Method for determining the coefficient of friction between wheels and roadway

Abstract

The invention relates to a method for determining a coefficient of friction between a wheel and a roadway, wherein at least one active vehicle unit that influences the longitudinal dynamics of the vehicle is controlled during driving in such a way that the wheel longitudinal force acting on at least one wheel is increased and / or the wheel support force is reduced while the course of the sum of all longitudinal forces acting on the vehicle remains unchanged.

Description

Technical Field

[0001] The invention relates to a method for determining a friction coefficient between wheels of a vehicle and a roadway on which the vehicle is traveling. Background Art

[0002] DE 10 2012 206 228 A1 describes a method for determining a friction coefficient, wherein a warning shock is automatically generated by actuating the brake system in order to indicate an imminent risk of collision, wherein an estimate of the friction coefficient of the roadway is carried out by evaluating the driving dynamics during the warning shock. The friction coefficient determination process is carried out during the time of the implementation of the warning shock and is based on an evaluation of the wheel slip.

[0003] DE 10 2013 227 083 A1 discloses a method for determining the friction coefficient between a wheel and a roadway, wherein the friction coefficient is graded into different friction coefficient grades according to the current driving situation and the friction coefficient grades are assigned the possibility for different driving situations. This allows the friction coefficient to be roughly divided into different friction coefficient grades according to the situation. Summary of the invention

[0004] By means of the method according to the invention, the friction coefficient between the wheel and the roadway can be determined during driving. In order to provide conditions for determining the friction coefficient with high accuracy, the wheel longitudinal force on at least one wheel, on which the current friction coefficient is to be determined, is increased and / or the wheel support force acting on this wheel is reduced. The currently used friction coefficient on the wheel is calculated from the ratio of the wheel longitudinal force to the wheel support force, wherein the increase in the wheel longitudinal force and / or the reduction in the wheel support force correspondingly leads to an increase in the actual currently used friction coefficient, which is to be determined by means of the method according to the invention. Based on the actual currently used friction coefficient, the maximum available friction coefficient can be estimated. In particular, in stable driving situations, the actual currently used friction coefficient is lower than the maximum available friction coefficient. In unstable driving situations, however, the forces acting on the wheel in the longitudinal direction or in the transverse direction are so high that these forces are no longer transmitted to the roadway in a force-locking manner.

[0005] The increase in the wheel longitudinal force and / or the reduction in the wheel contact force takes place automatically and without driver intervention by automatically actuating at least one active vehicle unit that influences the longitudinal dynamics of the vehicle.

[0006] The actual currently used friction coefficient is determined at the wheel with increased wheel longitudinal force and / or reduced wheel contact force and is calculated, for example, from sensor information of a wheel speed sensor. Due to the increased wheel longitudinal force and / or reduced wheel contact force, this wheel moves in the upper range of the wheel slip-friction coefficient curve, which allows a more accurate determination of the maximum available friction coefficient.

[0007] For the method according to the invention, the course of the sum of all longitudinal forces acting on the vehicle remains unchanged during the increase of the wheel longitudinal force and / or the reduction of the wheel support force on at least one wheel. The sum of the longitudinal forces acting on the vehicle consists of various internal and external forces, in particular of the engine drive force, the braking force in the vehicle's brake system, air resistance, rolling resistance and the engine drag torque. The current course of the sum of the longitudinal forces acting on the vehicle is maintained unchanged, while the wheel longitudinal force is increased and / or the wheel support force is reduced on at least one wheel, so that the subjective perception of the characteristics of the driving dynamics of the vehicle does not change for the driver. The change in force generated on the at least one wheel for the improved determination of the maximum available friction coefficient is thus not perceived by the driver. This also excludes a fear reaction of the driver, which may be caused by an intervention in the driving dynamics that occurs unexpectedly and can be perceived by the driver. The course of the longitudinal force is more precisely in line with the driver's expectations.

[0008] In order to keep the course of the sum of all longitudinal forces acting on the vehicle constant, it can be expedient to automatically actuate at least two active vehicle units that influence the longitudinal dynamics of the vehicle, such as at least two different brake units on the corresponding wheels or at least one drive unit for driving one or more wheels and simultaneously one or more brake units for braking one or more wheels. By simultaneously actuating a plurality of active vehicle units, a compensating effect is achieved, whereby the course of the sum of all longitudinal forces acting on the vehicle is kept constant in such a way that it would occur without automatic intervention in the vehicle units.

[0009] The sum of all longitudinal forces acting on the vehicle can optionally remain constant. This is the case in particular when driving at a constant speed, regardless of whether the vehicle is traveling straight or in a curve, or on a flat roadway, a downwardly sloping roadway, or an uphill roadway. In another embodiment, the sum of all longitudinal forces acting on the vehicle has a continuous profile during the determination of the friction coefficient. Embodiments are also conceivable in which the sum of the longitudinal forces increases or decreases, wherein there is advantageously continuity in the profile of the increase or decrease of the longitudinal forces.

[0010] However, in an alternative specific embodiment, a discontinuous course of the sum of all longitudinal forces acting on the vehicle can also be provided, while the currently used friction coefficient is determined at the wheel at which the longitudinal force is increased and / or the supporting force is reduced. A discontinuous course of the sum of the longitudinal forces acting on the vehicle is caused, for example, in the presence of an impact caused by a brake intervention that is not dependent on the driver, which is generated, for example, as a warning impact in a dangerous situation.

[0011] Advantageously, the friction coefficient at the wheel is determined within a time interval of an increase in the wheel longitudinal force and / or a decrease in the wheel support force of the wheel, during which the wheel longitudinal force of the wheel increases and / or the wheel support force of the wheel decreases. During this time interval, the relevant wheel has high dynamics. However, as an alternative, it can also be suitable that the time interval for determining the friction coefficient currently used immediately follows the end of the increase in the wheel longitudinal force and / or the reduction in the wheel support force, or that the time interval for determining the current friction coefficient extends not only over a part of the time interval of the increase in the wheel longitudinal force and / or the reduction in the wheel support force or over the time interval immediately following it. In the time interval immediately following the increase in the wheel longitudinal force and / or the reduction in the wheel support force, the wheel longitudinal force and the wheel support force on this wheel are advantageously constant.

[0012] In order to change the wheel force on the at least one wheel, not only the braking of the relevant wheel but also the driving thereof is taken into account. For example, two wheels on the same axle can be braked or accelerated in particular in the same way in order to achieve a symmetrical distribution of the forces to the two wheels of this axle. Thus, for example, a braking torque can be applied to the two rear wheels of a motor vehicle and a driving torque can be generated at the same time on the two front wheels of the motor vehicle in order to keep the speed of the vehicle constant or to set a speed profile that ensures a profile of the sum of all longitudinal forces acting on the vehicle that would be present on the axle even without automatically implemented driving and braking interventions.

[0013] In order to determine the friction coefficient currently used, both static or quasi-static driving conditions and dynamic driving conditions are taken into account. In static driving conditions, the vehicle moves, for example, at a constant speed on a flat, downhill or uphill section of road, wherein both straight-ahead driving and cornering driving are taken into account. In dynamic processes, the vehicle speed is in particular reduced or increased. Changes in the steering angle are also taken into account as state changes in driving dynamics.

[0014] As a dynamic process, a warning shock can be generated, for example, by braking a portion of the wheels, in particular the wheels on the rear axle of the vehicle, wherein all wheels can also be braked if necessary. This increases the wheel longitudinal force on the braked wheel, which is conducive to the estimation of the maximum available friction coefficient. If, for example, only the wheels on the rear axle are braked in order to generate a warning shock and without a regulatory intervention, such as an anti-lock braking system, it can be inferred from this that, based on the currently used friction coefficient, a double deceleration may be generated, for example, with respect to the warning shock deceleration. If, however, there is an intervention of a vehicle control system, such as an anti-lock braking system, the maximum deceleration of the vehicle is reached and known for the following braking process. If a warning shock deceleration is implemented on the wheel without a regulatory intervention, such as by an anti-lock braking system, then a four-fold total deceleration can be achieved, for example, at the vehicle level, with respect to the warning shock deceleration.

[0015] Overall, by actively distributing the wheel forces applied during a warning impact to one wheel or a small number of wheels, a range of the tire slip-friction coefficient curve that is beneficial for estimating the friction coefficient can be achieved.

[0016] Another dynamic driving situation is the desired vehicle deceleration when driving on a highway at a high speed and using driving resistance including air resistance, rolling resistance and motor drag torque. Such driving situations can be caused by the driver in a targeted manner to comfortably reduce the vehicle speed and, for example, catch up with a slower vehicle ahead or to maintain speed on a slope section. In these driving situations, the expected total deceleration caused by the driving resistance is simulated by applying a driving torque to a part of the wheels and a braking torque to another part of the wheels. Therefore, for example, the driving torque can be applied to the two wheels of the front axle and the braking torque can be applied to the two wheels of the rear axle, wherein the driving torque and the braking torque are coordinated with each other so that their sum is equivalent to the sum of the driving resistance in the absence of such driving torque and braking torque. The driving torque and the braking torque act in a defined time interval, during which the friction coefficient currently used can be determined by means of one or more wheels, and the braking torque is applied to the wheels.

[0017] When driving around a bend, for example, the wheel on the inside of the following bend can be braked in order to increase the wheel longitudinal force on this wheel. In addition, the combined pitching and swaying movement of the vehicle that occurs here reduces the wheel support force on the wheel on the inside of the following bend. As a further effect, the steering process through the bend is supported by the yaw moment that is generated. Additional comfort benefits are achieved because the steering effort for the driver is reduced.

[0018] The yaw moments generated by braking interventions at individual wheels, in particular when driving around a bend, can optionally be compensated by active vehicle units, such as torque vectoring or an active differential lock. In this case, the braking interventions can also be implemented in an enhanced manner.

[0019] After completion of the process of determining the currently utilized friction coefficient by automatic activation of one or more vehicle units that influence the driving dynamics of the vehicle, the automatically performed intervention in the vehicle units can be partially or completely canceled in such a way that the forces acting on the vehicle have the same course as they would have had without the active activation of one or more vehicle units.

[0020] The method is automatically implemented by actuating one or more active vehicle units, in particular a brake unit, a drive unit, a steering unit, a torque vectoring system, an active differential lock or an active chassis unit. No driver activity is required to implement the method. After determining the friction coefficient, intervention in the at least one active vehicle unit is advantageously canceled in order to restore the original driving state.

[0021] The invention also relates to a control unit for carrying out the method described above. Several method steps, in particular the generation of control signals for controlling the at least one active vehicle unit that influences the longitudinal dynamics, are generated in the control unit as a function of input signals, in particular sensor signals for determining the currently used friction coefficient, such as wheel speed information. The invention also relates to a vehicle having such a control unit and one or more active vehicle units for influencing the longitudinal dynamics of the vehicle.

[0022] The invention further relates to a computer program product having a program code, which is designed to carry out the method steps described above. The computer program product runs in a control unit described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Further advantages and advantageous embodiments can be gathered from the further claims, the description of the figures and the drawings, in which: Figure 1 Two diagrams are shown in , showing the time-dependent curve of the driving resistance on a motor vehicle and the curve of the longitudinal slip at the wheels. DETAILED DESCRIPTION

[0024] The two diagrams represent driving in a vehicle in a first phase with the accelerator pedal actuated for applying a drive torque at first a constant speed and driving in a subsequent phase without drive torque for rolling the vehicle without drive. In the time interval between 0 and t1, the vehicle is initially still driven with a constant drive torque, which is predefined by the driver by actuating the accelerator pedal. At time t1, the driver releases the accelerator pedal and ends the actuation of the accelerator pedal with the intention of rolling the vehicle without applying a drive torque. If no active vehicle units that influence the longitudinal dynamics of the vehicle are automatically actuated, the driving resistance M W The driving resistance acting on the vehicle includes air resistance, rolling resistance and motor drag torque. The driver predicts based on his experience that the driving resistance M W The torque trend.

[0025] By simultaneously increasing the drive torque M A Apply the braking torque M to the wheels of the front axle. B This method is used to simulate the driving resistance M applied to the wheels of the rear axle. W The purpose of this is to generate an increased wheel longitudinal force at the wheels of the rear axle, which is advantageous for the determination of the currently used coefficient of friction at the wheels of the rear axle. Furthermore, the load on the rear axle is relieved by the reduction in the speed of the vehicle, thereby reducing the wheel bearing forces at the wheels of the rear axle, which is also advantageous from the perspective of the determination of the currently used coefficient of friction at the wheels of the rear axle.

[0026] The coefficient of friction on the wheel is usually calculated from the following formula:

[0027]

[0028] where μ act Indicates the friction coefficient currently used, F x represents the wheel longitudinal force and F N Correspondingly, the friction coefficient μ currently used act By increasing the wheel longitudinal force F x and / or reduce the wheel support force F N The friction coefficient μ currently used act The maximum possible coefficient of friction is below that during steady-state driving.

[0029] By increasing the wheel longitudinal force F between times t1 and t2 x and at least slightly reduce the wheel support force FN In this way, the currently used friction coefficient μ on the wheels of the rear axle is increased overall. act Thus, a high percentage of the maximum friction coefficient is already utilized. This facilitates and improves the determination of the maximum friction coefficient and facilitates the prediction of the maximum friction coefficient. This information can be used, for example, to parameterize driver assistance systems such as electronic stability programs or similar systems.

[0030] At the time between t1 and t2, the automatically applied drive torque M A With the automatically applied braking torque M on the rear axle B The sum of the torque trend M W , which represents the sum of the driving resistances. This means that the driver does not notice anything from the automatic intervention in the braking system and drive system of the vehicle.

[0031] In the time interval between times t1 and t2, the motor drive torque M is increased at the wheels of the front axle. A When the longitudinal slip on the front axle wheel is FA Due to the applied braking torque M on the wheels of the rear axle B There, a negative longitudinal slip λ is generated. RA .

[0032] At time t2, the automatic intervention of the drive system and the brake system ends, so that the vehicle continues to roll without drive torque and brake torque. The forces and torques acting on the vehicle are reduced from the driving resistance M for air resistance, rolling resistance and engine drag torque. W and also corresponds to the course subjectively expected by the driver, as in the past, in the time interval between times t1 and t2, wherein the transition between t1 and t2 in the phase with automatic intervention in the drive system and brake system and the transition immediately after time t2 in the phase immediately following that take place continuously. This ensures that the forces acting on the vehicle have the expected course both at time t1 when the drive system and brake system are automatically activated and at time t2 when the drive system and brake system are automatically deactivated, so that the driver does not notice the activation and deactivation.

Claims

1. A method for determining a coefficient of friction between a wheel and a roadway, wherein at least one active vehicle unit that influences the longitudinal dynamics of the vehicle is controlled during driving in such a way that the wheel longitudinal force (F ) acting on at least one wheel is increased while the course of the sum of all longitudinal forces acting on the vehicle remains unchanged. x ) and reduce the wheel support force acting on this wheel (F N ), a driving torque is applied to the two wheels of the front axle and a braking torque is applied to the two wheels of the rear axle, wherein the driving torque and the braking torque are coordinated with each other in such a way that their sum corresponds to the sum of the driving resistances in the absence of such driving torque and braking torque, wherein the wheel longitudinal force (F x ) and the wheel support force (F N ) to obtain the actual friction coefficient currently used (μ act ).

2. The method according to claim 1, characterized in that When increasing the wheel longitudinal force (F x ) and / or reduce the wheel support force (F N ) is carried out at a time interval of act )’s process of obtaining .

3. The method according to claim 1 or 2, characterized in that At least one wheel is braked to determine the friction coefficient (μ act ).

4. The method according to any one of claims 1 to 3, characterized in that Brake or accelerate both wheels on the same axle.

5. The method according to any one of claims 1 to 4, characterized in that One part of the wheels is braked and a drive torque is applied to another part of the wheels.

6. The method according to any one of claims 1 to 5, characterized in that In calculating the friction coefficient (μ act ) during which the sum of all longitudinal forces acting on the vehicle has a continuous trend.

7. The method according to any one of claims 1 to 6, characterized in that In calculating the friction coefficient (μ act ) during which the sum of all longitudinal forces acting on the vehicle remains constant.

8. The method according to any one of claims 1 to 7, characterized in that During driving at a constant or nearly constant speed, a wheel longitudinal force (F x ) and / or wheel support force (F N ) changes.

9. The method according to any one of claims 1 to 7, characterized in that During the acceleration phase or the braking phase, the wheel longitudinal force (F x ) and / or wheel support force (F N ) changes.

10. The method according to any one of claims 1 to 9, characterized in that When driving straight ahead, the wheel longitudinal force (F x ) and / or wheel support force (F N ) changes.

11. The method according to any one of claims 1 to 9, characterized in that When cornering, the wheel longitudinal force (F x ) and / or wheel support force (F N ) changes. 12 . A control unit for actuating a vehicle unit that influences the longitudinal dynamics of a vehicle, for carrying out the method according to claim 1 .

13. A vehicle having a control unit according to claim 12 and a vehicle unit which influences the longitudinal dynamics of the vehicle. 14 . A computer program product having a program code which is designed to carry out the steps of the method according to claim 1 when the computer program product is run in a control unit according to claim 12 .

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