Abstract
A spindle drive (20) for an actuator of a steer-by-wire steering system (10). The spindle drive has a linearly displaceable threaded spindle (27). The spindle drive (20) is in the form of a roller screw drive. The roller screw drive has a firs efficiency in the driving direction and a second different efficiency in the reverse-driving direction.
Claims
1-9. (canceled)
10. A spindle drive (20) for an actuator (10) of a steer-by-wire steering system, comprising: a linearly displaceable threaded spindle (27), the spindle drive (20) being in the form of a roller screw drive, and the roller screw drive having different efficiencies in a driving direction and in a reverse-driving direction.
11. The spindle drive (20) according to claim 10, wherein in the driving direction the spindle drive (20) has an efficiency high enough so that the spindle drive (20) is not self-locking in the driving direction.
12. The spindle drive (20) according to claim 10, wherein the efficiency of the roller screw drive, in the reverse-driving direction, is low enough so that the spindle drive (20) is essentially self-locking.
13. The spindle drive (20) according to claim 12, wherein, when a maximum substantially axial force acts upon the threaded spindle (27) in the reverse-driving direction, a minimal torque is produced in the spindle drive (20) which corresponds approximately to a stall torque of the drive motor (22) and is equal to the stall torque of the drive motor (22).
14. The spindle drive (20) according to claim 10, wherein the roller screw drive is configured with a roller return.
15. The spindle drive (20) according to claim 10, wherein an inhibiting unit is provided, which is actuated electromagnetically to lock the spindle drive (20), and which produces at least one of an interlocking and a frictional coupling between the actuator (10) and at least one of the spindle and a spindle nut (25).
16. The spindle drive (20) according to claim 10, wherein the roller screw drive is designed to be play-free.
17. The spindle drive (20) according to claim 10, wherein the electric motor is a Vernier motor.
18. A steer-by-wire steering system for a motor vehicle made with an actuator having a spindle drive (20) according to claim 10.
19. A spindle drive for an actuator of a steer-by-wire steering system, the spindle drive comprising: a linearly displaceable threaded spindle, the spindle drive (20) being a roller screw drive having a first efficiency in a driving direction and a different second efficiency in a reverse-driving direction.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Below, the invention is described with reference to preferred embodiments and to the drawings, which show:
[0020] FIG. 1: A schematic view from above, of a rear axle of a vehicle,
[0021] FIG. 2: A schematic view of an actuator and a steer-by-wire steering system,
[0022] FIG. 3: A schematic exploded representation of the spindle drive, designed as a roller screw drive with roller return.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The schematic representation according to FIG. 1 shows a vehicle axle 1, in this case represented as a rear axle with a subframe 2, which is fixed to a vehicle structure or which forms the latter and is connected to the body of a motor vehicle. The wheels 5 and 6 are articulated to the subframe 2 by means of control arms 3. The control arms 3 form the wheel suspension for the wheels 5, 6. To the subframe 2 is attached an actuator of a steer-by-wire steering system. The actuator is fixed by its housing 21 to the subframe. In this embodiment, the actuator 10 is a central actuator with a through-going threaded spindle 27, which passes through the housing 21 of the actuator 10. At the ends of the threaded spindle 27 track-rods 23 are articulated, which at the end remote from the actuator 10 are in each case articulated to the wheel carrier (not shown) of the wheels 5 and 6. It can be seen clearly that axial displacement of the threaded spindle 27 to the left or right direction will bring about a change of the wheel steering angle, because the track-rods 23 form a forced connection between the wheel or wheel carrier and the actuator 10. To steer the wheels 5, 6, they are articulated rotatably about their vertical axis on the wheel suspension 3, as indicated by the curved double-arrows under the wheels 5, 6.
[0024] FIG. 2 shows a schematic representation of the actuator 10 already shown n FIG. 1. The actuator 10 has a housing 21, on which a drive motor 22 with its axis parallel is arranged. In the housing 21 is arranged the spindle drive 20, which comprises the spindle nut 25 and the threaded spindle 27. The spindle nut 25 is mounted positionally fixed but able to rotate relative to the housing by virtue of a roller bearing 29. The threaded spindle 27 passes through the spindle nut 25 coaxially with it. On the side of the spindle nut 25 facing away from the roller bearing 29, a belt wheel 30 is arranged positionally fixed on the spindle nut 25. The electric motor 22 has a drive pinion 32. A drive belt 34 in the form of a toothed belt is wrapped around both the drive pinion 32 and the belt wheel 30, so that when the electric motor or drive motor 22 rotates, the spindle nut 25 is caused to rotate about the longitudinal axis a. Depending on the rotational direction of the spindle nut 25, linear displacement of the threaded spindle 27 takes place in one direction or the other along the longitudinal axis a, as indicated by the double-arrow.
[0025] FIG. 3 shows an exploded representation of the roller screw drive with roller return conceived for use in the actuator 10 according to FIGS. 1 and 2. On its inside the spindle nut 25 has an internal thread 26. On the inside of the thread 26 there is in addition arranged an axially extending projection 49. A cage 46 has cut-outs 44a arranged concentrically relative to the longitudinal axis a, and which receive threaded rollers 44. The cut-outs 44a are arranged as elongated slots on the outside of the cage 46. When the cage 46 with the threaded rollers 44 is inside the spindle nut 25, the cage 46 and thus also the threaded rollers 44 are held in position relative to the spindle nut 25 by means of the holding rings 47a, 47b arranged at the ends or faces of the spindle nut 25. When the spindle nut 25 rotates around the threaded spindle 27, then due to the fact that the threaded rollers 44 engage both with the external thread of the spindle 27 and with the internal thread 26 of the spindle nut 25, an axial displacement of the threaded spindle 27 is enabled. This axial displacement corresponds to the driving direction of the threaded spindle. After one turn of the spindle nut 25 the threaded rollers 44, as a function of their thread pitch, will have covered a certain axial path relative to the spindle nut 25. Thus, these have to be returned. In this embodiment, they are lifted by the projection clear off the internal thread and returned axially by a further axial projection (not shown) on the holding ring 47a and thereafter again come into engagement with the internal thread 26 of the spindle nut 25.
[0026] In addition a wheel 5, 6 is shown schematically, which can be steered about a vertical axis h. The wheel 5, 6 is mounted rotatably on a wheel carrier 50. The wheel carrier 50 is articulated to a track-rod 23, which at its opposite end is articulated to the end of the threaded spindle 27. If now a lateral force F.sub.s acts on the threaded spindle 27, for example while driving round a curve, this is a force in the reverse-driving direction on the spindle drive 20 which, due to the low efficiency, produces only a very small torque on the spindle nut 25. That torque can be compensated by virtue of the stall torque of the electric motor, so that in the event of a lateral force the wheel steering angle does not change because in the reverse-driving direction the spindle drive is in that way designed to be self-locking. The spindle nut 25 is not rotated.
[0027] The actuator 10 shown as an example in FIGS. 1 and 2 is a central actuator which acts on both sides and can produce a steering movement on both of the wheels on an axle as shown in FIG. 1. However, the invention can be used just as well with a so-termed single actuator. In the case of a single actuator the spindle of the single actuator moves in and out of the housing of the single actuator, or in other words the length of the single actuator is variable. If the housing is fixed to the body, for example arranged on or supported by a subframe, then the length variation brings about an adjustment movement in the sense of a steering movement on the wheel carrier of a wheel so that a change of the wheel steering angle of the wheel concerned can take place. In this case a control arm can also be arranged between the wheel carrier and the single actuator.
Indexes
[0028] 1 Vehicle axle
2 Subframe, vehicle body
3 Control arm, wheel suspension
5 Wheel
6 Wheel
10 Actuator
[0029] 20 Spindle drive
21 Housing
[0030] 22 Drive motor, electric motor
23 Track-rod
25 Spindle nut
[0031] 26 Internal thread
27 Threaded spindle, spindle
29 Roller bearing
30 Belt wheel
32 Drive pinion
34 Drive belt
44 Threaded rollers
44a Cut-outs
45 Spindle nut
46 Cage
[0032] 47a Holding ring
47b Holding ring
49 Projection
[0033] 50 Wheel carrier
a Longitudinal axis
h Vertical axis
