Abstract:
A compact drive unit for actuating parking brakes is provided The drive unit has a reduction gear that is formed by a two-stage belt drive, in combination with a drive motor that is made up of a brushless direct current motor The two-stage belt drive preferably consists of two toothed belt drives functionally arranged in series; the direct current motor preferably has an external rotor with a rare-earth magnetic ring.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a drive unit for actuating a parking brake in a vehicle and having an actuating member that is coupled to the brake system. The drive unit has an electric drive motor and a rotatable force-transmission member to which the actuating member is connected. A reduction gear is arranged between the drive motor and the rotatable force-transmission member.  
         BACKGROUND OF THE INVENTION  
         [0002]    In luxury automobiles, such a drive unit replaces the hand or foot actuating means for the parking brake. This drive unit can be used to activate or deactivate the parking brake by means of manual actuation of an electric switch or else automatically, for example, via a tachometer signal or a tilt incline sensor.  
         SUMMARY OF THE INVENTION  
         [0003]    The invention provides a compact drive unit for actuating parking brakes which functions especially quietly with a high efficiency and which carries out the actuation procedure within a very short period of time. The drive unit according to the invention has a reduction gear that is formed by a two-stage belt drive, in combination with a drive motor that is made up of a brushless direct current motor. The two-stage belt drive preferably consists of two toothed belt drives functionally arranged in series, the direct current motor preferably has an external rotor with a rare-earth magnetic ring. The high efficiency of the two-stage belt drive offers the possibility of detecting and regulating the actuation force via the motor current without a separate force sensor. In combination with the high torque of the direct current motor that is used, very short actuation times of less than 0.5 seconds are achieved. This actuation time is sufficiently short to achieve an ABS function for the parking brake in case of emergency situations.  
           [0004]    Since the two-stage belt drive is reversible, i.e. it allows a reverse rotation, there is a need for a latching mechanism that maintains the activation state of the parking brake when the drive motor is switched off In a first embodiment, this latching mechanism is formed in that the first stage of the belt drive has a pulley with at least one latching recess on its circumference with which a spring-loaded ratchet interacts and which latches in the latching recess when the pulley rotates opposite to the drive direction. The ratchet can be disengaged from the latching recess by a cam disk that is coupled to the pulley so that it can rotate to a limited extent. Since the latching mechanism is situated on the first stage of the belt drive, just a few, for example two or three, latching positions distributed over the periphery of the pulley are sufficient to divide a total brake actuation stroke into adequately spaced partial strokes 
       
    
    
     SHORT DESCRIPTION OF THE DRAWINGS  
       [0005]    Additional characteristics and advantages of the invention ensue from the description below of a preferred embodiment and from the accompanying drawings The drawings show the following  
         [0006]    [0006]FIG. 1 a perspective view of the most important functional parts of the drive unit according to a first embodiment,  
         [0007]    [0007]FIG. 2 a sectional view of the drive unit,  
         [0008]    [0008]FIG. 3 a perspective partial view of the drive unit according to a second embodiment; and  
         [0009]    [0009]FIG. 4 a sectional view of the embodiment shown in FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0010]    Functional parts of a drive unit for parking brakes in vehicles as shown in FIG. 1 comprise a brushless direct current motor  10  with an external rotor, a reduction gear configured as a two-stage belt drive whose first stage consists of a pinion  12  connected to the external rotor of the direct current motor, a belt pulley  14  and a toothed belt  16 , and whose second stage consists of a pinion shaft  18  connected to the belt pulley  14 , a drum  20  and two parallel toothed belts  22 ,  24 , and a latching mechanism  26  that is situated on the belt pulley  14 . The axial ends of the drum have two toothed rings  28  that are shaped into the peripheral surface at an axial distance from each other and on which the toothed belts  22 ,  24  run The toothed belts  22 ,  24  are tightened around the drum  20  and the pinion shaft  18  by means of a shared, adjustable tension roller  30 . The axes of the direct current motor  10 , of the belt pulley  14  and of the pinion shaft  18 , which is coaxial thereto, as well as the drum are parallel, they can also lie in one shared plane.  
         [0011]    The reduction gear made up of the two-stage belt drive has an efficiency of over 80%. The brushless direct current motor  10 , whose rotor configured as an external rotor is equipped with a neodymium magnetic ring, develops a high starting torque of, for example, up to 2 Nm. This combination enables the drive unit to achieve extremely short actuation times in the range of less than one-half second.  
         [0012]    The latching mechanism  26  consists of two ratchets  32  that are situated diametrically opposed on the circumference of the belt pulley  14 —one of said ratchets  32  being shown in FIG. 1—and of corresponding latching recesses that have been left open on the outer circumference of the belt pulley  14 . The ratchets  32  are prestressed by suitable springs into the engaged position. A cam disk  34  is coupled to the belt pulley  14  so that it can rotate to a limited extent. On the outer circumference of the cam disk  34 , it has control cams on which the ratchet tips of the ratchets  32  slide which, at the same time, slide over the outer circumference of the belt pulley  14  that has been provided with the latching recesses.  
         [0013]    The drum  20  has a cable groove  36  (see FIG. 2) in its peripheral surface between the toothed rings  28  The cable groove  36  has a depth that is variable over the circumference, relative to the axis X-X of the drum. In the cable groove  36 , on the largest radius R 1  relative to the axis X-X, a traction cable  40  is connected by means of a pin  38  that passes axially through the drum  20  The traction cable  40  forms the actuation element of the parking brake of the vehicle. The side view in FIG. 2 shows the traction cable  40  diametrically opposed to the radius R 1  on a smaller radius R 2  of the cable groove  36 .  
         [0014]    The stator  42  of the direct current motor  10  is mounted on a printed circuit board  44  that also carries the electronic components of the control circuit of the direct current motor. The printed circuit board  44  is supported on a bearing plate  46  on which the bearings of the direct current motor  10 , of the pinion shaft  18  and of the drum  20  are arranged The described functional parts of the drive unit are accommodated in a housing  48  that has an outlet opening  50  for the traction cable  40  and a plug base  52  for a connector plug.  
         [0015]    In order to activate the parking brake, the direct current motor  10  is put into operation by manually actuating a switch provided in the vehicle for this purpose or by an automated function, as a result of which the drum  20  is rotated in the drive direction by means of the two-stage belt drive Due to the larger radius R 1  of the cable groove, at the beginning, a relatively large actuation path is traversed with a reduced actuation force; towards the end of the actuation path, the smaller radius&lt;R 2  of the cable groove becomes active, so that the actuation force is correspondingly larger and the actuation path is smaller. The latching recesses on the circumference of the toothed belt drive are configured in such a way that, together with the ratchets  32 , they form an anti-reverse lock that only allows rotation in the drive direction. The detection of the current consumed by the direct current motor  10  provides a criterion for the actuation force exerted on the traction cable  40  via the reduction gear. Once the necessary actuation force of, for example, 1000 N to 2500 N is reached after less than 0 5 seconds, the direct current motor  10  is switched off Due to the tensile stress now present in the traction cable  40 , the drum  20  and with it the belt pulley  14 —via the toothed belts  22 ,  24  and the pinion shaft  18 —are also made to rotate opposite from the drive direction, until the ratchets  32  latch in the next latching recess of the belt pulley  14 . In this state, the parking brake remains activated until, after a brief operation of the direct current motor  10 , it is released opposite from the normal drive direction  
         [0016]    In order to release the parking brake, the direct current motor  10  is briefly activated opposite from the normal drive direction. Through the control cam on the circumference of the cam disk  34 , the ratchets  32  are lifted out of the corresponding latching recesses of the belt pulley  14 , so that the latter can now rotate freely. Through the sustained tensile stress in the traction cable  40 , the drum  20  is now turned back into its starting position, which corresponds to that of the non-actuated parking brake. In this process, the direct current motor  10  is pulled along via the two-stage belt drive, as a result of which the return of the parking brake to the deactivated state is dampened so as to reduce noise.  
         [0017]    The embodiment of the drive unit shown in FIGS. 3 and 4 is generally similar to that of FIGS. 1 and 2 in that is also has a brushless dc motor  10  and a two-step belt transmission with a pinion  12 , a belt pulley  14 , a toothed belt  16 , a pinion shaft  18  and a drum  120 , the latter being configured for a single toothed belt  122  of an increased width. Drum  120  is mounted on a shaft  124  which also mounts a pulley  126  coaxially with and axially adjacent to drum  120 . Drum  120  and pulley  126  are connected for joint rotation, but a limited relative rotation is possible therebetween. Just as drum  20  in the preceding embodiment, pulley  126  has a cable groove  136  with a variable radius for connection of a brake cable  140  in a circumferential position of greatest radius.  
         [0018]    Pulley  126  has an outer periphery with ratchet teeth  142  A spring loaded ratchet pawl  144  is pivotally mounted in a frame for cooperation with ratchet teeth  142 . Adjacent ratchet teeth  142 , drum  120  has a peripheral cam structure  146  and ratchet pawl  144  axially spans both the ratchet teeth  142  and cam structure  146 . The geometry of the ratchet teeth  142  is such that drum  120  is allowed to rotate to wind up brake cable  140  when motor  10  rotates in a normal drive direction, but to block rotation of drum  120  in an opposite direction, thereby holding the parking brake in an activated state. To release the parking brake, motor  10  is shortly activated in a sense opposite to the normal drive direction to rotate drum  120  slightly relative to blocked pulley  126 . The peripheral cam structure  146  now urges ratchet pawl  144  out of engagement with ratchet teeth  142  to release pulley  126  from the blocked condition. The park brake is now free to return to a released condition.