Variable permeability magnetic field sensor and method

• US 20030151405A1
• Filed: 01/15/2003
• Published: 08/14/2003
• Est. Priority Date: 01/15/2002
• Status: Active Grant

• Current Assignee: National University of Singapore
• Original Assignee: National University of Singapore
• Inventors: Li, Xiaoping
• Granted Patent: US 6,853,186 B2
• Time in Patent Office: Days
• US Class Current: 324/244
• CPC Class Codes: G01R 33/04 using the flux-gate principle

Abstract

A magnetic field sensor relies on variations in permeability of magnetic material to detect an external field. An exemplary magnetic field sensor includes a magnetic material and two or more conductors, at least one of which is connected to an electrical energy source. Current flowing through at least one of the conductors establishes a magnetic field in the magnetic material at a magnitude at which there is a generally linear relationship between the magnetic field and the permeability of the material. An external field to be sensed influences the permeability of the material. Sensing variations in the permeability of the magnetic material allows the external magnetic field to be sensed. Preferably, the conductors are mutually coupled and exhibit a mutual inductance between each other so that detectable changes in the current distribution between the conductors caused by an external magnetic field may be detected while the net current flowing in the conductors remains sufficient to maintain a magnetic field of sufficient magnitude in the material. The sensitivity of the sensor may be increased by increasing the number of conductors. Sensors may be formed as thin films. Exemplary magnetic sensors are able to detect relatively weak external magnetic fields, such as those emitted by the human brain and other biological bodies and may be used in a variety of applications involving non-contact sensing and monitoring of brain activities or status of consciousness.

Claims

1. A magnetic field sensor, comprising:

a shell made of a magnetic material defining an interior;

a first conductor within said interior;

a second conductor within said interior, nested within said first conductor;

said first and second conductors exhibiting a mutual inductance between each other;

a source of electrical energy, interconnected with said first and second conductors, to establish an alternating electric current through at least one of said first and second conductors, and thereby a magnetic field in said shell;

a monitor, interconnected with one of said first and second conductors, to monitor changes in current therethrough as said shell is brought into proximity with an external magnetic field, to sense a change in permeability of said magnetic material.

2. The sensor of claim 1, wherein said first and second conductors are connected in parallel.

3. The sensor of claim 1, wherein said source provides a constant electrical current.

4. The sensor of claim 1, further comprising a plurality of conductors nested within said first conductor.

5. The sensor of claim 1, wherein said shell comprises a thin film.

6. The sensor of claim 4, wherein said shell comprises a thin film.

7. The sensor of claim 5, wherein said first and second conductors comprise thin conductive strips embedded within said shell.

8. A magnetic field sensor, comprising

a magnetic material;

two mutually-coupled conductors driven by a source of alternating electricity to generate a magnetic field in said magnetic material, and mutually coupled so that a change in permeability of said magnetic material results in re-distribution of current through said conductors;

a meter for measuring current through at least one of said first and second conductors to sense a change in permeability of said magnetic material caused by an external magnetic field, and thereby said external magnetic field.

9. A method of sensing an external magnetic field, comprising

inducing a magnetic field in a magnetic element by providing alternating current through first and second mutually-coupled electric conductors;

measuring a change in distribution of current in said two conductors, resulting from said external magnetic field varying permeability of said magnetic element.

10. A magnetic field sensor, comprising

a conductive core, surrounded by a magnetic layer;

a coil wound about said magnetic layer;

a source of electrical energy, providing electrical energy to said conductive core to establish a magnetic field in said magnetic layer, said magnetic field having a magnitude at which there is a linear relationship between said magnetic field and permeability of said magnetic layer;

a sensor for detecting changes in voltage across said coil, resulting from changes in said permeability of said magnetic layer, attributable to an external magnetic field.

11. A method of sensing a low frequency external magnetic field comprising:

exciting a magnetic element with a time varying magnetic field having a frequency and amplitude to maximize the sensitivity of the permeability of said magnetic element to magnetic field in said magnetic element;

sensing said external magnetic field by measuring variation of said permeability of said magnetic element, resulting from said external magnetic field.

12. The method of claim 11, wherein the said variation of said permeability is measured by measuring an impedance of said magnetic element connected to an AC circuit.

13. The method of claim 12, wherein said time varying magnetic field is circumferential to the longitudinal extent of said magnetic element.

14. The method of claim 11, wherein said variation of said permeability is measured by measuring an induced voltage in a coil coiling said magnetic element;

and said time varying magnetic field is parallel to the longitudinal extent of the said magnetic element.

15. The method of claim 11, wherein said frequency and amplitude are selected in dependence on magnetic properties of said magnetic element.