Patent Publication Number: US-2022212714-A1

Title: Steering system for a motor vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 102021200051.7, filed Jan. 6, 2021, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The disclosure relates to a steering system for a motor vehicle, which is designed as a steer-by-wire steering system. 
     BACKGROUND 
     Steering systems usually comprise a rack which is linearly displaceably mounted in order to adapt a wheel position. Originally, such a rack is coupled to the steering wheel via a handlebar, so that a linear displacement of the rack is achieved by rotating the steering wheel 
     In modern motor vehicles, a so-called steer-by-wire steering system (SBW steering system) can be used, in which there is no longer any mechanical connection between the steering wheel and the rack. The rack is displaced by an electric drive. Torsional forces can occur that act on the rack and cause undesired rotation of the rack. 
     Due to the no longer existing mechanical connection between the steering wheel and the rack, the mounting and guidance of the rack in a steer-by-wire steering system are different in comparison to conventional steering systems. 
     In particular, the requirements for the guidance of the rack change, since the electric drive in a steer-by-wire steering system can result in high thrust forces, for example higher than 13 kN, which must be taken into account when guiding the rack. 
     In addition, the electric drive transmits a torque to the rack. Torsional forces can occur that act on the rack and cause undesired rotation of the rack. In addition, the rack should in principle be guided with as little friction as possible, so that the rack can slide smoothly through the steering system when the motor vehicle is being steered. 
     Therefore what is needed is to provide a reliable steering system with which a rack can be adjusted, in particular in a steer-by-wire steering system. 
     SUMMARY 
     In accordance with one exemplary arrangement, a steering system for a motor vehicle is provided, which is designed as a steer-by-wire steering system. The steering system comprises a rack, an electric drive for longitudinal displacement of the rack, a worm gear and a drive shaft which is in toothed engagement with the rack. Here, the electric drive is connected to the drive shaft by the worm gear so as to transmit torque. 
     It has been recognized that through use of the worm gear, the electric drive can be coupled in a particularly reliable and quiet manner to the drive shaft so as to transmit torque, since the worm gear has a high load-carrying capacity compared to other toothed drives. 
     In this exemplary arrangement, the worm gear has a worm wheel and a worm shaft which engages with the worm wheel. The worm wheel is connected to the drive shaft so as to transmit torque and/or non-rotatably, and the worm shaft is connected to the electric drive so as to transmit torque and/or non-rotatably. This has the advantage that the worm shaft with the electric drive can be aligned on the worm wheel depending on the space available. The steering system can thus be flexibly adapted to different space requirements and is therefore particularly compact. 
     Furthermore, in one exemplary arrangement, it can be provided that the electric drive comprises a first electric motor and a second electric motor. In this way, the reliability can be further increased via redundancy. Additionally or alternatively, the two electric motors can be designed to be smaller in comparison to a single electric motor, so that the required installation space can be reduced or arranged more advantageously. 
     According to one exemplary arrangement, the first electric motor and the second electric motor are connected to the worm wheel by a common worm shaft so as to transmit torque. This arrangement makes the steering system particularly compact. 
     According to an alternative exemplary arrangement, the worm gear has a second worm shaft which engages with the worm wheel. The first worm shaft is connected to the first electric motor so as to transmit torque and/or non-rotatably and the second worm shaft is connected to the second electric motor so as to transmit torque and/or non-rotatably. Since the two electric motors are each connected to the worm wheel via their own worm shaft, the reliability of the steering system is further increased. Furthermore, the two worm shafts can be designed to be smaller in comparison with a single worm shaft, as a result of which the weight and/or the required installation space can be reduced. 
     Furthermore, the first worm shaft and the second worm shaft can be arranged on mutually opposite radial sides of the worm wheel, in particular the axis of rotation of the first worm shaft extending parallel to the axis of rotation of the second worm shaft. This ensures an effective distribution of forces and the worm shafts can act as bearings for the worm wheel, so that a bearing assigned to the worm wheel can be made more compact or can be omitted entirely. 
     In one exemplary arrangement, the steering system has two bearing bushings which are spaced apart from one another in the longitudinal direction of the rack. The bearing bushings together form a rack guide. Furthermore, the drive shaft is arranged between the two bearing bushings in the longitudinal direction of the rack. The rack is effectively supported against deflection by the bearing bushings, so that reliable and precise engagement of the drive shaft and the rack is ensured. Furthermore, the rack can be mounted with particularly low friction by the bearing bushings. 
     Furthermore, it can be provided that the drive shaft is arranged, in a position of the steering system, in one exemplary arrangement, in a neutral position of the steering system, centrally with respect to the rack in the longitudinal direction of the rack. In this way, the forces acting can be distributed particularly evenly over the rack and thus the load on the rack can be reduced. 
     The steering system can have a pressure piece which is arranged in the region of the drive shaft and which supports the rack. More precisely, the pressure piece presses the rack against the drive shaft in order to ensure that the drive shaft remains in toothed engagement with the rack. 
     The pressure piece is made in one exemplary arrangement, of plastics material and can thus be produced with a low mass and inexpensively. 
     In a further exemplary arrangement, the steering system has a wireless torque sensor which is provided for determining the torque of the drive shaft. In this way, the force that is transmitted via the rack can be precisely determined. 
     Furthermore, the rack can have a toothing, via which the drive shaft is in toothed engagement with the rack. The rack may have a constant cross section from a first end to the toothing and from the toothing to a second end opposite the first end. In this way, the rack can be designed to be particularly resilient, in particular with regard to the deflection. Furthermore, the rack can thereby be cut deeper or flattened in the region of the toothing in order to provide a larger contact region between the drive shaft and the rack. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Further advantages and features can be found in the following description and in the accompanying drawings, in which: 
         FIG. 1  is a schematic side view of a steering system according to an exemplary arrangement, 
         FIG. 2  is a schematic plan view of the steering system from  FIG. 1 , 
         FIG. 3  is a schematic sectional illustration of a section through the steering system along the line A-A in  FIG. 1 , and 
         FIG. 4  is a schematic plan view of a steering system according to another exemplary arrangement. 
     
    
    
     DETAILED DESCRIPTION 
     A steering system  10  for a motor vehicle is shown in  FIG. 1 . 
     The steering system  10  is a so-called steer-by-wire steering system in which there is no mechanical connection between a steering wheel  12 , which can be operated by a driver, and the wheels of the motor vehicle for transmitting the steering movement, 
     The steering system  10  has a rack  14  and an electric drive  16 . The electric drive  16  is set up for the longitudinal displacement, i.e. translational displacement, of the rack  14  in order to set a position of the wheels of the motor vehicle in accordance with a steering request by the driver. 
     In the exemplary arrangement shown, the electric drive  16  comprises a first electric motor  18  and a second electric motor  19 . 
     In principle, the electric drive  16  can have any number of electric motors, and in one particular exemplary arrangement, only a single electric motor  18 ,  19 . 
     The steering wheel  12  is coupled to the electric motors  18 ,  19  via a control unit  20  so as to transmit signals. As a result, there is only one electronic connection between the steering wheel  12  and the rack  14 . 
     For the sake of clarity, the steering wheel  12  and the control unit  20  are not shown in  FIG. 2  (or in  FIG. 4 ). 
     In order to couple the electric drive  16  to the rack  14  so as to transmit torque, the steering system  10  comprises a worm gear  22  having a worm wheel  24  and a worm shaft  26  which engages with the worm wheel  24 , as well as a drive shaft  28  which is in toothed engagement with the rack  14  via a toothing  30  (see  FIG. 2 ). 
     The worm wheel  24  is non-rotatably connected to the drive shaft  28  and the worm shaft  26  is connected to the electric drive  16  so as to transmit torque. 
     For this purpose, the two electric motors  18 ,  19  are arranged at opposite ends of the worm shaft  26  and are controlled accordingly via the control unit  20  in order to drive the common worm shaft  26 . 
     For the toothed engagement with the rack  14 , the drive shaft  28  has a pinion-like portion  32  (see  FIG. 3 ) in the contact region  34  with the toothing  30 . 
       FIG. 3  shows a section in the region of the drive shaft  28  through the steering system  10  along the line A-A in  FIG. 1 . 
     The steering system  10  also has a pressure piece  36  which supports the rack  14  on the side opposite the contact region  34 . As a result, deflection of the rack  14  is suppressed and thus a toothed engagement between the drive shaft  28  and the rack  14  is reliably ensured. Furthermore, the pressure piece  36  ensures effective suppression of noise. 
     In order to ensure a particularly stable toothed engagement, the steering system  10  can have a spring element  38  which acts on the pressure piece  36  with a spring force in the direction of the rack  14  or contact region  34 , as is illustrated in  FIG. 3  by an arrow. 
     In one exemplary arrangement, a contact surface  40  of the pressure piece  36  with the rack  14  is adapted to the geometry of the rack  14 , and in one particular arrangement, concave. 
     In one exemplary arrangement, the pressure piece  36  is made of a plastics material. 
     In the exemplary arrangement shown, the axis of rotation R of the drive shaft  28  is inclined by an angle a of less than  90   0  with respect to the longitudinal axis L of the rack  14 . As a result, the contact region  34  is particularly large, so that forces are transmitted particularly effectively from the drive shaft  28  to the rack  14 . 
     As shown in  FIG. 2 , the rack  14  extends from a first end  42  over a first end portion  44  to a toothed portion  46 , which has the toothing  30 , and from the toothed portion  46  over a second end portion  48  to a second end  50 , which is arranged opposite to the first end  42 . 
     Here, the toothing  30  is provided only in the toothed portion  46 . This means that the toothing  30  does not extend in the direction of the longitudinal axis L over the entire length of the rack  14 , but only over the toothed portion  46 . 
     In one exemplary arrangement, the length of the toothing  30  or the toothed portion  46  corresponds to a maximum translational movement range of the rack  14  in the displacement direction. 
     In one exemplary arrangement, the first end portion  44  and the second end portion  48  each have a constant circular cross section with the same diameter. 
     In principle, however, each of the end portions  44 ,  48  can have any desired cross section. 
     In order to guide the rack  14  in the motor vehicle, the steering system comprises a rack guide  52  having a first bearing bushing  54  and a second bearing bushing  55 , which each support the rack  14  on the periphery. 
     The first bearing bushing  54  is arranged in the first end portion  44  and the second bearing bushing  55  is arranged in the second end portion  48 . The contact region  34  is thus arranged between the toothing  30  and the drive shaft  28  in the direction of the longitudinal axis L between the two bearing bushings  54 ,  55 , and in one particular arrangement, in the middle. 
     In  FIGS. 1 and 2 , the steering system  10  is in a neutral position, i.e. in a position in which the wheels of the motor vehicle are oriented in the primary direction of movement of the motor vehicle, usually straight ahead. 
     In this neutral position, the drive shaft  28  is arranged centrally with respect to the rack  14  and the toothing  30  in the direction of the longitudinal axis L, so that the rack  14  can be adjusted equally in both directions when steering, i.e. to the left and to the right. 
     The location or relative position of the rack  14  with respect to the drive shaft  28  can be determined, for example, by an angle sensor or a rotary encoder which is connected to the control unit  20  so as to transmit signals. 
     In the exemplary arrangement shown, the steering system  10  also has a wireless torque sensor  56 , which is connected to the control unit  20  so as to transmit signals and is set up to determine a torque transmitted by the drive shaft  28  or acting in the drive shaft  28 . 
     Using the sensor data, the control unit  20  controls the electric drive  16  in order to adjust or translate steering movements of the steering wheel  12  into a corresponding relative position of the rack  14 . 
     Of course, in addition or as an alternative to steering movements of the steering wheel  12 , other control signals can be taken into account by the control unit, in particular control signals that support a driver, for example when keeping in lane, or directly predetermine a trajectory, for example in an autonomous driving or parking mode. 
     With reference to  FIG. 4 , a steering system  10  is now described according to a further exemplary arrangement. The same reference signs are used for the components which are known from the above arrangement and reference is made to the preceding explanations in this respect. 
     In contrast to the arrangement shown in  FIGS. 1 and 2 , the steering system  10  shown in  FIG. 4  comprises a worm gear  22  having a second worm shaft  58 , which is provided in addition to the first worm shaft  26  and which engages with the worm wheel  24 . 
     The first electric motor  18  is connected via the first worm shaft  26  and the second electric motor  19  via the second worm shaft  58  to the worm wheel  24  and thus to the drive shaft  28  so as to transmit torque. 
     The two worm shafts  26 ,  58  are arranged such that the worm wheel  24  is arranged between the two worm shafts  26 ,  58  and the axes of rotation E, F of the worm shafts  26 ,  58  extend parallel to one another. 
     In all exemplary arrangements shown, a steering system  10  is provided in this way, by which the relative position of the rack  14  can be set particularly reliably and precisely and the motor vehicle can thus be precisely steered. 
     The worm gear  22  ensures a particularly resilient, effective and low-noise transmission of the forces from the electric drive  16  to the drive shaft  28 . 
     Furthermore, the rack  14  and the rack guide  52  are designed in such a way that the rack  14  is particularly resistant to deflecting. 
     The disclosure is not limited to the disclosed arrangements. In particular, individual features of one exemplary arrangement can be combined as desired with features of other arrangements, in particular independently of the other features of the corresponding arrangements.