Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present invention claims priority to provisional application No. 60/450,248, filed on Feb. 26, 2003, and is related to Attorney Docket No. 201-1057/FGT-1780PA), filed simultaneously herewith, the disclosure of which is incorporated by reference. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    The present invention relates generally to dynamic control systems for automotive vehicles and, more specifically to a system that compensates wheel speed sensor signals to determine a vehicle reference velocity.  
           [0003]    It is a well-known practice to control various operating dynamics of a motor vehicle to achieve active safety. Examples of active safety systems include traction control, yaw stability control and roll stability control systems. A more recent development has been to combine all the available subsystems to achieve better vehicle safety and dynamics performance. The effective operation of the various control systems requires high-accuracy and fast-response-times in the determination of the operating states of the vehicle, regardless of road conditions and driving conditions. Such vehicle operating states include the vehicle longitudinal, lateral and vertical velocities measured along the body-fixed longitudinal, lateral and vertical axes, the attitude of the vehicle body, and the travel course of the vehicle.  
           [0004]    [0004] 4  One piece of basic information that forms the aforementioned vehicle state estimation is the linear velocity of the rotating centers of the four wheels. For example, this information can be used to assess the wheel slip used in anti-brake-lock controls and traction controls and to estimate the longitudinal velocity of the vehicle. In order to obtain the linear corner velocities, the wheel speed sensors are used. The wheel speed sensors output the products of the wheel rotational speeds and the rolling radii. The wheel rotational speeds are directly measured and the rolling radii are assumed their nominal values. During dynamic maneuvers, the variations of the wheel normal loading will affect the rolling radii. Hence, the nominal rolling radii may not reflect the actual rolling radii and thus cause errors in the calculation of the wheel speeds.  
           [0005]    It would, therefore be desirable to provide a more accurate way in which to determine the vehicle speed taking into consideration changes in rolling radii.  
         SUMMARY OF INVENTION  
         [0006]    The present invention provides an improved determination of the individual wheel speeds. In the present invention the individual wheel speed calculations may be compensated for by learning the rolling radii of the wheels. Thus, a more accurate determination of the vehicle reference velocity or the longitudinal velocity may be determined.  
           [0007]    In one aspect of the invention, a control system  24  for controlling a safety system  40  of an automotive vehicle includes a plurality of wheel speed sensors  30  generating a plurality of wheel velocity signals, a steering angle sensor  39  generating a steering actuator angle signal, a yaw rate sensor  28  generating a yaw rate signal, a lateral acceleration sensor  32  generating a lateral acceleration signal and a controller  26 . The controller  26  generates a final reference vehicle velocity in response to the plurality of wheel speed signals, the steering angle signal, the yaw rate signal and the lateral acceleration signal. The controller  26  controls the safety system in response to the final reference vehicle velocity.  
           [0008]    In a further aspect of the invention, a method of controlling a safety system for an automotive vehicle having a plurality of wheels includes determining a plurality of wheel velocities for the plurality of wheels, determining a preliminary longitudinal velocity of the vehicle from the plurality of wheel velocities, determining a plurality of correction factors for the plurality of wheel velocities for the plurality of wheels, determining a vehicle reference velocity in response to the plurality of correction factors, the plurality of wheel velocities and the preliminary longitudinal velocity, determining a lateral acceleration, determining a vehicle reference velocity correction factor in response to the lateral acceleration, determining a final reference velocity in response to the vehicle reference velocity correction factor and the vehicle reference velocity, and controlling the safety system in response to the final reference velocity.  
           [0009]    Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]    [0010]FIG. 1 is a top view of a motor vehicle illustrating various operating parameters of a vehicle experiencing a turning maneuver on a road surface.  
         [0011]    [0011]FIG. 2 is a side view of a motor vehicle wheel illustrating various operating parameters of the wheel.  
         [0012]    [0012]FIG. 3 is a block diagram showing a portion of a microprocessor interconnected to sensors and controlled devices, which may be included in a system according to the present invention.  
         [0013]    [0013]FIG. 4 is a control system block diagram in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    In the following figures the same reference numerals will be used to illustrate the same components.  
         [0015]    Referring now to FIG. 1, various operating parameters and variables used by the present invention are illustrated as they relate to the application of the present invention to a ground based motor vehicle  10  having wheels  12 ,  14 ,  16 ,  18 . Those skilled in the art will immediately recognize the basic physics represented by these illustrations, thereby making the adaptation to different types of vehicles easily within their reach. A lateral and longitudinal velocities of the center of gravity are denoted as V x  and V y  a yaw angular rate is denoted as ω x , a front wheel steering angle is denoted as δ, lateral acceleration is represented by a y , longitudinal acceleration is represented by a x .  
         [0016]    Using those vehicle motion variables, the velocities of the vehicle at the four corner locations, where the wheels are attached to the vehicle, can be calculated in the following form which are projected along the body fixed longitudinal and lateral directions  
           V   Ifx   =V   x −ω z   t   f   , V   Ify   =V   y +ω z   I   f    
           V   rfx   =V   x +ω z   t   f , V rfy   =V   y +ω z   I   f    (1)  
           V   Irx   =V   x −ω z   t   r   , V   Iry   =V   y −ω z   I   r    
           V   rrx   =V   x +ω z   t   r   , V   rry   =V   y −ω z   I   r    
         [0017]    where t f  and t r  are the half tracks for the front and rear axles, I f  and I r  are the distances between the center of gravity of the vehicle and the front and rear axles. The variables V If , V rf , V Ir  and V rr  are the linear velocities of the four corners along the wheel heading directions (left front, right front, left rear and right rear, respectively), which can be calculated as in the following  
           V   If   =V   Ifx  cos(δ)+ V   Ify  sin(δ)  
           V   rf   =V   rfx  cos(δ)+ V   rfy  sin(δ)   (2)  
         V Ir =V Irx    
         V rr =V rrx    
         [0018]    Referring now to FIG. 2, vehicle corner velocity along the wheel longitudinal direction is equal to the sum of the contact patch slip velocity v cp  and the product of the wheel rotational rate ω whI  and its rolling radius r 0 .  
         [0019]    Referring now to FIG. 3, stability control system  24  has a controller  26  used for receiving information from a number of sensors which may include a yaw rate sensor  28 , speed sensors  30  (at each wheel), a lateral acceleration sensor  32 , a roll rate sensor  34 , a steering angle (hand wheel position) sensor  35 , a longitudinal acceleration sensor  36 , a pitch rate sensor  37 , and steering angle position sensor  39 . Steering angle position sensor  39  senses the position of the steered road wheels. Lateral acceleration, longitudinal acceleration, yaw rate, roll orientation and speed may also be obtained using a global positioning system (GPS). Based upon inputs from the sensors, controller  26  controls the safety system  40 . Depending on the desired sensitivity, the type of safety system and various other factors, not all the sensors  28 - 39  may be used in a commercial embodiment. Other factors may be obtained from the sensors such as the surface mu and the vehicle side slip angle, β.  
         [0020]    Roll rate sensor  34  and pitch rate sensor  37  may sense the roll condition to be used with a rollover control system as an extension of the present application.  
         [0021]    Safety system  40  may be a number of types of safety systems including a roll stability control system, a yaw control system, antilock brakes, traction control, airbags, or active suspension system.  
         [0022]    Safety system  40  if implemented may control a position of a front right wheel actuator, a front left wheel actuator, a rear left wheel actuator, or a right rear wheel actuator. Although, as described above, two or more of the actuators may be simultaneously controlled as one actuator. Based on the inputs from sensors  28  through  39 , controller  26  determines the vehicle dynamic conditions and controls the safety system. Controller  26  may also use brake control coupled to front right brakes, front left brakes, rear left brakes, and right rear brakes to dynamically control the vehicle. By using brakes in addition to steering control some control benefits may be achieved. For example, yaw control and rollover control may be simultaneously accomplished.  
         [0023]    Speed sensor  30  may be one of a variety of speed sensors known to those skilled in the art. For example, a suitable speed sensor may include a sensor at every wheel that is averaged by controller  26 . As will be described below, the controller  26  translates the wheel speeds into the speed of the vehicle.  
         [0024]    Referring now to FIG. 4, a method of operating a safety system using a corrected vehicle velocity is determined. In step  60  the wheel speed sensors are read. In one embodiment each wheel has a separate speed sensor.  
         [0025]    The wheel speed sensor outputs usually are calibrated for providing the linear directional velocities V If , V rf , V Ir  and V rr  by multiplying the wheel rotational angular speeds with a nominal rolling radius of the wheels. The variables ω If-sensor , ω rf-sensor , ω Ir-sensor  and ω rr-sensor  are the wheel angular velocity at the left-front corner, right-front corner, left-rear corner and rear-right corner respectively. The nominal rolling radius (typically used in ABS) for calculating wheel speeds from the wheel rotational rates is r 0 . Thus, the linear directional velocities may be represented by:  
         v If =ω If-sensor r 0    
         v rf =ω rf-sensor r 0    (3)  
         v Ir =ω Ir-sensor r 0    
         v rr =ω rr-sensor r 0    
         [0026]    Notice that the wheels have different rolling radii than r 0 . Hence, in order to accurately calculate the actual linear velocities at the four corners, correction factors need to be added. The individual correction factors are denoted as K If , K rf , K Ir  and K rr  for the left-front, right-front, left-rear and rear-right corners, respectively. Thus, the linear directional velocities may then be represented by:  
         v If =K If ω If-sensor r 0    
         v rf =K rf ω rf-sensor r 0    (4)  
         v Ir =K Ir ω Ir-sensor r 0    
         v rr =K rr ω rr-sensor r 0    
         [0027]    Notice also that the wheels experience not only the rotational motion but also the linear sliding motion, or longitudinal slip. The slip is caused by the relative motion between the wheel and the road at the contact patch (CP). The longitudinal velocities of the relative motions at the contact patches are denoted as v cp-If , v cp-rf , v cp-Ir  and v cp-rr , then the vehicle corner velocities can be expressed as the sums of two speeds as in the following  
         
       V 
       If 
       =v 
       cp-If 
       +v 
       If  
     
           V   rf   =v   cp-rf   +v   rf    (5)  
         
       V 
       Ir 
       =v 
       cp-Ir 
       +v 
       Ir  
     
         
       V 
       rr 
       =v 
       cp-rr 
       +v 
       rr  
     
         [0028]    The longitudinal and lateral velocities of the vehicle may be determined in step  62  from the sensors, or they may be calculated as in Ford disclosure 201-1057 (Attorney Docket No. FGT-1780 PA) filed simultaneously herewith, or even a rough estimation by averaging certain variables calculated from wheel speeds. This may be a rough estimate or average but, as mentioned above, does not take into consideration the rolling radius or other factors. Consider  
           V   y   =V   x  tan(β)   (6)  
         [0029]    where βis the vehicle side slip angle V y  is the lateral velocity of the vehicle and V x  is the longitudinal velocity of the vehicle. In step  64 , the front steering angle  8  is determined. Then, the individual correction factors K If , K rf , K Ir  and K rr  for each wheel can be calculated in step  66  as  
                     κ   lf     =             V   x          [       cos        (   δ   )       +       tan        (   β   )                       sin        (   δ   )           ]       +       ω   z          [         l   f                     sin        (   δ   )         -       t   f                     cos        (   δ   )           ]             ω     lf   -   sensor            r   0         -                                v     cp   -   lf           ω     lf   -   sensor            r   0                       κ   rf     =             V   x          [       cos        (   δ   )       +       tan        (   β   )                       sin        (   δ   )           ]       +       ω   z          [         l   f                     sin        (   δ   )         +       t   f                     cos        (   δ   )           ]             ω     rf   -   sensor            r   0         -                                v     cp   -   rf           ω     rf   -   sensor            r   0                       κ   lf     =           V   x     -       ω   z          t   r             ω     lr   -   sensor            r   0         -       v     cp   -   lr           ω     lr   -   sensor            r   0                         κ   rr     =           V   x     +       ω   z          t   r             ω     rr   -   sensor            r   0         -       v     cp   -   rr           ω     rr   -   sensor            r   0                         (   7   )                               
 
         [0030]    The product term tan(β)sin(δ) is negligible in comparison to cos(δ), hence equation (7) may be further simplified to the following, which is independent of the vehicle side slip angle β 
                     κ   lf     ≈             V   x                     cos        (   δ   )         +       ω   z          [         l   f                     sin        (   δ   )         -       t   f                     cos        (   δ   )           ]             ω     lf   -   sensor            r   0         -       v     cp   -   lf           ω     lf   -   sensor            r   0                         κ   rf     ≈             V   x                     cos        (   δ   )         +       ω   z          [         l   f                     sin        (   δ   )         +       t   f                     cos        (   δ   )           ]             ω     rf   -   sensor            r   0         -       v     cp   -   rf           ω     rf   -   sensor            r   0                         κ   lr     =           V   x     -       ω   z          t   r             ω     lr   -   sensor            r   0         -       v     cp   -   lr           ω     lr   -   sensor            r   0                         κ   rr     =           V   x     +       ω   z          t   r             ω     rr   -   sensor            r   0         -       v     cp   -   rr           ω     rr   -   sensor            r   0                         (   8   )                               
 
         [0031]    In the case of small wheel longitudinal slip ratios, the longitudinal velocities v cp-If , v cp-rf , v cp-Ir  and v cp-rr  of the relative motions at the contact patches are close to zero, and equation (8) can be further simplified as the following  
                     κ   lf     ≈           V   x                     cos        (   δ   )         +       ω   z          [         l   f                     sin        (   δ   )         -       t   f                     cos        (   δ   )           ]             ω     lf   -   sensor            r   0                       κ   rf     ≈           V   x                     cos        (   δ   )         +       ω   z          [         l   f                     sin        (   δ   )         +       t   f                     cos        (   δ   )           ]             ω     rf   -   sensor            r   0                       κ   lr     =         V   x     -       ω   z          t   r             ω     lr   -   sensor            r   0                       κ   rr     =         V   x     +       ω   z          t   r             ω     rr   -   sensor            r   0                       (   9   )                               
 
         [0032]    The digital value of the above wheel speed individual correction factors K If , K rf , K Ir  and K rr  at the time instant t=kΔT are  
         K If     k   , K rf     k   , K Ir     k    and K rr     k   ,  
         [0033]    then learning algorithms can be used to calculate the average correction factors. The correction factors are determined using an iterative process that is updated every N calculation samples in the following learning example. Notice that this is a conditional computation which is conducted only if the wheel&#39;s longitudinal slip ratios are small.  
                 START                       if         k   &lt;   N                               κ   _       lf     k   +   1         =         κ   _       lf   k       +       κ     lf     k   +   1         N                                   κ   _       rf     k   +   1         =         κ   _       rf   k       +       κ     rf     k   +   1         N                                   κ   _       lr     k   +   1         =         κ   _       lr   k       +       κ     lr     k   +   1         N                                   κ   _       rr     k   +   1         =         κ   _       rr   k       +       κ     rr     k   +   1         N                               k   =     k   +   1               elseif         k   =   N                           k   =   0                               κ   _     lf     =       κ   _       lf     N   +   1                                     κ   _     rf     =       κ   _       rf     N   +   1                                     κ   _     lr     =       κ   _       lr     N   +   1                                     κ   _     rr     =       κ   _       rr     N   +   1                     go                 to                 START                             (   10   )                               
 
         [0034]    Using the above learning algorithm, corrected wheel speeds at each wheel can be determined in step  66  based upon the learned correction factor.  
         {circumflex over (v)} If     k   ={overscore (K)} If r 0 ω If-sensor     k      
         {circumflex over (v)} rf     k   ={overscore (K)} rf r 0 ω rf-sensor     k      (11)  
         {circumflex over (v)} Ir     k   ={overscore (K)} Ir r 0 ω Ir-sensor     k      
         {circumflex over (v)} rr     k   ={overscore (K)} rr r 0 ω rr-sensor     k      
         [0035]    Notice that the above learning algorithm only corrects the individual wheel speeds. There are cases when the average rolling radii of the four wheels are reduced together due to vehicle loading change. Feeding back the above corrected wheel speeds to the algorithms used in vehicle dynamics control will provide a vehicle reference velocity  
         {circumflex over (V)} ref    
         [0036]    in step  70  which needs to be further calibrated against the available vehicle longitudinal acceleration sensor signal.  
         [0037]    Consider that the actual vehicle reference velocity is  
         V ref =κ{circumflex over (V)} ref    (12)  
         [0038]    where κ is the global correction factor due to the total vehicle loading. κis usually a slow time varying parameters  
         κ{dot over ({circumflex over (V)})} ref   =a   x   −gθ   y    (13)  
         [0039]    where θ y  is the vehicle pitch angle generated from a pitch angle sensor or calculated from the pitch rate sensor signal.  
         [0040]    In step  72 , the longitudinal acceleration a x  is determined. Then, the following variables are defined  
                       V   ⋒     =     [             v   ⋒       ref   1                   v   ⋒       ref   2               ⋮               v   ⋒       ref   N             ]       ,               A   x     =     [           a     x   1                 a     x   2               ⋮             a     x   N             ]       ,             Θ   y     =     [           θ     y   1                 θ     y   2               ⋮             θ     y   N             ]                   (   14   )                               
 
         [0041]    Then a least square computation of the correction factor due to loading can be determined in step  74  as the following:  
         {circumflex over (κ)}=in v ( {circumflex over (V)}   T   {circumflex over (V)} ) {circumflex over (V)}   T   [A   x   −gΘ   y ]  (15)  
         [0042]    or in the following form  
               K   ^     =         ∑     k   =     M   +   1         M   +   N                v   ⋒       ref   k            (       a     x   k       -     g                   θ     y   k           )             ∑     k   =     M   +   1         M   +   N              v   ⋒       ref   k     2                 (   16   )                               
 
         [0043]    Notice that the global correction factor  
         {circumflex over (κ)} 
         [0044]    is updated every N computational samples when the wheels have small longitudinal slip ratios. The digital implementation of equation (16) can be obtained as in the following where V k+1  is the updated reference velocity determined in step  76 .  
           START                       if         k   &lt;   N                             A     k   +   1       =       A   k     +         v   ⋒       ref     k   +   1                         (       a     x     k   +   1         -     g                   θ     y     k   +   1             )                                   V     k   +   1       =       V   k     +         v   ⋒       ref     k   +   1                v   ⋒       ref     k   +   1                                     k   =     k   +   1               elseif         k   =   N                             K   ^     =       A     N   +   1         V     N   +   1                                 k   =   0               go                 to                 START                                           
 
         [0045]    The final corrected wheel speed sensor signals may be corrected by the aforementioned factors can also be obtained as the following:  
         {circumflex over (v)} If     k   ={circumflex over (K)}{overscore (K)} If r 0 ω If-sensor     k      
         {circumflex over (v)} rf     k   ={circumflex over (K)}{overscore (K)} rf r 0 ω rf-sensor     k      (17)  
         {circumflex over (v)} Ir     k   ={circumflex over (K)}{overscore (K)} Ir r 0 ω Ir-sensor     k      
         {circumflex over (v)} rr     k   ={circumflex over (K)}{overscore (K)} rr r 0 ω rr-sensor     k      
         [0046]    Once the corrected final vehicle reference velocity is determined, the safety system  40  may be controlled using the compensated velocity values.  
         [0047]    While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.

Technology Category: 7