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Physical Science Laboratory to test fiber-optic monitoring of helicopter parts

Physical Science Laboratory to test fiber-optic monitoring of helicopter parts



Jim Pate, a mechanical engineer at NMSU's Physical Science Laboratory, holds a spring fitted with a conventional strain gauge. Numerous conventional gauges can be replaced with a single fiber-optic strand, and fiber optics provide better data. (NMSU photo by Michael Kiernan)

Helicopters in the near future may be able to warn their pilots when a maneuver is causing dangerous stresses to rotors or other components. And the same "smart structure" technology may give transportation officials new options for weighing vehicles in motion and monitoring commercial traffic.

These are among the applications New Mexico State University's Physical Science Laboratory is exploring for a fiber-optic system being developed by NMSU civil engineers for monitoring bridges.

PSL program manager Robert Silver said the technology that civil engineering professor Rola Idriss and her colleagues have been testing on an Interstate 10 bridge in Las Cruces soon will be tried on Apache helicopter parts from Boeing. The idea in both cases is the same -- to accurately monitor the structure's response to stresses and strains.

"Whether the fiber-optic sensors are applied to an I-beam on a bridge or a helicopter rotor blade or an airplane wing, if the structure bends you get a measurement of the stress," Silver said. Improved data can help identify problems before structural fatigue or failure occurs, he said, and also may lead to improved manufacturing processes and more cost-efficient schedules for replacing parts.

The fiber-optic strands used by the NMSU researchers were developed by the U.S. Naval Research Laboratory. About the same diameter as a 30-pound-test fishing line, the fibers are etched with gratings that cause a reflection when a beam of light is sent down the fiber, Silver said. The wavelength of the reflection varies depending on the degree to which the fiber is bent, so the signals can be used to precisely measure stress.

The system offers significant advantages over conventional methods of measuring stress and strain, said James Pate, a mechanical engineer on the PSL technical team. One fiber can do the work of many mechanical strain gauges, the quality of data is better and the fibers are more durable, he said.

"If I have 64 conventional strain gauges on a structure, each one has three wires leading from it," Pate said. "That's an awful lot of wiring that something can go wrong with. I can replace all of them with one fiber."

Physical Science Laboratory to test fiber-optic monitoring of helicopter parts

Helicopters in the near future may be able to warn their pilots when a maneuver is causing dangerous stresses to rotors or other components. And the same "smart structure" technology may give transportation officials new options for weighing vehicles in motion and monitoring commercial traffic.

These are among the applications New Mexico State University's Physical Science Laboratory is exploring for a fiber-optic system being developed by NMSU civil engineers for monitoring bridges.

PSL program manager Robert Silver said the technology that civil engineering professor Rola Idriss and her colleagues have been testing on an Interstate 10 bridge in Las Cruces soon will be tried on Apache helicopter parts from Boeing. The idea in both cases is the same -- to accurately monitor the structure's response to stresses and strains.

"Whether the fiber-optic sensors are applied to an I-beam on a bridge or a helicopter rotor blade or an airplane wing, if the structure bends you get a measurement of the stress," Silver said. Improved data can help identify problems before structural fatigue or failure occurs, he said, and also may lead to improved manufacturing processes and more cost-efficient schedules for replacing parts.

The fiber-optic strands used by the NMSU researchers were developed by the U.S. Naval Research Laboratory. About the same diameter as a 30-pound-test fishing line, the fibers are etched with gratings that cause a reflection when a beam of light is sent down the fiber, Silver said. The wavelength of the reflection varies depending on the degree to which the fiber is bent, so the signals can be used to precisely measure stress.

The system offers significant advantages over conventional methods of measuring stress and strain, said James Pate, a mechanical engineer on the PSL technical team. One fiber can do the work of many mechanical strain gauges, the quality of data is better and the fibers are more durable, he said.

"If I have 64 conventional strain gauges on a structure, each one has three wires leading from it," Pate said. "That's an awful lot of wiring that something can go wrong with. I can replace all of them with one fiber."

The fiber-optic system can measure a wider range of strain, especially at lower levels, he said, "and repetitive low-level strains can add up and cause fatigue."

"What we would like to do eventually is mount these in a structure when it is built," Pate said. "It's called smart structures.' Nobody's been able to imbed conventional strain gauges in a structure and have them survive. But because this fiber is glass, it can be woven into a composite material."

A helicopter mounted with such a system would continually monitor the strains to which its rotors and other components are subjected, providing constant real-time feedback. "It would be nice for the aircraft to be able to tell the pilot, Hey, don't do this maneuver because we're getting a lot of stress,' " Pate said.

In the case of bridges, the fiber-optic sensors are intended mainly to allow for remote monitoring of structural conditions. But Pate and others involved in that project soon learned that the system also could be used to calculate the weight and speed of vehicles crossing the bridge.

"This could be part of an automated system" for monitoring traffic, Pate said. "If something super-heavy goes over a bridge, you could make a picture of it."

This capability also has other possibilities that state transportation officials are interested in, Silver said. PSL plans to develop a prototype weigh-in-motion system that could be either permanently imbedded in pavement or placed in a portable pad that could be rolled up and moved from place to place, he said.

The fiber-optic system is expected to be "cheaper, more reliable and more accurate" than existing weigh-in-motion systems, which are electro-mechanical devices set into pavement, Silver said.

The prototype will be tested at PSL's Border Technology Development Center, to be located at the Santa Teresa Port of Entry. The U.S. Department of Transportation earmarked $1 million for this fiscal year for PSL demonstration projects related to border technology.

Photo is available at http://kiernan.nmsu.edu/newsphoto.
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PHOTO: jim_pate.jpg

Karl Hill
January 6, 2000