Patent Publication Number: US-2021171090-A1

Title: Steering gear for a steer-by-wire steering system

Description:
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 . 
     In steer-by-wire steering systems, the position of the steered wheels is not directly coupled to the steering input means, for example, a steering wheel. There is a connection between the steering wheel and the steered wheels via electrical signals. The steering action desired by the driver is detected by a steering angle sensor and, depending on the steering action desired by the driver, the position of the steered wheels is controlled via a steering actuator. It is known to provide two steering actuators which are each associated with one of the steerable wheels and which are configured to adjust a steering angle of the respective wheel. There may also be provided a toothed rack steering gear which is in engagement with a pinion which is driven by a steering actuator. 
     In electromechanical steering gears, it is known to use axially parallel ball screw drives for converting an engine torque of an auxiliary drive into a linear movement of the toothed rack. The connection of the toothed rack to the steering pinion permits axial movements of the toothed rack, whilst rotations of the toothed rack are prevented. The ball screw drive is consequently constructed with a driven and rotatably supported ball nut. From the patent application DE 10 2012 015 181 A1 an electromechanical motor vehicle steering system is known and has parallel with the toothed rack an auxiliary drive with a spindle and a ball nut which is arranged thereon. The toothed rack and the spindle are connected to each other at the ends thereof by means of an end plate so that a displacement with respect to each other in an axial direction is not possible. A connection to the tie rods can be produced in the region of the ends. 
     Such ball screw drives (or roller screw drives or trapezoidal screw drives) absorb not only axial forces, but also radial forces and tilting moments which are introduced via the tie rods. There is thereby produced an unfavorable loading of the screw drive and consequently significantly greater dimensions than with a mainly axial loading. In spite of structural measures to reduce the sensitivity with respect to tilting (for example, by means of flexible connection of the ball screw drive using wave springs), the friction behavior of the steering gear worsens significantly as a result of these load components. 
     An object of the present invention is to provide a steering gear for a steer-by-wire steering system of a motor vehicle which has a small structural space requirement and which enables a better friction behavior of the steering gear. 
     This object is achieved with 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 . 
     Accordingly, a steering gear for a steer-by-wire steering system of a motor vehicle having an electric motor having a motor shaft is provided, wherein the motor shaft drives a spindle of a worm gear and the worm gear comprises a spindle nut in such a manner that a rotational movement originating from the motor shaft is converted into a linear movement of the spindle nut along an axis, wherein the spindle nut in order to steer wheels of the motor vehicle is connected to at least one tie rod. In contrast to electromechanical steering systems, the connection of the steering pinion is omitted in steer-by-wire systems. The driving is carried out according to the invention via a rotatably supported spindle, whereby structural space is saved. Preferably, the worm gear is a ball screw drive, a trapezoidal screw drive or a roller screw drive. There is preferably provision for the worm gear to be arranged in a housing in which the spindle is rotatably supported and the spindle nut is supported with an axial guide. 
     It is conceivable and possible for a rotary angle sensor to be provided on the spindle or the motor shaft so that as a result of the rotation of the motor shaft or the spindle a rotation angle can be measured. 
     It is further conceivable and possible for there to be provided a linear position sensor which is arranged on the spindle nut or on another axially movable component of the steering gear and which detects the movement thereof. 
     The direct connection of the spindle nut is preferably carried out to a single tie rod, in particular via a lever which is rigid in an axial direction, so that the linear movement of the spindle nut is transmitted to this tie rod. 
     Preferably, the lever is configured in such a manner that the axis of the linear movement extends parallel with the movement direction of the tie rod (or the articulation location of the tie rod). The lever is preferably linear and does not have an angle. 
     It is preferable for the lever to be connected to an inner thread of the tie rod. Furthermore, the lever may be connected to the tie rod by means of an articulation and may move it in an axial direction. The articulation is preferably an internal articulation. 
     Furthermore, a steer-by-wire steering system for a motor vehicle is provided, comprising:
         an electronically controllable steering actuator which acts on the steered wheels,   a control unit,   a feedback actuator which can be acted on by a driver with a driver&#39;s desired steering angle via a steering input means and which transmits a feedback signal to the steering input means as a response to the driver request and a travel state of the motor vehicle,   a device for signal transmission, which transmits the driver&#39;s request to the control unit,   wherein the control unit controls the steering actuator in order to transform the driver&#39;s request into a redirection of the steered wheels, and wherein a steering gear which comprises the steering actuator is configured as claimed in one of the preceding embodiments.       

     In an advantageous embodiment of the steering system, the movement of the tie rod which is directly connected to the spindle nut is transmitted to a second tie rod of the steering system by means of a coupling rod which extends parallel with the movement direction of the spindle nut. 
     Preferably, the electric motor is arranged in the region of the coupling rod. The spindle drive and the electric motor having the control unit are preferably orientated parallel with the coupling rod, which is enabled by the lever. This is a particularly space-saving embodiment. The electric motor has a motor shaft which is connected in a rotationally secure manner to a threaded spindle of the worm gear. The connection can preferably be carried out directly or by means of a coupling. It is preferably constructed in an aligned manner. There may also be provision for the motor to drive the threaded spindle via a step-down gear transmission. 
     It is advantageous for the worm gear or for the housing of the worm gear to be sealed toward the exterior by means of a bellows. 
     Preferably, the coupling rod is arranged in a housing which is sealed toward the exterior by means of at least one bellows. 
    
    
     
       A preferred embodiment of the invention is explained in greater detail below with reference to the drawings. Components which are identical or which have the same function are given the same reference numerals in the Figures, in which: 
         FIG. 1 : is a schematic illustration of a steer-by-wire steering system, 
         FIG. 2 : is a block diagram of a control of the steer-by-wire steering system, and 
         FIG. 3 : is a schematic illustration of a steering gear. 
     
    
    
       FIG. 1  shows a steer-by-wire steering system  1 . There is fitted to a steering shaft  2  a rotary angle sensor which is not illustrated and which detects the driver steering angle which is applied by turning a steering input means  3  which is constructed in the example as a steering wheel. However, a steering torque can also be detected in addition. A joystick may be used as the steering input means. There is further fitted to the steering shaft  2  a feedback actuator  4  which serves to simulate the reactions from the road  70  to the steering wheel  3  and consequently to provide the driver with feedback relating to the steering and travel behavior of the vehicle. The driver&#39;s steering request is transmitted via the rotation angle α of the steering shaft  2  measured by the rotary angle sensor via signal lines to a feedback actuator monitor unit  10 , as illustrated in  FIG. 2 . The feedback actuator monitor unit  10  transmits the driver&#39;s steering request to the control unit  60 . The feedback actuator monitor unit  10  preferably also takes over the control of the feedback actuator  4 . The feedback actuator monitor unit  10  may also be constructed integrally with the control unit  60 . The control unit  60  controls in accordance with the signal of the rotary angle sensor and other input variables an electrical steering actuator  6  which controls the position of the steered wheels  7 . The steering actuator  6  acts via a steering rod steering gear  8 , such as, for example, a toothed rack steering gear, and via tie rods  9  and other components directly on the steered wheels  7 . The detailed description of the steering gear  61  according to the invention is set out under  FIG. 3 . 
       FIG. 2  shows a control system of a steering actuator  6 . The steering actuator  6  receives the steering angle α from the control unit  60 . The toothed rack position  120  measured on a toothed rack  12  and other road information items  13  are transmitted to the control unit  60 . The control unit  60  determines the toothed rack force by means of known measurement or estimation methods and calculates from this a steering torque T ,fb . The feedback actuator  4  is controlled accordingly, whereby a steering sensation is produced. The control unit  60  further receives driver steering commands  51 , such as the steering angle status. The feedback actuator  4  receives signals via the signal line  50  inter alia from the rotary angle sensor which measures and stores the steering angle α, the steering angle acceleration and the steering angle speed. The feedback actuator  4  communicates with a feedback actuator monitor unit  10  which controls the feedback actuator  4 . The feedback actuator monitor unit  10  receives from the control unit  60  of the steering actuator  6  the actual steering angle β of the steered wheels  7 , and other variables which the control unit  60  has established. 
       FIG. 3  shows the steering gear  61  according to the invention. An electric motor  14  having a control unit (power pack) has a motor shaft  15  which is connected in a rotationally secure manner to a threaded spindle  16  (drive spindle) of a ball screw drive  17 . The connection can be carried out directly or via a coupling. It is preferably constructed in an aligned manner. There may also be provision for the motor to drive the threaded spindle via a step-down gear transmission. The threaded spindle  16  is rotatably supported about a longitudinal axis  160  at the two ends thereof in ball bearings  18 , in particular spindle bearings, in a housing  19 . There is preferably provision for the ball bearings  18  to be constructed at one side as a fixed bearing and at the other side as a movable bearing. The ball screw drive  17  has a ball nut  20  which is located on the threaded spindle  16  and which concentrically surrounds it. The ball nut  20  is retained at the outer side thereof in a linear guide  21 . The linear guide  21  is secured to the inner side of the housing  19  or constructed integrally with the housing  19 . The linear guide  21  determines a translational movement of the ball nut  20  along the longitudinal axis  160 . As a result of this support, there is produced a conversion of a rotational movement of the threaded spindle  16  into a linear movement of the ball nut  20  along the longitudinal axis  160 . The linear guide  21  may absorb radial and/or tilting loads. The ball nut  20  is connected to the tie rods  9  directly or by means of additional components. As a result of movement of the ball nut  20  along the longitudinal axis  160 , a steering movement is consequently produced. A non-central arrangement of the actuator  6  with respect to the wheels can be compensated for by the geometry of the connection elements. 
     In the embodiment shown in  FIG. 3 , the nut  20  of the spindle drive  17  is connected via a lever  22  to an articulation  23  of a first tie rod  9  and moves it in an axial direction. The articulation  23  is preferably an internal articulation which is arranged at the end of the tie rod  9  remote from the wheel. The movement of this tie rod is transmitted via a coupling rod  24  by means of which the inner articulations  23  of both tie rods  9  are connected, to the other side, to the second tie rod  9 . The coupling rod  24  is guided in a linear manner in a simple second housing  25  and absorbs the radial portions of the tie rod forces. The spindle drive  17  and the electric motor  14  or the power pack are located in the region of the coupling rod  24  and are orientated parallel therewith. The lever  22  is constructed in such a manner that the spindle drive  17  and the power pack are located in the direct vicinity of the coupling rod  24 , which makes the arrangement particularly compact. The lever  22  is preferably rigid in the longitudinal direction. In the simplest embodiment thereof, it is linear and does not have an angle. The lever  22  is preferably configured in such a manner that it connects the coupling rods  24  which are parallel with each other and the threaded spindle  16  over the shortest spacing. The spindle drive  17  or the housing  19  can be sealed externally with a bellows  26  which is, for example, secured to the motor housing. There is illustrated a seal with a bellows  26  which extends from the housing  19  in the direction toward the lever  22 . The housing  25  in which the coupling rod  24  is arranged is also preferably sealed externally at both ends with a bellows  27 . The two bellows  27  extend in this instance from the housing  25  toward the inner thread  23  of the corresponding tie rod  9 . 
     Since radial and tilting loads are absorbed by the linear guide  21 , the ball screw drive  17  is loaded almost exclusively axially and may be constructed to be significantly smaller, lighter and more cost-effective than in conventional constructions. With the same load-bearing capacity, there is produced a substantially improved friction behavior. The spindle length corresponds to the travel of the transmission  17 , plus additions for thread run-out and bearing. The bearing of the nut  20  is omitted, which affords an advantage with respect to structural space requirement in the radial direction which in many cases is significantly limited. As a result of the more favorable sizing of the ball screw drive  17 , the transmission ratio of upstream transmission stages (such as, for example, a belt drive) can be reduced, or it can even be dispensed with completely, which affords advantages with regard to costs and—decisively for the steer-by-wire system—provides operational reliability. Another advantage involves the lower rotational imbalance compared with solutions in which the nut rotates. 
     The invention is not limited to ball screw drives; worm gears, that is to say, roller screw drives or trapezoidal screw drives, can also generally be used.