CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 U.S. National Stage of International Application No. PCT/SE2012/050013, filed Jan. 11, 2012, which claims priority to European Patent Application No. EP 11000604.6, filed Jan. 26, 2011. The disclosures of the above applications are entirely incorporated by reference herein. 
     TECHNICAL FIELD OF THE INVENTION 
     When a motor vehicle is involved in a head-on collision with a pedestrian, the body and/or head of the pedestrian often strikes the windscreen of the vehicle. This can result in serious injury to the pedestrian, and significant damage to the vehicle. 
     In order to better protect the pedestrian, it has been proposed to provide a pedestrian airbag which, if vehicle sensors indicate that a relevant collision with a pedestrian is occurring or is likely to occur, inflates to cover a central portion of the windscreen, and/or the left and right A-pillars of the vehicle. Typically, the air-bag is stored beneath a rear edge of the vehicle&#39;s bonnet, or beneath a cowl which is provided between the base of the windscreen and the rear of the bonnet. The rear of the bonnet may be lifted, by a suitable air-bag, piston or other arrangement, to allow sufficient space for the air-bag to inflate. 
     The aim of a pedestrian airbag of this type is to provide an inflated cushion over the windscreen and/or A-pillars of the vehicle, thus cushioning the impact of a pedestrian with these parts of the vehicle. 
     One potential problem that can occur with the use of airbags of this type is that, once the airbag has been inflated, the driver of the vehicle is hindered to look out of the windscreen. If a vehicle collides with a pedestrian with sufficient force that the pedestrian strikes the windscreen of the vehicle, the vehicle is likely to be travelling at a considerable speed. If a pedestrian airbag inflates while the vehicle is travelling at speed, and subsequently obscures the driver&#39;s view for a long period of time, the result may be that the driver fails to avoid one or more further objects or hazards before bringing the vehicle to a complete stop. 
     PRIOR ART 
     One solution to this problem is presented in the generic JP 2006219046. This document discloses a pedestrian air-bag which has a pair of straps, which are attached to upper left and right corners of the air-bag. As the air-bag inflates the straps are drawn from a reel whereat tensioning a spring being connected to the reel such that the spring is stressed while the airbag is deploys. A locking device prevents the reel from turning into the unloading direction of the spring. The locking device can be brought into an unlocking state by means of a manually activated trigger. After manually activating the trigger, the deflating airbag is drawn from the windscreen allowing the driver to see clearly out of the windscreen again. 
     The problem of the solution presented in this document is, that usually the driver who is involved in a accident is too busy or too confused to operate the trigger in an early stage of the accident where it would be necessary. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved pedestrian airbag arrangement of the generic type. 
     The airbag arrangement according to the invention comprises an airbag covering at least a part of the windscreen and/or at least a part of at least one A-pillar of the vehicle when in deployed state, an inflator being in fluid connection to said airbag, and a retraction means for retracting said airbag, said retraction means comprising a triggering means and a retractor retracting the airbag when released by the triggering means. In order to solve the above defined problem, the triggering means is activated by the airbag. 
     The basic idea of this invention is to provide a triggering means which is automatically activated, namely by the deploying airbag. This means that the triggering means does not need a separate triggering signal especially in form of an electrical signal on the one hand but also no manual triggering on the other hand. This has several advantages: Since the triggering occurs automatically, the retraction of the airbag from the windscreen occurs to a predetermined time, especially not too late. Because it is not necessary to provide an electrical wiring the mounting is easy and cost-effective since the triggering means does not need to be connected to the electrical system of the vehicle. Further, the triggering of the retraction means by the deploying airbag makes the retraction means very reliable since there is no risk that the retraction means fail because of a broken electrical connection. 
     Preferably, the triggering means is connected to the airbag via a pull element like a strap. This pull element is called activation pull element. Usually, the retractor means is connected to the airbag also via a pull element like a strap. This pull element is called retraction pull element The retractor can especially comprise a spring biased reel. That means that a spring acting on this reel is provided with the spring being pre-loaded in the initial state of the retractor. 
     In the first basic concept the activation pull element and the retraction pull element are separate elements. This has the advantage that this may make it easier to adapt the retraction means more easily to the given geometry. 
     In a second basic concept the activation pull element and the retraction pull element are the same pull element. This has the advantage that fewer parts are needed. 
     It can in some applications be preferred that the retraction means, usually via the retraction pull element, does also influence the ventilation of the airbag, especially that it opens a ventilation opening. In this case the airbag is at the same time ventilated and retracted, leading to a quicker removal of the airbag from the windscreen. 
     Preferred embodiments and further advantages of the invention result from further sub-claims as well as from the preferred embodiments described hereinafter. 
     The invention will now be described by means of preferred embodiments in view of the accompanying drawings. 
     The drawings show: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  the front of a passenger car with a retraction means according to a first basic concept of the invention in an early deployment phase of the airbag in a schematic representation, 
         FIG. 2  what is shown in  FIG. 1  when the airbag is fully deployed, 
         FIG. 3  what is shown in  FIG. 2  after the airbag has been retracted, 
         FIG. 4  a retraction means that could be used in the first concept in a state where the retractor is locked, 
         FIG. 5  what is shown in  FIG. 4  after the retractor has been unlocked by a triggering device, 
         FIG. 6  a second basic concept of the invention in a representation according to  FIG. 1 , 
         FIG. 7  what is shown in  FIG. 6  in a state when the airbag is fully deployed, 
         FIG. 8  what is shown in  FIG. 7  after the airbag has been retracted, 
         FIG. 9  a triggering and retraction device which could be used in the second basic concept, 
         FIG. 10  a triggering and retraction device according to a second embodiment in a perspective representation, 
         FIG. 11  what is shown in  FIG. 10  in another angle of view, 
         FIG. 12  what is shown in  FIGS. 10 and 11  after the retractor has been released by the triggering means, 
         FIG. 13  what is shown in  FIG. 12  viewed from another angle, partially exploded 
         FIG. 14  a third embodiment of a triggering and retraction device in a locked state, 
         FIG. 15  what is shown in  FIG. 14  in an unlocked state, 
         FIG. 16  a section of an airbag with a ventilation opening being covered by the end of a strap, 
         FIG. 17  what is shown in  FIG. 16  after the ventilation opening has been opened, 
         FIG. 18  a fourth embodiment of a combined triggering and retraction device in a sectional view, 
         FIG. 19  what is shown in  FIG. 18  viewed from direction A, 
         FIG. 20  a fifth embodiment of a combined triggering and retraction device, 
         FIG. 21  a sixth embodiment of a combined triggering and retraction device, 
         FIG. 22  a top end of an airbag in a not completely deployed state and 
         FIG. 23  a variation of what is shown in  FIG. 22 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Basic Concept 
     The  FIGS. 1 to 3  show a front part of the vehicle. A pedestrian airbag device is provided as is generally known in the art. This pedestrian airbag device comprises an airbag module  20  having a housing  22 , a gas generator  24  and an airbag  26 . This airbag  26  is arranged and configured in order to cover at least a part of the windscreen  10  and/or one or both of the A-pillars. The housing can be located beneath the bonnet  12  or in a space between the rear end of the bonnet and the windscreen. This is not described in detail since this concept is known in the prior art. 
     In order to remove the airbag  20  from the windscreen at least in parts after it has been fully deployed in order to make it possible for the driver to look through the windscreen so that he may prevent a further accident, a retraction means for retracting said airbag is provided. This retraction means comprises a retractor  40  for retracting the airbag  26  and triggering means for triggering—this means releasing—the retractor  40 . The retractor  40  in this embodiment comprises a spring biased reel  42 . This means that the spring acting on the reel is pre-loaded in the initial state. A first end of a retraction strap  112  forming the retraction pull element is connected to an upper area of the airbag  26 , the second end of this retraction strap  112  is connected to this spring biased reel  42  of the retractor. In the first basic concept described in  FIGS. 1 to 3  a separate retractor  40  and a separate triggering device  30  are present. The triggering device  30  and the retractor  40  are connected via a mechanical connection. This mechanical connection is represented by the arrow  35 . An activation strap  110  forming the activation pull element connects the triggering device  30  with the airbag  26 . The activation strap  110  can be attached to the same point of the airbag as the retraction strap  112  or, as shown, to another point. 
     As one can see from  FIG. 2  the activation strap  110  (forming the activation pull element) is tensioned when the airbag is fully deployed and thus a force is applied from the airbag  26  via the activation strap  100  to the triggering device  30 . In this state the retraction starp  112  may not be tensioned. By this force the triggering device  30  is activated and triggers the retractor  40  via the mechanical connection  35 . By selecting the length of the activation strap  110 , one can determine at which deployment stage (at which position of the airbag) the triggering means is activated by the airbag. 
     After the retractor  40  has been triggered, the spring-biased reel  42  is released, the retraction strap  112  is wound onto a reel  42  and so the airbag  26  is pulled away from the windscreen. 
     One can see that the retraction means being comprised of the retractor  40 , the triggering device  30 , and the mechanical connection acting between the triggering device  30  and the retraction device  40  is independent from the electrical system of the vehicle and is exclusively controlled in a mechanical way by the deployment of the airbag  26  via the activation strap  110 . 
     The  FIGS. 4 and 5  show an embodiment how the triggering device  30 , the retractor  40  and mechanical connections  35  can be made: 
     The triggering device  30  comprises a bolt  50  that has a thread  52  at one end. A nut  54  is threaded on this thread  52  and the activation strap  110  is wound on the bolt  50 . A bar  60  forming the mechanical connection is rigidly connected to the nut  54 . Two guiding cylinders  64  extend from the bar  60  parallel to the bolt  50  into guiding bores  68  of a guiding block  66 . So, the bar  60  cannot pivot but only move into the axial direction of the bolt  50 . The bar  60  further comprises a locking bore  62 . 
     The retractor  40  has a reel  42  whose axis  42   a  is rotary held in bearings  44 . The axis  42   a  and the reel  42  are rigidly connected to each other. A pre-loaded constant force spring  45  acts on the axis  42   a  in a known manner. The retraction strap  112  is connected to the reel  42 . Finally, a locking pin  46  is connected to the reel  42  and extends through the locking bore  62  of the bar  60 . 
     When the airbag  26  pulls on the activation strap  110 , the activation strap  110  is unwound from bolt  50 . Since the nut  54  cannot rotate the nut  54  and the bar  60  move into the axial direction R and so the locking pin  46  is released from the locking bore  62  leading to a release of the reel  42  which is then driven by the constant force spring  45  and winds the retraction strap  112 . One can see that there is a certain time delay between the initial strengthening of the activation strap  110  and the release of the retractor  40 . 
     Second Basic Concept 
     The  FIGS. 6 to 8  show a second basic concept. Here, the triggering means is not located in a separate triggering device, here only one combined triggering and retraction device  36  is present combining both: the retractor and the triggering means. Also, only one sole strap  114  is provided instead of a separate activation strap and separate retraction strap. This sole strap  114  serves as activation pull element as well as retraction pull element. Once the sole strap  114  is tensioned ( FIG. 7 ) the sole strap  114  activates the triggering means that trigger the retractor which can also comprise a spring-biased reel  42 . 
       FIG. 9  shows a first embodiment of such a combined triggering and retraction device  36  in a schematic representation. The sole strap  114  is wound on the spring-biased reel  42  of the retractor, deflected by a deflector roller  72  and clamped between a locking member  76  and a release member  74 . The release member  74  is preferably breakable when the force applied to it is higher than a certain threshold value, but it can also be a pivotable or displaceable element which is removed from the locking member  76  when the force applied to it reached a predetermined value. By the clamping of the sole strap  114  between the release member  74  and the locking member  76  in the initial state, the spring-biased reel  42  is locked by the sole strap  114  itself. When the airbag is fully deployed ( FIG. 7 ), a tensile force is applied to the sole strap  114  and the sole strap  114  applies force to the release member  74  which then breaks. After this, the reel  42  is unlocked and winds the sole strap  114  which retracts the airbag (see  FIG. 8 ). The dashed line in  FIG. 9  shows the position of the sole strap  114  after the release member  74  serving as the triggering means was broken by the sole strap  114 . 
     The  FIGS. 10 to 13  show a second embodiment of a combined triggering and retraction device  36 . This combined retraction and triggering device  36  comprises a housing  80  in which a spring biased reel  42  serving as retractor is accommodated. As in the other embodiments described, a sole strap  114  is wound onto this spring biased reel  42 . The axis  42   a  of this reel  42  extend through the face side  82  of this housing as can be seen in  FIG. 12 . At some distance from this axis  42   a  two cone shaped protrusions  84  extend from the face side  82  of the housing  80  in axial direction of this axis  42   a . A first opening  86  and a second opening  87  are provided in the housing  80  and the groove  88  is provided between these two openings and encircling approximately a quarter of the circumference of the housing. 
     A locking arm  90  is pivotably held on the face side  82  of the housing by means of a bearing pin  98  which extends from the locking arm into a bearing hole extending into the face side  82  of the housing  80  between the two cone shaped protrusions  84 . The cone shaped protrusions  84  extend through bores  92  of this locking arm  90 . The end  94  of the arm extends vertically from the main part of the locking arm  90  into the groove  88  and shows an ear  96  through which the sole strap  114  projects. So, the sole strap  114  is guided from the first opening  86  through the ear  96  and from there through the second opening  87 . 
     In the initial state shown in the  FIGS. 10 and 11 , the locking arm  90  which servers as the triggering means, is lying onto the face side  82  of the housing  80 . In this state the axis  42   a  of the reel  42  is locked by the locking arm  90 . For this purpose the side of the locking arm  90  showing towards the face side  82  of the housing has a locking section locking the axis  42   a . This locking section cannot be seen in the drawings due to the chosen representation. When force is applied to the sole strap  114  because of the deploying airbag, the sole strap  114  pivots the locking arm  90  into the position shown in  FIG. 12 . Because of this pivotal movement the locking arm  90  is raised due to the cone shaped protrusions  84 . By this raising of the locking arm  90 , the locking section of the locking arm  90  loses contact to the axis  42 , the spring biased reel  42  is unlocked and the sole strap  114  is wound onto the reel. 
     The  FIGS. 14 and 15  show a third embodiment of a combined triggering and retraction device. Here, the spring biased reel  42  is mounted on a carrier  106  through which the axis of the spring biased reel  42  (the spring is not shown in the figures) extends. This carrier is movable in direction A on a base  100 . The axis  42   a  has flat ends  42   b  which are held in the initial state in locking grooves  104  of a locking bar  102  being rigidly connected to the base  100 . As in the last two embodiments the sole strap  114  is wound onto the spring biased reel which is locked in the initial state because the flat ends  42   b  are held in locking grooves  104 . 
     When force is applied by the sole strap  114  onto the spring biased reel  42  due to the deploying airbag the reel  42  moves into direction A, the flat ends  42   b  leave the locking grooves  104 , the spring biased reel  42  is unlocked and winds the sole strap  114 . Here, the movable spring biased reel  42  is both: a part of the retractor and a part of the triggering means. 
     In order to remove the airbag more quickly from the windscreen it can be preferred that the airbag is vented when retracted. A possibility how this can be achieved in an easy and safe way is shown in  FIGS. 16 and 17 . The retraction strap  112  or the sole strap  114  (depending on the embodiment) is sewn to the airbag via two different seams namely a permanent seam  122  and a tear seam  120 . A ventilation hole  28  is encircled by the tear seam  120  and when force is applied to the retraction strap/the sole strap by the retractor, the tear seam breaks and opens the ventilation hole  28  while the permanent seam  122  stays intact ( FIG. 17 ), so that the ventilation and the pulling away of the airbag from the windscreen occurs simultaneously. 
     The  FIGS. 18 and 19  show another embodiment of a combined triggering and retraction device which is in some respect similar to the one shown in  FIGS. 10 to 13 . Here, the reel  42  is held in a housing  120  with the housing being basically in a shape of a hollow cylinder in which the reel  42  is at least partially held in an coaxial way. This can be seen best in  FIG. 18 . The coaxial housing  120  shows an opening  122 , through which the sole strap  114 , which is fixed to the reel  42 , extends. In an area, where the reel  42  is located inside the housing, the reel  42  has a clamping protrusion  130  whose dimensions are such that it extends close to the inner surface of the housing  120  but is not in contact with the same. In the initial state shown in  FIGS. 18 and 19  the sole strap  114  extends from the reel  42  to said clamping protrusion  130  and a section of the sole strap  114  is located between the said clamping protrusion  130  and the inner side of the housing  120 , such that clamping occurs. From the clamping protrusion  130  the sole strap  114  extends to the airbag  26  as the described above. A pre-loaded spring is acting on the axis  42   a  of the reel  42  as is also described above (not shown in the Figures). Because a section of the sole strap  114  is located between the clamping protrusion and the inner wall of the housing clamping occurs and the reel  42  can not rotate. When the airbag deploys a pulling force is applied to the sole strap  114 , such that it is pulled away from the clamping protrusion so that no more clamping exists, the reel  42  rotates and the sole strap  114  is spooled onto the reel  42  as described above. 
     The  FIG. 20  shows an embodiment in which the reel  42  is directly blocked by the airbag  26 . For this purpose the reel  42  is rigidly connected to a blocking part  140  which has a flat surface  142 . This flat surface  142  lies against the airbag package, such that this blocking part  140  and thus the reel  42  cannot rotate, as is shown in  FIG. 20 . The geometry is chosen such that when the airbag is deployed the blocking part  140  is no longer blocked by the airbag package so that it can rotate. As an alternative the flat surface  142  can lie against a filling channel of the airbag, such that it is blocked as long as the airbag is in form of a package or the filling channel stays under pressure, such that the pulling back of the airbag does not take place before the airbag starts to lose its pressure. A pre-loaded spring is acting on the axis of the reel (not shown in the Figures). 
       FIG. 21  shows an embodiment which is to some respect similar to the one shown in  FIG. 20 . Here, we have a rotatable blocking part  140  with a flat surface  142  and an arm  144 . As in the embodiment in  FIG. 20  this blocking part is blocked as long as the airbag is not deployed, since the flat surface lies against the airbag package. The sole strap  114  is rigidly connected to the blocking part  140  and extends further to a spiral spring  150 . When the airbag is deployed the blocking part  140  can rotate and the necessary pulling force is basically applied to the sole strap  114  by the arm  144 . Additionally a pre-loaded spring can act on the axis of the reel (not shown in the Figures). 
       FIG. 22  shows a possibility how one can make sure that the triggering by the activation strap  110  or the sole strap  114  does not take place before the airbag  26  is completely deployed. For this purpose the activation strap  110  or sole strap  114  encircles the top end  26   a  of the airbag as is shown in  FIG. 22 . One can see clearly from the figure that this airbag will not apply its full force to the strap before the airbag  26  has reached its completely deployed state. 
       FIG. 23  shows an alternative to what is shown in  FIG. 22 . Here, the top end of the airbag  26  is folded inside the airbag. 
     LIST OF REFERENCE NUMBERS 
     
         
           10  windscreen 
           12  bonnet 
           20  Airbag module 
           22  housing 
           24  gas generator 
           26  airbag 
           26   a  top end 
           30  triggering device 
           35  mechanical connection 
           36  combined triggering and retraction device 
           40  retractor 
           42  reel 
           42   a  axis 
           42   b  flat end 
           44  bearing 
           45  constant force spring  45   
           46  locking pin 
           50  bolt 
           52  thread 
           60  bar 
           62  locking bore 
           64  guiding cylinder 
           66  guiding block 
           68  guiding bore 
           70  sole pull element 
           72  deflector role 
           74  release member 
           76  locking member 
           80  housing 
           82  face side 
           84  cone shaped protrusion 
           86  first opening 
           87  second opening 
           88  groove 
           89  bearing hole 
           90  locking arm 
           92  bore 
           94  end of the arm 
           96  ear 
           98  pin 
           100  base 
           102  locking bar 
           104  locking groove 
           106  carrier 
           110  activation strap 
           112  retraction strap 
           114  sole strap 
           120  coaxial housing 
           122  opening 
           130  clamping protrusion 
           140  blocking part 
           142  flat surface 
           144  arm
A pedestrian airbag arrangement for a vehicle includes an airbag covering at least a part of the windscreen and/or at least a part of at least one A-pillar of the vehicle when it is in deployed state, an inflator in fluid connection to the airbag, and a retraction means for retracting the airbag. The retraction means includes a triggering means and a retractor retracting the airbag when released by the triggering means. In order to provide an automatic retraction of the airbag, the triggering means is activated by the airbag.