Method for operating a steer-by-wire steering system of a vehicle, and steer-by-wire steering system

Abstract

The invention relates to a method (200) for operating a steer-by-wire steering system of a vehicle (1) for steering wheels (2, 3) of at least one rear axle (31) by means of at least one actuator (5). The method (200) is characterised by at least the following steps (205, 210, 215, 220, 225): • detecting wheel speeds (RDZ_mom 2, 3) at wheels (2, 3) of the front axle (21) and / or the rear axle (31); • detecting a steering wheel angle (LRW_mom); • determining a vehicle movement (veh_mv_mom) taking into account at least the wheel speeds (RDZ_mom 2, 3) and the steering wheel angle (LRW_mom) representing a first vehicle state (veh_comp1) of a compensated straight-line drive of the vehicle (1); • summing the respective detected steering wheel angles (LRW_mom) over time (t) depending on the first vehicle state (veh_comp1) in order to form an offset integral (LRW_int), • comparing the offset integral (LRW_int) with at least one first threshold value (LRW_int_krit1), wherein, when the first threshold value (LRW_int_krit1) is reached, a status (act_err) of the at least one actuator (5) of the rear axle is set, which status represents a fault state (act_cond_err) of the actuator.

Description

[0001] Method for operating a steer-by-wire steering system of a vehicle and steer-by-wire steering system

[0002] The present invention relates to a method for operating a steer-by-wire steering system of a vehicle according to the preamble of claim 1, to a device for carrying out such a method, to a steer-by-wire steering system for a vehicle which is operated according to such a method by means of such a device, to a computer program with program code means which is set up to carry out such a method and to a machine-readable storage medium on which such a computer program is stored.

[0003] In vehicles with four-wheel steering, mechanically decoupled steer-by-wire steering systems are used, which can adjust a wheel steering angle on a vehicle axle, from a steering handle that maps the driver's command for a direction of travel or a route. Actuators are used to actually adjust the wheel steering angle, for example on a rear axle, e.g. as known from DE 10 2014206 934 A1. These comprise an electric motor which drives a rotation-Z translation gear in order to translationally displace a steering rod in the form of a spindle, which is articulatedly coupled to the wheel carrier in order to rotate this and thus the wheels rotatably mounted on it about their vertical axis. The wheel carriers or wheels that are at least indirectly coupled to the steering rod can thus be steered by displacing the steering rod. A wheel steering angle on an axle of the vehicle can thus be adjusted.Depending on the installation space and design of the respective axle, one actuator can adjust the steering angle for both wheels of an axle simultaneously. It is also possible for a single actuator to adjust the steering angle for each wheel.

[0004] In order to precisely adjust the wheel steering angle, the position of the spindle or steering rod must always be known. This position can be determined using a rotor position sensor, which counts the angle of rotation or the revolutions of the electric motor's rotor, as well as using the known ratio of the rotation-to-translation gear. The rotor position sensor is designed in a known manner as a so-called multi-turn sensor, as known from DE 10 2018 213 112 A1 or DE 10 2020 114 871 A1. To check the signals from the rotor position sensor and also to initialize the steer-by-wire steering, an additional linear position sensor is used, which enables the determination of the absolute position of the steering rod or at least a specific position, usually the zero position. If the zero position is known, the rotor position sensor can be initialized to this value, if necessary after a loss of position.As an alternative to the linear position sensor, environmental sensors can be used, as is evident from the generic document DE 10 2017 209 231 A1, which discloses a method for operating a steer-by-wire steering system of a vehicle for steering the wheels of at least one rear axle. The unambiguous function of an actuator of a steer-by-wire steering system for precisely adjusting the wheel steering angles on the wheels of a vehicle is important for driving an intended path, also called a trajectory, as expected and safely.

[0005] DE 10 2014 117 926 A1 further teaches the calculation of an angle offset of a rear wheel steering while the vehicle is driving straight ahead.

[0006] Against this background, the present invention provides a method for operating a steer-by-wire steering system of a vehicle, a device for carrying out the method, a steer-by-wire steering system for a vehicle, a computer program with program code means, and a machine-readable medium, which are set out in the independent claims. Advantageous embodiments emerge from the subclaims and the following description.

[0007] The invention relates to a method for operating a steer-by-wire steering system of a vehicle for steering wheels of at least one rear axle. The steering movement is carried out by means of at least one actuator. The actuator can have a spindle drive. The spindle is translationally displaced by means of a rotary drive and a stationary spindle nut. The rotary drive is preferably designed as an electric motor and drives the spindle nut either directly, e.g. by means of a hollow rotor electric motor, or in a preferred variant indirectly by means of a gear, preferably a toothed belt gear. In the latter embodiment, the electric motor is arranged parallel to the longitudinal axis, preferably the common longitudinal axis of the spindle and the spindle nut, and is connected to the housing of the actuator, preferably screwed. If the spindle nut is turned or unscrewed in one direction or the other.set in rotation, the spindle, which with its external thread is permanently in engagement with the internal thread of the spindle nut, is translationally displaced in one direction or the other along its longitudinal axis. The moving thread consisting of the external and internal threads can be designed as a self-locking thread, preferably as a trapezoidal thread or as a thread with less self-locking or no self-locking. For this purpose, the pitch of the thread can be varied accordingly, or recirculating ball screws or roller screws or similar with little or no self-locking can be used. In its function as a steering rod, the spindle is at least indirectly coupled to the wheels, more precisely to wheel carriers that can pivot about their vertical axis. The wheels are rotatably mounted on the wheel carriers that can pivot about their vertical axis. The translational displacement of the spindle or steering rod allows a wheel steering angle to be adjusted on the relevant vehicle axle.

[0008] Due to a variety of influences, an actuator on the rear axle may set a wheel steering angle that does not lead to the expected trajectory. The basic consideration here is that the driver intends the vehicle to travel straight ahead because the path corresponds to a straight section of road. When traveling straight ahead, the vehicle generally moves along the vehicle's longitudinal axis and the longitudinal axis essentially follows the trajectory. Under ideal conditions on a level, non-inclined road surface without any other influences such as crosswinds, the wheel steering angles on both the front and rear axles correspond to an angle of 0°. In this case, the steering wheel angle, i.e. the angle of the steering handle, e.g. a steering wheel used to enter the desired path, is also 0°. When traveling straight ahead, all wheels on both the front and rear axles ideally have the same wheel speed or torque.Angular velocity. If there is a positioning error in at least one actuator for adjusting the wheel steering angle on the rear axle due to an actual position error of the steering rod or spindle or a fault in the gearbox, the wheel steering angle set by the actuator does not match the expected or required wheel steering angle. The driver, with the intention of driving straight ahead, will attempt to countersteer using the steering wheel on the front axle in order to be able to continue the intended straight-ahead journey. For example, when driving straight ahead, the wheel steering angle on the rear axle may actually be set to 1° to the right using the actuator instead of 0°. The driver would notice that the vehicle is not driving straight ahead, but in this example is yawing to the left. The driver would attempt to compensate for this by turning the steering wheel to the right. Depending on the steering ratio, the driver would, for example,Turn 10° to the right to set a steering angle of 1° on the front axle. The vehicle would now move straight ahead again, either pushing or in what is known as “crab steering”. The vehicle’s longitudinal axis would form an acute angle with the direction of travel, but would follow the “straight ahead” trajectory. After compensation on the front axle, all wheels on the front and rear axles would again have the same wheel speeds. The vehicle would move straight ahead again in relation to the intended path. However, a certain amount of counter-steering may be necessary to stay in lane due to driving on an inclined road surface and / or in crosswinds. The method according to the invention is intended to serve to reliably detect a fault in the rear axle steering actuator.The goal is to have a robust function that reliably detects an actuator fault only when it actually exists. Influences such as those mentioned above are to be masked out, so that only actual misalignments in the rear-axle steering actuator are detected.

[0009] According to the invention, the current rotational speeds of the wheels (wheel speeds) on the front and rear axles are recorded in a first step. Wheel speeds can be easily recorded, for example, using the ABS speed sensors located on the wheel carriers of a vehicle. The rotational speeds can be used to determine the vehicle's speed. Secondly, the rotational speeds can be compared with one another. This can, for example, involve comparing the left and right wheels of an axle, the wheels by axle or by side (left and right wheels of a vehicle), or even comparing all wheels with one another. As stated above, straight-line travel occurs when the wheels have the same rotational speeds. Deviations can be tolerated here, within the scope of measurement accuracy or due to a road surface that is not exactly level.The detected state can mean straight-ahead travel along the vehicle's longitudinal axis, with the wheels exhibiting the same steering angle of approximately 0°, preferably exactly 0°. Alternatively, a vehicle can offer a special driving mode to move the vehicle diagonally or sideways, for example, to exit a parking space or deliberately in a diagonal driving mode, also known as a crabwalk. Even with this special form of straight-ahead travel, the wheels should have the same speed within a tolerance, and the front and rear axles should also have the same steering angle.

[0010] In a further step, the current steering wheel angle is recorded. Based on a specified gear ratio, a wheel steering angle is determined for each axle. For example, turning the steering wheel by approximately 10° can result in a change in the wheel steering angle of approximately 1°.

[0011] Taking into account the currently recorded data, i.e. the wheel speeds of the wheels on at least one axle and the steering wheel angle, the current vehicle movement can be determined in a next step. If the wheel speeds are the same and the LRW corresponds to 0°, the vehicle should be traveling straight ahead along the longitudinal axis. It can therefore be assumed that the actuator for adjusting the wheel steering angle on the rear axle is working correctly and that the zero position of the spindle or steering rod in the actuator is consistent with the actual 0° position of the wheel steering angle. It can also be assumed that the position determination of the actuator or its steering rod using the rotor position sensor is functioning correctly. In the aforementioned case, the steering rod is in the "zero" position and the actuator has set a wheel steering angle of 0°.However, if, at the same wheel speeds, a steering wheel angle other than 0° is present due to counter-steering in order to be able to continue driving straight ahead, a momentary vehicle movement is determined, which is recognized as a first vehicle state of a compensated straight-ahead driving.

[0012] As already mentioned above, if the steering wheel angle is different from 0°, it is necessary to verify whether there is actually a fault in the rear axle actuator or whether, for example, crosswinds or other influences require compensation using the steering wheel in order to be able to follow the trajectory straight ahead. In a next step, the currently recorded steering wheel angle is therefore summed over time, preferably at intervals, preferably intervals of 5 to 50 ms, most preferably 10 to 20 ms. This happens depending on the detected first vehicle state of the compensated straight-ahead driving, i.e. as long as this state exists. This creates an offset integral which fills up with the steering wheel angles recorded at intervals during the compensated straight-ahead driving. In a further step, the offset integral is compared with a first threshold value, preferably at intervals.Only when this first threshold is reached is a status of the actuator set, which represents a fault condition. This advantageously makes it possible to detect an incorrectly set wheel steering angle by the actuator if the wheel steering angle set on the rear axle does not correspond to the expected value but is repeatedly incorrect over a certain period of time.

[0013] The first threshold is an empirically determined value, which is determined based on the vehicle type, the steering ratio on the front axle, and the rear axle, and the actuator used on the rear axle. If, for a given steering ratio, countersteering of 10° on the front axle is necessary, and this corresponds to a wheel steering angle of 1° on the front axle, this can compensate for a wheel steering angle of 1° on the rear axle. This countersteering on the front axle causes the vehicle to move straight ahead with a wheel steering angle of 1° at the front and rear, each shifting and compensating for the movement (direction of travel straight ahead).

[0014] The steering wheel angle is summed at intervals as long as compensated straight-ahead driving is detected. When the first threshold is reached, the incorrect position is unequivocally identified, and an actuator status is set accordingly. This status represents a fault condition of the actuator—in this case, on the rear axle. A controller can output this fault condition as a warning and / or change a steering function of the affected actuator. If the threshold is 300, for example, then, at least theoretically, the condition of straight-ahead driving with a 10° steering wheel angle must be present for 30 seconds to compensate, so that the error can also be detected by counter-steering during this time. However, real or realistic driving path or roadway situations have shown that there is a constant change in the road surface and driving path or trajectory.In fact, the condition of actual straight-ahead travel only exists for a short time. Therefore, the offset integral realistically only fills for a short time when the aforementioned conditions are present. It has been shown that short-term disturbances such as crosswinds, etc., can be masked out in this way. Surprisingly, it has been found that by checking at least the aforementioned conditions and calculating the offset integral, it is unlikely that a fault in the actuator or steering on the rear axle will be falsely detected.

[0015] The method presented here is intended to advantageously detect an incorrect wheel steering angle on a vehicle axle or a misalignment of at least one actuator on at least one rear axle of a vehicle in a cost-effective manner. This eliminates the need for an additional, complex and expensive linear position sensor for the spindle or steering rod of the vehicle steering system or steer-by-wire steering system.

[0016] The specified order of the aforementioned and subsequent procedural steps may vary where reasonably possible. Steps such as recording the wheel speeds and steering wheel angle can also be performed at very short intervals or simultaneously. Furthermore, the specified steps are executed continuously and thus repeatedly, with the data or signals being recorded and processed temporarily and / or at intervals.

[0017] Preferably, depending on the detected actuator state and also on the time until this first threshold value is reached, a status signal can be displayed to the vehicle driver, e.g. in the cockpit as an acoustic and / or visual signal in the form of a warning. This can be useful, for example, if the actuator is reinitialized due to the loss of the zero position of the actuator and the vehicle is therefore only to be moved at a low speed, e.g. in the range of 0 to 50 km / h. For example, a display for showing status messages can be arranged on a dashboard (cockpit) in the vehicle. In the event that there is a discrepancy between the expected and the set wheel steering angle, a corresponding warning can be displayed using the status of the error condition.Advantageously, a driver of the vehicle can be warned and, for example, take countermeasures, such as reducing speed, to increase safety for the vehicle and its occupants. The status message can take various forms, for example, depending on the current speed of the vehicle. Information can also be displayed when the vehicle is stationary, for example, "Rear axle steering is being initialized." At a speed of, for example, 20 to 50 km / h, this information can be linked to the note "Please drive slowly." At a speed of, for example, over 50 km / h, the warning message can, for example, have a visually striking background to clarify the hazard warning. Alternatively or additionally, a steering function of the actuator can be changed. For example, the actuator canbe temporarily switched off or the actuating speed limited.

[0018] The vehicle can be, for example, a passenger car, which can have, for example, an electric or hybrid drive. In addition, the vehicle can comprise a device in the form of a control unit, which can be designed to detect or read in various signals, such as sensor signals, and to control various vehicle functions directly or indirectly using the signals. Accordingly, in the method presented here, a sensor signal representing the expected wheel steering angle can be read in within the actuator. The sensor signal can, for example, be provided by a sensor unit, which can comprise at least one so-called multi-turn sensor. This can detect the rotations of a rotor of the drive motor of the actuator of the rear axle steering, e.g.count, whereby based on the rotations it is possible to determine the position of the steering rod and thus the steering angle of the wheels on the rear axle of the vehicle. Multi-turn sensors offer the advantage, particularly over linear position sensors, that they can detect rotation of the motor when switched off, even without power or with extremely low power consumption, and can count and store the number of revolutions. This means that the steering system can be initialized once, for example during production, while adjusting the track or when the vehicle is first put into operation (for example by approaching the end stops). Each time the rear axle steering is started, the absolute position of the steering rod, e.g. in the form of a spindle, can be calculated from the number of revolutions of the multi-turn sensor on the motor.

[0019] The method mentioned can, for example, also be used if there is doubt about the value of the multi-turn sensor when the vehicle is started. One possible reason for the spindle to wander away from a previously saved position when switched off is exceptional cases, for example when the vehicle is subject to dynamic influences, e.g. on a rough stretch of road, e.g. when it is being towed, or when the rear axle steering actuator issues an error while driving. In this case, the rear axle steering actuator could be deactivated initially and only enabled again once the absolute position of the spindle has been confirmed or determined by the method. An absolute position is understood to be an actual position, so that the actuator actually sets the expected wheel steering angle, e.g. when driving straight ahead it actually sets 0°.

[0020] The expected wheel steering angle can be determined using a detected rotation of the rotor of the rear axle steering motor in conjunction with a known gear ratio of a transmission into an expected wheel rotation angle of the rear axle.

[0021] The determination of vehicle motion can be further improved by considering additional vehicle parameters. Preferably, the step of determining vehicle motion can consider whether the wheels are being accelerated. A tolerance range is also preferably allowed here. A reasonable threshold value can be in the range of 4.5 to 6 m / s. 2 with a threshold value of 5.5 m / s being most preferred 2is taken into account. Below this threshold, negligible acceleration is assumed. Furthermore, it is advantageous if the deviation in the wheel speeds of the wheels on at least one axle, preferably the front axle, is recorded. It has been shown that a threshold value in the range of 1 to 3%, preferably 2%, deviation can be tolerated in order to be able to assume with sufficient certainty that the wheels have the same speed when driving straight ahead.

[0022] Furthermore, it is advantageous to also consider a threshold value for detecting the steering wheel angle. An actual, intended turning of the steering wheel for compensation can be assumed with sufficient certainty if the steering wheel angle exceeds the threshold value of 3° to 6°, preferably 5°, starting from the zero position of the steering wheel. Thus, it can be assumed with increasing accuracy that the vehicle movement corresponds to a compensated straight-ahead movement.

[0023] To determine the initial vehicle state of compensated straight-ahead travel even more reliably, additional parameters can be recorded and considered alternatively or additionally in the step of determining vehicle movement. These are listed below in a non-exhaustive list.

[0024] • Angular velocity of the steering wheel, in particular above a threshold value, preferably less than or equal to 507s and / or

[0025] • Lateral acceleration of the vehicle, especially above a threshold value, preferably less than or equal to 2 m / s 2 and / or

[0026] • Longitudinal acceleration of the vehicle, especially above a threshold value, preferably less than or equal to 2 m / s 2 and / or

[0027] • Yaw rate of the vehicle, especially below a threshold value, preferably less than or equal to 2 m / s 2 and / or

[0028] • Speed ​​of the vehicle, in particular above a threshold, preferably greater than or equal to 10 km / h.

[0029] Furthermore, the status of an ABS (anti-lock braking system: consideration of “brake of individual wheels not applied” or ABS not engaged) and / or the status of an ESP (Electronic Stability Program: consideration of “vehicle not subject to yaw movement” and / or braking of individual wheels or ESP engaged) can be advantageously taken into account.

[0030] Determining vehicle movement should reliably enable the precise determination of the first vehicle state of compensated straight-ahead driving. It has been shown that the more of the aforementioned parameters or states are recorded, the more accurate the determination becomes. Following an AND operation of the available states and parameters, an evaluation is performed in a corresponding unit of a control unit. If all states are met (all threshold values ​​are met and the queried states are met), compensated straight-ahead driving and thus the first vehicle state is reliably detected. The values ​​are then summed up in a corresponding unit of the control unit to form the offset integral. This ensures that an offset integral is only generated in the case of compensated straight-ahead driving.

[0031] The method is intended to detect an incorrect position in at least one actuator of the rear axle steering. The method is intended to be robust against influences that could complicate evaluation. The method is intended to prevent an incorrect position from being erroneously detected or indicated due to lateral influences as described above, e.g. crosswind or similar. In a further preferred embodiment, additional measures can be taken to further rule out misinterpretation depending on an additional vehicle state that is different from the first vehicle state (compensated straight-ahead driving). For this purpose, in addition to the summation and comparison with the first threshold value during compensated straight-ahead driving, a decay value is advantageously subtracted from the current offset integral.This subtraction is preferably carried out at intervals, in particular cyclically, preferably at intervals of 5 to 40 ms, most preferably 10 to 20 ms. In other words, during compensated straight-ahead travel, the values ​​are added up and during the sections driven which differ from the recorded compensated straight-ahead travel, e.g. cornering or similar, a decay value is subtracted step by step. The decay value is determined empirically depending on the vehicle fleet or vehicle. For example, for conventional cars, a decay value of 3 to 6, preferably 4, was determined, which is then cyclically subtracted from the current offset integral. It has also been shown that it is advantageous if the decay value is reduced the closer the offset integral approaches the first threshold value. A model comprising three threshold values ​​is preferred, whereby when aof the offset integral, the decay value is further reduced, preferably halved, when a second threshold is reached, and reduced again, preferably halved again, when a third threshold is reached. If the first threshold is exceeded, the decay value is reduced to a value close to zero, e.g., 0.05. This can further improve the detection of only actual errors without reducing the sensitivity of the method.

[0032] In addition to the accuracy and robustness of the method, measures are possible to reduce the time required to determine the compensated straight-ahead travel, especially in the case of high actuator deviations, which require a large steering wheel angle or large offset values. For this purpose, when the first vehicle state is present, a detected steering wheel angle in the range of 6° to 15°, preferably greater than 8°, and an undershoot of the angular velocity of the steering wheel in the range of 0.4 to 0.8 s, preferably less than 0.57 s, and preferably taking into account position information of the actuator (act_pos), if the position of the steering rod is in a range of -0.3 <act_pos<0,3 mm, bevorzugt in einem Bereich - 0,1 <act_pos<0,1 mm liegt, ein Produkt aus einem Verstärkungsfaktor und dem Lenkradwinkel in das Offset-Integral aufsummiert. Das Offset-Integral füllt sich dann schneller und eine große Abweichung bzw. Fehlstellung des Aktuators wird schneller erkannt.

[0033] In a preferred embodiment, after a fault condition has been detected, it can be made possible to return the actuator's deviation so that the actuator can once again reliably set the expected wheel steering angle. In other words, this embodiment can be used to reset the actuator so that the actuator can once again set the intended or expected steering wheel angle. This process is also referred to as (re-)initialization. For this purpose, information from so-called vehicle level sensors, for example, can be used to detect a misalignment (driving on an inclined surface, slanted body position, e.g. different spring deflection on the left and right sides) and the resulting influences on the process.For this purpose, for example, the deviation determined from the recorded steering wheel angle can be fed back into a control loop in order to slowly, i.e. gradually, compensate for the offset error of the rear axle steering actuator. This offers the advantage that any incorrect wheel position can be corrected directly, thereby increasing vehicle safety. Optionally, an observer / model can also be used which can detect the deviation in addition to or alternatively to the method presented. The model would calculate the required wheel steering angle at the rear wheel axle based, among other things, on the individual wheel speeds and the wheel steering angle at the front axle. If this deviates from the value set by the rear axle steering actuator, a safety status can be set and, if necessary, a warning can be issued to the driver. The unexpected wheel steering angle at the rear axle can then be reduced to a reasonable value.The spindle or steering rod can be returned in sensible, small increments of, for example, 0.1° per return interval. In other words, the rear-axle steering actuator is reset so that its zero position corresponds to the actual wheel steering angle of 0° for normal straight-ahead travel along the vehicle's longitudinal axis. The actuator can be reinitialized upon reaching the zero position. This can be particularly important for steer-by-wire steering, especially during system start-ups after the vehicle has been shut down and restarted, as well as in the event of errors, e.g., in the power supply, and during emergency operation.

[0034] The approach presented here further provides a device which is designed to carry out and control or implement the steps of the method presented here in corresponding devices.

[0035] A device can be an electrical device that processes electrical signals, for example sensor signals, and outputs control signals depending on them. The device can have one or more suitable interfaces, which can be embodied in hardware and / or software. In a hardware embodiment, the interfaces can, for example, be part of an integrated circuit in which functions of the device are implemented. The interfaces can also be separate integrated circuits or consist at least partially of discrete components. In a software embodiment, the interfaces can be software modules that are present, for example, on a microcontroller alongside other software modules. Such a device can be a control unit as part of the at least one actuator or at least part of a central control unit of a vehicle.

[0036] Furthermore, a steer-by-wire steering system for a vehicle, in particular a rear-axle steering system, is presented, wherein the steer-by-wire steering system comprises a previously presented device. With this combination of a steer-by-wire steering system, in particular a rear-axle steering system, and a device for controlling the method described above, all of the aforementioned advantages can advantageously be implemented in a simple manner.

[0037] Also advantageous is a computer program product comprising program code means which can be stored on a machine-readable medium such as a semiconductor memory, a hard disk memory or an optical memory and is used to carry out the method according to one of the embodiments described above when the program is executed on a computer or a device.

[0038] The invention is explained in more detail by way of example with reference to the accompanying drawings. They show:

[0039] Fig. 1 is a schematic representation of a vehicle with a steer-by-wire steering system;

[0040] Fig. 2 shows another view of the vehicle as shown in Fig. 1, driving straight ahead.

[0041] Fig. 3 shows another view of the vehicle like Fig. 1 or 1 with a positioning error on the rear axle

[0042] Fig. 4 shows another view of the vehicle. Fig. 2 shows compensated straight-ahead driving.

[0043] Fig. 5 is a flowchart of a method for operating a steer-by-wire steering system of a vehicle

[0044] In the following description of preferred embodiments of the present invention, the same or similar reference numerals are used for the elements shown in the various figures and having a similar effect, whereby a repeated description of these elements is omitted.

[0045] Figure 1 schematically shows a vehicle 1 with steer-by-wire steering with a steerable front axle 21 and a steerable rear axle 31. For steering the wheels 2 on the front axle 21, an actuator 4 is provided which can set or change a wheel steering angle via a steering linkage 41. A set wheel steering angle θvi is shown as an example on the right wheel 2 in Figure 1. On the rear axle 31, an actuator 5, via a steering linkage 51, ensures the setting or changing of the wheel steering angle θh on the rear wheels 3 (shown as an example on the left rear wheel). The actuator 5, shown in detail as an example in Fig. 1, has a housing 56 in which a spindle 52 is mounted so as to be translationally displaceable, acting as a steering rod. For this purpose, a spindle nut 53 is driven in a rotatable or rotational manner by an electric motor 54 and is rotatably mounted in the housing 56.The spindle nut 53 has an internal thread (not shown) that engages with an external thread (not shown) of the spindle 52, forming a motion thread. The rotational movement of the electric motor 54 is converted by the motion thread into the translational displacement or movement of the spindle 52. The spindle 52 is coupled to the aforementioned steering linkage 51 and, when displaced, causes a change in the wheel steering angle θh. The position of the spindle 52 can be detected by a rotor position sensor 55, knowing the ratio of the drive or transmission from the electric motor 54 via the spindle nut 53 to the spindle 52. In other words, the set wheel steering angle can be calculated using the known transmission ratio and the rotation-to-translation converter by means of the rotor position sensor 55, which is preferably designed as a so-called multi-turn sensor on the rotor of the electric motor 54.The details described for actuator 5 also apply in principle to actuator 4. In the further figures 2-4, the reference symbols for the above-mentioned components of actuator 5 are not shown again for identical components.

[0046] In the embodiment shown in Fig. 1, the wheels on the front axle 21 and the rear axle 31 are steered in opposite directions, so that at low speeds of, for example, less than 50 km / h, a reduced turning circle and improved parking options are advantageously achieved compared to vehicles which only have one steered (front) axle. Two steered axles also advantageously allow for more agile automated driving. The driver generally inputs his steering intention via a steering wheel 14, with the steering wheel angle LRW being detected by a sensor unit and, after the wheel steering angle has been calculated by a control unit SG, sent via a signal line to the actuator 4 and / or actuator 5, which sets the wheel steering angle on the front axle 21 and / or the rear axle 31. The sensor unit for transmitting the respective steering wheel angle LRW / LRW_mom is connected via a bus system 10 to the control unit SG of the steer-by-wire steering system orThe bus system 10 is electrically connected to the actuators 4, 5. The bus system 10 is, for example, a CAN bus or a Flexray bus or similar. Via the bus system, functions for a driver assistance system for autonomous driving (ADAS), correction functions for limiting a maximum steering angle, a parking assistant for automated parking, or even an electronic stability control (ESP) (shown here), etc., are electrically possible via the control unit SG and the steer-by-wire steering system via the associated actuators 4, 5. The control unit SG is provided with a unit that can execute a method using a computer program and control the actuators 4, 5 via the signal path, wherein the computer program is stored on a machine-readable storage medium as part of the control unit SG.

[0047] A yaw rate yaw_v measured at the center of gravity G of the vehicle is determined using a suitable sensor (angular rate sensor) at the center of gravity of the vehicle 1 and recorded and evaluated by the electronic stability control ESP. Sensors S are arranged at the outer ends of the vehicle 1 and are assigned to a sensor system and are used to detect the vehicle's surroundings. These can be temperature sensors, e.g. optical sensors that include a camera, or LiDAR or radar that are suitable for temperature, distance or optical detection, for example of the road surface. Weather conditions such as humidity or the temperature on the road surface, etc. can thus be recorded and fed to the control unit SG. The vehicle according to Fig. 1 follows a trajectory T, which is also shown schematically in Figure 1 at the front end of the vehicle in its direction of travel.Depending on the wheel positions of wheels 2 and 3 on the front axle 21 and rear axle 31, the trajectory T corresponds to a left turn. For this purpose, the front wheels 2 are turned to the left and the rear wheels 3 are turned to the right.

[0048] If the vehicle is now actually intended to drive straight ahead (the vehicle is traveling along its longitudinal axis and following a desired "straight ahead" trajectory), the driver sets a steering wheel angle of 0° so that wheels 2 on the front axle 21 and also wheels 3 on the rear axle 31 are set to a wheel steering angle of 0° (6V2 = 0; 6h2 = 0), as shown in Fig. 2. If the actuators 4, 5 are functioning properly, the known zero position of the spindle 52 or steering rod of both actuators 4, 5 is set based on a control signal from the control unit SG. This corresponds to a wheel steering angle of 0° in each case. The vehicle 1 can thus follow a trajectory T for straight-ahead travel along the longitudinal axis a as intended and shown in Fig. 2.

[0049] Figure 3 shows an example in which straight-ahead travel is intended, which is why a wheel steering angle of 0° is set on the front axle 21 (θV3 = 0), in accordance with the driver's input on the steering wheel (steering wheel angle = 0°). However, a wheel steering angle θh3 is set on the rear axle 31 (for clarity, much greater) than 0°. The wheels 3 on the rear axle 31 are turned to the right. This can occur if, due to a fault in the actuator 5 of the rear axle 31, the stored zero position does not match the actual zero position (=wheels in the straight-ahead position). Although straight-ahead travel is intended, vehicle 1 cannot travel straight due to the set wheel steering angle θV3 = 0°. Instead, it follows the specified trajectory T in the sense of a left turn.

[0050] In order to still be able to drive straight ahead, the driver will or must countersteer and use the steering wheel 14 on the front axle 21 to set a wheel steering angle 6v4 that corresponds to the wheel steering angle 6h4 on the rear axle 31. This is shown in Figure 4. In the situation shown, the driver has turned the steering wheel 14 to the right, as indicated by the arrow, in order to adjust the wheels in the same direction as the wheels on the rear axle. This results in compensated straight-ahead travel, in which the incorrect position of the actuator 5 for steering the rear axle 31 is compensated for. The longitudinal axis a forms an acute angle TT with the actual direction of travel according to the trajectory T shown. The vehicle 1 moves straight ahead with the set compensation. However, the vehicle is now moving in a so-called crab walk (pushing movement).It is obvious that when driving straight ahead, wheels 2, 3 rotate at at least approximately the same speed. The speeds can be detected, for example, using the ABS speed sensors located on the wheel carriers. The steering wheel angle LRW can be detected using a sensor in the steering column. The method according to the invention is described below using an exemplary embodiment.

[0051] Figure 5 shows a flowchart of a method 200 for operating a steer-by-wire steering system of a vehicle (1) according to an exemplary embodiment. The method 200 presented here can be carried out with a steer-by-wire steering system designed as a rear-axle steering system by means of an actuator 5 in a vehicle 1. In a first step, in addition to the current wheel speeds RDZ_mom of the wheels 2, 3 on the front axle 21 and on the rear axle 31, as well as the current steering wheel angle LRW_mom and the steering wheel angular velocity LRW_v, further parameters are recorded using suitable sensors. The longitudinal acceleration ax and the lateral acceleration ay of the vehicle are recorded, and the yaw rate of the vehicle yaw_v is recorded using the ESP. Furthermore, the anti-lock braking system ABS determines whether individual wheels are being braked and determines the speed veh_v of the vehicle, which is preferably calculated from the previously recorded wheel speeds.Also recorded is the calculated position of the steering rod (spindle 52), which is determined by taking into account the rotor position sensor 55 of the electric motor 54 of the actuator 5. From this, an expected wheel steering angle on the rear axle can be calculated, which is to be adjusted using the actuator 5 on the rear axle. The determined signals or data are transmitted from the corresponding sensors via the vehicle's bus system (e.g., CAN / Flexray or similar bus systems) to a control unit SG, or the control unit SG retrieves the determined signals or data via the bus system used. The control unit SG is schematically represented with a dashed border. Within the control unit SG, it is first determined whether the parameters lie within the limits or tolerances of the threshold values ​​or tolerances stored in the control unit SG (threshold values ​​and tolerances have already been mentioned above and are not listed again here).This is represented by the step "True?" and corresponds to an AND operation. This is necessary or is used here to determine the vehicle movement taking into account the aforementioned parameters. For example, if a steering wheel angle LRW_mom of 5° is present and the other parameters are within the aforementioned limits, the determination of the vehicle movement results in a first vehicle state veh_comp1 of compensated straight-ahead travel of the vehicle. While the first vehicle state veh_comp1 is present, the currently existing steering wheel angle LRW_mom is summed and an offset integral LRW_int is formed, which is filled at intervals with the currently existing steering wheel angle LRW_mom. After each addition, a check is made to determine whether the offset integral LRW_int has reached a first threshold value LRW_int_krit1.If this is the case, the control unit SG sets a status act_err with respect to the observed actuator 5. Reaching the first threshold value LRW_int_krit1 verifies that the first vehicle state veh_comp1 of compensated straight-ahead travel is actually present. The status act_err thus clearly represents an error state act_cond_err of the actuator 5 for steering the wheels 2, 3 on the rear axle 31. Depending on the time required to reach the first threshold value LRW_int_krit1, the control unit SG displays the status act_err to the driver in a display in the vehicle cockpit. Alternatively or additionally, the steering function of the actuator can be changed. Alternatively or additionally, the power (e.g., restriction of the maximum possible angle) of the actuator 5 can be reduced or it can be switched off at least temporarily.Depending on the status act_err, a correction function Corr can also be triggered for actuator 5. If, for example, a slight deviation in the wheel steering angle set by actuator 5 has been detected, the correction function Corr can be used to gradually reset or return actuator 5 at intervals. After each reset, the vehicle movement and thus the wheel steering angle set by actuator 5 and expected in this context are re-examined using the aforementioned steps. A detected error in actuator 5 can thus be automatically corrected within the framework of the method.

[0052] If, for example, the steering wheel angle LRW_mom is 0° and the other parameters result in straight-ahead travel, uncompensated, i.e. actual straight-ahead travel is assumed. The control unit detects proper, i.e. normal, vehicle movement veh_norm. For this vehicle movement and also for detected cornering, a decay value is subtracted as part of the method. This subtraction is preferably carried out at intervals, in particular cyclically, preferably at intervals of 10 ms. In other words, during compensated straight-ahead travel, the values ​​are summed up, and during the sections driven which differ from the detected compensated straight-ahead travel, e.g. cornering or similar, the decay value is subtracted. A decay value of 4 determined empirically for a conventional car is thus subtracted from the current offset integral in each case.Subtracting the decay value reduces the offset integral during the process. As mentioned above, this is intended to prevent the false detection of an actuator misalignment. This step-by-step subtraction effectively prevents this.

[0053] Reference symbol

[0054] 1 vehicle

[0055] 2 front wheel

[0056] 3 rear wheel

[0057] 4 Front axle steering actuator

[0058] 5 Rear axle steering actuator

[0059] 10 Bus system

[0060] 14 Steering wheel

[0061] 21 front axle

[0062] 31 Rear axle

[0063] 41 Steering linkage

[0064] 51 Steering linkage

[0065] 52 spindle, steering rod

[0066] 53 spindle nut

[0067] 54 electric motor

[0068] 55 Rotor position sensor

[0069] 56 housings

[0070] 60 ad

[0071] 200 Procedure for operating a steer-by-wire steering system a Longitudinal axis ax Longitudinal acceleration ay Lateral acceleration act_err Actuator status act_cond_err Actuator error status act_pos Position information (of the actuator) ABS Anti-lock braking system

[0072] Corr correction function

[0073] ESP Electronic Stability Program

[0074] G Focus

[0075] LRW steering wheel angle

[0076] LRW_Corr_sub decay value LRW_mom current steering wheel angle LRWJnt offset integral

[0077] LRW_int_krit_1 first threshold offset

[0078] LRW_int_krit_2 second threshold offset

[0079] LRW int crit 1 second threshold offset

[0080] LRW_v steering wheel angular velocity

[0081] RDZ wheel speed

[0082] RDZ_mom current wheel speed

[0083] SG control unit

[0084] S Sensors t Duration

[0085] T trajectory

[0086] TT angle (for compensated straight-ahead driving) veh_v vehicle speed veh_mv_mom current vehicle movement veh_comp1 first vehicle state (compensated straight-ahead driving) veh_norm further vehicle state yaw_v yaw rate

[0087] 6vi wheel steering angle front axle

[0088] 6hi Wheel steering angle rear axle

[0089] 6V2 Wheel steering angle front axle

[0090] 6h2 Wheel steering angle rear axle

[0091] 6V3 Wheel steering angle front axle

[0092] 6h3 Wheel steering angle rear axle

[0093] 6V4 Wheel steering angle front axle

[0094] 6h4 wheel steering angle rear axle

Claims

Patent claims 1 . Method (200) for operating a steer-by-wire steering system of a vehicle (1) for steering wheels (2, 3) of at least one rear axle (31) by means of at least one actuator (5), wherein the method (200) is characterized by at least the following steps • Detecting wheel speeds (RDZ_mom) on wheels (2, 3) of the front axle (21) and / or the rear axle (31), • Detecting a steering wheel angle (LRW_mom), • Determining a vehicle movement taking into account at least the wheel speeds (RDZ_mom) and the steering wheel angle (LRW_mom) representing a first vehicle state (veh_comp1) of a compensated straight-ahead travel of the vehicle (1), • Summing up the respectively recorded steering wheel angle (LRW_mom) after the time (t) depending on the first vehicle state (veh_comp1 ) to form an offset integral (LRWJnt), • Comparing the offset integral (LRWJnt) with at least one first threshold value (LRW_int_krit1 ), wherein upon reaching the first threshold value (LRW_int_krit1 ) a status (act_err) of the at least one actuator (5) is set, which represents an error state (act_cond_err) of the actuator (5).

2. Method according to claim 1, characterized in that depending on the detected actuator state (act_cond_err) and the time period (t) until the first threshold value (LRW_int_krit1) is reached, at least the status (act_err) of the actuator is output in a display unit of the vehicle (1) and / or a steering function of the steer-by-wire steering is changed at least for the actuator (5) of the rear axle (31).

3. Method according to one of the preceding claims, characterized in that in the step of determining the vehicle movement, the following is taken into account: • whether the wheels (2, 3) are subject to an acceleration below a threshold value, the threshold value being in the range of 4.5 to 6 m / s 2 lies, preferably less than or equal to 5.5 m / s 2 is and / or • whether a deviation of the wheel speeds (RDZ_mom) of the wheels (2, 3) of at least one axle (21, 31) is below a threshold value of 1 to 3%, preferably 2% and / or • whether the steering wheel angle (LRW) is considered to be equal to or greater than a threshold value of 3° to 6°, preferably equal to or greater than 5°.

4. Method according to one of the preceding claims, characterized in that for determining the first vehicle state (veh_comp1) for determining the vehicle movement at least further parameters are recorded and taken into account: • Angular velocity of the steering wheel, in particular above a threshold value, preferably less than or equal to 507s and / or • Lateral acceleration (ay) of the vehicle (1 ), in particular above a threshold value, preferably less than or equal to 2 m / s 2 and / or • Longitudinal acceleration (ax) of the vehicle (1 ), in particular above a threshold value, preferably less than or equal to 2 m / s 2 and / or • Yaw rate (yaw_v) of the vehicle (1 ), in particular below a threshold value, preferably less than or equal to 2 m / s 2 and / or • Speed ​​(veh_v) of the vehicle (1 ), in particular above a threshold value, preferably greater than or equal to 10 km / h and / or • Status of an ABS to determine whether active and / or individual wheels are being braked and / or • Status of an ESP to be active and / or to detect a yaw movement and / or to brake individual wheels 5. Method according to one of the preceding claims, characterized in that, depending on a present further vehicle state (veh_norm), which is different from the first vehicle state (veh_comp1), a subtraction of a decay value (LRW_corr_sub) from the instantaneous offset integral (LRWJnt) is carried out at intervals, in particular cyclically, wherein the decay value (LRW_corr_sub) is reduced when a second threshold value (LRW_int_krit2) with respect to the offset integral is reached and is further reduced when a third threshold value (LRW_int_krit3) is reached.

6. Method according to one of the preceding claims, characterized in that when the first vehicle state (veh_comp1 ) is present, a detected steering wheel angle (LRW_mom) in the range from 6° to 15°, preferably greater than 8°, and an angular speed of the steering wheel falling below the range from 0.4 to 0.8 7s, preferably less than 0.57s and taking into account the position information (act_pos) of the actuator, when the position of the handlebar is preferably in a range (- 0.1 <act_pos<0,1 ) mm liegt, ein produkt aus einem verstärkungsfaktor und dem lenkradwinkel (lrw_mom) in das offset-integral aufsummiert wird.

7. Method according to one of the preceding claims, characterized in that due to the detected error condition (act_cond_err) a step-by-step change of the wheel steering angle on the rear axle is carried out, wherein for this purpose the position of the steering rod of the actuator is changed step by step taking into account the offset integral (LRWJnt) and the detected steering wheel angle (LRW_mom) as well as the position information of the actuator (1).

8. Device (SG) which is configured to carry out the method (200) according to one of the preceding claims and to control the steer-by-wire steering system, at least designed as a rear axle steering system, accordingly.

9. Steer-by-wire steering system for a vehicle (1), at least designed as a rear-axle steering system, which is operated according to the method according to one of claims 1 to 7 by means of a device according to claim 8.

10. Computer program with program code means which is designed to carry out the method (200) according to one of the preceding claims 1 to 7 when the program is executed on a computer, in particular on the control unit (SG) according to claim 8. 11 . A machine-readable storage medium on which the computer program according to claim 10 is stored.

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