Abstract:
A method for shifting a transmission capable of operating in a hydrostatic power transmission mode or a blended hydrostatic and mechanical power transmission mode is provided. The transmission includes a mechanical portion and a hydraulic portion. The method comprises the steps of placing the transmission in the hydrostatic power transmission mode, reducing an amount of engagement of a primary clutch, adjusting a rotational speed of the hydraulic portion, increasing an amount of engagement of a secondary clutch, and engaging the secondary clutch and disengaging the primary clutch. The method for shifting the transmission minimizes torque interruption, increases a fuel efficiency of a vehicle, and increases a range of operating speeds of the vehicle the transmission is incorporated in.

Description:
CLAIM OF PRIORITY 
     The present application claims priority to and incorporates by reference U.S. Provisional Application No. 61/568,687 filed Dec. 9, 2011, entitled “Shifting Procedure for Powersplit Systems.” 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to power transmission systems. More particularly, this invention relates to a powersplit transmission for a vehicle, in which the powersplit transmission may be operated in a hydrostatic power transmission mode or a blended hydrostatic and mechanical power transmission mode. 
     Vehicles that incorporate the powersplit transmission, such as earth moving machinery, industrial equipment, and others may be operated in the hydrostatic power transmission mode or the blended hydrostatic and mechanical power transmission mode. In the hydrostatic power transmission mode, a speed of the vehicle or a motion of a hydraulically operated attachment coupled to the vehicle may be precisely controlled. Such a mode of operation is particularly useful when loading and unloading the vehicle, performing work with the hydraulically operated attachment, or maneuvering the vehicle in constrained areas, for example. In the blended hydrostatic and mechanical power transmission mode, a portion of an output of a prime mover of the vehicle bypasses a hydrostatic portion of the powersplit transmission. Such a mode of operation is useful to increase an efficiency of the vehicle and increase a range of operating speeds of the vehicle, for example. 
     An operator of the vehicle typically desires to change from the hydrostatic power transmission mode to the blended hydrostatic and mechanical power transmission mode. Such a process is usually performed while the vehicle is performing work, such as accelerating the vehicle. When the powersplit transmission changes from the hydrostatic power transmission mode to the blended hydrostatic and mechanical power transmission mode, the vehicle typically experience a torque interruption. 
     It would be advantageous to develop a method for shifting a powersplit transmission between modes of operation that minimizes torque interruption, increases a fuel efficiency of a vehicle, and increases a range of operating speeds of the vehicle the powersplit transmission is incorporated in. 
     SUMMARY OF THE INVENTION 
     Presently provided by the invention, a method for shifting a powersplit transmission between modes of operation that minimizes torque interruption, increases a fuel efficiency of a vehicle, and increases a range of operating speeds of the vehicle the powersplit transmission is incorporated in, has surprisingly been discovered. 
     In one embodiment, the present invention is directed to a method for shifting a transmission. The transmission includes a mechanical portion drivingly engaged with an output of the transmission, a hydraulic portion, a primary clutch, and a secondary clutch. The primary clutch is disposed between the hydraulic portion and the output of the transmission and the secondary clutch is disposed between the hydraulic portion and the mechanical portion. The method comprises the steps of providing the transmission capable of operating in a hydrostatic power transmission mode or a blended hydrostatic and mechanical power transmission mode; placing the transmission in the hydrostatic power transmission mode; reducing an amount of engagement of the primary clutch, thereby adjusting a torque applied to the output; adjusting a rotational speed of the hydraulic portion to facilitate driving engagement between the hydraulic circuit and mechanical portion, thereby adjusting a torque applied to the output; increasing an amount of engagement of the secondary clutch, thereby adjusting a torque applied to the output of the transmission; and engaging the secondary clutch and disengaging the primary clutch. 
     In another embodiment, the present invention is directed to a method for shifting a transmission. The transmission includes a mechanical portion having an epicyclic gearset drivingly engaged with an output of the transmission, a hydraulic portion including a variable displacement pump and a variable displacement motor, a primary clutch, and a secondary clutch; the primary clutch disposed between the hydraulic portion and the output of the transmission and the secondary clutch disposed between the hydraulic portion and the epicyclic gearset. The method comprises the steps of providing the transmission capable of operating in a hydrostatic power transmission mode or a blended hydrostatic and mechanical power transmission mode; placing the transmission in the hydrostatic power transmission mode; reducing an amount of engagement of the primary clutch by placing the primary clutch in a slip condition, thereby adjusting a torque applied to the output; adjusting a rotational speed of the hydraulic portion by adjusting a displacement of at least one of the variable displacement pump and the variable displacement motor to facilitate driving engagement between the hydraulic circuit and mechanical portion, thereby adjusting a torque applied to the output; increasing an amount of engagement of the secondary clutch by placing the secondary clutch in a slip condition, thereby adjusting a torque applied to the output of the transmission; and engaging the secondary clutch and disengaging the primary clutch. 
     In a third embodiment, the present invention is directed to a method for shifting a transmission. The transmission includes a mechanical portion having an epicyclic gearset drivingly engaged with an output of the transmission, a hydraulic portion including a variable displacement pump and a variable displacement motor, a primary clutch, and a secondary clutch; the primary clutch disposed between the hydraulic portion and the output of the transmission and the secondary clutch disposed between the hydraulic portion and the epicyclic gearset. The method comprises the steps of providing the transmission capable of operating in a hydrostatic power transmission mode or a blended hydrostatic and mechanical power transmission mode; placing the transmission in the hydrostatic power transmission mode; reducing an amount of engagement of the primary clutch by placing the primary clutch in a slip condition, thereby adjusting a torque applied to the output; actively adjusting a rotational speed of the variable displacement motor to stay between a speed of a portion of the epicyclic gearset and a speed of the output to facilitate driving engagement between the hydraulic circuit and mechanical portion, thereby adjusting a torque applied to the output; increasing an amount of engagement of the secondary clutch by placing the secondary clutch in a slip condition, thereby adjusting a torque applied to the output of the transmission; monitoring a sum of torques applied to the output for an equilibrium condition; and engaging the secondary clutch and disengaging the primary clutch. 
     Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a schematic illustration of a driveline equipped with a powersplit transmission, which may be shifted according to an embodiment of the invention; 
         FIG. 2  is a table showing a state of the components of the driveline illustrated in  FIG. 1  in various operational modes of the driveline; 
         FIG. 3A  graphically illustrates torque vectors associated with components of the driveline illustrated in  FIG. 1  in a forward hydrostatic mode;  FIG. 3B  graphically illustrates rotational speed associated with components of the driveline illustrated in  FIG. 1  in a forward hydrostatic mode; 
         FIG. 4A  graphically illustrates torque vectors associated with components of the driveline illustrated in  FIG. 1  in a forward powersplit mode; 
         FIG. 4B  is a speed diagram which graphically illustrates torque vectors associated with components of the driveline illustrated in  FIG. 1  in a forward powersplit mode; 
         FIG. 4C  graphically illustrates rotational speed associated with components of the driveline illustrated in  FIG. 1  in a forward powersplit mode; 
         FIG. 5A  graphically illustrates torque vectors associated with components of the driveline illustrated in  FIG. 1  during a shift procedure from the forward hydrostatic mode to the forward powersplit mode; 
         FIG. 5B  is a speed diagram which graphically illustrates torque vectors associated with components of the driveline illustrated in  FIG. 1  during a shift procedure from the forward hydrostatic mode to the forward powersplit mode; and 
         FIG. 6  graphically illustrates the shift procedure of the driveline illustrated in  FIG. 1  according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined herein. Hence, specific dimensions, directions or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless expressly stated otherwise. 
       FIG. 1  schematically illustrates a driveline  10  for a vehicle. The driveline  10  comprises a power source  12 , a powersplit transmission  14 , and a vehicle output  16 . 
     The power source  12  is drivingly engaged with an input  18  of the powersplit transmission  14 . An output  20  of the powersplit transmission  14  is drivingly engaged with the vehicle output  16 . 
     The power source  12  applies power to the input  18  of the powersplit transmission  14 . The power source  12  is, for example, an internal combustion engine; however, it is understood that the power source  12  may include an electric motor or another source of rotational output. It is understood that the power source  12  may be a hybrid power source including both an internal combustion engine and an electric motor. Further, it is understood that the power source  12  may include an output ratio adjusting device as known in the art. Further, it is understood that the power source  12  may include a clutch (not shown) as known in the art, for one of reducing and interrupting a rotational force transferred to the powersplit transmission  14 . 
     The powersplit transmission  14  is a power transmitting device comprising a mechanical portion  22  and a hydraulic portion  24 . The input  18  of the powersplit transmission  14  is drivingly engaged with the mechanical portion  22  and the hydraulic portion  24 . The input  18  may comprise a plurality of intermeshed gears; however, it is understood that the input  18  may comprise any device capable of splitting power between the mechanical portion  22  and the hydraulic portion  24 . The mechanical portion  22  and the hydraulic portion  24  are drivingly engaged with the output  20  of the powersplit transmission  14 . The mechanical portion  22  and the hydraulic portion  24  may share a housing (not shown) or the mechanical portion  22  and the hydraulic portion  24  may be respectively housed and disposed adjacent one another. Further, it is understood that at least a portion of the hydraulic portion  24  may be disposed elsewhere on the vehicle. 
     The mechanical portion  22  comprises a forward clutch  26 , a reverse clutch  28 , an intermediate portion  30 , and an epicyclic gearset  32 . It is understood that the forward clutch  26  and the reverse clutch  28  may be combined in a single unit. 
     The forward clutch  26  is a shift collar concentrically disposed about a portion of the input  18  and a portion of the intermediate portion  30 . The forward clutch  26  includes a set of teeth which may be engaged with corresponding teeth formed on a portion of the input  18  and a portion of the intermediate portion  30 ; however, it is understood the forward clutch  26  may be substituted with any clutching device that permits selective engagement between the input  18  and the intermediate portion  30 . 
     The forward clutch  26  may be positioned in an engaged position or a disengaged position by an actuator (not shown) as directed by a controller (not shown). In the engaged position, the forward clutch  26  facilitates driving engagement between a portion of the input  18  and a portion of the intermediate portion  30 , allowing power to be applied to the intermediate portion  30 . In the disengaged position, the forward clutch  26  is not drivingly engaged with one of the input  18  and the intermediate portion  30 . The forward clutch  26  may also include a synchronizer which facilitates meshing engagement between the forward clutch  26  and one of the input  18  and the intermediate portion  30 . The synchronizer is common and well known in the art. 
     The reverse clutch  28  comprises at least a shift collar concentrically disposed about a portion of the input  18  and a reverse gear assembly (not shown). The reverse gear assembly is drivingly engaged with a portion of the intermediate portion  30 . The reverse clutch  28  includes a set of teeth which may be engaged with corresponding teeth formed on a portion of the input  18  and a portion of the reverse gear; however, it is understood the reverse clutch  28  may be substituted with any clutching device that permits selective engagement between the input  18  and the intermediate portion  30  while reversing a rotation therebetween. 
     The reverse clutch  28  may be positioned in an engaged position or a disengaged position by an actuator (not shown) as directed by the controller. In the engaged position, the reverse clutch  28  facilitates driving engagement between a portion of the input  18  and a portion of the intermediate portion  30  while reversing a rotation therebetween, allowing power to be applied to the intermediate portion  30 . In the disengaged position, the reverse clutch  28  is not drivingly engaged with one of the input  18  and the intermediate portion  30 . The reverse clutch  28  may also include a synchronizer which facilitates meshing engagement between the reverse clutch  28  and one of the input  18  and the intermediate portion  30 . The synchronizer is common and well known in the art. 
     The intermediate portion  30  may comprise a plurality of intermeshed gears; however, it is understood that the intermediate portion  30  may comprise any device capable of facilitating driving engagement between the forward clutch  26  and the epicyclic gearset  32  and the reverse clutch  28  and the epicyclic gearset  32 . 
     The epicyclic gearset  32  includes an sun gear  34  which is drivingly engaged with the intermediate portion  30 . The sun gear  34  forms a portion of the epicyclic gearset  32 . As shown in  FIG. 1 , the sun gear  34  is a sun gear; however, it is understood that the sun gear  34  may form other portions of the epicyclic gearset  32 . The epicyclic gearset  32  also comprises the plurality of planet gears  36 , a carrier  38 , and a ring gear  40 . The sun gear  34  includes a plurality of teeth formed about an outer surface thereof which is drivingly engaged with a plurality of planet gears  36 . 
     The hydraulic portion  24  comprises a variable displacement pump  42 , a variable displacement motor  44 , a hydraulic output member  46 , a primary clutch  48 , and a secondary clutch  50 . It is understood that the primary clutch  48  and the secondary clutch  50  may be combined in a single unit. 
     The variable displacement pump  42  is a hydraulic axial piston pump having a movable swashplate (not shown). However, it is understood the variable displacement pump  42  may be any other type of variable displacement pump. The variable displacement pump  42  is drivingly engaged with the power source  12  through the input  18 . A first fluid port  52  of the variable displacement pump  42  is in fluid communication with a first fluid conduit  54 . A second fluid port  56  of the variable displacement pump  42  is in fluid communication with a second fluid conduit  58 . 
     The variable displacement motor  44  is a hydraulic axial piston motor having a movable swashplate (not shown). However, it is understood the variable displacement motor  44  may be any other type of variable displacement motor. The variable displacement motor  44  is drivingly engaged with the output  20  through the hydraulic output member  46  and the primary clutch  48  or the ring gear  40  of the epicyclic gearset  32  through the hydraulic output member  46  and the secondary clutch  50 . A first fluid port  60  of the variable displacement motor  44  is in fluid communication with the first fluid conduit  54 , facilitating fluid communication between the variable displacement pump  42  and the variable displacement motor  44 . A second fluid port  62  of the variable displacement motor  44  is in fluid communication with the second fluid conduit  58 , facilitating fluid communication between the variable displacement pump  42  and the variable displacement motor  44 . 
     The variable displacement pump  42 , the variable displacement motor  44 , the first fluid conduit  54 , and the second fluid conduit  58  form a fluid circuit as is known in the art. Further, it is understood that such a fluid circuit may include additional components such as a hydraulic cylinder, a directional valve, an accumulator, or a secondary motor. 
     The hydraulic output member  46  is drivingly engaged with the variable displacement motor  44  and a portion of the primary clutch  48  and the secondary clutch  50 . The hydraulic output member  46  may comprise a plurality of intermeshed gears; however, it is understood that the hydraulic output member  46  may comprise any device capable of splitting power between the variable displacement motor  44  and the primary clutch  48  and the secondary clutch  50 . 
     The primary clutch  48  is disposed about a portion of the hydraulic output member  46  and a portion of the output  20 . The primary clutch  48  facilitates variable engagement between a portion of the hydraulic output member  46  and a portion of the output  20 ; however, it is understood the primary clutch  48  may be substituted with any clutching device that permits selective engagement between the hydraulic output member  46  and the output  20 . As non-limiting examples, the primary clutch  48  may comprise a plurality of intermeshed plates, a cone clutch, or another style of clutch that may be variably engaged. 
     The primary clutch  48  may be positioned in an engaged position or a disengaged position by an actuator (not shown) as directed by the controller. In the engaged position, the primary clutch  48  facilitates driving engagement between a portion of the hydraulic output member  46  and a portion of the output  20 , allowing power to be applied to the output  20 . In the disengaged position, the primary clutch  48  is not drivingly engaged with one of the hydraulic output member  46  and the output  20 . 
     The secondary clutch  50  is disposed about a portion of the hydraulic output member  46  and a portion of a ring gear member  64 , the ring gear member drivingly engaged with the ring gear  40 . The secondary clutch  50  facilitates variable engagement between a portion of the hydraulic output member  46  and a portion of the ring gear member  64 ; however, it is understood the secondary clutch  50  may be substituted with any clutching device that permits selective engagement between the hydraulic output member  46  and the ring gear member  64 . As non-limiting examples, the secondary clutch  50  may comprise a plurality of intermeshed plates, a cone clutch, or another style of clutch that may be variably engaged. 
     The secondary clutch  50  may be positioned in an engaged position or a disengaged position by an actuator (not shown) as directed by the controller. In the engaged position, the secondary clutch  50  facilitates driving engagement between a portion of the hydraulic output member  46  and a portion of the ring gear member  64 , allowing power to be applied to the ring gear  40  through the ring gear member  64 . In the disengaged position, the secondary clutch  50  is not drivingly engaged with one of the hydraulic output member  46  and the ring gear member  64 . 
     The output  20  is drivingly engaged with the primary clutch  48  and the carrier  38  of the epicyclic gearset  32 ; however, it is understood that the output  20  may be drivingly engaged with other portions of the epicyclic gearset  32 . The output  20  may comprise a plurality of intermeshed gears; however, it is understood that the output  20  may comprise any device capable of facilitating driving engagement between the primary clutch  48  and the vehicle output  16  and the carrier  38  and the vehicle output  16 . 
     As shown in  FIG. 1 , the vehicle output  16  includes an axle differential  66 , which is drivingly engaged with a pair of axle half shafts  68 . The axle differential  66  and the pair of axle half shafts  68  are conventional and well known in the art. Alternately, the vehicle output  16  may be any other type of power transmission device. As non-limiting examples, the power transmission device may be a transmission, a drive shaft, or a transaxle. 
     In use, the powersplit transmission  14  may be operated in a hydrostatic power transmission mode or a blended hydrostatic and mechanical power transmission mode. 
     When the powersplit transmission  14  is operated in the hydrostatic power transmission mode, variable displacement motor  44  is drivingly engaged with the output  20  through the hydraulic output member  46  and the primary clutch  48 . The variable displacement motor  44  is fluidly driven by a variable displacement pump  42  through the first fluid conduit  54  and the second fluid conduit  58 . The variable displacement pump  42  is drivingly engaged with the power source  12  through the input  18 . 
     When the powersplit transmission  14  is operated in the blended hydrostatic and mechanical power transmission mode, the output  20  is driven by the variable displacement motor  44  through the hydraulic output member  46  and the primary clutch  48  and directly by the powers source  12  through one of the forward clutch  26  and the reverse clutch  28  and the epicyclic gearset  32 . As shown in  FIG. 1 , the forward clutch  26  or the reverse clutch  28  drives the sun gear  34  of the epicyclic gearset  32  through a respectively positive ratio and a negative ratio, which may be configured accordingly based on a vehicle type and intended use of the driveline  10 . 
     The variable displacement motor  44  may be engaged with one of the primary clutch  48  and the secondary clutch  50  through the hydraulic output member  46 . The primary clutch  48  may be drivingly engaged with the output  20 . The secondary clutch  50  may be drivingly engaged with the ring gear  40  of the epicyclic gearset  32  through the ring gear member  64 . When the primary clutch  48  is placed in the engaged position and the secondary clutch  50  is placed in the disengaged position, the variable displacement motor  44  is drivingly engaged with the output  20  with a negative ratio. When the primary clutch  48  is placed in the disengaged position and the secondary clutch  50  is placed in the engaged position, the variable displacement motor is drivingly engaged with the ring gear  40  of the epicyclic gearset  32  through the ring gear member  64 . 
     When the powersplit transmission  14  is operated in the blended hydrostatic and mechanical power transmission mode, the planet gears  36  are driven by both the ring gear  40  and the sun gear  34 . The carrier  38 , which is drivingly engaged with the output  20 , is driven by the planet gears  36 , which are rotatably disposed thereon. 
       FIG. 2  is a table showing a state of each of the power source  12 , the variable displacement motor  44 , the forward clutch  26 , the reverse clutch  28 , the primary clutch  48 , and the secondary clutch  50  when the driveline  10  is placed in one of five operational modes. The five operational modes are: a reverse powersplit mode, a reverse hydrostatic mode, a neutral mode, a forward hydrostatic mode, and a forward powersplit mode. 
     In the forward hydrostatic mode and the reverse hydrostatic mode, the variable displacement motor  44  drives the output  20  through the primary clutch  48 . The output  20  backdrives the carrier  38  of the epicyclic gearset  32 . To militate against a transfer of torque to the sun gear  34 , the secondary clutch  50  is placed in the disengaged position. 
     In the forward powersplit and the reverse powersplit mode, power from the mechanical portion  22  and the hydraulic portion  24  is combined using the epicyclic gearset  32 . Power from the power source  12  is applied to the forward clutch  26  or the reverse clutch  28  and from the variable displacement motor  44  though the secondary clutch  50 . The primary clutch  48  is placed in the disengaged position. 
     It is understood that the driveline  10  as illustrated in  FIG. 1  may be placed in other powersplit configurations not shown in  FIG. 2  or described herein. The operational modes as described herein are merely exemplary of some of the powersplit configurations that the driveline  10  illustrated in  FIG. 1  may be placed in. 
     Torque interruption may occur when shifting the powersplit transmission  14  from the forward hydrostatic mode to the forward powersplit mode. During the forward hydrostatic mode, the variable displacement motor  44  is providing power at a negative torque input. After the transition to the forward powersplit mode, the variable displacement motor  44  is absorbing power at positive torque. Because of this transition, during the shift, the torque changes from a negative to a positive value. When the torque change from a negative to a positive value, the torque applied to the variable displacement motor  44  is about equal zero. 
     In the forward hydrostatic mode and the forward powersplit mode, an output torque is related to the torque of the variable displacement motor  44 . As a non-limiting example, when the torque applied to the variable displacement motor  44  is about equal to zero, the output torque is about equal to zero. As such, when shifting, the output torque drops to zero during the shift and torque applied resumes after the shift. In some applications, the torque interruption is unacceptable. 
       FIG. 3A  graphically illustrates torque vectors of the variable displacement motor  44 , the primary clutch  48 , and the output  20  when the driveline  10  is placed in the forward hydrostatic mode. Reference letter “T” indicates a torque vector and reference letter “S” indicates a speed vector. An arrow pointed to the right indicates a positive vector, and an arrow pointed to the left indicates a negative vector. It should be noted that the torque vector applied to the variable displacement motor  44  is positive while the speed is negative, which may be interpreted as power being transferred from the variable displacement motor  44  to the output  20 . 
       FIG. 3B  graphically illustrates a speed of the output  20  and a speed of the variable displacement motor  44  with respect to time during a shift from the forward hydrostatic mode to the forward powersplit mode. It is understood that shifts from the forward powersplit mode to the forward hydrostatic mode, from the reverse hydrostatic mode to the reverse powersplit mode, and the reverse powersplit mode to the reverse hydrostatic mode may be adapted accordingly based on the known operative conditions. 
     In the forward hydrostatic mode, the variable displacement motor  44  is driving the output  20  with a negative ratio. When the speed of the variable displacement motor  44  is increased negatively by varying the displacement of the variable displacement pump  42  and/or the variable displacement motor  44 , the output  22  is positively sped up. 
       FIGS. 4A  graphically illustrates torque vectors of the variable displacement motor  44  when the driveline  10  is placed in the forward powersplit mode, with similar references to  FIG. 3A . In the forward powersplit mode, the variable displacement motor  44  is driving the ring gear  40  with a positive ratio through the secondary clutch  50  and the power source  12  is driving the sun gear  34  through the forward clutch  26  with a positive ratio. The carrier  38  is driven with a positive ratio to the output  20 . 
       FIG. 4B  illustrates a speed diagram of the epicyclic gearset  32  showing a speed of the ring gear  40  (which is directly related to the speed of the variable displacement motor  44 ) beginning at a negative speed and the speed of the sun gear  34  (which is directly related to the speed of the power source  12 ) being positive. As a result, a speed of the carrier  38  (which is directly related to a speed of the vehicle the driveline  10  is incorporated in) is positive. 
       FIG. 4B  also illustrates that when the speed of the variable displacement motor  44  is changed from a negative speed to a positive speed, a speed of the output  20  is also increased. 
     It should be noted that the torque vector acting on the variable displacement motor  44  begins negatively so that initially the variable displacement motor  44  is absorbing power from the power source  12  through the epicyclic gearset  32 . When a speed of the variable displacement motor  44  is about equal to zero, the variable displacement motor  44  is neither absorbing nor providing power so that all the power from the power source  12  is passed directly to the output  20  through the carrier  38 . When a speed of the variable displacement motor  44  is positive, the variable displacement motor  44  supplements the power source  12  in providing power to the output  20 . 
     When comparing  FIGS. 3A ,  4 A, and  4 B, it can be appreciate that a torque reversal of the variable displacement motor  44  from the forward hydrostatic mode to the forward powersplit mode occurs. The advantage of the driveline  10  is that shifting between the forward hydrostatic mode and the forward powersplit mode can be done synchronously, meaning that the variable displacement motor  44  does not need to change speed shifting from the forward hydrostatic mode to the forward powersplit mode. Conversely, the torque does change from positive to negative. 
       FIGS. 5A  graphically illustrates torque vectors applied to the variable displacement motor  44  by the primary clutch  48  and the secondary clutch  50  during a shift procedure from the forward hydrostatic mode to the forward powersplit mode, with similar references to  FIG. 3A . 
       FIG. 5B  illustrates a speed diagram of the epicyclic gearset  32  showing a speed of the ring gear  40  (which is directly related to the speed of the variable displacement motor  44 ) beginning at a negative speed and the speed of the sun gear  34  (which is directly related to the speed of the power source  12 ) being positive. As a result, the speed of the carrier  38  (which is directly related to a speed of the vehicle the driveline  10  is incorporate in) is positive. 
     To prevent torque interruption, the application of torque to the output  20  should be continuous. To facilitate continuous torque application, the primary clutch  48  and the secondary clutch  50  may be partially opened or closed so that they are placed into a slip condition. When the primary clutch  48  and the secondary clutch  50  are placed into a slip condition, it is possible to control an amount of torque applied to the output  20  by adjusting a pressure applied to the primary clutch  48  and the secondary clutch  50 . Further, it is possible to manipulate the speed of the variable displacement motor  44  (by adjusting the displacement of the variable displacement motor  44  or the variable displacement pump  42 ), instead of adjusting a pressure applied to the primary clutch  48  and the secondary clutch  50  to control whether the torque of the variable displacement motor  44  is positive or negative. Using the aforementioned techniques, the shift procedure can be developed that controls an amount of torque and whether the torque is positive or negative, and therefore it is possible to provide continuous torque to the output  22  despite the fact that the variable displacement motor  44  needs to provide zero torque at some point in time when shifting between the forward hydrostatic mode and the forward powersplit mode. 
       FIG. 6  graphically illustrates the shift procedure. A vertical positive axis displays the amount of pressure applied to the primary clutch  48  and the secondary clutch  50  versus time. The vertical positive axis also displays the speed of the output  20  versus time. A vertical negative axis displays the speed of the variable displacement motor  44  versus time. The vertical negative axis also displays the speed of the ring gear  40  versus time. The shift procedure is initiated prior to a synchronous shift point so that the torque vector of the variable displacement motor  44  may be adjusted accordingly using the following procedure. It is understood that the vertical values shown in  FIG. 6  indicate a direction of rotation, and that a negative value indicates a direction of rotation opposite a positive value. 
     As seen in  FIG. 6 , between t 0  and t 1 , the pressure applied to the primary clutch  48  is reduced to permit the primary clutch  48  to be placed in the slip condition. Consequently, the torque transferred through the primary clutch  48  is a function of the amount of pressure applied to the primary clutch  48 . The pressure applied to the primary clutch  48  is reduced until the torque of the output  20 , the torque transferred by the primary clutch  48 , and the torque applied to the variable displacement motor  44  is about equal. 
     Next, between t 1  and t 2 , the speed of the variable displacement motor  44  is controlled by adjusting the displacements of the variable displacement motor  44  and the variable displacement pump  42 , so that the speed of the variable displacement motor  44  is between the speed of the ring gear  40  and the speed of the output  20 . 
     Next, between t 2  and t 3 , the speed of the variable displacement motor  44  is actively adjusted to stay between the speed of the ring gear  40  and the speed of the output  20 . The pressure applied to the secondary clutch  50  is increased until torque is transferred from the variable displacement motor  44  to the ring gear  40 . Simultaneously, the pressure applied to the primary clutch  48  is decreased. Accordingly, the torque applied to the output  20  is about equal to the sum of the torque transferred to the ring gear  40  (multiplied by a ratio of the ring gear  40  to the carrier  38 ) and the torque transferred through the primary clutch  48 . 
     Further, the torque applied to the variable displacement motor  44  is about equal to the torque transferred through the primary clutch  48  minus the torque transferred through the secondary clutch  50 . 
     As shown in  FIG. 6 , where the amount of torque transferred through the primary clutch  48  is decreased by about the same amount of torque transferred through the secondary clutch  50  is increased; the sum of the torques applied to the output  20  will remain about constant. When sum of the torques applied to the output  20  remains about constant, an equilibrium condition occurs and the torque applied to the variable displacement motor  44  can be shifted from a positive torque to a negative torque without changing the torque of the output  20  or causing torque interruption. 
     Next, at t 3 , the primary clutch  48  is placed in the disengaged position, while the secondary clutch  50  is still in the slip condition. 
     Lastly, at t 4 , the output  20  has increased in speed so that the ring gear  40  is completely synchronized with the variable displacement motor  44 . Following the synchronization, the secondary clutch  50  can be placed in the engaged position, and the variable displacement motor  44  is now controlled to increase the speed of the vehicle the driveline  10  is incorporated in, as desired by an operator of the vehicle. 
     Further, in addition to the shift procedure as described above, it is understood that the shift procedure may be adapted to militate against a second torque interruption that may be caused by applying torque to the power source  12  through the sun gear  34  and one of the forward clutch  26  and the reverse clutch  28 . To reduce the second torque interruption, a speed of the power source  12  may be increased and one of the forward clutch  26  and the reverse clutch  28  may be placed in a slip condition. The slip condition of one of the forward clutch  26  and the reverse clutch  28  is proportional to the amount of pressure that is applied to the secondary clutch  50 . 
     The driveline  10  may also experience torque interruption when shifting from the blended hydrostatic and mechanical power transmission mode to a secondary blended hydrostatic and mechanical power transmission mode. It is understood that the method for shifting the powersplit transmission  14  as described hereinabove may be applied when shifting the powersplit transmission  14  from the blended hydrostatic and mechanical power transmission mode to a secondary blended hydrostatic and mechanical power transmission mode. The secondary blended hydrostatic and mechanical power transmission mode of operation may be useful to increase an efficiency of the vehicle and increase a range of operating speeds of the vehicle, for example. 
     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiments. 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.