WO2009074216A1

WO2009074216A1 - Electro hydraulic servo steering system and hydraulic accumulator for a servo steering system

Info

Publication number: WO2009074216A1
Application number: PCT/EP2008/009823
Authority: WO
Inventors: Heinz-Dieter Heitzer; Peter Zeiler
Original assignee: Trw Automative Gmbh
Priority date: 2007-12-10
Filing date: 2008-11-20
Publication date: 2009-06-18
Also published as: DE102007059440A1

Abstract

An electro hydraulic servo steering system (8) has a hydraulic circuit (10), comprising a pump (24), a cylinder/piston unit (27) and a hydraulic accumulator (36), wherein the pump (24) is designed as a reversible pump and can selectively apply hydraulic pressure to a first working chamber (28) or a second working chamber (30) of the cylinder/piston unit (27), and wherein on a suction connector (25, 26) of the pump (24) a preload pressure that can be set via the pressure storage (36) is present. A hydraulic accumulator (36) for a servo steering system, preferably for an electro-hydraulic servo steering system (8) as described above, comprises an accumulator housing (38), in which a pressure chamber (40) and a preload chamber (42) is configured, and a piston (44), which is pressurized in the direction of the pressure chamber (40) and can be moved between a first end position and a second end position and which separates the two chambers (40, 42) from each other, wherein a non-return valve (46), which connects the pressure chamber (40) and the preload chamber (42) in an open position, is integrated in the piston (44). The pressure chamber (40) comprises a pressure chamber connection (48), via which the accumulator (36) can be connected to the hydraulic circuit (10) of the servo steering system (8), wherein a collecting chamber (50) for gas bubbles is provided in the pressure chamber (40), and wherein the piston (44) assumes the first end position starting at a predeterminable hydraulic pressure in the pressure chamber (40), the pressure chamber (40) being connected to the preload chamber (42) via the collecting chamber (50) in said first end position.

Description

Electrohydraulic power steering system and hydraulic pressure accumulator for a power steering system

The invention relates to an electrohydraulic power steering system and 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, a piston acted upon in the direction of the pressure chamber, which is movable between a first end position and in a second end position and two chambers separated from each other, wherein in the piston, a check valve is integrated, which connects the pressure chamber and the biasing chamber in an open position, and wherein the pressure chamber has a pressure chamber port through which the pressure accumulator is connected to a hydraulic circuit of the power steering system.

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 to a rack of the steering system by means of a hydrostatic circuit. 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 cylinder. Such an electro-hydraulic power steering system is already known from US 6,152,254 A.

The necessary power assistance is determined in such a steering system according to the following principle: A steering torque sensor detects the applied by a driver on the steering wheel steering torque, from which a target value for the pressure difference is calculated on the steering cylinder. With the aid of an electronic control of the electric motor, the pressure difference detected by the pressure sensors in the working chambers is then adjusted to the current desired value. It has been found that a stiff and play-free coupling between the electric motor and the rack is necessary for proper operation of this loop. Therefore, the occurrence of cavitation phenomena in the hydrostatic circuit must be prevented as possible, and the possibly left in the filling or leakage air bubbles in the steering system should be able to escape automatically over time.

To avoid cavitation, power steering systems are already known from the prior art, in which a conventional, non-pressurized reservoir is replaced by a hydropneumatic accumulator to raise the pressure level. However, since the gas pressure in these hydropneumatic reservoirs depends to a considerable extent on the temperature, ensuring a largely constant biasing pressure at the usual temperature changes in the engine compartment is problematic. The object of the invention is therefore to provide a prestressed power steering system, which provides a largely constant biasing pressure in normal operation, can be filled with little effort and biased and allows an advantageous, automatic venting.

According to the invention, this object is achieved by an electrohydraulic power steering system comprising a hydraulic circuit comprising a pump, a cylinder / piston unit and a hydraulic pressure accumulator, the pump being designed as a reversible pump and optionally a first working chamber or a second working chamber of the cylinders. Piston unit can act on a hydraulic pressure, and wherein at a suction port of the pump is applied via the pressure accumulator adjustable biasing pressure. Due to this preload pressure at the suction connection of the pump cavitation phenomena in the power steering system are largely prevented.

Preferably, each working chamber is associated with a pressure sensor, wherein the smaller of the two sensed pressure values is defined as a biasing pressure. In one embodiment of the power steering system, a biasing pump is provided which is in communication with a reservoir under atmospheric pressure and a pressure chamber of the pressure accumulator, wherein is activated pump at a drop in the biasing pressure below a predetermined value and promotes hydraulic fluid from the reservoir into the pressure chamber. Thus, the biasing pump compensates for pressure fluctuations in the accumulator, which arise for example due to temperature changes, leaks in the hydraulic circuit and / or other changes in volume.

It has been found that for biasing the hydraulic circuit, a vibrating armature pump is particularly suitable and is therefore preferably used as a biasing pump. Since the pressure fluctuations are rather low when using a suitable pressure accumulator, an inexpensive and compact miniature version of the oscillating armature pump with a low delivery volume and a delivery pressure of a few bar is usually sufficient to compensate for these pressure fluctuations.

In a further embodiment, the hydraulic circuit has an electromagnetically actuated directional control valve and an electromagnetically actuated safety valve, wherein the hydraulic system can be filled and / or prestressed in an actuating position of the valves by means of the pump. By using the existing anyway hydraulic pump for this purpose, the filling or bias of the power steering system simplified considerably.

In another variant of the system, the hydraulic circuit has an electronically actuated directional control valve and an electromagnetically actuated safety valve, wherein the hydraulic system can be filled and / or prestressed in an actuating position of the directional control valve and an unconfirmed basic position of the safety valve by means of the pump.

In a particularly preferred system variant, the hydraulic circuit has a high-flow valve, which connects the working chambers of the cylinder / piston unit with each other and with the pressure accumulator in its unactuated basic position, wherein the hydraulic circuit can be filled and / or prestressed in this unactuated basic position of the high-flow valve by means of the pump , and wherein the high-flow valve is preferably pilot-operated. This design of the hydraulic circuit allows an advantageous realization of the filling or preload function and the fail-safe function with only a single high-flow valve. In addition, only two small, standardized low-flow valves are needed for active pilot control of the high-flow valve that results in a compact power steering system that can be produced inexpensively and can be operated particularly energy-saving as a result of the pilot control.

The high-flow valve is preferably a 6/2-way valve or a 5/2-way valve. Preferably, the high-flow valve is precontrolled by two solenoid-operated, designed as 2/2-way valves pilot valves, wherein an unconfirmed basic position of a pilot valve is an open position and an unconfirmed basic position of the other pilot valve is a blocking position. These pilot valves are designed for low hydraulic fluid flow and require only small actuation magnets with low energy consumption for a valve circuit.

Alternatively, the high-flow valve can be controlled directly. Compared with the pilot-operated variant, larger actuation magnets are required in this case, but the number of individual hydraulic components and consequently also the complexity of the overall system are reduced. Another object of the invention is to provide a pressure accumulator for a power steering system which ensures a substantially constant bias of the hydraulic circuit and enables easy filling or preloading of the power steering system.

According to the invention the object is achieved by a hydraulic accumulator of the type mentioned above, wherein in the pressure chamber of the pressure accumulator a collection space is formed for gas bubbles and wherein the piston occupies the first end position from a pre-definable hydraulic pressure in the pressure chamber, in which the pressure chamber via the collection space the preload chamber is in communication. By means of this pressure accumulator construction, an advantageous venting of the hydraulic circuit is possible in a simple manner.

The biasing chamber may be partially filled with hydraulic fluid, which is substantially below atmospheric pressure. Thus, the biasing chamber is used as a compensating volume, which receives as required hydraulic fluid from the pressure chamber or discharges into the pressure chamber to keep the pressure in the hydraulic circuit as constant as possible. In one embodiment of the pressure accumulator, the collection space in the second end position of the piston is separated from the pressure chamber port and the check valve can assume its open position. This ensures that the hydraulic circuit via the pressure chamber connection no gas bubbles from the collection chamber, but only sucks hydraulic fluid from the biasing chamber.

In this embodiment, a collection space bridging element may be provided in the pressure chamber substantially sealingly connecting the biasing chamber and the pressure chamber port in the second end position of the piston when the check valve is open.

Preferably, the biasing chamber is in the first end position of the piston via axial notches in a peripheral wall of the pressure accumulator housing with the

Collection room of the pressure chamber in connection. In the case of overpressure, excess hydraulic fluid can very easily be discharged from the hydraulic circuit into the preload chamber of the pressure accumulator through these notches.

Alternatively or additionally, a piston stop with an axial pin can be provided in the preload chamber, the pin in the first end position of the piston holding the check valve in the open position, so that the preload chamber communicates with the collection space of the pressure chamber. Thus, a release of excess hydraulic fluid from the hydraulic circuit via the check valve is possible, which is forced by the pin in its open position and establishes 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 drawings. In these show:

- Figure 1 shows the schematic circuit diagram of a power steering system according to the invention with a hydraulic circuit according to a first embodiment; - Figure 2 is a longitudinal section through a first embodiment of the pressure accumulator according to the invention; - Figure 3 is a longitudinal section through a second embodiment of the pressure accumulator according to the invention;

FIG. 4 shows a perspective sectional view of an accumulator piston for the pressure accumulator according to the invention, according to a third embodiment; - Figures 5a and 5b each show a detail section through a check valve of the pressure accumulator according to the invention;

FIG. 6 shows a (secondary) hydraulic circuit for filling or pretensioning the power steering system according to the invention;

- Figure 7 shows the schematic circuit diagram of a power steering system according to the invention with a hydraulic circuit according to a second embodiment;

- Figure 8 is a schematic circuit diagram of a power steering system according to the invention with a hydraulic circuit according to a third embodiment;

- Figure 9 shows the schematic circuit diagram of a power steering system according to the invention with a hydraulic circuit according to a fourth embodiment; and FIG. 10 shows a schematic detail section through a high-flow valve of the

Hydraulic circuit according to Figure 9 in an unactuated initial position and an actuating position.

In Figure 1 is a hydraulic circuit diagram for a power steering system 8 of a

Motor vehicle shown schematically. The circuit diagram shows concretely a preloaded hydraulic circuit 10 for an electrohydraulic power steering system 8.

The electro-hydraulic power steering system 8 comprises a steering shaft 11, which is coupled in a known manner via a steering gear 12 with a rack 14, which in turn cooperates with steerable wheels of the motor vehicle (not shown). At the steering gear 12 while a steering torque and / or steering angle sensor 16 is provided, which passes on its sensor data to an electronic control unit 18 of a motor-pump unit 20. The motor-pump unit 20 is the interface between the electrical and the hydraulic part of the electro-hydraulic power steering system 8 and, in addition to the electronic control unit 18, also has a motor 22 and a reversible pump 24. As a reversible pump 24 is in this Hang a pump with two pump ports 25, 26, wherein the pump has two different flow directions for hydraulic fluid and the pump ports 25, 26 uses depending on the conveying direction as Saugan- connection or pressure port. The power steering system 8 further comprises a cylinder / piston unit 27 with two working chambers 28, 30, which are pressurized depending on the conveying direction of the pump 24 and correspondingly cause or support an axial movement of the rack 14 in a first or an opposite second direction , The prevailing in the working chambers 28, 30 pressure is detected by pressure sensors 31 and transmitted to the electronic control unit 18. In the present example, the control unit 18 is additionally connected to a power supply 33 and further data acquisition units 35.

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

The basic structure of an electro-hydraulic vehicle steering system and its mode of operation are already generally known from the prior art, which is why in the following only details specific to the invention are discussed in greater detail.

A special feature of the power steering system 8 is the design of a pressure accumulator 36, which is shown schematically in Figure 1 and shown separately and in detail in Figure 2 according to a first embodiment. The pressure accumulator 36 comprises a pressure accumulator housing 38, in which a pressure chamber 40 and a biasing chamber 42 are formed, and a movable piston 44 which is acted upon in the direction of the pressure chamber 40 and the two chambers 40, 42 separated from each other. In the piston 44 is a check valve 46 integrated, which connects the pressure chamber 40 and the biasing chamber 42 in its open position.

The pressure chamber 40 has a pressure chamber connection 48, via which the pressure accumulator 36 can be connected to the hydraulic circuit 10 of the power steering system 8 (see FIG. 1). In the pressure chamber 40, a collection space 50 is formed for gas bubbles through which the pressure chamber 40 is in the first end position of the piston 44 with the biasing chamber 42 in communication, the piston 44 assumes the first end position from a predetermined hydraulic pressure in the pressure chamber 40. In the first embodiment according to FIG. 2, the collection space 50 is an annular space which is formed around a collection space bridging element 58 and is bounded on the outside by a peripheral wall 52 of the pressure storage housing 38. In the peripheral wall 52 axially aligned notches 54 are provided, which connect the biasing chamber 42 in the first end position of the piston 44 with the collection space 50 of the pressure chamber 40. According to FIG. 2, the first end position is identical to an upper stop position of the piston 44.

In the biasing chamber 42, a compression spring 55 is provided, which acts on the piston 44 in the direction of the pressure chamber 40. Furthermore, the biasing chamber 42 is at least partially filled with hydraulic fluid which is substantially at atmospheric pressure. According to Figure 2, a tank 56 is provided with a vent cover 57 to increase the Vorspannkammervolumens still, the vent cover 57 may be placed directly on the pressure accumulator housing 38 in other embodiments. If the piston 44 is moved counter to the force of the compression spring 55 by a growing pressure in the hydraulic circuit 10 in its first end position, so gas and / or hydraulic fluid from the pressure chamber 40 via the notches 54 in the biasing chamber 42 to flow. Since the collection space 50 first reaches the axial notches 54, initially predominantly gas fractions flow from the pressure chamber 40 into the biasing chamber 42, whereby the hydraulic circuit 10 is vented in the desired manner. The gas bubbles which accumulate in the collection space 50 remain, for example, when filling the power steering system 8 in the hydraulic circuit 10 or cavitation or leakage in the power steering system 8 penetrated. Particularly preferred is a Bottom of the piston 44 is slightly conical (not shown), so that the gas bubbles collect primarily on the peripheral wall 52 of the pressure accumulator housing 38 and discharged via the notches 54 in the biasing chamber 42. In the second end position, ie the lower stop position of the piston 44, the collection space 50 is separated from the pressure chamber connection 48. For this purpose, in the first embodiment of the pressure accumulator 36, the collection space bridging element 58 is provided in the pressure chamber 40 (see Figure 2), which substantially closes the preload chamber 42 and the pressure chamber port 48 in the second end position of the piston 44 (with the check valve 46 open) combines. In the event that the hydraulic circuit 10 sucks in hydraulic fluid in the second end position of the piston 44, it is thus ensured that no air from the collection chamber 50, but only hydraulic fluid from the preload chamber 42 is sucked in via the pressure chamber connection 48.

Since the hydraulic pressure accumulator 36 according to FIG. 2 has no closed gas volume in contrast to hydropneumatic pressure accumulators, the prestressing pressure defined by the pressure accumulator 36 is largely constant. This preload pressure is adjusted significantly by the compression spring 55. 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 venting 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. 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 of the accumulator housing 38, a piston stopper 60 is provided with an axial pin 62 in the biasing chamber 42 according to Figure 3, wherein the pin 62 in the first end position of the piston 44, the check valve 46 is kept open, so that the Biasing chamber 42 is in communication with the pressure chamber 40. In addition, a channel 64 is provided in the piston 44, the biasing chamber 42 with open check valve 46 with the collection space 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 assumes its open position both in the first end position and in the second end position of the piston 44 and allows a fluid flow between the pressure chamber 40 and the biasing chamber 42nd

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 previously described, second embodiment of the pressure accumulator 36 according to FIG. 3. The modified piston 44 has at its lower end face instead of the collection space. Bridging element 58 (see Figures 2 and 3) has a funnel-shaped recess 65, which tapers toward the check valve 46. Any existing gas bubbles collect in this funnel-shaped recess 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 biasing chamber 42. Thus, in this case, a desired automatic venting of the hydraulic circuit 10 is given at overpressure , For piston guide and piston seal in the cylindrical pressure accumulator housing 38, the piston 44 according to FIG. 4 has two separate guide rings 66 and a sealing ring 67. All pressure accumulator designs have in common that the pressure accumulator 36 allows a fluid flow between the pressure chamber 40 and the prestressing chamber 42 in both end positions of the piston 44. The hydraulic circuit 10 can thereby advantageously vacuum or vented hydraulic fluid.

Figures 5a and 5b show a particularly preferred embodiment of the check valve 46, which is integrated into the piston 44 of the accumulator 36, preferably screwed. The check valve 46 has a valve member 68, which is made of a hardened, polished steel ball. In a blocking position of the valve 46 according to FIG. 5 a, the valve member 68 with its spherical surface section seals against a valve seat 70. The valve member 68 is radially almost free of play but guided along a valve axis A movable in a piston bore 72. Recesses 74 in the valve member 68 ensure, in an open position of the check valve 46 (see FIG. 5b), a low-turbulence flow with a low pressure loss. The Asked valve construction is particularly advantageous because there are no valve parts in a flow space 73 downstream of the valve member 68, which could hinder fluid flow. The guide and the exact position of the valve member 68 are achieved by means of a guide sleeve 76, which adjoins the approximately hemispherical portion of the valve member 68. A closing spring 78 of the check valve 46 is arranged in the radial direction between the guide sleeve 76 and a peripheral wall of the piston bore 72 and acts on the valve member 68 in the blocking position according to FIG. 5a. An axial spring abutment serves as a valve sleeve 80, which is screwed into the piston bore 72 in the piston 44 for fastening the check valve 46.

6 shows a secondary hydraulic circuit 82, which ensures a particularly constant prestress in the primary hydraulic circuit 10 (see FIG. The secondary hydraulic circuit 82 has for this purpose a biasing pump 84 with a motor 86, wherein the biasing pump 84 is preferably a miniaturized oscillating armature pump. A suction side of the biasing pump 84 is connected via an unpressurized reservoir to the biasing chamber 42 and a pressure side of the biasing pump 84 to the pressure chamber 40 of the pressure accumulator 36. The non-pressurized reservoir is preferably the biasing chamber 42 or, as shown in Figure 6, the tank 56. Between the biasing pump 84 and the pressure chamber 40, a check valve 88 is provided which prevents hydraulic fluid flow from the pressure chamber 40 to the biasing chamber 42 with unconfirmed biasing pump 84 ,

Like the electric motor 22 of the primary hydraulic circuit 10 (FIG. 1), the motor 86 of the secondary hydraulic circuit 82 is also connected to the power supply 33 and is actuated by the electronic control unit 18 via a switch 90. The electronic control unit 18 checks on the basis of the pressure sensors 31, whether in each of the working chambers 28, 30 at least the predetermined, minimum bias pressure prevails. If this is not the case, the switch 90 is closed, and the biasing pump 84 delivers hydraulic fluid from the biasing chamber 42 into the pressure chamber 40 until at least the minimum bias pressure in each of the working chambers 28, 30 is established. The biasing pump 84 may also deliver up to a maximum biasing pressure at which the piston 44 is in its second end position and beneficial venting of the primary hydraulic circuit 10 occurs. Preferably, activation of the biasing pump 84 to adjust the biasing pressure occurs as part of a startup routine when starting the vehicle. During the driving operation, the switch 90 then remains open and is not activated by the electronic control unit 18. This rare but regular check and readjustment of the biasing pressure by the biasing pump 84 is sufficient to compensate for leakage in the primary hydraulic circuit 10 and to maintain the biasing pressure substantially constant.

In the following, embodiments of the electrohydraulic power steering system 8 will be discussed with reference to FIGS. 7 and 8, which allow a particularly advantageous filling, venting and / or pretensioning of the hydrostatic hydraulic circuit 10. 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. FIG. 7 shows the electrohydraulic power steering system 8 with a

(Primary) hydraulic circuit 10 according to a second embodiment, wherein in comparison to the first embodiment of Figure 1, an additional directional control valve 92, specifically a 4/2-way valve, and a connecting line 94 between the biasing chamber 42 and the hydraulic circuit 10 is provided. Furthermore, at the end stops of a piston 95, the cylinder / piston

Unit 27 Überströmnuten 96 is provided which produce a throttled connection between the working chambers 28, 30 in the stop positions of the piston 95. These overflow grooves 96 provide for a cushioning of the cylinder / piston unit 27, but on the other hand also allow a filling and bias of the primary hydraulic circuit 10 in the manner described below:

The electronic control unit 18 activates the valves 32, 92, so that they assume their Betätigungssteliung according to Figure 7. Thereafter, the pump 24 is operated so that it sucks hydraulic fluid from the preload chamber 42 of the pressure accumulator 36 via the connecting line 94 and delivers it to the first working chamber 28 of the cylinder / piston unit 27. The piston 95 moves due to this pressurization of the first working chamber 28 in its upper stop position (Figure 7), in the through the overflow 96 a Overflow of gas and / or hydraulic fluid from the first working chamber 28 into the second working chamber 30 is possible. From the upper working chamber 30, the gas and / or hydraulic fluid is introduced via the directional control valve 92 into the pressure chamber 40 of the pressure accumulator 36. When the pressure in the pressure chamber 40 is sufficiently high, overpressure relief takes place, in which gas and / or hydraulic fluid flows from the pressure chamber 40 into the prestressing chamber 42. According to FIGS. 2 to 4, the pressure accumulator 36 is designed in such a way that, in the first place, venting, ie an overflow of gas, takes place during the overpressure relief, before an overflow of hydraulic fluid takes place. Thus, the process described above is ideal for

Filling, venting and biasing the primary hydraulic circuit 10. After the hydraulic circuit 10 has reached the desired bias pressure, the safety valve 32 is operated as a conventional "fail-safe" valve and the spring-loaded directional control valve 92 is deactivated so that it is in its normal position (lower valve position in FIG The electro-hydraulic power steering system 8 is thereby converted back into its conventional operating function and can provide a desired power assistance in the following.

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

Section A in FIG. 7 shows an alternative embodiment of the directional control valve 92, in which a leakage connection 99 of the pump 24 is not blocked in the actuation position of the valve 92 but is connected to the pump connection 26.

Preferably, a plurality of components of the power steering system 8 to compact, pre-assembled components, such as the motor-pump unit 20 or a valve block 100 summarized, which is shown in Figure 7 by dashed rectangles. The described process for biasing the hydraulic circuit 10 in this second embodiment may also be in a startup routine at start up be integrated with the vehicle, so that the formation of a secondary hydraulic circuit 82 according to Figure 6 is unnecessary.

FIG. 8 shows the electrohydraulic power steering system 8 with the (primary) hydraulic circuit 10 according to a third embodiment, in which no overflow grooves 96 are provided in the cylinder / piston unit 27 in contrast to the second embodiment according to FIG. 7, so that no overflow of Gas and / or hydraulic fluid from the first working chamber 28 into the second working chamber 30 is possible. Nevertheless, the hydraulic circuit 10 can be vented and biased in an operating position of the directional control valve 92 and an unactuated basic position of the safety valve 32 (see FIG. The biasing of the power steering system 8 is exactly as described for Figure 7, except that the connection between the biasing chamber 42 and the pressure chamber 40 is not made by the overflow grooves 96 of the cylinder / piston unit 27, but by the safety valve 32.

A filling of the electro-hydraulic power steering system 8 is also possible in this third embodiment, but the piston 95 must be moved during the filling process by manually steering from one stop position to the other stop position to the present in the working chambers 28, 30 gas from the cylinder / piston Unit 27 to press.

In one embodiment, the pressure accumulator 36 is designed so that it is not necessary to resort to output signals of the pressure sensors 31 for biasing the (primary) hydraulic circuit 10 since these output signals may have a zero point shift (so-called drift) after a longer service life of the vehicle can. For this reason, the hydraulic circuit 10 is preferably biased, for example, every time the vehicle is started until the piston 44 of the pressure accumulator 36 reaches the axial notches 54, or until the axial pin 62 presses the check valve 46 in the piston 44 of the pressure accumulator 36. In this first end position of the piston 44, the biasing pressure in the hydraulic circuit 10 is clearly determined by the compression spring 55 of the pressure accumulator 36 and can be used to balance the two pressure sensors 31. FIG. 9 shows the electrohydraulic power steering system 8 with the (primary) hydraulic circuit 10 according to a fourth embodiment. This fourth embodiment is very similar to the second and third embodiments of the electrohydraulic power steering system 8, for which reason reference is also made to the above description of FIGS. 7 and 8. The essential difference is that the safety valve 32 and the directional control valve 92 are combined to form a single high-flow valve 102, wherein two low-flow valves in the form of pilot valves 104, 106 are provided for the pilot control of the high-flow valve 102. A valve designed for a large hydraulic fluid flow (eg for moving the rack 14) is referred to as a high-flow valve, whereas a valve designed for a low hydraulic fluid flow (eg for valve precontrol) is referred to as a low-flow valve. Of course, the safety valve 32 and the directional control valve 92 may be formed in FIGS. 7 and 8 as pilot-controlled high-flow valves in order to reduce the size of the control magnets and thus also the energy requirement for active valve control. Likewise, of course, variants are conceivable in which the high-flow valve 102 is directly controlled.

In FIGS. 9 and 10, the high-flow valve 102 is shown as a 6/2-way valve with the valve connections a to f and the pressure connections g and h. Alternatively, the valve connections a and b can also be combined to form a valve connection a ', which in the unactuated basic position is in communication with the valve connections d and e and in the actuation position with the valve connection f. The high flow valve 102 is then a 5/2-way valve. The high-flow valve 102 is acted upon by a spring element 108 in an unconfirmed basic position; according to FIG. 9, ie into the right-hand valve position, in which the working chambers 28, 30 of the cylinder / piston unit 27 are connected to one another and to the pressure chamber 40 of the pressure accumulator 36, the hydraulic circuit 10 in this unactuated basic position of the high-flow valve 102 by means of the reversible pump 24 can be filled and / or prestressed. This design of the hydraulic circuit 10 allows an advantageous realization of the filling or preload function and the fail-safe function with only a single high-flow valve 102. In addition, only two small, standardized low-flow valves, more precisely, an NC pilot valve 104th (normally closed) and a NO pilot valve 106 (normally opened) for the active pilot control of the high-flow valve 102 required. These pilot valves 104, 106 are in the present case electromagnetically actuated 2/2-way valves, as used, for example, in chassis stabilization systems or vehicle brake systems. When the pilot valves 104, 106 are deactivated, the pressure connections g and h of the high-flow valve 102 are each connected to the preload chamber 42 of the pressure accumulator 36, so that the high-flow valve 102 is in its unactuated basic position due to the spring action (FIG. 9). The power steering system 8 according to this fourth embodiment variant is overall very compact, particularly inexpensive to produce and can be operated extremely energy-saving due to the pilot control.

When activated by the respective drive magnets, the pilot valves 104, 106 are switched so that the pressure port h of the high-flow valve 102 remains connected to the biasing chamber 42, but the pressure port g is in communication with the pressure chamber 40. The resulting from the pressure difference between the pressure ports g and h is so great that it overcomes the force of the spring element 108 and the high-flow valve 102 switches to its (left) operating position. In this operating position, the valve ports a and f and c and e are connected to each other while the valve ports b and d are locked. On the direction of rotation and speed of the reversible pump 24 can thus be generated in the operating position, a desired rack power in the power steering system 8.

FIG. 10 shows the high-flow valve 102 according to FIG. 9 with the valve connections a to f and the pressure connections g and h in a schematic detail section. In this case, in the figure 10 above, the unactuated valve base position to see, and below the operating position in which a movable valve body 110 of the high-flow valve 102 relative to the valve base position in the direction of the arrow 112 is moved to the left.

Claims

claims

An electrohydraulic power steering system comprising a hydraulic circuit (10) including a pump (24), a cylinder / piston unit (27) and a hydraulic pressure accumulator (36), the pump (24) being a reversible pump and optionally one 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 pressure accumulator (36) adjustable Preload pressure is applied.

2. Power steering system according to claim 1, characterized in that each

Working chamber (28, 30) is associated with a pressure sensor (31), wherein the smaller of the two sensed pressure values is defined as a biasing pressure.

3. power steering system according to claim 1 or 2, characterized in that a biasing pump (84) is provided, which communicates with an under atmospheric pressure reservoir and a pressure chamber (40) of the pressure accumulator (36) in communication, wherein the biasing pump (84) at a decrease in the biasing pressure below a predetermined value is activated and hydraulic fluid from the reservoir into the pressure chamber (40) promotes.

4. power steering system according to claim 3, characterized in that the biasing pump (84) is an oscillating armature pump.

5. power steering system according to one of the preceding claims, characterized in that the hydraulic circuit (10) comprises an electromagnetically actuated directional control valve (92) and an electromagnetically actuated safety valve (32), wherein the hydraulic circuit (10) in an operating position of the valves (32, 92 ) by means of the pump (24) can be filled and / or prestressed.

6. power steering system according to one of claims 1 to 4, characterized in that the hydraulic circuit (10) comprises an electromagnetically operated directional control valve (92) and an electromagnetically actuated safety valve (32), wherein the hydraulic circuit (10) in an operating position of the directional control valve (92) and an unactuated basic position of the safety valve (32) by means of the pump (24) can be filled and / or prestressed.

7. power steering system according to one of claims 1 to 4, characterized in that the hydraulic circuit (10) has a high-flow valve (102), which connects the working chambers (28, 30) with each other and with the pressure accumulator (36) in its unactuated basic position the hydraulic circuit (10) in this unactuated basic position of the high-flow valve (102) by means of the pump (24) can be filled and / or prestressed.

8. power steering system according to claim 7, characterized in that the high-flow valve (102) is a 6/2-way valve or a 5/2-way valve.

9. power steering system according to claim 7 or 8, characterized in that the high-flow valve (102) by two electromagnetically actuated, as 2/2

Directional valves trained pilot valves (104, 106) is piloted, with an unconfirmed basic position of a pilot valve (106) is an open position and an unconfirmed basic position of the other pilot valve (104) is a blocking position.

10. power steering system according to claim 7 or 8, characterized in that the high-flow valve (102) is directly controlled.

11. Hydraulic pressure accumulator for a power steering system (8), 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

Preload chamber (42) are formed, and 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) separated from each other, wherein in the piston (44) a check valve (46) is integrated, which connects the pressure chamber (40) and the biasing chamber (42) in an open position, and wherein the pressure chamber (40) has a pressure chamber connection (48) via which the pressure accumulator (36) a hydraulic circuit (10) of the power steering system (8) is connectable, characterized in that in the pressure chamber (40) is formed a collection chamber (50) for gas bubbles, wherein the piston (44) from a predeterminable hydraulic pressure in the pressure chamber (40) assumes the first end position in which the pressure chamber (40) via the collection space (50) with the biasing chamber (42) is in communication.

12. Pressure accumulator according to claim 11, characterized in that the preload chamber (42) is at least partially filled with hydraulic fluid, which is substantially at atmospheric pressure.

13. Pressure accumulator according to claim 11 or 12, characterized in that the collection space (50) in the second end position of the piston (44) from the pressure chamber port (48) is separated and the check valve (46) can assume its open position.

14, pressure accumulator according to claim 13, 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 port (48) in the second end position of the piston (44) when open Check valve (46) connects substantially tight.

15. Pressure accumulator according to one of claims 11 to 14, characterized in that the preload chamber (42) in the first end position of the piston (44) via axial notches (54) in a peripheral wall (52) of the pressure accumulator housing (38) with the collection space ( 50) of the pressure chamber (40) is in communication.

16. Accumulator according to one of claims 11 to 14, characterized in that in the preload 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 ) holds the check valve (46) in the open position so that the biasing chamber (42) communicates with the collection space (50) of the pressure chamber (40).