Steering system and method for operating a steering system

Abstract

A steering system for a trailing axle of a motor vehicle includes an electric motor, a hydraulic pump, a first valve, and a working cylinder. The electric motor is configured to drive the hydraulic pump. The working cylinder has a central bore and a piston positioned in the working cylinder. The first valve is fluidically connected to the pump such that the first valve is configured to block flow between the central bore and the pump in response to pressure generated by the pump, and to release the flow in response to a non-pressurized state of the pump in order to divert hydraulic fluid from the working cylinder through the central bore via an adhesion-driven movement of the piston into a central position in the central bore. A method includes operating a steering system for a trailing axle of a motor vehicle of this type.

Claims

1. A steering system for a trailing axle of a motor vehicle, comprising: a working cylinder that is configured to pivot road wheels of a trailing axle of a motor vehicle, and that includes: a central bore; and a piston positioned in the working cylinder; a hydraulic pump connected to the working cylinder; and a first valve fluidically connected between the central bore and the hydraulic pump such that the first valve is configured to (i) block a flow of fluid between the central bore and the hydraulic pump in response to a pressure generated by the hydraulic pump, and (ii) release the flow of fluid in response to a pressureless state of the hydraulic pump in order to discharge hydraulic fluid from the working cylinder through the central bore via an adhesion-driven movement of the piston into a central position; an electric motor configured to drive the hydraulic pump; and a fluid tank fluidly connected between the first valve and the hydraulic pump so as to receive the hydraulic fluid discharged from the working cylinder through the first valve.

2. The steering system as claimed in claim 1, further comprising: a first hydraulic line connecting the hydraulic pump and a first connection of the working cylinder; a second hydraulic line connecting the hydraulic pump and a second connection of the working cylinder; and a second valve embodied as a shuttle valve and connected to the first hydraulic line and the second hydraulic line, the shuttle valve connected to the first valve to supply fluid pressure from the first hydraulic line or the second hydraulic line to the first valve so as to actuate the first valve with the fluid pressure.

3. A steering system for a trailing axle of a motor vehicle, comprising: a working cylinder that is configured to pivot road wheels of a trailing axle of a motor vehicle, and that includes: a central bore; and a piston positioned in the working cylinder; a hydraulic pump connected to the working cylinder; and a first valve fluidically connected between the central bore and the hydraulic pump such that the first valve is configured to (i) block a flow of fluid between the central bore and the hydraulic pump in response to a pressure generated by the hydraulic pump, and (ii) release the flow of fluid in response to a pressureless state of the hydraulic pump in order to discharge hydraulic fluid from the working cylinder through the central bore via an adhesion-driven movement of the piston into a central position; an electric motor configured to drive the hydraulic pump; a first hydraulic line positioned between the hydraulic pump and a first connection of the working cylinder; a second hydraulic line positioned between the hydraulic pump and a second connection of the working cylinder; and a second valve configured to pressurize the first valve with fluid pressure, the second valve embodied as a shuttle valve and connected to the first hydraulic line and the second hydraulic line, the second valve comprising: a first sealing seat; a second sealing seat; and an element that is axially movable between the first sealing seat and the second sealing seat, and that is configured to (i) selectively close the first sealing seat and the second sealing seat, depending on a direction of fluid flow, and (ii) pressurize the first valve with the fluid pressure.

4. The steering system as claimed in claim 1, further comprising: a first hydraulic line positioned between the hydraulic pump and a first connection of the working cylinder; a second hydraulic line positioned between the hydraulic pump and a second connection of the working cylinder; and a third valve and a fourth valve that are configured to pressurize the first valve with fluid pressure, the third and fourth valves embodied as check valves, the third valve connected to the first hydraulic line, and the fourth valve connected to the second hydraulic line.

5. The steering system as claimed in claim 1, further comprising: a first hydraulic line positioned between the hydraulic pump and a first connection of the working cylinder; a second hydraulic line positioned between the hydraulic pump and a second connection of the working cylinder; and a fifth valve positioned in series with the first valve between the central bore and the hydraulic pump, wherein the first valve is connected to the first hydraulic line and is actuated by fluid pressure from the first hydraulic line, and the fifth valve is connected to the second hydraulic line and is actuated by fluid pressure from the second hydraulic line.

6. The steering system as claimed in claim 5, wherein: the first valve and the fifth valve are each embodied as seat valves; and each of the first valve and the fifth valve includes: a respective compression spring; and a respective valve piston that is transferable into a basic position by the respective compression spring.

7. The steering system as claimed in claim 1, further comprising: a first hydraulic line positioned between the hydraulic pump and a first connection of the working cylinder; and a second hydraulic line positioned between the hydraulic pump and a second connection of the working cylinder; wherein: the first valve is embodied as a 2-port/3-position valve, and the first valve has: a first valve connection connected to the first hydraulic line such that fluid pressure in the first hydraulic line actuates the first valve in a first direction; and a second valve connection connected to the second hydraulic line such that fluid pressure in the second hydraulic line actuates the first valve in a second opposite direction.

8. The steering system as claimed in claim 2, wherein: the fluid tank is connected to the first hydraulic line and the second hydraulic line, and the steering system further comprises: at least one first filter positioned between the fluid tank and the first hydraulic line; a first feeder valve positioned between the fluid tank and the first hydraulic line; at least one second filter positioned between the fluid tank and the second hydraulic line; and a second feeder valve positioned between the fluid tank and the second hydraulic line.

9. The steering system as claimed in claim 8, further comprising: an orifice plate positioned upstream of the first valve between the central bore and the first valve and configured, in an open state of the first valve, to vary a volumetric flow of fluid from the central bore to the fluid tank.

10. The steering system as claimed in claim 1, wherein the hydraulic pump is embodied as a reversible pump.

11. A method for operating a steering system for a trailing axle of a motor vehicle, comprising: blocking, via a first valve, a flow of fluid between a central bore of a working cylinder and a hydraulic pump in response to a pressure generated by the hydraulic pump, wherein the central bore of working cylinder is fluidically connected to the hydraulic pump, wherein the hydraulic pump is driven by an electric motor, and wherein the working cylinder is configured to pivot road wheels of a trailing axle; and releasing the flow via the first valve in response to a pressureless state of the hydraulic pump, such that a piston positioned in the working cylinder undergoes an adhesion-driven return movement to a central position and causes hydraulic fluid to discharge from the working cylinder, wherein the flow is released via the first valve to a fluid tank that is fluidly connected between the first valve and the hydraulic pump.

12. The method as claimed in claim 11, wherein, when the piston is in the central position, the piston closes the central bore, and the road wheels of the trailing axle are fixed in a straight-ahead position.

13. The steering system as claimed in claim 2, wherein the second valve includes: a first sealing seat; a second sealing seat; and an element that is axially movable between the first sealing seat and the second sealing seat, and that is configured to (i) selectively close the first sealing seat and the second sealing seat, depending on a direction of fluid flow, and (ii) supply the first valve with the fluid pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings are intended to convey further understanding of the embodiments of the disclosure. They illustrate embodiments and serve in conjunction with the description for explaining principles and concepts of the disclosure.

(2) Other embodiments and many of the advantages mentioned are evident from the drawings. The elements of the drawings that are shown are not necessarily to scale in relation to one another.

(3) In the drawings:

(4) FIG. 1 shows a schematic representation of a motor vehicle with a steering system for a trailing axle according to a first embodiment of the disclosure;

(5) FIG. 2 shows a hydraulic diagram of the steering system for the trailing axle of the motor vehicle according to the first embodiment of the disclosure;

(6) FIG. 3 shows a hydraulic diagram of the steering system for the trailing axle of the motor vehicle according to a second embodiment of the disclosure;

(7) FIG. 4 shows a hydraulic diagram of the steering system for the trailing axle of the motor vehicle according to a third embodiment of the disclosure;

(8) FIG. 5 shows a hydraulic diagram of the steering system for the trailing axle of the motor vehicle according to a fourth embodiment of the disclosure; and

(9) FIG. 6 shows a flow diagram of a method for operating the steering system for the trailing axle of the motor vehicle according to the first to fourth embodiments of the disclosure.

DETAILED DESCRIPTION

(10) Unless anything to the contrary is stated, in the figures of the drawings the same designations denote elements, components or items that are the same or functionally the same.

(11) FIG. 1 shows a schematic representation of a motor vehicle F with a steering system 1 for a trailing axle A.sub.n according to a first embodiment of the disclosure.

(12) The motor vehicle F has in an advantageous way the steering system 1 for the trailing axle A.sub.n of the motor vehicle F. For detecting a steering angle α of road wheels R.sub.n on a front axle A.sub.v, a steering angle sensor F.sub.α is provided. For detecting a driving speed v of the motor vehicle F, a driving speed sensor S.sub.v is provided. The signals thereof are transmitted via a signal line (not shown in FIG. 1) to the steering system 1 of the trailing axle A.sub.n of the motor vehicle F. In particular, the signals are transmitted to a control unit (not shown in FIG. 1), which is designed for activating an electric motor (not shown in FIG. 1) and a reversible hydraulic pump that is operated therewith. The control unit is furthermore connected to a linear sensor (not shown in FIG. 1) of a working cylinder, which can move the road wheels R.sub.n of the trailing axle A.sub.n of the motor vehicle F in a defined caster angle.

(13) FIG. 2 shows a hydraulic diagram of the steering system 1 for the trailing axle A.sub.n of the motor vehicle F according to the first embodiment of the disclosure.

(14) The data determined by the steering angle sensor and driving speed sensor (not shown in FIG. 2) are transmitted to the control unit E. The control unit E calculates from the data a caster angle of the road wheels on the trailing axle of the motor vehicle and activates an electric motor M correspondingly.

(15) The electric motor M serves for driving a hydraulic pump P, which in turn is connected to the working cylinder 10 for pivoting the road wheels of the trailing axle. The working cylinder 10 has a central bore 11, via which hydraulic fluid can be removed from the working cylinder 10, so that a piston arranged in the working cylinder can be moved in an adhesion-driven manner into a central position, in which it closes the central bore, and the road wheels of the trailing axle are fixed in a straight-ahead position.

(16) In the case of this electrohydraulic steering system, the pivoting of the trailing axle takes place independently of the front axle, since a steering wheel of the motor vehicle is not mechanically connected to the trailing axle to be steered. Moreover, this system is decoupled from the internal combustion engine, so that on the one hand closed-loop control appropriate for requirements and on the other hand great spatial flexibility when fitting the system—as a result of the small number of components, which also can be placed freely—are ensured. At low speeds and at a standstill, active steering dependent on the steering angle of the front axle and on the driving speed is possible with this system. In particular, the road wheels of the trailing axle can also be brought out of any deflection angle into their straight-ahead position, and can be reliably fixed there, automatically, i.e. in an adhesion-driven manner, at any time even if there is a failure of the control unit E, the electric motor M and/or the hydraulic pump P.

(17) During the adhesion-driven movement of the piston of the working cylinder in the direction of the central bore, hydraulic fluid is discharged for example from a first cylinder space 10c in the direction of a fluid tank, while in a second cylinder space 10d without the use of a pump—replenishing fluid is fed in. As soon as the central bore is closed by the piston, further movement of it is blocked by the hydraulic fluid confined as it were on both sides, so that the wheels of the trailing axle are dependably fixed in their straight-ahead position.

(18) A trailing axle is intended to be understood here as meaning any axle which follows a deflection of a steered axle and which can be arranged to trail or lead a rigid rear or front axle, that is to say also as a leading axle. The steering system according to the disclosure can therefore also be used in the case of trailers, semitrailers or a second steered front axle.

(19) During normal operation of the steering system 1, the pump P draws via a first feeder valve 26, which is provided between a fluid tank 24 and a first hydraulic line 16, arranged between the pump P and a first connection 10a of the working cylinder 10. Preferably arranged between the fluid tank 24 and the first feeder valve 26 is a first filter 25, in order to filter contaminants that are possibly present in the fluid tank 24.

(20) During normal operation of the steering system 1, the pump P draws via a second feeder valve 28, which is provided between the fluid tank 24 and a second hydraulic line 18, arranged between the pump P and a second connection 10b of the working cylinder 10. Preferably arranged between the fluid tank 24 and the second feeder valve 28 is a second filter 27, in order to filter contaminants that are possibly present in the fluid tank 24.

(21) During a steering operation, the pump P delivers hydraulic fluid into the first cylinder space 10c, for example via the first hydraulic line 16 through a filter 30 and a check valve 31. Alternatively, the pump P may for example deliver hydraulic fluid into a second cylinder space 10d, via the second hydraulic line 18 through a filter 32 and a check valve 33.

(22) Assigned to the first cylinder space 10c is a return flow valve 34, which is closed during filling of the first cylinder space 10c. As a result of a pressure buildup in the first cylinder space 10c, a return flow valve 35 of the second cylinder space 10d is opened and, as a result, a return flow from the second cylinder space 10d into the fluid tank 24 is made possible.

(23) The second cylinder space 10d is assigned a return flow valve 35, which is closed during filling of the second cylinder space 10d. As a result of a pressure buildup in the second cylinder space 10d, the return flow valve 34 of the first cylinder space 10c is opened and, as a result, a return flow from the first cylinder space 10c into the fluid tank 24 is made possible.

(24) A first valve 12 is preferably provided in a fluid connection between the central bore 11 of the working cylinder 10 and the pump P. The first valve 12 is preferably fluidically connected to the pump P in such a way that it blocks a flow of fluid between the central bore 11 of the working cylinder 10 and the pump P when pressure is generated by the pump P. In a pressureless state of the pump P, the first valve 12 preferably releases a flow of fluid between the central bore 11 of the working cylinder 10 and the pump P. As a result, in an advantageous way hydraulic fluid from the working cylinder 10 via the central bore 11 of the working cylinder 10 can take place by an adhesion-driven return movement of the piston 14 arranged in the working cylinder 10 into a central position G.

(25) The various operating states are described below.

(26) Driving Straight Ahead, Higher Driving Speed

(27) When driving straight ahead at a higher driving speed, the trailing axle is not pivoted, but must be kept in the straight-ahead position. Since, when driving straight ahead, the pump P does not generate any pressure, the first valve 12 is not pressurized with a fluid pressure and is consequently open. Therefore, in the event of an angled position of the road wheels, hydraulic fluid can escape from the working cylinder 10 into the fluid tank 24 via the central bore 11. When the piston 14 reaches the central position G in the working cylinder 10, it is advantageously fixed and the road wheels are kept in the straight-ahead position.

(28) Active Steering, Low Driving Speed

(29) In the case of active steering, a steering angle of the front axle is detected by measuring instruments and transmitted to the control unit E of the steering system 1. With these and further parameters, such as for example the driving speed, a desired value of an angular position of the road wheels of the trailing axle is calculated and the electric motor M is activated by the control unit E. A.sub.n actual value is detected by way of a position sensor 36, arranged in the working cylinder 10, and is used for the control.

(30) In the present embodiment, the first valve 12 can be pressurized with fluid pressure via a second valve 15, which is designed as a shuttle valve. The second valve 15 is connected to a first hydraulic line 16, arranged between the pump P and the first connection 10a of the working cylinder 10. Furthermore, the second valve 15 is connected to the second hydraulic line 18, arranged between the pump P and the second connection 10b of the working cylinder 10.

(31) The second valve 15 preferably has an element 15a, which is axially movable between a first sealing seat 15b and a second sealing seat 15c. The element 15a preferably closes the first sealing seat 15b or the second sealing seat 15c, depending on the direction of fluid flow, and pressurizes the first valve 12 with a fluid pressure. The element 15a is preferably designed as a sphere. Alternatively, the element 15a may be of some other suitable form.

(32) If, for example, the pump P delivers a volumetric flow of the hydraulic fluid via the first hydraulic line 16, the second valve 15 is actuated and passes on a pump pressure to the first valve 12. At the same time, the second valve 15 prevents a volumetric flow of the hydraulic fluid to the second hydraulic line 18. The pump pressure that is present has the effect of actuating the first valve 12 and closing a connecting line between the central bore 11 and the fluid tank 24. Consequently, a loss of volumetric flow via the central bore 11 is prevented. As soon as the road wheels of the trailing axle have reached a predetermined desired position, the pump P is no longer driven and a fluid pressure in the first hydraulic line 16 drops, which has the consequence that the first valve 12 opens again. If there are deviations between desired and actual values, the steering system 1 makes adjustments, whereby the pump P builds up the required pressure, whereby the first valve 12 is closed again and a steering correction is performed.

(33) Failure of the Steering System

(34) If there is a failure of the steering system, a pump pressure previously built up during active steering drops. As a result, the first valve 12 assumes its basic position and connects the central bore 11 of the working cylinder 10 to the fluid tank 24, whereby the return of the axle is made possible.

(35) The first valve 12 is preferably additionally preceded by an orifice plate 29, by which a volumetric flow of the fluid from the central bore 11 to the fluid tank 24 is variable in the open state of the first valve 12. Consequently, a through-flow cross section can be varied, whereby a return speed of the trailing axle can be set. In addition, the pump P is preferably designed as a reversible pump.

(36) FIG. 3 shows a hydraulic diagram of the steering system 1 for the trailing axle A.sub.n of the motor vehicle F according to a second embodiment of the disclosure.

(37) The first valve 12 is preferably able to be pressurized with fluid pressure via a third valve 19 and a fourth valve 20, which are designed as check valves. The third valve 19 is preferably connected to the first hydraulic line 16, arranged between the pump P and the first connection 10a of the working cylinder 10. The fourth valve 20 is preferably connected to the second hydraulic line 18, arranged between the pump P and the second connection 10b of the working cylinder 10.

(38) FIG. 4 shows a hydraulic diagram of the steering system 1 for the trailing axle A.sub.n of the motor vehicle F according to a third embodiment of the disclosure.

(39) Preferably arranged between the central bore 11 of the working cylinder 10 and the pump P is a fifth valve 22, which is provided in series with the first valve 12. The first valve 12 is preferably connected to the first hydraulic line 16, arranged between the pump P and the first connection 10a of the working cylinder 10. The fifth valve 22 is preferably connected to the second hydraulic line 18, arranged between the pump P and the second connection 10b of the working cylinder 10.

(40) The first valve 12 and the fifth valve 22 are preferably designed as seat valves. A valve piston 12a of the first valve 12 and a valve piston 22a of the fifth valve are preferably transferable into a basic position by a respective compression spring 12b, 22b.

(41) FIG. 5 shows a hydraulic diagram of the steering system 1 for the trailing axle A.sub.n of the motor vehicle F according to a fourth embodiment of the disclosure.

(42) The first valve 12 is preferably designed as a 2 port/3-position valve. The 2 port/3-position valve is preferably connected at a first connection 12c to the first hydraulic line 16, arranged between the pump P and the first connection 10a of the working cylinder 10. Furthermore, the 2 port/3-position valve is connected at a second connection 12d to the second hydraulic line 18, arranged between the pump P and the second connection 10b of the working cylinder 10.

(43) FIG. 6 shows a flow diagram of a method for operating the steering system for the trailing axle of the motor vehicle according to the first to fourth embodiments of the disclosure.

(44) The method for operating the steering system for the trailing axle of the motor vehicle comprises providing S1 an electric motor for driving a hydraulic pump. Furthermore, the method comprises providing S2 a working cylinder connected to the pump for pivoting road wheels of the trailing axle. Moreover, the method comprises providing S3 a central bore of the working cylinder, which is fluidically connected to the pump. In addition, the method comprises providing S4 a first valve, which is provided in a fluid connection between the central bore and the pump, the first valve being fluidically connected to the pump in such a way that it blocks a flow of fluid between the central bore of the working cylinder and the pump when pressure is generated by the pump, and releases a flow of fluid between the central bore of the working cylinder and the pump in a pressureless state of the pump for discharging hydraulic fluid from the working cylinder via the central bore of the working cylinder by an adhesion-driven return movement of a piston arranged in the working cylinder into a central position.

(45) The method comprises moreover closing S5 the central bore by the piston in the central position in the working cylinder. The method comprises furthermore fixing S6 the road wheels of the trailing axle in a straight-ahead position.

(46) Although the present disclosure has been described above on the basis of preferred exemplary embodiments, it is not restricted to them but can be modified in various ways. In particular, the disclosure can be changed or modified in a variety of ways without departing from the essence of the disclosure.

(47) For example, valves of a different type that perform the same function may be used.

LIST OF DESIGNATIONS

(48) 1 Steering system 10 Working cylinder 10a First connection 10b Second connection 10c First cylinder space 10d Second cylinder space 11 Central bore 12 Valve 12a, 22a Valve piston 12b, 22b Compression spring 12c First connection 12d Second connection 14 Piston 14 Second valve 15a Element 15b First sealing seat 15c Second sealing seat 16 First hydraulic line 18 Second hydraulic line 19 Third valve 20 Fourth valve 22 Fifth valve 24 Fluid tank 25 First filter 26 First feeder valve 27 Second filter 28 Second feeder valve 29 Orifice plate 30, 32 Filter 31, 33 Check valve 34 First return valve 35 Second return valve 36 Position sensor A.sub.n Trailing axle E Control unit F Motor vehicle F.sub.α Steering angle G Central position M Electric motor P Pump Rn Road wheels of the trailing axle S.sub.v Vehicle speed V Vehicle speed α Steering angle