Coil and Hall device circuit for sensing magnetic fields

• US 4,945,306 A
• Filed: 10/25/1988
• Issued: 07/31/1990
• Est. Priority Date: 10/25/1988
• Status: Expired due to Fees

• Current Assignee: Atlantic Richfield Company Incorporated (BP PLC)
• Original Assignee: Atlantic Richfield Company Incorporated (BP PLC)
• Inventors: Lowther, Frank E.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US07/262,372
• Time in Patent Office: 644 Days
• Field of Search: 324/219-221, 324/235, 324/240, 324/251, 324/252, 324/258, 324/117 H, 307/309, 338/32 H, 73/DIG. 3
• US Class Current: 324/220
• CPC Class Codes: G01N 27/82 for investigating the presence of flaws

Abstract

Relatively minute changes in magnetic field intensity are sensed by a circuit including a wire coil connected to a Hall effect element to provide the bias current to the element. The Hall effect element is disposed in the magnetic field with the coil and the output signal of the element is proportional to the square of the intensity of the magnetic field. The circuit is included in a device for measuring magnetic anomalies such as stress or corrosion cracks in structures of magnetic material including fluid transmission pipelines.

Claims

1. A circuit for detecting relatively minute changes in the intensity of a magnetic field comprising:

a coil adapted to be disposed in said magnetic field and including conductor means for conducting a current in response to a change in the magnetic field sensed by said coil;

a first Hall effect element including means connected to said conductor means of said coil for receiving said current as a bias current imposed on said first Hall effect element;

said first Hall effect element being disposed in said magnetic field with said coil;

output conductor means connected to said first Hall effect element for producing a voltage signal which is proportional to the intensity of said magnetic field; and

at least a second Hall effect element disposed in said magnetic field and having means connected to said output conductor means of said first Hall effect element for receiving as a bias current imposed on said second Hall effect element a current produced by said voltage signal output by said first Hall effect element, said second Hall effect element including output conductor means for producing a voltage signal proportional to an exponential power of said magnetic field.

2. The circuit set forth in claim 1 including:

a noise bandwidth filter for filtering a selected frequency band of signals output from said coil in response to sensing changes in said magnetic field.

3. The circuit set forth in claim 1 including:

amplifier means interposed in said conductor means of said coil for amplifying the output signal of said coil responsive to a change in said magnetic field.

4. A device for sensing a relatively minute change in a magnetic field passing through a member formed of a magnetic material, which change is caused by a magnetic anomaly associated with said magnetic material, said device comprising:

a wire coil arranged to be disposed in said magnetic field and in proximity to said member;

means for producing a magnetic field passing through said member and said coil;

a Hall effect element disposed in said magnetic field, said Hall effect element including conductor means connected to said coil for conducting a current to said Hall effect element as a bias current and which is produced by a voltage signal generated in said coil as a result of a change in the intensity of said magnetic field;

signal output conductor means connected to said Hall effect element for producing a voltage across said signal output conductor means; and

an integrating circuit connected to said Hall effect element for integrating the signal output from said Hall effect element to provide a voltage across said output conductor means proportional to said change in the intensity of said magnetic field.

5. The device set forth in claim 4 including:

a bandwidth filter for filtering signals of a selected frequency band out of the signal provided at said signal output conductor means.

6. The device set forth in claim 4 wherein:

said means for providing said magnetic field comprises magnet means disposed within said coil.

7. The device set forth in claim 6 wherein:

said magnet means comprises a permanent magnet disposed in a cavity formed by said coil for producing a toroidal magnetic field with respect to the central axis of said coil.

8. The device set forth in claim 6 wherein:

said Hall effect element is disposed in said magnetic field adjacent to said coil.

9. The device set forth in claim 8 including:

means interposed in said magnetic field for interrupting said magnetic field at a predetermined cyclical rate.

10. The device set forth in claim 9 wherein:

said means for interrupting said magnetic field comprises a rotating disk including means forming an opening in said disk whereby a change in the intensity of said magnetic field is provided at a predetermined rate and magnitude to provide a repetitive signal of predetermined rate and magnitude superimposed on a second signal indicating the presence of said magnetic anomaly.

11. A device for use on a survey pig for detecting the presence of a crack in the wall of a fluid transmission pipeline, said pig including support means for supporting said device adjacent to said wall and means for generating a magnetic field imposed on said wall, said device comprising:

a wire coil disposed on said support means in said magnetic field and in proximity to said wall;

a Hall effect element disposed in said magnetic field, said Hall effect element including conductor means connected to said coil for conducting a current to said Hall effect element as a bias current and which is produced by a voltage signal generated in said coil as a result of a change in the intensity of said magnetic field; and

signal output conductor means connected to said Hall effect element for producing a voltage signal across said signal output conductor means proportional to said change in the intensity of said magnetic field.

12. The device set forth in claim 11 including:

a bandwidth filter for filtering signals of a selected frequency band out of the signal provided at said signal output conductor means.

13. The device as set forth in claim 11 wherein:

said means for generating said magnetic field comprises a permanent magnet disposed in a cavity formed by said coil for producing a toroidal magnetic field with respect to the central axis of said coil.

14. The device set forth in claim 11 including:

means interposed in said magnetic field for interrupting said magnetic field at a predetermined cyclical rate.

15. The device set forth in claim 14 wherein:

said means for interrupting said magnetic field comprises a rotating disk including means forming an opening in said disk whereby a change in the intensity of said magnetic field is provided at a predetermined rate and magnitude to provide a repetitive signal of predetermined rate and magnitude superimposed on a second signal indicating the presence of a crack in said wall.

16. A circuit for detecting relatively minute changes in the intensity of a magnetic field comprising:

a coil adapted to be disposed in said magnetic field and including conductor means for conducting a current in response to a change in the magnetic field sensed by said coil;

a Hall effect element including means connected to said conductor means of said coil for receiving said current as a bias current imposed on said Hall effect element, said Hall effect element being disposed in said magnetic field with said coil;

output conductor means connected to said Hall effect element for producing a voltage signal which is proportional to the intensity of said magnetic field; and

means interposed in said magnetic field for changing the effective intensity of said magnetic field sensed by said coil and said Hall effect element and of a magnitude and at a repetitive rate such that other changes in the intensity of said magnetic field produce a signal output of said Hall effect element which is superimposed on a signal output resulting from said means for changing said effective intensity.