WO2008077670A1

WO2008077670A1 - Method and device for determining a friction coefficient

Info

Publication number: WO2008077670A1
Application number: PCT/EP2007/061783
Authority: WO
Inventors: Ning Bian; Celine Gamulescu; Matthias Kretschmann; Andreas Mayer
Original assignee: Continental Automotive Gmbh
Priority date: 2006-12-22
Filing date: 2007-10-31
Publication date: 2008-07-03
Also published as: DE102006061249A1

Abstract

At least one inclination of at least one acceleration sensor is determined in relation to a predetermined plane of reference which is independent of a vehicle. At least one acceleration of the vehicle is detected in a plane by means of at least one acceleration sensor, said plane running substantially parallel to the driving path. The at least one acceleration of the vehicle which is detected is corrected according to the at least one inclination. A friction coefficient (μ_R) is determined according to the at least one corrected acceleration.

Description

description

Method and device for determining a friction index

The invention relates to a method and a device for determining a friction coefficient between at least one tire of a vehicle and a roadway.

Modern motor vehicles have control systems that intervene in predetermined situations in a control of the motor vehicle. The control system may be, for example, a vehicle dynamics control system and / or a driving dynamics comfort system. For example, an electronic stability system prevents a spin of the motor vehicle in the border area. The electronic stability system may include an anti-lock braking system whereby the braking effect is periodically released and reinserted when the wheels are locked. Further, the electronic stability system may include a traction control system which controls the internal combustion engine of the motor vehicle by temporarily limiting the torque transmitted to the wheels so that the wheels do not spin. For such interventions in the control of the motor vehicle, the knowledge of the coefficient of friction between the tires of the vehicle and the roadway is advantageous in order to avoid unnecessary or erroneous engagement.

DE 39 120 14 Al discloses a method for determining a coefficient of friction between a roadway and tires of a vehicle. When approaching a transverse dynamic critical driving condition, values for a steering angle, a driving speed, a yawing angular velocity and a lateral acceleration of the vehicle are respectively detected or determined. Depending on the steering angle and on the driving speed, a reference or desired yaw angular velocity is determined on the basis of a mathematical vehicle reference model. Further, a difference between the actual yaw rate of the vehicle and the reference or target vehicle yaw rate determined. The value of the lateral acceleration is determined as a measure of the coefficient of friction between the road surface and the tires of the vehicle at which the difference between the actual yaw rate and the reference yaw rate begins to increase sharply.

EP 1 627 790 A1 discloses a method for estimating a friction coefficient. A steering angle and a lateral acceleration are recorded. An estimated value of the friction coefficient is determined by means of a table. In the table, several areas for different steering angles and transverse accelerations are provided, which are assigned a plurality of friction coefficients. Depending on the detected steering angle and the detected lateral acceleration, the associated area of the table is selected and the assigned friction coefficient is used as an estimate of the friction coefficient.

US 2003/0074127 A1 discloses a system for calculating a road surface friction coefficient. This is estimated depending on a deviation of an estimated longitudinal or lateral acceleration with respect to a measured longitudinal or lateral acceleration of the vehicle.

The object of the invention is to provide a method and apparatus for determining a friction coefficient that is reliable.

The object is solved by the features of the independent claims. Advantageous developments of the invention are characterized in the subclaims.

According to a first aspect, the invention is characterized by a method and a corresponding device for determining a friction index between at least one tire of a vehicle and a roadway. At least one inclination of at least one acceleration sensor with respect to a vehicle-independent predetermined reference plane is determined. At least one acceleration of the vehicle is detected by means of the at least one acceleration sensor in a plane that runs essentially parallel to the roadway. The detected at least one acceleration of the vehicle is corrected as a function of the at least one inclination.

The friction coefficient is determined as a function of the corrected at least one acceleration.

The invention is based on the finding that in the presence of different inclinations of the road with respect to the predetermined reference plane, the detected at least one acceleration of the vehicle depending on a mounting direction of an acceleration sensor, which is provided for the detection of at least one acceleration, a different size Acceleration error caused by the gravitational acceleration. However, the acceleration error has a negative effect on the precision of the determined friction coefficient. Already an inclination of three degrees can lead to an acceleration error of 0.5 meters per second and significantly affect the precision of the determined coefficient of friction. By taking into account the at least one inclination of the at least one acceleration sensor and the gravitational acceleration, the influence of the gravitational acceleration on the detected at least one acceleration and the determined friction coefficient can be reduced or eliminated. The correction of the at least one acceleration thus takes place in particular with respect to the gravitational acceleration. The friction coefficient is therefore particularly precise and reliably determined.

The predetermined reference plane preferably has a fixed relation to the direction of gravitational acceleration. For example, the reference plane is perpendicular or parallel to the direction of gravitational acceleration. In particular, the predetermined reference plane is a horizontally extending, horizontal reference plane. However, the predetermined reference plane can also be specified differently. The at least one acceleration of the vehicle comprises in particular a longitudinal acceleration and / or a lateral acceleration of the vehicle. In addition, the at least one inclination of the roadway with respect to the predetermined reference plane comprises in particular a pitch angle and / or a roll angle.

In an advantageous embodiment, the at least one inclination is determined depending on the detected at least one acceleration, a yaw rate, at least one wheel speed and a steering wheel angle or steering angle by means of a predetermined physical vehicle model. The advantage is that the yaw rate, the wheel speeds and the steering wheel angle or steering angle are generally already available as measured variables in the vehicle, so that no additional sensors have to be provided for detecting these measured variables.

In a further advantageous embodiment, a pitch rate and / or a roll rate is detected. The at least one inclination is determined as a function of the pitch rate and / or the roll rate. In particular, the pitch angle is determined as a function of the pitch rate and the roll angle as a function of the roll rate. The advantage is that this allows a particularly precise and reliable determination of the at least one inclination and thus also of the friction coefficient. This applies in particular to special driving conditions, such as skidding or heavy slipping.

In this context, it is advantageous if the at least one inclination is determined by integration of the pitch rate or the roll rate. The advantage is that this is very easy.

In this context, it is further advantageous if, in the prevalence of at least one predetermined driving state of the vehicle, the at least one inclination determined by the predetermined physical vehicle model is assigned to integration as the initialization value. The advantage is that the initialization value contained in the at least one NEN predetermined driving condition is determined, is particularly reliable and by resetting the integration to the initial value diverging the integral can be easily and reliably prevented. The at least one predetermined driving state includes in particular normal driving without skidding or heavy slipping.

According to a second aspect, the invention is characterized by a method and a corresponding device for determining a friction index between at least one tire of a vehicle and a roadway. At least one differential angle is detected between a vehicle body and a chassis. At least one acceleration of the vehicle is detected in a plane that runs essentially parallel to the roadway. The detected at least one acceleration of the vehicle is corrected as a function of the at least one differential angle. The friction coefficient is determined as a function of the corrected at least one acceleration.

The invention is based on the finding that, when different differential angles between the vehicle body and the chassis are present, the detected at least one acceleration of the vehicle has a different magnitude of acceleration error as a function of a mounting direction of an acceleration sensor which is provided for detecting the at least one acceleration caused by the gravitational acceleration. However, the acceleration error has a negative effect on the precision of the determined friction coefficient. Even a difference angle of three degrees can lead to an acceleration error of 0.5 meters per second and significantly affect the precision of the determined coefficient of friction. By taking into account the at least one differential angle between the vehicle body and the chassis and the gravitational acceleration, the influence of the gravitational acceleration on the detected at least one acceleration and the determined friction coefficient can be reduced or eliminated. The correction of the at least one acceleration thus takes place in particular with respect to the gravitational acceleration. The friction characteristics The number is therefore particularly precise and reliably determinable. The at least one acceleration of the vehicle comprises, in particular, a longitudinal acceleration and / or a transverse acceleration of the vehicle.

In an advantageous embodiment of the first or the second aspect, the determination of the friction coefficient is suspended when an amount of the at least one inclination or the at least one differential angle is greater than a predetermined first inclination threshold or differential angle threshold, and resumed when the amount of at least one inclination or the at least one difference angle is smaller than a predetermined second inclination threshold value or difference angle threshold value. The first and the second inclination threshold value or the difference angle threshold value can in particular also be given the same or can be predetermined such that a switching hysteresis results. In the case of inclinations of the roadway which are greater than the predefined first inclination threshold value, it is possible for deformations of the tires to occur which may or may not be taken into account or are taken into account insufficiently in the physical model. By suspending the determination of the friction index, it can be achieved that only reliable friction coefficients are provided.

In a further advantageous embodiment of the first or the second aspect, a slip angle is detected or determined. A lateral coefficient of friction between the at least one tire of the vehicle and the roadway is determined by means of a model in which a functional relationship between the lateral friction value and the slip angle is predetermined in such a way that a nonlinear profile of the lateral friction value is dependent on the slip angle is from a recursively determined estimate of the friction coefficient. Depending on the lateral coefficient of friction, a model acceleration corresponding to the at least one acceleration of the vehicle is determined and a deviation between the at least determines an acceleration of the vehicle and the model acceleration. The recursion in determining the estimated value of the friction coefficient comprises that it is adjusted as a function of the determined deviation. The advantage is that the friction coefficient in this way can be determined simply and reliably as an estimated value.

Embodiments of the invention are explained below with reference to the schematic drawings. Show it:

FIG. 1 is a plan view of a vehicle;

FIG. 2 shows a side view of the vehicle on a longitudinally inclined roadway,

FIG. 3 shows a front view of the vehicle on a transversely inclined roadway,

FIG. 4 shows a further side view of the vehicle on the longitudinally inclined roadway,

FIG. 5 shows a first device for determining a friction index,

FIG. 6 shows a second device for determining the friction index,

FIG. 7 shows a first flowchart,

8 shows a second flowchart and

FIG. 9 shows a diagram of a lateral coefficient of friction.

Elements of the same construction or function are provided with the same reference numbers across the figures.

A vehicle has an arithmetic unit 1 and sensors (FIG. 1). The sensors are coupled to the arithmetic unit 1. The sensors are in particular designed for detecting driving dynamic measured variables. Such sensors are in particular a longitudinal acceleration sensor 2 for detecting a detected longitudinal acceleration ax sens and / or a lateral acceleration sensor 3 for detecting a detected lateral acceleration ay sens and a yaw rate sensor 4 for detecting a yaw rate w_z and a steering wheel angle sensor 7 for detecting a steering wheel angle γ. Instead of the steering wheel angle γ or in addition to this, a steering angle can be detected at the wheels, in particular in the presence of a superposition steering. Furthermore, a wheel speed sensor 8 is preferably arranged on each wheel of the vehicle for detecting a respective wheel speed rd. In addition, a roll rate sensor 5 for detecting a roll rate wx and / or a pitch rate sensor 6 for detecting a pitch rate w_y can also be provided. The sensors are in each case coupled to the arithmetic unit 1 and supply the respectively acquired measured values to them. Furthermore, other sensors may be provided.

FIG. 2 shows the vehicle in a side view on a roadway inclined in the longitudinal direction of the vehicle. The roadway is inclined with respect to a vehicle-independent predetermined reference plane BE by a pitch angle θ. Accordingly, the vehicle has an orientation in space that is rotated about a transverse axis of the vehicle. A LG FW of the

Vehicle also essentially has the pitch angle θ with respect to the predetermined reference plane BE.

FIG. 3 shows the vehicle in a front view on a roadway inclined in the transverse direction of the vehicle. The roadway is inclined with respect to the vehicle-independent predetermined reference plane BE by a roll angle φ. Accordingly, the vehicle has an orientation in space that is rotated about a longitudinal axis of the vehicle. The chassis FW of the vehicle also essentially has the roll angle φ with respect to the predetermined reference plane BE. The sensors and in particular the longitudinal acceleration sensor 2 and the lateral acceleration sensor 3 are generally fixedly mounted with a predetermined orientation in the vehicle. In general, the longitudinal acceleration sensor 2 and the lateral acceleration sensor 3 are arranged to detect an actual longitudinal acceleration ax or lateral acceleration a_y of the vehicle assuming a horizontal, horizontally running roadway. If, on the other hand, the roadway has an inclination, the longitudinal acceleration sensor 2 and / or the lateral acceleration sensor 3 detect not only a component of the actual longitudinal or lateral acceleration ax, ay, but also a component of a gravitational acceleration a_g. This leads to a longitudinal acceleration error ax err or to a lateral acceleration error a_y_err, that is, the detected longitudinal acceleration ax sens corresponds to a sum of the actual longitudinal acceleration a_x and the longitudinal acceleration error ax err and the detected lateral acceleration a_y_sens corresponds to a sum of the actual lateral acceleration ay and Lateral acceleration error a_y_err.

The predetermined reference plane BE is in particular given with a fixed relation to a direction of the gravitational acceleration a g, for example perpendicular or parallel to it. The predetermined reference plane BE is preferably a plane running at the location of the vehicle perpendicular to the direction of the gravitational acceleration a_g and thus extending horizontally and horizontally. However, the predetermined reference plane BE can also be specified differently.

The longitudinal acceleration sensor 2 and / or the lateral acceleration sensor 3 are preferably arranged in a vehicle body FA of the vehicle. The vehicle body FA and the chassis FW are coupled to one another, for example, via a damping system, which comprises, for example, springs. Because of this non-rigid coupling, the vehicle body FA and the chassis FW can have a pitch difference angle θ diff and / or a Have Wankdifferenzwinkel φ_diff to each other, which can be equal to zero, in particular during acceleration or deceleration of the vehicle, cornering or by loading the vehicle. The pitch difference angle θ_diff and / or the roll difference angle φ_diff may also be referred to as the difference angle. FIG. 4 shows the pitch difference angle θ_diff for the side view of the vehicle. However, the pitch difference angle θ diff and the roll difference angle φ_diff are small in most driving situations, so that the vehicle body FA therefore has substantially the inclination of the roadway with respect to a pitch or roll of the vehicle.

The vehicle preferably has spring travel sensors 9 or level sensors for determining the pitch difference angle θ_diff and the roll difference angle φ_diff.

FIG. 5 shows a first embodiment of a device for determining a friction coefficient μ_R. The coefficient of friction μ R characterizes the friction between the tire and the road and is defined as the quotient of a maximum of the tire on the road transferable force in the direction parallel to the road and a Aufstandskraft F_z or as a limit of this quotient for large slip values.

The device is formed for example by the arithmetic unit 1 or is a part of this. The device comprises a vehicle observer unit FB, which is designed to determine the inclination of the longitudinal and / or the lateral acceleration sensor 2, 3 with respect to the predetermined reference plane BE, for example as the sum of the pitch angle θ and the pitch difference angle θ diff and / or as the sum of the roll angle φ and the roll difference angle φ_diff. Preferably, these sums are determined as a function of the detected longitudinal acceleration a_x_sens and / or the detected lateral acceleration ay sens and the wheel speeds rd, the yaw rate wz and the steering wheel angle γ or steering angle by means of a predetermined physical vehicle model. Especially advantageous it is to determine pitch angle θ plus pitch difference angle θ_diff and / or roll angle φ plus rolling difference angle φ diff depending on the pitch rate w_y and / or the roll rate w_x.

The device further comprises an acceleration error determination unit ERR, which is designed as a function of the detected longitudinal acceleration a_x_sens and / or the detected lateral acceleration ay sens and the sum of the pitch angle θ and the pitch angle θ_diff and / or the sum of the roll angle φ and the roll difference angle φ diff to determine the longitudinal acceleration error a_x_err and / or the lateral acceleration error ay err. The determination takes place taking into account the acceleration of gravity a_g. The device is furthermore designed to determine the actual longitudinal acceleration a x as a function of the detected longitudinal acceleration a_x_sens and the longitudinal acceleration error a x err and to determine the actual lateral acceleration a_y in accordance with the detected lateral acceleration a y sens and the lateral acceleration error a y err.

The device further comprises an estimation unit EST, which is designed to determine the coefficient of friction μ R between the at least one tire of the vehicle and the roadway as an estimated value. The estimation unit EST is supplied with the actual longitudinal or lateral acceleration a_x, a_y. Further, the estimation unit EST is supplied with a slip angle α, the footing force F_z of the respective tire of the vehicle on the road, and a vehicle speed v. The slip angle α is defined as an angle between a wheel plane perpendicular to a wheel axis and a moving direction of a touch area in which the tire and the road touch.

Preferably, measured values of the measured quantities, for example the detected longitudinal or lateral acceleration a_x_sens, ay sens, the wheel speeds rd, the yaw rate wz and so on, are processed before these are sent to the vehicle observer unit FB, the acceleration error determination unit ERR or the Estimation unit EST be supplied. The processing of the respective measured values comprises, for example, a filtering and in particular a low-pass filtering and / or a correction or conversion of the acquired measured values. Furthermore, it may be provided to determine measured variables derived depending on the supplied measured values, for example taking vehicle-dynamics vehicle models into account or by simply calculating two or more of the measured values of different measured variables into a measured value of a measured variable derived therefrom. Such a derived measured variable is, for example, the vehicle speed v, which can be determined as a function of the detected wheel speeds rd, or the contact force F_z, which is exerted perpendicular to the roadway via a wheel of the vehicle. Furthermore, the slip angle α can be determined as a function of the detected yaw rate w_z. However, the said derived measured variables may also be determined or recorded differently.

FIG. 6 shows a second embodiment of the device for determining the coefficient of friction μ R, which differs from the first embodiment essentially in that the vehicle observer unit FB is designed, depending on sensor signals of the spring travel sensors 9 or level sensors, the pitch difference angle θ diff and / or Wankdifferenzwinkel φ_diff to determine, and the Beschleunigungsfehl erermittlungseinheit ERR is designed to determine depending on these difference angles and depending on the detected longitudinal acceleration ax sens and / or the detected lateral acceleration a_y_sens the longitudinal acceleration error a_x_err and / or the lateral acceleration error ay err. Otherwise, the second embodiment of the device substantially corresponds to the first embodiment of the device.

FIG. 7 shows a flow chart of a first program for determining the friction coefficient μ_R. The program is executed by the first embodiment of the device. The program starts in a step Sl. In a step S2, the yaw rate wz, the wheel speeds rd and the steering wheel angle γ or the steering angle detected or determined. A step S3 may be provided to detect the pitch rate wy and / or the roll rate w_x. Furthermore, the detected longitudinal acceleration a_x_sens and / or the detected lateral acceleration a_y_sens are detected in a step S4.

In a step S5, the sum of the pitch angle θ and the pitch difference angle θ_diff and / or the sum of the roll angle φ and the roll difference angle φ diff is preferably determined by the vehicle observer unit FB as a function of

Yaw rate w_z, the wheel speeds rd, the steering wheel angle γ or the steering angle and the detected longitudinal or lateral acceleration a_x_sens, a_y_sens and optionally depending on the pitch rate w y or the roll rate w x. The sum of the pitch angle θ and the pitch angle θ_diff and / or the sum of the roll angle φ and the roll difference angle φ diff are determined by means of the predetermined physical vehicle model.

In a step S6, it is preferably checked whether an amount of the sum of the pitch angle θ and the pitch difference angle θ diff is greater than a predetermined pitch angle threshold θ_th and / or an amount of the sum of the roll angle φ and the roll difference angle φ diff is greater than a given given roll angle threshold φ_th. The predetermined pitch angle threshold value θ th and the predetermined roll angle threshold value φ_th may also be referred to as predetermined tilt threshold values. These are for example about 20 degrees. However, they can also be larger or smaller than 20 degrees.

For the sum of the pitch angle θ and the pitch difference angle θ_diff and for the sum of the roll angle φ and the roll difference angle φ diff, in each case two predefined tilt threshold values can also be predetermined, in particular a predetermined first and a predetermined second tilt threshold value. If the predetermined first and second tilt threshold values are each given the same size, then they correspond this the predetermined pitch angle threshold θ_th or the predetermined roll angle threshold φ th. However, if the first and second inclination threshold values are predetermined differently, then a switching hysteresis can be specified. The determination of the friction coefficient μ R can thereby be suspended, for example, if the magnitude of the sum of the pitch angle θ and the pitch difference angle θ diff or the sum of the sum of the roll angle φ and the roll difference angle φ_diff is greater than the respective predetermined first tilt threshold, and resumed when the amount of the sum of the pitch angle θ and the pitch difference angle θ diff or the magnitude of the sum of the roll angle φ and the roll difference angle φ_diff is smaller than the respective predetermined second tilt threshold. In the case of the resumption of the determination of the coefficient of friction μ R, it is preferred that the friction coefficient μ_R determined last before the preceding suspension of the determination is used as the starting value.

If the condition in step S6 is met, that is, the

Amount of the sum of the pitch angle θ and the pitch difference angle θ diff is greater than the predetermined pitch angle threshold θ_th or the sum of the roll angle φ and the roll difference angle φ diff is greater than the predetermined roll angle threshold φ_th, then the program in

Step S2 continued. Otherwise, in a step S7 the longitudinal and / or the lateral acceleration error a_x_err, ay err is dependent on the detected longitudinal acceleration a_x_sens and the sum of the pitch angle θ and the pitch difference angle θ diff or dependent on the detected lateral acceleration a_y_sens and the sum of the roll angle φ and the roll difference angle φ diff determined. In a step S8, the actual longitudinal or lateral acceleration a_x, a_y is determined as a function of the longitudinal or lateral acceleration error a_x_err, a_y_err and the detected longitudinal or transverse acceleration a_x_sens, a_y_sens. The actual longitudinal or lateral acceleration ax, ay corresponds to one order the influence of gravitational acceleration corrected longitudinal or lateral acceleration.

In a step S9, the coefficient of friction μ R is determined as a function of the actual longitudinal or lateral acceleration a_x, a_y. The program is continued in step S2. The program is preferably continued only after a waiting period T W has elapsed. The waiting period T W, for example, corresponds to a sampling interval in which the measured values are detected or determined, or a predetermined time interval for adjusting and thus updating the estimated value of the friction coefficient μ R.

The determination of the sum of the pitch angle θ and the pitch difference angle θ_diff and / or the sum of the roll angle φ and the roll difference angle φ diff is preferably carried out in step S5 depending on the pitch rate w_y or the roll rate w x. The pitch rate wy and / or the roll rate w_x is preferably integrated numerically in order to be able to determine changes in the sum of the pitch angle θ and the pitch difference angle θ diff or the sum of the roll angle φ and the roll difference angle φ diff in a particularly precise manner, in particular even if the vehicle is in a particular driving condition, that is, the vehicle, for example, flings or slips heavily. An initialization value for the integration by the predetermined physical vehicle model is preferably determined as a function of the yaw rate w_z, the wheel speeds rd, the steering wheel angle γ or the steering angle and the detected longitudinal or lateral acceleration a_x_sens, ay sens. Furthermore, the integral is preferably reset to the currently determined initialization value in at least one predetermined driving state of the vehicle. This prevents divergence of the integral and the sum of the pitch angle θ and the pitch difference angle θ diff and / or the sum of the roll angle φ and the roll difference angle φ diff are durably reliably and precisely determinable. If the resetting of the integral to the current initialization value takes place sufficiently frequently, then the cor- Distorts the integral and thus the sum of the pitch angle θ and the pitch angle θ diff and / or the sum of the roll angle φ and the roll difference angle φ_diff only small and therefore do not affect the subsequent processing.

For the second embodiment of the device, a corresponding program is provided. Deviating from the first embodiment of the device, the sensor signals of the spring travel sensors 9 or the level sensors are detected in step S2. In step S5, the pitch difference angle θ diff and / or the roll difference angle φ diff is determined as a function of the detected sensor signals of the spring travel sensors 9 or the level sensors. The step S6 may be provided to check an amount of the pitch difference angle θ_diff and / or the roll difference angle φ diff to the exceeding of a predetermined differential angle threshold. Furthermore, in step S7 the longitudinal and / or the lateral acceleration error a_x_err, a_y_err is determined as a function of the detected longitudinal acceleration a x sens and the pitch difference angle θ_diff or dependent on the detected lateral acceleration a y sens and the roll difference angle φ_diff. Otherwise, the program essentially corresponds to that of the first embodiment of the device.

FIG. 8 shows a flow chart of a program for determining the friction coefficient μ_R by estimating according to step S9. The program is preferably executed by the estimation unit EST. The determination of the friction coefficient μ_R is shown as an example depending on the actual lateral acceleration a_y. However, it is also possible to determine the coefficient of friction μ R as a function of the actual longitudinal acceleration a_x accordingly. The program starts in a step Sil. In a step S12, the estimated value of the friction coefficient μ_R for a time t-1 is assigned a predetermined value, for example 1. In a step S13, the slip angle α, the contact force F z, the vehicle speed v are detected or determined as an input variable. In a step S14, a lateral coefficient of friction μ T is determined as a function of the slip angle α and of the estimated value of the friction coefficient μ_R at time t-1. The lateral friction coefficient μ_T is defined as the quotient of a lateral force F_y acting on the tire and the tire upright force F z on the roadway. In a step S15, a determined lateral acceleration a_y_c is determined as a vehicle dynamic model variable as a function of the determined lateral friction coefficient μ T, and is preferably smoothed by low-pass filtering. For example, the transverse force F_y is determined as the product of the determined lateral coefficient of friction μ_T and the contact force F z. The determined lateral acceleration a_y_c can then be determined as a function of the determined lateral force F y.

In a step S 16, the actual lateral acceleration a_y is determined or read as an input variable. In a step S17, it is preferably checked whether an amount of the detected lateral acceleration a_y is smaller than a predetermined lower transverse acceleration limit value a th. The predefined lower lateral acceleration limit value a_th is, for example, about 0.5 m / s ² , but may also be greater than or less than 0.5 m / s ² . Further, it is preferably checked in step S17 whether the vehicle speed v is smaller than a predetermined lower speed limit value v th. The predetermined lower speed limit v_th is for example about 5 m / s, but may also be greater than or less than 5 m / s.

Further, S17, it is preferably checked in step whether the actual lateral acceleration a_y within a predetermined tolerance band around the determined transverse acceleration ^a ^c is _Y_. A lower limit of the predetermined tolerance band is specified as a difference between an amount of the determined lateral acceleration a_y_c and a lower acceleration tolerance limit value a min. Accordingly, an upper re limit of the predetermined tolerance band by a sum of the amount of the determined lateral acceleration a_y_c and an upper acceleration tolerance limit a_max given. If the amount of the detected lateral acceleration a_y is within the predetermined tolerance band, then the program is ended in step S20. Otherwise, the estimated value of the friction coefficient μ_R is adjusted in a step S18. The adjustment of the estimated value of the friction coefficient μ R is effected such that the estimated value of the friction coefficient μ_R at a time t is a difference of the estimated value of the friction coefficient μ R at the time t-1 and a product of a factor K and a difference of the magnitude of the determined lateral acceleration a_y_c and the amount of the actual lateral acceleration ay is assigned. In a step S19, the estimated value of the friction coefficient μ_R at time t is also assigned to the estimated value of the friction coefficient μ R at time t-1 for a subsequent adaptation step according to steps S13 to S18. The program is ended in step S20. The step S12 is preferably executed only in a first execution of the program according to Figure 8 as an initialization. In the following executions of the program this is preferably carried out with the last determined friction coefficient μ R at time t-1 as the initialization value. In this way, the friction coefficient μ R is recursively determined as an estimated value. For example, the factor K is given as about 1.5 s / m ^• T_W. However, the factor K can also be specified differently.

The estimation unit EST is designed to execute the program according to FIG. 8 and to determine the estimated value of the friction coefficient μ_R between the at least one tire of the vehicle and the roadway, depending on the measured values acquired or determined. For estimating the friction coefficient μ_R, the lateral friction value μ_T is determined in step S14 as a function of the detected or determined slip angle α by means of a model. Between the lateral

Coefficient of friction μ T and the slip angle α in the model is a non-linear functional relationship, in a first curve kl and a second curve k2 for different Estimates of the friction coefficient μ_R in Figure 9 is shown by way of example. The estimated value of the friction coefficient μ_R is, for example, 0.5 for the first curve and k2 for the second curve 1. However, the functional relationship can also be specified differently.

It may also be provided that the determination of the actual longitudinal or lateral acceleration ax, ay according to the steps S2 to S8 and the determination of the determined lateral acceleration ayc as the vehicle dynamic model variable according to the steps S13 to S15 is parallel or quasi parallel to each other, that is, on the one hand, the steps S2 to S8 and on the other hand, the steps S13 to S15 are performed in parallel or quasi parallel to each other. The program according to FIG. 8 then does not end in step S20 but instead continues in step S13, if appropriate after expiration of the waiting period T_W.

In addition or as an alternative to the longitudinal acceleration sensor 2 and the lateral acceleration sensor 3, it is also possible to provide other or further acceleration sensors whose detected accelerations are corrected for the influence of the acceleration due to gravity of the respective acceleration sensor and are used to determine the friction coefficient μ_R can. In general, at least one inclination of at least one acceleration sensor with respect to the vehicle-independent predetermined reference plane BE is determined, and at least one acceleration of the vehicle is detected by means of the at least one acceleration sensor in the plane which is substantially parallel to the road surface or substantially parallel to the vehicle body FA , The detected at least one acceleration of the vehicle is corrected as a function of the at least one inclination and the friction coefficient μ R is determined as a function of the corrected at least one acceleration. Preferably, the at least one inclination comprises the pitch angle θ and / or the roll angle φ. Furthermore, the detected at least one acceleration preferably comprises the detected longitudinal acceleration a_x_sens and / or the detected lateral acceleration a_y_sens. However, the at least one inclination of the roadway can also be represented by a different angle, and the detected at least one acceleration can also be represented by another acceleration of the vehicle.

Claims

claims

Anspruch [en] A method for determining a friction coefficient (μ_R) between at least one tire of a vehicle and a roadway, in which

at least one inclination of at least one acceleration sensor with respect to a vehicle-independent predetermined reference plane (BE) is determined,

at least one acceleration of the vehicle is detected in a plane which runs essentially parallel to the roadway, by means of the at least one acceleration sensor,

the detected at least one acceleration of the vehicle is corrected as a function of the at least one inclination, and

- The friction coefficient (μ R) is determined depending on the corrected at least one acceleration.

2. The method of claim 1, wherein the at least one inclination is determined depending on the detected at least one acceleration, a yaw rate (w_z), at least one wheel speed (rd) and a steering wheel angle (γ) or steering angle by means of a predetermined physical vehicle model.

3. The method according to any one of the preceding claims, in which - a pitch rate (w_y) and / or a roll rate (w_x) is detected and

- The at least one slope is determined depending on the pitch rate (w_y) and / or of the roll rate (w x).

4. The method of claim 3, wherein the at least one

Inclination is determined by integration of the pitch rate (w_y) or the roll rate (w x).

5. The method of claim 4, wherein, when at least one predetermined driving state of the vehicle prevails, the at least one inclination determined by the predetermined physical vehicle model is assigned to integration as the initialization value.

6. A method for determining a friction coefficient (μ R) between at least one tire of a vehicle and a roadway, in which - at least one difference angle between a vehicle body (FA) and a chassis (FW) is detected,

- At least one acceleration of the vehicle is detected in a plane which is substantially parallel to the road surface, - the detected at least one acceleration of the vehicle is corrected depending on the at least one differential angle, and

- The friction coefficient (μ_R) is determined depending on the corrected at least one acceleration.

7. The method according to claim 1, wherein the determination of the friction coefficient (μ_R) is suspended when an amount of the at least one inclination or the at least one differential angle is greater than a predetermined first inclination threshold value or difference angle threshold value, and is resumed when the Amount of at least one inclination or the at least one differential angle is smaller than a predetermined second inclination threshold or Differenzwinkschwel- lenwert.

8. The method according to any one of the preceding claims, wherein

a slip angle (α) is detected or determined,

- A lateral coefficient of friction (μ_T) between the at least one tire of the vehicle and the roadway is determined by means of a model in which a functional relationship between the lateral coefficient of friction (μ T) and the slip angle (α) is predetermined such that a nonlinear course the lateral coefficient of friction (μ T) in relation to the slip angle (α) is dependent on a recursively determined estimated value of the friction index (μ R),

depending on the lateral coefficient of friction (μ_T), a model model corresponding to at least one acceleration of the vehicle acceleration is determined and a deviation between the at least one acceleration of the vehicle and the model acceleration is determined and

- the recursion in determining the estimated value of the friction coefficient (μ_R) includes that this depends on the determined

Deviation is adjusted.

9. A device for determining a friction coefficient (μ R) between at least one tire of a vehicle and a roadway, which is formed

for determining at least one inclination of at least one acceleration sensor with respect to a vehicle-independent predetermined reference plane (BE),

for detecting at least one acceleration of the vehicle by means of the at least one acceleration sensor in one

Plane that runs essentially parallel to the roadway,

- To correct the detected at least one acceleration of the vehicle depending on the at least one inclination and - to determine the friction coefficient (μ R) depending on the corrected at least one acceleration.

10. A device for determining a friction coefficient (μ_R) between at least one tire of a vehicle and a roadway, which is formed

for detecting at least one differential angle between a vehicle body (FA) and a chassis (FW),

for detecting at least one acceleration of the vehicle in a plane which is substantially parallel to the roadway,

- For correcting the detected at least one acceleration of the vehicle depending on the at least one differential angle and

- To determine the friction coefficient (μ R) depending on the corrected at least one acceleration.