DE102004053880A1 - Method for determination of coefficient of friction between vehicle tire and road surface involves determining maximum coefficient of friction with the help of values of instantaneous slip and instantaneous coefficient of friction - Google Patents

Abstract

The method involves determining maximum coefficient of friction by determining instantaneous slip (s) and instantaneous coefficient of friction (mu ). Value of instantaneous slip and instantaneous coefficient of friction is applied for the functional value of the curve in such a way that the required point is determined for certain value of s.

Description

The The invention relates to a method for determining the maximum coefficient of friction μ between a vehicle tire and a road surface. The knowledge of the maximum Friction between vehicle tires and road surface is in particular for the conditioning of driver assistance systems such as For example, anti-lock braking systems (ABS) or electronic stability programs (ESP) of considerable benefit. So it allows the knowledge of the maximum coefficient of friction, the control parameters of one of the aforementioned systems adapt in such a way that the system has its maximum effects can unfold.

In In the past, various approaches have become known, parameters, the type of contact between a vehicle tire and a road surface characterize, determine.

For example, in the German patent application DE 101 556 721 A1 proposed a device in which for the continuous measurement of the transmitted force and the coefficient of friction μ, a plurality of sensors arranged on the outer circumferential surface of a tire are used. In this case, the measurement signals are fed via an antenna arrangement to an evaluation unit. However, this approach proves to be extremely vulnerable and expensive, since it is necessary to install sensors in a typical heavily used environment and to supply the measured signals wirelessly to an evaluation unit.

Another approach, the German patent application DE 103 04 966 A1 is presented, it is assumed that from the known rotation speed of a wheel and the likewise known speed of the vehicle itself, the instantaneous slip can be determined. However, this approach does not allow the reliable determination of the maximum coefficient of friction, which has the above-described high relevance for the adjustment of the parameters of driver assistance and safety systems.

task It is therefore an object of the present invention to provide a method which allows, in a simple and reliable way, the maximum coefficient of friction between a vehicle tire and a road surface.

These The object is achieved by the method with the specified in claim 1 Characteristics solved. The features of the subclaims relate to advantageous embodiments and further developments of the invention.

Of the essential idea of the method according to the invention is that the maximum coefficient of friction is determined by the instantaneous Slip and the instantaneous coefficient of friction is determined from sensor data. When determining the maximum coefficient of friction, you do it yourself to take advantage of the fact that up to the maximum Coefficient of friction between the coefficient of friction μ and the slip S has a known functional relationship.

The relationship between slip S and coefficient of friction μ is in particular approximately linear, as in FIG 1 shown. The figure shows that the plot of the instantaneous coefficient of friction μ against the slip S yields an original straight line. Furthermore, it is known that the maximum coefficient of friction is achieved with a slip of approximately 0.15 to 0.2. It follows that it is sufficient to determine the maximum coefficient of friction, a single momentary coefficient of friction and the associated slip to determine. The knowledge of the corresponding point in the described plot makes it possible to define an original straight line. If one intersects this line of origin with a parallel to the ordinate, which intersects the abscissa in the range between 0.15 and 0.2, then the maximum coefficient of friction results as the ordinate of the intersection of the original line with the line parallel to the ordinate.

Depending on the external circumstances are also non-linear functional relationships between the slip S and the coefficient of friction μ conceivable.

From the above, it is immediately clear that it is necessary to determine the origin straight at a point to determine the slip S and the associated coefficient of friction μ. The slip is defined as the quotient of the difference between the wheel speed and the true speed of the vehicle over ground and the true speed, as shown in the formula below. there The wheel speed corresponds to the web speed of any point on the tread of the tire. The true speed over ground is the speed of the vehicle itself versus the road surface.

there let yourself For example, the true speed of the vehicle over ground by using a GPS system in a simple way. Such systems are now in a variety of standard vehicles especially integrated for the realization of navigation functionalities. In a similar way uncomplicated way determining the wheel speed by using it a speed sensor, as for conventional speed measurements is used. The knowledge of both sizes thus allows a simple determination of the momentary slip S.

Of the instantaneous coefficient of friction leaves derive in particular from the knowledge of the current driving force.

m₂:

Masse auf Antriebsachse

g:

Erdbeschleunigung

α:

Steigungswinkel

The relationship between momentary driving force K and friction coefficient μ is: K = m 2 g cos α μ With

m ₂ :

Mass on drive axle

G:

acceleration of gravity

α:

lead angle

there For example, the current driving force can be determined from a determination be made of the torque in the drive train. Here offers on the one hand, to carry out a measurement with corresponding sensors or the driving force from the current parameters of the injection system and derive a look-up table. Here is the look-up table For example, in a control unit for controlling the method according to the invention integrated; it contains the assignment of the parameters of the injection system to the corresponding Values for the driving force.

An alternative to this is to use the following relationship to determine the instantaneous coefficient of friction:
At each point in time, there is an equilibrium of forces between the propulsive force K on the one side and the acceleration, the downhill force as well as the air and rolling resistance. This relationship is illustrated in the following equation. m 2 g cosα μ = m 1 α + m 1 g sin α + K air + K roll

mit.

m₁:

Gesamtmasse des Fahrzeuges

m₂:

Masse auf Antriebsachse (kann beispielsweise unter Verwendung von Federbeinsensoren bestimmt werden)

g:

Erdbeschleunigung

α:

Steigungswinkel (aus Höhenänderung oder aus GPS-Daten in Verbindung mit einer digitalen Karte; auch die Verwendung eines Neigungssensors ist denkbar)

a:

Beschleunigung des Fahrzeuges (Bspw. aus GPS-Messung)

K_Luft:

Luftwiderstand (aus Berechnung über Geschwindigkeit und evtl. Lufttemperatur)

K_Roll:

Rollreibungskraft(aus empirisch bestimmten Werten bekannt)

The small angle approximation assuming α <10 ° gives cosα ≈ 1 and thus results from simple transformation:

With.

m ₁ :

Total mass of the vehicle

m ₂ :

Mass on drive axle (can be determined, for example, using strut sensors)

G:

acceleration of gravity

α:

Slope angle (from altitude changes or from GPS data in conjunction with a digital map, also the use of a tilt sensor is conceivable)

a:

Acceleration of the vehicle (eg from GPS measurement)

K _air :

Air resistance (from calculation over speed and possibly air temperature)

K _Roll :

Rolling friction force (known from empirically determined values)

Out these known sizes can be For example, in a microcomputer in a simple way after the above relationship determine the instantaneous coefficient of friction μ. Subsequently performs the described Plot of the thus determined friction coefficient in the representation of the Friction coefficient over the slip directly to determine the maximum coefficient of friction in the given situation. Through a constant update of the coefficient of friction will it be possible the assistance systems of a vehicle such as ABS or ESP constantly with the optimal parameters for one possible to provide an effective way of working. This is especially true commercial freight traffic For example, with trucks to ensure general road safety of considerable importance.

Claims (6)

Method for determining the coefficient of friction between at least one vehicle tire and a road surface, characterized in that the maximum coefficient of friction is determined by determining the instantaneous slip S and the instantaneous coefficient of friction μ and in a plot of μ over S the functional value of a curve through the thus determined point for a certain value for S is determined. Method according to claim 1, characterized in that the curve is an original straight line. Method according to one of the preceding claims, characterized characterized in that the particular value for S is in the range 0.15-0.2 lies. Method according to one of the preceding claims, characterized characterized in that the instantaneous slip S using a GPS sensors and at least one wheel speed sensor is determined. Method according to one of the preceding claims, characterized in that the coefficient of friction μ is determined from the driving force K. becomes. Method according to claim 5, characterized in that the coefficient of friction is derived from the relationship between the driving force, Downgrade force, inertial force, Roll friction and air resistance is determined.