DE102007059440A1 - Electrohydraulic power steering system and hydraulic pressure accumulator for a power steering system - Google Patents

Abstract

An electro-hydraulic power steering system (8) comprises a hydraulic circuit (10) comprising a pump (24), a cylinder / piston unit (27) and a hydraulic accumulator (36), the pump (24) being designed as a reversible pump and optionally a first working chamber (28) or a second working chamber (30) of the cylinder / piston unit (27) can act on a hydraulic pressure and wherein at a suction port (25, 26) of the pump (24) via the pressure accumulator (36 ) adjustable preload pressure is applied. A hydraulic pressure accumulator (36) for a power steering system, preferably for the electrohydraulic power steering system (8) described above, comprises a pressure accumulator housing (38) in which a pressure chamber (40) and a biasing chamber (42) are formed and one towards the pressure chamber (40) baufschlagten piston (44) which is movable between a first end position and a second end position and the two chambers (40, 42) separated from each other, wherein in the piston (44) a check valve (46) is integrated, which in a Open position, the pressure chamber (40) and the biasing chamber (42) connects. The pressure chamber (40) has a pressure chamber connection (48) via which the pressure accumulator (36) to the hydraulic circuit (10) of the power steering system (8) can be connected, wherein in the pressure chamber (40) is formed a collection chamber (50) for gas bubbles and wherein the piston (44) from a pre-definable hydraulic pressure in the pressure chamber (40) the ...

Description

The The invention relates to an electrohydraulic power steering system as well a hydraulic pressure accumulator for a power steering system, with a pressure accumulator housing in which a pressure chamber and a biasing chamber are formed, one in the direction of the pressure chamber acted upon piston between a first end position and is movable in a second end position and the two chambers separates, with a check valve in the piston is integrated, which in an open position, the pressure chamber and the biasing chamber connects, and wherein the pressure chamber has a pressure chamber port via the pressure accumulator to a hydraulic circuit of the power steering system is connectable.

As an electrohydraulic power steering system, a steering system is referred to below in which the power assistance generated by an electric motor is transmitted by means of a hydrostatic circuit to a rack of the steering system. The electric motor thereby drives a reversibly operating pump, wherein the pump has two connections which are each connected via hose lines with one of two working chambers of a steering cylinder. The pressure difference on the steering cylinder is thus determined by the drive torque of the pump or the output torque of the electric motor, wherein the working chambers are usually associated with pressure sensors that monitor the hydraulic pressure in the steering cylinders. Such an electro-hydraulic power steering system is already out of the US 6,152,254 A known.

The necessary power assistance is in such a steering system determined according to the following principle: A steering torque sensor detected the steering torque applied by a driver on the steering wheel, from the calculated a setpoint for the pressure difference at the steering cylinder becomes. With the help of an electronic control of the electric motor Then, the detected by the pressure sensors pressure difference adjusted in the working chambers to the current setpoint.

It it turned out that for a flawless Function of this loop a stiff and play-free coupling between the electric motor and the rack is necessary. Therefore the occurrence of cavitation phenomena in the hydrostatic Circle should be avoided if possible, and under circumstances remaining during filling or due to leakage Air bubbles in the steering system should become self-sufficient over time can escape.

to Avoidance of cavitation are already known from the prior art Servolenksysteme known in which a conventional, pressureless Reservoir is replaced by a hydropneumatic reservoir, to raise the pressure level. As the gas pressure in these hydropneumatic However, saving a significant amount of temperature depends on ensuring a largely constant Preload pressure at the usual temperature changes problematic in the engine compartment.

task The invention therefore provides a preloaded power steering system. that provides a largely constant biasing pressure in normal operation, can be filled and prestressed with little effort and an advantageous, automatic venting allows.

According to the invention this object is achieved by an electro-hydraulic power steering system, with a hydraulic circuit, a pump, a cylinder / piston unit and a hydraulic pressure accumulator, wherein the Pump is designed as a reversible pump and optionally a first Working chamber or a second working chamber of the cylinder / piston unit can act on a hydraulic pressure, and wherein at one Suction connection of the pump on via the pressure accumulator adjustable preload pressure is applied. By this biasing pressure At the suction connection of the pump cavitation phenomena largely prevented in the power steering system.

Preferably Each working chamber is assigned a pressure sensor, the smaller the two sensed pressure values are defined as prestressing pressure.

In An embodiment of the power steering system is a biasing pump provided with an under atmospheric pressure Reservoir and a pressure chamber of the pressure accumulator in conjunction stands, wherein the biasing pump at a drop in the biasing pressure is activated below a predeterminable value and hydraulic fluid from the reservoir into the pressure chamber promotes. Thus, the same Tension pump pressure fluctuations in the accumulator off, for example due to temperature changes, leaks in the hydraulic circuit and / or other volume changes occur.

It has been found that for biasing the hydraulic circuit a vibrating tank pump is particularly suitable and therefore preferred as Tension pump is used. Because the pressure fluctuations when using a suitable pressure accumulator are rather low, sufficient for balancing This pressure fluctuations usually a cheap and compact Miniature version of the vibration pump with low displacement and a discharge pressure of a few bar.

In Another embodiment, the hydraulic circuit an electromagnetically actuated directional control valve and an electromagnetic operated safety valve on, with the hydraulic system be filled in an operating position of the valves by means of the pump and / or can be prestressed. By using the existing anyway Hydraulic pump for this purpose simplifies the filling or bias of the power steering system considerably.

In another system variant, the hydraulic circuit has an electromagnetic actuated directional control valve and an electromagnetically actuated Safety valve on, with the hydraulic system in an operating position the directional valve and an unconfirmed basic position of the Safety valve fillable by means of the pump and / or can be prestressed.

A Another object of the invention is to provide a pressure accumulator for a power steering system that is a largely constant Bias of the hydraulic circuit ensures and easy filling or bias of the power steering system allows.

According to the invention the task by a hydraulic accumulator of the beginning solved type, wherein in the pressure chamber of the pressure accumulator a collection space is formed for gas bubbles and wherein the piston from a pre-definable hydraulic pressure in the pressure chamber the assumes the first end position in which the pressure chamber via the collection room with the header chamber is in communication. By This accumulator design is a simple way an advantageous Venting of the hydraulic circuit possible.

The Biasing chamber can be partially filled with hydraulic fluid be, which is substantially at atmospheric pressure. Thus, the biasing chamber is used as a compensating volume, the absorbs hydraulic fluid from the pressure chamber as required or into the pressure chamber to the pressure in the hydraulic circuit possible to keep constant.

In an embodiment of the accumulator is the collection room in the second end position of the piston separated from the pressure chamber connection and the check valve can take its open position. This ensures that the hydraulic circuit over the pressure chamber connection no gas bubbles from the collection room, but only hydraulic fluid from the preload chamber sucks.

In This embodiment can in the pressure chamber, a collection space bridging element be provided, which the biasing chamber and the pressure chamber port in the second end position of the piston with the check valve open essentially connects tightly.

Prefers the preload chamber overflows in the first end position of the piston axial notches in a peripheral wall of the accumulator housing with the collection space of the pressure chamber in conjunction. In overpressure case can be very easy due to these scores excess Hydraulic fluid from the hydraulic circuit in the preload chamber of Pressure accumulator are delivered.

alternative or additionally, in the biasing chamber, a piston stop be provided with an axial pin, wherein the pin in the first end position of the piston, the check valve in the Open position holds, so that the preload chamber with the collection space of the pressure chamber is in communication. This is a discharge of excess hydraulic fluid from the hydraulic circuit via the check valve possible, which by the pin in its open position is forced and makes a connection to the biasing chamber. The formation of axial notches in the peripheral wall of the pressure accumulator housing is therefore no longer necessary.

Further Features and advantages of the invention will become apparent from the following Description of preferred embodiments with reference to the painting. In these show:

1 the schematic circuit diagram of a power steering system according to the invention with a hydraulic circuit according to a first embodiment;

2 a longitudinal section through a first embodiment of the pressure accumulator according to the invention;

3 a longitudinal section through a second embodiment of the pressure accumulator according to the invention;

4 a sectional perspective view of an accumulator piston for the pressure accumulator according to the invention according to a third embodiment;

5a and 5b in each case a detail section through a check valve of the pressure accumulator according to the invention;

6 a (secondary) hydraulic circuit for filling or biasing the power steering system according to the invention;

7 the schematic circuit diagram of a power steering system according to the invention with a hydraulic circuit according to a second embodiment shape; and

8th the schematic circuit diagram of a power steering system according to the invention with a hydraulic circuit according to a third embodiment.

In 1 is a hydraulic circuit diagram for a power steering system 8th of a motor vehicle shown schematically. The circuit diagram shows a concrete biased hydraulic circuit 10 for an electrohydraulic power steering system 8th ,

The electrohydraulic power steering system 8th has a steering shaft 11 on, which in a known manner via a steering gear 12 with a rack 14 is coupled, which in turn interacts with steerable wheels of the motor vehicle (not shown). At the steering gear 12 is a steering torque and / or steering angle sensor 16 provided its sensor data to an electronic control unit 18 a motor-pump unit 20 passes. The engine-pump unit 20 is the interface between the electrical and the hydraulic part of the electrohydraulic power steering system 8th and points next to the electronic control unit 18 another engine 22 and a reversible pump 24 on. As a reversible pump 24 In this context, a pump with two pump connections 25 . 26 designated, wherein the pump has two different directions of delivery of hydraulic fluid and the pump ports 25 . 26 used as a suction port or pressure port depending on the conveying direction. The power steering system 8th further comprises a cylinder / piston unit 27 with two working chambers 28 . 30 , which depends on the conveying direction of the pump 24 be pressurized and correspondingly an axial movement of the rack 14 effect or assist in a first or an opposite second direction. The one in the working chambers 28 . 30 prevailing pressure is thereby from pressure sensors 31 detected and to the electronic control unit 18 transmitted. In the present example, the control unit 18 beyond that with a power supply 33 and other data acquisition units 35 connected.

The power steering system 8th according to 1 also has a safety valve 32 on which the working chambers 28 . 30 the cylinder / piston unit 27 in a first valve position according to 1 short circuits, so that in case of system failure, no hydraulic blockage. The motor vehicle steering then works purely mechanically in this case. The safety valve 32 is designed as an electromagnetically controlled directional control valve, wherein a spring element 34 Ensures that the safety valve 32 in the de-energized state in its basic position, that is moved in the illustrated, open valve position and takes over the function of a well-known "fail-safe" valve.

Of the basic structure of an electro-hydraulic vehicle steering system as well as its operation are already generally out of the state known in the art, which is why in the following only fiction relevant Specifics will be discussed in more detail.

A special feature of the power steering system 8th lies in the construction of a pressure accumulator 36 who in 1 schematically drawn and in 2 is shown separately and in detail according to a first embodiment. The accumulator 36 includes a pressure accumulator housing 38 in which a pressure chamber 40 and a biasing chamber 42 are formed, and a movable piston 44 moving in the direction of the pressure chamber 40 is charged and the two chambers 40 . 42 separates each other. In the pistons 44 is a check valve 46 integrated, which in its open position, the pressure chamber 40 and the preload chamber 42 combines.

The pressure chamber 40 has a pressure chamber connection 48 on, over which the accumulator 36 to the hydraulic circuit 10 of the power steering system 8th can be connected (see. 1 ). In the pressure chamber 40 is a collection room 50 designed for gas bubbles over which the pressure chamber 40 in the first end position of the piston 44 with the preload chamber 42 communicates with the piston 44 from a predetermined hydraulic pressure in the pressure chamber 40 takes the first end position. In the first embodiment according to 2 is the collection room 50 an annulus surrounding a collection room bridging element 58 is formed around and on the outside by a peripheral wall 52 the accumulator housing 38 is limited. In the peripheral wall 52 are axially aligned notches 54 provided, which is the preload chamber 42 in the first end position of the piston 44 with the collection room 50 the pressure chamber 40 connect. According to 2 is the first end position with an upper stop position of the piston 44 identical.

In the preload chamber 42 is a compression spring 55 provided the piston 44 towards the pressure chamber 40 applied. Furthermore, the preload chamber 42 at least partially filled with hydraulic fluid, which is substantially at atmospheric pressure. According to 2 is still a tank to increase the bias chamber volume 56 with a vent cover 57 provided, with the vent cover 57 in other embodiments also directly to the accumulator housing 38 can be attached. Will the piston 44 against the force of the compression spring 55 by a growing pressure in the hydraulic circuit 10 moved to its first end position, so gas and / or hydraulic fluid from the pressure chamber 40 about the notches 54 in the preload chamber 42 stream. Because the collection room 50 first the axial notches 54 he initially, predominantly gas fractions flow out of the pressure chamber 40 in the preload chamber 42 , causing the hydraulic circuit 10 is vented in the desired manner. The gas bubbles, which are in the collection room 50 accumulate, for example, when filling the power steering system 8th in the hydraulic circuit 10 remained or cavitation or leakage in the power steering system 8th penetrated. Particularly preferred is a bottom of the piston 44 slightly conical (not shown), so that the gas bubbles primarily on the peripheral wall 52 the accumulator housing 38 collect and over the notches 54 in the preload chamber 42 be delivered.

In the second end position, ie the lower stop position of the piston 44 is the collection room 50 from the pressure chamber connection 48 separated. For this purpose, in the first embodiment of the pressure accumulator 36 the collection room bridging element 58 in the pressure chamber 40 provided (cf. 2 ), which is the preload chamber 42 and the pressure chamber connection 48 in the second end position of the piston 44 (with check valve open 46 ) connects substantially tightly. In the event that the hydraulic circuit 10 in the second end position of the piston 44 Hydraulic fluid sucks, thus ensuring that the pressure chamber port 48 no air from the collection room 50 , but only hydraulic fluid from the biasing chamber 42 is sucked.

Since the hydraulic accumulator 36 according to 2 in contrast to hydropneumatic pressure accumulators has no completed gas volume, which is through the pressure accumulator 36 defined preload pressure largely constant. This preload pressure is thereby determined by the compression spring 55 set. A hydrostatic component from the pressureless hydraulic fluid in the biasing chamber 42 is usually negligible. Another advantage of the pressure accumulator 36 is the automatic bleeding of the hydraulic circuit 10 at overpressure, ie at a hydraulic pressure in the pressure chamber 40 which is above a maximum desired biasing pressure.

The 3 shows a second embodiment of the pressure accumulator 36 , which differs from the first embodiment described above, but only by a changed overpressure ventilation. Instead of the axial notches 54 in the peripheral wall 52 the accumulator housing 38 is according to 3 a piston stop 60 with an axial pin 62 in the preload chamber 42 provided, wherein the pin 62 in the first end position of the piston 44 the check valve 46 keeps open, so that the preload chamber 42 with the pressure chamber 40 communicates. Besides, in the piston 44 a channel 64 provided with the check valve open 46 the preload chamber 42 with the collection room 50 connects, so that in this second embodiment of the accumulator 36 a venting of the hydraulic circuit 10 in the first end position of the piston 44 is possible. The check valve 46 in this case takes both in the first end position and in the second end position of the piston 44 its open position and allows a fluid flow between the pressure chamber 40 and the biasing chamber 42 ,

The 4 shows the piston 44 for a third embodiment of the pressure accumulator 36 , Except for the piston 44 This third embodiment corresponds to the second embodiment of the pressure accumulator described above 36 according to

3 , The modified piston 44 has at its lower end face instead of the collection space bridging element 58 (see. 2 and 3 ) a funnel-shaped depression 65 on, going to the check valve 46 rejuvenated. Any existing gas bubbles collect in this funnel-shaped depression 65 and flow at an opening of the check valve 46 through the pin 62 in the first end position of the piston 44 in the preload chamber 42 , Thus, even in this case, a desired automatic venting of the hydraulic circuit 10 given at overpressure. For piston guide and piston seal in the cylindrical pressure accumulator housing 38 points the piston 44 according to 4 two separate guide rings 66 and a sealing ring 67 on.

All accumulator versions have in common that the pressure accumulator 36 in both end positions of the piston 44 a fluid flow between the pressure chamber 40 and the biasing chamber 42 allows. The hydraulic circuit 10 can thus be vacuumed or vented advantageously hydraulic fluid.

The 5a and 5b show a particularly preferred embodiment of the check valve 46 which is in the pistons 44 of the accumulator 36 integrated, preferably screwed. The check valve 46 has a valve member 68 on, which is made of a hardened, polished steel ball. In a blocking position of the valve 46 according to 5a lies the valve member 68 with its spherical surface portion sealing against a valve seat 70 at. The valve member 68 is radially almost free of play but along a valve axis A movable in a piston bore 72 guided. cutouts 74 in the valve member 68 provide in an open position of the check valve 46 (see. 5b ) for a low-turbulence flow with low pressure drop. The valve construction shown is therefore particularly advantageous because in a flow space 73 downstream of the valve member 68 no valve parts be find that could hinder fluid flow. The guide and the exact position of the valve member 68 be by means of a guide sleeve 76 reached, which is connected to the approximately hemispherical portion of the valve member 68 followed. A closing spring 78 the check valve 46 is in the radial direction between the guide sleeve 76 and a peripheral wall of the piston bore 72 arranged and acted upon the valve member 68 in the locked position according to 5a , As axial spring abutment serves a valve sleeve 80 used to attach the check valve 46 in the piston 44 into the piston bore 72 is screwed.

The 6 shows a secondary hydraulic circuit 82 which ensures a particularly constant preload in the primary hydraulic circuit 10 (see. 1 ). The secondary hydraulic circuit 82 has for this purpose a biasing pump 84 with a motor 86 on, with the biasing pump 84 is preferably a miniaturized vibration tank pump. One suction side of the priming pump 84 is via a pressureless reservoir with the biasing chamber 42 and a pressure side of the biasing pump 84 with the pressure chamber 40 of the accumulator 36 connected. The pressureless reservoir is preferably the biasing chamber 42 or, as in 6 represented, the tank 56 , Between the preload pump 84 and the pressure chamber 40 is a check valve 88 provided, which a hydraulic fluid flow from the pressure chamber 40 to the preload chamber 42 with uncontrolled pretensioning pump 84 in derogation.

Just like the electric motor 22 of the primary hydraulic circuit 10 ( 1 ) is also the engine 86 of the secondary hydraulic circuit 82 with the power supply 33 in conjunction and will have a switch 90 from the electronic control unit 18 driven. The electronic control unit 18 checked by the pressure sensors 31 whether in each of the working chambers 28 . 30 at least the predetermined minimum biasing pressure prevails. If this is not the case, then the switch 90 closed, and the biasing pump 84 promotes hydraulic fluid from the preload chamber 42 in the pressure chamber 40 until in each of the working chambers 28 . 30 at least the minimum biasing pressure sets. The pretensioning pump 84 can also promote up to a maximum preload pressure at which the piston 44 located in its second end position and an advantageous venting of the primary hydraulic circuit 10 takes place.

Preferably, the activation of the biasing pump 84 for adjusting the preload pressure in the context of a start routine when starting the vehicle. During driving, the switch then remains 90 opened and is not from the electronic control unit 18 driven. This rare but regular check and readjustment of the preload pressure by the biasing pump 84 is sufficient to prevent leakage in the primary hydraulic circuit 10 and to keep the biasing pressure substantially constant.

The following is based on the 7 and 8th on embodiments of the electro-hydraulic power steering system 8th received, which is a particularly advantageous filling, venting and / or bias of the hydrostatic hydraulic circuit 10 enable. To avoid repetition, reference is made to the understanding of the general principle of operation to the above description of the first embodiment, wherein corresponding components carry identical reference numerals.

The 7 shows the electro-hydraulic power steering system 8th with a (primary) hydraulic circuit 10 according to a second embodiment, wherein in comparison with the first embodiment according to 1 an additional directional valve 92 , Specifically, a 4/2-way valve, and a connecting line 94 between the preload chamber 42 and the hydraulic circuit 10 is provided.

Further, at the end stops of a piston 95 the cylinder / piston unit 27 overflow grooves 96 provided in the stop positions of the piston 95 a throttled connection between the working chambers 28 . 30 produce. These overflow grooves 96 ensure on the one hand for a cushioning of the cylinder / piston unit 27 On the other hand, they also allow filling and preloading of the primary hydraulic circuit 10 in the manner described below:
The electronic control unit 18 activates the valves 32 . 92 , so that they according to their operating position 7 taking. After that the pump will start 24 operated so that they over the connecting line 94 Hydraulic fluid from the preload chamber 42 of the accumulator 36 sucks and to the first working chamber 28 the cylinder / piston unit 27 emits. The piston 95 moves as a result of this pressurization of the first working chamber 28 in its upper stop position ( 7 ), in which through the overflow grooves 96 an overflow of gas and / or hydraulic fluid from the first working chamber 28 in the second working chamber 30 is possible. From the upper working chamber 30 the gas and / or hydraulic fluid is via the directional control valve 92 in the pressure chamber 40 of the accumulator 36 initiated. At a sufficiently high pressure in the pressure chamber 40 takes place overpressure relief, in the gas and / or hydraulic fluid from the pressure chamber 40 in the preload chamber 42 flows. According to the 2 to 4 is the accumulator 36 designed so that when the pressure relief initially primarily a vent, ie an overflow of gas takes place before an overflow of hydraulic fluid takes place.

Thus, the process described above is ideal for filling, venting and preloading of the primary hydraulic circuit 10 , After the hydraulic circuit 10 has reached the desired preload pressure, the safety valve 32 operated as a conventional "fail-safe" valve and the spring-loaded directional control valve 92 disabled, so that it is its home position (lower valve position in 7 ) occupies. The electrohydraulic power steering system 8th This converts it back into its conventional operating function and can subsequently provide a desired power assistance.

In the connection line 94 is particularly preferably a check valve 98 provided, which in normal operation at a pressurization of the second working chamber 30 an outflow of hydraulic fluid into the biasing chamber 42 of the accumulator 36 prevented.

The detail A in 7 shows an alternative embodiment of the directional control valve 92 in which a leakage connection 99 the pump 24 in the operating position of the valve 92 not locked, but with the pump connection 26 connected is.

Preferably, several components of the power steering system 8th to compact, preassembled modules, such. B. the motor-pump unit 20 or a valve block 100 summarized what is in 7 is shown by dashed rectangles.

The process described for biasing the hydraulic circuit 10 may also be integrated into a start routine when starting the vehicle in this second embodiment, so that the formation of a secondary hydraulic circuit 82 according to 6 unnecessary.

The 8th shows the electro-hydraulic power steering system 8th with the (primary) hydraulic circuit 10 According to a third embodiment, in which unlike the second embodiment according to 7 no overflow grooves 96 in the cylinder / piston unit 27 are provided so that no overflow of gas and / or hydraulic fluid from the first working chamber 28 in the second working chamber 30 is possible. Nevertheless, the hydraulic circuit 10 in an operating position of the directional control valve 92 and an unconfirmed initial position of the safety valve 32 (see. 8th ) are vented and biased. The preloading of the power steering system 8th It goes exactly as well 7 described, except that the connection between the biasing chamber 42 and the pressure chamber 40 not through the overflow grooves 96 the cylinder / piston unit 27 is produced, but through the safety valve 32 ,

A filling of the electro-hydraulic power steering system 8th is also possible in this third embodiment, wherein the piston 95 However, during the filling process by manual steering from one stop position to the other stop position must be moved to that in the working chambers 28 . 30 existing gas from the cylinder / piston unit 27 to press.

In one embodiment, the accumulator is 36 designed so that for biasing the (primary) hydraulic circuit 10 not on output signals from the pressure sensors 31 must be used, since these output signals may have a zero shift (so-called drift) after a longer life of the vehicle under certain circumstances. For this reason, the hydraulic circuit 10 z. B. preferably biased so far each time the vehicle is started until the piston 44 of the accumulator 36 the axial notches 54 reached, or until the axial pin 62 the check valve 46 in the piston 44 of the accumulator 36 imprints. In this first end position of the piston 44 becomes the preload pressure in the hydraulic circuit 10 from the compression spring 55 of the accumulator 36 clearly defined and can be used to balance the two pressure sensors 31 be used.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - US 6152254 A [0002]

Claims (12)

Electrohydraulic power steering system with a hydraulic circuit ( 10 ), which is a pump ( 24 ), a cylinder / piston unit ( 27 ) and a hydraulic accumulator ( 36 ), wherein the pump ( 24 ) is designed as a reversible pump and optionally a first working chamber ( 28 ) or a second working chamber ( 30 ) of the cylinder / piston unit ( 27 ) can act with a hydraulic pressure, and wherein at a suction port ( 25 . 26 ) of the pump ( 24 ) via the accumulator ( 36 ) adjustable preload pressure is applied. Power steering system according to claim 1, characterized in that each working chamber ( 28 . 30 ) a pressure sensor ( 31 ), wherein the smaller of the two sensed pressure values is defined as biasing pressure. Power steering system according to claim 1 or 2, characterized in that a biasing pump ( 84 ) provided with an under atmospheric pressure reservoir and a pressure chamber ( 40 ) of the pressure accumulator ( 36 ), wherein the biasing pump ( 84 ) is activated at a drop in the biasing pressure below a predeterminable value and hydraulic fluid from the reservoir into the pressure chamber ( 40 ) promotes. Power steering system according to claim 3, characterized in that the pretensioning pump ( 84 ) is a vibrating armature pump. Power steering system according to one of the preceding claims, characterized in that the hydraulic circuit ( 10 ) an electromagnetically actuated directional control valve ( 92 ) and an electromagnetically actuated safety valve ( 32 ), wherein the hydraulic circuit ( 10 ) in an actuated position of the valves ( 32 . 92 ) by means of the pump ( 24 ) can be filled and / or prestressed. Power steering system according to one of the preceding claims, characterized in that the hydraulic circuit ( 10 ) an electromagnetically actuated directional control valve ( 92 ) and an electromagnetically actuated safety valve ( 32 ), wherein the hydraulic circuit ( 10 ) in an actuating position of the directional control valve ( 92 ) and an unconfirmed initial position of the safety valve ( 32 ) by means of the pump ( 24 ) can be filled and / or prestressed. Hydraulic accumulator for a power steering system ( 8th ), preferably for an electro-hydraulic power steering system according to one of the preceding claims, with a pressure accumulator housing ( 38 ), in which a pressure chamber ( 40 ) and a biasing chamber ( 42 ) are formed, and one in the direction of the pressure chamber ( 40 ) acted upon piston ( 44 ) which is movable between a first end position and in a second end position and the two chambers ( 40 . 42 ), whereby in the piston ( 44 ) a check valve ( 46 ) is integrated, which in an open position, the pressure chamber ( 40 ) and the biasing chamber ( 42 ), and wherein the pressure chamber ( 40 ) a pressure chamber connection ( 48 ), over which the accumulator ( 36 ) to a hydraulic circuit ( 10 ) of the power steering system ( 8th ), characterized in that in the pressure chamber ( 40 ) a collection room ( 50 ) is designed for gas bubbles, wherein the piston ( 44 ) from a predeterminable hydraulic pressure in the pressure chamber ( 40 ) occupies the first end position in which the pressure chamber ( 40 ) over the collection room ( 50 ) with the preload chamber ( 42 ). Pressure accumulator according to claim 7, characterized in that the prestressing chamber ( 42 ) is at least partially filled with hydraulic fluid, which is substantially at atmospheric pressure. Pressure accumulator according to claim 7 or 8, characterized in that the collection space ( 50 ) in the second end position of the piston ( 44 ) from the pressure chamber connection ( 48 ) and the check valve ( 46 ) can take his open position. Pressure accumulator according to claim 9, characterized in that in the pressure chamber ( 40 ) a collection space bridging element ( 58 ) is provided which the biasing chamber ( 42 ) and the pressure chamber connection ( 48 ) in the second end position of the piston ( 44 ) with open check valve ( 46 ) connects substantially tightly. Pressure accumulator according to one of claims 7 to 10, characterized in that the preload chamber ( 42 ) in the first end position of the piston ( 44 ) over axial notches ( 54 ) in a peripheral wall ( 52 ) of the pressure accumulator housing ( 38 ) with the collection room ( 50 ) of the pressure chamber ( 40 ). Pressure accumulator according to one of Claims 7 to 10, characterized in that in the prestressing chamber ( 42 ) a piston stopper ( 60 ) with an axial pin ( 62 ) is provided, wherein the pin ( 62 ) in the first end position of the piston ( 44 ) the check valve ( 46 ) holds in the open position, so that the biasing chamber ( 42 ) with the collection room ( 50 ) of the pressure chamber ( 40 ) in Ver bond stands.