WO2008052541A1

WO2008052541A1 - A capacitive transducer with cutting areas

Info

Publication number: WO2008052541A1
Application number: PCT/DK2006/000608
Authority: WO
Inventors: Jorgen Mads Clausen; Peter Gravesen
Original assignee: Danfoss A/S
Priority date: 2006-11-03
Filing date: 2006-11-03
Publication date: 2008-05-08
Also published as: EP2092580A1

Abstract

The invention provides an actuator made from a layer of an elastomeric material (126) with an actuation area (129) in which a top surface and an opposite back surface are both covered by an electrically conductive material (128). To enable cutting of the actuator into smaller actuators, the elastomeric material comprises a cutting area (130) in which at least one of the top and back surfaces is not covered by electrically conductive material. In one embodiment, the elastomeric material is rolled to form a tubular body (124). The invention further provides a method of making a plurality of actuators by cutting an actuator into smaller pieces.

Description

A CAPACITIVE TRANSDUCER WITH CUTTING AREAS

INTRODUCTION

The present invention relates to a capacitive transducer capable of converting between electrical energy and mechanical energy. More particularly, the present invention relates to a capacitive transducer having corrugated electrodes divided into sections separated by totally or almost electrode free areas, the corrugation having been obtained without applying a pre-strain to a material holding the electrodes. The capacitive transducer of the present invention is formed by rolling a web of a dielectric material holding the set of electrodes, preferable rolled at an angle.

BACKGROUND OF THE INVENTION

Capacitive transcucers with electrodes for deflecting an elastomeric material, or elastomeric like material, are known to facilitate large deformation in a relatively low electric field, e.g. when compared with alternative dielectric materials. Typically, capacitive transducers are made by applying a conductive electrode, e.g. a carbon containing paint or a thin layer of gold to both sides of a film made from an elastomeric material e.g. silicone or polyurethane. The film is typically made in a moulding process. Laminated actuators are known, e.g. from US 5,977,685 which discloses layers of a polyurethane elastomere with electrodes on each side. The layers are laminated together to form a deformable sheet.

An electrical potential between two electrodes generates an electric field leading to a force of attraction or repulsion. As a result, the distance between the electrodes changes and the change leads to compression or tension of the elastomeric material which is thereby deformed. Due to the similarity between the principle of operation and the functioning of a muscle, an capacitive transducer is sometimes referred to as a Polymer Artificial Muscle (EPAM), electroactive elastomers or electroactive polymers. The capacitive transducer may operate as a generator, an actuator or a sensor. In the main embodiment the capacitive transducer is operating as an actuator and is therefore in the following mainly referred to as a capacitive actuator or just actuator.

E.g. in order to save space, elastomeric sheets can be rolled to form cylindrical actuators which could replace more traditional linear actuators in small-scale systems, e.g. in robotic applications wherein the cylindrical actuators may form gripping "fingers" e.g. of a robotic wrist. In the heretofore seen rolled actuators, the film is rolled into a tubular body which is fitted with mechanical connectors at axially opposite ends. Upon application of an electrical field to the electrodes, the sheet contracts or expands axially during deformation of the elastomeric material.

Capacitive actuators are normally made in batches of a small number and for very specific purposes. Often, the variation between batches merely relates to the length of cylindrical actuators but due to the available manufacturing technology, such simple variations are costly and time consuming. Normally, the elastomere is made in a thickness in the range of 10-100 μm, and the electrode is made in a thickness of 0,01-0,5 μm, e.g. by vaporising a thin layer of a conductive metal onto a surface of the elastomer. In an attempt to divide an actuator into smaller, customer specified pieces by cutting, one or both of the electrodes would normally be deformed, and short circuiting of the electrodes across the edge of the elastomer may therefore occur and render the actuator useless. DESCRIPTION OF THE INVENTION

It is an object of the invention to facilitate manufacturing of actuators, in particular in smaller batches with different lengths. Accordingly, the invention provides an actuator comprising a cutting area, in which at most one of the top and back surfaces is provided with an electrode.

Due to the cutting area, the actuator can be divided into several separate actuators without causing short circuiting between the top and the back electrode. Accordingly, an easier way of producing actuators in individual lengths, e.g. in accordance with specific customer demands, has been facilitated.

In this context, an electrode is a conductive surface portion located on one side of the elastomeric body, and which in combination with an adjacent electrode on the opposite side of the elastomeric body provides an expansion or contraction of the elastomeric body when an electrical field is applied between the electrodes. The electrodes may be made from any kind of electrically conductive material, it may have any shape, and it may be provided in various thicknesses. Typically, the electrode is provided with a connecting portion which facilitates application of the electrical field. The electrode could be divided into a plurality of electrode fields which are electrically connected by a connector. In this context, such electrode fields and the connector are considered to represent one electrode.

In accordance with the invention, one of the surfaces of the elastomeric body is not provided with an electrode in the cutting area, and preferably, not provided with any electrically conductive material at all.

The electrodes, i.e. e.g. the electrode fields and a connector, could be made by painting a conductive material onto the surface of the elastomeric layer, or a strip of a conductive material could be adhesively bonded to the surface of the elastomeric material, or as will be discussed later, the connector could form part of a core around which the actuator is coiled.

If the electrode is divided into separate fields, these fields are located outside the cutting area, and the connector which connects the fields should be located so that it covers an area wherein the opposite surface of the first layer is uncovered by electrically conductive material. The connector could be a separate connector which is arranged so that it connects the fields which constitute the electrode, or the electrode and the connector are formed as one uniform electrically conductive pattern on the surface of the first layer. As an example, the electrode could have the shape of a comb or an E or similar shapes with an axially extending connector and electrode fields extending perpendicularly to the connector. In this embodiment, the connectors of the top electrode and of the back electrode should simply be located on different locations of the first layer, i.e. not directly adjacent on opposite sides of the first layer. In a comb or E configuration of the electrodes, the top electrode could be rotated 180 degrees and shifted one half of the distance between each field in the direction in which the connector extends, relative to the back electrode so that the fields of the top electrode become interposed between the fields of the back electrode. A cutting area is thereby formed in which at most one of the surfaces of the first layer is covered by an electrode or by a connector, and it is therefore possible to separate the actuator into several smaller actuators by cutting through the connector in the cutting area. The cutting area could comprise one portion in which the top surface of the first layer is covered by the top electrode or a connector and another portion in which the back surface of the first layer is covered by the back electrode or a connector, as long as the electrically conductive material is only deposited on one of the surfaces. If the connector is deformed during the cutting process, it is not in danger of short circuiting the electrode of the opposite surface since this surface is not covered with electrically conductive material in that part of the cutting area.

An outer surface of the top electrode could be covered with an electrically isolating material and the actuator layer may thereafter be rolled around an axis to form a cylindrical actuator extending in a longitudinal direction. The actuator could be rolled around a core of an elastomeric material, and the core may comprise a conductive surface area which constitutes the conductor that electrically connects the above-mentioned electrode fields.

If a longitudinal direction is defined to be the longest edge of an oblong, e.g. rectangular, first layer, then each field may form an edge portion facing towards an adjacent field, and these edge portions may preferably extend at an angle different from 90 degrees to the longitudinal direction. It is thereby possible to form a spiral-shaped rolling which enables making of a cylindrical actuator which extends by a very long length in the longitudinal direction. As an example, the actuator may be produced as one "endless" cylindrical actuator from which pieces of a specific length are separated. When making the cylindrical actuator, care should be taken that a cutting area remains in which only one side of the first layer is covered with electrically conductive material.

It may be an advantage to provide on an outer surface of the elastomeric layer, indication lines for visually indicating positions of cutting areas on an outer surface of the rolled up actuator. The indication lines could be drawn up by ink or formed as a groove or elevation on the surface of the elastomeric layer

The electrically isolating layer could be constituted by a second layer of an elastomeric material, and in a preferred embodiment, the actuator is made from two layers of an elastomeric material, each provided with an electrode, e.g. vaporised onto one surface of the elastomeric layer. Subsequently, the layers are joined and the laminated structure is rolled to form a cylindrical actuator. In this embodiment, the top electrode forms part of, or is adhesively joined to one of the elastomeric layers and the back electrode forms part of, or is adhesively joined to the other elastomeric layer, and the layers, i.e. e.g. two, three, four or more layers, each consisting of an elastomeric layer with an electrode on one surface, are stacked to form a pile of alternating electrodes and elastomeric layers, In this embodiment, the invention applies equally by providing cutting areas in which an electrode is provided on at most one surface of each layer of elastomeric material in the stacked structure.

To avoid an increasing radial size of the spiral-shaped rolled cylinder actuator, the elastomeric layer(s) may comprise opposing curved edge portions forming the longest length of the layer, where the layer is rolled essentially transversely, e.g. perpendicularly, to the curved edge portion.

To prevent short circuiting of the electrodes across an edge of the elastomeric layer, it is an advantage to leave a rim portion of the elastomeric layer uncovered by electrically conductive material. In order to facilitate connection of the electrodes to a power source it may, however, be an advantage if one of the top and back electrodes or the connector covers a peripheral rim portion of the elastomeric material, or if both of the electrodes cover peripheral rim portions at different locations so that short circuiting of the covered rim portions is prevented.

The top and back electrodes are electrically conductive layers, e.g. made of metal. To improve the elasticity of the actuator in at least one direction, the elastomeric material may have a plurality of corrugations or have a wavy outer surface onto which the top or the back electrode is vaporised. The corrugations or waves could extend mutually parallel in the first direction and thus support deformation in the second direction being perpendicular to the first direction. The corrugated or wavy shape could be formed during manufacturing of the elastomeric body, e.g. in a moulding process. By vaporising a conductive material, e.g. gold, silver, copper, aluminium, or any similar conductive metal onto the body in a subsequent process, a very thin electrode may be formed onto the corrugations of the body. The relatively low elasticity of the electrode compared to the elasticity of the elastomeric material will effectively prevent deformation in the direction of the corrugations and thus concentrate the deformation on one specific direction being perpendicular to the corrugations.

In a specific drum shaped embodiment of the invention the rolled structure exposes a cavity which is limited in a radial direction by the rolled structure and which is limited in an axial direction by two closure parts fastened at axially opposite ends of the tubular portion. The closure parts may have the shape of axle journals, e.g. comprising fastening means for attaching the actuator to an application, e.g. a robotic handgrip. The core of an elastomeric material could be made by filling the cavity with an elastomeric material, e.g. in a liquid state. Subsequently, the liquid core is hardened, e.g. while the rolled structure is stretched axially.

In a second aspect, the invention provides a method of making an actuator comprising the steps of:

- providing a first layer of an elastomeric material,

- covering a top surface of an actuation area of the first layer with an electrically conductive top electrode ,

- covering a back surface of the actuation area with an electrically conductive back electrode, and - leaving an area of at least one of the top and back surfaces uncovered by electrically conductive material to form a cutting area.

In one embodiment one of the surfaces is covered with a conductive material in the cutting area while the opposite surface of the cutting area is uncovered by the electrically conductive material.

The actuator may be rolled to form a cylindrical actuator, and the cylinder may be made by applying the first conductive layer to the first elastomeric layer and the second conductive layer to a second elastomeric layer, the layers being laminated to form a structure of alternating elastomeric layers and conductive layers. Subsequently the laminated structure is rolled. The conductive layers could be connected to an electrical source via the conductive edge portions.

In a third aspect, the invention provides a method of making an actuator comprising the steps of:

- providing a first layer of an elastomeric material,

- covering a top surface of an actuation area of the first layer with a plurality of electrically conductive top electrode fields,

- covering a back surface of the actuation area with a plurality of electrically conductive back electrode fields, and

- cutting the first layer into separate pieces at locations between the fields of the top electrodes and between the fields of the back electrodes.

The last step of cutting the first layer into separate pieces may preferably take place after covering either the top electrode fields or the back electrode fields with an electrically isolating material, and preferably after rolling the layered structure of elastomeric material, electrodes and isolating material to form a cylindrical actuator.

Any of the features described in relation to the first aspect of the invention may apply equally in respect to the second and third aspects of the invention.

DETAILED DESCRIPTION

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

Fig. 1 is a perspective view of a portion of a composite according to an embodiment of the invention,

Figs. 2a-2f are cross sectional views of a portion of composites according to embodiments of the invention,

Fig. 2g is an enlarged section of Fig. 2a/2b/2c/2d/2e/2f,

Figs. 3a and 3b show an electroactive composite being exposed to zero electrical potential difference and being exposed to a high electrical potential difference,

Figs. 4a-4c illustrate the effect of exposing the electroactive composite of Fig. 3a to a high electrical potential difference as shown in Fig. 3b,

Figs. 5a and 5b illustrate an example of lamination of composites according to an embodiment of the invention, thereby forming an electroactive multilayer composite, Figs. 5c and 5d illustrate an electroactive multilayer composite being exposed to zero electrical potential difference and being exposed to a high electrical potential difference,

Figs. 6a and 6b illustrate a prior art multilayer composite before and after cutting,

Figs. 6c and 6d illustrate the multilayer composite with cutting areas before and after cutting,

Fig. 7 illustrate a two-layer composite with conductive materials across the cutting areas,

Fig. 8 illustrates a perspective view of two layers which can be joined to form a sheet according to the invention,

Fig. 9 illustrates the sheet in Fig. 1 being rolled up,

Fig. 10 illustrates a top view of an elastomeric layer with a row of electrodes,

Fig. 11 illustrates an alternative and preferred layout of the electrodes on the elastomeric layer,

Fig. 12 illustrates the sheet in Fig. 4 when it is rolled to form a tubular body

Fig. 13 illustrates a sheet with opposing curved edge portions, and

Fig. 14 illustrates a side-view of a laminated structure. Fig. 1 is a perspective view of a portion of a composite 1. The proportions of the composite are distorted in order to illustrate different elements of the composite 1. The composite 1 comprises a film 2 made of a dielectric material having a surface 3 provided with a pattern of raised and depressed surface portions, thereby forming a designed corrugated profile of the surface 3. An electrically conductive layer 4 has been applied to the surface 3, the electrically conductive material being deposited so that the electrically conductive layer is formed according to the pattern of raised and depressed surface portions. In terms of everyday physical things, the film 2 resembles in some aspects household wrapping film. It has a similar thickness and is comparably pliable and soft. However, it is more elastic than such a film, and has a marked mechanical anisotropy as will be explained in the following.

The dielectric material may be an elastomere or another material having similar characteristics.

Due to the pattern of raised and depressed surface portions, the electrically conductive layer 4 may even out as the film 2 expands, and recover its original shape as the film 2 contracts along the direction defined by arrows 5 without causing damage to the electrically conductive layer 4, this direction thereby defining a direction of compliance. Accordingly, the composite 1 is adapted to form part of a compliant structure capable of withstanding large strains.

As described above, the corrugated surface profile is directly impressed or moulded into the dielectric film 2 before the electrically conductive layer is deposited. The corrugation allows the manufacturing of a compliant composite using electrode materials of high elastic modulii, e.g. metal electrode. This can be obtained without having to apply pre-stretch or pre- strain to the dielectric film 2 while applying the electrically conductive layer 4, and the corrugated profile of the finished composite 1 does not depend on strain in the dielectric film 2, nor on the elasticity or other characteristics of the electrically conductive layer 4. Accordingly, the corrugation profile is replicated over substantially the entire surface 3 of the dielectric film 2 in a consistent manner, and it is possible to control this replication.

Furthermore, this approach provides the possibility of using standard replication and reel-to-reel coating, thereby making the process suitable for large-scale production. For instance, the electrically conductive layer 4 may be applied to the surface 3 of the dielectric film 2 using standard commercial physical vapour deposition (PVD) techniques. An advantage of this approach is that the anisotropy is determined by design, and that the actual anisotropy is obtained as a consequence of characteristics of the corrugated profile which is provided on the surface 3 of the dielectric film 2 and the electrically conductive layer 4 which follows the corrugated profile.

The composite 1 shown in Fig. 1 is designed to have an compliance in the range of the compliance of the dielectric film 2 in the direction defined by arrows 5, and a stiffness in the range of the stiffness of the electrically conductive layer 4 in a direction defined by arrows 6. The composite 1 may be produced in very long lengths, so called "endless" composites.

In Figs. 2a-2g, d denotes an average or representative corrugation depth, i.e. an average or representative distance between a raised portion and a neighbouring depressed portion of the pattern. H denotes an average thickness of the dielectric film 2, and h denotes an average thickness of the electrically conductive layer 4. In a preferred embodiment, the average thickness H of the dielectric film 2 is in the range of 10 μm - 100 //m. Figs. 2a-2c show composites 1 having different corrugation depth d, whereas the corrugation period P is substantially identical for the three composites shown. Comparing the composites 1 of Figs. 2d and 2e, the corrugation depth d is substantially identical, whereas the corrugation period P of the composite 1 in Fig. 2e is larger than the corrugation period P of the composite 1 shown in Fig. 2d. Compared hereto, the composite 1 of Fig. 2f has a smaller corrugation depth d and a larger corrugation period P.

The properties of the dielectric films 2 with anisotropic corrugated compliant metallic electrodes in the form of electrically conductive layers 4 as described in accordance with the present invention are optimised by design according to design rules developed by the inventors. These design rules take into consideration the dielectric and mechanical properties of the dielectric material and of the material of the electrically conductive layer.

The relative permittivity and breakdown field of the dielectric material on the one hand and electrical potential difference between electrodes on the other hand are the design parameters that determine the range of the average thickness, H of the dielectric film 2. The characteristic properties of the dielectric material are typically supplied by dielectric material manufacturers like Wacker-Chemie and Dow Corning.

Corrugation depth, d, is optimised with respect to the dielectric film thickness, H, in order to obtain a relatively uniform electric field distribution across the dielectric film situated between the electrodes. Such optimisation step is done using finite element simulations. A high d/H ratio corresponds to a non uniform electric field distribution and a low d/H ratio corresponds to a relatively uniform electric field distribution.

Anisotropy and compliance properties are the combined result of the shape and topology given to the surface of the dielectric film, e.g. an elastomer film, by a moulding process on one hand and the electrically conductive layer that takes up the corrugation shape on the other hand. Electrode layer thickness, h, and corrugation period, P, are optimised with respect to the corrugation depth, d, in order to obtain a dielectric film with metallic electrodes that is compliant in one 'in the plane' direction and almost not compliant in the transverse 'in the plane' direction. A film that is very compliant in one direction is a film that can be stretched or elongated very much in this direction by applying a relatively low level of forces in this direction without the risk of damaging the electrodes, and a film that will have very limited elongation in the transverse direction when a force is applied in this transverse direction. In order to optimise electrode compliance, the d/P and h/d ratios have to be optimized. High d/P ratios result in very compliant electrodes and low d/P ratios result in less compliant electrodes. High h/d ratios result in less compliant electrodes and low h/d ratios result in very compliant electrodes. The degree of anisotropy of the dielectric film with corrugated electrodes is determined by the compliance ratio between the direction in which the composite is compliant and the transverse direction in which the composite is almost not compliant. High compliance ratios result in very anisotropic structures and low ratios result in isotropic-like structures.

Once the ranges for the design parameters (H, d, h and P) are specified according to the above description, it is possible to predict the performance of the dielectric film with metallic electrodes in the form of electrically conductive layers in terms of how compliant and what maximum elongation in the compliant direction it can undergo and what the actuation forces will be. Stiffness in the transverse direction can be predicted as well. A refinement process for these parameters can be done if necessary.

It should be noted that for a given actuation force, actuators manufactured in accordance with the present invention, i.e. made from a dielectric material with electrodes deposited thereon, has a much lower weight, i.e. at least a factor five smaller, than conventional actuators, such as magnetic actuators, capable of providing a comparable actuation force. This is very important for applications where actuator volume and weight are of relevance.

Once all design parameters are optimised, a mould is designed according to the exact specifications for the corrugation topology.

Based on finite element electrostatic simulations, the inventors of the present invention have found that the ratio d/H should be in the range of 1/30 - /4. For example, having a ratio of 1/5 and a corrugation depth of approximately 4 μm, the thickness of the dielectric film 2 will be approximately 20 μm. Furthermore, the ratio between the corrugation depth d and the period P of the corrugations, d/P, and the ratio between the thickness h of the electrically conductive layer and the corrugation depth d, h/d, are important ratios directly affecting the compliance of the electrode. In preferred embodiments, the ratio d/P is in the range of 1/50 - 2, whereas the ratio h/d is in the range of 1/1000 - 1/50.

Another issue to take into consideration when defining the average thickness H of the dielectric film 2 is the so-called breakdown electric field related to dielectric materials. When an electrically conductive layer 4 is deposited on each surface of the dielectric film 2 thereby forming an electroactive composite, there is a maximum value for the voltage, V between these electrically conductive layers, for a given material thickness, H, i.e. a distance corresponding to the thickness, H, of the dielectric film 2, in order not to exceed the breakdown electric field, V/H, of the material. When the dielectric film 2 presents large variations in thickness across a surface area 3, then, for a given voltage between the electrically conductive layers, electric field and thickness variations will be of the same order of magnitude. As a consequence, parts of the dielectric film 2 having a higher local electric field will elongate more than those with a smaller local electric field. Furthermore, in situations where a transducer in which the composite 1 is operated close to a breakdown field, such variations may be damaging to the transducer, because parts of the dielectric film 2 will be subjected to electric fields which are larger than the breakdown field. Accordingly, it is very important to reduce the average thickness variations to the greatest possible extent when processing the dielectric film 2. For processing reasons a 10% average thickness variation is considered acceptable. When processing transducers with corrugated electrodes by design, i.e. in accordance with the present invention, these values can be controlled in a relatively accurate manner.

Figs. 3a and 3b illustrate an electroactive composite 1 comprising two electrically conductive layers 4 separated by a dielectric film 2 being exposed to zero electrical potential difference (Fig. 3a) and being exposed to a high electrical potential difference (Fig. 3b). As illustrated in Fig. 3b the dielectric film 2 is expanded, while the electrically conductive layers 4 are evened out, when exposed to an electrical potential difference. This is shown in detail in Figs. 4a-4c which illustrates portions of a section of the electroactive composite 1 at different steps in time, with hatchings omitted for the sake of clarity. A line of symmetry 10 is indicated at the bottom of each figure, illustrating that the composite 1 is an electroactive composite having an electrically conductive layer 4 deposited on each surface. Fig. 4a illustrate the electroactive composite 1 being exposed to zero electrical potential difference, the corrugation depth being the designed depth d and the corrugation period being the designed period P. In Fig. 4b it is illustrated that the dielectric film 2 is expanded in the compliance direction resulting in a reduced thickness H' of the film. Furthermore, the electrically conductive layer 4 is evened out resulting in a smaller corrugation depth d' and a larger corrugation period P'. Fig. 4c illustrate the electroactive composite 1 at a later time step, the thickness H" of the film 2 being even more reduced, the corrugation depth d" being even smaller and the corrugation period P" being larger.

It should be noted that capacitors produced in accordance with the present invention exhibit a 'self-healing' mechanism. A self-healing mechanism is characteristic of capacitors with very thin electrodes. It occurs when the dielectric material of the capacitor presents defects such as inclusions, pinholes, etc. For such a capacitor with a given thickness, when the applied potential difference between electrodes approaches the so-called breakdown voltage defined above, the average electric field approaches the critical breakdown field. However, in regions with defects, it will indeed exceed this critical breakdown field, and a cascading effect due to accelerated and colliding charges across dielectric film thickness at the positions of the defects occurs, thereby inducing a high in-rush transient current across the dielectric material. This results in a local transient over- heating with characteristic times in the microseconds range or much below, which is enough to "deplete/evaporate" the material of the very thin opposite electrodes at the positions of the defects and their close vicinity. This results in areas around defects where there is no more electrode material. Moreover the dimension of the areas with depleted electrode material increases with the local field. However, the capacitor as such is not damaged and continues to operate. Thus, the reference to 'self- healing'. As long as the depleted areas represent in total a very negligible fraction of the entire area of the capacitor, this will have very little consequence on the performance of the capacitor. Self-healing does not take place if the capacitor is made with thick electrodes, because the level of local over-heating is not sufficient to deplete the thick electrode material at the defects. In that case, when the critical breakdown is reached, consequent and instant damage of the capacitor occurs. In practice, the inventors of the present invention have made metallic electrodes with thickness up to 0.2 μm and always observed self-healing, even when operating the capacitor above breakdown. This does not cause any substantial damage to the capacitor, and the capacitor therefore continues to operate.

As shown in Fig. 5a, an electroactive multilayer composite 15, 16 comprises at least two composites 1 , each composite 1 comprising a dielectric film 2 having a front surface 20 and a rear surface 21 , the rear surface 21 being opposite to the front surface 20. The front surface 20 comprises a surface pattern 3 of raised and depressed portions and a first electrically conductive layer (not shown) covering at least a portion of the surface portion 3. Fig. 5a only show a portion of a multilayer composite 15 and 16, which portions having proportions out of order for illustration purposes.

Figs. 5a and 5b show an electroactive multilayer composite 15 having the first composite 1 arranged with its front surface 20 facing the rear surface 21 of the adjacent composite 1 , in the following referred to in general as a Front-to-Back multilayer composite 15. In this type of lamination process, the electrically conductive layer of the first composite 1 is in direct contact with the rear surface of the second composite 1. The composites 1 are laminated either by the use of an elastomer of the same type as used for producing the dielectric film 2 or alternatively, the two composites 1 are stacked without use of an adhesive. For some purposes it is preferred that the multilayer composite is made of stacked composites without the use of an adhesive. In these cases, the wave troughs are simply filled with air.

Due to the pattern of raised and depressed surface portions 3, the electrically conductive layer of each of the composites may even out as the film expands, and recover its original shape as the film contracts along the direction defined by arrows 5 (see Fig. 5b) without causing damage to the electrically conductive layers, this direction thereby defining a direction of compliance. Thus, the multilayer composite 15 shown in Fig. 5b is designed to be very compliant in the direction defined by arrows 5 and designed to be very stiff in the transverse direction defined by arrows 6.

Figs. 5c and 5d illustrate the electroactive multilayer composite 15 being exposed to zero electrical potential difference and being exposed to a high electrical potential difference. As can be seen from Fig. 5d the dielectric film is expanded, while the electrically conductive layers are evened out, when exposed to an electrical potential difference. It can further be seen that the depth of the wave troughs (the corrugation depth d) is reduced when the multilayer composite is exposed to an electrical potential difference. The composites can be bonded by applying a high electrical potential difference to the stacked composites, whereby the film of one composite and the electrically conductive layer of an adjacent composite adhere to each other without the use of an additional adhesive. Thus, they may be brought into intimate contact by electrostatic forces. Alternatively, they may adhere to each other by pressing them together, e.g. by the use of rollers, due to the characteristics of the dielectric film which may be slightly tacky when made of an elastomer.

Fig. 6a-6b shows the problem with prior art electroactive multilayer composite, where three electroactive composits 30 are stacked on top of each other. Fig. 6a shows the end portion, where there is a section 31 free of electrode 32. This insures a length (as illustrated by arrow 33) of elastomeric surface between two adjacent electrodes 32a and 32b sufficient to prevent short circuiting of the electrodes across the end-edge of the elastomeric layer. When cutting the layers at a the line 34 the situation at Fig. 6b often arises where the contact with the cutting tool has smeared the some of the material 35 of the electrodes 32 down the new end edges 31 of the elastomeres, Thereby the length electrode-free distance along elastomere surface (as illustrated by the arrow 36) has become significantly smaller, and given small thicknesses of the elastomeric films 30 in the range of 10 μm - 100 μm, the two electrodes 32a and 32b may easily short cut. Fig. 6c shows a first aspect of the invention, where almost electrode-free cutting areas is 37 is introduced. When cutting the layers at the line 38 none of the electrodes 32a or 32b is touched by the cutting tool, insuring that, though the elastomeres 32 still deforms as is seen on Fig. 6d, there still is a sufficient length (illustrated by arrow 39) elastomere surface distance around the end edges 31 between two adjacent electrodes 32a and 32b, to prevent any short circuiting of the electrodes across the end-edge.

Though the cutting areas at Fig. 6c are shown to be electrode-free, a conductive material may connect the electrodes across the cutting areas. This is illustrated at Fig. 7, where sections of two-layered elastomeres 40 and 41 is seen having electrodes 42a, 42b, 43a and 43b, where the electrode fields 42a and 43a aligns and the electrode fields 42b and 43b aligns. The electrode fields 42a and 42b on the elastomere 40 is connected by the conductive material 44 across the otherwise electrode- free cutting areas 46. Similar the electrode fields 43a and 43b on the elastomere 41 are connected by the conductive material 45 across the otherwise electrode-free cutting area 47. The two conductive materials 44 and 45 are seen to be off set in the sense that no conductive material 44 in the cutting area 46 is aligned with conductive material 45 in the cutting area 47.

In one embodiment one of the surfaces is covered with a conductive material in the cutting area while the opposite surface of the cutting area is uncovered by the electrically conductive material. Fig. 8 shows an actuator 101 comprising a first layer 102 of an elastomeric material and a second layer 103 of an elastomeric material. A top electrode forming a row of fields 104 is formed on the first layer, and a back electrode 105 is formed on the second layer. The back electrode comprises an edge portion 106 which covers a peripheral rim portion 107 and therefore facilitates connection of the electrode to a power source. Fingers 108 extend from the edge portion 106 and form areas 109 with no conductive material. The two layers are joined to form a laminated structure with the top electrode on one side of the first elastomeric layer and with the back electrode on the opposite side of the first layer, wherein the back electrode is covered by an electrically isolating layer constituted by the second layer of an elastomeric material. The structure comprises actuator areas where both sides of the first layer are covered by an electrode and cutting areas where only one side of the first layer is covered by electrically conductive material. The electrodes are applied to the elastomeric layers by use of a vapour deposition technique and the layers are glued together by use of regular glue.

In Fig. 9, the sheet is rolled into a cylindrical actuator 110 comprising sections 111 , 112, 113 between which the actuator can be separated into smaller actuators.

Fig. 10 shows one way of arranging electrode fields 114 on the elastomeric layer 115. According to the orientation of the electrodes and the shape of the spaces 116, the sheet should preferably be rolled in a direction indicated by the arrow 117. This direction is essentially perpendicular to the direction of the row of electrodes, indicated by the arrow 118.

Fig. 11 shows an alternative and preferred way of arranging the electrodes 119. The layers, and thus the sheet 120, extend mainly in a longitudinal direction indicated by the arrow 121. Each electrode comprises an edge portion 122 facing towards an adjacent electrode, which edge portion extends in an edge-portion-direction, indicated by the arrow 123. The edge-portion-direction forms an angle, α, being different from 90 degrees to the longitudinal direction.

Fig. 12 shows the sheet from Fig. 11 when the sheet is rolled around a centre axis 123 to form the cylindrical body 124. The sheet is rolled with an angle, β corresponding to (90-α) degrees between a longest edge 125 of the elastomeric layer 126 and the centre axis 123. Accordingly, the angle between the edges 127 of the electrode fields 128 and the centre axis of the rolled body becomes essentially equal to 90 degrees. In the actuation areas, i.e. where the shown top surface is covered by the electrode fields 128, the opposite back surface of the elastomeric layer 126 is covered with a similar electrode field. In the areas between the electrode fields, cutting areas exist in which at most one of the surfaces is covered by an electrically conductive material. Accordingly, the cylindrical actuator 124 comprises actuation areas 129 and cutting areas 130 in which the cylindrical actuator can be divided into separate actuators without causing short circuiting of the top and back electrodes.

Fig. 13 illustrates an embodiment of the invention wherein the laminated structure 131 comprises opposing curved edge portions 132, 133 and electrodes 134 which are electrically connected by a connector 135. Due to the opposing curved edge portions, the structure can be rolled up to form a tubular body without gradually increasing the radial size of the body with the number of turns by which the structure is rolled up.

Fig. 14 illustrates a side view of a structure made from two parts 136, 137, wherein each part comprises a layer of an elastomeric material 138, 139 with one of the top electrode 140 or the back electrode 141 located on a surface of the elastomeric material. The top or back electrode could e.g. be vaporised onto the surface. The elastomeric material may e.g. have a thickness in the range of 20-40 μm and the top and back electrodes may have a thickness in the range of 700-900 angstrom. As illustrated by the arrows 142, the two parts are joined, e.g. adhesively, and subsequently the structure is rolled to form a tubular body.

Claims

1. An actuator (101 ) comprising a first layer (102) of an elastomeric material, an electrically conductive top electrode (104) on a top surface of the first layer and an electrically conductive back electrode (105) on an opposite back surface of the layer, the actuator comprising at least one cutting area (109) in which at most one of the top and back surfaces is covered with one of the top and back electrodes.

2. An actuator according to claim 1 , in which at least one of the top and back surfaces is not covered by electrically conductive material in the cutting area (109).

3. An actuator according to claims 1-2, wherein an outer surface of the top electrode is covered with an electrically isolating material (103) and the actuator is rolled around an axis to form a cylindrical actuator (110) extending in a longitudinal direction.

4. An actuator according to claims 1-3, wherein one of the top electrode and the back electrode forms a plurality of mutually electrically isolated electrode fields (104) which are electrically connected by a connector.

5. An actuator according to claim 4, comprising a cylindrical core of an elastomeric material.

6. An actuator according to claim 5, wherein the cylindrical core comprises a conductive surface area.

7. An actuator according to claims 4-6, wherein the fields comprise edges (127) facing towards adjacent fields, the edges being essentially perpendicular to the longitudinal direction.

8. An actuator according to claims 3-8, wherein the electrically isolating layer is constituted by a second layer (103) of an elastomeric material.

9. An actuator according to claim 8, wherein the top electrode is deposited on a surface of the first layer, the back electrode is deposited on a surface of the second layer, and the layers are joined to form a structure of alternating elastomeric material and conductive material.

10. An actuator according to claims 8-9, wherein at least one of the top and back electrodes is located at a distance from a first peripheral rim portion of the first or second layer.

11. An actuator according to claims 8-10, wherein a peripheral rim portion (107) of at least one of the first or second layers comprises an electrically conductive material (106) connected with one of the electrodes.

12. An actuator according to any of the preceding claims, wherein the layers comprise opposing curved edge portions (132, 133).

13. A method of making an actuator comprising the steps of:

- providing a first layer of an elastomeric material,

- covering a back surface of the actuation area with an electrically conductive back electrode, and

- leaving an area of at least one of the top and back surfaces uncovered by electrically conductive material to form a cutting area.

14. A method according to claim 13, further comprising the step of rolling the laminated structure to form a tubular body.

15. A method according to claim 14, comprising the step of separating a smaller actuator from the tubular body by cutting the tubular body at the cutting area.

16. A method according to claims 13-15, wherein the top electrode is provided on the first layer, and wherein the back electrode is provided on a second layer of an elastomeric material, the layers being laminated to form a structure of alternating elastomeric material and conductive material.

17. A method of making a plurality of layered structures for making actuators, the method comprising the steps of:

- providing a first layer of an elastomeric material,

- cutting the layered structure into a plurality of layered structures at locations between the top electrode fields and the back electrode fields.

18. A method according to claim 17, wherein either the top electrode fields or the back electrode fields have been covered by an electrically isolating material prior to the cutting of the layered structure.

19. A method according to claims 17-18, wherein the layered structure is rolled around an axially extending axis to form a cylindrical body prior to the cutting of the layered structure.