Abstract:
In a method for controlling a motor vehicle via driving dynamics, using an auxiliary steering system, including a power steering assistance unit, a superposed transmission, and a final control element to correct a driver-steering angle by applying an auxiliary steering angle, an overall steering angle is formed to modify the wheel-steering angle of steered wheels with the aid of the superposed transmission, and a control and regulation unit assigned to the final control element determines a setpoint for the auxiliary steering angle. When an understeering state is detected, the setpoint of the auxiliary steering angle is modified such that the lateral wheel force is kept within a range of a maximally achievable maximum value for the lateral wheel force, which is dependent upon environmental influences, for the duration of the understeering state.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to Application No. 10 2007 000 995.1, filed in the Federal Republic of Germany on Nov. 28, 2007, which is expressly incorporated herein in its entirety by reference thereto. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for operating a superposed steering system in a motor vehicle. 
       BACKGROUND INFORMATION 
       [0003]    German Published Patent Application No. 197 51 125 describes a method for operating a steering system for a motor vehicle, which superimposes the steering motion initiated by the driver of the vehicle and the motion initiated by the final control element with the aid of a final control element and an auxiliary actuator, and a control signal, which is formed by superimposing at least two parallel and independent steering components, is generated for the final control element. 
       SUMMARY 
       [0004]    Example embodiments of the present invention provide a method for operating a superposed steering system in order to increase the driving safety during cornering. 
         [0005]    According to example embodiments of the present invention, when detecting an understeering state of the vehicle, the setpoint of the additional steering angle is modified with the aid of a control and regulation device, such that the lateral wheel force F y  is kept within a range of a maximum value for the lateral wheel force, which is assumed to be maximally achievable and affected by environmental influences (coefficient of friction, wheel parameters), for the duration of the detected understeering state. 
         [0006]    Further features and aspects of example embodiments of the present invention are described in more detail below with reference to the appended Figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  schematically illustrates the superposed steering system, located in a steering train, of a motor vehicle, to which the method according to example embodiments of the present invention are applicable. 
           [0008]      FIGS. 2   a  and  2   b  show the correlation between the slip angle and lateral guiding force or wheel return torque. 
           [0009]      FIG. 3  shows a configuration for determining the degree of the instantaneous understeering state. 
           [0010]      FIG. 4  shows an example embodiment of the present invention, which uses a differential value between the setpoint and the instantaneous yaw rate. 
           [0011]      FIG. 5  shows an example embodiment of the present invention, which uses the instantaneous transverse acceleration and an estimated rack force. 
           [0012]      FIG. 6  shows an implementation variant of the method according to example embodiments of the present invention, which uses wheel speeds and a virtual wheel-steering angle. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  schematically illustrates an auxiliary steering system of the type mentioned in the introduction, which includes a final control element  1 , which applies an auxiliary steering angle δ z  as specified by setpoint δ z, soll  into the steering train of the steering system with the aid of superimposed transmission  2 , and an overall steering angle δ G  is formed on the output side and conveyed to the electrically or hydraulically assisted steering gear  4  on the input side. Using a rack and tie rods, the overall steering angle is transmitted to steered wheels  5 , and a wheel steering angle δ R  is generated. A control and regulation unit  6  receives steering angle δ S  applied by the driver, and instantaneous driving speed v x  of the vehicle as input variables. A VSR (variable steering ratio) functionality implemented in control and regulation unit  6  uses the input variables to calculate a setpoint for final control element  1 . 
         [0014]    If a motor vehicle is cornering, a slip angle α v  is generated at the wheels—which have been abstracted to one wheel—of the steered front axle, and a corresponding slip angle α h  is generated at the rear axle. 
         [0015]    An understeering behavior during cornering is defined as α v −α h &gt;0, an oversteering behavior is defined as α v −α h &lt;0. During cornering, a motor vehicle generally tends to exhibit understeering behavior.  FIG. 1  shows slip angle α of the front axle in abstracted form at one steered wheel of the axle. Slip angle α is formed between speed vector v of the wheel and wheel steering angle δ R  when the vehicle exhibits understeering behavior. 
         [0016]    In motor vehicles equipped with a superposed steering system as described, for example, in German Published Patent Application No. 197 51 125, it is possible to implement autonomous dynamic-performance-related steering interventions for the purpose of restoring the vehicle&#39;s controllability. In this context, reference is also made to the pertinent publications by Anton van Zanten in connection with a vehicle dynamics control. 
         [0017]    According to example embodiments of the present invention, the understeering behavior of a heavily understeering vehicle is reduced with the aid of a superposed steering system. 
         [0018]    According to example embodiments of the present invention, in this state, the setpoint for the auxiliary steering angle is modified such that overall steering angle δ G  and, correspondingly, wheel steering angle δ R  is reduced according to the relation δ S +δ Z  and returned to, and kept within, a range of the maximum lateral guidance force F y,max  of the wheel. 
         [0019]    Thus, with the aid of the superposed steering system, an optimum wheel steering angle δ R  at which a maximally achievable lateral force is acting on the wheel is set, so that a maximally possible transverse acceleration of the vehicle is achieved. 
         [0020]    It is therefore provided to detect the maximum value of the lateral guide force with the aid of the estimated rack force. 
         [0021]    Wheel steering angle δ R  is produced by the additive superpositioning of a driver-steering angle δ S  applied by the driver, and an auxiliary steering angle δ Z  applied by the final control element, which results in an overall steering angle δ G  according to the relation δ S +δ Z . Overall steering angle δ G  is transmitted to the steered wheels with the aid of the steering gear and the tie rods and thus substantially corresponds to wheel steering angle δ R  of the wheels—abstracted to one wheel—of the steered front axle. 
         [0022]    When analyzing the correlation between lateral wheel force F y  and wheel steering angle δ R  or a slip angle α resulting therefrom, as shown in  FIG. 2   a,  then it becomes clear that, starting at a certain value, it is no longer possible to generate an additional lateral guide force. 
         [0023]    As wheel steering angle δ R  continues to increase, the lateral guide force decreases. 
         [0024]    This transition is denoted by point P in  FIG. 2   a.  To the right of this point, the vehicle is in an understeering state (shaded area). According to example embodiments of the present invention, the state in which a further increase of wheel steering angle δ R , i.e., a further increase in the wheel angle, no longer results in a further increase in the lateral wheel force, is detected. 
         [0025]      FIG. 2   b  illustrates the associated wheel return torque M R  of the wheel, or rack force F Z  acting on the rack according to the lateral force. With respect to slip angle α, maximum P for rack force F Z  or wheel return torque M R  manifests itself more clearly and earlier as a result of the wheel properties. Accordingly, point P of maximum lateral guide force F y,max  is in a range in which the rack force is decreasing again once the maximum denoted by point P′ has been exceeded. This recognition is quite helpful for the reliable detection of an understeering state. 
         [0026]    Since the maximum lateral guide force decreases as the coefficient of friction drops and accordingly, the wheel load differential as well, the tie-rod forces that are obtained are also lower because of the wheel properties. This results in threshold values as a function of the transverse acceleration. The instantaneous tie-rod force may be determined with the aid of an estimating algorithm, as described in German Published Patent Application No. 10 2006 036 751, which is expressly incorporated herein in its entirety by reference thereto. 
         [0027]    The understeering state may be identified by evaluating a previously determined understeering factor USF, as shown schematically in  FIG. 3 . 
         [0028]    Setpoint yaw rate Ψ soll , instantaneous yaw rate Ψ ist  and transverse acceleration a y  are forwarded to an arithmetic-logical functional unit  301 . 
         [0029]    These variables are offset internally and plausibilized with respect to each other, in order to determine a value that specifies the degree of understeering, USF %, therefrom. A subsequent evaluation and decision unit utilizes this as well as additional variables for a binary decision as to whether an understeering state is present. 
         [0030]      FIG. 4  shows an alternative method as a further exemplary embodiment. Steering angle δ S  applied by the driver, and vehicle velocity v x  are forwarded to a vehicle reference model  101 . From these, a setpoint yaw rate Ψ soll  is determined and compared to measured instantaneous yaw rate Ψ ist . 
         [0031]    A differential element  102  arithmetically determines a yaw-rate deviation value ΔΨ, and wheel-steering angle δ R  to be adjusted by the appropriate setting of the setpoint for the auxiliary steering angle δ Z , using an amplification element  102 , is specified accordingly. 
         [0032]    Functional block  301  may be stored as computer-implemented method in control and regulation unit  6 . 
         [0033]      FIG. 5  shows a further method according to an example embodiment of the present invention. Instantaneous transverse acceleration a y  is forwarded to functional block  501 , which converts lateral guide force F y  into a tie-rod force F S  based on vehicle-specific variables such as the center of gravity of the vehicle and the geometric axle and steering conditions. 
         [0034]    With the aid of internally known variables of the power steering system, in particular using information related to angle and torque, functional block  502 , which includes an estimation algorithm for determining tie-rod force F S  or rack force F Z , determines a rack force F Z  or tie-rod force F S  assumed to be real, which is acting on the rack. 
         [0035]    The output variables of both functional blocks  501 ,  502  are forwarded to a comparison device, the estimated tie-rod force determined with the aid of functional block  502  serving as actual value, and the tie-rod force coming from functional block  501  serving as setpoint. 
         [0036]    A subsequent regulation stage  504  determines a setpoint for auxiliary steering angle δ Z, soll  to be set, with mandatory consideration of the instantaneous driving state determined in functional block  503 , i.e., in the presence of a state evaluated as understeering state. Functional block  503  is used to determine the degree of understeering and operates according to the method described in connection with  FIG. 3 . 
         [0037]    An example embodiment of the present invention is shown in  FIG. 6 . 
         [0038]    The wheel speeds of the steered wheels of the front axle, RDZ vl , RDZ vr , are detected and transmitted to a functional block  601  for the calculation of a virtual wheel-steering angle δ R ′. For one, in wide ranges, virtual wheel-steering angle δ R ′ is practically identical to actually applied wheel-steering angle δ R , and for another, it also indicates the qualitative characteristic of transverse acceleration a y . 
         [0039]    It also is constant once the maximum transverse acceleration has been reached. 
         [0040]    Wheel-steering angle difference Δδ R  determined by comparison device  602  is forwarded to a subsequent regulation stage  603 , which determines a setpoint for auxiliary steering angle δ Z  in order to minimize an existing difference in the wheel-steering angle. Functional unit  604  is used to determine the degree of understeering and operates according to the method described in connection with  FIG. 3 . Depending on its input of understeering factor USF %, regulation stage  603  is switched into an active or inactive mode. A correction value for the auxiliary steering angle is calculated accordingly and either applied or set to zero. In this case, the information regarding the understeering state USF % need not necessarily be forwarded to regulation stage  603 . It is mainly used for a plausibility check.