Methods for determining the coefficient of friction and for operating a motor vehicle

Abstract

Method for determining a coefficient of friction prevailing between wheels (4, 5, 6, 7) of a motor vehicle and a road surface, characterized in that During a journey of the motor vehicle, steering angles of equal magnitude in opposite directions are set for the wheels (6, 7) of a steerable rear axle (3). the change in rotational speed and / or change in the rotational speed gradient resulting from this steering angle setting at at least one of the rear wheels (6, 7) is determined, The coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface is determined depending on the or each determined change in rotational speed and / or change in the speed gradient.

Description

[0001] The invention relates to a method for determining the coefficient of friction between the wheels of a motor vehicle and a road surface. Furthermore, the invention relates to a method for operating a motor vehicle depending on a determined coefficient of friction.

[0002] The coefficient of friction between the wheels of a motor vehicle and the road surface it travels on is an important parameter for improving, for example, the handling and safety of a motor vehicle. German patent application DE 103 19 662 A1 discloses a method for determining the coefficient of friction between the wheels of a motor vehicle and the road surface. According to this method, a current restoring torque acting on a steered wheel is determined during cornering. Furthermore, current vehicle dynamics values ​​are determined, from which a theoretical restoring torque can be calculated. The coefficient of friction is then determined from the current restoring torque and the current vehicle dynamics values. However, this prior art method can only be applied during cornering. Therefore, the method disclosed in DE 103 19 662 A1 is only conditionally suitable for determining the coefficient of friction.

[0003] From DE 10 2010 014 564 A1, another method for determining the coefficient of friction between the wheels of a motor vehicle and a road surface is known. According to this method, a first steering angle is set on a first tire and a second steering angle is set on a second tire. Sensors detect the forces required to set these steering angles, and an evaluation unit uses a predetermined algorithm to determine the coefficient of friction from the detected forces. This method requires additional sensors to determine the force needed to set a steering angle. Furthermore, this prior art method is only applicable if electric motors are used to set the steering angles.

[0004] There is a need for a novel method for determining the coefficient of friction between the wheels of a motor vehicle and a road surface, which can be used during a journey of the motor vehicle to determine the coefficient of friction without the need for additional sensors and regardless of which actuators are used to adjust the steering angle.

[0005] Based on this, the invention aims to create a novel method for determining the coefficient of friction and a method for operating a motor vehicle.

[0006] This problem is solved by a method for determining the coefficient of friction according to claim 1. During a journey of the motor vehicle, equal steering angles in opposite directions are set for the wheels of a steerable rear axle. The resulting change in rotational speed and / or change in the speed gradient at at least one of the rear wheels is determined. Depending on the determined change in rotational speed and / or change in the speed gradient, the coefficient of friction between the wheels of the motor vehicle and the road surface is determined.

[0007] The inventive method allows the coefficient of friction between the vehicle's wheels and the road surface to be determined while the vehicle is in motion. No additional sensors are required to determine the coefficient of friction. The inventive method is independent of the actuators used to adjust the steering angle at the rear axle wheels.

[0008] Following further training, the coefficient of friction is preferably determined during straight-line driving at a predominantly constant speed. In this case, the coefficient of friction can be determined with particular accuracy, namely in a steady-state or quasi-steady-state operating condition of the vehicle by evaluating both rear wheels. However, it is also possible to use the method when cornering and / or driving at a variable speed.

[0009] According to a further development of the invention, during straight-line driving at a constant speed, a first change in rotational speed is determined at a first wheel of the steerable rear axle between a rotational speed before the steering angle adjustment and a rotational speed after the steering angle adjustment, wherein a second change in rotational speed is determined at a second wheel of the steerable rear axle between a rotational speed before the steering angle adjustment and a rotational speed after the steering angle adjustment, and wherein, depending on the first change in rotational speed and the second change in rotational speed, the coefficient of friction between the wheels of the motor vehicle and the road surface is determined.The first change in rotational speed is compared with the second change in rotational speed in such a way that if the deviation between the first and second changes is less than a threshold value, the coefficient of friction between the vehicle's wheels and the road surface is determined; if the deviation between the first and second changes in rotational speed is greater than the threshold value, the coefficient of friction between the vehicle's wheels and the road surface is not determined. These features also serve to accurately determine the coefficient of friction between the vehicle's wheels and the road surface during straight-line driving without the need for additional sensors.Preferably, if the deviation between the first and second changes in rotational speed is less than the limit value, an average value between the first and second changes in rotational speed is determined, and the coefficient of friction between the wheels of the vehicle and the road surface is determined based on this average value. In this way, the coefficient of friction can be determined particularly advantageously.

[0010] Following further development, particularly during straight-line driving with a non-constant speed, a first change in the speed gradient between a speed gradient before the steering angle setting and a speed gradient after the steering angle setting is determined at a first wheel of the steerable rear axle, wherein a second change in the speed gradient between a speed gradient before the steering angle setting and a speed gradient after the steering angle setting is determined at a second wheel of the steerable rear axle, and wherein, depending on the first change in the speed gradient and the second change in the speed gradient, the coefficient of friction between the wheels of the motor vehicle and the road surface is determined.The first change in rotational speed gradient is compared with the second change in rotational speed gradient in such a way that if the deviation between the first and second changes in rotational speed gradient is less than a threshold value, the coefficient of friction between the vehicle's wheels and the road surface is determined; if the deviation between the first and second changes in rotational speed gradient is greater than the threshold value, the coefficient of friction between the vehicle's wheels and the road surface is not determined. These features also serve to accurately determine the coefficient of friction between the vehicle's wheels and the road surface during straight-line driving without the need for additional sensors.Preferably, if the deviation between the first and second changes in the speed gradient is less than the limit value, an average value between the first and second speed gradient changes is determined, and the coefficient of friction between the wheels of the vehicle and the road surface is determined based on this average value. In this way, the coefficient of friction can be determined particularly advantageously.

[0011] When driving straight ahead at a constant speed, the rotational speed gradients can be evaluated to determine the coefficient of friction, just as when driving straight ahead at a variable speed. However, when driving at a variable speed, evaluating the rotational speed gradients is necessary.

[0012] If the friction coefficient determination method is used during cornering, a change in rotational speed between a rotational speed before the steering angle adjustment and a rotational speed after the steering angle adjustment and / or a change in the rotational speed gradient between a rotational speed gradient before the steering angle adjustment and a rotational speed gradient after the steering angle adjustment is determined exclusively at a single wheel of the steerable rear axle, and the friction coefficient is determined accordingly.

[0013] Preferably, the coefficient of friction between the wheels of the vehicle and the road surface is determined based on the measured change in rotational speed and / or the change in the speed gradient, either using characteristic curves or maps. Determining the coefficient of friction based on the measured changes in rotational speed using characteristic curves or maps is simple and does not require a complex algorithm.

[0014] The characteristic map or curve is preferably tire-dependent and / or wheel load-dependent.

[0015] The inventive method for operating a motor vehicle depending on a coefficient of friction determined using the inventive method is defined in claim 12.

[0016] The inventive method for operating several motor vehicles is defined in claim 13.

[0017] Preferred embodiments are described in the dependent claims and the following description. Exemplary embodiments of the invention are explained in more detail with reference to the drawing, without being limited thereto. The drawing shows: Fig. 1 A highly schematic detail of a motor vehicle to illustrate the invention.

[0018] The invention relates to a method for determining the coefficient of friction prevailing between the wheels of a motor vehicle and a road surface. This coefficient of friction is also referred to as the static friction coefficient or road surface friction coefficient.

[0019] Fig. Figure 1 shows a highly schematic representation of a motor vehicle 1 with a front axle 2 and a rear axle 3. Two front wheels 4, 5 of the front axle 2 and two rear wheels 6, 7 of the rear axle 3 are shown.

[0020] Both the front wheels 5, 6 of the front axle 2 and the rear wheels 6, 7 of the rear axle 3 are steerable.

[0021] Fig. Figure 1 shows that the steering angles for the front wheels 4, 5 of the front axle 2 are set for straight-ahead driving of the motor vehicle 1.

[0022] In order to determine the coefficient of friction between the wheels of motor vehicle 1 and a road surface, during a journey of the motor vehicle, in particular during straight-ahead travel or alternatively during cornering, i.e., in particular when a steering angle for straight-ahead travel is set at the front wheels 4, 5 of the front axle 2, steering angles α of equal magnitude in opposite directions are set at the rear wheels 6, 7 of the steerable rear axle 3, i.e., the steering angle +α at rear wheel 6 and the steering angle -α at rear wheel 7. Fig. 1. The steering angles at the rear wheels 6, 7 are set so that the rear wheels 6, 7 converge in a direction away from the front wheels 4, 5. It is also possible to set the steering angles at the rear wheels 6, 7 so that the rear wheels 6, 7 converge towards the front wheels 4, 5. The adjustment of the equal-value steering angles α in opposite directions is carried out simultaneously at both rear wheels.

[0023] The changes in rotational speed and / or speed gradient resulting from this steering angle setting at the rear wheels 6 and 7 are then determined. Depending on these changes in rotational speed and / or speed gradient, the coefficient of friction between the vehicle's wheels and the road surface is calculated.

[0024] Fig. Figure 1 shows speed sensors 8, 9, which are assigned to the rear wheels 6, 7 of the rear axle 3 and with which the rotational speeds at the rear wheels 6, 7, and thus also the rotational speed gradients, can be measured. These speed sensors 8, 9 are sensors that are already installed in the vehicle and are part of a so-called ESC system (Electronic Stability Control system).

[0025] During a journey, a first change in rotational speed and / or a first change in the rotational speed gradient is determined at a first rear wheel 6 of the steerable rear axle 3 between a rotational speed or a rotational speed gradient before the steering angle adjustment and a rotational speed or a rotational speed gradient after the steering angle adjustment. At a second rear wheel 7 of the steerable rear axle 3, a second change in rotational speed and / or a second change in the rotational speed gradient is determined between a rotational speed or a rotational speed gradient before the steering angle adjustment and a rotational speed or a rotational speed gradient after the steering angle adjustment.

[0026] As previously explained, these steering angle settings are those used to set equal steering angles α in opposite directions at the rear wheels 6 and 7 of the steerable rear axle 3. The setting of these equal steering angles α in opposite directions is performed simultaneously at both rear wheels. Depending on the first change in rotational speed and / or the first change in the speed gradient, and the second change in rotational speed and / or the first change in the speed gradient, the coefficient of friction between the vehicle's wheels and the road surface is then determined.

[0027] When the method according to the invention is carried out at a non-constant driving speed, it is essential to work with speed gradients and changes in speed gradients. When the method according to the invention is carried out at a constant driving speed, it is possible to work with speeds and changes in speed and / or with speed gradients and changes in speed gradients.

[0028] Then, when the inventive method is used during cornering, the change in rotational speed or the speed gradient at only one of the rear wheels, i.e. either at the right rear wheel or at the left rear wheel, is evaluated to determine the coefficient of friction.

[0029] During straight-line driving, it is preferably provided that a deviation between the first change in rotational speed and / or first change in rotational gradient, measured at the first rear wheel 6, and the second change in rotational speed and / or first change in rotational gradient, measured at the second rear wheel 7, is determined. Then, if this deviation between the first and second changes in rotational speed and / or the deviation between the first and second changes in rotational gradient is less than a limit value, the coefficient of friction between the wheels of the vehicle and the road surface is determined.However, if the deviation between the first and second changes in rotational speed and / or the deviation between the first and second changes in the rotational speed gradient exceeds the limit, the coefficient of friction between the vehicle's wheels and the road surface is not determined; instead, the measured changes in rotational speed are discarded. This can occur, for example, if the vehicle drives through a puddle, over a patch of ice, or similar obstacle while determining the changes in rotational speed at the two rear wheels 6 and 7. If the deviations between at least two consecutive measurements are below the limit, this indicates a so-called µ-split, i.e., different coefficients of friction at the two rear wheels 6 and 7. This can occur, for example,This would be the case if, during the determination of the rotational speed gradients at the two rear wheels 6, 7, the motor vehicle is traveling with one rear wheel 6, 7 over a road surface that is snow-covered or icy on one side. In this case, an average value is calculated from the mean values ​​of each individual measurement, and the coefficient of friction between the wheels of the motor vehicle and the road surface is determined based on this average value.

[0030] Fig.Figure 1 shows a control device 10 which, based on the rotational speeds of the rear wheels 6, 7 provided by the rotational speed sensors 8, 9, determines the first change in rotational speed and / or first change in rotational speed gradient and the second change in rotational speed and / or second change in rotational speed gradient for the two rear wheels 6, 7 and determines the deviation between them, and then, if the deviation between the first change in rotational speed and the second change in rotational speed and / or the deviation between the first change in rotational speed gradient and the second change in rotational speed gradient is less than the limit value, determines the coefficient of friction depending on the changes in rotational speed and / or change in rotational speed gradient.

[0031] The determination of the coefficient of friction, depending on the determined first change in rotational speed and / or first change in rotational speed gradient of the first rear wheel 6 and the second change in rotational speed and / or second change in rotational speed gradient of the second rear wheel 7, is preferably carried out based on characteristic curves or maps. The greater the respective change in rotational speed, the greater the coefficient of friction.

[0032] The corresponding characteristic map or curve is determined in advance and stored in control unit 10.

[0033] The characteristic map or curve is preferably tire-dependent to account for the properties of different tires, for example, differences between summer and winter tires. The currently used tire type is known to the control unit, particularly if, after a tire change from summer to winter tires, a corresponding entry is made in the vehicle's control unit by the workshop.

[0034] The characteristic map or curve is preferably also dependent on wheel load. The vehicle mass is known to the control system.

[0035] Wheel loads change when a load is added. Using individual wheel load measurements is preferred. Especially when driving straight ahead, the total rear axle load can be split equally between the right and left rear wheels. The axle load on the rear axle can be measured indirectly, for example, using the headlight leveling sensors. These sensors are already installed in vehicles equipped with xenon headlights, so no additional sensors are required. When cornering, the right and left wheel loads differ significantly. In this case, if the total rear axle load is known, the individual wheel loads can be approximately estimated using the acceleration sensors.

[0036] As previously stated, the above determination of the coefficient of friction is preferably carried out during straight-line driving, particularly when driving at a constant speed. Therefore, for the determination of the coefficient of friction, stationary or quasi-stationary driving conditions are preferable during straight-line driving. While this is preferred, it is not mandatory. As already stated, the method can also be used when cornering and when driving at varying speeds.

[0037] This method can be used on front-wheel drive vehicles, rear-wheel drive vehicles, and all-wheel drive vehicles. On front-wheel drive vehicles, the rotational speed or slip at the rear wheels will decrease after the steering angle is adjusted. On rear-wheel drive vehicles, the rotational speed or slip at the rear wheels will increase after the steering angle is adjusted. All-wheel drive vehicles behave in the same way as rear-wheel drive vehicles in this respect.

[0038] It is also within the scope of the present invention to operate a motor vehicle 1 depending on a coefficient of friction between the wheels of the motor vehicle and the road surface, determined using the method described above.

[0039] In particular, it is provided that, depending on the coefficient of friction determined according to the invention between the wheels of the motor vehicle and the road surface, a drive torque of a drive unit is adjusted and / or, in the case of a hybrid vehicle, a recuperation torque is adjusted and / or, for example, a braking torque is adjusted for an autonomous braking maneuver and / or and / or a timely triggering of a braking maneuver is initiated depending on the prevailing coefficients of friction and / or, in the case of the brakes, a high coefficient of friction between the brake pad and the brake disc is maintained by periodic closing.

[0040] For example, if a decrease in the coefficient of friction is detected, this could indicate a wet road surface, snow, or similar conditions. In this case, the plan is to reduce the drive torque and / or the recuperation torque to increase driving safety. Furthermore, the braking torque can be adjusted for braking maneuvers. Damp or dirty brakes can be wiped clean periodically to increase their coefficient of friction.

[0041] According to a further aspect of the invention, it is provided that several motor vehicles can be operated depending on a coefficient of friction between the wheels of the first motor vehicle and the road surface, determined in the first motor vehicle using the method described above. It is provided that the coefficient of friction determined by the first motor vehicle is transmitted by the control unit 10 to a database cloud 11, stored therein, and then made available to at least a second motor vehicle for its operation. The second motor vehicle can then, like the first motor vehicle, adjust a drive torque and / or a recuperation torque and / or a braking torque and / or brakes by periodically wiping the brakes, depending on the provided coefficient of friction.

[0042] Then, if the second motor vehicle also has a steerable rear axle 3, the coefficient of friction provided by the database cloud can be verified in the second motor vehicle by carrying out the method according to the invention. A corresponding verification can then be transmitted back to the database cloud 11 to validate the coefficient of friction stored in the database cloud 11, which is made available to other motor vehicles. Reference sign 1 motor vehicle 2 Front axle 3 Rear axle 4-wheeler 5 wheels 6 wheels 7 wheel 8 Sensor 9 Sensor 10 Control unit 11 Database Cloud

Claims

Method for determining the coefficient of friction between the wheels (4, 5, 6, 7) of a motor vehicle and a road surface, characterized in that, during a journey of the motor vehicle, steering angles of equal magnitude in opposite directions are set for the wheels (6, 7) of a steerable rear axle (3), the change in rotational speed and / or change in the speed gradient resulting from this steering angle setting at at least one of the rear wheels (6, 7) is determined, and the coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface is determined depending on the or each determined change in rotational speed and / or change in the speed gradient. Method according to claim 1, characterized in that the coefficient of friction is determined during a journey at a constant driving speed, namely on the basis of the or any determined change in rotational speed and / or change in the rotational speed gradient. Method according to claim 1, characterized in that the coefficient of friction is determined during a journey with a non-constant driving speed, namely on the basis of the or any determined changes in the speed gradient. Method according to one of claims 1 to 3, characterized in that during straight-ahead driving, a first change in the speed gradient between a speed gradient before the steering angle adjustment and a speed gradient after the steering angle adjustment is determined at a first wheel (6) of the steerable rear axle, and that a second change in the speed gradient between a speed gradient before the steering angle adjustment and a speed gradient after the steering angle adjustment is determined at a second wheel (7) of the steerable rear axle, and that depending on the first and second speed gradient change, the coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface is determined. Method according to claim 4, characterized in that the first speed gradient change is compared with the second speed gradient change such that, if a deviation between the first speed gradient change and the second speed gradient change is less than a limit value, the coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface is determined; if the deviation between the first speed gradient change and the second speed gradient change is greater than the limit value, the coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface is not determined. Method according to one of claims 1 to 5, characterized in that during straight-ahead driving, a first change in rotational speed between a rotational speed before the steering angle adjustment and a rotational speed after the steering angle adjustment is determined at a first wheel (6) of the steerable rear axle, and that a second change in rotational speed between a rotational speed before the steering angle adjustment and a rotational speed after the steering angle adjustment is determined at a second wheel (7) of the steerable rear axle, and that depending on the first and second change in rotational speed, the coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface is determined. Method according to claim 6, characterized in that the first change in rotational speed is compared with the second change in rotational speed in such a way that, if a deviation between the first change in rotational speed and the second change in rotational speed is less than a limit value, the coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface is determined; if the deviation between the first change in rotational speed and the second change in rotational speed is greater than the limit value, the coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface is not determined. Method according to claim 5 or 7, characterized in that when a deviation between the first speed change and the second speed change and / or between the first speed gradient change and the second speed gradient change is less than the limit value, an average value between the first speed change and the second speed change and / or an average value between the first speed gradient change and the second speed gradient change is determined and, depending on the respective average value, the coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface is determined. Method according to one of claims 1 to 8, characterized in that, during cornering, a change in rotational speed between a rotational speed before the steering angle adjustment and a rotational speed after the steering angle adjustment and / or a change in the rotational speed gradient between a rotational speed gradient before the steering angle adjustment and a rotational speed gradient after the steering angle adjustment is determined exclusively at a single wheel of the steerable rear axle, and that the coefficient of friction is determined depending on this. Method according to one of claims 1 to 9, characterized in that the coefficient of friction is determined depending on the characteristic curve or map, depending on the or each determined change in rotational speed and / or the or each change in rotational speed gradient. Method according to claim 10, characterized in that the characteristic map or characteristic curve is tire-dependent and / or wheel load-dependent. Method for operating a motor vehicle depending on a coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface determined using the method according to one of claims 1 to 11, characterized in that, depending on the coefficient of friction between the wheels (4, 5, 6, 7) of the motor vehicle and the road surface, a drive torque is adjusted, and / or a recuperation torque is adjusted, and / or a braking torque is adjusted for a braking maneuver, and / or brakes are wiped by periodic closing. Method for operating several motor vehicles depending on a coefficient of friction between the wheels (4, 5, 6, 7) of the first motor vehicle and the road surface determined in a first motor vehicle using the method according to one of claims 1 to 11, characterized in that the coefficient of friction determined by the first motor vehicle is transmitted to a database cloud (11), stored in the same and made available to at least a second motor vehicle for its operation. Method according to claim 13, characterized in that in at least a second motor vehicle, a drive torque is adjusted depending on the coefficient of friction provided by the database cloud (11), and / or a recuperation torque is adjusted, and / or a braking torque is adjusted for a braking maneuver, and / or brakes are wiped by periodic closing. Method according to claim 13 or 14, characterized in that in at least a second motor vehicle the coefficient of friction is determined using the method according to one of claims 1 to 7 and verified with the coefficient of friction provided by the database cloud (11).

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