FR3143748A1 - METHOD FOR DETERMINING THE COEFFICIENT of tire grip on wet surfaces - Google Patents

Abstract

The invention relates to a method for obtaining the grip coefficient Mu of the tire on a wet surface in conditions of use on a vehicle comprising the following steps: Obtaining (S2) a reference longitudinal stiffness KXRef of the tire on a reference ground in wet conditions; Obtaining (S1) a adhesion coefficient MuRef of the tire on the reference ground in wet conditions; Determination of the meteorological parameters (S3) when using the tire on the vehicle; If the ground is in wet condition, determination (S4) of a longitudinal rigidity KXMes of the tire on the rolling ground; Evaluation (S5) of the adhesion coefficient Mu of the tire on the wet ground using the following formula: , where the coefficient n is a real number. Figure for abstract: Fig. 1

Description

METHOD FOR DETERMINING THE COEFFICIENT OF GRIP OF A TIRE ON WET GROUND Field of invention

The present invention relates to the determination of the driving conditions of a tire, in particular on wet ground, in order to improve the active safety of vehicles by improving information relating to the tire in real time.

Technological background

The present invention relates to methods for determining the coefficient of adhesion of the tire on the ground at the scale of the contact patch, i.e. the contact surface of the tire with the ground when the tire, mounted on a rim, is loaded and possibly inflated. Indeed, the overall coefficient of adhesion of the tire generally called Mu makes it possible to evaluate the potential adhesion of the tire on the ground. This makes it possible to optimize the active safety devices of the vehicle in order to prevent random behavior of the vehicle leading to potentially risky trajectories. This coefficient of adhesion of the tire Mu of course depends on the ground, both its nature, i.e. an asphalt road or sandy ground, and its condition, i.e. dry, damp, wet, snowy. For example, the same tire in terms of usage conditions, i.e. the same inflation pressure and the same applied static load, will see its adhesion coefficient Mu change depending on whether it is rolling on soft ground such as packed sand or snow or on hard ground such as an asphalt road. But its adhesion coefficient will also change depending on the condition of the ground. Thus, the adhesion coefficient Mu of a tire is higher on dry ground than on wet ground, due to the presence of water which modifies the adhesion with the tire. Furthermore, for example, one of the conditions strongly influencing the adhesion coefficient of the tire is the height of residual water on the ground which can cause partial or complete detachment of the contact between the tire and the ground leading to aquaplaning of the tire.

In addition, external factors such as ambient temperature can also influence the behavior of the tire's rubber compounds and therefore its adhesion potential. Therefore, obtaining information on the tire's adhesion coefficient according to the weather conditions makes it possible to adapt the trigger thresholds of the vehicle's active safety devices. This real-time adaptation of the trigger thresholds of the active safety devices makes driving the vehicle more peaceful. In particular, on black ground, including asphalt or bituminous roads for example, one of the conditions that strongly influences the tire's adhesion coefficient is the height of residual water on the ground, which can cause partial or complete separation of the contact between the tire and the ground, leading to aquaplaning of the tire.

The objects of the invention which follow aim to determine the evolution of the overall adhesion coefficient Mu of the tire in real time on the vehicle due to the sole presence of liquid water on the roadway, thereby determining the critical aquaplaning speed of the tire regardless of its condition and defining a safe linear rolling speed of the vehicle under the conditions experienced by the vehicle serving for example as trigger thresholds for active safety devices.

Description of the invention

• Obtention d’une rigidité longitudinale de référence KX_Refdu pneumatique sur un sol de référence en condition humide ;
• Obtention d’un coefficient d’adhérence Mu_Refdu pneumatique sur le sol de référence en condition humide ;
• Détermination des paramètres météorologiques lors de l’usage du pneumatique sur le véhicule pour identifier l’état du sol

• Si le sol est en condition mouillée, détermination d’une rigidité longitudinale KX_Mesdu pneumatique sur le sol de roulage ;

• Evaluation du coefficient d’adhérence du pneumatique sur le sol mouillé à l’aide la formule suivante :
• [Math 1], où le coefficient n est un nombre réel.

The invention relates to a method for obtaining the adhesion coefficient Mu of the tire on wet ground in conditions of use on a vehicle comprising the following steps:

• Obtaining a reference longitudinal stiffness KX _Ref of the tire on a reference ground in wet conditions;
• Obtaining a Mu _Ref adhesion coefficient of the tire on the reference ground in wet conditions;
• Determination of meteorological parameters during use of the tire on the vehicle to identify the condition of the ground

• If the ground is in wet condition, determination of a longitudinal rigidity KX _Mes of the tire on the rolling ground;

• Evaluation of the tire's coefficient of adhesion on wet ground using the following formula:
• [Math 1] , where the coefficient n is a real number.

Thus, the process first consists of obtaining reference quantities of the tire. These reference quantities correspond to use of the tire on so-called wet ground. The term "wet" here means that liquid water, although present on the roadway, does not form a film between the ground and the tire. As a result, the quantity of water is below a threshold such that water can infiltrate into the roughness of the ground without remaining overall on the surface of the same ground. Of course, this threshold is a function of the grain size of the ground, but being in wet conditions ensures two conditions: the presence of water at ground level, which intrinsically modifies the adhesion between the ground and the tire, and that the water is stored at ground level below its maximum height. That is to say, the interface between liquid water and air is located below the maximum height of the ground at the scale of the contact area, i.e. the contact surface between the ground and the tire in use conditions. Conversely, a wet ground is a ground where liquid water is present and for which the interface between water and air is greater than the maximum height of the ground at the scale of the contact area. Thus, a film of water is intercalated between the ground and the tire. Consequently, these data can be obtained on various types of ground by controlling the quantity of liquid water applied to the reference ground.

The necessary reference quantities are on the one hand the tyre's grip coefficient on wet ground Mu _Ref and on the other hand, the longitudinal rigidity of the tyre on this ground KX _Ref . These quantities can be fixed quantities. But they can be linked to the tyre, for example the seasonality of the tyre, i.e. a tyre typed as a summer tyre, winter tyre or 4-season tyre, or to the state of wear and ageing of the tyre but they are independent of the nature of the ground, more precisely, their dependence on the nature of the ground will be negligible compared to that of the tyre.

The tyre's wet grip coefficient Mu _Ref corresponds to the maximum level of the ratio between the shear forces and the normal force applied by the tyre on the ground at the contact patch. Beyond this threshold, the tyre begins to slide on the ground. Here, we are talking about a slip of the tyre on the scale of the contact patch and not on the scale of a material element of the tyre which will be similar to a micro-slip. The ground is in a wet state, that is to say in the presence of liquid water whose interface with the air is located below the maximum altitude of the ground on the scale of the contact patch.

The longitudinal stiffness KX _{R ef} corresponds to the slope at the origin of the curve linking the shear forces of the tire on the ground to the slip rate g% of the mounted assembly. For reasons of convenience and ease of calculation, the shear forces are generally limited to the longitudinal force FX, i.e. according to the direction of movement of the tire when it rotates around its natural axis of rotation.

The measurement of longitudinal stiffness KX requires both measuring the forces FX at the wheel center of the mounted assembly and the slip rate g% of the mounted assembly relative to the ground. Thus, it is necessary to obtain reliable information on these two quantities in real time and at the same time.

For the longitudinal forces FX at the wheel center of the mounted assembly, these can be estimated, for example, through the torques applied around the axis of rotation of the mounted assembly, whether they are motor or brake when the vehicle is moving in a straight line. This implies being able to go back to these data via the vehicle characteristics.

They can also be obtained, for example, through the static load of the vehicle and the longitudinal acceleration of the vehicle's center of gravity coupled with the distribution of the engine and braking forces between the front and rear axles. Optionally, the physical model used to derive the longitudinal forces FX at the wheel center of the mounted assembly takes into account various parameters including the slope of the road, the vehicle's forward speed, the vehicle's aerodynamic drag and the rolling resistance of the tire casing.

However, the FX forces can also be obtained using more direct measurements at the mounted assembly. It will be noted as a non-limiting illustrative example that the processing of at least two circumferential extension or contraction measurements in at least one side of the tire at two fixed points in space, located at different azimuths along the circumference, makes it possible to estimate the forces at the wheel center. This circumferential contraction or extension of the sidewalls is advantageously estimated by measuring the distance between the wires of the carcass ply of the sidewalls. Reference is made to patent document WO-A-03/014693 in the name of the applicants for a detailed description of this measurement of the characteristics of the mounted assembly.

The other essential characteristic for the evaluation of the longitudinal rigidity KX is the slip rate g% of the assembly mounted at the wheel center. This quantity can be estimated directly by the data provided by the electronic systems on board the vehicle such as the ABS system.

But it can also be evaluated through three elementary parameters which are the rotation speed W of the assembly mounted at the wheel center, the rolling radius Re of the mounted assembly and the forward speed V0 of the vehicle. The rotation speed W can be simply obtained by a wheel revolution encoder coupled to a clock. The rolling radius Re of the mounted assembly, which is not very sensitive to wear, is obtained using the distance traveled by the vehicle and the number of revolutions made by the mounted assembly to cover this distance. Finally, the forward speed V0 of the vehicle is obtained by means of a high-frequency measuring device such as the RT 3000 for example to have high precision or a GPS linked to the vehicle in elementary mode.

Obtaining the reference longitudinal rigidity then consists of carrying out a measurement or simulation of the longitudinal rigidity of the mounted assembly comprising the said tire on a vehicle or on a measuring bench on the scale of the mounted assembly provided that the rolling ground at the time of this measurement is in a wet state.

Recording the curve of the longitudinal shear force FX as a function of the slip rate makes it possible to obtain on the one hand the slope at the origin which is similar to the value of the KX _Ref and on the other hand, the maxima of the curve at the highest slip rate defines a value of the reference adhesion coefficient Mu _Ref of the said tire by combining it with the static vertical force applied to the mounted assembly.

Furthermore, the method requires obtaining the weather conditions at the time of the assessment in order to decide whether the wet ground condition is met. This must determine whether precipitation is occurring at the time of the measurement and with what intensity, i.e. what density of precipitation. A high intensity, having exceeded a certain threshold, indicates that the condition is likely to be wet. The nature of this precipitation must also be defined: snow, hail, rain which influence the state of the road. Indeed, driving on snow, hailstones or liquid water is not entirely comparable. Generally, in the event of hail or snow, the reduced visibility which results from it encourages the driver to reduce his speed naturally. On the other hand, depending on the intensity of the rain and the size of the water drops, the assessment of the quantity of water which will concentrate on the roadway is more random which requires the method of the invention to objectify this state.

The method requires a measurement of the longitudinal stiffness KX _Mes on the rolling surface if the rolling surface is considered to be in a wet state, which is determined by the intensity of precipitation and its nature.

Finally, the method evaluates in absolute terms the coefficient of adhesion of the tire on the rolling ground, which is wet, Mu by the ratio of the reference and measurement longitudinal rigidities according to the given formula and the adhesion potential of the tire in the absence of a water film interposed between the ground and the tire. Thus, the evaluation of the coefficient of adhesion of the tire on the rolling ground in a so-called wet state is carried out directly following the measurement of the measurement longitudinal rigidity, which makes its availability instantaneous.

The observed variation is mainly driven by the presence of a film of water at the interface between the ground and the tire. This estimation of the wet grip coefficient of the tire makes it possible to estimate, for example, the maximum longitudinal forces to be applied to the vehicle's wheels to optimize its braking distance. For the stability of the vehicle, it is preferable that no wheel slips on the ground or if it must slip that it is symmetrical between the two wheels of the same axle to guarantee the stability of the vehicle.

Preferably, the coefficient n is between 0.2 and 2.0, preferably between 0.5 and 1.0.

The inventor found that the range of 0.2 to 2.0 on the coefficient n of the formula makes it possible to obtain a good estimate of the coefficient of adhesion of the tire on wet ground on the scale of the contact patch according to the seasonality of the tires, their ranges and their dimensions. The restriction of the range to the interval between 0.5 and 1.0 is particularly relevant for passenger car and van tires.

Advantageously, the meteorological parameters are included in the group comprising the outside temperature, the level of intensity of precipitation.

The outside temperature is a quantity accessible on most transport vehicles at the vehicle level relating to its direct environment. Taking this quantity into account makes it easy to discriminate whether the precipitation seen by the vehicle is similar to snow or rain.

Very advantageously, the level of precipitation intensity is assessed by a sound measurement, a vibration measurement, an activation of vehicle devices sensitive to precipitation such as rain detectors on the windshields or the speed of windshield wipers.

The level of precipitation intensity makes it possible to estimate, on the one hand, the amount of precipitation in terms of volume via a number of impacts on a sensitive area of the vehicle such as a rain detector. But the intensity of precipitation can also assess the nature of precipitation using an assessment of the shocks of these impacts in relation to thresholds to discriminate the presence of hail or heavy raindrops. Of course, the amount of precipitation can be assessed, for example, by the speed of the windshield wipers which determine certain thresholds according to the amount of rain received by the windshield, for example. Finally, vibration or acoustic measurements at the level of the vehicle body, and in particular at the level of the cavity housing the vehicle's mounted assemblies, also make it possible to assess the quantity of mobile particles on the road but also their nature by analyzing the vibroacoustic signals they produce when they impact the vehicle.

According to a particular embodiment, the method comprises a step of determining a longitudinal rigidity KX' _Mes of the tire on the rolling ground when the state of the rolling ground is wet.

The wet condition can easily be identified using different thresholds than the wet condition using the same means of determination. It is then possible to carry out the same measurement of longitudinal stiffness as in wet using the same data except that the result obtained is a longitudinal stiffness of the tire on wet ground at the scale of the contact patch.

Advantageously, the reference longitudinal stiffness KX _Ref of the tire on wet ground is evaluated as the average of the longitudinal stiffnesses KX' _Mes on wet ground obtained on the tire mounted on the vehicle for a duration T.

Obtaining the reference longitudinal stiffness can then be done by averaging the longitudinal stiffnesses on wet ground that are achieved at the vehicle level. This makes it possible to adapt the flat rate value initially taken, which generally corresponds to a new condition of the tire, to the life cycle of the tire, i.e. by taking into account its wear and ageing implicitly.

According to another particular embodiment, the method comprises a step of identifying the tire mounted on the vehicle comprising at least the seasonality of the tire.

Preferably, the step of identifying the tire fitted to the vehicle includes obtaining the tire wear and/or tire aging.

Advantageously, the coefficient n depends on the tire.

Very advantageously, the reference longitudinal rigidity KX _Ref depends on the tire.

Flat-rate measurements of the reference quantities are always possible, in particular by entering values corresponding to a standard tire in new condition. However, information on the seasonality of the tires is necessary to improve the level of estimation of the tire's grip coefficient on wet ground. Of course, on a secondary scale, knowledge of the tire brand, its range and/or its dimensions makes it possible to improve the prediction of the grip coefficient by adapting the flat-rate values more finely to the identity of the tire. This degree of precision no longer makes sense if the nature of the ground varies too greatly compared to an average ground commonly encountered by the tire. Indeed, for example, the variations in the wet grip coefficient Mu generated by the nature of the ground become essential compared to the variations induced by the complete identity of the tire.

But taking into account the state of wear and the ageing of the tyre makes it possible to adapt the standard values of the reference quantities, in particular the reference longitudinal rigidity KX_Ref Who is more sensitive to these parameters than the reference grip coefficient of the tire on wet ground of which a flat-rate estimate is often sufficient. These two parameters influence at least one of the reference quantities and their consideration can be easy on the vehicle. Thus, the analysis of data such as the number of kilometers traveled and the time since the tire was installed on the vehicle, allow a good estimate of these parameters and therefore an update of the flat-rate values for the reference quantities, which improves the quality of evaluation of the tire's grip coefficient on wet ground and guarantees a better adaptation of the trigger thresholds of the vehicle's active safety devices.

In addition, the identification of the tire also makes it possible to adapt the coefficient n of the formula by modifying its value, which is initially taken as a flat rate by calibrating, for example, a standard tire in new condition.

• Détermination d’une vitesse longitudinale v de déplacement du véhicule ;
• Obtention du coefficient d’adhérence Mu_Refdu pneumatique sur un sol de référence en condition humide ;
• Obtention du coefficient d’adhérence Mu du pneumatique sur sol mouillé ;
• Evaluation de la vitesse critique d’aquaplaning Vcr du pneumatique à l’aide d’une fonction F comprenant les paramètres v, Mu et Mu_Refde la forme :
• [Math 2]

The invention also relates to a method for obtaining a critical aquaplaning speed v _cr of a tire on wet ground, mounted on a vehicle in rolling conditions, comprising the following steps:

• Determination of a longitudinal speed v of movement of the vehicle;
• Obtaining the Mu _Ref adhesion coefficient of the tire on a reference ground in wet conditions;
• Obtaining the tire's adhesion coefficient Mu on wet ground;
• Evaluation of the critical aquaplaning speed Vcr of the tire using a function F comprising the parameters v, Mu and Mu _Ref of the form:
• [Math 2]

• [Math 3], où β est un réel

Preferably, the function F is of the form:

• [Math 3] , where β is a real number

Very advantageously, β is between 0.1 and 1.0, preferably between 0.2 and 0.4.

From the estimation of the tyre's wet grip coefficient on the scale of the contact patch, an intrinsic quantity of the tyre by the reference wet grip coefficient of the tyre and the linear travel speed of the vehicle on which the tyre is mounted, it is possible to evaluate the critical aquaplaning speed of this tyre on the rolling surface. This critical aquaplaning speed then corresponds to the vehicle travel speed which results in a total loss of contact between the tyre and the wet surface by saturation of the tread groove network of the tyre due to too large a quantity of liquid water present above the maximum height of the macro-roughness of the ground on the scale of the contact patch. The groove network is then no longer able to evacuate the quantity of water which appears on the wet road. The higher the travel speed, the greater the flow of water to be evacuated and therefore the potential saturation of the tread groove network increases, leading to a progressive loss of contact between the tire and the ground, leading to aquaplaning of the tire.

To estimate this critical hydroplaning speed, it is necessary to have access to the linear travel speed v of the vehicle on wet ground, a value of the reference grip coefficient Mu _Ref of the tire on a standard wet ground and an evaluation of the grip coefficient of the tire on the wet ground on which the vehicle is traveling at speed v. From these quantities, the critical aquaplaning speed v _cr is evaluated for said tire on said wet ground. This makes it possible to inform the active safety devices of the vehicle in real time to activate the vehicle's speed limiter if necessary and to warn the driver of the situation, if there is one, or to inform the on-board systems so that they adapt the speed of the vehicle to the conditions. The formula identified on F is well suited for passenger car/van tires. The range of values of β covers all seasonalities of tires, i.e. summer, winter and all-season tires. The preferred β range is well suited for summer seasonal tires in premium ranges.

• Détermination d’un coefficient d’adhérence minimale du pneumatique sur sol mouillé Mu_Min
• Obtention d’un coefficient d’adhérence Mu_Refdu pneumatique sur un sol de référence en condition humide ;
• Détermination de la vitesse critique d’aquaplaning v_crdu pneumatique sur sol mouillé selon l’une des revendications 11 à 13 ;
• Evaluation de la vitesse de roulage sécuritaire V à l’aide d’une fonction H comprenant les paramètres v_cr, Mu_Minet Mu_Refde la forme :
• [Math 4]

The invention finally relates to a method for obtaining a safe rolling speed V on wet ground for a vehicle equipped with a pneumatic envelope comprising the following steps:

• Determination of a minimum coefficient of adhesion of the tire on wet ground Mu _Min
• Obtaining a Mu _Ref adhesion coefficient of the tire on a reference ground in wet conditions;
• Determination of the critical aquaplaning speed v _cr of the tire on wet ground according to one of claims 11 to 13;
• Evaluation of the safe driving speed V using a function H comprising the parameters v _cr , Mu _Min and Mu _Ref of the form:
• [Math 4]

• [Math 5], où γ est un réel, préférentiellement γ est compris entre 0,1 et 1,0.

Preferably, the function H is of the form:

• [Math 5] , where γ is a real number, preferably γ is between 0.1 and 1.0.

Finally, real-time knowledge of the critical aquaplaning speed v _cr of the tire on wet ground, the determination of a reference grip coefficient of the tire on wet ground generally encountered by the tire and a desired minimum grip coefficient Mu _Min makes it possible to define a maximum driving speed to avoid on the one hand any risk of aquaplaning and on the other hand, to ensure emergency maneuvers by minimizing the risks. These emergency maneuvers on the vehicle can be for example an untimely lane change or a trajectory on a tight bend, that is to say with a small radius of curvature. This ensures peaceful driving for the driver, that is to say without stress, if there is any, by directly controlling the active safety devices of the vehicle such as the cruise control, the speed limiter to optimize the stability devices of the vehicle.

Brief description of the drawings

• La présente un synoptique du procédé de détermination du coefficient d’adhérence Mu du pneumatique sur un sol mouillé, ainsi que la détermination de la vitesse critique v_crd’aquaplaning et de détermination de la vitesse de roulage sécuritaire V selon l’invention ;
• La présente l’évolution du coefficient d’adhérence Mu de pneumatiques sur sol mouillé en fonction du rapport de leur rigidité longitudinale KX sur le sol mouillé et sur un sol de référence dans un état humide ;
• La présente l’évolution de la vitesse critique d’aquaplaning de divers pneumatiques sur sol mouillé en fonction du coefficient d’adhérence Mu de pneumatique sur le sol mouillé ;
• La présente l’évolution de la vitesse de roulage V sécuritaire de pneumatiques sur sol mouillé en fonction du coefficient d’adhérence minimale Mu_Min de chaque pneumatique sur le sol mouillé.

The invention will be better understood on reading the following description, given solely as a non-limiting example and made with reference to the appended figures in which the same reference numbers designate identical parts throughout and in which:

• There presents a synopsis of the method for determining the coefficient of adhesion Mu of the tire on wet ground, as well as the determination of the critical speed v _cr of aquaplaning and the determination of the safe rolling speed V according to the invention;
• There presents the evolution of the adhesion coefficient Mu of tires on wet ground as a function of the ratio of their longitudinal rigidity KX on wet ground and on a reference ground in a wet state;
• There presents the evolution of the critical aquaplaning speed of various tires on wet ground as a function of the tire's adhesion coefficient Mu on wet ground;
• There presents the evolution of the safe rolling speed V of tires on wet ground as a function of the minimum grip coefficient Mu_Min of each tire on wet ground.

Detailed description of embodiments

There is a general synopsis of the methods according to the invention. First of all, the method for determining the coefficient of adhesion Mu of the tire on wet ground is defined through steps S1 to S5 with, optionally, steps O1, O2 and O3.

Step S2 consists of determining the reference longitudinal stiffness KX _Ref of the tire equipping the vehicle on a ground in a wet state. This value can be obtained as a flat rate initially, typically a value in a range between 20000 N/g% and 500000 N/g% is entirely admissible.

However, the determination of this fixed value can also depend on the identity of the tire through step O1 in order to be more precise. The priority identity information for this dependency comes from the seasonality of the tire at order zero. It is therefore appropriate to know whether it is a "summer", "winter" or "4 seasons" tire to refine the fixed value. However, the fixed value can also be linked to the tire range in a given seasonality or even the dimensions of the tire. However, this second dependency is of order 2 compared to other influential factors such as the state of wear of the tire or its state of aging which will rather be of order 1 in terms of influential factors. Finally, a last embodiment consists, as an option, in measuring longitudinal rigidities KX' _{M es} of the tire mounted on a vehicle corresponding to straight line rolling on wet ground and this whatever the nature of the ground. Thus, by averaging the measurements taken KX' _Mes over a short period on the scale of the change in tire wear or aging of the latter, we obtain a potentially more relevant value taking into account all the factors influencing the longitudinal stiffness KX of the tire without the optional step O1 of tire identification.

Step S1 consists of determining the reference grip coefficient Mu _Ref of the tire on wet ground. This value can be obtained as a flat rate initially; typically a value in the range between 0.5 and 1.3 is entirely acceptable.

However, the determination of this fixed value can also depend on the identity of the tire through step O1 in order to be more precise. The priority identity information for this dependency comes from the seasonality of the tire at order zero. It is therefore appropriate to know whether it is a “summer”, “winter” or “4 seasons” tire to refine the fixed value. However, the fixed value can also be linked to the tire range in a given seasonality or even the dimensions of the tire. However, this second dependency is of order 2 compared to other influential factors such as the state of wear of the tire or its state of aging which will rather be of order 1 in terms of influential factors.

These first two steps S1 and S2 can potentially be carried out before evaluating the various quantities of the invention in real time, which makes it possible to feed the calculator with these values. The calculator can be installed in the vehicle or remote from the vehicle. In this second solution, the data is fed via a cloud as is the restitution of the evaluations which will be transmitted to the vehicle.

While the vehicle is driving, the weather conditions must be determined during a step S3. This consists, for example, of estimating the vehicle's exterior temperature and estimating the level of precipitation. The first meteorological parameter, the exterior temperature, makes it possible to estimate whether the precipitation is potentially snow or water in a liquid state. The second meteorological parameter, the level of precipitation, makes it possible, depending on the sensors used on the vehicle, to assess the quantity of precipitation in terms of volume and/or to assess the quantity of precipitation in terms of mass falling on the vehicle. Thus, the speed of the automatic windshield wipers on the vehicle makes it possible to distinguish the quantity of precipitation in volume encountered by the rain sensor. And sound or vibration sensors on the vehicle make it possible to assess the quantity in volume of precipitation but also the mass quantity of precipitation by taking into account, if necessary, the longitudinal driving speed of the vehicle through the optional step O3. Thus, collecting all this data and analyzing it allows us to estimate whether the vehicle encounters a damp or wet road surface. In the second case, it does not matter how much water is stagnating above the road surface on the scale of the contact surface between the tire and the ground.

In the case where the analysis of the meteorological results defines a ground in a wet state, it is appropriate to move on to the next step S4. This consists of determining, during the rolling of the vehicle, the longitudinal rigidity KX _Mes of the tire equipping the vehicle on the rolling ground which is in a wet state. This measurement is to be carried out in a straight line, through the indication of the steering wheel angle for example, an evaluation of the slope of the points defined by the slip rate g% of the tire and the force FX applied to the tire, measured at the wheel center for example. The accumulation of several points each representing the force FX at the wheel center for a given slip rate g% makes it possible to estimate the slope at the origin of the point cloud which is similar to the longitudinal rigidity KX of the tire on the ground in a wet state.

Finally, an estimate of the tire's wet grip coefficient Mu can be obtained through step S5. The inputs of this step S5 are, on the one hand, the reference quantities Mu _Ref and KX _Ref which are the outputs of steps S1 and S2 and, on the other hand, the determination of the longitudinal stiffness KX _Mes obtained in step S4. Thus, it is possible in real time on the vehicle and during driving on wet ground to estimate the remaining grip potential of the tire linked to the wet state of the ground through the tire's grip coefficient Mu. An alert can then be sent to the driver or to the on-board driving systems to adapt the vehicle's driving conditions according to this grip coefficient Mu. For example, by measuring the remaining grip potential of the front axle tires, which will be most sensitive to the amount of liquid water on the road when driving forward, it is possible to avoid loss of contact of one of the tires located at the front of the vehicle with the ground or even of the entire front axle.

Then, the method for determining the critical aquaplaning speed v _cr of the tire on wet ground is defined through steps S1 and S5 to S7.

Steps S1 and S5 are those which have already been explained during the process of determining the adhesion coefficient Mu of the tire on wet ground.

In the case where the ground is in a wet state, the tire may lose rubber/ground contact by creating an intermediate water film between the two solid elements that are the tire and the ground, which corresponds to aquaplaning. In order to determine the limit speed from which rubber/ground contact is lost, which is called the critical aquaplaning speed vcr, it is first necessary to know the vehicle's rolling speed through step S6.

This step S6 takes the result of the optional step O3 if it was carried out to determine the grip coefficient Mu of the tire on wet ground in step S5. Otherwise, step S6 consists of determining the rolling speed v of the vehicle when the ground condition has been identified as wet. This determination of the speed can take various possibilities such as the speed defined by a GPS system (acronym in English for “Global Positioning System”), the rotation speed of the vehicle’s wheels by estimating a crushed radius Re associated with the measured wheel. Generally, the crushed radius Re of the tire is a function of the dimensions and range of tires as well as the static load applied and the inflation pressure of the tire. The determination of the speed can also be obtained through the vehicle’s on-board systems and in particular the instruments on the vehicle’s dashboard such as the speedometer.

Finally, the determination of the critical aquaplaning speed v _cr is obtained through step S7. The inputs of this step S7 are on the one hand the reference quantity Mu _Ref of step S1, the output of step S5, i.e. the grip coefficient of the tire on wet ground Mu and, on the other hand the determination of the vehicle's rolling speed v obtained in step S6. Thus, it is possible in real time on the vehicle and during rolling on wet ground to estimate the critical aquaplaning speed v _cr of the tire equipping the vehicle linked to the wet state of the ground. An alert can then be sent to the driver or to the on-board driving systems to adapt the vehicle's rolling conditions, in particular the rolling speed according to this critical aquaplaning speed v _cr . For example, by informing the driver or the vehicle's on-board driving systems to respect a speed limit of X% or Y speed units below the critical aquaplaning speed v _cr .

Finally, the method for determining a safe rolling speed V on wet ground is defined through steps S1 and S7 to S9.

Steps S1 and S7 are those which have already been explained during the process of determining the critical aquaplaning speed v _cr of the tire on wet ground.

The ground being in a wet state, it is possible that the tire may lose rubber/ground contact by creating an intermediate film of water between the two solid elements that are the tire and the ground, which corresponds to aquaplaning. In order to drive safely in relation to the presence of this film of water on the road, it is advisable to define for a minimum level of tire grip that we wish to maintain, which we call Mu _Min , the threshold rolling speed V not to be exceeded to guarantee this minimum level of grip Mu _Min .

Step S8 consists of determining the minimum grip coefficient Mu _Min of the tire according to the state of the ground which has been identified as wet. This determination of the minimum grip coefficient Mu _Min of the tire on wet ground is used to ensure easy and safe driving of the vehicle despite the potential presence of large quantities of liquid water on the roadway and this whatever the maneuvers of the vehicle including cornering for example.

This determination can be obtained as a flat rate initially; typically a value in the range between 0.4 and 0.8 is entirely acceptable.

However, the determination of this fixed value which is linked to the vehicle can also depend on the identity of the tire through step O1 in order to be more precise. The priority identity information for this dependency comes from the seasonality of the tire at order zero. It is therefore appropriate to know whether it is a “summer”, “winter” or “4 seasons” tire to refine the fixed value. However, the fixed value can also be linked to the tire range in a given seasonality or even the dimensions of the tire. However, this second dependency is of order 2 compared to other influential factors such as the state of wear of the tire or its state of aging which will rather be of order 1 in terms of influential factors.

Finally, the determination of the safe rolling speed V is obtained through step S9. The inputs of this step S9 are on the one hand the reference quantity Mu _Ref of step S1, the output of step S7, i.e. the critical aquaplaning speed v _cr of the tire on wet ground and, on the other hand the determination of the minimum grip coefficient Mu _Min of the tire on wet ground obtained in step S8. Thus, it is possible in real time on the vehicle and during rolling on wet ground to estimate the safe rolling speed V of the tire equipping the vehicle linked to the wet state of the ground. An alert can then be sent to the driver or to the on-board driving systems to adapt the vehicle's rolling conditions, in particular the rolling speed according to this safe rolling speed V. For example, by informing the driver or the vehicle's on-board driving systems to respect a speed limit of X% or Y speed units below the safe driving speed V.

There shows a representation of the wet grip coefficient Mu of the tire as a function of the longitudinal stiffness of the tire measured KX _Mes on wet ground during rolling. More precisely, the representation of the abscissas here is the ratio of the longitudinal stiffnesses KX of the same tire between the wet ground measured on the vehicle and a reference ground in the wet state.

The two curves shown 101 and 201 each correspond to a tire. 100 and 200. Each tire in the figure here belongs to a tire category of different seasonality. The tire 100 represented by the curve 101 in solid line is a tire of category "summer" while the tire identified by the curve 201 in dotted line is a tire 200 of the category "4 seasons". Each tire 100 and 200 is characterized by a reference adhesion coefficient Mu _Ref , here different. The mathematical representation of the two curves 101 and 201 is a function of the reference quantities Mu _Ref and KX _Ref of the tire as well as a power coefficient "n" which are all dependent on the tire 100 or 200.

Step S5 of the method consists of starting from a real-time KX _Mes , for a given 100 or 200 tire, identifying the point of adhesion coefficient Mu of the 100 or 200 tire on wet ground, regardless of the height of water on the ground.

For this purpose, at a KX level_My given the points K100 and K200, we draw a vertical line 102, 202 which intercepts the curves 101 and 201 of the at points 103 and 203 respectively. From points 103 and 203, we draw an orthogonal line 104 and 204 which intercepts the ordinate axis at a respective point Mu100 and Mu200. These two points Mu100 and Mu200 then represent the adhesion coefficient Mu of tire 100, respectively of tire 200, on wet ground for each of the tires and this whatever the height of water on the roadway.

There shows a representation of the critical aquaplaning speed v _cr of the tire on wet ground as a function of the grip coefficient Mu of the tire on wet ground during rolling.

The two curves shown 111 and 211 each correspond to a tire. 100 and 200. Each tire in the figure here belongs to a tire category of different seasonality. The tire 100 represented by the curve 111 in solid line is a tire of category "summer" while the tire identified by the curve 211 in dotted line is a tire 200 of the category "4 seasons". Each tire 100 and 200 is characterized by a reference coefficient of adhesion Mu _Ref , here different. The mathematical representation of the two curves 111 and 211 is a function of the reference quantity Mu _Ref , of the coefficient of adhesion Mu of the tire on wet ground, of the speed v of movement of the vehicle on which the tire is mounted as well as a coefficient of power "β" which are all dependent on the tire 100 or 200.

Step S7 of the process consists of starting from a Mu tire on wet ground in real time, for a given 100 or 200 tire, to identify the critical aquaplaning speed point v_crof the 100 or 200 tire, regardless of the height of water on the ground.

For this purpose, at a level of Mu given the points Mu100 and Mu200, we draw a vertical line 112, 212 which intercepts the curves 111 and 211 of the at points 113 and 213 respectively. From points 113 and 213, we draw an orthogonal line 114 and 214 which intercepts the ordinate axis at a respective point v100 and v200. These two points v100 and v200 then represent the critical aquaplaning speed v_crof tire 100, respectively of tire 200, for each of the tires.

There shows a representation of the safe driving speed V as a function of the minimum grip coefficient Mu _Min of the tire on wet ground that we want to have in all driving circumstances.

The two curves shown 121 and 221 each correspond to a tire. 100 and 200. Each tire in the figure here belongs to a tire category of different seasonality. The tire 100 represented by the curve 121 in solid line is a tire of category "summer" while the tire identified by the curve 221 in dotted line is a tire 200 of the category "4 seasons". Each tire 100 and 200 is characterized by a reference coefficient of adhesion Mu _Ref , here different. The mathematical representation of the two curves 121 and 221 is a function of the reference quantity Mu _Ref of the tire on wet ground, of the critical aquaplaning speed v _cr of the tire, of the minimum coefficient of adhesion Mu _Min of the tire on wet ground as well as a power coefficient "γ" which are all dependent on the tire 100 or 200.

Step S9 of the method consists of starting from a Mu_Minof tire on wet ground in real time, for a given 100 or 200 tire, to identify the safe rolling speed point V of the 100 or 200 tire, regardless of the height of water on the ground.

For this purpose, at a level of Mu_Min given, the Mu points_M100 and Mu_M200, we draw a vertical line 122, 222 which intercepts the curves 121 and 221 of the at points 123 and 223 respectively. From points 123 and 213, we draw an orthogonal line 124 and 224 which intercepts the ordinate axis at a respective point V100 and V200. These two points V100 and V200 then represent the safe rolling speed V of tire 100, respectively of tire 200, for each of the tires and whatever the height of water on the roadway.

Claims (15)

Method for obtaining the adhesion coefficient Mu of the tire on wet ground in conditions of use on a vehicle comprising the following steps:

• Obtaining (S2) a reference longitudinal rigidity KX _Ref of the tire on a reference ground in wet conditions;
• Obtaining (S1) a coefficient of adhesion Mu _Ref of the tire on the reference ground in wet conditions;
• Determination of meteorological parameters (S3) during use of the tire on the vehicle;
• If the ground is in wet condition, determination (S4) of a longitudinal rigidity KX _{M es} of the tire on the rolling ground;
• Evaluation (S5) of the adhesion coefficient Mu of the tire on wet ground using the following formula:

, where the coefficient n is a real number. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to claim 1 in which the coefficient n is between 0.2 and 2.0, preferably between 0.5 and 1.0. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to one of claims 1 to 2 in which the meteorological parameters (S3) are included in the group comprising the outside temperature, the level of intensity of precipitation. Method for obtaining the coefficient of adhesion Mu of the tire on wet ground according to claim 3 in which the level of intensity of the precipitation is evaluated by a sound measurement, a vibration measurement, an activation of devices of the vehicle sensitive to precipitation such as rain detectors on the windshields or the speed of sweeping of the windshield wipers. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to one of claims 1 to 4 in which the method comprises a step of determining a longitudinal rigidity KX' _{M es} of the tire on the rolling ground (O2) when the state of the rolling ground is wet. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to claim 5 in which the reference longitudinal rigidity KX _{R ef} (S2) of the tire on wet ground is evaluated as the average of the longitudinal rigidities KX' _{M es} on wet ground (O2] obtained on the tire mounted on the vehicle for a duration T. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to one of claims 1 to 6 in which the method comprises a step of identifying the tire (O1) mounted on the vehicle comprising at least the seasonality of the tire. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to claim 7 in which the step of identifying the tire (O1) mounted on the vehicle comprises obtaining the wear of the tire and/or the aging of the tire. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to one of claims 7 to 8 in which the coefficient n depends on the tire. Method for obtaining the coefficient of adhesion Mu of the tire on wet ground according to one of claims 7 to 9 in which the reference longitudinal rigidity KX _Ref (S2) depends on the tire. Method for obtaining a critical aquaplaning speed v_crof a tire on wet ground mounted on a vehicle in driving conditions comprising the following steps:

• Determination of a longitudinal speed v (S6) of movement of the vehicle;
• Obtaining a Mu _Ref adhesion coefficient of the tire (S1) on a reference ground in wet conditions;
• Obtaining the adhesion coefficient Mu of the tire (S5) on wet ground according to one of claims 1 to 10;
• Evaluation of the critical aquaplaning speed v _cr of the tire (S7) using a function F comprising the parameters v, Mu and Mu _Ref of the form:

Method for obtaining a critical aquaplaning speed v _cr of a tire on wet ground according to claim 11 in which the function F is of the form:
, where β is a real number Method for obtaining the critical aquaplaning speed v _cr of a tire on wet ground according to claim 12 in which β is between 0.1 and 1.0, preferably between 0.2 and 0.4. Method for obtaining a safe rolling speed V on wet ground for a vehicle equipped with a pneumatic envelope comprising the following steps:

• Determination of a minimum grip coefficient (S8) of the tire on wet ground Mu _{M in ;}
• Obtaining a Mu _Ref adhesion coefficient of the tire (S1) on a reference ground in wet conditions;
• Determination of the critical aquaplaning speed v _cr of the tire (S7) on wet ground according to one of claims 11 to 13;
• Evaluation of the safe rolling speed V (S9) using a function H comprising the parameters v _cr , Mu _Min and Mu _Ref of the form:

Method for obtaining a safe rolling speed V on wet ground according to claim 14 in which the function H is of the form:
, where γ is a real number, preferably γ is between 0.1 and 1.0.