Method and device for determining the friction coefficient of a road surface

Abstract

Method for estimating the coefficient of friction of a road surface (2) for planning emergency interventions in the event of a risk of collision for a motor vehicle (1), characterized by the steps Detection of a collision-threatening traffic situation, issuing a warning to the driver of the motor vehicle (1) by means of a predetermined braking jolt, Estimation of the current friction coefficient of the road surface (2) based on the braking jolt, performing a visual inspection of the road surface ahead starting with the braking jolt, and Planning and execution of an emergency maneuver based on the current friction coefficient and visual inspection of the road surface.

Description

[0001] The invention relates to a method for determining the coefficient of friction of a road surface according to the preamble of claim 1 and a corresponding device according to the preamble of claim 8.

[0002] Accident prevention is a key consideration in driver assistance systems, with emergency braking systems in current vehicles and evasive action functions in future vehicles becoming increasingly important. However, their effectiveness depends crucially on the coefficient of friction of the road surface in relation to the vehicle's tires. For example, the coefficient of friction on a wet road surface is significantly lower than on a dry road surface.

[0003] WO 2012 / 110030 A2 discloses a method and device for estimating the coefficient of friction using a 3D camera. The 3D camera captures an image of the vehicle's surroundings and uses the image data to create a height profile of the road surface in the entire area in front of the vehicle. The expected local coefficient of friction of the road surface can be estimated from the height profile. Additional information from the image data can be incorporated into the analysis.

[0004] The publication DE 10 2011 085 984 A1 describes a method for determining the coefficient of friction between a vehicle wheel and the road surface for each individual wheel. The wheel torque is specified according to a defined torque curve at a vehicle wheel, and the wheel speed is determined in response to the wheel torque. The current coefficient of friction of the road surface can be determined from the phase shift between the wheel torque and the wheel speed.

[0005] The publication DE 10 2009 041 566 A1 describes a method for determining the coefficient of friction of a road surface. On the one hand, a continuously available coefficient of friction is determined and updated during the journey, and on the other hand, a coefficient of friction is only updated depending on the situation and is therefore not constantly available. These two parameters are combined and evaluated to form a common friction estimate. The coefficient of friction, which is continuously updated during the journey, is determined based on a frequency analysis of the rotational and / or vibration behavior of the wheels, while the coefficient of friction, which is only updated depending on the situation, is determined in driving situations in which the coefficient of friction at at least one wheel can be estimated due to the use of slip control, high wheel deceleration, or high wheel acceleration.

[0006] The publication DE 10 2012 203 673 A1 describes a safety device for a motor vehicle in which a combined braking / evasive maneuver is performed predictively and situationally during a pre-collision phase to ensure operational safety. To plan the maneuver, the friction coefficient indication between the vehicle tires and the road surface is calculated based on an estimate, whereby the friction coefficient indication is determined from a driver-independent or driver-triggered braking intervention.

[0007] From DE 10 2010 028 384 A1 a method is known in which the current friction coefficient is taken into account during an evasive maneuver.

[0008] From DE 102 37 714 A1 a method for initiating an emergency braking is known, whereby it is provided that the friction coefficient is also determined in the case of a preliminary warning braking to warn the driver.

[0009] A method is known from DE 10 2010 027 449 A1, whereby the current friction coefficient is determined from the brake pressure during a warning braking operation.

[0010] From DE 10 2009 038 421 A1 a method is known in which a warning braking is carried out, which is perceived by the driver as a jolt, and in which the emergency braking is carried out in accordance with the determined coefficient of friction.

[0011] From DE 10 2012 206 228 A1 a method is known, whereby it is provided to provide a warning jolt as a pre-braking operation, to carry out a friction coefficient estimate during this warning jolt by evaluating the driving dynamics and then, if necessary, to carry out an emergency braking with brake pressure adapted to the friction coefficient estimate.

[0012] The disadvantage is that estimating the coefficient of friction using a camera does not provide reliable values, while determining the coefficient of friction based on wheel parameters, for example braking, can only provide a local statement.

[0013] The invention is therefore based on the object of improving the determination of the road friction coefficient for planning emergency interventions such as automatic evasive action or braking.

[0014] This object is achieved by a method for determining the coefficient of friction of a road surface having the features of claim 1 and by a corresponding device having the features of claim 8. Preferred embodiments of the invention are the subject of the dependent claims.

[0015] The method according to the invention for estimating the coefficient of friction of a road surface for planning emergency interventions in the event of a risk of collision with an obstacle ahead for a motor vehicle comprises the following steps: - Detection of a traffic situation that could result in a collision, - issuing a warning to the driver of the motor vehicle by means of a predetermined braking jolt, - Estimation of the current friction coefficient of the road based on the braking jerk, - Carry out a visual inspection of the road surface ahead, starting with the braking jolt, and - Planning and execution of an emergency maneuver based on the current friction coefficient and the visual inspection of the road surface.

[0016] By linking the current friction coefficient with the results of a visual inspection of the road surface ahead after the braking jolt, the current braking jolt can be projected into the maneuvering space of the emergency maneuver, thereby increasing the safety of the emergency maneuver.

[0017] Preferably, the road surface ahead is checked for changes compared to past images of the road surface, and the current friction coefficient is used to plan and execute the emergency maneuver for the road surface ahead, forming a maneuvering space for the emergency maneuver, if there is no change in the road surface condition. In other words, if the road surface has not changed, the locally determined friction coefficient is projected into the maneuvering space.

[0018] Further preferably, the road surface ahead is visually inspected for friction-reducing influences when changes occur compared to previous images of the road surface. Friction-reducing influences are those that reduce the friction coefficient of the road surface, such as water, ice, or snow on the road surface. Such friction-reducing influences can be detected using image processing methods or by comparison with previous images of the road surface.

[0019] It is also preferable to use a friction coefficient that is reduced compared to the current friction coefficient to plan the emergency maneuver if friction-reducing influences are detected.

[0020] Further preferably, the measured braking jerk is compared with a desired braking jerk and the emergency maneuver is initiated earlier if the measured braking jerk decreases compared to the desired braking jerk.

[0021] In particular, the braking jolt can be of a magnitude that corresponds to the braking process of the subsequent emergency maneuver. This simplifies the planning of the subsequent emergency maneuver.

[0022] Further preferably, a conclusion is drawn that the road surface has a reduced coefficient of friction when the vehicle's windshield wiper system is in operation. Since it is unlikely that a windshield wiper system will be activated without rain or snow, the operation of such a windshield wiper system is a strong indication of a road surface with a reduced coefficient of friction.

[0023] The device according to the invention for planning and carrying out an emergency maneuver based on a given collision probability of a vehicle with an obstacle ahead, wherein the device is set up and designed to carry out the method explained above, comprises - a device for situation recognition and determination of a collision probability, - a device for warning the driver of a possible impending collision by means of a braking jolt, - a device for estimating the current road friction coefficient based on the braking jerk, - a device for carrying out a visual inspection of the road surface ahead, and - a facility for planning and carrying out an emergency maneuver,

[0024] Preferably, the device for carrying out the visual inspection of the road surface ahead is implemented by a camera.

[0025] Further preferably, the device has a memory for storing images of the road surface.

[0026] A preferred embodiment of the invention is explained below with reference to the drawings. Fig. 1 a graphic representation of a planned emergency maneuver, Fig. 2 a normal braking jolt to estimate the friction coefficient, Fig. 3 a braking jerk curve with insufficient friction coefficient, and Fig. 4 a driver assistance system for planning and executing an emergency maneuver in a schematic representation.

[0027] Fig. 1 shows a typical traffic situation in which a collision is imminent and it is therefore to be investigated whether an emergency maneuver can be carried out without a collision. A vehicle 1 is traveling on a roadway 2 with a direction of travel or speed of 3, with an obstacle 4 located on the roadway 2, which obstacle the vehicle 1 will collide with if it continues traveling in direction 3. The potential risk of collision is detected by corresponding sensors of a driver assistance system or emergency maneuver system in the vehicle 1, and the question mentioned at the beginning arises as to whether an emergency maneuver is feasible or not. The emergency maneuver considered here to prevent a collision consists of avoiding the obstacle 4 along a trajectory 5, the course of which must be planned by the driver assistance system.In order to plan the course of trajectory 5, knowledge of the coefficient of friction between the vehicle's tires and the road surface 2 is helpful or even necessary. To warn the driver of an impending potential collision, the driver assistance system issues a warning at a predetermined time, allowing the collision to be avoided with an emergency maneuver such as emergency braking or an evasive maneuver. Such a warning can be provided, for example, by a brief braking jolt, which makes the driver aware of the urgency of the situation.

[0028] Fig. Figure 2 shows such a normal braking jolt as a function of time. In the upper part of the Fig. 2 the longitudinal delay a l as a function of time t. In the lower part of the Fig. 2 is the reduction of the wheel speed n as a function of time t due to the longitudinal deceleration a lof the braking jolt. It can be seen that the wheel speed n in response to the longitudinal deceleration a l From the decrease in wheel speed n over time t as a result of the course of the longitudinal deceleration a l of the braking jolt, a minimum value of the current friction coefficient between the tire and the road surface can be estimated. With the thus determined minimum available friction coefficient, the intended maneuver along the trajectory 5 of the Fig. 1, whereby the time of compulsory intervention by the driver assistance system is sufficiently later than the warning by the braking jolt in order to give the driver time to take his own measures if necessary.

[0029] Fig. 3 shows the case in which the desired braking jerk shown in dashed lines cannot be realized, but when a maximum value is reached, the longitudinal deceleration a ldrops to zero, as can be seen from the solid curve of the actually measured or realized braking jerk curve. With the decrease of the longitudinal deceleration a l to the value zero, the wheel speed n also drops to the value zero. In other words, the wheel locks and sticks after reaching the maximum value of the longitudinal deceleration a shown. l no longer on the road. In this case, too, a coefficient of friction and thus a maximum braking deceleration can be determined, which can be used to plan an emergency maneuver. However, the emergency maneuver may have to be initiated earlier due to the low coefficient of friction, or a collision may no longer be preventable, but only mitigated. An ABS system would prevent the wheels from locking. However, the current coefficient of friction can also be estimated from the requested deceleration at the time of ABS intervention.

[0030] In the Fig. Figure 4 shows the process sequence in a corresponding driver assistance system of a motor vehicle in the form of modules. In a first module 10, a situation detection of the surroundings of the motor vehicle is carried out using suitable sensors. If the situation detection 10 concludes that a collision with a vehicle in the Fig. If the obstacle 4 shown in Figure 1 could be imminent, appropriate measures are initiated or planned. After detecting a risk of collision, the first action is to issue a warning about the critical traffic situation to the driver of the motor vehicle. In this case, the warning to the driver of the motor vehicle is provided by a brief braking jolt, which is issued by another module 11 and is intended to sensitize the driver to the critical driving situation.

[0031] The braking jolt emitted by the warning module 11 not only serves to warn the driver, but the generated braking jolt with a predetermined course and longitudinal deceleration also serves in a friction coefficient estimation module 12 to determine or estimate the friction coefficient between the tyre and the road surface, as described above with reference to the Fig. 2 and Fig. 3. It is advantageous to use a braking jerk with the strength of the emergency maneuver that is subsequently intended. If this braking jerk cannot be fully represented, as is the case in Fig. 3, a current coefficient of friction can be determined in module 12 despite the loss of adhesion of the wheel with the road surface.

[0032] It is obvious that the currently determined coefficient of friction only applies to the location of the road surface at the time of the braking jolt. Therefore, in a subsequent module 13, a visual inspection of the road surface ahead is carried out using a suitable sensor to check, based on optical criteria, whether the road surface condition remains the same or changes after the braking jolt. This check determines whether the current surface condition of the road has changed compared to the surface condition at previous times or has remained the same. If the image has remained the same, it can be assumed that the surface condition will remain the same, so that the coefficient of friction currently determined using the braking jolt can be used for the maneuvering space of the future emergency maneuver.If, however, the image analysis reveals a change, for example, if the surface image contains light reflections that indicate a wet road ahead, it must be assumed when planning the emergency maneuver that it must be planned with a friction coefficient lower than the measured one. A memory module 15 is provided to compare the current image of the road ahead with past images of the road surface. This memory module serves to store and provide images of road surfaces at previous times.

[0033] After the visual inspection of the roadway ahead of the vehicle has been carried out in module 13, the emergency maneuver is planned and carried out in the emergency maneuver module 14 based on the friction coefficient estimate of module 12 and the visual inspection of module 13, as shown in the example of Fig.1 is illustrated by the motor vehicle 1 avoiding an obstacle 4 by means of an evasive maneuver given by the trajectory 5. List of reference symbols 1 vehicle 2 lanes 3 Speed / Direction 4 Obstacle 5 Avoidance trajectory a l Longitudinal delay n wheel speed t time 10 Module Situation Recognition 11 Brake jerk module 12 Friction coefficient estimation module 13 Module Visual Inspection 14 Emergency Maneuver Module 15 memory module

Claims

[1] Method for estimating the coefficient of friction of a road surface (2) for planning emergency interventions in the event of a risk of collision for a motor vehicle (1), characterized by the steps Detection of a collision-threatening traffic situation, issuing a warning to the driver of the motor vehicle (1) by means of a predetermined braking jolt, Estimation of the current friction coefficient of the road surface (2) based on the braking jolt, performing a visual inspection of the road surface ahead starting with the braking jolt, and Planning and execution of an emergency maneuver based on the current friction coefficient and visual inspection of the road surface. [2] Method according to claim 1, characterized bythat the road surface (2) ahead is checked for changes compared to past images of the road surface (2) and the current friction value is used to plan and carry out the emergency maneuver for the road surface ahead forming a maneuvering space for the emergency maneuver if there is no change in the surface condition of the road. [3] Method according to claim 2, characterized by that the road surface (2) ahead is checked for friction-reducing influences in the event of changes compared to past images of the road surface (2). [4] Method according to claim 3, characterized by that a friction coefficient reduced compared to the current friction coefficient is used to plan the emergency maneuver if friction-reducing influences are detected. [5] Method according to one of the preceding claims, characterized bythat the measured braking jerk is compared with a desired braking jerk and the emergency maneuver is initiated earlier if the measured braking jerk falls below the desired braking jerk. [6] Method according to one of the preceding claims, characterized by that the braking jolt has a strength that corresponds to or exceeds the braking process of the emergency maneuver intended later. [7] Method according to one of the preceding claims, characterized by that a road surface with a reduced coefficient of friction is inferred when the windscreen wiper system of the motor vehicle (1) is in operation. [8] Device for planning and carrying out an emergency maneuver based on a collision probability of a motor vehicle (1) with an obstacle (4) in front, wherein the device is set up and designed to carry out the method according to one of the preceding claims, with a device for situation recognition (10) and determination of a collision risk, a device (11) for warning the driver of a possible impending collision by means of a braking jolt, and a device (14) for carrying out an emergency maneuver, wherein the device further comprises: a device (12) for estimating the current road friction coefficient based on the braking jerk, and a device (13) for carrying out a visual inspection of the road surface ahead. [9] Device according to claim 8, characterized bythat the device (13) for carrying out the visual inspection of the road surface ahead is a camera. [10] Device according to one of claims 8 or 9, characterized by that the device has a memory (15) for storing images of the road surface.

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