GB2231153A - Electromechanical transducer - Google Patents

Abstract

An underwater acoustic pulse projector 20 comprising a planar coil 23 and plate-like copper piston 21 configuration overcomes problems associated with known constructions (Fig. 1 not shown), in which the coil is embedded in a thick epoxy housing to withstand reaction forces from electromagnetically displacing the piston whilst providing electrical insulation for underwater operation, due to poor heat transfer through the epoxy housing when the coil is energised at a high duty rate. The coil 23 is sheathed in a thin layer of GRP resin 24 which provides adequate electrical insulation and heat flow but not structural strength and this coil assembly 22 is mounted against a GRP reinforcing member 32 having apertures 37 and the housing 27, supporting coil assembly and reinforcing member, is open at 38 to the underwater environment which acts as a heat transfer fluid to remove heat from the surface of the coil assembly by way of the reinforcing member apertures. The opening 38 may be covered to protect the coil from damage and possibly not apertured, permitting a different heat transfer fluid. <IMAGE>

Description

ELECTROMECHANICAL TRANSDUCER This invention relates to electromechanical transducers and in particular to such transducers for delivering high mechanical power into a liquid in which immersed.

Transducers of such nature are employed for seismic and other purposes, being immersed in a body of water and caused to emit continuous or periodic signals by moving a diaphragm or piston against the water body.

One form of transducer with which the invention is particularly concerned is an acoustic pulse generator of a type comprising a flat, plate-like piston, often called a plate or boomer plate, of copper or aluminium movable to and from a correspondingly flat wound electrical coil in relation to which it is held by a resilient covering membrane or diaphragm whereby when a short duration high power electrical pulse is discharged into the coil the resultant electromagnetic induction explosively propels the piston plate away from the coil and transfers this motion into the water as a single broad band acoustic pressure pulse by way of the membrane.

It will be understood that the acoustic, that is mechanical, output is related to the electrical energy input at any particular hydrostatic pressure by the spacing, and uniformity of spacing, between the piston and coil, and to this end the transducer comprises a housing which supports both the piston and the coil so as to maintain a defined inoperative separation between them. It will also be understood that to function properly the coil must be both supported rigidly to prevent distortion of its planar form as reaction to the force exerted on the piston and electrically insulated from the piston and water environment of immersion.To this end it is known to mould the housing of epoxy resin in which the electrical coil is embedded and which provides for support of the piston, thereby achieving the objectives of supporting the coil rigidly, providing electrical insulation for it and defining its position relative to the piston.

The covering diaphragm is formed of rubber, Neoprene or the like and extends across the face of the piston plate remote from the coil so as to isolate the working parts of transducer from the effects of the water environment, offer little mechanical impedance to the transfer of motion of the propelled piston into the acoustic pressure pulse, retain the displaced piston and, when the propulsion force is removed, restore the piston to its rest position adjacent the coil.

As indicated the above described structure is well known because of its simplicity and ease of manufacture.

However there are disadvantages with this form of construction due to the thickness of epoxy resin enclosing the coil to provide it with the mechanical strength to resist operational reaction forces.

A first disadvantage is found by stresses set up unpredictably within the body of the resin during curing which may lead to distortion of the housing or unpredictable response to forces in the energised coild and the aforesaid reaction forces. There is a second disadvantage which becomes apparent if the transducer is energised on a continuous or frequent basis, that is, with a high energisation duty ratio.

Energisation of the electrical coil generates heat therein which is removed only by thermal conduction through the moulded epoxy transducer housing. Epoxy resin is known to be a poor thermal conductor and heat built up by continual energisation both alters the electrical properties of the coil and causes or exacerbates mechanical stresses and distortions in the mounting of the coil relative to the piston such that the nature of the emitted acoustic pressure pulse changes from the design form and derogates from the interpretation of echoes received at sensing transducers for which accuracy is based upon pre-supposed knowledge of the emission form.

It is an object of the present invention to provide an underwater electromechanical transducer which permits a greater duty ratio than hitherto known.

According to the present invention an electromechanical transducer adapted to operate immersed in a liquid environment comprises a coil assembly comprising an electromagnetic coil contained within a solid electrically insulating medium that is configured to provide low impedance to the transfer of heat from the coil, a piston operable to interact magnetically with the electromagnetic coil to produce a separating force between them, restoring means operable to bias the piston towards a rest position, housing means supporting the coil assembly and restoring means and defining an operating cavity containing the piston and a piston rest position adjacent the coil assembly, an apertured reinforcing member supported by the housing means adjacent the face of the coil assembly distal from the piston and operable to absorb operational reaction forces between the piston and coil assembly acting to displace the plane of the coil assembly with respect to the housing away from the piston and cooling means including at least in immersed operation a heat transfer fluid operable to transfer heat from the coil assembly by way of the apertured reinforcing member to the liquid environment of immersion.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a sectional elevation through an underwater electromechanical transducer of known form for producing acoustic pressure pulses, Figure 2 is a sectional elevation through an underwater electromechanical transducer according to the present invention for producing acoustic pressure pulses, Figure 3 is a plan view of the transducer of Figure 2, Figure 4(a) is a sectional elevation similar to Figure 2 showing a modification thereof comprising a protective cover, Figure 4(b) is a sectional elevation similar to Figure 4(a) showing an alternative arrangement of securing the protective cover, Figure 5 is a sectional elevation similar to Figure 4(a) but showing a further form of protective cover, Figure 6(a) is a sectional elevation through an alternative form of electromechanical transducer according to the present invention, and Figure 6(b) is a sectional elevation through a transducer similar to that of Figure 6(a) but adapted for operation at great depth.

Referring to Figure 1 this shows in sectional elevation an electromechanical transducer 10 comprising an acbustic pulse generator which is operated immersed in a liquid environment, usually the sea or some other large body of water, to emit periodic broad band acoustic pressure pulses.

To this end the transducer comprises a strong rigid housing 11 formed of an epoxy resin in which is embedded, conveniently during moulding of the housing, an electromagnetic coil 12 of substantially planar configuration, that is, a spiral, the length and other dimension of the coil providing a desired inductance and Q-factor.

A planar plate-like piston 13 of metal, preferably a non-ferromagnetic metal such as copper or aluminium, capable of electromagnetic interaction with the coil 12 when a current flows through the coil, is disposed parallel to one major face of the coil and located in a shallow recess 14, which may also be considered as an operating cavity, formed in the moulded housing 11 or on the surface thereof by separate spacers 15 and retained therein by a resilient diaphragm 16 of rubber or like material such a Neoprene. The diaphragm is secured to the housing 11 at its periphery, conveniently by an annular clamping ring 17 which also supports, and may even provide, spacers 15.

The known transducer 10 operates by having a short duration high power pulse discharged through the coil 12 from a remote energy source (not shown), the resultant rapidly changing magnetic field of the coil explosively propelling the piston away from the coil, the motion being transferred to the surrounding liquid environment as a single broad band acoustic pressure pulse, and the resilience of the diaphragm restoring the plate, displaced by the transient field, to its rest position in readiness for the next pulse.

Such transducers are frequently used in dirty or corrosive liquid environments, such as the sea, and the environment 18 in which they are immersed is kept from the piston 13 and the operating cavity 14, particularly the clearances between them, by the rubber diaphragm 16 whereas the electrical coil is protected both physically and electrically by the epoxy housing in which it is contained.

It will be appreciated that in operation the transducer is required to produce an acoustic pressure pulse of known power and characteristics so that the characteristic of reflections of such emitted pulses can by reference to the emission reveal valuable data regarding the reflector.

The characteristics of the emitted pressure pulse depend not only upon the electrical input to the coil but also the precise physical dimensions of the transducer components and the relative positions they take within it.

It will also be appreciated that in operation the electromagnetic force displacing the piston also produces a reaction on the housing 11 in which the coil is embedded and to avoid distortion of the housing due to such force the housing comprises a substantial block of resin.

This known construction has been found to have several disadvantages in practice.

Manufacture of the housing from a substantial body of resin results in stresses set up within the bulk during curing thereof which may lead to distortion of the housing or unpredictable response to the reaction forces causing departure of the intended dimensions and thus of the emitted pressure pulse characteristics.

The electrical current which passes through the coil and stressing set up thereby also generates heat which is removed by conduction through the body of epoxy resin comprising the housing and then to the liquid environment of immersion.

If the transducer is operated with a high duty rate, that is, producing a rapid sequence of pressure pulses, the poor thermal conductivity and thickness of the housing may be unable to transfer heat from the coil at a sufficient rate to prevent further stresses and distortions of the housing and corresponding deterioration in the characteristics of the emitted pressure pulse.

Referring to Figures 2 and 3 an electromechanical transducer 20 according to the present invention comprises a planar piston 21 of non ferromagnetic metal such as copper or aluminium. The transducer also comprises a coil assembly 22 consisting of a planar spiral wound configuration electromagnetic coil 23 that provides desired inductance and Q-factor contained within a solid electrically insulating medium 24 configured to provide low impedance to the transfer of heat from the coil.

The solid electrically insulating medium may comprise a coating of reinforced epoxy resin or the like applied to the coil before hardening and of sufficient thickness to provide electrical insulation and some mechanical support but insufficient to withstand the reaction forces of operation.

However it is difficult to define a suitably precise and minimal thickness for piston-coil separation by this method and preferably the solid electrically insulating medium 24 comprises two sheets 25, 25' of fibre reinforced plastics resin sandwiching the coil, the reinforcement material conveniently being glass. The sheets 25 and 25' are secured to the surface of the coil by an adhesive having good shear strength to resist the tendency of the coil spiral to 'unwind' when pulsed and thermal expansion consequent thereon. The sheets may be secured to each other at the periphery or by way of apertures in the coil, most practicably at the centre, such as indicated by fibre reinforced plastics plug 26, described again hereinafter, secured to each sheet.

The substantially planar coil assembly 22 and piston 21 are disposed with their planes parallel to each other.

Housing means 27 is formed by a filler reinforced epoxy resin moulding and supports the coil assembly in a recess 28 at the periphery of the assembly, conveniently by moulding the housing with the coil assembly in situ and providing a peripheral support and constraint for the coil against unwinding.

A shallow recess, or operating cavity, 29 is defined by a raised portion of the rigid housing means or, preferably, by peripheral spacer gasket 30 of resilient butyl material and contains the piston 21, providing a rest position for the piston adjacent the coil assembly. It will be seen that the piston is separated from the coil only by the thickness of the solid medium coating thereon and that to obtain a performance on the basis of a minimal and predetermined separation the use of a pre-formed sheet 25 which can be made uniform to close tolerances and both thickness and strength enables the piston to take up a predetermined separation with respect to the coil.

Restoring means, in the form of resilient diaphragm 31 supported on gasket 30 and secured to the housing by way of clamping means 31', extends across the piston and biases it into the rest position in the working cavity. The diaphragm is preferably formed of a Neoprene-Nylon-Neoprene composite known per se for pump diaphragms. In addition to the tautness it possesses to bias the piston tightly against the coil assembly on its rest position, but clearly no obstacle to piston motion when the coil is energised, the diaphragm protects the piston and working cavity from ingress of the water environment 18 and any contaminants it contains.

The housing means 27 also supports an apertured reinforcing member 32 which is disposed adjacent the face 33 of the coil assembly distal from the piston 21 to be abutted by the planar coil assembly 22 and absorb reaction forces between coil and diaphragm plate produced in operation.

The apertured reinforcing member conveniently, but not necessarily, comprises a plate of fibre, such as glass, reinforced plastics resin mounted at its periphery in the housing, conveniently by moulding the housing means around it during formation of the housing means, and taking the form of an annulus 34 having spokes 35 which extend to a common hub 36 and between the spokes apertures 37.

The coil assembly 22 may simply abut the face of the wheel-like reinforcing member so as to transfer reaction forces generated in operation thereto, being mounted in the housing recess 27 at its periphery only or may alternatively, or additionally, be secured to the adjacent face of the reinforcing member by an adhesive such as, or similar to, the resins forming the reinforcing member and coil assembly medium 24 or a toughened acrylic type of material.

Such adhesive securing provides for more uniform transfer of operational forces acting in a direction away from the diaphragm plate and also provides by its tensile strength support for the coil assembly against non-operational forces acting in a direction away from the reinforcing member, such as impacts on, or of, the transducer.

However, it will also be appreciated that whereas there is a wide choice of materials having properties of mechanical strength making them suitable for the reinforcing member, the ability to secure adhesively to the coil assembly may place constraints on the choice.

Alternatively, or in addition, the coil assembly may be secured to the reinforcing member mechanically such as by extending the plug 26, which may be formed of fibre reinforced plastics resin, through both the coil assembly 22 and reinforcing member 32. The plug may be bonded to the reinforcing member or simply clamp the coil assembly against it.

The transducer also comprises cooling means which, in transducer operation in a liquid environment of immersion, uses a heat transfer fluid able to pass through the apertures 37 of the reinforcing member to extract heat from the coil assembly, relatively efficiently conducted by the medium 24 by virtue of its thinness, and transfer it to the liquid of immersion 18.

The cooling means further comprises at least one cooling aperture in the housing means 27, conveniently a single large aperture 38 substantially coextensive with the reinforcing member, between the reinforcing member 32 and the environment 18 which proves a heat transfer path between the heat transfer fluid in contact with the coil assembly and the environmental liquid.

The cooling means may utilise the environmental liquid 18 as the heat transfer fluid, as shown in Figures 2 and 3, in which case when the transducer is immersed the liquid which forms its operating environment simply enters the cooling aperture 38 and a direct heat transfer path is provided between the coil assembly and the environmental liquid for heat removal.

A single aperture 38 may be considered as leaving the coil assembly, exposed by apertures 37 and 38, vulnerable to damage from outside the transducer. The housing may be formed with the plurality of smaller apertures in place of a large one 38, but this might restrict the heat transfer because of the poor thermal conductivity characteristics of the epoxy housing material from which the prior art suffers.

Preferably in such circumstances, and as shown in Figures 4(a) and 4(b), the housing is provided with a thermally conductive and impact resistant cover 40 having an array of apertures 41 therethrough to permit free passage of environmental liquid and spaced from the reinforcing member 39 to define a cavity cooling 42 so that impact on or of the cover will not be transmitted to the coil assembly.

It will be seen that although the effective area of the cooling aperture, in which there is direct contact between the heat transfer liquid in the cavity 42 and the environment 18, is reduced, there is still efficient transfer of heat by means of convection currents set up in the liquid and thermal conductivity of cover plate. The cover plate may be formed of a metal construction for maximum thermal conductivity in addition to such connection, although influence of such a metal body on the acoustic performance may have to be accommodated, or a relatively thin sheet of fibre reinforced plastics resin may offer adequate thermal conductivity whilst providing useful impact protection.

The cover 40 may be secured to the housing means as shown in Figure 4(a) or, as shown in Figure 4, may be secured to the reinforcing member 32 by way of studs 43 or other fastenings inserted into the reinforcing member during moulding thereof, such means of securing being essentially to retain the cover against removal without transmitting impacts to the coil assembly.

A cover such as 40 shown in Figure 4 need not be flat and one of convex, or domed, configuration may be provided, as illustrated at 50 in Figure 5, to increase the resistance to impacts and to some extent provide an increased surface area across which heat transfer is effected.

As an alternative to employing the environmental liquid as the heat transfer fluid the cooling aperture 38 may be sealed by an un-apertured cover plate 60 as illustrated in Figure 6(a) whereby heat transfer between the heat transfer fluid in cavity 42 and environment 18 is totally by way of conduction through the cover plate 60.

The heat transfer fluid may still be the same liquid 18 as the environment of immersion, possibly introduced upon immersion by a small duct (not shown) in the housing means 27, or any other fluid 61 having suitable properties related not only to the transfer of heat but also compatibility with the materials of the transducer and the changes of pressure inherent in operating within liquid environment at depth.

The heat transfer fluid 61 may conveniently be an oil of the type often used for cooling transformers employing similar materials as the transducer.

It may be desirable at shallow operating depths, and necessary at great depths, to equalise the pressures of separate heat transfer fluid and environmental liquid.

This may be achieved by having the cover plate flexible such that the environmental pressure is transmitted to the heat transfer fluid. However, if the cover plate requires a rigidty to give it strength in protecting the transducer from external damage the heat transfer fluid may be contained by a flexible membrane 62, similar to diaphragm 31 as shown in Figure 6(b) and the cover or housing apertured at 63 to permit the environmental liquid to exert the ambient pressure on the membrane and equalise the pressure of the heat transfer fluid with that of the environmental liquid.

The pressure in the operating cavity 29 also is desirably equalised with the environment at great operating depths and to this end ducting 64 illustrated in Figure 6(b) may be provided between the operating cavity 29 and cavity 42 through the housing or possibly through the coil assembly 22 itself, such as by way of plug 26.

The cover 60 may be domed also (not shown) in the manner of Figure 5 for added strength.

Although the above description has related to a particular configuration of coil and piston suited for producing individual acoustic pressure pulses, it will be appreciated by those skilled in the art that both components may differ whilst producing the same effect of piston movement in a pre-determined manner. With a planar, spiral coil, the piston may for example comprise a copper-or aluminium-containing composite with a non-metal which provides mechanical strength necessary to withstand the forces involved in producing a pressure pulse whilst having a lower inertia than if of solid metal.

Furthermore, the coil may take other than a planar form and may comprise a set of separate coils and the piston may comprise a ferromagnetic material or composite carrying ferromagnetic pole pieces for magnetic interaction with the coil or coils.

The transducer may be operated or optimised to operate in a continuous duty mode producing continuous wave acoustic signals rather than individual pulses.

Finally it is reiterated that the transducer is not limited by the type of liquid environment in which operated and requires only that the liquid contacting elements are unreactive therewith.

Claims (22)

Claims:

1. An electromechanical transducer adapted to operate immersed in a liquid environment comprising a coil assembly comprising an electromagnetic coil contained within a solid electrically insulating medium configured to provide low impedance to the transfer of heat from the -coil, a piston operable to interact magnetically with the electromagnetic coil to produce a separating force between them, restoring means operable to bias the piston towards a rest position, housing means supporting the coil assembly and restoring means and defining an operating cavity containing the piston and a piston rest position adjacent the coil assembly, an apertured reinforcing member supported by the housing means adjacent the face of the coil assembly distal from the piston and operable to absorb operational reaction forces between the piston and coil assembly acting to displace the plane of the coil assembly with respect to the housing away from the piston and cooling means including at least in immersed operation a heat transfer fluid operable to transfer heat from the coil assembly by way of the apertured reinforcing member to the liquid environment of immersion.

2. An electromechanical transducer as claimed in claim 1 in which the cooling means comprises at least one cooling aperture in the housing extending between the reinforcing member and the environments of immersion and arranged to provide a heat transfer path between the heat transfer fluid in contact with the coil assembly and the environmental liquid of immersion.

3. An electromechanical transducer as claimed in claim 2 in which the reinforcing member is mounted at its periphery in the housing and the cooling means comprises a single cooling aperture in the housing substantially coextensive with the exposed face of the reinforcing member.

4. An electromechanical transducer as claimed in claim 3 in which the cooling means includes a thermally conductive cover for the cooling aperture in the housing adapted to permit heat transfer across the cover from the heat transfer fluid, between the cover and the coil assembly, to the environment of immersion.

5. An electromechanical transducer as claimed in claim 4 in which the cover is apertured to permit the free passage of environmental liquid and said environmental liquid of immersion comprises the heat transfer fluid.

6. An electromechanical transducer as claimed in claim 4 in which the thermally conductive cover is adapted to define a cooling cavity separated from the environmental liquid of immersion containing the heat transfer fluid and provide a thermally conductive path between the heat transfer fluid and the liquid of immersion.

7. An electromechanical transducer as cla-imed in claim 6 in which the heat transfer fluid is oil.

8. An electromechanical transducer as claimed in any one of claims 4 to 7 in which the thermally conductive cover is a metal sheet.

9. An electromechanical transducer as claimed in any one of claims 6 to 8 including ducting extending between the opposite sides of the coil assembly for the passage of heat transfer fluid between the cooling and operating cavities.

10. An electromechanical transducer as claimed in any one of claims 6 to 9 in which the thermally conductive cover is flexible and responsive to changes in pressure of the liquid environmental of immersion to equalise the pressure of the heat transfer fluid with that of the liquid environment.

11. An electromechanical transducer as claimed in any one of the preceding claims in which the reinforcing member comprises a plate of fibre reinforced plastics resin.

12. An electromechanical transducer as claimed in claim 10 in which the fibre reinforcement is glass.

13. An electromechanical transducer as claimed in any one of the preceding claims in which the surface of the coil assembly is secured to the surface of the reinforcing member.

14. An electromechanical transducer as claimed in claim 13 in which the surface of the coil assembly is secured to the reinforcing member by adhesive.

15. An electromechanical transducer as claimed in any one of claims 11 to 14 in which the coil assembly is secured to the reinforcing member by fastening member extending through the coil assembly and reinforcing member.

16. An electromechanical transducer as claimed in any one of the preceding claims in which the coil assembly comprises a planar spiral electromagnetic coil sandwiched between two sheets of electrically insulating material.

17. An electromechanical transducer as claimed in claim 16 in which the coil is secured to the surface of each sheet of electrically insulating material by adhesive.

18. An electromechanical transducer as claimed in claim 16 or claim 17 in which the sheets of electrically insulating material are fibre reinforced plastics resin.

19. An electromechanical transducer as claimed in claim 17 in which the fibre reinforcement is glass.

20. An electromechanical transducer as claimed in any one of the preceding claims in which the housing comprises a moulding of epoxy resin into which the reinforcing member is moulded in formation of the housing.

21. An electromechanical transducer as claimed in any one of the preceding claims in which the restoring means comprises a diaphragm of Neoprene-Nylon-Neoprene secured at its periphery to the housing means and extending across the piston and covering the operating cavity.

22. An electromechanical transducer adapted to operate immersed in a liquid environment, the transducer being substantially as herein described with reference to, and as shown in, Figure 2 or 3 or any one of Figures 3, 4(a), 4(b), 5, 6(a) or 6(b) of the accompanying drawings.