DE102016222553A1 - Electronic slip-controlled brake system - Google Patents

Abstract

The invention relates to an electronically slip-controllable brake system (10). The brake system (10) comprises an actuatable master brake cylinder (16) to which at least one wheel brake (26a, 26b) is attached to a wheel (34a, 34b) of a front axle (36) of a vehicle and at least one wheel brake (26c; 26d) associated with a wheel (34c; 34d) of a rear axle (38) of the vehicle being detachably connected. An electronically controllable first actuator (30) of the brake system (10) is used to set and control mutually different brake pressures in the wheel brakes (26a - 26d) as a function of the prevailing slip conditions. An electronically controllable second actuator (24) causes adjustment and regulation of a uniform brake pressure at the wheel brakes (26a-26d) and a third actuator (42) limits the brake pressure generated by the second actuator (24) at the wheels (34c; 34d) of the rear axle (38) associated with wheel brakes (26c, 26d). An electronic control of the actuators (30, 34, 42) is effected by at least one electronic control unit (22). According to the invention, the third actuator (42) has at least one directional valve (52c, 52d) per wheel brake (26c, 26d) on the rear axle (38) 54c, 54d) and also a pressure fluid accumulator (56c, 56d). With this characteristic of the third actuator (42), the brake pressures at the wheel brakes (26c, 26d) of the rear axle (38) can be adjusted in the event of a fault of the first actuator (30). independently of each other and independent of the specifications of the second actuator (24) modulate and achieve a high vehicle deceleration with stable driving condition of the vehicle.

Description

State of the art

The invention relates to an electronically slip-controllable brake system according to the features of the preamble of claim 1.

An electronic slip-controllable brake system for a motor vehicle is known, for example from the DE 10 2009 001135 A1 ,

This known brake system comprises a first actuator, hereinafter referred to as primary actuators, in the form of a conventional ABS / ESP brake system for wheel-specific modulation of the brake pressure as a function of prevailing at the wheels slip conditions. The brake pressures of the individual wheel brakes can be set or regulated independently of one another by the first actuator system, which is why, in connection with a conventional 4-wheeled motor vehicle, this is also referred to as a 4-channel control actuator. The brake system is u.a. equipped with a hydraulic unit, which consists of a housing equipped with pumps and valves housing block and an electronic control unit, which controls these pumps and valves in response to sensor signals representing the slip conditions at the individual wheels. Each brake of the vehicle brake system are each assigned a brake pressure build-up valve and a respective brake pressure reduction valve for braking pressure control.

This primary actuation makes it possible to stabilize the driving state of a vehicle during a braking process, during start-up or while driving, by reducing the brake pressure at the wheel brakes at least until a possibly occurring wheel slip has assumed a controllable level or no longer occurs , The brake pressure is generated by the primary actuators and the driver together or independently of the driver. The primary actuators can therefore be operated in a so-called partially or in a fully active mode.

Furthermore, the known vehicle brake system has a second actuator or secondary actuator, for example in the form of an electromechanical brake booster. This secondary actuation is typically connected to the master cylinder and, in normal operation, enhances ride comfort by assisting the driver in establishing a brake pressure necessary for braking. An electromechanical brake booster for this purpose comprises an electronically controllable actuator, which provides an external force for actuating a master brake cylinder. The actuation of the master cylinder can be done solely by the external force of the secondary actuators or by a combination of this external force with a muscle power provided by the driver.

First and second actuators or primary actuators and secondary actuators therefore form two mutually redundant systems for generating and modulating a brake pressure in a vehicle brake system, wherein this brake pressure modulation can be carried out in each case with or without the driver's involvement. The two actuators thus fulfill an essential prerequisite for the realization and implementation of a partially or fully automated driving operation.

Because during such an automated driving operation, the driver only performs a control or monitoring function, there are particularly high demands with regard to the reliability of such electronically pressure-controllable vehicle brake systems, which are met by providing the two explained actuators.

However, in contrast to the primary actuation, the secondary actuation is only able to provide all the wheel brakes of the vehicle brake system connected thereto with a uniform brake pressure indirectly by actuation of the master brake cylinder or to uniformly modulate this brake pressure. This mode of operation is referred to in professional circles as a 1-channel control actuator. Nevertheless, a single-channel secondary device is sufficient to decelerate a vehicle in the event of a malfunction of the primary actuators while maintaining its directional stability to a standstill.

Minimum requirements for longitudinal or directional stabilization of the vehicle while maintaining a blocking order, that is, a brake pressure build-up such that the wheel brakes of the front axle reach their blocking limit in time before the wheel brakes of the rear axle, also a maintenance of the steerability of the vehicle and thus ensuring a maximum lock time the vehicle wheels and the possibility of an active or driver independent structure of a brake pressure.

In particular, the mentioned criterion of the blocking time limit of the wheels causes the maximum achievable deceleration values of the vehicle to depend on the braking power that can be converted by the wheel brakes of the rear axle. This convertible braking power at the rear axle is relatively low in the direction of the front axle due to the dynamic axle load displacement taking place during a braking operation for reasons of mass inertia. Since an axle load increase at the Front axle is forcibly associated with an axle load reduction of the rear axle, their wheels tend significantly earlier or at lower brake pressures to lock, as opposed to the more heavily loaded front wheels. A blocking of the rear wheels must be counteracted by a corresponding reduction in the brake pressure at the wheel brakes of the rear axle, if the driving stability of the vehicle should not be jeopardized.

Due to the explained property of the secondary actuators all existing wheel brakes can apply only with a uniform brake pressure, in combination with a low convertible by the wheel brakes of the rear brake pressure, without risk of locking the associated wheels, resulting in the case of a braking operation in which the brake pressure due to a incurred malfunction in the primary actuators of the secondary actuators is applied, the disadvantage that the convertible total braking power of the vehicle fails only relatively low or that consequently sets a relatively long braking distance of the vehicle. This has a particularly negative effect on vehicles in which the dynamic axle load displacement in the direction of the front axle during a braking operation, for example. Due to a high-lying vehicle center of gravity, particularly large fails.

To avoid this disadvantage, it is also known to equip generic electronic pressure-controllable vehicle brake systems with a further actuator, hereinafter referred to as third actuator.

This third actuator is another electronically controllable unit which is activated in case of failure of the primary and which adjusts the provided by the secondary brake pressure at the wheel brakes of the rear axle to the taking place during this braking weight reduction of the rear axle. Such a third actuator may be fluidly connected between the primary and secondary actuators or may alternatively be switched between the primary and the rear wheel brakes. It allows an electronic control of the secondary actuators such that they provide a higher brake pressure, which is fully implementable only by the wheel brakes of the heavily loaded front axle in a braking performance, by preventing an increase in the brake pressure at the wheel brakes of the rear axle, if this Brake pressure approaches a threshold, from which the brake pressure is no longer fully implemented in a braking performance. In other words, the third actuator system overrides the otherwise necessary limitation of the brake pressure level of the vehicle brake system to the lower brake pressure level which can still be converted by the wheel brakes of the rear axle and moreover makes it possible to fully utilize the braking power that can be transmitted by the wheel brakes of the front axle. As a result, an overall higher total brake line of the vehicle brake system and, consequently, a significant shortening of the braking process or the braking distance can be achieved in this way.

Advantages of the invention

An electronically slip-controllable vehicle brake system according to the features of claim 1 is in addition to the primary and the Sekundäraktuatorik equipped with a third actuator, with which the brake pressures at the wheel brakes of the rear axle, in the case of a brake pressure build-up by the 1-channel second actuator due to an existing fault at the first actuator, according to the invention can be modulated independently.

By means of the invention, therefore, a braking operation with a comparatively high vehicle deceleration and at the same time a high driving stability of the vehicle can be carried out even in the case of a faulty operation of the primary actuators.

According to the invention, the third actuator system per wheel brake of the rear axle comprises at least two directional control valves and also at least one pressure fluid accumulator. A first directional control valve in each case controls a first pressure medium connection existing between the assigned wheel brake and the first actuator, and a second directional valve each controls a second pressure medium connection existing between the associated wheel brake and the pressure medium reservoir.

The directional valves and the accumulator are components which are installed at least approximately identical in the primary actuators and therefore available in sufficient quantity and at low cost on the market.

By means of the respective first directional valve, a pressure build-up or a pressure holding a set brake pressure at one of the wheel brakes of the rear axle is controllable, while with the respective second directional control valve in combination with the downstream of it arranged pressure fluid storage a dependent of the volume of a storage chamber of the accumulator number of pressure reduction processes is controllable , A pressure reduction process takes place, for example, when the wheel slip at the, the wheel brakes of the rear axle associated wheels increases and thus reduces the transmissible by these wheel brakes braking performance.

In addition, by the respective second directional control valves the time and a period to emptying the pressure chambers filled with pressure medium controlled by these second way valves release the pressure medium connection of the storage chambers to the rest of the brake circuit as soon as no pressure on the wheel brakes of the rear axle pending or no braking is carried out.

In order to counteract as little flow resistance as possible to the outflowing pressure medium, a non-return valve is connected in parallel to the respective first directional control valve. The latter controls a bypass to each first directional control valve and opens under pressure control, that is, when the pressure level is higher on the wheel side of the check valve than on the side facing away from the wheel brakes of the rear axle side. Accordingly, backflowing pressure medium does not have to flow through the narrow cross sections of the respective first directional control valve and the emptying of the storage chamber takes place relatively quickly. Consequently, the third actuator is very quickly available for a subsequent, controlled by the second actuator braking process.

Further advantages or advantageous developments of the invention will become apparent from the dependent claims and / or the following description.

An activation of the third actuator is required only if a malfunction of the primary actuators has been detected and the brake pedal pressure generated by the Sekundäraktuatorik is higher than the convertible by the wheel brakes of the rear brake pressure, so if the wheel brakes of the rear axle due to the axle load shift towards the front axle, due the braking process, tend to uncontrolled blocking. With activation or activation of the third actuator, the brake pressure at the wheel brakes of the rear axle can be adjusted individually on the wheel so that the respective brake pressure from the respective wheel brakes can also be fully converted into a braking power, i. without causing uncontrolled locking of this wheel of the rear axle.

The third actuator can be built on the basis of a separate actuator housing separate from the primary and / or secondary actuators and uses its own associated fluid control components. On the one hand, this has the advantage that the components of the primary actuators are not exposed to an increased load profile for the purpose of extending the functional safety of a vehicle brake system according to the invention.

Furthermore, the electronic control unit of the primary actuators and / or the secondary actuators can be combined with that for the third actuator or it can be used structurally separate control units.

The invention thus allows a high degree of design freedom in the design implementation, at the same time manageable costs and yet compact design and ease of integration into an existing braking system.

list of figures

• 1 zeigt, schematisch vereinfacht, eine der Erfindung zugrunde liegende elektronisch schlupfregelbare Bremsanlage mit einer ersten Aktuatorik, einer zweiten Aktuatorik und einer dritten Aktuatorik zur Steuerung und Regelung des Bremsdrucks in Radbremsen einer Vorder- und einer Hinterachse eines Fahrzeugs.
• 2 zeigt darüber hinaus den erfindungsgemäßen Aufbau der dritten Aktuatorik sowie die strömungstechnische Kontaktierung der Komponenten dieser dritten Aktuatorik.

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

• 1 shows, schematically simplified, one of the invention underlying electronic slip controllable brake system with a first actuator, a second actuator and a third actuator for controlling and regulating the brake pressure in the wheel brakes of a front and a rear axle of a vehicle.
• 2 moreover shows the structure according to the invention of the third actuator as well as the fluidic contacting of the components of this third actuator.

Description of the embodiment

The electronic slip-controlled brake system 10 to 1 includes, inter alia, a. Via a pedal 12 by a driver by muscle power operable actuator 14 and one of this actuator unit 14 applied master cylinder 16 which is from an attached reservoir 18 is supplied for pressure medium. The operating unit 14 is with an electronically controllable actuator 20 equipped, by which the braking force applied by the driver can be strengthened by external force. With this actuator 20 is, for example, an electronically controllable electric motor with downstream transmission. An electronic control of the electric motor can in dependence of the operation of the pedal 12 or independently of a pedal operation. For the former case are pedal sensors 13 provided, which detect the actuation force and / or the actuation path and thus a braking request of a driver and their sensor signals from an electronic control unit 22 be evaluated for calculating the drive signals for the electric motor.

The electronically controllable brake booster or the actuating unit 14 forms in operative connection with the control unit 22 within the vehicle brake system, a so-called second actuator 24 or secondary actuators. The latter is by operation of the master cylinder 16 indirectly capable of wheel brakes 26a - 26d of the braking system 10 to apply a uniform brake pressure.

The second of this actuator 24 operated master cylinder 16 supplies two separate brake circuits with pressure medium and is via two lines assigned to the brake circuits 28a . 28b to a first actuator or primary actuators 30 connected.

In this primary actuators 30 it is the hydraulic unit of a well-known from the prior art slip-controllable vehicle brake system, which is also known under the name ABS / ESP brake system. To this first actuator or primary actuators 30 are druckmittlegitend the four wheel brakes 26a - 26d via an associated brake line 40a - 40d connected. Two of these wheel brakes 26a . 26b are provided to wheels if necessary 34a . 34b a front axle 36 or wheels 34c . 34d a rear axle 38 of the vehicle decelerate.

The primary actuators 30 is different from secondary 24 capable of the different wheel brakes 26a - 26d the vehicle brake system 10 to supply individually with different brake pressures. For this purpose, this primary features 30 Among other things, an unrecognizable, electric motor driven pressure generating unit and electronically controllable solenoid valves. A modulation of the brake pressure by these solenoid valves is carried out as a function of the slip conditions on the wheel in question 34 prevail and that of wheel sensors 35 be recorded. Their measuring signals represent the respective wheel speeds and are the electronic control unit 22 supplied and to drive signals for the pressure generator or regulating components of the first actuator 30 further processed.

Two brake lines 40c . 40d lead via a third actuator 42 the vehicle brake system 10 to the two wheel brakes 26c . 26d the rear axle 38 , This third actuatorics 42 is downstream of the primary actuators 30 and upstream of the wheel brakes 26c . 26d the rear axle 38 arranged. Primäraktuatorik 30 , third actuatorics 42 and the wheel brakes 26c . 26d the rear axle 38 are therefore connected in series or in series.

In normal operation of the vehicle brake system 10 is the necessary brake pressure by an adapted electronic control of the second actuator or secondary actuators 24 depending on an operation of the pedal 12 provided and possibly by electronic control of the first actuator or primary actuators 30 to the wheels 34a - 34d prevailing slip conditions adapted. The brake pressure build-up or the brake pressure modulation can be carried out with or without driver participation, for example, if the driving condition of the vehicle or the traffic conditions require it, or the vehicle is operated in an autonomous driving.

Despite all technical precautions are disturbances to such intended operation of the vehicle brake system 10 not be ruled out. In case of a disturbance on the primary actuators 30 is the secondary actuators 24 able to build a brake pressure automatically to bring the vehicle to a standstill. Primäraktuatorik 30 and secondary actuators 24 are therefore redundant to each other, or a failure of the first actuator 30 is through the second actuator 24 hedged.

As explained above, the third actuator is 42 present to the brake pressure level at the wheel brakes 26c . 26d the rear axle 38 opposite the brake pressure level at the wheel brakes 26a . 26b the front axle 36 lower, if in case of failure of the primary 30 the secondary actuators 24 is active and that of the secondary actuators 24 provided brake pressure from the wheel brakes 26c . 26d the rear axle 38 not fully implemented in a braking performance, so as soon as the wheels 34c . 34d the rear axle 38 a danger of locking threatens.

The third actuatorics 42 in this case is from the electronic control unit 22 so controlled that the brake pressure at the wheel brakes 26c . 26d the rear axle 38 an occurring during braking dynamic displacement of the axle load is adjusted. By such an adjustment of the brake pressure is premature blocking of the wheel brakes 26c . 26d the rear axle 38 as well as a forcibly accompanying unstable driving condition of the vehicle prevented. Furthermore, an extension of the vehicle braking path is counteracted. In other words, the third actuator technology lowers 42 the brake pressure at the wheel brakes 26c . 26d the rear axle 38 so far off that the wheel brakes 26c . 26d associated wheels 34c . 34d Just roll off or not permanently block uncontrolled.

Based on 2 in particular, the inventive structure of this third actuator 42 illustrated. In the following description are corresponding components of 1 and 2 provided with common reference numbers.

2 shows, at the top, the first actuator 30 , which about the two brake circuits of the brake system 10 assigned brake lines 28a . 28b supplied with pressurized fluid under brake pressure. To the first actuator 30 are via brake lines 40a - 40d a total of four wheel brakes 26a - 26d connected. It can be assumed that the wheel brakes 26a and 26b at the front axle 36 ( 1 ) of the vehicle while the wheel brakes 26c and 26d at the rear axle 38 ( 1 ) of the vehicle are arranged. Between the first actuator 30 and the wheel brakes 26c . 26d the rear axle 38 is the third actuator 42 , The latter includes one of the first actuators 30 separately executed actuator housing 50 , which has two times two pressure medium connections with the brake lines 40c and 40d and with the wheel brakes 26c and 26d is contacted.

On the actuator housing 50 are per wheel brake 26c . 26d one pressure build-up valve each 52c . 52d and in each case a pressure reduction valve 54c . 54d arranged. Furthermore, each is a wheel brake 26c . 26d in each case a pressure fluid reservoir 56c . 56d intended. It should be noted at this point that the two accumulator 56c . 56d possibly also be combined to form a common accumulator.

Further, each pressure build-up valve 54c . 54d a pressure operated check valve 58c . 58d connected in parallel. This check valve 58c . 58d is arranged in the pressure medium circuit such that there is a flow direction from the first actuator 30 to the wheel brake 26c . 26d locks or the opposite direction, so from the wheel brake 26c . 26d to the first actuator 30 releases when from the wheel brake 26c . 26d in the direction of the first actuator 30 a pressure gradient is present. Finally, on the actuator housing 50 at least one pressure sensor 60 present, which is the current brake pressure at one of the two wheel brakes 26c . 26d the rear axle measures and the corresponding measurement signal to the electronic control unit 22 supplies.

At the pressure build-up valves 52c . 52d These are preferably solenoid-operated directional control valves with exactly one inlet and one outlet, which can be switched by electronic control of a Ventilaktuators from a normally open basic position against a force of a return means in a blocking position. This embodiment is based on the symbolic representation of this way valves in the 2 removable. The pressure build-up valves 52c . 52d each control a first pressure medium connection 46 from the first actuatorics 30 to one of the wheel brakes 26c . 26d at the rear axle 38 of the vehicle.

The pressure lowering valves 54c . 54d are also electromagnetically actuated, but in contrast to the described pressure build-up valves 52c . 52d from a normally closed basic position, against the force of a return means, switchable to a passage position. In this passage position is the associated wheel brake 26c . 26d via a second pressure medium connection 48 with the accumulator 56c . 56d coupled.

The intended third actuator 42 is in trouble-free normal operation of the vehicle brake system 10 not active. Only when from the electronic control unit 22 a disturbance of the primary actuators 30 and that of the secondary actuators 24 provided brake pressure, due to the taking place during a braking axle load shift of the wheel brakes 26c . 26d the rear axle 38 can no longer be fully implemented in a braking performance, is also the third actuator 42 activated. The latter is from the wheel sensors 35 by the occurring speed changes of the wheels 34 a - d detectable.

When activated, the third actuator locks 42 by electronic control of the pressure build-up valves 52c . 52d the first pressure medium connection 46 the wheel brakes 26c . 26d the rear axle 38 with the second actuator 24 , With the blocking of this first pressure medium connection 46 remains at the wheel brakes 26c . 26d the rear axle 38 set brake pressure at the prevailing level and there is no further brake pressure buildup instead.

Regardless, can be parallel to this, another brake pressure build-up on the wheel brakes 26a . 26b the front axle 36 take place until these wheel brakes 26a . 26b also reach their blocking limit. However, it falls on the front axle 36 adjustable brake pressure significantly higher than at the rear axle 38 , because due to the dynamic axle load shifting an additional load on the front axle 36 with a corresponding relief of the rear axle 38 accompanied.

During a braking process, the slip conditions on the wheels change 34c . 34d the rear axle 38 in a direction which makes a reduction of the set brake pressure required, there is an electronic control of the pressure reduction valves 54c . 54d through the control unit 22 , The pressure lowering valves 54c . 54d open and give a second fluid connection 48 between the wheel brakes 26c . 26d and the pressure fluid stores 56c . 56d free. About this second pressure medium connection 48 can pressure fluid from the wheel brakes 26c . 26d in the direction of accumulator 56c . 56d flow off until the brake pressure in the wheel brakes 26c . 26d has fallen to a pressure level at which there is no longer a threat of permanent blockage.

The accumulator 56c . 56d For example, they may be in the form of bellows, bladder, diaphragm or piston accumulators. They thus include a biased by an elastic restoring force separating element which separates a loadable with pressure medium first storage chamber of a second storage chamber. The maximum volume consumption of a pressure medium reservoir 56c . 56d is naturally limited by the size of the pressure medium receiving accumulator chamber. Consequently must in the accumulator 56c . 56d cached volume as soon as possible be returned to the brake circuits, so that the accumulator 56c . 56d is fully available again for subsequent braking operations. An emptying of the accumulator 56c . 56d occurs at a time when the driver is pedaling 12 not actuated and to which also no driver-independent brake pressure build-up by the first actuator 30 and / or by the second actuator 24 is made. If these requirements are fulfilled, the pressure reducing valves will be 54c . 54d the third actuatorics 42 from the electronic control unit 22 are activated and are thereby switched to their passage position. Pressure medium from the pressure fluid reservoirs 56c . 56d can thus via the pressure reducing valves 54c . 54d through the wheel brakes 26c . 26d through to the already open pressure build-up valves 52c . 52d and from there on through the first actuator 30 through to the master cylinder 16 or the associated reservoir 18 flow away. After emptying the accumulator 56c . 56d becomes the electronic control of the pressure reducing valves 54c . 54d finished and the pressure reduction valves 54c . 54d return, due to the force of their return means, back into the locked position. The third actuatorics 42 then takes the in 2 shown basic position.

The pressure build-up valves 52c . 52d parallel check valves 58c . 58d serve in connection with the emptying of the accumulator 56c . 56d to counter the explained reflux of pressure medium as little as possible flow resistance and thereby drain the emptying process as quickly as possible. The check valves 58c . 58d open pressure-controlled and give a bypass to the respective pressure build-up valve 52c . 52d free. At least a large proportion of the flowing back pressure medium thus flows through the check valves 58c . 58d through and not from those in the pressure build-up valves 52c . 52d existing narrow flow cross sections hindered.

Through the check valves 58c . 58d In addition, there is also a rapid reduction of the brake pressure in the wheel brakes 26c . 26d , towards the end of one of the secondary actuators 24 initiated braking process.

Of course, changes or additions to the described embodiment are conceivable without departing from the spirit of the invention.

It should be noted again in this context that the mentioned in the description electronic control unit 22 does not necessarily have to be designed as a single control unit, but may very well comprise several individual control devices, each one or more actuators 24 . 30 . 42 can be assigned. In particular, for reasons of redundancy should at least two electronically separated control units 22 be provided, one of which is the first actuator 30 and / or the second actuator 24 controls, while a second controller is provided to the third actuator 42 to control.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102009001135 A1 [0002]

Claims (7)

An electronically slip-controlled brake system (10), in particular for a motor vehicle, comprising - an actuatable master brake cylinder (16) to which at least one wheel brake (26a; 26b) and a wheel (34a; 34b) of a front axle (36) of the vehicle and at least one A wheel brake (26c; 26d) associated with a wheel (34c; 34d) of a rear axle (38) of the vehicle is connected; - An electronically controllable first actuator (30) for setting and control of mutually different brake pressures in the wheel brakes (26a -26d) in dependence on each of the wheels (34a - d) present slip ratios; - An electronically controllable second actuator (24) for at least indirect setting and control of a uniform brake pressure to the wheel brakes (26a - 26d) of the vehicle brake system (10); - A between the second actuator (24) and the at least one wheel brake (26c, 26d) of the rear axle (38) connected third actuator (42) for limiting the brake pressure on the wheel brake (26c, 26d) of the rear axle (38) and - at least an electronic control unit (22), which controls the electronic control of the first, second and / or third actuators (30, 24, 42), characterized in that the third actuator (42) per wheel brake (26c, 26d) of the rear axle ( 38) is equipped with at least one directional control valve (52c, 52d; 54c, 54d) and at least one pressure medium reservoir (56c, 56d), wherein the at least one directional control valve (52c, 52d, 54c, 54d) between the associated wheel brake (26c, 26d) and the second actuator (24) existing first pressure medium connection and between the associated wheel brake (26c, 26d) and the pressure fluid reservoir (56c, 56d) existing second pressure medium connection (48) controls. Electronic slip-controlled brake system (10) after Claim 1 , characterized in that in the non-activated state of the directional control valve (52c, 52d, 54c, 54d), the first pressure medium connection open and the second pressure medium connection (48) is closed. Electronic slip-controlled brake system (10) after Claim 1 or 2 , characterized in that the third actuator (42) per wheel brake (26c, 26d) each having a pressure-actuated check valve (58c, 58d) which is connected in parallel to the directional control valve (52c, 52d, 54c, 54d) and the flow direction of the second actuator (24) to the wheel brake (26c, 26d) locks. Electronic slip-controlled brake system (10) according to one of Claims 1 to 3 , Characterized in characterized in that for loading and emptying of the accumulator (56c, 56d) with pressure medium, the directional valve (52c, 52d, 54c, 54d) is electronically controlled. Electronic slip-controlled brake system (10) according to one of Claims 1 to 4 , characterized in that at least one pressure sensor (60) in the third actuator (32) is provided which detects the brake pressure at one of the wheel brakes (26c, 26d) of the rear axle (38) of the vehicle. Electronic slip-controlled brake system (10) after Claim 5 characterized in that the directional control valve (52c, 52d; 54c, 54d), the accumulator (56c, 56d), the check valves (58c, 58d) and the pressure sensor (60) are arranged in a common actuator housing (50). Electronic slip-controlled brake system (10) according to one of Claims 1 to 6 , characterized in that the third actuator (42) per wheel brake (26c, 26d) of the rear axle (38) each have exactly one pressure medium connection, via which the third actuator (42) to one, a wheel brake (26c, 26d) of the rear axle (38) associated, connection of the first actuator (30) is connected.

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