Abstract:
A method for stabilizing a vehicle in a transverse direction, in which: for a first braking force distribution in which each vehicle wheel is braked using the maximum braking force transmittable to the road surface in the current driving situation, the yawing moment acting on the vehicle is ascertained; for at least a second braking force distribution that differs from the first braking force distribution in that at least one wheel is not braked using the maximum braking force, the yawing moment acting on the vehicle is ascertained; a setpoint yawing moment is ascertained; and from at least the first and second braking force distributions, the braking force distribution whose associated yawing moment comes closest to the setpoint yawing moment is set at the vehicle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Today, numerous motor vehicles have electronic stability programs, which increase driving safety by actively intervening in the vehicle motion. In this context, braking actions at the individual wheels are normally used to control the yawing motion of the vehicle in a stabilizing manner in the extreme range of driving dynamics, and thus, to keep the vehicle controllable for the driver. In so doing, the braking actions are set such that they are as unnoticeable as possible and result in only a slight reduction in speed. 
         [0002]    Published German patent application document DE 36 25 392 A1describes a closed-loop control system for preventing skidding movements of a motor vehicle, in which skidding movements are automatically correctible without the necessity of steering movements. This is achieved by selective deceleration and/or accelerations and/or a correction of the wheel angle of individual wheels. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    The present invention relates to a method for stabilizing a vehicle in a transverse direction or stabilizing a vehicle with respect to an angular motion about its vertical axis, in which
       for a first braking force distribution in which each vehicle wheel is braked using the maximum braking force transmittable to the road surface in the current driving situation, the yawing moment acting on the vehicle is ascertained;   for at least a second braking force distribution that differs from the first braking force distribution in that at least one wheel is not braked using the maximum braking force, the yawing moment acting on the vehicle is ascertained;   a setpoint yawing moment is ascertained; and   from at least the first and second braking force distributions, the braking force distribution whose associated yawing moment comes closest to the setpoint yawing moment is set at the vehicle.       
 
         [0008]    Known electronic stability programs decelerate the vehicle only slightly using their braking actions at the individual wheels. If an accident cannot be prevented in spite of these braking actions, then, in some situations, a stabilization action resulting in a sharp reduction in speed would have been advantageous. A stabilization action associated with a sharp reduction in speed has the following advantages:
       if a collision occurs in spite of the stabilization actions, then the lower vehicle speed results in a lower risk of injury.   a slower vehicle exhibits a higher damping of the yawing movement, which means that the stabilization is made easier for both the closed-loop control system and the driver.   the deceleration expands the handling options of the driver, since a longer period of time elapses up to a possible collision. In this manner, the driver has more time for actions preventing an accident or decreasing the severity of an accident.       
 
         [0012]    An advantageous refinement of the present invention is characterized in that the yawing moment acting on the vehicle is ascertained from the portions of the yawing moment generated by the individual wheels. In particular, the yawing moment may be ascertained in a simple manner from the sum of the yawing moment portions. 
         [0013]    An advantageous refinement of the present invention is characterized in that the yawing moment portions are ascertained from at least quantities describing the vehicle geometry, the wheel angle, the longitudinal force acting on the wheel and the transverse force acting on the wheel. 
         [0014]    An advantageous refinement of the present invention is characterized in that the maximum force transmittable by the tires to the road surface is ascertained from at least vehicle geometry data, the vehicle mass and the current transverse and longitudinal accelerations. 
         [0015]    An advantageous refinement of the present invention is characterized in that the maximum braking force transmittable to the road surface is ascertained from the maximum force transmittable to the road surface and the slip angle of the tire. The slip angle and numerous other variables are already available in conventional electronic stability programs. 
         [0016]    An advantageous refinement of the present invention is characterized in that the setpoint yawing moment is ascertained from at least the wheel angle and the longitudinal speed of the vehicle. 
         [0017]    Furthermore, the present invention includes a device having means that are configured to implement the method according to the present invention. 
         [0018]    The advantageous embodiments of the method according to the present invention also manifest themselves as advantageous embodiments of the device of the present invention, and vice versa. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  shows a flow chart for stabilization with maximum deceleration. 
           [0020]    For a simulated lateral rear-end collision by which the vehicle is set into rotation about its vertical axis, 
           [0021]      FIG. 2  shows the yawing moment portions of the individual wheels as a function of the reduction in braking force, starting from the maximum braking force. In the example shown, within the scope of the maximum forces transmittable by the tires, an actual yawing moment corresponding to the setpoint yawing moment is not able to be achieved in any braking-force reduction constellation. 
           [0022]    On the other hand, 
           [0023]      FIG. 3  shows an unstable driving situation, in which the setpoint yawing moment may be reached by reducing braking force at the right front wheel to stabilize the vehicle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The braking actions independent of the driver, which are carried out within the scope of an electronic stability program, are generally selected to bring about only a slight deceleration of the vehicle. In contrast to that, brake assist systems and emergency braking systems generate very high deceleration values up to the physical maximum, but in so doing, dispense with generating a stabilizing yawing moment. 
         [0025]    The present invention relates to a method and a device for setting a stabilizing yawing moment in a motor vehicle, the braking actions performed being simultaneously intended for generating a vehicle deceleration that is as large as possible. If an accident cannot be prevented by an action of an electronic stability program in which a vehicle is decelerated slightly, then it is possible for a collision to occur at a high speed and with a correspondingly high risk of injury. However, if a stabilization action is performed so that a rapid and sharp reduction in speed simultaneously occurs, then this has several advantages:
       if an accident occurs in spite of the stabilization actions, then a lower risk of injury is also produced by the lower speed.   a slow vehicle exhibits a higher damping of the yawing motion, which means that the stabilization is made easier for both the electronic stability program and the driver.   the options for action of the driver are expanded by the deceleration of the vehicle, since a longer time elapses up to the collision with an object at a fixed location. This may allow the driver to act to reduce a severity of the accident.       
 
         [0029]    Stabilization actions with simultaneous maximum deceleration are then appropriate, when a loss of control of the driver can be assumed, and therefore, a risk of collision exists whose severity may be reduced by decelerating the vehicle. In the simplest case, for example, it may be assumed that control is lost when an airbag is deployed, due to an initial collision, and the driver is thereby temporarily unable to act. 
         [0030]    The operation of a specific embodiment of the stabilization method according to the present invention is represented in  FIG. 1 . In this context, it is assumed that the setpoint yawing moment to be set is ascertained by a primary controller entity, which implements, for example, closed-loop yaw rate control. The objective of the method is to implement the requested yawing moment, such that the vehicle thereby undergoes maximum deceleration. This may be achieved by fully braking all four wheels in the initial state and, therefore, setting the maximum deceleration. The requested yawing moment is generated by selectively debraking the individual wheels out of this state. 
         [0031]    In  FIG. 1 , the method steps  1  through  6  are numbered on the left side. In step  1 , maximum forces F_max,i transmittable by the four wheels of the motor vehicle to the road surface are ascertained in block  102 , index i indicating the respective wheel. The forces are mainly a function of the coefficient of friction between tire and road surface and the normal force acting on the tire. The maximum transmittable forces are calculated from quantities provided by block  101 , which may include, e.g., vehicle geometry data, the vehicle mass and measured transverse and longitudinal accelerations. The normal forces are calculated, for example, from the static wheel loads, in view of the pitching and rolling moments resulting from transverse and longitudinal accelerations. The coefficient of friction may be estimated on the basis of the measured variables available in the electronic stability program. 
         [0032]    In step  2 , the maximum longitudinal force component Fx_max,i at all four wheels is determined in block  202 . In this context, the total force acting at a locked wheel is directed oppositely to the velocity vector at the center of tire contact. Maximum longitudinal force component Fx_max,i may be calculated from maximum force F max,i and the slip angle at the wheel. The slip angles are calculated on the basis of the measured and estimated variables available in the system. 
         [0033]    In step  3 , the longitudinal forces are varied, in block  302 , between zero and the maximum value calculated beforehand for each wheel. For each variation, the transverse force components remaining in the case of full utilization of the tire force potential are calculated using the Kamm&#39;s circle. 
         [0034]    In step  4 , in block  402 , the yawing moments generated by the wheels are calculated for these variations, from the transverse and longitudinal forces. In  FIGS. 2 and 3 , these are each plotted in the ordinate direction as a function of the debraking ΔFx plotted in the abscissa direction. A debraking of zero (ΔFx=0) means a maximum braking force present at the respective wheel. 
         [0035]    In the next step  5 , the debraking that minimizes the difference between the setpoint yawing moment and the sum of all of the yawing moment portions is ascertained for each wheel in succession. In  FIG. 2 , a high setpoint yawing moment Mz=MSet=1.5·104 Nm is requested. The result of this is that the left front (FL=“front left”) and right front (FR=“front right”) wheels remain fully braked, since debraking would reduce the yawing moments generated by these wheels. Right rear wheel RR (RR=“rear right”) generates the highest yawing moment when it is completely debraked, i.e., a braking force of zero is set. The left rear wheel (RL=“rear left”) generates a maximum yawing moment in the case of partial debraking. In the present case in  FIG. 2 , the requested yawing moment is not achievable via braking actions, as is apparent in light of total yawing moment MAct, which represents the sum of the four maximum achievable yawing moment portions. 
         [0036]    In order to arrive at a better understanding of  FIG. 2 , this is explained here again in detail. In the abscissa direction, debraking ΔFx is plotted in the physical unit N; in the ordinate direction, yawing moment Mz is plotted in the physical unit Nm. A debraking of zero (i.e., ΔFx=0) means that all of the wheels are braked at a maximum. In the driving situation present in  FIG. 2 , the vehicle was set into a clockwise rotation about the vertical axis due to a side collision at the rear end. The driving situation resembles the one in the case of a right-hand curve. The vehicle body tilts in the outer direction of the curve, and therefore, the wheels on the outside of the curve are also more heavily loaded. In the case of a right-hand curve, the wheels on the outside of the curve are the left wheels. The more highly loaded wheels may also exert greater braking forces. In  FIG. 2 , this manifests itself in that the curves of wheels RL (left rear wheel) and FL (left front wheel) may be unloaded up to a force of 7000 N (ΔFx=7000 N); therefore, the range covered by the two curves extends from ΔFx=0 to ΔFx=7000 N. Right wheels RR (right rear wheel) and FR (right front wheel) are less loaded, may each transmit a maximum braking force of 2000 N and may therefore be unloaded by a maximum of ΔFx=2000 N. 
         [0037]    The four operating points set at the wheels in  FIG. 2  are denoted by the symbol “+.” A maximum yawing moment is reached, when wheels FR and FL are not debraked (in this case, the maximum yawing moment is present at ΔFx=0), RL is debraked by approximately 3000 N and, with 2000 N, RR is debraked by the maximum value possible for this wheel, i.e., this wheel is not braked. However, sum MAct of the yawing moment portions setting in via the selected operating points is not enough to reach the desired setpoint yawing moment MSet, i.e., MAct&lt;MSet. 
         [0038]    On the other hand,  FIG. 3  shows a situation in which the requested yawing moment may be produced. In this case, the three wheels RR, FL and RL remain fully braked and only wheel FR must be debraked by app. 2300 N. Therefore, in this case, stabilization is implemented with simultaneous, sharp deceleration. 
         [0039]    In the following,  FIG. 1  is explained once more in detail. 
         [0040]    In the left column, steps  1  through  6  of the method are numbered consecutively. In the third column that includes blocks  102 ,  202 ,  302 ,  402 ,  502  and  602 , the steps of the method are represented. Using blocks  101 ,  201 ,  301  and  401 , the second column from the left shows the main variables used for the individual method steps. 
         [0041]    In block  102 , the calculation of maximum forces F_max,i transmittable at the individual wheels is performed. To that end, vehicle geometry data, the vehicle mass and the measured transverse and longitudinal accelerations are provided in block  101 . In block  202 , a calculation of maximum longitudinal force Fx_max,i transmittable at the individual wheels is subsequently performed. These are the forces transmittable in the case of full braking. For that purpose, the estimated slip angles are provided in block  201 . In block  302 , the longitudinal forces are varied. To that end, starting out from the state of full brake application, debraking instances between 0% (=full braking) and 100% (=wheel not braked) are considered. In addition, the transverse forces resulting in each instance are calculated. For this, the data of the Kamm&#39;s circle and a simple transverse force model for small slip angles are provided as additional input data in block  301 . 
         [0042]    In block  402 , the yawing moments generated by the individual wheels for the debraking variations considered in block  302  are calculated as a function of the longitudinal and transverse forces. In block  401 , the wheel angle and vehicle geometry data are provided as additional input data. 
         [0043]    In block  502 , the debraking variation that minimizes the deviation between the sum of all yawing moment portions and the setpoint yawing moment is selected. 
         [0044]    To that end, setpoint yawing moment Mz_set, which is ascertained by a superordinated vehicle dynamics controller, is provided by block  103 . 
         [0045]    In block  602 , the braking force distribution belonging to the debraking variation selected in block  502  is set at the individual wheels by a subordinate brake force regulator or slip controller. In particular, in an advantageous refinement, the brake force regulator may be configured such that locking of the wheels is prevented, i.e., the braking instances of the individual wheels are braking instances that prevent locking. 
         [0046]    In an advantageous embodiment, it is possible to execute the stabilization method of the present invention only after the presence of an initial collision of the vehicle, but not for stabilization in the absence of a collision. 
         [0047]    Of course, the maximum transmittable forces may also be ascertained using the sensor technology contained in “intelligent tires.”