Method for approximating a friction value
Patent Information
- Application Number
- EP2023817046
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-12-20
- Filing Date
- 2023-11-28
- Publication Date
- 2025-10-29
AI Technical Summary
Current methods for assessing the coefficient of friction between a vehicle's wheels and the road surface are unreliable, particularly in varying road conditions and lighting conditions, which can lead to unsafe driving, especially for inexperienced drivers and autonomous vehicles.
A method that determines the coefficient of friction by analyzing deviations between expected and actual steering variables and load characteristics using existing vehicle sensors, incorporating environmental indicators and lateral acceleration to provide a reliable and accurate approximation.
This method allows for a simple, cost-effective, and quick approximation of the coefficient of friction, enhancing safety by determining the maximum forces that can be transferred between the vehicle and the road, thereby improving driving stability and preventing accidents.
Smart Images
Figure 1.1
Abstract
Description
[0001] Method for approximating a coefficient of friction
[0002] The invention relates to a method for approximating a coefficient of friction between the wheels of a vehicle and a road surface. Furthermore, the invention relates to a driver assistance system, a vehicle, and a computer program product.
[0003] A vehicle's ability to change its speed or direction depends essentially on the forces that the vehicle's tires can transmit to the road surface. The most important factor influencing the transferable forces is the coefficient of friction between the road and the vehicle's tires. This coefficient of friction is influenced by the vehicle's tires and the properties of the road surface. In particular, the road surface properties can vary considerably over the course of a journey.
[0004] A human driver assesses road conditions visually through the vehicle's windshield and / or acoustically based on the rolling noise of the vehicle's wheels on the road. In doing so, a human driver uses experience and knowledge of the current tires and steering behavior of the vehicle and also takes current weather conditions into account. The current friction coefficient is essential for safe vehicle control, as this information can be used to adapt the driving style by comparing the intended vehicle movement with the actual vehicle movement. An experienced driver thus continuously estimates which longitudinal and lateral accelerations are safely possible for the vehicle. Many years of experience are essential for correctly estimating the forces that can be transferred to the road to control the vehicle and thus also the possible changes in the vehicle's movement.Inexperienced drivers in particular can misjudge the coefficient of friction between the vehicle's wheels and the road surface, which poses a significant risk of accidents. A reliable assessment of the coefficient of friction is also important for the safe operation of autonomous vehicles. Sensor-based approaches for the automated assessment of road conditions are well known. For example, optical sensors are available that optically detect the road ahead of the vehicle and evaluate the optically acquired image data to estimate the grip properties of the road surface. However, these sensors have several disadvantages. Firstly, the results are heavily influenced by the properties of the sensor and may not be applicable in all driving situations. For example, systems that use conventional cameras can only be used during the day due to poor lighting conditions.Furthermore, optical systems only take into account aspects of the road and neglect vehicle-specific aspects.
[0005] The object of the present invention is to provide a method for approximating a coefficient of friction between wheels of a vehicle and a roadway, a driver assistance system, a vehicle and / or a computer program product, which is preferably sufficiently accurate, enables improved safety and / or can be used reliably.
[0006] In a first aspect, the invention solves the aforementioned problem by means of a method for approximating a coefficient of friction between wheels of a vehicle in a current vehicle configuration and a roadway, the method comprising the following steps: determining at least one load characteristic of the current vehicle configuration; determining a target variable of the vehicle for a driving situation; determining an expected manipulated variable value that indicates a predicted value of a manipulated variable to be provided to a steering system for setting the target variable; determining an actual variable in the driving situation that corresponds to the target variable; determining an actual manipulated variable value that is provided to the steering system in the driving situation in order to control an actual variable; determining a manipulated variable deviation between the expected manipulated variable value and the actual manipulated variable value, and / or determining a target-actual deviation between the target variable and the actual variable;and approximating the coefficient of friction based on the determined load characteristic and based on the determined manipulated variable deviation and / or the determined target-actual deviation.;
[0007] The invention is based on the finding that the manipulated variable to be provided to set a specific output variable or actual variable on a steering system corresponds to the coefficient of friction between the vehicle's wheels and the road surface. Thus, the force to be provided to turn the wheels or the torque to be provided to turn the wheels is greater over wide operating ranges, the greater the coefficient of friction between the road surface and the steered wheels. The invention makes use of this finding to approximate the current coefficient of friction based on the actual variable and the corresponding manipulated variable. Furthermore, the invention is based on the finding that not only the torque applied to a steered wheel but also the load on the steered wheel is of crucial importance. This load on the steered wheel is taken into account by the load characteristic.The method allows for a very simple, cost-effective, and / or rapid approximation of the coefficient of friction, as the approximation is based on deviations between expected values and the quantities actually occurring during the driving situation. The method can be advantageously implemented using vehicle sensors (or their signals) that are already present in modern vehicles.
[0008] The coefficient of friction determines the maximum forces that can be transferred between the vehicle and the road. The driving situation is preferably a steering situation of the vehicle, i.e. a situation in which the position of the vehicle's wheels, the orientation of the vehicle and / or the yaw rate of the vehicle changes. For example, the driving situation is a vehicle cornering or a section of a cornering. The driving situation is not a discrete point in time, but a period of time. The driving situation comprises at least a period of time that is required to bring about a change in the actual variable by specifying a manipulated variable and / or to achieve an effect on the vehicle as a result of the change in the actual variable. The driving situation can preferably also include a standstill of the vehicle. For example, the driving situation can include test steering of a stationary vehicle.
[0009] The manipulated variable can be a variable that is directly applied to the tires by the steering system. However, the manipulated variable is preferably a physical variable that is provided to the steering system to control an actual variable corresponding to the manipulated variable.
[0010] The expected value of the manipulated variable is the value of the manipulated variable that, according to a forecast, must be provided to the steering system in order to control the target value intended for a driving situation. The actual value of the manipulated variable, on the other hand, is the value of the manipulated variable actually provided to the steering system in the driving situation. It should be understood that providing the actual value of the manipulated variable or a manipulated variable equal to the actual value of the manipulated variable in the driving situation does not necessarily mean that an actual variable that corresponds to the target value must be controlled. In the driving situation (actual situation), the actual variable can therefore be identical to or different from the target variable. Furthermore, in the actual situation, the actual manipulated variable can be identical to or different from the target manipulated variable. For example, both a target-actual deviation and a manipulated variable deviation can occur in the driving situation.It should be understood that a target-actual deviation can also be determined when the actual variable matches the target variable, and / or a manipulated variable deviation can also be determined when the actual manipulated variable matches the target manipulated variable. In this case, a value of zero is determined for the target-actual deviation or the manipulated variable deviation.
[0011] Preferably, the approximation of the friction coefficient is carried out based on the determined load characteristic and based on the determined manipulated variable deviation only if the manipulated variable actual value lies outside a manipulated variable tolerance around the manipulated variable expected value and / or based on the determined target-actual deviation only if the actual variable lies outside an actual variable tolerance around the target variable.
[0012] In a first preferred embodiment of the method, the target variable is or includes a target steering angle speed of the vehicle, and the actual variable is or includes an actual steering angle speed. The steering angle speed, i.e., the rate of change of the steering angle, which can be specified in s, for example, corresponds particularly directly to the coefficient of friction for a constant steering torque applied to adjust the steering angle and is therefore particularly suitable as a target or actual variable.
[0013] The control variable is preferably or comprises a steering torque provided at the steering system, in particular at a steering column of the steering system. This steering torque can be transmitted directly to the wheels or amplified by a power steering system. The steering torque preferably comprises the sum of all steering torques provided for steering the steered wheels. However, the control variable can also be, for example, a current control variable provided at a servomotor of the steering system. The steering system is preferably an active steering system that provides the actual steering torque at least partially based on electrical signals.
[0014] As a rule, the vehicle is steered in the driving situation in such a way that the actual steering angular velocity essentially corresponds to the target steering angular velocity, since a deviation of the actual steering angular velocity from the target steering angular velocity leads to a delayed or too rapid steering response of the vehicle. This in turn can cause the vehicle to deviate considerably from a planned path. Depending on the level or value of the friction coefficient, the actual manipulated variable required to achieve an actual steering angular velocity corresponding to the target steering angular velocity can vary greatly. On an icy road surface, for example, a significantly lower steering torque must be applied to turn the steered wheels than on wheels in contact with a rough road surface. As a rule, the manipulated variable deviation can therefore be determined to approximate the friction coefficient.However, it may also happen that an actual steering angle speed corresponding to the target steering angle speed cannot be achieved. This is the case, for example, if a maximum permissible steering torque would have to be exceeded. If such a target-actual deviation exists, this can also be used to approximate the friction coefficient. Of course, the friction coefficient can also be approximated based on the determined manipulated variable deviation and the determined target-actual deviation.
[0015] In a preferred development, the method further comprises determining a lateral acceleration of the vehicle in the driving situation, wherein the approximation of the coefficient of friction is preferably additionally carried out based on the lateral acceleration. When a vehicle negotiates a curve, a lateral acceleration always acts on the vehicle. This lateral acceleration causes the vehicle to roll about a vehicle's longitudinal axis. In the process, the outer wheels of the curve are loaded and the inner wheels are unloaded. This load change can affect the actual variable, particularly if the steering angular velocity is considered the actual variable. For example, as a result of a positive steering roll radius, which is typically present in trucks, a greater back-rotating moment can occur on an outer wheel of the curve than on an inner wheel.This additional lateral acceleration-dependent torque influences the steering angle velocity achievable by specifying a specific steering torque. The preferred approximation of the friction coefficient based on lateral acceleration allows for a more accurate approximation.
[0016] Preferably, the friction coefficient is only approximated if the lateral acceleration is below a lateral acceleration limit. The lateral acceleration limit is preferably less than or equal to 2 m / s. 2 The lateral acceleration limit can be used to limit the influence of lateral acceleration on the approximation of the friction coefficient. The method can be implemented with less effort and / or more precisely.
[0017] According to a preferred embodiment of the method, the load characteristic is or comprises a current axle load of a steering axle of the vehicle steered by the steering system. The current axle load is the axle load present on the steered steering axle in the driving situation. Here, the current axle load is the axle load present at the moment or during the period in which the actual variable is determined. However, it should be understood that the current axle load can already be determined before the driving situation. For example, the current axle load can be determined when the vehicle is activated, in particular while the vehicle is stationary, or while the vehicle is traveling straight ahead before the driving situation. It should be understood that the actual variable and the actual manipulated variable value are preferably determined at least partially simultaneously.The axle load on the steered axle corresponds particularly directly to the coefficient of friction, so that interference when approximating the coefficient of friction can be reduced. However, it can also be provided, for example, that the load characteristic includes a total vehicle mass, a partial total vehicle mass, a center of gravity position, a load weight of a vehicle's load, and / or a mass distribution of the vehicle.
[0018] Preferably, approximating the coefficient of friction comprises selecting a corresponding reference coefficient of friction from a coefficient of friction database comprising at least one reference coefficient of friction based on the load characteristic and based on the manipulated variable deviation and / or the target-actual deviation. The reference coefficient of friction is a coefficient of friction determined before the driving situation. The reference coefficient of friction corresponds to the current coefficient of friction if a reference load characteristic corresponding to the reference coefficient of friction lies within a load tolerance around the determined load characteristic, and if a reference manipulated variable deviation lies within the reference manipulated variable tolerance around the manipulated variable deviation and / or a reference target-actual deviation lies within a reference target-actual tolerance around the target-actual deviation.A current friction coefficient present in the driving situation can be particularly easily approximated using the previously described refinement of the method. This allows the load characteristics, the target-actual deviation, and / or the manipulated variable deviation, which are generally readily available during vehicle operation, to be used to reliably approximate the friction coefficient. For example, the actual manipulated variable and the actual steering angular velocity can be continuously determined and readily available by the vehicle's steering system. Selection is easily possible using the parameter combination of manipulated variable, actual variable, and load characteristics. The friction coefficient database preferably includes learned reference friction coefficients. The reference friction coefficients can, for example, be friction values approximated from driving situations that occurred earlier in the driving situation.For example, a friction coefficient can be approximated for a specific axle load, an associated steering torque, and a steering angular velocity occurring as a result of the steering torque, and this can then be stored as a reference friction coefficient in the friction coefficient database. The friction coefficient database can also be based entirely or partially on test drives and / or be pre-stored. The test drives can, for example, include a teach-in process for an ESC, particularly when the friction coefficient is high and the vehicle's lateral dynamics are low. It should be understood that the friction coefficient database does not have to be based on a large number of test drives and / or does not have to include a very large number of friction coefficients. For example, a maximum determined friction coefficient for an axle load present in the reference driving situation can be stored for several reference driving situations, which can also be driving situations that occur during normal vehicle operation.Assuming a linear dependence of the steering torque on the wheel load (or axle load) and an indirect proportional dependence of the steering torque on a vehicle speed, further reference values can then be inferred. If, for example, the manipulated variable (steering torque) required to achieve an actual variable (actual steering angle speed) deviates from a reference value determined in this way, the current friction coefficient can be inferred from the difference. In this way, the friction coefficient database can be updated with little effort. In this way, the friction coefficient database can be adapted to changes in the steering and / or the tires of the vehicle with comparatively little effort. In a preferred embodiment, the method further comprises: determining at least one environmental indicator; wherein the selection of a reference friction coefficient from the friction coefficient database is additionally based on the environmental indicator.The environmental indicator represents environmental conditions, particularly weather conditions. The environmental indicator can be taken into account to improve the selection of the reference friction coefficient.
[0019] Preferably, the environmental indicator is or represents a windshield wiper status of a vehicle's windshield wiper, a current ambient temperature, a current date, and / or a geographical location of the vehicle. For example, the environmental indicator can be generated by evaluating a windshield wiper signal. For example, a windshield wiper running at a high frequency generally indicates heavy precipitation, which in turn results in a reduced coefficient of friction compared to dry ambient conditions. The ambient temperature, the date, and the geographical location, particularly in conjunction with a windshield wiper signal, allow conclusions to be drawn, for example, as to whether icy conditions are to be expected.
[0020] In a preferred embodiment, the method further comprises determining a friction coefficient database. Preferably, determining the friction coefficient database comprises: determining a reference friction coefficient for a test cornering operation that precedes the driving situation in time; performing the test cornering operation; determining a reference load characteristic present in a test time period; determining a reference target value for the test cornering operation; determining an expected reference manipulated variable value that indicates a predicted value of a manipulated variable to be provided to a steering system for setting the reference target value, wherein the expected reference manipulated variable value is determined using the reference load characteristic; determining an actual reference variable for the test cornering operation that corresponds to the actual reference variable; determining an actual reference manipulated variable value for the test cornering operation.Determining a reference manipulated variable deviation between the reference manipulated variable expected value and the reference manipulated variable actual value; and / or determining a reference target-actual deviation between the reference target variable and the corresponding reference actual variable; and assigning a parameter combination from the reference target-actual deviation, the reference manipulated variable deviation, and the reference load characteristic to the reference friction coefficient in the friction coefficient database. When determining the friction coefficient database, a corresponding parameter combination is preferably assigned to a known reference friction coefficient.
[0021] The method preferably further comprises: detecting a control system intervention of a control system of the vehicle; determining a coefficient of friction using control system data provided by the control system; wherein the approximation of the coefficient of friction is alternatively or additionally carried out based on the coefficient of friction if a control system intervention is detected. The control system is preferably a stability control system of the vehicle, in particular a so-called Electronic Stability Control (ESC) and / or an anti-lock braking system (ABS) of the vehicle. Such stability control systems are provided in almost all modern vehicles. In the event of a control system intervention, stability control systems determine a large number of control system data that allow conclusions to be drawn about the coefficient of friction or that directly represent the coefficient of friction. The invention utilizes this in the preferred embodiment.
[0022] According to a preferred embodiment, the method further comprises: performing at least one follow-up operation using the approximated friction coefficient, wherein the follow-up operation is or includes providing a warning signal, placing a stability control system into a preventive control mode, redetermining a trajectory of the vehicle, determining a degree of freedom limit, limiting a degree of freedom of the vehicle, and / or validating a friction coefficient sensor. The follow-up operation is preferably only performed if the approximated friction coefficient falls below a friction coefficient limit. For example, a warning signal can only be output if the friction coefficient falls below the friction coefficient limit. This can be the case, for example, if the vehicle is driving on an icy road surface. The warning signal is preferably a visual, acoustic, and / or haptic warning signal.However, it can also be provided that the warning signal is an electrical warning signal that is provided to a control unit of the vehicle. The trajectory comprises at least one planned path that the vehicle must travel to fulfill a driving task. Furthermore, the trajectory comprises a driving dynamics specification. This driving dynamics specification is or preferably comprises a speed specified for traveling the path or a speed profile specified for traveling the path. The trajectory is determined by a fully or partially autonomous unit, such as an automatic distance control system or an autonomous control unit, which is also referred to as a virtual driver. Redetermining the planned trajectory can be a complete redetermination of the planned trajectory, a partial redetermination of the planned trajectory and / or an updating of the planned trajectory.Partial redetermination occurs, for example, when a trajectory curve or path included in the planned trajectory is retained and, at the same time, a speed profile corresponding to traveling along the trajectory curve and included in the planned trajectory is redetermined. During partial redetermination, preferably all information and / or data underlying the trajectory planning are redetermined. During updating, preferably only some of the information and / or data underlying the trajectory planning are redetermined. The determined friction coefficient and / or the determined driving dynamics limit value are preferably taken into account in the trajectory, which can increase safety when using the vehicle. Adhering to the driving dynamics limit value ensures safe and stable driving of the vehicle during normal operation.Preferably, the driving dynamics limit is or includes a maximum permissible vehicle speed, a maximum permissible lateral acceleration, a maximum permissible vehicle acceleration, a maximum permissible vehicle deceleration, a maximum permissible steering angle gradient, a maximum permissible steering frequency, or a minimum permissible turning radius of the vehicle. The friction coefficient sensor is preferably an optical and / or acoustic friction coefficient sensor.
[0023] In a second aspect, the invention achieves the aforementioned object with a driver assistance system configured to carry out the method according to the first aspect of the invention. The driver assistance system preferably comprises a control unit and an interface that can be connected to a vehicle network of the vehicle. The interface is preferably configured to receive vehicle signals that represent at least the load characteristic, the target variable, the actual variable, the expected value of the manipulated variable, and / or the actual value of the manipulated variable. It should be understood that one or more of the determination steps of the method can be performed by the driver assistance system based on such vehicle signals.The driver assistance system therefore does not have to determine the load characteristics directly itself, but can also determine them based on load signals provided by the vehicle's air suspension system on the vehicle network. In a third aspect, the invention achieves the aforementioned object by a vehicle with at least two axles, a braking system, a steering system, preferably an active steering system, and a driver assistance system according to the second aspect of the invention.
[0024] According to a fourth aspect of the invention, the object mentioned at the outset is achieved by means of a computer program product which has program code means stored on a computer-readable data carrier in order to carry out the method according to the first aspect of the invention when the computer program product is executed on a computing unit, in particular the control unit of the driver assistance system according to the second aspect of the invention.
[0025] It should be understood that the driver assistance system according to the second aspect of the invention, the vehicle according to the third aspect of the invention and the computer program product according to the fourth aspect of the invention have the same and similar sub-aspects as are particularly set out in the dependent claims to the method according to the first aspect of the invention.
[0026] Embodiments of the invention will now be described below with reference to the drawings. These are not necessarily intended to represent the embodiments to scale; rather, the drawings are schematic and / or slightly distorted where this is useful for explanation. With regard to additions to the teachings immediately apparent from the drawings, reference is made to the relevant prior art. It should be noted that numerous modifications and changes to the form and detail of an embodiment can be made without deviating from the general idea of the invention. The features of the invention disclosed in the description, in the drawings and in the claims can be essential for the further development of the invention, both individually and in any combination.Furthermore, all combinations of at least two of the features disclosed in the description, the drawings and / or the claims fall within the scope of the invention. The general idea of the invention is not limited to the exact form or detail of the preferred embodiments shown and described below, or limited to an object that would be limited compared to the object claimed in the claims. For specified dimensioning ranges, values lying within the stated limits are also intended to be disclosed as limit values and to be used and claimed as desired. For the sake of simplicity, the same reference numerals are used below for identical or similar parts or parts with identical or similar functions.
[0027] Further advantages, features and details of the invention will become apparent from the following description of the preferred embodiments and from the drawings, which show:
[0028] Fig. 1 is a plan view of a schematically illustrated vehicle;
[0029] Fig. 2 shows a driving situation of the vehicle according to Fig. 1, illustrated as cornering;
[0030] Fig. 3 is a schematic flow diagram of a method for approximating a coefficient of friction;
[0031] Fig. 4 is a schematic flow diagram illustrating a determination and selection of a reference friction coefficient and a prior determination of a friction coefficient database; and in
[0032] Fig. 5 is a schematic flow diagram illustrating a subsequent operation following the approximation of the friction coefficient.
[0033] Fig. 1 shows a vehicle 300 configured as a three-axle commercial vehicle 301. In addition to a front axle 302 and a rear axle 304, the vehicle 300 also includes a liftable auxiliary axle 306 arranged behind the rear axle 304 in the direction of travel 307. The liftable auxiliary axle 306 (lift axle 306 for short) can be raised or lifted so that the mass of the vehicle 300 or a weight force resulting from the load is distributed only across the front wheels 308 of the front axle 302 and the rear wheels 310 of the rear axle 304. When the lift axle 306 is lowered, the weight force of the vehicle 300 is additionally distributed across the auxiliary wheels 312 of the lift axle 306. The vehicle 300 includes a plurality of vehicle actuators 314 configured to influence the longitudinal and lateral dynamics of the vehicle 300. For this purpose, the vehicle actuators 314 influence several degrees of freedom of movement of the vehicle 300.To brake the vehicle 300, a braking system 316 is provided, which includes a brake control unit 318, a brake modulator 320, and a plurality of brake actuators 322. The brake actuators 322 are assigned to the wheels 308, 310, 312 of the vehicle 300 and are configured to provide a braking torque 313 to the wheels 308, 310, 312. For illustrative purposes, only the brake actuators 322 of the rear wheels 310 are connected to the brake modulator 320 in Fig. 1. To brake the vehicle 300, the brake modulator 320 provides braking pressure to the brake actuators 322, which then control brake slip at the wheels 308, 310, 312 of the vehicle 300.
[0034] As a further vehicle actuator 314, the vehicle 300 includes a steering system 324. The steering system 324 is configured to control steered wheels 326 of a steerable axle 328 of the vehicle 300 or to control a steering angle 330 on the steered wheels 326. In the commercial vehicle 301 according to Fig. 1, the front axle 302 represents the steerable axle 328, so that the front wheels 308a, 308b are the steered wheels 326. However, it can also be provided, for example, that the additional wheels 312 of the additional axle 312 are steerable, in which case the additional axle 312 is usually not liftable.
[0035] The steering system 324 here is an active steering system 332, i.e., an at least partially electronic steering system 332. With the active steering system 332, the steering angle 330 at the steerable wheels 326 is not adjusted purely mechanically, but rather at least partially based on electrical signals. For this purpose, the active steering system 332 has a steering control unit 334 connected to a servomotor 336. The servomotor 336 is arranged on a steering column 338 of the steering system 324 and is designed to provide a steering torque to the steering column 338. For this purpose, for example, an output shaft of the servomotor 336 (not shown in the figures) is connected to the steering column 338 by means of a gear. To provide the steering torque, the actuator motor 336 receives corresponding actuator control signals 342 from the steering control unit 334. The actuator control signals 342 can be provided directly in the form of an actuating current or an actuating voltage to the actuator motor 336.However, it can also be provided that the servomotor 336 has a servomotor controller that receives the servomotor control signals 342 and provides a corresponding actuating current or a corresponding actuating voltage. The steering control unit 334 can thus steer the vehicle 300 using the servomotor 336.
[0036] The partially electronic steering system 332 can be controlled not only by means of the steering control unit 334, but also manually. For this purpose, the steering system 324 has a steering wheel 344, which is connected to the steering column 338 via a torsion bar 346. A manual torque 348 provided by a human driver using the steering wheel 344 can be measured using the torsion bar 346 by means of a manual torque sensor 350. The manual torque sensor 350 detects a torsion of the torsion bar 346 and provides a corresponding manual torque signal 352. For this purpose, the manual torque sensor 350 is connected to the steering control unit 334. Furthermore, in the exemplary embodiment shown, the servomotor 336 also reports a provided servomotor torque signal 354 back to the steering control unit 334. The steering control unit 334 can determine a resulting steering torque 3 of the steering system 324 of the vehicle 300 using the actuator torque signal 354 and the manual torque signal 352.Steering torque 3 is the sum of manual torque 348, which is applied manually via the steering wheel 344, and a torque provided by the servomotor 336. The steering control unit 334 also takes into account a torque boost provided by a hydraulic steering torque amplifier 358. The steering torque amplifier 358 receives manual torque 348 and the torque of the servomotor 336 as input and applies a steering torque 3 to the steered wheels 326, which is boosted by a predetermined gain factor.
[0037] The steering torque 3 is a control variable 5 of the steering system 324, the specification of which leads to the control of the steering angle 330. The steering angle 330 can be determined by the steering control unit 334. A temporal rate of change of the steering angle 330 is a so-called steering angular velocity 7, which can also be determined here by the steering control unit 334. The steering angular velocity 7 thus indicates the extent by which the steering angle 330 changes per considered time period. In the preferred embodiment, the steering angular velocity 7 has a value in the unit degrees per second (° / s). Accordingly, if a steering angular velocity 7 of 10° / s is present at the steered wheels 326 for 2 s, then the steering angle 330 changes by 20° within the considered period of 2 s.In the illustrated embodiment, the steering control unit 334 is configured to determine both the manipulated variable 5, which here is the steering torque 3, and an actual variable 9, which is the steering angular velocity 7, caused by specifying the manipulated variable at the steering 324. The vehicle 300 travels on a roadway 366, with frictional contact between the wheels 308, 310, 312 of the vehicle 300 and the roadway 366. A coefficient of friction 2 between the roadway 366 and the steered wheels 326 of the vehicle 300 significantly influences which steering angular velocity 7 is achieved when a steering torque 3 is specified. Thus, with low friction and consequently also a low coefficient of friction 2 between the road surface 366 and the steered wheels 326, a significantly lower steering torque 3 must be provided to achieve the steering angular velocity 7 of 10° / s than when a high coefficient of friction 2 is present between the road surface 366 and the steered wheels 326.By providing the same manipulated variable 5 to the steering system 324, different actual variables 9 can also be controlled for different friction coefficients 2 between the road surface 366 and the steered wheels 326. For example, an icy road surface 366 opposes a rotation of the steered wheels 326 with a significantly lower torque to be overcome than a dry, rough road surface 366. The steering behavior of the vehicle 300 is significantly determined by the friction coefficient 2 present between the wheels 308, 310, 312 of the vehicle 300 and the road surface 300.
[0038] The vehicle 300 here is a semi-autonomous vehicle 300 and includes an autonomous unit 370 configured to control the vehicle 300. The autonomous unit 370 is connected to the steering control unit 334 via a vehicle network 372, which is a CAN bus system here. To control the vehicle 300, the autonomous unit 370, which can also be referred to as a virtual driver, provides control signals 374 on the vehicle network 372. The steering control unit 334 receives the control signals 374 from the vehicle network 372 and controls the servo motor 336 based on the control signals 374 such that a steering torque 3 corresponding to the control signals 374 is output. The control signals 374 include a target variable 11. In the exemplary embodiment considered, the autonomous unit 370 provides a desired steering angle velocity 13 as desired variable 11 on the vehicle network 372.This target steering angle speed 13 is a steering angle speed that the autonomous unit 370 specifies for a driving situation 15. The driving situation 15 is illustrated in Fig. 2 as an example of cornering of the vehicle 300. Fig. 2 shows the vehicle 300 at several positions in a curve 376, and is therefore intended to represent a temporal progression of the driving situation 15. At a curve entrance 378, the front wheels 308 of the vehicle are still aligned straight, so that the steering angle 330 has a value of 0°. At a curve apex 380, a steering angle 330 greater than 0° (approximately 20° in the example shown) is controlled at the front wheels 308 of the vehicle 300. This steering angle 330 is then reduced again towards a curve exit 382, so that the front wheels 308 again have a steering angle 330 of 0° at the curve exit 382.The autonomous unit 370 specifies a steering angle speed as the target steering angle speed 13, which, according to a forecast performed by the autonomous unit 370, is required to negotiate the curve 376. Between the curve start 378 and the curve apex 380, the steering angle speed has a positive value because the steering angle 330 is increasing. Similarly, the steering angle speed between the curve apex 380 and the curve exit 382 has a negative value.
[0039] The autonomous unit 370 specifies the steering angle speed 13 that it expects for the driving situation 15 as the target steering angle speed 13. The target steering angle speed 13 is selected such that the vehicle 300 follows the curve 376 and moves within defined limits of the roadway 366. Furthermore, the autonomous unit 370 also controls a drive motor of the vehicle 300 (not shown in the figures) such that the vehicle 300 is guided through the curve 376 at a safe speed 384 in the driving situation 15. For this purpose, the autonomous unit 370 determines in advance the target steering angle speed 13 and the speed 384 required for the driving situation 15. In the illustrated embodiment, this prediction is based, among other things, on the friction coefficient 2 between the steered wheels 326 and the roadway 366.If the actual friction coefficient 2 deviates from the friction coefficient 2 taken into account when determining the target steering angle velocity 13, the vehicle 300 may be unable to follow the curve 376. This poses a significant risk of an accident, as the autonomous unit 370 may not steer the vehicle 300 appropriately. For example, the autonomous unit 370 may steer the vehicle 300 into the curve 376 at a significantly excessive speed 384. If the road surface 366 is icy, the vehicle 300 may not be able to follow the course of the curve 376 and may be carried out of the curve 376. Knowledge of the friction coefficient 2 is therefore important for the safe operation of the vehicle 300.
[0040] To determine the coefficient of friction 2, the vehicle 300 includes an optical sensor 386, which here is embodied as a camera 388 that captures the road surface 366. However, the optical sensor 386 has the disadvantage that the coefficient of friction 2 can only be determined under sufficiently good lighting conditions. Therefore, in the exemplary embodiment shown, the vehicle 300 additionally includes a driver assistance system 200, which is configured to carry out method 1, explained below with reference to FIGS. 3 to 5, for approximating a coefficient of friction 2 between wheels 308, 310, 312 of the vehicle 300 and the road surface 366. The driver assistance system 200 can also verify a coefficient of friction 2 determined by the optical sensor 386. However, it should be understood that the vehicle 300 can also have only the driver assistance system 200 and no optical sensor 386.
[0041] The driver assistance system 200 comprises a control unit 202 and an interface 204. The interface 204 is connected to the vehicle network 372 and also receives sensor signals 390 from the optical sensor 386 via this network in order to then verify them.
[0042] In a first step of method 1 for approximating a current coefficient of friction 2 between the wheels 308, 310, 312 of the vehicle 300 in a current vehicle configuration 303 and the roadway 366, a load characteristic 19 of the current vehicle configuration 303 is determined 17. The current vehicle configuration 303 takes into account a current load of the vehicle 300. In the present exemplary embodiment, the load characteristic 19 of the current vehicle configuration 303 is an axle load 21 on the steerable axle 328 of the vehicle 300. The axle load 21 results not only from the dead weight of the vehicle 300 but also, among other things, from its load. The axle load 21 corresponds to a normal force acting on the steered wheels 326 in the direction of the roadway 366, which in turn significantly influences the coefficient of friction 2.Thus, lightly loaded wheels 326 can rotate significantly more easily on the roadway 366 than heavily loaded wheels 326 under otherwise identical conditions. By taking the axle load 21 into account, the quality of the approximation of the friction coefficient 2 can be improved. The axle load 21 is determined by an air suspension system of the vehicle 300 (not shown in the figures), wherein the air suspension system provides axle load signals 392 representing the axle load 21 on the vehicle network 372. The control unit 202 carries out the determination 17 of the load characteristic 19 using these axle load signals 392. Thus, signals already present on the vehicle network 372 can advantageously be used for the determination 17. Method 1 is particularly easy to implement.
[0043] As already explained above, the autonomous unit 370 determines the target variable 11 for the driving situation 15, which here is the target steering angle velocity 13, and provides it on the vehicle network 372. In doing so, the autonomous unit 370 preferably also takes into account the axle load 21 or other load characteristics of the vehicle 300. In a further step of the method 1, the control unit 202 of the driver assistance system 200 determines the target variable 11 (determination 23 in Fig. 3) using corresponding signals provided on the vehicle network 372. However, it can also be provided that the control unit 202 determines the target variable 11 directly.
[0044] The target steering angle speed 13 is available at the control unit 202 and at the steering control unit 334. The steering control unit 334 determines a manipulated variable expected value 25 from the provided target steering angle speed 13, which here is a steering torque expected value 27. To achieve an actual steering angle speed 29 (actual variable 9) that corresponds to the target steering angle speed 13, the steering control unit 334 initially controls a steering torque 3 as a manipulated variable 5, which corresponds to the steering torque expected value 27. At the beginning of the driving situation 15, the manipulated variable 5 corresponds to a manipulated variable expected value 25. However, if the friction coefficient, on the basis of which the steering torque expected value 27 is determined, deviates from the actual friction coefficient 2, then the provided steering torque 3 results in an actual steering angular velocity 29 that is different from the target steering angular velocity 13.The steering control unit 334 then adjusts the provided steering torque 3 or the manipulated variable 5 until the actual steering angular velocity 29 corresponds to the desired steering angular velocity 13. For example, in the case of an icy road surface 366, the steering control unit 334 reduces the steering torque 3 provided at the steering column 338 because the steered wheels 326 rotate more easily on the road surface 366 due to a low friction coefficient 2. A manipulated variable actual value 31, which here is an actual value of the steering torque 3, therefore deviates from the manipulated variable expected value 25 in the illustrated embodiment. The steering control unit 334 provides expected value signals 394 corresponding to the manipulated variable expected value 25 and actual manipulated variable signals 396 corresponding to the manipulated variable actual value 31 on the vehicle network 372.
[0045] The control unit 202 of the driver assistance system 200 receives the expected value signals 394 and, using the expected value signals 394, determines 33 the manipulated variable expected value 25. In a similar manner, the control unit 202 receives the manipulated variable actual value signals 396 and uses them to determine 35 the manipulated variable actual value 31. The determination 35 of the manipulated variable actual value 31 occurs here after the determination 33 of the manipulated variable expected value 25, but can in principle also occur simultaneously with or before the determination 33. In the present exemplary embodiment, the steering control unit 334 itself adjusts the manipulated variable 5 such that the actual variable 9, which here is the actual steering angular velocity 29, corresponds to the desired steering angular velocity 13. For this purpose, the steering control unit 334 continuously determines the value of the actual variable 9 (the actual steering angular velocity 29) and provides corresponding actual signals 398 on the vehicle network 372.In addition to the signals 394, 396, which relate to the manipulated variable 5, the steering control unit 334 in the embodiment shown also receives the actual signals 398 and determines the actual variable 9 therefrom during a determination 37.
[0046] The target variable 11 and the manipulated variable expected value 25 can already be determined (determination 23, 33) before the vehicle 300 actually enters driving situation 15. The determination 23, 33 of the target variable 11 and the manipulated variable expected value 25 can therefore already be carried out in the exemplary embodiment considered before the vehicle 300 negotiates curve 376. During or after driving situation 15, the control unit 202 can also determine the actual variable 9 relating to the actual vehicle state of the vehicle 300 and the manipulated variable actual value 31 for the driving situation (determination 35, 37 in Fig. 3). Two pairs of corresponding variables are thus available at the control unit 202 of the driver assistance system 200. A first pair is the target variable 11 and the associated actual variable 9 that actually occurred in driving situation 15.The expected manipulated variable value 25 and the actual manipulated variable value 31 actually provided to the steering system 324 in the driving situation 15 form a second pair of mutually corresponding variables. As described above, the steering control unit 334 controls the steering torque 3 such that the actual variable 9 in the driving situation 15 largely corresponds to the desired variable 11. The actual steering angular velocity 29 lies within a desired-actual tolerance 39 around the desired steering angular velocity 13. A desired-actual deviation 43 determined during a determination 41 between the actual variable 9 and the desired variable 11 (first pair of mutually corresponding variables) is therefore negligible in the present exemplary embodiment of method 1, wherein the desired-actual tolerance 39 is taken into account here in order to compensate for measurement errors included in the actual signals 398.A manipulated variable deviation 45 between the manipulated variable expected value 25 and the manipulated variable actual value 31 caused by the tracking of the actual variable 9 to the target variable 11 is determined in a further step of method 1 (determination 47 in Fig. 3). In the present exemplary embodiment, the manipulated variable deviation 45 therefore lies outside a manipulated variable tolerance 49 around the manipulated variable expected value 25, while the target-actual deviation 43 can be neglected. However, it should be understood that the target-actual deviation 43 can also have a significant value. This can be the case, for example, if the manipulated variable 5 is not adjusted quickly enough, or if the manipulated variable 5 cannot be adjusted such that the actual variable 9 corresponds to the target variable 11.
[0047] Based on the manipulated variable deviation 45, the target-actual deviation 43, and the determined load characteristic 19, an approximation 51 of the current friction coefficient 2 is performed in a subsequent step of method 1. In the exemplary embodiment considered, the control unit 202 of the driver assistance system 200 determines the friction coefficient 2 from the manipulated variable deviation 45, which is a difference between the expected steering torque value 27 and the steering torque 3 actually applied in the driving situation 15, and the axle load 21, wherein the control unit 202 takes into account that the target-actual deviation 43 is negligible. The quality of the approximation 51 is improved by using the load characteristic 19, since this takes into account a contact force of the steered wheels 326 on the road surface 366.
[0048] In the exemplary embodiment considered, method 1 further comprises determining 53 an environmental indicator 55, which is taken into account when approximating 51 the coefficient of friction 2. The control unit 202 of the driver assistance system 200 determines 53 the environmental indicator 55 based on environmental signals 400, which here are windshield wiper signals 402. A windshield wiper 404 of the vehicle 300 according to Fig. 1 provides the windshield wiper signals 402 on the vehicle network 372 so that they can be received by the control unit 202. The windshield wiper signals 402 represent a windshield wiper status of the windshield wiper 404 and thus allow conclusions to be drawn about the amount of precipitation prevailing in the driving situation 15. For example, the windshield wiper 404 generally runs at a high frequency when the precipitation is heavy, which in turn implies a low coefficient of friction 2.
[0049] Furthermore, in the exemplary embodiment shown, method 1 comprises determining 57 a lateral acceleration 59 of the vehicle 300 in driving situation 15. A control system 406 of the vehicle 300, which here is an Electronic Stability Control, intervenes to stabilize the vehicle 300 in the event of instability. For example, to generate a yaw moment acting toward the inside of the curve, the control system 406 causes the inside wheels 308, 310, 312 of the vehicle 300 to be braked more strongly than the outside wheels 308, 310, 312 when the vehicle 300 understeers. In order to reliably trigger such interventions, the control system 406 continuously detects the lateral acceleration 59 present on the vehicle 300 and provides corresponding control system signals 408 on the vehicle network 372.These control system signals 408 can be used by the control unit 202 of the driver assistance system 200 to determine 57 the lateral acceleration 59 of the vehicle 300 in the driving situation 15. The determined lateral acceleration 59 is then additionally used to approximate 51 the current friction coefficient 2.
[0050] Furthermore, based on the stability signals 408 of the control system 406, the driver assistance system 200 can detect a control system intervention 61 of the control system 406 (detection 63 in Fig. 3). The control system signals 408 include control system data 410 that are used in a determination 65 to determine a friction coefficient 67 between the wheels 308, 310, 312 of the vehicle 300 and the road surface 366. The control system 406 performs control system interventions 61 when the vehicle 300 is unstable. This is usually the case when insufficient forces can be transmitted between the vehicle 300 and the road surface 366, so that the available friction coefficient 67 is insufficient in these driving situations 15. The control system signals 408 can therefore advantageously be used to determine 65 the friction coefficient 67. For example, a lateral acceleration that just allows a stable ride for a known axle load of the vehicle 300 (iea lateral acceleration shortly before an instability occurs) can be used to determine the friction coefficient 67. Preferably, however, in addition to the friction coefficient 67, the target-actual deviation 43, the load characteristic 19 and / or the manipulated variable deviation 45 are also used to approximate 51 the friction coefficient 2.
[0051] According to Fig. 4, the approximation 51 of the current friction coefficient 2 involves selecting 69 a reference friction coefficient 71 from a friction coefficient database 79. In the present exemplary embodiment, a plurality of reference friction coefficients 71 are stored in the friction coefficient database 79, which were saved for a plurality of previous driving situations. During selection 69, a current friction coefficient 2 is determined by selecting a reference friction coefficient 71, which is assigned a reference manipulated variable deviation 81 that essentially corresponds to the manipulated variable deviation 45, and which is assigned a reference load characteristic 83 that essentially corresponds to the load characteristic 19 of the vehicle 300 prevailing in the driving situation 15. A required degree of agreement between the reference manipulated variable deviation 81 and the manipulated variable deviation 45, or between the reference load characteristic 83 and the load characteristic 19, can be defined depending on a size of the friction coefficient database 79.For example, a reference friction coefficient 71 can be selected as friction coefficient 2 if its reference load characteristic 83 deviates by 20% from a value of the load characteristic 19 if the friction coefficient database 79 is small. However, if the friction coefficient database 79 contains a large number of reference friction coefficients 71, then a reference friction coefficient 71 can, for example, only be selected as friction coefficient 2 if its reference load characteristic 83 deviates by a maximum of 5% from a value of the load characteristic 19.
[0052] Prior to the selection 69, method 1 further comprises determining 85 the friction coefficient database 79. During this determining 85, a test cornering 86 of the vehicle 300 is carried out (execution 87 in Fig. 4). However, the test cornering 86 can alternatively also be carried out with a comparison vehicle 300. The comparison vehicle 300 can, for example, be a vehicle of the same vehicle type as the vehicle 300 according to Fig. 1. A reference friction coefficient 71 for the test cornering 86 is determined separately during a determination 89 and is at least approximately known for the test cornering 86.During the test curve travel 86, a reference load characteristic 83 present in a test time period is determined 91, a reference manipulated variable expected value 95 is determined 93, a reference setpoint value 99 for the test curve travel 86 is determined 97, an actual reference manipulated variable value 103 for the test curve travel 86 is determined 101, and an actual reference variable 107 for the test curve travel 86 is determined 105. From the actual reference manipulated variable value 103 and the expected reference manipulated variable value 95, the reference manipulated variable deviation 81 can then be determined in a determination 109. A reference setpoint-actual deviation 111 is determined in a determination 113 based on the reference setpoint value 99 and the actual reference variable 107. The reference target-actual deviation 111, the reference manipulated variable deviation 81 and the reference load characteristic 83 are then assigned to the reference friction coefficient 71 (assignment 117 in Fig. 4).
[0053] In the exemplary embodiment of method 1, the current friction coefficient 2 is used following the determination 51 to carry out 119 a subsequent operation 121. The subsequent operation 121 here is the provision 123 of a warning signal 125 at a warning light 412 of the vehicle 300. Furthermore, an electrical warning signal 127 is provided by the control unit 202 of the driver assistance system 200 on the vehicle network 372. The electrical warning signal 127 is thus also present at the autonomous unit 370 and can be used by it to determine a trajectory. Furthermore, the electrical warning signal 127 can be used to place the control system 406 into a preventive control mode 414, in which the stability control system 380 can detect and compensate for any instabilities of the vehicle 300 at an early stage.In the present exemplary embodiment, however, the stability control system 380 is only switched to the preventive control mode 414 if the current friction coefficient 2 falls below a friction coefficient limit value. Thus, stabilizing interventions by the control system 406 are usually only necessary if the current friction coefficient 2 is comparatively low, as is the case, for example, with an icy road surface 366. Reference symbol (part of the.
[0054] Proceedings
[0055] Friction coefficient
[0056] Steering torque
[0057] Control variable
[0058] Steering angle speed
[0059] Actual size
[0060] Target size
[0061] Target steering angle speed
[0062] Driving situation
[0063] Determining a load characteristic
[0064] Load characteristics
[0065] axle load
[0066] Determine the target size
[0067] expected value of the manipulated variable
[0068] Steering torque expected value
[0069] Actual steering angle speed
[0070] Actual manipulated variable value
[0071] Determining the expected manipulated variable value
[0072] Determining the manipulated variable actual value
[0073] Determine the actual size
[0074] Target-actual tolerance
[0075] Determining a target-actual deviation
[0076] Target-actual deviation
[0077] Control variable deviation
[0078] Determining the manipulated variable deviation
[0079] Control variable tolerance
[0080] Approximating the coefficient of friction
[0081] Determining an environmental indicator
[0082] Environmental indicator
[0083] Determining a lateral acceleration
[0084] Lateral acceleration
[0085] Control system intervention Detecting a control system intervention
[0086] Determining a coefficient of friction
[0087] Friction coefficient
[0088] Selecting a reference friction coefficient
[0089] Reference friction coefficient
[0090] Friction coefficient database
[0091] Reference manipulated variable deviation
[0092] Reference load characteristics
[0093] Determining the friction coefficient database
[0094] Test cornering
[0095] Carrying out the test cornering
[0096] Determining the reference friction coefficient
[0097] Determine the reference load characteristics
[0098] Determining a reference manipulated variable expected value
[0099] Reference manipulated variable expected value
[0100] Determining a reference target size
[0101] Reference target size
[0102] Determining a reference manipulated variable actual value
[0103] Reference number I size - 1 weight
[0104] Determining a reference actual size
[0105] Reference actual size
[0106] Determining the reference manipulated variable deviation
[0107] Reference target-actual deviation
[0108] Determining the reference target-actual deviation
[0109] Assign to a reference friction coefficient
[0110] Performing a follow-up operation
[0111] Follow-up operation
[0112] Providing a warning signal
[0113] warning signal electrical warning signal
[0114] Driver assistance system
[0115] Control unit
[0116] interface
[0117] vehicle
[0118] Commercial vehicle front axle current vehicle configuration rear axle additional axle direction of travel, 308a, 308b front wheels
[0119] Rear wheels Additional wheels Braking torque Vehicle actuator Braking system Brake control unit Brake modulator Brake actuator Steering steered wheels Steerable axle Steering angle Active steering Steering control unit Actuator Steering column
[0120] Steering wheel actuator control signals
[0121] Torsion bar Hand torque Hand torque sensor Hand torque signal Actuator torque signal Steering torque amplifier Roadway autonomous unit Vehicle network Control signals Curve Curve entry Curve apex Curve exit Speed Optical sensor Camera
[0122] Sensor signals Axle load signals Expected value signals Actual value signals Actual signals
[0123] Environmental signals Windscreen wiper signals Windscreen wiper control system Control system signals Control system data
[0124] Warning light control mode
Claims
Patent claims 1 . Method (1) for approximating a coefficient of friction (2) between wheels (308, 310, 312) of a vehicle (300) in a current vehicle configuration (303) and a roadway (366), the method (1) comprising the following steps: Determining (17) at least one load characteristic (19) of the current vehicle configuration (303); Determining (23) a target value (11) of the vehicle (300) for a driving situation (15); Determining (33) a manipulated variable expected value (25) which indicates a predicted value of a manipulated variable (5) to be provided for setting the target variable (1 1 ) on a steering system (324), wherein the determining (33) of the manipulated variable expected value (25) is carried out using the load characteristic (19); Determining (37) an actual value (9) corresponding to the target value (11) in the driving situation (15); Determining (35) a manipulated variable actual value (31) which is provided to the steering (324) in the driving situation (15) in order to control the actual variable (9); Determining (47) a control variable deviation (45) between the control variable expected value (25) and the control variable actual value (31); and / or Determining (41) a target-actual deviation (43) between the target size (11) and the corresponding actual size (9); Approximating (33) the coefficient of friction (2) based on the determined load characteristic (5) and based on the determined manipulated variable deviation (25) and / or the determined target-actual deviation (29).
2. Method (1) according to claim 1, wherein the target variable (11) is or comprises a target steering angle speed (13) of the vehicle (300), and wherein the actual variable (9) is or comprises an actual steering angle speed (29).
3. Method (1) according to claim 1 or 2, wherein the manipulated variable (5) is or comprises a steering torque (3) provided at the steering (324).
4. Method (1) according to one of claims 1 to 3, further comprising: Determining (57) a lateral acceleration (59) of the vehicle (300) in the driving situation (15), wherein the approximation (51) of the adhesion value (2) is preferably additionally carried out based on the determined lateral acceleration (59).
5. Method (1) according to claim 4, wherein the approximation (51) of the friction coefficient (2) only takes place if the lateral acceleration (59) is below a lateral acceleration limit value.
6. Method (1) according to one of the preceding claims 1 to 5, wherein the load characteristic (19) is a current axle load (21) of a steering axle (328) of the vehicle (300) steered by the steering system (324).
7. Method (1 ) according to one of claims 1 to 6, wherein the approximation (51 ) of the friction coefficient (2) comprises: Selecting (69) a corresponding reference friction coefficient (71) from a friction coefficient database (79) comprising at least one reference friction coefficient (71), based on the load characteristic (19) and based on the manipulated variable deviation (45) and / or the target-actual deviation (43).
8. Method (1) according to claim 7, further comprising: Determining (53) at least one environmental indicator (55); wherein the selection (69) of a reference friction coefficient (71) from the friction coefficient database (79) is additionally carried out based on the environmental indicator (55).
9. The method (1) according to claim 8, wherein the environmental indicator (55) is or represents a windshield wiper status of a windshield wiper (404) of the vehicle (300), a current ambient temperature, a current date and / or a geographical location of the vehicle (300).
10. Method (1) according to one of claims 7 to 9, further comprising: determining (85) a friction coefficient database (79).
11. Method (1) according to claim 10, wherein determining (85) the friction coefficient database (79) comprises: Determining (89)) a reference friction coefficient (71) for a test curve (86) which precedes the driving situation (15); Carrying out (87) the test cornering (86); Determining (91) a reference load characteristic (83) present in a test period; Determining (97) a reference target value (99) for the test curve travel (86); Determining (93) a reference manipulated variable expected value (95) which indicates a predicted value of a manipulated variable (5) to be provided for setting the reference desired variable (99) on a steering system, wherein the determination (93) of the reference manipulated variable expected value (95) is preferably carried out using the reference load characteristic (83); Determining (105) a reference actual value (107) corresponding to the reference target value (99) for the test curve travel (86); Determining (101) a reference manipulated variable actual value (103) for the test curve travel (86); Determining (109) a reference manipulated variable deviation (81) between the reference manipulated variable expected value (95) and the reference manipulated variable actual value (103); and / or determining (113) a reference target-actual deviation (1 1 1) between the reference target variable (99) and the corresponding reference actual variable (107); and Assigning (117) a parameter combination from the reference target-actual deviation (1 1 1 ), the reference manipulated variable deviation (81 ) and the reference load characteristic (83) to the reference friction coefficient (71 ) in the friction coefficient database (79).
12. Method (1 ) according to one of claims 1 to 1 1 , further comprising: Detecting (63) a control system intervention (61) of a control system (406) of the vehicle (300); Determining (65) a coefficient of friction (67) using control system data (410) provided by the control system (406); wherein the approximation (51) of the coefficient of friction (2) is carried out alternatively or additionally based on the coefficient of friction (67) if a control system intervention (61) is detected.
13. Method (1) according to one of claims 1 to 12, further comprising carrying out (1 19) at least one subsequent operation (121) using the approximated coefficient of friction (2), wherein the subsequent operation (121) comprises providing (123) a warning signal (125, 127), setting a control system (406) into a preventive control mode (414); a re- averaging a trajectory of the vehicle (300), determining a degree of freedom of movement limit value, limiting a degree of freedom of movement of the vehicle (300) and / or validating a friction value sensor, wherein the subsequent operation (121) is preferably only carried out if the approximated friction value (2) falls below a friction value limit value.
14. Driver assistance system (200) for a vehicle (300) which is designed to carry out the method (1) according to one of the preceding claims 1 to 13.
15. Vehicle (300) having at least two axles (302, 304, 306), a braking system (316) and a steering system (324), wherein the vehicle (300) has a driver assistance system (200) according to claim 14.
16. A computer program product comprising program code means stored on a computer-readable data carrier for executing the method (1) according to any one of claims 1 to 13 when the computer program product is executed on a computing unit.