Method and apparatus for imaging thin structures

• US 6,182,512 B1
• Filed: 06/13/1997
• Issued: 02/06/2001
• Est. Priority Date: 06/13/1997
• Status: Expired due to Fees

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Lorraine, Peter William
• Primary Examiner(s): Williams, Hezron
• Assistant Examiner(s): FAYYAZ, NASHMIYA SAQIB
• Application Number:
  US08/876,175
• Time in Patent Office: 1,334 Days
• Field of Search: 73/655, 73/656, 73/657, 73/625, 73/626, 73/602, 356/357, 356/358
• US Class Current: 73/655
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids
• CPC Class Codes: G01N 2291/015 Attenuation, scattering
• CPC Class Codes: G01N 2291/0237 Thin materials, e.g. paper, membranes, thin films
• CPC Class Codes: G01N 2291/0422 Shear waves, transverse waves, horizontally polarised waves
• CPC Class Codes: G01N 2291/0423 Surface waves, e.g. Rayleigh waves, Love waves
• CPC Class Codes: G01N 2291/0427 Flexural waves, plate waves, e.g. Lamb waves, tuning fork, cantilever
• CPC Class Codes: G01N 2291/0428 Mode conversion
• CPC Class Codes: G01N 2291/102 one emitter, one receiver
• CPC Class Codes: G01N 29/069 Defect imaging, localisation and sizing using, e.g. time of flight diffraction [TOFD], synthetic aperture focusing technique [SAFT], Amplituden-Laufzeit-Ortskurven [ALOK] technique
• CPC Class Codes: G01N 29/2418 using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics photoacoustic cells G01N21/1702; measuring characteristics of vibrations by using radiation-sensitive means G01H9/00; acousto-optical conversion techniques for short-range imaging G01S15/8965; sound-producing devices using laser bundle G10K15/046
• CPC Class Codes: G01N 29/32 Arrangements for suppressing undesired influences, e.g. temperature or pressure variations , compensating for signal noise
• CPC Class Codes: G01S 15/8965 using acousto-optical or acousto-electronic conversion techniques
• CPC Class Codes: G01S 15/8968 using acoustical modulation of a light beam acousto-optical light control devices G02F1/11, G02F1/33

Abstract

An exemplary imaging apparatus includes a wave generator for generating a wave in an article; a detector for detecting a wave in the article; and a processor which: forms a matched filter based on a response of the article to a wave propagated through the article; directs the wave generator to generate a wave at a plurality of generation positions; directs the detector to detect a motion of the article at a plurality of detection positions; forms a scan data set from the detected motion of the article; multiplies the scan data set by the matched filter to produce a compensated data set; and coherently sums data points in the compensated data set to produce a focused image.

Claims

1. A method of forming an image of an article comprising the steps of:

propagating first waves in the article;

measuring a response of the article to the first waves;

forming a matched filter data set comprising first data points representing the response of the article to the first waves;

generating second waves in the article at a plurality of generation positions on the article, which second waves propagate through the article;

detecting a motion of the article at a plurality of detection positions to obtain second data points, the second data points forming a scan data set;

multiplying the scan data set by the matched filter data set to generate a compensated data set comprising a plurality of third data points; and

coherently summing the third data points in the compensated data set to form an image of the article.

2. The method of claim 1, wherein the first and second data points represent a surface displacement of the article as a function of time.

3. The method of claim 1, wherein the first and second data points represent a surface velocity of the article as a function of time.

4. The method of claim 1, wherein the matched filter data set contains information which compensates for propagation of multiple Lamb wave modes.

5. The method of claim 1, wherein the matched filter is a one-way matched filter.

6. The method of claim 1, wherein the matched filter is a pulse-echo matched filter.

7. The method of claim 6, wherein the pulse-echo matched filter is constructed with a reflector having reflective characteristics which simulate a defect in the article.

8. The method of claim 1, further comprising the step of reducing to zero a portion of the first data points of the matched filter data set.

9. The method of claim 8, wherein the portion of the first data points reduced to zero represents a mode of wave propagation.

10. The method of claim 1, wherein the matched filter data set contains information which compensates for frequency dispersion of the second waves.

11. The method of claim 1, further comprising the step of frequency filtering the matched filter to suppress modes which exist above a predetermined frequency.

12. The method of claim 10, wherein the second waves comprise Lamb waves.

13. The method of claim 1, further comprising the step of temporally filtering the matched filter to suppress modes having velocities greater than a predetermined velocity.

15. The method of claim 1, further comprising the steps of:

forming the matched filter to comprise first data points representing the response of the article as a function of a direction to a source;

detecting the motion of the article at the plurality of detection positions for a plurality of directions to obtain the second data points, the second data points forming the scan data set; and

multiplying the scan data set by the matched filter data set for a particular detection position and direction.

16. The method of claim 1, wherein the step of coherently summing the third data points in the compensated data set comprises:

calculating an adjusted distance for at least two detection positions based on a distance between a focal point and each of the at least two detection positions; and

summing the third data points having adjusted distances which are about equal.

17. The method of claim 1, wherein the step of coherently summing the third data points in the compensated data set is carried out in three dimensions to form a 3-dimensional volumetric image of the article.

18. The method of claim 1, wherein the step of coherently summing the third data points is carried out in two dimensions with reference to a surface of the article.

19. The method of claim 1, wherein the step of coherently summing the third data points in the compensated data set comprises:

selecting a subset of the third data points in the compensated data set to define a synthetic aperture; and

multiplying each of the third data points in the subset by a weighting factor based on a proximity of the respective third data point in the subset to a source.

20. The method of claim 1, wherein the step of coherently summing the third data points in the compensated data set comprises:

selecting a number of third data points in the compensated data set for coherent summing, the number based on a depth of a focal point, to form a subset which defines a synthetic aperture.

22. The method of claim 1, wherein the step of generating waves in the article comprises directing a source laser emitting a source laser beam at a surface of the article.

23. The method of claim 1, wherein the step of generating waves in the article comprises generating Lamb waves.

24. The method of claim 1, wherein the step of generating waves in the article comprises generating longitudinal and transverse waves.

14. A method of forming an image of an article comprising the steps of:

analytically deriving data points representing a response of the article as a function of the distance to a source;

forming a matched filter data set comprising first data points representing the response of the article as a function of the distance to a source;

generating waves in the article at a plurality of generation positions on the article, which waves propagate through the article;

detecting a motion of the article at a plurality of detecting positions to obtain second data points, the second data points forming a scan data set;

multiplying the scan data set by the matched filter data set to generate a compensated data set comprising a plurality of third data points; and

coherently summing the third data points in the compensated data set to form an image of the article.

21. An apparatus comprising;

a wave generator for generating a wave in an article;

a detector for detecting a wave in the article; and

a processor for;

sending first waves in the article;

measuring a response of the article to the first waves;

forming a matched filter based on the response of the article to the first waves;

directing the wave generator to generate second waves at a plurality of generation positions;

directing the detector to detect a motion of the article at a plurality of detection positions;

forming a scan data set from the detected motion of the article;

multiplying the scan data set by the matched filter to produce a compensated data set; and

coherently summing data points in the compensated data set to produce a focused image.

25. The apparatus of claim 21, wherein wave generator is adapted to generate a longitudinal wave and a transverse wave.

26. The apparatus of claim 21, wherein the wave generator comprises a first laser and the detector comprises a second laser.

27. The apparatus of claim 21, wherein wave generator is adapted to generate a Lamb wave.

28. The apparatus of claim 21, wherein the processor coherently sums the data points in the compensated data set by calculating a respective adjusted distance between a focal point and at least two of the detection positions and by summing the data points having adjusted distances which are about equal.

29. The apparatus of claim 21, wherein the detector is an interferometric detector.