Abstract:
In a motor vehicle, a friction clutch is disposed between a drive unit and a transmission. The value of the contact point of the clutch is adapted at predetermined points in time. A method for controlling the friction clutch includes the steps of determining a value of the contact point of the friction clutch, this value having been modified as a result of an adaptation thereof, and of continuously reducing the value of the contact point at a predetermined ramp rate, without letting this value drop below a predefined contact point reduction value.

Description:
BACKGROUND 
     The invention relates to a method for controlling a friction clutch. 
     One of the most important requirements for a clutch system is the precise momentum in the proximity of the contact point, because this has essential influence upon the start of crawling, driving, and the shifting comfort. 
     For precisely this reason the so-called contact point is learned directly in the transmission and saved in software. It is also attempted during the drive operation, whenever possible, to update this contact point and adapt the software. 
     The contact point can essentially be learned in two different fashions. On the one hand, it can be determined via the balance of the moment during crawling, start of driving, or also during the drive operation, generally via a Kalman filter. On the other hand, the contact point can be determined via a change in rotation of the inactive shaft after disengaging the gears, as shown for example in DE 10 2010 024 941 A1. 
     In the first case, the determination of the contact point based on the momentum is subject to a plurality of errors influencing the calculation, such as offsets of the engine torque and the dynamics of the clutch and/or the motor, or assumptions regarding the dimensional stability of the clutch. Therefore the contact point can usually be determined only with a precision of 1 mm, which is generally too vague. For this reason, the contact points of all dry duplex clutch systems are commonly determined via a second method based on transmission input shafts. 
     In the method based on the transmission input shaft, the inactive gear is rapidly disconnected in an inactive clutch that is open and in close proximity of the contact point. This acceleration of the inactive shaft can then be used for the determination of the momentum at the inactive clutch and thus the contact point. The measurement with an open clutch serves as a reference and/or determination of drag moment. 
     This method is disadvantageous in that constant driving operation is necessary in order to obtain a comparison. In case of traffic jams or driving in urban areas this can hardly be used. Therefore, frequently the momentum-based determination is activated as an “emergency strategy”. However it does not serve for any fine adjustment but rather to prevent major errors. 
     All of these strategies are based on the fact that the contact point changes only very slowly, noticeable over several 100 km. Rapid changes had to be pilot controlled, in principle. For example, the contact point can be shifted by 2 mm due to temperature. If this is not pilot controlled, after the vehicle was parked and the clutch cooled down, here a jerk or flare may result until the contact point has been learned again. In principle, errors are only learned with a time lag so that a fixed change rate of the contact point will always show an averaging error, see  FIG. 1 . However, it has shown that new clutch systems tend to settle in the vehicle within the first 100 km upon the initial start of operation of the clutch and here change the contact point by approximately 2 mm. This change can considerably contribute to tip-in strikes and body vibrations in the first kilometers upon engaging the clutch. 
     SUMMARY 
     The present invention is therefore based on the objective to provide an improvement of the precision of momentum in the first 100 km from the initial start of operation of the clutch in the motor vehicle and thus to avoid such engagement strikes. This objective is attained in the method with one or more features of the invention. 
     Thus, a method is provided to control a friction clutch arranged in a motor vehicle between a drive unit and a transmission, with a contact point value of the contact point of the clutch being determined via an adaptation at respective predetermined points of time, at which an adaptation of the contact point value is performed. Here it is provided that after the end of each adaptation of the contact point value, the contact point value determined by the most recent adaptation is reduced with a predetermined ramp rate, however it is not reduced by more than a predetermined contact point reduction value. The determination, if the adaptation of the contact point value shall be performed and the time of the actual adaptation of the contact point value, can be performed by other methods and respectively rendered available to the method. 
     In a preferred embodiment it is provided here that the method is performed until a predetermined condition regarding the initial start of operation of the clutch is no longer fulfilled. In another preferred embodiment it is provided here that the predetermined condition represents a predetermined distance shorter than the one traveled by the motor vehicle since the initial start of operation of the clutch. In another preferred embodiment it is provided that the predetermined distance ranges from 25 km to 1000 km and preferably amounts to 100 km. In another preferred embodiment it is provided that the contact point reduction value amounts to maximally 1.0 mm, preferably maximally 0.5 mm. 
     In another preferred embodiment it is provided here that the predetermined ramp rate is dependent on the distance traveled by the motor vehicle since the initial start of operation of the clutch. In another preferred embodiment it is here provided that the ramp rate reduces in value with increasing distance traveled by the motor vehicle since the initial start of operation of the clutch. In another preferred embodiment it is provided here that the reduction of the contact point value is performed with the predetermined ramp rate as the pilot control measure. 
     In other words, a friction clutch is arranged in a motor vehicle between a drive unit and a transmission. At predetermined points of time here an adaptation of the contact point value occurs for the contact point of the clutch. The method according to the invention for controlling the friction clutch comprises steps for determining a contact point value of the contact point of the friction clutch changed by adaptation and the constant reduction of the contact point value with a predetermined ramp rate, however not falling below a predetermined contact point reduction value. 
     Preferably the reduction occurs only until a predetermined condition is fulfilled regarding the initial start of operation of the friction clutch. This condition may include that the motor vehicle has traveled less than a predetermined distance since the initial start of operation of the friction clutch. The predetermined distance may range from 25 km to 1000 km, and amount preferably to 100 km. The contact point reduction value amounts maximally to 1.0 mm and preferably to 0.5 mm. The predetermined ramp rate may be dependent on the distance traveled by the motor vehicle since the initial start of operation of the friction clutch. The ramp rate may reduce in value with increasing distance traveled by the motor vehicle since the initial start of operation of the friction clutch. The reduction of the contact value may be implemented with the predetermined ramp rate as the pilot control measure. 
     With the above-described method advantageously the precision of the momentum during the first 100 km can be significantly improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is now described in greater detail with reference to the attached figures, in which 
         FIG. 1  shows an illustration of the method of  FIG. 1 , 
         FIG. 2  shows a flow chart of the method for controlling a friction clutch, and 
         FIG. 3  shows a motor vehicle with a friction clutch that can be controlled. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a diagram  100 , based on which the method according to the invention shall be explained schematically. The time is illustrated in the horizontal direction, a distance in the vertical direction. Values and ratios in both directions only serve exemplary purposes. The distance may be measured at the friction clutch and relates to the actuation of the friction clutch. For example the friction clutch may be activated over a distance from 0 mm to 18 mm, with the contact point being at a position between these values, at which then the friction clutch begins to transfer torque. The contact point can change over time under the influence of various parameters, such as temperature or aging, as shown in a first progression  105 . Usually the change of the contact point, the drift of the contact point, is greater during a predetermined phase after a first initial start of operation of the friction clutch, for example in a new motor vehicle leaving the factory, than after said phase. As a transition between the phase of the start of operation and a subsequent phase, here a travel distance of the motor vehicle of approx. 100 km is listed as an example. In other cases, the travel distance may range from approx. 25 km to approx. 1000 km. The first progression  105  reflects this as an example by it linearly dropping in the first phase and remaining constant in the subsequent phase. 
     A control of the friction clutch requires a contact point value, which shall reflect the given contact point as accurately as possible. For this purpose, the contact point value is newly calibrated by way of adaptations  110  and this way updated to the actual value. A second progression  115  shows the contact point value determined in this fashion. Here, as an example, adaptations  110  in regular time intervals are assumed; in a real motor vehicle the contact point value would generally at least partially be performed based on events, and thus show irregular time intervals. 
     Here the compensation of the drift of the contact point during the described phase of the start of operation occurs in consideration of the adaptation value such that the maximum difference of the contact point between the real contact point  105  and the calculated contact point  115  is minimized. 
     It is suggested to use a modified contact point value, with its progression  120  also being shown in  FIG. 1 . In order to provide the modified contact point  120  the adapted value of the contact point  115  is provided with a time varying offset, i.e. the value determined by the most recent adaptation  110  for the adaptive contact point is reduced by the offset changing over time. This reduction is reflected in the falling straight segments of the progression  120 , which respectively start for an adaptation  110  and/or an adaptation event. The offset is set to 0 for each adaptation  110 , thus in case of each adaptation event, and then starts immediately after each completed adaptation  100  once more at 0 with a predefined, negative ramp rate. Even if the modified contact point value  120 , as shown, is correct only at the points of time of the adaptation, overall it only deviates slightly from the real contact point  105 , so that an improved precision of control can be yielded. 
     Preferably the ramp rate is a function of the kilometers driven, thus the distance traveled by the motor vehicle since the initial start of operation of the clutch. This change is shown in the illustration of  FIG. 1  in that the above-mentioned straight segments show different inclines. 
     It is advantageous if the change of speed of the offset, thus the ramp rate, i.e. the incline of the falling offset value, shows initially a high value and then reduces with the distance traveled, thus the distance traveled by the motor vehicle since the initial start of operation of the clutch. In the illustration of  FIG. 1  the inclines of the straight segments reduce with increasing travel distance, which is achieved after increasing operating time of the motor vehicle. 
     The offset changing over time after each adaptation  110  is advantageously limited to a value that can be calibrated and predetermined in its amount to approx. 0.5 mm for example, which is shown in  FIG. 1  by a lower threshold  125  and an upper threshold  130  so that the adaptive contact point  120 , corrected by the offset, is not further reduced but remains constant until the next adaptation event, when the next adaptation  110  is performed.  FIG. 1  clearly shows this limitation for example in the straight segment of the modified contact point value  120  following the time  0 . 
     Due to the fact that the modification of the contact point value  120  occurs independent from any actual shift of the contact point  105  in a time and/or travel distance controlled fashion, this is called a pilot control. 
     If an adaptation event occurs and when an adaptation  110  shall be performed is generally determined by other methods and rendered available to the method according to the invention. It is discernible from  FIG. 1  that even only partial pilot controls lead to considerable improvements of the precision of the contact point. 
     In order to optimize the method the frequency of adaptations should also be maximized, thus adaptations  110  should occur as frequently as possible. A single setting of the contact point after the initial start of operation of the transmission and/or the clutch generally leads to considerably more distinct errors than the method described here. 
     Accordingly a method is suggested in which the contact point is pilot controlled for drift in a phase of the start of operation via a drive and flexibly limited for adaptation  110 . The drift is here preferably dependent on the distance traveled by the motor vehicle. 
       FIG. 2  shows a flow chart of a method  200  for controlling a friction clutch. A portion of the method  200 , which includes the steps  205  to  215 , is already known from prior art and is considered underlying the remaining part of the method  200  in the form shown or in an alternative one. 
     In a step  205  an event or a time is determined. In a step  210  it is determined if the event or the time set render any adaptation of the contact point value necessary, which shall reflect the contact point  105  of the friction clutch as closely as possible. If this is not the case, the method  200  can return to the start and run its course once more. Otherwise the adaptation  110  can occur in a step  215 . 
     A method  200  of prior art can also return to the start upon conclusion of the step  215  and then run its course again. Instead it is suggested to perform a few other steps  220  to  235  before the method  200  can be run once more. 
     In a step  220  the altered contact point value  115  is determined. At the time of the adaptation  110  this also represents the absolute value of the contact point  105 . It can be checked in an optional step  225  if the vehicle in which the friction clutch is installed has or has not yet traveled a predetermined distance since the initial start of operation of the friction clutch. If it has traveled said predetermined distance, it is outside a phase of the start of operation and an adaptation of the determined contact point value is not required. In this case, the method  200  can return to the start and run its course once more. Otherwise, the method can continue with the steps  230 ,  235 . The steps  220  and  225  may also be processed in the opposite sequence. 
     Before in a step  235  the contact point value  115  is reduced, optionally it can be checked in a step  230  if any reduction performed since the most recent adaptation  110  and/or  215  has already reached a predetermined value or exceeded it. These amounts are shown in  FIG. 1  under the reference character  125  and  130 . If this is the case, preferably no additional reduction of the contact point is performed and the method  200  can return to the start and run its course once more. Otherwise the above-mentioned reduction occurs in step  235 . Preferably the value of the reduction is dependent on the time passed since the most recent adaptation  215  and/or  110 . The speed by which the contact point is reduced in step  235  is preferably predetermined with a predetermined ramp rate. The ramp rate may particularly be newly set for each adaptation  110  and/or  215 . In particular the ramp rate can be determined in this case dependent on a distance traveled by the motor vehicle. The greater the distance traveled by the motor vehicle since the start of operation of the friction clutch the lower the ramp rate, so that with increasing distance traveled by the motor vehicle the time-dependent reduction of the contact point preferably becomes less steep. 
     It must be observed that the step  235  can also be performed parallel during the further progression of the method  200 , i.e. the reduction of the contact point  115  occurs in particular continuously and not only after an adaptation  110  has been performed in step  215 . 
       FIG. 3  shows a motor vehicle  300  with a drive train  305 . The drive train  305  comprises a drive unit  310  and a transmission  315 , between which a friction clutch  320  is arranged that can be controlled. The transmission  315  acts upon at least one drive wheel  325 . Preferably a control device  330  is provided to control the friction clutch  320 , which may particularly be arranged in order to implement the above-described method  200  partially or entirely. The friction clutch  320  may be controlled by an optional actuator, with the above-described distance, about which the friction clutch  320  is actuated, can be scanned at a transfer device between the actuator and the friction clutch  320 . For example a pulley or a hydraulic stroke may be provided as the transmission device between the actuator and the friction clutch  320 . 
     LIST OF REFERENCE CHARACTERS 
     
         
           100  diagram 
           105  first progression, contact point 
           110  adaptation 
           115  second progression, adaptive contact point value 
           120  third progression, modified contact point value 
           125  lower threshold 
           130  upper threshold 
           200  method 
           205  event/time control 
           210  adaptation required? 
           215  adaptation 
           220  detecting changed contact point value 
           225  travel distance of motor vehicle &lt;100 km? 
           230  maximum reduction reached? 
           235  time-dependent reduction of contact point value 
           300  motor vehicle 
           305  drive train 
           310  drive unit 
           315  transmission 
           320  friction clutch 
           325  drive wheel 
           330  control device