GB2331157A

GB2331157A - An arrangement for determining the geometry and gradients of magnetic fields

Info

Publication number: GB2331157A
Application number: GB8521154A
Authority: GB
Inventors: Gerhard Trenkler; Gernot Strmming; Wolfgang Bornhofft
Original assignee: Licentia Patent Verwaltungs GmbH
Current assignee: Licentia Patent Verwaltungs GmbH
Priority date: 1985-08-23
Filing date: 1985-08-23
Publication date: 1999-05-12
Anticipated expiration: 2005-08-23
Also published as: GB8521154D0; GB2331157B

Abstract

For determining the strength and gradient of magnetic fields caused by magnetic objects, sensors are are arranged around the object, and pairs 5 of these sensors are arranged in parallel at a predetermined spacing of their sensitive axes. They are electrically connected so that the difference in magnetic field strength which is detected by the individual sensors is formed and provides an indication of the gradient of the field and a sum signal is also derived.

Description

"An arrangement for determining the geometry and gradients qf magnetic fields." The invention relates to an arrangement for mapping the geometry and gradients of magnetic fields.

As is known, an approximate image of the magnetic field of magnetisable objects may be obtained in magnetic measurement systems, using sensors which are in a plane parallel to the earths surface and of which the sensitive axes extend normal to the earths surface.

These approximate images of the magnetic field are used for example in the German Navy and in navies of other countries, in order to ensure that the magnetic fields of ships do not exceed a predetermined value, thus reducing the danger from magnetic mines.

These images are also used to determin the effectiveness of such mines against ships in model simulations using ellipsoid models of the magnetic field of the ship, for example.

The differences which show themselves during such measurement as compared to the actual field image are so great that more modern systems use triple sensors which detect the field in three dimensions. In the case of objects with magnetic protection inherent in them, the field is still insufficiently detected particularly close to the object because there is no information about the reduction in intensity as the spacing or distance of the object increases. This is apparent for example with model simulations for determining the effectiveness of mines in which the input parameters for the simulated mines are obtained from ellipsoid models of the ships magnetic field.

These models can predict magnetic field at larger distances than the distances used during measurement to a sufficient degree of accuracy; however if the magnetic field is calculated at a smaller distance or spacing than the measurement spacing or distance, then the errors which occur are of the order of magnitude of the real magnetic field at this location.

Therefore this type of model fails to calculate the effectiveness of modern shallow water mines with a gradient ignition system and fails to enable measures to be taken to protect objects from these mines.

An improvement in simulation of magnetic fields can be achieved if a formula derived from the theory of electric fields is used.

The magnetic field of a magnetisable object can be represented outside the object in a range which does not include any magnetic sources by the scalar potential ; B = -grad * The Maxwell equation applies here: div B = as well as the Laplace equation:

Using the solution set =

subject to the secondary condition

the z-component of the magnetic field may be given thus:

On condition that the z-component of the magnetic field is very small for very large distances, then

so that the constant C2 is zero.

The z-component of the magnetic field is therefore

where Y is the gradient of the magnetic field in the z direction and can be approximated at the measurement location by taking

The two A and B brackets can be expanded in this case as Fourier series which include the constant C1 as a multiplication factor, taking account of the fact that the brackets only have to be calculated for a limited plane.

Therefore:

gives the z-component of the magnetic field.

The individual coefficients of the Fourier series A and B can be determined if the parameter z is inserted related to the measurement depth zero:

The parameters of the Fourier series A and B are able to be determined merely from the magnetic field measured in one plane, as is possible with measurement systems already available.

The gradient of the magnetic field cannot be determined, however, in this way.

The object of the invention is to provide an arrangement which avoids these disadvantages when measuring magnetisable objects and which makes it possible to determine the path of magnetic fields more accurately.

According to the invention, there is provided a device for simultaneous determination of the intensity and gradient of magnetic fields which are caused by a magnetisable and/or conductive object in the earths magnetic field, which comprises probes or sensors which are mounted in a common carrier and which deliver an electrical output signal in proportion to the said magnetic field, the probes or sensors being arranged at a number of spaced points in said carrier, wherein at each said point, two sensors are arranged with their sensitive axes parallel and at a pre-determined spacing, each pair of sensors being connected electrically so that the difference in the magnetic field strengths detected by the individual sensors is formed, which is a measure of the gradient of the said field,and also so that the sum of said field strengths is calculated.

The major advantage of the arrangement according to the invention is that the gradient of the field path of magnetic field is required in order to describe the field more accurately, and this can be ascertained thereby.

This is of particular importance for modern gradient type mines.

The invention will be described in greater detail by means of an examplary embodiment as shown in the drawings.

Fig.l shows an arrangement for determining the geometry and the gradient of magnetic fields, and Fig.2 shows the basic construction of the sensors and a block circuit diagram of a part of the measuring arrangement.

As seen in Fig.l the arrangement comprises a sensing area 2 having individual probes or sensors 3 which are located on a common sensor support 1.

This measuring arrangement may be arranged as shown on a sea bed, on the bed of a lake or river etc or at a predetermined depth in the water. The object which is to be measured, in this case a ship, is located above the measuring arrangement.

The spatial relationship shown in Fig.l between the object which is to be measured and the sensing area 2 need not necessarily look identical to that shown in Fig.l; the sensing area may also be located above, adjacent, in front of or behind the object which is to be measured. It is also possible for the sensing area to consist of only a single line of sensors.

Similar arrangements may be located on land for the measurement of ferro magnetic objects such as plant, components of plant, vehicles or tanks.

As seen in Fig.2 the probe or sensor 3 includes two sensing devices 5a and 5b which operate by way of example, using direct time encoding. The sensitive axes of the two sensors 5 lie in the longitudinal direction of the said probes. These probes 3 may be arranged in relation to their sensitive axes, adjacent to or on top of each other. If the probes 3 are vertically arranged then the vertical component of the magnetic field of the object to be measured is ascertained.

Another arrangement is horizontal alignment of the sensors 5 which may be arranged either end to end or in parallel adjacent each other.

The arrangement may be implemented longitudinally or in a transverse direction in relation to the object to be measured. Using longitudinal probes, transverse probes and a vertical probe, triple probes can be constructed which detect the three components of the magnetic field at a location as well as the spatial gradients in one field direction.

In another embodiment of the probe two or three sensors may be provided in each case in the probe at the location of the sensors 5 in order to detect the magnetic field and the geometric gradients of the magnetic field in two or three components.

Both sensors 5a and 5b of the probe 3 are controlled by an initial magnetising generator 6 in which the output voltage is passed or converted into a pulse length modulated digital signal with the aid of a comparator 7 said digital signal being converted in successively connected code conversion units 8 into a binary number in each case.

From the two output signals of the code converters 8 the difference S in magnetic field values at the location of sensors 5a and 5b is determined in a difference forming stage 9 corresponding to the gradient between the two sensors 5.

In a further embodiment of the invention the values of the magnetic field at the location of sensors 5a and 5b are added together in a further stage 10 from the two output signals of the code converters 8.

Claims

Patent Claims 1. A device for simultaneous determination of the intensity and gradient of magnetic fields which are caused by a magnetisable and/or conductive object in the earths magnetic field, which comprises probes or sensors which are mounted in a common carrier and which deliver an electrical output signal in proportion to the said magnetic field, the probes or sensors being arranged at a number of spaced points in said carrier, wherein at each said point, two sensors are arranged with their sensitive axes parallel and at a predetermined spacing, each pair of sensors being connected electrically so that the difference in the magnetic field strengths detected by the individual sensors is formed, which is a measure of the gradient of the said field,and also so that the sum of said field strengths is calculated.
2. A device according to claim 1 wherein that sensors having a magnetisable core material are used.
3. An arrangement according to claim 1 or 2 wherein sensors comprising semiconducting material are used.
4. An arrangement according to claim 1,wherein three pairs of sensors are connected together at each point to form an orthogonal triple.
5. An arrangement according to claim 5 characterised in that the triple of pairs of sensors is arranged in a plane below the object.
6. An arrangement according to claims 1 to 4 characterised in that the sensitive axes of the pairs of sensors are aligned in parallel with the normal to the earths surface and the points are in a plane parallel to the earth's surface.
7. An arrangement according to claims 1 to 4 characterised in that the pairs of sensors are aligned in parallel with the earth's surface and that their sensitive axes are on a straight line.

7. An arrangement according to claims 1 to 4 characterised in that the pairs of sensors are aligned in parallel with the earth's surface and that their sensitive axes are on a straight line.

Amendments to the claims have been fled as follows

1. A device for simultaneous determination of the intensity and gradient of magnetic fields which are caused by a magnetisable and/or conductive object in the earths magnetic field, which comprises an array of probes each having sensors which are mounted in a common carrier and which deliver an electrical output signal in proportion to the said magnetic field, the probes being arranged at a number of spaced points in said carrier, wherein at each said point, two sensors are arranged with their sensitive axes parallel and at a pre-determined spacing, each pair of sensors being connected electrically so that the difference in the magnetic field strengths detected by the individual sensors is formed, which is a measure of the gradient of the said field,and also so that the sum of said field strengths is calculated.

2. A device according to claim 1 wherein sensors having a magnetisable core material are used.

3. An arrangement according to claim 1 or 2 wherein sensors comprising semiconducting material are used.

4. An arrangement according to claim wherein three pairs of sensors are connected together at each point to form an orthogonal triple.

5. An arrangement according to claim 5 characterised in that the triple of pairs of sensors is arranged in a plane below the object.

6. An arrangement according to claims 1 to 4 characterised in that the sensitive axes of the pairs of sensors are aligned in parallel with the normal to the earths surface and the points are in a plane parallel to the earth's surface.