Patent Publication Number: US-11660912-B2

Title: Tire with a wireless indicator

Description:
PRIORITY 
     This application is a U.S. national application of the international application no. PCT/FI2018/050502 filed on Jun. 27, 2018, which claims priority of European application EP17397517.8 filed on Jul. 3, 2017, the contents of all of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to tires with electrical wear indicators. The invention relates to tires with wear indicators based on an LC or LCR resonator of which inductance and/or oscillation frequency in configured to change as a surface of the tire wears. 
     BACKGROUND 
     Remote monitoring systems employing LCR (inductance—capacitance —resistance) circuits are known e.g. from a document US 2005/0007239. In connection with an interrogation means, such circuits enable monitoring of a variety of properties, including strain, temperature, pressure, identification, performance, chemical phase transition (such as melting and state-of-cure), fluid level, wear, rotation rate, location and proximity. In general, the LCR circuit is passive, e.g. free from an electric source that converts chemical energy into electricity, even if the inductance itself is used to produce electricity by a varying magnetic field. However, the interrogation means is active, including an electric source that converts chemical energy into electricity. Typically the interrogation means is a hand held device or a device fixed to a system. The position of the interrogation means relative to the circuit can be reasonably freely chosen. However, the power consumption of the interrogation means depends on the reading distance. 
     SUMMARY 
     It has been found, that such remote monitoring systems are particularly feasible, when the measured surface, such as a wearing surface of a tire, needs to be leak-proof, either for water and/or other liquids or air and/or other gases. The issues is even more important if the liquid or gas is pressurized. In such a case, wiring from a measuring circuit would easily pose leakage problems. However, such issues are not present in wireless remote monitoring systems. 
     There are some problems when the system having the interrogator and the circuit is used to measure at least a property of a surface, e.g. wear of the tread of a tire. For example, the coil in such measurements is embedded in a piece of material that wears, e.g. a tread block of a tire. This affects the sensitivity of the measurements. It has been noticed that the sensitivity can be improved by applying the interrogation device to a suitable position. The interrogator is applied to suitable position with respect the circuit and the device, i.e. the tire, from which the property, such as wear, is measured. In some applications, the device, i.e. tire, naturally comprises some metal in between the circuit and the interrogator. In such systems, the wireless communication in between is deteriorated by the structure of the device. In particular in such cases, the mutual position between the circuit and the interrogator becomes important. In this description, the device is a tire having an electrical wear indicator. The tire may be a pneumatic tire. A tire or a pneumatic tire typically comprises a metallic reinforcing belt, e.g. a steel belt. The metallic belt may hinder the RF communication between the interrogator and the circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1   a    shows, as a side view, a wear indicator  190 , 
         FIGS.  1   b  and  1   c    show, as a side view, a device  100  having a wear indicator  190  for measuring wear of a surface  120  of the device  100 , 
         FIG.  1   d    shows wear (w, w1, w2) of the wear indicator of  FIG.  1     a,    
         FIG.  1   e    shows a worn wear indicator of  FIG.  1   c   , wherein the indicator has worn by the wear (w, w1, w2) of the surface  120 , 
         FIG.  1   f    shows, as a side view, a wear indicator  190  having two ferrite plates, 
         FIG.  1   g    shows, as a side view, a wear indicator, of which capacitive component comprises multiple capacitors, 
         FIGS.  2   a  to  2   e    show embodiments of wear indicators and corresponding devices, 
         FIG.  3    shows a capacitance c1 of a primary capacitive component  210  as function of wear w for some embodiments, 
         FIG.  4   a    indicates directions of magnetic fields generated and/or received by a primary inductive component  220  and a secondary inductive component  320 , 
         FIGS.  4   b  and  4   c    indicate magnetic fields generated and/or received by a primary inductive component  220  and a secondary inductive component  320 , when at least a reinforcing structure  150 ,  155  is arranged in the body  110 , 
         FIGS.  5   a  and  5   b    indicate positioning of a primary inductive component  220  relative to a secondary inductive component  320 , 
         FIGS.  6   a  to  6   i    illustrate embodiments of a primary capacitive component  210 , 
         FIG.  7    illustrates a system including a wear indicator, a gateway device  400 , and a cloud server  500 , 
         FIGS.  8   a  and  8   b    show a primary capacitive component  210  arranged in a blind hole  112  of a tire tread  120 , and 
         FIGS.  9   a  and  9   b    show a pneumatic tire  100  having the wear indicator. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, the embodiments are explained in connection with a wear indicator. However, in this description, a wear indicator serves for the purpose of an example of a more general device  100 , in particular a tire  100 , such as a pneumatic tire, having an embedded circuit  200  and in interrogator  300 .  FIG.  1   a    shows in a principal view a wear indicator  190 . The wear indicator comprises a circuit  200  and an interrogator  300 . The interrogator  300  is configured to interact with the circuit  200  wirelessly, as detailed below. This helps the leakage problems indicated above. 
     The wear indicator  190  is arranged in a tire  100  in such a way that a capacitive component thereof wears, as the tread  120  of the tire wears. The tire may be a pneumatic tire. However, the tire may be a non-pneumatic tire. Typically both pneumatic and non-pneumatic tires limit a cavity (e.g. a single cavity for pressurized air) or cavities (cavities within a non-pneumatic tire). The tire may be a tire for a passenger vehicle, such as a passenger car tire. The tire may be a heavy tire, i.e. a tire for a heavy machine, such as a forwarder, a loader, a truck, a caterpillar. The tire may be a tire for a motorcycle. 
     The circuit  200  comprises a primary capacitive component  210  and a primary inductive component  220 . The primary capacitive component  210  is electrically connected to the primary inductive component so as to form an electric oscillator. The circuit  200  may further comprise a resistive component (not shown). The oscillator is thus an LC or an LRC oscillator. The circuit  200  is energetically passive, i.e. it is free from a battery configured to convert chemical energy to electricity. The primary inductive component  220  transforms magnetic energy to electricity, which becomes temporarily stored in the primary capacitive component  210 , as per se known from an LC or an LCR oscillator. The oscillation frequency and/or the inductance of the circuit  200  is/are dependent on the capacitance of the primary capacitive component  210  and the inductance of the primary inductive component  220 . Typically, the angular resonant frequency of the circuit is expressed as ω=1/√(L1×c1), wherein L1 is the inductance of the primary inductive component  220  and c1 is the capacitance of the primary capacitive component  210 . As will be detailed below, in an embodiment, the primary capacitive component  210  is configured to wear in use, whereby its capacitance c1 changes. This affects e.g. the angular resonant frequency ω. This affects also the mutual inductance, in particular at a certain frequency, of the primary inductive component  220  and a secondary inductive component  320 . In this way, e.g. these quantities are indicative of how much the primary inductive component  220  has worn. However, also other quantities may affect the capacitance c1 of the primary capacitive component  210 . Thus, e.g. the aforementioned quantities may be indicative of also other parameters of the primary capacitive component  210  or the environment nearby the primary capacitive component  210 , such as moisture near the primary capacitive component  210  and/or moisture e.g. in between two electrodes ( 212 ,  214 ) of the primary capacitive component  210 . 
     The primary capacitive component  210  need not wear during measurements. It is possibly, for example, to measure the humidity of the environment, wherein the circuit is embedded. As known, the humidity affects the dielectric constant of a capacitor, and thus also the angular resonant frequency ω of the LC circuit. In addition or alternatively, the inductance of the primary inductive component  220  may be affected by the environment and/or use of the device  100 . For example, if the body  110  of the device  100  comprises magnetic material, the inductance of the primary inductive component  220  may change as the material of the body  110  wears. In addition or alternatively, the circuit  240  may comprise a primary sensor arrangement  240  for measuring some quantities. 
     The interrogator  300  comprises an electric source  330 , a communication circuit  310 , and a secondary inductive component  320 . The electric source is needed to power the interrogator. The electric source may be e.g. configured to transform mechanical and/or chemical energy to electric energy. As an alternative or in addition, the electric source may comprise a component configured convert magnetic energy into electricity. As an alternative or in addition, the electric source may comprise high-capacitance capacitor (e.g. a super capacitor) storing electric energy as such. Such a high-capacitance capacitor can be charged e.g. inductively or mechanically with a component transforming magnetic or mechanical energy, respectively, to electricity. A high-capacitance capacitor herein refers to a capacitor having a DC capacitance of at least 1 μF. 
     The secondary inductive component  320  is used to interrogate the circuit  200 . Thus, by forming a magnetic field to the secondary inductive component  320 , the magnetic field also penetrates the primary inductive component  220  thus affecting the mutual inductance of the interrogator  300  and the circuit  200 . In this way, the mutual inductance and/or the angular resonant frequency (or the resonant frequency) of the circuit can be measured. 
     The communication circuit  310  may be used to communicate the measured data to a gateway device  400  (see  FIG.  7   ). The communication circuit may comprise a control circuit for measuring the mutual inductance and/or the resonant frequency of the circuit. In the alternative, the interrogator  300  may comprise a separate control circuit for the purpose. In an embodiment, the interrogator  300  is configured to measure at least one of [i] a mutual inductance of the secondary capacitive component  320  and the circuit  200 , [ii] an inductance of the circuit  200 , and [iii] a resonance frequency of oscillation of the circuit  200 . 
     Referring to  FIG.  1   b   , such a wear indicator  190  can be used to measure wear of a first surface  120  of a body  110 , in particular wear of tire tread  120 , of which a part is formed by a tread block  110 . The first surface  120  is the surface that wears in use. When using such a wear indicator  190 , the circuit  200  is applied to a wearing surface  120 , e.g. a tread of a tire, in such a way that the primary capacitive component  210  also wears as the wearing surface  120  wears. The capacitor needs not to reach the surface of an unworn wearing surface, since it may suffice to measure the wear of only such surfaces that have worn a reasonable amount. However, preferably, only the primary capacitive component  210  wears, but not the primary inductive component  220 . Therefore and with reference to  FIG.  1   c   , in an embodiment, the circuit  200  is arranged in the body  110  in such a way that the primary capacitive component  210  configured to wear as the first surface  120  of the body  100  wears. Moreover, at least a part of the primary capacitive component  210  is arranged a first distance d1 apart from the first surface the body and inside the body  110 . Furthermore, at least a part of the primary inductive component  220  is arranged a second distance d2 apart from the first surface  120  of the body  110  and inside the body  110 . In a wear indicator, the second distance d2 is preferably greater than the first distance d1. In this way, as the first surface  120  wears, the primary capacitive component  210  starts to wear before the primary inductive component  220  starts to wear. Preferably the wear indicator  190  is arranged in such a way that, in normal use, the primary inductive component  220  does not wear. Moreover, as indicated above, in some other embodiments the second distance d2 may be less than the first distance d1, because neither of the primary capacitive and inductive components need to wear. 
     Moreover, the interrogator  300  is arranged, relative to the circuit  200 , in such a way that the secondary inductive component  320  is arranged on the same side of the first surface  120  as the primary inductive component  220 . The secondary inductive component  320  may be arranged inside the body  110  or on another side of the body  110 . Moreover, at least a part of the secondary inductive component  320  is arranged a third distance d3 apart from the first surface  120  the body, the third distance d 3  being greater than the second distance d2. This has the effect that also the secondary inductive component  320  does not start to wear until the primary inductive component  220  starts to wear (if it is to wear). This has the further effect that such a placement improves the magnetic coupling between the primary inductive component  220  and the secondary inductive component  320 . 
     Herein the body  110 , in combination with the wear indicator  190 , forms a tire  100  according various embodiments. Referring to  FIGS.  9   a  and  9   b   , in some embodiments, the body  110  is a body part of a pneumatic tire, whereby the device  100  is a pneumatic tire having an integrated electrical wear indicator. The body  110  may be e.g. a tread block of a tire  100 . 
     Referring to  FIG.  1   d   , in general, the amount of wear is referred to with a symbol w.  FIG.  1   d    indicates two values of wear, w1 and w2. In  FIG.  1   d   , the value of wear w1 refers to the value of wear w1 of the surface  120  of  FIG.  1   d   . The surface  120  may be e.g. unworn, and the value of wear w1 may be e.g. zero. 
       FIG.  1   e    shows the device  100  of  FIG.  1   d   , after the surface  120  has worn some amount. The value of wear of  FIG.  1   e    corresponds to w2. Thus, the surface  120  has worn by an amount of w2−w1 in between the  FIGS.  1   d    and  1   e.    
     Referring to  FIG.  1   b   , in an embodiment, the interrogator  300  is arranged on a second surface  130  of the object  110 , wherein the second surface  130  is opposite to the first surface  120 . The second surface may be a surface of a cavity limited by a tire  100 . For example, the second surface  130  may be a surface of an interior of a pneumatic tire  100 . 
     Since the primary capacitive component  210  is configured to wear by the same amount as the wearing surface  120 , preferably, the primary capacitive component  210  resists wear at most to the same degree as the body  110 . In other words, preferably, the material of the primary capacitive component  210  resists wear at most to the same degree as the material of body  110 . This ensures that the primary capacitive component  210  wears, in use, by the same amount as the wearing surface  120 ; at least when the surface  120  has worn to the limit where the primary capacitive component  210  starts to wear (see  FIG.  2   a   ). 
       FIGS.  2   a  to  2   e    indicate some embodiments of the device  100 . In these figures the primary capacitive component  210  comprises a first electrode  212  and a second electrode  214 . 
     As seen in  FIG.  2   a   , in an embodiment, when the first surface  120  is unworn, the primary capacitive component  210  is arranged a distance apart from the first surface  120 . In this way, the wear indicator is configured not to measure small values of wear, but only values greater than a limit. Such a limit is defined by the distance between the primary capacitive component  210  and the surface  120 . 
     In the embodiment of  FIG.  2   b   , the primary capacitive component  210  comprises a base capacitor  216 . The base capacitor  216  is configured not to wear in use. This has the effect that the capacitance of the primary capacitive component  210  remains sufficiently high throughout the design life of the wear indicator. The base capacitor  216  may comprise a part of the electrodes ( 212 ,  214 ; see  FIGS.  6   b  and  6   d   ). In addition or alternatively, the base capacitor  216  may comprise separate electrodes (see  FIG.  6   f   ). In addition or alternatively, the base capacitor  216  may comprise a separate capacitive component (see  FIG.  6   g   ). The separate capacitive component may be used also, when the primary capacitive component  210  comprises discrete capacitors  210   1 ,  210   2 ,  210   3 ,  210   4 ,  210   5 , and  210   6 , as indicated in  FIG.  1     g.    
     The purpose of such a base capacitor  216  is to tune the capacitance c1 and thus also the angular resonant frequency ω of the circuit  200 . This may improve the sensitivity of the circuit  200 . In particular, this may improve the sensitivity of the pair of the circuit  200  and the interrogator  300 , since the measurement electronics of the interrogator  300  may be designed to operate most efficiently on a defined frequency range. However, if the interrogator is designed differently, this issues noes not necessitate a use of a base capacitor  216 . 
     In an embodiment the base capacitor  216  (or  210   6 ) forms at least 25% of the capacitance c1 of the primary capacitive component  210 . For example, the base capacitor  216  may be arranged deeper in the body  110  (i.e. further away from the surface  120 ) than a wearing part of the primary capacitive component  210 . For example, the base capacitor  216  may be arranged e.g. on the other side  130  of the body  110  than the wearing part of the primary capacitive component  210 . 
     When the primary capacitive component  210  comprises multiple capacitors  210   1 ,  210   2 ,  210   3 ,  210   4 ,  210   5 , and  210   6 , as indicated in  FIG.  1   g   , the component  210   6  placed furthest away from the surface  120  may serve as the base capacitor  216  not designed to wear in use. However, in an embodiment according to  FIG.  1   g   , also the capacitors  210   6  may be designed to wear in use. 
     In the embodiment of  FIG.  2   c   , the components of the interrogator  300  are arranged within the body  110 . In the embodiment of  FIG.  2   d   , the first  212  and second  214  electrodes are wider at the wearing surface  120  than deeper inside the body. Such electrodes are shown in more detail e.g. in  FIGS.  6   c  and  6   d   . In the embodiment of  FIG.  2   e   , the interrogator  300  comprises, in addition to the secondary inductive component  320 , a secondary sensor arrangement  340 . Such a secondary sensor arrangement  340  may comprise sensor or sensors configured to measure the environment in which the interrogator  300  is. The secondary sensor arrangement  340  may comprise e.g. at least one of a temperature sensor, a pressure sensor, and an acceleration sensor. 
     Referring to  FIG.  2   d   , also the circuit  200  may comprise a primary sensor arrangement  240 . The primary sensor arrangement may comprise a sensor or sensors that require only a little electricity for functioning. The primary sensor arrangement may comprise e.g. at least one of a pressure sensor, a humidity sensor, and a temperature sensor. 
     It has been observed, that as the primary capacitive component  210  wears, the effects of capacitance changes for small values of wear may be hard to detect. The inventors assume that this is a results of the proportional capacitance change (change in proportion to the capacitance of the components  210 ) may initially be smaller than later, since later on the value of the capacitance is also smaller. This issue may be corrected to some extent with the base capacitor, as discussed above. However, preferably this issue is also corrected by careful design of the primary capacitive component  210 . Without going to details of the structure of the component at this point,  FIG.  3    shows capacitance values c1 as function of wear w for four different primary capacitive components  210 . 
     As shown by the curve  810  in  FIG.  3   , in an embodiment, the capacitance c1 of the primary capacitive component  210  may decrease with a constant slope for all values w of wear. Such a curve may be the result of the electrodes having the form of parallel plates ( FIG.  6   a   ) or co-centric electrodes ( FIG.  6   e   ) with or without a base capacitor  216  (see also  FIGS.  6   b  and  6   f   ). A corresponding effect can be achieved also by using separate capacitors  210   1 ,  210   2 ,  210   3 ,  210   4 , and  210   5 , (the capacitor  210   6  being the base capacitor) as indicated in  FIG.  1   g    the capacitors being equally spaced and equally large in terms of capacitance. However, since the capacitance c1 decreases also for small values of wear, the primary capacitive component is arranged to reach the wearing surface  120 , as in  FIGS.  1   b ,  1   g ,  2   b ,  2   d   , and  2   e.    
     As shown by the curve  820  in  FIG.  3   , in an embodiment, the capacitance c1 of the primary capacitive component  210  may decrease with a constant slope only for reasonably large values w of wear. Since the capacitance c1 does not decrease initially, the primary capacitive component is arranged a distance apart from the wearing surface  120 , as in  FIGS.  2   a  and  2   c   . Since the slope is constant thereafter, such a curve may be the result of the electrodes having the form of parallel plates ( FIG.  6   a   ) or co-centric electrodes ( FIG.  6   e   ) with or without a base capacitor  216  (see also  FIGS.  6   b  and  6   f   ). 
     As shown by the curve  830  in  FIG.  3   , in an embodiment, the capacitance c1 of the primary capacitive component  210  may decrease in such a way that the capacitance c1 changes as function of wear more rapidly initially than later. Formally, the capacitance c1 of the primary capacitive component  210  is a function of wear c1=c1 (w). Moreover, the rate of capacitance change is the derivative dc1/dw of the capacitance c1 with respect to the wear w. For some value w1 of wear, the derivative dc1/dw at this point is herein and commonly denoted by dc1/w|w1. As well known, the derivative is the slope of the tangent line at that point. The corresponding tangent line for the for the curve  830  depicted in the figure by the line  831 . For another value w2 of wear, the derivative dc1/dw at this point is herein and commonly denoted by dc1/w|w2. The corresponding tangent line for the for the curve  830  depicted in the figure by the line  832 . As indicated in the figures, the derivative is negative since the capacitance decreases as the surface wears. 
     As shown by the curve  830  in  FIG.  3   , in an embodiment, the absolute value of the derivate for small values of wear w is larger than for large values of wear w. Formally ∥dc1/w|w1∥&gt;∥dc1/w|w2∥ when w2&gt;w1. Herein ∥dc1/w|w1∥ denotes the absolute value of dc1/w|w1 and ∥dc1/w|w2∥ denotes the absolute value of dc1/w|w2. As known, the capacitance is proportional to area of the electrodes and inversely proportional to the distance between the electrodes. Thus, the curve  830  may be e.g. a result of the electrodes of  FIG.  6   c   , wherein the wider top of the electrodes  212 ,  214  is configured to wear earlier than the narrower bottom of the electrodes  212 ,  214 . Such a capacitance change may be achieved, in addition or alternatively, by arranging the top parts of the electrodes  212 ,  214  closer to each other than the bottom parts, as indicated in  FIG.  6   g   . A corresponding effect can be achieved also by using separate capacitors  210   1 ,  210   2 ,  210   3 ,  210   4 , and  210   5 , (the capacitor  210   6  being the base capacitor) as indicated in  FIG.  1   g   . In such a case, a capacitor  210   1  close to the surface  120  may have a capacitance greater than a capacitor  210   2  further away from the surface  120 . Moreover the capacitance of the base capacitor  210   6  may be higher than the capacitance of another capacitor  210   1 ,  210   2 ,  210   3 ,  210   4 , and  210   5 . 
     As indicated above, it may be beneficial to have a reasonably large capacitance c1. This value may be designed e.g. in such a way, that the resonant frequency of the circuit remains at reasonable level throughout the service life of the circuit. As shown by the curve  840  in  FIG.  3   , the capacitance c1 may be increased (relative to the curve  830 ). Such an increase may be achieved by a base capacitor  216 , e.g. the base capacitor of  FIG.  6   d   , or the capacitor  210   6  that is arranged deepest. 
     In an embodiment, the primary capacitive component  210  is configured such that for a first value of wear w1, the derivative of the capacitance c1 of the primary capacitive component  210  with respect to wear w has a first value of capacitance change dc1/dw|w1, and for a second value of wear w2, the derivative of the capacitance c1 of the primary capacitive component  210  with respect to wear w has a second value of capacitance change dc1/dw|w2. In an embodiment, the first value of capacitance change dc1/dw|w1 is different from the second value of capacitance change dc1/dw|w2. In a preferred embodiment, the first value of wear w1 is smaller than the second value of wear w2 and the first value of capacitance change dc1/dw|w1 is negative and smaller than the second value of capacitance change dc1/dw|w2. In practice, the derivatives can only be measured as a differential from two different measurements. The derivative dc1/w|w1 may be calculated as a differential measured from a range of 0.5 mm the range comprising the small value w1 of wear. The derivative dc1/w|w2 may be calculated as a differential measured from a range of 0.5 mm the range comprising the larger value w2 of wear. 
       FIGS.  6   a  to  6   i    show embodiments of the circuit  200 . The figures show only the primary capacitive component  210  and the primary inductive component  220 . The circuit  200  may further comprise resistive components. Moreover, the electrical wires in between the components have some resistance. 
     In  FIG.  6   a   , the primary capacitive component  210  is formed of a first plate forming a first electrode  212  and a parallel second plate forming a second electrode  214 . In between the electrodes  212 ,  214  is arranged some material  213  that is not electrically conductive. The electrical resistivity of such material  213  may be e.g. at least 10 Ωm at 20° C. In order to have mechanical stability, preferably the material  213  is solid dielectric material. Preferably, the solid dielectric material  213  is solid in at least typical use conditions, such as at the temperatures from −55° C. to +150° C., such as from −55° C. to +100° C. The dielectric material  213  may be solid also at other temperatures, however, preferably it does not melt or vaporize at the aforementioned temperature ranges. 
     In  FIG.  6   b   , a part of the electrodes  212 ,  214  form a base capacitor  216 . In  FIG.  6   c   , the capacitance change is designed to be initially larger than later on, as discussed in more detail above. In  FIG.  6   d   , a base capacitor  216  has been added to the electrodes of  FIG.  6     c.    
     In  FIG.  6   e   , the electrodes  212 ,  214  are arranged co-centrically. The outer electrode  212  has a shape of a generalized cylinder, such as an elliptic generalizer cylinder; preferably the outer electrode is a regular, i.e. circular, cylinder. The inner electrode  214  may be a bar or a cylinder. Preferably, some solid dielectric material  213  is arranged in between the inner electrode and the outer electrode. In  FIG.  6   f   , a base capacitor  216  has been added to the electrodes of  FIG.  6   e   . In  FIG.  6   g   , the diameter of the outer electrode  212  is less near a wearing surface than further away from the wearing surface. This has the effect that the capacitance change is designed to be initially larger than later on, as discussed in more detail above. Moreover, the embodiment of  FIG.  6   g    includes a base capacitor. 
     In  FIG.  6   h   , the primary capacitive component  210  comprises a first electrode  212 , which forms a capacitance with a ground electrode  214 , i.e. a second electrode. However, as indicated in  6   i , the circuit may function also without a ground electrode  214 . In this embodiment, a capacitance is formed in between the first electrode  212  and the environment wherein is arranged. However, it has been noticed, that the measurements are more accurate, when the primary capacitive component  210  comprises the first electrode  212  and the second electrode  214 . Measurements are accurate also when discrete capacitors are used (see  FIG.  1   g   ). 
     Referring to  FIG.  1   g   , the primary capacitive component  210  need not to comprise plates. For example, a primary capacitive component  210  may comprise capacitors  210   1 ,  210   2 ,  210   3 ,  210   4 ,  210   5 , and  210   6 , which may be e.g. discrete components. When the tire wears, the components and/or their wiring also wear, whereby the capacitance of the primary capacitive component  210  changes. In such a case, the capacitors are arranged electrically in parallel so that each one of the capacitors increases the capacitance of the component  210 . 
     Referring e.g. to  FIGS.  8   a  and  8   b   , in an embodiment, the body  110  comprises first material and limits a blind hole  112  for the circuit  200 . In the embodiment, the circuit  200  is arranged in the blind hole  112 . Moreover, the primary capacitive component  210  comprises (a,i) at least first electrode  212  or (a,ii) a capacitor  210   i  (i=1,2,3,4,5,6), and (b) dielectric material  213  such that at least some of the dielectric material  213  is left in between (c) a part of the body  110  and (d,i) the first electrode  212  or (d,ii) the capacitor  210   i  in a direction that is perpendicular to a normal N1 of the first surface  120 . Moreover, it is noted that an electrode  212 ,  214  forms at least a part of a capacitor in general. Furthermore, preferably the dielectric material  213  is not the same material as the first material. Such an embodiment may have been manufactured e.g. by forming the blind hole  112  into the wearing surface  120 , e.g. a tread  120  of a tire, and then inserting the circuit  200  into the blind hole  112 . Such a method for manufacturing is typically much easier than e.g. arranging the circuit  200  into the body  110  e.g. during polymerization of the body  110 . Moreover, forming a blind hole  112  to a cured or otherwise solid body  110  ensures that the circuit becomes arranged in a correct location and correct position. Such a blind hole can be formed e.g. during vulcanization of the tire, e.g. by using a tire mould. In the alternative, the blind hole can be manufactured, e.g. drilled, after vulcanization. 
     As indicated above and in  FIGS.  8   a  and  8   b   , in an embodiment, the primary capacitive component  210  comprises a second electrode  214 , and at least some of the dielectric material  213  is arranged in between the first electrode  212  and the second electrode  214 . As indicated in  FIGS.  8   a  and  8   b   , also some of the dielectric material  213  is left in between the body  110  and the first electrode  212  in a direction that is perpendicular to a normal N1 of the first surface  120 . 
     In a preferred embodiment, the primary inductive component  220  and the second inductive component  320  component are arranged relative to each other in such a way that their magnetic fields are strongly coupled. Moreover, in a preferable embodiment, the body  110  is formed of solid material, and the primary inductive component  220  and the second inductive component  320  are rigidly fixed to the body  110 . This has the effect that the mutual orientation and distance of the primary inductive component  220  and the second inductive component  320  remain constant in use, which significantly improves the sensitivity of the measurements and simplifies the analysis of the measured data. 
     Correspondingly, in an embodiment, the primary inductive component  220  is configured to form a primary magnetic field B1 and the secondary inductive component  320  is configured to form a secondary magnetic field B2. As known to a skilled person, the direction of such a magnetic field depends heavily on the point of observation. However, in the centre of the primary inductive component  220 , the primary magnetic field B1 is directed to a primary direction dB1. This applies at least in the centre of a primary coil  222  comprised by the primary inductive component  220 . Moreover, in the centre of the secondary inductive component  320 , the secondary magnetic field B2 is directed to a secondary direction dB2. This applies at least in the centre of a secondary coil  322  comprised by the secondary inductive component  320 . To have strong coupling between the magnetic fields B1 and B2, in and embodiment, e.g. in the embodiment of  FIG.  4   a   , an angle α between the primary direction dB1 and the secondary direction dB2 is less than 30 degrees or more than 150 degrees, preferably less than 15 degrees or more than 165 degrees. Moreover, it is understood that the angle α between two directions is always at most 180 degrees by definition. At least, of the multiple angels that are theoretically definable, one is in the range from 0 to 180 degrees, and that angle is herein referred to. 
     However, referring to  FIG.  4   c   , the angle α needs not to be small. For example, a primary core  224 , such a primary axle  224  can be used to guide the primary magnetic field B1. In a similar manner, a secondary core  324 , such a secondary axle  324  can be used to guide the secondary magnetic field B2. In  FIG.  4   c   , the secondary core  324  comprises a turning, whereby the secondary core  324  is configured to guide the secondary magnetic field B2 in such a way as to interact strongly with the primary magnetic field B1. In  FIG.  4   a   , the primary core  224 , i.e. the primary axle  224  is straight. However, a skilled person can easily shape the cores  224 ,  324  to increase magnetic interaction. In order to guide the primary magnetic field B1, in an embodiment, the primary core  224  comprises paramagnetic or ferromagnetic material. In order to guide the secondary magnetic field B2, in an embodiment, the secondary core  324  comprises paramagnetic or ferromagnetic material. 
     Moreover, in a preferred application, the primary direction dB1 is substantially parallel to a normal of the wearing surface  120 . For example, an angle β between the primary direction dB1 and a normal N1 of the first surface  120  may be less than 30 degrees or more than 150 degrees, such as less than 15 degrees or more than 165 degrees. Herein the normal N1 refers to a normal of the surface  120  at a point that is closest to the primary capacitive component  210 . This has the effect that, when the secondary inductive component  320  is arranged on an opposite side of the body  110  than the surface  120  of which wear the circuit  200  is configured to measure, the primary  220  and secondary  320  inductive components can be arranged close to each other. 
     Referring to  FIG.  4   b   , in an embodiment, the device  100  comprises a first reinforcing structure  150 . The purpose of the first reinforcing structure  150  is to reinforce the device  100 . For example, the first reinforcing structure  150  may be a metal coating of the body  110  arranged such that the first reinforcing structure  150  forms the second surface  130  (see e.g.  FIG.  1   b   ). In the alternative, the first reinforcing structure  150  may be a wire mesh or a belt arranged inside the body  110 . The first reinforcing structure  150  may be a belt of a tire  100 . Since the purpose of the first reinforcing structure  150  is to reinforce the body  100 , preferably, the reinforcing structure does not limit large apertures. More precisely, preferably, the first reinforcing structure  150  does not limit an aperture having an area of at least 0.5 cm 2 . A large aperture would weaken the reinforcing structure. However, when the first reinforcing structure  150  is free from apertures, in an embodiment, a part of the first reinforcing structure  150  is arranged in between the primary inductive component  220  and the secondary inductive component  320 . 
     Reinforcing structures such as belts typically comprise metal, since metals in general strong. However, metals in general also conduct electricity well, whereby they hinder the magnetic coupling between the primary and secondary inductive components ( 220 ,  320 ). In an embodiment, the first reinforcing structure  150  comprises material having an electrical resistivity of at most 1 Ωm at the temperature 23° C., such at most 10 −5  Ωm at the temperature 23° C. In particular in such a case, the mutual distance and arrangement between the inductive components ( 220 ,  320 ) becomes important. The first reinforcing structure  150  may comprise steel, or it may consist of steel. The first reinforcing structure  150  may comprise a steel mesh. 
     In addition or alternatively, the first reinforcing structure  150  such as a belt may comprise fibrous material. The fibrous material of first reinforcing structure  150  may comprise at least one of cotton, rayon, polyamide (Nylon), polyester, polyethylene terephthalate, and Poly-paraphenylene terephthalamide (Kevlar). 
     Referring to  FIG.  4   c   , in an embodiment, the device  100  comprises a second reinforcing structure  155 . Also a part of the second reinforcing structure  155  may be arranged in between the primary inductive component  220  and the secondary inductive component  320 . However, the first reinforcing structure  150  may provide sufficient reinforcement, whereby the second reinforcing structure  155  may limit a hole (i.e. an aperture), and not even a part of the second reinforcing structure  155  is left in between the primary inductive component  220  and the secondary inductive component  320 . 
     The second reinforcing structure  155  may comprise fibrous material. The fibrous material of second reinforcing structure  155  may comprise at least one of cotton, rayon, polyamide (Nylon), polyester, polyethylene terephthalate, and Poly-paraphenylene terephthalamide (Kevlar). 
     Referring to  FIG.  1   f   , the magnetic coupling between the inductive components  220 ,  320  can be improved by using one or two plates  225 ,  325  of ferromagnetic or paramagnetic material, such as ferrite or a metal comprising iron. The wear indicator  190  may comprise a primary plate  225  configured to enhance the magnetic field of the primary inductive component  220 . As indicated in  FIG.  1   f   , an imaginary axis, encircled by the primary inductive component  220 , penetrates the primary plate  225 . The imaginary axis may be parallel to the primary magnetic field generated B1 by the primary inductive components  220 , in particular a primary coil  222 , in its centre. In this way, the primary plate  225  is in magnetic connection with the primary coil  222 . As indicated in  FIG.  1   f   , preferably, the primary plate  225  is not arranged in between the primary inductive component  220  and the secondary inductive component  320 . 
     In addition or alternatively, the wear indicator  190  may comprise a secondary plate  325  configured to enhance the magnetic field of the secondary inductive component  320 . As indicated in  FIG.  1   f   , an imaginary axis, encircled by the secondary inductive component  320 , penetrates the secondary plate  325 . The imaginary axis may be parallel to the secondary magnetic field B2 generated by the secondary inductive component  320 , in particular a secondary coil  322 , in its centre. In this way, the secondary plate  325  is in magnetic connection with the secondary coil  322 . As indicated in  FIG.  1   f   , preferably, the secondary plate  325  is not arranged in between the primary inductive component  220  and the secondary inductive component  320 . 
     Referring to  FIG.  5   a   , in general, the primary inductive component  220  comprises a primary coil  222  wound about a primary axis AX1 and the secondary inductive component  320  comprises a secondary coil  322  wound about a secondary axis AX2. Such axes (AX1, AX2) may be clearly defined physical axles, e.g. comprising ferromagnetic or paramagnetic material. For example, in  FIG.  4   c   , the primary coil  222  is wound about a primary core  224 , which is an axle, thus forming the primary axis AX1 (compare to  FIG.  5   a   ). In this way, the primary core  224  is in magnetic connection with the primary coil  222 . Moreover, in this way, the secondary core  324  is in magnetic connection with the secondary coil  322 . Moreover, in  FIG.  4   c   , the secondary coil  322  is wound about a part of a secondary core  324 . The corresponding part (onto which the secondary coil  324  is wound), thus forms the secondary axis AX2. 
     However, a coil may be formed in planar form on a circuit board, whereby the centre of the coil would define the corresponding axis. Moreover a coil needs not to surround any solid material. As known to a skilled person, the direction of the primary axis AX1 is parallel (i.e. unidirectional or reverse) to the aforementioned primary direction dB1 and the direction of the secondary axis AX2 is parallel (i.e. unidirectional or reverse) to the aforementioned secondary direction dB2. 
     Referring to  FIG.  5   a   , the primary coil  222  has a primary cross section XS1 on a plane having a normal that is parallel to the primary axis AX1; and the secondary coil  322  has a secondary cross section XS2 on a plane having a normal that is parallel to the secondary axis AX2. As an alternative expression, the primary coil  222  is configured to form the primary magnetic field B1 that is in the centre of the primary coil  222  directed to a primary direction dB1, and the primary coil  222  has a primary cross section XS1 on a plane having a normal that is parallel to the primary direction dB1. In a similar way, the secondary coil  322  is configured to form the secondary magnetic field B2 that is in the centre of the secondary coil  322  directed to a secondary direction dB2, and the secondary coil  322  has a secondary cross section XS2 on a plane having a normal that is parallel to the secondary direction dB2. Herein the primary cross section XS1 is limited by the outermost perimeter of the primary coil  222 . In addition, the secondary cross section XS2 is limited by the outermost perimeter of the secondary coil  322 . The coils  222 ,  322  may be arranged on a printed circuit board, such as a multilayer printed circuit board. 
     Referring to  FIG.  5   b   , in order to have the strong coupling between the magnetic fields B1 and B2, in an embodiment, the primary cross section XS1 and the secondary cross section XS2 are arranged relative to each other in such a way that an imaginary straight line IML that is parallel to the primary direction dB1 and/or the secondary direction dB2 penetrates both the primary cross section XS1 and the secondary cross section XS2. This embodiment is shown in  FIG.  5     b.    
     As indicated in  FIG.  5   b   , preferably, the primary and secondary cross sections XS1, XS2 overlap by a reasonably amount. As indicated in  FIG.  5   b   , the directions dB1 and dB2 are parallel, the cross sections section XS1 and XS2 can be projected, in the direction dB1, onto a same plane P that has a normal in the direction dB1. The overlapping part XS12 of the cross sections is the intersection (intersection in the mathematical meaning, commonly denoted by XS1∩XS2) of the projections of the cross sections XS1 and XS2 on the plane P, as indicated in  FIG.  5   b   . In case the directions dB1 and dB2 are not parallel, the projections of XS1 and XS2 can be considered to be projected in either of the directions dB1 or dB2, onto a same plane P that has a normal in the direction dB1 or dB2, respectively. 
     As indicated in  FIG.  5   b   , the area Axs12 of the overlapping part XS12 is reasonably large compared to the area Axs1 of the primary cross section XS1 and/or to the area Axs2 of the secondary cross section XS2. It is also noted that the area Axs1 of the primary cross section XS1 is equal to the area Axs1 of the projection of the primary cross section XS1 on the plane. In a similar way, the area Axs2 of the secondary cross section XS2 is equal to the area Axs2 of the projection of the secondary cross section XS2 on the plane P. In a preferred embodiment, the area Axs12 of the overlapping part XS12 of the primary cross section XS1 and the secondary cross section XS2 is at least 25%, such as at least 33%, or at least half of the smaller of the following: the area Axs1 of the primary cross section XS1 and the area Axs2 of the secondary cross section XS2. More preferably, the area Axs12 of the overlapping part XS12 is at least 66%, at least 75%, or at least 90% of the smaller of Axs1 and Axs2. 
     In addition, the magnetic coupling of the coils  222 ,  322  has been observed to be good when the cross sectional size of the primary coil  222  is of the same order of magnitude as the cross sectional size of the secondary coil  322 . Therefore, preferably the ratio of the cross sectional areas of the coils  322 ,  222 , i.e. Axs2/Axs1, is from 0.1 to 10, such as from 0.2 to 5. 
     However, at least in some tires it may be preferable to keep the circuit  200  small. Thus, in an embodiment, the ratio Axs2/Axs1 of the area Axs2 of the secondary cross section XS2 to the area Axs1 of the primary cross section XS1 is at least 0.5 or at least 0.75 or at least 0.9. However, as indicated above, if the difference of the areas is too large, magnetic coupling starts to decrease. Thus, the ratio Axs2/Axs1 may be e.g. in the range from 0.5 to 10; or from 0.75 to 7; or from 0.9 to 5. 
     In addition, the magnetic coupling of the coils  222 ,  322  has been observed to be good when the distance d12 (see  FIG.  4   a   ) between the primary inductive component  220  and the secondary inductive component  320  is small. For example, in an embodiment, the distance d12 is at most 75 mm, such as at most 50 mm, at most 25 mm, at most 15 mm, or at most 10 mm. 
     Referring to  FIG.  7   , in an embodiment, the interrogator  300  is configured to communicate with a gateway device  400 . The gateway device  400  may be configured to display a value of wear, e.g. for a user. The gateway device  400  may be configured to compare a value of wear to a limit value. The gateway device  400  may be configured send an alarm signal when the value of wear exceeds the limit value. Such an alarm signal may be optical or visual. Such an alarm signal may be sent for a user. 
     In addition or in the alternative, the gateway device  400  may be configured to communicate with a service provider, such as a mobile phone network. For example, gateway device  400  may be configured to communicate with a cloud service via a mobile phone network. In the alternative, the interrogator  300  may communicate directly with a service provider, such as a mobile phone network, or for example via a mobile phone network. However, having a gateway device  400  reasonably near the interrogator  300  reduces the power consumption of the interrogator  300 . Typically this is beneficial, since the electric source  330  of the interrogator  300  may be hard to change or charge. 
     Preferably, the interrogator  300  is configured to send data to a gateway device  400  that is arranged at most 50 metres, preferably at most 20 metres, such as at most 10 metres away from the interrogator  300 . Preferably, the gateway device  400  is configured to send and receive data from a cloud server  500 . The interrogator  300  may be configured to communicate with the gateway device  400  through a Bluetooth technology. The interrogator  300  may be configured to communicate with the gateway device  400  wirelessly using radio waves at a frequency range from 2.4 GHz to 2.485 GHz. 
     In an embodiment, the interrogator  300  is configured to measure at least one of [i] a mutual inductance of the secondary capacitive component  320  and the circuit  200 , [ii] an inductance of the circuit  200 , and [iii] a resonance frequency of oscillation of the circuit  200 . Such measured data is indicative of wear w of the first surface  120  as detailed above. Moreover, in an embodiment, the interrogator  300  is configured to determine a value of wear w using the measured data (i.e. the data indicative of the wear). The interrogator  300  may send the value of wear to the gateway device  400  or directly to a cloud server  500 . In the alternative, the interrogator  300  may send the data indicative of the wear to the gateway device  400  or directly to the cloud server  500 . Correspondingly, the gateway device  400  or the cloud server  500  may be configured to determine a value of wear w using the received data indicative of the wear. 
     An embodiment of the invention is also a system for measuring a wear w of a surface  120 . Such a system comprises the device  100  (i.e. the tire with the circuit  200  and the interrogator  300  attached to it) and the gateway device  400 . The interrogator  300  of the device  100  is configured to send data to the gateway device  400 . The gateway device  400  is configured to receive data from the interrogator  300 . The gateway device  400  may be configured to communicate with the user as indicated above. The gateway device  400  may be configured to communicate with the cloud server  500  as indicated above. 
     It is possible to receive a wear indicator  190  (see  FIG.  1   a   ) comprising a separate circuit  200  and separate interrogator  300 . Moreover, the tire  100  with a wear indicator can be formed by arranging the circuit  200  and interrogator  300  relative to each other and the body  110  of the tire following the principles presented above. Correspondingly, the wear indicator  190  is arranged to a body  110 . As indicated above, the body  110  may be a tread or a tread block of a tire, e.g. a pneumatic tire. 
     An embodiment of such a method comprises receiving (e.g. arranging available) the wear indicator  190 . As indicated above, the wear indicator  190  comprises [i] a circuit  200  comprising a primary capacitive component  210  configured to wear and a primary inductive component  220  and [ii] an interrogator  300  comprising an electric source  330 , a communication circuit  310 , and a secondary inductive component  320 . In the method, at least a part of the primary capacitive component  210  of the circuit  200  is arranged into the body  110 , i.e. a tread of a tire. The tread may comprise tread blocks, and the primary capacitive component  210  may be arranged into a tread block. Moreover, an embodiment comprises arranging the primary inductive component  220  of the circuit  200  with respect to the body  110  such that at least part of the primary capacitive component  210  is closer to the wearing surface  120  than at least a part of the primary inductive component  220 . Moreover, the interrogator  300  is attached to the body  110  or to the circuit  200 . The interrogator  300  is attached such that at least a part of the secondary inductive component  320  is arranged further away from the wearing surface  120  than the part of the primary inductive component  220 . 
     A preferred embodiment comprises attaching the interrogator  300  onto another surface  130  of the tire  100 . The surface may be a surface  130  of a cavity of the tire. The surface  130  may be an inner surface of the tire, which is a pneumatic tyre. An embodiment comprises attaching the interrogator  300  at least partly into the tire  100 . 
       FIG.  9   a    shows a tire  100 , which is a pneumatic tire. As well known, a tire has a tread  120 . The tread  120  is an outer surface of the tire. The tread is formed as a surface of a tread block arrangement  114 . The tread block arrangement  114  includes tread blocks  110 , arranged in such a way that a groove or grooves are left in between the tread blocks  110 , as indicated in  FIG.  9   a   . Correspondingly, a single tread block  110  forms at least a part, typically only a part, of the tread  120 . The tread  120  is intended for a rolling contact against a surface  900  when the pneumatic tire  100  is used. The tread  120  has a surface normal substantially parallel to the radial direction SR of the tire  100 , the radial direction SR being perpendicular to the axis of rotation AXR of the tire  100 . 
     A pneumatic tire  100  is an example of the device  100  discussed above. The tread blocks of the pneumatic tire  100  form a body having a wearing surface  120 . In case of a pneumatic tire, the wearing surface  120  is the tread of the pneumatic tire  100 . 
     Referring to  FIG.  9   b   , the tire  100  comprises tread blocks  110 . At least a tread block is equipped with the circuit  200  as indicated above. The circuit  200  is arranged in a tread block in such a way that the primary capacitive component  210  wears as the tread  120  wears. When the tread  120  wears, also the part of the tread  120  that is formed by the surface of the tread block  110  having the circuit  200  wears. The primary capacitive component  210  is electrically coupled to the primary inductive component  220 . The interrogator  300  arranged on the inner surface  130  of the pneumatic tire  100 . The primary inductive component  220  is aligned with the secondary inductive component  320  in a way discussed in detail above. 
     In a pneumatic tire  100 , the distance d12 (see  FIG.  4   a   ) between the primary inductive component  220  and the secondary inductive component  320  is typically at most 75 mm, as indicated above. However, the magnetic coupling between the inductive components  220 ,  320  is typically the better the closer the inductive components  220 ,  230  are to each other. Thus, the distance d12 is preferably smaller, as indicated above. 
     In a pneumatic tire  100 , the tread block  110  comprises first material, such as rubber. Moreover, in an embodiment, the circuit  200  is arranged in a blind hole  112  of the tread block  110 . Thus, the tread block  110  limits a blind hole  112  for the circuit  200 . Before arranging at least a part of the circuit  200  or the whole circuit  200  into a blind hole of the tread block, a bind hole may be arranged into the tread block  110 . The blind hole may be manufactured in a mould during vulcanization of the tire  100 , or it may be manufactured, e.g. drilled, after vulcanization. 
     Referring also to  FIGS.  8   a  and  8   b   , in such an embodiment, at least some of the dielectric material  213  of the primary capacitive component  210  is left in between a part of the tread block  110  and the first electrode  212  or a capacitor  210   1  in a direction that is perpendicular to a normal N1 of the tread  120 . Preferably, the dielectric material  213  is not the same material as the first material. However, preferably, the primary capacitive component  210  is at most as resistant to wear as the tread  120 . Therefore, the dielectric material  213  may be reasonably soft. For example, the dielectric material  213  may be at most as resistant to wear as the tread  120 . 
     When the circuit  200  and the interrogator are arranged as parts of a pneumatic tire  100 , the gateway device  400  (see  FIG.  7   ) may be arranged in the car on which wheel the pneumatic tire is configured to be arranged. 
     Referring to  FIG.  9   b   , a pneumatic tire  100  typically comprises a reinforcing belt  150 . The reinforcing belt  150  comprises first cords. At least some of the first cords typically comprise metal, such as steel. The first cords may, in the alternative or in addition, comprise fibrous material, such as at least one of glass fibres, carbon fibres, aramid fibres and para-aramid fibres (i.e. Kevlar®). When the first cord comprise steel, the reinforcing belt  150  is commonly referred to as a steel belt  150 . Most typically, steel belts are used to reinforce the tire  100 . In an embodiment, the reinforcing belt  150  comprises material having an electrical resistivity of at most 1 Ωm at the temperature 23° C., such at most 10 −5  Ωm at the temperature 23° C. In particular in such a case, the mutual distance and arrangement between the inductive components ( 220 ,  320 ) becomes important. 
     Since the purpose of the reinforcing belt  150  is to reinforce, the reinforcing belt  150  is preferably integral, i.e. not provided with large holes. 
     Correspondingly, in an embodiment, a part of the reinforcing belt  150  is arranged in between the primary inductive component  220  and the secondary inductive component  320 . In particular, when the reinforcing belt  150  is arranged in between the inductive components  220  and  320  and the reinforcing belt  150  comprises steel, the mutual alignment of the primary inductive component  220  and the secondary inductive component  320  is important. The short mutual distance d12, as discussed above, and/or the having substantially parallel magnetic fields, as indicated by the directions dB1 and dB2 above, improves the coupling also in these cases. 
     As indicated in  FIG.  9   b   , an embodiment of the pneumatic tire  100  comprises a ply  155  or plies  155 . The ply or plies  155  comprise rubber as a matrix material and second cords integrated with the matrix. The second cords may comprise fibrous material. The fibrous material of the second cords may comprise at least one of cotton, rayon, polyamide (Nylon), polyester, polyethylene terephthalate, and Poly-paraphenylene terephthalamide (Kevlar). The second cords reinforce the ply or plies  155 . 
     In an embodiment, a part of the ply  155  or a part of at least one of the plies  155  is arranged in between the primary inductive component  220  and the secondary inductive component  320 . This has the effect that also the ply  155  or plies  155  may be made integral, i.e. not provided with large holes. Thus, the reinforcing effect of the plies is utilized in full. 
     However, the reinforcing effect of the belt  150  may be sufficient. In such a case, the ply  155  or plies  155  may limit a hole. In such an embodiment, the primary inductive component ( 220 ) and secondary inductive component  320  are arranged relative to the hole in such a way that the ply  155  or the plies  155  is/are not left in between the primary inductive component  220  and the secondary inductive component  320 . 
     In case the interrogator  300  is arranged inside a pneumatic tire  100 , the interrogator  300  preferably comprises the secondary sensor arrangement  340  as discussed above. The secondary sensor arrangement  340  may comprise e.g. (1) a pressure sensor, (2) an acceleration sensor, (3) a pressure sensor and an acceleration sensor, (4) a pressure sensor and a temperature sensor, (5) a pressure sensor, a temperature sensor, and an acceleration sensor; or any other combination of a pressure sensor, an acceleration sensor, and a temperature sensor. 
     In case the interrogator  300  is arranged inside a non-pneumatic tire  100 , the interrogator  300  preferably comprises the secondary sensor arrangement  340  as discussed above. The secondary sensor arrangement  340  may comprise e.g. (1) an acceleration sensor, (2) a temperature sensor, or (3) an acceleration sensor and temperature sensor. 
     A pneumatic tire  100  with a wear indicator  190  can be manufactured e.g. by arranging available the circuit  200  and the interrogator  300  as detailed above. Moreover, at least a part of the primary capacitive component  210  of the circuit  200  is arranged into the tread block  110  and the primary inductive component  220  is arranged into the pneumatic tire  100  such that at least part of the primary capacitive component  210  is closer to the tread  120  than at least a part of the primary inductive component  220 . Preferably also the primary inductive component  220  is arranged in the tread block  110 . As indicated above, the circuit  200  may be e.g. arranged in a bling hole  112  of a tread block. 
     Furthermore, the interrogator  300  is attached onto a surface of a cavity limited by the tire, e.g. an inner surface  130  of the pneumatic tire  100 , or at least partly into the tire  100  such that at least a part of the secondary inductive component  320  is arranged further away from the tread  120  than the part of the primary inductive component  220 . Preferably, the interrogator  300  is attached onto an inner surface  130  of the pneumatic tire  100 .