Eddy current probe having body of high permeability supporting drive coil and plural sensors

• US 5,399,968 A
• Filed: 09/24/1993
• Issued: 03/21/1995
• Est. Priority Date: 01/31/1992
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Northrop Grumman Corporation
• Inventors: Tam, Kent K., Sheppard, William R.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US08/126,944
• Time in Patent Office: 543 Days
• Field of Search: 324/226, 324/227, 324/232, 324/233, 324/235, 324/237-243, 324/262
• US Class Current: 324/242
• CPC Class Codes: G01N 27/902 by moving the sensors
• CPC Class Codes: G01N 27/904 with two or more sensors

Abstract

An eddy current probe for detection of cracks in multi-layered structures. The probe includes a body of a material having high permeability and is shaped to include a central core and an additional wall. A central core drive coil is wound around the central core of the body and a plurality of sense coils are located in an array in association with the additional wall. An outer drive coil is wound about the outside of the additional wall. The central core drive coil is excited with a first alternating current high frequency signal producing eddy currents primarily in the top of the layered structure. These eddy currents are sensed with the sense coil array and analyzed for structural defects in the top surface of the layered structure. The central core coil is excited with a second alternating current signal of a lower frequency than the first signal producing eddy currents in deeper layers of the structure. Such eddy currents are sensed by the sense coils and the signals so sensed are stored. The outer drive coil is excited with a low frequency signal and the sense coils operated to detect eddy currents in deeper layers of the structure. The signal from excitation of the outer drive coil is scaled with respect to the signal from the central core drive coil for separating signal indicative of defects from that indicative of differences in the structure of the underlying components of the layered structure. The probe may be circular or linear.

Claims

1. An eddy current probe comprising:

a body formed of a high permeability material, said body being shaped to include a central core and a circumferentially extending wall spaced from and concentric with said central core;

a central core drive coil wound around a surface of said central core facing said circumferentially extending wall, said central core drive coil generating a first magnetic field for inducing central eddy currents in a workpiece upon activation of a flow of alternating current through said central core drive coil;

a plurality of independent sense coils positioned in an array in association with said circumferentially extending wall; and

,

an outer drive coil wound around an outer surface of said circumferentially extending wall away from said central core, said outer drive coil generating a second magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flow of alternating current through said central core drive coil,

each of said independent sense coils independently sensing respective components of said central eddy currents and said peripheral eddy currents.

2. An eddy current probe comprising:

a body formed of a high permeability material, said body including a central core, a radially extending wall radiating from said central core, and a circumferentially extending wall extending from said radially extending wall concentrically with said central core, said circumferentially extending wall including a rim distal from said radial wall, said rim including a plurality of interstices dividing said rim into a plurality of partitions that are symmetrically located about said rim, said partitions being shaped so as to extend essentially parallel with an axis of said central core;

a central core drive coil wound around a surface of said central core facing said circumferentially extending wall, said central core drive coil generating a first magnetic field for inducing central eddy currents in a workpiece upon activation of a flow of alternating current through said central core drive coil;

a plurality of independent sense coils equal in number to the number of said partitions with a respective one of said independent sense coils being wound around a respective one of said partitions such that each of said partitions includes an associated independent sense coil wound around it; and

,

an outer drive coil wound around an outer surface of said circumferentially extending wall away from said central core, said outer drive coil generating a second magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

each of said independent sense coils independently sensing respective components of said central eddy currents and said peripheral eddy currents.

3. The eddy current probe of claim 2 wherein:

said flow of alternating current through said outer drive coil is activated independently of said flow of alternating current through said central core drive coil.

4. The eddy current probe of claim 2 wherein:

said plurality of interstices and plurality of partitions together form a castellated structure composed of essentially prismatic partitions separated by essentially straight sided crenelated openings.

5. An eddy current probe comprising:

a body formed of a high permeability material, said body including a central core, a radially extending wall radiating from said central core, and a circumferentially extending wall extending from said radially extending wall concentrically with said central core, said circumferentially extending wall including a rim distal from said radial wall, said rim including a plurality of interstices dividing said rim into a plurality of partitions that are symmetrically located about said rim, each of said partitions being shaped so as to include a portion thereof that extends in a plane that is essentially perpendicular to an axis of said central core;

a central core drive coil wound around a surface of said central core facing said circumferentially extending wall, said central core drive coil generating a first magnetic field for inducing central eddy currents in a workpiece upon activation of a flow of alternating current through said central core drive coil;

a plurality of independent sense coils equal in number to the number of said partitions with a respective one of said independent sense coils being wound around that portion of a respective one of said partitions that extends in said plane such that each of said partitions includes an associated independent sense coil wound around it,

each of said independent sense coils independently sensing a respective component of said central eddy currents; and

,

an outer drive coil wound around an outer surface of said circumferentially extending wall away from said central core, said outer drive coil generating a second magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flow of alternating current through said central core drive coil,

each of said independent sense coils independently sensing a respective component of said peripheral eddy currents.

6. An eddy current probe comprising:

a first body formed of a high permeability material, said first body including a central core, a radially extending wall radiating from said central core, and a circumferentially extending wall extending from said radially extending wall concentrically with said central core, said circumferentially extending wall including a solid lip distal from said radially extending wall;

a central core drive coil wound around a surface of said central core facing said circumferentially extending wall, said central core drive coil generating a first magnetic field for inducing central eddy currents in a workpiece upon activation of a flow of alternating current through said central core drive coil;

a second body mounted to said lip, said second body comprising a plurality of individual sections symmetrically arrayed with respect to said central core; and

,

a plurality of transducers, equal in number to the number of said sections of said second body, located on said second body, a respective one of said transducers being positioned in a respective one of said sections operatively associated with it, said transducers being positioned in at least one symmetrical array with respect to said central core,

each of said transducers independently sensing a respective component of said central eddy currents.

7. The eddy current probe of claim 6 wherein said each of said transducers comprises a sense coil embedded in said associated section.

8. The eddy current probe of claim 7 wherein at least a portion of said second body is formed of a high permeability material.

9. The eddy current probe of claim 8 wherein said portion of said second body comprises a plurality of sense coil cores formed of said high permeability material, each said sense coil core being embedded in said second body within a respective one of said sense coils.

10. The eddy current probe of claim 7, further comprising a plurality of ferrite rods, equal in number to the number of said sense coils, embedded in said second body, a respective one of said sense coils being wound around a respective one of said ferrite rods.

11. The eddy current probe of claim 7 wherein each of said sense coils is embedded in said second body such that said each of said sense coils is positioned in essentially parallel alignment with said central core of said first body.

12. The eddy current probe of claim 7 wherein each of said sense coils is embedded in said second body such that said each of said sense coils is positioned in a plane essentially perpendicular to an axis of said central core of said first body.

13. The eddy current probe of claim 6, further comprising an outer drive coil wound around an outer surface of said circumferentially extending wall away from said central core, said outer drive coil generating a second magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flow of alternating current through said central core drive coil,

each of said transducers independently sensing a respective component of said peripheral eddy currents.

14. The eddy current probe of claim 11, further comprising an outer drive coil wound around an outer surface of said circumferentially extending wall away from said central core, said outer drive coil generating a second magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flow of alternating current through said central core drive coil,

each of said sense coils independently sensing a respective component of said peripheral eddy currents.

15. The eddy current probe of claim 12, further comprising an outer drive coil wound around an outer surface of said circumferentially extending wall away from said central core, said outer drive coil generating a second magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flow of alternating current through said central core drive coil,

each of said sense coils independently sensing a respective component of said peripheral eddy currents.

16. The eddy current probe of claim 6, wherein:

said plurality of transducers are positioned in said second body in a first circular symmetrical array and a second circular symmetrical array,

said transducers of said first circular symmetrical array being positioned in alternating sequence with said transducers of said second circular symmetrical array at equal arcuate distances with respect to said central core,

said first circular symmetrical array being positioned at a first radial distance with respect to said central core,

said second circular symmetrical array being positioned at a second radial distance from said central core,

said second radial distance being different from said first radial distance.

17. The eddy current probe of claim 6, further comprising:

a central bore extending within said central core;

a central body adapted for retention in said central bore and having a central boss extending within said central bore concentric with and spaced from a surface of said central bore;

a central body coil wound around said central boss, said central body coil generating centering eddy currents in said workpiece upon activation of a flow of alternating current through said central body coil.

18. The eddy current probe of claim 6 wherein each of said transducers comprises a Hall effect sensor element.

19. The eddy current probe of claim 6 further comprising means engaging said circumferentially extending wall of said first body for guiding a motion of said first body in a closed orbit about a point that is off-center from the center of said central core.

20. The eddy current probe of claim 6, further comprising:

an axle mounted to said radially extending wall and extending away from said central core at a radial offset distance from an axis of said central core; and

,

means connected to said axle for rotating said probe about said axle.

21. The eddy current probe of claim 6, further comprising:

a case mounted to said second body along a perimeter of said second body, said first body being enclosed within said case and said second body;

an axle mounted to said radially extending wall and extending away from said central core through a clearance bore in said case at a radial offset distance from an axis of said central core; and

,

means connected to said axle for rotating said first body about said axle.

22. An eddy current probe comprising:

a body formed of a high permeability material, said body being shaped to include a central core, a radially extending wall radiating from said central core, and a circumferentially extending wall extending from said radially extending wall concentrically with said central core, said central core having a central axis;

a central core drive coil wound around a surface of said central core facing said circumferentially extending wall, said central core drive coil generating a first magnetic field for inducing central eddy currents in a workpiece upon activation of a flow of alternating current through said central core drive coil;

a plurality of transducer means for sensing eddy currents, each of said transducer means being positioned in operative association with said circumferentially extending wall in a symmetrical circular array, each of said transducer means independently sensing a respective component of said central eddy currents;

means engaging said body for moving said body in a prescribed closed orbit; and

,

an outer drive coil wound around an outer surface of said circumferentially extending wall away from said central core,

said outer drive coil generating a second magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flow of alternating current through said central core drive coil,

said each of said transducer means independently sensing a respective component of said peripheral eddy currents.

23. The eddy current probe of claim 22 wherein said means for moving said body in a prescribed closed orbit moves said body in an orbit about an off-center axis, said off-center axis being parallel to and radially displaced from said central axis of said central core.

24. The eddy current probe of claim 23 wherein:

said body includes an axle extending from said radially extending wall away from said central core in axial alignment with said off-center axis; and

,

said means for moving said body in a prescribed closed orbit engages said axle and rotates said body about said axle.

25. The eddy current probe of claim 23 wherein:

said means for moving said body in a prescribed closed orbit comprises a guide, said guide having a circular central aperture wall engaging said body; and

,

said means for moving said body in a prescribed closed orbit moves said body in said prescribed closed orbit about said off-center axis in contact with said central aperture wall without rotation of said body about said off-center axis.

38. The eddy current probe of claim 22, further comprising:

a central bore extending within said central core;

a central body adapted for retention in said central bore and having a central boss extending within said central bore concentric with and spaced from a surface of said central bore;

a central body coil wound around said central boss, said central body coil generating centering eddy currents in said workpiece upon activation of a flow of alternating current through said central body coil.

26. An eddy current probe comprising:

a first body formed of a high permeability material, said first body including a central core, a radially extending wall radiating from said central core, and a circumferentially extending wall extending from the radially extending wall concentrically with said central core, said circumferentially extending wall having a rim distal from said radially extending wall;

a central core drive coil wound around a surface of said central core facing said circumferentially extending wall, said central core drive coil generating a first magnetic field for inducing central eddy currents in a workpiece upon activation of a flow of alternating current through said central core drive coil;

a second body movably engaging said rim, said second body being symmetrically divided into a plurality of individual sections;

a plurality of transducer means, equal in number to the number of said sections of said second body, for sensing eddy currents, said transducer means being positioned on said second body with a respective one of said transducer means positioned in a respective one of said sections operatively associated with it, said plurality of transducer means being positioned in at least one symmetrical array with respect to said rim of said circumferentially extending wall, each of said transducer means independently sensing a respective component of said central eddy currents;

a case mounted to said second body along a perimeter of said second body, said case enclosing said first body; and

means for moving said first body in a prescribed closed orbit with respect to said second body within said case.

27. The eddy current probe of claim 26 wherein:

said second body includes a central axis, said central axis being located parallel to and radially displaced from an axis of said central core of said first body;

said first body includes an axle extending from said radially extending wall away from said central core through a clearance bore in said case in axial alignment with said central axis; and

,

said means for moving said first body in a prescribed closed orbit with respect to said second body engages said axle and moves said first body in an orbit centered on said central axis of said second body.

28. The eddy current probe of claim 27 wherein said means for moving said first body in a prescribed closed orbit rotates said first body about said central axis of said second body.

29. The eddy current probe of claim 27 wherein said central axis is an axis of symmetry of said symmetrical array, said means for moving said first body moving said central core and said central core drive coil in oscillating movement with respect to said case without rotation of said first body about said central axis of said second body.

39. The eddy current probe of claim 26, further comprising an outer drive coil wound around an outer surface of said circumferentially extending wall away from said central core, said outer drive coil generating a second magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flow of alternating current through said central core drive coil,

said each of said transducer means independently sensing a respective component of said peripheral eddy currents.

40. The eddy current probe of claim 26, further comprising:

a central bore extending within said central core;

a central body adapted for retention in said central bore and having a central boss extending within said central bore concentric with and spaced from a surface of said central bore;

a central body coil wound around said central boss, said central body coil generating centering eddy currents in said workpiece upon activation of a flow of alternating current through said central body coil.

30. An eddy current probe comprising:

a first body formed of a high permeability material, said first body being shaped to include a central core, a radial wall radially extending from said central core, and a circumferentially extending wall extending from the radial wall concentrically with said central core, said circumferentially extending wall including a lip distal from said radial wall;

a central core drive coil wound around a surface of said central core facing said circumferentially extending wall, said central core drive coil generating a first magnetic field for inducing central eddy currents in a workpiece upon activation of a flow of alternating current through said central core drive coil;

a second body mounted on said lip of said circumferentially extending wall; and

,

a plurality of Hall effect sensor elements positioned in said second body in at least one symmetrical array relative to said central core, each of said Hall effect sensor elements independently sensing a respective component of said central eddy currents.

31. The eddy current probe of claim 30 wherein said second body is a monolithic body having said Hall effect sensor elements integrally formed thereon.

41. The eddy current probe of claim 30, further comprising an outer drive coil wound around an outer surface of said circumferentially extending wall away from said central core, said outer drive coil generating a second magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flow of alternating current through said central core drive coil,

said each of said Hall effect sensor elements independently sensing a respective component of said peripheral eddy currents.

42. The eddy current probe of claim 30, further comprising:

a central bore extending within said central core;

a central body adapted for retention in said central bore and having a central boss extending within said central bore concentric with and spaced from a surface of said central bore;

a central body coil wound around said central boss, said central body coil generating centering eddy currents in said workpiece upon activation of a flow of alternating current through said central body coil.

32. An eddy current probe comprising:

a first body formed of a high permeability material, said first body being shaped to include a hollow central core, a radially extending wall radiating from said hollow central core, and a circumferentially extending wall extending from said radially extending wall concentrically with said hollow central core, said circumferentially extending wall including a rim distal from said radially extending wall, said rim including a plurality of interstices that divide said rim into a plurality of partitions that are symmetrically located about said circumferentially extending wall, said partitions being shaped so as to extend in essentially parallel alignment with said hollow central core;

a central core drive coil wound around a surface of said hollow central core facing said circumferentially extending wall, said central core drive coil generating a first magnetic field for inducing central eddy currents in a workpiece upon activation of a flow of alternating current through said central core drive coil;

a plurality of sense coils, equal in number to the number of said partitions, a respective one of said sense coils being wound around a respective one of said partitions; and

a centering coil located in a central bore in said hollow central core of said first body, said centering coil generating a centering magnetic field for inducing centering eddy currents in said workpiece upon activation of a flow of alternating current through said centering coil,

said flow of alternating current through said centering coil being activated independently of said flow of alternating current through said central core drive coil,

each of said sense coils independently sensing respective components of said central eddy currents and said centering eddy currents.

33. The eddy current probe of claim 32 further comprising:

a central core body adapted for retention in said central bore of said hollow central core,

said central core body having a central boss formed of a high permeability material extending within said central bore concentric with and spaced from a surface of said central bore,

said centering coil being wound around said central boss.

34. The eddy current probe of claim 32 further comprising:

an outer drive coil wound around an outer surface of said circumferentially extending wall away from said hollow central core, said outer drive coil generating a second magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flow of alternating current through said central core drive coil and said flow of alternating current through said centering coil,

said each of said sense coils independently sensing a respective component of said peripheral eddy currents.

35. An eddy current probe comprising:

a linear body formed of a high permeability material, said linear body having a first solid elongated section, a first plurality of individual partitions and a second plurality of individual partitions,

said first solid elongated section having opposing first and second sides that extend along the elongated dimension of said first solid elongated section,

said first plurality of individual partitions being located in a first linear array along said first side of said first solid elongated section,

said second plurality of individual partitions being located in a second linear array along said second side of said first solid elongated section,

said individual partitions of said first linear array each being spaced apart from one another and each orientated essentially perpendicular to said first solid elongated section and essentially mutually parallel to each other,

said individual partitions of said second linear array each being spaced apart from one another and each orientated essentially perpendicular to said first solid elongated section and essentially mutually parallel to each other;

a first drive coil wound around said first solid elongated section in the elongated dimension of said first solid elongated section between said first and second sides of said first solid elongated section, said first drive coil generating a first magnetic field for inducing first eddy currents in a workpiece upon activation of a flow of alternating current through said first drive coil; and

a plurality of individual sense coils, equal in number to the number of said second partitions, a respective one of said sense coils being wound around a respective one of said partitions of said second plurality of partitions, each of said sense coils independently sensing a respective component of said first eddy currents.

36. The eddy current probe of claim 35 further comprising an outer drive coil wound around a periphery of said first solid elongated section encompassing said first and second sides of said first elongated section,

said outer drive coil generating a peripheral magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flow of alternating current through said first drive coil,

each of said sense coils independently sensing a respective component of said peripheral eddy currents.

37. The eddy current probe of claim 35 further comprising:

a second solid elongated section and a third plurality of individual partitions,

said second solid elongated section having opposing mating and distal sides that extend along the elongated dimension of said second solid elongated section,

said mating side of said second solid elongated section being joined to said first solid elongated section along said second side of said first solid elongated section such that said first and second solid elongated sections are co-planar and parallel with each other and said second linear array is positioned at said joining of said first and said second solid elongated sections,

said third plurality of individual partitions being located in a third linear array along said distal side of said second solid elongated section,

said individual partitions of said third linear array each being spaced apart from one another and each orientated essentially perpendicular to said second solid elongated section and essentially mutually parallel to each other;

a second drive coil wound around said second solid elongated section in the elongated dimension of said second solid elongated section between said mating and distal sides of said second solid elongated section, said second drive coil generating a second magnetic field for inducing second eddy currents in said workpiece upon a flow of alternating current through said second drive coil; and

,

an outer drive coil wound around a periphery of said joined first and second solid elongated sections encompassing said first side of said first solid elongated section and said distal side of said second solid elongated section,

said outer drive coil generating a peripheral magnetic field for inducing peripheral eddy currents in said workpiece upon activation of a flow of alternating current through said outer drive coil,

said flow of alternating current through said outer drive coil being activated independently of said flows of alternating current through said first and second drive coils,

each of said sense coils independently sensing a respective component of said peripheral eddy currents.