A magnetic field sensor

Abstract

A magnetic field sensor includes a magnetic sensing element in the form of a coil surrounding magnetic material . Electric current is supplied to the coil in opposite directions , depending on the state of switches operated at times To, for a first direction , and Ts, for a reverse current direction . A voltmeter measures voltages across the coil during saturation , namely at least V1 at time T1, after T0, and V2 at time T2, after T5, with T1 – T0 = T2 – T5 = predetermined ΔT. A processor indicates V1 -V2, the magnitude and sign of which respectively indicate the strength and direction of a component of the ambient magnetic field.

Description

1 A magnetic field sensor

[0001] The present invention relates to a magnetic field sensor for determining the strength and / or direction of a component of an ambient magnetic field, comprising electrical circuitry which incorporates a magnetic sensing element, a voltmeter connected in the circuitry to provide a measure of the voltage across the magnetic sensing element, an electrical energy supply connector connected to the magnetic sensing element to supply an electrical current which passes through that element when the sensor is in use, and switches to reverse the direction of the flow of electrical current through the magnetic sensing element between successive measurements of the said voltage by the said voltmeter.

[0002] In one such magnetic field sensor which has already been proposed, the magnetic sensing element comprises a flux meter in which the current is passed through a primary winding, and the voltage is measured across a secondary winding. This requires a measurement of peak voltages which is of limited accuracy.

[0003] Another such magnetic field sensor is disclosed in US 5757184. In that sensor, a measurement is made of the time it takes for a response waveform to rise to a pre-determined threshold, and this measurement also is of limited accuracy.

[0004] The present invention seeks to provide a remedy.

[0005] Accordingly, a first aspect of the present invention is directed to a magnetic field sensor having the construction set out in the opening paragraph of the present specification, characterised in that the magnetic sensing element comprises a coil surrounding magnetic material, in that the electrical energy source connector and the said switches are connected to the said coil, in that the voltmeter is connected to measure the voltage across the said coil, and in that the circuitry further comprises a timer connected within the circuitry to cause the voltmeter to provide measurements <com:Math><semantics>V1<annotation encoding="application / x-tex">V_1< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>V2<annotation encoding="application / x-tex">V_2< / annotation>< / semantics>< / com:Math> respectively at times <com:Math><semantics>T1<annotation encoding="application / x-tex">T_1< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T2<annotation encoding="application / x-tex">T_2< / annotation>< / semantics>< / com:Math> which occur after successive operations of the switches respectively at times <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T5<annotation encoding="application / x-tex">T_5< / annotation>< / semantics>< / com:Math>, to initiate the passage of electrical current through the coil in respective opposite directions, such that <com:Math><semantics>T1−T0=T2−T5=ΔT<annotation encoding="application / x-tex">T_1 - T_0 = T_2 - T_5 = \Delta T< / annotation>< / semantics>< / com:Math>, where <com:Math><semantics>ΔT<annotation encoding="application / x-tex">\Delta T< / annotation>< / semantics>< / com:Math> is a predetermined period of time, and a processor connected in the circuitry to provide an indication of the value of <com:Math><semantics>V1−V2<annotation encoding="application / x-tex">V_1-V_2< / annotation>< / semantics>< / com:Math>, the magnitude of which provides an indication of the strength of the said component, and the sign of which provides an indication of the direction of the said component.

[0006] The timer may be a part or parts of the processor, which may be a microprocessor. This enables economy of manufacture.

[0007] The switches may be parts of the processor or microprocessor. Again, this affords an economy of parts in the manufacture of the sensor.

[0008] The timer may be provided with a memory in which is retained the value of <com:Math><semantics>ΔT<annotation encoding="application / x-tex">\Delta T< / annotation>< / semantics>< / com:Math>. Again, this affords an economy of parts.

[0009] The voltmeter may comprise an amplifier connected in series with an analogue-to-digital converter between the magnetic sensing element and the processor or microprocessor, and the voltmeter may be made as an integral part or parts of the microprocessor, again for economy of parts.

[0010] The present invention extends to a compass provided with a magnetic field sensor in accordance with the present invention. 3

[0011] According to a second aspect of the present invention, there is provided a method of determining the strength and / or direction of a component of an ambient magnetic field, comprising obtaining a measure of the voltage <com:Math><semantics>V1<annotation encoding="application / x-tex">V_1< / annotation>< / semantics>< / com:Math> across a magnetic sensing element, in the form of a coil surrounding magnetic material, of electrical circuitry of a magnetic field sensor by means of a voltmeter of the electrical circuitry at a time T<com:Sub>1< / com:Sub> after a time <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> when an electrical current through the coil in a first direction is initiated by switches of the circuitry, using a timer to equate <com:Math><semantics>T1−T0<annotation encoding="application / x-tex">T_1-T_0< / annotation>< / semantics>< / com:Math> to a predetermined period <com:Math><semantics>ΔT<annotation encoding="application / x-tex">\Delta T< / annotation>< / semantics>< / com:Math>, and obtaining a measure of the voltage <com:Math><semantics>V2<annotation encoding="application / x-tex">V_2< / annotation>< / semantics>< / com:Math> across the coil at a time <com:Math><semantics>T2<annotation encoding="application / x-tex">T_2< / annotation>< / semantics>< / com:Math> after a time <com:Math><semantics>T5<annotation encoding="application / x-tex">T_5< / annotation>< / semantics>< / com:Math> when an electrical current through the coil in the direction opposite to the said first direction is initiated by the switches, equating <com:Math><semantics>T2−T5<annotation encoding="application / x-tex">T_2-T_5< / annotation>< / semantics>< / com:Math> to the predetermined period <com:Math><semantics>ΔT<annotation encoding="application / x-tex">\Delta T< / annotation>< / semantics>< / com:Math>, and using a processor of the circuitry to provide an indication of the value of <com:Math><semantics>V1−V2<annotation encoding="application / x-tex">V_1-V_2< / annotation>< / semantics>< / com:Math>, the magnitude of which provides an indication of the strength of the said component, and the sign of which provides an indication of the direction of the said component.

[0012] The present invention extends to such a method using a sensor or a compass as set out in any one of the foregoing paragraphs between the one commencing "According to a first aspect" and the one commencing "According to a second aspect".

[0013] Examples of a sensor, compass and method embodying the present invention will now be described in greater detail with reference to the accompanying drawings, in which:

[0014] Figure 1 shows a circuit diagram of a sensor embodying the present invention,

[0015] Figure 2 shows a perspective end view of a magnetic sensing element of the circuitry shown in Figure 1; and

[0016] Figures 3 to 5 show respective explanatory graphs.

[0017] The magnetic field sensor 10 shown in Figure 1 comprises an electrical energy source connector 12 for connecting the sensor 10 to a source of electrical energy (not shown) at voltage V, and an earth connector 14 for connecting the sensor 10 to earth or equivalent. Between the connectors 12 and 14, from the connector 12 to the connector 14, are two resistances R<com:Sub>1< / com:Sub> and R<com:Sub>2< / com:Sub> connected in series with one another, followed by two switches S<com:Sub>1< / com:Sub> and <com:Math><semantics>S2<annotation encoding="application / x-tex">S_2< / annotation>< / semantics>< / com:Math> also connected in series with one another.

[0018] The interconnection 16 between the resistors R1 and R2 is connected to the input of an amplifier 18 the output from which is connected to the input of an analogue to digital converter 20, the output from which is connected to an input of a microprocessor 22 via a timer 24. An output 26 of the microprocessor 22 provides a digital output which can be connected to any other digital system, for example a meteorological system (not shown), or a navigation system (not shown).

[0019] Connected across the interconnection 16 and the interconnection 28 between the switches <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>S2<annotation encoding="application / x-tex">S_2< / annotation>< / semantics>< / com:Math> is a magnetic sensing element 30, via switches <com:Math><semantics>S3<annotation encoding="application / x-tex">S_3< / annotation>< / semantics>< / com:Math>, <com:Math><semantics>S4<annotation encoding="application / x-tex">S_4< / annotation>< / semantics>< / com:Math>, <com:Math><semantics>S5<annotation encoding="application / x-tex">S_5< / annotation>< / semantics>< / com:Math>, and <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math>.

[0020] As shown in Figure 2, the magnetic sensing element 30 comprises a coil 32 wound around and along a hollow cylindrical non-electrically conductive non- magnetic former 34 within and along the interior of which extends a solid cylindrical core 36 of a ferromagnetic material such as iron. 5

[0021] The coil has opposite ends 38 and 40 respectively connected to the interconnections (a) between switches S<com:Sub>3< / com:Sub> and S<com:Sub>5< / com:Sub> and (b) between switches S<com:Sub>4< / com:Sub> and <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math>. One side of each of the switches S3 and S4 is connected to the interconnection 16, and the other sides of the switches <com:Math><semantics>S3<annotation encoding="application / x-tex">S_3< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>S4<annotation encoding="application / x-tex">S_4< / annotation>< / semantics>< / com:Math> are connected respectively to the ends 38 and 40 of the coil 32. Similarly, one side of each of the switches S<com:Sub>5< / com:Sub> and S<com:Sub>6< / com:Sub> is connected to the interconnection between the switches <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>S2<annotation encoding="application / x-tex">S_2< / annotation>< / semantics>< / com:Math>, and the other sides of the switches S<com:Sub>5< / com:Sub> and S<com:Sub>6< / com:Sub> are connected respectively to the ends 38 and 40 of the coil 32.

[0022] All the switches <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> to <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math> are simple on-off switches connected to the microprocessor 22 for control thereby.

[0023] The magnetic sensing element 30 is one of three (the other two of which are not shown) each mounted orthogonally relative to the other two, and each with circuitry as shown in Figure 1 but sharing a common microprocessor 22 which addresses the three elements cyclically. Signals from the three sensors taken together are used in a manner known in itself to provide an indication of the strength and direction of the ambient magnetic field, as described for example with reference to Figure 20 of US 5757184.

[0024] With the connector 12 connected to a source of energy so that it is held at voltage <com:Math><semantics>V<annotation encoding="application / x-tex">V< / annotation>< / semantics>< / com:Math>, at time <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> the microprocessor 22 sets switches S2, S4 and S5 closed, and switches <com:Math><semantics>S3<annotation encoding="application / x-tex">S_3< / annotation>< / semantics>< / com:Math>, <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> open. After a predetermined period of time set by the microprocessor 22, at time <com:Math><semantics>T3<annotation encoding="application / x-tex">T_3< / annotation>< / semantics>< / com:Math>, switch <com:Math><semantics>S2<annotation encoding="application / x-tex">S_2< / annotation>< / semantics>< / com:Math> remains closed and switch <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> remains open, but the conditions of all the other switches <com:Math><semantics>S3<annotation encoding="application / x-tex">S_3< / annotation>< / semantics>< / com:Math> to <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math> are reversed, so that simultaneously switches S<com:Sub>3< / com:Sub> and S<com:Sub>6< / com:Sub> are closed, and switches S4 and S5 are opened. At time T4, being a predetermined period after time T<com:Sub>3< / com:Sub> equal to or about equal to the period between times <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T3<annotation encoding="application / x-tex">T_3< / annotation>< / semantics>< / com:Math>, the conditions of switches <com:Math><semantics>S3<annotation encoding="application / x-tex">S_3< / annotation>< / semantics>< / com:Math> to <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math> remain unchanged, but the conditions of switches <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>S2<annotation encoding="application / x-tex">S_2< / annotation>< / semantics>< / com:Math> are reversed, so that switch <com:Math><semantics>S2<annotation encoding="application / x-tex">S_2< / annotation>< / semantics>< / com:Math> is opened and switch <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> is closed.

[0025] At time <com:Math><semantics>T5<annotation encoding="application / x-tex">T_5< / annotation>< / semantics>< / com:Math>, being after time <com:Math><semantics>T4<annotation encoding="application / x-tex">T_4< / annotation>< / semantics>< / com:Math> by a period also about equal to that between times <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T3<annotation encoding="application / x-tex">T_3< / annotation>< / semantics>< / com:Math>, the conditions of switches <com:Math><semantics>S3<annotation encoding="application / x-tex">S_3< / annotation>< / semantics>< / com:Math> to <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math> again remain unchanged, but the conditions of the switches <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>S2<annotation encoding="application / x-tex">S_2< / annotation>< / semantics>< / com:Math> are reversed again back to the condition they had between times <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T3<annotation encoding="application / x-tex">T_3< / annotation>< / semantics>< / com:Math>.

[0026] Upon completion of a further period after time <com:Math><semantics>T5<annotation encoding="application / x-tex">T_5< / annotation>< / semantics>< / com:Math>, about the same as that between times <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T2<annotation encoding="application / x-tex">T_2< / annotation>< / semantics>< / com:Math>, at time <com:Math><semantics>T6<annotation encoding="application / x-tex">T_6< / annotation>< / semantics>< / com:Math>, the conditions of the switches <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>S2<annotation encoding="application / x-tex">S_2< / annotation>< / semantics>< / com:Math> remain unaltered, but those of switches <com:Math><semantics>S3<annotation encoding="application / x-tex">S_3< / annotation>< / semantics>< / com:Math> to <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math> are reversed back to the condition they had between times <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T3<annotation encoding="application / x-tex">T_3< / annotation>< / semantics>< / com:Math>.

[0027] Upon completion of a further such period after time <com:Math><semantics>T6<annotation encoding="application / x-tex">T_6< / annotation>< / semantics>< / com:Math>, at time <com:Math><semantics>T7<annotation encoding="application / x-tex">T_7< / annotation>< / semantics>< / com:Math>, the conditions of switches <com:Math><semantics>S3<annotation encoding="application / x-tex">S_3< / annotation>< / semantics>< / com:Math> to <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math> remain unchanged, but those of switches S<com:Sub>1< / com:Sub> and S<com:Sub>2< / com:Sub> are once again reversed. Finally, after a further such period, the conditions of switches S<com:Sub>3< / com:Sub> to S<com:Sub>6< / com:Sub> remain unchanged, but the conditions of switches <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>S2<annotation encoding="application / x-tex">S_2< / annotation>< / semantics>< / com:Math> are reversed to re- commence this cycle of switching the switches <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> to <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math>.

[0028] It will be appreciated that the resulting voltage applied across the combination of resistor R<com:Sub>1< / com:Sub> and the magnetic sensing element 30, with the direction of the voltage being dependent on the direction of this applied voltage relative to the coil 32, is as shown in Figure 3.

[0029] During successive cycles, the current flowing through the coil 32 is as shown in Figure 4. Which of the troughs and peaks shown at times <com:Math><semantics>T3<annotation encoding="application / x-tex">T_3< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T6<annotation encoding="application / x-tex">T_6< / annotation>< / semantics>< / com:Math> actually occurs, at around the time when the direction of applied voltage is reversed, depends upon the strength and direction of the ambient magnetic field. 7

[0030] Thus, during the period from <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> to <com:Math><semantics>T3<annotation encoding="application / x-tex">T_3< / annotation>< / semantics>< / com:Math>, the negative current passing through the coil 32 steadily increases as more and more of the respective magnetic fields of the magnetic domains in the core 36 align themselves with the magnetic field generated within the coil 32 by the electrical current passing through it. The current is considered negative because of the direction of flow of the current through the coil at this stage, so that an increase in the negative current is indicated by a fall in the plot of current as a function of time in the graph shown in Figure 4. When the magnetisation of the core 36 is saturated, the negative electrical current increases at an even greater rate. The point in time at which this happens will be dependent upon the ambient magnetic field, because that field will itself cause a certain degree of alignment or anti-alignment of the magnetic fields of those domains.

[0031] At the time <com:Math><semantics>T3<annotation encoding="application / x-tex">T_3< / annotation>< / semantics>< / com:Math>, the direction of the current through the coil 32 is reversed, and the plot of current against time reverses so that the plot of current versus time at this stage is more or less symmetrical about time T<com:Sub>3< / com:Sub>. This is so until the magnetic energy stored in the core 36 is almost zero at time T4 when the ends of the coil 32 are connected across the resistor R<com:Sub>2< / com:Sub> to completely remove the energy at which point the current passing through the coil 32 is zero.

[0032] The next half of the cycle of switching results in a similar process but with the current passing in a positive direction through the coil 32, so that the plot of current versus time in the right-hand side of Figure 4 is a vertical inversion of the plot of current versus time in the left-hand side of that Figure. However, for any non-zero value of the ambient magnetic field, it will be appreciated that the size of the negative peak and the positive peak at times <com:Math><semantics>T3<annotation encoding="application / x-tex">T_3< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T6<annotation encoding="application / x-tex">T_6< / annotation>< / semantics>< / com:Math> will be different for any actual plot of current versus time. In other words, the time it takes for saturation of the core 36 to be reached will be different for one direction of current flow than it is for the opposite direction of current flow.

[0033] Figure 5 shows a graph of the voltage at the interconnection 16 plotted against time. Which of the steeper falls at the right hand side of the plot actually occurs again depends upon the strength and direction of the ambient magnetic field. Because of the linearity of the function, and the spacing between successive steep lines corresponding to equal increments of strength in the ambient magnetic field, it can be seen that the value of the voltage at a time passed by all the steep lines varies in proportion to the strength of that field, in fact in inverse proportion, so that the stronger the ambient magnetic field, the lower the measured voltage at that time.

[0034] The actual times at which the voltage at interconnection 16 is made are at times <com:Math><semantics>T1<annotation encoding="application / x-tex">T_1< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T2<annotation encoding="application / x-tex">T_2< / annotation>< / semantics>< / com:Math>, respectively at equal periods of time <com:Math><semantics>ΔT<annotation encoding="application / x-tex">\Delta T< / annotation>< / semantics>< / com:Math> after times <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T5<annotation encoding="application / x-tex">T_5< / annotation>< / semantics>< / com:Math>, respectively just before times <com:Math><semantics>T3<annotation encoding="application / x-tex">T_3< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T6<annotation encoding="application / x-tex">T_6< / annotation>< / semantics>< / com:Math> when the applied voltage is reversed.

[0035] The period <com:Math><semantics>ΔT<annotation encoding="application / x-tex">\Delta T< / annotation>< / semantics>< / com:Math> is stored in the timer 24, which is restarted on every occurrence of the times <com:Math><semantics>T0<annotation encoding="application / x-tex">T_0< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>T5<annotation encoding="application / x-tex">T_5< / annotation>< / semantics>< / com:Math> in each cycle. When the count in the timer 24 equals <com:Math><semantics>ΔT<annotation encoding="application / x-tex">\Delta T< / annotation>< / semantics>< / com:Math>, the value of the voltage at interconnection 16 is read by the microprocessor 22 from the input it receives from the analogue to digital converter 20. The microprocessor 22 then calculates the value of the difference between two successive readings. The magnitude of the difference between two successive readings gives the strength of the component of the ambient magnetic field which is in alignment with the axis of the coil 32, and the sign of the difference gives the direction of that component.

[0036] With measurements from the three mutually orthogonal magnetic sensing elements 30, the microprocessor 22 programmed in a manner known in itself to cause the display 26 to display the direction of North, being the direction in which the field is indicated to be strongest and of the appropriate sign, the sensor acts as a compass.

[0037] Numerous variations and modifications to the illustrated sensor may occur to the reader without taking the resulting construction outside the scope of the present invention. For example, the switches <com:Math><semantics>S1<annotation encoding="application / x-tex">S_1< / annotation>< / semantics>< / com:Math> to <com:Math><semantics>S6<annotation encoding="application / x-tex">S_6< / annotation>< / semantics>< / com:Math>, the resistors <com:Math><semantics>R1<annotation encoding="application / x-tex">R_1< / annotation>< / semantics>< / com:Math> and <com:Math><semantics>R2<annotation encoding="application / x-tex">R_2< / annotation>< / semantics>< / com:Math>, the amplifier 18 and the analogue to digital converter 20 could all be incorporated in the microprocessor 22. .

Claims

1. A magnetic field sensor for determining the strength and / or direction of a component of an ambient magnetic field, comprising electrical circuitry which incorporates: a magnetic sensing element comprising a coil surrounding magnetic material, a voltmeter connected in the circuitry to provide a measure of the voltage across the magnetic sensing element, an electrical energy supply connector connected to the magnetic sensing element to supply an electrical current which passes through that element when the sensor is in use, and switches to reverse the direction of the flow of electrical current through the magnetic sensing element (30) between successive measurements of the said voltage by the said voltmeter, the electrical energy source connector and the said switches being connected to the said coil, wherein the voltmeter is connected to measure the voltage across the said coil, and the circuitry further comprises a timer connected within the circuitry to cause the voltmeter to provide measurements V 1 V_1 and V 2 V_2 respectively at times T 1 T_1 and T 2 T_2 which occur during saturation of the magnetic material after successive operations of the switches respectively at times T 0 T_0 and T 5 , to initiate the passage of electrical current through the coil in respective opposite directions, such that T 1 − T 0 = T 2 − T 5 = Δ T T_1 - T_0 = T_2 - T_5 = \Delta T , where Δ T \Delta T is a predetermined period of time, and a processor is connected in the circuitry to provide an indication of the value of V 1 − V 2 V_1-V_2 , the magnitude of which provides an indication of the strength of the said component, and the sign of which provides an indication of the direction of the said component.

2. A magnetic field sensor according to claim 1, characterised in that the timer is a part or parts of the processor.

3. A magnetic field sensor according to claim 1 or claim 2, characterised in that the switches are parts of the processor.

4. A magnetic field sensor according to any one of claims 1 to 3, characterised in that the voltmeter is a part or parts of the processor.

5. A magnetic field sensor according to any one of claims 1 to 4, characterised in that the timer is provided with a memory in which is retained the value of Δ T \Delta T .

6. A magnetic field sensor according to any one of claims 1 to 5, characterised in that the voltmeter comprises an amplifier connected in series with an analogue-to-digital converter between the magnetic sensing element and the processor.

7. A magnetic field sensor according to any one of claims 1 to 6, wherein the processor is a microprocessor.

8. A compass provided with a magnetic field sensor as claimed in any one of claims 1 to 7.

9. A method of determining the strength and / or direction of a component of an ambient magnetic field, comprising obtaining a measure of the voltage V 1 V_1 across a magnetic sensing element, in the form of a coil surrounding magnetic material, of electrical circuitry of a magnetic field sensor by means of a voltmeter of the electrical circuitry at a time T 1 T_1 during saturation of the magnetic material after a time T 0 T_0 when an electrical current through the coil in a first direction is initiated by switches of the circuitry, using a timer to equate T 1 T_1 - T 0 T_0 to a predetermined period Δ T \Delta T , and obtaining a measure of the voltage V 2 V_2 across the coil at a time T 2 T_2 also during saturation of the magnetic material after a time T 5 T_5 when an electrical current through the coil in the direction opposite to the said first direction is initiated by the switches, equating T 2 − T 5 T_2-T_5 to the predetermined period Δ T 1 \Delta T_1 and using a processor of the circuitry to provide an indication of the value of V 1 − V 2 V_1-V_2 , the magnitude of which provides an indication of the strength of the said component, and the sign of which provides an indication of the direction of the said component.

10. A method of determining the strength and / or direction of a component of an ambient magnetic field, using a magnetic field sensor as claimed in any one of claims 1 to 7.