GB2044460A

GB2044460A - Fluxgate magnetometers

Info

Publication number: GB2044460A
Application number: GB7935862A
Authority: GB
Original assignee: CA MINISTER NAT DEFENCE; Minister of National Defence of Canada
Current assignee: CA MINISTER NAT DEFENCE; Minister of National Defence of Canada
Priority date: 1979-01-11
Filing date: 1979-10-16
Publication date: 1980-10-15
Also published as: GB2044460B

Abstract

A fluxgate magnetometer sensor for simultaneously measuring, at the same point in space, the three mutually orthogonal components of a magnetic field comprises a ring core 1 of magnetizable material, with a drive winding wound toroidally around the core. A first secondary winding 3 is wound circumferentially surrounding the core. A second secondary winding 4 is wound toroidally over a portion of the core, and a third secondary winding 5 is wound toroidal]y over another portion of the core, preferably at 90 DEG to the second secondary winding. Windings 4, 5 are each split into two sections wound in opposition toroidally over opposite segments of the core. Alternatively 4, 5 may be wound completely and diametrically around the outside of the core (Fig. 2) or the core may be positioned in a non-magnetic cube with windings 3, 4, 5 wound on the outside of the cube (Fig 3). <IMAGE>

Description

SPECIFICATION Improvements in or relating to magnetometers.

This invention relates to the field of magnetic field detection, and in particular to a three axis fluxgate magnetometer sensor used in the detection of such fields.

Magnetometers are normally used for the detection of anomalies and variations in magnetic fields in the earth environment and more recently in outer space environments. In the earth environment magnetometers have been used for geophysical exploration, pipe location for utilities, location of buried ordnance, etc.

Highly sensitive magnetometers have previously been fabricated to detect fields as small as .01 nanotesla; a highly successful type of magnetometer used for detection and measurement of magnetic fields is a fluxgate magnetometer. This type of magnetometer utilizes a drive winding wound around portions of a magnetizable core. The core is driven in and out of saturation with a current which may be of square wave form. An ambient magnetic field passing through the core at an appropriate angle causes an imbalance in the signal sensed by a sense winding. This imbalance generates a second harmonic signal which is proportional to the ambient magnetic field, and which may be analyzed to obtain a determination of the field strength of the ambient magnetic field.

Typical prior art fluxgate magnetometers have sensed one vector component of an ambient magnetic field, although more recently, sensors which sense two perpendicular vector components of an ambient magnetic field have been designed. In order to sense the 3 orthogonal vector components of an ambient magnetic field, three separate cores were required, oriented in mutually orthogonal directions. Even the more recent form of sensor required at least two cores to detect the three vector components. However, if a gradient exists in the magnetic field, and the magnitude of the gradient is larger than the resolution required by the measurements, such structures are unsatisfactory for accurate measurements.Since the cores must necessarily be physically spaced from each other, the three field components are not measured at precisely the same point and there exists an error in the measurement.

The present invention is a three axis fluxgate magnetometer sensor which detects and measures the mutual orthogonal components of a magnetic field at a single point in space for what is believed to be the first time. Accordingly errors in measurement introduced by the physical spacing of detectors measuring the three orthogonal components of a magnetic field are eliminated.

In general, the invention is comprised of a ring core of magnetizable material having a central axis, a drive winding wound so as to evenly saturate the core when carrying a predetermined amount of current, and three secondary windings wound so as to have their respective magnetic axes mutually at 900 to each other at a central point on the central axis.

More particularly, the invention is comprised of a ring core of magnetizable material, with a drive winding wound toroidally around the core. A first secondary winding is wound surrounding the core circumferentially therewith. A second secondary winding is wound toroidally over a portion of the core, preferably over two quarter sections thereof, and a third secondary winding is wound toroidally over a second portion of the core at 900 to the second secondary winding, preferably over the remaining two quarter sections thereof.

A better understanding of the invention will be obtained by reference to the detailed description below, and to the following drawings, in which: Figures 1 A and 1 E are perspective views of various windings of a first embodiment of the invention, Figures 2A to 2E are perspective views of various windings of a second embodiment of the invention, Figure 3A is a perspective cut away-view of a portion of a third embodiment of the invention showing the drive winding, and Figure 3B is a perspective view of the secondary windings of the third embodiment of the invention.

Turning now to Figure 1A, a ring core 1 is shown, toroidally wound with a drive winding. The drive winding should be wound evenly around the core so as to evenly saturate the core when carrying a suitable current.

In this and the other embodiments of the invention, the core material should be of the type having a narrow hysteresis loop, with square corners and high magnetic flux capacity. The coercive force should be low, the effective permeability high, and the differential permeability high. The core should have a high resistivity, a low magnetostrictive constant, very thin laminations or tape, minimum hysteresis and eddy current losses, as small an air gap between layers as possible, and the material must be suitably heat treated.

Preferably the material should be 6-81.3 Molybdenum Permalloy (Trade Mark), which contains 6% molybdenum, 81.3% nickel and 12.7% iron. However 4-79 Molybdenum Permalloy (Trade Mark) which contains 4% molybdenum, 79% nickel and 17% iron is also suitable.

The first secondary winding 3 is shown in Figure 1 B. This winding is wound circumferentially so as to have its magnetic axis parallel to the central axis.

Figure 1 C shows the second secondary winding which is wound in two segments. The first segment is wound toroidally over one quarter section of the core. The second segment is wound toroidally over the opposite quarter section of the core. The two segments are series connected in a manner which will cancel the drive oscillator signal at the output of this secondary winding.

The third secondary winding 5, as shown in Figure 1 D, is wound toroidally over the two remaining quarter sections of the core. These two segments are also series connected in a manner which cancels the drive oscillator signal at the output of this secondary winding.

The magnetic axes of the three sense windings are orthogonal to each other.

In Figure 1 B, the magnetic field component parallel to the central axis, and in the X direction, will therefore be sensed by the first secondary winding. In Figure 1 C, the magnetic axis of the second sense winding intersects the central axis in the Y direction. The magnetic field component which is parallel to the magnetic axis of the second sense winding, and in the Y direction will be sensed by the second secondary winding.

In Figure 1 D, the magnetic axis of the third sense winding inte ects the X and Y intersection point in the Z direction. The magnetic field component which is parallel to the magnetic axis of the third sense winding and in the Z direction, is sensed by the third sense winding.

Accordingly it may be seen that the three secondary windings will be sensitive to the ambient magnetic field components in respective X, Y, and Z directions which appear on the central axis within the core. The magnetic axes of the seccndary windings intersects at a point which is the exact point of sensing of the ambient magnetic field.

Figure 1 E is a perspective view of the wound fluxgate magnetometer sensor. The top of the core 1 and drive winding 2 are illustrated, but in practice whould be hidden by the circumferential first secondary winding 3, and the second and third secondary windings 4 and 6.

For equal sensitivity on each axis, the number of turns may be determined from the following expression: Nxfld=(Ny) (od)=(Nz) (od), where Nx is the number of turns of the first secondary winding and N, and Nz are the number of turns respectively of the second and third secondary windings, d is th inner diameter of the core plus the thickness of the core, and od is the outside diameter of the core.

To ensure the same magnitude of noise for each axis, the thickness of the core should be equal to the height of the core. In one example, the drive winding carried sufficient current to produce an excitation field with a peak value of approximately 3 Oersteds. The diameter of the core was 1.000 inches, the thickness of the core was 0.125 inches, the height of the core was 0.125 inches, and the dimension "od" was 1.0625 inches. The driving oscillator carried a peak current of 100 milliamperes, and the drive winding contain 1 90 turns. The first secondary winding contained 500 turns, and each of the remaining secondary windings contained 1,478 turns.

Figures 2A-2E depict the components of a second embodiment of the invention. Figures 2A and 2B are similar to Figures 1 A and 1 B and show a toroidal drive winding 2 wound on ring core 1. Circumferential first secondary winding 3 is also wound around ring core 1.

However in Figure 2C, it may be seen that the second secondary winding 6 is wound diametrically around the core, covering opposite small segments thereof.

In Figure 2D, it may be seen that third secondary winding 7 is wound similarly as winding 6, but at 900 thereto.

Thus the windings are balanced with respect to the central axis, with first secondary winding 3 being sensitive the magnetic field component in the X direction, second secondary winding 6 being sensitive to the magnetic field component in the Y direction and the third secondary winding 7 being sensitive to the magnetic field component in the Z direction. The axis of the magnetic fields of each secondary winding intersect at a single central point.

Figure 2E is a perspective view of the magnetometer sensor completely wound. The drive winding, and first, second and third secondary windings are all wound on the core.

Each of the respective secondary windings is sensitive to the component of the ambient magnetic field which is parallel to the magnetic axis of the particular secondary winding. Since all three axes intersects at the centre of the core, it is the magnetic field at this intersection point which is sensed. Accordingly the exact magnetic field at a single particular point in space is accurately sensed.

Figures 3A and 3B depict an alternative form of the second embodiment of the invention. In this case, ring core 1 is wound with toroidial drive winding 2 as in the previous two embodiments.

However in the present case, the ring core is located centrally within an insulative cube or box, around which the first, second and third secondary windings are wound.

In Figure 3B, the first secondary winding 8 is wound around the periphery of the box 9. Second and third secondary windings are wound around the box across the top so as to pass diametrically across the core, in a manner similar to that of Figures 2C and 2D.

The resulting structure senses magnetic fields in the X, Y and Z direction in a manner similar to that of the embodiment of Figure 2E.

A person skilled in the art and understanding this invention may now conceive of other embodiments or modifications thereof, using the principles of this invention. All are considered within the sphere and scope of the invention as defined in the appended claims.

Claims

1. A three axis fluxgate magnetometer comprising a ring core of magnetizable material, having a central axis, a drive winding wound so as to evenly saturate the core when carrying a predetermined amount of current, and three secondary windings would so as to have their magnetic axes mutually at 900 to each other at a central point on the central axis.

2. A magnetometer sensor as defined in claim 1, in which each of a pair of said secondary windings is wound so as to have its magnetic axes orthogonal to the central axis, and mutually orthogonal to the other.

3. A magnetometer sensor as defined in claim 2, in which the remaining secondary winding is wound so as to have its magnetic axis parallel to the central axis.

4. A magnetometer sensor as defined in claim 3, in which the winding of each of said pair of secondary windings is split into two sections which are serially connected in a magnetic sense such as to cancel a magnetic field induced therein by said current carried by the drive winding, each pair of sections being toroidally wound on said core over opposite segments thereof, the segments of each section of said pair of secondary windings being located at 900 relative to the next.

5. A magnetometer sensor as defined in claim 3 in which the windings of each said pair of secondary windings are wound diametrically around the core at 900 to the other, and the remaining secondary winding being wound circumferentially surrounding the core.

6. A magnetometer sensor as defined in claims 3, 4 or 5 in which the drive winding is wound toroidally around the core.

7. A magnetometer sensor as defined in claims 3, 4 or 5 in which the number of turns of each of said pair of secondary windings is the same.

8. A magnetometer sensor as defined in claims 3, 4 or 5 in which the thickness of the core is equal to the length of the core, and in which the number of turns of each of the secondary windings is determined from the relationship Nx nd=(N,) (od)=(N2) (od) where Nx is the number of turns in said remaining secondary winding, N, andN are the number of turns in respective ones of each winding of said pair of secondary windings, d is the total of the inner diameter of the core plus the thickness of the core, and od is the outer diameter of the core.

9. A magnetometer sensor as defined in claim 4 in which a first of said pair of secondary windings is wound over two opposite quarter sections of the core, and the second of said pair of secondary windings is wound over the remaining two quarter sections of the core.

1 0. A magnetometer sensor as defined in claim 2 in which each of said pair of secondary windings is wound completely and diametrically around the outside of the core.

11. A magnetometer sensor as defined in claim 2, further including a nonmagnetic cube housing containing the ring core, the secondary windings being wound around the core outside of the housing.

12. A magnetometer sensor as defined in any one preceding claim in which the material of the core is comprised of 4% molybdenum, 79% nickel and 17% iron.

13. A magnetometer sensor as defined in any one of claims 1 to 11 in which the material of the core is comprised of 6% molybdenum, 81.3% nickel and 12.7% iron.

14. A magnetometer sensor as defined in any one preceding claim in which the thickness of the core wall is the same as the length of the core.

1 5. A magnetometer sensor substantially as described with reference to Figures 1 A to 1 E, Figures 2A to 2E, or Figures 3A to 3B of the accompanying drawings.