by David A. White

Government-funded research has spawned a new technology that has broad application in large-scale, high- precision, 3-D metrology. The technology is called coherent laser radar. The measurement system emerged after 10 years of demanding requirements and extensive use in the Department of Defense, Department of Transportation, Department of Energy, NASA, Boeing and privately funded research. More than $25 million has been invested in research and development. Today, only MetricVision, a subsidiary of the Thermo Electron Corp., has patent rights to this technology.

Why noncontact is significant

Coherent laser radar has practical benefits as an automation tool offering portable, noncontact precision 3-D measurement for the lab and the factory floor. Noncontact measurement that uses coherent laser radar speeds manufacturing, improves quality and lowers manufacturing costs while eliminating the need for photo-grammetry dots, laser tracker spherically mounted retroflectors or hand-held probes. In addition, it doesn't have to calculate offsets, because measurements are taken directly from part surfaces.

MetricVision uses coherent laser radar to directly measure surfaces and points or scan features. Software is used to analyze and compare the relationships of geometric entities or surface deformation from nominal shapes at high speed and with great accuracy. The measurement is achieved without mechanical or optical probe contact of any kind. MetricVision's Model 100B can be used to accurately align large parts during assembly, certify tooling and then monitor its repeatability during production, and measure metal, plastic, and composite parts and compare them to their computer-aided design models. It can also scan complex geometry that was impossible to scan before because it was too large, too hard to reach, too complex, too delicate or too labor-intensive. The system requires one operator to set it up, and then it runs unattended. It requires no special environment or expensive tooling. The system works indoors or out, in any lighting, and on any surface with a reflectivity of 1 percent or more.

How the technology works

The instrument directs a focused laser beam to a point on the piece to be meas-ured and recaptures a portion of the reflected light. The single large-aperture optical path maximizes signal strength and stability. As the laser light travels to and from the target, it also travels through a reference path of calibrated optical fiber in an environmentally controlled module. Heterodyne detection of the return optical signal mixed coherently with the reference signal produces the most sensitive radar possible. Extensive signal processing extracts the range-dependent signal frequency with great accuracy (see Figure 1). Measurement of the signal intensity and signal-to-noise ratio provides a built-in check that confirms measurement quality. The two paths are combined to determine the absolute range to the point.

Figure 1: Optical Frequency

MetricVision measures a distance and two angles to determine a point on a surface in space. This instrumentation uses a broadband frequency-modulated infrared laser (100 GHz modulation) that provides a robust, eye-safe signal. The enormous laser modulation bandwidth makes precise measurement possible in a millisecond. The up-sweep and down-sweep comparison provides simultaneous range and velocity data for measurements. Velocity data is important when it's necessary to meas-ure the dynamic response of a structure, i.e., vibration analysis.

Precise beam steering is delivered through a two-axis gimbal-mounted scan mirror that steers the laser radar's meas-urement beam through 360 degrees azimuth and 120 degrees elevation. Precision Heidenhain encoders provide 1/3 arc second resolution on the azimuth and elevation axes. Class 9 bearings and symmetric construction provide +/-1 arc second overall pointing accuracy, repeatable to ± 0.36 arc seconds (± 7 mm perpendicular to the beam at 10 m). The system measures range with interferometer-type accuracy (± 25 mm at 10 m, 1s).

Portable system components

A single sensor mounted to an instrument stand and a mobile workstation that integrates a computer with the laser radar make up the system. The workstation (cart) has two video monitors; one displays application software while the other shows a color video image of the area to be measured.

The stand-mounted instrument and the cart each occupy less than a square meter of floor space, making the system easy to move. The entire system is designed to be transported in a small truck or minivan. The robust scanner head is 40 kg. It integrates precision optics and mechanical engineering with electronics and software. The workstation weighs 190 kg and provides an ergonomic work area from which to control operations and direct the lasers. The eye-safe laser radar is Class I, and the optional red laser pointer is Class II. The cart also houses an industrial computer and modules to operate the scanner and coherent laser radar. The instrument is powered by 110 volts AC but incorporates a UPS to allow for international voltages from 80 to 240 VAC. The MetriVision100B communicates over 10/100 Base-T Ethernet. Its open interface "CLRIC" enables direct software control for embedded applications.

To support a host of different application software, a high-performance Pentium PC and peripherals (including a color printer) are part of the system. Data can be imported into leading third-party software to meet a broad range of inspection, quality assessment, reverse engineering and other special application needs. Software is available inter- nationally from many leading manufacturers, including Silicon Graphics (Alias Wavefront's EvalViewer), New River Kinematics (Spatial Analyzer), Imageware (Surfacer, Inspect It, Build It, Verdict), Perceptron (ScanWorks), Verisurf (all packages), Metris and Paraform.

Typical applications

Coherent laser radar can be used in a broad range of applications. Comparison to CAD, for example, is a very strong, much-requested feature. Coherent laser radar can rapidly sample as-built surfaces directly to an IGES or CATIA model and sample over extremely large areas in a contiguous coordinate system. This ability to accurately relocate the laser scanner in a single contiguous coordinate system eliminates the need to reassemble data clouds after the fact. Some of coherent laser radar's typical applications include:

 Model digitalization, including scanning artistic models, performing design lay-ups and reading data directly into a wide variety of CAD packages. The operator can gather data in uniform scans and control the scan density and area.

 Tool building and alignment, including locating and adjusting tool features in real time. The system can be driven to a specific point in space and continuously measure a part until it is positioned at its nominal location.

 Tool digitalization and documentation of as-built tools and die surfaces. The system can measure tools and surfaces for wear and monitor fiducial monuments and surfaces for tolerance and stability.

 In-process applications, such as aligning aircraft and automotive components and supporting robotic positioning. Coherent laser radar can correct for orientation, gap, flush and fit conditions, and can monitor tool and fixture stability during use so that data does not need to be collected manually. Several manufacturing workstations can be monitored automatically by a single machine without the need for operator intervention.

 Quality assurance applications, including comparing CAD to as-built parts and performing first-article inspections, incoming quality assurance, in-process quality assurance and outgoing quality assurance. Because the instrument can scan a part surface directly, the laser scanner is particularly well-suited to measuring tight pockets, small holes and otherwise inaccessible areas.

 Routine maintenance, such as performing static and dynamic inspections of aircraft, automotive and heavy-equipment tooling assemblies; monitoring deformation of building, tunnel and bridge structures without having to delay traffic for long periods, especially after seismic activity; and accurately measuring surface fractures that can provide early warning to possible fatigue and structural failure.

Comparing current technologies

Every tool in the metrology toolbox has value. Further, some visually acute operators have demonstrated better-than-average accuracy with older technology like theodolites. But new instruments can consistently demonstrate the highest accuracies. Figure 2 is an overview of qualitative comparisons that highlight the strengths of coherent laser radar to put this new technology in perspective.

Figure 2: Product Comparisons

The MV-100B is particularly well-suited to unattended checks that don't require the use of stick-on or hand-held cooperative targets. Once a measurement plan has been defined, the MetricVision scanner can monitor a sequence of events automatically. This is particularly valuable when large structures such as airframes are inspected on a routine basis for deformation against previous measurements or CAD models.

Future manufacturing

Manufacturing paradigms will change when 3-D vision is used broadly in automation. Current 2-D vision technology has been used extensively in high-speed, high-volume manufacturing applications with great success, but current 2-D vision systems rely on pattern recognition and the presence or absence of features. Their accuracy is only as good as their pixel density. Previous 3-D sensors were only close-range, small-area sensors. In the future, manufacturing lines will use 3-D vision sensors with the ability to measure the critical characteristics of features easily and quickly, without contact. New 3-D sensors don't need to be very close to the features of interest, nor do these sensors need to touch the objects. MetricVision sensors can use mirrors to gain a line of sight to hidden features on opposite sides of an object, and MetricVision records data in the current coordinate system.

Already, major manufacturers of aircraft, heavy machines and large automotive parts are investigating the integration of laser radar technology into their manufacturing processes. They don't want to wait until parts fail dimensional inspection to take corrective action. By embedding the laser radar technology into critical processes, they can eliminate scrap and increase production speed in a range of manufacturing areas.

About the author

David A. White worked for many years for DuPont in Delaware and later as a design and marketing consultant in Pennsylvania. As marketing director for SMX, he contributed to the broad acceptance of laser trackers worldwide. He has returned to consulting in Pennsylvania and is committed to the success of clients like MetricVision. Contact him by telephone at (610) 274-8674 or via e-mail at dwhite@qualitydigest.com . Visit MetricVision at its Web site at www.metricvision.com or call (703) 550-2945.