Abstract:
A front wheel power system which may enable independent control of power to each wheel as well as yield direct control over average and differential front wheel speeds.

Description:
FIELD OF THE INVENTION 
       [0001]    The disclosure relates to front wheel drives and, specifically, the control of front wheel drives on work vehicles such as motor graders. 
       BACKGROUND 
       [0002]    Conventional work vehicles such as, for example, motor graders, include ail wheel drive capabilities with at least one motor for driving the front wheels and a transmission for transferring power from the engine or, perhaps an electric motor, to the rear wheels. During turns of the vehicle, the front wheels may travel in arcuate or circular paths and may, for the sake of vehicular efficiency as well as operating experience, be required to rotate at greater speeds than the rear wheels when the front wheels are of a diameter equal to that of the rear wheels as the front wheels may travel greater distances. Also, a front wheel on the outer radius of the turn (an outer wheel) may be required to rotate at a greater speed than that of a front wheel on the inner radius of the turn (an inner wheel) as the path of the outer wheel has a greater radius than the path the inner wheel travels. 
         [0003]    Conventional work vehicles address these challenges with open differentials and variations of limited differential including: limited slip differentials; and differentials that are self locking, locked manually or locked via software at threshold differences between actual speeds and predicted speeds of left and right wheels (detection of slippage), etc. In efforts to address the obvious challenges presented by the arrangements noted above, some solutions monitor and independently control the rotational speeds of each of the front wheels at all times based on turning angles of the front wheels and, in the case of vehicles such as motor graders, the articulation angles of the vehicle. The latter solutions have various drawbacks that demand compromises. 
       SUMMARY 
       [0004]    The inventors have recognized that the mere independent control of the speeds of each of the front wheels may not provide direct control over independent response characteristics for average and differential speeds. Such an approach wheel. Thus, there is a tradeoff between acceleration and load acceptance smoothness on the one hand and steering and lateral traction on the other hand. The average speed of the front wheels, which is important for front wheel aggressiveness and slipping, is not controlled; it is a side effect of the loading and the control performance of the two loops. The differential speed of the two front wheels, which is important for steering performance, is not directly controlled; it is a side effect of the loading and the performance of the two speed loops. 
         [0005]    The invention may directly address the challenges presented above by directly and independently controlling the average and differential response characteristics of the front wheels. The invention may also control response characteristics of the rear wheels to improve overall efficiency and operating experience. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  illustrates an exemplary work vehicle utilizing the invention; 
           [0007]      FIG. 2  illustrates a schematic of a first exemplary embodiment of the wheel drive control system to be utilized in the exemplary work vehicle of  FIG. 1 ; 
           [0008]      FIG. 3  illustrates a schematic of a second exemplary embodiment of the wheel drive control system; 
           [0009]      FIG. 4   a  illustrates an exemplary flowchart for average speed determination in the exemplary drive system of  FIG. 2 ; 
           [0010]      FIG. 4   b  illustrates an exemplary flowchart for differential speed determination in the exemplary drive system of  FIG. 2 ; and 
           [0011]      FIG. 4   c  illustrates an exemplary flowchart for determination of right and left front wheel efforts. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    A description of exemplary embodiments of the invention will now be detailed. The same reference numbers will be used throughout the description as occasion allows. 
         [0013]      FIG. 1  illustrates an exemplary work vehicle, a motor grader  1 , which could make use of the invention. The motor grader  1  of  FIG. 1  may include: a cab  10  having a steering device  11  and a seat  12 ; a front portion  20  having a front frame  20   a,  a powered left front wheel  21 , a powered right front wheel  22 ; a rear portion  30  including a rear frame  30   a,  tandem devices  31 ; rear wheels  32 ,  33 ; and an articulation mechanism  40  including an articulation joint  41  and an articulation cylinder  42  for angular adjustments between the front and rear portions  20 ,  30 . Also included may be a tandem device  31  from which the rear wheels  32  receive motive power. The motor grader  1  may also include a work tool  50  for moving earth as the work vehicle  1  traverses the ground. 
         [0014]      FIG. 2  represents a schematic of a first exemplary embodiment of the wheel drive control system  100  for the left and right front wheels  21 ,  22  and the rear wheels  32 ,  33  of the motor grader of  FIG. 1 . As illustrated, the drive system  100  may, among other things, include: tandem devices  31  through which the rear wheels  32 ,  33  may receive motive power; a transmission  34 ; a transmission controller  110  which may be in communication with, and operatively connected to: the transmission  34 ; a left hydrostatic transmission  120 ; and a right hydrostatic transmission  130 . The transmission controller  110  may also be in communication with: an engine controller unit (ECU)  36 ; a left front wheel speed sensor  126 ; a left front wheel angle sensor  127 ; a right front wheel speed sensor  136 ; a right front wheel angle sensor  137 ; and a rear speed sensor  34   a.  An acceleration pedal  37  having a feature of detecting and communicating pedal positions may be in communication with the ECU  36 . As illustrated in  FIG. 2 , a vehicle speed sensor such as, for example, radar detector  160  may also be available and in communication with the transmission controller  110 . 
         [0015]    As illustrated, the left hydrostatic transmission  120  may include: a left hydraulic pump  121  with variable displacement; a left pump solenoid  122  to position a left pump swash plate  121   a;  a left hydraulic motor  123 ; a left motor solenoid  124  for positioning a left motor swash plate  123   a;  and a left pressure sensor  125  for sensing a pressure difference between the left hydraulic pump  121  and the left hydraulic motor  123 . The transmission controller  110  is in communication with the left pressure sensor  125  and operably connected to the left pump solenoid  122  and the left motor solenoid  124 . 
         [0016]    As with the left hydrostatic transmission  120 , the right hydrostatic transmission  130  may include: a right hydraulic pump  131  with variable displacement; a right pump solenoid  132  to position a right pump swash plate  131   a ; a right hydraulic motor  133 ; a right motor solenoid  134  for positioning a right motor swash plate  133   a;  and a right pressure sensor  135  for sensing a pressure difference between the right hydraulic pump  131  and the right hydraulic motor  133 . The transmission controller  110  is in communication with the right pressure sensor  135  and operably connected to the right pump solenoid  132  and the right motor solenoid  134 . 
         [0017]    As illustrated, the left and right hydrostatic transmissions  120 ,  130  may be mechanically connected to the engine  35 . They may also be mechanically connected to left and right front wheels  20 ,  30  respectively. 
         [0018]      FIG. 3  illustrates a schematic of a second exemplary embodiment of the wheel drive control system  100 ′. The differences between the first and second exemplary embodiments of the invention  100 ,  100 ′ may be attributed to rear transmission differences. The second exemplary embodiment of the wheel drive control system  100 ′ employs a rear hydrostatic transmission  60  in the stead of the geared transmission  34  of the first exemplary embodiment of the wheel drive control system  100 . As with the left and right hydrostatic transmissions  120 , 130  at the front of the work vehicle  1 , the transmission controller  110  may control the swash plates  61   a ,  62   a  of the respective pump and motor  61 ,  62  via operable connections to the respective pump and motor solenoids  63 ,  64 . Swash plate displacement may determine the average speed of the rear wheels  32 ,  33 . 
         [0019]      FIGS. 4   a,    4   b  and  4   c  illustrate an exemplary flowchart  200  for determining average and differential front wheel speed control efforts for the exemplary drive systems  100 ,  100 ′ of  FIGS. 2 and 3  and detailing the actions of the transmission controller  110  with respect to the transmissions  34 ,  60 , the rear speed sensors  34   a ,  66 , the left front wheel speed sensor  126 , the left front wheel angle sensor  127 , the right front wheel speed sensor  136 , the right front wheel angle sensor  137 , the articulation angle sensor  43 , the operator input device  140  and, possibly, a vehicle speed detector separate from the power train of the motor grader  1  such as, for example, the radar speed detector  160 . 
         [0020]    As illustrated in  FIG. 4   a,  the average front wheel speed control effort (ASCE) may be viewed as a function of nominal and feedback efforts. As illustrated, the drive system  100  is activated when the ignition is on and front wheel assist is turned on at step  201 . At step  202 , the controller  110  may determine an average primary drive speed as a function of detected rear speed via a rotational speed detector (such as, for example, rear speed detector  34   a,  rear speed detector  66  or the like); a detected vehicle ground speed via radar detector  160 ; or an anticipated speed via cross referencing rear speeds with displacements at the pump  61  and the motor  62 . The average front wheel speed target (ATS) may then be calculated, at step  203 , based upon primary drive average speed and the vehicle model (i.e., for example, a detected articulation angle, a detected steering angle, a length between the articulation joint  41  and the front wheels  21 ,  22  and a length between the articulation joint  41  and the rear wheels  32 ,  33 ). At step  204 , the average front wheel speed target (ATS) from step  203  may then be used to calculate a nominal (or feed forward) average front wheel speed control effort (NAE). In exemplary embodiments, the front wheels  21 ,  22  are driven by hydrostatic transmissions  120 ,  130  respectively. In such a case effort may be considered as a function of displacement. Thus, the NAE may be determined via the use of a lookup table cross referencing front wheel speeds with displacements for swash plates  121   a  and  131   a  or via equivalent formulae. Note that a displacement, especially a change in displacement, at hydrostatic transmission  60  may, under some circumstances, be considered to be an acceleration as the wheels  32 ,  33  may require a finite time to respond to the displacement with a speed adjustment. 
         [0021]    As illustrated, at step  206 , the average front wheel speed (AMS) may be determined from the speeds of the front wheels  21 ,  22  which may be detected by the left and right speed sensors  126 ,  136  at step  205 . The average front wheel speed error (ASE) may then be calculated at step  207  as a function of average front wheel speed as calculated at step  206  and front wheel average speed target as calculated at step  205  (e.g., ATS minus AMS). At step  208 , an average front wheel speed feedback effort (AFE) may be calculated as a function of ASE. At step  200  the average speed control effort may be calculated as the nominal feed forward front wheel speed effort plus the front wheel speed feedback effort (e.g., NAE plus AFE). 
         [0022]    Illustrated in  FIG. 4   b.  is a similar pattern used by the controller  110  to determine the associated differential front wheel speed control effort (DSCE). At step  210 , the controller  110  may determine differential front wheel speed (DFS) as detected right front wheel speed minus detected left front wheel speed. At step  211 , the controller  110  may calculate a target differential front wheel speed TDS as a function of average primary drive speed, and a vehicle model which may include: positions of the rear wheels  32 ,  33  with respect to the articulation joint  41 ; positions of the front wheels  21 ,  22  with respect to the articulation joint  41 ; the diameters of the front wheels  21 ,  22 ; the diameters of the rear wheels  32  , 33 ; the detected articulation angle Aa; and a turning angle Ta of the front wheels  21 ,  22 . The average primary drive speed may, among other things, be based upon a detected rotational speed via the rear speed sensor  34   a  or the detected ground speed of the vehicle  1  via a speed detector independent of rotational speeds such as, for example, the radar speed detector  160 . If the vehicle has a hydrostatic transmission  60 , the average primary drive speed may be determined with detected displacements at the hydrostatic transmission  60  via a lookup table or formula effectively cross referencing displacements and average primary drive speeds. At step  212 , the controller  110  may calculate the front wheel differential speed error (DSE) as a function of the detected differential front wheel speed (DFS) and the target differential front wheel speed (TDS), i.e., (DSE=TDS−DFS, and DFE=f(DSE)). At step  213 , the DFE is determined by cross referencing the DSE value with displacements in an appropriate lookup table or a suitable formula. At step  214 , the predicted or nominal front wheel differential speed feed forward effort (NDE) may be determined by using the TDS to find a corresponding displacement via a lookup table or formula cross referencing displacements and wheel speeds. Finally, at step  215 , the differential speed control effort (DSCE) may be calculated as the sum of the nominal differential speed feed forward effort and the differential speed feedback effort, i.e., DSCE=NDE+DFE. 
         [0023]    As illustrated in  FIG. 4   c,  at step  220  the controller  110  may determine right front wheel speed control effort (RSCE) as a function of average front wheel speed control effort (ASCE) and DSCE (e.g., RSCE=ASCE+DSCE). At step  221 , the left front wheel speed control effort (LSCE) may also be determined as a function of ASCE and DSCE (e.g., LSCE=ASCE−DSCE). The controller  110  may then send appropriate commands to control the speeds of the front wheels  21 ,  22 , i.e., signals controlling swash plate positions for the hydraulic pumps  121 ,  131  and motors  123 ,  133  of front wheels  21 ,  22 , and return to step  201 . 
         [0024]    An operator input device  170  may be used to communicate aggressiveness settings, i.e., settings of front wheel efforts and front wheel target speeds as a percentage or multiple of rear speeds detected via rear speed detector  34   a  or vehicle speed via, for example, the radar speed detector  160  and, thus control of aggressiveness by the transmission controller  110 . 
         [0025]    The exemplary drive system  100  may also allow control over torque windup by monitoring torque values at the left and right front wheels  21 ,  22 . In hydrostatic drives, hydrostatic or hydraulic pressure may be considered as proportional to torque. The transmission controller  110  may monitor torque values by monitoring the pressure signals from the left and right pressure sensors  125 ,  135  and determining the respective torques via an appropriate equation or a lookup table. The transmission controller  110  may then control windup by controlling the left and right hydrostatic transmissions  120 ,  130  such that the differences between the calculated left and right torques stay within a predetermined range. 
         [0026]    Having described the exemplary embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.