AU598908B2

AU598908B2 - A magnetometer and method for measuring and monitoring magnetic fields

Info

Publication number: AU598908B2
Application number: AU11618/88A
Authority: AU
Inventors: Ian Thomas Croser
Original assignee: CEA Technologies Pty Ltd
Current assignee: CEA Technologies Pty Ltd
Priority date: 1987-02-11
Filing date: 1988-02-10
Publication date: 1990-07-05
Anticipated expiration: 2008-02-10
Also published as: AU1161888A

Description

-i Ii i i 598908 FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1962 COMPLETE SPECIFICATION (Original) FOR OFFICE USE: Class Int. Class o 1 Application Number: Lodged: Complete Specification Lodged: Accepted: Published: 4 4O 4 4o 4 t.

Priority: 1 f -IL LA 0 MCI JM.; irs ,Ori,.iC Lit

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Related Art: Name of Applicant: CEA TECHNOLOGIES PTY LTD Address of Applicant: Unit 4 133 Gladstone Street Fyshwick Australian Capital Territory 2609 Actual Inventor(s): IAN THOMAS CROSER Address for Service: DAVIES COLLISON, Patent Attorneys, 1 Little Collins Street, Melbourne V 3000, Australia Complete Specification for the invention entitled: "A MAGNETOMETER AND METHOD FOR MEASURING AND MONITORING MAGNETIC FIELDS" The following statement is a full description of this invention, including the best method of performing it known to us 1 Flla- TITLE: "A MAGNETOMETER AND METHOD FOR MEASURING AND MONITORING MAGNETIC FIELDS" Technical field This invention concerns magnetometers. More particularly, it concerns magnetometers for the measurement of steady-state fields and for use in the detection of magnetic anomalies and transient variations in magnetic field.

Background art The need to measure magnetic fields accurately, and to detect small variations in magnetic fields, has long been recognised. A variety of magnetometers have been developed for these purposes, the most recent of which generate signals as a result of the rapid periodic variation of the magnetic field at a sensor. Such periodic variation can be effected by o' O changing the magnetic field that is experienced at the sensor, or by moving the sensor in the steady-state magnetic field. Examples of such magnetoeters are described in the specifications of Australian patents Nos 464,825 and 471,986.

The magnetometer disclosed in the specification of Australian patent No 464,825 has a magneto-transistor or Hall current element (or a similar device) as the sensor of the magnetic field. This sensor is held stationary while a ferromagnetic material (which acts as a concentrator of the magnetic field) is oscillated in front of it. The ferromagnetic 1I_~C_ i CIIIY_; ~--IIU4U~ 2 concentrator is a layer of ferromagnetic material which loads one side of a piezo-electric bimorph (a piezo-electric reed) that is cantilevered so that its free end is closely adjacent to the magnetic field sensor. The piezo-electric reed is driven by an alternating voltage at the resonant frequency of the reed. If the piezo-electric reed is made from lead zirconate and lead titanate and the reed has a length of about 4cm, the resonant frequency is about 200 Hz and a lateral movement of about imm is obtained at the free end of the cantilevered arrangement. The voltage signal developed by the sensor as a consequence of the alternating magnetic field is proportional to the magnetic field. The inventor of this magnetometer claims that it is able to measure field strengths or changes in field strength of about -4 4 gauss (10 gamma).

Unfortunately, the vibrating ferromagnetic material seriously distorts the field being measured, so there are problems in using the magnetometer of patent No 464,825 for accurate measurements of the absolute value of a magnetic field. In addition, that magnetometer cannot be used for precision measurements of the directional components of a magnetic field.

The magnetometer described and claimed in the specification of Australian patent No 471,986 relies upon the reciprocal movement of a conductor in a magnetic field to produce a signal that is indicative "T 3 of the field strength. The movement is effected by mounting the conductor on the surface of a body to which a surface elastic wave is applied. Typically, but not necessarily, the material of this body is a piezo-electric material. Preferably the conductor is in the form of a meander wire, with the spacing of the parallel strips in the meander wire being half the wavelength of the surface elastic wave. In the optimum design of the magnetometer, there are two meander wire conductors mounted on respective bodies, and the planes of the meander wire (and thus of the respective surface elastic waves) are at right angles to each other. As with the magnetometer of Australian patent No 464,825, the output signal of the sensor (in this case the meander wire or wires) is an analogue indication of the strength of the magnetic field being measured.

Although the magnetometer of patent No 471,986 is asserted to be an instrument which produces an accurate measurement of the absolute value of the strength of a directional component of a magnetic field, there is a problem with the stability of that magnetometer. The absolute value indicated depends upon the amplitude and frequency of the surface elastic wave and this depends upon factors such as the ambient temperature. In addition, the properties of the meander wire vary with ambient conditions. Since a small change in the frequency of oscillation of the meander wire affects the sensitivity of the magnetometer by a 4significant amount, there is a drift in sensitivity which is such that for precise measurements of magnetic field, that magnetometer has to be regarded as unstable.

Disclosure of the present invention It is an object of the present invention to provide a new form of magnetometer which is very sensitive, is stable, and is directional in its operation.

This objective is achieved by using the known technique of oscillating a sensing conductor within the magnetic field that is being measured, but mounting the sensor in this case a coil within a solenoid and passing a current through the solenoid until the magnetic field established by that current 6 15 is equal and opposite to the magnetic field that is

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o being measured. When the magnetic fields cancel each other, the sensor is moving in a region of zero magnetic field, and no signal is generated by the sensing coil. Preferably, a feedback loop is used to control the magnitude of the current through the solenoid. In such an arrangement, of course, the driver of the sensing coil must be non-magnetic.

Thus, according to the present invention, there is provided a method for measuring or monitoring a directional component of a magnetic field comprising:rr, c- aligning the axis of a solenoid with the direction of the directional component of the magnetic field; mounting a detector coil within and coaxial with the solenoid; vibrating a section of the detector coil to periodically increase and decrease the area thereof; monitoring the signal induced in the detector 10 coil; passing a DC current through the solenoid to 0 ogenerate therein a magnetic field which is o b opposed to the component of the magnetic field which is being monitored; and monitoring the current through the solenoid when the signal induced in the detector coil is zero.

Preferably the vibration of a section of the detector coil is effected by a piezo-electric driver and the current through the solenoid is controlled by a feed-back loop that is sensitive to the signal induced in the detector coil.

Also, according to the present invention, there is provided a magnetometer comprising: _i 1--i l- urc~ -6a solenoid nraew ite a-ma -l a detector coil mounted within the solenoid and on the axis thereof; non-magnetic means for cyclically vibrating part of the detector coil to periodically increase and decrease the area of the detector coil; means for monitoring signals induced in the detector coil; means for passing a current through the solenoid to generate a magnetic field on the axis thereof; and 0 means for monitoring the current through the solenoid.

Preferably the means to monitor the signals induced in the detector coil comprises an amplifier, filter and multiplier, in series. The detector coil output is thus amplified and filtered before being applied to the multiplier, which acts as a synchronous detector. In the multiplier, the amplified and filtered signal is multiplied with the voltage source used to power the vibrating means. The multiplier output is applied to an integrator, the output of which forms the current drive through the solenoid.

In this arrangement, the output from the multiplier is a DC voltage, the polarity of which is determined by the phase difference between the signals from the detector coil and from the voltage source powering I.0 7 the vibrator. The amplitude of the output from the multiplier is primarily determined by the magnitude of the detector coil output, for the voltage source for the vibrator has a substantially constant amplitude. The output from the multiplier will also have AC voltage components of twice the vibrating frequency and other harmonics of this frequency, but these AC components are filtered out to insignificant levels in the integrator, which acts as a first order low pass filter.

Typically, the non-magnetic means for vibrating at least part of the detector coil will be a piezo-electric transducer, driven by an oscillatory voltage source. The vibration of part of the detector coil will then be effected by bridging a section of the detector coil across the piezo-electric transducer.

The current through the solenoid when there is a zero output signal from the detector coil will give a direct indication of the strength of the component of the field being observed in the axial direction of the solenoid. Applying an appropriate calibration factor, this current can be converted into a digital signal, directly indicating the observed magnetic field strength. The direction of alignment of the axis of the solenoid can also be indicated in digital form, to show the direction of the component of the observed field that has been measured.

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8 When the magnetometer is indicating a balance of the observed and the generated magnetic fields, any perturbation of the field being observed will produce an output signal from the detector coil and hence a polarised error signal into the integrator. This signal can be used to produce an alarm and/or the system can be adjusted so that the change in magnetic field is measured and recorded.

These and other features (some optional) of the present invention will become more apparent from the following description of a preferred embodiment of magnetometer. In this description, reference will e made to the accompanying drawings.

Brief description of the drawings Figure 1 is a partly schematic block diagram of a o preferred embodiment of the magnetometer of the present invention.

Figure 2 is a sectional view (not to scale) of one form of detector coil that may be used in the present 20 invention.

Detailed description of the illustrated embodiment The basic principle utilised by the magnetometer of the present invention is that when a conductive wire is vibrated in a constant magnetic field of strength B which has a component that is perpendicular to both the line of the wire and the plane of vibration, an e.m.f, is induced in the wire, and that e.m.f is i 9 9 proportional to both the component of the field strength and the rate of vibration. ""'hen the wire is replaced by a section of a coil, which is mounted so that vibration of the section of the coil changes the area of the coil that is within the magnetic field, an e.m.f. is induced in the coil. This e.m.f. is also proportional to both the magnetic field and the rate of vibration. Thus a measurement of the e.m.f.

is a measurement of the field strength. (Note that if the entire coil should be vibrated in the magnetic field so that the area of flux enclosed by the coil remains constant, the e.m.f. induced in any part of the coil would be exactly cancelled by the e.m.f.

induced in another part of the coil; thus the net e.m.f. induced would be zero.) If a section of a coil is vibrated when the magnetic field perpendicular to the transverse plane of the coil and to the direction of vibration is zero, then there is no e.m.f, induced in the coil.

20 In the magnetometer illustrated in Figure i, a detector coil 10 is located within a solenoid 11.

This solenoid is positioned so that its axis is parallel to the component of the steady magnetic field that is to be measured. The detector coil is positioned on the axis of, and coaxial with, the solenoid 11 and part of the detector coil is oscillated in a direction transverse to the axis of the solenoid 11 and the coil 1 10 One construction that may be adopted to oscillate part of the detector coil is illustrated in Figure 2.

In this construction (which has been adopted in a prototype that has been built to test the present invention), a small plastic block 21 is mounted on a brass plate 22 that is bonded to one surface of a piezo-electric transducer 12. The transducer 12 is driven by a conventional circuit (not shown) powered by an alternating voltage source 13 (see Figure 1), tuned to the resonance frequency of the transducer.

The brass plate 22 is mounted in a support 24, which may be fabricated from any suitable non-magnetic material. The detector coil comprises a number of windings (30 turns were used in the prototype instrument) which tightly encircle the support 24 (which also acts as a former for the coil) and pass over the top of the block 21. When the piezo-electric transducer is activated, the plate 22 is driven into an oscillatory movement and the block 21 moves by the amount which the centre of the plate 22 moves. Movement of the block 21 causes a corresponding movement of that part of the windings of coil 10 which pass over the top of the block 21. In the prototype equipment, the block 21 was 5mm tall and 8mm wide, and moved, under the influence of the transducer 12, an amount of about microns.

11 When the top section of the coil 10 is oscillated within the magnetic field being observed, a voltage signal is generated in the detector coil 10 when the current through the solenoid 11 is zero or when the current in the solenoid does not produce an internal magnetic field that completely balances the steady state field. This alternating induced voltage signal is amplified by an amplifier 14, is then filtered by a band pass filter 15 (tuned to the frequency of the oscillator 13) and is synchronously detected by a *multiplier 16.

The voltage source (oscillator) 13 also generates a signal that is input into the multiplier 16. The output of the multiplier 16 is applied to an integrator 17 which produces a DC output current, in response to the signal applied to it. The DC output current from the integrator 17 is passed through the solenoid coil 11 and through an ammeter 18.

The current through the solenoid coil 11 anca -Jw o 20 ammeter 18 establishes a magnetic field on the ax of the solenoid coil 11. This established field is in a direction opposed to the direction of the steady-state magnetic field component that is being investigated.

12 As the established field increases, the e.m.f.

induced in the detector coil 10 falls, the amplitude of the output from the multiplier 16 falls and the integrated value of the DC current output of the integrator 1.7 increases more slowly. When the established field is equal in value to the steady-state field, the output of the integrator 17 is a steady value. The current then flowing through the ammeter 18 is an analogue signal which is directly proportional to the strength of the steady-state field.

This arrangement is equivalent to a first-order feedback system in which, under steady state conditions, the error signal, which is represented by the detected output of the coil 10, is zero.

The current through the ammeter 18 may be recorded, or may be converted into a digital output signal which is multiplied by the appropriate calibration factor to produce a direct reading of the field strength. This calibration factor is determined by the physical parameters of the solenoid coil 11.

If required, the magnetometer may be mounted so that the orientation of the axis of the solenoid coil 11 can be varied. In this arrangement, a signal which is indicative of the orientation of the solenoid axis may be generated (using known technology) and the direction of the magnetic field component that has

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lll_--C 13 been measured may be displayed (and/or recorded) in addition to the absolute value of the observed field strength.

If the magnetometer is to be used to detect and/or measure small variations in the steady state magnetic field (for example, in equipment for the detection of the presence of submarines in a monitored volume of water), a change in the strength of the steady state magnetic field can be recorded and/or may be used to trigger an alarm signal.

Electronic engineers will recognise that a benefit of the present invention is that the magnetometer components, apart from the detector coil, are all items that are available on an off-the-shelf basis or So that are well known in this field. No special new circuitry is required to construct and operate the magnetometer.

Although a specific embodiment of the present o invention has been illustrated and described above, those skilled in this art will appreciate that modifications to the magnetometer can be made without departing from the present inventive concept.

Industrial applications Some of the applications of the present invention have been noted already. However, the inherent stability and high degree of accuracy that can be obtained with the magnetometer of the present 14 invention makes it useful in any magnetic field measuring technique. Among the specific applications for which the invention is particularly suitable are the detection of minor perturbations of the earth's magnetic field due to the intrusion of ferromagnetic materials (such as submarines, noted above), airborne magnetic surveys (in which anomoly detection is important), portable land compass systems and small craft maritime compass applications. This list is not exhaustive.

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Claims

2. A method as defined in claim i, in which the vibration of a section of the detector coil is effected by a non-magnetic piezo-electric driver which is mounted within the detector coil. r i ~~II XI~V-IX.III. li-r- i~LV(-.iilii- I-L- ll._- 16
3. A method as defined in claim 1 or claim 2, in which the current through the solenoid is controlled by a feed back loop that is responsive to the signal induced in the vibrating detector coil.
4. A method as defined in any preceding claim, in which the magnetic field being measured or monitored is a steady state magnetic field, so C that the current through the solenoid when a zero signal is induced in the detector coil is a constant current, and including the step of observing when the constant current through the solenoid changes from its constant value, thus indicating a perturbation of the steady state magnetic field. A method as defined in claim 4, including the step of initiating an alarm signal in response to the change of the constant current through the solenoid.
6. A magnetometer comprising:- a solenoid: t--ed ith g-cic a detector coil mounted within the solenoid and on the axis thereof, with the axis of the detector coil aligned with the axis of the solenoid; 17 non-magnetic means for cyclically vibrating part of the detector coil to periodically increase and decrease the cross-sectional area of the detector coil; means for monitoring signals induced in the detector coil; means for passing a DC current through the solenoid to generate a magnetic field on the axis thereof; and means for monitoring the DC current through the solenoid.
7. A magnetometer as defined in claim 6, in which said means for passing a DC current through said solenoid includes a feedback loop that is responsive to the signal induced in the detector coil.
8. A magnetometer as defined in claim 6, in which: said means to monitor the signal induced i in the detector coil comprises a circuit which includes, in series, an amplifier and a filter, the output of said circuit providing a first input to a multiplier; (ii) said means for cyclically vibrating part of the detector coil is powered by an alternating current source, the output of which is also connected to said multiplier to provide a second input thereto; and 18 (iii) said means for passing a DC current through the solenoid comprises an integrator which generates an output DC signal which has a magnitude that is controlled by the output signal from said multiplier.
9. A magnetometer as defined in claim 6, claim 7 or claim 8, in which said non-magnetic means for cyclically vibrating part of the detector coil comprises a non-magnetic piezo-electric driver °which is mounted within the detector coil. A magnetometer as defined in any one of claims 6 1 to 9, operating to monitor the strength of a predetermined directional component of a steady state magnetic field, whereby the axis of the solenoid is aligned with the lines of force of the magnetic field, the DC current through the solenoid is a constant current and the signal induced in the detector coil is zero, the magnetometer including means responsive to a change in the constant current through the solenoid to provide an indication of a perturbation of the steady state magnetic field.
11. A magnetometer as defined in claim 10, in which said means responsive to a change in the constant current through the solenoid comprises means for initiating an alarm signal. 19
12. A method of measuring or monitoring a directional component of a magnetic field, substantially as hereinbefore described with reference to the accompanying drawings.
13. A magnetometer constructed substantially as hereinbefore described with reference to the accompanying drawings. to or indicated in the specification and/or claims and/or drawings -his application, Sindividually or cl ectively and any and all combin -s of any two or more of said parts, 0 0 o DATED this tenth day of February 1988 a CEA TECHNOLOGIES PTY LTD by its Patent Attorneys DAVIES COLLISON