GB2259222A - Underwater acoustic projector - Google Patents

Abstract

The acoustic projector has a fixed coil 12 and a moving permanent magnet 15 attached to the driver 16. This arrangement is chosen to allow easier cooling of the coil 12. The permanent magnet is preferably of the rare earth type e.g. Samarium-Cobalt or Neodymium-Iron. The projector is adapted for underwater use with a gas-filled casing 10 and means 18-22 to equalize the static pressures inside and outside the casing. <IMAGE>

Description

Acoustic Projector This invention relates to an acoustic projector, and specifically to an acoustic projector for use underwater.

A known underwater acoustic projector for use in a sonar system is shown in Figure 1. This comprises a rigid housing 1 with a flexible seal 2 forming one of its faces.

The projector is approximately symmetrical about an axis 3.

Within the housing 1 a diaphragm 4 has a tubular portion 4A with an electrical coil 5 wound around it, the diaphragm 4 is free to move along the axis 3 and slides around a fixed permanent magnet 6. The coil 5 is supplied with an input electrical signal along leads 7, which are linked to the coil 5 as it moves by a set of sliding contacts 8.

The housing 1 protects the projector from the fluid it is immersed in, generally seawater, while the flexible seal 2 allows acoustic energy generated by the moving diaphragm 4 to leave the housing 1. When an electrical current passes through the coil 5 it interacts with the magnetic field of the permanent magnet to move the coil 5 and diaphragm 4 axially. By supplying an A.C. current to the coil 5 via the leads 7 and sliding contacts 8 the diaphragm 4 can be to move back and forth, generating an acoustic signal.

There are two problems associated with projectors of this type, the first is that the acoustic power which can be transmitted by the projector is limited by overheating of the coil 5 and the second is that only small displacements of the diaphragm 4 are possible.

It has been attempted to overcome the first of these problems by cooling the coil 5, however this is not easy, filling the housing 1 with a liquid allows heat to be dissipated but increases the acoustic losses within the projector. Pumping a cooling liquid through the housing 1 can increase heat dissipation still further, but it is difficult to avoid interference between the pumping and the generating of acoustic pulses by the projector.

This invention was intended to produce an acoustic projector at least partially overcoming these problems.

This invention provides an acoustic projector comprising a fixed electrical coil and a moving permanent magnet.

Such a design overcomes the problem of coil overheating because the cross sectional area of the coil conductors can be increased, reducing ohmic heating for a given current level, without increasing the moving mass of the projector. Having a fixed coil also allows cooling of the coil to be achieved simply, if required. Furthermore, having a stationary coil allows multiple magnetic or electric circuits to be used to generate very large displacements.

An additional advantage is that no electrical contacts to the moving part of the projector are necessary, this makes the projector more reliable and robust because electrical connections between moving parts are always a weak point.

It was expected that mounting the permanent magnet on the moving part of the projector would increase the mass of the projector to such an extent that the efficiency of the projector would be unacceptably low. However it has been found that where the projector is operating in a relatively dense medium, for example water, this is not the case.

An underwater acoustic projector embodying the invention will now be described by way of example only with reference to the accompanying diagrammatic Figure 2.

Referring to Figure 2 an underwater acoustic projector according to the invention is shown.

The projector is contained within a sealed housing 10 and is roughly symmetrical about an axis 11. By roughly symmetrical it is meant that the bulk of the projector is symmetrical, but the gas and electrical input leads are not.

The fixed electrical coil 12 is situated on a fixed core 13 and is supplied with electrical signals along a pair of input leads 14.

An annular permanent magnet 15 is secured to a driver piston 16 which is free to move axially along the axis 11 relative to the fixed coil 12 and the core 13. The piston 16 is linked to the housing 10 by a flexible seal 17 which allows axial movement of the piston relative to the housing 10. When an electrical current is passed along the input leads 14 and through the coil 12 it interacts with the magnetic field of the permanent magnet 15 and moves the driver piston 16. This movement is used to generate an acoustic signal.

The interior of the projector is gas filled in order to balance the hydrostatic pressure load produced on the piston 16 and flexible seal 17 by the static water pressure outside the housing 1.

The static water pressure varies with depth, in order to alter the gas pressure within the housing 1 to remain equal to the static water pressure and so balance the static pressure load gas can enter and leave the housing 1 along a pipe 18.

The pipe 18 is linked to a valve unit 19 which controls gas outlet from the housing 10 to the atmosphere and gas input to the housing 10 from a high pressure gas reservoir 20. The valve unit 19 is linked to a pair of static pressure sensors 21 and 22 inside and outside the housing 10 respectively and operates to equalise the static pressure at the two sensors 21 and 22.

Since the coil 12 is no longer part of the moving mass of the projector the cross-sectional area of the conductors forming the coil 12 can be increased without any problems due to their increased mass, as a result the electrical resistance and heat generated by a current passing through the coil can be reduced. If cooling of the coil 12 is desired, it is relatively easy to arrange for coolant to be passed internally through the core 13 or in tubes through or around the coil 12 because the coil 12 is static.

In order to control or concentrate the magnetic field generated by the coil 12 the core 13 could be formed from soft iron or silicon steel and could be laminated to reduce eddy current formation.

The permanent magnet 15 is preferably formed from a rare earth magnetic material. The preferred materials being Samarium Cobalt or Neodynium Iron in sintered form, although moulded magnets may be preferred in some circumstances because they are cheaper although performance will be degraded.

If a very large displacement is required a number of coils 12 can be arranged along the core 13 and activated in turn to drive the permanent magnet 15 as a linear brushless D.C. motor.

The gas used to pressurise the projector will depend on the combination of acoustic impedance, electrical insulation and thermal properties required among other factors.

It may be preferred in some situations to fill the projector with a liquid rather than a gas.

Instead of valving gas in and out of the projector to balance the hydrostatic pressure, other techniques such as linking the gas inside the projector to a flexible gas reservoir, such as a gas filled bag, external the projector but exposed to the same hydrostatic pressure could be used to pressure balance the projector.

As well as or instead of using multiple switched coils to increase the displacement available, multiple permanent magnets could be used.

If desired a motion sensor could be attached to the driver piston to allow its movements to be tracked and compared with the movements desired to produce a required acoustic signal to allow error correction in use.

Claims (6)

1. An acoustic projector comprising a fixed electrical coil and a moving permanent magnet.

2. An acoustic projector as claimed in claim 1 and adapted for underwater use.

3. An acoustic projector as claimed in claim 2 in which the coil and permanent magnet are enclosed by a sealed housing.

4. An acoustic projector as claimed in claim 3 in which the sealed housing contains a gas.

5. An acoustic projector as claimed in claim 4 and additionally comprising means for varying the gas pressure within the housing to compensate for changes in the hydrostatic pressure on the projector.

6. An acoustic projector substantially as shown in or as described with reference to Figure 2 of the accompanying drawings.