Abstract:
A steer-by-wire steering system is proposed, having a steering wheel actuator and a braking device that acts on the steering column ( 3 ). The braking device is activated when the vehicle steering wheel is to be blocked or at least sharply braked, such as when the steering stop is reached or a wheel goes over the edge of a curb. The braking device functions like a switchable freewheel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE 01/04261 filed on Nov. 16, 2001. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a steer-by-wire steering system for a vehicle, having a manual steering device, in particular a steering wheel, connected to a steering column, and having a steering wheel actuator acting on the steering column, the steering wheel actuator having a rotor, connected to the steering column in a manner fixed against relative rotation, and a stator. 
   2. Description of the Prior Art 
   Steer-by-wire steering systems are distinguished in that in normal operation, there is no direct mechanical connection between the steering wheel and the steered wheels. Instead, the steering desired by the driver is detected via rotary angle sensors at the steering column and processed by a control unit, which transmits control signals accordingly to a steering actuator acting on the steered wheels of the vehicle. 
   A steering wheel actuator, via a moment, simulates the contact with the road for the driver. To that end, relatively low moment levels are required (approximately 5 Nm). For this reason, the electric motor that generates the moment can be integrated as a direct drive mechanism into the actuator, without an additional step-up gear. This makes a very good steering feel possible, with little fluctuation in moment and little noise development. 
   If higher moments are to be simulated for the driver, for instance by means of a steering stop or curb stop of the wheels, however, this cannot be done with that kind of electric motor without a gear. Since the steering stop, which in conventional steering systems is predetermined by the steering gear does not create any limitations to the rotary motion of the steering wheel in steer-by-wire steering systems, it is necessary to provide a separate steering stop for the steering wheel or steering column. This is necessary to prevent a flat spiral spring, for instance, in the steering column, which is required for actuating the horn, tripping an airbag, and/or actuating an automatic transmission, from being rotated excessively. If the flat spiral spring is rotated excessively, it becomes damaged. A curb stop can also be simulated by means of the rotary angle limitation of the steering column. 
   There are also steer-by-wire steering systems that have a so-called fallback plane. In terms of the present invention, fallback plane means a conventional mechanical connection between the steering wheel and the steering actuator that is activated if the steer-by-wire steering system fails. In the steer-by-wire mode, the mechanical connection between the steering wheel and the steering actuator is disconnected by a disengageable clutch, while for activating the fallback plane, the disengageable clutch is closed. 
   From German Patent Disclosure DE 43 30 658 A1, a steering wheel block for motor vehicles is known that uses clamping bodies. 
   The object of the invention is to furnish a steer-by-wire steering system braking device that is compact in structure, can be produced economically, and moreover has low energy consumption in the steer-by-wire mode. Moreover, the braking device should be triggerable simply and reliably. 
   In conjunction with the invention, the terms “clutch” and “braking device” differ in that a clutch transmits rotary motions and torques, while a braking device in the sense of the invention transmits only torque, since part of the braking device is connected to the vehicle in a manner fixed against relative rotation. 
   According to the invention, in a steer-by-wire steering system for a vehicle, having a manual steering device, in particular a steering wheel, connected to a steering column, and having a steering wheel actuator acting on the steering column, the steering wheel actuator having a rotor, connected to the steering column in a manner fixed against relative rotation, and a stator, this object is attained in that there is a braking device acting on the steering column; and that the braking device is actuated as a function of the torque transmitted from the stator to the steering column. 
   According to the invention, this object is also attained by a steer-by-wire steering system for a vehicle, having a manual steering device, in particular a steering wheel, connected to a steering column, having a clutch that couples a first portion to a second portion of a steering column, and having a steering wheel actuator acting on the steering column, the steering wheel actuator having a rotor, connected to the steering column in a manner fixed against relative rotation, and a stator, in that there is a clutch acting on the steering column; and that the clutch is actuated as a function of the torque transmitted from the stator to the steering column. 
   SUMMARY OF THE INVENTION 
   With the aid of the braking device acting on the steering column, the braking device being actuated as a function of the torque transmitted from the stator to the vehicle, it is possible in a simple way to assure that as needed, even major torques can be reported back from the steering column to the vehicle driver via the steering wheel. This can be the case for instance if the steering stop of the steer-by-wire steering system is reached, or if the vehicle with its steered wheels strikes a curb, and so forth. In all these situations, it suffices to exert a major braking moment on the steering column; there is no need for a rotary motion to be executed by the steering wheel actuator or the braking device. As a result, it is possible to dispense with the installation of a step-down gear in the steering wheel actuator, and this makes a very good steering feel with little moment fluctuation and low noise development possible. 
   In one embodiment of the steering system, it suffices to close the clutch, thus activating the mechanical fallback plane in order to make the steering wheel hard to move and giving the vehicle driver the feeling of a “stop”, or rotary angle limitation of the steering wheel. 
   One embodiment of the invention provides that the clutch is a braking device in the sense of the invention. This means that the output shaft of the clutch is connected to the vehicle in a manner fixed against relative rotation. 
   In a variant of the invention, it is provided that the braking device includes a switchable freewheel, which in a first switching state brings about a freewheel for both directions of rotation of the steering column and in a second switching state brings about a connection in a manner fixed against relative rotation of the steering column and the vehicle for both directions of rotation; that the freewheel has an outer race, connected to the vehicle in a manner fixed against relative rotation; that the outer race is disposed concentrically to the steering column; that of the faces, facing one another, of the outer race and the steering column, a first face is embodied cylindrically, while the second face has a diameter that increases and decreases in the circumferential direction; that clamping bodies are disposed between the outer race and the steering column; and that an actuating device is present, with which the clamping bodies can be stopped or rotated relative to the second face in the circumferential direction. 
   By embodying the braking device as a switchable freewheel, it is attained that major braking moments can be transmitted to the steering column in the smallest possible space. Each of the clamping bodies individually engages the steering column linearly, and moreover, many clamping bodies are distributed over the circumference of the steering column, so that excessively high loads are not put on the steering column when the braking device is activated. Notch effects or the like no longer need be feared, either. Moreover, freewheels can be produced quite economically. 
   In a further feature of the invention, it is provided that the clamping bodies are guided in a cage; that the clamping bodies are pressed away from the first face by spring elements integrated with the cage, or by the cage itself; that the stator is supported so that it swings; that the cage and the stator are connected to one another in a manner fixed against relative rotation; that the rotor of the steering wheel actuator is connected to the first face in a manner fixed against relative rotation; and that the rotation of the stator relative to the outer race is prevented, by means of at least one spring element that acts on both the stator and the outer race, until the tripping moment is reached. In this embodiment, the steering wheel actuator is integrated with the braking device, and the torque transmitted by the steering wheel actuator to the steering column is used directly to actuate the braking device of the invention. This purely mechanical embodiment is quite sturdy and is intrinsically independent of any interference that might occur in the control unit and of the supply voltage to the on-board electrical system of the vehicle. 
   It has proved especially reliable if at least one spring element is braced by a first end against the stator and a stop of the outer race and by a second end against the outer race. 
   In an alternative embodiment, the actuating member of the actuating device can be moved in the direction of the first face by the force of a spring, and the actuating member can be actuatable in the opposite direction by the force of a lifting magnet. In this embodiment, the electric motor of the steering wheel actuator need not be integrated with the braking device but can instead engage some other location on the steering column. Since in many models of electric motor, the electrical current through the motor is proportional to the moment output by the motor, whenever a torque that the steering wheel actuator cannot bring to bear on its own is to be impressed on the steering column, it is always possible, without separately measuring the moment, to activate the actuating device by triggering the lifting magnet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages and features of the invention will become apparent from the description contained herein below, taken in conjunction with the drawings, in which: 
     Shown are: 
       FIG. 1 : a schematic illustration of a steer-by-wire steering system with a mechanical fallback plane; 
       FIG. 2 : a schematic illustration of a steer-by-wire steering system without a mechanical fallback plane; 
       FIG. 3   a : a longitudinal section through a first exemplary embodiment of a clutch of the invention; 
       FIG. 3   b : a cross section taken along the line I—I in  FIG. 3   a;    
       FIG. 3   c : a cross section taken through a clutch of the invention along the line II—II. in  FIG. 3   a;    
       FIG. 4   a : a longitudinal section through a second exemplary embodiment of a clutch of the invention; 
       FIG. 4   b : a cross section taken along the line II—II in  FIG. 4   a ; and 
       FIG. 4   c : a cross section taken along the line I—I of  FIG. 4   a.    
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , a steer-by-wire steering system with a mechanical fallback plane is shown schematically. The task of every steering system is for the steering wish, expressed by turning a steering wheel  1 , to be transmitted to the steered wheels of the vehicle, which are not shown in  FIG. 1 . In conventional mechanical, hydraulic or electrical steering systems, this transmission is done via a steering column  3 , which is composed of multiple parts  3   a ,  3   b ,  3   c  and  3   d , and a steering actuator  5 , as a rule embodied as a steering gear, which converts the rotary motion of the steering column  3  into a linear motion. 
   In a steer-by-wire steering system, the steering column  3  is divided by a clutch  7  between the parts  3   a  and  3   b . It is understood that the clutch  7  can also be disposed at the parts  3   c  or  3   d.    
   In the steer-by-wire mode, that is, with the clutch  7  open, the driver&#39;s steering wish is picked up at the part  3   a  of the steering column that is connected directly to the steering wheel  1 , by a steering wheel rotary angle sensor, not shown in  FIG. 1 , and transmitted to a control unit, also not shown. The control unit in turn triggers the steering actuator  5 , which acts on the steered wheels, not shown, of the vehicle. 
   In the steer-by-wire mode, feedback from the roadway to the steered wheels is transmitted to the steering wheel  1  via a steering wheel actuator  9 . If an error is detected in the steer-by-wire steering system, the clutch  7  is closed, and the transmission of the driver&#39;s steering wish is done from the steering wheel  1  to the steered wheels in the conventional way, via the steering column  3  and the steering actuator  5 . 
   In  FIG. 2 , a steer-by-wire steering system in which there is no mechanical fallback plane is shown schematically. In this case, the steering actuator  5  is triggered as in the steer-by-wire mode of  FIG. 1 . 
   To enable limiting the steering wheel rotary angle, a braking device  10  according to the invention is provided on one end of the steering column  3 . The braking device  10  is connected to the vehicle in a manner fixed against relative rotation. The braking device  10  serves as a steering stop and serves to block the steering wheel  1 . Once the steering wheel  1  has been rotated through a predetermined angle in one direction, the braking device  10  is activated and thus prevents further rotation of the steering wheel  1 . The flat spiral spring, not shown in  FIG. 2 , in the steering column  3  is thus effectively protected against damage. The blocking of the steering wheel  1  can also be desired if the vehicle, with its steered wheels, strikes the edge of a curb and the steering wheel actuator  9  is unable to bring to bear a sufficiently major torque. The blocking of the steering wheel  1  also occurs if the vehicle is out of operation, so that the steering wheel  1  can be used by the driver as a hold on entering and leaving the vehicle. Furthermore, with the vehicle stopped, the blocking of the steering wheel  1  can also have the function of theft prevention. It is advantageous if the braking device  10  is closed in the currentless state. 
   The braking device  10  can be actuated as a function of the torque impressed on the steering column  3  by the steering wheel actuator  9 . In some models of motor for steering wheel actuators  9 , the torque output by these actuators is dependent on the current flow through the motor, so that the torque can be determined via a measurement of current. 
   In  FIG. 3   a , a first exemplary embodiment of a braking device  10  of the invention is shown in longitudinal section. Disposed coaxially to the steering column  3  is an outer race  40 , which is connected to the vehicle, not shown, in a manner fixed against relative rotation. 
   The steering column  3  has a cylindrical outer face  19 . Between an inner face  21  of the outer race  40  and the cylindrical outer face  19 , there are clamping bodies  45 . The clamping bodies  45  are fixed by a cage  44 . 
   An actuating device  29  is disposed in the outer race  40 . The actuating device  29  has an actuating member  31 , which is embodied as a cylindrical pin. When the actuating member  31  is moved in the direction of the cylindrical outer face  19  of the steering column  3 , it presses a portion  27  of the cage  44  against the cylindrical outer face  19  of the steering column  3 , so that the cage  44  takes on the rotary motion of the steering column  3 . As soon as the cage  44  is rotating relative to the outer race  40 , the clamping bodies  45  are likewise rotated relative to the outer race  40 , as a result of which the braking device  10  is activated. 
   The activation of the braking device  10  will now be explained in conjunction with  FIG. 3   b , which is a section taken along the line I—I of  FIG. 3   a.    
   It can be seen from  FIG. 3   b  that the steering column  3  has a cylindrical outer face  19 , and that the inner face  21  of the outer race  40  is not designed cylindrically. Instead, the inner face  21  has different radii. In  FIG. 3   b , the clamping bodies  45  are positioned at the locations of the greatest diameter of the inner face  21 . In this position, the clamping bodies  45  do not touch the outer face  19  of the steering column, and so no transmission of moments occurs between the outer race  40  and the steering column  3 . The clamping bodies  45  are held in the position shown by the cage  44 . Either the cage  44  itself is embodied to be so elastic that it presses the clamping bodies  45  outward, or it has spring elements (not shown), which press the clamping bodies  45  outward. 
   If the cage  44  is entrained by the steering column  3 , as already explained in conjunction with  FIG. 3   a , then the cage  44  moves the clamping bodies  45  out of the position shown in  FIG. 3   b , and the clamping bodies  45  roll over the inner face  21  of the outer race  40  and approach the cylindrical outer face  19  of the steering column  3 , until they finally touch it. As the outer race  40  and steering column  3  still continue to rotate relative to one another, the clamping bodies  45  clamp the outer race  40  and steering column  3  together. This creates a connection in a manner fixed against relative rotation between the outer race  40  and the steering column  3 . In this state, which is not shown in  FIG. 3   b , the braking device  10  is activated, and the steering wheel  1  is blocked. 
   Since the diameter of the inner face  21  of the outer race  40  decreases symmetrically to both sides, from the locations having the greatest diameter, the braking device  10  is engaged, regardless of the direction of the relative rotation of the outer race  40  and the steering column  3 , when the actuating member  31  presses the portion  27  of the cage  44  against the steering column  3 . In operation of the fallback plane, a certain amount of play in the steering column  3  when the braking device  10  is activated, upon a change of direction of rotation of the steering wheel  1 , is acceptable. 
   The function of the actuating device  29  will now be explained in further detail in conjunction with  FIG. 3   c . Compared to  FIGS. 3   a  and  3   b ,  FIG. 3   c  is shown enlarged. In it, the embodiment of the inner face  21  of the outer race  40  is seen especially clearly. Because of the variable spacing between the outer face  19  of the steering column  3  and the inner face  21  of the outer race  40 , clamping of the clamping bodies  45  between the steering column  3  and the outer race  40  occurs, as soon as the clamping bodies  45  move out of the positions shown in  FIGS. 3   b  and  3   c , relative to the outer race  40 . 
   The actuating device  29  has an actuating member  31 , which is pressed against the cage  44  via a spring  33 . The spring  33  is braced by one end on the outer race  40  and by the other on a collar  35  of the actuating member  31 . The actuating member  31  extends with its upper end into a lifting magnet  37 , which is embodied as an electromagnet. 
   If the braking device  10  is to be opened, the lifting magnet  39  is triggered, by a control unit not shown, in such a way that it exerts a force, which is contrary to and greater in amount than, the force of the spring  33  on the actuating member  31 . As a result, the actuating member  31  is lifted from the cage  44 , so that the steering column  3  and outer race  40  can rotate relative to one another. 
   As soon as the control unit, not shown, interrupts the flow of current through the lifting magnet  37 , the actuating member  31  drops onto the cage  44  as a result of the force of the spring  33  and thus brings about an activation of the braking device  10  of the invention. 
   The braking device  10  is always actuated whenever the steering wheel actuator  9 , for instance, is unable to furnish the requisite torque, or whenever the steering wheel  1  is to be activated for other reasons. As already noted, the current flow through the electric motor of the steering wheel actuator  9  can be used to determine the torque acting on the steering column. 
   It is naturally also possible to design the inner face  21  of the outer race  40  cylindrically, and to provide the steering column  3  with corresponding clamping faces. This embodiment, too, since it is based only on a geometric reversal, is encompassed by the scope of present invention. 
   In  FIG. 4   a , a second exemplary embodiment of a braking device  10  of the invention is shown in longitudinal section. In this exemplary embodiment again, the outer race  40  is connected to the vehicle (not shown) in a manner fixed against relative rotation. The outer race  40  has clamping faces  41 . In the outer race  40 , the stator  42  of the steering wheel actuator  9  is supported rotatably, for instance in swinging fashion. The rotor  43  of the electric motor is connected in a manner fixed against relative rotation to the steering column  3 . The cage  44  of the clamping bodies  45  protrudes into the stator  42  and is connected to it in a manner fixed against relative rotation. 
   The stator  42 , as can be seen from  FIG. 4   b , is fixed to the housing  40  via limit force springs  46   a  and  46   b  and a step  47 . The cage  44  is in turn connected to the stator  42 , for instance by means of a lug  48 . 
   If the steering wheel actuator generates a steering moment, the reaction moment is withstood at the stator  42 . If the forces resulting from the reaction moment exceed the prestressing of the limit force springs  46   a  and  46   b , then the stator  42  moves relative to the housing  40 . The limit force springs  46   a  and  46   b  are braced on one end on a protrusion  47  of the stator  42  and on a stop  49  of the outer race  40  and on the other end on the outer race  40 . 
   As soon as the stator  42  is in rotation, the cage  44  is rotated as well, and as can be seen from  FIG. 4   c , the clamping bodies  45  are shifted into the clamping position. In this exemplary embodiment, the braking device  10  is switched as a function of the moment output by the electric motor. If a steering stop is to be simulated, for instance, then the electric motor is triggered such that the forces resulting from the reaction moment overcome the prestressing of the limit force springs  46   a  and  b , and the steering column  3  is blocked. 
   If the switching of the braking device  10  functioning as a rotary angle limitation is not meant to occur at a certain reaction moment, but instead to be tripped by a control unit, for instance, then the stator  42  can in that case be connected (not shown) in a manner fixed against relative rotation to the housing  40 . The clamping bodies  45  are pressed resiliently into the clamping faces  41 . The actuation and engagement of the clutch are effected as described for  FIG. 3   c.    
   The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims. 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
                1 
               Steering wheel 
             
             
                 
                3 
               Steering column 
             
             
                 
                3a 
               Portion of the steering column steering column 
             
             
                 
                3b 
               Portion of the steering column steering column 
             
             
                 
                5 
               Steering gear 
             
             
                 
                7 
               Clutch 
             
             
                 
                9 
               Steering wheel actuator 
             
             
                 
               19 
               Cylindrical outer face 
             
             
                 
               21 
               Inner face 
             
             
                 
               27 
               Region 
             
             
                 
               29 
               Actuating device 
             
             
                 
               31 
               Actuating member 
             
             
                 
               33 
               Spring 
             
             
                 
               35 
               Collar 
             
             
                 
               37 
               Lifting magnet 
             
             
                 
               40 
               Outer race 
             
             
                 
               41 
               Clamping face 
             
             
                 
               42 
               Stator 
             
             
                 
               43 
               Rotor 
             
             
                 
               44 
               Cage 
             
             
                 
               45 
               Clamping bodies 
             
             
                 
               46a, b 
               Limit force springs 
             
             
                 
               47 
               Protrusion 
             
             
                 
               48 
               Lug 
             
             
                 
               49 
               Stop