METHOD AND CONTROL DEVICE FOR OPERATING AN ACTUATOR OF A STEER-BY-WIRE STEERING SYSTEM OF A MOTOR VEHICLE

Description

[0001] The invention relates to a method for operating and a control unit for controlling an actuator of a steer-by-wire steering system of a motor vehicle. Furthermore, the invention relates to a steer-by-wire steering system operated by such a method, a computer program for executing such a method, and a machine-readable storage medium on which such a computer program is stored.

[0002] From DE 10 2014 206 934 A1, an actuator with a stationary spindle nut and a spindle that can be displaced axially relative to it is known. At least one end of such an actuator, which is used in a steer-by-wire steering system, is connected to a wheel carrier by means of a steering linkage. The linear displacement of the spindle allows for a change in the steering angle of a wheel that is rotatably mounted on the wheel carrier. The steer-by-wire steering system, which can be actuated at least indirectly by a steering handle such as a steering wheel or operates independently of it, is controlled via the signal path, i.e., without mechanical coupling. Such a steering system must overcome the frictional resistance of the wheels against the road surface during a steering maneuver. Especially at very low speeds when maneuvering or parking until the vehicle comes to a standstill, comparatively high steering forces are required, which place a considerable load on the actuator.The actuator's spindle drive can be prone to resonant vibrations due to alternating static and sliding friction, and its spindle drive can reach high temperatures, which can lead to damage to the actuator and thus to the steer-by-wire steering system.

[0003] German patent DE 10 2014 017 127 A1 discloses a steer-by-wire steering system that can specify different steering ratios. At higher speeds, a lower steering ratio and thus a smaller steering angle range are selected. When the vehicle is stationary or at reduced speeds, a direct steering ratio is used, allowing unrestricted wheel steering up to full lock.

[0004] Against this background, it is an object of the invention to provide an improved method and an improved control unit for controlling an actuator of a steer-by-wire steering system of a motor vehicle when the vehicle is moved at very low speed during parking or maneuvering and increased prestresses occur between the tires and the road surface.

[0005] The solution to the above problem is provided by a method with the features of claim 1, by a steer-by-wire steering system with the features of claim 10, by a control unit with the features of claim 11, by a computer program with the features of claim 12, and by a machine-readable storage medium with the features of claim 13. Advantageous further developments are set forth in the dependent claims.

[0006] According to a first aspect of the invention, a method for operating an actuator of a steer-by-wire steering system of a motor vehicle at a very low speed compared to normal driving, from standstill to parking and / or maneuvering, wherein the method comprises at least the following steps: Determining the instantaneous speed of the motor vehicle; determining a limited steering angle as a function of at least the instantaneous speed; detecting a steering angle request; controlling the actuator to adjust a steering angle of at least one wheel, at least as a function of the steering angle request and taking into account the limited steering angle, so that loads within the actuator due to a build-up of preload between tire and road surface and / or between actuator and a steering linkage are reduced.

[0007] An actuator of a steer-by-wire steering system of a motor vehicle preferably has a housing in which a spindle and a rotatably and stationary spindle nut are mounted. The spindle and the spindle nut form a threaded connection and, within the housing, are part of a spindle drive for axially displacing the spindle relative to the spindle nut and thus also to the housing. For this purpose, the spindle has an external thread which engages with the internal thread of the spindle nut. If the spindle nut is driven by rotation, e.g., by an electric motor, preferably indirectly by a gearbox, preferably a belt drive, the threaded connection causes the spindle to be displaced axially along its longitudinal axis relative to the spindle nut and / or the housing.By axially displacing the wheel along its longitudinal axis, the steering angle of a wheel rotatably mounted on a wheel carrier, which is at least indirectly connected to one end of the spindle, can be changed.

[0008] The high actuating forces required for steering cause high friction in the actuator's gearbox, specifically in the threaded section of the actuator's spindle drive. Even with optimized lubricants, high friction occurs between the thread flanks within the threaded section, i.e., between the spindle and the spindle nut. Due to the static and kinetic friction occurring between the threaded components at the contact surfaces of the adjacent thread flanks, a so-called stick-slip effect can occur. This involves the alternating sticking and sliding of the thread flanks, which can lead to fluctuating torques between the spindle nut and the spindle. This can, for example, cause the spindle to vibrate, particularly torsional vibrations.Continuous or temporary excitation over a minimum period can cause the spindle or other components in the actuator to reach a resonant frequency. Furthermore, the vibrations cause thermal stresses that can negatively affect the lubricating properties of the lubricant. This can negatively impact the actuator's service life.

[0009] The aforementioned torsional vibration is also called a torsional oscillation. In contrast to translational vibration, torsional vibration occurs around the rotational degree of freedom of a system, in this case around the longitudinal axis of the spindle. In both cases, it is a mechanical vibration.

[0010] The term stick-slip effect is derived from the two English words "stick" and "slip." In physics and engineering, the stick-slip effect describes a generally undesirable, jerky sliding (standstill-slide-standstill-slide) of solid bodies moving against each other.

[0011] A steer-by-wire steering system is a steering system decoupled from the mechanical steering mechanism with a steering handle, such as a steering wheel. The driver's steering input via the steering handle is not transmitted mechanically, for example via a linkage, to the wheel hubs or wheels. Instead, a steering angle, or its change, is calculated for each wheel on an axle, for example in a control unit. This unit sends control signals to the actuator(s) of the steer-by-wire steering system, ultimately causing the steering angle change or adjustment at the respective wheel. The driver's steering angle request or a calculated steering angle change from the control unit may differ from the maximum adjustable steering angle at the axle in question, for example, it may be greater. In this case, only the maximum steering angle can be set.

[0012] The steering angle request consists of changing the steering angle from the existing steering angle to a desired steering angle, and this request is also dependent on time. In other words, the steering angle is changed within a certain time period, which is referred to as steering speed or steering gradient. For example, a driver can turn the steering wheel slowly, e.g., at 2° / s, or very quickly, e.g., at 20° / s, to make a steering angle change of, for example, 5°. In other words, the steering angle request encompasses both the change in angle and the rate of change of that angle.

[0013] Under normal operating conditions, steering angles are generally only changed by small adjustments when a vehicle is moving at a speed significantly above the aforementioned speed range from standstill to parking and / or maneuvering, for example, when driving in a built-up area at 30 to 50 km / h or on a rural road or motorway at even higher speeds. In these situations, steering angles are typically less than 1°. These small adjustments therefore require considerably less steering force, thus preventing torsional vibrations.

[0014] The approach presented here is based on the understanding that in certain situations, increased torque or force is required for steering, or in other words, for turning the respective wheel to the desired steering angle. The situation considered here assumes a very low vehicle speed, from standstill to parking and / or maneuvering. When completely stationary, the speed is 0 km / h. When parking and / or maneuvering, a speed of approximately 1 km / h or less is assumed. In the speed range of 0 to approximately 1 km / h, particularly high forces are required to achieve the desired steering angle. The lower the speed, the greater the expected steering forces that must be managed by the steer-by-wire actuator.This is because the entire weight of the vehicle rests on the tires mounted on the wheels. The contact between the tire and the road surface results from the tire's contact patch. The size of a tire's contact patch depends primarily on the wheel load and tire pressure, as the tire's internal pressure carries the majority of the wheel load. However, tire width, diameter, and sidewall stiffness also play a role. A greater force is required to steer a stationary wheel (i.e., to rotate it around its vertical axis) than when the wheel is rolling due to the vehicle's motion. As the rolling motion increases, less force is needed for steering. It is evident that, in addition to the vehicle's mass, the ambient temperature and tire temperature also have an influence, as they directly affect the friction between the tire and the road surface.The following additional parameters are listed here (not exhaustively): tire compound, tire type, tire friction coefficient, road surface and road condition (dry, wet, slippery, etc.).

[0015] A wheel's tire is typically made of rubber – an elastic material. When a force is applied to the wheel by the actuator of a steer-by-wire system, a preload is created due to static and sliding friction between the tire and the road surface. The tire is essentially pulled against the road surface and thus preloaded. Further preload is created between the actuator and the wheel hub by bearings and, if applicable, links such as a steering linkage, depending on the suspension design.

[0016] When, in the aforementioned low speed range, the steer-by-wire steering system steers from a large, preferably maximum, steering angle to a smaller one, the preload is initially reduced briefly, and then preload is reapplied. This preload increases as the vehicle's speed decreases or as it decelerates from rolling to a standstill. This constant switching occurs almost continuously during parking and / or maneuvering. When steering back from the previously set large steering angle, the direction of force changes in the steer-by-wire actuator. This leads to a load change within the actuator's gearbox or spindle drive, resulting in a change in stick-slip behavior. This can lead to vibrations and high thermal loads within the actuator or its drive threads.This behavior needs to be reduced or minimized.

[0017] Large steering angles, as used here, refer to steering angles that approach the maximum possible steering angles for the respective axle. Maneuvering and parking often involve frequently changing steering angles. Utilizing large, preferably the largest possible, steering angles makes it easier to enter parking spaces or maneuver with a trailer. It is particularly advantageous if the rear axle of the vehicle is also steerable, in addition to the front axle.

[0018] According to the invention, in the aforementioned method, the maximum possible steering angle is limited to a specific angle depending on the vehicle's current speed. Limiting the steering angle addresses the problem of excessive preload building up when steering in the aforementioned low speed range. If, during the initial steering angle request, a steering angle greater than the current limited angle is requested, only the currently limited steering angle can be set. The steer-by-wire actuator can only set a steering angle during the initial control step based on the requested steering angle and taking the limited steering angle into account. Therefore, any steering angle can be set if it is less than or equal to the previously determined limited steering angle.The actuator is preferably controlled by a control unit such as an ECU or controller. The ECU or controller is preferably part of the steer-by-wire steering system. However, the actuator can also be controlled by another control unit installed in the vehicle.

[0019] Advantageously, the invention allows the vibration behavior of the actuator or the components installed therein to be minimized without modifying an existing actuator of a steer-by-wire steering device by means of a controlled method.

[0020] In a preferred embodiment, the limited steering angle can be changed to a value less than a predefined maximum steering angle during the determination step. This consideration takes into account that a predefined maximum steering angle, which is structurally possible with the steer-by-wire steering system in the respective vehicle, can change due to boundary conditions, such as the aforementioned conditions regarding preload. The structurally maximum possible steering angle may, for example, not be practically achievable because a different tire size, such as wider tires or snow chains, is used on the steerable axle. Furthermore, the structurally maximum possible steering angle may not be adjustable due to a heavy vehicle load, because less space is available for steering movements when the wheels are compressed.This embodiment thus takes every vehicle situation into account, so that there can be no collision of the wheels or tires with the chassis or the vehicle body due to an excessively large steering angle.

[0021] Preferably, the limited steering angle is determined based on a characteristic curve during the determination step. This characteristic curve can specify that the limited steering angle is 50-80%, preferably 70%, of the predefined maximum steering angle. For example, if the maximum possible steering angle for a rear-axle steering system designed as steer-by-wire is 10°, the limited steering angle based on the characteristic curve can be 5-8°, preferably a maximum of 7°. Different characteristic curves can be defined for different vehicle situations. For example, the characteristic curves can be selected by a unit within a control unit and stored there. The characteristic curves are preferably selected depending on the vehicle's current speed, the steering angle requirement, taking into account the limited steering angle, the tire size or type used, the load condition, or other conditions.One or more parameters may be decisive.

[0022] The method is fundamentally based on determining the steering angle as a function of the vehicle's instantaneous speed. Essentially, a distinction is made between whether the vehicle is moving at very low speeds (speed from standstill to parking and / or maneuvering, see above) or at higher speeds. Preferably, in the step of determining the limited steering angle, a first steering angle range from 0° to less than or equal to the limited instantaneous steering angle and a second steering angle range greater than the limited instantaneous steering angle up to the predefined maximum steering angle are defined and preferably stored in a control unit. The steering angle to be set can assume any value within the first steering angle range for a vehicle speed within the first speed range, from standstill to a certain speed.The first speed preferably has a value in the range of 0.5 to 1 km / h, preferably about 0.7 km / h, most preferably exactly 0.7 km / h. In other words, the actuator can set any steering angle in the range between 0° and the limited steering angle when the vehicle is stationary or moving at a speed that can range from 0 km / h (stationary) up to a maximum of the first speed. This minimizes excessive stress on the actuator.

[0023] A steering angle of 0° is also referred to as the center position or neutral steering angle and corresponds to a vehicle driving straight ahead when a steering angle of 0° is set on each steered axle. The wheels are aligned parallel to the longitudinal direction of the vehicle.

[0024] Another embodiment is possible in which, after traveling at a speed greater than the initial speed range, the vehicle returns to and remains within the initial speed range. The vehicle then moves at the lower speed from standstill to, for example, 0.7 km / h, and a limited steering angle would be determined according to the method, depending on the current speed. Preferably, upon returning from the higher speed to the initial speed range, this last set maximum steering angle is set as the new limited steering angle. This is because, due to the previously higher speed, no preload was present when the steering angle was set.If, due to the predefined maximum steering angle, a maximum steering angle of 10° was possible and a limited steering angle of 7° was defined in the first speed range, then a steering angle of 7.5° could have been set outside the first speed range. When returning to the first speed range, the new limited steering angle of 7.5° is set and maintained in this embodiment.

[0025] If, according to the previous embodiment, the steering angle was maintained after returning to the first speed range, and a steering angle greater than 50-80%, preferably 70%, is requested again, then a maximum steering angle in the range greater than 50-80%, preferably 70%, of the predefined maximum steering angle up to the maintained limited steering angle can be set. For example, after returning to the first speed range, i.e., after the vehicle has come to a standstill, and a steering angle greater than 7° is requested, only an angle between the maintained value, e.g., 7.5° and 7°, can be set. In other words, in this embodiment, the maximum adjustable steering angle cannot be greater than the steering angle maintained from the previous journey at a higher speed.This extended limit is based on the fact that increasing the steering angle beyond the maintained value would again result in an unfavorable, excessive preload, because the vehicle is operating at low speeds. Surprisingly, no such preload occurs in the 7 to 7.5° range.

[0026] Preferably, the limited steering angle is lifted at speeds above the initial speed. This allows the steering angle to be set up to the predefined maximum angle after the steering angle request is detected during the actuator activation step. Above the initial speed, the vehicle's speed and wheel rotation are so high that no, or at least no detrimental, preload builds up in the tires relative to the road surface, nor in bearings or control arms such as the steering linkage.

[0027] When the vehicle is moving at a low speed, i.e., within the first speed range, the steering speed, expressed by a steering gradient, is preferably adjusted, and preferably reduced. The steering speed is considered as a parameter, for example, in degrees per second. In the first speed range, from standstill up to a maximum of 1 km / h, preferably 0.7 km / h, the steering speed can, for example, be in a range of 0–12° / s. Above the first speed, the steering gradient can be permitted to increase, for example, in a range of 12–18° / s. The steering gradient is adjusted depending on the steering angle requirement, so that a change in the steering angle can be made according to the time requirement.

[0028] To prevent abrupt steering inputs when leaving the initial speed range at which the limited steering angle is lifted, and with increasing speed—preferably a significant increase, such as during sudden high acceleration—a hard switch from the limited to the maximum steering angle is avoided as speed increases. Preferably, the limited steering angle is gradually adjusted from, for example, 7° to the maximum steering angle of 10°, depending on the current speed and / or acceleration, as well as the steering angle requirement. The aforementioned boundary conditions, e.g., regarding the vehicle's load, are preferably taken into account. To prevent a sudden change in the steering angle, the steering gradient is adjusted to a level manageable for the driver, whereby different driving modes, e.g.,Eco driving, sport driving, etc., can be taken into account. Preferably, the adjustment is achieved using at least one characteristic curve. The steering gradient can be, for example, 4° / s at 0 km / h (vehicle stationary), 2° / s at 10 km / h, and 0.25° / s at 250 km / h, depending on the vehicle's speed. This at least one characteristic curve thus affects the actuator's response speed, and the resulting steering angle change is smoothly adjusted, ensuring a seamless transition rather than a sudden change. This is advantageous in terms of driving safety and comfort.

[0029] In a further preferred embodiment, the acceleration of an actuator drive unit is changed, preferably reduced, depending on the current speed from standstill to parking and / or maneuvering during the step of determining the limited steering angle. The change is at least temporary, with the acceleration preferably being reduced. This reduces the aforementioned loads on the actuator, which can occur at large steering angles. At medium and small steering angles, a higher or even the maximum acceleration of the actuator drive for the steer-by-wire steering system is thus possible. Different limited accelerations can be set for different steering angle ranges, preferably based on at least one characteristic curve. The acceleration limitation has the advantage that the actuator starts up more slowly initially and keeps the loads low.After a slow start-up, acceleration is preferably applied to a higher steering gradient, so that, according to the steering angle requirement, the steering angle change occurs in the same time as if the acceleration were not limited.

[0030] The invention further relates to a steer-by-wire steering system with an actuator, which is operated according to the aforementioned method steps. A particularly advantageous embodiment is one in which, during the actuation step, an actuator is actuated by a steering system assigned to a steerable rear axle of the motor vehicle. If a steering angle is set on the rear axle that is opposite to the steering angles on the front axle, the vehicle can be maneuvered and parked more easily, in addition to a smaller turning circle. Besides the steerable rear axle, a front axle can also be configured as a steer-by-wire steering system.

[0031] According to another aspect, the invention relates to a control unit for controlling an actuator of a steer-by-wire steering system of a motor vehicle, wherein the control unit has the following features: An interface for detecting a speed, which represents the instantaneous speed of the motor vehicle; another interface for detecting a steering angle request, which represents an instantaneous requested change in the steering angle due to a driver request or a further change in the steering angle determined by a control unit; a unit for determining a limited steering angle, which represents an instantaneous maximum possible steering angle depending on at least the instantaneous speed; a unit for controlling the actuator for setting a steering angle of at least one wheel depending on at least the steering angle request and taking into account the limited steering angle.

[0032] The control unit is also capable of temporarily allowing the limited steering angle, i.e., for a specific period of time or duration. In addition to an instantaneous change in the steering angle due to a driver input, such as when the driver makes a steering movement using the steering wheel, a steering angle request can also be initiated by a control unit, such as an electronic stability program (ESP).

[0033] The control unit can be a control device, which might be, for example, an electrical device that processes electrical signals, such as sensor signals, and outputs control signals accordingly. The device can have one or more suitable interfaces, which can be implemented in hardware and / or software. In the case of a hardware implementation, the interfaces can, for example, be part of an integrated circuit in which the device's functions are implemented. The interfaces can also be their own integrated circuits or consist at least partially of discrete components. In the case of a software implementation, the interfaces can be software modules that are, for example, present on a microcontroller alongside other software modules.

[0034] A computer program product with program code that can be stored on a machine-readable medium such as semiconductor memory, hard disk memory or optical memory and is used to carry out the method according to one of the embodiments described above is also advantageous when the program is executed on a computer or control unit.

[0035] The invention is described below with reference to preferred embodiments and the drawing. The drawing shows: Fig. 1 shows a vehicle with a steer-by-wire steering system according to the known state of the art, and Fig. 2 shows a flowchart for the method. Fig. 3 shows a diagram of an embodiment of the method.

[0036] Figure 1Figure 1 schematically shows a vehicle 1 with a steerable front axle 21 and a steerable rear axle 31. A steer-by-wire steering system 4 is provided for steering the wheels 2 on the front axle 21. This system can adjust or change the wheel steering angle RLw v on the front axle via a steering linkage 41. This angle RLw v is located at the front of the right wheel 2 in the Figure 1The diagram illustrates this. On the rear axle 31, a steer-by-wire steering system 5, via a steering linkage 51, adjusts or changes the wheel steering angles RLw h at the rear wheels 3, as shown at the rear left wheel 3. As part of the steer-by-wire steering system, an actuator (not shown) is provided, which can apply a force to linearly move a steering rod or spindle. This rod, via the steering linkages 41, 51, interacts with the wheels 2, 3 to steer them. In the illustrated embodiment, the wheels on the front axle 21 and the rear axle 31 are steered in opposite directions, resulting in a reduced turning circle and improved maneuvering and parking capabilities at low speeds, e.g., less than 5 km / h, compared to vehicles with only one steered axle. Two steered axles also make it advantageous to perform automated journeys, as the vehicle can be maneuvered in a smaller space.The steering angle at the front axle 21 is generally adjusted via a steering wheel 14. The steering angle requested by the driver is detected by a sensor unit Lw and transmitted to a control unit SG via a signal line to the steer-by-wire steering system 4. Based on the steering angle request Lw_req submitted by the driver and taking into account the driving situation (load, vehicle speed, vehicle acceleration, yaw rate RG, etc.), the control unit calculates the steering angle RLw v and RLw h for the front and rear axles, respectively, and outputs the control signals to the steer-by-wire steering systems 4 and 5, which then adjust the corresponding steering angles at axles 21 and 31.

[0037] The control unit SG is connected to the steer-by-wire steering units 4 and 5 via signal lines shown schematically here. These units set the steering angles RLw v and RLw h calculated by the control unit SG. The sensor units (not shown) for transmitting the respective steering angles RLw v and RLw h of the wheels 2 and 3 are electrically connected to the control unit SG and the steer-by-wire steering units 4 and 5 via a bus system BUS. This bus system is, for example, a CAN bus or a FlexRay bus, or a similar bus system used in vehicles for data and signal transmission. Steering functions for a driver assistance system such as ADAS (Advanced Driver Assistance System), corridor functions (Korr) for limiting a maximum steering angle depending on driving situations, a PAS (Park Assist System) for assisted or automated parking, or an ESC (Electronic Stability Control System), etc., are transmitted via the BUS bus system.The control unit SG is electrically connected to the steer-by-wire steering systems 4 and 5 of the front axle 21 and the rear axle 31. The control unit SG is shown schematically and, as a central unit, outputs control signals to the steer-by-wire steering systems 4 and 5, which execute the steering angle changes or set the steering angle RLw v and RLw h.

[0038] Sensors S are arranged at the ends of the vehicle 1. These sensors are part of a sensor array and serve to detect the vehicle's surroundings. They can be temperature sensors, optical sensors (such as cameras), LiDAR, or radar, suitable for measuring temperature, distance, or other optical parameters, such as the road surface. Environmental conditions, such as humidity, road surface, and temperature, can thus be detected and transmitted to the control unit SG. The vehicle follows a trajectory T, which is defined in the Figure 1at the front end of the vehicle in its direction of travel, as shown schematically.

[0039] To rotate the wheels 2, 3 of vehicle 1 around their vertical axes in a steering movement using the respective steer-by-wire steering systems 4, 5, the friction of the tires of wheels 2, 3 against the road surface must be overcome. The lower the speed of the vehicle, the less the wheels 2, 3 move or roll in the intended direction of travel on the road surface. In the illustrated vehicle 1, its weight is borne by the four depicted wheels 2, 3, or rather, the tires. The contact between the tires and the road surface results from their contact patch. Due to the tire material, usually mostly rubber, and the road surface properties, friction occurs between the tires and the road surface. The lower the speed of the vehicle, the more force is required to steer the wheels 2, 3, because friction increases with decreasing speed. See also the detailed explanation above.

[0040] If a wheel 2, 3 on the steerable rear axle is steered to the right by an angle RLw h from a position parallel to the longitudinal axis L of the vehicle 1 (not shown), and the vehicle speed is in the range from standstill to approximately 1 km / h (speed v_0-1), a preload builds up in the tire relative to the road surface. The tire material is elastic and is essentially stretched relative to the road surface. Furthermore, preloads can arise due to the elasticity in the steering linkages 41, 51 and the joints (not shown). If, after the wheels 3 on the rear axle 31 have been steered to the right, the steering is reversed in the opposite direction, a change in the direction of force occurs for the steer-by-wire steering system 5 on the rear axle 31.The steering gear of the actuator in the steer-by-wire steering system 5 experiences at least a brief period of relief during steering return, followed by a significant load due to the force required for this return movement. The greater the required steering angle and the lower the speed, the greater the resulting preload. This can cause vibrations and thermal stresses in the steer-by-wire steering actuator, which can ultimately damage the actuator.

[0041] To make driving at speed v_0-1 more comfortable and safer, and to reduce the aforementioned preload, the steering angle of the wheels on each axle is limited. To determine the current speed, the control unit SG continuously records the instantaneous speed v_mom of the vehicle, preferably at intervals of 10 ms. This is located in Figure 1This is shown by way of example with a dashed line between the control unit SG and the right rear wheel 3. From Figure 2This document describes how a method for operating an actuator of a steer-by-wire steering system of a motor vehicle can be implemented to determine a compatible steering angle as a limited steering angle. The method comprises a step of acquiring an instantaneous speed v_mom of the motor vehicle 1, which, as mentioned above, serves to determine the current speed v_mom. If the vehicle is traveling at a speed v_0-1, preferably 0 < v_0-1 < 0.7 km / h, then in a step of determining a limited steering angle RLw_lim_mom, this angle is limited at least as a function of the instantaneous speed v_mom. Further parameters can be incorporated in the determination step, such as the condition of the road surface, which can be determined by sensors s in the vicinity of the vehicle while it is driving.If, in a further step of the acquisition process, a steering angle request Lw_req is acquired, which is triggered by a driver request or a control signal from the control unit SG, the limited steering angle RLw_lim_mom stored in the control unit SG is decisive, meaning that no larger steering angle can be set. The control unit SG may, for example, contain a characteristic curve, allowing the limited steering angle to be retrieved as a function of different speed levels. In a further step of actuating the actuator, a steering angle RLw v, RLw h is set at the wheels 2, 3 of the relevant axle 21, 31, depending on the steering angle request Lw_req and taking into account the previously determined limited steering angle RLw_lim_mom. If the vehicle remains within the speed range v_0-1, the previously determined limited steering angle RLw_lim_mom applies as the maximum possible steering angle.During this speed v_0-1, the adjustable steering angle RLw v , RLw h can take a value from 0° up to the determined limited steering angle RLw_lim_mom.

[0042] Above the aforementioned speed range v_0-1, the limited steering angle RLw_lim_mom is lifted, and the maximum steering angle specified by the design, or a steering angle limited by the vehicle's condition, can be set. A heavily loaded vehicle, for example, can represent such a situation, where the wheels are deeply recessed into the wheel wells, making it impossible to set the maximum steering angle specified by the design, as this could lead to collisions with the vehicle body or chassis.

[0043] Figure 3 The diagram shows the previously described relationship in detail. It depicts a section of a rear axle steering system according to... Figure 1A steer-by-wire steering system 5 is coupled to a wheel 3 via a steering linkage 51. Wheel 3 is steered to the right up to a steering angle defined as the limited steering angle RLw_lim_mom, which is determined based on the current speed v_mom and lies within the speed range v_0-1. Due to a renewed steering angle request Lw_req, initiated by the driver or calculated by the control unit, any steering angle within the range Lw1 can now be set while remaining within the aforementioned speed range v_0-1. The range Lw1 extends from straight-ahead driving (center position 0°) up to the limited steering angle RLw_lim_mom. This also applies analogously to a possible steering movement to the left, which is not considered here.

[0044] If the vehicle is traveling above the specified speed range (0.7 <= v1), any steering angle in the range Lw2, i.e. up to the maximum possible steering angle RLw_max, can be set, provided the vehicle situation allows this, for example due to the vehicle's load.

[0045] In a particular embodiment, the vehicle may initially have been moved at a higher speed (0.7 <= v1), i.e., outside the speed range v_0-1, so that the previously stored limited steering angle RLw_lim_mom is overridden. If the vehicle then slows down again, i.e., returns to the speed range v_0-1, the largest steering angle last set and stored, RLw_rec_Lw2, which can lie between RLw_lim_mom and RLw_max, is set as the new limited steering angle RLw_lim_Lw2 and stored in the control unit. If a steering angle request Lw_req is now above the steering angle RLw_lim_mom, any value in the range b between RLw_lim_mom and RLw_lim_Lw2 can be set. This is unproblematic because when the vehicle moves at (0.7 <= v 1 ), no relevant preload has built up in the tires or steering components, bearings or steering linkage.

[0046] For example, if a vehicle is moved during a parking maneuver in the speed range v_0-1, and a limited steering angle RLw_lim_mom of 7° has been stored due to this very low speed, and a maximum steering angle of 8° results from the subsequent vehicle movement at v1 >= 0.7 km / h, this value is stored in the control unit as RLw_rec_Lw2. If the vehicle then returns to the speed range v_0-1, and thus a limited steering angle of 7° applies to RLw_lim_mom, a steering angle between 7° and a maximum of 8° can be set upon a new steering angle request. Reference sign

[0047] 1 (Motor) vehicle 2 Front wheel 3 Rear wheel 4 Steer-by-wire steering 5 Steer-by-wire steering 14 Steering wheel 21 Front axle 31 Rear axle 41 Steering linkage 51 Steering linkage 200 Step of detection 220 Step of determination 240 Step of detection 260 Step of steering ADAS Driver Assistance System autonomous driving bArea BUSBus system ESC Electronic Stability Control Corr Corridor function Lw Sensor unit Lw_req Steering angle request Lw1 First steering angle range Lw2 Second steering angle range PAS Parking assistant RLw v (Wheel) steering angle front RLw h (Wheel) steering angle front RLw_lim_momb Limited steering angle RLw_lim_Lw2 Limited steering angle RLw_rec_Lw2 Limited steering angle RLw_max Maximum possible steering angle SG Control unit S Sensors T Trajectory v_0-1 First speed range v1 First speed v_mom Instantaneous speed

Claims

1. Method for operating an actuator of a steer-by-wire steering system of a motor vehicle (1) at a speed from a standstill to parking and / or manoeuvring, wherein the method comprises the following steps: - detecting a current speed (v_mom) of the motor vehicle - determining a limited steering angle (RLw_lim_mom) depending on at least the current speed (v_mom) - detecting a steering angle requirement (Lw_req) - activating the actuator to set a steering angle (RLwv, RLwh) of at least one wheel at least depending on the steering angle requirement (Lw_req) and taking into account the limited steering angle (RLw_lim_mom), so that loads within the actuator due to a build-up of prestress between tyre and roadway and / or between actuator and a steering linkage are reduced.

2. Method according to Claim 1, characterized in that, in the determining step, the limited steering angle (RLw_lim_mom) can be changed to a value lower than a predefined maximum steering angle (RLw_max).

3. Method according to either of the preceding claims, characterized in that, in the determining step, the limited steering angle (RLw_lim_mom) is defined based on a characteristic curve, preferably at 50 to 80%, most preferably at 70%, of the predefined maximum steering angle (RLw_max).

4. Method according to any of the preceding claims, characterized in that, in the step of determining the limited steering angle (RLw_lim_mom), a first steering angle range (Lw1) from 0° to less than or equal to the limited steering angle (RLw_lim_mom) and a second steering angle range (Lw2) of greater than the limited steering angle (RLw_lim_mom) up to the predefined maximum steering angle (RLw_max) are defined, wherein, at a speed of the motor vehicle in a first speed range (v_0-1) from a standstill to a first speed (v1), preferably 0.5 to 1 km / h, most preferably 0.7 km / h, the steering angle (RLwv, RLwh) to be set can assume any value in the first steering angle range (Lw1).

5. Method according to Claim 4, characterized in that, following the motor vehicle travelling at a speed of greater than the first speed (v1) and subsequently returning to and remaining in the first speed range (v_0-1), the last-set steering angle (RLw_rec_Lw2) from the second steering angle range (Lw2) is defined as the new limited steering angle (RLw_lim_Lw2).

6. Method according to Claim 5, characterized in that, following the motor vehicle returning to and remaining in the first speed range (v_0-1) and a renewed steering angle requirement (Lw_req) of greater than 50 to 80%, preferably greater than 70%, of the predefined maximum steering angle (RLw_max), a steering angle in the range of 50 to 80%, preferably 70%, of the predefined maximum steering angle (RLw_max) up to the maintained limited steering angle (RLw_lim_Lw2) can be set as a maximum.

7. Method according to any of the preceding Claims 4 to 6, characterized in that the limited steering angle (RLw_lim_mom, RLw_lim_Lw2) is cancelled at a speed greater than the first speed (v1), so that, following detection of the steering angle requirement (Lw_req), a steering angle (RLwv, RLwh) up to the predefined maximum steering angle (RLw_max) can be set in the step of activating the actuator.

8. Method according to any of the preceding claims, characterized in that setting of a new steering angle (RLwv, RLwh) is performed gradually starting from the previously set steering angle depending on the current speed (v_mom) and the steering angle requirement (Lw_req), preferably on the basis of a characteristic curve, in order to prevent a sudden change in steering angle.

9. Method according to any of the preceding claims, characterized in that an acceleration of a drive unit of the actuator is at least temporarily changed, preferably reduced, depending on the current speed (v_mom), in the step of determining the limited steering angle (RLw_lim_mom).

10. Steer-by-wire steering system comprising an actuator, preferably rear-axle steering system, which is operated in accordance with a method according to the preceding claims.

11. Control unit (SG) for controlling an actuator of a steer-by-wire steering system (4, 5) of a motor vehicle (1), wherein the control unit has the following features: • an interface for detecting a speed (v_mom), which represents a current speed of the motor vehicle, • a further interface for detecting a steering angle request (Lw_req), which represents a current change in steering angle due to a driver's request or due to a further change in a steering angle determined by a control unit (SG), • a unit for determining a limited steering angle (RLw_lim_mom), which represents a currently maximum possible steering angle, • a unit for activating the actuator for setting a steering angle (RLwv, RLwh) of at least one wheel depending on at least the steering angle requirement (Lw_req) and taking into account the limited steering angle (RLw_lim_mom), so that loads within the actuator due to a build-up of prestress between tyre and roadway and / or between actuator and a steering linkage are reduced.

12. Computer program, which is configured to carry out the method according to any of the preceding Claims 1 to 9.

13. Machine-readable storage medium, on which the computer program according to Claim 12 is stored.