted repeatedly if there were physical obstacles in the work area such a workers or cables.
The solution to this issue was first provided by Hexagon in 20XX when the PowerLock feature was first introduced to their Absolute Tracker range of laser trackers. However, laser trackers still required the skilled hand of a well-trained operator to deliver reliable results.
A Breakthrough Driven by Automotive
The next great development in the history of laser tracker systems came after a major automotive OEM challenged several metrology leaders to design a system that could track a handheld device capable of non-contact scanning a surface around an area the size of a car with tracker-like accuracy.
Although it wasn’t immediately met, this challenge was behind the introduction of the first large-volume wireless probe, which worked like a “walk around CMM” by allowing the operator to use its common stylus to measure a part in a way similar to using a CMM or portable measuring arm.
This breakthrough was made possible by the introduction of a new type of laser tracker that, rather than simple 3D measurement, could measure with “six degrees of freedom”. These “6DoF” laser trackers, the first of which was the landmark Leica Absolute Tracker AT901, were capable of measuring not just a single point, but an orientation around that point about a full six axes.
Most importantly, from a productivity standpoint, this new device allowed the measurement of hidden points within recesses, or simply points on the back side of the measurement object, without repositioning the laser tracker.
Early benchmarks showed that this new probing capability could provide an increase in throughput of up to 80 percent over traditional reflector measurement. This technology created such a dramatic shift in the way objects were measured that the reflector—the very tool that had until now been key to the functionality of the laser tracker—ended up being used far less often for measurement tasks.
The idea of surface digitisation with a laser tracker is nothing new; an operator in 1995 could be seen dragging a reflector over the surface of an aerostructure to create a simple point cloud. But the introduction of the 6DoF tracker opened up the possibility to take this a giant leap further.
But laser tracker based large-volume scanning has accelerated over the past six years. An example is a laser scanner with extreme speed that is tracked by a laser tracker and attached to a commercial of-the-shelf robot. This scanner-tracker integration effectively turns a standard robot into a very accurate shop floor measuring machine.
This fundamental shift in measuring from physically touching a part to measure it to “just scanning it” has allowed manufacturers to completely rethink their metrology workflows and equipment.
At around the same time that 6DoF probing and scanning was changing the workflows and typical applications of laser trackers, 3D terrestrial laser scanning was beginning to find its first applications in large scale manufacturing. This high-speed LIDAR scanning technology was originally deployed for geospatial land surveying, allowing an operator to collect millions of points very quickly in the course of capturing the surface of buildings or the surrounding landscape.
On the other end of the spectrum, there are handheld scanners with an ultra large stand-off area of up to three feet with a scan line of over two feet wide that captures huge amounts of data very rapidly. Other contemporary scanners allow the operator to measure objects the size of an average car from a single station (position) in less than 30 minutes. The need to scan very large objects quickly with metrology-grade accuracies has driven different manufacturers to integrate their laser trackers to several different scanners. In addition to the hand scanners described above, there are also examples of structured light scanners located by laser tracker, as well as terrestrial laser scanners using laser trackers to control their global accuracies.
The Industrialisation of Terrestrial Measurement
Laser trackers have the inherent ability to hold very tight tolerances over very large distances. This important feature renders the marriage of laser trackers and terrestrial laser scanners as a natural progression. Terrestrial laser scanners can measure millions of points very quickly, but it can be a challenge to register these point clouds together while maintaining metrology grade accuracies. It is exactly this need that lead the industry to another advancement in laser tracker technology—a scanning absolute distance meter that pushes laser trackers into the next level of usability. A scanning ADM that measures at an internal rate of over one million points per second is now integrated in a new line of laser trackers. The technology can register submillimetre noncontact surface scans with metrology grade SMR laser tracker measurements—all within a single battery powered IP54 sensor for factory floor usage or remote outdoor applications. This new product line effectively bridges the gap between laser trackers and lidar scanners.
Looking to the Future
Manufacturing has changed dramatically since that aerospace engineer was tasked with aligning the wings to the fuselage of the 747 more than 50 years ago. The modern airplanes replacing this legendary gem require an increasing amount of data-driven processes with an even higher level of precision was achievable before. In the past, some level of misalignment in the aerostructure could simply be “trimmed out” during flight testing, but today that equates to inefficiencies of the aircraft. To reach the fuel efficiency requirements of the burgeoning aerospace industry, new inspection processes and technology must continue to advance.
I have been involved with laser trackers since the early days and witnessed the evolution of this solution as it has grown and matured at a consistent rate. It has been amazing to watch some of the smartest minds in metrology push the power and usage envelope on this technology, considering its humble roots. Today, laser trackers are utilized in almost every type of large-scale manufacturing from aerospace to power generation. The emerging trend towards noncontact scanning is pioneering another giant leap for a technology that seems to have no limits.
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