Abstract:
A steering device for a superposition steering system is provided using at least one actuator. The device is based on the object of producing a superposition steering system which requires no additional installation space in the steering column. The actuator for the superposition steering system is therefore arranged in the steering wheel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of prior application number PCT/DE2006/001151, filed Jun. 29, 2006 and claims the benefit under 35 U.S.C. §119 of prior foreign application number DE 10 2005 034 636.7, filed Jul. 20, 2005. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a steering device for a superposition steering system. 
     BACKGROUND OF THE INVENTION 
     Nowadays, motor vehicles are equipped as a rule with power steering systems. It is an object of the power steering system to reduce the steering force requirement, that is to say the torque which is to be applied by the driver for a change in driving direction. In known power steering systems, this takes place as a function of the engine speed, since the pump for the assisting hydraulic system is driven by the vehicle motor. Furthermore, it is known to drive the pumps by actuable electric motors, as a result of which speed-dependent steering force assistance is made possible. Here, the assisting action of a power steering system of this type decreases as the speed increases. At high speeds, the driver therefore has to introduce a greater moment via the steering wheel for a change in the steering lock than at low speeds. In systems of this type, the input moment is adapted, that is to say regulated, to the respective conditions. 
     Superposition steering systems are characterized in that a further angle can be superimposed by an actuator onto steering angles which are input by the driver. For instance, DE 101 60 313 A1 discloses a superposition gear mechanism for a superposition steering system. This superposition gear mechanism is arranged between an input shaft which is connected to the steering wheel and an output shaft which is connected to a steering gear. This superposition gear mechanism is assigned an adjusting motor, and together they form the actuator. This actuator has the disadvantage that it is arranged between an input shaft and an output shaft, with the result that additional installation space is required in the region of the steering column and therefore a special steering column is required. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of providing a superposition steering system which requires no additional installation space in the steering column. Furthermore, it is to be possible for an additional assembly to be installed, with the aid of which autonomous driving is optionally to be possible. 
     According to the invention, this is achieved according to the features described hereinafter. 
     In a steering device for a superposition steering system, using at least one actuator for the superposition steering system, the latter is arranged in the steering wheel according to the invention. This results in the advantage that no special steering column is required for the superposition steering system, in which steering column the assembly for the superposition steering system is additionally installed in steering devices of the prior art. The arrangement according to the invention can therefore be incorporated more readily into existing steering systems, since a conventional steering column having a power steering system can be used. 
     The actuator preferably has a worm gear mechanism and at least one motor for driving a worm gear. In one embodiment, the worm gear mechanism has a large worm gear which is connected coaxially to a steering shaft and the large worm gear is assigned two smaller worm gears which are connected to in each case one electric motor. 
     It is expedient that the large worm gear has a sleeve which engages over the steering shaft for connection to the steering shaft, the rotationally secure connection being effected via a steering wheel journal. The motors are preferably arranged at least partially in the spokes of the steering wheel. 
     Furthermore, it is expedient that the lead of the worm gear mechanism is of sufficient magnitude that there is self locking. 
     It is likewise expedient that activation of the motors which is dependent on the speed of the vehicle is provided, the steering lock being reduced at high speed and the steering lock being increased at low speed. 
     Steering lock is to be understood as the angular change in the steered wheels in the event of a change in driving direction. 
     In one preferred embodiment, a power steering system having a controllable input moment which reacts to the steering angle difference of the superposition steering system is arranged behind the actuator for the superposition steering system. In particular, a usual steering sensation is produced as a result. Here, regulation is provided which reduces the input moment of the power steering system if steering angles are added and increases it if steering angles are subtracted. Steering angle is understood to be the rotational angle of the steering shaft. Depending on the actuation of the superposition steering system, said steering angle can be equal to, greater than or smaller than the angle change which is initiated by the driver on the steering wheel. 
     In one embodiment, a force-transmitting, form-fitting or frictional connection is provided between the steering wheel and a component which is fixed to the vehicle, in order to fix the steering wheel. 
     It is expedient that the actuator is arranged above a steering wheel base, and that an electromagnetic friction clutch is arranged as steering wheel fixing means between the underside of the steering wheel base and a steering wheel casing. 
     In one preferred embodiment, the friction clutch has an annular clutch part which is guided by means of guide pins in an inwardly protruding section of the steering shaft casing. 
     In a further embodiment, at least one additional, programmable servoactuator which is connected to a component which is fixed to the vehicle, for example to the steering column receptacle, is provided between the superposition steering system which is situated in the steering wheel and the power steering system. The programmable servoactuator acts directly on the steering shaft, for example via gearwheel mechanisms. The programmable servoactuator therefore actuates the steering gear directly via the upper steering shaft and the following power steering system. The vehicle can be controlled autonomously via this servoactuator. Assistance functions which intervene in the steering events, for example automatic lane maintaining or automatic parking, can therefore be realized by way of this servoactuator. Since the servoactuator lies in front of the power steering system, the servoactuator 
     can relieve the superposition steering system which is situated in the steering wheel, with the result that a smaller actuator can be installed for the superposition steering system. 
     There is provision in a further refinement for at least one actuating element to be assigned to the superposition steering system for setting the spacing between the axes of the worm gear and the worm of the actuator and/or to the housing, in which the worm gear and the worm are mounted, for introducing vibrations. For example, a piezoelectric actuator can be provided as actuating element. 
     The efficiency of the gear mechanism can be set as a result. 
     The piezoelectric actuator can therefore press the gear mechanism housing apart, in which the worm gears are mounted. This increases the axial spacing, as a result of which the play between the worm gears is likewise increased. As a result, the breakaway torque of the gear mechanism is reduced and the gear mechanism is set in motion more readily. As an alternative or in addition, the breakaway torque can be reduced by the vibration which is introduced by the piezoelectric actuator. 
     The housing can also be pressed together by means of the piezoelectric actuator. As a result, the axial spacing is reduced and the play between the worm gears is canceled completely, that is to say the gear mechanism is blocked and has reliable self locking. 
     Finally, the piezoelectric actuator can keep the worm gears at an optimum spacing for dynamic operation. In the favorable case, the axial spacing remains at the spacing which the gear mechanism assumes in the stressfree state of the actuator. Here, the gear mechanism runs with optimum play and optimum friction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is to be explained in one exemplary embodiment using drawings, in which: 
         FIG. 1  shows a perspective view of the steering wheel according to the invention with an actuator; 
         FIG. 2  shows a perspective view of the actuator from  FIG. 1 ; 
         FIG. 3  shows a section through the steering wheel according to  FIG. 1 ; 
         FIG. 4  shows a section through a steering wheel according to the invention with an electromagnetic friction clutch as steering wheel fixing means; 
         FIG. 5  shows the detail E of  FIG. 4  with an open clutch: 
         FIG. 6  shows the detail E of  FIG. 4  with a closed clutch; 
         FIG. 7  shows a diagrammatic illustration of the overall steering system; 
         FIG. 8  shows one embodiment with an actuator in a three-spoke steering wheel, viewed from above; 
         FIG. 9  shows a perspective view from the front of the embodiment according to  FIG. 8 ; 
         FIG. 10  shows a perspective view from the rear of the embodiment according to  FIG. 8 ; 
         FIG. 11  shows a side view of the embodiment according to  FIG. 8 ; 
         FIG. 12  shows one embodiment, in which the spacing between the axes of the worm gears of the actuator can be set, as viewed from above; 
         FIG. 13  shows the embodiment of  FIG. 12  with an open gear mechanism housing; 
         FIG. 14  shows a perspective view of the embodiment according to  FIG. 12 ; and 
         FIG. 15  shows a side view of the embodiment according to  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a steering wheel which has a steering wheel rim  1  and a steering wheel hollow  2  which is shown in section, in order to make an actuator visible. The steering wheel rim  1  and the steering wheel hollow  2  are connected to one another by spokes  3 ,  4 ,  5 ,  6 . The actuator has a large worm gear  7 , two small worm gears  8 ,  9  and electric motors  10 ,  11  which are assigned to the latter. The electric motor  10  is a constituent part of the spoke  5 , while the electric motor  11  is a constituent part of the spoke  4 . The large worm gear  7  has a sleeve  12  ( FIG. 2 ), by means of which the steering wheel can be fastened on a steering shaft  13  ( FIG. 3 ). The sleeve  12  engages over a steering wheel journal  14  and the two are connected to one another by means of a screw  15 . 
     Further details of the arrangement according to the invention can be seen from  FIG. 3 . The steering wheel hollow  2  has a steering wheel base  17 , in which a ball bearing  18  is provided for mounting the sleeve  12 . The steering shaft  13  is surrounded by a steering shaft casing  16 , on which an electromagnetic friction clutch  19  is arranged as steering wheel fixing means on the side which faces the steering wheel base  17 . Said electromagnetic friction clutch  19  is shown in detail in  FIGS. 4 to 6 . Furthermore, a steering angle sensor  23  is provided in the upper region of the steering shaft casing  16 . 
     It can be seen from  FIG. 4  that the friction clutch  19  has an annular clutch part  20  which is guided by means of guide pins  21  in an inwardly protruding section  22  of the steering shaft casing  16 . There is the possibility of autonomous driving or ESP intervention in the steering by means of this friction clutch  19 . The friction clutch fixes the steering wheel, which fixing is to be overcome by the driver. If autonomous steering is to take place, the clutch part  20  moves out of the section  22  of the steering shaft casing  16  and bears on the outside of the steering wheel base  17 , as shown in  FIG. 6 . There is now a frictional connection between the steering wheel base  17  and therefore the steering wheel and the steering shaft casing  26 . The necessary steering locks are then brought about by the superposition steering system. 
     If the driver suddenly wishes to input another steering angle, the system detects the friction or angle difference which is produced or the prevailing steering moment and moves the clutch back, as shown in  FIG. 5 . 
       FIG. 7  shows the overall steering system, in which this steering wheel works. The steering wheel having the steering angle sensor  23  is fastened on the steering shaft  13 , on which a further steering angle sensor  24  and sensors  25 ,  26  for the steering moment are provided. A servomotor  27  is arranged between the two last mentioned sensors. A steering gear  28  having drive shafts  29 ,  30  is provided at the end of the steering shaft  13  in a known manner. The steering angle sensors  23 ,  24 , the sensors  25 ,  26  for the steering moment and the servomotor  27  are assigned control electronics  31  with software. 
     In the embodiment of  FIG. 8 , a steering wheel is provided having a steering wheel rim  1  and three spokes  32 ,  33 ,  34 . The actuator which comprises a gear mechanism which is arranged in a gear mechanism housing  35  and an electric motor  36  is arranged in the center of the steering wheel. As can be seen from  FIG. 9 , the gear mechanism has a worm  37  and a worm gear  38  which is connected to the steering shaft  13 . 
     In the view of  FIG. 10  which shows the embodiment of  FIG. 8  perspectively from the rear, a second, so-called servoactuator having an electric motor  39  and an associated drive pinion  40  can be seen. In contrast to the first mentioned actuator, this servoactuator is arranged in a manner which is fixed to the vehicle in the region of the steering column (not shown). The drive pinion engages in a crown gear  41  which has an internal toothing system and is connected fixedly in terms of rotation to the steering shaft  13 . 
     The assignment of the essential components of the two actuators can be seen again from the side view of  FIG. 11 . Since the servoactuator lies in front of the power steering system (not shown), the servoactuator can relieve the superposition steering system which is situated in the steering wheel, with the result that a smaller actuator  36 ,  37 ,  38  can be installed for the superposition steering system. 
     In the embodiment of an actuator which is integrated in the steering wheel according to  FIG. 12 , the gear mechanism housing which is shown in the closed state in this figure has two regions. There are therefore a gear mechanism housing region  42  for a worm gear  43  ( FIG. 13 ) and a gear mechanism housing region  44  for a worm  45  ( FIG. 13 ). The gear mechanism housing regions  42 ,  44  are divided by a gap  46  which penetrates the gear mechanism housing completely in a direction parallel to the steering shaft axis  47  ( FIG. 15 ). The gear mechanism housing region  42  for the worm gear  43  is connected fixedly to the steering wheel (not shown). The worm  45  is assigned an electric motor  48 . 
     An actuable actuating element  49  which bridges the gap  46 , for example a piezoelectric actuator, is arranged on that end side of the gear mechanism housing which faces away from the electric motor  48 . The width of the gap  46  in a plane which is defined by the worm gear between the two gear mechanism housing regions  42 ,  44  and therefore the play between the worm gear  43  and the worm  45  can be set by the activation of the actuating element  49 . For this purpose, an actuator  50  extends in the direction of the gear mechanism housing region  42  from the actuating element  49  which is fastened to the gear mechanism housing region  44 , and is connected fixedly to said gear mechanism housing region  42  there, for example by means of a screw connection  51 . The width of the gap  46  can be changed by displacement of the actuator  50  in the actuating element  49  or by a change in the length of the actuator  50  if a piezoelectric actuator is used. 
       FIGS. 14 and 15  show the essential elements of the embodiment of  FIG. 12  once again, from other viewing directions.