METHOD FOR OBTAINING THE DEFORMATION OF A TIRE UNDER LOAD WHEN RUNNING

Abstract

A method for obtaining the deformation of a tire casing subjected to a load, rotating at a rotational speed W, comprising: Phase 1—Delimiting the first signal over a whole number of wheel revolutions to construct a first wheel revolution signal; and Determining a reference acceleration; and Phase 2—Delimiting the signal over a whole number of wheel revolutions to construct a second wheel revolution signal; Defining at least a first energy density S which is a function of the second wheel revolution signal, and of the reference acceleration, and which is denoted S+ when the wheel revolution signal is above a threshold value A, or is denoted S− when the wheel revolution signal is below or equal to said threshold value A; and Identifying the deformation as a function of the reference acceleration and of the first energy density S.

Claims

1.-13. (canceled)

14. A method for obtaining the deformation of a tire casing subjected to a load, in an inflated and laden state rotating at a rotational speed W, the tire casing having a crown, two sidewalls and two beads of revolution about a natural axis of rotation and a median plane, an intersection between the median plane and the natural axis of rotation defining a wheel center, the method comprising the following steps: fixing at least one sensor on the tire casing in line with the crown at a radial position R with respect to the natural axis of rotation, the at least one sensor being configured to generate at least one output signal proportional to acceleration experienced by the at least one sensor in the tire casing; acquiring a first abscissa signal u comprising at least an amplitude of the acceleration in a direction normal to the crown when running at the rotational speed W, at a sampling frequency fe the spatial discretization of which is less than 6 degrees; during a first phase: fixing a constant first sampling frequency fe1; delimiting the first signal over a whole number N.sup.TdR1 of wheel revolutions, N.sup.TdR1 being greater than or equal to 1, so as to construct a first wheel revolution signal Sig.sup.TdR1; determining a first reference acceleration γ.sup.reference1 as being a mean value of the wheel revolution signal Sig.sup.TdR1 with respect to one wheel revolution, using the following formula $\begin{array}{cc}{\gamma }^{\mathrm{reference}1}=\frac{\left({\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}1}}{\mathrm{Sig}}^{\mathrm{TdR}1}\right)}{N^{U1}},& \left[\mathrm{Math}1\right]\end{array}$ where N.sup.U1 is the number of points in Sig.sup.TdR1; and during a second phase: fixing a constant second sampling frequency fe2; delimiting the first signal over a whole number N.sup.TdR2 of wheel revolutions, N.sup.TdR2 being greater than or equal to 1, so as to construct a second wheel revolution signal Sig.sup.TdR2; defining at least a first energy density S which is a function of the second wheel revolution signal Sig.sup.TdR2, and of the reference acceleration γ.sup.reference1, and which is denoted S+ when the wheel revolution signal is above a threshold value A, or is denoted S− when the wheel revolution signal is below or equal to the threshold value A, using the following formulae $\begin{array}{cc}{S}^{+}=\left[{\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}2}>A}\left({\mathrm{Sig}}^{\mathrm{TdR}2}-{\gamma }^{\mathrm{Reference}1}\right)\right]*\frac{N^{\mathrm{TdR}2}}{N^{U2}},\mathrm{and}& \left[\mathrm{Math}2a\right]\\ S^{-}=\left[{\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}2}\le A}\left({\mathrm{Sig}}^{\mathrm{TdR}2}-{\gamma }^{\mathrm{Reference}1}\right)\right]*\frac{\stackrel{\mathrm{TdR}2}{N}}{N^{U2}},& \left[\mathrm{Math}2b\right]\end{array}$ where N.sup.U2 is the number of points in Sig.sup.TdR2; and identifying the deformation of the tyre casing Def %, which deformation is generated by the load as a function of the reference acceleration γ.sup.reference1 and of the first energy density S using one of the following formulae: $\begin{array}{cc}{\mathrm{Def}}_{\%}=\frac{N^{U1}}{N^{U2}}*\frac{\left[{\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}2}>A}\left({\mathrm{Sig}}^{\mathrm{TdR}2}-{\gamma }^{\mathrm{Reference}1}\right)\right]}{\left({\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}1}}{\mathrm{Sig}}^{\mathrm{TdR}1}\right)},\mathrm{or}& \left[\mathrm{Math}3a\right]\\ {\mathrm{Def}}_{\%}=\frac{N^{U1}}{N^{U2}}*\frac{\left[{\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}2}\le A}\left({\mathrm{Sig}}^{\mathrm{TdR}2}-{\gamma }^{\mathrm{Reference}1}\right)\right]}{\left({\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}1}}{\mathrm{Sig}}^{\mathrm{TdR}1}\right)},\mathrm{or}& \left[\mathrm{Math}3b\right]\\ {\mathrm{Def}}_{\%}=\frac{N^{U1}}{N^{U2}}*\frac{\begin{array}{c}\left[{\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}2}>A}\left({\mathrm{Sig}}^{\mathrm{TdR}2}-{\gamma }^{\mathrm{Reference}1}\right)\right]+\\ \left[{\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}2}\le A}\left({\mathrm{Sig}}^{\mathrm{TdR}2}-{\gamma }^{\mathrm{Reference}1}\right)\right]\end{array}}{\left(2*\left({\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}1}}{\mathrm{Sig}}^{\mathrm{TdR}1}\right)\right)}.& \left[\mathrm{Math}3c\right]\end{array}$

15. The method according to claim 14, wherein the acquisition of the first signal is performed for a rotational speed W greater than or equal to a threshold rotational speed W.sub.seuil defined by the following formula: $\begin{array}{cc}{W}_{\mathrm{seuil}}=\frac{12}{\sqrt{\mathrm{Dev}}},& \left[\mathrm{Math}4\right]\end{array}$ where Dev is the development of the tire casing.

16. The method according to claim 14, wherein the delimiting of the first signal over a number N.sup.TdR1 of wheel revolutions comprises the following steps during the first phase: identifying a first series of increments I, corresponding to the abscissa value u of the first signal at which the first signal crosses above or below a threshold value E; delimiting the first signal between a first increment I.sub.min and a second increment I.sub.max so as to construct a first wheel revolution signal Sig.sup.TdR1; and determining the number of wheel revolutions N.sup.TdR1 as being the difference I.sub.max minus I.sub.min.

17. The method according to claim 16, wherein the identification of the threshold E comprises the following steps before the first phase: fixing a sampling frequency fe0 for a first part of the first signal; identifying the maximum value MAX on the first part of the first signal; and defining the threshold value E which is a function of the value MAX.

18. The method according to claim 14, wherein the delimiting of the first signal over a number N.sup.TdR2 of wheel revolutions comprises the following steps during the second phase: identifying a first series of increments J, corresponding to the abscissa value u of the first signal at which the first signal crosses above or below a threshold value E; delimiting the first signal between a first increment J.sub.min and a second increment J.sub.max so as to construct a second wheel revolution signal Sig.sup.TdR2; and determining the number of wheel revolutions N.sup.TdR2 as being the difference J.sub.max minus J.sub.min.

19. The method according to claim 14, wherein the delimiting of the first signal over a number N.sup.TdR2 of wheel revolutions comprises the following steps: during the first phase: identifying the period T1 using the following formula $\begin{array}{cc}{T}_1=\frac{N^{U1}}{\mathrm{fe}1*N^{\mathrm{TdR}1}};& \left[\mathrm{Math}5\right]\end{array}$ and during the second phase: constructing a second wheel revolution signal Sig.sup.TdR2 beginning with the abscissa value u of the first signal situated at (1+M)/4 periods T1, M being a real positive number less than or equal to 2.0, after an end of the first wheel revolution signal Sig.sup.TdR1; delimiting the second wheel revolution signal Sig.sup.TdR2 over a duration t2 corresponding to the period T1 multiplied by a natural whole number N2, the number of wheel revolutions N.sup.TdR2 being equal to N2; identifying a first series of increments K, corresponding to the abscissa value u of the first signal beginning with the abscissa value u used to end the first wheel revolution signal Sig.sup.TdR1, at which the first signal crosses above or below a threshold value E′; and determining N′.sup.U2 as being the number of points in the first signal which are situated between the increments K.sub.1 and K.sub.N2+1, using the formula $\begin{array}{cc}{N}^{\prime U2}=\frac{\left(U_{K_{N2+1}}-U_{K_1}\right)}{N2}*N2,& \left[\mathrm{Math}6\right]\end{array}$  or which are situated between the increments K.sub.1 and K.sub.N2+2, using the formula $\begin{array}{cc}{N}^{\prime U2}=\frac{\left(U_{K_{N2+2}}-U_{K_1}\right)}{N2+1}*N2.& \left[\mathrm{Math}7\right]\end{array}$

20. The method according to claim 14, wherein the threshold value A for defining the first energy density is a function of the first reference acceleration γ.sup.reference1.

21. The method according to claim 20, wherein the threshold value A for defining the first energy density is a function of a factor C, the factor C being greater than or equal to 0.5 and less than or equal to 0.9, using the following formula
A=C*γ.sup.reference1  [Math 8].

22. The method according to claim 14, further comprising, having phased the second wheel revolution signal Sig.sup.TdR2 with respect to an angular position of the tire casing, making a correction Corr to the second wheel revolution signal Sig.sup.TdR2 to take account of the effect of the Earth's gravity.

23. The method according to claim 14, wherein the identification of the deformation of the tire casing Def % comprises the following steps during the second phase: determining a second reference acceleration γ.sup.reference2 associated with the second wheel revolution signal Sig.sup.TdR2, the latter being defined as being a mean value of the second wheel revolution signal Sig.sup.TdR2 with respect to one wheel revolution, using the following formula: $\begin{array}{cc}{\gamma }^{\mathrm{reference}2}=\frac{\left({\Sigma }_{{\mathrm{Sig}}^{\mathrm{TdR}2}}{\mathrm{Sig}}^{\mathrm{TdR}2}\right)}{N^{U2}};& \left[\mathrm{Math}9\right]\end{array}$ identifying a value O as being the value N.sup.U2 unless N′.sup.U2 exists, in which case the value of O is N′.sup.U2; identifying the deformation of the tire casing Def %, which deformation is generated by the load, using one of the following formulae $\begin{array}{cc}{\mathrm{Def}}_{\%}=\frac{N^{U1}}{O}*\frac{\left[S^{+}+\left({\gamma }^{\mathrm{reference}1}-{\gamma }^{\mathrm{reference}2}\right)*N^{\mathrm{TdR}2}\right]}{\left(N^{\mathrm{TdR}2}*{\gamma }^{\mathrm{reference}1}\right)},\mathrm{or}& \left[\mathrm{Math}10a\right]\\ {\mathrm{Def}}_{\%}=\frac{N^{U1}}{O}*\frac{\left(S^{-}\right)}{\left(N^{\mathrm{TdR}2}*{\gamma }^{\mathrm{reference}1}\right)},\mathrm{or}& \left[\mathrm{Math}10b\right]\\ {\mathrm{Def}}_{\%}=\frac{N^{U1}}{O}*\frac{\left[S^{+}+S^{-}+\left({\gamma }^{\mathrm{reference}1}-{\gamma }^{\mathrm{reference}2}\right)*N^{\mathrm{TdR}2}\right]}{2*\left(N^{\mathrm{TdR}2}*{\gamma }^{\mathrm{reference}1}\right)}.& \left[\mathrm{Math}10c\right]\end{array}$

24. The method according to claim 14, wherein the number of wheel revolutions N.sup.TdR2 of the second wheel revolution signal Sig.sup.TdR2 is unity, and the number of wheel revolutions N.sup.TdR1 of the first wheel revolution signal Sig.sup.TdR1 is unity.

25. The method according to claim 14 wherein, having made N.sub.i evaluations of the deformation of the tire casing Def.sup.i % over different first and second wheel revolution signals Sig.sup.TdR1, Sig.sup.TdR2 in the one same first signal, the deformation of the tire casing Def % is the mean of the deformations of the tire casing Def.sup.i % according to the following formula $\begin{array}{cc}{\mathrm{Def}}_{\%}=\frac{{\Sigma }_i{\mathrm{Def}}_{\%}^i}{N_i}.& \left[\mathrm{Math}11\right]\end{array}$

26. The method according to claim 25, wherein the N.sub.i evaluations are performed successively so that the second phase of the evaluation N.sub.i is the first phase of the evaluation N.sub.i+1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0088] The invention will be better understood on reading the following description, which relates to the case of application to pneumatic tyres. This application is given solely by way of example and made with reference to the attached figures in which:

[0089] FIG. 1 is an example of a first signal of the method.

[0090] FIG. 2 shows the first wheel revolution signal Sig.sup.TdR1 and how it is identified from the first signal.

[0091] FIG. 3 shows the second wheel revolution signal Sig.sup.TdR2 and how it is identified in a first embodiment.

[0092] FIG. 4 shows the second wheel revolution signal Sig.sup.TdR2 and how it is identified in another embodiment.

[0093] FIG. 5 shows the first and second wheel revolution signals Sig.sup.TdR1 and Sig.sup.TdR2 when running at a variable rotational speed W.

[0094] FIG. 6 shows the second wheel revolution signal Sig.sup.TdR2 when running at a rotational speed W that remains constant over a revolution.

DETAILED DESCRIPTION OF EMBODIMENTS

[0095] In order to implement the invention, a tyre casing has to be equipped with an electronic member comprising a sensor, a microcontroller, a clock, a memory space and an energy storage means, and radio frequency communications means capable of transmitting and possibly of receiving. The tyre casing comprises a crown, two sidewalls and two beads of revolution about a natural axis of rotation. The casing also comprises a median plane equidistant from the two beads, the intersection between the median plane and the natural axis of rotation defining a wheel centre.

[0096] The sensor is fixed to the tyre casing in line with the crown, in line with a rib or with a longitudinal groove which are regions of uniform stiffness, at a radial position R that is fixed with respect to the natural axis of rotation. The sensor is able to generate at least one output signal proportional to the acceleration normal to the crown that is experienced by the sensor inside the tyre casing. In fact, this sensor may be a single-axis sensor, in which case it needs to be positioned radially. It may also be made up of a plurality of single-axis sensors. In that case, the orientation of each of the single-axis sensors needs to be clearly identified with respect to the frame of reference of the tyre casing so as to reconstruct the acceleration normal to the crown of the tyre casing. Ideally, the sensor takes account of the continuous component and of the alternating component of the acceleration. In instances where only the alternating component is measured by the sensor, implementation of the method will require the continuous component to be constructed artificially. To do that, the rotational speed W of the tyre casing will need to be identified in real-time and the radial position R of the sensor will need to be precisely known. This is because the continuous component will be evaluated as being the centrifugal acceleration of the sensor with respect to the natural axis of rotation of the tyre casing. If the sensor takes account of the continuous component, this sensor may be an accelerometer using piezo-resistive or capacitive technology.

[0097] The electronic member is powered by the energy storage means, is controlled by the microcontroller with the aid of the clock, and also has implanted in it the calculation algorithms that make it possible for example to determine the state of deformation of the tyre by using the signals coming from the sensor element. The RF communications transmission means are used to transmit the calculated information and the reception means are used to receive operating instructions or information of use to the calculation algorithms. Ideally, this electronic member comprises or is associated with other measurement elements (measuring for example pressure, temperature, state of wear, distance travelled, etc.) so that components can be shared and operating costs optimized.

[0098] In this instance, the sensor is brought into operation by the microcontroller when the tyre casing is in a running condition. Of course, a threshold value for the rotational speed W beyond which acquisition of a sensor output signal is performed can be selected. The electronic member has a memory space suited to the type of analysis that is to be performed. In fact, the capacity of this memory space is predefined according to the use made of the electronic member. It is the microcontroller that controls the storage of the values from the sensor in the memory space. In addition, the microcontroller is able to perform elementary mathematical and logic operations on a reduced number of data. If the mathematical and logic operations are more complex, or if the number of data to be manipulated becomes great, the microcontroller is replaced by a microprocessor. Finally, the electronic member is powered by an energy storage means. The simplest energy storage means is the use of a battery. However, it is possible to envisage a large sized capacitor that can be recharged using a piezoelectric element.

[0099] The frequency range of the electronic member is able to cover a broad range of rotational speeds W with a spatial discretization of under 6 degrees. According to one particular embodiment, the sampling frequency is adaptive on demand or in response to a signal such as, for example, the rotational speed W of the tyre casing.

[0100] Optionally, the electronic member contains or is able to obtain the identification of the tyre casing. This information is useful for selecting a set of data that are useful for the calculation algorithms in the electronic member. If the electronic member needs to obtain the identification of the tyre or receive orders to take a measurement, the electronic member is equipped with a radiofrequency reception means. This operates in the low frequency range, ideally at the frequency of 125 kHz, so as to be free of the interference generated by the metallic regions of the tyre casing and its nearby environment in the vehicle.

[0101] According to one specific embodiment, the electronic member has radiofrequency transmission means, specifically in the UHF (ultra-high frequency) band, and particularly around 433 MHz or 900 MHz or what is known as the BLE (Bluetooth Low Emission) band which are free frequency bands. In addition, the UHF band makes it possible to have small antenna sizes making the electronic member easier to incorporate into the tyre casing.

[0102] This transmission communication is useful for transmitting the method data to the vehicle or to outside the vehicle. It is possible either to transmit the data string corresponding to the acquisition of the wheel revolution signal or to transmit the intermediate results that will have been calculated in the electronic member. This second transmission mode is necessarily less expensive in energy for the electronic member because the data stream is less intensive. Now, radiofrequency transmission consumes more energy than mathematical and logical operations.

[0103] FIG. 1 shows a first raw signal 1b in grey corresponding to the acceleration normal to the crown of a tyre casing of the heavy duty vehicle type running at a constant rotational speed W. Regularly, and periodically, the curve 1b passes through a near-zero value. This periodic phenomenon corresponds to the sensor passing through the contact patch of the tyre casing. The transition between the sensor passing through the contact patch of the tyre and the other part of the tyre casing occurs sharply at falling or rising fronts depending on whether the sensor is entering or leaving the contact patch. In addition, it will be noted that the first signal 1b on a scale of the order of one revolution of the wheel, follows a carrier wave, the first signal 1b oscillating about this carrier wave at a frequency higher than the frequency of revolution of the wheel. These oscillations correspond to the noise on the first signal 1b from the sensor, which noise is caused by the various unpredictable influences including the macroroughness of the road.

[0104] The curve indexed 1 in black represents the same accelerometer signal corrected only for the Earth's gravity, and which will be termed corrected first signal 1. The correction here is sinusoidal with the correction having being phased to a point situated at the centre of the contact patch, namely equal distances from the two wave fronts that delimit that part of the signal for which the value is near-zero. It may be seen that the first signal 1 is flatter between the regions characterizing the contact patch. It is preferable for the various steps of the method to be performed on this corrected first signal 1.

[0105] FIG. 2 shows the method for detecting the first wheel revolution signal 3. From the first signal 1, in this instance corrected in order to explain the example better, there is determined a threshold value E, illustrated by the dotted line 2. A series of increments I is identified, these being where the first signal 1 crosses the dotted line 2, for example from beneath, which physically corresponds to the sensor, securely connected in terms of rotation to the tyre casing, exiting the contact patch. The first wheel revolution signal 3 is then delimited as being between a first increment, in this instance I.sub.1, and a second increment, in this instance I.sub.3. The wheel revolution signal here represents the accelerometer signal from the sensor over two full revolutions of the wheel.

[0106] The threshold value E represented by the dotted line 2 has in this case been evaluated over part of the first signal 1 with a variable sampling frequency. The maximum discretized value obtained is extracted from this part of the first signal 1 and named MAX. The threshold value E is therefore a value comprised between 10 and 50% of the value MAX, and in this instance, this value is around 20%.

[0107] The first reference acceleration γ.sup.reference1 which is represented by the continuous line 4 in black, is calculated by way of the mean value of the first wheel revolution signal 3. It is evaluated in real-time by summing the values of the increments u of the first wheel revolution signal and then dividing, at the end of the wheel revolution signal, by the number of increments in the first wheel revolution signal.

[0108] FIG. 3 is an illustration of the delimitation of the second wheel revolution signal 7, in pale grey, from the first signal. This second wheel revolution signal 7 comes after the first wheel revolution signal which ends at the increment I.sub.3. In this case, it is the first embodiment that is used for this delimitation.

[0109] From the first reference acceleration γ.sup.reference1 illustrated by the continuous curve 4 in the preceding figure, a threshold value E′ is determined, this value in this instance being situated at half the first reference acceleration γ.sup.reference1. A series of increments J are then identified in the first signal corresponding to the crossing of the first signal through this threshold value E′. In the illustration it is the crossing of this threshold 1E′ from below that has been used to identify the increments J. The increments corresponding to the crossing of the threshold E′ from above could have been adopted. This threshold E′ is illustrated by the dotted line 5. The second wheel revolution signal 7 in grey is therefore delimited using a first increment, in this instance Ji, and a second increment, in this instance J.sub.3. This second wheel revolution signal 7 corresponds to a finite number of wheel revolutions, in this instance 2, give or take the discretization errors.

[0110] The second reference acceleration γ.sup.reference2 illustrated by the continuous line 6 is calculated by way of the mean value of the second wheel revolution signal 7, in the same way as before for γ.sup.reference1. This allows the calculation to be performed in real-time at the electronic member comprising the sensor by minimizing the memory and power resources of the electronic member.

[0111] FIG. 4 is an illustration of the delimitation of the second wheel revolution signal 7, in grey, from the first signal. This second wheel revolution signal 7 comes after the first wheel revolution signal which ends at the increment I.sub.3, which in this instance is the increment K.sub.1. In this case, it is the second embodiment that is used for this delimitation.

[0112] From the first wheel revolution signal, it is possible to calculate the period T of the first signal, with respect to one revolution of the wheel. Next, the method identifies the first increment u of the first signal which is situated between one quarter and three quarters of the period T situated after the end of the first wheel revolution signal delimited by the increment K1. In the illustration of FIG. 4, the second wheel revolution signal has been arbitrarily chosen to begin shortly after halfway through the period. That corresponds physically to the moment at which the sensor is situated opposite the contact patch defined by the tyre casing as it rotates as one with the tyre casing.

[0113] A second wheel revolution signal 7, in grey, is therefore constructed over a duration t corresponding to an integer multiple of the period T. The first increment u of the first signal situated after the duration t of this second wheel revolution signal 7 will not be included in the second wheel revolution signal 7.

[0114] From this second wheel revolution signal 7, the method determines a second reference acceleration γ.sup.reference2 as being the mean value of this second wheel revolution signal, represented by the continuous line 6.

[0115] Furthermore, the last increment u used for delimiting the first wheel revolution signal is used to define a series of increments K. The first increment K.sub.1 corresponds to the last increment u used to delimit the end of the first wheel revolution signal. The other increments K are calculated using a threshold value E′ represented by the dotted line 5 which will be less than or equal to half the first reference acceleration γ.sup.reference1 defined on the first wheel revolution signal. These increments allow the number of increments N′.sup.U2 to be identified.

[0116] FIG. 5 shows a first signal 1, previously corrected for the Earth's gravity and corresponding to the acceleration normal to the crown of a tyre casing of the heavy duty vehicle type running at a variable rotational speed W.

[0117] Here, the thresholds E and E′, represented respectively, for the first wheel revolution signal 3 in grey, and for the second wheel revolution signal 7 in pale grey, by the dotted lines 2 and 5, are determined.

[0118] The first threshold E makes it possible to identify the increments I corresponding, for example, to the sensor leaving the contact patch. In this analysis, the first wheel revolution signal is limited to one revolution of the wheel, as this is preferable in order to limit the errors associated with the variation in the rotational speed W of the tyre casing. The threshold E has been chosen so that it corresponds to half the reference acceleration of the first signal delimited over a whole number of wheel revolutions performed before the first wheel revolution signal 3. The first reference acceleration γ.sup.reference1 is also calculated, on this first wheel revolution signal 3, as being the mean value of this first wheel revolution signal, represented by the continuous line 4.

[0119] The threshold E′ for delimiting the second wheel revolution signal 7 here corresponds to half the first reference acceleration γ.sup.reference1 of the first wheel revolution signal. The second wheel revolution signal is delimited from these wave fronts on a single revolution of the wheel. The second reference acceleration γ.sup.reference2 is evaluated, on this second wheel revolution signal 7, as being the mean value of this second wheel revolution signal 7, represented by the continuous curve 6.

[0120] It will also be noted that, because the rotational speed W is variable here in the acceleration phase, the number of increments N.sup.U1 and N.sup.U2 between the first and the second wheel revolution signals 3 and 7 decrease substantially.

[0121] FIG. 6 is an illustration to explain the calculation of the energy densities that are positive S.sup.+ and negative S.sup.− on a second wheel revolution signal 10 corresponding to a single revolution of the wheel when the rotational speed W is constant. Of course, the method is the same if the rotational speed W is variable or if the wheel revolution signal is delimited over several revolutions of the wheel.

[0122] The threshold A is determined here as being the product of a value C, in this instance equal to 1.0 times the first reference acceleration γ.sup.reference1 identified on the first wheel revolution signal. This threshold is embodied by the continuous line 11. In fact, it is preferable on real signals, to adopt a value equal to 0.7 for C. If there is a lot of interference on the signals, then a C-value equal to 0.5 or 0.6 may be chosen. By contrast, for signals obtained on road surfaces that are smooth overall, a C-value of the order of 0.8 or 0.9 may be employed. This C-value needs to be fixed for all the steps of the method.

[0123] The positive energy densities S.sup.+ or negative energy densities S.sup.− are calculated as the sum of the absolute values of the differences between the second wheel revolution signal 10 and the first reference acceleration γ.sup.reference1 represented by the continuous curve 11. The area delimited by the areas S.sup.+ has to be equal to the area delimited by the areas S.sup.−, give or take the discretization errors.