STEERING GEAR FOR A STEER-BY-WIRE STEERING SYSTEM

Description

[0001] The present invention relates to a steering gear for a steer-by-wire steering system of a motor vehicle having the features of the preamble of claim 1 and a steer-by-wire steering system having the features of the preamble of claim 7.

[0002] In steer-by-wire steering systems, the position of the steered wheels is not directly coupled to the steering input device, such as a steering wheel. A connection exists between the steering wheel and the steered wheels via electrical signals. The driver's steering input is detected by a steering angle sensor, and the position of the steered wheels is controlled via a steering actuator depending on the driver's steering input. It is known to provide two steering actuators, each assigned to one of the steerable wheels and configured to adjust the steering angle of the respective wheel. A rack-and-pinion steering gear can also be provided, which engages a pinion driven by a steering actuator.

[0003] In electromechanical steering gears, it is known to use axially parallel ball screws to convert the motor torque of an auxiliary drive into a linear movement of the rack. The connection of the rack to the steering pinion allows axial movement of the rack, while preventing rotation of the rack. The ball screw is thus designed with a driven and rotatably mounted ball nut. German Patent Application DE 10 2012 015 181 A1 discloses an electromechanical motor vehicle steering system that has an auxiliary drive parallel to the rack, comprising a spindle and a ball nut mounted thereon. The rack and spindle are connected to each other at their ends via an end plate, preventing any axial displacement. A connection to the tie rods can be realized near the ends.

[0004] Such ball screws (or roller screws or trapezoidal spindle drives) absorb not only axial forces, but also radial forces and tilting moments introduced via the tie rods. This results in unfavorable loading of the screw drive and thus requires significantly larger dimensions than with primarily axial loading. Despite design measures to reduce tilt sensitivity (e.g., by compliant connection of the ball screw drive with wave springs), the friction behavior of the steering gear deteriorates significantly due to these load components.

[0005] From US 2008 / 0011537 A1, an electromechanical steering system, which can also be designed as a steer-by-wire steering system, is also known, in which an electric motor acts on the rack of the steering system via a ball screw to provide steering assistance.

[0006] Other electromechanical steering systems in which steering assistance is applied to the rack of the steering system by an electric motor via a screw drive are known from US 2007 / 0089926 A1 and US 4,742,882 A.

[0007] Document US4653602 discloses a steering gear according to the preamble of claim 1.

[0008] It is an object of the present invention to provide a steering gear for a steer-by-wire steering system of a motor vehicle which has a small installation space requirement and enables better friction behavior of the steering gear.

[0009] This object is achieved by a steering gear for a steer-by-wire steering system of a motor vehicle having the features of claim 1 and a steer-by-wire steering system of a motor vehicle having the features of claim 7.

[0010] Accordingly, a steering gear for a steer-by-wire steering system of a motor vehicle is provided with an electric motor having a motor shaft, wherein the motor shaft drives a spindle of a helical gear, and the helical gear comprises a spindle nut such that a rotary movement emanating from the motor shaft is converted into a linear movement of the spindle nut along an axis, wherein the spindle nut is connected to at least one tie rod for steering wheels of the motor vehicle. In contrast to electromechanical steering systems, steer-by-wire systems do not require a steering pinion connection. According to the invention, the drive is provided via a rotatably mounted spindle, thereby saving installation space. The helical gear is preferably a ball screw drive, a trapezoidal screw drive, or a roller screw drive.It is preferably provided that the screw gear is arranged in a housing in which the spindle is rotatable and the spindle nut is mounted with an axial guide.

[0011] It is conceivable and possible that a rotation angle sensor is provided on the spindle or the motor shaft so that the rotation of the motor shaft or the spindle can be measured.

[0012] It is further conceivable and possible that a linear position sensor is provided which is arranged on the spindle nut or on another axially movable component of the steering gear and detects its displacement.

[0013] The direct connection of the spindle nut is preferably made to a single tie rod, in particular via a lever that is rigid in the axial direction, so that the linear movement of the spindle nut is transmitted to this tie rod.

[0014] Preferably, the lever is designed such that the axis of linear movement extends parallel to the direction of movement of the tie rod (or the pivot point of the tie rod). The lever is preferably straight and has no angles.

[0015] It is preferred if the lever is connected to an internal thread of the tie rod. Furthermore, the lever can be connected to the tie rod via a joint, allowing it to move in the axial direction. The joint is preferably an internal joint.

[0016] Furthermore, a steer-by-wire steering system for a motor vehicle is provided, comprising: an electronically controllable steering actuator acting on the steered wheels, a control unit, a feedback actuator which can be acted upon by a driver via a steering input means with a driver request for a steering angle and which outputs a feedback signal to the steering input means in response to the driver request and a driving state of the motor vehicle, a signal transmission device which transmits the driver request to the control unit, wherein the control unit controls the steering actuator in order to transform the driver's request into a deflection of the steered wheels, and wherein a steering gear comprising the steering actuator is designed according to one of the preceding embodiments.

[0017] In an advantageous embodiment of the steering system, the movement of the tie rod directly connected to the spindle nut is transmitted to a second tie rod of the steering system by means of a coupling rod extending parallel to the direction of movement of the spindle nut.

[0018] The electric motor is preferably arranged in the region of the coupling rod. The spindle drive and the electric motor with the control unit are preferably aligned parallel to the coupling rod, which is made possible by the lever. This is a particularly space-saving design. The electric motor has a motor shaft that is rotationally fixedly connected to a threaded spindle of the helical gear. The connection can preferably be made directly or via a coupling. It is preferably aligned. It can also be provided that the motor drives the threaded spindle via a reduction gear.

[0019] It is advantageous if the helical gear or the housing of the helical gear is sealed to the outside with a bellows.

[0020] Preferably, the coupling rod is arranged in a housing which is sealed to the outside by at least one bellows.

[0021] A preferred embodiment of the invention is explained in more detail below with reference to the drawings. Similar or similarly functioning components are designated by the same reference numerals in the figures. They show: Fig. 1: a schematic representation of a steer-by-wire steering system, Fig. 2: a block diagram of a control of the steer-by-wire steering system, and Fig. 3: a schematic representation of a steering gear.

[0022] In the Figure 1A steer-by-wire steering system 1 is shown. A steering shaft 2 is fitted with a rotation angle sensor (not shown), which detects the driver's steering angle applied by turning a steering input device 3, which in the example is designed as a steering wheel. However, a steering torque can also be detected. A joystick can serve as the steering input device. Furthermore, a feedback actuator 4 is fitted to the steering shaft 2, which serves to simulate the reactions from the road surface 70 on the steering wheel 3 and thus provides the driver with feedback about the steering and handling behavior of the vehicle. The driver's steering request is transmitted via the rotation angle α of the steering shaft 2 measured by the rotation angle sensor via signal lines to a feedback actuator monitoring unit 10, as shown in the Figure 2The feedback actuator monitor unit 10 transmits the driver's steering request to the control unit 60. The feedback actuator monitor unit 10 preferably also controls the feedback actuator 4. The feedback actuator monitor unit 10 can also be formed integrally with the control unit 60. The control unit 60 controls an electric steering actuator 6, which controls the position of the steered wheels 7, depending on the signal from the angle of rotation sensor and other input variables. The steering actuator 6 acts indirectly on the steered wheels 7 via a steering rod steering gear 8, such as a rack and pinion steering gear, as well as via tie rods 9 and other components. The detailed description of the steering gear 61 according to the invention is given under Figure 3 .

[0023] Figure 2shows a control system for a steering actuator 6. The steering actuator 6 receives the steering angle α from the control unit 60. The rack position 120 measured on a rack 12 and other road information 13 are passed on to the control unit 60. The control unit 60 determines the rack force using known measuring or estimation methods and calculates a steering torque T ,fb from this. The feedback actuator 4 is controlled accordingly, thereby generating a steering feel. The control unit 60 also receives driver-side steering commands 51, such as the steering angle status. The feedback actuator 4 receives signals via the signal line 50, among other things, from the rotation angle sensor, which measures and stores the steering angle α, the steering angular acceleration, and the steering angular velocity. The feedback actuator 4 communicates with a feedback actuator monitoring unit 10, which controls the feedback actuator 4.The feedback actuator monitoring unit 10 receives from the control unit 60 of the steering actuator 6 the actual wheel steering angle β of the steered wheels 7, as well as other variables that the control unit 60 has determined.

[0024] Figure 3shows the steering gear 61 according to the invention. An electric motor 14 with a control unit (power pack) has a motor shaft 15 that is non-rotatably connected to a threaded spindle 16 (drive spindle) of a ball screw 17. The connection can be made directly or via a coupling. It is preferably designed to be aligned. It can also be provided that the motor drives the threaded spindle via a reduction gear. The threaded spindle 16 is rotatably mounted about a longitudinal axis 160 at both its ends in ball bearings 18, in particular spindle bearings, in a housing 19. It is preferably provided that the ball bearings 18 are designed as fixed bearings on one side and as loose bearings on the other side. The ball screw 17 has a ball nut 20 that sits on the threaded spindle 16 and concentrically surrounds it. The ball nut 20 is held on its outer side in a linear guide 21.The linear guide 21 is attached to the inside of the housing 19 or formed integrally with the housing 19. The linear guide 21 specifies a translational movement of the ball nut 20 along the longitudinal axis 160. This support converts a rotary movement of the threaded spindle 16 into a linear movement of the ball nut 20 along the longitudinal axis 160. The linear guide 21 can absorb radial and / or tilting loads. The ball nut 20 is connected to the tie rods 9 directly or by means of additional components. By moving the ball nut 20 along the longitudinal axis 160, a steering movement is thus generated. An off-center arrangement of the actuator 6 with respect to the wheels can be compensated for by the geometry of the connecting elements.

[0025] In the Figure 3In the embodiment shown, the nut 20 of the spindle drive 17 is connected via a lever 22 to a joint 23 of a first tie rod 9 and moves this in the axial direction. The joint 23 is preferably an inner joint arranged at the end of the tie rod 9 remote from the wheel. The movement of this tie rod is transmitted to the other side, to the second tie rod 9, by means of a coupling rod 24, to which the inner joints 23 of both tie rods 9 are connected. The coupling rod 24 is guided linearly in a simple second housing 25 and absorbs the radial components of the tie rod forces. The spindle drive 17 and the electric motor 14 or the power pack are located in the area of ​​the coupling rod 24 and are aligned parallel to it. The lever 22 is designed such that the spindle drive 17 and the power pack are in the immediate vicinity of the coupling rod 24, which makes the arrangement particularly compact. The lever 22 is preferably rigid in the longitudinal direction.In its simplest embodiment, it is straight and has no angles. The lever 22 is preferably designed such that it connects the parallel coupling rod 24 and the threaded spindle 16 over the shortest distance. The spindle drive 17 or the housing 19 can be sealed to the outside with a bellows 26, which is fastened, for example, to the motor housing. Shown is a seal with a bellows 26 that extends from the housing 19 to the lever 22. The housing 25 in which the coupling rod 24 is arranged is also preferably sealed to the outside at both ends with a bellows 27. The two bellows 27 each extend from the housing 25 to the internal thread 23 of the corresponding tie rod 9.

[0026] Since radial and tilting loads are absorbed by the linear guide 21, the ball screw drive 17 is almost exclusively subjected to axial load and can be made significantly smaller, lighter, and more cost-effective than conventional designs. With the same load capacity, friction behavior is significantly improved. The spindle length corresponds to the stroke of the gear 17, plus allowances for thread runout and bearings. The bearing of the nut 20 is eliminated, which offers an advantage in terms of installation space requirements in the radial direction, which is often very limited. The more favorable dimensioning of the ball screw drive 17 allows the transmission ratio of upstream transmission stages (such as a belt drive) to be reduced, or they can even be dispensed with entirely, which offers advantages in terms of cost and—crucial for the steer-by-wire system—functional safety.A further advantage is the lower rotational imbalance compared to solutions where the nut rotates.

[0027] The invention is not limited to ball screws; screw drives in general, including roller screws or trapezoidal screws, can be used.

Claims

1. A steering gear (61) for a steer-by-wire steering system (1) of a motor vehicle having an electric motor (14) having a motor shaft (15), wherein the motor shaft (15) drives a spindle (16) of a worm gear (17) and the worm gear (17) comprises a spindle nut (20) in such a manner that a rotational movement originating from the motor shaft (15) is converted into a linear movement of the spindle nut (20) along an axis (160), characterized in that the spindle nut (20) in order to steer wheels of the motor vehicle is connected to an articulation (23) of a first tie rod (9) via a lever (22), and that the steering gear (61) comprises a coupling rod (24) which extends parallel with the movement direction of the spindle nut (20) and which transmits the movement of the tie rod (9) which is directly connected to the spindle nut (20) to a second tie rod (9) of the steering system.

2. The steering gear as claimed in claim 1, characterized in that the worm gear (17) is a ball screw drive, a trapezoidal screw drive or a roller screw drive.

3. The steering gear as claimed in claim 1 or 2, characterized in that the worm gear (17) is arranged in a housing (19) in which the spindle (16) is rotatably supported and the spindle nut (20) is supported with an axial guide.

4. The steering gear as claimed in one of the preceding claims, characterized in that the spindle nut (20) is directly connected to a single tie rod (9) via a lever (22) as said lever (22) which is rigid in an axial direction so that the linear movement of the spindle nut (20) is transmitted to this tie rod (9).

5. The steering gear as claimed in claim 4, characterized in that the lever (22) is configured in such a manner that the axis of the linear movement (160) extends parallel with the movement direction of the tie rod (9).

6. The steering gear as claimed in claim 4 or 5, characterized in that the lever (22) is connected to an inner thread (23) of the tie rod (9).

7. A steer-by-wire steering system (1) for a motor vehicle, comprising: - an electronically controllable steering actuator (6) which acts on the steered wheels (7), - a control unit (60), - a feedback actuator (4) which can be acted on by a driver with a driver's desired steering angle via a steering input means (3) and which transmits a feedback signal to the steering input means (3) as a response to the driver's request and a travel state of the motor vehicle, - a device for signal transmission which transmits the driver's request to the control unit (60), wherein the control unit (60) controls the steering actuator in order to transform the driver's request into a redirection of the steered wheels (7), characterized in that the steering actuator (6) is part of a steering gear (61) which is configured as claimed in one of the preceding claims 1 to 6.

8. The steer-by-wire steering system as claimed in claim 7, characterized in that the electric motor (14) is arranged in the region of the coupling rod (24).

9. The steer-by-wire steering system as claimed in one of the preceding claims 7 or 8, characterized in that the worm gear (17) or the housing (19) of the worm gear is sealed toward the exterior by means of a bellows (26).

10. The steer-by-wire steering system as claimed in one of the preceding claims 7 to 9, characterized in that the coupling rod (24) is arranged in a housing (25) which is sealed toward the exterior by means of at least one bellows (27).