Abstract:
A method for executing a downshift in a transmission includes starting disengagement of a second control element after starting disengagement of a first element. Disengagement of the second element starts before starting engagement of a fourth element. A third element is forced to synchronous speed by beginning engagement of the fourth element before engaging the third element. Engagement of the third and fourth elements is completed at the end of the downshift.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to automatic transmissions for automotive vehicles, in particular to transmissions comprising planetary gearsets operated by friction control elements. 
     2. Description of the Prior Art 
     It is difficult to achieve acceptable shift quality on sequential, i.e., continuous downshifts, such as a 6-4-3 or 5-3-2 downshift, in an automatic transmission because a torque disturbance may occur during the transition from the first to the second portion of the shift. In order to execute the transition smoothly, the offgoing control element, a clutch or brake, for the second shift must slip before the oncoming element of the first shift gains torque capacity. 
     The shifts are difficult to calibrate robustly. If the offgoing element slips too soon, a neutral interval occurs near the end of the shift. Conversely, if the offgoing element slips too late, a torque bump occurs as the oncoming element of the first shift gains capacity. 
     To achieve acceptable feel, the oncoming and offgoing elements must be closely synchronized. But precise synchronization is difficult to achieve under all operating conditions. 
     The period required to execute successive downshifts between adjacent gears using conventional control techniques is unacceptable approximating 1.2 seconds to complete such downshifts. There is a need in the industry for a control strategy that permits successive downshifts to be completed smoothly and within an acceptably short period. 
     SUMMARY OF THE INVENTION 
     A method for executing a sequential downshift in a transmission includes disengaging a first element and starting disengagement of a second element, forcing a third element toward synchronous speed by increasing to a low capacity a torque capacity of the fourth element before engaging the third element, and engaging the third and fourth elements. 
     With this control strategy, there is no need to precisely synchronize the oncoming element of the first shift with the offgoing element of the second shift. Shift time is equivalent to that of a 6-2 direct downshift, providing greater consistency among the power on downshifts. 
     The control maintains output torque during the ratio change and allows for change of mind to the intermediate gear. Should the driver tip out early enough in the shift, the first oncoming element is applied and the second shift is cancelled. In addition, the final on coming element may be pre-staged to allow a continuous ratio change if the driver tips into a 6-4 or 5-3 downshift in progress. 
     Early application of the second on-coming element increases energy dissipation. The time the clutch applied, however, is significantly less than during equivalent downshifts using another control strategy. 
     It is no longer necessary to perform single step interlocked downshifts to achieve high shift quality. Shift time is short and provides greater consistency among downshifts. The control is robust, easy to calibrate and provides fast smooth downshifts. 
     This control strategy eliminates the need for close synchronization, allowing the offgoing element of the second shift to be released late enough to avoid a neutral interval. In addition, torque from the final oncoming element helps the ratio change to progress through the intermediate gear ratio. This approach maintains output torque and allows for change of mind to the intermediate gear. Should the driver tip out early enough in the shift, the first oncoming element is applied and the second shift is cancelled. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram showing the kinematic arrangement of an automatic transmission; 
         FIG. 2  shows the selective table of friction elements; 
         FIGS. 3A and 3B  schematically represent the Ravigneaux gearset of  FIG. 1 ; 
         FIGS. 4A and 4B  schematically represent a simple planetary portion of the Ravigneaux gearset of  FIG. 1 ; 
         FIGS. 5A and 5B  schematically represent a compound planetary portion of Ravigneaux gearset of  FIG. 1 ; 
         FIG. 6  is a lever representing the kinematics of the planetary portion of the Ravigneaux gearset of  FIG. 1 ; 
         FIG. 7  is a lever representing the kinematics of the compound planetary portion of Ravigneaux gearset of  FIG. 1 ; 
         FIG. 8  is a lever that represents the Ravigneaux gearset of  FIG. 1  and derived from  FIGS. 6 and 7 ; 
         FIG. 9  shows the lever of  FIG. 6  with the numeric relationships of a automatic transmission application; and 
         FIG. 10  is a lever diagram representing the Ravigneaux gearset of  FIG. 1  with the numeric relationships of the transmission application of  FIG. 9 ; 
         FIG. 11  is a graph showing an abrupt output torque disturbance in the Ravigneaux gearset of  FIG. 1  during a downshift; 
         FIG. 12  is a graph showing the variation of clutch and brake torques in the Ravigneaux gearset of  FIG. 1  during a downshift wherein CL/B gains capacity later than CL/A; 
         FIG. 13  is a graph showing the variation of clutch and brake torques in the Ravigneaux gearset of  FIG. 1  during a downshift wherein CL/A closes rapidly; 
         FIG. 14  is a graph showing the variation of output torque in the Ravigneaux gearset of  FIG. 1  during a downshift wherein CL/B gains capacity earlier than CL/A; 
         FIG. 15  is a graph showing the variation of clutch and brake torque in the Ravigneaux gearset of  FIG. 1  during a downshift wherein CL/B gains capacity earlier than CL/A; 
         FIG. 16  is a graph showing the variation of element speeds in the Ravigneaux gearset of  FIG. 1  during a downshift; 
         FIGS. 17-20  are lever diagrams in the Ravigneaux gearset of  FIG. 1  showing progressive variation of element speeds and element torques during the downshift illustrated in  FIG. 16 ; and 
         FIG. 21  is diagram of the control logic showing the steps for controlling a downshift in a transmission. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in  FIG. 1  the kinematic arrangement of an automatic transmission. The torque converter  10  includes an impeller wheel  12  connected to the crankshaft  14  of an internal combustion engine, a bladed turbine wheel  16 , and a bladed stator wheel  18 . The impeller, stator and turbine wheels define a toroidal fluid flow circuit, whereby the impeller is hydrokinetically connected to the turbine. The stator  18  is supported rotatably on a stationary stator sleeve shaft  20 , and an overrunning brake  22  anchors the stator to the shaft  20  to prevent rotation of the stator in a direction opposite the direction of rotation of the impeller, although free-wheeling motion in the opposite direction is permitted. 
     The torque converter assembly includes a lockup or bypass clutch  24  located within the torque converter impeller housing  25 . When clutch  24  is engaged, the turbine and impeller are mechanically connected; when clutch  24  is disengaged, they are hydrokinetically connected and mechanically disconnected. Fluid contained in the torque converter  10  is supplied from the output of an oil pump assembly  30  and is returned to an oil sump, to which an inlet of the pump is connected hydraulically. 
     Planetary gearing includes a first simple planetary gear set  32  and a second Ravigneaux planetary gear set  34 . The first gear unit  32  includes a sun gear  38 , ring gear  40 , carrier  42 , and planetary pinions  44 , supported on carrier  42  in meshing engagement with sun gear  38  and ring gear  40 . Sun gear  38  is fixed against rotation. Ring gear  40  is continually connected to an input shaft  45  and to an overdrive clutch, i.e., CL/E. Carrier is continually connected to a forward clutch, i.e., CL/A, and to a direct clutch, i.e. CL/B, which is connected to an intermediate brake, i.e., CL/C. 
     The second gear set  34  includes first and second sun gears  46 ,  47 , ring gear  48 , carrier  50 , and first and second sets of planetary pinions  52 ,  53  rotatably supported on carrier  50 . Pinions  53  are in meshing engagement with sun gear  47 . Pinions  52  are in meshing engagement with sun gear  46 , ring gear  48  and pinions  53 . Sun gear  46  is continually connected to intermediate brake CL/C. Ring gear  40  is continually connected to an output shaft  58 . Carrier  50  is continually connected to a low-reverse brake, i.e., CL/D, and to CL/E. Sun gear  47  is continually connected to forward clutch, CL/A. Direct clutch CL/B is connected to intermediate brake CL/C and forward clutch CL/A. 
     Referring to  FIGS. 1 and 2 , the first forward gear is produced when clutch CL/A and brake CL/D are engaged. The sun gear  47  is driven at the speed ratio produced by gearset  32 , and carrier  50  is held against rotation. Output  58  is driven at the low reduction ratio of the double planetary gearset  34 . 
     In the second forward gear, clutch CL/A and brake CL/C are engaged. The sun gear  47  is driven at the speed ratio produced by gearset  32 , and sun gear  46  is held against rotation. Output  58  is driven at the intermediate reduction ratio of the double planetary gearset  34 . 
     In third forward gear, clutches CL/A and CL/B are engaged. Sun gears  46 ,  47  are driven at the speed ratio produced by gearset  32 . Gearset  34  is locked up, and output  58  is driven at the speed ratio produced by gearset  32 . 
     In fourth forward gear, clutches CL/A and CL/E are engaged. The sun gear  47  is driven at the speed ratio produced by gearset  32 , and carrier  50  is driven at the speed of input  45 . Output  58  is driven at an intermediate speed ratio. 
     In fifth forward gear, clutches CL/B and CL/E are engaged. Carrier  50  is driven at the speed of input  45 , and sun gear  46  is driven at the speed ratio produced by gearset  32 . Output  58  is driven at an intermediate overdrive ratio through gearset  34 . 
     In sixth gear, clutch CL/E and brake CL/C are engaged. Carrier  50  is driven at the speed of input  45 , and sun gear  46  is fixed against rotation by brake CL/C. Output  58  is driven at the entire overdrive ratio of gearset  34 . 
     In reverse drive, clutch CL/B and brake CL/D are engaged. Brake CL/D holds carrier  50  fixed against rotation, and sun gear  46  is driven at the speed ratio produced by gearset  32 . Output  58  is driven at the reverse drive ratio of gearset  34 . 
     Each upshift from the current gear to the next higher gear or to the gear that it next higher, and each downshift from the current gear to the next lower gear or to the gear that is next lowest is produced throughout by changing only one of the two friction elements that are engaged in the current gear. 
     A 6-4-3 downshift begins in sixth gear with clutch CL/E and brake CL/C engaged, advances to fourth gear by disengaging brake CL/C, engaging clutch CL/A and maintaining clutch CL/E engaged, and ends in third gear by disengaging clutch CL/E, engaging clutch CL/B and maintaining clutch CL/A engaged. 
     For the 6-4-3 downshift, the first control element is brake CL/C, the second control element is clutch CL/E, the third control element is clutch CL/A, and the fourth control element is clutch CL/B. 
     A 5-3-2 downshift begins in fifth gear with clutches CL/B and CL/E engaged, advances to third gear by disengaging clutch CL/E, and engaging clutches CL/A and CL/B, and ends in second gear by disengaging clutch CL/B, engaging brake CL/C and maintaining clutch CL/A engaged. 
     For the 5-3-2 downshift, the first control element is clutch CL/E, the second control element is clutch CL/B, the third control element is clutch CL/A, and the fourth control element is brake CL/C. 
       FIGS. 3A-9  illustrate the kinematics of multi-step downshifts produced by the Ravigneaux gearset  34  in relation to a lever analogy.  FIGS. 3A and 3B  show that gearset  34  is formed by combining the two gearsets shown in  FIGS. 4A-4B  and  5 A- 5 B. 
     Let: 
     
         
         N S1 =Number of teeth on sun  1 . 
         θ s1 =Angular displacement of S 1 . 
         N S2 =Number of teeth on sun  2 . 
         θ s2 =Angular displacement of S 2 . 
         N R =Number of teeth on ring. 
       
    
     From  FIGS. 3A and 3B , if carrier  50  is grounded, i.e., held against rotation, and Sun  1  (sun gear  46 ) turns θ s1  radians, the circumferential distance Sun  1  travels is θ s1 *N S1 . Since no slipping occurs between the gears, Sun  2  (sun gear  47 ) must also travel the same circumferential distance (θ s1 *S 1 ), but in the opposite direction. The angular displacement ratio between Sun  1  and Sun  2  can be represented as follows: 
     
       
         
           
             
               
                 
                   
                     
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                       → 
                       
                         
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                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
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                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     From  FIGS. 4A and 4B , a lever can be constructed as shown in  FIG. 6 . If C (carrier  50 ) is held and Sun  1  is the input, then R (ring gear  48 ) is the output and the angle ratio of this gearset is: 
     
       
         
           
             
               
                 
                   
                     
                       θ 
                       R 
                     
                     
                       θ 
                       
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                         N 
                         
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                     Equation 
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                     2 
                   
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     From  FIGS. 5A and 5B , if C is held, Sun  2  and R rotate in the same direction or Sun  2  and R are on the same side of the lever). Also, the tangential velocity of R is less than the tangential velocity of Sun  2 . Thus the lever for the gear set of  FIG. 5A  is constructed as shown in  FIG. 7 . 
     The angular displacement can be written as follows: 
                       θ   R       θ     S   ⁢           ⁢   2         =       N     S   ⁢           ⁢   1       X             (     Equation   ⁢           ⁢   3     )               
From Equations 2 and 3:
 
                       Equation   ⁢           ⁢   2       Equation   ⁢           ⁢   3       =         θ     S   ⁢           ⁢   2         θ     S   ⁢           ⁢   1         =     -     X     N   R                   (     Equation   ⁢           ⁢   4     )               
Substitute Equation 4 in Equation 1:
 
     
       
         
           
             
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                 X 
                 
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                 → 
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                       1 
                     
                   
                   
                     N 
                     
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     Finally, from  FIGS. 6 and 7 , the lever  60 , which represents the Ravigneaux gearset  34 , is shown in  FIG. 8 . 
       FIG. 9  shows the numerical relationships for a particular transmission application. Since the ring gear  48  is connected to the output shaft  58 , any relative motion within gearset  34  causes the lever  60  to pivot about the output  58 . 
     The lever diagram of  FIG. 10  is helpful in understanding the mechanism by which early application of the final oncoming element negates the torque disturbance due to the torque transfer of the first oncoming element.  FIG. 10  shows the geometric relationships for the Ravigneaux gearset  34 . 
     Since ring gear  48  is connected to the output shaft  58 , any relative motion within gearset  34  causes the lever  60  to pivot about ring gear  48  at point  62 . As indicated in  FIG. 2 , clutch CL/B, brake CL/C and sun gear  46  are connected to lever  60  at point  64 . Clutch CL/E and carrier  50  are connected to lever  60  at point  66 . Clutch CL/A and sun gear  47  are connected to lever  60  at point  68 . Clutch CL/B and brake CL/C have significant mechanical advantage about the output  58  compared to either clutch CL/A or brake CL/E. 
     As  FIG. 11  shows, a significant torque disturbance occurs if clutch CL/A gains torque capacity before clutch CL/B. Since clutch CL/E has significant torque capacity, clutch CL/A pulls the transmission back toward the fourth gear. In the figures, DS_TRQ means driveshaft torque and TQT_WO_TQMOD means transmission input torque without torque modulation. 
     As  FIG. 12  shows, clutch CL/B gains torque capacity significantly later than clutch CL/A. 
     As  FIG. 13  shows, clutch CL/A closes rapidly, causing the torque disturbance of  FIG. 11 , and clutch CL/B closes shortly after clutch CL/A closes. 
     The simulation torque trace of  FIG. 14  closely matches the vehicle torque trace with clutch CL/B applied early.  FIG. 14  illustrates a 6-4-3 shift with early application of clutch CL/B at about 4 psi higher pressure than the stroke pressure of clutch CL/B. 
       FIG. 15  shows that clutch CL/B gains some torque capacity well before the torque transfer onto clutch CL/A, which closes at low torque capacity as speeds pass through synchronous speed. 
       FIG. 16  shows the speeds of sun gear  46 , sun gear  47  and carrier  50  near the end of the shift. Torque from clutch CL/B causes the Ravigneaux gearset  34  to move toward the third gear synchronous ratio. In fact, clutch CL/B has closed when the torque transfer onto clutch CL/A begins at approximately 14.75 sec. 
       FIGS. 16-20  show progressively near the end of a 6-4-3 downshift the variation of element speeds and element torques of the Ravigneaux gearset  34  as it shifts into third gear from fourth gear. As  FIGS. 16 and 17  show, at 14.7 sec. after recordation of data begins, the speed of clutch CL/A and sun gear  47  is 1325 rpm, and the speed of clutch CL/B, brake CL/C and sun gear  46  is 2582 rpm, which speeds continue to diverge as shown in  FIG. 16 . The net torque on gearset  34  is −39.2 ft-lbs. 
     Due to its mechanical or lever advantage, the torque on gearset  34  due to torque from clutch CL/B nearly equals the torque carried by clutch CL/E. At this point, torque from clutch CL/A aids clutch CL/B in moving the gearset toward a final ratio (1:1). 
     As  FIGS. 16 and 18  show, at 14.75 sec, clutch CL/B has sufficient torque capacity for the speeds of clutches CL/A and CL/B to converge. The net torque on gearset  34  is +430 ft-lbs. 
     As  FIGS. 16 and 19  show, at 14.8 sec, clutch CL/A closes at low torque capacity as gearset element speeds pass through synchronous speed. The speed of clutch CL/A has not changed from 1240 rpm at 14.75 sec. Torque from clutch CL/B has sufficient capacity to oppose the torque from clutch CL/A, which has now changed to a positive direction. The net torque on gearset  34  is +493.5 ft-lbs. 
     As  FIGS. 16 and 20  show, at 14.85 sec, the downshift is nearly complete. Torque from clutch CL/B has sufficient capacity to oppose the rising torque from clutch CL/A. The net torque on the gearset is +512.4 ft-lbs. 
     Referring to the logic flow diagram of the control steps of  FIG. 21 , at step  70 , a transmission controller issues a command for a sequenced downshift, such as a 6-4-3 downshift. 
     At step  72 , a check is made to determine whether the commanded downshift requires control of a disturbance of output torque. If the result of test  72  is logically false, control advances to step  74 , where a conventional downshift control is executed. 
     If the result of test  72  is logically true, at step  76  the commanded downshift begins by disengaging the second element (clutch CL/E) after disengaging the first element (brake CL/C). Actuating pressure in the latter oncoming element of the target third gear (clutch CL/B) is boosted after boosting the actuating pressure in the initial oncoming element of the target third gear (clutch CL/A). Boosting pressure, i.e., stroke pressure, causes the piston of the respective element to move in its servo cylinder toward the clutch discs substantially closing all clearances in the servo but without developing torque transmitting capacity in the element. 
     At step  78 , the fourth element (clutch CL/B) is brought to low torque capacity after disengaging the second offgoing element (clutch CL/E). 
     Engagement of the fourth element (clutch CL/B) at torque low capacity begins before engagement of the third element (clutch CL/A), thereby forcing the third element (clutch CL/A) toward the synchronous speed for the target gear prior to full engagement of the third element (clutch CL/A) and fourth element (clutch CL/B). 
     At step  80 , a check is made to determine whether the vehicle operator has caused a change of mind shift before a downshift to the intermediate gear, i.e., fourth gear has been completed. If the result of test  80  is logically true, control advances to step  82  where clutch CL/A is brought to holding torque capacity and subsequent shifts are cancelled while executing the sequenced downshift control strategy. 
     If the result of test  80  is logically false, at step  84  the commanded downshift is completed by fully engaging the third element (clutch CL/A) and the fourth element (clutch CL/B) at high capacity, preferably concurrently. Before fully engaging the third element (clutch CL/A) and the fourth element (clutch CL/B) at high capacity, a single torque modulation event is executed by reducing engine output torque to about 50-60 percent of current engine torque for about 100 Msec. 
     The control strategy for a sequential downshift, such as a 6-4-3 or 5-3-2 downshift, maintains output torque during the gear ratio change and allows for a change of mind shift to the intermediate gear. Should the driver tip-out of the accelerator pedal early enough during the downshift, the first oncoming element is applied and the second shift is cancelled. In addition, the final oncoming control element may be pre-staged to allow a continuous ratio change if the driver tips into a 6-4 or 5-3 downshift while the earlier downshift is in progress. 
     Early application of the second, oncoming control element (clutch CL/B) increases energy dissipation. The period during which that control element is applied, however, is significantly shorter than it would be during an equivalent downshift using a conventional control strategy. 
     The solution provides means to calibrate continuous downshifts and to reduce significantly the torque disturbance. Opposing torque from the final oncoming element, clutch CL/B, is used to negate the initial portion of the torque transfer onto the first oncoming element, clutch CL/A. The oncoming element of the second downshift, i.e., clutch CL/B, is boosted and brought to a low torque capacity just before the torque transfer at the end of the first shift. Since the oncoming element of the second shift, clutch CL/B, has low capacity, it only negates the initial portion of the torque transfer. The offgoing element of the second downshift, brake CL/E, must begin the second ratio change before the oncoming element of the first shift, clutch CL/B, gains significantly greater torque capacity than the oncoming element of the second shift. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.