WO1987007767A1 - Piezoelectric transducers - Google Patents

Abstract

A film (10) of a polymeric piezoeletric material such as polyvinylidene fluoride is connected to an external circuit (18) without the use of any electrically conductive connection to the film (10). Conductive elements (14) are spaced from the film (10) by a dielectric material (12), conveniently in the form of an adhesive which also secures these members together. In preferred embodiments, the elements (14) are mechanically compliant, such as conductive cloth or foam.

Description

Piezoelectric transducers

This invention relates to piezoelectric transducers, particularly but not exclusively those making use of polymeric piezoelectric materials.

Various materials of this nature are known, one being polyvinylidene fluoride (PVDF) which will be used herein as an example. Known PVDF films can exhibit high piezoelectr activity, for example those produced from a homopolymer of vinylidene fluoride by melt extrusion followed by mechanical stretching to produce orientation and electrical polarisation or poling in a high intensity field. An example of such a material is KYNAR (trade mark) film by Pennwalt Corporation. Such materials produce electrical energy when deformed by bending, stretching or compression, or can be operated in reverse by applying a voltage to produce mechanical movement.

When used as a transducer, it has hitherto been the practice to provide electrodes on the surfaces of the PVDF film, which electrodes couple the charge produced in the film on deformation to an external circuit, thereby producing current flow in the circuit. Such electrodes may be applied to the PVDF film by silk-screening with a conductive ink, for example silver ink, or by metallisation by vapour deposition or sputtering.

The present invention in its broadest aspect is based on the discovery that useful devices can be realised without electrodes or conductors contacting the piezoelectric material. The invention also provides devices making use of this principle.

A practical drawbick of PVDF and other polymeric piezo- electric materials is that heating above a transition temperature causes the piezoelectric characteristics to be lost. This limits the manufacturing operations which can be applied to the materials. The present invention is also useful in overcoming this problem. The invention is defined in the appended claims.

Embodiments of the invention will now be described. by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a perspective view, partly in section, of a device embodying the invnetion and illustrating the principle thereof;

Fig. 2 is a like view of a modified device; Fig. 3 is a perspective view, partly broken away, of a further embodiment;

Fig. 4 is a cross-section of another embodiment; and Fig. 5 is a similar cross-section of yet another embodiment.

Referring to Fig. 1, a transducer comprises a sheet 10 of PVDF film each face of which is covered with an insulating layer 12 of a dielectric material which in turn is covered by a conductive layer 14. When the film 10 is deformed by compression, bending or stretching, electric charge is created within it, which is capacitively voupled to the conductive layers 14 and in turn causes a current to flow via conductors 16 through an external circuit indicated at 18 A convenient means of providing the insluating layers

12 is in the form of an insulating adhesive which also serves to secure the assembly together. This is the case in the embodiment of Fig. 2 in which a PVDF film 20 is secured to and electrically insulated from conductive members 22 by adhesive layers, e.g. of epoxy. In this embodiment, the conductive members 22 are sheets of an electrically conductiv mechanically compliant foam material; one suitable material is ENSOLITE CEC (trade mark) by Uniroyal, which is a vinyl nitrile foam loaded with carbon. Other plastics or rubbers in foam or sheet form and containing dispersed conductive material such as carbon, copper, aluminium, or silver would also be suitable. By way of example, ENSOLITE CEC has a

4 surface resistivity of about 10 ohms/square, which has been found to be suitable. The use of conductive foams in this manner has the advantage that a relatively small applied force causes stretching of the film to produce a relatively high signal. Fig. 3 illustrates an embodiment for dealing with a situation where it is desired to detect the occurrence and/or the intensity of a force occuring within a relatively large area, but where the location of the force within that area is not of significance.

Such a requirement could be approached, in principle, by covering the area with a single large sheet of PVDF, but

(a) there is a fairly low limit to the size of sheet availab

(b) such an arrangement would have a high capacitance leadin to poor signal levels, and (c) the applied force would be likely to damage the film by creasing or tearing.

It would also be possible in principle to cover the area of interest with a plurality of smaller pieces of metallised PVDF film. This approach has two disadvantages. One is that there are serious difficulties in accomodating the necessary conductors. Secondly, if the pieces are electrically interconnected, much of the energy produced by a force acting on one is dissipated in the others, leadin again to poor signal strength. In fig. 3, the area of interest is covered with a numbe of PVDF sheets 30 which are sandwiched betweeen conductive foam sheets 32 and 34, for example of the material specified above. It is necessary for the PVDF sheets 30 to be insulated from the conductive sheets 32 and 34; it is also necessary for the cinductive sheets 32 and 34 to be insulated from each other in those areas where no PVDF material is present. This is suitably achieved by coating one side of the PVDF sheets 30 with an insulating adhesive, securing these in position on one of the foam sheets 32, covering the whole surface of the other foam sheet 34 with an insulating adhesive, and adhering this over the first assembly.

Conductors 36 attached to the sheets 32, 34 connect the transducer to an external circuit indicated at 38. A force applied to any part of the transducer will produce a signal to the circuit 38representative of the occurrence and magnitude of the force, althought here will of course be no indication of the location of the force on the transducer area. Fig. 4 shows an embodiment in which a number of discrete PVDF films 40 are secured between common conductive sheets 42 and 44, and separated therefrom by non-conductive adhesive layers 46. In this embodiment, however, each PVDF film is provided on its major faces with metallised coverings 48, which may for example be of aluminium. Metalli PVDF film is well known per se and is commercially available The advantage of including a metallised layer of this nature is that charge created within the PVDF material by the application of a force is spread across the metallised surface which provides a larger area for capacitive coupling to the outer conductor 42 or 44.

The conductive layers 42, 44 may be metal foil, conductive impregnated cloth, or conductive foam, and the adhesive 46 may conveniently be an epoxy or a double-sided pressure-sensitive tape. Similar materials may also be used in other embodiments.

Fig. 5 is concerned with providing a piezoelectric transducer which is transparent, or at least partially transmissive, to light. This obviously requires the use of light transmissive electrodes. Transparent electrodes are well known- in other fields, but difficulties can arise in applying them to PVDF and similar materials. A particular useful material for transparent electrodes is indium-tin oxide (ITO) , but deposition of ITO on a plastic substrate involves the use of temperatures above the transition temperature for PVDF. The transducer of Fig. 5 comprises a PVDF film 50 secured by layers of insulating adhesive 52 between sheets 54 each comprising a transparent layer of ITO 56 deposited on a MYLAR (trade mark) film substrate; methods of metallising MYLAR in this way are well known to those skilled in the art. Thus, the invention makes it possible to keep the production of electrodes quite separate from the piezo material. The invention provides other advantages. Large arrays of complex shapes of piezo material can be achieved without complex conductor arrangements. Solder connections may be made to non-piezo electrode sheets before assembling the transducer. The thickness of, and the material constituting, the insulating layers can be selected to provide a desired value of capacitance; this can be used in conjunction with the characteristics of the piezo material itself to produce desired electrical properties for the transducer, for example by altering resonant frequencies.

Claims

1. A transducer comprising a piezoelectric element and two conductive elements associated with opposed faces of the piezoelectric element, at least one of said conductive elements being adjacent said face and spaced therefrom by a dielectric medium without any conductive connection between said element and said face.

2. The transducer of claim 1, in which said piezoelectric element is a polymeric piezoelectric film.

3. The transducer of claim 2, in which said conductive elements are secured to said faces by an adhesive forming a dielectric layer.

4. The transducer of claim 3, in which the faces of said film in contact with said adhesive are provided with a metallised coating.

5. The transducer of claim 3 or claim 4, in which said conductive elements comprise mechanically compliant, electrically conductive material.

6. The transducer of claim 5, in which said material is a resilient foam having a conductive material dispersed therein.

7. The transducer of claim 1, in which a plurality of piezoelectric elements share a common conductive element on at least one side.

8. A transducer assembly comprising: a first sheet of mechanically compliant, electrically conductive material, a second sheet of a mechanically compliant, electrically conductive material coextensive with said first sheet, a plurality of planar pieces of a polymeric piezoelectric material each having one face secured to said first sheet by an electrically insulating adhesive and the other face secured to the second sheet by an electrically insulating adhesive, and means insulating from each other those areas of the mutually-facing surfaces of said first and second sheets not engaged with said pieces of piezoelectric material.

9. the assembly of claim 8, in which said faces of the piezoelectric material are provided with a conductive coating.