Method for steering a vehicle, controller for a motor vehicle, and motor vehicle

Abstract

A method of steering a vehicle having a front axle steering mechanism and, in particular, a rear axle steering mechanism. It is proposed that interventions are carried out at the rear axle to maintain the vehicle on course, at the same time, interventions are carried out at the front axle steering mechanism to avoid a restoring self-alignment of the front wheels.

Claims

1. A method of steering a motor vehicle having at least one front axle steering mechanism and a rear axle steering mechanism (5), the method comprising: carrying out interventions at a rear axle by adjusting a rear wheel steering angle of rear wheels and producing a yaw torque on the motor vehicle to maintain a desired trajectory of the motor vehicle; and as intervention are carried out at the rear axle, carrying out interventions at the front axle steering mechanism by fixing alignment of front wheels of the motor vehicle along a set front wheel steering angle and preventing a restoring self-alignment of the front wheels from the set front wheel steering angle.

2. The method according to claim 1, further comprising changing the rear wheel steering angle of the rear wheels by virtue of the interventions at the rear axle by actuating the rear axle steering mechanism.

3. The method according to claim 1, further comprising influencing, by virtue of the interventions at the rear axle, the yaw torque on the motor vehicle by applying at least one of drive torque and braking torque at the rear wheels with either an active rear axle differential or individual wheel drives.

4. The method according to claim 1, further comprising influencing, by virtue of the interventions at the rear axle, the yaw torque on the motor vehicle by changing a braking torque on the rear wheels with a driving dynamics regulation system.

5. The method according to claim 1, further comprising applying, by virtue of the interventions at the front axle steering mechanism, either a torque or a force to at least one of the front axle steering mechanism (4) and the front wheels which is counter to the restoring self-alignment of the front wheels.

6. The method according to claim 1, further comprising fixing the front wheels at the set front wheel steering angle with the front axle steering mechanism, by virtue of the interventions at the front axle steering mechanism.

7. The method according to claim 1, further comprising implementing the intervention at, at least one of the front axle steering mechanism and the rear axle steering mechanism, by one of: a superimposed steering system, an electric steering system (steer-by-wire), an electromechanical steering system, and an electro-hydraulic steering system.

8. The method according to claim 1, further comprising providing the motor vehicle with both a steerable front axle and a steerable rear axle, and a control unit that carries out at least one of the interventions at the rear wheels of the rear axle and interventions at the front wheels of the front axle, as a function of vehicle parameters relating to at least one of a vehicle speed, a yaw rate, an acceleration, a front wheel steering angle and the rear wheel steering angle of the front and the rear wheels, respectively, in order to maintain the motor vehicle on course.

9. The method according to claim 8, further comprising maintaining the motor vehicle on course by evaluating with the control unit additional signals from a vehicle environment recognition system in order to ensure that correct tracking of the motor vehicle is maintained.

10. A motor vehicle having a control unit, a front axle steering mechanism and a rear axle steering mechanism that facilitate steering of the motor vehicle at a front axle and at a rear axle, respectively, the control unit carrying out interventions at rear wheels of the rear axle and front wheels of the front axle, as a function of vehicle parameters relating to at least one of a vehicle speed, a yaw rate, an acceleration and wheel steering angles of the front and the rear wheels (2, 3), respectively, in order to maintain the motor vehicle on course, and the front axle steering mechanism being connected to the front wheels and controlled by the control unit to carry out interventions at the front wheels by fixing alignment of the front wheels at least temporarily at a set front wheel steering angle while interventions are carried out at the rear wheels by at least one of steering the rear wheels with the rear axle steering mechanism, and influencing a yaw torque of the motor vehicle by at least one of an active rear axle differential and a driving dynamics regulation system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Below, the invention is described with reference to preferred embodiments referring to the drawing, which shows:

(2) FIG. 1: A schematic representation of a vehicle, showing the influence of the restoring self-alignment torque on the front wheels,

(3) FIG. 2: A schematic representation of a vehicle with intentional suppression of the restoring self-alignment torque at the front wheels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) FIGS. 1 and 2 shown in each case a schematically represented vehicle 1 with two front steered wheels 2 and two rear steered wheels 3. The front wheels 2 and the rear wheels 3 are steered by the schematically represented steering mechanisms 4, 5 by means of a central actuator or a central force supporting mechanism. The respective wheels 2, 3 are linked by linkages 41, 51 such as the usual track rods, and control signals are sent by the control unit SG to the linkages. The front and rear axle steering mechanisms 4, 5 are kinematically independent of one another. In the middle of the vehicle 1 is the center of gravity S of the vehicle. By virtue of interventions at the front axle VA or the rear axle HA, such as the steering, braking or acceleration of individual wheels, a yaw torque M.sub.G is produced about the center of gravity S. The interventions take place as the result of signals from a control unit SG, which in the present case receives data from an ESC (Electronic Stability Program), such as vehicle speed, acceleration and yaw rate.

(5) FIG. 1 shows the following situation: Due to a control signal from the control unit SG the rear wheels 3 have been steered to the right by the steering mechanism 5 via the track rods 51. At the rear axle, the rear wheels are deflected through a wheel steering angle of .sub.h. This has produced a yaw torque M.sub.G, which acts on the vehicle 1 counterclockwise about the vertical axis or center of gravity S. Owing to this yaw torque M.sub.G, due to the lateral force produced thereby the front wheels 2 have experienced a restoring self-alignment torque M.sub.R, which brings about a change of the wheel steering angle .sub.v in the same direction, to the right as shown here. The intentional steering angle correction applied by the control unit SG at the rear axle HA is therefore almost without effect, or at least of little efficacy.

(6) FIG. 2 shows approximately the same situation, wherein the front wheels are held in position by the steering mechanism 4 in co-operation with the track rod system 41. In this temporary situation, i.e. during the intervention at the rear axle HA (a steering correction), which is carried out by the control unit SG by means of the steering mechanism 5 at the rear axle HA, the wheel steering angle .sub.v currently set at the front axle VA remains unchanged.

(7) By comparing FIGS. 1 and 2 the radii rv1 and rv2 and rh1 and rh2 indicated can also be seen. Here, the radii rv are perpendiculars extending from the wheel plane of the front wheels 2. The radii rh are perpendiculars extending from the wheel plane of the rear wheels 3. The radii rv and rh intersect at a point. It is evident that if the wheel steering angle .sub.v in FIG. 1 is not maintained, a larger curve radius rv, rh is obtained than in FIG. 2, because in the latter case the wheel steering angle .sub.v is maintained. For the sake of simplicity, the consideration of the curve radius rv, rh is limited to one track.

(8) Below, as an example embodiment of the invention a driving maneuver with rear axle steering HA for correct tracking/maintaining the trajectory of a vehicle is described: The start of the deviation from the trajectory T required by the driver is detected by acceleration sensors and a driving environment recognition system F present in the vehicle 1, for example at least by means of a sensor such as a camera, radar or Lidar. A control unit SG evaluates the sensor signals and by means of a steering mechanism 5 a wheel steering angle .sub.h is set at the rear wheels 3 (intervention at the rear axle), so that the trajectory T is maintained. At the same time, at the front axle steering mechanism VA, preferably at the steering wheel or at the front wheels 2, a torque M.sub.H is applied for the at least temporary maintenance of the wheel steering angle .sub.v at the front wheels 2, so that effective action can take place by virtue of the intervention at the rear axle HA.

(9) If such an intervention were not to be carried out at the front axle steering mechanism VA, thencaused kinematically due to the caster configurationa restoration self-alignment of the front wheels would be produced, which would bring about a torque M.sub.R on the front wheels 2 which in the extreme case would be expressed as an oblique position of the vehicle 1, a so-termed crabwise motion (see FIG. 2). Despite the setting of a rear wheel steering angle .sub.h the vehicle 1 would essentially continue following the set course T, i.e. straight ahead orif driving round a bendfarther on the same curve radius rv, rh that corresponds to the currently set wheel steering angle .sub.v of the front wheels 2. The effect of the intervention at the rear axle HA would be almost nothing, or only undesirably small.

(10) Thus, in order to achieve the desired change of direction the front axle wheel steering angle .sub.v must be temporarily maintained. Thus, the torque M.sub.H applied at the front axle steering mechanism 4 prevents the kinematic self-alignment of the front wheels 2, so that the steering wheel angle remains in the currently set position and for the driver there is no noticeable change. Alternatively to the torque produced, the intervention can also take place by means of a counterforce, with the same effect.

(11) Alternatively to the torque or the counterforce, the intervention can also be effected by setting a defined wheel steering angle .sub.v at the front axle linkage VA, preferably in that a movement or an angular position is set at the steering wheel, the steering linkage, the track rod or the wheels. In this way too the front axle steering mechanism VA can be decoupled from the effects of the rear axle steering mechanism HA.

(12) It is understood that the above-mentioned features of the invention can be used not just in the combination indicated in each case, but also in other combinations or in isolation, without going beyond the scope of the invention.

INDEXES

(13) 1 Vehicle 2 Front wheels 3 Rear wheels 4 Front axle steering mechanism 41. Linkage Rear axle steering mechanism 51. Linkage rv Curve radius rh Curve radius S Center of gravity VA Front axle HA Rear axle M.sub.G Yaw torque M.sub.R Restoring, self-alignment torque M.sub.H Holding torque SG Control unit ESC Electronic Stability Control .sub.v Front wheel steering angle .sub.h Rear wheel steering angle F Vehicle environment recognition system T Trajectory, path, driving lane