Abstract:
A method for selectively coupling a motor to a drive train of a motor vehicle using a clutch having a first clutch part driven by the motor, a second clutch part associated with the drive train and an actuator, the method includes the steps of: accelerating the first clutch part while simultaneously activating the actuator during a first actuation phase without engaging the first and second clutch parts; suspending activation of the actuator until a predefined rotational speed difference is reached between the first and second clutch parts; and engaging the first and second clutch parts during a second actuation phase upon reaching the predefined speed difference.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of German Application No. 10 2011 010 616.2 filed Feb. 28, 2011 and U.S. Provisional Application No. 61/367,181 filed Jul. 23, 2010. The entire disclosure of each of the above applications is incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates to a method of operating a drive train of a motor vehicle. 
     BACKGROUND OF THE INVENTION 
     Drive trains which can be adapted to the respective driving conditions present are used in motor vehicles to improve driving dynamics. In this respect, cut-off clutches are frequently used which serve inter alia selectively to couple specific components of the drive train with one another or to cut them off from one another. For example, it can be advantageous in certain driving situations with hybrid vehicles to cut off an electric motor of the hybrid drive from the other components of the drive train, for instance when an internal combustion engine of the hybrid drive delivers the required drive torque and the drive power of the electric motor is not required. A safety shut-down in the event of operating problems can also be ensured by a cut-off clutch. Ultimately, such cut-off clutches thus contribute to the improvement of the total efficiency of the drive train, of the driving dynamics, of the response of the vehicle on an ESP engagement and of the response on an operating problem in the electrical traction components. 
     The efficient control of the cut-off clutch is of material importance so that its advantages come into effect to the greatest possible extent. An engagement of the clutch should in particular take place as fast and as reliably as possible so that the drive train can be adapted fast to changing driving conditions. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a reliable and highly dynamic method of operating a drive train which can be selectively coupled to a motor by a clutch. 
     In accordance with the invention, the method of operating a drive train of a motor vehicle having a motor which can be selectively coupled to the drive train by means of a clutch which is actuable by an actuator and which has a first clutch part associated with a drive shaft of the motor and a second clutch part associated with the drive train, comprises the following steps on the engagement of the clutch:
         accelerating the first clutch part by the motor for reducing a rotational speed difference between the first clutch part and the second clutch part with a simultaneous activation of the actuator during a first actuation phase for overcoming a dead space of the clutch, wherein the clutch parts are not yet in engagement with one another;   suspending the activation of the actuator after overcoming the dead space and before bringing the clutch parts into engagement as long as a predefined rotational speed difference has not yet been reached;   activating the actuator again during a second actuation phase for bringing the clutch parts into engagement when the predefined rotational speed difference has been reached or is fallen below.       

     In other words, the rotational speed of the first clutch part is first synchronized with the rotational speed of the second clutch part in that the motor accelerates the first clutch part. A completed synchronization is to be understood as a state in which the two clutch parts have a predefined rotational speed difference which is selected as a rule as comparatively small as possible to allow a problem-free engagement of the clutch. The predefined rotational speed difference is in particular to be dimensioned such that the engagement procedure can be carried out without impairing the driving comfort and simultaneously an actuation time is kept small. 
     On operation of a vehicle and in an open state of the clutch, the first clutch part as a rule does not rotate or only rotates slowly, whereas the second clutch part rotates with the rotational speed of the drive train. The rotational speed of the first clutch part must therefore be increased. 
     To improve the dynamic response of the clutch, the actuator actuating the clutch is already activated during the acceleration of the first clutch part (first actuation phase) to overcome the dead space of the clutch. I.e. the time required for the synchronization is simultaneously utilized to overcome the dead space of the clutch. 
     A “dead space” of the clutch is to be understood, for example, as a spacing between the clutch parts in an open state of the clutch which has to be passed through before the clutch parts are brought into contact with one another and an engagement of the clutch parts takes place. 
     As long as the synchronization has not yet been completed, and the predefined rotational speed difference has not yet been reached or fallen below, the activation of the actuator is interrupted in an actuation state in which the clutch parts are just before engagement. In this state, the dead space of the clutch has already almost been completely overcome. 
     The actuator is activated again (second actuation phase) when the predefined rotational speed difference is reached or fallen below so that the clutch parts are brought into engagement and the engagement process is thus completed. 
     Ultimately, a time period which is required for accelerating the first clutch part for reaching a predefined difference between the rotational speed of the first church part and the rotational speed of the second clutch part is utilized for overcoming the dead space of the clutch to increase the actuation dynamics of the clutch. If the dead space of the clutch has already been overcome before the predefined rotational speed difference was reached, the activation of the actuator is suspended just before a bringing into engagement of the clutch parts. 
     Further embodiments of the invention are set forth in the description, in the dependent claims and in the drawings. 
     A monitoring device can be associated with the actuator with which a monitored signal is produced at least during the second actuation phase which indicates a situation preventing the bringing into engagement of the clutch. With a dog clutch, such a situation can, for example, be an out-of-mesh position of the two clutch parts. The monitoring device monitors the engagement of the clutch at least during the second actuation phase and delivers a monitored signal which can be utilized, for example, to optimize the control of the clutch or of the actuator actuating it. 
     The actuator in particular includes an electrically operated actuator motor. In this case, the voltage applied to the actuator and/or the current applied to the actuator can be determined by the monitoring device for producing the monitored signal. 
     The activation of the actuator can be restricted for at least so long during the second actuation phase as long as the monitored signal exceeds or is below a threshold value. A blocking of an actuation mechanism of the actuator on the presence of the above-described situation preventing the bringing into engagement of the clutch is avoided by this procedure. It is in particular monitored whether a power increase can be observed at the actuator motor which would, for example, indicate an out-of-mesh position of the clutch parts in a dog clutch. 
     In accordance with an embodiment, an energy store is associated with the actuator which converts at least some of the actuation movement produced by the actuator into actuation energy and stores said actuator energy during a situation preventing the bringing into engagement of the clutch—in particular during the restricted activation of the actuator. The actuation energy taken up by the energy store can be output as soon as the situation preventing the bringing into engagement of the clutch is cancelled. The load on the clutch parts and on the components of the actuator is thereby reduced whereas simultaneously a fast engagement of the clutch is ensured when the situation preventing the bringing into engagement of the clutch—for example an out-of-mesh position of the clutch parts of a dog clutch—is no longer present. 
     To improve the engagement dynamics of the clutch, the dead space of the clutch to be overcome can be determined on the basis of data of a sensor with which a position of the first clutch part and/or of the second clutch part can be determined. For example, with a suitable embodiment of the sensor, the actually present spacing between the two clutch parts in a disengaged state of the clutch can be determined and from this—while taking account of the geometrical circumstances of the clutch parts, for instance the design of the teeth of a dog clutch—a dead space to be overcome can be calculated. It is also possible to determine the position of only one of the two clutch parts if, for example, the other clutch part is arranged in an axially fixed respect. The sensor can, for example, include a rotary encoder which determines the position of a drive shaft of the motor—and thus indirectly the position of the first clutch part—or the position of the first clutch part. 
     The invention furthermore relates to a drive train of a motor vehicle having a motor which can be selectively coupled to the drive train by means of a clutch which can be actuated by an actuator and which has a first clutch part associated with a drive shaft of the motor and a second clutch part associated with the drive train, wherein a motor control unit is associated with the motor and an actuator control unit is associated with the actuator, said actuator control unit being designed so that at least one of the above-described embodiments of the method in accordance with the invention can be carried out. 
     The motor control unit and the actuator control unit can form an assembly, in particular an integrated control unit. 
     As already mentioned above, the clutch can be a dog clutch. 
     The motor is in particular an electric motor which is, for example, a component of a hybrid drive of the motor vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described in the following purely by way of example with reference to advantageous embodiments and to the enclosed drawings. There are shown: 
         FIG. 1  a perspective view of a part of an embodiment of the drive train in accordance with the invention with a dog clutch and an actuator associated with the clutch; 
         FIG. 2  a sectional view through the clutch and the actuator of  FIG. 1 ; 
         FIG. 3  a perspective view of a part of a further embodiment of the drive train in accordance with the invention with a dog clutch and an actuator associated with the clutch; and 
         FIG. 4  a sectional view through the clutch and the actuator of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIGS. 1 and 2  show a dog clutch  10  with a clutch part  10   a  and a clutch part  10   b . The clutch parts  10   a ,  10   b  each have complementary teeth  12   a  or cut-outs  12   b . The dog clutch  10  is in engagement in the state shown in  FIG. 1 . 
     The clutch part  10   a  is—as shown in FIG.  2 —rotationally fixedly connected to a toothed wheel  14  which can be driven, for example by an electric motor, not shown, of a hybrid drive of a motor vehicle, to make a rotary movement. In the shown engaged state of the clutch  10 , this movement is transferred to the clutch part  10   b  which is in turn connected rotationally fixedly, but axially displaceably, to a drive shaft  16  drive-effectively coupled with components of a drive train of the motor vehicle. In the embodiment shown, the drive shaft  16  is rotationally fixedly coupled with a wheel flange  17 . The drive shaft  16  is furthermore supported by bearings  18  at the motor vehicle. 
     It is understood that the drive shaft  16  can be connected to the electric motor in alternative embodiments of the drive train, whereas the clutch part  10   a  is in drive-effective connection with further components of the drive train. 
     On a decoupling of the clutch  10 —for instance when a contribution of the electric motor to the forward drive of the vehicle is no longer required—the electric motor driving the toothed wheel  14  is, for example, switched over from torque regulation to rotational speed regulation and the clutch  10  thus ideally runs without load. In real operation, however, residual torques are also applied to the clutch in this state which have to be overcome to disengage the clutch  10  from an actuator  20  to separate the clutch parts  10   a ,  10   b  from one another. 
     The actuator  20  includes an actuator motor  22  whose rotational drive movement is converted via a spindle  24  and a nut  34  into a pivoting of a shift fork  26  designed as a pivot fork. The shift fork  26  is supported approximately centrally by a support pin  28 . A connection of the shift fork  26  to the clutch part  10   b  includes a grooved ball bearing  30  ( FIG. 2 ) which takes up relative movements between the fork  26  and the clutch part  10   b . The groove ball bearing  30  is in turn connected to the shift fork  26  via a swivel joint  31 . At the spindle side, the shift fork  26  is provided with a slit  32  in which the nut  34  with an integrated sliding piece mounted on the spindle  24  is used. The nut  34  is preferably made from plastic for a simplified manufacture and to minimize friction. 
     On an activation of the actuator motor  22 , the shift fork  26  is pivoted toward the actuator motor  22  by a movement of the nut  34  so that the clutch part  10   b  is moved away from the clutch part  10   a . The clutch part  10   b  is for this purpose connected in the manner of a sliding collar by a toothed arrangement rotationally fixedly, but axially displaceably, to the drive shaft  16 . 
     The activation of the actuator motor  22  takes place, for example, for a predefined time period so that the clutch part  10   b  adopts a predefined position with respect to the clutch part  10   a  in an open state of the clutch  10 . 
     If the clutch should be closed again, the clutch parts  10   a ,  10   b  must be synchronized, i.e. they must be brought to substantially the same rotational speed level. 
     For this purpose, the clutch part  10   a  which is idling or is rotating more slowly than the clutch part  10   b  coupled with the drive train is accelerated by the electric motor. To utilize the time required for this purpose efficiently, the actuator motor  22  is simultaneously activated to move the clutch part  10   b  toward the clutch part  10   a . If a predefined rotational speed difference has not yet been reached between the clutch parts  10   a ,  10   b , although the dead space of the clutch  10  has already substantially been passed through and an engagement of the teeth  12   a  or cut-outs  12   b  is just before completion, the activation of the actuator motor  22  and the spacing reached up to then is maintained between the clutch parts  10   a ,  10   b . In the meantime, the acceleration of the clutch part  10   a  is continued until the predefined rotational speed difference between the clutch parts  10   a ,  10   b  is reached or fallen below. The engagement of the clutch  10  can then be continued, i.e. the actuator motor  22  is activated again, to complete the engagement of the clutch  10 . 
     On the engagement procedure of the clutch  10 , the situation can arise that the teeth  12   a  of the clutch parts  10   a ,  10   b  hit one another so that an engagement of the dog clutch  10  is not easily possible. This situation can be recognized, for example, by a current increase at the actuator motor  22 . To prevent damage to the components of the actuator  20  and the clutch  10 , the voltage applied to the actuator motor  22  is limited and thus the actuation restricted. In particular a blocking of the spindle/nut mechanism is thereby prevented. 
     The actuator  20  can have a buffer mechanism or energy storage mechanism which takes up the actuation movement generated by the actuator motor  22  on a reduced or restricted activation thereof and stores it for so long until the teeth  12   a  are opposite complementary cut-outs  12   b . The then possible engagement of the clutch  10  is assisted by an output of the actuation movement stored in the buffer mechanism, for example in the form of a deformation of an elastic element. 
       FIGS. 3 and 4  show an alternative embodiment  20 ′ of the actuator with an axially displaceable—but not pivotable—shift fork  26 ′. The shift fork  26 ′ cooperates with a nut  34 ′ which is connected to a spindle  24 ′ via a movement thread. The nut  34 ′ can be directly connected to the fork  26 ′. Alternatively, a spring (not shown) can be provided, for example, between the two named components which acts as a buffer mechanism of the kind described above. 
     A sensor  36  which includes a sensor pin  36   a  and a sensor element  36   b  is provided to improve the actuation dynamics of the clutch  10 . The sensor element  36   b  installed in a stationary position allows the determination of a relative position of the sensor pin  36   a  and thus of the shift fork  26 ′. Ultimately, the position in which the clutch part  10   b  is located relative to the axially fixedly arranged clutch part  10   a  can be determined by the measured data of the sensor element  36   b . Since the geometry of the teeth  12   a  and of the cut-outs  12   b  is known, the dead space of the clutch  10  to be passed through can be determined which is present in an open state of the clutch  10 . 
     It is understood that the sensor  36  can be provided in an analog manner for the position determination of the shift fork  26  of  FIGS. 1 and 2 . Alternative or additional sensors for determining the position of the clutch part  10   b  and/or of the clutch part  10   a  are, for example, rotary encoders at the actuator motor  22  or at the spindle  24 ,  24 ′. 
     As can be seen from  FIG. 1 , the teeth  12   a  have flanks which are arranged not fully parallel to the axis of rotation of the clutch parts  10   a ,  10   b  and of the drive shaft  16 . The flank angles of the jaw toothed arrangement are designed so that the force equilibrium of the axial force from a jaw toothed arrangement designed in a repelling manner and from the frictional force in the insertion toothed arrangement of the clutch part  10   b  on the drive shaft  16  is balanced under all friction conditions. It can thus be ensured that the actuator  20 ,  20 ′ only has to take up small axial forces in a closed and loaded state and therefore only small holding currents are required at the actuator motor  22 . Furthermore, the axial force required for opening the clutch  10  under load can be influenced by a suitable selection of the flank angles of the teeth  12   a  and can thus be adapted to different demands. 
     Reference Numeral List 
     
         
           10  dog clutch 
           10   a ,  10   b  clutch part 
           12   a  tooth 
           12   b  cut-out 
           14  toothed wheel 
           16  drive shaft 
           17  wheel flange 
           18  bearing 
           20 ,  20 ′ actuator 
           22  actuator motor 
           24 ,  24 ′ spindle 
           26 ,  26 ′ shift fork 
           28  bearing pin 
           30  grooved ball bearing 
           31  swivel joint 
           32  slit 
           34 ,  34 ′ nut 
           36  sensor 
           36   a  sensor pin 
           36   b  sensor element