Abstract:
A method and system for inhibiting torque steer in a vehicle equipped with steerable wheels that are power driven. The method determines a maximum engine torque limit, determines an estimated driver-desired torque, and controls the actual torque, by adjustment of the throttle angle, to be the smaller of the maximum engine torque limit and the estimated driver-desired torque. Sensors measure steering angle and transmission gear position and a calculator determines the maximum engine torque limit based upon the steering angle and transmission gear position. Further sensors measure engine speed, throttle angle, and atmospheric pressure, and a calculator estimates driver-desired torque based upon the measured engine speed, throttle angle, and atmospheric pressure. A comparator selects the lower of the maximum engine torque limit and the driver-desired engine torque and uses the selected torque to control throttle angle to inhibit torque steer.

Description:
BACKGROUND OF THE INVENTION  
       FIELD OF THE INVENTION  
         [0001]    The present invention is generally directed toward a method and system for inhibiting torque steer in a vehicle equipped with steerable wheels that are power driven.  
         DESCRIPTION OF RELATED ART  
         [0002]    Vehicles equipped with steerable wheels that are power driven such as front-wheel drive vehicles and four-wheel drive vehicles have the potential to generate a difference in left/right side tire longitudinal force under the application of engine torque. This difference in left/right side tire longitudinal force can be observed in most vehicles, but is especially noticeable in vehicles equipped with a traction enhancement device such as a limited slip differential or another type of torque splitting control device. The mismatch in left/right driving torque creates a difference in the suspension restoring torque between the left side and the right side of the vehicle that ultimately leads to perturbations in steering wheel torque, which is commonly referred to as “torque steer”.  
           [0003]    The dynamic conditions that operate to cause torque steer in a vehicle equipped with power-driven, steerable wheels, are well known in the art. Generally, when the vehicle&#39;s power-driven steerable wheels are turned to the left under the application of engine torque, the left side tire longitudinal force is smaller than the right side tire longitudinal force. This translates into a torque steer that the driver of the vehicle feels in the steering wheel as a pull to the left. Factors such as the amount of engine torque applied and the transmission gear selected contribute to the overall level of driving torque delivered to the front axle, the resulting left/right driving torque difference amount, and the resulting level of torque steer.  
           [0004]    A variety of traction control systems are known that control the slip rate of the driving axle in order to enhance vehicle stability and maneuverability. These known traction control systems generally become active upon the occurrence of a wheel-slip condition or upon the occurrence of a difference in driving wheel speed. Upon sensing such a condition, such systems may incorporate engine throttle or torque control and/or brake system control to improve traction and to mitigate torque steer. The intent of such systems is to intervene in the event of excessive wheel slip so as to keep the tire slip rate within a desired range.  
           [0005]    One such system is described in Schmitt et al., U.S. Pat. No. 6,154,546. This patent discloses a method and device for controlling traction in a motor vehicle in which a maximum transmittable driving torque is calculated as a function of various operating parameters of the vehicle and its turning performance. When a skidding tendency of at least one driving wheel occurs, the system engages and reduces engine torque to a calculated maximum transmittable torque value.  
           [0006]    In many driving situations however, the left/right difference in longitudinal tire force can lead to a persistent torque steer before the onset of appreciable wheel slip. This situation is especially problematic when high levels of engine torque are applied as a vehicle is being steered in a direction other than straight on a high adhesion surface. In such situations, the torque steer condition occurs before a typical wheel-slip-based traction control system activates. Thus, conventional wheel-slip based traction control systems are generally ineffective to mitigate or inhibit torque steer before a wheel-slip condition occurs.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method and system for inhibiting torque steer in a vehicle equipped with steerable wheels that are power driven. The method and system according to the invention inhibit torque steer by limiting the actual amount of engine torque applied to the wheels to the lower of an estimated driver-requested engine torque and a maximum engine torque limit. The method and system effectively inhibits torque steer before appreciable wheel-slip occurs, and thus operates to inhibit torque steer before activation of a conventional wheel-slip based traction control device.  
           [0008]    The method according to the invention includes the steps of determining a maximum engine torque limit as a function of steering angle and transmission gear position, comparing the maximum engine torque limit with an estimated driver-requested engine torque, and adjusting or controlling engine operation so as to have actual engine torque be substantially equal to the lower of the maximum engine torque limit and the estimated driver-requested engine torque. The controlling step preferably includes providing a calculated engine throttle angle signal to an engine throttle controller, which adjusts engine throttle position. The torque steer inhibiting throttle command can be subordinate to one or more higher priority commands sent to the engine control system, such as a traction control throttle command sent to the throttle controller by a wheel-slip-based traction control system.  
           [0009]    The system according to the invention comprises sensors that measure steering angle and transmission gear position, one or more controllers that calculate the maximum engine torque limit and estimated driver-requested engine torque, a comparator that selects a lower of the maximum engine torque and the estimated driver-requested engine torque, and a throttle angle calculator that determines the throttle angle based upon the selected engine torque and the engine speed. Because the system determines the maximum engine torque limit as a function of steering angle, the system allows for greater straight-line acceleration performance as compared to when the vehicle is being steered in a direction other than in a straight line. The invention improves the overall steering feel of vehicles equipped with steerable wheels that are power driven. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    These and further features of the invention will be apparent with reference to the following description and drawings, wherein:  
         [0011]    [0011]FIG. 1 schematically illustrates a torque steer inhibiting system according to the present invention;  
         [0012]    [0012]FIG. 2 is a graph showing an exemplary plot of the maximum transmittable torque limit as a function of steering angle for three transmission gear positions in a vehicle; and,  
         [0013]    [0013]FIGS. 3 a  through  3   f  are graphs comparing selected operating conditions as a function of time in a vehicle equipped with a system according to the invention (FIGS. 3 b ,  3   d , and  3   f ) with the same operating conditions as a function of time in a vehicle that is not equipped with a system according to the invention (FIGS. 3 a ,  3   c ,  3   e ). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    With reference to FIG. 1, the system  10  according to the present invention includes a steering angle sensor  12 , a gear position sensor  14 , an atmospheric pressure-sensor  16 , a throttle angle sensor  18 , an engine speed sensor  20 , a torque limit calculator  22 , a throttle adjustment angle adjustment calculator  24 , an engine torque estimator  26 , a comparator/selector  28 , a throttle angle calculator  30 , and an engine throttle controller  32 . As will be appreciated from the following description, the engine speed sensor  20 , throttle angle sensor  18 , atmospheric pressure sensor  16 , throttle angle adjustment calculator  24 , and engine torque estimator  26  define an estimated engine torque portion  10   a  of the system  10 , whereas the gear position sensor  14 , steering angle position sensor  12 , and torque limit calculator  22  define a maximum calculated torque portion  10   b  section of the system  10 .  
         [0015]    The steering angle sensor  12  measures steering angle. Steering angle can, but need not be, measured in terms of a positive or negative angle of steering wheel rotation from a neutral position, which is straight ahead driving (0° steering angle). In such an arrangement, a steering wheel turned a quarter revolution to the left from the neutral position would be a −90° steering angle. Likewise, a steering wheel turned a half revolution to the right from the neutral position would be a +180° steering wheel angle. The steering angle sensor  12  senses the position of the steering wheel relative to neutral and generates a steering angle signal  12   a  that is transmitted to the torque limit calculator.  
         [0016]    The transmission gear position sensor  14  measures or detects transmission gear position. Transmission gear position is typically measured as an integer, where the first transmission gear is  1 , the second transmission gear is  2 , and so on. The transmission gear position sensor  14  senses the transmission gear position and generates a transmission gear position signal  14   a  that is transmitted to the torque limit calculator  22 .  
         [0017]    The torque limit calculator  22  receives the steering angle signal  12   a  from the steering angle sensor  12  and the transmission gear position signal  14   a  from the transmission gear position sensor  14  and uses this data, in combination with a software algorithm containing vehicle-specific parameters, to calculate a maximum engine torque limit. The torque limit calculator  22  transmits a maximum torque limit signal  22   a  to the comparator  28 .  
         [0018]    The maximum engine torque limit is the maximum amount of engine torque that can be applied in the particular transmission gear at the particular steering angle without producing an unacceptable amount of torque steer. This value must be calculated for each vehicle design, and will vary from vehicle to vehicle due to different suspension set-ups, weights, drag, steering ratios, etc. In all cases, however, the maximum engine torque limit will be much higher when the steering wheel is a neutral position for straight ahead driving (e.g., steering angle=0°) than when the steering wheel is turned away from the neutral position (e.g., steering angle is greater than or less than 0°).  
         [0019]    [0019]FIG. 2 shows an exemplary plot of the maximum engine torque limit for a vehicle as a function of steering angle from 0° to 180° (e.g., a right turn) in three transmission gear positions. It will be appreciated that the maximum engine torque limit for a right turn may be the same as for a left turn, or may be different. However, the maximum engine torque limit for a vehicle will always be higher when the steering wheel is at or near a neutral position as compared to when the steering wheel is turned significantly to the right or left of the neutral position.  
         [0020]    Driver-requested engine torque is the amount of torque demanded or requested by the driver at any given moment in time. Driver-requested engine-torque is typically related to accelerator pedal position, but will vary due to factors that affect ngine performance. While it may, in some circumstances, be acceptable to employ a sensor that senses accelerator pedal position for estimating driver-requested engine torque, it is more accurate and preferable for the system to employ a plurality of sensors that measure various engine operating and environmental conditions, and to use the sensed conditions to estimate the driver-requested engine torque.  
         [0021]    In the illustrated and preferred embodiment of the invention, the estimated torque calculating portion  10   a  of the system  10  includes the engine speed sensor  20  that measures engine speed and generates an engine speed signal  20   a , the atmospheric pressure sensor  16  that measures atmospheric pressure and generates an atmospheric pressure signal  16   a , and the throttle angle sensor  18  that measures driver-requested throttle position or angle and generates a driver-requested throttle angle signal  18   a . The atmospheric pressure signal  16   a  and the throttle position signal  18   a  are fed to the throttle angle adjustment calculator  24 , which calculates an atmospheric pressure-adjustment for the throttle angle, and outputs a throttle angle adjustment signal  24   a  to the engine torque estimator  26 . The engine torque estimator  26  receives the throttle angle adjustment signal  24   a  and the engine speed signal  20   a , and outputs an estimated driver-requested engine torque signal  26   a  to the comparator  28 .  
         [0022]    The comparator  28  receives the maximum torque limit signal  22   a  from the torque limit calculator  22  and the estimated driver-requested engine torque signal  26   a  from the engine torque estimator  26 . The comparator  28  compares the maximum engine torque limit with driver-requested engine torque and passes a torque signal  28   a  corresponding to the lower of the estimated engine torque (driver-requested engine torque signal) and the calculated maximum torque (maximum torque limit signal) to the throttle angle calculator  30 .  
         [0023]    The throttle angle calculator  30  receives the torque signal  28   a  from the comparator  28  and the engine speed signal  20   a  from the engine speed sensor  20 , and calculates the engine throttle angle that would produce the selected torque at the given engine speed. A calculated engine throttle angle signal  30   a  is transmitted from the throttle angle calculator  30  to the engine throttle control system  32 . The engine throttle controller  32 , in turn, adjusts the throttle angle to correspond with the calculated engine throttle setting and thereby controls the actual engine torque to substantially approximate the value of the torque signal  28   a  passed by the comparator  28 .  
         [0024]    The sensors  12 ,  14 ,  16 ,  18 ,  20  used in the system according to the invention can be utilized exclusively by the system or can be shared with other vehicle systems. Preferably, the sensors measure and transmit data continuously so that calculations and adjustments are made on a real time basis. Further, the calculators  22 ,  24 ,  30 , estimator  26 , comparator  28 , and controller  32  are preferably provided in one or more microprocessors incorporating or utilizing appropriate control software, as will be appreciated by those skilled in the art, and may be dedicated to the system  10  or shared by other vehicle systems. Thus, the system is dynamic, and allows for immediate adjustments in throttle angle and, hence, actual engine torque in response to changes that are being made to steering angle, transmission gear position, and/or driver requested engine torque. The throttle angle adjustment signal  30   a  sent by the throttle angle calculator  30  can be granted a priority, which is either superior to or subordinate to one or more engine throttle commands sent to the engine throttle control system by other vehicle systems (i.e., the wheel-slip based traction control system).  
         [0025]    The preferred method of inhibiting torque steer according to the present invention involves determining a maximum engine torque limit as a function of steering angle and transmission gear position, comparing the maximum engine torque limit with driver-requested engine torque, and controlling or adjusting actual engine torque (by adjustment of the throttle angle) to the lower of the maximum engine torque limit and the driver-requested engine torque. Unlike conventional methods, the method of the present invention effectively inhibits torque steer before a wheel-slip condition occurs.  
         [0026]    It will be appreciated that the torque steer inhibiting system and method according to the present invention can be used on vehicle that is equipped with a conventional wheel-slip based traction control system. In such situations, the torque steer inhibiting system will be operational before the wheel-slip based traction control system.  
         [0027]    It is preferable that the throttle angle adjustment signal  30   a  sent by the throttle angle calculator  30  be subordinate to, or to be given a lower priority than, any throttle commands that may be sent to the engine throttle control system  30  by the wheel-slip-based traction control system. Thus, the throttle control system of the present invention will be operable before any traction control system but, when a wheel-slip condition occurs, throttle commands transmitted to the engine throttle control system  32  or the like by the wheel-slip based traction control system take precedence over throttle commands  30   a  transmitted to the engine throttle control system  32  by the throttle angle calculator  30 .  
         [0028]    [0028]FIGS. 3 a  through  3   f  are graphs comparing selected operating conditions as a function of time in a vehicle equipped with a system according to the invention (FIGS. 3 b ,  3   d , and  3   f ) with the same operating conditions as a function of time in a vehicle that is not equipped with a system according to the invention (FIGS. 3 a ,  3   c , and  3   e ). FIGS. 3 a  and  3   b  show accelerator pedal position and engine throttle position as a function of time. In FIG. 3 a , engine throttle position tracks accelerator pedal position. In FIG. 3 b , engine throttle position initially tracks the accelerator pedal position until a point “A” at which throttle position or angle is retarded relative to accelerator pedal position due to operation of the system of the present invention. More specifically, and as will be appreciated from the foregoing description, at the point “A” the user requested engine torque exceeds the maximum engine torque limit and, therefore, the throttle angle is controlled so that the actual engine torque does not exceed the maximum permissible engine torque as embodied in the torque signal  28   a . Accordingly, the throttle signal  30   a  to the engine throttle controller  32  serves to adjust the throttle position and, thus, actual engine torque to the maximum transmittable torque limit for the particular steering angle and transmission gear position and thereby inhibits torque steer.  
         [0029]    [0029]FIGS. 3 c  and  3   d  show the torque steer experienced by the driver of the vehicle under the same conditions and time as in FIGS. 3 a  and  3   b . It is noted that a significant amount of torque steer is created or experienced in the vehicle of FIG. 3 c  (in which the inventive system is not employed), whereas torque steer is substantially prevented or inhibited in the vehicle depicted in FIG. 3 d , wherein the system of the present invention is utilized.  
         [0030]    [0030]FIGS. 3 e  and  3   f  compare driver-requested engine torque, actual engine torque, and limit torque value under conditions similar to those of FIGS. 3 a - 3   d . In FIG. 3 e , it is noted that the driver-requested, actual, and limit torques are equal to one another. However, FIG. 3 f  shows that, in a vehicle equipped with the system of the present invention, actual engine torque is limited to the lower of driver-requested engine torque and the maximum engine torque limit (maximum permissible torque). Thus, the actual engine torque tracks the driver-requested engine torque until point “A” at which the maximum engine torque limit is lower than the driver-requested engine torque, at which point the actual engine torque tracks the maximum engine torque limit.  
         [0031]    The system according to the invention limits actual engine torque to a value that prevents or minimizes torque steer, and does so well before a wheel-slip based traction control system could activate and intervene. Thus, the system of the present invention is proactive rather than reactive. Furthermore, since the system and method of the invention operate before a wheel-slip based traction control systems can intervene, the system can effectively inhibit torque steer at low levels of transvers acceleration when wheel-slip conditions do not occur. This means that the vehicle need not be at its limit of turning performance for the system to operate to inhibit torque steer.  
         [0032]    The system according to the present invention may operate much more frequently to inhibit torque steer than wheel-slip-based traction control systems, especially under high driver-requested engine torque conditions on high adhesion surfaces. On low adhesion surfaces, sufficient wheel slip may occur before the torque steer limit torque is reached, and the wheel-slip based traction control system therefore may become active such that torque steer function limit control is not used.  
         [0033]    While the preferred embodiment of the present invention has been disclosed herein, the present invention is not limited thereto. Rather, the method of the present invention is capable of numerous modification and improvements and, therefore, the scope of the present invention is only defied by the claims appended hereto.