FIELD OF THE INVENTION  
       [0001]     The present invention relates to a vehicle dynamics control system, and a method for stabilizing a vehicle in critical driving situations according to the present invention.  
       BACKGROUND INFORMATION  
       [0002]     Vehicle dynamics control systems, such as ESP (electronic stability programs), are used for the purpose of improving the controllability of motor vehicles in critical driving situations, for example, upon oversteering when negotiating curves, and stabilizing the vehicle. Known vehicle dynamics control systems include a control unit, in which a control algorithm for executing a float angle and/or yaw rate regulation is stored, as well as an array of sensors which provide measured values about the current driving condition of the vehicle. Different setpoint variables are calculated from the driver selection, in particular the steering wheel position, the accelerator pedal position, and the brake operation. In the event of too high a deviation of the actual behavior from the setpoint behavior of the vehicle, the electronic stability program intervenes in the vehicle operation and produces a compensating yaw moment, which counteracts the yaw movement of the vehicle. For this purpose, the vehicle dynamics control system typically operates the vehicle brakes and/or the engine controller as final controlling elements.  
         [0003]     Modern vehicles also increasingly include active rear-wheel steering systems, which may also intervene in the vehicle operation for the purpose of vehicle stabilization. Systems of this type typically include a separate control unit and a steering control element, using which the steering angle of the rear wheels may be adjusted. The control algorithm of the rear-wheel steering system typically also determines different setpoint values of driving condition variables, such as a setpoint yaw rate or a setpoint float angle, and calculates a required stabilization intervention (the superimposed steering angle) from the system deviation. The calculated steering angle changes are implemented using a steering control element and influence the driving behavior of the vehicle.  
         [0004]     Since both the electronic stability program ESP and the active rear-wheel steering system (RWS) perform stabilization interventions, this may result in the two systems mutually impairing one another and, in the worst case, the driving safety being endangered.  
       SUMMARY OF THE INVENTION  
       [0005]     It is therefore an object of the present invention to coordinate the stabilization interventions of the vehicle dynamics control system and the rear-wheel steering system.  
         [0006]     An aspect of the present invention is providing an expanded vehicle dynamics control system (VDM), which may also address a steering control element of the rear-wheel steering system, in addition to the brake system and the engine controller, and providing this VDM system with only one single control algorithm, which generates a controller output variable (e.g., a yaw moment), from which both an actuation request for a final control element (i.e., the brake system or the engine controller) of the vehicle dynamics control system and also for the steering control element of the rear-wheel steering system are generated. This type of central regulation may be implemented particularly easily and is safe and reliable.  
         [0007]     The corresponding control algorithm may be implemented, for example, in the control unit of the vehicle dynamics control system. The currently existing electronic stability program algorithm (ESP) must only be slightly supplemented and adapted for this purpose. Preferably, no separate electronic stability program is executed in the control unit of the rear-wheel steering system.  
         [0008]     The expanded vehicle dynamics controller (VDM) preferably includes a distributor unit, which generates both an actuation for the brake system and the engine controller and an actuation for the steering control element of the rear-wheel steering system from the controller output variable.  
         [0009]     The control unit of the expanded vehicle dynamics control system (VDM) and the control unit of the rear-wheel steering system (RWS) are preferably connected to a shared data bus (e.g., a chassis CAN), via which different types of steering angle information are transmitted. In addition, the VDM control unit is preferably connected to a further bus (e.g., a PT CAN), via which different types of sensor information of the VDM sensor system are transmitted in particular. Through the separate transmission of steering angle and other sensor information, the data transmission speed may be elevated and system security may be improved.  
         [0010]     In an expanded electronic stability program (VDM), as was described above, some special control technology characteristics arise, which will be explained in greater detail in the following:  
         [0000]     1. Adaptation of the Control Behavior of the Yaw Rate and Float Angle Regulators  
         [0011]     Known vehicle dynamics controllers typically include a yaw rate regulator and a float angle regulator, which generate a regulator output variable as a function of their associated system deviation, from which the manipulated variable for the brake system (hydraulic modulator) and/or the engine controller (motronics) is calculated. The expanded vehicle dynamics control system (VDM) according to the present invention also calculates a manipulated variable for the rear-wheel steering system as a function of the system deviation. The following set of problems thus results in regard to the yaw rate and float angle regulation:  
         [0012]     Basically, a control intervention of the yaw rate regulator in the rear-wheel steering simultaneously also has an influence on the float angle of the vehicle. In contrast to a braking intervention, the intervention in the rear-wheel steering causes an increase in the float angle while reducing the yaw rate. A control intervention of the float angle regulator in the rear-wheel steering, in contrast, causes an increase in the yaw rate while reducing the float angle. The interventions of the two regulators thus act in precisely opposite directions.  
         [0013]     In both cases, the operating point of one regulator is shifted as a function of the actuator intervention of the other regulator. The two regulators may thus mutually amplify one another and impair the stability of the vehicle. It is therefore suggested that the regulation behavior of the yaw rate regulator preferably be adapted as a function of the share of the slip angle regulator in the manipulated variable for the rear-wheel steering system and the regulation behavior of the slip angle regulator be adapted as a function of the share of the yaw rate regulator. For this purpose, for example, the sensitivity of the regulators may be varied correspondingly.  
         [0014]     To influence the regulator sensitivity, either the control threshold of the regulator may be adapted or the system deviation itself may be corrected. Thus, for example, the system deviation of the yaw rate regulator may be set as a function of the share of the float angle regulator in the manipulated variable for the rear-wheel steering system and the system deviation of the float angle regulator may be set as a function of the share of the yaw rate regulator.  
         [0015]     A correction unit is preferably provided for adapting the system deviation, which generates a corrected system deviation from the system deviations of each of the regulators, which then forms the basis for the yaw rate and/or float angle regulation. The correction unit preferably defines a dead zone, i.e., a range of the system deviation in which the system deviation is set to zero, and a range in which the original system deviation is reduced by a predefined absolute value. The operating point displacements of one regulator because of the control interventions of the particular other regulator are thus compensated for.  
         [0000]     2. Activation/Deactivation of the Yaw Rate Regulator in Specific Ranges as a Function of the Slip Angle  
         [0016]     In the event of high slip angles of the rear axle wheels, only a very weak effect or even no effect may be exerted on the driving behavior of the vehicle through the change of the rear-wheel steering angle, since the tire transverse forces change only slightly at large slip angles. In the range of larger slip angles (e.g., between 3° and 5°), in particular at a low coefficient of friction (e.g., on snow), hardly any further steering effect may be achieved by turning the steering wheel further. In order to nonetheless be able to stabilize the vehicle, it is therefore suggested that in a range of larger slip angles (which may differ depending on the road surface substrate), the electronic stability program be allowed to intervene more strongly in the vehicle operation via the brake system and/or the engine controller than in a range of smaller slip angles.  
         [0017]     According to a preferred embodiment of the present invention, the yaw rate regulator or float angle regulator is thus activated in a predefined slip angle range (in particular at large slip angles) and deactivated in another range (in particular at small slip angles), or at least its effect is reduced. According to the present invention, in particular, stabilizing interventions in the rear-wheel steering are not suppressed, since in this case the steering behavior of the vehicle would change. This would no longer be controllable by the driver or would at least place great demands on the driver. The stability interventions in the rear-wheel steering are therefore preferably not interrupted in the range of higher slip angles.  
         [0018]     For the purpose of activating/deactivating braking interventions, for example, a clearing signal (CRS) may be generated, using which stability interventions in the brake system are permitted and/or suppressed. The clearing signal is preferably a function of the slip angle having maximum adhesion at a predefined road surface coefficient of friction. (The road surface coefficient of friction is typically estimated by the control algorithm.) The slip angles having maximum adhesion at different coefficients of friction are preferably mathematically approximated using a characteristic curve.  
         [0000]     3. Calculating the Superimposed Steering Angle from the Regulator Output Variable  
         [0019]     The expanded vehicle dynamics control system (VDM) preferably includes a computer unit, using which the corresponding manipulated variable (superimposed steering angle) is calculated from the component of the moment relative to the center of gravity which is to be implemented by the rear-wheel steering. In order to ensure that this manipulated variable does not assume too high or incorrect values in any case, thus endangering the driving safety, one or more of the following processing steps may be executed:  
         [0020]     The raw value of the superimposed steering angle obtained from the conversion of torque into superimposed steering angle is preferably scaled and limited as a function of the estimated coefficient of friction. For this purpose, a device is preferably provided which defines a dead zone, i.e., sets the superimposed steering angle to zero for small steering angle changes and reduces the superimposed steering angle by a predefined value in the remaining range.  
         [0021]     The size of the dead zone is preferably also a function of the (estimated) road surface coefficient of friction.  
         [0022]     Through the cited measures, the robustness of the electronic stability program and the steerability of the vehicle may be improved in particular.  
         [0000]     4. Special Embodiment of the Yaw Rate Regulator  
         [0023]     The yaw rate regulator of the expanded VDM system is preferably implemented as a PID regulator. The stabilization behavior may thus be significantly improved in relation to a typical P regulator.  
         [0024]     The regulating behavior of the I and D components of the yaw rate regulator also brings a certain set of problems with it, which are based in particular on the fact that the output signal of the I component must be set back to zero as rapidly as possible after an adjustment and the D component is relatively noise-sensitive. In order to prevent too strong a stabilization intervention by the I and D components of the PID regulator, the influence of the I and D regulator is preferably reduced as a function of the coefficient of friction. Disproportionately strong control interventions may thus be avoided in the event of road surfaces having low coefficients of friction in particular.  
         [0025]     In addition, a device is preferably provided which permits relatively high manipulated variables of the I and D regulator at the beginning of a stability regulation and reduces the components of the I and D regulator as a function of the coefficient of friction after a first steering intervention. This has the advantage that, in particular during steady-state straight travel, during which the coefficient of friction of the road surface may be estimated relatively poorly by the regulator, a relatively high regulator amplification of the I and D components is first permitted at the beginning of the stability regulation and the regulator amplification is stabilized at lower regulator amplifications as a function of the coefficient of friction in the course of the stability regulation.  
         [0026]     According to a preferred embodiment of the present invention, the regulating behavior of the PID regulator is therefore designed as a function of the coefficient of friction. In addition to the regulator amplification, other regulator parameters may also be a function of the coefficient of friction. 
     
    
     SUMMARY OF THE INVENTION  
       [0027]      FIG. 1  shows a schematic block diagram of an expanded vehicle dynamics control system VDM having an active rear-wheel steering system RWS.  
         [0028]      FIG. 2  shows a more precise illustration of different regulator components of the expanded vehicle dynamics controller.  
         [0029]      FIG. 3  shows the signal flow between the control unit of the expanded vehicle dynamics control system VDM and the control unit of the rear-wheel steering system RWS.  
         [0030]      FIG. 4  shows a yaw rate regulator and a float angle regulator of the electronic stability program.  
         [0031]      FIG. 5  shows a block diagram for calculating dead zones for the yaw rate and float angle regulation.  
         [0032]      FIG. 6  shows the correction of the system deviations as a function of the dead zone.  
         [0033]      FIGS. 7   a ,  7   b  shows the tire identifiers of a tire in the longitudinal and transverse directions of the tire for different road surfaces.  
         [0034]      FIG. 8  shows a characteristics map of the slip angle at maximum adhesion for different road surface coefficients of friction.  
         [0035]      FIG. 9  shows the generation of a clearing signal for braking interventions.  
         [0036]      FIG. 10  shows the calculation of the superimposed steering angle for the rear-wheel steering.  
         [0037]      FIG. 11  shows the coarse structure of the electronic stability program.  
         [0038]      FIG. 12  shows a more precise illustration of the electronic stability program of  FIG. 11 .  
         [0039]      FIG. 13  shows the structure of an I and D regulator of the yaw rate regulator.  
         [0040]      FIG. 14  shows the calculation of a reduction factor for the amplification of the I and D component of the yaw rate regulator as a function of the coefficient of friction. 
     
    
     DETAILED DESCRIPTION  
       [0041]      FIG. 1  shows the regulator structure of an expanded vehicle dynamics control system VDM, which, for stability purposes, is capable of activating a steering control element of an active rear-wheel steering system  8   a  (RWS) in addition to the brake system and the engine controller (combined in block  8   b ). The VDM system includes a control algorithm which is shown schematically by blocks  3 - 6 . In this case, reference numeral  3  identifies an “observer,” reference numeral  4  identifies a unit for setpoint value calculation, in which a setpoint yaw rate is determined in particular, and reference numeral  5  identifies a state regulator, whose regulator output variable ΔM z  is a yaw moment or a proportional variable.  
         [0042]     Furthermore, the control algorithm includes a distributor unit  6 , which converts regulator output variable ΔM z  into the components ΔLwHA, p WheelSet  for individual subsystems  8   a  (rear-wheel steering system) and  8   b  (hydraulic unit and motronics), ΔLwHA being a superimposed steering angle (in the form of a steering angle change) for the rear-wheel steering and p WheelSet  being a brake pressure for hydraulic system  15 ,  18 .  
         [0043]     Individual actuating requests ΔLwHA, p WheelSet  are transmitted via interfaces  7   a ,  7   b  to control unit  1  of rear-wheel steering system  8   a  and electronic system  15  ( FIG. 2 ) of active brake system  8   b . Circuits  1 ,  15  then activate final control element  18 ,  20  ( FIG. 2 ) accordingly, reference numeral  18  identifying a disk brake and reference numeral  20  identifying the steering control element. The new, altered actual status of vehicle  10  is recorded using sensor system  11  and supplied to control algorithm  3 - 6 .  
         [0044]      FIG. 2  shows a more detailed view of expanded vehicle dynamics control system VDM from  FIG. 1 . The entire system includes vehicle  10  as the controlled system, sensors  11  for determining the regulator input variables, final control elements  18 - 20  for influencing the drivability, and a hierarchically structured regulator  29  (having components  3 - 6 ,  9 ,  13 ), including a higher-order vehicle dynamics controller  5  (state regulator) and a lower-order braking and drive slip regulator  13 . The regulator functions are implemented in control unit  2  of vehicle dynamics control system VDM.  
         [0045]     The structure and function of such a vehicle dynamics controller are sufficiently known from the related art (e.g., Bosch, Kraftfahrtechnisches Handbuch [Automotive Handbook], 23rd edition), so that in the following only the functions and, in particular, the differences from known regulators will be discussed: the actual values of the regulated status variables (yaw rate, float angle) are determined in “observer”  3 . The setpoint values of the status variables are calculated in unit  4  for setpoint value calculation.  
         [0046]     Higher-order state regulator  5  executes a yaw rate and float angle regulation in a known way and generates a regulator output variable ΔM z  in the form of a yaw moment or a variable proportional thereto. A part of regulator output variable ΔM z  is converted into a setpoint slip lambda So , which is supplied to lower-order braking and drive slip regulator  13 . Setpoint slip lambda So  calculated for the individual wheels is converted into corresponding manipulated variables p WheelSet , M SoEng  for brake system  15 ,  18  and engine controller  16 ,  19 , which set the required braking and/or driving forces at the individual wheels.  
         [0047]     Furthermore, distributor unit  6  generates a partial center of gravity moment ΔM zx , which is to be implemented by rear-wheel steering  17 ,  20 . This center of gravity moment ΔM zx  is then converted by a computer unit  14  into a superimposed steering angle ΔLwHA. Superimposed steering angle ΔLwHA is finally added at steering control element  20  to the current rear-wheel steering angle.  
         [0048]     The weighting of individual control components p WheelSet , M SoEng , ΔM zx  calculated from regulator output variable ΔM z  of state regulator  5  may basically be selected arbitrarily, depending on how strong the desired intervention of individual subsystems  8   a ,  8   b  is. Preferably, their distribution is a function of the coefficient of friction or the slip angle of the rear wheels, however.  
         [0049]      FIG. 3  shows the signal flow between control unit  2  of vehicle dynamics control system VDM and control unit  1  of rear-wheel steering system RWS. To calculate a setpoint yaw moment, vehicle dynamics controller  29  needs rear-wheel steering angle Lw_dr desired by the driver, which is generated by a steering function  27 . In addition, vehicle dynamics controller  29  needs actual rear-wheel steering angle LwHA in order to be able to calculate the slip angle of the rear wheels. Actual rear-wheel steering angle LwHA is typically measured.  
         [0050]     If necessary, further signals (not shown), such as an “operating status” signal or a “status” signal may also be transmitted between control units  1  and  2  for the security software of vehicle dynamics controller  29  and a clearing software, using which vehicle dynamics controller  29  may be activated and/or deactivated.  
         [0051]     RWS control unit  1  includes a control function  27 , which calculates rear-wheel steering angle Lw_dr desired by the driver as a function of set steering wheel angle LwS and wheel velocity v Wheel . As long as vehicle  10  is in a stable state, this steering angle Lw_dr is set by a steering angle regulator  28  at the rear axle. In contrast, if vehicle  10  is in an unstable situation, vehicle dynamics controller  29  generates a superimposed steering angle ΔLwHA in the form of a steering angle change which is transmitted to RWS control unit  1 , where it is linked to rear-wheel steering angle Lw_dr desired by the driver. Rear-wheel steering angle Lw So  resulting therefrom then forms the new setpoint value for steering angle regulator  28 .  
         [0052]     Cited steering angle information Lw_dr, LwHA, ΔLwHA is transmitted via a data bus, which is also referred to as a chassis CAN. VDM control unit  2  is additionally connected to a second data bus PT CAN, via which different sensor signals of ESP sensor system  11  are input in particular. The separate bus connection between both control units  1 ,  2  allows particularly rapid and reliable transmission.  
         [0053]      FIG. 4  shows a more precise illustration of vehicle dynamics controller  29 . This includes a yaw rate regulator  30  and a float angle regulator  31 . Float angle regulator  31  is designed for technical reasons as a regulator which limits slip angle alpha of the wheels on the rear axle. Limiting the slip angle on the rear axle has the same effect in vehicle dynamics as regulating the vehicle float angle (beta) or regulating the vehicle lateral velocity, so that the name “float angle regulator” is used here.  
         [0054]     The two regulators  30  and  31  receive associated system deviation evGi (yaw rate) and eBeta (slip angle) and they generate a corresponding center of gravity moment ΔM zGi  or ΔM zBeta , respectively. Regulator output variables ΔM zGi  and ΔM zBeta  are processed in block  32 , and a center of gravity moment ΔM z  is generated therefrom, which is typically a setpoint yaw moment ΔM GiSo .  
         [0055]     Finally, distributor unit  6  distributes center of gravity moment ΔM z  to the individual subsystems, specifically the brake system and the engine controller (combined in block  8   b ) and rear-wheel steering system  8   a , actuation requests being output in the form of a center of gravity moment ΔM z1  and a setpoint slip lambda So . Variables ΔM z1 , lambda So  are then converted into corresponding manipulated variables ΔLwHA, p WheelSet , M SoEng  in blocks  13  and  14 .  
         [0056]     In an expanded electronic stability program (VDM), as was described above, some special control technology characteristics arise, which will be explained in greater detail in the following:  
         [0000]     1. Adaptation of the Regulating Behavior of the Yaw Rate and Float Angle Regulators.  
         [0057]     In principle, a control intervention of yaw rate regulator  30  in the rear-wheel steering also has an influence simultaneously on float angle and/or slip angle alHA of vehicle  10 . In contrast to braking interventions, the intervention in the rear-wheel steering causes an increase in float angle beta and/or slip angle alHA while reducing yaw rate vGi. A regulating intervention in float angle regulator  31  on the rear-axis steering, in contrast, causes an increase in yaw rate vGi while reducing the float angle. The interventions of both regulators  30 ,  31  thus act in precisely opposite directions. This will become clearer from the following example:  
         [0058]     In the event of too high a yaw rate vGi of vehicle  10 , the rear wheels are influenced in the same direction as the front wheels in order to reduce yaw rate vGi. However, the float angle and slip angle alHA on the rear axle are thus increased. This means that operating point deviations occur at float angle regulator  31 . This may in turn result in float angle regulator  31  intervening in the driving operation and causing a deflection of the rear wheels in the opposite direction to reduce slip angle alHA. Regulators  30 ,  31  may thus mutually amplify one another and endanger the driving safety.  
         [0059]     To coordinate both regulators  30 ,  31 , it is suggested that the regulating behavior of yaw rate regulator  30  be set as a function of the share of slip angle regulator  31  and control request ΔLwHA for the rear-wheel steering and the regulating behavior of slip angle regulator  31  be set as a function of the share of yaw rate regulator  30 . A possibility for coordinating both regulators  30 ,  31  is illustrated in  FIGS. 5 and 6 .  
         [0060]      FIGS. 5 and 6  show a method in which system deviations evGi and eBeta of yaw rate regulator  30  and float angle regulator  31  are modified as a function of the level of the control intervention of the other regulator  31  or  30 , respectively. The sensitivity of regulators  30 ,  31  is thus influenced. (The regulator sensitivity may alternately also be adapted by changing the control thresholds at the operating point deviations.)  
         [0061]      FIG. 5  shows the calculation of dead zones ToZoGi and ToZoBeta, which are used for correcting the operating point deviations of yaw rate regulator  30  and slip angle regulator  31 , respectively. The actual correction functions are shown in  FIG. 6 .  
         [0062]     Two correction units  26   a  and  26   b  are provided for correcting system deviations evGi and eBeta, respectively; the correction units calculate a corrected system deviation evGi, eBeta from actual system deviations evGi 0 , eBeta 0 , which are then supplied to regulators  30 ,  31 . Correction units  26   a ,  26   b  define a dead zone ToZo, i.e., a range of the system deviation in which system deviation evGi, eBeta is set to zero, and a range in which the actual system deviation is reduced by a predefined absolute value. If actual system deviation evGi 0  or eBeta 0  is located within the dead zone, whose boundaries are predefined by the values ±ToZoGi and ±ToZoBeta, corrected system deviations evGi and eBeta supplied to regulators  30 ,  31  are set to zero. Outside the dead zone, actual system deviations evGi 0  and eBeta 0  are reduced by value ToZoGi and ToZoBeta, respectively. Operating point deviations of yaw rate regulator  30  and float angle regulator  31  may thus be compensated for.  
         [0063]     The calculation of dead zones ToZoGi and ToZoBeta is schematically illustrated in  FIG. 5 . The calculation includes a block  21 , in which components ΔLwHABeta, ΔLwHAvGi of both regulators  30 ,  31  of superimposed steering angle ΔLwHA are calculated. In this case, ΔLwHABeta is the share of float angle regulator  31  and ΔLwHAvGi is the share of yaw rate regulator  30  in overall superimposed steering angle ΔLwHA.  
         [0064]     The change of yaw rate ΔvGI and the change of float angle and/or slip angle ΔalHA are calculated in blocks  22  and  23 . Because of an intervention of float angle regulator  31  yaw rate deviation ΔvGI results in this case from: 
 
 ΔvGI=−ΔLwHA Beta *vGi   So /( Lw−LwHA )  (1) 
 
 and because of an intervention of yaw rate regulator  30  slip angle deviation ΔalHA results in: 
 
ΔalHA=−ΔLwHAvGi  (2) 
 
         [0065]     In this case, Lw is the front-wheel steering angle, LwHA is the rear-wheel steering angle, and vGi So  is the yaw rate without the adjustment of float angle regulator  31 .  
         [0066]     Actuators  20  of the active rear-wheel steering typically operate very rapidly; however, the operating point deviations are not established immediately. The inertia of the actuator system and the entire vehicle may be simulated by a suitably adapted low-pass filtering  24 ,  25 . Operating point deviations ΔvGI and ΔalHA are therefore each supplied to a low-pass filter  25   a ,  25   b , at whose output values ToZovGi and ToZoBeta for the above-mentioned dead zones are output. Filter time constant tau is set as a variable here as a function of the curve of operating point deviations ΔvGI, ΔalHA using units  24   a  and  24   b . In this case, different time constants tau are selected in particular for signals ΔvGI and ΔalHA, which become larger and smaller.  
         [0067]     The operating point deviations from equations (1) and (2) may, for example, be added directly to the corresponding setpoint values. Preferably, however, the absolute value of each of operating point deviations ΔvGI and ΔalHA is determined and the value for a dead zone ToZovGi and ToZoBeta, respectively, is calculated therefrom. In this case: 
 
 ΔvGI=|LwHA Beta *vGiso /( Lw−LwHA )|= ToZovGi   (3) 
 
and 
 
Δ alHA=|ΔLwHAvGi|=ToZo Beta  (4) 
 
         [0068]     In this case, vGi So  is the setpoint yaw rate, Lw is the front axle steering angle, and LwHA is the rear axle steering angle.  
         [0069]     Equations (1)-(4) may be derived from the known linear single track model. Accordingly, the following equation applies for setpoint yaw rate vGi So : 
 
υ Gi   So =(( Lw−LwHA )* v )/(1*(1+(υ/υ ch ) 2 ))  (5) 
 
 with υ being vehicle velocity, 1 being wheelbase, and υ ch  being characteristic velocity. 
 
         [0070]     Through differentiation, the following equation results from equation (5): 
 
 ΔυGi   So =(−Δ LwHA*v )/(1*(1+(υ/υ ch ) 2 ))  (6) 
 
 with 
 
 υGi being yaw rate change and ΔLwHA being change of the rear-wheel steering angle. 
 
         [0071]     After rearranging and equating the equations (5) and (6), the change of yaw rate ΔvGi So  as a function of a steering angle change at the rear axle results: 
 
 ΔvGi   So   =−ΔLwHA*vGi   So /( Lw−LwHA )  (7) 
 
         [0072]     The equations of the linear single track model also provide a statement about the slip angle at the rear axle, in which the following applies: 
 
 alHA=−LwHA+ Beta− vGi   Actual *1 HA/v   (8) 
 
 where 
    Beta is the float angle of the vehicle center of gravity     vGi Actual  is the measured yaw rate and     1HA is the distance of the rear axle relative to the center of gravity.    
 
         [0076]     After differentiation, the following equation results for the slip angle change, i.e., float angle change ΔalHA as a result of a steering angle change ΔLwHA at the rear axle: 
 
ΔalHA=−ΔLwHA  (9) 
 
 2. Activation/Deactivation of the Yaw Rate Regulator in Specific Ranges as a Function of the Slip Angle 
 
         [0077]      FIGS. 7-9  show the generation of a clearing signal CRS, using which a control intervention of brake hydraulics  15 ,  18  may be permitted or suppressed. Clearing signal CRS is generated in such a way that in the range of higher slip angles alHA, stabilization interventions are permitted both using the active rear-wheel steering and also using engine controller  16 ,  19  or brake system  15 ,  18 . In contrast, in the range of smaller slip angles alHA, stabilization interventions by brake system  15 ,  18  and/or engine controller  16 ,  19  are suppressed, and only rear-wheel steering system  17 ,  20  is used for vehicle stabilization. Alternately, ESP stability interventions in this range may also only be strongly reduced.  
         [0078]      FIG. 7   a  shows the tire identifier (μ/slip characteristic curve) in the longitudinal direction of the tire for different road surfaces. In this case, reference numeral  61  identifies the coefficient of friction curve for a dry road surface, reference numeral  62  for a wet road surface, reference numeral  63  for snow, and reference numeral  64  for ice.  
         [0079]      FIG. 7   b  shows the tire identifier (μ/slip angle characteristic curve) in the transverse direction of the tire for different road surfaces. In this case, reference numeral  65  identifies a dry road surface, reference numeral  66  identifies snow, and reference numeral  67  identifies ice.  
         [0080]     Characteristic curves  65 - 67  are continuous, having a positive gradient starting from the origin until a maximum coefficient of friction is reached and then are essentially flat or have a negative gradient. Those slip angles alpha at which the tires have a maximum adhesion in the transverse direction are referred to in this case as alHAmax.  
         [0081]     For stability regulation using rear-wheel steering, this characteristic curve means that at small slip angles (alpha&lt;alHAmax) the lateral forces may be expediently modulated, while at large slip angles (alpha&gt;alHAmax) hardly any change or no change of the tire transverse forces may be achieved through a steering angle change ΔLwHA, since the gradient of characteristic curves  65 - 67  is nearly zero in this range. In the range of larger slip angles, it is therefore necessary to permit stronger ESP stability interventions in brake system  15 ,  18  and in engine controller  16 ,  19 . The steering intervention in rear-wheel steering  17 ,  20  is expressly not suppressed in this case, since an interruption of the active rear-wheel steering would result in an altered drivability of the vehicle and would irritate the driver.  
         [0082]     The individual steps of the calculation of clearing signal CRS are illustrated in  FIGS. 8 and 9 . In this case,  FIG. 8  shows a characteristic curve  68 , which approximates the curve of slip angle alHAmax having maximum adhesion at different road surface coefficients of friction μ.  
         [0083]      FIG. 9  shows the actual functions for generating clearing signal CRS. In this case, block  33  determines slip angle alHAmax having maximum adhesion at a predefined coefficient of friction μ according to the characteristic curve of  FIG. 8 . Coefficient of friction μ is typically an estimated variable of vehicle dynamics controller  29 , which is determined from the center of gravity accelerations of the vehicle in the longitudinal and transverse directions.  
         [0084]     With operational rear-wheel steering, whose status signal Stat is taken into account at node  34 , function block  35  generates clearing signal CRS through simple threshold value comparison. If current slip angle alHA is greater than threshold value alHAmax, signal CRS is set to “true” and therefore ESP interventions are permitted. Otherwise, signal CRS is set to “false” and ESP interventions are thus suppressed. Signal CRS is a Boolean signal.  
         [0000]     3. Calculating the Superimposed Steering Angle from the Regulator Output Variable  
         [0085]      FIG. 10  shows the calculation of superimposed steering angle ΔLwHA from regulator output variable ΔM z  of state regulator  5 . The algorithm shown is particularly robust for regulation. In addition, activation signals ΔLwHA for the rear-wheel steering system which are understood as plausible by the driver are generated.  
         [0086]     The algorithm includes a low-pass filter  36 , which is implemented here as a Pt1 filter and generates a filtered moment signal ΔM zF . The low-pass filtering of the center of gravity moment change ΔM z  is shown using a constant filter time constant, but may also be optionally implemented as a function of the coefficient of friction. Signal ΔM zF  is converted into a raw value ΔLwHA 0  for steering angle change ΔLwHA using function  37 . Raw value ΔLwHA 0  of the superimposed steering angle is then scaled as a function of the coefficient of friction using a function  38 , smaller values ΔLwHA Sc  basically being generated in the event of larger coefficients of friction μ at node  42 . Scaling  38  allows adaptation specific to the customer and vehicle in particular.  
         [0087]     Scaled superimposed steering angle ΔLwHA Sc  is finally reduced using a function  40 , which in turn defines a dead zone ToZo, in which superimposed steering angle ΔLwHA is set to zero. The size of dead zone ToZo is a function of the coefficient of friction, it basically being smaller at larger coefficients of friction than at smaller coefficients of friction. The functional relationship between the size of dead zone ToZo and coefficient of friction μ is predefined by a function  39 . Dead zone ToZo causes robustness of the regulation against signal noise and parameter oscillations in particular.  
         [0000]     4. Special Embodiment of the Yaw Rate Regulator  
         [0088]      FIGS. 11 through 14  show an embodiment of vehicle dynamics controller  29 , in which yaw rate regulator  30  is implemented as a PID regulator. The quality of the stabilization may thus be improved in comparison to a simple P regulator.  
         [0089]      FIG. 11  shows the coarse structure of state regulator  5 , yaw rate regulator  30  having a P component  43 , an I component  44 , and a D component  45 . Regulator components  43 - 45  each generate their own output variable in the form of a moment ΔM zP , ΔM zI , ΔM zD  relative to the center of gravity from system deviation evGi between actual and setpoint yaw rates dPsi/dt.  
         [0090]     Float angle regulator  31  is implemented as P regulator  47  and generates a moment change ΔM zBeta  from system deviation eBeta between actual and setpoint slip angles alHA. The regulator components of regulator  30  and  31  are processed in block  46  and a resulting moment ΔM z  relative to the center of gravity is calculated. This moment ΔM z  is then in turn distributed to the individual subsystems.  
         [0091]      FIG. 12  shows a possible embodiment of a yaw rate regulator  30 , in which system deviations evGi and eBeta are first multiplied by amplification factors pvGi  55  and pBeta  48 , respectively, of purely proportional regulator components  43 ,  47  (nodes  50 ,  53 ). The amplified system deviation is then multiplied by reduction factors RedBeta  49  and RedvGi  56 , respectively, in favor of the regulator components of I and D regulators  44 ,  45 . Instead of multiplication by factors pBeta, pvGi and the subsequent reduction by reduction factors  49 ,  46 , multiplication by one single amplification factor may also be performed. However, the implementation shown allows a standard layout of PID regulator  30  and an application-specific reduction on the basis of reduction factors  49  and  56 .  
         [0092]     P components ΔM zP , eBeta, D component ΔM zD , and I component ΔM zI  are added at node  57 . A linkage of components ΔM zP , ΔM zD , and ΔM zI  as a function of the driving situation or a calculation of amplification factors  48 ,  49 ,  55 ,  56  as a function of the driving situation may also be provided, for example. In this case, the cited variables may be linked as a function of the coefficient of friction, the vehicle velocity, or other status variables, for example.  
         [0093]     The addition at node  57  results in a raw value ΔM z0 , which is limited at node  58  as a function of vehicle velocity vFz. In this way, in particular at low vehicle velocities, manipulated variable ΔM z  may be reduced and therefore smaller control interventions may occur. A corresponding reduction function is shown in block  52 . The resulting signal is also limited by a limiting function  59  for reasons of safety. In this way, it may be ensured that impermissibly large control requests may be suppressed. The overall regulator may also be deactivated and/or cleared at node  60  using a signal F. Finally, variable ΔM z  may be provided for the further computing sequence within vehicle dynamics controller  29  and, as described above, manipulated variables ΔLwHA, p WheelSet , and M SoEng  may be derived for the different subsystems.  
         [0094]      FIGS. 13 and 14  show the generation of I and D components ΔM zI , ΔM zD  of regulator output variable ΔM z . I and D components ΔM zI , ΔM zD  are a function of the coefficient of friction in particular in this case. Time constant T HP  of I regulator  44  is also a function of coefficient of friction μ.  
         [0095]      FIG. 13  shows an algorithm  70  on the top left, which, as a function of clearing signal F, lateral acceleration ay, and taking into consideration a parameter P ayI , generates a resulting clearing signal RIC. This signal RIC determines overall whether a regulator component ΔM zI  of I regulator  44  and a regulator component ΔM zD  of D regulator  45  is generated or not. Signal RIC is then supplied to an algorithm  71  and an algorithm  74 .  
         [0096]     Algorithm  71  is used to reduce system deviation evGi and may, for example, include a function having a dead zone ToZo. The size of the dead zone is in turn predefined by a parameter P ToZo    75 . The resulting value of system deviation evGi is then linked at node  76  to a reduction factor Red ID  and a signal evGi′ is generated. Signal RIC also has an influence on the size of dead zone ToZo in algorithm  71 . In addition, the size of dead zone ToZo may also be determined as a function of the driving condition or other influencing variables.  
         [0097]     Subsequently, undesired signal components are filtered out of signal evGi′ using filters  77  and  79 . The resulting signals are then limited to maximum values by functions  78  and  80 . This is again performed for reasons of safety.  
         [0098]     The actual regulator functions of I regulator  44  and D regulator  45  are illustrated in blocks  81  and  82 . The D regulating algorithm is implemented here as a second-order low-pass filter. Regulator parameters natural frequency omega 0  and damping d are supplied by block  83  and  84 .  
         [0099]     I regulator  81  is implemented here as a first-order high-pass filter. Time constant T HP  is variable and is predefined by block  74  as a function of signal evGi′, which is in turn a function of the coefficient of friction. Time constant T HP  is essentially determined as follows: a base signal for the selection of high-pass filter time constant T HP  is generated from signal evGi′ using absolute value calculation  73  and differentiation  72 . The selection algorithm is shown as block  74 . It is first queried therein whether the status of the RIC signal is active. If not, a higher value T HP2  is selected for time constant T HP  of high-pass filter  81 . If clearing signal RIC is low and/or inactive, it is checked whether signal evGi′ has a positive or negative gradient. In the case of a positive gradient, very small value T HP0  is selected for time constant T HP , and in the case of a negative gradient, a larger value T HP1  is selected for time constant T HP , with T HP2 &gt;T HP1 &gt;T HP0 .  
         [0100]     Filter functions  77 ,  79  upstream from I and D regulators may, for example, have constant parameters. Alternately, it is also possible to set one or more of the filter parameters as a function of the driving situation, in particular vehicle velocity vFz, lateral acceleration ay, or another driving condition. Resulting signal components ΔM zI  and ΔM zD  are then supplied to vehicle dynamics controller  29  for further processing.  
         [0101]      FIG. 14  shows the determination of reduction factor Red ID , by which system deviation evGi is multiplied. The function of reduction factor Red ID  is to prevent excessively high integral regulator components ΔM zI  in particular. The algorithm for calculating reduction factor Red ID  essentially has two branches. The upper branch includes a block  87  having a function, using which a reduction factor Red ID1  is calculated as a function of estimated coefficient of friction μ. Coefficient of friction μ may be estimated by vehicle dynamics controller  29  from the lateral and longitudinal accelerations of vehicle  10 , for example.  
         [0102]     Since the coefficient of friction is determined from the vehicle acceleration, the signal value in the event of steady-state straight-line travel is approximately zero. Only in the event of stronger longitudinal or lateral acceleration does coefficient of friction μ assume the actual value near 1. This behavior is rather unfavorable for determining a suitable regulator amplification.  
         [0103]     The second branch includes a function  85 , using which a setpoint yaw rate vGi So  is calculated according to the linear single track model. In this model, vehicle velocity vFz and front axle steering angle lw enter as input variables. The signal of the setpoint yaw rate is then limited in block  86  as a function of the coefficient of friction via complex filtering algorithms and an output signal LimvGi is generated. The input and output signals of filter algorithm  86  are subjected in block  89  to an expanded quotient calculation, which prevents division by zero and keeps the value range from being exceeded. In case of travel on a road surface having a high coefficient of friction, the quotient from block  89  results in values near one. In block  90 , these values are weighted to make calibration possible and reduction factor Red ID2  is generated. Finally, in block  88 , the maximum value is selected from both reduction factors Red ID1  and Red ID2  and output as a value Red ID .  
         [0104]     This method has the advantage in particular that at the beginning of a stability regulation, in particular starting from steady-state straight-line travel, value Red ID  is not too low and is stabilized at an exact function of the coefficient of friction in the course of the stability regulation. The extent of the amplification reduction is settable during the calibration of the control algorithms in this case.  
       List Of Reference Numerals  
       [0000]    
       
           1  RWS control unit  
           2  ESP control unit  
           3  observer  
           4  unit for setpoint value calculation  
           5  state regulator  
           6  distributor unit  
           7  interfaces  
           8  RWS algorithm  
           9  unit for setpoint slip calculation  
           10  vehicle  
           11  sensor system  
           13  braking and drive slip regulator  
           14  unit for superimposed steering angle calculation  
           15  electronics of the brake system  
           16  motronics  
           17  RWS electronics  
           18  disk brake  
           19  final control elements of the engine controller  
           20  steering control element  
           21  determination of the steering angle components  
           22  calculation of the operating point deviation for the yaw rate  
           23  calculation of the operating point deviation for the slip angle  
           24  determination of the filter time constant  
           25  low-pass filter  
           26   a  correction of the system deviation of the yaw rate  
           26   b  correction of the system deviation of the float angle  
           27  rear-wheel steering function  
           28  rear-wheel steering angle regulator  
           29  vehicle dynamics controller  
           30  yaw rate regulator  
           31  float angle regulator  
           32  coordination of the regulator components  
           33  determination of the slip angle at maximum adhesion  
           34  multiplication node  
           35  generation of clearing signal CRS  
           36  Pt1 filter  
           37  conversion function for the superimposed steering angle  
           38  scaling function  
           39  characteristic curve for the dead zone  
           40  limiting function  
           41  multiplication node  
           42  multiplication node  
           43  P regulator  
           44  I regulator  
           45  D regulator  
           46  coordination of the regulator components  
           47  P regulator  
           48  amplification factor of the float angle regulator  
           49  reduction factor of the float angle regulator  
           50  multiplication node  
           51  multiplication node  
           52  limitation as a function of vehicle velocity  
           53  multiplication node  
           54  multiplication node  
           55  amplification factor of P regulator  43   
           56  reduction factor of the P regulator  
           57  addition node  
           58  multiplication node  
           59  limiting function  
           60  multiplication node  
           61 - 64  tire identifiers in longitudinal direction  
           65 - 67  tire identifiers in transverse direction  
           68  curve of the slip angle at maximum adhesion  
           69  parameter  
           70  generation of clearing signal RIC  
           71  limiting function  
           72  differentiation  
           73  absolute value calculation  
           74  algorithm for selecting time constant T HP    
           75  dead zone  
           76  multiplication node  
           77  integrator  
           78  limiting of the I component  
           79  filter  
           80  limiting of the D component  
           81  I regulator function  
           82  D regulator function  
           83  natural frequency  
           84  damping  
           85  linear single track model  
           86  filtering  
           87  determination of a reduction factor  
           88  selection of the maximum value  
           89  quotient calculation  
           90  determination of reduction factor RED ID2    
          ay lateral acceleration  
          evGi system deviation of the yaw rate  
          RED ID  reduction factor  
          evGi′ signal value of the system deviation of the yaw rate  
          ΔM zI  I component  
          ΔM zD  D component  
          ΔM zP  p component  
          ΔM z  regulator output variable  
          RED ID1  reduction factor  
          RED ID2  reduction factor  
          μ coefficient of friction  
          Lw front axle steering angle  
          eBeta system deviation of the float angle  
          CRS clearing signal  
          F clearing signal  
          ToZo dead zone  
          ΔLwHA superimposed steering angle  
          ΔLwHA 0  raw value of the superimposed steering angle  
          ΔLwHA Sc  scaled superimposed steering angle  
          alHA slip angle rear axle  
          alHA max  slip angle at maximum adhesion  
          P parameter  
          Stat status signal  
          ToZoGi dead zone of the yaw rate regulator  
          ToZoBeta dead zone of the float angle regulator  
          ΔvGI operating point deviation of the yaw rate regulator  
          ΔalHA operating point deviation of the float angle regulator  
          tau time constant  
          evGi 0  raw value of the system deviation  
          eBeta 0  raw value of the system deviation  
          p WheelSet  manipulated variable  
          m SoEng  manipulated variable  
          Lw_dr desired rear-wheel steering angle  
          LwHA measured rear-wheel steering angle
A device is described for stabilizing a vehicle in critical driving situations, including a vehicle dynamics control system having a control unit, in which a vehicle dynamics controller is stored, at least one final control element, and a sensor system for measuring different driving condition variables, and including a rear-wheel steering system having a control unit and a final control element. The electronic stability program may be integrated into a control unit if the electronic stability program algorithm includes a distributor unit which, from a regulator output variable, generates both a setpoint requirement for the final control element of the vehicle dynamics control system and also a setpoint requirement for the final control element of the rear-wheel steering system.