A NASA-developed fiber optic sensor provides the kind of detailed feedback that could guide the direction of flexible wings and other next-generation aerospace parts. The multi-core fiber (MCF) contains light reflectors, known as fiber Bragg gratings, that reveal shape and position in three dimensions.
For analysis of stiff, long-established aircraft structures, NASA traditionally relied upon surface-bonded fiber optic strain sensors. As today’s airframes move to lighter, more flexible configurations, new structures like morphing aircraft wing flaps and adaptable control surfaces provide too much unpredictable behavior for a traditional strain gauge to manage.
Jason Moore, a fiber optics sensor engineer at NASA Langley Research Center, currently tests and develops fiber optic technologies, including a multicore fiber that senses in-flight structural shape change in the more advanced and flexible aircraft technology.
Moore focuses his research on how to use the fiber Bragg grating measurements to deduce the shape of the multicore fiber.
“We’re trying to get these shape sensors into or onto these flexible flaps, to feed back to the control system what these flaps are actually doing,” said Moore.
Changing the Shape of Flight
NASA’s Adaptive Compliant Trailing Edge project, a collaboration begun in 2014 between NASA, the Air Force Research Laboratory (AFRL), and the Ann Arbor, MI-based aerospace manufacturer FlexSys, is an effort to create shape-changing flaps that bridge the wing’s gaps to create a seamless, bendable surface. The trailing-edge wing flaps aim to improve aircraft aerodynamic efficiency and reduce takeoff and landing noise.
Fiber optic sensors, previously employed to sense environmental characteristics like temperature and pressure, now provide an important understanding of an advanced aircraft’s structural shape. The multicore fiber developed by Moore, once bonded, bends along with the moving surface. By measuring the bending and twisting of the MCF, using the embedded gratings, Moore can determine the shape of the multi-core fiber.
“You basically track the bending and the changing shape of the fiber, and that will directly tell what the shape of the structure is that it is secured to,” said Moore.
The fiber Bragg grating sensors, contained in each core of the fiber, are embedded wavelength-specific reflectors. By tracking the specific wavelength reflected from each “FBG” strain sensor, the effect on the fiber can be determined. If the fiber is stretched, for example, the wavelength reflected from the sensor increases, demonstrating strain.
The many cores are spaced equally throughout the fiber, both in an angular and radial sense. Even a small bend in the fiber will compress some of the cores, while stretching some of the others.
Think of a garden hose. If a fiber were run on the inside of a bend, the fiber would be compressed, showing a negative strain. If the fiber were on the outside of the bend, positive strain measurement would occur as the hose stretches.
“With many cores, we take up the measurements from all the gratings at each individual section down the fiber and get bend measurements,” said Moore. “From those bend measurements, there’s more math that can kick out an actual three-dimensional shape of the fiber.”
When optical fibers were first used to provide shape-sensing measurements, bending had to be estimated at sequential points along the fiber, leading to errors and a poor indication of actual fiber position in three-dimensional space.
NASA’s patent-pending algorithms and apparatus incorporate the curvature, bend direction, and twisting information of the fiber to obtain an accurate 3D location and shape characterization.
Achieving a Location, in Three Dimensions
The MCF’s fiber Bragg grating strain sensors determine how any point along the fiber is positioned in space. The characteristics of optical fibers and the fiber Bragg gratings vary with curvature. By sensing the relative change of the fiber sensors in each of three or more fiber cores, the three-dimensional change in position can be determined.
Measurement of each core’s strain occurs at specific axial locations along the fiber. When a multi-core fiber is bent, the strain imposed in each core, relative to one another, provides position information.
The sensing system also includes an interrogator, a measurement instrument that contains a zero-point reference of the fiber shape after installation. The lightweight fiber optic cables lead from the interrogator out to the shape-sensing fiber, which is taped down or bonded at locations of interest.
The interrogator measures the stretch or compression in each core’s grating along the fiber. A laser sweeps over each core of the multi-core fiber, recording individual back-reflections. Through some signal processing, the measurements of each fiber Bragg grating are produced.
Beyond flexible aircraft flaps, NASA’s fiber optic sensor could also be used to monitor the deflection of NASA’s inflatable re-entry shields and the agency’s tethered satellites, which feature long cables that sweep through magnetic fields to generate electricity. The bonded sensors could measure the shape and behavior of the cable while it is being deployed.
Additionally, the fiber optic shape sensors could also be used inside catheters for very precise surgical maneuvers. Fiber optic-based, shape-sensing catheters, placed in the human body, could be located without the use of X-ray technology.
Moore’s primary application for the sensor, however, is to support new flexible aircraft and provide a measurement that might not otherwise be possible.
“These fiber optic shape sensors are just about the only way to get so much feedback in so much detail into what these control surfaces are doing that could ultimately steer the direction of these next-gen control surfaces,” said Moore.
Moore is presently coordinating the installation of some multi-core fibers on wing models for wind tunnel testing. In a future flight test, the Langley team also plans to test the sensors’ ability to monitor the effects of flutter, an unstable oscillation of an aircraft caused by aerodynamic forces.
In addition to creating equipment to test accuracy and refine the shape-sensing algorithms, Moore is also looking for more cost-effective ways of producing a multi-core fiber that contains more densely populated fiber Bragg gratings.