NZ718193B2 - Nondestructive, absolute determination of thickness of or depth in dielectric materials - Google Patents
Nondestructive, absolute determination of thickness of or depth in dielectric materials Download PDFInfo
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- NZ718193B2 NZ718193B2 NZ718193A NZ71819314A NZ718193B2 NZ 718193 B2 NZ718193 B2 NZ 718193B2 NZ 718193 A NZ718193 A NZ 718193A NZ 71819314 A NZ71819314 A NZ 71819314A NZ 718193 B2 NZ718193 B2 NZ 718193B2
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- thickness
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B15/00—Measuring arrangements characterised by the use of electromagnetic waves or particle radiation, e.g. by the use of microwaves, X-rays, gamma rays or electrons
- G01B15/02—Measuring arrangements characterised by the use of electromagnetic waves or particle radiation, e.g. by the use of microwaves, X-rays, gamma rays or electrons for measuring thickness
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N22/00—Investigating or analysing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimetre or more
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N22/00—Investigating or analysing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimetre or more
- G01N22/02—Investigating the presence of flaws
Abstract
Enhanced measurement of thickness in bulk dielectric materials is disclosed. Microwave radiation is partially reflected at interfaces where the dielectric constant changes (e.g., the back wall of a part). The reflected microwaves are combined with a portion of the outgoing beam at each of at least two separate detectors. A pair of sinusoidal or quasi-sinusoidal waves results. Thickness or depth measurement is enhanced by exploiting the phase and amplitude relationships between multiple sinusoidal or quasi-sinusoidal standing waves at detectors sharing a common microwave source. These relationships are used to determine an unambiguous relationship between the signal and the thickness or depth. wo separate detectors. A pair of sinusoidal or quasi-sinusoidal waves results. Thickness or depth measurement is enhanced by exploiting the phase and amplitude relationships between multiple sinusoidal or quasi-sinusoidal standing waves at detectors sharing a common microwave source. These relationships are used to determine an unambiguous relationship between the signal and the thickness or depth.
Description
NONDESTRUCTIVE, ABSOLUTE DETERMINATION OF
THICKNESS OF OR DEPTH IN DIELECTRIC MATERIALS
Jack R. Little, Jr.
This invention was made with government support under contract numbers
FA865008C5306 and 12C5109 awarded by the United States ment of
Defense (United States Air . The United States Government has certain rights
in this invention.
The benefit of the 25 September 2013 filing date of United States
provisional patent application serial number 61/882,288 is claimed under 35 U.S.C.
§ 119(e) in the United States; and is claimed under applicable es and
conventions in all other countries. The complete disclosure of priority ation
61/882,288 is hereby incorporated by reference in its entirety.
TECHNICAL FIELD
This invention pertains to an tus and method for the non-destructive
determination of the depth of features in a dielectric material, the thickness of a
dielectric material, and the use of thickness information so determined in
nondestructive evaluation (NDE) of bulk dielectric materials.
BACKGROUND ART
There is an unfilled need for improved, nondestructive means to test bulk
dielectric materials for flaws, defects, larities, and other features; and
particularly to determine the absolute thickness of bulk dielectric materials when
given access to only one side of a part under inspection. Additionally, there is an
unfilled need for improved, nondestructive means to ine variations in the
density (or porosity) when the thickness of a bulk tric material is known. For
example, there is an unfilled need for improved, nondestructive means for examining
dielectric materials in three dimensions, volumetrically, and measuring both thickness
and changes in thickness. For a manufactured tric component that has been in
service for some time, the remaining thickness is often important as an indicator of
the component’s remaining life; but it can be difficult to measure thickness when only
one surface of the component is accessible. Density can also be a major indicator of
the serviceability of manufactured dielectric components, because the density often
s ly to the strength of the component. The ions of a manufactured
part are often known or are easily measured, but it is more difficult to determine
density and variations in density. There is an ed need for improved means for
the nondestructive determination of density and changes in density of a bulk
dielectric material when its thickness is known.
For example, there is an unfilled need for enhanced, nondestructive
means for ing the ing wall thickness in dielectric tanks and pipes.
(This invention has numerous applications, and is not limited to the inspection of
tanks and pipes.)
Modern chemical processing often involves the use of components made
of dielectric materials. Common tric material product forms include fiber
reinforced plastic (often called “fiberglass” or “FRP”) pipes and vessels. These
materials are also commonly used in modern infrastructure, such as drinking water
and waste water processing. There exists an unfilled need for improved means to
e the thickness of such materials nondestructively, especially for means that
may be employed while the produce remains in-service, and where there is access
to only one side of the tric component. (This invention has numerous
applications, and is not limited to the inspection of FRP.)
Due to the corrosive or abrasive nature of the fluids that are often used in
various processes, the wall thickness often diminishes over time as a direct result of
service-induced degradation. These service-induced thickness changes are difficult
to detect nondestructively through conventional means.
It is highly desirable that a testing method should be nondestructive, and
that it should be usable whether a facility is running or idle. Furthermore, because
the access space outside the component can be limited, and the geometry of a
component can be complex, any portion of the detection machinery that must be in
contact with the ent (or in the vicinity of the ent) should be small
enough to accommodate the available space and geometry.
When the component to be tested is made of metal, then well-established
ultrasonic inspection techniques can be used. However, ultrasonic inspection cannot
be used effectively for reinforced dielectric materials, because the structural fibers
scatter nearly all sound energy, and return little usable signal. The mesh or fabric of
a composite material so ly scatters and disperses onic waves that the
resulting reflection is ely noisy. Eddy current measurements or magnetic
measurements do not work well in these materials either, because they do not
conduct electricity.
Neither is radiography particularly helpful. X-ray radiography can be used
to detect changes in bulk y or to detect changes in thickness, but it requires
access to both sides of the component under inspection. This renders X-ray
radiography ctive for in-service inspection of many components.
Another example of an unfilled need for ed methods to measure
density lies in the field of engineered ceramic composites. In such composites both
the reinforcing fibers and the matrix are made of a ceramic material. Typically, the
fibers are woven or otherwise arranged into a t structure into which the matrix
is deposited by chemical methods. The matrix is typically ted iteratively. The
chemical reaction that results in the deposition occurs in sequential steps, with each
step ting additional c material into the tices between reinforcing
fibers. Since the location of the fibers and the outer boundary of the part do not
change, the porosity of the part decreases (and its density correspondingly
increases) with each iteration. When the parts are highly engineered and their
physical dimensions are closely controlled, the al thickness, measured in
inches or mm, is generally known within close tolerances. Because the th of a
part is typically a function of its density, it would be highly desirable to have improved
nondestructive means to e density. Ultrasonic methods are not effective for
determining density in such materials, due to the scattering of sound waves by the
reinforcing fibers. Neither can eddy current or magnetic methods be used, as the
ceramic composites are bulk non-conductors. Although changes in density can be
detected by radiography, the changes of interest in ceramic composite applications
are typically too small to be resolved by conventional raphy. Additionally,
radiography requires access to both sides of the part, for that reason is not an
acceptable method in many circumstances.
An ew of microwave testing techniques is given in A. Bahr,
Microwave Nondestructive Testing Methods (1982).
Several microwave nondestructive testing techniques are disclosed in A.
Lucian et al., "The Development of Microwave NDT logy for the Inspection of
Nonmetallic Materials and Composites," pp. 2 in Proceedings of the Sixth
Symposium on Nondestructive tion of Aerospace and s Systems
Components and Materials (San o, TX 1967).
J. Kurian et al., "Microwave Non-Destructive Flaw/Defect Detection
System for Non-Metallic Media Supported by Microprocessor-Based
Instrumentation," J. Microwave Power and Electromagnetic , vol. 24, pp. 74-
78 (1989) discloses a method for detecting defects in a tire by measuring
transmission of microwaves from a dipole transmitting antenna inside the tire,
through the treads of the tire, with transmission detected by a linear array of
detectors. Differential rates of transmission were correlated with changes in
thickness or with defects.
C. Howell et al., The Use of Low Cost Industrial AM—CW ’Microwave
Distance Sensors’ for Industrial Control Applications (no date) discloses a microwave
distance sensor to measure ces to an object from about 15 centimeters to
about 6 meters away, by measuring the phase angle of a returned amplitude-
modulated microwave signal reflected from the .
United States patent no. 3,278,841 discloses a microwave flaw detection
system, particularly for use with large, solid-propellant rocket motors. Microwaves
were transmitted from inside the propellant, reflected off a metal casing, and
detected by a receiver displaced from the microwave transmitter. lrregularities in the
th of the received signal were correlated with cracks or other flaws in the
propellant.
United States patent no. 4,520,308 discloses a system for measuring the
ess of a dielectric material by measuring the phase shift of microwaves
transmitted along a microwave strip line conductor adjacent to the material whose
thickness is being measured. See also United States patent no. 4,123,703.
United States patent no. 2,999,982 ses a Doppler-effect-based
method for microwave detection of inhomogeneities in compact als such as
polished glass. Relatively high scanning speeds were used to produce a Doppler
effect. In the one example given, the relative speed of the glass versus the detector
was 650 centimeters per second.
United States patent no. 3,144,601 discloses a method for microwave
detection of inhomogeneities in nducting als such as glass sheets and
plates. Detection was performed by simple measurement of the echoes of the
reflected microwaves; by measuring losses in intensity following transmission
through the object; or by mixing incident and ted waves to create beats,
particularly when the material being examined was ing (i.e., ing Doppler
shifts in the frequency of the reflected microwaves).
United States patent no. 3,271,668 discloses the use of microwaves to
measure the rate of progressive attrition from a surface of a body of a solid dielectric
material; for example, measuring the burning profile in a solid rocket motor.
Microwaves were transmitted h the fuel (or other material), the surface of
which reflected some of the microwaves back to a detector. The relative phase of
incident and reflected microwaves varied as the distance from the microwave
transmitter to the e of the burning fuel changed, allowing the distance to the
surface of the fuel to be determined as a function of time.
United States patent no. 4,707,652 discloses a technique for detecting
impurities in a bulk material by measuring changes in the scattering of microwave
radiation incident on the bulk material.
United States patent no. 4,514,680 discloses a method for detecting knots
in lumber, by transmitting microwaves through the lumber from two sources of the
same intensity, but with a 180-degree phase shift. Transmitted microwaves are
detected on the opposite side of the lumber. If the lumber is knot-free, there is a null
in the microwave field at the detectors, but if a knot is present the phase and
amplitude of microwave radiation at the detectors are altered.
United States patent no. 4,581,574 discloses a method for determining the
average tric constant of a dielectric material having a conductive e, by
transmitting microwaves from two transducers into a sheet of the material, and
making measurements of the energies of reflected microwaves. By measuring
average tric constants along a plurality of paths in the plane of the sheet,
locations of variations within the sheet may be identified.
United States patent no. 4,274,288 ses an acoustic, interferometric
method for measuring the depth of a surface flaw such as a crack.
United States patent no. 4,087,746 discloses a method for determining
optical anisotropy in a dielectric material by ing changes in the polarization of
microwaves transmitted through the material.
United States patent no. 6,172,510 discloses the probing of targeted
portions of a layered material by ave ion focused onto the targeted
portion by adjustment of antenna position and ation establishing a single
oblique incidence path for reflection of antenna emitted probing radiation. Signal
measurements of the radiation along the oblique incidence path are obtained to
provide for tion and detection of defects in the targeted portion of the ure
being probed.
A. Khanfar et al., “Microwave near-field tructive detection and
characterization and disbands in concrete structures using fuzzy logic techniques,”
Composite Structures Elsevier UK, vol. 62, pp. 335-339 (2003) discloses a near-field
ave nondestructive testing technique for disbond/crack detection and
evaluation in a concrete structure. The frequency of operation and standoff ce
could be optimized to achieve maximum sensitivity to the presence of a disband,
which is viewed as an additional layer and which changes the properties of the
effective tion cient (phase and magnitude). The change depends on the
thickness and location of the disbond. Multiple frequency measurements could be
used to obtain disbond on and thickness information. A fuzzy logic model was
described relating the phase of reflection coefficient, frequency of operation, and
standoff distance to the disbond thickness and depth.
S. Ganchev et al., “Microwave ion of defects in glass reinforced
polymer composites,” Proc. SPIE — International Society for Optical Engineering
USA, vol. 2275, pp. 11-20 (1994) discloses the use of microwaves for defect and flaw
ion in glass reinforced polymer composites. The ff distance and the
ncy were studied as means of increasing detection sensitivity.
A prior microwave method for the nondestructive testing of dielectric
components employs virtual standing waves. See US. Patents 6,359,446,
7,777,499, 6,653,847, and 8,035,400 These methods, while effective for detecting
and characterizing thickness or density changes over a small range (plus or minus 1/:
of the wavelength “A” in the al being inspected), can give ambiguous results in
some circumstances. Several different values for the thickness or density can
correspond to a single value of the measured output. Despite the improvements
represented by these earlier methods, the 8,035,400 patent frankly acknowledged:
“There can be ambiguity in interpreting an interferometric signal, as points within the
specimen that are spaced an integral number of half-wavelengths apart may not
initially be distinguished from one another, due to the identical phase of the waves
reflected from such points (where the wavelength in question is that within the
material, which generally differ from the wavelength in air or vacuum, depending on
the index of refraction).” One solution proposed was that “if a frequency is chosen to
reduce the number of wavelengths needed to traverse the thickness of the
specimen, one may enhance the sensitivity at a selected depth range with minimal
ambiguity. In the special case where the specimen thickness is less than (preferably
substantially less than) half the wavelength, then the imaging may be optimized for a
single, very narrow band of the thickness within the specimen.” However, no solution
was proposed for the more general problem of ing these ambiguities when the
ess of the specimen can be several multiples of a wavelength. There is an
unfilled need for ed testing methods that can resolve such ities in
measurements of bulk dielectric thickness, density, or features.
See also US. s 5,539,322, 5,574,379, 5,216,372, 6,005,397,
3,025,463, 4,344,030, 214, 5,384,543, 7,190,177; Japanese patent abstract
61274209; and published international application WO9710514.
DISCLOSURE OF INVENTION
l have discovered an improved high resolution method and apparatus to
determine depth and ess in bulk tric als. l have discovered a novel
way to resolve the ambiguity in depth or thickness that was left unresolved in earlier
interferometry-based nondestructive measurement techniques. The novel method
can unambiguously resolve depth and thickness with high precision. Monochromatic
radiation, preferably microwave radiation, more ably microwaves in the 5-50
gigahertz ncy range, is used to interrogate a sample. The microwaves are
partly reflected at each e where the dielectric constant changes (e.g., to
measure thickness changes as the microwave beam encounters the back wall of the
specimen under inspection, with g ce between the back wall of the
specimen and the microwave source and detectors). In a preferred ment, the
apparatus comprises a single ave source, and two or more ors. The
distance(s) between the detectors (and therefore their phase onship) is known
(or can be measured). A portion of the transmitted beam is combined with the signal
reflected by the specimen under inspection. These two signals have the same
frequency, but may differ in amplitude and phase. The signals combine at the
location of each detector to produce an interference pattern, a pattern that changes
as the thickness of the specimen changes, or as the position of the specimen
changes relative to the or. For each detector, the interrogating ion may
be thought of as a sinusoidal (or quasi-sinusoidal) standing wave. If one used only a
single detector, then the relationship n the or output and the sample
thickness would produce ambiguous thickness measurements, with identical output
values occurring every 1/: wavelength in thickness (or A/4) as the thickness changes
(assuming that all other parameters remained unchanged). Simply repeating the
measurements with multiple detectors does not resolve the ambiguity, regardless of
the spacing of the detectors (whether spacing is measured in distance or in phase).
As used in the specification and claims, unless context clearly indicates
otherwise, terms such as “thickness,” “depth,” and the like should be understood as
referring in the first ce to distance as measured in units of the wavelength (A) of
the microwave energy that is used to perform the inspection, where the wavelength
is the effective wavelength in the material under inspection, which in l will
differ from the wavelength in air (or vacuum). Conversion to other convenient units
(e.g., mm, cm, in) may easily be performed where desired. Fundamentally, an initial
determination in accordance with the present invention determines the number of
unit wavelengths in the material being inspected, with sion into measurements
in other units being secondary or d from the number of wavelengths thus
determined.
l have discovered a method of “combining” the output of multiple detectors
into a novel phase plot, a phase plot that can resolve the ambiguity that is otherwise
inherent in measurements of thickness, depth, etc. A simple example will illustrate:
er an embodiment with two detectors spaced A/4 apart (based on A in air).
(The technique can be generalized to detector numbers greater than two, and to
detector gs other than A/4. In general, increasing the number of detectors will
improve resolution.) For the purposes of this illustration, the distance n the
microwave source (and detectors) to the front surface of the specimen will be held
constant. Thus the phase relationship between the front surface and the detectors is
constant. Additionally, the contribution of the front e reflection to the signal at
each detector remains constant in both phase and amplitude, even as the thickness
varies. The output signals from the two detectors vary periodically in the ess
domain, either idally or quasi-sinusoidally. For an individual detector, this
periodic behavior produces an ambiguity in the inferred thickness. The present
invention allows this ambiguity to be resolved. It is preferred that the spacing
between the two ors be chosen so that the absolute value of one detector’s
output is a maximum when the output of the other detector is y between its
own maximum and minimum. (This point may or may not be equal to zero,
depending on where the null is set in a particular case.) Alternatively, it is preferred
that the spacing between two detectors be chosen so that the absolute value of the
slope of a line tangent to the standing wave is a maximum on one or when the
slope of a line tangent to the standing wave for the other detector is zero. To
illustrate, in a hypothetical ideal case where the output signal is precisely sinusoidal,
the preferred g between the detectors would be such that the phase difference
between the two detectors is 90° + (n x 180°), where n is an integer (which may be
positive, ve, or zero).
When the output signals of the two ors are plotted against one
another (not necessarily as a direct function of time, nor necessarily as a direct
function of distance, but against one r) — for example with the voltage at the
“A” detector as the “X” value and the voltage at the “B” detector as the corresponding
“Y” value in an (X,Y) data pair — then the resulting plot will generally be an ellipse (or
quasi-ellipse), as depicted schematically in Figure 1. (The ellipse could even be a
circle if the relative sensitivities of the two detector outputs were identical.) Each
time the ess of the material changes by 1/2 A, the (X,Y) data point repeats and
passes around the ellipse. This combination of the output from two detectors
extends the unambiguous range for measuring thickness by a factor of 2, from 1A A to
1/2 A. If a straight line is drawn from any point on the ellipse to the , the angle
from that line to the x-axis (or any other fixed line passing h the origin)
ponds to the thickness within a range of 1/2 A.
Actually, the idealized elliptical phase plot shown in Figure 1 is
oversimplified since it disregards the loss of microwave energy that also occurs with
changes in thickness. For a particular type of dielectric material, the attenuation
increases as a function of sample thickness. As depicted in Figure 2, the
attenuation losses convert the theoretically lossless phase plane ellipse of Figure 1
into ing more r to an elliptical . These losses actually are
beneficial for the measurement process, because they provide additional information
that can be extracted. In the phase plane spiral of Figure 2, note that the values of
(X,Y) do not repeat. The phase plane spiral curve does not cross itself, g that
the periodic ambiguity in thickness depicted in Figure 1, which exists when only the
relative phase of multiple detectors is considered, is eliminated by plotting the (X,Y)
coordinates represented by the signal from the multiple detectors in phase plane
space, with the amplitude of the signal decreasing with increasing sample thickness.
(Actually, as shown in Figure 5 of Appendix A of priority application 61/882,288, even
with the novel phase plane analysis there can still sometimes be s of ambiguity
arising from internal reflections of microwaves from boundaries. Even when such
complications exist, most measurements in the phase plane analysis still produce
unambiguous determinations of ess / depth.)
Apparent changes in thickness, measured in units of the microwave
wavelength in the material, can result either from actual changes in dimension (e.g.,
measured in inches or centimeters), or from density changes (which cause s
in refractive index and therefore ngth). Unless context clearly indicates
otherwise, as used in this disclosure and in the Claims, the term “thickness” should
be understood to refer to the nt thickness of a material, as measured in units
of wavelength of the ogating radiation. In other words, the “thickness” is the
apparent thickness, which can be a function both of the actual dimensions of an
object, and its density and refractive index, which may vary as a function of position.
The ion provides an apparatus and method for the non-destructive
determination of specimen thickness (or feature depth), measured in units of
wavelength within the inspected material, and the use of thickness information so
determined in nondestructive evaluation (NDE) of bulk dielectric materials. The
refractive index of a material depends on its chemical composition. The refractive
index also varies as a function of the density, even with a constant al
ition. The dependence of refractive index upon density results in a change in
the wavelength of the electromagnetic energy as it propagates through regions of
varying density. Thus the ability to determine s in the position of a standing
electromagnetic wave in a specimen, the dimensions of which are known, permits
determination of the refractive index, and hence the density (or porosity, which is
inversely related to the density).
The detector may be scanned relative to the specimen at any desired
speed, and the scanning speed need not even be m. The novel ion
technique is not based on Doppler-shifts in frequency, which result from motion, but
rather is based on interference between reflected and reference microwaves that
have substantially the same frequency, where the interference is caused by s
in location (independent of motion per so).
The novel technique can detect thickness changes and changes in
dielectric constant (which in dielectrics may, for example, result from changes in
density or porosity), in essentially any dielectric materials. The technique can also
be sfully used on composite materials containing conductive ents, but
whose construction makes them overall nonconductors -- for e, carbon fiber
composites.
The novel method and apparatus have been successfully tested in a
prototype ment. The microwave transmitter/detector was small, and readily
suited for use in environments in which access space may be limited.
The computed thickness value from the processed signal (from the
ors) may be plotted as "Z" in a nsional plot, where "X" and "Y" are
Cartesian coordinates on the surface of a specimen, to produce a map of thickness.
(Other coordinate systems may also be used in lieu of an orthogonal Cartesian
system, as convenient for the shape of the particular en being inspected, for
example cylindrical coordinates, toroidal coordinates, spherical polar coordinates,
etc.) In an alternative embodiment, a fourth dimension may be added to a plot, using
color palette ions to indicate the presence of s in the specimen.
If desired, one may ine whether a through-thickness inspection is
feasible for a particular specimen with a particular transducer by placing the
transducer against one surface of the specimen and moving an object on the far side
of the specimen. If the microwave energy fully penetrates the specimen, a change in
the position of the object on the far side of the specimen should produce changes in
the observed transducer . In such a case, a thickness ement should be
possible.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 depicts a phase plane plot showing detector A and B output
voltages as (X,Y) data pairs in two dimensions, for a case with two detectors and a
detector g of A/4. This plot is for a hypothetical, idealized case in which no
attenuation in the material occurs.
Figure 2 depicts a phase plane plot showing detector A and B output
voltages as (X,Y) data pairs in two dimensions, for a case with two detectors and a
detector spacing of A/4. This plot is also for a hypothetical, idealized case, but it
represents a more realistic system in which microwave energy is lost in the inspected
material by attenuation (as a function of what is often termed the “loss tangent” of the
material).
Figure 3 depicts schematically the distance from the Phase Plane spiral
to its center as a function of sample thickness.
Figure 4 depicts an experimentally ed thickness domain plot using
two detectors.
Figure 5 depicts an experimentally measured phase plane plot.
Figure 6 s tically, in cross section, a lass wedge that
was used for testing the prototype ment over a range of .
Figure 7 depicts schematically a prototype embodiment of an apparatus in
accordance with the present invention.
MODES FOR CARRYING OUT THE INVENTION
Substances such as fiberglass that produce noisy reflection patterns in
ultrasonic ques may be inspected at low noise levels with the novel microwave
technique. For example, the novel technique readily detects thickness changes in
fiberglass, or in ceramic composites.
There are many potential fields of use for the invention. As one illustration,
the invention may be used to inspect fiber-reinforced plastic (FRP) pipe.
Commercially available FRP pipe is a complex composite structure, typically
containing many layers of varying composition, density, and dielectric constant.
When aves are directed towards an FRP pipe, reflections return from all
interfaces between materials of different dielectric constant. The returning signal is a
superposition of many different waveforms, essentially cal in frequency, but
generally differing in phase and amplitude. In general, the full thickness of the
material is inspected, and all interfaces upon which the microwaves impinge
contribute to the returning waveform. The present invention is capable of measuring
the remaining thickness in such materials with a high degree of precision, accuracy,
and repeatability.
Mixing a portion of the outgoing radiation with the reflected rm
results in a complex standing rm. (The waveform is “standing,” i.e.,
unvarying, similar to a vibrating string, so long as the relative positions of the
transducer and the specimen remain unchanged, but in general it will vary as those
positions vary.) The standing waveform that corresponds to a single detector passes
from the inspection device (transducer), then usually through an intervening medium
such as air, and then into the specimen. As the waveform passes through
components of the specimen having differing indices of refraction, the ngth
changes, while the frequency remains nt. The higher the index of refraction,
the slower the propagation of electromagnetic , and the shorter the
wavelength s.
The t invention is capable of determining thickness unambiguously,
using a ation of phase and amplitude measured by multiple detectors when
the specimen is irradiated from a common microwave source.
Figure 7 depicts schematically a prototype embodiment of an apparatus in
accordance with the present invention. The prototype apparatus comprised a
transducer with a single microwave source (transmitter) and two detectors. The
detectors output two ls of SIGNAL information (not shown). The detectors
WO 47931
were separated by approximately 0.12 inch (0.30 cm) in the direction of propagation,
corresponding to imately 1/4 wavelength. The SIGNALS from the two
detectors were transferred to signal conditioning electronics, where they were
amplified, filtered, and ioned prior to being sent to an analog-to-digital
converter (ADC). There were two position encoders, “X-Pos” and “Y—Pos” (not
shown), the outputs of which were also sent to the ADC. The ADC transmitted digital
data, containing SIGNAL information for both channels, and X and Y location data, to
a processing computer, which then created images for chosen regions of interest.
In the prototype apparatus, the output voltage from the 2 separate
detectors was ed in a display and analysis computer. In future embodiments,
this data processing will be performed in a dedicated processor located on the
transducer itself.
A fiberglass wedge was constructed for prototype testing; a cross section
is illustrated schematically in Figure 6. The wedge was scanned to confirm that
able data could be collected, and that the prototype embodiment worked as
expected.
The Transducer
A red ucer was a microwave transceiver based on a Gunn
diode. See, e.g., B. Hale (ed.), The 1989 ARRL Handbook for the Radio Amateur,
pp. 32-57 & 32-58 (66th ed., 1988); The Microwave exer: An uction
(various authors, no date); MIA-COM Semiconductor Products, Varactor Tuned
Gunnplexer Transceiver "Front End" (1985); Microwave Associates, Varactor Tuned
Gunn Oscillator Transceivers for cial Applications (1977). The transducers
that were used in prototype embodiments of the invention were tunable 10, 25, or 35
gigahertz transceivers (frequencies could be higher or lower if desired, e.g., 5 - 50
GHz). The transceiver could be used with or without a waveguide section. The
detector had two microwave frequency diodes orated as part of the assembly.
The detector diodes were located inside the outgoing radiation beam, between the
aperture and the front surface of the transducer housing. The transducer was
frequency-stable, and ed only a 5-10 Volt DC power supply to produce the
desired microwave output es. It was mounted in a housing, which could either
be moved by hand, or in future embodiments will preferably be moved by an
automated tion device (a robot).
The transducer included mechanical means to control the stand-off
distance; the stand-off distance is preferably held nt. The transducer was
connected to signal processing electronics, data acquisition hardware, and an
imaging and analysis er via a multiple-conductor cable.
The transducer was also connected to a position-encoder system for
determining the X and Y position of the transducer. The position encoder outputs
were fed to the computer for use in imaging and analysis of the specimen.
Signal sing and Power Supply
In the prototype embodiment, the detection diodes were located at fixed
positions within the path of the outgoing microwave beam, so that the output signal
maintained a constant amplitude and frequency as seen by each detection diode.
Alternatively, the positions of the detection diodes could be made variable,
independent of varying the stand-off distance. Microwaves radiated from the
transducer to the specimen being tested. Each time the microwave beam came to
an interface between materials of different dielectric constants (e.g., the interface
between air and the specimen, or the interface between the bulk specimen and a
flaw or feature within it), a portion of the microwave energy was transmitted, and a
portion was reflected. The portion that was reflected depended on the angle of
incidence, the difference in the dielectric constants n the materials (which is
related to the index of tion), the surface e, and other factors. Some of the
reflected portion of the interrogating beam returned to the transducer, where it was
detected by the detector diode(s). The reflected signal and the transmitted signal
were of identical frequency, but (in l) differed in both amplitude and phase.
These simple sinusoids or quasi-sinusoids added together (were mixed) at the
detecting diode(s), to produce a DC voltage that changed as the sample (or n
of sample) under inspection changed. In most specimens there are many interfaces,
producing many reflected signals. However, regardless of the complexity of the
reflected signal, the detector diode(s) output produced a constant DC voltage when
the on of the transducer relative to the specimen and the interrogating
frequency were both held constant. This constant DC voltage is sometimes referred
to as the "SIGNAL." The “SIGNAL” may comprise multiple components, from
multiple detectors.
The SIGNAL was erred to signal sing electronics via a wired
connection. The observed SIGNAL was lly on the order of 1-100 olts at
the input of the signal processing electronics. The SIGNAL was converted from
analog to l form by the data acquisition system described r. The analog
SIGNAL was digitized for maximum resolution of the SIGNAL voltage.
Routing the SIGNAL directly to the data acquisition system would have
diminished the resolution for extremely small defects that the intrinsic frequency
stability and low noise of the transducer would ise permit. An amplifier was
therefore included in the signal processing components, prior to the ADC. The
amplifier improved SIGNAL resolution by a factor greater than 100, while maintaining
an acceptable signal-to-noise ratio.
The data acquisition system supported eight differentially-connected
analog input channels, each with its own negative signal connection. At least two
analog channels were used to input amplified SIGNAL. Additional digital channels
were used for input from the X and Y position encoders. In general, it is preferred to
collect on information for both the X and Y ons of the transducer.
However, it sometimes suffices to collect position information from a single position
r. For example, when collecting data for a specimen having the shape of a
right circular cylinder, the transducer may revolve radially around the cylinder while
progressing axially down the cylinder at a known rate. Then the Y position is a direct
on of the X position, and a single position encoder may suffice.
When the data from a scan over multiple locations is yed graphically
at an appropriate scale, the resulting image shows thickness changes in the
specimen. Typically, the collected t contains far more detail than is
conveniently represented in a single image. The regions of interest are therefore
selected, and an image is created by ng the scale and color (or gray scale)
gradient for the SIGNAL for a clear visual display of the features of st. The
stand-off spacing is selected to obtain the depth resolution desired, which is a
function of the frequency of the microwaves, and the index of refraction of the
specimen. When a transducer with multiple detectors is used (rather than a single
or), then the number of scans may be reduced, as multiple images optimized
at different depths may be created with data from a single scan.
The power supply for the microwave generator comprised a regulated,
low-voltage power supply between 5 and 12 VDC, e of supplying current
ient to drive the Gunn diode. The 5-12 volts were delivered to the transducer
housing, where power was delivered to the ucer. The same power supply was
configured to provide the required voltages for the amplifiers, position encoders, and
data acquisition system. Power for the data acquisition system could also be
provided by the notebook computer via the USB interface.
Signal Analysis and Determination of Thickness
The novel technique is based, in part, on the principle that interfaces
between materials with different dielectric constants (including, for example, l
thickness changes) act as microwave reflectors. When a scan is made by measuring
the SIGNAL at different X and Y positions, and the data are used to create an image,
these thickness changes can be displayed directly (as in a thickness map of the
specimen).
During scanning with the prototype device, information was aneously
gathered for values of the two SIGNAL channels, the X location, and the Y location.
These data were processed by computer to e a two-dimensional image of the
SIGNAL.
Obstacles overcome by the present invention.
The distance from a point on the Phase Plane spiral to its center, termed
the r magnitude,” changes more-or-less exponentially with thickness
(becoming longer at lower esses, and shorter at higher thicknesses — see Fig.
3). This ideal exponential behavior occurs when the effective channel gains are
identical and the effective phase difference between the two detectors is precisely
A/4 (A in air). Since a combination of vector magnitude and phase angle is used to
determine thickness unambiguously, ideally the transducer is positioned so that the
effective phase angle n detectors is A/4.
If the microwave propagation could indeed be described accurately as a
simple plane wave propagation, then the optimum distance between the two
detectors (in the direction of propagation) would simply be A/4. However, when an
actual transducer was built with a simple A/4 cement n the detectors
and tested, the observed results were not as expected. The propagation of
microwaves inside a finite transducer is in fact neither an idealized cal wave
front, nor an idealized planar wave front, but instead is a complex hybrid between
these two idealized cases. As a result, the m distance between the detectors
is not simply A/4, as one might lly expect. Instead, the optimum distance may be
empirically determined for a particular ngth and a particular waveguide.
When the detectors are optimally positioned, the vector magnitude varies
approximately exponentially with the thickness, and does not oscillate substantially
as the thickness changes.
An additional complication is that the Phase Plane plot is truly ric
only when the effective gains used to amplify the signals from all detectors are equal.
However, unlike for the case for incorrect physical placement of the detectors, when
different gains are used the unequal gain ratios can be corrected in post-processing.
With previous microwave inspection ques, it has sometimes been
necessary to adjust the “null” or offset voltage of a detector signal to prevent signal
saturation and clipping. When the null e is changed for either or both
detectors, the center of the phase plane plot will move as well. This complication
has made it difficult or impossible to determine thickness unambiguously using prior
microwave inspection techniques, because for any real sample only a very small
portion of the phase plane spiral is available. (Indeed, if the thickness does not
, only a single point is known.) If the location of the center is not known, then
the vector magnitude cannot be ated and the thickness cannot be determined.
By contrast, in the current invention the null or offset voltages are preferably
maintained constant, so that the center of the phase plane spiral is known, and the
vector magnitude and thickness can be determined unambiguously.
A preferred method for practicing the invention is to plot points in phase
plane space as described above, and to correlate those points with unique distances
empirically. However, those of skill in the art will recognize that other methods of
achieving the same result are mathematically equivalent to the preferred method.
For es of the present specification and , any mathematically equivalent
method is considered to be identical to the preferred method, and to be within the
scope of the invention as defined.
The te disclosures of all references cited in this ication,
including ty application 61/882,288 and the ix to the priority application,
are hereby incorporated by reference. In the event of an othen/vise irreconcilable
conflict, however, the present specification shall control.
Claims (19)
1. A method for nondestructively and unambiguously measuring the thickness of a bulk tric material, or measuring the depth of a feature in a bulk dielectric material, or both; said method sing the steps of: (a) generating microwaves from a microwave source, wherein the microwaves have substantially nt frequency; (b) directing a first portion of the generated microwaves to the material to produce a standing wave; wherein the standing wave is a function of the wavelength of the microwaves, of the distance from the source to the nearest e of the material, and of the distance from the source to farthest surface of the al or the distance from the source to a feature within the material; (c) mixing the ted microwaves, in each of at least two different ors, with a second portion of the generated microwaves to produce an interference signal for each of the detectors; wherein the detectors are spatially displaced from one another; wherein the interference signal is a on of the thickness of the material, or a on of the depth of a feature within the material, or both; and wherein there is a phase difference between the different detectors at the frequency of the microwaves as a consequence of the spatial displacement between the detectors; (d) determining, for each of one or more locations on or in the material, a point in a phase plane space whose coordinates correspond to the magnitude and sign of the interference signals that are produced at each of the detectors for each of the one or more locations; and (e) correlating, for each of the one or more locations on or in the material, the point that is determined in the phase plane space with a unique thickness for the material at each location, or with a unique depth for a e within the material, or both.
2. The method of Claim 1, wherein said method is used to measure the thickness of the material.
3. The method of Claim 1, wherein said method is used to measure the depth of a feature in the material.
4. A method sing repeating the steps of Claim 1 for a plurality of locations on or in the material, and forming an image that displays graphically the s in thickness, or that ys graphically the depths of features, or both for each of the plurality of locations; y the image visually depicts the thickness of the material, or the locations of features within the material, or both.
5. The method of Claim 4, wherein the image is two-dimensional.
6. The method of Claim 4, wherein the image is three-dimensional.
7. The method of Claim 4, wherein the image is three-dimensional, and wherein the image additionally depicts a fourth dimension via changes in the color of the image, wherein variations in the color indicate the presence of defects in the material.
8. The method of Claim 1, wherein the resolution of the thickness measurements, depth ements, or both is substantially smaller than the ngth of the microwaves.
9. The method of Claim 1, wherein the material is a composite material.
10. The method of Claim 1, wherein the phase difference between at least one pair of the detectors is about urth the wavelength of the aves.
11. The method of Claim 1, wherein said method is repeated at each of a plurality of different microwave frequencies, to enhance the resolution of discrimination between different substances that have differential responses to radiation as a function of microwave frequency.
12. An tus for nondestructively and guously measuring the thickness of a bulk dielectric material, or measuring the depth of a feature in a bulk dielectric material, or both; said apparatus sing: (a) a tor of microwaves of substantially constant frequency; wherein said generator is d to direct a first portion of the generated microwaves to the material to create a standing wave; wherein the standing wave is a function of the wavelength of the microwaves, of the distance from the source to the nearest surface of the material, and of the distance from the source to farthest e of the material or the distance from the source to a feature within the material; (b) at least two different detectors displaced spatially from one another, wherein each of said detectors is adapted to add the reflected microwaves with a second portion of the generated microwaves to produce an interference pattern for each of said detectors; wherein the interference pattern is a function of the thickness of the al, or a function of the depth of a feature within the material, or both; n there is a phase difference between the different said detectors at the frequency of the microwaves as a consequence of the displacement between said detectors; and (c) a computer mmed to determine, for one or more ons on or in the material, a point in a phase plane space whose coordinates correspond to the magnitude and sign of the interference signals that are produced at each of said detectors for the one or more locations; and to correlate, for each of the one or more locations, the point that is determined in the phase plane space with a unique thickness for the material at each location, or with a unique depth for the feature within the material, or both.
13. The apparatus of Claim 12, wherein the computer is programmed to determine, for a ity of locations on or in the material, the thickness of the material, or the depth of a feature in the material, or both; and to form an image that displays cally the changes in thickness or that displays cally the depths of features, or both for each of the plurality of locations, whereby the image visually depicts the thickness of the material, or the locations of features within the al, or both.
14. The apparatus of Claim 13, n the image is two-dimensional.
15. The apparatus of Claim 13, wherein the image is three-dimensional.
16. The apparatus of Claim 13, wherein the image additionally depicts a fourth dimension of information via changes in the color of the image, wherein ions in the color indicate the presence of defects in the material.
17. The apparatus of Claim 12, wherein said apparatus is adapted to measure thickness, feature depth, or both with a resolution that is substantially smaller than the wavelength of the microwaves.
18. The apparatus of Claim 12, wherein the phase difference between at least one pair of said detectors is about urth the wavelength of the microwaves.
19. The apparatus of Claim 12, n said apparatus is programmed to repeat the thickness or depth measurements at each of a plurality of different microwave frequencies, to enhance the resolution of the discrimination between different substances that have differential responses to radiation as a on of microwave frequency. nmfid gesuma, mama “mg“ flu u.3 fl;31‘“8 nua1.W
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US201361882288P | 2013-09-25 | 2013-09-25 | |
US61/882,288 | 2013-09-25 | ||
PCT/US2014/056730 WO2015047931A1 (en) | 2013-09-25 | 2014-09-22 | Nondestructive, absolute determination of thickness of or depth in dielectric materials |
Publications (2)
Publication Number | Publication Date |
---|---|
NZ718193A NZ718193A (en) | 2020-09-25 |
NZ718193B2 true NZ718193B2 (en) | 2021-01-06 |
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