EP0623506B1 - Method for the automatic compensation of the residual magnetism of a ferromagnetic tow - Google Patents

Description

The present invention relates to methods which make it possible to automatically compensate for the residual magnetization which persists in a ferromagnetic type dredge when it has been put out of service. This makes it possible to remove from it, when necessary, all efficiency and therefore any danger in order to be able to handle and / or store it without risk.

Dredges adapted to the operating principle of these devices are used to dredge mines. For example, in the case of rudder mines, mechanical devices intended to cut the ruts are towed behind the dredger, and in the case of acoustic mines of submerged vehicles comprising sound sources intended to actuate the firing device. sufficient distance from the dredger. It is also known in the case of mines with magnetic release to use different systems towed behind the dredger, itself treated so as not to emit a disturbing magnetic field, a device creating a magnetic field allowing there also to activate the devices of firing mines. These firing devices have of course been improved so as not to be deceived by the relatively coarse simulation devices used at the start. These improvements relate essentially to the implementation of means making it possible to differentiate the magnetic signature of the lure towed by the dredger, from the magnetic signature of a real building. We then perfected the lure to obtain a magnetic signature as close as possible to that of a real building.

One of the known systems used for this is described in European patent application No. 0 130 767 filed on June 25, 1984 under the title "Improvements in or relating to magnetic assemblies" and benefiting from a priority filing in Great Britain under No. 83 18111 dated July 4, 1983.

The device described in this patent application comprises a series of ferromagnetic elements towed in single file behind a minesweeper. Each of these elements is itself composed of 19 cores surrounded by coils. These windings are associated with current pulse generators adjustable in direction and in amplitude. These pulses make it possible to generate within each ferromagnetic core separately, a magnetic field which makes it possible to magnetize at saturation the core in one direction or the other and to demagnetize it, at least to a certain extent, enabled by the hysteresis cycle of the material used to build the nucleus.

The combination of the magnetic fields thus generated by each of the cores makes it possible to obtain for each element a magnetic field which is adjustable in steps in intensity and in direction. As the magnetization provided by each of the elements can be adjusted in this way, the magnetic signature of a boat can be simulated in a relatively fine manner.

There are, however, times when it is necessary to cancel the residual magnetization of each of the elements as perfectly as possible. This is in particular the case when, outside dredging periods for example, it is desired to store the device on the dredger deck, in particular to facilitate the maneuver of the latter, in particular by increasing its speed, which is considerably slowed down by the tensile force of the device when it is towed.

To do this, we bring the device back on board and put the elements on the dredger deck.

If then there remains a residual magnetization, this one, even if it is weak for each element, can reach a critical value taking into account the gathering of the elements in a restricted space. We then run the risk of detonating a mine over which the dredger would pass, whereas this one, because of its particular characteristics, would not have triggered it on its own.

To reduce the magnetization of each element as much as possible, 18 of its nuclei can be brought to magnetic saturation by grouping them in such a way that two consecutive nuclei have opposite magnetization directions and by demagnetizing the 19th nucleus located in the center. However due to a number of effects, in particular differences in constructions, variations in the hysteresis cycle, and the very great difficulty of obtaining a really zero magnetization for the 19th element, there can remain a residual magnetization which is all the more dangerous as the residual magnetizations of all the elements can add up.

If we consider for example that a dredging element is capable of providing a maximum magnetization equal to 70,000 Am ² , a residual magnetization of 5%, current value, or 3,500 A m ² , generates a magnetic field of 300 nT at a distance 9 m, corresponding to the standard water height adopted for this type of device. In the example described in the reference patent application, the dredge has 6 identical elements and the residual field generated by these 6 elements grouped together in one place on the dredger deck can then reach a value of 1,800 nT at this distance from 9 m. However, we know that we are striving to reach a residual magnetic field of less than 100 nT for dredgers, in order to allow them to navigate with sufficient safety over mines with magnetic release submerged by 9 m of background.

It is therefore clear that the storage on deck of the dredger of the device described in this reference patent application is capable of completely annihilating the effect of the means used to obtain sufficient security for the dredger.

French patent 2,559,303 discloses a method for compensating the magnetic fields of any object in which in particular differential magnetometers are used to measure the field to be compensated. However, this compensation is essentially carried out by the action of a loop traversed by an electric current and whose field is opposed to that to be compensated. The modification of the magnetization of the object which can possibly be caused by the loop is only partial and it is necessary to maintain a permanent current in this loop.

To overcome this drawback, the invention provides a method according to the appended claims.

• la figure 1: une vue en coupe d'un élément de drague selon l'art connu ; et
• la figure 2 : une vue en coupe longitudinale d'un élément de drague munie selon l'invention d'un gradientmètre et de ses organes annexes.

Other features and advantages of the invention will appear clearly in the following description given by way of nonlimiting example with reference to the appended figures which represent:

• Figure 1: a sectional view of a dredge element according to the prior art; and
• Figure 2: a longitudinal sectional view of a dredge element provided according to the invention with a gradient meter and its associated bodies.

There is shown in Figure 1 a cross section of a ferromagnetic dredge element. This element is formed by 19 cores 101 distributed over two concentric crowns, an outer crown comprising 12 cores plus an inner crown comprising 6 cores, plus a single central element forming the 19th nucleus. Each core is itself formed by a set of bars 102, joined together in the form of an elongated packet enclosed by a coil 103 enabling them to be magnetized in the desired manner by causing this coil to pass through an adequate current.

The assembly is contained in an envelope 104 and the space between the cores inside this envelope is filled with a compound 105 making it possible to maintain the cores in the desired arrangement while participating in the hydrostatic balance of the element. .

According to this arrangement, the cores of the two crowns can be grouped in pairs, which allows one of them to saturate in one direction and the other to saturation in the other direction to cancel the total magnetization overall. of the two crowns of 18 cores. The demagnetization of the central core is obtained by running the winding which surrounds it with a current which reduces this magnetization to 0.

Now we know that it is very difficult to obtain in this way a true cancellation of the magnetization of a piece of isolated ferromagnetic material. The easiest and most common way is to subject this material to decreasing cycles of hysteresis nested one inside the other by circulating in the external winding an alternating current which decreases progressively from the value making it possible to bring the core at saturation up to zero.

This method is not practical at all in particular because it is long, it consumes a lot of energy and it causes a noticeable heating of the whole. It is in particular not very compatible with the operation of the dredger for which provision has been made, among other things to simplify the control circuits, to use current pulses in the windings. This is also the reason why we prefer to demagnetize the dredge by grouping the outer cores in pairs, each element of which is brought to saturation in the opposite direction to the other.

There is shown in Figure 2 a simplified longitudinal sectional view of the element of Figure 1 provided with additional members including in particular the means of the invention.

According to the invention, a magnetic field gradient meter 201 is used, located substantially on the axis of the central core at a distance d from the end of this core. In a practical embodiment example, where the cores are approximately 2 m long, the distance d will be equal to 0.7 m.

This gradient meter 201 is a known device which measures the gradient of the field at a point in the space conventionally located at the center of the gradient meter and in a determined direction. Currently, to obtain this gradient, two probes 202 and 203 are used, of the so-called "flux-gate" type for example, aligned along the measurement axis. The difference in the magnetic field measurements given by these probes makes it possible to obtain, as a function of their spacing, which is for example equal to 0.2 m, the value of the gradient at the central point between these two probes. This device is in principle polarized along an axis which according to the invention will be the axis of the large magnet formed by all of the cores of the dredger element.

The gradient meter 201 is connected to an electronic unit 204 which supplies it with various service signals and analyzes the signals from the probes to determine the magnetic field gradient at the location where the gradient meter is positioned.

These electronic circuits also include means making it possible, from these measurement signals, to control a power supply device 205 for the windings 103 of the cores 101.

This device 204 operates in feedback so as to minimize, preferably to cancel, the magnetic field gradient measured by the magnetometer 201. Thus, therefore, when the demagnetization operation of the dredge element is launched by commanding a sequence d current pulses in the windings 103, by means not shown and known which come to control the power means 205 and which can themselves be contained in the members of the electronic unit 204, the magnetic field generated by the drag element outside of itself tends to cancel out, especially at the location where the gradient meter is located. The gradient of this field is then also canceled.

This sequence then being completed, the residual magnetization is detected by the gradient meter which then controls via the box 204 the emission of complementary currents by the power box 205 in the windings, and in any case in at least one of them.

A simpler, but crude, way to achieve this would be to use a simple magnetometer in place of the gradient meter. We immediately see that this solution is far from being entirely satisfactory, since we would then tend to cancel not only the field attributable to the dredge element, but also the earth's magnetic field, thus keeping a residual magnetization in the element, this magnetization being almost opposite to that of the terrestrial field. As the magnetic mines are intended precisely to explode by noting a variation of the magnetic field compared to the normal value corresponding to the terrestrial field, one could not obtain a good result. However, this solution is at least usable at first to rough the cancellation of the residual field.

The gradient of the Earth's magnetic field, if it is not always zero, is however often very low, except perhaps in the presence of certain anomalies due to ferromagnetic rocks. The gradient coming from the dredger element is on the other hand much greater since one is at a distance d from this element relatively small compared to its dimensions. If therefore we manage to cancel this gradient, we are much safer to have canceled the residual magnetic field from the element.

From this principle, two separate methods of controlling demagnetization currents can be used.

In a first method, we start from the conventional method of global demagnetization of the element by saturation in opposite directions of the contiguous nuclei and demagnetization of the central nucleus without control at this level. The servo system is then started up on the value 0 of the measurement of the gradientmeter and the latter comes to finely adjust the magnetization of the central core around the value in principle zero obtained previously. Finally, this process is stopped when the value of the gradient is minimum, that is to say when there is no longer any variation in the gradient in at least two successive iterations of the process. In principle the value of the magnetization of the central core is then zero but in practice this will not be the case since there will be a small residual magnetization making it possible to compensate for the residual magnetization originating from the irregularities in the compensation two by two of the cores of the crowns.

The disadvantage of this first method is that obtaining a value 0 of the gradient at a point on the axis of the system does not guarantee a zero field in any space with sufficient precision. This could only be achieved if the drag element could be assimilated to a perfect magnetic dipole, which is not the case since the magnetizations of the cores arranged in a crown are not zero and are only compensated two by two. This compensation clearly does not give a homogeneous magnetic field and therefore zero at all points.

A second method then consists in first demagnetizing all the nuclei in the same way as the central nucleus was demagnetized in the first method. In a second step, the magnetization of each of the nuclei is varied slightly one after the other, cyclically from the first to the last, each time minimizing the field gradient determined by the gradient meter 201. When this cyclic process is finished, if the gradient is not low enough, this action is repeated a certain number of times until the desired result is obtained.

This second method gives superior results from the point of view of homogeneity in the space of the resulting zero field, but it has the disadvantage of being longer to implement since it is necessary to act on each of the nuclei one after the other.

The invention naturally extends to the cases of dredges which operate in the three dimensions of the species, each element of the dredge comprising three generally identical octagonal elements. It is then necessary to place a gradient meter per element and to implement the method for each of these elements separately.

Claims (3)

1. Process for the automatic compensation of the magnetization of a magnetized element with the aid of a coil surrounding this element, in which an electric current is passed through this coil, the strength of the current being determined by measuring the gradient of the magnetic field outside and in proximity to this element so as to minimize the value of this gradient, characterized in that this element is an element of a ferromagnetic sweep designed to be towed by a ship, in that this element includes an odd-numbered set of cores (101) arranged with a central core and at least one ring including an even number of cores surrounding this central core, in that the value of the gradient is measured (201) on the magnetic axis of the central core, in that the cores of the ring are magnetized to saturation in alternate directions in respect of two contiguous cores, in that the magnetization of the central core is restored substantially to 0, and in that the magnetization of the central core is modified slightly about this 0 value in order to minimize the value of the measured gradient.
2. Process for the automatic compensation of the magnetization of a magnetized element with the aid of a coil surrounding this element, in which an electric current is passed through this coil, the strength of the current being determined by measuring the gradient of the magnetic field outside and in proximity to this element so as to minimize the value of this gradient, characterized in that this element is an element of a ferromagnetic sweep designed to be towed by a ship, in that this element includes an odd-numbered set of cores (101) arranged with a central core and at least one ring including an even number of cores surrounding this central core, in that the value of the gradient is measured (201) on the magnetic axis of the central core, in that all the cores are firstly demagnetized and in that the magnetization of each core is then successively modified slightly about the zero value thus obtained in order to minimize the value of the measured gradient.
3. Process according to Claim 2, characterized in that the procedure is iteratively repeated until the substantially zero value of the finally obtained gradient no longer varies from one iteration to the next.

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