DE102016200691B4 - Motor vehicle with a braking device and emergency braking function - Google Patents

Abstract

Motor vehicle (1) with a braking device which, for the front wheels (2) of the motor vehicle (1) and for the rear wheels (3) of the motor vehicle (1), has a combination brake cylinder (7) for actuating a respective friction brake (4) of the respective wheel (2, 3), with each combination brake cylinder (7) having a first pressure chamber (8) for a service brake function, by means of which the friction brake (4) can be actuated by ventilation, and a second pressure chamber (10) for a parking brake function, by means of which the friction brake ( 4) can be actuated by venting and is provided and wherein the braking device has a service brake circuit (9) for controlled ventilation of the respective first pressure chamber (8) of the combination brake cylinder (7) and a parking brake circuit (11) to prevent rolling away during parking operation ) comprises the combination brake cylinder (7) for the controlled venting of the respective second pressure chamber (10), whereby the parking brake circuit (11) for the combination brake cylinders (7) of the front wheels (2) on the one hand and for the combination brake cylinders (7) of the rear wheels (3) on the other hand each have at least one separate control valve (21, 22) for venting and re-venting the respective second pressure chamber ( 10) the combination brake cylinder (7) and a valve control device (18) is set up to set the control valves (21, 22) independently of one another, characterized in that a load estimation device is set up to estimate a load of the motor vehicle (1), and the valve control device (18) is set up to set the control valves (21, 22) differently as a function of an estimation signal from the load estimation device in such a way that the rear wheels (3) are not locked at all or at least only after the front wheels (2) are locked, and / or that the distribution of the braking forces on the front and rear axles is up to a predetermined tolerance corresponds to the distribution of the load on the front and rear axles.

Description

The invention relates to a motor vehicle with a braking device according to the preamble of claim 1 and a method for operating a motor vehicle according to the preamble of claim 12.

For example, this is from the DE 199 35 979 A1 a vehicle brake system is known in which the friction brakes of the rear wheels of the motor vehicle can each be actuated by a combination brake cylinder. For the parking brake, a spring-loaded cylinder of the combination brake cylinder is used to close the friction brakes of the rear wheels so that the motor vehicle is prevented from rolling away.

From the DE 10 2005 042 673 A1 A compressed air brake system is known for a motor vehicle, which has a redundancy line in order to continue to provide pressure for actuating the friction brakes of the front wheels from a rear brake circuit by switching a two-way valve in the event of failure of a brake circuit for front wheels.

Such redundancy measures can compensate for the failure of half of the service brake circuit system in a brake circuit system for the service braking, that is to say in the service brake circuit system. If, however, in a brake circuit operating system, for example, its control unit fails or the compressor fails, the motor vehicle is no longer able to enable a speed reduction while driving.

Efforts are therefore being made to introduce an additional level of redundancy into the braking function. For example, it can be provided that the parking brake circuit is activated in the event of a complete failure of the service brake circuit, so that the motor vehicle is brought to a standstill by means of the anti-roll device. This is brought about by switching on the anti-roll device so that those wheels of the motor vehicle that are detected by the anti-roll device brake the motor vehicle to a standstill and then hold it there.

The disadvantage of this solution is that the motor vehicle must be brought to a standstill immediately upon detection of a defect in the service brake circuit, since the parking brake circuit can only switch on the anti-roll device in binary mode and thereby activate the friction brakes. This is particularly undesirable in the case of autonomous vehicles, which would then simply be brought to a standstill if a defect in the service brake circuit were detected without further measures to be taken to protect road traffic, since there is no person in the motor vehicle. In the case of light and heavy trucks in particular, this can represent a traffic obstacle and even a traffic hazard. In addition, the brakes of the parking circuit usually act on the rear wheels of the motor vehicle. With purely digital braking, there is a risk of blockage on the rear wheels of the motor vehicle and thus a considerable risk of skidding.

From the DE 10 2008 018 622 A1 , the DE 10 2008 003 379 A1 , the DE 32 03 730 A1 and the DE 29 10 929 A1 It is known to use spring-loaded brake cylinders of the front brakes and the rear brakes in a truck to brake the truck in the event of a failure of the service brakes. Care is taken to generate a greater braking torque with the front brakes than with the rear brakes in order to prevent the rear wheels from locking. In one case, an ABS control system is used for the control.

The invention is based on the object of providing a redundant brake system in a motor vehicle, in particular an autonomously driving motor vehicle.

The object is achieved by the subjects of the independent claims. Advantageous developments of the invention are given by the features of the dependent claims, the following description and the figure.

The invention provides a motor vehicle which has a braking device. For the front wheels of the motor vehicle and for the rear wheels of the motor vehicle, the braking device each has a tristop cylinder for actuating a respective friction brake of the respective wheel. In a manner known per se, each Tristop cylinder provides a first pressure chamber for a service brake function, by means of which the friction brake can be actuated by venting, i.e. by increasing the pressure in the chamber, and a second pressure chamber for a parking brake function, by means of which the friction brake is vented , that is, can be actuated by reducing the pressure in the second pressure chamber. By reducing the pressure in the second pressure chamber, it is possible, for example, for a pretensioned spring to operate the friction brake. By increasing the pressure in the second pressure chamber, the spring is compressed again and the friction brake is released. The service brake is provided for reducing the driving speed, that is to say for braking the motor vehicle, while driving, ie when driving. Correspondingly, the braking device for the speed reduction has a service brake circuit for the controlled ventilation of the respective first pressure chamber of the Tristop cylinder. Furthermore, the parking brake of the motor vehicle is also implemented by the Tristop cylinder. Correspondingly, the braking device for anti-roll protection or parking braking during parking operation of the motor vehicle has a parking brake circuit for the controlled venting of the respective second pressure chamber of the Tristop cylinder. Parking operation means that the motor vehicle is at a standstill when the motor vehicle is switched off, for example.

In order to provide a redundant brake system with a good brake force distribution for driving, the parking brake circuit for the tristop cylinders of the front wheels on the one hand and for the tristop cylinders of the rear wheels on the other hand each have at least one control valve for venting and re-venting the respective second pressure chamber of the tristop cylinder. Thus, not only can the respective first pressure chamber of the Tristop cylinder be specifically ventilated and vented in a known manner by means of the service brake circuit and a different or different brake pressure can be built up or provided in the service brake circuit for the front wheels and the rear wheels, but the parking brake circuit is also in one front brake circuit for the front wheels and a rear brake circuit for the rear wheels, whereby the respective friction brake for the front wheels on the one hand and for the rear wheels on the other hand by the respective control valve for the front Tristop cylinder on the one hand and the rear Tristop cylinder on the other hand can be operated separately or independently of each other. Correspondingly, in the motor vehicle according to the invention, a valve control device is also set up to set the control valves independently of one another. The parking brake circuit thus realizes a parking brake with two brake circuits.

The invention has the advantage that both by means of the service brake circuit and by means of the parking brake circuit, the friction brakes for reducing speed during driving, i.e. while the motor vehicle is rolling, can be controlled in such a way that the brake pressure for the front wheels is set differently from the brake pressure for the rear wheels can be. In the case of commercial vehicles in particular, this is advantageous due to the large spread of the axle load on the rear axle between empty and fully loaded. This was recognized as a necessary prerequisite for the fact that full braking of a motor vehicle while driving is also possible by means of a parking brake. The motor vehicle can therefore continue to be driven even if the service brake circuit fails. Otherwise, excessive braking can block the rear wheels without the front wheels being able to generate a sufficiently large or optimal braking torque.

The invention provides for an adaptive division of the braking force between the front wheels and rear wheels as a function of the load. A load estimation device is set up to estimate a load of the motor vehicle, and the valve control device is set up to set the control valves differently depending on an estimation signal from the load estimation device in such a way that the rear wheels are not locked at all or at least only after the front wheels are locked and / or that the distribution of the braking forces on the front and rear axles corresponds to the distribution of the load on the front and rear axles up to a predetermined tolerance level. The brake pressure in the second pressure chambers of the tristop cylinders of the rear wheels is preferably maximized. Said load estimation includes the estimation of the load to the effect that the load weight and / or the load distribution in the motor vehicle are determined. A device can be provided as the load estimation device, for example, which determines a compression travel of the motor vehicle, i.e. the change in travel of shock absorbers and / or leaf springs of the motor vehicle due to a load compared to the empty state of the motor vehicle. In vehicles with an air suspension system, the air pressure within the bellows can be used as a measure of the payload in a known manner. The setting of the valves can take place, for example, by means of an assignment rule which assigns a braking torque distribution for the front wheels and the rear wheels to the determined load. The assignment rule can be implemented as a characteristic curve.

The invention also includes optional developments, the features of which result in additional advantages.

A further development of the invention makes it possible to fine-tune the speed reduction by means of the parking brake circuit in such a way that only a partial speed reduction takes place, i.e. the motor vehicle is not fully braked with the braking force provided for the anti-roll protection. In this development, each of the control valves is implemented by a proportional valve. Such a proportional valve can be set steplessly or with more than two steps, so that a division or distribution of the brake pressure between the second pressure chambers of the tristop cylinders of the front wheels on the one hand and the tristop cylinders of the rear wheels on the other hand can be graded or divided, and here also the pressure in every second pressure chamber by means of the Control valves can be changed gradually or in steps.

A further development of the invention even makes it possible to use the parking brake circuit to completely redundantly replace the service brake circuit, i.e. to brake the motor vehicle using the parking brake circuit in the same way during driving by braking maneuvers or to reduce the speed as with the service brake circuit. In this development, the valve control device is set up to receive a deceleration signal which specifies a target deceleration for the motor vehicle and to effect the target deceleration via the control valves of the parking brake circuit by means of a pressure control for the respective second pressure chambers to reduce speed during driving. The pressure in the second pressure chambers can thus be regulated so that the motor vehicle can be braked in a targeted manner with the required target deceleration. Said deceleration signal can be generated, for example, by a system for driverless driving of the motor vehicle or by a brake pedal sensor.

According to a further development of the invention, the parking brake circuit is used as a redundant brake circuit or backup brake circuit in the event of a failure of the actual service brake circuit. In this development, the valve control device is set up to detect an error signal during driving, which signals a defect in the service brake circuit, or the absence of a regular signal, and depending on the error signal or the absence of the regular signal during driving, when a brake signal is received To bring about speed reduction by setting the control valves of the parking brake circuit. If the service brake circuit fails, a fully-fledged replacement is available with the parking brake circuit in the event that the failure is detected and signaled by the error signal or the absence of the regular signal. The defect in the service brake circuit can be identified using means known per se from the prior art.

Said brake signal can be, for example, the signal for said target deceleration. The brake signal can therefore include a specification for a braking torque and / or a braking acceleration (that is to say a deceleration or negative acceleration). The valve control device is preferably set up to set the specification for the braking torque and / or the braking acceleration by setting a larger braking torque on the front wheels than on the rear wheels. This is now possible through the control valves of the parking brake circuit. The braking force distribution with a greater proportion on the front wheels than on the rear wheels results in the advantage that the braking force or the braking torque can be maximized by means of the parking brake circuit without the rear wheels blocking and the vehicle or the vehicle train then breaking out.

According to a further development of the invention, there is even a reaction in the event that the rear wheels or the front wheels and the rear wheels are blocked. In this development, a blockage detection device is provided at least for the rear wheels, which is set up to generate a blocking signal in the event of a wheel blockage during driving operation. The valve control device is set up to release a signaled wheel blockage by setting the control valves. This results in an ABS functionality (ABS - anti-lock braking system) even for the parking brake circuit.

According to a development of the invention, a wheel speed sensor can be provided as a blockage detection device on each wheel to be monitored, which is provided in particular as an additional wheel speed sensor exclusively for the blockage detection described. In addition or as an alternative to wheel speed sensors, a comparison unit can be provided for comparing a transmission speed with a driving speed signal. In a transmission, a mean value of the speeds of the rear wheels, for example, can be determined. This is possible by coupling the rear wheels to the gearbox via the differential. A driving speed sensor can, for example, determine the average speed of all wheels of the motor vehicle. The comparison unit can be, for example, a program module in a microcontroller that determines a deviation between the mean value of the rear wheel speeds on the one hand and the driving speed on the other. A wheel blockage can also be recognized here.

A further development of the invention prevents a wheel blockage in advance or at least counteracts the wheel blockage. In this refinement, the valve control device is set up to actuate the friction brakes while driving during a speed reduction by setting the control valves of the parking brake circuit in such a way that the rear wheels maintain a higher speed than the front wheels. This at least ensures that the front wheels are blocked first, if at all, before the rear wheels also block. A different wheel speed results from a different slip of the wheels with respect to the driving surface.

A further development solves the problem that if the brake control device fails, the brake circuits can no longer be activated at all. In this further development of the invention, the valve control device, which is provided in the aforementioned manner for controlling the control valves of the parking brake circuit, is provided by a brake parking control device that differs from a service brake control device. The service brake control device controls the control valves of the service brake circuit, which are not described in detail here. In the prior art, the brake parking control device is only responsible for the anti-roll protection described. However, it can also be expanded with little effort as the valve control device for the control valves of the parking brake circuit. This results in complete redundancy also with regard to the control logic. If the service brake control device fails, the motor vehicle can still carry out braking maneuvers while driving on the basis of the brake parking control device.

In this case, the brake parking control device is preferably connected to a different electrical energy source than the service brake control device. This further increases the redundancy. The service brake circuit and the parking brake circuit are preferably supplied with electrical energy from different electrical voltage sources. For example, the service brake circuit can be supplied by a first battery and the parking brake circuit can be supplied by a second electrical battery. If the first battery fails, the vehicle can still be braked by the parking brake system, which is electrically supplied from the second battery.

As already stated, it is necessary, particularly in the case of driverless or driverless motor vehicles, not to simply stop the motor vehicle directly on the road in the event of a defect in the service brake circuit. In this context, a further development of the invention provides that the motor vehicle has a system for driverless driving of the motor vehicle and this system is set up to reduce the speed by means of the service brake circuit during driving operation in a manner known per se and, however, in the event of a defect in the Service brake circuit to continue driving the motor vehicle (i.e. not simply to stop) and to bring about the speed reduction by means of the valve control device via the parking brake circuit. As a result, the system for driverless driving of the motor vehicle can, for example, still maneuver the motor vehicle into a parking lot and park it safely there.

The motor vehicle according to the invention is preferably designed as a motor vehicle, in particular as a truck, rail vehicle or passenger car or passenger bus, so that it can be equipped, for example, with a system for driverless driving, i.e. freight or passengers can be transported by motor vehicle without personnel costs.

The invention also provides a method for operating a motor vehicle. The method results from the operation of the motor vehicle according to the invention. In the aforementioned manner, tristop cylinders are provided on the front and rear wheels, with each tristop cylinder having a first pressure chamber for a service brake function, by means of which the friction brake of each wheel is actuated by ventilation, and a second pressure chamber for a parking brake function, by means of which the friction brake of the respective Wheel is actuated by venting, is provided. The braking device uses a service brake circuit to ventilate the respective first pressure chamber of the tristop cylinder during driving operation to reduce speed and by means of a parking brake circuit ventilates the respective second pressure chamber of the tristop cylinder during parking operation or for parking operation in order to provide protection against rolling away. The parking brake circuit for the tristop cylinders of the front wheels on the one hand and for the tristop cylinders of the rear wheels on the other hand each has at least one control valve for venting and re-venting the respective second pressure chamber of the tristop cylinders. A valve control device adjusts the control valves independently of one another in order to obtain the aforementioned advantages, particularly during driving operation, for a speed reduction by means of the parking brake circuit.

The invention also includes further developments of the method according to the invention which have features as they have already been described in connection with the further developments of the motor vehicle according to the invention. For this reason, the corresponding developments of the method according to the invention are not described again here.

An exemplary embodiment of the invention is described below. For this purpose, the single figure (FIG.) Shows a schematic representation of an embodiment of the motor vehicle according to the invention.

The exemplary embodiment explained below is a preferred embodiment of the invention. In the exemplary embodiment, the described components of the embodiment each represent individual features of the invention which are to be considered independently of one another and which also further develop the invention in each case independently of one another. Furthermore, the embodiment described is also through further features of the invention already described can be added.

The figure shows a motor vehicle 1 which can be, for example, a truck or a tractor unit for a tractor-trailer or a taxi or a passenger bus. The car 1 is in particular as an autonomously driving vehicle with a system for driverless driving of the motor vehicle 1 .

Front wheels are shown 2 and rear wheels 3 , each with a friction brake 4th have, of which a brake disc in the figure 5 and a caliper 6th are shown with brake pads not shown in detail. To operate the respective friction brake 4th is with the motor vehicle 1 one Tristop cylinder each 7th with a first pressure chamber 8th for a pneumatic service brake circuit 9 and a second pressure chamber 10 for a pneumatic parking brake circuit 11 provided. Another name for Tristop cylinder is also combination brake cylinder or combination cylinder for short.

The service brake circuit 9 can be configured in a manner known per se, for example as a function of an actuation of a brake pedal of the motor vehicle 1 or as a function of a braking request signal of a system for automatically driving the motor vehicle 1 in the first pressure chamber 8th of every Tristop cylinder 7th to increase a pressure, i.e. the first pressure chamber 8th to ventilate, creating a push rod 12th in a known manner on the friction brake 4th acts and thereby a braking torque on the front wheels 2 or the rear wheels 3 generated.

If the motor vehicle 1 is to be turned off, can by means of the parking brake circuit 11th the second pressure chamber 10 be vented. In other words, the second pressure chamber is provided for driving 10 is under pressure, that is, is ventilated, so that this causes the push rod 12th not through the second pressure chamber 10 on the friction brakes 4th works. By venting the second pressure chamber 10 acts a spring 13th on the push rod 12th and this in turn on the friction brakes as described 4th so that the friction brakes 4th the front wheels 2 or the rear wheels 3 brake and thus the parked vehicle rolling away 1 impede.

From the parking brake circuit 11th are a compressor 14th , an air tank 15th , a check valve 16 and a parking brake controller 17th with a valve control device 18th shown. The valve control device 18th can for example be implemented on the basis of a processor device, such as a microcontroller, and a program module for the processor device.

In the motor vehicle 1 represents the valve control device 18th two control valves 19th , 20th to thereby reduce the pressure in the second pressure chambers 10 the Tristop cylinder 7th the front wheels 2 regardless of the pressure in the second pressure chambers 10 the Tristop cylinder 7th the rear wheels 3 to adjust. During a parking operation with the motor vehicle stationary and switched off 1 can, for example, an operator using a parking brake button 21 the second pressure chambers in a known manner 10 vent.

During driving operation, it is now additionally based on the valve control device 18th possible the control valves 19th , 20th to be controlled separately from each other and thereby also by fully or partially venting the second chambers 10 the car 1 to brake, that is, to reduce its speed without skidding the motor vehicle 1 to cause.

In particular, the control valves are for this purpose 19th , 20th designed as proportional valves. Tristop cylinders on all four wheels 7th are provided and the parking brake circuit 11th by means of the control valves 19th , 20th can be braked in a metered manner, the parking brake circuit 11th as redundancy for the service brake circuit 9 can be used. When the service brake circuit 9 is not functional due to a fault, the braking of the motor vehicle 1 while driving by means of the parking brake circuit 11th be performed. All four wheels can be braked 2 , 3 achieve a maximum braking effect without affecting the motor vehicle 1 goes into a skid.

This is the case with the motor vehicle 1 possible because of the parking brake circuit 11th at least two control valves 19th , 20th and thus the parking brake circuit 11th in two pitch circles or pneumatic channels 22nd , 22 ' is split to the front wheels 2 one hand and the rear wheels 3 on the other hand to brake with an independent brake pressure. The first pneumatic channel 22nd is for the two rear wheels 3 the rear axle and the second pneumatic duct 22 ' for the two front wheels 2 the front axle provided to their Tristop cylinder 7th via the second pressure chambers 10 to operate. You get a parking brake system with a variable brake force distribution. This can be done by the valve control device 18th can be set. In this way, the influence of wheel load differences can be compensated for by loading. When the vehicle is unladen 1 the rear wheels are allowed 3 only slightly when the vehicle is loaded 1 in relation to this, more (with the same braking deceleration) are braked.

Optionally on each rear wheel 3 a stand-alone wheel speed sensor 23 be provided whose signals with the parking brake control device 17th read in and by the valve control device 18th be evaluated. This can be done by increasing the pressure in the pneumatic duct 22nd the rear axle of a possible wheel blockage of one or both of the rear wheels 3 be counteracted.

The valve control device is preferred 18th , for example the parking brake control unit 17th , to another source of electrical energy 24 connected as a service brake control device of the service brake circuit 9 . Optionally, in a transmission of the motor vehicle 1 an average of the two rear wheel speeds can be used with a vehicle speed sensor as a signal for blockage detection.

Overall, the example shows how the invention can provide a two-channel parking brake.

List of reference symbols

1

Motor vehicle

2

Front wheel

3

Rear wheel

4th

Friction brake

5

Brake disc

6th

Caliper

7th

Tristop cylinder

8th

First pressure chamber

9

Service brake circuit

10

Second pressure chamber

11th

Parking brake circuit

12th

Push rod

13th

feather

14th

compressor

15th

Air tank

16

check valve

17th

Parking brake control unit

18th

Valve control device

19th

Control valve

20th

Control valve

21

Parking brake button

22nd

Pneumatic duct

22 '

Pneumatic duct

23

Wheel speed sensor

24

Energy source

Claims (12)

Motor vehicle (1) with a braking device which, for the front wheels (2) of the motor vehicle (1) and for the rear wheels (3) of the motor vehicle (1), has a combination brake cylinder (7) for actuating a respective friction brake (4) of the respective wheel (2, 3), with each combination brake cylinder (7) having a first pressure chamber (8) for a service brake function, by means of which the friction brake (4) can be actuated by ventilation, and for a parking brake function a second pressure chamber (10), by means of which the friction brake ( 4) can be actuated by venting and is provided and wherein the braking device has a service brake circuit (9) for controlled ventilation of the respective first pressure chamber (8) of the combination brake cylinder (7) and a parking brake circuit (11) to prevent rolling away during parking operation ) for controlled venting of the respective second pressure chamber (10) of the combination brake cylinder (7), wherein the parking brake circuit (11) for the combination brake cylinders (7) of the front wheels (2) on the one hand and for the combination brake cylinders (7) of the rear wheels (3) on the other hand at least one separate control valve (21, 22) for venting and re-venting the respective second pressure chamber ( 10) the combination brake cylinder (7) and a valve control device (18) is set up to set the control valves (21, 22) independently of one another, characterized in that a load estimation device is set up to estimate a load of the motor vehicle (1), and the valve control device (18) is set up to set the control valves (21, 22) differently as a function of an estimation signal from the load estimation device in such a way that the rear wheels (3) are not blocked at all or at least only after the front wheels (2) are blocked and / or that the distribution of the braking forces on the front and rear axles is up to a predetermined T Tolerance corresponds to the distribution of the load on the front and rear axles. Motor vehicle (1) according to Claim 1 wherein each of the control valves (21, 22) is provided by a proportional valve. Motor vehicle (1) according to one of the preceding claims, wherein the valve control device (18) is set up to receive a deceleration signal, which indicates a target deceleration for the motor vehicle (1), and to reduce the target speed while driving. Delay via the control valves (21, 22) by regulating the pressure for the respective second pressure chambers (10). Motor vehicle (1) according to one of the preceding claims, wherein the valve control device (18) is set up to detect an error signal during driving operation, which signals a defect in the service brake circuit (9), or the absence of a regular signal and depending on the error signal or failure of the regular signal during driving operation when receiving a brake signal to bring about the speed reduction by setting the control valves (21, 22) of the parking brake circuit (11). Motor vehicle (1) according to Claim 4 , wherein the brake signal includes a specification for a braking torque and / or a braking acceleration and the valve control device (18) is set up to implement the specification by setting a greater braking torque on the front wheels (2) than on the rear wheels (3). Motor vehicle (1) according to one of the preceding claims, wherein a blockage detection device is provided at least for the rear wheels (3), which is set up to generate a blocking signal in the event of a wheel blockage during driving operation, and the valve control device (18) is set up to signal a wheel blockage by setting the control valves (22). Motor vehicle (1) according to Claim 6 , wherein the blockage detection device has a wheel speed sensor (23) on each wheel (3) to be monitored and / or a comparison unit for comparing a transmission speed with a driving speed signal. Motor vehicle (1) according to one of the preceding claims, wherein the valve control device (18) is set up to actuate the friction brakes (4) during driving operation during the speed reduction by setting the control valves (21, 22) in such a way that the rear wheels (3) Maintain a higher speed than the front wheels (2). Motor vehicle (1) according to one of the preceding claims, wherein the valve control device (18) is provided by a brake parking control device (17) which is different from a service brake control device. Motor vehicle (1) according to Claim 9 , wherein the brake parking control device (17) is connected to a different electrical energy source (24) than the service brake control device. Motor vehicle (1) according to one of the preceding claims, wherein the motor vehicle (1) has a system for automated driving of the motor vehicle (1) and the system is set up to effect the speed reduction by means of the service brake circuit (9) during driving operation and in the case of a Defect in the service brake circuit (9) to continue driving the motor vehicle (1) and to reduce the speed by means of the valve control device (18) via the parking brake circuit (11). A method for operating a motor vehicle (1) with a braking device which has a combination brake cylinder (7) for actuating a respective friction brake (4) of the respective for the front wheels (2) of the motor vehicle (1) and for the rear wheels (3) of the motor vehicle (1) Wheel (2, 3), with each combination brake cylinder (7) each having a first pressure chamber (8) for a service brake function, by means of which the friction brake (4) is actuated by ventilation, and a second pressure chamber (10) for a parking brake function by means of which the friction brake (4) is actuated by venting is provided, and wherein the braking device ventilates the respective first pressure chamber (8) of the combination brake cylinder (7) during driving operation for a speed reduction by means of a service brake circuit (9) and by means of a parking brake circuit (11) the respective second pressure chamber (10) of the combination brake cylinder (7) is vented during parking operation for anti-roll protection, wherein the parking brake circuit (11) for the combination brake cylinders (7) of the front wheels on the one hand and for the combination brake cylinders (7) of the rear wheels (3) on the other hand, each has at least one separate control valve (21, 22) for venting and re-venting the respective second pressure chamber (10) of the Combination brake cylinder (7) and a valve control device (18) sets the control valves (21, 22) independently of one another, characterized in that a load estimation device estimates a load of the motor vehicle (1) and the valve control device (18) controls the control valves (21, 22) Depends on an estimation signal of the load estimation device so that the rear wheels (3) are not locked at all or at least only after the front wheels (2) are locked and / or that the braking forces are distributed between the front and rear axles up to a predetermined tolerance level corresponds to the distribution of the load on the front and rear axles.

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