METHOD FOR OBTAINING THE DISTANCE TRAVELLED BY A ROLLING TIRE

Abstract

A method for obtaining the distance travelled by a tire comprises fixing a sensor, to the right of the crown with a radial position Rc, capable of generating a signal proportional to the acceleration experienced; rolling the tire at a rotation speed W, subject to a load Z; acquiring, after a time T, a first signal Sig.sup.i comprising the acceleration amplitude in the direction normal to the crown, wherein the values below a threshold N represent less than 40 percent of the length of the first signal; identifying a reference value V.sub.i.sup.reference, being the square root of the average value of the first signal Sig.sup.i; and determining the distance travelled D during the time T from the following formula: D=A*T*V.sub.i.sup.reference, where A is proportional to the square root of the rolling radius of the tire.

Claims

1.-15. (canceled)

16. A method for obtaining a distance travelled by a tire casing in a state mounted on-wheel in order to form a mounted assembly, the tire casing having a crown equipped with a tread able to make contact with a ground, two sidewalls and two beads of revolution about a natural axis of rotation and a median plane, an intersection of the median plane and of the natural axis of rotation defining a wheel center, comprising the following steps: fastening to the tire casing plumb with the crown at least one sensor having a radial position R.sub.C with respect to the natural axis of rotation in the on-wheel mounted state and able to generate at least one output signal proportional to an acceleration experienced by the sensor in the tire casing; placing the mounted assembly under conditions in which it may rotate at a rotational speed W and is loaded with a load Z; acquiring, after a time interval T, a first signal Sig.sup.i comprising at least an amplitude of the acceleration in a direction normal to the crown, values of the first signal Sig.sup.i less than a threshold N representing less than 40 percent of a length of the first signal Sig.sup.i; identifying a first reference quantity V.sub.i.sup.reference defined as a square root of a mean value of the first signal Sig.sup.i; determining the distance D travelled during the time interval T using the following formula:
D=A*T*V.sub.i.sup.reference, where A is dependent on the tire casing, and at least proportional to a square root of the rolling radius corresponding to a smallest distance between the wheel center and the ground under loaded conditions.

17. The method according to claim 16, wherein, the time interval T remaining the same between each measurement, the reference quantity V.sub.i.sup.reference takes a set value V.sup.set, and a total distance D.sub.f travelled by the tire casing is determined using the following formula:
D.sub.f=T*Σ.sub.i=0.sup.n(A*V.sub.i.sup.reference).

18. The method according to claim 17, wherein, the function A being a constant, the total distance D.sub.f travelled is determined using the following formula:
D.sub.f=T*A*Σ.sub.i=0.sup.nV.sub.i.sup.reference.

19. The method according to claim 16, wherein the first signal Sig.sup.i is demarcated over a number N.sub.TdR of wheel revolutions, N.sub.TdR being greater than or equal to 1, in order to construct a wheel-revolution signal Sig.sup.TdR.sub.i, and the reference quantity V.sub.i.sup.reference is a square root of the mean value of the wheel-revolution signal Sig.sup.TdR.sub.i over one revolution.

20. The method according to claim 19, wherein, having identified a first series of increments I corresponding to the abscissa u of at least a first signal Sig.sup.i where the first signal Sig.sup.i crosses a threshold value B, the first signal Sig.sup.i is demarcated between a first increment I.sub.min and a second increment I.sub.max with max equal to min+2*k, k being a strictly positive natural integer, over the integer number N.sub.TdR of wheel revolutions defined by the formula: $N_{\mathrm{TdR}}=\frac{\left(\max -\min \right)}{2}.$

21. The method according to claim 20, wherein the first series of increments I is identified using the following steps: defining the threshold value B, which is a value between 0.1 and 0.5 of the at least one maximum of the at least one portion of the first signal Sig.sup.i; determining a second signal of abscissa u depending on the at least one portion of the first signal Sig.sup.i and the threshold value B; and identifying the first series of increments I corresponding to the abscissa u of the at least one portion of the first signal Sig.sup.i where the second signal crosses a threshold E.

22. The method according to claim 20, wherein identification of the increments comprises the following steps: creating a second series of increments J corresponding to a median abscissa u between the abscissae u of increments I of consecutive and identical parities; and constructing the wheel-revolution signal Sig.sup.TdR.sub.i between a first increment J.sub.min and a second increment J.sub.max, min and max being of same parity.

23. The method according to claim 16, wherein the first signal Sig.sup.i is acquired if the rotational speed W of the tire casing is greater than a threshold W.sub.threshold defined by the following formula: $W_{\mathrm{threshold}}=\frac{12}{\sqrt{\mathrm{Dev}}},$ where Dev is a distance travelled by the tire casing in one revolution.

24. The method according to claim 16, wherein the time interval T between two evaluations of the reference quantity V.sub.i.sup.reference is less than or equal to 10 minutes.

25. The method according to claim 16, wherein, the tire casing defining a rolling radius R.sub.P, the function A is proportional to the following ratio B: $B=\frac{R_P}{\sqrt{R_c}}.$

26. The method according to claim 25, wherein the rolling radius R.sub.P is dependent on the load Z borne by the tire casing.

27. The method according to claim 25, wherein, the tire casing being inflated to an inflation pressure P, the rolling radius R.sub.P and the radial position R.sub.C are dependent on the inflation pressure P of the tire casing.

28. The method according to claim 25, wherein the rolling radius R.sub.P is dependent on the total distance travelled by the tire casing.

29. The method according to claim 16, wherein the first signal Sig.sup.i is acquired at a constant sampling frequency.

30. The method according to claim 16, wherein a spatial discretization of sampling of the first signal Sig.sup.i is less than 10 degrees.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] 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 drawings in which:

[0065] FIG. 1 shows an example of first signals Sig.sup.i according to two embodiments of the method.

[0066] FIG. 2 shows a plurality of first signals Sig.sup.i of the method, depending on the sampling frequency and the length of the signal.

[0067] FIG. 3 shows the wheel-revolution signal Sig.sup.TdR and its identification with a first embodiment.

[0068] FIG. 4 shows the wheel-revolution signal Sig.sup.TdR and its identification with another embodiment.

[0069] FIG. 5 shows the wheel-revolution signal Sig.sup.TdR when rotating at a variable rotational speed W.

[0070] FIG. 6 shows the speed profile of a road vehicle in daily use.

[0071] FIG. 7 shows differences in daily distances travelled on the one hand as evaluated by the method and on the other hand as measured in the vehicle, as a function of the time interval T of the method.

DETAILED DESCRIPTION OF EMBODIMENTS

[0072] In order to implement the invention, a tyre casing has to be equipped with an electronic unit 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.

[0073] The sensor is fastened to the tyre casing plumb with the crown, facing a protruding piece of material or a longitudinal groove, these being regions of uniform stiffness, in a radial position R.sub.C that remains constant with respect to the natural axis of rotation in its on-wheel mounted state. The sensor is able to generate at least one output signal proportional to the acceleration normal to the crown 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. The sensor takes account of the continuous component of the acceleration. The sensor may be an accelerometer employing a piezoresistive or capacitive technology.

[0074] The electronic unit, which is powered by the energy storage means, is controlled by the microcontroller with the aid of the clock, said microcontroller also having installed in it computing algorithms that allow, for example, to reference quantity V.sup.reference of the tyre casing to be determined using the signals generated by the sensor element. The radiofrequency-communications transmission means are used to transmit the computed information, and the radiofrequency-communications reception means are used to receive operating instructions or information of use to the computing algorithms. Ideally, this electronic unit comprises or is associated with other measuring elements (such as means for evaluating inflation pressure, the temperature in the internal cavity of the mounted assembly, the state of wear of the tread, etc.) so that components may be shared and operating costs optimized.

[0075] Here, the sensor is turned on by the microcontroller when the tyre casing is under rolling conditions. Of course, it is possible to select for the rotational speed W a threshold value from which a signal output by the sensor is acquired. The electronic unit has available to it a memory space suitable for the type of analysis that it is desired to performed. In fact, the capacity of this memory space is predefined depending on how the electronic unit is to be used. 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 small 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. Lastly, the electronic unit is supplied with power by a storage means. The simplest storage means is a battery. However, a capacitor of large size rechargeable using a piezoelectric element could be envisioned.

[0076] The sampling frequency of the electronic unit allows a wide range of rotational speeds W to be covered with a spatial discretization of less than 10 degrees. According to one particular embodiment, the sampling frequency is adaptable on demand or in response to a signal such as, for example, the rotational speed W of the tyre casing.

[0077] Optionally, the electronic unit contains or is able to obtain the identification of the tyre casing. This information is useful with regard to selecting a set of data useful to the computing algorithms employed in the electronic unit. If the electronic unit needs to obtain the identification of the tyre casing or receive orders to take a measurement, the electronic unit is equipped with a radiofrequency reception means. This means operates in the low-frequency range, and ideally at the frequency of 125 kHz, so as to be free of the interference generated by the metal regions of the tyre casing and its nearby environment in the vehicle.

[0078] According to one specific embodiment, the electronic unit comprises radiofrequency transmission means, specifically transmitting in the UHF band (UHF standing for ultra-high frequency), and particularly in the vicinity of 433 MHz or 900 MHz or what is known as the BLE band (BLE standing for Bluetooth Low Emission), these being free frequency bands. In addition, the UHF band makes it possible to have small antenna sizes making the electronic unit easier to incorporate into the tyre casing.

[0079] This transmission communication is useful for transmitting the data of the method to the vehicle or to outside the vehicle. It is possible either to transmit the data string corresponding to the acquisition of the first signal Sig.sup.i or of the wheel-revolution signal Sig.sup.TdR, or to transmit intermediate results that will have been computed in the electronic unit. The last two modes of transmission necessarily cost the electronic member less in terms of power because the flow of data is less substantial. Specifically, radiofrequency transmission consumes more power than mathematical and logic operations.

[0080] FIG. 1 shows a first raw signal 1b in grey corresponding to the acceleration normal to the crown of a truck tyre casing rotating at a constant rotational speed W. Regularly, and periodically, the curve 1b passes through a low, 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 casing 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 lb, on a scale of the order of one wheel revolution, follows a carrier wave, the first signal 1b oscillating about this carrier wave at a frequency higher than the frequency of wheel revolution. 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.

[0081] 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 is here sinusoidal, the correction having being applied to a point located at the centre of the contact patch, i.e. at equal distance from the two edges that demarcate the portion of the signal the value of which is almost zero. It may be seen that the first signal 1 is flatter between the regions characterizing the contact patch. Although not essential, it is preferable for the various steps of the method to be performed on this corrected first signal 1.

[0082] FIG. 2 illustrates the impact of the spatial discretization and the length of the first signal Sig.sup.i on the reference acceleration value γ.sup.reference on which the reference quantity V.sup.reference is based and the method for evaluating the distance D and D.sub.f travelled by the tyre casing. Thus, the continuous curve in dark grey shows a first signal referenced 1 corresponding to the normal acceleration corrected for gravity with an angular step of one degree extending over a plurality of wheel revolutions, the representation of which has been deliberately limited to approximately one revolution. The average value of this signal, which by definition tends towards the reference acceleration γ.sup.reference, has been represented by the continuous straight line 4. Whether the signal is corrected for gravity or not, the same average value would have been obtained over an integer number of wheel revolutions. This value is used as a gauge, and it is equal to 100 by default.

[0083] A second first signal Sig.sup.i, represented by the black triangles referenced 10, corresponds to the same signal 1; however, in this case the spatial discretization of the signal is 10 degrees. It is possible to restrict the length of this signal to a single wheel revolution by combining samples located at the near-zero value, in order to form a wheel-revolution signal Sig.sup.TdR. This signal 10 teaches us that this level of spatial discretization allows, at the very least, the accelerometric signal to be isolated to one wheel revolution since one sample will necessarily have an almost zero value per contact patch, allowing this event to be detected under the standard conditions of use of a conventional tyre casing. Of course, beyond this maximum angular discretization, the passage of the sensor through the contact patch could potentially not be identifiable using the accelerometric signal. If consideration is limited to one complete wheel revolution Sig.sup.TdR, a mean value of this signal equal to 99% of the value of the reference acceleration γ.sup.reference is obtained, this being highly satisfactory.

[0084] Lastly a third first signal Sig.sup.i represented by the light-grey circles and referenced 11 corresponds to the same signal 1; however, in this case the spatial discretization of the signal is 10 degrees and sampling is limited to the first five increments, i.e. to a fraction of one wheel revolution. In FIG. 2, the straight dashed line referenced 3 represents a threshold N. Here, the threshold N corresponds to fifty percent of the maximum value of the accelerometric signal over more than one revolution without correction for gravity. It will be noted that few of the points of the signals 10 and 11 are below this threshold N. Furthermore, the length of the signals 10 and 11 is chosen so that at least 60 percent of the length of the signal is located above this threshold N. In the case of the signal 11, precisely 60 percent of the length of the signal 11 is located above this threshold since the length of the signal 11 is equal to 5 and two values are below this threshold N (representing 40% of the signal). In this case, the mean value of the signal 11 is equal to 90% of the reference acceleration γ.sup.reference this being satisfactory as a valid estimate of the distance travelled may be obtained therewith. If this signal 11 had been limited to the first four values, in which case the condition regarding the length of the signal above N would not have been met, the mean value would have fallen to 79 percent of the reference acceleration. Furthermore, if the last four values of the signal 11 had been used, in which case the condition with respect to the threshold N would have been met, the average value would have been 110 percent of the reference acceleration, this still being highly satisfactory. Likewise, it will be noted that a modification of the threshold N would not have changed the result much. However, this threshold N must be carefully selected if the raw accelerometric signal not corrected for gravity is used, as then, because of the value of gravity, the signal might not pass through zero. FIG. 3 shows the method for determining a wheel-revolution signal Sig.sup.TdR (referenced 2 in the figure) over an integer number of wheel revolutions. From the first signal Sig.sup.i, here corrected to allow a better explanation of the example, a threshold E, illustrated by the dashed line 3, is determined. Here, the threshold E is set to half the maximum amplitude of the first signal Sig.sup.i without any corrections. A series of increments I is identified, these being where the first signal Sig.sup.i crosses the dashed line 3, for example from below, which physically corresponds to the sensor, which rotates as one with the tyre casing, exiting from the contact patch. Thus, here, crossings from above of the threshold E by the first signal Sig.sup.i, which correspond to entry into the contact patch, and which would have generated intermediate increments I, are neglected. The first signal Sig.sup.i is thus limited to a wheel revolution signal Sig.sup.TdR (referenced 2) between a first increment, here I.sub.1, and a second increment, here I.sub.3. The wheel-revolution signal Sig.sup.TdR here represents the accelerometric signal of the sensor over two complete wheel revolutions.

[0085] The threshold value E represented by the dashed line 3 has in this instance been evaluated in one portion of the first signal Sig.sup.i with a variable sampling frequency. The maximum discretized value obtained is extracted from this portion 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 about 50%.

[0086] The mean value of the wheel-revolution signal Sig.sup.TdR (referenced 2) over a single revolution, which corresponds to the reference acceleration y.sup.reference which has been represented by the continuous black line 4, is computed. It is evaluated in real time, by summing the values of the increments u of the first wheel-revolution signal Sig.sup.TdR, the sum being divided, at the end of the wheel-revolution signal Sig.sup.TdR, by the number of increments of the first wheel-revolution signal Sig.sup.TdR. To do this, it is enough just to find the first crossing from below of the threshold E by the first signal, which determines the starting point of the wheel-revolution signal Sig.sup.TdR. Of course, the computation may also be done after the whole recording of the wheel-revolution signal Sig.sup.TdR (referenced 2) has been recorded and stored in memory.

[0087] FIG. 4 is an illustration of the wheel-revolution signal Sig.sup.TdR (referenced 7 and shown in grey) being demarcated from the accelerometric signal. Here, it is the second embodiment that is used for this demarcation.

[0088] From the signal delivered by the sensor, here corrected to allow a better explanation of the example, a threshold B, illustrated by the dashed line 5, is determined. A series of increments I is identified, these being where the first signal crosses the dashed line 5, which physically corresponds to the sensor, which rotates as one with the tyre casing, entering into or exiting from the contact patch. Next, considering only odd-numbered increments I in this illustration, a series of increments J located equidistantly from the odd-numbered increments I is constructed. These increments are identified by vertical lines of dots in FIG. 3. Of course, the method may be applied as long as the selected increments are between one eighth and seven eighths of the length of the signal comprised between the two increments I.sub.i and I.sub.i+1.

[0089] The wheel-revolution signal Sig.sup.TdR (referenced 7) is then demarcated between a first increment, here J1, and a second increment, here J3. The wheel-revolution signal Sig.sup.TdR (referenced 7) here represents the accelerometric signal delivered by the sensor over two complete wheel revolutions.

[0090] The threshold value B, which has been represented by the dashed line 5, has in this instance been evaluated in one portion of the accelerometric signal, with a variable sampling frequency. From this portion of the accelerometric signal, the obtained discretized maximum value, which is called MAX, is extracted. The threshold value B is therefore a value comprised between 10 and 50% of the value MAX, and in this instance this value is around 50%.

[0091] The reference acceleration γ.sup.reference, which has been represented by the continuous line 6 in black, is determined by computing the mean value of the first wheel-revolution signal Sig.sup.TdR (referenced 7). It is evaluated in real time by summing the values of the increments u of the wheel-revolution signal between the increments J1 and J3, the sum then being divided, at the end of the wheel-revolution signal, by the number of increments u in the wheel-revolution signal Sig.sup.TdR (referenced 7).

[0092] This second embodiment is the better method because the discretization errors at the extremities of the wheel-revolution signal Sig.sup.TdR cause only a small variation in the computation of the reference acceleration. Specifically, at these extremities, the sensitivity of the signal is low with respect to the sensitivity of the signal level with the increments I.

[0093] FIG. 5 shows an accelerometric signal, previously corrected for the Earth's gravity and corresponding to the acceleration normal to the crown of a truck tyre casing rotating at a variable rotational speed W.

[0094] Here, a threshold E, represented by the dashed line 3, is determined for the wheel-revolution signal Sig.sup.TdR (shown in light grey and referenced 2).

[0095] The threshold value E makes it possible to identify increments I, which for example correspond to the sensor exiting from the contact patch. In this analysis, the wheel-revolution signal Sig.sup.TdR is limited to one wheel revolution, 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 was chosen to correspond to half the reference acceleration of the first signal demarcated over an integer number of wheel revolutions before the wheel-revolution signal 2. The reference acceleration γ.sup.reference is also determined, from the wheel-revolution signal 2, by computing the mean value of the wheel-revolution signal 2, which has been illustrated by the continuous curve referenced 4.

[0096] It will also be noted that, because the rotational speed W is variable, here in an acceleration phase, the period associated with the wheel revolution decreases, resulting in increasingly closely-spaced rising edges or falling edges.

[0097] FIG. 6 is an illustration of the speed profile of a truck in regional use in Europe. The vehicle was equipped with a mounted assembly comprising a sensor according to the invention, the assembly being mounted at the front of the vehicle. The response of the sensor was recorded at regular time intervals T of the order of one minute. During a measurement phase, the sampling frequency of the sensor was chosen so that the conditions of signal length and of minimum angular step size were met in the entire use-related range of speeds. The daily distance travelled was estimated in the integrated device with a measurement frequency of 1 minute. To do this, the function A was chosen to be the ratio B. The rolling radius R.sub.P and the positioning radius R.sub.C of the sensor were calibrated beforehand on a testbed under loaded and unloaded conditions by applying the rules of the ETRTO. Moreover, the signals were stored in an external memory space, which made it possible to modify the time interval T between the measurement phases and thus carry out other evaluations. Moreover, the mileage covered by the vehicle during the day was recorded directly by the speedometer of the vehicle and via a commercially available GPS device.

[0098] FIG. 7 is a summary of the differences between the mileage recorded by the vehicle and various evaluations of the method that differed only in the time interval T between the measurement phases, which was varied from 1 minute to 40 minutes in steps of 1 minute. The first evaluation at 1 minute was that of the device integrated into the mounted assembly. The other evaluations were made by eliminating raw data to increase the time interval T between the measurements. Here, in a given evaluation of daily distance travelled, the time interval T remained constant.

[0099] It will be noted that if the time interval T is less than 10 minutes, the difference in distance between the distance recorded by the vehicle and the distance evaluated by the method does not exceed 5 percent. If, for this specific itinerary, the time interval is set to 20 minutes; the error in the estimate still does not exceed 10 percent. In addition, the shorter the time interval T, the smaller the difference between the reference value recorded by the vehicle and the evaluation made according to the method. The error is even insignificant below a value of 5 minutes for the time interval T between the measurement phases.