Patent Publication Number: US-6988605-B2

Title: Method and device for operating a clutch

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method and a device for operating a clutch between an internal combustion engine and at least one driven wheel of a vehicle, a torque being transmitted between the internal combustion engine and the driven wheel by pressing the clutch together via an application force or an application pressure. 
   BACKGROUND INFORMATION 
   If a clutch is operated with slip, it is possible to draw inferences concerning the clutch torque transmitted if the coefficient of friction is known. The intended use of this torque information is to determine the transmission input torque. Precise knowledge of the transmission input torque is of particular significance for continuously variable transmissions (CVT) so that the safety pressure when controlling the belt tension of belt transmissions can be reduced and the transmission efficiency can be increased. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to improve the operation of a clutch. 
   The object is achieved by a method and a device for operating a clutch between an internal combustion engine and at least one driven wheel of a vehicle, a torque being transmitted between the internal combustion engine and the driven wheel to operate a clutch between an internal combustion engine and at least one driven wheel of a vehicle by pressing the clutch together via an application force or an application pressure, the application force or the application pressure being controlled or regulated as a function of a clutch slip in the clutch when the torque is transmitted between the internal combustion engine and the driven wheel, and a setpoint clutch slip. 
   In an advantageous embodiment of the present invention, the application force or the application pressure is controlled or regulated as a function of the difference between the clutch slip and the setpoint clutch slip. 
   In a further advantageous embodiment of the present invention, the application force or the application pressure is regulated by a slip controller. 
   In a further advantageous embodiment of the present invention, the application force or the application pressure is regulated by an inverse clutch model which calculates the application force or the application pressure as a function of the torque transmitted via the clutch. 
   The device according to the present invention for operating a clutch between an internal combustion engine and at least one driven wheel of a vehicle, in which a torque is transmitted between the internal combustion engine and the driven wheel by pressing the clutch together via an application force or an application pressure, is provided with a slip controller to regulate the application force or the application pressure as a function of a clutch slip in the clutch when the torque is transmitted between the internal combustion engine and the driven wheel, and a setpoint clutch slip. 
   In an advantageous embodiment of the present invention, the slip controller has an inverse clutch model to calculate the application force or the application pressure as a function of the torque transmitted via the clutch. 
   In a further advantageous embodiment of the present invention, the slip controller has a regulator to calculate a differential torque as a function of the clutch slip and the setpoint clutch slip. 
   In a further advantageous embodiment of the present invention, the input variable of the inverse clutch model is a function of the differential torque. 
   In a further advantageous embodiment of the present invention, the sum of the differential torque and the engine torque generated by the internal combustion engine is an input variable of the inverse clutch model. 
   In a further advantageous embodiment of the present invention, the coefficient of friction of the clutch is a parameter of the inverse clutch model. 
   In a further advantageous embodiment of the present invention, an adapter is provided to adapt the coefficient of friction of the clutch. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a drive unit for a motor vehicle. 
       FIG. 2  shows a clutch controller. 
       FIG. 3  shows a slip regulator. 
       FIG. 4  shows a flow chart. 
       FIG. 5  shows a coefficient of friction-slip characteristic curve. 
       FIG. 6  shows a flow chart. 
       FIG. 7  shows an illustration of the flow chart of FIG.  4 . 
       FIG. 8  shows an illustration of the flow chart of FIG.  6 . 
       FIG. 9  shows an advantageous exemplary embodiment of a clutch controller. 
       FIG. 10  shows an alternative exemplary embodiment for a slip controller. 
       FIG. 11  shows a graph of slip plotted over time. 
       FIG. 12  shows a graph of slip plotted over time. 
       FIG. 13  shows a clutch. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a drive unit for a motor vehicle. Reference symbol  1  identifies an internal combustion engine which is connected to an automatic transmission  2  via a shaft  4 . Automatic transmission  2  is designed in a particularly advantageous manner as a belt transmission. Automatic transmission  2  is connected via a clutch input shaft  5 , a clutch  3 , a clutch output shaft  6 , and a differential  7  to driven wheels  8 ,  9  for the purpose of propelling the motor vehicle. By pressing clutch  3  together with an application pressure p, it is possible to adjust the torque which is transmitted via clutch  3 . In order to adjust the torque transmitted via clutch  3 , a clutch controller  12  is provided, which by specifying a setpoint application pressure p*, adjusts the application pressure in clutch  3 . The application pressure is synonymous with an application force with which clutch  3  is pressed together. 
   Input variables in clutch controller  12  include rotational speed n E  of clutch input shaft  5  which is measured by a rotational speed sensor  10 , rotational speed n A  of clutch output shaft  6  which is measured by a rotational speed sensor  11 , transmission ratio i of automatic transmission  2  and a setpoint value Δn* for the clutch slip of clutch  3  (setpoint clutch slip) as well as optionally torque T M  of internal combustion engine  1  as well as information ΔT M  relating to the inaccuracy of the information relating to torque T M  of internal combustion engine  1 . Clutch slip Δn is defined as
 
Δ n=n   E   −n   A  
 
   Torque T M  of internal combustion engine  1  and information ΔT M  relating to the inaccuracy of the information relating to torque T M  of internal combustion engine  1  are provided, for example, by an engine controller which is not illustrated. 
     FIG. 2  shows clutch controller  12 . It has a subtracter  20 , a slip controller  21  and an adapter  22 . Slip controller  21  is explained in greater detail with reference to FIG.  3  and the adapter with reference to FIG.  4 . The subtracter determines clutch slip Δn, which is the input variable in slip controller  21 . Additional input variables of slip controller  21  include setpoint clutch slip Δn*, engine torque T M , transmission ratio i of automatic transmission  2  and coefficient of friction μ. Coefficient of friction μ is formed by adapter  22 . Input variables in adapter  22  include setpoint clutch slip Δn*, transmission ratio i of automatic transmission  2 , torque T M  of internal combustion engine  1 , information ΔT M  relating to the inaccuracy of the information relating to torque T M  of internal combustion engine  1  as well as a differential torque T R  which is formed by slip controller  21 . In addition to coefficient of friction μ, a corrected engine torque T MK  is an additional reference quantity of adapter  22 . Slip controller  21  also forms setpoint application pressure p*. 
     FIG. 3  shows the internal structure of slip controller  21 . Slip controller  21  has a filter  31  for the purpose of filtering clutch slip Δn. An adder  36  is used to produce the difference between setpoint clutch slip Δn* and clutch slip Δn which is filtered by filter  31 . This difference is negated by a negater  32  and is an input variable in a regulator  33 , which in an advantageous embodiment, is designed as a PID controller. Differential torque T R  is the output variable of controller  33 . 
   A filter  34  is used to filter engine torque T M . Engine torque T M , which is filtered in this manner, is multiplied by transmission ratio i of automatic transmission  2  by a multiplier  70  and added to differential torque T R  by an adder  37 . The sum of differential torque T R  and the engine torque, filtered and multiplied by transmission ratio i of automatic transmission  2 , is clutch torque T K  to be transmitted by clutch  3 , which together with coefficient of friction μ, is an input variable in an inverse clutch model  35 . In inverse clutch model  35 , the following equation is implemented in an exemplary embodiment: 
         p   *     =       1     A   R       ⁢     (         T   K       μ   ·   r   ·     Z   R         +     F   0       )           
 
   A is the piston surface of clutch  3 , Z R  the effective friction radius of clutch  3 , Z R  the number of friction surfaces of clutch  3  and F 0  is the minimum force required for transmitting torque via clutch  3 . 
     FIG. 4  shows a flow chart as an implementation of adapter  22 . Reference symbol  40  identifies the start of the sequence and reference symbol  49  the end of the sequence. In step  41 , information T M  relating to the engine torque, information ΔT M  relating to the inaccuracy of the information relating to engine torque T M , differential torque T R , setpoint clutch slip Δn* and application pressure p are input. 
   In a subsequent step  42 , a coefficient of friction μ is formed from setpoint clutch slip Δn* and application pressure p. In an advantageous manner, this is achieved by a coefficient of friction-slip characteristic curve which is a function of application pressure p. A characteristic curve of this type is illustrated for example in FIG.  5  and is identified by reference symbol  50 . 
   Step  42  is followed by interrogation  43  inquiring whether
 
ΔT M ≦T 1  
 
where T 1  is a (first) tolerance value. If
 
ΔT M ≦T 1  
 
then step  44  follows in which a new coefficient of friction μ of the clutch is formed according to 
       μ   =     μ   +         T   M     ·   i           T   M     ·   i     +     T   R               
 
and a corrected engine torque T MK  is formed according to
 
T MK =T M  
 
   Step  44  is followed by step  45  in which the coefficient of friction-slip characteristic curve  50  as a function of the application pressure is modified in such a manner that the new value for coefficient of friction μ and setpoint clutch slip Δn* form a pair of values on modified coefficient of friction-slip characteristic curve  51 . Step  45  is illustrated in FIG.  5 . Reference symbol μ 1  identifies the value for coefficient of friction μ for the relevant application pressure prior to execution of step  45  and μ 2  identifies the value of coefficient of friction μ for the relevant application pressure after execution of step  45 . Coefficient of friction μ 1  is formed using characteristic curve  50  as a function of setpoint clutch slip Δn* (see step  42 ). In step  45 , coefficient of friction-clutch slip characteristic curve  50  is modified in such a manner that a coefficient of friction-clutch slip characteristic curve  51  is produced, on which value  12  and setpoint clutch slip Δn* are a pair of values. 
   If
 
ΔT M ≦T 1  
 
is not fulfilled, then instead of step  44 , step  48  follows in which a corrected engine torque T MK  is equated to the sum of engine torque T M  generated by internal combustion engine I and differential torque T R  divided by transmission ratio i of automatic transmission  2 :
 
 T   M   =T   M   +T   R   /i  
 
   Step  46  or  48  is followed by an interrogation  47  inquiring whether the preceding sequence is to be repeated. If this is the case, then step  41  follows. If this is not the case, the sequence is terminated. 
     FIG. 6  shows a modification of the flow chart of FIG.  4 . Interrogation  43  is not followed by step  48  but rather by an interrogation  60 . Interrogation  60  inquires whether
 ΔT M &gt;T 2    
is fulfilled, T 2  being a second tolerance value. If this condition is fulfilled, then step  48  follows. However if the condition is not met, step  46  is performed.
 
     FIGS. 7 and 8  illustrate the differences between the flow charts as shown in FIG.  4  and FIG.  6 . Information ΔT M  relating to the inaccuracy of the information relating to engine torque T M  of internal combustion engine  1  is shown on the abscissa. The ordinate in  FIG. 7  and  FIG. 8  indicates which steps are executed. The value −1 symbolizes the execution of steps  44  and  45 , the value 1 symbolizes the execution of step  48 , and the value 0 represents neither the execution of steps  44  and  45  nor of step  48 . Interrogation  43  in  FIG. 4  corresponds to a binary switch. The combination of interrogations  43  and  60  in  FIG. 6  corresponds to a three-point switch. Instead of these two straightforward switch types, it is naturally also feasible to perform complicated switching procedures, such as flowing transitions, which can be performed, e.g., by fuzzy techniques. 
     FIG. 9  shows an advantageous exemplary embodiment of a clutch controller  79  which can be used as a substitute for clutch controller  12  of FIG.  1 . Clutch controller  79  in  FIG. 9  has a slip controller  80  and a protection device  81  to protect the drive unit, automatic transmission  2  in particular, against torque shocks. Shock torque T S  is the output variable of protection device  81 . In an advantageous embodiment, shock torque T S  is calculated according to the following equation 
         T   S     =       T   C     -       Σ   l     ⁢       J   l     ·       2   ⁢     π   ·   Δ     ⁢           ⁢     n   max         Δ   ⁢           ⁢   t                   
where
     J 1  is the moment of inertia of a 1st component of the drive unit on the side of clutch  3 , on which internal combustion engine  1  is situated.   Δn max  is the maximum permissible clutch slip   T C  is a constant torque   Δt is the period of time, in which a torque shock results in an increase in slip.   
   The introduction of torque shocks, in particular torque shocks which are introduced into the drive unit by virtue of driven wheels  8  and  9 , may cause damage to automatic transmission  2 . It is particularly critical to protect, for example, a variator of a CVT (continuously variable transmission). Even a brief period of slip in this type of belt transmission due to a torque shock can result in permanent damage to the belt transmission. Torque shocks of this type occur, for example, in a change from a roadway surface having a low coefficient of friction to a roadway surface having a high coefficient of friction. Examples include the change from an ice-covered roadway to a dry roadway or when crossing railroad tracks. 
   If the duration of slip Δt is of secondary importance, then shock torque T S  may be made equal to constant torque T C . 
   In an advantageous embodiment, it is possible to transmit shock torque T S  to a transmission controller so that, for example, the application pressure can be increased accordingly in a belt transmission. The application pressure required in the belt transmission is to be increased as a function of shock torque T S . 
     FIG. 10  illustrates slip controller  80  in detail. Slip controller  80  differs from slip controller  21  in that it has a minimum value generator  82 . Minimum value generator  82  compares differential torque T R  and shock torque T S  and outputs the smaller torque as an output variable. 
     FIG. 11  shows a corresponding level of slip Δn plotted over time t when using a clutch controller  79  as shown in FIG.  9 . Point in time t 1  identifies the point in time at which the maximum permissible slip Δn max  is reached and t 2  identifies the point in time at which the slip caused by the torque shock has decayed. The period of time between points in time t 2  and t 1  is slip time Δt.  FIG. 11  shows the progression of clutch slip Δn if setpoint clutch slip Δn* is equal to zero. In the event that setpoint clutch slip Δn* does not equal zero,  FIG. 12  shows the variation of clutch slip Δn. In this case clutch slip Δn at point of time t 2  is equal to setpoint clutch slip Δn*. 
   In order to protect clutch  3  from thermal overload, slip time Δt is advantageously adjusted as a function of the thermal loading in clutch  3 . For this purpose, the temperature of clutch  3  is estimated using a thermodynamic model. If the estimated temperature of clutch  3  exceeds a critical temperature limit, then setpoint clutch slip Δn* is reduced to zero. Moreover, in an advantageous embodiment, a reserve application pressure is increased. This may be achieved, for example, by increasing value F 0 . Alternatively, a reserve torque may be increased. This is achieved, for example, by increasing value T C . 
     FIG. 13  shows a clutch  3  in an exemplary embodiment. Reference symbol  83  identifies a lubricating oil supply for hydraulic oil, reference symbol  84  an outer driver, reference symbol  85  an inner driver, reference symbol  86  an outer blade, reference symbol  87  an inner blade, reference symbol  88  a restoring spring, reference symbol  93  a cylinder, reference symbol  94  a piston, reference symbol  95  a pressure plate and reference symbol  96  a pressure medium supply. Outer driver  84 , which is connected to clutch input shaft  5 , is provided with outer blades  86 , and in an advantageous embodiment, with steel blades without a friction lining. Inner driver  85 , which is connected to clutch output shaft  6 , accommodates inner blades  87  which are coated with a friction lining. Upon the introduction of hydraulic oil at a defined pressure level via pressure medium supply  96  into cylinder  93 , piston  94  moves against the force of restoring spring  88  in the direction of pressure plate  95  and presses together the blade package which has inner and outer blades  87  and  86 . In order to cool the blade package, hydraulic oil is directed to inner and outer blades  87  and  86  via lubricating oil supply  83 . 
   LIST OF REFERENCE SYMBOLS 
   
       
         1  engine 
         2  transmission 
         3  clutch 
         4  shaft 
         5  clutch input shaft 
         6  clutch output shaft 
         7  differential 
         8 ,  9  drive wheels 
         10 ,  11  rotational speed sensors 
         12 ,  79  clutch controller 
         20  subtracter 
         21 ,  80  slip controller 
         22  adapter 
         31 ,  34  filter 
         32  negater 
         33  regulator 
         35  inverse clutch model 
         36 ,  37  adder 
         40  start of the sequence 
         41 ,  42 ,  44 , step 
         45 ,  46 ,  48 , 
         43 ,  47 ,  60 , interrogation 
         49  end of the sequence 
         50 ,  51  coefficient of friction-slip characteristic curve 
         70  multiplier 
         81  protection device 
         82  minimum value generator 
         83  lubricating oil supply 
         84  outer driver 
         85  inner driver 
         86  outer blade 
         87  inner blade 
         88  restoring spring 
         91  engine torque setpoint generator 
         93  cylinder 
         94  piston 
         95  pressure plate 
         96  pressure medium supply 
       n E  rotational speed of the clutch input shaft 
       n A  rotational speed of the clutch output shaft 
       T M  information relating to the engine torque 
       ΔT M  inaccuracy of the information relating to the engine torque 
       T R  differential torque (regulator output) 
       T K  clutch torque 
       T 1  first tolerance value 
       T 2  second tolerance value 
       Δn clutch slip 
       Δn* setpoint clutch slip 
       i transmission ratio of the transmission 
       p application pressure 
       p* setpoint application pressure 
       μ, μ 1 , μ 2  coefficient of friction 
       J 1  moment of inertia of the drive unit on the side of clutch  1 , on which the internal combustion engine is situated. 
       Δn max  maximum permissible clutch slip 
       T C  constant torque 
       Δt time period in which a torque shock causes an increase in slip 
       A R  friction surface of the steel blades of the clutch 
       Z R  number of friction surfaces of the clutch 
       T MK  corrected engine torque 
       F 0  minimum force required for transmitting torque via the clutch 
       T S  shock torque 
       t 1  point in time 
       t 2  point in time 
       r effective friction radius of the clutch