DE102020111520B3 - Method and device for determining the coefficient of friction between a vehicle wheel and the roadway - Google Patents

Abstract

A method is provided for determining the coefficient of friction between a vehicle wheel and the roadway, comprising the steps: reducing a wheel load acting on a first vehicle wheel; Setting a first toe angle on the first vehicle wheel and a second toe angle on a second vehicle wheel mounted on the same vehicle wheel axle such that the first toe angle is greater than the second toe angle and that a lateral force exerted by the first vehicle wheel is a lateral force exerted by the second vehicle wheel cancels; Determining a preliminary coefficient of friction between the first vehicle wheel and the roadway from the quotient of the longitudinal wheel force and the reduced wheel load, the coefficient of friction corresponding to the maximum coefficient of friction; and calculating a coefficient of friction between the first vehicle wheel and the road surface from the quotient of the longitudinal wheel force and the wheel load applied to the first vehicle wheel in the normal state on the basis of the preliminary coefficient of friction.

Description

The present invention relates to a device and a method for determining a coefficient of friction between a vehicle wheel and the roadway on which the vehicle is moving.

The coefficient of friction or coefficient of friction between the roadway and the tire is an important variable for the driving operation of a vehicle. A determination or at least an estimate of this variable which is as precise as possible is advantageous, in particular for autonomously operated vehicles. For example, precise knowledge of the coefficient of friction is of great importance for active driving stability systems in vehicles and is required for optimal execution of electronic stability programs. From the point of view of a vehicle, the maximum achievable coefficient of friction is a parameter that is relatively difficult to measure and also a parameter that changes significantly depending on the type of roadway. Its value depends largely on the nature of the surface on which the vehicle is moving and is mostly estimated. An estimate of the coefficient of friction carried out within the stability programs can, however, only relatively seldom provide a reliable value, namely in the case of high friction slip values which occur when the vehicle is braking or accelerating. In these highly dynamic phases, the coefficient of friction and the slip ratio are determined in order to obtain information about the maximum coefficient of friction.

Due to the fact that a vehicle in road traffic is usually not operated in the range of kinematic limit values, for example in the range of the maximum coefficient of friction, the measured values are used to determine relatively low coefficients of friction, which are well below the actual coefficient of friction. The maximum coefficient of friction is then determined using the course of the ⌷ curve, which describes the relationship between slip and coefficient of friction. In order to obtain an estimated value that is as accurate as possible, it would be advantageous to work with measured values that are as close as possible to the actual maximum coefficient of friction. However, this is seldom the case because, as already mentioned, motor vehicles in road traffic usually do not penetrate into the area of high coefficients of friction. As a result, the coefficients of friction that are determined on the basis of such estimates are often not accurate enough.

In driving situations outside of strong braking, acceleration and steering maneuvers, the information about the road surface coefficient of friction is very imprecise. There are therefore solutions to be able to determine the coefficient of friction in other driving situations in order to increase the availability of current coefficients of friction for dynamic driving stability systems.

For example, if rear axle steering (HAL) is available for individual wheels, the coefficient of friction can be estimated by bringing the rear axle wheels into strong toe-in. The side forces of the rear wheels that occur in this case, which are selected in such a way that they cancel each other out in total, can be measured in a suitable manner in order to determine the coefficient of friction on their basis. However, the problem here is that the vehicle is prone to instabilities during the measurement. During the measurement, the entire rear axle is close to the grip limit or close to the maximum coefficient of friction, so that the vehicle can become unstable even with small steering or evasive maneuvers. Therefore, this procedure is not recommended for security reasons.

From block letters DE 10 2018 200 180 A1 a system for determining the coefficient of friction between wheel and road is known in which the longitudinal wheel force acting on at least one vehicle wheel is increased by means of a vehicle unit influencing the longitudinal dynamics of the vehicle and / or the wheel contact force is reduced. Due to the increased longitudinal wheel force or reduced wheel contact force, the vehicle wheel moves in a higher area of the wheel slip coefficient of friction curve, which allows the maximum available coefficient of friction to be determined more precisely.

In block letters DE 10 2010 014 564 A1 describes a method for determining a coefficient of friction for a vehicle on the basis of detecting a force required for setting a first predetermined steering angle of a first tire of the vehicle and a second predetermined steering angle of a second tire of the vehicle.

In block letters DE 10 2016 220 692 A1 is a method for determining a coefficient of friction between the wheels of a motor vehicle and a roadway, in which steering angles of the same magnitude are set in opposite directions on wheels of a steerable rear axle for the wheels of the rear axle while the vehicle is driving. The change in speed and / or change in the speed gradient resulting from this steering angle setting on at least one of the rear wheels is determined and the coefficient of friction is determined from this.

Pamphlet DE 10 2018 205 904 A1 discloses 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, with at least one actuation of an actuator, a steering angle of the wheel is changed, as a result of which a current driving state is influenced and a change in the driving state resulting from the activation and the change in the coefficient of friction are estimated as a function of the change.

Against this background, the invention is based on the object of providing a method for determining the coefficient of friction between tires and road, by means of which the coefficient of friction can be determined as precisely as possible in all driving states without the measurement method itself representing a safety risk for the operation of the vehicle.

This object is achieved by means of the method for determining the coefficient of friction between a vehicle wheel and the roadway and by means of the corresponding device according to the independent claims. Further preferred embodiments can be found in the dependent claims.

It is known that the coefficient of friction μ, also referred to as a force fit, is a function of the slip. The general basic form of the traction-slip curve is also known, which initially increases linearly, then flattens slightly to reach a maximum and then decreases again. For the optimal setting of the chassis of the vehicle, the maximum coefficient of friction µ _max is always of interest, which characterizes the strength of the maximum longitudinal wheel force (also referred to as wheel circumferential force) F _{RL, max} = µ _max * F _z , which is transmitted to the road surface in a non-positive manner can, where F _Z denotes the wheel load, which is also referred to as the wheel contact force and acts on the vehicle wheel under consideration. Although the terms coefficient of friction and coefficient of friction are used synonymously in the specialist literature, the term coefficient of friction is used in the following description for the maximum coefficient of friction.

In various embodiments, a method for determining the coefficient of friction between a vehicle wheel and the roadway is provided. In particular, the method is used to determine the current coefficient of friction between a vehicle wheel and the roadway. Strictly speaking, the coefficient of friction between a vehicle wheel and the road surface means the coefficient of friction between the corresponding tire and the road surface, which significantly determines the dynamic limits of a vehicle.

In a first step, the method includes reducing a wheel load acting on a first vehicle wheel. Depending on the design of the vehicle and the position of its center of gravity, a second vehicle wheel, which is mounted on the same vehicle wheel axle as the first vehicle wheel, takes on part of this load. In other words, according to the invention, the wheel load distribution on a vehicle wheel axle is changed in such a way that the first vehicle wheel is relieved and the second vehicle wheel is thereby additionally loaded. Therefore, while the method according to the invention is being carried out, the wheel load acting on the second vehicle wheel is greater than the wheel load acting on the first vehicle wheel. In the normal basic state of the vehicle, the two wheel loads are usually approximately the same. The first and second vehicle wheels are vehicle wheels which are mounted on the same vehicle wheel axle, that is to say front or rear wheels. With regard to the correspondence between the vehicle side (left or right) and the first and second vehicle wheels, the method is symmetrical: The first vehicle wheel can be the left or the right vehicle wheel, so that the second vehicle wheel is accordingly the other vehicle wheel with the same wheel axle is involved.

In a further step, the method includes setting a first toe angle (also referred to as a slip angle) on the first vehicle wheel and a second toe angle on the second vehicle wheel such that on the one hand the first toe angle is greater than the second toe angle and that on the other hand a side force exerted by the first vehicle wheel cancels a side force exerted by the second vehicle wheel. Setting the toe angle can in particular mean changing an already existing toe angle, for example an enlargement of a toe-in or toe-out that exists in the basic state of the vehicle.

In the context of the method according to the invention, the first toe angle of the first vehicle wheel, which is the unloaded vehicle wheel, is set to at least one relatively high angle value (in relation to the second toe angle) so that the first vehicle wheel can be brought close to the coefficient of friction. The second toe angle, on the other hand, is set to a relatively small angle value (in relation to the first toe angle), so that the second rear wheel is operated with a relatively small coefficient of friction (in relation to the first vehicle wheel). The individually different settings of the toe angles of the first and second vehicle wheels can be achieved in the case of a rear axle with a wheel-specific rear axle steering (HAL), by means of which the left and right rear wheels can be steered individually. According to the invention, the toe angles of the vehicle wheels are basically set in the same direction, that is to say either both in toe-in or both in toe-out, but with a different angle value in each case.

Furthermore, the toe angles of the first and second vehicle wheels are set in such a way that overall the side forces acting on the two vehicle wheels cancel each other out. A side force acting on a vehicle wheel can be understood to mean a force component which acts along the associated vehicle wheel axis. This force component can be determined by means of suitable sensors, for example by means of strain gauges on axle components or by changing the operating parameters of servomotors, such as the power consumption that is used in the axle steering, in particular the rear axle steering, for setting the steering angle of the vehicle wheels. Since, in total, the lateral forces of the vehicle wheels cancel each other out due to their suitable setting, the associated vehicle wheel axle remains far away from the coefficient of friction, so that the vehicle remains in a stable state in terms of driving dynamics. This driving dynamically stable state is largely maintained by the overloaded vehicle wheel, which compensates for the wheel load shift away from the first vehicle wheel and is operated with a very low coefficient of friction.

In a further step, the method includes determining a preliminary coefficient of friction between the first vehicle _{wheel and the road surface from the quotient of the longitudinal wheel force F RL} and the wheel contact force F _z . The longitudinal wheel force F _RL can be determined from the square root of the sum of a first force component F _x squared and a second force component F _y squared when the vehicle wheel is not aligned parallel to the vehicle longitudinal axis. The first force component F _x can correspond to the x force component which acts along the longitudinal axis of the vehicle. The first force component F _x can be determined from the portion of the sum of kinematic factors that is attributable to the corresponding vehicle wheel. The kinematic factors can take into account the longitudinal acceleration of the vehicle, the gradient or gradient, the current braking force distribution and the driving resistance. Alternatively, the first force component F _x can be calculated from the drive torque applied to the corresponding vehicle wheel and the toe angle or steering angle. The second force component F _y can correspond to the y force component, which acts in a direction perpendicular to the longitudinal axis of the vehicle and thus corresponds to the side force already mentioned.

The momentary coefficient of friction can be calculated from the forces acting on the first vehicle wheel for each set first toe angle. By scanning the course of the coefficient of friction by sequentially setting different first toe angles, the traction-slip curve can in principle be measured, from which a more precise preliminary coefficient of friction can be determined. The coefficient of friction is referred to as provisional because it corresponds to a non-natural driving condition and is converted in a further step to the natural driving condition without vehicle wheel relief. In other words, within the scope of the inventive method, one of the vehicle wheels of a vehicle wheel axle is briefly relieved and used as a probe to scan the traction-slip curve, whereas the other vehicle wheel of the same vehicle wheel axle acts as an opposite pole and stabilizes the vehicle wheel axle in terms of driving dynamics. As a result, the vehicle remains safe in terms of driving dynamics and can, if necessary, carry out acceleration and / or steering maneuvers.

In a final step, the method includes calculating a coefficient of friction between the first vehicle wheel and the road surface from the quotient of the longitudinal wheel force and the wheel load applied to the first vehicle wheel in the normal state based on the preliminary coefficient of friction. In order to determine the coefficient of friction with normal wheel load of the first vehicle wheel and thus with the usual wheel load distribution from the provisional coefficient of friction determined on the basis of the relieved first vehicle wheel, empirically determined correction factors can be used which indicate the ratio in which, depending on the wheel load reduction, the provisional coefficient of friction for the coefficient of friction is available. The correction factors can be determined by means of corresponding measurements on a test bench environment.

In a further embodiment of the method, reducing the wheel load acting on the first vehicle wheel can include reducing the distance between the center point of the vehicle wheel and the vehicle chassis. The relieving of the load on the first vehicle wheel can include a lifting of the first vehicle wheel by means of an active damping element through a corresponding control by an active damping controller (ADR). The active damping element can consequently be set up as an actuator and enable the vehicle wheel to be raised vertically, that is to say vertically to the longitudinal and transverse axes of the vehicle. Relieving the load on the first vehicle wheel does not require any adaptation of the longitudinal kinematics of the vehicle, such as braking or acceleration. In particular, the deliberately induced relief of the first vehicle wheel is not based on a pitching and / or rolling movement of the vehicle, so that the relief of the first vehicle wheel can be carried out without having to adjust the drive torques on the other wheels of the vehicle. In the case of a front-heavy vehicle The method can preferably be carried out in relation to the rear wheels. In general, however, the method can be carried out both in relation to the front wheels and the rear wheels of a vehicle.

In a further embodiment of the method, the setting of the first toe angle can include that the first toe angle is set to an end value on the basis of a start value. Between the start value and the end value there is a critical value at which the coefficient of friction is maximum. This value corresponds to the preliminary coefficient of friction. The starting value can correspond to the standard set first toe angle of the first vehicle wheel in the basic state or to a value that is higher. The adjustment of the first toe angle from the start value to the end value can take place continuously or at predetermined angular intervals, for example adapted to the slope of the curve, with the instantaneous coefficient of friction being calculated at the same time. This allows the force-fit / slip curve to be scanned. As soon as the maximum value of the coefficient of friction has been determined reliably and precisely enough, that is, when the maximum value has been reached and the values become smaller again, the method can be sniffed out.

In a further embodiment of the method, the second toe angle of the second vehicle wheel can be set such that the second vehicle wheel is operated at a coefficient of friction which is at least 50%, preferably at least 80%, even more preferably at least 90% lower than that prevailing coefficient of friction in the normal state. As a result, the second vehicle wheel is operated far away from the coefficient of friction or the adhesion limit and exerts a stabilizing effect on the associated vehicle wheel axle. In other words, with regard to the utilization of the coefficient of friction on the second vehicle wheel, a low utilization takes place (e.g. in the range of 10% of the coefficient of friction), whereas a high utilization takes place on the first vehicle wheel (e.g. in the range of 90% to 100% of the coefficient of friction). It should be noted, however, that the almost complete utilization of the coefficient of friction on the first vehicle wheel takes place in an "incorrect", artificially induced driving state, which does not correspond to the natural basic state of the vehicle, in which the two vehicle wheels under consideration are subjected to the same wheel load as a first approximation are. In other words, within the scope of the method according to the invention, the coefficient of friction in a “wrong” driving state is determined in order to determine the coefficient of friction in the “correct” or normal driving state of the vehicle.

In a further embodiment of the method, the set first toe angle and the set second toe angle can be either positive or negative with respect to the longitudinal axis of the vehicle. In other words, a toe-in or a toe-out can be set on the corresponding vehicle wheel axle for the duration of the implementation of the method according to the invention, the first toe angle differing from the second toe angle, as already explained.

In a further embodiment of the method, this can be carried out several times during a journey, with the vehicle wheels being exchanged with regard to their role as the first and second vehicle wheel when the method is carried out before when the method is subsequently carried out. By alternately using the left and right vehicle wheel of a corresponding wheel axle as the first vehicle wheel, which is exposed to larger toe angles (or to which the toe angle already set during normal operation of the vehicle is increased), the wear on the vehicle wheels can be kept in balance.

The method according to the invention can generally be carried out at fixed time intervals, for example every 5 minutes, at fixed route intervals, for example every 10 km, or else depending on the situation. The situation-dependent implementation can take place, for example, on the basis of an evaluation of the environmental parameters and can be triggered, for example, by precipitation (can be derived from rain sensor data or camera data), wind (can be derived from the dynamics of the chassis) or other events of this type. Furthermore, the implementation can generally be triggered if there is a suspicion, derived from data from the vehicle camera, that the road surface is changing. The condition of the road surface can be determined or estimated, for example, on the basis of sensor data from the vehicle (e.g. vehicle camera, sound sensor in the wheel arch, rain sensor), operating parameters of vehicle parts (e.g. condition parameters of the damping system) and / or route information based on navigation. Information from ongoing navigation can be used to determine the geographic embedding of the route, i.e. to determine whether, for example, leaves, split, bumps or deer can be expected on the route. Furthermore, on the basis of the navigation-supported information, it can be prevented that the method is carried out on unsuitable route sections, for example on very winding routes or on routes whose roadway property only lasts for a short distance.

In a further embodiment of the method, the method can be carried out when an evaluation of environmental parameters of the vehicle shows that a hazard potential threshold value has been exceeded, the environmental parameters including traffic volume and / or weather conditions and / or route and / or route characteristics. In order to determine the hazard potential value, each currently recorded environmental parameter can be assigned a hazard potential number which reflects the hazard potential. As a result, the method can be carried out in particular when, in a particular risk potential situation (e.g. deer crossing in a wooded area where the road has a slope and where it has rained shortly before), the most accurate possible knowledge of the current coefficient of friction is of great importance On the one hand, to possibly warn the driver accordingly and, on the other hand, to optimally adapt the vehicle systems to a driving environment classified as critical.

The hazard potential, which can be determined by the volume of traffic, weather conditions, route, etc., can also be monitored continuously. The method according to the invention can be activated as a function of the identified hazard potential. This ensures that the process is not activated in unfavorable driving situations, e.g. when turning into a curve, and that the process is not active in an avoidable dangerous situation. This embodiment can be used particularly preferably in autonomous vehicles.

In the event of an unforeseen dangerous situation during the measurement, the changed toe angles can be returned to the basic state as quickly as possible and the corresponding vehicle wheels can be loaded again synchronously. This will return the vehicle to its normal driving condition.

In further embodiments, a device for determining the coefficient of friction between a vehicle wheel and a road surface is provided, which is set up to carry out the method according to the invention in a vehicle. The device can be in the form of a functional module or control device which is embedded in the vehicle electronics and controls the required vehicle units (e.g. rear axle steering and active damper elements) by means of appropriate control signals so that the method is carried out. The invention also relates to a vehicle having said device.

The features mentioned above and those yet to be explained below can be used not only in the respectively specified combination, but also in other combinations or on their own, without departing from the scope of the present invention.

Further advantages and embodiments of the invention emerge from the description and the accompanying drawing.

1 illustrates the possible configuration of the vehicle wheels which can be used for carrying out the method according to the invention.

Using the in 1 sketched vehicle shown in a top view 1 a possible course of the method according to the invention is to be explained below by way of example. In the exemplary scenario, the vehicle is moving 1 straight ahead what by the arrow 2 is indicated. The one by the arrow 2 The direction shown also corresponds to the longitudinal axis of the vehicle. The white cross 3 gives the center of gravity of the vehicle 1 at. As a result, the in 1 sketched vehicle 1 around a vehicle with a front-cumbersome center of gravity. With the sketched vehicle 1 the method for determining the riding value between a vehicle edge and the roadway is carried out on the rear axle on which the right rear wheel is located 5 and the left rear wheel 6th are stored. The choice of the rear axle for carrying out the method is advantageous for vehicles with a front-heavy center of gravity. The right front wheel 4th and the left front wheel 7th or their kinematics are not influenced during the process. On the right rear wheel 5 and on the left rear wheel 6th is the longitudinal axis of the vehicle 8th drawn. Also on the right rear wheel 5 the right longitudinal axis of the tire 9 and on the left rear wheel 6th the left longitudinal axis of the tire 10 drawn. The angle between the vehicle's longitudinal axis 8th and the right longitudinal axis of the tire 9 corresponds to the first toe angle α of the first, in this case the right rear wheel 5 and the angle between the vehicle's longitudinal axis 8th and the left longitudinal axis of the tire 10 corresponds to the second toe angle β of the second, in this case the left rear wheel.

At the beginning of the method according to the invention, a vehicle degree is relieved. In the example shown, the right rear wheel is 5 relieved by lifting it by means of the associated active damper element, i.e. moving it in the opposite direction to the effective direction of the wheel load, so that the corresponding wheel load is reduced. By relieving the load on the right rear wheel 5 also becomes the left rear wheel 6th burdened to a greater extent. The first toe angle then becomes α and the second toe angle β adjusted. In the example shown, the two rear wheels 5 , 6th a toe-in is set, the first toe angle α being greater than the second toe angle β . To determine the preliminary coefficient of friction, the first toe angle α runs through values between a start value and an end value, the coefficient of friction being calculated continuously or at predetermined angular intervals during this time. This enables the course of the adhesion-slip curve to be recorded. The area around the maximum of the curve, which corresponds to the preliminary coefficient of friction, is particularly important here. As soon as this area of the curve has been reproduced and the provisional coefficient of friction has been determined accordingly, the right rear wheel becomes 5 returned to its original load condition and the first toe angle α and the second toe angle β will also assume their original values corresponding to the basic condition of the vehicle.

From the provisional coefficient of friction determined during the measurement, the actual coefficient of friction can then be determined, taking into account a corresponding correction factor, which in the case of one on the right rear wheel 5 usually applied wheel load is present.

It should be noted that only one exemplary embodiment of the method according to the invention has been described here to illustrate it. The method can also be carried out differently, for example by on the two rear wheels 5 , 6th a toe-out is set instead of a toe-in. The method according to the invention can also be carried out with the participation of the front wheels 4th , 7th carried out, especially on vehicles 1 with a tail-heavy center of gravity 3 .

Claims (9)

Method for determining the coefficient of friction between a vehicle wheel and the road, comprising: Reducing a wheel load acting on a first vehicle wheel; Setting a first toe angle on the first vehicle wheel and a second toe angle on a second vehicle wheel mounted on the same vehicle wheel axle in such a way that i) that the first toe angle is greater than the second toe angle, and ii) that a side force exerted by the first vehicle wheel cancels a side force exerted by the second vehicle wheel; Determining a preliminary coefficient of friction between the first vehicle wheel and the roadway from the quotient of the longitudinal wheel force and the reduced wheel load, the coefficient of friction corresponding to the maximum coefficient of friction; and Calculating a coefficient of friction between the first vehicle wheel and the road surface from the quotient of the longitudinal wheel force and the wheel load applied to the first vehicle wheel in the normal state on the basis of the preliminary coefficient of friction. Procedure according to Claim 1 wherein reducing the wheel load on the first vehicle wheel comprises decreasing the distance between the center point of the vehicle wheel and the vehicle chassis. Procedure according to Claim 1 or 2 The setting of the first toe angle can include that the first toe angle, starting with a start value, is adjusted to an end value, with a critical value between the start value and the end value at which the determined preliminary coefficient of friction is at a maximum. Method according to one of the Claims 1 until 3 , the second toe angle of the second vehicle wheel being set in such a way that the second vehicle wheel is operated at a coefficient of friction which is at least 50%, preferably at least 80% lower than the coefficient of friction prevailing in the normal state. Method according to one of the Claims 1 until 4th , wherein the adjusted first toe angle and the adjusted second toe angle are either positive or negative. Method according to one of the Claims 1 until 5 , the method being carried out several times during a journey, the vehicle wheels being exchanged with regard to their role as the first vehicle wheel and the second vehicle wheel when the method is carried out before the method is carried out. Method according to one of the Claims 1 until 6th The method is carried out when an evaluation of environmental parameters of the vehicle shows that a hazard potential threshold value has been exceeded, the environmental parameters including traffic volume and / or weather conditions and / or route and / or route characteristics. Device for determining the coefficient of friction between a vehicle wheel and a roadway, which is set up according to one of the Claims 1 until 7th perform. Vehicle with a device according to Claim 8 .

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