METHOD FOR DETERMINING A COEFFICIENT OF FRICTION BETWEEN A WHEEL OF A MOTOR VEHICLE AND A ROAD SURFACE USING TORQUE VECTORING

Description

[0001] The present invention relates to a method for determining a coefficient of friction between a wheel of a motor vehicle and a road surface with the features of the preamble of claim 1 and a steer-by-wire steering system of a motor vehicle with the features of the preamble of claim 8.

[0002] In steer-by-wire steering systems, the position of the steered wheels is not directly linked to the steering input device, such as a steering wheel. Instead, the steering wheel and the steered wheels are connected via electrical signals. The driver's steering input is detected by a steering angle sensor, and the position of the steered wheels is adjusted accordingly by a steering actuator.

[0003] To avoid unstable driving situations such as oversteer or understeer, a system called "torque vectoring" is used. This system distributes drive torque between the left and right wheels of the vehicle using a specially designed differential to counteract understeer or oversteer. In an unstable driving situation, one wheel is braked while the drive force is directed to the wheel with the greatest reduction in yaw moment. In other words, if understeer occurs, the control system applies more drive force to the outside wheel via a separate clutch, effectively pushing the vehicle into the turn.

[0004] The coefficient of friction between the road surface and the wheels is required for controlling vehicle dynamics and driver assistance systems. Determining the coefficient of friction allows, for example, ABS and traction control systems to be controlled with exceptional precision. It is known to determine the coefficient of friction between the wheels and the road surface using an estimation method that considers the lateral or longitudinal dynamics of the vehicle. Such a method is disclosed, for example, in German patent application DE 10 2007 007 282 A1. In this method, a frictional contact is determined axle-specifically from lateral forces and axle loads calculated in a model, and the coefficient of friction of the road surface is estimated by comparing the two frictional contact values.

[0005] From EP 3 106 360 A1, a method for estimating a coefficient of friction is known in which a positive torque is applied to both wheels of a first axle and an opposite, negative torque is applied to both wheels of a second axle. Under these conditions, the tire rotational speeds are measured and the tire forces are estimated. The coefficient of friction between the tires and the road surface is then estimated from these values ​​using a tire model. The applied torque is reduced as soon as the utilization of the friction approaches an assumed maximum coefficient of friction.

[0006] EP 2 351 678 A1 describes a method for estimating the friction state of a road surface with which a vehicle is in contact. The output variable is a grip characteristic value determined as a function of two input variables, which are a ratio of a force acting on a tire and a degree of slip in two different directions.

[0007] DE 10 2013 011 883 A1 relates to a method for operating the steering of a motor vehicle in which the wheel drive torques are distributed to the steerable wheels in such a way as to reduce the difference between an actual steering angle and a target steering angle. Individual wheel drives are assigned to the steerable axle.

[0008] WO 2008 / 028697 A2 concerns a method for monitoring the noise of a sensor and its application in the automotive sector for determining a coefficient of friction between the tires of a motor vehicle and a road surface.

[0009] WO 2009 / 060075 A2 describes a method for determining the coefficient of friction between a motor vehicle tire and the surface of a roadway, in which the coefficient of friction is determined from a first and a second friction parameter using a recursive estimation algorithm. The first friction parameter is determined using a model in which a functional relationship between the first friction parameter and the slip of the motor vehicle tire is specified. The second friction parameter is determined from the quotient between a longitudinal force and a contact force of the motor vehicle tire.

[0010] The dissertation "Improvement potential of stabilization systems in passenger cars through friction coefficient sensors" by Dipl.-Ing. Ingo Weber, TU Darmstadt, 21.12.2004 deals, among other things, with the determination of the friction coefficient in the vehicle and presents various possibilities for estimating the currently used friction coefficient based on vehicle-internal state variables and for friction coefficient measurement using sensors.

[0011] The object of the present invention is to provide an improved method and a steer-by-wire steering system for determining the coefficient of friction between a vehicle wheel and a road surface, with which the coefficient of friction can be determined more accurately, simply and reliably.

[0012] This problem is solved by a method for determining a coefficient of friction between a wheel of a motor vehicle and a road surface with the features of claim 1 and a steer-by-wire steering system of a motor vehicle with the features of claim 7.

[0013] Accordingly, a method is provided for determining a coefficient of friction between a steerable wheel of a motor vehicle and a road surface, wherein the motor vehicle has an individual wheel drive designed to steer the wheel

[0014] to drive the wheel independently of at least one other steerable wheel of the motor vehicle, wherein wheel drive motors (15, 150) are each connected to the steerable wheels (7, 70) via drive shafts (27) and the wheel drive motors (15, 150) are electric motors.

[0015] The process includes the following steps: Applying a wheel drive torque to the wheel of the motor vehicle via the individual wheel drive, such that the wheel is made to slip, and determining the coefficient of friction using the friction torque, which results from the difference between the wheel drive torque and a moment of inertia of the wheel.

[0016] This method allows for the simple, accurate, and reliable determination of the coefficient of friction between the road surface and the vehicle wheel. No estimation is necessary.

[0017] According to the invention, the force acting on the wheel in the direction of travel is derived from the frictional torque by means of the relationship F X = M Re ibung r Rad The force is determined by r, where r is the radius of the vehicle wheel. This force is then used in the calculation of the coefficient of friction, such that the coefficient of friction is the quotient of the force acting on the wheel in the direction of travel and a normal force.

[0018] Preferably, the slip for determining the coefficient of friction lies in a range between 0.1 and 0.8.

[0019] Preferably, the slip time is in a range between 0.1s and 0.3s.

[0020] It is advantageous if the wheel drive torque is applied to the wheel without changing the vehicle's direction.

[0021] According to the invention, the application of the wheel drive torque to the wheel of the motor vehicle is preceded by a stable straight-line journey of the motor vehicle at a constant speed and a yaw rate that does not exceed a predefined value.

[0022] It is conceivable and possible that the existing steering and braking system assists with driving straight ahead at a constant speed. This allows vehicle reactions to be suppressed, enabling the identification of the friction coefficient of the vehicle's wheel.

[0023] This stable straight-line driving is preferably maintained for a duration of more than 5 seconds.

[0024] In an advantageous embodiment, the coefficient of friction is determined between a maximum and a minimum value and set to either the maximum or the minimum value, depending on which of the two values ​​is closer to the coefficient of friction. Preferably, the maximum value is 1.2 and the minimum value is 0.3. The method can be performed sequentially so that current friction values ​​are always available during driving.

[0025] According to the invention, a steer-by-wire steering system of a motor vehicle is provided with a steerable front axle having two steerable wheels, wherein the front axle has an individual wheel drive which drives the wheel drives assigned to the steerable wheels individually by means of a drive control, and wherein the drive control is designed to carry out the previously described method for determining the coefficient of friction.

[0026] A preferred embodiment of the invention is explained in more detail below with reference to the drawings. Similar or functionally equivalent components are designated with the same reference numerals in the figures. The figures show: Fig. 1: a schematic representation of a steer-by-wire steering system with two separate wheel drives on the front axle, Fig. 2: a schematic representation of a steer-by-wire steering system in top view, and Fig. 3: a schematic representation of the forces acting on the wheels to determine the coefficient of friction between the road surface and the wheels.

[0027] In the Figure 1Figure 1 shows a steer-by-wire steering system. A rotary angle sensor (not shown) is attached to a steering shaft 2. This sensor detects the steering angle applied by the driver turning a steering input device 3, which in this example is designed as a steering wheel. A steering torque can also be detected. Furthermore, a feedback actuator 4 is attached to the steering shaft 2. This actuator simulates the feedback from the road surface to the steering wheel 3, thus providing the driver with feedback about the vehicle's steering and handling behavior. The driver's steering input is transmitted via signal lines to a steering control unit 5 using the rotary angle of the steering shaft 2 measured by the rotary angle sensor. Depending on other input variables, the steering control unit 5 actuates an electric steering actuator 6, which controls the position of the steered wheels 7 and 70.The steering actuator 6 acts indirectly on the steered wheels 7, 70 via a steering linkage 8, such as a rack and pinion steering gear, as well as via tie rods 9 and other components. Drive motors 10 are assigned to the steerable wheels 7, 70, which drive the wheels 7, 70 separately in the form of an individual wheel drive. A drive control unit 11 determines the drive torques for the steerable wheels 7, 70 based on the rotation angle of the steering shaft 2 measured by the rotation angle sensor and other signals, and controls the respective drive motor 10 accordingly.

[0028] In Figure 2The diagram schematically depicts the motor vehicle with two axles, with the drive for the steerable wheels 7, 70 located on the front axle 12. The front axle 12 comprises, relative to one direction of travel, a left steerable wheel 7 and a right steerable wheel 70, which are connected to each other via the rack 13 of the rack and pinion steering gear. When the rack 13 is displaced transversely to the right or left relative to the direction of travel, the wheels pivot about a respective pivot point 14, 140. A left drive motor 15 is located on the left side of the vehicle and a right drive motor 150 is located on the right side. The wheel drive motors 15, 150 are each connected to the steerable wheels 7, 70 via drive shafts 27. The wheel drive motors 15, 150 are electric motors. The drive control 11 controls the left drive motor 15 via a first signal line S1 and the right drive motor 150 via a second signal line S2.The drive control unit 11 also receives information about the state of the rear axle via a signal line S3. The vehicle moves in the direction of travel x at a speed v and around the vertical axis at an angular velocity Ψ (yaw rate).

[0029] In Figure 3The determination of the coefficient of friction is illustrated. When the vehicle travels straight ahead in the x-direction at a constant speed v, at least one wheel (7.70) is briefly subjected to a wheel drive torque M impulse without changing the vehicle's direction. This causes the wheel (7.70) to accelerate or decelerate in such a way that it slips. If, for example, the front right wheel is made to slip, the yaw rate and longitudinal acceleration of the wheel must be close to zero to avoid changing the vehicle's direction. A corresponding longitudinal force and yaw moment in the opposite direction of rotation of the front right wheel are then applied to one or more other wheels (other than the right wheel) through braking and steering operations. Figure 3The example of acceleration is shown. The term "slip" refers to the condition when, during acceleration or deceleration of the vehicle wheel 7.70, the surface speed of the wheel differs from the vehicle speed v. The slip is the magnitude of the quotient of the difference between the surface speed of the wheel 7.70 and the vehicle speed v. The slip is therefore 1 when the wheel is locked. Preferably, for determining the coefficient of friction, the slip lies in a range between 0.1 and 0.8. The slip is preceded by stable straight-line driving at a constant speed and a yaw rate that does not exceed a predefined value, for example, 0.05° / s. This condition preferably lasts for a duration of more than 5 s. The subsequent slip has a duration that preferably lies in a range between 0.1 s and 0.3 s.The coefficient of friction µ between the wheel and the road surface can be determined from the short-term wheel drive torque: . μ = F X F Z , where FX is the force acting in the x-direction and FZ is the normal force acting perpendicular to it. FX is given by: F X = M Re ibung r Rad , where M friction is the frictional torque and r wheel is the radius of the wheel. M friction is further defined by: M Friction = M In the pulse - M inertia , where M inertia is the moment of inertia.

[0030] Since the moment of inertia of the rotating mass and the normal force (during stable straight-line travel) on the driven wheel are known and constant, the frictional torque Mfriction and from this the coefficient of friction µ between the wheel and the road surface can be determined. More precisely, the vehicle travels at a constant speed, there is no dynamic load change, and the normal force corresponds to the value when the vehicle is stationary. The static weight distribution and mass distribution are known. The moment of inertia is defined by: M inertia = T inertia * α wheel , where T inertia is the moment of inertia and α wheel is the wheel angular acceleration, which is determined from the measured wheel angular velocity.

[0031] The coefficient of friction is preferably determined between a maximum and a minimum value. In one embodiment, the maximum value is 1.2 and the minimum value is 0.3. After determining the coefficient of friction, it is set to either the maximum or the minimum value for further use, depending on which of the two values ​​is closer.

[0032] The friction coefficient defined in this way can be used in electromechanical power steering systems and / or steer-by-wire steering systems, for example for: Adjustment of steering feel in steer-by-wire steering systems, estimation of driving conditions, addition of optical friction coefficient detection, determination of permissible steering operations (e.g. in an autonomously driving vehicle, lane keeping, etc.).

[0033] To improve the accuracy of determining the coefficient of friction, a tire model can also be used.

Claims

1. Method for determining a coefficient of friction (µ) between a steerable wheel (7, 70) of a motor vehicle and a road surface (16), wherein the motor vehicle has an individual wheel drive which is designed to drive the steerable wheel (7, 70) independently of at least one other steerable wheel (7, 70) of the motor vehicle, wherein wheel drive motors (15, 150) are connected to the steerable wheels (7, 70) via drive shafts (27) and the wheel drive motors (15, 150) are electric motors, characterised in that the following process steps are provided: · Applying a wheel drive torque (MImpuls) to the wheel (7, 70) of the motor vehicle by means of the individual wheel drive in such a way that the wheel (7, 70) is brought into slip, and · determining the coefficient of friction (µ) by means of the friction torque (MReibung), which is calculated from the difference between the wheel drive torque (MImpuls) and an inertia torque (MTrägheit) of the wheel (7, 70) according to the formula μ = F X F Z = M Impuls − M Trägheit r Rad ∗ F Z where rRad is the radius of the wheel (7, 70), FX is the force acting on the wheel in the direction of travel, and FZ is the normal force, and wherein the moment of inertia (MTrägheit) is defined by MTrägheit = TTrägheit * αRad, wherein TTrägheit is the moment of inertia and αRad is the wheel angular acceleration, which is determined from a measured wheel angular velocity, wherein the application of the wheel drive torque (MImpuls) to the wheel (7, 70) of the motor vehicle is preceded by stable straight-line travel of the motor vehicle at a constant speed and a yaw rate that does not exceed a predefined value.

2. Method according to claim 1, characterised in that the slip for determining the coefficient of friction (µ) is in a range between 0.1 and 0.8.

3. Method according to one of the preceding claims, characterised in that the wheel (7, 70) is brought into slip for a defined period of time, which is in a range between 0.1 s and 0.3 s.

4. Method according to one of claims 1 to 3, characterised in that the stable straight-line travel is preferably maintained for a period of more than 5 seconds.

5. Method according to one of the preceding claims, characterised in that the coefficient of friction (µ) is determined between a maximum value and a minimum value and the coefficient of friction (µ) is set to the maximum value or the minimum value, depending on which of the two values is closer to the coefficient of friction.

6. Method according to claim 5, characterised in that the maximum value is 1.2 and the minimum value is 0.3.

7. Steer-by-wire steering system for a motor vehicle with a steerable front axle (12) having two steerable wheels (7, 70), wherein the front axle (12) has an individual wheel drive which, by means of a drive control (11), individually drives the wheel drives (15, 150) assigned to the steerable wheels (7, 70), characterised in that the drive control (11) is designed to execute the method according to one of claims 1 to 6.