Abstract:
A method and system for controlling a transmission in a work machine is provided. The output of a hydrostatic transmission having a source of pressurized fluid is combined with the output of a mechanical transmission having at least one engaged clutch. An operational speed of an engine that provides an input to both the hydrostatic transmission and the mechanical transmission is sensed. The at least one clutch of the mechanical transmission is disengaged and the displacement of the source of pressurized fluid is modified when the operational speed of the engine drops below a stall limit. The disengagement of the at least one clutch and the modification of the displacement of the source of pressurized fluid allow the operational speed of the engine to rise above the stall limit.

Description:
TECHNICAL FIELD  
         [0001]    The present invention is directed to a control system and method for a transmission. More particularly, the present invention is directed to a system and method for controlling a torque output of a transmission in a work machine.  
         BACKGROUND  
         [0002]    Work machines, such as, for example, wheel loaders, track loaders, bulldozers, and backhoes, typically use a transmission to translate the rotational speed of an engine shaft into a drive speed. These transmissions are typically operable to provide a series of gear ratios that translate the speed of the engine shaft into different drive speeds. The gear ratios usually include forward and reverse speeds that range from low to high to provide different powers and speeds for the work machine as different operating conditions are encountered.  
           [0003]    Some work machines are designed to work in low speed ranges and require precise speed control through the low speed ranges. To achieve this speed control, a split torque transmission may be used to convert the rotational speed of the engine shaft. This type of transmission combines the outputs of a hydrostatic transmission and a mechanical transmission to rotate a drive shaft and move the vehicle.  
           [0004]    A split torque transmission may be operated with one or both of the hydrostatic and mechanical transmissions. Typically, the work machine operates on the hydrostatic transmission alone when the machine is operating in the low speed range, such as, for example, when digging or loading operations are performed. The work machine will usually engage the mechanical transmission to supplement the hydrostatic transmission when higher speeds are required. For example, a wheel loader may operate on the hydrostatic transmission when moving at speeds up to two miles per hour. When moving at speeds above two miles per hour, the wheel loader will typically engage the mechanical transmission and operate on a combination of the hydrostatic and mechanical transmissions.  
           [0005]    As described in U.S. Pat. No. 5,682,315, a control system for a split torque transmission will rely on several indicators to determine when and how to adjust the transmission to achieve a desired speed. To determine the appropriate transmission settings, the control system may monitor the position of several operator inputs, such as, for example, the position of a speed pedal, a range lever, and a direction lever. In addition, the control system may monitor several operating conditions in the work machine, such as, for example, the engine speed, the mechanical transmission output speed, and the hydrostatic transmission output speed. By monitoring these indicators, the control system will be able to determine when and how to adjust the transmission to achieve the desired speed.  
           [0006]    This type of control system may not, however, account for unexpected operating conditions, such as, for example, a severe engine underspeed situation. An engine underspeed situation may occur, for example, when the work machine encounters a heavy load, such as a work pile, when moving at a significant ground speed. Encountering the heavy load will rapidly decrease the groundspeed of the work machine and the operational speed of the engine.  
           [0007]    If the transmission is not adjusted to account for the increased output load and to allow the engine to resume an acceptable operating speed, the engine may stall. If the drop in engine speed is rapid enough, the control system may not be able to adjust the transmission quickly enough to prevent the engine from stalling. If the engine stalls, the operator will be forced to restart the engine before resuming work, thereby causing an inconvenient interruption in the work process. This, of course, translates to a loss of machine efficiency.  
           [0008]    The transmission control system of the present invention solves one or more of the problems set forth above.  
         SUMMARY OF THE INVENTION  
         [0009]    One aspect of the present invention is directed to a method for controlling a transmission in a work machine. The output of a hydrostatic transmission having a source of pressurized fluid is combined with the output of a mechanical transmission having at least one engaged clutch. An operational speed of an engine is sensed. The engine provides an input to both the hydrostatic transmission and the mechanical transmission. At least one clutch of the mechanical transmission is disengaged and the displacement of the source of pressurized fluid is modified when the operational speed of the engine drops below a stall limit. The disengagement of the at least one clutch and the modification of the displacement of the source of pressurized fluid allow the operational speed of the engine to rise above the stall limit.  
           [0010]    In another aspect, the present invention is directed to a control system for a transmission that combines a mechanical transmission having at least one clutch and a hydrostatic transmission having a source of pressurized fluid. The control system includes a sensor configured to sense the operational speed of an engine that provides an input to the transmission. A control is configured to disengage at least one clutch and to modify the displacement of the source of pressurized fluid when the operational speed of the engine drops below a stall limit. The disengagement of at least one clutch and the modification of the displacement of the source of pressurized fluid allows the speed of the engine to increase above the stall limit.  
           [0011]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings:  
         [0013]    [0013]FIG. 1 is a schematic and diagrammatic illustration of a control system for a transmission in accordance with one exemplary embodiment of the present invention;  
         [0014]    [0014]FIG. 2 is a graphic illustration depicting the displacement of a pump as a function of machine ground speed for a transmission; and  
         [0015]    [0015]FIG. 3 is a flowchart illustrating a method of controlling a transmission in accordance with one exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]    Reference will now be made in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
         [0017]    An exemplary embodiment of a transmission  12  for a work machine  10  is diagrammatically illustrated in FIG. 1. Transmission  12  may be referred to as a split torque transmission. Alternatively, transmission  12  may be a continuously variable transmission with a hydrostatic pump, an electrical transmission, or another transmission known to those having ordinary skill in the art.  
         [0018]    As illustrated, work machine  10  includes an engine  14 . Engine  14  is operable to generate a torque through a rotation of an engine crankshaft  16 . Engine  14  typically includes a rated speed (e.g. rotational output speed of the crankshaft), which provides an indication of the standard operational speed of the engine. For example, an engine in a wheel loader may have a rated speed of approximately 1800 rpm.  
         [0019]    Engine crankshaft  16  may transmit the torque generated by engine  14  to both a hydrostatic transmission  17  and a mechanical transmission  18 . Although, it is envisioned that an electrical transmission may be used as an alternative to the hydrostatic transmission. An input gear  21  is connected to and rotates with engine crankshaft  16 . A hydrostatic input gear  42  engages input gear  21 . A rotation of input gear  21  results in a corresponding rotation of hydrostatic input gear  42 , which provides the driving input to hydrostatic transmission  17 .  
         [0020]    As illustrated in FIG. 1, hydrostatic transmission  17  includes a source of pressurized fluid  44  that may be operated to generate a flow of pressurized fluid. Source of pressurized fluid  44  may be, for example, a variable displacement pump or any other device readily apparent to one skilled in the art as having a variable displacement capability. Source of pressurized fluid  44  is driven by the rotational input of hydrostatic input gear  42 .  
         [0021]    Source of pressurized fluid  44  may generate a fluid flow that is variable in both direction and rate and include, for example, an actuation device  45  that controls the rate and direction of the generated fluid flow. For example, actuation device  45  may be a solenoid activated swash plate. Actuation of the swash plate in a first direction generates a first flow of pressurized fluid through a first fluid line  48 . Actuation of the swash plate in a second direction generates a second flow of pressurized fluid through a second fluid line  50 . The magnitude of movement of the swash plate controls the rate of the generated fluid flow. For example, a partial movement of the swash plate in the first direction generates a partial displacement of fluid in the first direction. A full movement of the swash plate in the first direction generates a maximum displacement of fluid flow in the first direction.  
         [0022]    First fluid line  48  and second fluid line  50  connect the source of pressurized fluid  44  with a fluid motor  46 . Fluid motor  46  is operable to rotate a motor output shaft  51  based on the rate and direction of the fluid flow generated by source of pressurized fluid  44 . For example, when the source of pressurized fluid  44  generates the first flow of fluid through first fluid line  48 , fluid motor  46  rotates motor shaft  51  in a first direction at a speed that corresponds to the flow rate of the first flow of fluid. When the source of pressurized fluid  44  generates the second flow of fluid through second fluid line  50 , fluid motor  46  rotates motor shaft  51  in a second, or opposite, direction at a speed that corresponds to the flow rate of the second flow of fluid.  
         [0023]    Motor shaft  51  drives a hydrostatic output gear  52  that is engaged with a coupling gear  54 . A clutch  56  may be engaged to fix coupling gear  54  to an output shaft  58 . The engagement of clutch  56  creates a linkage between motor shaft  51  and output shaft  58 . Thus, when clutch  56  is engaged and source of pressurized fluid  44  is activated to provide a flow of pressurized fluid to fluid motor  46 , the resulting rotation of motor shaft  51  will cause a corresponding rotation of output shaft  58 . By reversing the direction of fluid flow to fluid motor  46 , the direction of rotation of output shaft  58  may also be reversed. The rotational speed of output shaft  58  may be varied by changing the displacement of source of pressurized fluid  44  to change the flow rate of fluid to fluid motor  46 . In this manner, hydrostatic transmission  17  may be operated to provide a desired speed and direction of rotation of output shaft  58 .  
         [0024]    As illustrated in FIG. 1, mechanical transmission  18  may include a first gear assembly  19  to provide forward speeds and a second gear assembly  20  to provide reverse speeds. First gear assembly  19  may include a first clutch  24  and a forward drive gear  26  that is engaged with a mechanical output gear  32 . Engagement of first clutch  24  drivingly connects forward drive gear  26  with engine crankshaft  16 . When first clutch  24  is engaged, the rotational input of engine crankshaft  16  is translated to a corresponding rotation of mechanical output gear  32 .  
         [0025]    Second gear assembly  20  may include a reverse input gear  22  that is engaged with input gear  21 , a second clutch  28 , and a reverse drive gear  30  that is engaged with mechanical output gear  32 . Engagement of second clutch  28  drivingly connects reverse drive gear  30  with engine crankshaft  16 . When second clutch  28  is engaged, the rotational input of engine crankshaft  16  is translated to a corresponding rotation of mechanical output gear  32 . The inclusion of reverse input gear  22  causes a reverse rotation of mechanical output gear  32  in response to a rotation of engine crankshaft  16 . Thus, mechanical output gear  32  will rotate in one direction when first clutch  24  is engaged and in the opposite direction when second clutch  28  is engaged.  
         [0026]    As also illustrated in FIG. 1, a summing gear assembly  34  is provided to selectively combine the outputs of hydrostatic transmission  17  and mechanical transmission  12 . Summing gear assembly  34  of this exemplary embodiment includes a ring gear  36 , a planet gear  38 , and a sun gear  40  that is connected to output shaft  58 . Output shaft  58  is connected to a power train  62  that may be used to move work machine  10 .  
         [0027]    In summing gear assembly  34 , the relative rotational speeds and directions of ring gear  36  and planet gear  38  control the resulting rotational speed and direction of sun gear  40  and, therefore, output shaft  58 . For example, if ring gear  36  is held stationary, the rotational speed and direction of planet gear  38  will determine the speed and direction of rotation of sun gear  40 . If ring gear  36  is rotated in the same direction as planet gear  38 , the rotational speed of sun gear  40  may be decreased accordingly. If ring gear  36  is rotated in the opposite direction of planet gear  38 , the rotational speed of sun gear  40  may be increased accordingly.  
         [0028]    The rotational speed and direction of ring gear  36  is controlled by hydrostatic transmission  17  through the engagement of coupling gear  54  and hydrostatic output gear  52 . The rotational speed and direction of planet gear  38  is controlled by mechanical transmission  18  through the connection with mechanical output gear  32 . Thus, by controlling the outputs of hydrostatic transmission  17  and mechanical transmission  18 , the rotational speed and direction of output shaft  58  may be controlled to thereby control the speed of work machine  10 .  
         [0029]    As illustrated in FIG. 1, transmission  12  may include a control system  64 . Control system  64  may include a computer, which has all the components required to run an application, such as, for example, a memory  66 , a secondary storage device, a processor, such as a central processing unit, and an input device. One skilled in the art will appreciate that this computer can contain additional or different components. Furthermore, although aspects of the present invention are described as being stored in memory, one skilled in the art will appreciate that these aspects can also be stored on or read from other types of computer program products or computer-readable media, such as computer chips and secondary storage devices, including hard disks, floppy disks, CD-ROM, or other forms of RAM or ROM.  
         [0030]    Control system  64  may be configured to govern the operation of transmission  12 . Control system  64  may govern transmission  12  by transmitting activation signals to activation device  45  and by transmitting appropriate signals to clutches  24 ,  28 , and  56 . In this manner, control system  64  may activate source of pressurized fluid  44  to provide a desired fluid flow to fluid motor  46  and may engage or disengage clutches  24 ,  28 , and  56  to control mechanical transmission  18 .  
         [0031]    As also shown in FIG. 1, a series of sensors may be disposed in transmission  12  to provide information to control system  64  regarding the operational speed of engine  12 . For example, an engine speed sensor  68  may be disposed adjacent engine crankshaft  16  to provide information regarding the rotational speed of engine crankshaft  16 . In addition, an output speed sensor  60  may be disposed adjacent output shaft  58  to provide information regarding the rotational speed of output shaft  58 . Other speed sensors may be positioned within transmission  12  to monitor the rotational speed of other shafts. These sensors may be any type of device readily apparent to one skilled in the art as capable of sensing rotational speed, such as, for example, potentiometers, thermistors and/or magnetic speed pickups or other conventional electrical transducers.  
         [0032]    In an exemplary embodiment, control system  64  may be used to vary the displacement of source of pressurized fluid  44  to achieve a particular ground speed of the machine (FIG. 2). Referring to FIG. 2, a plot  70  illustrates an exemplary rate and direction of fluid displacement of source of pressurized fluid  44  to achieve a first and a second speed ranges  72 ,  74  of transmission  12 . As an example, first speed range  72  may provide speeds from 0 to 3.2 km/h (2 mph) and second speed range  74  may provide speeds from 3.2 km/h (2 mph) to 12.9 km/h (8 mph). Alternative embodiments of transmission  12  may provide for additional speed ranges and/or for smaller or larger speed ranges. For example, a third speed range  76  may be provided to achieve speeds above 12.9 km/h (8 mph).  
         [0033]    When an operator requests that exemplary work machine  10  be accelerated through first speed range  72  and second speed range  74 , control system  64  will engage clutch  56  and disengage first and second clutches  24 ,  28  to drive work machine  10  via hydrostatic transmission  17  (FIG. 1) alone. It may be seen that the pump displacement corresponding to the first speed range  72  is from 0 to a maximum negative displacement  73 .  
         [0034]    When source of pressurized fluid  44  reaches a peak negative displacement  73 , control system  64  controls mechanical transmission  18  by increasing the speed of work machine  10  pursuant to second speed range  74 . Accordingly, control system  64  disengages clutch  56  and engages first clutch  24 . This combines the outputs of hydrostatic transmission  17  and mechanical transmission  18  to increase the ground speed of work machine  10 . At the beginning of second speed range  74 , ring gear  36  and planet gear  38  are rotating in the same direction. Accordingly, in the second speed range  74 , the speed of work machine  10  is increased by coinciding with the decrease of the displacement of source of pressurized fluid  44  to thereby decrease the rotational speed of ring gear  36 .  
         [0035]    Control system  64  continues to increase the speed of work machine  10  through second speed range  74  by reducing the displacement of source of pressurized fluid  44  to zero, and thereafter, changing the direction of displacement of source of pressurized fluid  44 . The change in direction of the displacement of source of pressurized fluid  44  will change the direction of rotation of ring gear  36  so that ring gear  36  is rotating in the opposite direction of planet gear  38 . When ring gear  36  is rotating in a direction opposite to planet gear  38 , an increase in the magnitude of the difference in rotational speeds between ring gear  36  and planet gear  38  will result in an increase in the rotational speed of output shaft  58 . Thus, by increasing the displacement of source of pressurized fluid  44  when ring gear  36  and planet gear  38  are rotating in opposite directions, the speed of work machine  10  may be increased. The end of second speed range  74  is reached when source of pressurized fluid  44  reaches a peak positive displacement  75 .  
         [0036]    When an operator requests that the speed of work machine  10  be decreased, control system  64  will reverse the events described above. For example, if work machine  10  is operating at the higher end of second speed range  74 , control system  64  will decrease the magnitude of displacement of source of pressurized fluid  44  to decrease the rotational speed difference between ring gear  36  and planet gear  38 , to thereby decrease the rotational speed of output shaft  58 . Control system  64  will continue to decrease the displacement of source of pressurized fluid  44  to zero and then reverse the direction of the displacement. The reversal of direction of displacement will cause ring gear  36  to begin rotating in the same direction as planet gear  48 , to further reduce the rotational speed difference and the rotational speed of output shaft  58 .  
         [0037]    When source of pressurized fluid  44  reaches its maximum displacement  73 , control system  64  will disengage mechanical transmission  18  to return to first speed range  72 . Accordingly, control system  64  will disengage first clutch  24  and engage clutch  56 . If desired, the speed of work machine  10  may then be reduced to zero by reducing the displacement of source of pressurized fluid  44  to zero.  
         [0038]    Under some circumstances, work machine  10  may encounter an underspeed situation. This may occur, for example, when work machine  10  is climbing an incline or when work machine  10  encounters a heavy load, such as a wheel loader engaging a work pile. In these situations, the torque generated by engine  14  may not be sufficient to maintain the ground speed of work machine  10 , and the speed of engine  14  will decrease as the work machine  10  slows. Unless transmission  12  is adjusted or the force exerted on work machine  10  decreases, the engine speed will continue to decrease until engine  14  stalls.  
         [0039]    The flowchart of FIG. 3 illustrates an exemplary method ( 90 ) for handling underspeed situations in a work machine  10 . Control system  64  (FIG. 1) monitors the rotational speed of engine  14  (Step  92 ). If work machine  10  is traveling at first speed  78  (referring to FIG. 2) and encounters a load that causes a decrease in engine speed, control system  64  detects the change in engine speed (Step  94 ).  
         [0040]    Control system  64  determines if the engine speed has dropped below a stall limit (Step  96 ). The stall limit is a threshold value that indicates engine  14  is nearing a stall condition. A machine owner, manufacturer, or operator may determine the stall limit for a particular piece of equipment. The stall limit may depend upon, inter alia, the particular operating characteristics of engine  14  and may be calculated as a percentage of the rated speed of engine  14 . For example, a wheel loader engine having a rated speed of approximately 1800 rpm may have a stall limit that is approximately 75% of the rated limit, or approximately 1400 rpm.  
         [0041]    If the current engine speed is within an acceptable range of the rated speed, control system  64  will engage the standard downshift procedure described above (Step  98 ). In this situation, which may be typical of a work machine climbing an incline, the standard downshift process of transmission  12  will respond to the reduced engine speed by modifying the transmission settings to increase the engine speed and prevent engine  14  from stalling.  
         [0042]    If, however, the current engine speed drops below the stall limit, the standard downshift process may not change transmission  12  quickly enough to prevent engine  14  from stalling. In this situation, which may occur when a work machine engages a work pile, control system  64  may override the standard downshift process and follow a forced downshift process.  
         [0043]    In the forced downshift process, control system  64  adjusts transmission  12  to “jump” to a new speed range, instead of following the standard downshift. In the disclosed embodiment, control system  64  disengages mechanical transmission  18  by disengaging engaged clutches  24  and  28  and engaging clutch  56  (Step  100 ). Control system  64  may also modify the displacement of source of pressurized fluid  44  by adjusting the flow rate and/or direction of the generated fluid flow (Step  102 ) In this manner, control system  64  adjusts the settings of transmission  12  to “jump” from, for example, first speed  78  to second speed  80  (referring to FIG. 2) instead of following the standard downshift process. This jump will quickly reduce the torque requirements of engine  14  and may prevent engine  14  from stalling due to an underspeed situation.  
         [0044]    In certain work machines, such as, for example, a wheel loader, the forced downshift process may only be encountered when the wheel loader engages a work pile. Accordingly, control system  64  may enable additional functions after the forced downshift process. For example, control system  64  may lock mechanical transmission  18  to prevent mechanical transmission  18  from being re-engaged until work machine  10  is operated in the reverse direction. This will prevent a “hunting” situation where transmission  12  moves between speed ranges to find the appropriate configuration. In addition, control system  64  may assume that the operator has engaged a work pile with the intention of digging a load of earth. Accordingly, control system  64  may activate an automatic dig function after the forced downshift process to improve the efficiency of the operation.  
         [0045]    Industrial Applicability  
         [0046]    As will be apparent from the foregoing description, the present invention provides a control system for a transmission that may prevent the engine of a work machine from stalling. The control system monitors the operational speed of the engine and determines when the engine is nearing a stall point. When the engine is in danger of stalling, the control system jumps the transmission to a new speed setting that may prevent the stall from occurring.  
         [0047]    The control system of the present invention may be implemented into any work machine that utilizes a split torque transmission or a continuously variable transmission with a hydrostatic pump to convert the rotational speed of an engine into a drive speed for the work machine. The control system of the present invention may be implemented into an existing work machine without any major modifications or the addition of expensive hardware. The control system of the present invention may improve the overall efficiency of a work machine by preventing inconvenient and work-interrupting engine stalls.  
         [0048]    It will be apparent to those skilled in the art that various modifications and variations can be made in the control system of the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and their equivalents.