GB2222026A

GB2222026A - Cancelling the magnetic field of waterborne vessels

Info

Publication number: GB2222026A
Application number: GB8819779A
Authority: GB
Inventors: Terence Warren Nichols; Peter Harry Maddocks
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1988-08-19
Filing date: 1988-08-19
Publication date: 1990-02-21
Anticipated expiration: 2008-08-19
Also published as: EP0356146A3; NO893308L; DK405689D0; NO893308D0; GB2222026B; DK405689A; GB8819779D0; FI893896A0; JPH02114088A; AU3999089A; FI893896A; EP0356146A2

Description

1 MAGNET ASSEMBLY This invention relates to suitable, for example, for

signature of a ship or submarine.

a magnet assembly reducing the magnetic The ferro-magnetic content of a modern submarine gives rise to a well defined magnetic signature. This signature can be detected by a suitable sensor. Thus, magnetic sensing can be used either as a location aid or to trigger the detonating device in a variety of submarine threat weapons. As the sensitivity of sensors has increased, the submarine designer has been forced to incorporate systems, normally called degaussing systems, to reduce the magnetic signature. The desired maximum level of residual signature is dictated by threat analysis. Normally, this level is determined by the specific threat in a specific location, and not from the level necessary for safe operation for the majority of the voyage.

Patent number 2142781 discloses a degaussing system comprising a plurality of variable magnetic moment magnets (VMM,s) the magnetisation saturation state of each of which can be selectively changed in direction by the application of a current to a solenoid surrounding the permanent magnets. A ship degaussing system is 2n,22026 I/7647/CCL 2 - disclosed in which the variable magnets are contained in buoyant casings so that they float around the ship. The magnets are intended to create a magnetic field approximately equal and opposite to that produced by the ship. However, while this may reduce the magnetic signature, there is no means of ensuring that the signature is minimized, or controlled to a predetermined level.

The present invention provides a magnet assembly comprising a rigid, preferably non-magnetic, frame locatable around a vessel, a plurality of magnetic elements supported by the frame, a plurality of magnetic sensors mounted rigidly on the frame but spaced therefrom and control means for controlling the magnetisation state of the magnetic elements according to the magnetic field detected by the magnetic sensors.

At least some of the magnetic elements may be electro-magnets, for example coils of the type used conventionally in ship degaussing, or. in the form of solenoids wound around a soft ferro-magnetic core. At least some of the magnetic elements may comprise a permanent magnet and means for switching the permanent magnet from one magnetisation saturation state to the opposite magnetisation saturation state. Such switching I/7647/CCL - 3 means may comprise a solenoid wound around the permanent magnet and a control circuit arranged to supply an electric current to the solenoid for a sufficient time, and in the appropriate direction, to achieve the desired magnetisation saturation state of the permanent magnet.

The assembly may be constructed in a form suitable for temporary attachment to a vessel, such as a submarine, when it is travelling in a high risk area.

Alternative applications for the magnet assembly include a ship's magnetic signature simulator for research a magnetic mine sweeper or an arrangement for de-magnetisation, of purposes, Ilde-perming" ships.

i. e.

permanent The magnetic sensors provide feedback to a controller which in turn selectively controls the magnetisation states of the various magnetic elements so as to oppose the field of the vessel. The magnetic elements are preferably located in a U-shaped framework in three orthogonal directions. The controller software can incorporate algorithms suited for each specific class of vessels, allowing magnetic modelling of the vessel and thus the derivation of the settings for the magnetic elements. The assembly of the invention will a significant reduction in residual signature of a vessel.

I/7647/CCL 4 In the best case this may be below lOnT.

Reference is made to the drawing, which is a diagrammatic perspective view of a magnet assembly according to one embodiment of the invention. The assembly is suitable for use, for example, with a submarine, and comprises a U-section body 10 formed of glass reinforced plastics material in which are embedded arrays 11 of magnetic elements 12, the arrays Ila in the sides of the body 10 consisting of elements extending vertically and horizontally, while those 11b in the base of the body 10 extending transversely of the body in a horizontal direction.

Magnetic detectors 13 are mounted below the body 10 by rigid support arms 14 extending for a distance below the body 10 approximately equal to the beam width of the submarine to be accommodated in the assembly. The detectors 13 consist of three-axis flux gate magnetometer sensors.

The magnetic elements 12 consist of a permanent magnet core surrounded by a solenoid connected to a control system (not shown). The core may be a solidrod or a plurality of thinner rods, the number being chosen to ensure maximum magnetic moment. Suitably, the number I/7647/CCL - 5 of thinner rods will be less than seven. The lengths of the magnetic elements 12 will typically be from 1 to 5 metres.

The body 10 is preferably provided with flotation chambers so that its buoyancy may be controlled. A small power supply may also be incorporated in the body 10 or mounted thereon to ensure that the control system may be operated before the body is coupled to a submarine.

measurements measurements determine In use, the assembly is submerged sufficiently to enable a submarine to enter into the body, and are made with the magnetic sensors 13 to sense local field conditions. This enables the effect of the local field conditions to be taken into account when measurements are made with the submarine in place.

The submarine then enters the assembly, which is clamped to the submarine so as to move therewith. Further are made with the magnetic sensors 13 to the initial magnetic signature of the submarine. The control system then operates to set the polarity of the magnetic elements individually or in groups so as to reduce the measured signature to a minimum.

I/7647/CCL - 6 The power for switching the variable moment magnets may be obtained from the submarine itself. The control system continues to monitor the magnetic signature of the vessel and to adjust it to maintain a minimum magnetic signature by appropriately. This will be necessary, for example, when the submarine is moving and changes its heading, thereby changing the induced magnetisation of the submarine.

switching magnetic elements When the submarine and attached assembly are clear of the high threat area, the assembly is released to be towed back to port, for example, by a tug. The assembly may be formed in separate sections connectable together so as to accommodate submarines of different length, for example.

sensors in denth Although the magnetic sensors 13 are shown as arranged at a uniform depth below the body and aligned longitudally thereof, it may be desirable to stagger the or athwartships, or both. Where operation in shallow water is desirable, the sensors may be mounted above or on the sides of the body 10, the control algorithm being modified to predict from the measurements the likely values beneath the body.

A t I/7647/CCL - 7 The magnetic elements described with reference to the drawing are variable moment magnets (VMM's). In an alternative arrangement, conventional degaussing coils may be embedded in or mounted n the body 10 and serving to reduce the greater part of the magnetic signature, with YMM's being provided to supplement the effect of the coil by reducing localised features of the magnetic signature. Another alternative is the use of solenoids cores in place of some or all of and soft-iron with soft-iron VMM's. While require a whereas VMM's switched, the such element therein.

can be the coils cored solenoids continuous current flow to be effective, merely require pulses when the polarity is strength of the field produced by each varied by varying the current I/7647/CCL 8 -

Claims

1. A magnet assembly comprising a rigid frame locatable around a vessel, a plurality of magnetic elements supported by the frame, a plurality of magnetic sensors mounted rigidly on the frame but spaced therefrom, and control means for controlling the magnetisation state of the magnetic elements according to the magnetic field detected by the magnetic sensors.

2. A magnet assembly according to Claim 1, wherein at least some of the magnetic elements are electromagnets.

3. A magnet assembly according to Claim 2, wherein each electro-magnet comprises a solenoid wound around a soft ferro-magnetic core.

4. A magnet assembly according to Claim 1, 2 or 3, wherein at least some of the magnetic elements comprise a permanent magnet and means for switching the permanent magnet from one magnetisation saturation state to the opposite magnetisation saturation state.

5. A magnet assembly according to Claim 4, wherein the switching means comprises a solenoid wound around the permanent magnet, and a control circuit arranged to I/7647/CCL - 9 supply an electric current to the solenoid for a sufficient time, and in the appropriate direction, to achieve the desired magnetisation saturation state of the permanent magnet.

6. A magnet assembly according to any preceding claim wherein the rigid frame comprises a body of glass reinforced plastics material enclosing the magnetic elements.

7. A magnet assembly according to any preceding claim, wherein the rigid frame is of generally U-shaped section.

8. A magnet assembly according to any preceding claim, wherein means are provided for controlling the buoyancy of the assembly in water.

9. A magnet assembly according to any preceding claim, wherein at least some of the magnetic sensors are mounted beneath the assembly.

10. A magnetic assembly according to Claim 9, wherein the magnetic sensors are spaced from the assembly by a distance approximately equal to the width of the assembly.

I/7647/CCL - 10 A magnet assembly, substantially as described with reference to the drawing.

1 I Published 1990 atThePatent Office. State House, 66'71 High Holborn. London WC111 4TP. Further copies maybe obtamedfrom, The Patent officeSales Branch, St Mary Cray. Orpingtop. Kei-t BR5 3F.D. Pr,ri:ed by Tkihiplex techniques ltAl, St Mary Cray, Kent. Con. 1187