Abstract:
A method for shifting a transmission from a current gear to a target gear following release of an accelerator pedal, the transmission having first and second clutches for alternately connecting and disconnecting a power source and first and second input shafts, respectively, first couplers for alternately connecting and disconnecting a transmission output and a first set of gears driveably connected to the first clutch, and second couplers for alternately connecting and disconnecting the output and a second set of gears driveably connected to the second clutch. The steps include disengaging the first and second clutches, using a first coupler to maintain a drive connection between the output and the current gear, determining a target gear, using the target gear to determine a preselected gear, actuating a coupler to produce a drive connection between the output and the preselected gear, and engaging one of the first and second clutches such that said clutch driveably connects the power source to the output through said coupler and preselected gear.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to the control of gear changes in an automatic transmission for a motor vehicle. More particularly, it pertains to a strategy for controlling gear changes following a change-of-mind event in a powershift transmission. 
   A change-of-mind shift occurs when the operator of a motor vehicle equipped with an automatic transmission depresses the accelerator pedal, called a “tip-in,” and immediately releases the pedal, called a “tip-out” or “back-out.” A change-of-mind shift can occur also when the vehicle operator releases the accelerator pedal, then immediately depresses the pedal. If the transmission were operating in third gear when the change-of-mind event occurs, an upshift through several gears to sixth gear might be immediately followed by a downshift to third gear. Change-of-mind gear shifts are very difficult to handle in a powershift transmission, and they require a relatively long period to complete due to the need for coordinated engagement and disengagement of several synchronizers or couplers during the shift. In some cases, the transmission performs a single upshift, such as a 6-3 shift, when the driver tips-in. Such shifts cause the operator to sense a lengthy, unacceptable delay in completing the shift, and the shift may be accompanied by a bump or harsh shift feel. Vehicle occupants regard such sensations as unacceptable shift quality. 
   There is a need promptly to stabilize gear changes in a powershift transmission following a change-of-mind event. There is a need to eliminate harshness and delay in completing gear changes following the event that initiates automatic gear changes in the transmission. 
   SUMMARY OF THE INVENTION 
   The control strategy of this invention employs a neutral idle concept during back-out and coasting to handle change-of-mind shifts. Neutral idle enables the transmission to stay in a neutral state while the accelerator pedal is closed, to preselect preferred next gear, and to disposition the transmission for a rapid gear change to the preselected gear for the next tip-in. While neutral idle is enabled, the engine speed drops to idle speed without upshifts, the clutch torque capacity is zero, and the clutch is in the stroke position. Therefore, most of the change-of-mind shifts are eliminated and the transmission shifts from the current gear to a higher gear following a tip-in when higher vehicle speeds are expected by the operator. Engine torque control is used during these tip-in events to provide a smooth transition without creating excessive slip. During a tip-in event, clutch slip is controlled by engine torque, instead of by clutch torque, which is the conventional technique for controlling clutch slip. After the operator driver stays in coast mode long enough, a higher gear will be selected by engaging a synchronizer or coupler based on the vehicle speed. If the driver tip-ins from a coast condition, the appropriate clutch will engage, thereby producing the current gear without a shift. 
   The control of this invention eliminates change-of-mind shifts instead of improving them. It enables the powershift transmission to provide the best change-of-mind shift quality and the desired wheel torque the driver wants and expects. It also enables the transmission to provide the best response to the driver demanded tip-in and tip-out. 
   A method for shifting a transmission from a current gear to a target gear following release of an accelerator pedal, the transmission having first and second clutches for alternately connecting and disconnecting a power source and first and second input shafts, respectively, first couplers for alternately connecting and disconnecting a transmission output and a first set of gears driveably connected to the first clutch, and second couplers for alternately connecting and disconnecting the output and a second set of gears driveably connected to the second clutch. The steps include disengaging the first and second clutches, using a first coupler to maintain a drive connection between the output and the current gear, determining a target gear, using the target gear to determine a preselected gear, actuating a coupler to produce a drive connection between the output and the preselected gear, and engaging one of the first and second clutches such that said clutch driveably connects the power source to the output through said coupler and preselected gear. 
   The scope of applicability of the present invention 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 within the spirit and scope of the invention will become apparent to those skilled in the art. 

   
     DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
       FIG. 1  is a schematic diagram of a twin clutch powershift automatic transmission to which the shift control strategy of this invention may be applied; 
       FIG. 2  is chart showing the coupler and the corresponding gears of  FIG. 1  with which they are associated; 
       FIG. 3  is a schematic diagram of an electronic system for controlling the transmission following a change-of-mind event; 
       FIG. 4  is a logic flow diagram of the strategy for controlling a change-of-mind event; and 
       FIG. 5  is a graph showing the relation between displacement of the accelerator pedal and vehicle speed for each of the upshifts and downshifts of the transmission; 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , the powershift transmission  10  includes a first input shaft  12  associated with the odd-numbered forward speed ratios, and a second input shaft  14  associated with the even-numbered forward speed ratios and reverse drive. Input shaft  14  is a sleeve shaft surrounding input shaft  12 . A dual clutch mechanism  16  produces a drive connection between the crank shaft  18  of an engine or another power source to the first and second input shafts  12 ,  14 . The clutch mechanism  16  includes a flywheel  20 , which is driveably connected to crankshaft  18  and is alternately driveably connected to and disconnected from input shaft  12  when clutch  22  is engaged and disengaged, respectively. Flywheel  20  is alternately driveably connected to and disconnected from input shaft  14  when clutch  24  is engaged and disengaged, respectively. 
   Preferably input shaft  12  is formed with pinions  26 ,  27 ,  28  for the first, second, and third speed ratios. Each pinion  26 - 28  is in meshing engagement with a corresponding gear  29 ,  30 ,  31 , each gear being journalled on an output shaft  32 . Similarly, the second input shaft  14  is preferably formed with pinions  32 ,  33 ,  34  for the second, fourth, and sixth forward speed ratios, and a reverse pinion  38 . Each pinion  32 - 34  is in meshing engagement with a corresponding gear  35 ,  36 ,  37 , each gear being journalled on output shaft  32 . The reverse drive pinion  38  is in meshing engagement with a reverse idler (not shown), which meshes with a reverse output gear  39 , journalled on the output shaft  32 . A final drive pinion  40 , secured to output shaft  32 , meshes with a final drive gear  41 , which transmits power to the axles of the driven wheels of the vehicle. 
   A synchronizer  42  alternately driveably connects the first speed gear  29  with the output shaft  32  when the sleeve of the synchronizer is displaced rightward from the neutral position shown in  FIG. 1  and disconnects gear  29  from shaft  32  when the sleeve is in the neutral position. A second synchronizer  44  alternately connects and disconnects the third speed gear  30  and the fifth speed gear  31  with output shaft  32  depending on the axial position of the selector sleeve of synchronizer  44 . Similarly, synchronizer  46  alternately connects and disconnects the second speed gear  35  and the fourth speed gear  36  with output shaft  32  depending on the axial position of the selector sleeve of synchronizer  46 . Synchronizer  48  alternately connects and disconnects the sixth speed gear  37  and the reverse gear  39  to the output shaft depending on the axially position of its selector sleeve. In  FIG. 1 , all of the synchronizers  42 ,  44 ,  46 ,  48  are shown with their selector sleeves in the neutral position. 
   Clutches  22  and  24  are preferably normally open clutches, which may be actuated electro-mechanically, but those clutches may be normally-closed. A first, electromechanical actuator  50  engages and disengages clutch  22 ; a second actuator  52  engages and disengages clutch  24 . The selector sleeves of synchronizers  42  and  44  are actuated by an electromechanical actuator  54 ; the selector sleeves of synchronizers  46  and  48  are actuated by an electromagnetic actuator  56 . 
   The transmission is controlled such that it produces a current or active gear upon engaging one of the input clutches  22 ,  24  after the selector sleeve of the corresponding synchronizer has been moved to a state that produces a drive connection between current gear and the output shaft  32 . In addition, the transmission is controlled to produce a preselected gear, which is produced by changing the position of the selector sleeve of the corresponding synchronizer to produce a drive connection between the output shaft and the preselected gear and by disengaging the clutch  22 ,  24  associated with the preselected gear. 
   The chart of  FIG. 2  shows that the sleeve of synchronizer  42  moves rightward from the neutral position to connect the first gear  29  to the output shaft  32 . The sleeve of synchronizer  44  moves rightward from the neutral position to connect third gear  30  to the output shaft  32  and moves leftward from the neutral position to connect the fifth gear  31  to the output shaft. The sleeve of synchronizer  46  moves rightward from the neutral position to connect gear  35  to the output shaft  32  and moves leftward from the neutral position to connect the fourth gear  36  to the output shaft. The sleeve of synchronizer  48  moves rightward from the neutral position to connect the sixth gear  37  to the output shaft and moves leftward from the neutral position to driveably connect the reverse gear  39  to the output shaft. 
     FIG. 3  shows the arrangement of an electronic system for controlling the actuation of clutches  22  and  24  through operation the clutch actuators  50 ,  52 , and for actuating the synchronizers  42 ,  44 ,  46 ,  48  through operation on the synchronizer actuators  54 ,  56 . A transmission control unit (TCU)  60 , which is accessible to electronically stored, coded algorithms, responds to the various inputs, executes the control algorithms, and produces electronic command signal to the actuators  50 ,  52 ,  54 ,  56 . The position or displacement of the actuators is carried back as input to the TCU  60 . The position of a transmission shift lever  62  is communicated on CAN  64  to the TCU input, and communications among various sensors, the TCU and actuators is carried on the CAN. Speed sensors  66 ,  67 ,  68  produce electronic signals representing the speed of output  32 , the speed of input  12 , and speed of input  14 . Other TCU inputs include state of a transmission park switch  70 , the position or state  71  of clutch  22 , and the engaged or disengaged state  72  of clutch  24 . The magnitude of displacement of an accelerator pedal  102  is another TCU input. 
     FIG. 4  is a logic flow diagram illustrating the steps for controlling engagement and disengagement of the clutches  22 ,  24  and for controlling the connections made by the synchronizers or couplers  42 ,  44 ,  46 ,  48  during a sequence of displacements of the accelerator pedal  102  by the vehicle operator. That sequence, called a “change-of-mind” event, begins with an abrupt release or tip-out of the pedal followed shortly thereafter by a depression or tip-in of the pedal. In an automatic transmission, whose kinematic arrangement is a dual clutch-layshaft or a conventional torque converter-planetary gearset, that sequence normally induces a gear ratio change or shift. 
   The control strategy for a change-of-mind event begins at step  100  upon the release of the accelerator pedal  102 . Displacement of the pedal from a released position is represented by counts produced electronically by a sensor and supplied to the input of the TCU  60 . The control system continually monitors the accelerator pedal displacement sensor  102  to determine whether the pedal is released. 
   When the test at step  104  is true, clutches  12  and  14  are placed in a stroke position at step  106 , i.e., the torque capacity of both clutches is reduced to zero by actuators  50 ,  52 , but each clutches is in position to be immediately engaged upon minimal displacement of the respective actuator. 
   At step  108 , the control determines a target gear.  FIG. 5  is a graph showing, for the operating state of the motor vehicle represented by vehicle speed and displacement of the accelerator pedal, where each upshift and downshift is commanded by the TCU  60 . When the operating state of the motor vehicle crosses one of the gear change lines, the TCU  60  commands a change from the current gear to the target gear. In this way at step  108 , the target gear is determined. 
   However, when the gear changes are controlled by the strategy of the present invention, instead of immediately commanding a gear ratio change to the target gear when the accelerator pedal is released, at step  110  the control strategy determines a preselected gear, which may be different from the target gear. For example, if the current gear is the third gear and the accelerator pedal is released, the target gear determined from schedule of  FIG. 5  may be the sixth gear and the preselected gear will also be the sixth gear. However, if the transmission is operating in the third gear and the target gear is fifth gear, the preselected gear will be fourth gear because the fifth gear is associated with clutch  22  and input shaft  12 , the same clutch and same input shaft with which third gear is associated. If, the target gear is the fourth gear, then the preselected gear will become the fourth gear because that fourth gear is associated with clutch  24  and input shaft  14 , a clutch and input shaft that are different from those with which third gear is associated. 
   After the preselected gear is determined at step  110 , at step  112  one of the actuators  54 ,  56  will actuate the coupler that will driveably connect the preselected gear to the output  32 . For example, if fourth gear is the preselected gear, the selector sleeve of coupler  46  is moved leftward by actuator  56  to connect gear  36  to output shaft  32 , but clutch  24  remains disengaged. The current gear, third gear, remains driveably connected to the output shaft  32  through its coupler  44 . But input shafts  12 ,  14  are disconnected from crankshaft  18  because both clutches  22 ,  22  are in the stroke position. 
   The control continues to monitor displacement of the accelerator pedal  102 . At step  114 , if accelerator pedal position is depressed, as represented by a count greater than zero, control passes to step  116  where the control determines whether the coupler actuated in step  112  has completed making the drive connection of the preselected gear to the output  32 . If the drive connection has not been completed, control passes to step  118  where the TCU  60  issues a command to re-engage the clutch associated with the current gear. For the example referred to above, if the third gear is the current gear, then clutch  22  is re-engaged. Thereafter, upshifts from the current gear to the target are performed sequentially by disengaging the engaged clutch  12  and engaging the disengaged clutch  14  while manipulating the couplers so that stepwise, sequential gear changes occur between the current gear and the target gear. 
   If the coupler actuated at step  112  has completed the drive connection of the preselected gear to the output  32 , control passes to step  120  where the control causes clutch  14 , which is not associated with the current gear, third gear, becomes engaged and driveably connects the preselected gear, fourth gear, to the output  32 . If the current gear is the preselected gear, then the clutch that is associated with the current gear is engaged instead. 
   In the example referred to throughout the description, the current gear is the third gear and the preferred target gears are fourth gear and sixth gear. If this is the case, clutch  24  is engaged at step  120 . However, if the preselected gear is the third gear, then at step  120 , clutch  22  is re-engaged. 
   Throughout the description reference is made to the couplers being synchronizers, but they could be dog clutches, which produce no speed synchronization before the couplers driveably connect the output gears to the output shaft  32 . 
   In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.