GB2194054A - Magnetometer - Google Patents

Abstract

A magnetometer operates by measuring the change in Youngs Modulus of an amorphous metal sensor part (1) when a magnetic field (9) is present. This measurement is made by forming an oscillator (4) from the amorphous metal (6) (7) (8) and measuring the changes in its oscillation when a magnetic field is present. The oscillator may be caused to vibrate by a light beam from pulsed laser diode (10) of a photoacoustic drive measures (2). The vibrations of the amorphous metal are detected by detecting at (12) a reflected light beam from laser diode (13) which is modulated by the vibrating amorphous metal. Alternatively Youngs modulus may be determined by measuring the speed of sound through the metal on applying a static stress and sensing deformation. <IMAGE>

Description

SPECIFICATION A magnetometer This invention relates to magnetometers.

Known magnetometers are electrically read and powered so they can be unsuitable for use in areas where sparks could cause explosions or in areas, such as generating plants, where high levels of electromagnetic interference nay be encountered. Flux gate magnetometers, which are one of the most common types of magnetometer, also have the drawbacks that they require a high current supply, which it can be difficult and inconvenient to provide, and that they contain bulky coils which makes it difficult to miniaturize them.

This invention developed from the realisation that it would be possible to construct a magnetometer which does not suffer from any of these drawbacks by the use of an amorphous metal exhibiting the bE effect. This is the name given to the effect where the Youngs modulus (E) of an amorphous metal varies with the strength and direction of the applied magnetic field.

This invention provides a magnetometer including an element comprising an amorphous metal having a Youngs modulus dependent on an applied magnetic field and detecting means for producing a signal dependent on the Youngs modulus of the amorphous metal element and thus the applied magnetic field.

By employing the invention it is possible to avoid the need for high currents and bulky coils because the Youngs modulus of a metal can be measured without them.

The Youngs modulus of a material is the ratio of stress to strain so changes in it can be sensed by applying stress and sensing some resultant depending on strain. There are a number of methods of doing this.

It is preferred to measure the Youngs modulus of the amorphous metal by forming a resonant element from the amorphous metal and measuring changes in the movement of the resonant element associated with changes in the Youngs modulus. Other methods of measuring the Youngs modulus of the amorphous metal would be to measure the speed of sound through it or to apply a static stress and sense changes in the deformation produced which is a function of strain.

Preferably the resonant element is a balanced oscillator because a balanced oscillator is efficient to drive. Also a balanced oscillator is "isolated" from its supports: that is to say there is very little coupling, in a perfectly balanced. oscillator none, between the oscillating and non-oscillating parts of the system so any changes in the mounting of the system will not effect the oscillation.

The resonant frequency of the element is preferably measured and this measurement used to calculate the applied magnetic field.

This technique is preferred because it allows very accurate measurements of Youngs modulus. The resonant element could alternatively be arranged to be driven at a constant frequency and the changes in the amplitude of oscillation, and thus the Youngs modulus, deduced.

It would be possible to drive the resonant element capacitively by charging the resonant element and an adjacent plate so that electrostatic forces between them cause the resonant element to oscillate, but preferably the resonant element is powered optically, using the photo acoustic effect, and its movements are measured optically. This is advantageous because all links between the magnetometer sensor and its driving and reading systems are made by optical fibres and are as a result immune to electro-magnetic interference and will not cause sparks under any circumstances.

It is preferred to form the resonant element from a piece of amorphous metal. A layer of amorphous metal on a substrate might be preferable in some circumstances but this can give problems due to differential thermal expansion of amorphous metal and substrate.

If a layer of amorphous metal on a substrate were to be used quartz would be preferred as the substrate because the piezoelectric properties of quartz would allow a piezoelectric drive to be employed. A piezoelectric drive could comprise gold electrodes on two faces of the resonant element arranged such that when an alternating voltage is applied between them the electric field set up would cause the resonant element to oscillate at the frequency of the alternating voltage.

The amorphous metal preferred for the resonant element is an alloy consisting of 80% iron 13% phosphorous and 7% carbon because it has the largest change in Youngs modulus relative to applied magnetic field of any amorphous metal tested. It is of course possible that some other amorphous metal may be preferred in some circumstances.

An embodiment of the invention will now be described with reference to the accompanying drawing of a magnetometer shown partiy as a perspective view and partly as a schematic block diagram.

The magnetometer comprises two parts, a sensor assembly 1 and a driving and reading assembly 2. The sensor assembly 1 can be remote from the driving and reading assembly 2 and is linked to it by a single optical fibre 3.

The sensor assembly 1 comprises a balanced oscillator 4 formed entirely from an alloy of 80% Fe 13% P and 7% C that has been annealed in a high magnetic field. This alloy is formed into the balanced oscillator 4 by etching with Ferric Chloride. The oscillator 4 consists of a support 5 and three arms 6,7 and 8 arranged so that arms 7 and 8 oscillate in anti-phase to the central arm 6. The oscillator 4 is sensitive only to changes of Youngs modulus in the direction along the arms i.e.

the direction of arrow 9, and so the magnetic sensor assembly can only detect the strength of the magnetic field in this direction.

The balanced oscillator 4 is caused to resonate by modulated pulses of light from a pulsed laser diode 10 which are directed onto the central arm 6. The pulses of light from laser diode 10 travel along an optical fibre 11.

These pulses of light cause balanced oscillator 4 to resonate due to the photo-acoustic effect, i.e. the pulses of light cause local heating of the central arm 6 and the expansion due to this heating causes vibration of the balanced oscillator 4 at the modulation frequency of the pulses of light. It is preferred to use a pulsed laser diode pulsating at a high frequency for convenience, since the pulsation rate of such a diode will usually be far higher than the resonant frequency of the resonant element the pulsed laser diode will be modulated. If the frequency of modulation of the light is the same as the resonant frequency of balanced oscillator 4 it will resonate.

The resonation of balanced oscillator 4 is measured by a photo-detector 12 and a continuous wave laser diode 13, at a different frequency to, and lower power than, pulsed laser diode 10.

The light from laser diode 13 is fed down optical fibre 14 to a unidirectional coupler 15 where it is combined with the pulses of light from laser diode 10. The combined signals then pass through a second unidirectional coupler 16 and along a fibre 3 which terminates at the sensor assembly 1.

The end of the fibre 3 is supported so that the light from it falls on the central arm 6 of the balanced oscillator 4.

The light from the laser diode 13 is reflected from the central arm 6 back along a path which, because of the vibration of the arm 6, is periodically swept across the end of the fibre 3. Light returning along the fibre 3 is thus modulated in dependence on the movement of the central arm 6. This modulated light travels along fibre 17 and through an optical filter 18, to the photo-detector 12. The filter 18 is a narrow band pass filter centred on the frequency of laser diode 13 and prevents the pulsed light from laser diode 10 reaching the photo-detector 12 after being reflected from the arm 6.

The signals produced by photo-detector 12 are fed to a timing circuit 19. Timing circuit 19 works out the frequency at which central arm 6 is resonating from the modulation of the light from continuous wave laser diode 13 and uses this information to hold the pulse modulation frequency of pulsed laser diode 10 at the resonant frequency of balanced oscillator 4 as this resonant frequency changes.

The signals from timing circuit 19 are also supplied to a computing unit 20 which uses an algorithm to calculate the magnetic field strength from the resonant frequency of the balanced oscillator 4.

The field strength calculated by computer 20 is then displayed by a display 21.

This system is only capable of sensing the magnetic field strength in direction 9. A more practicai system would have three balanced oscillators sensing magnetic field strength in three orthogonal directions, each oscillator being separately driven and read. The three field strengths could be calculated together or calculated separately and combined by trigonometry.

It should be noted that the driving and reading assembly 2 is the subject of a co-pending application.

Claims (10)

1. A magnetometer including an element comprising an amorphous metal having a Youngs modulus dependent on an applied magnetic field and detecting means for producing an output signal dependent on the Youngs modulus of the amorphous metal element and thus the applied magnetic field.

2. A magnetometer as claimed in claim 1 in which the element has a resonant frequency depending on the Youngs modulus, in which drive means is included to cause the element to vibrate, and in which the detecting means is constructed and arranged to detect the vibration and to produce the output signal as a function of a characteristic of the vibration.

3. A magnetometer as claimed in claim 2 in which the drive means is such as to cause the element to vibrate at its resonant frequency and in which the output signal is a function of the frequency of vibration.

4. A magnetometer as claimed in claim 2 or 3 in which the element is a balanced oscillator.

5. A magnetometer as claimed in any one of claims 2 to 4 in which the drive means is photo-acoustic.

6. A magnetometer as claimed in any one of claims 2 to 5 in which the detecting means is adapted to detect a beam of light modulated by vibration of the element.

7. A magnetometer as claimed in any one of claims 2 to 6 in which the element comprises a layer of an amorphous metal on a substrate.

8. A magnetometer as claimed in claim 7 in which the substrate is quartz.

9. A magnetometer as claimed in any preceding claim in which the amorphous metal is an alloy containing Iron, Phosphorous and Carbon.

10. A magnetometer substantially as illustrated in figure 1 of the accompanying illustrations and substantially as described with reference thereto.