PIEZOELECTRIC ACTUATORS WITH INCREASED DEFORMATION

Abstract

The invention relates to an electromechanical actuator with increased deformation, including, on the one hand, at least one drive element connected to a source of AC voltage so as to produce a deformation of the drive element and, on the other hand, a plate configured for amplifying the amplitude of the vibration that the drive element must transmit to a substrate to be actuated. A first face of the plate is rigidly fixed to the drive element and a second face of the plate, opposite to the first face, is fixed by means of an actuation lug to the substrate to be actuated.

Claims

1-20. (canceled)

21. An electromechanical actuator with increased deformation, comprising, on one hand, at least one drive element connected to a source of AC voltage so as to produce a deformation of said drive element and, on the other hand, a plate configured for amplifying the amplitude of the vibration that the drive element must transmit to a substrate to be actuated, wherein a first face of the plate is rigidly fixed to the drive element and a second face of the plate, opposite to the first face, is fixed by means of an actuation lug to the substrate to be actuated.

22. The electromechanical actuator as claimed in claim 21, wherein the actuation lug is small compared to the plate and is centered with respect to the plate.

23. The actuator as claimed in claim 21, wherein said drive element is a piezoelectric actuator, a magnetic actuator or an electrostatic actuator.

24. The actuator as claimed in claim 21, wherein the plate is configured for generating a deformation with maximum flexure at a main resonance frequency in the range between 20 kHz and 200 kHz.

25. The actuator as claimed in claim 21, wherein the shapes, dimensions and material of the plate are chosen such that the amplitude of the vibration generated by the plate at the resonance frequency is greater by an amplification factor in the range between 4 and 50 than the amplitude of the vibration generated by the drive element alone.

26. The actuator as claimed in claim 21, wherein the plate has the form of a disk of diameter in the range between 9 mm and 12 mm and of thickness in the range between 0.2 mm and 1 mm for an unloaded operating frequency of around 60 kHz.

27. The actuator as claimed in claim 26, wherein the plate has the form of a disk of diameter 11 mm and of thickness 0.5 mm.

28. The actuator as claimed in claim 21, wherein the plate takes the form of a parallelepiped of length L, of width B and of thickness h.

29. The actuator as claimed in claim 28, wherein the length L of the plate is in the range between 9 mm and 11 mm, the width B is in the range between 4 mm and 6 mm, and the thickness h is in the range between 1 mm and 2 mm, for an unloaded operating frequency of around 70 kHz.

30. The actuator as claimed in claim 28, wherein the plate has a length L of 10 mm, a width B of 5 mm, and a thickness h of 1.5 mm.

31. The actuator as claimed in claim 28, wherein the amplification factor for the movement of each drive element is a function of the ratio b/L between the width b of the actuation lug and the length L of the plate.

32. The actuator as claimed in claim 31, wherein the ratio b/L of the width b of the actuation lug over the length L of the plate is in the range between 0.1 and 0.45.

33. The actuator as claimed in claim 21, wherein the length L of the plate is substantially greater than its width B, and the plate comprises several drive elements on one of its faces and, on its opposite face, an actuation lug of the same length L as the plate and of width b less than the width B of the plate.

34. The actuator as claimed in claim 21, wherein the plate has a closed surface in the form of a flat or 3D ring and comprises spaced out drive elements situated on one of the faces of the plate and an actuation lug situated on the opposite face of the plate and extending over the length L of the plate and having a width b less than the width B of the plate.

35. The actuator as claimed in claim 33, wherein the amplification factor for the movement of each drive element is a function of the ratio b/B between the width b of the actuation lug and the width B of the plate.

36. The actuator as claimed in claim 21, wherein the drive element is a slab made of piezoelectric ceramic of square, rectangular or circular shape whose largest dimensions (length, width, diameter) are in the range between 6 mm and 8 mm for an operating frequency of 64 kHz.

37. The actuator as claimed in claim 21, being configured such that the amplitude of its deformation in the direction perpendicular to the substrate at the main resonance frequency is in the range between 10 microns and 30 microns for the actuator not coupled to a substrate, and that the amplitude of the deformation transmitted by the actuator to the substrate in coupled mode is in the range between 1 and 2.5 microns.

38. The actuator as claimed in claim 21, wherein the drive element and the plate are rigidly attached with a strong adhesive with low energy dissipation, notably an Epoxy adhesive.

39. A device comprising a substrate to be actuated by a haptic effect, comprising at least one actuator as claimed in claim 21.

40. The device as claimed in claim 39, comprising a substrate to be actuated, a mounting plate fixed perpendicularly to said substrate, and one or more actuators fixed by means of their actuation lug onto said mounting plate.

41. The device as claimed in claim 39, wherein said substrate is a display screen.

Description

DETAILED DESCRIPTION

[0039] The invention will be described in more detail with reference to the appended figures, in which:

[0040] FIG. 1 shows schematically three known examples of piezoelectric actuators with increased deformation;

[0041] FIG. 2 shows schematic diagrams of a first embodiment of an actuator according to the invention, as an exploded perspective view, as an assembled perspective view and as a cross-sectional perspective view;

[0042] FIG. 3 shows several schematic diagrams of a second embodiment of an actuator according to the invention;

[0043] FIG. 4 shows a cross-sectional view of one exemplary assembly of an actuator according to the invention on a substrate to be actuated;

[0044] FIG. 5 shows a distribution diagram of multiple actuators according to the invention on a surface to be actuated;

[0045] FIG. 6 shows a graph illustrating the effect of amplification of the amplitude of the movement of an actuator according to the invention.

[0046] FIG. 7 shows a curve showing the maximum amplitude of deformation of a substrate of the actuator as a function of the ratio between the width of the actuation lug and the length of the amplification plate of the piezoelectric actuator.

[0047] FIGS. 8A to 8C show perspective views of one variant of an actuator according to the invention, seen on its own from above (FIG. 8A), from below (FIG. 8B), and seen from above with its modal shape (FIG. 8C), respectively;

[0048] FIGS. 9a and 9b show digital 3D finite element simulations (COMSOL) showing the modal shape 41 obtained by calculation of the natural modes of the assembly consisting of an actuator according to FIG. 8 and a vibrating surface for two different geometries of the surface to be actuated;

[0049] FIGS. 10A and 10B show two variants of an actuator using an amplification plate in the form of a ring;

[0050] FIGS. 11A and 11B show, respectively, a perspective and cross-sectional view of a device incorporating an actuator according to FIG. 8.

[0051] Reference is now made to FIG. 1 relating to three types of known devices allowing the deformation of a piezoelectric actuator to be amplified.

[0052] It is recalled that there exist two main types of piezoelectric actuators: [0053] non-amplified actuators, in which the displacement obtained is directly equal to the deformation of the piezoelectric material under the application of an electrical voltage. [0054] amplified actuators, in which a mechanical device is used to amplify the movement of the piezoelectric material, typically by a factor of 2 to 20.

[0055] At the present time, ceramic multilayers are conventionally used as drive elements in amplified piezoelectric actuators. The integration of this type of material imposes specific precautions, such as for example the necessity for providing a mechanical pre-stressing or of avoiding torsional forces. Under the condition of a good design and correct use, piezoelectric actuators are extremely reliable and robust.

[0056] FIG. 1a corresponds to one example of a known amplified piezoelectric actuator 1a, composed of an assembly of layers of stacked elementary piezoelectric ceramics 2. The amplification effect is obtained by the multiplication of the deformation of one elementary ceramic 2 by the number of unitary ceramics present in the device. It can accordingly be seen that this can be effective from the point of view of the amplitude of the vertical movement generated, shown schematically by a bidirectional arrow, but the stacking implies that the dimensions of this actuator become too large and exclude this device for certain applications requiring actuators of limited size. Furthermore, this device needs a very rigid fixed attachment point 3, which is also itself quite large.

[0057] FIG. 1b corresponds to a piezoelectric actuator 1b known as a flextensional actuator, which is based on the principle of the stacking of unitary ceramics 2 in FIG. 1a, but by tilting the stack and enclosing it in a mechanical structure 4 which transforms the deformation of the ceramics 2 into a perpendicular deformation, shown schematically by the vertical double arrow. However, this known variant does not solve the problems inherent in the variant in FIG. 1a.

[0058] FIG. 1c corresponds to an actuator 1c known as a bimorphic actuator, comprising two piezoelectric ceramics 2, 2′ operating in opposition. This structure has the advantage of offering a large displacement, but also requires a fixed and rigid reference point 3, and in the end is not suitable for actuating a surface, such as the surface of a screen for example.

[0059] FIG. 2 shows an amplified actuator 20 according to a first embodiment of the invention, as an exploded perspective view (a), as an assembled perspective view (b) and as a cross-sectional perspective view (c).

[0060] This first embodiment is composed of a piezoelectric drive element 21 in the top part, of an amplification plate 22, here of circular shape, rigidly attached to the drive element 21 which will set it into vibration in flexure mode, and of an actuating lug or pin 23 rigidly fixed to the plate 22. This actuation lug is designed to transmit the movements of the composite assembly formed by the drive element 21 and the plate 22 to a surface to be actuated (not shown in this figure). The actuation lug 23 is situated on the face of the plate 22 opposite to that carrying the drive element 21.

[0061] The drive element 21 is preferably, but not necessarily, an elementary piezoelectric actuator in the form of a ceramic slab. It is shown in the shape of a square, but it could be circular or of another shape. In a known manner, the ceramic slab 21 has two metal electrodes (not shown) for the application of a power supply voltage allowing the piezoelectric effect to be obtained, namely a deformation of the thickness of the ceramic as a function of the applied electrical voltage.

[0062] According to the invention, the ceramic slab 21 is bonded as rigidly as possible onto the amplification plate 22, for example by means of a layer of Epoxy adhesive (not shown), in order to avoid as far as possible the dissipations of energy at the interface between the ceramic slab 21 and the plate 22. When the adhesive used is electrically insulating, it is necessary to apply the excitation voltage directly onto the electrodes of the piezoelectric slab. In contrast, when the adhesive used is electrically conducting and when the plate 22 is metal, for example made of brass, steel or zinc, it is possible to apply the excitation voltage between the upper electrode of the piezoelectric slab 21 and the plate 22.

[0063] According to the first embodiment shown in FIG. 2, the plate 22 takes the form of a circular disk. In order to obtain a resonance frequency in the ultrasonic range targeted, namely from 20 kHz to 200 kHz, its thickness may for example be in the range between 0.2 mm and 1 mm, for example 0.5 mm, and its diameter may be in the range between 9 mm and 12 mm, for example 11 mm, as a function notably of the resonance frequency sought, for example 60 kHz unloaded, in other words prior to coupling with a surface to be actuated, and also as a function of other parameters such as notably the density of the material used for the plate.

[0064] According to one advantageous embodiment, the piezoelectric ceramic 21 has a surface area in the form of a square of around 7 mm on a side inscribed in the surface area of the plate 22, and with a thickness of around 0.5 mm, in the case of a targeted operating frequency of around 64 kHz.

[0065] The actuation lug 23 is composed either of a rigid element, for example made of metal or of glass mounted onto and rigidly attached to the plate 22 (FIG. 2a), or of an extension of the material of the plate 22 (FIG. 2c). This second solution allows additional energy dissipations to be avoided since there is no adhesive interface between the plate 22 and the actuation lug 23.

[0066] When the actuator 20 is fixed to a surface to be actuated (not shown in FIG. 2), the deformations of the ceramic slab 21 are transmitted to the plate 22 which amplifies them and transmits them to said surface to be actuated by means of the actuation lug 23.

[0067] FIG. 3a shows a second practical embodiment of an actuator 20 according to the invention. In this configuration, rectangular parallelepipedic structures are used for the drive element 21 and for the plate 22 whose flexure behaves like a beam of rectangular cross-section.

[0068] The dimensions of the actuation lug 23 are small with respect to the plate and it is centered with respect to the plate.

[0069] In the embodiment shown, the plate 22 has a width B, a length L, and a thickness h.

[0070] The transverse bending movements of a beam of rectangular cross-section, denoted y(x, t), are expressed by the following wave equation:

E.I.∂.sup.4.y(x,t)/∂x.sup.4+ρ.∂.sup.2.y(x,t)/∂t.sup.2=0  [Math 1]

[0071] where E denotes Young's modulus,

[0072] [Math 2] I=B.h.sup.3/L.sup.2 denotes the quadratic moment along the y-axis, and ρ denotes the density of the beam. For a harmonic solution (sinusoidal regime), the equation [Math 1] becomes:

(E.I.δ.sup.4/δx.sup.4−ω.sup.2.ρ).Y(x)=0  [Math 3]

[0073] where Y (x) denotes the amplitude of the transverse vibrations along the x-axis. In free-free condition, the solution of the equation [Math 3] leads to the following expression [Math 4] in which [Math 5] β.sub.n=.sup.4√ρ.ω.sup.2.sub.n represents the wave number:

Y.sub.n(X)=cos β.sub.nx+cos hβ.sub.nx−((cos β.sub.nL−cos hβ.sub.nL))/(sin β.sub.nx+sin hβ.sub.nx))/(sin β.sub.nL−sin hβ.sub.nL)  [Math 4]

[0074] where the index n indicates the number of the vibration mode. The frequency of each natural vibration mode of the beam alone in flexure mode is given by the following formula [Math 6]:

f.sub.n=(2n).sup.−1(β.sub.n.I).sup.2√(E.I/ρ.I.sup.4)  [Math 6]

[0075] It is therefore observed that, in order to obtain a given frequency of vibration, several geometries (length, width, thickness) of the plate 22 in the form of a beam could be suitable.

[0076] FIGS. 3b and 3c respectively show a side view and a perspective view of one particular embodiment of a parallelepipedic actuator 20 of width B and of length L, in the flexed position corresponding to a particular moment in time for the actuator in operation. In the example shown, the width and the length of the ceramic slab 21 are equal to those of the plate 22 but this is not obligatory. The actuation lug 23 has a length B equal to the width of the plate, and a width b less than the length L of the plate.

[0077] FIG. 4 shows schematically the installation of an amplified actuator 20 according to the invention on one face of a substrate 40 to be actuated, taking the form of a plate. The actuator 20 is fixed (bonded) onto the lower face 42 of the substrate 40 to be actuated via the free face 24 of the actuation lug 23. Thus, the plate 22 of the actuator is located between the actuation lug 23 and the ceramic slab 21. In order to limit the losses by dissipation of energy, the mounting used to fix the actuation lug 23 onto the lower face 42 of the substrate 40 to be actuated must also be as rigid as possible, using for example an Epoxy adhesive. The terminals of the piezoelectric slab 21 are then supplied with an excitation signal at an ultrasonic frequency by means of electrodes (not shown).

[0078] The amplitude of the movement of the uncoupled amplified actuator 20 may then be of the order of 10 to 30 microns, greater by a factor 4 to 50 than the amplitude of the known piezoelectric actuators.

[0079] Furthermore, the amplitude of the movement caused by the actuator 20 on the surface of the substrate 40 to be actuated via the actuation lug 23 is also amplified by a factor 4 to 50 with respect to that which would be transmitted by a non-amplified actuator according to the prior art.

[0080] The increased deformation of the substrate 40 then has an amplitude of the order of 1 to 2.5 microns at the ultrasonic resonance frequency, and is then clearly felt by the finger 41 of a user positioned on the upper face of the substrate 40 to be actuated, even if the latter is made of a viscoelastic material such as a plastic, wood, or equivalent material.

[0081] When it is necessary to produce a haptic effect differentiated at several points of a substrate 40 to be actuated, or when the substrate to be actuated exhibits too great a loss of the vibrations, it is then possible to equip one face of this substrate 40 with an array of amplified actuators 20 or 50 (FIG. 8), for example a matrix array of actuators 20 or 50 as shown schematically in FIG. 5.

[0082] FIG. 6 shows the results of a simulation of the amplitude of displacement of an amplified piezoelectric actuator 20 according to the invention, coupled to a substrate 40 to be actuated, and operating at an ultrasonic frequency of 71.3 kHz.

[0083] In the left-hand part of FIG. 6, the actuator used is a simple non-amplified piezoelectric slab 21. The amplitude of the bending deformation between its edges and the center, which corresponds to the maximum deformation point, is approximately equal to 2.10.sup.−8 meter, or 0.02 microns.

[0084] In the right-hand part of FIG. 6, the actuator used is an amplified actuator 20 according to the invention. The amplitude of the bending deformation between the edges and the center, which corresponds to the maximum deformation point, is approximately equal to 10.sup.−6 meter, or 1 micron. The invention has therefore allowed, in this particular case, an amplitude barely detectable by a human finger (0.02 microns) to be amplified by a factor 50 into a much more detectable amplitude of displacement of the surface of the plate of 1 micron.

[0085] Tests have shown that, for a given frequency of operation and a given type of surface, the amplitude of displacement of a substrate 40 obtained by the amplified actuator 20 according to the invention is highly dependent on the ratio between the dimensions of the amplification plate 22 and those of its actuation lug 23. The results of the tests are reproduced in the curve in FIG. 7, whose ordinate gives the amplitude of the deformation of the actuator according to the invention as a function of the ratio denoted b/L between the width b of the actuation lug and the length L of the plate 22 of the actuator in FIG. 3, as abscissa.

[0086] As can be seen, this amplitude of deformation remains greater than a micron as long as the aforementioned ratio b/L remains within a range of values included between around 0.1 and 0.45. The maximum amplitude is even close to 2.5 microns when said dimensional ratio b/L is of the order of 0.3, for a single frequency and a single substrate hence in coupled mode.

[0087] The principle of the invention may furthermore be implemented in embodiments other than those in FIGS. 2 and 3.

[0088] Thus, as shown in FIGS. 8A, 8B, one particularly advantageous embodiment consists in adding several drive elements 21 on one face of a plate 22 again of rectangular shape but with a length L much greater than its width B. A series of drive elements 21, notably made of ceramic, are fixed onto one face of this plate. The drive elements 21 are separated by regions with no drive element, which allows the surface area of drive element needed, a fairly costly component, to be economized with no detriment to the result obtained. As previously, an actuation lug 23 is disposed on the face of the plate 22 opposite to that carrying the drive elements 21. However, in this embodiment, the actuation lug takes the form of a longitudinal rail of width b running along the length L of the rectangular plate 22 (rather than the width B of the plate as in FIG. 3). The actuation lug or rail 23 here is again either separate and rigidly fixed onto the plate 22, or directly formed by a protrusion of the plate 22. In this embodiment, it is the ratio between the width b of the actuation lug 23 and the width B of the plate 22 which allows the amplification factor to be controlled.

[0089] FIG. 8c shows the profile of the modal shape 41 which allows the vertical displacement of the actuation lug to be maximized, for the uncoupled actuator 50.

[0090] FIGS. 9a and 9b show the vibrational mode analysis carried out on two coupled systems for an actuator 50 with two different structures 40 to be actuated (one in the form of a rectangular plate in FIG. 9a, the other in the form of a semi-circular plate in FIG. 9b), and it is observed that the profile of the modal shape 41 in FIG. 8c is again seen in each of the two configurations in FIGS. 9a and 9b. In other words, the resonant mode of the free actuator 50 is preserved for the coupled system, independently of the geometry of the plate to be actuated.

[0091] Another embodiment of an actuator 60 according to the invention is shown schematically in FIG. 10. It uses a plate 22 in the form of a flat ring (FIG. 10A), or a plate 22 in the form of a 3D ring (FIG. 10B). In these scenarios, the drive elements 21 may be situated on one of the faces of the plate 22, and the actuation lug 23 is then situated on the opposite face of the plate 22 and extends around the plate over its whole circumference. In this embodiment, it is again the ratio between the width b of the actuation lug and the width B of the plate 22 which allows the amplification factor to be controlled. An actuator 60 in the form of a 3D ring may notably be used to actuate a sphere, a cylinder, a lever, etc.

[0092] Reference is now made to FIG. 11 which shows one advantageous use of the actuator 50 in FIG. 8, in order to cause a rectangular surface to vibrate, which is for example a surface of a substrate 40 forming part of a screen on which it is desired to generate a haptic feedback effect by ultrasonic lubrication.

[0093] In this embodiment, the actuator 50 in FIG. 8 may be turned at 90° and mounted onto a mounting plate 43 (FIG. 11B) coupled by a rigid attachment 42 to the surface of the substrate 40 to be made to vibrate. This allows a gain in the compactness, the surface of the actuator 50 no longer being situated in the plane of the substrate 40 to be actuated, but in a plane perpendicular to it.

[0094] It should be noted that it would also be possible to fix onto the mounting plate 43 several elementary actuators 20 according to FIG. 2 or to FIG. 3, and/or several actuators 50 in FIG. 8, instead and in place of the single actuator 50 shown in FIG. 11.

[0095] In the configuration of FIG. 11, the flexure waves in the substrate 40 to be made to vibrate are generated by the creation of a bending moment. The latter is maximized by the presence of the actuator 50, which creates a vibration of high amplitude independently of the shape of the surface to be made to vibrate, as mentioned in relation with FIG. 9. An optimization of this vibration may be obtained by adjusting the various dimensions of the actuator 50.

ADVANTAGES OF THE INVENTION

[0096] The invention meets the aims targeted and allows high amplitudes of haptic feedback to be obtained including on viscoelastic surfaces exhibiting a strong attenuation of ultrasonic waves, such as plastics, wood, or touchscreens with optical adhesive bonding between the surface glass and the display.

[0097] The invention allows ultrasonic displacements to be created that are greater than with the known piezoelectric actuators, sufficient to obtain the effect of ultrasonic lubrication, namely the modification of the friction of the finger on surfaces excited by a standing wave, even if these surfaces are of the type that dissipate vibrational energy.

[0098] The invention also allows the need for a bulk mechanical reference to be obviated, which allows an enhanced miniaturization of the amplified actuator.

[0099] The invention therefore allows actuators with increased deformation to be used directly on surfaces with friction programmed by ultrasonic lubrication.

[0100] The novel structure is particularly simple and inexpensive to implement, which allows large surface areas, or surfaces which heretofore could not benefit from a significant haptic feedback effect, for example rear faces of screens, to be equipped with actuators according to the invention.