Patent Publication Number: US-2023162578-A1

Title: Haptic interface with flexible hinges

Description:
TECHNICAL FIELD 
     The present invention concerns human-machine interfaces and more particularly those producing haptic effects. 
     PRIOR ART 
     A haptic interface enables the user to interact with the environment through the sense of touch. Today the haptic effect is increasingly used in numerous applications, for example on a smartphone when the latter generates a slight vibration when a key displayed on the screen is pressed to simulate the impression of pressing a button. Haptic applications also exist for virtual or augmented reality devices, in particular to enable greater immersion in video games. Haptic interfaces such as tactile pads, generating a haptic effect on a tactile surface when a user contacts it, can in particular be integrated into numerous technologies such as computers, tablets and/or smartphones. 
     It is known to generate varied and relatively complex haptic effects on a rigid tactile surface thanks to ultrasound transducers emitting ultrasound waves that propagate on the rigid surface. 
     With the introduction to the market of flexible commercial products such as foldable mobile telephones and roll-up televisions, it is desirable to have flexible haptic interfaces available that can be integrated into such products. 
     The use of a simple flexible support carrying ultrasound transducers is however limited because the waves emitted by the transducers are in part absorbed by the flexible materials; the haptic effect generated is then difficult for a user to perceive on applying pressure with their finger or their hand on such a support. 
     It is moreover known to use pneumatic actuation to generate haptic effects on a flexible structure. The patent U.S. Ser. No. 10/240,688B2 discloses a flexible haptic interface including pneumatic actuators for generating a topological effect. 
     The paper by Yu et al. “Skin-integrated wireless haptic interfaces for virtual and augmented reality” (Nature, 2019) discloses a flexible haptic interface including magnetic actuators for generating diverse haptic effects. However, the spatial resolution of the haptic effects obtained with this kind of interface is relatively low and the use of magnetic actuators leads to high electrical energy consumption. 
     Finally, non-Newtonian fluids have already been used to generate simple haptic effects such as button effects and relief effects. The paper by Lochtefeld “Towards real organic user interfaces—using non-Newtonian fluids for self-actuated displays” (CHI13 workshop, 2013) describes a haptic device using a rheothickening non-Newtonian fluid between two flexible films placed on a fixed network of loudspeakers. A topology that can be perceived by the finger is generated when a soundwave propagates in the fluid and hardens it locally. 
     STATEMENT OF INVENTION 
     There exists a need for further improvement of flexible haptic interfaces, in particular in order to obtain a tactile interface capable of generating varied haptic effects and having good spatial resolution over the whole of the interface. 
     SUMMARY OF THE INVENTION 
     The invention aims to respond to this need, and achieves this, in accordance with a first of its aspects, thanks to a flexible haptic interface defining a tactile surface able to be contacted by a user, the interface including:
         a plurality of rigid tactile elements connected by a flexible support forming at least one hinge extending between at least two adjacent elements, and   in contact with each element, at least one actuator, the actuators being arranged to transmit a mechanical excitation to the rigid tactile elements in order to drive in vibration the hinge situated between them with a vibration amplitude detectable by touch.       

     By “detectable by touch” must be understood that the amplitude of vibration of the hinge exceeds an amplitude of the order of 1 micron under a load comprised between 0.05 and 0.5 N. 
     By “transmitting a mechanical excitation to the elements” is meant inducing a vibration of the elements with an amplitude detectable by touch, the amplitude of vibration of the elements possibly being different from, in particular lower than, that of the hinge. 
     Thanks to the invention, because the vibration of the hinge participates, in addition to that of the excited rigid tactile elements, in the haptic effect generated by the haptic interface, it is possible to obtain haptic effects having a relatively good spatial resolution. 
     The benefit is therefore obtained of a haptic interface having some flexibility, whilst offering varied haptic effects generated with good spatial resolution, in particular with no significant discontinuity between the excited elements, suitable for numerous applications. 
     The interface preferably includes a control circuit configured to modulate the signals sent to the actuators in order to mechanically induce a vibration of the elements and of the hinges and to generate a haptic sensation on the tactile surface. 
     The vibration of the elements and of the hinges can generate a variation of friction detectable by touch by a user moving their finger over the tactile surface. This effect, known as the squeeze-film effect, can give the user the impression of touching a surface including reliefs or having different textures. 
     The vibration of the elements and of the hinges may further generate a pulse detectable by touch by a user exerting a static contact on the tactile surface. This effect for example makes it possible to give the user the impression of pressing a button. 
     Rigid Tactile Elements 
     The invention is not limited to the excitation of individual tactile elements of a particular material or having a particular shape. However, certain shapes and certain materials can facilitate the manufacture of the interface. 
     Accordingly, at least some of the elements, better still all the elements, may have a polyhedral, preferably parallelepipedal, general shape. The elements may in particular takes the shape of rectangular, in particular square, blades when seen from the front. 
     At least some of the elements, better still all the elements, are preferably made of a relatively hard and widely used material, such as glass, preferably borosilicate glass. 
     At least some of the elements, better still all the elements, preferably have a substantially constant thickness, preferably comprised between 50 microns and 5 mm, better still comprised between 200 and 700 microns. 
     At least some of the elements, better still all the elements, may have a surface area of at least 1 cm 2 , for example an upper surface with dimensions of at least 10 mm by at least 15 mm, when the elements have a rectangular blade shape as seen from the front. 
     All the elements may be identical. Alternatively, there may be combined, in the same interface, individual tactile elements of different sizes, as a function for example of their location relative to the tactile surface and/or of the application. 
     Support and Hinges 
     By “support” is meant any single-layer or multilayer structure that holds the rigid tactile elements in position and assures their cohesion in the haptic interface. The support is preferably formed by one or more polymer materials, which can facilitate manufacturing and obtaining the desired flexibility of the haptic interface, whilst generating a deformation with the desired amplitude at the level of the hinges. 
     By “hinge” is meant the flexible portion connecting two adjacent tactile elements together. The hinges may include one or more layers depending on the structure of the support, the layers being mechanically coupled in vibration in the case of a multilayer hinge. 
     The support may include polycarbonate (PC), polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN), polyetheretherketone (PEEK), KAPTON or any other appropriate material. 
     The thickness of the support may be chosen as a function of its flexibility, the softer the support the greater its thickness can be whilst enabling the desired flexibility to be preserved. 
     The support is preferably of substantially constant thickness at the level of the hinges, the thickness preferably being comprised between 100 and 800 microns. 
     The support may have an equivalent Young&#39;s modulus comprised between 0.5 and 9 GPa, better still comprised between 2 and 5 GPa. 
     By “equivalent Young&#39;s modulus” is meant, for a multilayer support, the mean value of the Young&#39;s moduli of the various layers weighted by the thicknesses of said layers. 
     The tactile elements are advantageously made of a material that is more rigid than that or those of the support. The ratio of the Young&#39;s modulus of each tactile element to that of the support is for example greater than or equal to 8, better still greater than or equal to 20, even better still greater than or equal to 50, more preferably comprised between 80 and 90, in particular equal to approximately 85. 
     The tactile elements may be integrated into the support in various ways, depending on the structure of the support. 
     The support may in particular include a supporting layer and a layer for compensating the thickness of the elements. This compensation layer extends between the elements on top of the supporting layer. 
     When interconnected at their interface between two adjacent elements the compensation layer and the supporting layer form multilayer hinges, in particular dual-layer hinges, between those elements. 
     The supporting layer may also serve as a layer protecting the tactile elements and/or the actuators with which the latter are provided. 
     The supporting layer preferably has a substantially constant thickness, preferably comprised between 50 nm and 500 microns, better still comprised between 25 and 80 microns, for example 50 μm. 
     The supporting layer may on its own provide the strength of the tactile elements within the haptic structure. The thickness compensation layer may also have this function, in combination with the supporting layer. 
     The tactile elements may all be situated on the same side of the supporting layer. 
     The compensation layer is preferably of substantially constant thickness, preferably comprised between 100 microns and 5 cm, preferably comprised between 500 μm and 5 mm, for example 1 mm. 
     At least some of the tactile elements, preferably all the tactile elements, may have an exterior surface leading onto the tactile surface. 
     This exterior surface of the tactile elements can be flush with the exterior surface of the compensation layer, the exterior surface of the elements and that of the compensation layer then defining a substantially smooth surface that can serve as a tactile surface for the interface. 
     The user then comes directly into contact with the tactile elements when they place their finger in contact with the tactile surface. 
     Alternatively the tactile elements and the compensation layer can be covered with a protection layer that may or may not be identical with the supporting layer of the support. 
     In this way it is possible to obtain hinges including three layers when the vibration of the protection layer is mechanically coupled to that of the underlying layers. 
     The thickness of the protection layer is preferably relatively small so as not to unduly affect the sensation of the tactile effect at the level of the elements. 
     The support may include housings in which the tactile elements at least partially extend, the latter elements being for example continuously covered by a flexible film defining the tactile surface. 
     In one embodiment the support includes a tactile element receiving layer, the receiving layer forming cup-shape housings, the support also including a layer covering the elements defined for example by the aforementioned flexible film. 
     In this embodiment it is considered that the covering layer forms part of the hinges when its vibration is coupled to that of the underlying receiving layer in the regions between the tactile elements. 
     The covering layer in particular makes it possible for the outside of the tactile surface to be uniform and to prevent spurious tactile sensations that could otherwise arise if the user were in direct contact with the elements and/or with the adjoining regions, the latter elements possibly forming a heterogeneous surface when touched. 
     Arrangement of the Tactile Elements 
     The tactile elements may be distributed in various ways on the support, depending in particular on the application. 
     The spacing between two adjacent elements may be chosen as a function of the flexibility of the support and of its thickness, so as to obtain the desired vibration amplitude for the hinge whilst preserving the overall flexibility. 
     The softer the support and/or the smaller its thickness, the closer together the elements may be whilst preserving an amplitude of vibration of the hinge sufficient to be detectable by touch. 
     The spacing between two adjacent elements (as measured edge to edge) is preferably comprised between 1 and 10 mm, better still comprised between 3 and 8 mm. 
     The support preferably has an equivalent Young&#39;s modulus comprised between 0.5 GPa and 8.5 GPa, and the ratio e c /L c  of the thickness e c  at the level of the hinges to the spacing L c  between the two adjacent elements is preferably comprised between 0.023 and 0.128. This makes it possible to obtain good results for the haptic interface. 
     In particular, the support may have an equivalent Young&#39;s modulus comprised between 0.5 GPa and 5 GPa, in particular of 2.2 GPa plus or minus 20%, better still of 2.2 GPa plus or minus 10%, and the ratio e c /L c  of the thickness e c  of the support at the level of the hinges to the spacing L c  between two adjacent elements may be comprised between 0.03 and 0.08. 
     Alternatively, the ratio e c /L c  of the thickness e c  of the support at the level of the hinges and the spacing L c  between two adjacent elements may be comprised between 0.03 and 0.1, in particular equal to 0.05 plus or minus 20%, better still 0.05 plus or minus 10%, the equivalent Young&#39;s modulus being comprised between 0.8 and 8.5 GPa. 
     At least some of the elements may be arranged in lines and/or in columns, in particular on the same plane when the support is laid flat, preferably in a regular array. 
     Alternatively, the elements are disposed in accordance with a concentric or other distribution. 
     Actuators 
     Each tactile element may be provided with at least one actuator extending over at least one face of the element, for example the face situated on the side of the support and/or its opposite face. The actuator may be disposed on a vibration antinode of the element. 
     Each actuator may be of piezoelectric, ferroelectric, electromagnetic or thermic type, preferably of piezoelectric type. In particular, each actuator may be a ceramic piezoelectric actuator. 
     The shape and the arrangement of the actuators are preferably chosen so as to obtain the desired mode or modes of vibration on the tactile element. 
     The actuator or actuators associated with a tactile element may each have an elongate shape, preferably in the direction of the width of the element when the latter has a rectangular shape. 
     The actuator or actuators may be disposed on respective opposite sides of a median plane of the tactile element. The actuator or actuators may be disposed at the level of vibration nodes or antinodes. 
     The actuators may be of diverse shapes; each has for example a polygonal, in particular rectangular or square, general shape, or a circular or annular shape. 
     A plurality of shapes of actuators may be combined on the same tactile element in order to be able to generate a plurality of different vibration modes, for example alternately, which makes it possible to obtain varied haptic effects. 
     The invention further has for object, independently of or in combination with the above, a haptic interface including at least one rigid tactile element, preferably of rectangular shape when seen from the front, including:
         a first actuator, preferably of annular shape, preferably disposed at the centre of the element and able to generate a first vibration mode on the tactile element, in particular a bending mode,   at least one actuator, preferably of elongate shape, in particular elongate in the direction of the width of the element if the latter is of rectangular shape, preferably disposed at opposite ends of the element, and able to generate on the element a second vibration mode different from the first mode, in particular a Lamb oscillatory mode.       

     Ancillary Systems 
     The interface may include a system for detecting contact of the user with the tactile surface, in particular a capacitive detection structure, which is if applicable integrated into the haptic structure. 
     The interface may include a system enabling at least partial superposition of an image on the tactile surface. The interface may therefore include a screen, preferably a screen integrating the haptic structure. 
     The interface may include at least one actuator enabling selective conformation of the interface in accordance with at least two distinct shapes. This may be useful for example for changing the shape of the interface as a function of the tactile sensation to be produced, for example to improve the quality of the simulation. 
     The invention further has for object an item of clothing equipped with a haptic interface in accordance with the invention as defined above. 
     The invention further has for object a mobile device equipped with an interface in accordance with the invention as defined above. 
     Method for Generating a Haptic Effect 
     The invention further has for object a method for generating a haptic effect with a flexible haptic interface as described above, in which method rigid tactile elements are excited mechanically thanks to the actuators disposed in contact with the elements, in order to drive in vibration the hinge situated between them with a vibration amplitude that is detectable by touch. 
     The features of the interface described hereinabove apply to the method in combination with one another or individually. 
     The method in accordance with the invention may include a step consisting in first detecting the position of the contact of a user on the haptic interface thanks to a detection system and then, thanks to a control circuit, modulating signals sent to the actuators as a function of the detected position in order to excite the elements. 
     It is therefore possible to generate the desired haptic effect in the detected contact zone. 
     Excitation of the Rigid Tactile Elements 
     The invention is not limited to any particular mechanical excitation of the rigid tactile elements. However, certain types of excitation, and in particular certain modes of vibration of the tactile elements, facilitate causing the hinge situated between them to vibrate and enable generation of haptic effects in a relatively reliable manner. 
     Accordingly, the rigid tactile elements may be mechanically excited in accordance with a first so-called “bending mode” vibration mode, in particular at a frequency going from 25 to 40 kHz, in particular around 33 kHz, so as to generate bending of the hinge situated between them. 
     Alternatively, the rigid tactile elements may be excited mechanically in accordance with a Lamb anti symmetrical mode, in particular at a frequency comprised between 55 and 100 kHz, for example approximately 65 kHz. 
     More generally, the rigid tactile elements are mechanically excited at a frequency preferably comprised between 20 and 100 kHz. 
     The invention further has for object a method for generating a haptic sensation with the aid of an interface of the kind defined hereinabove, in which method only some of the actuators are excited selectively at a given time so as to generate a predefined vibration mode, in particular a bending or a Lamb mode; the actuators may be excited alternately to go from a bending mode to a Lamb mode and vice-versa; the actuators may also be excited differently depending on the zones of the interface, when the interface includes a plurality of such rigid elements in different zones of the interface, so that in certain zones the vibratory mode is a bending mode and in others a Lamb mode. 
     Production of an Individual Tactile Element 
     An individual tactile element provided with piezoelectric-type actuators usable in an interface as defined above may be manufactured by the method including the steps consisting in:
         depositing, preferably by screenprinting, at least one first layer of a conductive material onto the upper face of a rigid support, preferably a glass support,   fixing piezoelectric actuators to the first layer formed in this way so as to establish a first electric connection with the actuators,   depositing a layer of an insulator on the layers previously deposited,   thinning the insulating layer in order to expose the upper face of each piezoelectric actuator,   depositing a layer of a conductive material on the actuators exposed in this way so as to establish a second electrical connection with the actuators,   cutting the support so as to obtain tactile elements each provided with one or more piezoelectric actuators.       

     This method in particular enables simple manufacture of a plurality of tactile elements based on the same support, for example a glass plate. 
     The tactile elements obtained in this way can then be mounted on a flexible support to form a haptic interface. 
     A haptic interface as defined above is manufactured for example by the method including the steps consisting in:
         depositing a first layer of flexible material, preferably of polymer, on a manufacturing support,   fixing tactile elements provided with one or more actuators onto the layer deposited in this way, the actuators being disposed on the exterior face of the elements,   electrically connecting each actuator to the control circuit, preferably by masking and spraying a conductive material,   covering the tactile elements and actuators with a flexible material, preferably a polymer film, and   separating, in particular peeling, the assembly produced in this way from the manufacturing support so as to expose the lower face of the first layer.       

     This first layer may constitute the aforementioned cover layer and the flexible material deposited on the elements may constitute the aforementioned receiving layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention might be better understood upon reading the following detailed description of non-limiting embodiments thereof and examining the appended drawings, in which: 
         FIG.  1    represents partially and schematically an example of a haptic interface in accordance with the invention, 
         FIG.  2    represents partially and schematically an example of a haptic interface including tactile elements arranged in a matrix, 
         FIG.  3    illustrates partially and schematically in perspective a haptic structure including two tactile elements connected by a portion of a flexible support forming a hinge, 
         FIG.  4   a    illustrates the causing of the hinge from  FIG.  3    to vibrate based on a bending mode of the tactile elements, 
         FIG.  4   b    illustrates the causing of the hinge from  FIG.  3    to vibrate based on a Lamb mode of the tactile elements, 
         FIG.  5    illustrates the possibility of generating two vibration modes on a tactile element provided with varied piezoelectric actuators, 
         FIG.  6    represents partially and schematically a flexible hinge situated between two elements and subjected at rest to a vertical force at its centre, 
         FIG.  7   a    illustrates partially and schematically an example of integration of the tactile elements into the interface, 
         FIG.  7   b    is a view analogous to  FIG.  7   a    of another example of the integration of the tactile elements, 
         FIG.  8    represents partially and schematically an example of a haptic interface equipped with an ancillary system, 
         FIG.  9    is a block schematic illustrating an example of the operation of the haptic interface in accordance with the invention, 
         FIG.  10   a    illustrates partially and schematically the possibility of generating a button tactile effect based on an interface in accordance with the invention, 
         FIG.  10   b    illustrates partially and schematically the possibility of generating a relief tactile effect based on an interface in accordance with the invention, 
         FIG.  10   c    illustrates partially and schematically the possibility of generating a texture tactile effect based on an interface in accordance with the invention, 
         FIG.  11   ,  FIG.  12    and  FIG.  13    illustrate partially and schematically successive steps of an example of a method for manufacturing tactile elements with their actuators, 
         FIG.  14   ,  FIG.  15   ,  FIG.  16    and  FIG.  17    illustrate partially and schematically successive steps of an example of a method for manufacturing a haptic interface, 
         FIG.  18   a    represents an example of an item of clothing equipped with a flexible haptic interface in accordance with the invention, 
         FIG.  18   b    illustrates an example of a mobile device equipped with a flexible haptic interface in accordance with the invention, and 
         FIG.  19    illustrates the possibility of modifying the shape of the interface by means of actuators. 
     
    
    
     DETAILED DESCRIPTION 
     There is shown in  FIG.  1    an example of a flexible haptic interface  1  usable for the execution of the method in accordance with the invention. The interface  1  includes a haptic structure  2  defining a tactile surface S adapted to be contacted by a user and on which a haptic effect can be generated. 
     The haptic structure  2  includes rigid tactile elements  3  carried by a flexible support  4 . The tactile elements  3  are for example made of glass, in particular borosilicate glass, or silicon. 
     The flexible support  4  forms hinges  40  that extend between the tactile elements  3 . 
     In the example considered here the tactile elements  3  are provided with a plurality of actuators  5 , preferably piezoelectric actuators, the latter being connected to a control circuit  6  by means of electrical connections  50 . 
     The control circuit  6  is configured to modulate the signals sent to the actuators  5  in order to induce vibrations of the latter mechanically and to excite the tactile elements  3  mechanically. 
     Excitation of two adjacent tactile elements  3  causes the hinge  40  situated between them to vibrate. 
     The disposition of the tactile elements  3  on the flexible support, and in particular the spacing L c  between two adjacent pixels, is chosen so that when the tactile elements  3  are excited mechanically by means of the actuators  5  the resulting amplitude of vibration of the hinges  40  is detectable by touch. 
     Thanks to the invention the hinges  40  and the tactile elements  3  may be acted upon at the same time to generate the required haptic effect, which, if required, makes it possible to obtain for the user in contact with the tactile surface S a substantially continuous tactile perception over the contact zone and/or of enhanced resolution. 
     The haptic structure  2  may be of any shape and of varied size. Its dimensions are for example, depending on the application, of the order of the cm, of the dm or of the meter. 
     The tactile elements  3  may have a varied shape as seen from the front, for example a rectangular, square, circular or other shape. Their greatest dimension is for example comprised between a few hundred microns and a few cm per side, preferably from 2 mm to 1 cm per side. 
     The tactile elements  3  may be distributed over the support  4  in accordance with diverse configurations. 
     For example  FIG.  2    shows identical tactile elements  3  distributed in lines and columns on the support  4  in a regular array. 
     In other embodiments (not shown) the interface  1  includes tactile elements of different sizes and/or shapes, or again tactile elements  3  disposed in accordance with a concentric distribution or a quincunx. 
     In particular the size of the tactile elements may be varied in order to generate haptic effects producing tactile simulation of larger or smaller patterns. 
     The tactile elements  3  are disposed on the support  4  so that the haptic structure  2  preserves an overall flexibility in at least one direction, and better still in all directions. 
     Nevertheless, as mentioned above, this disposition is subject to certain constraints to obtain effect perceptible by the user by touch. It is considered that vibration of the hinges or of the tactile elements is perceptible by touch if its amplitude exceeds 1 micron under a load comprised between 0.05 and 0.5 N. 
     The amplitude of vibration of a tactile element  3  depends, inter alia, on its dimensions, on its stiffness and on the vibration mode in which it is activated. 
     The amplitude of vibration of a flexible hinge  40  situated between two excited tactile elements depends for example on its thickness e c , on the characteristics of the material or materials constituting the flexible support  4  at the level of that hinge, and on the spacing L c  between the two elements. 
     There will now be described with reference to  FIGS.  3  to  7     a  and  7   b  an example of the dimensioning of a flexible hinge  40  situated between two rigid tactile elements  3 . 
     As shown in  FIG.  3   , for this example there are considered two tactile elements  3  carried by a flexible support  4 , the hinge being formed by the portion of the support that extends between the elements  3 . 
     The tactile elements  3  are identical, for example made of glass and of rectangular blade shape of width Width of approximately 10 mm, length Length of approximately 15 mm, and thickness e p  of approximately 500 microns. 
     The flexible support  4  is for example a polymer film of thickness e c , disposed on the elements  3  and forming between them a flexible hinge  40  of width L c  and of length approximately equal to the width Width of the elements  3 . 
     The support  4  is for example a polymer of mass per unit volume p, Young&#39;s modulus E and Poisson&#39;s coefficient v. 
     The tactile elements  3  are each provided with one or more actuators  5 , as can be seen in  FIG.  7   a   . Thanks to the actuators  5  the tactile elements  3  can be made to vibrate in vibration modes adapted to generate haptic effects at the level thereof. 
     The tactile elements  3  are excited for example in a first, so-called “bending mode”, vibration mode, for example at a frequency of 33 kHz. The tactile elements  3  excited in this way can then drive bending of the flexible hinge  40 , as shown in  FIG.  4   a   . In this way there is obtained a surface S of the haptic structure  2  vibrating all over, that is to say at the level of the hinges and of the tactile elements  3 . 
     Alternatively, the tactile elements  3  are excited in a Lamb antisymmetric mode, for example a Lamb mode at 65 kHz, driving the flexible hinge  40  in vibration with an amplitude greater than that of the vibration of the tactile elements  3 , as shown in  FIG.  4     b.    
     The vibration modes such as those that have just been described can be obtained using piezoelectric actuators, for example lead zirconate titanate (PZT) actuators. 
     The dimensions of the actuators and their disposition on the tactile elements  3  can be adapted by means of numerical simulations, for example using finite element type methods on commercial software such as COMSOL or ANSYS. 
     There may also be employed the method described in the paper by Casset, F. et al. “Low voltage actuated plate for haptic applications with PZT thin-film” (Proceedings of Transducers, 2013). 
     For example some actuators  5  may be disposed on a tactile element  3  so as to be able to generate a plurality of vibration modes, for example the aforementioned two vibration modes, namely a bending mode M 1  and a Lamb mode M 2  as shown in  FIG.  5   . 
     In this example the tactile element  3  is provided with a central actuator  500  of circular shape and two actuators  550  of elongate shape at its ends, the latter extending in the direction of the width Width of the actuator. 
     The actuators  500  and  550  may be energised in turn, for example by a potential difference of 50 V, in order to exploit a certain vibration mode. 
     The circular actuator  500  makes it possible for example to use the bending mode of the tactile element  3 , while the actuators  550  generate the Lamb mode, for example at a frequency of 50.7 kHz. 
     These two vibration modes enable a displacement of at least 1 micron at the centre of the tactile elements  3 . 
     Of course, other actuator configurations are possible for obtaining the desired vibration modes. Rectangular actuators are preferably used to obtain a Lamb mode and square and/or circular actuators to obtain an out-of-plane mode such as the bending mode. 
     The support  4  being preferably made of a polymer material, that is to say is much more flexible than the tactile elements  3 , a vibration amplitude of the hinges  40  is naturally obtained that is higher than that of the tactile elements  3  that are rigid. 
     Nevertheless, for the vibrations to be perceptible by touch, the hinges  40  must have some rigidity so as to be able to be deformed even when they are subjected to a force exerted by the user at the moment of contact. 
     To determine the material and the dimensions of the hinges enabling an effect of this kind, a static analysis is effected for example with the hinge considered as an elastic beam built in at both ends subject to a vertical force F, as shown in  FIG.  6    for the system described above. 
     According to beam theory, the deformation equation of the hinge is: 
         FL   c   3 =192 E I   gz   f    
     where 
     
       
         
           
             
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     is the quadratic moment of the hinge about the deformation axis, Width being the length of the hinge, e c  its thickness and L c  its width. E is the Young&#39;s modulus of the hinge and f the deformation at its centre. 
     In this example it is considered that the force F applied by the finger to the hinge is comprised between 0.05 N and 0.5 N, the tactile detection threshold for a finger exerting a normal force on a tactile surface being approximately 0.05 N, as described in the paper by King et al. “Perceptual Thresholds for Single vs. Multi-Finger Haptic Interaction” (2010 IEEE Haptics Symposium). 
     Thus the deformation equation may be rewritten as follows: 
     
       
         
           
             
               
                 
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     For example, f&gt;3 μm, that is to say more than three times the minimum deformation detectable by touch, and Width=10 mm. 
     Using the above equation, it is possible to determine the remaining parameters of interest, namely the Young&#39;s modulus of the hinge and its dimensions L c  and e c , in various ways. 
     For example the material to be used is imposed and a calculation rule is obtained for the ratio 
     
       
         
           
             
               
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     For example an equivalent Young&#39;s modulus E comprised between 0.5 GPa and 8.5 GPa is set so as to encompass a plurality of polymers.
 
From the deformation equation established hereinabove, it is possible to obtain the minimum and maximum values of the ratios
 
     
       
         
           
             
               
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                 c 
               
             
             , 
           
         
       
     
     that is to say those corresponding to a normal force of 0.05 N to 0.5 N, respectively, as summarised in the table below. 
                             TABLE 1                 E (GPa)               e   c       L   c       ⁢         for         0.05       N                       e   c       L   c       ⁢         for         0.5       N                                                    0.5   0.059   0.128       1   0.047   0.101       2.5   0.035   0.075       8   0.024   0.051       8.5   0.023   0.05                    
Thus the overall minimum and maximum values of the ratio e c /L c  can be obtained for an equivalent Young&#39;s modulus E comprised between 0.5 GPa and 8.5 GPa:
 
     
       
         
           
             
               
                 0 
                 . 
                 0 
               
               ⁢ 
               2 
               ⁢ 
               3 
             
             &lt; 
             
               
                 e 
                 c 
               
               
                 L 
                 c 
               
             
             &lt; 
             
               
                 0 
                 . 
                 1 
               
               ⁢ 
               2 
               ⁢ 
               8 
             
           
         
       
     
     In particular, for a support the equivalent Young&#39;s modulus of which is equal to 0.5 GPa, for example polyethylene (PE), the ratio e c /L c  is comprised between 0.059 and 0.128, which for some hinge lengths makes it possible to determine the minimum thickness value e c     min    and the maximum thickness value e c     max   . The table below summarises such values for L c  comprised between 1 and 7 mm. 
                             TABLE 2               L c  (mm)   e c     min    (μm)   e c     max    (μm)                                            1   59   128       2   119   255       3   178   383       4   237   511       5   296   639       6   356   766       7   415   895                    
For a support with an equivalent Young&#39;s modulus of 8.5 GPa, for example a composite material consisting of a carbon or glass fibre reinforced polymer, the ratio e c /L c  is, as indicated in the first table, comprised between 0.023 and 0.050. The following values for the minimum and maximum thickness of the hinge are obtained for L c  comprised between 2 and 8 mm:
 
                             TABLE 3               Lc (mm)   e c     min    (μm)   e c     max    (μm)                                            2   46   99       3   69   149       4   92   199       5   115   248       6   138   298       7   161   348       8   184   397       9   207   447       10   231   497                    
For a support the equivalent Young&#39;s modulus of which is equal to 2.5 GPa, for example a polycarbonate (PC) support, the ratio
 
     
       
         
           
             
               e 
               c 
             
             
               L 
               c 
             
           
         
       
     
     is comprised between 0.035 and 0.075 which yields the values summarised in the table below for the minimum and maximum thickness of the hinge with L c  comprised between 1 and 10 mm. 
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 L c  (mm) 
                 e c     min    (μm) 
                 e c     max    (μm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 35 
                 75 
               
               
                 2 
                 69 
                 149 
               
               
                 3 
                 104 
                 224 
               
               
                 4 
                 139 
                 299 
               
               
                 5 
                 173 
                 373 
               
               
                 6 
                 208 
                 448 
               
               
                 7 
                 243 
                 523 
               
               
                 8 
                 277 
                 598 
               
               
                 9 
                 312 
                 672 
               
               
                 10 
                 347 
                 747 
               
               
                   
               
            
           
         
       
     
     Other values for the dimensions of the hinge and/or its Young&#39;s modulus make it possible to obtain the sought haptic effect. For example, for a length L c  comprised between 3 and 8 mm and a thickness e c  comprised between 100 and 700 microns, a polyethyleneterephthalate (PET) type polymer film may be used, for example a Melinex® or Mylar® film, having a Young&#39;s modulus of approximately 3.5 GPa, or a polyethylenenaphthalate (PEN) type polymer film with a Young&#39;s modulus of approximately 5 GPa. 
     A tactile element  3  can be integrated into the structure  2  in ways other than that which has just been described. 
     In the example shown in  FIG.  7   a    the support includes a supporting film  410  that may serve as a protection layer and a layer  400  for compensation of the thickness of the elements  3 . 
     The tactile elements  3  are carried by the film  410  and their entire thickness is embedded in the compensation layer  400 . 
     In the example considered here the flexible hinges  40  are dual layer hinges; they include the layer portions  400  and  410  situated in the regions between two adjacent elements  3 , said layers being mechanically coupled in vibration when the elements  3  are excited. The hinges  40  for example have a thickness e c  less than or equal to 800 microns. 
     The tactile elements  3  have an exterior surface  3   a  leading onto the tactile surface S. As shown, the hinges  40  may have an exterior surface  40   a  that comes to be flush with the exterior surface  3   a  of the tactile elements, to form a smooth surface with the latter that is substantially plane when the haptic structure  2  is laid flat. The compensation layer  400  may have substantially the same thickness as the tactile elements  3 . 
     The user is able to touch the exterior surface  3   a  of the tactile elements  3  and/or the surface  40   a  of the hinges  40  of the interface directly on placing their finger in contact with the tactile surface S. 
     Alternatively, it is possible to cover the tactile elements  3  and the hinges  40  with a thin protection layer (not represented). This makes it possible to render the tactile surface uniform and to prevent the user feeling unwanted texture effect caused by the difference between the material of the elements and that of the hinges  40 . 
     In the example shown in  FIG.  7   a    the actuators  5  are preferably situated on the face  3   b  of the tactile elements  3  situated on the side of the supporting film  410 , as shown. The tactile elements  3  provided with the actuators  5  are for example fixed to the support  4  using an insulative glue  55 . 
     In the variant shown in  FIG.  7   b    the support  4  includes a layer  420  to receive the elements  3  and a covering layer  430 . 
     The receiving layer  420  forms housings  425  in the form of cups, each of which receives a tactile element  3 . The covering layer  430 , for example a flexible polymer film, covers the receiving layer  420  and the tactile elements  3  continuously, thus closing the housing  425  at the top. 
     The covering layer  430  also serves as a film protecting the tactile elements  3  for example. 
     In the example considered here, the free face  430   a  of the covering layer  43  defines the tactile surface S. The covering layer  430  is sufficiently thin to propagate the vibratory effect of the tactile elements  3  to the tactile surface S. 
     In this example the hinges  40  are multilayer hinges; they are defined as the portions of the support  4  situated between the elements  3  and including the layers  420  and  430 . The hinges  40  have for example a thickness e c  less than 800 microns. 
     The covering layer  430  has for example a thickness comprised between a few microns and few tens of microns. 
     In this variant the actuators  5  may be situated anywhere on the lower face of the element  3 , that is to say between the tactile element  3  and the bottom of the housing  425  in the receiving layer  420 , and/or the upper face of the tactile element  3 , that is to say between the tactile element  3  and the covering layer  430 . 
     When the actuators  5  are present on the opposite faces of an element  3 , their excitation is preferably controlled in such a manner as to have the benefit of constructive vibratory interference between them. 
     The interface in accordance with the invention may further include, as shown in  FIG.  8   , a detection system  7 , for example a capacitive system known in the prior art, enabling the detection of at least one point of contact of the finger of the user with the surface S. 
     The detection system has been schematically represented in  FIG.  8    in the form of a member covering the tactile elements  3  and the support  4 , but it may form part of the support  4  or be integrated into the tactile elements  3 . 
     The interface  1  may further include one or more ancillary devices for enhancing the experience of the user, in particular by making it a multisensory experience. 
     Thus the interface  1  may include a flexible screen (not represented) which makes it possible to superpose an image on the tactile surface S and for example to give the user the impression of virtually touching what they observe on the screen. The interface  1  may further include one or more loudspeakers (not represented) to add an audio effect. 
     A haptic interface  1  in accordance with the invention can function in accordance with the steps shown in  FIG.  9   . 
     In the step  81  the interface is on standby, in the absence of a user in contact with the tactile surface S, and the tactile elements  3  are at rest. 
     In the step  82  the detection 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 haptic effect or effects must be generated. 
     In the step  83  the control circuit  6  sends the appropriate control signals to actuate at least some of the actuators  5  so as to generate the desired haptic effect. 
     In the step  84  the actuators  5  concerned contract or expand by virtue of a reverse piezoelectric effect in accordance with an actuation amplitude corresponding to the signal received, generating by unimorph effect a vibration of the rigid element  3  with which they are in contact. The hinges  40  of the flexible support  4  situated between two excited tactile elements  3  are driven in vibration in turn. 
     In the step  85  the vibration of the tactile element or elements induced by the actuators and that of the hinges situated between two activated actuators are perceived by touch by the user on the surface S. 
     In the step  86  the user breaks contact with the tactile surface S and the interface may return to standby, for example if energy saving is desired. 
     The perception by touch generated in the step  85  comes from feeling vibration of the tactile elements  3  and the hinges  40 , which can give the user the impression of touching a vibrating surface or a surface with a relief. 
     By actuating more or fewer tactile elements  3  it is possible to create relatively complex effects. Some are described hereinafter and shown in  FIGS.  10   a ,  10   b    and  10   c.    
     There is considered in the following examples a matrix  10  of identical tactile elements  3  of circular shape distributed uniformly over the matrix  10 , in a quincunx. The tactile elements  3  each include an actuator  5  placed at their centre. 
     In the example shown in  FIG.  10   a    a group  30  of tactile elements  3  belonging to the matrix  10  is actuated by the control circuit  6 . The whole of the group  30 , including the tactile elements  3  and the hinges  40 , is caused to vibrate so as to transmit to a user U exerting a static contact on the surface a pulse corresponding to a tactile perception  60  of a “button effect”, that is to say giving the user the impression of pressing a keyboard key. 
     In another example shown in  FIG.  10   b    a plurality of groups  31  and  32  including tactile elements  3  and hinges  40  are caused to vibrate simultaneously or alternately in order to generate tactile perceptions  61  and  62  of a pseudo-relief effect, that is to say giving the user moving their finger in contact with the interface the impression of brushing reliefs, for example the peak of a mountain on a map displayed on a screen. 
     In another example shown in  FIG.  10   c    groups  33  and  34  of tactile elements  3  and the corresponding hinges  40  are excited simultaneously or alternately in order to generate tactile perceptions  63  and  64  of a texture effect, that is to say giving the user moving their 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 watch strap. 
     By moving their finger, the user is able to perceive a variation of friction, known as “squeeze-film effect”, caused by the tactile elements  3  that for example generate an ultrasound mode or an evanescent wave when they are excited by the actuators  5 . 
     In the examples that have just been described the tactile elements  3  of the same group may be activated with a vibration mode that is identical for all the tactile elements of the group or specific to each tactile element, according to the required effect. 
     The deformation of the hinges situated between the activated elements  3  depends on the vibration modes chosen. 
     Tactile elements  3  provided with piezoelectric actuators  5  may be produced by the steps shown in  FIGS.  11  to  14    and described hereinafter. 
     First there is deposited, for example by screenprinting, a first layer  24  of an electrically conductive material, for example gold, on the upper face of a rigid plate  22 , in particular a glass plate. 
     The layer  24  is discontinuous: it includes for example a plurality of separate sections  24   a  and  24   b  that form the electrical power supply tracks of the actuators  5  that will come into contact with the tactile elements  3 . 
     The thickness of the plate  22  is for example 500 microns. The thickness of the layer  24  is for example 300 nm. 
     There are then added piezoelectric actuators  5  including for example a lower electrode  52 , a piezoelectric or ferroelectric layer  54  and an upper electrode  56 . Alternatively, the actuators  5  include only the layer  54 . 
     The layer  54  that is situated between the two electrodes is for example of lead zirconate titanate (PZT) type. It may also consist of aluminium nitride (AIN), zinc oxide (ZnO) or any other piezoelectric or ferroelectric material that is appropriate. The layer  54  may furthermore be thinned and adjusted to the desired thickness. 
     In particular a commercial piezoelectric ceramic may be used, or the actuator  5  may be formed by depositing thin layers and shaping them on the plate  22 , as described in the patent FR3082997. 
     As shown in  FIG.  12   , the lower electrode  52  of the piezoelectric actuators  5  may be connected to the track  24  by means of a layer  51  of conductive glue, for example silver paste, for example approximately 40 μm thick. The deformation of the piezoelectric layer  54  by the effect of a potential difference applied between the electrodes is transmitted to the plate  22  by unimorphic effect in the layer  51 . 
     Each actuator  5  is then covered with an electrical insulation layer  58 , for example made of a polymer material, in order to retain the actuator  5  and to insulate the electrodes  52  and  56 . 
     Only a portion of the power supply track  24   a  remains uncovered, as shown in  FIG.  12   . 
     The insulating layer  58  is then thinned from the top in order to uncover the electrode  56  in order to deposit by inkjet printing or some other appropriate method a layer of conductive material  26  connecting the upper electrode  56  with the power supply track  24   a  as shown in  FIG.  13   . 
     Alternatively, the insulating layer  58  and the piezoelectric layer  54  may be thinned to the desired thickness and then a layer of conductive material  26  forming both the upper electrode and the connection to the track  24   a  deposited. If necessary the layer  26  is obtained by using a stencil and spraying gold and its thickness is for example 300 nm. 
     The plate  22  can then be cut in order to obtain tactile elements  3  of the desired size and provided with the intended number of actuators  5 . 
     The tactile elements  3  obtained in this way may then be mounted on and fixed to a flexible substrate  4  to form a haptic structure  2 , for example by the production steps described hereinafter with reference to  FIGS.  14  to  17   . In particular, a haptic structure such as that described above with reference to  FIG.  7   b    may be obtained by the following steps. 
     First the tactile elements  3  are placed at the desired locations on a manufacturing support  100 , for example a silicone plate, on which will have been formed beforehand a flexible layer  42 , for example of SINK polymer, in particular by lamination, intended to form for example the aforementioned covering layer. 
     The tactile elements  3  are for example held in place by means of a layer  44  of rigid glue, for example of epoxy type, for example 40 microns thick, as shown in  FIG.  14   . 
     A layer  46  of conductive material is then deposited, for example a layer of gold 300 nm thick obtained by masking and spraying, in order to form the electrical connectors between each actuator  5  and the control circuit  6 , as shown in  FIG.  15   . 
     An insulating layer  45 , for example a bead of polymer, may be deposited around the tactile elements  3  beforehand. 
     The combination of the tactile elements  3  and their actuators  5  is then continuously covered, for example by lamination or by gluing, by a layer  42 , in particular a soft polymer film, in order to form flexible bridges between the tactile elements  3 , as shown in  FIG.  16   . The layer  42  is intended to form the aforementioned receiving layer. 
     Alternatively, the tactile elements  3  may be coated with a soft polymer. 
     Finally the manufacturing support is peeled away in order to expose the covering layer  43 , the exposed surface defining the tactile surface S of the flexible haptic interface. The tactile surface may equally, as an alternative, be defined by the upper surface  2   a  of the haptic structure. 
     The interface  1  in accordance with the invention may be integrated into an item of clothing, for example a glove, as shown in  FIG.  18     a.    
     It may alternatively be integrated into a mobile device, for example a flexible telephone, as shown in  FIG.  18     b.    
     In other examples the haptic structure  2  may change shape according to the desired haptic effect, the interface  1  given its flexibility being a reconfigurable tangible object, as shown in  FIG.  19   . In that figure it is seen that the interface may be mechanically coupled to one or more actuators  11  that enable it to be deformed to impart to it a predefined shape corresponding to that of the object touching which it is required to simulate.