Method for operating a steer-by-wire steering system of a motor vehicle, and control device, computer program and steer-by-wire steering system

Abstract

A method for operating a motor vehicle steer-by-wire system, wherein at least one steering function requests one or more steering angle changes as theoretical steering angles at at least one vehicle axle, wherein an arbitration unit (AE) weights the steering angle changes to theoretical steering angles (SLw_i) and determines a total theoretical steering angle (G_SLw) after taking into account at least one of the theoretical steering angles (SLw_i). The invention is characterized in that it is continuously checked whether the following condition is met before the total theoretical steering angle (G_SLw) is set by actuators of the steer-by-wire system: the gradient of the total theoretical steering angle (G_SLw) is less than or equal to the gradient of at least one of the theoretical steering angles (SLw_i).

Description

TECHNICAL FIELD

[0001] The present application relates to a method for operating a steer-by-wire system, a control device, a computer program having program code means and a steer-by-wire system. BACKGROUND

[0002] Steer-by-wire systems are used in motor vehicles to steer the wheels on one or more axles. It is important here that, for steer-by-wire systems, there is a mechanical decoupling between the steering input by the driver and the actual steering movement. For example, a steering movement by the driver on the steering wheel is recorded by a sensor system and forwarded to a control device, which evaluates the data. Depending on the vehicle parameters and, if applicable, other prevailing environmental conditions, a control signal is then forwarded to the actuators of the steer-by-wire system, which finally execute the steering movement of the wheels, or in other words, change the steering angle of the wheels.

[0003] For a safe operation, or in other words, for a safe operation of the limbs and life of the vehicle occupants and the vehicle environment, functions are required which meet specific safety designs or safety standards. In the automotive industry requirements are determined for this, which are divided into different levels. For this, there is the so-called automotive safety integrity level, in which the level, i.e. the level, is based on a hazard analysis and risk assessment. For this, the potential hazards of a system are analyzed and the possible faults that can occur in a specific driving situation are divided into different safety requirement levels from A to D, respectively, according to the estimated risk or requirement. Therein, the A level is considered the lowest level and the D level is considered the highest level (ASIL-A to ASIL-D). The vehicle chassis must meet the high to highest safety requirements. The wheels of the motor vehicle are suspended on its chassis. The steering system, which belongs to the chassis, is designed to guide the wheels. In straight driving or in a curve, the wheels must be guided in terms of their wheel steering angle so that they do not change their wheel steering angle due to lateral forces acting on the wheels or only within a low permissible limit, otherwise it is not possible to curve or to maintain the curve radius. This is particularly challenging because, simply put, the wheels are constantly trying to change the wheel steering angle. This is evident, for example, when the steering wheel is released while curving. Then, due to the camber or caster angle at the chassis resulting from the design, the wheels will try to return to the neutral position in the straight-ahead direction. In straight driving, the wheel steering angle is 0 degrees. If the wheels are now steered on at least one axle by a steer-by-wire system, this steer-by-wire system must be designed to guide the wheels. Depending on the driving situation, the steer-by-wire system must be able to perform slow, gentle or fast steering movements even under maximum lateral forces. A minimum steering movement or wheel steering angle change is sufficient to cause the directional behavior of the vehicle, for example when driving long distances quickly at 100 km / h. Conversely, large steering angle changes are required during parking, where high adjustment forces are required due to high wheel support forces and low wheel rotation movements. An average driver has little chance of controlling an erroneous steering movement of the steer-by-wire system, especially at high vehicle speeds. Therefore, the highest safety level according to ASIL-D is predestined for the steering system and also for the steer-by-wire system.

[0004] For a steer-by-wire system, the adjustment signal and the associated steering angle change are usually calculated on the control device of the steer-by-wire system. In absolute terms, the total theoretical steering angle is calculated. On the basis of, for example, the steering angle and the vehicle speed, the calculated adjustment signal is forwarded to the actuators of the steer-by-wire system, which implement the steering movement. The steer-by-wire system itself monitors here whether the maximum steering angle is not exceeded, so that the steering angle is limited and a collision with, for example, chassis components or components of the vehicle body does not occur. This function can be implemented by means of a so-called corridor function. If the steer-by-wire system is combined with other steering functions, for example, of a merging driver assistance system, it must be ensured that steering angle changes generated externally on the steer-by-wire system or its control device, if necessary, are passed through the corridor function. In order to comply with the ASIL-D standard, it must be ruled out in all cases here that the steer-by-wire system obtains a false signal and changes the steering angle. SUMMARY

[0005] The object of the application is to make possible the necessary safe operation of a steer-by-wire system in a vehicle with different steering functions.

[0006] The object is achieved by a method for operating a steer-by-wire system of a motor vehicle. To achieve this object, a control device and a computer program with a program code field and a steer-by-wire system are also claimed.

[0007] According to a first aspect of the application, a method for operating a steer-by-wire steering system of a motor vehicle is presented. In the vehicle, at least one steering function requests one or more steering angle changes on at least one wheel on at least one axle. An arbitration unit obtains the steering angle changes on a signal path, for example by means of a bus, for example a CAN bus, Flexray, etc. In a first step, the arbitration unit weights the requested steering angle changes, which are presented as theoretical steering angles of the respective steering function. These theoretical steering angles are prioritized in the weighting. For example, in the case of an auxiliary function being activated by the driver for parking, i.e. for parking the vehicle in an existing parking space, a requested or preset theoretical steering angle is present. A corridor function, which is present as another steering function, allows the wheels on the front axle and the rear axle to be steered in opposite directions at low speeds, for example below 25 km / h. However, the maximum steering angle is limited, whereby, for example, the wheels do not come into contact with the chassis or wheel arches. A first theoretical angle results from this function. Due to the activated auxiliary function for parking, it can be assumed that a high steering angle is required for parking the vehicle, which should be set by the actuators of the steer-by-wire steering system. Another theoretical steering angle results. The arbitration unit must now assess and allow the steering in opposite directions, on the one hand, while permitting the maximum theoretical steering angle in consideration of the maximum adjustment angle. In consideration of the weighting, the arbitration unit gives one of the theoretical steering angles priority from the current theoretical steering angles and then determines a total theoretical angle from this, which is to be set for the successful execution of the parking process. The arbitration unit performs this action and the associated calculation of the total theoretical steering angle continuously and intermittently, in the aforementioned example, until the parking process is complete and the vehicle is stationary. In other words, the arbitration unit continuously receives different theoretical steering angles and weights them. Here, the respective prioritized theoretical steering angle is constantly switched, since this theoretical steering angle information is sent to the steer-by-wire steering system, more precisely to its control device, and thus to the arbitration unit from different sources, for example the corridor function and the auxiliary function for parking.

[0008] According to the application, it is provided that, before the total theoretical angle is actually set by the actuators of the steer-by-wire steering system, a check is continuously performed to determine whether the following conditions are met:

[0009] The gradient of the total theoretical angle is smaller than or equal to the gradient of the theoretical steering angle.

[0010] The process is understood to mean that the method, more precisely the method steps, are continuously, i.e. without interruption, performed during the operation of the vehicle or the operation of the steer-by-wire steering system. Here, the checks or other steps can be performed at the same or different time intervals, also referred to as intervals.

[0011] The gradient of the total theoretical angle and the gradient of the theoretical steering angle (derivation of the angle with respect to time) here each result in a steering angle speed.

[0012] Despite the most careful selection of components of the steer-by-wire system, for example the control devices or actuators, and the safety-critical design of the signal path from the control device of the vehicle to the control device of the steer-by-wire system and the most careful programming of the control devices, it is theoretically possible that an erroneous calculation of the total theoretical steering angle can occur. This small possibility of an erroneous change in the calculated total theoretical angle should be avoided in order to avoid a dangerous situation. If the above-mentioned condition, i.e. the gradient of the total theoretical angle is greater than the gradient of the theoretical steering angle in the individual steering functions previously determined by the arbitration unit, is not met, the result is set to an error. In this error case, for example, the steering movement of the actuators of the steer-by-wire system can be temporarily frozen, or the actuators move the wheels, for example, to a wheel steering angle of 0 degrees, which corresponds to a straight-ahead driving. In any case, a situation can arise in which even an untrained driver can control safely. In other words, a safe state of the steer-by-wire system is initiated in the event of an error. At this point, it is again mentioned that the high safety level according to ASIL for vehicle steering systems mentioned at the beginning makes such a check meaningful or necessary. The function referred to here as steering function requires at least one theoretical steering angle, even if, for example, the corridor function does not directly participate in the actual steering.

[0013] Preferably, the steering angle change is required depending on the driving situation of the motor vehicle. The corridor function allows a maximum adjustment angle, for example, at low speeds, for example less than or equal to 25 km / h. In this area, it also allows steering in the opposite direction. In addition, an assistance function for parking can also be activated in this area. If the driving situation is observed, the check according to the application can be carried out more purposefully with this.

[0014] Preferably, a safety factor is added to the gradient of the theoretical steering angle before the check. The value of the safety factor is designed to be variable depending on the vehicle speed and can be deposited or stored in one or more characteristic curves in the steer-by-wire system, more precisely in its control device. Preferably, the value of the safety factor decreases with increasing vehicle speed. It is obvious that even a small steering movement, which is significantly below 1 degree, has a greater impact at high driving speeds than at low driving speeds in the walking speed range, for example, when parking. In other words, the negative effects of an erroneously calculated total theoretical angle are much more severe at high speeds than at very low speeds. The safety factor is likewise an angular velocity which also takes into account the adjustment speed of the actuators of the steer-by-wire system.

[0015] In a preferred embodiment, the check is carried out at intervals. Sufficient results can be obtained with intervals of 1 to 20 milliseconds, preferably with intervals of 4 milliseconds. Each check of the gradient is here a fixed gradient comparison in time and is therefore not an unsolicited check. In this way, the check can be carried out purposefully and depending on the driving situation.

[0016] In another preferred embodiment, the arbitration unit additionally performs a gradual alternation of the first theoretical steering angle to at least one further theoretical steering angle, preferably depending on the driving situation of the motor vehicle. The consideration of the gradual alternation of the theoretical steering angle in the calculation of the overall theoretical steering angle means a gradual change. With this, a step should be avoided, so that no sudden steering movement occurs. This can be the case if the difference between the theoretical steering angle of the first steering function and the further steering function, for example an automatic driving with an assistance system, the steering angle of the driver, is so great that without the gradual alternation a sharp and sudden change in the steering angle would occur. It is obvious that in the sense of the driving situation, the vehicle speed also plays a certain role here. Due to the low vehicle speed, the gradual change of the wheel steering angle during a parking process occurs more slowly than, for example, a small change in the wheel steering angle during a passing process by the driver changing the vehicle lane. However, the gradual change does not mean that the change from the actual steering angle to the theoretical steering angle must always be slow. Advantageously, the gradual change is performed depending on, for example, the vehicle speed and the parameters of the respective intentional steering function. Here, the arbitration unit also intervenes in a similar way as in the case of checking the gradient of the overall theoretical angle by comparison with the gradient of the theoretical steering angle. In other words, in addition to changing the steering angle to be set, the arbitration unit likewise performs and continuously checks the gradual alternation.

[0017] For determining the driving situation of the motor vehicle, at least one of the following parameters will preferably be read:

[0018] - steering wheel angle

[0019] - steering wheel angle speed

[0020] - steering wheel angle acceleration

[0021] - vehicle speed

[0022] - yaw rate

[0023] - lateral acceleration

[0024] - longitudinal acceleration

[0025] - wheel steering angle of at least one wheel on the axle to be steered

[0026] The parameter driving mode takes into account the current state in which the vehicle is located. It is thus possible to adopt a manual driving mode in which all steering inputs are determined by the driver. For example, the driver can activate a driving mode for parking (parking assistance system) by means of which the vehicle independently or in other words autonomously drives into a parking space. Such an assistance system can be activated, for example, only up to a speed of 10 km / h. Likewise, the driver can activate an assistance system for fully automatic driving by means of which the vehicle can be driven automatically within the vehicle's capabilities or within a range selected by the driver. When the driver selects the respective driving mode, the interface requests a change in driving mode. It is then checked on the vehicle side whether the mode can be switched.

[0027] The steering wheel angle is detected in a sensoric manner at the steering wheel or steering handle or steering column of the vehicle. The angle of rotation provides information about the wishes of the driver, i.e. his steering intention. The steering wheel angle velocity and the steering wheel angle acceleration can be calculated in a simple manner from the detected steering wheel angle by means of the first or second derivative. By means of these two parameters more detailed conclusions about the steering intention can be drawn, for example sudden steering to avoid etc. The yaw rate provides information about the rate of rotation of the vehicle about its vertical axis. This is detected, for example, by means of a rotation rate sensor in an electronic stability control system (ESC). The parameters lateral acceleration and longitudinal acceleration are also of great benefit for the analysis of the driving situation. The longitudinal acceleration provides information about which acceleration acts on the vehicle, for example when braking (negative acceleration) or when accelerating by giving gas (positive acceleration in the direction of travel). A lateral acceleration occurs in the vehicle, for example, when switching the driving direction or after entering a curve when turning. By means of the latter parameter conclusions can also be drawn, for example, about the lateral forces and side forces acting on the chassis or vehicle. In combination with information about the friction coefficient conclusions can be drawn about the side slip angle or, if necessary, about understeering or oversteering.

[0028] The axle to be steered is the axle steered by the steer-by-wire system. It is therefore advantageous to know the current wheel steering angle of the respective steered wheel on the axle to be steered before changing the wheel steering angle. The wheel steering angle of the wheels of the motor vehicle provides information, for example, about whether the vehicle is driving straight or is in a turn. In other words, the trajectory of the vehicle can be determined by reading the position of the wheels from the wheel steering angle.

[0029] The list of parameters listed is not exhaustive. Other parameters, for example the determination of the road state using suitable, in particular optical sensors or cloud-based data or data from a navigation device, are conceivable, as are other parameters not mentioned here, which can advantageously be used to determine the driving situation.

[0030] Preferably, in order to determine the driving situation of the motor vehicle, signals from at least one control device, in particular not belonging to the steer-by-wire system, are read by the arbitration unit and taken into account. Therein, the signals are read at intervals, in particular in the range of 5 to 40 milliseconds, preferably at intervals of 10 milliseconds. Thus, the steer-by-wire system preferably has its own control device. The above-mentioned steering functions can be mapped, for example, to one or more control devices. These control devices can forward the desired steering angle to the arbitration unit via a bus system present in the vehicle, for example a CAN bus or a Flexray system. The arbitration unit itself can be implemented, for example, on the control device of the steer-by-wire system.

[0031] In a preferred embodiment, the values determined from the check gradient comparison are debounced in the arbitration unit, in particular before an actual error is derived in the calculation of the total desired steering angle. In order to obtain reliable values and to exclude possible mechanical and / or electrical disturbances, the result of the gradient comparison is compared several times. If the error persists for a certain time, the error is set and the above-mentioned measures are initiated. If the interval for checking the gradient comparison is performed at an interval of 4 ms and, for example, the consistency of 5 checks is compared to each other, the verification of the too large total desired steering angle can be performed within a very short time of 20 milliseconds. The high safety requirements for a steer-by-wire system as a steering system are thus met. Other error sources are advantageously eliminated thereby.

[0032] Preferably, the at least one steering function is one of the following exemplary mentioned functions. The mentioned steering functions come from a non-exhaustive list. Other similar functions are also possible to use. Preferably, these functions are present in the motor vehicle and are implemented on one or more control devices, but they are not implemented on the control device of the steer-by-wire system, although these functions are processed by the control device.

[0033] - corridor function

[0034] - parking function

[0035] - fallback function

[0036] - driver assistance function

[0037] - ESC

[0038] A first corridor function for low speeds monitors the maximum adjustment angle of individual wheels of a vehicle axle, for example. Thus, if a large wheel steering angle change occurs while driving slowly and countersteering is permitted, a collision of the wheel with a component of the vehicle body or a component of the chassis can be prevented. A further corridor function for high speeds can mean that the wheel steering angle is limited to a few minutes or less of change depending on the speed. In this case, the corridor function likewise monitors, for example, depending on the driving situation, whether the steering directions at the front axle and the rear axle of the motor vehicle are the same in order to increase the driving stability and agility of the vehicle, for example, when overtaking.

[0039] A parking function is an auxiliary function that can be selected by the driver, such that, for example, after activation of the parking function, the vehicle independently parks in a parking space in the sense of a driver assistance system. This function can interact with the corridor function, for example, at low speeds.

[0040] A fallback function can intervene, for example, when an error is detected, for example, a false signal or a preset theoretical value, for example, an angle change. For example, depending on the speed of the vehicle, for example, below 50 km / h, an ESC signal is expected. If this signal is not present, a signal can be replaced by the fallback function, so that driving can continue within the limit. ESC is a function of an electronic stability program. Here, steering or braking interventions can be carried out on individual wheels in order to maintain the trajectory of the vehicle. This function is activated, for example, only above a vehicle speed of 25 km / h.

[0041] A driver assistance function is a superordinate term. Here, reference is made to driver assistance functions in different stages for automated driving in the sense of automated driving. The driver can activate the automated driving, which allows partial automated driving to complete automated driving at different levels.

[0042] The different functions described above can be activated individually or simultaneously and thus request different changes in the theoretical steering angle at the same time, respectively. Depending on the driving situation, these theoretical steering angles are weighted by an arbitration unit and, after determining the priority, a total theoretical steering angle is determined.

[0043] According to a further aspect of the present application, a control device is used, which is set up for carrying out the method according to one of the preceding embodiments. The control device can be designed, for example, to work in a digital manner and to execute the method thereon by means of a computer program with a program medium. According to a further aspect, the present application relates to a computer program with a program code field in order to carry out the method according to one of the above-mentioned embodiments when the program is stored on a computer, in particular on the aforementioned control device.

[0044] Finally, the present invention relates to a steer-by-wire system having the aforementioned control device. In a preferred embodiment, the steer-by-wire system is designed as a rear axle steering system for a motor vehicle. Therefore, the vehicle may have a mechanically or electrically supported mechanical steering system on the front axle, which receives steering signals from the driver via a steering handle (e.g., a steering wheel). In this case, the rear axle steering system is an additional steering system mechanically decoupled from the front axle steering system, which provides significant advantages when turning in the opposite direction due to a smaller turning circle and greatly improved stopping capability. The rear axle steering system can be used advantageously to improve agility and driving safety by increasing driving stability through turning in the same direction at speeds, for example, above 50 km / h. However, it is not excluded that all axles of the vehicle may be equipped with only one steer-by-wire system. Therefore, the front axle, as a steer-by-wire system, can also be operated using adjustment signals from the control device. The unnecessary mechanical components (steering column for the steering gear transmission of the front axle) can advantageously reduce weight and cost. Attached Figure Description

[0045] The present invention will now be described with reference to the accompanying drawings and preferred embodiments. The drawings show:

[0046] Figure 1 A vehicle with a steer-by-wire system is shown;

[0047] Figure 2 A flowchart of a method according to the prior art is shown; and

[0048] Figure 3 A flowchart of the method according to the present invention is shown. Detailed Implementation

[0049] Figure 1 A vehicle 1 with a steerable front axle 21 and a steerable rear axle 31 is schematically shown. A steering system 4 is provided to steer the wheels 2 at the front axle 21. This steering system can adjust or change the front wheel steering angle δ of the wheels 2 via a steering guide rod 41. v The angle δ v exist Figure 1 The example is marked at the right wheel 2. At the rear axle 31, the steer-by-wire system 5 sets or changes the wheel steering angle δ at the rear wheel 3 via the steering guide rod 51. hIn the illustrated embodiment, the wheels at the front axle steering system 4 and the rear axle steering system 5 are steered in opposite directions, thereby advantageously reducing the turning radius and improving the parking possibility at low speeds, for example below 50 km / h, compared to a vehicle having only one steered axle. The automated driving can also be advantageously improved with two steered axles. The steering angle at the front axle 21 is adjusted here essentially by the driver via the steering wheel 14, wherein the steering angle Lw is detected by a sensor unit and transmitted via a steering angle signal line to the steer-by-wire system 4, which adjusts the steering angle. The sensor unit for transmitting the steering angle Lw is electrically connected to the control device of the steer-by-wire system 5 via a bus system. The bus system is, for example, a CAN bus or a Flexray bus. Via this bus system, the steering function for the driver assistance system automated driving ADAS, the correction function for limiting the maximum steering angle Korr, the parking assistance PAS for automated parking and the electronic stability control system ESC are all electrically connected to the control device SG of the steer-by-wire system of the rear axle. The control device SG is provided with an arbitration unit AE, which continuously performs a check of the gradient dSLw_i / dt of the theoretical steering angle and the gradient dG_SLw / dt of the total theoretical steering angle G_SLw determined by the arbitration unit. To this end, in the control device Figure 1 The control device is drawn in dotted lines in order to indicate that the arbitration unit belongs to the control device SG of the rear steer-by-wire system 5.

[0050] The yaw rate R G measured at the center of gravity S G of the vehicle 1 is acquired by a suitable sensor (rotation rate sensor) at the center of gravity of the vehicle 1 and detected or evaluated by the electronic stability control system ESC. The sensor S is arranged at the outer end of the vehicle 1 and is assigned to a sensor system and serves to identify the vehicle surroundings. This can be a temperature sensor, an optical sensor, for example comprising a camera, or also a laser radar or radar, which are suitable for the optical or distance detection of, for example, the temperature of the road. Thus, weather conditions such as the moisture or temperature on the road can be detected and transmitted to the control device SG. The vehicle follows a trajectory T, which is also shown schematically in the figure at the front end of the vehicle in its driving direction.

[0051] Figure 2The diagram illustrates the signal path from the steering handle or steering wheel 14 to the actual change in steering angle, or the setting of the total theoretical steering angle G_SLw, according to the prior art STEER-BY-WIRE-LENKUNG steer-by-wire system. The steering angle Lw is detected by a sensing unit due to the driver's steering movement at the steering wheel 14. On the signal path, the theoretical steering angle SLw is forwarded via a vehicle bus system, such as a CAN bus, to the control unit SG SBWL of the steer-by-wire system. The control unit SG SBWL of the steer-by-wire system SBWL takes into account the corridor function Korr operating thereon, thus ensuring that the maximum steering angle is not exceeded. Therefore, collisions between the steering wheels and chassis or body components are particularly prevented. The control unit SG SBWL then determines the total theoretical steering angle G_SLw and sends it to the steer-by-wire system SBWL, which changes the wheel steering angle via actuators installed therein. The described arrangement and the method of execution therein do not allow for the inclusion of other steering functions.

[0052] Figure 3 A flowchart schematically illustrates the apparatus or method according to the invention. A bus system in a vehicle, such as a CAN bus or Flexray for signal transmission, is shown approximately centered. Different steering functions are shown on the left side of the bus. (Previously, regarding...) Figure 1 The functions already described—driver steering angle Lw, corridor function Korr, parking assist PAS, driver assistance system for autonomous driving ADAS, and electronic stability control system ESC—request theoretical steering angles SLw_1 to SLw_5 according to demand and send them to the bus. Other steering functions Lx are exemplarily given, requesting theoretical steering angle SLw_x and representing steering functions not mentioned here and the theoretical steering angle requested by them. Driving conditions, indicated by the denot Fzg_S, also flow into the bus as signals and can be read from them by the arbitration unit AE. All theoretical steering angles SLw_1 to SLw_5 are read as separate signals by the arbitration unit AE, which includes the sequential alternation function B. The arbitration unit AE weights the individual theoretical steering angles (SLw_1 to SLw_x) and prioritizes theoretical steering angle SLw_i. In other words, the arbitration unit determines the source corresponding to one of the aforementioned steering functions. It is also possible that information from two sources is mixed together, at least temporarily. However, the final arbitration unit AE determines the theoretical steering angle SLw_i by taking into account the current driving situation Fzg_S, and thereby determines the total theoretical steering angle G_SLw, which is continuously calculated.

[0053] To avoid sudden steering movements, the arbitration unit can gradually transition from a first specific theoretical steering angle to a subsequently determined theoretical steering angle through the gradually alternating functions assigned to it. This also occurs continuously.

[0054] In a further step, it is continuously checked at intervals of 4 ms whether the gradient of the respective total theoretical steering angle G_SLw is less than or equal to the gradient of the respective theoretical steering angle SLw_i plus the respective increased safety factor v_safe. The result of this check is saved separately and it is checked with the next calculated value whether the same result occurs several times. For example, if the condition is true five times in succession, the total theoretical steering angle G_SLw is transmitted (set) to the steer-by-wire system. The actuators of the steer-by-wire system then adjust this total theoretical steering angle G_SLw. If the check is not fulfilled (true? = no) if it is checked several times, for example five times in succession within 20 ms, the steer-by-wire system is brought into (set to) an error mode or a safety mode SLw_safe. In this case, depending on the driving situation, the steer-by-wire system will either freeze the current steering angle or drive gradually towards a steering angle of 0°, which corresponds to straight-ahead driving. In this way, it is ensured that, when the calculation of the total theoretical steering angle G_SLw is faulty, no situation arises which cannot be controlled by the normal driver. This therefore advantageously gives a steer-by-wire system or a method for operating a steer-by-wire system which meets the high safety requirements according to ASIL.

[0055] List of reference signs

[0056] 1 vehicle

[0057] 2 front wheels

[0058] 3 rear wheels

[0059] 4 front axle steering system

[0060] 5 rear axle steering system

[0061] 14 steering angle of the driver

[0062] 21 front axle

[0063] 31 rear axle

[0064] 41 steering spindle

[0065] 51 steering spindle

[0066] AE arbitration unit

[0067] ADAS driver assistance system automated driving

[0068] B gradual alternation function

[0069] BUS bus system (CAN, Flexray)

[0070] ESC electronic stability control system

[0071] Korr Corridor function

[0072] PAS Parking assistance

[0073] R G Yaw rate

[0074] SG Control device

[0075] S G Center of gravity

[0076] S Sensor system

[0077] T Trajectory

[0078] delta v Front wheel steering angle

[0079] delta h Rear wheel steering angle

[0080] Fzg_S Driving situation

[0081] Lw Driver's steering angle

[0082] Lx Other steering function

[0083] SLw(_1bis_5) Theoretical steering angle SLw_1 to SLw_5

[0084] SLw_x Other theoretical steering angle

[0085] SLw_i Theoretical steering angle (determined by AE)

[0086] SG Control device of SBWL steer-by-wire system

[0087] G_SLw Total theoretical steering angle

[0088] SBWL Steer-by-wire system

[0089] dG_SLw / dt Total theoretical steering angle gradient

[0090] dSLw_i / dt Theoretical steering angle gradient

[0091] SLw_safe Steer-by-wire system safety mode

[0092] v_safe Safety factor.

Claims

1. A method for operating a steer-by-wire system of a motor vehicle, wherein At least one steering function request as one or more steering angle changes of at least one theoretical steering angle at a vehicle axle, wherein an arbitration unit (AE) weights the steering angle changes as a theoretical steering angle (SLw_i) and determines a total theoretical steering angle (G_SLw) taking into account at least one theoretical steering angle (SLw_i), characterized in that it is continuously checked before the total theoretical steering angle (G_SLw) is set by actuators of the steer-by-wire system whether the following condition is met: The gradient dG_SLw / dt of the total theoretical steering angle (G_SLw) is smaller than or equal to the gradient dSLw_i / dt of at least one theoretical steering angle (SLw_i).

2. The method of claim 1, wherein, The steering angle changes are requested depending on a driving situation (Fzg_S) of the motor vehicle.

3. The method according to claim 1 or 2, characterized in that, Before the check, a safety factor (v_safe) is added to the gradient of the theoretical steering angle (SLw_i), respectively, the value of which is variable depending on the vehicle speed.

4. The method of claim 3, wherein, The value of the safety factor decreases with increasing vehicle speed.

5. The method according to claim 1 or 2, characterized in that, The check is performed at intervals.

6. The method of claim 5, wherein, The check is performed at intervals of 1 to 20 ms.

7. The method of claim 5, wherein, The check is performed every 4 ms.

8. The method of claim 2, wherein, The arbitration unit (AE) performs a gradual alternation (B) when determining the total theoretical steering angle (G_SLw) by taking into account a first theoretical steering angle (S_Lw1) and at least one further theoretical steering angle (S_Lw2).

9. The method of claim 8, wherein, The arbitration unit (AE) performs a gradual alternation (B) depending on a driving situation (Fzg_S) of the motor vehicle.

10. The method of claim 2, wherein, For determining the driving situation (Fzg_S) of the motor vehicle, at least one of the following parameters is taken into account: - driving mode - steering wheel angle - steering wheel angle velocity - steering wheel angle acceleration - vehicle speed - yaw rate - lateral acceleration - longitudinal acceleration - wheel steering angle of at least one wheel on the axle to be steered.

11. The method of claim 10, wherein, For determining the driving situation (Fzg_S) of the motor vehicle, the arbitration unit takes into account signals from at least one control device, wherein the signals are read at intervals.

12. The method of claim 11, wherein, For determining the driving situation (Fzg_S) of the motor vehicle, the arbitration unit takes into account signals from at least one control device not belonging to the steer-by-wire system, wherein the signals are read at intervals.

13. The method of claim 11, wherein, The signals are read at intervals of 5 to 40 ms.

14. The method of claim 11, wherein, The signals are read at intervals of 10 ms.

15. The method of claim 1 or 2, wherein, The result of the check is debounced to obtain a reliable evaluation of the condition total theoretical steering angle (G_SLw) <= theoretical steering angle (SLw_i) or total theoretical steering angle (G_SLw) <= theoretical steering angle (SLw_i) + safety factor (v_safe).

16. The method of claim 1 or 2, wherein, The at least one steering function is at least one of the following functions, wherein the functions are not executed on a control device of the steer-by-wire system: • walk-along function • parking function • back-up function • driver assistance function.

17. Control device for carrying out a method according to any one of the preceding claims.

18. Computer program having a program code section for performing, when the program is executed on a computer, the method according to any one of the preceding claims 1 to 8.

19. Computer program according to claim 18, which is executed on a control device of the steer-by-wire system.

20. Steer-by-wire system having a control device according to claim 17.

21. Steer-by-wire system according to claim 20, which is a rear axle steering system of a motor vehicle.

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