Steer-by-wire steering system and method for operating a steer-by-wire steering system with vehicle stabilization

Abstract

The present invention relates to a method for operating a steer-by-wire steering system (1) in a motor vehicle, wherein a steering input is detected and a steering input wheel steering angle (δdrv) is determined from the detected steering input, a set actual wheel steering angle (δrw), a vehicle speed (V) and a yaw rate (r) of the motor vehicle are detected, and taking these quantities (δrw, V, r) into account, a driving condition of the motor vehicle is evaluated with regard to the occurrence of understeer or oversteer.When understeer or oversteer is detected, a supporting steering signal (δast) is provided to counteract the understeer / oversteer. Taking into account the steering input and the supporting steering signal (δast), a steering actuator unit (9) is provided with a wheel steering angle (δreq) to be set, which the steering actuator unit (9) converts into a wheel steering angle of the steerable wheels (14). For determining the supporting steering signal (δast), an adapted steering ratio and a wheel steering angle limit are determined depending on the degree of understeer / oversteer. Furthermore, the invention relates to a steer-by-wire steering system that can be operated according to the above method.

Description

[0001] The invention relates to a steer-by-wire steering system comprising an inertial measuring unit configured to determine a yaw rate during the operation of a motor vehicle, a speed determination unit configured to determine a vehicle speed during the operation of a motor vehicle, a steering actuator unit configured to set a wheel steering angle of steerable wheels and to determine a set wheel steering angle, a feedback actuator unit configured to detect a steering input and to determine a steering input wheel steering angle from a detected steering input, and a computing unit.

[0002] Furthermore, the invention relates to a method for operating a steer-by-wire steering system in a motor vehicle, wherein a steering input is detected and a steering input wheel steering angle is determined from the detected steering input, a set actual wheel steering angle, a vehicle speed and a yaw rate of the motor vehicle are detected, taking into account the set actual wheel steering angle, the vehicle speed and the yaw rate, a driving state of the motor vehicle is evaluated with regard to the occurrence of understeer or oversteer of the motor vehicle, if understeer or oversteer is detected, a supporting steering signal is generated which counteracts the understeer or oversteer of the motor vehicle, and a steering actuator unit is provided with a set wheel steering angle taking into account the steering input and the supporting steering signal.which the steering actuator unit translates into a wheel steering angle of the steerable wheels of the motor vehicle.

[0003] US Patent 7,032,705 B2 discloses a system for compensating oversteer and understeer in vehicles with a steer-by-wire steering system. The system comprises a driver interface for inputting a steering angle, a sensor system for detecting the input steering angle and for recording vehicle parameters, a control unit, and an actuator system for adjusting the steering angle of the steerable wheels. The control unit compares a detected yaw rate and a detected lateral acceleration with corresponding calculated target values. In case of deviations from the calculated target values, the system detects oversteer or understeer and, using a proportional-integral controller, generates a speed-dependent assisting steering signal to counteract the oversteer or understeer.

[0004] Furthermore, generic US 7,099,759 B2 describes a method and device for enabling real-time control of vehicle dynamics, thereby improving the vehicle's stability and handling. The vehicle may have a steer-by-wire steering system. It is described that an estimated understeer or oversteer indicator is calculated based on the general steering equation. This calculation takes into account vehicle speed, steering angle, yaw rate, and lateral acceleration. The indicator enables real-time control of the vehicle's yaw motion by coordinating the control of the brakes, steering, drive, and suspension.

[0005] Furthermore, US 7,997,373 B2 discloses a steering control device for vehicles which adjusts the steering angle of a wheel under different friction conditions between the right and left sides of the road (µ-split conditions) independently of the driver's operation, thus preventing oversteer or understeer.

[0006] Furthermore, DE 10 2019 214 225 A1 describes a method for operating a vehicle with dynamic variation of a usable wheel steering angle range, whereby the wheel steering angle range can be adjusted, in particular, depending on the degree of oversteer or understeer of the vehicle. The yaw rate of the vehicle can also affect the wheel steering angle range.

[0007] Furthermore, DE 10 2005 012 548 A1 discloses a method for increasing the driving stability of a vehicle while driving through a curve. This method involves adjusting the steering ratio by means of a superimposed steering system, which combines the steering movement specified by the driver with an additional steering movement generated by a control unit. The vehicle's behavior is monitored for the occurrence of understeer, and if understeer is detected, the steering ratio is made more indirect with increasing steering wheel angle.

[0008] Against this background, it is an object of the present invention to provide an improved steer-by-wire steering system and an improved method for operating a steer-by-wire steering system in which, advantageously, in normal driving situations, the driver's steering movement is directly translated into a request for the steering angle of the wheels, but advantageously, in the event of oversteer or understeer of a motor vehicle, a steering intervention to stabilize the motor vehicle takes place, which in particular follows the intention of the driver, but advantageously does not directly execute his steering input.

[0009] To solve this problem, a method for operating a steer-by-wire steering system in a motor vehicle according to claim 1 and a steer-by-wire steering system according to the dependent claim are proposed. Further advantageous embodiments of the invention are described in the dependent claims and the description and are illustrated in the figures.

[0010] The proposed solution provides a method for operating a steer-by-wire steering system in a motor vehicle, whereby a steering input is detected and a steering input wheel angle is determined from this input. Additionally, a set actual wheel angle, vehicle speed, and yaw rate of the vehicle are detected. Taking into account the set actual wheel angle, vehicle speed, and yaw rate, the vehicle's driving condition is evaluated with regard to the occurrence of understeer or oversteer. If understeer or oversteer is detected, a supporting steering signal is provided to counteract the understeer or oversteer, respectively.A steering actuator unit is then provided with a set wheel steering angle, taking into account the steering input and the supporting steering signal. The steering actuator unit then translates this angle into a steering angle for the steerable wheels of the vehicle. To determine the supporting steering signal, an adapted steering ratio and a wheel steering angle limit are calculated based on whether the vehicle is exhibiting understeer or oversteer.

[0011] The steer-by-wire system advantageously offers the possibility of independently controlling the set steering angle, particularly through the position of the steering linkage, and the position of the steering wheel. Advantageously, driving condition parameters, especially the currently set steering angle, the currently determined vehicle speed, and the currently determined yaw rate of the vehicle, are used to increase vehicle stability and improve driving safety.The method advantageously allows the driver of the motor vehicle to drive undisturbed in most normal driving situations, particularly in situations without understeer or oversteer, and advantageously provides assistance only in the event of a loss of stability, particularly oversteer or understeer, especially to avoid an accident, while advantageously preserving the driving pleasure during normal driving. Thus, the method advantageously enables a steering movement to be directly translated into a corresponding steering angle of the steered wheels in normal driving situations without oversteer or understeer.In abnormal driving situations involving oversteer or understeer, the steering input resulting from the steering movement is advantageously adjusted for the wheel steering angle, in particular in such a way that the intended direction of travel is followed, but the detected steering command is advantageously not executed directly.

[0012] In particular, it is intended that the steering input is given by the driver of the motor vehicle via a steering hand. However, the steering input can alternatively also be given by a driver assistance system, especially an AD system (AD: Autonomous Driving).

[0013] According to the invention, evaluating the driving condition includes generating an evaluation signal. This signal represents the degree of understeer or oversteer of the vehicle. The evaluation signal thus provides, in particular, a measure of any understeer or oversteer occurring. This advantageously allows for a more effective and adapted response to understeer or oversteer. Advantageously, the generated evaluation signal is ±0 in a normal driving condition. This means that the evaluation signal is zero or approximately zero in a normal driving condition where no understeer or oversteer occurs. When the vehicle is oversteering, the evaluation signal is advantageously less than zero, and its magnitude advantageously increases with the degree of oversteer.In the case of understeer by the vehicle, the evaluation signal is advantageously greater than 0, and the evaluation signal advantageously increases with the degree of understeer. In particular, it is provided that the evaluation signal can take values ​​from the interval [-20, 20].

[0014] Furthermore, the procedure provides for the generation of a limitation signal based on the evaluation signal. This limitation signal influences the wheel steering angle limitation and, in particular, the determination of the required supporting steering signal. Advantageously, the limitation signal can be used to weaken the implementation of a steering input wheel steering angle, so that a wheel steering angle can be specified instead and given greater consideration by the steering actuator unit during implementation, thus contributing to stabilizing the vehicle's driving state.

[0015] Advantageously, each value of the generated evaluation signal is assigned a corresponding value for the limitation signal, particularly using a lookup table. A particularly advantageous value assignment is described in Fig. Figure 3 illustrates this. In particular, the limiting signal is designed as a function of the evaluation signal, with the limiting signal corresponding, at least section by section, to a parabola. Advantageously, such a mapping allows for good results in limiting or attenuating the steering input wheel steering angle.

[0016] According to a further advantageous embodiment, a scaling signal is generated based on the evaluation signal, particularly in addition to the limiting signal. The scaling signal advantageously influences the adjustment of the steering ratio. Advantageously, a value for the scaling signal is assigned to each value of the generated evaluation signal, particularly using a lookup table. Specifically, the scaling signal is assigned a value λ where 0 ≤ λ ≤ 1, with a particularly advantageous mapping of values ​​of the evaluation signal to values ​​of the scaling signal in Fig. Figure 4 illustrates this. The scaling signal is based, in particular, on the idea of ​​reducing the yaw rate resulting from the steering input when oversteer or understeer increases. Advantageously, the steering input wheel steering angle is limited using the scaling signal, so that a steering movement of the steering wheel is advantageously not, or at least not completely, translated into a corresponding wheel steering angle by the steering actuator.

[0017] A further advantageous development of the method provides that, based on the steering input wheel steering angle and the scaling signal, a target yaw rate is determined, in particular a target yaw rate that, on the one hand, takes the driving direction input into account and, on the other hand, contributes to the stabilization of the vehicle. Advantageously, taking into account the detected actual wheel steering angle, the detected vehicle speed, the detected yaw rate, and the determined target yaw rate, a target wheel steering angle is determined that is required to achieve the target yaw rate. The determination of this target wheel steering angle is carried out, in particular, using a control unit, specifically a control unit of a vehicle motion control system (VMS) that is communicatively connected to the steer-by-wire steering system.The target steering angle determined in this way is specifically not intended to be transmitted to the steering actuator as the steering angle to be set. Advantageously, the target steering angle represents an intermediate result, which is, however, advantageously taken into account when determining the steering angle to be set.

[0018] In particular, it is provided that a differential wheel steering angle is determined by subtracting the steering input wheel steering angle from the determined target wheel steering angle. To generate a steering assistance wheel steering angle, a saturation function is advantageously applied to the determined differential wheel steering angle, limiting the differential wheel steering angle to a range defined by the previously determined limiting signal, where the steering assistance wheel steering angle is advantageously the assisting steering signal.

[0019] Advantageously, the steering input wheel steering angle and the power steering assist wheel steering angle are then added to obtain the wheel steering angle to be set. This means that the sum of the steering input wheel steering angle and the power steering assist wheel steering angle advantageously yields the wheel steering angle to be set, which is advantageously transmitted to the steering actuator unit for setting the wheel steering angle of the steered wheels. For normal driving conditions, in which neither oversteer nor understeer occurs, the power steering assist wheel steering angle is advantageously set to 0, so that the steering input wheel steering angle is then advantageously directly implemented as the wheel steering angle to be set.

[0020] In particular, a method for operating a steer-by-wire steering system in a motor vehicle is proposed, in which a target yaw rate is calculated based on a detected steering input and a vehicle model, taking into account current driving state parameters. Advantageously, the vehicle stability of the motor vehicle with respect to the occurrence of oversteer or understeer is evaluated, in particular by determining and estimating the current state of the motor vehicle and the steering system. Advantageously, a control unit, in particular a control unit of a vehicle motion control system of the motor vehicle, reduces the steering assistance for a driver steering input and increases the intervention by the control unit to stabilize the motor vehicle, based on the current stability state of the motor vehicle.The driver's steering input, derived in particular from the steering wheel movement, is advantageously taken into account to calculate a target trajectory that advantageously keeps the vehicle stable. Once the vehicle is stabilized, the driver's steering inputs are advantageously transmitted directly to the steering actuator, allowing the vehicle to execute them directly. The adjustable multiplier characteristics for the scaling signal and the limiting signal are of particular importance in this context.

[0021] The scaling signal is advantageously used to scale the steering ratio. If oversteer or understeer increases, a driver-defined target yaw rate is advantageously reduced. The steering wheel angle is advantageously limited with respect to a steering input, so that the steering gear does not move along with the steering wheel; in particular, a larger steering input does not result in a larger wheel steering angle.

[0022] The limiting signal advantageously serves to limit the steering assistance. The control unit, in particular the control unit of the vehicle motion control system, advantageously compares the steering input wheel steering angle resulting from the steering input with the target wheel steering angle, based on the determined vehicle states and the driver's steering direction input. This target wheel steering angle enables adherence to the target trajectory and simultaneously stabilizes the vehicle. The greater the difference between the target wheel steering angle and the steering input wheel steering angle during a detected vehicle instability, the more the control unit advantageously reduces the transmission of the steering input wheel steering angle to the steering actuator and instead transmits the calculated target wheel steering angle with greater weighting.Advantageously, the driver remains largely undisturbed by the interventions, the driver's steering requirements are advantageously translated into a steering movement of the steerable wheels in the best possible way, and the vehicle stability and steerability of the motor vehicle are advantageously maintained.

[0023] The steer-by-wire steering system proposed to solve the aforementioned problem comprises an inertial measuring unit configured to determine a yaw rate during the operation of a motor vehicle, a speed determination unit configured to determine a vehicle speed during the operation of a motor vehicle, a steering actuator unit configured to set and determine a set steering angle of steerable wheels, a feedback actuator unit configured to detect a steering input and determine a steering input wheel steering angle from a detected steering input, and a computing unit. Advantageously, the steer-by-wire steering system is configured to be operated according to a method developed according to the invention. This results in the advantages described in connection with the proposed method.

[0024] According to an advantageous embodiment, the computing unit, which may also be configured as a computing system, comprises a vehicle stability evaluation unit configured to generate the evaluation signal; a first evaluation unit configured to generate the limiting signal; a second evaluation unit configured to generate the scaling signal; a target yaw rate generation unit; a control unit configured to determine the target wheel steering angle; a first summing unit configured to subtract the steering input wheel steering angle from the target wheel steering angle; a limiting unit configured to generate the steering assistance wheel steering angle as the assisting steering signal; and a second summing unit configured to add the steering assistance wheel steering angle and the steering input wheel steering angle.and a communication system for transmitting the generated signals, in particular for appropriate processing and / or further processing of the signals and / or parameters by the aforementioned units;

[0025] Further advantageous details, features and embodiments of the invention are explained in more detail in connection with the exemplary embodiments illustrated in the figures (hereinafter Fig.: figure). The figures show: Fig. 1 in a simplified perspective view an embodiment of a steer-by-wire steering system designed according to the invention; Fig. 2 a block diagram to illustrate a further embodiment of a steer-by-wire steering system designed according to the invention in the implementation of an embodiment of a method designed according to the invention; Fig. 3 in a diagram representation an exemplary embodiment of an assignment of a limitation signal to an evaluation signal; Fig. 4 in a diagram representation an exemplary embodiment of an assignment of a scaling signal to a rating signal; and Fig. 5 in a diagram representation an embodiment for determining the steering assist wheel steering angle.

[0026] In the various figures, identical parts are usually marked with the same reference symbols and are therefore sometimes only explained in connection with one of the figures.

[0027] In Fig. Figure 1 shows an embodiment of a steer-by-wire steering system 1 for a motor vehicle designed according to the invention. The steer-by-wire steering system 1 comprises a steering column with a steering shaft 2 and a feedback actuator unit 5. A steering handle 3, designed as a steering wheel, is fixedly mounted at one end of the steering shaft 2. A vehicle operator can make a steering input via the steering handle 3. The feedback actuator unit 5 is configured to detect a steering input and to determine a steering input wheel steering angle δdrv from the detected steering input. Furthermore, in this embodiment, the feedback actuator unit 5 is configured to exert a torque, or steering resistance torque, on the steering shaft 2, in particular to provide steering feel. This steering resistance torque is perceptible to a vehicle operator of the motor vehicle as steering resistance via the steering handle 3.To generate the torque, the feedback actuator 5 comprises a feedback actuator electric motor 52 with an associated feedback actuator control unit 51, which, in addition to determining the steering input wheel steering angle δdrv, is designed in particular to control the electric motor 52.

[0028] The steering handle 3 of the steer-by-wire steering system 1 can be rotated in a known manner to transmit a steering input to the steering shaft 2, which is detected by the feedback actuator unit 5. For this purpose, the feedback actuator unit 5 can, in particular, include a first steering angle sensor unit assigned to the steering shaft 2 (in Fig. 1 not explicitly shown) and a second steering angle sensor unit (in Fig. 1 (also not explicitly shown) include components designed to capture a steering angle of the steering shaft 2 set by means of the steering handle 3 as a steering input wheel steering angle δdrv.

[0029] Furthermore, the steer-by-wire steering system 1 includes a steering actuator 9 configured to set a wheel steering angle of steerable wheels 14 according to a specified setting of a wheel steering angle δreq and to determine a set wheel steering angle δrw. In this embodiment, the steering actuator 9 comprises a coupling element 12 designed as a rack, upon which the steering actuator 9 acts to set a wheel steering angle of the steerable wheels 14. To determine the set wheel steering angle δrw, the steering actuator 9 can use a coupling element position sensor unit (in Fig. 1 not explicitly shown) as a means of determining the position of the coupling element 12, wherein the set wheel steering angle δrw of the steered wheels 14 can then be determined from the position of the coupling element 12.

[0030] In this embodiment, the steering actuator unit 9 comprises a steering actuator control unit 91 and a steering actuator electric motor 92. The steering actuator unit 9 is further designed to convert a specified wheel steering angle δreq into a steering angle of the steerable wheels 14 via a steering gear 6 by controlling the steering actuator electric motor 92. In this embodiment, the steering actuator electric motor 92 acts via a transmission belt 61 on a spindle drive 62, which is operatively connected to the coupling element 12 designed as a rack. By appropriately controlling the steering actuator electric motor 92 using the steering actuator control unit 91, the spindle drive 62 is driven to convert the specified wheel steering angle δreq into a steering movement of the steerable wheels 14. In this embodiment, the steerable wheels 14 are connected to the coupling element 12 in a known manner via tie rods 13.The tie rods 13 themselves are connected in a known manner via steering knuckles to each steered wheel 14.

[0031] Furthermore, the steer-by-wire steering system 1 includes a Fig. 1 inertial measurement unit not explicitly shown, also in Fig. 1 speed determination unit not explicitly shown and a computing unit 30, wherein the inertial measuring unit, the speed determination unit and the computing unit 30 need not be explicitly and especially not exclusively assigned to the steer-by-wire steering system 1, but in particular may be assigned to the motor vehicle for several systems, in particular for the steer-by-wire steering system 1.

[0032] The inertial measuring unit is configured to determine the yaw rate r during the operation of a motor vehicle, and the speed determination unit is configured to determine the vehicle speed V during the operation of a motor vehicle. The measured quantities r, V, δdrv and δrw are transmitted to the processing unit 30 via a communication system 40.

[0033] In this embodiment, the steer-by-wire steering system 1 is configured to operate such that a steering input made by the driver is detected, and a steering input wheel steering angle δdrv is determined from this input. Furthermore, taking into account the currently set actual wheel steering angle δrw of the steered wheels 14, the currently detected vehicle speed V, and the currently detected yaw rate r, the vehicle's driving condition is evaluated with regard to the occurrence of understeer or oversteer using the processing unit 30. If understeer or oversteer is detected, a supporting steering signal δast is provided to counteract the understeer or oversteer of the vehicle.The steering actuator unit 9 is then provided with a settable wheel steering angle δreq, taking into account the steering input wheel steering angle δdrv resulting from the steering input and the supporting steering signal δast. The steering actuator unit 9 then converts this wheel steering angle into a wheel steering angle of the steerable wheels 14 of the vehicle. To determine the supporting steering signal δast, an adapted steering ratio and a wheel steering angle limit are calculated using the processing unit 30, depending on whether the vehicle is exhibiting understeer or oversteer.

[0034] Fig. Figure 2 shows a further advantageous embodiment of a steer-by-wire steering system 1 in a block diagram. The steer-by-wire steering system 1 comprises an inertial measuring unit 20 configured to determine a yaw rate r during the operation of a motor vehicle 100, a speed determination unit 21 configured to determine a vehicle speed V during the operation of a motor vehicle 100, a steering actuator unit 9 configured to set a wheel steering angle of steerable wheels 14 and to determine a set wheel steering angle δrw, a feedback actuator unit 5 configured to detect a steering input and to determine a steering input wheel steering angle δdrv from a detected steering input, and a computing unit 30.

[0035] This computing unit 30, which may in particular be part of a vehicle motion control system of the motor vehicle, comprises in this embodiment a vehicle stability evaluation unit 31, a first evaluation unit 32, a second evaluation unit 33, a target yaw rate generation unit 34, a control unit 35, a limiting unit 38, a first summing unit 36, a second summing unit 37 and a communication system 40.

[0036] When the steer-by-wire steering system 1 in the motor vehicle 100 is operating, the feedback actuator unit 5 detects a steering input and determines a steering input wheel steering angle δdrv from the detected steering input. In addition, the steering actuator unit 9 detects a set actual wheel steering angle δrw of the steered wheels of the motor vehicle 100. Furthermore, the current vehicle speed V of the motor vehicle 100 is determined by means of the speed determination unit 21, and the current yaw rate r of the motor vehicle 100 is determined by means of the inertial measurement unit 20.

[0037] The measured values ​​δrw, V, r are transmitted via the communication system 40, which can be a CAN bus (CAN: Controller Area Network), to the vehicle stability evaluation unit 31. Taking into account the actual wheel steering angle δrw, the vehicle speed V, and the yaw rate r transmitted to it, the vehicle stability evaluation unit 31 assesses the current driving state of the vehicle 100 with regard to the occurrence of understeer or oversteer. As part of this assessment, the vehicle stability evaluation unit 31 generates an evaluation signal γ in this embodiment. The evaluation signal γ is a measure of the degree of understeer or oversteer of the vehicle 100, whereby the generated evaluation signal γ is 0 or at least approximately 0 in a normal driving state, less than 0 in the case of oversteer, and greater than 0 in the case of understeer.This evaluation signal γ is then transmitted via the communication system 40 to the first evaluation unit 32 and the second evaluation unit 33.

[0038] The first evaluation unit 32 determines a limiting signal δlim based on the evaluation signal γ. For this purpose, a lookup table assigns a corresponding value for the limiting signal δlim to each value of the evaluation signal γ. In this embodiment, the limiting signal δlim is designed as a function of the evaluation signal γ, which corresponds piecewise to a parabola. The limiting signal δlim is intended to define a wheel steering angle limit.

[0039] An advantageous embodiment of such an assignment of a value of the limiting signal δlim to a value of the evaluation signal γ shows Fig. 3. As shown there, the values ​​for the evaluation signal γ lie in an interval [-20, 20], whereby in this embodiment, values ​​less than -10 for the evaluation signal γ are always assigned the value 30 and values ​​greater than 10 for the evaluation signal γ are always assigned the value 20. Between the values ​​-10 and 10 for the evaluation signal γ, the limiting signal δlim in this embodiment essentially has a parabolic shape, whereby the magnitude of the slope of the limiting signal δlim in the interval [-10, 0] is greater than in the interval [0, 10]. If the evaluation signal γ has the value 0, then the limiting signal δlim also has the value 0, which means that no wheel steering angle limitation occurs. As already mentioned, negative values ​​(<0) for the generated evaluation signal γ result in the event of oversteering of the vehicle and positive values ​​(>0) in the event of understeering of the vehicle.

[0040] The second evaluation unit 33 determines a scaling signal λ based on the evaluation signal γ, whereby a value for the scaling signal λ is assigned to each value of the evaluation signal γ using a lookup table. The scaling signal λ is intended to adjust a steering ratio. In this embodiment, the scaling signal λ is determined such that for a value of the evaluation signal γ, it has a value λ with 0 ≤ λ ≤ 1. An advantageous embodiment for determining the scaling signal λ as a function of the evaluation signal γ is shown. Fig. 4. The values ​​for the evaluation signal γ are, as in Fig. 3, between -20 and 20, where negative values ​​(<0) for the evaluation signal γ result in oversteering of the vehicle and positive values ​​(>0) in understeering of the vehicle. The shape of the scaling signal λ indicates that in cases of strong oversteering or understeering of the vehicle, the steering ratio is significantly adjusted, whereas at values ​​around 0, in this embodiment for approximately -2.5 ≤ γ ≤ 1.5, the scaling factor λ is set to λ = 1, thus leaving the steering ratio unaffected. The scaling factor λ determined by the second evaluation unit 33 is then transmitted via the communication system 40 to the target yaw rate generation unit 34.

[0041] The target yaw rate generation unit 34 determines a target yaw rate r_drv based on the steering input wheel steering angle δdrv determined from the detected steering input, which is also transmitted to the target yaw rate generation unit 34 via the communication system 40, and the scaling signal λ generated for the motor vehicle 100 by means of the second evaluation unit.

[0042] The target yaw rate r_drv, the yaw rate r detected for vehicle 100, the vehicle speed V detected for vehicle 100, and an actual wheel steering angle δrw set for the vehicle are then transmitted via the communication system 40 to the control unit 35, which determines a target wheel steering angle δctr from these input values ​​r_drv, r, V, δrw, which is required to achieve the target yaw rate r_drv. The control unit 35 can, in particular, be configured as a corresponding control device for this purpose.

[0043] Using the first summing unit 36, the steering input wheel steering angle δdrv is subtracted from the target wheel steering angle δctr determined by the control unit 35, thus forming a differential wheel steering angle δdiff. Using the limiting unit 38, a saturation function is applied to the determined differential wheel steering angle δdiff to form a steering support wheel steering angle δast, limiting the differential wheel steering angle δdiff to a range defined by the limiting signal δlim. Fig. Figure 5 shows an embodiment for an assignment of a steering support wheel steering angle δast to a differential wheel steering angle δdiff, wherein the steering support wheel steering angle δast is limited if the value for the differential wheel steering angle δdiff is less than -δlim or greater than +δlim.

[0044] The steering assistance wheel steering angle δast determined in this way is then added to the steering input wheel steering angle δdrv using the second summing unit 37, thus generating the steering angle to be set δreq, which is transmitted to the steering actuator unit 9 via the communication system 40. The steering actuator unit 9 then sets the steering angle of the steered wheels according to the received steering angle to be set δreq.

[0045] The steer-by-wire steering system 1 according to this embodiment is thus advantageously designed such that a steering input from the driver, here a steering input wheel steering angle δdrv, is directly translated into a set wheel steering angle δreq in a normal driving situation in which no understeer or oversteer occurs, because the evaluation signal γ is 0 or approximately 0 in that case and thus the limiting signal δlim is determined to 0 and the scaling signal λ is determined to 1.

[0046] In understeer or oversteer situations, the limiting signal δlim is not equal to 0 and the scaling signal λ is not equal to 1, so that an intervention occurs with the then determined steering assistance wheel steering angle δast, causing the vehicle 100 to follow only the driver's intended steering input instead of following the direct steering input wheel steering angle δdrv according to the driver's steering input. The evaluation signal γ is used to describe the degree of understeer or oversteer, with the assistance limit ±δlim determined by the limiting signal δlim depending on the evaluation signal γ. The target wheel steering angle δctr requested by the control unit 35 is calculated to achieve the reference yaw rate r_drv, which is preferably generated model-based based on the driver input δdrv. The target yaw rate generation unit 34 converts the driver input δdrv into a yaw rate request r_drv.The driver input is scaled by the scaling signal λ with 0 ≤ λ ≤ 1 as a factor, where the scaling signal λ also depends on the evaluation signal γ, so that in understeer or oversteer situations the driver's steering input is dampened.

[0047] The embodiments shown in the figures and explained in connection with them serve to illustrate the invention and are not limiting to it. Reference symbol list 1 Steer-by-wire steering system 2 Steering shaft 3 Steering handle 5 Feedback actuator unit 51 Feedback actuator control unit 52 feedback actuator electric motor 6 Steering gear 61 Transmission belts 62 Spindle drive 9 Steering actuator unit 91 Steering actuator control unit 92 Steering actuator motor 12 coupling element 13 Tie rod 14 steerable wheels 20 inertial measurement units 21 Speed ​​determination unit 30 computing units 31 Vehicle Stability Assessment Unit 32 first evaluation unit 33 second evaluation unit 34 Target Yaw Rate Generation Unit 35 Control unit 36 first summation unit 37 second summation unit 38 Limiting unit 40 Communication system 100 motor vehicles δdrv Steering input wheel steering angle drw Actual Wheel Steering Angle steering support wheel steering angle (supporting steering signal) δreq wheel steering angle to be set δlim Limiting signal δctr Target wheel steering angle δdiff Differential wheel steering angle V Vehicle speed r yaw rate r_drv Target yaw rate γ rating signal λ Scaling signal

Claims

[1] Method for operating a steer-by-wire steering system (1) in a motor vehicle, wherein a steering input is recorded and a steering input wheel steering angle (δdrv) is determined from the recorded steering input, a set actual wheel steering angle (δrw), a vehicle speed (V) and a yaw rate (r) of the motor vehicle are recorded, Taking into account the set actual wheel steering angle (δrw), the vehicle speed (V) and the yaw rate (r), a driving condition of the motor vehicle is evaluated with regard to the occurrence of understeer or oversteer of the motor vehicle. When understeer or oversteer is detected, a supporting steering signal (δast) is provided to counteract the understeer or oversteer of the motor vehicle. and a steering actuator unit (9) is provided with a steering angle to be set (δreq) taking into account the steering input and the supporting steering signal (δast), which the steering actuator unit (9) converts into a steering angle of the steerable wheels (14) of the motor vehicle, characterized by , that For the determination of the assisting steering signal (δast), an adapted steering ratio and a wheel steering angle limit are determined depending on the degree of understeer or oversteer of the motor vehicle. wherein the evaluation of the driving condition includes generating an evaluation signal (γ), wherein the evaluation signal (γ) represents the degree of understeer or oversteer of the motor vehicle, and a limitation signal (δlim) is generated based on the evaluation signal (γ), whereby the limitation signal (δlim) influences the wheel steering angle limitation. [2] Method according to claim 1, characterized by , that the generated evaluation signal (γ) is ±0 in a normal driving condition, < 0 in oversteer and > 0 in understeer. [3] Method according to claim 1 or claim 2, characterized by , that each value of the evaluation signal (γ) is assigned a value for the limitation signal (δlim), in particular by means of a lookup table. [4] Method according to any of the preceding claims, characterized by , that the limiting signal (δlim) is formed as a function of the evaluation signal (γ), wherein the limiting signal (δlim) in particular corresponds at least piecewise to a parabola. [5] Method according to any of the foregoing claims, characterized by , that based on the evaluation signal (γ) a scaling signal (λ) is generated, whereby the scaling signal (λ) influences the adjustment of the steering ratio. [6] Method according to claim 5, characterized by, that each value of the evaluation signal (γ) is assigned a value for the scaling signal (λ), in particular by means of a lookup table. [7] Method according to claim 6, characterized by , that the scaling signal (λ) is assigned a value λ with 0 ≤ λ ≤ 1. [8] Method according to any one of claims 5 to 7, characterized by , that a target yaw rate (r_drv) is determined based on the steering input wheel steering angle (δdrv) and the scaling signal (λ). [9] Method according to claim 8, characterized by , that taking into account the recorded set actual wheel steering angle (δrw), the recorded vehicle speed (V), the recorded yaw rate (r) and the target yaw rate (r_drv), a target wheel steering angle (δctr) is determined which is required to realize the target yaw rate (r_drv). [10] Method according to claim 9, characterized by, that the steering input wheel steering angle (δdrv) is subtracted from the determined target wheel steering angle (δctr) to determine a differential wheel steering angle (δdiff), wherein a saturation function is applied to the determined differential wheel steering angle (δdiff) to form a steering support wheel steering angle (δast), which limits the differential wheel steering angle (δdiff) to a range defined by the limiting signal (δlim>), where the steering support wheel steering angle (δast) is the supporting steering signal. [11] Method according to claim 10, characterized by , that the steering input wheel steering angle (δdrv) and the steering support wheel steering angle (δast) are added to the wheel steering angle to be set (δreq). [12] Steer-by-wire steering system (1) comprising an inertial measuring unit (20) configured to determine a yaw rate (r) during the operation of a motor vehicle (100), a speed determination unit (21) configured to determine a vehicle speed (V) during the operation of a motor vehicle (100), a steering actuator unit (9) configured to set a wheel steering angle of steerable wheels (14) and to determine a set wheel steering angle (δrw), a feedback actuator unit (5) configured to detect a steering input and to determine a steering input wheel steering angle (δdrv) from a detected steering input, and a computing unit (30), characterized by , that the steer-by-wire steering system (1) is designed to be operated in a motor vehicle (100) according to a method according to one of claims 1 to 11. [13] Steer-by-wire steering system (1) according to claim 12, characterized by, that the computing unit comprises (30): a vehicle stability evaluation unit (31) configured to generate the evaluation signal (γ); a first evaluation unit (32) which is configured to generate the limiting signal (δlim); a second evaluation unit (33) which is configured to generate the scaling signal (λ); a target yaw rate generation unit (34); a control unit (35) designed to determine the target wheel steering angle (δctr); a first summing unit (36) configured to subtract the steering input wheel steering angle (δdrv) from the target wheel steering angle (δctr); a limiting unit (38) configured to generate the steering assistance wheel steering angle (δast) as the assisting steering signal; a second summing unit (37) configured to add the power steering wheel steering angle (δast) and the steering input wheel steering angle (δdrv); and a communication system (40) for transmitting the generated signals (γ, r, V, λ, δdrv, δrw, δast, δreq, δlim, δctr, δdiff).

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