February 1997

Virtual Assembly

Continued from page 1

By Larry Krumenaker

Resolving Ambiguities

Boeing's first application of the technology has been in its "wire shop," where workers assemble wire bundles that connect circuits from one section of an aircraft to another. Each plane has about 1,000 such bundles, each of which must be preassembled on 3-by-8-foot pressboard sheets studded with pegs. The conventional assembly technique, which relies on myriad markings on plotter paper glued to the pressboard, is cumbersome because the bundles have to be assembled in hundreds of different ways depending on the aircraft model. "Storing 1,000 unique boards for bundles requires a lot of space," says Mizell, "and many are changed for individual customers." With AR, wiring configurations for all models are stored in the computer. When the aircraft model is called up, the computer displays how the bundles should be assembled one wire at a time using guide lines that appear over the blank board. This eliminates ambiguity and improves efficiency, he says. Moreover, any board can now be used for any bundle.

As promising as the results have been so far, researchers caution that the technology has been tested only under carefully controlled conditions and is not yet ready for prime time. The main limitation, says Neumann, is with tracking. The system often has difficulty repositioning the overlaid information in exactly the right spot over the workpiece when the camera's view is distorted by bad lighting or occluded, he explains. For example, if an operator blocks the camera lens with a wrench while tightening a bolt, the onscreen overlay could drift as the operator's head turns. "It isn't very helpful if an arrow is suddenly pointing to the wrong place," he says.

Neumann says the consensus is that hybrid trackers, units that have two or more different tracking technologies built in, will be required to solve the problem. One system, which is being tested in combination with the video pattern-recognition approach used by McDonnell Douglas and Boeing, is called magnetic tracking. This technique relies on an electrical device containing huge magnetic coils that generate three magnetic fields aligned at right angles to one another in the space surrounding the work area. A sensor in the AR helmet measures the relative strength of each field to divine the camera's precise orientation with respect to the workpiece.

Eric Foxlin, chairman and vice-president of research and development at InterSense Corp. in Cambridge, Mass., is developing prototypes of an "acoustic inertial" hybrid tracker for a range of potential AR tasks, including Boeing's "wire shop" application. In one such unit, three ultrasonic speakers placed at right angles to each other on the helmet send out ultrasonic chirps or pulses to microphones placed around the work area. By measuring the time it takes for the pulses to reach the microphones, the computer can calculate how far the headset is from each one and determine its precise location and orientation. The tracking unit also uses gyroscopes and accelerometers to measure the worker's head movements to help keep track of the camera's position.

Any technique, by itself, has limitations, says Foxlin. Metal objects can distort magnetic fields, any solid object can distort ultrasonic signals, and slightly inaccurate readings from the gyroscopes and accelerometers can accumulate rapidly and cause drift. But with two systems providing continual updates, he says, one can compensate when the other fails.

As more prototype experiments prove the concept and developers home in on remedies to technical shortcomings, interest in AR appears to be growing. The early work, though limited, says Neumann, "has brought more people and different technologies to the field." In anticipation, AR developers such as Honeywell are now targeting a wide range of tasks beyond assembly and manufacturing, including maintenance, construction, military, and even medical and surgical applications.