Electromagnetic acoustic transducer with recessed coils

• US 20030205088A1
• Filed: 05/12/2003
• Published: 11/06/2003
• Est. Priority Date: 11/15/2000
• Status: Active Grant

• Current Assignee: Frank Passarelli Jr
• Original Assignee: Frank Passarelli Jr
• Inventors: Passarelli, Frank Jr.
• Granted Patent: US 6,951,133 B2
• Time in Patent Office: Days
• US Class Current: 73/643
• CPC Class Codes: G01N 2291/014 Resonance or resonant frequency
• CPC Class Codes: G01N 2291/0422 Shear waves, transverse waves, horizontally polarised waves
• CPC Class Codes: G01N 29/223 Supports, positioning or alignment in fixed situation mounting transducers per se G10K11/004
• CPC Class Codes: G01N 29/2412 using the magnetostrictive properties of the material to be examined, e.g. electromagnetic acoustic transducers [EMAT]; investigating the presence of flaws using eddy currents G01N27/90, magnetostrictive transducers B06B1/08, measuring magnetostrictive properties G01R33/18
• CPC Class Codes: G01N 29/348 with frequency characteristics, e.g. single frequency signals, chirp signals measuring frequency of mechanical vibrations or acoustic waves in general G01H1/06, G01H3/04; measuring frequency or analysing frequency spectra G01R23/00
• CPC Class Codes: G01N 29/42 by frequency filtering or by tuning to resonant frequency

Abstract

The disclosed invention mounts transmitting and receiving electrical coils within channels or chambers formed by the notched ends of magnets closest to the metallic object under test where said magnets are arranged to form substantially annular arrays of an electromagnetic acoustic transducer pair applicable in non-destructive testing. The detection, preferably by tuned coils, of one or more electromagnetically-induced resonant frequencies at shifted locations indicates the presence of one or more flaws in the metallic structure under test.

Claims

1. An apparatus for nondestructive testing of metallic structures comprising:

(A) a housing having an opening surrounding an exterior wall of a metallic object having a longitudinal axis;

said metallic object translatable, along its longitudinal axis, through the housing; and

(B) a first electromagnetic acoustic transducer comprising;

(1) a first plurality of substantially similar permanent magnets mounted in the housing and arranged in a first array and positioned substantially in planar radial fashion, about a first radial center, around the opening at substantially equal intervals with ends of adjacent magnets possessing opposite polarity;

said first array positioned transverse to, and with the first radial center substantially colinear with, the longitudinal axis of said metallic object;

each of said first plurality of substantially similar permanent magnets comprising;

(a) a first end adjacent to the opening and

(b) a second end substantially perpendicular to and spaced from the wall wherein the second end is notched in the plane substantially colinear with the longitudinal axis of the metallic object; and

(2) a first electrically conductive wire coil mounted in the housing and positioned within an annular channel formed by the notched second ends of the first plurality of substantially similar permanent magnets.

2. The apparatus claimed in claim 1 wherein the opening is elliptical.

3. The apparatus claimed in claim 1 wherein the opening is substantially circular with a substantially flat sector.

4. The apparatus as claimed in claim 1 further comprising a second electromagnetic acoustic transducer comprising:

(A) a second plurality of permanent magnets, substantially similar to the first plurality of substantially similar permanent magnets, mounted in the housing and arranged in a second array and positioned substantially in planar radial fashion, about a second radial center, around the opening at substantially equal intervals with ends of adjacent magnets possessing opposite polarity;

the second array being substantially parallel and proximate to the first array;

said second array plurality positioned transverse to, and with the second radial center substantially colinear with, the longitudinal axis of the metallic object, each of said second plurality of magnets comprising;

(1) a first end adjacent the opening and (2) a second end substantially perpendicular to and spaced from the wall wherein the second end is notched in the plane substantially colinear with the longitudinal axis of the metallic object; and

(B) a second electrically conductive wire coil mounted in the housing and positioned within an annular channel formed by the notched second ends of the second plurality of permanent magnets.

5. The apparatus claimed in claim 4 wherein the opening is elliptical.

6. The apparatus claimed in claim 4 wherein the opening is substantially circular with a substantially flat sector.

7. The apparatus as claimed in claim 4 wherein the first array is rotationally offset in alignment relative to the second array.

8. The apparatus as claimed in claim 7 wherein spacing of each of the second end of the magnets is substantially consistent permitting the longitudinal travel of the metallic object without mechanically contacting the apparatus.

9. The apparatus as claimed in claim 8 wherein the first array and the second array are each comprised of twenty-six magnets.

10. An apparatus for nondestructive testing of metallic structures as claimed in claim 2 wherein the first electromagnetic acoustic transducer is separated from the second electromagnetic acoustic transducer by a dielectric spacer.

11. The apparatus as claimed in claim 4 further comprising

(A) a signal generator coupled to the first electrically conductive wire coil;

(B) a signal detector coupled to the second electrically conductive wire coil;

whereby the signal generator applies an electrical excitation signal to the first electrically conductive wire coil mounted in the channel of the first array, and the signal detector senses the electrical response signal in the second electrically conductive wire coil mounted in the channel of the second array.

12. The apparatus as claimed in claim 1 wherein an electrical excitation signal applied to the first electrically conductive wire coil induces vibrations in the metallic object.

13. The apparatus as claimed in claim 4 wherein acoustical vibrations in the metallic object electrically excite the second electrically conductive wire coil.

14. The apparatus as claimed in claim 4 wherein the first electromagnetic acoustic transducer is separated from the second electromagnetic acoustic transducer by a dielectric spacer.

15. An electromagnetic acoustic transducer comprising:

(A) a plurality of magnets mounted in a housing and arranged in an array and positioned substantially in a planar radial fashion, about a radial center, at substantially equal intervals around an opening of the housing with ends of adjacent magnets possessing opposite polarity;

said opening providing a pass-through for a metallic object;

said array positioned transverse to, and with the radial center substantially collinear with, a longitudinal axis of the metallic object, each of said plurality of magnets comprising;

(1) a first end adjacent to the opening and

(2) a second end substantially perpendicular to and spaced from an exterior longitudinal wall of the metallic object wherein the second end is notched in the plane substantially collinear with the longitudinal axis of the metallic object; and

(B) an electrically conductive wire coil mounted in the housing and positioned within an annular channel formed by the notched second ends of the plurality of magnets.

16. A method of tuning a resonant EMAT coil possessing a tuning capacitor;

the tuning method comprising the steps of;

a. establishing at least one electrical reactance goal and at least one performance frequency;

b. selecting a wire strand of a wire gauge and winding said wire strand within an EMAT with a nominal initial number of turns;

c. inserting a test specimen into the EMAT proximate to the resonant EMAT coil;

d. measuring at least one impedance for each of the at least one performance frequencies;

e. comparing at each of the at least one performance frequencies, the at least one electrical reactance goal to measured impedance;

f. if, for each of the at least one performance frequencies, a difference between the at least one electrical reactance goal and the measured impedance is large, reselecting a wire gauge of the wire strand and adjusting the turns of the wire strand of the reselected wire gauge; and

g. if, for each of the at least one performance frequencies, a difference between the at least one electrical reactance goal and the measured impedance is small, adjusting the tuning capacitor;

whereby maximal real impedance may be determined.

17. A method of end detection of test specimens passing an EMAT receiving coil;

the method comprising the steps of;

a. selecting a reference voltage level substantive enough to result in an interrupt signal when compared with an amplified and conditioned signal emanating from the EMAT receiving coil when devoid of a test specimen;

b. amplifying the signal emanating from the EMAT receiving coil;

c. conditioning the amplified signal;

d. comparing the conditioned, amplified signal with a reference voltage source; and

e. generating a difference signal;

whereby a high difference signal exceeds and threshold and thereby interrupts digital signal processing of amplified, demodulated and digitized signals emanating from the EMAT receiver coil.

18. An apparatus for end detection of one or more test specimens passing an EMAT comprised of:

a. a receiving coil of an EMAT;

b. an amplifier for amplifying amplitude modulated signals emanating from the EMAT coil;

c. a detector for conditioning the amplified signals;

d. a reference voltage source set at a level sufficient to result in a signal of sufficient magnitude that when the reference voltage is compared to the conditioned signals of the receiving coil of an EMAT devoid of a test specimen;

e. a voltage comparator for comparing the reference voltage source with the conditioned signals; and

f. a threshold trigger triggerable by a voltage high signal of the voltage comparator;

whereby digital signal processing of the amplified, demodulated, and digitized signals emanating from the EMAT coil may be interrupted.

19. A method of inspection using EMAT-based acoustic resonance detection comprising the steps of:

(a) mounting and centering a test article relative to an EMAT;

(b) generating excitation signals emanating as EM field variations in the EMAT;

(c) electrically and acoustically exciting the test article by way of EM field variations in the EMAT;

(d) receiving, by a coil, EM field variations as induced by the excited test article and outputting by the coil of resulting signals;

(e) receiving the coil signals;

(f) demodulating and filtering the coil signals;

(g) thresholding the coil signals;

(h) mapping, in an array using a digital computer, the signals that have passed threshold such that each amplitude data point is correlated with its corresponding frequency point; and

(i) determining a position of a resonance in a sweep of the mapped signals;

20. The method of fault detection in EMAT-based acoustic resonance detection as claimed in claim 19 wherein the signals are digitized before the step of demodulation.

21. The method of fault detection in EMAT-based acoustic resonance detection as claimed in claim 19 wherein the signals are digitized after the step of demodulation and filtering.

22. The method of fault detection in EMAT-based acoustic resonance detection as claimed in claim 19 wherein the step of determining position of resonance in the sweep is detecting a peak for a resonance exhibiting an exponentially decaying sinusoidal waveform, whereby the peak amplitude point is extracted from an averaged, rectified waveform.

23. The method of fault detection in EMAT-based acoustic resonance detection as claimed in claim 19 wherein the step of determining position of resonance within the sweep is averaging weighted peak power points of a resonance.