This presentation provides an overview of fiber optic sensing technology development activities performed at NASA Dryden in support of Unmanned Aircraft Systems. Examples of current and previous work are presented in the following categories: algorithm development, system development, instrumentation installation, ground R&D, and flight testing. Examples of current research and development activities are provided

Bakalyar, John

Chan, Patrick

Ko, William L.

Parker, Allen R.

Piazza, Anthony

Richards, Lance

NASA Technical Reports Server

Advanced Fiber Optic-Based Sensing
T h l f U d Ai ft
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Dr. Lance Richards, Allen R. Parker, Anthony Piazza,
Dr. William L. Ko, Dr. Patrick Chan, and John Bakalyar
Dryden Flight Research Center, Edwards, CA
UAS Payloads Conference
San Diego, CA
11/16/2011
https://ntrs.nasa.gov/search.jsp?R=20120002715 2019-08-30T19:18:51+00:00Z
Fiber Optic Sensing for UAS Applications
Advantages over Conventional Measurements
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n ? Unrivaled density of sensors for spatially distributed measurements
? Measurements immune to EMI, RFI and radiation
? Lightweight sensors
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? Typical installation is 0.1 - 1% the weight of conventional gage installations (based on past trade
studies)
? 1000?s of sensors on a single fiber (up to 80 feet per fiber)
? No copper wires
With i l d l d l ith th d t i t f l
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displacement and load at points along the fiber
? Small fiber diameter
? Approximately the diameter of a human hair
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? Unobtrusive installation
? Fibers can be bonded externally or applied as a ?Smart Layer? top ply
? Single calibration value for an entire lot of fiber
? Wide temperature range (cryo – 550F) Fib ti
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Strain gage
er op c
sensor
Fiber for
628
FOSS
sensors
Wires for 21
Fiber optic
sensor
strain gage
measurements
Fiber Optic Sensing for UAS Applications
Anticipated Impact
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? Potential to revolutionize
UAV design and
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performance throughout
the life-cycle Source: COTS Journal
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Production
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Test and Evaluation
In-flight operation
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Off-nominal flight
End of life-cycle decisions
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defenseindustrydaily.com
Fiber Optic System Operation Overview
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n Fiber Optic Sensing with Fiber Bragg Gratings
? Immune to electromagnetic / radio-frequency interference and
radiation
? Lightweight fiber-optic sensing approach having the potential of Grating region
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embedment into structures
? Multiplex 100s of sensors onto one optical fiber
? Fiber gratings are written at the same wavelength
? Uses a narrowband wavelength tunable laser
Laser tuning
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source to interrogate sensors
? Typically easier to install than conventional
strain sensors
? In addition to measuring strain and temperature these sensors
start stop
Tuning
direction
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can be use to determine shape
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iiR nLkCosRI )2(
Ri ? spectrum of ith grating
n ? effective index
L ? path difference
2k
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Reflector
L1
Laser light
Loss light
Reflected light
k ? wavenumber
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L3
L2(IR)
Fiber Optic System Operation Overview
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n ? Fourier transforms (both forward and inverse) are used to
discriminate between gratings
? The Fourier transform separates the IR waveform into sinusoids
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of different frequency which sum to the original waveform
FFT iFFT
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Optical Wavelength( ) > Length(L) Length(L) > Wavelength( )
Spectral Mapping
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1 2
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6 0 0 0
8 0 0 0
FFT
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01 5 57 . 5 1 5 58 1 5 5 8. 5 1 5 59 1 5 5 9. 5 1 5 60 1 5 60 . 5 1 5 61 1 5 6 1. 5 1 5 6 2
-80 0 0
Wavelength( ) domain Length(L) domain
Fiber Optic System Operation Overview
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n ? By bandpass filtering around a specific frequency (grating location)
within the length domain and performing an iFFT, the spectrum of each
grating can be independently measured and strain inferred (FM radio)
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iFFT
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Length(L) domain (inches) Wavelength( ) domain
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? Using a centroid function the center wavelength can be resolved
? The wavelength change is proportional to the induced strain
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K K – proportionality constant (0.7-0.8)
Interrogation Process
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Tunable Laser Perform FFTSignal Conditioning and A/D
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1548 to 1552nm
S/C A/D
Wavelength
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Length
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Perform
Windowing Perform iFFT
Filtering and
Centroid
Centroid to
Strain
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Conversion
WavelengthL th
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Domain
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Domain
Research and Technology Development Areas
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?Algorithm Development
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?Flight Testing
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Research and Technology Development Areas
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?Algorithm Development
Real-time wing shape measurement
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using fiber optics sensors
(Ko, Richards; Patent 7,715,994)
Real-time applied loads on complex
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(Richards, Ko; Patent 7,520,176)
Data processing algorithms
(P k US P t t P di )
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? Flight Testing
Real-time Wing Shape Measurement
Motivation – Helios UAV
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Helios Mishap Report – Lessons Learned
Helios wing dihedral on takeoff In-flight breakup
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? Measurement of wing dihedral in real-time should be accomplished with a
visual display of results available to the test crew during flight
? Procedure to control wing dihedral in flight is necessary for the Helios class
of vehicle
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Real-time Wing Shape Measurement
Theoretical Development
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Deflection of a Single Fiber:
( l)2
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Slope:
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Typically the first
station is at the root:
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Real-time Wing Shape Measurement
Global Observer – Algorithm Validation Testing
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n ? Strain gages
? Validate the FBGs
? Not used for shape prediction used for structural evaluation
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? Photogrammetry
? Provided validation information for wing shape prediction
M t l di l t t t t t i t
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p ? easures ac ua sp acemen  vec ors a arge  po n s
? 10 photogrammetry images taken per load condition
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Real-time Wing Shape Measurement
Global Observer – Algorithm Validation Testing
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100% DLL
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FWD AFT
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Over the entire wing span, the predicted displacements of
fiber 3 closely match the actual for every load condition.
Real-Time Externally-Applied Loads
Approach
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n ? Bending moment calculated at each analysis station
? Cross-sectional properties calculated by applying known load
? EI/c term backed out at each evaluation station
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? With properties known, strain can be directly related to bending
moment
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EIM
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Get properties at
each station
a cu a e momen
at each station
K t
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nown momen
Real-Time Externally-Applied Loads
Swept Plate Loads Testing
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Cross-sectional properties calculated using Uniform load calibration
U if L d C Si l P i t L d C
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Calculated Moment
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Calculated Moment
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0 5 10 15 20 25 30 35
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Wing Shape and Externally-Applied Loads
Results
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? Different test articles
? Different load cases
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? Different load magnitudes
? Load results will be improved
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? Developing methods to further use FOSS measurements
? Angle-of-twist
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? Improved deflection and load
? Torque
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NASA Technology FOSS Systems (4DSP)
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n ? Technical Highlights
? 4DSP has licensed NASA technology to
commercially develop FOSS systems
? http://www.4dsp.com/RTS150.php
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? Single laser greatly reduces cost per sensor
? High fiber count systems
? Modular design with 8 channels per card
? Expandable
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? Up to sensing 80 feet per fiber
? 11? x 7? x 12?
? 100 Hz max sample rate
Li ht i ht t f ltit d f
Ground system
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? Approximately 25 lbs
? Cost
? 8 fiber system approx $100K
U t 16 000
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? 32 fiber system approx $150K
? Up to 64,000 sensors
? System can be flight-certified (+$30K)
? Low power requirements (<10 Amps at 28 Volts DC)
? Applications
? Transport Aircraft, Ships, Civil Structures
Flight system
Compact FOSS (cFOSS) System
In Development
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? Lightweight, ruggedized system
? Packaged within a 6? cube
? Targeted specifications:
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? Fiber count:  8
? Max Fiber length:  80 ft
? Max # sensors/system:15,360
Max Sample rate: 100 Hz
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? Power: 50W @ 28Vdc
? Weight: <10 lbs
? Size: 5 x 6 x 6 in ~5?
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? Vibration and Shock: NASA Curve B
? Altitude: 65kFt
? Applications:
Fi ht i ft
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? UAVs
? Launch vehicles
? Spacecraft
T t t t $50K? arge  sys em cos :
? Availability: End of 2012
8 Fiber Card
Large Scale FOSS (LsFOSS) Technology
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n ? Technical Highlights
? Single laser greatly reduces cost per sensor
? High fiber count systems
? Up to 16 fibers monitored simultaneously/system
FOSS Ground System
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? Each fiber can be up to 40ft long
? Each fiber at 40ft long can have up to 2000
measurements (total of 32,000 /system)
? Up to 8 systems can be networked together
yielding approx 1 mile of sensing distance (1/4?
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spacing, 256,000 measurements)
? 11? x 7? x 12?
? 100 Hz max sample rate
? Lightweight system for multitude of sensors
Data Server
FOSS 1
FOSS 2
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? Approximately 25 lbs
? Applications:
? Transport Aircraft FOSS 4
FOSS 3
Display PC 1
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? Ships
? Civil Structures
? Ground Testing
? Structures Laboratory
FOSS 5
FOSS 6
Display PC 2
Display PC 3
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FOSS 8
Display PC ND
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FOSS Installation Advantages and Challenges
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n Installation Advantages
? Greatly reduced installation time compared to conventional strain gages
? 2 man-days for 40? fiber (1000 strain sensors for a continuous surface run)
? Multiple sensors installed simultaneously
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? Same surface preparation and adhesives as conventional strain gages
? Minimal time spent working on vehicle
? All connectors can be added prior to installation, away from part
? No soldering
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? No clamping pressure required
? Circular cross-section eliminates possibility of trapping air between sensor and part
(eliminates repeat installations)
? Can be installed with little or no impact to OML
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Installation Challenges
? Optical fiber more fragile than strain gages
? Some measurement locations not practical
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due to fiber minimum bend radius
? Not practical if only interested in spot
measurements Optical Fiber
Research and Technology Development Areas
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Embedment of Fiber Optic Sensors within Composites
Biological Inspiration of FOSS
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? Four yards of nerve fibers One square-inch of human skin
Human Skin
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? 600 pain sensors
? 1300 nerve cells
? 9000 nerve endings
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? 75 pressure sensors
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? 100 sweat glands
? 3 million cells
3 d f bl d l
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Source: Biswas, Aman. Explore the Human Body.
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Embedment of Fiber Optic Sensors within Composites
The Multidisciplinary Challenge
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n ? Fiber Optic Sensors embedded within Composite
Overwrapped Pressure Vessels
? Goal is to understand embedded FBG sensor response
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? Requires comprehensive, multi-disciplinary approach
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Courtesy: M Emmons, GP Carman, UCLA
Embedment of Fiber Optic Sensors within
Composite Overwrapped Pressure Vessels (COPVs)
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n The Goal:  Characterize measurement response
of fiber Bragg sensors embedded in COPVs
? Determine overall sensor accuracy as a function of its
orientation relative to the layered materials in the structure
Theoretical
de elopment
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? Use finite element techniques to understand the
thermal/mechanical loads present in the fiber optic, lenticular
resin rich region, and the adjacent composite material as well
as issues related to ingress/egress.
Coupon testingv
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? Experimentally evaluate the accuracy and long term durability
of the embedded sensor / host material system when subjected
to quasi-static thermal mechanical loading
The Approach: Evaluate accuracy
Analysis and Modeling
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and long term durability of a
fiber optic sensors embedded
within COPVs Sensor Installation
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? Analytical modeling of the fiber optic sensor
? Epoxy composite fabrication
? Quasi-static testing of coupons
? Long term fatigue testing
? Testing of representative aerospace
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Embedding / Fabrication Failure Testing
AeroVironment’s Global Observer
Wing Loads Tests at NASA Dryden
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? Validate strain predictions along the wingspan
? Measured strain distribution along the centerline top and
bottom as well as along the trailing edge top and bottom.
p
a
c
e
 
A
d
m
i
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? FO Strain distribution measurements are being used to
interpret shape using Dryden?s single fiber shape algorithm
A 24 fib t d i d f hi h 18 fib 40ft
c
s
 
a
n
d
 
S
p ? - er sys em was es gne  o  w c er
(~17,200 gratings) fibers were used to instrument this wing
A
e
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a
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N
a
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a
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27
Research and Technology Development Areas
12 nl
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?Algorithm Development
1
0 6)13(6 i
nin innc
y
p
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?FBG System Development
c
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a
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S
p
? Instrumentation
A
e
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a
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?Ground Testing / R&D
N
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a
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A
?Flight Testing
28
Flight Test Results
Predator-B
n
i
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t
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n ? Flight validation testing
? 18 flights tests conducted; 36 flight-hours logged
? Conducted first flight validation testing April 28, 2008
p
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d
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? Believed to be the first flight validation test of FBG strain and wing shape sensing
? Multiple flight maneuvers performed
? Two fiber configurations
Fib ti d ti l t i h ll t t
c
s
 
a
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S
p ? er op c an  conven ona  s ra n gages s ow exce en  agreemen
? FBG system performed well throughout entire flight ? no issues
A
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N
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29
Ikhana in Flight
Video clip of flight data superimposed on Ikhana photograph
AeroVironment’s Global Observer
Flight Testing
n
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t
r
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? Validate strain predictions along the left wing using 8, 40ft fibers
? An aft fuselage surface fiber was installed to monitor fuselage and tail
movement
p
a
c
e
 
A
d
m
i
n
? Strain distribution were measured along the left wing centerline top and
bottom as well as along the trailing edge top and bottom.
c
s
 
a
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S
p
? 8 of the 9 total fibers are attached to the system at any give time
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30
Concluding Remarks
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Fiber Optic Wing Shape Sensing toward UAS applications involves five major areas
? Algorithm development
? Real-time wing shape and applied loads algorithms using fiber optics sensors were in good agreement
with conventional measurements
p
a
c
e
 
A
d
m
i
n
? FBG system development
? Current Flight Systems in Operation: 4 and 8 Fiber Systems
? Flown on Ikhana and Global Observer, resp.
? Future Systems underdevelopment:
c
s
 
a
n
d
 
S
p
? 64 Fiber ?Large-Vehicle? System
? 4 Fiber ?Compact? System
? Instrumentation
? Installation Advantages
A
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o
n
a
u
t
i
c
? Greatly reduced installation time compared to conventional strain gages
? Installation Challenges
? Optical fiber more fragile than strain gages
? Ground Testing / R&D
N
a
t
i
o
n
a
l
A
? A 24-fiber system was used on Global Observer; 18 fiber 40ft (~17,200 gratings) fibers were to measure
strain and wing shape in real-time
? Flight Testing
? Predator-B; Ikhana
31
? Real time fiber Bragg strain measurements successfully acquired and validated in flight (4/28/2008)
? Real-time fiber optic wing shape sensing successfully demonstrated in flight
? Global Observer


Advanced Fiber Optic-Based Sensing Technology for Unmanned Aircraft Systems

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