This application is a continuation of prior International Application PCT/DE2007/000124, filed Jan. 25, 2007, which is hereby incorporated by reference herein. 
     The invention relates to a method and device for referencing the position of an actuating element of a functional unit, especially a clutch in the drivetrain of a motor vehicle. 
     BACKGROUND 
     In the motorized actuation of functional units using an actuating element in which the function of the functional unit changes, e.g., in proportion to the position of the actuating element, it is frequently necessary to reference the position of the actuating element so that the precise position of the actuating element is known in a control unit. An example of this is the motorized actuation of a clutch in which the position of a clutch lever determines the torque transferability of the clutch. A precise knowledge of the torque transferability of the clutch is important to precisely control or regulate starting and shifting. 
     Referencing is especially essential when for example an electric motor is used as the actuator that drives the actuating element via a shaft whose rotary position is detected with an incremental sensor. The incremental sensor cannot detect absolute positions of the actuating element, but merely changes in position so that it is necessary to determine the absolute position of the actuating element or the rotary position of the output shaft of the electric motor by means of referencing. 
     A control element for a motor vehicle friction clutch is known from DE 44 33 825 C2 in which the output shaft of an electric motor drives a segment gear on which an actuating element for the clutch is mounted. The rotational range of the segment gear is limited by stops that can be approached for referencing. In this manner, the actuating element&#39;s absolute position can be determined from which the displacement path can be detected by means of an incremental sensor. 
     A device for shifting a gear is known from DE 100 27 330 A1 in which an electric motor whose rotary position is detected by means of an incremental sensor drives an actuating element of the transmission that can be moved into an end position against a stop. The actuating element is additionally equipped with a surface profile that engages with a spring-loaded pickup component. The profile of the actuating element is such that the force required to displace the actuating element depends on the relative position between the surface profile and the pickup component. The slope of shape surfaces of the profile can be such that the force applied by the elastic pretension of the pickup component on the actuating element is greater in sections than the holding torque of the electric motor so that one can determine if the actuating element is within a predetermined control range when a small voltage is applied to the electric motor. 
     With referencing that uses stops and thereby recognizes that the actuating element does not move despite the force exerted by the actuator on the actuating element, there is a danger of incorrectly identifying a blockage as a reference position in a transmission path in which the actuating element is located. Particularly when actuating clutches, this can cause hazardous operating conditions since the vehicle may for example unintentionally drive away. 
     SUMMARY OF THE INVENTION 
     An object of the present invention provides an method and device for increasing the safety of referencing the position of an actuating element of a functional unit. 
     An embodiment of the present invention provides a method for referencing the position of an actuating element of a functional unit, especially a clutch in the drivetrain of a motor vehicle in which the attainment of a reference position of the actuating element is assessed for plausibility by checking whether a variable applicable to the actuating element changes in a predetermined manner when the actuating element is at a specific distance from the reference position. 
     The variable applicable to the actuating element may be advantageously the actuation force required to position the actuating element. 
     Alternatively, the variable applicable to the actuating element can be the speed of the actuating element that exists when a constant actuation force is applied by the actuating element. 
     In one embodiment of the method according to the invention in which the variable applicable to the actuating element may be changed in a predetermined manner when a plausibility position is reached, the variable may be determined when the actuating element moves before and/or after the plausibility position is reached, a mutual relationship may be established for the determined variables, and the plausibility position may be evaluated when the mutual relationship meets specific criteria. In the above-cited embodiment of the method according to the invention, the plausibility position itself may also be evaluated for plausibility to increase the overall reliability of the referencing. 
     Another embodiment according to the invention provides a device for referencing the position of an actuating element of a functional unit, especially a clutch in the drivetrain of a motor vehicle, and the device has an actuator to adjust an actuating element, an incremental sensor that detects the change of the position of the actuating element, a referencing device defining a reference position of the actuating element, a plausibility device defining the plausibility position at a predetermined distance from the reference position, a control unit for controlling the actuator, said control unit having a counter to count the increments detected by the incremental sensor, a reference position detection device to detect the reference position of the actuating element, a plausibility position detection device to detect the plausibility position of the actuating element, a memory for saving counts of the incremental sensor and a target plausibility number of the increments that lie between the reference position and the plausibility position, and a comparator for comparing the target plausibility number with an actual plausibility number that is determined together with the referencing, and the referencing is saved as valid when the actual plausibility number deviates from the target plausibility number by less than a predetermined value. 
     The actuator can for example be an electric motor. 
     The plausibility device can be designed so that it changes in a predetermined manner the force to be applied by the actuator that is required to move the actuating element in the plausibility position. 
     The plausibility position detection device can for example detect the power consumption of the actuator. 
     The control unit of the device according to the invention can be designed such that the plausibility of the plausibility position may be checked by determining a variable having an influence on the actuation of an actuating element before and/or after the plausibility position is reached, a mutual relationship may be established between the determined quantities, and the plausibility position may be held to be plausible when a predetermined relationship is identified. 
     In a preferred embodiment, the plausibility device has a pickup component guided along a track that moves relative to the pickup component when the actuating element is moved, and the force required to move the track relative to the component in the plausibility position of the actuating element changes in a predetermined manner. 
     The pickup component may be advantageously elastically pretensioned toward the track, and the elastic pretension may change when the track moves relative to the pickup component. 
     In one embodiment of the device according to the invention, the actuator may actuate a linearly movable pickup element that swings the actuating element when it moves along a shaped surface of a swingable actuating element, and the plausibility device may be formed by a sudden change in slope of the shaped surface. 
     The referencing device can for example be formed by a stop. 
     In another embodiment, the referencing device may be formed by a referencing position of the shaped surface that the pickup element contacts when the clutch independently opens. 
     The invention that can be advantageously used wherever the position of an actuating element of a functional unit needs to be referenced will be explained in the following example of a motorized actuation device for a motor vehicle clutch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures show the following: 
       
         
           
                 
                 
               
             
                 
                     
                 
               
               
                 
                   FIG. 1 
                   A plan view of part of an actuation device of a clutch; 
                 
                 
                   FIGS. 2 and 3 
                   Examples of tracks that are picked up by a pickup  
                 
                 
                     
                   component; 
                 
                 
                   FIG. 4 
                   A flowchart explaining the referencing; 
                 
                 
                   FIG. 5 
                   Another exemplary embodiment of a track with a pickup  
                 
                 
                     
                   element; 
                 
                 
                   FIG. 6 
                   A half-length section of a double clutch; 
                 
                 
                   FIG. 7 
                   A section from FIG. 6 to explain the actuation device; and 
                 
                 
                   FIG. 8 
                   A force path characteristic to explain referencing according  
                 
                 
                     
                   to the invention. 
                 
                 
                     
                 
               
            
           
         
       
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , an electric motor  12  is mounted to the housing  10  of a clutch actuation unit, and the output shaft  14  of the electric motor rotatably drives a segment gear  16  rotatably mounted in the housing  10  on an axis A. The segment gear  16  has a peripheral area with teeth  18  that meshes with the helical teeth  20  of the output shaft  14 . The rotary position of the output shaft  14  is detected by an incremental sensor  21 . 
     An operation rod  24  is mounted in a bearing  22  to the segment gear  16 , and the movement of the operation rod that can be a linear movement by means of the articulated subdivision of the operation rod is transferred to a release lever of a clutch with a generally known construction. The operation rod  24  can be directly connected to the clutch lever, or for example be connected to the clutch lever by means of a hydraulic transmission path. The rotatability of the segment gear  16  on axis A is limited by a stop  26  fixed to the housing that, in one or the other end position, contacts stop surfaces  28  and  30  of the segment gear  16 . On its outer perimeter, the segment gear  16  has a profile that forms a track  32  for a pickup roller  34  that is guided to move linearly in a housing extension and is elastically pretensioned via a spring  36  against the track  32 . 
     The electric motor  12  is controlled by an electronic control unit  38  that contains a microprocessor with associated memory devices and an input connected to the incremental sensor  21 , as well as other inputs that are supplied with input variables such as the position of the gas pedal, the vehicle speed, the position of a gear, etc. that are relevant for operating the clutch. An output of the control unit  38  is connected to the electric motor  12  and controls its operation. 
       FIGS. 2 and 3  show two schematic representations of the contour of the track  32 . In the embodiment in  FIG. 2 , the track  32  maintains a constant radial distance from rotary axis A of the segment gear  16 , but is provided with a bump or a projection  40  that is at a predetermined distance “a” from the position at which the contact surface  28  of the segment gear  16  contacts the stop  26 . In the embodiment in  FIG. 3 , the track  32  has a bottom position  42  at which the distance between the track  32  from the axis A is minimal. On both sides of the bottom position  42 , the track  32  has a slope, and the slope toward the stop  26  has an angle α that is larger than the angle β of the opposing slope. 
     In the following, the functioning of the device will be explained with reference to the flowchart in  FIG. 4 . 
     Assuming that in step  60 , a program is activated in the control unit  38  in which the position of the operation rod  24  or the clutch lever geometrically coupled to the operation rod is referenced. In step  62 , the segment gear  16  is rotated counterclockwise, wherein the electric motor  12  is for example driven at a constant output so that the segment gear  16  rotates at a speed dependent on the counterforce of the clutch. In step  64 , the rotational speed is evaluated, and in step  66 , it is evaluated whether the rotational speed passes through a minimum which occurs when the pickup roller  34  passes over the projection  40 . When a minimum is identified in step  66 , that count is saved that a counter in the control unit  38  counting the increments of the incremental sensor  21  has when the minimum is passed through. The rotation of the segment gear  16  stops until it is determined in step  70  that the segment gear  16  is not rotating despite the force applied the electric motor  12  such that the stop  26  is reached. In step  72 , the count Z A  of the counter is saved that the counter has when the stop is reached. Step  74  checks if the difference between count Z A  and Z min  corresponds to distance “a”. If this is the case, the referencing is held to be plausible or valid in step  76 , and the count Z A  is held to correspond to the reference position. When the difference between Z A  and Z min  does not corresponds to distance “a”, the referencing is held to be implausible or wrong, and a corresponding display can be output. The components or functions for detecting the stop position accordingly form a reference position detecting device. The components or functions for determining the position of the pickup roller  34  traveling over the projection  40  form a plausibility position detection device. The number of increments between the plausibility position and the stop position is a target or actual plausibility count. 
     The distance “a” of the projection  40  from the stop position can differ due to manufacturing tolerances so that it is advantageous to teach distance “a” to the system after the clutch control system is mounted, for example at the end of the assembly line in a vehicle manufacturing plant. 
     The plausibility check occurs in a similar manner with the track shown in  FIG. 3 . The speed of the output shaft  14  then suddenly increases while the applied output of the electric motor  12  remains constant when the pickup roller  34  passes through the lowest position  42  and continues to move along the steeper ramp. Of course, the segment gear  16  reaches the stop  26  before the end of the steeper ramp is reached. In this case as well, the distance between the lowest position  42  and reaching the stop  26  is known beforehand. The movement of the pickup roller  34  along the steeper ramp can also be identified in that the electric motor  12  is operated at a constant speed, and the power consumption of electric motor  12  is measured. This power consumption lies within a predetermined range while the pickup roller  34  rolls on the steeper ramp, and the range lies between the output necessary for displacement when the segment gear  16  travels on a section of the track  32  with a constant radius, and the value that the output or voltage of the electric motor  12  assumes when the stop is reached. In a similar way, the output of the electric motor can be determined when the pickup roller travels on the flat ramp toward the stop  26  which is less than the power consumed by the electric motor while the pickup roller travels on a segment of the track  32  with a constant radius. 
     In the embodiment in  FIG. 2  as well, the electric motor can be operated at a regulated, constant speed, and its power consumption can be determined when it travels over the projection  40 . 
     The distance between the lowest position  42  and the stop is also previously known in the embodiment in  FIG. 3  and is advantageously learned only after the assembly of the actuation device. 
     The invention can be altered in many ways. For example, the track or a shaped surface can be on the operation rod  24  or another component within the transmission of motion between the electric motor and clutch. Alternately, the pickup roller or the pickup element can be mounted on a component moved by the electric motor and can pick up a track fixed to the housing. There is a wide range of options, especially for the geometric design, for determining a specific location on the track that is at a predetermined distance from another specific location such as a stop. Instead of the stop position itself, the specific position on the track such as the lowest position  42  in  FIG. 3  can be used as a reference position whose plausibility is then checked by identifying the stop  26  or another specific position on the track at a predetermined distance from the above-cited specific location. 
       FIG. 5  shows another modified embodiment of a device according to the invention. A track is indicated by reference number  32  on which a pickup roller  34  rests. The track  32  has a stop  44  that limits the relative mobility between the pickup roller  34  and the track  32  in the “clutch open” direction. A stop at the opposite end of the track  32  for limiting the relative movement between the pickup roller  34  and track  32  in the “clutched closed” direction is not shown. 
     In the area before the stop  44 , the track  32  has a recess  46  that transitions via a steep elevation or edge  48  into the remaining track. The pickup roller  34  can be directly mounted on a clutch lever or on a component between the clutch actuator and the clutch lever. 
     The plausibility of the position of the pickup roller  34  on the stop  44  can be checked by a small traveling motion of the clutch towards the closed position in which the pickup roller  34  contacts the edge  48  (drawn as a dashed line). This position of the pickup roller  34  can be detected by a sudden decrease in speed while the current applied to the actuator remains constant, or by a sudden increase in current or power consumption by the actuator while the actuator is regulated to maintain a constant speed. With this type of plausibility check, a slight movement of the clutch out of the assumed reference position into a position at which there is a significant but easily overcomeable resistance is sufficient to reliably determine the plausibility of the reference position. Of course, the plausibility can also be checked by detecting the sudden reduction of actuation force of the clutch before the stop  44  is reached; a flag is simultaneously set, and it is checked whether the stop  44  has been reached within the predetermined additional actuation path (width of the recess  46 ). 
     Since the plausibility check of the reference position that can be performed in a similar manner for the “clutched closed” reference position is only associated with a small actuation movement, it is easy to perform when, for example, the start position is lost due to a controller reset or error, or the actuator stops at an unanticipated position due to a mechanical defect or hits a stop. 
       FIG. 6  shows a half lengthwise section of a double clutch  160  known per se whose housing  162  is fixedly connected to a crankshaft of an internal combustion engine. On the left of a baseplate  164  of the housing  162  is a first clutch disc  172  of a first clutch K 1 , and a clutch disc  174  of a second clutch K 2  is on the right side of the baseplate  164 . 
     In the following, only the actuation of the right-side clutch disc  174  will be explained. The left-side clutch disc  172  is actuated in a corresponding manner using a further actuation device. 
     The clutch disc  174  can be pressed against the base plate  164  using a pressure plate  176  to form a friction lock so that torque is transferable from the housing  162  via the clutch disc  174  to a second input shaft  178  of the double clutch transmission. To axially move the pressure plate  176 , a plate-spring-like clutch lever  180  is used that is supported between the housing  162  and the pressure plate  176  and is shaped so that it independently moves the clutch into the open position. 
     To move the clutch lever  180  so that the pressure plate  176  presses the clutch disc  174  against the base plate  164 , a transmission element  182  is provided that is supported against the clutch lever  180  on one side via a bearing, and at its other end, it contacts an operating lever  184 . 
     The operating lever  184  contacts a roller  186  against which it is pressed by a spring  188  that abuts a permanently affixed stop  190 . 
     The roller  186  is mounted on a spindle nut  192  whose thread engages with a spindle  194  that is rotated by an electric motor  196 . To detect the rotary position of a component that rotates when the electric motor  196  is actuated, an increment sensor  198  is provided that is connected to an input of an electronic controller. 
     To actuate the clutch lever  180 , the electric motor  196  rotates the spindle  194  along which the spindle nut  192  moves linearly which causes the roller  186  to move along a shaped surface of the operating lever  184  and to swing the operating lever corresponding to the slope that the shaped surface  100  has relative to the direction of movement of the roller  186 . 
     The entire arrangement causes the plate-spring-like clutch lever  180  to move the transmission element  182  to the right from a closed clutch position due to its intrinsic force, wherein the idle torque of the electric motor  196  is overcome when the operating lever  184  is swung counterclockwise, and the roller  186  moves upward while rotating the spindle  194 . The clutch is hence pushed by the electric motor into its closed position and independently moves into its open position when the electric motor shuts off. 
       FIG. 7  illustrates the forces interacting with the operating lever  184  whose shape slightly differs from the lever shown in  FIG. 6 . 
     F 1  identifies the force that is exerted by the spring  188  on the operating lever  184 . F 2  identifies the force of the transmission element  182  acting on the operating lever  184 . L is the distance between the lines of action of the forces. X is the distance of the contact point between the roller  186  and the operating lever  184  from the line of action of force F 1 . 
     The following relationship holds true:
 
 F   2 ×( L−X )= F   1   ×X.  
 
     When the electric motor  196  is not powered, the system assumes the position shown in  FIG. 7  under the force of the clutch lever  180 . 
       FIG. 8  shows the force to be applied by the electric motor  196  on the spindle nut  192  or the roller  186  as a function of (X−X 0 ) wherein X 0  is the position shown in  FIG. 7  in which the clutch moves into an open position. This position is characterized in that the slope of the shaped surface  100  is reversed relative to the direction of motion. At the top in  FIG. 7 , i.e., beyond the open position of the clutch, the angle of inclination is comparatively large so that the negative force to overcome the open position rises strongly. Starting from open position X 0  (shown in  FIG. 7 ), the shaped surface  100  relative to the direction of movement of the roller  186  is initially only sloped slightly along section “c” and then transitions over a steep step into a similarly angled section. 
       FIG. 8  shows the following relationship of the forces: 
     When X=X 0 , the force F reverses its sign. To close the clutch, first a slightly increasing force is necessary as X increases that, after section c is overcome and the step is reached at X 1 , increases suddenly and then increases less strongly according to the slope. Depending on the design of the step at position X 1 , the force F can be more or less elevated after overcoming the step. 
     The following explains how the stepped design of the shaped surface  100  can be used for a reliable probability check of the referencing. 
     When the actuator  196  is powerless, the system independently moves to the position in  FIG. 7  in which the roller  186  is at distance X 0  from the line of action of force F 1 . When the clutch is moved in the closed direction by the electric motor  196  to secure or check the plausibility of the referencing, the force has the characteristic in  FIG. 8 , i.e., it only slightly increases from an initial level while the roller  186  moves along section c and then increases suddenly. This sudden increase and the associated deflection of the roller  186  or section c are measured for example by measuring the power consumption of the electric motor or, when this is operated with constant power, by measuring the revolutions per unit time of the electric motor and the section c (using the incremental sensor  198 ). When section c lies within a plausible range, the referencing is held to be okay. 
     The attainment of plausibility position X 1  can be further ensured by checking if the force/path curve in  FIG. 8  actually follows the specified course. This can be done for example as follows: 
     While the spindle nut  192  is driven by the electric motor  196  at a constant regulated speed or revolutions per unit time, the (filtered) maximum value of the voltage (U max ) required by the electric motor is saved. The respective (filtered) position XU max  is also saved at which this maximum motor voltage occurs. 
     The current maximum voltage (U zw ) is saved at a predetermined position a bit before reaching the step or position X 1 . 
     After the spindle nut  192  has moved sufficiently far beyond step X 1 , i.e., has safely moved past the plausibility position provided in the system, the ratio is formed between the two noted voltages (U max /U zw ), wherein the value of U max  corresponds to the peak of force F shown in  FIG. 8 . A target value for U max /U zw  is saved in the electronic controller. The attainment of plausibility position X 1  is only held to be acceptable when the measured ratio U max /U zw  deviates less than a predetermined value from the saved target value. 
     The plausibility check of plausibility position X 1  that must be successful to accept the referencing can also be performed in a different manner: 
     The voltage difference can be used instead of the ratio between the two voltages. The advantage of this is that the signal is largely independent of the base load. 
     Since the force/path characteristic is specified, the voltage gradient can also be evaluated that occurs when the step is being passed over. 
     Instead of evaluating the voltage applied to the electric motor, the voltage through the electric motor can also be detected. The advantage is that the imprecisely known electric resistance in the motor circuit is largely without influence. 
     When the current is evaluated, the ratio, difference or gradient of the measured currents or measured current characteristic can be evaluated. 
     In an altered embodiment, the actuation lever  184  in  FIG. 7  above position X 0  of the roller can be bent to be approximately vertical so that it forms a stop. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 Reference number list 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 Housing 
               
               
                 12 
                 Electric motor 
               
               
                 14 
                 Output shaft 
               
               
                 16 
                 Segment gear 
               
               
                 18 
                 Teeth 
               
               
                 20 
                 Helical teeth 
               
               
                 21 
                 Increment sensor 
               
               
                 22 
                 Bearing 
               
               
                 24 
                 Operation rod 
               
               
                 26 
                 Stop 
               
               
                 28 
                 Stop surface 
               
               
                 30 
                 stop surface 
               
               
                 32 
                 Track 
               
               
                 34 
                 Pickup roller 
               
               
                 36 
                 Spring 
               
               
                 38 
                 Control unit 
               
               
                 40 
                 Projection 
               
               
                 42 
                 Lowest position 
               
               
                 44 
                 Stop (clutch open) 
               
               
                 46 
                 Recess 
               
               
                 48 
                 Edge 
               
               
                 60 
                 Referencing 
               
               
                 62 
                 Rotation 
               
               
                 64 
                 Evaluation 
               
               
                 66 
                 Evaluation of rotational speed 
               
               
                 68 
                 Saving 
               
               
                 70 
                 Stop reached 
               
               
                 72 
                 Saved counts 
               
               
                 74 
                 Check distance 
               
               
                 76 
                 Referencing 
               
               
                 100 
                 Shaped surface 
               
               
                 160 
                 Double clutch 
               
               
                 162 
                 Housing 
               
               
                 164 
                 Baseplate 
               
               
                 172 
                 Left clutch disk 
               
               
                 174 
                 Clutch disk 
               
               
                 176 
                 Pressure plate 
               
               
                 178 
                 Input shaft 
               
               
                 180 
                 Clutch lever 
               
               
                 182 
                 Transmission element 
               
               
                 184 
                 Operation lever 
               
               
                 186 
                 Roller 
               
               
                 188 
                 Spring 
               
               
                 190 
                 Stop 
               
               
                 192 
                 Spindle nut 
               
               
                 194 
                 Spindle 
               
               
                 196 
                 Electric motor 
               
               
                 198 
                 Increment sensor 
               
               
                 A 
                 Axis 
               
               
                 a 
                 Distance 
               
               
                 α 
                 Angle 
               
               
                 β 
                 Angle 
               
               
                 Z a   
                 Count 
               
               
                 Z min   
                 Count 
               
               
                 K1 
                 First clutch 
               
               
                 K2 
                 Second clutch
In a method for referencing the position of an actuating element of a functional unit, in particular of a clutch in the drivetrain of a vehicle, the attainment of a reference position of the actuating element is assessed for plausibility in that it is checked whether a variable which can be applied to the actuating element varies in a predetermined way in a predetermined spacing of the position of the actuating element from the reference position.