Patent Application
20160233412
The invention relates to an active actuator able to provide a planar shear movement. It relates more particularly to actuators using piezoelectric fibres.
Flat actuators, formed from layers of piezoelectric fibres parallel to one direction and sandwiched between layers carrying electrodes oriented perpendicularly to these fibres in order to activate them, have already been developed in order to slide two structure elements with respect to each other. The applications sought in aeronautics are for example the controlled deformation of an aerodynamic element in order to adapt it according to its variations in incidence.
In this type of application, the performance of the actuator in the shear direction is expressed by the value of the corresponding angular distortion.
The applicant has already proposed, in the document FR 2 893 783, a rectangular flat actuator producing a shear in the direction of its long edge, consisting of an active layer of piezoelectric fibres sandwiched between two layers carrying parallel electrodes, themselves each covered with a layer of fabric with rigid warp and weft oriented in two directions forming a mesh. The weft and warp define a mesh of juxtaposed deformable parallelograms. The piezoelectric fibres form an angle of 45° with the long edge and 90° with the electrodes. The various layers are adhesively bonded to each other so as best to transmit the shear movement of the planar layer of piezoelectric fibres. The layers of fabric with rigid warp and weft provide both the transmission of the shear movement to the members on which the actuator is installed and the rigidity of the device in the other directions.
The drawback of this device is the limited amplitude of the angular distortion allowed by an elementary module of this type of actuator. Whatever the piezoelectric material used in the fibres, the tests carried out by the applicant result in a mean angular distortion of 1256 μdef at 45° for a fibre deformation of 800 μdef.
The object of the invention is to obtain much greater values of the angular distortion with an actuator using a layer of piezoelectric fibres. This requires firstly increasing the value of the angular distortion produced by the movement of the piezoelectric fibres, secondly guaranteeing the level of force produced by the actuator in the shear direction.
To this end, the invention relates to a piezoelectric actuator with a large shear movement in a chosen direction, having a sandwich structure comprising at least one active layer comprising piezoelectric fibres parallel to one another and inclined with respect to said direction, said active layer being placed between at least two layers carrying electrodes disposed so as to be able to cause on command a variation in length of said fibres. This actuator is characterised in that:
The combination of non-deformable elements and the matrix consisting of the incompressible elastic material in the inter-fibre spaces makes it possible to ensure that the active layer reacts to the tensile forces exerted in its plane, substantially like a succession of deformable parallelograms with rigid edges between two non-deformable elements.
In this configuration, it can be shown that inclining the piezoelectric fibres in a direction of less than 40° with respect to the direction of the non-deformable elements makes it possible to obtain angular distortions greater than the diagonal inclination at 45° by a factor of at least 2.
Finally, the transmission of the forces to the members to which the actuator is applied, passing through the electrode-carrying layer, takes place entirely through the rigid elements, through the rigid glues. These elements must therefore have a high tensile modulus in order to withstand, without deforming, the forces induced by the fibres. On the other hand, this transmission mode eliminates the losses caused by the shear strength in the layers of glue responsible for transmitting the forces between the active layers and the other elements of the sandwich, as exist in the prior embodiments.
Advantageously, the angle of inclination of said piezoelectric fibres with respect to said chosen direction is less than 10°. This is because the angular distortion increases greatly when the inclination of the fibres decreases and is theoretically four times greater than the value obtained for 45°, starting from this inclination.
Preferably, the piezoelectric fibres are connected to the electrode-carrying layer by an elastic material having a shear modulus of less than 10 MPa. This layer of elastomer makes it possible to avoid electrical breakdowns by keeping the distance between the piezoelectric fibres and the electrodes substantially constant. On the other hand, its low modulus means that it opposes a low shear strength to the movement of the fibres and that it therefore reduces the force exerted by the actuator only a little.
Preferably, the electrode-carrying layer is produced from non-conductive material manufactured from polyamide. This type of material withstands well the angular shearing imposed by the actuator in operation.
Advantageously, the two edges of the actuator that are parallel to the chosen direction of said active layer are each formed by one of said non-deformable elements. This configuration has the advantage, inter alia, that the movements can be transmitted to the actuated members only by these two non-deformable elements.
The invention also relates to a method for producing a piezoelectric actuator with a large shear movement in a chosen direction, having a sandwich structure comprising at least one active layer comprising piezoelectric fibres parallel to one another and inclined with respect to said direction, said active layer being placed between at least two electrode-carrying layers disposed so as to be able to cause on command a variation in length of said fibres. This method is characterised by the following operations:
Advantageously, the step of positioning the piezoelectric fibres is performed by sawing a sheet of the material making up said piezoelectric fibres manufactured substantially to the dimensions of the active layer.
Preferably, the non-deformable elements are placed in grooves formed in the shearing direction in a sheet substantially to the dimensions of the active layer composed, over its entire extent, of piezoelectric fibres separated by bands of elastic material.
According to these last two features, producing a block makes it possible to save on time while ensuring regular positioning of the elements and evenness of the actuator on its surface.
The invention also relates to an at least partially deformable aerodynamic element comprising a piezoelectric actuator as described above.
The present invention will be understood better and other details, features or advantages of the present invention will become more apparent from reading the following description with reference to the accompanying drawings, in which:
An example of a planar actuator, shown in
The central layer 1 (
The filiform bars 8 have a rectangular cross section, with a width of around 350 μm, and are parallel to the long edge in the direction X and spaced apart, for example, by 1 to 2 mm. These filiform bars 8 are made from highly insulating materials with a high tensile modulus, of at least 200 GPa, which makes them non-deformable elements. Materials with a high dielectric strength and a high tensile modulus such as silicon carbide and boron are preferably used.
Between these filiform bars 8, piezoelectric fibres 2 with a rectangular cross section are disposed, forming an angle θ with the direction X. This angle is chosen in the range between 2° and 40°. The properties of the actuator according to the value chosen are disclosed later, during the description of its functioning. For example, the width of these fibres is around 150μ. The material of these fibres 2 is of the MFC (macro-fibre composite) type, preferably to be taken from the following list: PZT-SA, PZT-5H or PMN-32% PT single crystal.
The piezoelectric fibres 2 are adhesively bonded at both ends thereof to the filiform bars 8 with epoxy rigid glue 9.
The gap between two fibres 2 is filled with an elastomer 10, which is preferably incompressible, with a low shear modulus, preferably less than 20 MPa. The dielectric strength of this elastomer 10 is also preferably greater than that of the piezoelectric fibres 2. The width of the band of elastomer 10 between two fibres 2 is preferably approximately 55μ. The minimum width of these gaps is limited firstly by the need for the elastomer to withstand the deformation of the actuator, and secondly by the dielectric strength of this elastomer 10, so that there is no electrical breakdown between the regions with opposite polarities on the piezoelectric fibres 2.
The planar layers 3, 4 carrying the electrodes 5 are formed by a thin film of polyamide material of the Kapton (registered trade mark) type. Preferably, the thickness of the film is approximately 0.3 mm. The electrodes are embedded in the film of the planar layer and are formed by two series of elongate interdigitated electrodes (51 and 52 for the top layer and 3, 53 and 54 for the bottom layer 4). These electrodes 5 are oriented perpendicular to the piezoelectric fibres 2.
The intermediate layers 6 and 7, providing the connection between the layer 1 of piezoelectric fibres and the electrode-carrying layers 3 and 4, are composed, as depicted in
The thickness of these layers, for example around 20 is limited to what is necessary to ensure the rigid connection between the filiform bars 8 and the electrode-carrying layers 4 and 5.
The functioning of the actuator according to the invention is such that, when the piezoelectric fibres 2 are energised by means of the interdigitated electrodes 51, 52, 53 and 54, secured to the electrode-carrying layers 4 and 5, made from Kapton, the fibres 2 elongate while bearing at each of their ends on the filiform bars 8, which move in opposite directions.
Unexpectedly, the assembly formed by the filiform bars 8 and the elastomer 10 placed between the piezoelectric fibres 2 behaves substantially as a deformable frame having rigid edges, the diagonal of which is inclined by an angle θ with respect to the long edges consisting of filiform bars 8. When such a frame, depicted in
Moreover, these results illustrate the fact that, for a given elongation of the piezoelectric fibres 2, it is possible to obtain appreciably greater angular distortions when the inclination of the fibres is small. In addition, this shearing is transmitted to the electrode-carrying Kapton layers since they are adhesively bonded to the filiform bars 8 by the epoxy glue 12 with a high rigidity modulus.
Moreover, the lower the forces opposing the elongation of the fibres, the greater is such elongation. The shearing created by the actuator at its centre is therefore greater when:
Because of the evaluation of the rigidity of the assembly opposing the work of the fibres 2 and because of the angular distortion value γ for small angles θ, an actuator according to the invention is preferably produced with an inclination of the fibres of less than 10°. However, the smaller this angle, the smaller the number of fibres 2 for a given actuator surface area. It therefore appears that there will be a value below which the force exerted by the fibres will not be sufficient to overcome the shearing of the elastomer 9 between the filiform bars 8. It is for this reason that a limit 2° less than this inclination is fixed here for a practical embodiment.
In order to illustrate the performance that can be expected of a sensor according to the invention, calculations were carried out to model the response of an elementary module formed by two filiform bars 8 connected by piezoelectric fibres inclined at 10°. The two modules differ on account of their lengths and widths: 5.2×1.2 mm for the first, 10.5×2.2 mm for the second. Otherwise, the other characteristics, summarised in the following table, are identical:
The other dimensional characteristics are those already cited as an example in the description of the embodiment. The piezoelectric fibres 2 are made from 1000 volt PZT5A so as to obtain a fibre elongation, denoted ∈L, of around 800 μdef. The electrode-carrying layers 3 and 4 are made from Kapton.
The angular distortion is evaluated at the centre of the Kapton film forming an electrode-carrying layer 3. This value is compared with that obtained with a sensor according to the prior art, the fibres of which are inclined at 45° and also subjected to a voltage of 1000 V. This sensor has a length of 85 mm and a width of 28 mm. The results are set out in the following table:
Moreover, the method for manufacturing the example of an actuator described previously is illustrated in
A first step A consists of sawing this sheet in order to form parallel strips 2 of piezoelectric fibres, inclined by an angle θ with respect to the long edge of the sheet. The separation of the fibres 2 corresponds to their separation in the actuator. Moreover, complete sawing of the polymer film 14 supporting the assembly is avoided.
In a step B, the interstices between piezoelectric fibres 2 are filled with the elastomer 10 of the active layer 1.
In a step C, sawing is carried out in the direction of the length of the assembly, fibres 2 and elastomer 10, without attacking the polymer film 14, in order to form longitudinal grooves in as many locations as there will be filiform bars 8.
Step D consists of depositing a protective cover 15 on the longitudinal bands comprising the fibres 2.
Step E consists of placing the filiform bars 8 in the longitudinal locations and adhesively bonding them to the piezoelectric fibres 2 with the epoxy glue 9.
Step F consists of removing the protective covers 15.
Step G consists of placing the films of epoxy glue 12 on the top face of the active layer 1 thus created, opposite the filiform bars 8, and placing the elastic films 11 opposite the bands containing the piezoelectric fibres 2.
Step H consists of placing the electrode-carrying layer 3 with its electrodes 5 and then connecting it to the active layer 1 by a polymerisation of the films 12 and 11.
During a step I, the polymer film is removed and then in a step J the two layers thus obtained are turned over.
Next, steps G and H are performed on the other side of the active layer 1 in order to place the second electrode-carrying layer 4.
This type of actuator can advantageously be used in aeronautical applications, in particular for modifying the shape of an aerodynamic element. For example, the patent application FR 2 924 681 describes the use of planar actuators inserted in the structure of a blade of a rotating wing in order to deform it torsionally by a relative sliding of the parts situated around a slot oriented substantially in the direction of the wing span. In addition, the amplitude of the sliding must be controlled during the rotation of the blade. The actuator according to the invention can easily be substituted for the planar actuator of the piezoelectric type already envisaged in this application. In general terms, the invention can be applied whenever a part of an aerodynamic element must perform sliding or rotation movements about a given position and the space allocated to the actuator is small.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1359055 | Sep 2013 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2014/052249 | 9/11/2014 | WO | 00 |
