Estimating adhesion potential by assessing rolling radius

Abstract

A method is provided for determining an adhesion potential of a tire mounted on a wheel and travelling over ground. The method utilizes a database constructed to contain information obtained by evaluating experimental data on an evolution of a rolling radius of the tire as a function of predetermined rolling conditions of the tire on ground of variable and known adhesion. Based on the information in the database, an estimation model (M.sub.adpot) of an adhesion potential is established by determining a function linking an adhesion potential (μ.sub.max) of the tire to a rolling radius (RRt) of the tire and to vehicle parameters. The rolling radius (RRt) of the tire is determined while the tire is rolling. The adhesion potential (μ.sub.max) of the tire is evaluated by application of the estimation model (M.sub.adpot) and as a function of the vehicle parameters.

Claims

1. A method for determining an adhesion potential (μ.sub.max) of a tire mounted on a wheel of a vehicle travelling over ground, the method comprising steps of: (a) constructing a database of information obtained by evaluating experimental data on an evolution of a rolling radius (RRt) of the tire as a function of predetermined rolling conditions of the tire on ground of variable and known adhesion; (b) based on the information in the database, establishing an adhesion-potential estimation model (M.sub.adpot) by determining a function linking an adhesion potential (μ.sub.max) of the tire to the rolling radius (RRt) of the tire and to vehicle parameters; and (c) determining the rolling radius (RRt) of the tire while the tire is rolling, and determining the adhesion potential (μ.sub.max) of the tire as a function of the vehicle parameters by applying the adhesion-potential estimation model (M.sub.adpot).

2. The method according to claim 1, wherein the rolling radius (RRt) of the tire is determined by establishing a ratio between a speed of movement (V.sub.sol) of the vehicle relative to the ground and a speed of rotation (Ω) of the wheel about an axis of the wheel according to:
RRt=V.sub.sol/Ω.

3. The method according to claim 2, wherein the rolling radius (RRt) of the tire is determined by averaging rolling-radius measurements of the tire acquired during a mean period.

4. The method according to claim 3, wherein the mean period is between 1 and 10 seconds, inclusive, with between 2.5 and 3.5 seconds, inclusive, being preferable for the mean period.

5. The method according to claim 4, wherein the rolling radius (RRt) of the tire is determined by using the speed of movement (V.sub.sol) of the vehicle relative to the ground obtained using a GPS system onboard the vehicle and the rotation speed (Ω) of the wheel obtained using a coder that generates a plurality of pulses on each wheel revolution.

6. The method according to claim 5, wherein the rolling radius (RRt) of the tire is determined on each pulse generated by the coder.

7. The method according to claim 1, wherein the adhesion-potential estimation model (M.sub.adpot) is applied when the rolling radius (RRt) of the tire is below a predetermined threshold.

8. The method according to claim 1, wherein the adhesion-potential estimation model (M.sub.adpot) estimates the adhesion potential (μ.sub.max) according to:
μ.sub.max=α.sub.1e.sup.(RRt)+α.sub.2Ze.sup.(RRt)+α.sub.3Pe.sup.(RRt)+α.sub.4Z+α.sub.5P+α.sub.6PZ+α.sub.7, where RRt represents the rolling radius of the tire, Z represents a load applied to the wheel, P represents a tire pressure value, and α.sub.1, α.sub.2, α.sub.3, α.sub.4, α.sub.5, α.sub.6, α.sub.7 are constants.

9. The method according to claim 8, wherein the tire pressure value (P) is given by:
P=P.sub.TPMS+α.sub.8V.sub.sol.sup.2, where P.sub.TPMS represents a value of a pressure given by a sensor accommodated inside the tire, V.sub.sol represents the speed of movement of the vehicle relative to the ground, and a.sub.8 is a constant.

10. The method according to claim 8, further comprising a step of: (d) determining a free load (Z.sub.free) applied to the wheel by carrying out the steps (a), (b), and (c) during which, when the vehicle is not engaged in a turn and is not subject to any transverse or longitudinal acceleration, when a zero drive or braking torque is applied to the wheel, and when the wheel is under free rolling conditions: detecting that the ground on which the vehicle is travelling is dry, estimating the rolling radius (RRt) of the tire, and searching for a value of Z that causes the adhesion-potential estimation model (M.sub.adpot) to determine the adhesion potential (μ.sub.max) to be equal to 1, the value of Z corresponding to the free load (Z.sub.free).

11. The method according to claim 10, further comprising a step of determining a mean free load (Z.sub.meanfree) during a period when the ground on which the vehicle is travelling is dry.

12. The method according to claim 10, wherein the adhesion potential (μ.sub.max) is determined using the adhesion-potential estimation model (M.sub.adpot), and based on the rolling radius (RRt) of the tire, the tire pressure value (P), and the free load (Z.sub.free) or the mean free load (Z.sub.meanfree).

13. The method according to claim 10, wherein the adhesion potential (μ.sub.max) is determined using the adhesion-potential estimation model (M.sub.adpot) and based on the rolling radius (RRt), the tire pressure value (P), and the load (Z) equal to an instantaneous load (Z.sub.inst) applied to the wheel and calculated using a function defining a dynamic of the vehicle corresponding to vehicle data at a given moment that includes any one or any combination of two or more of: the free load (Z.sub.free) on the wheel or the mean load (Z.sub.meanfree), a drive or braking torque, a drift angle, transverse and longitudinal accelerations, and a camber angle.

14. The method according to claim 8, wherein the load (Z) is determined as a function of a measurement of a distance between a point situated on a chassis of the vehicle and a point situated on a support of the wheel and a stiffness of a suspension connecting the support to the chassis.

15. The method according to claim 1, wherein the adhesion potential (μ.sub.max) of the tire determined in the step (c) corresponds to a tire mounted on a non-steered and non-driving wheel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood on reading the appended figures, which are provided by way of example in the case of a passenger vehicle and have no limiting character, in which figures:

(2) FIG. 1 represents an experiment plan demonstrating the evolution of the rolling radius as a function of a variation of the adhesion potential.

(3) FIG. 2 represents an experiment plan similar to the previous one under different conditions of speed, load and pressure.

(4) FIG. 3 represents the variation of the rolling radius as a function of the adhesion potential for different loads and pressures.

(5) FIG. 4 represents a distribution of the adhesion potential values calculated from measured radius values as a function of a given real adhesion potential.

(6) FIG. 5 represents an evolution of the error margin as a function of the rolling radius.

(7) FIG. 6 represents an evolution of the calculation of the predicted adhesion potential when travelling under variable adhesion conditions.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(8) FIG. 1, showing the phenomenon on which the invention relies, makes it possible to visualise the measurement of the rolling radius of a non-driving and non-steered wheel when successively travelling over wet ground between 0 and T.sub.1 and over dry ground between T.sub.1 and T.sub.2 and again over wet ground beyond T.sub.2 in the case of a vehicle travelling in a straight line at constant speed.

(9) It is seen that the rolling radius RRt increases by 0.5 mm on the transition from travelling over wet ground to travelling over dry ground and decreases by the same amount on passing from dry ground to wet ground.

(10) FIG. 2 reproduces this experiment plan for different speeds, real load conditions and inflation pressures. There is again seen a variation of the rolling radius on passing from dry ground (represented by crosses) to wet ground (represented by circles) and vice versa.

(11) The method then consists in building a relation between the rolling radius and the adhesion potential as a function of the vehicle parameters having the greatest influence, such as the speed, the load, the tire inflation pressure, the braking or driving torque or the transverse forces exerted on the tire when the latter is subject to a drift angle or a camber angle.

(12) The first order vehicle parameters liable to modify the rolling radius, and therefore the determination of the adhesion potential, are the load and the tire pressure. The longitudinal forces linked to the existence of a braking torque or a driving torque and the transverse forces linked to the tire starting to drift or to the camber angle are involved only for calculating the load. Accordingly, considering the longitudinal and transverse forces to be virtually zero, which is the case if the vehicle is not being subjected to a particular manoeuvre, it is possible to evaluate the adhesion potential of the tire and therefore to know the level of adhesion that is available should the vehicle brake or make a turn at a certain speed.

(13) For this reason, a non-driving and non-steered wheel, such as rear wheel on a front wheel drive type vehicle, for example, is of more particular interest. It will therefore be verified that the brakes are not activated and that the steering wheel angle is equal to zero before measuring the rolling radius when rolling. This information is generally available at any time on modern vehicles equipped with driving assistance systems such as an electronic stability program (ESP).

(14) In accordance with one preferred embodiment, a box is plugged into the diagnostic socket of the vehicle; this box may contain the GPS system and also makes it possible to access the signals of the driving assistance systems of the vehicle, notably the electronic trajectory corrector or ESP.

(15) FIG. 3 shows the evolution of the rolling radius RRt as a function of the value of the adhesion potential μ.sub.max for different load and pressure conditions represented by different deflection values, respectively for a deflection of 30 mm, 18 mm and 15 mm. Here the deflection is equal to the difference between the radius of the inflated tire in the free state and the radius of the tire under load.

(16) All of the data measured experimentally then makes it possible to determine a model M.sub.adpot relating the adhesion potential μ.sub.max, and the rolling radius and a function of the load and the pressure. These measurements are effected at speeds between 30 km/h and 110 km/h inclusive.

(17) The model M.sub.adpot obtained is of the general form:
μ.sub.max=α.sub.1e.sup.(RRt)+α.sub.2Ze.sup.(RRt)+α.sub.3Pe.sup.(RRt)+α.sub.4Z+α.sub.5P+α.sub.6PZ+α.sub.7
where Z represents the load supported by the tire, P represents a tire pressure value and the values α.sub.1, α.sub.2, α.sub.3, α.sub.4, α.sub.5, α.sub.6, α.sub.7, represent coefficients determined experimentally.

(18) It is seen that the adhesion potential has an impact on the rolling radius that increases as that potential decreases.

(19) The accuracy of the determination of the adhesion potential will then depend on the accuracy of the measurement of the value of the rolling radius and also on an accurate knowledge of the value of the pressure and of the load applied to the wheel and of the tire pressure. To be more specific, the rolling radius varies within a range of a few tenths of a millimetre and this therefore requires that this value be evaluated with a controlled error margin.

(20) The rolling radius is determined in a simple manner by establishing the ratio between the speed of movement of the vehicle relative to the ground (V.sub.sol) and the rotation speed of the wheel about its axis (Ω).
RRt=V.sub.sol/Ω

(21) The ground speed V.sub.sol is obtained from data collected by the GPS navigation system, for example with an accuracy of 0.1 km/h.

(22) The rotation speed (Ω) of the wheel about its axis may be determined using the wheel coder used by the antilock braking system (ABS) and typically producing 196 pulses per wheel revolution.

(23) It is possible to increase the accuracy of the rolling radius calculation by averaging rolling radii measured over a predetermined mean duration. A mean duration of 3 seconds makes it possible to obtain an acceptable error margin. However, it appears necessary to limit this mean duration to around ten seconds so as not to penalise the time to make information on the adhesion potential available and to guard against the consequences of rapid evolution of the rolling conditions.

(24) The rolling radius is then calculated on each pulse from the wheel coder. Accordingly, for a mean duration of 3 seconds at a speed of 90 km/h, the rolling radius is estimated 7350 times and an accuracy in terms of the determination of the rolling radius of the order of +/−0.15 mm can be expected with a confidence of 95%.

(25) Alternatively, it is possible to measure a distance (d) travelled during a given time and to measure the angular variation (a) of the wheel during the same time interval. The rolling radius is obtained by establishing the ratio between these two values: RRt=d/a.

(26) The load and pressure data comprises vehicle parameters that are not normally liable to vary greatly during a particular journey. By journey is meant the movement of the vehicle between stopping twice or between the engine being stopped twice.

(27) The tire pressure is obtained by acquiring data originating from the pressure sensor (TPMS) accommodated in the tire. It is possible to achieve an accuracy of the order of 0.1 bar with a confidence of 95%.

(28) To take account of certain centrifugal effects, the pressure may be corrected as a function of the speed in the following manner:
P=P.sub.TPMS+α.sub.8V.sub.sol.sup.2
where P.sub.TPMS represents the pressure measured by the pressure sensor accommodated in the tire, V.sub.sol represents the speed of the vehicle relative to the ground and a.sub.8 is a coefficient determined experimentally.

(29) The load Z may be determined in various ways.

(30) A first and direct way is to measure a distance variation between a fixed point on the chassis of the vehicle and a point on the support of the wheel. The load is deduced from this taking account of the stiffness of the suspension connecting the wheel support to the chassis. However, this method is impacted by instantaneous variations caused by irregularities of the road surface and necessitates filtering of the acquired data.

(31) Another method consists in using the adhesion prediction model M.sub.adpot to predict adhesion under particular conditions, making a hypothesis as to the value of the adhesion potential and seeking the value Z.sub.free the load yielding this value on the basis of the of rolling radius and the pressure measured when travelling under the conditions explained above.

(32) This hypothesis is easily to verify when the vehicle is travelling over dry ground and the adhesion potential is generally equal to or greater than 1. This condition can be met using an acoustic detector of the noise generated by the rolling tire, for example, or information relating to activation or deactivation of the windscreen wipers.

(33) It is then detected that the wheel is not subjected to any braking or driving torque, for example by observing the brake or accelerator pedal, and that the vehicle is not making a turn and is not subject to transverse or longitudinal accelerations, which can be verified using accelerometers placed in the vehicle, for example, or by observing the steering angle; the wheel is under free rolling conditions. The rolling radius and the tire pressure are then measured under the conditions stated above. Then the load Z.sub.free giving an adhesion potential equal to 1 is looked for by applying the model M.sub.adpot for determining the adhesion potential.

(34) This measurement may be carried out as many times as necessary while the vehicle is travelling over dry ground to obtain a mean free load Z.sub.meanfree calculated with a good confidence.

(35) This method makes it possible to determine the load borne by the wheel with an accuracy of +/−50 daN and a confidence of 95%. This free load value, or the mean free load value, which varies little during the same journey, is then stored to serve as a basis for subsequent calculations, in particular in the situation where the vehicle encounters rolling conditions on wet or slippery ground during this journey. After verifying that the braking or driving torque applied to the wheel is zero and that the vehicle is not making a turn and is not subjected to transverse and/or longitudinal accelerations, the model M.sub.adpot for determining the adhesion potential is applied taking the value of the load Z as equal to the free load Z.sub.free or to the mean free load Z.sub.meanfree, the value of which is immediately available without calling for additional calculations.

(36) In a complementary way it is also possible to determine on the basis of the free load Z.sub.free or the mean free load Z.sub.meanfree a value Z.sub.inst of the instantaneous load taking into account the transverse and longitudinal loads imposed on the wheel. It is then necessary to acquire additional information available on the vehicle BusCAN, such as the transverse or longitudinal acceleration, the driving or braking torque, the steering wheel angle or drive angle, and the camber angle, and to apply a dynamic model specific to the vehicle making it possible to calculate the instantaneous load Z.sub.inst applied to the wheel.

(37) The aforementioned box plugged into the diagnostic socket of the vehicle can facilitate accessing this information periodically.

(38) These models, well known to the person skilled in the art, are not part of the present invention and may be generated on the basis of the following publications, for example: “Fundamentals of vehicle dynamics”, 1992, by T. D. GILLESPIE, “Les liaisons au sol”, 1995, by T. HALCONRUY, “A new tire model for vehicle dynamics simulations” by BAYLE, FORISSIER and LAFON of the company submitting the present application, on the basis of the model proposed by PACEJKA.

(39) An adhesion potential μ.sub.max representing the instantaneous rolling conditions of the tire is then determined using the model M.sub.adpot.

(40) With the stated measurement accuracies, the adhesion potential is estimated with an accuracy of the order of +/−0.2 with a confidence of 95%.

(41) FIG. 4 makes it possible to appreciate the spread of the evaluation of the calculated adhesion potential μ.sub.maxcalc as a function of the real adhesion potential μ.sub.maxreal

(42) It is seen that this spread is greater for high values of the adhesion potential μ.sub.max, in other words, when the vehicle is travelling under conditions far from conditions liable to place it in hazard. Also, the accuracy increases for lower values of μ.sub.max, which makes it possible to obtain the most accurate information when the vehicle moves onto more slippery terrain.

(43) FIG. 5 shows in the same way the evolution of the accuracy of the value of the adhesion potential predicted using the model as a function of the rolling radius. This error increases with the rolling radius and the adhesion potential. The greatest errors correspond to the situation in which the deflection is the smallest, i.e. when the pressure is high and/or the load low. This is rare in real life situations.

(44) Also, to improve detection reliability, it may prove necessary to limit the application of the model to configurations in which the rolling radius is below a particular threshold, such that the adjustment error is typically less than 0.3, for example. In the situation shown in FIG. 5, use of the model will therefore be limited to rolling radii less than 306.5 mm.

(45) FIG. 6 shows a recording of the value of the rolling radius μ.sub.max measured when travelling over ground having variable adhesion coefficients during a journey of 15 minutes. At the beginning of the recording, the ground offers high adhesion (μ.sub.max=1); after two minutes the adhesion conditions and the rolling radius are strongly degraded (μ.sub.max=0.2).

(46) The proposed method therefore makes it possible to obtain reliable information on the evolution of the adhesion potential, a knowledge of which is essential for the safety of the vehicle, taking into account a phenomenon impacting directly on this parameter.

(47) The embodiments of the invention providing a basis for the present description are therefore not limiting on the invention and, as already shown, may be implemented differently provided that this makes it possible to obtain the technical effects described and claimed.

(48) The method of the invention has been illustrated in the case of passenger vehicle tires, but also applies to any type of tire, notably for agricultural vehicles, heavy goods vehicles, two-wheeled vehicles and civil engineering plant.