Abstract:
A method of calculating feed-forward lateral acceleration of a moving vehicle is provided. The method includes the use of a corrected steering angle of the vehicle. The steering angle is corrected by using the Ackerman steer angle of the vehicle. Depending upon the speed at which the vehicle is traveling the steering angle may be corrected using the full Ackerman steer angle, a fraction of the Ackerman steer angle or not at all.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to methods for estimating a vehicle&#39;s feed-forward lateral acceleration and, more particularly, to methods using a measured parameter of the vehicle&#39;s steering system to refine a feed-forward lateral acceleration estimation technique. 
   2. Description of Related Art 
   A vehicle develops an acceleration component lateral to the longitudinal axis of the vehicle when the vehicle begins to turn. During these situations it is normal in vehicles that provide an adjustability feature, to adjust the torque distribution between the wheels (front-to-back, side-to-side, etc.) in order to allow for effective handling of the vehicle. Lateral acceleration is one factor that is used in calculations that control the torque distribution amongst the wheels. 
   To enhance control response, it is advantageous to estimate the lateral acceleration of the vehicle from specific driver inputs. An estimated lateral acceleration may be used in a feed-forward control system to adjust for a condition caused by turning the vehicle, in combination with changing vehicle speed or alone, prior to the turn having a major affect on the stability of the vehicle. This estimated lateral acceleration is known as feed-forward lateral acceleration. 
   With reference to  FIG. 3 , a known feed-forward lateral acceleration estimation method is schematically illustrated. In this known method, the measured vehicle speed  10  is input into a gain table  12  to arrive at a signal  14  that is lateral acceleration per degree of steering angle. In multiplier  17  the measured steering angle  16  is multiplied with the signal  14  to provide a lateral acceleration signal  18 , which is then converted to a magnitude value (by taking the absolute value  20 ) and its output is limited by passing this result through a saturation table  22  (to ensure the calculated value does not rise above actual vehicle cornering limits) to derive a normalized lateral acceleration signal  24 . In multiplier  29 , the normalized lateral acceleration signal  24  is multiplied by the sign of the steering angle,  26 , to arrive at the estimated feed-forward lateral acceleration signal  28 , which is used to control the front-to-rear and/or side-to-side torque applied to the vehicle wheels. 
   Unfortunately, in situations of low vehicle speed and tight turning, the known method for estimating feed-forward lateral acceleration become less accurate. More specifically, the feed-forward lateral acceleration is overestimated. It is believed that this overestimation is primarily due to the fact that the high turning angle (steering angle) tends to dominate the calculation. As a result, in a drive torque system utilizing feed-forward control, high rates of activation or shifting of drive torque may be implemented when they are, in fact, not required. Therefore, there exists a need in the art to correct the measured steering angle to compensate the feed-forward estimate of the lateral acceleration in low speed, tight turning situations. 
   SUMMARY OF THE INVENTION 
   The present invention is directed toward a method and apparatus for compensating for overestimations to feed-forward lateral acceleration estimates at low speeds and tight turning radii. 
   In accordance with the present invention, a correction factor based on a known quantity in the state of the art of vehicle dynamics is used. In one embodiment of the invention a method is provided that measures the speed of the moving vehicle, measures the steering angle of the moving vehicle, calculates an Ackerman steer angle of the moving vehicle, corrects the steering angle using the Ackerman steer angle, and finally calculates the feed-forward lateral acceleration of the vehicle using the speed of the vehicle and the corrected steering angle. 
   In further accordance with the present invention, the measured vehicle steering angle is corrected by subtracting the calculated Ackerman steer angle, and the so-corrected or adjusted vehicle steering angle is used to estimate the feed-forward lateral acceleration. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and further features of the invention will be apparent with reference to the following description and drawings, wherein: 
       FIG. 1  schematically illustrates a feed-forward lateral acceleration calculation technique using an Ackerman steer angle correction according to the present invention; 
       FIG. 2  is a schematic representation of the position of wheels on a vehicle during a turn; and, 
       FIG. 3  schematically illustrates a conventional feed-forward lateral acceleration estimating technique. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As used in this description and in the appended claims, the following terms will have the definitions indicated thereafter:
         “feed-forward lateral acceleration” means a calculated estimation of a vehicle&#39;s actual lateral acceleration (excluding any delays associated with the natural build-up of lateral acceleration following a steering angle input);   “steering angle” means the angular displacement of the steering wheel itself by the driver; and   “vehicle overall steering ratio” means the number of turns (rotations) of the steering wheel of the vehicle for one turn (rotation) of the vehicle road wheel about a vertical axis.       

   An improved method of calculating feed-forward lateral acceleration of a vehicle is provided. This improved method uses measurements of a vehicle speed and measurements of a vehicle steering angle, corrected using a calculated Ackerman steer angle, to calculate feed-forward lateral acceleration. The present invention provides an estimated feed-forward lateral acceleration that is more accurate, and minimizes or eliminates the problems of over-actuation from which the known methods suffer. Accordingly, the present invention, to be described hereafter in greater detail, provides greater accuracy, especially in situations when the vehicle is traveling at a low speed and making a tight turn. 
   Referring to  FIG. 2 , the wheels  100 ,  102 ,  104  of a vehicle  106  are illustrated in a turning orientation. The wheels include a front left wheel  100 , a front right wheel  102 , and a pair of rear wheels  104 . As the vehicle  106  makes any type of turn, acceleration is created in a direction lateral to the vehicle&#39;s centerline  108 . This component, called lateral acceleration, is estimated by the method of the present invention, described hereinafter, and is used as part of a feed-forward control system to adjust the front-to-rear and side-to-side drive torque distribution to the wheels  100 ,  102 ,  104  for greater stability. As previously stated, feed-forward lateral acceleration estimation may lose its accuracy in conditions of low vehicle speed and tight turning. The improved estimation method and system of the present invention employs a correction factor to maintain accuracy. 
   With reference to  FIG. 1 , the improved method of calculating or estimating feed-forward lateral acceleration is described. It is noted that although several of the components used in the improved method are identical to those used in the conventional method described hereinbefore with regard to  FIG. 3 , these components are described in more detail hereinafter and given different reference numerals. 
   Readings of vehicle speed  200  and vehicle steering angle  202  are made. These readings are made by one or more sensors, as is known in the art. The sensed vehicle speed is input into a lookup table referred to hereinafter as a lateral acceleration gain table  204 . The lateral acceleration gain table includes values that are experimentally determined and serve to correlate input vehicle speed to lateral acceleration per steering angle degree. Therefore, the lateral acceleration gain table  204  serves to convert the measured vehicle speed into an anticipated or predicted lateral acceleration signal  206 , which has units of: (lateral acceleration)/(steering angle). 
   The output  206  of the lateral acceleration gain table  204  is supplied to a multiplier  208 , which also receives the corrected steering angle signal  210 , described hereinafter. 
   The vehicles measured steering angle  202  is reduced by the Ackerman steer angle  212  to arrive at the corrected steering angle. Calculation of the Ackerman steer angle  212  is described in more detail below. 
   Preferably, the measured steering angle  202  is corrected by subtracting the Ackerman steer angle  212  from the measured value in a subtraction block  214  to provide the corrected steering angle  210 . The corrected steering angle  210  is then multiplied by the predicted lateral acceleration gain signal  206  to provide a signal referred to hereinafter as the estimated lateral acceleration  216 . The absolute value of the estimated lateral acceleration is modified into a normalized lateral acceleration signal  218  by using a saturation table  220 . 
   The saturation table  220  contains values that are experimentally determined, and takes into account that the vehicle is not capable of generating lateral accelerations above a certain level due to natural tire adhesion limits. Therefore, the feed-forward lateral acceleration using the present method is reduced or limited so as not to produce lateral acceleration levels that are not physically possible from normal tire adhesion limits. Accordingly, the saturation table  220  includes a range of factors that vary based on the absolute value of the calculated lateral acceleration,  216 , prohibiting it from achieving a level that is not possible based on tire lateral adhesion. Rather than a simple saturation function (which assumes a 1:1 correspondence of the output to the input up to the saturation limit value) a shaping saturation function is used to allow progressive growth of the input parameter (calculated lateral acceleration) up to a defined maximum value. 
   The normalized lateral acceleration  218  output by the saturation table  220  is multiplied in the multiplier  221  by the sign of the corrected steering angle  210  to provide feed-forward lateral acceleration  222 . The feed-forward lateral acceleration  222  is used to adjust the front-to-back and/or side-to-side wheel torque distribution of the vehicle. Insofar as use of feed-forward lateral acceleration to adjust wheel torque distribution is known in the art, and is not part of the present invention, it will not be discussed further hereinafter. 
   With reference to  FIG. 2 , for correct Ackerman steering the steer angle A i  of the inner wheel  100  is greater than the steer angle A o  of the outer wheel  102 , such that the axes  101 ,  103  of the inner and outer wheels intersects at a single point  107  on the projection  105  of the rear axle. Accordingly, the Ackerman steer angle may be thought of the difference between the inner wheel steering angle A i  and the outer wheel steering angle A o  necessary to make the aforementioned condition exist. Although the Ackerman steer angle is considered to be known in the art, it will be described hereinafter to assist understanding of the disclosure. 
   The Ackerman steer angle δ may be referred to as the front road wheel input required to steer a normally front steer vehicle around a tight turn at or near zero velocity. The Ackerman steer angle of the vehicle may simply be defined as the vehicle&#39;s wheelbase divided by the turning radius of the vehicle. The turning radius of the vehicle may be described by other vehicle characteristics as follows:
 
 R=V/Ψ 
         R=Vehicle turning radius   V=Vehicle speed   Ψ=Vehicle yaw rate       

   The yaw rate is estimated by known vehicle parameters in combination with other measured outputs of the vehicle, such as described below. Alternatively, the yaw rate may be measured by a yaw rate sensor. One way to estimate the yaw rate is as follows: 
   
     
       
         
           Ψ 
           = 
           
             
               2 
               TR 
             
             × 
             
               ( 
               
                 
                   V 
                   OUT 
                 
                 - 
                 
                   V 
                   IN 
                 
               
               ) 
             
           
         
       
     
       
       
         
           Ψ=Vehicle Yaw Rate 
           V OUT =Speed of outside wheel in a turn 
           V IN =Speed of inside wheel in a turn 
           TR=Vehicle Track Width 
         
       
     
  
   An estimate of the vehicle speed is the average of the speed of an inside wheel of the vehicle and an outside wheel of the vehicle, shown as follows: 
   
     
       
         
           V 
           = 
           
             
               1 
               2 
             
             × 
             
               ( 
               
                 
                   V 
                   OUT 
                 
                 + 
                 
                   V 
                   IN 
                 
               
               ) 
             
           
         
       
     
   
   The equations are rewritten in combination to provide a calculation for the Ackerman steer angle: 
   
     
       
         
           δ 
           = 
           
             
               2 
               × 
               L 
               × 
               
                 ( 
                 
                   
                     V 
                     OUT 
                   
                   - 
                   
                     V 
                     IN 
                   
                 
                 ) 
               
             
             
               TR 
               × 
               
                 ( 
                 
                   
                     V 
                     OUT 
                   
                   + 
                   
                     V 
                     IN 
                   
                 
                 ) 
               
             
           
         
       
     
   
   The equation for measuring the Ackerman steer angle may be scaled by the overall vehicle steer ratio thus allowing the Ackerman steer angle to be expressed in a manner that allows for direct correction of the measured steering angle: 
   
     
       
         
           δ 
           = 
           
             
               
                 2 
                 × 
                 L 
                 × 
                 
                   ( 
                   
                     
                       V 
                       OUT 
                     
                     - 
                     
                       V 
                       IN 
                     
                   
                   ) 
                 
               
               
                 TR 
                 × 
                 
                   ( 
                   
                     
                       V 
                       OUT 
                     
                     + 
                     
                       V 
                       IN 
                     
                   
                   ) 
                 
               
             
             × 
             SR 
           
         
       
     
       
       
         
           δ=Ackerman steer angle 
           V OUT =Speed of outside wheel in a turn 
           V IN =Speed of inside wheel in a turn 
           TR=Vehicle Track Width 
           L=vehicle wheelbase 
           SR=Vehicle overall steering ratio 
         
       
     
  
   In a preferred method of practicing the invention, a correction based on Ackerman steer angle  212  is limited with speed in order to mitigate any failure risk of certain sensors. Before applying the calculated Ackerman steer angle  212  to a correction of the measured steering angle  202 , the Ackerman steer angle  212  is passed through a speed-based correction permission table. Since correction is only required at speeds below a certain level a diminishing correction effect with increasing speed is created to ensure system failsafe robustness. For speeds below a certain value (40 kph), the Ackerman steer angle  212  is multiplied by “1” and fully used in correction. Within a speed range of 40–50 kph, the multiplying factor is gradually decreased to “0”, such that above 50 kph, the Ackerman steer angle  212  no longer takes effect. 
   The vehicle includes a number of sensors used to measure the parameters of the vehicle while in operation. A known speedometer is used to measure the overall speed of the vehicle. Velocity sensors that are part of known anti-lock braking systems (ABS) are used to measure individual wheel velocities. The vehicle&#39;s steering angle may be measured by a sensor in the steering column or by other known means. 
   Although the invention has been shown and described with reference to certain preferred and alternate embodiments, the invention is not limited to these specific embodiments. Minor variations and insubstantial differences in the various combinations of materials and methods of application may occur to those of ordinary skill in the art while remaining within the scope of the invention as claimed and equivalents.