METHOD FOR DETERMINING A GEAR FOR A POWER STEERING SYSTEM AS A FUNCTION OF A VEHICLE SPEED AND A STEERING WHEEL ANGLE

Abstract

A method for determining a gear ratio (VGR) for a power steering system of a vehicle, said power steering system comprising a steering wheel determining a steering wheel angle (A.sub.v) and a rack determining a rack position (X.sub.c), said rack position (X.sub.c) varying between a lower limit rack position and an upper limit rack position (X.sub.csup), the gear ratio (VGR) defining a ratio between the rack position (X.sub.c) and the steering wheel angle (A.sub.v), or conversely, characterized in that the method comprises a definition step in which the gear ratio (VGR) is defined as a function of a vehicle speed (V1, V2, V3) and of the steering wheel angle (A.sub.v) so that the upper limit rack position (X.sub.csup) corresponds to a single upper limit steering wheel angle (A.sub.vsup).

Claims

1. A method for determining a gear ratio (VGR) for a power steering system of a vehicle, said power steering system comprising a steering wheel determining a steering wheel angle (A.sub.v) and a rack determining a rack position (X.sub.c), said rack position (X.sub.c) varying between a lower limit rack position and an upper limit rack position (X.sub.csup), the gear ratio (VGR) defining a ratio between the rack position (X.sub.c) and the steering wheel angle (A.sub.v), or between the steering wheel angle (A.sub.v) and the rack position (X.sub.c), wherein the method comprises a definition step in which the gear ratio (VGR) is defined as a function of a vehicle speed (V1, V2, V3) and the steering wheel angle (A.sub.v) so that the upper limit rack position (X.sub.csup) corresponds to a single upper limit steering wheel angle (A.sub.vsup).

2. The method according to claim 1, wherein the power steering system is of the steer-by-wire type or of the variable gear reduction mechanical type.

3. The method according to claim 1, wherein the upper limit steering wheel angle (A.sub.vsup) is comprised between: $\frac{X_{\mathrm{csup}}}{5}$
and
X.sub.csup.Math.2 where: X.sub.csup is the upper limit rack position.

4. The method according to claim 1, wherein the definition step comprises, for a considered vehicle speed (V1, V2, V3): a phase of determining a limit variation in which the limit variation of the gear ratio (VGR) as a function of the steering wheel angle (A.sub.v) is determined so as to guarantee a controllability of the vehicle below this limit variation; a phase of determining a grip steering wheel angle (A.sub.vad_V2, A.sub.vad_V3) in which the grip steering wheel angle (A.sub.vad_V2, A.sub.vad_V3), corresponding to the steering wheel angle (A.sub.v) above which the vehicle no longer exhibits controllability, is determined; a characterization phase in which the gear ratio (VGR) is defined so that below the grip steering wheel angle (A.sub.vad_V2, A.sub.vad_V3), the variation of the gear ratio (VGR) is less than or equal to the limit variation.

5. The method according to claim 4, wherein the gear ratio (VGR) is defined during the characterization phase so that above the grip steering wheel angle (A.sub.vad_V2, A.sub.vad_V3), the variation of the gear ratio (VGR) is greater than the limit variation.

6. The method according to claim 4, wherein the phase of determining a limit variation and/or a grip steering wheel angle (A.sub.vad_V2, A.sub.vad_V3) is carried out by means of a mathematical model or a physical test.

7. The method according to claim 4, wherein the grip steering wheel angle (A.sub.vad_V2, A.sub.vad_V3) is determined when the vehicle is in at least one grip condition favorable to the controllability of the vehicle.

8. The method according to claim 7, wherein the at least one favorable grip condition is a dry ground.

9. A vehicle comprising a power steering system implementing a gear ratio (VGR) determined according to claim 1.

Description

[0055] The invention will be better understood, thanks to the description below, which relates to an embodiment according to the present invention, given by way of non-limiting example and explained with reference to the appended schematic drawings, in which:

[0056] FIG. 1 is a diagram of a rack position as a function of a steering wheel angle by implementing a gear ratio according to the state of the art;

[0057] FIG. 2 is a diagram of a gear ratio as a function of the steering wheel angle according to the invention;

[0058] FIG. 3 is a diagram of the rack position as a function of the steering wheel angle by implementing the gear ratio according to the invention.

[0059] Only the elements necessary for understanding the invention have been represented. To facilitate reading of the drawings, the same elements carry the same references from one figure to another.

[0060] A mechanical type steering system with variable gear reduction, also called «Active Front Steering», or an electric power steering system without a mechanical link, called «steer-by-wire», comprises several elements including a steering wheel determining a steering wheel angle A.sub.v, a rack determining a rack position X.sub.c, and two wheels each connected to a tie-rod. The rack is displaced between two stops of a casing, said stops defining an upper limit rack position X.sub.csup making it possible to reach a maximum angle of orientation of the wheels, and a lower limit rack position. The stops can be real or virtual. In FIG. 3, the rack position X.sub.c varies by an angle ranging from −530° to 530°. However, only the part from 0° to 530° has been represented, the 0° being a center of symmetry.

[0061] A gear ratio VGR is defined as a ratio between the rack position X.sub.c and the steering wheel angle A.sub.v, or between the steering wheel angle A.sub.v and the rack position X.sub.c according to the formula:

[00004]$\begin{array}{cc}\mathrm{VGR}=\frac{A_v}{X_c}& \left[\mathrm{Math}5\right]\end{array}$

[0062] With:

[0063] VGR: the gear ratio,

[0064] A.sub.v: the steering wheel angle

[0065] X.sub.c: the rack position

[0066] Or by the formula [Math 6]

[00005]$\mathrm{VGR}=\frac{X_c}{A_v}$

[0067] With:

[0068] VGR: the gear ratio,

[0069] A.sub.v: the steering wheel angle

[0070] X.sub.c: the rack position

[0071] It is generally accepted that the formula Math 2 is the inverse of the gear ratio. Thus, in the rest of the description, it is considered that the gear ratio is calculated with the formula Math 1.

[0072] The rack position X.sub.c corresponds to an image value of the rack position that can be measured directly on the rack or obtained by deduction or calculation. For example, the image of the rack position can be obtained by a position of an assistance motor modifying the rack position X.sub.c, or by an orientation angle of the wheels or yaw angle of the wheels.

[0073] Similarly, the steering wheel angle A.sub.v corresponds to an image value of the steering wheel angle which can be measured directly on the steering wheel or obtained by deduction or calculation. For example, the image of the steering wheel angle can be obtained by a position of a motor exerting a torque on an axis of a column supporting the steering wheel or by an absolute angle sensor positioned on said axis of the column supporting the steering wheel.

[0074] The method according to the invention implements a definition step. During this step, the gear ratio VGR is defined as a function of a vehicle speed V1, V2, V3 and of the steering wheel angle A.sub.v so that the upper limit rack position X.sub.csup corresponds to a single upper limit steering wheel angle A.sub.vsup.

[0075] More precisely, the definition step comprises, for a considered vehicle speed V1, V2, V3, that is to say a given vehicle speed V1, V2, V3, a phase of determining a limit variation, a phase of determining a grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3 and a characterization phase.

[0076] In the phase of determining a limit variation, the limit variation of the gear ratio VGR as a function of the steering wheel angle A.sub.v is determined so as to guarantee controllability of the vehicle below this limit variation. The limit variation of the gear ratio VGR corresponds to the steering coefficient of the curve of the gear ratio VGR as a function of the steering wheel angle A.sub.v as represented, for example, in FIG. 2, for the considered vehicle speed V1, V2, V3. The limit variation is a threshold beyond which the gear ratio VGR varies too rapidly for the controllability of the vehicle to be guaranteed at the considered speed V1, V2, V3. In other words, if the gear ratio VGR varies by a value lower than the variation limit, the vehicle can be controlled, while if the gear ratio VGR varies by a value greater than the variation limit, the vehicle is not necessarily controllable, that is to say there is a significant risk of loss of controllability. In an embodiment, the limit variation is determined when the vehicle is in at least one grip condition not favorable to the controllability of the vehicle or/and the limit variation is determined according to at least one life condition of the vehicle.

[0077] During the phase of determining a grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3, the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3, corresponding to the steering wheel angle A.sub.v above which the vehicle no longer exhibits controllability, is determined. The grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3 is a threshold corresponding to a vehicle grip limit for a considered vehicle speed V1, V2, V3. In other words, beyond the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3, the vehicle is not controllable, that is to say it cannot be controlled, at the considered speed V1, V2, V3.

[0078] According to an embodiment, the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3 is determined when the vehicle is in at least one grip condition favorable to the controllability of the vehicle. For example, the at least one favorable grip condition is a dry ground.

[0079] According to an embodiment, the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3 is determined according to at least one life condition of the vehicle.

[0080] The characterization phase defines the gear ratio VGR for the considered vehicle speed V1, V2, V3. More specifically, the gear ratio VGR is defined so that below the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3, the variation of the gear ratio VGR is less than or equal to the limit variation. The limit variation and the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3 represent two limits to define the gear ratio VGR in such a way as to guarantee the controllability of the vehicle. In other words, below the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3, the gear ratio variation must be less than or equal to the limit variation so that the driver can control the According to an embodiment, the gear ratio VGR is also defined, during the characterization phase, so that above the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3, the variation of the gear ratio VGR is greater than the limit variation.

[0081] According to an embodiment, the phase of determining a limit variation and/or a grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3 is carried out by means of a mathematical model or a physical test.

[0082] Thus, according to the invention, the gear ratio VGR depends on the vehicle speed V1, V2, V3. This ensures good maneuverability at low speed by determining a low gear ratio, and good controllability at high speed with a high gear ratio.

[0083] According to the invention, the gear ratio VGR also depends on the steering wheel angle A.sub.v. Thus, the gear ratio VGR is adapted as a function of the steering wheel angle A.sub.v so that the upper limit rack position X.sub.csup corresponds to a single upper limit steering wheel angle A.sub.vsup. In other words, whatever the speed of the vehicle V1, V2, V3, the steering wheel angle A.sub.v varies over a range, an upper limit of which is the upper limit steering wheel angle A.sub.vsup.

[0084] In this way, when the driver turns the steering wheel so as to position the steering wheel according to the upper limit steering wheel angle A.sub.vsup, and therefore the rack at the upper limit rack position X.sub.csup, a decrease or an increase in the vehicle speed V1, V2, V3 does not modify the driver feeling. In other words, the steering wheel remains positioned at the upper limit steering wheel angle A.sub.vsup. There is therefore no steering wheel torque-steer, or return effect as in the state of the art. Finally, when there is a loss of grip in an oversteer, it is easy to find a suitable counter-steering steering wheel angle A.sub.v.

[0085] FIG. 2 illustrates a diagram representing the gear ratio VGR as a function of the steering wheel angle A.sub.v according to the invention according to three different vehicle speeds V1, V2, V3.

[0086] FIG. 3 illustrates the rack position X.sub.c as a function of the steering wheel angle A.sub.v for each of the three vehicle speeds V1, V2, V3.

[0087] For each vehicle speed V1, V2, V3, the limit variation of the gear ratio VGR was determined as well as the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3. In FIG. 2, the grip steering wheel angle for the speed V1 is not represented because at the speed V1, the vehicle is controllable over the entire range of the steering wheel angle. For the speed V2, the grip steering wheel angle A.sub.vad_V2 is equal to 45° and for the speed V3, the grip steering wheel angle A.sub.vad_V3 is equal to 18°. Before the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3, the variation of the gear ratio VGR is less than or equal to the limit variation. After the grip steering wheel angle A.sub.vad_V2, A.sub.vad_V3, the variation of the gear ratio VGR is selected so that the upper limit rack position X.sub.csup corresponds to the upper limit steering wheel angle A.sub.vsup, whatever the value of the variation of the gear ratio VGR.

[0088] Of course, the invention is not limited to the embodiments described and shown in the appended figures. Modifications remain possible, in particular from the point of view of the constitution of the various elements or by substitution of technical equivalents, without departing from the scope of protection of the invention.