Note: Descriptions are shown in the official language in which they were submitted.
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Title of the Invention
Active Strain Gages for Earthquake Damage Assessment
1. Field of the Invention
This invention relates generally to optical waveguide sensors and more
specifically to temperature insensitive optical waveguide sensors which can
monitor
strain in excess of 2000 ~,s.
2. Description of the Related Art
In some regions of countries, it is necessary to assess the condition of vital
infrastructure elements (viz. roads, bridges and buildings under normal
loading
1 o conditions and after catastrophic events such as earthquakes and floods.
For example,
state highway departments and construction companies could use such
information to
minimize repair cost, reduce service disruptions and avoid or detect
catastrophic failures.
Such data could also be used to alert emergency response teams to dangerous
conditions
within structures, provide warning of imminent structural failure and/or give
detailed
damage information for use in structural repair. Automated diagnostic
monitoring
systems could monitor the integrity of a structure and assess the damage with
little
human intervention through real time processing of the information.
Electrical strain gages, including resistive type strain gages and their
equivalents
such as piezoelectric, semiconductor, and capacitance gages, are fairly
compact and
2o highly accurate strain measuring devices. However, they have the
disadvantage of
rather small dynamic range and some hysteresis. Traditional electric
resistance strain
sensors have gage factors less than four. Environmental conditions such as
moisture and
temperature markedly affect the performance of resistance strain gages and
thus, those
gages need frequent calibration. If the baseline electrical resistance changes
sufficiently
due to environm.,ntal effects, the gage will not develop its stated
calibration factor and
measuring errors will be introduced. For traditional strain measurement
methods, the
problem of gage stability due to temperature changes can be reduced if the
period of
observation is kept short, otherwise the response must be considered to be a
function of
temperature (hence time) in addition to strain. Comparisons with traditional
electrical
3o resistance strain gages show that the optical methods are cost effective
because of their
high gage factors. There are also compatible with fiber optic communication
systems
and can be accessed from remote locations in a reliable manner.
Optical fiber based sensors have been applied to many of applications
including
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displacement (position), temperature, pressure, sound, and strain. In
application,
optical sensor data collection can generally be divided into two basic
categories: phase-
modulated and intensity-modulated. Intensity modulated sensors are usually
associated
with displacement or some other physical perturbation that interacts with the
sensor. The
perturbation causes a change in received light intensity and the intensity is
related to the
monitored parameter. Phase-modulated sensors compare the phase of light in a
sensing
path to the phase in a reference. Phase difference can be measured with
extreme
sensitivity, but frequently requires sophisticated electronics for signal
processing. Also,
phase modulated sensors have been used to measure temperature, thus they need
to be
to calibrated for temperature when used in a strain application. Application
of this type of
optical sensor includes in situ monitoring of thin film deposition thickness.
Phase-
modulated sensors are generally more accurate than intensity-modulated
sensors.
However, they are usually more expensive and extremely sensitive to
environmental
effects, such as temperature.
The optical waveguide sensor of the invention overcomes the drawbacks of the
aforementioned prior art strain gages
Brief Summary of the Invention
Broadly, the invention comprises an optical waveguide sensor, which comprises
a
housing having an interior and exterior surface. At least two layers are
applied to the
2o exterior surface of the housing. The first layer comprises a low refractive
index material
and the second layer comprises a highly reflective material. First and second
optical
fibers are in communication with the housing. A beam of light of known
intensity is
passed through the first optical fiber through the housing and received by the
second
optical fiber. The beam is attenuated according to how many 'bounces' or
reflections it
experiences as it passes through the housing which is determined by the
conformation of
the housing. The conformation of the housing is directly related to the
bending strain
that the housing experiences. Means for detecting the change in the intensity
of the
beam of light is in communication with the second optical fiber which allows
for the
monitoring of up to at least 2000 ps.
3o In a preferred embodiment of the invention the optical waveguide sensor
comprises a flexible, hollow, glass tube with an absorptive layer of polyimide
deposited
on the outsid., followed by the deposition of a layer of high optical
reflection, such as
aluminum. The parameter that is monitored is the intensity of the exiting
light after the
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beam has passed through the sensor tube. The beam is attenuated according to
how
many 'bounces' or reflections it experiences which in turn is a function of
the radius of
curvature of the hollow tube sensor. The radius of the curvature of the tube
is directly
related to the bending strain that the tube experiences and so applied strain
can be
inferred by monitoring the exit light beam intensity. The stability,
ruggedness and
simplicity of the present invention facilitate its use for remote sensing
applications.
Since optical fiber technology can be used to both send and receive the light
signals,
instantaneous strain can be monitored and relayed immediately or stored for
later
retrieval via a transmission link.
1o In another preferred embodiment of the invention the optical waveguide of
the
invention has a gage factor of about 500 for strains in excess of 2000 ~s. The
housing is
comprised of a hollow, glass wave-guide of dimensions of about 0.5 mm ID X 0.8
mm
OD X 100 mm long.
The geometry of the housing of the optical waveguide sensor is compatible with
standard telecommunications thereby facilitating the incorporation of the
housing into
smart system arrays for damage assessment in structures such as buildings,
roads, and
bridges. Optical fibers bring the excitation light signal to and the response
signal from
the housing. In a preferred embodiment, the housing is a glass tube having a
small
diameter. The small diameter glass tubes act as the substrate for multiple
thin film layers
2o that can be optimized to provide the maximum dynamic range for a
predetermined strain
excursion. The optical wave guide sensor of the invention which comprises a
glass tube
coated with the thin film layers responds to bending strain by attenuating the
optical
intensity of the excitation signal and exhibits little or no hysteresis.
The optical waveguide sensors of the invention have a large gage factor of
about
500, and are temperature insensitive, i.e. the sensors do not respond to
temperature
changes over the normal range of outdoor temperatures (-20 to 50 °C),
inexpensive to
manufacture, not affected by electromagnetic fields, chemically inert to
environmental
conditions such as moisture and acid rain thereby making it possible to embed
the
sensors in a concrete structure with no fear of chemical reaction with the
concrete and
3o are readily interfaced with fiber optics communications equipment, i.e. the
connection
between the optical fibers and the sensing area can be hermetically sealed
between the
fiber and the capillary tube.
Brief Description of the Drawings
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Fig. 1 is a schematic of an embodiment of the optical waveguide
sensor of the invention.
Fig. 2 is a schematic of the layout used for testing an embodiment of the
optical
waveguide sensor.
Fig. 3 is a graph illustrating the strain response of optical waveguide
sensors
comprised of uncoated, aluminum coated, polyimide coated, and polyimide plus
aluminum coated capillaries.
Fig. 4 is a graph illustrating the strain response of an optical waveguide
sensor
comprised of an ITO and aluminum coated capillary.
Fig. 5 is a table of gage factors for the various types of sensors shown in
Figs. 3
and 4.
Description of the Preferred Embodiments)
Referring to Fig. 1, the optical waveguide sensor 10 of the invention is
generally
shown. A housing 12 has an interior surface 14 and an exterior surface 16. The
exterior
surface 16 is comprised of at least one layer of low index of refraction
material 24 and at
least one layer of highly reflective material 26. The housing 12 , and layers
28 and30
have very small coefficients of expansion, e.g. 9X106 in/in °C. The
basic dimensions of
the sensor 10 in the direction of strain do not change over the temperature
ranges
typically encountered environmentally. The housing 12 is in communication with
a first
optical fiber 28 and a second optical fiber 30. Means for detecting (not
shown) the
change in the intensity of light when light is passed through the housing 12,
reflected and
refracted within the housing 12 and received by the second optical fiber 30,
is in
communication with the second optical fiber 30.
In the preferred embodiment of the invention, the low index of refraction
material
of the first layer 24 is polyimide and the highly reflective material of the
second layer 26
is aluminum.
EXPERIMENTAL
The invention comprises a robust waveguide strain sensor capable of monitoring
strain of up to about at least 2000~s. The active strain elements for the
sensors comprise
3o hollow glass tubes onto which thin film, optically active materials are
deposited.
lOcm long, hollow glass wave guides were obtained from commercial suppliers.
The tube type sizes evaluated were; plain glass tubes, 5mm ID with 0.20mm wall
thickness (Fisher Scientific, Pittsburgh PA) and ~16 - 35 ~,m polyimide coated
glass
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tubes with wall thickness, 0.09mm, 0.175mm, 0.1075mm and tube inside
diameters of 0.32mm, 0.45mm, and 0.53mm respectively (Alltech Company,
Deerfield,
NY). The tubes were cleaned with a commercial ammonia based glass cleaner and
then
with acetone, methanol and deionized water rinses followed by blow drying in
filtered
5 nitrogen gas. Subsequent to cleaning they were placed in an ozone plasma
chamber for
two hours to remove any residual organic surface contaminants. Three different
thin
film coatings were evaluated for the optically sensitive layer; polyimide,
indium tin
oxide and zinc oxide in thicknesses of 0.1 to 40 Vim. Other coatings believed
suitable for
purposes of the ir_vention include silicon and germanium.
1 o Polyimide coatings were 'standard' films provided by gas chromatography
supply
houses for column capillaries. The latter two materials were deposited by RF
reactive
sputtering. After applying the active coating, a reflective outer layer of
aluminum
(~O.S~,m) was deposited, also by reactive sputtering. Other reflective layers
believed to
be suitable include silver, platinum, and palladium. The source and detector
optical
fibers were then epoxy bonded into either end of the tubes. The ends of the
source and
detector fibers 28 and 30 respectively were prepared, using standard industry
techniques,
to produce a flat surface normal to the fiber and sensor axis.
Referring to Fig. 2., the main components of the experimental apparatus were 1
)
a HeNe laser light source 40 emitting light energy in a range of 632 to 633
nm, 2) a
2o microscope objective lens in a micro manipulator mount 42 to focus the
laser light onto
the source optical fiber 28, 3) a reference photodiode beam detector 44, 4)a
sensing beam
photodiode detectors 46, 5) a four point bending apparatus 48 with built in
position
sensing 50 and a comparator 52. Beam chopping and frequency sensitive
amplification
was used to stabilize the system against line power and laser fluctuations.
About 4% of
the laser light was directed to the reference diode detector 44 via a beam
splitter 52 in the
optical path. The reference signal was electronically divided into the sensor
output
signal to further reduce random noise. Commercial multimode optical fibers
were used
to bring the light source to the sensor input and carry the light exiting the
sensor to the
output detector. Strain was induced using the four point bending apparatus
ASTM C-
1341-97 with a programmable stepper motor. Typical step size was 0.25mm.
The relationship between strain and deflection for a rod or tube in four point
bending is:
__ s
s 8a +az d (1)
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Where 8 is deflection from equilibrium of the center of the tube, d is the
outside
diameter of the tube, and a is half the distance between the two movable
(inner) pins of
the four point bending rig (2a=2.92cm in this case).
Equation ( 1 ) gives a measurement of strain based on the geometry of the four-
point bending device. The quantity 8 can be measured directly or can be
determined
accurately by constructing a calibration curve of 8 versus inner pin
displacement. Over
the range of bending necessary to attain 2000~s, 8 is linear with displacement
and so can
be directly inferred from the linear displacement of the inner pins.
RESULTS
1o The smart optical strain sensor employs a hollow glass waveguide support
with
the active sensing material located between the glass outer wall and the
reflective
(aluminum) over coating. The gage factor or response to strain was calculated
as
follows:
G_ DID 1
I° DE
(2)
where DI is the change in intensity as measured by the light detector diode at
two strain
levels, Os is the change in strain, and I° is the intensity in the
unstrained condition. Since
the intensity versus strain response was found to be essentially linear over
the strain
ranges tested, the gage factor was calculated by dividing the slope of the I
versus E curve
2o by the I-axis intercept of the straight line that best fit the data. In
practice the parameter
measured is the output voltage of the amplifier used to measure the response,
to intensity
changes, of the sensor output detector diodes. That voltage was divided by the
output of
the reference diode amplifier and the ratio of the two voltages plotted as a
function of
strain.
When an ITO layer is added to the sensor construction, the gage factor is
reduced
but the range increases significantly. The particular structure, used for the
response
curve in Figure 4, was formed by sputtering 0.9~.m of ITO followed by O.S~,m
of
aluminum onto the polyimide coated 0.53mmm ID tubes. The gage factor for that
sensor
configuration was 410. The change in signal when going from unstrained to
2000~E was
3o extrapolated to 'ue 90% (reduction). This is in contrast to the typical 50%
signal
reduction observed over the same strain range for the various specimens
without ITO
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active layer (Figure 3). The associated gage factors for the configurations
presented in
Figures 3 and 4 are summarized in the table of Figure 5.
Cyclic straining of the sensor was done to test the reproducibility of the
optical
sensors. In four cycles from zero to maximum strain, for identical thin film
structures
fabricated on tubes of the same ID, the strain gages had reproducibility
better than 1 %.
DISCUSSION
The optical strain waveguide sensors are based on the loss of light that
occurs
when the laser beam hits the inner wall of the waveguide, traverses the wall
(and any
coatings thereon), reflects from the mirrored outer layer (aluminum) and
traverses the
1 o coatings and wall of the waveguide once more. When light impinges upon the
inner
surface of the capillary, some is reflected, some is scattered (non-specular
reflection) and
some is transmitted (refracted) into the tube wall. These processes occur at
each
subsequent interface as well with an reflection/transmission ratio that
depends, according
to Fresnel and Snell Laws, upon indices of refraction of each material in the
stack. In
addition, each material absorbs some of the light in accordance with its
individual
absorptivity. So after one interaction event, the initial intensity, Io, is
reduced to I1 = (1-
f)Io. Where 'f is the fractional loss for one interaction. If there are 'N'
interactions,
then the final intensity is IN = (1-f)NIo. With the highly reflective coating
in place on the
outside, only absorption and scattering actually reduce the intensity of the
light beam
2o since reflection and refraction only affect the phase of the light waves as
they arrive at
some point in space.
As bending stresses are applied to the sensor, the curvature increases thereby
increasing the number of 'bounces' or interactions between the light beam and
the
waveguide with associated losses for each bounce. The equation for the number
of
bounces, N, as function of radius of curvature, R, is given by (11):
N - 1 + sL,
2dcos-' '
(3)
And d is the outer diameter of the tube, s is the strain and L is the gage
length.
When an aluminum reflective layer is used as the outer coating little light
can
3o escape, thus the gage factor is reduced for that configuration. This is
supported by the
fact that the smallest gage factor occurred for the plain glass capillaries
coated with an
aluminum reflector. Almost all the light that is launched into the tube
reaches the
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detector fiber at the exit end. On the other hand plain glass, without any
coatings at
all, loses the most intensity on each bounce since light can escape the tube
completely.
So the totally uncoated configuration has the largest gage factor of all. A
similar
scenario applies to the polyimide coated capillaries with and without the
reflecting outer
coatings. One might conclude, therefore, that the 'best' sensor is the plain
uncoated
glass and this might be so for applications where the maximum strain is less
than
1000~s. (For instance, counting the traffic passage on a highway with optical
sensors
imbedded in the roadway would require sensors with responses optimized for
something
like 400ps.) However, there are other considerations. First of all the plain
glass tubes
are not robust enough to reliably record 2000~,E without failure. If the walls
were thinner
they could withstand more bending but then they become too fragile to handle.
The
addition of the polyimide layer was an enhancement developed for the gas
chromatography industry. The polyimide layer increased the flexibility of the
capillaries
permitting wall thickness reductions and much smaller bend radii without
failure. As
part of the evaluation, three sizes of polyimide coated tubes were tested.
It is believed a sensor which has an ID of 0.53~0.012 mm a wall thickness of
0.085~0.012mm and a polyimide layer 24~4~.m thick. is a convenient match for
the
mufti-mode fibers that are currently used. With this particular capillary,
strain in excess
of SOOO~,s can easily be attained. Smaller, thinner walled polyimide coated
capillary
2o tubes are available should measurements at even higher strains be required.
The addition of absorptive layers and the tailoring of their thickness can be
used
to expand the dynamic range of the optical sensors design and end use,
tailored for a
specific dynamic range. Ideally the maximum strain should result in a
reduction of
intensity to a few percent of the initial value.
The waveguide sensor of the invention is a robust, chemically and thermally
stable waveguide. The sensor can survive strain in excess of 2000~,s and is
readily
incorporated into optical fiber data collection systems. The optical
properties of the
active coatings on the sensor can be optimized to give the maximum dynamic
range for a
specific maximum strain criterion. Polyimide-coated capillaries can be
strained at least
3o to SOOO~s and are supplied with better tolerance control.
The foregoing description has been limited to a specific embodiment of the
invention. It will be apparent, however, that variations and modifications can
be made to
the invention, with the attainment of some or all of the advantages of the
invention.
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Therefore, it is the object of the appended claims to cover all such
variations and
modifications as come within the true spirit and scope of the invention.
Having described my invention, what I now claim is:
