Abstract:
A method and apparatus for controlling torque delivery independently, i.e., asymmetrically, to the two rear axles and wheels of a vehicle with front wheel drive provides improved vehicle handling and performance. The apparatus includes a prime mover, transaxle, power takeoff, rear axle having a pair of independently controllable modulating clutches driving respective rear axle and wheels, various vehicle sensors and a microprocessor. The method, embodied in software in the microprocessor, senses wheel speeds, yaw rate, lateral acceleration, throttle position and steering wheel angle, determines various reference values and oversteer and understeer conditions and activates one or both of the two clutches.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates generally to a method and apparatus for controlling torque delivery to the rear wheels of a front wheel drive vehicle and more particularly to a method and apparatus for controlling independent engagement of clutches disposed at the rear of a front wheel drive vehicle that asymmetrically provide drive torque to the respective rear wheels. 
   Extensive effort has been directed to many aspects of vehicle control and performance in vehicles equipped with adaptive four-wheel drive systems. Since the opportunity exists in vehicles equipped with four-wheel drive systems to monitor and control torque application to all four vehicle wheels rather than simply two wheels, as in many vehicles, the opportunity to significantly enhance the performance and performance characteristics of such vehicles also exists. 
   Many patented systems address and exploit the capabilities of four-wheel drive systems in manners intended to, for example, provide skid control, provide optimum acceleration and deceleration, or provide maximum acceleration and deceleration subject to maintaining vehicle control. 
   In addition to skid or slip sensing and control, a recent area of patent activity can be characterized as control of vehicle yaw, that is, motion of the vehicle about its Z or center, vertical axis. 
   For example, U.S. Pat. No. 5,332,059 teaches a four-wheel drive vehicle control system having a steering angle sensor and a clutch disposed across a rear differential. The clutch inhibits differentiation in response to sensed vehicle speed, steering angle and longitudinal and lateral acceleration. 
   U.S. Pat. No. 5,341,893 discloses a four-wheel drive system for a vehicle such as a tractor wherein a front differential drives left and right front wheels and torque is supplied to the rear wheels through individual clutches. 
   U.S. Pat. No. 6,076,033 teaches a process for controlling yaw in a motor vehicle through the generation of mutually exclusive braking and driving forces on the left and right wheels of a vehicle. 
   Another four-wheel drive system appears in U.S. Pat. No. 6,145,614 which discloses a four-wheel drive system having a center differential with a differentiation inhibiting device disposed across the differential and a second differential at the primary axle which also has a differentiation inhibiting clutch disposed there across. The system also includes a turn sensor and means for adjusting the extent of differentiation inhibition depending upon the speed difference between the right and left main drive wheels. 
   From the foregoing survey of patents directed to motor vehicle yaw control, it is apparent that improvements to the subject art are desirable. 
   BRIEF SUMMARY OF THE INVENTION 
   A method and apparatus for controlling torque delivery independently, i.e., asymmetrically, to the two rear axles and wheels of a vehicle with front wheel drive provides improved vehicle handling and performance. The apparatus includes a prime mover, transaxle, power takeoff, rear axle having a pair of independently, controllable modulating clutches driving respective rear axle and wheels, various vehicle sensors and a microprocessor. The method, embodied in software in the microprocessor, senses wheel speeds, yaw rate, lateral acceleration, throttle position and steering wheel angle, determines various reference values and oversteer and understeer conditions and activates one or both of the two clutches. 
   Thus it is an object of the present invention to provide a method for asymmetrically delivering torque through twin clutches to the rear axles and wheels of a front wheel drive motor vehicle. 
   It is a further object of the present invention to provide a method for independently controlling twin clutches in a rear axle of a front wheel drive motor vehicle to provide improved vehicle handling and control. 
   It is a still further object of the present invention to provide a method for controlling independent activation of left and right clutches of a rear axle of a front wheel drive motor vehicle based upon sensed wheel speeds, yaw rate, lateral acceleration, throttle position and steering wheel angle. 
   It is a still further object of the present invention to provide an apparatus for asymmetrically delivering drive torque to the rear axles and wheels of a front wheel drive vehicle. 
   It is a still further object of the present invention to provide an apparatus for independently delivering drive torque to the rear wheels of a front wheel drive motor vehicle having a twin clutch rear axle, sensors for wheel speeds, yaw rate, lateral acceleration, throttle position and steering wheel angle and a microprocessor. 
   It is a still further object of the present invention to provide an apparatus for a front wheel drive motor vehicle having a prime mover, transaxle, power takeoff, twin, independently modulatable clutches in a rear axle, various sensors monitoring vehicle operating conditions and a microprocessor having an output driving the modulating clutches. 
   Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings wherein like reference numbers refer to the same component, element or feature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic, plan view of a front wheel drive motor vehicle driveline incorporating the present invention; 
       FIG. 2  is a full, sectional view of a twin modulating clutch rear axle according to the present invention; 
       FIG. 3  is a block diagram of the control architecture residing in a microprocessor according to the present invention; 
       FIG. 4  is a block diagram of one of the traction controller modules residing in the microprocessor according to the present invention; 
       FIG. 5  is a block diagram of the dynamics controller module residing in the microprocessor according to the present invention; 
       FIG. 6  is a block diagram of one of the smart actuator modules residing in the microprocessor according to the present invention; 
       FIG. 7  is a flow chart illustrating the sequence of steps undertaken by the dynamics controller module illustrated in  FIG. 5 ; 
       FIG. 8  is a flow chart of the yaw rate calculation illustrated in  FIG. 5 ; 
       FIGS. 9A and 9B  are flow charts of the clutch selector logic illustrated in  FIG. 5 ; 
       FIG. 10  is a flow chart relating to the left turn understeer detection subroutine illustrated in  FIG. 9B ; 
       FIG. 11  is a flow chart relating to the right turn understeer detection subroutine illustrated in  FIG. 9B ; 
       FIGS. 12A and 12B  are flow charts relating to the right turn oversteer detection subroutine illustrated in  FIG. 9B ; 
       FIGS. 13A and 13B  are flow charts relating to the left turn oversteer detection subroutine illustrated in  FIG. 9B ; and 
       FIGS. 14A ,  14 B and  14 C are flow charts relating to the arbitrator module illustrated in  FIG. 3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , an adaptive four-wheel vehicle drive train is diagrammatically illustrated and designated by the reference number  10 . The four-wheel vehicle drive train  10  includes a prime mover  12  such as a gasoline, diesel or natural gas fuel internal combustion engine which is coupled to and directly drives a transaxle  14 . The output of the transaxle  14  drives a primary or front driveline  20  and a second or rear driveline  30 . The primary driveline  20  comprises a front or primary propshaft  22 , a front or primary differential  24 , a pair of live front axles  26  and a respective pair of front tire and wheel assemblies  28 . It should be appreciated that the front or primary differential  24  is conventional. 
   The transaxle  14 , through a power takeoff  16 , also provides drive torque to the secondary or rear drive line  30  comprising a secondary propshaft  32  having appropriate universal joints  34 , a rear or secondary axle assembly  36 , a pair of live secondary or rear axles  38  and a respective pair of secondary or rear tire and wheel assemblies  40 . As utilized herein with regard to the secondary axle assembly  36 , the term “axle” is used to identify a device for receiving drive line torque, distributing it to two generally aligned, transversely disposed drive axles and accommodating rotational speed differences resulting from, inter alia, vehicle cornering. As such, the term “axle” is intended to include the present invention which provides these functions but which does not include a conventional caged differential gear set. 
   The foregoing and following description relates to a vehicle wherein the primary drive line  20  is disposed at the front of the vehicle and, correspondingly, the secondary drive line  30  is disposed at the rear of the vehicle, such a vehicle commonly being referred to as a (primary) front wheel drive vehicle or adaptive four-wheel drive vehicle. 
   Associated with the vehicle drive train  10  is a controller or microprocessor  50  which receives signals from a plurality of sensors and provides two independent control, i.e., actuation, signals to the rear or secondary axle assembly  36 . Specifically, a steering angle sensor  52  senses the angular position of the steering column  54  and steering wheel and provides an appropriate signal to the microprocessor  50 . Since there is typically a direct and positive linkage between the steering column  54  and the front (steering) tire and wheel assemblies  28 , the angular position of the front tire and wheel assemblies can be directly inferred and, in fact, computed from the information provided by the steering angle sensor  52 . Thus, it should be understood that rotation of the steering column  54  and movement of the steering angle sensor  52  will always correspond, according to a known mathematical relationship, to the angular movement of the front tire and wheel assemblies  28 . This is true of even variable ratio steering systems. Scaling factors in the microprocessor  50  can readily convert angular position of the steering column  54  to angular position of the front (steering) tire and wheel assemblies  28 . 
   Due to such ready conversions when referring to “steering angle,” such reference is to the angular position of the steering column  54  and attached steering wheel, it being understood that both the angle of the steering column  54  and angle of the front tire and wheel assemblies  28  of a given vehicle are related by a known relationship or ratio, as noted above, and that either may be sensed, if desired, and scaled and converted to the other as appropriate. In this regard, either a linear sensor (not illustrated) operably linked to a steering rack or other steering component exhibiting linear motion or an angular sensor having limited motion linked to a steering component having limited motion will function in this system. Lastly, in steer-by-wire systems, the microprocessor  50  may be fed a signal from the steering angle sensor  52  of the steer-by-wire system. All of these sensor types, sensor locations and system configurations are deemed to be within the scope of this invention. It should be appreciated, however, that the relatively significant extent of rotation of the steering column  54 , typically at least three turns (1080°) lock-to-lock, provides better angular definition in the output signal of the sensor  52  relative to a sensor location exhibiting less rotational or linear movement. 
   The vehicle drive train  10  also includes a first variable reluctance or Hall Effect sensor  56  which senses the rotational speed of the left primary (front) tire and wheel assembly  28  and provides a signal to the microprocessor  50 . A second variable reluctance or Hall Effect sensor  58  senses the rotational speed of the right primary (front) tire and wheel assembly  28  and provides a signal to the microprocessor  50 . A third variable reluctance or Hall effect sensor  60  associated with the left secondary (rear) tire and wheel assembly  40  senses its speed and provides a signal to the microprocessor  50 . Finally, a fourth variable reluctance or Hall effect sensor  62  associated with the right secondary (rear) tire and wheel assembly  40  senses its speed and provides a signal to the microprocessor  50 . It should be understood that the speed sensors  56 ,  58 ,  60  and  62  may be independent, i.e., dedicated, sensors or may be those sensors mounted in the vehicle to provide signals for anti-lock brake systems (ABS) or other speed sensing and traction control systems. It is also to be understood that an appropriate and conventional counting or tone wheel (not illustrated) is associated with each of the respective tire and wheel assemblies  28  and  40  in proximate sensing relationship with each of the speed sensors  56 ,  58 ,  60  and  62 . A throttle position sensor  64  and a yaw rate and lateral accelerator sensor  65 , which may be unitary or separate devices, also provide signals to the microprocessor  50 . The microprocessor  50  includes software which receives and may condition the signals from the steering angle sensor  52 , the wheel speed sensors  56 ,  58 ,  60  and  62 , the throttle position sensor  64  and the yaw and lateral acceleration sensor  65 . 
   Referring now to  FIG. 2 , the rear or secondary axle assembly  36  includes an input shaft  70  which receives drive torque from the secondary propshaft  32 . The input shaft  70  may include a flange or cup  72  or similar component which forms a portion of, for example, a universal joint  34  or other connection to the secondary propshaft  32 . The flange  72  may be retained on the input shaft  74  by a lock nut  74  or similar device. The input shaft  70  is received within a centrally disposed, axially extending center housing  76  and is surrounded by a suitable oil seal  78  which provides a fluid impervious seal between the housing  76  and the input shaft  70  or an associated portion of the flange  72 . The input shaft  70  is preferably rotatably supported by a pair of anti-friction bearings such as the tapered roller bearing assemblies  80 . The input shaft  70  terminates in a hypoid or bevel gear  82  having gear teeth  84  which mate with complementarily configured gear teeth  86  on a ring gear  88  secured to a flange  92  on a centrally disposed tubular drive member  94  by suitable threaded fasteners  96 . 
   The tubular drive member  94  is rotatably supported by a pair of anti-friction bearings such as ball bearing assemblies  102 . The tubular drive member  94  is hollow and defines an interior volume  104 . A pair of scavengers or scoops  106  extend radially through the wall of the tubular drive member  94  and collect a lubricating and cooling fluid  108  driving it into the interior volume  104 . The lubricating and cooling fluid  108  is then provided to components in the rear differential assembly  36  through passageways  110  in communication with the interior volume  104  of the tubular drive member  94 . 
   The rear or secondary axle assembly  36  also includes a pair of bell housings  112 A and  112 B which are attached to the center housing  76  by threaded fasteners  114 . The housings  112 A and  112 B are mirror-images, i.e., left and right, components which each receive a respective one of a pair of modulating clutch assemblies  120 A and  120 B. But for the opposed, mirror-image arrangement of the two modulating clutch assemblies  120 A and  120 B, the components of the two clutch assemblies  120 A and  120 B described below are identical. Accordingly, and for purposes of clarity in  FIG. 2 , numerical component callouts may appear in either or both of the left and right clutch assemblies  120 A and  120 B, it being understood that such components reside in and such callouts refer to both assemblies. 
   Both of the modulating clutch assemblies  120 A and  120 B are driven by the input shaft  70  through the bevel gears  82  and  88  and the tubular drive member  94 . Specifically, the ring gear  88 , which as noted above, is secured to the tubular drive member  94 . A tubular extension  122  of the ring gear  88  includes external or male splines  124 , which mate with internal or female splines or gear teeth  128 A, formed on a left drive collar  130 A. The left drive collar  130 A also includes external or male splines or gear teeth  132 A which mate with complementarily configured internal or female splines or gear teeth  134 A on a clutch end bell  140 A. With regard to the drive to the right modulating clutch assembly  120 B, the tubular drive member  94  includes external or male splines or gear teeth  136 , which engage complementarily configured female splines or gear teeth  128 B and the drive collar  130 B. Correspondingly, the drive collar  130 B includes male or external splines or gear teeth  132 B which are complementary to and engage internal or female splines or gear teeth  134 B formed on a clutch end bell  140 B. 
   The clutch end bells  140 A and  140 B are identical but disposed in mirror image relationship. Each of the clutch end bells  140 A and  140 B includes internal splines  142  which drivingly engage complementarily configured external splines  144  on a first plurality of larger diameter friction clutch plates or discs  146 . Interleaved with a first plurality of larger diameter friction clutch plates or discs  146  is a second plurality of smaller diameter friction clutch plates or discs  148 . At least one face of each of the friction clutch plates or discs  146  and  148  includes suitable friction clutch material. Each of the smaller diameter friction clutch plates or discs  148  includes internal or female splines  150  which engage complementarily configured male or external splines  152  on a circular collar or hub  154 . The hub  154  is, in turn, coupled by internal or female splines or gear teeth  156  to male splines or gear teeth  158  on respective left and right output shafts  160 A and  160 B for rotation therewith. 
   The modulating clutch assemblies  120 A and  120 B also include ball ramp actuator assemblies  170 A and  170 B. The ball ramp actuator assemblies  170 A and  170 B each include a circular apply plate  172  which includes female splines or internal gear teeth  174  which mate with the male splines  152  on the collar or hub  154 . The apply plate  172  thus rotates with the second plurality of clutch plates  148  and may move axially relative thereto. The apply plate  172  may include a shoulder  176  which positions and receives a flat washer  178  which engages an armature  182 . The armature  182  includes male splines  184  about its periphery which are complementary to and engage the female splines  142  on the interior of the end bells  140 A and  140 B. Thus, the armature  182  rotates with the end bell  140 A and the first plurality of clutch plates  146 . The armature  182  is disposed adjacent a U-shaped circular rotor  186 . The rotor  186  partially surrounds a stationary housing  192  which contains an electromagnetic coil  194 . The stationary housing  192  and the coil  194  are preferably secured to the bell housings  112 A and  112 B by a plurality of threaded studs and fasteners  196 . Electrical energy may be provided to the electromagnetic coils  194  through respective left and right electrical conductors  66  and  68 . 
   Coupled to the rotor  186  by any suitable means such as weldments, interengaging splines or an interference fit is a first circular member  202 . The first circular member  202  defines a loose, freely rotating fit about the output shafts  160 A and  160 B and thus the first circular member  202  and the rotor  186  are free to rotate about the output shafts  160 A and  160 B and the housings  192  of the electromagnetic coils  194 . The first circular member  202  includes a plurality of curved ramps or recesses  204  arranged in a circular pattern about the axis of the output shaft  160 B. The ramps or recesses  204  represent oblique sections of a helical torus. Disposed within each of the recesses  204  is a load transferring ball  206  or similar load transferring member which translates along the ramps defined by the oblique surfaces of the recesses  204 . 
   A second circular member  208  is disposed in opposed relationship with the first circular member  202  and includes a like plurality of complementarily sized and arranged recesses  212 . The load transferring balls  206  are thus received within the pairs of opposing recesses  204  and  212 , the ends of the recesses  204  and  212  being curved and much steeper in slope than the interior regions of the recesses such that the load transferring balls  206  are effectively trapped therein. A plurality of wave washers or Belleville springs  214  are disposed between the second circular member  208  and the hub or collar  154  and bias the second circular member  208  toward the first circular member  202 . 
   It will be appreciated that the recesses  204  and  212  and the load transferring balls  206  may be replaced with other analogous mechanical elements which cause axial displacement of the circular members  202  and  208  in response to relative rotation therebetween. For example, tapered rollers disposed in complementarily configured conical helices may be utilized. 
   The second circular member  208  includes a plurality of female splines or gear teeth  215  which are complementary to and engage the male splines or external gear teeth  158  on the output shaft  160 B. The axial position of the first circular member  202  is established by a thrust bearing assembly  216 . Adjacent the thrust bearing assembly  216  is an anti-friction bearing such as a ball bearing assembly  218  which rotatably supports and axially locates the output shaft  160 B. The ball bearing assembly  218  is retained by a pair of snap rings  222  and axially positions the output shaft  160 B relative to the bell housing  112 B. Adjacent the ball bearing assembly  218  and the terminus of the output shaft  160 B is an oil seal  224 . The terminal portion of the output shaft  160 B may include male splines  226 , a flange or other component which facilitates driving connection to the adjacent rear axle  38 . The opposite ends of the drive shafts  160 A and  160 B are rotatably supported in a cylindrical journal bearing, a bushing or a roller bearing assembly  228  received within the tubular drive member  94 . 
   Referring now to  FIG. 3 , the microprocessor  50  includes several modules which receive data from one or more of the various sensors, including the steering wheel angle sensor  52 , the wheel speed sensors  56 ,  58 ,  60 , and  62 , the throttle position sensor  64  and the yaw rate and lateral acceleration sensors  65 . The microprocessor  50  includes six modules or building blocks including left and right traction controller modules  250 A and  250 B which are described in greater detail in  FIG. 4 , a dynamics controller module  252  which is described in greater detail in  FIG. 5 , an arbitrator module  256  which is described in greater detail in  FIGS. 14A ,  14 B and  14 C, and left and right smart actuator modules  258 A and  258 B which are described in greater detail in  FIG. 6 . 
   Turning then to  FIG. 4 , the right and left traction controller modules  250 A and  250 B are the same and thus only the left traction controller module  250 A will be described. Both modules  250 A and  250 B read the vehicle speed by being provided with the speed from all four wheel speed sensors  56 ,  58 ,  60  and  62 , and also receive a signal from the steering wheel angle sensor  52 , and the throttle position sensor  64 . The slip error signal is optional and, when utilized represents the difference between actual wheel slip and calculated or expected wheel slip. From the vehicle speed and throttle position signals, a torque demand is determined by a subroutine  262 . The output of the torque demand subroutine  262  is provided both to a second subroutine  264  which determines a target torque and also to a third subroutine  266  which provides a torque transition signal. The throttle position from the throttle position sensor  64  is also provided to a conditioning or filtering subroutine  268  which provides a filtered throttle signal to the target torque subroutine  264 . The steering wheel angle from the steering angle sensor  52  is provided to a subroutine  272  which provides a signal relating to the status of the turn, either right or left, which is provided to both the second subroutine  264  and the third subroutine  266 . The optional slip error signal in the line  260  is also provided to the subroutines  264  and  266 . The outputs of both the left traction controller module  250 A and the right traction controller module  250 B are provided to the arbitrator module  256 , as illustrated in  FIG. 3 . 
   Referring now to  FIG. 5 , the dynamics controller module  252  illustrated in  FIG. 3 , is shown in greater detail. The dynamics controller  252  accepts signals from the vehicle speed sensors  56 ,  58 ,  60 ,  62 , from the steering wheel angle sensor  52  and the yaw rate and lateral acceleration sensor  65 . It also receives a signal relating to the left and right turn speed condition which will be described subsequently. Several of the signals are provided to the yaw rate reference calculation subroutine  276  which is described in greater detail in  FIG. 8 . A proportional integral differential (PID) controller module  278  which receives a signal directly from the yaw rate sensor  65  and an output signal from the subroutine  276  is illustrated in greater detail in  FIG. 14C . The yaw rate reference module  252  also includes an oversteer detection module  282  which receives a signal from the yaw rate reference subroutine  276 . The oversteer detection subroutines are presented in  FIGS. 10 ,  11 ,  12 A,  12 B,  13 A and  13 B. The dynamics controller module  252  also includes a driving torque detection subroutine  284  which receives the left and right turn speed condition signal. The outputs of the subroutines  278 ,  282  and  284  are provided to a clutch selector logic subroutine  286  which is illustrated in  FIGS. 9A and 9B . The clutch selector logic subroutine  286  provides right and left torque control outputs to the smart actuator modules  258 A and  258 B, respectively, as illustrated in  FIG. 3 . 
   Referring now to  FIG. 6 , one of the two smart actuator modules  258 A is illustrated, it being understood that the modules  258 A and  258 B, but for their separate and dedicated nature to provide electrical energy to the right and left electromagnetic clutch assemblies  120 A and  120 B, respectively, are the same. The module  258 A receives a signal from the electrical system of the motor vehicle indicating the currently available voltage to be supplied to the electromagnetic coil  194 A. The smart actuator module  258 A also receives a signal from the clutch selector logic subroutine  286  which is the level of the torque application requested. The module  258 A also receives a signal indicating the input speed of the clutch which may be derived by averaging the signals from the front wheel speed sensors  56  and  58  and also a clutch output speed signal which may be derived by averaging the speeds of the rear wheel speed sensors  60  and  62 . Alternatively, a single sensor (not illustrated) sensing the speed of the secondary propshaft  32  or a directly coupled component may be utilized to sense the clutch input speed. A clutch torque controller  292  received the clutch torque requested, conditions the torque level request and provides it to a coil current controller  294  which also is provided with the presently available electrical system voltage. The coil current controller  294  provides an output signal in the line  66  to the left coil  194  of the electromagnetic clutch assembly  120 A and may utilize a pulse width modulation (PWM) control scheme or any other control scheme capable of providing a modulating, i.e., proportional, electrical signal to the coil  194 . A coil current estimator  296  also receives the control voltages and drives, with the clutch input and output speeds, a clutch torque estimator  298  which provides a signal representing an estimated torque level. 
   Referring now to  FIG. 7 , a flow chart presenting the steps of the dynamics controller module  252  illustrated in both  FIGS. 3 and 5  is illustrated. The dynamics controller module  252  receives inputs from the four vehicle speed sensors  56 ,  58 ,  60  and  62  which then may be utilized to compute the vehicle speed and also receives signals from the steering wheel angle sensor  52 , the throttle position sensor  64  and the yaw rate and lateral acceleration sensor  65 . A signal representing the current state of the arbitrator module  256  is also provided. This data is initialized or stored as needed at an initialization point  300  and utilized in a process step  302  to calculate a yaw control torque according to the flow chart illustrated in  FIG. 8 . The calculated yaw control torque is then utilized in a process step  304  to determine and enable the clutch selector logic set forth in the flow charts appearing in  FIGS. 9A and 9B . In turn, this data is utilized in a process step  306 . 
   The process step  306  provides a yaw control left torque request which is the product of a left clutch flag (one if the flag is set or zero if the flag is not set) times the left clutch control torque. In other words, if the left clutch flag is not set, the yaw control left torque request will be zero. If the left clutch flag is set, i.e., is equal to one, the yaw control left torque request will be the left clutch control torque. This is followed by a similar process step  308  which correspondingly determines the yaw control right torque request which is the product of the right clutch flag (either zero for off or one for on) times the right clutch control torque. Thus, if the right control clutch flag is set, i.e., is equal to one, the yaw control right torque request equals the right clutch control torque. If the right clutch flag is not set, the yaw control right torque request equals zero. These signals and an oversteer flag represent the output of the dynamics controller module  252 . 
   Turning then to  FIG. 8 , which explains in detail the process step/subroutine  302  of the dynamics controller module  252  illustrated in  FIG. 7 , the calculation of the yaw control torque utilizes the signals from the steering wheel angle sensor  52 , the vehicle speed which may be the average of the four sensors  56 ,  58 ,  60  and  62  or vehicle speed information determined by other sensors and processes, the yaw rate sensor  65  and a fixed value (stored in memory) relating to the wheelbase of the vehicle which are all read and stored in an initializing step  310 . The subroutine  302  then moves to a process step  312  which calculates a yaw rate reference value. Using a PID controller, such as the controller  278  illustrated in  FIG. 5 , requires that an error signal be calculated which requires that a reference signal also be calculated. 
   The yaw rate reference used in the PID controller  278  is a linear representation of yaw rate for a neutral steer vehicle which is represented in the equation 
           ψ   .     ⁢           ⁢   ref     =         δ   f     ⁢   V     I         
 
wherein ψ equals yaw rate, δ f  equals the angle of the front wheels of the vehicle, which as noted above, may be calculated from the steering wheel angle sensor  52 , V equals the speed of the vehicle and I equals the wheel-base.
 
   This equation may be multiplied by a gain K to permit tuning of the vehicle characteristics from understeer to oversteer: 
           ψ   .     ⁢           ⁢   ref     =       k   ref     ⁢           ⁢           δ   f     ⁢   V     I     .           
 
This reference signal is accurate at relatively low lateral accelerations and is sufficient for use in this system. Greater accuracy and higher lateral acceleration can be achieved by representing the curvature response to a steer angle through use of the equation 
           ψ   .     ⁢           ⁢   ref     =           δ   f     ⁢   V       I   ⁡     (     1   +     KV   2       )         .         
 
The subroutine  302  then moves to a process step  314  which takes the derivative of the calculated yaw rate to provide a yaw acceleration reference value. In the process step  316 , a yaw rate error is calculated through use of the equation ψ error=ψ ref −ψ measured, the ψ being calculated in the process step  312 . Next, the subroutine  302  moves to a process step  318  which reads the sign of the yaw rate error. If the reference yaw rate is greater than the measured yaw rate, this value is positive; if the reference yaw rate I is less than the measured yaw rate, this value is negative. In a process step  322 , the yaw acceleration is calculated and these various values are used in a process step for the PID controller  278  to generate the right or left torque request according to the equation 
         T   request     =         K   p     ⁢     ψ   .     ⁢           ⁢   error     +     K   ⁢     ∫         ψ   ⁢             .     ⁢   error         ⁢           +         K   D     ⁡     (         ⅆ               ⅆ   t       ⁢     ψ   .     ⁢           ⁢   error     )       .           
 
The subroutine  302  then provides a right clutch control torque, a left clutch control torque as well as the calculated values of the yaw acceleration, the yaw acceleration reference value and the yaw error sign.
 
   Returning briefly to  FIG. 7 , the subroutine  302  then moves to the subroutine  304  which is the clutch selector logic which is described in greater detail in  FIGS. 9A and 9B . 
   Referring then to  FIGS. 9A and 9B , the clutch selector logic subroutine  304  utilizes input data from the right front wheel speed sensor  58 , the left front wheel speed sensor  56 , the right rear wheel speed sensor  62  and a left rear wheel speed sensor  60 , the yaw rate error sign, the yaw acceleration, the yaw acceleration reference yaw rate, and data from the steering wheel angle sensor  52 . This information is provided to a process step  332  which sets the yaw sign as the sign of the yaw rate. The subroutine  304  then moves to a second process step  334  which sets the sign of the front wheel angle as the sign of the steering wheel angle. This is simply a positive or negative sign depending upon the current left of center or right of center position of the steering column  54  and the convention (either Society of Automotive Engineers (SAE) or the International Standards Organization (ISO) utilized. According to the SAE standard or convention left of center is positive and right of center is negative. The ISO standard is the opposite. The process step  336  determines the average front wheel speed by adding the speed of the right front wheel and the speed of the left front wheel and dividing by two. 
   The subroutine  304  then moves to a process step  338  which sets the yaw acceleration band to a constant value. The subroutine  304  then branches and determines both left and right understeer and oversteer. Specifically, left turn understeer is detected in a subroutine  340  which is illustrated in  FIG. 10 . Right turn oversteer is detected in a subroutine  342  illustrated in  FIG. 11 . Similarly, right turn understeer is detected in a subroutine  344  illustrated in  FIGS. 12A and 12B  and left turn oversteer is detected in a subroutine  346  illustrated in  FIGS. 13A and 13B . When either left turn understeer or right turn understeer has been detected, a decision point  350  is exited at TRUE and a process step  352  sets a right clutch flag. If the decision point  350  determines that there is no left turn understeer or right turn oversteer, the decision point  350  is exited at FALSE and the right clutch flag is set at zero in a process step  354 . Correspondingly, if right turn understeer or left turn oversteer is detected, a decision point  360  is exited at TRUE and a left clutch flag is set in a process step  362 . Contrariwise, if the decision point  360  is exited at FALSE, since no right turn understeer or left steer oversteer is detected, a process step  364  sets a left clutch flag at zero. The clutch selector logic subroutine  304  output thus includes right and left clutch flags which may be set or not set and oversteer flags which may be set or not set as will be described with regard to  FIGS. 10 ,  11 ,  12 A,  12 B,  13 A and  13 B. 
   Referring now to  FIG. 10 , the left turn understeer detection subroutine  340  is illustrated. The left turn understeer detection subroutine  340  begins within an initialization step  366  which reads the right rear wheel speed from the sensor  62 , reads the average front wheel speed which may be recomputed or computed from the process step  336  illustrated in  FIG. 9A , reads the yaw sign and reads the yaw error sign. The subroutine  340  then moves to a process step  368  which reads a constant predetermined value as a speed overage value. The speed overage value is a tunable parameter which may be empirically or experimentally determined is typically in a range of from zero to a percent of the maximum vehicle speed. 
   The subroutine  340  then moves to a process step  372  which determines a front wheel speed limit by adding together the previously computed average front wheel speed plus the speed overage value read in the process step  368 . Next, a decision point  374  is entered which determines whether the right rear wheel speed is less than or equal to the front speed limit. If this proposition is TRUE, the left turn speed condition is set at one or logic high in a process step  376 . If this proposition is FALSE, a left turn speed condition is set to zero and the subroutine  340  proceeds to a second decision point  382  which determines if the left turn speed condition equals one and the yaw sign equals minus one and the yaw error equals minus one. If all three of these conditions are TRUE, the decision point  382  is exited at TRUE and the left turn understeer value is set to one or logic high in a process step  384 . If the inquiry of the decision point  382  is not true, it is exited at FALSE and proceeds to a process step  386  which sets the left turn understeer at zero. The output  388  of the left turn understeer detection subroutine  340  is thus a positive, logic high or one if there is left turn understeer and a logic low zero or null value if there is not left turn understeer. 
   Turning then to  FIG. 11 , the right turn understeer detection subroutine  342  is illustrated. Essentially, the right turn understeer detection subroutine  342  includes the same process steps and decision points as the left turn understeer detection subroutine  340  described directly above. Nonetheless, for reasons of clarity and completeness, it will be fully described. The right turn understeer detection subroutine  342  is provided with data and signals including the left wheel rear speed from the sensor  60 . The average front wheel speed as computed in the clutch selector logic subroutine  304  and the yaw sign and the yaw error sign are provided in the initialization step  390 . The subroutine  342  then moves to a process step  392  which reads a speed overage value which is equal to a predetermined constant. In a process step  394 , a front wheel speed limit is determined which is the sum of the average front wheel speed and the speed overage value. 
   The subroutine  340  then moves to a decision point  396  which determines whether the left rear wheel speed is less than or equal to the front wheel speed limit determined in the step immediately above. If this condition is TRUE, a process step  398  is entered which sets the right turn speed condition equal to logic high or one. If this condition is not true, the decision point  396  is exited at FALSE and the subroutine  342  moves to a process step  402  which sets the right turn speed condition equal to logic low, zero or null. Next, a decision point  404  is entered which determines whether the right turn speed condition is equal to one, and the yaw sign is equal to minus one and the yaw error is equal to minus one. If all of these conditions are true, the decision point  404  is exited at TRUE and the subroutine  342  moves to a process step  406  which sets a right turn understeer flag or value at logic high or one. If not all of these conditions are TRUE, the decision point  404  is exited at FALSE and a process step  408  sets a right turn understeer value or flag at logic low, zero or null. An output  410  of the right turn understeer detection subroutine  342  provides this right turn understeer value or flag to other subroutines and systems as necessary. 
   Turning now to  FIGS. 12A and 12B , it will be appreciated that the right turn oversteer detection subroutine  344  which is referenced in  FIG. 9B  includes an initialization step  422  which reads the yaw sign, the yaw error sign, the yaw acceleration, the yaw acceleration reference, the yaw acceleration band and the right turn understeer output  410  from the right turn understeer detection subroutine  342 . Next, a process step  424  is entered which computes the yaw acceleration limit which is the product of the yaw acceleration reference and the yaw acceleration band which was set in the process step  338  in the clutch selector logic subroutine  304 . Next, a decision point  426  is entered which determines whether the currently detected yaw acceleration is greater than the just computed yaw acceleration limit. If it is, the decision point  426  is exited at TRUE and in a process step  428 , a yaw acceleration flag or value is set to one or TRUE. If the yaw acceleration is not greater than the yaw acceleration limit, the decision point  426  is exited at FALSE and the yaw acceleration flag is set to zero or null in a process step  432 . Next, a decision point  434  is entered which determines whether either the yaw acceleration flag is set or equal to one or the yaw error sign is minus one. If either of these statements is true, the decision point  434  is exited at TRUE and the process step  436  sets a yaw error and acceleration flag on or equal to one. If the inquiry in the decision point  434  is not true, it is exited at FALSE, a process step  438  is entered and the yaw error and acceleration flag is set to zero or null. 
   Continuing on to  FIG. 12B , a decision point  442  is entered and an inquiry is made which determines whether the yaw error and acceleration flag is set equal to one and the yaw sign is equal to one and there is not right turn understeer. If this statement is true, the decision point  442  is exited at TRUE and a right turn oversteer flag or value is set to one in a process step  444 . If the statement in decision point  442  is FALSE, a process step  446  is entered which sets a right turn oversteer value or flag to zero. The right turn oversteer detection subroutine  344  ends in an output value or signal for the right turn understeer which is either zero or one and this value is provided to the clutch selector logic illustrated in  FIG. 9B . 
   Turning now to  FIGS. 13A and 13B , a left turn oversteer detection subroutine  346  is illustrated which is essentially similar to the right turn oversteer detection subroutine  344  illustrated in  FIGS. 12A and 12B . However, for purposes of clarity and completeness, the left turn oversteer detection subroutine  346  will be fully described. In an input and initializing step  452 , signals representing the yaw sign, the yaw error sign, yaw acceleration, the yaw acceleration reference, the yaw acceleration band and left turn understeer flag or value are provided and initialized. The left turn oversteer detection subroutine  346  then enters a process step  454  which computes a yaw acceleration limit as the product of the yaw acceleration reference and the yaw acceleration band. Next, a decision point  456  is entered which determines whether the yaw acceleration is greater than the yaw acceleration limit. If it is, the decision point  456  is exited at TRUE and a process step  458  is entered which sets a yaw acceleration flag or value equal to one. If the yaw acceleration is not greater than the yaw acceleration limit, the decision point  456  is exited at FALSE and a process step  462  sets a yaw acceleration flag or value equal to zero. 
   Next, a decision point  464  is entered which determines whether the yaw acceleration flag is set and equal to one or the yaw error sign equals minus one. If either of these statements are TRUE, the decision point  464  is exited at TRUE and a yaw error and acceleration value or flag is set at one in a process step  466 . If the interrogation in the decision point  664  is answered in the negative, it is exited at FALSE and a process step  468  sets a yaw error and acceleration flag or value to zero. Continuing on to  FIG. 13B , a decision point  472  is entered which inquires whether the yaw error and acceleration flag is set to one, whether the yaw sign is set to one and that there is not a left turn understeer. If all of these conditions are true, the decision point  472  is exited at TRUE and a process step  474  sets a left turn oversteer value or flag to one. If the statement is not true, the decision point  472  is exited at FALSE and a process step  476  sets a left turn oversteer value or flag to zero. An output step  478  provides a left turn understeer value or flag to the clutch selector logic illustrated in  FIG. 9B . 
   Referring now to  FIGS. 3 ,  14 A,  14 B and  14 C, the arbitrator module  256  selects how the overall system will operate and brokers or arbitrates the data provided by the left and right traction controller modules  250 A and  250 B and the dynamics controller module  252 . As such, it includes a selector  502  which determines the mode of operation of the arbitrator module  256 . In mode one, the multi-port switch  504  is in position one and selects data based upon speed and steering wheel angle. In operational mode two, it utilizes reference yaw rate with speed and in operational mode three, it utilizes reference yaw rate. As such, it is provided with data from the steering wheel angle sensor  52  of which the absolute value is taken by appropriate mathematical manipulation in a device  506 . This absolute value is provided to a comparator or relational operator  508 . A decision steering wheel angle threshold or reference  510  also provides a value to the relational operator  508 . The decision steering wheel angle reference  510  is a tunable parameter that may be empirically or experimentally selected or determined from a range of zero up to the maximum steering wheel angle. The relational operator  508  determines whether the steering wheel angle from the sensor  52  is less than the decision steering wheel angle threshold or reference  510 . If it is, a one or positive logic signal is provided to a logical OR operator  512 . If the value of the steering wheel angle from the sensor  52  is more than the decision steering wheel angle threshold or reference  510  a zero or null logic signal is outputted by the relational operator  510 . 
   Similarly, the vehicle speed from a previous calculation or an average from the four speed sensors  56 ,  58 ,  60  and  62  and a decision vehicle speed threshold or reference  514  also provides a vehicle speed value to a relational operator or comparator  518 . The decision vehicle speed threshold  514  is a tunable parameter that may be empirically or experimentally selected or determined from a range of zero up to the nominal or actual maximum vehicle speed. The relational operator  518  determines whether the current vehicle speed is less than or equal to the decision vehicle speed threshold  514 . If it is, a one or positive logic signal is provided to an input of the logical OR operator  512  and one input of another logical OR operator or device  522 . If the current vehicle speed is greater than the decision vehicle speed threshold  514 , the relational operator  518  outputs a logic zero or null signal to the logical OR operators  512  and  522 . 
   Both the logical OR operator  512  and the logical OR operator  522  operate conventionally and provide a positive or logic high or one output when either or both of their inputs receive positive or logic high or one inputs. 
   Signals from the logical OR operators  512  and  522  are provided to mode positions one and two, respectively, of the multi-port switch  504 . Depending upon the selected arbitrator mode, the selected output of the multi-port switch  504  is then provided to and controls a switch  524  which selects either slip control torque from the traction controller modules  250 A and  250 B or yaw control torque from the dynamics controller module  252 . The output of the switch is the arbitrated torque which is provided to the smart actuator modules  258 A and  258 B. 
     FIG. 14B  illustrates additional components of the arbitrator module  256 . The previously computed reference yaw rate is provided to an absolute value operator  532  and the absolute value of the reference yaw rate is provided to one input of a relational operator or comparator  534 . A yaw reference gain value  536  is also provided to the relational operator  534 . The yaw reference gain is a tunable parameter that may be empirically or experimentally selected or determined and has a value between zero and a maximum value. If the absolute value of the reference yaw rate is les than the yaw reference gain, the relational operator  534  provides a positive logic high or one value to one input of a logical OR operator  538 . If the absolute value of the reference yaw rate is greater than the yaw reference gain value  536 , the relational operator  534  outputs a logic zero or null signal to one input of the logical OR operator  538 . 
   Similarly, the current yaw rate is provided to an absolute value operator  542  and this absolute value is then provided to one input of a relational operator or comparator  544 . Provided to the other input of the relational operator  544  is the yaw reference gain value  536 A described directly above. This, as noted, is an empirically or experimentally determined value and preferably is the same as but may be different from the yaw reference gain value  536 . If the absolute value of the yaw rate is less than the value of the yaw reference gain  536 A, the relational operator  544  provides a positive logic or one value to the other input of the logical OR operator  538 . When one or both logic inputs to the logical OR operator  538  are positive or one, the logical OR operator provides a positive or one logic signal to one input of a three input logical AND operator  548 . A logical OR operator  552  having three inputs receives the logic output from the three input logical AND operator  548  on one of its three inputs 
   Turning now to  FIG. 14C , the proportional, integral derivative (PID) controller  278  illustrated in  FIG. 5  which also relates to the process step  324  appearing in  FIG. 8  is presented. This utilizes a yaw rate signal which is then provided to an absolute value operator  562  and yaw acceleration and a yaw acceleration limit  568  are provided to a relational operator  566 . The relational operator  566  determines whether the yaw acceleration is less than or equal to the empirically or experimentally generated value of the yaw acceleration limit  568 . If it is less, a yaw acceleration flag is set and the data is provided to the logical AND operator  548  illustrated in  FIG. 14B . In the lower portion of  FIG. 14C , right oversteer flags and left oversteer flags are set and this data is provided to the logical OR operator  552  illustrated in  FIG. 14B . If all three signals to the logical AND operator  548  are positive or TRUE, a signal is provided to one of the inputs of the logical or operator  552 . If any one of the inputs of the logical or operator  552  are a logic high, a logic high output is provided to logical or operator  522  illustrated in  FIG. 14A  as well as the third portion of the multi-port switch  504 . 
   The foregoing disclosure is the best mode devised by the inventors for practicing this invention. It is apparent, however, that apparatus incorporating modifications and variations will be obvious to one skilled in the art of motor vehicle rear axle components and control systems. Inasmuch as the foregoing disclosure is intended to enable one skilled in the pertinent art to practice the instant invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the spirit and scope of the following claims.