Variable permeability magnetometer systems and methods for aerospace applications

• US 10,036,784 B2
• Filed: 04/01/2015
• Issued: 07/31/2018
• Est. Priority Date: 11/17/2014
• Status: Active Grant

• Current Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Original Assignee: United States of America as represented by the Administrator of NASA
• Inventors: Shams, Qamar A., Sutton, John F.
• Primary Examiner(s): Assouad, Patrick
• Assistant Examiner(s): YODICHKAS, KHRISTOPHER J
• Application Number:
  US14/676,172
• Publication Number:
  US 20160139213A1
• Time in Patent Office: 1,217 Days
• Field of Search: 324253
• CPC Class Codes: G01R 33/022 Measuring gradient
• CPC Class Codes: G01R 33/028 Electrodynamic magnetometers
• CPC Class Codes: G01V 3/081 the magnetic field is produced by the objects or geological structures characterised by the method of magnetic field measurement G01R33/00
• CPC Class Codes: G01V 3/10 using induction coils

Abstract

A magnetometer configured to measure low field strength magnetic fields is provided. Certain embodiments of the magnetometer include a cylindrical coil assembly having a variable permeability core and terminals disposed at both ends. A current source circuit may be operably connected to the terminals and configured to apply a voltage controlled current across the terminals. A voltage readout circuit may be operably connected to the terminals and configured to measure a voltage across the terminals due to the applied current from the current source. An inductance of the coil assembly directly varies as an ambient magnetic field strength varies a permeability of the variable permeability core, and a voltage across the terminals varies directly with the inductance such that the measured voltage across the terminals is a direct measure of the ambient magnetic field strength.

Claims

1. A magnetometer comprising:

a cylindrical coil assembly extending from a first end to an opposing second end, having a variable permeability core configured to measure low strength magnetic fields having a frequency response range between 0 and 700 hertz, the cylindrical coil assembly further comprising terminals, including at least a first terminal at the first end and a second terminal at the second end of the coil assembly, wherein the variable permeability core includes a plastic coil form and a foil strip constructed of a variable permeability magnetic material that is wrapped within the plastic coil form, the variable permeability core further including a search coil having a sense winding that is wound over the plastic coil form, the search coil further including a secondary winding configured for feedback control or calibration of the magnetometer, the secondary winding being disposed over the sense winding;

a current source circuit operably connected to the terminals, and including a crystal oscillator configured to provide a high-frequency clock signal to drive the current source circuit and thereby apply a plurality of electric currents to the cylindrical coil assembly, including a voltage-controlled sense current across the terminals, a feedback current applied to the secondary winding to cancel out variation in the Earth'"'"'s field caused by a change in orientation of the cylindrical coil assembly, and a biasing current across the terminals; and

a voltage readout circuit operably connected to the terminals and configured to measure a voltage across the terminals, with direct current offsets eliminated, due to the applied voltage-controlled currents from the current source circuit,

wherein an inductance of the cylindrical coil assembly directly varies as an ambient magnetic field strength varies a permeability of the variable permeability core, and a voltage across the terminals varies directly with the inductance such that the measured voltage across the terminals is a direct measure of the ambient magnetic field strength.

2. The magnetometer of claim 1, wherein the foil strip is comprised of a cobalt-alloy magnetic material.

3. The magnetometer of claim 1, wherein the sense winding is comprised of an enameled copper wire having at least 500 turns.

4. The magnetometer of claim 1, wherein the voltage readout circuit includes a double synchronous demodulator, a low pass filter, and an integrator, and wherein the feedback current is fed to the secondary windings through each of the double synchronous demodulator, the low pass filter, and the integrator.

5. The magnetometer of claim 1, wherein the current source circuit further includes a digital frequency divider configured to divide the clock signal of the crystal oscillator from 1 MHz to a lower frequency of 10 kHz.

6. The magnetometer of claim 5, wherein the current source circuit further includes a signal shaper configured to shape the lower frequency signal of the digital frequency divider into a sinusoidal form.

7. The magnetometer of claim 1, wherein the current source circuit is configured to adjust the applied biasing current across the terminals to bias the variable permeability core to an optimum location of a core permeability curve.

8. A method of measuring a low magnetic field strength having a frequency response range of between 0 and 700 hertz comprising:

applying a plurality of currents to a winding of a coil assembly having a variable permeability core, including a sinusoidal, voltage-controlled current across two opposing terminals of the coil assembly having the variable permeability core, the variable permeability core including a foil strip constructed of a variable permeability magnetic material wrapped within the plastic coil form, and a search coil wound over the plastic coil form;

measuring a voltage applied across the terminals due to the sinusoidal, voltage-controlled current;

calculating the low magnetic field strength from the measured voltage; and

detecting a presence of a geophysical or atmospheric event using the calculated low magnetic field strength.

9. The method of claim 8 further comprising:

applying a feedback signal to the winding of the coil assembly as one of the plurality of currents;

enhancing a receiving cross section of the coil assembly; and

measuring an adjusted voltage across the terminals.

10. The method of claim 9, further comprising removing, by the feedback signal, variations in ambient magnetic field strength due to a change in a coil assembly orientation.

11. The method of claim 8, wherein measuring the voltage includes applying a double synchronous detector for detecting a measured voltage that substantially eliminates l/f noise, switch noise, and the direct current offsets.

12. The method of claim 11, wherein measuring the voltage further includes applying a low pass filter.

13. The method of claim 8, wherein applying the sinusoidal, voltage-controlled current source includes providing a clock signal, by a crystal oscillator, to drive a high-output impedance current source.

14. The method of claim 13, wherein applying the sinusoidal, voltage-controlled current source further includes:

dividing the clock signal of the crystal oscillator to a lower frequency signal; and

shaping the lower frequency signal into a sinusoidal form.

15. The method of claim 8, further comprising:

enclosing each of a pair of the coil assemblies in a corresponding wooden box;

burying the enclosed coil assemblies to a depth of six to nine inches below a ground surface a minimum distance of 2,000 miles apart; and

wirelessly transmitting the measured voltage to a remote readout screen located above the ground surface.

16. The method of claim 8, wherein the foil strip of a variable permeability magnetic material is constructed of cobalt-alloy magnetic material.

17. The method of claim 8, wherein applying the plurality of currents includes applying each of:

a 10 kHz sine oscillator signal as the sinusoidal, voltage-controlled current;

a variable biasing current operable for biasing the core to a predetermined location of a core permeability curve; and

a feedback current applied directly to the search coil.