Method and device for determining the coefficient of friction for a vehicle

Abstract

Method for determining the coefficient of friction for a vehicle (100), characterized in that a longitudinal acceleration (a x ) of the vehicle (100) is determined (302), wherein a wheel torque (M) of a wheel (102) of the vehicle (100) is determined (304), wherein a wheel speed (ω) of the wheel (102) is determined (306), wherein a derivative (ω̇) of the wheel speed is determined (308), wherein a contact force (FN) and a longitudinal force (Fx) are determined depending on the wheel torque (M) of the wheel (310), and wherein a coefficient of friction (µx) is determined depending on the slip stiffness (kx) (316), characterized in that a quantity (x) is determined depending on the longitudinal acceleration (ax), the wheel speed (ω), and the derivative (ω̇) of the wheel speed (312), which defines a change in a slip stiffness (kx), wherein the slip stiffness (kx) depends on the quantity (x), a rolling radius (r) of the wheel, the insurgent force (F N ) and the axial force (F x ) is determined (314).

Description

[0001] The invention relates to a method and a device for determining the coefficient of friction for a vehicle.

[0002] A vehicle interacts with the surface it is driving on via its tires. The coefficient of friction describes the relationship between the contact force and the frictional force for the power transmission between the tire and the surface.

[0003] Conventional methods for determining the coefficient of friction calculate a coefficient of friction as a function of slip stiffness. For example, a vehicle speed and the contact force and wheel torques of one wheel, or the contact forces and wheel torques of several wheels, are determined, and the coefficient of friction is estimated as a function of the slip stiffness. Conventional methods use, for example, Kalman filters. One such method is known, for instance, from German patent application DE 10 2012 000 097 A1.

[0004] It would be desirable to improve the determination of the coefficient of friction.

[0005] This is achieved through the subject matter of the independent claims.

[0006] A method for determining the coefficient of friction for a vehicle involves determining the vehicle's longitudinal acceleration, the wheel torque of a wheel, the wheel rotational speed, and the derivative of that wheel rotational speed. Depending on the longitudinal acceleration, wheel rotational speed, and the derivative of that wheel rotational speed, a quantity is determined that defines a change in slip stiffness. Depending on the wheel torque, a contact force and a longitudinal force are also determined. The slip stiffness is then determined based on the magnitude of the contact force, the wheel's rolling radius, the contact force, and the longitudinal force. The method for determining the coefficient of friction does not directly estimate the slip stiffness, but rather considers its change over time. This change over time is estimated based on the accelerations within the vehicle.For this purpose, the vehicle's acceleration sensors are used, for example. The inertial measurement unit (IMU), a spatial combination of several acceleration and yaw rate sensors, provides longitudinal acceleration values ​​that allow for a particularly accurate estimation of the coefficient of friction, even without reliably knowing the vehicle's longitudinal speed. This determination of the coefficient of friction is especially advantageous in all-wheel-drive vehicles. In vehicles with a single driven axle, the vehicle's longitudinal speed can often be determined with sufficient accuracy depending on the axle being driven. Wheel speed sensors on the driven axle can be used for this purpose. This is not possible in all-wheel-drive vehicles because all driven tires are constantly slipping, causing the wheel speed sensors to deliver inaccurate readings.

[0007] Preferably, the quantity that defines a change in slip stiffness is dependent on the differential equation. x˙=axω−ω˙ωx The change in slip stiffness, i.e., the time derivative of the slip stiffness, is determined based on the accelerations in the vehicle, in particular the longitudinal acceleration, i.e., the acceleration in the longitudinal direction x.

[0008] In one aspect, slip stiffness defines a linear relationship between slip and the coefficient of friction. This linear relationship represents a slip characteristic curve. The slip characteristic curve exhibits a different slope, i.e., slip stiffness, for different coefficients of friction.

[0009] Preferably, the longitudinal acceleration is measured by an inertial measuring unit. This increases the accuracy of the estimate, especially for all-wheel drive vehicles.

[0010] Preferably, the longitudinal acceleration is determined based on a spatial combination of accelerations detected by multiple accelerometers. Using several spatially arranged accelerometers increases the accuracy of the resulting longitudinal acceleration and the reliability of the friction coefficient determination.

[0011] Preferably, the longitudinal acceleration is determined as a function of a rotation rate detected by a rotation rate sensor. This further increases reliability.

[0012] Preferably, the vehicle is controlled depending on the coefficient of friction.

[0013] A device for determining the coefficient of friction for a vehicle comprises an input for data or signals defining a longitudinal acceleration of the vehicle, a wheel torque of a wheel of the vehicle and a wheel speed of the wheel, wherein the device comprises a computing unit and a memory configured to execute the described method depending on the longitudinal acceleration, the wheel torque and the wheel speed.

[0014] Preferably, the device comprises an inertial measuring unit configured to detect the longitudinal acceleration at the input. This ensures a particularly reliable determination of the longitudinal acceleration.

[0015] Preferably, the device comprises a yaw rate sensor configured to detect the vehicle's yaw rate, and the device is configured to determine the longitudinal acceleration as a function of the yaw rate. This further increases reliability.

[0016] Preferably, the device includes an actuator designed to control the vehicle depending on the coefficient of friction.

[0017] Further advantageous embodiments will become apparent from the following description and the drawing. The drawing shows Fig. 1. Parts of a vehicle, Fig. 2 parts of a device for determining the coefficient of friction, Fig. 3 steps in a procedure for determining the coefficient of friction.

[0018] In Fig. Figure 1 shows a schematic representation of parts of a vehicle 100. The vehicle 100 comprises several wheels 102. In an all-wheel-drive vehicle 100, all wheels 102 are driven. In a single-axle-driven vehicle, the wheels of the driven axle are driven.

[0019] The following describes the determination of the coefficient of friction for a driven wheel 102. Determining the coefficient of friction for individual wheels allows for more precise control of the force distribution at each wheel. For some applications, determining the coefficient of friction for the vehicle 100 may be useful. For this purpose, the coefficients of friction of the individual wheels can be determined and, for example, averaged.

[0020] The force distribution at a driven wheel 102 is in Fig. 1 schematically represented. The driven wheel 102 has a rolling radius r and rotates at a speed ω, driven by a wheel torque M. Due to a ground force F N , which depends on a mass of the vehicle 100, the driven wheel 102 can exert a longitudinal force F x transferred, with which the vehicle 100 with a longitudinal acceleration a x The vehicle 100 comprises an inertial measuring unit 104, which measures the longitudinal acceleration a xrecorded.

[0021] The vehicle 100 is located on a surface 106. Depending on the nature of the surface 106, the driven wheel 102 can experience more or less longitudinal force F. x transferred to substrate 106.

[0022] In the case of propulsion, the driven wheel 102 rotates at a vehicle speed v x with a slip λ=rωvx.

[0023] A slip curve defines a linear relationship between the slip λ and a coefficient of friction µ for movement in the longitudinal direction x. x .

[0024] A device 200 for determining the coefficient of friction for the vehicle 100 is in Fig. 2 schematically represented.

[0025] The device 200 includes an input 202 for the longitudinal acceleration a x of the vehicle 100, for the wheel torque M of the driven wheel 102 of the vehicle 100 and for the wheel speed ω of the wheel 102.

[0026] The device 200 comprises a computing unit 204 and a memory 206, which are configured depending on the longitudinal acceleration a x The following procedure is to be carried out using the vehicle 100, the wheel torque M of the driven wheel 102 of the vehicle 100 and the wheel speed ω of the driven wheel 102 at the input 202.

[0027] The device 200 can include the inertial measuring unit 104, which is configured to measure the longitudinal acceleration a x for input 202. The device 200 can include a yaw rate sensor configured to detect the yaw rate of the vehicle 100. In this case, the device 200 is configured to detect the longitudinal acceleration a x to determine depending on the yaw rate. A vehicle model calculation can be used for this purpose, which establishes a relationship between yaw rate and longitudinal acceleration a. x defined.

[0028] The device 200 can include a wheel speed sensor that measures the wheel speed ω. The device can include a torque sensor that measures the wheel torque M. These quantities can also be dependent on other quantities, for example, a drive torque, the longitudinal acceleration a. x or a motor speed that is measured by other sensors or determined by means of modeling.

[0029] In this example, memory 206 contains a family of curves with a large number of slip curves. Fig. 2 are a first slip curve µ h with a high coefficient of friction for a surface 106 consisting of asphalt and a second slip curve µ l with a low coefficient of friction compared to the high coefficient of friction for a surface 106 consisting of snow or gravel.

[0030] In general, the coefficient of friction is µ. x defined in the longitudinal direction x as μx=FxFz=kxλ.

[0031] The slip stiffness k x defines the slope of the slip characteristic curve. A first slip stiffness k h the first slip curve µ h In the example, it is greater than a second slip stiffness k l the second slip curve µ l. .

[0032] The procedure for determining the coefficient of friction stipulates that in step 302 the longitudinal acceleration a x The vehicle's 100 is determined. The longitudinal acceleration a x For example, it is measured by an inertial measuring unit 104. The longitudinal acceleration a x It can also be determined depending on a spatial combination of the accelerations detected by multiple accelerometers. The longitudinal acceleration a x one aspect can be determined depending on the rotation rate detected by the rotation rate sensor.

[0033] In step 304, the wheel torque M of the driven wheel 102 is determined. The wheel torque M is measured, for example, at the wheel, or calculated from the longitudinal acceleration a. x or a drive torque of an engine of the vehicle estimated at 100.

[0034] In step 306, the wheel speed ω of the driven wheel 102 is determined. The wheel speed ω is measured, for example, by a wheel speed sensor.

[0035] In step 308, a derivative ω̇ of the wheel speed of the driven wheel 102 is determined. The derivative ω̇ of the wheel speed ω is calculated, for example, from the wheel speed ω, or determined from incremental signals from the wheel speed sensor. If the derivative ω̇ of the wheel speed is determined from incremental signals, the wheel speed ω can be determined by integrating the derivative ω̇ of the wheel speed ω.

[0036] Steps 302 to 308 can be performed in the order described or in any other order.

[0037] In step 310, the ground force F is determined depending on the wheel torque M of the driven wheel 102. N and the axial force F x determined. The reactive force F N For example, the number of wheels and the vehicle's mass are both factors, with 100 being the quotient of mass and number of wheels. Dynamic load shifts during acceleration can be corrected by adjusting the contact force F. N depending on the longitudinal acceleration a x must be taken into account. The axial force F x is determined, for example, depending on the wheel torque M and the rolling radius of the wheel r: Fx=Mr.

[0038] In step 312, depending on the longitudinal acceleration a x, the wheel speed ω and the derivative ω̇ of the wheel speed determine a quantity x that represents the change in slip stiffness k x defined.

[0039] The quantity x, which defines the change in slip stiffness, depends on the differential equation. x˙=axω−ω˙ωx in particular determined by integration of the differential equation.

[0040] In step 314, the slip stiffness k is determined. x depending on the size x, the rolling radius r, the ground force F N and the axial force F x for example, determined as follows: kx=FxFz(rx−1)

[0041] In step 316, the coefficient of friction µ is determined. x depending on the slip stiffness k x certainly.

[0042] The coefficient of friction µ xThis concerns the longitudinal direction x for a driven wheel. A coefficient of friction for the vehicle 100 or for the surface 106 can be determined, for example, by determining the coefficient of friction of all wheels and averaging them.

[0043] In one aspect, it is provided that the vehicle 100 or the driven wheel 102, for which the coefficient of friction µ x The device is determined to be controlled by a further step of the process depending on this coefficient of friction. In this case, the device includes an actuator that controls the vehicle accordingly. For example, the coefficient of friction µ x a quantity for stability control on different substrates or serves to determine a parameter for this stability control.

Claims

[1] Method for determining the coefficient of friction for a vehicle (100), characterized by , that a longitudinal acceleration (a x ) of the vehicle (100) is determined (302), wherein a wheel torque (M) of a wheel (102) of the vehicle (100) is determined (304), wherein a wheel speed (ω) of the wheel (102) is determined (306), wherein a derivative (ω̇) of the wheel speed is determined (308), wherein a contact force (FN) and a longitudinal force (Fx) are determined depending on the wheel torque (M) of the wheel (310), and wherein a coefficient of friction (µx) is determined depending on the slip stiffness (kx) (316). characterized by , that depending on the longitudinal acceleration (ax), the wheel speed (ω) and the derivative (ω̇) of the wheel speed, a quantity (x) is determined (312) which defines a change in a slip stiffness (kx), where the slip stiffness (kx) depends on the quantity (x), a rolling radius (r) of the wheel, the contact force (F) N ) and the axial force (F x ) is determined (314). [2] Method according to claim 1, characterized by , that the quantity (x) that defines a change in slip stiffness depends on the differential equation x˙=axω−ω˙ωx is determined. [3] Method according to claim 2, characterized by , that the slip stiffness (k x ) a linear relationship between slip (λ) and the coefficient of friction (µ) x ) defined. [4] Method according to any one of the preceding claims, characterized by , that the longitudinal acceleration (a x ) is measured by an inertial measuring unit (104). [5] Method according to any one of the preceding claims, characterized by , that the longitudinal acceleration (a x ) is determined depending on a spatial combination of the accelerations detected by multiple acceleration sensors. [6] Method according to claim 5, characterized by , that the longitudinal acceleration (a x) is determined depending on a rotation rate detected by a rotation rate sensor. [7] Method according to any of the preceding claims, characterized by that the vehicle depends on the coefficient of friction (µ) x ) is controlled. [8] Device (200) for determining the coefficient of friction for a vehicle (100), characterized by that the device (200) includes an input (202) for data or signals that represent a longitudinal acceleration (a x ) of the vehicle (100), a wheel torque (M) of a wheel (102) of the vehicle (100) and a wheel rotation speed (ω) of the wheel (102), wherein the device comprises a computing unit (204) and a memory 206 which are configured depending on the longitudinal acceleration (a x ), the wheel torque (M) and the wheel speed (ω) to carry out the method according to one of claims 1 to 6. [9] Device (200) according to claim 8, characterized by, that the device (200) comprises an inertial measuring unit (104) configured to measure the longitudinal acceleration (a x ) for the entrance (202). [10] Device (200) according to claim 8 or 9, characterized by , that the device (200) comprises a yaw rate sensor configured to detect a yaw rate of the vehicle, wherein the device (200) is configured to detect the longitudinal acceleration (a x ) depending on the rotation rate. [11] Device according to any one of claims 8 to 10, characterized by that the device includes an actuator designed to steer the vehicle depending on the coefficient of friction (µ) x ) to head towards.

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