Note: Descriptions are shown in the official language in which they were submitted.
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MATERIAL EROSION MONITORING SYSTEM AND METHOD
FIELD OF THE INVENTION
The present invention relates to systems and methods for evaluating the status
of a material. More particularly, the present invention relates to systems and
methods
for determining refractory bricks- material interface using electromagnetic
waves.
BACKGROUND OF THE INVENTION
Evaluation methods and systems exist within various industries for measuring
the properties during and after formation of certain materials. The surface
characteristics, internal homogeneity, and thickness of a material are some of
the
important attributes that may require evaluation. In particular, the wall
thickness of
glass and plastic containers using non-contact reflective and/or absorptive
techniques
by deploying sensors and emitters to direct radiation towards the container
have been
addressed in the prior art, as described in U.S. Pat. App. No. 20130268237 by
Wolfe
et al. However, these methods are primarily aimed to evaluate the thickness of
manufactured glass and plastic containers by means of using radiation capable
to pass
through those materials without sustaining significant losses in the levels of
such
radiation or accessing more than just one external surface of such materials.
On a bigger scale, some industries such as the glass, steel, and plastic
industries use large furnaces to melt the raw material used for processing.
These
furnaces may reach a length equivalent to the height of a 20-story building.
Thus, they
are a key asset for manufacturers in terms of costs and operational
functionality. In
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order to minimize the internal heat loss at high operating temperatures, these
furnaces
are constructed using refractory material, having very high melting
temperatures and
good insulation properties, to create a refractory melting chamber. However,
the inner
walls of the refractory chamber of the furnace will degrade during operation.
The
effects of this degradation include inner surface erosion, stress cracks, and
refractory
material diffusion into the molten material.
Currently, there is no well-established method of deterministically measuring
the thickness and erosion profile of the walls of such furnaces. As a result,
manufacturers experience either an unexpected leakage of molten material
through the
furnace wall or conservatively shut down the furnace for re-build to reduce
the
likelihood of any potential leakage, based on the manufacturer's experience of
the
expected lifetime of the furnace. The lifetime of a furnace is affected by a
number of
factors, including the operational age, the average temperature of operation,
the
heating and cooling temperature rates, the range of temperatures of operation,
the
number of cycles of operation, and the type and quality of the refractory
material as
well as the load and type of the molten material used in the furnace. Each of
these
factors is subject to uncertainties that make it difficult to create accurate
estimates of
the expected lifetime of a furnace. Moreover, the flow of molten material,
such as
molten glass, at high temperatures erodes and degrades the inner surface of
the
refractory material and creates a high risk for molten glass leakage through
the
refractory wall. A major leak of molten glass through the gaps and cracks in
the
furnace walls may require at least 30 days of production disruption before the
furnace
can be restored to operating mode because it needs to be cooled down,
repaired, and
fired up again. Furthermore, a leak of molten glass may cause significant
damage to
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the equipment around the furnace and, most importantly, put at risk the health
and life
of workers. For these reasons, in most cases furnace overhauls are conducted
at a
substantially earlier time than needed. This leads to significant costs for
manufacturers in terms of their initial investment and the reduced production
capacity
over the operational life of the furnace.
Another important issue is that the material used to build the refractory
chamber of the furnace may have internal flaws not visible by surface
inspection. This
could translate into a shorter life of the furnace and pose serious risks
during furnace
operation. Accordingly, on the one hand the refractory material manufacturer
would
like to have a means to evaluate the material during manufacture to be able to
qualify
the material for furnace construction following quality standards to deliver
material
with no flaws. On the other hand, the customer purchasing the refractory
material
would like to have a means for performing internal inspections of such
material
before constructing a furnace.
Previous efforts have been made to use microwave signals to measure the
thickness of materials such as furnace walls, as described in U.S. Pat. No.
6,198,293
to Woskov et al. and U.S. Pat. App. No. 20130144554 by Walton et al. However,
these efforts have faced certain challenges and limitations. In particular,
attempts
made to determine furnace wall thickness on hot furnaces have been generally
unsuccessful because of the large signal losses involved in evaluating the
inner
surface of refractory materials, especially at relatively high frequency
bands.
Likewise, at relatively low frequency bands signals still experience losses
and are
limited in terms of the bandwidth and resolution required by existing systems.
Moreover, in placing system components close to the surface of the refractory
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material to be evaluated, spurious signal reflections make it difficult to
isolate the
reflected signal of interest, thus further complicating the evaluation of the
status of
either the inner surface or the interior of such materials. A major challenge
is that
furnace walls become more electrically conductive as temperature increases.
Therefore, signals going through a hot furnace wall experience significant
losses
making the detection of these signals very challenging.
Thus, there remains a need in the art for systems and methods capable of
remotely evaluating the status of such refractory materials, through
measurements of
propagating electromagnetic waves, that avoid the problems of prior art
systems and
methods.
SUMMARY OF THE INVENTION
An improved system and method to evaluate the status of a material is
disclosed herein. One or more aspects of exemplary embodiments provide
advantages
while avoiding disadvantages of the prior art. The system and method are
operative to
identify flaws and measure the erosion profile and thickness of different
materials,
including refractory materials, using electromagnetic waves. The system is
designed
to reduce a plurality of reflections associated with the propagation of
electromagnetic
waves launched into the material under evaluation, by a sufficient extent so
as to
.. enable detection of electromagnetic waves of interest reflected from remote
discontinuities of the material. Furthermore, the system and method utilize a
configuration and signal processing techniques that reduce clutter and enable
the
isolation of electromagnetic waves of interest. Moreover, the launcher used in
the
system is impedance matched to the material under evaluation, and the feeding
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mechanism is designed to mitigate multiple reflection effects to further
suppress
clutter.
The system launches electromagnetic waves into a near surface of a material
to be evaluated. The electromagnetic waves penetrate the material and reflect
from
discontinuities inside and from both the near and a remote surface of the
material. The
reflected electromagnetic waves are received by a computer-based processor and
timed, using as reference the wave reflected from the near surface of the
material. The
computer-based processor determines the delay in time between the reference
wave
and other reflected electromagnetic waves, which include undesired clutter.
Where the
to magnitude of the clutter is below the magnitude of the electromagnetic
waves
reflected from remote discontinuities of the material, the computer-based
processor
identifies a peak level of magnitude associated with these discontinuities and
determines the distance from such discontinuities to the near surface of the
material
associated with the reference wave. One or more evaluations over an area of
the
material provides the thickness of the material and the location of flaws
inside the
material at each evaluation to create an erosion profile of the remote surface
of the
material.
The system also includes an electromagnetic wave launcher designed and
adapted to reduce a plurality of reflections that significantly contribute to
the clutter
received by the computer-based processor. The launcher provides levels of
clutter
reduction by a sufficient extent so as to enable detection of electromagnetic
waves of
interest that otherwise might not be possible. The launcher may be used in
evaluation
of the refractory walls of hot furnaces to create an erosion profile of the
surface of the
inner walls in an operational furnace.
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The method to evaluate the status and measure the erosion profile and
thickness of different materials includes the step of setting up the
electromagnetic
wave launcher conformally contiguous to the near, outer surface of the
material under
evaluation. The method further includes the steps of launching electromagnetic
waves
into the material and measuring, over a frequency band, the amplitude and the
phase
of waves reflecting from discontinuities from said material. The method also
includes
transforming measured data to time domain, calibrating the data to distance
domain,
and identifying data associated with reflected electromagnetic waves of
interest; in
particular, waves reflected from the inner, remote surface of the material
under
evaluation to determine the thickness of such material.
By significantly reducing the level of clutter caused by reflections and
ringing
of propagating electromagnetic waves, as compared to standard techniques, and
by
determining the location of remote discontinuities from the material under
evaluation,
the system and method are able to identify flaws and measure the erosion
profile of
the remote surface of such material.
BRIEF DESCRIPTION OF THE DRAWINGS
The numerous advantages of the present invention may be better understood
by those skilled in the art by reference to the accompanying drawings in
which:
Fig 1 shows a schematic view of an exemplary embodiment of a system using
a rolled-edge electromagnetic wave launcher.
Figs 2A to 2D show various aspects of an electromagnetic wave launcher with
two rolled edges in accordance with one embodiment.
Fig 3 shows a design of a feeding transitioning section.
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Fig 4 shows a plot of the magnitude of the noise, clutter, and reflected
electromagnetic waves of interest in accordance with a hypothetical scenario.
Fig 5 shows a plot of the magnitude of the noise, clutter, and reflected
electromagnetic waves of interest using a launcher with and without rolled
edges.
Fig 6 shows a perspective view of a planar electromagnetic wave launcher in
accordance with another embodiment.
Fig 7 shows a perspective view of a planar electromagnetic wave launcher
with curved edges in accordance with another embodiment.
Fig 8 shows a schematic view of a method for computing the thickness of a
dielectric material according to any of the embodiments of the invention.
Fig 9 shows a perspective view of an electrically-small electromagnetic wave
launcher in accordance with an embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The following description is of a particular embodiment of the invention, set
out to enable one to practice an implementation of the invention, and is not
intended
to limit the preferred embodiment, but to serve as a particular example
thereof. Those
skilled in the art should appreciate that they may readily use the conception
and
specific embodiments disclosed as a basis for modifying or designing other
methods
and systems for carrying out the same purposes of the present invention. Those
skilled in the art should also realize that such equivalent assemblies do not
depart
from the spirit and scope of the invention in its broadest form.
In accordance with certain aspects of an embodiment of the invention, a
material evaluation system is shown in Fig 1. The system is configured to
evaluate a
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status of a refractory material used as a furnace wall. Thus, the refractory
material has
an outer surface and an inner surface opposite the outer surface. The inner
surface of
the refractory material is contiguous to (i.e., in contact with) a molten
material, such
as glass, plastic or steel or any other material contained within the furnace.
An
electromagnetic (EM) wave launcher 10, comprising a feeding end 12, a
launching
end 14, and an elongated section 16 in between and adjoining feeding end 12
and
launching end 14, is disposed contiguous to an area of the outer surface of
the
refractory material to be evaluated. EM wave launcher 10 is designed to
operate at a
frequency band large enough to cover the operational frequency band of the
system.
1() Specifically, and as discussed in greater detail below, the dimensions
of the
rectangular cross section (width and height) at the launching end of EM wave
launcher 10, the length of the launcher (or alternatively the width and height
flare
angles and the length), and the dielectric properties of the material
occupying the
internal volume of EM wave launcher 10 are all selected to cause EM wave
launcher
10 to operate at a sufficiently large frequency band to cover the operational
frequency
band of the system, and with regard to certain aspects of an embodiment of the
invention, in the frequency band from 0.5 GHz to 10 GHz. Likewise, EM wave
launcher 10 is designed to tolerate the required temperature range of the
near, outer
surface of a furnace wall. More particularly, the material that is used to
form EM
wave launcher 10 is selected to allow EM wave launcher 10 to withstand such
high
temperatures (the area of the launcher exposed to the highest temperature
being the
area placed contiguous to the furnace outer surface). For example, the
conductive
material on the sides and on the rolled edges of the launcher is selected so
as to have a
melting temperature point larger (including some appropriate safety margin as
may be
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selected by those skilled in the art) than the temperature of the furnace
outer surface.
Likewise, with regard to the dielectric material occupying the internal volume
of the
launcher as discussed in greater detail below, typical ceramic-type materials
withstand
temperatures much higher than the maximum expected temperature of the furnace
.. outer surface. With regard to certain aspects of an embodiment of the
invention, the
dielectric substrate material also has similar properties to those of the
ceramic
material, in terms of temperature of operation. Finally, in the case that a
variable
conductivity material is used (again as discussed in greater detail below),
the
protecting layers of adhesive provide temperature isolation to the variable
1() .. conductivity material. Preferably, the selection of such materials will
allow use of the
EM wave launcher 10 against a surface having a temperature as high as 1600 F
for a
few seconds, which is sufficient enough to take the necessary data for
operation.
However, for longer duration operation, such materials should be able to
withstand an
ambient temperature limit of approximately 700 F, with the surface reaching
.. temperatures up to approximately 1000 F.
As used herein, "near" surface is also intended to refer to the outer surface
of
the material under evaluation that is contiguous to launching end 14 of EM
wave
launcher 10. Likewise, "remote" surface is also intended to refer to the inner
surface
of the material under evaluation opposite the near surface immediately
adjacent
.. launching end 14 of EM wave launcher 10. Thus, in the case of a furnace,
the remote
surface comprises the inner surface of the outer wall of the furnace, and the
near
surface comprises the outer surface of the outer wall of the furnace.
Feeding end 12 includes a feeding transition section 18 electrically connected
to a radiofrequency (RF) transmission line, such as a coaxial cable 20. A
computer-
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based processor 22 is also electrically connected to coaxial cable 20.
Accordingly,
coaxial cable 20 is electrically connected at a first end to computer-based
processor
22, and at a second end to feeding transition section 18. Coaxial cable 20 is
selected to
have a physical length from computer-based processor 22 to feeding transition
section
18, such that a propagation time of an EM wave propagating between first end
and
second end of coaxial cable 20 is larger than a propagation time of the EM
wave from
feeding transition section 18 to the remote inner surface of the refractory
material
under evaluation and back to the near, outer surface of the material. In other
words,
the propagation time of the EM wave propagating throughout the length of
coaxial
.. cable 20 is larger than the propagation time of the EM wave propagating
throughout
EM wave launcher 10 plus the propagation time of the EM wave propagating back
and forth through the thickness of the refractory material.
Computer-based processor 22 comprises an RF subsystem 23, a signal
processing subsystem, and executable computer code or software. RF subsystem
23
comprises a tunable signal source, such as a voltage controlled oscillator or
a
frequency synthesizer, preferably operable in a frequency band going somewhere
from 0.25 GHz to 30 GHz; at least one directional coupler; a coherent
detector; and at
least one analog-to-digital converter. The signal processing subsystem
comprises data
storage and data processing algorithms. Referring again to FIG. 1, it is noted
that
components of computer-based processor 22 have not been shown as these
components are not critical to the explanation of this embodiment. Those of
ordinary
skill in the art will realize that various arrangements of RF subsystem 23
components
may be possible and additional components, such as filters, impedance matching
networks, amplifiers, non-coherent detectors and other test instrumentation
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used as different ways to implement RF subsystem 23 functions of computer-
based
processor 22 as are known in the prior art.
Launching end 14 of EM wave launcher 10 is placed in physical contact with
the refractory material to be evaluated. More specifically, launching end 14
is
.. preferred to be physically conformal to the area of the near surface of the
refractory
material with which launching end 14 is in physical contact (i.e., is
configured so as to
minimize spacing between launching end 14 and the surface under examination).
In
other words, it is not desired to have any gap or clearance larger than 2 mm
between
the surface of launching end 14 and the area of the near surface of the
refractory
1() material with which launching end 14 is in physical contact.
Elongated section 16 of EM wave launcher 10 is preferably selected to have a
physical length from feeding end 12 to launching end 14 such that a
propagation time
of an EM wave propagating from feeding end 12 to launching end 14 is larger
than a
propagation time of said EM wave propagating from the near, outer surface of
the
refractory material under evaluation to the remote, inner surface of the
material. In
other words, the propagation time of the EM wave propagating along the EM wave
launcher 10 is preferred to be larger than the propagation time of the EM wave
propagating through the thickness of the refractory material. Typical
thickness values
of refractory material of furnace walls range from 0.5 inches to 12 inches.
Accordingly, depending on the target range of thickness measurements, the
length of
elongated section 16 of EM wave launcher 10 typically ranges somewhere from 2
inches to 15 inches.
Figs 2A to 2D show various aspects of one version of EM wave launcher 10,
used in Fig 1. In this embodiment, Fig 2A illustrates a perspective view of EM
wave
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launcher 10, physically structured as a truncated, two-edge flared pyramid
with a
rectangular cross-section from feeding end 12 to launching end 16. Figs 2B and
2C
show side views of EM wave launcher 10 having rectangular cross-section
dimensions of 0.2 inches x 0.13 inches at feeding end 12 and 2.5 inches x 4.25
inches
at launching end 14. Accordingly, four side plates 24a, 24b, 24c, and 24d form
EM
wave launcher 10. Each side plate 24a, 24b, 24c, and 24d is preferably made of
a
dielectric or conductive material. Typically, a conductive material having a
thickness
in the range of 0.01 inches and 0.25 inches, and more preferably between 0.05
inches
and 0.1 inches is used. In the particular embodiment shown in Fig 2D, a
conductive
material approximately 0.078-inches thick was used. Thus, more specifically,
side
plates 24a, 24b, 24c, and 24d of EM wave launcher 10 form a structure that
surrounds, without fully enclosing, an internal volume of EM wave launcher 10.
Side
plates 24a, 24b, 24c, and 24d of EM wave launcher 10 do not surround the
internal
volume at feeding end 12 and launching end 14 of EM wave launcher 10.
Referring again to Fig 2A, at any cross-sectional view, four edges 26a, 26b,
26c, and 26d form the rectangular cross section of EM wave launcher 10. The
dimensions of such rectangular cross-section of EM wave launcher 10 linearly
increase from feeding end 12 to transition points 28a, 28b, 28c and 28d, which
are
located along elongated section 16 in between feeding end 12 and launching end
14.
Accordingly, the shape of EM wave launcher 10, from feeding end 10 to
transition
points 28a, 28b, 28c, and 28d, corresponds to the shape of a regular
rectangular cross-
section pyramid. However, from transition points 28a, 28b, 28c, and 28d to
launching
end 14, the dimension of each end of opposite edges 26a and 26c of the
rectangular
cross-section of EM wave launcher 10 increases following a curve described by
a
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circular function with a 0.78 inches radius of curvature, as shown in Fig 2D.
More
specifically, the structure of EM wave launcher 10 corresponds to the
structure of a
truncated rectangular cross-section pyramid having two elliptically-flared or
elliptically-rolled opposite edges. Typical values of a thickness of launching
end 14
may range between 0 and 0.25 inches. In this particular embodiment, launching
end
14 has a thickness of 0.078 inches. Likewise, the rolling of edges 26a and 26c
starts at
a point where a separation between transition points 28a and 28b or
equivalently
between transition points 28c and 28d is 2.9 inches. Accordingly, transition
points
28a, 28b, 28c, and 28d are located approximately 0.63 inches from launching
end 14.
Furthermore, EM wave launcher 10 is physically configured to have an
impedance at launching end 14 that substantially matches an impedance of the
near
surface of the refractory material. The internal volume of EM wave launcher 10
may
be at least partially filled with a solid ceramic filling material having an
impedance
that substantially matches a predetermined impedance of the refractory
material under
the normal operating conditions of the furnace. This predetermination may be
obtained by measuring the dielectric properties of the refractory material at
various
temperatures using methods well known in the prior art. Alternatively, the
manufacturer of the refractory material may provide data about the dielectric
properties of the material at different temperatures. These data can be used
to
determine the impedance of the material. The impedance of the refractory
material is
primarily determined by both a relative dielectric permittivity of the
material and a
tangent loss of the material. Typically, the relative dielectric permittivity
may range
from 1 to 25 depending on the specific type of material and temperature of the
material. Thus, the internal volume of EM wave launcher 10 may be partially or
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completely filled with a dielectric filling material of similar relative
dielectric
permittivity to that of the refractory material to substantially match the
impedance of
the refractory material.
The filling material used to fill the internal volume of EM wave launcher 10
may be air, liquid or solid. Preferably the filling material is a mixture of
solid powder
or granulated material in which the maximum dimension of each grain is desired
to be
no larger than ten percent of a wavelength of an EM wave propagating in EM
wave
launcher 10 at the lowest frequency of operation. More preferably, the filling
material
is a solid ceramic piece of material or the like adapted to fit into the
internal volume
of EM wave launcher 10. Alternatively, the internal volume of EM wave launcher
10
may be layered, from feeding end 12 to launching end 14, so that each layer is
filled
with a filling material that has a slightly different dielectric permittivity
to the
dielectric permittivity of the filling material of any adjacent layer to
structure multiple
layers of different dielectric permittivity in an arrangement that gradually
adjust an
impedance from feeding end 12 to the impedance of the refractory material to
be
evaluated at launching end 14. Whenever necessary a lid or cap may be placed
at
feeding end 12 and launching end 14 to prevent the filling material from
exiting the
internal volume of EM wave launcher 10 during manipulation or operation of EM
wave launcher 10. Those skilled in the art realize that a cap placed at
launching end
14 must be made of a material having similar dielectric characteristics as
those as the
filling material to prevent a substantial discontinuity to an EM wave
propagating
through said cap. Likewise, a cap placed at feeding end 12 must be made of a
material
according to a specific design of feeding transition section 18.
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Fig 3 shows a design of a feeding transitioning section 18 using a cap 30
formed by a shell of a conductive material having a thickness of approximately
0.1
inches. Cap 30 forms an air-filled cavity surrounded by the shell, having a
semicircular cross-section in a first dimension, and a rectangular cross-
section in a
second dimension normal to said first dimension. In this embodiment, the
semicircular cross-section is defined by a semicircular section 32, having an
internal
radius of approximately 0.75 inches, and a linear section of approximately 1.6
inches,
comprising a first section 34a and a second section 34b of substantially the
same
dimensions, separated by a gap 35, whereas said rectangular cross-section is
defined
by said linear section, defining a width of approximately 1.6 inches, and
another
linear section, defining a length of approximately 1.3 inches (not shown in
Fig 3).
Cap 30 has a first circular opening at one side of semicircular section 32
large
enough to just allow coaxial cable 20 to enter inside of the cavity. Outer
conductor 36
of coaxial cable 20 is electrically connected to both semicircular section 32
of cap 30
.. and conductive side plate 24a of EM wave launcher 10 at feeding end 12. A
pin or
probe 38 is formed by extending a center conductor of coaxial cable 20 beyond
outer
conductor 36 of coaxial cable 20 inside of the cavity; in this case the pin
length is
approximately 0.1 inches. Likewise, gap 35 of cap 30 defines a second opening
that
separates linear section 34a from linear section 34b. The dimensions of gap 35
are
large enough just to allow the tip of the truncated end of EM wave launcher
10, that is
closer to feeding end 12, to fit into the cavity. In this embodiment, side
plates 24a and
24c of EM wave launcher 10 are made of conductive material. Accordingly, side
plate
24a of EM wave launcher 10 is electrically connected to second section 34b,
and side
plate 24c of EM wave launcher 10 is electrically connected to first section
34a. Also,
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outer conductor 36 of coaxial cable 20 is electrically connected to first
section 34a.
Additionally, pin 38 is electrically connected to second section 34b. In this
way, EM
wave launcher 10 may be excited by pin 38 of coaxial cable 20 in a cavity-
backed
feeding pin configuration. Typically, pin 38 is located at a distance from the
cap equal
to a quarter wavelength corresponding to a center frequency of the frequency
band of
the EM waves propagating along EM wave launcher 10.
Those skilled in the art will realize that semicircular section 32 may be
shaped
following different configurations, such as elliptical, planar or other smooth
function.
Likewise, one or more sections of cap 30 may be removed in certain
configurations,
and the cavity may be filled with dielectric material. Furthermore, the
dimensions of
linear sections 34a and 34b may be designed in combination with feeding end 12
of
EM wave launcher 10 to reduce undesirable ringing effects.
Operation
In accordance with further aspects of an embodiment of the invention, the
manner of using the material evaluation system of Fig 1 is based on the
fundamentals
of EM wave propagation. Computer-based processor 22 controls the tunable RF
signal source, operating in a frequency band that properly penetrates the
refractory
material with low enough loss, preferably somewhere between 0.25 GHz and 30
GHz,
and more preferably operating in a frequency range somewhere between 0.25 GHz
and 10 GHz. The RF signal source is carried by coaxial cable 20 to feeding
transition
section 18 in order to excite at least one propagation mode within EM wave
launcher
10 such that a number of EM waves are able to propagate from feeding end 12 to
launching end 14 at the frequency range of interest. The bandwidth of the EM
waves
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propagating in EM wave launcher 10 is typically selected to be at least 2 GHz
to
permit the resolution required by the user.
Upon reaching EM wave launcher 10, the RF signal source from computer-
based processor 22 will experience an initial discontinuity at feeding
transition section
18 resulting from adapting EM fields of the RF signal source propagating along
coaxial cable 20 to EM fields of propagating modes excited inside EM launcher
10.
This initial discontinuity causes a part of the RF signal source to reflect
back to
computer processor 22.
Additionally, once EM waves propagating along EM wave launcher 10 reach
the near, outer surface of the refractory material, a first part of the EM
waves will
penetrate through the near, outer surface of the material and propagate inside
the
material until reaching the remote, inner surface of the material. A second
part of the
EM waves will reflect back, from the near, outer surface of the refractory
material, to
EM wave launcher 10 and a part of the reflected EM waves will propagate until
reaching computer processor 22. Upon the first part of the EM waves reaching
the
remote, inner surface of the refractory material, a third part of the EM waves
will
penetrate through and propagate inside the molten material contained within
the
furnace. A fourth part of the EM waves will reflect back, from the remote,
inner
surface of the refractory material, to EM wave launcher 10 and a part of the
reflected
EM waves will propagate until reaching computer processor 22. The second part
of
the EM waves reflect as a result of the waves propagating through a media
discontinuity between internal volume of EM wave launcher 10 at launching end
14
and the refractory material. Likewise, the fourth part of the EM waves reflect
as a
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result of the waves propagating through a media discontinuity between the
refractory
material and the molten material.
Furthermore, EM waves propagating through the refractory material may
experience discontinuities resulting from a presence of an inhomogeneous
region or a
flaw inside the refractory material. As such, a part of the EM waves will
reflect back,
from the flaw inside the refractory material, to EM wave launcher 10 and a
part of the
reflected EM waves will propagate until reaching computer processor 22.
Even further, EM waves propagating along EM wave launcher 10 will
experience an additional edge discontinuity at launching end 14. More
specifically,
the edge discontinuity will occur at edges 26a, 26b, 26c, and 26d
corresponding to
launching end 14, as shown in Fig 2A, as a result of the waves propagating
through a
media discontinuity between the internal volume of EM wave launcher 10 at
launching end 14 and media surrounding the edges, such as the near, outer
surface of
the refractory material, and the medium surrounding EM wave launcher 10, such
as
air. Accordingly, part of the EM waves will reflect back from the edges to EM
wave
launcher 10 and a part of the reflected EM waves will propagate until reaching
computer processor 22.
Moreover, the EM waves reflected from edges 26a, 26b, 26c, and 26d
corresponding to launching end 14 may reach one or more of the other edges
multiple
times to create an undesirable "ringing" or "reverberation" effect due to
multiple edge
reflections of the EM waves. Eventually, part of the multiple reflected EM
waves will
reach computer processor 22.
Likewise, any reflected wave within EM wave launcher 10 within the
refractory material or between feeding end 12 and computer processor 22 will
be
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affected by any discontinuity at the near, outer surface of the refractory
material,
launching end 14, and feeding end 12. In other words, the effects of a
discontinuity
will affect propagating EM waves regardless of the direction of propagation of
the
EM waves, either from computer processor 22 to the remote, inner wall of the
refractory material or from the remote, inner wall of the refractory material
to
computer processor 22. Accordingly, multiple EM wave reflections occur that
may
create ringing effects and adversely affect an ability of computer processor
22 to
detect a reflected EM wave of interest. In other words, a number of spurious
signals
or undesired EM wave reflections are inherently present that may cause serious
performance issues of the material evaluation system. A term commonly used to
refer
to the aggregated effects of such spurious signals or undesired EM wave
reflections is
"clutter."
In particular, a first EM wave of interest to evaluate the status of the
refractory
material is an initial reflected EM wave from the discontinuity between
launching end
14 and the near, outer wall of the refractory material to establish a
reference for
determining the thickness of the refractory material or determining the
location of a
flaw inside said material. A second EM wave of interest is an initial
reflected EM
wave from the discontinuity between the remote, inner wall of the refractory
material
and the molten material within the furnace to determine the thickness of the
refractory
material. A third EM wave of interest is an initial reflected EM wave from a
discontinuity of a flaw inside the refractory material to determine the
location of the
flaw.
Correspondingly, a number of different terms are major contributors to the
overall clutter in the system. A first term corresponds to the reflected RF
signal from
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feeding transition section 18 to computer-based processor 22. A second term
corresponds to the multiple RF signal reflections or ringing between feeding
transition
section 18 and computer-based processor 22. A third term corresponds to the
reflected
EM wave from edges 26a, 26b, 26c, and 26d at launching end 14 to computer-
based
processor 22. A fourth term corresponds to the multiple edge reflections or
ringing of
EM waves from edges 26a, 26b, 26c, and 26d at launching end 14 to computer-
based
processor 22. A fifth term corresponds to the multiple reflections or ringing
of EM
waves between the near, outer wall of the refractory material and the remote,
inner
wall of the refractory material that reach computer-based processor 22. A
sixth term
1() .. corresponds to the multiple reflections or ringing of EM waves between
a flaw inside
the refractory material and the near, outer wall of the refractory material
that reach
computer-based processor 22. A seventh term corresponds to the multiple
reflections
or ringing of EM waves between a flaw inside the refractory material and the
remote,
inner wall of the refractory material that reach computer-based processor 22.
An
.. eighth term corresponds to the multiple reflections or ringing of EM waves
between
feeding end 12 and the near, outer wall of the refractory material that reach
computer-
based processor 22. A ninth term corresponds to the multiple reflections or
ringing of
EM waves between feeding transition section 18 and feeding end 12 that reach
computer-based processor 22.
In this embodiment, an RF signal or EM wave that is received by computer-
based processor 22 goes through a coherent detector that provides voltages
proportional to the in-phase (I) and quadrature-phase (Q) components of the
received
RF signal or EM wave relative to a reference version of the original RF signal
source;
thus permitting both amplitude and relative phase to be measured. The
reference
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version of the original RF signal source is provided by a sample obtained by
means of
a directional coupler. Analog-to-digital converters output digital data
proportional to
the I voltage and the Q voltage outputs of the coherent detector. The digital
data is
then read, stored, and processed by computer-based processor 22. Optionally,
.. computer-based processor 22 further adapts the processed data to display
the results to
the user. Computer-based processor 22 has executable computer code configured
to
measure reflected EM waves received to produce frequency domain data and
transform the frequency domain data to time domain data. Furthermore, computer-
based processor 22 calibrates the time domain data to distance domain data,
identifies
.. a peak in the distance domain profile associated with an EM wave of
interest reflected
from the refractory material, and determines a distance traveled by the EM
wave of
interest.
Thus, computer-based processor 22 is capable of determining a relative time
delay between a received RF signal or EM wave and the original RF signal
source.
The time domain data can be used to determine the relative time of arrival of
each EM
wave of interest and the clutter terms. Of particular importance is that any
EM wave
of interest will be received during an interval of time between the arrival of
the first
EM wave of interest, used as a reference, and the arrival of the second EM
wave of
interest. In other words, any information of the status of the refractory
material will
arrive to computer-based processor 22 during this interval of time.
Accordingly, the
only clutter terms that may arrive during this interval of time at computer-
based
processor are those corresponding to the second, third, fourth, sixth, eighth,
and ninth
clutter terms.
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Furthermore, by selecting the length of coaxial cable 20 such that the
propagation time of an EM wave propagating throughout the length of coaxial
cable
20 is larger than the propagation time of the EM wave propagating throughout
EM
wave launcher 10 plus the propagation time of the EM wave propagating back and
forth through the thickness of the refractory material, the multiple
reflections
corresponding to the second clutter term will arrive at computer-based
processor 22
later than any EM wave of interest. Likewise, by selecting the length of
elongated
section 16 of EM wave launcher 10 such that the propagation time of an EM wave
propagating along EM wave launcher 10 is larger than the propagation time of
the EM
wave propagating through the thickness of the refractory material, the
multiple
reflections corresponding to the eighth clutter term will arrive at computer-
based
processor 22 later than any EM wave of interest.
The ringing effects produced by the sixth clutter term, i.e., the ringing of
EM
waves between a flaw inside the refractory material and the near, outer wall
of the
refractory material that reach computer-based processor 22, will arrive at
computer-
based processor 22 at the same interval time as the EM waves of interest only
if the
flaw is located closer to the near, outer wall of the refractory material than
to the
remote, inner wall of the material. However, this effect will be noticeable
only when a
flaw is present at a distance from the near, outer wall of the material that
is smaller
than half of the thickness of the material. Those skilled in the art will
realize that
measurements at multiple frequencies and known signal processing techniques
may
allow determining when this situation occurs.
As shown in Fig 3, the use of a cavity-backed feeding transitioning in this
embodiment may reduce the effects of the ninth clutter term, i.e., the ringing
of EM
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waves between feeding transition section 18 and feeding end 12 that may reach
computer-based processor 22 during the same time interval as an EM wave of
interest.
Because of the inherent wideband requirement of EM wave launcher 10, the
critical
quarter-wavelength distance is difficult to maintain over the whole frequency
band of
operation. Accordingly, the ringing effects may partly, although still
significantly, be
removed.
Therefore, the most relevant and at the same time the most difficult clutter
terms to remove from the system are those related to edges 26a, 26b, 26c, and
26d at
launching end 14. These are the third and fourth clutter terms as described
above.
The computer executable code of computer-based processor 22 allows
calibration of the time domain data to a distance domain data based upon a
known
velocity of an EM wave travelling along coaxial cable 20 and EM wave launcher
10
and through the refractory material under evaluation. Also, the reference or
zero
distance value corresponds to the transition between launching end 14 of EM
wave
launcher 14 and the near, outer surface of the refractory material. Fig 4
shows a plot
of the magnitude of the received EM waves at computer-based processor 22 as a
function of distance. This represents a possible scenario for the system shown
in Fig
1, wherein a flaw within the refractory material is present. The effect of the
clutter
terms in determining the EM waves of interest at computer-based processor 22
may
be noticed. The solid line curve represents the magnitude of the EM waves of
interest
plus the system noise. The dashed line curve represents the magnitude of the
clutter
plus the system noise. Note also that the distance interval of interest is
only the
distance corresponding to the thickness of the refractory material, in this
case
approximately 6 inches. Where the magnitude of the clutter plus noise is about
the
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same or larger than the magnitude of the EM waves of interest associated with
both
the flaw and the thickness of the refractory material, as shown in Fig 4,
those EM
waves of interest cannot be detected by computer-based processor 22. Thus,
neither
the EM wave of interest associated with the flaw of the refractory material,
showed
.. approximately at a distance of 4 inches in Fig 4, nor the EM wave of
interest
associated with the remote, inner wall of said material, showed approximately
at a
distance of 6 inches, can be detected due to the clutter effects. Accordingly,
the
thickness of the refractory material cannot be determined. In this case, only
the
magnitude of the EM wave of interest associated with the near, outer surface
of the
refractory material can be determined because it is above the magnitude of the
clutter
plus noise. However, determining the magnitude of only the EM wave of interest
associated with the near, outer surface of the material is not very useful.
Hence, it is of utmost importance to reduce the magnitude of the clutter plus
noise to a level below the magnitude of the EM waves of interest associated
with the
flaw or the thickness of the refractory material to be able to determine the
status of the
material. Typically, in most applications involving the evaluation of a
refractory
material, the clutter is so large that a material evaluation system becomes
unreliable
and, in general, unable to determine the status of the material. In addition,
known
techniques such as those based on subtraction of measurements of reflected EM
waves taken at different locations on the surface of the furnace wall are
ineffective to
reduce the clutter. The reason for the ineffectiveness of the techniques is
the
variability of the clutter component associated with each of the measurements,
caused
by variations of the surface temperature, tangent loss, and set up of EM wave
launcher
10 and the surface of the furnace wall, from measurement to measurement.
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Figs 1 and 2 show a design of EM wave launcher 10 that significantly reduces
the clutter terms related to edges 26a, 26b, 26c, and 26d at launching end 14.
As
previously indicated, the clutter terms related to edges 26a, 26b, 26c, and
26d at
launching end 14 are the most relevant and at the same time the most
challenging
clutter terms to suppress from the system. Fig 5 shows actual measurement data
of a
10-inches thick refractory material installed on an operating furnace. In this
case, the
thickness of the refractory wall was selected to be free of flaws and be so
thick that
there are no reflected EM waves from flaws and the reflected EM waves from the
remote, inner surface of the furnace wall are so attenuated that they do not
reach
computer-based processor 22. Thus, Fig 5 shows only results of clutter plus
noise
measurements for an EM wave launcher 10 with rolled edges and a substantially
similar EM wave launcher 10 with no rolled edges. A solid line curve
represents the
magnitude of clutter plus noise of the processed time domain data using an EM
wave
launcher 10 with rolled edges, as described above. The dashed line curve
represents
the magnitude of clutter plus noise where a substantially similar EM wave
launcher
10 without rolled edges is used. As seen in Fig 4, an effect of using an EM
wave
launcher with rolled edges is a reduction in clutter plus noise of around 20
to over 30
dB in a region where the reflected EM wave of interest associated with the
near, outer
surface of the furnace wall would be expected to appear, such as the region
where
time is larger than 1 nanosecond.
Another effect of using an EM wave launcher with rolled edges is a reduction
in clutter plus noise of as much as 10 dB for the reflected EM wave of
interest
associated with the near, outer surface of the furnace wall. Also, because the
system
noise is substantially similar in both cases, when using EM wave launcher 10
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and without rolled edges, the reductions in clutter plus noise levels observed
in Fig 5
correspond primarily to reductions in clutter levels.
With regard to Fig 1, in which a single EM wave launcher 10 is used, such a
system is commonly referred to as a mono-static configuration. Optionally, an
additional EM wave launcher 10 may be added to only receive reflected EM
waves.
In such configuration, commonly known as a bi-static configuration, a first
"active"
EM wave launcher 10 will be used to launch the EM waves into the material
under
evaluation as shown in Fig 1. A second "passive" EM wave launcher 10 is placed
next
to the first EM wave launcher 10. The second EM wave launcher 10 will only
receive
reflected EM waves. Thus, the reflected EM waves return to computer-based
processor 22 using a different path from the path used by the launched EM
waves.
This provides an inherent separation between launched and received EM waves.
Unlike Fig 1, this bi-static configuration does not require an additional
component,
such as a directional coupler, to separate transmitted and received EM waves
coming
.. from and going to computer-based processor 22 to perform a coherent
detection of the
reflected EM waves.
Preferably, in a bi-static configuration, a center point of an imaginary plane
containing launching end 14 of the first EM wave launcher 10 is placed as
close as
possible to a corresponding center point of a plane containing launching end
14 of the
second EM wave launcher 10, having both launching ends conformally placed in
contact with the near, outer surface of the refractory material. One reason
for a
preferred minimum separation between EM wave launchers in this configuration
is
that the distance traveled by the reflected EM waves is shorter, which results
in less
losses. A second reason is that the second EM wave launcher will be able to
receive
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more reflected EM waves, especially those EM waves reflected at angles near
180
degrees with respect to the launched EM waves. Furthermore, to receive the
reflected
EM waves having a substantially same electric field polarization as an
electric field
polarization of the launched EM waves, in certain situations, an orientation
of the
first EM wave launcher 10 with respect to the second EM wave launcher 10 may
be
selected to have edges 24a, 24b, 24c, and 24d at launching end 14 of the first
EM
wave launcher 10 be substantially parallel to edges 24a, 24b, 24c, and 24d at
launching end 14 of the second EM wave launcher 10. Those skilled in the art
will
recognize that a relative orientation of the first EM wave launcher 10 with
respect to
the second EM wave launcher 10 may need to be adjusted to receive the
reflected EM
waves having a substantially desired electric field polarization ¨ such as co-
polarized,
cross-polarized, or any combination thereof ¨ as compared to an electric field
polarization of the launched EM waves. Furthermore, the second EM wave
launcher
is not required to be identical or similar to the first EM wave launcher.
With regard to still further aspects of the invention, where transverse
electric
and magnetic (TEM) waves are exclusively used, EM wave launcher 10 may be
configured to have only two opposite side plates made of conductive material.
In
other words, in a first configuration only side plates 24a and 24c are made
using a
conductive material. In a second configuration, only side plates 24b and 24d
are made
using a conductive material. The preferred thickness dimensions for these two
different configurations are the same as for the configuration having four
conductive
side plates, as shown in Fig 2. Thus, more specifically, a first group of two
opposite
side plates of EM wave launcher 10 are made of conductive material, and a
second
group of two opposite side plates may be removed, be made of a dielectric or
other
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material as known in the prior art, or simply be replaced by opposite surfaces
of a
solid filling dielectric material such as ceramic.
Further, EM wave launcher 10 may alternatively be provided at least two
opposite side plates in which a material having a variable conductivity is
disposed,
instead of being made using a conductive material. Those skilled in the art
will realize
that one or more coating applications of a conductive material applied to a
dielectric
material filling the internal volume of EM wave launcher 10 may be used to
achieve a
desired profile of variable conductivity along the side plates. Alternatively,
a film,
uniform in thickness and having a variable conductivity may be disposed
between
feeding end 12 and launching end 14. More specifically, the variable
conductivity
material may be disposed on at least side plates 24a and 24c or at least side
plates 24b
and 24d. In this alternative embodiment, the internal volume of EM wave
launcher 10
is filled with a solid dielectric, preferably ceramic. A variable conductive
film is
disposed on two opposite side surfaces of the dielectric, going from feeding
end 12 to
launching end 14, to form side plates 24a and 24c or 24b and 24d of EM wave
launcher 10. In this configuration, a first end of the variable conductivity
material is
disposed closer to feeding end 12, and a second end of the variable
conductivity
material is disposed closer to launching end 14. Thus, electromagnetic waves
propagate in EM wave launcher 10 within a region partly surrounded by the
variable
conductivity material, wherein the conductivity varies as a function of the
distance
from a point on the variable conductivity material to launching end 14.
Alternatively,
multiple sections of conductive films, each having a different conductivity,
may be
arranged sequentially from lower to higher conductivity to create an
increasing
conductivity profile as a function of distance from the first to the last of
the sections.
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The thickness of each individual layer of conductive film is preferred to be
in the
range of between 0.001 inches and 0.1 inches.
Typically, a sheet resistance characterizes the degree of conductivity of a
thin
film layer of material of uniform thickness. A larger sheet resistance
corresponds to a
lower conductivity and vice versa. In the configuration described immediately
above,
the sheet resistance of the variable conductivity material increases following
an
exponential function from the first end of the variable conductivity material,
closer to
feeding end 12, to the second end of the variable conductivity material,
closer to
launching end 14.
In particular, the lowest value of sheet resistance of the variable
conductivity
material at the first end, closer to feeding end 12, is preferred to be below
1 Ohm per
square. More preferably, the lowest value of sheet resistance is similar to
the sheet
resistance of a conductive material such as copper or silver. On the other
hand, the
highest value of sheet resistance of the variable conductivity material at the
second
end, closer to launching end 14, is preferred to be in a range somewhere
between 50
Ohms per square and 1000 Ohms per square. More preferably, the lowest value of
sheet resistance is similar to the sheet resistance of a dielectric material
such as
ceramic. In other words, the variable conductivity material behaves as a
conductive
material closer to feeding end 12 and gradually transitions to have the
preferred
maximum sheet resistance value as the variable conductivity material gets
closer to
launching end 14. This variable conductivity profile provides a significant
reduction
of reflections of EM waves from the edges at launching end 14. Accordingly,
the
variable conductivity profile provides a significant reduction of clutter
resulting from
EM waves reflecting from the edges at launching end 14.
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The above described variable conductivity profile is substantially the same
for
each of at least two opposite side plates 24a and 24c or 24b and 24d of EM
wave
launcher 10. However, those skilled in the art will realize that different
profiles in
each side plate may be used. In general, the profile of the sheet resistance
of the
variable conductivity material may increase following a step, elliptical,
exponential,
or a smooth transitioning function, or any combination thereof, optimally
designed to
reduce the clutter, from the first end of the variable conductivity material,
closer to
feeding end 12, to the second end of the variable conductivity material,
closer to
launching end 14.
A critical issue in using a resistive film disposed relatively close to
launching
end 14 is that, under normal operating conditions, the refractory material may
reach
temperatures of several hundred degrees Fahrenheit at the near, outer surface
of the
furnace. Launching end 14 is in physical contact with the hot material. Hence,
most
likely, the film may be physically damaged unless protected. A conductive film
may
be sandwiched in between two layers of high-temperature adhesive to protect
the film.
This three-layer structure may be disposed on at least two opposite side
surfaces of a
dielectric material filling the internal volume of EM wave launcher 10, going
from
feeding end 12 to launching end 14, to form side plates 24a and 24c of EM wave
launcher 10. In the present embodiment, the dielectric material and the three-
layer
structure was cured at a temperature of approximately 300 degrees Fahrenheit
for a 2-
hour period. Preferably, the film and each of the layers of adhesive has a
thickness
ranging somewhere between 0.001 inches and 0.01 inches.
More preferably, the layers of adhesive have similar electrical properties as
the
electrical properties of the dielectric material. Furthermore, high-
temperature ceramic
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cement or other equivalent material may be placed on top of the three-layer
structure
for increased protection. In this manner, a compact packaging is provided to
not only
protect the film from physical damage due to the high temperatures experienced
by
launching end 14 and from manipulation during set up and operation of EM wave
launcher 10, but also to hold the film in place during operation. Those
skilled in the
art will realize that various types of adhesives and cement materials
commercially
available may be used, typically having a curing time between one hour and
three
hours at temperatures ranging from 200 to 500 degrees Fahrenheit.
The effects of configuring EM wave launcher 10 using a variable conductivity
material as described are so significant in reducing clutter terms related to
the edges
of the launching end 14 of EM wave launcher 10 that an embodiment using the
variable conductivity material may not require flared or rolled edges at
launching end
14. Thus, either a first configuration using an EM launcher with rolled edges
or a
second configuration using an EM wave launcher having at least two side plates
with
.. a variable conductivity material may be used to significantly reduce edge
reflections
in most applications. Of course, a third configuration combining both
techniques to
reduce edge reflections will provide further improvement to the material
evaluation
system.
Launching end 14 of EM launcher 10 may extend following a topology of the
near, outer surface of the material to be evaluated. Alternatively, the rolled
edges of
launching end 14 of EM wave launcher 10 may follow a circular function or
other
function that smoothly extends away sufficiently enough from transition points
28a,
28b, 28c, and 28d so as to reduce the effects of edge reflections.
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Optionally, the entire material evaluation system may be packaged into a
single portable unit in which an operator triggers the launch of EM waves,
over a
frequency band, by activating a switch. More specifically, the entire material
evaluation system may be enclosed in a single hand held unit. The unit may
evaluate
the status of the furnace wall at a single point and record the information in
a built-in
memory. Alternatively, the EM wave launcher along with a subset of components
of
the material evaluation system may be integrated into a single assembly to
launch the
EM waves and to only measure, record, and store the amplitude and phase of the
EM
waves coming into the EM wave launcher. Then the stored data may be
transferred to
computer-based processor 22 using a portable memory drive or by means of a
flexible
cable for evaluating the status, or ultimately determining the thickness, of
the subject
material under evaluation. Alternatively, the data may be transferred
wirelessly in real
time or at a convenient opportunity. Furthermore, the hand held unit may
include data
processing components and a display to show the thickness of the furnace wall
and/or
the distance from the outer, near surface of the refractory material to a
discontinuity
embedded in the material under evaluation. The portable unit may be designed
to scan
by hand an area of the furnace wall while taking measurements at multiple
locations.
Moreover, EM wave launcher 10 may be periodically used for one or more
evaluations of said material under evaluation, or may be installed permanently
and
fixed onto the outer, near surface of the material under evaluation to
continuously
monitor the status of the material under evaluation. Alternatively, a region
of the
outer, near surface of the material under evaluation may be scanned, by moving
the
EM wave launcher, during operation, over and while maintaining physical
contact
with the outer, near surface of the material under evaluation.
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The RF front-end of RF subsystem 23 of computer-based processor 22 may be
integrated with feeding transition section 18 of EM wave launcher 10. In other
words,
coaxial cable 20 may be removed from the system as it is no longer required.
In this
situation, any multiple reflections between the RF front-end and feeding
transition
.. section 18 will arrive to computer-based processor 22 before any of the
reflected EM
waves of interest. Alternatively, coaxial cable 20 may be disposed following a
predetermined physical route to produce maximum stability of the RF signal or
the
EM wave travelling in the cable. Furthermore, such stability may be
accomplished by
mechanically attaching the cable to a supporting structure, so as to minimize
any
movement of coaxial cable 20. Likewise, preventing coaxial cable 20 from
following
a route requiring the cable to bend beyond a certain angle from a straight-
line routing
may help in reducing the overall clutter in the system.
Those skilled in the art will recognize that EM wave launcher 10 may be
implemented using multiple devices and materials in various configurations
that
include one or more of an antenna, a waveguide, a dielectric material, a
conductive
material, a material having a variable conductivity, a metamaterial, or any
combination thereof configured in different geometrical arrangements.
In particular, Fig 6 shows an optional configuration of a planar EM wave
launcher 60 comprising a bow-tie antenna having a first layer 62a of
conductive
material and a second layer 62b of conductive material, wherein the edges of
both of
layers 62a and 62b are linearly tapered to have a triangular shape and are
disposed on
a top surface of dielectric substrate 64. EM wave launcher 60 is typically fed
by a
balanced-to-unbalanced device, referred to as a "balun," that adapts an
impedance of
an unbalanced transmission line, such as a coaxial cable, to an input
impedance of the
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bow-tie antenna. In this configuration, the input impedance of the bow-tie
antenna is
substantially matched to the impedance of the near, outer surface of the
refractory
material. Substrate 64 has an underside surface, with a layer of conductive
material
disposed over all of the underside surface to form a ground plane, and two
openings to
allow the balun to feed the bow-tie antenna. Typically, these openings are
made
through the smallest dimension or thickness of substrate 64 and are large
enough to
just allow a wire to go through each opening and electrically connect the
balun to
each layer 62a and 62b at points where the layers are at its closest distance,
approximately 0.1 inches in this case, as it is well understood by those
skilled in the
art. In this configuration, the dimensions of substrate 64 are 4 inches long,
3 inches
wide, and 0.27 inches thick. A maximum width of each layer 62a, 62b is
approximately 2.7 inches, and a length of approximately 1.95 inches. The
thickness of
each layer 62a, 62b is typical of those previously described corresponding to
a film or
coating of conductive material applied to a dielectric substrate.
Additionally, substrate
64 may have a dielectric permittivity somewhere between 1 and 150, and a
tangent
loss between 0 and 1.
In a typical evaluation of a material, the top surface of substrate 64,
containing
the bow-tie antenna, is conformally placed against the near, outer surface of
the
refractory material to launch EM waves, coming from computer-based processor
22,
into the refractory wall and to receive reflected EM waves going back to
computer-
based processor 22. Those skill in the art will realize that layers 62a and
62b can be
implemented by means of a variable conductivity material as described in
previous
embodiments of EM wave launcher 10. Likewise, the shape of layers 62a and 62b
can
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be other than triangular, having straight edges, curved edges that follow a
particular
function, or a combination thereof.
Similarly, Fig 7 shows a configuration for planar EM wave launcher 60 of Fig
6, having substrate 64 consisting of a first planar section 64a, a first
curved-edge
section 64b, a second curved-edge section 64c, a second planar section 64d,
and a
third planar section 64e. First planar section 64a extends over a plane from
the bow-
tie feeding area in a first dimension along the width of substrate 64 until
reaching the
width of substrate 64, in this case approximately 3 inches, and in a second
dimension
along the length of substrate 64 until reaching transition points 66a, 66b,
66c, and
66d; in this case, the distance between transition points 66a and 66b and
between
transition points 66c and 66d is approximately 4 inches.
As the first curved-edge section 64b and the second curved-edge section 64c
extend away from the feeding point of the bow-tie antenna along the length of
substrate 64, sections 64b and 64c bend towards the underside surface of
substrate 64
following a circular path with a radius of curvature of approximately 1.6
inches for a
quarter of circumference to reach transition points 68a, 68b, 68c, and 68d. In
other
words, the distance along the curved path of substrate 64 between transition
points
66a and 68a is approximately 2.51 inches. This is substantially the same
distance
between transition points 66b and 68b, transition points 66c and 68c, and
transition
points 66d and 68d, respectively. Likewise, this is the same length of section
64b and
section 64c along the curved path of substrate 64. At transition points 68a
and 68c,
second planar section 64d begins to extend the length of substrate 64 by
approximately 0.5 inches. Correspondingly, at transition points 68b and 68d,
third
planar section 64e begins to extend the length of substrate 64 by
approximately 0.5
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inches. As such, second planar section 64d, and a third planar section 64e are
substantially perpendicular to first planar section 64a.
In the configurations shown in Figs 6 and 7, the strongest clutter terms
correspond to multiple reflections at the edges of the bow-tie antenna. The
dimensions
of the curved edges of the configuration of Fig 7 are selected to extend the
propagation time of the EM wave propagating on the surface of substrate 64
such that
the time is longer than the propagation time of an EM wave propagating from
the
near, outer surface of the refractory wall to the remote, inner surface of the
refractory
wall. In this manner, the clutter effects associated with the multiple
reflections of EM
waves from the edges of the bow-tie antenna are significantly reduced. Those
skilled
in the art will realize that, in the configuration of Fig 7, the edges of
layers 64b and
64c may be tapered to follow an elliptical function, an exponential function,
a smooth
transitioning function, or any combination thereof. In addition, the length of
sections
64d and 64e may be adjusted with the ultimate goal of reducing the clutter.
Furthermore, in each of the above-described configurations, a person of
ordinary skill in the art will realize that a particular single signal
processing method
may be selected according to an estimated thickness of the material to be
evaluated.
For example, a signal processing method based on a Fourier Transform may be
used
to process the data received by computer-based processor 22, especially
related to the
evaluation of walls with thickness larger than 6 inches. On the other hand,
signal
processing methods based on super resolution algorithms would be preferred for
evaluation of walls with thickness below 3 inches. Alternatively, a hybrid
signal
processing method comprised of one or more single signal processing methods
may
be used according to additional factors including the frequency of operation
and
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bandwidth of the system, the temperature of operation of the furnace, and the
type and
quality of the refractory material.
Likewise, in each of the above-described configurations, the launching end of
EM wave launcher 10 is, as discussed elsewhere in this specification,
impedance
matched to the material under evaluation, which further helps to suppress
clutter.
Regarding each of the above-described configurations, a method depicted in
Fig 8 for determining the thickness of the subject material under evaluation,
such as
refractory material, may be performed according to the following:
1. At step 810, setting up an EM wave launcher by placing a launching
.. end of the EM wave launcher conformally contiguous to an outer, near
surface of the
material under evaluation to maximize physical contact, which corresponds to
minimizing gaps, between the launching end of the EM wave launcher and the
outer,
near surface of the material under evaluation, such that upon operation of the
EM
launcher, EM waves are launched into the outer, near surface of the material
under
evaluation.
2. Next, at step 820, launching EM waves from the EM launcher into the
outer surface of the material under evaluation by exciting EM wave propagating
modes inside the EM wave launcher over a transmit frequency range, and
correspondingly generating EM waves propagating inside the EM wave launcher
from
a feeding end of the EM wave launcher to the launching end of the EM wave
launcher, over said frequency range.
3. Next, at step 830, measuring the amplitude and the phase of EM waves
coming into the EM wave launcher over the frequency range, as a result of
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propagation of the EM waves launched by the EM wave launcher into the outer
surface of the material under evaluation.
4. Next, at step
840, storing the measured amplitude and phase frequency
domain data of the EM waves coming into the EM wave launcher.
5. Next, at step
850, transferring the recorded frequency domain data to a
computer-based data processor.
6. Next, at step 860, transforming the recorded frequency domain data to
time domain data by performing a mathematical inverse Fourier transform or
other
model-based inverse spectral transformation method, using the computer-based
data
processor.
7. Next, at step 870, calibrating the time domain data to distance domain
data, according to the known or estimated phase velocity of the EM waves in
the
material under evaluation, and defining a reference point in a distance domain
profile,
based on a peak value over a clutter plus noise level of the calibrated
distance domain
data, that corresponds to the physical length between the feeding end of the
EM wave
launcher and the outer, near surface of the material under evaluation; wherein
the
reference point may be associated with an EM wave reflected into the EM wave
launcher from the outer, near surface of the material under evaluation.
8. Next, at step 880, evaluating the calibrated distance domain data to
identify a peak value, over the clutter plus noise level, between the
reference point
and a known original thickness of the material under evaluation, which may be
associated with an EM wave reflected into the EM wave launcher from the inner,
remote surface of the material under evaluation.
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9. Last,
at step 890, determining a distance from the identified peak value
at step 880 to the reference point; the distance corresponding to the
thickness of the
material under evaluation (distance between the outer, near surface and the
inner,
remote surface of the material under evaluation).
Those of ordinary skill in the art will recognize that the steps above
indicated
can be correspondingly adjusted for specific configurations and other
constraints such
as measurement equipment, operating frequency band, type of EM wave launcher,
operational conditions, surrounding environment, and available area and
location for
implementation of the material evaluation system for a given application. In
particular, measurements of the amplitude and the phase of EM waves, required
over
a high dynamic range (in some cases in excess of 90 dB), may be accomplished
in
multiple ways, such as through use of a network analyzer, to measure the Si 1
scattering parameter, over a frequency band, using a monostatic configuration
(a
single device to both launch EM waves and receive EM waves) or to measure the
521
scattering parameter, over a frequency band, using a bistatic configuration (a
first
device to launch EM waves and a second device to receive EM waves).
Additionally, those skilled in the art will recognize that, while evaluating
the
calibrated distance domain data, intermediate peak values over the clutter
plus noise
level may appear between the reference point, associated with an EM wave
reflected
from the outer, near surface of the material under evaluation, and the peak
value
associated with an EM wave reflected from the inner, remote surface of the
material
under evaluation; it being understood that the intermediate peak values may be
associated with flaws of the material under evaluation existing between the
outer, near
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surface of the material under evaluation and the inner, remote surface of the
material
under evaluation.
Furthermore, the calibration of the time domain data to distance domain data
includes the subtraction of the delay time (distance) associated with the EM
wave
launcher and cables. Moreover, the frequency dispersion effects of the EM wave
launcher and the material under evaluation may be removed, if necessary, by
normalizing the measured data of the material under evaluation with respect to
another set of measured data corresponding to a reference configuration, by
way of
non-limiting example, of a known characteristic and thickness of a material
similar to
1() the material under evaluation, through processes well known to those
skilled in the
art.
Still further, it is noted that the material evaluation system as described in
Fig
1 operates in the frequency domain by launching EM waves at specific
frequencies
within the frequency band of interest. Then the recorded frequency domain data
is
transformed to time domain data for further processing. However, the system
may be
implemented to operate in the time domain as well. Preferably, in the time
domain
operation mode, the EM wave launcher may launch a plurality of EM waves in the
frequency domain, such that the time domain representation of this plurality
of EM
waves corresponds to an RF waveform of short duration, for example a Gaussian,
Rayleigh, Hermitian, Laplacian pulse or of the like or a combination thereof.
Alternatively, the EM wave launcher may be directly fed with such type of
pulse by a time domain RF waveform generator. More preferably, the duration of
the
RF waveform is not larger than 5 nanoseconds. Accordingly, the EM wave
launcher
will transmit and receive time domain pulses and the system will measure the
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amplitude and time of arrival of these pulses, store the measured data, and
transfer the
stored data to the computer-based processing unit in the time domain. Then the
steps
may proceed of calibrating the time domain data to distance domain data,
evaluating
the calibrated domain data to identify the location of flaws on the inner,
remote
surface of the material under evaluation, and determining the thickness of the
material
under evaluation.
In each of the above-described configurations, as applicable, the length of
the
elongated section of the EM wave launcher 10, between feeding end 12 and the
portion of the launching end farthest away from feeding end 12, is selected to
be long
enough such that the multiple reflections corresponding to the eighth clutter
term and
other potential multiple reflections will arrive at computer-based processor
22
distinctively later than any EM wave of interest. In this situation, the
propagation time
of an EM wave propagating along the EM wave launcher 10 is distinguishably
larger
than the propagation time of the EM wave propagating through the thickness of
the
refractory material under evaluation.
Accordingly, it is of utmost importance to configure the EM wave launcher 10
such that the time of receipt of an EM wave of interest, reflected from a
remote
discontinuity of the material under evaluation, is distinguishable from the
time of
receipt of spurious signals, reflected from the near surface of the material
under
evaluation. Hence, the length of the elongated section 16 of EM wave launcher
10,
between feeding end 12 and the portion of the launching end farthest away from
feeding end 12, may be shortened. The shortened length is selected to be short
enough
such that the multiple reflections corresponding to the eighth clutter term
and other
potential multiple reflections, including at least part of those corresponding
to the
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fourth clutter term, will arrive at the computer-based processor earlier than
any EM
wave of interest. In this case, the propagation time of an EM wave propagating
along
EM wave launcher 10 is distinguishably shorter than the propagation time of
both the
EM wave propagating through the thickness of the refractory material under
evaluation and multiple reflections from the launching end 14.
Fig 9 shows a perspective view of an electrically-small EM wave launcher 90,
comprising a feeding end 92 and a launching end 94. In particular, the length
between
feeding end 92 and launching end 94 is electrically small enough such that
multiple
EM wave reflections between launching end 94 and feeding end 92 arrive at a
time
distinguishably earlier than the time of arrival of any EM wave of interest.
Likewise,
multiple EM wave reflections from the near, outer wall of the refractory
material
under evaluation (not shown) will arrive at a time distinguishably earlier
than the time
of arrival of any EM wave of interest.
Feeding end 92 includes a feeding transition section 96 that electrically
connects to a radiofrequency (RF) transmission line (not shown) through a
coaxial
connector 98. In this configuration, feeding transition section 96 consists of
a coaxial
cable-fed cavity-backed feed. In particular, feeding transition section 96 is
formed by
an air-filled box of conductive material having rectangular cross-sections
along each
dimension of the box. The design characteristics of feeding transition section
96,
corresponding to a coaxial cable-fed cavity-backed feed with rectangular cross-
sections, are well-known in the prior art and are equivalent to those of
feeding
transition section 18, as described above in reference to Fig 3. Those skilled
in the art
will realize that the RF transmission line electrically connected to feeding
transition
section 96 may comprise a coaxial cable, a coplanar waveguide, a stripline or
a
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microstrip. In an alternative configuration a coplanar waveguide is configured
to
impedance-match and directly connect to at least one component of an RF
transmitter
or an RF receiver.
Accordingly, EM wave launcher 90 may be used in place of EM wave
launcher 10 as indicated in the above-described embodiments, as applicable.
More
specifically, in this embodiment, EM wave launcher 90 consists of a modified
pyramidal horn antenna with a rectangular cross section. EM wave launcher 90
structurally comprises a first flared plate 91a and a second flared plate 91b
disposed
opposite to plate 91a, like in a standard pyramidal horn antenna, having a
rectangular
cross section, in which two opposite smaller plates are removed. In this
configuration,
first and second flared plates 91a, 91b are made of a conductive material.
The thickness of first and second flared plates 91a, 91b at launching end 94
defines a first edge 93a and a second edge 93b each of which may range in
length
from 0.01 to 1 inch. Importantly, prior art EM wave launchers, such as
standard horn
.. antennas, have a typical thickness-to-length ratio of the flared plates in
the order of
less than 5%. However, in a preferred configuration, the thickness of first
and second
flared plates 91a, 91b is uniform, the thickness-to-length ratio of flared
plates 91a,
91b is in the range of 15% to 85%, and the length of each edge 93a, 93b is at
least
0.25 inch to smoothen the discontinuity experienced by an EM wave propagating
.. along EM wave launcher and reaching a third edge 93c at launching end 94.
More preferably, one or more edges of first and second flared plates 91a, 91b
at launching end 94 may be rolled following an elliptical function or other
smooth
function to reduce the effects of edge reflections, for example, as described
above in
reference to Figs 1 and 2A-2D. Most preferably, at least a portion of one or
more of
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edges of first and second flared plates 91a, 91b at launching end 94,
including edges
93a, 93b, or corresponding edges adjacent or opposite to edges 93a, 93b, have
a
rolled configuration.
Alternatively, the thickness of first and second flared plates 91a, 91b need
not
be uniform from feeding end 92 to launching end 94 and may have a variable
thickness profile. In the case where the thickness of first and second flared
plates 91a,
91b is variable, a preferred thickness profile may include a gradually
increasing
thickness at each of first and second flared plates 91a, 91b, having the
thickest portion
at edges 93a, 93b of launching end 94. Furthermore, first and second flared
plates 91a,
91b of EM wave launcher 90 may comprise a material having a variable
conductivity
instead of being made of a conductive material, as described above in
reference to
Figs 2A-2D.
In the particular configuration shown in Fig 9, the volumetric region wherein
the EM waves propagate within the EM wave launcher, in between first and
second
flared plates 91a, 91b, from feeding end 92 to launching end 94 is filled with
a
dielectric material 95, which may be a gas, liquid, or solid. Preferably,
dielectric
material 95 is a solid material having an impedance that substantially matches
a
predetermined impedance of the refractory material under the normal operating
conditions of the furnace, as explained in the above-described embodiments,
particularly in reference to Figs 2A-2D.
More preferably, the volumetric region defined by dielectric material 95 is
larger than the above-referenced volumetric region wherein the EM waves
propagate
within the EM wave launcher, such that a portion 95a of dielectric material 95
and a
corresponding identical opposite portion (not shown) at least partly protrude
only
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from the sides of first and second flared plates 91a, 91b going along from
feeding end
92 to launching end 94. Accordingly, an edge 95b of dielectric material 95 is
aligned
with edges 93a and 93b, such that the side of EM wave launcher 90, at
launching end
94, and edges 93a, 93b, 93c, and 95b all lie in the same plane. Most
preferably,
portion 95a of dielectric material 95 and the corresponding identical opposite
portion
(not shown) of dielectric material 95 symmetrically protrude on the sides of
EM wave
launcher 90 from feeding end 92 to launching end 94, each having the shape of
a
wedge with a thick end at feeding end 92 of at least 0.05 inch. This
configuration of
dielectric material 95 contributes to the mitigation of both multiple EM wave
1()
reflections from the edges of flared plates 91a, 91b and coupling to and from
external
devices and other components.
Those skilled in the art will recognize that dielectric material 95, portion
95a,
and the corresponding identical opposite portion (not shown) may each comprise
different shapes, sizes, and types of materials.
Preferably, EM wave launcher 90 is designed to operate in the frequency
range from 0.25 GHz to 6 GHz. As a result, the dimensions of the rectangular
cross
section (width and height) at launching end 94 of EM wave launcher 90, the
length of
EM wave launcher 90, and the dielectric properties of dielectric material 95
are all
selected to enable EM wave launcher 90 to operate within this frequency range
as
well known in the prior art. In addition, EM wave launcher 90 is designed to
tolerate
the required operating temperature range of the near, outer surface of a
furnace wall.
Optionally, EM wave launcher 90 can be used standalone (mono-static) or in
array of more than one unit (multi-static) configuration, as described above
in
reference to Fig 1 and well known in the prior art. Likewise, EM wave launcher
90
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may be part of an entire material evaluation system packaged into a single
portable
unit, a single hand held unit, or integrated into a single assembly as
described above.
In general, the various above-described configurations and method may be
implemented to collect measurement data directly as part of a time domain-
based
material evaluation system. Accordingly, one or more time-domain pulses are
transmitted and the corresponding reflected pulses are recorded for data
processing
and material evaluation. In particular, and with reference to Fig 9, EM wave
launcher
90 may be implemented to operate in a time domain-based material evaluation
system. In this case, the length of flared plates 91a, 91b is preferably
dimensioned
such that the propagation time of a transmitted pulse from feeding end 92 to
launching
end 94 of EM wave launcher 90 is less than 1 nanosecond.
The various embodiments have been described herein in an illustrative
manner, and it is to be understood that the terminology used is intended to be
in the
nature of words of description rather than of limitation. Any embodiment
herein
disclosed may include one or more aspects of the other embodiments. The
exemplary
embodiments were described to explain some of the principles of the present
invention so that others skilled in the art may practice the invention.
Obviously, many
modifications and variations of the invention are possible in light of the
above
teachings. The present invention may be practiced otherwise than as
specifically
described within the scope of the appended claims and their legal equivalents.
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