N242-093 TITLE: Distributed Acceleration Sensor for Integrated Flight and Structural Control

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Microelectronics; Space Technology; Sustainment

OBJECTIVE: Develop distributed sensors and associated electronics to measure real-time structural acceleration of an airborne platform’s mode shapes. The low-cost, low-SWAP, embeddable measurement system will enable tightly integrated flight and aeroelastic control on future platforms with an otherwise unachievable combination of speed, endurance, and agility.

DESCRIPTION: The distributed sensor must directly measure acceleration of an airframe’s mode shape in real-time. Currently, accelerations at distinct points can be measured using a collection of conventional accelerometers, and these sensors can all be sampled and processed to estimate a mode shape. The effectiveness of this approach is dependent on accelerometer location relative to the mode shape, and therefore, depends on a priori knowledge of the mode shapes. Alternatively, fiber optic sensors can provide distributed measurements of structural displacement. The fiber optic sensor’s capability for distributed measurements is compelling, but the fact that these sensors generally measure displacement means that the signals lag acceleration by 180 degrees. In terms of control of an agile yet flexible aircraft, 180 degrees of phase lag poses significant stability challenges. Simply differentiating the displacement signal is not a solution because differentiation would amplify noise. Accelerometers can be installed at various locations, but the measurements must be individually sampled and collectively processed to estimate the acceleration of a mode shape. This SBIR topic seeks sensors and electronics that will have the utility of a single, distributed measurement device, such as a fiber optic displacement sensor, and produce real-time acceleration measurements of airframe structural modes. This approach practically eliminates the need for a priori knowledge of the mode shapes. Many state-of-the-art accelerometers use a small, calibrated mass attached to a piezo-electric or piezo-resistive crystal. As the calibrated mass accelerates along with the sensor, a corresponding force on the crystal produces a proportional voltage or resistance change depending on the mode of operation. These sensors have the advantage of being robust and compact, but they produce measurements at a point. On the other hand, fiber optic sensors leverage light scattering phenomena correlated to changes in temperature or displacement of the fiber itself. Measurements of light scattered from throughout the fiber can be correlated to displacement and/or temperature variations along the length of the fiber. Thus, fiber optic sensors have the advantage of measuring transverse displacement along the length of fiber. These sensors can be attached to large structures under observation, such as aircraft or bridges. However, fiber optic sensors do not intrinsically measure acceleration, which would be more suitable for integrated flight and structural control due to the phase advantages. The focus of this SBIR topic is to create a sensor that intrinsically measures structural acceleration for spans greater than 40 feet in length, e.g., the span of a large, tailless sensor platform. Sensor measurements will exploit material phenomena that correlate to structural accelerations. Double differentiation of a displacement signal, which would serve to amplify high frequency noise, is not an acceptable solution for measuring acceleration.

PHASE I: Produce a conceptual design for the objective sensor. First order modeling and simulation (M&S) of the underlying phenomena should support the merit of the design. Provide a baseline for the materials, electronics, and software needed to produce a prototype as well as an estimated cost to build and test the prototype. Model potential test conditions and sensor output for a prototype sensor as well as the predicted output for a sensor installed on an operational aircraft.

PHASE II: Fabricate and test the prototype sensor designed in Phase I. Test the sensor in a controlled laboratory environment. Refine the M&S tools from Phase I and use them to predict the sensor response under the test conditions. Test measurements should be compared with predictions. Conventional accelerometers should be used to spot check distributed measurements from the sensor. Conduct experiments to quantify the precision, accuracy, range, drift, noise, and bandwidth of the sensor. Work with government, industry, and/or academia to identify potential air platforms to test the sensor in an operational environment. Estimate the cost of operational test and validation.

PHASE III DUAL USE APPLICATIONS: Work with DoD air platform providers to assess the potential of using the sensor in a structural and flight dynamic closed-loop feedback system to enable future tailless air platforms capable of high-speed dash, long-endurance loiter, and agile maneuvering. Work with commercial civil and aerospace engineering firms to assess potential sensor use in structural monitoring and condition based maintenance.

REFERENCES:

1. Beranek, J., Nicolai, L., Buonanno, M., Burnett, E., Atkinson, C., Holm-Hansen, B., & Flick, P. (2010, September). Conceptual design of a multi-utility aeroelastic demonstrator. In 13th AIAA/ISSMO Multidisciplinary Analysis Optimization Conference (p. 9350).
2. "How to Measure Acceleration." https://www.omega.com/en-us/resources/accelerometers
3. "Fiber Optic Sensing." VIAVI Solutions. https://www.viavisolutions.com/en-us/fiber-optic-sensing
4. Burnett, E. L.; Beranek, J. A.; Holm-Hansen, B. T.; Atkinson, C. J. and Flick, P. M. "Design and flight test of active flutter suppression on the X-56A multi-utility technology test-bed aircraft." The Aeronautical Journal, 120(1228), 2016, pp. 893-909.
5. Burnett, E., Atkinson, C., Beranek, J., Sibbitt, B., Holm-Hansen, B., & Nicolai, L. (2010, August). Ndof simulation model for flight control development with flight test correlation. In AIAA Modeling and Simulation Technologies Conference (p. 7780).
6. Ryan, J. J., & Bosworth, J. T. (2014). Current and future research in active control of lightweight, flexible structures using the X-56 aircraft. In 52nd Aerospace Sciences Meeting (p. 0597).
7. Ouellette, J. A., Boucher, M. J., & Suh, P. (2023). Using Distributed Fiber-optic Strain Sensing to Estimate Generalized Modal Coordinates from Flight-test Data. In AIAA SCITECH 2023 Forum (p. 2069).
8. Miller, C. J., Schaefer, J., Boucher, M., Ouellette, J., & Howe, S. (2022). X-56 Flight-test Approach for Envelope Expansion Past Open-loop Flutter Instability. NATO Science and Technology Organization (STO), STO-MP-AVT-211, 18-1.

KEYWORDS: Distributed Accelerometer; Fiber Optic Sensor; Structural Control; Aeroelastic Control; Aeroservoelastic Control; Structural Modes; Structural Acceleration; Flight Control

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