Abstract:
A drive for a hood on a motor vehicle, in particular an engine hood, which is movable between a closed position and a lifted position that is abruptly achieved in the case of an accident. The drive comprises a drive unit having an electric drive motor and being effective between a vehicle structure and the hood. The drive motor moves the hood into the lifted position in the case of an accident.

Description:
TECHNICAL FIELD 
     The invention relates to a drive for a hood on a motor vehicle, in particular an engine hood. 
     BACKGROUND OF THE INVENTION 
     Such hoods are movable between a closed position and a lifted position, the drive comprising a drive unit active between a vehicle structure and the hood. 
     A pedestrian is able to be protected in a collision with a passenger car against injuries by lifting the engine hood, which enhances the deformation path thereof. 
     It is desirable that such lifting of the engine hood can be performed in case of an accident very rapidly and, if no collision occurs, repeatedly without the replacement of components, and further, that the engine hood can readily be transferred again into its closed position. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a drive for a hood on a motor vehicle, which allows in a simple manner a repeated, rapid lifting of the hood as well as a simple return into the closed position thereof. This is achieved in a drive which comprises a drive unit with an electric drive motor which is active between a vehicle structure and the hood. The drive motor moves the hood rapidly into a lifted position in case of an accident. Using the electric motor as energy source to lift the hood allows a lifting action without single-use parts like pyrotechnical units. Further, the movement of the hood can be predetermined very exactly and oscillations of the hood on reaching of the lifted position can be avoided. 
     Preferably, the energy for opening the hood is made available exclusively by the electrical motor. However, it is possible to provide an additional energy storing device that supports the electric motor during the initial movement of the hood. The drive motor preferably brings about in a dual function the releasing of the traveling stop, so that the hood can be lifted, as well as the lifting of the hood into the lifted position. Additionally, the motor can also return the hood from the lifted position into the closed position. If an energy storing device is provided, the latter movement is performed preferably in such a manner that the drive motor operates against the energy storing device. 
     The hood preferably concerns an engine hood, like in the example initially mentioned; the invention, however, is also transferable to other hoods in the vehicle such as trunk lids and so on. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a vehicle hood including an energy storing device and a drive according to the invention, in a closed position of the hood; 
         FIG. 2  shows the hood of  FIG. 1  in a lifted position; 
         FIG. 3  shows the hood of  FIG. 1  in a maintenance or end position; 
         FIG. 4  is an exploded partial view of a drive unit according to the invention; 
         FIG. 5  is a schematic, perspective partial view of a drive unit used with the invention, as seen from a first side with an opened housing; 
         FIG. 6  shows the drive unit of  FIG. 5  from an opposite side with an opened housing; 
         FIG. 7  is a schematic, exploded partial view of the drive unit used with the invention without showing the housing; 
         FIG. 8  is a schematic, perspective partial view of the drive unit of  FIG. 5  without the housing; 
         FIG. 9  shows a section along line IX—IX of  FIG. 5 ; 
         FIGS. 10  to  13  are schematic illustrations of the drive unit at various moments during releasing the traveling stop without showing the housing; 
         FIG. 14  shows the movement of the hood versus time in turning angle of the drive motor; and 
         FIG. 15  shows the motor turning speed versus time during the opening of the hood. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A hood  10  of a motor vehicle, an engine hood in the example described here, is engaged by a drive unit  14  designed as a closed assembly unit. The drive unit  14  has a crank mechanism  16  with a first arm  18  and a second arm  20 , the free end of second arm  20  being connected with the underside of the hood  10 . In the closed position, the end of the first arm connected with the second arm is oriented so as to point away from the hood. 
     Optionally, the hood is connected with an energy storing device  12  separate from the drive unit  14 , which is realized here in the form of a compression spring (helical spring) and which in the closed position of the engine hood ( FIG. 1 ) exerts a force F onto the hood  10  at a place remote from the drive unit  14 . The energy storing device  12  has its maximum energy, i.e. the spring is under tension, in the closed position. 
     In the event of a collision, e.g. with a pedestrian, the hood  10  is brought into a lifted position (FIG.  2 ). The energy for that process is taken mainly from an electrical drive motor  44  that is part of the drive unit  14  and will be described later on in greater detail. The energy storing device  12  supports only the initial movement of the hood  10 . In this lifted position, the first arm  18  of the crank mechanism is rotated by 90° (in anti-clockwise direction here) with respect to its position when closed. The hood  10  is now lifted with respect to the closed position. Lifting the hood  10  is performed so rapidly that e.g. a pedestrian, which is struck by the passenger car, will hit the hood  10  which is already lifted. The drive unit and the hood  10  are designed such that the hood  10  can be deformed in order to take up the energy of the pedestrian. If no collision occurs or after the impact, the hood  10  can again be transferred into the closed position by the drive unit  14 . 
     The drive unit  14  may also be used for lifting the hood  10  into a maintenance position that goes beyond the lifted position and is shown in FIG.  3 . In one embodiment of the invention, this position can also be the end position for the lifting movement, i.e. the lifted position. The crank mechanism is moved so far until the end of the first arm  18  that is connected with the second arm  20  is rotated by approximately 180° with respect to the closed position, and the crank mechanism arrives at a dead center position. 
     The drive unit  14  has a housing  22  with an outer side to which the crank mechanism  16  is mounted. Provided on the housing  22  are stops  24  for the first arm  18 . 
     Arranged in the interior of the housing  22  is a first toothed wheel  26  which is non-rotatably connected with the crank mechanism  16  by means of a shaft  28 . This toothed wheel  26  meshes with a second toothed wheel  30  which is connected with a first locking disc  34  by means of a shaft  32 . A second locking disc  36  is rotatably provided on the shaft  32 . In the closed position of the hood  10 , a locking pawl  38  rests against the first locking disc  34  (see stop  50  in FIG.  11 ), so that the shaft  32  is prevented from rotating in one direction. The locking pawl  38  is pivotally mounted on the housing and is biased in a direction towards the first locking disc  34  by means of a restoring spring. By means of a toothed belt  40 , which is applied to the locking disc  36 , and a third toothed wheel  42 , the second locking disc  36  realized as a toothed wheel can be driven by the electric drive motor  44 . The drive motor  44  is a known high-speed, brushless, low inertia DC servo motor. The rotor of the motor  44  is bell-shaped so as to reduce its inertia further. The motor  44  is able to provide the energy to lift a 20 kg engine hood in about 60 ms over a lifting distance of about 60 mm. 
     The toothed wheels  42 ,  36 ,  30  and  26  are components of a two-stage reduction gear. 
     The locking pawl  38  together with the two locking discs  34 ,  36  forms a traveling stop which blocks in the closed position a rotation of the first toothed wheel  26  and, hence, of the crank mechanism  16 . Since the hood  10  is connected with the second arm  20 , a lifting of the hood  10  e.g. by the energy storing device is prevented in this position. The traveling stop as well as the mode of releasing it (see also  FIGS. 7  to  13 ) will be described in detail in the following. 
     The first locking disc  34  has four protrusions  46   a ,  46   b  which project into corresponding openings  48   a ,  48   b  in the second locking disc  36 . Helical springs  52  are arranged between the protrusions  46   b  and the edge of the openings  48   b . The protrusions  46   a ,  46   b  are inserted in the openings  48   a ,  48   b  so as to have a specific play, so that the second locking disc  36  can be rotated with respect to the first locking disc  34  by a specific angle and against the restoring force of the springs  52 . It is only the first locking disc  34  which is non-rotatably connected with the shaft  32 . 
     On its outer edge, the first locking disc  34  has a stop  50  against which the locking pawl  38  rests in the closed position (FIG.  10 ). The second locking disc  36  has a slope  54  in the region of the contact surface of locking pawl  38 . 
     If the motor  44 , which has a very quick run-up, is in operation, then force is transferred through the toothed belt  40  to the second locking disc  36 , and the second locking disc  36  is slightly rotated (see arrow in  FIGS. 11 and 12 ) with respect to the first locking disc  34 , because the first locking disc  34  is retained by the locking pawl  38 . Due to the rotation of the second locking disc  36 , the slope  54  moves with respect to the locking pawl  38 , so that the locking pawl  38  is lifted off from the stop  50  of the first locking disc  34  (FIG.  12 ). The traveling stop is released now, and the protrusions  46   b  rest against circumferential wall portions of the openings  48   b , so that the locking disc  6  drives the locking disc  34  (FIG.  12 ). No external mean for releasing the locking pawl  38  is provided. 
     A programmable electronic actuation unit  60  ( FIG. 1 ) coupled to a crash sensor (not shown) actuates the drive motor  44  on signal of the sensor. The motor is then accelerated very rapidly to initiate the movement of the hood into its lifted position (FIG.  15 ). The inertia of the motor  44  and drive  14  are to be neglected in comparison with the inertia of the hood  10 . After the drive motor  44  has accelerated the hood  10 , the actuation unit  60  controls the motor  44  in a manner that the hood is decelerated over the final part of the lifting distance, so that the hood reaches the lifted position with a relatively slow speed and no abrupt stop of the movement occurs. Oscillations of the hood can be avoided. The travelling curve of the hood is shown in FIG.  14 . During the last part of the lifting distance the motor  44  effectively acts as a brake as the actuation unit  60  is appropriately programmed to amend the controlling of the motor after the initial acceleration of the hood. In the lifted position, the hood is preferably locked against movement back into the closed position by the crank mechanism  16 , which e.g. in a known way is provided with a snap-in locking device, which can be released by further movement of the crank mechanism (not shown), or which is in a dead center position shown in FIG.  3 . 
     For transferring the hood  10  from the lifted position back into the closed position, the motor  44  is operated in the reverse direction of rotation. The motor power is transferred to the hood  10  through the gear and the crank mechanism  16 ; with the result that the hood is moved back into its closed position against the force of the energy storing device  12 . In so doing, energy is returned to the energy storing device  12 . If the hood  10  is in the closed position again, the motor is switched off and the locking pawl  38  again comes to rest against the stop  50 . As long as the motor  44  does not provide a rotation of the second locking disc  36 , the locking pawl  38  continues to rest against stop  50 , and the hood  10  remains in its closed position. 
     In order to lift the hood  10  beyond the lifted position into the maintenance position, the motor  44  is operated in the same direction as for releasing the traveling stop, if the hood  10  is in the lifted position. The crank mechanism  16  can be extended so far until the first arm  18  rests against its housing stop  24 . When this position is the end position of the hood lifting movement, the motor  44  moves the crank mechanism  16  until the dead center position is reached.