Method for operating a steering system of a vehicle

Abstract

Method for operating a steering system (10) of a vehicle (12), in particular a motor vehicle, wherein the steering system (10) is designed as a steer-by-wire steering system and comprises an operating unit (14) with at least one steering handle (16) and with at least one feedback actuator (18) cooperating with the steering handle (16), as well as at least one wheel steering angle adjuster (20) operatively connected to the operating unit (14) for changing a wheel steering angle of at least one vehicle wheel (22, 24), wherein an operation of the feedback actuator (18) is monitored by means of a monitoring function (26), and in at least one operating state in which a malfunction and / or failure of the feedback actuator (18) is determined by means of the monitoring function (26), a steering characteristic of the steering system (10) is changed, and wherein a steering ratio between the operating unit (14) and the wheel steering angle adjuster (20) is modified to change the steering characteristic. becomes,characterized in that , - when changing the steering characteristics, a current driving situation and / or an imminent driving situation is taken into account, and / or - when changing the steering characteristics, a dynamic and / or an absolute value of a driver target specification for the wheel steering angle adjuster (20) is taken into account.

Description

State of the art

[0001] The invention is based on a method for operating a steering system of a vehicle according to the preamble of claim 1. The invention also relates to a computing unit for carrying out such a method, a steering system with such a computing unit and a vehicle with such a steering system.

[0002] Vehicles with conventional steering systems are known from the prior art, in which a steering handle, for example in the form of a steering wheel, is mechanically connected via a steering column to a wheel steering angle adjuster in the form of a steering gear. Furthermore, vehicles with steer-by-wire steering systems are known, which do not require a direct mechanical connection between a steering handle and the steered vehicle wheels and in which a driver command and / or steering command are transmitted exclusively electrically. Such a steer-by-wire steering system generally comprises an operating unit with a steering handle and a feedback actuator, as well as at least one wheel steering angle adjuster that is mechanically separated from the operating unit and can be designed, for example, as a central adjuster or as an individual wheel adjuster.

[0003] Furthermore, such steer-by-wire steering systems are generally designed redundantly for reasons of operational safety. With regard to the control unit, one option, for example, is to design the control unit as fail-safe or fail-operational. Furthermore, the control unit can also be designed as fail-operational with regard to detecting a driver's desired input and fail-safe with regard to a feedback torque provided by the feedback actuator. With a corresponding design of the control unit, a malfunction and / or failure of the feedback actuator can lead to unintentional steering movements of the steering handle due to the sudden loss of the feedback torque. These movements are interpreted by the steering system as a driver's desired input and / or steering input and can consequently lead to undesired vehicle reactions.Possible procedures for handling such error cases can be found, for example, in DE 10 2016 009 684 A1 and DE 10 2018 222 442 A1.

[0004] Similar methods are also disclosed in DE 10 2019 135 047 A1, DE 10 2016 005 938 A1, DE 101 01 827 A1 and the unpublished DE 10 2022 122 122 A1.

[0005] A generic method for operating a steering system of a vehicle is also described in DE 10 2021 206 791 B3.

[0006] Based on this, the object of the invention is, in particular, to provide a method with improved properties with regard to functionality. This object is achieved by the features of claims 1, 10, 11, and 12, while advantageous embodiments and further developments of the invention can be found in the subclaims. Disclosure of the invention

[0007] The invention is based on a method for operating a steering system of a vehicle, in particular a motor vehicle, wherein the steering system is designed as a steer-by-wire steering system and comprises an operating unit with at least one steering handle and with at least one feedback actuator interacting with the steering handle and at least one wheel steering angle adjuster operatively connected to the operating unit for changing a wheel steering angle of at least one vehicle wheel, wherein an operation of the feedback actuator is monitored by means of a monitoring function and in at least one operating state in which a malfunction and / or a failure of the feedback actuator is determined by means of the monitoring function, a steering characteristic of the steering system is changed.

[0008] It is proposed that, in order to change the steering characteristics, a steering ratio between the control unit and the wheel steering angle adjuster is modified. The operating state in which a malfunction and / or failure of the feedback actuator is determined by means of the monitoring function therefore corresponds in particular to a fault operating state. Furthermore, the steering characteristics are changed by modifying the steering ratio in particular such that, in the operating state and in particular during a transition of the feedback actuator from an active and / or fully functional state to a passive and / or degraded state, a substantially constant steering behavior is provided and / or achieved, and an unintentional steering movement by a driver due to the malfunction and / or failure of the feedback actuator does not lead to an undesirable vehicle reaction.This configuration, in particular, allows for improved functionality, advantageously improving the controllability and / or manageability of the vehicle in the event of a fault or when the active feedback torque of the feedback actuator is lost. Furthermore, an advantageously adaptive and / or variable method can be provided, in which the steering characteristics can be flexibly adapted to current operating conditions. Furthermore, advantageous maneuverability of the vehicle can be achieved and operational reliability increased.

[0009] The steering system is embodied as a steer-by-wire steering system, in which a driver's desired input and / or steering input, in particular from a driver, is advantageously transmitted purely electrically to the vehicle wheels. For this purpose, the steer-by-wire steering system comprises the control unit, which is in particular redundant, and at least one wheel steering angle adjuster, which is mechanically separated from the control unit and in particular redundant. Preferably, the control unit and the wheel steering angle adjuster are at least partially fail-operational.A "wheel steering angle adjuster" is understood to mean an actuator unit coupled to at least one vehicle wheel, which is intended to transmit a driver target input and / or steering input, in particular a driver's input, to the vehicle wheel by changing a wheel steering angle of at least one vehicle wheel, and thereby advantageously control at least one orientation of the vehicle wheel and / or influence a direction of travel of the vehicle. For this purpose, the wheel steering angle adjuster advantageously comprises at least one steering actuating 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 actuating element. The wheel steering angle adjuster can be designed as a central actuator and can be assigned to at least two vehicle wheels, in particular steerable ones and preferably designed as front wheels.Alternatively, however, the wheel steering angle adjuster can also be designed as an individual wheel adjuster and assigned to exactly one vehicle wheel, in particular a steerable one and preferably designed as a front wheel. Furthermore, a "feedback actuator" should be understood to mean an actuator unit, in particular different from the wheel steering angle adjuster and in particular in direct mechanical connection with the steering handle, which is intended to detect signals, forces and / or torques from the steering handle, in particular directly, and / or to transmit them to the steering handle, in particular directly. In the present case, the feedback actuator is intended, in a normal operating state, at least to provide an active feedback torque and thereby to generate a steering resistance and / or a restoring torque on the steering handle. Furthermore, the feedback actuator is intended in this context to adapt a steering feel, which is in particular perceivable via the steering handle.For this purpose, the feedback actuator can comprise at least one additional electric motor. "A malfunction and / or failure of the feedback actuator" is to be understood in particular as a malfunction and / or failure of the feedback actuator itself and / or of a peripheral assembly interacting with the feedback actuator, such as a power supply, and a malfunction of the feedback actuator caused thereby. Furthermore, the wheel steering angle is fundamentally equivalent to other variables between the steering actuator and the vehicle wheel, such as a deflection of the steering actuating element and / or a deflection position of the wheel steering angle actuator and / or a motor movement. The same applies to a deflection of the steering handle, which is equivalent to other variables between the steering handle and the feedback actuator, such as a steering column angle and / or a motor angle.For the torque quantities at the vehicle wheel and at the steering handle, the same equivalence of the quantities between the vehicle wheel / steering handle and the respective connected actuator applies.

[0010] Furthermore, the vehicle and preferably the steering system comprise at least one computing unit, which is intended to carry out the method for operating the steering system. A "computing unit" is to be understood as an electrical and / or electronic unit which has an information input, an information processing unit, and an information output. Advantageously, the computing unit further comprises at least one processor, at least one operating memory, at least one input and / or output means, at least one operating program, at least one control and / or regulating routine, at least one calculation routine, at least one determination routine, at least one evaluation routine, and / or at least one adaptation routine. In particular, in the present case, the computing unit comprises at least one monitoring function for monitoring operation of the feedback actuator.Furthermore, the computing unit is particularly provided to monitor and evaluate operation of the feedback actuator by means of the monitoring function. Furthermore, the computing unit is provided to change a steering characteristic of the steering system in at least one operating state in which a malfunction and / or failure of the feedback actuator is determined by means of the monitoring function, and to modify a steering ratio between the control unit and the wheel steering angle adjuster for this purpose. In this context, the computing unit can be particularly provided to use an error signal provided by the monitoring function to adapt the steering characteristic. The computing unit is preferably integrated into a control unit of the vehicle, for example a central vehicle control unit, or advantageously a control unit of the steering system, particularly in the form of a steering control unit.A "steering ratio" is to be understood as a gear ratio, in particular a virtual one, between the control unit and the wheel steering angle adjuster, which defines a correlation between a driver's desired input and / or steering input on the steering handle and a wheel steering angle of the vehicle wheel and / or wheels. The steering ratio therefore indicates at least how a deflection of the steering handle or a steering angle of the steering handle affects a deflection of the vehicle wheel and / or wheels or a steering angle of the vehicle wheel and / or wheels. In this case, based on the driver's desired input and / or steering input on the steering handle and the steering ratio, a desired input for the wheel steering angle adjuster is thus obtained, whereby the wheel steering angle adjuster is intended to implement the desired input. "Intended" is to be understood in particular as specially programmed, designed and / or equipped.The fact that an object is intended for a specific function should be understood in particular to mean that the object fulfils and / or executes this specific function in at least one application and / or operating state.

[0011] In this case, when changing the steering characteristics and, in particular, modifying the steering ratio, a current driving situation and / or an imminent driving situation can be taken into account, thereby enabling, in particular, a situationally adapted change in the steering characteristics. In particular, the steering characteristics in this case are changed depending on the current driving situation and / or the imminent driving situation. The current driving situation and / or the imminent driving situation can be determined and / or predicted, for example, based on at least one vehicle variable, such as a yaw rate, a deflection of the steering handle, and / or a steering movement.Alternatively or additionally, the current driving situation and / or the imminent driving situation can also be derived from route planning of a navigation system of the vehicle and / or a corresponding sensor system, for example in the form of a camera system, of the vehicle.

[0012] Advantageously, the operating state corresponds to cornering. Preferably, the steering characteristics are only changed when the steering angle of the vehicle wheel is not equal to zero and, advantageously, is at least 0.1°, particularly in terms of magnitude. Thus, particularly preferably, the steering characteristics are changed when the vehicle is cornering, while the steering characteristics are not changed when the vehicle is traveling straight ahead. This can increase operational reliability, particularly in critical driving situations. Furthermore, a particularly efficient method can advantageously be provided.

[0013] Alternatively or additionally, when changing the steering characteristics and in particular when modifying the steering ratio, a dynamic and / or an absolute value of the driver's target value for the wheel steering angle adjuster, for example a maximum deflection of the steering handle and / or a steering speed of the steering handle, can be taken into account.

[0014] The dynamics and / or the absolute value of the driver's desired input can be determined, for example, using another monitoring function, in particular the computing unit. Furthermore, it is conceivable to change the steering characteristics by modifying the steering ratio only if the dynamics and / or the absolute value of the driver's desired input exceeds a limit. This makes it possible, in particular, to detect an overreaction by the driver when the active feedback torque of the feedback actuator is lost, and to adjust the steering characteristics accordingly.

[0015] It is further proposed that the steering ratio be increased in the operating state to change the steering characteristics. Preferably, the steering ratio is increased from a normal or current steering ratio, particularly one used in a fault-free normal operating state, to a modified steering ratio, wherein the modified steering ratio is at least 10%, preferably at least 20%, and particularly preferably at least 30% higher than the normal steering ratio. This reduces the vehicle's reaction and makes the situation more manageable for the driver.

[0016] According to a particularly preferred embodiment, it is proposed that in the operating state and in particular for changing the steering characteristics, a change from the normal or current steering ratio to the modified steering ratio takes place by means of a cross-fade and in particular using a moving average. For this purpose, a blend factor is advantageously defined which initially has the value 0 and at the end of the cross-fade or blend phase has the value 1. An actual steering ratio then results during the cross-fade or blend phase from a superposition of the normal or current steering ratio and the modified steering ratio, as well as taking the blend factor into account. This makes it possible, in particular, to achieve a particularly harmonious transition from the normal or current steering ratio to the modified steering ratio.

[0017] The normal or current steering ratio and / or the modified steering ratio can be selected, for example, depending on a vehicle speed and / or a deflection of the steering handle and / or a deflection of the steering actuator, for example in the form of a rack stroke, and / or a wheel steering angle of at least one vehicle wheel. However, it is advantageously proposed that the modified steering ratio have a fixed value. This advantageously allows a defined and pre-applied vehicle response to be established in the operating state. Furthermore, the required computing resources can be minimized.

[0018] It is preferably further proposed that when changing the steering characteristics and in particular when modifying the steering ratio, a steering speed of the steering handle is taken into account and a duration of the blending is varied depending on the steering speed. In this case, at least the blending factor is advantageously a dynamic value that can be influenced by the steering speed. In addition, the steering speed can be offset against a further factor, which can, for example, be constant or dependent on the vehicle speed or other vehicle variables. This embodiment advantageously allows for a more or less rapid blending from the normal or current steering ratio to the modified steering ratio depending on the steering dynamics.

[0019] A particularly simple implementation of the crossfade in software terms can be achieved in particular if an integrator is used for the crossfade. In particular, the computing unit can comprise the integrator. Preferably, the integrator is activated in the operating state by an error signal provided by the monitoring function and / or released by an enable signal. In this context, the integrator is preferably only activated when a fault and / or failure of the feedback actuator is detected by the monitoring function. Accordingly, the integrator is preferably deactivated in the normal operating state, which can advantageously reduce resource requirements. In addition, the integrator can initially be locked after activation, so that an additional release must occur via the enable signal.

[0020] Preferably, the enable signal is only generated if the normal or current steering ratio is smaller than the modified steering ratio and / or a sign of a steering handle deflection and a sign of the steering handle's steering speed match. In this context, the integrator is thus only enabled under the condition that the normal or current steering ratio is smaller than the modified steering ratio and / or under the condition that the driver actually increases their desired steering input or steering input and thus, when cornering, steers further into the curve. This can, in particular, further increase operational reliability.

[0021] Furthermore, it is proposed that a modified absolute value of the steering speed be used as the input variable of the integrator, with a deadband being used to modify the steering speed. In particular, the deadband ensures that a reaction only occurs when the driver initiates significant steering movements or the steering speed reaches a corresponding threshold. This can advantageously compensate for an overreaction by the driver when the active feedback torque of the feedback actuator is lost.

[0022] In a further embodiment, it is also proposed that a vehicle speed be taken into account when changing the steering characteristics, and in particular when modifying the steering ratio. Preferably, the vehicle speed is determined and evaluated at least to determine the normal or current steering ratio and / or the modified steering ratio. Advantageously, the normal or current steering ratio and / or the modified steering ratio are therefore dependent on the vehicle speed. This allows the steering characteristics to be adapted particularly flexibly to current operating conditions.

[0023] The method for operating the steering system is not intended to be limited to the application and embodiment described above. In particular, the method for operating the steering system may comprise a number of individual elements, components, and units that differs from the number stated herein to fulfill a functionality described herein. Drawings

[0024] Further advantages will become apparent from the following description of the drawings. The drawings illustrate an embodiment of the invention.

[0025] They show: Fig. 1a-b a vehicle with a steering system designed as a steer-by-wire steering system in a simplified representation, Fig. 2 example diagrams of various signals for the operation of the steering system and Fig. 3 an exemplary flow chart with main process steps of a method for operating the steering system. Description of the embodiment

[0026] The Fig. 1a and Fig. 1b shows a simplified representation of a vehicle 12, embodied as a passenger vehicle, with a plurality of vehicle wheels 22, 24 and a steering system 10. The steering system 10 has an operative connection with the vehicle wheels 22, 24 and is provided for influencing a direction of travel of the vehicle 12. Furthermore, the steering system 10 is embodied here as a steer-by-wire steering system, in which a driver command or steering command is electrically transmitted to the vehicle wheels 22, 24 in at least one operating state.

[0027] The steering system 10 has an operating unit 14, which can be actuated in particular by a driver and / or passenger. 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 in a normal operating state at least to provide an active feedback torque and thereby to generate a steering resistance and / or a restoring torque on the steering handle 16. For this purpose, the feedback actuator 18 comprises at least one electric motor (not shown), in particular designed as a permanently excited synchronous motor. Furthermore, the operating unit 14 is designed to be fail-operational with regard to detecting the driver's desired input, in particular via the steering handle 16, and fail-safe with regard to the feedback torque provided by the feedback actuator 18.Alternatively, a steering handle could also be designed as a joystick, a steering lever, and / or a steering ball, or the like. Furthermore, a feedback actuator could comprise multiple electric motors. Furthermore, an operating unit could also be designed to be fail-safe or fail-operational.

[0028] Furthermore, the steering system 10 comprises a conventional wheel steering angle adjuster 20. The wheel steering angle adjuster 20 is mechanically separated from the control unit 14. The wheel steering angle adjuster 20 is connected purely electrically to the control unit 14. Furthermore, the wheel steering angle adjuster 20 is embodied, for example, as a central adjuster. The wheel steering angle adjuster 20 has an operative connection with at least two of the vehicle wheels 22, 24, in particular two front wheels, and is intended to convert the driver's desired input or steering input into a steering movement of the vehicle wheels 22, 24. For this purpose, the wheel steering angle adjuster 20 comprises a steering actuating element 36, embodied, for example, as a rack, and a steering actuator 38 that interacts with the steering actuating element 36.In this case, the steering actuator 38 comprises at least one further electric motor (not shown), particularly embodied as a permanent magnet synchronous motor, and is provided for controlling the steerable vehicle wheels 22, 24. In principle, a steering system could, of course, also comprise several wheel steering angle adjusters, particularly embodied as individual wheel adjusters. Furthermore, a steering actuator could comprise several electric motors.

[0029] In addition, the vehicle 12 has a control unit 40. In the present case, the control unit 40 is designed as a steering control unit and is therefore part of the steering system 10. The control unit 40 has an electrical connection to the wheel steering angle adjuster 20. The control unit 40 also has an electrical connection to the operating unit 14. The control unit 40 is provided at least for controlling the operation of the steering system 10. In the present case, the control unit 40 is provided to control the steering actuator 38 as a function of a signal from the operating unit 14, for example as a function of the driver's desired input or steering input and / or a manual torque. The control unit 40 can also be provided to control the feedback actuator 18 as a function of a signal from the wheel steering angle adjuster 20.

[0030] For this purpose, the control unit 40 comprises a computing unit 34. The computing 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). In addition, the computing unit 34 comprises at least one operating program stored in the operating memory with at least one calculation routine, at least one determination routine, at least one evaluation routine, and at least one adaptation routine. In addition, the computing unit 34 in the present case comprises at least one monitoring function 26. In principle, however, a control unit could also be different from a steering control unit and, for example, be designed as a single, central vehicle control unit with a central computing unit. It is also conceivable to provide separate control units and / or computing units for a wheel steering angle adjuster and for an operating unit and to connect these to one another in a communicating manner.

[0031] In addition, the vehicle 12 and / or the steering system 10 may comprise further components and / or assemblies not shown, such as, for example, an internal vehicle sensor system for detecting at least one vehicle variable, for example a yaw rate, an external sensor system, for example in the form of a camera system, and / or a navigation system known per se.

[0032] In the event of a malfunction and / or failure of the feedback actuator 18, under certain circumstances and / or in certain driving situations, such as cornering, the sudden loss of the feedback torque during a transition of the feedback actuator 18 from an active and / or fully functional state to a passive and / or degraded state may result in unintended steering movements of the steering handle 16. These movements may be interpreted by the steering system 10 as a driver command and / or steering command and may consequently lead to undesirable vehicle reactions. In this context, it is assumed that the passive behavior of the feedback actuator 18 with regard to torque feedback is sufficient to operate the vehicle 12 safely, and only the transition from the active to the passive case may pose a challenge in terms of controllability.The reason for this is that the feedback torque suddenly decreases in the event of a corresponding malfunction and / or failure of the feedback actuator 18, since the inherent passive friction in the steering system 10 is significantly smaller than the feedback torque in the normal operating state. Particularly during cornering, a sudden reduction in the feedback torque and consequently a counter-torque on the steering handle 16 can lead to safety-critical situations, since the driver's reaction time means they can only readjust their holding force with a time delay and thus steer further into the curve than intended. Depending on the steering ratio, a more indirect ratio can increase controllability even after the transition to the passive state.

[0033] To avoid such safety-critical situations, a method for operating the steering system 10 is proposed below. In this case, the computing unit 34 is provided to execute the method and, in particular, has a computer program with corresponding program code means for this purpose. Alternatively, however, a computing unit of a control unit assigned to an operating unit could also be provided to implement the method.

[0034] According to the invention, an operation of the feedback actuator 18 is monitored by means of the monitoring function 26 and, in at least one operating state in which a malfunction and / or failure of the feedback actuator 18 is determined by means of the monitoring function 26, a steering characteristic of the steering system 10 is changed by modifying a, in particular virtual, steering ratio between the operating unit 14 and the wheel steering angle adjuster 20.In the present case, the steering characteristic is changed by modifying the steering ratio in such a way that in the operating state and in particular during a transition of the feedback actuator 18 from an active and / or fully functional state to a passive and / or degraded state, a substantially constant steering behavior is provided and / or achieved and an unintentional steering movement of a driver due to the malfunction and / or failure of the feedback actuator 18 does not lead to an undesirable vehicle reaction.

[0035] In this case, a current driving situation and / or an imminent driving situation are taken into account, and the steering characteristics are changed depending on the current driving situation and / or the imminent driving situation. Preferably, a corresponding change in the steering characteristics only occurs when cornering or when the vehicle 12 is detected to be cornering. Accordingly, the operating state corresponds to cornering, with the steering characteristics only being changed if the wheel steering angle of the vehicle wheels 22, 24 is not equal to zero.

[0036] Furthermore, when changing the steering characteristics and, in particular, modifying the steering ratio, a dynamic and / or an absolute value of the driver's target input for the wheel steering angle adjuster 20 is taken into account, for example, a maximum deflection of the steering handle 16 and / or a steering speed of the steering handle 16. The dynamic and / or the absolute value of the driver's target input can be determined, for example, by means of a corresponding vehicle sensor system and / or by means of a further monitoring function (not shown) of the computing unit 34.In this context, the steering characteristics can be changed by modifying the steering ratio, for example, if the dynamics and / or the absolute value of the driver's target input exceeds a limit value, whereby an overreaction of the driver when the active feedback moment of the feedback actuator 18 is eliminated can be determined and the steering characteristics can be adapted depending on this.

[0037] In addition, the steering ratio is increased in the operating state, in particular from a normal or current steering ratio 28, which is used in particular in the fault-free normal operating state, to a modified steering ratio 30 (cf. in particular also Fig. 2). The normal or current steering ratio 28 has a fixed value. The same applies to the modified steering ratio 30. Furthermore, the normal or current steering ratio 28 and the modified steering ratio 30 in this case are selected depending on the vehicle speed. In principle, however, constant values ​​for a normal or current steering ratio and / or a modified steering ratio are also possible. Increasing the steering ratio is particularly useful if, based on the current driving situation and / or the imminent driving situation and / or the dynamics and / or the absolute value of the driver's target input, it is determined that the driver is steered further into the curve than intended. Increasing the steering ratio in this case reduces the vehicle's reaction and makes the situation more manageable for the driver.In principle, controllability increases by increasing the steering ratio, as the loss of driver steering precision due to the lack of feedback can be at least partially compensated for by a reduction in vehicle response. For example, if the steering ratio is increased from 10 to 15, a deflection of the steering handle 16 to 85° results in a wheel steering angle of only 5.67°. This means that the driver only needs to correct or redirect the steering handle 16 by 40° to achieve the required 3° wheel steering angle for cornering.

[0038] A change from the normal or current steering ratio 28 to the modified steering ratio 30 is also achieved by means of a crossfade and using a moving average. For this purpose, a blend factor is defined, which initially has the value 0 and at the end of the crossfade or blend phase has the value 1. An initial steering ratio or actual steering ratio is then determined during the crossfade or blend phase by superimposing the normal or current steering ratio 28 and the modified steering ratio 30, as well as taking the blend factor into account. In this context, the following applies to the initial steering ratio or actual steering ratio: VSR=VSRnorm⋅(1−KP)+VSRmod⋅KP

[0039] VSR describes the initial steering ratio or actual steering ratio, VSR norm the normal or current steering ratio 28 and VSR modthe modified steering ratio 30, while KP represents the glare factor.

[0040] The blend factor is also a dynamic value that can be influenced by a steering speed of the steering handle 16, thereby determining the duration of the blending. Accordingly, when changing the steering characteristics and, in particular, modifying the steering ratio, the steering speed of the steering handle 16 is taken into account, and the duration of the blending varies depending on the steering speed. This advantageously allows for a more or less rapid blending from the normal or current steering ratio 28 to the modified steering ratio 30, depending on the steering dynamics.

[0041] Furthermore, an integrator 32 is used for cross-fading. The integrator 32 is 0 and / or deactivated in the normal operating state and is only activated in the operating state by an error signal provided by the monitoring function 26. Consequently, the integrator 32 is only activated when a fault and / or failure of the feedback actuator 18 is detected by the monitoring function 26. Furthermore, in the present case, the integrator 32 must also be enabled by an enable signal, particularly after activation by the error signal. The enable signal is preferably only generated when the normal or current steering ratio 28 is smaller than the modified steering ratio 30 and a sign of a deflection of the steering handle 16 and a sign of the steering speed of the steering handle 16 match one another. In this context, the integrator 32 is therefore only enabled under the condition that the normal orThe current steering ratio 28 is smaller than the modified steering ratio 30, and under the condition that the driver actually increases his or her target steering input and thus steers further into the curve when cornering. In principle, however, activation and / or enabling of the integrator could also be omitted.

[0042] The actual input variable of integrator 32 is a modified absolute value of the steering speed. The steering speed is modified via a deadband, so that a reaction only occurs when the driver initiates significant steering movements or the steering speed reaches a corresponding threshold. Scaling can also be performed to select the duration of the fade or the dynamics of the fade accordingly.

[0043] Fig. 2 shows exemplary diagrams of various signals for operating the steering system 10.

[0044] An ordinate axis 42 is designed as a magnitude axis. A time in [s] is shown on an abscissa axis 44. A curve 46 shows a hypothetical course of a deflection of the steering handle 16, in this case in particular in the form of an actual steering wheel angle, without changing the steering characteristics by modifying the steering ratio. A curve 48 shows a course of a deflection of the steering handle 16, in this case in particular in the form of an actual steering wheel angle, with a change in the steering characteristics by modifying the steering ratio. A curve 50 shows a hypothetical course of a driver target specification for the wheel steering angle adjuster 20 or a wheel steering angle of the vehicle wheels 22, 24 without changing the steering characteristics by modifying the steering ratio. A curve 52 shows a course of a driver target specification for the wheel steering angle adjuster 20 ora wheel steering angle of the vehicle wheels 22, 24 with a change in the steering characteristics by modifying the steering ratio. Curve 54 shows the normal steering ratio 28 as well as a hypothetical curve of the normal steering ratio 28 without a change in the steering characteristics by modifying the steering ratio. Curve 56 shows the modified steering ratio 30.

[0045] Based on Fig. 2 shows that the driver initially enters a curve and then drives at a constant radius until the feedback actuator 18 fails at second 10. Curves 50 and 52 clearly show the reduced steering angle of the vehicle wheels 22, 24 achieved by the method according to the invention and the associated reduced vehicle reaction. Furthermore, curves 46 and 48 show that after a reaction time, the correction angle required to drive the same radius as before the failure of the feedback actuator 18 can be smaller.

[0046] Fig. 3 finally shows an exemplary flow chart with main process steps of the method for operating the steering system 10.

[0047] A method step 60 corresponds to a normal operating state, in particular a fault-free one. In this case, the steering ratio corresponds to a normal steering ratio 28 and can, for example, have a value of 10. Furthermore, the operation of the feedback actuator 18 is monitored by means of the monitoring function 26.

[0048] In a method step 62, a fault and / or failure of the feedback actuator 18 is determined by means of the monitoring function 26.

[0049] In a subsequent method step 64, a steering characteristic of the steering system 10 is changed by modifying a, in particular virtual, steering ratio between the control unit 14 and the wheel steering angle adjuster 20. For this purpose, the steering ratio is advantageously increased and changed from the normal steering ratio 28 to a modified steering ratio 30, which may, for example, have a value of 15.

[0050] The example flow chart in Fig. 3 is intended merely as an example to describe a method for operating the steering system 10. In particular, individual method steps may vary, or additional method steps may be added. In this context, it is conceivable, for example, to consider a current driving situation and / or an imminent driving situation and / or a dynamic and / or an absolute value of a driver target specification for the wheel steering angle adjuster 20. Furthermore, a change from the normal steering ratio 28 to the modified steering ratio 30 can be achieved by means of a crossfade.

Claims

[1] Method for operating a steering system (10) of a vehicle (12), in particular a motor vehicle, wherein the steering system (10) is designed as a steer-by-wire steering system and comprises an operating unit (14) with at least one steering handle (16) and with at least one feedback actuator (18) cooperating with the steering handle (16), as well as at least one wheel steering angle adjuster (20) operatively connected to the operating unit (14) for changing a wheel steering angle of at least one vehicle wheel (22, 24), wherein an operation of the feedback actuator (18) is monitored by means of a monitoring function (26) and, in at least one operating state in which a fault and / or failure of the feedback actuator (18) is determined by means of the monitoring function (26), a steering characteristic of the steering system (10) is changed, and wherein, in order to change the steering characteristic, a steering ratio between the operating unit (14) and the wheel steering angle adjuster (20) is modified, characterized by, that - when changing the steering characteristics, a current driving situation and / or an imminent driving situation is taken into account, and / or - when changing the steering characteristics, a dynamic and / or an absolute value of a driver target specification for the wheel steering angle adjuster (20) is taken into account. [2] Method according to claim 1, characterized by that the operating state corresponds to cornering and the steering characteristic is only changed if the wheel steering angle of the vehicle wheel (22, 24) is not equal to zero. [3] Method according to one of the preceding claims, characterized by that the steering ratio is increased in the operating state. [4] Method according to one of the preceding claims, characterized bythat in the operating state a change from a normal or current steering ratio (28) to a modified steering ratio (30) takes place by means of a cross-fade and in particular using a moving average. [5] Method according to claim 4, characterized by that the modified steering ratio (30) has a fixed value. [6] Method according to claim 4 or 5, characterized by that when changing the steering characteristic, a steering speed of the steering handle (16) is taken into account and a duration of the cross-fading is varied depending on the steering speed. [7] Method according to one of claims 4 to 6, characterized by that an integrator (32) is used for cross-fading, which integrator is activated in the operating state by an error signal provided by the monitoring function (26) and / or released by an enable signal. [8] Method according to claim 7, characterized bythat the release signal is only generated if the normal or current steering ratio (28) is smaller than the modified steering ratio (30) and / or a sign of a deflection of the steering handle (16) and a sign of the steering speed agree with each other. [9] Method according to claim 7 or 8, characterized by that a modified absolute value of the steering speed is used as the input variable of the integrator (32), wherein a dead zone is used to modify the steering speed. [10] Computing unit (34) for carrying out a method according to one of the preceding claims. [11] Steering system (10), in particular a steer-by-wire steering system, with an operating unit (14) which comprises at least one steering handle (16) and at least one feedback actuator (18) cooperating with the steering handle (16), with at least one wheel steering angle adjuster (20) operatively connected to the operating unit (14) for controlling a wheel steering angle of at least one vehicle wheel (22, 24), and with a computing unit (34) according to claim 10. [12] Vehicle (12), in particular motor vehicle, with a steering system (10) according to claim 11.

Images