U.S. patent application number 09/770323 was filed with the patent office on 2001-11-29 for flaw detection system using acoustic doppler effect.
This patent application is currently assigned to Massachusetts Institute of Technology. Invention is credited to Wooh, Shi-Chang.
Application Number | 20010045130 09/770323 |
Document ID | / |
Family ID | 21843991 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010045130 |
Kind Code |
A1 |
Wooh, Shi-Chang |
November 29, 2001 |
Flaw detection system using acoustic doppler effect
Abstract
A flaw detection system using acoustic Doppler effect for
detecting flaws in a medium wherein there is relative motion
between the medium and system includes a transducer, spaced from
the medium to be inspected, for introducing to and sensing from the
medium an acoustic signal that propagates in the medium at a
predetermined frequency; and a detector, responsive to the sensed
propagating acoustic signal, for detecting in the sensed acoustic
signal the Doppler shifted frequency representative of a flaw in
the medium.
Inventors: |
Wooh, Shi-Chang; (Bedford,
MA) |
Correspondence
Address: |
Iandiorio & Teska
260 Bear Hill Road
Waltham
MA
02451-1018
US
|
Assignee: |
Massachusetts Institute of
Technology
|
Family ID: |
21843991 |
Appl. No.: |
09/770323 |
Filed: |
January 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09770323 |
Jan 26, 2001 |
|
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09028536 |
Feb 24, 1998 |
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Current U.S.
Class: |
73/600 ;
73/636 |
Current CPC
Class: |
G01N 2291/2623 20130101;
G01N 29/2418 20130101; G01N 29/42 20130101; G01N 2291/101 20130101;
G01N 29/265 20130101; G01N 29/40 20130101; G01N 29/2412 20130101;
G01N 2291/017 20130101; B61K 9/10 20130101; G01N 2291/102
20130101 |
Class at
Publication: |
73/600 ;
73/636 |
International
Class: |
G01N 029/02; G01N
009/24 |
Claims
What is claimed is:
1. A flaw detection system using acoustic Doppler effect for
detecting flaws in a medium wherein there is relative motion
between the medium and system comprising: transducer means, spaced
from the medium to be inspected, for introducing to and sensing
from the medium an acoustic signal that propagates in said medium
at a predetermined frequency; and means, responsive to the sensed
propagating acoustic signal, for detecting in the sensed acoustic
signal the Doppler shifted frequency representative of a flaw in
the medium.
2. The flaw detection system using acoustic Doppler effect of claim
1 in which said transducer means includes a separate transmitter
and receiver.
3. The flaw detection system using acoustic Doppler effect of claim
1 in which said transducer means is an ultrasonic transducer and
said acoustic signal is an ultrasonic signal.
4. The flaw detection system using acoustic Doppler effect of claim
1 in which said transducer transmits an acoustic signal for
propagation in said medium.
5. The flaw detection system using acoustic Doppler effect of claim
1 in which said transducer transmits optical energy for inducing
the acoustic signal in said medium.
6. The flaw detection system using acoustic Doppler effect of claim
5 in which said transducer includes a laser for transmitting said
optical energy.
7. The flaw detection system using acoustic Doppler effect of claim
1 in which said transducer includes an acoustic receiver.
8. The flaw detection system using acoustic Doppler effect of claim
1 in which said transducer includes an electromagnetic acoustic
transmitter and receiver for inducing an acoustic signal into said
medium and sensing the Doppler shifted acoustic signal.
9. The flaw detection system using acoustic Doppler effect of claim
1 in which said means for detecting includes a spectrum analyzer
for distinguishing the Doppler effect frequency.
10. The flaw detection system using acoustic Doppler effect of
claim 9 in which said means for detecting includes a thresholding
circuit for identifying a preselected level as a flaw.
11. The flaw detection system using acoustic Doppler effect of
claim 1 in which said means for detecting includes a bandpass
filter and a peak detector for distinguishing the Doppler effect
frequency.
12. The flaw detection system using acoustic Doppler effect of
claim 11 in which said means for detecting includes a thresholding
circuit for identifying a preselected level as a flaw.
13. The flaw detection system using acoustic Doppler effect of
claim 1 in which said means for detecting includes an A/D converter
and a digital filter for distinguishing the Doppler effect
frequency, and a thresholding circuit for identifying a preselected
level as a flaw.
14. The flaw detection system using acoustic Doppler effect of
claim 1 in which said medium is a railroad rail.
15. The flaw detection system using acoustic Doppler effect of
claim 1 in which said transducer means transmits to the surface of
the medium and receives from the surface of the medium an acoustic
signal and the flaws detected are surface flaws.
16. The flaw detection system using acoustic Doppler effect of
claim 1 in which said transducer means induces an acoustic signal
internally in the medium and the flaws detected are internal
flaws.
17. The flaw detection system using acoustic Doppler effect of
claim 1 in which said transducer means includes a laser vibrometer
interferometer for sensing the acoustic signal propagating in the
medium.
18. A flaw detection system using acoustic Doppler effect for
detecting surface flaws in a medium wherein there is relative
motion between the medium and system comprising: acoustic
transducer means, spaced from the medium to be inspected, for
transmitting an acoustic signal to and receiving the reflected
acoustic signal at a predetermined frequency from the surface of
the medium to be inspected; and means, responsive to the reflected
acoustic signal, for distinguishing the Doppler shifted frequency
in the reflected acoustic signal from the predetermined frequency
of the transmitted acoustic signal representative of a surface flaw
in the medium.
19. A flaw detection system using acoustic Doppler effect for
detecting flaws in a medium wherein there is relative motion
between the medium and system comprising: transducer means, spaced
from the medium to be inspected, for inducing an acoustic signal to
propagate in the medium at a predetermined frequency and sensing
the propagating acoustic signal in the medium; and means,
responsive to the sensed propagating acoustic signal, for
distinguishing the Doppler shifted frequency representative of a
flaw in the medium.
20. The flaw detection system using acoustic Doppler effect for
detecting flaws of claim 19 in which said transducer means includes
an electromagnetic acoustic transducer for inducing and sensing the
acoustic signal.
21. The flaw detection system using acoustic Doppler effect for
detecting flaws of claim 19 in which said transducer means includes
a transmitter and a receiver and said transmitter includes a laser
for locally heating the medium to generate acoustic signals.
Description
FIELD OF INVENTION
[0001] This invention relates to a flaw detection system using
acoustic Doppler effect for detecting flaws in a medium to be
inspected wherein there is relative motion between the system and
medium.
BACKGROUND OF INVENTION
[0002] Railroads provide both efficiency and economy in passenger
and freight transportation. Like other transportation modes,
however, they are prone to various problems. Statistics show that
over the course of this century, the average carload and trainload
tonnage has increased significantly. There is also an increasing
concentration of traffic on fewer main line tracks. The average
length of haul has also risen. Unfortunately, these trends have not
been offset with a proportional increase in the amount of new rail
laid. Consequently, the stress on rails and fatigue related
failures may continue to increase. With the new demands, it is
important to assess the rail integrity by detecting rail defects
nondestructively and speedily.
[0003] Typical defects often found in railroad tracks include
transverse and longitudinal defects in the rail head, web defects,
base defects, surface defects as well as other miscellaneous damage
such as head wear, corrosion, crushed head, burned rail, bolt hole
cracks, head and web separation.
[0004] Nondestructive evaluation of rail tracks may be approached
by continuous monitoring or detailed inspection. In the context of
rail assessment, continuous monitoring results in global evaluation
of the rail whereas detailed inspection focuses on a particular
area to locate and/or characterize a defect in detail.
[0005] In continuous monitoring, some techniques for inspection of
rail flaws at an intermediate speed are currently available, but
the technology lacks efficient monitoring techniques at a high
speed comparable to the speed of a passenger car. One of the
limitations on speed is the need for the transducer to be in
contact with the rail. Furthermore, existing detailed inspection
techniques have limited capabilities, primarily due to poor sensor
performance and the requirement of contact with the rail
surface.
[0006] Currently, surface defects are detected by means of a device
called a track circuit. This device uses the track as part of an
electric circuit and uses the resistivity of the rail as an
indication of surface discontinuities. Another approach is the use
of ultrasonic probes in contact with the track surface by a rolling
wheel. These techniques require contact with the sensor and the
rail. Therefore, they are not quite suitable for high-speed
monitoring.
[0007] Improved inspection systems are needed in many other
applications, for example, in which there is relative motion
between the system and medium to be inspected such as conveyors,
cables, ropes and roadbeds. Presently inspection techniques tend to
be slow and not so reliable because they typically use a change in
the amplitude of the probe signal to identify a defect or flaw.
Amplitude data is not easily repeatable or reliable.
SUMMARY OF INVENTION
[0008] It is therefore an object of this invention to provide a
flaw detection system using acoustic Doppler effect for detecting
flaws in a medium to be inspected.
[0009] It is a further object of this invention to provide such a
flaw detection system using acoustic Doppler effect which is faster
and more reliable.
[0010] It is a further object of this invention to provide such a
flaw detection system using acoustic Doppler effect which is
adapted for detecting flaws in a variety of moving and stationery
mediums such as conveyors, cables, ropes, railroad tracks and
roads.
[0011] It is a further object of this invention to provide such a
flaw detection system using acoustic Doppler effect which utilizes
a change in frequency not amplitude to identify a flaw.
[0012] It is a further object of this invention to provide such a
flaw detection system using acoustic Doppler effect which is
capable of extremely high speed operation and improves its
resolution with speed.
[0013] It is a further object of this invention to provide such a
flaw detection system using acoustic Doppler effect which operates
in a remote or non-contact mode spaced from the medium to be
inspected.
[0014] It is a further object of this invention to provide such a
flaw detection system using acoustic Doppler effect which can be
used to detect surface or internal flaws.
[0015] It is a further object of this invention to provide such a
flaw detection system using acoustic Doppler effect in which
stronger signals can be obtained in surface flaw inspection due to
air coupling of acoustic signals.
[0016] It is a further object of this invention to provide such a
flaw detection system using acoustic Doppler effect which enables
continuous non-stop monitoring.
[0017] The invention results from the realization that a truly
elegant yet extremely reliable continuous and high speed detection
system for detecting a flaw in a medium such as a conveyor belt,
cable, rope, railroad track or road can be effected by sensing a
Doppler shift in a carrier signal caused by a flaw.
[0018] This invention features a flaw detection system using
acoustic Doppler effect for detecting flaws in a medium wherein
there is relative motion between the medium and system. There are
transducer means, spaced from the medium to be inspected, for
introducing to and sensing from the medium an acoustic signal that
propagates in said medium at a predetermined frequency. There are
also means, responsive to the sensed propagating acoustic signal,
for detecting in the sensed acoustic signal the Doppler shifted
frequency representative of a flaw in the medium.
[0019] In a preferred embodiment the transducer means may include a
separate transmitter and receiver. The transducer may be an
ultrasonic transducer and the acoustic signal an ultrasonic signal.
The transducer may transmit an acoustic signal from propagation in
the medium or the transducer may transmit optical energy for
inducing the acoustic signal in the medium. The transducer may
include a laser for transmitting that optical energy. The
transducer may include an acoustic receiver. The transducer may
include an electromagnetic acoustic transmitter and receiver for
inducing an acoustic signal into the medium and sensing the Doppler
shifted acoustic signal. The means for detecting may include a
spectrum analyzer, or a bandpass filter and a peak detector, or an
A to D converter and a digital filter for the purpose of
distinguishing the Doppler effect frequency. In addition there may
be a thresholding circuit identified with any one of the options
for identifying a preselected label as a flaw. The medium to be
inspected may be a railroad rail. The transducer may transmit to
the surface of the medium and receive from the surface of the
medium an acoustic signal and the flaws detected may be surface
flaws. Or the transmitter may induce an acoustic signal internally
in the medium and the flaws detected may be internal flaws. The
transducer means may include a laser vibrometer interferometer for
sensing the acoustic signal propagating in the medium.
[0020] The invention also features a flaw detection system using
acoustic Doppler effect for detecting surface flaws when there is
relative motion between the medium and system. There is an acoustic
transducer means spaced from the medium to be inspected for
transmitting an acoustic signal to and receiving the reflected
acoustic signal at a predetermined frequency from the surface of
the medium to be inspected. Means responsive to the reflected
acoustic signal distinguish the Doppler shifted frequency in the
reflected acoustic signal from the predetermined frequency of the
transmitted acoustic signal representative of a surface flaw in the
medium.
[0021] The invention also features a flaw detection system using
acoustic Doppler effect for detecting flaws in a medium wherein
there is relative motion between the medium and system. There are
transducer means spaced from the medium to be inspected for
inducing an acoustic signal to propagate the medium at a
predetermined frequency and sensing the propagated acoustic signal
in the medium. Means, responsive to the sensed propagating acoustic
signal, distinguish the Doppler shifted frequency representative of
a flaw in the medium.
[0022] In a preferred embodiment the transducer means may include
an electromagnetic acoustic transducer for inducing and sensing the
acoustic signal. The transducer means may include a transmitter and
a receiver and the transmitter may include a laser for locally
heating the medium to generate acoustic signals.
DISCLOSURE OF PREFERRED EMBODIMENT
[0023] Other objects, features and advantages will occur to those
skilled in the art from the following description of a preferred
embodiment and the accompanying drawings, in which:
[0024] FIG. 1 is a schematic block diagram of a flaw detection
system using acoustic Doppler effect detection system according to
this invention adapted to inspect for defects in a railroad
rail;
[0025] FIG. 2 is an enlarged detailed view of the acoustic
transducer of FIG. 1 implemented with separate receiver and
transmitter in the ultrasonic range;
[0026] FIG. 3 illustrates the output signal from the acoustic
transducer;
[0027] FIG. 4A illustrates the output carrier signal reflected from
the unflawed surface;
[0028] FIG. 4B illustrates the Doppler shifted return signal
reflected from a surface flaw;
[0029] FIG. 4C is the magnitude spectrum of the signal reflected
from the unflawed surface;
[0030] FIG. 4D is the magnitude spectrum of the reflection from the
flaw;
[0031] FIG. 5 is a more detailed block diagram of one
implementation of the Doppler detection circuit of FIG. 1;
[0032] FIG. 6 is a more detailed block diagram of another
implementation of the Doppler detection circuit of FIG. 1;
[0033] FIG. 7 is a more detailed block diagram of another
implementation of the Doppler detection circuit of FIG. 1;
[0034] FIG. 8 illustrates the peak magnitude of a Short Time
Fourier Transform implementation of the programmable digital signal
processor of FIG. 7;
[0035] FIG. 9A illustrates the magnitude spectrum of the Doppler
window;
[0036] FIG. 9B illustrates the inverse Fourier transform of the
magnitude spectrum of FIG. 9A;
[0037] FIG. 9C is the result of convoluting the inverse Fourier
transform of FIG. 9B with the input signal of FIG. 3;
[0038] FIGS. 10A, B and C are schematic diagrams showing the
acoustic transducer implemented with an electromagnetic acoustic
transducer using separate spaced transmitter and receiver, separate
adjacent transmitter and receiver, and a single combined
transmitter/receiver unit, respectively; and
[0039] FIG. 11 is a schematic view of another form of transducer
using a laser acoustic transmitter.
[0040] An advantageous feature of the high-speed flaw detection
system using acoustic Doppler effect according to this invention is
that the transducer need not, and in fact preferably is not, in
contact with the rail or other medium to be inspected. Instead, the
transducer remotely senses the discontinuities through the air.
There are several devices that operate in a non-contact mode
including electromagnetic acoustic transducers (EMAT) (G.A. Alers,
Railroad Rail Flaw Detection System Based on Electromagnetic
Acoustic Transducers, U.S. Department of Transportation Report
DOT/FRA/ORD-88-09 (1988) and laser-based acoustic or ultrasound
(LBU) (C. B. Scruby and L. E. Drain, Laser-Ultrasonics: Techniques
and Applications, Adam Hilger, Briston, UK (1990)). More recently,
air-coupled piezoelectric transducers have shown interesting
results in some materials (A. Safaeinili, O. I. Lobkis, nd D. E.
Chimenti, "Air-coupled Ultrasonic Characterization of Composite
Plates", Materials Evaluation, Vol. 53, 1186-1190 (October 1995)).
Air-coupled transducers are attractive because they allow
ultrasound to propagate through gaseous media without requiring
mechanical contact between the transducer and the medium to be
inspected. When used for inspecting railroad tracks the acoustic
impedance mismatch between the steel and air is used to great
advantage since it reflects most of the energy from the steel
surface back to the transducer. When the invention is employed in
railroad rail monitoring, a typical car speed for monitoring the
rail may reach above sixty miles per hour and in fact, increased
car speed leads to more pronounced Doppler effects and better
overall efficiency.
[0041] There is shown in FIG. 1 a flaw detection system using
acoustic Doppler effect system 10 according to this invention
mounted on a railroad car 12, indicated generally in phantom, which
moves in either direction as indicated by arrow 14 along railroad
rail 16, which contains a flaw 18 on its surface. System 10
includes a non-contact acoustic transducer 20 which beams out an
acoustic signal 22, referred to as a carrier signal, which reflects
back from rail 16 as the returned or reflected signal 24. When
signal 22 strikes the smooth portion of rail 16 the carrier signal
comes back with its frequency unchanged, but when acoustic signal
22 strikes flaw 18 the return signal 24 will contain Doppler
shifted frequency. Depending upon the position of the moving
vehicle 12 and/or the direction of the acoustic signal output 22,
the Doppler shift may be an increase or a decrease in frequency.
The return acoustic signal 24 is sensed and transduced to an
electric signal and submitted to Doppler detection circuit 26 which
extracts the Doppler frequency and then submits it to a threshold
or comparator circuit 28. If the Doppler frequency is above a
preselected reference level a defect alarm output is provided by
threshold circuit 28.
[0042] When a transmitter and receiver are used in a bistatic
arrangement, i.e., the transmitter and receiver are separated by a
distance and if the angles between the transducers and the target
(flaw) are .psi., the Doppler frequency shift can be expressed as:
1 f = 2 f s ( V s C ) cos ( 1 )
[0043] where f.sub.S is the frequency of the input signal, .DELTA.f
is the difference between the input frequency and the Doppler
shifted frequency, v.sub.S is the relative speed between the system
and the medium to be inspected, in this case for example it may be
the railroad car carrying the system traveling at for example sixty
miles an hour, c is the wave speed in the medium, air in this case,
and .psi. is the angle between the direction of motion and the
direction to the receiving transducer from the notch.
[0044] Acoustic transducer 20 may be a single transducer which acts
as both transmitter and receiver, or it may be two separate units,
one a transmitter, the other a receiver. In FIG. 2 transducer 20a
includes such discrete devices where a transmitter 30 transmits an
ultrasonic output beam 22a and ultrasonic receiver 32 receives the
reflected ultrasonic signal 24a.
[0045] The output 40, FIG. 3, of transducer 20 shows a general
smooth amplitude profile over time in the areas 42 but shows
distinctive characteristics 44 where a defect such as defect 18 has
been seen. Typically the output acoustic signal 22b, FIG. 4A, is in
the range of 100 kHz. Upon hitting a defect or flaw the return wave
appears as at 24b in FIG. 4B. The magnitude spectrum 22bb, FIG. 4C,
of output signal 22b shows a marked rise at 100 kHz while the
magnitude spectrum 24bb, FIG. 4D for the return signal 24b is
accompanied by a very distinct peak 50 at about 115 kHz which is
the Doppler shifted frequency resulting from the Doppler effect
caused by the flaw 18.
[0046] Doppler detection circuit 26 may be implemented in any
number of ways. For example, detection circuit 26a, FIG. 5, may
include an analog bandpass filter 60 which provides a bandpass
window centered on 115 kHz where the Doppler shift is expected at a
relative speed of 60 miles an hour between the medium and system.
The output from filter 60 is then selected by gated peak detector
62 so that any signals appearing in that band above a certain level
will be accepted as a flaw detection. Alternatively, Doppler
detection circuit 26b, FIG. 6, may include a spectrum analyzer 64
which directly provides the Doppler shifted frequency output.
[0047] In another implementation 26c, FIG. 7, Doppler detection
circuit 26 may include an analog to digital converter 66 which
converts the analog signal to a digital signal and then submits it
to a programmable digital signal processor 68. The programmable
digital signal processor may be programmed in a number of different
ways. For example, it may be programmed to operate as a Short-Time
Fourier transform. Beginning with the signal as shown in FIG. 3,
the Short-Time Fourier Transform 2 S ( f ) = 1 2 - .infin. +
.infin. s ( t ) - ( 2 f t ) W ( t ) t ( 2 )
[0048] results in discrete and prominent features 44a, FIG. 8,
corresponding to each of the flaws or defects 44 in FIG. 3.
[0049] Alternatively, the programmable digital signal processor 68
may be programmed to produce a bandpass 70, FIG. 9A, in the range
of 105 to 115 kHz then obtain the inverse transform 72, FIG. 9B, of
response 70 and convolve it with the return or reflected signal as
shown in FIG. 3 in accordance with the input signal designated x(n)
and the filter coefficient h(n) in the discrete-time domain
directly as shown in the following expression: 3 y ( n ) = k = 0 N
- 1 h ( k ) x ( n - k ) ( 3 )
[0050] wherein y is the filtered output signal, N is the number of
points, h is the filter coefficient, k and n are index variables,
and x is the input signal. The result of that convolution is shown
in FIG. 9C wherein each of the flaws or defects 44, FIG. 3, creates
a discrete and very prominent feature 44b.
[0051] Although as disclosed herein the acoustic signals are
continuous wave signals, this is not a necessary limitation of the
invention as pulse or spike pulse signals can also be used. For
monitoring internal flaws 18b, c, d, FIGS. 10A, 10B and 10C, an
electromagnetic acoustic transducer or EMAT may be used. Such a
transducer 20b, FIG. 10A, may include an electromagnetic
transmitter 30b and receiver 32b for monitoring internal flaws such
as flaw 18b. While transmitter and receiver 30b and 32b are spaced
apart, FIG. 10A, this is not a necessary limitation for as shown in
FIG. 10B, EMAT transmitter 30c and receiver 32c may be adjacent to
one another and it is not necessary that the receiver and
transmitter be separate, for as shown in FIG. 10C an EMAT
transducer 20d which both transmits and receives can be used. As is
well known, the EMAT transmitter 30b establishes a varying magnetic
field 31b which induces the acoustic signal 22b in rail 16. EMAT
receiver 32b through its magnetic field 33b senses the acoustic
return signal 24b.
[0052] Yet another transducer 20e, FIG. 11, is shown in which the
transmitter may be a laser 30e that either provides its energy
directly over beam 22e or through optical fiber cable 90 delivers
its energy to rail 16 where it induces an acoustic wave that
propagates in rail 16. The laser should be a powerful one such as a
Q-switched Nd:YAG laser. The receiver 32e may be an EMAT transducer
or an acoustic transducer as already disclosed or may be an
interferometer vibrometer device using a Fabry-Perot technique, for
example.
[0053] Although specific features of this invention are shown in
some drawings and not others, this is for convenience only as each
feature may be combined with any or all of the other features in
accordance with the invention.
[0054] Other embodiments will occur to those skilled in the art and
are within the following claims:
* * * * *