Method for determining at least one coefficient of friction, device

Abstract

Method for determining at least one coefficient of friction between at least one wheel (4, 5) of a motor vehicle (1) having a plurality of wheels (4, 5, 6, 7) and a roadway (10) on which the wheels (4, 5) are positioned, wherein by controlling at least one actuator for influencing a current driving state, a change in the driving state resulting from the control and, as a function of the change, the coefficient of friction are estimated, wherein, for determining the coefficient of friction, at least one steering actuator (8, 9), which is assigned to only one wheel (4, 5) of the motor vehicle (1), is controlled as an actuator in order to change a steering angle (α) of the wheel (4, 5), wherein a torque necessary for changing the steering angle (α) is detected and, as a function of the detected torque, the coefficient of friction is estimated, wherein two steering actuators (8, 9), each assigned to only one wheel (4, 5) of the motor vehicle (1), wherein the two wheels (4,5) are arranged on opposite longitudinal sides of the motor vehicle (1), are controlled to determine the coefficient of friction in such a way that they change the steering angles (α) of the wheels (4, 5) in opposite directions to the same extent, and wherein the steering angles (α) are adjusted several times in opposite directions, alternating between a toe-in and a toe-out.

Description

[0001] The invention relates to a method for determining at least one coefficient of friction between at least one wheel of a motor vehicle having a plurality of wheels and a roadway on which the wheels stand, wherein by controlling at least one actuator for influencing a current driving state, a change in the driving state resulting from the control and, as a function of the change, the coefficient of friction are estimated.

[0002] Furthermore, the invention relates to a device for carrying out the above-mentioned method. State of the art

[0003] Modern motor vehicle safety systems react depending on current friction values to ensure optimal hazard avoidance or defusing. For example, ABS or ESP systems regulate braking force according to a currently detected friction value in order to prevent one or all of the vehicle's wheels from spinning and to stabilize the vehicle. However, the current friction value of the road, or the friction value acting between the road and the wheel, is only detected when the ABS or ESP system intervenes. Thus, an estimate of the friction value only takes place when control is active and braking intervention is already necessary. This means that the friction value is only detected when the actual limits are exceeded, which also has a disadvantageous impact on comfort for vehicle occupants, as the intervention of the braking system inevitably leads to a deceleration of the vehicle.It should therefore be avoided to activate the braking system only to determine the coefficient of friction.

[0004] Furthermore, DE 10 2010 014 564 A1 discloses a method for determining at least one coefficient of friction between at least one wheel of a motor vehicle and a roadway, wherein, to determine the coefficient of friction, at least one steering actuator which is assigned to only one wheel of the motor vehicle is controlled as an actuator, wherein a torque required to change the steering angle is detected and the coefficient of friction is estimated as a function of the detected torque, and wherein two steering actuators, each assigned to only one wheel of the motor vehicle, are controlled to determine the coefficient of friction in such a way that they change the steering angles of the wheels to the same extent in opposite directions and the steering angles are adjusted several times in opposite directions.

[0005] Furthermore, DE 10 2015 220 211 A1 discloses a method for determining the coefficient of friction by adjusting a wheel.

[0006] Furthermore, DE 10 2016 014 547 A1 discloses instructions for sharing measured friction values with an external database. Disclosure of the invention

[0007] The method according to the invention with the features of claim 1 has the advantage that a friction coefficient estimate is made without intervening in the braking system or without the braking system intervening in the driving state, which can be carried out unnoticed, in particular, by the occupants of the motor vehicle. Today's motor vehicles are often equipped with an electromechanical steering system, which is coupled to two wheels arranged opposite one another on the longitudinal sides of the motor vehicle in order to steer the motor vehicle. While previously a steering system required a mechanical coupling between the steering wheel and the wheels, steering systems are now being developed that do away with the mechanical coupling. Instead, an electrical coupling is used. Such systems are referred to as steer-by-wire systems.The method according to the invention makes use of such a system with individual wheel actuators, since such a system allows wheels to be controlled individually, so that each steering angle is adjusted individually. Thus, the method according to the invention provides that, to determine the coefficient of friction, at least one steering actuator assigned to only one wheel of the motor vehicle is controlled as the at least one actuator in order to change a steering angle of the wheel, wherein a torque necessary to change the steering angle is detected and the coefficient of friction is estimated depending on the detected torque. If, for example, when driving straight ahead, the steering angle of only one wheel is changed within a limited range, this does not necessarily affect the overall driving operation, apart from the fact that as the steering angle on one wheel increases, the force decelerating the motor vehicle increases.In this respect, the steering angle is preferably only changed within a limited range by the method, preferably by less than 1°, preferably less than 0.5°. Thus, changing the steering angle does not affect driving operation and, in particular, occurs unnoticed by the occupants of the motor vehicle. However, the force required to adjust the steering angle depends on the coefficient of friction acting between the wheel and the road surface. This allows the currently acting coefficient of friction to be estimated depending on the torque required for the adjustment. Thus, the method according to the invention makes it possible to estimate the coefficient of friction while driving, even during rolling or coasting operation of the motor vehicle, depending on the torque required to set the desired steering angle, without this impairing the comfort of the vehicle occupants.

[0008] In this case, there are two steering actuators, each assigned to only one wheel of the motor vehicle. These two wheels are arranged on opposite longitudinal sides of the motor vehicle. To determine the coefficient of friction, the steering actuators are controlled in such a way that they change the steering angles of the wheels in opposite directions by the same amount. By changing the steering angles in opposite directions, it is ensured that changing the steering angles does not initiate cornering, but rather that the steering angle of one wheel is balanced by the steering angle of the other wheel, for example, and straight-line travel is maintained. This minimizes the effect on the vehicle occupants and the vehicle as a whole and further increases comfort.

[0009] According to a preferred development of the invention, the steering angles of these two wheels are adjusted to set a toe-in or a toe-out, in particular alternating between a toe-in and a toe-out. This ultimately results from the opposing control of the steering angles. In particular, they are adjusted to set a toe-in or toe-out depending on a current driving situation, for example, fast or slow straight-ahead travel, forward or backward movement, or the like.

[0010] According to a preferred development of the invention, the torques required to change the respective steering angle are recorded and compared. By comparing the recorded torques, a statement about a relative friction coefficient difference between the two wheels can be determined without determining or estimating a current friction coefficient, so that a μ-split situation can be easily detected without the intervention of the braking system.

[0011] According to a preferred development of the invention, the torque curve of the respective steering actuator is also recorded, and the recorded steering curves are compared with each other. This offers the advantage that statements regarding the driving condition can be derived even with small changes in the steering angle. For example, if the required steering torque decreases on one side of the vehicle at a low vehicle speed, low grip is detected on that side, and this information is taken into account for controlling the steering and / or braking processes of the vehicle.

[0012] Furthermore, the operating current of the respective steering actuator is preferably recorded, and the required torque is determined based on the recorded operating current. Because the steering angle to be set is specified, the operating current required to achieve this steering angle depends on the coefficient of friction of the road surface. Thus, the required torque can be easily determined based on the operating current. In particular, a characteristic curve or characteristic map of the steering actuator is used for this purpose, which assigns torque values to the operating current.

[0013] Furthermore, it is preferably provided that the method is carried out regularly. This checks, in particular at regular intervals, whether the coefficient of friction of the road surface is changing. This ensures that timely information about the current coefficient of friction and / or a change in the coefficient of friction is available and can be used by the other system of the motor vehicle.

[0014] According to an advantageous further development, the frequency of performing the procedure is preferably varied depending on a recently determined friction coefficient. If, for example, a change in the friction coefficient is detected, the procedure is performed at an earlier or later time than originally planned during the next run to accommodate the potentially increased or reduced risk.

[0015] In particular, the frequency of the test is increased as the coefficient of friction decreases. This ensures that as the driving risk increases, monitoring of the coefficient of friction is increased, and the operation of the vehicle, especially the safety-relevant systems, is adjusted promptly and according to the risk. As the coefficient of friction increases, the frequency is preferably reduced to save energy and prevent wear.

[0016] Furthermore, it is preferably provided that a detected friction coefficient is transmitted to nearby motor vehicles and / or a central database. This also informs other road users about the current friction coefficient, so that their safety systems can be adapted to the driving situation at the relevant location, for example, without having to determine the friction coefficient themselves. For this purpose, the detected friction coefficient is transmitted to neighboring motor vehicles and / or the central database, in particular along with current position information, which is provided in particular by a navigation system of the motor vehicle.

[0017] The device according to the invention with the features of claim 10 is characterized in that it is specifically designed to carry out the method according to the invention. This results in the aforementioned advantages. In particular, at least two wheels of the motor vehicle are each assigned a steering actuator, with the two wheels being arranged, in particular, on opposite longitudinal sides of the motor vehicle, so that the steering angle of at least these two wheels can be individually adjusted by the respective steering actuator.

[0018] Further advantages and preferred features and combinations of features arise in particular from what has been described above and from the claims.

[0019] The invention will be explained in more detail below with reference to the drawings. Fig. 1 a motor vehicle in a simplified plan view and Fig. 2 a flow chart to explain an advantageous method.

[0020] Fig. 1 shows a simplified plan view of a motor vehicle 1, which in the present case has a front wheel axle 2 and a rear wheel axle 3, wherein both wheel axles each have two wheels 4, 5 and 6, 7.

[0021] The wheels 4, 5 of the front wheel axle 2 are designed to be steerable. For this purpose, each of the wheels 4, 5 is assigned its own steering actuator 8, 9, which is designed, in particular, as an electric motor, hydraulic, pneumatic, and / or electromagnetic actuator and is coupled to the respective wheel 4, 5 in such a way that it can individually adjust the steering angle of the respective wheel 4, 5.

[0022] For reasons of clarity, a drive system and a braking system of the motor vehicle 1 are shown in Fig. 1 not shown.

[0023] By means of the method described below, at least one coefficient of friction acting between the motor vehicle 1 and a roadway 10 along which the motor vehicle 1 is moving is determined during operation of the motor vehicle 1.

[0024] Fig. Figure 2 shows a flow chart that begins in step S1 with the commissioning of the motor vehicle 1. At regular intervals, which will be discussed again later, the method is started in step S2. In the following steps S3_8 and S3_9, the actuators 8, 9 are initially controlled to adjust the respective associated wheel 4, 5 by a predetermined steering angle α. The actuators 8, 9 are controlled in such a way that the steering angles are adjusted in opposite directions (+α / -α), so that, for example, the Fig. 1. Alternatively, a toe-in can also be set. Preferably, the steering angles are adjusted several times in opposite directions, for example alternating between a toe-in and a toe-out, in particular sinusoidally. In this case, a current steering angle is adjusted by no more than 1°, in particular by less than or equal to 0.5°. In a subsequent step S4_8 and S4_9, the operating currents of the respective actuator 8, 9 required for each adjustment of the steering angle are monitored and, depending on the respectively detected operating currents and a characteristic map associated with the respective actuators 8, 9, a required actuator torque is determined in a step S5_8 and S5_9. The required torque is understood to be the torque that must be applied by the actuators 8, 9 in order to adjust the respective wheel by the desired steering angle α.

[0025] Subsequently, the necessary moments of the two actuators 8, 9 are compared with each other in a step S6.

[0026] If a subsequent query S7 shows that the torques do not deviate from one another (n), the coefficient of friction acting between wheels 4, 5 and the road surface 10 is determined in a step S8 depending on the determined torques and a characteristic map that was previously created, for example, through tests. The process is then restarted in step S2, if necessary after a predefined period of time. If, however, query S7 shows that the torques of the two actuators 8, 9 deviate from one another (j), a µ-split situation is detected in a subsequent step S9. A measure of the µ-split situation is then checked in a query S10. If the detected split falls below a predefinable limit value (n), the system returns to step S8 and monitoring is restarted.

[0027] However, if the detected µ-split value exceeds the predeterminable limit value in step S10 (j), in a subsequent step S11, a friction coefficient currently acting between roadway 10 and vehicle 1 is determined as a function of the µ-split and the detected moments by means of a characteristic map which, as already described above, was preferably created by previous tests or calculations.

[0028] This coefficient of friction determined in this way is then transmitted to other systems of the motor vehicle, for example driver assistance systems in a step S13, an autonomous driving computer in a step S14, a display for presenting the situation to the driver of the motor vehicle in a step S15 and / or in a step S16 to a central database or neighboring motor vehicles.

[0029] The advantageous method ensures that information for the steerable wheels 4, 5 is derived by the individual wheel actuators or steering actuators, whereby instead of a two-wheel steering, as in the embodiment of Fig. 1, four-wheel steering or all-wheel steering can also be provided. Optionally, the tire pressure of the respective wheel 4, 5, 6, 7 can also be taken into account when determining the currently effective friction coefficient and / or the µ-split situation, provided that appropriate sensors are installed.

[0030] The steering angles are expediently changed synchronously, i.e., simultaneously, to avoid impairing the driving behavior. Information regarding the road surface condition is derived from the steering torque curves, preferably for minor lane changes. For example, if the required torque drops on one side at low vehicle speed, for example, at wheel 5, low grip is detected there, and the vehicle's steering or braking system can take this into account for steering and / or braking operations and control the respective actuators accordingly, as shown in Fig.2. It is also conceivable, in a simplified form, to compare only the torques for the rotation of wheels 4, 5 around the vertical axis with the individual wheel position in order to detect a µ-split situation. In more complex versions, the implemented system can also determine or estimate the specific values based on the machine characteristic map of the respective actuators 8, 9.

[0031] To check the coefficient of friction, staggered or different time intervals can be provided for restarting driving in step S2. For example, with a normal coefficient of friction on all wheels, a shorter interval for executing the method can be provided than with a reduced coefficient of friction. As soon as a deviation is detected, in particular a decrease in the coefficient of friction, for example, due to ice formation on the road surface, the interval is preferably increased, since a critical friction situation has now been identified and driving safety should be increased.

[0032] The advantage over optical systems is that it is independent of current lighting conditions and the contamination of optical inspection equipment. Furthermore, the condition of the road surface does not need to be estimated based on an image, thus saving computing power and energy.

Claims

[1] Method for determining at least one coefficient of friction between at least one wheel (4, 5) of a motor vehicle (1) having a plurality of wheels (4, 5, 6, 7) and a roadway (10) on which the wheels (4, 5) are located, wherein by controlling at least one actuator for influencing a current driving state, a change in the driving state resulting from the control and, as a function of the change, the coefficient of friction are estimated, wherein for determining the coefficient of friction, at least one steering actuator (8, 9) which is assigned to only one wheel (4, 5) of the motor vehicle (1) is controlled as an actuator in order to change a steering angle (α) of the wheel (4, 5), wherein a torque necessary for changing the steering angle (α) is detected and, as a function of the detected torque, the coefficient of friction is estimated, wherein two steering actuators (8, 9), each assigned to only one wheel (4, 5) of the motor vehicle (1), wherein the two wheels (4,5) are arranged on opposite longitudinal sides of the motor vehicle (1), are controlled to determine the coefficient of friction in such a way that they change the steering angles (α) of the wheels (4, 5) in opposite directions to the same extent, and wherein the steering angles (α) are adjusted several times in opposite directions, alternating between a toe-in and a toe-out. [2] Method according to claim 1, characterized by that the steering angles (α) are adjusted several times in opposite directions so that a sinusoidal change in the steering angles (α) is achieved. [3] Method according to claim 1 or 2, characterized by that the steering actuators (8,9) are each assigned to a wheel (4,5) of a steerable front axle (2). [4] Method according to one of the preceding claims, characterized by that the moments necessary to change the respective steering angle (α) are recorded and compared with each other. [5] Method according to one of the preceding claims, characterized bythat a torque curve of the respective steering actuator (8,9) is recorded and the steering curves are compared with each other. [6] Method according to one of the preceding claims, characterized by that an operating current of the respective steering actuator (8,9) is detected and the necessary torque is determined as a function of the operating current. [7] Method according to one of the preceding claims, characterized by that the procedure is carried out regularly, whereby the frequency of implementation is varied depending on a last determined friction value. [8] Method according to claim 7, characterized by that with decreasing friction coefficient the frequency increases and / or with increasing friction coefficient the frequency decreases. [9] Method according to one of the preceding claims, characterized by that a detected friction value is / are sent to at least one motor vehicle (1) located nearby and / or to a central database. [10] Device, in particular control device, for operating a motor vehicle (1) having a plurality of wheels (4, 5), wherein at least one of the wheels (4, 5) is assigned a steering actuator (8, 9) for setting a steering angle (α) of only this wheel (4, 5), wherein at least two wheels (4, 5) are each assigned a steering actuator (8, 9), wherein the two wheels (4, 5) are arranged on opposite longitudinal sides of the motor vehicle (1), and wherein the device is specially designed to carry out the method according to one of claims 1 to 9.

Images