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 torques.

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 all 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 includes two control loops; one for the right wheel and the other for the left front wheel. Thus, there is a tradeoff between acceleration and lead acceptance smoothness on the one hand and steering and lateral traction on the other hand. The average speed at 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; and 
           [0009]      FIG. 4  illustrates an exemplary flowchart for average torque determination in the exemplary drive system of  FIGS. 2 and 3 ; 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    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. 
         [0011]      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. 
         [0012]      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 or throttle  37  having a feature of detecting and communicating pedal positions may be in communication with the ECU  36  for throttling an engine  35 , 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 . As illustrated, a conventional articulation angle sensor  45  may be available for detecting the articulation angle between the front and rear portions  20 ,  30 . 
         [0013]    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 awash plate  121   a ; a left hydraulic motor  123 ; a left motor solenoid  124  for positioning a left motor awash 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 . 
         [0014]    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 awash plate  131   a , a right hydraulic motor  133 ; a right motor solenoid  134  for positioning a right motor swath 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 . 
         [0015]    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. 
         [0016]      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 illustrated, a speed sensor  34   a  may remain. As with the left and right hydrostatic transmissions  120 ,  130  at the front of the work vehicle the transmission controller  110  may control the awash 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 . The ECU  36  may determine current engine torque (CET) as a function of current fuel usage rate, current engine speed and current operating load), i.e., f(current fuel usage rate, current engine speed, current operating load) using a conventional engine performance formula or table, all of which are detected by the ECU  36  via conventional means. 
         [0017]      FIG. 4  illustrates an exemplary flowchart  200  for determining average and differential front wheel torque control for the exemplary drive systems  100 ,  100 ′ of  FIGS. 2 and 3  and, with respect to  FIG. 3  detailing the actions of the transmission controller  110  with respect to the engine controller unit (ECU)  36 , the rear hydrostatic transmission  60 , the rear pressure sensor  65 , the rear hydraulic pump solenoid  63 , the left pump solenoid  122 , the left pressure sensor  125 , the left front wheel angle sensor  127 , the right pump solenoid  132 , the right pressure sensor  135 , 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 . With respect to  FIG. 2 , the mechanical transmission  34  and speed sensor  34   a  replace the rear hydrostatic transmission  60  and its associated parts. 
         [0018]    As illustrated in  FIG. 4 , torque control begins when an exemplary d rive system  100  is started at  201 , At step  202  the rear drive torque (RDT) may be estimated, via alternative  1 , by subtracting known parasitic engine loads such as, for example, fans, etc., from the CET. At step  203  the transmission controller  110  may determine a target front wheel torque (TFT) as a percentage of the RDT. 
         [0019]    As illustrated in  FIG. 4 , at step  204 , the transmission controller  110  may estimate front wheel torques using pressure and displacement measurements at each of the left and right transmissions  120 ,  130 . The pressure measurements may be taken from the left and right pressure sensors  125 ,  135  and the displacement measurements may be determined from displacement adjustments to the left and right hydrostatic transmissions  20 ,  30  from the transmission controller  110 . Total front torque measured/feedback (TFTM) may then be determined, at step  205 , by adding the torques calculated at the left and right front wheels  22 ,  32 . At step  206 , torque error (TE) may be determined as a difference between TFT and TFTM while a correction command (CC) may be determined as a function of TE via a conventional formula or lookup table. 
         [0020]    At step  207 , the nominal can command (NMC), i.e., the signal for adjusting displacement, may be determined as a function of rear speed, operator inputs, and vehicle geometry such as, for example articulation angle, wheel turning angle, etc., i.e., nominal mean command=f(rear speed, operator inputs, vehicle geometry). The mean command (MC) for each of the front wheels  20 ,  30  may then, at step  208 , be determined as the sum of NMC and CC, i.e., MC=NMC+CC. 
         [0021]    At step  209 , the differential torque measured/feedback (DTM) may be determined by taking the difference between the torques measured at the left and right front wheels  22 ,  32 , i.e. the difference between the left front wheel torque (LWT) and the right front wheel torque (RWT). At step  210 , the differential torque reference/target (DTT) may be determined via operator inputs and vehicle geometry. At step  211 , differential torque error (DTE) may be calculated as DTT−DTM and a differential torque correction command (DCC) may be determined as a function of DTE via a conventional formula or lookup table. 
         [0022]    At step  212 , a nominal differential torque command (NDTC) may be determined as a function of rear speed, operator inputs and vehicle geometry. At step  213  the differential command (DC) may be calculated as the sum of NDTC and DCC. 
         [0023]    Finally, at step  214  the left displacement command for the left hydrostatic transmission  20  may be determined as MC+DC and the right displacement command for the right hydrostatic transmission  30  may be determined as MC−DC. 
         [0024]    Please note that step  202  RDT may be estimated via alternative  2  which is by multiplying current displacements and measured pressures at the rear hydrostatic transmission  60 , an estimation which is relevant to the alternative exemplary drive system of  FIG. 3 . 
         [0025]    Also, note that displacement determinations may be made with zero (“0”) or non-existent values for front wheel turning angles and articulation angles. Thus, although turning and articulation angle sensors  127 ,  45  are preferred for greater estimate accuracy, the torque control system may function without a turning angle sensor  127  or an articulation angle sensor  45 . 
         [0026]    Having described the exemplary embodiments above, it will become apparent that various modifications can be made without departing from the scope of the mention as defined in the accompanying claims.