STEERING SYSTEM FOR AN AGRICULTURAL VEHICLE

Description

[0001] The present invention relates to a steering system for an agricultural vehicle according to the preamble of claim 1.

[0002] EP 0 974 508 B1 discloses a steering system for an agricultural vehicle of the type mentioned above. The vehicle has at least one primary-steered axle and at least one further axle, each of which is assigned at least one steering cylinder for changing a wheel steering angle. The double-acting steering cylinder for actuating the wheels is assigned to the wheel pairs arranged on the axles, wherein the steering cylinders are actuated by a fluid supplied by a valve unit, which is fed to the valve unit by a hydraulic pump. The valve unit consists of an electro-hydraulic steering control and switching unit and a hydraulic steering control and switching unit, which has a hydraulic device, a so-called orbitrol, connected to a steering wheel of the vehicle. Each of the steering control and switching units has switching valves.The switching valves of the respective switching units are controlled individually to enable different steering types when controlling the axles. One steering type provides for the control of only the primary controlled axle, another steering type provides for all-wheel steering. With all-wheel steering, a steering cylinder is each controlled by the first and second steering control units and the associated switching units. By controlling the switching valves of the switching units, a steering mode can be set in which the fluid flows directly back to the hydraulic device, bypassing the steering cylinders, and the displaced fluid flow has no influence on the steering angle of the wheels. This generates a steering wheel counter-torque on a steering wheel operatively connected to the hydraulic device, which corresponds to that when the steering cylinders are controlled by actuating the hydraulic device.The steering wheel, connected to an electric rotary encoder, serves as a steering angle setpoint transmitter, with the vehicle driver experiencing the same steering feel. In this steering mode, the steering wheel can continue to be turned even when a double-acting piston located in the steering cylinder has reached its maximum position, which corresponds to a steering stop. The driver therefore receives no feedback from the steering system informing them that the maximum steering stop has been reached. Furthermore, in this steering mode, the driver does not receive any force feedback from the steering in the form they would expect from a hydraulic steering system using the hydraulic device or the orbitrol.

[0003] Furthermore, US 2018 / 0297632 A1 discloses, with respect to the independent claim, a steering system for an agricultural vehicle, comprising: at least one primary-controlled axle and at least one further axle, each of which is assigned at least one steering cylinder for changing a wheel steering angle of the agricultural vehicle, a valve unit connected to a supply unit for pressurizing the steering cylinders, wherein the valve unit comprises a first steering control and switching unit with a proportional unit and a locking unit and a second steering control and switching unit with a hydraulic device and at least one switching valve, at least one control device which is configured to control the first steering control and switching unit, wherein the steering system is operable in a first steering mode and at least one further steering mode, wherein the at least one control device is configured toin the first steering mode, to sensorily detect and evaluate a steering movement of a steering wheel connected to the second steering control and switching unit in order to control the first steering control and switching unit based on the evaluation, wherein the at least one control device is configured to control the switching valve in the first steering mode by means of a singular, in particular electrical, signal in order to adjust at least one actuation behavior of the steering wheel.

[0004] Based on the above-mentioned prior art, the object of the invention is to further develop a steering system for an agricultural vehicle of the above-mentioned type, which is characterized by an adjustable actuation behavior of a steering wheel.

[0005] From a device-technical perspective, this problem is solved by the features of the independent claim. The dependent claims that follow thereafter describe advantageous developments of the invention.

[0006] According to the invention, a steering system for an agricultural vehicle is proposed, comprising: at least one primary-controlled axle and at least one further axle, each of which is assigned at least one steering cylinder for changing a wheel steering angle of the agricultural vehicle, a valve unit connected to a supply unit for pressurizing the steering cylinders, wherein the valve unit comprises a first steering control and switching unit with a proportional unit and a locking unit and a second steering control and switching unit with a hydraulic device and at least three switching valves, at least one control device which is designed to control the first steering control and switching unit, wherein the steering system is operable in a first steering mode and at least one further steering mode, wherein the at least one control device is configured to sensor-detect and evaluate a steering movement of a steering wheel connected to the second steering control and switching unit in the first steering mode in order to control the first steering control and switching unit based on the evaluation.

[0007] According to the invention, the at least one control device is configured to simultaneously control the at least three switching valves in the first steering mode by means of a singular, in particular electrical, signal in order to adjust at least one actuation behavior of the steering wheel. The at least one control device generates a singular electrical signal, which is used to simultaneously control the at least three switching valves.

[0008] The invention is based on the idea that in the first steering mode, hereinafter also referred to as the fully electronic steering mode, in which the steering movement of the steering wheel connected to the second steering control and switching unit is detected by sensors and transmitted to the first steering control and switching unit as a steering angle target signal, the driver of the vehicle is easily given the option of adjusting at least one actuation behavior of the steering wheel. The term actuation behavior of the steering wheel describes the behavior resulting from actuating the steering wheel to adjust a steering angle, which is characterized by the presence or absence of haptic perception by the driver. In the first steering mode, outputs of the hydraulic device or of the orbitrol are connected to one another, so that when the steering wheel is turned, the orbitrol conveys its volume flow directly towards the tank.

[0009] The agricultural vehicle is, in particular, a tractor. In particular, the tractor has two steerable axles.

[0010] The at least one control device can be designed with a memory unit in which characteristic curves for controlling the first steering control and switching unit are stored, and with a computing unit for processing the characteristic curves.

[0011] According to the invention, the second steering control and switching unit comprises an electrically controlled actuating valve which, when actuated by the singular electrical signal of the at least one control device, provides a switching pressure that simultaneously actuates the three switching valves. When appropriately actuated, the actuating valve provides a switching pressure that simultaneously changes the switching position of the three switching valves.

[0012] Applying the switching pressure of the actuating valve to the three switching valves interrupts the hydraulic connection between the second steering control and switching unit and the steering cylinders. This action activates the fully electronic steering mode, whereby the steering wheel can be moved or rotated beyond the end stop of the steering cylinders as one of the actuating behaviors.

[0013] Preferably, two of the switching valves of the second steering control and switching unit can be designed as normally open valves (NO valves), and one of the switching valves can be designed as a normally closed valve (NC valve). The two switching valves designed as NO valves are in the open switching position when no switching pressure is applied. The switching valve designed as an NC valve is in the closed switching position when no switching pressure is applied.

[0014] The switching valve, designed as an NC valve, can be arranged between hydraulic lines that connect the hydraulic system to the steering cylinders via the respective switching valve, designed as an NO valve. By switching the switching valve, designed as an NC valve, the outputs of the orbitrol can be connected to each other, i.e., short-circuited.

[0015] When the actuating valve is electrically controlled by the at least one control device, the two switching valves designed as NO valves are moved to their closed switching position by the application of control pressure, while the switching valve designed as an NC valve is moved to its open switching position. When the two NO valves are in the closed switching position, the hydraulic connection between the hydraulic device or the Orbitrol and the steering cylinders is interrupted. When the NC valve is in the open switching position, it establishes a hydraulic connection between the Orbitrol outputs, so that the Orbitrol directs its volume flow back towards the tank when the steering wheel is turned. As a result, in fully electronic steering mode, the steering wheel is actuated in a manner that allows the driver to turn the steering wheel endlessly.

[0016] According to a preferred development, an electrically controlled throttle valve can be connected upstream of the switching valve designed as an NC valve. By controlling the electrically controlled throttle valve, further actuation behaviors of the steering wheel can be adjusted. The control takes place depending on the actuation of the steering wheel in order to adjust the steering angle.

[0017] In particular, the throttle valve can be configured to throttle the volume flow between the hydraulic device and a tank proportional to the current supply down to a minimum flow rate. The electrically controlled throttle valve serves to allow the volume flow diverted from the Orbitrol directly into the tank in the first steering mode or fully electronic steering mode to pass through unthrottled in the de-energized state and to throttle it down to a minimum flow rate with increasing current supply. The throttle valve is preferably energized depending on the steering angle resulting from the steering movement of the steering wheel. By throttling the short-circuited volume flow, an increase in force on the steering wheel that the driver can feel can be achieved, which represents a further actuation behavior of the steering wheel.The minimum flow rate is set to a value that ensures that the driver can turn the steering wheel even at maximum current to ensure the steerability of the vehicle even if the throttle valve fails.

[0018] Alternatively or additionally, the throttle valve can be designed to throttle the volume flow between the hydraulic device and a tank in proportion to the current supply until the flow is interrupted. The only throttle valve can be designed to throttle both as the current supply increases down to a minimum flow rate and proportional to the current supply until the flow is interrupted. The characteristic of the control of the throttle valve, i.e. that the only throttle valve throttles until the minimum flow rate is reached or until the flow is interrupted, can be specified by the at least one control device. When the end stop of the hydraulic cylinder(s) is reached, the current supply to the throttle valve is at its maximum, resulting in an interruption of the flow between the Orbitrol and the tank.By blocking the connection between the two outputs of the Orbitrol, the steering wheel can no longer be turned. This allows the steering wheel to be simulated as a further actuation behavior, preventing further rotation.

[0019] For operational safety reasons, it may be advisable to provide a pressure switch that switches depending on the pressure applied by operating the steering wheel to cancel the flow interruption. This prevents the steering wheel from becoming inoperable if the throttle valve fails at the moment the flow is interrupted. In such a situation, the pressure switch can open to cancel the fully electronic steering mode. To do this, the pressure switch can be triggered to reestablish the connection between the Orbitrol outputs and the NO valves of the second steering control and switching unit.

[0020] In particular, the at least one control device can be configured to adjust the current supply to the electrically controlled throttle valve depending on the set steering angle. Thus, as the steering angle increases and the hydraulic cylinder(s) approach the end stop, the throttling of the volume flow can increase until the set minimum flow rate is reached or the flow is interrupted.

[0021] According to a preferred development, an operating element having a self-holding function for controlling the actuating valve can be assigned to the at least one control device. Thus, electrical control of the actuating valve can be achieved, for example, by electrical self-holding after actuation of an operating element designed as a button or switch. By actuating the operating element, the driver can switch to fully electronic steering mode. The electrical self-holding can be canceled by actuating the operating element again or by actuating another element, for example a road drive switch or by selecting another steering program, so that the steering system switches to or reverts to another steering mode, in particular the orbital roll steering mode.

[0022] An actuation behavior of the steering wheel can be a continued actuation of the steering wheel regardless of reaching an end stop of the steering cylinder of the primary controlled axle.

[0023] An operating behavior of the steering wheel can be the induction of increasing resistance when operating the steering wheel.

[0024] An actuation behavior of the steering wheel can simulate reaching an end stop of the steering cylinder.

[0025] In particular, the actuating valve and / or the throttle valve can be arranged in a common housing of the first steering control and switching unit and / or the second steering control and switching unit or in a separate housing. Arranging the actuating valve and / or the throttle valve in a separate housing has the advantage that the components can be retrofitted or optionally installed in vehicles that use the fully electronic steering mode.

[0026] According to one embodiment, exactly one control device can be provided for controlling the first steering control and switching unit, which, in addition to the proportional unit and the locking unit, controls at least the actuating valve. Furthermore, the exactly one control device can be configured to also control the second steering control and switching unit.

[0027] Alternatively, two control devices can be provided, one of which serves to control the first steering control and switching unit, and the other of which serves to control the second steering control and switching unit. It can further be provided that one of the two control devices is configured to control at least the actuating valve of the second steering control and switching unit.

[0028] The present invention is explained in more detail below with reference to embodiments shown in the drawings.

[0029] They show: Fig. 1 a block diagram of a steering system; Fig. 2 a block diagram of the steering system according to Fig. 1 according to a further embodiment; and Fig. 3 a block diagram of the steering system according to Fig. 1 according to a third embodiment.

[0030] An agricultural vehicle, in particular a tractor, has at least two axles, a front axle and a rear axle, with wheels arranged thereon. A steering system 1 is provided for steering the vehicle, as shown in Fig. 1shown. The steering system 1 enables multi-axle steering, which is used where different purposes of agricultural vehicles require the vehicle's characteristics to be adapted in terms of maneuverability and directional stability. These increased requirements can be met using various steering types. In addition to the generally common front-axle steering, in which only the front wheels are turned while the rear wheels remain in the straight-ahead position, agricultural vehicles have a type of steering known as all-wheel steering, in which the front and rear wheels are turned in opposite directions. All-wheel steering is preferably used where the vehicle has to be moved in a small space. By turning the front and rear wheels in opposite directions, particularly small turning circles can be achieved for the vehicle, for example when maneuvering.Another type of steering is known as crab steering, in which the front and rear wheels are turned in the same direction.

[0031] The representation in Fig. 1shows the steering system 1 according to the invention, which comprises at least one primary-controlled axle, here and preferably the front axle, and at least one further axle, here the rear axle, each of which is assigned at least one steering cylinder 2 for changing a wheel turning angle of the agricultural vehicle. Furthermore, the steering system 1 comprises a valve unit 4 connected to a supply unit 3 for pressurizing the steering cylinders 2, wherein the valve unit 4 comprises a first steering control and switching unit 5 with a proportional unit 6 and a locking unit 7 and a second steering control and switching unit 8 with a hydraulic device 9, hereinafter referred to synonymously as an orbitrol, and at least three switching valves 10, 11, 12. The steering system 1 also comprises at least one control device 13 and / or 18.At least the control device 13 can be designed with a memory unit 14 in which characteristic curves for controlling the first steering control and switching unit 5 are stored, and with a computing unit 15 for processing the characteristic curves stored in the memory unit 14.

[0032] The steering cylinders 2 are designed as double-acting hydraulic cylinders. The supply unit 3, designed as a hydraulic pump, is connected to the first steering control and switching unit 5 and the steering cylinder 2 via a first supply line 16. Furthermore, the supply unit 3 is connected to the second steering control and switching unit 5 and the steering cylinder 2 via a second supply line 17.

[0033] The vehicle is steered hydraulically by means of the first steering control and switching unit 5. The proportional unit 6 comprises at least one electrically controlled proportional valve, and the blocking unit 7 comprises two electrically controlled switching valves. The electrical control of the first steering control and switching unit 5 can be performed by the at least one control device 13. Alternatively, the first steering control and switching unit 5 can comprise the control device 18, designed as an electronic module, for its electrical control.

[0034] A steering movement 20 generated by a steering wheel 19 arranged in a driver's cab of the vehicle is detected by sensors, for example, by a steering wheel sensor 21. The control device 13 is configured to evaluate the measurement signals of the steering wheel sensor 21 and, depending on the evaluation, to generate control signals with which the first steering control and switching unit 5 is controlled. The first steering control and switching unit 5 can be controlled directly by the control device 13 or by the control device 18 embodied as an electronic module. Alternatively, the control device 18 can perform this task instead of the control device 13, so that the control device 13 as such could be dispensed with.

[0035] For this purpose, the switching valves 10, 11, 12 are in their Fig. 1shown switching position. The two switching valves 10, 11 are designed as normally open valves (NO valves) and are in the open switching position when there is no switching pressure applied. The third switching valve 12 is designed as a normally closed valve (NC valve) and is in the closed switching position when there is no switching pressure applied. According to the shown switching position of the switching valves 10, 11, 12, outputs 9a, 9b of the orbitrol 9 are connected to the steering cylinder 2 via hydraulic lines 22, 23. For this purpose, the third switching valve 12 is arranged between a hydraulic line 25, which connects the hydraulic lines 22, 23 assigned to the outputs 9a, 9b of the orbitrol 9. According to the switching position of the switching valves 10, 11, 12 shown, the driver of the vehicle receives haptic feedback when the end stop of the steering cylinder 2 is reached.This steering mode is called orbital steering.

[0036] Secondly, the vehicle is steered by means of the second steering control and switching unit 8. By applying a control pressure to the switching valves 10, 11, 12, they change their switching position, whereby the switching valves 10, 11, when subjected to appropriate pressure, cause the separation of the Orbitrol 9 from the steering cylinder 2. When subjected to a control pressure, the switching valve 12 establishes a connection between the outputs 9a, 9b of the Orbitrol 9. The Orbitrol 9 feeds the volume flow supplied by the supply unit 3 through the supply line 17 back into a tank T. By "short-circuiting" the Orbitrol 9, a driver can turn the steering wheel 19 endlessly, even if the steering cylinder 2 has already reached its end stop.

[0037] The steering system 1 is operable in a first steering mode and at least one further steering mode, such as the orbital roll steering mode. The control device 13 or the control device 18 is configured to detect the steering movement 20 of the steering wheel 19 connected to the second steering control and switching unit 8 by means of sensors in the first steering mode and to transmit it as a steering angle target signal to the first steering control and switching unit 5 for controlling the latter. The first steering mode, in which the vehicle is steered by means of the second steering control and switching unit 8, is referred to as the fully electronic steering mode.

[0038] To switch to the first steering mode, or fully electronic steering mode, it is necessary to control the at least three switching valves 10, 11, 12, which, according to the prior art, is achieved by individually controlling the switching valves. The control device 13 or the control device 18 is configured to simultaneously control the at least three switching valves 10, 11, 12 in the fully electronic steering mode using a singular, in particular electrical, signal in order to adjust at least one actuation behavior of the steering wheel 19.

[0039] For this purpose, the second steering control and switching unit 8 comprises an electrically controlled actuating valve 24. When actuated by the electrical signal generated by the control device 13 or the control device 18, the electrically controlled actuating valve 24 provides a switching pressure that simultaneously actuates the three switching valves 10, 11, 12. This results in an actuating behavior of the steering wheel 19 that is a continued actuation of the steering wheel 19 regardless of whether the steering cylinder 2 of the primary-controlled axle reaches an end stop. The actuating valve 24 is signal-connected to the switching valves 10, 11, 12 via a control line 26. In the switching position shown, the control line 26 is connected to the tank T.By actuating the control device 13 or the control device 18, the actuating valve 24 is transferred to a switching position in which the control line 26 is connected to the supply line 17. Thus, the three switching valves 10, 11, 12 are simultaneously subjected to their control pressure in order to change at least one actuation behavior of the steering wheel 19.

[0040] In Fig. 2 is a block diagram of the steering system 1 according to Fig. 1 according to a further exemplary embodiment. This embodiment makes it possible to adjust an actuation behavior of the steering wheel 19, which is the induction of increasing resistance when actuating the steering wheel 19. This generates an increase in force on the steering wheel 19 that the driver can feel.

[0041] For this purpose, the second steering control and switching unit 8 additionally includes an electrically controlled throttle valve 27. The electrically controlled throttle valve 27 is connected upstream of the switching valve 12, which is designed as an NC valve, in the hydraulic line 25. The throttle valve 27 is configured to throttle the volume flow between the hydraulic device 9 and the tank T proportional to the current supply down to a minimum flow rate. The current supply to the throttle valve 27 depends on the steering angle resulting from the steering movement 20 of the steering wheel 19.

[0042] The electrically controlled throttle valve 27 is used to allow the volume flow diverted from the Orbitrol 9 directly into the tank T in the first steering mode or fully electronic steering mode to pass unthrottled in the de-energized state and to throttle it down to a minimum flow rate as the current increases. By throttling the short-circuited volume flow, an increase in force on the steering wheel 19 can be achieved that is noticeable to the driver, which represents a further actuation behavior of the steering wheel. The minimum flow rate is set to a value that ensures that the driver can turn the steering wheel 19 even at maximum current, in order to ensure the steerability of the vehicle even if the throttle valve 27 fails.

[0043] In Fig. 3 is a block diagram of the steering system 1 according to Fig. 1according to a third exemplary embodiment. This embodiment makes it possible to set an actuation behavior of the steering wheel 19 which, in addition to causing increasing resistance when actuating the steering wheel 19, simulates the reaching of an end stop of the steering cylinder 2 as an additional actuation behavior of the steering wheel 19.

[0044] For this purpose, the second steering control and switching unit 8 is provided with a throttle valve 28, which is also electrically controlled and is designed to throttle the volume flow proportionally to the current supply until the flow is interrupted. The electrically controlled throttle valve 28 serves to allow the volume flow diverted from the Orbitrol 9 directly into the tank T in the first steering mode or fully electronic steering mode to pass through unthrottled in the de-energized state and to throttle it with increasing current supply until the flow is interrupted. In this way, an end stop of the steering wheel 19 can be simulated as a further actuation behavior. The throttle valve 28 used instead of the throttle valve 27 can be designed to both throttle down to a minimum flow rate with increasing current supply and to throttle proportionally to the current supply until the flow is interrupted.The characteristic of the control of the throttle valve 28, ie that only one throttle valve 28 throttles until the minimum flow rate is reached or until the flow is interrupted, can be specified by the control device 13.

[0045] For reasons of operational safety, it may be advisable to also provide a pressure switch 29, which switches depending on the pressure applied by actuating the steering wheel 19 in order to cancel the flow interruption. This prevents the steering wheel 19 from becoming inoperable if the throttle valve 28 fails at the moment the flow is interrupted. In such a situation, the pressure switch 29 can open to cancel the fully electronic steering mode. For this purpose, the connection between the outputs 9a, 9b of the orbitrol 9 and the switching valves 10, 11, designed as NO valves, of the second steering control and switching unit 8 can be restored by triggering the pressure switch.For this purpose, when a limit value for the pressure applied by actuating the steering wheel 19 is reached, the control device 13 can transmit a signal to the actuating valve 24, in which the latter assumes a switching position in which the connection between the control line 26 and the supply line 17 is interrupted.

[0046] Preferably, the actuating valve 24 and / or the throttle valve 27, 28, along with the pressure switch 29, can be arranged together with the first steering control and switching unit 5 and the second steering control and switching unit 8, along with their associated components, in a common housing 30. An alternative embodiment provides that the first steering control and switching unit 5 and / or the second steering control and switching unit 8 can each be arranged in a separate housing. Arranging the actuating valve 24 and / or the throttle valve 27, 28, along with the pressure switch 29, in a separate housing has the advantage that these components can be retrofitted or optionally installed in vehicles that use the fully electronic steering mode.

[0047] The control device 13 is designed to adjust the current supply to the electrically controlled throttle valve 27 or 28 depending on the set steering angle.

[0048] Another feature common to the exemplary embodiments is that the control device 13 is assigned an operating element 31 for controlling the actuating valve 24, which has a self-holding function. Thus, electrical control of the actuating valve 24 can occur, for example, through electrical self-holding after the driver has actuated the operating element 31, which is designed as a button or switch. By actuating the operating element 31, the driver can switch to the fully electronic steering mode. The electrical self-holding can be canceled by actuating the operating element 31 again or by actuating another element, for example a road drive switch or by selecting another steering program, so that the steering system 1 switches to or reverts to another steering mode, in particular the orbital roll steering mode. List of reference symbols

[0049] 1 Steering system 2 steering cylinder 3 supply unit 4 valve unit 5 First steering control and switching unit 6 Proportional unit 7 Locking unit 8 Second steering control and switching unit 9 Hydraulic device / Orbitrol 9a Exit from 9 9b Exit from 9 10 Switching valve (NO valve) 11 Switching valve (NO valve) 12 Switching valve (NC valve) 13 Control device 14 storage unit 15 Computing unit 16 supply line 17 supply line 18 Electronic module 19 steering wheel 20 Steering movement 21 Steering wheel sensor 22 Hydraulic line 23 Hydraulic line 24 Actuating valve 25 Hydraulic line 26 control line 27 throttle valve 28 throttle valve 29 pressure switch 30 Housing 31 Control element T tank

Claims

1. A steering system (1) for an agricultural vehicle, comprising: - at least one primary controlled axle and at least one further axle, each being associated with at least one steering cylinder (2) for changing a wheel steering angle of the agricultural vehicle, - a valve unit (4) connected to a supply unit (3) for pressurizing the steering cylinders (2), wherein the valve unit (4) comprises a first steering control and switching unit (5) with a proportional unit (6) and a blocking unit (7) as well as a second steering control and switching unit (8) with a hydraulic device (9) and at least three on-off valves (10, 11, 12), - at least one control device (13, 18), which is configured to control the first steering control and switching unit (5), wherein the steering system (1) can be operated in a first steering mode and at least one further steering mode, wherein in the first steering mode, the at least one control device (13, 18) is configured to detect and evaluate, by means of a sensor, a steering movement (20) of a steering wheel (19) connected to the second steering control and switching unit (8) in order to control the first steering control and switching unit (5) on the basis of the evaluation, wherein in the first steering mode, the at least one control device (13, 18) is configured to control the at least three on-off valves (10, 11, 12) simultaneously by means of a singular, in particular electrical, signal in order to set at least one actuation characteristic of the steering wheel (19), wherein the second steering control and switching unit (8) comprises an electrically controlled actuating valve (24) which, when controlled by the electrical signal, provides a switching pressure to the at least one control device (13, 18) which simultaneously pressurises the three on-off valves (10, 11, 12).

2. The steering system (1) according to claim 1, characterized in that the pressurisation of the three on-off valves (10, 11, 12) with the switching pressure of the actuating valve (24) results in the interruption of the hydraulic connection of the second steering control and switching unit (8) to the steering cylinders (2).

3. The steering system (1) according to claim 1 or claim 2, characterized in that two of the on-off valves (10, 11) are constructed as normally open valves (NO valves) and one of the on-off valves (12) is constructed as a normally closed valve (NC valve).

4. The steering system (1) according to claim 3, characterized in that the on-off valve (12) constructed as a NC valve is disposed between hydraulic lines (22, 23) which connect the hydraulic device (9) to the steering cylinders (2) via the respective on-off valve (10, 11) constructed as a NO valve.

5. The steering system (1) according to claim 4, characterized in that an electrically controlled flow control valve (27, 28) is connected upstream of the on-off valve (12) constructed as a NC valve.

6. The steering system (1) according to claim 5, characterized in that the flow control valve (27) is configured to restrict the volumetric flow between the hydraulic device (9) and a tank (T) proportionally to the energisation down to a minimum flow rate.

7. The steering system (1) according to 5 or claim 6, characterized in that the flow control valve (28) is configured to restrict the volumetric flow between the hydraulic device (9) and a tank (T) proportionally to the energisation until the flow is interrupted.

8. The steering system (1) according to claim 7, characterized in that a pressure switch (29) is provided which switches as a function of a pressure exerted by the actuation of the steering wheel (19) in order to cancel the interruption of the flow.

9. The steering system (1) according to one of claims 5 to 8, characterized in that the at least one control device (13, 18) is configured to adjust the energisation of the electrically controlled flow control valve (27, 28) as a function of the set steering angle.

10. The steering system (1) according to one of the preceding claims, characterized in that the at least one control device (13, 18) is associated with an operating element (31) with a self-retaining function for controlling the actuating valve (24).

11. The steering system (1) according to one of the preceding claims, characterized in that an actuation characteristic of the steering wheel (19) is a continued actuation of the steering wheel (19) independently of reaching an end stop of the steering cylinder (2) of the primary controlled axle.

12. The steering system (1) according to one of the preceding claims, characterized in that an actuation characteristic of the steering wheel (19) is the generation of an increasing resistance when the steering wheel (19) is actuated.

13. The steering system (1) according to one of the preceding claims, characterized in that an actuation characteristic of the steering wheel (19) simulates reaching an end stop of the steering cylinder (2).

14. The steering system (1) according to one of the preceding claims, characterized in that at least the actuating valve (24) and / or the flow control valve (27, 28) is or are disposed in a common housing (30) of the first steering control and switching unit (5) and / or or of the second steering control and switching unit (8) or are disposed in a separate housing.