Apparatus for detecting ferromagnetic material

• US 4,449,115 A
• Filed: 08/02/1983
• Issued: 05/15/1984
• Est. Priority Date: 10/15/1980
• Status: Expired due to Term

• Current Assignee: 3M Company
• Original Assignee: 3M Company
• Inventors: Koerner, Ralph J.
• Primary Examiner(s): Swann, III, Glen R.
• Application Number:
  US06/519,214
• Time in Patent Office: 287 Days
• Field of Search: 340/38 L, 340/568, 340/551
• US Class Current: 340/941
• CPC Class Codes: G01V 3/101 by measuring the impedance of the search coil; by measuring features of a resonant circuit comprising the search coil measuring impedance or characteristics derived therefrom G01R27/00, e.g. quality factor G01R27/26
• CPC Class Codes: G08G 1/042 using inductive or magnetic detectors

Abstract

A convertor is used having an impedance responsive to the magnetic field intensity of the earth. The convertor may be coupled to a detector for monitoring the impedance of the convertor and generating a call signal responsive to the change in impedance. Generally, the impedance is primarily inductance and the detector is an oscillator using the convertor in a resonant circuit with a frequency monitor coupled to the detector. The convertor is an open magnetic structure having a non-linear BH curve biased by the earth'"'"'s magnetic field. The open magnetic structure is a core of a highly permeable material which approaches saturation under the influence of the earth'"'"'s magnetic field, with the axis of the core at least partially aligned with the magnetic flux lines of the earth'"'"'s magnetic field, and a coil wound around the axis of the core.

Claims

1. A passive device adapted to be connected to a detector for detecting a ferromagnetic material, said device comprising a non-linear impedance responsive to a change in the magnetic field intensity of the earth.

2. A device as in claim 1 wherein said change in impedance is primarily due to a change in inductance.

3. A device as in claim 2 wherein said converter comprises an open magnetic structure having a B-H curve with a non-linear portion biased by said magnetic field intensity near said non-linear portion of said B-H curve.

4. A device as in claim 3 wherein said open magnetic structure comprises:

a core having an axis at least partially aligned with the magnetic flux lines of the earth'"'"'s magnetic field, said core consisting essentially of a highly permeable material which approaches saturation under the influence of said magnetic field intensity; and

a coil having a conductor surrounding said axis of said core.

5. A passive device adapted to be connected to a detector for detecting a ferromagnetic material in the influence of magnetic flux lines of the magnetic field intensity of the earth, comprising:

a core having a major axis along its largest dimension and having orthogonal cross-sectional axes, said major axis of said core being at least partially aligned with the magnetic flux lines of the earth'"'"'s magnetic field, said core consisting essentially of a permeable material which approaches saturation under the influence of said magnetic field intensity, said major axis being at least one hundred times greater than the smaller of said orthogonal cross-sectional axes;

a coil having a conductor surrounding said major axis of said core.

6. A device as in claim 5 wherein said major axis of said core is larger than either of said orthogonal cross-sectional axes.

7. A device as in claim 5 wherein said core has a coercivity of less than said magnetic field intensity.

8. A device as in claim 7 wherein said coercivity is less than 0.2 oersteds.

9. A device as in claim 5 wherein said material is comprised of an iron-nickel alloy.

10. A device as in claim 9 wherein said iron-nickel alloy is approximately 80% nickel.

11. A passive apparatus for detecting a ferromagnetic material in a predetermined area comprising:

conversion means having a non-linear impedance responsive to the magnetic field intensity of the earth for converting a change in said magnetic field intensity into a change in said impedance; and

detection means coupled to said conversion means for monitoring said impedance and for generating a signal indicative of the detection of said ferromagnetic material in said predetermined area responsive to said change in said impedance.

12. An apparatus as in claim 11 wherein said change in said impedance is primarily due to a change in inductance.

13. An apparatus as in claim 11 wherein said conversion means comprises an open magnetic structure having a B-H curve with a non-linear portion biased by said magnetic field intensity near said non-linear portion of said B-H curve.

14. An apparatus as in claim 13 wherein said open magnetic structure comprises:

a core having an axis at least partially aligned with the magnetic flux lines of the earth'"'"'s magnetic field consisting essentially of a highly permeable material which approaches saturation under the influence of said magnetic field intensity; and

a coil having a conductor surrounding said axis of said core.

15. A passive apparatus for detecting a ferromagnetic material comprising:

a core having a major axis along its largest dimension and having orthogonal cross-sectional axes, said major axis of said core being at least partially aligned with the magnetic flux lines of the earth'"'"'s magnetic field, said core consisting essentially of a permeable material which approaches saturation under the influence of said magnetic field intensity, said major axis being much larger than the smaller of said orthogonal cross-sectional axes;

a coil having a conductor surrounding said major axis of said core;

said coil and said core having an impedance measured from said coil; and

detection means coupled to said coil for monitoring impedance and for generating a signal indicative of the detection of said ferromagnetic material responsive to a change in said impedance.

16. An apparatus as in claim 15 wherein said major axis of said core is longer than either of said orthogonal cross-sectional axes.

17. An apparatus as in claim 15 wherein said major axis is at least one hundred times greater than the smaller of said orthogonal cross-sectional axes.

18. An apparatus as in claim 17 wherein said core has a coercivity of less than said magnetic field intensity.

19. An apparatus as in claim 18 wherein said coercivity is less than 0.2 oerstads.

20. An apparatus as in claim 18 wherein said core is a strip of material wound on the inside of a nonmagnetic tube.

21. An apparatus as in claim 20 wherein said strip has a thickness of about 1 mil and a width of about 62 mils wound into a helix forming a wound length of about one and one-half inches.

22. An apparatus as in claim 17 wherein said material is comprised of an iron-nickel alloy.

23. An apparatus as in claim 22 wherein said iron-nickel alloy is approximately 80% nickel.

24. An apparatus as in claim 15 wherein said detection means comprises:

an oscillator operatively coupled to said conversion means for oscillating at a frequency dependant on said impedance; and

a detector operatively coupled to said oscillator for detecting changes in said frequency.

25. A passive apparatus for detecting a vehicle in a surveillance area of a roadway comprising:

conversion means having a non-linear impedance responsive to the magnetic field intensity of the earth for converting changes in said magnetic field intensity into a change in said impedance; and

detection means coupled to said conversion means for monitoring said impedance and for generating a signal indicative of the detection of said ferromagnetic material in said predetermined area responsive to said change in said impedance.

26. An apparatus as in claim 25 wherein said change in said impedance is primarily due to a change in inductance.

27. An apparatus as in claim 26 wherein said conversion means comprises an open magnetic structure having a B-H curve with a non-linear portion biased by magnetic field intensity near said non-linear portion biased by magnetic field intensity near said non-linear portion of said B-H curve.

28. A passive apparatus for detecting a vehicle in a surveillance area of a roadway comprising:

a core having a major axis along its largest dimension and having orthogonal cross-sectional axes, said major axis of said core being at least partially aligned with the magnetic flux lines of the earth'"'"'s magnetic field, said core consisting essentially of a permeable material which approaches saturation under the influence of said magnetic field intensity, said major axis being at 100 times the smaller of said orthogonal cross-sectional axes;

a coil having a conductor surrounding said major axis of said core;

said coil and said core having a impedance measured from said coil; and

detection means coupled to said coil for monitoring said impedance and for generating a signal indicative of the detection of said ferromagnetic material responsive to a change in said impedance.

29. An apparatus as in claim 28 wherein said major axis of said core is larger than either of said cross-sectional axes.

30. An apparatus as in claim 28 wherein said core has a coercivity of less than said magnetic field intensity.

31. An apparatus as in claim 30 wherein said coercivity is less than 0.2 oerstads.

32. An apparatus as in claim 28 wherein said core is a strip of material wound on the inside of a nonmagnetic tube.

33. An apparatus as in claim 32 wherein said strip has a thickness of about 1 mil and a width of about 62 mils wound into a helix forming a wound length of about one and one-half inches.

34. An apparatus as in claim 33 wherein said material is comprised of an iron-nickel alloy.

35. An apparatus as in claim 34 wherein said iron-nickel alloy is approximately 80% nickel.

36. An apparatus as in claim 28 wherein said change in said impedance is due to a decrease in said inductance as said vehicle arrives in said predetermined area of said roadway.

37. An apparatus as in claim 36 wherein said conversion means is located in vertical juxtaposition to said surveillance area.

38. An apparatus as in claim 28 wherein said change in said impedance is due to an increase in said inductance as said vehicle arrives in said surveillance area.

39. An apparatus as in claim 38 wherein said roadway has a pair of lanes each having a surveillance area located within each of said pair of lanes and wherein said conversion means is located between said pair of lanes.

40. An apparatus as in claim 28 wherein said detection means comprises:

an oscillator operatively coupled to said conversion means for oscillating at a frequency dependent on said impedance of said conversion means; and

a detector operatively coupled to said oscillator for detecting changes in said frequency.