Abstract:
A method for detecting wear on a tire at one or more predetermined wear thresholds in tread pattern elements of the tire includes the steps of allocating a discrete wear variable to the tread pattern element to a discrete impedance variable representing the impedance of a passive tuned circuit contained in at least one of the elements. A system according to the invention for implementing this method comprises a detection unit intended to be incorporated in a tire and containing the passive circuit which is covered with rubber, the passive circuit comprising a coil and n (n≧1) capacitor or capacitors connected in parallel to the terminals of the coil, and an active interrogation circuit to which the passive circuit is coupled, which is mounted fixed to the wheel and which comprises a frequency-sweep energy generator and a means of detecting the tuning frequencies between the passive circuit and the interrogation circuit.

Description:
This application is a Division of U.S. Ser. No. 10/902,558 filed on Jul. 29, 2004, now U.S. Pat. No. 7,204,135 issued on Apr. 17, 2007, and which is a continuation of International PCT/EP03/00888 filed on Jan. 29, 2003. 

   BACKGROUND AND SUMMARY OF THE INVENTION 
   The present invention concerns a method of detecting when a tire reaches one or more predetermined wear thresholds, which may be detected when the tire is stopped or while running. The invention also concerns a detection unit intended to be incorporated in such a tire for implementing this method, a tread pattern element for such a tire comprising this unit, a tire tread provided with this unit, and a tire comprising this tread. The invention also comprises a system of implementing this method comprising this unit, a tire/wheel assembly for a motor vehicle and such a vehicle comprising this system. 
   It is known how to provide wear detectors for tread patterns on tires for motor vehicles. 
   The German patent document DE-A-197 45 734 concerns a wear detector for a tire which forms part of the tire (see  FIGS. 2 and 3  of this document). More precisely, in the mass of a tread pattern strip on the tire, metallic wires are provided, forming electrically conductive loops which extend at different heights from the top of the bar and which are connected to a detection circuit contained in the tread on the tire, radially below the tread strip. 
   When a vehicle equipped with this tire is running, these loops are broken successively because of the wear on the strip, so that they form open switches, and the detection circuit delivers a signal representing these breaks to an assessment unit present in the vehicle. 
   One major drawback of the wear detector described in this document is that the detection circuit can comprise electrically active components in the mass of the tire. 
   One aim of the present invention is to propose a method of detecting one or more predetermined wear thresholds on a tire comprising tread pattern elements, which remedies this drawback. 
   To this end, the detection method according to the invention comprises the assigning a discrete wear variable representing the wear on the element to a discrete impedance variable representing the impedance of a passive tuned circuit contained in at least one of the tread pattern elements. 
   According to another characteristic of the invention, the method comprises the measurement of resonant frequencies of the tuned circuit which represent, respectively, values of the impedance variable and a comparison of each of these measured frequencies with one or more reference resonant frequencies. The reference resonant frequencies each represent a wear state on the element which does not reach at least one of the thresholds. 
   According to another characteristic of the invention, the tuned circuit comprises a coil and at least a quantity n (n≧1) capacitor(s) connected in parallel to the terminals of the coil, the impedance variable being a function of the inductance of the coil or of the capacitance of the capacitor or capacitors, the inductance (the first case) or the capacitance (the second case) being designed to vary discretely when the wear threshold or thresholds are reached. 
   In a first embodiment, this method is such that a discrete increase in the inductance represents the tire reaching a wear threshold or thresholds, measured by a discrete reduction in the resonant frequency. 
   According to another aspect of the first embodiment, and considering that the tread pattern elements are intended to be in contact with the ground through their respective external faces, the method provides the coil with one or more short-circuit conductive lines each comprising a vertex which is contained inside a tread pattern element and which is directed towards the external face of the tread pattern element, the respective vertices being situated at different heights in the radial direction of the tire, a wear threshold being reached when the external face of a tread pattern element has broken the vertex of a conductive line. 
   According to a second embodiment, the method includes the step of determining that a discrete reduction in the capacitance has occurred, the discrete reduction representing the tire reaching a threshold or one of the thresholds, this reaching then being measured by a discrete increase in the resonant frequency. 
   Further to the second embodiment, and considering again that the tread pattern elements are intended to be in contact with the ground through their respective external faces, the method comprises the step of determining the discrete reduction in capacitance measured in at least two capacitors, each provided with at least one conductive branch, wherein two or more branches are arranged in parallel, the at least one branch comprising a vertex which is contained inside the tread pattern element and which is directed towards the external face of 
   the tread pattern element, two adjacent capacitors being connected together by at least one branch, the vertices respectively relating to pairs of adjacent capacitors being situated at different heights in the radial direction of the tire, and the wear threshold or thresholds being reached when the external face has broken the vertex of the branch or one of the branches. 
   Advantageously, the impedance variable is measured over all or part of the width of the tire (i.e. the axial dimension thereof. 
   It should be noted that this method according to the invention makes it possible to indirectly detect the breaking of the short-circuit line or lines or parallel branches by measuring resonant frequencies, providing only electrically passive components in the tire. 
   In the present description, “vertex” means the portion of the short-circuit line or lines or parallel branch which is closest to the external face of the corresponding tread pattern element, in the radial direction of the tire. 
   According to one embodiment of the invention, the or each short-circuit line or parallel branch forms substantially an inverted U, whose connecting web (which may be straight or curved for example) forms the aforementioned vertex and is contained inside one of the tread pattern elements so that the web or its tangent are substantially parallel to the external face of the element. 
   According to a variant embodiment of the invention, the at least one short-circuit line or parallel branch forms substantially an inverted V, whose vertex is consequently practically at one point. 
   Another aim of the invention is to propose a detection unit intended to be incorporated in a tire for implementing the method according to the invention for detecting the reaching of one or more predetermined wear thresholds. 
   To this end, a tire casing according to the invention contains a passive electrical circuit which is covered with a rubber composition which can be used in the tread of the tire and which comprises a coil printed on a substrate and a quantity n (n≧1) capacitors connected in parallel to the terminals of the coil, and in that: 
   in a first embodiment, the coil is provided with one or more short-circuit lines each comprising a vertex which is contained inside the unit, the vertices being situated at different heights on the coil, or 
   in a second embodiment, each capacitor is provided with one or more branches in parallel, each comprising a vertex which is contained inside the casing, two adjacent capacitors being connected together by one or more branches, the vertices respectively relating to adjacent pairs of capacitors situated at different heights on the capacitors. 
   According to an example embodiment of the invention relating to the first case, the coil is provided with a single short-circuit line connecting a terminal of the coil to a turn thereof 
   According to another example embodiment of the invention relating to the first case, the coil is provided with a quantity m (m≧2) short-circuit lines connecting the same terminal of the coil to a same quantity m turns thereof, the respective heights of the line vertices as from the coil decreasing in the direction of the turns approaching the terminal. 
   According to an example embodiment of the invention relating to the second case, the passive circuit comprises two capacitors which are connected in parallel to the terminals of the coil and which are connected together by the branch in parallel or by a pair of such branches, the respective vertex or vertices of which occupy one and the same height from the capacitance, for detecting a single wear threshold on the tread pattern element containing the vertex or vertices. 
   According to another example embodiment of the invention relating to the second case, the passive circuit comprises at least three capacitors which are connected in parallel to the terminals of the coil and which are connected in pairs to each other by a branch in parallel or by a pair of such branches, so that the circuit comprises at least two branches whose respective vertices occupy different heights from the capacitors for detecting at least one intermediate wear threshold on the tread pattern element containing the vertices. 
   Another aim of the invention is to propose a tread pattern element for a tire intended to be attached to the tread of the tire, for implementing the aforementioned method according to the invention. 
   To this end, a tread pattern element according to the invention comprises a detection unit as defined previously, the tread pattern element comprising a base intended to be fixed to the tread, an external face intended to be in contact with the ground and the passive electrical circuit between the base and the external face, the vertex of the at least one short-circuit line or branch in parallel being adjacent to the external face and being situated so as to be recessed from the external face. 
   “Tread pattern element” means in the present description any element in relief which is intended to be in contact at one time or another with the ground following its incorporation in the tread, i.e. at the very start of running, or after wear on this element has begun. This element can thus consist of a block, for example, substantially parallelepiped or cylindrical in shape, or a circumferential rib with a variable transverse section (i.e. extends over all or part of the circumference of the tread). 
   It should be noted that the tread pattern element according to the invention can advantageously consist of a “wear indicator”, for example, in the form of a block or rib with a height substantially less than that of the tread pattern elements which are intended to be in contact with the ground from the start of running. 
   Another aim of the invention is to propose a tread for a tire and such a tire, for implementing the method according to the invention for detecting the reaching of one or more wear thresholds. 
   According to a first embodiment of the invention, a tread according to the invention is such that it comprises at least one tread pattern element such as the aforementioned, and a tire according to the invention is such that it comprises the tread. 
   It should be noted that a tire according to the invention might not include the unit according to the invention but only the passive electrical circuit according to the invention, which would then be without the rubber composition which coats this electrical circuit in order to form the unit. 
   Advantageously, the tread according to the invention also comprises a conductive short-circuit loop which is substantially parallel to the external face of the tread and which connects together the terminals of the tuned circuit for measuring the wear on all or part of the width of the tread when the loop is cut by the external face because of the wear. 
   According to a second embodiment of the invention, a tire for implementing the wear detection method comprises a carcass reinforcement which is radially surmounted by a crown reinforcement and which is extended laterally by side walls, a tread comprising tread pattern elements radially surmounting the crown reinforcement. This tire comprises the detection unit according to the invention, this unit being substantially in the shape of a tack whose head is connected to a radially internal face of the tire facing one of the tread pattern elements and whose stem contains said short-circuit line or lines or branch or branches in parallel and extends radially through the tire from the radially internal face and as far as the inside of the tread pattern element. 
   It should be noted that the tires according to these first or second modes comprise only electrically passive components in their mass. 
   Another aim of the invention is to propose a tire/wheel assembly for a motor vehicle comprising a wheel and a tire mounted on the wheel, for implementing the aforementioned wear detection method according to the invention. 
   To this end, the tire/wheel assembly according to the invention is such that the tire corresponds to the first or second aspects according to the invention, and that the passive electrical circuit in the tire is coupled to an active interrogation circuit which is fixedly mounted to the wheel and which comprises a frequency-sweep energy generator and means for detecting the tuning frequencies between the passive circuit and the interrogation circuit. 
   According to a preferred embodiment of the invention, the interrogation circuit comprises, in addition to the generator, a capacitor, a resistance and a coil coupled to the coil of the passive circuit, and the tuning frequency detection means is then connected to the terminals of the resistance to measure the voltage at these terminals. 
   According to an example embodiment of the invention, the interrogation circuit is mounted on an inflation valve of the wheel (i.e. an instrumented valve). 
   According to a variant embodiment of the invention, the interrogation circuit is mounted on a module for measuring the internal pressure of the tire mounted on the wheel. 
   According to another variant embodiment of the invention, the interrogation circuit is mounted directly on the wheel rim. 
   According to another characteristic of the invention, the interrogation circuit is provided with transmission means intended to transmit tuning frequency information to a central unit which is portable or mounted inside the vehicle, which central unit is intended to inform a user that at least one predetermined wear threshold of the tire has been reached. 
   Another aim of the invention is to propose a motor vehicle comprising a chassis, tire/wheel assemblies and a suspension device providing a flexible connection between the chassis and the tire/wheel assemblies, for implementing the aforementioned wear detection method according to the invention. 
   To this end, the vehicle according to the invention is such that it is equipped with at least one tire according to the first or second aspects according to the invention, and comprises on the chassis or on the suspension device an active interrogation circuit to which there is coupled the passive electrical circuit contained in the tire, the interrogation circuit comprising a frequency-sweep energy generator and a means of detecting the tuning frequencies between the passive circuit and the interrogation circuit. 
   According to another preferential characteristic of this vehicle, this interrogation circuit also comprises a capacitor, a resistance and a coil coupled to the coil of the passive circuit, and this tuning frequency detection means is connected to the terminals of this resistance in order to measure the voltage at these terminals. 
   According to one example embodiment of the invention, the interrogation circuit is mounted on a wall of the chassis corresponding to the wheel passage area. 
   According to a variant embodiment of the invention, the interrogation circuit is mounted on the suspension device, for example, on a suspension arm or on a damper cup forming a stop, on which a damper spring of the suspension arm bears. 
   According to another characteristic of this vehicle according to the invention, the interrogation circuit is provided with transmission means intended to transmit tuning frequency information to a central unit which is portable or mounted inside the vehicle, which central unit is intended to inform a user that a predetermined wear threshold of the tire has been reached. 
   Another aim of the present invention is to propose a detection system for implementing the method according to the invention for detecting that one or more predetermined wear thresholds of a tire have been reached. 
   This system comprises the detection unit according to the invention and an active interrogation circuit to which the passive circuit of the unit is coupled, this interrogation circuit comprising a frequency-sweep energy generator and a means of detecting the tuning frequencies between the passive circuit and the interrogation circuit. 
   The interrogation circuit preferably comprises, in addition to this generator, a capacitor, a resistance and a coil coupled to the coil of the passive circuit, and this tuning frequency detection means is then connected to the terminals of the resistance in order to measure the voltage at these terminals. 
   Also preferentially, this interrogation circuit is provided with transmission means intended to transmit tuning frequency information to a central unit, which may be portable or mounted inside a vehicle and intended to inform a user that a predetermined wear threshold of the tire has been reached. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned characteristics of the invention, as well as others, will be understood better from a reading of the following description of an example embodiment of the invention given by way of illustration and non-limitingly, the description to be read in conjunction with the accompanying drawings, amongst which: 
       FIG. 1  is a schematic perspective view of a coil printed on a substrate which is used for manufacturing a tuned circuit forming part of a wear measuring system according to the invention, 
       FIG. 2  is a schematic perspective view of a tuned circuit of the LC type according to one example embodiment of the invention, which is manufactured from the printed coil of  FIG. 1 , 
       FIG. 3  is a schematic perspective view of a tuned circuit of the LC type according to a further example embodiment of the invention, which is manufactured from the printed coil of  FIG. 1 , 
       FIG. 4  is a schematic perspective view of the tuned circuit according to the example in  FIG. 2  which is covered with a coating composition, 
       FIG. 5  is a partially exploded schematic view in cross-section of a part of a tire according to the invention illustrating selected various tread pattern elements which are instrumented according to different embodiments of the invention and which each comprise a tuned circuit according to the invention, 
       FIG. 6  depicts simplified electrical diagrams of an interrogation circuit and of a tuned circuit according to the invention coupled electromagnetically, in the wear measuring system according to the invention, 
       FIG. 7  is a schematic view in section illustrating two other variant embodiments of a tuned circuit included in a tread pattern element according to the invention, and 
       FIG. 8  is a schematic partial front view of a suspension device provided with an interrogation circuit according to the invention to which there is coupled a tuned circuit included in the adjacent tire for measuring the wear thereon. 
   

   DETAILED DESCRIPTION 
     FIG. 1  depicts a coil  1  comprising terminals  1   a  and  1   b  which is printed in a known manner on a flat substrate  2 . 
     FIG. 2  depicts a tuned circuit  3  of the LC type which is obtained from the printed coil  1  according to an example embodiment of the invention. This electrically passive tuned circuit  3  comprises two capacitors  4  and  5  which are connected in parallel by a pair of conductive bypass wires  6  and  7  and which are electrically connected to the coil  1 . As can be seen in  FIG. 2 , one of the capacitors  4  is connected directly to the terminals  1   a ,  1   b  of the coil  1  while the other capacitor  5  is connected in parallel with respect to the capacitor  4 . 
   Each capacitor  4 ,  5  consists in a known manner of two conductive plates  8  connected together by a dielectric  9 . In the example in  FIG. 2 , one of the bypass wires  6 , for example made from copper, connects the two capacitors  4  and  5  together (i.e. on only one side of the substrate  2 ) whilst forming a loop in an inverted U, the wings  6   a  and  6   b  of which are respectively fixed to the facing plates  8  of the capacitors  4  and  5  and the web  6   c  of which forming a vertex is substantially parallel to the plane of the substrate  2 . 
     FIG. 3  depicts a tuned circuit  10  of the LC type according to a variant embodiment of the invention, which is also obtained from the printed coil  1  of  FIG. 1 . This tuned circuit  10  comprises three capacitors  11 ,  12  and  13  which are connected in parallel by means of two pairs of conductive bypass wires  14 ,  15 ,  16  connecting them in pairs together and which are electrically connected to the coil  1 . As can be seen in  FIG. 3 , one of the capacitors  11  is directly connected to the terminals  1   a ,  1   b  of the coil  1  whilst the other two capacitors  12  and  13  are connected in parallel with respect to the capacitor  11 . 
   Each capacitor  11 ,  12 ,  13  consists of two conductive plates  17  connected together by a dielectric  18 . 
   In the example in  FIG. 3 , one of the bypass wires  14 , for example made from copper, connects together the adjacent capacitors  11  and  12  on one side of these like the wire  6  in  FIG. 2 , forming a loop in an inverted U, whose wings  14   a  and  14   b  are respectively fixed to the facing plates  17  of the capacitors  11  and  12  and the web  14   c  of which is substantially parallel to the plane of the substrate  2 . 
   On the same side as the substrate  2 , another bypass wire  15 , for example made from copper, connects together the adjacent capacitors  12  and  13  forming a loop in an inverted U whose wings  15   a  and  15   b  are respectively fixed to the plates  17  of the capacitors  12 ,  13  and whose web  15   c  is substantially parallel to the plane of the substrate  2 . 
   As can be seen in  FIG. 3 , the wings  14   a  and  14   b  of the wire  14  have a different height than the wings  15   a  and  15   b  of the wire  15 , which means that the webs  14   c  and  15   c  of the wires  14  and  15 , which are substantially parallel to the plane of the substrate  2 , are situated at different distances from the substrate  2 . 
     FIG. 4  depicts a unit  19  comprising the tuned circuit  3  of  FIG. 2  and a rubber coating composition  20  which covers the circuit  3  on at least one of the faces  2   a  of the substrate  2 . The unit  19  comprises an external face  19   a  and is intended to be incorporated in a tread  21  of a tire  22  (see  FIG. 5 ) with a view to detecting wear information on the tread  21 , as will be explained below. 
   The coating composition  20  is identical to the composition that constitutes the tread. 
   Naturally, the detection unit  19  could comprise in the same way the tuned circuit  10  according to  FIG. 3 , or any other tuned circuit according to the invention comprising more than two capacitors connected in parallel and electrically connected to the coil  1 . 
   In the example in  FIG. 4 , the detection unit  19  has a parallelepiped shape. It will be seen below that this unit  19  can have any geometry adapted to the chosen type of incorporation in the tread  21 . 
     FIG. 5  illustrates, by the arrow A, a first embodiment of an instrumented tread pattern element  23  of the tread  21  according to the invention. 
   This instrumented tread pattern element  23  has the shape of a parallelepiped block or rib, and is typically delimited by a base  24 , an external face  25  intended to contact and wear on the ground during the running of the tire  22  and by lateral faces  26  connecting the external face  25  to the base  24 . According to the invention, this element  23  comprises the aforementioned detection unit  19 . 
   The tread pattern element  23  thus comprises the tuned circuit  3  or  10  and the rubber coating composition  20 , which is identical to the composition of the tread  21 . 
   It should be noted that this element  23  must be such that the web  6 c of the wire  6  or the webs  14   c ,  15   c  of the wires  14 ,  15  form respectively the aforementioned loop or loops as inverted U&#39;s which are situated below the external face  25  of the element  23  (i.e. these webs  6   c ,  14   c ,  15   c  are not flush with the surface of the external face  25 ). 
   In the example of the tuned circuit  3  in  FIG. 2  comprising only one wire  6  in an inverted U, it should be noted that the distance between the web  6   c  of the wire  6  and the external face  25  of the element  23  is designed so as to define a critical wear level to be detected for the tread  21  (this distance being in particular determined by the height of the loop formed by the wire  6 ). 
   In the example of the tuned circuit  10  in  FIG. 3  comprising two wires  14  and  15  in an inverted U, it will be noted that the distance d, between the web  15   c  of the wire  15  which is furthest away from the substrate  2  and the external face  25  of the element  23  is designed to define an intermediate wear level to be detected for the tread  21 , whilst the distance d 2  (greater than d 1 ) between the web  14   c  of the other wire  14  (which is closest to the substrate  2 ) and the external face  25  is designed to define the critical wear level to be detected (these distances being in particular determined by the heights of the loops respectively formed by wires  14  and  15 ). 
   This tread pattern element  23  was incorporated in the tread  21  when the latter was manufactured, by connecting the base  24  of the element  23  with a location  27  on the tread  21  which is provided for this purpose. As can be seen in  FIG. 5 , this location  27  has for example the form of a cavity with a rectangular cross-section which is intended to receive the element  23 . 
   This incorporation of the tread pattern element  23  in the tread  21  can be effected before the curing of the latter, by a suitable positioning of the element  23  inside the curing mould (not shown), or after curing, using known techniques peculiar to the retreading of tires (by means of special adhesive and heat treatment). 
   The arrow B in  FIG. 5  illustrates a second embodiment of an instrumented tread pattern element  28  according to the invention. 
   This tread pattern element  28  consists of a “wear indicator”, i.e. an element for example in the form of a block or rib with a height substantially less than that of the tread patterns on the tread  21 . 
   Like the tread pattern element  23  according to the first embodiment, the element  28  comprises the aforementioned detection unit  19 , i.e. the tuned circuit  3  or  10  as well as the rubber coating composition  20 , which is still identical to the composition of the tread  21 . 
   It should be noted that this element  28  must be such that the web  6 c of the wire  6  or the webs  14   c ,  15   c  of the wires  14 , 15  forming respectively the aforementioned loop or loops in an inverted U which it comprises are not flush with the surface of the external face  28   a  of the element  28 . 
   As with the aforementioned first embodiment, it should be noted that, in the example of the tuned circuit  3  of  FIG. 2  comprising a single wire  6  in an inverted U, the distance between the web  6   c  of the wire  6  and the external face  28   a  of the element  28  is designed to define a critical wear level to be detected for the tread  21  and that, in the example of the tuned circuit  10  in  FIG. 3  comprising two wires  14  and  15  in an inverted U, the distance d 1  between the web  15   c  of the wire  15  which is furthest away from the substrate  2  and the external face  28   a  is designed to define an intermediate wear level to be detected for the tread  21 , whilst the distance d 2  (greater than d 1 ) between the web  14   c  of the other wire  14  (which is closest to the substrate  2 ) and the external face  28   a  is designed to define the critical wear level to be detected. 
   This tread pattern element  28  was incorporated in the tread  21  during the manufacture thereof, like the tread pattern element  28  of the first embodiment, for example by adhesive bonding to the bottom of a groove of the tread  21 . 
   The arrow C in  FIG. 5  illustrates a third embodiment of an instrumented tread pattern element  29  according to the invention. 
   This tread pattern element  29  consists of a block with a parallelepiped shape or a rib, and is provided in its mass with the detection unit  19 , which extends radially through the tire  22  from the radially internal face  22   a  of the tire  22  (corresponding to the internal rubber compound) and as far as the inside of the block or rib forming the element  29 . 
   As can be seen in  FIG. 5 , the unit  19  has substantially the shape of a tack (i.e. a flat head  30  on which a stem  31  is mounted), so that the substrate  2  of the tuned circuit  3  or  10  is contained in the head  30  and the wire  6  or the wires  14 ,  15  in an inverted U extend from the head  30  along the stem  31 . 
   Inside the unit  19 , the whole of the tuned circuit  3 ,  10  is covered with the coating composition  20 , which is for example identical to that of the tread  21 . 
   As before, it should be noted that this instrumented tread pattern element  29  should be such that the web  6   c  of the wire  6  or the webs  14   c ,  15   e  of the wires  14 ,  15  which it comprises are not flush with the surface of the external face  29   a  of the element  29 . 
   As with the aforementioned first and second embodiments, it should be noted that, in the example of the tuned circuit  3  in  FIG. 2  comprising a single wire  6  in an inverted U shape, the distance between the web  6   c  of the wire  6  and the external face  29   a  of the element  29  is designed to define a critical wear level to be detected for the tread  21  and that, in the example of the tuned circuit  10  in  FIG. 3  comprising two wires  14  and  15  in an inverted U shape, the distance d 1  between the web  15   c  of the wire  15  which is further away from the substrate  2  and the external face  29   a  is designed to define an intermediate wear level to be detected for the tread  21  whilst the distance between the web  14   c  of the other wire  14  (which is closer to the substrate  2 ) and the external face  29   a  is designed to define the critical wear level to be detected. 
   The head  30  of the unit  19  is mounted in contact with the radially internal face  22   a  of the tire  22  by means of a shoulder formed by its internal fixing face  30  with the stem  31 . 
   The head  30  can have any appropriate geometry (circular, rectangular, etc cross-section) provided that it matches the shape of this internal face  22   a  at the location provided for its fixing. This head  30  can advantageously exhibit a broadening of its section in the direction of its internal fixing face (frustoconical or pyramidal shape, for example). 
   As for the stem  31  of the unit, this can also have various geometries, for example prismatic or cylindrical in shape. 
   The unit  19  was incorporated in the tire  22  when the latter was manufactured, by connecting the shoulder  30 a of the head  30  and the stem  31  with a location in the tire  22  which is provided for this purpose. This location has for example the form of a cavity with a rectangular or circular cross-section which is intended to receive the stem  31 . 
   This incorporation of the unit  19  in the tire  22  can be effected before the latter is cured, by means of a suitable positioning of the element  23  inside the curing mould, or after curing, by means of a particular adhesive. 
   In  FIG. 7  two other variant embodiments of a tuned circuit  103  of the LC type according to the invention are illustrated, each variant being shown included in a tread pattern element  123  according to the invention. 
   The tuned circuit  103  according to the first variant (left-hand part of  FIG. 7 ) comprises a coil  101  at the terminals  101   a  and  101   b  of which a capacitor  104  is connected. This coil  101  is printed on a flat substrate (not shown) and is provided with four conductive short-circuit lines  105  to  108  connecting one and the same terminal  101   a  of the coil  101  to four turns thereof, respectively. Each line  105  to  108  forms a loop in an inverted U or V comprising a vertex  105   a  to  108   a  which is contained inside the tread pattern element  123  and which is directed towards the external face  123   a  of the latter. 
   It can be seen in  FIG. 7  that these vertices  105   a  to  108   a  are situated at different heights in the radial direction of the element  123 , the radial heights of these vertices  105   a  to  108   a  decreasing in the direction of the lines  105  to  108  which are connected to turns close to the terminal  101   a , and to the lines  105  to  108 . 
   The tuned circuit  203  according to the second variant (right-hand part of  FIG. 7 ) comprises a coil  201  at the terminals  201   a  and  201   b  of which a capacitor  204  is connected. This coil  201  is also printed on a flat substrate (not shown) and is provided with a single conductive short-circuit line  205  connecting a terminal  201   a  of the coil to a turn thereof. This line  205  comprises a vertex  205   a  which is contained inside the tread pattern element  123  and which is directed towards the external face  123   a  of the latter. 
   In relation to these two variant embodiments of tuned circuits  103 ,  203 , the external face  123   a  of the tread pattern element  123  corresponding to a critical wear level whose effect on the tuned circuit  103 ,  203  will be commented on below has been shown in dotted lines in  FIG. 7 . 
   It should be noted that the aforementioned description in relation to  FIG. 5  of these tread pattern elements  23 ,  28 ,  29  according to three embodiments of the invention (arrows A, B or C) applies mutatis mutandis to the tuned circuits  103  or  203  of  FIG. 7 , replacing the webs  6   c ,  14   c ,  15   c  of the wires  6 ,  14 ,  15  with the vertices  105   a ,  106   a ,  107   a ,  108   a  or  205   a  of the bypass lines  105 ,  106 ,  107 ,  108  or  205 . 
     FIG. 6  depicts in a simplified manner an interrogation circuit  40  and a tuned circuit according to the invention, such as the circuit  10 , which are coupled electromagnetically in order to form the wear measurement system according to the invention. In this example, the aforementioned tuned circuit  10  is shown in relation to  FIG. 3  comprising the three capacitors  11 ,  12 ,  13  connected in pairs to each other by the bypass branches  14  and  15 , the capacitor  11  being connected to the terminals of the coil  1 . It will be understood that any other tuned circuit according to the invention (such as the circuit  103  or  203  for example) could figure in place of this circuit  10 . 
   The interrogation circuit  40  can be mounted on a fixed or movable part of the vehicle which is situated facing the tire  22  according to the invention. 
   As a fixed part, it is possible for example to cite part of the chassis of the vehicle, and the interrogation circuit  40  can then be mounted on a wall of the chassis corresponding to the wheel passage area, such as the “mudguard”. 
   With reference to  FIG. 8 , it is also possible to cite by way of the fixed part of the chassis a part of a suspension device  50 , which is intended to provide a flexible connection between the chassis of a motor vehicle (not shown) and each wheel of the vehicle, on which the tire  22  is mounted (the corresponding wheel is not visible in  FIG. 8 ). An axle  51  on which the wheel is mounted by means of a journal  52  is also depicted partially in  FIG. 8 . 
   The suspension device  50  comprises essentially a suspension arm  53  provided with a damping spring  54  whose seat bears on a damper cup  55  forming a fixed stop for the spring  54  (the impact stop of this spring adjacent to the chassis is not shown), the interrogation circuit  40  being mounted on this cup  55 . 
   With a movable part, it is possible to cite for example the wheel on which the tire  22  is mounted, and the interrogation circuit  40  can then be mounted directly on the wheel rim, on the valve of the wheel or on an internal pressure measurement module with which the wheel is provided. 
   In the example embodiment in  FIG. 6 , the interrogation circuit  40  comprises a frequency-sweep energy generator  41 , a resistance  42 , a capacitor  43  and an antenna  44  formed by a coil which is intended to be coupled to the coil  1  of the tuned circuit  3  or  10 . It should be noted that this circuit  40  could be without a resistance  42 , i.e. comprising a capacitor  43  associated with a coil  44 . 
   The interrogation circuit  40  is provided with a means  45  of detecting the tuning frequencies (also referred to as resonant frequencies) between the tuned circuit  3  or  10  and the interrogation circuit  40 . This detection means  45  is advantageously connected to the terminals of the resistance  42  and is for example intended to measure the amplitude of the voltage at these terminals, which passes through an optimum (energy absorption) when there is frequency tuning between the two circuits  3  or  10  and  40 . 
   In addition, the interrogation circuit  40  is provided with transmission means designed to transmit tuning frequency information to a central unit mounted inside the vehicle according to the invention, which central unit is intended to inform the driver of the wear on the tread  21 . 
   The wear measuring system according to the invention, which comprises a passive tuned circuit  3 ,  10 ,  103 ,  203  (with reference to one of  FIGS. 2 ,  3 ,  5 ,  7 ) and an interrogation circuit  40 , such as the one in  FIG. 6 , functions as follows. 
   The supply frequency of the generator  41  is varied and the voltage at the terminals of the resistance  42  is measured in order to determine the tuning frequency of the tuned circuit  3 ,  10 ,  103 ,  203  with the interrogation circuit  40  which is coupled to it. 
   According to the invention, there is derived from the tuning frequency value detected at a given moment wear information on the tread  21  comprising the tuned circuit  3 ,  10 ,  103 ,  203 , as will be detailed below. 
   In the example in  FIG. 2  where the tuned circuit  3  comprises a single wire  6  in an inverted U connecting together the two capacitors  4  and  5  with respective capacitances C 1  and C 2 , whilst the wear on the external face  25 ,  28   a ,  29   a  of the tread pattern element  23 ,  28 ,  29  does not have the effect of cutting the web  6   c  of the wire  6 , the detection means  45  of the circuit  40  detect a tuning frequency f 1  given by the following equation:
 
 f   1 =1/[2π√{square root over ( L ( C   1   +C   2 ))}],
 
   L being the inductance (expressed in henrys) of the coil  1  of the tuned circuit  3 . 
   The transmission means of the circuit  40  transmit this tuning frequency information (value f 1 ) to the central unit of the vehicle or to a portable unit, which on request informs the user that the tread  21  of the tire  22  has not yet reached a predetermined critical degree of wear. 
   After the breaking of the web  6   c  of the wire  6  by wear on the external face  25 ,  28   a ,  29   a , the detection means  45  of the circuit  40  detect a tuning frequency f 2  greater than f 1 , which is given by the following equation (the capacitor  5  being cut off, only the capacitor  4  is then connected to the terminals of the coil  1 ):
 
 f   1 =1/[2π√{square root over ( LC   1 )}].
 
   The transmission means of the circuit  40  transmit this tuning frequency information (value f 2 ) to the central unit of the vehicle, which informs the driver that the tread  21  of the tire  22  has reached the predetermined critical degree of wear. 
   In the example in  FIG. 3  where the tuned circuit  10  comprises two wires  14  and  15  in an inverted U connecting the three capacitors  11 ,  12 ,  13  with respective capacitances C ′2 , C′ 3  in pairs to each other as long as the wear on the external face  25 ,  28   a ,  29   a  of the tread pattern element  23 ,  28 ,  29  has not had the effect of cutting the web  15   c  of the wire  15  which is closest to the external face  25 ,  28   a ,  29   a  (i.e. the web  15   c  furthest away from the substrate  2 ), the detection means  45  of the circuit  40  detect a tuning frequency f 3  given by the following equation:
 
 f   1 =1/[2π√{square root over ( L ( C   1   ′+C′   2   +C′   3 ))}].
 
   The transmission means of the circuit  40  transmit this tuning frequency information (value f 3 ) to the central unit of the vehicle or to a portable unit, which on request informs the user that the tread  21  of the tire  22  has not yet reached a predetermined intermediate wear threshold (corresponding to the radial position of the web  15   c  of the wire  15  in the tread  21 ). 
   After this web  15   c  is broken by wear on the external face  25 ,  28   a ,  29   a  and as long as the web  14   c  of the wire  14  which is furthest away from the external face  25 ,  28   a ,  29   a  (i.e. the closest to the substrate  2 ) is not broken, the detection means  45  of the circuit  40  detect a tuning frequency f 4  greater than f 3 , which is given by the following equation (the third capacitor  13  being cut off, only the capacitors  11  and  12  are then connected to the terminals of the coil  1 ):
 
 f   1 =1/[2π√{square root over ( L ( C′   1   +C′   2 ))}].
 
   The transmission means of the circuit  40  transmit this tuning frequency information (value f 4 ) to the central unit, which informs the user that the tread  21  of the tire  22  has reached the predetermined intermediate wear threshold. 
   After the cutting of the web  14   c  of the wire  14  by increased wear on the external face  25 ,  28   a ,  29   a , the detection means  45  of the circuit  40  detect a tuning frequency f 5  greater than f 4 , given by the following equation (the second capacitor  12  being in its turn cut off, only the capacitor  11  is connected to the terminals of the coil  1 ):
 
 f   5 =1/[2π√{square root over ( L ( C′   1 ))}].
 
   The transmission means of the circuit  40  transmit this tuning frequency information (value f 5 ) to the central unit, which informs the user that the tread  21  of the tire  22  has reached a predetermined critical wear threshold (corresponding to the radial position of the web  14   c  of the tread  21 ). 
   In the first variant embodiment of the example in  FIG. 7  where the tuned circuit  103  comprises four short-circuit lines  105  to  108  connected to the coil  101 , as long as the wear on the external face  123   a  of the tread pattern element  123  has not had the effect of cutting the vertex  108   a  of the short-circuit line  108  which is closest to the external face  123   a , the detection means  45  of the circuit  40  detect a maximum tuning frequency f 0  (corresponding to a minimum value for the inductance of the coil  101 ). 
   The transmission means of the circuit  40  transmit this tuning frequency information f 0  to the central unit of the vehicle or to a portable unit, which on request informs the user that the tread  21  of the tire  22  has not yet reached a first predetermined intermediate wear threshold corresponding to the radial position of the vertex  108   a  in the tread  21 . 
   After the breaking of this vertex  108   a  by wear on the external face  1   23   a  and as long as the radially lower adjacent vertex  107   a  has not been cut, the detection means  45  of the circuit  40  detect a tuning frequency  11  lower than f 0  (corresponding to a higher value of the inductance). The transmission means of the circuit  40  transmit this tuning frequency information f 1  to the central unit, which informs the user that the tread  21  has reached the first predetermined intermediate wear threshold. 
   After the breaking of the vertex  107   a  of the short-circuit line  107  by further increased wear on the external face  123   a  and as long as the immediately lower vertex  106   a  has not been broken, the detection means  45  detect a tuning frequency f 2  lower than f 1  (corresponding to an even higher value of the inductance). The transmission means of the circuit  40  transmit this tuning frequency information f 2  to the central unit, which informs the user that the tread  21  has reached a second predetermined intermediate wear threshold corresponding to the radial position of the vertex  107   a  in the tread  21 . 
   After the breaking of the vertex  106   a  of the short-circuit line  106  by further increased wear on the external face  123   a  and as long as the immediately lower vertex  105   a  has not been broken, the detection means  45  detect a tuning frequency f 3  lower than f 2  (corresponding to an even higher value of the inductance). The transmission means of the circuit  40  transmit this tuning frequency information f 3 to the central unit, which informs the user that the tread  21  has reached a third predetermined intermediate wear threshold corresponding to the radial position of the vertex  106   a  in the tread  21 . 
   After the breaking of the vertex  105   a  of the short-circuit line  105  by further increased wear on the external face  123   a  (which then appears in dotted lines in  FIG. 7 ), the detection means  45  detect a tuning frequency f 4  lower than f 3  (corresponding to an even higher value of the inductance). The transmission means of the circuit  40  transmit this tuning frequency information f 4  to the central unit, which informs the user that the tread  21  has reached a fourth and last (critical) predetermined wear threshold corresponding to the radial position of the vertex  105   a  in the tread  21 . 
   In the second variant embodiment of the example in  FIG. 7  where the tuned circuit  103  comprises only one short-circuit line  205  mounted on the coil  201 , as long as the wear on the external face  123   a  of the tread pattern element  123  has not had the effect of breaking the vertex  205   a  of this line  205 , the detection means  45  of the circuit  40  detect a maximum tuning frequency f 0  (corresponding to a minimum value of the inductance of the coil  201 ). The transmission means of the circuit  40  transmit this tuning frequency information f 0  to the central unit, which on request informs the user that the tread  21  of the tire  22  has not yet reached a predetermined critical wear threshold corresponding to the radial position of the vertex  205   a  in the tread  21 . 
   After the breaking of this vertex  205   a  by wear on the external face  123   a  (which then appears in dotted lines in  FIG. 7 ), the detection means  45  of the circuit  40  detect a tuning frequency f 1  less than f 0  (corresponding to a higher value of the inductance). The transmission means of the circuit  40  transmit this tuning frequency information f 1  to the central unit, which informs the user that the tread  21  has reached the predetermined critical wear threshold. 
   In conclusion, it should be noted that the wear measurement system according to the invention has the advantage of comprising only one passive electrical circuit inside the tire  22 , because the wear detection is effected at a distance by the detection means  45  (whether on a fixed part of the vehicle or on the wheel).