Method for controlling motor vehicle in event of failure of front-axle steering, and emergency steering system

Abstract

The invention relates to a method for controlling a motor vehicle having a steer-by-wire system with a front-axle steering unit (2) and a rear-axle steering unit (3) in the event of a fault (25) being detected in the front-axle steering unit (2), vehicle signals relating to a steering input (721) of the motor vehicle, a position of a steering rack of the front-axle steering unit (2) and a yaw velocity (751) are detected, the detected vehicle signals (721, 751) are made available for a control unit (6), and the control unit (6) generates a first control signal for controlling a steering regulator (84) of the rear-axle steering unit (3) in order to implement the steering input, a first control signal is generated taking into account a steering input (721), a position of a steering rack of the front-axle steering unit (2) and a yaw velocity (751), and a steering adjuster (84) of the rear-axle steering unit (3) is controlled by the generated first control signal (94). The invention also relates to an emergency steering system which is designed in particular for implementing such a method.

Description

[0001] The present invention relates to a method for controlling a motor vehicle having a steer-by-wire system with a front axle steering system and a rear axle steering system in the event of a fault in the front axle steering system, wherein vehicle signals related to the steering specification of the motor vehicle, the position of the rack and pinion of the front axle steering system, and the yaw rate are detected, the detected vehicle signals are provided to a control unit, and the control unit generates a control signal. The present invention also relates to an emergency steering system for providing steering capability for a motor vehicle having a steer-by-wire system with a front axle steering system and a rear axle steering system in the event of a failure of the front axle steering system.

[0002] Steer-by-wire systems have been described several times in the prior art. For example, DE 10 2018 114 988 A1 discloses a steer-by-wire system having a steering handle, a feedback actuator, and a steering actuator, wherein the steering specification can be predefined via the steering handle and converted by the steering actuator into a steering movement of the steerable wheels of the motor vehicle. The challenges of steer-by-wire systems involve keeping the motor vehicle controllable even in the event of a fault in the steer-by-wire system. In this regard, DE 102020 100 719 A1 proposes to deactivate the defective steering system of a motor vehicle having a front axle and a rear axle steering system in the event of a fault in the front axle or rear axle steering system, and to enable a steering movement by means of an autonomous driving mode or a derived expected movement. DE 10 2019 217 588 A1 also discloses that, after a vehicle collision, due to one or more wheels of an axle no longer being able to be fully steered or even no longer being able to be steered, steering is performed by means of a functional steering axle, and a braking signal is transmitted to one of the wheels connected to the steering axle.

[0003] DE 10 2018 107 612 A1 also discloses a motor vehicle having a front axle and a rear axle steering system and torque vectoring on the rear axle, wherein a comparison of the actual motor vehicle state with the desired motor vehicle behavior of the motor vehicle is set. The rear axle steering system and the rear wheel drive are used to control the rear wheels, in which case the actual motor vehicle state approaches the desired motor vehicle behavior. It is further disclosed that, in the event of a failure of the front axle steering system and the front axle drive, the rear axle steering system and the rear axle drive can take over the control of the motor vehicle.

[0004] WO 2017 / 001045 A1 also describes a motor vehicle having a front axle steering system and a rear axle steering system. In this case, the front axle steering system is provided with automatic control for autonomous driving. In this case, a failure recognition device can recognize a failure of the front axle steering system, whereupon the steering of the vehicle is then carried out by the rear axle steering system. In addition, in the event of a failure of the front axle steering system, the front wheels are braked so that the central return of the front axle steering system is carried out.

[0005] EP 2 072 374 A2 also discloses a device for damping a rear axle steering system, wherein an electric motor of a steering actuator acting on the rear axle is operated to damp the rear axle steering system during generator operation.

[0006] On this basis, it is also necessary to keep a motor vehicle with a steer-by-wire system controllable in the event of a fault in the steering system or on the steering system, and thus further reduce the risk of personal injury.

[0007] In this context, the object of the present invention is to improve a motor vehicle having a steer-by-wire system, which steer-by-wire system comprises a front axle steering system and a rear axle steering system, and in particular to keep the motor vehicle in a steerable state in an improved manner in the event of a fault in the front axle steering system.

[0008] To achieve this object, an emergency steering system according to the independent claims and a method for controlling a motor vehicle having a steer-by-wire system, which steer-by-wire system has a front axle steering system and a rear axle steering system, are proposed. Additional preferred configurations of the invention are described in the dependent claims and the description and are shown in the drawings.

[0009] The proposed solution provides a method for controlling a motor vehicle having a steer-by-wire system with a front axle steering system and a rear axle steering system in the event of a recognized failure of the front axle steering system, wherein vehicle signals related to the steering specification of the motor vehicle, the position of the rack and pinion of the front axle steering system, and the yaw rate are detected, the detected vehicle signals are provided to a control unit, and the control unit generates a first control signal for controlling the steering actuator of the rear axle steering system in order to convert the steering specification. The first control signal is generated taking into account the steering specification, the position of the rack and pinion of the front axle steering system, and the yaw rate according to the invention, and the steering actuator of the rear axle steering system is controlled using the generated first control signal. Preferably, by taking into account the steering specification and the position of the rack and pinion and additionally taking into account the yaw rate of the motor vehicle, an improved ability to steer the motor vehicle in the event of a failure related to the front axle steering system is achieved. In particular, the failure of the ability to control the front axle steering system is recognized as a failure of the front axle steering system, wherein the rack and pinion of the front axle steering system can in particular move freely. However, in particular, a blocked or damped steering movement is also recognized as a failure of the front axle steering system. In particular, a failure of the front axle steering system is recognized when the predefined steering specification can no longer be converted by means of the front axle steering system. In this case, the failure of the front axle steering system can in particular be attributed to an accident of the vehicle. In this case, the vehicle preferably remains steerable by means of this method until the vehicle is safely stopped. In particular, it is still possible to bypass possible obstacles until the vehicle is safely stopped. The motor vehicle is in particular a two-track motor vehicle, in particular a two-track passenger vehicle having two front wheels and two rear wheels.

[0010] According to a preferred embodiment of the method, the control unit generates a second control signal for controlling the front brakes of the motor vehicle, at least one front wheel brake of the motor vehicle, and / or the rear brakes of the motor vehicle, at least one rear wheel brake of the motor vehicle in order to convert the steering specification, wherein the front brakes and / or the rear brakes are preferably controlled by the generated second control signal. In particular, in this case it is provided that the travel direction of the motor vehicle is intended to be influenced in the context of the detected steering specification by the control of the front brakes and the rear brakes. Preferably, the front brakes and the rear brakes are used in a direction-dependent manner by means of the second control signal, in particular according to the steering specification performed by the driver or the automatic motor vehicle control unit. In another preferred manner, it is taken into account during the generation and use of the second control signal whether the vehicle is to be braked. If a yaw moment acting on the vehicle is generated by using the front brakes and / or the rear brakes by means of the second control signal, a compensation torque is preferably provided which preferably maintains the vehicle speed.

[0011] A preferred further development of the method provides for additionally detecting vehicle signals related to the acceleration specification, in particular actuation of the brake pedal and / or actuation of the accelerator pedal, wherein, taking into account the acceleration specification, a braking pressure for the front brake and / or the rear brake is calculated as a second control signal. In particular, a braking pressure is pre-set for each wheel of the vehicle using the second control signal. In particular, it is also preferably taken into account whether the driver wishes to brake the vehicle. In particular, it is also set that when no braking is detected as an acceleration specification, a compensation torque for maintaining the vehicle speed is provided for the yaw moment generated by controlling the front brake and / or the rear brake. When braking is detected as an acceleration specification, this setting of the compensation torque is preferably omitted.

[0012] According to another preferred embodiment of the method, vehicle signals related to the vehicle speed are additionally detected, wherein, taking into account the vehicle speed and the calculated braking pressure, a reference value for the yaw angular velocity of the motor vehicle is established. Preferably, the reference value is used for the yaw angular velocity of the motor vehicle by a yaw regulator of the motor vehicle, in particular by an active yaw regulation system, which is preferably used in an auxiliary manner in order to convert the steering specification in the event of a front axle steering system failure. Preferably, a further improved steering ability can thus be achieved.

[0013] Another preferred embodiment provides for generating a steering actuator signal for a rear axle steering system taking into account the steering specification and the position of the rack and pinion of the front axle steering system, the yaw regulator generating a control signal for determining the torque distribution at the respective wheels of the motor vehicle taking into account the yaw angular velocity of the motor vehicle, and generating a first control signal based on the superposition of the steering actuator signal and the control signal. Preferably, the steering ability of the motor vehicle when the front axle steering system has failed is further improved thereby. In particular, in this case, the rear wheel steering system becomes the main actuator for steering in the event of a defect in the front axle steering system, in particular in the event of a defect in the front wheel rack and pinion, wherein preferably the rear wheel steering system is controlled by the front steering components, in particular the steering specification and the position of the rack and pinion of the front wheel steering system, and the superimposed yaw regulator. The superimposed yaw regulator, in particular the control signal generated by the yaw regulator, preferably reduces interference effects, such as in particular μ-split braking. It is also preferably set that the difference between the detected vehicle signal related to the yaw angular velocity of the motor vehicle and the established reference value of the yaw angular velocity is provided as an input variable to the yaw regulator.

[0014] In another preferred manner, a specification of a desired yaw rate to be achieved for a motor vehicle is provided, and the command can correspond in particular to a reference value of the yaw rate of the established motor vehicle, and a braking specification for at least one of the wheels of the motor vehicle is determined taking into account the current actual yaw rate of the motor vehicle so that the actual yaw rate is closer to the desired yaw rate. Preferably, the braking application is performed according to the determined braking specification. In particular, a method for assisting in steering a two-rail motor vehicle having two front wheels and two rear wheels is proposed, in which a specification of a desired yaw rate of the motor vehicle to be achieved is provided, the current yaw rate of the motor vehicle is taken into account, a braking specification for at least one of the wheels of the motor vehicle is determined so that the actual yaw rate approaches the desired yaw rate, and the braking application is performed according to the determined braking specification.

[0015] Providing a specification of a desired yaw rate to be achieved for a motor vehicle, determining a braking specification for at least one of the wheels of the motor vehicle taking into account the current actual yaw rate of the motor vehicle so that the actual yaw rate approaches the desired yaw rate, and providing a braking application according to the determined braking specification is based on the concept that a moment about the vertical axis is generated by introducing a braking torque unidirectionally to a two-rail motor vehicle having two front wheels and two rear wheels, and this moment is preferably used to assist a malfunctioning steer-by-wire system of the motor vehicle when performing a change in the traveling direction, and / or to control the motor vehicle individually thereby, especially when no steering ability is provided additionally. If the braking torque acts mainly on the front axle of the two-rail motor vehicle, it appears to the motor vehicle in a manner of understeering, while a stronger braking torque acting on the rear axle of the motor vehicle appears to the motor vehicle in a manner of oversteering. This characteristic is preferably used to optimize the steering behavior of the vehicle, especially in the case of at least partial failure of the steer-by-wire system of the motor vehicle, especially in the case of a malfunction of the front axle steering system. Preferably, the actual yaw rate is measured. This can be performed in particular by the measuring device of this ESP (Electronic Stability Program) or by an additional sensor unit. This angular velocity is preferably used together with other values, especially the steering angle, lateral acceleration, and / or wheel speed, to achieve a more accurate determination of the true actual yaw rate.

[0016] According to a preferred embodiment of the method, a specification of the desired yaw rate is provided taking into account the steering specification. In this case, the steering specification can be pre-determined by the driver via the steering handle or by a driver assistance system during at least partial autonomous operation of the motor vehicle. Preferably, in this case, the braking torque is introduced in such a way that the motor vehicle better follows the steering specification.

[0017] In particular, it is provided that the desired yaw rate corresponds to a reference value of the yaw rate of the motor vehicle established. According to one variant, the desired yaw rate is determined based on the desired steering angles of the front and rear wheels. In this case, the desired steering angles are determined in particular based on the detected steering specifications, wherein, in the case of a steering specification by means of the steering wheel, the desired steering angles are calculated in particular based on the steering wheel angle and the steering wheel rotation speed, in particular further taking into account the vehicle speed, a fixed variable with respect to the wheel base and / or an inherent steering gradient.

[0018] In particular, the desired yaw rate can be determined according to the functional relationship set forth below:

[0019]

[0020] where:

[0021] ψ d = desired yaw rate;

[0022] v = vehicle speed;

[0023] l = wheel base;

[0024] EG = inherent steering gradient; and

[0025] δ t = atan(tan(δ f ) - tan(δ r ))), where

[0026] δ f = desired steering angle of the front wheels; and

[0027] δ r = desired steering angle of the rear wheels.

[0028] A preferred further development provides for determining the above-mentioned braking specifications based on a two-track model, wherein the two-track model particularly describes the stable and unstable lateral dynamics of the motor vehicle. The two-track model is preferably solved for its stable state such that the desired yaw rate, as an input to the two-track model, preferably results in the braking torque difference required to achieve the desired yaw rate as an output. In particular, a specification regarding the desired yaw rate is provided as an input variable to the two-track model, and the two-track model for the assumed stable state of the motor vehicle is used to establish the resulting braking torque difference between the wheels as an output variable, wherein the braking specifications are preferably determined based on the braking torque difference. In this case, a braking torque difference is formed in particular between the left wheel and the right wheel.

[0029] According to a further preferred development, a scaling factor is applied to the established braking torque difference, where the scaling factor is less than 1. This embodiment is provided especially for assisting steering, that is, especially when the steering of the motor vehicle is at least still partially functional, namely especially when the rear axle steering system or the rear axle steering system and the front axle steering system to at least a certain extent are still functional.

[0030] It has been established in tests that, depending on the manoeuvre, this scaling factor is preferably in the range between 0.05 and 0.2, more preferably in the range between 0.05 and 0.1. Thus, another preferred embodiment provides for evaluating the current driving manoeuvre based on items of driving state information, namely especially information items related to the current driving state of the motor vehicle, especially the vehicle speed. In this case, depending on the evaluation of the driving manoeuvre, the scaling factor is preferably determined as a value in the range from 0.05 to 0.2, especially in the range from 0.05 to 0.1. In this case, driving manoeuvres in the marginal range preferably result in a smaller value of the scaling factor, while so-called "limp-home" driving manoeuvres preferably result in a larger value of the scaling factor. In another preferred manner, the scaling factor is especially speed-dependent, where a high travelling speed preferably results in a high scaling factor within the determined range, and a low travelling speed preferably results in a low scaling factor within the determined range.

[0031] In another preferred manner, in the case of identifying at least partial failure of the steer-by-wire system of the motor vehicle, an embodiment of the method with brake application is applied according to the determined braking specification, where the higher the value preferably determined for the scaling factor, the lower the steering action that can still be provided by the steering system. In this case, due to the selective brake application, the following state can preferably be achieved: the motor vehicle follows the steering specification better than would be the case if there were only the steering action that could still be provided by the steering system. In this regard, a method for assisting the steering of a two-axle motor vehicle with two front wheels and two rear wheels is especially proposed, where at least partial failure of the steer-by-wire system is identified, the detected steering specification is taken into account, a specification regarding the desired yaw rate to be achieved for the motor vehicle is provided, the current actual yaw rate of the motor vehicle is further taken into account, a braking specification for at least one of the wheels of the motor vehicle is determined so that the actual yaw rate approaches the desired yaw rate, and a brake application is carried out according to the determined braking specification, where especially the determination of the braking specification is carried out based on a two-axle model, especially as described above.

[0032] In particular, a force difference ΔF between the front axle and the rear axle of the motor vehicle is determined X, the braking specifications are determined based on a two-track model, wherein, based on the determined force difference, in particular the braking pressure is determined, and the motor vehicle is selectively braked by means of this braking pressure. In this case, the determination of the braking pressure is preferably based on the established force difference, taking into account the wheel diameters of the individual wheels of the motor vehicle and the so-called c p value. In this case, the c p value is established experimentally for the respective vehicle model by comparing the measured braking pressure with the occurring braking torque.

[0033] According to a preferred embodiment, the force difference ΔF X is determined according to the following formula:

[0034]

[0035] where:

[0036] yaw acceleration,

[0037] c α : lateral tire stiffness,

[0038] l f : distance between the front axle of the motor vehicle and the center of mass of the motor vehicle,

[0039] l r : distance between the rear axle of the motor vehicle and the center of mass of the motor vehicle,

[0040] m: vehicle mass, and

[0041] v: local speed (vehicle speed).

[0042] A preferred variant provides for the calculation of the braking specifications, wherein, preferably, the desired yaw moment to be achieved is determined according to the desired yaw angular velocity and the desired yaw moment is proportional to the desired yaw angular velocity, and wherein the desired yaw moment is preferably multiplied by the value of the current travel speed of the motor vehicle. In particular, in this case, it is provided to calculate the braking pressure by multiplying the desired yaw moment by the vehicle speed, the desired yaw moment being proportional to the desired yaw angular velocity, which is preferably determined according to the steering angle of the steering handle of the motor vehicle. In particular, in this embodiment, the determination of the braking pressure is based on the determination of the force difference ΔF X for the following formula:

[0043]

[0044] where:

[0045] yaw acceleration,

[0046] v: local speed (vehicle speed), and

[0047] C: a constant, in particular where C = [80…100], more particularly where C = 95.

[0048] Thus, as a preferred embodiment for determining the braking specification, it is set that the braking pressure for at least one specific wheel among the wheels of a motor vehicle is determined as the braking specification, particularly for the wheels of an axle of a motor vehicle. Preferably, the brakes of the motor vehicle are actuated using the determined braking pressure. Preferably, the higher the traveling speed of the motor vehicle, the greater the determined braking pressure.

[0049] In particular, it is set that a method for determining the braking specification is executed in the case of identifying at least partial failure of the steer-by-wire system of the motor vehicle, where such a partial failure of the steering system, particularly in the case of a full-wheel steering system, is the failure of the steering ability of the front axle of the motor vehicle by means of the steering system to control the corresponding steering actuator. Preferably, under such a partial failure of the steering system, auxiliary steering is performed by preferably additionally providing braking application. According to another preferred embodiment of the method proposed for achieving the purpose mentioned in the introduction, a vehicle signal related to the response of the front brakes of the motor vehicle is additionally detected and provided to the control unit, where the control unit generates a braking torque compensation as a third control signal considering the response of the front brakes of the motor vehicle, and controls the driveline of the motor vehicle using the third control signal. In particular, the fact that the motor vehicle follows the steering specification is thereby further improved, particularly further considering the acceleration specification.

[0050] Another preferred embodiment provides damping of the movement of the rack and pinion of the front axle steering system, particularly when the rack and pinion can move freely. Preferably, the steering influence caused by a defective front axle steering system is thereby reduced. In particular, it is set that the control unit generates a fourth control signal for controlling the short-circuit circuit of the electric motor of the steering actuator of the front axle steering system, where the phases of the electric motor are short-circuited by means of the control using the fourth control signal, and thus the movement of the rack and pinion of the front axle steering system is damped.

[0051] According to another aspect, generally, damping of the movement of a malfunctioning axle steering system is provided, in particular to avoid an undesired wheel steering angle of the wheels of the malfunctioning axle steering system being externally forced to be adjusted. Preferably, in particular, when adjusting the wheel steering angle in favor of the steering specification, the movement of the malfunctioning axle steering system is damped. In particular, in this case, the properly functioning axle steering system is used to convert the steering specification. Preferably, here, the properly functioning axle steering system can be assisted by selective brake application, introduced drive torque, and / or additional control interventions. In particular, it is set to apply a damping with respect to a significant damping torque to the electric motor of the road wheel actuator of the steer-by-wire system, in particular to the electric motor of the steering actuator of the steer-by-wire system, preferably by short-circuiting the electric phases of the electric motor. The resulting damping force is preferably used to keep the wheel steering angle of the malfunctioning axle steering system, in particular the wheel steering angle of the front road wheels, constant, such that in particular the motor vehicle can be steered via the rear axle steering system.

[0052] Preferably, in this variant, the motor phase short-circuit is activated by a control unit, in particular by a driving state determination device, more particularly by a driving dynamics control unit, which preferably identifies the state of the motor vehicle and steers after the axle has been affected by a malfunction and failed. If additional damping is used particularly by motor phase short-circuit, then the undesired steering movement of the wheels that can no longer be selectively steered due to dynamic lateral forces is preferably braked by friction and the generated damping. Preferably, this effect can limit the interference response of the still properly functioning axle steering system, in particular the properly functioning rear axle steering system, wherein preferably, slow steering via an alternative steering system is possible by braking or driving and this slow steering can be preferably selectively used. Thereby, preferably, the ability to control the motor vehicle in the case of a defective steering system can be further improved. The proposed embodiment can be very useful especially in the case of an emergency brake - during which, despite a defective steering system, it is intended to maintain the lane - or in the case of a slow avoidance movement. Preferably, the control unit identifies the failure of the electric steering actuator of the front axle steering system and activates the phase short-circuit at its electric servo motor, such that preferably the wheel steering angle of the front wheels is fixed. Then, the control unit preferably transmits reference position information to the rear axle steering system based on the measured steering wheel position.

[0053] In particular, it is also set that the damping and alternative steering function of a defective axle steering system are carried out, in particular by selectively braking the wheels of the motor vehicle and / or by steering the still steerable wheels of the properly functioning axle steering system, so as to work in parallel. In this case, preferably, the damping function is selectively switched on and off, so as to be able to indirectly adjust the desired wheel steering angle and thus further improve the steering control.

[0054] According to a further preferred development of the method, the driving state determination device determines the driving state of the motor vehicle taking into account the available vehicle signals, in particular taking into account all available driving signals. In this case, the determination of the driving state preferably includes an assessment as to whether the motor vehicle is traveling in urban traffic, traveling across the countryside or traveling on a highway. The driving state determined by the driving state determination device is preferably provided to the control unit. Preferably, the additional control signal is adjusted according to the determined driving state. The control unit takes particular account of the driving state for generating at least one of the control signals, in particular for generating the first control signal, the second control signal, the third control signal and / or the fourth control signal. Preferably, the steering ability is thus adapted to the current driving state in an improved manner as the case may be. In particular, further improved results regarding yaw regulation can thus be obtained, and in particular the appropriate braking pressure for the front brakes and / or the rear brakes can be pre-determined in a better adapted manner.

[0055] Another preferred embodiment provides for additionally detecting a first steering angle of the front wheels and using a rear axle steering system to convert a steering specification to adjust a second steering angle of the rear wheels, determining a first desired steering angle for the front wheels based on the detected steering specification, and determining a second desired steering angle for the rear wheels, and determining the second steering angle taking into account the determined first desired steering angle, the determined second desired steering angle, and the detected first steering angle. In particular, a method for operating a steer-by-wire system of a motor vehicle is provided, the steer-by-wire system having a first steering system for steering a first wheel on a first axle of the motor vehicle, in particular a front axle steering system, and a second steering system for steering a second wheel on a second axle of the motor vehicle, in particular a rear axle steering system, wherein a steering specification for the motor vehicle is detected, a first steering angle of the first wheel, in particular the front wheels, is detected, and the second steering system is used to adjust the second steering angle of the second wheel, in particular the rear wheels, for converting a steering command, wherein a first desired steering angle for the first wheel of the motor vehicle, in particular the front wheels of the motor vehicle, and a second desired steering angle for the second wheel of the motor vehicle, in particular the rear wheels, are determined based on the detected steering specification, and the second steering angle is determined taking into account the determined first desired steering angle, the determined second desired steering angle, and the detected first steering angle. Since in many steering maneuvers, both steering systems are not used, but instead only the first steering system, in particular the front axle steering system, is used to convert the steering specification, the second desired steering angle can thus be determined in particular at 0° for these steering maneuvers, and thus direct use of the second steering system, in particular the rear axle steering system, can be dispensed with for these steering maneuvers. By taking into account the first desired steering angle, the second desired steering angle, and the detected first steering angle, it is preferably possible to determine the second steering angle of the wheels of the second steering system, in particular the rear axle steering system, preferably such that the motor vehicle follows the steering specification as precisely as possible, in particular when the first steering system, which preferably corresponds to the front axle steering system, is no longer suitable for converting the steering specification or is at least only suitable for converting the steering specification in a limited manner due to a functional failure. Thus, in particular when the first steering system of a passenger vehicle, in particular the front wheel steering system, is no longer fully functional after a collision or for other reasons and the first wheel, in particular the front wheels, can still only be steered in a limited manner, and thus in particular only a steering angle smaller than a predetermined value can be adjusted, the application of this method is provided.

[0056] To detect the steering angle of the first wheel, it is specifically provided to measure the steering angle of the first wheel, in particular by means of a correspondingly configured sensor unit. Alternatively or additionally, to detect the steering angle of the first wheel, it is provided to estimate the steering angle of the first wheel, in particular by means of a state estimator. In this case, the estimation is preferably carried out such that the rear axle position is determined based on the measured position at the second steering system. Based on the measured rear axle position, the introduced yaw rate, and by means of the measured wheel speeds, the first steering angle position is preferably estimated.

[0057] According to a particularly preferred embodiment, it is provided to determine the steering angle of the second wheel, where

[0058] β adapt = atan(tan(α Soll ) + tan(β Soll ) - tan(α Ist ))), where

[0059] β adapt = the steering angle of the second wheel (to be adjusted),

[0060] α Soll = the first desired steering angle for the first wheel (front wheel);

[0061] β Soll = the second desired steering angle for the second wheel (rear wheel); and

[0062] α Ist = the (detected) steering angle of the first wheel.

[0063] Preferably, the problem of determining the steering angle of the second wheel is solved by the following method: calculating the corresponding kinematic radius according to the steering specification, and determining the first desired steering angle and the second desired steering angle, and then preferably, additionally considering the measured actual steering angle of the first wheel, preferably calculating the steering angle of the second wheel necessary for converting the steering specification according to the above formula.

[0064] In particular, a first steering system can be provided, in particular a front axle steering system, which is the main steering system of a motor vehicle. During normal operation of the motor vehicle, in particular in a collision-free state, the steering specification is converted by means of this main steering system. In particular, in a normal operation mode, the steering specification is converted solely by means of this main steering system, that is to say without using a second steering system, in particular a rear axle steering system. In this case, the second steering system, namely in particular the rear axle steering system, is preferably an auxiliary steering system which, during normal operation, in particular in a collision-free state, can assist the main steering system in predetermined driving situations in order to support the steering specification, in particular to improve the driving behavior of the motor vehicle compared to a motor vehicle comprising only the main steering system, in particular to reduce the turning circle of the motor vehicle during a parking operation, to increase the dynamics of the driving behavior in a movement mode of the motor vehicle and / or to stabilize the motor vehicle in a driving situation-dependent manner. In this case, the main steering system is in particular the front axle steering system of the motor vehicle, and the auxiliary steering system is in particular the rear axle steering system of the motor vehicle, which preferably together form a so-called all-wheel steering system during normal operation. In the event of a malfunction of the front axle steering system, in particular in the case of a collision-related failure of the front axle steering system (wherein a particular first desired steering angle can in particular no longer or no longer fully be adjusted by the front axle steering system), the motor vehicle is preferably kept in a steerable state by means of the auxiliary steering system, namely the rear axle steering system, and in particular can then preferably execute the steering specification. In particular, it is provided that in the event of a failure of the front axle steering system, the rear axle steering system, which is still fully functional, can additionally assist during the conversion of the steering specification by selectively controlling actuators acting on the wheels of the vehicle, such as brakes and / or drive units.

[0065] Preferably, during operation of a steer-by-wire system of a motor vehicle, the function of at least the first steering system is monitored, in particular the function of the first steering system and the function of the second steering system, and a failure of the first steering system is identified, and after identifying the failure of the first steering system, the steering system of the motor vehicle is operated such that a steering specification for the motor vehicle is detected, a first steering angle of a first wheel is detected, a first desired steering angle of the first wheel of the motor vehicle and a second desired steering angle of a second wheel of the motor vehicle are determined based on the detected steering specification, a second steering angle of the second wheel is determined taking into account the determined first desired steering angle and the determined second desired steering angle and the detected first steering angle, and the determined second steering angle of the second wheel is adjusted using the second steering system in order to convert the steering specification.

[0066] In particular, a steering specification pre-determined by the driver is set to steer the motor vehicle via a steering handle, in particular via a steering wheel, in this method. However, as a preferred variant, a steering specification is set by a driver assistance system, in particular by a driver assistance system configured to safely stop the motor vehicle, in particular to avoid a further collision, after a collision of the motor vehicle, in order to steer the motor vehicle. By means of the method according to the invention, such a further collision can preferably be prevented, in particular since the motor vehicle remains steerable and it is also preferably possible to determine the steering movement to be converted of the steering wheels of the second axle in a relatively precise manner.

[0067] A further development of the above-described embodiment of the method provides that, in the normal operating mode of a steer-by-wire system, in particular in a normal operating mode comprising a plurality of normal operating modes of the steer-by-wire system, only the front axle steering system is used to convert the detected steering specification, and thus the second desired steering angle for the rear wheels is determined or fixed at 0° in this normal operating mode. Thus, in this normal operating mode, which preferably covers many kilometers of travel of the motor vehicle, the second desired steering angle remains unconsidered. Thus, in this specific normal operating mode, when a malfunction of the front axle steering system is recognized, the second wheel steering angle is preferably determined in a simplified manner, where

[0068] β adapt = atan(tan(α Soll ) - tan(α Ist ))。

[0069] The following emergency steering system is provided as a solution further proposed for the problems mentioned in the introduction. The emergency steering system is used to provide the ability to steer a motor vehicle having a steer-by-wire system with a front axle steering system and a rear axle steering system in the event of a failure of the front axle steering system. The emergency steering system includes a control unit. Among them, the steering actuator of the rear axle steering system and the actuators of the motor vehicle outside the steer-by-wire system are associated with the control unit, and among them, a sensor unit for detecting vehicle signals related to steering specifications and an additional sensor unit for detecting additional vehicle signals are associated with the control unit. In this case, the control unit of the emergency steering system is configured to: receive vehicle signals from the associated sensor unit, and generate control signals for the steering actuator associated with the control unit of the rear axle steering system and the actuators of the motor vehicle outside the steer-by-wire system according to the received vehicle signals in order to convert the received steering specifications. Preferably, with the emergency steering system, the ability to steer a motor vehicle is improved because not only the still-functional rear axle steering system is used to convert the steering specifications (which may be insufficient in many cases), but also at least one additional actuator of the motor vehicle that was initially not associated with the steering system is controlled, such as in particular the front brakes of the motor vehicle, especially the brake units associated with the respective front wheels, and / or the rear brakes of the motor vehicle, especially the brake units associated with the respective rear wheels, and / or at least one drive unit of the motor vehicle. In this regard, the emergency steering system 1 preferably includes at least one actuator of the steer-by-wire system and at least one additional actuator of the motor vehicle that was initially not included in the steer-by-wire system. Preferably, the vehicle auxiliary system connected to the actuator is used to control at least one additional actuator of the motor vehicle and is preferably controlled by the control unit of the emergency steering system.

[0070] The additional sensor unit associated with the control unit of the emergency steering system particularly includes at least one of the following sensor units: a sensor unit for detecting the actuation of a braking operation element, particularly a brake pedal; a sensor unit for detecting the actuation of an acceleration operation element, particularly an accelerator pedal; a sensor unit for detecting the position of the toothed rack of the front axle steering system; a sensor unit for detecting the vehicle speed; a sensor unit for detecting the yaw angular velocity of the motor vehicle. According to a preferred embodiment, all of the above sensor units are associated with the control unit of the emergency steering system. Instead of the sensor unit for detecting the position of the toothed rack of the front axle steering system, an estimation unit can also be particularly provided, by means of which an estimation can preferably be used to establish a position of the toothed rack of the front axle steering system. Preferably, the control unit is configured to accurately establish the driving state by means of the vehicle signals detected by the sensor unit, and to control the steering actuator of the rear axle steering system and the additional actuators of the motor vehicle, so that the vehicle follows the steering specifications in an improved manner in the event of a failure of the front axle steering system.

[0071] The additional actuators of the motor vehicle associated with the motor vehicle, particularly outside a steer-by-wire system, particularly include at least one of the following actuators: the front brakes of the motor vehicle, particularly at least one front wheel brake of the motor vehicle; the rear brakes of the motor vehicle, particularly at least one rear wheel brake of the motor vehicle; the driveline of the motor vehicle, particularly at least one motor provided for driving the motor vehicle, more particularly a hub motor of a wheel associated with each wheel of the motor vehicle. According to a preferred embodiment, all of the above actuator units are associated with the control unit of the emergency steering system, so that in the event of a fault in the front axle steering system being recognized, all of the actuator units can preferably be controlled by the control unit of the emergency steering system to convert the steering specifications.

[0072] Another preferred embodiment provides that a short-circuit circuit of the electric motor of the steering actuator of the front axle steering system is additionally associated with the control unit of the emergency steering system, wherein, in the case of actuation, the short-circuit circuit is configured to short-circuit the phases of the electric motor and thus damp the movement of the toothed rack of the front axle steering system. If, in the event of a fault in the front axle steering system, the toothed rack of the front axle steering system can move freely, then preferably the steering affected by the front axle can be reduced thereby, and thus the steering movement of the motor vehicle can be controlled in an improved manner.

[0073] In another preferred embodiment, the driving state determination device is also associated with the control unit of the emergency steering system. Preferably, the driving state determination device is configured to determine the driving state of the motor vehicle by taking into account the detected vehicle signals, and provide the driving state as an additional input signal to the control unit. Further preferably, the control unit is configured to generate at least one control signal by taking into account the provided driving state. In this case, the driving state of the motor vehicle is specifically related to the association regarding whether the motor vehicle is traveling in urban traffic, across the countryside or on a highway. Therefore, preferably, the steering maneuver can be performed in an improved manner according to the situation.

[0074] Preferably, the control unit is further configured to control the brakes associated with the respective wheels. Preferably, the control unit or the processing unit associated with the control unit is configured to determine the braking specifications, particularly the braking pressure as the braking specification. For this purpose, the fixed values required, particularly the distances of the front and rear axles from the center of gravity, are preferably stored in the control unit or the processing unit, and the variable values, particularly the vehicle speed, are provided to the control unit or the processing unit by the sensors of the motor vehicle. In particular, one configuration provides that the control unit is configured to provide a specification of the desired yaw rate to be achieved for the motor vehicle by taking into account the detected steering specifications and the state of the steering system, determine the braking specifications for at least one of the wheels of the motor vehicle by taking into account the current yaw rate of the motor vehicle, so that the actual yaw rate approaches the desired yaw rate, and perform the braking application according to the determined braking specifications. Preferably, the emergency steering system is thus supported by the control unit in order to convert the steering specifications.

[0075] In another preferred embodiment, the control unit is configured to determine a first desired steering angle for the front wheels and a second desired steering angle for the rear wheels according to the steering specifications. In addition, preferably, the emergency steering system includes a sensor unit for detecting the current first actual wheel steering angle of the front wheels. In this case, the control unit is preferably further configured to, in the case of at least partial failure of the front axle steering system, control the steering actuator of the rear axle steering system by taking into account the first desired steering angle for the front wheels and / or the second desired steering angle for the rear wheels and the first actual wheel steering angle, so that the steering actuator of the rear axle steering system adjusts the second wheel steering angle for the rear wheels in order to convert the steering specifications. Preferably, the emergency steering system configured in this way thus enables the motor vehicle to continue to be maneuverable in a further improved manner, where the front axle steering system has failed, particularly such that the steering specifications can no longer be converted solely by the front axle steering system. Therefore, preferably, the operational reliability of the motor vehicle having such an emergency steering system is improved, and thus the users of such a motor vehicle are better protected from harm.

[0076] According to a preferred embodiment of the steering system, the front axle steering system is the main steering system for a motor vehicle, which is specifically configured to convert the steering specification during normal operation, particularly in a normal operation mode from a plurality of normal operation modes, and particularly without additionally using the rear axle steering system of the steering system. The rear axle steering system is preferably an auxiliary steering system for the motor vehicle in a predetermined driving situation during normal operation, which is specifically configured to assist the main steering system in order to convert the steering specification. In this configuration, in this regard, only the first desired steering angle for the front wheels is generally established according to the steering specification, and the second desired steering angle for the rear wheels is fixed at 0°, so that in this regard, during normal operation, the steering specification is generally only converted by the front axle steering system. The rear axle steering system is preferably used only in a specific driving situation during normal operation in an auxiliary manner relative to the front axle steering system, particularly to obtain a smaller turning radius, increase flexibility in a predetermined driving situation, and / or stabilize the motor vehicle in a specific driving situation. Preferably, the control unit is also configured to use the rear axle steering system to convert the detected steering specification in the event of a failure of the front axle steering system, particularly as described above. Preferably, the control unit is also configured to: in the event of a failure of the front axle steering system, in addition to adjusting the second wheel steering angle, control the actuators acting on the wheels of the motor vehicle, particularly the brakes acting on the wheels and / or the drive units acting on the wheels, in order to convert the steering specification. Therefore, due to braking and / or drifting movements, the vehicle is intended to follow the steering specification in a further improved manner.

[0077] The emergency steering system is preferably configured to perform the method configured according to the present invention, wherein the emergency steering system is preferably configured to perform the above method steps individually or in combination.

[0078] Other preferred details, features, and configuration details of the present invention will be described in more detail in connection with the exemplary embodiments shown in the accompanying drawings (Figure), in which:

[0079] Figure 1 A plan view of an exemplary embodiment of the emergency steering system configured according to the present invention is shown in a highly simplified manner;

[0080] Figure 2 A block diagram of another exemplary embodiment of the emergency steering system configured according to the present invention is shown, and the emergency steering system is configured to perform the method configured according to the present invention;

[0081] Figure 3 A schematic view of an exemplary embodiment of a motor vehicle having an emergency steering system configured according to the present invention is shown;

[0082] Figure 4 A schematic diagram showing braking pressure, steering angle, and target lateral acceleration in an additional exemplary embodiment of a method configured according to the present invention, wherein the braking pressure is determined based on a stable equilibrium state;

[0083] Figure 5 A schematic diagram showing braking pressure, steering angle, and target lateral acceleration in an additional exemplary embodiment of a method configured according to the present invention, wherein the braking pressure is determined;

[0084] Figure 6a A simplified schematic diagram showing an additional exemplary embodiment of an emergency steering system configured according to the present invention for a motor vehicle having steerable front wheels and non-steerable rear wheels in a normal operating mode;

[0085] Figure 6b Shows a simplified schematic diagram of a steering system of a motor vehicle in the case of failure of the front axle steering system according to Figure 6a ;

[0086] Figure 7a A simplified schematic diagram showing an additional exemplary embodiment of an emergency steering system configured according to the present invention for a motor vehicle having steerable front wheels and steerable rear wheels in a normal operating mode; and

[0087] Figure 7b Shows a simplified schematic diagram of a steering system of a motor vehicle in the case of failure of the front axle steering system according to Figure 7a ;

[0088] In the drawings, the same reference numerals are used for the same components, and thus are sometimes explained only in conjunction with one of the drawings.

[0089] Figure 1 Shows a motor vehicle having a steer-by-wire steering system and a powertrain, which particularly includes a front-wheel drive 4 and a rear-wheel drive 5. The steer-by-wire steering system has a front axle steering system 2 with a steering actuator 20 and a rear axle steering system 3 with a steering actuator 84. The left front wheel FL and the right front wheel FR can be steered using the front axle steering system 2 during normal operation. The left rear wheel RL and the right rear wheel RR can be steered using the rear axle steering system 3 during normal operation. In this case, each of the front wheels FL, FR has a front wheel brake as a front brake 81, and each of the rear wheels RL, RR has a rear wheel brake as a rear brake 81. In Figure 1In the exemplary embodiment shown, the occurrence of a fault 25 associated with the front axle steering system 2 is now schematically depicted. This fault 25 can in particular be caused by an accident of a motor vehicle, where the fault 25 results in the failure of the front axle steering system 2. In order for the motor vehicle to still be set up to stop safely and support steering, the motor vehicle includes an emergency steering system 1, which is configured to provide the ability to steer in the event of the failure of the front axle steering system 2.

[0090] The emergency steering system 1 includes a control unit 6, which can in particular be the same as the control unit of the steer-by-wire system of the motor vehicle. Here, the steering actuators 84 of the rear axle steering system 3 and the actuators 81, 82, 4, 5 of the motor vehicle are associated with the control unit 6 outside of the original steer-by-wire system, and a sensor unit for detecting vehicle signals related to the steering specification 721 and an additional sensor unit for detecting additional vehicle signals are associated with the control unit 6. Due to the association of the steering actuators 84 of the rear axle steering system and the additional actuators 81, 82, 4, 5 with the control unit 6, the control unit 6 can control these associated actuators 84, 81, 82, 4, 5 and use them to convert the steering specification 721. In this regard, the emergency steering system 1 is in particular an extension of a malfunctioning steer-by-wire system and, in addition to the front axle steering system 2 affected by the fault 25, particularly includes the rear axle steering system 3, the additional actuators 81, 82, 4, 5 associated with the control unit 6 of the emergency steering system 1, and the sensors associated with the control unit 6 of the emergency steering system 1.

[0091] In particular, according to Figure 1 the emergency steering system 1 of the exemplary embodiment shown is configured to identify the fault 25 of the front axle steering system 2 and, in the event of the fault 25 of the front axle steering system 2 being identified, detect vehicle signals, in particular vehicle signals related to the steering specification 721 of the motor vehicle, the position of the rack and pinion of the front axle steering system 2, and the yaw rate 751, using Figure 1 the associated sensor unit not explicitly shown in

[0092] and provide these detected vehicle signals to the control unit 6. The control unit 6 of the emergency steering system 1 is configured to: receive the vehicle signals 721, 751 and generate control signals for the steering actuators 84 of the rear axle steering system 3 and the additional actuators 81, 82, 4, 5, namely in particular the front brakes 81, the rear brakes 82, the front wheel drive 4 and the rear wheel drive 5, taking into account the received vehicle signals 721, 751, and control these actuators 84, 81, 82, 4, 5 such that the received steering specification 721 is converted.

[0093] Another particularly preferred exemplary embodiment of the emergency steering system 1 is shown in Figure 2 in the form of a block diagram, in which an exemplary execution of a method configured according to the invention is also illustrated with reference to this block diagram.

[0094] In this case, Figure 2 the emergency steering system 1 is shown, which is configured to provide steering capability for a motor vehicle with a steer-by-wire system having a front axle steering system and a rear axle steering system by means of a control unit 6 in the event of failure of the front axle steering system, wherein a large number of sensor units 7 and a large number of actuators 8 are associated with the control unit. In this exemplary embodiment, a sensor unit 71 for detecting actuation of the brake pedal, a sensor unit 72 for detecting actuation of the accelerator pedal, a sensor unit 73 for detecting the position of the rack of the front axle steering system, a sensor unit 74 for detecting the vehicle speed, and a sensor unit 75 for detecting the yaw angular velocity of the motor vehicle are associated with the control unit 6 as the sensor units 7. In this exemplary embodiment, a front brake 81, a rear brake 82, a damper unit 83 of the front wheel steering system, a steering actuator 84 of the rear axle steering system, and a powertrain 85 of the motor vehicle, in particular a front-wheel drive and a rear-wheel drive of the motor vehicle, are associated with the control unit 6 as the actuators 8. In this case, in this exemplary embodiment, it is provided that the electric motor of the steering actuator of the front axle steering system can be controlled via a short-circuit circuit such that the phases of the electric motor are short-circuited and thus the movement of the rack of the front wheel steering system is damped. Furthermore, a driving state determination device 10 is associated with the control unit 6, which can preferably access all vehicle signals of the motor vehicle and is particularly used for operating vehicle assistance systems of the motor vehicle, in particular an autopilot mode, so-called autopilot. The driving state determination device 10 is configured to provide additional input variables, in particular input variables related to the driving state of the motor vehicle, for the control unit 6 of the emergency steering system 1.

[0095] The control unit 6 itself comprises different units for processing the signals received by the control unit 6. Thus, in this exemplary embodiment, the control unit 6 comprises a yaw regulator 61, a unit 62 for establishing a steering actuator signal, a unit 63 for establishing a reference value for the yaw angular velocity of the motor vehicle, a unit 64 for establishing a brake torque compensation, a unit 65 for calculating the respective brake pressures of the front brake 81 and the rear brake 82, a unit 66 for determining the activation or deactivation of the damping of the mobility of the rack of the front axle steering system 2, and two units 67 for linking signals, in particular for adding or subtracting signals.

[0096] In the case of a detected fault in the front axle steering system of a motor vehicle, in this case, in this exemplary embodiment, a method for controlling the motor vehicle performed by the emergency steering system 1 is set as described below. The sensor unit 7 detects vehicle signals 711 related to the acceleration specification, vehicle signals 721 related to the steering specification, vehicle signals 731 related to the position of the rack and pinion of the front axle steering system, vehicle signals 741 related to the vehicle speed, and vehicle signals 751 related to the yaw angular velocity of the motor vehicle, and provides these vehicle signals 711, 721, 731, 741, 751, 761 to the control unit 6. In addition, vehicle signals 761 related to the response of the front brakes 81 of the motor vehicle are provided to the control unit 6. In addition, different vehicle signals 101, 102, 103, 104 related to the driving state of the motor vehicle are provided to the control unit 6 by the driving state determination device 10. The driving state determination device 10 particularly determines whether the braking torque must be compensated. In addition, in the driving state determination device 10, it is determined whether a switch between the front-wheel drive and the rear-wheel drive must be performed with respect to the powertrain 85. In addition, the lane condition determination is performed by the driving state determination device 10, and particularly determines whether the motor vehicle is moving on a highway, across the countryside, or in urban traffic.

[0097] Here, vehicle signals 711, 721, 731, 741, 751, 761, 101, 102, 103, 104 detected by control unit 6 are provided to different units 61, 62, 63, 64, 65, 66 of control unit 6, and then these units generate control signals 91, 92, 93, 94, 95 for actuators 81, 82, 83, 84, 85 and control actuators 81, 82, 83, 84, 85 accordingly. Thus, the vehicle signal 711 related to the acceleration specification is transmitted to unit 65 for calculating the corresponding braking pressures of front brake 81 and rear brake 82. This unit generates a control signal 91 for controlling front brake 81 and a control signal 92 for controlling rear brake 82, with additional consideration of vehicle signal 101 related to the driving state of the motor vehicle and the reference value established by unit 63 for establishing a reference value of the yaw rate. Then, front brake 81 and rear brake 82 are controlled according to the corresponding control signals 91, 92. In this case, front brake 81 and rear brake 82 are used in a direction-related manner according to the acceleration specification and steering specification indicating whether the vehicle should be braked. If a braking input is used to generate a yaw moment on the vehicle, a compensation torque can be provided. For this purpose, unit 64 for establishing braking torque compensation generates a corresponding control signal 95 for driveline 85. The compensation torque generated by the controller maintains the speed of the vehicle and this compensation torque comes from driveline 85 of the vehicle, in particular from a front-wheel drive or a rear-wheel drive. Here, a switching is determined in driving state determination device 10. For this purpose, unit 64 for establishing braking torque compensation receives corresponding vehicle signal 103 from driving state determination device 10, and vehicle signal 761 related to the response of front brake 81 from front brake 81, and generates a control signal 95 for driveline 85 considering these vehicle signals 103, 761.

[0098] However, in the case of a defective front axle steering system, the main actuator for steering is the rear axle steering system, in which the steering angle of the steering wheels of the rear axle is adjusted via the steering actuator 84 of the rear axle steering system, and the steering actuator 84 is controlled by a control signal 94 generated by the control unit 6. In this exemplary embodiment, here, the steering actuator 84 is in principle controlled by the front steering component and the superimposed yaw regulator, which damps disturbances such as, in particular, μ-split braking. The unit 62 for establishing the steering actuator signal 621 establishes the steering actuator signal 621 for this purpose, which is superimposed with the control signal 611 established by the yaw regulator 61 to form the control signal 94. In this case, the unit 62 for establishing the steering actuator signal 621 takes into account the received vehicle signal 721 related to the steering specification and the received vehicle signal 731 related to the position of the rack of the front axle steering system to establish the steering actuator signal 621. The yaw regulator 61 takes into account, for establishing the control signal 611, the superimposed signal of the reference value 631 of the yaw angular velocity - which is established by the unit 63 for establishing the reference value of the yaw angular velocity - and the yaw angular velocity 751 detected by the sensor. In this case, the yaw angular velocity 751 detected by the sensor is subtracted from the reference value 631 of the yaw angular velocity, and the result is provided as the input variable 610 to the yaw regulator 61. For establishing the reference value 631 of the yaw angular velocity, the unit 63 for establishing the reference value of the yaw angular velocity takes into account the vehicle signal 741 related to the vehicle speed and the signal provided by the unit 65 for calculating the respective braking pressures of the front brake 81 and the rear brake 82, in particular the signal related to the calculated braking pressures.

[0099] Furthermore, a control signal 92 is generated by the unit 66 for determining the activation or deactivation of the damping of the front axle steering system, and the damper unit 83 of the front axle steering system is activated or deactivated by this control signal. In order to determine whether the damper unit 83 should be activated and thus the steering movement of the front axle should be damped or whether the damper unit 83 should not be activated or should be deactivated, the unit 66 for determining the activation or deactivation of the damping of the front axle steering system evaluates the vehicle signal 102 provided by the driving state determination device 10 and related to the driving state of the motor vehicle. In this case, the activation or deactivation of the damper unit 83 depends in particular on the driving maneuvers detected by the driving state determination device 10 as "desired".

[0100] In order to enable the vehicle to steer in the event of a failure of the front axle steering system, in this exemplary embodiment, the emergency steering system 1 uses a rear axle steering system via the steering actuator 84 of the rear axle steering system, in particular a yaw regulator for improving the control of the steering actuator 84, a selective braking application to the front brakes 81 and the rear brakes 82 by steer-by-brake, an applicable torque distribution by selectively applying drive torque via the powertrain 85, and a brake pressure compensation if necessary.

[0101] Referring to Figure 3 , a further exemplary embodiment of a two-axle motor vehicle with a left front wheel FL, a right front wheel FR, a left rear wheel RL, a right rear wheel RR, and an emergency steering system 1 will be explained in more detail. The emergency steering system 1 includes a steer-by-wire system 11, wherein the steer-by-wire system 11 includes a control unit 6, which can in particular be in the form of a driver assistance system and is configured to perform, in an assisting manner, a method for steering a motor vehicle, in particular in the case where the steer-by-wire system 11 is affected by a fault and, in particular due to a malfunction of the front axle steering system, the steering specifications cannot or at least cannot be converted solely by controlling the steering actuator of the steer-by-wire system 11, which acts on the steerable wheels of the motor vehicle via a connecting rod.

[0102] In order to steer in an assisting manner, the control unit 6 includes a unit 68 for determining braking specifications, which is configured to control the brakes 81L, 81R, 82L, 82R associated with the respective wheels FL, FR, RL, RR of the motor vehicle, thereby achieving a desired yaw movement of the motor vehicle according to the steering specifications. For this purpose, in this exemplary embodiment, the steering specifications introduced by the driver via the steering handle are detected by means of the steer-by-wire system 11, and the control unit 6 provides a desired yaw angular velocity taking into account the detected steering specifications.

[0103] Using a sensor 7 connected to a control unit 6 and configured to detect different items of driving state information, determine the current actual yaw rate of the motor vehicle, and taking into account the determined actual yaw rate, determine a braking specification for brakes 81L, 81R, 82L, 82R such that due to the braking application - i.e., in particular the selective actuation of brakes 81L, 81R associated with the front wheels FL, FR or brakes 82L, 82R associated with the rear wheels RL, RR, the actual yaw rate is brought close to the desired yaw rate according to the determined braking specification. In this case, in this exemplary embodiment, the control unit 6, in particular the unit 68 associated with the control unit 6 for determining the braking specification, determines the braking pressure for brakes 81L, 81R, 82L, 82R associated with wheels FL, FR, RL, RR according to the braking specification. In this case, in this exemplary embodiment, depending on the situation, the left brakes 81L, 82L are set to be actuated with the determined braking pressure, and the right brakes 81R, 82R are set to be actuated with a braking pressure of zero, or the right brakes 81R, 82R are set to be actuated with the determined braking pressure, and the left brakes 81L, 82L are set to be actuated with a braking pressure of zero. This is based on the concept that a force difference relative to the longitudinal force F x should be generated to enable the desired expected yaw rate to be achieved.

[0104] According to Figure 3 a first variant of the exemplary embodiment described above, in this case it is set to determine the braking specification based on a two-track model, i.e., in this exemplary embodiment, determine the braking pressure for brakes 81L, 81R, 82L, 82R of the motor vehicle. For this purpose, Figure 4 by way of example, the following is elaborated: three different steering angles SA1, SA2, SA3, which are shown in the graph of (b) of Figure 4 as an angle in degrees relative to the vehicle speed in km / h (km: kilometer, h: hour); corresponding desired yaw rates TLA1, TLA2, TLA3, which are shown in (c) of Figure 4 as an acceleration in m / s 2 per unit (m: meter, s: second) relative to the vehicle speed in km / h; and the determined braking pressures BP1, BP2, BP3, which are shown in the graph of (a) of Figure 4 as a pressure in bar relative to the vehicle speed in km / h. Thus, from the steering specification according to the steering angle SA1, the desired yaw rate TLA1 and the braking pressure BP1 are generated. Thus, in this variant, for a smaller steering angle, a smaller braking pressure is also generated. It can also be seen that in this variant, the braking pressure decreases at higher driving speeds.

[0105] In this case, the braking pressure is generated based on a two-rail model according to the longitudinal force F x and the force difference ΔF X where the force difference is determined as follows:

[0106]

[0107] Taking into account

[0108] δ f ≈ 0,

[0109] δ r ≈ 0,

[0110]

[0111] F y = c α * α,

[0112]

[0113] where the lateral load displacement F y1 = F y2 and F y3 = F y4

[0114] (where F x1 or F x2 is equal to 0)

[0115] and the steady state is solved when the equations are related,

[0116] F x * r = p * c P

[0117] F x = ΔF x , because depending on whether ΔF x > 0 or ΔF x < 0, the brake is actuated at only one wheel and F x = 0 at the other wheel,

[0118] N b = F x * r (as a requirement for the braking torque N b ), and (for determining the braking pressure)

[0119] In this case, the following applies to these equations:

[0120] F y : Lateral force,

[0121] F x : Longitudinal force,

[0122] CoG: Center of gravity,

[0123] α: Lateral displacement angle of the tire,

[0124] δ: Steering angle of the wheel,

[0125] v: Local velocity,

[0126] β: Kingpin inclination angle of the chassis,

[0127] Yaw velocity,

[0128] Yaw acceleration,

[0129] m: Vehicle mass,

[0130] c α : Lateral tire stiffness,

[0131] b: Track width,

[0132] l: Spacing from the center of gravity,

[0133] N b : Braking torque,

[0134] r: Wheel radius,

[0135] p: Braking pressure,

[0136] c P : Braking pressure ratio to braking torque coefficient (to be established through experiments),

[0137] Index "Yin": Front,

[0138] Index "Yin": Rear,

[0139] Index "Yin": Left front,

[0140] Index "Yin": Right front,

[0141] Index "Yin": Left rear,

[0142] Index "Yin": Right rear.

[0143] The control unit 6 or a determination unit 68 associated with the control unit 6 recalculates the force difference established according to the above formula in consideration of the wheel diameters of the wheels FL, FR, RL, RR and the so-called c_p value (braking pressure ratio (:) torque) to form a braking pressure. The braking pressure is scaled by a scale factor less than 1, and a scale factor having a value between 0.05 and 0.1 has been found to be particularly preferred. In this case, in this variant, the control unit 6 is further configured to evaluate the current driving maneuver based on the following vehicle information items, which can in particular be established by means of the sensor 7, and the scale factor is fixed according to the evaluation of the driving maneuver.

[0144] In Figure 3 a second variant of the exemplary embodiment shown, different from the first variant, the braking specification is calculated, wherein a desired yaw moment proportional to the desired yaw angular velocity is determined according to the desired yaw angular velocity, and the desired yaw moment is multiplied by the value of the current driving speed of the motor vehicle. In this second variant, the control unit 6 is configured to determine the longitudinal force F x of the force difference ΔF X :

[0145] where C is a constant used as an adjustment factor. Then, again according to the braking pressure is determined.

[0146] According to this second variant, Figure 5 by way of example, the following is illustrated: three different steering angles SA1, SA2, SA3, which are shown in the Figure 5 (b) graph as an angle in degrees relative to the vehicle speed in km / h (km: kilometer, h: hour); corresponding desired yaw angular velocities TLA1, TLA2, TLA3, which are shown in the Figure 5 (c) as an acceleration in m / s 2 (m: meter, s: second) relative to the vehicle speed in km / h; and the determined braking pressures BP1, BP2, BP3, which are shown in the Figure 5 (a) graph as a pressure in bar relative to the vehicle speed in km / h. In this case, the constant C is fixed at the value 95. The braking pressure required here increases with the speed, so that the driver has a stronger feeling of understeering of the vehicle. This is preferred if the main steering system is based on the rear wheels rather than the front wheels, especially if there is no more ability to steer the front wheels in a four-wheel steering system due to a fault, for example due to an accident.

[0147] Figures 6a to 6bAn exemplary embodiment of a steering system of a motor vehicle is shown. The steering system of the motor vehicle has a front axle steering system 2 as a first steering system and a rear axle steering system 3 as a second steering system. Here, the steering system is a steer-by-wire system 11, which can in particular be included in an emergency steering system, as already described. The front axle steering system 3 of the steer-by-wire system 11 includes a first steering actuator 20 for steering the front wheels FL, FR of the motor vehicle. The rear axle steering system 3 of the steer-by-wire system 11 includes a second steering actuator 84 for steering the rear wheels RL, RR of the motor vehicle. In addition, the steer-by-wire system 11 includes a steering wheel as a steering handle 29. In particular during normal operation of the steer-by-wire system 11, the desired steering angle is preferably calculated based on the speed of the motor vehicle and the position of the steering handle 29. In particular in the case of at least partial failure of the front axle steering system 2, the kinematic curve radii R1, R2 are associated with the steering specification, which are in particular used as a basis for determining the required steering angle correction on the fault-free rear axle steering system 3.

[0148] Via the steering handle 29, the driver of the motor vehicle can pre-determine the steering specification for steering the motor vehicle, where, as a result of the steering specification, the kinematic radii R1, R2 followed by the motor vehicle are preferably associated with the steering specification. Alternatively, the steering specification can also be pre-determined by the driver assistance system 28 of the motor vehicle.

[0149] The steer-by-wire system 11 of the motor vehicle further includes: a first sensor unit 76 for detecting the current first wheel steering angle α_Ist of the front wheels FL, FR, i.e., the wheel steering angle actually occupied by the front wheels FL, FR; and a second sensor unit 77 for detecting the current second wheel steering angle of the rear wheels RL, RR, i.e., the wheel steering angle actually occupied by the rear wheels RL, RR. In this case, the sensor units 76, 77 are connected to the control unit 6 of the steer-by-wire system 11. The control unit 6 is configured to determine a first desired steering angle α_Soll for the front wheels FL, FR of the motor vehicle and a second desired steering angle β_Soll for the rear wheels RL, RR according to the detected steering specification. In addition, the control unit 6 is configured to, in the case of a malfunction of the front axle steering system 2, consider the first desired steering angle α_Soll for the front wheels FL, FR, consider the second desired steering angle β_Soll for the rear wheels RL, RR, and consider the first wheel steering angle α_Ist detected by the first sensor unit 76, and control the steering actuator 84 of the rear axle steering system 3 of the steer-by-wire system 11 in such a way as to adjust the second wheel steering angle β_adapt for the rear wheels RL, RR, so as to convert the steering specification. In Figures 6a to 7b the steer-by-wire system 11 in different driving situations is shown.

[0150] In this case, Figure 6a a steer-by-wire system 11 in a faultless normal operating mode is shown, in which the detected steering specification with the associated kinematic radius R1 is converted solely by the front axle steering system 2. The rear axle steering system 3 does not change the wheel steering angles of the rear wheels RL, RR for the purpose of converting the steering specification. Thus, in Figure 6a the second desired steering angle β_Soll for the rear wheels RL, RR is determined to be 0°. In this case, the desired steering angle α_Soll determined for converting the steering specification for the front wheels FL, FR corresponds to the first wheel steering angle α_Ist detected by the sensor.

[0151] For example, due to a collision in which the steering gear mechanism of the steer-by-wire system 11 has been damaged, the following situation may now occur: the front axle steering system 2 is no longer fully functional, and the front wheels FL, FR can no longer be adjusted via the steering actuator 20 such that they may assume the first desired steering angle α_Soll for converting the detected steering specification. In Figure 6b such a situation is shown. Here, for performing the steering specification, it is intended to convert the same kinematic radius R1 as shown in Figure 6a However, due to the damage to the front axle steering system 2, the front wheels FL, FR do not assume the desired steering angle α_Soll determined by the control unit 6, but instead only assume the wheel steering angle α_Ist detected by the sensor unit 76. Here, the control unit 6 recognizes the impairment of the front axle steering system 2 and then takes into account the previously determined first desired steering angle α_Soll - which corresponds to the angle α_Soll for the front wheels FL, FR shown in Figure 6a and takes into account the actual first wheel steering angle α_Ist detected by means of the sensor unit 76 to determine the wheel steering angle β_adapt for the rear wheels RL, RR and adjusts this angle β_adapt by means of the steering actuator 84 of the rear axle steering system 3. In this case, the determination of the wheel steering angle is performed according to the following equation:

[0152] β adapt = atan(tan(α Soll ) - tan(α Ist ))

[0153] If this angle can be adjusted precisely, the motor vehicle can be steered according to the steering specification with the same kinematic radius R1 as shown in Figure 6a Otherwise, at least the steering behavior of the motor vehicle in the case of Figure 6b can be improved in a manner that corresponds to the case of Figure 6aThe steering behavior of the motor vehicle in the case of, especially in the case not shown here, i.e., when the determined steering angle β_adapt is greater than the adjustable steering angle. In this case, the steering movement of the motor vehicle is preferably further approximated to the initially desired steering movement of the motor vehicle, and thus to the initial steering specification, by selective braking and intentionally introduced yaw rate (especially as described in the exemplary embodiments already explained).

[0154] Figure 7a The steer-by-wire system 11 in another normal operating case is shown, wherein, in order to convert the steering specification with the associated kinematic radius R2, both a first desired steering angle α_Soll different from 0° for the front wheels FL, FR and a second desired steering angle β_Soll different from 0° for the rear wheels RL, RR are determined. Then, during fault-free operation, the determined desired steering angles α_Soll, β_Soll are adjusted accordingly by means of the steering actuators 20, 84.

[0155] If now a malfunction of the front axle steering system 2 occurs here such that the detected first wheel steering angle α_Ist is less than the first desired steering angle α_Soll determined for the front wheels FL, FR, the control unit 6 of the steer-by-wire system 11 determines an applicable second steering angle β_adapt taking into account the first desired steering angle α_Soll determined for the fault-free case for converting the steering specification, the second desired steering angle β_Soll determined for the fault-free case for converting the steering specification, and the first wheel steering angle α_Ist detected by means of the sensor unit 76. This applicable second steering angle β_adapt is intended to be adjusted by means of the steering actuator 84 of the rear axle steering system 3 and replaces the desired specification for the initially established desired steering angle β_Soll. The second steering angle β_adapt is determined by the following formula:

[0156] β adapt = atan(tan(α Soll ) + tan(β Soll ) - tan(α Ist ))

[0157] In this case, as already explained with respect to Figure 6a it also applies that at least when the second steering angle β_adapt can be adjusted, the kinematic radius R2 associated with the steering specification can be precisely converted. Otherwise, at least in an improved manner, especially in the corresponding manner already explained for the failure cases described with reference to Figure 6b the steering specification is approximated.

[0158] Exemplary embodiments shown in and described in connection with the accompanying drawings are used to illustrate the present invention and not to limit the present invention.

[0159] List of Reference Numerals

[0160] 1 Emergency steering system

[0161] 11 Steer-by-wire system

[0162] 2 Front axle steering system

[0163] 20 Steering actuator of the front axle steering system

[0164] 25 Fault of the front axle steering system

[0165] 28 Driver assistance system

[0166] 29 Steering handle

[0167] 3 Rear axle steering system

[0168] 4 Front-wheel drive

[0169] 5 Rear-wheel drive

[0170] 6 Control unit

[0171] 61 Yaw regulator

[0172] 610 Input variables of the yaw regulator (61)

[0173] 611 Control signal of the yaw regulator (61)

[0174] 62 Unit for establishing a steering actuator signal

[0175] 621 Steering actuator signal

[0176] 63 Unit for establishing a reference value of the yaw angular velocity

[0177] 631 Established reference value of the yaw angular velocity

[0178] 64 Unit for establishing a braking torque compensation

[0179] 65 Unit for calculating the corresponding braking pressures of the front brake (81) and the rear brake (82)

[0180] 66 Unit for determining the activation / deactivation of the damping of the front axle steering system (2)

[0181] 67 Unit for linking signals

[0182] 68 Unit for determining braking specifications

[0183] 7 Sensor unit

[0184] 71 Sensor unit for detecting the actuation of the brake pedal / accelerator pedal

[0185] 72 Sensor unit for detecting the steering specification

[0186] 73 Sensor unit for detecting the position of the toothed rack of the front axle steering system (2)

[0187] 74 Sensor unit for detecting the vehicle speed

[0188] 75 Sensor unit for detecting the yaw angular velocity of the motor vehicle

[0189] 76 Sensor unit for detecting the current first actual wheel steering angle of the front wheels

[0190] 77 Sensor unit for detecting the current second actual wheel steering angle of the rear wheels

[0191] 711 Vehicle signal related to the acceleration specification

[0192] 721 Vehicle signal related to the steering specification

[0193] 731 Vehicle signal related to the position of the toothed rack of the front wheel steering system

[0194] 741 Vehicle signal related to the vehicle speed

[0195] 751 Vehicle signal related to the yaw angular velocity of the motor vehicle

[0196] 761 Vehicle signal related to the response of the front brake (81)

[0197] 8 Actuator of the motor vehicle

[0198] 81 Front brake

[0199] 81L Left front brake

[0200] 81R Right front brake

[0201] 82 Rear brake

[0202] 82L Left rear brake

[0203] 82R Right rear brake

[0204] 83 Damper unit of the front axle steering system (2)

[0205] 84 Steering actuator of the rear axle steering system (3)

[0206] 85 Powertrain

[0207] 91 Control signal for the front brake (81)

[0208] 92 Control signal for the rear brake (82)

[0209] 93 Control signal for the damper unit (83) of the rack and pinion for the front axle steering system

[0210] 94 Control signal for the steering actuator (84) for the rear axle steering system (3)

[0211] 95 Control signal for the powertrain (85)

[0212] 10 Driving state determination device

[0213] 101 Vehicle signal related to the driving state of the motor vehicle

[0214] 102 Vehicle signal related to the driving state of the motor vehicle

[0215] 103 Vehicle signal related to the driving state of the motor vehicle

[0216] 104 Vehicle signal related to the driving state of the motor vehicle

[0217] FL Left front wheel

[0218] FR Right front wheel

[0219] RL Left rear wheel

[0220] RR Right rear wheel

[0221] L Axle spacing between the first axle (5) and the second axle (8)

[0222] R1, R2 Kinematic curve radius associated with the steering specification

[0223] α_Soll First desired steering angle for the first wheel

[0224] α_Ist First (measured) wheel steering angle of the first wheel

[0225] β_Soll Second desired steering angle for the second wheel

[0226] β_adapt Second wheel steering angle of the second wheel

Claims

1. A method for controlling a motor vehicle having a steer-by-wire system with a front axle steering system (2) and a rear axle steering system (3) in the event of an identified fault (25) in the front axle steering system (2), Among them, detecting vehicle signals related to the steering specification (721) of the motor vehicle, the position (731) of the toothed rack of the front axle steering system (2), and the yaw angular velocity (751), and providing the detected vehicle signals (721, 731, 751) to a control unit (6), the control unit (6) generating a first control signal (94) for controlling a steering actuator (84) of the rear axle steering system (3) in order to convert the steering specification, wherein the first control signal (94) is generated taking into account the steering specification (721), the position (731) of the toothed rack of the front axle steering system (2), and the yaw angular velocity (751), and the steering actuator (84) of the rear axle steering system (3) is controlled by the generated first control signal (94).

2. The method according to claim 1, wherein The control unit (6) generates second control signals (91, 92) for controlling the front brakes (81) of the motor vehicle and / or the rear brakes (82) of the motor vehicle in order to convert the steering specification, wherein the front brakes (81) and / or the rear brakes (82) are controlled by the generated second control signals (91, 92).

3. The method according to claim 2, wherein Additionally detecting a vehicle signal related to the acceleration specification (711), wherein, taking into account the acceleration specification (711), a braking pressure for the front brakes (81) and / or the rear brakes (82) is calculated as the second control signals (91, 92).

4. The method according to claim 3, wherein Additionally detecting a vehicle signal related to the vehicle speed (741), wherein, taking into account the vehicle speed (741) and the calculated braking pressure, a reference value (631) for the yaw angular velocity of the motor vehicle is established.

5. The method according to one of the preceding claims, characterized in that, Taking into account the steering specification (721) and the position (731) of the toothed rack of the front axle steering system (2), a steering actuator signal (621) for the steering actuator (84) of the rear axle steering system (3) is generated, a yaw regulator (61) generates a control signal (611) for determining the torque distribution at each wheel (FL, FR, RL, RR) of the motor vehicle taking into account the yaw angular velocity of the motor vehicle, and the first control signal (94) is generated based on the superposition of the steering actuator signal (621) and the control signal (611).

6. The method according to claim 4 and claim 5, characterized in that, The difference between the detected vehicle signal related to the yaw angular velocity (751) of the motor vehicle and the established reference value (631) for the yaw angular velocity is provided as an input variable (610) to the yaw regulator (61).

7. The method according to one of the preceding claims, characterized in that, Provide a specification of a desired yaw rate to be achieved for the motor vehicle, taking into account the current actual yaw rate of the motor vehicle, determine a braking specification for at least one of the wheels (FL, FR, RL, RR) of the motor vehicle so that the actual yaw rate is closer to the desired yaw rate, and perform a braking application according to the determined braking specification.

8. The method according to claim 7, wherein Determine the braking specification based on a two-track model, wherein a specification of the desired yaw rate is provided as an input variable to the two-track model, and a braking torque difference resulting therefrom is established between the wheels (FL, FR, RL, RR) using the two-track model for an assumed steady state of the motor vehicle as an output variable, wherein the braking specification is determined according to the braking torque difference.

9. The method according to claim 7 or claim 8, characterized in that, Apply a scaling factor to the determined braking torque difference, wherein the scaling factor is less than 1 and is particularly determined as a value in the range from 0.05 to 0.2, and wherein the higher the value to which the scaling factor is preferably set, the lower the steering action that can still be provided by the steering system.

10. The method according to one of the preceding claims, characterized in that, A vehicle signal related to the response (761) of the front brake (81) of the motor vehicle is additionally detected and provided to the control unit (6), wherein the control unit (6) generates a braking torque compensation as a third control signal (95) taking into account the response (761) of the front brake (81) of the motor vehicle, and the powertrain (85) of the motor vehicle is controlled by the third control signal (95).

11. The method according to one of the preceding claims, characterized in that, Damp the movement of the toothed rack of the front axle steering system (2).

12. The method according to one of the preceding claims, characterized in that, The driving state determination device (10) determines the driving state (101, 102, 103, 104) of the motor vehicle considering available vehicle signals (711, 721, 731, 741, 751, 761), wherein the determined driving state (101, 102, 103, 104) is provided to the control unit (6).

13. The method according to claim 12, wherein The control unit (6) takes into account the driving state (101, 102, 103, 104) for generating at least one of the control signals (91, 92, 93, 94, 95).

14. The method according to one of the preceding claims, characterized in that, Additionally detect a first steering angle (α_Ist) of the front wheels (FL, FR), and use the rear axle steering system (3) to adjust a second steering angle (β_adapt) of the rear wheels (HL, HR) so as to convert the steering specification (721), determine a first desired steering angle (α_Soll) for the front wheels according to the detected steering specification (721) and determine a second desired steering angle (β_Soll) for the rear wheels (RL, RR), and determine the second steering angle (β_adapt) considering the determined first desired steering angle (α_Soll), the determined second desired steering angle (β_Soll) and the detected first steering angle (α_Ist).

15. The method according to claim 14, characterized in that, The second steering angle (β_adapt) is determined by β adapt = atan(tan(α Soll ) + tan(β Soll ) - tan(α Ist ))), where β adapt = Second steering angle α Soll = the first desired steering angle for the front wheel; β Soll = the second desired steering angle for the rear wheel; and α Ist = The detected first-round steering angle.

16. An emergency steering system (1) for providing the ability to steer a motor vehicle having a steer-by-wire system with a front axle steering system (2) and a rear axle steering system (3) in the event of failure of the front axle steering system (2), the emergency steering system comprising a control unit (6), wherein, The steering actuator (84) of the rear axle steering system (3) and the actuators (8) of the motor vehicle outside the steer-by-wire system are associated with the control unit (6), and a sensor unit (72) for detecting vehicle signals related to steering specifications and additional sensor units (71, 73, 74, 75) for detecting additional vehicle signals are associated with the control unit (6), wherein the control unit (6) is configured to receive vehicle signals (711, 721, 731, 741, 751) from the associated sensor units (7), and generate control signals (91, 92, 93, 94, 95) for the steering actuator (84) of the rear axle steering system (3) associated with the control unit (6) and the actuators (8) of the motor vehicle outside the steer-by-wire system according to the received vehicle signals (711, 721, 731, 741, 751) to convert the received steering specifications (721).

17. The emergency steering system (1) according to claim 16, characterized in that, The additional sensor units (71, 73, 74, 75) include at least one of the following sensor units: a sensor unit (71) for detecting the actuation of a brake operating element; a sensor unit (71) for detecting the actuation of an acceleration operating element; a sensor unit (73) for detecting the position of the toothed rack of the front axle steering system; a sensor unit (74) for detecting the vehicle speed; a sensor unit (75) for detecting the yaw angular velocity (751) of the motor vehicle.

18. The emergency steering system (1) according to claim 16 or claim 17, characterized in that, The actuators of the motor vehicle outside the steer-by-wire system include at least one of the following actuators: the front brakes (81) of the motor vehicle; the rear brakes (82) of the motor vehicle; the driveline (85) of the motor vehicle.

19. The emergency steering system (1) according to one of claims 16 to 18, characterized in that, The short-circuit circuit of the electric motor of the steering actuator (20) of the front axle steering system (2) is also associated with the control unit (6), wherein the short-circuit circuit is configured to short-circuit the phases of the electric motor in the case of actuation and thus damp the movement of the toothed rack of the front axle steering system (2).

20. The emergency steering system (1) according to one of claims 16 to 19, characterized in that, The driving state determination device (10) is also associated with the control unit (6), wherein the driving state determination device (10) is configured to determine the driving state of the motor vehicle considering the detected vehicle signals (711, 721, 731, 741, 751), and provide the driving state of the motor vehicle as an additional input signal for the control unit (6), wherein the control unit (6) is also configured to consider the provided driving state for generating at least one of the control signals (91, 92, 93, 94, 95).

21. The emergency steering system (1) according to one of claims 16 to 20, characterized in that, The emergency steering system (1) further comprises a sensor unit (76) for detecting a current first wheel steering angle (α_Ist) of the front wheels (FL, FR), wherein the control unit (6) is further configured to: determine a first desired steering angle (α_Soll) for the front wheels (FL, FR) of the motor vehicle and / or a second desired steering angle (β_Soll) for the rear wheels (RL, RR) of the motor vehicle according to the steering specification (721), and is further configured to control a steering actuator (84) of the rear axle steering system (3) in the case of at least partial failure of the front axle steering system (2) taking into account the first desired steering angle (α_Soll) for the front wheels (FL, FR) and / or taking into account the second desired steering angle (β_Soll) for the rear wheels (RL, RR) and the detected first wheel steering angle (α_Ist), such that the steering actuator (84) adjusts a second wheel steering angle (β_adapt) for the rear wheels (7) in order to convert the steering specification.

22. The emergency steering system (1) according to one of claims 16 to 21, characterized in that, The emergency steering system (1) is configured to carry out the method according to one of claims 1 to 15.

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