Abstract:
A method to improve the braking behavior of a vehicle is provided, in which it is not permitted to exceed a maximum allowable braking pressure difference between the braking pressures on the two wheels of one axle, and in which the maximum allowable braking pressure difference between the wheels of one axle is a function of at least one variable describing the vehicle dynamics. When an unstable behavior of the vehicle is recognized, the maximum allowable braking pressure difference between the wheels of the axle is maintained or reduced.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for improving the braking behavior of a vehicle. 
     BACKGROUND INFORMATION 
     When braking on a road surface having widely different coefficients of friction with respect to the vehicle (this situation is referred to below as μ-split), it is possible that widely different braking forces may be applied in the vehicle. This results in a yaw moment about the vertical axis of the vehicle, which may result in skidding. 
     The driver now has the possibility to counteract the yaw moment by countersteering. Since as a rule, the yaw moment builds up very rapidly, the driver does not have enough time to react. Braking systems such as ABS (antilock braking system) or vehicle dynamics control systems such as ESP (electronic stability program) therefore limit the braking pressure on the high-μ side of the vehicle as a function of the low-μ side after the μ-split situation is recognized (the high-μ side is the side of the vehicle or of the road surface on which the higher coefficient of friction exists between the tires and road surface). This occurs based on the maximum allowable differential braking pressure. 
     A method for braking the wheels of a vehicle is shown in German Published Patent Application No. 42 25 983, wherein the buildup of braking pressure is influenced on at least one wheel to reduce a yaw moment produced by an ABS. In doing so, the braking pressure on the wheels of one axle is influenced such that the difference of the braking pressures of one axle does not exceed a maximum allowable value. This maximum allowable value is determined as a function of the speed of the vehicle and the transverse acceleration. 
     SUMMARY 
     The present invention is directed to a method for improving the braking behavior of a vehicle, in which it is not permitted to exceed a maximum allowable braking pressure difference between the braking pressures on the two wheels of one axle, and in which the maximum allowable braking pressure difference between the wheels of one axle being a function of at least one variable describing the vehicle dynamics. 
     The advantage of the present invention is that when an unstable behavior of the vehicle is recognized, the maximum allowable braking pressure difference between the same wheels is maintained or reduced. 
     In a braking operation on a μ-split road surface, there is a conflict of objectives between driving stability of the vehicle and a short braking distance. 
     Driving stability is attained by keeping the allowable differential pressure low as a result of which the braking forces applied to the wheels of an axle stay nearly the same. However, the result of this is that the braking distance is lengthened since the maximum possible braking forces are not applied. However, for an optimum braking distance, the differential pressure should not be limited so that the maximum possible braking force may be set on each wheel. In μ-split braking, however, this results in a yaw moment about the vertical axis, which may result in skidding. By maintaining and/or reducing the maximum allowable braking pressure difference between the wheels of an axle when a slightly unstable behavior of the vehicle is recognized (in particular if there is still no skidding), it is possible to restabilize the vehicle. 
     An advantageous embodiment is characterized by reducing the maximum allowable braking pressure difference only if having previously maintained the maximum allowable braking pressure difference did not result in stabilizing the vehicle. Maintaining the maximum allowable braking pressure is intended to give the driver time to countersteer. Only if this measure is inadequate or comes too late is the maximum allowable braking pressure difference reduced. 
     In this connection, it is of advantage if when a first stage of unstable behavior is recognized, the maximum allowable braking pressure difference is maintained, and when a second stage of unstable behavior is recognized, the maximum allowable braking pressure is reduced. 
     The first stage of unstable behavior is recognized by an analysis of the deviation of the actual yaw rate of the vehicle from the desired yaw rate of the vehicle, and the second stage of unstable behavior is recognized by an analysis of the yaw acceleration. 
     The variables “actual yaw rate” and “desired yaw rate” are available in a vehicle equipped with a vehicle dynamics control system so that no significant additional expense is required for the present invention. The difference between these two yaw rates represents an indicator of an unstable state of vehicle dynamics. The yaw acceleration may be obtained from the yaw speed (or yaw rate, the terms “yaw speed” and “yaw rate” describe the same thing) by simple time differentiation. The yaw acceleration may be selected because it may be considered an indicator for an instability in a μ-split braking. The relationship between yaw acceleration and yaw moment is essentially given by Newton&#39;s law of motion. 
     In an exemplary embodiment of the invention, if the absolute value of the deviation between the actual yaw rate determined for the vehicle and the desired yaw rate exceeds a specifiable threshold value, a first stage of unstable behavior is recognized. A check based on a threshold value is in particular simple to implement. 
     Furthermore, if the steering angle exceeds a specifiable threshold value, the first stage of unstable behavior is recognized. This is related to the fact that the yaw moment should not be increased further if considerable steering has already been performed. This prevents the vehicle from suddenly turning in the other direction if the friction characteristics between the road surface and the tires change. 
     In an exemplary embodiment, the second stage of unstable behavior is recognized if at least the yaw acceleration exceeds a specifiable threshold, and the difference between the braking pressures on the two wheels of one axle exceeds a specifiable threshold value, and the transverse acceleration of the vehicle remains below a specifiable threshold value. 
     These three conditions represent necessary conditions for the presence of the second stage of unstable behavior. In order to avoid an incorrect activation of the logic, the differential pressure may only be adapted if the transverse acceleration (ay) is lower than a limit value. 
     Furthermore, the second stage of unstable behavior may be recognized if at least the yaw acceleration exceeds a specifiable threshold value, and the braking pressure difference between the braking pressures on the two wheels of one axle exceeds a specifiable threshold value, and the transverse acceleration of the vehicle remains below a specifiable threshold value and a rotation of the vehicle about its vertical axis to the side to which the higher braking pressure is applied is detected. 
     It is thus determined if the rotation of the vehicle is at all attributable to the braking pressure difference. 
     It is also possible that the second stage of unstable behavior is recognized if at least the yaw acceleration exceeds a specifiable threshold value, and the braking pressure difference between the braking pressures on the two wheels of one axle exceeds a specifiable threshold value, and the transverse acceleration of the vehicle remains below a specifiable threshold value, and a rotation of the vehicle about its vertical axis to the side to which the higher braking pressure is applied is detected, and the presence of a braking operation on a μ-split road surface is recognized. 
     The device of the present invention for improving the braking behavior of a vehicle contains an arrangement for controlling the braking pressure whereby it is ensured that a maximum allowable braking pressure difference between the braking pressures on the two wheels of an axle may not be exceeded, the maximum allowable braking pressure difference between the wheels of an axle being a function of at least one variable describing the vehicle dynamics, and is characterized in that the arrangement to control the braking pressure maintain or reduce the maximum allowable braking pressure difference between the wheels of an axle if an unstable behavior is recognized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph illustrating a principle of a controlled increase of differential pressure. 
         FIG. 2  is a flowchart of a first phase of the differential pressure control. 
         FIG. 3  is a flowchart of a second phase of the differential pressure control. 
         FIG. 4  is a flowchart of the first phase of the differential pressure control, illustrating the control of the maximum allowable pressure difference. 
         FIG. 5  is a flowchart of the second phase of the differential pressure control, illustrating the control of the maximum allowable pressure difference. 
     
    
    
     DETAILED DESCRIPTION 
     In a braking operation on a μ-split road surface, there is a conflict of objectives between driving stability of the vehicle and a short braking distance. Driving stability is attained by keeping the allowable differential pressure low as a result of which the braking forces applied to the wheels of one axle stay nearly the same. However, the result of this is that the braking distance is lengthened since the maximum possible braking forces are not applied. However, for an optimum braking distance, the differential pressure should not be limited so that the maximum possible braking force may be set on each wheel. In μ-split braking, however, this results in a yaw moment about the vertical axis, which may result in skidding. 
     The basic procedure for controlling and/or regulating the differential pressure is illustrated in  FIG. 1 . In it time t is plotted in the direction of the x-axis, the maximum allowable differential pressure dP is plotted in the direction of the y-axis. 
     In other systems, after μ-split braking is recognized (point in time t 0  in  FIG. 1 ), the allowable differential pressure is limited to a small starting value ( FIG. 1 : P 0 ). This value is then increased to a maximum value ( FIG. 1 : P 1 ) as a function of time. Maximum value P 1  may possibly also be speed dependent. This limited pressure buildup on the high-μ wheel gives the driver enough time to reduce the yaw moment by countersteering. 
     The system may be tuned such that even an inexperienced driver or a driver who has been surprised by a sudden μ-split situation does not begin to skid. Therefore a small pressure increase gradient δp (see  FIG. 1 ) may be selected. 
     In contrast to this, the present invention takes the behavior of the vehicle into account. The controlled differential pressure increase is based on a regulation which influences the allowable differential pressure as a function of the yaw acceleration, the system deviation of the yaw speed (yaw rate) and of the steering angle. The result is a better compromise between braking distance and driving stability for most driving situations. 
     The great advantage of the present invention is that pressure increase gradient δp may be selected to be lower so that the braking distance is shortened. Should, however, the yaw moment about the vertical axis of the vehicle become too high, the present invention prevents the vehicle from starting to skid. The braking behavior of a vehicle is thus improved by the vehicle behavior influencing the maximum allowable braking pressure difference or braking force difference between the two wheels of an axle. 
     For a good compromise between braking distance and driving stability, it is critical to find an optimum differential pressure increase gradient. This is, however, a function of the road surface conditions and driver responses. The present invention includes a regulation based on brake control or brake regulation, which in turn is made up of two parts: a) a precontrol which prevents a further increase of the pressure difference and b) a regulation which is capable of reducing the pressure difference. 
     Precontrol (First Phase): 
     The precontrol is activated when a first stage of unstable behavior is recognized. In vehicle dynamics control systems, a desired yaw speed (vGiAck) is generally first determined from steering angle Lw and vehicle speed (vx) using characteristic speed vch (this is a vehicle constant). Using transverse vehicle acceleration (ay), longitudinal vehicle acceleration (vx), and additional auxiliary variables if necessary, desired yaw speed (vGiSo) is determined therefrom. 
     Yaw speed deviation evGi is determinable from the difference:
 
 evGi=vGiSo−vg 1  (1)
 
     If evGi becomes too great in a μ-split braking operation, this is an indication that the vehicle is becoming unstable. As a rule, the driver has not been able to reduce the yaw moment by countersteering. The yaw moment may not increase further in order to give the driver time to countersteer. The pressure increase gradient must therefore be set to zero for a specific period of time. Subsequently, the yaw moment should not increase further if considerable steering has already been performed so that the vehicle does not suddenly lurch in the other direction if the coefficient of friction circumstances change. 
     This situation is illustrated in a block diagram in  FIG. 2 . For the sake of clarity, only the “yes” outputs are drawn in the blocks in  FIG. 2 . Of course, blocks  200 ,  202 ,  204  and  205  also have “no” outputs. If one of the conditions checked in these blocks is not fulfilled (i.e., the “no” output is activated), then the sequence is interrupted or branched back to block  200 , i.e., the sequence starts anew. 
     In block  200 , it is checked whether μ-split braking is present; this is denoted by the symbol “μ” in block  200 . 
     If μ-split braking is present, it is checked in block  202  whether the amount of the deviation |evGi| of the yaw speed exceeds a specific threshold value (Lim 1 ). Threshold value Lim 1  is determined as a function of longitudinal vehicle acceleration vx. Lim  1  is determined in block  201 . For this purpose, longitudinal vehicle acceleration vx is used as an input variable for block  201 . Deviation evGi is determined in subtraction block  203 : evGi=vGiSo−vGi. 
     As another condition, it is checked in block  205  whether steering angle Lw exceeds a specifiable threshold value Lim 2 : Lw&gt;Lim 2 . For this purpose, block  205  receives the steering angle as an input signal (from a steering angle sensor, for example). 
     The output signals of comparison blocks  202  (|evGi|&gt;Lim 1 ?) and  205  (Lw&gt;Lim 2 ?) are fed to block  204 . 
     If at least one of the conditions is fulfilled, i.e., 
     |evGi|&gt;Lim 1  OR Lw&gt;Lim 2 , 
     then in block  205  the differential pressure gradient δp (see  FIG. 1 ) is set to zero; otherwise it stays at the original value. If the deviation again remains below threshold value Lim 1  and the steering angle again remains below threshold value Lim 2 , the differential pressure may increase again.
 
Regulation (Second Phase):
 
     The regulation is activated if a second stage of unstable behavior is recognized. Should the described precontrol be insufficient or comes too late, the yaw moment must be reduced actively, i.e., without any action by the driver. Yaw acceleration DvGi may be considered an indicator of an excessively high yaw moment. This is calculated using time differentiation of the yaw speed:
 
 DvGi=d ( VGi )/ dt  
 
The differential pressure should only be reduced if the vehicle rotates too rapidly to the high-μ side in μ-split braking. To avoid an incorrect activation of the logic, the differential pressure may only be adapted if transverse vehicle acceleration (ay) is less than a limit value (Lim 3 ).
 
     The braking pressure on the front wheels is used to reliably recognize the high-μ side in μ-split braking: 
     For example, the high-μ side may be the left side of the vehicle. On the high-μ side, braking pressure pvL (pVL=braking pressure of the left front wheel) is higher by a specific amount (Off 1 ) than braking pressure pvR of the other side (pVR=braking pressure of the right front wheel). If the vehicle rotates simultaneously in this direction (DvGi&gt;Lim 4 ), allowable differential pressure dP is adjusted according to the following regulating equation:
 
 dP=dP−C*|DvGi| 
 
     C is a constant that may be applied as a function of the vehicle. 
     The reduction lowers the braking pressure in the high-μ wheel and, as a consequence, the yaw moment about the vertical axis is reduced. After the vehicle has become stabilized, the differential pressure may be increased again. 
     The regulation is described with reference to a block diagram ( FIG. 3 ): 
     In query block  300 , it is determined whether μ-split braking is present (denoted by the symbol “μ” in block  300 ) and whether transverse vehicle acceleration (ay) simultaneously remains below a threshold value Lim 3 . If this is the case, two queries are made simultaneously:
         query  1  takes place in block  301 . A check is made there whether braking pressure pvL of the left front wheel exceeds braking pressure pvR of the right front wheel by more than value Off 1  (pvL&gt;pvR=Off 1 ) and whether DvGi&gt;Lim 4  at the same time.   query  2  takes place in block  302 . A check is made there whether braking pressure pvR of the right front wheel exceeds braking pressure pvL of the left front wheel by more than value Off 1  (pvR&gt;pvL=Off 1 ) and whether DvGi&lt;Lim 4  at the same time.       

     These two queries include both possible equivalent cases as to whether a yaw moment is occurring to the right or left about the vertical axis of the vehicle. Each “yes” output of the two gating queries  301  and  302  is fed to OR query  303 . If one of the two conditions  301  and  302  is fulfilled, then OR query  303  is fulfilled at the same time. The “yes” output of OR query  303  is therefore fed to block  304 . In block  304 , product C* |DvGi| is formed. The result of this block is fed to gating block  305 . At the same time the maximum allowable pressure difference (differential pressure) dP is fed to block  305 . From these two variables (namely variable C*|DvGi| and (old) variable dp), the new variable
 
 dP=dP−C*|DvGi| 
 
is formed, i.e., dP is reduced by value C*|DvGi|. New variable dP is on the left side of the equation; old variable dP is on the right side of the equation, i.e., the equation is to be understood as an iteration method.
 
     The control of the maximum allowable pressure difference is illustrated in  FIG. 4  in the form of a flow diagram. After the start in block  400 , it is checked in block  402  whether μ-split braking is present. If this is not the case, then branching occurs back to block  400 . If it is the case, then it is checked in block  404  whether condition |evGi|&gt;Lim 1  is fulfilled. If this condition is not fulfilled, then it is checked in block  406  whether Lw&gt;Lim 2  is fulfilled. If condition  404  is fulfilled, branching also takes place to block  408 . If condition  406  is not fulfilled, branching occurs back to block  400 . In block  408 , the maximum allowable braking pressure difference is maintained, i.e., δp=0. 
     The method for regulating the maximum allowable pressure difference is illustrated in  FIG. 5  in the form of a flow diagram. After the start in block  500 , it is checked in block  501  whether μ-split braking is present and whether ay&lt;Lim 3  at the same time. If this condition is not fulfilled, branching occurs back to block  500 . If both conditions are fulfilled in block  501 , then it is checked in block  502  whether (pVL&gt;pVR+Off 1 ) and (DvGi&lt;Lim 4 ) are fulfilled simultaneously or whether (pVR&gt;pVL+Off 1 ) and (DvGi&lt;−Lim 4 ) are fulfilled simultaneously. If this is not the case, i.e., neither condition is fulfilled, then branching occurs back to block  500 . If, however, the query in block  502  is fulfilled, then variable C*|DvGi| is formed in block  503  and maximum allowable pressure difference dP is reduced by this value. 
     The abbreviations used in the description are listed once more below: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 ay 
                 transverse vehicle acceleration 
               
               
                   
                 dP 
                 maximum allowable pressure difference 
               
               
                   
                 DvGi 
                 yaw acceleration 
               
               
                   
                 evGi 
                 yaw rate deviation 
               
               
                   
                 high-μ side 
                 side of the vehicle with a high coefficient of 
               
               
                   
                   
                 friction (of the road surface) 
               
               
                   
                 low-μ side 
                 side of the vehicle with a low coefficient of 
               
               
                   
                   
                 friction (of the road surface) 
               
               
                   
                 Lw 
                 steering angle 
               
               
                   
                 P0, P1 
                 braking pressures 
               
               
                   
                 PVL, pVR 
                 wheel braking pressure, left front, left right 
               
               
                   
                 t 
                 time 
               
               
                   
                 vch 
                 characteristic speed (constant) 
               
               
                   
                 vGi 
                 yaw rate 
               
               
                   
                 vGiAck 
                 desired raw value for yaw rate 
               
               
                   
                 VgiSo 
                 desired yaw rate 
               
               
                   
                 vx 
                 longitudinal vehicle acceleration 
               
               
                   
                 δp 
                 pressure increase gradient