FIELD OF THE INVENTION

The present invention relates to an electronic radio-identification device or radiofrequency transponder able to be fastened to an object to be identified that, in particular in service, is subjected to high thermal-mechanical stresses, such as a tyre.

TECHNOLOGICAL BACKGROUND

In the field of RFID devices (RFID being the acronym of RadioFrequency Identification), passive radiofrequency-identification transponders are conventionally used to identify, track and manage objects. These devices allow more reliable and faster automated management.

These radiofrequency-identification transponders generally consist of at least one electronic chip and one antenna formed by a magnetic loop or a radiating antenna, which is fastened to the object to be identified.

The communication performance of the radiofrequency transponder is expressed in terms of the maximum distance of communication of the radiofrequency transponder with a radiofrequency reader, for a given signal communicated to or by the radiofrequency reader.

In the case of highly extensible products such as, for example, tyres, there is a need to identify the product throughout its life from its manufacture to its removal from the market and, in particular, during use thereof. Thus, in order to facilitate this task, in particular under the conditions of use, a high communication performance is required, which is expressed in terms of the ability to interrogate the radiofrequency transponder at a large distance (several meters) from the product, via a radiofrequency reader. Lastly, it is desired for the manufacturing cost of such a device to be as competitive as possible.

A passive radiofrequency transponder able to meet the needs of tyres is known in the prior art, and in particular from document WO 2016/193457A1. This transponder consists of an electronic chip, connected to a printed circuit board to which is electrically connected a first primary antenna. This primary antenna is electromagnetically coupled to a single-strand helical spring that forms a radiating dipole antenna. The communication with an external radiofrequency reader uses radiowaves and in particular the UHF band, UHF being the acronym of Ultra-High Frequency. Therefore, the characteristics of the helical spring are adjusted for the chosen communication frequency. Thus, the disappearance of the mechanical junction between the printed circuit board and the radiating antenna improves the mechanical resistance of the radiofrequency transponder.

However, such a radiofrequency transponder has weaknesses. Although this radiofrequency transponder is suitable for operating at the communication frequency of the external radiofrequency reader in particular in an environment such as a tyre, it is tricky to position the electronic portion with respect to the radiating antenna and to keep it working therewith, and hence, if the communication performance of the radiofrequency transponders is not to be degraded, it is necessary for them to be handled with precision, impacting the production cost of the radiofrequency transponder. In addition, it is also necessary to not adversely affect other aspects of the performance of the radiofrequency transponder, such as its physical integrity, or the endurance of the object, such as a tyre casing, to be identified accommodating the radiofrequency transponder.

The present invention relates to a passive radiofrequency transponder and its production process aiming to improve the performance compromise of the radiofrequency transponders employed, inter alia, in the tyre industry.

DESCRIPTION OF THE INVENTION

The invention relates to a passive radiofrequency transponder intended to be integrated into an object to be identified made of a highly extensible material such an elastomer composition or blend. This radiofrequency transponder firstly comprises a radiating dipole antenna. The latter consists of a single-strand helical spring and has an axis of revolution, a winding diameter, a helix pitch, a median plane and a wire diameter defining inside and outside diameters of the radiating antenna. This radiating antenna has a length L0suitable for communication in a frequency band with a radiofrequency-transmitting reader. This radiofrequency transponder also comprises an electronic portion located inside the radiating antenna. This electronic portion comprises an electronic chip and a primary antenna electrically connected to the electronic chip and electromagnetically coupled to the radiating antenna. The primary antenna firstly has an axis substantially parallel to the axis of revolution of the radiating antenna and secondly a median plane, which is substantially superposed with the median plane of the radiating antenna. Lastly, the primary antenna is circumscribed by a cylinder the axis of revolution of which is parallel to the axis of the primary antenna and the diameter of which is larger than or equal to one third of the inside diameter of the radiating antenna. This radiofrequency transponder is characterized in that, in a first region of the radiating antenna, in which the radiating antenna is not located plumb with the electronic portion, a first inside diameter of the radiating antenna corresponding to the inside diameter of the radiating antenna in this first region is smaller than a second inside diameter of the radiating antenna corresponding to the inside diameter of the radiating antenna in a second region located plumb with the electronic portion and in that the electronic portion is circumscribed by a cylinder the axis of revolution of which is parallel to the axis of revolution of the radiating antenna and the diameter of which is larger than or equal to the first inside diameter of the radiating antenna.

Here, the term “elastomer” is understood to mean all the elastomers including TPEs (acronym of ThermoPlastic Elastomers), such as for example diene polymers, i.e. polymers comprising diene units, silicones, polyurethanes and polyolefins.

Here, the term “electromagnetic coupling” is understood to mean coupling via electromagnetic radiation, i.e. the transfer of energy without physical contact between two systems including, on the one hand, inductive coupling and, on the other hand, capacitive coupling. The primary antenna is the preferably comprised in the group comprising: a coil, a loop or a wire segment or a combination of these conductive elements.

Here, the term “substantially parallel” is understood to mean that the angle generated by the axial directions of each antenna is smaller than or equal to 30 degrees. In this case, the electromagnetic coupling between the two antennas is optimal, notably improving the communication performance of the radiofrequency transponder.

Here, the median plane of the coil and of the helical spring should first be defined. By definition, it is a fictional plane separating the object into two equal portions. In our case, this median plane is perpendicular to the axis of each antenna. Lastly, what is meant here by “substantially superposed” is that the relative distance between the median planes is smaller than one tenth of the length L0of the radiating antenna.

Thus, since the electrical current is of maximum magnitude at the centre of the radiating antenna, the magnetic field induced by this current is also maximum at the centre of the radiating antenna, and thus it is ensured that the inductive coupling between the two antennas is optimal, thereby improving the communication performance of the radiofrequency transponder.

By defining the relative dimensions of the primary antenna with respect to the characteristics of the helical spring of the radiating antenna, it is ensured that the distance between the two antennas will be smaller than the diameter of the primary antenna in the case where the primary antenna is located inside the radiating antenna. Thus, the inductive coupling between the two antennas and therefore the communication performance of the radiofrequency transponder are optimized in transmission and reception.

Lastly, by ensuring that the cylinder that circumscribes the electronic portion has an axis of revolution parallel to the axis of revolution of the radiating antenna and a diameter larger than or equal to the first inside diameter of the radiating antenna, the first region of the radiating antenna therefore forms a stop with respect to the axial movement of the electronic portion. The fact that this first region is located on either side of the region of the radiating antenna located plumb with the electronic portion, because of the centred position of the electronic portion with respect to the radiating antenna, ensures that two mechanical stops axially located externally to the electronic portion are then provided, limiting any axial movement of the electronic portion of the radiofrequency transponder. In addition, since the diameter of the cylinder circumscribing the electronic portion is located inside the radiating antenna in the second region, this diameter is necessarily smaller than the second inside diameter of the radiating antenna. Thus, the radial movement of the electronic portion is limited by the second inside diameter of the radiating antenna. In conclusion, the movement of the electronic portion is limited, this allowing the communication performance of the radiofrequency transponder to be ensured while ensuring a physical integrity of the electronic portion and of the radiating antenna of the radiofrequency transponder. Lastly, the endurance of the object accommodating this radiofrequency transponder is also not impacted by this choice of design. The handling of the radiofrequency transponders is greatly facilitated thereby, without any need for additional precautions. This passive radiofrequency transponder may be, in particular, an identification transponder, called RFID tag.

In one particular embodiment, the first inside diameter of the radiating antenna is constant or continuously variable although solutions of piecewise variable type may also be envisaged with variations in inside diameter of small amplitude from one piece to the contiguous piece.

This makes it possible to ensure, in the first region of the radiating antenna and from a mechanical point of view, the absence of any singular points that would impact the physical integrity of the radiating antenna.

Preferably, the primary antenna of the electronic portion being connected to the terminals of a circuit board comprising the electronic chip, the electrical impedance of the primary antenna is matched to the electrical impedance of the circuit board of the radiofrequency transponder.

The term “electrical impedance of the circuit board” is understood to mean the electrical impedance across the terminals of the primary antenna, this representing the electrical impedance of the circuit board comprising at least one electronic chip and a printed circuit board to which the electronic chip is connected.

By matching the impedance of the primary antenna to that of the circuit board, the radiofrequency transponder is optimized at the communication frequency by improving the gain and achieving a circuit board of more selective form factor and narrower passband. Thus, the communication performance of the radiofrequency transponder is improved for a given amount of energy transmitted to the radiofrequency transponder. This in particular results in an increase in the read distance of the radiofrequency transponder. The impedance match of the primary antenna is obtained by adjusting at least one of the geometric features of the primary antenna, such as, for example, the diameter of the wire, the material of this wire and the length of the wire.

The impedance match of the primary antenna may thus be obtained by adding an impedance-matching circuit made up of additional electronic components between the primary antenna and the electronic circuit, such as, for example, filters based on an inductor, capacitors and transmission lines.

The impedance match of the primary antenna may also be obtained by combining features of the primary antenna and features of an impedance-matching circuit.

According to one particular embodiment, the electronic chip and at least one portion of the primary antenna are embedded in a stiff and electrically insulating material, such as, for example, high-temperature epoxy resin. This assembly forms the electronic portion of the radiofrequency transponder.

Thus, the electronic portion comprising at least one portion of the primary antenna and the electronic chip connected to the printed circuit board is stiffened, making the mechanical connections between the components thereof more reliable with respect to the thermomechanical stresses to which the object to be identified is subjected.

This also allows the electronic portion of the radiofrequency transponder to be manufactured independently of the radiating antenna or of the object to be identified. In particular, for example, using a micro-coil of a number of turns as primary antenna allows miniaturization of the electronic component comprising the primary antenna and the electronic chip to be envisaged.

According to another embodiment, the portion of the primary antenna not embedded in the stiff mass is coated with an electrically insulating material.

Thus, if the primary antenna is not entirely contained in the stiff and electrically insulating mass of the electronic portion, it is useful to insulate it via a coating made of an electrically insulating material, such as those employed for the insulating sheaths of electrical cables.

Advantageously, the ratio between the helix pitch and the winding diameter for at least one loop of the helical spring located in the first region of the radiating antenna is higher than 0.8.

Specifically, outside of the region of the radiating antenna that is located plumb with the electronic portion and therefore with the primary antenna, a ratio of the helix pitch to the winding diameter higher than 0.8 for a loop of the radiating antenna has the advantageous effect of stretching the helical spring. Thus, the length of wire needed to make an elementary span of radiating antenna is decreased. Thus, the resistance of the radiating antenna is decreased. Therefore, for a given electric field, the electrical current flowing through the radiating antenna is of higher magnitude at the natural frequency of the radiating antenna, this allowing the communication performance of the radiofrequency transponder to be improved. In addition, stretching the helical spring allows the efficiency of the radiating antenna to be improved by improving the ratio between the radiation resistance and loss resistance. This stretching thus allows, for a given flow of electrical current through the radiating antenna, the electric field radiated by the radiating antenna to be maximized. Lastly, for radiating antenna of given helix pitch, stretching the radiating antenna allows the volume occupied by the helical spring to be decreased. Thus, in a constrained dimensional environment, such as the thickness of a tyre casing, it is possible to increase the thickness of insulating rubber surrounding the radiating antenna in the first region thereof. This electrical insulation minimizes losses and therefore improves the communication performance of the radiofrequency transponder, both in transmission and in reception. Of course, it is ideal for each of the loops of the helical spring of the first region of the radiating antenna to be elongate, this correspondingly improving the communication performance of the radiofrequency transponder.

Preferably, the ratio between the helix pitch and the winding diameter of each of the loops of the spring in the first region of the radiating antenna is lower than 3, and preferably lower than 2.

Although it is advantageous to improve the radioelectric performance of the radiating antenna, it is necessary to also not neglect the other functions that it must perform. In particular, the helical spring is a supple structure suitable for bearing the three-dimensional stresses to which the radiofrequency transponder will necessarily be subjected in its tyre application. Thus, it is recommended to limit the amount by which the radiating antenna is stretched in this first region in order to ensure the radiating antenna preserves a sufficient suppleness on the whole and thus to ensure the physical integrity of the radiofrequency antenna.

According to one particular embodiment, at least one turn of the two last loops of the helical spring in the first region of the radiating antenna, i.e. the two loops that are located axially outermost with respect to the electronic portion, adjoin.

This then avoids the radiating antennas of the radiofrequency transponders from becoming interlaced during the handling of the latter. Thus, the handling of the radio-frequency transponders is facilitated, allowing the cost price of the radiofrequency transponders to be optimized. The fact of limiting this effect to the loops located axially outermost with respect to the electronic portion affects only slightly the radioelectric performance of the radiating antenna.

According to one preferred embodiment, in the second region of the radiating antenna in which the radiating antenna is located plumb with the electronic portion, the ratio between the helix pitch and the winding diameter for each loop of the helical spring is lower than or equal to 0.8.

Specifically, in this second region of the radiating antenna, and more particularly in the region located plumb with the primary antenna, the effect expected from the radiating antenna is electromagnetic coupling with the primary antenna of the electronic portion. In particular, the coupling is mainly inductive if this primary antenna is a coil of a number of turns. Thus, a first lever for improving this coupling is to increase the inductance of the radiating antenna in this second region, this amounting to contracting the helical spring. In addition, contracting the radiating antenna in this second region also promotes the transfer of energy between the primary antenna and the radiating antenna by increasing, for a given length of the primary antenna located facing the radiating antenna, the area of exchange furnished by the radiating antenna. This improvement in energy transfer leads to a better communication performance being obtained from the radiofrequency transponder.

According to one specific embodiment, the radioelectric communication with the radiofrequency-transmitting reader occurs in the UHF band and most specifically in the range comprised between 860 and 960 MHz.

Specifically, in this frequency band, the length of the radiating antenna is inversely proportional to the communication frequency. Furthermore, outside of these frequency bands, radioelectric communication is highly perturbed or even impossible through elastomeric materials. Thus, this is the best compromise between the size of the radiofrequency transponder and its radioelectric communication, in particular in the far-field, allowing communication distances that are satisfactory for the tyre industry to be obtained.

According to another particular embodiment, the length L0of the radiating antenna is comprised between 30 and 50 millimetres.

Specifically, in the frequency range between 860 and 960 MHz and depending on the relative dielectric permittivities of the elastomer blends surrounding the radiofrequency transponder, the half-wavelength of the radioelectric waves transmitted or received by the radiofrequency transponder is located in the interval between 30 and 50 millimetres, ideally between 35 and 45 millimetres for conventional materials used for designing tyres. In order to optimize the operation of the radiating antenna at these wavelengths, it is perfectly recommended to tailor the length of the radiating antenna to this half-wavelength.

Advantageously, the winding diameter of the helical spring in the first region of the radiating antenna is comprised between 0.6 and 2.0 millimetres, and preferably between 0.6 and 1.6 millimetres.

This allows the volume occupied by the radiating antenna to be limited and therefore the thickness of electrically insulating elastomer blend around the radiofrequency transponder to be increased. Of course, this diameter of the helical spring in the first region of the radiating antenna may be constant, variable, continually variable or piecewise variable. It is preferable from a point of view of the mechanical integrity of the radiating antenna for the diameter to be constant or continuously variable.

According to one preferred embodiment, the helix pitch of at least one loop of the radiating antenna in the first region of the radiating antenna is comprised between 1 and 4 millimetres, preferably between 1.5 and 2 millimetres.

This makes it possible to ensure that the ratio of the helix pitch to the winding diameter of the spring, or at least one loop, in the first region of the radiating antenna is lower than 3, guaranteeing a minimum of elongation of the helical spring. In addition, this pitch may also be constant or variable throughout the first region of the radiating antenna. Of course, it is preferable for the pitch to be continuously variable or variable with small transitions in variation in order to avoid singular points in the radiating antenna that would form mechanical weaknesses for the structure.

According to one advantageous embodiment, the diameter of the wire of the radiating antenna is comprised between 0.05 and 0.25 millimetres, and ideally between 0.12 and 0.22 millimetres.

In this wire range loss resistance is certain to be low, thus improving the radioelectric performance of the radiating antenna. In addition, limiting the diameter of the wire allows the distance between the radiating antenna and the electrical conductors to be increased by increasing the thickness of the electrically insulating elastomer blends. However, it is necessary for the wire to preserve a certain strength in order to be able to bear the thermomechanical stresses that it will undergo in a highly stressed environment such as a tyre casing, without optimizing the breaking stress of the material of these wires, which is generally mild steel. This makes it possible to ensure the radiating antenna represents a satisfactory technical/economical compromise.

Another subject of the invention is an identification tag consisting of a radiofrequency transponder embedded in a supple and electrically insulating mass of elastomer blends.

The term electrically insulating is understood here to mean that the electrical conductivity of the elastomer blend is below the conductive charge transfer threshold of the blend.

Thus, an identification tag that facilitates the installation of the radiofrequency transponder in objects to be identified, which comprise portions made of material based on elastomer, is formed. A conventional bonding-rubber layer will possibly be employed if necessary, to secure the identification tag to the object, such as a tyre, to be identified.

In addition, the stiffness and electrical-conductivity characteristics of the elastomer blend ensure a quality mechanical insertion and electrical insulation of the radiofrequency transponder within the components of the object to be identified. Thus, the operation of the radiofrequency transponder is not perturbed by the object to be identified.

Another subject of the invention is a process for manufacturing the radiofrequency transponder, which comprises the following steps:an electronic chip is electrically connected to a printed circuit board in order to form a circuit board;a coil forming a primary antenna is produced using a conductive wire;the primary antenna is electrically connected to the circuit board;at least one portion of the primary antenna and the circuit board are embedded in a stiff and electrically insulating mass such as a heat-curable resin, in order to form the electronic portion of the radiofrequency transponder;a primary portion of a radiating antenna of length L0and of axis of revolution U consisting of a helical spring is produced, according to one specific embodiment starting with the loop located outermost with respect to the median plane of the radiating antenna, this portion comprising the first half of the first region of the radiating antenna and at least a first portion of the second region of the radiating antenna;the electronic portion of the radiofrequency transponder is positioned axially and radially with respect to the primary portion of the radiating antenna, according to one specific embodiment by placing the electronic portion in abutment against the loop of the first region of the radiating antenna which is located innermost with respect to the median plane of the radiating antenna, in such a way that the primary antenna of the electronic portion is circumscribed by a cylinder having an axis and a median plane that is perpendicular thereto, the axis of the primary antenna and the axis of revolution U of the radiating antenna being substantially parallel and the median plane of the primary antenna being substantially centred with respect to the middle of the length L0of the radiating antenna;the secondary portion of the radiating antenna is finalized, advantageously starting with the second portion of the second region of the radiating antenna, this secondary portion comprising at least the second half of the first region of the radiating antenna.

Thus, manufacture of the radiofrequency transponder is simplified by producing the electronic portion and the radiating antenna of the radiofrequency transponder separately. In addition, the step of assembling the two components requires no mechanical or electrical connection between the two components, this drastically decreasing the cost of manufacture of the radiofrequency transponder.

The step of producing the radiating antenna is performed in two phases. Between the two phases, the previously produced electronic portion is positioned with respect to the primary portion of the radiating antenna produced. Next, the secondary portion of the antenna is produced in the presence of the electronic portion. This process is easily automatable.

The invention also relates to another process for manufacturing the radiofrequency transponder, which comprises the following steps:an electronic chip is electrically connected to a printed circuit board in order to form a circuit board;a coil forming a primary antenna is produced using a conductive wire;the primary antenna is electrically connected to the circuit board;at least one portion of the primary antenna and the circuit board are embedded in a stiff and electrically insulating mass such as a heat-curable resin, in order to form the electronic portion of the radiofrequency transponder;a radiating antenna of length L0and of axis of revolution U consisting of a helical spring comprising a second region, defined by a helix pitch P2and a winding diameter D2, is produced between two portions of a first region of the radiating antenna, which region is defined by a helix pitch P1and a winding diameter D1;a region, comprising at least one turn of the helical spring, of transition between one portion of the first region and the second region of the radiating antenna is identified using means, for example optical means or mechanical or magnetic distance sensors, for detecting the variation in the winding diameter and/or the variation in the helix pitch;a bending point located in the turn of the region of transition is identified, this point preferably being located in the half of the turn that is located closest to the second region of the radiating antenna;the helical spring is bent by a first angle, which is preferably comprised between 70 and 110°, about an axis perpendicular to the axis of revolution U passing through the bending point between the portions of the helical spring that are located upstream and downstream of the bending point, in order to create an opening in the upstream portion of the helical spring, which comprises the second region of the radiating antenna, the inscribed diameter of which is larger than or equal to the diameter of the cylinder circumscribing the electronic portion;the electronic portion is inserted into the upstream portion of the spring of the helical spring via the opening, so that that axial end of the cylinder circumscribing the electronic portion which is located outermost with respect to the upstream portion of the helical spring is located at a distance smaller than the helix pitch P2above a plane the normal of which is the axis of revolution of the upstream portion passing through the bending point;the helical spring is bent again at the bending point by a second angle opposite to the first angle about the axis perpendicular to the axis of revolution of the upstream portion of the helical spring passing through the bending point and the amplitude of which is at least equal to or larger than the amplitude of the first angle, so that the axes of revolution of the upstream and downstream portions of the helical spring are collinear.

This second embodiment allows the step of producing the helical spring forming the radiating antenna to be dissociated from the step of assembling the electronic portion with the radiating antenna, this allowing the industrial manufacturing yield of the helical spring to be optimized. The assembly may thus be carried out at a dedicated station.

Depending on the nature of the metal from which the helical spring is made and on its elasto-plastic properties, it is recommended to choose the second bending angle so that the two portions located upstream and downstream of the bending point are collinear after the second time the helical spring is bent, in order to ensure the helical spring of axis of revolution U is reformed. The two axes of revolution of the upstream and downstream portions are aligned to within acceptable tolerance limits, so that the radiofrequency of the operation of the radiating antenna is not disrupted, this corresponding to an angle between the axes of revolution of the upstream and downstream portion of the helical spring smaller than 20°.

The potential localized increase in the plasticity of the helical spring in proximity to the bending point does not adversely affect the mechanical behaviour of the helical spring, which by nature has a large range of movement. This region must not in principle remain outside the domain of plasticity of the material of the helical spring.

The location of the bending point, in the transition region, in proximity to the second region of the radiating antenna, makes it possible to ensure a centrality and a coaxiality of the electronic portion with respect to the second region of the helical spring, this being favourable to achieving correct electromagnetic operation between the primary and radiating antennas.

In addition, whatever the manufacturing process, at the manufacturing station where the springs are wound, it is easy to modify both the helix pitch and the winding diameter of the helical spring of each loop using, for steel wires, a given wire of diameter comprised between 0.05 and 0.25 millimetres, coated or not. This allows, in particular, adjoining turns to be produced as necessary.

According to one particular embodiment, any one of the manufacturing processes according to the invention also comprises a step of coating the primary antenna portion that is not embedded into the stiff and electrically insulating mass of an electrically insulating material.

This feature is useful as it allows a quality electromagnetic coupling to be obtained between the radiating antenna and the primary antenna while preventing any physical contact between the conductive portions of the two antennas.

Preferably, any one of the manufacturing processes according to the invention comprises a step of adding an additional electronic circuit comprising inductive and/or capacitive elements to the circuit board, and ideally a galvanic circuit of meander shape in order to match the electrical impedance of the primary antenna to the electrical impedance of the circuit board.

This allows the efficacy of the primary antenna to be improved in the case where the way in which the primary antenna is produced allows the electrical impedance of the circuit board to be approached, but not to be perfectly matched.

Another subject of the invention is a process for manufacturing the identification tag, in which a radiofrequency transponder is incorporated into a mass of at least one supple and electrically insulating elastomer blend using an injection-moulding, compression-moulding or extrusion process.

Thus, whatever the, green or cured, state of the elastomer, it is easy to incorporate the identification tag into an object to be identified, such as a tyre, comprising elastomer products using, where necessary, conventional elastomer/elastomer adhesion techniques. This incorporation may take place either during the phase of manufacture of the object, in which case the tag is for example incorporated into a green unfinished tyre, in particular before the elastomers are cured or vulcanized, or during a subsequent step of the process for manufacturing the object to be identified, in which case the tag is for example placed directly on an internal or external face of the tyre.

