Abstract:
When a vehicle driver selects reverse while a vehicle is moving forward, a transmission controller applies friction shift elements within the transmission to create a partial tie-up condition to decelerate the vehicle. Once the vehicle slows below a threshold speed, the controller engages a selectable one way brake and releases some of the friction shift elements to engage a reverse gear ratio.

Description:
TECHNICAL FIELD 
     This disclosure relates to the field of automatic transmission controls. More particularly, the disclosure pertains to a method of engaging a controllable one way clutch to establish an opposite direction power flow path. 
     BACKGROUND 
     Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. Transmission speed ratio is the ratio of input shaft speed to output shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Generally, transmissions include at least one negative speed ratio which is engaged when the driver selects reverse. 
     Many automatic transmissions implement a discrete number of different transmission ratios in which each ratio is establish by engaging a particular subset of friction shift elements and/or other shift elements. Shift elements may include devices that couple two rotating elements to one another, commonly called clutches, and devices which couple a rotating element to a stationary element, commonly called brakes. To shift from one speed ratio to another speed ratio, one shift element, called the off-going element, is released and another shift element, called the oncoming element, is engaged. Some types of shift elements, such as dog clutches, have no capability to absorb energy during engagement. When the oncoming element is a shift element of this type, the shafts to be coupled must be at very nearly the same speed before engagement. 
     When the vehicle is stationary, the gearbox input is also stationary even for very high speed ratios. Since an internal combustion engine cannot generate torque at zero crankshaft speed, a launch device is necessary to permit the engine to rotate and transmit torque to the gearbox input. Many automatic transmission utilize a torque converter having an impeller driven by the engine crankshaft and a turbine driving the gearbox input shaft. Torque is transferred from the impeller to the turbine whenever the impeller rotates faster than the turbine. Torque is transferred in the opposite direction when the turbine rotates faster than the impeller. 
     SUMMARY OF THE DISCLOSURE 
     A transmission includes input and output shafts, a switchable shift element, a plurality of friction shift elements, and a controller. The transmission may also include a torque converter. The switchable shift element is configured to switch between a lock-lock state in which two components are coupled and a lock-free state in which relative rotation between the components is allowed in only one direction. The plurality of friction shift elements are engageable in combinations of two to establish a plurality of power flow paths including at least one reverse power flow path. The reverse power flow path is established by engagement of a reverse friction shift element and placing the switchable device in the lock-lock state. The controller is programmed to respond to a shift into reverse with the switchable shift element overrunning by at least partially engaging two of the friction shift elements to exert negative torque on the output shaft to slow the vehicle. In some embodiments, the controller may also at least partially engage a third friction shift element, which may be the reverse friction shift element. The controller may be further programmed to switch the switchable shift element into the lock-lock state after the output shaft decelerates below a threshold speed. 
     A method of controlling a transmission includes responding to a shift into reverse by at least partially engaging at least three friction elements to exert negative torque on a transmission output shaft and a turbine shaft and then commanding a switchable one way brake to prevent rotation of an internal shaft. After commanding the switchable one way brake to change state, the method may also include releasing all but one friction shift elements to establish a reverse power flow path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a vehicle powertrain. 
         FIG. 2  is a schematic representation of a gearbox arrangement. 
         FIG. 3  is a flow chart for a first method of engaging reverse of the gearbox arrangement of  FIG. 2  while a vehicle is moving forward. 
         FIG. 4  is a graph illustrating vehicle acceleration and vehicle speed during execution of the method of  FIG. 3 . 
         FIG. 5  is a flow chart for a second method of engaging reverse of the gearbox arrangement of  FIG. 2  while a vehicle is moving forward. 
         FIG. 6  is a graph illustrating vehicle acceleration and vehicle speed during execution of the method of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     A front wheel drive (FWD) vehicle powertrain is illustrated schematically in  FIG. 1 . Mechanical connections are indicated by solid lines and signals are indicated by dashed lines. Power is provided by internal combustion engine  10 . Torque converter  12  includes an impeller driven by the engine crankshaft and a turbine. The turbine is fixed to the input shaft of gearbox  14 . The torque converter also includes a bypass clutch which selectively couples the impeller to the turbine. When the bypass clutch is engaged, torque is transferred by the bypass clutch. When the bypass clutch is disengaged, torque is transferred hydro-dynamically between the impeller and the turbine. Gearbox  14  includes a number of planetary gear sets and shift elements interconnected to establish a variety of power flow paths, each with a distinct speed ratio, by selective engagement of the shift elements. Power is transferred from an output element of gearbox  14  to differential  16 . The power may be transferred by means of meshing gears or by means of a chain. The transfer may also multiply the torque and reduce the speed by a fixed final drive ratio. Differential distributes the power to left and right front wheels  18  and  20  allowing slight speed differences as the vehicle turns a corner. The torque converter, gearbox, and differential may collectively be called a transaxle  22  or a transverse transmission. A rear-wheel drive vehicle powertrain has similar components although the engine, torque converter, gearbox, and differential are located along the vehicle centerline and drive rear wheels. The present invention is applicable to both front wheel drive and rear wheel drive powertrain configurations. 
     The engine and gearbox respond to commands from controller  24 . The controller sends signals to gearbox  14  to apply particular shift elements. The controller sends signals to engine  10  indicating what amount of torque to produce. Controller  24  receives signals from a variety of sensors, including a shift lever  26  and an accelerator pedal  28 . The driver moves shift lever  26  among several positions to indicate the desired direction of travel. A D position indicates a desire to move forward. An R position indicates a desire to move backwards. An N position indicates a desire for neutral. A P position indicates a desire to engage park. The term shift lever is used here to represent any user interface element intended to indicate these choices including, for example, a console mounted lever, a steering wheel mounted lever, or a touch screen. Controller  24  may be implemented, for example, as a single micro-processor or as multiple communicating micro-processors. 
     An exemplary arrangement of gearbox  14  is illustrated in  FIG. 2 . Gearbox  14  includes an input shaft  30  driven by the torque converter turbine, an output element  32 , and a transmission case  34  fixed to vehicle structure. Shafts other than the input shaft and output shaft may be called internal shafts. A first simple planetary gear set includes sun gear  42 , ring gear  44 , carrier  46  fixedly coupled to output element  32  and supporting a number of planet gears  48  that each mesh with both sun gear  42  and ring gear  44 . A second simple planetary gear set includes sun gear  52  fixedly coupled to the input shaft  30 , ring gear  54 , carrier  56  fixedly coupled to ring gear  44  and supporting a number of planet gears  58  that each mesh with both sun gear  52  and ring gear  54 . The third planetary gear set includes a carrier  62  fixedly coupled to ring gear  54  and supporting a number of stepped planet gears  64 . Each stepped planet gears include a set of gear teeth meshing with both sun gear  64  and ring gear  66  and a second smaller diameter set of gear teeth meshing with sun gear  68 . 
     Gearbox  14  further includes a set of hydraulically actuated friction shift elements  70 ,  72 ,  74 ,  76 ,  78 , and  80 . A friction shift element may be in a disengaged state, a partially engaged state, or a fully engaged state. In a disengaged state, the selectively coupled elements are free to rotate at different speeds with negligible torque transfer. In the partially engaged state, torque is transferred between elements rotating at different speeds. The amount of torque transferred is equal to the shift element torque capacity. In the fully engaged state, the elements are constrained to rotate (or not rotate) as a unit. A shift element remains in the fully engaged state as long as the torque capacity exceeds the torque required to keep the elements rotating as a unit. Carrier  62  and ring gear  54  are selectively coupled to input shaft  30  by clutch  70 . Sun gear  68  is selectively coupled to input shaft  30  by clutch  72  and selectively held against rotation by brake  74 . Sun gear  68  is selectively held against rotation by brake  76  and selectively coupled to carrier  62  and ring gear  54  by clutch  78 . Sun gear  42  is selectively held against rotation by brake  80 . 
     Selectable One Way Brake (SOWB)  82  is actively controlled to be in one of two states. In a lock-lock state, SOWB  82  holds carrier  62  and ring  54  against rotation in both directions. In a lock-free state, SOWB  82  passively restrains carrier  62  and ring gear  54  from rotating in a reverse direction, but permits rotation in a forward direction. Forward direction is defined as the direction of rotation of input shaft  30  when the transmission is transmitting power from the engine to the wheels. Unlike a friction shift element, SOWB does not have a partially engaged state in which it can transmit torque between elements that have relative speed. Consequently, transitioning from the lock-free state to the lock-lock state in the presence of relative rotation causes an abrupt change in element speed, resulting in a high short duration torque at the transmission output. The magnitude of this torque pulse depends upon the inertia of the elements that change in speed and the torque transfer path to the output element. In extreme cases, components may break. 
     The shift elements of gearbox  16  are engaged in combinations of two to establish nine forward speed ratio power flow paths and one reverse speed ratio power flow path as shown in Table 1. (SOWB is considered engaged when it is transmitting torque as opposed to overrunning.) Note that SOWB  82  is in the lock-lock state only in the reverse power flow path. SOWB  82  is in lock-free state in all of the forward power flow paths. In the 1st gear power flow path, SOWB transmits torque whenever the power is transferred from input shaft  30  output shaft  32 . This power flow path is incapable of transmitting power from output  32  to input  30  with SOWB  82  in the lock-free state because SOWB  82  overruns. (Some transmission may implement an operating mode in which SOWB  82  is in the lock-lock state when in 1st gear to provide engine braking.) Use of a one way brake simplifies the control of an upshift where the selectable device is the off-going element. This is most valuable for a shift from 1st gear to 2nd gear because any torque disturbances created from inaccurate control are magnified by the torque ratio. In some gearing arrangements, the device may be a selectable one way clutch that, in one state, allows relative rotation in only direction between two rotating shafts and, in another state, couples the two rotating shafts. 
     
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 70 
                 72 
                 74 
                 76 
                 78 
                 80 
                 82 
                 Ratio 
                 Step 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Rev 
                   
                 X 
                   
                   
                   
                   
                 lock-lock 
                 −2.96 
                 63% 
               
               
                 1st 
                   
                   
                   
                   
                   
                 X 
                 lock-free 
                 4.69 
               
               
                   
                   
                   
                   
                   
                   
                   
                 (engaged) 
               
               
                 2nd 
                   
                   
                   
                 X 
                   
                 X 
                 lock-free 
                 3.31 
                 1.42 
               
               
                   
                   
                   
                   
                   
                   
                   
                 (overrunning) 
               
               
                 3rd 
                   
                   
                 X 
                   
                   
                 X 
                 lock-free 
                 3.01 
                 1.10 
               
               
                   
                   
                   
                   
                   
                   
                   
                 (overrunning) 
               
               
                 4th 
                   
                   
                   
                   
                 X 
                 X 
                 lock-free 
                 2.45 
                 1.23 
               
               
                   
                   
                   
                   
                   
                   
                   
                 (overrunning) 
               
               
                 5th 
                   
                 X 
                   
                   
                   
                 X 
                 lock-free 
                 1.92 
                 1.27 
               
               
                   
                   
                   
                   
                   
                   
                   
                 (overrunning) 
               
               
                 6th 
                 X 
                   
                   
                   
                   
                 X 
                 lock-free 
                 1.45 
                 1.33 
               
               
                   
                   
                   
                   
                   
                   
                   
                 (overrunning) 
               
               
                 7th 
                 X 
                 X 
                   
                   
                   
                   
                 lock-free 
                 1.00 
                 1.45 
               
               
                   
                   
                   
                   
                   
                   
                   
                 (overrunning) 
               
               
                 8th 
                 X 
                   
                 X 
                   
                   
                   
                 lock-free 
                 0.75 
                 1.34 
               
               
                   
                   
                   
                   
                   
                   
                   
                 (overrunning) 
               
               
                 9th 
                 X 
                   
                   
                 X 
                   
                   
                 lock-free 
                 0.62 
                 1.21 
               
               
                   
                   
                   
                   
                   
                   
                   
                 (overrunning) 
               
               
                   
               
             
          
         
       
     
     When shift lever  26  is moved from the drive position to the reverse position, controller  24  needs to command engagement of friction clutch  72 , command SOWB  82  to enter the lock-lock state, and release all other friction shift elements in order to establish the power flow path for reverse. When the vehicle is stationary with the 1st gear power flow path established (brake  80  engaged), there is no relative speed across SOWB  82 , so the transition is straight forward. Specifically, since output  32  is stationary, carrier  46  is also stationary. Since brake  80  is holding sun gear  42  stationary, ring gear  44  and carrier  56  will also be stationary. Since the torque converter impeller is rotating at least at engine idle speed, it will apply positive torque to sun gear  52  as long as the turbine is rotating slower than the impeller and clutches  70  and  72  are both released. This positive torque on sun gear  52  with carrier  56  stationary will tend to force ring gear  54  and carrier  62  to rotate in the opposite direction. However, SOWB  82  restrains them from rotating backwards. Even when the vehicle is moving forward at a slow speed, SOWB  82  will be engaged. However, if the speed of output  32  exceeds the engine speed divided by the 1st gear ratio, then SOWB will overrun. 
     If the driver moves shift lever  26  into the reverse position while SOWB  82  is overrunning, then the controller must ensure that carrier  62  is stopped or almost stopped before switching SOWB to the lock-lock state. Otherwise, the sudden deceleration of carrier  62  will cause a torque impulse that is disturbing to vehicle occupants and may even cause transmission components to fail. SOWB  82  may be overrunning either because the transmission is in a forward gear higher than 1st gear, or because the vehicle speed is high enough that the SOWB overruns in 1st gear. The controller could simply disengage all friction shift elements except  80  and wait for the vehicle speed to drop sufficiently to permit switching SOWB  82  to the lock-lock state. However, a driver expects negative torque soon after moving the shift lever to reverse and will be dissatisfied if the vehicle does not respond for too long. 
       FIG. 3  is a flow chart for a process of managing reverse engagements that avoids long delays even when the vehicle is initially moving forward.  FIG. 4  illustrates vehicle speed and acceleration as a function of time when the method of  FIG. 3  is executed. The method begins at  90  with shift lever  26  in a drive position and the transmission shift elements engaged to establish one of the forward driving gear ratios. The method progresses to  92  in response to the driver moving shift lever  26  to the reverse position. If SOWB  82  is not overrunning at  92 , then control moves to  94  where the state of SOWB  82  is switched to the lock-lock state. The controller may determine whether it is overrunning using a speed sensor on carrier  62  or by computing the speed of carrier  62  based on other speed sensors. At  96 , brake  80  is released. Simultaneously, at  98 , clutch  72  is engaged. (This assumes the transmission was previously in 1st gear, which would ordinarily be the case any time SOWB  82  is not overrunning.) Once these engagements and disengagements have completed, the transmission is in final state  98  with reverse gear engaged. 
     If SOWB  82  is determined to be overrunning at  92 , the controller takes affirmative action to slow the vehicle. At  100 , the controller engages clutches  72  and  78  and releases all other friction elements. Engaging clutch  72  prepares the transmission for eventual engagement of reverse gear. Engaging clutch  78  couples carrier  62 , sun gear  64 , and sun gear  68  to output element  32 . With clutch  78  engaged, brakes  74  and  76  both act to slow the vehicle. At  102 , one or both of brake  74  and  76  are partially engaged to slow the vehicle. As shown before time A in  FIG. 5 , the objective is to control the speed profile such that the vehicle reaches zero speed smoothly. This is accomplished by using a speed measurement as a closed loop feedback signal. This continues until the output speed falls below a threshold as determined at  104 . The threshold is selected such that any torque impulse resulting from switching SOWB  82  to the lock-lock state with multiple friction elements engaged is acceptable. At  106 , the state of SOWB  82  is switched to the lock-lock state. This occurs with the vehicle at a steady speed very close to zero between A and B in  FIG. 5 . At  108 , shift elements  74 ,  76 , and  78  are all released placing the transmission in reverse at  98 . (Brakes  74  and  76  would already have been at very close to zero torque capacity.) Once in reverse, the vehicle begins to accelerate backwards as shown after time B in  FIG. 5 . Ideally, the time between A and B is as short as possible such that the maneuver feels to vehicle occupants like a continuous deceleration. 
       FIG. 5  is a flow chart for an alternative process of managing reverse engagements that also avoids long delays even when the vehicle is initially moving forward.  FIG. 5  illustrates vehicle speed and acceleration as a function of time when the method of  FIG. 4  is executed. Like the method of  FIG. 3 , the method begins at  90  with shift lever  26  in a drive position and the transmission shift elements engaged to establish one of the forward driving gear ratios and progresses to  92  in response to the driver moving shift lever  26  to the reverse position. If SOWB  82  is not overrunning at  92 , then the transition to reverse is accomplished in the same manner as the method of  FIG. 3 . The method differs if SOWB  82  is determined to be overrunning at  92 . Like the method of  FIG. 3 , the controller takes affirmative action to slow the vehicle. At  110 , the controller engages clutch  78  and releases all other friction elements, including clutch  72 . With clutch  78  engaged, brakes  74  and  76  both act to slow the vehicle. At  102 ′, one or both of brake  74  and  76  are partially engaged to slow the vehicle. This step differs from  102  in the method of  FIG. 3  only in that the target speed profile is slightly different. As shown before time A in  FIG. 6 , the objective is to control the speed profile such that the vehicle smoothly reaches a low, positive speed. This continues until the output speed falls below a threshold as determined at  104 ′. This step differs from  104  in the method of  FIG. 3  in that the threshold is not as close to zero. Since clutch  72  is not engaged, the turbine speed tends toward the engine speed. At  112 , Shift elements  74 ,  76 , and  78  are released and brake  80  is partially engaged. Partial engagement of brake  80  with the vehicle slow (and therefore carrier  46  slow) tends to slow ring  44  and carrier  56 . With the turbine and sun gear  52  rotating near engine speed, slowing carrier  56  causes ring gear  54  and carrier  62  to slow down until SOWB  82  engages to prevent it from rotating backwards. When engagement of SOWB  82  is detected at  114 , its state is changed at  94  as shown between A and B in  FIG. 6 . Then, reverse is engaged and the vehicle begins to accelerate backwards as shown after time B in  FIG. 6 . Ideally, the time between A and B is as short as possible such that the maneuver feels to vehicle occupants like a continuous deceleration. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.