FR3080698A1 - DEVICE FOR MAINTAINING A TOUCH SURFACE AND TACTILE INSTALLATION - Google Patents

Abstract

This device for holding a tactile surface (10, 12) by embedding comprises a holding frame (16) having an upper rim (16A) and a lower rim (16B), between which the tactile surface is intended to be embedded. (10, 12), damping means (18) arranged between an inner face of the upper rim (16A) and the recessed touch surface (10, 12), and means (20) for detecting a movement of the touch surface (10, 12), arranged between an inner face of the lower edge (16B) and the touch surface (10, 12) recessed. It further comprises at least one spacer (22) arranged: against a lower face of the damping means (18) and having a first free face (24A) in contact with the touch surface (10, 12), with a coefficient of substantially lower friction than this underside of the damping means (18); and against an upper face of the means (20) for detecting displacement and having a second free face (24B) in contact with the touch-sensitive surface (10, 12), with a coefficient of friction substantially lower than this upper face of the means (20 ) motion detection.

Description

The present invention relates to a device for holding in place a tactile surface. It also relates to a touch-sensitive installation comprising a touch-sensitive surface and a support for at least three support devices for embedding the touch-sensitive surface.

Numerous objects with a tactile surface are known, in particular mobile telephones or other portable devices for personal digital assistance. Their touch interface is usually a small, flat, rectangular screen with which a user can interact using a stylus or one or more of their fingers. Each of these objects implements a method of locating at least one touch using one or more detection techniques, such as the deployment of a capacitive film on the surface to be made tactile or the laying of an infrared frame around this surface. In the first case, illustrated for example by patent application WO 2011/098854 A1, a plastic film in which is engraved an electrical network connected to a computing unit is spread over the surface. When touched with a finger, the measured capacity is locally disturbed, which makes it possible to locate the touch. But this technology harms the transparency of the surface on which the mesh film of electric wires is deployed and the deployment of such a plastic film on a relief or large surface can quickly prove problematic. In the second case, the frame is made up of emitting and receiving infrared light-emitting diodes arranged respectively opposite one another in rows and columns so as to generate a grid of the surface. When a finger or any other object cuts the beams in rows and columns, it is located. However, this technology is very sensitive to sunlight, which is loaded with infrared radiation, as well as to the environment (dirt). In addition, the surface must be flat. Furthermore, in the two aforementioned cases, the cost price is high and increases rapidly depending on the size of the surface to be made tactile. If we wish to reduce this cost price, we at the same time very significantly reduce the performance of the touch interface.

Other technologies used are based on the propagation of acoustic waves, as for example in patent application US 2009/0116339 A1. But these technologies are sensitive to the environment, especially to temperature, and may require binding learning before they are used. In addition, the need to transmit energy to the touch interface from a touch to be detected limits the possibilities of interpretation of this touch.

In addition, the complexity of learning increases rapidly depending on the size of the surface to be made tactile.

Another solution is then to have a certain number of mechanical stress sensors against the surface to be made tactile regardless of its size and to apply a method based on a measurement of the respective stresses exerted on each sensor during a touch in order to deduce therefrom. , by barycentric calculation, the location of the touch. This solution is in particular particularly suitable for transparent tactile surfaces of large sizes and any shape, in particular flat or in relief, such as interactive glass surfaces of tables or interactive transparent ceiling lights of motor vehicles.

For these reasons, the invention relates more specifically to an installation with a tactile surface at the periphery of which are arranged means for detecting displacements, for example with mechanical stress sensors. Such a method is for example disclosed in US Patent 3,657,475. A minimum of two sensors is necessary for a one-dimensional location of the touch relative to an axis. A minimum of three sensors is necessary for a two-dimensional localization of touch on a surface, knowing that four sensors arranged at the four corners of a flat rectangular surface allow satisfactory results to be obtained with good overall stability, at the risk of 'a problem of maintenance by hyperstatism.

US Patent 3,657,475 describes four sensors interposed between a fixed support and a touch surface at the four corners of the latter. These sensors are strain gauges or piezoelectric sensors. They must, on the one hand, support the empty weight of the tactile surface and, on the other hand, remain sensitive to touches whose force or pressure is added to the weight of this tactile surface. The force of the touch cannot therefore be too weak relative to the weight of the surface so as not to induce problems of sensitivity as well as of precision. In addition, the sensors used in this document are quite expensive and the piezoelectric sensors in particular are sensitive to temperature variations.

It is therefore advantageously preferred to use displacement detection means with mechanical stress sensors such as capacitive sensors deemed inexpensive, as for example taught in the international patent application WO 2015/189166 A1 in particular for applications in the automotive industry (ie overhead lights and other tactile interaction surfaces). So that, in a context of tactile surface sensitive to touch, the capacitive sensors themselves do not pose sensitivity or precision problems such as those mentioned previously, a device is provided in this document for embedding the surface to make it tactile, comprising:

- a support frame having an upper rim and a lower rim, between which is intended to embed a peripheral portion of the tactile surface,

- damping means, disposed against an inner face of the upper edge of the support frame, between this inner face of the upper edge and the peripheral portion of the touch surface when it is embedded, and

means for detecting a displacement of the peripheral portion of the touch surface, disposed against an inner face of the lower edge of the support frame, between this inner face of the lower edge and the peripheral portion of the touch surface when it is recessed.

Such a device is modular, allows the tactile surface to be maintained at several peripheral points to increase its apparent rigidity and limit any deformation, all of this at lower cost. It is particularly suitable for making a large transparent ceiling light tactile in the passenger compartment of a motor vehicle. However, such an application to the automotive industry adds strong constraints of resistance to vibrations and temperature variations as well as compatibility with mechanical production tolerances. For example, with regard to temperatures, it is generally considered that variations between -40 ° C and +80 ° C must be able to be tolerated. The device as proposed in document WO 2015/189166 A1 is therefore not sufficient to meet all these constraints even if its damping means effectively reduce the effect of vibrations. In particular, any expansion of the touch surface inevitably generates significant bias in the detection of touches.

Solutions exist to meet this or that constraint. For example, US Patent 7,933,738 B2 proposes filtering possible vibrations by adding an accelerometer. Also for example, patent 8,270,148 B2 proposes the insertion of a suspension membrane to maintain the tactile surface laterally in its main plane without hampering its normal movement under the effect of a touch. However, none of these documents proposes a general solution to simply satisfy all of the aforementioned constraints.

It may thus be desired to provide a device for supporting by embedding a tactile surface which makes it possible to simply overcome at least a large part of the aforementioned problems and constraints.

It is therefore proposed a device for maintaining by embedding a tactile surface comprising:

- a support frame having an upper rim and a lower rim, between which is intended to embed a peripheral portion of the tactile surface,

- damping means, disposed against an inner face of the upper edge of the support frame, between this inner face of the upper edge and the peripheral portion of the touch surface when it is embedded,

means for detecting a displacement of the peripheral portion of the touch surface, disposed against an inner face of the lower edge of the support frame, between this inner face of the lower edge and the peripheral portion of the touch surface when it is recessed, further comprising at least one intermediate piece arranged:

- against a lower face of the damping means and having a first free face in contact with the tactile surface, with a coefficient of friction substantially lower than this lower face of the damping means, and

- Against an upper face of the displacement detection means and having a second free face of contact with the tactile surface, with a coefficient of friction substantially lower than this upper face of the displacement detection means.

Thus, not only the device proposed is designed to reduce the effect of vibrations, but also it responds effectively to the constraints of temperature variations by reducing the effects of any expansion and to the constraints of automobile construction by tolerance to play, thanks to the resumption of friction contacts by the intermediate piece which has free contact surfaces with substantially reduced friction. By "appreciably" is meant a perceptible effect beyond the vagaries and biases linked to the mechanical tolerances of construction and materials in the automotive sector.

Optionally, the first and second free faces in contact with the tactile surface are made of a material with a coefficient of friction less than or equal to that of polyoxymethylene or polytetrafluoroethylene.

Also optionally, said at least one intermediate piece comprises a rigid monobloc piece with two branches extending in a "U" section, in particular made of aluminum or aluminum alloy, each of the first and second free faces in contact with the touch-sensitive surface consisting of a coating arranged against an internal face of one of the two branches of the monobloc part with a “U” section.

Optionally also:

one of the first and second free faces in contact with the touch surface is made up of a rigid coating directly in contact with the internal face of the corresponding branch of the rigid monobloc piece, and

the other of the first and second free faces of contact with the tactile surface consists of a rigid coating indirectly in contact with the internal face of the corresponding branch of the rigid monobloc piece, more precisely in contact with this internal face by the 'through a layer of foam and a possible metal stiffening plate.

Also optionally, the damping means comprise a layer of foam.

Also optionally, the displacement detection means comprise a capacitive sensor with a dielectric foam layer.

Also optionally, the damping means comprise a capacitive sensor with a dielectric foam layer.

Also optionally, each layer of foam used is made of polyurethane or silicone.

It is also proposed a tactile installation comprising:

- a tactile surface,

a support comprising at least three support devices for embedding according to the invention, for maintaining the touch surface in the support by embedding at least three peripheral portions of the touch surface,

- At least one tangential force recovery blade that the touch surface can undergo during any interaction, this tangential force recovery blade connecting the support to a non-embedded middle part of the touch surface in a pivot spring connection preventing any relative displacement of the tactile surface parallel to a main plane of the support.

Optionally, each embedded peripheral portion of the tactile surface is provided, on each side of contact with the intermediate piece, with a pad with free surface in sphere portion for contact of each embedded peripheral portion with three degrees of freedom of rotary movements with the first and second free faces of the intermediate piece of each holding device.

The invention will be better understood with the aid of the description which follows, given solely by way of example and made with reference to the appended drawings in which:

FIG. 1 schematically represents in section the general structure of a device for holding a touch-sensitive surface by embedding, according to a first embodiment of the invention,

FIG. 2 schematically shows in section the general structure of a device for holding a touch-sensitive surface by embedding, according to a second embodiment of the invention,

FIG. 3 schematically represents in perspective the general structure of a tactile installation according to a first embodiment of the invention, and

- Figure 4 shows schematically in perspective the general structure of a tactile installation according to a second embodiment of the invention.

The installation illustrated in section (or side view) in FIG. 1 comprises a tactile surface 10, partially illustrated, of which a peripheral portion 12 is embedded in a holding device 14.

This device 14 comprises a retaining frame 16 having an upper rim 16A and a lower rim 16B, between which is embedded the peripheral portion 12 of the touch surface 10. It further comprises damping means 18 arranged against an inner face of the upper rim 16A of the retaining frame 16, between this inner face of the upper rim 16A and the peripheral portion 12 of the tactile surface 10. It further comprises means 20 for detecting a displacement of the peripheral portion 12 of the tactile surface 10, arranged against an inner face of the lower edge 16B of the holding frame 16, between this inner face of the lower edge 16B and the peripheral portion 12 of the touch surface 10.

The damping means 18 are more precisely constituted in this example of a foam damping element, in particular a layer of foam resistant to high temperatures such as a polyurethane or silicone foam. By "resistant to high temperatures" is meant a foam retaining its initial shape memory even in the event of high temperatures, in particular temperatures up to +80 ° C.

The detection means 20 are more precisely constituted in this example of a capacitive sensor with two electrodes separated by a dielectric layer, in particular a layer of dielectric foam resistant to high temperatures such as a polyurethane or silicone foam. When a user exerts a point pressure T against the tactile surface 10, more precisely against the lower face of the tactile surface 10 according to the illustration of FIG. 1, that generates a displacement of the embedded peripheral portion 12 and a deformation in thickness of the dielectric foam layer of the capacitive sensor 20, more precisely an expansion generating a measurable variation in capacity allowing in turn to participate in a barycentric calculation of touch location when the touch surface 10 is thus embedded by at least three holding devices such as that illustrated in FIG. 1. In a preferred embodiment, the electrodes can be flexible according to a flexible circuit design.

The thicknesses of the foams constituting the damping means 18 and the dielectric layer of the capacitive sensor 20 can advantageously be chosen to ensure a first mechanical prestress of 0.5 mm +/- 0.25 mm. This gives better sensitivity to the installation, in particular by facilitating the deformation of the dielectric foam layer of the capacitive sensor 20 when pressed against the touch surface 10.

But in accordance with the present invention and contrary to the teaching of document WO 2015/189166 A1, the embedded peripheral portion 12 of the touch surface 10 is not directly in contact with the damping means 18 and the detection means 20 Indeed, the holding device 14 further comprises at least one intermediate piece 22 comprising, in general, a first intermediate portion 22A disposed against a lower face of the damping element 18 and a second intermediate portion 22B disposed against an upper face of the capacitive sensor 20. More precisely, the first intermediate portion 22A has a first upper face in contact with the lower face of the damping element 18 and a first free lower face 24A of contact with the peripheral portion 12 of the surface tactile 10. More precisely also, the second intermediate portion 22B has a second underside in contact with the upper face of the capacitive sensor 20 and a second free upper face 24B of contact with the peripheral portion 12 of the touch surface 10.

Advantageously, the first free lower face 24A of the first intermediate portion 22A and the second free upper face 24B of the second intermediate portion 22B are formed from a material with a coefficient of friction substantially lower than the lower face of the damping element 18 and than the upper face of the capacitive sensor 20. Thus, the tactile surface 10 is maintained, thanks to the intermediate piece 22, with reduced friction and with recovery of games and problems of hyperstatism.

In a preferred embodiment, the intermediate piece 22 consists of a single rigid monobloc piece with two branches (22A and 22B) which extend in a "U" section. It is for example mainly made of aluminum or aluminum alloy to ensure good rigidity of the assembly, while the first free lower face 24A and second free upper face 24B of contact with the touch surface 10 are made of '' a coating, for example two plates of polyoxymethylene (POM), polytetrafluoroethylene (PTFE), or any other material with a coefficient of friction less than or equal to that of polyoxymethylene or polytetrafluoroethylene, respectively glued against the internal faces of the two branches 22A and 22B of the one-piece part. More precisely, given that in FIG. 1, the touch surface 10 is liable to undergo the pressure of a touch from the bottom upwards in the direction of the arrow T, the first free lower face 24A is a rigid plate of POM or PTFE directly in contact with the internal face of the branch 22A of the intermediate piece 22, so that the pressure as perceived by the peripheral portion 12 is indeed fully transmitted to the capacitive sensor 20. The second free upper face 24B is a rigid plate of POM or PTFE indirectly in contact with the internal face of the branch 22B of the intermediate piece 22, more precisely in contact with this internal face by means of a foam damping element 26, in particular a layer of polyurethane foam or silicone. This damping element also fulfills a function of recalling the tactile surface 10 against the intermediate piece 22, the foam thus being able to be replaced by any other system capable of providing this recall function. A metal stiffening plate (not illustrated in FIG. 1, generally called a metallic “foil”) can advantageously be added between the foam layer 26 and the rigid POM or PTFE 24B plate to avoid any deformation of the rigid POM plate or PTFE 24B. This sandwiching of the touch surface 10 in the intermediate piece 22 can be done by design with a thickness of foam 26 chosen to also provide a second mechanical prestress, for example less than the first previously mentioned.

It will be noted in particular that when the intermediate piece 22 consists of a single rigid monobloc piece, it makes it possible not to impose the first mechanical prestress of 0.5 mm +/- 0.25 mm, due to the thicknesses of the foams of the elements 18 and 20, to the rigid plates 24A and 24B. This makes it possible to guarantee a weak pinching of the peripheral portion 12 to ensure freedom of lateral movement of the tactile surface 10.

Optionally but advantageously, in order to make the contact between the tactile surface 10 and the rigid plates 24A, 24B, as punctual and free as possible, the peripheral portion 12 can be provided, on each side of contact with the intermediate piece 22, of a patch 28 with a free surface in a portion of a sphere for contact of the peripheral portion 12 when it is embedded in three degrees of freedom of rotary movements with the rigid plates 24A and 24B. In this case, the addition of a metal stiffening plate is very advantageous for reinforcing the robustness of the rigid plate 24B. It will also be noted that the second weaker mechanical prestressing provided by the foam 26 makes it possible to guarantee an absence of play between the pads 28 and the peripheral portion 12.

Thus, the effort of holding the tactile surface 10 by pinching is independent of the prestressing of the damping means 18 and of detection 20, as well as of any pressure exerted on the tactile surface 10 during its use. This guarantees an easy lateral displacement of the tactile surface 10 in all circumstances and therefore the cancellation of the influence of the majority of the tangential forces.

In another preferred embodiment and illustrated in FIG. 2, the foam layer of the damping element 18 is replaced by a capacitive sensor with a dielectric foam layer, for example a capacitive sensor identical to the sensor 20. Two capacitive sensors per holding device improve the sensitivity of the measurement.

In an interactive automotive ceiling application context, a touch-sensitive installation 30 is schematically illustrated in perspective in FIG. 3.

It comprises a touch surface 32 such as the touch surface 10, of any shape whatsoever, a rigid monobloc support 34 and at least three support devices for embedding such as that of FIG. 1 or 2 bearing the reference 14. More precisely , four holding devices 36, 38, 40, 42 are fixed to the support 34 for hyperstatic embedding of the touch surface 32 at four of its peripheral portions, in particular at its four corners. One of these peripheral portions, referenced 44 and embedded in the holding device 38, is clearly visible in FIG. 3. When a touch pressure T is exerted from the bottom upwards against the underside of the touch surface 32 , its result in each peripheral holding zone is picked up by each holding device so as to allow localization of the touch by barycentric calculation or prior learning.

But on the occasion of a touch exerted to interact with the tactile surface 32, the latter can also undergo involuntary tangential forces liable to disturb the measurements and the location, in particular when it is not flat. To reduce or even eliminate this risk, an advantageous option is to provide at least one blade for taking up any tangential forces, this blade connecting the support 34 to a non-embedded middle part of the touch surface 32 according to a pivoting spring connection preventing any displacement relative to the touch surface 32 parallel to the main plane of the support 34.

More specifically, in the nonlimiting example of FIG. 3, four blades are provided. Two of them, referenced 46 and 48, are fixed in the extension of one another in the left lateral median part of the installation 30, on the one hand to a local left lateral protuberance 50 of the tactile surface 32 at their two opposite ends opposite, and on the other hand, at their respective two distant ends, to a left side wall 52 itself fixed to the support 34 and extending from the holding device 36 to the holding device 42 The two other blades, referenced 54 and 56, are fixed in the extension of one another in the right lateral median part of the installation 30, on the one hand to a local right lateral protuberance 58 of the tactile surface 32 at their two opposite ends opposite, and on the other hand, at their two respective distant ends, to a right side wall 60 fixed to the support 34 and extending from the dispositi f holding 38 to the holding device 40. These four blades 46, 48, 54 and 56 thus form two pairs of lateral suspensions capable of absorbing, by pivot spring-loaded connection laterally constrained between the touch surface 32 and the walls 52, 60, tangential constraints involuntarily applied during an interaction.

To increase the efficiency of these tangential force recovery blades by reducing the torques and leverage they induce, it is advantageous to bring them as close as possible to the interaction surface in the tactile surface 32, for example in a median plane of the relief. In the example of FIG. 3, the touch surface 32 is curved downwards, so that the effective interaction surface is shifted downward relative to the plane of arrangement of the blades 46, 48, 54 and 56.

The installation 30 can then be simply adapted as illustrated in FIG. 4, so as to bring the plane of arrangement of the blades 46, 48, 54 and 56 closer to the interaction surface. It suffices to add studs or extensions downwards, under the lateral walls 52, 60 and under the local lateral protuberances 50, 58, facing the fixing ends of the blades. Thus, for example, a central stud 62 extends downwards from the left lateral local protuberance 50 (not visible in FIG. 4) to the two close fixing ends of the blades 46 and 48 and two distant studs 64, 66 extend respectively downwards from the left side wall 52 to the two distant fixing ends of the blades 46 and 48. Likewise, a central stud 68 extends downwards from the local right side projection 58 to at the two close fixing ends of the blades 54 and 56 and two remote studs 70, 72 extend respectively downwards from the right side wall 60 to the two distant fixing ends of the blades 54 and 56.

It clearly appears that a device for holding a touch-sensitive surface by embedding such as one of those described above allows an effective recovery of the tactile stresses undergone by a contact face of the tactile surface, thus allowing a precise localization of touches. possible, while ensuring robustness to vibrations, machining tolerances and expansions / contractions due to the temperature variations inherent in a good part of the applications which may be envisaged.

In particular, the presence of the different layers of foam, for the damping means, the displacement detection means and the fixing of one of the POM or PTFE plates, limits the resonance modes of the tactile surface.

Note also that the invention is not limited to the embodiments described above.

Thus, as a variant, the whole of the intermediate piece could be made of a material with a coefficient of friction substantially lower than the underside of the damping means and the top face of the displacement detection means.

In another variant, a coating can be added to the contact surface to limit the friction coefficients upstream.

It will more generally appear to those skilled in the art that various modifications can be made to the embodiments described above, in the light of the teaching which has just been disclosed to him. In the detailed presentation of the invention which has been made previously, the terms used should not be interpreted as limiting the invention to the embodiments set out in the present description, but must be interpreted to include all the equivalents the prediction of which is within the reach of the skilled person in applying his general knowledge to the implementation of the teaching which has just been disclosed to him.

Claims (10)

1. Device for embedding a tactile surface (10) comprising: a support frame (16) having an upper rim (16A) and a lower rim (16B), between which is intended to embed a peripheral portion (12) of the touch surface (10), damping means ( 18), arranged against an inner face of the upper edge (16A) of the retaining frame (16), between this inner face of the upper edge (16A) and the peripheral portion (12) of the touch surface (10) when it is recessed, and means (20) for detecting a displacement of the peripheral portion (12) of the touch surface (10), disposed against an inner face of the lower edge (16B) of the holding frame (16), between this inner face of the lower rim (16B) and the peripheral portion (12) of the touch surface (10) when it is embedded, characterized in that it also comprises at least one intermediate piece (22) arranged: against a lower face of the damping means (18) and having a first free face (24A) of contact with the tactile surface (10), with a coefficient of friction substantially lower than this lower face of the damping means (18) , and against an upper face of the displacement detection means (20) and having a second free face (24B) of contact with the tactile surface (10), with a coefficient of friction substantially lower than this upper face of the means (20) displacement detection. 2. device for holding in place a touch surface (10) according to claim 1, in which the first and second free faces (24A, 24B) of contact with the touch surface (10) are made of a coefficient material. friction less than or equal to that of polyoxymethylene or polytetrafluoroethylene. 3. Device for embedding a tactile surface (10) according to claim 1 or 2, wherein said at least one intermediate piece (22) comprises a rigid monobloc piece with two branches (22A, 22B) extending along a “U” section, in particular made of aluminum or aluminum alloy, each of the first and second free faces (24A, 24B) of contact with the tactile surface (10) consisting of a coating disposed against an internal face of one of the two branches (22A, 22B) of the one-piece part with a “U” section. 4. Device for holding a touch-sensitive surface (10) according to claim 3, in which: one (24A) of the first and second free faces (24A, 24B) of contact with the tactile surface (10) consists of a rigid coating directly in contact with the internal face of the corresponding branch (22A) of the part rigid monobloc, and the other (24B) of the first and second free faces (24A, 24B) of contact with the tactile surface (10) consists of a rigid coating indirectly in contact with the internal face of the corresponding branch (22B ) of the rigid one-piece part, more precisely in contact with this internal face by means of a layer of foam (26) and of a possible metal stiffening plate. 5. Device for holding a touch-sensitive surface (10) according to any one of claims 1 to 4, in which the damping means (18) comprise a layer of foam. 6. Device for embedding a tactile surface (10) according to any one of claims 1 to 5, in which the means (20) for detecting a displacement comprise a capacitive sensor with a dielectric foam layer. 7. Device for holding in place a tactile surface (10) according to any one of claims 1 to 6, in which the damping means (18) comprise a capacitive sensor with a dielectric foam layer. 8. Device for holding a touch-sensitive surface (10) according to any one of claims 4 to 7, in which each layer of foam used (18, 20, 26) is made of polyurethane or silicone. 9. Tactile installation (30) comprising: a touch surface (32), a support (34) comprising at least three recessed holding devices (36, 38, 40, 42) according to any one of claims 1 to 8, for holding the touch surface (32 ) in the support (34) by embedding at least three peripheral portions of the touch surface (32), at least one blade (46, 48, 54, 56) for taking up tangential forces which the touch surface can undergo ( 32) during any interaction, this 5 tangential force recovery blade connecting the support (34) to a non-recessed median part of the touch surface (32) according to a spring-loaded pivot connection preventing any relative movement of the touch surface (32) parallel to a main plane of the support (34). 10. Tactile installation (30) according to claim 9, in which each 10 recessed peripheral portion of the touch surface (32) is provided, on each side of contact with the intermediate piece (22), of a pad (28) with free surface in sphere portion for contact of each peripheral portion recessed at three degrees of freedom of rotary movement with the first and second free faces (24A, 24B) of the intermediate piece (22) of each holding device (36, 38, 40, 42).