Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2002-08-01
2004-11-02
Chapman, John E. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S600000
Reexamination Certificate
active
06810743
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Description of the Related Art
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.
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.
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, N.Y., 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.
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.
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.
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.
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.
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.
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.
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.
It is yet another object of the present invention to provide a nondestructive method and apparatus for evaluating the condition of electrical wire conductors.
It is yet another object of the present invention to provide a nondestructive method and apparatus for evaluating the condition of wire coatings.
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
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.
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 t
Anastasi Robert F.
Madaras Eric I.
Chapman John E.
Edwards Robin W.
The United States of America as represented by the Administrator
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