WO2009132653A1

WO2009132653A1 - A transducer comprising a composite material with fiber arranged in a pattern to provide anisotropic compliance

Info

Publication number: WO2009132653A1
Application number: PCT/DK2009/000103
Authority: WO
Inventors: Michael Tryson; Mohamed Benslimane; Hans-Erik Kiil; Mike Zumbrum
Original assignee: Danfoss A/S
Priority date: 2008-04-30
Filing date: 2009-04-30
Publication date: 2009-11-05

Abstract

The invention provides an elastomer transducer for converting between electrical energy and mechanical energy. The transducer comprises a composite material with a body (2, 4) and a film (3) of an elastomer material, the body comprising at least an electrically conductive portion and an anisotropic body, in the anisotropic body, fibers are arranged in a pattern to provide inferior flexure resistance and an anisotropic compliance to stretch in a first direction.

Description

A TRANSDUCER COMPRISING A COMPOSITE MATERIAL WITH FIBERS ARRANGED IN A PATTERN TO PROVIDE ANISOTROPIC COMPLIANCE

INTRODUCTION

The invention relates to an elastomer transducer for converting between mechanical and electrical energies. In particular, the invention relates to a transducer comprising a film of an elastomer material arranged between first and second electrically conductive layers and being elastically deflectable in response to repulsion or attraction of the layers. The invention further relates to a composite material for such a transducer and to a method of manufacturing such a composite material.

BACKGROUND OF THE INVENTION

An electrical potential difference between two electrodes located on opposite surfaces of an elastomer body may generate an electric field leading to a force of attraction and thus a deflection of the elastomer body under influence of Coulomb forces between the electrodes. Such transducers are referred to as electroactive polymer transducers (EAP-transducers), or artificial muscles.

US 6,376,971 discloses a compliant electrode which is positioned in contact with a polymer in such a way, that when applying a potential difference across the electrodes, the electric field arising between the electrodes contracts the electrodes against each other, thereby deflecting the polymer. Since the electrodes are of a substantially rigid material, they must be made textured in order to make them compliant.

US 6,376,971 discloses a planar compliant electrode being structured and providing one-directional compliance, where metal traces are patterned in parallel lines over a charge distribution layer, both of which cover an active area of a polymer. The metal traces and charge distribution layer are applied to opposite surfaces of the polymer. The charge distribution layer facilitates distribution of charge between metal traces and is compliant. As a result, the structured electrode allows deflection in a compliant direction perpendicular to the parallel metal traces. In general, the charge distribution layer has a conductance greater than the electroactive polymer but less than the metal traces.

A document US 2005/0040733 discloses a transducer constructed of rolled polymers with electrodes on the two opposing surfaces, and introducing a general-purpose layer rolled between such a set of polymer and electrodes, where this layer may be a thin fiber composite with orientation around the circumference of the disclosed transducer, wherein when bonding a stretched polymer layer to the general purpose layer, the movement can be constrained in the circumferential direction, while allowing compliance in the axial direction. These insulating fibers of the general-purpose layer may be embedded in a soft elastomeric matrix, and when rolled into the transducer, are squeezed between two electrode layers of two adjacent layers of the rolled polymers with electrodes. The insulating fibers therefore does not form an integral part of the construction of the set of polymer with electrodes itself, but are connected as it is rolled between such layers.

Another example is described in US 5,977,685 where, in order to prevent the displacement in the width direction and provide displacement only in the length direction, it is suggested onto the outside of the electrodes, to put bares or anisotropic materials, which are easily stretchable in the length direction or less stretchable in the width direction, such as a woven fabric or canvas,. Again, these anisotropic materials positioned onto the electrodes, to increase the anisotropic nature, having no indication to be conductive.

Other EAP transducers are described e.g. in US 2004/0012301 in which a waved section is provided in a body of an elastomer material. The waved shape provides compliance of the transducer in a specific direction. DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a transducer with an alternative composite and method of making a composite for a transducer. In particular, it is an object to provide a transducer which can be manufactured in an efficient way.

According to a first aspect, the invention provides a transducer which comprises an anisotropic body formed of a material comprising fibers which are arranged in a predetermined woven or non-woven pattern whereby the material becomes deformable and the body becomes stretchable in an anisotropic manner.

By stretchable in an anisotropic manner is meant that the body has stretching properties that differ according to the direction of measurement. As an example, it may not be possible to stretch the body in one specific direction or it may at least be very difficult to stretch the body in one specific direction relative to other directions.

The ability of the body material to deform in specific directions may concentrate the deformation of the film into one or more specific directions. This may improve the performance of the transducer.

The film may be fixed to the body e.g. adhesively and e.g. by embedding at least a part of the film in a void space between the fibers.

The body may comprise a polymer material, e.g. a PTFE material.

The fibers may be arranged in a more or less compact manner whereby a void space may constitute e.g. between 20 and 80 pet of the volume of the body, and the film could e.g. fill up at least 5 pet or possibly in a range between 10-80 pet of the void space.

Each electrically conductive layer may comprise a layer of a conductive material, e.g. a layer applied to a surface of the anisotropic body. For this purpose, a conductive polymer, such as a polyaniline-based conductive polymer coating could be used, or a layer of a conductive metal could be applied e.g. in an evaporation process. As an example, the conductive portion could comprise a metal selected from a group consisting of silver, gold and nickel.

The conductive layers may have a thickness in the range of 0.01-0.1 μm, and the anisotropic body may have a similar thickness. The film may have a thickness between 10 μm and 200 μm, such as between 20 μm and 150 μm, such as between 30 μm and 100 μm, such as between 40 μm and 80 μm.

Each electrically conductive layer may have a resistivity which is less than 10^"4 Ω-cm. By providing the conductive layers with a low resistivity, the response time for conversion between mechanical and electrical energy can be maintained at an acceptable level while allowing a large surface area of the composite material.

The elastomer of the film may e.g. have a resistivity which is larger than 10¹⁰ Ω-cm. Preferably, the resistivity of the elastomer material is much higher than the resistivity of the conductive layers, preferably at least 10¹⁴-10¹⁸ times higher. The resistivity of the elastomer may also be higher than that of the anisotropic body. The elastomer may e.g. comprise a material selected from a group consisting of block copolymers and block-selective oligomers.

The fibers may be individual and un-connected fibers, or the fibers may be connected in nodes in a micro-porous structure wherein voids or "bubble-like" structures form a relatively large part of the volume of the body, e.g. more than 50 pet of the body. The porous structure may e.g. be a polymer structure, e.g. as described in US 6,673,455. If the fibers electrically conductive, or if they are coated with an electrically conductive coating, it may be an advantage if the fibers are in electrically conductive contact with each other. If the fibers are unconnected fibers, they may therefore preferably be arranged directly against other fibers so that the anisotropic body can form a one homogeneously electrically conductive body. In the following description, fibers describe individual fibers which are oriented in a predetermined manner and which form part of a woven or non-woven structure whereas fibrils describe "fiber-like" bridges between nodes in a porous structure.

The compliance may in particular be provided by the ability of the fibers or fibrils to reorient relative to each other. As an example, the fibrils may be allowed to rotate around the nodes or the fibers may be allowed to slide relative to each other.

As deflection of the fibers or fibrils in a lengthwise direction along their axis typically will requires much greater force than that which is required to deflect or rotate the fibers or fibrils in other directions, a controlled orientation of the fibers or fibrils may provide the anisotropy of the body.

The anisotropy may be provided by having a larger fraction of the fibers or fibrils being oriented in one common direction, where this direction then becomes less compliant to stretching than other directions.

The fibers or fibrils may be provided so that a sum of the lengths of fibers or fibrils extending in one specific direction becomes higher or lower than a comparable sum of the lengths of fibers or fibrils extending in other directions.

In one embodiment, a first group of fibers or fibrils are oriented in a first direction, and a second group of fibers or fibrils are oriented in a second direction being non-perpendicular to the first direction. The fibers or fibrils overlap each other and compliance and anisotropy is facilitated by allowing one of the groups of fibers or fibrils to reorient relative to the other group of fibers or fibrils thereby changing the angle between the two groups of fibers or fibrils in a direction away from perpendicular.

In a porous structure, the anisotropy can be provided by different structures not only of the fibrils but also of the nodes in which the fibrils are joined. In a woven or non-woven structure, the anisotropy can be provided not only by the direction of the fibers but also by use of fibers with different elasticity in different directions. As an example, the body may comprise fibers which require a high force to elongate and fibers which require less force to elastically elongate, where the last mentioned fibers are oriented primarily in a compliant direction of the body and the first mentioned fibers are oriented primarily in a less compliant direction of the body.

The fibers or fibrils could have a relatively small cross-sectional size e.g. in the range of 10 micrometer up to 1 mm in diameter if the cross-section is circular, and as a largest dimension if the cross-section is non-circular. The individual fibers may e.g. have a length between 1 mm and 30 mm, and the fibrils may e.g. have a length between 100 micrometer and 10 mm - i.e. the distance between the nodes in a microporous structure may be between 100 micrometer and 10 mm.

The film may substantially encapsulate at least a part, and possibly the entire anisotropic body so that the body becomes located completely within the film, or the film may at least extend from one side of the body, through the body and into the opposite side of the body. The two opposite outer surfaces of the composite material may thus both be formed by the film. In such an embodiment, a large part of the void between the fibrils may be filled out by the elastomer material of the film.

The transducer may comprise an additional anisotropic body formed similar to the already mentioned anisotropic body. The film could then be arranged between the two anisotropic bodies. In this case, the electrically conductive layers may form part of different anisotropic bodies or they may be attached to different anisotropic bodies, e.g. by filling out a part of a void space between the fibers.

At least one of the electrically conductive layers may penetrate partly into one of the anisotropic bodies without extending all the way through thickness of the anisotropic body. In one embodiment, one of the conductive layers completely penetrate one of the anisotropic bodies but leaves a certain amount of the void between the fibers free so that the film can penetrate into this remaining void.

The elastomer material mentioned throughout this text may e.g. be a silicone material such as a weak adhesive silicone. A suitable elastomer is Elastosil RT 625, manufactured by Wacker-Chemie. Alternatively, Elastosil RT 622 or Elastosil RT 601 , also manufactured by Wacker-Chemie may be used. As an alternative, other kinds of polymers may be chosen.

In one embodiment of the transducer, the fibers or fibrils are made from an electrically conductive material whereby the anisotropic body may itself form at least a part of one of the electrically conductive layers.

At least one of the layers of the transducer may be a relatively flat layer. In an alternative embodiment, the layers could be tubular.

In a second aspect, the invention provides a composite material e.g. for a transducer. The composite material comprises a first layer and a second layer, the first layer comprising at least an electrically conductive portion and an anisotropic body formed of a material comprising fibers which are arranged in a predetermined woven or non-woven pattern whereby the material becomes deformable and the body becomes stretchable in an anisotropic manner.

In a third aspect, the invention provides a method of making a composite material for an EAP transducer, the method comprising:

- providing a first layer by depositing an electrically conductive material on a body which comprises fibers arranged in a predetermined woven or non- woven pattern whereby the material becomes deformable and the body becomes stretchable in an anisotropic manner; and

- providing a layer of an elastomer material on a surface of the first layer. In one embodiment, the anisotropic body is made from a PTFE porous membrane, e.g. an expanded and porous PTFE membrane which forms a backing layer for support of a conductive layer which is applied on one surface of the membrane, and which forms a backing layer for support of the film which is applied to the opposite surface, e.g. in the form of a liquid silicone material which is partly cured or solidified on the surface.

The substrate backing layer may e.g. be a non-woven polyester material approximately 0.004-0.008 inches in thickness, and having an average pore size of 50-400 um. The pores may form in the range of 50-90 pet of the volume and the density of the backing layer could e.g. be between 0.12 and 0.20 gm/cc. Such a material is available e.g. from E.I. DuPont Company, Inc., Wilmington, DE 19898.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a preferred embodiment of the invention will be described in further details with reference to the drawing in which:

Fig. 1 illustrates a transducer according to the invention;

Fig. 2 illustrates body for a transducer according to the invention wherein the fibers are connected in nodes;

Figs. 3-5 illustrate various patterns of fibers in a body for a transducer;

Fig. 6 illustrates an alternative embodiment of a transducer according to the invention;

Fig. 7 illustrates a composite material for a transducer; and

Fig. 8 illustrates two layers of a composite material arranged in a stack. Fig. 1 illustrates a transducer 1 for converting between electrical energy and mechanical energy. The transducer comprises a body 2, a film 3, and an additional body 4. The bodies 2 and 4 comprise electrically conductive portions and thereby form electrically conductive layers on opposite sides of the film 3. The bodies 2 and 4 each has a structure which renders the layer compliant to stretch in a first direction and less compliant to stretch in a perpendicular direction.

The bodies comprise fibers arranged in a predetermined non-woven pattern. The fibers are individual fibers, and a major part of the fibers extend in the same direction whereby the bodies become non-compliant to deform in this direction. The fibers may be formed in a body of an elastomer material, e.g. a silicone material, or the fibers may be joined to form a non-woven fabric, e.g. a felt type fabric with a majority of the fibers in one direction or in specific directions. The film 3 comprises an elastomer material which is adhered to the bodies or which is at least partly embedded in a void between the fibers.

Fig. 2 illustrates fibers which are connected in nodes in a microporous structure.

Figs. 3-5 illustrate various patterns of fibers in a fabric type anisotropic body. Fig. 3 is a felt type fabric, whereas Figs. 4 and 5 illustrate two different kinds of woven fabrics.

Fig. 6 illustrates a transducer for converting between electrical energy and mechanical energy, the transducer comprising a film 6 of an elastomer material arranged between first and second layers 7, 8 of an electrically conductive material and being elastically deflectable in response to repulsion or attraction of the layers. The transducer comprises an anisotropic body 9 formed of a material comprising fibers which are arranged in a predetermined woven or non-woven pattern whereby the material becomes deformable and the body becomes stretchable in an anisotropic manner. The body is arranged inside the film 6. Fig. 7 illustrates a composite material comprising a first layer 10 and a second layer 11 , the first layer 10 comprising at least an electrically conductive portion forming an electrode, (not shown) and an anisotropic body formed of a material comprising fibers which are arranged in a predetermined woven or non-woven pattern whereby the material becomes deformable and the body becomes stretchable in an anisotropic manner. The second layer is a film of an elastomer material which is easily deformable relative to the first layer. The first layer is arranged inside the second layer.

Fig. 8 illustrates that two or more layers of the composite material may be arranged in a stack so that layers of the film are between electrodes whereby the film can be deformed when an electrical field is applied between two adjacent electrodes.

The stack could e.g. be rolled to form a cylindrical transducer with the film between electrodes.

Claims

1. A transducer for converting between electrical energy and mechanical energy, the transducer comprising a film of an elastomer material arranged between first and second layers of an electrically conductive material and being elastically deflectable in response to repulsion or attraction of the layers, wherein the transducer comprises an anisotropic body formed of a material comprising fibers which are arranged in a predetermined woven or non-woven pattern whereby the material becomes deformable and the body becomes stretchable in an anisotropic manner.

2. A transducer according to claim 1 , wherein the anisotropic body comprises a polymer material.

3. A transducer according to any of the preceding claims, wherein the anisotropic body comprises PTFE.

4. A transducer according to any of the preceding claims, wherein the anisotropic body is located within the film.

5. A transducer according to any of the preceding claims, comprising an additional anisotropic body formed of a material comprising fibers which are arranged in a predetermined woven or non-woven pattern whereby the material becomes deformable and the body becomes stretchable in an anisotropic manner.

6. A transducer according to claim 5, wherein the film encapsulates at least some of the fibers.

7. A transducer according to any of the preceding claims, wherein at least one of the electrically conductive layers forms part of one of the anisotropic bodies.

8. A transducer according to any of the preceding claims, wherein at least one of the electrically conductive layers comprises an electrically conductive material deposited on a surface of the anisotropic body.

9. A transducer according to claim 8, wherein the conductive material penetrates into but not through the anisotropic body.

10. A transducer according to any of the preceding claims, wherein the elastomer material is a silicone material.

11. A composite material comprising a first layer and a second layer, the first layer comprising at least an electrically conductive portion and an anisotropic body formed of a material comprising fibers which are arranged in a predetermined woven or non-woven pattern whereby the material becomes deformable and the body becomes stretchable in an anisotropic manner.

12. A method of making a composite material for an EAP transducer, the method comprising:

- providing a layer of an elastomer material on a surface of the first layer.