Abstract:
A method of preemptively applying torque to a secondary axle of an all wheel drive vehicle is provided. A determination is made of a preemptive torque value based at least upon throttle rate. A determination is made if a minimum throttle rate has been met. If the minimum throttle rate is met, a controller preemptively applies preemptive torque value to the secondary axle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/779,937, filed Mar. 7, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for controlling torque transferred by an engine to a secondary axle of an all wheel drive (AWD) vehicle and a vehicle so controlled. 
       BACKGROUND OF THE INVENTION 
       [0003]    All wheel drive vehicles have a primary axle and a secondary axle. For purposes of fuel consumption, during normal vehicle operation the primary axle is typically exclusively powered by the engine. For improved vehicle handling purposes, certain vehicle operating conditions will cause torque to be delivered through a coupling to a secondary axle. Usually the amount of torque delivered to the secondary axle is adjustable and is controlled by a controller. The conditions which cause torque to be delivered to the secondary axle can include loss of traction due to poor road conditions, an apportioning of torque to the secondary axle for better handling due to the speed of the vehicle, a loss of traction of tire wheels on the primary axle due to vehicle acceleration. 
         [0004]    Typically when the control system delivers torque to the secondary axle due to tractional losses during vehicle acceleration, the torsional engagement of the secondary axle occurs only after sensors on the primary axle wheels notice a slip condition. Accordingly, there is a slight delay before torque is transferred to the secondary axle to alleviate a primary axle slip condition. It is desirable to provide an AWD system wherein the aforementioned delay can be materially reduced or eliminated. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides an AWD system wherein the activation delay can be materially reduced or eliminated when the vehicle is accelerated into a potential primary axle slip condition. 
         [0006]    Other features of the present invention will be more apparent to those skilled in the art as the invention is further described in the accompanying drawings and detail description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is schematic diagram of a motorized vehicle in accordance with an embodiment of the present invention. 
           [0008]      FIG. 2  is a schematic flowchart of the logic for the timer of the control system of the present invention. 
           [0009]      FIG. 3  is a schematic flowchart of the preempt torque reset logic of the present invention. 
           [0010]      FIG. 4  is a flowchart of the preempt torque set logic according to the present Invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]    Referring to  FIG. 1 , a vehicle implementing a preemptive torque control system is generally shown at  10 . The vehicle  10  has an engine  12  which is operably connected to a front axle  14  and a rear axle  16 . The vehicle  10  is an all-wheel drive (AWD) vehicle, wherein the engine  12  transfers torque to both the front axle  14  and rear axle  16 . In an AWD vehicle the engine  12  provides torque to a primary axle or front axle  14  and a secondary axle or rear axle  16 . However, it should be appreciated that the primary axle can be the rear axle  16  and the secondary axle to be the front axle  14 . By way of explanation and not limitation, for description purposes below, the front axle  14  is the primary axle and the rear axle  16  is the secondary axle. 
         [0012]    Wheels  18  are placed at both ends of the front axle  14  and rear axle  16 . Thus, as torque is applied to the axles  14 ,  16  from the engine  12 , the axles  14 ,  16  rotate which causes the wheels  18  to rotate and allows the vehicle  10  to move. A coupling  20  is placed on a drive shaft  22  between the engine  12  and the rear axle  16  for operably connecting the engine  12  and the rear axle  16 . A control unit  24  is then used to control the amount of torque applied to the rear axle  16  through the coupling  20 . Further, sensors  26  are placed on the vehicle  10  in order to determine vehicle operating conditions, with which the data from the sensors  26  is then transmitted to the control unit  24 . Thus, the sensors  26  are interfaced or connected to the control unit  24 . The control unit  24  determines the amount of torque applied by the engine  12  to the front axle  14  and rear axle  16 . The amount of torque transferred from the engine  12  to the axles  14 ,  16  is controlled by a throttle  27  which is typically operated via a pedal by a driver of the vehicle  10 . Thus, depending on the position of the throttle  27  and the rate of change of the position of the throttle  27 , otherwise known as the throttle  27  rate, the amount of torque transferred from the engine  12  to the axles  14 ,  16  is altered. 
         [0013]      FIG. 2  illustrates the control system that controls the timer  50  for the preemptive torque. The preempt torque timer logic circuit starts within the starting of the engine at the start function  7 . Start function  7  then goes to a decision function  30 . Decision function  30  inquires if the preemptive torque timer is to be reset (preempt reset=true). The default value for decision function  30  is false. If decision function  30  is false, then the logic will go to decision function  32 . Decision function  32  inquires if the timer  50  is running. The default position for decision function  32  is false; accordingly, the logic will proceed to decision block  34 . Decision function  34  inquires if the rate of activation of the throttle is beyond a predetermined value. The default position for decision function  34  is no. It should be noted that as used in this invention the throttle rate generally corresponds to the rate of angular travel of the accelerator pedal although they need not be a directly proportional relationship. Decision function  34  has a default value of being false and accordingly if the vehicle driver does not push the accelerator pedal down fast enough, logic will revert back to decision box  30 . If the vehicle driver has caused the vehicle throttle to be increased beyond a predetermined value, decision function  34  will render a yes response and will accordingly turn on the increment timer  50 . The increment timer  50  will then signal to the decision function  30  that it is running. Decision function  30  will only change to a true if the preemptive torque timer reset has started. The preemptive torque timer reset is controlled by the flowchart logic shown in  FIG. 3 . The default value for preemptive torque timer reset is false so accordingly as the timer  50  is turned on decision function  30  loop down to decision function  32 . Since the timer  50  is now running, decision function  32 &#39;s logic will proceed back to the timer  50  and this will cause a continuous loop to occur until the preemptive torque timer reset has been activated to true. When the preemptive torque timer reset has been activated to true, decision function  30  will yield a yes in logic going to decision function  40  causing the timer  50  to be reset to zero. Accordingly, the timer  50  will be started by the throttle being accelerated beyond a predetermined rate of acceleration and the timer  50  will only be reset to zero upon the satisfaction of one of the control logic functions provided in  FIG. 3 . 
         [0014]      FIG. 3  illustrates the logic used in the control system to reset the clock  50 . The logic of the control system will start upon the starting of the engine shown as the start function  7 . Typically both the timer logic and the reset preemptive torque time logic will be recalculated at five millisecond intervals. In the decision function  60  an inquire will be made if the preemptive torque timer set is false and if the preemptive torque timer set during the previous sample time period was true. The default value for decision function  60  will be negative and decision function  60  will only be true if the timer  50  has run past a predetermined time period as to be explained later. If the result of decision function  60  is negative then the logic goes to decision function  70 . Decision function  70  will only be positive if the vehicle is moving and the vehicle was not moving during the previous sample time period which as explained previously will typically be around five milliseconds. Decision function  70  will only be activated to a yes if the vehicle starts to move. If the vehicle is not moving then the logic from decision function  70  proceeds onto decision function  80 . Decision function  80  will only give a yes result if the vehicle was moving during the previous five milliseconds and the vehicle is now stopped. Accordingly, decision function  80  will only be activated when the vehicle comes to a stop. The logic then proceeds onto decision function  90 . Decision function  90  will be no unless three separate conditions are met. The first condition at decision function  90  is that the vehicle must be moving. The second condition of decision function  90  is that the throttle position is equal to zero. The third condition for decision function  90  is that the preemptive torque request as to be explained later is equal or less than zero. This condition will only occur when the vehicle operator has pulled their foot up on the accelerator to cause the accelerator pedal to come to the zero position. If the result of decision function  90  is negative the logic will go to the preemptive torque reset being false function  100 . The effective result of the preemptive reset being false is to allow the timer  50  to continue to run. If there is a yes result to decision functions  60 ,  70 ,  80  or  90 , the effect will be to cause the preemptive reset to be true thereby effect is that of turning off the timer function  110 . 
         [0015]      FIG. 4  is a flowchart of the logic for the preemptive torque set logic. Preemptive torque set is essentially the logic used to set the amount of torque which is delivered to the secondary axle via the coupling  20 . The preempt set logic is started with the starting of the engine and cycles approximately every five milliseconds. In decision function  120 , an inquiry is made is the vehicle moving. If the vehicle is not moving then the logic goes to the decision block  130 . At decision function  130  the first Inquiry is if the timer  50  is started. The second thing that must be true for the output of decision function  30  to be true is that the time counted by the timer  50  is below a first predetermined or standing hold time. The standing hold time will typically be a period long enough to be significant, but short enough such that several operator inputs to the vehicle operation cannot be made in a shorter period. Practice has shown a typical time period of about 0.5 seconds is a preferred value for the standing hold time. If a timer  50  is running in the standing hold time of 0.5 second has not expired, the logic will then go down to decision function  140  wherein the preempt set will be set as true. If the timer has exceeded the first predetermined time frame then the logic will proceed to decision function  150  setting the preemptive set to be false. A preempt set equal to false will cause the output preempt set in decision function  160  to be zero. A preempt set true precedes through function  140  will cause certain calculations to be made in the output preempt set function  160 . Going back to decision function  120  if a vehicle is moving the logic will then proceed to decision function  170 . A decision function  170  two Inquiries are made which must true. The first inquiry is the timer  50  activated. The second inquiry is has the timer value less than a second predetermined value which is the moving hold time. If the timer is running and has not counted past the second predetermined time, the result of decision block  170  will be yes and the preemptive set true function block  180  will then be fed to the output preemptive set  160 . If the timer has not been turned on or if the timer has exceeded the second predetermined value the output from decision function  170  will go through decision function  190  and the preempt set will be false. If the predetermined or the preempt set is true from function  180  or  140  the control system will calculate the pd torque request. The pd torque request is a function which it equal kp times throttle position+kd times the throttle position rate. Kp and kd can differ from one another or be equal at a given speed. The pd torque request is limited to a range of zero to X, a tunable value typically in a range of 300 Newton meters for a typical passenger vehicle however, it may be lower or greater for certain vehicles. The kp is a function of the vehicle speed, the kd is a function of the vehicle speed. Both kp and kd are table based values which are selected for certain vehicles. From calculation box  200 , the pd torque request is then submitted to the pd torque request raw calculation box  210 . Pd torque request raw is equal to the greater of the current pd torque request or the previous pd torque request taken five milliseconds prior. The net effect of the pd torque request raw is that pd torque request will always be equal or greater as time proceeds even though a current value of the torque request may fluctuate up or down during any given time. The pd torque request is then multiplied by a SWA factor in multiplier function  220 . Multiplier function  220  calculates a SWA factor which is based upon the steering wheel angle of the vehicle. Since most all wheel drive vehicles do not have a differential between the two separate axles but only has differentials between the wheels on each axle, it is desirable to limit excessive torque to the secondary axle under hard steering angles maneuvering operations to prevent tire skid or turning skid on the wheels. After proceeding through the SWA factor, which will be 1 if the vehicle is going straight and will be less than 1 if the steering wheel is being turned, the pd torque request will then be multiplied by a rate limit in function block  230 . Function block  230  will usually have no rate limit for positive increases in requested preemptive torque. Request for lowering of the preemptive torque will be rate limited so that there is not a sudden release of torque supplied to the secondary axle. The lowering request occurs when the preemptive set equals false from functions  150  or  180 . 
         [0016]    In operation the timer  50  will not be started until the throttle is pushed down beyond a predetermined rate (decision function  34 ). If the vehicle is stationary when the timer is started, the preempt set logic will go from decision function  120  to decision function  130 . When the timer  50  is first started, decision function  30  will have a time on the timer which is less than that of the standing hold time which typically is 0.5 seconds. Decision function  130  will give a true response causing decision function  140  to set the preempt set to be true proceeding onto the output preempt set  160 . A pd torque request will be calculated in function  200 . The above noted calculation will then be used to calculate a preemptive torque request raw in calculator function  210 . A SWA factor will modify the pd raw torque request based upon the steering wheel angle in function  220 . If the request is continually increasing, there will be no rate limit in calculation function  230  and the coupling  20  will be engaged to meet the request. The above noted preemptive torque request will be continuously calculated up to the expiration of the standing hold time. When the standing hold time is met, decision function  60  will activate the preempt reset  110  which causes the timer  50  to be cut off. Also, movement of the vehicle will cause the preemptive reset to be true causing the timer  50  to be cut off. If the vehicle starts to move and the throttle position is not returned to zero or if the preemptive torque request is not equal to zero then the timer will be restarted and logic box  120  will have a yes response causing the timer to be restarted by decision function  180  to the second hold time. Preemptive torque is continually applied by the coupling  20  until the expiration of the second predetermined time. Preemptive torque will only be applied for a maximum of the first and second predetermined time periods which will be approximately 1.1 seconds. After such time, other control systems will apply torque to secondary wheels as required by the remainder of the AWD control system for the vehicle. The benefit of the preemptive torque is that torque will be applied to the secondary axle before any sense of slipping in the wheels is experienced by the sensors. 
         [0017]    While preferred embodiments of the present invention have been disclosed, it is to be understood it has been described by way of example only, and various modifications can be made without departing from the spirit and scope of the invention as it is encompassed in the following claims.