Introduction

This invention relates to a fluxgate sensor comprising a ferromagnetic core, an excitation coil and a pick-up coil. More specifically, this invention relates to a low intensity (<1 mT, i.e. 10 G), DC or low frequency (<1 kHz) magnetic field transducer of the fluxgate type.

Fluxgate sensors are commonly used for sensing weak magnetic fields such as the geomagnetic field. These fluxgate sensors are often used as transducers for electronic compasses and have applications in numerous devices including compasses, precise magnetometers, gradiometers and like devices. Typically, the fluxgate sensor comprises a ferromagnetic core, an excitation coil wound around the core and a further pick-up coil wound around the core. The ferromagnetic core is periodically excited to magnetic saturation by an alternating current in the excitation coil. A magnetic flux inside the core is a result of interaction of this excitation current and a weak measured magnetic field. The time derivative of this magnetic flux induces a voltage in the pick-up coil which may be measured and evaluated. The voltage measured from the pick-up coil contains signals proportional to the sensed weak magnetic field resulting from its interaction with the strong excitation field in the ferromagnetic sensor core. From this information, it is possible to derive the magnitude and polarity of the vectorial projection of the geomagnetic or other magnetic field with respect to the sensor's axis of sensitivity defined by the core and pick-up coil shape .

Numerous different constructions of fluxgate sensors have been proposed over the years. One such fluxgate sensor is that described in GB Patent Application No. 2,386,197, in the name of Samsung Electro-Mechanics Co., Ltd. This Patent Application describes a fluxgate sensor constructed using printed circuit board techniques. The sensor has a drive (excitation) coil and a pick-up coil. The magnetic core may be an amorphous, permalloy, or supermalloy material in a single strip, a pair of parallel strips, or a rectangular frame. Another fluxgate sensor is described in GB Patent Application No. 2,386,198, also in the name of Samsung Electro-Mechanics Co., Ltd. This Patent Application describes an alternative construction of fluxgate sensor that has a plurality of drive coils and an excitation coil. The fluxgate sensor has a pair of magnetic cores stacked one above the other in two layers of the PCB structure. The fluxgate sensor offers an alternative construction to that proposed in GB 2,386,197.

US Patent Application No. US 2003/0173962, in the name of Samsung Electro- Mechanics Co., Ltd., describes a single axis fluxgate sensor. The fluxgate sensor has separate excitation and pick-up coils and it is formed in a PCB structure. The fluxgate sensor described uses two parallel bar-shaped cores, or alternatively a rectangular core.

US Patent Application No. US 2004/0027121 , in the name of Choi, describes a fluxgate sensor similar in construction to the one disclosed in US 2003/0173962. US 2004/0027121 , however, discloses a fluxgate sensor having windings for both X and Y magnetic field detection. This is commonly referred to as a two-axis fluxgate sensor. The excitation and pick-up coils are again provided by separate coils.

PCT Patent Publication No. WO 9308450, in name of APAC, Inc. describes a method of manufacturing a thin-film compass using a fluxgate sensor. The compass comprises a sensor having flat windings and core components. The windings are separated into excitation and pick-up windings. This Patent Application describes one of the earlier implementations using thin-film microfabrication techniques including winding patterns and vias to construct the compass. Various configurations of core including octagonal and Y-shaped cores are described in this specification.

US Patent Application No. US 2004/0027120, in the name of Rippingale, discloses a fluxgate sensor that has separate excitation and pick-up coils. The magnetic core is embedded into PCB and the excitation coil is implemented using PCB technology.

US Patent No. US 6,404,192, in the name of Asulab S.A., describes a magnetic sensor implemented using CMOS techniques. The sensor has separate drive and pick-up coils. This document describes the use of pulsed excitation signals as well as detecting induced voltage signals at higher harmonics.

In addition to single and two axis fluxgate sensors, various three axis fluxgate sensors have been disclosed in the past. One such three axis fluxgate sensor is described in US Patent No. US 4,462,165 in the name of The Boeing Company. This Patent describes a three axis sensor for detecting the magnetic field in the X, Y and Z axes. The sensor has a simple drive (excitation) winding wound around the core and three separate pick-up coils.

US Patent Application No. US 2002/0056202, in the name of Tamura, describes a three axis sensor constructed from a two axis fluxgate sensor and a Hall effect device for monitoring the third axis perpendicular to the first two axes. The fluxgate sensor has separate drive and pick-up coils. The fluxgate sensor is constructed using PCB techniques and a toroidal core.

Although each of these fluxgate sensors has its own advantages and qualities, it is an object of the present invention to provide a fluxgate sensor that is simple to manufacture and has a simplified construction over at least some of the existing constructions of fluxgate sensors.

Statements of Invention

According to the invention there is provided a fluxgate sensor comprising a ferromagnetic core, an excitation coil and a pick-up coil, characterised in that the excitation coil and the pick-up coil are implemented using a common coil.

By having such a fluxgate sensor, the construction of the sensor is significantly simplified and this will reduce manufacturing complexity and cost of producing the sensor. If the fluxgate sensor is implemented using PCB technology or magnetics on silicone technology, the number of metal layers required is reduced thereby reducing manufacturing complexity and cost. Alternatively, in PCB and magnetics on silicone technology implementations, additional layers equivalent to the number of layers of existing devices could be provided in order to provide more turns for each of the coils and thus reduce power consumption and improve sensitivity of the fluxgate sensor - A -

when compared to known devices.

In one embodiment of the invention the common coil is partitioned into two serially connected sections, each section having a common middle terminal with the other section and an equal number of windings to the other section.

In one embodiment of the invention the fluxgate sensor is a two axis fluxgate sensor having a pair of common coils, each common coil comprising an excitation coil and a pick-up coil.

In one embodiment of the invention the fluxgate sensor is a two axis fluxgate sensor having a common coil, the common coil comprising a pair of excitation coils and a pair of pick-up coils.

In one embodiment of the invention, the common coil is partitioned into four serially connected sections, each section having a common middle terminal with one other section and an equal number of windings to that other section.

In one embodiment of the invention the fluxgate sensor is a three axis fluxgate sensor having three common coils, each common coil comprising an excitation coil and a pick-up coil.

In one embodiment of the invention the fluxgate sensor is a three axis fluxgate sensor having a common coil, the common coil comprising three excitation coils and three pick-up coils.

In one embodiment of the invention, the common coil is partitioned into six serially connected sections, each section having a common middle terminal with one other section and an equal number of windings to that other section.

In one embodiment of the invention some of the common coils sections are arranged mutually perpendicular to each other.

In one embodiment of the invention the core is a racetrack core having a pair of straight sections and a pair of curved sections.

In one embodiment of the invention the common coil is only wound around the pair of straight sections.

In one embodiment of the invention the common coil is wound around both of the straight sections and both of the curved sections.

In one embodiment of the invention the core is a double strip core. Alternatively, the core is a double rod core. In another alternative embodiment, the core is a rectangular core. In a further alternative embodiment the core is a square core.

In one embodiment of the invention the core is a toroidal core. Alternatively, the core is a hexagonal core. In another alternative embodiment, the core is an octagonal core.

In one embodiment of the invention the sensor is realized using printed circuit board (PCB) technology and the common coil is implemented using a plurality of layers of conductive material and vias electrically connecting the layers of the conductive material.

In one embodiment of the invention the sensor is realized using magnetics on silicon technology and the common coil is implemented using a plurality of layers of conductive material and a plurality of interconnections electrically connecting the layers of conductive material.

In one embodiment of the invention the common coil is implemented using two layers of conductive material.

In one embodiment of the invention there is a pair of common coils and the common coils are implemented using four or more layers of conductive material.

In one embodiment of the invention there is provided a fluxgate sensor in which for each common coil, there is provided an extraction circuit, the extraction circuit comprising a pair of filters and an amplifier.

In one embodiment of the invention, each of the pair of filters is a high pass filter.

In one embodiment of the invention the high pass filters each comprise a capacitor and a resistor.

In one embodiment of the invention the amplifier is a summing inverting amplifier.

In one embodiment of the invention the amplifier is implemented using an operational amplifier. Alternatively, the amplifier is implemented using an instrumentation amplifier.

In one embodiment of the invention the sensor is excited with a sine wave excitation current.

In one embodiment of the invention the sensor is excited with short current pulses.

In one embodiment of the invention the fluxgate is a miniature fluxgate. Alternatively, the fluxgate is a micro-fluxgate.

Detailed Description of the Invention

The invention will now be more clearly understood from the following description of some embodiments thereof, given by way of example only in which:

Figure 1 is a diagrammatic representation of a fluxgate sensor known in the art;

Figure 2 is a schematic representation of the first stage of an extraction circuit known in the art;

Figure 3 is a diagrammatic representation of a fluxgate sensor according to the present invention; Figure 4 is a schematic representation of an extraction circuit for use with the fluxgate sensor shown in Fig. 3;

Figures 5(a) to 5(g) inclusive are diagrammatic representations of a plurality of fluxgate sensors used to verify the qualities of the present invention against state of the art configurations;

Figure 6 is a diagrammatic representation of a two axis fluxgate sensor known in the art;

Figure 7 is a diagrammatic representation of a two axis fluxgate sensor according to the present invention;

Figure 8 is an alternative construction of two axis fluxgate sensor according to the present invention;

Figure 9 is a diagrammatic representation of a first embodiment of three axis fluxgate sensor according to the present invention;

Figure 10 is an alternative embodiment of three axis fluxgate sensor according to the present invention;

Figure 11 is a diagrammatic representation of an alternative embodiment of fluxgate sensor according to the present invention; and

Figure 12 is a schematic representation of an extraction circuit for use with the fluxgate sensor shown in Fig. 11.

Figure 13 is a diagrammatic representation of a two axis fluxgate sensor known in the art realized with spiral coils;

Figure 14 is a diagrammatic representation of a two axis fluxgate sensor according to the present invention realized with spiral coils; and Figure 15 is a diagrammatic representation of a fluxgate sensor according to the present invention realized with spiral coils.

Referring to Figure 1 , there is shown a fluxgate sensor that is known in the art, indicated generally by the reference numeral 1 comprising a core 3, an excitation coil 5 wound around the core 3 and a pick-up coil 7 wound around the core 3. The fluxgate sensor 1 further comprises a current source 9 to provide an alternating current to the excitation coil and measurement circuitry (not shown) to measure the voltage V ₁ across the pickup coil. A letter H and an arrow below the letter H indicate the sensor axis of sensitivity i.e. direction in which the magnetic field is measured by this sensor.

In use, an alternating current is supplied from the current source 9 to the excitation coil 5 which drives the core into magnetic saturation. A magnetic flux inside the core is a result of interaction of this excitation current and a weak measured magnetic field. The time derivative of this magnetic flux induces a voltage in the pick-up coil which may be measured and evaluated. The induced voltage across the pick-up coil is measured and this voltage is evaluated as it contains signals proportional to the sensed weak magnetic field component parallel to the sensor axis of sensitivity in which the magnetic sensor is located. The excitation coil 5 and pick-up coil 7 are formed using two separate coil sections electrically insulated from each other. The excitation coil 5 is wound in such arrangement that the sum of excitation magnetic flux created by this coil in the ferromagnetic core is zero in every cross-section plane perpendicular to the axis of sensitivity along the core length. The pick-up coil 7 encompasses the core and its longitudinal axis is identical to the sensitivity axis of the sensor.

Referring to Figure 2, there is shown a schematic representation of an extraction circuit indicated generally by the reference numeral 11 for the fluxgate sensor 1. The extraction circuit 11 comprises a filter 13 which in turn comprises a capacitor 15 and a resistor 17. The extraction circuit 11 further comprises a non-inverting amplifier 19 implemented using an operational amplifier (op-amp) 21 and a pair of resistors 23, 25.

Further signal detection circuitry (not shown) will be provided for the fluxgate sensor which may include a band pass filter, synchronous detection circuitry, or gated differential integrators and a low pass filter. This additional circuitry however is not fundamental to the present invention. Referring to Figure 3, there is shown a fluxgate sensor, indicated generally by the reference numeral 31 , according to the present invention. The fluxgate sensor 31 comprises a core 33 and a common coil 35 wound around the core. The common coil 35 is a single coil that comprises the excitation and pick-up coils of the fluxgate sensor. The common coil 35 is divided into two halves 37, 39 , otherwise referred to as sections, with a common middle terminal 41. The two halves each have an equal number of turns and they are spread along an equal part of the core section length. A current source 43 is provided to deliver an alternating current to the common coil 35. Effectively therefore, the excitation coil is divided into two serially connected sections 37, 39, each wrapped around one part of the sensor core.

In use, the voltage induced in these two sections 37, 39 of serially connected sections of common coil is summed and evaluated. Useful signals normally present (and detected) at the pick-up coil are easily extracted from the voltages induced in these two sections 37, 39 of the excitation coil. By having such a fluxgate sensor, the construction complexity is substantially reduced by removing the transducer pick-up coil and using a single coil as both excitation and pick-up coil at the same time, while maintaining excellent excitation field homogeneity and transducer sensitivity.

By common coil, what is meant is a single, continuous coil that incorporates both the excitation coil and the pick-up coil. The whole coil is used as both the excitation coil and the pick up coil. In the embodiment shown in Figure 3, there is essentially a single three terminal coil instead of two separate coils as is known in the prior art. Alternatively, the whole coil is used as the excitation coil and suitable parts of the coil are used as the pick up coil simultaneously. In the embodiment shown in Figure 11 , there is essentially a single five terminal coil instead of two separate coils as is known in the prior art. In the claims and description, when reference is made to a common coil comprising one or more excitation coils and/or one or more pick-up coils, it will be understood that there is a single common, unitary coil that performs the function of the one or more excitation coils and one or more pick-up coils. The common unitary coil is used as both the excitation coil and the pick-up coil.

The core 33 is a so-called racetrack core having a pair of longitudinal sections 45, 47 and a pair of curved sections 49, 51. One half 37 of the common coil 35 is wrapped around one of the pair of longitudinal sections 47 and the other half 39 of the common coil 35 is wrapped around the other longitudinal section 47 of the racetrack core 33. In an alternative embodiment, one half 37 of the common coil 35 could be wrapped around one of the longitudinal sections 45 and around part of one or both of the curved sections 49 and 51 and the other half 39 of the common coil 35 could be wrapped around the other longitudinal section 47 and around the same amount of the one or both curved sections 49 and 51 so that the coil halves 37 and 39 are placed substantially symmetrically along the core longitudinal symmetry axis (shown as an alternating dotted/dashed line in Fig. 3). In a further alternative embodiment, it is envisaged that the one half 37 of the common coil 35 could be wrapped around one of the curved sections 49 and the other half 39 of the common coil 35 could be wrapped around the other curved section 51 of the core 33.

Referring now to Figure 4, there is shown an extraction circuit for use with the fluxgate sensor shown in Fig. 3. The extraction circuit, indicated generally by the reference numeral 61 , comprises a pair of high pass filters 63, 65 and a summing inverting amplifier 67. The high pass filter 63 comprises a capacitor 69 and a resistor 71 and the high pass filter 65 comprises a capacitor 73 and a resistor 75. The summing inverting amplifier 67 comprises an opamp 77 and a resistor 79. Again further signal detection circuitry following the extraction circuit would be provided including, but not limited to a band pass filter, synchronous detection circuitry, gated differential integrators, or a low pass filter, but these are not central to the present invention.

Referring generally to Figures 5(a) to 5(e) inclusive, there is shown a range of racetrack core fluxgate sensors realised with PCB technology, some with a common coil configuration and some with a coil configuration known in the art. The fluxgate sensors shown in Figures 5(a), 5(c) and 5(d) are realised with PCB technology with a common coil configuration. The fluxgate sensors shown in Figures 5(b) and 5(e) are realised with PCB technology with a configuration known in the art utilising separate coils for the excitation coil and the pick-up coil.

Referring generally to Figures 5(f) and 5(g), there is shown a pair of rectangular core configurations, one with a common coil configuration according to the present invention and the other with a coil configuration known in the art. The fluxgate sensor shown in Figure 5(f) has a common coil configuration whereas the fluxgate sensor shown in Figure 5(g) has the prior art configuration with separate excitation coil and two pick-up coils.

Referring specifically to Figure 5(a), there is shown a fluxgate sensor, indicated generally by the reference numeral 80, comprising a sensor core 83 with coils 85 realised with copper tracks and layer interconnections, all of which are in turn mounted on a printed circuit board (PCB) 87. The core length is 20mm and the coils are arranged in a common coil configuration with 78 turns around the core and a common middle terminal 41.

Referring specifically to Figure 5(b), where like parts have been given the same reference numeral as before, there is shown a fluxgate sensor, indicated generally by the reference numeral 81. The core length is 20mm and the coils 85 are arranged in a configuration known in the art with a separate excitation coil and a separate pick-up coil. There are 55 turns in total, 32 turns for the excitation coil and 23 turns for the pickup coil.

Referring specifically to Figure 5(c), where like parts have been given the same reference numeral as before, there is shown a fluxgate sensor, indicated generally by the reference numeral 82. The core length is 30mm and the coils are arranged in a common coil configuration with 122 turns and a common middle terminal 41.

Referring specifically to Figure 5(d), where like parts have been given the same reference numeral as before, there is shown a fluxgate sensor, indicated generally by the reference numeral 84. The core length is 30mm and the coils are arranged in a common coil configuration with 116 turns and a common middle terminal 41.

Referring specifically to Figure 5(e), where like parts have been given the same reference numeral as before, there is shown a fluxgate sensor, indicated generally by the reference numeral 86. The core length is 30mm and the coils 85 are arranged in a configuration known in the art with a separate excitation coil and a separate pick-up coil. There are 87 turns in total, 50 turns for the excitation coil and 37 turns for the pick- up coil.

Referring specifically to Figure 5(f), where like parts have been given the same reference numeral as before, there is shown a fluxgate sensor, indicated generally by the reference numeral 88. The core length along all sides is 10mm and the coils are arranged in a common coil configuration with 52 turns, 26 turns for the "X" axis and 26 turns for the "Y" axis. In the embodiment shown, there are two common coils, one for the "X" axis and one for the "Y" axis and each common coil is provided with a common middle terminal 41.

It is worthwhile noting that for ease of testing, both sections of the common coil were realised with separate terminals and by appropriate connection of these terminals, the sensor shown schematically in either Figure 3 or Figure 7 is obtained. Furthermore, in relation to the sensor shown in Figure 5(f), the middle terminal may result as a connection of two coil ends on the opposite corners of the core or as a connection of two coil ends on adjacent corners of the core depending on the coil turn orientation.

Referring specifically to Figure 5(g), where like parts have been given the same reference numeral as before, there is shown a fluxgate sensor, indicated generally by the reference numeral 89. The core length along all sides is again 10mm and the coils 85 are arranged in a configuration known in the art with separate excitation coils and separate pick-up coils. There are 88 turns in total, 44 turns for the "X" axis and 44 turns for the "Y" axis. Of the 44 turns for each of the "X" and "Y" axes, there are 28 turns for the excitation coil and 16 turns for the pick-up coil.

The coils of the sensors in Figures 5(a), 5(b), 5(c), 5(d), 5(e) and 5(f) were realized on two metallic layers. The coils of sensor in Figure 5(g) were realized on six metallic layers, making such a sensor more costly. The sensors in Figures 5(a), 5(b), 5(c), 5(d) and 5(e) are single axis sensors capable of sensing the magnetic field in the longitudinal axis of the sensor core. The sensors in Figures 5(f) and 5(g) are dual axis sensors capable of sensing the magnetic field in two perpendicular directions in the plane of the sensor. In each of the sensors shown in Figures 5(a) to 5(g) inclusive, the sensor core 83 comprises a single layer of 25 μm thick high permeability amorphous alloy foil. For verification of the present invention, the common coil configuration sensors were compared against the prior art coil configuration sensors with the same core size. The summary of the results is shown in Table 1 and Table 2 below. All results have been recalculated so that the output signal amplification of the operational amplifier in the case of the common coil sensors is excluded. The RMS (root mean square) of sensor noise in frequency band 0.1 to 10 Hz was measured ("Noise r m s") as well as noise power spectral density at 1 Hz ("Noise PSD@1 Hz") for each sensor layout.

Table 1

Table 2

The sinewave excitation current amplitude was adjusted to achieve maximum sensitivity in all cases and the excitation frequency was 25 kHz in all cases. Referring to Table 1 with single axis sensors, there is shown that while using only two metallic layers for all sensors, the sensors made according to the present invention have higher sensitivity than sensors with the same core size realized with separate excitation and pick-up coils. The hysteresis error for zero measured field in all measured samples was below the fluctuations of the ambient field which reached 0.04% Full Scale (FS) range ±50 μT. The linearity error of the common coil sensors is either smaller or comparable to prior art sensors with the same core size. The noise of the common coil sensors is considerably lower than the noise of prior art sensors. Referring to Table 2 relating to the dual axis sensors, it can be seen that the dual axis sensor which is realized with a common coil according to the present invention with only two metallic layers provides comparable performance to the sensor with six metallic layers and with separate excitation coils and pick-up coils in terms of sensitivity, maximum linearity error and noise properties. The hysteresis error for zero measured field was below 0.04 % FS in both measured sensor samples.

Referring to Figure 6, there is shown a two axis fluxgate sensor known in the art, indicated generally by the reference numeral 91. The fluxgate sensor has an excitation coil 93 which is wound around the entire rectangular core 95 in a continuous fashion and is fed by a current source 97. A pair of pick-up coils 99, 101 are provided and each pick-up coil is wound around the rectangular core back and forth across two opposite sides of the rectangular core 95. The pick-up coils 99, 101 are arranged substantially mutually perpendicular to each other. This is a commonly known configuration of two axis fluxgate sensor that may be used as a relatively simple compass. The excitation coil 93 is used to periodically cause the magnetic core 95 to go into saturation and the pick-up coil 99 is used to determine the magnetic field in which the sensor is placed along the X axis, whereas the pick-up coil 101 is used to determine the magnetic field in which the sensor is placed along the Y axis.

Referring to Figure 7, there is shown a two axis fluxgate sensor according to the present invention indicated generally by the reference numeral 111. The two axis fluxgate sensor 111 comprises a pair of common coils 113, 115, each of the common coils 113, 115 comprises an excitation and pick-up coil similar to those described in relation to Fig. 3 above. A current source 117 delivers alternating current to the common coil 113 and a second current source 119 supplies alternating current to the common coil 115. An extraction circuit (not shown) similar to those shown in Fig. 4 above, would be provided for each common coil. By having such a fluxgate sensor, the number of coils is reduced from three to two compared with the prior art device shown in Fig. 6. Furthermore if the fluxgate sensor shown in Fig. 7 is implemented using PCB technology, or magnetics on silicone technology, the number of layers required would be reduced.

For example, if the fluxgate sensor shown in Figure 6 were implemented using PCB technology, six metallic layers would be required for the windings of the sensor. Two metallic layers for the excitation coil 93, two metallic layers for the pick-up coil 99 and two metallic layers for the second pick-up coil 101. A layer is required for the top and bottom of each wire section and vias connect the top and bottom sections so that the wire is wound around a core 95. However, the two axis fluxgate sensor 111 only requires two layers of metallic layers to be implemented. Additional layers, for example four or six layers or indeed more layers could be provided in order to provide more turns for each of the coils and thus further reduce power consumption and increase sensitivity but this is optional.

Referring to Figure 8, there is shown an alternative construction of two axis fluxgate sensor according to the present invention. The two axis fluxgate sensor, indicated generally by the reference numeral 121 comprises a common coil 123 wound around the whole core 125. The common coil 123 is supplied by a current source 127. The common coil 123 essentially comprises a pair of excitation coils and a pair of pick-up coils. The voltages induced in opposing sections of the common coil 123 are measured and the magnetic field in both the X and Y axis may then be deduced. It is envisaged that the winding of the coil will be divided equally among each leg of the core so that one quarter of the total number of coil windings will be wound around each leg.

Referring now to Figure 9, there is shown a still further alternative construction of fluxgate sensor according to the present invention, indicated generally by the reference numeral 131. The fluxgate sensor 131 comprises a core 133 with core sections in the "X", "Y" and "Z" planes. A common coil winding 135, 137, 139 is provided on each pair of the opposing legs of the core. A single common coil 135 provides both the excitation and the pick-up coil on the opposing leg sections 141a, 141 b of the core 133. These are used to measure the "X" component of the magnetic field. A single common coil winding 137 is wound around the opposing legs 141c, 141 d of the core 133 and is used to determine the "Y" component of the magnetic field. Finally, a common coil winding 139 is wound around the opposing leg sections 141e and 141f of the core and provides both the excitation coil and the pick-up coil in a single common coil 139 to measure the "Z" component of the magnetic field. Each of the common coils 135, 137 and 139 is provided with a current source 143, 145, 147 respectively to provide an alternating current to that common coil. Extraction circuits similar to those shown in Fig. 4 above will be provided for each of the common coils 135, 137, 139.

Referring to Figure 10, there is shown an alternative construction of fluxgate sensor indicated generally by the reference numeral 151 in which like parts have been given the same reference numerals as before. In this instance, the core 133 is similar to that shown in Fig. 9. However, in this instance a single common coil 153 is provided and wound around all legs of 141a-141f of the core 133. A current source 155 provides an alternating current to the common coil 153. The voltages of the coil sections surrounding opposing leg pairs 141a and 141 b, 141c and 141d, 141e and 141f respectively are measured and summed together to determine the magnetic field in the "X" plane, "Y" plane and "Z" plane respectively. In such an embodiment, a single common coil is required to obtain the magnetic field information. It is envisaged that the common coil will be divided so that a sixth of the total coil turns will be wound around each of the legs 141a - 141f of the core 133.

Referring now to Figure 11 , there is shown an alternative embodiment of fluxgate sensor, indicated generally by the reference numeral 161 , where like parts have been given the same reference numeral as before. The sensor 161 comprises a core 33 having a common coil 163 wound thereon. The common coil 163 is divided into four sections 165, 167, 169, 171 , each of which is wrapped around one of the distinctive longitudinal sections 45 and 47 and curved sections 49 and 51 respectively. In the embodiment shown, the straight sections 165, 167 of the common coil are used as an excitation and a pick-up coil and the curved sections 169, 171 of the common coil are used as an excitation coil only. It is envisaged that such a configuration will have the advantage that there will be homogenous excitation around the entire core while having the common coil excitation and pick up coils around the longitudinal sections 45, 47 only.

Referring to Figure 12, there is shown an extraction circuit for use with the fluxgate sensor shown in Figure 11 , indicated generally by the reference numeral 181 , where like parts have been given the same reference numerals as before. The extraction circuit 181 is identical to the circuit shown in Figure 4 with the exception of the coil configuration of the fluxgate sensor.

Referring to Figure 13, there is shown another embodiment of fluxgate sensor, indicated generally by the reference numeral 191 , which is known in the art. The fluxgate sensor 191 comprises a cross-shaped core 193 and a plurality of spiral shaped windings including a spiral shaped excitation winding 195 and a spiral shaped pick-up winding. The spiral shaped excitation winding 195 further comprises four sub- spiral windings, 196(a), 196(b), 196(c) and 196(d), one for each leg 199 of the cross shaped core 193, connected together in series. The spiral shaped pick-up winding comprise two separate windings 197(a) and 197(b), one winding 197(a) for the "X" direction and one winding 197(b) for the "Y" direction. The spiral shaped pick-up windings 197(a) and 197(b) each comprise two pick-up sub-spiral windings, 198(a), 198(b), 198(c) and 198(d), two of which 198(a) and 198(b) are for the "X" direction and two of which 198(c) and 198(d) are for the "Y" direction. Each of the pick-up sub-spiral windings 198(a), 198(b), 198(c) and 198(d) is connected to only one other pick-up sub- spiral winding 198(a), 198(b), 198(c) and 198(d). In this case, pick-up sub-spiral winding 198(a) is connected to pick-up sub-spiral winding 198(b) and pick-up sub- spiral winding 198(c) is connected to pick-up sub-spiral winding 198(d).

Referring to Figure 14, there is shown another embodiment of fluxgate sensor according to the present invention, indicated generally by the reference numeral 201 , comprising a cross-shaped core 203 and a pair of common coils 205, 207 each comprising an excitation coil and a pick up coil. Each of the common coils 205, 207 is spiral shaped and each comprises two spirals connected in series with each other. The two spirals 206(a), 206(b) of the common coil 205 are wound around opposing, parallel legs of the cross-shaped core 203 to detect magnetic field in the "X" direction and the two spirals 208(a), 208(b) of the common coil 207 are wound around opposing, parallel legs of the cross-shaped core 203 to detect magnetic field in the "Y" direction. A pair of current sources 209 are provided, one for each common coil 205, 207. A common middle terminal 210 is provided for each of the common coils 205, 207.

Referring to Figure 15, there is shown another embodiment of fluxgate sensor according to the present invention, indicated generally by the reference numeral 211 , comprising a single strip core 213 and a common coil 215. The common coil 215 comprises a pair of spiral windings, 216(a), 216(b), one of which is wrapped around one end of the single strip core and the other of which is wrapped around the other end of the single strip core. A current source 217 is provided for the common coil. A common middle terminal 219 is further provided. The advantage of the spiral winding as shown in Figures 14 and 15 is that the devices will be easier to manufacture, particularly when micro-fabrication techniques (for example, those techniques involving Silicon) are used. The devices shown in Figures 14 and 15 will have the further advantage over the embodiment shown in Figure 13 in that they will have a reduced number of metallic layers to form the coils. The embodiment shown in Figures 14 and 15 would require only two metallic layers whereas the embodiment shown in Figure 13 would typically require at least three metallic layers unless the available area is divided between excitation and pick-up coils. If the available area is divided between excitation and pick-up coils, there is a performance trade-off and the sensor would have lower sensitivity and higher noise characteristics.

It will be understood that various other core configurations could be used with the common coil configurations described above. For example, a ring (toroidal) core, a square core, a rectangular core, a double strip core, a double rod core, an octagonal core or any other suitably shaped core could be used with the common coil configuration. In all of the embodiments shown, the cross-section of the core is constant along the whole length of the core. It is envisaged that the common coil configuration could also find useful application in wire wound fluxgate sensors and is not limited to application in PCB or magnetics on silicon implementations.

In addition to the above, reference is made throughout to the sensor detecting magnetic fields. It will be understood that in some instances, the fluxgate sensor may be used to detect magnetic fields that are caused by the presence of a current. In such instances, the fluxgate sensor is by extension a current sensor.

In the specification the terms "comprise, comprises, comprised and comprising" and the terms "include, includes, included and including" are all deemed totally interchangeable and should be afforded the widest possible interpretation.

The invention is in no way limited to the embodiments hereinbefore described but may be varied in both construction and detail with the scope of the appended claims.