The strain gauge, a device commonly used in the aerospace industry to detect stress and deformation, has its limitations. The three copper wires of the strain gauge often lead to labor-intensive efforts; a large, complicated structure requiring 100 strain measurements, for example, means 300 lead wires. As the implementation becomes more complex, the wire bundle itself gets bigger and heavier. Strain gauges are also susceptible to electronic magnetic interference, and the sensors must be spaced out at distant intervals.
A real-time fiber optic sensor from NASA’s Armstrong Flight Research Center aims to reduce the amount of time needed to install a sensor, as well as dramatically increase the amount of data collected with a single system.
Thousands of the Armstrong fiber optic sensors can be epoxied to a material at less than half-inch intervals. The sensors can also be placed in previously inaccessible locations — for example, within bolted joints or embedded in a composite structure.
Using NASA-patented methods, the sensors calculate a variety of critical parameters, including shape, stress, temperature, pressure, cryogenic liquid level, operational loads, and accelerations. The technology processes 16,000 measurements, each at rates of 100 times per second.
A new embodiment of the technology samples dozens of the 16,000 sensors at 5,000 times per second, to capture the dynamic characteristics of structures, including natural frequencies, mode shapes, and damping. The real-time monitoring capability enables an immediate response in the event of an emergency.
“With our fiber optics sensor, we’re able to have thousands of measurements along a single fiber, which is about as big in diameter as a human hair, and it takes us a fraction of a time to install those 1,000 sensors as it would to install one single strain gauge,” said Armstrong Project Manager Jeff Bauer, who provides organizational support for the technology.
The current system can run 1 to 8 fibers. The sensing length of the fiber ranges from 40-80 feet long, allowing over two thousand sensors per fiber. The fiber, which contains etched fiber Bragg gratings, is the same kind typically used in telecommunications networks. An avionics element of the system, about the size of a shoe box, includes the electronics and laser.
For aerospace applications, NASA uses the sensor to measure local strain of structures during operation. The strain information validates designs and models of how structures — NASA’s flexible wings, for example — are expected to perform. Structural engineers use the shape information to confirm material property predictions, and aerodynamicists use the data to validate lift and other performance-related parameters.
“In fact, for one of the vehicles in flight, based on the wing’s shape, you’re actually able to infer exactly how much fuel is onboard the aircraft,” said Bauer. By knowing the shape of the wing at a known flight condition, the gross weight of the aircraft can be determined and thus the amount of available fuel.
The sensor is currently being used by NASA to support a test of composite pressure vessels for the Advanced Propellant Loading Project. Recently, the fiber optic technology also provided data acquisition on the satellite clamp band for the Magnetosphere Multiscale Mission (MMS). Through MMS, four identical spacecraft will orbit around Earth to study the phenomenon of magnetic reconnection.
A promising new fiber optic sensor is additionally being developed to detect and characterize magnetic fields that satellites experience in operation.
“Satellites are very sensitive instruments that are subject to electromagnetic interference. If you could have a sensor in that area, you could tell very quickly if your science sensor was being subjected to electronic magnetic interference,” said Bauer. “Then you could quickly pinpoint the source of that emission and shut it down so you don’t damage your valuable satellite.”
Outside of NASA, the technology has other possibilities beyond aerospace. The fiber optic sensors can monitor the structural integrity of high-rise buildings, bridges, and pipelines, or ensure precise placement of tiny catheters.
There is also interest from the oil and gas industry, says Bauer, a sector that can use the sensor to determine how much fluid is in a storage tank, and more importantly, how much of that fluid is oil versus other liquid materials. In addition, NASA is entering into a licensing agreement with a company that will be using the cryogenic-level sensor for large cryogenic storage tanks.
“We think it has the potential to give much more precision than what is currently available in a cryogenic-level sensor,” said Bauer.
US spaceflight company Virgin Galactic has also been working with the sensor system, purchased from the Austin, TX-based 4DSP, now Sensuron (who licenses the technology from NASA). The NASAlicensed system obtains both strain and deflection measurements on various elements of the Virgin carrier aircraft.
NASA is in discussions with several companies inside and outside of the aerospace industry who are interested in licensing the technology — and perhaps abandoning the strain gauge.
This article was written by Billy Hurley, Associate Editor at NASA Tech Briefs. For more information, visit www.nasa.gov/centers/armstrong/home .