Abstract:
A transmission control method improves shift feel during kickdown shifts. The release clutch is fully released at the initiation of the kickdown shift. The release clutch is then reapplied when the volume of the release clutch reaches a threshold capacity. The volume of the release clutch is slowly ramped down, thereby increasing turbine speed. When the turbine speed reaches a threshold, the apply clutch is actuated. The apply clutch is actuated by controlling the volume of the apply clutch according to a target volume.

Description:
FIELD OF THE INVENTION 
     The present invention relates to automotive transmissions, and more particularly to controlling kickdown shifts in automotive transmissions based on accumulator feedback. 
     BACKGROUND OF THE INVENTION 
     Due to relatively high instances of system inertia and delay in automotive transmissions, feedback control of various components in automotive transmissions is not appropriate for certain transient elements. Control of transmission turbine speed during a kickdown shift is one example of a transient condition in automotive transmissions. During a kickdown shift, such as a drop from 4 th  gear to 3 rd  gear, or from 3 rd  gear to 2 nd  gear, the speed of the turbine must increase to correspond to a targeted gear ratio. Additionally, the acceleration of the turbine must be controlled to correspond to a targeted acceleration according to current vehicle acceleration. In such transient cases, feedforward control can be used to anticipate system changes. For example, mixed feedforward and feedback control can be used for a smooth kickdown shift without causing significant “feel” issues for the driver, thereby improving overall shift quality. Shift quality has been shown to be an important factor for driver satisfaction. 
     Automotive transmissions may use accumulators to absorb apply pressure fluid during certain shift operations. The presence of the accumulator reduces sensitivity of torque variations in torque phase during shifts. However, accumulators cause the pressure response to be slower and more difficult to predict since the solenoid current directly controls the flow rate and indirectly controls the pressure. With reference to  FIG. 1 , a typical accumulator  10  includes one or more springs  12  and a piston  14 . Fluid fills the accumulator  10  and compresses the spring  12 . The volume of the accumulator  10  varies over the usable range of the spring  12 , and is indicative of the volume of a particular shift element in the transmission. The volume of the shift element is a further indicator of the capacity of the shift element, which may be used for control purposes. Target volume kickdown logic determines a target volume for the shift element, and subsequently calculates a change in shift element volume required for proper control. Control based on target volume can be used to calculate changes in element capacity or volume that are required to achieve target acceleration. As a result, excessive runaway or harshness during shifting is prevented. 
     Target volume control can be determined according to desired volume change due to turbine inertia force and/or desired volume change due to engine inertia force. Conventionally, empirical methods are used to determine target volume control. For example, change in volume can be calculated according to relationships between turbine inertia force, engine inertia force, accumulator pressure, and/or release element clutch pressure. However, such empirical methods are not particularly accurate in practice because turbine acceleration and engine acceleration each belong to independent dynamic systems. Therefore, the release element clutch cannot directly control engine acceleration. When the release element clutch is used to control turbine acceleration, turbine torque from the engine must be assumed as a fixed input through the torque converter and is a function of slip speed between the engine and the turbine. 
     If only the engine dynamic system is considered, the engine resistance torque, or turbine torque, can be changed to control engine acceleration if the throttle opening is fixed. However, turbine torque, or engine resistance torque, that is required to control the engine acceleration into a desired acceleration is different from the fixed turbine torque when turbine acceleration is controlled into a desired value. The control may be overcompensated because the torque required to change the engine acceleration is much larger than the turbine torque received from the engine. Therefore, it is desirable to provide optimized control during a kickdown shift to further improve shift quality. A continuous variable and speed-based desired acceleration method to provide consistent and accurate transmission control based in part on accumulator pressure is proposed. 
     SUMMARY OF THE INVENTION 
     A vehicle transmission comprises a plurality of gears. A torque converter assembly transmits torque between an engine and the plurality of gears through a plurality of engagement elements. A plurality of solenoids are operable to actuate the plurality of engagement elements. An accumulator is indicative of a pressure of at least one of the engagement elements. A controller calculates a torque of the at least one engagement element based on a first relationship between a volume of the accumulator and the pressure, and controls the torque based on a second relationship between the torque and a duty cycle of at least one of the solenoids. 
     In another aspect of the invention, a transmission control method for kickdown shifts comprises releasing a release engagement element. The release engagement element is applied when a volume of the release engagement element reaches a threshold capacity of the release engagement element. The volume of the release engagement element is decreased, thereby increasing transmission turbine speed. A volume of an apply engagement element is increased when the transmission turbine speed reaches a first threshold. A target volume of the apply engagement element is determined. The volume of the apply engagement element is controlled according to the target volume. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  illustrates an accumulator according to the prior art; 
         FIG. 2  is a functional block diagram of a transmission control system according to the present invention; 
         FIG. 3  illustrates a vehicle transmission according to the present invention; and 
         FIG. 4  illustrates a transmission kickdown control method according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     The present invention uses a model-based approach to identify speed and torque dynamics for each transmission element during transmission shift operations. Referring now to  FIG. 2 , a transmission control system  20  includes an engine  22 , a torque converter  24 , an automatic transmission  26 , and a controller  28 . The engine  22  drives the automatic transmission  26  through the torque converter  24 . The transmission  26  drives a vehicle through a gear ratio. The controller  28  communicates with various sensors and controls transmission shifting. For example, an engine speed sensor  30  generates an engine speed signal. An accumulator  32  fills with oil, varying the volume of the accumulator  32 , which changes clutch pressure. The controller  28  determines required torque of the transmission element clutches according to engine speed, volume of the accumulator  32 , and additional factors of the torque converter  24  and the transmission  26 , such as torque converter transferred torque, inertia for the engaged elements of the transmission  26 , and desired turbine acceleration. The controller  28  further calculates a control duty cycle for the transmission  26  based on a relationship between each individual element clutch torque and pressure, and a relationship between accumulator pressure and accumulator volume change. 
     Kickdown shifts are controlled based on target volume control and continuous variable, speed based desired acceleration. Referring now to  FIG. 3 , an exemplary automotive transmission  40  includes planetary gears  42 ,  44 ,  46  and element clutches  48 ,  50 ,  52 ,  54 ,  56 , and  58 . One or more of the clutches interact with one or more of the planetary gears in order to select a gear ratio of the transmission  40 . For example, when clutch  54  is in contact with planetary gear  42 , and clutch  56  is in contact with planetary gears  42  and  44 , 4th gear is selected. However, in order to select 3rd gear, clutch  48  must be in contact with planetary gear  46  and clutch  56  must be in contact with planetary gears  42  and  44 . Therefore, in order for the transmission  40  to downshift from 4th gear to 3rd gear, clutch  54  must release planetary gear  42  and clutch  48  must be applied to planetary gear  46 . In any particular downshift, the element clutches that are releasing are referred to as “release element clutches.” Conversely, element clutches that are applied during a downshift are referred to as “apply element clutches.” 
     During the inertia phase of a kickdown shift, the torque required for releasing an element clutch is determined. Hereinafter, all references to the release clutch refer to clutch  54  with respect to a 4-3 kickdown shift wherein the clutch  54  is the release element clutch and clutch  48  is the apply element clutch. Although the following equations refer to a 4-3 kickdown shift, it should be understood that analogous calculations can be applied to other kickdown shifts. For a 4-3 kickdown shift (from 4th gear to 3rd gear), the torque for release element clutch  54  is: 
         T     4   ⁢   c       =       1   4     ⁡     [       T   t     -     3   ⁢     T   ud       -       (       I   1     +     4   ⁢     I   2       +     I   3     +     16   ⁢     I   4       +     9   ⁢     I   5         )     ⁢     α   t       +       (       6   ⁢     I   2       +     12   ⁢     I   4       +     6   ⁢     I   5         )     ⁢     α   o         ]           
 
where T t  is turbine output torque, T ucl  is torque at element clutch  48 , α t  is turbine acceleration, α 0  is output vehicle acceleration, and I 1  through I 5  are the inertia of each transmission element clutch as indicated in  FIG. 3 . The inertia of the release element clutch  54  is not considered. Because α 0  is much smaller than turbine acceleration due to significant vehicle inertia, output inertia force (6I 2 +12I 4 +6I 5 )α 0  and the torque at element clutch  48  can be removed, resulting in: 
               T     4   ⁢   c       =       1   4     ⁡     [       T   t     -       (       I   1     +     4   ⁢     I   2       +     I   3     +     16   ⁢     I   4       +     9   ⁢     I   5         )     ⁢     α   t         ]               (     Equation   ⁢           ⁢   1     )             
 
     In a pulse width modulated solenoid system, the indication of clutch torque is accumulator volume. According to the relationship between the accumulator volume and the clutch pressure, equation 1 becomes: 
     T 4c =P 4C A p μ f R eff n 4C , and subsequently, 
               P     4   ⁢   C       =       1     4   ⁢     μ   f     ⁢     A   p     ⁢     R   eff     ⁢     n     4   ⁢   C           ⁡     [       T   t     -       (       I   1     +     4   ⁢     I   2       +     I   3     +     16   ⁢     I   4       +     9   ⁢     I   5         )     ⁢     α   t         ]               (     Equation   ⁢           ⁢   2     )             
 
where P 4C  is the clutch pressure, A P  is the friction material area, μ f  is the coefficient of friction, R eff  is the effective radial, and n 4C  is the number of friction surfaces. The relationship between the accumulator volume and the clutch pressures is expressed as: 
               V     4   ⁢   C       =           A   A       K   A       ⁢     {         1     4   ⁢     μ   f         ⁡     [       T   t     -       (       I   1     +     4   ⁢     I   2       +     I   3     +     16   ⁢     I   4       +     9   ⁢     I   5         )     ⁢   dt       ]       -     P   pre       }       +     V     A   ⁢           ⁢   min                 (     Equation   ⁢           ⁢   3     )             
 
     and 
           V   A     =           A   A       K   A       ⁡     [       P   A     -     P   pre       ]       +     V     A   ⁢           ⁢   min           ,       
 
where V A  is current accumulator volume, A A  is accumulator piston area, K A  is the accumulator spring coefficient, P A  is accumulator pressure, P pre  is pre-loaded accumulator pressure, and V Amin  is the minimum accumulator volume.
 
     Equation 1 is the required clutch torque during steady state conditions. Additionally, equation 1 is the theoretical initial value for feedback controls. In a transient case, the torque change required for acceleration can be estimated by taking the derivative of equation 1 as follows: 
                 ⅆ     T     4   ⁢   c           ⅆ   t       =         1   4     ⁡     [         ⅆ     T   t         ⅆ   t       -       (       I   1     +     4   ⁢     I   2       +     I   3     +     16   ⁢     I   4       +     9   ⁢     I   5         )     ⁢       ⅆ     α   t         ⅆ   t           ]       .             (     Equation   ⁢           ⁢   4     )             
 
     This differential equation is discretized as: 
             T     4   ⁢   C     des     -     T     4   ⁢   C     c       dt     =       1   4     ⁢       {           T   t   i     -     T   t     i   -   1           Δ   ⁢           ⁢   t       +       (       I   1     +     4   ⁢     I   2       +     I   3     +     16   ⁢     I   4       +     9   ⁢     I   5         )     ⁢         α   t     -     α   dt         Δ   ⁢           ⁢   t           }     .           
 
     However, torque is not the actual control actuator in the preferred embodiment. Instead, the duty cycle of the solenoid is the control force used to change the torque in the element clutches. Therefore, the relationship between clutch torque and the duty cycle of the solenoid must be determined. The relationship between clutch torque and the duty cycle of the solenoid is based in part on a relationship between accumulator pressure and the flow rate: 
           Q   DC     =         ⅆ     V   a         ⅆ   t       =         A   a       K   a       ⁢       ⅆ     P     4   ⁢   C           ⅆ   t             ,       
 
where Q DC  is the transmission oil flow rate, V a  is accumulator volume, A a  is accumulator area, K a  is the accumulator spring coefficient, and P 4C  is the clutch pressure of clutch  54 . Torque on the clutch  54  can be calculated based on accumulator pressure according to T 4C =P 4C A p μ f R eff n 4C , substituting the relationships between the clutch and the accumulator into the control equation, which is equation 1, results in a formulation of target volume control duty cycle flow rate as: 
               Q   DC     =         3   ⁢     A   a   2         4   ⁢     μ   f     ⁢     K   a     ⁢     R   eff     ⁢     N     4   ⁢   c       ⁢     A   p         ⁢     {           T   t   i     -     T   t     i   -   1           Δ   ⁢           ⁢   t       +       (       I   1     +     4   ⁢     I   2       +     I   3     +     16   ⁢     I   4       +     9   ⁢     I   5         )     ⁢         α   t     -     α   dt         Δ   ⁢           ⁢   t           }               (     Equation   ⁢           ⁢   5     )             
 
     The first term in equation 5 is the torque required to overcome the torque input change from the torque converter. The second term is torque required to change the turbine and planetary gear inertias. Therefore, 
             δ   ⁢           ⁢     V   t         t   tv       =         3   ⁢     A   a   2         4   ⁢     μ   f     ⁢     K   a     ⁢     R   eff     ⁢     N     4   ⁢   c       ⁢     A   p         ⁢     (       I   1     +     4   ⁢     I   2       +     I   3     +     16   ⁢     I   4       +     9   ⁢     I   5         )     ⁢         α   t     -     α   dt         Δ   ⁢           ⁢   t           ,       
 
and 
             δ   ⁢           ⁢     V   e         t   ev       =         3   ⁢     A   a   2         4   ⁢     μ   f     ⁢     K   a     ⁢     R   eff     ⁢     N     4   ⁢   c       ⁢     A   p         ⁢         T   t   i     -     T   t     i   -   1           Δ   ⁢           ⁢   t           ,       
 
where 
         δ   ⁢           ⁢     V   t         t   tv         
 
is desired volume change due to turbine inertia force over time and 
         δ   ⁢           ⁢     V   e         t   ev         
 
is desired volume change due to engine inertia force over time.
 
     Input torque is equal to engine flywheel torque when the converter clutch is in lock-up and/or partial lock positions. When the converter is in an unlock position, the input torque can be calculated by a torque converter slip regression model: 
         T   t   i   =└C   0 N e   i +C 1 (N e   i −N t   i )┘N e   i  for N t &lt;0.85N e , otherwise:           T   t   i     =       [           C   0     0.15     ⁢     (       N   e   i     -     N   t   i       )       +       C   1     ⁡     (       N   e   i     -     N   t   i       )         ]     ⁢     N   e   i         ,         
     where C 0  and C 1  are constants, N e   i  is engine speed, and N t   i  is turbine speed. 
     Using the above models, the present invention determines transmission kickdown control according to a release phase  60 , a target volume control phase  62 , an apply element fill phase  64 , and an apply element control phase  66  as shown in  FIG. 4 . The transmission control as described relates to N i , or current turbine speed  68 , N j , or target turbine gear speed  70 , and N t , or turbine acceleration  72 . In the release phase  60 , T 4C  is calculated according to equation 1. When the capacity of clutch  54  (as shown in  FIG. 3 ) is less than the required torque, turbine speed will increase from its original gear speed N j . The acceleration of the turbine speed depends on the input torque and the control torque in clutch  54 :
 
( I   1 +4 I   2   +I   3 +16 I   4 +9 I   5 )α t   =T   t −3 T   UD −4 T   4C .
 
     At the beginning of the kickdown shift, clutch  54  is released quickly. The clutch  54  is reapplied when the track volume V 4C  reaches the calculated volume from Equation 3. Then, V 4C  is slowly ramped down until the turbine speed reaches a desired acceleration. Thereafter, the character time of  96  is increased to satisfy the condition: 
         α   d     &lt;     -           T   t     -     4   ⁢       (     T     4   ⁢   C       )     min             I   1     +     4   ⁢     I   2       +     I   3     +     16   ⁢     I   4       +     9   ⁢     I   5           .           
 
During the release phase  60 , the turbine speed begins to increase from the turbine speed  68  toward the target gear speed  70  as the turbine acceleration  72  decreases.
 
     In the target volume control phase  62 , turbine speed approaches and/or reaches desired initial turbine acceleration 
         α   d     =           N   j     -     N   i       τ     .         
 
     Actual target volume control activates according to a target gear turbine speed and desired acceleration 
           α   d     =       -         N   j     -     N   i     +     Δ   ⁢           ⁢   N           τ   2     ⁡     (     1   -     ⅇ       -     τ   1         τ   2           )           ⁢     ⅇ       -   t       τ   2             ,       
 
where τ 1  is a desired time for the turbine to travel from the current gear speed to the desired gear speed and τ 2  is the decal rate of the desired acceleration.
 
     When t&gt;τ 2 −t f , where t f  is the required apply element fast fill clutch volume time, the apply element clutch begins to fill. As shown in  FIG. 4 , the turbine acceleration  72  decreases as the turbine speed  68  increases toward the target gear speed  70 . 
     In the apply element fill phase  64 , DC t  is applied to the apply element clutch after N t &gt;N j . In other words, as the turbine speed  68  surpasses the target gear speed  70 , torque is applied to the apply element clutch. In a 4-3 kickdown shift, the apply element clutch  48  pressure is: 
     P UD =T t −4T 4C −(I 1 −2I 2 +I 3 +4I 4 +3I 5 )α 0 +P rs , where P UD  is the apply element clutch  48  pressure and P rs  is pre-loaded accumulator spring pressure. The targeted volume to achieve this pressure is 
           V   UD     =       A     K   S       ⁢     (     PA   -   P     )         ,       
 
where A is accumulator piston area and K S  is spring stiffness.
 
     In the apply element control phase  66 , the turbine speed  68  begins to exhibit a negative slope. The release element is fast-vented in order to rapidly dump the pressure to the release element. Torque is managed to quickly ramp the apply element to full pressure. Therefore, the release element clutch is fully released based on the values of N t &gt;N j  and α t −α j . In this manner, the release element is fully released and the apply element is fully applied, completing the gear change. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.