The invention relates to magnetic assemblies and in particular _» though not exclusively to magnets for use in mine- sweeping or ship degaussing systems. 5 ,' Many ships have degaussing systems built into their structures. These systems comprise electrical coils which can be energised so that they produce a magnetic field which is equal and opposite to that of the ship so that the magnetic field of the ship is neutralised. Some ships, though, do not have built-in degaussing 10 measures and situations may arise where such ships have to pass through mined waters. Thus a system is required which will enable such ships to be degaussed temporarily to allow them to pass through the dangerous areas.

There are a number of conventional minesweeping methods. 15 The most common are the systems where the required magnetic fields are produced by electrically pulsed loops towed behind a ship. The loop may either be a closed loop system consisting of a large area horziontal plane loop or an open loop system con¬ sisting of two or more electrodes with electric currents driven 20 througn the sea water between them. Other methods include a simple dc electromagnet, with a fixed magnetic moment, towed from a helicopter, and a fixed magnetic moment permanent magnet towed behind a small ship for precursor magnetic sweeping. The object of the present invention is to provide a 25. versatile magnetic system which can be used with a fixed or varying magnetic moment so as to be suitable for use in ship degaussing and also minesweeping systems.

The magnet system must be capable of operating in three modes: 0 a) as a single moment magnet; b) with a gradually varying magnetic moment; and c) with a pulsing, continuously varying magnetic moment as the situation requires.

The invention provides a magnet assembly which comprises a 35 plurality of permanent magnets, each of which can be switched froia one magnetisation saturation state (positive or negative)

to ύhe υ _j juoilc ^* __ιg-ictisation saturation state, such that the over¬ all magnetic moment of the assembly can be va-ricd stepwise. Thus the magnet assembly will be referred to as a "variable permanent magnet". Such magnet assemblies would be particularly suitable for use in minesweeping and ship degaussing systems.

Each of the permanent magnets has a positive or negative magnetic moment depending on whether the material is positively or negatively saturated. Each magnet can then be switched from one saturation state to the other by driving the magnet into the opposite saturation state. Thus, if one permanent magnet is switched to its opposite saturation state, the overall magnetic moment of the system increases or decreases by the change in the magnetic moment of that permanent magnet.

The switching ^" means for each permanent magnet preferably comprises a solenoid winding. Preferably a solenoid is wound around each permanent magnet and can produce a field which will force the permanent magnet into positive or negative saturation in dependence on the direction of the solenoid current. An electric pulse through the solenoid produces a magnetic field which drives the permanent magnet into saturation such that the magnet is switched from one saturation state to the opposite saturation state. Preferably there is a control circuit to connect each individual solenoid to a dc power source for sufficient time to achieve magnetic saturation. Each permanent magnet preferably comprises a plurality of rods or cylinders. The rods or cylinders are of a permanent magnetic material and they may be arranged in a bundle.

The amount of magnetic material in the variable permanent magnet depends on the maximum total magnetic moment required. For a larger magnetic moment requirement more magnetic material must be used in the system.

As each permanent magnet bundle has its magnetic moment switched the overall magnetic moment alters stepwise. The size of the step depends on the size of the magnetic moment of the bundle, thus for a certain overall magnetic moment finer steps are achieved by using more bundles, each of less material, and coarser steps are achieved by using fewer bundles, each of more material.

Thus the amount of magnetic material in each bundle is determined by the overall magnetic moment and step size re¬ quirements and from this the number of rods or cylinders in each bundle can be determined. This is limited though by the mechanical properties of the magnetic material.

Pre erably the number of bundles used and the switching sequence for the. solenoids are controlled by computer programme. The solenoid pulsing sequence depends on the required magnet application: the magnet moment may be preset to ^* a fixed value; it may vary slowly to compensate for varying conditions; or it may ^* be continuously pulsed to give a particular waveform.

Preferably the magnetic material used is a chromium steel with between 1 ₂ and 12% chromium. A preferred steel contains 6% Cr and 1?_C. Iron/carbon/aiuminiu steels have acceptable ^" magnetic properties, and also the advantage of being non- strategic materials, but in most cases their mechanical pro¬ perties are not good enough. Some tool steels can also be used. Advantageously the material is a permanent magnet-type with remanence and coercivity values being as high as possible. Preferably the remanence is not less than about 7000 gauss ^• -■ and in practice a remanence in the range 7000 to 9500 gauss may be used. The coercivity is preferably not less than 60 oersted In practice a value of about 100 oersted has been found suitable.

The variable permanent magnet is preferably able to produce

4 a maximtra total magnetic moment of a least 6x10 Am in each

5 2 magnet direction. A magnetic field of up to 10 Am is desirable but this is dependent on the material coercivity. The material and its assembly within the variable permanent magnet are pre¬ ferably also strong enough to withstand explosions when used as a mine-CQuntermeasure. The material should also be able to be .. formed into long rods or cylinders with very little variation from straightness over their length. Preferably it should be able to be made into rods which are out of straightness by less than about 0.1?. of their length. Preferably the variable permanent magnet comprises a number of ^* permanent magnet bundles enclosed in a casing which does not affect the magnetic fields produced and is strong enough to - withstand mine explosions. Conveniently the casing may be

made of glass reinforced plastic (GRP) .

Preferably the casing is such as to make the magnet assembly buoyant so that it will float..

There is no critical arrangement of the permanent magnet bundles within the casing. Conveniently they maybe arranged symmetrically about the long axis, or alternatively they may be arranged asymmetrically so as to render the system "bottom heavy" so that it will float only one way up.

The variable permanent magnet assembly may be used singly or in groups of up to 60 or more individual magnets.

Pre erably each complete system uses only one external source of power and only one solenoid sequence programme- for any number of individual magnets. Preferably each magnet has external connections so that power and commands can be received and passed onward to other similar magnets.

- Preferably each individual variable permanent magnet can be constructed in the form of a 3-axis magnet to produce orthogonal magnetic fields.

The variable permanent magnet system may be used to simulate a ship's magnetic signature, or the signature of .-another object, to degauss ships or other objects, or for a minesweeping system either towed behind a ship or as part of a remote controlled precursor magnetic-, sweeping system.

In order that the invention may be more fully understood one embodiment thereof will now be described, by way of example only, with reference to the drawings of which:

Figure 1 illustrates one possible arrangement for a variable permanent magnetj Figure 2 is a block diagram of a variable permanent magnet system,* Figure is a diagram of a variable permanent magnet system for use as a ship signature simulatorj Figures 4_L & B shov/ a variable permanent magnet system for degaussing a ship, in side elevation and plan vievj and Figure 5 shows a variable permanent magnet system for minesweeping. Figure 1 shows a magnet assembly of a variable permanent magnet which comprises 19 bund3.es or switchable permanent magnets 1 each of 7 rods 2. Each rod 2 must not vary from the straight by more than about 0.1% of its length. The rods 2 are made of 6% chromium, Λ carbon, steel which has been normally heat treated and quenched wit}

rods 2 being restrained during que.-.ching to prevent bending.

The magnetic material has a remanence of at least 7000 gauss and a coercivity of about 100 oersted. The total magnetic field

4 2 produced by the assembly is about 6x10 Am .

A magnet casing 3, made of GRP, encloses the bundles 1.

5 Solenoid windings 4 are close-wound over the length of each bundle and can produce afield of 25 Ampere turns/mm which ensures that the steel is driven into saturation.

A pulse current of 27A maximum, at 600V, is used to fully saturate the magnetic material. A pulse length of 76ms is used

10 to allow time for full saturation. The pulse length limits the speed of variation of the moment by limiting the switching cycle time.

. There is a further time delay introduced, making the cycle tir.-.e εbout 150ns, such that errors in the cycle are not able to

- 15 . cause two solenoids to be switched on at the sε__e time.

If the bundle 1 is positively saturated the associated solenoid 4 can be pulsed to switch it to negative saturation and vice versa.

20 Each magnet has two saturation states, at the positive and negative remanence points.

Figure 2 shows a generalised arrangement of a variable permanent magnet system for use in minesweeping or ship de¬ gaussing systems which comprises a generator 5, a control box 6

25 and a series of variable permanent 3-axis magnets 7. As many as 603-axis magnets may be used and the control box must be capable of controlling the switching sequence of each one. Each 3-axis magnet has electronic circuitry to enable instructions to be accepted. The distance from the control box 6 to the 3-axis

30 magnets 7 is limited only by the power drop in the cable link. In a system of 603-axis magnets there may be 300m or more from the first to the last magnet and the distance from the control box to the first magnet can be 100m or more and so the system must be designed to cope with this.

35 The magnet system is very versatile and can be used in three modes: a) the magnetic moment of each variable permanent magnet is preset to a fixed value; b) the magnetic moment is varied

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slowly; and c) the magnetic moment is varied continuously. Examples of uses for the magnet system of Figure 2 in the three modes are illustrated by Figures 3, A and 5.

Referring to Figure 3, a system for use as a ship signature simulator is shown. Ship signature simulation is a method of minesweeping wherein the magnet system is used to produce a ship— ype magnetic signature. A number of variable permanent 3-axis magnets 7 are arranged in a line, separated by spacers 8 with their magnetic moments preset at different fixed values so as to create the same field pattern. as a ship. The magnet system is towed by a vessel 9 and when it passes near a mine the mine detects an apparent ship's magnetic signature and so e_q?lodes. I a typical arrangement 6-10 magnets are used. Another variation of the preset signature system is target simulation for testing of magnetic anomaly detectors.

Referring to Figures 4A and 4B, a ship degaussing system is shown using a variable permanent magnet degaussing system for use with ships which do not have their own degaussing systems. A number of variable permanent magnets 7 is placed around a ship:, the number used depending on the size of the ship and the total magnetic moment required. Each magnet 7 is a 3-axis magnet with the axes being vertical, across the ^■ ship and along the length of the ship. The magnets are contained in buoyant casings or placed on inflatable rafts so that they float around the ship. They are kept apart from each other by spacers 8 and kept at a distance of 1-2 a froπ the ship's sides by fender/spacers 11, bow yoke 12 and spacer struts 13 to prevent the magnets touching the ship. The generator 5 and control box 6 are arranged in the magnet line on a buoyant raft (not shown). The -magnets create a magnetic field, approximately equal and opposite to that produced by the ship, to neutralise the effect of the ship's magnetic field. As the ship changes heading its field changes and so the field produced by the magnets is also changed to keep the resultant magnetic field to a minimum.

Referring to Figure 5» a variable permanent magnet system for use as a minesweeper is shown. 0ns or more J>-β3±s variable permanent magnets 7 are towed behind a minesweeping ship 1 . The magnet solenoids are pulsed to produce a continuously varying magnetic moment of a desired waveform. The generator 5 and control box 6 may be carried on the ship. The variable permanent magnets have the advantage of being buoyant and fairly small and, as only one solenoid at a time is energised, they only require small generators. Thus smaller vessels can tow the minesweep than with conventional minesweeps. Alternatively, if ^' one or more of the 3-ojά.s variable permanent magnets, conveniently two of them, are connected by an overhead raft with a generator 5 asd control box 6 on the raft, the buoyancy of the magnets will cause the system to float. The raft may be fitted . with an outboard motor and radio control equipment and thus can form a- remotely controlled precursor magnetic sweeping device.

The variable permanent magnet has many advantages over existing magnets. It is more versatile in that it can be used as a single moment magnet or a variable moment magnet. It is fairly small and requires only a small generator, thus it can be transported easily. The magnetic moment can. be reduced to aero so the magnets can be transported by air without affecting navigation devices. Thus if an area, for example an island, is surrounded by mines the magnets can be flown in by plane and any suitable ship, for example a fishing vessel, can be used to tow the magnets as a minesweep, giving a very versatile minesweeping capability. If neces3ary the magnets can be used to degauss a vessel for the purpose.

The magnets have possible industrial applications for adjusting or neutralising magnetic fields and as a magnetic ^' field source for calibration purposes. '

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