Method for operating a steering system of a vehicle - Patents.com

Abstract

The invention starts from a method for operating a steering system (10) of a vehicle (12), in particular of a motor vehicle, configured as a steer-by-wire steering system and including an operating unit (14) with at least one steering wheel (16) and at least one feedback actuator (18) cooperating with the steering wheel (16), and at least one wheel steering angle regulator (20) operatively connected to the operating unit (14) for varying the wheel steering angle of at least one wheel (22, 24), the operation of the feedback actuator (18) being monitored by a monitoring function (26) and the steering characteristic of the steering system (10) being modified in at least one operating state in which a fault and / or failure of the feedback actuator (18) has been identified by the monitoring function (26). To modify the steering characteristic, it is proposed to modify the steering gear ratio between the operating unit (14) and the wheel steering angle regulator (20).

Description

[Technical field]

[0001] The invention starts from a method for operating a steering system of a vehicle according to the preamble of claim 1. Furthermore, the invention relates to a computing unit for implementing such a method, to a steering system comprising such a computing unit and to a vehicle comprising such a steering system. [Background technology]

[0002] From the prior art, vehicles are known with conventional steering systems, in which a steering wheel, for example in the form of a steering wheel, is mechanically fixedly connected to a wheel steering angle regulator in the form of a steering gear via a steering column. Furthermore, vehicles are known with steer-by-wire steering systems, which can be operated without a direct mechanical connection between the steering wheel and the steered wheels and in which the driver's desired settings and / or steering settings are transmitted exclusively electrically. Such steer-by-wire steering systems usually comprise an operating unit with a steering wheel and a feedback actuator, and at least one wheel steering angle regulator mechanically separated from the operating unit, which can be configured, for example, as a central regulator or as an individual wheel regulator.

[0003] Furthermore, such steer-by-wire steering systems are essentially configured redundantly for reasons of operational reliability. With regard to the operating unit, it is possible to configure the operating unit, for example, in a fail-safe or fail-operational manner. Furthermore, it is also possible to configure the operating unit in a fail-operational manner with respect to the detection of the driver setpoint and in a fail-safe manner with respect to the feedback torque provided by the feedback actuator. In the event of a fault and / or failure of the feedback actuator in a corresponding embodiment of the operating unit, unintended steering movements at the steering wheel may result due to a sudden disappearance of the feedback torque, which may be interpreted by the steering system as a driver setpoint and / or a steering setpoint, resulting in an undesirable vehicle response. Possible methods for handling such error cases are taken, for example, from DE 102016009684 A1 and DE 102018222442 A1. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] DE 102016009684 A1 [Patent Document 2] DE 102018222442 Summary of the Invention [Problem to be solved by the invention]

[0005] Starting from the above, the object of the present invention is to provide a method with improved properties, in particular with regard to functionality. This object is achieved by the features of claims 1, 12, 13 and 14, whereas advantageous embodiments and developments of the invention follow from the dependent claims. [Means for solving the problem]

[0006] Disclosure of the Invention The invention starts from a method for operating a steering system of a vehicle, in particular of an automobile, which is configured as a steer-by-wire steering system and comprises an operating unit with at least one steering wheel and at least one feedback actuator cooperating with the steering wheel, and at least one wheel steering angle regulator operatively connected to the operating unit for varying the wheel steering angle of at least one wheel, in which the operation of the feedback actuator is monitored by a monitoring function, and in at least one operating state in which a fault and / or failure of the feedback actuator has been identified by the monitoring function, the steering characteristic of the steering system is changed.

[0007] In order to change the steering characteristic, it is proposed to modify the steering gear ratio between the operating unit and the wheel steering angle regulator. The operating states in which the monitoring function has identified a fault and / or failure of the feedback actuator thus correspond in particular to error operating states. Furthermore, the steering gear ratio is modified so that, in particular in the above-mentioned operating states, in particular when the feedback actuator passes from an active and / or fully functional state to a passive and / or degraded state, a substantially unchanged steering behavior is provided and / or achieved and no undesired vehicle response is caused by an unintended steering movement of the driver due to a fault and / or failure of the feedback actuator. This embodiment allows in particular to improve the functionality, advantageously to improve the controllability and / or masterability of the vehicle in error cases or when the active feedback torque of the feedback actuator disappears. Furthermore, it is possible to provide an advantageously adaptive and / or variable method in which the steering characteristic can be flexibly adapted to the current operating conditions. Furthermore, advantageous maneuverability of the vehicle can be achieved and operational reliability can be increased.

[0008] The steering system is here designed as a steer-by-wire steering system, in particular in which the driver's desired settings and / or steering settings are transmitted to the wheels, preferably purely electrically. For this purpose, the steer-by-wire steering system comprises an operating unit, in particular configured redundantly, and at least one wheel steering angle regulator, in particular configured redundantly, mechanically separated from the operating unit. Preferably, in this case, the operating unit and the wheel steering angle regulator are at least partially configured fail-operationally. By "wheel steering angle regulator" is to be understood an actuator unit coupled to at least one wheel, which is configured to transmit in particular the driver's desired settings and / or steering settings to the wheels by changing the wheel steering angle of the at least one wheel, thereby preferably controlling the orientation of at least one of the wheels and / or influencing the driving direction of the vehicle. For this purpose, the wheel steering angle regulator advantageously comprises at least one steering adjustment element, for example in the form of a rack, and at least one steering actuator, for example in the form of an electric motor, operatively connected to the steering adjustment element. In this case, the wheel steering angle regulator can be configured as a central regulator and associated with at least two wheels, which are in particular steerable and preferably configured as front wheels. Alternatively, however, it is also possible for the wheel steering angle regulator to be configured as an individual wheel regulator and associated with exactly one wheel, which is in particular steerable and preferably configured as a front wheel. Furthermore, a "feedback actuator" is to be understood as an actuator unit, in particular different from the wheel steering angle regulator, which is in particular mechanically connected directly to the steering wheel and which is configured to detect, in particular directly, and / or transmit, in particular directly, steering wheel signals, forces and / or torques to the steering wheel. In the present specification, a feedback actuator is provided for providing at least an active feedback torque in normal operating conditions, thereby generating a steering resistance and / or a restoring torque for the steering wheel.Furthermore, a feedback actuator is provided in this connection to adapt the steering feel perceptible in particular via the steering wheel. For this purpose, the feedback actuator may comprise at least one further electric motor. By "faults and / or failures of the feedback actuator" is to be understood in particular faults and / or failures of the feedback actuator itself and / or of peripheral assemblies cooperating with the feedback actuator, such as, for example, an energy supply, and a failure of the feedback actuator caused thereby. Furthermore, the wheel steering angle is essentially equivalent to other parameters between the steering actuator and the wheels, such as, for example, the displacement of the steering adjustment element and / or the displacement position of the wheel steering angle regulator and / or the motor movement. The same applies for the displacement of the steering wheel, which is equivalent to other parameters between the steering wheel and the feedback actuator, such as, for example, the steering column angle and / or the motor angle. For the torque parameters at the wheels and steering wheel, a similar equivalence of parameters between the wheels / steering wheel and the respective connected actuators applies.

[0009] Furthermore, the vehicle, preferably the steering system, comprises at least one calculation unit, which is provided for implementing the method for operating the steering system. By "calculation unit" is to be understood an electrical and / or electronic unit having an information input, an information processing and an information output. Advantageously, the calculation unit further comprises at least one processor, at least one operating memory, at least one input means and / or output means, at least one operating program, at least one control and / or adjustment routine, at least one calculation routine, at least one identification routine, at least one evaluation routine and / or at least one adaptation routine. In particular, the calculation unit comprises in this case at least one monitoring function for monitoring the operation of the feedback actuator. Furthermore, the calculation unit is provided for monitoring and evaluating, in particular, the operation of the feedback actuator by means of the monitoring function. Furthermore, the calculation unit is provided for modifying the steering characteristic of the steering system in at least one operating state in which a fault and / or failure of the feedback actuator has been identified by the monitoring function, and for this purpose for modifying the steering gear ratio between the operating unit and the wheel steering angle regulator. In this connection, a calculation unit may be provided for utilizing an error signal provided by the monitoring function, in particular for the purpose of adapting the steering characteristic. Preferably, the calculation unit is in this case integrated into a control device of the vehicle, for example into a central vehicle control device, or advantageously into a control device of the steering system, in particular in the form of a steering control device. A "steering gear ratio" is to be understood as a conversion ratio, in particular a virtual one, between the operating unit and the wheel steering angle regulator, which defines the correlation between the driver's desired settings and / or steering settings at the steering wheel and the wheel steering angle of one and / or several wheels. The steering gear ratio therefore indicates at least how a displacement of the steering wheel or a turning of the steering wheel acts on the displacement of one and / or several wheels or on the turning of one and / or several wheels.Therefore, in this specification, a target setpoint for the wheel steering angle regulator is generated based on the driver target setpoint and / or steering setpoint at the steering wheel and the steering gear ratio, and the wheel steering angle regulator is provided to realize this target setpoint. "Provided" should be understood to mean, in particular, specially programmed, designed, and / or installed. An object being provided for a certain function should be understood to mean, in particular, that the object realizes and / or executes this certain function at least in a use state and / or an operating state.

[0010] It is further proposed to take into account the current and / or impending driving situation when the steering characteristic is changed, in particular when the steering gear ratio is modified, so that the steering characteristic can be changed in a particularly situation-specific manner. In particular, in this case, the steering characteristic is changed depending on the current and / or impending driving situation. The current and / or impending driving situation can be determined and / or predicted, for example, on the basis of at least one vehicle parameter, for example the yaw rate, the steering wheel displacement and / or the steering movement. However, alternatively or additionally, the current and / or impending driving situation can also be derived from a route plan of a navigation system of the vehicle and / or from a corresponding sensor system of the vehicle, for example in the form of a camera system.

[0011] Advantageously, the operating state corresponds to cornering. In this case, the steering characteristic is preferably changed only if the wheel steering angle of the wheel is not equal to zero, preferably at least 0.1° in absolute value. Particularly preferably, therefore, a change in the steering characteristic is carried out when the vehicle is cornering, whereas no change in the steering characteristic is carried out when the vehicle is driving straight ahead. This allows for increased operational reliability, particularly in critical driving situations. Furthermore, advantageously, a particularly efficient method can be provided.

[0012] It is further proposed to take into account the dynamics and / or absolute value of the driver's setpoint for the wheel steering angle regulator, i.e. for example the maximum displacement of the steering wheel and / or the steering speed of the steering wheel, when the steering characteristic is changed, in particular when the steering gear ratio is modified. The determination of the dynamics and / or absolute value of the driver's setpoint can be carried out, for example, in particular by means of further monitoring functions of the calculation unit. Furthermore, it is conceivable to change the steering characteristic by modifying the steering gear ratio only if the dynamics and / or absolute value of the driver's setpoint exceed a limit value. This makes it possible to identify an overreaction of the driver, in particular when the active feedback torque of the feedback actuator disappears, and to adapt the steering characteristic accordingly.

[0013] Furthermore, it is proposed to change the steering gear ratio, that is, to increase the steering gear ratio in the above-mentioned operating conditions. Preferably, the steering gear ratio is increased in this case from the normal or current steering gear ratio used in the normal operating conditions, particularly without errors, to a corrected steering gear ratio, where the corrected steering gear ratio is at least 10%, preferably at least 20%, particularly preferably at least 30% higher than the normal steering gear ratio. This reduces the vehicle response and makes the situation more manageable for the driver.

[0014] According to a particularly preferred embodiment, it is proposed that in the abovementioned operating states, in particular for changing the steering characteristic, the switchover from the normal or current steering gear ratio to the corrected steering gear ratio is carried out by cross-fading, in particular using a moving average value. For this purpose, a fade factor is preferably defined, which has the value 0 at the beginning and has the value 1 at the end of the cross-fading or fade phase. Then, during the cross-fading or fade phase, the actual steering gear ratio is obtained based on the overlap of the normal or current steering gear ratio and the corrected steering gear ratio, taking the fade factor into account. This makes it possible to achieve a particularly harmonious transition from the normal or current steering gear ratio to the corrected steering gear ratio.

[0015] The normal or current steering gear ratio and / or the corrected steering gear ratio can be selected, for example, depending on the vehicle speed and / or the displacement of the steering wheel and / or the displacement of a steering adjustment element, for example in the form of a rack stroke and / or the wheel steering angle of at least one wheel. However, it is preferably proposed that the corrected steering gear ratio has a fixed value, which allows a preferably defined pre-applied vehicle response to be defined in the abovementioned operating states. Furthermore, the required computational resources can be kept to a minimum.

[0016] It is furthermore preferably proposed to take into account the steering speed of the steering wheel when the steering characteristic is changed, in particular when the steering gear ratio is modified, and to modify the duration of the cross-fading in dependence on the steering speed. Advantageously, in this case, at least the fade coefficient is a dynamic value that can be influenced by the steering speed. Furthermore, the steering speed can also be calculated using further coefficients, which can be, for example, constant or dependent on the vehicle speed or other vehicle parameters. This embodiment advantageously allows a more or less rapid cross-fading from the normal or current steering gear ratio to the modified steering gear ratio depending on the steering dynamics.

[0017] If an integrator is used for the cross-fading, the cross-fading can be realized particularly simply in software technology. In particular, the calculation unit can in this case include an integrator. Preferably, the integrator is activated by an error signal provided by the monitoring function in the abovementioned operating state and / or is triggered by a trigger signal. Preferably, the integrator is only activated in this context if a fault and / or a failure of the feedback actuator is identified by the monitoring function. The integrator is therefore preferably deactivated in the normal operating state, which can advantageously reduce resource demands. Furthermore, the integrator can also be temporarily switched off after activation, so that an additional triggering by a trigger signal has to be performed.

[0018] Preferably, the trigger signal is generated only if the normal or current steering gear ratio is smaller than the corrected steering gear ratio and / or if the sign of the steering wheel displacement and the sign of the steering wheel steering speed are mutually consistent. In this context, therefore, the integrator is triggered only under the condition that the normal or current steering gear ratio is smaller than the corrected steering gear ratio and / or the driver actually increases his driver's desired setpoint or his steering setpoint, and thus, when driving around a curve, the driver displaces further in the direction of the curve. This can further increase the reliability of operation in particular.

[0019] Furthermore, it is proposed to use the corrected absolute value of the steering speed as the input parameter of the integrator, and to use a dead band to correct the steering speed. The dead band ensures that the response is only carried out when the driver starts a particularly noticeable steering movement or when the steering speed reaches a corresponding threshold value. This can advantageously compensate for the driver's overreaction when the active feedback torque of the feedback actuator disappears.

[0020] In a further embodiment, it is further proposed to take the vehicle speed into account when the steering characteristic is changed, in particular when the steering gear ratio is modified. Preferably, in this case, the vehicle speed is determined and evaluated in order to determine at least the normal or current steering gear ratio and / or the modified steering gear ratio. Advantageously, the normal or current steering gear ratio and / or the modified steering gear ratio are thus dependent on the vehicle speed. This allows a particularly flexible adaptation of the steering characteristic to the current operating conditions.

[0021] The method for operating a steering system herein is not limited to the applications and embodiments described above, in particular, the method for operating a steering system to achieve the functionality described herein may have a different number of individual elements, components and units than those listed herein.

[0022] Further advantages will be apparent from the following description of the drawings, in which an embodiment of the invention is shown. [Brief description of the drawings]

[0023] [Figure 1a] FIG. 1 is a schematic diagram of a vehicle with a steering system configured as a steer-by-wire steering system. [Figure 1b] FIG. 1 is a schematic diagram of a vehicle with a steering system configured as a steer-by-wire steering system. [Diagram 2] 4 is an exemplary graph of various signals for operating a steering system. [Diagram 3] 3 is an exemplary flow chart with main method steps of a method for operating a steering system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Description of the embodiment 1a and 1b show schematic diagrams of a vehicle 12, exemplarily configured as a passenger car, with a number of wheels 22, 24 and a steering system 10. The steering system 10 has an operative connection with the wheels 22, 24 and is provided for influencing the direction of travel of the vehicle 12. Furthermore, the steering system 10 is configured here as a steer-by-wire steering system, in which case the driver target or steering settings in at least one operating state are electrically transmitted to the wheels 22, 24.

[0025] The steering system 10 comprises an operating unit 14, which can be operated in particular by the driver and / or passengers. The operating unit 14 comprises a steering handle 16, for example in the form of a steering wheel, and a feedback actuator 18, which is in particular mechanically coupled to the steering handle 16. In the present case, the feedback actuator 18 is provided for providing at least an active feedback torque in normal operating conditions, thereby generating a steering resistance and / or a restoring torque for the steering handle 16. For this purpose, the feedback actuator 18 comprises at least one electric motor (not shown), which is in particular configured as a permanent magnet excited synchronous motor. Furthermore, the operating unit 14 is in the present case configured in a fail-operational manner in particular with regard to the detection of a driver setpoint via the steering handle 16 and in a fail-safe manner in relation to the feedback torque provided by the feedback actuator 18. Alternatively, the steering handle may be configured as a joystick, a steering lever and / or a steering ball or the like. Furthermore, the feedback actuator may comprise a plurality of electric motors. Furthermore, it is also possible for the operating unit to be configured in a fundamentally fail-safe or fail-operational manner.

[0026] The steering system 10 further comprises a wheel steering angle regulator 20, which is known per se. The wheel steering angle regulator 20 is configured mechanically separated from the operating unit 14. The wheel steering angle regulator 20 is connected to the operating unit 14 purely electrically. Furthermore, the wheel steering angle regulator 20 is configured, for example, as a central regulator. The wheel steering angle regulator 20 has an operational connection with at least two wheels 22, 24, in particular the two front wheels, and is provided for converting a driver's desired setpoint or a steering setpoint into a steering movement of the wheels 22, 24. For this purpose, the wheel steering angle regulator 20 comprises a steering regulator element 36, which is configured, for example, as a rack, and a steering actuator 38 which cooperates with the steering regulator element 36. The steering actuator 38 comprises at least one further electric motor (not shown), which in this case is configured, in particular, as a permanently excited synchronous motor, and is provided for driving the steerable wheels 22, 24. In principle, the steering system can of course also comprise a number of wheel steering angle regulators, in particular configured as individual wheel regulators. Furthermore, the steering actuator can comprise a number of electric motors.

[0027] Furthermore, the vehicle 12 comprises a control device 40, which in this case is configured as a steering control device and is therefore part of the steering system 10. The control device 40 has an electrical connection with the wheel steering angle adjuster 20. The control device 40 further has an electrical connection with the operating unit 14. The control device 40 is provided for controlling at least the operation of the steering system 10. The control device 40 is provided here for driving the steering actuator 38 in dependence on a signal of the operating unit 14, for example in dependence on a driver target set value or steering set value and / or a manual torque. Furthermore, the control device 40 may also be provided for driving the feedback actuator 18 in dependence on a signal of the wheel steering angle adjuster 20.

[0028] For this purpose, the control device 40 comprises a calculation unit 34. The calculation unit 34 comprises at least one processor (not shown), for example in the form of a microprocessor, and at least one operating memory (not shown). Furthermore, the calculation unit 34 comprises at least one operating program stored in the operating memory, which comprises at least one calculation routine, at least one identification routine, at least one evaluation routine and at least one adaptation routine. Furthermore, the calculation unit 34 comprises in the present case at least one monitoring function 26. However, the control device can basically also be different from the steering control device and can be configured, for example, as an individual central vehicle control device with a central calculation unit. Furthermore, it is also conceivable to provide separate control devices and / or calculation units for the wheel steering angle regulator and the operating unit, which are communicatively connected to each other.

[0029] Furthermore, the vehicle 12 and / or the steering system 10 may include further components and / or assemblies not shown, such as an internal vehicle sensor for detecting at least one vehicle parameter, e.g. yaw rate, an external sensor system, e.g. in the form of a camera system, and / or a navigation system known per se.

[0030] In case of faults and / or failures of the feedback actuator 18, the sudden disappearance of the feedback torque when the feedback actuator 18 goes from an active and / or fully functional state to a passive and / or degraded state in certain circumstances and / or in certain driving situations, for example when driving around curves, may result in unintended steering movements at the steering wheel 16, which may be interpreted by the steering system 10 as driver target and / or steering settings, resulting in undesired vehicle responses. In this context, it is assumed that the passive behavior of the feedback actuator 18 with respect to the torque feedback is sufficient to ensure reliable operation of the vehicle 12, and only the transition from the active to the passive case may be a controllability issue. The reason for this is that in case of a corresponding fault and / or failure of the feedback actuator 18, the feedback torque becomes abruptly smaller, since the passive friction inherent in the steering system 10 is much smaller than the feedback torque in normal operating conditions. Particularly when cornering, a sudden drop in the feedback torque and thus in the counter torque at the steering wheel 16 can lead to safety-critical situations, since the driver can only readjust his holding force with a time delay due to his reaction time, which results in steering further into the curve than intended. In this case, depending on the steering gear ratio, an indirect gear ratio can increase the controllability even after the transition to a passive state.

[0031] Therefore, in the following, a method is proposed for operating the steering system 10 in order to avoid such safety-critical situations. In this description, in particular a calculation unit 34 is provided for carrying out the method and for this purpose has a computer program with in particular corresponding program code means. However, instead, a calculation unit of the control device, which is associated with an operating unit, can also be provided for carrying out the method.

[0032] According to the invention, the operation of the feedback actuator 18 is monitored by a monitoring function 26 and, in at least one operating state in which a fault and / or failure of the feedback actuator 18 is identified by the monitoring function 26, the steering characteristic of the steering system 10 is changed, in particular by modifying the virtual steering gear ratio between the operating unit 14 and the wheel steering angle regulator 20. In the present case, in the above-mentioned operating states, in particular when the feedback actuator 18 goes from an active and / or fully functional state to a passive and / or degraded state, the steering characteristic is changed by modifying the steering gear ratio in such a way that a substantially unchanged steering behavior is provided and / or achieved and that unintended steering movements of the driver due to a fault and / or failure of the feedback actuator 18 do not result in undesired vehicle responses.

[0033] For this purpose, the current and / or impending driving situation is taken into account and the steering characteristic is changed depending on the current and / or impending driving situation. Preferably, in this case, a corresponding change of the steering characteristic is only performed when the vehicle 12 is turning a curve or if it is determined that the vehicle 12 is about to turn a curve. The above-mentioned operating state thus corresponds to turning a curve and the steering characteristic is only changed if the wheel steering angle of the wheels 22, 24 is not equal to zero.

[0034] Furthermore, when the steering characteristic is changed, in particular when the steering gear ratio is modified, the dynamics and / or absolute value of the driver setpoint for the wheel steering angle regulator 20 are taken into account, i.e. for example the maximum displacement of the steering wheel 16 and / or the steering speed of the steering wheel 16. The determination of the dynamics and / or absolute value of the driver setpoint can be performed, for example, by means of corresponding vehicle sensors and / or by means of further monitoring functions (not shown) of the calculation unit 34. In this connection, for example, the steering characteristic can be changed by modifying the steering gear ratio if the dynamics and / or absolute value of the driver setpoint exceed a limit value, so that an over-reaction of the driver in the event of loss of the active feedback torque of the feedback actuator 18 can be identified and the steering characteristic can be adapted accordingly.

[0035] Furthermore, the steering gear ratio is increased in the above mentioned operating conditions, more specifically from the normal or current steering gear ratio 28 used in particular in normal operating conditions without errors to a modified steering gear ratio 30 (see also in particular FIG. 2). The normal or current steering gear ratio 28 has a fixed value. The same applies to the modified steering gear ratio 30. Furthermore, the normal or current steering gear ratio 28 and the modified steering gear ratio 30 are selected in the present case depending on the vehicle speed. However, in principle, constant values ​​are also possible for the normal or current steering gear ratio and / or the modified steering gear ratio. In particular, it is sensible to increase the steering gear ratio if it is determined based on the current and / or impending driving situation and / or based on the dynamics and / or absolute values ​​of the driver's target setpoints that the driver is steering in a more curved direction than intended. In this case, the vehicle response is reduced by increasing the steering gear ratio, making the situation more controllable for the driver. In principle, the controllability is increased by increasing the steering gear ratio. This is because the loss of driver steering precision due to the lack of feedback torque can be at least partially compensated for by the reduced vehicle response. If the steering gear ratio is increased, for example, from a value of 10 to a value of 15, a wheel steering angle of only 5.67° occurs when the steering wheel 16 is displaced 85°, so that the driver only needs to correct or restore 40° at the steering wheel 16 to set the wheel steering angle of 3° required for the exemplary curve maneuver.

[0036] Furthermore, the switchover from the normal or current steering gear ratio 28 to the modified steering gear ratio 30 is performed by cross-fading using a moving average value. For this purpose, a fade coefficient is defined, which has the value 0 at the beginning and the value 1 at the end of the cross-fading or fade phase. Then, during the cross-fading or fade phase, based on the overlap of the normal or current steering gear ratio 28 and the modified steering gear ratio 30, taking the fade coefficient into account, an initial or actual steering gear ratio is obtained. In this connection, for the initial or actual steering gear ratio: VSR=VSR norm (1-KP)+VSR mod ·KP holds true.

[0037] In this case, VSR stands for the initial steering gear ratio or the actual steering gear ratio, VSR norm represents the normal or current steering gear ratio 28, VSR mod represents the modified steering gear ratio 30, while KP represents the fade coefficient.

[0038] Furthermore, the fade factor is a dynamic value that can be influenced by the steering speed of the steering wheel 16, whereby the fade factor defines the duration of the cross-fading. Thus, when the steering characteristic is changed, in particular when the steering gear ratio is modified, the steering speed of the steering wheel 16 is taken into account and the duration of the cross-fading is modified depending on the steering speed, which advantageously allows a more or less rapid cross-fading from the normal or current steering gear ratio 28 to the modified steering gear ratio 30 depending on the steering dynamics.

[0039] Furthermore, an integrator 32 is used for the cross-fading. The integrator 32 is zero and / or deactivated in normal operating conditions and is only activated by an error signal provided by the monitoring function 26 in the above-mentioned operating conditions. As a result, the integrator 32 is only activated if the monitoring function 26 identifies a fault and / or a failure of the feedback actuator 18. Furthermore, the integrator 32 must be triggered by a trigger signal in addition, after being activated in this case in particular by an error signal. Preferably, in this case, the trigger signal is generated only if the normal or current steering gear ratio 28 is smaller than the corrected steering gear ratio 30 and if the sign of the displacement of the steering wheel 16 and the sign of the steering speed of the steering wheel 16 coincide with each other. In this context, triggering of the integrator 32 is therefore only performed under the condition that the normal or current steering gear ratio 28 is smaller than the corrected steering gear ratio 30 and under the condition that the driver actually increases his driver target setpoint or his steering setpoint and thus displaces the driver further in the direction of the bend when driving around a bend. In principle, however, it is also possible to omit starting and / or triggering the integrator.

[0040] The actual input parameter of the integrator 32 is the corrected absolute value of the steering speed. Here, the steering speed is corrected by a dead band so that the response is only performed when the driver starts a significant steering movement or when the steering speed reaches a corresponding threshold value. Furthermore, scaling can be performed to allow the cross-fading duration or cross-fading dynamics to be selected accordingly.

[0041] FIG. 2 shows an example graph of different signals for operating the steering system 10.

[0042] The vertical axis 42 is configured as a parameter axis. The horizontal axis 44 shows the time in [s]. The curve 46 shows the hypothetical course of the displacement of the steering wheel 16, in this case in particular in the form of the actual steering wheel angle, when no change in the steering characteristic by a steering gear ratio modification is implemented. The curve 48 shows the course of the displacement of the steering wheel 16, in this case in particular in the form of the actual steering wheel angle, when no change in the steering characteristic by a steering gear ratio modification is implemented. The curve 50 shows the hypothetical course of the driver's target set value for the wheel steering angle regulator 20 or the wheel steering angle of the wheels 22, 24, when no change in the steering characteristic by a steering gear ratio modification is implemented. The curve 52 shows the course of the driver's target set value for the wheel steering angle regulator 20 or the wheel steering angle of the wheels 22, 24, when no change in the steering characteristic by a steering gear ratio modification is implemented. The curve 54 shows the normal steering gear ratio 28 and a hypothetical progression of the normal steering gear ratio 28 when no change in steering characteristics due to steering gear ratio modification is implemented. The curve 56 shows the modified steering gear ratio 30.

[0043] 2, it can be seen that the driver first makes a deflection in the direction of the curve and then travels at a constant radius until the feedback actuator 18 fails at the time 10 seconds. The curves 50 and 52 clearly show the reduction in the wheel steering angle of the wheels 22, 24 achieved by the method according to the invention and the associated reduction in the vehicle response. Furthermore, the curves 46 and 48 show that after the reaction time has elapsed, a smaller correction angle is required to travel the same radius as before the failure of the feedback actuator 18.

[0044] Finally, FIG. 3 shows an exemplary flow chart with the main method steps of a method for operating the steering system 10.

[0045] Method step 60 corresponds in particular to a normal operating state without errors. In this case, the steering gear ratio corresponds to the normal steering gear ratio 28 and may have, for example, a value of 10. Furthermore, the operation of the feedback actuator 18 is monitored by the monitoring function 26.

[0046] In method step 62 , a fault and / or failure of the feedback actuator 18 is identified by the monitoring function 26 .

[0047] In the subsequent method step 64, the steering characteristic of the steering system 10 is changed, in particular by modifying the virtual steering gear ratio between the operating unit 14 and the wheel steering angle regulator 20. For this purpose, the steering gear ratio is advantageously increased and switched from the normal steering gear ratio 28 to a modified steering gear ratio 30, which may have a value of, for example, 15.

[0048] The exemplary flow chart of Fig. 3 is merely an exemplary description of a method for operating the steering system 10. In particular, individual method steps may be modified or additional method steps may be added. In this connection, it is conceivable, for example, to take into account the current and / or impending driving situation and / or the dynamics and / or absolute value of the driver's desired settings for the wheel steering angle regulator 20. Furthermore, the switchover from the normal steering gear ratio 28 to the modified steering gear ratio 30 can be performed by cross-fading.

Claims

1. A method for operating a steering system (10) of a vehicle (12), in particular of an automobile, comprising the steps of: The steering system (10) is configured as a steer-by-wire steering system and includes an operating unit (14) having at least one steering wheel (16) and at least one feedback actuator (18) cooperating with the steering wheel (16), and at least one wheel steering angle adjuster (20) operatively connected to the operating unit (14) for varying a wheel steering angle of at least one wheel (22, 24), The operation of the feedback actuator (18) is monitored by a monitoring function (26); and modifying a steering characteristic of the steering system in at least one operating condition in which a fault and / or failure of the feedback actuator is identified by the monitoring function. In the method, To change the steering characteristic, a steering gear ratio between the operating unit (14) and the wheel steering angle adjuster (20) is modified. A method comprising:

2. When changing the steering characteristic, the current driving situation and / or the impending driving situation are taken into account. The method of claim 1.

3. The operating state corresponds to curve driving, the steering characteristic is changed only if the wheel steering angle of the wheels (22, 24) is not equal to zero; The method according to claim 1 or 2.

4. When the steering characteristic is changed, the dynamics and / or the absolute value of the driver's desired setting for the wheel steering angle adjuster (20) are taken into account.

4. The method according to claim 1 .

5. The steering gear ratio is increased in the operating state.

5. The method according to any one of claims 1 to 4.

6. In said operating state, the switching from the normal or current steering gear ratio (28) to the modified steering gear ratio (30) is performed by cross-fading, in particular by using a moving average value.

6. The method according to any one of claims 1 to 5.

7. The modified steering gear ratio (30) has a fixed value. The method according to claim 6.

8. When the steering characteristic is changed, a steering speed of the steering wheel (16) is taken into consideration, and the duration of the cross-fading is changed depending on the steering speed. The method according to claim 6 or 7.

9. An integrator (32) is used for the cross-fading, the integrator (32) being activated by an error signal and / or triggered by a trigger signal provided by the monitoring function (26) in said operating state; 9. The method according to any one of claims 6 to 8.

10. the trigger signal is generated only if the normal or current steering gear ratio (28) is smaller than the corrected steering gear ratio (30) and / or if the sign of the displacement of the steering wheel (16) and the sign of the steering speed correspond to each other.

10. The method of claim 9.

11. a corrected absolute value of the steering speed is used as an input parameter of the integrator (32) and a dead band is used to modify the steering speed; 11. The method according to claim 9 or 10.

12. A computing unit (34) for carrying out the method according to any one of claims 1 to 11.

13. A steering system (10), in particular a steer-by-wire steering system, comprising: an operating unit (14) including at least one steering handle (16) and at least one feedback actuator (18) cooperating with said steering handle (16); at least one wheel steering angle adjuster (20) operatively connected to said operating unit (14) for controlling the wheel steering angle of at least one wheel (22, 24); A calculation unit (34) according to claim 12, A steering system (10) comprising:

14. A vehicle (12), in particular a motor vehicle, comprising: A vehicle (12) comprising a steering system (10) according to claim 13.

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