U.S. patent application number 10/212563 was filed with the patent office on 2003-02-27 for non-destructive evaluation of wire insulation and coatings.
Invention is credited to Anastasi, Robert F., Madaras, Eric I..
Application Number | 20030037615 10/212563 |
Document ID | / |
Family ID | 32996318 |
Filed Date | 2003-02-27 |
United States Patent
Application |
20030037615 |
Kind Code |
A1 |
Madaras, Eric I. ; et
al. |
February 27, 2003 |
Non-destructive evaluation of wire insulation and coatings
Abstract
The present invention uses the generation and detection of
acoustic guided waves to evaluate the condition of the insulation
on electrical wiring. Low order axisymmetric and flexural acoustic
modes are generated in the insulated wire and travel partially in
the center conductor and partially in the outer insulation. The
stiffness of the insulation and the insulation's condition affect
the overall wave speed and amplitude of the guided wave. Analysis
of the received signal provides information about the age or useful
life of the wire insulation. In accordance with the present
invention, signal transmission occurs at one location on the
electrical wire to be evaluated, and detection occurs at one or
more locations along the electrical wire. Additional receivers can
be used to improve measurement accuracy. Either the transmission
transducer or one or more receiver transducers may be angled at
other than 90 degrees to the wire. Generation of the guided waves
can be accomplished by imparting a pressure pulse on the wire.
Alternative embodiments include generation via a laser, such as a
Q-switched laser or a laser diode.
Inventors: |
Madaras, Eric I.; (Yorktown,
VA) ; Anastasi, Robert F.; (Hampton, VA) |
Correspondence
Address: |
NASA Langley Research Center
MS 212
3 Langley Blvd.
Hampton
VA
23602
US
|
Family ID: |
32996318 |
Appl. No.: |
10/212563 |
Filed: |
August 1, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60311967 |
Aug 1, 2001 |
|
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60396498 |
Jul 17, 2002 |
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Current U.S.
Class: |
73/598 ;
73/643 |
Current CPC
Class: |
G01N 29/11 20130101;
G01N 2291/2626 20130101; G01R 31/1272 20130101; G01N 29/2418
20130101; G01N 2291/02827 20130101; G01N 2291/102 20130101; G01N
2291/048 20130101; G01N 2291/0231 20130101; G01N 29/07 20130101;
G01R 31/58 20200101; G01N 2291/0421 20130101; G01N 2291/103
20130101; G01N 29/045 20130101 |
Class at
Publication: |
73/598 ;
73/643 |
International
Class: |
G01N 029/08 |
Claims
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. A method for evaluating wire comprising: transmitting an
ultrasonic wave into the wire at a first location; receiving the
ultrasonic wave at one or more one predetermined locations along
the wire; processing the received ultrasonic wave to evaluate the
wire.
2. The method of claim 1, wherein the wire is electrical wire.
3. The method of claim 1, wherein the wire comprises an insulated
conductor.
4. The method of claim 1, wherein the wire is coated.
5. The method of claim 1, wherein the wire is stranded.
6. The method of claim 3, wherein the processing step comprises
evaluation of the insulation.
7. The method of claim 3, wherein the processing step comprises
evaluation of the conductor.
8. The method of claim 3, wherein the processing step comprises
evaluation of the insulation and the conductor.
9. The method of claim 1, wherein the ultrasonic wave is a surface
wave.
10. The method of claim 1, wherein the ultrasonic wave is a body
wave.
11. The method of claim 1, wherein the ultrasonic wave is
transmitted by a transducer.
12. The method of claim 1, wherein the transducer is clamped to the
wire.
13. The method of claim 11, wherein the transducer is a
piezoelectric transducer.
14. The method of claim 1, wherein the ultrasonic wave is
transmitted by a laser.
15. The method of claim 14, wherein the laser is a Q-switched
laser.
16. The method of claim 14, wherein the laser is a laser diode.
17. The method of claim 1, wherein the ultrasonic wave is received
by one or more transducers.
18. The method of claim 17, wherein the one or more transducers are
piezoelectric transducers.
19. The method of claim 1, wherein the ultrasonic wave is received
by a laser.
20. The method of claim 1, wherein the ultrasonic wave is received
at the same location as it is transmitted.
21. The method of claim 1, wherein the ultrasonic wave is received
at one or more locations separate from the location of
generation.
22. The method of claim 9, wherein the transmitter transducer is
angled at other than 90 degrees to the wire.
23. The method of claim 17, wherein the receiver transducers are
angled at other than 90 degrees to the wire.
24. The method of claim 1, wherein the step of processing the
received wave comprises the step of calculating the phase velocity
of the wave.
25. The method of claim 1, wherein: the step of transmitting the
ultrasonic wave comprises transmitting the wave at a predetermined
angle with respect to the wire; and the step of receiving the
ultrasonic wave comprises receiving the wave at an angle with
respect to the wire that is substantially the same as the
predetermined angle.
26. The method of claim 1, wherein: the step of transmitting the
ultrasonic wave comprises transmitting the wave at a predetermined
angle with respect to the wire; and the step of receiving the
ultrasonic wave comprises receiving the wave at an angle with
respect to the wire that is different than the predetermined
angle.
27. The method of claim 1 further comprising the step of generating
a pulse, and wherein the transmitting step further comprises the
step of applying the pulse to the wire.
28. The method of claim 1 further comprising the step of generating
a waveform, and wherein the transmitting step further comprises the
step of applying the waveform to the wire.
29. The method of claim 1 wherein the receiving step comprises the
step of converting the received wave into an electrical
waveform.
30. The method of claim 1 wherein the step of processing comprises
evaluation of the phase velocity of the received wave.
31. The method of claim 1 wherein the step of processing comprises
evaluation of the waveform of the received wave.
32. The method of claim 1, wherein the step of processing further
comprises comparison of the received wave to a pre-defined look-up
table of baseline properties for the wire.
33. The method of claim 1, wherein the step of processing further
comprises comparison of the received ultrasonic wave velocity
properties to a predefined look-up table of baseline velocity
properties for the wire.
34. The method of claim 1, wherein the step of processing further
comprises comparison of the received ultrasonic wave attenuation
properties to a predefined look-up table of baseline attenuation
properties for the wire.
35. The method of claim 1, wherein the step of processing further
comprises use of modeling selected from the group consisting of
ultrasonic propagation, finite element and finite difference.
36. An apparatus for evaluating wire comprising: a transmitting
device for generating an ultrasonic wave into the wire at a first
location; one or more receiving devices for receiving the
ultrasonic wave at one or more predetermined locations along the
wire; a processing device for processing the received ultrasonic
wave to evaluate the wire.
37. The apparatus of claim 36, wherein the wire is electrical
wire.
38. The apparatus of claim 36, wherein the wire comprises an
insulated conductor.
39. The apparatus of claim 36, wherein the wire is coated.
40. The apparatus of claim 36, wherein the wire is stranded.
41. The apparatus of claim 36, wherein the processing step
comprises evaluation of the insulation.
42. The apparatus of claim 36, wherein the processing step
comprises evaluation of the conductor.
43. The apparatus of claim 36, wherein the processing step
comprises evaluation of the insulation and the conductor.
44. The apparatus of claim 36, wherein the ultrasonic wave is a
surface wave.
45. The apparatus of claim 36, wherein the ultrasonic wave is a
body wave.
46. The apparatus of claim 36, wherein the transmitting device is
an ultrasonic transducer.
47. The apparatus of claim 46, wherein the transducer is clamped to
the wire.
48. The apparatus of claim 46, wherein the transducer is a
piezoelectric transducer.
49. The apparatus of claim 36, wherein the transmitting device is a
laser.
50. The apparatus of claim 49, wherein the laser is a Q-switched
laser.
51. The apparatus of claim 49, wherein the laser is a laser
diode.
52. The apparatus of claim 36, wherein each receiving device is a
transducer.
53. The apparatus of claim 52, wherein each transducer is a
piezoelectric transducer.
54. The apparatus of claim 36, wherein each receiving device is a
laser.
55. The apparatus of claim 36, wherein the ultrasonic wave is
received at the same location as it is transmitted.
56. The apparatus of claim 36, wherein the ultrasonic wave is
received at one or more locations separate from the location of
transmission.
57. The method of claim 46, wherein the transducer is angled at
other than 90 degrees to the wire.
58. The apparatus of claim 53, wherein each transducer is angled at
other than 90 degrees to the wire.
59. The apparatus of claim 36, wherein the processing device is a
computer.
60. The apparatus of claim 36 further comprising a pulse
generator.
61. The apparatus of claim 36 further comprising a waveform
generator.
Description
CLAIM OF BENEFIT OF PROVISIONAL APPLICATION
[0001] Pursuant to 35 U.S.C. .sctn.119, the benefit of priority
from provisional application Serial No. 60/311,967, entitled
"Method and Apparatus for Evaluating Insulated Wire", with a filing
date of Aug. 1, 2001, application Serial No.______, entitled
"Method and Apparatus for Evaluating Insulated Wire", with a filing
date of Apr. 24, 2002, and application Ser. No. 60/396,498,
entitled "Method and Apparatus for Evaluating Insulated Wire", with
a filing date of Jul. 17, 2002, is claimed for this non-provisional
application.
ORIGIN OF INVENTION
[0002] The invention described herein was made by employees of the
United States Government and may be used by or for the Government
for governmental purposes without the payment of any royalties
thereon or therefor.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates broadly to the field of
nondestructive examination and more specifically to the
nondestructive examination of wiring. Even more specifically, the
present invention relates to the nondestructive examination of wire
insulation and coatings.
[0005] 2. Description of the Related Art
[0006] Electrical wiring is critical to the operation of most
modern day equipment and, in its operation, is subjected to heat,
cold, moisture, vibrations, tension and other environmental
conditions which eventually may cause the wire insulation and even
the wire conductor to fail. In most cases, these environmental and
operational conditions are modest and wiring is used for years, but
in some cases these conditions are extreme and cause the insulation
to become brittle and crack. The cracks expose the underlying wire
conductor and become a potential source for short circuits and
fire.
[0007] There are few available methods to evaluate the condition of
the insulation on electrical wiring. Typical wire inspections are
done visually and often after the fact, in response to an
instrument or system failure. A visual inspection often fails to
detect many cracks and flaws because the cracks and flaws are not
visible or are located in spaces that are difficult to see.
Furthermore, a visual inspection offers little quantitative
information about the condition of the wire insulation. Some
techniques require a section of wire to be removed for laboratory
testing. These techniques are undesirable due to their destructive
nature. There are also techniques that involve application of
voltage to the wire to detect current leakage. The current leakage
is indicative of an insulation failure, such as cracking, but does
not provide predictive information on the state of the insulation.
Some of the voltage application techniques are conducted in air,
while others imbed the wires in a conductive medium. Additionally,
some involve high voltage while others have been designed to detect
leakage at low voltages.
[0008] Meeker, T. R., and Meitzler, A. H., "Guided Wave Propagation
in Elongated Cylinders and Plates," Physical Acoustics--Principles
and Methods, edited by W. P. Mason, Academic Press, NY, Vol. 1,
Part A., 1964, pp.111-167; Thurston, R. N., J. Acoust. Soc. Am.,
64, 1, 1-37, (1978); McNiven, H. D., Sackman, J. L., and Shah, A.
H., J. Acoust. Soc. Am., 35, 10,1602-1609, (1963), and Abramson, H.
N., J. Acoust. Soc. Am., 29, 1, 42-46, (1957) examined acoustic
guided wave propagation in cylindrical geometry. Madaras, E. I.,
and Anastasi, R. F., "Pseudo-Random Modulation of a Laser Diode for
Generation Ultrasonic Longitudinal Waves," 26 Annual Review of
Progress in Qualitative Nondestructive Evaluation, Montreal,
Quebec, Canada, July 1999, and Anastasi, R. F. and Madaras, E. I.,
"Pulse Compression Techniques for Laser Generated Ultrasound," IEEE
International Ultrasonics Symposium-1999, edited by S. C. Schneider
and B. R. McAvoy, IEEE Ultrasonics, Ferroelectronics, and Frequency
Control Society, 1999, both incorporated herein by reference,
examined ultrasonic guided waves for characterization of wire.
[0009] There are numerous methods for wire nondestructive
examination that involve investigation of the conductor. One method
is Time Domain Reflectometry (TDR) and another is Standing Wave
Reflectometry (SWR). These methods and related variants are
sensitive to the conductor but are only mildly affected by the
condition of the insulation. Furthermore, these methods only detect
insulation failure.
[0010] U.S. Pat. No.4,380,931 (Frost, et al.), utilizing a
plurality of noncontacting ultrasonic transducers in cooperation
with a magnetic field, is applicable only to conductive wires, and
more specifically only to solid cylindrically shaped objects, not
stranded wires with insulation. Furthermore, only torsional waves
are produced in a solid conductor. U.S. Pat. No. 5,457,994 (Kwun et
al.) utilizes the magnetoresistive effect to generate and detect
acoustic waves to measure the condition of conducting wires, but
does not detect the surrounding materials' condition. U.S. Pat. No.
4,593,244 (Summers et al.) is limited to measuring the thickness of
conductive coatings that are on ferromagnetic substrates. In
general, electrical wires that are usually of interest do not
utilize a conductive coating and, in addition, the thickness of a
wire coating is, in general, not the only concern that faces most
electrical wire users.
[0011] U.S. Pat. Nos. 4,659,991 (Weischedel), 4,929,897 (Van Der
Walt), 4,979,125 (Kwun et al.), and 5,456,113 (Kwun et al.) teach
methods that are applicable only to ferromagnetic materials. None
of the aforementioned patents teach non-destructive examination of
wire insulation. U.S. Pat. No. 4,659,991 (Weischedel), detects
shape changes in a cable and uses magnetic fields to sense the
shape changes, but is not relevant to wire insulation. U.S. Pat.
No. 4,929,897 (Van Der Walt), also detects shape changes in a cable
and also uses magnetic fields from a different sensor geometry than
Weishedel to sense the shape changes, and again is not relevant to
wire insulation. U.S. Pat. No. 4,979,125 (Kwun et al.) tests a
cable, rope or metal strand (which are not insulated) by first
striking the cable with an impulse such as a hammer or
electromagnetically driven plunger, and then detecting the
resulting vibrations with a magnetic sensor. U.S. Pat. No.
5,456,113 (Kwun et al.) tests ferromagnetic cables and ropes (which
are not insulated) by inducing and detecting acoustic/ultrasonic
waves by a magnetorestrictive means.
[0012] It is therefore an object of the present invention to
provide a nondestructive method and apparatus for evaluating the
condition, both prior to and subsequent to failure, of the
insulation on electrical wiring.
[0013] It is another object of the present invention to provide a
nondestructive method and apparatus for evaluating the condition of
wire insulation quantitatively, giving the user information on the
expected safe remaining life of the wire.
[0014] It is another object of the present invention to provide a
nondestructive method and apparatus for evaluating the condition of
either ferromagnetic or nonferromagnetic insulation on electrical
wiring.
[0015] It is yet another object of the present invention to provide
a nondestructive method and apparatus to utilize ultrasonic wave
generation to evaluate the condition of electrical wire
insulation.
[0016] It is yet another object of the present invention to provide
a nondestructive method and apparatus for evaluating the condition
of electrical wire conductors.
[0017] It is yet another object of the present invention to provide
a nondestructive method and apparatus for evaluating the condition
of wire coatings.
[0018] Still other objects and advantages of the present invention
will in part be obvious and will in part be apparent from the
specification.
SUMMARY OF THE INVENTION
[0019] The present invention uses the generation and detection of
acoustic guided waves to evaluate the condition of the insulation
on electrical wiring. Low order axisymmetric and flexural acoustic
modes are generated in the insulated wire. These modes travel
partially in the center conductor and partially in the outer
insulation. The stiffness of the insulation and the insulation's
condition affect the overall wave speed and amplitude of the guided
wave. Thus, the measurement of wave speed will in part be a
measurement of material stiffness and, in part, be a measurement of
insulation condition. Analysis of the received signal provides
information about the age or useful life of the wire
insulation.
[0020] Although there are other, higher order modes that are
generated, the two lowest order modes mentioned are generally the
easiest to excite. The flexural mode is one of the largest
generated. Although the axisymmetric mode is generally small, it is
easy to measure, and thus desirable to use. Little or no
axisymmetric mode is generated with the laser generation method, to
be discussed later, most likely reflecting the small area of
generation in contrast to the larger area of a transducer. The
particular mode to be utilized is determined based on the ease of
generation, low attenuation, and sensitivity to the damage being
tested for. Some testing of a baseline sample will generally be
needed to determine which mode to utilize.
[0021] The wave speed and attenuation of the waves are measured and
provide information about the physical condition of the insulation.
The speed measurement is related to the stiffness and density of
the material components. The attenuation measurement is related to
the structure and microstructure of the component materials, such
as microcracks in the insulation. In general, wire insulations are
of a polymer base and have much lower stiffness characteristics
than the center conductor, which is usually copper or aluminum.
Because copper and aluminum have a much higher wave speed than
polymers, the effect of wrapping insulation on a cylindrical shaped
conductor will be to lower the wave speed of the guided wave. As
the insulation is aged, it will loose its plasticity and harden,
which will lead to cracks, exposing the center conductor, which
could lead to electrical shorting. As the insulation hardens, the
coating material will stiffen, which will cause the wave speed to
be greater. The frequency content and amplitude information provide
an indication of the insulation's condition, such as chaffing,
cuts, nicks, cracks and flaws. Each of these conditions will
attenuate the signal. Both flaws and degradation will affect the
signals. For example, a nick in the insulation changes the
frequency content of the signal, whereas degradation alters the
signal speed and attenuation. The present invention is applicable
to any conductor material, with the details of the wave motion
depending on the relative constituents.
[0022] In accordance with the present invention, signal
transmission occurs at one location on the electrical wire to be
evaluated, and detection occurs at one or more locations along the
electrical wire. The number and position of detection locations
depends on the user's preference. In one embodiment, transmission
and detection occurs at one location, which is especially effective
for evaluating the termination points of wire, such as at
connectors, as well as for detecting signals reflected from flaws.
For connectors, one transducer can be used to transmit the signal
to the connector and detect the reflected signal. The transducer
would be positioned as close as possible to the connector.
Evaluation can consist of viewing the waves or estimating the wave
velocity based on the distance of the transducer from the
connector. With a flaw, the existence of the flaw would produce a
signal anomaly.
[0023] In another embodiment, detection occurs at one or more
locations separate from the transmitting location. This
configuration generally has good signal to noise. The positioning
of the transducers is dependent upon the anticipated region of
criticality. Often certain areas are more suspect than others and
should be inspected with more detail and frequency. General areas
could be spot checked if desired. Two simultaneous measurements can
be taken to generate both attenuation and speed values. If the
distances between any two pairs of transmit or detect transducers
are not equal, then the difference between the time of the received
signals divided into the differences in transducer spacing will
give the velocity of the ultrasonic wave. Additional receivers can
be used to improve measurement accuracy.
[0024] In further alternative embodiments, either the transmitter
transducer or one or more receiver transducers may be angled at
other than 90 degrees to the wire.
[0025] Generation of the guided waves can be accomplished by
imparting a pressure pulse on the wire. Alternative embodiments
include generation via a laser, such as a Q-switched laser or a
laser diode.
[0026] The detected signal can be further processed to extract the
material properties of interest with respect to the wire
insulation. One method of processing is to apply the generation and
detection to wires exhibiting a range of conditions, both
acceptable and unacceptable, to produce a look-up table of velocity
or attenuation properties for that specific wire type that could
then classify an unknown wire specimen. Another method is to apply
modeling. By setting up the differential equations for the particle
motions and stresses and strains, and matching the boundary
conditions at the interface of the conductor to insulation and the
insulation to air, an ultrasonic propagation model for a wire
covered by insulation can be developed. A further general
description of such modeling can be found in the earlier references
to Meeker, T. R., and Meitzler, A. H.,; Thurston, R. N.,; McNiven,
H. D., Sackman, J. L., and Shah, A. H.,; and Abramson, H. N. Then,
using known properties for the conductor and the dimensions of the
wire, the properties of the insulation can be inferred utilizing
mathematical techniques via a computer. Examples of commercially
available software include Disperse, as well as general software in
commercial packages such as Numerical Methods in C, Mathematica,
MatLab, and IDL. In a similar method, finite element method or
finite difference modeling software can be used to accomplish the
same result, although generally more expensive.
[0027] The present invention provides the capability to measure
velocities, frequencies, and magnitudes. The system is adapted to
measure characteristics that are relevant to the flaw/degradation
being tested for. For example, the signal's frequency content would
be significantly changed and the attenuation would be worse for
severe chafing.
[0028] In addition to the evaluation of insulation, the present
invention can also be used for evaluating coatings, as well as the
conductor itself. It can also be used to evaluate stranded wire.
The system is adapted, frequency for example, to measure the
particular constituent condition. The invention can be used for any
layered media, including cylindrical or rectangularly shaped
structures, and including media that is not conductive. The
stiffness of various layers would determine the ultimate efficacy
of any testing. At the lowest frequencies, it would test the whole
structure, but at higher frequencies, it would tend to test the
layers with the lowest stiffness.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] These and other features, aspects, and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings
where:
[0030] FIG. 1 shows a schematic of an embodiment of the present
invention having a single transmitter transducer and a separate
single receiver transducer;
[0031] FIG. 2 shows transducers clipped to a wire;
[0032] FIG. 3 shows a schematic of an embodiment utilizing
transducers and incorporating pre-amplification, filtering and
automation via digitizer and computer;
[0033] FIGS. 4A-4B show bare aluminum rod and polymer-coated
aluminum rod test articles;
[0034] FIG. 5 illustrates a typical ultrasonic signal in the bare
aluminum rod of FIG. 4A;
[0035] FIG. 6 illustrates axisymmetric and flexural mode amplitudes
as a variation of detection angle;
[0036] FIG. 7 shows a wire test article;
[0037] FIG. 8 shows experimental results for MIL-W-22759/34
wire;
[0038] FIG. 9 shows experimental results for MIL-W-81381 wire;
[0039] FIG. 10 shows baseline values of MIL-W-81381 velocity
measurements compared to modulus-derived velocity;
[0040] FIG. 11 shows baseline values of MIL-W-81381 velocity
measurements compared to normalized modulus measurements;
[0041] FIG. 12 illustrates a heat treatment profile;
[0042] FIG. 13 shows velocity measurement for heat-damaged
MIL-W-81381 wire compared to modulus-derived velocity using the
heat treatment profile of FIG. 12;
[0043] FIG. 14 shows velocity measurement for heat-damaged
MIL-W-81381 wire compared to normalized modulus measurement using
the heat treatment profile of FIG. 12;
[0044] FIG. 15 shows axisymmetric phase velocity of Kapton.RTM.
insulated wire;
[0045] FIG. 16 shows axisymmetric phase velocity of aromatic
polyimide insulated wire;
[0046] FIG. 17 shows a MIL-W-22759/34 AWG 20 wire sample;
[0047] FIGS. 18A-18C illustrate experimental results for the
MIL-W-22759/34 AWG 20 wire sample;
[0048] FIG. 19 is a schematic of an embodiment having one
transmitter transducer and two receiver transducers;
[0049] FIG. 20 illustrates the time difference between received
signals for the embodiment shown in FIG. 19;
[0050] FIG. 21 is a schematic of an embodiment utilizing a laser
diode for signal generation;
[0051] FIG. 22 shows typical experimental results obtained using
laser-diode signal generation;
[0052] FIGS. 23A-23C illustrate the experimental set-up and results
for a pulsed piezoelectric transducer and a modulated piezoelectric
transducer;
[0053] FIGS. 24A-24B illustrate experimental set-up and results
obtained using a modulated laser diode;
[0054] FIG. 25 shows a detailed schematic of an embodiment
utilizing a laser diode; and
[0055] FIG. 26 shows experimental results for laser-diode
generation in MIL-W-22759/34 wire.
DETAILED DESCRIPTION
[0056] Referring now to the drawings, and more particularly to FIG.
1, an embodiment of the present invention is shown and referenced
generally by numeral 10. In describing the embodiments of the
present invention, like numerals used in the various figures refer
to like features of the invention. In the embodiment illustrated,
signal generation occurs at a single location along wire 12 and
detection occurs at a single separate location along wire 12. A
piezoelectric transducer 14 generates the guided waves in
combination with an ultrasonic pulser or waveform generator 18.
Other transducers are also acceptable, but piezoelectric
transducers are commonly used and function well for this purpose.
Use of the ultrasonic pulser or waveform generator 18 imparts a
pressure pulse on the wire 12. The use of an ultrasonic pulser will
generate many frequencies at once, whereas a waveform generator
would be used to generate a more specific set of frequencies. The
pressure pulse application will set up numerous flexural and
axisymmetric waves that will transmit through the length of wire 12
in both directions. These modes travel partially in the center
conductor and partially in the outer insulation.
[0057] Use of low frequency, wide band transducers, as shown in
FIG. 2, clamped onto the wire allow for reliable, repeatable
coupling to overcome transducer coupling. The transducers 30 are
mounted in holders 32 that can be clamped to the wire 12. The
holder 32 holds the wire across the center of the transducer 30
face. Contact between the transducer 30 and the wire 12 is critical
to producing a repeatable measurement. The clamping allows for
control of where the transducer touches the wire so that a reliable
signal can be reproduced and to provide a solid contact for both
generation and detection.
[0058] The frequency range of interest for the transducers 30 will
depend in part on the flaws being tested for and the general
dimensions of the wire 12. An example of a suitable transducer is a
3/8" acoustic emission transducer, which is small, sensitive to low
frequencies (.about.50KHz), and wide band (up to .about.2 MHz).
Examples of suitable pulsers are Panametrics and Metrotek pulsers.
An example of a suitable waveform generator is a LeCroy arbitrary
waveform generator. One example of a suitable transmitter
transducer 14 and receiver transducer 16 is a broadband acoustic
emission piezoelectric transducer that operates in the 200-300 kHz
range. These piezoelectric transducers are capable of generating a
signal that transmits a fairly long distance without much
attenuation. The signals that are created in the wire would also be
in the 200-300 kHz range with lower frequency signals traveling
better (with less attenuation) than higher frequency signals. The
shape and type of wire 12 under evaluation determines what
frequencies are generated. In general, the larger the wire, the
lower the frequencies that are used. The signal is detected by
receiver transducer 16 and amplified by pre-amplifier 20 prior to
viewing on oscilloscope 22.
[0059] In its basic geometry, the insulated wire may be considered
a cylindrical wave-guide or perhaps more descriptively, a clad rod,
where the wire conductor is the core and the insulation is the
cladding. In general, many acoustic waves will propagate in an
isotropic cylinder. The lowest mode of vibration is the
axisymmetric mode, which can be divided into axial-radial and
torsional modes. The next order of vibration is the flexural mode,
and higher modes are screw modes. The lowest branch of the
axial-radial mode extends to zero frequency where the limiting
phase velocity is called the bar velocity. In the low frequency
regime this mode is nearly non-dispersive. As frequency increases
the phase velocity drops to a value slightly below the Raleigh
velocity and then approaches the Rayleigh velocity from below at
higher frequencies.
[0060] Another embodiment, shown in FIG. 3 and referenced generally
by numeral 40, incorporates an additional amplifier 42, filters 44,
digitizer 46 and computer 48. The signal is detected by receiver
transducer 16, amplified by pre-amplifier 20 and amplifier 42 and
filtered 44 to capture the acoustic wave or waves of interest.
Examples of suitable pre-amps 20 are Panametrics (20-2000 KHz, with
40 or 60 dB of gain) or Digitial Wave (40-4000 KHz, with 30 dB).
Examples of suitable amplifiers are Panameterics 5052 or Digital
Wave's filter/amplifier which controls the frequency with high pass
and low pass filters and with gain from 0 to 42 dB. The detected
signal can then be digitized 46 and passed to computer 48 for
processing. Pre-amplification is often needed for signal
amplification. Filtering is helpful when suppression of higher
modes is desired. Automation of the system requires the digitizer
46 and computer 48. The ultrasonic signals from different points on
the wire can then be compared via analytic methods to measure the
phase velocity and/or signal loss from the different modes.
Suitable analytical methods include comparison to a preexisting
look-up table of velocity or attenuation properties for the
specific wire type, utilization of an ultrasonic propagation model,
and finite element or finite difference modeling.
[0061] In one experimental example, referring to FIGS. 4A and 4B,
the test articles consisted of a bare solid aluminum rod, as shown
in FIG. 4A, and a solid aluminum rod having a polymer coating, as
shown in FIG. 4B. The bare aluminum rod, simulating a wire
conductor, had a 3.23 mm (0.127 in.) diameter. The polymer coating,
simulating the insulation, had a thickness of 0.57 mm (0.0225 in.),
resulting in an overall diameter of 4.37 mm (0.172 in.). The length
of each rod was 762 mm (approximately 30 in.) The coating was
thermoplastic heat-shrink Polyolefin. TABLE I shows the properties
of the conductor and insulator. The experimental set-up shown in
FIG. 1 was utilized.
1 TABLE I Properties* Aluminum Thermoplastic Long. Velocity (m/s)
6320 1868.sup.+ Shear. Velocity (m/s) 3130 -- Bar Velocity (m/s)
5119 -- Density (gm/cm.sup.3) 2.700 0.971 Poisson's Ration 0.338
0.458 Young's Modulus (Gpa) 70.76 1.2 *published values
.sup.+measured value
[0062] Piezoelectric transmitter transducer 14 and piezoelectric
receiver transducer 16 were separated by between approximately 3 to
30 cm, although other distances could be used. The transducers 14
and 16 each had a frequency range of 50 kHz to 1.5 MHz and were
mechanically clipped to the test article, as shown in FIG. 2. The
frequencies were chosen by the naturally generated frequencies that
the wire tended to generate. A typical ultrasonic signal in the
bare aluminum rod is shown in FIG. 5. The smaller amplitude wave at
about 50 ps is the first axisymmetric wave mode and the larger
amplitude wave initiating at about 75 ps is the first flexural mode
wave. The amplitude difference between the axisymmetric and
flexural wave modes is consistent with the geometry of the
ultrasonic generation. Since the transmitter transducer 14 is
located on the side of the rod, a larger amplitude bending force is
applied to the rod, and thus a larger amplitude flexural mode is
generated. To further investigate, the signal was examined as a
function of rotational angle between the transmitting and receiving
transducers. The transmitter transducer 14 was held stationary
while the receiver transducer 16 was rotated around the aluminum
rod in increments of 10 degrees. A plot of the resulting
axisymmetric and flexural mode amplitudes is shown in FIG. 6. The
axisymmetric mode amplitude is constant while the flexural mode
amplitude follows a cosine-squared shape with a minimum at 90
degrees. Signals similar to those shown in FIG. 5 were observed
when the distance between the transmitter transducer 14 and
receiver transducer 16 was varied. The resulting signals, as a
function of distance, demonstrate that the frequency content of the
axisymmetric mode remains constant while the frequency content of
the flexural mode varies and contains higher order modes. These
higher order modes were evident in the signal as small changes or
variation in the sinusoidal shape of the wave, and changed as the
distance between the transducers was varied.
[0063] The phase velocity of the axisymmetric mode was determined
by taking a series of measurements of a constant phase point as a
function of transducer separation. Because the axisymmetric mode is
faster than the flexural mode and arrives first, it is easy to
isolate and measure. The separation distance of the two transducers
varied from 50 mm to 250 mm, over which 10 to 12 measurements were
taken. The phase point in time was plotted against the distance and
a linear curve fit was applied to the data. The slope of the linear
fit was the measure of the phase velocity. The phase velocity of
the bare rod and the polymer coated aluminum rod were 5128 m/s and
4663 m/s, respectively. This phase velocity measurement in the bare
aluminum rod is consistent with a calculated bar velocity of 5119
m/s. The measured changes in phase velocity between the bare and
coated aluminum rod demonstrate the effect of the coating. This
example illustrates the sensitivity of the lowest order
axisymmetric mode to stiffness changes in the wire insulation. At
the lowest frequencies of the flexural mode, there is less effect
of the insulating material/coating on the wave speed. The
sensitivity is not as great as in the low frequency axisymmetric
mode.
[0064] In a further experimental example, 12, 16 and 20 gauge
Tefzel.RTM. coated MIL-W-22759/34 wire samples were heat-damaged.
FIG. 7 shows the wire test article and TABLE II shows the diameter,
strand number, and strand gauge as a
2 TABLE II Gauge 12 16 20 Overall Dia. (mm) 2.78 1.90 1.51 Wire
Bundle Dia. (mm) 2.05 1.29 0.81 Insulation Thk. (mm) 0.365 0.305
0.35 Wire strands per Bundle 37 19 19 Wire Strand Gauge 28 29
32
[0065] function of wire gauge. Three samples of each gauge wire
were cut to a length of approximately 60 cm. One sample of each
gauge was used for a baseline measurement, one sample of each gauge
was heated in an oven at 349.degree. C. for one hour, and one
sample of each gauge was heated in an oven at 399.degree. C. for
one hour. The insulation on the baseline sample was smooth,
flexible, and off-white in color. The samples that were heat
damaged at 349.degree. C. remained smooth and flexible but the
color changed to gray. The samples heat damaged at 399.degree. C.
became brittle, cracked, and the color approached black. The phase
velocity in each of these samples was measured following the same
procedures described in the earlier experimental example and the
results are shown in FIG. 8. As shown, the axisymmetric phase
velocity measurement is able to distinguish between the baseline
and heat-damage conditions.
[0066] In the experimental example shown in FIG. 7 and TABLE III,
12, 16 and 20 gauge MIL-W-81381 wire samples were heat-damaged. The
insulation type on these samples was polyimide/FEP laminated tapes.
FIG. 9 shows the results for each of the gauge wires at the
baseline condition, after heating at 399 degrees C for one hour and
after heating at 399 degrees C for 49 hours.
3 TABLE III Gauge 12 16 20 Mil spec variant /12 /21 /7 Overall Dia.
(mm) 2.50 1.63 1.29 Wire Bundle Dia. (mm) 2.09 1.33 0.94 Insulation
Thk. (mm) 0.21 0.15 0.17 Wire Strands per Bundle 37 19 19 Wire
Strand Gauge 28 29 32 Conductor Type Ni coated Sn coated Ag coated
copper copper copper Max. Operating Temp. 200.degree. C.
200.degree. C. 200.degree. C.
[0067] Additional experimental examples were conducted to compare
the ultrasonic damage measurement to the mechanically measured
damage. For the mechanical measurement, a small table top
electromechanical load frame was used, having a 1000 pound load
cell, 1 inch extensometer, and load ranges of up to 100 pounds for
12 gauge wire and up to 40 pounds for 20 gauge wire. FIG. 10 shows
the baseline values of MIL-W-81381 velocity measurements compared
to the velocity derived from modulus measurements. FIG. 11 shows
the baseline values of MIL-W-81381 velocity measurements compared
to normalized modulus measurements. The ultrasonic guided wave
velocity of the lowest order axisymmetric mode for 12 gauge wires
were measured as 3352, 3596 and 3712 m/s for baseline, one hour and
49 hours at 399 degrees C, respectively. Mechanically, the tensile
moduli of these wires were 8020, 10882 and 15894 ksi for baseline,
one hour, and 49 hours at 399 degrees C, respectively.
[0068] FIGS. 13 and 14 show additional results obtained for the
MIL-W-81381 wire using the heat treatment profile shown in FIG. 12.
The profile illustrates the temperature ramp up to 370 degrees C,
the dwell at 370 degrees C, and the temperature cool down. FIGS. 15
and 16 show additional experimental results for Kapton.RTM.
insulated wire at dwell temperature 370.degree. C. and aromatic
polyimide wire insulation at dwell temperature 400.degree. C.,
respectively. The wire cores were stranded copper. For the 12
gauge, the copper was a nickel-coated wire, for the 16 gauge the
copper was tin-coated wire, and for the 20 gauge wire, the copper
was silver coated.
[0069] In addition to providing quantitative information concerning
degradation of wire insulation, the present invention is also
useful for detecting actual flaws in the insulation, such as a cut.
A signal loss would be apparent using the pitch and catch method.
Use of the pulse echo method would allow one to see a reflected
signal from the flaw. FIGS. 18A through 18C are illustrative of the
amplitude change resulting from a flaw such as a cut. More
specifically, FIGS. 18A-18C show the results for the wire shown in
FIG. 17 undamaged and damaged by a cut approximately 0.2 in. in
length and extending through to the conductor. The experimental
set-up illustrated in FIG. 1 with an input signal of a 100 kHz, 5
cycle Gaussian enveloped sine wave was used. The frequencies were
determined in a manner similar to earlier discussions. The number
of cycles was determined based on general knowledge. The wire had
19 strands, each approximately 0.2 mm in diameter, and two layers
of insulation.
[0070] In an alternate embodiment, as shown in FIG. 19, three
transducers are used, where the transmitter transducer 14 is
located between two outer receiver transducers 16a and 16b. If the
distances D1 and D2 are not equal, then the difference between the
time of the received signals, as shown in FIG. 20, divided into the
differences in transducer spacing will give the velocity
V=(D1-D2)/.sub..DELTA.t). This eliminates the need to move the
receiver or transmitter transducer to obtain various measurements
from which the wave velocity is calculated. Additional receivers
can be used to try to improve accuracy. Also, this embodiment lends
itself to a phase wave measurement technique. In the phase wave
technique, either one transducer is used in a pulse echo manner
with the distance to two well-defined reflection points that are
known, or two transducers (one transmitting and one receiving) are
needed for accurate velocity measurements. The phase wave
measurement technique is discussed in more detail in Wolfgang
Sachse and Yih-Hsing Pao, "On the Determination of Phase and Group
Velocities of Dispersive Waves in Solids," J. Appl. Phys., Vol.
49(8), pp 4320-4327, August 1978. The greater the spacing, the more
accurate the velocity measurements. More than two receiver
transducers can be used but each each measurement affects the
signal, so that more transducers will measure a modified signal. To
facilitate movement of the transducers, all three transducers can
be clamped as a unit onto the wire.
[0071] In alternative embodiments, either the transmitter
transducers or the one or more receiver transducers may be angled
at other than 90 degrees to the wire. The angling of the
transmission transducers produce surface waves instead of body
waves. In this embodiment, although the signal may be small due to
less efficiency in generating the signal, detection efficiencies
are improved since the wave spends most of its time in the
insulating material and more efficiently interacts with a flaw.
Angling of the receiver transducers might be beneficial in
evaluating damage such as cracks and surface damage to the
insulation.
[0072] In another alternate embodiment, the ultrasonic waves are
laser generated. The laser generation allows for non-contacting
measurements to be made at a distance. The heat created by the
laser causes a deformation in the cable insulation, which generates
a detectable acoustic signal. The laser may be a low-power laser
diode or a Q-switched laser. In general, Q-switched lasers are used
for ultrasonic wave generation. For the purpose of the lower
frequencies, the modulated laser diode may generate lower
frequencies better. The use of a laser diode to generate ultrasound
is attractive because of its low cost, small size, lightweight,
simple optics and modulation capability. The laser diode generates
via a coded low power signal so that little or no damage to the
wire insulation occurs. Using cross correlation techniques, the
ultrasonic signal can be recovered. Cross correlation techniques
are known in the art and are described in numerous publications
such as Cook, C. E., M. Bernfeld, and C. A. Palmieri, "Matched
Filtering, Pulse Compression and Waveform Design," Radars, Volume
3: Pulse Compression, edited by D. K. Barton, Artech House,
Massachusetts, 1975; and Furgason, E. S., V. L. Newhouse, N. M.
Bilgutay, and G. R. Cooper, "Application of Random Signal
Correlation Techniques to Ultrasonic Flaw Detection," Ultrasonics,
January 1975. Laser detection can also be utilized. A discussion of
laser detection in general can be found at C. B. Scruby and L. E.
Drain "Laser Ultrasonics, Techniques and Applications," Adam
Hilfer, NY, 1990. These laser generation and/or detection
embodiments would be desirable in an area where a non-contacting
measurement is desired, such as in an area that is more remote and
difficult to reach. The use of a Q-switched laser (high frequency)
versus a laser diode (low frequency) is based on the frequency
desired.
[0073] In an experimental example, having the set-up shown in FIG.
21, a 150 mW modulated laser diode 50 was used to generate
ultrasound and a conventional piezoelectric transducer 16 was used
as the receiver. Generally, the highest power that will not damage
the material should be used. A conventional ultrasonic signal was
recovered by signal correlation. The laser-diode beam incident on
the wire insulation was 2 mm in diameter and had a power density of
17.83 mW/mm2. The power was measured with a calibrated power
detector. A frequency generator modulated the laser diode drive
current and the beam intensity in a frequency swept pattern from 1
kHz to 100 kHz. The insulation became damaged (slightly blackened)
when the power density reached 20 mW/mm2. The previously discussed
12-gauge MIL-W-22759/34 baseline sample and the 12 gauge sample
that was heat damaged at 349 degrees C for one hour were examined.
A typical ultrasonic signal recovered from correlating the
transmitted and received signals is shown in FIG. 22. The first
flexural mode can be seen initiating at about 12 ps. The phase
velocity of this flexural mode was measured by taking a series of
measurements of a constant phase point as a function of generation
point and receiver separation. The laser diode 50 was translated in
millimeter increments while the piezoelectric ultrasonic receiver
transmitter 16 was held in a fixed position. The phase point in
time was plotted against the translation stage displacement and a
linear curve fit was applied. The slope of the linear fit
represents the flexural phase velocity. The baseline flexural phase
velocity was 529 m/s while the heat-damaged sample had a phase
velocity of 548 m/s. The flexural mode phase velocity measured with
the laser is much lower than the axisymmetric mode phase velocity
measured with the transducers. This is consistent with dispersion
curve relations for cylindrical rods. These relations show the
flexural mode phase velocity approaches zero as frequency
approaches zero while the axisymmetric mode phase approaches the
bar velocity as frequency approaches zero. Laser generation tends
to generate very little of the axisymmetric mode, whereas the
transducers tend to produce some axisymmetric mode, and because it
arrives early, it is easy to isolate and measure. FIGS. 23 and 24
illustrate a comparison between pulsed piezoelectric transducer,
modulated piezoelectric transducer and modulated laser diode
generated ultrasound in a solid copper wire without insulation.
FIGS. 23B and 23C show results obtained using pulsed and modulated
piezoelectric transducers using the set-up shown in FIG. 23A. As
shown, the two methods produce the same general signal. FIG. 24B
shows results obtained using a modulated laser diode using the
set-up shown in FIG. 24A.
[0074] FIG. 25 illustrates a more detailed schematic of an
embodiment utilizing a laser diode for ultrasound generation. The
Function Generator generates a swept frequency tone burst. A
suitable Function Generator is a LeCroy Arbitrary waveform
generator. That signal is used to drive the laser diode driver
which controls the laser diode. A commercially available high-speed
laser amplifier is suitable for the driver. The output from the
modulated laser diode may be focused by one or more lenses to focus
the incident beam. This focusing should produce a beam generally on
the order of a few tens of microns in diameter. A thermoelectric
temperature controller can be used to provide maximum stability to
the laser diode and prevent mode hopping. The controller can also
be helpful in extending the lifetime of the laser diode by
operating at lower junction temperatures. The incident beam
transmits an acoustic wave into the wire, which is received at
piezoelectric receiver transducer 16. As discussed earlier, this
embodiment is not limited to a single receiver transducer. The
received signal is amplified prior to processing. Suitable for
signal acquisition is a LeCroy digitizing oscilloscope which has
signal processing capabilities to capture the signals and internal
processing capabilities to perform cross correlation.
[0075] FIGS. 26A and 26B illustrate experimental data for 12 gauge
MIL-W-22759/34 wire. The laser diode beam diameter was 2 mm, the
power was 17.83 mW/mm2 and the modulation was 1 kHz to 100 kHz. The
wire was heat damaged for one hour at 349 degrees C. The flexural
mode phase velocity was determined to be 529 m/s and 548 m/s for
the baseline and heat damages samples, respectively. This increase
in velocity for the heat-damaged sample is consistent with the
axisymmetric wave measurements made using piezoelectric
transducers. The "translation stage" was a small manually operated
motion controller that allowed the sample to be translated a small
distance.
[0076] The present invention can be used for any layered media,
including cylindrical or rectangularly shaped structures, and
including media that is not conductive. In addition to the
evaluation of insulation, it can also be used for evaluating
coatings, as well as the conductor itself. It can also be used to
evaluate stranded wire. The system is adapted, frequency for
example, to measure the particular constituent condition. The
stiffness of various layers would determine the ultimate efficacy
of any testing. At the lowest frequencies, it would test the whole
structure, but at higher frequencies, it would tend to test the
layers with the lowest stiffness.
[0077] Additional discussion and experimental examples can be found
in Robert F. Anastasi and Eric I. Madaras, Investigating the Use of
Ultrasonic Guided Waves for Aging Wire Insulation Assessment,
SPIE's 7th Annual International Symposium on NDE for Health
Monitoring and Diagnostics, San Diego, Calif., Mar. 17-21, 2002;
and Eric I. Madaras and Robert F. Anastasi, Investigating the Use
of Ultrasound for Evaluating Aging Wiring Insulation, 5th Joint
NASA/FAA/DoD Conference on Aging Aircraft, Orlando, Fla., Sep.
10-13, 2001; and Robert F. Anastasi and Eric I. Madaras, Ultrasonic
Guided Waves for Aging Wire Insulation Assessment, 28th Annual
Review of Progress in Quantitative Nondestructive Evaluation
(QNDE), Brunswick, Me., Jul. 29-Aug. 3, 2001, all herein
incorporated by reference.
[0078] Although our invention has been illustrated and described
with reference to the preferred embodiments thereof, we wish to
have it understood that it is in no way limited to the details of
such embodiment, but is capable of numerous modifications for many
mechanisms, and is capable of numerous modifications within the
scope of the appended claims.
* * * * *