METHOD FOR DETERMINING THE COEFFICIENT OF TIRE ADHESION ON WETNER SURFACES

Abstract

The invention relates to a method for obtaining the coefficient of friction Mu of a tire on a wet surface under operating conditions on a vehicle, comprising the following steps: Obtaining (S2) a reference longitudinal stiffness KXRef of the tire on a reference surface in wet conditions; Obtaining (S1) a coefficient of friction MuRef of the tire on the reference surface in wet conditions; Determining the meteorological parameters (S3) during the use of the tire on the vehicle; If the surface is wet, determining (S4) a longitudinal stiffness KXMes of the tire on the running surface; Evaluating (S5) the coefficient of friction Mu of the tire on the wet surface using the following formula: , where the coefficient n is a real number. Figure for the abstract: Fig. 1

Description

Title of the invention: METHOD for DETERMINING THE COEFFICIENT of Tire Adhesion on Wet Surfaces Field of the invention

[0001] The present invention relates to determining the driving conditions of a tire, particularly on wet surfaces, in order to improve the active safety of vehicles by enhancing real-time tire information. Technological background

[0002] The present invention relates to methods for determining the coefficient of friction of a tire on the ground at the contact patch scale, that is, the contact area of ​​the tire with the ground when the tire, mounted on a rim, is loaded and possibly inflated. Indeed, the overall coefficient of friction of the tire, generally called Mu, allows for the evaluation of the tire's potential grip on the ground. This makes it possible to optimize the vehicle's active safety systems in order to prevent unpredictable vehicle behavior leading to potentially risky trajectories. This coefficient of friction of the tire, Mu, naturally depends on the ground, both its nature, i.e., an asphalt road or sandy terrain, and its condition, i.e., dry, damp, wet, or snowy.For example, the same tire, under the same conditions of use—that is, the same inflation pressure and the same applied static load—will have its coefficient of friction (Mu) change depending on whether it is driven on a soft surface like packed sand or snow, or a rigid surface like an asphalt road. But its coefficient of friction will also change depending on the condition of the road surface. Thus, the coefficient of friction (Mu) of a tire is higher on dry ground than on wet ground, due to the presence of water which alters the contact with the tire. Furthermore, for example, one of the conditions that strongly influences the tire's coefficient of friction is the amount of residual water on the road surface, which can cause partial or complete separation of the contact between the tire and the road, leading to aquaplaning.

[0003] Furthermore, external factors such as ambient temperature can also influence the behavior of the tire's rubber compounds and therefore its grip potential. Consequently, obtaining information on the tire's coefficient of friction according to weather conditions allows for the adjustment of the activation thresholds of the vehicle's active safety systems. This real-time adjustment of the activation thresholds of the active safety systems makes driving the vehicle safer. In particular, on black surfaces, including the On asphalt or bituminous roads, for example, one of the conditions that strongly influences the coefficient of adhesion of the tire is the height of residual water on the ground, which can cause a partial or complete separation of the contact between the tire and the ground, leading to aquaplaning of the tire.

[0004] The objects of the invention that follow aim to determine the evolution of the overall coefficient of friction Mu of the tire in real time on a vehicle due solely to the presence of liquid water on the road surface, thereby determining the critical aquaplaning speed of the tire regardless of its condition and defining a safe linear driving speed of the vehicle under the conditions experienced by the vehicle, which serve, for example, as activation thresholds for active safety devices. Description of the invention

[0005] The invention relates to a method for obtaining the coefficient of friction Mu of the tire on a wet surface under conditions of use on a vehicle, comprising the following steps: • Obtaining a reference longitudinal stiffness KXRef of the tire on a reference ground in wet conditions; • Obtaining a MuRef coefficient of adhesion of the tire on the reference ground in wet conditions; • Determining meteorological parameters during tire use on the vehicle to identify soil conditions • If the ground is in wet condition, determination of a longitudinal stiffness KXMes of the tire on the running surface; • Evaluation of the tire's coefficient of friction on wet surfaces using the following formula: [Math 1] Mu = MuKef . n, where the coefficient n is a real number. KXMes X KXRef)

[0006] Thus, the process consists first of all in obtaining reference dimensions for the tire. These reference dimensions correspond to tire use on a so-called wet surface. The term "wet" here means that the liquid water, although present on the road surface, does not form a film between the surface and the tire. Therefore, the quantity of water is below a threshold such that the water can infiltrate the soil's irregularities without remaining largely on the surface. Of course, this threshold depends on the soil particle size, but being in wet conditions ensures two things: the presence of water at the soil scale, which intrinsically modifies the adhesion between the soil and the tire, and that the water is stored at ground level below its maximum height. That is to say, the interface between the liquid water and the air is located below the maximum soil height at the surface scale. The contact area refers to the surface area of ​​contact between the ground and the tire under operating conditions. Conversely, a wet surface is defined as one where liquid water is present and where the water-air interface is greater than the maximum ground height at the contact area scale. Thus, a film of water is interposed between the ground and the tire. Consequently, this data can be obtained on various types of soil by controlling the amount of liquid water applied to the reference surface.

[0007] The necessary reference values ​​are, on the one hand, the coefficient of friction of the tire on a wet surface MuRef and, on the other hand, the longitudinal stiffness of the tire on this surface KXRef. These values ​​can be fixed values. However, they can also be related to the tire, for example, the seasonality of the tire, i.e., a tire designed for summer, winter, or all-season use, or to the tire's wear and aging condition. But they are independent of the nature of the surface; more precisely, their dependence on the nature of the surface will be negligible compared to that of the tire.

[0008] The coefficient of friction of the tire on wet ground, MuRef, corresponds to the maximum level of the ratio between the shear forces and the normal force applied by the tire to the ground at the contact patch. Beyond this threshold, the tire begins to slip on the ground. Here, we are referring to a slip of the tire at the scale of the contact patch and not at the scale of a material element of the tire, which would be akin 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 ground level at the scale of the contact patch.

[0009] The longitudinal stiffness KXRef corresponds to the slope at the origin of the curve relating 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, that is, along the direction of movement of the tire when it rotates around its natural axis of rotation.

[0010] The longitudinal stiffness measurement KX requires measuring both the forces FX at the wheel center of the assembled unit and the slip rate g% of the assembled unit relative to the ground. Therefore, reliable information for these two quantities must be obtained in real time and simultaneously.

[0011] For the longitudinal forces FX at the wheel center of the assembly, these can be estimated, for example, through the torques applied around the axis of rotation of the assembly, whether driving or braking, when the vehicle is moving in a straight line. This implies being able to retrieve this data through the vehicle's characteristics.

[0012] They can, for example, also be obtained through the static load of the vehicle and longitudinal acceleration of the vehicle's center of gravity coupled with the distribution of driving and braking forces between the front and rear axles. Optionally, the physical model used to calculate the longitudinal forces FX at the wheel center of the assembled vehicle takes into account various parameters including the road gradient, the vehicle's forward speed, the vehicle's aerodynamic drag, and the tire's rolling resistance.

[0013] However, the FX forces can also be obtained using more direct measurements at the level of the assembled vehicle. As a non-limiting illustrative example, it should be noted that processing 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 threads of the sidewall carcass ply. Reference should be made to patent document WO-A-03 / 014693 filed by the applicants for a detailed description of this measurement of the characteristics of the assembled vehicle.

[0014] The other essential characteristic for evaluating the longitudinal rigidity KX is the slip rate g% of the assembly mounted at the wheel center. This quantity can be estimated directly from data provided by the vehicle's onboard electronic systems, such as the ABS system.

[0015] However, it can also be evaluated using three elementary parameters: the rotational speed W of the assembly mounted at the wheel center, the rolling radius Re of the assembly, and the forward speed V0 of the vehicle. The rotational speed W can be easily obtained using a wheel revolution encoder coupled to a clock. The rolling radius Re of the assembly, which is relatively insensitive to wear, is obtained using the distance traveled by the vehicle and the number of revolutions made by the assembly to cover that distance. Finally, the forward speed V0 of the vehicle is obtained using a high-frequency measuring device such as the RT 3000, for example, to achieve high accuracy, or a GPS linked to the vehicle in basic mode.

[0016] Obtaining the reference longitudinal stiffness then consists of carrying out a measurement or simulation of the longitudinal stiffness of the assembled tire on a vehicle or on a measuring bench at the scale of the assembled tire, provided that the road surface at the time of this measurement is wet.

[0017] Recording the longitudinal shear force curve FX as a function of the slip rate allows us to obtain, on the one hand, the initial slope, which is similar to the value of KXRef, and on the other hand, the maximum of the curve at the highest slip rate defines a value for the reference friction coefficient MuRef of the tire by combining it with the static vertical force applied to the assembly mounted.

[0018] Furthermore, the method requires obtaining the meteorological conditions at the time of the evaluation in order to determine whether the wet road condition is met. This must determine if precipitation is occurring at the time of measurement and with what intensity, i.e., what the precipitation density is. A high intensity, exceeding a certain threshold, indicates that wet conditions are likely. The nature of this precipitation—snow, hail, or rain—must also be defined, as these affect the road surface. Indeed, driving on snow, hailstones, or liquid water is not entirely comparable. Generally, in the case of hail or snow, the resulting reduced visibility naturally prompts the driver to reduce their speed.However, depending on the intensity of the rain and the size of the raindrops, assessing the amount of water that will concentrate on the road surface is more uncertain, which necessitates the method of the invention to objectively determine this state.

[0019] The method requires a measurement of the longitudinal stiffness KXMes on the running surface if it is assumed that the running surface is in a wet state, which is determined by the intensity of the precipitation and its nature.

[0020] Finally, the method evaluates the absolute coefficient of adhesion of the tire on the wet running surface, Mu, by the ratio of the reference and measurement longitudinal stiffnesses according to the given formula and the potential adhesion of the tire in the absence of a film of water interposed between the surface and the tire. Thus, the evaluation of the tire's coefficient of adhesion on the running surface in a so-called wet state is performed directly following the measurement of the measurement longitudinal stiffness, making its availability instantaneous.

[0021] The observed variation is primarily driven by the presence of a film of water at the interface between the road surface and the tire. This estimation of the tire's wet grip coefficient allows, for example, the estimation of the maximum longitudinal forces to be applied to the vehicle's wheels to optimize its braking distance. For vehicle stability, it is preferable that no wheel slip on the road, or if slippage is necessary, that it be symmetrical between the two wheels on the same axle to guarantee vehicle stability.

[0022] Preferably, the coefficient n is between 0.2 and 2.0, preferably between 0.5 and 1.0.

[0023] The inventor has found that the range of 0.2 to 2.0 for the coefficient n in the formula provides a good estimate of the tire's coefficient of friction on wet surfaces at the contact patch scale, taking into account the seasonality of the tires, their ranges, and their dimensions. Restricting the range to the interval between 0.5 and 1.0 is particularly relevant for passenger car and light truck tires.

[0024] Advantageously, meteorological parameters are included in the group including the outside temperature and the level of precipitation intensity.

[0025] The outside temperature is a parameter accessible on most transport vehicles at the vehicle level, relating to its immediate environment. Taking this parameter into account makes it easy to distinguish whether the precipitation seen by the vehicle is equivalent to snow or rain.

[0026] Most advantageously, the level of precipitation intensity is assessed by a sound measurement, a vibration measurement, an activation of precipitation-sensitive vehicle devices such as rain detectors on windshields or the sweeping speed of windshield wipers.

[0027] The precipitation intensity level allows for the estimation of the amount of precipitation in terms of volume by counting the number of impacts on a sensitive area of ​​the vehicle, such as a rain sensor. However, precipitation intensity can also be used to assess the nature of the precipitation by evaluating the impact of these shocks against thresholds to distinguish between hail and heavy raindrops. Of course, the amount of precipitation can also be assessed, for example, by the windshield wiper sweep speed, which determines certain thresholds based on the amount of rain received by the windshield.Finally, vibration or acoustic measurements at the vehicle body level, 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 as well as their nature through the analysis of the vibroacoustic signals they produce when they impact the vehicle.

[0028] According to a particular embodiment, the method includes a step of determining a longitudinal stiffness KX'Mes of the tire on the running surface when the condition of the running surface is wet.

[0029] The wet condition can easily be identified using thresholds different from those of the wet condition, employing the same means of determination. It is then possible to perform the same longitudinal stiffness measurement as in the wet condition using the same data, except that the result obtained is a longitudinal stiffness of the tire on a wet surface at the scale of the contact patch.

[0030] Advantageously, the reference longitudinal stiffness KXRef 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 period T.

[0031] The reference longitudinal stiffness can then be obtained by averaging the longitudinal stiffness measurements taken on wet surfaces at the vehicle level. This allows the initial fixed value, which generally corresponds to a new tire, to be adapted to the tire's life cycle, i.e., implicitly taking into account its wear and aging.

[0032] According to another particular embodiment, the method includes a step of identifying the tire mounted on the vehicle, including at least the seasonality of the tire.

[0033] Preferably, the step of identifying the tire mounted on the vehicle includes obtaining the tire wear and / or tire aging.

[0034] Advantageously, the coefficient n depends on the tire.

[0035] Very advantageously, the reference longitudinal rigidity KXRef depends on the tire.

[0036] Standard measurements of reference values ​​are always possible, particularly by entering values ​​corresponding to a standard tire in new condition. However, information on tire seasonality is necessary to improve the estimation of the tire's coefficient of friction on wet surfaces. Of course, on a secondary level, knowledge of the tire brand, its range, and / or its dimensions improves the prediction of the coefficient of friction by adapting the standard values ​​more precisely to the tire's specific characteristics. This level of precision becomes meaningless if the nature of the surface varies too significantly compared to an average surface commonly encountered by the tire. Indeed, for example, the variations in the coefficient of friction on wet surfaces (Mu) generated by the nature of the surface become paramount compared to the variations induced by the complete tire specifications.

[0037] However, taking into account the wear and aging of the tire allows for the adjustment of the default values ​​of the reference parameters, in particular the reference longitudinal stiffness KXRef, which is more sensitive to these parameters than the reference coefficient of adhesion of the tire on wet surfaces, for which a default estimate is often sufficient. These two parameters influence at least one of the reference parameters, and their consideration can be easily implemented 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 allows for a good estimation of these parameters and therefore an update of the default values ​​for the reference parameters, which improves the quality of the evaluation of the tire's coefficient of adhesion on wet surfaces and ensures better adaptation of the activation thresholds of the vehicle's active safety devices.

[0038] 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 fixed amount by basing it, for example, on a standard tire in new condition.

[0039] The invention also relates to a method for obtaining a critical aquaplaning speed vcr of a tire on a wet surface, mounted on a vehicle in driving conditions, including the following steps: • Determination of a longitudinal speed v of vehicle movement; • Obtaining the MuRef coefficient of friction of the tire on a surface of reference in wet conditions; • Obtaining the coefficient of adhesion Mu of the tire on wet ground; • Evaluation of the critical aquaplaning speed Ver of the tire using a function F comprising the parameters v, Mu and MuRef of the form: [Math 2] vCr = F( v, Mu, MuRe^)

[0040] Preferably, the function F is of the form: • [Math 3], xv, where B is a real number LJF(v, Mu, MuRe^ =-------------j 1 (f 1 _ Mu \YJ

[0041] Most advantageously, [3 is between 0.1 and 1.0, preferably between 0.2 and 0.4.

[0042] By estimating the tire's wet grip coefficient at the contact patch scale, an intrinsic tire property based on the tire's reference wet grip coefficient, and the linear speed of the vehicle on which the tire is mounted, it is possible to evaluate the critical aquaplaning speed of this tire on the road surface. This critical aquaplaning speed corresponds to the vehicle speed at which a total loss of contact between the tire and the wet surface occurs due to saturation of the tread grooves caused by an excessive amount of liquid water present above the maximum height of the surface's macro-roughness at the contact patch scale. The groove network is then no longer able to evacuate the amount of water present on the wet road.The higher the speed of travel, the greater the flow of water to be evacuated, and consequently the potential saturation of the tread groove network increases, leading to a progressive loss of contact between the tire and the ground, ultimately resulting in aquaplaning.

[0043] To estimate this critical hydroplaning speed, it is necessary to have access to the linear speed v of the vehicle on the wet surface, a value of the reference coefficient of friction MuRef of the tire on a standard wet surface, and an evaluation of the coefficient of friction of the tire on the wet surface on which the vehicle is traveling at speed v. From these values, the critical aquaplaning speed vcr for said tire on said wet surface is calculated. This allows the vehicle's active safety systems to be informed 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 Inform the vehicle's onboard systems so they can adapt the vehicle's speed to the conditions. The formula identified on F is well-suited for passenger car / van tires. The range of values ​​of [3] covers all tire seasons, i.e., summer, winter, and all-season tires. The preferred range of [3] is well-suited for summer tires in premium ranges.

[0044] The invention also relates to a method for obtaining a safe driving speed V on wet surfaces for a vehicle equipped with pneumatic tires, comprising the following steps: • Determination of a minimum coefficient of friction for the tire on wet surfaces MuMin • Obtaining a MuRef coefficient of adhesion of the tire on a reference surface in wet conditions; • Determination of the critical aquaplaning speed vcr 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 vcr, MuMin and MuRef of the form: • [Math 4] v = H(Vcr>MuM.n>MuRef)

[0045] Preferably, the function H is of the form: [Math 5] H(vcr,MuMin,MuRef) = \y, where v is a real number, preferred-MuRefJ tiellement y is between 0.1 and 1.0.

[0046] Finally, real-time knowledge of the critical aquaplaning speed (vCT) of the tire on wet surfaces, the determination of a reference coefficient of friction for the tire on wet surfaces typically encountered by the tire, and a desired minimum coefficient of friction (MuMin) makes it possible to define a maximum driving speed that avoids any risk of aquaplaning and ensures emergency maneuvers while minimizing risks. These emergency maneuvers on the vehicle might include, for example, an unexpected lane change or a sharp turn, i.e., one with a small radius of curvature. This ensures a smooth, stress-free driving experience for the driver, if any, by directly controlling the vehicle's active safety systems, such as cruise control and the speed limiter, to optimize vehicle stability. Brief description of the drawings

[0047] The invention will be better understood upon reading the following description, given solely by way of non-limiting example and made with reference to the attached figures. in which the same reference numbers designate identical parts everywhere and in which: • Fig. 1 presents a synoptic diagram of the method for determining the coefficient of adhesion Mu of the tire on a wet surface, as well as the determination of the critical speed vcr of aquaplaning and the determination of the safe driving speed V according to the invention; • Fig. 2 shows the evolution of the coefficient of adhesion Mu of tires on wet ground as a function of the ratio of their longitudinal stiffness KX on the wet ground and on a reference ground in a wet state; • Fig. 3 shows the evolution of the critical aquaplaning speed of various tires on wet ground as a function of the coefficient of adhesion Mu of the tire on the wet ground; • Fig. 4 shows the evolution of the safe rolling speed V of tires on wet ground as a function of the minimum coefficient of adhesion Mu Minde of each tire on the wet ground. Detailed description of implementation methods

[0048] Figure 1 is a general overview of the methods according to the invention. First, the method for determining the coefficient of adhesion Mu of the tire on a wet surface is defined through steps S1 to S5 with, optionally, steps 1, 2 and 3.

[0049] Step S2 consists of determining the reference longitudinal stiffness KXRef of the tire fitted to the vehicle on a wet surface. This value can initially be obtained using a fixed value; typically, a value in the range of 20,000 N / g% to 500,000 N / g% is perfectly acceptable.

[0050] However, determining this fixed value can also depend on the tire's identification through step 01 in order to be more precise. The primary identification information for this dependency comes from the tire's seasonality at the zeroth order. It is therefore necessary to know whether it is a "summer," "winter," or "all-season" tire to refine the fixed value. However, the fixed value can also be linked to the tire range within a given season or even the tire dimensions. This second dependency, however, is of the second order compared to other influential factors such as the tire's wear or aging state, which are of the first order. Finally, an optional embodiment consists of measuring the longitudinal stiffness KX'Mes of the tire mounted on the vehicle, corresponding to straight-line driving on a wet surface, regardless of the type of surface.Thus, by averaging the KX'Mes measurements over a short period on the scale of the change in tire wear or its aging, we obtain an even value. actually more relevant taking into account all factors influencing the longitudinal stiffness KX of the tire without the optional step 01 of tire identification.

[0051] Step SI consists of determining the reference coefficient of friction MuRef of the tire on a wet surface. This value can initially be obtained using a fixed value; typically, a value in the range of 0.5 to 1.3 is perfectly acceptable.

[0052] However, determining this fixed value may also depend on the tire's identity, as determined in step 01, to be more precise. The primary identification information for this dependency comes from the tire's seasonality (zero order). It is therefore necessary to know whether it is a "summer," "winter," or "all-season" tire to refine the fixed value. The fixed value may also be linked to the tire range within a given season, or even to the tire's dimensions. However, this second dependency is of second order compared to other influential factors such as the tire's wear or age, which are of first order.

[0053] These first two steps, S1 and S2, can potentially be performed before evaluating the various parameters of the invention in real time, thus feeding these values ​​to the computer. The computer can be installed in the vehicle or remotely. In this second solution, the data is fed via a cloud service, as is the transmission of the evaluations to the vehicle.

[0054] During vehicle operation, the weather conditions must be determined during a step S3. This step involves, for example, estimating the vehicle's outside temperature and the level of precipitation. The first meteorological parameter, the outside temperature, allows for an estimation of whether the precipitation is potentially snow or liquid water. The second meteorological parameter, the level of precipitation, allows, depending on the sensors used on the vehicle, the evaluation of the amount of precipitation in terms of volume and / or the amount of precipitation falling on the vehicle in terms of mass. Thus, the speed of the automatic windshield wipers on the vehicle allows the rain sensor to determine the volume of precipitation encountered.And sound or vibration sensors on the vehicle allow for the evaluation of both the volume and mass of precipitation, taking into account, if necessary, the vehicle's longitudinal speed through optional step 03. Thus, the collection and analysis of all this data makes it possible to estimate whether the vehicle is encountering a damp or wet road surface. In the latter case, the amount of water remaining above the road surface is irrelevant. contact between the tire and the ground.

[0055] If the analysis of meteorological data indicates a wet surface, proceed to the next step, S4. This step involves determining, during vehicle operation, the longitudinal stiffness KXMes of the tire fitted to the vehicle on the wet surface. This measurement is performed in a straight line, using, for example, the steering angle, an evaluation of the slope of the points defined by the tire's slip rate g% and the force FX applied to the tire, measured at the wheel center. Accumulating several points, each representing the force FX at the wheel center for a given slip rate g%, allows estimation of the slope at the origin of the point cloud, which is similar to the longitudinal stiffness KX of the tire on the wet surface.

[0056] Finally, an estimate of the tire's coefficient of friction on wet surfaces, Mu, can be obtained through step S5. The inputs to this step S5 are, on the one hand, the reference values ​​MuRef and KXRef, which are the outputs of steps S1 and S2, and on the other hand, the determination of the longitudinal stiffness, KXMes, obtained in step S4. Thus, it is possible, in real time on the vehicle and during driving on wet surfaces, to estimate the remaining tire friction potential related to the wet condition of the surface through the tire's coefficient of friction, Mu. An alert can then be sent to the driver or the vehicle's onboard driving systems to adapt the vehicle's driving conditions according to this coefficient of friction, Mu.For example, by measuring the remaining grip potential of the front axle tires, which will be the most sensitive to the amount of liquid water on the road when driving forward, it is possible to prevent loss of contact between one of the tires located at the front of the vehicle and the ground, or even the entire front axle.

[0057] Next, the method for determining the critical aquaplaning speed vcr of the tire on a wet surface is defined through steps Slet S5 to S7.

[0058] Steps SI and S5 are those which have already been explained during the process of determining the coefficient of adhesion Mu of the tire on wet ground.

[0059] When the road surface is wet, the tire may lose contact with the road surface due to the formation of a film of water between the two solid elements, the tire and the road, which is known as aquaplaning. To determine the speed limit at which contact with the road surface is lost, called the critical aquaplaning speed, it is first necessary to know the vehicle's speed, as determined in step S6.

[0060] This step S6 takes the result of the optional step 03 if the latter was carried out to determine the coefficient of friction Mu of the tire on wet surfaces in step S5. Otherwise, step S6 consists of determining the vehicle's speed v When the ground condition has been identified as wet, speed can be determined in various ways, such as using a GPS (Global Positioning System) speed, or calculating the vehicle's wheel rotation speed by estimating the compressed radius (Re) associated with the measured wheel. Generally, the compressed radius (Re) of the tire depends on its dimensions and range, as well as the applied static load and tire inflation pressure. Speed ​​can also be determined using the vehicle's onboard systems, particularly the instrument panel such as the speedometer.

[0061] Finally, the critical aquaplaning speed vcr is determined in step S7. The inputs to this step S7 are, on the one hand, the reference value MuRef from step S1, the output of step S5, i.e., the tire's wet grip coefficient Mu, and, on the other hand, the vehicle's rolling speed v obtained in step S6. Thus, it is possible, in real time on the vehicle and during driving on wet surfaces, to estimate the critical aquaplaning speed vcr of the tire fitted to the vehicle, taking into account the wet condition of the surface. An alert can then be sent to the driver or the vehicle's onboard driving systems to adapt the vehicle's driving conditions, in particular the rolling speed, according to this critical aquaplaning speed vcr.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 (vcr).

[0062] Finally, the method for determining a safe driving speed V on wet ground is defined through steps SI and S7 to S9.

[0063] Steps SI and S7 are those which have already been explained during the process of determining the critical aquaplaning speed vcr of the tire on wet ground.

[0064] When the road surface is wet, the tire may lose contact with the road surface due to the formation of a film of water between the two solid elements, the tire and the road, which is known as aquaplaning. To ensure safe driving with respect to this water film on the road surface, it is necessary to define, for a minimum desired level of tire grip, denoted MuMin, the threshold driving speed V that must not be exceeded to guarantee this minimum grip level MuMin.

[0065] Step S8 consists of determining the minimum coefficient of friction MuMin of the tire according to the condition of the ground, which has been identified as wet. This determination of the minimum coefficient of friction MuMin of the tire on wet surfaces serves to guarantee easy and safe driving of the vehicle despite the potential presence of large quantities of liquid water on the road surface, regardless of the vehicle maneuvers, such as taking a turn.

[0066] This determination can be obtained initially on a flat-rate basis; classically, a value within a range between 0.4 and 0.8 is perfectly acceptable.

[0067] However, determining this fixed value, which is linked to the vehicle, can also depend on the tire's identity through step 01 in order to be more precise. The primary identity information for this dependency comes from the tire's seasonality at the zeroth order. It is therefore necessary to know whether it is a "summer," "winter," or "all-season" tire to refine the fixed value. However, the fixed value can also be linked to the tire range within a given season or even the tire's dimensions. Nevertheless, this second dependency is of the second order compared to other influential factors such as the tire's wear or age, which are of the first order.

[0068] Finally, the safe driving speed V is determined in step S9. The inputs to this step S9 are, on the one hand, the reference value MuRef from step S1, the output of step S7, i.e., the critical aquaplaning speed vcr of the tire on wet surfaces, and, on the other hand, the determination of the minimum coefficient of friction MuMin of the tire on wet surfaces obtained in step S8. Thus, it is possible, in real time on the vehicle and during driving on wet surfaces, to estimate the safe driving speed V of the tire fitted to the vehicle, taking into account the wet condition of the surface. An alert can then be sent to the driver or the on-board driving systems to adapt the vehicle's driving conditions, in particular the driving speed, according to this safe driving 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.

[0069] Figure 2 shows a representation of the coefficient of friction Mu of the tire on a wet surface as a function of the longitudinal stiffness of the tire measured KXMes on a wet surface during driving. More precisely, the abscissa here represents the ratio of the longitudinal stiffness KX of the same tire between the wet surface measured on the vehicle and a reference surface in a wet state.

[0070] The two curves shown, 101 and 201, each correspond to a tire, 100 and 200. Each tire in the figure belongs to a different seasonal tire category. Tire 100, represented by curve 101 (solid line), is a "summer" tire, while tire 201 (dashed line) is a "4-season" tire, 200. Each tire, 100 and 200, is characterized by a coefficient. The reference adhesion value MuRef is different here. The mathematical representation of the two curves 101 and 201 is a function of the reference quantities MuRef and KXRef of the tire as well as a power coefficient "n" which are all dependent on the tire being 100 or 200.

[0071] Step S5 of the process consists of using a real-time KXMes, for a given 100 or 200 tire, to identify the point of coefficient of adhesion Mu of the 100 or 200 tire on wet ground regardless of the height of water on the ground.

[0072] To this end, at a given level KXMesdonné, points K100 and K200, a vertical line 102, 202 is drawn which intersects the curves 101 and 201 of [Fig. 2] at points 103 and 203 respectively. From points 103 and 203, an orthogonal line 104 and 204 is drawn which intersects the ordinate axis at points Mu100 and Mu200 respectively. These two points Mu100 and Mu200 then represent the coefficient of friction Mu of tire 100, and of tire 200 respectively, on a wet surface for each of the tires, regardless of the water depth on the road.

[0073] Fig. 3 shows a representation of the critical aquaplaning speed vcr of the tire on wet ground as a function of the coefficient of adhesion Mu of the tire on wet ground during driving.

[0074] The two curves shown, 111 and 211, each correspond to a tire, 100 and 200. Each tire in the figure belongs to a different seasonal tire category. Tire 100, represented by curve 111 (solid line), is a "summer" tire, while tire 211 (dashed line) is a "200" tire, an "all-season" tire. Each tire, 100 and 200, is characterized by a reference coefficient of friction, MuRef, which differs here. The mathematical representation of the two curves, 111 and 211, is a function of the reference quantity MuRef, the coefficient of friction Mu of the tire on the wet surface, the speed v of the vehicle on which the tire is mounted, and a power coefficient "[3], all of which depend on whether the tire is 100 or 200.

[0075] Step S7 of the process consists of using a real-time wet ground Mupneumatic, for a given 100 or 200 tire, to identify the critical speed point of aquaplaning vcr of the 100 or 200 tire regardless of the water level on the ground.

[0076] To this end, at a level of Mudonné, points Mu100 and Mu200, a vertical line 112, 212 is drawn which intersects the curves 111 and 211 of [Fig. 3] at points 113 and 213 respectively. From points 113 and 213, an orthogonal line 114 and 214 is drawn which intersects the y-axis at points v100 and v200 respectively. These two points v100 and v200 then represent the critical aquaplaning speed vcr of the tire 100, respectively tire 200, for each of the tires.

[0077] Fig. 4 shows a representation of the safe driving speed V as a function of the minimum coefficient of adhesion MuMin of the tire on wet ground that one wishes to have in all driving circumstances.

[0078] The two curves shown, 121 and 221, each correspond to a tire, 100 and 200. Each tire in the figure belongs to a different seasonal tire category. Tire 100, represented by curve 121 (solid line), is a "summer" tire, while tire 221 (dashed line) is a "200" tire, an "all-season" tire. Each tire, 100 and 200, is characterized by a reference coefficient of friction, MuRef, which differs here. The mathematical representation of the two curves, 121 and 221, is a function of the reference value MuRef of the tire on wet surfaces, the critical aquaplaning speed vcr of the tire, the minimum coefficient of friction MuMin of the tire on wet surfaces, and a power coefficient "y," all of which depend on whether the tire is 100 or 200.

[0079] Step S9 of the process consists of, from a real-time MuMin of the tire on wet ground, for a given tire 100 or 200, identifying the safe rolling speed point V of the tire 100 or 200 regardless of the water level on the ground.

[0080] To this end, at a given level of Mum100 and Mum200, a vertical line 122, 222 is drawn which intersects the curves 121 and 221 of [Fig. 4] at points 123 and 223 respectively. From points 123 and 213, an orthogonal line 124 and 224 is drawn which intersects the ordinate axis at points V100 and V200 respectively. These two points V100 and V200 then represent the safe driving speed V of tire 100, and of tire 200 respectively, for each of the tires and regardless of the water level on the road.

Claims

Claims

1. 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 KXRef of the tire on reference ground in wet conditions; - Obtaining (S 1 ) an adhesion coefficient MuRef of the tire on the reference ground in wet conditions; - Determining the meteorological parameters (S3) during use of the tire on the vehicle; - If the ground is in wet conditions, determining (S4) a longitudinal rigidity KXMes of the tire on the rolling ground; - Evaluating (S5) the adhesion coefficient Mu of the tire on wet ground using the following formula: [Math.l] Mu = MuRef ■ , where the coefficient n is a real number.

2. Method for obtaining the coefficient of adhesion 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.

3. Method for obtaining the coefficient of adhesion 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 the precipitation.

4. 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.

5. Method for obtaining the coefficient of adhesion 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'Mes tudinale of the tire on the rolling ground (02) when the condition of the rolling ground is wet.

6. Method for obtaining the adhesion coefficient Mu of the tire on wet ground according to claim 5 in which the reference longitudinal rigidity KXRef (S2) of the tire on wet ground is evaluated as the average of the longitudinal rigidities KX'Mes on wet ground (02] obtained on the tire mounted on the vehicle for a duration T.

7. 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 (01) mounted on the vehicle comprising at least the seasonality of the tire.

8. 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 (01) mounted on the vehicle comprises obtaining the wear of the tire and / or the aging of the tire.

9. Method for obtaining the coefficient of adhesion Mu of the tire on wet ground according to one of claims 7 to 8 in which the coefficient n depends on the tire.

10. 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 KXRef (S2) depends on the tire.

11. Method for obtaining a critical aquaplaning speed vcr of a tire on wet ground mounted on a vehicle in rolling condition comprising the following steps: - Determining a longitudinal speed v (S6) of movement of the vehicle; - Obtaining a coefficient of adhesion MuRef of the tire (SI) on a reference ground in wet condition; - Obtaining the coefficient of adhesion Mu of the tire (S5) on wet ground according to one of claims 1 to 10; - Evaluating the critical aquaplaning speed vcr of the tire (S7) using a function F comprising the parameters v, Mu and MuRef of the form: [Math.2] VCr = v,Mu,MuRef)

12. Method for obtaining a critical aquaplaning speed vcr of a tire on wet ground according to claim 11 in which the function F is of the form: [Math.3] F(y, Mu, Mu^^ = ((._ Mu \\ MuRef) ) , where [3 is a real

13. Method for obtaining the critical aquaplaning speed vcr of a tire on wet ground according to claim 12 in which [3 is between 0.1 and 1.0, preferably between 0.2 and 0.

4.

14. Method for obtaining a safe rolling speed V on wet ground of a vehicle equipped with a pneumatic casing comprising the following steps: - Determination of a minimum adhesion coefficient (S8) of the tire on wet ground MuMin. - Obtaining an adhesion coefficient MuRef of the tire (SI) on a reference ground in wet conditions; - Determination of the critical aquaplaning speed vcr 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 vcr, MuMin and Mu Ref of the form: [Math.4] V = H( vcr, MuMin,MuRef)

15. Method for obtaining a safe rolling speed V on wet ground according to claim 14 in which the function H is of the form: [Math.5] HÇvcr> MuMin, MuRef) = v„. * ( 1 M"' ) , where y is a real number, preferably y is between 0.1 and 1.0.

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