DE19703776A1 - Brake pressure setter with brake force amplification - Google Patents

Abstract

The brake pressure setter, for a hydraulic brake system, has an amplifier piston operated by the brake pedal and able to be further amplified by the pressure of an amplifier pump (28). The force on the amplifier piston is transmitted to the pressure circuit piston (10) of the main brake cylinder (2), which is connected to the wheel brakes (9). The pressure can be built up by the amplifier pump in the amplifier chamber (13). It is controllable by a pressure control valve (29), the valve body (32) of which can be operated electromagnetically by a proportional magnet (36).

Description

The invention relates to a brake pressure sensor for a hydraulic brake system for motor vehicles with one a wheel brake cylinder connectable master cylinder, one hydraulic brake booster with a one booster chamber-limiting booster piston for actuating the Master cylinder, one driven by a motor Booster pump and one operated by a brake pedal Control device with a pressure control valve for control of the booster pressure acting on the booster piston.

In a brake pressure transmitter known from DE 44 43 869 A1 of the type mentioned is the pressure control valve in the Ver connected to a working piston of the master cylinder booster piston arranged and the brake pedal is through a Push rod mechanically with the valve body of the pressure control connected valve. In the brake release position it is Pressure control valve open, it with the booster chamber a chamber connected to a trailing tank binds into which the booster piston can be moved. From this chamber leads to the suction side of the amplifier kerpump, the pressure side of the booster chamber is closed. To generate a brake pressure, Actuation of the push rod closed the pressure control valve. At the same time, the Booster pump turned on and the chamber through a Solenoid valve separated from the waste tank. The Ver Stronger pump receives to build the required amplifier  pressure that is proportional to the foot force from the chamber pressed volume and conveys this into the amplifier chamber. One connected to the suction side of the booster pump Floating piston arrangement takes up excess volume and ensures that a quick start of the Brake necessary, increased volume requirements quickly covered can be. The structure of this known brake pressure sensor is comparatively complex and requires everyone Braking in addition to the actuation of the pressure control valve Control of a solenoid valve. An automatic actuation the brake for driving stability or traction control the known brake pressure sensor does not allow.

A brake pressure transmitter is known from DE 44 20 148 A1, at which the pressure piston of a master brake cylinder through the Booster piston of a hydraulic brake booster can be actuated. The booster piston is through the one Variable pump generated pressure in a booster chamber and additionally by the pressure in a transmission chamber movable, the pressure in the transfer chamber through a control piston which can be actuated with the aid of the brake pedal can be generated. The adjustment pump is adjusted with Help the difference between the pressures in the over control chamber and the amplifier chamber controlled such that the ratio of the pressures to one another remains the same. Also this known brake pressure sensor is not suitable for one automatic brake actuation for driving stability or Drive slip control.

The invention has for its object a brake pressure to create donors of the type mentioned at the outset automatic brake actuation by an electronic rule setup in a simple way.  

This object is achieved in that Brake pressure transmitter of the type mentioned the pressure control Valve electromagnetically through a proportional magnet can be actuated and that the excitation current of the proportional according to the measured values of the actuating force of the brake pedal detecting electrical measuring device and / or the measured values from wheel speed sensors electronic control device for controlling the brake slip, drive slip and / or driving stability a motor vehicle is controllable.

The brake pressure transmitter according to the invention enables simple and advantageously the interaction of a hydraulic Brake booster and an electronic Control device for regulating the brake pressure in the wheel brake cylinders and for automatic brake actuation. By the electromagnetic actuation of the pressure control valve with simple means one from the operation of the brake pedal independent control of the hydraulic Brake booster reached. The master cylinder can therefore with the help of the brake booster by an electronic Control device are operated automatically, one such activity does not affect the position of the Brake pedal and the initiation of a pedal-operated Braking has. The pressure builds up at automatic actuation in the same way as for a Brake actuation using the brake pedal. It is therefore for Traction control or driving stability control none Special valves in the brake system required, but for Regulation of the brake pressure in the wheel brake cylinders can same hydraulic components that are used are suitable for brake slip control. Also the Operation of the hydraulic components of a brake slip control system can when using a fiction  brake pressure sensor in the event of traction slip or Driving stability control can be maintained unchanged. It is therefore, for example, not a special construction of the back Delivery pump of the brake slip control device required. In the control of the brake slip offers the fiction appropriate brake pressure transmitters still have the option of using the excitation current of the electronic control device Proportional magnets and thus the boost pressure reducing magnetic force and thereby ver strengthening pressure temporarily below that caused by pedal force certain value in order to reduce the Master cylinder generated brake pressure the control requirement to adapt. With the device according to the invention not only the desired one, usually the Achieve pedal force proportional brake force boost, but it is without changes to the device different control radio would have to be made by yourself possible.

A more detailed embodiment of the brake pressure according to the invention donor can be done in different ways. So it can electrical measuring device for recording the actuating force of the brake pedal according to the invention be a force transducer that is preferably arranged in the actuating rod the brake pedal with the booster piston or the piston of the Master cylinder connects. According to the invention, the Actuating rod supported directly on the booster piston be.

Another advantageous embodiment can consist in that the electrical measuring device is a pressure sensor, the one generated by pressing the brake pedal ed, the pressure proportional to the actuating force is beatable.  

According to a further proposal of the invention can also be done be seen that the pressure control valve through the brake pedal hydraulically with a force proportional to the pedal force and without actuating the brake pedal to drive slip or Driving stability control using a proportional magnet can be actuated, the excitation current of an electronic Control device depending on measured values of Wheel speed sensors can be generated. This configuration of the Invention has the advantage that the brake pressure at Braking operations determined by the driver's will is only controlled hydraulically, and not by is dependent on electronic measured value processing.

To generate a pressure proportional to the pedal force According to the invention in the housing of the master cylinder on the Brake pedal facing side of the booster chamber one Control chamber can be provided on one side of a connected to the brake pedal by an operating rod Spool and on the other side from one to the Booster piston supported actuating piston is limited. The control piston generates a brake application in the Control chamber a pressure proportional to the pedal force, the is measurable by the pressure sensor or to a seam mer cylinder for hydraulic actuation of the pressure control valve can be transferred. The tax chamber is in front preferably in the manner of a master cylinder by an off same bore or one in its effect this corresponds valve connected to a reservoir.

The generation of the boost pressure of the hydraulic ver starters can be designed according to the circulation principle, in which one from the booster pump via the pressure control valve in the Circuit promoted pressure medium flow when the Pressure control valve is throttled until the pump  output a gain corresponding to the control force pressure is reached. A is suitable for this mode of operation Pressure control valve in which the valve body by the force of the proportional magnet or the actuating force of the Neh merzylinders in the closing direction and through the reinforcement pressure is movable in the opening direction. Is the pressure tax valve can be actuated electromagnetically and hydraulically it is advantageous if the slave cylinder on the Pressure control valve facing away from the proportional magnet is arranged, the actuating piston of the slave cylinder through a guide pin of the magnet armature is formed, in the manner of a plunger in a Wall of the slave cylinder is guided and sealed. By Use a Teflon seal on the bushing Guide pin in the slave cylinder can be a smaller one Hysteresis and accordingly better control behavior can be achieved.

Is the dynamic behavior of the booster pump enough for one Pressure generation based on the recirculation principle is not sufficient Supply of the brake booster a low pressure accumulator be provided regularly by the booster pump is loaded. It is advantageous if the between the booster chamber and the booster pump outlet arranged pressure control valve designed as a slide valve is the hydraulic one in its initial position Connection between the amplifier chamber and the Booster pump or the low pressure accumulator interrupts and connects the booster chamber to the reservoir that through the magnetic force of the proportional magnet into a Open position is movable, in which the amplifier chamber the storage container separately and with the booster pump or the low pressure accumulator is connected and its Valve slide has an end face that is opposite to the  Magnetic force with the boost pressure can be applied.

The valve spool of the slide valve can according to the invention have three separators that the cylinder bore of the Divide the valve housing into four valve chambers Switching area of the middle separator in the wall of the Cylinder bore one with the booster chamber hydraulically connected, annular recess is provided, which in Starting position of the valve spool with one on one Storage container connected valve chamber and in switching position of the valve spool with one at the outlet of the Booster pump connected valve chamber connection.

The invention is explained below with reference to exemplary embodiments play explained in more detail, which is shown in the drawing are. Show it

Fig. 1 is a brake pressure transducer with a pressure transducer by a controlled electromagnetic Actuate the supply of a pressure control valve,

Fig. 2, a brake pressure transducer with a hydraulically and electromagnetically actuated pressure control valve

Fig. 3 is a brake pressure sensor with a controlled by a force transducer electromagnetic actuation supply of a pressure control valve and

Connectable Fig. 4 is a brake pressure transducer according to FIG. 1 a to a low-pressure accumulator, designed as a slide valve pressure control valve.

The brake pressure transducer shown in Fig. 1, in a common housing 1 to a master cylinder 2, a brake booster 3 and a hydraulic control device 4, which can be actuated by a brake pedal 5. At the master cylinder 2 two brake circuits 6 , 7 are separated from each other in the usual way by a floating piston, which connect the master cylinder 2 in a known manner via a central control unit 8 with the wheel brakes 9 of a vehicle.

The master cylinder 2 is actuated by means of a pressure piston 10 which , with its end protruding from the master cylinder 2, bears against an intensified piston 12 which is displaceably guided and sealed in a cylinder bore 11 . The booster piston 12 divides the Zylin derbohrung 11 into an booster chamber 13 and an equalization chamber 14th A cylinder bore 15 adjoins the booster chamber 13 , the diameter of which is smaller than the diameter of the cylinder bore 11 . In the Zylin derbohrung 15 , a supply piston 16 and a control piston 17 are arranged at a distance from each other and sealed against the bore wall. The actuating piston 16 is formed by a stepped extension of the amplifier piston 12 . The control piston 17 projects out of the housing 1 with a guide sleeve 19 and is connected to the brake pedal 5 by an actuating rod 20 . The gap between the actuating piston 16 and the control piston 17 forms a control chamber 21 which is connected to an electrical pressure sensor 23 by a bore 22 .

The master cylinder 2 has a reservoir 24 to which the compensation chamber 14 is connected via a line 25 . Via a ver through the control piston 17 compensating bore 26 and a line 18 , the control chamber 21 is connected to the compensation chamber 14 and thus also to the reservoir 24 . The United booster chamber 13 is connected through a line 27 to the output of an amplifier pump 28 which promotes the circuit via a pressure control valve 29 and a return line 30 connected to the reservoir 24 . The pressure control valve 29 has a valve housing 31 with a valve seat 33 which can be closed by a valve body 32 . The valve body 32 is arranged in a valve chamber 34 which is closed to the return line and is held in a starting position lifted off the valve seat 33 by a compression spring 35 . To actuate the valve body 32 , the valve housing 31 is connected to a proportional magnet 36 , by means of the armature of which the valve body 32 can be moved in the direction of the valve seat 33 against the pressure in the line 27 and against the force of the compression spring 35 upon excitation.

The proportional magnet 36 is excited by a current which is generated by an electronic control device (not shown) and which is linearly dependent on the measured value of the pressure sensor.

When the brake pedal 5 is operated, the control piston is moved by the force transmitted from the actuating rod 20 pedaling force Fp in the control chamber 21 17 and generates a pressure after closing of the balancing bore 26 in the control chamber 21 through which the actuating piston 16 and with this, the booster piston 12 and the pressure piston 10 are moved in the direction of the master cylinder 2 and a braking process is initiated. Simultaneously, the electronic control unit receives a measuring signal corresponding to this pressure of the acted upon by the pressure in the control chamber 21 pressure transducer 23rd The control unit then switches on the drive of the booster pump 28 and generates an electric current proportional to the measured pressure, which excites the proportional magnet 36 and generates a magnetic force Fm proportional to the pedal force Fp, by means of which the valve body 32 is moved against the valve seat 33 . As a result, the flow of the booster pump 28 is throttled or temporarily interrupted until a booster pressure is built up in the line 27 , which acts on the valve body 32 with a counterforce corresponding to the magnetic force Fm. The boost pressure passes via the line 27 into the boost chamber 13 and pressurized therein, the annular surface of the booster piston 12, whereby the latter acts with an additional boost force to pressure the rotary piston 10 and increases the braking pressure in the brake circuits 6, 7 accordingly.

If an automatic actuation of individual wheel brakes to regulate the drive slip or the driving stability is to take place, the proportional magnet 36 is activated directly by the electronic control device provided for this purpose and is excited with a current which is required to generate the desired brake pressure. The simultaneously activated booster pump 28 delivers pressure medium with the pressure determined by the pressure control valve 29 into the booster chamber 13 , as a result of which the booster piston 12 is moved in the direction of the master cylinder 2 and actuates the pressure circuit piston 10 . The position of the brake pedal 5 remains unaffected, since the increase in volume caused by the stroke of the piston 12 with the actuating piston 16 in the control chamber 21 is compensated for by the supply of pressure medium via the line 18 and the compensating bore 26 . The brake pedal 5 can therefore also be operated in the usual manner during automatic braking, in which case the signal from the pressure sensor 23 naturally takes precedence and exclusively takes over the further control of the pressure control valve 29 .

The control of the brake pressure on the wheel brakes 9 takes place in a traction control or driving stability control by the control unit 8 in accordance with the control commands of an electronic control device which processes the rotational speeds of the vehicle wheels measured by sensors. The control unit 8 can also be used for brake slip control during a brake application initiated by the brake pedal 5 .

When the brake pressure generator according to FIG. 2 1, the pressure control valve 29 is controlled in a introduced by the brake pedal 5 braking operation directly by the pressure in the control chamber 21 hydraulically in contrast to the braking pressure generator of FIG.. For this purpose, the pressure control valve 29 on the side facing away from the valve body 32 of the proportional magnet 36 has a slave cylinder 37 integrated into the housing 31 , which is connected by a line 38 to the control chamber 21 . The actuating piston 39 of the slave cylinder 37 is formed by one with the anchor 40 of the proportional solenoid 36 associated guide pin which is rei guided in a bore in an end wall 41 of the slave cylinder 37 by means of a Teflon seal 42 bungsarm and sealed.

In this embodiment of the brake pressure transmitter, the proportional magnet 36 is only activated when automatic braking processes are carried out. If, on the other hand, the brake pressure transmitter is actuated by the brake pedal 5 , the proportional magnet remains de-energized and the amplification pressure is controlled exclusively by the slave cylinder 37 connected to the control chamber. The electronic control device detects an actuation of the brake pedal by the switch-on signal of a brake light switch 43 actuated by the brake pedal. This switch-on signal also serves to switch on the drive of the amplifier pump 28 .

In the example shown in Fig. 3 the braking pressure generator, the pedal force F is transmitted via the actuating rod 20 directly on the booster piston 12. The hydraulic Steuerein device 4 is omitted and the guide sleeve 19 connected to the actuating rod 20 protrudes directly into the reinforcing chamber 13 and is connected to the booster piston 12 there. In the actuating rod 20 there is a force transducer 44 which controls a current proportional to the pedal force Fp for exciting the proportional magnet 36 in a manner analogous to the pressure transducer 23 in the embodiment according to FIG. 1. Otherwise, the construction and operation of the brake pressure generator according corresponds to Fig. 1 3 that of the braking pressure generator of FIG..

Fig. 4 shows a modification of the embodiment of FIG. 1, in which instead of a pump circuit open in the rest position according to the circulation principle, a pump circuit closed in the rest position is provided with a low-pressure accumulator 45, which is charged by the booster pump 28 . Such an arrangement is advantageous in order to achieve high actuation dynamics with a relatively small pump. The pressure control valve 29 is designed here as an electromagnetically operable slide valve and has a valve housing 47 with a cylinder bore 48 in which a valve slide 49 with three separating bodies 50 , 51 , 52 arranged at a distance from one another can be moved, which separates the cylinder bore in valve chambers 53 , 54 , 55 , 56 divide. The valve chambers 54 and 56 are connected to a return line 30 which leads to the suction side of the booster pump 28 and to the reservoir 24 . A line 27 leads from the valve chamber 55 to the outlet of the booster pump 28 . The low-pressure accumulator 45 is connected to the line 27 via an electromagnetically actuated shut-off valve 57 . In the stroke area of the middle separating body 51 , an annular groove 58 is formed in the cylinder bore 48 , which is connected to the booster chamber 13 of the brake booster 3 via a radial connection bore and a line 59 . The valve chamber 53 is also connected to the line 59 .

The valve spool 49 is held in the starting position shown by a compression spring 60 arranged in the valve chamber 53 . In this position, the pressure-side valve chamber 55 is separated from the annular groove 58 . In contrast, the suction-side valve chamber 54 is open to the annular groove 58 , whereby the booster chamber 13 is connected to the reservoir 24 . If the proportional magnet is excited by the initiation of a braking process, the valve slide 49 is shifted to the right against the force of the compression spring 60 , as a result of which the valve chambers 54 are separated from the annular groove 58 and the valve chamber 55 is connected to the annular groove 58 . As a result, the pressure medium conveyed by the booster pump 18 and, by opening the shut-off valve 57, the pressure medium stored in the low-pressure accumulator 45 can reach the booster chamber 13 and act on the booster piston 12 . The pressure built up in the booster chamber 13 also acts on the end face of the separating body 50 in the valve chamber 53 and thereby generates a compressive force which is opposite to the magnetic force Fm. As soon as this compressive force exceeds the magnetic force Fm, the valve slide 49 is moved far back to the left in the direction of the starting position sc until the separating body 51 closes the connection between the valve chamber 55 and the annular groove 58 and thereby prevents a further increase in pressure. In this position, the valve slide 49 remains only as long as the magnetic force Fm is in equilibrium with the pressure force and the spring force which act on the valve slide 49 . If, for example, the magnetic force Fm increases due to greater actuation of the brake pedal 5 , the connection between the valve chamber 55 and the annular groove 58 is opened again, as a result of which the pressure in the booster chamber 13 increases further. If the magnetic force Fm decreases, for example by reducing the pedal force Fp, the connection between the valve chamber 54 and the annular groove 58 is opened, whereby pressure medium escapes from the booster chamber 13 to the reservoir 24 and the pressure in the booster chamber 13 drops. In this way, the pressure in the booster chamber 13 is respectively adjusted value proportional to a force Fm of the magnet or the pedal force Fp.

The excess pressure medium conveyed by the booster pump 28 during the pressure control can first be taken up by the low-pressure accumulator 45 . If the low pressure accumulator is filled, a pressure limiting valve 61 connected in parallel to the amplifier pump 28 opens, as a result of which the excess pressure medium reaches the suction side of the amplifier pump 28 .

The advantages of the brake pressure sensors described are summarized mainly to see that for braking power boost no vacuum booster is required and Independence from a vacuum source exists; thus redu adorns the installation space decisively. The hydraulic Gain is due to the high power density of the Hydraulics achieved. In addition to the brake booster can several control functions can be implemented. The drive slip and the driving stability control are only through Commissioning of the booster pump causes a normal Standard low pressure pump can be what is noisy reducing and reducing costs. There is no need  Use of special valves and a special construction of the Pump in the hydraulic control unit Brake slip control, as this occurs during an automatic Braking is not used to generate brake pressure and the pump on the suction side is not subjected to pressure. By not using special valves, the ventilation of the Control unit simplified.

In addition, it is pointed out that the booster pump 28 presented in the previous exemplary embodiments according to FIGS. 1-4 can also be represented by means of a piezoelectric pump drive.

Reference list

1

casing

2nd

Master cylinder

3rd

Brake booster

4th

Control device

5

Brake pedal

6

Brake circuit

7

Brake circuit

8th

Control unit

9

Wheel brake

10th

Pressure piston

11

Cylinder bore

12th

Booster piston

13

Amplifier chamber

14

Compensation chamber

15

Cylinder bore

16

Actuating piston

17th

Control piston

18th

management

19th

Guide sleeve

20th

Operating rod

21

Tax chamber

22

drilling

23

Pressure transducer

24th

Storage container

25th

management

26

Compensating hole

27

management

28

Booster pump

29

Pressure control valve

30th

Return line

31

Valve body

32

Valve body

33

Valve seat

34

Valve space

35

Compression spring

36

Proportional magnet

37

Slave cylinder

38

management

39

Actuating piston

40

anchor

41

Front wall

42

Teflon seal

43

Brake light switch

44

Load cell

45

Low pressure accumulator

47

Valve body

48

Cylinder bore

49

Valve spool

50

Separator

51

Separator

52

Separator

53

Valve chamber

54

Valve chamber

55

Valve chamber

56

Valve chamber

57

Check valve

58

Ring groove

59

management

60

Compression spring

61

Pressure relief valve

Claims (13)

1. Brake pressure sensor for a hydraulic brake system for motor vehicles with a master cylinder that can be connected to a wheel brake cylinder, a hydraulic brake booster with a booster piston that limits a booster chamber for actuating the master brake cylinder, a booster pump that is driven by a motor, and a control device that can be actuated by a brake pedal and that has a pressure control valve Control of the booster pressure acting on the booster piston, characterized in that the pressure control valve ( 29 ) can be actuated electromagnetically by a proportional magnet ( 36 ) and that the excitation current of the proportional magnet ( 36 ) corresponds to the measured values of an electrical measuring device which detects the actuating force of the brake pedal ( 5 ) and / or the measured values from wheel speed sensors of an electronic control device for controlling the brake slip, the drive slip and / or the driving stability of the F ahrzeugs is generated. 2. Device according to claim 1, characterized in that the electrical measuring device is a force transducer ( 44 ). 3. Apparatus according to claim 1, characterized in that the electrical measuring device is a pressure sensor ( 23 ) which can be acted upon by a pressure which can be generated by the actuation of the brake pedal ( 5 ) and which is proportional to the actuating force. 4. The device according to claim 1, characterized in that the pressure control valve ( 29 ) by the brake pedal ( 5 ) hydraulically with a force proportional to the pedal force (Fp) and for traction control or driving stability control by a proportional magnet ( 36 ), the excitation current of which can be actuated can be generated by an electronic control device as a function of measured values from wheel speed sensors. 5. The device according to claim 2, characterized in that the brake pedal ( 5 ) via an actuating rod ( 20 ) is connected directly to the booster piston ( 12 ) that the force transducer ( 44 ) between the brake pedal ( 5 ) and the actuating rod ( 20 ) is arranged and that an excitation current corresponding to the measured value of this force transducer ( 44 ) can be generated for the proportional magnet ( 36 ) of the pressure control valve ( 29 ) in an electronic control device. 6. Device according to one of claims 3 or 4, characterized in that the booster piston ( 12 ) is designed as a stage piston and with its smaller diameter stage forms an actuating piston ( 16 ) which delimits a control chamber ( 21 ) and that the brake pedal ( 5 ) is connected via an actuating rod ( 20 ) to a control piston ( 17 ), by means of which a pressure proportional to the pedal force (Fp) can be generated in the control chamber ( 21 ). 7. The device according to claim 5, characterized in that the pressure in the control chamber ( 21 ) by the pressure sensor ( 23 ) can be detected and that in an electronic control device a measured value of the pressure sensor ( 23 ) corresponding excitation current for the proportional magnet ( 36 ) of the pressure control valve ( 29 ) can be generated. 8. The device according to claim 5, characterized in that the control chamber ( 21 ) with a hydraulic slave cylinder ( 37 ) for actuating the valve body ( 32 ) of the pressure control valve ( 29 ) is connected. 9. The device according to claim 7, characterized in that the slave cylinder ( 37 ) on the pressure control valve ( 29 ) facing away from the proportional magnet ( 36 ) is arranged and has an actuating piston ( 39 ) by a low-friction seal ( 36 ) in the End wall ( 41 ) of the slave cylinder ( 37 ) is sealed. 10. Device according to one of the preceding claims, characterized in that the pressure control valve ( 29 ) is an open seat valve in its rest position, the sen valve seat ( 33 ) on the input side to the pressure side of the booster pump ( 28 ) and the booster chamber ( 13 ) is connected and connected on the output side via a return line ( 30 ) to a reservoir ( 24 ) or the suction side of the booster pump ( 28 ) and its valve body ( 32 ) by the magnetic force (Fm) of the proportional magnet ( 36 ) or the actuating force of the Slave cylinder ( 37 ) is movable in the closing direction against the boost pressure. 11. Device according to one of claims 1 to 8, characterized in that the pressure control valve ( 29 ) is designed as a slide valve, which is arranged between the United booster chamber ( 13 ) and the pressure side of the amplifier pump ( 28 ) and in its rest position the hydraulic Interruption between the booster chamber ( 13 ) and the booster pump ( 28 ) and connects the booster chamber ( 13 ) to the reservoir ( 24 ), which can be moved by the magnetic force (Fm) of the proportional magnet ( 36 ) into an open position in which the booster chamber ( 13 ) is separated from the storage container ( 24 ) and connected to the booster pump ( 28 ) or the low-pressure accumulator ( 45 ) and the valve slide ( 49 ) has an end face which can be acted upon by the booster pressure against the magnetic force (Fm). 12. The apparatus according to claim 10, characterized in that the valve slide ( 49 ) of the slide valve has three separating bodies ( 50 , 51 , 52 ), the tion ( 48 ) of the valve housing ( 47 ) in four valve chambers ( 53 , 54 , 55) , 56 ) subdivide, in the switching region of the middle separating body ( 51 ) in the wall of the cylinder bore ( 48 ), an annular recess ( 58 ) hydraulically connected to the booster chamber ( 13 ) is seen before, which in the initial position of the valve spool ( 49 ) with a valve chamber ( 54 ) connected to a storage container ( 24 ) and in the switching position of the valve slide ( 49 ) with a valve chamber ( 55 ) connected to the outlet of the booster pump ( 28 ). 13. Device according to one of claims 10 or 11, characterized in that the pressure side of the booster pump ( 28 ) is connected via a valve in the starting position ( 57 ) to a low-pressure accumulator ( 45 ) and this can be switched on if necessary.