GB2438057A

GB2438057A - Magnetic sensor with electromagnetic radiation compensation

Info

Publication number: GB2438057A
Application number: GB0708642A
Authority: GB
Inventors: Paul Andrew Robertson
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-05-12
Filing date: 2007-05-04
Publication date: 2007-11-14
Anticipated expiration: 2027-05-04
Also published as: GB0708642D0; GB2461207A; GB0609439D0; GB2461437B; GB2438057B; GB0917782D0; GB0917780D0; GB2461437A; GB2461207B

Abstract

A magnetic sensor with integral electromagnetic radiation compensation coils. The invention provides a high sensitivity and wide bandwidth measurement system for magnetic fields and electric currents utilising a simple magneto-inductive sensor, such as a single-core fluxgate device, comprising a sensing coil 13 and a core 15, driven with a radio frequency excitation current in combination with a separate compensation coil 14 driven with a related alternating current of appropriate magnitude and phase such that external electro-magnetic fields emitted by the system are minimised. Means is also provided for detecting overload of the sensor as the core approaches continuous saturation and for minimising any deleterious effects of the ohmic resistance of the sensor coil on the effectiveness of the flux nulling feedback system.

Description

Magnetic Sensor The invention relates to the electronic sensing of

magnetic fields and electrical currents with fluxgate, magneto-inductive and magneto-impedance sensors such that the electromagnetic emissions from such devices may be minimised and the response of the sensor improved.

Magnetic sensors such as fiuxgate, magneto-impedance and magneto-inductive devices rely on the interaction of an alternating excitation magnetic field with a high permeability core wherein that interaction is altered by the presence of an external magnetic field which also couples into the core. Such systems are well known in the prior art, for example, in the measurement of the Earth's magnetic field for electronic compasses.

In the case of ring-core, racetrack and twin core fluxgate sensors the excitation field produced external to the sensor is relatively small and such devices are unlikely to cause interference with neighbouring systems or instniments. However, such devices are complex to manufacture, often comprising an assembly of several coils and core parts, and are not well suited to production by low cost micro-fabrication techniques. It is for this reason that single solenoidal coil sensors with a single core are popular in high volume applications such as electronic compasses and, as described in patent GB23 19621, a miniature probe for measuring magnetic currents in nearby conductors.

One of the drawbacks of single coil sensors is that they can produce an external electromagnetic field at the excitation frequency which can couple into external circuits.

Whilst this may not be a problem with an electronic compass, where the signal bandwidth is small and the excitation frequency low, for electronic instrumentation applications a high excitation frequency typically over 1 MHz may be employed to achieve a wide sensing signal bandwidth (the excitation frequency is typically several times the maximum signal bandwidth required). Thus the excitation frequency produces an external alternating magnetic field which may interfere with the operation or observation of nearby electronic circuitry, such as a printed circuit board (PCB) track in which current is being measured.

It is an object of this invention to provide a magnetic sensor system based on a simple solenoid coil arrangement and yet minimise the electromagnetic fields emitted by said system by providing a compensation coil in proximity to the sensor which produces a magnetic flux in anti-phase to that from the sensor coil.

It is a further object of the invention that the compensation coil can also contain a magnetic core substantially similar to that of the sensor such that the magnetic field gradient may be measured in addition to the magnetic field strength or magnetic flux density.

A further object of the invention is to provide a means of overload detection and indication in the case of the sensor being a fluxgate magnetometer or other magneto-inductive sensing device which can indicate a false near-zero reading under conditions of sensor saturation.

A further object of the invention is to include a current feedback path to the sensor incorporating a high-pass filter such that the impedance of the sensor at low frequency, particularly due to the electrical resistance of the coil and lead wires, does not compromise the closed-loop feedback applied to maintain the sensor at a null flux balance as determined by the difference between the positive and negative peak amplitudes produced by the creation of even harmonics of the excitation frequency.

Another object of the invention is to provide an automatic means of changing the output scale, range and offset by automatic means when the sensor system is alternatively used as an open field sensor or when operating in a closed magnetic circuit, such as can be advantageous for example when measuring the current flow in a conductor by surrounding said conductor with a toroid of magnetic material interrupted with an air gap to accommodate a magnetic sensor.

The invention will now be described with reference to the following figures: Figure 1 illustrates a sensor coil and compensation coil arranged for external flux cancellation, Figure 2 illustrates a dual sensing and compensation coil arrangement, Figure 3 illustrates a schematic block diagram of electronics to drive the sensor and compensation coils, Figure 4 illustrates a schematic block diagram of electronics to drive a dual sensing and compensation coil arrangement, Figure 5 illustrates a schematic block diagram of interface electronics for a fluxgate sensor incorporating a high pass filter into the nulling feedback circuit, Figure 6 illustrates an overload detection circuit incorporates into a fluxgate interface circuit, Figure 7 illustrates a range and scale switching arrangement such that the output of the sensor circuit automatically changes to a convenient scale factor when a flux-concentrator for a current carrying conductor is fitted on to the sensor system.

With reference to figure 1, a sensing coil 13 with a core of high permeability, soft magnetic material 15 is located in close proximity and with its axis in parallel to that of a compensation coil 14 such that the flux produced by the sensing coil, as a result of the excitation current passed through it via the leads 16, is substantially cancelled by an anti-phase current passing though the compensation coil via its leads 18. The two coils can either be wound in a similar sense and their currents in anti-phase or alternatively, the currents can be in-phase and the coils either wound or connected in opposite directions. An advantage of using currents in anti-phase is that the supply current to the associated drive electronics will carry less ripple since the sum of the two currents will be closer to zero.

With reference to figure 2, a pair of similar axially parallel coils 22 and 26 are used, both carrying a magnetic core 20 or 24 respectively. In this case, driving each coil through its respective leads 28 or 30 with balanced anti-phase currents will result in very low RF emission if the cores and coils arc closely matched in their physical properties since higher hannonics produced by the non-linear magnetic cores will also effectively cancel at a distance away from the coils of several times the coil dimensions. Either or both coils may be used as sensors in this case, where the difference signal will be a measure of the magnetic field gradient and the sum or average will give an indication of the magnitude of the magnetic field detected, with an improved signal to noise ratio over that achieved with a single sensing coil.

It should be noted that this arrangement is similar to the standard twin core fluxgate magnetometer except that there is no requirement for a third detection / null feedback coil enclosing both coils and cores, as would be the case in the standard fluxgate configuration.

Instead, each coil may be used for the purposes of excitation, harmonic detection and feedback if desired, as described in the following paragraphs and figures referred to.

With reference to figure 3, a schematic block diagram of an example sensor and compensation coil interface circuit is shown. A radio frequency oscillator 2 drives a sensing coil 4 through an impedance 3. The positive and negative peak amplitudes are demodulated by demodulators 9 and 8 and summed at the amplifier 10. The demodulators may comprise a diode, capacitor, resistor network or may be realised using active components such as semiconductor switches, transistors or mixer circuits. An external magnetic field coupling with the sensor causes a difference in the amplitudes of positive and negative peaks due to the presence of even harmonics with the fundamental excitation waveform and this in turn causes a net d.c. component in the output 12 of the amplifier 10. If the gain of the amplifier 10 is a large negative value then its output 12 may be connected back to the sensor coil 4 through an impedance 11 such that there is net negative feedback and the circuit maintains the sensor core at substantially zero static flux ie. it is a null feedback system. Alternatively, for open-loop operation, the impedance 11 may be left open circuit. A low-pass filter 27 may be included to minimise the excitation frequency component in the output signal 29. Thus the components 8, 9, 10, 11 and 27 provide a sensor waveform amplitude monitoring and feedback block 25 with a voltage output signal 29 proportional to the external magnetic field coupling with the sensor. Further details of the operation of such a sensing system are given in patent (3B23 19621. In order to provide the current drive to the compensation coil 5, the voltage across the sensor is monitored though an optional phase shifting network I by the amplifier 6, the output of which drives the compensation coil 5 through an impedance 7. The gain of the amplifier 6, the phase shift through the network 1 and the value of impedance 7 are selected such that the net alternating magnetic flux observed some distance from the coils is close to zero.

With reference to figure 4, a schematic block diagram of a dual sensor I compensation coil interface circuit is shown. A radio frequency oscillator 40 drives one sensor coil 41 through an impedance 42 whilst the other sensor coil 43 is driven by an amplifier 44 through an impedance 45 such that the magnitude and phase of the excitation currents produce a minimal external net magnetic flux as observed some distance away from the sensor coils. To permit fine adjustment for optimum flux cancellation, an optional phase shift network 46 may be introduced. Amplitude monitoring and feedback loops 47 and 48, as previously described and illustrated in figure 3 as block 25, are connected to one or both sensor coils, each operating in either open or closed loop mode. The output signals 49 and 50 may be combined to give an indication of the magnetic field gradient, by taking their difference, or the average magnitude

of the magnetic field, by taking their sum.

With reference to figureS, a schematic block diagram of an improved single core, single coil fluxgate interface circuit is shown incorporating a high-pass filter 63 into the signal path between the sensor 69 coil voltage and the demodulators 64 and 65. The sensor 69 is driven with an excitation current from the RF oscillator 61 through impedance 62. In previous examples described without the filter 63, when operating in null feedback mode a d.c. voltage will appear across the sensor coil equal to the feedback current multiplied by the resistance of the coil and its lead wires. In cases where the coil resistance is more than about I ohm, this d.c. voltage can have a significant effect on the effectiveness of the feedback circuits to null balance the static sensor core flux -resulting in degraded sensing linearity and a reduced dynamic range. By introducing a high pass filter 63, the demodulators 64 and 65 are isolated from the d.c. voltage across the sensor coil and the null feedback is more effective. It can be advantageous to allow some degree of d.c. coupling through the filter 63 however to optimise the d.c. loop stability. The demodulator outputs are summed by a feedback amplifier 66, the output of which provides a nulling current through the feedback impedance 67. The output signal 68 is derived by filtering the amplifier 66 output with a low-pass filter 70.

With reference to figure 6, an overload detection circuit is shown for a fluxgate, magneto-impedance or magneto-inductive sensor 70 driven by an RF oscillator 72 through impedance 71. Demodulation and feedback circuits, as illustrated and described previously herein are not shown in this case, although can be implemented if a linear output signal is required. When the sensor 70 is overloaded such that its magnetic core is at or near saturation, there may only be a small amplitude of even hannonics of the excitation waveform produced, which in turn will give a sub-linear output signal level. Under this condition ie. when the sensor system is over-range, the impedance of the sensor will be seen to reduce as its inductance falls, due to a decrease in the effective permeability of the core. As the sensor impedance falls, the peak-to-peak voltage across the coil terminals due to the excitation current also falls. This change may be monitored with a demodulator (envelope detector) 74 which produces a d.c. signal related to the peak-to-peak amplitude of the alternating voltage across the sensor coil. This signal may be compared with a fixed threshold, or preferably, a threshold level derived from the excitation oscillator 72 amplitude by means of a second envelope detector 73 and scaling means, such as a pair of resistors 75 and 76. In the linear operating range of the sensor, the d.c. voltage from demodulator 74 is higher than the voltage from demodulator 73 scaled by resistors 75 and 76. This pair of voltages is compared with a comparator 77 or other means and may be used to drive a visual, audio or other indicator 78, such as a light emitting diode, though a resistor 79. Under sensor saturation or overload conditions, the voltage from demodulator 74 falls causing the output of comparator 77 to change state. The comparator 77 may preferably have some hysteresis and / or pulse stretching circuitry included so that momentary overloads may be more easily noticed.

With reference to figure 7, means for measuring the current flowing in a conductor 80 by means of a magnetic sensor 82 is illustrated wherein the magnetic flux around the conductor is concentrated in an air gap 87 within a toroid of magnetic material 84. The magnetic sensor 82 is located in the air gap 87 so as to monitor the magnetic field produced as a result of the current flowing in conductor 80. In order for the output of the sensor system to be scaled in convenient units when measuring magnetic fields or currents, a feature 85 attached to the toroid 84 or its housing, communicates with a second feature 86 attached to the sensor housing 89. The feature 86 communicates to the sensor interface electronics such that the scale factor of the system may be altered. Examples of features 85 and 86 respectively include a mechanical feature and a microswitch or a small magnet and a magnetic sensor, such as a reed switch, magneto-resistive or Hall effect switch, or a mechanical feature and an opto-electronic device. For example, the scale factor of the system may then be changed automatically from 1V/mT or 1OV/G, when reading magnetic flux density or field strength, to 1 V/A when reading cunent, when the toroid 84 is put in place.

It will be appreciated by those skilled in the art that other combinations of sensors, coils and interface electronics, beyond those illustrated by way of example herein, are possible. The examples described herein are intended for illustration and not to limit the scope of this patent specification, which is defined by the following claims.

Claims

Claims 1. A magnetic field sensing system based on a sensor or sensors

excited with a radio-frequency alternating current and which incorporates one or more compensation coils driven with alternating current such that the external electro-magnetic field emitted by the sensor and compensations coils is minimised.

2. A magnetic sensor system according to claim 1 where the compensation coil or coils contains a magnetic core or cores substantially similar to the core or cores contained within the sensing coil or coils.

3. A magnetic sensor system wherein a plurality of coils with magnetic cores are connected to suitable excitation, demodulation and null feedback circuitry such that each coil and core combination may act as both a magnetic sensor and field compensation device such that multiple measurements of the magnetic field may be taken simultaneously, including the derivation of magnetic field gradient, where the magnitude and phase of the coil excitation currents are arranged to minimise the external electro-magnetic field emitted by the coils.

4. A fluxgate magnetic sensor system in which additional circuitry monitors the time-averaged impedance of the sensor such that magnetic field overload conditions resulting in excessive magnetic saturation of the sensor core may be detected.

5. A magnetic sensor system according to claim 4 wherein the overload detection circuitTy comprises one or more demodulators and threshold detection means to detect the fall in impedance experienced as the sensor approaches saturation.

6. A magnetic sensor system utilising a fiuxgate, magneto-inductive or magneto-impedance sensor driven with a radio frequency alternating current and provided with a null feedback current to maintain the sensor core at substantially zero net flux, derived from the amplitude of the positive and negative voltage wavefomi peaks, incorporating a high pass filter between the sensor and demodulation circuits such that the voltage produced as a result of the coil and lead wire resistance canying the feedback current has minimal effect on the null balance point achieved.

7. A sensor system comprising a magnetic field sensor which can optionally be used with a detachable flux concentrator for the measurement of current in a conductor passing through said flux concentrator characterised by detection means for the presence of the flux concentrator enabling the system to automatically set an appropriate output scale and or calibration settings for use with the flux concentrator.

8. A current sensor system according to claim 7 in which the flux concentrator comprises an air-gapped ring or loop of magnetic material such as ferrite and the presence of the concentrator is detected by a mechanical, opto-electronic or magnetic switching means.