DE102013224776A1 - Brake system for a motor vehicle - Google Patents

Abstract

The invention relates to a brake system for a motor vehicle having at least two axles, wherein the wheels of a drive axle are connected to an electric drive which has at least one electric machine which can be operated as a motor or generator and which has a first brake circuit with at least one hydraulic wheel brake, in particular two wheel brakes , a master brake cylinder operable by the driver via a brake pedal and connected to the wheel brakes of the first brake circuit, a brake actuation sensor, and a second brake circuit having at least one hydraulic wheel brake, in particular two wheel brakes. According to the invention, the second brake circuit is hydraulically connected only to an autonomous pressure source, the wheel brakes of the second brake circuit being associated with the drive axle and the wheel brakes of the first brake circuit being associated with another axle. Furthermore, the invention relates to a method for controlling a brake system.

Description

The invention relates to a brake system according to the preamble of claim 1, a method for controlling a brake system according to the preamble of claim 14, and the use of a brake system in a motor vehicle having an internal combustion engine.

Motor vehicles with an at least temporary electric drive are becoming increasingly important, and it is desirable in view of the range of the electric drive not only to charge the battery of the motor vehicle while standing, but also to do so during operation. By operating the electric drive or the at least one electric machine as a generator, electrical energy can be obtained during braking of the motor vehicle by conversion of the kinetic energy and stored in the battery.

The braking torque developed by the recuperation, ie the recovery of electrical energy by the generator, depends on the speed of the motor vehicle and the level of the battery. In order to be able to apply sufficient braking deceleration in any case, brake systems have e.g. for hybrid vehicles that combine electric drive and combustion engine, in addition to the electric machine additional braking means. These additional braking means, which are usually implemented in two legally prescribed brake circuits, include, for example, electromechanical or hydraulic wheel brakes based on the friction of a brake pad against a disc or drum connected to the wheel.

In particular, the interaction of hydraulic friction brakes and a generator, which is desirable because of low system costs and moderate system complexity, causes some technical challenges: For efficient recuperation the maximum possible braking torque of the generator should be used in the respective driving situation. Furthermore, a transition between a deceleration of the motor vehicle by the braking deceleration of the generator and a deceleration of the motor vehicle by friction brakes should be possible without causing jerky changes in the deceleration or a critical in terms of driving stability braking force distribution. In addition, it must be avoided that arise by the decoupling of the electric regenerative braking by the generator of the brake pedal for the driver an unusual pedal feel, so that he can estimate the braking effect achieved by him bad.

From the DE 10 2011 003 346 A1 a braking system of a motor vehicle is known, which has at least one electrically regenerative brake and a hydraulic brake system with friction brakes, wherein the individual wheels associated wheel brakes are arranged in at least two brake circuits which are acted upon by a tandem master cylinder with pressure medium, and wherein each brake circuit at least one pressure accumulator and electrically controllable hydraulic valves comprises. The brake system comprises an electronic control and regulating unit, by means of which, when braking with the electric regenerative brake, at least one hydraulic valve is activated in such a way that pressure medium is discharged into a predetermined pressure reservoir at a point in time. In general, the total deceleration of the vehicle is composed of deceleration rates of the wheel brakes and the generator. This generic brake system allows comfortable for the driver pedal feel and efficient recuperation, but the control is quite complex and the life of the brake system by the number of possible switching cycles of the solenoid valves limited, as these - unlike a conventional braking system - even during "normal" Brakes are actuated.

The object of the present invention is to provide an alternative braking system for a motor vehicle with an electrically driven axle, which allows efficient recuperation with a pleasant pedal feel for the driver and preferably has a moderate system complexity.

This object is achieved by a braking system according to claim 1.

Thus, a brake system for a motor vehicle having at least two axles is provided, wherein the wheels of a drive axle are connected to an electric drive which has at least one electric machine which can be operated as a motor or generator and which has a first brake circuit with at least one hydraulic wheel brake, in particular two Wheel brakes, with an operable by the driver via a brake pedal master cylinder, which is connected to the wheel brakes of the first brake circuit, a brake actuation sensor, and a second brake circuit with at least one hydraulic wheel brake, in particular two wheel brakes comprises. According to the invention, the second brake circuit is hydraulically connected only to an autonomous pressure source, the wheel brakes of the second brake circuit being associated with the drive axle and the wheel brakes of the first brake circuit being associated with another axle. With an autonomous or driver-independent pressure source is here meant that for a pressure build-up Energy required not from the braking force of the driver, but an external source such as the vehicle battery is removed and thus a driver-independent pressure build-up can take place.

By only the wheel brakes of the non-driven axle are directly operated by the driver via a brake pedal, the braking torque to the drive axle can be completely built by the electric drive, without a complex hydraulic control is required. As long as the available drag torque of the electric machine / s is sufficient in the generator mode, there is expediently no pressure build-up by the autonomous pressure source. The widespread use of proven hydraulic components, the system complexity remains limited and ensures high reliability at relatively low cost.

According to a preferred embodiment of the invention, the autonomous pressure source is designed as an electro-hydraulic actuator in which a piston is driven by an electric motor with a downstream rotation-translation gear. Such an actuator allows a stepless pressure build-up with a high dynamic, as well as a uniform pressure reduction in all connected wheel brakes.

According to an alternative preferred embodiment of the invention, the autonomous pressure source comprises a driven by an electric motor pump, in particular a piston pump with preferably more than 2 pistons, preferably a pulsation damper, in particular connected to the outlet side of the pump elastic element, a pressure medium reservoir, a between the suction side the first pump disposed on the pump and the pressure medium reservoir and a second valve disposed between the outlet side of the pump and the pressure medium reservoir. Electrically driven piston pumps are particularly inexpensive and robust. By means of a pulsation damper and / or a pump with, in particular, 6 pistons, only slight pressure fluctuations or pulsations occur during pressure buildup.

Conveniently, the autonomous pressure source comprises a pressure sensor and an electronic control unit, wherein the electronic control unit is connected to the brake actuation sensor and has an interface to a first vehicle data bus. The pressure sensor expediently allows a more accurate pressure control. By the electronic control unit is connected to a brake actuation sensor, the autonomous pressure source can ensure a sufficient braking function independently of other control devices. A cooperative control of the braking behavior between a plurality of control devices is possible via the vehicle data bus, wherein preferably an electrohydraulic control device of the first brake circuit passes on brake requests to the autonomous pressure source.

It is particularly useful when the electronic control unit of the autonomous pressure source is connected via the first vehicle data bus with an electro-hydraulic control unit, which comprises the first brake circuit associated solenoid valves and a first pressure accumulator, in particular each wheel brake of the first brake circuit arranged between the master cylinder and the wheel brake Inlet valve and an arranged between the wheel brake and the first pressure accumulator discharge valve is associated. By the electro-hydraulic control unit, which is mature and inexpensive available, a brake slip control can be done on the wheel brakes of the first brake circuit.

It is particularly expedient if the electrohydraulic control unit has an electrically driven pump, in particular a piston pump, which is connected to the first pressure accumulator on the suction side, and an isolating valve arranged between the master cylinder and inlet valve and a reversing valve arranged between the first pressure accumulator and the master cylinder comprises, wherein the outlet side of the electrically driven pump is arranged in particular between the isolation valve and inlet valve / s, and / or comprises a pressure sensor associated with the first brake circuit. Such a configuration also makes it possible to provide a driver-independent pressure build-up at the wheel brakes of the first brake circuit, as is required, for example, for vehicle dynamics control.

Furthermore, it is particularly expedient if the electrohydraulic control unit is connected via a second vehicle data bus to a drive control unit which activates the electrical machine / s associated with the drive axle. The connection to a drive control unit allows coordination between the generator and the friction brake or wheel brakes. Appropriately, the drive control unit reports the maximum available generator braking torque; the electro-hydraulic control unit requests a braking torque to the generator and / or autonomous pressure source in accordance with the driver's brake application.

Furthermore, it is particularly useful if the electro-hydraulic control unit comprises the second brake circuit associated with solenoid valves and a second pressure accumulator, in particular each wheel brake of the second brake circuit between the autonomous pressure source and the Wheel brake arranged inlet valve and an outlet valve disposed between the wheel brake and the second pressure accumulator is assigned. This is advantageous in terms of dynamic control of the braking torque at individual wheels of the drive axle.

In particular, the electrohydraulic control device expediently comprises a second separating valve arranged between the autonomous pressure source and the inlet valve (s) of the second brake circuit and a second switching valve arranged between the second pressure reservoir and the autonomous pressure source. By an analog control of the isolation valve can be a uniform control of the brake pressure in the drive axle.

Preferably, the electro-hydraulic control unit is connected to at least one wheel speed sensor, wherein in particular each wheel brake of the first brake circuit, preferably also each wheel brake of the second brake circuit, a wheel speed sensor is associated. With the wheel speed sensors, excessive slippage of individual wheels can be detected and a vehicle deceleration can be measured.

It is advantageous if the wheel brake / s of the first brake circuit is / are designed as drum brakes, in particular associated with the rear axle, in which case the master brake cylinder is preferably actuated without auxiliary power. The fact that drum brakes are self-reinforcing and comparatively low braking torques are required on the rear axle according to an ideal braking force distribution, can be dispensed with an auxiliary power assistance. Thus, the brake system can be made particularly inexpensive and compact.

Preferably, the wheel brake / s of the second brake circuit are / is designed as disc brakes and in particular assigned to the front axle. At the wheel brakes of the front axle higher braking torques are required. In terms of metering and dissipation of heat occurring during braking disc brakes are advantageous.

It is particularly expedient if the autonomous pressure source comprises the second brake circuit associated solenoid valves and a second pressure accumulator, in particular each wheel brake of the second brake circuit between a pressure generating unit of the autonomous pressure source, in particular an outlet side of a piston pump or a pressure chamber of an electro-hydraulic actuator, and the wheel brake arranged inlet valve and disposed between the wheel brake and the second pressure accumulator exhaust valve is associated, and wherein the electronic control unit is connected to at least one wheel speed sensor, in particular each wheel brake of the second brake circuit is associated with a wheel speed sensor. This allows a brake slip control and / or a vehicle dynamics control even if the brake circuit of the rear axle is constructed hydraulically simpler. Thus, in small vehicles and if in particular a vehicle dynamics control is to take place only in the event of oversteer, a particularly simple and cost-effective braking system can be used on the non-driven axle.

The invention further relates to a method for controlling a braking system according to the invention, in which a braking operation is implemented by the driver on the drive axle by a generator operation of the electric machine / n, and the autonomous pressure source is activated when a measured actual deceleration by more than one predetermined threshold is below a determined in accordance with the brake actuation target delay. Thus, a consistent and comfortable braking behavior for the driver is ensured regardless of the availability of the generator.

Furthermore, the invention relates to the use of a brake system according to the invention in a motor vehicle with an internal combustion engine, wherein the wheels of the drive axle and / or the wheels of the other axle can be driven by the internal combustion engine.

Further preferred embodiments will become apparent from the following description of an embodiment with reference to figures.

Show it

1 A first embodiment of a brake system according to the invention,

2 A second embodiment of a brake system according to the invention, and

3 a particularly preferred embodiment of the first embodiment.

1 shows a schematic representation of a brake system according to the invention in a first embodiment of a four-wheeled motor vehicle.

braking system 1 has four hydraulic wheel brakes 8a . 8b . 8c . 8d on, which are arranged in two brake circuits I, II. Here, on the left front wheel FL is the wheel brake 8a , on the right front wheel FR the wheel brake 8b , on the left rear wheel RL the wheel brake 8c and on the right Rear wheel RR the wheel brake 8d arranged. Every wheel or wheel brake 8th has an associated wheel speed sensor 9 on. The wheel brakes 8c . 8d the rear axle are arranged in a first brake circuit I and preferably designed as drum brakes, while the wheel brakes 8a . 8b the front axle are arranged in a second brake circuit II and are preferably designed as disc brakes. The wheels FR, FL of the front axle are with an electric drive 2 connected, which can be operated as a generator during braking. The electric drive 2 may comprise an electric machine connected to both wheels or two electrical machines connected to each wheel, in particular wheel hub motors.

The brake 8c . 8d of the first brake circuit I are with a master cylinder 5 connected by the driver via a brake pedal 4 can be operated. The master cylinder 5 has a single pressure chamber and is preferably operated without auxiliary power, in particular without an upstream vacuum brake booster. In the de-energized state is the master cylinder 5 with a storage container 6 connected, the pressure medium under normal atmospheric pressure or contains brake fluid. As soon as the driver depresses the brake pedal 4 actuated and the piston of the master cylinder is moved into the pressure chamber, the connection to the reservoir is interrupted. By means of a brake actuation sensor 7 for example, as a pedal angle sensor or in the master cylinder 5 arranged inductive displacement sensor can be realized, it is measured how much or by which way the driver operates the brake pedal. In the hydraulic line between master cylinder 5 and wheel brakes 8c . 8d an electrohydraulic control unit is arranged, which consists of a hydraulic control unit (HCU) 12 and an electronic control unit (ECU) 11 consists. The electronic control unit 11 can solenoid valves of the hydraulic control unit 12 drive to the wheel brakes 8c . 8d to separate from the master cylinder and perform a pressure reduction, preferably by dissipating pressure medium in a low-pressure accumulator. The electro-hydraulic control unit or the electronic control unit 11 is with the wheel speed sensors 9c and 9d connected and can for example perform a brake slip control on the wheel brakes of the first brake circuit.

The brake 8a . 8b of the second brake circuit II are with an autonomous pressure source 3 connected to a pressure control unit (MECU) 31 , a pressure generating unit (Akt.) 30 and a pressure medium reservoir (Res.) 32 includes. The pressure control unit 31 is connected to the brake actuation sensor and controls the pressure generating unit 30 in accordance with the detected brake actuation. Via a first vehicle data bus B-CAN are the electronic control unit 11 and the pressure control unit 31 connected. In addition, the generator 2 or an electric motor control unit with the electronic control unit 11 connected. Between autonomous pressure source 3 and wheel brakes 8a . 8b of the second brake circuit II is a part of the hydraulic control unit separated from the first brake circuit I. 12 arranged. This allows, for example, a brake slip control of the wheel brakes 8a . 8b of the second brake circuit via in the hydraulic control unit 12 arranged solenoid valves. For this purpose, the electronic control unit 11 , which controls the solenoid valves, preferably also with the wheel speed sensors 9a and 9b connected.

When the driver operates the brake pedal, it is expediently provided that initially only the electric drive on the front axle 2 is operated as a generator and thus builds up a drag torque. The brake 8c . 8d Build due to the direct connection between the brake pedal 4 and master cylinder 5 a braking torque on the rear axle. Since the operation of the driver is not supported by auxiliary power and at a suitable stability of braking points for braking force distribution, the braking torque or drag torque at the front axle is significantly higher, thus efficient recuperation can be done. When the electric drive 2 can not be operated as a generator (eg because the battery is full), so is a braking torque in the wheel brakes 8a . 8b the front axle built by the autonomous pressure source 3 is activated. It may be provided that the electronic control unit 11 the pressure control unit 31 specifies a desired braking torque or a desired pressure. A lack of delay by the generator may be based on a message from the electric motor control unit via the vehicle data bus V-CAN to the electronic control unit 11 be reported. Even at low speeds decreasing generator power is expediently by an activation of the autonomous pressure source 3 compensated.

The autonomous pressure source 3 In a first variant, it may be embodied as an electrohydraulic actuator, in which an electric motor displaces a piston in a pressure chamber via a rotational-translation gear in order to build up a pressure and, if appropriate, subsequently to reduce it by retracting the piston. Conveniently, a transmission gear can be provided in order to build a comparatively large torque with a small electric motor high speed. It is also expedient if the autonomous pressure source contains a brake fluid reservoir, which is connected in particular in the unactuated state with the pressure chamber; the reservoir can also be connected via a connecting line to the reservoir 6 of the master cylinder 5 be realized.

In a second variant, the autonomous pressure source 3 comprise a driven by an electric motor piston pump. By using a pump with multiple pistons, in particular 6 Piston, and / or a pulsation damper, which reduces pressure fluctuations occurring, for example, via an elastic element attached to the outlet side, such as a membrane, ensures a virtually constant delivery rate. Furthermore, it can be expediently provided on the suction side of the pump to install a solenoid valve, which allows an opening or closing a suction control of the pump. For a pressure equalization, the autonomous pressure source 3 in this variant, preferably arranged between the suction side and outlet side solenoid valve. The suction side of the pump is connected to a brake fluid reservoir, which also via a connecting line to the reservoir 6 of the master cylinder 5 can be realized.

2 shows a schematic representation of a brake system according to the invention in a second embodiment of a four-wheeled motor vehicle.

In all figures, the reference numerals have been retained, for example, the wheel brake of the left front wheel FL in 2 With 8a as well as in 1 the case was. The details already described should therefore not be discussed further, but the differences are explained below.

This in 2 shown embodiment differs from that in 1 shown that the second brake circuit II not with the hydraulic control unit 12 connected is. For example, for a brake slip control, it is therefore appropriate that the autonomous pressure source the wheel brakes 8a . 8b the solenoid valve associated with the first brake circuit and with the corresponding wheel speed sensors 9a . 9b connected is. The central control in a recuperative braking, in which the electric drive 2 is operated as a generator, advantageously takes over the electronic control unit 11 which is connected via a first vehicle data bus B-CAN to the control unit of the autonomous pressure source and via a second data bus V-CAN to the electric drive. For an increase in operational safety, it is useful here, as in the first embodiment, when the brake actuation sensor is designed to be redundant and via more than one line to the control unit of the autonomous pressure source 3 connected is.

3 shows a particularly preferred embodiment of a brake system according to the invention according to the first embodiment of a four-wheeled motor vehicle.

braking system 1 includes a driver-operable brake pedal 4 , which without auxiliary power assistance on a master brake cylinder 5 acts, which with the wheel brakes 8c . 8d a first brake circuit I is connected. The executed with only one pressure chamber master cylinder is connected in the unactuated state with a reservoir for brake fluid or pressure medium and has an inductive displacement sensor 7 on, which detects the distance by which the piston has been moved into the pressure chamber of the master cylinder. Downstream of the master cylinder is a pressure sensor 16-I and a separating valve 17-I arranged, which is designed as a normally open solenoid valve. Preferably, between the master cylinder 5 and isolation valve 17-I a simulator chamber 15 arranged. This may for example be designed as an elastic element, such as a hat or a membrane and allows adjustment of the pedal feel for the driver by a higher volume absorption is simulated.

Downstream of the isolation valve 17-I are two preferably designed as drum brakes wheel brakes 8a . 8b arranged, which are assigned to the wheels RL, RR of the rear axle. Due to the self-amplification of the drum brakes they have a high efficiency, ie the required actuation energy per braking torque is low. Since only comparatively low braking torques are required according to an optimum braking force distribution on the rear axle with regard to driving stability, auxiliary power assistance can be dispensed with without sacrificing comfort. Between isolation valve 17-I and wheel brake 8a . 8b is each an inlet valve Ia-18 . 18-Ib arranged, which is designed as a normally open solenoid valve. Furthermore, the wheel brakes 8a . 8b via one outlet valve each Ia-19 . 19-Ib , which are designed as normally closed solenoid valves, with a low-pressure accumulator 20-I get connected. Via a changeover valve 21-I , Which is designed as a normally closed solenoid valve, the low-pressure accumulator 20-I with the master cylinder 5 get connected.

Furthermore, brake circuit I has an electrically driven piston pump 13 , in particular a two-piston pump, which on the suction side with the low-pressure accumulator 20-I connected is. The outlet of piston pump 13 is between the isolation valve 17-I and the intake valves Ia-18 . 18-Ib arranged. Appropriately, is on the outlet side of the piston pump 13 furthermore a pulsation damper 14 arranged, which for example via a elastic membrane is realized. The ones with the wheel brakes 8c . 8d The wheels RR and RL connected to the first brake circuit I each have a wheel speed sensor 9c . 9d on. pump 13 , Low pressure accumulator 20-I and solenoid valves 17-I . Ia-18 . 18-Ib . Ia-19 . 19-Ib . 21-I are in a hydraulic control unit (HCU) 12 arranged. The evaluation of the signals from pressure sensor 16-I and wheel speed sensors 9c . 9d carried out in an electronic control unit (ECU) 11 , which continue the solenoid valves 17-I . Ia-18 . 18-Ib . Ia-19 . 19-Ib . 21-I controls.

The electronic control unit 11 has an arithmetic unit, in particular a kernredundanten microcontroller, drive circuits for the solenoid valves, evaluation circuits for sensor signals, in particular wheel speed sensor signals and interfaces to vehicle data buses and is via the electrical system 10 supplied with electrical energy. Furthermore, expediently a transverse acceleration sensor 25 , a longitudinal acceleration sensor 26 and a yaw rate sensor 27 integrated in the electronic control unit. Thus, comfort functions such as hill start assist, vehicle dynamics control such as yaw moment control and slip control such as an antilock braking system may be provided by the electronic control unit driving the pump and solenoid valves and / or issuing a request via a vehicle data bus B-CAN, V-CAN. Particularly preferred are electronic control unit 11 and hydraulic control unit 12 integrated in a common housing.

Via a first vehicle data bus B-CAN is the electronic control unit 11 with an autonomous pressure source 3 connected. This includes a pressure control unit MECU, an electromechanical actuator or a pump. For detecting the brake operation, the pressure control unit is also connected to the brake actuation sensor. In the example shown, a piston in a pressure chamber is moved in an electromechanical actuator. The piston is moved by an electric motor with a downstream rotation-translation gear, whereby the rotor position of the electric motor and / or a displacement of the piston are measured by sensors. The pressure control unit expediently comprises a drive circuit for the electric motor and is for power supply to the electrical system 10 connected. Instead of a pressure medium reservoir has the autonomous pressure source 3 a connecting line to the reservoir 6 on, via a check valve, a formation of negative pressure in the brake circuit is prevented.

The pressure chamber of the autonomous pressure source is via a normally open isolation valve 17-II and one normally open inlet valve 18-II-a . 18-II-b with two wheel brakes 8a . 8b connected, which are assigned to the wheels FL, FR of the front axle. The wheels FL, FR continue to have wheel speed sensors 9a . 9b on, which preferably with the electronic control unit 11 are connected. Via outlet valve 19-II-a respectively. 19-II-b can brake fluid from the wheel brake 9a respectively. 9b in a low-pressure accumulator 20-II be derived. Low-pressure accumulator 20-II can via a normally closed switching valve 21-II be connected to the pressure chamber. The solenoid valves 17-II . 18-II-a . 18-II-b . 19-II-a . 19-II-b . 21-II , the low-pressure reservoir 20-II and one between isolation valve 17-II and autonomous pressure source 3 arranged pressure sensor 16-II are conveniently in the hydraulic control unit 12 integrated and in particular by the electronic control unit 11 driven.

The electronic control unit 11 is via a second vehicle data bus V-CAN with an electric drive 2 connected. This includes a drive control unit 22 , a power electronics (PEB) 23 and one or more electrical machines (EMR) 24 , The power electronics include, for example, per phase winding of the electric machine, two power switches and a microcontroller for controlling. The drive control unit can also be designed to control an internal combustion engine and with the power electronics 23 be connected via a third vehicle data bus E-CAN. Via the vehicle data buses, the various control devices expediently exchange status information (such as the availability or the presence of an error) and requirements.

It is advantageous if, in the case of a braking request by the driver, the electronic control unit 11 determined suitable braking torques for front and rear axles and to the electric drive 2 or the autonomous pressure source 3 sends corresponding requests. Here, the braking torque on the front axle is preferably generated at least partially by the electric drive in the generator mode. Also, a brake slip or vehicle dynamics control can be done by methods known per se, wherein the braking torque at one or more wheels of the front axle also combined by a drag torque of the electric drive 3 and a pressure build-up by the autonomous pressure source 3 can be implemented. Thus, regardless of the filling state of the vehicle battery, a consistent braking behavior can be ensured.

By eliminating a brake booster, the brake system according to the invention can be made compact. By a known per se electro-hydraulic control unit with adapted software is used, the brake system according to the invention is comparatively simple and inexpensive to implement.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 102011003346 A1 [0005]

Claims (15)

Brake system ( 1 ) for a motor vehicle with at least two axles, wherein the wheels (FL, FR) of a drive axle with an electric drive ( 2 ), which has at least one electric machine which can be operated as a motor or generator, comprising a first brake circuit (I) with at least one hydraulic wheel brake ( 8c . 8d ), in particular two wheel brakes ( 8c . 8d ), with one from the driver via a brake pedal ( 4 ) operable master cylinder ( 5 ), with the wheel brakes ( 8c . 8d ) of the first brake circuit (I) is connected to a brake actuation sensor ( 7 ), and a second brake circuit (II) with at least one hydraulic wheel brake ( 8a . 8b ), in particular two wheel brakes ( 8a . 8b ), characterized in that the second brake circuit (II) only with an autonomous pressure source ( 3 ) is hydraulically connected, wherein the wheel brakes ( 8a . 8b ) of the second brake circuit (II) are assigned to the drive axle and the wheel brakes ( 8c . 8d ) of the first brake circuit (I) are assigned to another axle. Braking system according to claim 1, characterized in that the autonomous pressure source ( 3 ) is designed as an electro-hydraulic actuator, in which a piston is driven by an electric motor with a downstream rotation-translation gear. Braking system according to claim 1, characterized in that the autonomous pressure source ( 3 ) driven by an electric motor pump, in particular a piston pump with preferably more than 2 Piston, preferably a pulsation damper, in particular a connected to the outlet side of the pump elastic element, a pressure medium reservoir, arranged between the suction side of the pump and the pressure medium reservoir first valve and disposed between the outlet side of the pump and the pressure medium reservoir second valve comprises. Braking system according to one of the preceding claims, characterized in that the autonomous pressure source ( 3 ) a pressure sensor and an electronic control unit ( 31 ), wherein the electronic control unit ( 31 ) with the brake actuation sensor ( 7 ) and has an interface to a first vehicle data bus (B-CAN). Braking system according to claim 4, characterized in that the electronic control unit ( 31 ) of the autonomous pressure source ( 3 ) via the first vehicle data bus (B-CAN) with an electro-hydraulic control unit ( 11 . 12 ), which associated with the first brake circuit (I) associated solenoid valves ( 17-I . Ia-18 . 18-Ib . Ia-19 . 19-Ib . 21-I ) and a first accumulator ( 20-I ), wherein in particular each wheel brake ( 8c . 8d ) of the first brake circuit (I) a between the master cylinder ( 5 ) and the wheel brake ( 8c . 8d ) inlet valve ( Ia-18 . 18-Ib ) and one between the wheel brake ( 8c . 8d ) and the first accumulator ( 20-I ) arranged outlet valve ( Ia-19 . 19-Ib ) assigned. Brake system according to claim 5, characterized in that the electro-hydraulic control device ( 11 . 12 ) an electrically driven pump associated with the first brake circuit (I) ( 13 ), in particular a piston pump, the suction side with the first pressure accumulator ( 20-I ) and between a master cylinder ( 5 ) and inlet valve (s) ( Ia-18 . 18-Ib ) arranged separating valve ( 17-I ) and one between first accumulator ( 20-I ) and master cylinder ( 5 ) arranged switching valve ( 21-I ), wherein the outlet side of the electrically driven pump ( 13 ) between isolation valve ( 17-I ) and inlet valve (s) ( Ia-18 . 18-Ib ) is arranged, and / or a pressure sensor associated with the first brake circuit (I) ( 16-I ). Braking system according to claim 5 or 6, characterized in that the electro-hydraulic control device ( 11 . 12 ) via a second vehicle data bus (V-CAN) with a drive control unit ( 22 . 23 ) connected to the drive axle associated electrical / n machine / ( 24 ). Brake system according to one of claims 5 to 7, characterized in that the electro-hydraulic control device ( 11 . 12 ) the second brake circuit (II) associated with solenoid valves ( 17-II . 18-II-a . 18-II-b . 19-II-a . 19-II-b . 21-II ) and a second accumulator ( 20-II ), wherein in particular each wheel brake ( 8a . 8b ) of the second brake circuit (II) between the autonomous pressure source ( 3 ) and the wheel brake ( 8a . 8b ) inlet valve ( 18-II-a . 18-II-b ) and one between the wheel brake ( 8a . 8b ) and the second pressure accumulator ( 20-II ) arranged outlet valve ( 19-II-a . 19-II-b ) assigned. Braking system according to claim 8, characterized in that the electro-hydraulic control unit ( 11 . 12 ) between the autonomous pressure source ( 3 ) and the inlet valve (s) ( 18-II-a . 18-II-b ) of the second brake circuit (II) arranged second isolation valve ( 17-II ) and between the second accumulator ( 20-II ) and the autonomous pressure source ( 3 ) arranged second switching valve ( 21-II ). Brake system according to one of claims 5 to 9, characterized in that the electro-hydraulic control device ( 11 . 12 ) with at least a wheel speed sensor ( 9a . 9b . 9c . 9d ), in particular each wheel brake ( 8c . 8d ) of the first brake circuit (I), preferably also each wheel brake ( 8a . 8b ) of the second brake circuit (II), a wheel speed sensor ( 9a . 9b . 9c . 9d ) assigned. Brake system according to one of the preceding claims, characterized in that the wheel brake / n ( 8c . 8d ) of the first brake circuit (I) is designed as a drum brakes / are, in particular the rear axle is assigned / are, wherein the master cylinder ( 5 ) is preferably operated without auxiliary power. Brake system according to one of the preceding claims, characterized in that the wheel brake / n ( 8a . 8b ) of the second brake circuit (II) is designed as a disc brakes / are. Braking system according to claim 4, characterized in that the autonomous pressure source ( 3 ) the second brake circuit (II) associated with solenoid valves ( 17-II . 18-II-a . 18-II-b . 19-II-a . 19-II-b . 21-II ) and a second accumulator ( 20-II ), wherein in particular each wheel brake ( 8a . 8b ) of the second brake circuit (II) between a pressure generating unit of the autonomous pressure source ( 3 ), in particular an outlet side of a piston pump or a pressure chamber of an electro-hydraulic actuator, and the wheel brake ( 8a . 8b ) inlet valve ( 18-II-a . 18-II-b ) and one between the wheel brake ( 8a . 8b ) and the second pressure accumulator ( 20-II 9 arranged outlet valve ( 19-II-a . 19-II-b ), and wherein the electronic control unit ( 31 ) with at least one wheel speed sensor ( 9b . 9c ), in particular each wheel brake ( 8a . 8b ) of the second brake circuit (II) a wheel speed sensor ( 9b . 9c ) assigned. Method for controlling a brake system ( 1 ) according to any one of claims 1 to 13, characterized in that a braking operation is implemented by the driver on the drive axle by a generator operation of the electric machine / s, and that the autonomous pressure source ( 3 ) is activated when a measured actual deceleration is more than a predetermined threshold below a target deceleration determined in accordance with the brake application. Use of a braking system ( 1 ) according to one of claims 1 to 13 in a motor vehicle with an internal combustion engine, wherein the wheels (FR, FL) of the drive axle and / or the wheels (RR, RL) of the other axle are driven by the internal combustion engine.

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