Inspection apparatus and method

• US 8,274,279 B2
• Filed: 10/16/2007
• Issued: 09/25/2012
• Est. Priority Date: 10/17/2006
• Status: Active Grant

• Current Assignee: Athena Industrial Technologies, Inc.
• Original Assignee: Athena Industrial Technologies, Inc.
• Inventors: Gies, Paul D.
• Primary Examiner(s): Hollington, Jermele M
• Assistant Examiner(s): NGUYEN, TRUNG Q
• Application Number:
  US12/445,802
• Publication Number:
  US 20100207620A1
• Time in Patent Office: 1,806 Days
• Field of Search: 324/240
• US Class Current: 324/240
• CPC Class Codes: G01N 27/9006 Details, e.g. in the structure or functioning of sensors

Abstract

An apparatus and method are disclosed for detecting flaws in electrically conductive materials by observing properties of the back-EMF of the eddy current field generated by driving magnetic flux through the object to be examined. The input signal may include sweeps at several frequencies, and may do so at one time under the principle of wave superposition. The sectorial observations of eddy currents summations may be compared to a known datum for a defect free material, the presence of anomalies in eddy field back EMF divergence tending to provide an indication of an irregularity in the underlying eddy field, and hence in the underlying material itself. The portable unit may have a number of different configurations depending on the nature of the object to be examined, be it a flat or large radius plate, a flange, a rail, or some other structural element.

Claims

1. A portable anomaly detection apparatus for inspecting an electrically conductive object, the anomaly detection apparatus comprising:

an electromagnetic field emitter operable to pass a time varying magnetic flux field through at least a portion of the electrically conductive object, and thereby to generate an electrical eddy current field in the electrically conductive object, said electromagnetic field emitter including a magnetic flux focusing core;

eddy current monitoring circuitry operable to survey at least first and second portions of the eddy current field, the first and second portions being non-coterminous;

said eddy current monitoring circuitry including a first eddy field divergence sensor coil operable directly to sense eddy field divergence in the electrically conductive object; and

signal processing apparatus connected to said eddy current monitoring circuitry, said signal processing apparatus being operable to compare data received from said eddy current monitoring circuitry observed at said first and second portions of the eddy current field.

2. The portable anomaly detection apparatus of claim 1 wherein said first eddy field divergence sensor coil has first and second legs extending radially relative to said core, and third and fourth legs extending on a circumferential arc relative to said core, said third leg extending circumferentially relative to said core at a closer radial distance to said core than said fourth leg.

3. The portable anomaly detection apparatus of claim 1 wherein said first sensing member overlaps said second sensing member.

4. A mobile anomaly detector operable to survey an electrically conductive object, said anomaly detector comprising:

a shoe, said shoe having a sole for engagement with the object;

an electromagnetic field emitter mounted within said shoe;

said electromagnetic field emitter including a magnetic flux focusing core;

said magnetic flux focusing core having first and second soles at said sole, said poles being spaced apart by a gap;

a power source connected to drive said emitter, said power source being operable at a plurality of frequencies;

said emitter being operable to induce an eddy current field in the object when said sole is placed adjacent thereto;

a plurality of eddy current sensors arrayed adjacent said electromagnetic field emitter, members of said plurality of eddy current sensors including eddy current field divergence sensor coils, each of said eddy current field divergence sensor coils being operable to sense directly eddy field differential induced in the electrically conductive object, each of said eddy current field divergence sensor coils being located to monitor a different portion of the eddy current field at said sole adjacent to said electromagnetic field emitter; and

at least one of (a) a recorder operable to store recording data from said eddy current sensors for subsequent signal processing operations; and

(b) a processor operable to interpret data obtained from said eddy current sensors.

5. The mobile anomaly detector of claim 4 wherein said sole has a profile constraining said detector to a single degree of freedom in translation, and one of:

(a) said anomaly detector includes a plurality of said emitters;

(b) said anomaly detector includes a plurality of said electromagnetic field emitters, and said sole has a profile conforming to a rail road track rail; and

(c) said anomaly detector includes a plurality of said electromagnetic field emitters, and said plurality of electromagnetic field emitters includes at least a first emitter and a second emitter, said first and second emitters being movable relative to one another to permit said mobile anomaly detector to girdle the electrically conductive object.

6. The mobile anomaly detector of claim 4 wherein each of said eddy current field divergence sensor coils has first and second legs extending radially relative to said core, and third and fourth legs extending on a circumferential arc relative to said core, said third leg extending circumferentially relative to said core at a closer radial distance to said core than said fourth leg.

7. The mobile anomaly detector of claim 4 wherein one of said sensor coils overlaps one other of said sensors coils.

8. A hand portable eddy field anomaly detector for use in detecting anomalies in an electrically conductive object, said eddy field anomaly detector comprising:

a magnetic field emitter operable to induce an eddy current field in an adjacent electrically conductive object to be examined;

said magnetic field emitter including a magnetic flux focusing core;

said magnetic flux focusing core having first and second poles for placement, in use, next to the electrically conductive object, said first and second poles being separated by a gap;

sensing apparatus operable, while said anomaly detector is in one location, to sample, at first and second sensing regions of said gap, back EMF of the eddy current field;

said sensing apparatus operable to sample at said first sensing region including a first eddy field divergence sensor coil operable to sense directly eddy field differential in the electrically conductive object;

said first sensing region being different from said second sensing region;

at least one of

(a) a recorder operable to store data observed with respect to said first and second sensing regions; and

(b) a signal processor operable to compare at least one of

(i) data observed with respect to said first sensing region and data observed with respect to said second sensing region; and

(ii) data observed with respect to at least one of said first and second sensing regions and a reference datum; and

said first and second sensing regions satisfying at least one condition selected from the set of conditions consisting of

(a) said first sensing region includes at least a portion not covered by said second sensing region, and said first and second sensing regions are non-concentric;

(b) said first sensing region includes at least a portion not covered by said second sensing region, and said second sensing region includes at least another portion not covered by said first sensing region; and

(c) said first and second sensing regions are non-concentric and at least one of said first and second sensing regions is eccentric relative to said magnetic field emitter. field emitter.

9. The hand portable eddy field anomaly detector of claim 8 wherein said sensing apparatus includes at least one of:

(a) an array of discrete sensors each having a different footprint;

(b) an array of discrete sensors each having a different footprint, said array of discrete sensors extending in a substantially annular pattern;

(c) an array of discrete sensors each having a different footprint, said array of discrete sensors including a plurality of said discrete sensors defining segments of a substantially annular footprint;

(d) an array of discrete sensors each having a different footprint, and said array of discrete sensors includes a plurality of said discrete sensors defining n segments about a central axis, said n segments being spaced on an angular pitch of 360/n degrees about said central axis;

(e) an array of discrete sensors each having a different footprint, and said array of discrete sensors includes a plurality of said discrete sensors defining n segments about a central axis, said n segments each subtending an angular arc of 360/n degrees +/−

20%;

(f) a movable sensor locatable in a plurality of different sampling positions while anomaly detector remains in one place; and

(g) a movable sensor locatable in a plurality of different sampling positions while said anomaly detector remains in one place, said magnetic field emitter includes a magnetically permeable core having a centerline axis, and said movable sensor is angularly displaceable on an arc centered on said centerline axis.

10. The hand portable eddy field anomaly detector of claim 8 wherein said anomaly detector includes one of:

(a) a magnetically permeable core having a pole piece and a co-operating peripheral wall spaced from said pole piece; and

(b) a magnetically permeable core having a pole piece and a co-operating peripheral wall spaced from said pole piece, and said anomaly detector has a foot for placement against the object to be examined, and said foot is selected from the set of feet consisting of

a substantially planar base;

(ii) a chamfered base;

(iii) a chamfered base that includes a first portion and a second portion, each of the first and second portions of said foot including members of said sensing apparatus, and said first and second portions lying to either side of said chamfer;

(iv) a base having a contoured profile;

(v) a base having a profile defining a portion of a cylindrical surface; and

(vi) a base having a profile defining a portion of a circular cylindrical surface.

11. The hand portable eddy field anomaly detector of claim 8 comprising a hand held sensing module for passing next to the object to be examined, and a display, and one of:

(a) a processor operable to present anomaly identification indicia on said display;

(b) a processor operable to present anomaly identification indicia on said display, and said display includes an anomaly depth indicator;

(c) a processor operable to present anomaly identification indicia on said display, and said display includes an anomaly position indicator; and

(d) a processor operable to present anomaly identification indicia on said display, and said display includes an anomaly direction indicator.

12. The hand portable eddy field anomaly detector of claim 8 further comprising any of:

a photographic recording medium;

an audio recording medium;

telemetry apparatus; and

incremental change of position apparatus.

13. The hand portable eddy field anomaly detector of claim 8 wherein said anomaly detector apparatus comprises a chassis having a carrying handle, a power supply mounted to the chassis, a display mounted to the chassis, and a removable hand sensor.

14. The hand portable eddy field anomaly detector of claim 8 further comprising any one of:

(a) a controller operable to drive said magnetic field emitter at more than one frequency; and

(b) a controller operable to drive said magnetic field emitter at more than one frequency, said controller being operable to drive said magnetic field emitter at a plurality of frequencies simultaneously.

15. The hand portable eddy field anomaly detector of claim 8 wherein said apparatus includes any of:

(a) a sensor mounted to measure total emitted magnetic flux from said magnetic fieldemitter;

(b) a sensor located to monitor total eddy current back EMF; and

(c) a sensor mounted to measure total emitted magnetic flux from said magnetic fieldemitter, a further sensor located to monitor total eddy current back EMF; and

a processor operable to compare total emitted magnetic flux to total eddy current back EMF.

16. The hand portable eddy field anomaly detector of claim 8 wherein one of:

(a) said anomaly detector weighs up to 20 lbs.; and

(b) said anomaly detector has a sensing module for passing next to object to be examined, and said sensing module weighs up to 5 lbs.

17. The hand portable eddy field anomaly detector of claim 8 wherein said first eddy field divergence sensor coil has first and second legs extending radially relative to said core, and third and fourth legs extending on a circumferential arc relative to said core, said third leg extending circumferentially relative to said core at a closer radial distance to said core than said fourth leg.

18. The hand portable eddy field anomaly detector of claim 8 wherein the detector includes a second eddy field divergence sensor coil operable to sense directly eddy field differential in the electrically conductive object, and said first and second eddy field divergence sensor coils overlap.

19. A method of detecting anomalies in an electrically conductive object, said method comprising the steps of:

establishing a time varying eddy current field in the electrically conductive object using an electromagnetic field emitting member that includes a magnetic flux focusing core, said magnetic flux focusing core having first and second soles that are, in use, placed next adjacent to the electrically conductive object, said first and second poles being spaced apart by a gap;

monitoring at least first and second regions of the eddy current field at the gap, the first and second regions of the eddy current field at the gap being non-coterminous and non-concentric, the step of monitoring including using a first eddy field divergence sensor coil associated with the first region of the eddy current field at the gap directly to sense eddy field differential in the electrically conductive object at the gap; and

comparing at least one of

(a) eddy currents monitored in said first region with eddy currents monitored in said second region; and

(b) eddy currents monitored in both said first region and said second region with a reference standard for said first region and said second region respectively.

20. The method of claim 19 wherein said method includes one of

(a) monitoring total eddy current flux through a zone to be examined, and comparing that total eddy current flux to a sum of eddy current fluxes observed in discrete portions of said zone;

and

(b) employing a magnetic flux emitting apparatus to establish said time varying eddy current field;

monitoring total eddy current flux through a zone adjacent to the object to be examined, and comparing that total eddy current flux with a measured total emitted flux from the magnetic flux emitting apparatus.

21. The method of claim 19 wherein the first eddy field divergence sensor coil has first and second legs extending radially relative to said core, and third and fourth legs extending on a circumferential arc relative to said core, said third leg extending circumferentially relative to said core at a closer radial distance to said core than said fourth leg, and said step of monitoring includes sensing eddy field differential between said third and fourth legs.

22. The method of claim 19 wherein there is a second eddy field divergence sensor coil associated with the second region, the first region overlaps the second region, and said step of monitoring includes sensing eddy field differential in both said first and second eddy field divergence sensor coils.

23. A process of detection of anomalies in an electrically conductive object, said process comprising:

providing an anomaly detection apparatus having an electromagnetic field emitting member, said electromagnetic field emitting member including a flux focusing core that includes a first portion and a second portion;

said first and second portions of said flux focusing core each having a respective proximal end and a distal end;

said proximal end of said first portion and said proximal end of said second portion being linked together on a continuous magnetic core path,

said distal end of said first portion and said distal end of said second portion being separated from each other by a gap and defining respective poles of said flux focusing core;

a power source connected for powering said electromagnetic field emitting member,

said power source being operable to cause said electromagnetic field emitting member to emit a time varying electromagnetic field;

said power source having at least a first frequency and a second frequency;

an array of electromagnetic field sensing members, the members of said array being mounted adjacent to said electromagnetic field emitting member, the field sensing members including a first eddy field divergence sensor coil operable directly to sense eddy field differential in the gap, adjacent to the electrically conductive object;

monitoring circuitry connected to the members of said array,

said monitoring circuitry being operable to sense variation of signals as between members of said array; and

a processor operable to compare signals sensed at said members of said array in response to electromagnetic signals emitted at said first frequency and at said second frequency;

passing said anomaly detection apparatus adjacent to the electrically conductive object with said distal ends of said first and second portions of said flux focusing core being positioned nearer to the electrically conductive object than said proximal ends of said first and second portions; and

emitting electromagnetic field signals at more than one frequency from said electromagnetic field emitting member to cause eddy currents to be generated in the electrically conductive object;

monitoring eddy currents sensed directly by the array of electromagnetic field sensing members at a plurality of positions at the gap; and

comparing data monitored at the plurality of positions at the gap and at the first and second frequencies to provide an indication of the location of an anomaly in the electrically conductive object.

24. The process of claim 23 wherein the first eddy field divergence sensor coil has first and second legs extending radially relative to said core, and third and fourth legs extending on a circumferential arc relative to said core, said third leg extending circumferentially relative to said core at a closer radial distance to said core than said fourth leg and said process includes using said first eddy field divergence sensor coil to sense eddy field differential directly.

25. The process of claim 23 wherein the array of electromagnetic field sensing members also includes a second eddy field divergence sensor coil operable directly to sense eddy field differential, said first eddy field divergence sensor coil overlaps said second eddy field divergence sensor coil, and said process includes sensing eddy field differential with said first and second eddy field divergence sensor coils.

26. An anomaly detection apparatus for placement next to an electrically conductive object, said anomaly detection apparatus comprising:

an electromagnetic field emitting member, said electromagnetic field emitting member including a magnetic flux focusing core;

a power source connected to said electromagnetic field emitting member, said power source being operable to drive said electromagnetic field emitting member to emit a time varying electromagnetic field;

said power source having at least first and second driving frequencies;

an array of electromagnetic field sensing members, the members of said array being mounted adjacent to said electromagnetic field emitting member;

said array including at least first and second field sensing members, said first and second field sensing members being oriented toward non-coincident regions adjacent to said field emitting member, at least said first electromagnetic field sensing member being non-concentric with said electromagnetic field emitting member, said first electromagnetic field sensing member including an eddy field divergence sensor coil operable to sense directly eddy field differential in the electrically conductive object;

monitoring circuitry connected to the members of said array, said monitoring circuitry being operable to observe differential variation of signals as between members of said array; and

a processor operable to compare signals sensed at said members of said array in response to electromagnetic signals emitted at said first frequency and at said second frequency.

27. The anomaly detection apparatus of claim 26 wherein:

said magnetic flux focusing core includes a first portion and a second portion;

said first portion has a proximal end and a distal end;

said second portion has a proximal end and a distal end;

said proximal end of said first portion and said proximal end of said second portion being linked together on a continuous magnetic core path;

said distal end of said first portion and said distal end of said second portion being separated from each other by a gap;

in operation, said distal ends of said first and second portions being positioned nearer to the electrically conductive object than said proximal ends of said first and second portions; and

said first field sensing member being positioned to measure eddy field divergence at said gap.

28. The anomaly detection apparatus of claim 27 wherein:

said eddy field divergence sensor coil of said first electromagnetic field sensing member includes a first portion that underlies said distal end of said first portion of said magnetic flux focusing core, such that, in use, said first portion of said first eddy field divergence sensor coil is located between said distal end of said first portion of said magnetic flux focusing core and the electrically conductive object; and

said eddy field divergence sensor coil of said first electromagnetic field sensing member includes a second portion that underlies said distal end of said second portion of said magnetic flux focusing core, such that, in use, said second portion of said first eddy field divergence sensor coil is located between said distal end of said second portion of said magnetic flux focusing core and the electrically conductive object.

29. The anomaly detection apparatus of claim 27 wherein:

said distal end of said first portion of said magnetic flux focusing core defines a first pole of said magnetic flux focusing core;

said distal end of said second portion of said magnetic flux focusing core defines a second pole of said magnetic flux focusing core; and

said eddy field divergence sensor coil of said first electromagnetic field sensing member is mounted substantially flush with said first and second poles of said magnetic flux focusing core.

30. The anomaly detection apparatus of claim 27 wherein:

said distal end of said first portion of said magnetic flux focusing core defines a first pole of said magnetic flux focusing core;

said distal end of said second portion of said magnetic flux focusing core defines a second pole of said magnetic flux focusing core; and

said eddy field divergence sensor coil of said first electromagnetic field sensing member is mounted substantially co-planar with said first and second poles of said magnetic flux focusing core.

31. The anomaly detection apparatus of claim 27 wherein:

said first portion of said magnetic flux focusing core is a pole piece;

said second portion of said magnetic flux focusing core is a surrounding wall extending peripherally about said pole piece;

said gap is a peripheral gap between said pole piece and said surrounding wall; and

said eddy field divergence sensor coil of said first electromagnetic field sensing member is positioned to define a closed loop covering a sector of said peripheral gap.

32. The anomaly detection apparatus of claim 27 wherein:

said first portion of said magnetic flux focusing core is a pole piece;

said second portion of said magnetic flux focusing core is a peripheral wall surrounding said pole piece;

said peripheral wall being circular and concentric to said pole piece;

said gap having the shape of an annulus centered on said pole piece; and

said eddy field divergence sensor coil of said first electromagnetic field sensing member is positioned to define a closed loop monitoring a sector of said annular gap.

33. The anomaly detection apparatus of claim 32 wherein:

the magnetic flux focusing core has an axial direction, a radial direction, and a circumferential direction; and

said eddy field divergence sensor coil of said first electromagnetic field sensing member has the form of a four-sided loop that includes a first leg extending in the circumferential direction adjacent to said pole piece, a second leg extending in the circumferential direction adjacent to said peripheral wall, a third leg extending in the radial direction from the first leg to the second leg, and a fourth leg extending in the radial direction from the second leg to the first leg.

34. The anomaly detection apparatus of claim 27 wherein said array of electromagnetic field sensing members includes an electromagnetic field sensing member having an eddy field divergence sensor coil that overlaps the eddy field divergence sensor coil of said first electromagnetic field sensing member.

35. The anomaly detection apparatus of claim 27 wherein said anomaly detection apparatus includes a drive reference coil operable to measure a magnetic field induced in said magnetic flux focusing core by said power source.

36. The anomaly detection apparatus of claim 27 wherein:

said anomaly detection apparatus includes a winding extending about said first portion of said magnetic flux focusing core;

said winding being positioned in said gap substantially axially flush with said distal ends of said first and second portions of said magnetic flux focusing core; and

said winding being operable to sense radial flux thereat.

37. The anomaly detection apparatus of claim 27 wherein:

said distal end of said second portion surrounds said distal end of said first portion;

said gap extends peripherally about said distal end of said first portion; and

each of said electromagnetic field sensing members includes a sensing coil defining a closed loop, and each said closed loop covers at least a sector of said gap, all sectors of said gap being associated with at least one said closed loop.

38. The anomaly detection apparatus of claim 27 wherein:

said first portion of said magnetic flux focusing core is a pole piece;

said second portion of said magnetic flux focusing core is a surrounding wall extending peripherally about said pole piece;

said gap is a peripheral gap between said pole piece and said surrounding wall; and

said eddy field divergence sensor coil of said first electromagnetic field sensing member is a closed loop positioned to monitor a sector of said peripheral gap.

39. The anomaly detection apparatus of claim 27 wherein:

the magnetic flux focusing core has a radial direction and a circumferential direction; and

said first portion of said magnetic flux focusing core is a pole piece, said distal end thereof defining a first pole of said magnetic flux focusing core;

said second portion of said magnetic flux focusing core is a peripheral wall surrounding said pole piece, said distal end thereof defining a second pole of said magnetic flux focusing core;

said peripheral wall being a circular wall concentric to said pole piece;

said gap having the shape of an annulus centered on said pole piece;

said eddy field divergence sensor coil of said first electromagnetic field sensing member is a closed loop positioned to monitor a first sector of said peripheral gap;

said eddy field divergence sensor coil of said first electromagnetic field sensing member has the form of a four-sided loop;

said four-sided loop includes a first leg that underlies said distal end of said first portion of said magnetic flux focusing core, such that, in use, said first leg of said eddy field divergence sensor coil is located between said first pole of said magnetic flux focusing core and the electrically conductive object; and

said four-sided loop includes a second leg that underlies said distal end of said second portion of said magnetic flux focusing core, such that, in use, said second portion of said eddy current field divergence sensor coil is located between said second pole of said magnetic flux focusing core and the electrically conductive object;

said four-sided loop includes a third leg extending in the radial direction from the first leg to the second leg;

said four-sided loop includes a fourth leg extending in the radial direction from the second leg to the first leg; and

said four-sided loop is mounted substantially flush and co-planar with said first and second poles of said magnetic flux focusing core.

40. The anomaly detection apparatus of claim 27 wherein:

the magnetic flux focusing core has a radial direction and a circumferential direction; and

said first portion of said magnetic flux focusing core is a pole piece, said distal end thereof defining a first pole of said magnetic flux focusing core;

said second portion of said magnetic flux focusing core is a peripheral wall surrounding said pole piece, said distal end thereof defining a second pole of said magnetic flux focusing core;

said peripheral wall being a circular wall concentric to said pole piece;

said gap having the shape of an annulus centered on said pole piece;

said eddy field divergence sensor coil of said first electromagnetic field sensing member is positioned to define an enclosed loop covering a sector of said annular gap;

said eddy field divergence sensor coil of said first electromagnetic field sensing member has the form of a four-sided loop;

said four-sided loop includes a first leg, said first leg extending in the circumferential direction adjacent to said first portion of said magnetic flux focusing core;

said four-sided loop includes a second leg, said second leg extending in the circumferential direction adjacent to said second portion of said magnetic flux focusing core;

said four-sided loop includes a third leg extending in the radial direction from the first leg to the second leg;

said four-sided loop includes a fourth leg extending in the radial direction from the second leg to the first leg; and

said four-sided loop is mounted substantially flush and co-planar with said first and second poles of said magnetic flux focusing core.

41. The anomaly detection apparatus of claim 27 wherein:

said anomaly detection apparatus includes a drive reference coil operable to measure magnetic field induced in said flux focusing core by said power source;

said anomaly detection apparatus includes a winding extending completely around said first portion of said magnetic focusing core;

said winding being positioned in said gap substantially axially flush with said distal ends of said first and second portions of said magnetic flux focusing core; and

said winding being operable to sense total radial flux thereat.

42. The anomaly detection apparatus of claim 27 wherein:

said distal end of said second portion surrounds said distal end of said first portion;

said gap extends peripherally about said distal end of said first portion; and

each of said electromagnetic field sensing members includes a sensing coil defining a closed loop, and each said closed loop is positioned to monitor a different sector of said gap, all sectors of said gap being monitored by at least one said closed loop.

43. The anomaly detection apparatus of claim 26 wherein:

said magnetic flux focusing core includes a first portion and a second portion;

said first portion has a proximal end and a distal end;

said second portion has a proximal end and a distal end;

said proximal end of said first portion and said proximal end of said second portion being linked together on a continuous magnetic core path;

said distal end of said first portion and said distal end of said second portion being separated from each other by a gap;

said distal end of said first portion of said magnetic flux focusing core defines a first pole of said magnetic flux focusing core;

said distal end of said second portion of said magnetic flux focusing core defines a second pole of said magnetic flux focusing core;

in operation, said distal ends of said first and second portions being positioned nearer to the electrically conductive object than said proximal ends of said first and second portions; and

said eddy field divergence sensor coil of said first field sensing member is mounted substantially flush and co-planar with said first and second poles of said magnetic flux focusing core.

44. The anomaly detection apparatus of claim 26 wherein said eddy field divergence sensor coil has first and second legs extending radially relative to said core, and third and fourth legs extending on a circumferential arc relative to said magnetic flux focusing core, said third leg extending circumferentially relative to said magnetic flux focusing core at a closer radial distance to said magnetic flux focusing core than said fourth leg.

45. The anomaly detection apparatus of claim 26 wherein said first electromagnetic field sensing member overlaps said second electromagnetic field sensing member.