Method and braking system for electronically setting the brake force distribution and motor vehicle having such a braking system

A method for electronically setting the brake force distribution of a desired total braking force in partial braking forces to the axle of a motor vehicle in dependence on the differential slip is provided. The differential slip is detected as the difference of the slip values of a variable representing the slip at the respective axle and is assigned to a relevant pair of axles. One of the axles is selected as a reference axle and the respective differential slip of a pair of axles is determined as the difference of the slip value at the reference axle and of the respective slip value of one of the further axles.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a U.S. National Stage Application under 35 U.S.C. § 371 of International Application No. PCT/EP2018/060400 filed on Apr. 24, 2018, and claims priority to German Patent Application No. DE 10 2017 005 501.7 filed on Jun. 9, 2017. The International Application was published in German on Dec. 13, 2018 as WO 2018/224216 A1 under PCT Article 21(2). The International Application and Publication are hereby incorporated by reference herein.

FIELD

The present invention relates to electronic braking including, for example, electronically setting the distribution of a desired total braking force and partial braking forces for axles of a motor vehicle.

BACKGROUND

DE 38 29 951 A1 discloses a method for carrying out a load-dependent regulation of the braking pressure on a commercial vehicle, which utilizes the components of a present antilocking system in order thus to realize an automatic load-dependent braking function (ALB). In the known methods, the braking pressure and thus the brake force distribution is intended to be regulated axle-specifically below the locking limit, wherein an inter-axle braking pressure distribution is automatically regulated, in accordance with the evaluation of the wheel rotational speed signals delivered by the rotational speed sensors, in a slip range which lies below the range in which the antilocking function comes effective.

DE 10 2006 045 317 A1 discloses an electronic brake force distribution in which the braking forces are distributed to the vehicle axles in dependence on the vehicle deceleration instead of in dependence on the differential slip. DE 10 2011 118 130 B4 discloses an automatic regulation of the brake force distribution for commercial vehicles, in which the setting of the distribution of a desired total braking force in partial braking forces to the axles of the commercial vehicle occurs in dependence on the differential slip or a differential speed.

SUMMARY

In an embodiment, the present invention provides a method for electronically setting the brake force distribution of a desired total braking force in partial braking forces to the axle of a motor vehicle in dependence on the differential slip. The differential slip is detected as the difference of the slip values of a variable representing the slip at the respective axle and is assigned to a relevant pair of axles. One of the axles is selected as a reference axle and the respective differential slip of a pair of axles is determined as the difference of the slip value at the reference axle and of the respective slip value of one of the further axles. A threshold value is prescribed for the differential slip and, in the presence of differential slip below the prescribed threshold value:

In a first theorem, the desired total braking force of the motor vehicle is assumed as the sum of the partial braking forces of the axles considered in the brake force distribution and a slip-force ratio is prescribed or established for the axles.

In a second theorem, the differential slip is assumed as the difference of the respective products of a slip-force ratio and of the partial braking force at the respective axle considered in the brake force distribution, and at least one desired differential slip determined in advance is prescribed as differential slip for the respective pairs of axles.

From a linking of the first theorem with the second theorem, with consideration of the slip-force ratios of the axles and of the prescribed desired differential slip, the partial braking force at the reference axle is established and set.

DETAILED DESCRIPTION

The electronic brake force distribution (EBD) is a system for stabilizing motor vehicles. By contrast to conventional braking systems having an antilocking system (ABS), the electronic brake force distribution distributes the braking force to be provided overall by setting braking pressures between the axles of the vehicle and thus stabilizes the motor vehicle. The electronic brake force distribution allows the brakes of a motor vehicle to be designed in a manner appropriate to the requirements. In the case of monitored stability behavior of the motor vehicle, the rear axles can be used more strongly for the overall deceleration of the motor vehicle.

In the electronic brake force distribution, an electronic control device continuously calculates the slip differences at the front and rear wheels. if in a braking operation the slip ratio exceeds a prescribed stability limit value, the closing of ABS pressure inlet valves prevents a further increase of the braking pressure at the critical axle and/or increases the braking force at other axles. In the case of an increasing braking requirement, for example if the driver raises the brake pedal force and thus the braking requirement, the slip at the front wheels also generally increases. The ratio of the slip values at the wheels of the front axle and of the critical axle in question becomes smaller again and the pressure inlet valve is now opened again, with the result that the braking pressure at the wheels of the relevant axle can increase again.

nthe brake force distribution by regulating the braking pressure, the braking force at the critical axle is reduced and, where appropriate, the braking forces at all other axles are increased in order to obtain optimum brake force distribution according to the specifications of the regulation. After each regulating intervention, the system waits and checks whether the correction was successful, that is to say the current state corresponds to the specifications. If necessary, further adaptations are performed, that is to say further regulating interventions follow. Here, the brake force distribution occurs in a plurality of small adaptation steps. Particularly in the case of commercial vehicles having a plurality of axles, the brake force distribution becomes more complicated with an increasing number of axles, with the result that the setting of the brake force distribution is often associated with very many small regulating interventions. The readjustment often has a turbulent action and is perceived as unpleasant. Furthermore, a correspondingly large supply of compressed air has to be provided for carrying out the adaptation measures by means of pressure increase and venting for the purpose of pressure reduction.

An object on which the invention is based is to reduce the complexity in the continuous setting of the brake force distribution in motor vehicles having an arbitrary number of axles.

According to an embodiment of the invention, one of the axles is selected as a reference axle and the respective differential slip of a pair of axles is determined as the difference of the slip value at the reference axle and of the respective slip value of one of the further axles. In the presence of differential slip between the axles below a predetermined threshold value, a partial braking force for the selective reference axle is established and set. This reference axle is one of the axles of the motor vehicle and each pair of axles which is considered for determining the partial braking force of the reference axle is formed with the reference axle as a pair component. For the setting, according to an embodiment of the invention, of the distribution of the total braking force to the axles, the respective differential slip between the selected reference axle and one of the remaining axles of the motor vehicle is thus established and evaluated. In other words, the partial braking force is set for a certain reference axle common to all pairs of axles, wherein differential slip for respective pairs of axles is considered with respect to the reference axle and taken into consideration in establishing the partial braking force, in particular with respect to the front axle of the motor vehicle as reference axle.

With prescribed or established slip-force ratios for the axles and also desired differential slip, determined in advance, for each pair of axles, an optimized partial braking force for the reference axle can be established, and this in an arbitrary number of further axles of the vehicle. An embodiment of the invention makes it possible with these specifications to determine the partial braking force at the reference axle below the prescribed threshold value under advantageous conditions of the prescribed values. In this embodiment, the invention has recognized that the ratio between the braking force and the slip at an axle extends substantially linearly in the stable region of the μ-slip curve. Below the prescribed threshold value, which is prescribed such that the stable region of the μ-slip curve is not left, a characteristic slip-force ratio is present for the respective axle. This is continuously established or established in advance and prescribed.

With a determined desired differential slip for each pair of axles, it is thus possible, in the case of differential slips below the prescribed threshold value, for the partial braking force at the reference axle to be established. The desired differential slip for each respective pair of axles is prescribed according to prescribed optimization requirements, for example with view to a wear which is as low as possible. The partial braking forces corresponding to the specifications then result in the desired wear optimization. At the same time, the maximum permissible values for the differential slip are not exceeded.

An embodiment of a method according to the invention with consideration of a prescribed and hence prognosticated desired differential slip and slip-force ratios for the axles is provided only below a prescribed threshold value for the differential slip. If the differential slip exceeds this threshold value, there occur stability interventions, with, for example, an antilocking system setting the braking pressures.

To determine the partial braking force at the reference axle, in a first theorem the desired total braking force of the motor vehicle is assumed as the sum of the partial braking forces of the axles considered in the brake force distribution. The follow equation thus results for the first theorem:
FTotal=F1+F2+F3(formula 1)
where
FTotaldenotes total braking force
F1denotes partial braking force at reference axle
F2denotes partial braking force at second axle
F3denotes partial braking force at third axle
Fndenotes braking force at n-th axle.

In a second theorem, the differential slip is assumed as the difference of the respective products of a slip-force ratio and of the partial braking force at the respective axle considered in the brake force distribution. The second theorem thus corresponds to the following equation:

Reformulating this equation gives the following equation for the partial braking force at the n-th axle:

By virtue of the linking according to an embodiment of the invention of the first theorem with the second theorem with consideration of the slip-force ratios of the axles and of the prescribed desired differential slip, the partial braking force at the reference axle can be precisely determined corresponding to these optimal specifications. The following equation results for the determination of the partial braking force at the reference axle:

The braking operation with a prescribed desired differential slip and individual slip-force ratios for the axles results, in the case of differential slips below the threshold hold prescribed according to an embodiment of the invention, in the number of adjustment measures required for setting the brake force distribution being drastically reduced. In this way, the air consumption of the braking system is reduced and moreover ensures smooth and thus stable braking.

In an advantageous embodiment of the invention, the partial braking forces of the further axles are determined with consideration of the already established partial braking force for the reference axle according to the second theorem or the reformulation of the second theorem that is indicated above as formula 3. Here, it is possible to establish, for an arbitrary number of axles, partial braking forces of the respective axles with optimized distribution of the total braking force.

The slip-force ratios for the determination according to an embodiment of the invention of the partial braking force for the reference axle are preferably prescribed corresponding to the gradient in the stable region of the respective static friction coefficient-slip curves (μ-slip curves) of the wheels at the respective axles. The gradient of the static friction coefficient/slip curve is approximately constant in the stable region, with the result that the value of the gradient represents the characteristic of the respective wheel braking and can be used in a particularly suitable manner for the purposes of the determination according to an embodiment of the invention of the partial braking force at the reference axle.

Reliability of the calculation of the partial braking forces within the context of the brake force distribution is provided with specification of the threshold values of the differential slip below 10%. The brake force distribution advantageously occurs in a calculation on the basis of the desired differential slip, which is established in advance or, in other words, estimated, for each pair of axles below a threshold value of approximately 5%, with the result that a corresponding threshold value of 5% is prescribed. Since for most tires the maximum of the μ-slip curves in the case of a slip is around approximately 10%, the specification of the threshold value of 5% ensures that the slip-force ratios for the axles that are established in advance and prescribed in the calculation are realistically estimated and therefore the brake force distribution is decisively determined from the prescribed desired differential slip. This desired differential slip is optimized in advance for example with a view to wear which is as low as possible.

In each case according to an embodiment, a desired differential slip for each pair of axles is advantageously established or prescribed, wherein the position of the respective axle in the sequence of the axles of the vehicle is taken into consideration.

Establishing the partial braking forces is simplified if the reference axle used is the axle of the vehicle that is situated in the front in the direction of travel, that is to say the vehicle front axle.

In a braking system for electronically setting the brake force distribution with consideration of slip-force ratios for the axles which can be determined in advance and a desired differential slip for all pairs of axles which is determined in advance, a brake control unit is assigned sensors for the axle-by-axle detection of information for determining slip values of a variable representing the slip at the respective axle. Use is preferably made for this purpose of the rotational speed sensors of the respective wheels which are present for present electronic braking systems or at least for an antilocking system. The slip value of the respective wheel is established from the information of the rotational speed sensors. The slip values of two wheels of the same axle can be averaged to determine the differential slip of two axles. As an alternative to using the slip of the respective axle, the speed or the differential speed can be used from the information of the rotational speed sensors.

FIG. 1shows an electric-pneumatic plan of the braking system4of a motor vehicle5, namely of a commercial vehicle30. In the exemplary embodiment, the motor vehicle has three axles1,2,3, namely an axle1situated to the front in the forward direction (front axle), a second axle2and a third axle3. Each axle1,2,3respectively has two wheels6which are arranged on both sides of the motor vehicle5. Each wheel6is respectively assigned a pneumatically actuatable wheel brake7. The wheel brakes7each generate, under the action of braking pressure, partial braking forces F1, F2, F3(FIG. 2) on the respective axle1,2,3.

The setting of the braking pressure and thus of the braking forces is monitored by an electronic brake control unit8. Each wheel6is assigned a pressure control valve9which can be individually activated by the brake control unit8. It is possible by means of a corresponding activation of the respective pressure control valve9for the brake control unit8to individually regulate the braking pressure at each wheel6of the motor vehicle5and thus the braking force of the respective axle.

The brake control unit8and the pressure control valves9are key elements of an antilocking system (ABS) or of an electronic braking system (EBS), which also includes rotational speed sensors10at the respective wheels6. Each rotational speed sensor10is electrically connected to the brake control unit8and continuously signals the rotational speed measurement values11detected by it to the brake control unit8. The brake control unit8establishes the slip λ1, λ2, λ3of the respective axles1,2,3via the rotational speed measurement values11. Here, the results of the individual wheels6of the respectively identical axles can be averaged to a result which represents the respective axle1,2,3, that is to say the slip λ1, λ2, λ3(FIG. 2) for the respective axle1,2,3.

In order to set a defined braking force, the brake control unit8generates electrical actuating signals12for the respective pressure control valves9. Here, depending on the requirement, the braking pressure at each wheel brake7can be increased, maintained or lowered as required in order in this way to generate a defined braking force at the respective wheels6. The brake control unit8is designed to set the braking forces in dependence on differential slips. Accordingly, differential slips δλ2, δλ3 are established from the slip values λ1, λ2, λ3for the individual axles and assigned to a relevant pair15,16of axles1,2,3. Here, the differential slip δλ2 refers to the slip difference between the front axle1and the second axle2, and the differential slip δλ3 refers to the slip difference between the front axle1and the third axle3.

The brake control unit8also sets, via the pressure control valves9, the distribution of a desired total braking force in partial braking forces to the axles1,2,3of the motor vehicle5(brake force distribution22inFIG. 2). For this purpose, specified values are held in a memory element13for the brake control unit8, namely in each case a slip-force ratio for the axles of the motor vehicle5, that is to say, in the exemplary embodiment of a motor vehicle5having three axles, correspondingly three slip-force ratios (δλ/δF)1, (δλ/δF)2, (δλ/δF)3. Moreover, values determined in advance for the desired differential slip λ12, λ13are held in the memory element13. The electronic setting of the distribution of a desired total braking force in partial braking forces to the axles1,2,3of the motor vehicle5is explained in more detail below with reference toFIG. 2.

In the exemplary embodiment illustrated inFIG. 2of the electronic setting of the brake force distribution, it is the case that, for each axle1,2,3, the rotational speed measurement values11of the rotational speed sensors10are fed for evaluation14in order to calculate a slip. Accordingly, a slip value λ1, is established for the axle1, a slip value λ2is established for the second axle2and slip value λ3is established for the third axle3.

In order to establish a partial braking force, one of the axles1,2,3is selected as the reference axle29and the respective differential slip δλ2, δλ3 of a pair15,16of axles1,2,3is determined as the difference of the slip value λ1at the reference axle29and of the respective slip value λ2, λ3of one of the further axles2,3. In the exemplary embodiment, the reference axle29is the axle1of the motor vehicle5situated at the front in the direction of travel, that is to say the vehicle front axle. The differential slips δλ2, δλ3 for each pair15,16of axles are thus related to the axle1selected as the reference axle29for which the partial braking force F1is established. All pairs15,16of axles1,2,3which are considered for establishing the distribution of the total braking force with respect to the respective differential slip δλ2, δλ3 are formed with the reference axle29as respective pair component.

In a comparison step17, the continuously established differential slips δλ2, δλ3 are compared with a prescribed threshold value18for the differential slip δλ2, δλ3. The threshold value18is preferably below 10% slip, namely, in the exemplary embodiment shown, approximately 5% slip. In case of differential slips δλ2, δλ3 above the prescribed threshold value18, a regulation19of the braking pressure or of the braking forces occurs in dependence on the continuously established differential slips δλ2, δλ3.

If both differential slips δλ2, δλ3 lie below the prescribed threshold value18for the currently established differential slips δλ2, δλ3, there occurs a switching20into an estimating mode21in which, for the setting of the brake force distribution22in partial braking forces F1, F2, F3, the desired differential slip λ12, λ13established in advance is prescribed for each pair of axles15,16, and slip-force ratios (δλ/δF)1, (δλ/δF)2, (δλ/δF)3are prescribed for the axles1,2,3.

FIG. 3shows a schematic μ-slip curve26or static friction coefficient/slip curve. The slip-force ratio (δλ/δF)1, (δλ/δF)2, (δλ/δF)3corresponds qualitatively in each case to the gradient27in the stable branch28of the respective μ-slip curve26for the corresponding wheel.FIG. 4illustrates, in the detail IV according toFIG. 3, the substantially linear profile in the stable branch28with a gradient27. The substantially constant gradient27is used for the purposes of estimating the slip-force ratio and thus for the brake force distribution22. The stable branch28of the μ-slip curves26extends to close to the maximum of the curve26, which is approximately 10%, with the result that it is ensured with the specification of the threshold value18of approximately 5% that the specification of realistic slip ratios occurs.

In the brake force distribution22, as first theorem23the desired total braking force FTotalof the motor vehicle is assumed as the sum of the partial braking forces F1, F2, F3of the axles1,2,3considered in the brake force distribution22. This corresponds to the following relationship:
FTotal=F1+F2+F3

na second theorem24, the differential slip is determined as the difference of the respective products of a slip-force ratio (δλ/δF)1, (δλ/δF)2, (δλ/δF)3and of the partial braking force F1, F2, F3at the respective one of the three axles1,2,3. This results in the following relationship for the second theorem24:

Reformulating this second theorem24results in the following equation for the partial braking force at the n-th axle:

This equation takes account of the fact that the slip-force ratios (δλ/δF)1, (δλ/δF)2, (δλ/δF)3are each determined by the gradient27of the respective μ-slip curve.

From a linking25of the first theorem23with the second theorem24with consideration of the prescribed slip-force ratios (δλ/δF)1, (δλ/δF)2, (δλ/δF)3and of the prescribed desired differential slip λ12, λ13, there results, for the exemplary embodiment of a motor vehicle5having three axles (1,2,3), the following relationship for establishing the partial braking force F1at the reference axle29, that is to say the front axle designated by the reference sign1:

With consideration of the established partial braking force F1for the reference axle29, that is to say the front axle1in the exemplary embodiment, the respective partial braking force F2, F3at the further axles is determined according to the second theorem24.

The values for the desired differential slip λ12, λ13are established in advance for the respective pairs15,16of axles for example under the point of view of as low a wear as possible.

LIST OF REFERENCE SIGNS

1. (Front) axle2. (Second) axle3. (Third) axle4. Braking system5. Motor vehicle6. Wheel7. Wheel brake8. Brake control unit9. Pressure control valve10. Rotational speed sensor11. Rotational speed measurement values12. Actuating signals13. Memory element14. Evaluation15. Pair of axles16. Pair of axles17. Comparison step18. Threshold value19. Regulation20. Switching21. Estimating mode22. Brake force distribution23. First theorem24. Second theorem25. Linking26. μ-Slip curve27. Gradient28. Stable branch29. Reference axle30. Commercial vehicleF1Partial braking force at reference axleF2Partial braking force at second axleF3Partial braking force at third axleFTotalTotal braking forceλ1Slip value at reference axleλ2Slip value at second axleλ3Slip value at third axleδλ2 Differential slip between second axle and reference axleδλ3 Differential slip between third axle and reference axleλ12Desired differential slip between second axle and reference axleλ13Desired differential slip between third axle and reference axle(δλ/δF)1Slip/force ratio of reference axle(δλ/δF)2Slip/force ratio of second axle(δλ/δF)3Slip/force ratio of third axle