DE102021213941B3 - Actuator of a steer-by-wire steering and steer-by-wire steering - Google Patents

Abstract

Actuator (10) of a steer-by-wire steering system, comprising a spindle (124, 224) with an external thread (224a), a spindle nut (50, 250) with an internal thread (250i1, 250i2), the external thread (224a) and the Internal threads (250i1, 250i2) are designed as a motion thread for translational displacement of the spindle (124, 224). The actuator is characterized by a movement thread, which is designed as a flat thread, the spindle (124, 224) being offset axially parallel to the spindle nut and the offset (V) being smaller than the maximum thread depth (t).

Description

The invention relates to an actuator for a steer-by-wire steering system and a steer-by-wire steering system and a rear-axle steering system according to the preambles of the independent patent claims.

From the DE10 2014 206 934 A1 an actuator of a steer-by-wire steering system with a spindle drive as a rotation/translation converter consisting of a spindle and a spindle nut with a self-locking trapezoidal thread for a rear-wheel steering system of a motor vehicle became known. Due to this purpose, the spindle is not only loaded by axial forces, but also subjected to bending due to lateral forces occurring in the chassis. In the self-centering trapezoidal thread of the spindle drive, the bending causes an increased notch effect in the motion thread. Despite the selection of suitable lubricants, the flanks of the spindle and nut threads can become wedged, which can result in jamming and increased resistance, ie increased drive torque or increased wear and tear and other disadvantages. Due to the self-centering, a continuous supply of lubricant between the flanks is suboptimal, since lubricant can be deposited that is no longer available for the spindle and nut thread.

Furthermore, from the U.S. 2021/0 309 277 A1 an adjusting drive of a steering column for a motor vehicle is known, which has a movement thread with a flat thread. The DE 10 2008 002 289 A1 discloses a power steering system for a vehicle and also uses a flat thread. DE 20 2012 010 203 U1 discloses a lead screw with integrated bearing units in the nut. The nut can be designed as a two-part nut unit with a flat thread.

It is an object of the invention to specify an improvement in the movement thread for an actuator of a steer-by-wire steering system.

The problem on which the invention is based is solved by the independent patent claims. Advantageous configurations result from the dependent claims.

According to a first aspect of the invention, an actuator of a steer-by-wire steering system is provided, which comprises a spindle with an external thread and a spindle nut with an internal thread. The internal thread, together with the external thread of the spindle, forms a motion thread for translational displacement of the spindle along its longitudinal axis when the spindle nut, which is mounted in a stationary manner relative to the actuator, is set in rotary motion. The spindle in its function as a steering rod is coupled with at least one end indirectly via at least one joint or directly via at least one link and a joint to at least one wheel carrier, which rotatably accommodates a wheel. The translatory displacement of the spindle allows at least one wheel carrier of an axle and thus at least one wheel to be steered about its vertical axis.

A steering system in a motor vehicle must generally be able to handle the high forces in the chassis of a motor vehicle which act on the steered vehicle axle and thus on the wheel steering due to lateral forces, for example when cornering or lateral acceleration of the vehicle. These high forces of 5 to 15 kN, for example, act equally on a steer-by-wire steering system and the associated actuator, which causes the setting movement or steering movement. There is a requirement for wheel steering that it must be designed to guide the wheel so that the set wheel steering angle and thus the intended travel path can be maintained. This means that a wheel steering angle must always be maintained when high lateral and/or lateral forces and the resulting forces and moments act on the steering. This is especially the case while driving. The spindle of an actuator is therefore designed as a wheel-guiding component. If the actuator fails, it must be able to hold the wheel steering angle in the current position. In the case of steer-by-wire steering, for example, acting as an additional steering system on the rear axle in addition to the usual steering system on the front axle, a return to a steering angle of 0 degrees, ie in the direction of the longitudinal axis of the vehicle, may be required in the event of a fault. This steering angle must then also be maintained after the return, because otherwise the steering angle on the rear axle could change in an uncontrolled manner due to dynamic influences and the vehicle is more difficult to control. This retention can be provided by a lock within the actuator, but this adds weight and cost. Preferably, the movement thread is therefore designed with such an efficiency that if the spindle drive fails, there is self-locking, which is why trapezoidal threads are used in a known manner for movement threads in actuators of a steer-by-wire steering system, for example according to DIN 103. Such trapezoidal threads have per These have a low level of efficiency and therefore a steer-by-wire steering system can be designed to be self-locking in order to limit, reduce or completely prevent the unwanted adjustment of the steering angle in the event of a fault. Because of their geo geometry of the flanks, the spindle and the spindle nut of a trapezoidal thread center themselves under load in the axial direction, i.e. along their longitudinal axis. However, during operation of the actuator, the lubricant used tends to accumulate radially outwards in the direction of the root circle of the internal thread of the spindle nut. This means that the lubricant is not optimally available in the movement thread between the flanks of the spindle and the spindle nut.

In order to retain the advantages of self-locking and at the same time at least maintain the efficiency of the motion thread, it has surprisingly been found that a flat thread can be used as a motion thread for the spindle drive of the actuator of a steer-by-wire steering system. In contrast to a trapezoidal thread, a flat thread has a profile shape such that the flanks of the spindle and spindle nut run almost, preferably exactly, parallel to one another. The flank angle is preferably less than 3°, most preferably in the range of 2°-0°. There are no filigree edges such as with a conventional trapezoidal or saw thread. In contrast to the trapezoidal thread according to the DIN103 standard with a flank angle of 30°, a notch effect is therefore hardly possible with a flat thread, so that much higher forces can be absorbed or transmitted with the same dimensions of the movement thread. Due to the almost or exactly parallel flanks, a spindle drive with a flat thread is very little or no longer self-centering under axial load. The latter applies to a flank angle of 0°, which corresponds to the parallel course of the flanks. Due to the engagement of the flanks of the spindle and the spindle nut, however, a parallel course from the spindle nut to the spindle is forced in an advantageous manner. This is favored by the rotation of the spindle nut around the spindle while the actuator is in operation.

Surprisingly, it has been shown that in the case of an actuator of a steer-by-wire steering system, forming the movement thread as a flat thread can improve the supply of lubricant. Due to the parallel thread partner, the spindle can be arranged offset to the stationary spindle nut in the actuator without any problems, since such an influence of a centering as with a standard-compliant trapezoidal thread with a 30° flank angle does not exist. In other words, the spindle is deliberately arranged in the actuator in such a way that its longitudinal axis runs offset parallel to the longitudinal or rotational axis of the spindle nut. The spindle is thus offset parallel to the axis relative to the spindle nut. The flat thread has a symmetrical thread cross-section and can therefore be loaded equally in both axial directions.

Due to the axially parallel arrangement mentioned above, the flanks of the spindle nut do not mesh concentrically with the flanks of the spindle. In a first partial area, the overlapping of the flanks of the spindle nut with flanks of the spindle is at its maximum. In this area, the spindle and the spindle nut are closest to each other, or in other words at a minimum distance from each other. The overlap is minimal in a radially opposite second area. In this area, the spindle and the spindle nut are at a maximum distance from each other. Lubricant, which could be deposited in the second area, is caught at this point by the flanks of the internal thread of the spindle nut and transported along with it every time the spindle nut rotates around the spindle. The circulating movement of the internal thread of the spindle nut around the external thread of the spindle during operation of the actuator causes the lubricant to be constantly transported in the sense of an even distribution in the movement thread. The aforementioned deposit of the lubricant in the second area is therefore also of a more theoretical nature. Advantageously, a constant supply of lubricant to the movement thread is ensured.

The offset is formed depending on the thread depth. The offset corresponds approximately, preferably at most, to the thread depth of the spindle. Taking into account the usual tolerances when producing the movement thread as a flat thread with a view to cost-effective production of the thread, ie the internal thread of the spindle nut and the external thread of the spindle, the offset is preferably designed to be slightly smaller than the maximum thread depth. The distance between the tip circle of the spindle and the root circle of the spindle nut is preferably tolerated with a clearance fit in this area. This counteracts contact and possible jamming of the movement thread on the tip circle of the external thread with the root circle of the spindle nut thread (internal thread) in the aforementioned first area.

The rotary movement of the spindle nut is preferably brought about by means of an electric motor, preferably with the interposition of a gear, in particular a belt drive, preferably with a toothed belt. When using a toothed belt, slipping and/or skipping of the belt is effectively counteracted. The unit consisting at least of spindle nut and spindle is also called spindle drive or spindle called drive. The actuator preferably has a housing in which the spindle nut is mounted in a rotatable and stationary manner by means of a roller bearing. The spindle is arranged in a rotationally fixed manner relative to the housing and can only be displaced along its longitudinal axis. The electric motor can be arranged coaxially to the spindle or parallel to it. In the case of the parallel arrangement, the motor can be coupled to the housing, preferably in a non-positive manner, and the motor is preferably screwed to the housing.

The internal thread of the spindle nut is preferably not continuous but divided. In relation to the axial extent of the spindle nut, at least two partial areas are provided, which provide threads which are arranged axially one behind the other but are separate from one another. The partial areas can be arranged, for example, in the outer areas of the spindle nut. This design allows the friction in the movement thread to be further reduced, so that the efficiency of the movement thread in the actuator can be further increased. In a further embodiment, a partial area of the thread can be formed in a threaded ring. The threaded ring, in turn, can be arranged so that it can move axially within the spindle nut and is supported in relation to the spindle nut by means of an energy accumulator. A split spindle nut can thus be formed, with the support of the threaded ring relative to the spindle nut being able to minimize any play between the two threads of the spindle and spindle nut.

The respective thread of the spindle and/or the spindle nut is preferably produced by means of a cutting process, such as thread cutting or thread milling or thread turning, which is also used in the production of trapezoidal threads. A grinding step can preferably follow the above-mentioned methods in order to obtain a required accuracy. The whirling process can also be used especially for the external thread. This results in good wear resistance, tensile strength and reverse bending strength for the spindle as a steering rod. The grinding results in a high surface quality, which means less susceptibility to corrosion. Since steering systems for motor vehicles are produced in large quantities, economical production is also possible using high-quality methods for producing spindles and nuts in an actuator of a steer-by-wire steering system. A high profile accuracy of the thread and also a high accuracy of the flank diameter (parallelism) can be produced by means of final grinding. The thread rolling thus results in a cost-effective, high-quality motion thread, as is necessary for an actuator of a steer-by-wire steering system.

According to a further aspect of the invention, a steer-by-wire steering system is proposed which includes an actuator according to one or more of the embodiments described above. The steer-by-wire steering is preferably used in a rear-wheel steering system of a motor vehicle. Since steering systems of this type are safety-relevant systems, a fail-safe system for steer-by-wire steering can be provided when using an actuator with a movement thread designed as a flat thread.

A steer-by-wire steering system is a mostly electromechanical unit that is mechanically decoupled from a mechanical steering handle, e.g. a steering wheel. Steering signals are generated in a control unit based on steering signals and one or more parameters such as vehicle speed, steering wheel angle, current steering angles on the front and/or rear axle, yaw acceleration and/or lateral acceleration of the vehicle, etc. The steering movement takes place by means of at least one actuator of the steer-by-wire steering, which receives steering signals from the control unit. As mentioned above, a spindle or steering rod can be linearly displaced in the actuator by means of a spindle drive, which is directly or indirectly coupled in an articulated manner to wheel carriers. By shifting the spindle, the wheel carriers can be pivoted about their vertical axis, so that the wheels rotatably mounted on the wheel carriers can be subjected to a change in the wheel steering angle of the respective wheel carrier.

Exemplary embodiments of the invention are shown in the drawing and are described in more detail below, with further features and/or advantages being able to result from the description and/or the drawing. The invention is not limited to the following explanations.

• 1 eine Fahrzeugachse mit Steer-by-wire-Lenkung,
• 2 einen Aktuator einer Hinterachslenkung mit einem Spindelantrieb,
• 3 eine geschnittene Darstellung eines Spindelantriebes mit einem Flachgewinde

Show it

• 1 a vehicle axle with steer-by-wire steering,
• 2 an actuator of a rear axle steering with a spindle drive,
• 3 a sectional view of a spindle drive with a flat thread

1 shows a schematic representation of a vehicle axle 1 with a steer-by-wire steering 12 in plan view. The vehicle axle is designed here as a rear axle, which is attached to a vehicle body by means of an auxiliary frame 2 of a motor vehicle is attached. The wheels 5 and 6 are articulated on the subframe 2 by means of links 3, 4, designed here as wishbones. The links 3, 4 are part of the wheel suspension for the wheels 5, 6. An actuator 10 of a steer-by-wire steering system 12 is arranged on the auxiliary frame 2. The actuator 10 is attached to the auxiliary frame 2 with its housing 22 . In the present embodiment, the actuator 10 has a steering rod 24 in the form of a spindle as a central actuator, which is guided through the housing 22 of the actuator 10 and at the end of which articulated connections 27 are provided for connection to tie rods 23, 25. The drive motor in the form of an electric motor 29 is arranged parallel to the steering rod 24 . The axial displacement of the steering rod 24 takes place due to a rotation/translation converter, which 2 is shown as a spindle drive 30. In the event of an axial displacement, i.e. a displacement of the steering rod 24 along its longitudinal axis s in one direction or the other, the wheel steering angle changes because the tie rods 23, 25 represent a forced connection between the wheel 5, 6 or wheel carrier 7 and the actuator 10 (for simplification only the wheel carrier of the left wheel 5 is shown). To steer the wheels 5, 6, they are pivoted by means of wheel carriers 7 about their vertical axis to the wheel suspension by means of links 3, 4, as indicated by double arrows (direction of rotation 8, 9), which show the rotation about the vertical axis. This results in a steering movement for the wheels 5, 6.

2 12 shows the actuator 10 of the steer-by-wire steering system 12 in a partially cut-out detailed view with a housing 22 which can be fastened to the vehicle body on the vehicle side by means of the tabs 22a, 22b shown. A spindle drive 30 is arranged inside the housing 22 and has a spindle nut 50 which can be driven in the direction of rotation and is in engagement with the spindle 124 . Spindle and spindle nut thus form a moving thread. The spindle nut 50 can be driven in the direction of rotation by an electric motor 29 via a belt drive 60 and is mounted stationary and rotatable in the housing 22 via a roller bearing 40 . By rotating the spindle nut 50, the spindle 124 can be displaced in one direction or the other. The spindle 124 is connected at both ends to bearing sleeves 70, 75 guided on the housing side, which in turn are connected to articulated connections 27, 28 arranged at the ends of the housing 22 in the form of pivot pins. The actuator 10 is designed as a so-called central actuator, ie it acts simultaneously on the steering of both rear wheels and can be arranged, for example, in the center of the vehicle.

3 shows a sectional view of the spindle drive 30 of an actuator 2 , shown here as a spindle drive 230, a spindle nut 250 with its divided internal thread 250 _i1 , 250 _i2 being in engagement with the external thread 224a of the spindle 224 . The spindle nut 250 is stationarily supported in the actuator (not shown) by means of a roller bearing 240 . The split internal thread 250 _i1 , 250 _i2 of the spindle nut 250 and the spindle 224 form a movement thread of the spindle drive 230 . Seals 244 , 245 in the form of shaft sealing rings are arranged on the end face of the spindle nut 250 , which minimize the escape of lubricant outside the limits of the spindle nut 250 . The motion thread is designed as a flat thread with a 0° flank angle. This results in a parallel alignment of the spindle nut 250 with respect to the spindle 224. This results in a profile shape in which the flanks of the spindle 224 and the spindle nut 250 run parallel to one another. Thus, on the contacting flanks of the engaged threads of spindle nut 250 and spindle 224, there are no filigree edges in contact, as is the case, for example, with a trapezoidal or buttress thread. A notch effect is not possible here, so that the movement thread as a flat thread is well suited to the bending stresses. Due to the engagement of the flanks of the threads of spindle 224 and spindle nut 250, there is additionally and advantageously a parallel course from spindle nut 250 to spindle 224, because the flank angle here is 0°.

Due to the parallel thread partner, the spindle 224 can be arranged offset to the stationary mounted spindle nut 250 in the actuator. In other words, the spindle 224 is not concentrically arranged in the actuator with respect to the spindle nut. Instead, the spindle 224 is deliberately arranged in such a way that its longitudinal axis s runs offset parallel to the longitudinal or rotational axis m of the spindle nut 250 . The spindle 224 is thus arranged axially parallel to the spindle nut 250 . Due to this arrangement, the flanks of the spindle nut 250 do not mesh concentrically with the flanks of the spindle 224. In a first partial area T ₁ , T ₂ the overlapping of the flanks of the spindle nut 250 with flanks of the spindle is at its maximum. In this area, the spindle 224 and the spindle nut 250 are closest to each other or the distance is minimal here. In a radially opposite second partial area T ₃ , T ₄ the overlap is minimal and consequently the distance from the spindle 224 to the spindle nut 250 is at a maximum here. Lubricant, which could be deposited in the second partial area T ₃ , T ₄ , is caught and transported at this point by the flanks of the internal thread 250 _i1 , 250 _i2 of the spindle nut 250 with each revolution of the spindle nut 250 around the spindle 224 . The circumferential movement of the internal thread 250 _i1 , 250 _i2 of the spindle nut 250 around the external thread 224a enables and promotes constant transport of the Lubricant in terms of a uniform distribution in the motion thread during operation of the actuator. Advantageously, a constant supply of lubricant to the motion thread is ensured, so that the actuator can be operated with as little friction as possible and for a long service life.

The longitudinal axis m of the spindle nut 250 is spaced by the offset V from the longitudinal axis s of the spindle 224 . The possible offset V is designed depending on the thread depth. The offset V corresponds here approximately to the maximum thread depth of the spindle. Taking into account the usual tolerances in the manufacture of the moving thread, the offset V is designed to be minimally smaller than the maximum thread depth. Contact of the movement thread between the tip circle of the external thread 224a and the root circle of the spindle nut thread 250 _i1 , 250 _i2 (internal thread) in the first partial area T ₁ , T ₂ is counteracted. For better understanding, the tip circle diameter d _K of the spindle 224, the root circle diameter d _F of the spindle 224 and the root circle diameter D _F of the spindle nut 250 are shown, the latter being larger than the tip circle diameter d _K of the spindle 224. The distance between the tip circle of the spindle 224 and the root circle of the spindle nut 250 is tolerated with a loose fit in the partial area T ₁ , T ₂ , so that contact between the tip circle of the spindle 224 and the root circle of the spindle nut 250 is ruled out. Jamming in the first partial area T ₁ , T ₂ is thus excluded.

Seen overall, the result is an actuator for a steer-by-wire steering system which is suitable for high loads in the chassis of a motor vehicle due to lateral and transverse forces. By improving the supply of lubricant, a durable and low-friction actuator can be created.

Reference List

1

vehicle axle

2

subframe

3, 4

handlebars

5, 6

wheel

7

wheel carrier

8, 9

sense of rotation

10

actuator

12

Steer-by-wire steering

22

Housing

23

tie rod

24

handlebar

25

tie rod

27, 28

joint connection

29

electric motor

22a, 22b

tabs

30

spindle drive

40

roller bearing

50

spindle nut

60

belt drive

70, 75

bearing sleeve

124

spindle

224

spindle

224a

external thread

230

spindle drive

240

roller bearing

244, 245

seals

250

spindle nut

250i1

inner thread

250i2

inner thread

dK

tip circle diameter, spindle

dF

root diameter, spindle

DF

root diameter, nut

m

Longitudinal axis/axis of rotation Spindle nut

s

Longitudinal axis of the spindle

V

offset

T1, T2

subarea

T3, T4

subarea

Claims (6)

Actuator (10) of a steer-by-wire steering (12), comprising a spindle (124, 224) with an external thread (224a), a spindle nut (50, 250) with an internal thread (250 _i1 , 250 _i2 ), the external thread (224a) and the internal thread (250 _i1 , 250 _i2 ) are designed as a movement thread for the translational displacement of the spindle (124, 224), characterized in that the movement thread is designed as a flat thread, the spindle (124, 224) being opposite to the Spindle nut (50, 250) is offset parallel to the axis and the offset (V) is smaller than the maximum thread depth. Actuator (10) after a steer-by-wire steering (12). claim 1 , characterized , that the flat thread has a flank angle of less than 3°, preferably in the range from 2° to 0°, most preferably 0°. Actuator (10) of a steer-by-wire steering system (12) according to one of the preceding claims, characterized in that the internal thread (250i1, 250i2) of the spindle nut (250) is of divided design. Actuator (10) of a steer-by-wire steering system (12) according to one of the preceding claims, characterized in that the external thread (224a) of the spindle (224) and/or the internal thread (250i1, 250i2) of the spindle nut (250) produced by means of a cutting process, preferably thread cutting or milling or turning, preferably produced with subsequent grinding. Steer-by-wire steering (12), in particular for steering a motor vehicle, characterized by an actuator (10) according to one of the preceding claims. Rear axle steering designed as a steer-by-wire steering (12). claim 5 .

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