"Measuring Across Space and Time," Feature Article, January 2003

measuring across space and time

Large-scale metrology moves GPS in out of the rain.

by Paul Sharke, Associate Editor

Bringing together the sections of an aircraft fuselage takes large, beefy fixtures to move the parts into position while holding their alignment. According to a Boeing mechanical engineer, Jim Cobb, "When assembled, airplanes are rigid enough to fly." In pieces, however, they're really quite flexible, he said.

A typical main assembly process relies on steel structures—with some tools reaching as high as three stories—to hold the cylindrical fuselage barrels in alignment. Aided by linear bearings, the tools join the parts together in a sequence that goes, "Push together, check fit, drill holes, pull apart, clean scrap, reassemble, and rivet," Cobb said.

Arc Second Inc. of Dulles, Va., has conceived a way to reduce some of the expense associated with fixed tooling by bringing the tools and techniques of the Global Positioning System under the roof of an aircraft assembly building.

Cobb, who is employed in Boeing Commercial
Airplanes Manufacturing R&D, has demonstrated the system—dubbed Constellation 3Di by its maker, Arc Second—at several Boeing production sites around the country.

Boeing originally investigated using radio-frequency GPS transmitters throughout its factories, Cobb said. But radio waves wouldn't propagate well inside buildings.

A large assembly, such as the fuselage of this Boeing 777-300ER, relies on heavy tools to keep panels aligned until they're joined. Eventually, indoor GPS may relieve such tools of that responsibility.

Instead, the Constellation 3Di system uses laser light to project what amounts to a three-dimensional coordinate grid onto a factory space. Receivers key onto this grid to determine their locations. Like GPS, the transmitters can serve an unlimited number of receivers.

The system works on the principle of triangulation—using four points of data from two transmitters to calculate x, y, and z coordinates at each receiver. Just as a lighthouse has no idea how many, or which, ships are using its signal, the indoor GPS transmitters remain unaware of the number, or types, of receivers that are homing in on their transmissions.

A setup routine relates the positions of the transmitters to one another. A scaling sequence places two receivers at a known distance to calibrate the transmitters. Then, the system begins relaying azimuth and elevation data to any number of receivers within sight and range of any two transmitters.

Up to a point, the accuracy of positional information improves with additional transmitters, said Arc Second's engineering vice president, Tom Hedges. Two transmitters are necessary to establish a minimally acceptable level of precision. Three or four can reduce the uncertainty of that measurement by 10 or 15 percent. Five transmitters can reduce the uncertainty by another 5 percent. Four transmitters yield a good balance of accuracy and cost.

The typical high-bay area could have 15 or so transmitters in the ceiling, Hedges said. Of course, only a few of them might be within sight of a receiver at any given time.

The transmitter sends out three light signals. Two beams, generated by infrared lasers, fan out as they pass through rod lenses and rotate continuously through the measuring space. A third signal from an infrared LED strobe makes an index pulse for determining the horizontal angle, or azimuth, at each receiver.

Every transmitter rotates at a unique speed. Photo detectors on the receivers clock the time interval between vertical pulses to measure elevation, or detect the strobe flash when determining azimuth.

Receivers can be put almost anywhere: on parts, on tools, on fixtures.

Arc Second's president, Ed Barrientos, likened the system to a robotic theodolite. It replaces a man and an instrument making angular measurements sequentially, with many, many stations capable of determining their own positions all at the same time.

Indeed, similar systems from Sunnyvale, Calif.-based Trimble Navigation Ltd., which supplies the transmitters to Arc Second, have been seen on construction sites for some time. In the 1990s, GPS systems began replacing the classic optical survey tools and traditional two-man surveying teams for long-range measuring, Barrientos explained. Taking GPS inside is the next logical step.

But the tolerances around a construction site are quite forgiving compared to what's maintained on the aircraft assembly floor. This was one of the first objections Boeing personnel raised when they began considering the Constellation 3Di system.

That investigation began about two years ago, Cobb said. So far, the system has been tested extensively only in the laboratory. The results there have been "an order of magnitude better" than anyone was expecting, he said. The company is still evaluating how the system will do under actual factory conditions.

Arc Second had the system independently tested in two ways. In one test, the accuracy of the system's generated coordinates was compared with several linear measurements taken through a laser interferometer. According to the company, its Constellation 3Di system showed an uncertainty of 4 to 8 parts per million, compared with 1.4 ppm for the laser interferometer and 10 ppm of uncertainty for a typical laser tracker.

Above the waterline of an America's Cup hull, a probe and receiver records the coordinates of a measuring point from signals beamed off transmitters.

In another test, 3-D coordinate data was compared to monuments whose positions were known within 10 or 20 mils. The Constellation data measured the monuments within 20 mils with a two-sigma standard deviation.

Arc Second reported that the accuracy of its system depends largely on the accuracy of the means used to scale it. An interferometric scale bar will return a higher precision than will a scale provided by a laser tracker, the company said.

Like GPS, adding a single fixed receiver, or base station, improves Constellation's reliability. The addition gives a nearly instantaneous indication that the system has wandered off tolerance. Through multiple fixed stations, the system can continuously correct for errors in the measurement field.

Eliminating hard tooling may be just the beginning. For substantially lower cost than a laser tracker, a three- or four-transmitter system opens the possibility of myriad measurements being made simultaneously, Cobb said.

Arc Second envisions those same uses, and many others, Barrientos said: not only for tracking the location of two subassemblies as they come together, but also for measuring continuously the symmetry of wings during assembly or detecting distortion in two fuselage barrels during joining.

A drill, fitted with three receivers, could tell a tech exactly when he'd found the right spot for a hole and the correct orientation at which to bore it. An instrumented torque wrench could document bolt data as it happened. In non-destructive evaluation, a probe anointed with receivers could generate an image of the part undergoing inspection.

The list goes on: The system could track the positions and movements of cranes to keep them from colliding with any airframes under assembly. Scaffolding so instrumented could avoid getting in the way of moving production lines.

Automakers, shipbuilders, and railroaders have demonstrated interest in the system, too, Hedges said. So have machine tool manufacturers. At a large eastern university, researchers are investigating its application to robots, he added.