METHOD FOR DETERMINING A LIMIT SPEED FOR DRIVING

Description

FIELD OF THE INVENTION

[0001] The present invention relates to the field of motor vehicles, and in particular to the field of systems and devices for assisting the driving of such vehicles.

[0002] Today's motor vehicles are equipped with a wide range of features to improve the safety of the driver and passengers. These include brake assist systems (ABS), which prevent wheels from locking up under heavy braking. Electronic stability control (ESP) systems are also available, which help prevent vehicles from skidding by controlling the trajectory.

[0003] Furthermore, we now see that a significant number of road accidents are still linked to loss of control of a vehicle, particularly when cornering, when the grip conditions are poorly estimated by a driver, who therefore does not realize that he is driving unsafely. However, the current systems mentioned above, such as ABS and ESP, do not always prevent loss of control of the vehicle, for example due to cornering at excessive speed.

[0004] It therefore appears useful to be able to recommend to a driver a speed limit to respect for the next event planned on his route, in order to avoid a loss of grip.

[0005] A system developed by Pirelli is known in the state of the art, which estimates in real time a safe speed linked to a local curvature of the road, and compares it with an instantaneous speed of the vehicle. However, this system has several drawbacks.

[0006] On the one hand, this system uses a constant grip potential, depending on the type of tire installed on the vehicle, without taking into account environmental conditions, the condition of the tire or the speed of the vehicle. However, we know that these elements have a strong influence on the grip potential; for example, on a wet road, the grip potential at high speed is much lower than at low speed, especially for a worn tire.

[0007] Furthermore, the proposed system does not allow for a speed recommendation for an upcoming travel event, but indicates to the driver that he is in a risky situation in terms of vehicle grip. This system therefore does not allow the driver to anticipate the next event, for example a bend, in safe driving conditions.

[0008] Document EP 2817189 A1 is also known, which proposes controlling the vehicle speed according to a grip threshold between the vehicle's tire and the ground. This grip threshold can be detected by conventional systems for measuring slippage or measuring the reaction moment applied to the center of the tire's wheel.

[0009] The present invention aims to overcome these drawbacks by proposing a solution allowing the real-time determination of a driving speed limit, and its use to provide a vehicle and / or a driver with relevant driving assistance information. BRIEF DESCRIPTION OF THE INVENTION

[0010] Thus, the invention relates to a method for determining a driving speed limit according to claim 1.

[0011] By "journey event" we mean, in the remainder of the description, any characteristic portion of a journey to come, for example but not exclusively: a bend, a segment of road with a steep slope or a steep incline, a segment of road with particularly poor grip, for example due to a patch of ice, a gravelly area or any other event.

[0012] It is also specified that, by "available adhesion potential", we mean the maximum adhesion potential available at a given time. I - Estimation of available adhesion potential

[0013] The estimation of a grip potential can be carried out in real time, that is to say that the potential of the tire / ground torque, on which the vehicle is traveling, is estimated at the instant when the estimation is made. In a particular embodiment, the grip potential estimated at an instant n is considered to be identical to an instant n+1 if the parameters having an influence on the grip have not changed between instants n and n+1.

[0014] The estimation can also be carried out in anticipation of a known upcoming route, either because the route has been recorded in a vehicle's navigator, or because it is possible, from a GPS position of the vehicle, to know the road on which the vehicle is traveling. I-1-Determination of influential parameters

[0015] In a particular embodiment, a method according to the invention comprises an initial step of determining the parameters influencing the grip potential, these parameters being included in the group comprising the grip number of the road or other standardized indicators of road grip, the height to sand of the road or average texture depth called PMT, the height of water on the road, the temperature of the ground and the surrounding air, the driving speed and all the characteristics of the tire having an influence on the grip, in particular but not exclusively the pressure of the tire, its tread height, its load, and the type of tire.

[0016] These influential parameters are provided directly by the vehicle, and / or provided by an external system, and / or measured in real time. Thus, for example, the grip number of the road or other standardized indicators of road adhesion can be provided by pre-existing maps. The temperature can be measured in real time, for example by sensors available today on a vehicle.

[0017] Tire parameters, such as pressure, tread height, load and driving speed, can be determined by systems embedded in the vehicle or in the tires.

[0018] In an embodiment which is not part of the present invention, the initial step of determining influential parameters comprises a step of measuring the sound power generated by the tire during rolling, and a step of determining the height of water on the road, and the height of the tread pattern as a function of this sound power. In another particular embodiment, the macro-texture of the road is also determined as a function of this sound power. For this purpose, the vehicle in which a method according to the invention is implemented must be equipped with a microphone installed at the level of the tires or in the front or rear bumpers of the vehicle.

[0019] The number of parameters that have a potential impact on tire noise can be significant. However, it appears that some parameters have a weak or secondary influence on the nature of the noise generated by the tire. This may be the case, for example, with the internal pressure of the tire or the tire load.

[0020] Thus, it appears that the meteorological state of the road, characterized by the height of water on the road, seems to be a primary parameter. Its impact on tire noise is very significant and, above all, weakly dependent on all other parameters such as the state of the road surface, the state of wear of the tire or the type of tire tread. These other parameters are also likely to a lesser extent to vary the rolling noise, provided that we can discern their specific acoustic signatures.

[0021] Regarding this water height, a distinction is made between a dry road and a wet road, characterized by a water height that is level with the natural roughness of the road surface, or a wet road where the water height exceeds the level of the natural roughness of the road surface.

[0022] In an exemplary embodiment, this height of water on the road is estimated using one of the means included in the group comprising: fixed weather stations installed at the roadside, and including means of communicating to vehicles traveling on the road the water level as they pass, optical sensors on board the vehicle, analytical models making it possible to estimate a residual water level based on known meteorological data (quantity of precipitation, sunshine, etc.) or data concerning the road (drainability, slope, traffic, etc.).

[0023] Regarding the average texture depth, a coating is considered closed when it takes on a smooth appearance without macro-roughness, such as bitumen that has evaporated after being subjected to high heat. A coating will be considered open when the macro-roughness is significant, such as that of a worn coating or that of a country road quickly repaired using a surface coating made by spraying pebbles onto bitumen. A medium coating describes all coatings in an intermediate state between the two previous states and more specifically qualifies new coatings. The different macro-textures can thus be categorized as follows: a coating with a closed macro-texture has a PMT of between 0 and 0.4 millimeters. A coating with a medium macro-texture has a PMT of between 0.4 and 1.1 millimeters, and a coating with an open macro-texture has a PMT greater than 1.1 millimeters.It is known that the macro-roughness of a road surface has a strong influence on the noise generated by the tire. In particular, the phenomenon of pumping air trapped between the ground and the tire tread pattern is all the more pronounced when the road surface is closed. Real-time knowledge of the condition of a road can be useful in the case where, for example, this information is returned by a large number of vehicles or a fleet of dedicated vehicles to a centralized system for monitoring and maintaining the road network.

[0024] Regarding the tread height of the tire, which characterizes its state of wear, we distinguish between the new state, the worn state, and an intermediate state considered here as the state of the tire at mid-wear. Information on the evolution over time of the wear characteristic is useful, especially if it is coupled with information on the meteorological state of the road. Indeed, it is known that a vehicle equipped with worn tires that drives on a wet surface is more likely to lose its grip than if it had new tires.

[0025] In one embodiment, the estimation of a remaining sculpture height is given in real time by one of the means included in the group comprising: Sensors embedded in the tire, Optical sensors evaluating the evolution of the tread height, these sensors being embedded in the vehicle or installed on the ground, Magnetic sensors, Embedded wear models taking into account the mileage traveled, use, vehicle type, wear measurement points carried out by mechanical or optical means, etc.

[0026] The type of tire tread pattern is, for example, a summer tread pattern or a winter tread pattern. These two types of tires are essentially distinguished by treads having different tread patterns, more directional, heavily notched and siped in the case of winter tread patterns, less directional and less notched in the case of summer tread patterns, as well as by the nature of the materials forming the tread, softer in the case of winter tires, and harder in the case of summer tires. These characteristics are not without influence on the behavior and road holding of the vehicle, and therefore on its grip. I-2 - Method using a mathematical model

[0027] Knowing the influencing parameters, several embodiments are possible for determining the available grip potential. In a first preferred embodiment, a mathematical formula is used to estimate this grip potential as a function of speed. Thus, the potential can be calculated as follows: µ max = f(micro-roughness of the ground, speed, height to sand, water height, tread height, inflation pressure, load).

[0028] This function is specific to each tire available on the market and approved. All or part of the parameters indicated in this expression have previously been measured and / or determined beforehand, as previously described. I-3 - Method using abacuses

[0029] In another embodiment, the step of determining an adhesion potential as a function of speed is carried out by implementing predetermined adhesion level charts. In this embodiment, several steps are implemented, and will subsequently be described in detail using figures: We construct grip level charts based on influential parameters and their availability, We measure, in real time, the values ​​of these parameters, We select the chart corresponding to the values ​​of the parameters, and We read, on the chart, the estimated value of the grip potential, for the actual driving speed. I-4 - Rolling radius method

[0030] According to the present invention, the step of determining an available adhesion potential comprises the following steps: the evolution of a rolling radius of the tire is evaluated as a function of predetermined rolling conditions of said tire on known and variable grip surfaces, to constitute an experimental database, from the experimental database, a model for estimating the grip potential is established by determining a function linking the grip potential to the rolling radius and to vehicle parameters, while the tire is rolling, the rolling radius is determined and, by applying said model, and as a function of the vehicle parameters, the grip potential of said tire is evaluated.

[0031] Once the available grip potential has been estimated, a method according to the invention provides for determining a grip requirement, depending on the driving situation of the vehicle, and the driving characteristics of the driver of the vehicle. II - Estimating a need for adhesion II - 1 - Estimation of a next journey event

[0032] In one embodiment, to determine a grip requirement on an upcoming path segment, changes in geometric properties of an upcoming path segment, such as curvature, slope, or gradient, are detected or identified. The position of the next path event, such as a turn, intersection, or mandatory stop, is also detected.

[0033] In order to detect these elements, it is useful to know the route, or at least part of the route, taken by the vehicle. In an advantageous embodiment, the route is programmed into the vehicle's GPS navigator, for example to assist the driver's navigation, and it is easy to know the next route event.

[0034] In another embodiment, the route is not programmed into the vehicle, and it is then necessary to detect the road on which the vehicle is traveling, using a process called "map-matching", or cartographic recalibration, which makes it possible to regularly compare GPS positions of the vehicle with cartographic data, in order to determine the route taken.

[0035] Knowing the coordinates of the next journey event, it is then possible to determine a set of geopositioned points, describing this next journey event; we then have a table of points describing an approximate trajectory which will be taken by the driver. II - 2 - Estimation of the need for adhesion on the next event

[0036] Knowing the approximate trajectory taken by the driver on the next travel event, it is then possible to estimate the grip requirement on this event. Indeed, we know that a vehicle of mass M subjected to an event is the seat of an acceleration γ(V) which is exerted in the plane of the vehicle. Using an analytical model of the complete vehicle, we can deduce, among other things, from this acceleration γ(V), the wheel center forces F x , F y and F z for each tire of the vehicle, and consequently calculate the grip requirement µ tire (V) at each tire on this travel event defined by: μ pneu = F x 2 + F y 2 F z

[0037] It is specified here that, in the tire reference frame, the axis OX will be used to designate the axis representing the circumferential direction of the tire, OY the axis parallel to the tire's axis of rotation or transverse axis, and OZ the axis normal to the tire's axis of rotation, or radial axis.

[0038] In one embodiment, a first simplifying hypothesis will be used which makes it possible to obtain a grip requirement with a precision of order 0. In the case where the next event is a curve, we know that when a vehicle of mass M is registered at a given constant speed in a bend of radius of curvature R, the acceleration undergone by the vehicle, and the consequent force of which is taken up by the roadway, is defined by γ y (V) = V 2 < / R, in the absence of slope and cant.

[0039] According to a second simplifying hypothesis, considering the case where all the tires are identical and undergo the same conditions, we know that the principle of balance of forces between those produced by the vehicle on the ground and those transmitted by the tire / road contact makes it possible to establish the formula g.µ(V) = γ (V), g being the acceleration of gravity (in m / s 2< ), γ(V) being the acceleration in the plane of the vehicle (in m / s 2< ).

[0040] It is also useful, whether the next event is a bend or something else, to know the slope or the gradient of the road in order to estimate in the same way the acceleration undergone by the vehicle, and therefore the need for grip.

[0041] In one embodiment, the next travel event is a low-traction travel segment, for example, a patch of ice or other element on the roadway. The vehicle may be aware in advance of the presence of a low-traction segment by receiving external data, including but not limited to another vehicle, and / or roadside devices, and / or maps and / or data from road operators.

[0042] It is specified here that, in a simplified embodiment, it will be considered that the grip conditions on the upcoming route are the same as the grip conditions at the location where the vehicle is located when the method is implemented in anticipation.

[0043] However, in some cases it is advantageous to take into account the specific grip conditions on the upcoming route, for example in the case of a road whose Grip Number changes. III - Determination of the driving speed limit

[0044] When the upcoming travel event is a curve, at each point a curvature can be estimated and a safe speed limit can be determined by the available tire grip limit condition. Thus, the speed limit on an event is the one that achieves equality between grip requirement and grip potential, i.e. µ(V) = µ max (V)

[0045] In the case where all the tires are identical and subject to the same conditions, and in the absence of slope and gradient, the principle of balancing the forces between those produced by the vehicle on the ground and those that the tire / road contact is capable of transmitting makes it possible to establish the formula: γ y V lim = V lim 2 / R = g μ max V lim , where g is the acceleration of gravity (in m / s 2< ). Real-time resolution of this implicit equation makes it possible to estimate the safe speed limit V lim on the target bend. This method, based on simplifying assumptions, nevertheless makes it possible to fix the orders of magnitude and to initialize a target speed.

[0046] In one embodiment, this safe speed limit will be corrected with a safety margin. Indeed, in the absence of knowledge of the future accelerations γ x, resulting from the driver's braking or acceleration actions and / or a road slope, it is useful to reduce the safe speed limit in order not to risk a loss of grip which would be induced by a consumption of longitudinal grip which in fact reduces the available transverse grip.

[0047] In another exemplary embodiment where the next travel event is, for example, a straight line with a steep slope and / or a steep incline, the consequent acceleration experienced by the vehicle will be estimated, and the speed limit not to be exceeded will then be determined so that this acceleration allows compliance with the grip limits. The same applies to any other travel event.

[0048] Furthermore, and considering that the information of a constantly fluctuating speed limit is not relevant for the driver, the following assumptions were used in the chosen embodiment: It is assumed that in the steepest areas of a bend (the apex), the acceleration is overwhelmingly transverse, which is fairly well verified. The margin of error induced by this assumption is reported on the error bar of the speed limit display. The equation is solved for all local maxima of curvatures detected over the projected distance, and it is assumed that around the speed limit, all tires are at the maximum available grip potential. IV- Driver alert and information device

[0049] The speed limit thus determined can be indicated to the driver visually, to allow him to adapt his driving. In the case where the next journey event is a bend, the display of the information will be different depending on whether the next bend to come on the journey is an independent bend, or if it is linked to one or more other bends. Faces will be considered as linked if the distance between them is less than a certain threshold, for example 40 meters.

[0050] So, in the case of an independent turn, we will display, for example, the speed limit for this turn. We can also, optionally, display the distance to reach this turn, and the direction of the turn.

[0051] In the case of a series of turns, for example, the lowest speed limit of all the speed limits of the turns making up the series will be displayed, and optionally the distance to reach the start of the series. The displayed speed limit is updated as the sequence progresses, to always be equal to the minimum of the speed limits of the remaining turns in the series.

[0052] Furthermore, since the safe speed limit is determined in advance, it is useful, in one embodiment of the invention, to take into account the potential grip available in real time to warn the driver of a risk of ultimately not respecting this condition. Thus, in certain situations, it is useful to indicate to the driver of the vehicle that it is already time to brake if he wants to be able to respect the recommended speed for the next bend. This warning can take the form of an audible or visual alert, for example by flashing the speed display which will be described later using figures. BRIEF DESCRIPTION OF THE FIGURES

[0053] Other objectives and advantages of the invention will appear clearly in the following description of a preferred but non-limiting embodiment, illustrated by the following figures in which: There figure 1shows an example of charts that can be used in a method according to the invention, for determining an available adhesion potential as a function of speed, the Figures 2a, 2b and 2c show examples of the display in a vehicle of safe speed limit information, and The Figures 3a and 3b show examples of implementation of a method according to the invention in a vehicle. DESCRIPTION OF THE BEST MODE FOR CARRYING OUT THE INVENTION

[0054] There figure 1 shows an example of charts that can be used in a method according to the invention, for determining an adhesion potential.

[0055] Preferably, the charts are parameterized according to the influential parameters which are available during an implementation of the invention, for example the water height and the tread height, and are plotted according to the rolling speed.

[0056] The number of charts to create is therefore a function of the number of values ​​of these quantities that we are going to measure. Typically, if we know how to measure 2 levels of wear (new / worn) and 2 levels of humidity (dry / wet), we will create 4 charts corresponding to the possible combinations of these 2 states. If certain combinations lead to very similar adhesion results, we can reduce the number of charts.

[0057] The charts are constructed in several steps: First, a statistical distribution of each of the model inputs is determined. As many charts are calculated as there are combinations of these parameters, but each is calculated with a very small standard deviation around the value of the measured parameter. Typically, identifying a worn tire reduces the distribution to 2mm±1.5mm instead of 5mm±4mm in the absence of this information. The accuracy of the grip estimation is thus greatly improved. We make a random draw of N combinations chosen from these distributions of input parameters (typically N = 1000 or 10000). We calculate N variants of µ with these N combinations for P speed cases (typically 11 speed cases varying from 30 to 130 km / h in steps of 10 km / h). We thus plot N µ(V) curves, each corresponding to a draw from among N combinations of input parameters. We finally extract from this bundle of curves the percentile that interests us according to the targeted estimation risk, for example the limit of the lowest 10% or 1% of µ. At the output of this step, we therefore have a µ(V) curve for each possible combination of influential parameters.

[0058] So, the figure 1 shows four curves on which: S1 is the available grip for a new tire on a wet road, S2 is the available grip for a worn tire on a wet road, S3 is the available grip for a new tire on a wet road, and S4 is the available grip for a worn tire on a wet road.

[0059] It is clear from the example below that this information creates 3 significantly different classes of adhesion potential.

[0060] This approach makes it possible to reduce the amount of information to be stored without losing prediction quality since the precise calculations are carried out upstream of exploitation.

[0061] On this figure 1 Also shown, as a black curve, is the grip requirement for a future travel event, as a function of the driving speed. For a known radius of curvature R, the grip requirement as a function of speed is a parabola defined by V 2 < / R.

[0062] For each of the situations represented in curves S1 to S4, the safe speed limit corresponds to the intersection of the black curve with each of these curves. We then see on the figure 1 that with worn tires on a wet road, the safe speed limit to pass the next journey event is 60 km / h. This speed increases to 95 km / h in the case of new tires on a wet road, and to 120 km / h in the case of a wet road, regardless of the condition of the tires.

[0063] Once this safe speed has been determined, the invention provides, in an advantageous embodiment, a display, several examples of which are illustrated on the Figures 2a to 2c . There Figure 2ashows an example of a display when approaching an independent bend, i.e. one not included in a series of bends. In this case, the speed to be respected for the next bend is displayed in a circular panel. The distance to reach the next bend and the direction of curvature can also be displayed in a triangular panel.

[0064] There Figure 2b shows an example of a display approaching a series of turns. The speed sign indicates the lowest safe speed among all the turns in the series. The triangular sign indicates the distance separating the vehicle from the first turn in the series, and the fact that the turns in the series are not all in the same direction.

[0065] There Figure 2cshows an example of a display during a turn sequence. The speed sign indicates the lowest safe speed among all the turns remaining in the sequence, and the triangular sign indicates the distance to be traveled before the end of the sequence.

[0066] THE Figures 3a and 3b show examples of implementation of a method according to the invention in a vehicle. The Figure 3a shows more particularly an implementation using a connected device not integrated into the vehicle, such as a Smartphone or tablet. Thus, in this embodiment, the device 100 comprises GSM communications means for receiving external data such as maps or information concerning traffic or other things. In addition, the device 100 comprises means for receiving information allowing GPS geolocation of the vehicle or determination of the vehicle speed.

[0067] The vehicle also comprises a computer 101a installed in the vehicle, and connected to various sensors such as microphones 102 and 103. This computer 101a comprises means for processing the signals from the sensors 102 and 103 in order to obtain information concerning a height of water on the road, a depth of texture of the road, a state of wear or pressure of the tire. In another embodiment, not shown in the figure, the vehicle further comprises other sensors, such as temperature, wear, pressure sensors, etc. installed directly on the vehicle and / or on the tires. In this case, the computer 101a comprises means for processing the signals from all the sensors

[0068] After processing, the information is sent from the computer 101a to the device 100, which implements a method according to the invention to determine a driving speed limit, and display it on a screen integrated in the device 100.

[0069] In the example shown on the Figure 3b , a method according to the invention is implemented directly in the computer 101b of the vehicle. As previously, this computer is connected to sensors 102 and 103 which have the same functions as in the example of the Figure 3a . On the other hand, in this example, the computer 101b is connected to the vehicle's CAN bus, in order to read information such as the vehicle's speeds and / or accelerations.

[0070] The display and control module 200 integrated into the vehicle includes GSM communications means for receiving external data such as maps or information concerning traffic or other things and means for receiving information allowing GPS geolocation of the vehicle. It can make this information available to the computer 101b on the CAN communication bus.

[0071] This module 200 is also equipped with display means for displaying the determined driving speed limit, in order to inform the driver.

[0072] In an exemplary embodiment, the characteristics of the tire are taken into account for the implementation of the method. For this purpose, these characteristics are stored in a memory of the vehicle, and / or a tire identifier is read by an RFID type reader, and associated with characteristics stored in a database, and / or the grip model implemented in the computer 101b or in the device 100, is chosen from a set of grip models as a function of a tire identifier.

Claims

1. Method for determining a limit speed for driving for a driver at the steering wheel of a vehicle, the method comprising the following steps during which: - the grip potential available at a given instant between a tyre of the vehicle and the roadway on which the tyre is running is estimated as a function of known and / or measured influencing parameters, - a need for grip on a future route event is determined as a function of the driving situation of the vehicle and of the driving characteristics of the driver of the vehicle, - as a function of this need for grip and of the estimated grip potential, a limit speed for driving on the forthcoming route event is determined that makes it possible to not exceed the grip potential, - -the method comprising an initial step of determining parameters influencing the grip potential, these parameters comprising, at least, the water depth on the roadway, and characterized in that the step of estimating an available grip potential comprises the following steps: - the progression of a rolling radius of the tyre is assessed as a function of predetermined conditions for running of said tyre on grounds of variable and known grips, in order to form an experimental database, - the experimental database is used to establish a model for estimating the grip potential by determining a function connecting the grip potential to the rolling radius and to vehicle parameters, - during running of the tyre, the rolling radius is determined and, by application of said model, and as a function of the vehicle parameters, the grip potential of said tyre is assessed.

2. Determining method according to Claim 1, wherein the step of determining a need for grip comprises the following sub-steps: - the step of identifying a forthcoming route event, and - the step of determining a set of geopositioned points forming the event.

3. Determining method according to Claim 2, further comprising: - the step of determining a radius of curvature of this forthcoming future event, and - the step of determining the need for grip as a function of this radius of curvature and of an analytical model of the vehicle.

4. Determining method according to one of the preceding claims, wherein the limit speed for driving is corrected by a correction factor which makes it possible to take into account driver actions on the forthcoming route event.

5. Determining method according to Claim 1, comprising an initial step of determining parameters influencing the grip potential, these parameters being of the group comprising the grip number or standardized indicators of grip for the roadway, the sand depth of the road, the temperature, the tyre pressure, the tread pattern depth thereof, the load thereof, and the driving speed.

6. Method according to one of the preceding claims, wherein the step of determining a grip potential as a function of speed is carried out by applying a mathematical formula implemented by an electronic computer present on the vehicle.

7. Method according to one of Claims 1-5, wherein the step of determining a grip potential as a function of speed is carried out by using predetermined graphs for grip level.

8. Method according to Claim 1, wherein the step of determining the limit speed for driving corresponds to a step of determining the intersection between the need for grip expressed as a function of the driving speed and the available grip potential expressed as a function of the driving speed.

9. Method according to one of the preceding claims, comprising a step of informing the vehicle driver of the determined limit speed for driving.