Method for operating a steering system of a vehicle and steering system

Abstract

The invention relates to a method for operating a steering system of a vehicle having a variable steering gear ratio, in which the following steps are carried out: determining an actual radius of curvature of the vehicle as a function of a currently set vehicle steering angle; determining a target curvature radius of the vehicle according to the target trajectory; determining a driving envelope on the basis of an offset that is constant over the target radius of curvature or is dependent on the driving condition; comparing whether the actual radius of curvature is within or outside the driving envelope; wherein a more direct steering gear ratio is achieved when the actual radius of curvature lies outside the driving envelope than when the actual radius of curvature lies within the driving envelope.

Description

TECHNICAL FIELD

[0001] The invention relates to a method for operating a steering system of a vehicle according to claim 1 and to a steering system according to the dependent claim 7. For the state of the art reference is made to DE 103 23 975 Al. BACKGROUND

[0002] Variable steering transmission ratios of vehicle steering systems are known in the art. Variable steering transmission ratio is understood as the ratio between the angle of rotation of the steering wheel and the angle of pivoting of the steered wheels.

[0003] It is known here to change the steering transmission ratio depending on the driving situation. Thus, at high speeds the transmission ratio can be designed indirectly in order to control the vehicle more precisely. In turn, at low speeds and in situations in which a handover on the steering wheel is often required due to the high steering lock, the steering transmission ratio can be designed more directly. Thus, for example, it is possible for the driver to achieve a parking maneuver or a tight turn without a handover. Such a system is known, for example, from DE 103 23 975 Al.

[0004] A disadvantage in fast, situation-dependent steering transmission ratios is that the driver must always adapt or rather adjust again. As a result, the vehicle is perceived by the driver as very nervous. The precision in guiding the vehicle is also affected by the fast changing steering transmission ratio in such a way that, for example, the steering is too much when the steering transmission is applied more directly than usual. SUMMARY

[0005] It is therefore the task of the invention to specify a method for operating a steering system with a variable steering transmission ratio, which eliminates the above-mentioned disadvantages.

[0006] The solution to the stated task is given by a method for operating a steering system of a vehicle with a variable steering transmission ratio having the features of claim 1 and by a steering system for carrying out the method according to claim 9. Advantageous design solutions and further improvements are the content of the dependent claims.

[0007] A method for operating a steering system of a vehicle with a variable steering transmission ratio is proposed.

[0008] The method is carried out in a plurality of steps, but these steps do not necessarily have to be carried out in the order stated below and are also not necessarily carried out sequentially.

[0009] Here, the actual radius of curvature of the vehicle is determined from the currently set vehicle steering angle. The vehicle steering angle is preferably one or more axle steering angles, in particular the front axle steering angle (in the case of front wheel steering). If the vehicle additionally comprises a rear axle steering, the effective overall steering angle is taken into account. The actual radius of curvature here describes the radius of curvature at which the vehicle will now travel at the current vehicle steering angle.

[0010] In a further step, a target radius of curvature of the vehicle is determined from the target trajectory.

[0011] The target trajectory of the vehicle is known in a plurality of assistance functions or is continuously calculated adaptively up to a certain time horizon and event horizon. It is therefore particularly preferred to provide, for each section of the driving trajectory, i.e. at each time instant, an explicit target radius of curvature calculated from the data of one or more driver assistance systems and thus a corresponding matching turning angle of the wheels. In particular, data from environment-based driver assistance systems, such as a cornering assistant, a lane change assistant, an emergency brake assistant or an evasion assistant, can be used in determining the target trajectory.

[0012] Data from the environment detection devices of the vehicle, such as radar or lidar, can also be used in determining the target trajectory.

[0013] It can furthermore be provided that a driving envelope is determined on the basis of a constant or driving condition-dependent offset from the target radius of curvature. Here, such a driving envelope relates to the feasible or practical region of the radius of curvature which is due to different feasible target trajectories or tolerances within the target trajectory.

[0014] Here, the driving envelope is designed in particular such that, viewed along the target trajectory, it widens with increasing prediction horizon.

[0015] Thus, only a small tolerance from the target radius of curvature is allowed at the motor vehicle itself, while this tolerance becomes increasingly wider with increasing prediction horizon of the motor vehicle.

[0016] Here, the offset forming the driving envelope can be chosen to be constant or driving condition-dependent. In the case of a driving condition-dependent offset, for example the vehicle speed can be a factor.

[0017] The driving envelope is thus a region in which the vehicle has reached a "target corridor" with respect to the target trajectory to this extent. Precise steering within the driving envelope is therefore preferred.

[0018] In a further method step, it is compared whether the actual radius of curvature lies within the driving envelope or outside it.

[0019] A more direct steering transmission ratio is achieved if the actual radius of curvature lies outside the driving envelope than if it lies within the driving envelope.

[0020] A reduction in the steering transmission ratio means that the steering becomes more direct, that is to say that, at a reduced steering transmission ratio, a steering movement on the steering wheel is converted into a greater turning angle on the wheels. Correspondingly, an increase in the steering transmission ratio means that the steering becomes more indirect, that is to say that, at an increased steering transmission ratio, a turning movement on the steering wheel is converted into a smaller turning angle on the wheels.

[0021] The steering transmission ratio is therefore set to be more direct if it is determined that the actual radius of curvature lies outside the driving envelope and the vehicle is therefore (also) not in the target corridor, that is to say that a steering movement on the steering wheel is converted into a greater turning angle on the wheels than if the actual radius of curvature lies within the driving envelope. Once the vehicle is in the target corridor or the driving envelope, more precise steering is desired and the steering transmission ratio becomes more indirect.

[0022] In this way, the steering characteristic can be designed more directly in accordance with the target trajectory and at the same time the driving precision is improved by the target-precise lane guidance.

[0023] Here, the recalculation of the target trajectory on the basis of changes in the vehicle motion and the environment is preferably carried out continuously in each calculation step with trajectory planning. Thus, the driver is again enabled to steer directly onto the new trajectory.

[0024] The steps of the method are preferably coupled directly to one another continuously. The method is therefore preferably carried out in real time. Thus, a new target trajectory is constantly or continuously calculated, the comparison between the target radius of curvature and the actual radius of curvature is carried out and the steering transmission ratio is adjusted accordingly.

[0025] It is preferably provided that, when the actual radius of curvature reaches the driving envelope, the steering transmission ratio is gradually adjusted to be more indirect. The driver is then slowly adjusted to the changing steering transmission ratio. The driver is therefore not surprised by the changing steering transmission ratio.

[0026] When the actual radius of curvature again leaves the driving envelope, for example when a re-centering has to be carried out, it is provided that the steering transmission ratio is readjusted to be more direct.

[0027] It is particularly preferred that the steering transmission ratio is not adjusted to be more direct abruptly, but gradually, so that the transition cannot be felt suddenly by the driver.

[0028] However, preferably, the more direct steering transmission ratio is again achieved only when leaving the travel envelope when the driver's steering movement is directed to the target trajectory or when the (preferably continuously newly determined) target trajectory leaves the current working point movement. This means that the direct steering transmission ratio is again set only when the target trajectory should be traveled with high probability or the travel envelope should be reached. If the vehicle deviates from the travel envelope (according to the driver's wishes), thus not always adhering to the target trajectory (for whatever reason), it is preferred to remain in the indirect steering transmission ratio. This further enables the driver to make more precise steering.

[0029] Additionally possible is an adjustment of the steering transmission ratio again depending on the vehicle speed.

[0030] Further proposed is a steering system of a vehicle with variable steering transmission ratio for implementing the method according to any one of claims 1 to 8.

[0031] Here, the steering system is preferably a so-called steer-by-wire steering system.

[0032] The solution proposed here offers many advantages. During an evasive maneuver, for example, the vehicle can react significantly more quickly on the target trajectory of the evasive aid due to the much more direct steering transmission ratio. Nevertheless, precise evasive maneuvers are possible because the steering transmission ratio is again adjusted to be significantly more indirect when the target trajectory (or target corridor) is reached by the solution. Thus, even in dynamic driving situations, the driver is supported in following the target trajectory with high precision.

[0033] These and other features, in addition to what is apparent in this specification, are derived from the figures, each of which can be taken separately or in multiple combinations in subcombinations in embodiments of the present application and can be advantageous and protectable embodiments in their own right, to which protection is hereby claimed. BRIEF DESCRIPTION OF DRAWINGS

[0034] The application is further explained below with the aid of an embodiment.

[0035] Here, Figure 1 An embodiment of the application is shown in which a vehicle carries out a turning process. Here, all the features described in detail can be important for the application.

[0036] In Figure 2 An exemplary characteristic curve for the steering transmission ratio in relation to the current vehicle steering angle is shown in DETAILED DESCRIPTION

[0037] As can be seen in Figure 1 The vehicle is in a typical traffic scenario of a turning process, as can be seen in

[0038] The target trajectory for an intended right turn is calculated in real time and communicated to the steering system. The target curvature radius K Soll Can be determined or considered by presetting the target trajectory by one or more driver assistance systems, such as an evasion assistant.

[0039] The driving envelope is formed based on the target curvature radius K Soll with a driving situation-dependent offset K Soll ± ΔK Off .

[0040] Since the current actual curvature radius K Ist of the vehicle lies outside the driving envelope, the turn-in to the calculated target steering angle can be effected with a more direct steering transmission ratio. Ideally, the transmission ratio is applied at this point such that the driver does not have to change hands until the steering angle is reached. In further course, i.e. when the actual curvature radius K Ist falls within the driving envelope, the vehicle behavior is ensured not to be perceived as nervous and the steering is not overstepped in these situations by a more indirect steering transmission ratio.

[0041] After driving through the bend, the target trajectory is changed again by the driver assistance towards the direction of "straight ahead". The steering movement to this new working point is assisted in the return by a more direct transmission ratio at the beginning (because the actual curvature radius K Ist is outside the driving envelope) and is changed to a less direct or indirect steering behavior (because the actual curvature radius K Ist is again within the driving envelope) when the straight-ahead position is reached.

[0042] In the case of a steer-by-wire steering system, the synchronicity of the two independent steer-by-wire systems "steering wheel" and "steering mechanism" must be ensured by the adaptability of the steering transmission ratio in the return.

[0043] An exemplary characteristic curve for the steering transmission ratio as a function of the current driver steering angle δ L is shown in Figure 2 . The exemplary steering transmission ratio characteristic curve corresponding to the front axle steering angle range δ VA,soll（t） ± Δδ VA corresponding to the target curvature radius is shown in the case of maintaining the applied steering transmission ratio after reaching the target value in the working point AP.

[0044] As can be seen here, a direct steering transmission ratio is given as long as the current driver steering angle δ L does not fall within the front axle steering angle range δ VA,soll（t） ± Δδ VA . As soon as the current driver steering angle δ Lfalling within the range of the front axle steering angle δ VA,soll（t） ± Δδ VA If the steering angle δ is within the range of the front axle steering angle δ In this specification, the steering transmission ratio is gradually adjusted to an indirect transmission ratio which has its minimum value in the working point AP.

[0045] The direct characteristic is only adjusted again when the trajectory calculation motion moves away from the current working point AP. The recalculation of the working point AP is continuously carried out with the trajectory planning in each calculation step on the basis of changes in the vehicle motion and the environment. Thus, the driver is again able to steer directly onto the new trajectory.

Claims

1. Method for operating a steering system of a vehicle having a variable steering transmission ratio, wherein The following steps are performed: - determining the actual radius of curvature (K Ist ) of the vehicle from the current set steering angle (δ L ) of the vehicle, - determining a target curvature radius (K Soll ) of the vehicle from the target trajectory, - determining a driving envelope based on a constant or driving condition dependent offset (K Soll ±ΔK Off ) on the target radius of curvature, - comparing said actual radius of curvature (K Ist ) is located inside or outside said driving envelope, - wherein a more direct steering transmission ratio is achieved when the actual radius of curvature (K Ist ) lies outside the driving envelope than when the actual radius of curvature (K Ist ) lies within the driving envelope.

2. The method of claim 1, wherein, The steps are repeated continuously.

3. The method of claim 1 or 2, wherein, Viewed along the target trajectory, the driving envelope widens with increasing predicted field of view of the vehicle.

4. The method according to any of the preceding claims, wherein, At the actual radius of curvature (K Ist ) reaches the driving envelope, the steering ratio is gradually adjusted to be more indirect.

5. The method according to any of the preceding claims, wherein, at the actual radius of curvature (K Ist ) leaving the driving envelope, the steering ratio is gradually adjusted to be more direct.

6. The method according to any one of the preceding claims, wherein, at the actual radius of curvature (K Ist ) leaving the driving envelope, the steering ratio is again adjusted to be more direct only when the driver's steering motion is directed to the target trajectory.

7. The method according to any of the preceding claims, wherein, The steering transmission ratio is adjusted in relation to the speed.

8. The method of any of the preceding claims, wherein, The target trajectory is determined from data of at least one driver assistance system of the vehicle.

9. Steering system of a vehicle with variable steering transmission ratio, the steering system being configured to implement a method according to any one of claims 1 to 8.

10. The steering system of claim 9, wherein, The steering system is a steer-by-wire steering system. The steering system is a steer-by-wire steering system.

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