Abstract:
A method is described for classifying a clutch unit for a drivetrain of a motor vehicle, wherein the clutch unit has at least one friction clutch for the controllable transmission of a torque from an input element to an output element and has an actuator for actuating the friction clutch. Here, the clutch unit is controlled on the basis of a predefined characteristic curve which describes a predefined dependency of the clutch torque to be transmitted on an actuator control variable.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for classifying a clutch unit for a drivetrain of a motor vehicle, wherein the clutch unit has at least one friction clutch for the controllable transmission of a torque from an input element to an output element and has an actuator for actuating the friction clutch, and wherein the clutch unit is controlled on the basis of a predefined characteristic curve which describes a predefined dependency of the clutch torque to be transmitted on an actuator control variable. 
     A clutch unit of said type serves, for example in a transfer box of a motor vehicle with all-wheel drive, for the controllable transmission of a drive torque to a primary axle and/or a secondary axle of the motor vehicle. In a so-called “torque on demand” transfer box, the wheels of the primary axle are permanently driven, while a part of the drive torque can be selectively transmitted to the wheels of the secondary axle by means of said clutch unit. The transfer box may also be designed as a controllable central differential in which the clutch unit is assigned to a differential lock in order to set the distribution of the drive torque in the longitudinal direction of the vehicle. A clutch unit of said type may also be used in a torque transmitting device which, in a motor vehicle with a permanently driven front axle, enables a part of the drive torque to be transmitted to the rear axle, with the unit being arranged for example on the front axle differential or on the rear axle differential. Such different applications and arrangements are known from U.S. Pat. No. 7,111,716 B2. 
     A clutch unit of the type mentioned in the introduction may also act in the transverse direction of the motor vehicle, for example for a differential lock of an axle differential or in a torque superposition arrangement of an axle differential (so-called “torque vectoring”). In all of the above cases, the clutch unit can connect a rotating input element (for example input shaft) and a rotating output element (for example output shaft) to one another in a frictionally engaging manner, in particular in order to transmit a drive torque. Alternatively, the clutch unit may be configured as a brake, with a stationary input element or a stationary output element, in particular in order to transmit a braking torque. 
     In the abovementioned applications of the clutch unit, the clutch unit is arranged downstream of the main transmission of the drivetrain (that is to say downstream of the manual or automatic shift transmission or CVT) with regard to the power flow direction. The clutch torque—that is to say the torque transmitted by the friction clutch—is conventionally variably adjusted as a function of the respective driving situation. Depending on the driving dynamics requirements, which may for example be dependent on the driving situation or on environmental influences (for example slippery roadway surface with slipping drive wheels), a change in the torque to be transmitted by the clutch unit thus takes place. For this purpose, not only is a controlled engagement of the friction clutch required, but rather often also a relatively long period of operation with a precisely set clutch torque. 
     The clutch unit comprises a friction clutch and an actuator for actuating the friction clutch. The friction clutch is typically a multiplate clutch, that is to say a multi-disk clutch. The actuator may have an electric motor. The actuator may additionally comprise a gearing device for a speed reduction of a rotational movement of a motor shaft of the electric motor. Furthermore, the actuator may have a deflecting device which converts a rotational movement of the actuator (for example motor shaft or gearing element) into a translatory movement of the friction clutch (for example pressure piston). Alternatively, an electromagnetic, hydraulic or electrohydraulic actuator may however also be provided. 
     A clutch unit of the type mentioned in the introduction and a method for classifying a clutch unit of said type are known from WO 2003/025422 A1 (corresponding to U.S. Pat. No. 7,032,733 B2), the content of which is expressly incorporated into the content of disclosure of the present application. As described in more detail in WO 2003/025422 A1, it is not imperatively necessary for direct torque regulation (with the measured actual clutch torque as a regulating variable) to be provided for setting a determined desired clutch torque. In fact, as a result of a corresponding calibration of the clutch unit, it is possible for the friction clutch to be controlled via the indirect route of position regulation of the actuator. To set the desired torque to be transmitted, for example the rotational angle of the electric motor or some other position variable of the actuator is thus taken into consideration as a regulating variable and set to a value which corresponds to the desired clutch torque. For this purpose, a clutch torque/actuator position dependency is empirically determined which is stored as a characteristic curve for example in the form of a look-up table (LUT) or a function (that is to say a calculation rule). Therefore, for a certain torque demand, the corresponding nominal value of the relevant position variable of the actuator (for example rotational angle) is determined and set via regulation on the basis of said dependency. Said characteristic curve may be determined individually for each clutch unit or torque transmitting arrangement at the factory. 
     Here, it is essential that the clutch torque/actuator control variable dependency described by the characteristic curve be brought into the closest possible correlation with the actual clutch torque/actuator control variable dependency. This may in principle take place by virtue of the profile of the characteristic curve for each individual clutch unit being specified and stored according to the actual clutch torque/actuator control variable dependency. This however entails a high level of calculation expenditure and a large memory requirement. Since the differences in the clutch characteristic between the clutch units of a series are usually only relatively small, one possible approach is therefore not to establish a complete characteristic curve but rather to suitably modify a characteristic curve (base characteristic curve) specified uniformly for all clutch units of the same type. For example, the gradient of the characteristic curve may be modified, with then only a single value for the modified gradient being assigned to the clutch unit and correspondingly stored. 
     To further reduce the memory requirements, every clutch unit may be graded into one of a plurality of discrete classes according to the determined value for the modification. Such a process is referred to as classification. The determined gradient or a corresponding corrective value may for example be stored in a non-volatile memory in the clutch unit. Alternatively, the classification may also be carried out by means of an electrical circuit arrangement which is assigned to the respective clutch unit. For example, the clutch unit may be provided with a coding plug, the circuitry of which relates to the corrective value. A coding plug of said type is disclosed for example in WO 2005/009797 A1 (corresponding to U.S. Pat. No. 7,129,716 B2). 
     In the case of classification carried out at the factory, the conventional approach is for a predefined rotational speed difference to be set between the input element and the output element of the clutch unit to be classified on a test stand, for a predefined nominal value of the clutch torque to be set on the basis of the base characteristic curve, and for the actually transmitted clutch torque, that is to say the actual value of the clutch torque, to be detected by means of suitable sensors. The deviation between the nominal value and the actual value is subsequently determined. A corrective value for adapting the base characteristic curve can be determined on the basis of the deviation. 
     In said type of classification, it is possible to firstly set the rotational speed difference, to then set the actuator corresponding to the nominal value of the predefined clutch torque, and to subsequently measure the actual value of the clutch torque. This calibration method may be referred to as a dynamic method. Alternatively, it is also possible for firstly the actuator to be set corresponding to the nominal value of the clutch torque, for the predefined rotational speed difference to then be set and for the actual value of the clutch torque to subsequently be detected. This calibration method may be referred to as a static method. It has been found that the actual value of the clutch torque determined by means of the static method can deviate from the actual value determined by means of the dynamic method for the same rotational speed difference and the same nominal value of the clutch torque. The determined actual dependency between the actuator control variable and the clutch torque is thus dependent on the sequence of setting steps during the calibration. Depending on the driving situation, it can be the case that both a characteristic curve adapted according to the dynamic process and also a characteristic curve adapted according to the static method do not sufficiently accurately represent the real setting behavior, which leads to undesired impairment of the setting accuracy. In particular, in certain driving situations, for example during a starting process, it can be the case that inadmissibly high clutch torques are transmitted, which can result in damage to clutch components. 
     It is therefore an object of the invention to improve the calibration of clutch units of the stated type such that the setting accuracy is increased and the clutch components are protected from excessively high clutch torques. 
     SUMMARY OF THE INVENTION 
     The foregoing object is achieved by providing a calibrating method comprising the following steps: 
     (a) specifying at least one first and one second value range of the clutch torque to be transmitted; 
     (b) determining at least one first corrective value, wherein firstly a predefined rotational speed difference is set between the input element and the output element, wherein subsequently, at the predefined rotational speed difference, the actuator is set to a value of the actuator control variable which, according to the predefined characteristic curve, corresponds to a nominal value of a clutch torque lying within the first value range, and wherein after the setting of the actuator, an actual value of the clutch torque is detected by measurement, wherein the first corrective value is determined as a function of a deviation between the nominal value and the actual value; 
     (c) determining at least one second corrective value, wherein firstly the actuator is set to a value of the actuator control variable which, according to the predefined characteristic curve, corresponds to a nominal value of a clutch torque lying within the second value range, wherein after the setting of the actuator, a predefined rotational speed difference is set between the input element and the output element, and wherein an actual value of the clutch torque is detected by measurement after the setting of the rotational speed difference, wherein the second corrective value is determined as a function of a deviation between the nominal value and the actual value; and 
     (d) adapting the predefined characteristic curve by means of the first and the second corrective value. 
     According to the invention, it has been recognized that a clutch unit calibrated by means of the dynamic method has a higher setting accuracy in certain ranges of the clutch torque than a clutch unit calibrated by means of the static method, while in other ranges of the clutch torque, a clutch unit calibrated by means of the static method has a higher setting accuracy than a clutch unit calibrated by means of the dynamic method. If two separate value ranges of the clutch torque to be transmitted are now specified, and for each value range a calibration according to the method which is particularly suitable for said range is carried out, then it is possible overall to obtain an improved setting accuracy of the calibrated clutch unit. According to the invention, the two different calibration methods are thus advantageously combined to form an overall calibration method. Each of the calibration methods can be used for the range for which it yields the better setting accuracy results. As a result, this enables a more precise representation of the actual conditions by the characteristic curve. 
     The stated actuator control variable is in particular an actuator position variable (for example rotational angle). Alternatively, the actuator control variable may for example be represented by a hydraulic pressure. 
     In a second embodiment, the first and second value ranges are specified as a function of a control strategy of the motor vehicle, which control strategy is defined in particular by a dependency of the clutch torque to be transmitted on the rotational speed difference between the input element and the output element. The value ranges are thus adapted to the real conditions of the associated vehicle. In this way, it is possible for a high setting accuracy of the clutch unit to be classified to be obtained over the entire operating range. 
     In a further embodiment, the first value range corresponds to low clutch torques and the second value range corresponds to high clutch torques. Specifically, it has been found that the dynamic calibration method leads to better setting accuracy results than the static calibration method in the range of low clutch torques, while the static calibration method is more suitable, with regard to setting accuracy, for the range of high clutch torques. This is associated with the fact that, during clutch operation according to the conventional control strategy, low clutch torques are set via regulation in various driving situations, while high clutch torques, for example for starting, are set via pilot control. 
     At least two corrective values may be determined for one or both value ranges. This increases the accuracy of the characteristic curve adaptation. 
     Preferably, for the adaptation of the characteristic curve, a gradient and/or an offset of the characteristic curve is modified. In particular, for the adaptation of the characteristic curve, a gradient adaptation value and an offset adaptation value may be determined as a function of the first and second corrective values. The stored characteristic curve itself remains unchanged in such a process, since only two parameters associated with the characteristic curve are updated. A complete re-establishment of the characteristic curve with corresponding calculation and memory expenditure is avoided in this way. The stated gradient adaptation value and the stated offset adaptation value may be stored in simple look-up tables. 
     The predefined characteristic curve—that is to say the abovementioned base characteristic curve—is preferably stored in a control device which is connected to the clutch unit, wherein the determined gradient adaptation value and the determined offset adaptation value are stored in a non-volatile memory which is assigned to the control device. The assignment may be realized for example by means of an electrical connection. Therefore, the predefined characteristic curve, which is the same for all clutch units of a certain type, is advantageously stored in the control device, such that the control devices can be manufactured uniformly, separately from the clutch units. The gradient adaptation value and the offset adaptation value which bear the individual calibration information are stored in a separate memory which, when the clutch unit is fully assembled, can be read out by the control device. Since said memory must only store individual adaptation values, it may be of correspondingly small design. 
     It is for example possible for a non-volatile memory to be provided within the control device which is designed as an add-on unit. The non-volatile memory may also be designed to store further parameter sets relating to the correction. Alternatively, the clutch unit may be assigned a coding plug with fixed circuitry, the circuitry of which coding plug relates in a predetermined way to the gradient adaptation value and the offset adaptation value. 
     The gradient adaptation value and/or the offset adaptation value is preferably selected from a group of several predefined class values. In other words, it is possible for a plurality of possible classes of different gradient adaptation values and offset adaptation values to be specified, one of which is selected on the basis of the first and second corrective values. The memory requirement can be further reduced in this way. 
     In a further embodiment, the predefined characteristic curve is divided into two characteristic curve sections which correspond to the first and the second value range, and a respective gradient adaptation value and a respective offset adaptation value is determined for each of the characteristic curve sections. The characteristic curve is thus modified in sections, wherein for each section, a corrective value determined by means of the optimum calibration method for the respective value range is taken into consideration for the modification. Should a particularly pronounced correction be required for one of the value ranges while only a relatively minor correction is required for the other value range, this situation can be easily handled as a result of the division of the characteristic curve on the basis of the value ranges. 
     It is also possible for more than two value ranges of the clutch torque to be transmitted to be specified, in order to thereby further increase the adaptation of the characteristic curve to the actual conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A clutch unit or torque transmission arrangement calibrated according to the invention may be used in different arrangements for transmitting a torque along a drivetrain of a motor vehicle, as explained in the introduction. The invention is explained below, with reference to the drawings, merely by way of example in connection with a “torque on demand” transfer box. 
         FIG. 1  shows a schematic view of a drivetrain of a motor vehicle. 
         FIG. 2  shows a schematic view of a transfer box. 
         FIG. 3  shows a cross-sectional view of the transfer box according to  FIG. 2 . 
         FIG. 4  shows a schematic view of a clutch actuator. 
         FIG. 5  shows a flow diagram of a classification method. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows a drivetrain of a motor vehicle having switchable all-wheel drive. The drive torque generated by an internal combustion engine  11  is supplied via a main transmission  13  (manual shift transmission or automatic transmission) to a transfer box  15 . A first output of the transfer box  15  is coupled via a cardan shaft  17  to a rear axle differential gear  19 . The wheels  21  of the rear axle  23  are permanently driven in this way. The rear axle  23  therefore forms the primary axle of the vehicle. A second output of the transfer box  15  is coupled via a cardan shaft  25  to a front axle differential gear  27 . In this way, a part of the drive torque of the internal combustion engine  11  can selectively be transmitted to the wheels  29  of the front axle  31 . The front axle  31  thus forms the secondary axle of the vehicle. 
       FIG. 1  also shows a driving dynamics regulating unit  33 . The latter is connected to wheel rotational speed sensors  35 ,  37  which are assigned to the wheels  21  of the rear axle  23  and to the wheels  29  of the front axle  31  respectively. The driving dynamics regulating unit  33  is also connected to further sensors  39 , for example a yaw rate sensor. As a function of the signals of the sensors  35 ,  37 ,  39 , the driving dynamics regulating unit  33  generates a control signal which is supplied to a control device (not shown in  FIG. 1 ) of the transfer box  15  in order to hereby set a certain distribution of the drive torque between the two axles  23 ,  31  of the vehicle. Said control signal is in particular a nominal value of a clutch torque, that is to say a torque demand for a clutch unit of the transfer box  15 . 
       FIG. 2  shows a schematic cross-sectional view of the transfer box  15  according to  FIG. 1 . The transfer box  15  has an input shaft  41 , a first output shaft  43  and a second output shaft  45 . The first output shaft  43  is formed coaxially with respect to and is rotationally fixed with respect to—and preferably formed in one piece with—the input shaft  41 . The second output shaft  45  is arranged parallel and offset with respect to the input shaft  41 . 
     The transfer box  15  has a clutch unit  47  with a friction clutch  49  and an actuator  51 . The friction clutch  49  has a clutch cage  53  which is rotationally fixedly connected to the input shaft  41  and to the first output shaft  43  and bears a plurality of clutch plates. The friction clutch  49  also has a rotatably mounted clutch hub  55  which likewise bears a plurality of clutch plates which engage, in an alternating arrangement, into the plates of the clutch cage  53 . The clutch hub  55  is rotationally fixedly connected to a drive input gearwheel  57  of a chain drive  59 . A drive output gearwheel  61  of the chain drive  59  is rotationally fixedly connected to the second output shaft  45 . Instead of the chain drive  59 , a gear train may be provided, for example with an intermediate gearwheel between said gearwheels  57 ,  61 . 
     By actuating the actuator  51  in the direction of engagement of the friction clutch  49 , an increasing proportion of the drive torque introduced into the transfer box  15  via the input shaft  41  can be transmitted to the second output shaft  45 . 
       FIG. 3  shows details of the transfer box  15  according to  FIG. 2  in a cross-sectional view. It can be seen in particular that the actuator  51  has a support ring  63  and an actuating ring  65 , which are mounted so as to be rotatable with respect to the rotational axis A of the input shaft  41  and of the first output shaft  43 . The support ring  63  is supported axially on the drive input gearwheel  57  via an axial bearing. In contrast, the adjusting ring  65  is mounted so as to be axially movable. At the sides facing toward one another, the support ring  63  and the adjusting ring  65  have in each case a plurality of ball channels  67  and  69 . These run in the circumferential direction with respect to the axis A and are inclined in a ramped fashion in the circumferential direction relative to a plane normal to the axis A, that is to say the ball channels  67 ,  69  have a varying depth in the circumferential direction. In each case one ball channel  67  of the support ring  63  and a ball channel  69  of the adjusting ring  65  are situated opposite one another and in this way enclose an associated ball  71 . By rotating the support ring  63  and the adjusting ring  65  relative to one another, it is therefore possible to effect an axial movement of the adjusting ring  65 , with the adjusting ring  65  interacting via an axial bearing with a pressure ring  73  of the friction clutch  49 . The pressure ring  73  is preloaded in the release direction of the friction clutch  49  by means of a plate spring arrangement  75 . 
     A respective actuating lever  77  and  79  is integrally formed on the support ring  63  and on the adjusting ring  65 . A respective roller  81  and  83  is rotatably mounted on the free end of each lever  77 ,  79 . Via the rollers  81 ,  83 , the actuating levers  77 ,  79  interact with the two end sides  85 ,  87  of a control disk  89  which is rotatable about an axis C. The end sides  85 ,  87  have an inclined profile in the circumferential direction in relation to a plane normal to the axis C, that is to say the control disk  89  is of wedge-shaped design in cross section. By rotating the control disk  89 , the actuating levers  77 ,  79  can therefore be moved in a scissors-like manner in order to rotate the support ring  63  and the actuating ring  65  relative to one another. The control disk  89  has an integrally formed spline toothing extension  91 . By means of the latter, the control disk  89  can be drive-connected to an electric motor and to an associated step-down gearing (not shown in  FIG. 3 ). 
     It is therefore possible, by means of corresponding activation of said electric motor, for the control disk  89  to be driven in a rotational movement in order to hereby pivot the actuating levers  77 ,  79  relative to one another. The rotation of the support ring  63  and of the adjusting ring  65  relative to one another which is caused in this way generates an axial movement of the adjusting ring  65 . The pressure ring  73  therefore generates an engagement of the friction clutch  49  or—assisted by the plate spring arrangement  75 —a disengagement of the friction clutch  49 . 
       FIG. 4  shows the actuator  51  according to  FIGS. 2 and 3  in a schematic view. The actuator  51  has a controllable electric motor  93  with an armature shaft  95 , a step-down gearing  97  with a worm  99  and a worm gear  101 , and also a deflecting device  103 . By means of the deflecting device  103 , a rotational movement of an output shaft  105  of the step-down gearing  97  is converted into a translatory, that is to say rectilinear movement of the pressure ring  73  ( FIG. 3 ). The deflecting device  103  comprises the control disk  89  and the support ring  63  and the adjusting ring  65  with the actuating levers  77 ,  79  and the balls  71  according to  FIG. 3 . A sensor  107  is arranged on the armature shaft  95  of the electric motor  93 , which sensor  107  is designed for example as an incremental encoder. As shown in  FIG. 4 , the sensor  107  may alternatively also be arranged as sensor  107 ′ on the output shaft  105 . 
     The sensor  107  generates a signal which corresponds to an actuator position value. In the exemplary embodiment shown, this is the rotational angle actual value α′ of the armature shaft  95 . Said signal α′ is supplied to a control device  109  of the transfer box  15 . From the driving dynamics regulating unit  33  of the motor vehicle ( FIG. 1 ), the control device  109  also receives a torque demand M, that is to say a nominal value of the clutch torque. As will be explained in detail below, the control device  109  determines a rotational angle nominal value α on the basis of a clutch torque/rotational angle characteristic curve  111  which is stored in a non-volatile memory  113  of the control device  109 , on the basis of the torque demand M. As a function of the difference between the rotational angle nominal value α and the rotational angle actual value α′, the control device  109  generates a control signal for the electric motor  93  in order to adjust the friction clutch  49  ( FIGS. 2 and 3 ) correspondingly. The control device  109  thus acts as a position regulator. 
     Said clutch torque/rotational angle characteristic curve  111  is based on an empirically determined average clutch torque/rotational angle dependency for a certain type of clutch unit  47 . Said characteristic curve  1 , which is stored in the memory  113  of the control device  109 , thus forms a base characteristic curve. To compensate tolerance-related individual deviations of the actual clutch characteristic from the predefined characteristic curve  111  based on average values, however, every transfer box  15  which is produced is measured at the factory. Said measurement is carried out on a test stand on which firstly a predefined rotational speed difference can be set between the first output shaft  43  and the second output shaft  45  and secondly the actually transmitted clutch torque can be measured with relatively high accuracy, for example by means of torsion sensors. To set the rotational speed difference, the approach in practice is for the input shaft  41  or one of the output shafts  43 ,  45  to be held fixed while the other output shaft  45 ,  43  which is not held fixed is driven with a defined rotational speed by an electric motor. Alternatively, both input shafts  43 ,  45  or the input shaft  41  and the output shaft  45  could be driven in order to produce the desired rotational speed difference. 
     The characteristic curve  111  is then adapted to the actual transmission behavior of the respective transfer box  15  or of the clutch unit  47  using the dynamic calibration method and the static calibration method, as will be explained by way of example below with reference to  FIG. 5 . 
     According to  FIG. 5 , in a step S 1 , the predefined characteristic curve  111  is accessed from the associated memory. In a step S 2 , a boundary is specified which divides the operating range of the clutch unit  47  into a lower value range and an upper value range of the clutch torque to be transmitted. 
     In a step S 3 , a corrective value for the lower value range is then determined by means of the dynamic calibration method. For this purpose, a predefined rotational speed difference is set between the first output shaft  43  and the second output shaft  45 , with the actuator  51  not yet being actuated. Only after the desired rotational speed difference has been set is the actuator  51  actuated, and the electric motor  93  is set to a rotational angle which, according to the predefined characteristic curve  111 , corresponds to a nominal value of a clutch torque lying within the lower value range. After the rotational angle is set, the transmitted clutch torque is measured in order to obtain an actual value of the clutch torque. The deviation between the nominal value and the actual value is subsequently determined, which deviation indicates the degree to which the predefined characteristic curve  111  must be adapted. Accordingly, a suitable corrective value for the lower value range is determined on the basis of the determined deviation. Depending on requirements, the method according to step S 3  may be repeated for further nominal values of the clutch torque within the lower value range in order to obtain a plurality of respective corrective values. 
     In a step S 4 , a corrective value for the upper value range is determined by means of the static calibration method. For this purpose, the actuator  51  is actuated by virtue of the electric motor  93  being set to a rotational angle which, according to the predefined characteristic curve  111 , corresponds to a nominal value of a clutch torque lying within the upper value range. During the adjustment, the first output shaft  43  and the second output shaft  45  do not rotate relative to one another, that is to say the rotational speed difference is zero. After the electric motor  93  is set to the corresponding rotational angle, that is to say after the actuator  51  has assumed the desired position, one of the output shafts  43 ,  45  is driven in order to set a predefined rotational speed difference between the first output shaft  43  and the second output shaft  45 . After the rotational speed difference has reached the desired value, the transmitted clutch torque is measured in order to obtain an actual value of the clutch torque. Subsequently, the deviation between the nominal value and the actual value is again determined, and a corrective value for the upper value range is determined on the basis of the determined deviation. The method according to step S 4  may also be repeated if required for further nominal values of the clutch torque within the upper value range in order to obtain a plurality of respective corrective values for the upper value range. 
     In a step S 5 , a gradient adaptation value and an offset adaptation value are determined as a function of the determined corrective values. The gradient adaptation value and the offset adaptation value are selected such that the characteristic curve  111 , after a modification, is adapted to the best possible extent to the determined actual values of the clutch torque. By means of the predefined characteristic curve  111 , the gradient adaptation value and the offset adaptation value, a characteristic curve which is adapted to the clutch unit  47  to be classified is defined. Said adapted characteristic curve is assigned to the clutch unit  47  by virtue of the gradient adaptation value and the offset adaptation value being stored in a non-volatile memory (step S 6 ). 
     After the classification is carried out, the clutch unit  47  is delivered and connected to a control device  109 . To control the clutch unit  47 , firstly the predefined characteristic curve  111  stored in the memory  113  of the control device  109  is accessed. The control device  109  is capable of communicating with said non-volatile memory of the clutch device  47  and accessing the gradient adaptation value and the offset adaptation value. For example, said non-volatile memory may be electrically connected to the control device, or said non-volatile memory forms a part of the memory  113  of the control device  109 . On the basis of the predefined characteristic curve  111  and the accessed adaptation values, the control device  109  takes into consideration an adapted characteristic curve, which is ultimately utilized for controlling the clutch device  47 . 
     In the adaptation of a clutch characteristic curve by means of a gradient adaptation value and an offset adaptation value, it is fundamentally possible for a nominal value of the clutch torque to be multiplied with the gradient adaptation value in order to determine a modified nominal value of the clutch torque, with a preliminary nominal value of the rotational angle being determined on the basis of the characteristic curve  111  as a function of the modified nominal value of the clutch torque, and with the offset adaptation value being added to the preliminary nominal value of the rotational angle in order to determine a nominal value of the rotational angle. The stored predefined characteristic curve  111  itself remains unchanged in such a process, since only two parameters which are assigned to the characteristic curve  111  are updated. A complete re-establishment of the characteristic curve  111 , with corresponding calculation and memory expenditure, is avoided in this way. The gradient adaptation value may also be smaller than one, such that the multiplication is equivalent to a division. Likewise, the offset adaptation value may be negative, such that the addition is equivalent to a subtraction. 
     While the invention can be used particularly advantageously in a transfer box with electromechanical actuation of the friction clutch, the invention is not restricted to the exemplary embodiment explained above. Other arrangements in the drivetrain of a motor vehicle are also possible, as explained in the introduction. Furthermore, the actuator  51  may be of some design other than that explained above in conjunction with the figures. For example, a different type of step-down gearing  97  or a different type of deflecting device  103  may be provided. Instead of the electromechanical actuation of the friction clutch  49  shown, it is for example also possible for an electromagnetic, hydraulic or electrohydraulic actuating means to be provided.