Abstract:
A transmission system ( 14 ) comprising an input shaft ( 112 ); a first clutch ( 114 ) configured to selectively couple a first gear ( 116 ) to the input shaft ( 112 ); an output shaft ( 126 ); a second gear ( 138 ) engaging with the first gear ( 116 ), —and a second clutch ( 136 ) configured to couple the second gear ( 138 ) to the output shaft ( 126 ) when the first gear ( 116 ) rotates the second gear ( 138 ) faster than the output shaft ( 126 ).

Description:
[0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/156,042 filed on Feb. 27, 2009 and to co-pending application Ser. No. PCT/US10/25324 filed on Feb. 25, 2010 which are both herein incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    A torque converter is used for transferring rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load, such as a vehicle. Like a basic fluid coupling, the torque converter normally takes the place of a mechanical clutch, allowing the load to be separated from the power source. 
         [0003]    The torque converter has three stages of operation. During a stall stage, the engine is applying power to a torque converter pump but a torque converter turbine cannot rotate. For example, in an automobile, this would occur when the driver has placed the transmission in gear but prevents the vehicle from moving by continuing to apply the brakes. During an acceleration stage, the vehicle is accelerating but there still is a relatively large difference between pump and turbine speed. During a coupling stage when the vehicle is moving, the turbine reaches a larger percent of the speed of the pump. 
         [0004]    The torque converter is used for smoothing the engagement of the engine to the drive train. However, torque converters are generally inefficient and much of the wasted energy is expended in the form of heat. For example, there is zero efficiency during the stall stage, efficiency generally increases during the acceleration phase, and it is still moderately inefficient during the coupling stage. 
       SUMMARY 
       [0005]    A multi-speed transmission system replaces a torque converter by controlled clutch slipping. The multi-speed transmission is also designed to replace the torque amplification normally provided by torque converters at low speeds. The transmission system uses one-way bearings that provide smooth transitions between gears and significantly improve the efficiency of the transmission to the equivalent of a manual transmission while eliminating the drag normally associated with hydraulic clutch packs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic diagram of a multi-speed transmission system. 
           [0007]      FIG. 2  is a schematic cut-away view of a one-way bearing used in the transmission system of  FIG. 1 . 
           [0008]      FIG. 3A-3C  are isolated views of a portion of the transmission system shown in  FIG. 1 . 
           [0009]      FIG. 4  is a block diagram of a control system used in conjunction with the transmission system of  FIG. 1 . 
           [0010]      FIG. 5  is a flow diagram showing in more detail how the transmission system in  FIG. 1  operates when the vehicle is in a stopped position. 
           [0011]      FIG. 6  is a state diagram showing in more detail how the transmission system in  FIG. 1  shifts between gears. 
           [0012]      FIG. 7  is a flow diagram showing in more detail how the transmission system in  FIG. 1  operates when the vehicle is in a stopped position on a grade. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  is a schematic of a portion of a vehicle  10  that includes a multi-speed transmission system  14 . The transmission system  14  uses a clutch pack overrun system  110  that eliminates some of the inefficiencies associated with torque converters. The vehicle  10  in one embodiment is an industrial lift truck. However, the transmission system  14  can be used in a variety of different vehicles. 
         [0014]    The vehicle  10  includes an engine  12  that is connected to a drive axle assembly  34  through the transmission system  14 . The engine  12  rotates an input shaft  112  that then through clutch pack overrun system  110  selectively applies torque and rotates an output shaft  126 . The output shaft  126  couples the transmission system  14  with the drive axle assembly  34  and causes the drive axle assembly  34  to rotate wheels  39 . 
         [0015]    One embodiment the drive axle assembly  34  is conventional. In another embodiment, the drive axle assembly  34  uses a drive axle clutch system  142  that includes different travel direction hydraulic clutches and gears to rotate wheels  39  in different directions and move the vehicle  10  in different forward, reverse, and turning directions. The drive axle clutch system  142  is described in U.S. Provisional Patent Application Ser. No. 61/156,042 filed on Feb. 27, 2009, and U.S. patent application Ser. No. ______ which have both been incorporated by reference in their entirety. 
         [0016]    It should be understood that the transmission system  14  can operate with any conventional axle assembly and vehicle direction control system. The transmission system  14  is not required to be used in conjunction with the drive axle clutch system  142  described above, and can operate independently of the drive axle clutch system  142 . However, at least one embodiment below describes how the transmission system  14  operates in conjunction with drive axle clutch system  142 . 
         [0017]    The transmission system  14  includes a first drive gear  116  selectively connected to the input shaft  112  through a first hydraulic clutch pack  114 . A second drive gear  120  is selectively connected to the input shaft  112  through a second hydraulic clutch pack  118  and a third drive gear  122  is rigidly connected to the input shaft  112 . 
         [0018]    A first driven gear  138  engages with first drive gear  116  and engages with the output shaft  126  through a first one-way bearing  136 . A second driven gear  134  engages with a second drive gear  120  and is engaged with the output shaft  126  through a second one-way bearing  132 . A third driven gear  128  engages with a third drive gear  122  and is selectively connected to the output shaft  126  by a third hydraulic clutch pack  130 . 
         [0019]    Hydraulic clutches  114 ,  118 , and  130  operate similar to hydro-mechanical clutches in power shift transmissions. The hydraulic clutches  114  and  118  can selectively lock the gears  116  and  120 , respectively, to the input shaft  112  when rotating. The hydraulic clutch  130  can selectively lock the gear  128  to the output shaft  126 . Each hydraulic clutch is provided with a proportional electro-hydraulic valve and hydraulic pressure sensor to provide for control and feedback (see  FIG. 4 ). Alternative sensors, such as torque sensors can be used in place of pressure sensors for closed feedback loop control. 
         [0020]    Torque is transferred from input shaft  112  to output shaft  126  when first hydraulic clutch  114  locks first drive gear  116  to input shaft  112  and first one-way bearing  136  locks first driven gear  138  to output shaft  126 . Torque is also transferred from input shaft  112  to output shaft  126  when second hydraulic clutch  118  locks second drive gear  120  to input shaft  112  and second one-way bearing  132  locks second driven gear  134  to output shaft  126 . Torque is also transferred from input shaft  112  to output shaft  126  when third hydraulic clutch  130  locks third driven gear  128  to output shaft  126 . 
         [0021]    The one-way bearings  136  and  132  lock the gears  138  and  134 , respectively, to the output shaft  126  when turning in only one direction of shaft rotation. The one-way bearings  136  and  132  allow the output shaft  126  to free wheel inside the driven gears  138  and  134 , respectively, if the output shaft  126  turns faster than the driven gear. 
         [0022]      FIG. 2  is a simplified sectional view showing some of the elements in one of the one-way bearings  136  or  132 .  FIG. 2  uses first one-way bearing  136  as an example. The first one-way bearing  136  is coupled to the output shaft  126  and includes bearings  150  that press against an inside wall of the driven gear  138 . 
         [0023]    When the first driven gear  138  has a rotational speed  152  that is faster than the rotational speed  154  of output shaft  126 , the first one-way bearing  136  automatically locks the first driven gear  138  to the output shaft  126 . The first one-way bearing  136  automatically releases the first driven gear  138  from the output shaft  126  when the output shaft  126  starts rotating at a faster rotational speed  152  than the first driven gear  138 . This unlocked one-way bearing state is alternatively referred to as free-wheeling. 
         [0024]    When the first driven gear  138  is overrun by the output shaft  126 , the first drive gear  116  in  FIG. 1  cannot transfer torque from the input shaft  112  to the output shaft  126 . There is also very low drag when the first one-way bearing  136  is in the unlocked free-wheeling state. The one-way bearing is used to accomplish an up or down shift. Again, it should be noted that  FIG. 2  is a simplified representation of a one-way bearing, and other one-way bearing configurations can also be used in transmission system  14 . 
         [0025]      FIGS. 3A-3C  describe in more detail how the transmission system  14  operates. The rotational states  160 A- 160 H refer to different rotational states of the shafts  112  and  126  and different rotational or locking states of the one-way bearings and hydraulic clutches. 
         [0026]    Referring first to  FIG. 3A , the hydraulic clutch  114  is activated and hydraulic clutches  118  and  130  are deactivated. Plates in the first hydraulic clutch  114  press together in the active state, coupling the first driven gear  116  to the input shaft  112 . Activating first hydraulic clutch  114  causes a rotation  160 B in first drive gear  116 . Depending on the current state of the vehicle  10  either in a stopped or moving condition, the first hydraulic clutch  114  may be slipped to gradually engage the input shaft  112  with first drive gear  116  or the first hydraulic clutch  114  may be locked. 
         [0027]    The first drive gear  116  has a relatively low rotational speed  160 B, creating a rotational speed  160 C in first driven gear  138 . However, output shaft  126  is currently not rotating and the faster rotation  160 C of first driven gear  138  causes the first one-way bearing  136  to lock first driven gear  138  to output shaft  126 . The locking of first one-way bearing  136  allows the first drive gear  116  to apply torque to the output shaft  126  and start output shaft  126  rotating with a rotational speed  160 H. 
         [0028]    The second hydraulic clutch  118  is currently not activated, so the second drive gear  120  is unlocked and has no rotational speed  160 D and the second driven gear  134  is unlocked and has no rotational speed  160 E. Since the output shaft  126  is rotating faster than stationary second driven gear  134 , the second one-way bearing  132  does not engage and the output shaft  126  freewheels inside of the second driven gear  134 . In this stage, the second drive gear  120  does not apply any torque to the output shaft  126 . 
         [0029]    The third drive gear  122  is permanently attached to the input shaft  112  and has a rotational speed  160 F and limited torque that rotates the third driven gear  128 . However, the third hydraulic clutch  130  is currently not activated and therefore the third drive gear  122  also does not apply torque to the output shaft  126 . 
         [0030]    The high gear ratio of driven gear  138  to drive gear  116  provides high torque to the output shaft  126  for pushing. The first hydraulic clutch  114  can also be used as an inching clutch for starting and fine positioning. However, any of the other clutches may be also used for inching control. 
         [0031]      FIG. 3B  shows how the transmission system  14  operates during a transition from first drive gear  116  to the second drive gear  120 . The second hydraulic clutch  118  is activated causing input shaft  112  to rotate second drive gear  120  with rotational speed  160 D. In this example, the first hydraulic clutch  114  is shown still activated and the third hydraulic clutch  130  is still not activated. However, the first hydraulic clutch  114  may be released sometime after the second hydraulic clutch  118  is activated. 
         [0032]    The second drive gear  120  rotates the second driven gear  134  faster than the first drive gear  116  rotates the first driven gear  138  and output shaft  126 . Accordingly, the second one-way bearing  132  locks the second driven gear  134  to output shaft  126  and the second drive gear  120  starts applying torque and a rotational speed  160 H to the output shaft  126 . Output shaft  126  is now rotating faster than the first driven gear  138  causing the first one-way bearing  136  to release the first driven gear  138  from output shaft  128 . Output shaft  128  then starts free-wheeling inside of the first driven gear  138  and the first drive gear  116  no longer applies torque to the output shaft  126 . The third hydraulic clutch  130  is still deactivated and the third drive gear  122  still does not apply torque to the output shaft  126 . 
         [0033]    One advantage of the transmission system  14  is the simple relatively smooth transitions between different gears. For example, the first one-way bearing  136  automatically disengages when the second one-way bearing  132  engages. Thus, the disengagement of the first hydraulic clutch  114  does not have to be precisely coordinated with the engagement of the second hydraulic clutch  118 . The use of a high gear ratio with gears  116  and  138  also eliminates the need for a torque converter. The engine  12  ( FIG. 1 ) also does not need to be revved up as high to prevent stalling when transitioning to lower gear ratios. 
         [0034]      FIG. 3C  shows how the transmission system  14  operates during another transition from second drive gear  120  to the third drive gear  122 . The third hydraulic clutch  130  is activated locking the third driven gear  128  to output shaft  126 . The third drive gear  122  has a rotational speed  160 F and applies torque and rotates the output shaft  126 . In this example, the second hydraulic clutch  118  is shown still activated and first hydraulic clutch  114  is shown deactivated. However, any combination of the hydraulic clutches  114  and  118  may be released or not released after hydraulic clutch  130  is activated. For example, it is possible for all three hydraulic clutches to be engaged without gears binding. 
         [0035]    The third drive gear  122  generates a rotational speed  160 G in the third driven gear  128  which in turn creates a rotational speed  160 H in the output shaft  126 . In this embodiment the gear ratio between the third driven gear  128  and the third drive gear  122  is lower than the gear ratio between the second driven gear  134  and the second drive gear  120 . The gear ratio between the second driven gear  134  and the second drive gear  120  is lower than the gear ratio between the first driven gear  138  and the first drive gear  116 . Thus, the rotational speed  160 H will be faster than both the rotational speed of the first driven gear  138  and faster than the rotational speed of the second driven gear  134 . Accordingly, the second one-way bearing  132  disengages the second driven gear  134  from output shaft  126  and the first one-way bearing  136  keeps the first driven gear  138  disengaged from output shaft  126 . Thus, the transmission system  14  moves into third drive gear  122  without having to mechanically coordinate the disengagement of the other gears  116  and  120 . 
         [0036]    A reverse process is used to downshift from the drive gear  122  back down to gears  120  or  116 . Conventional transmission systems have to simultaneously modulate both the deactivation of one gear clutch and the activation of another gear clutch requiring a high degree of coordination to achieve smooth shifting. However, in the transmission system  14 , different hydraulic clutches can remain engaged during upshifting and downshifting operations because of the overrunning capability of the associated one-way bearings. As the transmission system  14  shifts, one gear starts to transmit torque and stops overrunning as another gear is disengaged. 
         [0037]    For example, the second hydraulic clutch  118  can be engaged while the third hydraulic clutch  130  is disengaged. This allows the second drive gear  120  to eventually start rotating the second driven gear  134  faster than the output shaft  126 . The second one-way bearing  132  then engages the second driven gear  134  with the output shaft  126  and allows the second drive gear  120  to start applying torque to the output shaft  126 . 
         [0038]    Similarly, the first hydraulic clutch  114  can be engaged while the second hydraulic clutch  118  is disengaged. The first one-way bearing  136  locks the first driven gear  138  with output shaft  126  when the rotational speed of the first driven gear  138  overtakes the rotational speed of output shaft  126 . The first drive gear  116  then starts applying torque to the output shaft  126 . 
         [0039]    There is a relatively smooth transition from the third drive gear  122  to the second drive gear  120  and from the second drive gear  120  to the first drive gear  116 . This is due to the one-way bearings  132  and  136  only locking the output shaft  126  with driven gears  134  and  138 , respectively, when the speed of the driven gears overtake the rotational speed of output shaft  126 . Thus, the vehicle jerking that normally occurs in conventional transmission systems when transitioning between gears may be reduced. 
         [0040]    During “free wheeling” when going down a grade in first drive gear  116 , the output shaft  126  may overrun the first driven gear  138 . One control strategy is to shift to third drive gear  122  and let third driven gear  128  provide some degree of engine braking. The clutch system  142  located in the drive axle assembly  34  in  FIG. 1  can also be used for braking the vehicle  10 . 
       Control System 
       [0041]      FIG. 4  shows a control system for the vehicle  10  and transmission system  14  previously shown in  FIGS. 1-3C . A Central Processing Unit (CPU)  40  controls the activation of hydraulic clutch packs  114 ,  118 , and  130  in the transmission system  14  according to different vehicle parameters. A control valve  16  in the transmission  14  controls fluid pressure that controls the activation of the clutch packs  114 ,  118 , and  130 . 
         [0042]    The CPU  40  receives a vehicle speed and direction signal  18  from a vehicle speed sensor  200  that indicates the Transmission Output Shaft rotational Speed (TOSS) and direction of the output shaft  126 . An Engine Rotations Per Minute (ERPM) signal  30  is generated from an engine speed sensor  204  and indicates how fast the input shaft  112  ( FIG. 1 ) connected to the engine  12  is rotating. An engine governor control signal  32  controls a throttle valve  206  that controls the speed of engine  12 . A transmission temperature signal  28  is generated by a temperature sensor  208  and identifies the temperature of the transmission fluid in the transmission  14 . 
         [0043]    The CPU  40  receives a brake pedal position signal  42  from a brake pedal position sensor  210  on brake pedal  43 . An accelerator pedal position signal  44  is received from an accelerator pedal position sensor  212  on accelerator pedal  50 . The accelerator pedal position can alternatively correspond to a throttle value, acceleration value, or deceleration value. 
         [0044]    A forward-reverse direction signal  46  is generated by a direction lever or pedal  52  and indicates a forward or backward direction the vehicle operator selects for the vehicle  10 . An internal or external memory  48  contains mapped parameters identifying clutch pressure values and other control and speed parameters used for performing different braking and shifting operations. Some of the parameters stored in memory  48  are described in more detail below in  FIGS. 5-7 . 
         [0045]    The hydraulic clutches  114 ,  118 , and  130 , in combination with one-way bearings  136  and  132  selectively engage and disengage the input shaft  112  with the output shaft  126  as described above. The engaging force of the hydraulic clutches  114 ,  118 , and  130  are controlled by changing the oil pressure in the clutch chambers. The oil pressure in the clutch chambers is controlled by the control value  16  which is controlled by the CPU  40 . 
         [0046]    Control valve clutch signal  22  controls the oil pressure in the first hydraulic clutch pack  114 , control valve signal  24  controls the oil pressure in the second hydraulic clutch pack  118 , and control valve signal  26  controls the oil pressure in the third hydraulic clutch pack  130 . Where the drive axle clutch system  142  in  FIG. 1  is used, one or more signals  70  control the oil pressure(s) for the clutch system  142  ( FIG. 1 ) in the drive axle assembly  34 . 
         [0047]    Pressure sensor signal  56  indicates the amount of pressure applied by the control valve  16  in the hydraulic clutch pack  114 . Pressure sensor signal  60  indicates the amount of pressure applied in the hydraulic clutch pack  118  and pressure sensor signal  64  indicates the amount of pressure applied by the control valve  16  to the hydraulic clutch pack  130 . When hydraulic clutch packs are used in the drive axle  34 , one or more pressure sensor signals  72  indicate the amount of pressure applied to the hydraulic clutch packs  142 . When a conventional drive axle is used, pressure sensor signal  72  is not used. 
         [0048]    The CPU  40  uses the signals  56 ,  60 , and  64  to determine the amount of slipping in the hydraulic clutch packs  114 ,  118 , and  130 , respectively. When any of the clutch pressures are zero, the particular hydraulic clutch  114 ,  118 , or  130  disengages that associated gear from the input shaft  112  or output shaft  126 . When the clutch pressure for any of the hydraulic clutch packs is at a maximum pressure, the corresponding clutch pack maximizes the engaging force between the associated shaft and gear (locking). When the clutch pack pressure is between zero and the maximum value, the corresponding clutch pack is partially engaged. The partially engaged condition is referred to as “clutch pack slipping.” 
         [0049]    As mentioned above, the drive axle  34  can be a conventional drive axle that does not use hydraulic clutch packs. However, when located in the drive axle assembly  34 , the clutch system  142  permits the application of torque from the engine  12  to be separated from clutch pack braking. This permits engine speed control independent of ground speed. For example, an operator may wish to speed up the engine  12  for hydraulic operations while decreasing the vehicle travel speed. This can be performed automatically by having the CPU  40  disengage the transmission  14  and apply clutch pack braking in the drive axle assembly  34 . 
         [0050]      FIG. 5  is a flow diagram describing one way the control system in  FIG. 4  operates when the vehicle  10  is stopped in state  300 . In operation  302 , the CPU  40  receives a command to move the vehicle  10 . For example, the CPU  40  may receive the accelerator pedal position signal  44  responsive to the Accelerator Pedal Position (APP) of accelerator pedal  50 . In operation  304 , the first hydraulic clutch  114  is slipped by the CPU  40  by controlling the amount of pressure supplied by control valve  16  via signal  22 . The slipping of the first hydraulic clutch  114  limits torque, preventing engine  12  from stalling, and reduces drive gear engagement shock to the drive axle assembly  34  and the vehicle operator. This clutch pack slipping replaces at least one of the functions of a torque converter, namely preventing the engine  12  from stalling when the vehicle  10  starts moving from a stopped position. 
         [0051]    Selected clutches in clutch system  142  when used in the drive axle  34  are also engaged in operation  304  according to the selected travel direction and slope of the vehicle  10 . For example, a first set of clutches in clutch system  142  may be selected for engagement by CPU  40  via signals  70  to move the vehicle  10  in a forward direction and a second set of clutches in clutch system  142  may be selected for engagement by the CPU  40  to move the vehicle  10  in the reverse direction. The direction of the vehicle  10  may be determined by the CPU  40  via the direction sensor signal  46 . 
         [0052]    In operation  306 , the CPU  40  continues to increase the pressure supplied by control valve  16  to the first hydraulic clutch  114  and correspondingly increases the amount of torque supplied by the engine  12  to the drive axle  34  according to operator intent. For example, the CPU  40  continuously monitors the position of accelerator pedal  50  to determine how much pressure and associated slipping to apply in the first hydraulic clutch pack  114  using signal  22  and to determine what speed to run engine  12  using signal  32 . 
         [0053]    The CPU  40  in operations  304  and  306  continues to increase pressure until engagement of the first hydraulic clutch  114  is finished. For example, when the operator stops depressing accelerator pedal  50 , the CPU  40  may determine that the first hydraulic clutch  114  has the correct amount of slippage and the engine  12  is providing the correct amount of torque to drive axle  34 . 
         [0054]    The CPU  40  may continue to increase pressure to the first hydraulic clutch  114  in operations  304  and  306  until the first hydraulic clutch  114  completely locks input shaft  112  to the first driven gear  116  in operation  308  and while the drive axle clutches in clutch system  142  remain engaged. The vehicle  10  is now moving and the start up sequence for the vehicle  10  performed by the CPU  40  is completed in operation  310 . 
         [0055]      FIG. 6  is a state diagram further explaining how the transmission system  14  shifts between different gears. The example described below shows transitions between three different gears. However, more or fewer than three gears can be used in the transmission system  14 . Transitions between additional gears would operate similarly to the transitions between the second and third gears as described below.  FIG. 6  illustrates normal up and down shifting and also shows how the first gear is torque limited to prevent engine stalling and to prevent overloading the drive axle. 
         [0056]    In one embodiment, the operations described in  FIG. 6  are controlled by the CPU  40  previously shown in  FIG. 4 . Example control valve pressures are used in  FIG. 6  for illustrative purposes but alternative pressures can be used to provide similar clutch pack modulations. In this example, a 0 pounds per square inch (psi) pressure is associated with a completely unlocked hydraulic clutch. A 20 psi hydraulic clutch pressure is associated with a touch point where the clutch is just starting to transfer torque to the drive axle  34 . A 40 psi pressure represents a clutch that is lightly engaged (slipping) and transfers only a partial amount of torque to reduce impact on the vehicle when a one-way bearing is initially engaged. A 140 psi pressure is associated with a fully locked hydraulic clutch. 
         [0057]    The CPU  40  can determine from the gear ratios currently being used in the transmission system  14 , Engine Rotations Per Minute (ERPM)  30 , and Transmission Output Shaft Speed (TOSS)  18  (see  FIG. 4 ) when there is zero slip in a particular hydraulic clutch  114 ,  118 , or  130 . Travel downshift speed values and travel upshift speed values as described below are predetermined variables based on accelerator pedal position  44  and ERPM  30 . 
         [0058]    The vehicle  10  and transmission system  14  are initially in a neutral state  320 . A vehicle move command condition  321  moves the transmission system into a first gear slip state  322 . The pressure in the first hydraulic clutch  114  is decreased if the ERPM is less than a predetermined engine stall speed. Otherwise, the pressure in the first clutch is increased. Varying the clutch pressure is alternatively referred to as modulation. 
         [0059]    When the measured clutch slip in condition  323  is zero, the first clutch  114  is locked by increasing the clutch pressure to 140 psi. The transmission system also moves into a first gear locked state  324 . If the ERPMs drop down below a predetermined downshift speed # 1  in condition  325 , the CPU moves the transmission system back into first gear slip state  322 . The CPU uses a first gear modulation chart in memory  48  to determine what pressures to then apply to the first clutch  114 . In this example, the CPU starts at 40 psi to reduce the torque on the engine  12  and then varies the clutch pressure according to the accelerator pedal position  44 , ERPM  30 , and TOSS  18 . 
         [0060]    Otherwise, the transmission system stays in the first gear locked state  324  until the ERPM rises above a predetermined upshift speed # 1  in condition  326 . When the ERPM rises above the upshift speed # 1  value, the CPU moves the transmission system into second gear slip state  327 . In this example, the CPU starts the pressure in the second hydraulic clutch  118  at 20 psi while keeping the first hydraulic clutch  114  in a fully locked condition. While in the second gear slip state  327 , the CPU increases or modulates the pressure based on mappings of the accelerator pedal position  44 , ERPM  30  and TOSS  18  in the second gear modulation chart. 
         [0061]    If the ERPM drops below the down shift speed # 1  value in condition  328 , the CPU  40  moves the transmission system back into the first gear slip state  322  and the first gear modulation pressure starts at 40 psi in the first gear modulation chart. The clutch pressure is set to 40 psi to quickly move the hydraulic clutch  118  to a beginning initial slipping condition. 
         [0062]    While in second gear slip state  327 , the CPU continues to increase the pressure in hydraulic clutch  118  until the second clutch  118  has zero slip in condition  329 . The pressure is then set to 140 psi to solidly hold the hydraulic clutch  118  in a second gear locked state  340 . The pressure in the first hydraulic clutch  114  is also set to 40 psi allowing the transmission system to quickly respond to any downshift back to first gear slip state  322 . 
         [0063]    In second gear locked state  340 , a reduction of the ERPM below a predetermined down shift speed # 2  value in condition  341  causes the CPU to move back to first gear slip state  322 . The pressure in clutch  118  is reduced down to 0 psi and the first clutch  114  is entered at 40 psi in the first gear modulation chart. The controlled reduction of the pressure in the second clutch  118  down to 0 psi reduces vehicle jolt that could happen if the second gear were instantly disengaged. If the ERPM rises above a predetermined upshift speed # 2  in condition  342 , the CPU moves the transmission system into a third gear slip state  343  and starts hydraulic clutch  130  at 20 psi in a third gear modulation chart. 
         [0064]    The CPU continues to modulate/increase the pressure in the hydraulic clutch  130 . If the ERPM drops below a predetermined downshift speed # 3  value in condition  344 , the CPU moves the transmission back to the second gear slip state  327  and starts with 40 psi in the second gear modulation chart. Otherwise, the CPU in third gear slip state  343  continues to increase the pressure in the third hydraulic clutch  130  until there is zero clutch slip in condition  345 . The CPU then sets the pressure in hydraulic clutch  130  to 140 psi and moves into third gear locked state  346 . The second gear pressure is also reduced down to 40 psi to provide a quick response to any transmission transitions back to second gear slip state  327 . 
         [0065]    The transmission system stays in the third gear locked state  346  unless the ERPM falls below the downshift speed # 3  value in condition  347 . In this case, the transmission system moves back into second gear slip state  327 , the pressure in the hydraulic clutch  130  is modulated down to 0 psi, and the CPU starts the second clutch  118  at 40 psi in the second gear modulation chart. 
         [0066]    The system described above provides relatively simple transitions between gears without requiring precise synchronized engagement and disengagement of different clutches during gear transitions. Clutches do not have to be fully disengaged during a gear transition therefore partially engaged or disengaged clutches will not create unnecessary heat and reduce the overall efficiency of the transmission system  14 . Additional gears and equivalent modulation states could be included in the transmission system  14 . The different down shift values, upshift values, gear modulation charts, and psi pressures can vary for different types of vehicles and different types of transmission systems. 
         [0067]      FIG. 7  is a flow diagram describing in more detail how the CPU  40  controls the hydraulic clutches when the vehicle  10  is located on an inclined grade. As mentioned above, the transmission system  14  can operate with any type of conventional drive axle with directional control. However, in one embodiment the transmission system  14  may operate in conjunction with the clutch system  142  shown in  FIG. 1 . 
         [0068]    A base braking torque is defined in a look up table contained in memory  48  ( FIG. 4 ) as the minimal braking torque. The base braking torque value may be determined by experimenting with the lowest value that prevents the vehicle  10  from rolling on a grade with a given slope. The purpose of the minimal fixed torque is to stop the vehicle  10  on flat terrain and prevent or limit rolling on a grade. 
         [0069]    When the vehicle  10  is stopped in operation  350 , the CPU  40  in operation  352  disengages the hydraulic clutches in the transmission system  14  and fully engages all the direction clutches in the clutch system  142  in the drive axle  34 . When the vehicle  10  is commanded to move forward or reverse in operation  354 , the CPU  40  slips the first hydraulic clutch  114  and keeps the selected clutch engaged in operation  356  and at the same time decreases and modulates the opposing travel direction clutches in the clutch system  142  to a minimal value to prevent the vehicle  10  from jerking in the selected direction. 
         [0070]    The vehicle  10  then starts moving. If the vehicle  10  starts moving in the opposite direction in operation  358 , the CPU  40  increases the engagement of the first hydraulic clutch  114  in operation  360 . If the vehicle  10  continues to move in the opposite direction in operation  362 , the CPU  40  further increases engagement of the first hydraulic clutch  114  in operation  360 . When the vehicle  10  starts moving in the selected direction and the acceleration pedal position is greater than a threshold value, the CPU  40  fully releases (neutralizes) the opposing direction clutch(es) in clutch system  142  in operation  366 . The CPU  40  in operation  368  fully engages the first hydraulic clutch  114  when the vehicle speed indicated by speed and direction signal  18  is greater than a calculated engine stall speed. 
         [0071]    Thus, control system shown above controls torque to prevent engine stall and clutch damage due to overheating. Torque control is accomplished by slipping the selected direction clutches in the clutch system  142 , or in the first hydraulic clutch  114 . The clutch pressure is derived from a calculated engine stall torque. 
         [0072]    The engine  12  will not be reduced below a minimum speed which maintains enough torque plus a pre-set safety margin to prevent stalling. If the clutch energy exceeds a limit, the torque capacity of the clutch is reduced by reducing clutch pressure or fully disengaging the clutch to prevent damage. Engine speed will be commanded by the CPU  40  to a minimum torque without stalling. 
         [0073]    If the clutch energy limit is exceeded, then slipping may alternate between the selected direction clutch(es) in clutch system  142  and the first hydraulic clutch  114 , while maintaining a constant transmitted torque. A software clutch energy estimator that monitors clutch heat can be implemented by CPU  40  according to oil temperature, clutch pressure, cooling rate, and slip rate measured via the CPU  40  and the sensors in  FIG. 4 . When the estimated clutch energy is reduced to an acceptable value, then clutch torque can be increased smoothly within thermal limits to fully re-engage normal driving torque and vehicle performance During subsequent engine braking, the highest third gear hydraulic clutch  130  can be engaged to connect the engine  12  with output shaft  126 . 
         [0074]    The system described above can use dedicated processor systems, micro controllers, programmable logic devices, or microprocessors that perform some or all of the operations. Some of the operations described above may be implemented in software and other operations may be implemented in hardware. 
         [0075]    For the sake of convenience, the operations are described as various interconnected functional blocks or distinct software modules. This is not necessary, however, and there may be cases where these functional blocks or modules are equivalently aggregated into a single logic device, program or operation with unclear boundaries. In any event, the functional blocks and software modules or features of the flexible interface can be implemented by themselves, or in combination with other operations in either hardware or software. 
         [0076]    Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention may be modified in arrangement and detail without departing from such principles. I/we claim all modifications and variation coming within the spirit and scope of the following claims.