WO2007068571A1

WO2007068571A1 - Method and device for determining a maximum coefficient of friction on a wheel of a stationary vehicle

Info

Publication number: WO2007068571A1
Application number: PCT/EP2006/068917
Authority: WO
Inventors: Damiano Molfetta
Original assignee: Siemens Aktiengesellschaft
Priority date: 2005-12-15
Filing date: 2006-11-27
Publication date: 2007-06-21
Also published as: DE102005060023A1

Abstract

The problem in vehicles equipped with an electrically controllable parking brake is that a wheel (11) that is to be braked cannot be decelerated individually in a way that would be useful based on the actual static friction thereof when the vehicle is at a standstill or parked. On a slippery uphill or downhill stretch of road (12), for example, the wheel (11) is blocked because of the low static friction thereof, thus being unable to contribute substantially to retaining the vehicle. According to the invention, a maximum coefficient of friction between the wheel (11) and the ground is therefore determined for at least one wheel (11) that is to be braked on a stationary vehicle. Said maximum coefficient of friction is determined by first fully engaging the operating brake or the parking brake and then continuously releasing the same until the wheel (11) that is to be braked or the vehicle begins to move.

Description

description

Method and device for determining a maximum coefficient of friction on a wheel of a stationary vehicle

The invention relates to a method and a device for determining a maximum coefficient of friction on at least one wheel to be braked a stationary vehicle according to the preamble of independent claims 1 and 7. In vehicles that are equipped in particular with an electrically controllable parking brake, there is the problem that At standstill or a parked vehicle a wheel to be braked can not be braked as individually as it would be useful due to its actual coefficient of friction. For example, the wheel blocked in a slippery surface of a slope or downhill due to its low coefficient of friction and thus can not contribute significantly to the holding of the vehicle. Consequently, a wheel that can only apply a low coefficient of friction does not need to be braked as much as a wheel with a high one

Coefficient of friction. On the other hand, with known coefficients of friction, the holding force of the wheel can be optimized by adding a corresponding adhesive reserve. This is interesting for example in vehicles that are located on parking spaces, the slope may change, for. B. in a duplex garage, on a ferry in heavy waves, etc.

It is already known that modern motor vehicles are designed with a complex sensor system with which, for example, the driving dynamics of the motor vehicle can be regulated. In this case, corresponding sensors, such as acceleration sensors, yaw rate sensors, wheel sensors or systems such as ABS, ESP detect the movements occurring during the travel around the three main axes of the vehicle: longitudinal axis, transverse axis and vertical axis. From the ones supplied by the sensors Data driving dynamics is controlled or regulated, ie by braking one or more wheels or throttling the engine power breaking the vehicle from the traffic lane to be prevented. However, this known method can be applied only to a moving vehicle.

Furthermore, measuring systems or methods are known with which the static or rolling friction of a wheel is determined when the vehicle is moving on its lane. This happens, for example, by detecting the braking torque and the slip when the wheels or the vehicle is in motion. In a stationary vehicle, however, the known method for determining a coefficient of friction is not applicable.

The invention is based on the object to determine a maximum coefficient of friction for a wheel to be braked a vehicle, the vehicle is on a slope or downhill and should not roll away. This object is achieved with the characterizing features of the independent claims 1 and 7.

In the case of the method or the device for determining a maximum coefficient of friction on at least one wheel of a vehicle to be braked, there is the advantage that new control strategies for braking the vehicle can be designed when the coefficient of friction between the wheel and its ground is known. This is important, for example, for a car, but also for large and heavy vehicles, such as buses and trucks, as may be significant

Braking forces must be transmitted to the individual wheels. This applies in particular to a vehicle that is parked on a steeper slope or downhill slope and thereby has to be braked safely so that an undesired rolling away is prevented with certainty. When Particularly advantageous is considered that when parking or parking the vehicle, the braking force can be distributed individually to the individual wheels according to the friction coefficient actually occurring. Therefore, according to the invention, the wheel is only braked to a maximum extent, as permitted by the determined coefficient of friction. A wheel that has a low coefficient of friction and thus can absorb only low holding forces, therefore need only be slowed down less.

The measures listed in the dependent claims advantageous refinements and improvements of the independent claims 1 and 7 method and the device are given. It is considered to be particularly advantageous that based on the determined maximum coefficient of friction a minimum braking force is determined, with which the wheel is braked. The wheel can thus be kept blocked in most cases with the minimum braking force. This reduces the wear on the brake system, so that a longer life, especially the mechanically loaded parts of the brake system can be achieved.

According to the invention, it is provided to determine the maximum coefficient of friction successively for at least one further wheel to be braked, preferably for the wheels of an axle.

It also appears to be advantageous to apply an adhesive reserve to the minimum braking force in order to prevent the vehicle from rolling backwards or backwards, in particular when there is a changing gradient, as may be the case with a duplex garage or on a ferry in heavy seas secure when the slope changes. This further improves the stability of the vehicle. Furthermore, it is provided that the method for determining the maximum coefficient of friction can be canceled manually, for example when the service brake is actuated, or automatically when a dangerous situation is detected.

For the device according to the invention, it is provided that the program-controlled computing unit determines a minimum braking force with the aid of the algorithm and the determined maximum coefficient of friction for the corresponding wheel.

If a maximum friction coefficient is determined for each wheel to be braked, then all wheels can be braked individually and with minimal braking forces.

To determine the stoppage of the vehicle is a

Motion sensor used, which is already present on the vehicle. Usually, a modern vehicle is equipped with a plurality of motion sensors with which the standstill of a wheel or of the vehicle can be determined. Such a motion sensor, for example, a wheel sensor, a

Acceleration sensor, a rotation rate sensor, a parking sensor or the like. Furthermore, control devices for vehicle dynamics, for example an ABS (antilock brake system) or ESP system (electronic stability program) can be used to determine the stoppage of the vehicle.

An embodiment of the invention is illustrated in the drawing and will be explained in more detail in the following description.

FIG. 1 shows in a simplified representation a block diagram of an inventive device for determining a coefficient of friction on a wheel of the vehicle, FIG. 2 shows a diagram with an inclined plane which has a pitch angle α,

FIG. 3 shows a first flow chart with a test run A and

FIG. 4 shows a second flowchart with a test run

1 shows a schematic representation of a device 10 according to the invention, with which a braking force can be determined taking into account a coefficient of friction between a wheel to be braked of a vehicle and its ground. As FIG. 1 further shows, the device 10 (evaluation device) has a program-controlled computing unit 1, a memory 2 and an interface 3. For example, measured data and comparative values for coefficients of friction etc. are stored in the memory 2. Furthermore, the control program for the arithmetic unit 1 is stored in the memory 2. The control program contains an algorithm with which the method for determining the braking force is controlled. The algorithm will be explained later essentially with reference to FIGS. 3 and 4.

The evaluation device 10 is connected on the input side with one or more motion sensors 6 to 9. This may be, for example, an acceleration sensor 6, a yaw rate sensor 7, a wheel sensor 8, a parking sensor 9 and the like. Furthermore, the evaluation device 10 may be connected to an ABS system and / or an ESP system. On the basis of the signals sent by the motion sensors 6 to 9, the evaluation device 10 first determines a standstill of the vehicle, for example a car, bus or the like, and then activates if appropriate, the determination of the coefficient of friction or the braking force on at least one wheel of the vehicle to be braked.

In Figure 2, the operation of the invention to

Underlying method for determining the coefficient of friction on a wheel of a vehicle explained in more detail. In many vehicles, especially in luxury vehicles, an electrically operated parking brake is used instead of the usual hand or foot brake. By using an electric parking brake constructive advantages for the vehicle can be achieved, in particular simplifies the structure. However, known electrically operated parking brakes have the disadvantage that the braking force, i. the power of the

Brake system is applied and acts on the brake caliper, is designed to the maximum actuating force of a servomotor, even if this force is not always required in full. This increases the wear on the moving or the braking force receiving components, so that they must be more strongly dimensioned.

In the method according to the invention, a maximum coefficient of friction is first individually determined for each wheel to be braked and, for example, a corresponding one

Braking force derived for a wheel to be braked, with the wheel brake is actuated. The braking force is determined as a minimum braking force as a function of a maximum possible coefficient of friction, which acts between the wheel and its base and builds up a corresponding frictional force or holding force on the gradient or gradient. The maximum friction value or a maximum holding force is achieved when the brake is released continuously during a test run until the vehicle moves slightly downhill. Blocking a wheel, it means that the coefficient of friction between this wheel and its surface less than at the still rotating wheels. The blocking wheel has then reached its maximum holding force according to its maximum coefficient of friction. The method works automatically and can alternatively be started by the driver of the vehicle, for example by pressing a button.

Furthermore, it is provided that the program for determining the coefficient of friction can be aborted, for example, by actuation of the service brake by the driver or automatically, in particular when a dangerous situation occurs.

As in the plane no automatic activation of the parking brake is needed, or here the

Braking force can remain very low, this method is essentially applied to gradients or slopes where an accidental rolling of the vehicle for safety reasons must be prevented. Another prerequisite for the application of the method according to the invention is that the vehicle is at a standstill. The determination of the coefficient of friction and in particular the braking force is therefore carried out on a stationary vehicle. In order to be able to use the braking force optimally, furthermore, a distribution of the wheel loads of the vehicle must be known and each wheel must be individually brakable.

According to the invention, it is provided that the determined coefficient of friction can be used not only to determine the braking force but also for other vehicle functions.

For example, in the case of a known coefficient of friction, a maximum vehicle weight or a maximum payload for a certain gradient distance can be calculated. Alternatively, with a known coefficient of friction, the maximum pitch angle for a travel path can be specified. The embodiment relates to a vehicle with four wheels to be braked with the parking brake. For more or fewer wheels, the algorithm must be adjusted accordingly. As already mentioned, the method for determining the braking forces is based on first tightening the service brake or alternatively the parking brake and then releasing it continuously until the vehicle moves. Blocking a wheel, then that is an indication that this wheel is the coefficient of friction is less than on a rolling wheel. It is doing only a minimal taxiway, for example, only a few centimeters are needed. Under no circumstances should the vehicle touch an obstacle or endanger someone. To determine the movement, at least one signal from one of the connected motion sensors 6 to 9 is evaluated. With the aid of the algorithm according to the invention, in which known vehicle parameters such as the vehicle weight, the weight distribution and the gradient angle of the road are taken into account, the coefficient of friction and / or the braking force for the relevant wheel is calculated. The test run is run at least once for each wheel to determine the coefficients of friction or braking forces on all wheels to be braked.

Figure 2 shows a diagram with an inclined plane, which is modeled on a slope or slope distance. On the inclined plane two wheels 11 are shown, which are mounted on two correspondingly arranged axles of the vehicle. According to Figure 2, the slope of the inclined plane has the pitch angle α, which acts on the vehicle. The vehicle has the vehicle weight G with the two components Gl or G2, which is distributed to the two axes of the wheels 11 and the two wheels left and right of the two axis and the Normal forces Ni and N ₂ generated. For braking the vehicle, which has, for example, i = 4 wheels to be braked, the friction or holding force of the four wheels F _Bp i, F _Bp 2, F _B p3, F _B p4 is thus required. Due to the physical relationships thus results for the holding force:

F _B p = 2 !, Vi ₁ -N ₁ = YF _Bpι ≥ G-sinα, holding condition (1) * = i 'ι = \

μ _; corresponds to the maximum friction coefficient on the wheel i, the pitch angle α is> 0 and the normal forces N ₁ correspond to the normal forces Ni, N ₂ , N ₃ and N ₄ corresponding to the vehicle weight force G.

By transforming the above holding condition (1), the holding condition (2) can be derived therefrom:

tgα, holding condition (2]

with the 'minimum required coefficients of friction' μ ^* and the weight distribution J ₁ .

A method for calculating the weight distribution J ₁ is known for example from DE 19603430 Al. Here, the weight force distribution J ₁ can be calculated when the vehicle is in the plane, ie, the pitch angle α is 0. In said publication, the weight distribution is determined by means of a yaw rate sensor by evaluating the pitching motion of the motor vehicle. The axle loads and / or the wheel contact forces of the vehicle can be calculated from the sensor signals. Of Furthermore, in a control computer the

Angle velocity in the vehicle longitudinal direction integrated up to the pitch angle and integrated the angular velocity in the vehicle transverse direction up to the roll angle. From the pitch angle, the roll angle, the terrain slope, the left and right wheelbase, the front and rear lane and the vehicle speed can then calculate the axle load distribution.

The above method for the determination of

Axle load distribution in the plane still has to be converted for the vehicle, which according to the invention is located on the inclined ground. The background has the pitch angle α. The pitch angle α is measured by a pitch angle sensor 4, with which the device according to the invention is also connected.

The determination of the maximum coefficient of friction μ _; for a wheel i now takes place according to the following inequality (3), if the brake force distribution proportional to the

Weight distribution is, ie when μ _; = constant:

μ _; > tga, V ie [l; 4], inequality (3)

In the inequality (3), V ie [l; 4] for all elements of i for the wheels i = 1 to i = 4. The above inequality (3) is determined for all wheels that are to be braked in the stationary vehicle.

The algorithm underlying the invention will now be explained in more detail below with reference to the two flow charts corresponding to Figures 3 and 4. First, the program is started with a test run A, as shown in Figure 3. After a start in position 20 will First, the service brake or alternatively the parking brake fully actuated and then reduced in proportion to the wheel load, the braking force continuously on all wheel brakes. In position 22 it is checked whether inequality (3) is fulfilled for all wheels. If this is the case, then it is determined that the vehicle is not moving with "yes" in position 23. In position 24, it is checked whether the minimum braking force with which the vehicle is just to be kept on the slope has been reached. If this is not the case, then the program jumps back to position 21 with "no". In the other case, if "yes" is started at position 28 with the test run B2, as will be explained in more detail later in accordance with FIG.

If the query in position 22 has been decided as "no", ie the inequality (3) is not fulfilled for all wheels, then according to position 25 it means that the vehicle rolls or the wheel on which the inequality (3) does not roll In this case, the friction value of the locked wheel is calculated in position 26, as will be shown later with equation 6. Similarly, the maximum holding force of the wheel or a minimum braking force to be applied can be determined 27 is started with the test run B1 Before each test run, the braking force is initialized, ie, the braking force is increased in proportion to the load distribution to all wheels to keep the vehicle safe on the uphill or downhill.

For a better understanding, the flowchart of the test run B (1/2) is first explained in more detail in accordance with FIG. The program starts in position 40. In contrast to test run A, a wheel to be tested is selected during test run B (1/2), where the braking force is increased. In return, the braking force is reduced at the other three wheels. The

Sum of the braking forces remains constant and corresponds to the minimum braking force of the individual wheels for the slope. The braking force distribution is proportional to the wheel load.

According to FIG. 4, in position 40a the braking force is initially initialized on all wheels, after that in

Position 41 a test wheel i selected. In position 42, the braking force is continuously increased on the test wheel i. At the same time the braking force is reduced on the remaining three wheels i according to position 43. In position 44 the program is over or it starts again in position 40.

The test run B (1/2) distinguishes two cases Bl and B2. These two cases Bl, B2 are shown in the flow chart of Figure 3 and are explained in more detail below.

In Case B2, no blocked wheel was detected in Test Run A, that is:

Vie [l; 4]; μ _; > tga

In the flowchart of Figure 3 is continued: According to the position 31 is now checked whether the vehicle is moving. If the vehicle does not move, then if "no" in position 32, it is checked whether the minimum force (minimum braking force) has been reached

"Yes" in position 34, the coefficient of friction of the test wheel with the equation 9 to be explained later.

If, on the other hand, the minimum force in position 32 has not been reached, then in position 33 the braking force on the test wheel is increased and the braking force on the other wheels is reduced. Thereafter, the program jumps back to position 31. However, if the test wheel is locked in position 31 and it is determined that the vehicle is moving, then the friction coefficient of the test wheel is calculated with equation 8 in equation (8), as will be explained in more detail later on, after which the program jumps to position 36 Here it is checked whether the coefficients of friction are calculated on all wheels to be braked, if this is the case then with "yes" the program in position 37 is ended. Otherwise, a new test wheel is selected at "No" in position 30. In position 29, it is checked whether it is the test run case B1 or B2. If it is Bl, then the program jumps to position 27 On the other hand, if the case is B2 then the program returns to position 28.

The case Bl means that a wheel in the test run has blocked A, as will be explained in more detail later to the equations (10a) and (10b).

In the case of Bl, according to the holding condition, it becomes

Equation (10a) and (10b), the test continued analogous to the case B2 in position 31. The test procedure until the end of the program in position 37 is the same as that described above for the case B2, so that a repetition does not appear necessary.

According to the two test runs A and B1 / B2 described above, the algorithm according to the invention will now be explained in more detail with reference to the equations presented below. According to position 21 (Figure 3), the test run A begins. While the braking forces are continuously reduced to a minimum and the braking force distribution remains proportional, the validity of inequality 3 on all wheels is checked (position 22). If the vehicle begins to move, which is caused for example by a motion sensor, Speed sensor, vehicle dynamics acceleration sensor or the like is signaled, then it is determined with the help of the speed sensors, which wheel blocked. In the case of the blocked wheel, the inequality (3) is no longer satisfied, that is to say for the coefficient of friction:

μ _; <tga.

The wheel with the lowest coefficient of friction is thus detected. Thus, now the coefficient of friction and consequently the maximum

Friction force or holding force on this wheel from the known variables (pitch angle α, weight distributions J ₁ and JJ *) are calculated. For the calculation of the coefficient of friction μ, the conditions of equations (4) and (5) apply:

Σμ, ^* -Y, = Σμ, ^* -Y, + K ¹ Y ₁ = tg &, equation (4, ι = l ι = ll ≠ l

= μ ^* 'ΣY _* + μ, -Y ₁ = ^? & ^α > equation (5; ι = ll ≠ l

The result for the friction coefficient according to equation (6) is: tgα -μ _* • £ γ _,

I = I μ _; = -, equation (6;

with μ ^* as current, minimum required coefficient of friction.

The test run A is completed when a blocked wheel was detected and its maximum coefficient of friction or the maximum holding force has been calculated or if in test run A the braking force has been reduced to a minimum, with which the vehicle is just kept on the slope, and the vehicle has not moved (see Fig. 3, position 24). Then continue with the test run B corresponding to the positions 27 and 28 respectively. Depending on the test run A, the two different cases result:

Case B2: It did not lock a wheel in Test Run A and Case Bl: It blocked a wheel.

If no wheel has moved or all wheels are rolled at the same time at set minimum force, then the inequality (3) is fulfilled on all wheels.

According to the case B2, if in test run A no wheel is blocked, the formula is:

Vie [l; 4]; μ _; > tga inequality (3).

The holding condition (2) still retains its validity. The wheel to be tested is indexed with i. The holding condition can thus be reshaped to the following form:

4 4 ₄ JX "Υ" ⁼ Σ μ ^* -. Y. ⁺ VT - IΪ - μ ^{* • Σγ,} + μ ₇ ^• Y ₇ = tga, ^Gl eic ^h clothes ^⁽⁷⁾

I = I i = l _{ι = l}

where μ ≦ tga, since the braking force on the non-tested wheels is reduced and the braking force on the wheel i is increased. The sum of the four braking forces on the four wheels must again correspond to at least the minimum braking force in order to To keep the vehicle on the slope or downhill slope, the following two states can occur:

1. The vehicle is moving, ie the wheel i is blocked and the wheels i ≠ i roll because they are less braked than their braking potential μ _; > tga is.

The friction coefficient μ ~ of the wheel i then calculates to

4 tga - μ ^{* ■} Σγ, ^{ι =} i, μ ~ = ^{1 ≠ 1} > _t g _a , equation (8)

2. The vehicle does not move as long as the minimum required coefficient of friction μ ^{~ *} > 0.

The friction coefficient μ ~ of the wheel / is then calculated as μ ~> - ^, equation (9)

'Y ~

This test procedure is now repeated for the remaining wheels three more times.

In the case of Bl, in Test Run A a wheel has stalled, i. :

Vz ^' e [l; 4], i ≠ i: (μ _; > tga) Λ (μ _; = value <tga), Equation 9a

will be completed . The procedure is analogous to Case B2. However, after the first coefficient of friction is known, only the coefficients of friction of now three wheels must be determined. The holding condition for the second wheel results to

Σμ, ^* -γ, = Σμ, ^* -γ, + μ, -γ, + μr-γ? ⁼ tga> ^for μ ^* > μ, / c ⁱ . αoa

Σμ, ^* -γ, = Σμ, ^* -γ, ⁺ μr-γ? ⁼ & ^α ' ^for μ, ^* ≤μ, Eq. (10b)

wherein the coefficient of friction μ _; from the equation 6 is known.

This procedure is also repeated for the remaining two wheels. According to the algorithm described above, the friction coefficient is determined on each wheel and from this the braking force.

In a further embodiment of the invention, it is provided to determine the minimum braking force for the wheels of an axle.

When determining the minimum braking force of a wheel, it must be taken into account that the frictional forces that can be absorbed by a tire can be determined according to the adhesion characteristic diagram (Kamm's circle). From the adhesion characteristic diagram results that with rising

Longitudinal forces the cornering forces sink. It must therefore be ensured that the tire or the wheel is able to absorb the cornering forces. This problem is solved by the invention in that the minimally present coefficients of friction or the minimum required longitudinal forces are determined. The determined longitudinal forces are thus always smaller than the maximum possible longitudinal forces. This results in an advantageous manner that the wheel is always able to absorb the cornering forces.

In a further embodiment of the invention it is provided that the test runs A, B (1/2) can be interrupted automatically and / or manually if a

Abbruch signal is generated by a motion sensor, distance sensor or by the driver himself. For example, the termination of the test run can be interrupted upon actuation of the service brake. On the other hand, it is provided that the test run A, B (1/2) is started automatically and / or manually. In the worst case, the test run must be repeated four times in a vehicle with four wheels, namely for each wheel is a test run. This means that the motion sensor must detect a movement of the vehicle four times. Since only a few centimeters of driving distance are required to detect a movement of the vehicle, it is possible, for example, to monitor from a distance sensor whether the range of motion for the vehicle to a possible obstacle is still free.

In a further embodiment of the invention, it is provided to determine with the aid of the algorithm, up to which pitch angle the vehicle can be safely parked. If the angle of inclination is too great, for example if the vehicle is parked on a ramp, a signal can be generated from the determined coefficients of friction which gives the driver of the vehicle an appropriate indication. In the other case, if the coefficients of friction are sufficiently high, there is a certain reserve of unbraked mass. This reserve can be used for additional loading of the vehicle. In this case, a corresponding information can be output to the driver, which indicates how high his maximum payload can be at a given pitch angle, or what reserves his vehicle still has.

Claims

claims

1. A method for determining a maximum coefficient of friction for at least one wheel to be braked (11) of a vehicle, wherein the vehicle is at a standstill, for example on a slope or downgrade (12) and is formed with a motion sensor (6 to 9) with which a standstill or movement of the wheel (11) or of the vehicle can be detected, and wherein the wheel (11) or the vehicle is equipped with an electrically controllable parking brake with which the braking force for the wheel to be braked (11) is adjustable , characterized,

- that the service brake or the parking brake is continuously released until the vehicle moves a short distance,

- Is checked during the solution of the brake, if at least one wheel (11) is blocked and

- That for the blocked wheel (11) a maximum friction coefficient is determined.

2. The method according to claim 1, characterized in that based on the determined maximum coefficient of friction for the wheel (11) a minimum braking force is determined, with which the wheel (11) is braked.

3. The method according to claim 1 or 2, characterized in that the determination of the maximum coefficient of friction is determined successively for at least one further wheel to be braked (11), preferably for the wheels (11) of the same vehicle axle.

4. The method according to any one of the preceding claims, characterized in that for braking the wheel (11) the minimal braking force is applied to an adhesive reserve.

5. The method according to any one of the preceding claims, characterized in that the activation of the parking brake is done automatically and / or manually.

6. The method according to any one of the preceding claims, characterized in that the determination of the maximum coefficient of friction is manually canceled, for example when operating the service brake or automatically upon detection of a dangerous situation.

7. Device for determining the maximum coefficient of friction on at least one wheel to be braked (11) of a vehicle for a method according to one of the preceding claims, with a controllable parking brake for the wheel to be braked (11), with at least one motion sensor (6 to 9) for detecting the standstill of the wheel to be braked (11) or the vehicle and with an evaluation device (10), characterized

- that the evaluation device (10) has a program-controlled computing unit (1) and a memory (2),

- That in the memory (2) the friction values influencing information, such as a Radlastenverteilung and a program are stored with an algorithm and

- That the arithmetic unit (1) is adapted to determine with the help of the stored information and the algorithm a maximum coefficient of friction between the wheel (11) and the ground for the wheel to be braked (11).

Apparatus according to claim 7, characterized in that the program-controlled computing unit (1) is formed, with the aid of the algorithm and the determined maximum Friction coefficient for the wheel (11) to determine a minimum braking force with which the wheel (11) is braked.

9. Apparatus according to claim 7 or 8, characterized in that the computing unit (1) is designed to determine an individual maximum coefficient of friction for each wheel (11) to be braked.

10. Device according to one of claims 7 to 9, characterized in that 10 for determining the stoppage of

Vehicle as motion sensor (6 to 9), a wheel sensor (8), an acceleration sensor (6), a rotation rate sensor (7), a parking sensor (9) or the like can be used.

1511. Device according to one of claims to 10, characterized in that the controllable parking brake is automatically and / or manually be actuated by the driver of the vehicle.

2012. Device according to one of claims 7 to 11, characterized in that the determination of the maximum coefficient of friction when changing the inclination angle of the road, change the vehicle weight, change the axle load distribution or the like can be determined again.

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