All-wheel steering system for a motor vehicle, motor vehicle, and method of operating an all-wheel steering system

Abstract

An all-wheel steering system for a motor vehicle is described which has an active front axle steering system and an active rear axle steering system. The front axle steering system includes a gear ratio unit which is coupled to a steering wheel. The gear ratio unit is configured to define a front axle steering angle as a function of a steering wheel angle and a front axle steering ratio. In addition, the gear ratio unit is coupled to the rear axle steering system in such a way that the front axle steering ratio is adjustable as a function of a rear axle steering angle. Further, a motor vehicle having such an all-wheel steering system is discussed. Also presented is a method of operating an all-wheel steering system for a motor vehicle.

Claims

1. A method of operating an all-wheel steering system for a motor vehicle comprising the steps of: (a) analyzing stored map data and/or camera image data to detect a presence of an upcoming bend in a road on which the motor vehicle is traveling, (b) when the presence of the upcoming bend is detected, adjusting a front axle steering ratio in the motor vehicle as a function of a rear axle steering angle and the presence of the upcoming bend prior to the motor vehicle reaching the upcoming bend, and (c) turning front wheels of the motor vehicle by a front axle steering angle, the front axle steering angle being a function of a steering wheel angle and the front axle steering ratio in the motor vehicle, the front axle steering ratio being a ratio of the steering wheel angle to the front axle steering angle, wherein the front axle steering ratio is further adjusted as a function of angular positions of a steering wheel and at least one of the front axle steering angle and the rear axle steering angle is adjustable for driving dynamics control, wherein the rear axle steering angle is changed and the front axle steering ratio is adjusted based on the changed rear axle steering angle and the presence of the upcoming bend.

2. The method according to claim 1, wherein the detected presence of the upcoming bend results in a reduction in the front axle steering ratio.

3. The method according to claim 1, wherein the stored map data is provided by a navigation system and the camera image data is provided by a vehicle-side camera.

4. The method according to claim 1, wherein the front axle steering ratio in the motor vehicle is adjusted before the front wheels of the motor vehicle are turned.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows a motor vehicle according to the invention having an all-wheel steering system according to the invention, which can be operated by means of a method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) A motor vehicle 10 includes an all-wheel steering system 12.

(3) The all-wheel steering system 12 comprises an active front axle steering system 14 and an active rear axle steering system 16, the rear axle steering system 16 being in the form of a steer-by-wire system.

(4) Here, a front axle 18, on which the front axle steering system 14 acts, has two front wheels 18a, 18b arranged thereon, which are turned or deflected by a front axle steering angle δ.sub.F in FIG. 1.

(5) Similarly, a rear axle 20, on which the rear axle steering system 16 acts, has two rear wheels 20a, 20b arranged thereon, which are turned or deflected by a rear axle steering angle δ.sub.H.

(6) In FIG. 1, the front axle steering angle δ.sub.F and the rear axle steering angle δ.sub.H have been drawn in so as to make them clearly visible. In comparison with reality, the size of the illustrated angles may be very much larger or smaller.

(7) For reasons of clarity, the rear axle steering angle δ.sub.H is illustrated as an actually present angle. However, it may just as well be an angle demanded by an associated control unit.

(8) The front axle steering angle δ.sub.F is adjusted using a steering wheel 22 at which a steering wheel angle φ can be adjusted and which is coupled to a gear ratio unit 24 of the front axle steering system 14.

(9) The front axle steering angle δ.sub.F is thus fixed as a function of the steering wheel angle φ and a front axle steering ratio implemented by the gear ratio unit 24.

(10) The gear ratio unit 24 is also coupled to the rear axle steering system 16 so that the front axle steering ratio is adjustable as a function of the rear axle steering angle δ.sub.H.

(11) The rear axle steering angle δ.sub.H is adjusted in the interests of as high a vehicle stability as possible. In the illustrated embodiment, this is effected without a direct dependence on the front axle steering angle δ.sub.F. To this end, use is made of methods known from the prior art. As already mentioned, the rear axle steering angle δ.sub.H may be an actual or a demanded angle here.

(12) In the illustrated embodiment, a small front axle steering ratio is set for large rear axle steering angles δ.sub.H. As a result of this, a driver of the motor vehicle 10 can set a relatively large front axle steering angle δ.sub.F by means of a relatively small steering wheel angle φ. In other words, the driver needs to rotate the steering wheel 22 only a little, which means that his/her steering effort is low.

(13) In addition, a bend detection unit 26 is coupled to the gear ratio unit 24. The coupling is configured in such a way here that the front axle steering ratio is adjustable as a function of a signal of the bend detection unit 26.

(14) For generating such a signal, the bend detection unit 26 is adapted to process angular positions of the steering wheel 22, that is, the steering wheel angle φ, and/or path information which, in the illustrated embodiment, it receives from a prediction system, which is not shown in more detail. In this way, the bend detection unit 26 detects whether a driver of the motor vehicle 10 wishes to take a bend or just starts to take a bend.

(15) Alternatively or additionally, a vehicle-side camera, not shown in more detail, may also be used for supplying path information.

(16) In the illustrated embodiment, in such a case the front axle steering ratio is reduced.

(17) The result is that, in comparison to a front axle steering ratio that has not been reduced, the driver of the motor vehicle 10 only needs to set a smaller steering wheel angle φ at the steering wheel 22 in order to take the identical bend. Thus, the steering effort for the driver is low.

(18) The front axle steering ratio is always adjusted continuously here.

(19) Furthermore, the all-wheel steering system 12 has an interface to a driving dynamics control unit 28.

(20) The driving dynamics control unit 28 can adjust the front axle steering angle SF and/or the rear axle steering angle δ.sub.H by means of appropriate signals; for this purpose, it uses methods known from the prior art.

(21) Since both the front axle steering system 14 and the rear axle steering system 16 are so-called active steering systems, this can be effected without a change in the steering wheel angle φ.

(22) Operation of the all-wheel steering system 12 can proceed as follows.

(23) It is assumed that the all-wheel steering system 12 is in an operating mode in which the front axle steering angle δ.sub.F and the rear axle steering angle δ.sub.H are oriented in the same direction. This is usually the case if the speed of the motor vehicle 10 is higher than approx. 50 km/h.

(24) The operating mode with the front axle steering angle δ.sub.F and the rear axle steering angle δ.sub.H being oriented in the same direction is also illustrated in FIG. 1.

(25) Utilizing information about the current steering wheel angle φ and by evaluating path information provided through the prediction system, not shown in more detail, or the vehicle-side camera, not shown in more detail, the bend detection unit 26 now detects that the driver of the motor vehicle 10 wishes to take a bend.

(26) Now the currently present rear axle steering angle δ.sub.H is acquired, which was fixed by means of a known method in the interests of a maximum possible vehicle stability, and therefore also a maximum yaw rate damping. This may have been done by means of the driving dynamics control unit 28.

(27) Then, utilizing the rear axle steering angle δ.sub.H and the signals of the bend detection unit 26, a front axle steering ratio is defined that is suitable for the current driving situation. In particular, for large rear axle steering angles δ.sub.H a small front axle steering ratio is also selected here.

(28) The front axle steering angle δ.sub.F is then always obtained as a function of the steering wheel angle φ, which is set by the driver, and the front axle steering ratio.

(29) This means that in the present case the driver only needs to set a relatively small steering wheel angle φ even in the case of large rear axle steering angles δ.sub.H, in order to obtain a comparatively large front axle steering angle δ.sub.F.

(30) The driver can therefore take a relatively sharp bend without having to turn the steering wheel 22 heavily. In other words, when cornering, the steering effort for the driver is low.

(31) In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained.