DE69100832T2 - METHOD FOR THE AUTOMATIC COMPENSATION OF THE MAGNETIC MUST OF A FERROMAGNETIC MATERIAL INDUCED BY THE MAGNETIC FIELD OF THE EARTH, ESPECIALLY A SHIP. - Google Patents

Description

The present invention relates to methods that allow the automatic compensation of the magnetizations induced by the Earth's magnetic field in structures made of ferromagnetic materials.

In particular, the present invention relates to a method for calculating the conductor loop effects due to vertical, transverse and longitudinal magnetizations in ferromagnetic structures.

It is known that the presence of ferromagnetic materials in a ship makes that ship detectable by means of detecting its "magnetic signature", which may be integrated, for example, in mines or in other ships.

The "magnetic signature" of a ship is formed by its permanent magnetization and its induced magnetization.

The permanent magnetization of a structure is due to the ferromagnetic materials from which it is formed. It is essentially constant, only changing over time and is related to the nature of the material used.

The induced magnetization, on the other hand, is significantly variable. In the case of a ship, it depends on its orientation in the Earth's field, its course and its inclination due to rolling and pitching motion.

The ship's magnetic signature therefore makes it possible to locate it, track it and possibly guide or detonate guided missiles designed to destroy it.

It is therefore very important to minimize this "magnetic signature", namely to eliminate it, in order to prevent its detection by the magnetic process.

This so-called "demagnetization" operation is carried out in a known manner by generating a magnetic field in the volume of the ship that compensates for that of the ship in order to eliminate its magnetic signature. For this purpose, the ship is provided with a set of conductor loops called "demagnetization loops" through which electrical currents flow.

The dimensions and arrangement of the loops and the currents flowing in them are determined in such a way as to best minimize the "magnetic signature" of the ship, regardless of its orientation in the Earth's magnetic field, i.e. regardless of its heading and of its inclination due to rolling and pitching. These demagnetization loops are distributed in three directions corresponding to the roll axis, the yaw axis and the pitch axis, which are conventionally called "R, G, N" or "L, V, T" (longitudinal, vertical, transverse).

A magnetic measuring station is used to determine the currents to be sent through the demagnetization loops. This station measures the magnetic signature of the ship and comprises, for example, two linear networks of magnetic sensors. These networks are installed on the seabed, one of them oriented north-south and the other east-west. A station installed on land analyses the measurements taken to determine the currents to be sent into the various loops and their polarity.

Since the permanent magnetization is constant regardless of the position of the ship and its course, the current to be sent into each loop to compensate for the permanent magnetization is constant.

On the other hand, the induced magnetization is variable, so it is necessary to superimpose on the already calculated currents another variable component, which is determined according to the course and the geographical latitude of the ship, which in turn are obtained, for example, by gyroscopic or optical measuring devices. The total magnetization (permanent + induced) is written as follows:

PL + PV + PT + IV + ILcosθ + ITsinθ

where: P is the permanent magnetization,

I is the induced magnetization,

L, V and T are the three axes along which the demagnetization loops are arranged,

θ represents the magnetic heading of the ship.

The measuring station must determine these six components. To do this, the ship travels the same trajectory twice over the magnetic pickup network, on opposite courses. The permanent magnetization associated with the ship rotates with it, while the induced magnetization does not. To know the induced magnetization, it is sufficient to subtract the measurements in the two opposite directions. If the induced magnetization and the total magnetization are thus known, the permanent magnetization is obtained directly. Thus, a voyage in a north-south direction and then in a south-north direction allows the determination of IL, after which a voyage in an east-west direction and then in a west-east direction allows the calculation of IT.

However, this procedure does not allow the determination of IV.

The determination of the vertical induced magnetization is then carried out by empirical methods. One of these methods, which is used in most cases, is based on the consideration that the vertical induced magnetization represents a certain fixed percentage of the total vertical magnetization measured at a given location.

However, this method remains a very close approximation. In addition, due to the presence of machines, engines, propulsion systems, etc., certain parts of the ship are more magnetic than others, which is why the results of this evaluation method are erroneous.

Another method is to perform these adjustments twice at two very different latitudes and to make approximations with respect to the measurement differences.

This method has the disadvantage that it requires a long and expensive voyage at sea for the ships to be demagnetized and assumes that everything else remains unchanged.

Therefore, a third method is also used, which consists in spatially reproducing a magnetic field environment different from the Earth's magnetic field and managing the difference in the measurements obtained. This method does not require long journey, but it entails the operation of very expensive equipment designed to generate a different magnetic field throughout the entire vicinity of a ship.

The invention aims in particular to eliminate the above-mentioned disadvantages.

More precisely, a first object of the invention is to apply to ships with ferromagnetic structures a method that allows a non-empirical compensation calculation of the induced vertical magnetization without special infrastructure.

A second object of the invention is to be able to apply this calculation to the compensation of the induced magnetizations in the horizontal and transverse planes.

A third object of the invention is to enable the calculation of such a compensation without a long voyage of the ship and at low cost.

A fourth object of the invention is to calculate the magnetization to be compensated, regardless of whether the position and shape of the demagnetization loops contained in the ship are known or not.

These objects, as well as others which will become clear hereinafter, are achieved by means of a process for the automatic compensation of the magnetizations induced by a directional component of the terrestrial magnetic field in a ferromagnetic structure, in particular a shift, which consists in causing compensating currents to flow in at least one demagnetization loop located in a plane substantially perpendicular to said directional component, characterized in that it comprises the following stages:

- the conductor loop effect (EDCN)i of each of the said demagnetising loops with core at a known height Ho and for a given reference current (Ici) is obtained by measurement;

- by calculation, the air loop conductor loop effect (EDCN)i is obtained for each of the said loops (i) at the height Ho for the same reference current Ici;

- the currents (Io)i to be introduced into each of the demagnetization loops (i) are calculated in such a way that the sum of their air loop-conductor loop effects at the level of the structure to be compensated is equal to the said directional component of the magnetic field to be compensated;

- to introduce the reference currents Ioi, the magnetic field (IM) caused by the magnetization induced along the axis in question is calculated according to the formula:

where k is the number of loops arranged in the plane under consideration;

- the currents Ii that should flow in the loops (k) surrounding the ferromagnetic structures are optimized by calculation in order to compensate for the magnetic field IM induced in the plane under consideration; and

- the currents Ii optimized in this way are introduced into the loops (k).

Preferably, the calculation of the conductor loop effect of an air loop (EDCA)i at the height Ho, in the case where the dimensions and coordinates of the loop are known, is carried out according to the Biot-Savart law with the current Ic.

Advantageously, in the case where the dimensions and the coordinates of a loop are not known, the calculation of the air loop conductor loop effect (EDCA)i of the said demagnetization loop (i) at the height Ho is carried out under the assumption that the loop with unknown dimensions and unknown coordinates is equivalent to a loop with a given simple geometry, e.g. of the polygonal or elliptical type, and that the dimensions, the coordinates and the current of the equivalent loop are optimized such that the correlation coefficient between the measured conductor loop effect of the loop with unknown dimensions and unknown coordinates is as large as possible.

According to a preferred embodiment of the invention, the calculation of the currents (Ioi) represents an optimization, in particular in the least squares sense, of a field equivalent to a field generated by a fictitious loop surrounding a ship containing the said structures and whose current is such that the field at the center of this conductor loop is equal to the directional component of the earth's magnetic field to be compensated.

Preferably, the optimization of the currents (Ii) to flow in the loops (k) surrounding the ferromagnetic structures is achieved by minimizing

and by sending the currents (Ii) into the compensation loops (i) surrounding the said structures.

The method is advantageously applied to the compensation of magnetizations induced in the vertical, transverse and/or longitudinal directions.

Further features and advantages of the invention will become clear from reading the description of a preferred embodiment of the invention given by way of example and not by way of limitation and the accompanying drawings in which:

- Fig. 1 shows the three directions of arrangement of the demagnetization loops in a ship;

- Fig. 2A shows the conductor loop effect of a loop with core;

- Fig. 2B shows the conductor loop effect of the same loop without core;

- Fig. 3A shows an example of the arrangement of "V" loops in a ship;

- Fig. 3B shows a section along AA in Fig. 3A;

- Fig. 4 shows a compensation of an induced magnetization of metallic structures using several loops;

- Fig. 5 is a flow chart showing the calculation method according to the invention for a compensation of the magnetic field by means of k loops.

Fig. 1 shows the three directions in which the demagnetization loops are arranged in a ship.

The ship 1 contains ferromagnetic structures surrounded by demagnetization loops 2,3,4. These loops are arranged in three different planes. One loop 2 serves to compensate the longitudinal magnetization and is usually called "L" loop, another loop 3 serves to compensate the vertical magnetization and is denoted "V", finally the third loop 4, which has the function of compensating the transverse magnetization, is denoted "T".

A loop is characterized by its coordinates (x,y,z) with respect to a given point, by its dimensions, by its resistance, by its shape, by the maximum current that can flow through it and by the number of turns present.

Of course, the arrangement of Fig. 1 is only illustrative, the demagnetization of all ferromagnetic structures contained in a ship requiring the arrangement of numerous loops in different planes around the structures to be compensated.

The present invention has the particular object of compensating the vertical induced magnetization by means of at least one "V" loop 3 arranged in a plane parallel to the waterline of the vessel 1.

Now the principle of the procedure is explained.

Figure 2A shows the conductor loop effect of a loop with a ferromagnetic core, designated EDCN. A ferromagnetic structure 20 is surrounded by a single loop 22, the center 21 of which is also the center of gravity of the structure.

A conductor loop effect is a measure of the difference in magnetic fields that are measured on the one hand when current is flowing through the coil and on the other hand when current is not flowing through the coil.

The conductive loop effect of the loop 22 is measured for a direct current Ic 23 of known magnitude, obtaining a curve 25 representing the magnetic field vector 24 related to the magnetization induced in the core by the loop 22.

In order to obtain the magnetic field due to the magnetization induced by the loop 22 in the structure 20, the conductor loop effect of the loop 22 alone without the core 20, denoted by EDCA, must be known. Such a loop is called an air loop.

Fig. 2B shows the conductor loop effect of an air loop. Two cases must be considered: If the coordinates and dimensions of the loop are known, this exact loop is adopted and its conductor loop effect is calculated without a core. If the characteristics of the loop are unknown, the characteristics of a loop with simple geometry are optimized.

In the case where the coordinates and dimensions of the loop are known, the conductor loop effect of the loop without core is calculated, but this conductor loop effect cannot be measured. It is therefore calculated with knowledge of the geometry of the loop 22 and their position in the vessel for the current Ic 23 and for the distance at which the measurement of the conductor loop effect of the loop with core was carried out.

This calculation is carried out, for example, using the Biot-Savart law, with which the conductor loop effect 26 of the air loop sought can be obtained, namely:

(EDCN)Ic - (EDCA)Ic

The magnetic field to be compensated is equal to the difference between the measured conductor loop effect with the structure to be compensated and the conductor loop effect of the air loop.

To obtain this field so that it is equal to that generated by the vertical component of the Earth's magnetic field, it is sufficient to inject a current Io so that the conducting loop effect of the air loop at the level of the structure to be compensated is equal to the vertical component of the Earth's magnetic field. Thus, the vertical induced magnetic field, denoted IVM, is given by:

[(EDCN)Ic - (EDCA)Ic]×Io/Ic

induced.

The magnetic field induced by the current Io flowing in the loop 22 surrounding the structure 20 to be compensated therefore compensates the earth's magnetic field and makes the ferromagnetic structure invisible to detection systems based on this principle by generating a resultant magnetic field in the vicinity of the structure that is approximately zero.

This approximation is more accurate the larger the loop is in relation to the structure to be compensated.

In the case where the coordinates and dimensions in the loop are not known, for example in the case of old ships for which there are no precise plans indicating the arrangement of the existing loops, it is necessary to approximate the size and coordinates of this loop. The conductor loop effect of the air loop is then estimated assuming that its measured conductor loop effect with core is equivalent to the conductor loop effect of the same conductor loop without core, but through which a current of different magnitude flows. It is also assumed that the loop can be reduced to a loop with a simple geometry, for example a polygonal or elliptical geometry.

The parameters of the air loop, namely the current intensity IcA, the coordinates of the center and the dimensions are optimized so as to reproduce the field of the conductor loop effect (EDCN)Ic measured at a height Ho for a known current Ic. This optimization consists in particular in varying the coordinates of the center of the loop (x,y,z) and its dimensions so that the correlation coefficient between the measured (EDCN)Ic and the calculated EDCA is equal to 1. It can be carried out, for example, using the least squares method.

You get:

(EDCA)IcA (EDCN)Ic

Because of

(EDCA)Ic = (EDCA)IcA x Ic/IcA

surrendered:

(EDCA)Ic (EDCN)Ic x Ic/IcA

This optimization also preserves the geometry of the loop so that the current Io is obtained as in the previous case. If (EDCN)Ic, (EDCA)Ic, Io and Ic are known, IVM is obtained as above.

As stated above, a ship contains several demagnetization loops, in particular the "V" loops, with Figure 3A showing an example of arrangements of "V" loops in a ship.

The demagnetization loops 31, 32, 33, 34, 35 are located in the vessel 30 at different heights. The loop 36 is a fictitious loop representing the totality of the loops 31 to 35. In the following description, i is the index referring to a loop.

The conductor loop effects (EDCN)i are measured at the station during the vessel’s settings for currents Ice and for the conductor loops in the horizontal plane.

If k "V" conductor loops are present, the field IVM is given by the previous teachings as follows:

Because (EDCN)i and Ici are known, it remains to calculate EDCAi and Ioi for the k conductor loops.

If the arrangement and geometry of the "V" loops are known, the (EDCA)i are calculated as indicated above. The currents Ioi are calculated by optimizing, for example in the least squares sense, these currents in order to generate with the air loops a field equivalent to the field generated by the fictitious loop 36 surrounding the vessel, through which a current flows, so that it generates a field at the center equal to the vertical component of the earth's field. The vertical position of this loop 36 is preferably located at the magnetic center of gravity of the ferromagnetic parts or, if this position is unknown, at the mean value of the heights of the "V" loops or on the waterline of a surface vessel.

If the location and geometry of the "V" loops are not known, the (EDCA)i as well as the Ioi are obtained by reducing to the previous case as described above.

Once the vertical induced magnetic field IVM has been obtained, the currents Ii to be sent into the loops are determined, for example, by minimizing

receive.

The method can be extended to compensations of transverse ("T") and longitudinal ("L") induced fields by applying the invention to demagnetizing conductor loops in the vertical plane along the ship's axis and to demagnetizing conductor loops in the vertical planes perpendicular to the ship's axis.

The adjustment of all degaussing conductor loops therefore requires no more than a single voyage of the vessel on any known course.

Fig. 4 shows an example of compensation of induced magnetization of metallic structures contained in a ship using multiple loops ("V", "T" and/or "L").

In this example, the ship contains five loops between its bow 40 and its stern 41. The conductor loop effects 42, 43, 44, 45 and 46 have the function of compensating the magnetic signature 47 of the ship.

Since the magnetic fields induced in the ferromagnetic structures evolve with the course of the ship and with its geographical position, it is necessary to continuously compensate these magnetic fields, regardless of whether they are longitudinal, transverse or vertical.

Therefore, the vessel in question is equipped with a three-axis magnetometer 6, 7, 8, fixed to a rigid mast 5 connected to the vessel 1 and located sufficiently far from the vessel so that the latter has no influence on the measurements carried out.

The triaxial magnetometer 6, 7, 8 provides information on the magnetic fields measured along the three axes: transverse, longitudinal and vertical. This information is transmitted to a processing unit which makes it possible to obtain the currents Ii which must be sent to the various loops of the vessel 1, as has just been described.

Thus, the compensation of the various components of the Earth's magnetic field is carried out continuously depending on the course, orientation and inclination of the ship.

It goes without saying that the greater the number of loops, the better the compensation, and that this compensation requires knowledge of the different conductor loop effects of each of the loops.

Fig. 5 is a flow chart showing the calculation method according to the invention for a compensation of the magnetic field by means of k loops.

The first operation 50 to be carried out is the measurement of (EDCN)i, i.e. the conductor loop effect of a loop i (with core) for all the loops contained in a plane of the vessel containing ferromagnetic structures whose magnetizations are to be compensated.

This measurement of (EDCN)i is performed for a given current Ici.

The following step 51 is a test step which consists in asking whether the coordinates of the centre, the dimension and the placement of the loop i are known.

If the answer is yes, step 52 consists in calculating (EDCA)i, i.e. the conductor loop effect of loop i without core, for example according to the Biot-Savart law, as well as the current Ioi.

If the answer is no, step 53 consists in approximating the geometry of loop i such that the correlation coefficient between the measured (EDCN)i and the calculated (EDCA)i is 1. (EDCA)i and then Ioi are obtained.

Once (EDCA)i is calculated, step 54 consists in determining the magnetic field IM induced in the plane under consideration according to

for all of the k loops that are in the same plane ("L", "V" or "T").

The following step 55 consists in determining the currents Ii to be sent through the k loops by minimizing

to compensate for the magnetic field IM induced in the plane under consideration.

Numerous other applications of the invention can be envisaged, which will be readily apparent to those skilled in the art.

Claims (6)

1. Method for the automatic compensation of the magnetizations induced by a directional component of the earth's magnetic field in a ferromagnetic structure, in particular a ship, which consists in allowing compensating currents to flow in at least one demagnetization loop located in a plane substantially perpendicular to said directional component, characterized in that it comprises the following stages: - obtained by measuring (50) the conductor loop effect (EDCN)i of each of said demagnetizing loops with core at a known height Ho and for a given reference current (Ici); - Obtaining by calculation (52, 53) the air loop conductor loop effect (EDCA)i for each of said loops (i) at the height Ho for the same reference current (Ici); - Calculation of the currents (Ioi) to be introduced into each of the demagnetization loops (i) such that the sum of their air loop-conductor loop effects at the level of the structure to be compensated is equal to the said directional component of the magnetic field to be compensated; - Calculation (54) of the magnetic field (IM) caused by the magnetization induced along the axis considered, according to the formula: where k is the number of loops arranged in the plane under consideration to introduce the reference currents 10; - optimization (55) by calculating the currents Ii that should flow in the loops (k) surrounding the ferromagnetic structures in order to compensate for the magnetic field (IM) induced in the plane under consideration; and - Introduction of the optimized currents Ii into the mentioned loops (k). 2. Method according to claim 1, characterized in that the calculation (52) of the conductor loop effect of an air loop (EDCA) at the height Ho, in the case where the dimensions and the coordinates of the loop are known, is carried out by the Biot-Savart law with the current Ic. 3. Method according to claim 1, characterized in that in the case where the dimensions and the coordinates of a loop are not known, the calculation (53) of the air loop conductor loop effect (EDCA)i of said demagnetization loop (i) at the height Ho is carried out assuming that the loop with unknown dimensions and unknown coordinates is equivalent to a loop with a given simple geometry, e.g. of the polygonal or elliptical type, by calculating the correlation coefficient between the measured conductor loop effect [(EDCN)Ic] of the loop with unknown dimensions and unknown coordinates and the air loop conductor loop effect (EDCA)i of the equivalent loop and by optimizing the dimensions, the coordinates and the current of the equivalent loop so that the correlation coefficient is as large as possible. 4. Method according to one of claims 1 to 3, characterized in that the calculation of the currents (Ioi) represents an optimization, in particular in the least squares sense, of a field equivalent to a field generated by a fictitious loop surrounding a ship containing said structures and whose current is such that the field at the center of this conductor loop is equal to the directional component of the Earth's magnetic field to be compensated. 5. Method according to claim i, characterized in that the optimization (55) of the currents (Ii) to flow in the loops (k) surrounding the ferromagnetic structures is carried out by minimizing and is carried out by sending the currents (Ii) into the compensation loops (i) surrounding the said structures. 6. Method according to one of claims 1 to 5, characterized in that it is applied to the compensation of magnetizations induced in the vertical, transverse and/or longitudinal direction.