Abstract:
The present invention provides a method for improving cornering performance. From a given vehicle speed and lateral acceleration, in instances of a decrease in drive torque, individual wheel torque is shifted from inside wheels to outside wheels to increase vehicle yaw forces and counteract the understeer phenomenon. Similarly, in instances of increasing drive torque, individual wheel torque is shifted from outside wheels to inside wheels on a common axle to reduce vehicle yaw forces and counteract the oversteer phenomenon. Actual implementation of side-to-side torque shifting is done using a correction factor multiplied by other factors within a general torque shift equation.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is generally directed toward a method for improving cornering performance in a four wheel drive vehicle by scaling the side-to-side drive torque bias in response to changes in total driveline torque. 
   2. Description of Related Art 
   In the area of electronic drive torque distribution control, it is possible to shift drive torque to various driven wheels in accordance with specific vehicle operational conditions and sensed driver needs. During cornering it is advantageous to send more drive torque to the outside wheel of a given drive axle to enhance the transient characteristics of the vehicle&#39;s turning behavior. 
   During situations of combined acceleration and turning it can be particularly difficult to achieve consistent line trace performance when the drive torque is increased and/or decreased either by driver demand and or variations in system functioning. In an attempt to maintain consistent line trace while the drive torque is varied, it is important to maintain a relatively constant yawing moment from the distributed drive torque. 
   In various state of the art driveline control systems, there is a consistently varying yaw moment from the drive torque at a given trimmed lateral acceleration. As the drive torque increases and/or decreases, the amount of yawing moment changes and the overall line trace is not maintained. 
   Current practice is illustrated in  FIG. 1 . The drive torque distribution on a given axle (side-to-side distribution) is biased toward the outside wheel (as represented by different arrow sizes on the wheels) in order to provide enhanced traction and line trace capability. This side-to-side bias (or torque split) creates an “inward” turning moment on the vehicle body and helps to direct the vehicle inward in the turn as drive torque increases. As the total drive torque increases and/or decreases, and the side-to-side bias ratio does not change, the amount of inward yawing moment changes according to the delivered level of drive torque. Because of this behavior, perturbations in the line trace behavior occur. Starting from a quasi-equilibrium position at a prescribed lateral acceleration, as total drive torque is increased the inward yawing moment increases and the vehicle begins to “tighten its line”. Conversely, starting from a quasi-equilibrium position at a prescribed lateral acceleration where there is an existing high level of drive torque and drive torque is lessened, the vehicle begins to “loosen its line” as the inward turning moment is released. Both situations cause noticeable and undesirable disturbances in the driving trajectory. A method and system that avoids these situations is desired. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method for improving cornering performance. From a given vehicle speed and lateral acceleration, in instances of a decrease in drive torque, individual wheel torque is shifted from inside wheels to outside wheels on a common axle to increase vehicle yaw forces and counteract an understeer phenomenon. Similarly, in instances of increasing drive torque, individual wheel torque is shifted from outside wheels to inside wheels on a common axle to reduce vehicle yaw forces and counteract an oversteer phenomenon. Actual implementation of side-to-side torque shifting is done using a scaling factor (0.0 to 1.0) multiplied by other factors within a general torque shift equation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and further features of the invention will be apparent with reference to the following description and drawings, wherein: 
       FIGS. 1   a  and  1   b  are schematic diagrams of a vehicle illustrating, in general, distribution of drive torque and turning behavior with respect to variations in total drive torque; 
       FIG. 2   a  is a schematic diagram of a vehicle illustrating the turning behavior utilizing axle side-to-side bias to generate an inward turning moment in an instance of large total drive torque; 
       FIG. 2   b  is a schematic diagram of a vehicle illustrating the turning behavior utilizing axle side-to-side bias to generate an inward turning moment in an instance of small total drive torque; 
       FIG. 3  is a schematic representation of the integration of a side-to-side bias scaling factor depending on total estimated drive torque; and 
       FIG. 4  is a graphical representation of an alternative side-to-side bias scaling factor depending on measured vehicle lateral acceleration. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The method according to the invention comprises the steps of estimating the total drive torque generated by the vehicle. The estimated total drive torque is then used to modify the side-to-side distribution of actual drive torque by using one or more scaling factors in addition to any other side-to-side torque distribution means already in place. Proper distribution of drive torque maintains a constant vehicle inward yaw moment and as a result produces predictable line trace performance. The method is described in greater detail below. 
   As used in this description and in the appended claims, the following terms will have the definitions indicated thereafter:
         “drive torque” means torque generated by the vehicle engine and transmission and distributed between the four vehicle wheels;   “total drive torque” means the sum of the drive torque applied to all four vehicle wheels;   “inside wheels” mean the front and rear wheels on a common side of a vehicle that define a smaller radius during a vehicle turn;   “outside wheels” mean the front and rear wheels on a common side of a vehicle that define a larger radius during a vehicle turn; and   “side-to-side bias” means the ratio between the drive torque distributed to the inside wheels and the drive torque distributed to the outside wheels on a single axle of the vehicle.       

   The total drive torque of a vehicle varies during operation of the vehicle. Variations in total drive torque are caused by one or more of the following factors: operator torque demand, increased/decreased vehicle speed, traction requirements, braking requirements and other stability requirements. Factors such as traction requirements cause drive torque to be distributed differently amongst the vehicle&#39;s four wheels. Additionally, as previously stated, side-to-side distribution of torque between inside and outside wheels can, in part, be determined by the steering angle of the vehicle to provide increased traction. The turning of the vehicle also creates a measurable component of lateral acceleration. 
   In order to maintain similar levels of inward yawing moment for a given vehicle condition (i.e. speed and lateral acceleration), the side-to-side bias of drive torque must be large for small total drive torque outputs and small for large total drive torque outputs. Referring to  FIG. 2   a , a vehicle is shown turning to the left; and due to an asymmetric drive torque distribution of the total drive torque amount, a yawing moment M cg  is generated. Large arrows on the vehicle wheels represent the magnitude of drive force applied at each wheel, respectively. The drive force at each wheel is related to each wheel&#39;s drive torque through the loaded radius of the tire/wheel assembly, thus, the arrows on the wheels are also representative of drive torque. The sum of the magnitudes of each individual arrow is equal to the total drive torque of the vehicle. 
   The side-to-side bias of drive torque is represented by the size relationship of the arrows on the right and left sides of the vehicle on a common axle, namely the ratio of the length of the larger to the length of the smaller. Referring again to  FIG. 2   a  which illustrates a side-to-side bias of 4:3, the relative sizes of the arrows on the right and left wheels of the rear axle, as explained below, also indicate the applied yaw moment created by the left-right drive torque difference and can be denoted by the quantity dM (assuming lateral tire force is not significantly influenced by the presence of these longitudinal driving forces). 
   To further illustrate the calculation of the applied yaw moment,  FIG. 2   a  also shows moment arms ( 20   a ,  20   b ) representing the perpendicular distance from the line of action of each wheel&#39;s drive force to the vehicle center of gravity. Moment arm  20   a  is the vehicle&#39;s front axle half-track width and is symmetric between the left and right side. Moment arm  20   b  is the vehicle&#39;s rear axle half-track width and is symmetric between the left and right side. The track width is the lateral distance between two tire contact patch center on a given axle. The moment generated about the vehicle center of gravity by the front axle wheel force components perpendicular to the moment arms,  20   a  on each side cancel each other because the applied forces at each front wheel are equal. On the rear axle the moment generated by the force component perpendicular to the moment arm  20   b  of the right wheel exceeds that generated by the force component perpendicular to the moment arm  20   b  of the left wheel by a factor that is equal to the length difference, dM, between the representative arrows shown on the rear axle. Thus, in comparing the ration of the lengths of the arrows, the side-to-side bias of the rear axle illustrated in  FIG. 2   a  is 4:3 and creates a defined yaw moment M cg . The quantity dM represents the difference in the two rear wheel drive torque values. 
   Referring to  FIG. 2   b , the side-to-side bias of drive torque represented by arrows on the wheels of the rear axle is 2:1, which is larger than the ratio of 4:3 shown in  figure 2   a . However, the applied yaw moment, also depicted by the quantity M cg , is the same as that of  FIG. 2   a  since the arrow difference length of the two rear axle drive torques is also equal to dM. The side-to-side bias is larger than in  FIG. 2   a , but the resulting yaw moment is the same. Because of the lower total drive torque represented by the sum of all arrow lengths (compared to  FIG. 2   a ), a larger side-to-side bias is needed to create the same yaw moment M cg . Thus, the larger side-to-side bias creates an equivalent yaw moment contribution and as a result the yaw moment is constant between the illustrated instances in  FIG. 2   a  and  FIG. 2   b.    
   It will be understood that additional yaw moment can be created by varying the side-to-side bias between wheels on the vehicle front axle. It will also be understood that in an instance where the vehicle is turning in an opposite direction, the side-to-side bias is reversed. 
   Referring to  FIG. 3 , integration of the side-to-side bias based on total drive torque with other vehicle side-to-side bias calculations in a programmable four wheel drive system is illustrated for a single vehicle axle. In step  120 , existing state of the art calculates a side-to-side bias coefficient (X) based on various input parameters such as lateral acceleration (lateral G), vehicle speed and estimated driver intent. The range of the bias coefficient is between a value of 0 to 0.5. A value of 0 indicates no additional torque transfer to the outside wheel, or the two wheels on a given axle have the same intended torque. A value of 0.5 indicates that all the torque on a given axle is intended to be transferred to the outside wheel. In step  100  an estimation of the total drive torque is made. In step  110 , depending on the estimated total drive torque, a side-to-side bias scaling factor is taken from a stored table of pre-determined values. At small values of estimated total drive torque, the bias scaling factor from operation  110  approaches 1.0 (meaning that there is no intended modification to the side-to-side bias coefficient calculated in operation  120 ), so that when multiplied by the result of operation  120  in operation  130 , the end result is the side-to-side bias factor itself. As the estimated drive torque increases, the bias scaling factor reduces in magnitude such that when multiplied to the result of operation  120  in operation  130 , the modified side-to-side bias command is smaller, indicating a smaller amount of torque transfer between the two wheels on a given axle. Application of the modified side-to-side bias command to the programmable vehicle control system provides a condition where the yaw moment of the vehicle is maintained generally constant in a cornering situation with variable total drive torque and the vehicle does not tend to tack inward or outward. 
   Referring to  FIG. 4 , as an additional method of pictorially showing the effect of maintaining a consistent yaw moment, the side-to-side bias coefficient is scaled according to measured or calculated lateral acceleration of the vehicle. Once the lateral acceleration of the vehicle reaches a threshold value, preferably 0.2 g, side-to-side bias is increased to the outside wheel(s). As lateral acceleration increases, the side-to-side bias to the outside wheel(s) can be increased quickly or gradually. The rate of increase will depend on the vehicle speed as well as the previously disclosed method for changing the side-to-side bias command depending on estimated total drive torque. For the case of low estimated total drive torque, the gradient of the rise is steep indicated by the label in  FIG. 4 . For the case of high estimated total drive torque the rate of increase is more gradual. The region in-between the two lines represents the total operational area of the side-to-side transfer. It may be noted that the gradient of either line (low total drive torque or high total drive torque) may also be scaled by vehicle speed or a variety of other operational parameters. 
   Although the invention has been shown and described with reference to certain preferred and alternate embodiments, the invention is not limited to these specific embodiments. Minor variations and insubstantial differences in the various combinations of materials and methods of application may occur to those of ordinary skill in the art while remaining within the scope of the invention as claimed and equivalents.