Abstract:
A vehicle steering wheel ( 20 ) includes a movable airbag module ( 24 ) which can be depressed by a limited axial stroke for generating a horn signal. The vehicle steering wheel ( 20 ) further includes a sensor system ( 22   a,    22   b ) operating in a contactless manner for determining the stroke.

Description:
TECHNICAL FIELD 
   The invention relates to a vehicle steering wheel comprising a movable airbag module. 
   BACKGROUND OF THE INVENTION 
   Floatingly mounted airbag modules which can be depressed by a limited axial stroke for generating a horn signal, undertake the function of a horn actuating element. Such modules are as also designated as “floating horn” modules. They have the advantage, compared with horn button switches arranged on the steering wheel, that the airbag module always keeps its central position independently of the steering wheel turning angle, so that the horn actuation in dangerous situations is simplified. The module is movable against the force of compression springs so far downwards that at least one of the contacts provided on the base of the module mounting is closed by the depressing of the airbag module. 
   In order to ensure a triggering of the horn in case of a decentral pressure exertion onto the airbag module, several contact sites are provided, and the airbag module must be able to tilt laterally. This requires a relatively large distance between the side wall of the airbag module and the lateral guide of the airbag mounting, which contradicts the requirement for as small gap dimensions as possible and in addition promotes undesired vibrations of the airbag module. 
   German Patent No. 196 25 722 shows a vehicle steering wheel with a movable airbag module which can be depressed by a limited axial stroke for generating a horn signal. The distance between the airbag module and the lateral guides of the steering structure is provided by laterally projecting extensions of the airbag module. 
   A further disadvantage of the hitherto conventional floating horn modules is the very costly electrical connection of the plurality of horn contacts which requires several cables and/or printed circuit boards. 
   It is an object of the invention to provide a vehicle steering wheel with a floating horn module which, with few components, ensures a reliable triggering of the horn. 
   BRIEF SUMMARY OF THE INVENTION 
   The steering wheel according to the invention comprises a movable airbag module which can be depressed by a limited axial stroke for generating a horn signal. The vehicle steering wheel further comprises a sensor system operating in a contactless manner is provided for determining the stroke. According to the invention, the triggering of the horn therefore does not take place by mechanical closing of contacts, but rather as a function of the stroke of the airbag module as determined by the sensor system. This has the advantage that the requirement of the plurality of horn contacts and compression springs is dispensed with. The stroke is only measured at one point which, in addition, is also virtually freely selectable. The contactless detection of the stroke has the advantage furthermore that no wear or ageing of the contacts takes place (e.g. through spark erosion). 
   The measurement of the stroke can be facilitated in that the airbag module is guided parallel to a steering axis of the steering wheel. The airbag module then performs an axial movement (i.e. not a tilting movement) independently of the actuation position (site of pressure exertion), such stroke being readily measurable. In addition, small gap dimensions can be achieved through the parallel guiding. 
   The parallel guiding of the airbag module can be achieved or at least assisted by several guide elements arranged radially between the airbag module and an airbag module mounting. 
   According to a first alternative, the sensor system comprises a continuous path measurement device yielding an analog output signal. The evaluation of the path measurement takes place in this case preferably in a microcontroller with which the path measurement device is connected. In the case of multifunctional steering wheels advantageously a microcontroller can be used which is already present anyhow. In such a microcontroller, the data and sequences necessary for triggering the horn can be integrated without difficulty. 
   According to a second alternative, the sensor system comprises a threshold value path measurement device yielding a digital output signal. This simpler and more favourably priced variant commends itself in steering wheels not including a microcontroller. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a ,  1   b  show diagrammatically a steering wheel with a floating horn airbag module according to the prior art; 
       FIG. 2  shows a transparent perspective view of a steering wheel according to the invention; 
       FIG. 3  shows a further transparent perspective view of the steering wheel of  FIG. 2  without outer covering; 
       FIG. 4  shows a perspective rear view of the steering wheel of  FIG. 2 ; 
       FIG. 5  shows a perspective detail view of a stroke limiter of the steering wheel of  FIG. 2 ; 
       FIG. 6  shows diagrammatically a parallel-guided floating horn airbag module; 
       FIG. 7  shows a block diagram for a measurement of the stroke of the airbag module with a digital sensor; and 
       FIG. 8  shows a block diagram for a measurement of the stroke of the airbag module with an analog sensor. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1   a  and  1   b  show in two-dimensional representation by way of example a conventional steering wheel  10  with a floating horn airbag module  12  in a mounting  14  of the steering wheel structure. The airbag module  12  is mounted on several compression springs  16  at an axial distance from several button switches  18 . A sufficiently great radial distance d must be provided between the airbag module  12  and the mounting  14 , in order to ensure an actuation of a button switch  18  (see  FIG. 1   b ) with a decentral depressing of the airbag module  12 , without the airbag module  12  jamming in the mounting  14 . 
     FIGS. 2 to 5  show a steering wheel  20  according to the invention with a sensor device  22   a ,  22   b  operating in a contactless manner (illustrated only symbolically), for detecting the stroke in case of a depressing of the floating horn airbag module  24 . The airbag module  24  is held in a cylindrical outer covering  26  (not illustrated in  FIGS. 3 and 5 ), which is closed by a base  28  on the side facing away from the vehicle occupant. The airbag module  24  is mounted on a guide device  30 , arranged on the base  28 , by means of only one compression spring  32 . The guide device  30  provides for a guiding of the airbag module  24  largely parallel to the axis of rotation of the steering wheel  20 . The parallel guiding is assisted by several guide elements  34  provided between the airbag module  24  and the outer covering  26 . The guide elements  34  can be fastened either on the airbag module  24  or on the outer covering  26 . In particular, polyoxymethylene (POM), which has proved to be successful as a sliding bearing material and which is suitable for dry operation, is suited as material for the guide elements  34 . 
   The stroke of the airbag module  24  is delimited in both axial directions by stops  36 ,  38 , which are illustrated on an enlarged scale in  FIG. 5 . The shape of the stops  36 ,  38  which are fastened to the outer covering  26 , in addition prevents a rotation of the airbag module  24 . 
   The parallel guiding of the airbag module  24 , which is illustrated diagrammatically again in  FIG. 6 , permits a reproducible movement of the airbag module  24 , independently of the actuation position on depressing of the airbag module  24 . The movement of the airbag module  24  can be reliably detected by the sensor device  22   a ,  22   b . The particular characteristic of the sensor device  22   a ,  22   b  lies in that the stroke is detected in a contactless manner, and the horn is triggered or not depending on the determined stroke. Button switches or similar contact devices, which are possibly susceptible to wear, are not necessary. The detection of the stroke can take place either in a threshold manner or continuously, as is explained below with the aid of  FIGS. 7 and 8 . 
     FIG. 7  shows a block diagram for a threshold path measurement of the stroke of the airbag module  24  with a digital sensor system. The force exerted by the vehicle occupant by pressing onto the airbag module  24  is converted, as described above, by means of the parallel guidance into a largely linear axial movement of the airbag module  24 . A path measurement device  40  determines the stroke of the airbag module and emits an altered output signal on exceeding of a given threshold value. This altered output signal can, for example, be a voltage generated by means of a Schmitt trigger on a defined level, which is increased compared with a defined initial voltage present when the threshold value has not been exceeded. The horn is then triggered or not depending on this digital output signal (“high” or “low”). 
     FIG. 8  shows a corresponding block diagram for a continuous path measurement with an analog sensor system. The path measurement device  40 ′ here, in contrast to the previously described threshold measurement, does not emit a digital but rather an analog output signal, changing with the stroke of the airbag module  24 . This signal is supplied to a microcontroller  42  which by means of a stored mathematical function calculates therefrom the stroke of the airbag module  24 . In addition to the mathematical function which describes the dependence of the stroke on the output signal of the path measurement device  40 , a threshold value is also stored. If the calculated stroke of the airbag module  24  exceeds this threshold value, the horn is triggered. 
   The evaluation of the analog output signal of the path measurement device  40 ′ in the microcontroller  42  is particularly simple when the path measurement device  40 ′ generates a voltage proportional to the displacement of the airbag module  24 , so that a straight line is produced as output characteristic of the path measurement device  40 ′. In this case, only the incline and the axis intercept of the straight line have to be determined through reference measurements of the output voltage in the non-displaced state (position of rest) and on maximum stroke of the airbag module  24  (the knowledge of the maximum stroke is presupposed). 
   The use of the analog sensor system has the advantage of a better resolution compared with the digital system. The position of the airbag module  24  is known at any time. Furthermore, the analog system offers the possibility of adapting the parameters deposited in the microcontroller  42 , which can also take place within the scope of a self-diagnosis. The functionality necessary for triggering the horn can be integrated without difficulty into an available microcontroller of a multifunctional steering wheel. 
   In the following table, various possibilities are set out for a contactless measurement of the stroke of the airbag module. 
   
     
       
             
             
             
           
         
             
                 
             
             
               Measurement 
                 
                 
             
             
               Principle 
               Realization 
               Output Signal 
             
             
                 
             
           
           
             
               Capacitive 
               capacity change 
               analog 
             
             
                 
               resonant circuit damping 
               analog 
             
             
               Inductive 
               inductivity change 
               analog 
             
             
                 
               resonant circuit damping 
               analog 
             
             
                 
               transformatory 
               analog 
             
             
               Magnetic 
               Reed contact 
               digital 
             
             
                 
               Hall sensor 
               digital 
             
             
                 
               Magnetic field sensor 
               analog 
             
             
               Optic 
               light barrier 
               digital 
             
             
                 
               lateral effect diode 
               analog 
             
             
                 
               diode as transmitter, 
               analog 
             
             
                 
               phototransistor system as receiver 
             
             
               acoustic 
               ultrasonics 
               analog