GB2289145A

GB2289145A - Magnetic field cancelling system

Info

Publication number: GB2289145A
Application number: GB9509178A
Authority: GB
Inventors: Denis Frank Spicer
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-05-05
Filing date: 1995-05-05
Publication date: 1995-11-08
Anticipated expiration: 2015-05-05
Also published as: WO1995030906A1; GB2289145B; GB9408942D0; GB9509178D0; AU2413295A

Abstract

A magnetic field cancelling system comprises a flux gate, bias coils, flux sensing means 30 and a feedback means. The initial value of magnetic field as registered at the flux gate by the flux sensing means 30 is maintained by measuring subsequent variations in magnetic field and using the feedback means to vary the current in cables 34, 36, 38 and produce a field fluctuation equal and opposite to subsequent fluctuations. The magnetic field cancelling system can be connected to a magnetic field production system 42 to enable an area around equipment 40 to be kept at a constant field value. A reset arrangement couples the feedback means to the bias coils to produce an initial zero field at the sensing means 30. <IMAGE>

Description

Title: Magnetic Field Cancellina Svstem Field of Invention This invention relates to a magnetic field cancelling system, wherein a DC capability to offset constant magnetic fields is provided.

Backaround to the Invention The existence of stray magnetic fields around, for example, electrical power installations has led to the development of systems for cancelling these magnetic fields to improve the performance of instruments sensitive to stray magnetic field effects. Known magnetic field cancelling systems are AC systems, that is they cancel magnetic fields only within a range of magnetic field frequencies, for example, between 0.5Hz and 5000Hz. This range is very suitable for cancelling fields due to electrical power installations (typically 50 or 60 Hz plus harmonics) and fields due to rotating machinery (fans, motors etc at 5 to 50 Hz).

Typically magnetic field cancelling systems use a negative feedback principle to cancel the field. The magnetic flux passing through a sensor is continuously sensed and a current proportional to the sensed flux is fed back to the field coils to oppose the sensed flux. Typically the feedback loop has an open loop gain of 100 times (ie 1 unit of sensed flux results in 100 units of flux fed back), so the sensed flux is reduced by a factor of 100 when the feedback loop is closed. This system is exactly analogous to the "virtual earth11 negative feedback configuration used with operational amplifiers (Op-amps) in electronic circuit design.

Since the system is closed loop, it must obey the standard frequency response rules which apply to all closed loop control systems in order to be unconditionally stable at all frequencies. If these rules are not obeyed, the system will oscillate or ring at certain frequencies. These rules are essentially the same as used by an electrical engineer when designing an op-amp. The rules determine the allowed frequency response of the system in open loop (ie when there is no negative feedback) in order to make it unconditionally stable in closed loop. An AC cancelling system would typically have an open loop frequency response as shown in Figure 1.

The 6dB/octave roll-off regions at high and low frequencies are essential for unconditional stability (ie no oscillation) and fastest setting of the feedback loop.

It is possible to extend the low frequency range limit of AC cancelling systems by engineering trade-offs in the analog integrator circuit. These extended systems enable reasonable cancelling of fields caused by objects made of magnetic materials moving through the earth's magnetic field.

To achieve the extended low frequency range the noise and stability performance is traded off. Hence although extended systems can cancel to lower frequencies, the field remaining after cancelling is greater because the sensor noise (which sets the limit) is greater. There are also severe practical difficulties in achieving the required 6/dB octave low frequency roll-off and a system which cancels down to .01 Hz may take an hour to stabilise after switch on.

Summary of the invention According to the present invention, a magnetic field cancelling system is provided comprising flux detection means, a magnetic field production system, a bias magnetic field production system placed around the flux detection means and feedback means, wherein flux detection means produces a signal representing the initial value of magnetic field, the signal is sent via the feedback means to the bias magnetic field production system so as to generate a field to oppose that generated by the field production system and so create a substantially zero field at the flux detection means so that the flux detection means detects small variations in magnetic field value against this substantially zero field.

This magnetic offsetting of the operating point of the flux detection system to a base value, ideally zero, ensures that any instabilities in the flux gate signal are only multiplied by small differences in the field value between the field production system and bias field production system. This results in a lesser contribution of the instabilities to the field changes sensed by the flux detection means and results in more accurate sensing of variations in field.

By not attempting to cancel the earth's magnetic field but merely to cancel any changes in field value from a base value, the power dissipation of the system is reduced.

The variations detected by the flux detection means may be used to control a further magnetic field production system contained within the field production system so as to create a region that is maintained at constant field value.

The further magnetic field production system preferably produces magnetic field components for three independent orthogonal axes. This is typically achieved by the use of independent current carrying means, each generating a field value for one axis.

The bias magnetic field production system may be provided about one, two or three axes of the flux detection means. In a preferred embodiment the system is provided for three independent orthogonal axes.

Preferably the operating point of the bias field production system is determined at intervals so as to correct for long term magnetic drift in the signal produced by the flux detection means.

The flux detection means preferably further comprises a flux gate magnetometer with a drive coil and a sensing means, wherein the drive coil is wound onto the flux gate to allow alternate magnetic saturation and non-saturation of the flux gate by use of a suitable drive current and the sensing means detects the incident magnetic field.

The sensing means may be provided with a sense winding to detect magnetic field in the x axis direction, with a further sense winding to detect magnetic field in the y axis direction with a circumferential winding to detect magnetic field in the z axis direction.

Alternatively magnetic field in the z axis direction may be detected by a second flux gate suitably positioned relative to the first flux gate.

Preferably the flux gate is provided with flux concentration means so as to enhance the sensitivity of field fluctuation detection. The flux gate is typically cylindrical in shape with a central aperture.

Preferably the feedback means further comprises an analogue to digital signal conversion means, a memory means to store the digital signal, and a digital to analogue conversion means so that an analogue signal representing the initial value required to generate the bias magnetic field is stored as a digital signal within the memory means and is converted to an analogue signal by the digital to analogue conversion means to provide a signal to control the bias field until the operating point of the flux detection means is determined at the next interval.

Sampling of the initial field value by the flux detection means is thus used to provide a stored signal to control the generation of the bias field until a further zero field condition is determined. This sample and hold method allows the bias field to be generated at a stable value indefinitely, the value being updated periodically.

The feedback means may be applied to individual axes or to all axes. If it is applied to all axes it is preferable that the determination of the control values sent to each axis of the bias magnetic field production system occurs simultaneously.

The invention will now be described by way of example with reference to the following drawings in which: Figure 1 is a graph depicting the typical frequency response of an AC magnetic field cancelling system; Figure 2(a) and 2(b) depict the flux lines through a flux gate 2(a) when in a ferromagnetic state and 2(b) when in a saturated state; Figure 3(a) and 3(b) show schematic views of a sense winding and drive winding as wound onto the flux gate; Figure 4 illustrates a drive current applied to cause magnetic saturation in the flux gate; Figure 5 is a graph depicting the frequency response of a magnetic field cancelling system in accordance with a preferred embodiment of the invention; Figure 6(a) is side view, partially sectioned, of the flux gate and flux concentrators; Figure 6(b) is a plan view of the flux lines present around the flux gate and flux concentrators; Figure 7 is a perspective view of the flux gate and flux compensators with compensatinq bias coils; Figure 8 is a perspective view of the magnetic cancelling system as applied to an enclosed space containing sensitive equipment; Figure 9 shows a schematic diagram of a reset feedback loop as used in the magnetic field cancelling system shown in Figure 5; and Figure 10 shows a schematic diagram of negative feedback field cancelling.

Detailed description of a preferred embodiment In Figure 5 the frequency response of a magnetic cancelling system with DC capability is shown. The frequency response of a typical AC system is shown in Figure 1. There is no low frequency roll-off when using the system with DC capability.

To achieve DC capability a flux gate magnetometer of suitable design is used.

It must be noted that the earth has a generally steady magnetic field (hence magnetic compasses) on which a cancelling system must operate. Also, certain equipments (such as NMR body scanners) create steady magnetic fields which add to the earth's field. These steady fields are ignored by AC field cancelling systems. DC cancelling systems can detect these steady fields and will attempt to cancel them.

Although it is quite possible to cancel these steady fields it is generally not desirable (since large amounts of power would be required) and generally not necessary since it is chanaes in a magnetic field which cause most of the problems to sensitive equipment. Measurement of the magnetic field at the start (time 0) and then holding it constant at that value as long as is required by cancelling any changes, achieves a constant field value.

Measurement of magnetic fields is achieved by use of a magnetometer, see Figure 3. The "flux gate" is a particular type of magnetometer. One realisation of a flux gate magnetometer uses a ring of magnetic material 12.

When the ring of ferro-magnetic material 12 is placed in a uniform magnetic field 14 the flux lines are drawn into the ring on one side, pass through the ring and exit on the other side, as in Figure 2(a). Figure 2(b) shows what happens when a ring of non-magnetic material is placed in the same field.

The flux lines pass through without distortion.

Ferro-magnetic materials exhibit magnetic saturation. That is, when the number of flux lines in the material becomes high, the magnetic properties are lost or degraded. The flux gate magnetometer works by alternately saturating the ring to cause it to switch between the two states of Figure 2(a) and Figure 2(b).

Switching between the two states of Figure 2(a) and 2(b) is achieved by use of a sense winding 16 and a drive winding 18.

The sense winding 16 has two sections, on opposite sides of the ring 12. The two sections are connected in anti-phase as shown, to sense the change in flux lines when the ring 12 switches between the states 2(a) and 2(b). The drive winding 18 is wound uniformly around the ring 12 (called a toroidal winding). An alternating current is passed through the drive winding 18 which is sufficient to saturate the ring 12.

The circuit arrangement and waveforms applied to the drive winding 18 are shown in Figure 4.

The drive voltage and period and the number of turns on the drive winding 18 are chosen so that the ring 12 is driven into saturation for about 25% of the drive period. When the ring 12 is in the linear inductance region the flux lines are like 2(a). When the ring 12 is magnetically saturated the flux lines are like 2(b). The ring 12 is driven into and out of saturation twice each drive period. Each time the ring 12 goes into or out of saturation an induced voltage (due to the flux change) occurs in the sense winding 16. The signal induced in the sense winding 16 is amplified and synchronously rectified to recover the DC components of the measured field. The drive frequency and its harmonics are removed by a low pass filter.

Preferably the one axis (X) magnetometer is extended to two axes (X & Y) by adding a second (orthogonal) sense winding.

A third (Z) axis can be sensed by using a second ring or with a circumferential sense winding.

While it would be possible to construct a DC magnetic field cancelling system using currently available flux gate magnetometers, using the principles described here, the system would not be very useful because of the restricted performance of the magnetometer. The new magnetometer described here has approximately one order of magnitude improvement in bandwidth and signal to noise and two orders of magnitude in DC stability.

A magnetometer 22 as used in the DC magnetic field cancelling system is shown in Figures 6(a) and 6(b) and Figure 7.

Improvements in performance are achieved by: 1. The use of small ring cores 12 at high drive frequency 2. The use of flux concentrators 24 for improved signal to noise 3. The use of precision field bias coils 26, 28 for improved DC stability.

The magnetometer upper frequency limit is set by the flux gate drive frequency and the need to filter the drive frequency from the output. To achieve good cancelling an upper limit of about 2 kHz is required and 5 kHz is desirable. Typically, a drive frequency 10 times the upper limit is necessary to achieve satisfactory filtering.

The use of a high drive frequency requires the use of a small ring 12. In order to drive the ring 12 into saturation, power must be dissipated in the driving circuit (including the series resistor 20 in Figure 4). This power dissipation, for a given size core is (to a first order) proportional to frequency. The power dissipated in the ring 12 due to hysteresis losses is also proportional to frequency. For a given drive frequency, the power dissipation is roughly proportional to the volume of the ring 12. A ring 12 of volume 430mm3 requires a drive power of about 1 watt at 25 kHz drive frequency.

The signal amplitude obtained from the flux gate sense winding 16 is a complex function of the magnetic properties of the ring 12. Low incremental permeability in saturation is more important than high permeability in the linear inductance region. Prior art teaches the use of high permeability nickel alloys (eg Mu metal) for the ring material. For small rings, used at high frequencies, it is preferable to use Manganese Zinc ferrite cores. These have essentially zero eddy current losses, a much squarer hysteresis loop and low incremental permeability in saturation.

With reference to Figure 2(a) it is clear that the flux which passes through the ring 12 is proportional to the size of the ring 12. Hence for best signal amplitude the ring 12 should be as large as possible, in direct opposition to the requirement for high drive frequency. However, since the signal acquisition is essentially a sampling process, the signal amplitude (after rectification and filtering) is directly proportional to the drive frequency. Hence the disadvantage of the small ring 12 is minimal. The noise level is set by the thermal noise performance of the amplifier chain (determined by the best state of the art) and is not really a variable.

The effective size of the ring 12 can be increased by using flux concentrators 24. These enable small rings to achieve better signal amplitude by collecting flux and concentrating it into the ring 12. Figure 6 shows a suitable design of flux concentrator 24 for a two axis flux gate. The fins 26 are made of a high permeability material such as Mu metal. The effect on the flux lines of a uniform field is shown in Figure 6b.

To be useful, a DC magnetic field cancelling system needs te be able to hold the field stable to around + 0.1 milliGauss(mG). The earth's field produces a flux density of around 500 mG so 0.1 mG represents a stability level of 1 part in 5000 in the presence of the earths field.

Typically state of the art flux gates can measure fields with an accuracy and stability of reading of 1 or 2 percent. This is 50-100 times less stable than the system developed. The flux gates sense the full field and any offsets are subtracted by electronic systems. The instabilities of the flux gates are multiplied by the full field value and are present in the signal used to represent the field value.

Magnetic field bias coils as shown in Figure 7 are used to increase the sensitivity of the system. The bias coils achieve increase in the sensitivity by magnetically offsetting the operating point of the flux gates to zero. The instabilities of the flux gates are only multiplied by the small difference between the external magnetic field and the bias field. The contribution of the instabilities to the signal used to represent the field value is thus reduced.

The flux gate 12 and the flux concentrators 24 are placed in the centre of the bias coils 26, 28 as shown. The bias coils 26, 28 are similar to Helmholtz coils with the spacing S chosen to give the best field uniformity in the centre. Each bias coil would typically comprise many turns of fine wire to enable it to be driven with low power circuits.

Field uniformity is not essential to the invention and other arrangements of bias coils are possible.

The bias coils 26, 28 are used to create a local magnetic field (around the flux gate 12) equal and opposite to the field present at the flux gate 12. The field present at the flux gate is typically the earth's field plus any field from equipment such as NMR body scanners.

The field produced by the bias coils 26, 28 is the result of currents passed through the coils 26, 28. The currents to be passed through the bias coils 26, 28 are determined during a "reset" process.

In use, as shown in Figure 8, a system 32 in accordance with the invention with a magnetic field sensor 30 and electrical cables is positioned to form a cage 42 around magnetically sensitive equipment 40 within an enclosed space. Cancelling of magnetic field variations within the cage is achieved by feedback from the magnetic field cancelling system 32, altering the current supplied to the cables forming the cage 42.

The start up condition (time O) is established by a "reset" function which establishes the operating point of the magnetic field sensor 30. Figure 9 shows a "reset" feedback loop which is used in the magnetic field cancelling system to establish the "reset" function. Figure 9 shows one axis of the system, which an equivalent set up used for the other axes. In Figure 9 the "reset" function measures the field at the flux gate magnetometer 22. The "reset" function then supplies a current to the bias coil 26 to produce a field equal and opposite tc that registered at the flux gate magnometer 22.

The circuit loop in Figure 9 contains switches 44, 46, 48, an analogue to digital convertor (ADC) 50, a memory means 52 associated with the magnetic field sensor 30 and a digital analogue convertor (DAC) 54, together with signal amplifiers shown by conventional symbols. When switches 44, 46, 48 are set to the "reset" position the field cancelling is interrupted for a few seconds, that is, there is no current driven through the field cable 34 in Figure 8. The flux gate output is now fed back to the bias coil 26. This feedback loop sets the output of the flux gate magnetometer 22 to zero and determines the correct current to be driven through the bias coil 26. The value of the bias coil current is then converted to digital data via ADC 50 and stored in memory means 52. Thus the magnetic sensor 30 has a stored value for the bias coil current in the memory means 52 until the next "reset" occurs.

At the end of the "reset" period, the switches 44, 46, 48 are set to the run position and the bias coil 26 is driven by current from the DAC 54 using the digital information stored in memory means 52. This sample and hold method enables the bias field to be generated at a stable value indefinitely.

In multiple axis systems, the bias coil of one axis may create a small field in the direction of the other axes. The convergence of the feedback loops of such a multiple axis system is improved by ensuring that all axes are reset at the same time so as to compensate for any such interactions.

Following "reset", the currents through the bias coils 26, 28 are held constant (until the next "reset"). Hence the field produced by the bias coils remains constant (until the next "reset").

When cancelling resumes following "reset", the flux gate 12 has only to sense the small changes in field which occur. The flux gate accuracy of 1 or 2 percent is quite satisfactory for this purpose. The flux gate 12 is essentially working in a null detection mode.

The stability of this arrangement is principally determined by the stability of the bias field not by the flux gate 12. The stability of the bias field is determined by the stability of the bias coil currents and the mechanical stability of the former on which the bias coils 26, 28 are constructed. It is not difficult to hold the bias coil currents stable to 1 part in 100,000 with state of the art electronic components. The coil former could be made of highly stable material such as "Zerodurt' (a zero temperature coefficient glass ceramic) or stability could be achieved using a much lower cost plastic material in conjunction with electronic temperature control.

Figure 10 illustrates how the invention can be considered as being analogous to an electrical negative feedback cancelling system. A magnetic flux sensor 60 detects the flux present and converts this to an electrical signal which is fed to a power amplifier 62. The amplifier 62 drives a field coil 58 which generates flux to counteract the flux detected by the sensor 60. Negative feedback arises because the flux generated by the coil 48 contributes to the flux detected by the sensor 60. Thus 56 can be considered as representing a notional input for flux which would be detected by the sensor 60 in the absence of any current in the coil 58, whilst the broken line 64 represents the contribution of the coil 58.

Claims

1. A magnetic field cancelling system comprising flux detection means, a magnetic field production system, a bias magnetic field production system placed around the flux detection means, and feedback means, wherein the flux detection means produces a signal representing the initial value of magnetic field, the signal is sent via the feedback means to the bias magnetic field production system so as to generate a field to oppose that generated by the field production system as so create a substantially zero field at the flux detection means so that the flux detection means detects small variations in magnetic field value against this substantially ero field.

2. A magnetic field cancelling system according to claim 1, wherein the variations detected by the flux detection means are used to control a further magnetic field production system contained within the field production system so as to create a region that is maintained at constant field value.

3. A magnetic field cancelling system according to claim 2, wherein magnetic field components are produced for three independent orthogonal axes.

4. A magnetic field cancelling system according to claim 1, claim 2, or claim 3, wherein the bias magnetic field production system is provided about one axis of the flux detection means.

5. A magnetic field cancelling system according to claim 1, claim 2, or claim 3, wherein the bias magnetic field production system is provided about two axes of the flux detection means.

6. A magnetic field cancelling system according to claims 1, 2, or 3, wherein the bias magnetic field production system is provided about three axes of the flux detection means.

7. A magnetic field cancelling systems according to claims 1, 2 or 3, wherein the bias magnetic field production system is provided about three independent orthogonal axes.

8. A magnetic field cancelling system according to any of claims 1 to 7, wherein the operating point of the bias field production system is determined periodically so as to correct for long term magnetic drift in the signal produced in the flux detection means.

9. A magnetic field cancelling system according to any of claims 1 to 8, wherein the flux detection means further comprises a flux gate meter with a device coil and a sensing means wherein the drive coil is wound onto the flux gate to allow alternate magnetic saturation and non-saturation of the flux gate by use of the suitable drive current and the sensing means detects the instant magnetic field.

10. A magnetic field cancelling system according to claim 9 wherein the sensing means comprises a sense winding to detect magnetic field in the x axis direction, and a further sense winding to detect magnetic field in the y axis direction.

11. A magnetic field cancelling system according to claim 10, wherein the sensing means is provided with a circumferential winding to detect magnetic field in the z axis direction.

12. A magnetic field cancelling system according to claim 10, wherein the magnetic field in the z axis direction is detected by a second flux gate suitably positioned relative to the first flux gate.

13. A magnetic field cancelling system according to any of claims 9 to 12, wherein the flux gate has flux concentration means so as to enhance the sensitivity of the field fluctuation detection.

14. A magnetic field system according to any of the proceeding claims, wherein the feedback means further comprises an analogue to digital signal conversion means, a memory means to store a digital signal, and a digital to analogue conversion means, so that an analogue signal representing the initial value required to generate the bias magnetic field is stored as a digital signal within the memory means and is converted to an analogue signal by the digital to analogue conversion means to provide a signal to control the bias field until the operating point of the bias field is determined at the next interval.

15. A magnetic cancelling system according to any of the proceeding claims, wherein feedback means is provided for each individual axis.

16. A magnetic field cancelling system according to any of claims 1-14, wherein feedback means is provided for three independent orthogonal axes.

17. A magnetic field cancelling system according te claim 16, wherein the operating point of all the axes of the flux detection means is determined simultaneously.

18. A magnetic field cancelling system constructed, arranged and adapted, to operate substantially as herein described with reference to the accompanying drawings.