Laser Scanners Demand Attention as Technology Improves
Manufacturing Engineering Magazine October 2019
By Bruce Morey
Laser line scanners are starting to be used across the board, according to Joel Martin, laser tracker product manager for Hexagon Manufacturing Intelligence, North Kingstown, R. I. New users are finding the main attractions of laser scanners—speed and ease of use. “You wave them over the part. You end up with a point cloud. Everybody is happy,” said Martin. What prevented more widespread use in the past were laser scanners’ perceived tradeoffs. Using one usually meant sacrificing accuracy or working with noisy data. There were limitations in materials as well, with lasers having difficulty with some surfaces that were shiny or had strong contrasts.
However, those old perceptions are becoming misperceptions under the onslaught of technology advances.
Improvements include optics, computing power, software and algorithms as well as the quality of lasers today, such as low-speckle, blue-light lasers, according to Martin. Noise and accuracy have improved. Even speed, their main attraction, has improved as well, with collection rates measured in the millions of points per second in some high-end models. New models are lightweight and easy to handle, often weighing a pound or less. New uses range across aerospace, automotive, heavy industry, and construction equipment industries.
“If the last time you looked at a laser scanner was only two years ago, look again,” said Martin. “The technology is moving so fast that what you saw [then] is already better.”
New Capabilities, Applications
Each category of improvement—more speed, less noise and more surfaces—has opened new applications. “For example, you are now seeing laser line scanners in applications where low noise is really critical,” said Martin.
One such application now ripe for laser line scanners is reverse engineering or part-to-CAD, measuring parts produced in the days of paper drawings that need 3D CAD models today for retrofits and improvements, according to Martin. This application is especially sensitive to higher levels of noise and new lasers are up to the task.
As easy as laser line scanners are to use, their performance specifications can still be tailored to tune them for specific applications. Higher accuracy over large areas is achieved by combining a portable laser scanner with an interferometry laser tracker picking up a reflector on the scanner. If line-of-sight is maintained to the tracker device, large areas like car bodies or aircraft structures are easily measured quite accurately, to within 50-100 µm, building complete data sets rapidly. Hexagon is also a supplier of laser tracker devices and offers complete systems of trackers and scanners.
There are tradeoffs between collection width and resolution of the point cloud. “If you are trying to collect data for an airplane wing to reverse engineer it, you don’t want to use a scanner with a 3″ (76.2-mm) wide scan line, that would be like painting your house with a 3″ brush,” explained Martin. “So, for example, for bigger parts like airplane wings, we offer the LAS XL with a 2′ (0.6-m) scan line and a 3′ (0.9-m) depth of field, combined with a laser tracker for higher accuracy.” Hexagon now has identified so many different applications that it offers ten different laser scanners with tuned performance specs.
The sum of the improvements has also made them simpler to use, even for novices. That means no more futzing with settings to get an optimal collection. The newest systems automatically adjust as easily as a camera in a smartphone. In fact, that is no coincidence.
“The vast improvements in the commercial market for collection optics and detectors has helped us” in the industrial market, said Martin. He pointed out the features of the company’s latest laser line scanner offering, the RS6, take advantage of newly available technologies. Designed to be used with its Absolute Arm seven-axis PCCM, Hexagon developed an algorithm called SHINE (Systematic High-Intelligence Noise Elimination), which uses the high refresh rates of today’s image sensors and fast computing to combine multiple exposures of the laser reflection for a higher quality and more accurate measurement, according to the company.
“It is similar to the HDR function in a cell phone camera, combining a high-exposure and low-exposure image for a better collection,” he said, “but without the image duplication that HDR normally brings. For a laser scanner, that’s important because it means that the frame rate never slows down on hard-to-scan surfaces, unlike other laser scanners.”
The RS6 combines the best of metrology by including a touch probe along with its non-contact laser scanner. Enhancing usability, it weighs a mere 14 oz (400 g). The combined RS6 and Absolute Arm accuracy is certified according to ISO 10360-8 Annex D to provide values as low as 41 µm, depending on the length of the arm. “It is important to state the accuracy of the combined arm and sensor,” explained Martin. “Not each by itself.”
Good for Complex Parts
Matthew Gibbons, applications manager for Nikon Metrology Inc., Brighton, Mich., observed that the best applications for laser line scanning are parts with contours and compound shapes, such as stamped sheet metal, injected plastics, and castings.
“With castings, the tolerances are more open, making them especially good for lasers,” he said. “Soft parts are also ideal, such as car interiors and rubber components, where a touch probe may deform the part. The laser will not impact the measurement.” He agreed that advances in laser technology opened many more of these applications as speed increased, accuracy improved, and advances in discrimination meant more materials could be measured, such as black or glossy surfaces.
Nikon offers three different models of handheld scanners, each aimed at different applications. Following improvements in the field, Nikon developed its Enhanced Sensor Performance (ESP3) algorithm for use on its KScan and MMDx handheld scanners.
An improved ESP4 is used in its H120. ESP offers real-time dynamic adjustment of the laser intensity for every point, according to the company. It allows for scanning across parts with strong color transitions and varying reflectivity, and requires no user interaction, making it easy to use. Intelligent reflection control is another feature, and is used to measure shiny or polished materials.
While the MMDx has a frame rate of 150 Hz, the H120 boasts 450 Hz (as well as a low-speckle blue laser source.) Both the MMDx and H120 are fitted with a touch probe for greater flexibility.
“The H120 collects up to 450,000 points per second, and the blue-light laser definitely helps reduce noise and increase accuracy,” said Gibbons. He also pointed to the optics available from Nikon, one of the prime providers of commercial-grade lenses for cameras. “This means we can scan very fine details, like the edges of sheet metal parts,” he said.
Gibbons also believes that most potential users are comfortable with the technology. It has lost its novelty, and most are well informed of its capabilities. “Today, most customers want to replicate [existing systems] exactly, [such as] what they do with their CMM with a touch probe,” he said. “In a lot of cases, we can achieve that.” Not always, though. That includes line-of-sight issues, where getting deep into small openings might be difficult compared to specialty touch probes. Clear parts also remain a problem.
For accuracy specifications, Nikon also reports the accuracy of a combined arm and sensor in a defined procedure. For example, Gibbons said that the H120 fitted to an MCAx25+ portable arm has an accuracy of 32 µm to 2 σ, meaning at least 95 percent of the measured points will be within the 32 µm specification.
He agreed with others that future developments in laser scanners will follow existing trends, including faster collections, working with an even greater assortment of challenging surfaces and better noise suppression. New applications for laser line scanning include those that are new themselves, like the emerging additive manufacturing world.
“We are seeing a lot of interest in laser scanning of additive manufactured parts now that these are going straight into production, into manufacturing and assembly,” said Gibbons. Such parts have complex surfaces and laser scanning seems ideal to capture them for measurement.
Versatility for Process Control
According to Nadir Shah, director of engineering for Automated Precision Inc. (API) Rockville, Md., scanners offer economy of efforts, resources, and time. “Scanners can measure any contour, any shape, [quickly], wherever the part is, including in the production line,” he said.
Shah agreed that the quality of scanning has improved to the point where even parts with rough surfaces are now measured accurately with laser scanners. Not only have optics, lasers, and processing improved, but engineers have innovated with architectures and configurations. For example, there are now laser line scanners that use multiple lines for better angle coverage. “They also vary the width and distance, along with multiple projection lines” to improve accuracy and reduce noise, he said. Noise rejection algorithms have improved as well.
“Most automotive OEMs or parts manufacturers are implementing line scanner and the area scanner [structured light] solutions inline, where the parts are being made and inspected 100 percent,” Shah said. He also noted that they are moving beyond simply using it for in-line quality, rejecting parts as they are made, and are using scanner data for process control. Some, for example, are using scanned data to ask if welding robots are in the right place and other process questions. This kind of rapid, in-line inspection is difficult to do with the tried-and-true CMM because of the need for expensive fixturing and the speed of the device itself, according to Shah.
He too noted that as scanners have improved, new applications are available, especially in aerospace where more composites are used in mixed-material parts. “A door of an aircraft will have aluminum, maybe some other alloys, maybe some composites. We have a customer using one of our scanners that is looking at a large assembly that is 80 percent composites, 15 percent aluminum, and the rest steel and other alloys,” he explained. With one of API’s scanners, the customer is now doing 100 percent inspection on this mixed material part, with a single set-up.
“Before they used scanners, they did not do 100 percent inspection, instead relying on checking for quality issues when they were assembling the aircraft,” he said. That is late in the process to correct errors. Scanning data for process control is only going to grow, according to Shah.
API offers two versions of laser scanners. One is a line scanner called the iScan that works with API’s Radian laser tracker for improved accuracy. The other is a 3D scanner designed for large-scale scanning and architectural work, like Building Information Modeling (BIM). The iScan is offered as an option with its Radian line of laser trackers, weighs only 1.2 kg, and scans at speeds up to 100,000 points per second. It scans both reflective and dark surfaces and has a swivel head that provides 360º of yaw and roll for “infinite sensor positioning,” according to API. Its 2 σ accuracy is due to its pairing with the Radian laser tracker and is better at closer range. Advertised accuracies are 50 µm at 7 m and 80 µm at 15 m standoff.