Patent Description:
Such a steering system is known, for example, from <CIT>. A vehicle equipped with such a steering system can be steered under the control of the electro-hydraulic steering valve on the one hand or under the control of a conventional steering unit on the other hand.

<CIT>, <CIT> and <CIT> relate to steering systems.

The object underlying the invention is to provide a steering system that has a good comfort.

This object is solved with a steering system according to claim <NUM>.

The steering command arrangement comprises a steering wheel connected to a hydraulic pumping unit, wherein the pumping unit is part of a hydraulic circuit having means for creating a flow resistance, wherein the hydraulic circuit is connected to the steering valve by means of a safety valve establishing or interrupting a hydraulic flow path between the hydraulic circuit and the steering valve.

In such a system the electro-hydraulic steering valve is the principle means for controlling the flow to and from the working port arrangement, i.e. the means for determining the amount of hydraulic fluid which is delivered to a steering motor connected to the working ports of the working port arrangement. The hydraulic circuit is used for two purposes: in an undisturbed condition it creates a sort of "steering feel" since the pumping means which are driven by the steering wheel pump hydraulic fluid through the circuit and the circuit shows a certain flow resistance for the pumped fluid. Thus, rotating the steering wheel has to overcome a certain torque and the driver experiences a feeling which is comparable to the feeling which is produced when the vehicle is steered by a mechanical steering unit. Under normal circumstances the fluid from the pumping unit is not used for steering purposes and this fluid is only "pumped around".

In embodiment of the invention the safety valve establishes a hydraulic connection between the hydraulic circuit and the working port arrangement in case of a lack of pressure at the supply port. Thus, the safety valve is operated automatically, so that steering of the vehicle equipped with the steering system is always possible.

The safety valve is connected to the working port arrangement via the steering valve. Thus, no additional connections to the working port arrangement are necessary.

In an embodiment of the invention the means for creating the flow resistance have a variable orifice. The variable orifice is the main element for creating the flow resistance. When the orifice is variable, the flow resistance can also be varied. Thus, the steering feeling can be adapted to different driving situations. It is, for example, possible to have a low flow resistance when the speed of the vehicle is low and that the flow resistance is high when the speed of the vehicle is high.

In an embodiment of the invention the means for creating the flow resistance are at least partly arranged in the safety valve. Thus, no additional means are necessary.

In an embodiment of the invention a flow resistance of the means for creating the flow resistance depends on a valve position of the safety valve. The safety valve can have, for example, a spool which is moveable in a housing or within a sleeve. The spool defines with a corresponding counterpart at least one orifice and the size of the orifice is then defined by the position of the spool.

In an embodiment of the invention the hydraulic circuit comprises a steering feeling valve connected to the supply port arrangement. The steering feeling valve is able to supply additional hydraulic flow into the hydraulic circuit. Thus, such a flow can either support the rotation of the steering wheel or can produce a larger counter torque. Furthermore, the steering feeling valve can be used for self-centering of the vehicle, i.e. can be used to bring the angular position of the steering wheel and the angular position of the steered wheels into line.

In an embodiment of the invention the hydraulic circuit comprises a relief valve arrangement in a line connecting two ports of the pumping unit. The pumping unit drives hydraulic fluid from one port to the other port, wherein the direction of the flow depends on the direction in which the steering wheel is rotated. The relief valve arrangement can be used to define a maximum torque of the steering wheel. Furthermore, it can simulate an end-stop steering wheel position in which the steering wheel cannot be rotated further.

In an embodiment of the invention the relief valve arrangement comprises two relief valves, wherein each relief valve is controlled by a pressure difference over the relief valve and is bridged by a check valve opening in a direction away from the other relief valve. Thus, the two relief valves can be arranged in a common line. The pressure on the upstream side of each relief valve corresponds to the pressure produced by the output side of the pumping unit and the pressure on the opposite side of the relief valve corresponds basically onto the pressure at the input side of the pumping unit. A pressure drop at the check valve is in this case neglectable.

In an embodiment of the invention the steering command arrangement comprises in addition to the steering wheel a steering command device, wherein the steering wheel is connected to the pumping unit by means of a releasable coupling. The steering command device can be, for example, a joystick or the like. When the vehicle is steered by means of the joystick, it is not necessary that the steering wheel rotates upon steering the vehicle. Such a rotation can be prevented by the use of the releasable coupling.

In an embodiment of the invention the steering valve comprises safety valve ports connected to the safety valve, wherein the steering valve comprises a main fluid path arranged for each direction of steering and a check valve arrangement, the check valve arrangement allowing a flow of hydraulic fluid from the safety valve to one of the working ports and away from the other working port to the safety valve depending on the valve position of the steering valve. Thus, if needed, the flow of hydraulic fluid produced by the rotation of the steering wheel can be used to increase the flow delivered to the working port arrangement and to increase the steering speed.

In an embodiment of the invention in a neutral position of the steering valve the check valve arrangement closes a connection between the safety valve ports and the return port. This is an additional safety measure. There is no possibility that hydraulic fluid escapes to the return port. All hydraulic fluid flow produced in an emergency case by the rotation of the steering wheel and the pumping unit is supplied to the working port arrangement.

In an embodiment of the invention the safety valve comprises actuating means allowing a remote actuation of the safety valve. The safety valve can be, for example, electrically operated by means of a solenoid arrangement or hydraulically by means of corresponding pressures.

In an embodiment of the invention the pumping unit is a measuring motor. In this case the steering system can be equipped with a conventional mechanical steering unit.

In an alternative embodiment the pumping unit is a hydraulic pump, in particular in form of an orbitrol. Thus, only a part of the conventional steering unit is used in such a steering unit.

Embodiments of the invention will now be described in more detail with reference to the drawing, in which:.

<FIG> shows schematically a steering system <NUM> comprising a supply port arrangement having a supply port P and a return port T. The supply port P is connected to a pump <NUM> which can be operated, for example, by the combustion engine of the vehicle to be steered by the steering system. The return port T is connected to a tank <NUM>.

The steering system <NUM> comprises a steering wheel <NUM> which forms at least part of a steering command arrangement. The steering wheel <NUM> is connected to a steering sensor <NUM> and to a column <NUM> by means of which it is connected to a pumping unit <NUM>. The pumping unit <NUM> can be in form of a measuring motor of a conventional hydraulic steering unit or it can be simply a hydraulic pump, for example in form of an orbitrol <NUM>' (as shown in <FIG>).

The steering system furthermore comprises an electro-hydraulic steering valve <NUM> controlled by a controller <NUM>, wherein the controller <NUM> receives sensor signals of the steering sensor <NUM>. The steering valve <NUM> comprises a working port arrangement having two working ports L, R which are connected to a steering motor <NUM>.

The steering valve <NUM> is used to establish a connection between the supply port P and one of the working ports L, R and at the same time to establish a connection between the other of the working ports R, L and the return port T. The direction of the flow of fluid from the pump <NUM> to the steering motor <NUM> depends on the direction of rotation of the steering wheel <NUM> which is detected by the steering sensor <NUM>. The steering sensor <NUM> cannot only detect the angle by which the steering wheel <NUM> is rotated, but also the speed of rotation.

The pumping unit <NUM> is connected to a hydraulic circuit <NUM> having means for creating a flow resistance, in particular an orifice <NUM>, as shown in <FIG>. Furthermore, the hydraulic circuit <NUM> is connected to the steering valve <NUM> by means of a safety valve <NUM>. The safety valve <NUM> establishes or interrupts a hydraulic flow path between the hydraulic circuit <NUM> and the steering valve <NUM>.

Thus, in "normal" condition, the vehicle equipped with the steering system according to <FIG> is steered exclusively under the control of the steering valve <NUM> which in turn is actuated depending on the rotation of the steering wheel <NUM>. Only in an emergency case, for example, when the pressure of the pump <NUM> is not sufficient to actuate the steering motor <NUM>, a connection between the pumping means <NUM> and the steering motor <NUM> is established via the safety valve <NUM> and the hydraulic circuit <NUM>, so that the rotation of the steering wheel <NUM> drives hydraulic fluid in a controlled way to the steering motor <NUM>.

Further details of the steering system <NUM> are shown in <FIG>. The same elements are denoted with the same reference numerals. <FIG> does not show the steering sensor <NUM>.

The pumping unit <NUM> comprises two ports L1, R1 which are used as input port and output port for the pumping unit <NUM> depending on the direction of rotation of the steering wheel <NUM>. The two ports L1, R1 are connected to the hydraulic circuit <NUM>. The hydraulic circuit <NUM> comprises a variable orifice <NUM> which is arranged in the safety valve <NUM>. <FIG> shows the position of the safety valve <NUM>, in which a hydraulic connection between the hydraulic circuit <NUM> and the steering valve <NUM> is interrupted. In this case, there is a loop formed between the two ports L1, R1 of the pumping unit <NUM> and this loop comprises the flow resistance produced by the orifice <NUM>, so that hydraulic fluid is only "pumped around" and produces a counter torque with respect to the torque which is used to rotate the steering wheel <NUM>.

It should be noted that the orifice <NUM> can be a variable orifice. The flow resistance of the orifice <NUM> can be varied by changing the position of the safety valve <NUM>. To this end the safety valve <NUM> can be equipped, for example, with a spool which is displaceable in a housing. The orifice <NUM> can be formed by respective openings in the spool and in the housing, so that the displacement of this spool in the housing changes the flow resistance of the orifice <NUM>.

The safety valve <NUM> can be actuated electrically. To this end it comprises one or more solenoids <NUM>, <NUM> which can be used to displace, for example, the spool in the housing. In an alternative embodiment the safety valve <NUM> can be actuated hydraulically.

The actuation of the safety valve <NUM> can be made automatically, for example, when the pressure of the pump <NUM> decreases under a pre-defined level.

The hydraulic circuit <NUM> comprises furthermore a steering feeling valve <NUM> which is connected to the supply port P and to the return port T. The steering feeling valve <NUM> can be used, for example, for self-centering of the vehicle, i.e. for the electrically follow-up of the steering wheel position. Furthermore, it can be used to change the pressure conditions in the hydraulic circuit <NUM>. When, for example, the supply port P is connected to the left-hand port L1 of the pumping means <NUM> and the steering wheel <NUM> is rotated into the left-hand direction, the driver has to produce a larger torque to rotate the steering wheel <NUM>. On the other hand, when the driver rotates the steering ring wheel <NUM> under the same conditions to the right-hand side, he needs less torque than previously. Thus, the steering feeling valve <NUM> can change the feed-back of the vehicle which experiences the driver at the steering wheel <NUM>.

Furthermore, the hydraulic circuit <NUM> comprises a relief valve arrangement in a line <NUM> connecting the two ports L1, R1 of the pumping unit <NUM>. The relief valve arrangement comprises two relief valves <NUM>, <NUM>. Each relief valve <NUM>, <NUM> is loaded in an opening direction by a pressure at the port L1, R1, respectively of the pumping unit <NUM> to which the respective relief valve is connected. The relief valves <NUM>, <NUM> are loaded in a closing direction by the pressure at the respective outlets of the relief valves <NUM>, <NUM>. Furthermore, each relief valve <NUM>, <NUM> is bridged by a check valve <NUM>, <NUM> opening in a direction away from the other relief valve <NUM>,<NUM>.

Thus, the relief valve arrangement having the two relief valves <NUM>, <NUM> can be used to define a maximum torque which has to be overcome by the steering wheel <NUM>. When this torque is exceeded, for example, when the pressure at the left port L1 of the pumping means <NUM> is too high, the relief valve <NUM> opens and allows fluid to escape via the check valve <NUM> of the other relief valve <NUM> to the right port R1 of the pumping unit <NUM>. safety valve port L2. The same is true for the other safety valve port R2 which is connected to check valves <NUM>, <NUM> depending on the position of the steering valve <NUM>. In the neutral position of the steering valve <NUM>, the left safety valve port L2 is connected to a check valve <NUM> and the right safety valve port R2 is connected to a check valve <NUM>. Both check valves <NUM>, <NUM> are closed by a pressure at the respective safety valve port L2, R2. Furthermore, a connection between the two check valves <NUM>,. <NUM> is connected to a further check valve <NUM> which closes in a direction to the return port T.

Claim 1:
Steering system (<NUM>) comprising a supply port arrangement having a supply port (P) and a return port (T), a steering command arrangement having a steering sensor (<NUM>), a controller (<NUM>) connected to the steering sensor (<NUM>), an electro-hydraulic steering valve (<NUM>) controlled by the controller (<NUM>), and a working port arrangement having two working ports (L, R), wherein the working port arrangement is connected to the supply port arrangement by means of the steering valve (<NUM>), wherein the steering command arrangement comprises a steering wheel (<NUM>) connected to a hydraulic pumping unit (<NUM>, <NUM>'), wherein the pumping unit (<NUM>, <NUM>') is part of a hydraulic circuit (<NUM>) having means for creating a flow resistance, wherein the hydraulic circuit (<NUM>) is connected to the steering valve (<NUM>) by means of a safety valve (<NUM>) establishing or interrupting a hydraulic flow path between the hydraulic circuit (<NUM>) and the steering valve (<NUM>),
characterized in that the safety valve (<NUM>) is connected to the working port arrangement via the steering valve (<NUM>).