Method for determining grip coefficient of tyre on wet road surface

Abstract

The invention relates to a method for determining a grip coefficient Mu of a tyre on a wet road surface in a vehicle use state, comprising the following steps:-determining (S2) a reference longitudinal stiffness KXRef of the tyre on a reference road surface in a wet state; -acquiring (S1) a grip coefficient MuRef of the tyre on a reference road surface in a wet state; -determining a weather parameter (S3) during use of the tyre on the vehicle; determining (S4) the longitudinal stiffness, KXMes, of the tyre on the road surface if the road surface is in a wet state; -evaluating (S5) the grip coefficient Mu of the tyre on the wet road surface using formula (I), where the coefficient n is a real number between 0.2 and 2.0.

Description

Technical Field

[0001] The present invention relates to determining the driving state of a tire, in particular the driving state on a wet ground, so as to improve the active safety of a vehicle by real-time improving information related to the tire. Background Art

[0002] The present invention relates to a method for determining the grip coefficient of a tire on the ground on the scale of the contact patch (i.e., the contact area between the tire and the ground when the tire mounted on the rim is loaded and optionally inflated). Specifically, the overall grip coefficient of the tire (commonly referred to as Mu) can evaluate the potential grip of the tire on the ground. This enables optimizing the active safety devices of the vehicle to prevent possible dangerous trajectories caused by the random behavior of the vehicle. Of course, the grip coefficient Mu of the tire depends on the nature (i.e., asphalt road surface or sand) and state (i.e., dry, wet, moist, snow-covered) of the ground. For example, the grip coefficient Mu of tires that are the same in terms of usage state (i.e., the same inflation pressure and the same applied static load) will vary depending on whether the tire is traveling on a soft ground such as compacted sand or snow, or on a hard ground such as an asphalt road surface. However, the grip coefficient of the tire also varies according to the ground state. Therefore, since the presence of water changes the adhesion of the tire, the grip coefficient Mu of the tire on a dry ground is higher than that on a wet ground. In addition, for example, one of the states that strongly affects the grip coefficient of the tire is the height of the residual water on the ground, which may cause partial or complete loss of contact between the tire and the ground, resulting in tire hydroplaning.

[0003] In addition, external factors such as ambient temperature may also affect the properties of the rubber compound of the tire, thereby affecting its grip potential. Therefore, obtaining information about the grip coefficient of the tire according to the weather state enables adjusting the triggering threshold of the active safety device of the vehicle. This real-time adjustment of the triggering threshold of the active safety device makes vehicle driving more comfortable. Especially on a black ground (e.g., including an asphalt or tarmac road surface), one of the states that strongly affects the grip coefficient of the tire is the height of the residual water on the ground, which may cause partial or complete loss of contact between the tire and the ground, resulting in tire hydroplaning.

[0004] The object of the following subject matter of the present invention is to determine the real-time change of the overall grip coefficient Mu of the tire on the vehicle caused only by the presence of liquid water on the road, thereby determining the critical hydroplaning speed of the tire (regardless of its state), and defining the linear safe driving speed of the vehicle under the state the vehicle undergoes (e.g., used as the triggering threshold of the active safety device). Summary of the Invention

[0005] The present invention relates to a method for obtaining the grip coefficient Mu of a tire on a wet ground in a vehicle use state, which comprises the following steps:

[0006] · Obtain the reference longitudinal stiffness KX of the tire on a reference ground in a wet state Ref ;

[0007] · Obtain the grip coefficient Mu of the tire on a reference ground in a wet state Ref ;

[0008] · Determine weather parameters during the use of the tire on the vehicle to identify the state of the ground;

[0009] · If the ground is wet, determine the longitudinal stiffness KX of the tire on the driving road surface Mes ;

[0010] · Evaluate the grip coefficient of the tire on the wet ground by means of the following formula:

[0011] · [Mathematical formula 1] where the coefficient n is a real number between 0.2 and 2.0.

[0012] Therefore, the method first includes obtaining reference variables of the tire. These reference variables correspond to the use of the tire on a so-called wet ground. The term "wet" is understood to mean that although there is liquid water on the road, it does not form a water film between the ground and the tire. Therefore, the water volume is below the threshold value, so that the water can enter the irregular areas of the ground and usually does not remain on the ground. Of course, this threshold value depends on the particle size of the ground, but the fact that it is in a wet state ensures two states: there is water on the ground scale, which essentially changes the adhesion between the ground and the tire, and the water is stored at the ground, below its maximum height. This means that the interface between the liquid water and the air is below the maximum height of the ground on the scale of the ground contact surface (i.e., the contact area between the ground and the tire in the use state). On the contrary, the term "wet ground" is used for a ground with liquid water, in which case the interface between the water and the air is higher than the maximum height of the ground on the scale of the ground contact surface. Therefore, a water film is between the ground and the tire. Therefore, by monitoring the amount of liquid water on the reference ground, these data can be obtained on various types of ground.

[0013] The required reference variables are, on the one hand, the grip coefficient Mu of the tire on the wet ground Ref , and on the other hand, the longitudinal stiffness KX of the tire on this ground RefThese variables can be fixed variables. However, they may be related to the tire, such as the seasonality of the tire (i.e., summer tire, winter tire, or all-season tire) or the wear and aging state of the tire, but they are independent of the nature of the ground; more precisely, compared with the nature of the tire, their dependence on the nature of the ground can be ignored.

[0014] The grip coefficient Mu of the tire on a wet ground Ref Corresponds to the maximum ratio between the shear force and the normal force that the tire exerts on the ground in the contact patch. Exceeding this threshold, the tire will start to slip on the ground. In this case, it is considered that the tire slips on the scale of the contact patch, rather than on the scale of the tire material elements, which is similar to micro-slip. The ground is in a wet state, which means that there is liquid water, and on the scale of the contact patch, the interface between water and air is below the maximum height of the ground.

[0015] The longitudinal stiffness KX Ref Corresponds to the original slope of the curve connecting the shear force of the tire on the ground and the slip rate g% of the mounting assembly. For reasons of convenience and ease of calculation, the shear force is usually limited to the longitudinal force FX, that is, the force along the direction of movement of the tire when the tire rotates around its natural axis of rotation.

[0016] The measurement of the longitudinal stiffness KX requires measuring the force FX at the wheel center of the mounting assembly and the slip rate g% of the mounting assembly relative to the actual ground. Therefore, it is necessary to obtain reliable information related to these two variables simultaneously and in real time.

[0017] For the longitudinal force FX at the wheel center of the mounting assembly, for example, it can be estimated by the torque applied around the axis of rotation of the mounting assembly, whether it is the driving torque or the braking torque when the vehicle is moving straight. This means that these data can be traced back through the characteristics of the vehicle.

[0018] For example, they can also be obtained by the static load of the vehicle, the longitudinal acceleration of the vehicle's center of gravity, combined with the distribution of the driving force and the braking force between the front and rear axles. Optionally, the physical model capable of tracing back the longitudinal force FX at the wheel center of the mounting assembly takes into account different parameters, including the slope of the road, the forward speed of the vehicle, the aerodynamic resistance of the vehicle, and the rolling resistance of the tire tread.

[0019] However, the longitudinal force FX can also be obtained by making a more direct measurement on the mounting assembly. By way of non-limiting illustrative example, it will be noted that by processing at least two measurements of the circumferential contraction or expansion of at least one sidewall of the outer tire at two spatially fixed points located at different azimuths along the circumference, the force at the wheel center can be estimated. Such circumferential expansion or contraction of the sidewall is advantageously estimated by measuring the distance between the cords of the carcass ply of the sidewall. For a detailed description of such measurements in relation to the characteristics of the mounting assembly, reference can be made to the patent document WO-A-03 / 014693 in the name of the applicant.

[0020] Another characteristic essential for evaluating the longitudinal stiffness KX is the slip ratio g% at the wheel center of the mounting assembly. This variable can be directly estimated from data provided by on-vehicle electronic systems such as an ABS system.

[0021] But it can also be evaluated by three basic parameters, namely, the rotational speed W of the wheel center of the mounting assembly, the rolling radius Re of the mounting assembly, and the forward speed V0 of the vehicle. The rotational speed W can be simply obtained by a wheel rotation encoder coupled to a clock. The rolling radius Re of the mounting assembly is less sensitive to wear and is obtained from the distance traveled by the vehicle and the number of revolutions made by the mounting assembly in traveling that distance. Finally, for example, for high precision, the forward speed V0 of the vehicle is obtained by a type RT 3000 high-frequency measuring device, or the forward speed V0 of the vehicle is obtained by a GPS connected to the vehicle in a basic mode.

[0022] Thus, obtaining the reference longitudinal stiffness involves measuring or simulating the longitudinal stiffness of the mounting assembly including the tire on a test bench at the scale of the vehicle or of the mounting assembly, provided that the road surface is in a wet state during the measurement.

[0023] Recording the curve of the longitudinal shear force FX as a function of the slip ratio can obtain the raw slope related to the value of KX Ref In addition, the maximum value of the curve at the maximum slip ratio, in combination with the static vertical force applied to the mounting assembly, defines the reference grip coefficient Mu Ref of the tire.

[0024] In addition, the method requires obtaining the weather status during evaluation to determine whether the wet ground state is met. This requires determining whether precipitation has occurred and its intensity at the time of measurement, i.e., the intensity of precipitation. A stronger intensity exceeding a certain threshold indicates a possible wet state. The types of precipitation affecting the road state also need to be defined: snow, hail, rain. This is because driving on snow, hail, or liquid water is not fully analogous. Generally, in the case of hail or snow, the reduced visibility prompts the driver to naturally slow down. In contrast, depending on the intensity of the rain and the size of the water droplets, the assessment of the amount of water accumulated on the road is more random, so an objective determination of this state is required according to the method of the present invention.

[0025] If it is considered that the driving road surface is in a wet state (which is determined by the intensity and type of precipitation), the method requires measuring the longitudinal stiffness KX on the driving road surface Mes 。

[0026] Finally, the method evaluates the grip coefficient Mu of the tire on the wet driving road surface in absolute value according to a given formula, based on the reference longitudinal stiffness and the measured longitudinal stiffness, and the grip potential of the tire in the absence of a water film between the ground and the tire. Therefore, the evaluation of the grip coefficient of the tire on the so-called wet driving road surface is carried out directly after measuring the longitudinal stiffness, making it immediately available.

[0027] The changes found are mainly due to the presence of a water film at the interface between the ground and the tire. This estimation of the wet grip coefficient of the tire enables the estimation of, for example, the maximum longitudinal force applied to the vehicle wheels to optimize its braking distance. For the stability of the vehicle, preferably, no wheel slips on the ground, or if a wheel has to slip, it is symmetric between the two wheels on the same axle to ensure the stability of the vehicle.

[0028] Preferably, the coefficient n is between 0.5 and 1.0.

[0029] The inventors have found that a range of 0.2 to 2.0 for the coefficient n in the formula enables a good estimation of the grip coefficient of the tire on a wet ground at the scale of the contact surface, depending on the seasonality, range, and size of the tire. Limiting the range to an interval between 0.5 and 1.0 is particularly applicable to passenger car tires and truck tires.

[0030] Advantageously, the weather parameters are included in the group including the external temperature and the precipitation intensity level.

[0031] The external temperature is a variable related to the direct environment of the vehicle that can be obtained inside most transport vehicles. Considering this variable, it is easy to distinguish whether the precipitation seen by the vehicle is equivalent to snow or rain.

[0032] Very advantageously, the precipitation intensity level is evaluated by sound measurement, vibration measurement, activation of precipitation-sensitive vehicle devices such as rain detectors on the windscreen or the wiping speed of the windscreen wipers.

[0033] The precipitation intensity level enables the estimation of the precipitation amount in terms of volume by the number of impacts on vehicle-sensitive areas such as rain detectors. However, the precipitation type can also be evaluated by comparing the vibrations of these impacts with a threshold value to distinguish whether there is hail or large raindrops. Of course, the precipitation amount can be evaluated, for example, by the wiping speed of the windscreen wipers. For example, the windscreen wipers determine a specific threshold based on the amount of rain falling on the windscreen. Finally, the vibration or acoustic measurements at the vehicle body, especially at the chamber accommodating the vehicle's mounting components, can also evaluate the number and type of moving particles on the road by analyzing the vibro-acoustic signals generated when the moving particles on the road impact the vehicle.

[0034] According to a specific embodiment, the method includes determining the longitudinal stiffness KX' of the tire on the driving road surface when the driving road surface is in a wet state Mes step.

[0035] By using the same determination device, the wet state can be easily identified by a threshold different from that of the wet state threshold. Therefore, the longitudinal stiffness of the same nature as that in the wet state can be measured with the same data, except that the obtained result is the longitudinal stiffness of the tire on the wet ground at the scale of the ground contact surface.

[0036] Advantageously, the reference longitudinal stiffness KX of the tire on the wet ground Ref is evaluated as the average value of the longitudinal stiffness KX' of the tire on the wet ground obtained on the tires mounted on the vehicle within time T Mes average value.

[0037] Therefore, the reference longitudinal stiffness can be obtained by the average value of the longitudinal stiffness on the wet ground measured on the vehicle. This enables the initially adopted fixed value (usually corresponding to when the tire is new) to adapt to the tire's life cycle, that is, implicitly taking into account its wear and aging.

[0038] According to another specific embodiment, the method includes the step of identifying the tires mounted on the vehicle, at least including the seasonality of the tires.

[0039] Preferably, the step of identifying the tires mounted on the vehicle includes obtaining the wear and / or aging of the tires.

[0040] Advantageously, the coefficient n depends on the tire.

[0041] Very advantageously, the reference longitudinal stiffness KX RefDepends on the tire.

[0042] A fixed measurement of the reference variables is still possible, especially by finding the values corresponding to the new standard tires. However, in order to improve the estimation level of the tire's grip coefficient on wet ground, information about the tire's seasonality is necessary. Of course, at a secondary scale, knowing the brand, range, and / or size of the tire can improve the prediction of the grip coefficient by making the fixed values more finely adapted to the tire's characteristics. If the type of ground varies too much compared to the average ground encountered by the tire, this precision is no longer meaningful. Specifically, for example, compared to the variations brought about by the complete characteristics of the tire, the variation in the grip coefficient Mu on wet ground caused by the type of ground becomes crucial.

[0043] However, considering the wear state and aging state of the tire, the fixed values of the reference variables can be adjusted, especially the reference longitudinal stiffness KX Ref , which is more sensitive to these parameters than the reference grip coefficient of the tire on wet ground. For wet ground, fixed estimated values are usually sufficient. These two parameters affect at least one of the reference variables and can be easily taken into account on the vehicle. Therefore, the analysis of data such as the number of kilometers traveled and the time since the tire was installed on the vehicle can well estimate these parameters, so that the fixed values of the reference variables can be updated, thereby improving the evaluation quality of the tire's grip coefficient on wet ground and ensuring better adjustment of the threshold for triggering the vehicle's active safety devices.

[0044] In addition, identifying the tire can also adjust it by changing the value of the coefficient n of the formula, which was initially set, for example, by fixed measurement on new standard tires.

[0045] The present invention also relates to a method for obtaining the critical hydroplaning speed v of a tire mounted on a vehicle on wet ground during a driving state cr , which includes the following steps:

[0046] · Determine the longitudinal moving speed v of the vehicle;

[0047] · Obtain the grip coefficient Mu of the tire on a reference ground in a wet state Ref ;

[0048] · Determine the grip coefficient Mu of the tire on wet ground;

[0049] · Evaluate the critical hydroplaning speed v of the tire by means of a function F in the following form including the parameters v, Mu, and Mu Ref : cr :

[0050] Mathematical formula 2 v Cr = F(v, Mu, MuRef )

[0051] Preferably, the function F is in the following form:

[0052] Mathematical formula 3 where β is a real number between 0.1 and 1.0.

[0053] Very advantageously, β is between 0.2 and 0.4.

[0054] Based on the estimation of the grip coefficient of the tire on a wet ground at the ground contact scale, the estimation of the inherent variables of the tire through the reference grip coefficient on a wet ground, and the estimation of the linear moving speed of the vehicle equipped with the tire, the critical hydroplaning speed of the tire on the driving road surface can be evaluated. Then, the critical hydroplaning speed corresponds to the moving speed of the vehicle, which causes the complete loss of contact between the tire and the wet ground. This is because at the ground contact scale, excessive liquid water is located above the maximum height of the ground macro-roughness, resulting in the saturation of the void network in the tire tread. Thus, the void network can no longer drain the amount of water on the wet road. The higher the moving speed, the more water needs to be drained. Therefore, the potential saturation of the void network in the tread is higher, leading to the gradual loss of contact between the tire and the ground until the tire starts to hydroplane.

[0055] To estimate the critical hydroplaning speed, it is necessary to obtain the linear moving speed v of the vehicle on a wet ground, the reference grip coefficient Mu Ref of the tire on a standard wet ground, and the evaluated value of the grip coefficient of the tire on the wet ground when the vehicle is moving at speed v. Based on these variables, the critical hydroplaning speed v cr of the tire on the wet ground is evaluated. This enables the active safety device of the vehicle to be notified in real time so that the speed limiter of the vehicle can be activated if necessary, and the driver (if any) can be warned of the situation, or the on-vehicle system can be notified so that the on-vehicle system can adjust the speed of the vehicle according to the situation. The formula determined by F is very suitable for the tires of passenger cars / trucks. The range of the β value covers all seasons of the tires, namely summer tires, winter tires, and all-season tires. The preferred range of β is very suitable for high-end summer tires.

[0056] Finally, the present invention relates to a method for obtaining the safe driving speed V of a vehicle equipped with tires on a wet ground, which includes the following steps:

[0057] · Determine the minimum grip coefficient Mu Min of the tire on a wet ground;

[0058] · Obtain the grip coefficient Mu Ref of the tire on a reference ground in a wet state;

[0059] · Determine the critical aquaplaning speed v of the tyre according to any one of claims 11 to 13 on a wet road surface cr ;

[0060] · Evaluate the safe driving speed V by means of a function H in the following form which includes the parameters v cr , Mu Min and Mu Ref :

[0061] Mathematical formula 4 V = H(v cr , Mu Min , Mu Ref )

[0062] Preferably, the function H is in the following form:

[0063] Mathematical formula 5 where γ is a real number, preferably between 0.1 and 1.0.

[0064] Finally, by knowing in real time the critical aquaplaning speed v of the tyre on a wet road surface cr , determining the reference grip coefficient and the required minimum grip coefficient Mu of the tyre on the wet road surface usually encountered by the tyre Min , the maximum driving speed can be defined so as to avoid any aquaplaning risk and ensure the minimization of risks during emergency manoeuvres. These emergency manoeuvres of the vehicle may be, for example, an untimely lane change or a trajectory of a sharp turn (i.e., a trajectory with a small radius of curvature). By directly controlling the active safety devices of the vehicle, such as a speed regulator, a speed limiter, to optimize the stability devices of the vehicle, this ensures smooth driving for the driver (if any), which means no stress. Description of the drawings

[0065] The present invention will be better understood by reading the following description which is given by way of non - limiting example only and with reference to the drawings, in which the same reference numerals always denote the same parts and in which:

[0066] Figure 1 shows an overview of a method for determining the grip coefficient Mu of a tyre on a wet road surface, determining the critical aquaplaning speed v cr and determining the safe driving speed V according to the present invention;

[0067] Figure 2 shows the variation of the grip coefficient Mu of the tyre on a wet road surface according to the ratio of the longitudinal stiffness KX of the tyre on a wet road surface to that on a reference road surface in the wet state;

[0068] Figure 3Shows the change in the critical hydroplaning speed of various tires on a wet ground according to the coefficient of grip Mu of the tire on the wet ground;

[0069] Figure 4 Shows the safe driving speed V of the tire on the wet ground according to the minimum coefficient of grip Mu of each tire on the wet ground Min change. Detailed implementation mode

[0070] Figure 1 Is an overall overview of the method according to the present invention. First, a method for determining the coefficient of grip Mu of a tire on a wet ground is defined by steps S1 to S5 and optional steps O1, O2 and O3.

[0071] Step S2 includes determining the reference longitudinal stiffness KX of the tire installed on the vehicle on the ground in a wet state Ref . This value can be obtained in a fixed manner in the first stage; generally, values in the range between 20000 N / g% and 500000 N / g% are very acceptable.

[0072] However, for greater precision, this fixed value can also be determined according to the characteristics of the tire by step O1. The priority characteristic information of this dependence is derived from the zero - order seasonality of the tire. Therefore, in order to refine the fixed value, it is necessary to know whether the tire is a "summer" tire, a "winter" tire or an "all - season" tire. However, the fixed value can also be related to the range of tires of a given seasonality, or even to the tire size. However, this second dependence is a second - order dependence prior to other influencing factors (such as the wear state or aging state of the tire), and other influencing factors will be closer to the first order in terms of influencing factors. Finally, the last embodiment optionally includes measuring the longitudinal stiffness KX' of the tire installed on the vehicle Mes , which corresponds to straight - line driving on a wet ground and is independent of the ground type. Therefore, by averaging the measured values KX' obtained in a short period of time on the scale of changes in tire wear or aging Mes , a possibly more relevant value considering all influencing factors of the tire longitudinal stiffness KX is obtained without the optional step O1 of identifying the tire.

[0073] Step S1 includes determining the reference coefficient of grip Mu of the tire on the wet ground Ref . This value can be obtained in a fixed manner in the first stage; generally, values in the range between 0.5 and 1.3 are very acceptable.

[0074] However, for greater precision, this fixed value can also be determined according to the characteristics of the tire in step O1. The priority characteristic information of this dependence is derived from the zero - order seasonality of the tire. Therefore, in order to refine the fixed value, it is necessary to know whether the tire is a "summer" tire, a "winter" tire or an "all - season" tire. However, the fixed value can also be related to the range of tires of a given seasonality and even to the tire size. However, this second dependence is a second - order dependence prior to other influencing factors (such as the wear state or aging state of the tire), and the other influencing factors will be closer to the first - order in terms of influencing factors.

[0075] These first two steps S1 and S2 can be performed before the real - time evaluation of the various variables of the present invention, so that these values can be provided to the calculator. The calculator can be installed inside the vehicle or located outside the vehicle. In the second solution, the data is provided via the cloud, and the output of the evaluation is also transmitted to the vehicle via the cloud.

[0076] When the vehicle is moving, it is necessary to determine the weather conditions in step S3. For example, this includes estimating the temperature outside the vehicle and estimating the precipitation level. The first weather parameter (outside temperature) can estimate whether the precipitation is likely to be snow or liquid water. The second weather parameter (precipitation level) can evaluate the precipitation volume and / or the precipitation mass falling on the vehicle using sensors on the vehicle. Therefore, the speed of the automatic windshield wipers on the vehicle can distinguish the precipitation volume in terms of the impact on the rain sensor. In addition, the sound or vibration sensors on the vehicle can evaluate the precipitation volume and can also evaluate the precipitation mass, and if necessary, consider the longitudinal driving speed of the vehicle through the optional step O3 to evaluate the precipitation. Therefore, by collecting and analyzing all this data, it can be estimated whether the vehicle is in contact with a wet or moist road. In the second case, at the scale of the contact area between the tire and the ground, the amount of water retained on the road is not important.

[0077] When the analysis of the weather results determines that the ground is in a wet state, it is necessary to proceed to the subsequent step S4. This step S4 includes determining the longitudinal stiffness KX of the tire installed on the vehicle on a wet driving road surface when the vehicle is moving. Mes . This measurement is carried out on a straight line (for example, through the indication of the steering angle), and the slope of the point defined by the slip ratio g% of the tire and the force FX applied to the tire measured, for example, at the wheel center is evaluated. By accumulating multiple points representing the force FX at the wheel center for a given slip ratio g%, the original slope of the point cloud related to the longitudinal stiffness KX of the tire on a wet ground can be estimated.

[0078] Finally, an estimated value of the grip coefficient Mu of the tire on a wet ground can be obtained through step S5. The input of this step S5 is the reference variable Mu which is the output of steps S1 and S2.Ref and KX Ref , and the longitudinal stiffness KX obtained in step S4 Mes Determination. Therefore, when driving on a vehicle and on a wet ground, the remaining grip potential of the tire related to the wet state of the ground can be estimated in real time through the grip coefficient Mu of the tire. Therefore, a warning can be sent to the driver or the in-vehicle driving system to adjust the driving state of the vehicle according to the grip coefficient Mu. For example, by measuring the remaining grip potential of the front axle tires that are most sensitive to the amount of liquid water on the road during forward driving, loss of contact between one of the tires located at the front of the vehicle and the ground or even the entire front axle can be avoided.

[0079] Next, a method for determining the critical hydroplaning speed v of the tire on a wet ground is defined through steps S1 and S5 to S7 cr of the tire.

[0080] Steps S1 and S5 have been explained in the method for determining the grip coefficient Mu of the tire on a wet ground.

[0081] When the ground is in a wet state, by forming a water film between the two solid elements composed of the tire and the ground, the tire may lose rubber / ground contact, which corresponds to hydroplaning. To determine the limiting speed at which rubber / ground contact is lost (referred to as the critical hydroplaning speed v cr ), first, the driving speed of the vehicle needs to be known through step S6.

[0082] If the optional step O3 is executed in step S5 to determine the grip coefficient Mu of the tire on a wet ground, then step S6 adopts the result of the optional step O3. Otherwise, step S6 includes determining the driving speed v of the vehicle when the ground state is identified as wet. This speed determination can utilize various possibilities, such as the speed defined by the Global Positioning System (GPS), or by estimating the compression radius Re related to the measured wheel to determine the rotational speed of the vehicle wheel. Generally, the compression radius Re of the tire depends on the size and range of the tire, as well as the static load applied and the inflation pressure of the tire. Therefore, the speed can be determined by the in-vehicle system of the vehicle (especially the dashboard instruments of the vehicle such as the speedometer).

[0083] Finally, the critical hydroplaning speed v cr is determined through step S7. The input of this step S7 is the reference variable Mu of step S1 Ref, the output of step S5 (i.e., the grip coefficient Mu of the tire on the wet ground) and the determination of the driving speed v of the vehicle obtained in step S6. Therefore, when driving on the vehicle and on the wet ground, the critical hydroplaning speed v of the tire installed on the vehicle related to the wet state of the ground can be estimated in real time. cr . Therefore, a warning can be sent to the driver or the in-vehicle driving system to adjust the driving state of the vehicle, especially the driving speed, according to the critical hydroplaning speed v. cr . For example, inform the driver or the in-vehicle driving system of the vehicle to comply with a speed limit that is X% or Y speed units lower than the critical hydroplaning speed v. cr

[0084] Finally, the method for determining the safe driving speed V on the wet ground is defined by steps S1 and S7 to S9.

[0085] Steps S1 and S7 have been explained in the method for determining the critical hydroplaning speed v of the tire on the wet ground. cr

[0086] When the ground is in a wet state, by forming a water film between the two solid elements composed of the tire and the ground, the tire may lose rubber / ground contact, which corresponds to hydroplaning. In order to drive completely safely when this water film exists on the road, it is necessary to define a threshold driving speed V for the minimum grip level of the tire (referred to as Mu) that needs to be maintained to ensure this minimum grip level Mu without exceeding it. Min ) Min

[0087] Step S8 includes determining the minimum grip coefficient Mu of the tire when the ground state is identified as wet. Min Determining the minimum grip coefficient Mu of the tire on the wet ground Min is used to ensure that the vehicle can drive easily and without risk regardless of how the vehicle is maneuvered (such as turning), even though there may be a large amount of liquid water on the road.

[0088] This determination can be obtained in a fixed manner in the first stage; generally, values in the range between 0.4 and 0.8 are very acceptable.

[0089] However, for greater precision, this fixed value can also be determined according to the characteristics of the tire by step O1. The priority characteristic information of this dependency originates from the zero - order seasonality of the tire. Therefore, in order to refine the fixed value, it is necessary to know whether the tire is a "summer" tire, a "winter" tire, or an "all - season" tire. However, the fixed value can also be related to the range of tires of a given seasonality and even to the tire size. However, this second dependency is a second - order dependency prior to other influencing factors (such as the wear state or aging state of the tire), and the other influencing factors will be closer to the first - order in terms of influencing factors.

[0090] Finally, the safe driving speed V is determined by step S9. The input of this step S9 is the reference variable Mu of step S1 Ref , the output of step S7 (i.e., the critical hydroplaning value v of the tire on a wet road surface cr ) and the determination of the minimum grip coefficient Mu of the tire on a wet road surface obtained in step S8 Min . Therefore, when driving on a vehicle and on a wet road surface, the safe driving speed V of the tire installed on the vehicle related to the wet state of the road surface can be estimated in real - time. Therefore, a warning can be sent to the driver or the in - vehicle driving system to adjust the driving state of the vehicle, especially the driving speed, according to this safe driving speed V. For example, inform the driver or the in - vehicle driving system of the vehicle to comply with a speed limit that is X% or Y speed units lower than the safe driving speed V.

[0091] Figure 2 Shows the grip coefficient Mu of the tire on a wet road surface according to the longitudinal stiffness KX of the tire measured during driving on a wet road surface Mes . More specifically, in this case, the horizontal axis shows the ratio of the longitudinal stiffness KX of the same tire between the wet road surface measured on the vehicle and the reference road surface in a wet state.

[0092] The two curves 101 and 201 depicted respectively correspond to the tires 100 and 200. In this case, each tire in the figure belongs to a tire category with a different seasonality. The tire 100 represented by the solid line 101 is a "summer" tire, while the tire represented by the dashed line 201 is an "all - season" tire 200. The tires 100 and 200 are characterized by different reference grip coefficients Mu Ref in this case. The mathematical representation of the two curves 101 and 201 is a function of the reference variables Mu Ref and KX Ref of the tire and the power coefficient "n", all of which depend on the tire 100 or 200.

[0093] Step S5 of this method includes, for a given tire 100 or 200, based on the real - time KX Mes, the points for identifying the grip coefficient Mu of tire 100 or 200 on a wet ground are independent of the height of water on the ground.

[0094] In this regard, at a given KX Mes (i.e., points K100 and K200), draw vertical lines 102 and 202, which intersect the Figure 2 curves 101 and 201 in at points 103 and 203 respectively. Based on points 103 and 203, draw orthogonal lines 104 and 204, which intersect the vertical axis at corresponding points Mu100 and Mu200. Therefore, these two points Mu100 and Mu200 respectively represent the grip coefficient Mu of tire 100 and tire 200 for each tire on a wet ground, independent of the height of water on the road.

[0095] Figure 3 Shows the critical hydroplaning speed v of the tire on a wet ground cr According to the representation of the grip coefficient Mu of the tire on a wet ground during driving.

[0096] The two depicted curves 111 and 211 respectively correspond to tire 100 and 200. In this case, each tire in the figure belongs to a tire category with different seasons. The tire 100 represented by the solid line 111 is a "summer" tire, while the tire represented by the dashed line 211 is a "all-season" tire 200. Each tire 100 and 200 is characterized by a different reference grip coefficient Mu Ref in this case. The mathematical representation of the two curves 111 and 211 is a function of the reference variable Mu Ref , the grip coefficient Mu of the tire on a wet ground, the moving speed v of the vehicle on which the tire is installed, and the power coefficient "β", all of which depend on tire 100 or 200.

[0097] Step S7 of the method includes, for a given tire 100 or 200, identifying the critical hydroplaning speed v of tire 100 or 200 based on the real-time Mu of the tire on a wet ground cr of the point, independent of the height of water on the ground.

[0098] In this regard, at a given Mu (i.e., points Mu100 and Mu200), draw vertical lines 112 and 212, which intersect the Figure 3 curves 111 and 211 in at points 113 and 213 respectively. Based on points 113 and 213, draw orthogonal lines 114 and 214, which intersect the vertical axis at corresponding points v100 and v200. Therefore, these two points v100 and v200 respectively represent the critical hydroplaning speed v of tire 100 and tire 200 for each tire cr .

[0099] Figure 4 shows the safe driving speed V as a function of the minimum grip coefficient Mu of the tire on wet ground, which is desired in all driving situations. Min representation.

[0100] The two curves 121 and 221 depicted correspond to tires 100 and 200 respectively. In this case, each tire in the figure belongs to a tire category with different seasons. The tire 100 represented by the solid line 121 is a "summer" tire, while the tire represented by the dashed line 221 is a "four-season" tire 200. The tires 100 and 200 are characterized by different reference grip coefficients Mu in this case. Ref The mathematical representation of the two curves 121 and 221 is a function of the reference variable Mu of the tire on wet ground, Ref the critical hydroplaning speed v of the tire, cr the minimum grip coefficient Mu of the tire on wet ground, Min and the power coefficient "γ", all of which depend on the tire 100 or 200.

[0101] Step S9 of the method includes, for a given tire 100 or 200, identifying the point of the safe driving speed V of the tire 100 or 100 based on the real-time Mu of the tire on wet ground, Min regardless of the height of the water on the ground.

[0102] In this regard, at a given Mu Min (i.e., points Mu M 100 and Mu M 200), vertical lines 122 and 222 are drawn, which intersect the curves 121 and 221 in Figure 4 at points 123 and 223 respectively. Based on points 123 and 213, orthogonal lines 124 and 224 are drawn, which intersect the vertical axis at the corresponding points V100 and V200. Therefore, these two points V100 and V200 represent the safe driving speed V of the tire 100 and the tire 200 for each tire respectively, regardless of the height of the water on the road.

Claims

1. A method for obtaining the grip coefficient Mu of a tire on a wet ground in a vehicle usage state, comprising the following steps: - Obtain (S2) the reference longitudinal stiffness KX of the tire on a reference ground surface in a wet state Ref ; - Obtain (S1) the grip coefficient Mu of the tire on a reference ground in a wet state Ref ; - During the use of the tire on the vehicle, determine the weather parameters (S3); - If the ground is wet, determine (S4) the longitudinal stiffness KX of the tire on the driving road surface Mes ; - Evaluate (S5) the grip coefficient Mu of the tire on the wet ground by means of the following formula: Mathematical formula 1 Among them, the coefficient n is a real number between 0.2 and 2.

0.

2. The method for obtaining the grip coefficient Mu of a tire on a wet ground according to claim 1, wherein, The coefficient n is between 0.5 and 1.

0.

3. The method for obtaining the grip coefficient Mu of a tire on a wet ground according to any one of claims 1 and 2, wherein, The weather parameters (S3) are included in the group including the external temperature, precipitation intensity level.

4. The method for obtaining the grip coefficient Mu of a tire on a wet ground according to claim 3, wherein, Evaluate the precipitation intensity level by sound measurement, vibration measurement, activation of a precipitation-sensitive vehicle device such as a rain detector on the windshield or the wiping speed of the windshield wiper.

5. The method for obtaining the grip coefficient Mu of a tire on a wet ground according to any one of claims 1 to 4, wherein, The method includes determining a longitudinal stiffness KX' of a tire on a driving road surface when the driving road surface is in a wet state Mes in step (O2).

6. The method for obtaining the grip coefficient Mu of a tire on a wet ground according to claim 5, wherein, Reference longitudinal stiffness KX of a tire on a wet ground Ref (S2) is evaluated as the average value of the longitudinal stiffness KX’ on a wet ground obtained at time T on a tire mounted on a vehicle Mes (O2).

7. The method for obtaining the grip coefficient Mu of a tire on a wet ground according to any one of claims 1 to 6, wherein, The method includes the step (O1) of identifying the tire mounted on the vehicle, at least including the seasonality of the tire.

8. The method for obtaining the grip coefficient Mu of a tire on a wet ground according to claim 7, wherein, The step (O1) of identifying the tire mounted on the vehicle includes obtaining the wear and / or aging of the tire.

9. The method for obtaining the grip coefficient Mu of a tire on a wet ground according to any one of claims 7 and 8, wherein, The coefficient n depends on the tire.

10. The method for obtaining the grip coefficient Mu of a tire on a wet ground according to any one of claims 7 to 9, wherein, Reference longitudinal stiffness KX Ref (S2) Depends on the tire.

11. A method for obtaining the critical hydroplaning speed v of a tire mounted on a vehicle on a wet ground in a driving state cr comprising the following steps: - Determine the longitudinal moving speed v of the vehicle (S6); - Obtain the grip coefficient Mu of the tire on a reference ground in a wet state Ref (S1); - Obtain the grip coefficient Mu of the tire on the wet ground according to any one of claims 1 to 10 (S5); - Evaluate the critical hydroplaning speed v of the tire by means of a function F of the following form including the parameters v, Mu and Mu Ref where the function F includes the parameters v, Mu and Mu cr (S7): Mathematical formula 2 v Cr = F(v, Mu, Mu Ref )。 12. The method for obtaining the critical hydroplaning speed v of a tire on a wet ground according to claim 11, wherein, cr The function F is in the following form: ​ Mathematical formula 3 Among them, β is a real number between 0.1 and 1.

0.

13. The method for obtaining the critical hydroplaning speed v of a tire on a wet ground according to claim 12, wherein, cr β is between 0.2 and 0.

4.

14. A method for obtaining the safe driving speed V of a vehicle equipped with a tire tread on a wet ground, comprising the following steps: - Determine the minimum grip coefficient Mu of the tire on a wet ground Min (S8); - Obtain the grip coefficient Mu of the tire on a reference ground in a wet state Ref (S1); - Determine the critical aquaplaning speed v of the tire according to any one of claims 11 to 13 on a wet ground cr (S7); -Evaluate the safe driving speed V(S9) by means of a function H in the following form, including parameters v cr , Mu Min and Mu Ref : Mathematical formula 4 V = H(v cr , Mu Min , Mu Ref ).

15. The method for obtaining the safe driving speed V on a wet ground according to claim 14, wherein, The function H is in the following form: Mathematical formula 5 Where γ is a real number, preferably, γ ranges from 0.1 to 1.0.

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