Directed Energy Target Failure Sensors
Summary:
Luna Innovations Incorporated (Blacksburg, VA) has developed fiber-optic-based strain and temperature sensors. Luna was awarded an MDA SBIR Phase II to develop the fiber-optic sensors for characterizing stainless steel targets under the attack of high-energy laser weapons. Besides this primary application, Luna's sensors can be used in hypersonic wind tunnel instrumentation or structural health monitoring of high-temperature engine components.
Technology Description:
Luna's fiber-optic sensors measure distributed strain and temperature across the surface of a structure. The sensors and their associated cabling are fabricated using fiber optics. Luna uses extrinsic Fabry-Perot interferometry to do the measurements. To measure both strain and temperature, an 850-nanometer wavelength light is transmitted to the cleaved end of a single-mode fiber.
To perform the strain measurements, upon reaching the end of the fiber, the light is partially reflected, while the remaining light travels past the end of the fiber and is bounced off a secondary reflector. The reflectors (also fibers) are aligned with the main fiber in a capillary tube and attached to a substrate. The two reflected light signals interfere with each other forming a fringe pattern. As the substrate strains, the distance between the two fiber end-faces vary, causing the fringe pattern to change. Using a spectrometer, the changing gap is measured to obtain the strain. The sensor is less prone to failure because the fiber itself is not being strained by the substrate.
The temperature sensor has a small, single crystal chip on the end of the fiber. The two faces of the chip are reflectors. Precise temperature changes can be measured by two phenomena. The coefficient of thermal expansion of the chip causes the length of the crystal to vary with temperature, and the changes in the index of refraction differ based on temperature.
Luna developed a new bonding technique for the strain and temperature sensors. When testing the thermal shock capability of the bond, Luna made an interesting discovery. To test the bond line, the company put the sensors in a furnace and heated them to 660°C. In addition to the bond line not breaking, Luna's sensors tracked the change in heat when the furnace went from room temperature to 660°C. So, the company performed many more tests to learn how fast the sensors could be heated and still track the temperature. The sensors accurately tracked the temperatures of propane torches, wind tunnels, gas drains, and machining laser beams. The maximum response rate Luna demonstrated was when the sensors tracked the heat of a 100-Watt carbon dioxide machining laser beam from room temperature to 1200°C in half a millisecond. The temperature rose 2.4 million degrees of heat per second. The company went the other direction also. It plunged the sensors into liquid nitrogen; the temperature dropped so fast it looked like a square wave, an abrupt 90° drop on the readout system. But, upon taking a closer look, Luna could see the sensor reacted fast enough to track the boiling dynamics of the liquid nitrogen. Distinct regions of varying cool-down rates where bubbles were forming on the end of the sensor, breaking off, and nucleating could be seen. The cool-down rate accelerated when the liquid nitrogen contacted the sensor; it would decrease as the nitrogen gas bubble formed. Luna found the temperature sensor to have millisecond response time and a dynamic range of -269°C to 1400°C when tracking temperature.
MDA Origins:
Luna was awarded an MDA SBIR Phase II MDA to develop the strain and temperature sensors to characterize stainless steel targets under the attack of high-energy laser weapons such as the Airborne Laser.
Spinoff Applications:
Besides the primary application of measuring directed energy laser effects on targets, Luna's sensors can be used in hypersonic wind tunnel instrumentation or structural health monitoring of high-temperature engine components.
Hypersonic Aircraft
High-response-rate sensors are essential for measuring the temperature change on the surface of hypersonic vehicles. The temperature of the air at the front of hypersonic vehicles can range up to 3000°C. Dealing with such high heat levels, researchers and manufactures need to accurately know the total temperature of the air around a hypersonic aircraft. Luna developed a probe, which captures the air for measurement by the temperature sensor.
Turbine Engines
The high-temperature strain sensors can be applied to the structural health monitoring of gas turbine engines in aircraft as well. Conventional strain sensors are based on piezo silicon carbide, which degrades at around 700°C to 750°C. A turbine engine runs around 1200°C to 1400°C. Luna's strain sensors have a high-temperature measurement capability demonstrated at more than 1400°C.
Commercialization:
Luna tested prototypes of its strain and temperature sensors at Kirtland Air Force Base. Currently, the company manufactures and sells them. The strain sensors have been sold to manufacturers of carbon-carbon composite aircraft structures for high temperature strain measurements at more than 1100°C. Lower temperature strain sensors have been used for fatigue testing on aircraft structures. Luna has also been in discussions with a manufacturer interested in using the strain sensors on gas turbine aircraft engines. Luna, in conjunction with the Air Force Arnold Engineering and Development Center, is using its temperature sensors in hypersonic aircraft research.
Whereas the strain sensors are a standard product, the temperature sensors are still developmental. Luna is improving the sensors' high-speed readout system, which is part of the MDA Phase II SBIR project. The basic design is complete, but the hardware still needs testing.
Company Profile:
In 1990, a company called Fiber and Sensor Technologies (F&S) was founded to develop an accelerometer/high performance strain gauge technology for monitoring the health of aircraft designs. In 1998, the company refocused itself and F&S changed its name to Luna Innovations Incorporated. Today, Luna Innovations has more than 110 employees, five locations, and five new spinoff companies, several with technologies derived through SBIR programs. The company identifies market opportunities and integrates them with new, cutting-edge technologies that move the concepts from the lab to the marketplace.
Contact Information:
Robert Fielder
Luna Innovations Incorporated
2851 Commerce Street
Blacksburg, VA 24060
Tel:(540) 552-5128
Fax:(540) 951-0760
email: solutions@lunainnovations.com
web: www.lunainnovations.com
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