EP0356146A2

EP0356146A2 - Magnet assembly

Info

Publication number: EP0356146A2
Application number: EP89308355A
Authority: EP
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: 1989-08-17
Publication date: 1990-02-28
Also published as: EP0356146A3; FI893896A0; GB8819779D0; FI893896A; AU3999089A; GB2222026B; NO893308L; GB2222026A; NO893308D0; DK405689A; DK405689D0; JPH02114088A

Abstract

A magnet assembly comprises a rigid non-magnetic frame (10) which can be located around a vessel such as a submarine. Embedded in the frame (10) is a plurality of magnetic elements (12), suitably variable magnetic moment magnets. A plurality of magnetic sensors (13) is mounted rigidly on the frame (10) but spaced from it, and control means are provided for controlling the magnetisation state of the variable moment of magnets according to the magnetic field detected by the magnetic sensors. The magnetic signature of the submarine can thereby be reduced to a minimum.

Description

This invention relates to a magnet assembly suitable, for example, for reducing the magnetic signature of a ship or submarine.
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 selectivity changed in direction by the application of a current to a solenoid surrounding the permanent magnets. A ship degaussing system is 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 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 appropriate direction, to achieve the desired magnetisation saturation state of the permanent magnet.
Alternatively, the control circuit may be arranged to supply current such that the permanent magnet is set to a magnetisation state less than saturation, thus permitting a more precise control of the pattern of the magnetic field provided by the assembly.
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 purposes, a magnetic mine sweeper or an arrangement for "de-perming", i.e. permanent de-magnetisation, of ships.
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 vehicles, allowing magnetic modelling of the vessel and thus the derivation of the settings for the magnetic elements. The assembly of the invention will permit a significant reduction in residual signature of a vessel to be achieved.
In the best case this may be below 10nT.
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 11a 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 solid rod or a plurality of thinner rods, the number being chosen to ensure maximum magnetic moment. Suitably, the number 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.
In use, the assembly is submerged sufficiently to enable a submarine to enter into the body, and measurements 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 measurements are made with the magnetic sensors 13 to determine 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.
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 switching magnetic elements appropriately. This will be necessary, for example, when the submarine is moving and changes its heading, thereby changing the induced magnetisation of the submarine.
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.
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 sensors in depth 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.
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 VMM'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 with soft-iron cores in place of some or all of the VMM's. While coils and soft-iron cored solenoids require a continuous current flow to be effective, whereas VMM's merely require pulses when the polarity is switched, the strength of the field produced by each such element can be varied by varying the current therein.

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 electro-magnets.

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 setting the magnetisation state of the permanent magnet to a desired level and direction.

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 supply an electric current to the solenoid for a sufficient time, and in the appropriate direction, to achieve the desired magnetisation 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.

6. A magnet 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.