Abstract:
An anti-skid brake control system and its control method for vehicle has a signal collection unit and an electronic control unit. The electronic control unit includes a data receiving module, a data processing module and a data control module. The data receiving module receives the data collected by the signal collection unit. The data processing module calculates sideslip angles and slip rates for wheels. The data control module sets target slip rates based on the slip rates corresponding to the friction coefficient μ for different sideslip angles. The device emits control commands after comparing the calculated slip rate to the set target slip rate, so that the ABS control is real time.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a vehicle anti-skid brake (ABS) control system and its control method, which belongs to the field of control of vehicle ABS. 
       BACKGROUND 
       [0002]    In existing vehicle control systems, by means of obtaining some parameters (e.g., sideslip angle or slip rate of the vehicle etc.) at the instantaneous time while the vehicle runs can control the vehicle more effectively. On automobiles in early stage, the running parameters were measured by means of onboard sensors; as GPS technology is widely used, the technique of utilizing GPS technology to calibrate the sensors and thereby obtaining vehicle running parameters has been proposed extensively. 
         [0003]    U.S. Pat. No. 6,681,180 provided a method for judging whether the vehicle wheels slip by GPS on the basis of a two-wheel vehicle model; with the method, the sideslip angle of the vehicle can be obtained dynamically. Said method employs an apparatus comprising a signal collection unit and an electronic control unit; said signal collection unit comprises a GPS configured to obtain speed and position signals of the vehicle, a gyro configured to obtain angular speed signal of the vehicle, and an acceleration sensor configured to obtain acceleration signal of the vehicle; said electronic control unit comprises a data receiving module configured to obtain the data of the signal collection unit and a data processing module configured to calculate sideslip angles for the wheels of the vehicle. The data receiving module of said electronic control unit obtains speed and position signals of the vehicle from the GPS, the angular speed signal from the gyro, and the acceleration signal from the accelerometer, and the data processing module calculates the centroidal speed, the speeds of front and rear wheels in direction x and y, and the sideslip angles of front and rear wheels. With above apparatus and method, the instantaneous sideslip angle can be measured while the vehicle runs. Though the apparatus and method can partially reduce the occurrence of wheel lockup during braking while the vehicle turns, on one hand, the wheels often can&#39;t take full advantage of ground braking force and therefore the braking length is increased while the vehicle turns, on the other hand, wheel lockup still occurs often. 
       SUMMARY 
       [0004]    The object of the present invention is to overcome the drawback that the wheels can&#39;t take full advantage of ground braking force and are often locked up completely during braking while the vehicle turns in the prior art. The present invention provides a vehicle anti-skid brake (ABS) control system and its control method that enables the wheels to take full advantage of ground braking force and prevent complete wheel lockup during braking while the vehicle turns. 
         [0005]    The inventor of the present invention found that: though the sideslip angles for wheels can be calculated and thereby the slip rate can be obtained by using the method provided in U.S. Pat. No. 6,681,180, the patent didn&#39;t disclose how to utilize sideslip angle for ABS control for the vehicle. In existing vehicle ABS control systems and control methods, a constant slip rate threshold is set first, and when the vehicle brakes during turning, the sideslip angle is measured in real time so that the slip rate is calculated. Commands are given to the braking device in accordance with the relationship between the slip rate and the slip rate threshold. However, actually, there is close relationship between the slip rate threshold and the sideslip angle. A constant slip rate threshold often results in that it is unable to provide enough braking force while the vehicle brakes during turning, and such control system may result in wheel lockup. 
         [0006]    In the prior art, the ABS control is to estimate parameters on the basis of a two-wheel vehicle model. Furthermore, the method provided in U.S. Pat. No. 6,681,180 is also based on the two-wheel vehicle model, i.e. the same control method is applied to left and right wheels. However, when the vehicle turns, the sideslip angles of the wheels are different, i.e. even left and right wheels in front or in rear may have different sideslip angles. Therefore, such method may cause wheel lockup when the vehicle brakes during turning. 
         [0007]    In view that the prior art doesn&#39;t take into account the problem that different wheel sideslip angles require different slip rate thresholds in vehicle ABS control. The present invention provides an vehicle ABS control system and method, which can set the slip rate threshold depending on the sideslip angle in real time, and implement vehicle ABS control by comparing the variable slip rate threshold with the measured slip rate, so as to solve the wheel lockup problem in existing ABS control systems. Another object of the present invention is to provide a method for calculating the sideslip angles and slip rates for each wheel on four-wheel vehicle respectively on the basis of a four-wheel vehicle model, in view that the two-wheel vehicle model adopted in the prior art can&#39;t prevent wheel from locking up when the vehicle brakes during turning. 
         [0008]    The system provided in the present invention comprises a signal collection unit and an electronic control unit, said electronic control unit comprises a data receiving module, a data processing module, and a data control module, said data receiving module is configured to obtain the data from the signal collection unit, wherein said data processing module is configured to calculate sideslip angles and slip rates for wheels, said data control module sets a slip rate threshold in accordance with the slip rate corresponding to the respective peak of braking force coefficient p at different sideslip angles, and outputs a control command in accordance with the result of comparison between the calculated slip rate and the set slip rate threshold to perform ABS control in real time. 
         [0009]    The method provided in the present invention comprises: 
         [0010]    step 1: a data collection unit collecting vehicle data; 
         [0011]    step 2: an electronic control unit receiving the vehicle data collected by the data collection unit, and calculating sideslip angles and slip rates for the vehicle wheels from said data; 
         [0012]    step 3: the electronic control unit setting the slip rate threshold for respective sideslip angle in accordance with the slip rate corresponding to the respective peak of braking force coefficient p at different sideslip angles; and 
         [0013]    step 4: the electronic control unit comparing the calculated slip rate with the set slip rate threshold, and outputting an ABS control command in accordance with the result of comparison. 
         [0014]    With the system and method provided in the present invention, separate ABS control can be applied to each wheel of the vehicle. In the prior art, a constant slip rate threshold is used for vehicle ABS control; however, since the braking force coefficient μ and the corresponding slip rate vary with the sideslip angle, such an ABS control method can&#39;t apply braking force as required, and may cause the wheels unable to take full advantage of ground braking force. Therefore, the present invention sets the slip rate threshold in accordance with the relationship between slip rate corresponding to different sideslip angles and peak of braking force coefficient, so as to apply braking force as required and ensure the wheels can take full advantage of ground braking force. Furthermore, in the prior art, the vehicle ABS control system is based on a two-wheel vehicle system model, i.e., equal braking force is applied to left and right wheels. However, when the vehicle turns, each wheel may have a sideslip angle, i.e. the sideslip angles of left wheel and right wheel may be different; if equal braking force is applied to left and right wheels, the braking force may be inappropriate and thereby may cause wheel lockup. In view of that problem, the present invention establishes a four-wheel vehicle model and applies ABS control separately for each wheel in accordance with the sideslip angle and slip rate for each wheel, so that each wheel can take full advantage of ground braking force. With the system and method provided in the present invention, the ABS control system can prevent lockup of any wheel while the vehicle brakes during turning, and minimize the braking length and improve vehicle safety while preventing the vehicle from off-tracking. The ABS control method is also effectively even the vehicle runs straightly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0015]      FIG. 1  is a structural block diagram of the vehicle ABS control system in accordance with the present invention; 
           [0016]      FIG. 2  is a flow diagram of the control subprogram of the data control module in the electronic control unit; 
           [0017]      FIG. 3  is a curve chart of braking force coefficient at different sideslip angles; 
           [0018]      FIG. 4  is a schematic diagram of vehicle model when the vehicle turns; 
           [0019]      FIG. 5  is an operating flow diagram of the electronic control unit; 
           [0020]      FIG. 6  is a structural block diagram of the vehicle ABS control system in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Hereunder the present invention will be further detailed with reference to the accompanying drawings. 
         [0022]    The present invention provides an vehicle ABS control system  10 , comprising a signal collection unit  20 , an electronic control unit  30 , as shown in  FIG. 1 ; said signal collection unit can be implemented with the exiting technology of signal processing unit in the prior art, and it comprises: a GPS  21  configured to obtain centroidal speed of the vehicle, a gyro  22  configured to obtain angular speed, an accelerometer  23  configured to obtain acceleration, wheel speed sensors  24  configured to obtain wheel rotation speeds, and a steering wheel angle sensor  25  configured to obtain steering wheel angle. Said electronic control unit  30  comprises a data receiving module configured to obtain the data from the signal collection unit for the electronic control unit, a data processing module configured to calculate wheel sideslip angles and slip rates, and a data control module configured to perform control. Said data control module sets a slip rate threshold in accordance with the slip rate corresponding to the respective peak of braking force coefficient μ at different sideslip angles, and outputs a control command in accordance with the result of comparison between the calculated slip rate and the slip rate threshold to perform ABS control in real time. 
         [0023]      FIG. 2  is a flow diagram of the data control module. As shown in  FIG. 2 , the data control module of said electronic control unit  30  executes the four parallel control submodules corresponding to the four wheels in step S 800 . On the basis of the sideslip angle α i  of the i th  wheel (i=1, 2, 3, or 4), the electronic control unit  30  sets the lower limit s i   min  and upper limit s i   max  of the slip rate threshold for the i th  wheel in real time. When the vehicle brakes, if the electronic control unit  30  determines the slip rate s i  for the i th  wheel is greater than the limit s i   max , it will output an command to decrease the braking force applied on the i th  wheel; if s i  is smaller than the limit s i   min , it will output an command to increase the braking force applied on the i th  wheel; and if s i  is between s i   min  and s i   max , it will require to keep the braking force unchanged. 
         [0024]    As shown in  FIG. 2 , in step S 810 , the electronic control unit  30  sets the lower limit s i   min  and the upper limit s i   max  of slip rate threshold for ABS control for the i th  wheel in accordance with the sideslip angle α i  of the i th  wheel. For different sideslip angles, the curve of braking force coefficient μ vs. slip rate s is also different, and the greater braking force coefficients correspond to different slip rate ranges; therefore, different upper and lower limits of slip rate threshold should be set for the wheel at different sideslip angles.  FIG. 3  shows the curve of braking force coefficient vs. sideslip angle. R.PUSCA et al provided the emulation curve chart in “Slip Control Strategy of an Electrical Four Wheel Drive Vehicle”. Hereafter an embodiment of the present invention will be provided referred to the curve. ABS control is executed for slip rate ±0.02 corresponding to the peak of braking force coefficient μ; since the range of slip rate s varies with the sideslip angle, different slip rate thresholds should be set at different sideslip angles; a look-up table method (i.e., the table of slip rate thresholds vs. sideslip angles as shown in Table 1, which is prepared on the basis of  FIG. 3 ) can be used to obtain the slip rate thresholds; at other sideslip angles, the corresponding slip rate thresholds can be determined with an interpolation method. 
         [0025]    In step S 820 , determining whether the slip rate s i  for the i th  wheel is within the range of slip rate threshold; if s i   min &lt;s i &lt;s i   max , proceed to step S 840 ; otherwise proceed to step S 830 . 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 sideslip angle (°) 
                 lower limit of slip rate 
                 upper limit of slip rate 
               
               
                   
               
             
             
               
                 0 
                 0.03 
                 0.07 
               
               
                 3 
                 0.05 
                 0.09 
               
               
                 6 
                 0.07 
                 0.11 
               
               
                 9 
                 0.11 
                 0.15 
               
               
                 12  
                 0.13 
                 0.17 
               
               
                   
               
             
          
         
       
     
         [0026]    In step S 830 , determining whether the slip rate s i  for the i th  wheel is greater than s i   max  again; if s i &gt;s i   max , proceed to step S 850 ; otherwise proceed to step S 860 . 
         [0027]    In step S 840 , the electronic control unit  30  outputs a command to keep the braking force applied on the i th  wheel unchanged. 
         [0028]    In step S 850 , the electronic control unit  30  outputs a command to decrease the braking force applied on the i th  wheel. 
         [0029]    In step S 860 , the electronic control unit  30  outputs a command to increase the braking force applied on the i th  wheel. 
         [0030]    In the system described in the present invention, the method used by the data processing unit to calculate the sideslip angle and slip rate for each wheel of a four-wheel vehicle can be an existing method; a preferred method is to obtain the centroidal speed of the vehicle, in that way, the four wheels of the vehicle will center around the centroid; that method can simplify the calculation. In order to obtain the centroidal speed, the preferred method in the present invention is to mount GPS  21  of said signal collection unit on the top of the vehicle, right above the centroid of the vehicle, so as to obtain the centroidal speed of the vehicle. 
         [0031]    The operating principle of the embodiment of the present invention is as follows: the vehicle model in turning state is shown in  FIG. 4 . The electronic control unit  30  is the core of the entire system.  FIG. 5  shows the operating flow diagram of the electronic control unit  30 . With the system described in the present invention mounted on the vehicle, when the vehicle runs, the centroidal speed signal of the vehicle can be obtained from GPS  21 , and the angular speed signal can be obtained from the gyro. The data receiving module in said electronic control unit  30  executes step S 200  to obtain the centroidal speed and angular speed signals of the vehicle. The data processing module in said electronic control unit  30  executes steps S 300 -S 700 : firstly, by using the obtained signals, a yaw Kalman filter is utilized to execute step S 300  to calculate the GPS measured value β GPS  of sideslip angle and estimated yaw rate r at the centroid of the vehicle, wherein β is the sideslip angle at centroid, and r is the yaw rate of vehicle, while the Kalman filter is a filter implemented with a well-known mathematical algorithm. 
         [0032]    Preferably, according to the present invention, said data processing module comprises: a submodule for calculating the sideslip angle and the estimated yaw rate at the centroid; a submodule for calculating the GPS measured value of longitudinal and lateral velocities at the centroid; a submodule for calculating the estimated longitudinal and lateral velocities at the centroid; a submodule for calculating the longitudinal and lateral velocities for each wheel; a submodule for calculating the sideslip angle for each wheel; and a submodule for calculating the slip rate for each wheel. Hereafter, the specific calculation method and process will be introduced. 
         [0033]    The data processing module in said electronic control unit  30  executes step S 400  to calculate the GPS measured values of longitudinal and lateral velocities V x   GPS  and V y   GPS  at the centroid of the vehicle according to the equations (1) and (2). 
         [0000]        V   x   GPS   =V   cg   GPS  cos(β GPS )  (1) 
         [0000]        V   y   GPS   =V   cg   GPS  sin(β GPS )  (2) 
         [0034]    Where, V cg   GPS  is the GPS measured value of centroidal speed of the vehicle that is obtained directly by GPS  21 . 
         [0035]    The data receiving module in said electronic control unit  30  obtains the accelerometer signal from the acceleration sensor  24 . With the measured GPS values of longitudinal and lateral velocities V x   GPS  and V y   GPs  at the centroid of the vehicle and the acceleration signal, said data processing module utilizes a Kalman filter for speed to execute step S 500  to calculate the estimated longitudinal and lateral velocities V x  and V y  at the centroid. 
         [0036]    The data receiving module of said electronic control unit  30  obtains the steering wheel angle value from the steering wheel angle sensor  25 . With the calculated V x  and V y  and the obtained steering wheel angle signal, on the basis of the vehicle model in turning state as shown in  FIG. 4 , the data processing module in said electronic control unit  30  executes step S 600  to calculate the longitudinal velocity V ix , lateral velocity V iy , and sideslip angle α i  at the center of each wheel according to the equations (3)-(10), wherein, V ix  (i=1, 2, 3, or 4) is the longitudinal component of wheel speed (i.e., longitudinal velocity) at the center of the i th  wheel, V iy  (i=1, 2, 3, or 4) is the lateral component of wheel speed (i.e., lateral velocity) at the center of the i th  wheel, and α i  (i=1, 2, 3, or 4) is the sideslip angle of the i th  wheel (note: only V 1x , V 1y  and α 1  for the first wheel are shown in the drawing). The equations are: 
         [0000]        V   1x   =V   x   +r ( d   2   +e  cos(δ))  (3) 
         [0000]        V   1y   =V   y   +r ( l   f   +e  sin(δ))  (4) 
         [0000]        V   2x   =V   x   −r ( d   2   −e  cos(δ))  (5) 
         [0000]        V   2y   =V   y   +r ( l   f   −e  sin(δ))  (6) 
         [0000]        V   3x   =V   x   +r ( d   2   +e )  (7) 
         [0000]        V   3y   =V   y   +rl   r   (8) 
         [0000]        V   4x   =V   x   −r ( d   2   +e )  (9) 
         [0000]        V   4y   =V   y   −rl   r   (10) 
         [0037]    Where, e is the distance from the steering knuckle spindle to the tire tread centre; d 2  is the lateral distance from the centroid to the steering knuckle; and l f  and l r  are the distance from the centroid to the front axle and the distance from centroid to rear axle, respectively. 
         [0038]    As shown in  FIG. 4 , the sideslip angle α 1  is the included angle between center speed V 1  for the first wheel and wheel plane. 
         [0000]    
       
         
           
             
               
                 
                   
                     α 
                     1 
                   
                   = 
                   
                     δ 
                     - 
                     
                       arctan 
                        
                       
                         
                           V 
                           
                             1 
                              
                             
                                 
                             
                              
                             y 
                           
                         
                         
                           V 
                           
                             1 
                              
                             
                                 
                             
                              
                             x 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0039]    Similarly, the sideslip angles for other wheels can be obtained: 
         [0000]    
       
         
           
             
               
                 
                   
                     α 
                     2 
                   
                   = 
                   
                     δ 
                     - 
                     
                       arctan 
                        
                       
                         
                           V 
                           
                             2 
                              
                             
                                 
                             
                              
                             y 
                           
                         
                         
                           V 
                           
                             2 
                              
                             
                                 
                             
                              
                             x 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
             
               
                 
                   
                     α 
                     3 
                   
                   = 
                   
                     arctan 
                      
                     
                       
                         V 
                         
                           3 
                            
                           
                               
                           
                            
                           y 
                         
                       
                       
                         V 
                         
                           3 
                            
                           
                               
                           
                            
                           x 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
             
               
                 
                   
                     α 
                     4 
                   
                   = 
                   
                     arctan 
                      
                     
                       
                         V 
                         
                           4 
                            
                           
                               
                           
                            
                           y 
                         
                       
                       
                         V 
                         
                           4 
                            
                           
                               
                           
                            
                           x 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
         [0040]    Where, δ is the steering angle of wheel (converted from the turning angle of steering wheel, which is obtained from the steering wheel angle sensor  25 ). 
         [0041]    Hereafter the slip rate s i  for each wheel will be calculated with the signal from the wheel speed sensor  24  and above calculated result. 
         [0042]    With said longitudinal velocity V ix , lateral velocity v iy , wheel rolling radius r w , and wheel rotation speed ω i  of the i th  wheel, said electronic control unit executes step S 700  to calculate the slip rate s i  (i=1, 2, 3, or 4) for each wheel according to the equation (15); 
         [0000]    
       
         
           
             
               
                 
                   
                     s 
                     i 
                   
                   = 
                   
                     
                       
                         
                           
                             
                               V 
                               ix 
                               2 
                             
                             + 
                             
                               V 
                               iy 
                               2 
                             
                           
                         
                          
                         
                           cos 
                            
                           
                             ( 
                             
                               α 
                               i 
                             
                             ) 
                           
                         
                       
                       - 
                       
                         
                           r 
                           w 
                         
                          
                         
                           ω 
                           i 
                         
                       
                     
                     
                       
                         
                           
                             V 
                             ix 
                             2 
                           
                           + 
                           
                             V 
                             iy 
                             2 
                           
                         
                       
                        
                       
                         cos 
                          
                         
                           ( 
                           
                             α 
                             i 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
         [0043]    Using above method and combining GPS and sensors, the sideslip angle and slip rate for each wheel of the four-wheel vehicle can be calculated. 
         [0044]    The data control module in said electronic control unit  30  executes step S 800 . The execution method has been described above. 
         [0045]      FIG. 6  is a system structural diagram in accordance with an embodiment of the present invention. The system is added with an ABS actuator unit. Said ABS actuator unit  40  comprises an ABS actuator for the first wheel  41 , an ABS actuator for the second wheel  42 , an ABS actuator for the third wheel, and an ABS actuator for the fourth wheel. Said vehicle ABS control system is based on a four-wheel vehicle model; the ABS actuators for four wheels in said wheel ABS actuator unit are independent to each other. 
         [0046]    Said electronic control unit is electrically connected to the GPS, the gyro, the accelerometer, the wheel speed sensors, and the steering wheel angle sensor in said signal collection unit, as well as the ABS actuator for the first wheel, the ABS actuator for the second wheel, the ABS actuator for the third wheel, and the ABS actuator for the fourth wheel in said wheel ABS actuator unit. 
         [0047]    With the vehicle ABS control method utilized in said vehicle ABS control system provided in present invention, after said system in the embodiment is mounted on the vehicle and the above calculation and control method is loaded in the electronic control unit, the onboard GPS and other sensors measure the sideslip angle and slip rate for each wheel, the system sets the upper limit and lower limit s min  and s max  of slip rate for each wheel at different sideslip angles, and outputs control commands to the wheel ABS actuator unit in accordance with the relationship between the measured slip rates and the set slip rate thresholds to accomplish ABS control for each wheel. 
         [0048]    While the present invention has been illustrated and described with reference to some preferred embodiments, the present invention is not limited to these. Those skilled in the art should recognize that various variations and modifications can be made without departing from the spirit and scope of the present invention as defined by the accompanying claims.