METHOD FOR MEASURING THE THICKNESS OF A LAYER OF RUBBER-LIKE MATERIAL

Abstract

A method is provided for measuring a thickness of a layer of rubber-like material. The layer of rubber-like material includes a free face in contact with air and a face joined to an adjacent reinforcement made of elements electrically insulated from one another. Each of the elements includes at least one hysteretic material having a magnetic permeability greater than the magnetic permeability of air. According to the method, a sensitive element, which emits an alternating magnetic field, is brought towards the layer of rubber-like material whose thickness is to be measured, hysteretic losses in the adjacent reinforcement are measured at terminals of the sensitive element, and a thickness of the layer of rubber-like material is evaluated based on the hysteretic losses.

Claims

1-10 (canceled)

11: A method for measuring a thickness of a layer of rubber-like material that includes a free face in contact with air and a face joined to an adjacent reinforcement made of elements electrically insulated from one another, each of the elements containing at least one hysteretic material having a magnetic permeability greater than a magnetic permeability of air, the method comprising steps of: causing a sensitive element, which emits an alternating magnetic field, to be moved towards the layer of rubber-like material; measuring hysteretic losses in the adjacent reinforcement, the hysteretic losses being measured at terminals of the sensitive element; and evaluating a thickness of the layer of rubber-like material based on the hysteretic losses.

12: The method according to claim 11, wherein the step of causing the sensitive element to be moved towards the layer of rubber-like material includes applying to the free face of the layer of rubber-like material a housing in which the sensitive element is installed.

13: The method according to claim 12, wherein the housing includes, in addition to the sensitive element, an electronic measuring device.

14: The method according to claim 11, wherein the sensitive element is a solenoid formed as one of: a printed circuit, a copper wire wound with a ferromagnetic support, and a copper wire wound without a ferromagnetic support.

15: The method according to claim 11, further comprising steps of: supplying the sensitive element with an alternating electrical signal; and causing a frequency of the alternating electrical signal to vary.

16: The method according to claim 15, wherein the frequency of the alternating electrical signal is below a cut-off frequency of the sensitive element.

17: The method according to claim 15, further comprising a step of determining an appropriate excitation frequency of the sensitive element.

18: The method according to claim 11, wherein the sensitive element is formed from turns of a coil, and wherein the step of causing the sensitive element to be moved towards the layer of rubber-like material includes positioning the sensitive element in such a way that a plane parallel to the turns of the coil is parallel to a surface of the free face of the layer of rubber-like material.

19: The method according to claim 11, wherein an electrical conductivity of the adjacent reinforcement is such that a resistivity measured between two points located on two separate elements, or a resistivity measured between two points located on a same non-metallic element, is greater than one megaohm meter.

20: The method according to claim 11, wherein an electrical conductivity of the adjacent reinforcement is such that a resistivity measured between two points located on two separate elements, or a resistivity measured between two points located on a same non-metallic element, is greater than ten megaohm meter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Other objects and advantages of the invention will be clearly apparent from the following description of some preferred, but non-limiting, embodiments, illustrated by the following drawings, in which:

[0041] FIGS. 1a and 1b detail a first example of a sensitive element used in a method according to the invention, and curves representative of the frequency response of a sensitive element of this type.

[0042] FIGS. 2a and 2b show a second example of a sensitive element used in a method according to the invention, and curves representative of the frequency response of a sensitive element of this type.

[0043] FIG. 3, described above, shows the magnetic response of a material forming an adjacent tyre reinforcement, in the context of the execution of the method according to the invention.

DESCRIPTION OF THE BEST EMBODIMENT OF THE INVENTION

[0044] FIG. 1a shows a first example of a sensitive element used in a method according to the invention. In this example, the sensitive element is a coil 200, connected to an electronic measurement circuit 100. Advantageously, these two elements are installed in a housing, not shown in the drawing, designed to be positioned against the layer of material to be measured.

[0045] This housing may take the form of a portable element, which a user brings manually towards a tyre whose thickness is to be measured. In another example, the housing may take the form of a retarder installed on a roadway, over which retarder a vehicle, fitted with a tyre whose wear is to be measured, is made to pass. In another embodiment, the housing may be integrated into the roadway so as not to retard the vehicles whose tyre wear is to be measured.

[0046] In the example of FIG. 1a, the coil 200 must be supplied with an alternating electrical signal in order to measure the thickness of the layer of rubber-like material. For this purpose, the excitation frequency of said coil should be set to an appropriate value.

[0047] This coil 200 may be modelled by the combination of a pure inductance Ls and a pure resistance with a value of Rs. This pure resistance Rs is equivalent, for a certain frequency range of the supply signal, to the sum of the ohmic resistance of the inductance and a resistive component proportional to the losses generated by the magnetic field emitted by the coil 200, in the material forming the aforementioned adjacent reinforcement.

[0048] FIG. 1b shows the variation of the respective values of Ls (curve 300) and Rs (curve 400) when the supply frequency of the coil 200 is made to vary, in the absence of an adjacent reinforcement. Thus three areas of operation of the coil 200 are seen to appear.

[0049] The first area 500 corresponds to an operating mode in which the modelling of the coil 200 by an inductance placed in series with a resistance is valid.

[0050] The second area 600 corresponds to an area in which the inductance 200 acts as an anti-resonant circuit, since the value of the resistance Rs increases to such a point that said coil 200 can no longer be supplied. The positioning of the frequency Fc defining this area depends directly on the characteristics of the coil 200, which for example include, but are not limited to, the number of its turns, its ohmic resistance, the diameter of the conducting wires used to form the turns, or the nature of the material forming the turns.

[0051] The third area 700 is an area of operation in which the coil 200 is no longer simply similar to a pure inductance placed in series with a pure resistance. In this case, a capacitance Cs must be added to the model. According to the modelling schemas, this capacitance Cs may be placed in parallel with the pure inductance Ls or in parallel with the pair formed by the combination of the inductance Ls and the resistance Rs.

[0052] The area 500 is the preferred area of application of the method for measuring the thickness of a layer of rubber-like material according to the invention.

[0053] FIG. 2a shows the execution of a method according to the invention for measuring the thickness of a layer of material 10, having an adjacent reinforcement 30 on the face opposite the free face. This reinforcement 30 has been formed from steel cords assembled parallel to one another by means of a matrix of very highly resistive rubber-like material. The reinforcement formed in this way has a very low electrical conductivity, since the cords are not in contact with one another, and are connected mechanically by means of a resistive matrix. However, this reinforcement is a good magnetic field conductor, because the cables are ferromagnetic.

[0054] The first step of a method according to the invention is to position a flat coil 20 against the free face of the layer of material 10. This coil is then supplied with a supply signal whose frequency is below the cut-off frequency Fc of the coil. The variation of the resistance seen at the terminals of the coil is then measured.

[0055] As mentioned above, the coil 20 is equivalent to the combination of a pure inductance Ls and a resistance Rs. When the frequency of the supply signal of the coil 20 is made to vary, for a given distance between the coil 20 and the reinforcement 30, a variation is seen in the value of Rs.

[0056] This variation is the combination of the variation found in the absence of a reinforcement at a frequency below Fc, as shown in FIG. 1b, on the one hand, and the variation due to the presence of the reinforcement 30 near the coil 20 on the other hand.

[0057] FIG. 2b shows the variation of the quantity ΔRs, which corresponds to the portion of the variation of Rs which is solely due to the presence of the reinforcement 30 near the coil 20. In FIG. 2b, this variation is shown as a function of the frequency of the supply signal of the coil 20.

[0058] Thus the curve 41 shows the variation of ΔRs found when the thickness of the rubber-like material 10 is one millimetre.

[0059] The curve 51 shows the variation of ΔRs found when the thickness of the rubber-like material 10 is five millimetres.

[0060] The curve 61 shows the variation of ΔRs found when the thickness of the rubber-like material 10 is ten millimetres.

[0061] Finally, the curve 71 shows the variation of ΔRs found when the thickness of the rubber-like material 10 is twenty millimetres.

[0062] In all cases, the frequency of the supply signal used remains below the frequency Fc defined in the example of FIGS. 1a and 1b.

[0063] It has been found that, if the supply frequency is set at a level F1 a long way below the frequency Fc, it is difficult to separate the variations of ΔRs obtained at this frequency F1 for a variation of layer thickness varying between one and twenty millimetres.

[0064] However, if the frequency of the supply signal is set at a higher level F2, it may be found that the variation of ΔRs can provide much better sensitivity when the thickness of the layer of rubber-like material varies between one and twenty millimetres.

[0065] In this case, it becomes possible to use the variation of ΔRs for measuring the thickness of rubber-like material.

[0066] In a variant of the method according to the invention, for high values of supply frequency of the coil 20, it is also possible to use the variation of ΔLs and ΔRs simultaneously in order to measure the layer thickness.

[0067] This is because, in this case, in the absence of eddy currents, ΔLs increases when the distance between the coil 20 and the reinforcement 30 decreases. This variation therefore follows the same direction as the variation of ΔRs, because ΔRs increases with a decrease in the distance between the coil 20 and the reinforcement 30.

[0068] Advantageously, the variation of ΔRs may be used on its own, or combined with the variation of ΔLs. This embodiment is notably, but not exclusively, used when the supply frequency of the coil 20 exceeds ten percent of the frequency Fc defined above.

[0069] Alternatively, the variation of Ls and Rs may be used directly, separately or in combination, to provide a measurement of the thickness of the layer of rubber-like material. The distance measured in this way corresponds to the distance between a free face of a layer of rubber-like material and a metallic reinforcement present in the layer. Thus, by bringing a sensitive element towards the top of a tread pattern, the tyre wear may be determined by comparing an initial distance between the top of the tread pattern and the metallic reinforcement with a measured distance.