Steering system for a trailing axle of a vehicle

Abstract

Steering system (LS) for at least one trailing or leading axle of a vehicle, comprising: - a steering angle sensor (S α ) for measuring a steering angle of wheels of a front axle of the vehicle, - a vehicle speed sensor (S v ) to measure the driving speed of the vehicle, - an electric motor (1.2) driving a hydraulic pump (2), - a working cylinder (11) for steering the wheels of the trailing axle, which is connected to the hydraulic pump (2) via supply lines (7 and 7.1), - a control unit (1.1) which uses the data from the steering angle sensor (S α ) and the vehicle speed sensor (S v ) determines a caster angle of the wheels on the trailing axle of the vehicle and controls the electric motor (1.2) accordingly, - wherein the working cylinder (11) has a central position bore (11.3) through which hydraulic fluid can be discharged from the working cylinder (11), - wherein the center position bore (11.3) of the working cylinder (11) is connected to a center position valve (12) through which hydraulic fluid can flow back into an oil tank (6), and - wherein a piston (14.1) closes the center position bore (11.3) when the wheels of the trailing axle are in the straight-ahead position, characterized in that the piston (14.1) has a piston seal (14.2) and a piston guide, wherein the center position bore (11.3) can only be closed by the piston seal (14.2), that the piston guide is taken over directly by an area on the piston (14.1) and wherein diagonal or axial grooves are introduced into this area, which ensure a sufficient flow of oil between the piston guide and the cylinder tube into the center position bore (11.3), or that the piston (14.1) has a guide ring (14.3) for the piston guide, in which guide ring diagonal or axial grooves are introduced, which ensure a sufficient flow of oil between the piston guide and the cylinder tube into the center position bore (11.3).

Description

The present invention relates to the technical field of steering systems and, more particularly, to a steering system for a trailing axle of a vehicle according to claim 1.Prior ArtHeavy vehicles--in particular commercial vehicles--often have more than two axles, so-called trailing axles (NLA). If the NLAs are rigid, the vehicles have a large turning circle. Therefore, in addition to a front axle steering system, a steerable NLA is often additionally installed. The NLA can be positively steered or adhesively steered, i.e. steered by the restoring movement of the wheels themselves. This additional NLA steering allows smaller radii of curvature, thereby achieving higher maneuverability. In addition, the slip angle on the tires is reduced, thereby reducing the tire wear of the vehicle.However, active steering of the NLA is only desirable at low speeds. At higher vehicle speeds, steering of the NLA is not desired, as this negatively affects stable driving. The NLA must be fixed from a certain vehicle-dependent speed in order not to cause an unstable driving state. In such systems, it is advantageous that in the event of a failure or at higher travel speeds, the axle can be held in the straight-ahead position.It is known in the art that the NLA is linked via a hydraulic cylinder. The oil is pumped into one or the other cylinder chamber via a pump, which is driven by an electric motor, depending on how the valves are switched. Since the electric motor can drive equivalently in both directions, one or the other cylinder space can be acted upon by a reversible pump depending on the direction of rotation.DE 103 51 482 A1 shows a steering system in which a hydraulically articulated rear axle of the vehicle is held in the current position or is articulated with adhesion by means of an additional blocking device and is then locked in the middle position. However, this requires additional components, requires additional installation space and is therefore expensive.DE 10 2006 008 436 A1 shows a mechanically coupled multi-axle steering system in which a steering force is only applied to the additional steering axle when this steering force is also actively required-i.e. during steering angle. However, this system for a rear axle steering system, which is to be locked from a certain speed range in straight travel, can only be realized with a great deal of effort.DE 10 2010 024 847 A1 discloses a steering spindle which has a piston. The piston is provided with a sealing insert and a sliding insert.Finally, DE 10 2012 105 976 A1 discloses, as a generic document, a steering system for a trailing axle having an electronic control in which the articulation of the trailing axle takes place independently of the front axle. The pump is driven by an electric motor, whereby the system operates in an energy-efficient manner. The blocking function is realized in the simplest way in that, during the adhesion-driven return movement of the piston, hydraulic fluid is discharged from the working cylinder through a central position bore. When the piston reaches this central position bore, it closes it and thus blocks further movement.However, there is the problem that the piston already closes the central position bore before reaching the theoretical central position. As a result, in the fault mode, sufficiently accurate return of the axle into the straight-ahead position is not possible. The reason for this is the close piston play in the cylinder tube, in particular the close play between the guide band and the cylinder tube, which does not allow sufficient oil flow to the central position bore.The object of the present invention is to avoid the above-mentioned disadvantage, i.e. to provide a suitable piston mounting in the cylinder tube, which ensures that only the piston seal closes the central position bore and otherwise a sufficient oil flow is ensured.The object is achieved by a steering system according to claim 1. The piston guide is provided with a plurality of grooves, so that a sufficient oil flow between the guide ring and the cylinder tube to the central position bore is ensured.The grooves can be aligned both diagonally and axially. It is also conceivable to align the grooves radially and then to arrange additional transverse grooves in the axial direction.The object is alternatively achieved in that the piston guide is taken over directly by the piston seal.The invention is explained with reference to the following figures: FIG. 1 is a functional diagram of a vehicle. FIG. 2 shows the hydraulic circuit diagram of the entire steering system. FIG. 3 shows an embodiment of the piston guide according to the invention FIG. 4 shows a second embodiment of the piston guide according to the invention FIG. 5 shows a third embodiment of the piston guide according to the inventionThe functional diagram of FIG. 1 shows that a steering angle sensor Sα is provided for detecting a steering angle of road wheels of a front axle, and a travel speed sensor S v is provided for detecting a travel speed v. Their signals are transmitted via a signal line to the steering system LS of the trailing axle.FIG. 2 shows the hydraulic circuit diagram of the entire steering system. The data obtained from a steering angle sensor which measures the steering angle at the front axle and from a vehicle speed sensor are input into a control unit 1.1. The control unit 1.1 calculates a trailing angle of road wheels on the trailing axis of the vehicle from the data and correspondingly controls an electric motor 1.2.The electric motor 1.2 serves to drive a hydraulic pump 2, which in turn is connected to at least one working cylinder 11 for the articulation of the running wheels of the trailing axle. The working cylinder 11 has a central position bore 11.3, via which hydraulic fluid can be discharged from the working cylinder 11, so that the piston can be moved in an adhesion-driven manner into a central position in which it closes the central position bore 11.3, and the running wheels of the trailing axle are blocked in a straight-ahead position.In this electrohydraulic steering system, the steering of the trailing axle takes place independently of the front axle since the steering wheel is not mechanically connected to the axle to be steered. In addition, this system is decoupled from the internal combustion engine, so that on the one hand regulation according to requirements and on the other hand, due to the few and additionally freely placeable components, high spatial flexibility is ensured during its installation. At low speeds and at a standstill, this system makes active steering possible as a function of the steering angle of the front axle and of the travel speed. In particular, the wheels of the trailing axle can be brought automatically, i.e. externally driven, from each deflection angle into their straight-ahead position at all times even in the event of failure of the electronics and / or of the hydraulics and can also be reliably blocked there without additional electronics and / or hydraulics being necessary for this purpose.During the adhesion-driven movement of the piston in the direction of the central position bore, the hydraulic fluid is discharged from one cylinder chamber in the direction of the oil container, while fluid is drawn in into the other cylinder chamber-without the use of a pump. As soon as the central position bore is closed by the piston, its further movement is blocked by the hydraulic fluid, which is confined on both sides, so to speak, so that the wheels of the trailing axle are held securely in their straight-ahead position. In the event of failure of the hydraulic and / or electric system at low speeds, there is thus the possibility that the trailing axis is driven in a damped adhesion-guided manner into the central position and is held there.A trailing axis is to be understood here as any axis which follows the deflection of a steered axis and which can be arranged following or leading to a rigid rear or front axis, that is to say also as a leading axis. The steering system according to the invention can thus also be used in trailers, trailers or a second steered front axle. The steering system according to the invention has a valve which is connected in a fluid connection between the central position bore and the oil container and which, in a basic position, releases a fluid flow between the central position bore of the working cylinder and the oil container, and which, in a working position, prevents a fluid flow between the central position bore of the working cylinder and the oil container, and in which the control device is designed to detect faults and, in the event of a fault, to switch the valve into its basic position, such that the piston can be moved in an adhesion-driven manner as far as its central position, in which the piston closes the central position bore, and the running wheels of the trailing axle are blocked in a straight-ahead position. In the event of a failure of the pump, for example, the system can be transferred to a safe state by appropriate switching of the valve. While the valve is in its working position, for example, during trouble-free operation of the steering system, in which the piston is movable only via the pump, it can then, if a trouble occurs, permit its movement into the central position by opening the central position bore. In this basic position, the valve is de-energized and can thus be kept energy-free and thus particularly reliable, wherein the deactivated pump also no longer requires additional energy. The central position of the piston can be held reliably, since it closes the central position bore of the cylinder and a further discharge of hydraulic fluid is no longer possible.The control device can be designed to switch the engine off when a predeterminable driving speed is exceeded. The trailing axle is thus automatically moved--starting from a certain presettable speed--i.e. driven by adhesion--into its central position and held there, so that its running wheels are in the straight-ahead position. Since this center position can be held without further energy expenditure, the power consumption of the system is minimal.The pump is preferably designed as a reversibly operable pump or as a combination of a unidirectionally operable pump with a valve block. A reversibly operable pump makes the lowest demands on the space requirement of the steering system, while a pump operable in one sense requires a simpler electric drive.The pump 2 draws oil from the oil container 6 via the suction valves 3, 3.1. Since coarse dirt can be in the oil container 6, two filters 4, 4.1 are arranged in the suction line for safety purposes.During the steering process, the pump 2 delivers oil via the feed line 7, 7.1 through the filters 8, 8.1 and the check valves 9, 9.1 into a cylinder chamber 11.1, 11.2.Each cylinder chamber 11.1, 11.2 is assigned a return valve 10, 10.1, which is closed when the cylinder chamber 11.1, 11.2 is filled. As a result of the pressure build-up in one cylinder chamber 11.1, 11.2, the return valve 10, 10.1 of the respective other cylinder chamber 11.1, 11.2 is opened and the return flow from the other cylinder chamber 11.1, 11.2 through the filter 5 to the oil container 6 is thereby made possible.The central position valve 12 has a defined flow cross section for the open switching position, whereby the return speed of the follow-up axle can be set exactly.FIG. 3 shows a piston 14.1 with a guide ring 14.3 attached. The guide ring 14.3 has grooves arranged diagonally all around it, the cross section and groove spacing of which have been selected such that when passing over the central position bore, a sufficient oil flow between the guide ring and the cylinder tube into the central position bore is achieved. Thus, the piston guide has no influence on the oil flow via the central position bore and it is ensured that only the piston seal 14.2 can close the central position bore.FIG. 4 shows that instead of the guide ring, the piston guide can also be taken over directly by a correspondingly designed region on the piston 14.1. In this case, the grooves for the oil flow can also be introduced directly into the piston 14.1.FIG. 5 shows an embodiment in which the piston guide is taken over directly by the piston seal 14.2. For this purpose, the piston seal is made wider and the piston is deposited directly next to the seal groove. Thus, the transverse piston forces can be transmitted directly via the primary piston seal to the cylinder wall. The secondary piston seal lying underneath cannot therefore be deformed, which in this case would lead to a metallic run-on of the piston on the cylinder running surface.The various operating states are described below.Straight-ahead travel, higher travel speedWhen traveling straight ahead at a higher travel speed, the trailing axis is not linked but must be held in the straight-ahead position. The axle can be actively held by the motor here, but as a result energy is consumed. Preferably, therefore, the axle is held by oil trapped in the working cylinder 11. In this case, the axle is actively returned by the motor to the straight-ahead travel position and the steering system is subsequently switched passive, i.e. the middle position valve 12 is closed and thus locks the cylinder spaces together with the nonreturn valves 9 and 9.1. The advantage here is that no more energy is required by the motor.Active Steering, Low Vehicle SpeedWhen steering is active, the steering angle of the front axle is detected by measurement technology and transmitted to the control unit 1.1 of the rear axle steering system. Using these and further parameters, for example the vehicle speed, the setpoint value of the rear axle is calculated and the engine 1.2 is controlled by the control unit 1.1. The actual value is detected via the position sensor 13 and used for regulation.The latter directly drives a reversible pump 2. This conveys, for example, in the direction of the feed line 7 to the cylinder side 11.1. The pump 2 always draws oil filtered via the suction valves 3 and 3.1 from the oil container 6.The engine 1.2 is controlled by a corresponding control algorithm until the setpoint value of the rear axle is reached.Failure of the Steering SystemIf a failure of the steering system occurs (e.g. a fault of the motor 1.2, sensor 13 or control unit 1.1), motor 1.2 is switched off, the central position valve 12 goes into its basic position and the axle is returned via the wheel forces and the desired damping via the central position bore 11.3 into the straight-ahead travel position, i.e. safe state. There, the axle is held as explained below.Failure in straight-ahead driving, higher driving speedIf the system fails when driving straight ahead, this has no effect, since the piston seal has closed the central position bore and the check valves 9 and 9.1 do not allow the oil to flow back out of the working cylinder 11, i.e. the cylinder is hydraulically blocked.Assisted steering failure, low vehicle speedIf the system fails when steering is active, the solenoid of the middle position valve 12 is de-energized, whereby it assumes its basic position. If the piston is in the straight-ahead travel position, it cannot be moved any further-accordingly, the axle remains held.If the axis is deflected, there is no possibility in many fault cases of actively moving the axis into the middle position. When cornering, the deenergization of the middle position valve 12 prevents the axle from deflecting further than it is at this moment. The axle restoring forces attempt to move the cylinder in the direction of straight travel, this movement being made possible by the central position bore 11.3 in the working cylinder 11 until the piston reaches the central position. After reaching the central position, the axle is held in this position, since the piston closes off the central position bore 11.3.The speed of the axle movement in the event of a failure can be adjusted by the middle position bore 11.3 or a throttle in the middle position valve 12 in such a way that no critical driving state arises.For the return movement of the piston in the direction of the central position, oil must be sucked in from the oil container 6. This takes place via the suction valves 3 and 3.1 and the check valves 9 and 9.1.List of reference characters1.1 Control unit 1.2 Electric motor 2 Pump 3 / 3.1 Suction valve 4 / 4.1 Filter 5 Filter 6 Oil container 7 / 7.1 Feed line 8 / 8.1 Filter 9 / 9.1 Nonreturn valve 10 / 10.1 Nonreturn valve 11 Working cylinder 11.1 / 11.2 Cylinder chamber 11.3 Central position bore 12 Central position valve 13 Position sensor 14.1 Piston 14.2 Piston seal 14.3 Guide ring LS Steering system Sα Steering angle sensor Sv Travel speed sensor

Claims

Steering system (LS) for at least one trailing or leading axle of a vehicle, comprising: - a steering angle sensor (Sα) for measuring a steering angle of road wheels of a front axle of the vehicle, - a travel speed sensor (S v) for measuring a travel speed of the vehicle, - an electric motor (1.2) which drives a hydraulic pump (2), - a working cylinder (11) for steering the road wheels of the trailing axle, which working cylinder is connected to the hydraulic pump (2) via feed lines (7 and 7.1), - a control unit (1.1), which determines a trailing angle of road wheels on the trailing axle of the vehicle with the aid of the data of the steering angle sensor (Sα) and of the vehicle speed sensor (S v) and controls the electric motor (1.2) accordingly, - wherein the working cylinder (11) has a central position bore (11.3), via which hydraulic fluid can be discharged from the working cylinder (11), - wherein the central position bore (11.3) of the working cylinder (11) is connected to a central position valve (12), via which hydraulic fluid can flow back into an oil container (6), and - wherein a piston (14.1), in the straight-ahead position of the road wheels of the trailing axle, closes the central position bore (11.3), characterized in that the piston (14.1) has a piston seal (14.2) and a piston guide, wherein the central position bore (11.3) can be closed only by the piston seal (14.2), that the piston guide is taken over directly by a region on the piston (14.1) and wherein diagonal or axial grooves are introduced into this region, which ensure a sufficient oil flow between the piston guide and the cylinder tube into the central position bore (11.3), or that the piston (14.1) for the piston guide has a guide ring (14.3), in which diagonal or axial grooves are introduced, which ensure a sufficient oil flow between the piston guide and the cylinder tube into the central position bore (11.3).

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