METHOD FOR OPERATING A POWER STEERING SYSTEM OF A VEHICLE, POWER STEERING SYSTEM AND VEHICLE

Abstract

A method for operating a power steering system (12) of a vehicle (10) comprises the following steps: a) preparation and/or calculation of a characteristic curve (a), which indicates a power assistance (F.sub.1) to be applied by the power steering system (12) as a function of a driver manual torque (M), b) determination of an initial steering angle s (x.sub.1) of a steering wheel (24), c) determination of a disturbing force, which acts on a steering train (22) of the vehicle (10) due to external forces, d) compensation for the disturbing force by means of an applied compensating force (F.sub.3) in such a way that a steering rack force (F.sub.2) at the initial steering angle (x.sub.1) assumes an initial value, in particular the initial value zero, and e) application of a synthesized steering rack force (F.sub.4) to produce a desired driving feel.

A power steering system and a vehicle comprising such a power steering system are furthermore described.

Claims

1. A method for operating a power steering system (12) of a vehicle (10), comprising the following steps: determination of an initial steering angle s (x.sub.1) of a steering wheel (24), determination a disturbing force acting on a steering train (22) of the vehicle (10) due to external forces, compensation for the disturbing force by means of an applied compensating force (F.sub.3) in such a way that a steering rack force (F.sub.2) at the initial steering angle (x.sub.1) assumes an initial value, in particular the initial value zero, and application a synthesized steering rack force (F.sub.4) to produce a desired driving feel.

2. The method as claimed in claim 1, wherein the magnitude of the applied synthesized steering rack force (F.sub.4) varies around the initial steering angle (x.sub.1) and is selected in such a way as to produce a desired steering resistance characteristic, in particular wherein the desired characteristic of the synthesized steering rack force (F.sub.4) corresponds to the disturbing force characteristic to be expected around a neutral steering angle (x.sub.0), wherein the neutral steering angle (x.sub.0) corresponds to the neutral position of the steering wheel (24).

3. The method as claimed in claim 1, wherein the method additionally comprises the following steps: preparation and/or calculation of a characteristic curve (a), which indicates a power assistance (F.sub.1) to be applied by the power steering system (12) as a function of a driver manual torque (M), measurement of the driver manual torque (M) applied, determination of the power assistance (F.sub.1) by means of the characteristic curve (a) on the basis of the driver manual torque (M) applied, and application of the power assistance (F.sub.1) to the steering train (22) of the vehicle (10).

4. The method as claimed in claim 1, wherein the disturbing force is compensated for in such a way that the sum of the disturbing force and the compensating force (F.sub.3) assumes the initial value for all steering angles (x).

5. The method as claimed in claim 1, wherein the characteristic curve (a) is prepared and/or calculated on the basis of the current driving maneuver, in particular on the basis of the speed of the vehicle (10).

6. The method as claimed in claim 1, wherein the power assistance (F.sub.1), the compensating force (F.sub.3) and/or the synthesized steering rack force (F.sub.4) is exerted on the steering rack (20) by means of an actuator (18).

7. The method as claimed in claim 1, wherein the disturbing force results, at least in part, from interference to the lateral control of the vehicle (10).

8. The method as claimed in claim 1, wherein the compensating force (F.sub.3) is selected so that at least elements of the disturbing force are at least partially and in particular fully compensated for and/or elements of the disturbing force elements are amplified.

9. The method as claimed in claim 1, wherein the disturbing force is determined by means of a disturbance variable monitor, which in particular comprises sensor, and/or by means of a reference value.

10. The method as claimed in claim 1, wherein the vehicle (10) comprises a steering angle control module (17), which sets the initial steering angle (x.sub.1).

11. The method as claimed in claim 10, wherein the steering angle control module (17) determines an assistance force (F.sub.5), which is likewise applied to the steering train (22) of the vehicle (10) and which corresponds to a steering lock from the neutral position to the initial steering angle (x.sub.1).

12. The method as claimed in claim 9, wherein the steering angle control module (17) is capable of exerting the assistance force (F.sub.5) on the steering train (22) by means of an actuator (18), in particular wherein the assistance force (F.sub.5) and the power assistance (F.sub.1) are exerted by the same actuator.

13. The method as claimed claim 10, wherein the steering angle control module (17) comprises a closed-loop with no integral component.

14. A power steering system having a control module (14), a steering angle sensor (16) and an actuator (18) for generating a power assistance (F.sub.1), a compensating force (F.sub.3) and/or a synthesized steering rack force (F.sub.4), wherein the power steering system (12) is designed to perform the method as claimed in claim 1.

15. The power steering system as claimed in claim 13, wherein the control module (14) comprises a driver manual torque module (26) for determining the driver manual torque (M) applied and/or a power assistance module (28) for determining the power assistance (F.sub.1), the compensating force (F.sub.3) and/or the synthesized steering rack force (F.sub.4).

16. The power steering system as claimed in claim 14, wherein the power steering system (12) comprises a steering angle control module (17).

17. A vehicle comprising a power steering system (12) as claimed in claim 14.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Further features and characteristics of the invention emerge from the following description and from the drawings, in which:

[0043] FIG. 1 shows a motor vehicle according to the invention having a power steering system according to the invention,

[0044] FIG. 2 shows diagram of the power assistance generated by an EPS as a function of the driver manual torque,

[0045] FIG. 3 shows a diagram of the steering rack force characteristic as a function of the steering angle,

[0046] FIG. 4 shows a flow chart of the method according to the invention,

[0047] FIG. 5 shows a block diagram of the contributors to the force applied by the power steering system in FIG. 1, and

[0048] FIG. 6 shows a block diagram of the control structure of the power steering system in FIG. 1.

DESCRIPTION

[0049] FIG. 1 schematically shows a vehicle 10 according to the invention having a power steering system 12 according to the invention.

[0050] The power steering system 12 comprises a control module 14, a steering angle sensor 16, a steering angle control module 17 and an actuator 18 for generating a force F.

[0051] The vehicle 10 comprises a steering rack 20, which is connected to a steering train 22. The steering train 22 can be operated by a driver (not shown) via a steering wheel 24.

[0052] The steering angle sensor 16 monitors the position of the steering wheel 24 and is therefore able to determine a steering angle x of the steering wheel 24.

[0053] The actuator 18 is, for example, an electric motor and can therefore act on the steering train 22, in order to steer the vehicle 10. To put it another way, the actuator 18, by means of the force F applied, is able to produce a movement, in particular a rotational movement, of the steering train 22.

[0054] The control module 14 further comprises a driver manual torque module 26 which can be used to determine the driver manual torque M applied by the driver to the steering wheel 24.

[0055] The control module 14 further comprises a power assistance module 28 that serves to determine a power assistance F.sub.1 which in a steering movement is applied to the steering train 22 by means of the actuator 18.

[0056] In basic operation of the vehicle 10 the power steering system 12 provides a power assistance F.sub.1, which varies as a function of the driver manual torque M according to the characteristic curve a represented in FIG. 2. The power assistance F.sub.1 is in this case equal to the total force F applied by the power assistance system 12.

[0057] Conventional EPS-systems provide the driver with power assistance F.sub.1 that increases symmetrically around a zero point, an ever more rapid increase in the power assistance F.sub.1 being observable with increasing driver manual torque M, as shown in FIG. 2.

[0058] The zero point of the characteristic curve a usually coincides with a neutral position of the steering wheel 24. The driver of the vehicle 10 therefore receives the same power assistance F.sub.1 for the same driver manual torque M irrespective of the steering direction

[0059] In FIG. 3 the steering rack force F.sub.2 of the steering rack 20 is represented as a function of the steering angle x of the steering wheel 24. It can be seen that in the neutral position of the steering wheel 24, which corresponds to a neutral steering angle x.sub.0, the steering rack force F.sub.2 is minimal and increases symmetrically around the steering angle x.sub.0 as a function of the steering angle x, as is illustrated by the characteristic line b.

[0060] The characteristic line b accordingly describes the characteristic of the steering rack force F.sub.2 around the neutral steering angle x.sub.0, which is a measure of the steering resistance around the neutral steering angle x.sub.0.

[0061] The driving feel experienced by the driver of the vehicle 10 results from the interplay between the driver manual torque M applied, and hence also the power assistance F.sub.1, and the steering rack force F.sub.2.

[0062] The method according to the invention for operating the power steering system 12 of the vehicle 10 serves to transfer this driving feel to any steering angle x, for example to the initial steering angle x.sub.1 represented in FIG. 3.

[0063] FIG. 4 shows a flow chart of the method according to the invention, which is explained in more detail below.

[0064] In a first mode of the power steering system 12 it is to be operated so that the driving feel experience by the driver is independent of disturbing forces acting on the vehicle 10.

[0065] For this purpose, an initial steering angle x.sub.1 of the steering wheel 24 is first determined and an initial value is defined for the steering rack force F.sub.2 which the driver is intended to experience at the initial steering angle x.sub.1 (step S1). The initial steering angle x.sub.1 is, for example, the current steering angle and is determined, for example, by means of the steering angle sensor 16.

[0066] A disturbing force which acts on the steering train 22 of the vehicle 10 due to external forces is then determined (step S2). The disturbing force results, at least in part, from interference to the lateral control of the vehicle 10, for example by steep road surfaces, cross winds and/or uneven weight distributions of the vehicle 10. The characteristic of the steering rack force F.sub.2 is substantially predetermined by the disturbing force.

[0067] A disturbance variable monitor 29 (cf. FIG. 6) and/or a reference value may be used for determining the disturbing force. The reference value may likewise be vehicle-based and/or vary as a function of the speed.

[0068] Now a compensating force F.sub.3 is determined, which is intended to offset the disturbing force to the initial value. To put it another way, the compensating force is selected so that the sum of the disturbing force and the compensating force produces the initial value, in particular for all steering angles.

[0069] The compensating force is then applied, and the calculated disturbing force is thus at least partially and in particular fully compensated for (step S3). Here the steering rack force F.sub.2, at least at the initial steering angle x.sub.1, but in particular for all steering angles, assumes a fixed initial value, which in the embodiment shown is equal to zero.

[0070] In principle, however, any other desired initial value could also be selected. This is of particular interest if specific contributions to the disturbance variable are even to be amplified, for example in order to provide the driver of the vehicle 10 with information on the road surface condition.

[0071] In addition, a synthesized steering rack force F.sub.4 is applied to the steering train 22 of the vehicle 10, for example by the actuator 18 (step S4), so as to produce a desired characteristic of the steering rack force F.sub.2 corresponding to the curve c around the initial steering angle x.sub.1 (cf. FIG. 3). The characteristic of the curve c around the initial steering angle x.sub.1 here corresponds to the expected characteristic of the curve b around the neutral steering angle x.sub.0, as had resulted in step S2 before applying the compensating force F.sub.3—that is to say the expected characteristic of the disturbing force. A desired steering resistance characteristic around the initial steering angle x.sub.1 is therefore generated via the synthesized steering rack force F.sub.4.

[0072] It is also feasible for the desired characteristic produced by the synthesized steering rack force F.sub.4 to deviate from the expected characteristic of the disturbing force, in order to tailor the driving feel and/or the steering resistance.

[0073] Here the desired characteristic may always be based on the characteristic of the disturbing force, since the driver expects a steering resistance or a driving feel that does not deviate too much from the disturbing force.

[0074] The magnitude of the synthesized steering rack force F.sub.4 may alternatively or additionally be selected on the basis of driving parameters, such as the vehicle speed, the lateral acceleration etc.

[0075] The magnitude of the compensating force F.sub.3 and/or the synthesized steering rack force F.sub.4 to be applied may likewise be determined by the power assistance module 28. Alternatively, a separate module could be provided for determining the compensating force F.sub.3 and/or the synthesized steering rack force F.sub.4.

[0076] The power assistance F.sub.1 to be applied by the power steering system 12 may optionally be calculated as a function of the driver manual torque M. For this purpose, a characteristic curve, for example the characteristic curve a in FIG. 2, is first prepared and/or calculated (step S5).

[0077] The characteristic curve may be prepared and/or calculated, for example, as a function of the vehicle model, the current driving maneuver and/or the current speed of the vehicle 10.

[0078] The use of a characteristic curve as in FIG. 2, i.e. a characteristic curve of a conventional EPS, presents itself, since drivers of modern vehicles nowadays expect a power assistance which conforms to this characteristic curve.

[0079] Then, the driver manual torque M applied is additionally measured, in order to determine the steering intention of the driver (step S6). This is done, for example, by the driver manual torque module 26 of the control module 14.

[0080] The driver manual torque M determined is used as a basis for determining, via the characteristic curve a and after compensation for all disturbing variables and application of the synthesized steering rack force, the power assistance F.sub.1 needed in order to give the driver the same driving feel around the initial steering angle x.sub.1 as he would expect around the neutral position of the steering wheel 24 in the previously explained basic operation of the power steering system 12 (step S7).

[0081] Finally, the power assistance F.sub.1 determined is applied to the steering train 22 of the vehicle 10, for example via the actuator 18 (step S8).

[0082] The driver in his steering movement is therefore supported by the power assistance F.sub.1 in precisely the way he would expect, that is to say on the basis of the characteristic curve a in FIG. 2. This represents a considerable difference compared to the prior art, in which no compensating force F.sub.3 and no synthesized steering rack force F.sub.4 is applied. In the prior art the driver must himself apply a corresponding steering torque to compensate for the disturbing force, in order to maintain his course. If he now performs a steering movement to change course, the power assistance F.sub.1 would not be calculated from the origin of the characteristic curve a, however, but from the steering torque already applied. This leads to an asymmetrical power assistance F.sub.1, which the driver is not used to and/or does not expect.

[0083] The initial steering angle x.sub.1, particularly in this first mode of the power steering system 12, may also correspond to the steering angle x.sub.0. In this case the method according to the invention serves primarily to compensate for the disturbing forces acting on the vehicle 10, since the contribution of the synthesized steering rack force F.sub.4 in this case may be zero.

[0084] However, with the aid of the steering angle control module 17 the power steering system 12 according to the invention can also be operated in a second mode.

[0085] In the second mode the steering angle control module 17 sets the initial steering angle x.sub.1. For example, the steering angle control module 17 is designed as a driver assistance system, in particular as a lane-keeping assist system and/or an autonomous or semi-autonomous driving assistance system.

[0086] Accordingly, in this case the driver of the vehicle 10 may happen to intervene in a steering wheel 24 deflected from the neutral position. In this case the driver, immediately on grasping the steering wheel 24, will obtain the driving feel which he would expect in the neutral position of the steering wheel 24.

[0087] For this purpose, the steering angle control module 17 calculates an assistance force F.sub.5, which corresponds to a steering lock from the neutral position to the initial steering angle x.sub.1. This force is applied to the steering train 22 of the vehicle even prior to step S1 or in step S8 in addition to the power assistance F.sub.1.

[0088] The different proportions of the total force F applied to the steering train 22 applied are illustrated schematically in FIG. 5, in which the contributions in basic operation, in the first mode and in the second mode are listed from bottom to top.

[0089] FIG. 6 schematically represents the interaction of the various components of the vehicle 10 and the power steering system 12.

[0090] The steering train 22 is connected to the steering rack 20.

[0091] Steering rack forces F2 act on the steering rack 20. In addition, disturbing forces act on the steering rack 20 and the steering train 22, in particular disturbing forces acting on the lateral control of the vehicle 10, that is, for example, in the same direction as the steering rack force F.sub.2 indicated by an arrow in FIG. 6.

[0092] Fitted to the steering train 22 is the steering wheel 24, the steering angle x of which can be determined by a steering angle sensor 16. The currently prevailing steering angle x is transmitted as initial steering angle x.sub.1 to the power steering system 12.

[0093] In addition, the driver manual torque M applied to the steering wheel 24 is determined by means of a driver manual torque sensor 30 and likewise relayed to the power steering system 12.

[0094] In addition, further parameters P to be taken into account can be transmitted to the power steering system 12, for example the current speed of the vehicle 10.

[0095] The power steering system 12, in accordance with the method described above, calculates a force F which may comprise both the power assistance F.sub.1 and also—depending on the mode in which the power steering system 12 is being operated—contributions by the compensating force F.sub.3, the synthesized steering rack force F.sub.4 and the assistance force F.sub.5.

[0096] The actuator 18 may apply the force F to the steering train 22.

[0097] In the embodiment shown in FIG. 6 an additional second steering angle sensor 32 is provided, which serves to monitor the steering angle x after application of the force F by the actuator 18.