Patent Publication Number: US-11650668-B2

Title: Flexible haptic interface

Description:
TECHNICAL FIELD 
     The present invention relates to human-machine interfaces, and more particularly those producing haptic effects. 
     PRIOR ART 
     A haptic interface allows the user to interact with his environment via his sense of touch. At the present time, haptic effects are increasingly being used in many applications, for example in smartphones, the latter generating a slight vibration when a key displayed on the screen is pressed in order to simulate the impression of pressing on a button. Haptic applications also exist in virtual- or augmented-reality devices, in particular in order to allow a better immersion in video games. Haptic interfaces such as touch panels, which generate a haptic effect on a touch surface when a user makes contact therewith, may especially be integrated into many technologies such as desktop or laptop computers, tablet computers and/or smartphones. 
     It is known to generate various and relatively complex haptic effects on a stiff touch surface by virtue of ultrasonic transducers that emit ultrasonic waves that propagate over the stiff surface. 
     With the appearance on the market of flexible commercial products, such as foldable smartphones or even rollable television sets, it would be desirable to provide flexible haptic interfaces able to be integrated into such products. 
     Use of a simple flexible carrier bearing ultrasonic transducers is however limited because the waves emitted by the transducers are partially absorbed by the flexible material; the generated haptic effect is then difficult for a user applying a pressure with his finger or his hand on such a carrier to perceive. 
     It is moreover known to use a pneumatic actuator to generate haptic effects on a flexible structure. U.S. Ser. No. 10/240,688B2 discloses a flexible haptic interface comprising pneumatic actuators allowing a topology effect to be generated. 
     The article by Yu et al. “Skin-integrated wireless haptic interfaces for virtual and augmented reality” (Nature, 2019) discloses a flexible haptic interface comprising magnetic actuators allowing various haptic effects to be generated. However, the spatial resolution of the haptic effects obtained with such an interface is relatively low, and the use of magnetic actuators leads to a high power consumption. 
     Lastly, non-newtonian fluids have already been used to generate simple haptic effects such as button effects and relief effects. The article by Lochtefeld “Towards real organic user interfaces—using non-Newtonian fluids for self-actuated displays” (CHI13 workshop, 2013) describes a haptic device using a shear-thickening non-newtonian fluid sandwiched between two flexible sheets placed on a fixed array of loudspeakers. A topology perceptible with the finger is generated when a sound wave propagates through the fluid and hardens it locally. 
     The article by Poncet et al. “Static and dynamic studies of electro-active polymer actuators and integration in a demonstrator” (Actuators Journal, 2017) describes a device comprising piezoelectric electro-active polymer actuators arranged so as to obtain flexible-membrane-based vibrotactile buttons. These buttons are borne by a stiff carrier. 
     DISCLOSURE OF THE INVENTION 
     There is a need to further improve flexible haptic interfaces, especially in order to provide a touch interface capable of generating various haptic effects and having a good spatial resolution. 
     SUMMARY OF THE INVENTION 
     The invention aims to meet this need, and does so, according to a first of its aspects, by virtue of a flexible haptic interface comprising a haptic structure defining a touch surface touchable by a user, the structure comprising:
         a plurality of stiff touch pixels borne by a flexible carrier,   at least one actuator that makes contact with each pixel and that is arranged to transmit, to the pixel, a mechanical excitation that leads to an effect perceptible tactilely by the user.       

     By virtue of the invention, the haptic structure has a certain flexibility, while still allowing satisfactory haptic effects to be generated, with a spatial resolution and a reliability suitable for many applications. 
     The interface may comprise any control circuit configured to modulate the signals sent to the actuators in order to mechanically induce a vibration of the pixels and to generate a corresponding haptic sensation on the touch surface. 
     Touch Pixels 
     The invention is not limited to touch pixels having a particular shape. However, certain shapes may facilitate manufacture. Thus, at least certain of the pixels, and better still all of the pixels, may have a polyhedral, and preferably parallelepipedal, general shape. The pixels may especially take the form of rectangular pads. 
     At least certain of the pixels, and better still all of the pixels, are preferably made of a relatively hard and commonplace material, such as glass, and preferably borosilicate glass. 
     At least certain of the pixels, and better still all of the pixels, preferably have a substantially constant thickness, preferably comprised between 50 microns and 5 mm, and better still between 200 and 700 microns. 
     All of the pixels may be identical. As a variant, it is possible to combine, within the same interface, touch pixels of different sizes, depending for example on their location relative to the touch surface, or on the application. 
     Carrier and Integration of the Touch Pixels 
     By “carrier”, what is meant is any mono- or multi-layer structure that ensures the touch pixels are held in position within the haptic structure, and the cohesion of the touch pixels within the haptic structure. 
     The carrier is preferably formed from one or more polymeric materials, this possibly facilitating manufacture and the obtainment of the flexibility sought for the haptic structure. 
     The pixels are advantageously made of a material that is stiffer than that or those of the carrier. The ratio of the Young&#39;s modulus of each touch pixel to that of the carrier is for example higher than or equal to 8, and better still higher than or equal to 20, and even better still higher than or equal to 50, and more preferably comprised between 80 and 90, and especially equal to about 85. 
     The choice of the thickness of the carrier may be made depending on the flexibility thereof: the softer the carrier, the larger its thickness may be while allowing the sought flexibility to be preserved. 
     The touch pixels may be integrated into the carrier in various ways, depending on the structure of the carrier. 
     The carrier may especially comprise a bearing layer and a layer for compensating for the thickness of the pixels. This compensating layer lies between the pixels, above the bearing layer. 
     The bearing layer may also play a role as a protective layer protecting the touch pixels and/or the actuators with which the latter are equipped. 
     Preferably, the bearing layer has a substantially constant thickness, preferably comprised between 50 nm and 500 microns, and better still between 25 and 80 microns, and for example of 50 μm. 
     The bearing layer may alone ensure the touch pixels are held in place within the haptic structure. The thickness-compensating layer may also play this role, in association with the bearing layer. 
     The touch pixels may all be located on the same side of the bearing layer, and the compensating layer comprises flexible segments, also called “flexible bridges” or “flexible hinges”, extending between the pixels. 
     The compensating layer is preferably of substantially constant thickness, preferably comprised between 100 microns and 5 cm, and preferably between 500 μm and 5 mm, and for example of 1 mm. 
     At least some of the touch pixels, and preferably all of the touch pixels, may have an exterior surface that emerges onto the touch surface. 
     This exterior surface of the touch pixels may lie flush with the exterior surface of the compensating layer, the exterior surface of the pixels and that of the compensating layer then defining a substantially smooth surface that may serve as touch surface for the interface. 
     The user then makes direct contact with the touch pixels when he places his finger in contact with the touch surface. 
     As a variant, the touch pixels and the compensating layer may be covered with a protective layer, which may be identical to the bearing layer of the carrier, or not. The thickness of this protective layer is relatively small, so as not to affect the perception of the touch effect unduly. 
     The carrier may comprise housings in which the touch pixels at least partially lie, the latter for example being continuously covered with a flexible sheet defining the touch surface. 
     In one example of embodiment, the carrier comprises a layer for receiving the touch pixels, the receiving layer for example forming basin-shaped housings, and the carrier also comprising a layer for covering the pixels, which layer is for example defined by the aforementioned flexible sheet. 
     The covering layer especially allows the exterior uniformity of the touch surface to be increased, and parasitic touch sensations that could otherwise be felt if the user made direct contact with the pixels and/or neighboring regions, since these might form to the touch a nonuniform surface, to be avoided. 
     Arrangement of the Touch Pixels 
     The touch pixels may be distributed in various ways over the carrier, especially depending on the application. 
     The spacing between two adjacent touch pixels is preferably comprised between 10 microns and 5 mm, and better still between 1 and 2 mm. A small spacing between the pixels especially allows the spatial resolution of the generated haptic effect to be increased, if it is desired to do so. 
     At least some of the pixels are for example arranged in rows and/or columns, and especially in the same plane when the carrier is flat, and preferably in a regular grid. 
     As a variant, the pixels may be placed with a concentric distribution, inter alia. 
     Actuators 
     Each touch pixel may be equipped with a plurality of actuators extending over one face of the pixel, for example the face located on the side of the carrier and/or its opposite face. 
     The one or more actuators associated with a touch pixel may each have a shape that is elongate, preferably in the direction of the width of the pixel in the case where the latter has a rectangular shape. 
     The actuators may be placed on either side of a median plane of the touch pixel, preferably at vibration nodes or anti-nodes. 
     The actuators may each have a polygonal general shape, and especially a rectangular or square shape, or have a circular shape. 
     Each actuator may be piezoelectric, ferroelectric, electromagnetic or thermal. In particular, each actuator may be a piezoelectric ceramic. 
     Associated Systems 
     The interface may comprise a system for detecting a contact of the user on the touch surface, and especially a capacitive detecting structure, which is where appropriate integrated into the haptic structure. 
     The interface may comprise a system allowing an image to be at least partially superposed on the touch surface. The interface may thus comprise a screen, and preferably a screen incorporating the haptic structure. 
     The interface may comprise at least one actuator allowing the interface to be selectively shaped into at least two distinct shapes. This may for example be useful to change the shape of the interface depending on the touch sensation to be reproduced, in order for example to improve the quality of the simulation. 
     Another subject of the invention is an article of clothing equipped with a haptic interface according to the invention and such as defined above. 
     Another subject of the invention is a mobile device equipped with an interface according to the invention and such as defined above. 
     Tactile Stimulus 
     Yet another subject of the invention is a method for generating at least one tactile stimulus capable of being felt by a user touching an interface according to the invention, comprising the steps consisting in:
         detecting the position of the contact of the user on the flexible touch surface of the interface by virtue of a detecting system,   modulating, by virtue of a control circuit, the signals sent to the actuators depending on the detected position in order to mechanically induce a vibration of the pixels and to generate an effect perceptible tactilely by the user on the touch surface.       

     The vibration of the pixels may generate a variation in friction perceptible tactilely by a user moving his finger over the touch surface. This effect, which is also called the squeeze-film effect, may give the user the impression of touching a surface comprising reliefs, or having various textures. 
     The vibration of the pixels may even generate a pulse perceptible tactilely by a user exerting a static contact on the touch surface. This effect for example allows the user to be given the impression of pressing on a button. 
     Production of an Touch Pixel 
     Another subject of the invention is a process for manufacturing an touch pixel equipped with piezoelectric actuators and usable in an interface such as defined above, comprising the steps consisting in:
         depositing, preferably by screen-printing, at least a first layer of a conductive material on the upper face of a stiff carrier, preferably a carrier made of glass,   fastening piezoelectric actuators to the first layer thus formed so as to establish a first electrical connection with the actuators,   depositing a layer of an insulator on the layers deposited beforehand,   thinning the insulating layer in order to uncover the upper face of each piezoelectric actuator,   depositing a layer of a conductive material on the actuators thus uncovered, so as to establish a second electrical connection with the actuators,   dicing the carrier so as to obtain touch pixels each equipped with one or more piezoelectric actuators.       

     This process especially allows a plurality of touch pixels to be manufactured, simply, using the same carrier, a glass plate for example. 
     The touch pixels thus obtained may then be transferred to a flexible carrier to form a haptic interface. 
     Yet another subject of the invention is a process for manufacturing a haptic interface such as defined above, comprising the steps consisting in:
         depositing a first layer made of flexible material, and preferably made of polymer, on a manufacturing carrier,   fastening touch pixels equipped with one or more actuators to the layer thus deposited, the actuators being placed on the exterior face of the pixels,   electrically connecting each actuator to the control circuit, preferably by masking and sputtering of a conductive material,   covering the touch pixels and actuators with a flexible material, preferably a polymer film, and,   separating, especially by peeling, the assembly thus produced from the manufacturing carrier so as to uncover the lower face of the first layer.       

     This first layer may form the aforementioned covering layer, and the flexible material deposited on the pixels may form the aforementioned receiving layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will possibly be better understood on reading the following detailed description of non-limiting examples of implementation thereof, and on examining the appended drawing, in which: 
         FIG.  1    partially and schematically shows an example of a flexible haptic interface according to the invention, 
         FIG.  2    partially and schematically shows an example of a haptic structure comprising touch pixels arranged in a matrix array, 
         FIG.  3    partially and schematically illustrates an example of integration of the touch pixels into the interface, 
         FIG.  4    is a view analogous to the one shown in  FIG.  3    of another example of integration of the touch pixels, 
         FIG.  5    partially and schematically shows an example of a flexible haptic interface equipped with an associated system, 
         FIG.  6    is a partial and schematic perspective view of an example of a touch pixel equipped with two actuators, 
         FIG.  7    illustrates a Lamb mode generated on the touch pixel of  FIG.  6    by the actuators making contact therewith, 
         FIG.  8    is a graph illustrating the resonant frequencies of the pixel of  FIG.  6    as a function of its thickness, 
         FIG.  9    is a partial and schematic perspective view of a haptic structure showing two touch pixels, 
         FIG.  10    is a schematic and partial cross section through the thickness of the haptic structure of  FIG.  9   , 
         FIG.  11    is a graph illustrating vibration of the haptic structure of  FIG.  9    when the carrier is made of polyimide (PI), 
         FIG.  12   a    partially and schematically illustrates vibration of the haptic structure of  FIG.  9    when the carrier is made of polyethylene naphthalate (PEN), 
         FIG.  12   b    is a graph showing the amplitude of vertical deformation of a touch pixel in cross section XII of  FIG.  12     a,    
         FIG.  13   a    partially and schematically shows a haptic structure comprising a matrix array of touch pixels, 
         FIG.  13   b    partially and schematically shows the possibility of making only some of the touch pixels of the haptic structure of  FIG.  13   a    vibrate, 
         FIG.  14    is a block diagram illustrating one example of operation of the haptic interface according to the invention, 
         FIG.  15   a    partially and schematically illustrates the possibility of generating a button touch effect using an interface according to the invention, 
         FIG.  15   b    partially and schematically illustrates the possibility of generating a relief touch effect using an interface according to the invention, 
         FIG.  15   c    partially and schematically illustrates the possibility of generating a texture touch effect using an interface according to the invention, 
         FIG.  16   ,  FIG.  17    and  FIG.  18    partially and schematically illustrate successive steps of an example of a process for manufacturing touch pixels with their actuators, 
         FIG.  19   ,  FIG.  20   ,  FIG.  21    and  FIG.  22    partially and schematically illustrate successive steps of an example of a process for manufacturing a haptic structure, 
         FIG.  23   a    shows an example of an article of clothing equipped with a flexible haptic interface according to the invention, 
         FIG.  23   b    illustrates an example of a mobile device equipped with a flexible haptic interface according to the invention, and 
         FIG.  24    illustrates the possibility of modifying the shape of the interface using actuators. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    illustrates one example of a flexible haptic interface  1  according to the invention. The interface  1  comprises a haptic structure  2  defining a touch surface S. The haptic structure  2  comprises touch pixels  3  that are borne by a flexible carrier  4 . The touch pixels  3 , which are called pixels by analogy with the optically active pixels of a screen, are stiff and placed on the carrier  4  so that the haptic structure  2  preserves an overall flexibility in at least one direction, and better still in every direction. 
     The touch pixels  3  are equipped with one or more actuators  5 , preferably piezoelectric actuators, the latter being connected to a control circuit  6  by virtue of electrical connections  50 . The control circuit  6  is configured to modulate the signals sent to the actuators  5  in order to mechanically induce vibration thereof and to generate a haptic sensation on the touch surface S. 
     The haptic structure  2  may be of any shape, and of various sizes. Its dimensions are for example, depending on the application, of the order of one cm, of one dm or of one meter. 
     The touch pixels  3  may be various shapes, and for example rectangular, square, circular, inter alia. Their largest dimension is for example comprised between a few hundred microns and a few cm in side length, and preferably from 2 mm to 1 cm in side length. 
     The touch pixels  3  are for example made of glass, and especially borosilicate glass, or of silicon. 
     The touch pixels  3  may be distributed over the carrier  4  in various configurations. 
     For example, in  FIG.  2    identical touch pixels  3  distributed in rows and columns over the carrier  4 , in a regular array, have been illustrated. 
     In other embodiments (not illustrated), the interface  1  comprises touch pixels of different shapes and/or sizes, or even touch pixels  3  placed with a concentric distribution or staggered. 
     The size of the touch pixels may in particular be adjusted in order to generate haptic effects that simulate tactilely larger or smaller patterns. 
     It is also possible to adjust the spacing between the touch pixels in order to generate haptic effects of higher or lower spatial resolution. The distance d between two touch pixels is preferably comprised between a few tens of microns and a few mm, and in particular between 1 and 2 mm. 
     The carrier  4  holds the touch pixels  3  and participates in the cohesion of the haptic structure  2 . 
     The carrier  4  may be mono-layer or multi-layer. It is preferably made of polymer, and for example of polyethylene naphthalate (PEN), polyimide (PI) or even polycarbonate (PC). It may furthermore comprise one or more thin layers of metal oxides, of silicon or of other metals. The thickness of the carrier is set depending on the nature of the material, so as to preserve the desired flexibility. 
     A touch pixel  3  may be integrated into the structure  2  in a number of ways. 
     In the example illustrated in  FIG.  3   , the carrier comprises a bearing film  41 , which may serve as protective layer, and a layer  40  for compensating for the thickness of the pixels  3 . 
     The pixels  3  are borne by the film  41  and the entire thickness thereof is embedded in the compensating layer  40 , which defines flexible segments  400  between the pixels, which segments are also called “hinges”. 
     The touch pixels  3  have an exterior surface  3   a  that emerges onto the touch surface S. As illustrated, the compensating layer  40  may have an exterior surface  40   a  that lies flush with the surface  3   a  of the touch pixels, so as to form therewith a smooth surface, which is substantially planar when the haptic structure  2  is flat. The compensating layer  40  may have substantially the same thickness as the touch pixels  3 . 
     The user may touch the surface  3   a  of the touch pixels  3  of the interface directly when he brings his finger into contact with the touch surface S. 
     As a variant, it is possible to cover the touch pixels  3  and the compensating layer  40  with a thin protective layer (not shown). This allows the uniformity of the touch surface to be increased, and prevents the user from feeling an undesired texture effect due to the difference between the material of the pixels and that of the compensating layer  40 . 
     In the example illustrated in  FIG.  3   , the actuators  5  are preferably located on the face  3   b  of the touch pixels  3  that is located on the side of the bearing film  41 , as illustrated. The touch pixels  3  equipped with the actuators  5  are for example fastened to the carrier  4  by virtue of an insulating adhesive  55 . 
     In the variant illustrated in  FIG.  4   , the carrier  4  comprises a layer  42  for receiving the pixels  3  and a covering layer  43 . 
     The receiving layer  42  forms basin-shaped housings  420 , which each accommodate one touch pixel  3 . The covering layer  43 , which is for example a flexible polymer sheet, continuously covers the receiving layer  42  and the touch pixels  3 , thus closing the housings  420  from above. 
     The covering layer  43  for example also serves as a protective sheet protecting the touch pixels  3 . 
     In the example in question, the free face  43   a  of the covering layer  43  defines the touch surface S. The covering layer  43  is thin enough for the vibrational effect of the touch pixels  3  to be able to propagate to the touch surface S. The covering layer  43  for example has a thickness comprised between a few microns and a few tens of microns. 
     In this variant, the actuators  5  may immaterially be located on the lower face of the pixel  3 , i.e. between the touch pixel  3  and the bottom of the housing  420  of the receiving layer  42 , and/or the upper face of the touch pixel  3 , i.e. between the touch pixel  3  and the covering layer  43 . When some actuators  5  are present on the opposite faces of an pixel  3 , their excitation is preferably controlled so as to take advantage of constructive vibrational interference therebetween. 
     In the example illustrated in  FIG.  5   , the interface comprises a detecting system  7 , for example a known prior-art capacitive system, allowing at least one point of contact of the finger of the user with the surface S to be detected. 
     The detecting system has been shown in  FIG.  5    schematically in the form of a member covering the touch pixels  3  and the carrier  4 , but it may form part of the carrier  4  or be integrated into the touch pixels. 
     The interface  1  may furthermore comprise one or more associated devices allowing user experience to be improved, and especially it to be made a multisensory experience. 
     The interface  1  may thus comprise a flexible screen (not shown) that allows an image to be superposed on the touch surface S and for example the user to be given the impression of touching, virtually, what he is able to see on the screen. The interface  1  may further comprise one or more loudspeakers (not shown) with a view to adding an audio effect. 
     The actuators  5  may be of various shapes and sizes. The number, arrangement and size of the actuators  5  on a touch pixel  3  depends on the sought-after effect. 
       FIG.  6    illustrates an example of a touch pixel  3  taking the form of a rectangular pad of width W of about 1 cm and of length L of about 1.5 cm. The thickness e of the pixel  3  in the example in question is 500 microns. 
     The pixel  3  bears two strip-shaped actuators  5 , each of a width of about 2000 microns for example, and which are each for example placed at a distance m of 2250 microns from a lateral edge of the pixel. 
     The actuators  5  are for example 2 microns in thickness, and for example have an active layer based on aluminum nitride (AlN), lead zirconate titanate (PZT) or any other suitable ferroelectric or piezoelectric material. 
     The size and position of the two actuators  5  are for example chosen such as to generate a Lamb oscillatory mode in the touch pixel, for example at a frequency of about 61 kHz, according to the methodology described in the article by Casset, F., et al. “Low voltage actuated plate for haptic applications with PZT thin-film” (Proceedings of Transducers, 2013). 
     As illustrated in  FIG.  7   , the actuators  5  may be located at the anti-nodes of the vibrational mode that they generate. However, other actuation configurations, for example comprising actuators positioned at vibration nodes, are possible. The embodiment of  FIG.  7    for example allows a variation in friction on the touch pixel  3  that is tactilely perceptible by a user moving his finger over the touch surface to be generated. 
     It is possible to obtain a haptic effect similar to the one that has just been described with a touch pixel of different thickness. 
     In particular, a plurality of vibrational modes may be generated at frequencies that depend on the thickness of the touch pixel  3 . It is possible to generate, in the touch pixel  3 , a Lamb mode, as described above, or the first vibrational mode in the dimension of the length L of the pixel, or even the first vibrational mode in the dimension of the width W of the pixel. 
     The various vibrational modes will possibly be obtained by adapting the piezoelectric actuators using the method described in the article by Poncet et al, “Design and realization of electroactive polymer actuators for transparent and flexible haptic feedback interfaces” (EuroSime, 2016). 
     The graph of  FIG.  8    shows examples of resonant frequencies corresponding to the three aforementioned vibrational modes, as a function of the thickness of the pixel for a touch pixel of planar dimensions of 1×1.5 cm 2  comprising two strip-shaped actuators such as shown in  FIG.  6   . 
     It may be seen from this figure that if it is desired to use the Lamb mode (indicated by circles in the graph) to generate the sought-after haptic effect, a thickness of the touch pixel comprised between 200 and 700 microns will advantageously be chosen, in order to remain at frequencies inaudible to the human ear, i.e. higher than about 20 kHz, and at frequencies lower than 100 kHz, so as to keep the control electronics relatively simple. 
     Likewise, if it is desired to exploit the first vibrational mode in the dimension of the length L of the pixel (indicated by triangles in the graph), a thickness of the touch pixel larger than 900 microns will be chosen. 
     Lastly, if it is desired to exploit the first vibrational mode in the dimension of the width W of the pixel (indicated by squares in the graph), a thickness of the touch pixel between 500 and 900 microns will be chosen. 
     The thickness of the pixel  3  is in addition constrained by the stiffness sought to perceive the touch effect. Specifically, a low stiffness of the touch pixel  3 , induced, inter alia, by a small thickness, may decrease the force exerted by the touch pixel  3  on the user and therefore the effect felt. A larger thickness of the touch pixel  3  in contrast decreases the amplitude of vibration of the pixel, and may therefore attenuate the haptic effect felt. 
     A thickness comprised between 50 microns and a few mm, and preferably comprised between 200 and 700 microns, may be a good compromise. 
     In one variant, the actuators  5  are distributed over the touch pixel  3  in a matrix-array arrangement. In one example, actuators  5  of square shape of about 5×5 mm 2 , spaced apart regularly by 2 mm, are distributed in this way over the pixel  3 . In another example, the touch pixel  3  is equipped with a single actuator  6 , which is for example placed at the center of one of its main faces. 
     The stiffness of the carrier  4  bearing the touch pixels  3 , with respect to that of the touch pixels  3 , also has an impact on the quality of the generated haptic effect. 
     Results of simulation of the vibrations of a haptic structure  2 , such as shown in  FIG.  9   , comprising a carrier  4  bearing two touch pixels  3  similar to that of  FIG.  6    are described below. These results were obtained with the commercially available software package COVENTOR, but any other software package, especially allowing a finite-elements analysis, is usable. 
     In the example in question and as shown in  FIG.  10   , the touch pixels  3  were embedded in the carrier  4  and had an exterior surface  3   a  that emerged onto the touch surface S. The structure of the carrier  4  was such as described with reference to  FIG.  3   . 
     The touch pixels  3  were for example slips of borosilicate glass of about 500 microns in thickness. Their Young&#39;s modulus was of the order of 65 to 70 GPa. 
     The carrier had a length L S  of 3.5 cm, a width W S  of 2.5 cm and a total thickness h E +h P  equal to 525 microns, the thickness h E  of the compensating layer  40  being 500 microns, and the thickness h P  of the bearing film  41  being 25 microns. 
       FIG.  11    illustrates the amplitude of vibration simulated for such a haptic structure in the X-, Y- and Z-directions, for a frequency comprised between 60 and 70 kHz, when the carrier  4  that served as comparative example was made of PI, which material had a Young&#39;s modulus of the order of 7.5 GPa. The sought-after vibrational mode was not obtained under these conditions, and no advantageous haptic effect was able to be generated. The carrier  4  made of PI being too rigid, it applied mechanical boundary conditions to the perimeter of the touch pixels and restricted their vibration. 
     In contrast, when the carrier  4  was made of PEN, material the Young&#39;s modulus of which was of the order of 0.75 GPa, it was possible to generate a natural Lamb mode in the haptic structure  2 , as shown in  FIG.  12     a.    
     As illustrated in  FIG.  12   b   , the corresponding amplitude A of deformation of the touch pixels  3  was able especially to reach+/−2 microns when the corresponding actuators  5  were subjected to an AC voltage of +/−30 V, this allowing a marked haptic effect to be obtained. 
     A ratio of the Young&#39;s moduli of about 65 GPa/7.5 GPa, i.e. of the order of 8, therefore did not allow the desired haptic effects to be produced under the simulated conditions. In contrast, a ratio of the order of 86 (corresponding to 65 GPa/0.75 GPa) allowed the desired effects to be produced. 
     Of course, other thicknesses h P  and h E  are envisionable. For example, for a polymer such as PEN, PI or PC, a thickness comprised between 100 microns and a few cm, and preferably a few mm, allows the carrier to provide mechanical strength while remaining flexible. 
     The thickness h P  is preferably small enough, especially with respect to h E , to not disrupt the electromechanical behavior of the touch pixels  3 . It is for example comprised between a few tens of nm and a few hundred microns, and preferably between 25 and 90 microns. 
     The carrier  4  may be multi-layer or mono-layer, as described above. In the case of a multi-layer carrier, the Young&#39;s modulus E T  and the total thickness h T  of the carrier will possibly be obtained using the following approximations, which are known in the prior art: 
     
       
         
           
             
               
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                 T 
               
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                   i 
                 
               
             
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                 E 
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                       h 
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     With h i  and E i  the thickness and the Young&#39;s modulus of layer i, respectively. 
     The one or more actuators of each touch pixel  3  of an interface  1  may be addressed independently by the control circuit  6 , this allowing haptic effects that are relatively localized on the touch surface S to be generated. 
     It is for example possible to obtain a perceptible deformation of the touch surface S solely at its center, as illustrated in  FIG.  13   b   , by solely activating the central touch pixel  300  of a matrix array of 3×3 touch pixels, i.e. a matrix array such as shown in  FIG.  13     a.    
     A haptic interface  1  according to the invention may operate according to the steps illustrated in  FIG.  14   . 
     In step  81 , the interface is on standby, in the absence of a user touching the touch surface S, and the touch pixels  3  are at rest. 
     In step  82 , the detecting system  6  detects at least one point of contact of the user with the surface S and the control circuit  6  determines the location where the one or more haptic effects must be generated. 
     In step  83 , the control circuit  6  sends suitable control signals to actuate at least some of the actuators  5  so as to generate the desired haptic effect. 
     In step  84 , the actuators  5  in question contract or lengthen via an inverse piezoelectric effect, with an actuation amplitude corresponding to the received signal, generating, via a unimorphic effect, a vibration of the stiff pixel  3  with which they make contact. 
     In step  85 , the vibration of the one or more touch pixels that is induced by the actuators is perceived tactilely by the user on the surface S. 
     In step  86 , the user breaks contact with the touch surface S and the interface may return to standby, if it is desired to save energy for example. 
     The tactile perception generated in step  85  for example results from feeling a vibration of the touch pixels, this possibly giving the user the impression of touching a vibrating surface or a surface having a relief. 
     By actuating more or fewer touch pixels, it is possible to create relatively complex effects. Certain are described below and illustrated in  FIGS.  15   a ,  15   b    and  15   c.    
     A matrix array  10  of identical touch pixels  3 , of circular shape and distributed uniformly over the matrix array  10 , staggered, will be considered in the following examples. The touch pixels each comprise an actuator  5  placed at their center. 
     In the example illustrated in  FIG.  15   a   , a group of touch pixels  30  belonging to the matrix array  10  is actuated by the control circuit  6  in order to transmit to a user U exerting a static contact on the surface a pulse corresponding to a tactile perception  60  of a “key-click effect”, i.e. one giving the user the impression of having pressed on a keyboard key. 
     In another example, illustrated in  FIG.  15   b   , a plurality of groups of actuators  31  and  32  are actuated simultaneously or alternately in order to generate tactile perceptions  61  and  62  of a pseudo-relief effect, i.e. one giving the user, who is moving his finger in contact with the interface, the impression of brushing against reliefs, for example the peak of a mountain of a map displayed on a screen. 
     In another example, illustrated in  FIG.  15   c   , groups of touch pixels  33  and  34  are activated simultaneously or alternately in order to generate tactile perceptions  63  and  64  of a texture effect, i.e. one giving the user, who is moving his finger in contact with the interface, the impression of touching the smooth surface  64  of the dial of a watch or the rougher surface  63  of its strap. 
     The user, by moving his finger, is able to perceive a variation in friction, which is also called the squeeze-film effect, due to the touch pixels  3 , which for example generate an ultrasonic mode or an evanescent wave when they are excited by the actuators  5 . 
     In the examples that have just been described, the touch pixels  3  of a given group may be activated in a vibrational mode that is identical for all the touch pixels of the group, or that is specific to each touch pixel, depending on the sought-after effect. 
     Touch pixels  3  equipped with piezoelectric actuators  5  may be produced by following the steps illustrated in  FIGS.  16  to  18    and described below. 
     Firstly, for example by screen-printing, a first layer  24  of an electrically conductive material, of gold for example, is deposited on the upper face of a stiff plate  22 , especially a plate made of glass. 
     The layer  24  is discontinuous: it for example comprises a plurality of separate sections  24   a  and  24   b  that form the electrical supply tracks of the actuators  5  that will make contact with the touch pixels  3 . 
     The thickness of the plate  22  is for example 500 microns. The thickness of the layer  24  is for example 300 nm. 
     Next, piezoelectric actuators  5  comprising, for example, a lower electrode  52 , a ferroelectric or piezoelectric layer  54  and an upper electrode  56 , are transferred. As a variant, the actuators  5  comprise only the layer  54 . 
     The layer  54  that is located between the two electrodes is for example made of lead zirconate titanate (PZT). It may also be made of aluminum nitride (AlN), of zinc oxide (ZnO) or of any other suitable ferroelectric or piezoelectric material. The layer  54  may furthermore be thinned and adjusted to the desired thickness. 
     It is especially possible to use a commercially available piezoelectric ceramic, or to form the actuator  5  by depositing thin layers and patterning them on the plate  22 , as described in patent FR3082997. 
     As illustrated in  FIG.  16   , the lower electrode  52  of the piezoelectric actuators  5  may be connected to the track  24  by virtue of a layer of conductive adhesive  51 , a silver paste for example, for example of about 40 μm in thickness. The deformation of the piezoelectric layer  54  under the effect of a potential difference applied between the electrodes is transmitted to the plate  22  via a unimorphic effect in the layer  51 . 
     Next, each actuator  5  is covered with an electrically insulating layer  58 , which is for example made of a polymer, in order to keep the actuator  5  in place and to insulate the electrodes  52  and  56 . 
     Only one portion of the supply track  24   a  remains uncovered, as illustrated in  FIG.  17   . 
     Next, the insulating layer  58  is thinned from above in order to uncover the electrode  56 , with a view to depositing, by inkjet printing or another suitable method, a layer of conductive material  26  to connect the upper electrode  56  with the supply track  24   a , as illustrated in  FIG.  18   . 
     As a variant, the insulating layer  58  and the piezoelectric layer  54  may be thinned to the desired thickness, then a layer of conductive material  26  forming both the upper electrode and the connection to the track  24   a  may be deposited. Where appropriate, the layer  26  is obtained using a stencil and sputtering gold, and its thickness is for example 300 nm. 
     The plate  22  may then be diced in order to obtain touch pixels  3  of the desired size, and equipped with the intended number of actuators  5 . 
     The touch pixels  3  thus obtained may subsequently be transferred and fastened to a flexible substrate  4  with a view to forming a haptic structure  2 , by following for example the production steps described below with reference to  FIGS.  19  to  22   . In particular, a haptic structure such as that described above with reference to  FIG.  4    may be obtained by following the following steps. 
     Firstly, the touch pixels  3  are placed in the desired locations on a manufacturing carrier  100 , for example a silicone plate, on which a flexible layer  42 , for example of SINR polymer, which is intended for example to form the aforementioned covering layer, will have been formed beforehand, especially by lamination. 
     The touch pixels  3  are for example held in place by virtue of a layer  44  of stiff adhesive, an epoxy adhesive for example, for example of a thickness of 40 microns, as illustrated in  FIG.  19   . 
     Subsequently, a layer of conductive material  46 , for example a layer of gold of 300 nm thickness obtained by masking and sputtering, is deposited in order to form the electrical connection between each actuator  5  and the control circuit  6 , such as illustrated in  FIG.  20   . 
     An insulating layer  45 , such as for example a bead of polymer, may have been deposited around the touch pixels  3  beforehand. 
     Next, for example by rolling or adhesive bonding, all of the touch pixels  3  and their actuators  5  are continuously covered with a layer  42 , especially a film of soft polymer, in order to form flexible bridges between the touch pixels  3 , as illustrated in  FIG.  21   . The layer  42  is intended to form the aforementioned receiving layer. 
     As a variant, the touch pixels  3  may be coated with a soft polymer. 
     Lastly, the manufacturing carrier is removed by peeling in order to uncover the covering layer  43 , the uncovered surface defining the touch surface S of the flexible haptic interface. The touch surface may also, as a variant, be defined by the upper surface  2   a  of the haptic structure. 
     The interface  1  according to the invention may be integrated into an article of clothing, a glove for example, as illustrated in  FIG.  23     a.    
     It may as a variant be integrated into a mobile device, a flexible telephone for example, as illustrated in  FIG.  23     b.    
     The haptic structure  2  may also, in other examples, change shape depending on the sought-after haptic effect, the interface  1  possibly, on account of its flexibility, being a reconfigurable tangible object, as illustrated in  FIG.  24   . In this figure, it may be seen that the interface may be mechanically coupled to one or more actuators  11  that allow it to be deformed in order to give it a predefined shape, corresponding to that of the object that it is sought to simulate tactilely.