WO1992018256A1

WO1992018256A1 - Device for acoustic wave generation

Info

Publication number: WO1992018256A1
Application number: PCT/GB1992/000727
Authority: WO
Inventors: Richard Warren Jones
Original assignee: Jones Richard W
Priority date: 1991-04-20
Filing date: 1992-04-21
Publication date: 1992-10-29
Also published as: GB9108490D0

Abstract

An ultrasonic transducer comprises a working member (12) of piezo-electric material having at least two surface regions (13, 14) separated by a boundary surface region (15); a film layer (16, 17) of conductive material on each surface region (13, 14); a conductor (18, 19) whereby a voltage can be applied to each film (16, 17); a protective non-conducting mask (22, 23), for each of the film layers and in intimate contact therewith, the mask (22, 23) comprising a fibre reinforced material. In addition to the transducer methods are described of fabricating the transducer with intimate contact between film layer and mask. At least one passage (50, 68) can be provided through the working member (12) and a web (50A, 68A) of the fibre reinforced material located through the, or each, passage (50, 68) so that the or each web (50A, 68A) has one end integral with one mask (22, 23) and the other end integral with the other mask (23, 22) to form a mechanical link between different regions of one mask or between two masks. This arrangement provides for a stronger mechanical linkage of the protective mask (22, 23) to the body (12) of the transducer. It also serves to preserve the shape of a flexible working member (12) which is distorded into a predetermined shape prior to the mask material (22, 23) polymerising to form a rigid structure. The transducer (51) can include a backing member (60) serving to direct the acoustic output of the transducer predominantly in a particular direction (A) either by reflecting or attenuating acoustic output that would otherwise be generated in any other direction that the particular direction (A).

Description

DEVICE FOR ACOUSTIC WAVE GENERATION

Technical field

This invention relates to an acoustic transducer and a method of fabricating such a transducer.

Vibrations can be generated by applying an alternating voltage to a piezo-electric material. Such a material is characterised in that when an alternating voltage at a particular frequency is applied between two active surfaces of the material then the surfaces vibrate. Conversely if mechanical vibration is applied to two active surfaces of such a material an alternating voltage is induced between the active surfaces.

There are a number of piezo-electric materials available. Whatever material is used to generate a selected acoustic frequency it is necessary to apply a driving voltage to discrete and separate active surfaces of the material. This serves to generate a voltage gradient through the body of the material between the surfaces so inducing electro-strictive movement of the active surfaces of the piezo-electric material. The movement is proportional to: the voltage applied to the material; the material thickness; and electro-strictive properties of the material.

Piezo-electric materials respond to, and generate, voltage gradients and are good insulators. In fabricating a transducer it is necessary to ensure the active surfaces are insulated from each other. For the applied voltage to be used to best effect to generate vibration it is necessary to ensure the active surface are good electrical conductors. This is usually achieved by depositing a thin film of a conductive material (such as aluminium) across each active surface which can then be used to apply an excitation voltage uniformly across the surface.

Waves of high frequency mechanical vibrations are directional and can be propagated along a predetermined path. This enables them to be used to measure properties of materials through which the waves are propagated and further to detect non-homogenous regions (such as objects or discontinuities) along the path of wave propagation. This is achieved by detecting vibrations reflected back along the path to the generating device resulting in the reflected mechanical vibrations inducing a voltage between the active surfaces of the propagating piezo-electric device.

It is generally impractical to locate an active surface directly on the medium since the film needs to be electrically insulated from a surrounding medium and also to be protected from abrasion or other mechanical damage. Any material used for protection should not degrade the performance of the transducer and in particular should neither adversely affect the mechanical coupling between the active surface and the propagating path in the working medium nor limit the ability of the transducer to resonate at its natural frequency. To this end the protective material should be applied in the form of a thin, uniform layer.

Background Art

Transducers making use of piezo-electric material and layers of different materials are known.

PCT Application WO/85/0 477 in the name of Jones discloses an electro mechanical transducer having two layers of a polarized piezoelectric polymer coated on each face with a conductive coating. The two layers are arranged back to back and secured by a double-sided adhesive tape with their positively charged conducting coatings facing each other. As the protective layers of insulation and a protective layer of sheath are provided. The inner conductive coatings are interconnected by a pin passing through the polymer layers with the outer conductive coating being omitted in the vicinity of the pin. The outer conductive coatings are likewise interconnected by a pin connection to an external screened cable being made through these pins with one pin being connected to the outer screen of the cable and the other pin being connected to the inner signal wire. Because the device is piezoelectric no external power supply is required and interconnection of the two inner conductive coatings enhances the signal generated by the device while the two outer coatings act as an electromagnetic and electrostatic screen to reduce extraneous and unwanted spurious signals which would otherwise affect operation of the device.

UK Patent Application 2 151 434 in the name of the Raytheon Company describes a polymer transducer consisting of multiple layers disposed symmetrically about a stiffener layer. This layer prevents flexural modes in the operating frequency band and provides a mounting structure for acceleration noise cancellation. Piezoelectric polymer layers are attached to the stiffener layer directly. Other piezoelectric polymer layers are attached to the stiffener layer through intervening layers which provide electrical and mechanical vibration isolation of the outer piezoelectric layers from the inner and stiffener layers. The directly attached piezoelectric layers provide signals primarily indicative of strain in the stiffener layer and are used to eliminate the effect that mechanical vibration of the array has on signals produced by the outer piezoelectric layers.

Both of these known transducers are relatively complex in form and do not seek to provide a transducer of simple construction which includes means for protecting delicate parts of the transducer against mechanical damage. Applications which will be mentioned hereafter particularly require the provision of a transducer embodying such means. Disclosure of Invention

According to a first aspect of the present invention there is provided an acoustic transducer characterised by:

1 a working member (12) of piezo-electric material having at least two surface regions (13, 14) separated by a boundary surface region (15);

2 a film layer (16, 17) of conductive material on each surface region (13, 14);

3 a conductor (18, 19) whereby a voltage can be applied to each film layer (16, 17);

4 a protective non-conducting mask (22, 23), for at least one of the film layers (16, 17) and in intimate contact therewith, the mask (22, 23) comprising a fibre reinforced material of uniform thickness impregnated with a polymerised material.

According to a first preferred version of the first aspect of the present invention the acoustic transducer is characterised by at least one passage (50, 68) extending through the working member (12) and a web (50A, 68A) of the mask extends through the, or each, passage (50, 68) the web having one end integral with one mask (22, 23) and the other end integral with the other mask (23, 22) to form a mechanical link between different regions of one mask (22, 23) or between two masks (22, 23).

According to a second preferred version of the first aspect of the present invention or the first preferred version thereof the acoustic transducer is characterised in that the body 12 is of a piezoelectric ceramic material such as barium titanate or a plastics material such as PVDF.

According to a third preferred version of the first aspect of the present invention or any preceding preferred version thereof the acoustic transducer is characterised by a backing member (60) providing for the acoustic output to be transmitted from substantially one direction (A) from the transducer by either absorbing or reflecting vibrations emitted by the transducer (51) which, but for the existence of the backing member (60), would tend to be emitted in a direction other than the one direction (A). Preferably in this case the acoustic transducer as is characterised in that the backing member (60) is provided with an envelope (61) corresponding in form, function and material to the non-conducting mask (22, 23).

According to a second aspect of the present invention there is provided a method of fabricating an acoustic transducer according to the first aspect or any preferred version thereof is characterised by the steps of:

1 forming on a member (12) of piezo-electric material two surface regions (13, 14) separated by a boundary surface region (15);

2 depositing on each surface region (13, 14) a film layer (16, 17) of conductive material;

3 securing a discrete conductor (18, 19) to each film layer (16, 17);

4 locating a matrix layer of non-conductive material in intimate contact with one of the regions;

5 applying a polymer in a liquid state to the matrix layer and causing the polymer to polymerise so as to form a continuous mask (22, 23) of substantially non-conducting material over the film layer (16, 17) of the region (13, 14).

According to a first preferred version of the second aspect of the present invention the method of fabrication is characterised in that prior to the step of locating the matrix layer at least one passage (50, 68) is formed in the member (12) and causing a web (50A, 68A) of matrix and polymer material extends through the, or each, passage (50, 68); the or each web (50A, 68A) having each end integral with a mask (22, 23) so as to form a mechanical link by way of the web between either different regions of one mask (22, 23) or between two masks (22, 23). According to a second preferred version of the present invention the method of fabrication of an acoustic transducer according to the first aspect or any preferred version thereof is characterised by the steps of:

1 forming on a member (64) of plastics piezo-electric material two surface regions (65, 66) separated by a boundary surface region (67);

2 depositing on each surface region (65, 66) a film layer (69, 70) of conductive material;

3 securing a discrete conductor (71, 72) to each film layer (69, 70);

4 positioning a protective layer (73, 74) of non-conducting tissue material either impregnated, or subsequently impregnated, with a polymerisable material over each film layer (69, 70);

5 juxtaposing the member (64) and its associated film layers (69, 70) and protective layers (73, 74) between a complementary pair of shaped platens (77, 78);

6 displacing the platens (77, 78) towards one another to drive the protective layer (73, 74) into intimate contact with its associated film layer (69, 70) and to form the body member (64) and its associated layers (69, 70, 73, 74) into a predetermined generally dished shape;

7 enabling the polymerisable material to polymerise so as cause the protective layers (73, 74) to form into a rigid shell serving to retain the body member (64) in its dished shape on subsequent separation of the platens (77, 78) to release the fabricated transducer (63).

Typically the method of fabrication according to the second preferred version is characterised in that prior to the step of positioning a protective layer (73, 74) bores (68) are provided in the film layers (69, 70) and the body member (64) and filled with a bonding material and prior to the step of enabling the polymerisable material to polymerise bonding material projecting from the bores (68) and the polymerisable material are caused to mix so that in the enabling step the bonding material projecting from the bores (68) and the polymerisable material become integrated by rigid webs extending through the bores (68) so providing for rigid mechanical linkage between the layers (73, 74) serving to hold the body member (64) in its dished shape.

According to a third aspect of the present invention an acoustic sound generator comprising a transducer according to the first aspect of the present invention coupled to a generator whereby a voltage pulse or a train of pulses or a continuous pulse output can be applied to the conductors (18, 19).

This invention is particularly concerned with the provision of a protective material as a coating on a fragile film layer without undue increase in mechanical mass. Whilst the transducer is intended for use at any appropriate acoustic frequency it is particularly applicable to use in connection with ultrasonic frequencies. It may also be used to generate a mechanical impulse for an applied voltage pulse. Thus the transducer can be used either to generate a single output pulse, whether of simple or complex form, a continuous steam of pulses or discrete packets of pulses at predetermined intervals.

Brief Description of Drawings

Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings of ultrasonic transducers of which:

Figures 1 shows an exploded view; and

Figure 4 an assembled view of a first embodiment;

Figures 2 and 3 show steps in the fabrication of the transducer described in connection with Figures 1 and 4;

Figures 5, 6 and 9 show respectively second third and fourth embodiments of acoustic transducers; and

Figures 7 and 8 show steps in the fabrication of the transducer described in connection with Figure 9. The figures are diagrammatic and are intended to show the location and function of the components involved. However the figures do not represent the relative thicknesses of the transducer components involved. Modes for carrying out the Invention

Figures 1, 2, 3 and 4

These variously show a transducer 11 with a body member 12 of piezo-electric material on which is formed parallel flat faces 13, 14 with a common peripheral boundary 15 which serves to electrically isolate the faces 13, 14 from one another. Conducting films 16, 17 are deposited on, respectively, faces 13, 14 to enable a voltage pulse or pulses of appropriate frequency to be applied to the films, and so the body member 12, by way of conductors 18, 19. Layers 20, 21 of non-conductive tissue form the matrices of protective layers 22, 23 for the relatively delicate layers 16, 17 and related items as will be described hereafter.

Figures 2 and 3 show the fabrication of the transducer 11. The body member 12 has faces 13, 14 prepared, coated with conducting films 16, 17 and the conductors 18, 19 attached. Layers 22, 23 of non-conducting tissue material now impregnated with a self curing bonding material are positioned on either side of the body member 12. The assembled items are then placed between platens 24, 25 of a moulding press of conventional design. The platens 24, 25 are flat and coated with a release agent. They are driven towards one another to cause the impregnated layers 22, 23 to be pressed into intimate contact with their respective film layers 16, 17. The tissue material is thoroughly wetted by the bonding material so as to ensure that no air bubbles are trapped in the layers or between the layers and the film material. Once polymerisation of the bonding material has occurred the press platens are separated and the completed transducer removed. The use of pressure applied by platens enables a uniform thickness to be achieved for the completed layers 22, 23 to be established over the protective coating to give the coating a thickness approximately equal to that of the fibre tissue: the polymer occupying the interstices of the matrix material of fibre tissue. Figure 5

This shows a transducer 41 similar in many respects to that described in connection with Figures 1 - 4. Thus a body 42 of piezo-electric material with active surfaces 43, 44 has an isolating periphery 45 separating them. The surfaces 43, 44 are provided with, respectively, film layers 46, 47 protected by layers 48, 49 of non¬ conducting material. However in the case of this embodiment the body 42 is pierced by a number of bores 50 which serve to link surfaces 43, 44. In coating the film layers care is taken to ensure that no film material enters the bores 50 which could provide a low resistance conducting path between the film layers 46, 47 when a voltage difference is established across the two active surfaces 43, 44. Bonding agent is poured to fill the bores 50 and the impregnated material to form non-conductive layers 48, 49 are located in place on the surface 43, 44. When the material polymerises, webs (such as web 50A in bore 50) provide a mechanically rigid material link between the two layers 48, 49 ensuring a strong integral direct mechanical connection between layers 48, 49.

Figure 6

This shows a transducer 51 similar to that described in connection with Figures 1 to 4 with a body 52 of piezo-electric material with active surfaces 53, 54 with isolating periphery 55 separating them. The surfaces 53, 54 are provided with, respectively, film layers 56, 57 protected by layers 58, 59 of non-conducting material. In this embodiment the transducer 51 incorporates a member 60 and a protective envelope 61 which serves to provide protection for the transducer 51. The member 60 whether by reflection or attenuation causes the acoustic output of the body 52 to be predominantly directed in the direction of arrow A. The material of envelope 61 is chosen so that it does not prejudice the operating frequency of the transducer such as by resonating at a different frequency. In this example the protective layer 59 is not essential to the operation of the transducer 51 provided that the member 60 and envelope 61 offer protection for the film layer 57 on the body 52.

Figures 7. 8 and 9

These are concerned with the provision of a shaped transducer providing for a directed acoustic output.

These show a transducer 63 with a body member 64 of plastics piezo-electric material such as PVDE on which is formed parallel faces 65, 66 with a common peripheral boundary 67 which serves to electrically isolate the faces 65, 66 from one another. As described above in connection with Figure 5 and to the same end the body member 64 is pierced by bores 68. Conducting films 69, 70 are deposited on, respectively, faces 65, 66 and have voltages of appropriate frequency applied to them by way of conductors 71, 72. Layers 73, 74 of non-conductive fibre reinforced plastic are formed as will be described hereafter.

Figures 7 and 8 show the steps in fabricating the dished transducer 63 shown in Figure 9. The body member 64 which is of a plastics piezo-electric material has faces 65, 66, prepared, coated with conducting film layers 69, 70 and the conductors 71, 72 attached. The faces 65, 66 are separated by a boundary surface region 67. The bores 68 are drilled through the layers 69, 70 and the body member 64 and the bores 68 are filled with a bonding material care being taken to avoid the entrapment of air in the bores. Layers 73, 74 of non-conducting tissue material impregnated with the self curing bonding material are positioned on either side of the body member 64. The assembled items are then placed between shaped platens 77, 78 of a moulding press of conventional design. The shaped platens 77, 78 are coated with a release agent and then driven towards one another to cause the impregnated layers 73, 74 to be pressed into intimate contact with their respective film layers 69, 70 and to enable bonding material in the outer ends of the bores 68 to intimately blend with the impregnated layers 73, 74. The shaped platens form the flexible body member 64 into the desired dished form. On polymerising the bonding material in the bores 68 form rigid webs (typically web 68A) between the layers 73, 74 providing for a strong direct mechanical link between layers 73, 74 serving to hold the body member 64 in its dished configuration.

Once polymerisation has occurred the platens 77, 78 are opened and the formed complete transducer 63 removed. The use of platens provides for the exact form of the dished or other selected form to be reproduced with accuracy.

The dished configuration provides for a focused sonic beam to be generated by way of the transducer on its concave side in the direction of the arrow A⁷.

Industrial Applicability

The transducers described according to the present invention are applicable to a number of industrial applications. In particular units of the type described, and manufactured as described, are used in instrumentation, particularly portable instrumentation, for use in the water and effluent industries for the measurement of flow and flow change along channels. In such applications it is necessary for the units used to withstand unsympathetic usage and maintenance during location on site while providing accurate output signals for local or remote interpretation.

Claims

1 An acoustic transducer characterised by:

2 a film layer (16, 17) of conductive material on each surface region (13, 14);

An acoustic transducer as claimed in Claim 1 characterised by at least one passage (50, 68) extending through the working member (12) and a web (50A, 68A) of the mask extends through the, or each, passage (50, 68) the web having one end integral with one mask (22, 23) and the other end integral with the other mask (23, 22) to form a mechanical link between different regions of one mask (22, 23) or between two masks (22, 23).

An acoustic transducer as claimed in Claim 1 or Claim 2 characterised in that the body 12 is of a piezoelectric ceramic material such as barium titanate or a plastics material such as PVDF.

An acoustic transducer as claimed in any preceding claim characterised by a backing member (60) providing for the acoustic output to be transmitted from substantially one direction (A) from the transducer by either absorbing or reflecting vibrations emitted by the transducer (51) which, but for the existence of the backing member (60), would tend to be emitted in a direction other than the one direction (A). An acoustic transducer as claimed in Claim 4 characterised in that the backing member (60) is provided with an envelope (61) corresponding in form, function and material to the non-conducting mask (22, 23).

A method of fabricating an acoustic transducer as claimed in any preceding claim characterised by the steps of:

3 securing a discrete conductor (18, 19) to each film layer (16, 17);

A method of fabrication as claimed in Claim 6 characterised in that prior to the step of locating the matrix layer at least one passage (50, 68) is formed in the member (12) and causing a web (50A, 68A) of matrix and polymer material extends through the, or each, passage (50, 68); the or each web (50A, 68A) having each end integral with a mask (22, 23) so as to form a mechanical link by way of the web between either different regions of one mask (22, 23) or between two masks (22, 23).

A method of fabrication an acoustic transducer as claimed in any of preceding Claims 1 to 5 characterised by the steps of:

3 securing a discrete conductor (71, 72) to each film layer (69, 70);

A method of fabrication an acoustic transducer as claimed in Claim 8 characterised in that prior to the step of positioning a protective layer (73, 74) bores (68) are provided in the film layers (69, 70) and the body member (64) and filled with a bonding material and prior to the step of enabling the polymerisable material to polymerise bonding material projecting from the bores (68) and the polymerisable material are caused to mix so that in the enabling step the bonding material projecting from the bores (68) and the polymerisable material become integrated by rigid webs extending through the bores (68) so providing for rigid mechanical linkage between the layers (73, 74) serving to hold the body member (64) in its dished shape.

An acoustic sound generator comprising a transducer as claimed in any of preceding claims 1 to 5 together with a generator whereby a voltage pulse or pulses can be applied to the conductors (18, 19).