WO2000047458A1 - Method and device for determining a brake force exerted in the contact area of a wheel of a vehicle - Google Patents

Abstract

The invention relates to a method and a device for determining a brake force (FB) exerted in the contact area of a wheel (1) of a vehicle for a brake force regulation system. The device co ntains a clamping force-determining module which determines a clamping force (FSP) effecting a braking operation and a factor-determining module which determines a brake transmission factor ( tau B) that represents a proportionality between the clamping force (FSP) and the brake force (FB). The device also contains a brake force-determining module (12) which determines the brake force (FB) in dependence on the clamping force (FSP) and the brake transmission factor ( tau B). According to the invention, the brake force (FB) is determined with a high degree of accuracy.

Description

 Method and device for determining a braking force acting in the footprint of a wheel of a vehicle

The invention relates to a method and a device for determining a braking force acting in the footprint of a wheel of a vehicle for a braking force control, which determine the braking force with high accuracy.

Modern braking systems in motor vehicles are equipped with higher-level control systems (ABS, ASR, ESP, etc.). These control systems influence vehicle dynamics with the aim of adhering to the deceleration and course specified by the driver.

The state of the art in these systems is that the higher-level control (eg wheel slip control / wheel deceleration control, float angle control, etc.) is subordinate to a clamping force control. In some systems, the clamping force applied to the wheel brake is reconstructed and regulated using a hydraulic pressure model. As shown in DE 197 42 920 AI, the clamping force can be measured, for example, in conventional hydraulic brake systems and in the case of electromechanical brakes can be determined by measurement or by reconstruction.

In general, the braking force acting in the wheel contact area can be used as a feedback variable for the control of vehicle brakes, since it is the variable that directly influences the vehicle dynamics. Theoretically, the braking force can be determined from the wheel contact force via a coefficient of friction between the road surface and the tire. This wheel friction coefficient depends on many variable factors such as the road surface, the intermediate medium between the tire and the road, the tire tread etc. The wheel friction coefficient can therefore change quickly, which is why it is difficult to determine it precisely. Accordingly, the braking force dependent thereon cannot be determined exactly.

An object of the invention is to specify a method and a device for determining a braking force acting in the footprint of a wheel of a vehicle for a braking force control, which determine the braking force with a high degree of accuracy.

This object is achieved with the features of the independent claims. Dependent claims are directed on preferred embodiments of the invention.

According to the invention, a clamping force determination module can determine a clamping force of a brake that brings about braking. A factor determination module can determine a brake transmission factor, which can represent a proportionality between the clamping force and the braking force. Furthermore, a braking force determination module can determine the braking force as a function of the clamping force and the brake transmission ratio. determine factor. The brake transmission factor is advantageously estimated using a parameter estimation method. By determining the braking force as a function of precisely ascertainable variables, the braking force is also highly accurate.

The tensioning force can bring about braking during a brake tensioning process and / or a brake release process. Braking can be effected hydraulically or electromechanically. It can e.g. be exercised via a brake disc or a brake drum.

Various embodiments of the invention will now be explained in more detail by way of example with reference to the accompanying schematic drawings. Show it:

1 is a simplified wheel model,

2 is a diagram for explaining the relationship between the coefficient of friction and the slip of a wheel,

3 shows a block diagram of an embodiment of the device according to the invention,

4 shows a flow diagram of an embodiment of the method according to the invention, and

5 shows a comparison of results according to the method according to the invention (estimated) with comparative measurements (measured).

A model of a wheel 1 is shown in simplified form in FIG. 1. A brake disc 3 is indicated within the wheel 1, on which a brake pad 2 can rest. A corresponding brake pad is advantageously present on the side of the brake disk 3 facing away from the brake pad 2 shown. On the brake pad 2, for example, vertically act as a tensioning force FSP. The friction effect of the brake pad 2 can be summarized in an action point 6 on a friction radius r- _ß . Like the brake disc 3, the wheel 1 can rotate about the axis 5 at the wheel angular velocity COR. The wheel 1 can be assumed to be a rigid disk with an inertia ΘR. Furthermore, the dynamic tire radius r ^ yn can be regarded as constant. Between the wheel 1 and the roadway 4, the wheel contact force F _z preferably acts perpendicular to the roadway 4. When braking, a braking force Fg is generated counter to the direction of travel. The braking force Fg can be determined, for example, from the tension force F _$ p using the following equation:

Θ. ω, Θ _τ ω _τ

F _B = ^F SP ~ FSP ~~ (1)

^L dyn ^L dyn ^L dyn ^L dyn

with the internal gear ratio C *, which for disc brakes can be made up of twice the friction coefficient μ- _ß between the brake pad and disc (C * = 2-U _ß ), the wheel angle acceleration ώ and the brake gear ratio i _ß , which is the result of the internal gear ratio C * and the effective friction radius r _ß .

All of the quantities listed in equation (1) can be determined in a known manner except for the braking force Fg and the brake transmission factor T _β . To determine the brake transmission factor t _ß , the friction coefficient μ _ß can be determined. This depends on many influencing factors such as type of lining and composition, lining and disc temperature, relative speed between the friction partners, contact pressure, brake disc material and intermediate medium / foreign substances (water, abrasion, dirt, Etc . ). An exact pre-calculation of the friction coefficient μ _ß depending on these influencing factors is not possible. In Burckhardt: "Chassis technology: brake dynamics and car brake systems", Vogel-Verlag, 1991, an approach is given to estimate its value:

The influence of the relative speed is small and is neglected. μ _SS, max i ^st thereby the maximum Belagreibwert, T _ß the friction surface T _ß * the friction surface at μß.max 'Phyd the hydraulic pressure Phyd * a reference hydraulic pressure (usually the maximum allowable for the system pressure), and CT and Cp are dimensionless material constants. The friction surface temperature _ß can be determined using the differential method also given there.

The effective friction radius r _ß changes, for example, due to the expansion of the brake's fist under tension as well as through oblique wear of the brake pads or the brake disc. Thus, the friction coefficient μ _ß and the effective friction radius r _{ß are subject to} large fluctuations, which are reflected in the brake transmission factor i _ß . It is known to experts in the field that the friction coefficient μ _β can scatter by + 30%, for example, under extreme loads. The long-term fluctuations in the friction coefficient μ _ß due to wear and material changes are much smaller (<10%) than the short-term fluctuations due to temperature changes. Based on the friction coefficient μ _ß and the effective friction radius r _ß can therefore only be concluded from the behavior of the brake transmission factor T _ß , but this itself cannot be determined exactly in the manner described.

To determine the brake transmission factor, equation (1) can be solved according to the brake transmission factor τ_ _\ and the braking force F _ß by the wheel friction coefficient between the tire and the road μ _X and the wheel contact force F ₂ ; be expressed so that, for example, with the wheel bearing friction torque MLR. -be straight ahead applies:

_ F _z ^• μ _R χ (λ _B χ) ^{• r} dyn + ^M RL, R + ®R '<> _R τ _B -. (j)

The wheel friction coefficient μ _X can be dependent on the brake _slip λ _ßX , the relationship in turn being subject to influencing parameters such as road surface, intermediate medium, tire profile, etc. The parameters of road surface and intermediate medium can change very quickly during driving. 2 shows the relationship between the wheel friction coefficient μ _X and the brake _slip λ _βX for three different _combinations of influencing factors. The result is approximately the same curve, the maxima of which depend, for example, on the different influencing factors mentioned. Common to all is essentially the slope of the curves from the origin to the onset of so-called sliding slip, represented by the start of sliding in FIG. 2. This slope, the so-called longitudinal stiffness k of the tire, depends exclusively on the type of tire and the tread depth, that is to say State of wear from (see Eichhorn and Roth: "Non-positive connection between tires and road surface - influencing variables and detection", VDI reports No. 916, VDI- Verlag 1993). This area of the coefficient of friction / slip curve therefore changes only very slowly and can therefore be suitable, for example, as a parameter for determining the brake transmission factor i _β .

For the wheel friction coefficient μ _X before the start of sliding, the following simple relationship can be represented with the longitudinal stiffness k:

μ _Rx ^■ = * ^• λ _Bx • (4)

Equation (4) can be used in equation (3) so that the following results for the brake transmission ratio:

F _z • k • λ _Bx • r _d + M _{RL (R} + Θ _R • ώ _R ^τ B = ^• ( ⁵ )

^F SP

The wheel contact force F _z , the brake _slip l _ßX and the longitudinal stiffness k are still to be determined. The wheel contact force F _z can be determined, for example, using a known method (Tiemann: "Investigations into the braking behavior of cars with ABS on an uneven road surface, taking particular account of the influence of the vibration damper, progress reports VDI series 12 no. 204, VDI publishing house, 1994).

The slip λ can be determined, for example, using the following equation from the vehicle speed ^{VKfz unc} * of the respective wheel angular speed:

V _K f

Braking situation: λ _Bx z = z ~ ^ω R ^{• • *} -dyn

/ ^v vehicle

(6) ω _R ^• r _dyr i - V motor vehicle

Drive case: λ _τx = ω _R • -'- dyn

A determination of wheel slip over the speed in the wheel contact surface instead of the vehicle speed V fz i ^st also conceivable. To determine the longitudinal stiffness k according to equation (4), the wheel friction coefficient μ is required before the start of sliding. This can be determined during the vehicle drive from the wheel horizontal force F _x in the contact area in the longitudinal direction of the tire and the wheel contact force F _z :

since the wheel horizontal force F _x , at least on the wheels of the driven axle, can be determined in the drive phases of the vehicle from the moment of the internal combustion engine. The wheel horizontal force F _x can be determined from the following equation:

_ M _AR - M _{RL> R} - Θ _R ώ _R ^h x - ^ Roll ^• (ö) ^r dyn ^F Roll i ^st rolling friction, the AR Radantriebsmoment ^M and RL, the R Radlagerreibmoment. In modern vehicles, the torque delivered by the vehicle engine to the transmission can be measured with a high degree of accuracy and taking into account the power consumed by the auxiliary units in the engine control unit (e.g. engine control ME 7 from Bosch, Unland et al .: "New efficient application methods for the physically based motor control ME 7 ", - MTZ Motortechnische Zeitschrift 59, No. 11, pp. 744-751, 1998") or using a torque sensor (Siemens: "New sensor for torque measurement", ATZ Automobiltechnische Zeitschrift 100, no. 4, p. 268, 1998 ") between the engine and transmission. The wheel drive torque M ^ R can be calculated using the known gear ratios and differential ratios as well as the efficiencies and the inertia of the components of the drive train. However, the wheel horizontal force F _x cannot be determined in this way during braking. It can thus first only determine the longitudinal stiffness k of the driven wheels during the drive.

For the following explanations it can be assumed that the tires of an axle are of the same type (prescribed by law in most countries) and have the same wear condition. In modern vehicles with tire pressure monitoring systems, this can be checked by the electronics, since the tire sensors transmit a tire identifier to the control unit (Normann and Festenberg: "Electronic tire pressure monitoring", progress reports VDI, series 12, No. 362, VDI publishing house , 1998 "). This allows differences in the tires to be taken into account or compensated for. Thus, for the tires an axis of the same longitudinal stiffness k can be assumed. In the following, a vehicle with a driven rear axle HA is considered as an example. The longitudinal stiffness of the driven rear wheels k ^ can thus be determined as follows:

-χ, HA

--HA (9)

-z, HA λ Tx, HA

where the index "HA" stands for the rear axle. The drive slip λ-χ _{X /} H _A ^and the wheel contact force F _Z _HA can be the mean of the respective values for the left and right wheels of the rear axle HA. The wheel horizontal force F _X _HA can correspond to the wheel horizontal force F _x from equation (8), since the differential gear ensures an equal distribution of the moments on the two wheels of the driven axle. In order to increase the accuracy of the longitudinal stiffness kjj _A according to equation (9), this can be determined, for example, while driving using a parameter estimation method, preferably a recursive least-squares (RLS) estimation method (see, for example, Isermann: "Identification of Dynamic Systems", Vol. 1, Springer-Verlag, 1992). By introducing a forgetting factor that is almost 1, the change in the longitudinal stiffness kg _A due to the wear of the tread and the aging of the tire rubber can be taken into account, whereby the forgetting factor can be selected depending on the sampling time constant. A high forgetting factor of almost 1 means that many previous values contribute to the estimate (see Isermann, loc. Cit.). In all-wheel drive vehicles, the determination of the longitudinal stiffness ky _{A can be carried out} ^on the Front wheels VA are carried out analogously to the estimation of the longitudinal stiffness kH _A on the rear wheels HA.

If the drive slip λι> _x and the brake _slip λ _{ßX are considered} according to equation (6), then their amounts are at least the same for the same pairing of tires and road surface. Thus, the longitudinal stiffness of the driven wheels determined in the drive can kg ^ ^for the case of braking the wheels are applied. The following can apply to the braking of the drive axle wheels:

^k _H Λ = τ ^h rz, HA ^ ^' f ^Λ Bx, HA' ⁽¹⁰⁾

the wheel horizontal force F _X can correspond to the braking force F _ß when braking.

There remains the longitudinal stiffness ky _{A of} the non-driven wheels, in this example that of the front wheels. This is preferably done during a braking operation, since only then are horizontal forces transmitted from the non-driven wheels to the road. The braking force of the non-driven wheels F _ß A can be determined from the difference between the total braking force F _ßf g _{es of} the vehicle and the braking force F _ßf H _{A of} the driven wheels determined, for example, using equation (10):

^* ^ B, VA ⁼ ^ B.ges ^_ ^ B.HA '(H)

^• ^ B.ges ^ z.HA ^' - ^ HA ^' ^ Bx.HA k- VA - (12) ^* ^ z, VA '^ Bx.VA The brake _slip λ _ßXf v _A ^and the wheel standing force F _{Z /} VA can result from the respective mean value of the left and right wheels of the front axle VA.

The total braking force F _ßf g _{es of} the vehicle can be determined from the total _deceleration or negative longitudinal acceleration x _{A of} the vehicle and the mass of the vehicle mχf _z taking into account the air resistance force in the longitudinal direction FL _X and the total rolling resistance force F ₀ n _f g _es as follows:

The longitudinal stiffness of the non-driven wheels kv _A , like the longitudinal stiffness of the driven wheels kf _{f A} , can be determined during driving operation to improve the accuracy, ie, for example, to suppress interference, using a parameter estimation method, for example the RLS method. A forgetting factor can be selected in each case, which is preferably in a range between 0.9 and 1, for example 0.9999.

The brake transmission factors i _β for the individual wheels 1 of the vehicle can be determined according to one of the following equations: -z, vl VA λ Bx, vl ^* -dyn M RL, R, vl + θ R, v ω R, vl

^• B, from left

^F SP, from left

^L Fz, vr ^' k _V A ^• ^ Bx, vr ^{' r} dyn ^{+ M} RL, R, vr ⁺ ®R, v '®R, vr ^τ B, vr

^F SP, vr

(14)

^F z, hl 'πA ^' ^ Bx, hl ^{' ' r} dyn ^{+ M} AR + ^M RL, R, hl ⁺ θ _R , _h ' ^ώ R, hl

^L B, St.

^F SP, St.

^F z, hr 'kfjA ^' ß, hr ^'r dyn ^{+ M} AR ^{+ M} RL, R, hr ⁺ θ _R , _h ^{' ώ} R, hr ^τ B, hr

^F SP, hr

The index "vl" identifies the left front wheel, the index "vr" the right front wheel, the index "hl" the left rear wheel and the index "hr" the right rear wheel, as well as the index "v" front and the index "h "behind.

In order to achieve high accuracy for the brake transmission factor i _β , for example to suppress disturbances and / or measurement errors, this can be estimated using a parameter estimation method, preferably an RLS method, with a forgetting factor, which is preferably in a range between 0.9 and 1 . The forgetting factor can be chosen lower than when estimating the longitudinal stiffness k, since the brake transmission factor i _ß (eg due to temperature fluctuations) can change faster than the longitudinal stiffness k.

With the estimated brake transmission factors, e.g. the wheel braking forces are determined using equation (1).

A possible implementation of the device according to the invention is based on the example of a rear-driven vehicle shown schematically in Fig. 3. A plurality of inputs 17 are shown therein, via which, for example, all input variables and / or constants or parameters required for carrying out the method according to the invention can be entered. These can be, for example, the clamping forces F§p of the individual wheels, the vehicle speed VKfz _/ the longitudinal acceleration of the vehicle x _A and the angular velocities ωR. Furthermore, a wheel contact force module 8 can determine the wheel contact forces of the respective wheels and output them to a general data and / or control line 16 for further processing. The wheel contact force module 8 can also have inputs, not shown. For example, the wheel contact forces of the rear wheels F _Zf hl and F _Zf hr and the vehicle speed VKfz can be fed via line 16 to an axle drive torque module 7. The axle drive torque module 7 can, for example, determine the axle drive torque MAR during the drive of the vehicle and output it on the line 16. It can also determine the wheel horizontal force F _x H i wheel longitudinal direction and output it to a first longitudinal stiffening module 9.

The first longitudinal stiffening module 9 can receive the wheel contact forces of the rear wheels F _Zf hl and F _{Z /} hr as well as the corresponding wheel rotation speeds COR ^ I and α) R _f hr and the vehicle speed VKfz as further input variables. From the input variables mentioned, the first longitudinal _{stiffening module} 9 can determine the brake _slip for the rear wheels λ _βX HA ^and the corresponding longitudinal stiffness π _A and output it, for example, to a second longitudinal _{stiffening module} 11. The first longitudinal stiffness module 9 can, for example, determine the longitudinal stiffness of the driven wheels kπ _A according to equation (9), preferably using a parameter estimation method. The second longitudinal stiffening module 11 can be used as input variables via the cables device 16, the respective wheel angular velocities of the individual wheels COR. _V I, .omega.R _Vr, ω _f hl and .omega.R _f r _/ the vehicle speed Vκf _z, the longitudinal acceleration of the vehicle x _A and the respective wheel contact forces of the individual wheels F _zv ι, F _{ZfVr, Zf} hl ^un d F _Zf hr obtained. It can then determine the brake _{slip of} the front wheels λ _ßX VA and the associated longitudinal stiffness ky _A from its input variables and output them to a factor determination module 10. The second longitudinal stiffness module 11 can determine the longitudinal stiffness of the non-driven wheels ky _{A in} accordance with equation (12), preferably using a parameter estimation method.

The factor determination module 10 receives the longitudinal stiffness of the rear wheels kgA as a further input variable from the first longitudinal stiffness module 9. Furthermore, the tensioning forces FS, the wheel contact forces F _z and the wheel angular velocities ωR of the respective wheels as well as the vehicle speed Vκf _z and the axle drive torque MR can be fed via the line 16 become. The factor determination module 10 can then determine the brake transmission factors X _{β of} the individual wheels from its input variables, preferably according to a parameter estimation method, and output them to a brake force determination module 12.

The braking force determination module 12 can determine the braking forces F _{ß of} the respective wheels from the corresponding input variables from the line 16 and the brake transmission factors i _ß and output them to a braking force control 13, which can then output appropriate signals for setting the respective brakes.

The device according to the invention and the method according to the invention can also be used, for example, in braking force control 13. be tegrated. Furthermore, memories can be provided in each module for storing required and / or determined quantities. The individual modules can also be connected directly to one another via corresponding lines. If necessary, further inputs and outputs, not shown, can be provided for control by a control unit, not shown, and / or for data transmission. The features of the device according to the invention and of the method according to the invention are preferably implemented in digital technology, the longitudinal stiffness k being able to be determined, for example, with a time constant in the millisecond to second range and the brake transmission factor with an equally large or a smaller time constant.

The longitudinal stiffness of the driven wheels kgA can advantageously only be determined with traction slip values of, for example, λτx, HA <0.03. This can ensure, for example, that the relationship between the wheel friction coefficient μR _X and the drive slip λτ _{x is} linear. The longitudinal stiffness of the non-driven wheels ky _A is preferably only determined when the front axle has a brake _slip of, for example, λ _βXf v _A <0.03 and / or the rear axle brake _slip of, for example, λ _{βX /} H _A <0.03. This also ensures that the relationship between the wheel friction coefficient μR _X and the respective brake _slip λ _{ßX is} linear. Accordingly, the brake transmission factors T _ß can be determined, for example, during braking only when the front axle is _slipping, for example λ _ßX v _A <0.03 and / or the rear axle is _slipping , for example λ _ßX , HA <0.03. In the other cases, the brake transmission factor i _ß can be taken over, for example, from a memory in which the last and / or several previously determined brake transmission factors i _ß are stored. The Connected relationships apply in particular to the straight driving of the vehicle. When the wheels are turned in, the brake transmission factors can also be adopted from a memory, for example.

FIG. 4 shows a flow chart of an embodiment according to the invention using the example of a rear-driven vehicle. The speed of the vehicle VKfz, the longitudinal acceleration x _{A of} the vehicle and the tensioning force Fgp and the wheel angular velocity ω _{R of} one or more wheels 1 are first determined there in step 18. Then, in step 19, the wheel standing force of one or more wheels 1 is determined. Then in step 20 it is queried whether the vehicle is being driven or braked. This can be done, for example, by evaluating the vehicle speed V fz. If the vehicle is neither driven nor braked (not shown), a possibility can be provided that the method for the current controller loop is ended.

If the vehicle is driven, the motor drive torque MAR and the wheel horizontal force in the longitudinal direction F _{x are} determined in step 21. Subsequently, in step 22, the drive slip of the driven wheels iτx, H _A and the corresponding longitudinal stiffness kπ _{A are} determined. Thereafter, it is checked in step 23 whether the traction slip of the driven wheels lτx, H _A i ^m valid range, that is, z. B. is less than 0.03. If this is the case, its value is saved. If the question in step 23 is answered in the negative, the method is ended.

If it turns out in step 20 that the vehicle is being braked, in step 25 the brake slip of the non-driven nen wheels l _ß , v _A and the corresponding longitudinal stiffness kv _A determined. Then, in step 26, a query is made as to whether the brake slip of the non-driven wheels l _{β -v A is} less than 0.03. If this is the case, the brake transmission factor t _{β of} one or more wheels 1 is determined in step 27, the value of which is stored in step 28. In the subsequent step 29, the braking force F _{β of} one or more wheels 1 is determined. If the query in step 26 is answered in the negative, the process moves on to step 29 immediately.

In step 25, for example, the brake _{slip of} the driven wheels l _ßX , H _A can also be determined. This can then also be checked in a further step to determine whether it is less than 0.03, for example. Accordingly, the respective brake transmission factor t _{β of} the driven wheels and / or the non-driven wheels can be determined if the condition is met.

To illustrate the accuracy of the invention, FIG. 5 shows a comparison of results according to the method according to the invention (estimated) with comparison measurements (measured) for the brake transmission factor of the left front wheel X _β , _v l (a) and the right front wheel X _β , vr (b ). The brake transmission factors shown were estimated using an RLS method with a forgetting factor of 0.9999. There is a very good agreement between the estimated and the measured values. The values of the estimate for X _ß , _v l 0,0 0.089 and X _ß , _V r 0,0 0.083 that occur within approximately 300 s are close to the values of i _ß = 2 μ _ß • r _ß »0.088 specified by the manufacturer. The measured brake transmission factors were calculated from the measured clamping forces FSP and by using Torque measuring figs measured braking torques M _ß determined, the relationship between braking torque M _ß and braking force F _ß can be given by

M _B = F _B ^• r _dyn . (15)

The features of the device according to the invention and of the method according to the invention make it possible to determine the braking forces on the individual wheels of a motor vehicle that influence vehicle dynamics. This can be done by determining the clamping forces and corresponding proportionality factors, i.e. Brake transmission factors happen. It is taken into account that the brake transmission factors change during driving, this change happening slowly. The brake ratio factors can each be newly adapted while driving, which enables braking force determination with high accuracy.

Claims

Expectations 1. A method for determining a braking force (F _ß ) acting in the footprint of a wheel (1) of a vehicle for a braking force control (13), characterized by the following steps: Determining a braking force (Fgp) of a brake causing braking, - Determining a brake transmission factor (t _ß ), which represents a proportionality between the clamping force (Fsp) and the braking force (F _ß ), and - Determine the braking force (F _ß ) depending on the clamping force (FSP) and the brake transmission factor (t _B ). 2. The method according to claim 1, in which a wheel contact force (F ₂ ) acting in the contact area is determined and the brake transmission factor (t _β ) is determined as a function of the wheel contact force (F _z ). 3. The method according to claim 1 or 2, in which a coefficient of friction (Rx) between the wheel (1) and a roadway (4) is determined and the brake transmission factor (t _β ) is determined as a function of the coefficient of friction (HIR _X ). 4. The method according to at least one of claims 1 to 3, in which a brake _slip (l _ßX ) and / or a drive slip (lτx) of the wheel is determined and the brake transmission factor (t _ß ) as a function of the brake _slip (l _ßX ) and / or the traction slip (lτχ) is determined. , Method according to at least one of Claims 1 to 4, in which the gradient (ky _A ) of a linear initial characteristic of the coefficient of friction (ΠIR _X ) as a function of the brake slip (l _ß ) and / or the gradient (kπA) of a linear initial characteristic of the coefficient of friction (ΠIR _X ) are determined as a function of the drive slip (lτx) and the brake transmission factor (t _ß ) is determined as a function of the gradient (s) determined (ky _{A /} kgA). 6. The method of claim 5, wherein one or both of the slopes (v _A ^ -H) are each estimated using a parameter estimation method. 7. The method according to at least one of claims 1 to 6, in which an axle drive torque (MAR) is determined and the brake transmission factor (t _β ) is determined as a function of the axle drive torque (MAR). 8. The method according to at least one of claims 1 to 7, in which the brake transmission factor (t _β ) is estimated on the basis of a parameter estimation method. 9. The method according to at least one of claims 1 to 8, wherein the braking force (F _ß ) according to the equation F fc _B - - ^{Tß •} vh _sp ^Θ R ^{• ώ} R dyn ^L dyn and / or the brake transmission factor of the left front wheel (t _ßfV ι), the right front wheel (t _ßfVr ), the left rear wheel (t _ß hl) and the right rear wheel (tß _f hr) each according to one of the estimation equations ^F z, vl ^' k _V A ^' ^ Bx, vl ' ^r dyn + ^M RL, R, vl ⁺ "R, v ^{' ω} R, vl ^L B, from left ^F SP, from left F 'k _V A. 1 ^ Bx ^ r ^'r dyn ^{+ M} RL, R, vr ⁺ "R, v ^{* ω} R, vr ^l B, vr ^F SP, vr ^F z, l 'k _H A ^' ^ Bx, hl ^"r dyn ^{+ M} AR ^{+ M} R, R, hl ⁺ ^ _R , '® _{R (hl} ^L B, St. ^F SP, St. ^F z, hr ^' _H A' λßx, hr ^'r dyn ^{+ M} AR ^{+ M} R, R, hr ⁺ ®R, b. ^' ®R, hr "Brhr- -SP, hr are determined, where t _ß the brake transmission factor, ^r dyn the effective tire radius, FSP the clamping force, Θ _R the moment of inertia of the wheel (1), Θ _R , _V , θ _{R / h} the respective moments of inertia of the front and rear wheels, ώ _R the wheel angle acceleration, ®R, vi ® _R , vr ι ® _R , i '^ _yhr the respective wheel angle accelerations front left, front right, rear left and rear right, ^F z, vl / ^F z, vr / ^F z, hlr ^F z, hr the respective wheel contact forces front left, front right, rear left and rear right, kv _A f kπ _A the respective gradients of the linear characteristic curve of the coefficient of friction depending on the slip (l _ßX , lτχ) for the wheels of the front axle (VA) and the rear axle (HA), ! BX, V1! Bx, vr, lßx, hl, iBx, hr the respective brake slips front left, front right, rear left and rear right, MRL, R, _V 1 M _RL , _{R Vr} , M _RL , _{R hl} , M _{RLfR / hr are} the respective wheel bearing _{friction torques} at the front left, front right, rear left and rear right, MAR are the final drive torque. 10.Device for determining a braking force (F _ß ) acting in the footprint of a wheel (1) of a vehicle for a braking force control (13), characterized by a clamping force determination module (14) which determines a clamping force (Fgp) of a brake causing braking, a factor determination module (10), which determines a brake transmission factor (t _ß ), which represents a relationship between the clamping force (FSP) and the braking force (F _ß ), and a braking force determination module (12), which depends on the braking force (F _ß ) the clamping force (Fgp) and the brake transmission factor (t _ß ).