N151-006 TITLE: Low Power, Low Cost, Lightweight, Multichannel Optical Fiber Interrogation Unit for Structural Health Management of Rotor Blades

TECHNOLOGY AREAS: Air Platform, Sensors

ACQUISITION PROGRAM: PMA 261

OBJECTIVE: Develop an innovative optical fiber interrogator of low weight, small factor, and low power draw for integration into composite rotorcraft blade Structural Health and Usage Monitoring systems (SHUMs).

DESCRIPTION: The main rotor blades and associated rotating hardware are some of the highest dynamically loaded parts found on rotorcraft. These dynamic parts have historically been hard to instrument without a significant weight penalty and are often inspected at intervals. A system capable of monitoring true strains, as well as damaging impacts during rotorcraft operation, without the usually associated weight penalties would have enormous benefits. Usage information taken from this system would enable health and usage monitoring (HUM) of the rotor system, allowing maintainers to be alerted when components are about to show signs of degradation, resulting in increased safety and reduction in unnecessary maintenance. Additionally, faster maintenance turnaround would translate into improved aircraft availability and lower life cycle costs.

Optical fiber sensors could be used for the monitoring of strain levels, vibrations, and temperature in a rotor blade. In order to perform impact detection, degradation diagnostics, and fatigue damage monitoring, the low weight of the fiber sensor, and its immunity to electrical interference, are major benefits to this sensing method. In addition, these optical fibers can be embedded into composite fiber blades during their construction, giving them a layer of protection from environmental factors. Optical fibers can measure much larger strain ranges than traditional foil strain gages. An optical fiber system could also be used to assist blade tracking. By embedding these sensors into a rotor blade, the safety and cost of rotorcraft operations would be greatly improved. This condition based maintenance functionality is in line with current Navy programs like the CH-53K Integrated Hybrid Structural Management Systems (IHSMS), which is an effort aimed at developing rotorcraft airframe and rotor system Structural Health Management (SHM) capabilities.

The sensor interrogator is the major component within the optical fiber system which drives the weight and power requirements. Mission-ready helicopter load-outs avoid slip rings due to their unnecessary weight and complexity; a fiber system, therefore, must be able to use the limited power available from energy harvesting methods. With a weight of several pounds (and high power requirements), commercial interrogator units are unusable in the dynamic rotorcraft environment. An interrogator that is much lighter and smaller than these commercial units is desired. The system must be of low volume (less than 200 cm3) and weight (no greater than 0.33 kg), and be capable of interrogating an optical fiber containing 15 sensing locations in a single blade, and have no moving parts. The sensor interrogator should also be able to withstand the high vibrations and loads found in a Naval rotor system in which it will be installed. The interrogator must be able to accurately resolve the large blade strains produced by a helicopter blade, and be able to obtain data from each sensor at a rate of at least 1 kHz. The interrogator must also be able to operate efficiently, drawing no more than 3 watts of power.

PHASE I: Determine and demonstrate the feasibility of a multi-function optical fiber sensor system that can, at minimum, meet all of the stated requirements listed in the description. For Phase I efforts only, the requirements for the system can be scaled up to 30 sensing locations or down to 5 sensing locations as long as all of the other requirements (power draw, size, weight, etc.) scale similarly.

PHASE II: Develop and mature technologies, fabricate and deliver a prototype of the proposed optical fiber sensing system capable of surviving in the vibratory environment similar to one found on a Naval helicopters rotor system, and demonstrate its ability to meet the stated requirements.

PHASE III: Integrate the optical fiber sensing system, resulting from the Phase II effort, on a Navy helicopter rotor equipped with wireless sensor and power systems. Perform field testing to show the robustness of the system and to resolve issues regarding the interrogator integration with embedded sensors and on-board communication networks. Transition into use on appropriate Navy platforms and commercial applications.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Potential to integrate into structural health usage management/monitoring systems (HUMS) for use in engineering projects such as buildings, bridges, robotics, commercial aviation and helicopters.

REFERENCES:
1. Annamdas V.G.M., Yang Y. & Liu H. (2008). Current development in fiber Bragg grating sensors and their Applications. Proceedings of SPIE, San Diego, California, USA, 6932, 69320D (paper no: 6932-15). http://www.academia.edu/2195126/Current_development_in_fiber_Bragg_grating_sensors_and_their_applications

2. Todd M.D., Johnson G.A. & Vohra S.T. (2001). Deployment of Fiber Bragg Grating-Based Measurement System in a Structural Health Monitoring Application. Smart Materials and Structures, 2001, 10, 534-539. http://iopscience.iop.org/0964-1726/10/3/316/pdf/0964-1726_10_3_316.pdf

3. MIL-STD-810G, Environmental Engineering Considerations and Laboratory Tests (31 Oct 2008).

KEYWORDS: Wireless; Blade; Uav; Hums; fiber interrogator; strain monitoring

** TOPIC AUTHOR (TPOC) **

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