DE102016203545A1 - Method of determining road grip classes - Google Patents

Abstract

Method for determining road grip classes from measured values of a circumferential force on at least one vehicle wheel comprising the following steps: a) determining pairs of values of the circumferential force acting on the vehicle wheel and a tire slip generated by this circumferential force on the vehicle wheel, b) determining values for a high friction roadway, c) Calculating tire slip stiffness values, d) defining a road grade class indicative of a high friction roadway, and e) classifying an actual tire slip stiffness value as a value indicating a high friction roadway.

Description

The invention relates to a method for determining road grip classes from measured values of a circumferential force on at least one vehicle wheel of a vehicle traveling on a roadway and of the wheel slip caused by this circumferential force.

For an improved function of a modern driver assistance system, such as an emergency braking system, it is necessary to estimate a coefficient of adhesion between the tire and the road.

During manual driving, the estimation of the coefficient of adhesion is the responsibility of the driver. An essential part of the driving task is to estimate the road condition and to adapt the driving style or style of driving. With the introduction of highly automated systems and driving functions, the driving task is gradually transferred from the driver to the vehicle. The essential basis for this is to determine the driving environment or the driving conditions as exactly as possible, to describe them and make them available to the system. The maximum adhesion coefficient between the tire and the road surface determines the physical limit for the vehicle's realizable driving dynamics. Thus, the information about the adhesion coefficient forms an essential part of the environment representation. With regard to automated driving reliable information about the road condition or derived from the available maximum adhesion coefficient is an indispensable amount of information to secure the driving function. But even in the area of so-called Driver Assistance Systems and Advanced Driver Assistance Systems, knowledge of the available maximum coefficient of force coefficient results in a significant safety benefit.

From the DE 40 10 507 C1 is a method for monitoring the adhesion between the road and tires of driven vehicle wheels known. In this method, it is assumed that a certain coefficient of friction with associated maximum braking torque can be assigned to different initial gradients. Thus, the wheel slip λ and at the same time an effective Radumfangskraft the driven wheels during stationary driving conditions continuously determined. These value pairs from the determined Radumfangskraft and the wheel slip are stored to form a Radumfangskraft wheel slip characteristic field, so that so that the linear part of the slip characteristic is known. The knowledge of the curve increase of the currently effective slip characteristic also provides the knowledge of the maximum effective coefficient of friction or the magnitude of the maximum wheel peripheral force possible in this driving state, since according to this known method there is a substantially fixed relationship between the linear rise and the maximum curve of the slip characteristics.

Also the WO 2014/199328 A1 describes a method for friction coefficient estimation and proceeds from the recognition that slip characteristics with a low maximum adhesion coefficient (also called μ-peak) leave the linear region at a lower slip value than is the case for slip characteristics with a higher maximum adhesion coefficient.

Furthermore, also describes the DE 10 2012 217 772 A1 a method for determining the maximum adhesion coefficient by also determining the initial slope of a slip characteristic and from this the maximum adhesion coefficient is determined. For this purpose, first the current traction coefficient on the vehicle wheel and the associated slip is determined by measurements and from this the value of the initial slope is calculated. By comparison with the reference origin line, the range for a maximum adhesion coefficient is determined.

In the area of the initial slope of a slip characteristic, both tire forces and tire slip are very low. In order to obtain a sufficient degree of definition in stationary or quasi-stationary states, a robust and, above all, well-resolving signal is required both for the braking torque or the braking force and for the slip. If the scattering is very large and / or the resolution is low only for one of the two signals, it is no longer possible to achieve an exact and, above all, reproducible result.

Moreover, from the DE 10 2004 044 788 B4 a method of friction coefficient determination between a tire of a vehicle and a roadway is known in which the wheel speed and the absolute speed of the vehicle are measured, from this wheel speed and the absolute speed of the slip is determined, then a force acting on the vehicle driving or braking torque is measured and Finally, a coefficient of friction between the tire and the roadway is determined by means of an algorithm, are taken into account as the input of the slip and the drive or braking torque.

The object of the invention is to provide a method for determining Fahrbahngriffigkeitsklassen, with which the roadway on which the vehicle is moved, can be classified in terms of the coefficient of friction and the information required for this purpose not only during an acceleration process of the vehicle are generated, but in particular during an engine drag torque operation or braking operation of individual wheels of the vehicle.

This object is achieved by a method for determining road grip classes with the features of patent claim 1.

• a) Ermitteln von Wertepaaren jeweils aus dem Wert einer am Fahrzeugrad wirkenden Umfangskraft und einem durch diese Umfangskraft erzeugten Reifenschlupf an dem Fahrzeugrad, indem aus einer gemessenen Raddrehzahl des Fahrzeugrades und der gemessenen Fahrzeuggeschwindigkeit der Reifenschlupf bestimmt und die Umfangskraft aus einem an dem Fahrzeugrad oder aus einem an einer Fahrzeugachse des Fahrzeugs wirkenden Motorantriebsmoment, aus einem an dem Fahrzeugrad oder aus einem an einer Fahrzeugachse des Fahrzeugs wirkenden Motorschleppmoment, aus einem Bremsmoment an dem Fahrzeugrad oder aus einem an einer Fahrzeugachse des Fahrzeugs wirkenden Bremsmoment bestimmt wird,
• b) Bestimmen einer eine Hochreibwertfahrbahn anzeigende Wertegruppe aus den ermittelten Wertepaaren von Umfangskraft und Reifenschlupf, wobei im Ermittlungszeitpunkt dieser Wertepaare von Fahrdynamikgrößen des Fahrzeugs größer als ein vorgegebener Wert bei gleichzeitig fehlenden Eingriffen eines Fahrstabilitätssystems und/oder von optischen Sensoren des Fahrzeugs und/oder von externen Datenquellen des Fahrzeugs eine Hochreibwertfahrbahn angezeigt wird bzw. werden,
• c) Berechnen von Reifenschlupfsteifigkeitswerten aus den Wertepaaren durch Quotientenbildung von Umfangskraft und Radschlupf,
• d) Definieren einer eine Hochreibwertfahrbahn anzeigenden Fahrbahngriffigkeitsklasse durch Bestimmen einer mittleren Reifenschlupfsteifigkeit aus den Reifenschlupfsteifigkeitswerten der Wertepaare der Wertegruppe und der Streuung dieser Wertepaare um die mittlere Reifenschlupfsteifigkeit mittels eines statistischen Auswerteverfahrens, und
• e) Klassifizieren eines aktuellen Reifenschlupfsteifigkeitswertes als ein eine Hochreibwertfahrbahn anzeigender Wert, falls dieser Wert in der die Hochreibwertfahrbahn anzeigenden Fahrbahngriffigkeitsklasse liegt, andernfalls als ein eine Niedrigreibwertfahrbahn anzeigender Wert.

In the method according to the invention for determining road grip classes from measured values of a circumferential force on at least one vehicle wheel of a vehicle traveling on a roadway and of the tire slip caused by this circumferential force, the following steps are carried out:

• a) determining pairs of values in each case from the value of a circumferential force acting on the vehicle wheel and a tire slip on the vehicle wheel generated by this circumferential force by determining the tire slip from a measured wheel speed of the vehicle wheel and the measured vehicle speed and determining the circumferential force from one on or off the vehicle wheel A motor drive torque acting on a vehicle axle of the vehicle is determined from a motor drag torque acting on the vehicle wheel or from an engine drag torque acting on a vehicle axle of the vehicle, a brake torque on the vehicle wheel or a brake torque acting on a vehicle axle of the vehicle.
• b) determining a value group indicating a high friction road from the determined value pairs of circumferential force and tire slip, wherein at the time of determination of these pairs of vehicle dynamics values of the vehicle greater than a predetermined value with simultaneous lack of intervention of a driving stability system and / or optical sensors of the vehicle and / or external Data sources of the vehicle a Hochreibwertfahrbahn is or will be,
• c) calculating tire slip stiffness values from the value pairs by quotient of circumferential force and wheel slip,
• d) defining a road grade class indicating a high friction roadway by determining a mean tire slip stiffness from the tire slip stiffness values of the value pairs of the value group and the dispersion of these pairs of values around the average tire slip stiffness by a statistical evaluation method, and
• e) classifying an actual tire slip stiffness value as a value indicative of a high friction roadway value if that value is in the roadway grade class indicating the high friction roadway, otherwise as a value indicative of a low friction roadway.

In this method according to the invention, a comparison standard indicating a high friction value is first generated by generating a data series from tire slip stiffnesses, calculated as quotients of value pairs of circumferential force of the vehicle wheel and associated wheel slip, during the detection of a high friction roadway. This data series is subjected to a statistical evaluation method for determining an average tire slip stiffness and calculated based on the average tire slip stiffness, the associated value dispersion, so determines the spread. In order for a Hochreibwertfahrbahn indicating lane grip class is defined with a coefficient of friction μ of preferably μ ≥ 0.5. Thus, all currently determined tire slip stiffnesses with values within the range of average tire slip stiffness indicate such a high friction value, in the other case, when these values are outside this roadway grade, a low friction value.

The circumferential force of a vehicle wheel can be determined in this method according to the invention both during an acceleration process and during a braking operation of individual vehicle wheels or axles, in particular during an engine towing operation. As a result, the scope of the available information is increased, whereby the accuracy of the statistical evaluation method and thus also the accuracy of the determination of the road grade class are increased.

By comparing the current tire slip stiffnesses against the comparison standard indicating a high friction value, tire-specific relationships, parameters and, in particular, changes can be detected and eliminated, since the reference standard is continuously adapted when a reference signal is present, thereby realizing a self-learning behavior of the system via the tire life, in particular in the Correlation of tire changes and tire wear, even in changed environmental conditions.

The behavior of the vehicle control system in the determination times of these value pairs is observed in order to determine a value group from the determined wheel slip wheel torque value pairs that indicates a high friction roadway. In fact, it can be deduced from the behavior of the vehicle control system whether the vehicle is moving on a high friction road or a low friction road.

The display of a Hochreibwertfahrbahn can be done using various data sources. For example, from driving dynamics of the vehicle, by exceeding a predetermined acceleration value or a predetermined delay value such a reference signal is generated. This information can also be provided by evaluating a road condition detected by a camera as an optical sensor or a different type of optical sensor, for example a laser sensor scanning the roadway, if, for example, a dry roadway is detected. In addition, external data sources which make available, for example, local weather data or road condition data (eg a backend connection, car-to-X communication or also dynamic safety data) can be used by networking to generate such a reference signal.

• d1) aus jeweils einer Datenreihe von zeitlich aufeinanderfolgenden Wertepaaren aus Umfangskraft und Radschlupf, deren Reifenschlupfsteifigkeitswerte eine Niedrigreibwertfahrbahn anzeigen, eine Anfangssteigung anhand einer Ausgleichsgeraden als linearer Teil der Schlupf-Umfangskraft-Kennlinie ermittelt wird, und
• d2) die Steigung einer jeden Ausgleichsgeraden als mittlere Reifenschlupfsteifigkeit einer eine Niedrigreibwertfahrbahn anzeigenden Fahrbahngriffigkeitsklasse mit einer mittels eines statistischen Auswerteverfahrens bestimmten Streuung der Wertepaare um die ermittelte Ausgleichsgerade definiert wird.

An advantageous embodiment of the invention provides the following method steps, in which:

• d1) from each of a data series of temporally successive value pairs of circumferential force and wheel slip whose tire slip stiffness values indicate a low friction line, an initial slope is determined using a straight line as a linear part of the slip-circumferential force characteristic, and
• d2) the slope of each equalizer straight line is defined as average tire slip stiffness of a road grade class indicative of a low-friction roadway having a distribution of the value pairs about the determined equalization line determined by a statistical evaluation method.

With these method steps d1 and d2, the tire slip stiffness values lying outside the road grip class indicating a high friction roadway are classified by the generation of further roadway grip classes. For this purpose, a data series of temporally successive pairs of values is used by determining, for the tire slip stiffness values by means of a statistical evaluation method, the slope of a compensation straight line in the circumferential force-slip diagram representing the mean tire slip stiffness and the associated scattering. Thus, for each equalization straight line generated in this way, a low-friction roadway class indicative roadway class is defined whose mean tire slip stiffness corresponds to the slope of the equalization straight line. Thus, it is possible to define a road grip class indicating a snow lane and a lane grip class indicating an icy lane. Thus, the tire slip stiffness values may be assigned to one of these road grade classes indicating, for example, a snow track or an icy road, if it is not in the road grade class indicating a high friction road.

A particularly preferred development of the invention consists in that, for the classification of a tire slip stiffness value indicating a low-friction-value roadway, the curve shape approximated by a mathematical compensation calculation to the associated data series is taken into account. Thus, for example, a curvature points to a snow road, while discontinuities, for example, sudden tearing point to an icy roadway.

Furthermore, it is particularly advantageous if, according to further development, the method steps d, d1 and d2 are carried out only in the presence of magnitude-positive slip gradients and driving states of the vehicle without vehicle control system interventions. This ensures that hysteresis phenomena are excluded by the high and return of the tire slip.

According to a further preferred development of the invention, in the implementation of method steps d, d1 and d2, the classification of the tire slip stiffness values is plausibilized on the basis of the intervention behavior of a vehicle control system by making plausibility checks on a road grade class indicative of high and low vehicle lashes in the absence of vehicle control system interventions above predetermined longitudinal and / or lateral accelerations Control systems interventions below predetermined longitudinal and / or transverse accelerations, a low friction roadway indicating lane grip class is plausibility.

According to a further advantageous embodiment of the invention, the method steps d, d1 and d2 are carried out both during acceleration and during a deceleration of the vehicle, wherein for the acceleration case and the deceleration case, the average tire slip stiffness are equated in the same roadway grip class. This is possible because the initial slope in the tire slip-circumferential force characteristic is the same for the drive case and the brake case. Thus, the accuracy of calculating the mean tire slip stiffness can be improved by multiple nodes.

Furthermore, it is advantageous if the amounts of the circumferential forces and the slip are used for the calculation of the tire slip stiffness according to further development. This ensures that the evaluation can take place in the same quadrant of the Radschlupf-circumferential force diagram and thus increases the point density by superposition of drive and braking or Schleppmomentenfälle. This makes the statement more accurate and increases availability.

According to a further advantageous embodiment of the invention, the method steps d, d1 and d2 within a predetermined Slip window between a slip value λ _min and a slip value λ _max performed, take place at a slip greater than λ _max control system interventions of a vehicle control system and at the same time such control system interventions are used for plausibility of Fahrbahngriffigkeitsklassifikation. In the slip region defined thereby, the slip is determined continuously and thus the slope of the regression line defining a road grip class can be determined more accurately, because if the absolute values of wheel torque and wheel slip are too small taking into account a finite measurement accuracy, the quotient formation for the determination of the slip stiffness becomes inaccurate and if the slip values are too high the waveform but also possible control system interventions would adversely affect the classification.

Finally, it is advantageous if, for further development according to the invention, tire slip is detected for both vehicle axle wheels subjected to circumferential force on a vehicle axle, and the tire slip stiffness values are determined by the quotient of the circumferential force and the tire slippage of the most slipping vehicle wheel or the mean value all Wheels of a vehicle axle is calculated. In an axial loading of drive or braking torque, the torque is distributed in a first approximation evenly on the drive wheels. The slip value required to determine the slip stiffness quotient is either averaged over all applied wheels or the higher slip value of the two wheels is used for the calculation.

According to a last advantageous embodiment of the invention, the averaging and / or filtering of the value pairs of circumferential force and tire slip takes place over a full wheel revolution or over integer multiples of individual wheel revolutions.

The method according to the invention is suitable for use in vehicles which have an engine control unit, a brake control unit, a speed detection device for the vehicle wheels, a vehicle dynamics control system and a device for determining a wheel torque or a braking torque as a circumferential force of the vehicle wheel.

The method according to the invention will be described below on the basis of exemplary embodiments with reference to the attached figures. Show it:

1 a wheel torque-wheel slip diagram with entered measured values,

2 a tire slip stiffness time plot with the readings 1 .

3 a wheel torque-wheel slip diagram with registered and classified with regard to the road condition measured values,

4 a tire slip stiffness time plot with the readings 3 .

5 a wheel drag torque-wheel slip diagram with measured values acquired and recorded during drag torque operation,

6 a Radomoment-Radschlupf diagram for determining a slip offset, and

7 a Radomoment-Radschlupf diagram with equal wheel slip stiffnesses for the drive and the brake case.

The detection of a road grip (coefficient of friction) according to the method according to the invention is carried out by the determination of road grip classes from the tire slip stiffness of a vehicle wheel of a vehicle and the Radumfangskraft-Radschlupf characteristic of the vehicle wheel. The inventive method allows the differentiation between a Hochreibwertfahrbahn with a coefficient of friction of, for example, ≥ 0.5 and a low friction line, wherein a low friction line has a low grip, such as in snow or ice. The coefficient of friction of snow is approximately <0.5, that of ice is approximately ≤ 0.2, the corresponding value of dry asphalt is approximately at 1.0.

The wheel peripheral force (hereinafter referred to as circumferential force) includes both the drive torque during an acceleration phase of the vehicle and the braking torque during use of the friction brake or a Rekuperationsvorganges for energy recovery, for example via an electric motor, or during a drag torque operation of the vehicle.

To determine the Radumfangskraft-Radschlupf characteristic pairs of values are each determined from the value of a circumferential force acting on the vehicle wheel and a tire slip generated by this circumferential force on the vehicle by determined from a measured wheel speed of the vehicle wheel and the measured vehicle speed of tire slip and the circumferential force of a Motor drive torque, a motor drag torque or a braking torque is determined. The determination of the circumferential force from the engine drag torque is first the engine torque on the vehicle axle of the vehicle and then only on this vehicle calculated acting moment. The same applies to the determination of the circumferential force from the braking torque.

The detection of the drag torque can for example be done directly on the engine torque signal of the engine, or be read from a speed / gear stage drag torque curve.

By averaging and / or filtering each Radschlupf / Umfangsskraft value pair a highly precise determination of slip in the per thousand range is achieved over a whole Radumdrehung or integer multiples of individual Radumdrehungen, which even with low engine drag torque precise statement is possible. Value fluctuations of the tire slip values can also be compensated for by a classification of the tire slip. The wheel slip circumferential force value pairs are distributed in a wheel slip circumferential force diagram according to the 1 and 5 , It puts 1 the situation that the circumferential force is a wheel torque T, namely a driving or braking torque during 5 represents the drag torque mode with a wheel drag torque T. Because the 1 If both drive and braking torques are recorded as wheel torque, the amounts | T | of wheel torque T and | λ | registered by Radschlupf λ. The clusters H of the wheel slip-wheel drag torque value pairs in 5 arise through different gears and speeds. However, so-called slip returns in the direction of the origin of coordinates with decreasing vehicle acceleration or with decreasing vehicle deceleration are not taken into account.

Further, from the wheel slip-wheel torque value pairs, the tire slip stiffness B as the quotient of wheel torque | T | and wheel slip | λ | calculated. The representation of these values B of the tire slip stiffness along the time axis is shown in the diagram 2 shown.

The vehicle is equipped with an electronic driving stability control system, such as an ABS, TCS or AYC, commonly called ESC (hereinafter referred to as vehicle control system).

The behavior of the vehicle control system in the determination times of these value pairs is observed in order to determine a value group from the determined wheel slip wheel torque value pairs that indicates a high friction roadway. In fact, it can be deduced from the behavior of the vehicle control system whether the vehicle is moving on a high friction road or a low friction road.

If the progression of driving dynamics parameters over an observation period, such as, for example, the longitudinal and / or lateral acceleration is or are greater than a predefined value and no control system interventions take place, this behavior of the vehicle control system indicates a high-friction roadway. The wheel slip wheel moment value pairs determined over this observation period are combined to form a value group indicating a high friction roadway.

Control system interventions, on the other hand, would indicate a low friction value at low acceleration, deceleration, or lateral acceleration. In this way, a road grade class indicating a low-friction roadway can be made plausible on the basis of such control system interventions.

Displaying a Hochreibwertfahrbahn can also be realized by means of optical sensors, for example. A mono or stereo camera of the vehicle. Here, the classification of the road surface by means of an image analysis of the detected by the camera road surface. Another way to display a Hochreibwertfahrbahn is access to external data sources, such as. Weather data from the example. The absence of snow or ice can be closed. Also, vehicle control system interventions such as ABS, TCS or AYC (ESC) can be used to determine the actual coefficient of friction between the tire and the roadway, thus indicating a high friction roadway.

Such a value group indicating a high friction roadway represents the wheel slip / wheel moment value pairs determined up to a time t _{1 of} an observation period, whose tire slip stiffness values B in 2 are shown.

By means of a statistical evaluation method (statistical distribution function ΔB ₁ = f (σ _(B1) )), a mean value ΔB ₁ with associated scattering is determined from the value pairs of the value group and thus a lane grip class A indicative of a high friction lane is defined. The class boundaries determined by the dispersion are in 2 with ΔB _1o as the upper limit and ΔB _1u as the lower limit. The mean value ΔB ₁ is in 1 the slope of a regression line g ₁ represents, as is the lower limit and the upper limit .DELTA.B _1u .DELTA.B _1o as the slope of a straight line g _1u or as a slope of a straight line g in _1o 1 shown. Out 1 It can be seen that these pairs of values indicating the roadway class A have only a small spread around this balancing line g ₁ .

All tire slip stiffness values B or wheel slip wheel torque value pairs can thus be used be classified. If these values or value pairs lie in this roadway grip class A, the vehicle moves on a high-friction roadway; in the other case, if these values or value pairs do not lie in the roadway grip class A, they indicate a low-friction roadway. Thus, this Fahrbahngriffigkeitsklasse A represents a high friction indicative comparison standard.

By comparing the current tire slip stiffnesses against the comparison standard indicating a high friction value, tire-specific relationships and parameters can be identified and eliminated, since this comparison standard can be adapted continuously and thus a self-learning behavior of the system can be realized, especially in the context of tire changes and tire wear, even if changed Environmental conditions.

When determining the roadway grip class A, it is advantageous for the tire slip stiffness B to be within given slip limits λ _min and λ _max as shown 1 to determine. The definition of these slip limits also defines the working range of the method according to the invention. Thus, λ _max should be in the range of wheel slip values at which electronic driving stability control systems such as ABS, TCS, ESC become active on a lane with low road grip, such as a roadway with snow or ice. Such control system interventions are used for plausibility of the roadway classification. Thus, the consideration of such interventions of driving stability control systems, such as. Radschlupf control systems to increased accuracy in the definition of a Fahrbahngriffigkeitsklasse. The size λ _min should not be chosen too close to the value zero in order not to jeopardize the mathematical division operation.

The above related to the 1 and 2 described method is also performed in the engine drag torque operating case. The wheel slip wheel drag torque value pairs detected in such an operating case are in 5 shown. From the displaying during a Hochreibwertfahrbahn reference signal value pairs generated the tire slip stiffness, and B (by means of a statistical evaluation function .DELTA.B = f (σ _(B)) determined a mean .DELTA.B and the associated scattering. The corresponding regression line g ₁ together with the class boundaries g _1u and of the y-axis determine the roadway grip class A.

Thus, all tire slip stiffness values B and wheel slip-wheel circumference force value pairs can be classified, wherein the wheel peripheral force is present as drive, brake or drag torque. If these values or value pairs lie in this roadway grip class A, the vehicle moves on a high-friction roadway; in the other case, if these values or value pairs do not lie in the roadway grip class A, they indicate a low-friction roadway.

The roadway grip class A thus represents a comparison standard lying at high friction for the tire slip stiffness values B or wheel slip-wheel circumference force value pairs (| λ |, | T |).

By using a statistical evaluation function, the tire slip stiffness values B or the wheel slip circumferential force value pairs (| λ |, | T |) lying outside the road grip class A can be classified not only as a value indicating a low friction road, but also as such by determining further road grip classes whether the vehicle is driving on a snow-covered or ice-covered road, for example.

So be according to 4 the determined subsequent to the time t1 tire slip stiffness B as data series of temporally successive tire slip stiffness values B by means of a statistical evaluation method (.DELTA.B ₂ = f (σ _(B2) and .DELTA.B ₃ = f (σ _(B3)) is an average .DELTA.B ₂ with associated scattering and a mean value .DELTA.B ₃ with associated scattering and thus determines two further Fahrbahngriffigkeitsklassen B and C with the class limits .DELTA.B ₂ as a lower limit and .DELTA.B _2o as the upper limit for a Fahrbahngriffigkeitsklasse B and .DELTA.B _3u as the lower limit and .DELTA.B _3o as an upper limit for another Fahrbahngriffigkeitsklasse C according to 4 Are defined. These mean values ΔB ₂ and ΔB ₃ are the initial slopes of equalization lines g ₂ and g ₃ as slip-circumferential force characteristics in FIG 3 in which the class limits ΔB _2u and ΔB _2c define the range limiting the road grip class B and the class limits _{ΔB 3u} and ΔB _3o define the range limiting the road grip class _C, and in FIG 3 are _plotted as straight lines g _2u and g _2o as well as g _3u (x-axis) and g _3o .

Furthermore, from the 3 and 4 it can be seen that the class limits ΔB _1u and ΔB _2u and the class limits ΔB _2u and ΔB _3o coincide.

The road grip class B shows a snow-covered road, while the road grip class C after the time t ₂ (see. 4 ) indicates an icy roadway.

The above related to the 3 and 4 The method described is also carried out in the drag torque operating case. The wheel slip-wheel drag torque value pairs detected in such an operating case are in 5 shown. From the wheel slip-wheel drag torque value pairs lying outside the roadway grip class A, the road grip classes B for snow-covered roadways and the roadway grip class C for ice-covered roadways are determined in the same way. The corresponding _{regression lines} g ₂ and g ₃ together with the class _limits g _2u and g _2o or g _3o and g _3u (x-axis) determine the road _{grip classes} B and C.

Thus, all tire slip stiffness values B and wheel slip-wheel circumference force value pairs can be classified, wherein the wheel peripheral force is present as drive, brake or drag torque. If these values or value pairs lie in this roadway grip class A, the vehicle moves on a high friction roadway; in the other case, if these values or value pairs are not in the roadway grip class A, they lie either in the roadway class B indicating a snowy roadway or in one ice-covered roadway grade C.

From the 3 and 4 It can be seen that the wheel slip wheel torque value pairs of the road grip classes B and C, and thus also the associated tire slip stiffness values, scatter significantly more strongly than the values of the road grip class A. This is also from the Radschlupf Radschlepp moment diagram after 5 to recognize. This behavior can be advantageously used to assign the pairs of values to a Fahrbahngriffigkeitsklasse. Furthermore, it can advantageously be exploited for the classification of a value indicative of a road grip class B or C (either as a wheel slip circumferential force value pair or as a tire slip stiffness value) to also take into account the curve shape of the associated data series from the wheel slip circumferential force value pairs. Thus, for example, a curvature points to a snow road, while discontinuities, for example, sudden tearing point to an icy roadway.

The due to a drive or braking torque according to 3 in inverse representation or due to a Radschleppmomentes according to 5 certain regression lines g ₁ , g ₂ and g ₃ with the associated limits are each merged into a single regression line, ie the mean tire slip stiffnesses are equated in the same Fahrbahngriffigkeitsklasse, as described below with reference to 7 is explained.

This equalization is possible because the initial slopes of the tire slip-circumferential force characteristic of a tire for the drive case and the brake case - as from the Schleppmomenten representation after 7 shown - are the same. This fact is used to improve the accuracy of the calculation of the average tire slip stiffness by multiple nodes and at the same time not necessarily run the best-fit line through the zero point. A zero offset d, z. B. by inaccurate or missing slippage calibration - for example, after adjustment of the tire air pressure or after tire change - is not a hindrance in this process. Here is a very significant advantage that makes the process robust against calibration deviations.

In this diagram according to 7 the 1st quadrant shows the drive case with the average tire slip stiffness of the roadway grip classes A, B and C, which is referred to as the compensation straight lines g ₁ , g ₂ and g _3. The third quadrant shows the drag torque case representative of the general braking situation, with the corresponding equalization straight lines having the same gradient as those in the 1st quadrant and therefore also referred to as the straight line g ₁ , g ₂ and g ₃ and thus are the same average Reifenschlupfsteifigkeiten.

It is also possible to determine these equalization lines g ₁ , g ₂ and g ₃ separately for a driven or braked vehicle wheel.

Finally, to detect μ-split situations, tire slip is determined for both vehicle wheel circumferentially applied to a vehicle axle of the vehicle, and the tire slip stiffness values are calculated by the quotient of circumferential force and tire slip of the most slipping vehicle wheel.

The determination of the slip offset S ₀ takes place in torque-free as well as in (quasi) stationary driving maneuvers. The slip offset results, for example, from different rolling circumferences of the tires z. B. caused by manufacturing tolerances, different air pressures or wear of individual tires and is not a slip in the true sense. The slip offset occurs in torque-free driving maneuvers and thus can not be determined directly in normal driving normally. So that value pairs of wheel slip and wheel torque or circumferential force are collected in the method according to the invention, a regression line can be continuously determined from these value pairs. The intersection of the regression line with the slip axis corresponds to the slip offset S ₀ . The 6 shows the calculation of the slip offset by linear regression.

• – ein Motorsteuergerät,
• – ein Bremssteuergerät,
• – eine Drehzahlerfassungseinrichtung für die Fahrzeugräder des Fahrzeugs,
• – ein Fahrdynamik-Regelsystem, und
• – eine Einrichtung zur Bestimmung eines Radmomentes oder eines Bremsmomentes als Umfangskraft des Fahrzeugrades.

A vehicle for carrying out the method according to the invention comprises the following components:

• An engine control unit,
• A brake control device,
• A speed detection device for the vehicle wheels of the vehicle,
• - a vehicle dynamics control system, and
• - A device for determining a wheel torque or a braking torque as the circumferential force of the vehicle wheel.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 4010507 C1 [0004]
• WO 2014/199328 A1 [0005]
• DE 102012217772 A1 [0006]
• DE 102004044788 B4 [0008]

Claims (12)

Method for determining road grip classes from measured values of a circumferential force on at least one vehicle wheel of a vehicle traveling on a roadway and the tire slip caused by this circumferential force on the same vehicle wheel, comprising the following steps: a) determining pairs of values in each case from the value of a circumferential force acting on the vehicle wheel and a tire slip on the vehicle wheel generated by this circumferential force by determining the tire slip from a measured wheel speed of the vehicle wheel and the measured vehicle speed and determining the circumferential force from one on or off the vehicle wheel A motor drive torque acting on a vehicle axle of the vehicle is determined from a motor drag torque acting on the vehicle wheel or from an engine drag torque acting on a vehicle axle of the vehicle, a brake torque on the vehicle wheel or a brake torque acting on a vehicle axle of the vehicle. b) determining a value group indicating a high friction road from the determined value pairs of circumferential force and tire slip, wherein at the time of determination of these pairs of vehicle dynamics values of the vehicle greater than a predetermined value with simultaneous lack of intervention of a driving stability system and / or optical sensors of the vehicle and / or external Data sources of the vehicle a Hochreibwertfahrbahn is or will be, c) calculating tire slip stiffness values from the value pairs by quotient of circumferential force and wheel slip, d) defining a road grade class indicating a high friction roadway by determining a mean tire slip stiffness from the tire slip stiffness values of the value pairs of the value group and the dispersion of these pairs of values around the average tire slip stiffness by a statistical evaluation method, and e) classifying an actual tire slip stiffness value as a value indicative of a high friction roadway value if that value is in the roadway grade class indicating the high friction roadway, otherwise as a value indicative of a low friction roadway. The method of claim 1, wherein d1) from each of a data series of temporally successive value pairs of circumferential force and wheel slip whose tire slip stiffness values indicate a low friction line, an initial slope is determined using a straight line as a linear part of the slip-circumferential force characteristic, and d2) the slope of each equalizer straight line is defined as average tire slip stiffness of a road grade class indicative of a low-friction roadway having a distribution of the value pairs about the determined equalization line determined by a statistical evaluation method. A method according to claim 1 or 2, wherein the tire slip stiffness values are assigned to one of the road friction class indicative of a low friction road, if its value is in that road grade class. Method according to one of the preceding claims, wherein for classifying a tire slip stiffness value indicative of a low friction roadway, the curve shape approximated by a mathematical equalization calculation to the associated data train is taken into account. Method according to one of the preceding claims, in which the method steps d, d1 and d2 are carried out only in the presence of amount-positive slip gradients and driving states of the vehicle without vehicle control system interventions. Method according to one of the preceding claims, wherein in the implementation of the method steps d, d1 and d2, the classification of the Reifenschlupfsteifigkeitswerte on the engagement behavior of a vehicle control system is plausibility by plausibility in non-vehicle control system interventions above predetermined longitudinal and / or transverse accelerations a Hochreibwertfahrbahn indicating lane classification class is plausibility and in the case of control system interventions, a lane friction class indicating a low-friction-value lane is checked for plausibility below predetermined longitudinal and / or lateral accelerations. Method according to one of the preceding claims, wherein the method steps d, d1 and d2 are carried out both during acceleration and during a deceleration of the vehicle, wherein for the acceleration case and the deceleration case the average tire slip stiffnesses are equated respectively with the road grip class. A method according to any one of the preceding claims, wherein the amounts of circumferential forces and slip are used to calculate tire slip stiffnesses. Method according to one of the preceding claims, wherein the method steps d, d1 and d2 are performed within a predetermined slip window between a slip value λ _min and a slip value λ _max , wherein at a slip greater than λ _max Control system interventions of a vehicle control system take place and at the same time such control system interventions are used for plausibility of the roadway classification. Method according to one of the preceding claims, wherein for detecting μ-split situations, the tire slip for both on a vehicle axle with a circumferential force applied vehicle wheels of the vehicle determined and the tire slip stiffness values by the quotient of the circumferential force and the tire slip of the most slipping vehicle wheel or Average of all wheels of a vehicle axle is calculated. Method according to one of the preceding claims, wherein the averaging and / or filtering of the value pairs of circumferential force and tire slip in each case over a full wheel revolution or over integer multiples of individual wheel revolutions takes place. Vehicle for carrying out the method according to one of the preceding claims, comprising An engine control unit, A brake control device, A speed detection device for the vehicle wheels of the vehicle, - a vehicle dynamics control system, and - A device for determining a wheel torque or a braking torque as the circumferential force of the vehicle wheel.

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