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# Seeking the Finer LightPUBLIC ACCESS

The Newest Laser-based Sensing Devices Strive to Improve Processes from Construction Engineering Management to Steelmaking.

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Mechanical Engineering 121(05), 50-53 (May 01, 1999) (4 pages) doi:10.1115/1.1999-MAY-1

## Abstract

Manufacturers, engineers, architects, and researchers are using the precision, speed, and noncontact processes and achieve more control. Riegl Laser Systems of Horn, Austria, designed its 3-D LMS-Z210 imaging scanner to generate 240,000 laser measurement points within 30 seconds to create highly accurate images of complex structures without the need for conventional surveying. The National Institute of Standards and Technology in Gaithersburg, Maryland, is designing a laser-based system to track the myriad of tools, materials, and equipment on construction sites. Riegl Laser Measurement designed its LMS-Z210 3-D Imaging Scanner to serve in three-dimensional measurement applications that are too complex for traditional theodolites to survey, including shipyards, quarries, vineyards, and crime scenes. Riegl USA expects to begin several projects using the 3-D scanner to continuously monitor bulk material piles in the wood and corn processing industries to improve process management and control.

## Article

Manufacturer's, engineers, architects, and researchers are using the precision, speed, and noncontact properties of lase r beam sensors to improve processes and achieve more control. This is most notable in three laser sensor devices launch ed in the pas t eight months, and in a system still being developed. They incorporate the latest developments in laser technology to enhance their capabilities.

Riegl Laser Systems of Horn, Austria, designed its 3-D LMS-Z210 imaging scanner to generate 240,000 laser measurement points within 30 seconds to create highly accurate images of complex structures without the need for conventional surveying. Engineers at TSI Inc. in St. Paul, Minn., developed the Laservec diode velocimeter to take flow measurements more easily and conveniently than with costlier and bulkier research laser velocimeters.

Honeywell 's Micro Switch division in Freeport, Ill., introduced its convergent beam SVP2 Smart Value photoelectric sensors to provide a small er, more precise light beam to serve the booming electronic assembly market.

The National Institute of Standards and Technology in Gaith ersburg, Md., is designing a laser-based system to track the myriad of tools, mater ials, and equipment on construction sites.

At the dawn of the 21st century, most architects, builders, and engineers continue to rely on the traditional theodolite to survey objects and scenes. Today's theodolites consist of a laser rangefinder mounted on a tilt-and-pan platform that uses two axis encoders to provide range, azimuth, and elevation measurements. Although the theodolites yield accurate results, they are manually operated and relatively slow. In addition, the complexity of large-scale subjects can exceed the capabilities of the theodolite.

Riegl Laser Measurement designed its LMS-Z210 3-D lnnging Scanner to serve in three-dimensional measurement applications that are too complex for traditional theodolites to survey, including shipyards, quarries, vineyards, and crime scenes. Riegl introduced the scanner commercially in Europe last October, and in the United States this past February, through Riegl USA, a subsidiary based in Orlando, Fla.

The new 3-D scanner determines distance by emitting pulses of laser light to a surface along two axes over any field of view, and measuring the time it takes for the energy to return to a sensor. By generating nearly 8,000 such time-of-flight measurements per second, the scanner creates a high-fidelity, three-dimension al point cloud to represent the scene. This point cloud can be processed further to yield more traditional data formats by using standard computer-aided-design software packages, such as VR.ML or DXF.

The LMS-Z210 scanner is tripod-mounted to face the target object. When the device is activated, an electronic pulse generator periodically drives a semiconductor laser diode that emits the transmit beam. A rotating, three-sided mirror directs the range-finder beam over an 80-degree vertical line scan axis. The entire mirror assembly is rotated about the orthogonal frame axis by a high-precision stepper motor to provide a sharply defined 80 x 340- degree horizontal scan over the scene. The company claims the process yields measurement accuracy to within 25 mm at peak measurement rates of 20,000 Hz.

A portion of the outgoing light energy is reflected back from the target surface to the scanner's range-finder receiver lens. The lens focuses the energy to a photo diode receiver to generate an electric signal. A quartz-stabilized clock frequency measures the time interval between the transmitted and received laser pulse. An internal, eightbit parallel data output microprocessor converts the data for display and output as an image on a personal computer or laptop. Equipping a PC with Riegl-Scan software provides data acquisition and real-time display.

The scanner provides surveying data that include range, bearing angle, inclination angle, and the distance between any two points. In addition, the laser scanner is able to color code its images because it can measure the intensity of the reflected optical signal. For example, black means that no laser measurement could be performed because of low reflectivity, signifying that the objects are close to the ground, while blue means images are more than 77 meters away. Operators can adjust range limits, varying from 7 mm to 70 meters, to improve the resolution of color coding.

Bouillon Inc. of Seattle used a single Riegl scanner to measure the dimensions of ship hulls in dry dock. This task previously required the installation of a vast array of reflectors over a ship 's hull to enable measurements to be taken by a theodolite, in what was ordinarily a six-week process. The LMS-Z210 did the job for an estimated cost of$5,000, a fraction of the previous cost of$300,000.

Ferrotron, a German provider of measurement equipment to the steel industry, used an LMS-Z210 scanner to measure the interior dimensions of vessels that transport molten steel. This enables the steel manufacturer to maximize the service life of each vessel at a significant cost saving. Units also have been sold to clients in Austria, Spain, Israel, Australia, and Japan.

Riegl USA expects to begin several projects using the 3-D scanner to continuously monitor bulk material piles in the wood and corn processing industries to improve process management and control. Reality Capture Technologies Inc. in San Fran cisco, a group of engineers working under contract for the National Aeronautics and Space Administration, used the 3-D scanner to capture virtual reality scenes of NASA's Mars Lander vehicle. The scanner eventually may be used to replace the stereoscopic systems currently installed on the Lander.

There are also signs that the LMS-Z210 scanner may be going Hollywood, because an emerging entertainment market for the 3-D scanner is acquiring scenes for the virtual reality program used in making movies.

NIST researchers created this image of their agency's National Construction Automation Testbed in Gaithersburg, Md., by means of a laser mapping system they designed to keep track of tools, materials, and equipment on construction sites in real-time measurement.

TSI's laservec diode velocimeter captures laser light scattered by particles at a point where two laser beams cross to measure the airflow in a wind tunnel at the University of Minnesota .

## One Head is Better than Two

Engineers at TSI Inc., in St. Paul, Minn., combined the laser transmitter and receiver in a single optical head on their Laservec Diode Velocimeter, introduced last September. Like similar devices, the Laservec uses the principle of laser Doppler velocimetry to measure the flow of solids, gases, or liquids. "Briefly, this involves crossing two laser beams to measure velocity at the area they cross," explained Rajan Menon, an aerospace engineer and marketing engineer at TSI.

The Laservec is aimed at the flow diagnostic applications in chemical and pharmaceutical processing, and the electronic cooling and testing in wind and water tunnels that are often served by argon ion laser velocimeters. The latter devices produce the tri-colored beams-green, blue, and violet- needed to differentiate the three axes of velocity, namely up and down, in and out, and side to side.

Argon ion lasers are typically large and expensive instruments that often require a special power source and water-cooling. TSI designed the Laservec, at 56 cm long, to be less than half the size and price (it costs 37,000) of argon ion laser velocimeters, and to plug into standard plant electrical grids. A diode laser in the Laservec's 27-cm-long optical head emits a 50-mW beam possessing a 690-nm wavelength, at the bottom of the red/infrared visible range. The beam is split in two, with one part sent through a Bragg cell that enables the system to measure positive and negative flow direction. A lens focuses both beams onto the target flow, and the region crossed by the beams, ranging from 0.2 to 0.3 mm, is the area of measurement. Micron-size particles moving through the flow, either ambient or introduced by the operator, scatter the laser light, creating miniature bursts of light. The receiving lens of the Laservec collects the scattered light and amplifies it. This signal is processed, and the results are transferred to a computer via a TSI interface card, to get velocity maps of the flow field. The Laservec signal converter uses fast Fourier technique to differentiate true light bursts from background noise to ensure accurate velocity measurements. "Simply put, the Laservec focuses on the brightest region of the measurement area to take measurements as accurate as a fraction of a millimeter per velocity," Menon said. TSI engineers kept the Laservec compact by means of its optics and the single head design, which made for a smaller package. They also addressed the problem of the diode drifting from its calibration over time by using a path length matching technique. "This involved using hardware to keep the distance the two laser beams must travel the same, to cancel out drift," said Menon. A European truck manufacturer is using the Laservec to measure airflow inside a scale model of a refrigerated cargo area. The aim is to ensure good circulation and hence reasonably uniform low temperature. The need to measure in spaces between boxes led the manufacturer to use noninvasive, laser-based, velocity-measuring techniques. In addition to this, there is also an interest in measuring the airflow around the truck scale models, to design vehicles with reduced wind resistance and to reduce noise caused by air turbulence. Previously, the truck company's researchers mounted mechanic al probes on their scale models to measure airflow. This approach involved mounting multiple probes, or continually remounting probes to take flow measurements at different points on the model. Both methods were labor- intensive practices. There were other drawbacks in using mechanical probes. If the probes extended too far from the surface of the truck model, the vibration caused by the flow could affect the accuracy of measurements. On the other hand, running the probes nearly flush to the model surface is complicated. In either case, the probes had to be recalibrated to compensate for changes in temperature or humidity. Today, the truck manufacturer's research and development team aims the Laservec at any point of the scale models under study, without concerns about temperature, humidity, or the complications of mounted probes. The National Institute of Standards and Technology's laser tracking system may eventually contribute real-time data, as shown on the left, while structures are being built. ## Precision Assembly Honeywell 's Micro Switch division in Freeport, Ill., introduced its convergent beam SVP2 Smart Value photo electric sensors last November to provide a smaller, precise light beam to serve the booming electronic assembly market. According to Steve Dye, photoelectric product manager at the Micro Switch division, "We make a full range of photoelectric sensors for positioning applications." He said Honeywell wanted to use its vertical cavity surface emitting laser, or VCSEL, technology originally developed for telecommunications, to gain leverage in the industrial assembly market. The SVP2's ho using measures 1 % x 2 x 5/ 8 inches. The housing can be screw mounted or bracket mounted to face the assembly it is measuring. At the heart of the sensor is the VCSEL, a silicon wafer etched, cut, and crafted into a chip specifically for emitting a laser beam. An electrical pulse is induced through contact pins, causing the chip to emit light. "This produces a highly collimated beam, so that standard optics can focus very precisely onto the target," said Dye. In addition to its sharp beam pattern, the VCSEL beam is 0.1 mm wide, a tenth the size of many laser diode beams, enabling the SVP2 to measure more precisely. The light contacts the target and is reflected back to the SVP2 receiving lens, which focuses it onto a Honeywell photo diode. The photo diode produces an electrical output proportional to the power of the return beam, rather than measuring time of flight. The electrical signal is sent to an integrated circuit that processes it and compares it to a reference signal level. Based on this processed signal, the sensor output is turned on and off. The signal is sent to a personal computer or a programmable logic controller. Depending on the application and status of the sensor, the PC or PLC will adjust the process or alert operators to do so as needed. Converting a telecommunications technology to small part position measurement was challenging. For example, in telecommunications, the VCSEL was designed to respond in nanoseconds, but in parts assembly, the laser technology's response had to slow to microseconds. The SVP2's optical designers worked closely with the staff members who developed the VCSEL chip to balance the new requirements of the SVP2 with the chip's capabilities. The SVP2 is being used in wafer processing applications, where it determines if silicon wafers are properly aligned for etching. A 300-mm wafer can cost up to10,000. A printer uses the SVP2 to count bundles of medical labels packed so tightly together that the gap between labels cannot be read easily by the naked eye. A manufacturer of metal filters uses the SVP2 to count the hundreds of ribs, each slimmer than a millimeter, in metal filters incorporated by chemical processors.

Honeywell's SVP2 laser sensor measures position to determine whether silicon wafers are properly aligned for etching.

## Where Did I Put that Trowel?

Optics and automation experts continue to extend the application for laser sensors. One promising technology being developed by the National Institute of Standards and Technology is a laser system that will keep track of materials, machines, and tools on construction sites. This project is headed by William C. Stone, leader of the Construction Metrology and Automation Group at NIST, who said the cost of tracking equipment and material that is moved on a daily basis is considerable.

"Several members of my team are members of technical committees for the Construction Industry Institute based in Austin, Texas, an industry/ owner-led consortium of about 100 of the country's largest contractors," Stone said. "At a recent meeting in Houston, where we discussed a typical $100 million oil refinery upgrade, we determined that it cost approximately$500,000 for workers to count materials and keep track of about 10,000 parts."

NIST is using infrared pulsed lidar technology that measures. the time of flight of reflected laser light to calculate distance. "Our research focuses on faster data acquisition rates, higher data density, and wireless communication links that can accommodate the necessary real-time bandwidth," explained Stone.

The lidar instrumentation Stone's team is using emits a beam that contacts a target surface. and captures the reflected light to produce a two-and- one-half dimensional image of the site; that is, the devices obtain information on the X and Y position and the front face range of objects within the imaging instrument's field of view. Line-of-sight restrictions mean the lidars cannot see what is behind a physical object. By scanning, the systems can create a cloud made of up of tens of thousands of data points to form a detailed picture.

"By taking such pictures from many different viewpoints, we can extract important knowledge about the construction site—for example, the cut-and-fill status of landscapers and excavators," Stone said. "In fact, we are developing an autonomous vehicle for the lidar system that can collect this data without the need to have a survey team on the site." In addition to lidars, the NIST researchers are using other sophisticated instruments, including global positioning systems, and a new approach 'developed at NIST known as non-line-of-sight, or NLS, surveying for tracking the location of objects through solid walls. "NLS is based on sending time-synchronized, wideband electromagnetic impulses through the walls to a roving measurement system inside the building," explained Stone.

There are several prototype tracking systems in use, with the field-mobile lidar system, human guided at first, scheduled for deployment this summer. "We will be using that system for a full-scale field test during the excavation and assembly of a \$6 million process plant at NIST," Stone said. "We will be uplinking live status data to the prime building contractor in this case."

According to Stone, commercial applications for the tracking system should open up in the next three to five years. He noted that a pressing need for full commercialization is the development of both field metrology and dynamic construction database communication protocols and standards. To that end, NIST is seeking cooperative research and development partners from the metrology and construction areas to develop database communication protocols and standards.

"These standards will pave the way for third-party software development to leverage the utility of live construction data, much like the computer-aided design data exchange standards being adopted by most software houses," said Stone.

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