WO2004109817A2

WO2004109817A2 - Electroactive polymer contractible actuator

Info

Publication number: WO2004109817A2
Application number: PCT/IB2004/001868
Authority: WO
Inventors: Federico Carpi; Danilo De Rossi
Original assignee: Universita Di Pisa
Priority date: 2003-06-09
Filing date: 2004-06-09
Publication date: 2004-12-16
Also published as: WO2004109817A3; ITPI20030043A1

Abstract

A electromechanical actuator (1) for converting electric energy into mechanical energy or vice-versa comprising at least a first and at least a second deformable electrode (11 and 12) and a dielectric element (20) that can be deformed located between the electrodes in order to fill the space between the coils. The two deformable electrodes (11 and 12) have curvilinear shape extending as coils so that each coil of the first electrode is alternalted to a coil of the second electrode, and in particular have helical shape. They can be made of resilient material in which particles of conductive material are integrated and are arranged coaxially so that each coil of the first electrode (11) is alternated to a coil of the second electrode (12). Between coils of the electrodes (11 and 12) a dielectric element (20) is located made of electro-active polymer capable of warping or deflecting. When a voltage AV is applied to the couple of deformable electrodes (11) and (12), the electrical field generated produces an inner stress that causes the resilient material (20) to deform according to this voltage. Thus electric energy is converted into mechanical energy when between said deformable electrodes a voltage generating an electrical field causes the dielectric material to deform. Conversely mechanical energy is converted into electric energy when the actuator is made of a material capable of generating electrical field when a deformation is applied. The actuator can be used also as sensor of deformation, as electric generator or as condenser at high specific capacity.

Description

TITLE

ELECTRO-ACTIVE POLYMER CONTRACTIBLE ACTUATOR

Field of the invention The present invention relates to a device that can be used in particular, but not exclusively, in the field of technologies for converting electric energy into mechanical energy or vice-versa.

In particular, the invention relates to an electromechanical actuator integrating an electro-active polymer.

Background of the invention

As known, a variety exists of electromechanical actuators, for example piezoelectric, electrostatic and electromagnetic. However, such types of actuators, if used for devices of small dimensions, have limited performances versus capacity of deformation, pressure applied, response rate, energy consumption, etc.

To this end electromechanical actuators exist made of special polymers so-called "electroactive", since they are capable to warp if subject to a suitable electrical stimulation. Dielectric elastomers belong to this category of polymers, such as, for example, silicone elastomers, acrylic elastomers, polyurethane elastomers, etc., and some thermoplastic material, such as some PVDF copoly ers .

Electro-active polymers, used in devices capable of converting electric energy into mechanical energy, or vice- versa, have good performances also in devices of small size, being their performances independent from scale. For this reason, electro-active polymers are studied and used for different micro-and macro- technological applications, such as for example robotics and bioengineering (for making "artificial muscles") .

For example, elementary actuators exist comprising an electro-active polymer layer acting as dielectric layer between two planar deformable electrodes . When a certain voltage is applied between the two electrodes, the electro- active polymer warps reducing its thickness and increasing its width and length owing to electrostatic forces. The deformation of the electro-active polymer can be then used for converting electric energy into mechanical energy. A structure studied as electro-active actuator consists of a stack of elementary planar actuators, which are connected in series mechanically and in parallel electrically and which are operated in a direction parallel to the electrical field created between each couple of electrodes.

However, the embodiment of a stack of planar actuating elements as above described is expensive, owing to the geometric-structural features of the configuration, as well as to the discontinuity of the electrodes, which requires short-circuiting separately two series of electrodes, i.e. connecting the respective series of electrodes to each other for all the elementary actuators . Such a solution, furthermore, has further drawbacks, since the electrical contacts for each couple of electrodes of a single element have to be executed in a small space. Each actuator must in fact have a thickness within certain limits for achieving sufficient electrical fields.

Furthermore, the electrical connections between the electrodes of each series normally affect the deformation of the whole stack.

Summary of the invention

It is therefore a feature of the present invention to provide an electromechanical actuator structurally much easier than the devices of prior art and with performances at least comparable to them versus force development, capacity of deformation, response rate, etc. These and other features are accomplished with one exemplary electromechanical actuator for converting electric energy into mechanical energy, or vice-versa, according to the present invention, whose characteristic is that it comprises: - at least a first and a second deformable electrode having curvilinear continuous shape extending as coils so that each coil of the first electrode is alternated to a coil of the second electrode;

- a dielectric element that can be deformed located between the electrodes in order to fill the space between the coils.

Preferably, the or each first and the or each second deformable electrode have substantially helical shape and are arranged coaxially. In particular, if between the deformable electrodes a voltage is applied the electrical field generated causes the dielectric material to deform obtaining then the conversion of electric energy into mechanical energy. More in detail, during the conversion the dielectric material is subject to an axial contraction and a radial expansion.

The particular continuous structure of the electrodes is substantially equivalent to a structure comprising a plurality of electrodes arranged mechanically in series and interconnected electrically in parallel, but with the advantage of being executed indeed without discontinuity and without impeding the free deformation of the actuator.

Preferably, the dielectric resilient element is made of an electro-active polymer, for example chosen among silicone elastomers, acrylic elastomers, polyurethane elastomers, a thermoplastic polymers, etc.

Furthermore, the electromechanical actuator can be subject to a deformation suitable for generating an electrical field. In other words, alternatively, for operating as actuator, the device can also be used as sensor of deformation or electric generator, capable of converting mechanical energy into electric energy. To this end, advantageously, the dielectric can also be material with piezoelectric properties. The actuator can furthermore, work also as sensor of deformation of piezoresistive type, i.e. having inside a conductive element whose electrical resistance varies versus the deformation of the structure. This piezoresistive element can be made either by the two electrodes of the actuator, or by a further deformable conductor having a predetermined shape, for example helical or strip-like, and embedded in the dielectric material or mounted on the side surface.

The actuator can, furthermore, work also as piezocapacitive sensor, whose electrical capacity varies versus the deformation of the structure.

The operation of the device as sensor can be carried out at the same time as the operation as actuator or in alternative to it. The device furthermore, has a structure and a configuration suitable for its use as electric condenser having high specific capacity and predetermined stiffness.

Advantageously, the or each first and the or each second deformable electrode can be made of conductive resilient material, for example an elastomer with embedded conductive particles (as grafite, lampblack, nanotubes of carbon, silver, gold, etc.) or a conductive polymer.

In an exemplary embodiment of the invention, the or each first and the or each second deformable electrode are arranged so that two adjacent coils have the surfaces facing each other and overlapped axially.

Alternatively, said deformable electrodes are coupled with at least a first electrode arranged outside and at least a second electrode arranged inside, wherein the coils extend according to a spiral and/or a helix.

Preferably, the electrodes have size substantially alike.

Advantageously, the electromechanical actuator can be subject to a pre-tensioning step during which the device is deformed in one or more preferential directions . This way, by applying a voltage between the electrodes, the electromechanical actuator warps in one or more preferential directions. So, it is possible to use the electromechanical actuator for particular types of applications that require a controlled deformation of the device. The pre-tensioning step, furthermore, improves in certain cases also the electro-active polymer's response rate, dielectric stiffness, developed pressure, etc. According to another aspect of the invention, a method for making an electromechanical actuator capable of converting electric energy into mechanical energy or vice- versa, in particular that can be used as sensor of deformation or as condenser or as electric generator, comprises the following steps:

- prearranging at least a first and a second deformable electrode having continuous curvilinear shape extending as coils arranged so that each coil of the first electrode is alternated to a coil of the second electrode;

- interposing a dielectric element that can be deformed between the electrodes in order to fill the space between the coils. Brief description of the drawings

Further characteristics and advantages of the present invention will be made clearer with the following description of possible exemplary embodiments, with reference to the attached drawings, in which like reference characters designate the same or similar parts, throughout the figures of which

- figure 1 shows a perspective elevation front view of a couple of electrodes according to the invention; - figure 2 shows a cross sectional view according to arrows II-II of the electrodes of figure 1;

- figure 3 shows a perspective elevation front view of an electromechanical actuator according to the invention executed connecting the electrodes of figure 1 with a dielectric material in a rest position, i.e. without that a voltage is applied;

- figure 4 shows a perspective elevation front view of the electromechanical actuator of figure 3 in the deformed configuration applying a voltage between the electrodes; - figures from 5 to 7 show diagrammatically some possible operations to obtain the electromechanical actuator, according to the invention.

- figure 8 shows a perspective elevational side view of an helix of an electro-active polymer ^• obtainable by the process of figures 5-6 or 7;

- figure 9 shows a perspective elevational side view of the electrodes made on the two main surfaces of an helix having the inner and outer surfaces preliminarily coated;

- figure 10 shows a perspective elevational side view of the step of matching the helix of an electro-active polymer of figure 8 with the helix of the electrodes on the two main surfaces of figure 9.

Description of a preferred exemplary embodiment In figures from 1 to 4 a first exemplary embodiment is shown of an electromechanical actuator 1 that can be used as converter of electric energy into mechanical energy or vice-versa and as sensor of deformation, according to the present invention.

In particular, the electromechanical actuator 1 is made assembling a first deformable electrode 11 and a second deformable electrode 12 having substantially helical shape as shown in figure 1. The two electrodes 11 and 12 can be made of resilient material in which particles are integrated of conductive material. They are arranged coaxially and so that each coil of the first electrode 11 is alternated to a coil of the second electrode 12.

Anong two adjacent coils of the electrodes 11 and 12 a dielectric element 20 is located capable of warping, or deflecting. For example, an electro-active polymer can be used chosen among, for example, silicone elastomers, acrylic elastomers, polyurethane elastomers, thermoplastic polymers, etc., owing to their property of having good performances irrespective of the dimensions, in particular also for small size of a device incorporating them.

In particular, if to the couple of deformable electrodes 11 and 12 a voltage is applied ΔV, the electrical field generated produces an electrical field that causes the resilient material 20 to deform. This way a conversion of electric energy into mechanical energy is achieved.

As shown in particular in figures 3 and 4, the peculiar structure used for deformable electrodes 11 and 12 is substantially equivalent, concerning the electrical connection, to the structure that would be obtained assembling in parallel many electrodes, but with the advantage of having a single part capable of developing a free deformation of the actuator 1.

In particular, actuator 1 turns from a rest position without a voltage applied between electrodes 11 and 12 (figure 3) , where the distance between two adjacent coils is di and the lateral size is L_x, to a shrunk configuration where two adjacent coils are at a distance d₂ while the lateral size changes into L₂ when a voltage is applied ΔV between the two electrodes (figure 4) . The actuator 1 is therefore capable of shrinking in the direction of its own axis 2; more in detail, the direction of the electrical field and the working direction of the actuator 1 tend to be parallel to each other. This causes a high deformability of the actuator 1 and then of the force that it can develop. Furthermore, the performance of the actuator 1, in particular versus deformation % and actuating tension, tends to improve as the pitch of the helix of the electrodes 11 and 12 decreases.

According to the invention, the electromechanical actuator 1 can also be used for converting mechanical energy into electric energy by applying a mechanical deformation that causes the generation of an electrical field. This way, the device can also work as sensor of deformation or electricity generator.

Furthermore, the device can work also as sensor of deformation of piezoresistive type, exploiting the effect induced by an applied deformation versus variation of the resistance of one or both the electrodes of the device, or of a further deformable conductor (not shown in the figures) , for example with helical or strip-like shape, embedded in the dielectric or mounted on the surface of the device. A further use of the device as sensor of deformation exploits a piezocapacitive effect, according to a variation of the electrical capacity of the device caused by a deformation thereof.

In addition to the function as actuator, sensor and generator, the device presents a structure and a configuration suitable for its use as electric condenser having high specific capacity and with predetermined stiffness. The operation as sensor can be carried out contemporaneously, or alternatively, with respect to an operation as actuator.

Some possible methods diagrammatically shown in figures from 5 to the 10 to make the electromechanical actuator as above described are indicated below.

A first step of this process provides making a tubular element 60 in electro-active polymer, for example a silicone elastomer, obtained by injection or extrusion. Tubular element 60 is then used as starting material to make the helical shape of the electro-active polymer (figures 8 and 9) . This is obtained by the combination of the motion of tubular element 60 with respect to a cutting tool 55 that cuts it radially, figure 5. In particular, tubular element 60 rotates about its own axis 62 operated by a motor 80 and at the same time is cut by tool 55 movable in a direction substantially parallel to the axis 62 of tubular element 60 same. In particular, motor 80 moves about axis 62 a tubular support 61 on which tubular element 60 is put. Cutting tool 55, for example a metal blade (figure 6) , or a laser (figure 7), is slidingly mounted along a guide 50 parallel to the axis 62 of tubular element 60. The combination of the translation of cutting tool 55 and of the rotation of tubular element 60 provides the helical shape (figures 8 and 9) . In particular, the geometry of the helix can be modified within determined limits by adjusting the speed of rotation of tubular element 60 and the speed of tool 55.

Another possible technology that can be used to obtain the electromechanical actuator, according to the invention, provides a process for moulding by casting. In particular, a mould of plastic material can be used, for example of Teflon, or metal, in particular of aluminum, made by known technologies . The mould can be obtained by means of traditional techniques of the positive/negative type, or, alternatively, by creating a model of helix in plastics or metal and made with the technique of the stereolithography or by means of micromechanical production. The material of a raw electro-active polymer (for example silicone rubber) is then put into moulds obtaining an helix of electro-active polymer. Suitable electrodes (for example made of silicone with embedded conductive particles or sprayed with grafite) are created on the two main surfaces of an helix having an inner surface 102 and an outer surface 103 previously coated. The electrodic material, dissolved in suitable solvents (for example trichloroethylene or ether for carbon loaded with silicone) , is put on the helix 100 also for coating by dipping, painting or spraying. A removal of the lining allows to obtain two continuous and distinct electrodes (figure 9) . Another process for the production of the electrodes is described hereafter. The electrodic material is mounted on the whole surface of an helix 100 by dipping, painting or spraying. The coated helix 101 is then shrunk to reach the shape of a hollow cylinder, whose inner and outer surfaces are cut away (by means a circular hollow , cutter) in order to remove from the side portions of the helix 101 the deposited electrodic material. This way, the inner surface 102 and the outer surface 103 of the helix 101 are not coated any more by an electrodic material, whereas the other two surfaces are two continuous and different electrodes. The structure of the device is completed by combining the electrodized helix 100 with the non- electrodized helix 100 (figure 10) . The two helical elements 100 and 101 are then welded together by the same materials used to make them. For example, if a silicone elastomer is used as electro-active polymer for the production the helical elements, the same raw elastomer (non-reticular) is preliminarily arranged on the whole surface of at least one of the two helical elements. The two helical elements are then shrunk and the welding material is treated, thus finally assembling the device.

The foregoing description of a specific embodiment will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such an embodiment without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiment. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Claims

1. Electromechanical actuator for converting electric energy into mechanical energy or vice-versa characterised in that it comprises: - at least a first and at least a second deformable electrode having curvilinear continuous shape extending as coils so that each coil of the first electrode is alternated to a coil of the second electrode; - a dielectric element that can be deformed located between the electrodes in order to fill the space between the coils.

2. Electromechanical actuator, according to claim 1, wherein said at least first and second deformable electrodes have substantially helical shape and are arranged coaxially.

3. Electromechanical actuator, according to claim 1, characterised in that electric energy is converted into mechanical energy when between said at least first deformable electrode and said at least second deformable electrode a voltage is applied whereby an electrical field is generated that causes the dielectric material to deform, causing an axial contraction and a radial expansion.

4. Electromechanical actuator, according to claim 1, characterised in that mechanical energy is converted into electric energy when a deformation is applied suitable for generating an electrical field, in particular said electromechanical actuator acting as sensor of deformation or electric generator.

5. Electromechanical actuator, according to claim 1, wherein said dielectric element is made of an electro- active polymer.

6. Electromechanical actuator, according to claim 1, wherein said at least first and second deformable electrodes are made of conductive resilient material, selected from the group: - an elastomer with embedded conductive particles; — a conductive polymer.

7. Electromechanical actuator, according to claim 1, wherein said at least first and second deformable electrodes are arranged so that two adjacent coils have the surfaces facing each other and overlapped axially.

8. Electromechanical actuator, according to claim 1, wherein said at least first electrode is arranged outside and is associated to said at least second electrode arranged inside and wherein said coils extend according to a spiral or an helix.

9. Electromechanical actuator, according to claim 1, wherein said at least first and second deformable electrodes have a size substantially alike.

10. Electromechanical actuator, according to claim 1, characterised in that it is pre-tensioned in one or more preferential directions, said actuator being deflected by said pre-tensioning in one or more preferential directions.

11. Electromechanical actuator, according to claim 1, characterised in that it can work also as a sensor of deformation of piezoresistive type, where the deformation causes a variation of the electrical resistance of the two electrodes, or of a further deformable conductor.

12. Electromechanical actuator, according to claim 11, characterised in that a further elastomer is arranged inside with embedded conductive particles having function of variable electrical resistance that can be used as piezoresistive sensor, said further conductor having shape selected from the group of helical shape or strip-like shape, embedded in the dielectric material or mounted on a side surface thereof.

13. Uso of an electromechanical actuator, according to the previous claims, for making a electric condenser at high specific capacity and with predetermined stiffness .

14. Method for making an electromechanical actuator for converting electric energy into mechanical energy or vice-versa, in particular that can be used as sensor of deformation or as condenser, characterised in that it comprises the following steps:

- prearranging at least a first and a second deformable electrode having shape curvilinear extending as coils arranged so that each coil of the first electrode is alternated to a coil of the second electrode;

- interposing a dielectric element that can be deformed between the electrodes in order to fill the space between the coils.