Method and control unit for operating a steering by wire, and steering by wire

Abstract

The invention relates to a method for operating a steer-by-wire device of a motor vehicle at a speed from stationary to parking and / or shunting. The method comprises the following steps: ascertaining a rotational speed (rsmcur) of a spindle nut (25) of the spindle drive (20); -ascertaining a vehicle speed (vcur); determining a limit value (rsmlim) of the rotational speed (rsmcur) of the spindle nut at least as a function of the vehicle speed (vcur); -actuating an actuator to adjust the steering angle (RLwv, RLwh) of at least one wheel (5, 6) using the limit value (rsmlim) of the rotational speed of the spindle nut.

Description

Technical Field

[0001] The invention relates to a method and a control unit for operating a steer-by-wire device and to a steer-by-wire device for a motor vehicle according to the preambles of the independent claims. Background Art

[0002] An actuator is known from DE 10 2014 206 934 A1, which has a fixedly supported spindle nut and a spindle fixed in a rotationally fixed manner and axially displaceable relative to the spindle nut. At least one end of this actuator for a wire-controlled steering device is connected to the wheel support via a steering tie rod. The wheel steering angle of the wheel rotatably supported at the wheel support can be changed by the linear displacement of the spindle. The wire-controlled steering device that can be at least indirectly operated by a steering handle (for example a steering wheel) or works independently of the steering handle is controlled via a signal path, that is, without mechanical coupling. This steering device must overcome the friction resistance of the wheel relative to the lane during the steering process and the friction resistance in the spindle drive. In particular, at very low speeds, when the motor vehicle is maneuvering or parking until it is stationary, a relatively high steering force is required, which puts a large load on the actuator. Due to the alternating static and sliding friction between the thread flanks of the screw nut and the thread flanks of the screw, the screw in the screw drive of the actuator may resonate and the screw drive may reach high temperatures, which may lead to damage to the actuator and thus to damage to the steer-by-wire device. Summary of the invention

[0003] Against this background, the object of the present invention is to provide an improved method and a control unit for operating a steer-by-wire device and a steer-by-wire device for a motor vehicle when one or more wheels on an axle of the vehicle are steered at very low speeds when stationary or in parking or maneuvering situations.

[0004] According to a first aspect of the invention, a method for operating a steer-by-wire device of a motor vehicle is proposed, the motor vehicle being operated at a much lower speed from rest to parking and / or maneuvering than in normal driving. The method comprises at least the following steps:

[0005] - Get the rotation speed of the screw nut of the screw drive;

[0006] -Get vehicle speed;

[0007] - determining a limit value for the rotational speed of the spindle nut at least as a function of the vehicle speed;

[0008] The actuator is controlled to adjust the steering angle of at least one wheel on the axle using a limit value of the rotation speed of the spindle nut.

[0009] The steer-by-wire device comprises an actuator having a screw drive for axially displacing a screw. The actuator of the steer-by-wire device of a motor vehicle preferably has a housing in which a screw and a screw nut arranged rotatably and fixedly are supported. The screw and the screw nut form a movement thread and are part of a screw drive in the housing for axially displacing the screw relative to the screw nut and therefore also relative to the housing. For this purpose, the screw has an external thread that engages with the internal thread of the screw nut. If the screw nut is driven in rotation, for example by an electric motor, preferably indirectly by a transmission, preferably by a belt drive, most preferably by a toothed belt drive, the movement thread causes the anti-rotationally fixed screw to be axially displaced along its longitudinal axis relative to the screw nut or the housing. By axial displacement along its longitudinal axis, the wheel steering angle of a wheel rotatably arranged on a wheel carrier, which is at least indirectly connected to the end of the screw, can be changed.

[0010] The large adjustment forces required for steering cause large frictional forces in the actuator gearbox, in particular in the moving thread of the actuator's lead screw drive. Even with optimized lubricants and materials, large frictional forces still occur between the thread flanks in the moving thread (i.e. between the lead screw and the lead screw nut). Due to the static friction and sliding friction that occurs between the two sides of the thread on the contact surface of the thread flanks that are abutting each other, the so-called stick-slip effect may occur. This involves alternating adhesion and sliding of the thread flanks, which on the one hand may lead to torque fluctuations between the lead screw nut and the lead screw. In this case, for example, the lead screw may be stimulated to vibrate, in particular to rotate. Continuous or temporary excitation within a minimum time period can cause the lead screw or other components in the actuator to reach a resonant frequency. In addition, the vibrations can generate thermal loads, which can put negative pressure on the lubricating properties of the lubricant. This can have a negative impact on the service life of the actuator.

[0011] The above-mentioned rotational vibrations are also called torsional vibrations. In contrast to translational vibrations, in rotational vibrations the vibrations occur about the rotational degree of freedom of the system, here about the longitudinal axis of the screw. In both cases, they are mechanical vibrations.

[0012] The term stick-slip effect is derived from the two English words "stick" and "slip". In physics and technology, the stick-slip effect describes the usually undesirable and sudden sliding of solid bodies moving relative to each other (rest-slip-rest-slip).

[0013] A steer-by-wire device is a steering device that decouples a mechanical steering device from a steering handle (e.g., a steering wheel). The steering device can also be operated completely by a control unit, for example in an autonomous vehicle. The steering movement of the driver via the steering handle is not transmitted to the wheel carrier or wheel along a mechanical path (e.g., via a connecting rod). Instead, the steering angle (also called wheel steering angle) or the change in the steering angle of the corresponding wheel of the axle is calculated, for example, in a control unit, which sends a control signal to one or more actuators of the steer-by-wire device and ultimately causes a change in the steering angle on the corresponding wheel or an adjustment of the steering angle. Here, the driver's steering angle request or the calculated steering angle change may deviate (e.g., be greater than) the maximum adjustable steering angle on the relevant axle. In this case, at most the maximum possible steering angle can be set. The steering angle request includes changing the steering angle from the existing steering angle to the desired steering angle, wherein the steering angle request is also time-dependent. As an example, it is mentioned here that the driver can turn the steering wheel slowly (e.g., at 2° / s), but can also turn the steering wheel very quickly (e.g., at 20° / s) in order to perform a steering angle change of, for example, 5°. In other words, we need to obtain both the change in angle and the speed of the angle change.

[0014] In normal operation of the steering system, smaller changes in the wheel steering angle are mostly performed when the vehicle is moving at speeds significantly above the above range, for example in closed residential areas at, for example, 30 to 50 km / h, or at higher speeds on country roads or motorways. In this case, it can usually be assumed that the steering angle changes are less than 1°. Therefore, the control forces required for these small changes are significantly smaller in comparison, so that almost no or no torsional vibrations occur.

[0015] The method proposed here is based on the recognition that in certain situations an increased torque or an increased force is required to steer, or in other words, to turn the corresponding wheel to the desired steering angle or wheel steering angle. The situation considered here assumes that the vehicle has a very low speed from rest to parking and / or maneuvering. At complete rest, the speed is equal to 0 km / h. When parking and / or maneuvering, a speed of less than or equal to 5 km / h is assumed here. In the speed range from 0 to about 1 km / h, particularly large forces are required to turn the desired steering angle. The lower the speed, the greater the steering force to be expected, which must be achieved by the actuator of the steer-by-wire device. This is because the entire weight of the vehicle rests on the tires. The contact between the tire and the roadway is determined by the tire contact patch. The size of the tire contact patch depends primarily on the wheel load and the tire pressure, because the internal pressure of the tire bears a large part of the wheel load. But the tire width, tire diameter and the stiffness of the tire wall also play a role. The force required for steering, i.e. for rotating the wheel about its vertical axis, is greater when the wheel is stationary than when the wheel is rolling due to the movement of the vehicle. As the rolling motion increases, i.e. the vehicle speed increases, the forces required for steering become smaller. It can be seen that in addition to the vehicle mass, the ambient temperature and the tire temperature also have an influence, since they directly influence the friction between the tire and the road surface. The following other parameters are mentioned here in a non-exhaustive manner: tire compound, tire type, tire friction coefficient, road pavement and road condition (dry, wet, dirty, icy, etc.).

[0016] The tires of the wheels are usually made of rubber, an elastic material. If the force from the actuator of the steer-by-wire system acts on the wheel to steer the wheel, a prestress is generated due to the static or sliding friction between the tire and the road surface. The tire is lifted up to a certain extent relative to the road surface and is thus prestressed. Additional prestress is generated between the actuator and the wheel carrier via the bearings arranged between them and, if necessary, connecting rods (for example, steering rods), depending on the design of the chassis.

[0017] If, in the aforementioned low-speed range, the steering is now switched back to a smaller steering angle by means of the steer-by-wire system from a large steering angle, preferably from the maximum possible steering angle, the prestress is first briefly reduced and then re-established. The prestress increases as the vehicle speed decreases, or as the vehicle speed decreases from rolling to coming to a standstill. When parking and / or maneuvering, the situation changes almost constantly. When turning back from a previously set large steering angle, the direction of the forces in the actuator of the steer-by-wire system changes. This leads to load changes in the transmission or spindle drive of the actuator, which in turn leads to changes in the stick-slip behavior. This can lead to vibrations and high thermal loads in the actuator or its moving thread. This behavior needs to be reduced or minimized.

[0018] According to the invention, in the method described above, the rotation speed of the lead screw nut of the actuator of the steer-by-wire device is limited according to the current speed of the vehicle, preferably based on the nominal rotation speed at which it normally operates. In the lead screw drive of the actuator, improved properties of both friction sides in the actuator (e.g., the thread flanks of the moving thread that abut against each other) can be achieved by limiting the rotation speed. As a result, the above-mentioned rotational vibrations and thermal loads are minimized or not generated at all.

[0019] The speed limitation is performed by a step for determining a limit value for the speed of the spindle nut. The limit value is determined at least according to the current vehicle speed. The speed of the spindle nut is acquired continuously, preferably at certain intervals, preferably at intervals of 10 ms. The current vehicle speed is acquired continuously, preferably at certain intervals, preferably at intervals of 10 ms, which is present, for example, due to a request by the driver by depressing the accelerator pedal. The acquisition of the speed of the spindle nut is preferably performed by a sensor system. The speed can be acquired preferably in a contactless manner by a sensor preferably arranged in a housing. For this purpose, an incremental sensor can be used, for example. In a preferred embodiment, the speed of the spindle nut is determined using a rotor position sensor of a drive-type electric motor. Such a rotor position sensor is preferably configured as an incremental sensor. In short, the number of revolutions of the motor is counted, in particular according to time. When the spindle nut is driven indirectly by a transmission, preferably a belt transmission, preferably a toothed belt transmission, the speed of the spindle nut can be determined by knowing the speed of the electric motor. In particular, the determination of the speed is implemented in a calculation manner in the control unit.

[0020] For example, during parking, the maximum possible steering angle may have been set at the front axle so that the wheels are turned as far as possible in one direction, for example to the left. Due to this steering angle request, the maximum steering angle of the wire steer device is set by the control unit at the rear axle in the opposite direction, here to the right. Usually, the vehicle stops after such a steering maneuver when parking. The actuator of the wire steer device also stops and temporarily does not make any steering angle adjustments. At this time, the driver turns the steering wheel in the opposite direction again and / or controls the vehicle speed change according to the calculated changes or the changed accelerator pedal position. Alternatively, it may be that a driving assistance system (such as a parking assistance system) causes these changes. In addition, these changes are carried out at a certain speed. These parameters are acquired by the control unit and in a limit value determination step, the limit values ​​of the rotation speed of the spindle nut are limited according to the vehicle speed. Finally, the actuator of the wire steer device is controlled to adjust the steering angle by taking into account at least the aforementioned parameters and using the limit values. This occurs when the rotation speed of the spindle nut is different from the nominal rotation speed used in other cases, preferably lower. This results in the friction characteristics of the friction sides in the screw drive or in the moving thread changing due to the relative speed between the flanks of the internal thread of the screw nut and the flanks of the external thread of the screw in normal operation, which deviate from the nominal speed. The above-mentioned stick-slip characteristics change and torsional vibrations in the screw drive are reduced or prevented. The control of the actuator is preferably carried out by a control unit such as a controller or a control unit. The control unit is preferably part of the steer-by-wire device. However, the control of the actuator can also be carried out by another controller installed in the vehicle, which is not part of the steer-by-wire device.

[0021] Without such limitation by using limit values, the drive (e.g. electric motor) would reach the nominal speed by a predetermined acceleration, which is determined in the design of the steer-by-wire system for adjusting the steering angle. Due to the speed of the drive, the speed of the spindle nut is generated, if necessary with a transmission connected in between. In other words, the nominal speed leads to a predetermined adjustment speed associated with the transmission or the spindle drive of the actuator, which ultimately leads to the steering speed and is also called the steering gradient. The steering gradient indicates how much angle the correspondingly steered wheel can be adjusted around its vertical axis per unit time, i.e., for example, in degrees per second. Different steering gradients can be determined for different driving situations, including different vehicle speeds and boundary conditions (e.g., vehicle load, type of tire used or road conditions, etc.). The actuator can have, for example, a control speed at the above-mentioned nominal speed, so that for a vehicle ready to drive and unloaded with its wheels on a dry road, the wheel can be adjusted by the steer-by-wire system with a steering gradient of, for example, up to 18° / s. Here, at low speeds, for example in parking and / or maneuvering situations, the steering speed is reduced due to the friction between the tires and the roadway, and there is a small steering gradient, for example 2-8° / s. However, at very high speeds, for example 200 km / h, the steering gradient is reduced or limited to, for example, 0.25° / s to avoid sudden steering movements, which could lead to dangerous driving situations.

[0022] In this context, a large steering angle is understood to be a steering angle that is in the range of the maximum steering angle that is structurally possible for the respective axle. When maneuvering or parking, the vehicle speed and the steering angle at the wheels often change. By using a large steering angle, preferably the maximum possible steering angle, it is easier to maneuver into a parking space or to maneuver with a trailer, for example. It is therefore particularly advantageous if, in addition to the front axle, the rear axle of the motor vehicle is also steerable.

[0023] A steering angle of 0° is also referred to as the center position or neutral steering angle and corresponds to straight-ahead driving of the vehicle if a (wheel) steering angle of 0° is set at each steered axle or at each steered wheel. In this case, the wheels are oriented parallel to the longitudinal direction of the vehicle.

[0024] In a preferred embodiment, the limit value is used at least temporarily. When the vehicle speed is reached, the limit value is cancelled, from which, for example, it is no longer necessary to limit the rotational speed of the spindle nut. The limit value can also be used for a predetermined period of time, so that after the period of time has elapsed, the limit value is reset to a nominal value or a maximum value, or the limit value is cancelled. Depending on the expected vehicle speed, the previously determined limit value can also be changed, in particular reduced from a previously set value, in order to take into account the resulting changed driving situation. The reasons for this can be a change in the position of the accelerator pedal or a change in the calculated or required speed due to a change in the pavement of the road (slippery road, with different friction coefficients, for example on each side of the vehicle).

[0025] It is also preferred that the limit value is set to a predetermined minimum value in the determination step. For example, such a predetermined minimum value can be equal to half or three quarters of the nominal speed of the rotational speed of the screw nut, at which a structurally determined movement of the screw can be achieved within a predetermined time. It is thus advantageously ensured that the rotational speed is always sufficient so that the desired steering movement can be adjusted under every conceivable condition or driving situation. In other words, it is ensured that the wheel steering angle can be changed without trouble while taking into account every driving situation of the motor vehicle, without causing harmful temperature increases or resonances in the actuator. The nominal speed of the screw nut is a structurally determined value that is decisive for the adjustment speed of the actuator of the wire steering device. As will be described later, the nominal speed can be exceeded at least temporarily in order to compensate for the so-called offset. Therefore, even in the case of temporarily using the limit value of the rotational speed of the screw nut, the adjustment speed of the actuator can still be maintained, so that the adjustment speed of the actuator is not affected or is not significantly affected. The nominal speed can be exceeded up to the maximum value of the rotational speed of the screw nut and is, for example, structurally limited by the transmission and / or the electric motor.

[0026] The limiting of the rotation speed is performed in the step of determining the limit value at least according to the current vehicle speed. In addition to the current steering angle request, the position of the screw and / or the vehicle state can also be taken into account in an advantageous manner. Regarding the steering angle request, it has been mentioned above that this may be different. Large steering angle changes can be made, for example, when parking, the maximum steering angle changes from the left to the right. This can be manipulated by the steering handle at a large change speed according to the driver's request. In this case, the position of the screw in the housing of the actuator is changed to the greatest extent. It is also possible that very wide tires are used on the vehicle and the vehicle load is large. The ambient temperature (the temperature of the lubricant in the screw drive, the track and the tire) also plays an important role, because at low temperatures, higher friction values ​​will be generated due to, for example, changes in the viscosity of the lubricant. By additionally considering the steering angle request and / or the screw position and / or the vehicle state, the determination of the limit value of the rotation speed of the screw nut can be further optimized in terms of quality.

[0027] In another preferred embodiment, the vibrations of the actuator, preferably the impending resonant vibrations, can be detected and taken into account in the step of determining the limit value. This can be performed by a suitable sensor system, for example a structure-borne sound sensor, preferably by a micromechanical piezoelectric signal collector. At least one signal collector is, for example, firmly connected to the housing of the actuator and is preferably arranged in the housing. The connection to the control unit can be wireless or formed by a signal line. If the vibration reaches a level stored in the control unit, this is taken into account when determining the limit value for the rotational speed of the spindle nut.

[0028] Furthermore, a steering angle range can be determined, wherein a specific limit value is assigned to the respective range. In the case where the current steering angle is less than 50-80% of the maximum possible steering angle, a further limit value can be determined in the determination step due to being outside this range. Preferably, the range can be limited to a steering angle less than 65-70% of the maximum possible steering angle. For example, a first range is obtained in which the steering angle is 0-70% of the maximum possible steering angle and a second range is obtained in which the steering angle is greater than 70% of the maximum possible steering angle up to the maximum possible steering angle. For example, the structurally maximum steering angle at the rear axle can be 10°. If this maximum steering angle is set and turned back to, for example, 8°, the rotation speed of the spindle nut is limited to the second range. For example, in the first range of 0 to 70%, or here from 0 to 7°, no limit can be specified, because at steering angles of this magnitude, the prestress is so low that rotational vibrations in the actuator are negligible or do not occur. In other words, the limit value can preferably be determined to be above 70% to 100% of the maximum steering angle. Preferably, the limit value is canceled when the steering angle changes within or reaches the range of 0 to 70%. The division of the range can be advantageously stored as a characteristic curve in the control unit so that the corresponding limit value can be called up or assigned in the determination step based on the characteristic curve.

[0029] Different characteristic curves can be stored in the control unit for different driving situations. This consideration takes into account that the predefined maximum steering angle that is structurally feasible for the steer-by-wire device in the corresponding motor vehicle may change due to boundary conditions. For example, such driving situations may arise due to the loading of the vehicle. For example, the wheels are sunk deeper into the wheel housing due to the loading, and the maximum steering angle must be limited, otherwise the wheels or tires will collide with parts of the chassis or the vehicle body if the structurally maximum possible steering angle is set. This loading also leads to greater wheel loads, which require greater forces for steering. The loading of the vehicle can be detected in a known manner by suitable sensor systems, such as height detection devices. For example, a characteristic curve with a possibly smaller maximum steering angle can be selected based on the height detection device. For example, a limited maximum steering angle can also be obtained based on the use of wider tires or the use of snow chains, etc. Due to the available installation space in the wheel housing area, it may also be necessary here to limit the maximum available steering angle, for example, by a characteristic curve, because there is not enough space for the steering movement. For this purpose, the tires or wheels can be equipped with an RFID sensor system, for example, which sends a tire condition signal to the control unit. This possibility therefore also makes it possible to provide a support point for a limit value for limiting the acceleration which deviates from the normal state of the vehicle and is sufficient for the driving situation.

[0030] The loading and the resulting greater wheel loads are preferably taken into account in at least one characteristic curve.

[0031] If the vehicle is traveling at the above-mentioned low speed, the limit value, i.e. the limit rotation speed of the spindle nut, will lead to a slower change in the wheel steering angle. In order to achieve a target steering angle based on the steering angle request, the actuator is essentially operated at a predetermined adjustment speed so that the steering angle at the wheels of the axle can be adjusted within a certain time according to the steering angle request. Due to the limited rotation speed, a deviation occurs, so that the requested steering angle is only reached later than without the limitation. In order to compensate for this deviation, the steering speed or the steering gradient is at least temporarily changed, preferably increased. For this purpose, the predetermined adjustment speed is increased while taking into account the requested steering angle and / or the currently set steering angle and / or the current vehicle speed. In other words, after the spindle nut is turned more slowly using the limit value, it is at least temporarily increased to a higher rotation speed and thus accelerated to a higher steering gradient. In this way, in an advantageous manner, despite the limited rotation speed, the change in the steering angle is achieved by the wire control device in the same time as with the nominal rotation speed without the limitation. The steering gradient can be in the range of 0-12° / s at a speed of the vehicle from rest up to a maximum of 1 km / h, preferably up to 0.7 km / h. Depending on the steering angle request, the steer-by-wire system is operated at the speed required for the adjusted steering gradient. In the case of a temporary increase in the steering gradient, the steering gradient can be increased by 20% to 70%, preferably by 30% to 50%. Preferably, the maximum steering gradient can be temporarily increased from 12° / s here by 4° / s-6° / s to 18° / s, so that the desired steering angle change can be carried out within the desired time, i.e. as far as possible according to the steering angle request. This increase is preferably controlled in such a way that the range that is critical for vibration excitation is excluded as much as possible.

[0032] When leaving a low speed and increasing the speed, preferably when the speed of the vehicle increases sharply due to, for example, a sudden increase in the acceleration of the vehicle, in order not to cause a sudden change in the rotation speed and thus a sudden steering movement, it can be provided that, as the speed increases, no hard switching of the rotation speed and therefore also no hard switching of the steering gradient is performed. The limit value for limiting the rotation speed is preferably not changed suddenly. Instead, a gradual regulation is preferably performed in the sense that the previously changed rotation speed smoothly transitions to the target value or nominal rotation speed. This is advantageous for the driving safety or controllability of the vehicle and the driving comfort.

[0033] In another preferred embodiment, in a further step, the current speed of the motor vehicle is acquired, in particular at certain intervals, preferably at intervals of 10 ms. Here, in the step of determining the limit value, the current speed is taken into account, wherein the limit value is cancelled above the limit speed. The limit speed indicates the departure from the aforementioned low speed and is therefore greater than 1 km / h, preferably 1.1 km / h, preferably above 0.7 km / h, most preferably 0.71 km / h. In the method according to the invention, the limit value for the limited rotational speed is preferably determined as a function of the current steering angle of the wheels on the respective axle of the motor vehicle. If the steering angle is within the aforementioned first range, i.e. 0-70% of the maximum possible steering angle at the relevant axle, then in this embodiment, the rotational speed is preferably not limited in the step of determining the limit value. In the sense of additional redundancy or in order to be able to carry out the method in view of a more precise determination of the limit value and with greater safety for the vehicle occupants, the operation of the steer-by-wire device can be further improved in an advantageous manner.

[0034] From the above embodiments, various vehicle parameters are known, some of which can be acquired using sensors. As mentioned above, for various different vehicle situations, there are some ranges. For example, depending on the vehicle speed, characteristic curves can be stored in the control unit, which reflect specific vehicle parameters within specific ranges. When determining the limit values ​​for the rotation speed of the spindle nut, the characteristic curves can be used in an advantageous manner in the determination step. These characteristic curves simplify the determination of the limit values ​​and ultimately the adjustment of the steering angle of at least one wheel on the axle using the limit values ​​for the rotation speed of the spindle nut.

[0035] A particularly advantageous embodiment is that in the step of actuating the actuator, the steering angle at a steerable rear axle of the motor vehicle is actuated. If a steering angle can be set at the rear axle, which extends in the opposite direction to the steering angle at the front axle, this results in a smaller turning radius at low speeds compared to a vehicle with an unsteered rear axle. Due to the steering of the rear wheels, the vehicle can be maneuvered or parked better. The steerable rear axle is preferably designed as a steer-by-wire system.

[0036] According to another aspect, the invention relates to a control unit for controlling an actuator of a steer-by-wire device of a motor vehicle, wherein the control unit has at least the following features:

[0037] An interface for at least indirectly acquiring the rotational speed of the spindle nut (25), which represents the current rotational speed (r_sm_cur);

[0038] An interface for obtaining the vehicle speed, which represents the current speed of the motor vehicle (v_cur);

[0039] An interface for acquiring a steering angle, which represents the current steering angle (RLw) of at least one wheel (5, 6) of the motor vehicle v _mom, RLw h _mom);

[0040] An interface for obtaining a steering angle request (Lw_req_cur) representing a current steering angle change due to a driver request or a change in the steering angle determined by the control unit (SG) or another control unit;

[0041] a unit for determining a limit value for the rotational speed of a spindle nut (25) of an actuator (10) of a steer-by-wire device (12), the limit value representing a limit value (r_sm_lim);

[0042] An actuator for controlling a steer-by-wire device to adjust a steering angle (RLw) of at least one wheel (5, 6) using a limit value (r_sm_lim) v RLw h ) unit.

[0043] Here, the control unit can also limit the limited rotational speed temporarily, i.e. temporarily or within a certain time period. The control unit can determine, preferably calculate, the current rotational speed of the spindle nut based on the rotational speed of the electric motor. The rotational speed of the electric motor is preferably obtained by a rotor position sensor built into the electric motor, which is preferably designed as an incremental sensor. If necessary, the control unit takes into account the existing transmission ratio resulting from the transmission ratio of the transmission. The control unit can also preferably determine the screw position by means of the rotor position sensor of the electric motor. A linear distance sensor can be used in addition to check the plausibility of the screw position, which preferably works in a contactless manner.

[0044] The control unit may be a controller, which may be, for example, an electrical device that processes electrical signals, such as sensor signals, and outputs control signals accordingly. The device may have one or more suitable interfaces, which may be designed according to hardware and / or software. In the case of hardware design, the interface may, for example, be a part of an integrated circuit that implements the function of the device. The interface may also be an independent integrated circuit or at least partially composed of discrete components. In the case of software design, the interface may be designed as a software module in the form of a computer program or a part of a computer program, which is implemented on a microcontroller, for example, together with other software modules.

[0045] Also advantageous is a computer program product with a program code which can be stored on a machine-readable data carrier such as a semiconductor memory, hard disk memory or optical memory and is used for performing a method according to one of the above-described embodiments when the program is executed on a computer or a control unit.

[0046] Advantageously, the invention can cause a minimization of the vibration behavior of the actuator or the components contained therein by the control according to the method without changing the mechanical structure of the existing actuator of the steer-by-wire device. In combination with an improvement in the lubricant supply, the overall service life of the steer-by-wire device can thus be cost-effectively increased. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The present invention will be described below with reference to the accompanying drawings by means of preferred embodiments. In the accompanying drawings:

[0048] Figure 1 An axle with steer-by-wire is shown;

[0049] Figure 2 An actuator for a steer-by-wire device is shown;

[0050] Figure 3 A schematic diagram of the method according to the invention is shown. DETAILED DESCRIPTION

[0051] Figure 1A wire-controlled steering device 12 known in the prior art is shown on an axle 1, which is shown here in a top view as a rear axle with a subframe 2, which is fixed to the body frame or belongs to the body frame and is connected to the chassis (running gear) of the body of the motor vehicle. However, the present invention is not limited to the rear axle. The wheels 5 and 6 are hinged to the subframe 2 via connecting rods 3, 4. The connecting rods 3, 4 are part of the wheel suspension for the wheels 5, 6. The actuator 10 of the wire-controlled steering device 12 is arranged on the subframe 2. The actuator 10 is fixed to the subframe 2 via its housing 21. In the present embodiment, the wire-controlled steering device 12 is configured as a central steering device acting on the two wheels 5, 6 of the axle. It has a through-type steering tie rod in the form of a screw 27, which passes through the housing 21 of the actuator 10 and is configured as an axially movable screw. The electric motor 22 is arranged axially parallel to the screw 27. At the end of the longitudinally movable threaded spindle 27, a tie rod 23, 24 is articulated, which is articulated with the end facing away from the actuator 10 to the wheel carrier (not shown) of the wheels 5 and 6. Obviously, with the axial displacement, i.e. the displacement of the threaded spindle 27 along the longitudinal axis s in one direction or the other, the wheel steering angle 8, 9 changes. This is because the tie rods 23, 24 form a positive connection between the respective wheel 5, 6 or wheel carrier and the actuator 10. In order to steer the wheels 5, 6, these wheels are connected to the wheel carrier so that they can rotate about the vertical axis of the wheel or wheel carrier.

[0052] Figure 2 Shows Figure 1Detailed schematic diagram of the actuator 10 shown in FIG. The actuator 10 has a housing 21, and the electric motor 22 is arranged on the housing 21 in parallel with the axis. The screw drive 20 consisting of a screw nut 25 and a screw 27 designed as a screw with an external thread is arranged in the housing 21. The screw is fixed to prevent rotation (not shown). The screw nut 25 and the screw 27 are engaged and form a motion thread. The screw nut 25 is supported in a fixed and rotatable manner relative to the housing 21 by a rolling bearing 29. The screw (steering tie rod 27) is guided through the screw nut 25 and arranged coaxially with it. On the side of the screw nut 25 facing away from the rolling bearing 29, the pulley 30 is arranged on the screw nut 25 in a non-rotatable manner. The electric motor 22 has a drive pinion 32. The drive belt 34 in the form of a toothed belt surrounds both the drive pinion 32 and the pulley 30, so that when the electric motor 22 rotates, the screw nut 25 rotates around the longitudinal axis s without slipping. The drive pinion 32, the pulley 30 and the drive belt 34 form a transmission. Depending on the direction of rotation of the spindle nut 25, the spindle 27 is linearly displaced or moved along the longitudinal axis s in one direction or the other, depending on the direction of rotation of the electric motor 22. Due to the rotation of the electric motor, the rotation speed of the spindle nut 25 results from the rotation speed of the electric motor 22 and the transmission ratio of the belt drive. The force F acting essentially axially on the spindle 27, for example due to the lateral force of the wheel, is applied to the spindle 27. ext , which may lead to an increase in friction in the moving thread. This is because the surface pressure between the side of the external thread of the screw 27 and the side of the internal thread of the screw nut 25 is due to the force F ext The lead screw may be stimulated to vibrate due to friction. As a result, the smooth operation of the lead screw drive 20 may be deteriorated due to the reaction of the lead screw nut 25 and the lead screw or the steering tie rod 27 on both sides of the friction.

[0053] Figure 3The method according to the present invention is schematically shown in a flow chart. For example, the current vehicle speed v_cur, the current steering angle request LW_req_cur, the screw position S_pos, the vehicle state c_sit, the vibration value vib_cur of the actuator 10 and the rotation speed r_sm_cur of the screw nut 25 are provided to the control unit SG via a signal bus in the vehicle, such as a CAN bus or a Flexray bus. For this purpose, interfaces 100, 110, 120, 130, 140, 150 are respectively provided. The signal flow of the above parameters flowing to the control unit SG is simplified by arrows. In the first step 200, at least the rotation speed r_sm_cur of the screw nut 25 of the screw drive 20 and the current vehicle speed v_cur are acquired in the control unit SG. These parameters are continuously acquired at intervals of 10ms in the control unit. In the next step 220, a comparison is made with the characteristic curve stored in the control unit. The relationship between the above parameters within a specific range is stored by the characteristic curve. For example, the change of the friction coefficient with respect to the tire size or tire width in the speed range of 0-1km / h can be stored. In another characteristic curve, the change of the screw position S_pos with respect to the rotational speed of the spindle nut 25 can be stored. In another characteristic curve, the change of the friction coefficient in the moving thread with respect to the screw position S_pos relative to the spindle nut 25 can be stored. In another characteristic curve, the influence of the friction in the spindle drive 20 or in the moving thread on the vibration behavior of the spindle (steering tie rod) 27 can be stored. In a further step 240, a limit value r_sm_lim of the rotational speed of the spindle nut 25 is determined at least as a function of the vehicle speed v_cur. In this case, the control unit SG can access the characteristic curve stored there and take into account the value pairs stored there, which were mentioned above as an example.

[0054] If the limit value r_sm_lim is determined, then in a further step 250 the actuator 10 is controlled to adjust the steering angle RLw of at least one wheel 5 , 6 on the axle. v RLw h, wherein the limit value r_sm_lim of the rotation speed of the spindle nut 25 is used. In step 300, the actuator 10 then adjusts the steering angle at a rotation speed that deviates from the nominal rotation speed, i.e., at a lower rotation speed, due to the previously determined limit value r_sm_lim. Since the rotation speed of the spindle nut 25 is lower than the nominal rotation speed, resonance of the spindle 27 is prevented in the current case. The limit value r_sm_lim is maintained at least until the vehicle speed v_cur increases above a threshold value, for example, greater than or equal to 1 km / h. If it is stored in the controller, for example, via a characteristic curve, from which the rotation speed limited by the limit value r_sm_lim can be increased to the nominal rotation speed again, the control unit controls the actuator 10 so that the rotation speed of the spindle nut 25 gradually increases from the limited rotation speed r_sm_lim to the nominal rotation speed. Depending on the duration of the rotation speed limitation, the rotation speed of the spindle nut 25 is temporarily increased to a rotation speed that exceeds the nominal rotation speed. As a result, the change in the steering angle now requested can be compensated, as if the speed limit value r_sm_lim was not used and the spindle nut was already running at the nominal speed.

[0055] The consideration of the vehicle parameters shown above for determining the limit value of the rotation speed of the spindle nut 25 only shows one possibility in the sense of an exemplary embodiment. The other parameters mentioned here can also be considered individually or in any combination to determine the limit value of the rotation speed of the spindle nut 25.

[0056] The method steps according to the invention can be performed repeatedly and in a sequence different from that given. Therefore, the invention is not limited to the sequence mentioned here.

[0057] List of Reference Numerals

[0058] 1 Axle

[0059] 2 Subframe

[0060] 3 Connecting rod

[0061] 4 Connecting rod

[0062] 5. Wheels

[0063] 6 Wheels

[0064] 8 Wheel steering angle

[0065] 9 Wheel steering angle

[0066] 10 Actuator

[0067] 12 Steer-by-wire

[0068] 20 Screw drive

[0069] 21 Shell

[0070] 22 Electric Motor

[0071] 23 Tie rod

[0072] 24 Tie rod

[0073] 25 Screw nut

[0074] 27 Steering rod, lead screw

[0075] 29 Rolling bearings

[0076] 30 Pulley

[0077] 32 Drive pinion

[0078] 34 Traction device, (toothed) belt

[0079] 100 Interface

[0080] 110 Interface

[0081] 120 Interface

[0082] 130 Interface

[0083] 140 Interface

[0084] 150 Interface

[0085] 200 Get Steps

[0086] 220 Comparison steps with characteristic curves

[0087] 240 Steps for determining the limit value (rotation speed of the screw nut)

[0088] 250 Actuator Control Procedure

[0089] 300 Adjust steering angle

[0090] c_sit Vehicle status

[0091] F ext force

[0092] Lw_req_cur Steering angle request

[0093] r_sm_cur Speed ​​of the screw nut

[0094] r_sm_lim speed limit

[0095] RL v Front (wheel) steering angle

[0096] RL h Rear (wheel) steering angle

[0097] RLv _mom front (wheel) steering angle

[0098] RL h _mom rear (wheel) steering angle

[0099] S Longitudinal axis

[0100] s_1 sensor

[0101] s_2 sensor

[0102] SG Control Unit

[0103] S_pos Screw position

[0104] v_cur speed

[0105] vib_cur vibration value

Claims

1. A method for operating a steer-by-wire device (12) of a motor vehicle at a speed from a standstill to a parking and / or maneuvering speed, wherein: The steer-by-wire device comprises an actuator (10) having a screw drive (20) for axially moving a screw (27), wherein the method comprises at least the following steps: - obtaining the rotation speed (r_sm_cur) of the screw nut (25) of the screw drive (20); -Get vehicle speed (v_cur); - determining a limit value (r_sm_lim) for the rotation speed of the spindle nut (25) at least as a function of the vehicle speed (v_cur); - controlling the actuator to adjust the steering angle (RLw) of at least one wheel (5, 6) on the axle using the limit value (r_sm_lim) v RLw h ).

2. The method according to claim 1, characterized in that In the determination step, the limit value (r_sm_lim) is at least temporarily changed, preferably reduced or cancelled.

3. The method according to any one of the preceding claims, characterized in that In the determining step, the limit value (r_sm_lim) is set to a predetermined minimum value or maximum value.

4. The method according to any one of the preceding claims, characterized in that In a further step, at least one steering angle request (Lw_req_cur) and / or a spindle position (S_pos) and / or a vehicle state (c_sit) is determined, which is to be taken into account in the step of determining the limiting value (r_sm_lim).

5. The method according to any one of the preceding claims, characterized in that Vibrations of the actuator (10) are detected by at least one sensor (s_1, s_2) and are taken into account in the step of determining the limit value (r_sm_lim).

6. The method according to any one of the preceding claims, characterized in that In determining the limit value (r_sm_lim), it is preferred that at least one of the current steering angle request (Lw_req_cur) and / or the current set steering angle (RLw v _mom, RLw h _mom) and / or the current vehicle speed (v_cur) to temporarily change, preferably increase, the steering gradient.

7. The method according to any one of the preceding claims, characterized in that The rotational speed (r_sm_cur) and / or the current speed of the vehicle (v_cur) are detected at least at intervals, wherein a limit value (r_sm_lim) for the rotational speed of the spindle nut (25) is eliminated as a function of the vehicle speed (v_cur).

8. The method according to claim 7, characterized in that After the limit value (r_sm_lim) is cancelled, preferably according to the currently set steering angle (RLw v _mom, RLw h _mom) and / or the expected change in the steering angle request (Lw_req_cur) and / or the vehicle speed gradually returns to the target value of the rotational speed of the spindle nut.

9. The method according to any one of the preceding claims, characterized in that In order to determine the limiting value (r_sm_lim) in the determination step, vehicle parameters are used which are determined on the basis of at least one characteristic curve, which is in particular stored in the control unit (SG).

10. A control unit (SG) for operating a steer-by-wire device (12) of a motor vehicle according to the method as claimed in any one of the preceding claims, wherein: The control unit has the following features: An interface for at least indirectly acquiring the rotational speed of the spindle nut (25), which represents the current rotational speed (r_sm_cur); An interface for obtaining a vehicle speed representing the current speed of the motor vehicle (v_cur); An interface for acquiring a steering angle, which represents the current steering angle (RLw) of at least one wheel (2, 3) of the motor vehicle v _mom, RLw h _mom); an interface for obtaining a steering angle request (Lw_req_cur) representing a current steering angle change due to a driver request or a steering angle change determined by the control unit (SG) or another control unit; a unit for determining a limit value for the rotational speed of a spindle nut (25) of an actuator (10) of a steer-by-wire device (12), the limit value representing a limit value (r_sm_lim); an actuator for controlling the steer-by-wire device to adjust the steering angle (RLw) of at least one wheel (5, 6) using the limit value (r_sm_lim) v RLw h ) unit.

11. A steer-by-wire device, the steer-by-wire device being operated according to the method of any one of the preceding claims 1 to 9, wherein: The steer-by-wire device (12) is preferably designed as a rear-axle steering device.

12. Computer program, arranged to perform the method according to any one of the preceding claims 1 to 9, wherein: The computer program is preferably executed on a control unit (SG) according to claim 10 .

13. A machine-readable storage medium having stored thereon a computer program according to claim 12.

Images