Note: Descriptions are shown in the official language in which they were submitted.
- ~ 1327995
FIELD OF INVENTION
This invention relates to a fluxgate sensor, and
more particularly relates to a parallel fluxgate sensor.
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BACRGRQU~D OF TE~E INVENTION
Magnetic field sensors can be realized in many
forms. A variety of magnetic fiel~ sensors have been
developed, many of them employing various wound-coil
configurations. Other sensors rely on different physical
principles. Thus, Hall-effect sensors, Xagnetoresistive
sensors and Magnetic wire sensors, as well as a version of
an electron tube have been used. The present invention is
concerned with wound-coil sensors.
A fluxgate sensor is used to detect and measure
a magnetic field. Early fluxgate sensors consisted of a
length of wire wound on a hollow tubular bobbin with a
magnetically sensitive material placed in the centre of
the bobbin, to form the core of the sensor. When the
sensor is driven into saturation by alternate positive and
negative pulses, the response of the sensor coil is a
measure of the magnetic field strength parallel to the
sensor~s axis. Consequently, this type of sensor is
referred to as a ~parallel fluxgate sensor~. Sensors can
be configured to provide an llorthogonal fluxgate sen60r".
Such sensors are u~ed in a ~'magnetometer~, which
is a device that measures the strength of a magnetic
field, and a "gradiometer" which is a device that measures
the difference in magnetic field strength or gradient of
the magnetic field.
There are several practical design
considerations when using a sensor of the type ~ust
described. Principal considerations are sensitivity and
balance. Balance i8 the most critical design consideration
for a gradiometer, since several sensors are used to
determine the strength of the magnetic field gradient.
Sensitivity i8 also important, but it is much more easily
' ~ controlled. If the sensors of a magnetic gradiometer are
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not exactly balanced, then the gradiometer can give a
false output, i.e. it can indicate that there is a
difference in the magnetic field when no such difference
exists, or alternatively fail to detect if a magnetic
difference does occur. Similarly, a device must be
sensitive enough for its intended use.
In all known conventional sensors, a sensor has
a coil wound about a bobbin and a core located in the
bobbin. The core entirely fills the core of the bobbin and
` 10 does not extend beyond the ends of the core. The core
cannot be manipulated, and it is conventional to provide
balance and compensation coils to achieve a balance
between the sensor coils.
SUMMARY OF THE_PRESENT INVENTION
In accordance with the present invention, there
is provided a parallel fluxgate sensor, which comprises:
a coil wound on the body around the axis; and an
' adjustable core formed from an elongate strip of
.i ferromagnetic material, said strip being generally
rectangular and of uniform thickness and said core having
~ a generally zig zag profile; wherein said core is located
!,j in said bore and said core extends along the bore axis
beyond either end of said coil, and wherein the position
of said core relative to said coil can be adjusted to
j~ 25 balance the sensor relative to another similar sensor.
;i It has been discovered that, by increasing the
i length of the core material so that it extends out beyond
;I the ends of the coil, sensitivity of the sensor to a
magnetic field is greatly increased. It is believed that
this is because the coil's magnetic field pattern lines
are more easily perturbed, as compared to a standard
fluxgate sensor where the core material does not extend
i beyond the ends of the coil.
' A further advantage of the core shape of the
present invention is that in a preferred form described
below, it can be used to balance a pair of fluxgate
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sensors. In a gradiometer, one has two separate sensors, fordetermining the magnetic field gradient.
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Known fluxgate sensors employ multiple windings
around the core material. In general, there is a drive
winding, a sense winding and a balance winding, which
`~makes for a complex structure. The drive winding is used
-5 to saturate the core material in a periodic fashion. The
sense winding is used to detect the response of the core
material to the excitation by the drive winding and the
external magnetic field. The balance winding is used to
match the sensor characteristics of the two sensors, to
ensure that no incorrect reading is given in the complete
gradiometer. If the balance winding was not used, then
almost certainly due to usual tolerances etc., the two
;sensors would detect a difference in magnetic field even
where one did not exist.
15In a preferred form of the present invention,
~ the core is formed from a non-circular section strip and
; is mounted in the bore 80 as to be rotatable about its
axis. Then a small twist of the core can be used to ad~ust
each sensor. As well, the cores can be mounted for
longitudinal or axial ad~ustment. Consequently, the
sensors according to the present invention do not require
balance winding and a minor physical mechanical ad~ustment
~ of each or one core will enable the two sensors of the
`~ device to be matched perfectly.
~ 25Additionally, it is possible for the sensor of
f~ the present invention to have ~ust a single winding,
' rather than separate sense and drive windings. When
combined with appropriate circuitry, one can still obtain
the nece~sary detection of magnetic fields etc.
3 0 BRIE~ 5C~IpTION OF THE p~AWIN~S
For a better understanding of the present
invention and to show more clearly how it may be carried
into effect, reference will now be made, by way of
example, to the accompanying drawings, in which:
35Figure 1 is a perspective view of a tip of a
gradiometer in accordance with the present invention;
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Figures 2a and 2b are perspective views of a
~ conventional fluxgate sensor and a fluxgate sensor in
- accordance with the present invention showing the magnetic
field patterns;
5Figures 3a and 3b are perspective views of the
cores of the two sensors of the gradiometer of Figure l;
and
Figure 4 is a schematic view of a circuit for
use with sensors of the present invention.
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D~SCRIPTION OF TE~ PREFERR~D EMBODIMENT
In Figure 1, there is shown the tip of a
gradiometer device, the tip being indicated at 1. The rest
of the device can be largely conventional, and hence is
not shown in detail. In the tip 1, there are two parallel
fluxgate sensors indicated at 2 and 4. The sensors 2, 4
are aligned along a common axis 6.
Referring to Figures 2a and 2b, Figure 2a shows
a conventional sensor indicated at 10. This ~ensor 10
includes a coil body 12 which is generally cylindrical,
and on which a coil 14 is wound. A core 16, which is in
the form of a generally rectangular slab is located within
the body 12. It i8 to be noted that the core 16 does not
extend beyond the ends of the coil 14, and indeed is of
substantially the same length as the coil 14.
25The magnetic field line pattern for this
conventional sensor 10 are indicated at 18. It can be seen
that the field line pattern is relatively dense at the
ends of the core 16 so that it is not overly sensitive to
, external influences.
j 30Figure 2b show~ one form of a sensor in
,` accordance with the present invention. The ~ensor 20 again
' ha8 a coil body 22 and a coil 24 mounted on it. Here, a
~ core 26 is again generally rectangular but the core 26
tl extends beyond the ends of the coil 24 as indicated by
portions 28 outside of the coil 24.
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This gives a field pattern 29 as ~hown. The
- field pattern is not so concentrated at the ends of the
core 26, and hence is more susceptible to influence by an
external magnetic field.
Referring to Figures 3a and 3b, there are shown
two profiles for the sensors 2 and 4 of Figure 1,
respectively.
For the first sensor, sensor 2, a core 30 is
formed from a rectangular strip material. The core 30
includes tips 31 and 32 which are at 90 degrees relative
to one another. Extending from the tip 31, there are a
series of zig zag or V-shaped portions 34.
The core for the sensor 4 is shown in Figure 3b
and indicated at 36. It corresponds closely to the core
30. It has tips 37, 38 which are at right angles to one
another and arranged at 45 to an axis of the device 1 and
` ~ hence at 45 to the tips 31, 32 of the core 30. Again,
various zig zag or ~-shaped portions 39 are provided
between the tips 37, 38.
Whilst the pairs of tips of each core 30 or 36
need not be at 90 to one another, it has been discovered
this angle gives the best benefits for balancing.
Similarly, the angle of 45 between the tips of the two
sensor cores is not essential, but it has been found to be
optimum.
The zig-zag profile provides for economic
manufacture, whilst providing a core that is uniformly
distributed on either side of the axis. The core can be
' mounted in the coil body and manipulated both axially and
radially. An alternative arrangement would be to provide
-I a holder for the core in the form of a tube located within
the bore of the coil body, so that the holder and core are
manipulated together. Then, the core could be, for
example, a flat strip.
In use, the cores are manipulated by twisting
each core, or one of the cores, slightly to achieve a
balanced condition in certain strategic directions. As
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well, the cores can be adjusted longitudinally or axially.
The cores are then secured in position by adhesive or some
mechanical securing means.
Reference will now be made to Figure 4 which
shows a schematic block diagram of a circuit, indicated
generally at 50, for a gradiometer in accordance with the
present invention. One end of each of the coils of the
sensors 2, 4 is connected through to ground, as indicated
at 52. The other ends of the coils of the sensors 2, 4
serve as inputs and are connected to the outputs of
drivers 54, which in turn are driven by an oscillator 56.
The inputs of the sensors are also connected
through rectifier and average units 58, 59. The rectifier
and average unit 58 for the first sensor 52 is connected
- 15 directly through to an adder 60. The other unit 59 is
~ connected through an inverting amplifier 62 to the adder
- 60. Consequently, the adder serves to determine the
difference between the signal at the inputs of the two
sensors 2, 4. This in turn is related to the difference in
the magnetic field sensed by the two sen~ors which gives
a determination of the gradient in the magnetic field.
~ he output of the adder 60 is connected through
an absolute value unit 64 through to a voltage controlled
oscillator 66. The voltage controlled oscillator 66 in
turn i8 connected to an audio amplifier 68 which drives an
audio transducer 70. Thus, the output of the audio
transducer 70 is dependent upon the gradient of the
magnetic field sensed by the sensors 2, 4.
Each sensor when coupled to the electronic drive
and sense circuitry will have a unique zero offset and
gain characteristic. Electrically, the sensor(s) are
matched at various Inulll directions to ensure the
response from both sensors under all conditions are the
same. The electrical nulling does not effect the sensors
directly; it ad~usts the electronic circuits to compensate
for a difference in sensor response.
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The electrical balance circuits are standard,
and hence are not described here. They compensate for both
the sensor differences as well as the circuit component
deviations. The mechanical balance provided by the cores
` 5 compensates for the true mechanical differences between
the two sensors. The sensors must be aligned as closely as
possible in all three cartesian directions in order to
cancel common fields correctly.
A common fluxgate sensor requires that the core
be forced into alternate positive and negative saturation.
This fact makes a squarewave excitation voltage ideal
since saturation will ~e obtained swiftly and completely.
However, several different waveforms have been used
- including a sinewave, triangle wave and some very complex
compo~ite waves. For the present invention, a square wave
is suggested, although this i8 not essential.
When the sensor core reàches saturation, the
response of the sen~or will include a component from the
ambient magnetic field present. The phenomenon is well
known, although the fluxgate mechanism i8 not completely
- understood.
It has been found that contrary to conventional
i teaching, it is not essential for the sensor cores to be
fully saturated during their operation.
25In the present cores 26, there is not complete
saturation, particularly at the tips. This behaviour has
been established by observing the responqe of the sensor
to the driving waveform. If the sensors were saturated,
then the response waveform would not resemble the driving
; 30 waveform since it would be translated by the non-linear
portion of the B-H curve of the sensor. Some saturation
does occur, but it is not complete.
The present sensor 20 must be driven close, if
Inot completely, into saturation for proper operation. The
,f35 circuit 50 exploits the fact that saturation in the entire
core is not required for proper fluxgate operation. For an
instrument of this class, the sensitivity gained by
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extending the core exceeds the fact that the entire core
is not saturated perfectly and does not provide an ideal
response.
It is not presently known if there is an ideal
ratio of sensor core length to sensor coil length. It has
been found, mainly through experimentation, that extending
the core does increase the sensitivity of the sensor when
- driven and detected in the same manner as the equivalent
core with a coil-sized core. For sen~ors tested, it has
10 been determined that having a core-to-coil ratio of 2:1 i6
optimum given the drive/~ense circuitry, frequency, etc.
employed.
It is known to use the drive winding as the
sense winding as well. A separate sense winding exists in
15 most sensors to increase sensitivity and optimise coil
characteristics. The drive winding can be optimised for
saturating the sensor core and requiring a small amount of
power *o achieve this. The sense winding can be optimised
to inçrease the sensed signal by its turns ratio with
20 respect to the drive winding, and at the same time, have
good impedance characteristic6 60 circuitry interface is
simple.
However, it i~ conventional to provide ~eparate
;~ balance and compensating windings. The present invention
s 25 eliminates the need for a "balance" winding and a
~compensating" windlng which are commonly used for the
matching or balancing of two fluxgate sensors in a
gradiometer. Our matching is accomplished mainly through
the mechanical matchLng o the senoor~ a~ descrLbed above.
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