United States Patent 
6,581,014

Sills
, et al.

June 17, 2003

Apparatus and method for analysis of guided ultrasonic waves
Abstract
Method and apparatus for using a guided wave to determine the location of
one or more flaws in an inspected object are disclosed. An ultrasonic
guided wave is launched into the object using conventional ultrasound
methods, and the reflected/received guided wave is sampled to capture a
series of individual reflected waveforms. The individual reflected
waveforms are then partitioned according to the sampling time. Each of the
partitioned acquired waveforms is compared with a selected timevarying
dispersionmodeled reference waveform associated with the unique geometry
of the inspected object, a multiplicity of "theoretical" flaw locations,
and the characteristics of the original ultrasonic guided wave. To make a
comparison, the reference waveform is also generated as a series of
partitioned waveforms which model the shape of a wave that may be expected
to be reflected from a multiplicity of theoretical flaws located in the
object. The shape of the reflected waveforms is correlated with the shape
of the reference waveforms, and a high level of correlation indicates the
presence of a real flaw at the theoretical flaw location.
Inventors:

Sills; James A. (San Antonio, TX);
Schwartz; Christian J. (San Antonio, TX)

Assignee:

Southwest Research Institute (San Antonio, TX)

Appl. No.:

828640 
Filed:

April 9, 2001 
Current U.S. Class: 
702/39 
Intern'l Class: 
G01B 005/28 
Field of Search: 
702/39
73/644,622,637,618,602
235/151
128/660

References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Barlow; John
Assistant Examiner: Lau; Tung S
Attorney, Agent or Firm: Gunn, Lee & Hanor
Parent Case Text
REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application for
Patent No. 60/219,762 titled "TimeVarying Matched Filter Method for
Analysis of Guided Ultrasonic Waves" filed on Jul. 20, 2000, is related
thereto, is commonly assigned therewith, and the subject matter thereof is
incorporated herein by reference in its entirety.
Claims
What is claimed is:
1. A method for using a selected ultrasonic guided wave having a
characteristic to determine a location of a real flaw in an inspected
object having a geometry and a multiplicity of theoretical flaw locations,
comprising the steps of:
selecting a timevarying dispersionmodeled reference signal associated
with the geometry of the inspected object, the multiplicity of theoretical
flaw locations within the inspected object, and the characteristic of the
guided wave;
launching the selected ultrasonic guided wave into the inspected object;
receiving a reflected signal generated by the interaction of the selected
ultrasonic guided wave and the geometry of the inspected object including
any real flaw located therein;
comparing the selected timevarying dispersionmodeled reference signal
with the received reflected signal;
determining the location of the real flaw in the inspected object if the
timevarying dispersionmodeled reference signal resembles the received
reflected signal;
wherein the timevarying dispersionmodeled reference signal comprises a
plurality of partitioned reference waveforms, and wherein the step of
comparing the timevarying dispersionmodeled reference signal with the
received reflected signal includes the steps of:
partitioning the received reflected signal according to a time of reception
so as to produce a plurality of partitioned reflected signal waveforms;
and
comparing the plurality of partitioned reflected signal waveforms with the
plurality of partitioned reference waveforms to determine a level of
correlation.
2. The method of claim 1, wherein each one of the plurality of partitioned
reflected signal waveforms has a corresponding reflected signal wave
shape, wherein each one of the plurality of partitioned reference
waveforms has a corresponding reference signal wave shape, and wherein the
step of comparing the plurality of partitioned reflected signal waveforms
with the plurality of partitioned reference waveforms to determine a level
of correlation includes the step of comparing the corresponding reflected
signal wave shape with the corresponding reference signal wave shape.
3. The method of claim 1, wherein each one of the plurality of partitioned
reflected signal waveforms has a corresponding reflected signal envelope,
wherein each one of the plurality of partitioned reference waveforms has a
corresponding reference signal envelope, and wherein the step of comparing
the plurality of partitioned reflected signal waveforms with the plurality
of partitioned reference waveforms to determine a level of correlation
includes the step of comparing the corresponding reflected signal envelope
with the corresponding reference signal envelope.
4. The method of claim 1, wherein the step of determining the location of
the real flaw in the inspected object if the timevarying
dispersionmodeled reference signal is substantially similar to the
reflected signal includes the steps of:
comparing the level of correlation with a predetermined correlation level;
and
providing an indication of the location of the real flaw in the inspected
object if the level of correlation is greater than about the predetermined
correlation level.
5. The method of claim 1, wherein the step of partitioning the received
reflected signal according to a time of reception of each waveform therein
so as to produce a plurality of partitioned reflected signal waveforms
includes the steps of:
sampling the received reflected signal at a preselected sampling time
interval to obtain a single partitioned reflected signal waveform;
storing the single partitioned reflected signal waveform; and
repeating the steps of sampling and storing for a predetermined acquisition
time period.
6. A method for using a selected ultrasonic guided wave having a
characteristic to determine a location of a real flaw in an inspected
object having a geometry and a multiplicity of theoretical flaw locations,
comprising the steps of:
selecting a timevarying dispersionmodeled reference signal associated
with the geometry of the inspected object, the multiplicity of theoretical
flaw locations within the inspected object, and the characteristic of the
guided wave;
launching the selected ultrasonic guided wave into the inspected object;
receiving a reflected signal generated by the interaction of the selected
ultrasonic guided wave and the geometry of the inspected object including
any real flaw located therein;
comparing the selected timevarying dispersionmodeled reference signal
with the received reflected signal;
determining the location of the real flaw in the inspected object if the
timevarying dispersionmodeled reference signal resembles the received
reflected signal;
wherein the step of comparing the timevarying dispersionmodeled reference
signal with the received reflected signal includes the steps of:
partitioning the received reflected signal according to a time of reception
so as to produce a plurality of partitioned reflected signal waveforms;
selecting a single partitioned reflected signal waveform from the plurality
of partitioned reflected signal waveforms associated with a predetermined
time of acquisition;
generating a single partitioned reference signal waveform associated with
the predetermined time of acquisition; and
comparing the single partitioned reflected signal waveform with the single
partitioned reference waveform to determine a level of correlation.
7. The method of claim 6, wherein the step of determining the location of
the real flaw in the inspected object if the timevarying
dispersionmodeled reference signal is substantially similar to the
received reflected signal includes the steps of:
comparing the level of correlation with a predetermined correlation level;
and
providing an indication of the location of the real flaw in the inspected
object if the level of correlation is greater than about the predetermined
correlation level.
8. A method for using a selected ultrasonic guided wave having a
characteristic to determine a location of a real flaw in an inspected
object having a geometry and a multiplicity of theoretical flaw locations,
comprising the steps of:
selecting a timevarying dispersionmodeled reference signal associated
with the geometry of the inspected object, the multiplicity of theoretical
flaw locations within the inspected object, and the characteristic of the
guided wave;
launching the selected ultrasonic guided wave into the inspected object;
receiving a reflected signal generated by the interaction of the selected
ultrasonic guided wave and the geometry of the inspected object including
any real flaw located therein;
comparing the selected timevarying dispersionmodeled reference signal
with the received reflected signal;
determining the location of the real flaw in the inspected object if the
timevarying dispersionmodeled reference signal resembles the received
reflected signal;
wherein the timevarying dispersionmodeled reference signal represents one
of a plurality of reference propagation modes, and wherein the step of
comparing the timevarying dispersionmodeled reference signal with the
received reflected signal includes the steps of:
selecting at least one reference propagation mode from the plurality of
reference propagation modes, wherein the selected at least one reference
propagation mode includes a corresponding plurality of partitioned
reference signal waveforms;
partitioning the received reflected signal according to a time of reception
so as to produce a plurality of partitioned reflected signal waveforms;
and
comparing the plurality of partitioned reflected signal waveforms with the
corresponding plurality of partitioned reference waveforms to determine a
level of correlation.
9. An apparatus for using a selected ultrasonic guided wave having a
characteristic to determine a location of a real flaw in an inspected
object having a geometry and a multiplicity of theoretical flaw locations,
comprising:
a means for selecting a timevarying dispersionmodeled reference signal
associated with the geometry of the inspected object, the multiplicity of
theoretical flaw locations therein, and the characteristic of the selected
ultrasonic guided wave;
a means for launching the selected ultrasonic guided wave into the
inspected object;
a means for receiving a reflected signal generated by the interaction of
the selected ultrasonic guided wave and the geometry of the inspected
object including any real flaw located therein;
a means for comparing the timevarying dispersionmodeled reference signal
with the received reflected signal;
a means for determining the location of the real flaw in the inspected
object if the timevarying dispersionmodeled reference signal resembles
the received reflected signal;
wherein the means for comparing the timevarying dispersionmodeled
reference signal with the received reflected signal includes a means for
correlating the timevarying dispersionmodeled reference signal with the
received reflected signal.
10. An apparatus for using a selected ultrasonic guided wave having a
characteristic to determine a location of a real flaw in an inspected
object having a geometry and a multiplicity of theoretical flaw locations,
comprising:
a means for selecting a timevarying dispersionmodeled reference signal
associated with the geometry of the inspected object, the multiplicity of
theoretical flaw locations therein, and the characteristic of the selected
ultrasonic guided wave;
a means for launching the selected ultrasonic guided wave into the
inspected object;
a means for receiving a reflected signal generated by the interaction of
the selected ultrasonic guided wave and the geometry of the inspected
object including any real flaw located therein;
a means for comparing the timevarying dispersionmodeled reference signal
with the received reflected signal;
a means for determining the location of the real flaw in the inspected
object if the timevarying dispersionmodeled reference signal resembles
the received reflected signal;
wherein the timevarying dispersionmodeled reference signal comprises a
plurality of partitioned reference waveforms, and wherein the means for
comparing the timevarying dispersionmodeled reference signal with the
received reflected signal further includes a means for partitioning the
received reflected signal according to a time of reception so as to
produce a plurality of partitioned reflected signal waveforms.
11. The apparatus of claim 10, wherein each one of the plurality of
partitioned reflected signal waveforms has a corresponding reflected
signal wave shape, wherein each one of the plurality of partitioned
reference waveforms has a corresponding reference signal wave shape, and
wherein the means for comparing the plurality of partitioned reflected
signal waveforms with the plurality of partitioned reference waveforms to
determine a level of correlation includes a means for comparing the
corresponding received reflected signal wave shape with the corresponding
reference signal wave shape.
12. The apparatus of claim 10, wherein each one of the plurality of
partitioned reflected signal waveforms has a corresponding reflected
signal envelope, wherein each one of the plurality of partitioned
reference waveforms has a corresponding reference signal envelope, and
wherein the means for comparing the plurality of partitioned reflected
signal waveforms with the plurality of partitioned reference waveforms to
determine a level of correlation includes a means for comparing the
corresponding reflected signal envelope with the corresponding reference
signal envelope.
13. An apparatus for using a selected ultrasonic guided wave having a
characteristic to determine a location of a real flaw in an inspected
object having a geometry and a multiplicity of theoretical flaw locations,
comprising:
a means for selecting a timevarying dispersionmodeled reference signal
associated with the geometry of the inspected object, the multiplicity of
theoretical flaw locations therein, and the characteristic of the selected
ultrasonic guided wave;
a means for launching the selected ultrasonic guided wave into the
inspected object;
a means for receiving a reflected signal generated by the interaction of
the selected ultrasonic guided wave and the geometry of the inspected
object including any real flaw located therein;
a means for comparing the timevarying dispersionmodeled reference signal
with the received reflected signal;
a means for determining the location of the real flaw in the inspected
object if the timevarying dispersionmodeled reference signal resembles
the received reflected signal;
wherein the means for determining the location of the real flaw in the
inspected object if the timevarying dispersionmodeled reference signal
resembles the received reflected signal includes a means for comparing a
level of correlation between the timevarying dispersionmodeled reference
signal and the received reflected signal with a predetermined correlation
level, wherein a level of correlation approximately greater than or equal
to the predetermined correlation level provides an indication of the
location of the real flaw in the inspected object.
14. The apparatus of claim 10, wherein the means for partitioning the
received reflected signal according to a time of reception so as to
produce a plurality of partitioned reflected signal waveforms includes:
means for sampling the reflected signal at a preselected sampling time
interval to obtain a single partitioned reflected signal waveform;
means for storing the single partitioned reflected signal waveform; and
means for repeating the steps of sampling and storing for a predetermined
acquisition time period.
15. An apparatus for using a selected ultrasonic guided wave having a
characteristic to determine a location of a real flaw in an inspected
object having a geometry and a multiplicity of theoretical flaw locations,
comprising:
a means for selecting a timevarying dispersionmodeled reference signal
associated with the geometry of the inspected object, the multiplicity of
theoretical flaw locations therein, and the characteristic of the selected
ultrasonic guided wave;
a means for launching the selected ultrasonic guided wave into the
inspected object;
means for receiving a reflected signal generated by the interaction of the
selected ultrasonic guided wave and the geometry of the inspected object
including any real flaw located therein;
a means for comparing the timevarying dispersionmodeled reference signal
with the received reflected signal;
a means for determining the location of the real flaw in the inspected
object if the timevarying dispersionmodeled reference signal resembles
the received reflected signal;
wherein the means for comparing the timevarying dispersionmodeled
reference signal with the received reflected signal includes:
means for partitioning the received reflected signal according to a time of
reception so as to produce a plurality of partitioned reflected signal
waveforms;
means for selecting a single partitioned reflected signal waveform from the
plurality of partitioned reflected signal waveforms associated with a
predetermined time of acquisition;
means for generating a single partitioned reference signal waveform
associated with the predetermined time of acquisition; and
means for comparing the single partitioned reflected signal waveform with
the single partitioned reference waveform to determine the level of
correlation.
16. The apparatus of claim 15, wherein the means for determining the
location of the real flaw in the inspected object if the timevarying
dispersionmodeled reference signal is substantially similar to the
received reflected signal includes:
means for comparing the level of correlation with a predetermined
correlation level; and
means for providing an indication of the location of the real flaw in the
inspected object if the level of correlation is greater than about the
predetermined correlation level.
17. An apparatus for using a selected ultrasonic guided wave having a
characteristic to determine a location of a real flaw in an inspected
object having a geometry and a multiplicity of theoretical flaw locations,
comprising:
a means for selecting a timevarying dispersionmodeled reference signal
associated with the geometry of the inspected object, the multiplicity of
theoretical flaw locations therein, and the characteristic of the selected
ultrasonic guided wave;
means for launching the selected ultrasonic guided wave into the inspected
object;
means for receiving a reflected signal generated by the interaction of the
selected ultrasonic guided wave and the geometry of the inspected object
including any real flaw located therein;
a means for comparing the timevarying dispersionmodeled reference signal
with the received reflected signal;
a means for determining the location of the real flaw in the inspected
object if the timevarying dispersionmodeled reference signal resembles
the received reflected signal;
wherein the timevarying dispersionmodeled reference signal represents one
of a plurality of reference propagation modes, and wherein the means for
comparing the timevarying dispersionmodeled reference signal with the
received reflected signal includes:
means for selecting at least one reference propagation mode from the
plurality of reference propagation modes, wherein the selected at least
one reference propagation mode includes a corresponding plurality of
partitioned reference signal waveforms;
means for partitioning the reflected signal according to a time of
reception so as to produce a plurality of partitioned reflected signal
waveforms; and
means for comparing the plurality of partitioned reflected signal waveforms
with the corresponding plurality of partitioned reference waveforms to
determine a level of correlation.
18. The apparatus of claim 9, wherein the means for correlating the
timevarying dispersionmodeled reference signal with the received
reflected signal includes a timevarying matched filter module executed on
the apparatus.
19. The apparatus of claim 10, wherein the means for partitioning the
reflected signal according to a time of reception so as to produce a
plurality of partitioned reflected signal waveforms includes a partition
module executed on the apparatus.
Description
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to nondestructive testing and, more
particularly, to a method and apparatus for analyzing guided waves in
inspected objects using timevarying matched filters.
2. History of Related Art
Ultrasonic wave inspection techniques are useful for many NonDestructive
Evaluation (NDE) applications. These techniques typically involve
transmitting a narrow band ultrasonic frequency interrogation signal down
the length of an object and analyzing the reflected or "inspection" signal
for the presence of material boundaries or flaws (e.g., surfaces, joints,
welds, cracks, etc.) in the object. Defects in the object that cannot be
seen by visual inspection can often be detected by analyzing the
inspection signal. Thus, ultrasonic wave inspection techniques can provide
a cost effective solution for detecting defects in many objects such as
railroad rails, stranded cables, pipes, and the like, from a single set up
location.
Generally, in the field of acoustics, there are two fundamental types of
waves that propagate through material: pressure waves, and shear waves.
These waves are called "bulk" waves and they propagate through the
material at a constant velocity over all frequencies, including ultrasonic
frequencies. An incident ultrasonic bulk wave transmitted along an object
will be reflected from one end of the object so as to arrive at a fixed
time at the transmission location according to a predictable, fixed travel
time period.
Ultrasonic bulk waves are typically used as the incident waves in
nondestructive evaluation applications. As the bulk waves enter an object
and propagate along the length thereof, they reflect between the surfaces
of the object. In objects of continuous cross section, the interaction of
the bulk waves with one another and the object's surfaces produces
envelopes of disturbance, called Lamb waves or guided waves, which also
propagate along the object. Guided waves, unlike bulk waves, have
velocities that vary depending on the frequency components of the waves.
Thus, the time of arrival for a guided wave envelope reflected from the
end of a pipe is often different for each envelope.
Furthermore, whereas there are only two types of bulk waves, there are an
infinite number of guided waves that can exist for an object of a given
geometry, such as a pipe. These different types of guided waves are
distinguished by their modes; each mode has its own velocity vs. frequency
relationship. Moreover, in a typical guided wave inspection, it is
virtually impossible to ensure that only one mode will propagate. To the
contrary, it is more likely that two or more modes will be present,
thereby producing multiple reflections from the same material boundary or
flaw, each having different velocities and, therefore, different times of
arrival.
The phenomenon of the velocity of a signal being dependent on its frequency
is called "dispersion." The effect of dispersion on guided waves is to
cause their waveforms to change with time, generally becoming more
elongated as they propagate down the length of the examined object. Guided
waves have varying amounts of dispersion depending on the wave mode. For
example, "longitudinalone" or L(0, 1) guided waves are dispersive over
virtually all frequencies, whereas "longitudinaltwo" or L(0, 2) guided
waves have a short band of frequencies over which they are not dispersive.
Within this short band of frequencies, the velocity of the L(0, 2) guided
wave is essentially constant and, therefore, the distance traveled over a
given time period may be more readily determined. For this reason, L(0, 2)
guided waves are commonly used in locating flaws and defects in piping.
Distinguishing guided wave modes among multiple reflections, however, can
be quite a complicated process. As mentioned previously, guided waves of
many different modes are produced by the interaction of bulk waves with
object boundaries and flaws. The presence of these modes can lead to
multiple detections of the same boundary or flaw within the object and
different levels of sensitivity to the boundary or flaw based on the
specific reflected mode. The presence of noise in the inspection signal
can mask reflections to make the task of identifying individual modes even
more difficult. Analysis of the inspection signal can therefore become a
very complex task that requires extensive knowledge and time.
Conventional methods used to analyze guided wave inspection signals apply
joint timefrequency analysis techniques in an attempt to observe
dispersive behavior in reflected guided waves and then match the behavior
to the modes theoretically predicted by such behavior. However, the time
when a reflection begins (i.e., the onset of reflection) can be unclear,
and dispersion of the narrow frequency band initiation pulse used to
produce guided waves tends to decrease resolution due to elongation or
"widening" of the waveform over time. Furthermore, a dispersive guided
wave mode can sometimes appear to be nondispersive, such that a portion
of the L(0, 1) mode may resemble a portion of the L(0, 2) mode.
One commonly used joint timefrequency analysis technique is the ShortTime
Fourier Transform (STFT). The STFT display, or spectrogram, can make
evident velocity differences between frequency components of an examined
portion of the inspection signal. The STFT provides useful results, but
has several limitations. First, due to its limited resolution in both the
time and frequency domains, the STFT result becomes difficult to
accurately interpret as the distance traveled by the guided wave
increases. As mentioned earlier, dispersive guided wave modes elongate in
the time domain as they propagate down the length of the examined object.
The resulting elongated shape in the STFT can interfere with other
reflections. The presence of noise in the signal further complicates STFT
interpretation. Therefore, techniques based on the STFT can have
difficulty pinpointing the exact onset of a reflection signal due to the
limited resolution of the STFT. Thus, analysis of the STFT typically
requires tedious labor by skilled analysts with extensive experience, and
is difficult to automate.
Accordingly, it is desirable to provide a more reliable and robust method
and apparatus for analyzing guided wave inspection signals. Specifically,
it is desirable to provide a signal processing method and apparatus that
can more effectively accommodate the dispersive nature of guided wave
modes so as to aid in the reliable characterization of inspected object
geometric boundaries and flaws.
SUMMARY OF THE INVENTION
The present invention is directed to a method and apparatus for analyzing
guided waves using a timevarying matched filter to correlate received
guided waves with a timevarying dispersionmodeled reference wave to
determine the location of a real flaw in an inspected object.
In general, in one aspect, the method includes the steps of selecting a
timevarying dispersionmodeled reference signal associated with the
geometry of an inspected object, a multiplicity of theoretical flaw
locations located within the inspected object, and one or more
characteristics of the selected guided wave; launching the selected guided
wave signal into the inspected object; receiving a reflected signal
generated by the interaction of the selected guided wave and the geometry
of the inspected object (including any real flaw located therein);
comparing the timevarying dispersionmodeled reference signal with the
received reflected signal; and determining the location of the real flaw
in the inspected object if the timevarying dispersionmodeled reference
signal is substantially similar to the received reflected signal. The
comparison can be conducted over the entire waveform for the reference and
reflected signals during each sample interval. The comparison can also be
conducted using only the detected wave envelope for each reference and
reflected signal.
In general, in another aspect, the apparatus includes a means for selecting
a timevarying dispersionmodeled reference signal associated with the
geometry of an inspected object, a multiplicity of theoretical flaw
locations located within the object, and the characteristics of the
selected guided wave; a means for launching the selected guided wave
signal into the inspected object; a means for receiving a reflected signal
generated by the interaction of the selected guided wave and the geometry
of the inspected object; a means for comparing the timevarying
dispersionmodeled reference signal with the received reflected signal;
and a means for determining the location of a real flaw in the inspected
object if the timevarying dispersionmodeled reference signal is
substantially similar to the received reflected signal.
The means for selecting the timevarying dispersionmodeled reference
signal associated with the geometry of the inspected object, the
multiplicity of theoretical flaw locations, and the characteristics of the
guided wave may include a workstation or desktop computer capable of
simulating the dispersive behavior of a guided wave in the inspected
object, such as a pipe, as it interacts with the geometry of the inspected
object. The computer typically includes a memory unit, a processor unit,
and a storage unit for storing one or more program modules to generate
individual reference signals which correspond to each sample of the
reflected signal to be compared.
The means for launching the guided wave signal into the inspected object
and the means for receiving the reflected signal generated by the
interaction of the guided wave and the geometry of the inspected object
may be an ultrasonic signal generator and a transducer, respectively.
The means for comparing the timevarying dispersionmodeled reference
signal with the reflected signal and the means for determining the
location of the real flaw in the inspected object if the timevarying
dispersionmodeled reference signal is substantially similar to the
reflected signal may also comprise a desktop computer, workstation, or
other data processing apparatus, as are well known to those skilled in the
art.
The method and apparatus operate under the assumption that a theoretical
flaw exists at every point in time during which a waveform is acquired. If
the waveform of the acquired reflected signal has a high level of
correlation with the waveform of the generated reference signal
corresponding to that point in time, then the hypothesis or assumption of
a flaw at that location in the inspected object is verified. If there is a
low level of correlation, then the hypothesis fails, and no flaw is
detected.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the method and apparatus of the present
invention may be had by reference to the following detailed description
when taken in conjunction with the accompanying drawings wherein:
FIG. 1 is system level diagram of an exemplary apparatus for analyzing
guided wave inspection signals according to one embodiment of the present
invention;
FIG. 2 is a functional block diagram of an exemplary guided wave analysis
application according to one embodiment of the present invention;
FIG. 3 is flowchart of an exemplary method of identifying guided wave modes
according to one embodiment of the present invention;
FIG. 4 is a functional block diagram of the system used to derive the
timevarying matched filter of the present invention;
FIG. 5 is a flowchart of an exemplary method of implementing a timevarying
matched filter according to one embodiment of the present invention;
FIG. 6 is a matrix of timevarying dispersionmodeled reference waveforms;
FIG. 7 is a plot of a simulated inspection signal containing reflections of
L(0, 1) and L(0, 2) guided wave modes;
FIG. 8 is a plot of the output of the timevarying matched filter of the
present invention biased for the L(0, 1) mode;
FIG. 9 is a plot of the output of the timevarying matched filter of the
present invention biased for the L(0, 2) mode;
FIG. 10 is a plot of the signal shown in FIG. 7 with the L(0, 2) mode only;
and
FIG. 11 is a plot of the signal shown in FIG. 7 with the L(0, 1) and
remaining noise mode only.
DETAILED DESCRIPTION OF PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS
Following is a detailed description of the preferred embodiments of the
present invention and its advantages with reference to the drawings,
wherein like numerals are used for like and corresponding parts.
The invention is generally directed to using a guided wave to determine the
location of one or more flaws in an inspected object, such as a pipe,
rail, etc. In essence, an ultrasonic guided wave signal is launched into
the object using conventional ultrasound methods, and the
reflected/received signal is sampled to capture a series of individual
reflected (guided) waves. The reflected signal is then partitioned into
individual waveforms. A timevarying dispersionmodeled reference signal
is selected based on the unique geometry of the inspected object, a
multiplicity of "theoretical" flaw locations within the object, and the
characteristics of the original ultrasonic guided wave. To make a
comparison, the reference signal is also generated as a series of
partitioned waveforms which model the shape of the waveforms that are
expected to be reflected from each theoretical flaw location in the
object. The shape of the reflected waveforms is correlated with the shape
of the reference waveforms, and a high level of correlation indicates the
presence of a real flaw at the theoretical flaw location.
FIG. 1 illustrates an exemplary system for analyzing ultrasonic guided
waves according to one exemplary embodiment of the present invention. A
data processing apparatus 10, such as one or more highend desktop
computers, workstations, or servers, is coupled to an ultrasonic
instrument 11 for receiving ultrasonic guided waves therefrom. The
ultrasonic instrument 11 may be any commercially available ultrasonic
instrument suitable for the purpose, such as a Krautkramer.TM. USIP20. A
transmission line 12 connects the ultrasonic instrument 11 to an
ultrasonic transducer 13, which is in turn attached to an object 14 to be
inspected. The ultrasonic transducer 13 may be any commercially available
ultrasonic transducer, such as a piezoelectric transducer, capable of
generating and receiving ultrasonic waves. The transmission line 12
carries transmitted electric signals 68 from the ultrasonic instrument 11
to the transducer 13, which cause the transducer 13 to generate narrowband
ultrasonic pulses of a predetermined frequency and duration. The
ultrasonic pulses propagate through the object 14 as guided waves 15 until
they encounter a material boundary or flaw 16. Some of the guided waves 15
are then reflected back from the material boundary or flaw 16 and returned
to the transducer 13. The reflected guided waves 69 (dashed lines) are
received by the transducer 13 and converted to equivalent received
electric signals 67 thereby. These electric signals 67 are thereafter
carried by the transmission line 12 back to the ultrasonic equipment 11
for processing. The ultrasonic equipment 11 processes the electric signals
by, for example, amplifying, sampling, digitizing, and storing the
signals, repeating the process as needed, and provides the processed
signals as digital data 66 to the data processing apparatus 10 to be
analyzed thereby.
The data processing apparatus 10 has a number of functional components for
analyzing the data 66 from the ultrasonic instrument 11 including a
processor unit 17, a memory unit 18, and a storage unit 19. The processor
unit 17 is responsible for executing various software applications which
may reside within the data processing apparatus 10 including an operating
system therefor and any data analysis applications residing thereon. The
memory unit 18 serves to temporarily store data that may be needed by the
processor unit 17 during execution of the various software applications.
Long term storage of the various software applications including the
operating system and any data analysis software applications as well as
raw and/or processed data from the ultrasonic instrument 11 is provided by
the storage unit 19. One of the data analysis applications stored in the
storage unit 19 and executed by the processor unit 17 within the apparatus
10 is a guided wave analysis application 20 for comparing reference
signals with received reflected signals and determining the location of a
material boundary or real flaw in an inspected object.
Referring now to FIG. 2, in one embodiment of the invention, the guided
wave analysis application 20 includes a number of program modules for
analyzing the received guided wave data including a partition unit 21
module, a timevarying matched filter unit 22 module, and an output unit
23 module. These program modules 21, 22, and 23 operate in conjunction
with one another to identify certain desired guided wave modes in the
reflected guided waves so as to provide an indication of the location of a
possible material boundary and/or real flaw. Each of the program modules
21, 22, and 23 is described below.
The partition unit 21 receives data representing an incoming guided wave
signal 24 obtained during a predetermined acquisition period and divides
this signal into smaller segments, each segment having one or more guided
waves therein. The segments may be defined on a time basis, e.g.,
according to the start time of each reflected guided wave, or
alternatively, at a regular, predetermined time intervals based on the
sampling rate of the ultrasonic instrument 11 (see FIG. 1). A typical
sampling rate is about 500 kHz, and selection of the sampling rate and
size of the segment (i.e., number of samples) may depend on the particular
needs of the application. For example, a smaller segment with a high
sampling rate may provide higher resolution, but requires more processing
capacity, whereas a larger segment (and lower sampling rate) may provide
lower resolution, and requires less processing capacity.
In some embodiments, the partition unit may be configured to isolate a
predetermined portion of the incoming signal 24 occurring during a certain
time interval in order to capture a single reflected wave. The waveform of
this single wave may then be compared to the waveforms of the reference
signal by the timevarying matched filter 22 to determine a level of
correlation.
Operation of the timevarying matched filter 22 is similar to a
conventional matched filter in that an incoming signal 24 is compared to
the reference signal 25, and the degree of correlation between the two
waveforms thereof is provided, after processing by the output unit 23, as
an output signal 26. The correlation may also be performed on a wave
envelope basis, i.e., based on the shapes outlined by the peaks and
troughs of one or more consecutive waves, instead of the amplitudes of the
individual waves. In either case, if there is a high degree of correlation
(i.e., a match) between the incoming signal 24 and the reference signal
25, then the timevarying matched filter 22 will output a value
corresponding to the degree of correlation. Higher values signify a higher
degree of correlation.
However, unlike conventional matched filters where the input signals are
time invariant, the incoming guided wave signals 24 have waveforms that
may change over time due to the dispersive nature of the guided waves. The
actual amount of dispersion may depend on a number of factors, including:
the desired wave mode (e.g., L(0, 1), L(0, 2), etc.), the geometry (e.g.,
length, thickness, etc.) and material (e.g., steel, titanium, etc.) of the
object through which the guided waves are propagating, the location of the
material boundary and/or flaw from which the guided waves are reflected,
and the frequency or frequencies of the incident wave. Thus, in accordance
with an exemplary embodiment of the invention, the timevarying matched
filter 22 must be able to account for the timevarying, dispersive nature
of the incoming guided wave signal 24 in order to effectively compare it
to the reference signal 25.
In this exemplary embodiment, the reference signal 25 is a timevarying
dispersionmodeled reference signal (hence, the term "timevarying"
matched filter). Such a timevarying reference signal 25 may then be
properly compared to the incoming guided wave signals 24 to more
accurately identify the presence of certain guided wave modes therein. The
reference signal 25 is generated by the processing apparatus 10 (see FIG.
1) and may be derived, in large part, from the predicted dispersive
behavior of selected guided wave modes in objects having known geometries
and material properties, the locations of theoretical flaws in such
objects, and the characteristics of the incident (transmitted) ultrasonic
pulse. For example, the dispersive behavior of L(0, 1) and L(0, 2) mode
guided waves in steel pipes, rails, and the like are well known and
understood by those of ordinary skill in the art. In addition, the various
types and locations of flaws and defects that may occur within those
objects are also well known and documented. This information may be
compiled and employed to simulate a dispersive wave of a particular mode
in an object examined using well known simulation software, such as
Disperse.TM. Software v. 1.0, available from the Imperial College,
University of London. The reference signal 25 may then be modeled after
the selected simulated dispersive wave mode and generated by the
processing apparatus 10. Preferably, the reference signal 25 is generated
as one or more time segments of the same size as those selected for the
incoming signal 24.
After correlation, the output unit 24 generates an output signal 26 that
corresponds to the correlation results received from the timevarying
matched filter 22. Generally, the output unit 24 generates a level of
output based on degree of correlation. In addition, the output unit 24 may
process the correlation result of the timevarying matched filter 22 by,
for example, normalizing and performing a Hilbert transform thereon prior
to generating the output signal 26.
Referring now to FIG. 3, a method of identifying a guided wave mode
according to one embodiment of the present invention is shown in a general
sense. At step 30, the particular timevarying dispersionmodeled
reference signal can be selected for the desired mode and the particular
object to be inspected using the guided wave analysis application. An
ultrasonic pulse (guided wave) is launched into the object to be inspected
at step 31, typically using an ultrasonic instrument and a transducer. At
step 32, guided waves that are reflected off a material boundary or flaw
are received by the ultrasonic instrument and the transducer. The
reflected guided wave signal is partitioned based on some predetermined
segment size related to the extent of dispersion of a particular mode. For
instance, 300 points for L(0, 1) in a 4.5 inch OD steel pipe. At step 34,
the reflected guided wave signal is compared to the timevarying
dispersionmodeled reference signal, either on a waveform basis or on a
wave envelope basis. At step 35, it is determined the degree of
correlation, or substantial similarity, between the waveforms/envelopes of
the reflected signal and the reference signal. In some embodiments, the
degree of correlation between the waveforms/envelopes of the two signals
should meet or exceed a predefined threshold level of correlation in order
to be considered substantially similar. This threshold level would be
chosen at a level that avoids detection of harmlesslysmall flaws. Such a
determination would be based on a safety and economic decision by the
pipeline operator. If there is substantial similarity, then it is
determined that the desired guided wave mode is present in the reflected
guided wave signal at step 36. This information may then be used according
to known techniques to determine the location of material boundaries or
flaws within the inspected object at step 37. If substantial similarity
does not exist, then it is determined that the desired guided wave mode
and flaw location is not present in the reflected signal at step 38.
Under the above arrangement, a theoretical flaw is assumed to exist at
every point in time where a reflected wave is acquired, and if the
acquired reflected wave has a high level of correlation with the reference
wave corresponding to that point in time, than the hypothesis or
assumption of a flaw at that location in the inspected object is verified.
If there is a low level of correlation, then it is assumed that no flaw is
detected for the selected guided wave mode. The process can thereafter be
repeated using as many different selected guided wave modes as needed to
detect other flaws.
The following is a derivation of a mathematical model of the timevarying
matched filter of the present invention. Referring to FIG. 4, the
objective of such a filter is to be able to detect the components, which
are the reflections, of a signal s(t) generated from a system 40 with an
impulse response function h(t), and an input u(t). The signal s(t) can be
given by the wellknown convolution integral:
##EQU1##
(See, e.g., Oppenheim, A. V., and Schafer, R. W., DiscreteTime Signal
Processing, Prentice Hall, Englewood Cliffs, N.J. 1989.)
The system 40 can be decomposed as shown into several component systems 42,
each having an impulse response function h(t, .tau..sub.i), as expressed
by the following summation:
##EQU2##
Each component system 42 can be further decomposed into a cascade composed
of a pure delay 44, having a transfer function .delta.(t.tau..sub.i),
followed by a dispersive system 46 having an impulse response function
h.sub.i (t), as shown.
It follows that h(t, .tau..sub.i)=0 for all t.ltoreq..tau..sub.i such that
the signal s(t) is comprised of N components and can be expressed as:
##EQU3##
and .tau..sub.i is the arrival time of the i.sup.th component or reflection
s(t, .tau..sub.i). In other words, s(t, .tau..sub.i) denotes a dispersive
component/reflection that is present in the signal s(t) beginning only at
time t=.tau..sub.i. Seven of these are shown in FIG. 7 as S.sub.1 through
S.sub.7. S.sub.1, S.sub.5, S.sub.6 are L(0, 1)'s and S.sub.2, S.sub.3,
S.sub.4, S.sub.7 are L(0, 2)'s.
The objective is to detect each reflection and estimate the unknown arrival
times .tau..sub.i from the received signal r(t), which is given by
r(t)=s(t)+n(t), where n(t) is Gaussian noise. Those of ordinary skill in
the art can assume that the impulse response function h(t, .tau..sub.i)
for the component systems 42 is known for all t and .tau..sub.i for the
specific geometries and material properties of the inspected objects that
carry the guided wave. The input signal u(t) is likewise known from the
characteristics of the ultrasonic pulse used to produce reflected waves
(i.e., the initiation signal).
As alluded to earlier, the conventional technique for detecting a signal
and estimating its arrival time is the matched filter, which performs
correlation processing between the received signal r(t) and the desired
signal s(t). Normally, the matched filter is time invariant. However, in
this case, due to the dispersive nature of guided waves, the desired
signal s(t) has been expressed as a function of .tau..sub.i and,
therefore, the optimal matched filter should also be timevarying.
Accordingly, the received signal r(t) is passed through a linear
timevarying filter with Green's function g(t, .sigma.) to generate the
response y(t) given by:
##EQU4##
where the noise component y.sub.n (t) is given by:
##EQU5##
and the signal component y.sub.s (t) is given by:
##EQU6##
Green's function g(t, .sigma.) is the system response at time t to an
impulse applied at time .sigma.. (See, e.g., Stakgold, I., Green's
Function and Boundary Value Problems, Wiley Press, New York, 1998.)
Using the expression for s(t) in Equation (3), y.sub.s (t) can be written
as:
##EQU7##
Commuting the order of integration with summation results in the following:
##EQU8##
It follows from Equation (9) that the signal component y.sub.s (t) can be
written as:
##EQU9##
The objective in designing a matched filter is to maximize the
signaltonoise ratio for each reflection. (See, e.g., McDonough, R. N.,
and Whalen, A. D. Detection of Signals in Noise, Academic Press, San
Diego, 1995.) The signaltonoise ratio for the i.sup.th reflection can be
defined by:
##EQU10##
at the specific time t=t.sub.i =.tau..sub.i +T.sub.i, where T.sub.i is the
duration of time over which s(t, .tau..sub.i) is nonzero, and r.sub.yn (t,
t) can be expressed as:
r.sub.yn (t,t)=.EPSILON.[y.sub.n (t)y.sub.n (t)] (12)
Using the results from Equation (5), the signaltonoise ratio R(t,
.tau..sub.i) defined by Equation 11 will be maximized at t=t.sub.i
=.tau..sub.i +T.sub.i if the following is true:
g(t,.sigma.)=s(.sigma.,tT.sub.i) (13)
Thus, by setting Green's function equal to the i.sup.th
component/reflection of the desired signal s(t), the signaltonoise ratio
R(t, .tau..sub.i) will be maximized. The timevarying matched filter
response will then exhibit peaks at t=t.sub.i =.tau..sub.i +T.sub.i that
are indicative of the presence of a reflection and its time of arrival.
Note that often the matched filter response will be shifted by T.sub.i
such that the peaks in y(t) will align with energy spikes in the received
signal r(t).
Following herein is a simulated example showing the performance of the
timevarying matched filter of the present invention based on the above
model. To implement the timevarying matched filter and evaluate its
performance, a simulated guided wave signal r(t) as it propagates within a
pipe was created using a computer program. This simulation was produced
using mathematical results from Gazis, D. C., J. Accoust. Soc. Am. 31,
568573 (1959), incorporated herein by reference, and the dispersion
relationship produced by the Disperse.TM. software (mentioned above) for a
nominal cylinder diameter and wall thickness. This information is normally
sufficient to construct the system model h(t, .tau..sub.i) given by
Equation (2). A Gaussianwindowed sinusoidal pulse is used for u(t),
representing the initial ultrasonic pulse induced in the examined pipe.
A second program, the exemplary steps of which are shown in FIG. 5, was
written using Equation (5) to implement the timevarying matched filter of
the present invention. At step 50, a sufficient amount of memory space is
allocated. A predetermined input signal having the characteristics, such
as a narrow frequency band, Gaussianwindowed sinusoid, of an ultrasonic
pulse is loaded at step 51. At step 52, a Fourier transform is performed
on the input signal. Dispersion data is applied to the Fourier transform
of the input signal to simulate a dispersive guided wave propagating
through the pipe at step 54. The dispersion data for a pipe is well known
and is based on a number of factors including the characteristics of the
input signal and the geometry and material properties of the pipe.
At step 55, a matrix of adequate size is generated for storing the impulse
responses h(t, .tau.), and is then filled with simulated waveforms. An
exemplary matrix is shown in FIG. 6 with M rows and N columns. Each row
t.sub.i represents the begin time for the reflections from a particular
theoretical flaw, with each subsequent row t.sub.i+1 representing flaws
that are located progressively further and further down the length of the
examined object. Each column .tau..sub..iota. represents the arrival time
of the i.sup.th reflection associated with a particular reflection start
time t.sub.i, with each subsequent column .tau..sub..iota.+1 representing
reflections that arrive progressively later and later in time. Thus, each
row t.sub.i in the matrix is associated with a vector of .tau.'s
representing arrival times for reflections that began at time t for that
row. The waveforms used to fill the matrix are generated from simulations
that use the Fourier transform of the input signal, the reflection begin
time t, and the dispersion behavior for the desired guided wave mode. Time
t is the time step index in the sampled data. For each time step, the time
t is used to generate a reference signal that would have begun at that
time. Time t is simply equal to the size of the time step=t.sub.1 t.sub.0
or t.sub.n1 t.sub.n.
At step 56, the completed h(t, .tau.) matrix becomes Green's function g(t,
.sigma.) in Equation (5). The received guided wave data to be analyzed is
then partitioned at step 57 by collecting a vector of the guided wave data
corresponding to each time t of the matrix g(t, .sigma.) and having the
same length as a row of the g(t, .sigma.) matrix. This vector of data
becomes r(.sigma.) in Equation (5).
At step 58, the guided wave data to be analyzed is correlated with the h(t,
.tau.) (i.e., g(t, .tau.)) matrix data. More specifically, the correlation
is performed by multiplying the appropriate row of the g(t, .sigma.)
matrix with the corresponding vector r(.sigma.) for every time t in the
matrix. The result of the correlation, which is y(t) in Equation (5), is
provided at step 59. At step 60, a Hilbert transform is performed to
remove the sinusoidal nature of the output y(t) and to produce all
positive values therefor. The output y(t) for every time t is thereafter
plotted at step 61.
FIG. 7 shows the simulated received signal r(t) 100, which includes the
simulated mode reflections (i.e., S.sub.1 . . . S.sub.7) along with
colored and white noise to create a signal representative of field
inspection data. The horizontal axis 110 is time and the vertical axis 120
is the amplitude. Time zero indicates the initial or launch time of the
input signal. The various simulated mode reflections of the received
signal r(t) 100 are labeled S.sub.1 to S.sub.7 and represent possible
material boundaries and/or flaws.
The responses y.sub.i (t) given by Equation (5) are shown in FIGS. 8 and 9
for detection of L(0, 1) and L(0, 2) guided wave modes of reflection,
respectively. Note that the responses of the timevarying matched filter
as shown in FIGS. 8 and 9 have been normalized by the factor:
##EQU11##
In most evaluations involving pipe geometry, L(0, 2) is the preferred mode
for inspection because it is nondispersive over a sizeable frequency
range. Thus, absolute detection of L(0, 1) mode reflections is important
so as to avoid their misinterpretation as L(0, 2) reflections. As shown in
FIGS. 8 and 9, both timevarying matched filter responses exhibit peaks in
response to the received inspection signal r(t) 100. This is due to the
fact that L(0, 1) and L(0, 2) mode reflections have some similarity,
especially early in the signal before the L(0, 1) wave has dispersed.
However, upon closer examination, it is evident that for the same signal
location, the peak of the timevarying matched filter response of FIG. 9
appears sharper. The peak sharpness can be used to identify the particular
mode of reflection. Sharper peaks indicate the degree of likelihood that
the chosen mode was detected at that point. For this data, both the L(0,
1) and L(0, 2) TVMF's were run. For a particular reflection of interest in
the inspection signal, the sharpnesses of the TVMF peaks were compared for
the L(0, 1) and L(0, 2) outputs. The sharper of the two indicated which
mode the reflection belonged to. Furthermore, the temporal location of the
tip of a peak in the timevarying matched filter response can be used to
determine the time of arrival for the reflected wave.
FIG. 10 shows the L(0, 2) reflections as constructed using the dispersion
model and parameters generated from the timevarying matched filter
response. The L(0, 2) reflections from FIG. 10 were then subtracted from
the inspection signal in FIG. 7 to isolate the L(0, 1) reflections and
noise, as can be seen in FIG. 11.
TABLE 1 lists the timeofarrival estimations using the timevarying
matched filter for all of the mode reflections in the received inspection
signal r(t) 100 shown in FIG. 7. As can be seen, the estimated times of
arrival (Estimated .tau..sub.i) very closely track the actual time of
arrival (Actual .tau..sub.i) in this simulation. The result confirms the
validity and accuracy of the model of the timevarying matched filter
described above.
TABLE 1
Reflection Actual .tau..sub.i (ms) Estimated .tau..sub.i (ms) Mode of
Reflection
S.sub.1 0.750 0.752 L(0,1)
S.sub.2 1.000 1.003 L(0,2)
S.sub.3 1.350 1.356 L(0,2)
S.sub.4 1.500 1.502 L(0,2)
S.sub.5 1.900 1.910 L(0,1)
S.sub.6 2.900 2.920 L(0,1)
S.sub.7 3.000 3.004 L(0,2)
Although the invention has been described with reference to specific
embodiments, this description is not meant to be construed in a limited
sense. The various modifications of the disclosed embodiments, as well as
alternative embodiments of the invention, will become apparent to persons
skilled in the art upon reference to the description of the invention. It
is, therefore, contemplated that the appended claims will cover such
modifications that fall within the scope of the invention, or their
equivalents.
* * * * *