Method for evaluating the dynamic load supported by a tire by measuring pressure and longitudinal acceleration

Abstract

A method, for evaluating the variation in the dynamic load borne by a tire comprising a front axle and a rear axle, comprises steps during which: the pressure of the air contained inside the tire is measured, a difference in load borne by the tire is determined as a function of the transfer of the total load of the vehicle between axles and of the position of the tire on the vehicle and, using a tire model that is preestablished, a pressure differential is determined, a corrected pressure value is determined, on the basis of the corrected pressure values, a reference pressure is determined, and the difference between the measured pressure in the reference pressure is calculated, and the variation in dynamic load borne by the tire is determined on the basis of the model of the tire.

Claims

1. A method for evaluating variation in the dynamic load ΔF.sub.ZD Pneu borne by a tire of a vehicle comprising a front axle and a rear axle, the method comprising, during a running period, the steps of: (a) measuring a pressure P.sub.M of air contained inside the tire; (b) at each pressure measurement point, determining a difference in load ΔF.sub.ZPneu borne by the tire as a function of transfer ΔF.sub.Z of a total load of the vehicle F.sub.ZTot between axles and of the position of the tire on the vehicle and, using a preestablished tire model that connects a difference in load with a difference in pressure, determining a pressure differential ΔP associated with the difference in load borne by the tire; (c) determining a corrected pressure value P.sub.C by calculating an algebraic sum of the value of the measured pressure P.sub.M and the value of the pressure differential ΔP; (d) determining, on the basis of corrected pressure values P.sub.C, a reference pressure P.sub.R corresponding to the pressure of the same tire subjected to the same temperature conditions and at a determined static reference load F.sub.ZSRef and (e) calculating the difference between the measured pressure P.sub.M and the reference pressure P.sub.R and determining the variation in dynamic load ΔF.sub.ZD Pneu borne by the tire on the basis of the model of the tire.

2. The method according to claim 1, wherein the value of the transfer αF.sub.Z between axles of the total load F.sub.ZTot borne by the vehicle is estimated on the basis of a predetermined law connecting the value of the total load borne by the vehicle, (F.sub.Z Av+F.sub.Z AR)=F.sub.zTot, the height h of the center of gravity of the vehicle, the value E of a wheelbase separating the front axle and the rear axle, and the value of the longitudinal acceleration γ.sub.l.

3. The method according to claim 2, wherein the value of the longitudinal acceleration γ.sub.l is estimated by connecting the vehicle position data obtained from a GPS system with a known profile of a route taken by the vehicle or by calculating a gradient by taking into consideration the variations in altitude as measured by the GPS system.

4. The method according to claim 2, wherein the value of the longitudinal acceleration γ.sub.l is estimated using an accelerometer mounted on the vehicle.

5. The method according to claim 2, wherein the predetermined law is of the type: $\Delta F_Z=\frac{h}{E}*\frac{\left(F_{Z\mathrm{AV}}+F_{Z\mathrm{AR}}\right)}{g}*{\gamma }_l.$

6. The method according to claim 1, wherein the reference pressure P.sub.R is obtained by determining a best-fit polynomial P.sub.R(t) fitted to corrected pressure values P.sub.C.

7. The method according to claim 1, wherein the running period comprises at least successively an unladen running cycle and a laden running cycle which respectively delimit an unladen running time interval and a laden running time interval.

8. The method according to claim 7, wherein, at the end of step (a) and prior to the start of step (b), time intervals corresponding to laden and unladen running cycles, during which the total load F.sub.ZTot of the vehicle remains constant, are identified.

9. The method according to claim 7, further comprising the step: (f) evaluating a payload F.sub.zc borne by the tire during a time interval corresponding to a laden running cycle using the model of the tire and using a difference in pressure between an unladen reference pressure P.sub.RV and a laden reference pressure P.sub.RC.

10. The method according to claim 9, wherein the values of the unladen reference pressure for a time interval corresponding to a laden running cycle are determined by interpolating or by extrapolating the reference pressure values obtained during the unladen running cycles prior and subsequent to the laden running cycle.

11. The method according to claim 9, wherein the unladen reference pressure values P.sub.RV for a time interval corresponding to a laden running cycle are determined by adjusting, by translation, the pressure values obtained using the best-fit polynomial P.sub.R(t) determined during the time interval corresponding to this laden running cycle, so that the reference pressure values at the start and at the end of this time interval more or less align with, respectively, the reference pressure values obtained at the end of the time interval corresponding to the previous unladen running cycle, and the reference pressure values obtained at the start of the time interval correspond to the next unladen running cycle.

12. The method according to claim 9, further comprising the step: (g) calculating the value of the total laden weight F.sub.ZTot of the vehicle by calculating the sum of the payloads F.sub.zc borne by each of the tires and the total unladen weight F.sub.zv of the vehicle and determining a new evaluation of the height h of the center of gravity.

13. The method according to claim 12, wherein steps (b), (c), (d), (e), (f) and (g) are carried out as many times as necessary using, for the value of the total laden weight F.sub.ZTot of the vehicle, the value obtained in step (f), and using, for the height h of the center of gravity, the value obtained in step (g), until the value of the total laden weight and the value of the height of the center of gravity converge toward stable values.

14. A device for implementing the method according to claim 1 comprising: at least one computer processing unit; data interchange means for exchanging data between the computer processing unit and sensors capable of acquiring values for pressure P.sub.M and for longitudinal acceleration γ.sub.l; and coded instructions stored and loaded into the computer processing unit to cause execution of the steps of the method.

15. A non-transitory computer readable storage medium comprising programmed code elements for implementing the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention will be understood better from studying the appended figures, which are provided by way of example and are in no way limiting, and in which:

(2) FIG. 1 schematically illustrates a civil engineering vehicle moving over inclined ground.

(3) FIG. 2 represents an extract of a recording of the inflation pressure, measured in a tyre as a function of time.

(4) FIG. 3 is a graph illustrating the result of the processing intended to identify the time intervals corresponding to laden or unladen running cycles.

(5) FIG. 4 is a graph illustrating how the corrected pressure and the reference pressure evolve as a function of time for a front right (AD) tyre.

(6) FIG. 5 depicts reference-pressure curves superimposed on the signal from FIG. 2, for each of the time intervals.

(7) FIG. 6 depicts the reference curve superimposed on the signal from FIG. 3, corresponding to a constant load equal to the static load when the vehicle is unladen.

(8) FIG. 7 depicts a curve evaluating the dynamic load borne by a tyre as a function of time for a static reference load equal to the static load borne by the tyre when the vehicle is running unladen.

DETAILED DESCRIPTION

(9) The civil engineering vehicle illustrated in FIG. 1 is of the “dumper” type used for transporting heavy loads over made-up routes. These vehicles are used in particular in open-cast mines comprising a track which for example allows the vehicle to descend empty and re-ascend laden with the ore extracted from the cutting face and that makes up the payload. In order to improve the efficiency of the mining operations, the vehicles are being made to transport increasingly heavy loads. The total static load F.sub.ZTot transported, which comprises the unladen mass of the vehicle F.sub.ZV and the payload F.sub.ZC, may therefore reach or even exceed 600 tonnes.

(10) These site vehicles comprise a front wheelset axle assembly comprising two steered wheels, and a rigid rear wheelset axle assembly generally comprising four twinned driven wheels arranged in pairs on each of the sides of the vehicle. A wheelset is defined as being all of the pneumatic tyre elements providing the ground-contact system between the ground and the bearing chassis of the vehicle. The distance between the front wheelset and the rear wheelset defines the wheelbase E of the vehicle. The total load (F.sub.ZTot) is distributed between the front wheelset (F.sub.ZAV) and the rear wheelset F.sub.ZAR and then, according to their position, between the six tyres (F.sub.ZS Pneu) that form the running gear, the vehicle being stationary on flat ground. During running, this static load is accompanied by a dynamic load that is variable, which may go so far as to double the instantaneous load borne by a tyre.

(11) The tyres of the vehicle illustrated in FIG. 1 comprise pressure sensors 40, 41 which are connected to a computer processing unit 20. An accelerometer 30, able to measure the longitudinal acceleration γ.sub.l experienced by the vehicle, is also connected to the said computer processing unit 20. Data interchange means for exchanging data between the computer processing unit and the pressure and longitudinal-acceleration sensors are also provided.

(12) Alternatively, it is also possible to determine the value of γ.sub.l by connecting the vehicle positioning data obtained by a GPS (Global Positioning System) system to the known profile of the route followed by the vehicle, and by assigning to each point on this route a value for the inclination of the gradient encountered by the vehicle. The gradient may also be calculated directly by taking account of altitude variations measured by the GPS system.

(13) The period of running time during which the method is implemented is broken down into several time intervals within which the load borne by the vehicle is constant. As a general rule, these time intervals correspond to the times of the unladen-journey cycles and to the times of the laden-journey cycles during which the total laden weight corresponds to the total unladen weight increased by the weight of the payload. It will be noted that the weight of this payload may vary from one laden-journey cycle to another.

(14) Step a in the implementation of the method according to the invention consists in measuring a value P.sub.M for the air pressure prevailing inside the tyre. This measurement may be taken continuously or at a given frequency. Steps are therefore taken to ensure that this sampling frequency is higher than 10 Hz and preferably higher than 20 Hz so as to be very much higher than the frequency at which the dynamic load varies.

(15) FIG. 2 illustrates a recording 1 of the measured pressure P.sub.M within a front right tyre during a time period comprising several distinct time intervals corresponding to unladen-running cycles and to laden-running cycles.

(16) During step b in the implementation of the method, the value of the difference in load seen by a tyre during the time interval under consideration according to the profile of the route taken by the vehicle during this time interval is calculated for each pressure measurement point.

(17) In order to do this, a value for the total load (F.sub.Z AV+F.sub.Z AR) borne by the vehicle is predetermined, as too is the value of the height h of the centre of gravity G during this time interval.

(18) The value for the total load can be obtained by weighing the vehicle before and after it is unloaded. When these data are not accessible, a first assumption based on likely observations is made. For an unladen vehicle, the value of the unladen static load (F.sub.ZV) and the position of the centre of gravity, unladen, are generally contained in the technical information available from the vehicle manufacturer. The estimate of the payload F.sub.ZC may usefully be taken from the manufacturer's technical documentation or from statistical observations made previously. The height h of the centre of gravity will vary according to whether the vehicle is running unladen or laden.

(19) In order to improve the predetermination of the total load of the vehicle during a time interval, it is possible, at the end of step a, and as a preliminary to the starting of step b, to identify from the recording of the measured pressure and in the running period under consideration, the various time intervals that correspond to different running cycles and during which the total load of the vehicle and the load borne by the tyre are different. When the vehicle is running unladen, the payload F.sub.ZC is equal to zero.

(20) To do this, use may beneficially be made of bandpass filters. The time intervals during which the vehicle is laden correspond to the durations for which the measured pressure values are above a given threshold, and the time intervals during which the vehicle is running unladen correspond to the durations during which the measured pressure values are below this threshold.

(21) The result of this processing is illustrated in FIG. 3 in which it is possible to distinguish the lower-pressure zones 2, 4, 6, which correspond to unladen-running cycles, and the higher-pressure zones 3, 5, 7, which correspond to laden-running cycles.

(22) The static load borne by a tyre F.sub.ZSPneu will therefore be dependent on the position of the centre of gravity, on the position of the tyre on the vehicle, and on the total load F.sub.ZTot of the vehicle, which is the sum of the unladen load F.sub.ZV and of the payload F.sub.ZC. This calculation calls upon models of the functional model or multi-body model type, both known to those skilled in the art.

(23) When the vehicle is placed on inclined ground, there is a transfer of load ΔF.sub.Z between the front and rear axles.

(24) This transfer of load between axles can be calculated simply using a predetermined law of the type:

(25) $\begin{array}{cc}\Delta F_Z=\frac{h}{E}*\frac{\left(F_{Z\mathrm{AV}}+F_{Z\mathrm{AR}}\right)}{g}*{\gamma }_l& \left(L\right)\end{array}$
Where ΔF.sub.Z represents the value of the load transfer between axles, where (F.sub.Z AV+F.sub.Z AR) represents the sum of the load borne by the front axle and of the load borne by the rear axle, this sum being equal to the total static load of the vehicle F.sub.Z Tot, where g represents the value of the acceleration due to gravity, h the height of the centre of gravity during this time interval, and γl the value of the transverse acceleration.

(26) To a first approximation, it is considered that this load transfer is split evenly between the tyres of the front axle and those of the rear axle. So it is considered that each of the tyres of the front wheelset experiences a difference in load ΔF.sub.ZPneu, equal to

(27) $\Delta F_{Z\mathrm{Pneu}}\mathrm{AV}=\frac{\Delta F_Z}{2},$
and that each of the tyres of the rear wheelset experiences a difference in load equal to

(28) $\Delta F_{Z\mathrm{Pneu}}\mathrm{AR}=\frac{\Delta F_Z}{4}.$

(29) The algebraic value of the difference in load ΔF.sub.ZPneu experienced by a tyre is dependent on the direction of the gradient and on the position of the tyre. Thus, for a tyre arranged on the front wheelset when the vehicle is facing in the direction of ascent, the value of the difference in load decreases the value of the load borne by the said tyre when the vehicle is placed on flat ground. The value of the difference in load ΔF.sub.ZPneu for a tyre arranged on the rear wheelset when the vehicle is facing in the direction of ascent, represents an increase to the load borne by that tyre when the vehicle is placed on flat ground.

(30) On flat ground and at constant speed, the value of γ.sub.l is zero, and the transfer of load between axles ΔF.sub.Z is equal to zero.

(31) For each pressure value measured during the time interval under consideration, the predetermined law L and the instantaneous value of the longitudinal acceleration γ.sub.l are used to calculate the value of the difference in load ΔF.sub.ZPneu for the tyre under consideration.

(32) And, using a model of the tyre established beforehand and which connects a difference in load with a variation in pressure with respect to a known initial state, a pressure differential ΔP connected with the difference in load seen by the tyre, is determined.

(33) This model of the tyre, as a general rule, takes the form of a look-up table, of one or more graphs, or else of a mathematical relationship, all of which are specific to a size of tyre moving under controlled conditions. Advantageously, these data are obtained for known rim and tyre temperatures. Likewise, it is preferable for this model to be determined on ground similar to the ground on which the vehicle is intended to operate, in order to account for the potential deformation of the ground liable to influence the variation in pressure inside the tyre, particularly when the ground is loose ground.

(34) The data derived from the model of the tyre are stored in the memory of the computer processing unit 20.

(35) In step c, the measured pressure value P.sub.M is corrected for the pressure differential ΔP determined previously in step b. This correction is illustrated in FIG. 4, in which the measured-pressure values P.sub.M are surrounded by a first ellipse S.sub.1, and the corrected pressure values P.sub.C by a second ellipse S.sub.2. It may be seen that, for a front right tyre, arranged in its mounted position, the corrected pressure values P.sub.C are higher than the measured pressure values P.sub.M.

(36) At the end of step c, pressure values substantially equivalent to those that would have been obtained for the same load and internal-air-temperature conditions, had the vehicle run over flat ground, are obtained for the whole of the running period.

(37) In step d, and in a way similar to that of the method described in publication EP 2 593 317, a reference pressure value P.sub.R is determined for each time interval and at each corrected pressure point P.sub.C. This reference pressure P.sub.R corresponds to the pressure that the tyre would have if subjected to the same temperature variations and to a determined constant reference static load F.sub.ZSRef for this time interval. It makes it possible to highlight slow changes in pressure associated with the changes in temperature, and get around the high-frequency variations which are associated with the variations in dynamic load.

(38) Without implying any limitation, this static reference load F.sub.ZSRef may correspond to the static load borne by the tyre during the time interval under consideration and be equal to the static load F.sub.ZSPneu determined in step b, or else to the static load F.sub.ZVPneu borne by the tyre when the vehicle is running unladen.

(39) The reference pressure values P.sub.R may be obtained using a lowpass filter to eliminate the variations associated with the dynamic response of the vehicle or, for preference, using a best-fit polynomial P.sub.R(t) that defines the reference pressure values as a function of time within each of the time intervals considered during the course of the running period. In this way, reference-pressure-value segments visible as 8, 9 and 10 in FIG. 4 or as 8, 9, 10, 11, 12 and 13 in FIG. 5, are defined.

(40) It is possible to improve this result by eliminating the pressure values obtained at the start and end of each time period, in order to get around abnormal values associated with the loading and unloading operations.

(41) When the static reference load F.sub.ZSRef corresponds to the load actually borne by the tyre F.sub.ZSPneu when the vehicle is running alternately laden or unladen, as is illustrated in FIG. 5, the values given by the best-fit polynomials P.sub.R(t) constitute the reference pressure P.sub.R made up alternatively from segments 8, 10, 12 which corresponds to a time interval during which the vehicle is running unladen, and from segments 9, 11, 13 which corresponds to a time interval during which the vehicle is carrying a payload. These segments are disjointed.

(42) When the static reference load F.sub.ZSRef corresponds to the unladen static load F.sub.ZVPneu, it is necessary to determine the reference pressure values for the time intervals corresponding to laden-running cycles. These reference pressure values represent the unladen reference pressure P.sub.RV.

(43) In order to perform this processing, a first method involves interpolating or extrapolating the reference pressure values obtained during the time periods immediately prior and subsequent to the said laden-running cycle under consideration and which correspond to unladen-running cycles. This interpolation can be done easily using the best-fit polynomials P.sub.R(t) for these two unladen-running cycles.

(44) A second method involves adjusting, by translation, the pressure values obtained using the best-fit polynomial P.sub.R(t) determined during the time interval corresponding to this laden-running cycle. These values represent the laden reference pressure P.sub.RC. This translation is performed in such a way that the reference pressure values at the start and end of this time interval more or less align with, respectively, the reference pressure values obtained at the end of the time interval corresponding to the previous unladen-running cycle, and at the start of the time interval corresponding to the next unladen-running cycle. These translated values correspond to the unladen reference pressure P.sub.RV.

(45) This adjustment is illustrated in FIG. 4 in which the curve portion 9, which features the reference pressure values during the time interval corresponding to a laden-running cycle, is translated downwards (see curve 14) so that it is connected by its two ends to the curved portions 8 and 10 that feature the reference pressure values obtained during the time intervals corresponding to two, one prior and one subsequent, unladen-running cycles. FIG. 6 illustrates how the unladen reference pressure evolves by causing the curve portions representing the reference pressures 8, 14, 10, 15, 12, and 16 to join up. The curve portion 14 corresponds to the translation of the curve portion 9, the curve portion 15 corresponds to the translation of the curve portion 11, and the curve portion 16 corresponds to the translation of the curve portion 13.

(46) In step e, the difference between the measured pressure P.sub.M and the reference pressure P.sub.R obtained in step d is calculated at each pressure measurement point, and, using the model of the tyre that dictates the correspondence between a pressure difference and difference in load, this is used to deduce the value of the dynamic load ΔF.sub.ZD Pneu seen by the tyre at this instant.

(47) FIG. 7 shows how the dynamic load ΔF.sub.ZD Pneu of the right front tyre varies by considering, by way of static reference load, F.sub.ZS Ref the load borne by the tyre when the vehicle is running unladen F.sub.ZVPneu.

(48) The method described hereinabove also makes it possible to estimate the static payload F.sub.ZC borne by the vehicle during a given time interval corresponding to a laden-running cycle when this data item is not accessible from a weighing means. This total load corresponds to the sum of the value of the static payloads borne by each of the tyres.

(49) During step f, the value of the payload borne by a tyre during a time interval corresponding to a laden-running cycle is determined using the said model of the tyre and using a difference in pressure between the unladen reference pressure P.sub.RV and the laden reference pressure P.sub.RC, both obtained according to the method described hereinabove.

(50) The sum of the values of the payloads borne by each of the tyres then makes it possible, during a step g, to evaluate the total payload F.sub.ZC borne by the vehicle. And, by calculating the sum of this payload and of the total unladen weight of the vehicle F.sub.ZV, the total laden weight F.sub.ZTot of the vehicle during the time interval considered is obtained.

(51) Knowing the payload and the static unladen load borne by each tyre, it is possible easily to determine the load transfer per axle. And, by inverting the predetermined law (L) mentioned hereinabove, it is then possible to calculate the height h of the centre of gravity G for each time interval of the running period.

(52) If the values of the total laden weight of the vehicle F.sub.ZTot and of the height h differ significantly from the predetermined values estimated during step b, it is then possible to rerun steps b, c, d, e, f, and g as many times as necessary until these values converge towards stabilized values.

(53) At the end of these calculations, it is then possible to determine the variation in dynamic load ΔF.sub.ZD Pneu, of the tyre with a good level of accuracy on the basis of the pressure and longitudinal-acceleration values alone.

(54) Because of the high amplitudes observed, it is possible to gain a better understanding of certain phenomena that lead to premature tyre degradation and to implement the corrective solutions by altering the design of the tyres or the conditions in which the vehicle is operated, such as for example track maintenance or route profile.

(55) The invention also relates to the software containing the programmed code elements for implementing the method as described hereinabove, when the said software is loaded into the computer processing unit 20 and executed by the said computer processing unit, or when the said software is recorded onto a support that can be read on a computer processing unit.

Terminology

(56) 2, 4, 6 Time intervals corresponding to unladen-running cycles. 3, 5, 7 Time intervals corresponding to laden-running cycles. 8, 10, 12 Reference pressure corresponding to unladen-running cycles. 9, 11, 13 Reference pressure corresponding to laden-running cycles. 20 Computer processing unit. 30 Accelerometer. 40, 41 Pressure sensors. G Centre of gravity. E Wheelbase between axles. h Height of the centre of gravity with respect to the ground. F.sub.ZAV Static load borne by the front axle. F.sub.ZAR Static load borne by the rear axle. F.sub.ZV Total static load of the vehicle when unladen. F.sub.ZC Static working load borne by the vehicle. F.sub.ZTot Total static load of the vehicle (F.sub.Z Tot=F.sub.ZAV+F.sub.ZAR=F.sub.ZV+F.sub.ZC) ΔF.sub.Z Transfer of load between axles. F.sub.ZS Pneu Static load borne by a tyre. F.sub.ZcPneu Working load borne by a tyre. ΔF.sub.ZD Pneu Variation in the dynamic load borne by a tyre. ΔF.sub.ZPneu Difference in load experienced by a tyre associated with the transfer of load between axles. F.sub.ZSRef Static reference load borne by a tyre. F.sub.ZVPneu Unladen static load borne by a tyre. γ.sub.l Longitudinal acceleration. ΔP Pressure differential associated with the transfer of load between axles. P.sub.M Measured pressure. P.sub.C Corrected pressure. P.sub.R Reference pressure. P.sub.RV Unladen reference pressure. P.sub.RC Laden reference pressure. P.sub.R(t) Best-fit polynomial.