U.S. patent application number 11/942305 was filed with the patent office on 2008-03-20 for fluxgate.
This patent application is currently assigned to BARTINGTON INSTRUMENTS LIMITED. Invention is credited to Geoffrey William Bartington.
Application Number | 20080068010 11/942305 |
Document ID | / |
Family ID | 32117573 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080068010 |
Kind Code |
A1 |
Bartington; Geoffrey
William |
March 20, 2008 |
FLUXGATE
Abstract
A fluxgate including at least two ferromagnetic or
ferrimagnetic, flexible cores, and at least two sets of a plurality
of windings of an electrically conductive material. At least one
set of windings is wound around each of the cores. The fluxgate
further includes an electrically conductive, flexible shield
enclosing the cores and the windings.
Inventors: |
Bartington; Geoffrey William;
(Witney-Oxfordshire, GB) |
Correspondence
Address: |
THOMPSON HINE L.L.P.;Intellectual Property Group
P.O. BOX 8801
DAYTON
OH
45401-8801
US
|
Assignee: |
BARTINGTON INSTRUMENTS
LIMITED
10 Thorney Leys Business Park
Witney
GB
OX8 7GE
|
Family ID: |
32117573 |
Appl. No.: |
11/942305 |
Filed: |
November 19, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10592409 |
Sep 11, 2006 |
7298141 |
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11942305 |
Nov 19, 2007 |
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PCT/GB2005/050020 |
Feb 22, 2005 |
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11942305 |
Nov 19, 2007 |
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Current U.S.
Class: |
324/253 |
Current CPC
Class: |
G01R 33/04 20130101 |
Class at
Publication: |
324/253 |
International
Class: |
G01R 33/04 20060101
G01R033/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 2004 |
GB |
0405617.2 |
Claims
1. A fluxgate comprising: at least two ferromagnetic or
ferrimagnetic, flexible cores; at least two sets of a plurality of
windings of an electrically conductive material, at least one set
of windings being wound around each of said cores; and an
electrically conductive, flexible shield enclosing said cores and
said windings.
2. A fluxgate according to claim 1, comprising a first flexible
insulating means for electrically insulating said cores from said
windings.
3. A fluxgate according to claim 2, wherein the first insulating
means comprises at least two flexible, electrically insulating
tubes, wherein each core is surrounded by a said tube and said sets
of windings are wound around said tubes.
4. A fluxgate according to claim 1, wherein the electrically
conductive material of said windings is coated with, or surrounded
by, electrically insulating material.
5. A fluxgate according to claim 1, further comprising a flexible
insulating jacket provided around said shield.
6. A fluxgate according to claim 1, wherein said magnetic cores
comprise a high magnetic permeability saturable material.
7. A fluxgate according to claim 1, wherein said cores comprise a
soft ferromagnetic or ferrimagnetic material.
8. A fluxgate according to claim 1, wherein said cores are
sufficiently thin so that the magnetic properties of said cores are
not substantially altered by stress/strain induced by flexure.
9. A fluxgate according to claim 1, wherein said cores comprise Mu
metal.
10. A fluxgate according to claim 9, wherein said cores comprise
strips of Mu metal foil.
11. A fluxgate according to claim 1, wherein said cores comprise Mu
metal of 0.2 mm thickness or less.
12. A fluxgate according to claim 1, wherein said cores comprise Mu
metal of 0.1 mm thickness or less.
13. A fluxgate according to claim 1, wherein said cores comprise Mu
metal wires.
14. A fluxgate according to claim 1, wherein said cores comprise an
evaporated film of ferromagnetic or ferrimagnetic material.
15. A fluxgate according to claim 1, wherein said windings are
helical windings.
16. A fluxgate according to claim 1, wherein said windings are
single layer helical windings.
17. A fluxgate according to claim 1, wherein the length of the
fluxgate is at least 0.5 m.
18. A fluxgate according to claim 1, wherein the fluxgate is
sufficiently flexible so as to be able to be arranged substantially
in the form of a loop.
19. A fluxgate according to claim 1, wherein the fluxgate has first
and second axial ends and is sufficiently flexible so as to be able
to be arranged such that the first axial end is in close proximity
to the second end.
20. A fluxgate, arranged in an open loop.
21. A fluxgate comprising: at least two flexible cores; at least
two sets of a plurality of windings, at least one set of windings
being wound around each of said cores, the cores and windings
interacting with each other by magnetic coupling between a
ferromagnetic or ferrimagnetic material and an electrically
conductive material; and an electrically conductive, flexible
shield enclosing said cores and said windings.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Ser. No.
10/592,409, which is in turn the United States national phase entry
of PCT International Application No. PCT/GB2005/050020 filed Feb.
22, 2005, and claims priority to Great Britain Application No.
0405617.2 filed Mar. 12, 2004. The entire contents of all of those
applications are hereby incorporated by reference.
[0002] The present invention relates to fluxgates and fluxgate
magnetometers, and more particularly, to fluxgate magnetometers
that can be used to form an access control system.
BACKGROUND
[0003] A fluxgate and its associated electronics can be used to
convert static or alternating magnetic field information into an
electrical signal for the purpose of measurement and control. The
principles of operation are known and can be found in the
literature, for example in Bartington G. 1994 Magnetic Fields
report for the IEE Colloquium Low Level Low Frequency Magnetic
Fields, London 2; 1-9.
[0004] A typical prior art fluxgate consists of two magnetic,
rigid, cylindrical cores, each of which is surrounded by helical
windings of wire over the length of the cores. Both cores and their
respective windings are surrounded by a conductive, rigid
electrical shield. The typical length of a conventional fluxgate is
10 or 20 cm.
[0005] The present inventor has appreciated that for many
applications a rigid fluxgate is unsuitable, or impractical.
[0006] The inventor has also appreciated that the relatively short
length of conventional fluxgates limits their use.
[0007] The inventor has also appreciated that in certain
applications a fluxgate which is not straight may be of use.
[0008] The present invention has been made in view of these
limitations.
SUMMARY
[0009] The inventor has appreciated that making the entire fluxgate
flexible provides unexpected technical advantages. The flexibility
of the fluxgate according to the present invention is desirable in
itself. In addition, owing to the flexibility a number of
embodiments become possible which might otherwise be impractical.
In particular, the fluxgate of embodiments of the invention can
take a form similar to a shielded cable, and accordingly the
fluxgate can be made relatively long without becoming impractical
to handle. Whilst a rigid prior art fluxgate might for many
purposes become impractical if its length exceeds e.g. 0.5 m, the
flexible fluxgate according to the present invention can in some
embodiments have a length of many meters, potentially several
kilometres.
[0010] Owing to its flexibility, the fluxgate can be formed into a
loop, and this may in some embodiments be used to protect the
perimeter of an object. Hence the fluxgate can be used in an access
control system. E.g. if the fluxgate is arranged in a loop, a
magnetic object such as a vehicle entering the loop would have an
influence on the magnetic field sensed by the fluxgate, and the
resulting measurements can be processed so that the access control
system can provide an output in response to the magnetic object
entering the loop.
[0011] An access system can however also be formed by using one or
more fluxgates according to the present invention without
necessarily being arranged in a loop.
[0012] Aspects of the invention are set out in the independent
claims. Apparatus aspects corresponding to method aspects disclosed
herein are also provided, and vice versa.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Some illustrative embodiments of the invention will now be
described by way of example only and with reference to the
accompanying drawings, in which:
[0014] FIG. 1 shows the structure of a fluxgate according to an
embodiment of the present invention;
[0015] FIG. 2 shows the structure (slightly simplified) of a
fluxgate magnetometer according to an embodiment of the present
invention;
[0016] FIG. 3 shows the structure (slightly simplified) of a
fluxgate magnetometer according to another embodiment of the
present invention;
[0017] FIG. 4 shows the structure (slightly simplified) of a
fluxgate magnetometer according to another embodiment of the
present invention;
[0018] FIG. 5 shows the relationship between the magnetic field and
the magnetic flux so as to aid in the understanding of the
invention; and
[0019] FIG. 6 shows the structure (strongly simplified) of an
access control system according to an embodiment of the present
invention.
DETAILED DESCRIPTION
[0020] FIG. 1 shows the mechanical structure of a fluxgate
according to an embodiment of the present invention.
[0021] Two high magnetic permeability saturable, flexible cores 1,
2 are sheathed in loose fitting and electrically insulating,
flexible tubes 3, 4. The tubes 3, 4 are each over wound with a set
of helical windings 5, 6 of insulated wire over the entire length
of the tubes. Only some of the helical windings are shown
explicitly, whilst the windings in the middle portion of the figure
are indicated by dots. The wire may be coated with insulation or it
may be surrounded by insulation material. In one case there is only
one layer (in radial direction) of windings on each core. A
conductive electrical shield 7 encloses the cores, tubes and
windings. The entire assembly is enclosed in a protective,
insulating jacket 8. Materials suitable for the cores 1, 2, the
tubes 3, 4, the windings 5, 6, the shield 7 and the jacket 8 will
be apparent to those skilled in the art. All of these materials may
be mechanically flexible (or bendable or non-rigid). The cores can
be made flexible by making them sufficiently thin, e.g.
sufficiently thin in one or two dimensions. By making the cores
thin it is possible to use materials which are normally (i.e. if
made thicker) not flexible. One suitable core material is Mu metal.
A typical composition of Mu metal is 78% Nickel, 20% Ferrous Iron
and 2% Molybdenum. Reasonable results have been achieved in trials
with thin strips of Mu metal foil of 0.2 mm or less thickness. The
flexibility was improved by reducing the thickness to 0.1 mm or
less, and further to 0.05 mm or less, 0.02 mm or less and 0.01 mm
or less. Depending on the thickness used it may be advantageous to
apply the high magnetic permeability material to a flexible support
material. The high magnetic permeability material can for example
take the form of an evaporated film. It is also conceivable to make
some parts of the fluxgate from a plurality of rigid portions which
are connected in a flexible way, e.g. somewhat similar to the
scales of a fish.
[0022] Electrical connections for the windings 6, windings 5 and
the shield 7 are made at each end of the fluxgate. The ends of the
fluxgate are identified as first and second axial ends 15, 16, or
remote and local ends 15, 16. In use as a fluxgate magnetometer the
local end 16 of the fluxgate is connected to driving and processing
circuitry, as explained below. The connections at the local end
carry reference numbers 9, 10, 11 (respectively for the winding 6,
winding 5 and the shield 7), and the corresponding connections at
the remote end carry reference numbers 12, 13 and 14.
[0023] FIG. 2 shows a first embodiment of a fluxgate magnetometer.
Only the windings 5, 6 and the shield 7 are shown, but in strongly
simplified representation. The fluxgate of this and the following
embodiments of fluxgate magnetometers are preferably as explained
in connection with FIG. 1.
[0024] In this embodiment, the connections 12, 13, 14 at the remote
end 15 are all terminated with a short circuit 17, 18 or a
connector or connecting means and at the local end 16 the windings
5, 6 only are terminated with the secondary winding 21 of a
transformer. An alternating excitation electrical current is
supplied to the primary winding 20 of the transformer at a
frequency F Hz by means of AC source 19 or supplying means. The
local end terminal 11 of shield 7 is connected to one input of a
voltage sensor 22 or sensing means, and a point (in one case the
mid-point of the secondary winding 21 of the transformer) is
connected to the other input of the voltage sensor 22. The
transformer and AC source 19 may together constitute a non-DC
current source, or means for generating a non-DC electrical current
in the windings 10, 11.
[0025] The alternating excitation electrical current is of
sufficient magnitude to generate a cyclically saturating magnetic
field in the helical windings 5, 6. In the presence of a magnetic
field H to be sensed and resulting in magnetic flux B=H cos .phi.
in the cores 1, 2 a voltage Vs having a frequency of 2 F Hz will
appear across the inputs of voltage sensor 22.
[0026] The relationship between magnetic flux B and magnetic field
H is shown in FIG. 5. The orientation of magnetic field H to be
sensed is not necessarily parallel to the cores 1, 2 of the
fluxgate, but forms an angle .phi. with the cores. This results in
a magnetic flux B parallel to the cores 1, 2 of magnitude B=H cos
.phi..
[0027] The magnitude of the sensed voltage Vs will be substantially
proportional to the vector sum of the magnetic field flux density
at all points along the flexible fluxgate. This embodiment hence
enables spatial integration along the length of the fluxgate.
[0028] The proportionality is degraded or lost if the measured flux
density B exceeds the magnitude of the excitation flux. To counter
this, as a refinement of this embodiment a flux may be fed back to
the fluxgate via a current in the windings 5, 6 to oppose the
measured field and thus increase the accuracy of the measurement
under some conditions. This could in some applications be achieved
by adding a DC component, or a means for generating a magnetic
flux, to the AC excitation provided by source 19. Suitable
processing equipment (not shown) includes equipment which first
performs an initial measurement (without opposing field), then
calculates the required current in the windings 5, 6 for opposing
the measured field, then controls source 19 to achieve this current
and then performs a refined measurement (with opposing field
applied).
[0029] FIG. 3 shows a second embodiment of a fluxgate magnetometer.
Again, only the windings 5, 6 and the shield 7 are shown, but in
strongly simplified representation. The electrical connections and
mode of operation of this embodiment are similar to those of the
first embodiment. The differences to the first embodiment are:
[0030] a) The windings 5, 6 are each terminated at their ends with
an impedance Z 23, 24, 25, 26, which may be either real or complex.
The impedances 25, 26 at the local end can be selectively variable
in value.
[0031] b) The frequency of the excitation signal F is chosen to be
sufficiently high so as to cause standing electromagnetic waves to
appear along the fluxgate sensor.
[0032] By manipulation of the termination impedances 25, 26 at the
local end 16 the position of nodes and anti-nodes of the standing
wave along the sensor can also be manipulated. In this way the
region or regions of the core where cyclic saturation occurs can be
manipulated.
[0033] The position or positions of the cores 1, 2 where a measured
signal can originate (contributing to sensed voltage Vs) are
therefore known and provide spatial discrimination of signal
sources.
[0034] A range of measurements with different values of impedances
25, 26 can be performed and the measurement results collated and
processed.
[0035] FIG. 4 shows a third embodiment of a fluxgate magnetometer.
Again, only the windings 5, 6 and the shield 7 are shown, but in
strongly simplified representation. The electrical connections and
mode of operation of this embodiment are similar to those of the
second embodiment. The differences to the second embodiment
are:
[0036] a) The terminating impedances 23-26 may be fixed for both
ends 15, 16.
[0037] b) The continuous wave excitation source 19 has been
replaced by a narrow pulse source 27.
[0038] By selecting the appropriate impedance value for the remote
and local ends 15, 16 of the sensor a travelling wave or pulse can
be launched back and forth along the sensor. This relies upon the
principle of signal reflection at the remote end 15 back to the
local end 16 of the sensor. The portion or portions of the sensor
in cyclic saturation at a given point in time and responsible for
production of a signal Vs are known as a function of time. This
embodiment hence also enables spatial discrimination.
[0039] FIG. 6 shows an example of application in simplified
representation. The fluxgate, arranged in a loop with local and
remote ends 16, 15 in close proximity to each other, forms part of
an access control system. Other parts of the access control system
such as the circuitry connected to the local and remote ends 16, 15
are not shown. The circuitry of the fluxgate magnetometer according
to the third embodiment may be used. Additionally, a processor or
similar (not shown) is provided to translate the sensed voltage Vs
into information regarding access to the system.
[0040] This system can be used for example for controlling or
detecting access to the interior of the loop, e.g. object 28 in
FIG. 6. E.g. if the loop is arranged in or on the ground and a
magnetic object such as a car passes over the perimeter defined by
the loop of the fluxgate so as to "enter" the loop this will
normally result in a variation of the magnetic field near the
fluxgate, and this can be sensed by the system. If the fluxgate
magnetometer according to the third embodiment is used it is
possible to determine at which position the magnetic object has
entered the loop since the time of the variation of the magnetic
field sensed by the fluxgate can be translated into positional
information, owing to the spatial resolution capabilities of the
system.
[0041] As an alternative, the fluxgate does not necessarily have to
be flexible. A rigid, loop shaped fluxgate could also employed in
the access control system.
[0042] In other embodiments an access control system is provided
using a fluxgate which is not arranged in a loop. For example, a
substantially straight or a curved fluxgate can be used to detect
magnetic objects passing across the fluxgate.
[0043] Several substantially straight or curved fluxgates can also
be arranged next to each other, e.g. end-to-end.
[0044] In all of the above embodiments and alternatives thereto,
whether or not the fluxgate is flexible or not, and whether it is
loop shaped or not, it is envisaged that the length of the fluxgate
may be more than 0.5 m, or more than 1 m, more than 2 m, or more
than 5 m. The fluxgate can be longer than 50 m, or at least 100 m,
or at least 1 km, or several hundreds of metres or several
kilometres.
[0045] Whilst the cores 1, 2 may be surrounded by tubes 3, 4, so as
to insulate the cores from the windings 5, 6, it is also possible
to omit the tubes 3, 4, and purely to rely on insulation provided
on the wire material of the windings 5, 6. This insulation may take
the form of a coating or an insulating sleeve.
[0046] In order to provide flexible cores the particular material
described in connection with the first embodiment can be used.
Other materials and dimensions may be used. However, the choice of
materials and dimensions for the cores may be such that the core
material is sufficiently thin so that the magnetic properties are
not substantially altered by stress/strain induced by flexure.
[0047] Although the invention has been described in terms of
various embodiments as set forth above, it should be understood
that these embodiments are illustrative only and that the claims
are not limited to those embodiments. Those skilled in the art will
be able to make modifications and alternatives in view of the
disclosure which are contemplated as falling within the scope of
the appended claims. Each feature disclosed or illustrated in the
present specification may be incorporated in the invention, whether
alone or in any appropriate combination with any other feature
disclosed or illustrated herein.
* * * * *