Motor vehicle and method for operating a motor vehicle in the event of a detected malfunction of the steer-by-wire steering system

Abstract

The present invention relates to a method for operating a motor vehicle (1) with a steer-by-wire steering system (10), a drive system (60), a braking system (70), and a spring-damping system (80), wherein controllable actuators (16, 19, 61, 62, 71, 72, 73, 74, 81, 82, 83, 84) are assigned to the systems (10, 60, 70, 80), and the steer-by-wire steering system (10) comprises a steering actuator (16) acting on a coupling element (15) as a controllable actuator, wherein a wheel steering angle (α) of steered wheels (7) of the motor vehicle (1) can be adjusted via the coupling element (15), and wherein, during driving operation of the motor vehicle (1), a desired trajectory (2) for the motor vehicle (1) is determined, and in the event of a detected malfunction of the steer-by-wire steering system (10), a subset of the functional actuators (71, 72, 73, 74) is controlled in such a way that the motor vehicle (1) continues to follow the determined target trajectory (2),and the steering actuator (16) is also controlled in such a way that the steering actuator (16) selectively dampens or allows a change in the steering angle (α) of steered wheels (7) of the motor vehicle (1) to reduce a deviation from the intended trajectory (2). Furthermore, the invention relates to a motor vehicle (1) that is designed for operation according to such a method.

Description

2 Furthermore, a method for trajectory control of a motor vehicle with a steer-by-wire steering system is known from DE102019200716A1. It is described that two steering actuators are provided for steering the motor vehicle, with each actuator being assigned to a steerable wheel. By detecting and evaluating saturation indicators and actuator efficiency variables in both actuators, the functionality of both actuators is monitored. If the function of one of the two actuators is impaired, a larger steering angle is requested from the second actuator and a maximum possible steering angle from the first actuator in order to follow a target trajectory. Furthermore, US2003 / 0221894A1 discloses a method for operating a steer-by-wire steering system in a motor vehicle in the event of a functional impairment of the steering actuator unit.It is provided that by controlling the brakes of the motor vehicle, a change in the direction of the motor vehicle is caused, whereby vibrations of the steerable wheels are dampened by a short circuit of a winding of the motor of the steering actuator unit.15 In DE102022119973A1, it is further disclosed for a motor vehicle with a steer-by-wire steering system, in the event of a malfunction of the front axle steering, a steering motor vehicle using a torque vectoring device, whereby a phase short circuit of the electric motor of the front axle steering causes a damping of steering movements of the steerable wheels20. Also from WO2024 / 133152A1, damping of a movement of a rack of the front axle steering is known for a vehicle with a steer-by-wire steering system. 25 Furthermore, EP0999117A2 describes a motor vehicle with a steer-by-wire steering system, whereby it is provided that steering interventions can also be achieved by targeted braking interventions using a braking system of the motor vehicle, especially in the event of a malfunction of the steering system.30 Starting from this, the objective of the present invention is to further reduce the risk of an accident for a motor vehicle in the event of a malfunction in relation to the steer-by-wire steering system and to improve the vehicle stability and controllability of the motor vehicle in the event of an impairment of the steer-by-wire steering system. 35 BE2024 / 5839 3 To solve this objective, a method for operating a motor vehicle according to claim 1 and a motor vehicle according to the dependent claim are proposed. Further advantageous embodiments of the invention are described in the dependent claims and the description and are illustrated in the figures. 5 The proposed solution provides a method for operating a motor vehicle with a steer-by-wire steering system, a drive system, a braking system and a spring-damping system.The systems, in particular the steer-by-wire steering system, the drive system, the braking system, and the suspension system, are assigned controllable actuators with which the respective system can advantageously be controlled.10 The steer-by-wire steering system of the motor vehicle comprises, as a controllable actuator, a steering adjuster acting on a coupling element, in particular on a rack, whereby the steering angle of steered wheels of the motor vehicle can be set via the coupling element. The method further provides that, during operation of the motor vehicle, a target trajectory for the motor vehicle is determined taking into account a current driving instruction from a vehicle user or a current driving instruction from a driver assistance system. In addition, the functionality of the actuators is checked, in particular continuously at preferably predetermined time intervals.In the event of a detected malfunction of the steer-by-wire steering system, particularly a malfunction that restricts the steering capability of the steer-by-wire steering system, at least a subset of the 20 functional actuators are controlled in such a way that the vehicle continues to follow the specified target trajectory, at least to a large extent. "To a large extent" means, in particular, that the vehicle essentially follows the target trajectory, but deviations may occur, so that an actual trajectory does not necessarily have to be identical to the target trajectory, but in particular a direction of travel resulting from the target trajectory is maintained. Furthermore, the procedure provides that the steering actuator acting on the coupling element is additionally controlled in such a way that the steering actuator specifically dampens or allows a change in the steering angle of steered wheels of the motor vehicle to reduce a deviation from the target trajectory30.In particular, the steering actuator is controlled accordingly, especially by a control unit, and furthermore especially by a control unit of a vehicle motion control system of the motor vehicle. Thus, for example, if a currently set wheel steering angle is suitable so that the motor vehicle continues to follow the target trajectory, the steering actuator advantageously dampens movement of the connecting rod and thus of the steered wheels 35 BE2024 / 5839 4 and thus advantageously ensures that the set wheel steering angle is essentially maintained unchanged.If, for example, a currently set wheel steering angle is unsuitable for the vehicle to continue following the intended trajectory, depending in particular on the functionality of the other actuators, it is advantageous either to control the steering actuator in such a way that it dampens a movement of the connecting rod, 5 especially to make the driving behavior of the vehicle deterministic and thus to increase the controllability of the vehicle, which is particularly advantageous with small wheel steering angles, and furthermore especially with wheel steering angles between 0° and 5°., or further advantageously, according to the invention, the steering actuator acting on the coupling element is further controlled in such a way that a change in the steering angle of the steered wheels is permitted, 10 in particular if, by controlling further actuators, especially brake, drive and / or spring-damping actuators, the steering angle can be adjusted to maintain the target trajectory, whereby a steering angle then set is advantageously maintained again by damping the coupling element by means of the steering actuator. In particular, it is also provided that, for the adjustment of the steering angle 15 by means of the further actuators, especially the brake, drive and / or spring-damping actuators, a change in the steering angle is repeatedly damped or permitted alternately by the steering actuator. By damping, the The connecting rod is fixed in a current position, in particular to reduce negative influences that counteract the setting of the desired wheel steering angle, especially due to an uneven road surface.Advantageously, by controlling the steering actuator, the vehicle stability and controllability of the motor vehicle are improved in the event of a malfunction of the steer-by-wire steering system, which reduces the risk of accidents as a result of the malfunctions. Depending on the degree of steering capability still obtained through appropriate control of the functioning actuators, it may be provided that the motor vehicle still performs a so-called "limp-home maneuver". The functionality of the actuators is advantageously checked by querying a status signal from the actuators, whereby the query is carried out in particular at recurring intervals. The status signal can be provided in particular by a diagnostic unit assigned to the respective actuator. Additionally or alternatively, it is advantageously provided that the functionality of the actuators is checked by comparing a performance requested by a respective actuator with a performance provided by the respective actuator.In particular, if a requested BE2024 / 5839 5 performance deviates significantly from a provided performance, a functional failure of the corresponding actuator unit is advantageously recognized. In particular, it is provided that the steering actuator comprises a multi-phase electric motor, especially a three-phase electric motor. A change in the wheel steering angle or a movement of the connecting rod is then advantageously dampened by a short-circuiting of phases. By a disconnecting short circuit of the phases, i.e., an interruption of the conductive connection between the phases created by the short circuit, a change in the wheel steering angle or a movement of the connecting rod is advantageously permitted. A further advantageous embodiment of the proposed method also provides that the steer-by-wire steering system further comprises a rear-wheel steering system with a rear-wheel steering actuator as a further controllable actuator.In the event of a detected malfunction of the steer-by-wire steering system, the rear-wheel steering actuator is advantageously controlled, assuming its functionality, as an actuator from the subset of functioning actuators in such a way that the vehicle continues to follow the determined target trajectory, at least to a large extent. In particular, it is provided that the front-wheel steering of the steer-by-wire steering system, with the steering actuator acting on the coupling element, is primarily responsible for converting a steering input into a corresponding wheel steering angle in normal, trouble-free operation, while the rear-wheel steering contributes to the conversion of the steering input depending on the driving situation during normal operation. According to a further advantageous design, the braking system includes brake actuators assigned to the wheels of the vehicle as additional controllable actuators.In the event of a detected malfunction of the steer-by-wire steering system, the brake actuators, while functioning as actuators from the subset of functioning actuators, are controlled asymmetrically in such a way that a yaw moment is generated, so that the vehicle continues to follow the intended trajectory, at least to a large extent. In particular, it is provided that, especially for setting a wheel steering angle, in particular for increasing or decreasing a set wheel steering angle, a change in the wheel steering angle is permitted by the steering actuator in the event of a malfunction of the steer-by-wire steering system, so that movement of the coupling element is not damped, and by targeted braking of the front left wheel or the front right wheel, the wheel steering angle is influenced in such a way that the vehicle continues to follow the intended trajectory with improved accuracy.35 BE2024 / 5839 6 Furthermore, an advantageous development of the proposed method provides that the vehicle's drive system includes drive actuators assigned to the vehicle's wheels as additional controllable actuators, in particular an electric motor assigned to each wheel as drive actuators. In the event of a detected malfunction of the steer-by-wire steering system, the drive actuators, assuming proper functionality, are controlled as actuators from the subset of functioning actuators in such a way that a yaw moment is generated, so that the vehicle continues to follow the intended trajectory, at least to a large extent. Furthermore, it is advantageous that, in the event of a malfunction of the steer-by-wire steering system, a set wheel steering angle can also be changed, in particular by targeted control of the drive actuator assigned to the front left wheel or the front right wheel. especially enlarged or decreased.For changing the wheel steering angle, in particular for increasing or decreasing a set wheel steering angle, movement of the coupling element by the steering actuator is advantageously permitted, i.e., the movement of the coupling element is not damped. By selectively applying a drive torque to the front left wheel and / or the front right wheel, the wheel steering angle is advantageously influenced in such a way that the vehicle follows the intended trajectory with further improvement. Furthermore, the spring-damping system of the vehicle advantageously comprises damping actuators assigned to each wheel of the vehicle, in particular semi-active dampers or active dampers, as additional controllable actuators.In the event of a detected malfunction of the steer-by-wire steering system, the damping actuators, assuming proper functionality, are advantageously controlled as actuators from the subset of functioning actuators in such a way that the steering angle of the steerable wheels is specifically influenced, in particular increased or decreased, so that the vehicle continues to follow the intended trajectory, at least to a large extent. Here, too, movement of the coupling element is advantageously dampened or permitted by the steering actuator, depending on the requirements. According to a further advantageous embodiment, in the event of a detected malfunction of the steer-by-wire steering system, at least one actuator from the subset of functioning actuators is controlled in such a way that the vehicle is brought to a standstill. In particular, it is intended that the functional actuators of the motor vehicle are controlled in such a way that the motor vehicle performs a so-called "limp-aside maneuver".In this process, the vehicle advantageously continues to follow the intended trajectory in essence during the reduction of the vehicle's speed, thereby advantageously preventing a collision with an obstacle. In particular, it is provided that the braking system includes brake actuators assigned to the wheels of the vehicle as further controllable actuators, whereby, in the event of a detected malfunction of the steer-by-wire steering system, the brake actuators, provided they are functioning correctly, are controlled as actuators from the subset of functioning actuators in such a way that the vehicle is brought to a standstill. Advantageously, the control of the brake actuators to maintain the intended trajectory has priority.10. Advantageously, the drive system further comprises drive actuators assigned to the wheels of the motor vehicle, in particular electric motors assigned to the wheels, as additional controllable actuators, whereby in the event of a detected malfunction of the steer-by-wire steering system, the drive actuators, provided they are functioning correctly, are controlled as actuators from the subset of functioning actuators in such a way that the motor vehicle is brought to a standstill. Advantageously, the drive actuators are used as brakes in generator mode. Advantageously, controlling the drive actuators to maintain the desired trajectory takes precedence over decelerating the motor vehicle by corresponding control of the drive actuators. 20 The motor vehicle proposed to solve the aforementioned problem further comprises a steer-by-wire steering system, a drive system, a braking system and a spring damping system, wherein controllable actuators are assigned to the systems.The steer-by-wire steering system comprises a controllable actuator and a steering adjuster acting on a coupling element, in particular a rack, whereby the steering angle of the steered wheels of the motor vehicle can be adjusted via the 25 coupling elements. Furthermore, the motor vehicle comprises a vehicle motion control system, which is advantageously connected to all actuators for signal transmission. The motor vehicle is designed to be operated according to a method configured according to the invention, in particular using the vehicle motion control system. Advantageously, each of the 30 actuators is assigned corresponding actuator control units, which are configured to control the actuators, in particular according to instructions specified by the vehicle motion control system. Advantageously, the proposed Motor vehicles realize the advantages explained in connection with the design of the proposed procedure BE2024 / 5839 8.The features of the method are also applicable to motor vehicles in a corresponding manner. Further advantageous details, features, and design details of the invention are explained in more detail in connection with the five exemplary embodiments shown in the figures (hereinafter referred to as Fig.: figure). Fig. 1 shows, in a schematic representation, an exemplary embodiment for a motor vehicle designed according to the invention during operation according to an exemplary embodiment for a method designed according to the invention; Fig. 2 shows, in a simplified perspective view, an advantageous design of a steer-by-wire steering system of an exemplary embodiment for a motor vehicle designed according to the invention; Fig. 3 shows, in a simplified circuit diagram, an exemplary embodiment for a circuit arrangement for controlling the steering actuator according to an exemplary embodiment for a method designed according to the invention; and Fig.Figure 4a shows a schematic representation of a section of a steer-by-wire steering system 20 and, to illustrate a control of the steering actuator according to an embodiment, Figure 4b shows a method designed according to the invention. In the various figures, identical parts are generally provided with the same reference numerals and are therefore sometimes only explained in connection with one of Figures 25. Figure 1 shows an advantageous embodiment for a motor vehicle 1. The motor vehicle 1 comprises a steer-by-wire steering system 10, a drive system 60, a brake system 70, and a spring-damping system 80, which are each shown only schematically 30. In addition, the motor vehicle comprises a vehicle motion control system 3, which is connected to the systems 10, 60, 70, and 80 of the motor vehicle 1 for the transmission of signals. Furthermore, the Vehicle Motion Control System receives 3 values ​​from sensors (not explicitly shown in Fig. 1) relating to the current driving condition of the motor vehicle 1.The Vehicle Motion Control System 3 is specifically designed to react to disturbances in the systems 35 BE2024 / 5839 9 10, 60, 70, 80 and to operate the motor vehicle in emergency mode, thereby at least partially compensating for the failure of actuators by controlling other actuators. Advantageously, the Vehicle Motion Control System 3 specifies target values ​​that are to be implemented by the corresponding actuators using an actuator control unit assigned to each of these actuators (not explicitly shown in Fig. 1). 5 In this embodiment, damping actuators 81, 82, 83, 84 at the respective wheel suspensions are controllable by the spring-damping system 80 of the motor vehicle. assigned, whereby the damping actuators 81, 82, 83, 84 can be actively controlled, in particular via solenoid valves and hydraulic elements. In a trouble-free normal operation 10, the damping actuators 81, 82, 83, 84 can be used to actively counteract, in particular, heaving, pitching and rolling movements of the motor vehicle 1.In addition, the ground clearance of the motor vehicle can be advantageously varied as needed in a trouble-free normal operating mode by means of the active spring damping system80. 15 Furthermore, the braking system 70 of the motor vehicle 1 comprises individually controllable brake actuators 71, 72, 73, 74, each assigned to the wheels 7, 8 of the motor vehicle 1. The drive system 60 of the motor vehicle 1, in this embodiment, comprises a drive actuator 61 assigned to the rear wheels 8 and a drive actuator 62 assigned to the front wheels 7. The drive actuators 61, 62 are 20 each designed as electric motors in this embodiment, with which the motor vehicle can be driven at all four wheels 7, 8. The drive torque to be provided to the wheels 7, 8 is adjustable, in particular, depending on a setting made by a vehicle user or a driver assistance system, especially also the vehicle motion control system 3.25 The steer-by-wire steering system 10, which is further comprised of the motor vehicle 1, includes in this embodiment a front-wheel steering system with a steering actuator 16 acting on a coupling element 15 as a controllable actuator. In normal, trouble-free operation, the steering actuator 16 can adjust a steering angle of the steered wheels 7 of the motor vehicle 1 via the coupling element 15, in particular depending on a steering command given by a vehicle user or a driver assistance system. As indicated in Fig. 1, the steer-by-wire steering system 10 can optionally also have a rear-wheel steering system with a rear-wheel steering actuator 19, with which the rear wheels can then also be steered, and thus an all-wheel steering system. In particular, it is intended that with the 35 BE2024 / 5839 10 Rear-wheel steering allows only comparatively small steering angles to be set, and all-wheel steering is operated as proportional steering, especially in normal operation.A detailed embodiment of an advantageous design of the steer-by-wire steering system 10 of the motor vehicle 1 is shown in Fig. 2. In this embodiment, the steer-by-wire steering system 10 comprises a steering column with a steering shaft 11 and a feedback actuator 13. A steering handle 12, designed as a steering wheel, is fixedly mounted at one end of the steering shaft 11. A vehicle operator can issue a steering command via the steering handle 12. The feedback actuator 13 is designed to exert a torque or steering resistance torque on the steering shaft 1110, in particular to provide a steering feel. This steering resistance torque is transmitted by a vehicle operator of the motor vehicle 1 via the steering handle 3 as Steering resistance is perceptible. To generate the torque, the feedback actuator 5 comprises a feedback actuator electric motor with an associated feedback actuator control unit for controlling the feedback actuator electric motor.15 The steering handle 12 of the steer-by-wire steering system 10 can be rotated in a known manner to introduce a steering command into the steering shaft 11, which is detected by sensors. For this purpose, the steer-by-wire steering system 10 in this embodiment comprises a first steering angle sensor unit (not explicitly shown in Fig. 2) associated with the steering shaft 11, in particular an absolute angle sensor unit 20, and a second steering angle sensor unit (also not explicitly shown in Fig. 2), in particular a relative angle sensor unit, which are configured to detect an angle set by means of the steering handle 12 as the steering angle of the steering shaft 11. A detected steering command is transmitted via a signal line 18 to a steering actuator unit 16 of the steer-by-wire steering system 10, which acts on a coupling element 15 designed as a rack to adjust a wheel steering angle of the steerable wheels 7 corresponding to the steering command 25. By means of Fig.2 position sensors that are not explicitly shown can advantageously be used to determine the set wheel steering angle of the steered wheels 7 from the position of the coupling element 12. 30 The steering actuator 16 in this embodiment comprises a steering actuator electric motor 17, which is designed as a three-phase electric motor, and in particular as a permanent magnet synchronous motor. The steering actuator 16 is designed to convert a detected steering input into a steering angle α of the steerable wheels 7 via a steering gear 14 by controlling the steering actuator electric motor 17. In this 35 BE2024 / 5839 11 embodiment, the steering actuator electric motor 17 acts via a transmission belt 141 on a spindle drive 142 which is operatively connected to the rack and pinion. By appropriately controlling the steering actuator electric motor 17, the spindle drive 142 is driven to convert a steering input into a steering movement of the steerable wheels 14.The steering actuator 16 acts on the coupling element 15 via the spindle drive 5, which is driven by the steering actuator electric motor 17, and sets the required steering angle of the steerable wheels 7 of the motor vehicle 1 in normal, trouble-free operation. In this embodiment, the steerable wheels 7 are connected to the coupling element 15 in a known manner via tie rods 6. The tie rods 6 themselves are connected to each steered wheel 7 in a known manner via steering knuckles. 10 During operation of the motor vehicle 1, a target trajectory 2 is determined for the motor vehicle 1, which the motor vehicle 1 is to follow, taking into account a current driving instruction that can be given by a vehicle user and / or a driver assistance system of the motor vehicle 1. The driving instruction includes a steering command. 15 Advantageously, the driving specification also takes into account a current acceleration or deceleration specification, which then advantageously also contributes to the determination of the target trajectory2.In normal, trouble-free operation, the actuators 16, 61, 62, 71, 72, 73, 74, 81, 82, 83, 84 of the motor vehicle are controlled so that the motor vehicle 1 moves according to the target trajectory 2. In addition, the functionality of the actuators 16, 61, 62, 71, 72, 73, 74, 81, 82, 83, 84 is constantly checked during normal operation. The functionality of the actuators 16, 19, 61, 62, 71, 72, 73, 74, 81, 82, 83, 84 is checked by querying a status signal of the actuators 16, 19, 61, 62, 71, 72, 73, 74, 81, 82, 25, 83, 84, whereby the status signal in this embodiment is generated by a diagnostic unit (not explicitly shown) assigned to the respective actuator. Additionally, the functionality of the actuators 16, 19, 61, 62, 71, 72, 73, 74, 81, 82, 83, 84 is checked during activation by comparing a power requested by a respective actuator 16, 19, 61, 62, 71, 72, 73, 74, 81, 82, 83, 84 with a power provided by the respective actuator 30 16, 19, 61, 62, 71, 72, 73, 74, 81, 82, 83, 84.If a significant deviation is detected in this comparison, an impairment of the functionality of the actuator in question is recognized. BE2024 / 5839 12 If, during normal operation of the motor vehicle 1, a malfunction of the steer-by-wire steering system 10 is detected, which may also be the case due to a collision of the motor vehicle, the actuators that continue to function, or at least a selection of the actuators that continue to function, of the steer-by-wire steering system 10, the drive system 60, the braking system 70 and the suspension system 80 are controlled in such a way 5, in particular using the vehicle motion control system 3, that the control intervention on the actuators is aimed at ensuring that the motor vehicle 1 continues to follow the intended trajectory 2 as accurately as possible and that the motor vehicle 1 is also brought to a controlled standstill.A malfunction of the steer-by-wire steering system 10 is detected in particular when the steer-by-wire steering system 10 cannot translate a steering input 10 into a steering movement of the steerable wheels 7 of the motor vehicle 1 as intended, especially due to a faulty response of the steering actuator 10 or due to a problem with one of the steerable wheels 7. If the brake actuators 71, 72, 73, 74 remain functional, they are controlled 15 in such a way that the vehicle 1 is decelerated by braking interventions, but a skidding of the vehicle 1 is prevented. In particular, if the vehicle 1 is to follow a curve according to the target trajectory 2, the brake actuators 71, 72, 73, 74 are also controlled masymmetrically to generate a yaw moment G, whereby the yaw moment G to be generated is derived from the target trajectory 2. Through this 20 control of the brake actuators 71, 72, 73, 74, the vehicle 1 essentially continues to follow the target trajectory 2 until it comes to a standstill.If the drive actuators 61, 62 of the drive system 60 are still functional, they are controlled to further decelerate the vehicle's movement 1 and contribute to achieving the desired yaw moment (G). 25 Optionally, the damping actuators 81, 82, 83, 84 can also be controlled to steer the vehicle's direction of travel 1 to maintain the target trajectory 2. The damping actuators 81, 82, 83, 84 are controlled in such a way as to induce a body movement, in particular a roll movement, which specifically influences the steering angle 30 of the steerable wheels 7. The situation determines, in particular by the Vehicle Motion Control System 3, which of the functional actuators should be controlled and in what way in order to bring the motor vehicle 1 to a standstill following the target trajectory 2.35 BE2024 / 5839 13 In addition, the steering actuator 16 acting on the coupling element 15 is controlled, in particular triggered by the Vehicle Motion Control System 3, in such a way that the steering actuator 16 selectively dampens or allows a change in the current wheel steering angle of the steered wheels 7 of the motor vehicle 1 by allowing or blocking movement of the coupling element 15 5, so that a deviation from the target trajectory 2 is counteracted. The electric motor 17 of the steering actuator 16 and the steering actuator 16 as well as its control are explained in more detail with further reference to Fig. 3 or Fig. 4a and Fig. 4b. 10 In this embodiment, the electric motor 17 of the steering actuator 16 comprises a three-phase electric motor 17, wherein a movement of the coupling element 15 and thus a change in the steering angle of the steerable wheels 7 is damped by short-circuiting phases 21, 22, 23 of the electric motor.By interrupting the short circuit of phases 21, 22, 23, i.e., by interrupting the short circuit, a movement of the coupling element 15 and thus a change in the steering angle of the steerable wheels 7 is permitted. Fig. 3 shows an exemplary design for the electric motor 17 and its control. Fig. 3 shows a circuit arrangement 30 with a control unit 31, an inverter 32 comprising three 20 half-bridges via which the phases 21, 22, 23 of the electric motor 17 are controlled, and a feedback branch 33 via which the control unit 31 receives feedback on the currents applied to the phases 21, 22, 23. In addition, a switching unit 35 is provided, which is designed to electrically connect the phases 21, 22, 23 of the electric motor 17 to each other, i.e., to short-circuit the phases 21, 22, 23, or 25 to disconnect the electrically conductive connection of the phases 21, 22, 23 again, i.e., to interrupt the short circuit of the phases 21, 22, 23.The switching unit 35 provides controllable switching elements 36, in particular self-conducting MOSFETs, connecting the phases 21, 22, 23 of the electric motor 17. The switching unit 35 and the switching elements 36 are shown schematically in Fig. 3. The switching elements 36 allow the phases 21, 22, 23 of the electric motor 17 to be electrically short-circuited to dampen the movement of the coupling element 15 and thus to dampen a change in a set wheel steering angle α. The switching elements 36 of the switching unit 35 are controlled by the control unit 31. The control unit 31 communicates in particular with the Vehicle Motion Control System 3.35 BE2024 / 5839 14 In Fig. 4a and Fig. 4b, a schematic representation is shown in the event of a steer-by- Wire steering system the effect of the open switching elements 36 (no short circuit of phases 21, 22, 23 of the electric motor 17; Fig. 4a) and the closed switching elements 36 (short circuit of phases 21, 22, 23 of the electric motor 17; Fig. 4b) is shown.If the 5 switching elements 36 are open, and the phases 21, 22, 23 of the electric motor 17 of the steering actuator 16 are not short-circuited accordingly, the coupling element 15 is essentially freely movable in the event of a fault in the steer-by-wire steering system 10, as symbolically represented by the arrows 151. This means that, for example, when driving over a pothole, a different steering angle of the steered wheels 7 can be established. This free movement of the coupling element 10 is also used to influence the steering of the steerable wheels 7 in a desired direction by controlling functional and available actuators 61, 62, 71, 72, 73, 74, 81, 82, 83, 84 of the vehicle systems 60, 70, 80. to specifically influence the wheel steering angle α in the event of a disturbance of the steer-by-wire steering system 10 in a manner that the motor vehicle 1 continues to follow the target trajectory 2.If a more advantageous wheel steering angle α is set for compliance with the target trajectory, the switching elements 36 are closed, thus short-circuiting phases 21, 22, 23 of the electric motor 17 of the steering actuator 16, as symbolically shown in Fig. 4b. Due to the short circuit, the electric motor 17 acts as a motor brake and largely fixes the coupling element 15 in its current position, thus damping any change in the set wheel steering angle α. A change in the wheel steering angle α can occur in several steps, meaning that the switching elements 36 alternately switch between open and closed positions in conjunction with the control of the other actuators, in particular the left front brake actuator 71 or the right front brake actuator 72, and the state of the Coupling element 15 is therefore freely movable 151 and blocked 152. 25 In the illustration in Fig.1, in which a malfunction of the steer-by-wire steering system 10 is supposed to be present, and the motor vehicle 1 is nevertheless supposed to continue following the specified target trajectory 2, it can be recognized that the current wheel steering angle α of the steered wheels 7 is not suitable so that the target trajectory 2 can be maintained. Specifically, in the situation shown as an example in Fig. 1 30, it is provided that all brake actuators 71, 72, 73, 74 are controlled, whereby the arrows F1, F2, F3, F4 shown at the wheels 7, 8 symbolize the different strengths of the brake interventions. In addition, the rear drive actuator 61 is controlled to generate a drive torque FA. Through this control of the actuators 71, BE2024 / 5839 15 72, 73, 74, 61 A yaw moment is generated that forces the vehicle 1 to continue following the target trajectory 2. A yaw rate control can be used to generate the yaw moment G. The change in the wheel steering angle α is dampened when the yaw rate control is applied, thus closing the switching elements 36.In particular, additionally or alternatively, a stronger braking force F1 can be achieved with the front left brake actuator 71 than with the front right brake actuator 72 and the steering actuator 16, such that, with undamped movement of the coupling element 15, the wheel steering angle α is changed and the steerable wheels 7 turn to the left. At a suitable wheel steering angle, the switching elements 36 are then actuated again so that they close, and thus the movement of the coupling element 15 is largely 10 prevented. In a motor vehicle 1 with a steer-by-wire steering system 10, which also has rear-wheel steering with a rear-wheel steering actuator 19, if a fault is detected in the steer-by-wire steering system 10 affecting the front-wheel steering, it is advantageously 15 provided that the functional rear-wheel steering actuator 19 is also used and controlled so that the motor vehicle 1 continues to follow the determined target trajectory 2.