Title of Invention: SYSTEM AND METHOD FOR DETERMINING A STEERING ANGLE FOR A VEHICLE AND SYSTEM AND METHOD FOR CONTROLLING A VEHICLE BASED ON SAME Abstract: A system for determining an angular position of a steerable wheel of a vehicle can include a plurality of sensors and a controller in electrical communication with each of these sensors. The controller can be configured to compare a first data set and a second data set to respective straight path conditions, wherein the first data set includes data from a first set of the sensors and the second data includes data from a second set of the sensors. The controller can be configured to determine a substantially straight path of travel condition when the comparison of at least one of the first data set and the second data set satisfies the respective straight path conditions, and can then associate that data acquired from the steering angle sensor(s) with a neutral position. Detailed Description: CROSS REFERENCE TO RELATED APPLICATIONS This application claims the priority benefit under 35 U.S.C. §120 and is a continuation-in-part of co-pending U.S. patent application Ser. No. 12/857,359 filed on Aug. 16, 2010. BACKGROUND 1. Field The presently disclosed subject matter relates to devices, systems, and processes useful as an assistance feature to supplement an operator's control input to a vehicle. 2. Description of the Related Art Vehicles have typically included a system(s) that assists an operator's input for the control of the vehicle. This assistance can supplement the operator's control of the vehicle to enhance comfort for the operator/passenger(s) and/or to enhance dynamic performance of the vehicle. Examples of an operator assistance system can include, but are not limited to, power steering systems, anti-lock braking systems (ABS), traction control systems (TCS), and stability assistance systems (aka, VSA, VSC or ESP)—which can be, active suspension systems, all-wheel drive systems (AWD) and four-wheel steering systems (4ws). Some of these operator assistance systems can signal the operator to alter their input through visual, aural or tactile notification. Examples of these systems can include, but are not limited to, active braking systems, lane departure warning systems, parking assistance systems and obstacle avoidance systems. Each of these operator assistance systems are generally known in the art. The operator assistance system can include a controller—also called an electronic control unit (ECU) or a central processing unit (CPU)—at least one sensor, and at least one actuator in electrical communication with the ECU. The ECU often relies on data generated by a steering angle sensor so that the ECU can interpret the operator's input and send an appropriate signal(s) to the appropriate actuator(s) to supplement the operator's input. In many of these operator assistance systems, it can be advantageous for the ECU to rely on data from the steering angle sensor to determine whether the operator is attempting to steer the vehicle to the left or to the right relative to a neutral position, as well as to determine the magnitude of the directional change (relative to the neutral position) intended by the operator. Typically, the neutral position corresponds to the position of the steering system components and the steering angle sensor when the vehicle travels along a substantially straight path. It is known in the art to position the steering angle sensor within the steering system of the vehicle such that the steering angle sensor can detect the operator's steering input. The steering angle sensor can be an absolute position or a relative position sensor. Both types of steering sensors and their implementation are known in the art. An absolute position sensor can indicate, at any time, the position of the steering wheel relative to the neutral position. As a result, an absolute steering position sensor can be a complicated device and can be expensive to produce. Further, in some of these operator assistance systems, it may be unnecessary to determine the neutral position for appropriate operation of the system. Thus, an absolute position sensor can increase the cost and/or complexity of the operator assistance system with no corresponding benefit. In contrast, a relative position sensor can be less complex in design and less costly compared to an absolute position sensor. However, a relative position sensor can detect only a change in position of the steering wheel relative to the previously detected position. Thus, the raw sensor data can neither indicate the direction relative to the neutral position to which the operator has turned the steering wheel nor indicate how far from the neutral position the operator has turned the steering wheel. In order to translate the raw sensor data into absolute position data, additional electronic circuitry and/or software can be provided in the sensor assembly, or in the ECU, so that the neutral position can be determined either by the sensor or the ECU. This additional circuitry can offset or exceed the cost and/or the complexity advantage(s) that the relative position sensor can have over an absolute position sensor. Operator assistance systems known in the art that employ a relative position sensor and also rely on determination of the neutral position typically do not store the neutral position of the steering angle sensor after the vehicle is shut down. Thus, the neutral position determination algorithm is performed each time the vehicle is started. Operation of this algorithm can be dependent on a particular travel path(s) taken by the vehicle. As such, it can take a finite period of time to complete. Thus, the vehicle assistance system can be rendered ineffective prior to completion of the neutral position determination algorithm. There is, and has been, a need to provide a system and method that can obtain and process real-time data indicative of the operator's steering direction input (i.e., left or right) and the magnitude of the steering input (i.e., how far from neutral the driver has rotated the steering wheel) via a relative position sensor as close to vehicle start-up as possible, where the system and method are relatively simple in design and have a minimum cost. In order to provide the operator assistance system utilizing a relative position steering angle sensor with relevant steering angle data, there is a need to provide an estimated steering angle that can approximate the actual angle of the steerable wheel(s) of the vehicle until the neutral position algorithm is complete. Further, a failure of the steering sensor or interruption of data from the sensor can cause the operator input assistance system to shut down (or otherwise be impaired to a certain degree) until the failure or interruption is resolved, thereby canceling the assistance to the operator's input. Thus, there is a need to provide a failure mode where the steering angle can be approximated from at least one other vehicle dynamic sensor so that the operator input assistance system can operate in a sufficient manner despite a failure of the steering angle sensor. SUMMARY According to one aspect of the disclosure a system for determining an angular position of a pair of steerable wheels of a vehicle having a plurality of wheels can include a relative position steering angle sensor, a yaw rate sensor, a lateral acceleration sensor and a plurality of speed sensors. The relative position steering angle sensor can be configured to output data indicative of a change in position of the pair of steerable wheels. The yaw rate sensor can be configured to output data indicative of an angular velocity of the vehicle about a vertical axis of the vehicle. The lateral acceleration sensor configured to output data indicative of an acceleration of the vehicle in a substantially transverse direction of the vehicle. Each wheel speed sensor can be configured to output data indicative of a rotational velocity of a respective one of the plurality of wheels. The controller can be in electrical communication with each of the pair of wheel speed sensors, the lateral acceleration sensor, and the yaw rate sensor. The controller can be configured to: compare a first data set and a second data set to respective straight path conditions, wherein the first data set includes data from the yaw rate sensor and data from the lateral acceleration sensor and the second data includes data from the plurality of wheel speed sensors; determine a substantially straight path of travel condition when the comparison of at least one of the first data set and the second data set satisfies the respective straight path conditions; and associate data acquired from the steering angle sensor with a neutral position when the controller determines the substantially straight path of travel condition. According to another aspect of the disclosed subject matter, a method for determining a steering angle of a pair of steerable wheels of a vehicle having a plurality of wheels can include: comparing a first data set and a second data set to respective straight path conditions, wherein the first data set includes data from a yaw rate sensor and data from a lateral acceleration sensor and the second data includes data from a plurality of wheel speed sensors; determining a substantially straight path of travel condition when the comparison of at least one of the first data set and the second data set satisfies the respective straight path conditions; and associating data acquired from a steering angle sensor with a neutral position when the controller determines the substantially straight path of travel condition. According to still another aspect of the disclosed subject matter, a control system for an all-wheel drive powertrain of a vehicle having a power source configured to produce torque, a steerable pair of wheels configured to be driven by the torque, and a second pair of wheels configured to be selectively driven by the torque, the control system can include a first clutch assembly, a second clutch assembly, a yaw rate sensor, a lateral acceleration sensor, a plurality of wheel speed sensors, and a controller. The first clutch assembly can be selectively connecting a first wheel of the second pair of wheels to the power source. The second clutch assembly can be selectively connecting a second wheel of the second pair of wheels to the power source. The yaw rate sensor can be configured to output data representative of an angular velocity of the vehicle about a vertical axis of the vehicle. The lateral acceleration sensor can be configured to output data indicative of an acceleration of the vehicle in a substantially transverse direction of the vehicle. Each wheel speed sensor can be configured to output data indicative of a rotational velocity of a respective one of the plurality of wheels. The controller can be in electrical communication with each of the sensors and the first and second clutch assemblies. The controller can be configured to: determine a neutral position for the steering angle sensor; determine an estimated steering angle until the controller determines the neutral position; modulate the clutches according to the data from the steering angle sensor after the controller determines neutral position; and modulate the clutches according to the estimated steering angle until the controller determines the neutral position. According to yet another aspect of the disclosed subject matter, a method for controlling an on-demand all-wheel drive system of a vehicle to supplement an operator's input to the vehicle comprising: providing a power source, a pair of steerable wheels driven by the power source, a pair of rear wheels selectively driven by the power source, a pair of rear clutch assemblies selectively connecting a respective one of the pair of rear wheels to the power source, a yaw rate sensor configured to output data representative of an angular velocity of the vehicle about a vertical axis of the vehicle, a lateral acceleration sensor configured to output data indicative of an acceleration of the vehicle in a substantially transverse direction of the vehicle, and a plurality of wheel speed sensors configured to output data representative of a rotational speed of a respective one of the pair of steerable wheels and the pair of rear wheels; determining a neutral position for the steering angle sensor; determining an estimated steering angle until the neutral position is determined; modulate the clutches according to the data from the steering angle sensor after the neutral position is determined; and modulate the clutches according to the estimated steering angle until the neutral position is determined. BRIEF DESCRIPTION OF THE DRAWINGS The disclosed subject matter of the present application will now be described in more detail with reference to exemplary embodiments of the apparatus and method, given by way of example, and with reference to the accompanying drawings, in which: FIG. 1 is a schematic view of an exemplary powertrain for a vehicle made in accordance with principles of the disclosed subject matter. FIG. 2 is a flowchart depicting an exemplary algorithm in accordance with principles of the disclosed subject matter. FIG. 3 is a flowchart depicting an exemplary algorithm usable as a subroutine of the algorithm of FIG. 2. FIG. 4 is a flowchart depicting an exemplary algorithm usable as a subroutine of the algorithm of FIG. 3. FIG. 5 is a table of parameters usable in the algorithm depicted in FIG. 4 to estimate when a vehicle could be traveling along a straight path. FIG. 6 illustrates graphs of lateral threshold versus speed data usable with the algorithm of FIG. 4. FIG. 7 illustrates graphs of yaw threshold versus speed data usable with the algorithm of FIG. 4. FIG. 8 is a schematic depicting various physical parameters of the vehicle of FIG. 1 traversing a curved path. FIG. 9 is a flowchart depicting an exemplary algorithm usable as a subroutine of the algorithm of FIG. 3, which algorithm can be executed in parallel to that depicted in FIG. 4. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS FIG. 1 illustrates an embodiment of a powertrain 10 for a vehicle 12, where the vehicle 12 has a longitudinal direction L, a transverse (or lateral) direction T perpendicular to the longitudinal direction, and a vertical direction V perpendicular to both the longitudinal direction L and the transverse direction T. The powertrain 10 can be configured as an on-demand, part-time, all-wheel drive system in accordance with the principles of the disclosed subject matter. This exemplary powertrain 10 can be configured such that the steerable front wheels 14L, 14R are the primary drive wheels and the rear wheels 16L, 16R are selectively driven automatically when additional tractive effort is appropriate for the given vehicle conditions. However, the powertrain 10 can be configured such that the rear wheels 16L, 16R are the primary drive wheels and the front wheels 14L, 14R are driven only when additional tractive effort is appropriate. In other embodiments, the powertrain 10 can be configured as: a full-time, all-wheel drive system; a manually-engageable, part-time all-wheel drive system; a front-wheel drive system; or a rear-wheel drive system, all of which are generally known in the art. The powertrain 10 can include a power source 18, a transmission 20, the pair of front wheels 14L, 14R, the pair of rear wheels 16L, 16R, a front differential assembly 22, a pair of front driveshafts 24L, 24R, a power-take-off assembly 26, a propeller shaft 28, a rear differential assembly 30, a pair of rear driveshafts 32L, 32R and a control system 34 arranged in a manner known in the art. The control system 34 can be configured to automatically engage/disengage the rear wheels 16L, 16R with the power source 18, as appropriate. An exemplary control system is disclosed in co-pending U.S. patent application Ser. No. 12/847,880, (Attorney Docket No. 3009-0108), entitled “Control System and Method for Automatic Control of Selection of On-Demand All-Wheel Drive Assembly for A Vehicle Drivetrain”, and filed Jul. 30, 2010, which is incorporated herein by reference in its entirety. The control system 34 also can be configured to determine a steering angle signal, δAWD, which can correspond with the real-time toe angle of the front wheels 14L, 14R targeted by the vehicle operator and can control the vehicle 12 based on the steering angle signal, δAWD, in accordance with principles of the disclosed subject matter. The control system 34 can include one or more of a controller 36, a steering angle sensor 38, a yaw rate sensor 40, a pair of rear wheel speed sensors 42L, 42R, a pair of front wheel speed sensors 43L, 43R, a transmission output shaft speed sensor 44, a lateral acceleration sensor 45 and a pair of clutch assemblies 46L, 46R. The controller 36 can be in electrical communication with each of the sensors 38, 40, 42L, 42R, 43L, 43R, 44 and each of the clutch assemblies 46L, 46R. The controller 36 can be referred to as an electronic control unit (ECU) or as a central processing unit (CPU). The pair of clutch assemblies 46L, 46R can be mounted within the rear differential assembly 30. The steering angle sensor 38 can be a relative position sensor such that the sensor 38 indicates position data relative to the previous position and not relative to a predetermined constant position such as the neutral position of the steering wheel. The steering angle sensor 38 can be located adjacent a component of the steering system (not illustrated) that lies between the steering wheel (not illustrated) and either one of the front wheels 14L, 14R. The steering angle sensor 38 can measure movement of this component when the vehicle operator rotates the steering wheel. For example, the steering angle sensor 38 can lie adjacent the steering column (not illustrated) to detect rotary movement of the steering column. In another example, the steering angle sensor 38 can lie adjacent the steering rack (not illustrated) to detect linear movement of the steering rack. In yet another example, the steering angle sensor 38 can be associated with an electric power steering (EPS) motor (not illustrated) to detect movement of the EPS motor relative to the steering rack. Other configurations of the steering angle sensor 38 are also possible. The data from the steering angle sensor 38 can be processed by the steering angle sensor 38 or by the ECU 36 to determine the steering angle signal, δAWD. From the steering angle signal, δAWD, the ECU 36 can determine the operator's intended directional target for the vehicle 12. Further details regarding the determination and usage of the steering angle signal, δAWD, by the ECU 36 are provided below. The yaw rate sensor 40 can be mounted on an appropriate portion of the vehicle 12 to detect angular rotation of the vehicle 12 about an axis extending in the vertical direction V. The raw data from the yaw rate sensor 40 can be processed by the yaw rate sensor 40 or by the ECU 36 to indicate an angular velocity of the vehicle 12 about its vertical axis V. Yaw rate sensors and their use in vehicular control systems are generally known in the art. The wheel speed sensors 42L, 42R can be mounted on an appropriate portion of the vehicle 12 to detect rotation of the respective rear wheel 16L, 16R (or the respective rear driveshaft 321, 32R) and are generally known in the art. The raw data from the wheel speed sensors 42L, 42R can be processed by one or both of the wheel speed sensors 42L, 42R or by the ECU 36 to indicate a rotational velocity of the respective rear wheels 16L, 16R. The wheel speed sensors 42L, 42R can be any sensor known in the art to provide the appropriate data. Alternatively, instead of with the ECU 36, the wheels speed sensors 42L, 42R can be in electrical communication with an additional controller (not illustrated) that is different from the ECU 36. This additional controller can be in electrical communication with the ECU 36. This additional controller can be utilized to affect a different control system of the vehicle 12, such as, but not limited to, an anti-lock brake system (not illustrated). This additional controller can be configured to receive processed data from the wheel speed sensors 42L, 42R or to receive the raw data from the wheel speed sensors 42L, 42R and process the raw data to indicate a rotational velocity of the respective wheels 12L, 16R. Then, this additional controller can electrically communicate the processed data to the ECU 36. The transmission output shaft speed sensor 44 can be adjacent an output shaft (not shown) of the transmission 20. The transmission output shaft speed sensor 44 can detect rotation of the output shaft. The raw data can be processed by the transmission output shaft speed sensor 44 or by the ECU to indicate a rotational velocity of the output shaft and its use is generally known in the art for conversion into a travel velocity of the vehicle. The transmission output shaft speed sensor 44 can be any sensor known in the art to provide the appropriate data. Alternatively, the output shaft speed sensor 44 can be omitted from the control system 34 and data from the wheel speed sensors 42L, 42R can be used to indicate the travel velocity of the vehicle 12. In this alternate embodiment, the vehicle 12 further can include front wheel speed sensors 43L, 43R. The signals from the wheel speed sensors 42L, 42R, 43L, 43R can be averaged, or they can be subject to a minimum function analysis, or another appropriate process can be employed to determine on which one of the wheel speed sensors 42L, 42R, 43L, 43R to rely. It is also possible assign only one of the sensors 42L, 42R, 43L, 43R to provide data indicative of the vehicle travel velocity. This assignment can be fixed, randomly selectable, or selected based on some operating condition, such as but not limited to, the fault status of each sensor 42L, 42R, 43L, 43R, or the relative magnitudes of the data from each wheel speed sensor 42L, 42R, 43L, 43R, or the turning direction of the vehicle 12, or whether the front wheels 14L, 14R are driving the vehicle 12, or whether the front wheels 14L, 14R and the rear wheels 16L, 16R are driving the vehicle 12. The lateral acceleration sensor 45 can be mounted on an appropriate portion of the vehicle 12 to sense acceleration of the vehicle 12 in the transverse direction T. The raw data from the lateral acceleration sensor 45 can be processed by the lateral acceleration sensor 45 or by the ECU 36 to indicate an acceleration of the vehicle 12 in the transverse direction T. Based on data received from any combination of the sensors 38, 40, 42L, 42R, 43L, 43R, 44, 45 and any other inputs known in the art to be appropriate, the ECU 36 can manipulate the clutch assemblies 46L, 46R to vary the distribution of torque from the power source 18 to a respective one of the rear wheels 16L, 16R in a manner that can enhance the performance of the vehicle 12 as it enters, traverses and/or exits a curve, or experiences other driving conditions that warrant operator assistance. That is, the ECU 36 can supplement the steering input to the front wheels 14L, 14R by driving the appropriate one of the rear wheels 16L, 16R (e.g., the outside wheel) with more torque than the other of the rear wheels 16L, 16R (e.g., the inside wheel). Thus, the control system 24 can manipulate torque distribution to the rear wheels 16L, 16R in order to reduce, correct and/or prevent an understeering or oversteering condition of the vehicle 12 as it enters, traverses and/or exits a curve. FIG. 2 depicts an exemplary algorithm that the ECU 36 can follow to modulate the clutch assemblies 46L, 46R as mentioned above during its control of the all-wheel drive system (AWD). The ECU 36 can begin the AWD control algorithm at step S100. At step S102, the ECU 36 can determine the steering angle signal, δAWD, based on data received from the array of sensors 38, 40, 42L, 42R, 44. Details of an exemplary determination of the steering angle signal, δAWD, by the ECU 36 will be explained below with reference to FIGS. 3-5. After the ECU 36 determines the steering angle signal, δAWD, the ECU 36 can proceed to step S104 where the ECU 36 can determine whether torque vectoring through the rear wheels 16L, 16R is appropriate for the given dynamic conditions of the vehicle 12. That is, the ECU 36 can determine at step S104 whether to supplement the operator's input to the steering wheel by driving one of the rear wheels 16L, 16R with more torque that the other of the rear wheels 16L, 16R. In general, when the steering angle signal, δAWD, indicates the neutral position, the ECU 36 can determine that the vehicle 12 is traveling along a substantially straight path or that the operator intends to direct the vehicle along a substantially straight path. When the steering angle signal, δAWD, does not indicate the neutral position, the ECU 36 can determine that the operator of the vehicle 12 is either attempting to alter the direction in which the vehicle 12 is traveling or the operator is directing the vehicle along a curved path. Thus, the steering angle signal, δAWD, can be analyzed by the ECU 36 in conjunction with other dynamic data, such as, but not limited to, yaw rate, lateral acceleration, longitudinal acceleration and individual wheel speeds, to determine whether to supplement the driver's steering input with torque vectoring through the wheels 16L, 16R. Torque vectoring control is generally known in the art and will not be further described. An example of this function of the controller 36 can be found in U.S. Patent Application Publication No. 2007/0260388, which is incorporated herein by reference in its entirety. If the ECU 36 turns on the torque vectoring feature of the control system 34 at step S104, then the ECU 36 can proceed to step S106. At step S106, the ECU 36 can modulate the torque distribution individually through each of the clutch assemblies 46L, 46R such that an appropriate one of the rear wheels 16L, 16R is driven with more torque from the power source 18 than the other of the rear wheels 16L, 16R. The torque differential between the left rear wheel 16L and the right rear wheel 16R can be a function of the steering angle signal, δAWD, as is generally known in the art—see, for example, U.S. Patent Application Publication No. 2007/0260388, referenced above. The ECU 36 can then proceed to step S108 where ECU 36 can exit the AWD control algorithm. If the ECU 36 turns off the torque vectoring feature of the control system 34 at step S104, then the ECU 36 can proceed to step S110. At step S110, the ECU 36 can modulate the torque distribution equally through each of the clutch assemblies 46L, 46R such that both rear wheels 16L, 16R are driven with substantially equal torque when no torque modulation or vectoring is applied. The ECU 36 can then proceed to step S108 where ECU 36 can exit the AWD control algorithm. The determination by the ECU 36 of the steering angle signal, δAWD, can be assisted by data received from the steering angle sensor 38, which data can indicate to the ECU 36 the operator's targeted direction of travel for the vehicle 12. The steering angle sensor 38 can provide data representative of the direction and magnitude of any movement of the steering wheel by the vehicle operator in real-time. The steering angle sensor 38 can be positioned in appropriate proximity to an appropriate component of the steering system of the vehicle 12 in any manner described above or known in the art. As discussed above, the steering angle sensor 38 can be an absolute position sensor or a relative position sensor. In this exemplary embodiment, the steering angle sensor 38 can be a relative position sensor 38. As such, the steering angle sensor 38 can provide data that indicates the direction and magnitude of movement from the last position transmitted to the controller 36. Hence, the steering angle sensor 38 does not provide the ECU 36 with sensor position data relative to the neutral position. As will be described in detail below, the ECU 36 or the steering angle sensor 38 can be configured with hardware and/or software to enable the ECU 36 or the steering angle sensor 38 to translate the raw sensor data into data representing the magnitude and direction of the operator's steering input relative to the neutral position. FIG. 3 illustrates a flowchart of an algorithm that the ECU 36 can perform in order to determine a real-time determination of the steering angle signal, δAWD, referenced in step S102 of FIG. 2. The ECU 36 can begin the steering angle determination algorithm at step S102, and then can proceed to step S112. At step S112, the ECU 36 can determine whether the ECU 36 has started its calibration of the steering angle sensor 38 so that the neutral position of the steering angle sensor 38 can be determined. Calibration of the sensor 38 can be advantageous after a shut-down of the power source 18, a power interruption to the ECU 36 and/or the steering angle sensor 38, a temporary malfunction of the ECU 36 and/or the steering angle sensor 38, and/or other similar event where prior calibration data can be deleted/lost from a memory device of the control system 34. If the ECU 36 determines at step S112 that the calibration algorithm has not been started, then the ECU 36 can proceed to step S114 where the ECU 36 can start the steering angle calibration algorithm. The steering angle sensor calibration algorithm of step S114 can take multiple iterations to complete. As a result, the ECU 36 can be configured to operate this algorithm in parallel with the steering angle determination algorithm of FIG. 3. This can permit the ECU 36 to advantageously manipulate the rear clutch assemblies 46L, 46R even though the neutral position of the steering angle sensor 38 is undetermined. The steering angle sensor calibration algorithm represented by step S114 will be described with later reference to FIGS. 4-8. After the ECU 36 starts the steering angle sensor calibration algorithm at step S114 or after the ECU 36 determines at step S112 that the steering angle sensor calibration algorithm has been started, the ECU 36 can proceed to step S116. As will be explained in further detail below, after each iteration of the steering angle calibration algorithm, the ECU 36 can store a value that can indicate the status (e.g., complete or incomplete) of the steering angle calibration algorithm. At step S116, the ECU 36 can determine whether the steering angle calibration algorithm has been completed by the ECU 36 by retrieving and comparing this value during step S116. In an alternate embodiment, the steering angle sensor 38 can be configured to perform the steering angle sensor calibration algorithm of step S114 (instead of the ECU 36). In this alternate arrangement, the steering angle sensor 38 can signal to the ECU 36 the start and status (e.g., complete or incomplete) of the steering angle algorithm. If the ECU 36 determines at step S116 that the steering angle sensor calibration algorithm is complete, then the ECU 36 can proceed to step S118. At step S118, the ECU 36 can employ data from the steering angle sensor calibration algorithm and real-time data from the steering angle sensor 38 to determine the steering angle signal, δAWD, in real-time. An example of the calculation useful for the determination of the steering angle signal, δAWD, can be made according to the equation: δAWD=δsensor−δneutral where: δsensor is the real-time data from the steering angle sensor 38; and δneutral is the value of the neutral position of the steering angle sensor 38 obtained from the steering angle sensor calibration algorithm initiated at step S114. If the ECU 36 determines at step S116 that the steering angle sensor calibration algorithm is not yet complete, then the ECU 36 can proceed to step S120. At step S120, the ECU 36 can follow an algorithm that permits the ECU 36 to estimate the steering angle signal, δAWD, based on data received by the ECU 36 from vehicle sensors other than the steering angle sensor 38. This estimate of the steering angle signal, δAWD, can provide an approximation of the real-time toe angle of the front wheels 14L, 14R. Thus, the ECU 36 can employ the steering angle signal, δAWD, with improved accuracy as compared to a pre-defined constant value. Details of this algorithm will be described below with reference to FIGS. 8 and 9. After completion of either step S118 or step S120, the ECU 36 can move to step S122. Here, the ECU 36 can return to the AWD control algorithm at step S102 and continue through the algorithm beginning at step S104, as described above, to modulate the rear clutch assemblies 46L, 46R based on the steering angle signal, δAWD, determined at step S118 or at step S120. When the control system 34 employs a relative position steering angle sensor 38, the ECU 36 can enhance its determination of an appropriate steering angle signal, δAWD, by determining how far and in which direction from its neutral position the operator has moved the steering wheel (not shown). As, mentioned above, the steering angle calibration algorithm of step S114 can permit the ECU 36 to make this determination. FIG. 4 illustrates an exemplary embodiment of the steering angle sensor calibration algorithm. Generally, the vehicle 12 will travel along a substantially straight path when the operator positions the steering wheel in its neutral position. This relationship between steering wheel neutral position and a substantially straight path of travel by the vehicle 12 can form a basis for this exemplary steering angle sensor calibration algorithm. That is, the ECU 36 can determine that data acquired from the relative position steering angle sensor 38 when the vehicle 12 follows a substantially straight path represents a neutral position value, δneutral. The steering angle sensor calibration algorithm can permit the ECU 36 to determine the neutral position value, δneutral, from data provided by sensors other than the steering angle sensor 38, such as but not limited to, the yaw rate sensor 40, the wheel speed sensors 42L, 42R, 43L, 43R, the output shaft speed sensor 44, the lateral acceleration sensor 45, a longitudinal acceleration sensor (not illustrated), and a rotational acceleration sensor (not illustrated). The steering angle sensor calibration algorithm can begin at step S114. Then the ECU 36 can proceed to step S138. At step S138, the ECU 36 can store internally or externally data indicative of the status (e.g., started, complete, incomplete) of the steering angle sensor calibration algorithm. This data can be later retrieved by the ECU 36 at steps S112 and S116 of the steering angle determination algorithm described above. The ECU 36 can proceed to step S140 where the ECU 36 can make a judgment regarding the travel path of the vehicle 12 using data from various sensors in electrical communication with the ECU 36. In particular, step S140 can permit the ECU 36 to estimate when the vehicle 12 is traveling along a substantially straight path. While the vehicle 12 travels along a substantially straight path, then the ECU 36 can determine that a particular steering sensor data value, δsensor, n, can correspond to a neutral position value, δneutral, for the steering angle sensor 38. Exemplary details of this judgment will be described with reference to FIGS. 5-8. The ECU 36 can follow two separate and independent algorithms in order to determine when the vehicle 12 follows a substantially straight path of travel. Each of these algorithms can permit the ECU 36 to estimate when a substantially straight travel path occurs independent of the other algorithm. The first of these two independent algorithms (first straight judgment) can rely on data obtained by the wheel speed sensors 42L, 42R, 43L, 43R, the yaw rate sensor 40, and the lateral acceleration sensor 45 along with laboratory data compiled into look-up tables. The second of these two independent algorithms (second straight judgment) can rely on data obtained from the rear wheel speed sensors 42L, 42R, the yaw rate sensor 40, and optionally the output shaft speed sensor 44. The ECU 36 can execute these two straight judgment algorithms in any order or simultaneously. Also, the ECU 36 can forgo execution of one of these two straight judgment algorithms if the ECU 36 estimates from the other straight judgment algorithm that the vehicle is traveling along a substantially straight path. Using two separate and independent algorithms, the ECU 36 can process a respective unique grouping of data provided by these alternate sensors. These two independent algorithms can complement each other when: (a) the wheel speed is stable and not accelerating; (b) the yaw rate and lateral acceleration data fall within respective straight driving ranges; (c) the determined neutral position value, δneutral, is within a predetermined tolerance of a zero degree position for the steering wheel; and (d) the steering angle sensor data, δsensor, is stable over a predetermined period of time (i.e., the steering angle sensor data, δsensor, indicates little or no movement of the steering wheel during the predetermined period of time). Thus, these two independent assessments of a substantially straight path of travel by the vehicle 12 can improve the speed and reliability for the calibration of the steering angle sensor 38. FIG. 5 lists a plurality of exemplary data comparisons that can be utilized by the ECU 36 when following each of these separate and independent judgment algorithms. This listing of comparisons can be amended by addition, replacement and/or deletion in order to meet the intended performance specifications for the system. The first straight judgment algorithm can rely on data from the rear wheel speed sensors 42L, 42R, the front wheel speed sensors 43L, 43R, the yaw rate sensor 40, and the lateral acceleration sensor 45. The second straight judgment can rely on data from the rear wheel speed sensors 42L, 42R, the yaw rate sensor 40 and the output shaft speed sensor 44. Thus, the ECU 36 can judge a substantially straight path of travel by the vehicle 12 based on two independent data sets. The first straight judgment algorithm can evaluate five parameters: 1) wheel slip; 2) cruise status (e.g., substantially no acceleration); 3) estimated straight travel as a function of yaw rate; 4) estimated straight travel as a function of lateral acceleration; and 5) minimum speed to achieve precision for estimated straight travel. The analysis of these parameters can be performed in any order and the threshold values relied upon for comparison of these parameters can be set in accordance with the desired performance specifications for the vehicle 12. In another embodiment, any one or any combination of these parameters can be omitted or can be replaced with any other parameter(s) appropriate to achieve the desired performance specifications for the vehicle 12. In another embodiment, parameters in addition to those described can be considered in order to achieve the desired performance specifications for the vehicle 12. In the first data comparison of the first straight judgment outlined in FIG. 5, a plurality of wheel slip ratios can be analyzed by the ECU 36 in accordance with step S140 of FIG. 4. Each wheel slip ratio analyzed can indicate the percent difference between speeds of different pairs of the wheels 14L, 14R, 16L, 16R. When the vehicle 12 travels along a straight path, each of the wheels 14L, 14R, 16L, 16R can rotate at substantially the same speed. Thus, a slip ratio substantially equal to zero can be indicative of a substantially straight travel path by the vehicle 12. A slip threshold value that corresponds to a substantially straight path can be predetermined according to the desired precision and accuracy for the straight travel estimation and the dynamic performance desired for the vehicle 12. In an exemplary embodiment, the slip threshold can be set at 4%. Thus, if the ECU 36 determines at step S140 that the slip ratios described above are each less than the slip threshold, then the ECU 36 can determine that one possible parameter of straight travel is met. During step S140 of FIG. 4, the ECU 36 can use real-time data from the wheel speed sensors 42L, 42R, 43L, 43R to determine the slip ratio between: a) the front wheels 14L 14R; b) the rear wheels 16L, 16R; and c) the average of the two front wheel speeds and the average rear wheel speeds. The ECU 36 can compare each of these slip ratios to the slip threshold. The value of the slip threshold can be identical or unique for each of the determined slip ratios, as required to obtain the desired performance specification of the vehicle 12. In an alternate embodiment, the slip ratio of one or more pairs of diagonal wheels can be analyzed by the ECU 36 during step S140. This diagonal ratio comparison can be in addition to or in place of any one or combination of the slip ratio comparisons described above. In the second data comparison of the first straight judgment, the ECU 36 can compare an acceleration value to an acceleration threshold value. The acceleration threshold value can represent a cruising condition for the vehicle 12 where the acceleration of the vehicle 12 is substantially zero. Thus, this comparison can indicate that the wheel speed sensors 42L, 42R, 43L, 43R are providing stable wheel speed data. The acceleration threshold value can be any appropriate predetermined value that can provide the desired precision and accuracy for the straight travel estimate and the dynamic performance desired for the vehicle 12. An exemplary acceleration threshold value can be 0.3 g, where g represents the acceleration value of gravity. The acceleration value can indicate the acceleration of the front axle, the rear axle, or the propeller shaft 28. The axle acceleration value can be derived from any combination of the wheel speed sensors 42L, 42R, 43L, 43R. In an alternate embodiment, the axle acceleration value can be provided to the ECU 36 by an accelerometer (not illustrated)—or other appropriate sensor known in the art—associated with an appropriate component of the drivetrain, such as but not limited to the front differential assembly 22 or any of the shafts 24L, 24R, 28, 32L, 32R, in any manner known in the art. In another alternate embodiment, the ECU 36 can acquire the acceleration value from a longitudinal acceleration sensor (not shown) mounted on an appropriate portion of the vehicle 12 in any manner known in the art. In the exemplary listing of FIG. 5, the front axle acceleration can be used for this comparison where the ECU 36 can be configured to derive the front axle acceleration value from data acquired from one or both of the front wheel speed sensors 43L, 43R. In the third data comparison of the first straight judgment of FIG. 5, the ECU 36 can compare a lateral acceleration value to a lateral threshold. The lateral acceleration sensor 45 can provide the ECU 36 with either raw data or processed data, which the ECU 36 can utilize for comparison to the lateral threshold. The lateral threshold can be a predetermined value that can indicate substantially straight travel in accordance with the desired precision and accuracy for the straight travel estimate and the dynamic performance desired for the vehicle 12. If the lateral acceleration sensor data is less than the lateral threshold, then ECU 36 can determine at step S140 of FIG. 4 that at least one parameter indicates that the vehicle 12 is probably traveling along a substantially straight path. The lateral threshold can be a constant predetermined value or it can be a variable predetermined value. Generally, lateral acceleration varies as a function of vehicle speed. Thus, using a lateral threshold that can vary as a function of speed can enhance the precision and accuracy of the straight travel estimation during step S140. FIG. 6 illustrates two plots of lateral acceleration versus wheel speed taken empirically—one with the steering angle, δ, held at approximately 6° and one with the steering angle, δ, held at approximately 25°—for a particular embodiment of the vehicle 12. As shown by FIG. 6, the plot of the lateral acceleration data when the steering angle, δ, is maintained at 6° is unique from the plot of the lateral acceleration data when the steering angle, δ, is maintained at 25° . Thus, the steering angle, δ, can be estimated from speed data in combination with lateral acceleration data. A threshold value for the steering angle, δ, for implementing the data available from FIG. 6 can be predetermined to correspond to a substantially straight path of travel in accordance with the desired precision and accuracy for the straight travel assessment and the dynamic performance desired for the vehicle 12. In an exemplary embodiment, a steering angle, δ, of less than 6° can correspond to a substantially straight path of travel for the vehicle 12. Thus, the range of lateral acceleration values within the shaded region A of FIG. 6 can be used as lateral threshold values which can be interpreted by the ECU 36 as indicative of a substantially straight path. The empirical plots of lateral acceleration data can be unique to a particular embodiment of the vehicle 12 due to many variables, such as but not limited to vehicle weight distribution, and suspension and tire characteristics. Thus, the shaded region A can be unique to each particular embodiment of the vehicle 12. In the fourth data comparison of the first straight judgment, the ECU 36 can compare a yaw rate value to a yaw threshold. The yaw rate sensor 40 can provide the ECU 36 with either raw data or processed data, which the ECU 36 can utilize for comparison to the yaw threshold. The yaw threshold can be a predetermined value that can indicate substantially straight travel in accordance with the desired precision and accuracy for the straight travel assessment and the dynamic performance desired for the vehicle 12. If the yaw rate data is less than the yaw threshold, then ECU 36 can determine at step S140 that at least one parameter indicates that the vehicle 12 can be traveling along a substantially straight path. The yaw threshold can be a constant predetermined value or it can be a variable predetermined value. Generally, yaw rate varies as a function of vehicle speed. Thus, using a yaw threshold that can vary as a function of speed can enhance the precision and accuracy of the straight travel assessment. FIG. 7 illustrates two plots of yaw rate versus wheel speed taken empirically—one with the steering angle, δ, held at approximately 6° and one with the steering angle, δ, held at approximately 25°—for a particular embodiment of the vehicle 12. As shown by FIG. 7, the plot of the yaw rate data when the steering angle, δ, is maintained at 6° is unique from the plot of the yaw rate data when the steering angle, δ, is maintained at 25° . Thus, the steering angle, δ, can be estimated from speed data in combination with yaw rate data. A threshold value for the steering angle, δ, for implementing data of FIG. 7 into step S140 can be predetermined to correspond to a substantially straight path of travel in accordance with the desired precision and accuracy for the straight travel assessment and the dynamic performance desired for the vehicle 12. This threshold value can be the same as or different from that used to with respect to the data of FIG. 6. In an exemplary embodiment, a steering angle, δ, of less than 6° can correspond to a substantially straight path of travel for the vehicle 12. Thus, the range of yaw rate values within the shaded region B of FIG. 7 can be used as yaw threshold values which can be interpreted by the ECU 36 as indicative of a substantially straight path. The empirical plots of yaw rate data can be unique to a particular embodiment of the vehicle 12 due to many variables, such as but not limited to vehicle weight distribution, and suspension and tire characteristics. Thus, the shaded region B can be unique to each particular embodiment of the vehicle 12. In the fifth data comparison of the first straight judgment of FIG. 5, the ECU 36 can compare a minimum speed value to a speed threshold. Any combination of the wheel speed sensors 42L, 42R, 43L, 43R and the output shaft speed sensor 44—or any other appropriate sensor associated with a component(s) of the vehicle 12, such as but not limited to the front differential assembly 22, the propeller shaft 28, and any combination of the driveshafts 24L, 24R, 32L, 32R—can provide the ECU 36 with either raw data or processed data, which the ECU 36 can utilize for comparison to the speed threshold. As noted above and illustrated in FIGS. 6 and 7, the shaded regions A and B might not be applicable to certain speeds in order to meet the desired
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