Method for determining the grip coefficient of a tire on wet roads

Abstract

A method for obtaining the tire grip coefficient Mu on a wet road surface under vehicle operating conditions, comprising: Ref Step (S2) to obtain the tire grip coefficient Mu on the reference road surface in wet conditions. Ref (S1) a step of determining weather parameters while the tire is mounted on the vehicle; and (S3) a step of determining the longitudinal stiffness KX of the tire on the road surface when the road surface is wet. Mes and (S5) estimating the tire's grip coefficient Mu on a wet road surface using Equation 1, 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 running state of tires, in particular on wet roads, in order to improve the active safety of vehicles by improving information about the tires in real time. [Background technology]

[0002] The present invention relates to a method for determining a tire's grip coefficient at the scale of the contact patch, i.e., the area where the tire comes into contact with the road surface when the tire is mounted on a rim and is loaded and optionally inflated. Specifically, the tire's overall grip coefficient (commonly referred to as Mu) allows for evaluation of the tire's potential grip on the road surface. This allows for the optimization of vehicle active safety devices to prevent random vehicle behavior, which may potentially lead to dangerous trajectories. This tire's grip coefficient Mu, of course, depends on both the nature of the road surface, i.e., asphalt or sand, and its condition, such as dry, damp, wet, or snow-covered. For example, the grip coefficient Mu of the same tire, even under the same tire pressure and static load, will vary depending on whether the tire is driven on a soft surface, such as compacted sand or snow, or on a hard surface, such as asphalt. However, the grip coefficient also varies depending on the road surface condition. The tire's grip coefficient Mu is higher on dry roads than on wet roads because the presence of moisture changes the tire's adhesion to the road. Furthermore, for example, one of the conditions that strongly affects the grip coefficient of a tire is the height of water remaining on the road surface, which can cause partial or complete loss of contact between the tire and the road surface, leading to aquaplaning of the tire.

[0003] Furthermore, external factors such as ambient temperature can also affect the behavior of a tire's rubber compound, which in turn can affect tire potential. Consequently, obtaining information about a tire's grip coefficient depending on weather conditions can allow for the adaptation of thresholds for activating a vehicle's active safety devices. In this way, adapting thresholds for activating active safety devices in real time can make the vehicle more comfortable to drive. Specifically, on black road surfaces, including asphalt or bitumen roads, one of the conditions that strongly affects a tire's grip coefficient is the height of water remaining on the road surface, which can cause partial or complete loss of contact between the tire and the road surface, leading to tire aquaplaning. Summary of the Invention [Problem to be solved by the invention]

[0004] The following subject matter of the present invention has the objective of determining in real time on a vehicle the change in the overall grip coefficient Mu of a tire due simply to the presence of liquid water on the road surface, and as a result, determining the critical aquaplaning speed of the tire regardless of the state of the tire, and defining a linear safe driving speed of the vehicle under the conditions to which the vehicle is subjected, which may serve as a threshold for the activation of, for example, active safety devices. [Means for solving the problem]

[0005] The present invention relates to a method for obtaining the grip coefficient Mu of a tire on a wet road surface under vehicle use conditions, said method comprising the steps of: Reference longitudinal stiffness KX of a tire on a reference road surface in wet conditions Ref and obtaining Grip coefficient Mu of a tire on a reference road surface in wet conditions Ref and obtaining determining meteorological parameters while the tire is mounted on the vehicle in order to identify road surface conditions; When the road surface is wet, the vertical stiffness of the tire on the road surface KX Mesdetermining a The tire's grip coefficient on a wet road surface is calculated using the following formula: [Formula 1] evaluating using JPEG2025542019000002.jpg1340; where the coefficient n is a real number between 0.2 and 2.0.

[0006] Therefore, the method first involves obtaining tire reference variables. These reference variables correspond to the use of a tire on a so-called wet road surface. The term "wet" is understood to mean that liquid water is present on the road surface, but no film is formed between the road surface and the tire. As a result, the amount of water is below a threshold, and the water can generally enter the road surface irregularities without remaining on the road surface. Of course, this threshold depends on the granularity of the road surface, but the fact that the road surface is in a wet state guarantees two conditions: the presence of water at the scale of the road surface (which essentially changes the adhesion between the tire and the road surface), and the fact that the water is stored at a level below the maximum height of the road surface. This means that the liquid water / air interface is below the maximum height of the road surface at the scale of the contact patch, i.e., the contact area between the road surface and the tire in use. Conversely, the term wet road is used when, on a road surface with liquid water, the water / air interface is above the maximum height of the road surface at the scale of the contact patch. Therefore, a film of water is trapped between the road surface and the tire. These data can therefore be obtained on various types of road surfaces by monitoring the amount of liquid water on a reference road surface.

[0007] The necessary reference variables are, on the one hand, the tire's grip coefficient on a wet road, Mu Ref On the other hand, the vertical stiffness of the tire on this road surface is KX RefThese variables can be fixed variables, but they can also be related to the tire, for example the seasonality of the tire, i.e. summer tire, winter tire, all-season tire, or the state of wear and age of the tire, but are independent of the nature of the road surface, or more precisely, their dependence on the nature of the road surface will be negligible compared to that of the tire.

[0008] Tire grip coefficient Mu on wet roads Ref corresponds to the maximum level of the ratio of shear force to normal force that the tire exerts on the road surface at the contact patch. Above this threshold, the tire will begin to slip on the road surface. In this case, the tire is said to slip at the scale of the contact patch, not at the scale of the tire's material components, which would be relevant for microslip. A wet road surface refers to the presence of liquid water, where the liquid water-air interface is below the maximum height of the road surface at the scale of the contact patch.

[0009] Vertical stiffness KX Ref corresponds to the slope at the origin of the curve connecting the shear force of the tire on the road surface and the slip ratio g% of the mounting assembly. For reasons of convenience and ease of calculation, the shear force is generally restricted to the longitudinal force FX, i.e., in the direction of movement of the tire as it rotates about its natural axis of rotation.

[0010] Measuring the longitudinal stiffness KX requires measuring both the force FX at the wheel center of the mounting assembly and the slip ratio g% of the mounting assembly relative to the actual road surface. Therefore, reliable information on these two variables must be obtained simultaneously in real time.

[0011] With respect to the longitudinal forces FX at the wheel center of the mounting assembly, these can be estimated by torques applied about the axis of rotation of the mounting assembly, whether they are drive torques or braking torques when the vehicle is moving straight ahead, including the ability to trace these data back through the characteristics of the vehicle.

[0012] These may be obtained, for example, from the static load of the vehicle, the longitudinal acceleration of the vehicle's center of gravity, and the distribution of drive and braking forces between the front and rear axles. Optionally, the physical model that allows tracing back to the longitudinal force FX at the wheel center of the mounting assembly takes into account various 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 casing.

[0013] However, the force FX can also be obtained by more direct measurements on the mounting assembly. It should be noted that, by way of non-limiting example, the force at the wheel center can be estimated by processing at least two measurements of the circumferential contraction or expansion of at least one sidewall of the casing at two spatially fixed points located at different circumferential orientations. This circumferential expansion or contraction of the sidewall is conveniently estimated by measuring the distance between the threads of the carcass ply of the sidewall. For a detailed description of this measurement of the characteristics of the mounting assembly, reference can be made to the applicant's WO 03 / 014693.

[0014] Another characteristic essential for assessing the longitudinal stiffness KX is the slip ratio g% of the mounting assembly at the wheel center. This variable can be estimated directly from data provided by electronic systems installed in the vehicle, such as the ABS system.

[0015] However, it can also be assessed by three basic parameters: the rotational speed W of the mounting assembly at the wheel center, the rolling radius Re of the mounting assembly, and the forward speed V of the vehicle. The rotational speed W can be easily obtained by a wheel rotation encoder coupled to a speedometer (clock). The rolling radius Re of the mounting assembly is less susceptible to wear and is obtained by the distance traveled by the vehicle and the number of revolutions made by the mounting assembly to cover this distance. Finally, the forward speed V of the vehicle is obtained, to achieve high accuracy, via a high-frequency measuring device, for example of the RT3000 type, or via a GPS linked to the vehicle in basic mode.

[0016] Therefore, the reference vertical stiffness is obtained by measuring or simulating the vertical stiffness of the mounting assembly constituting the tire on a vehicle or a test bench, on the condition that the road surface being traveled on is wet at the time the measurement is made.

[0017] The curve of longitudinal shear force FX as a function of slip ratio is recorded as KX Ref and the maximum value of the curve at the maximum slip ratio, in combination with the static normal force applied to the mounting assembly, gives the reference grip coefficient Mu of the tire. Ref Specifies the value of

[0018] Furthermore, this method requires obtaining the weather conditions at the time of evaluation to determine whether wet road conditions are met. It is necessary to confirm whether precipitation is occurring at the time of measurement and at what intensity, i.e., precipitation intensity. High precipitation intensity exceeding a predetermined threshold likely indicates the presence of wet conditions. It is also necessary to clarify the type of precipitation affecting the road surface conditions, such as snow, hail, or rain. This is because driving on snow, hail, and liquid water is completely incomparable. In general, when there is hail or snow, drivers naturally slow down due to reduced visibility. In contrast, the assessment of the amount of water accumulated on the road is more random, depending on the intensity of the rain and the size of the droplets. Therefore, the method of the present invention is required to objectify this condition.

[0019] In this method, when the road surface is judged to be in a wet state, the longitudinal stiffness KX of the road surface is Mes This requires measurements of the precipitation intensity and type.

[0020] Finally, the method evaluates the tire's grip coefficient Mu on a wet road surface in absolute terms, based on the reference and measured longitudinal stiffnesses according to a predetermined formula, and the tire's grip potential when there is no water film between the road surface and the tire. Therefore, the evaluation of the tire's grip coefficient in so-called wet conditions is performed directly after measuring the longitudinal stiffness and is therefore immediately available.

[0021] The variations found are mostly due to the presence of a film of water at the interface between the road surface and the tire. Estimating the wet grip coefficient of a tire in this way makes it possible to estimate the maximum longitudinal force acting on the vehicle's wheels, for example to optimize braking distance. To ensure vehicle stability, it is desirable that no wheel slips on the road surface, or that any slippage is symmetrical between the two wheels on the same axle.

[0022] Preferably, this coefficient n is between 0.5 and 1.0.

[0023] The inventors have found that by limiting the coefficient n of the formula to a range between 0.2 and 2.0, it is possible to obtain a good estimate of the tire's grip coefficient on wet roads, with a contact patch size that depends on the seasonality of the tire, its range and its size. The restriction to a range between 0.5 and 1.0 is particularly relevant for passenger car tires and van tires.

[0024] Advantageously, the weather parameters are included in the group consisting of: outside temperature, precipitation intensity.

[0025] The outside temperature is a variable that is accessible inside most transport vehicles and is related to the vehicle's immediate environment. By taking this variable into account, it is possible to easily distinguish whether precipitation observed in the vehicle is snow or rain.

[0026] Very advantageously, the level of precipitation intensity is assessed by sound measurements, vibration measurements, the activation of precipitation-sensitive vehicle devices such as rain detectors on the windscreen, or the wiping speed of windscreen wipers.

[0027] The level of precipitation intensity allows the volumetric estimation of precipitation by multiple impacts on the vehicle's sensitive zone, such as a rain detector. However, precipitation intensity also allows the evaluation of the type of precipitation by evaluating these impacts in comparison with a threshold for distinguishing between hail and heavy rain. Of course, the amount of precipitation can also be evaluated, for example, by the wiper speed, which determines a certain threshold depending on the amount of rain falling on the windshield. Finally, vibration or acoustic measurements on the vehicle body, and in particular on the cavity that houses the vehicle's mounting assembly, make it possible to evaluate not only the amount but also the type of moving particles on the road by analyzing the vibro-acoustic signals generated when the vehicle is impacted.

[0028] According to one particular embodiment, the method comprises: determining a longitudinal stiffness KX' of the tire on the road surface when the road surface is in a wet state; Mes The method includes determining:

[0029] A wet condition can be easily identified using the same determination means but with different thresholds than a wet condition, and therefore with the same data it is possible to make the same measurement of longitudinal stiffness as in wet conditions, except that the result obtained is the longitudinal stiffness of the tire on a wet road at the scale of the contact patch.

[0030] Advantageously, the reference longitudinal stiffness of the tire on a wet road surface KX Ref is the longitudinal stiffness KX' on a wet road obtained with a tire mounted on a vehicle Mes It is evaluated as the average of the time T.

[0031] In this way, the reference longitudinal stiffness can be obtained using the average value of the longitudinal stiffness measured on a vehicle on a wet road, so that the fixed value, which is typically measured when the tire is new, can be adapted to the life cycle of the tire.

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

[0033] Preferably, the step of identifying the tyres fitted to the vehicle comprises obtaining tyre wear and / or tyre aging.

[0034] Advantageously, the factor n is tire dependent.

[0035] Very advantageously, the reference longitudinal stiffness KX Ref depends on the tire.

[0036] It is possible to measure the reference variable as a fixed value, especially by finding values ​​corresponding to standard tires when new. However, to improve the level of estimation of a tire's grip coefficient on wet surfaces, information about the tire's seasonality is necessary. Of course, as a secondary measure, knowing the tire brand, range, and size allows the fixed value to be more closely adapted to the tire's specifics, improving the accuracy of the grip coefficient prediction. Such accuracy is meaningless if the type of road surface is significantly different compared to the average road surface the tire encounters. In particular, for example, the variation in the wet grip coefficient Mu caused by the type of road surface becomes most significant compared to the variation introduced by complete tire uniformity.

[0037] However, by taking into account the state of tire wear and aging, the fixed values ​​of the reference variables, specifically the reference longitudinal stiffness KX Ref This standard longitudinal stiffness KX RefThe tire's grip coefficient on wet roads is more sensitive to these parameters than the reference coefficient of tire grip on wet roads, for which a fixed estimate is often sufficient. These two parameters affect at least one reference variable and may be easier to take into account on the vehicle. Therefore, by analyzing data such as the number of kilometers traveled and the time since the tire was fitted, it is possible to properly estimate these parameters and update the fixed values ​​of the reference variables, thereby improving the quality of the assessment of the tire's grip coefficient on wet roads and ensuring a better adaptation of the thresholds for activating the vehicle's active safety devices. do.

[0038] Furthermore, the identification of the tire also makes it possible to adapt the coefficient n of the formula by changing the value of the coefficient n, which is fixedly determined, for example by initializing it to a new standard tire.

[0039] The present invention also provides a method for determining the critical aquaplaning speed v of a tire mounted on a vehicle on a wet road surface under driving conditions. cr The method for obtaining determining a longitudinal velocity v of the vehicle; Grip coefficient Mu of a tire on a reference road surface in wet conditions Ref and obtaining determining a grip coefficient Mu of the tire on a wet road surface; The following formula: [Formula 2] Parameters v, Mu, and Mu of JPEG2025542019000003.jpg842 Ref Using a function F containing cr and evaluating Includes.

[0040] Preferably, the function F is: [Formula 3] JPEG2025542019000004.jpg1859, where β is a real number between 0.1 and 1.0.

[0041] Very advantageously, β is between 0.2 and 0.4.

[0042] Based on an estimate of the tire's grip coefficient on a wet road at the contact patch scale, an estimate of the tire's intrinsic variables according to the reference grip coefficient on a wet road, and an estimate of the linear travel speed of a vehicle equipped with the tire, it is possible to evaluate the critical aquaplaning speed for this tire on the road surface. This aquaplaning speed corresponds to the travel speed of the vehicle, which leads to a complete loss of contact between the tire and the wet road surface due to saturation of the spatial network of the tire tread as a result of an excessive amount of liquid water above the maximum height of the macro-roughness of the contact patch. The spatial network is no longer able to evacuate the amount of water present on the wet road surface. The higher the travel speed, the greater the amount of water that is evacuated, which in turn increases the potential saturation of the spatial network of the tread, leading to a gradual loss of contact between the tire and the road surface and the tire beginning to aquaplan.

[0043] To estimate this critical aquaplaning speed, we need to calculate the vehicle's linear speed on a wet road, v, and the tire's grip coefficient on a standard wet road, Mu. Ref and an estimate of the tire's grip coefficient on a wet road with the vehicle traveling at speed v. Based on these variables, the tire's critical aquaplaning speed on a wet road, v, is calculated. cr is evaluated. This allows the vehicle's active safety devices to be notified in real time, activating the vehicle's speed limiter if necessary, and alerting the driver if the situation arises or informing the on-board system to adapt the vehicle's speed to the situation. The formula specified by F is suitable for passenger car / van type tires. The range of values ​​for β covers all tire seasonalities, i.e., summer tires, winter tires, and all-season tires. The preferred range of β is suitable for premium range summer tires.

[0044] Finally, the invention relates to a method for obtaining a safe driving speed V on a wet road surface of a vehicle equipped with a tire casing, said method comprising the steps of: Minimum tire grip coefficient Mu on wet roads Min determining a Grip coefficient Mu of a tire on a wet road Ref determining a According to any one of claims 11 to 13, the limit aquaplaning speed v of the tire on a wet road surface is cr determining a Safe driving speed V is calculated using the following formula. [Formula 4] Parameter v of JPEG2025542019000005.jpg844 cr , Mu Min , Mu Ref evaluating using a function H including Includes.

[0045] The function H is expressed as follows: [Formula 5] JPEG2025542019000006.jpg1171, where γ is a real number, preferably γ is between 0.1 and 1.0;

[0046] Finally, the limiting aquaplaning speed of a tire on a wet road, v cr Knowing in real time, the reference grip coefficient of the tire on wet road surfaces that the tire typically encounters, and the desired minimum grip coefficient Mu Min By determining the maximum driving speed, it is possible to define a maximum driving speed at which any risk of aquaplaning can be avoided and emergency maneuvering can be ensured while minimizing the risk. Such emergency maneuvers by the vehicle can be, for example, an untimely lane change or a trajectory around a sharp curve, i.e., a curve with a small curvature radius. This ensures a stress-free and gentle driving for the driver by directly controlling the active safety devices of the vehicle, such as a speed regulator or a speed limiter, in order to optimize the vehicle's stability.

[0047] The invention will be better understood on reading the following description, given only by way of non-limiting example. [Brief explanation of the drawings]

[0048] [Figure 1] 1 shows an overview of a method for determining the grip coefficient Mu of a tire on a wet road surface, a method for determining the critical aquaplaning speed vcr, and a method for determining the safe driving speed V according to the present invention. [Figure 2] 1 shows the variation of the tire's grip coefficient Mu on a wet road surface as a function of the ratio of the tire's longitudinal stiffness KX on a wet road surface to that on a damp reference road surface. [Figure 3] The variation of the critical aquaplaning speed for different tires on wet roads as a function of the tire's grip coefficient Mu on wet roads. [Figure 4] The variation of the safe running speed V of a tire on a wet road surface as a function of the minimum value MuMin of the grip coefficient of each tire on a wet road surface is shown. DETAILED DESCRIPTION OF THE INVENTION

[0049] A general overview of the method according to the invention is given in Figure 1. Firstly, the method for determining the grip coefficient Mu of a tire on a wet road surface is defined by steps S1 to S5, and optionally steps O1, O2 and O3.

[0050] Step S2 is the reference longitudinal stiffness KX of the tire mounted on the vehicle. Ref This value can be obtained as a fixed value in a first step, and values ​​in the range between 20,000 N / g% and 500,000 N / g% are usually accepted.

[0051] However, the determination of this fixed value may depend on the tire's identification information from step O1 for greater accuracy. The primary identification information for this dependency arises from the tire's zero-order seasonality. Therefore, 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, this fixed value may also be related to the tire range for a given season, or to the tire size. However, this second dependency is a second-order dependency prior to other influencing factors such as the tire's wear state or aging, which are more primary influencing factors. Finally, a final embodiment may optionally determine the longitudinal stiffness KX' of a tire fitted to a vehicle travelling in a straight line on a wet road, regardless of the type of road surface. Mes Therefore, the KX' measured in a short period of time is used to measure the extent of tire wear or aging. Mes By averaging these values, a more appropriate value that takes into account all factors that affect the tire's longitudinal stiffness KX may be obtained without the optional step O1 of identifying the tire.

[0052] Step S1 is to calculate the tire grip coefficient Mu on a wet road surface. Ref This value can be obtained as a fixed value in the first step. Usually, values ​​in the range between 0.5 and 1.3 are perfectly acceptable.

[0053] However, the determination of this fixed value may depend on the tire's identification information from step O1 for greater accuracy. The priority identification information for this dependency arises from the tire's zero-order seasonality. Therefore, 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, this fixed value may also be related to the tire range for a given season, or to the tire's size. However, this second dependency is a secondary dependency that precedes other influencing factors, such as the tire's wear state or aging state, and is a more primary influencing factor.

[0054] These two first steps S1 and S2 can be carried out before the real-time evaluation of the various variables of the present invention, making it possible to provide these values ​​to a computer that can be installed in the vehicle or located outside the vehicle. In this second solution, the data is provided via the cloud, as is the output of the evaluation, which will be sent to the vehicle.

[0055] While the vehicle is moving, the weather conditions must be determined in step S3. For example, this involves estimating the outside temperature of the vehicle and the amount of precipitation. The first weather parameter, the outside temperature, allows for estimating whether the precipitation is potentially snow or liquid water. The second weather parameter, the amount of precipitation, allows for the volumetric evaluation of the precipitation and / or the mass of precipitation falling on the vehicle, using sensors installed on the vehicle. Thus, the speed of the vehicle's automatic wipers makes it possible to distinguish the volumetric amount of precipitation hitting the rain sensor. In addition, sound or vibration sensors on the vehicle can be used to evaluate the volumetric amount of precipitation, but if necessary, the precipitation can also be evaluated by mass by taking into account the vehicle's longitudinal speed in optional step O3. Therefore, collecting all this data and analyzing it allows for an estimation of whether the vehicle is in contact with a damp or wet road surface. In the second case, the amount of water remaining on the road at the scale of the tire-road contact patch is less important.

[0056] If the meteorological analysis result indicates that the road surface is wet, it is necessary to proceed to the next step S4, which is to calculate the longitudinal stiffness KX of the tires mounted on the vehicle on the wet road surface while the vehicle is running. MesThis measurement is carried out by evaluating the gradient of the points on the line of the steering angle indication, defined for example by the tire slip ratio g% and the force FX applied to the tire measured at the wheel center. The accumulation of a number of points, each representing the force FX at the wheel center for a given slip ratio g%, makes it possible to estimate the gradient of the origin of the point cloud, which is related to the longitudinal stiffness KX of the tire on a wet road surface.

[0057] Finally, in step S5, an estimate of the tire grip coefficient Mu on a wet road surface is obtained. 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 vertical stiffness KX obtained in step S4 Mes The tire grip coefficient Mu is the determined value. Therefore, when a vehicle is traveling on a wet road surface, the tire's remaining grip potential relative to the wet condition of the road surface can be estimated in real time using the tire's grip coefficient Mu. A warning can then be sent to the driver or the vehicle's driving system so that the vehicle's driving conditions can be adapted according to the grip coefficient Mu. For example, when traveling forward, measuring the remaining grip potential of the front axle tires, which are most sensitive to the amount of liquid water on the road surface, can prevent one of the tires located at the front of the vehicle or the entire front axle from losing contact with the road surface.

[0058] Next, steps S1, S5 to S7 calculate the critical aquaplaning speed v of the tire on the wet road surface. cr A method for determining the

[0059] Steps S1 and S5 have already been described in the method for determining the grip coefficient Mu of a tire on a wet road surface.

[0060] When the road surface is wet, a film of water can form between the two solid elements, the tire and the road, causing the tire to lose rubber / road contact, which corresponds to aquaplaning. crTo determine the critical speed at which rubber / road contact is lost, which is called the limit speed, it is first necessary to know the vehicle's traveling speed in step S6.

[0061] This step S6 utilizes the results of the optional step O3, if performed in step S5, to determine the tire's grip coefficient Mu on a wet road surface. Otherwise, step S6 determines the vehicle's traveling speed v when the road surface is identified as wet. This speed determination can utilize various possibilities, such as a speed determined by a GPS (Global Positioning System) or the rotational speed of the vehicle's wheels by estimating the measured compression radius Re associated with the wheels. In general, the compression radius Re of a tire depends on the size and area of ​​the tire, the applied static load, and the tire inflation pressure. Therefore, the speed can be determined by the vehicle's on-board systems, in particular the vehicle's dashboard instruments, such as the speedometer.

[0062] Finally, step S7 determines the critical aquaplaning speed v cr The input of this step S7 is the reference variable Mu Ref , the output of step S5, i.e., the tire grip coefficient Mu on the wet road surface, and the determined value of the vehicle's running speed v obtained in step S6. Therefore, when the vehicle is running on a wet road, the critical aquaplaning speed v of the tires mounted on the vehicle related to the wet condition of the road surface can be obtained in real time. cr Therefore, the critical aquaplaning speed v cr Depending on the critical aquaplaning speed v, a warning can be sent to the driver or the vehicle's driving system so as to adapt the vehicle's driving conditions, in particular the driving speed. cr The vehicle driver or on-board navigation system is notified to adhere to a speed limit that is X% or Y speed units lower.

[0063] Finally, the method for determining a safe driving speed V on a wet road surface is defined by steps S1 and S7 to S9.

[0064] Steps S1 and S7 are performed to calculate the critical aquaplaning speed v of the tire on a wet road surface. cr The method for determining the

[0065] When the road surface is wet, a film of water is created between the two solid elements, the tire and the road, which can cause the tire to lose rubber / road contact, a phenomenon known as aquaplaning. Mu, which must be maintained to drive completely safely when this film of water is present on the road surface, Min Regarding the minimum grip level of the tire, called the minimum grip level Mu Min To guarantee this, it is necessary to define a threshold driving speed V that must not be exceeded.

[0066] Step S8 is to calculate the minimum tire grip coefficient Mu when the road surface is determined to be in a wet state. Min This determines the minimum grip coefficient Mu of the tire on the wet road surface. Min The determination of serves to ensure easy and risk-free driving of the vehicle despite the possible presence of large amounts of liquid water on the road surface and despite maneuvering the vehicle, for example cornering.

[0067] This determination can be taken as a fixed value in the first stage, typically with values ​​in the range between 0.4 and 0.8 being perfectly acceptable.

[0068] However, the determination of this fixed value related to the vehicle may depend, for greater accuracy, on the tire's identification information from step O1. The priority identification information for this dependency arises from the tire's zero-order seasonality. Therefore, 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, this fixed value may also be related to the tire range for a given season, or to the tire size. However, this second dependency is a second-order dependency prior to other influencing factors, such as the tire's wear state or aging, and is a more primary influencing factor.

[0069] Finally, the safe driving speed V is determined in step S9. The input of this step S9 is the reference variable Mu Ref , the output of step S7, i.e., the critical aquaplaning value v of the tire on the wet road surface. cr , and the minimum grip coefficient Mu of the tire on the wet road surface obtained in step S8. Min Therefore, when a vehicle is traveling on a wet road, it is possible to estimate in real time the safe driving speed V of the tires mounted on the vehicle related to the wet condition of the road surface. For example, a warning can be sent to the driver or the on-board driving system to observe a speed limit that is X% or Y speed units lower than the safe driving speed V.

[0070] Figure 2 shows the relationship between the tire grip coefficient Mu on a wet road surface and the tire's longitudinal stiffness KX on a wet road surface during driving. Mes More specifically, the horizontal axis in this case represents the ratio of the vertical stiffness KX of the same tire measured on a wet road surface from the vehicle to a reference road surface in a wet condition.

[0071] Each of the two illustrated curves 101 and 201 corresponds to a tire 100 and a tire 200. In this case, each tire in the diagram belongs to a different seasonal tire category. Tire 100, represented by the solid line 101, is a "summer" tire, while tire 200, represented by the dashed line 201, is an "all-season" tire. Each tire 100, 200 has a different reference grip coefficient Mu, in this case. Ref The mathematical representation of the two curves 101 and 201 is characterized by the tire reference variable Mu Ref and KX Ref , and the power coefficient "n", all of which depend on the tire 100 or 200.

[0072] Step S5 of the method calculates the KX for a given tire 100 or 200 in real time. Mes , and identifies the point of grip coefficient Mu of tire 100 or 200 on a wet road surface, regardless of the height of water on the road surface.

[0073] In this regard, KX Mes At given levels, points K100 and K200, vertical lines 102, 202 are drawn, which intersect the curves 101 and 201 of Figure 2 at points 103 and 203, respectively. Starting from points 103 and 203, perpendicular lines 104 and 204 are drawn, which intersect the vertical axis at points Mu100 and Mu200, respectively. These two points Mu100 and Mu200 therefore represent the grip coefficient Mu of tire 100 and the grip coefficient Mu of tire 200 on a wet road surface, respectively, regardless of the water level on the road surface.

[0074] Figure 3 shows the critical aquaplaning speed v of a tire on a wet road. cr as a function of the tire's grip coefficient Mu on a wet road surface during driving.

[0075] The two illustrated curves 111 and 211 correspond to tires 100 and 200, respectively. In this case, each tire in the diagram belongs to a different seasonal tire category. Tire 100, represented by solid line 111, is a "summer" tire, while tire 200, represented by dashed line 211, is an "all-season" tire. Each tire 100, 200, in this case, has a different reference grip coefficient Mu Ref The mathematical representation of the two curves 111 and 211 is characterized by the reference variable Mu Ref , the tire's coefficient of grip Mu on a wet road surface, the speed v of the vehicle on which the tire is fitted, and the power coefficient "β", all of which depend on the tire 100 or 200.

[0076] Step S7 of the method calculates, for a given tire 100 or 200, the critical aquaplaning speed v of the tire 100 or 200, starting from Mu of the tire on a wet road surface in real time, regardless of the height of water on the road surface. cr This identifies the point.

[0077] In this regard, at a given level Mu, points Mu100 and Mu200, vertical lines 112, 212 are drawn, which intersect the curves 111 and 211 of FIG. 3 at points 113 and 213, respectively. Starting from points 113 and 213, perpendicular lines 114 and 214 are drawn, which intersect the vertical axis at points v100 and v200, respectively. These two points v100 and v200 therefore correspond to the critical aquaplaning speed v of the tire 100, respectively. cr and the critical aquaplaning speed v of tire 200 cr represents.

[0078] FIG. 4 shows the relationship between the safe driving speed V and the minimum grip coefficient Mu on a wet road surface that a tire is desired to have under all driving conditions. Min It is shown as a function of .

[0079] The two illustrated curves 121 and 221 correspond to tires 100 and 200, respectively. In this case, each tire in the diagram belongs to a different seasonal tire category. Tire 100, represented by solid line 121, is a "summer" tire, while tire 200, represented by dashed line 221, is an "all-season" tire. Each tire 100, 200, in this case, has a different reference grip coefficient Mu Ref The mathematical representation of the two curves 121 and 221 is characterized by the reference variable Mu of the tire on a wet road surface. Ref , the critical aquaplaning speed of the tire v cr , the minimum grip coefficient of the tire on a wet road Mu Min , and the power coefficient “γ”, all of which depend on the tire 100 or 200.

[0080] Step S9 of the method calculates, for a given tire 100 or 200, the tire's grip coefficient Mu on a wet road surface in real time. Min Starting from V, the point of safe driving speed V for tire 100 or 200 is identified, regardless of the water level on the road surface.

[0081] In this regard, at a given level, Mu Min , point Mu M 100 and Mu M 4 at points 123 and 223. Starting from points 123 and 213, perpendicular lines 124 and 224 are drawn which intersect the vertical axis at points V100 and V200, respectively. These two points V100 and V200 therefore represent the safe driving speed V for tire 100 and the safe driving speed V for tire 200, respectively, regardless of the water level on the road surface.

Claims

1. A method for obtaining a tire grip coefficient Mu on a wet road surface under vehicle use conditions, comprising: Reference longitudinal stiffness KX of the tire on a reference road surface in a wet condition Ref a step (S2) of acquiring Grip coefficient Mu of the tire on the reference road surface in a wet state Ref A step (S1) of acquiring determining weather parameters during the time that said tire is mounted on said vehicle (S3); When the road surface is wet, the vertical stiffness KX of the tire on the road surface Mes (S4) determining The grip coefficient Mu of the tire on the wet road surface is calculated using the following formula: [Formula 1] (S5) evaluating the Including, A method for obtaining the tire's grip coefficient Mu on a wet road surface, where the coefficient n is a real number between 0.2 and 2.

0.

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

0.

3. 3. The method for obtaining the grip coefficient Mu of a tire on a wet road surface according to claim 1 or 2, wherein said meteorological parameters (S3) are included in the group comprising: outside temperature, precipitation intensity level.

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

5. When the road surface is wet, the vertical stiffness KX' of the tire on the road surface Mes 5. A method for obtaining the grip coefficient Mu of a tire on a wet road surface according to claim 1, further comprising the step (O2) of determining:

6. The reference longitudinal stiffness KXRef(S2) of the tire on a wet road surface is calculated by calculating the reference longitudinal stiffness KX' on a wet road surface obtained over a time T for the tire mounted on the vehicle. Mes 6. The method for obtaining the grip coefficient Mu of a tire on a wet road surface according to claim 5, wherein the grip coefficient Mu is evaluated as an average value of (O2).

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

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

9. 9. A method for obtaining a grip coefficient Mu of a tire on a wet road surface according to claim 7 or 8, wherein the coefficient n depends on the tire.

10. The reference vertical stiffness KX Ref 10. The method for obtaining a grip coefficient Mu of a tire on a wet road surface according to claim 7, wherein (S2) depends on the tire.

11. The critical aquaplaning speed v of a tire mounted on a vehicle on a wet road under driving conditions cr A method for obtaining determining a longitudinal moving speed v of the vehicle (S6); Grip coefficient Mu of the tire on a reference road surface in wet conditions Ref A step (S1) of acquiring a step (S5) of acquiring the grip coefficient Mu of the tire on a wet road surface according to any one of claims 1 to 10; The following formula: [Formula 2] The parameters v, Mu and Mu Ref (S7) estimating the critical aquaplaning speed vcr of the tire using a function F comprising: The critical aquaplaning speed v of the tire on a wet road surface, including cr How to get it.

12. The function F is expressed by the following formula: [Formula 3] 12. The critical aquaplaning speed v of a tire on a wet road surface according to claim 11, wherein β is a real number between 0.1 and 1.

0. cr How to get it.

13. 13. The critical aquaplaning speed v of a tire on a wet road surface according to claim 12, wherein β is between 0.2 and 0.

4. cr How to get it.

14. 1. A method for obtaining a safe driving speed V on a wet road surface of a vehicle having a tire casing, comprising: The minimum grip coefficient Mu of the tire on a wet road surface Min (S8) determining Grip coefficient Mu of the tire on a reference road surface in wet conditions Ref A step (S1) of acquiring According to any one of claims 11 to 13, the critical aquaplaning speed v of the tire on a wet road surface is cr (S7) determining The safe driving speed V is calculated by the following formula: [Formula 4] The parameter v cr , Mu Min , Mu Ref (S9) evaluating using a function H including A method for obtaining a safe driving speed V on a wet road surface, comprising:

15. The function H is expressed by the following formula: [Formula 5] 15. The method for obtaining a safe driving speed V on a wet road surface according to claim 14, wherein γ is a real number, preferably γ is between 0.1 and 1.0.

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