Method for operating a steer-by-wire steering system

Abstract

The invention relates to a method for operating a steer-by-wire steering system, wherein the steering request of a vehicle occupant by way of the actuating of a steering handle is detected and is set on a toothed rack of a steering transmission as an assigned toothed rack position, comprising the following steps: detecting an axial load on the toothed rack due to a different force ratio between a left and a right wheel of an axle of the vehicle, wherein the toothed rack is moved based on this load.

Description

[0001] Method for operating a steer-by-wire steering system

[0002] The invention relates to a method for operating a steer-by-wire steering system according to the preamble of claim 1. For the prior art, reference is made, for example, to DE 10 2014 211 815 A1.

[0003] So-called steer-by-wire steering systems for vehicles are already known in various forms from the state of the art. One such steer-by-wire steering system is described, for example, in DE 10 2014 211 815 A1.

[0004] This differs from conventional steering systems in that the steering of the wheels is completely decoupled from the driver's mechanical steering movement and takes place solely through a purely electronic transmission. The conventional mechanical transmission devices are omitted; instead, the driver generates data through their steering movement on the steering handle or steering wheel, which is fed into an electronic control unit. This control module evaluates the data and converts it into appropriate steering commands. This controls the steering gear, which executes the desired steering movement by moving the rack and pinion. If there is no such active control of the steering gear, the rack and pinion also moves - i.e. the rack remains stationary under all other circumstances.

[0005] However, this has the negative consequence that the load spectrum on the components of a steer-by-wire steering gear increases over the vehicle's lifetime due to road surface loads. Therefore, a steer-by-wire system must be dimensioned more robustly to meet the increased requirements, which results in additional costs, weight, and space requirements in the vehicle.

[0006] It is therefore the object of the invention to solve the above-mentioned problem.

[0007] The solution to the problem arises from a method having the features of claim 1 and from a steer-by-wire steering system having the features of the independent claim 5. Advantageous developments and further developments are the subject of the dependent claims.

[0008] To implement the method, a steer-by-wire steering system is proposed. As already explained in the introductory section, the steering input of a vehicle occupant is detected by actuating a steering handle and is adjusted on a rack of a steering gear as the assigned rack position. The steering of the wheels is completely mechanically decoupled from the driver's steering movement.

[0009] The aim is that with a steer-by-wire steering gear (due to the rack position control) no higher axial load peaks occur on the rack than with a conventional (mechanically coupled) steering gear without steer-by-wire.

[0010] It is therefore intended that an axial load or force on the rack is detected due to a different force ratio between a left and right wheel of a common axle of the vehicle.

[0011] Such a different force ratio between a left and right wheel of the vehicle occurs, for example, when the vehicle suddenly drives up a curb with one wheel or when the vehicle is driving over an uneven road surface. This results in different forces acting on the left and right wheels. Such a different force ratio at the two wheels of a common axle (on which the steering rack is also located) in turn leads to a load on the steering gear. The steering gear therefore absorbs such a load, particularly in the axial direction.

[0012] In particular, such load measurement is performed by a so-called rack force estimator. The current rack force or rack load is determined based on measured currents in the steering gear's servo motor and other physical variables.

[0013] Alternatively or additionally, the detection can also be performed using suitable sensors, measurements, calculations, estimates, or the like. Subsequently, a slight rack movement is provided depending on the detected rack load, particularly as a reaction to mitigate load peaks.

[0014] It is further preferably provided that after the load has ended, the rack is moved back to its original position.

[0015] Such rack movement, dependent on the rack load, allows at least partial absorption of the loads on the steering gear, thus relieving its load. This control strategy allows rack impulses—due, for example, to road surface excitations—to be cushioned by a permissible positional deviation of the rack. This eliminates the need to consider increased load collectives in the steering gear dimensioning.

[0016] This rack movement is then preferably so slight that it is preferably not reported back to a vehicle driver (for example at the steering handle).

[0017] It is further preferably provided that the rack movement is carried out by a rack position controller. The controller stiffness is selected such that a position deviation of the rack is permitted when a detected rack load is applied. If such a load is detected, the position controller gives way (within a defined stiffness) and allows a certain deviation in the rack position. In an operating situation without a specified load on the steering gear, the rack position is always fixed. Only when a load is detected does the position controller give way, so that at least part of the forces acting on the steering gear are cushioned by a rack movement. After the load is removed, this rack movement is then preferably corrected back to the original position.

[0018] In an advantageous embodiment of the invention, a rack movement only occurs when the detected load exceeds a threshold value. For example, if the force acting on the steering gear is less than 50 N, no rack movement occurs; however, if the force exceeds 50 N, a rack movement occurs. Furthermore, a steer-by-wire steering system of a vehicle for implementing the method according to one of claims 1 to 4 is proposed.

[0019] The steering system comprises a steering gear with a rack, controlled by a control unit. The movement of the rack essentially implements the driver's steering input. This rack position is then maintained by the rack position controller and only corrected accordingly when the driver's steering input is adjusted. If axial forces or loads act on the rack (especially shocks), the rack position controller allows a defined deviation (for example, after exceeding a threshold value described above) from the target position as a cushioning reaction to the axial shocks. This deviation represents a slight rack movement.

[0020] Furthermore, at least one detection unit is provided for detecting an axial load on the rack due to a different force ratio between a left and a right wheel of a common axle of the vehicle. Such a unit can be, for example, one or more sensor units and / or a control unit for estimating, calculating, or simulating the excitation. In particular, the detection unit is a rack force estimator as described above.

[0021] The rack is designed in such a way that it can perform a slight movement as a result of the detected load.

[0022] Particularly preferably, the steering system comprises a rack position controller for controlling the movement of the rack. The movement of the rack in the event of the aforementioned load on the transmission is then preferably based on a selected stiffness setting of the position controller, which allows for a position deviation of the rack depending on the load.

Claims

Patent claims 1. A method for operating a steer-by-wire steering system, wherein the steering request of a vehicle occupant is detected by actuating a steering handle and is set on a rack of a steering gear as an assigned rack position, comprising the following steps: Detecting an axial load on the rack due to a different force ratio between a left and a right wheel of an axle of the vehicle, characterized in that a rack movement occurs depending on this load.

2. Method according to claim 1, wherein the rack movement is carried out by a rack position controller, the controller stiffness of which is selected such that a position deviation of the rack is permitted when a detected load on the rack is applied.

3. Method according to claim 1 or 2, wherein after the loading has ended the rack is moved back to its original position.

4. Method according to one of the preceding claims, wherein a rack movement only occurs when the detected axial load exceeds a threshold value.

5. Steer-by-wire steering system of a vehicle for carrying out the method according to one of claims 1 to 4, comprising a rack controllable by a control unit, and at least one detection unit which detects an axial load on the steering gear, wherein the rack is configured to execute a slight movement as a result of the detected load.

6. Steer-by-wire steering system according to claim 5, comprising a rack position controller for adjusting the rack, the stiffness of which is selected such that a position deviation of the rack is permitted when a detected axial load on the rack.

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