Abstract:
A method of stabilizing a vehicle combination including a towing vehicle and a trailer, includes determining at least one vehicle state variable describing the state of the vehicle from a comparison with an assigned nominal value. A correcting variable is produced, which is supplied to an actuator in the vehicle. The correcting variable acts upon a steering actuator in order to adjust the wheel steering angle.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for stabilizing a vehicle combination, comprising a towing vehicle and a trailer. 
     For stabilizing truck and trailer combinations, which include a towing vehicle and a trailer hitched by a tow bar, DE 100 30 128 A1 discloses detecting differences between the course desired by the driver and the actual movement of the vehicle by means of sensing technology in the towing vehicle, and, thereupon, to braking individual wheels of the trailer axle in order to avoid dynamic driving instabilities by these means. As a result of braking the trailer, the vehicle combination extends, reducing the danger of jackknifing between the towing vehicle and the trailer. 
     SUMMARY OF THE INVENTION 
     Starting out from this state of the art, it is an object of the invention to stabilize a vehicle combination, which comprises a towing vehicle and a trailer, with simple means. This is to be accomplished basically without activating the braking system of the vehicle combination. 
     Pursuant to the invention, this objective is accomplished by an approach which includes determining an actual value of at least one vehicle state variable describing a state of the vehicle and comparing the actual value of the at least one vehicle state variable with a nominal value of the at least one vehicle state variable corresponding thereto. A correcting variable is produced based on the comparing which is supplied to a steering actuator in the vehicle, and the correcting variable is made to act upon a steering actuator for changing a relevant setting thereof based on the comparing and adjusting a wheel steering angle accordingly at least at one steerable wheel of the vehicle. 
     For the method of stabilizing the vehicle combination in accordance with the invention, at least one vehicle control variable, describing the driving state, is determined, either by measurement or by calculation, for example, with the help of an observer, and is used as a basis for a comparison with an assigned nominal value. From this comparison, a vehicle control variable is produced, which is supplied to an actuator in the vehicle for changing the current setting. Pursuant to the invention, provisions are made so that the control variable acts upon a steering actuator, as a result of which the angle of the steering wheel is changed at least at one steerable wheel of the vehicle. 
     In the case of a threatening instability of the vehicle composite or of one that has already taken place, stabilization can be carried out in this way solely by affecting the steering. Additional stabilizing measures may be provided in the vehicle. However, they are not necessarily required for avoiding rolling motion of the vehicle combination. For example, it may be appropriate to intervene in the braking process and/or in the engine control in addition to affecting a steering. In principle, however, intervening in the steering is sufficient for the stabilization. 
     Intervention in the steering is accomplished basically over the steerable wheels of the towing vehicle. Alternatively or additionally, it is, however, also possible to intervene in the steering at the trailer, provided that the latter has steerable wheels. 
     In particular, the yaw rate is called upon as vehicle control variable, which can be affected as a process variable. Additionally or alternatively, the trailer angle, which describes the angular deviation between the longitudinal axes of the towing vehicle and of the trailer, can also be taken into consideration as a process variable. In general, any vehicle state variable, from which information concerning the transverse dynamics of the vehicle can be inferred, comes into consideration as a process variable, that is, any vehicle state variables or a combination hereof related to the transverse dynamics of the vehicle. This may optionally also be the steering angle, the transverse velocity or the transverse acceleration or the angular velocity difference at the wheels of the vehicle. 
     Linear, as well as nonlinear, control formulations can be used for the control, which is required for stabilizing the vehicle combination. Since the vehicle-trailer model, which forms the basis for the control as a mathematical replacement model, is nonlinear, a linearization about a defined operating point is required for use as a linear controller. However, the use of nonlinear controls, for example, of compensation controllers (also known as feedback or feedback linearization) which is based on the fundamental principle of an inverse vehicle model, is also possible. If the quality of the model is adequate, the real behavior of the vehicle is compensated for by the model and a linear control relationship results. 
     It may also be appropriate to provide a limit before and/or after the controller, in order, on the one hand, to let differences between the nominal value and the actual value be effective only for the case in which the difference exceeds a relevant threshold, and, on the other, to provide a lid for the difference between the nominal and actual values at a maximum value, in order to avoid excessively large control interventions. For a limiter downstream from the controller, the magnitude of the control interventions is limited to a maximum value. 
     The difference between the nominal and actual values, which are to be supplied to the controller, may optionally be subjected to integration, in order to take the dynamics of the driving behavior into consideration by these means. This can be accomplished by adapting the limits in the limiter. For example, if the vehicle is unstable over a certain period of time, the limit can be reduced, so that the control algorithm is run through more frequently and engages with a larger amplitude. Conversely, the limit can be increased if the vehicle, including the trailer, runs stably for a minimum period of time. 
     Advantageously, the limit is related to the difference between the nominal value and the actual value of the vehicle control variable under consideration. In order to establish the magnitude of the limit of the vehicle control value in question, further vehicle state variables and/or parameters may flow in. For example, the attitude angle, the tire forces or the maximum friction may flow into the limit. 
     The method is realized in a steering system, which has at its disposal at least one device for manipulating the steering, usually a steering wheel, a steering linkage and a steering actuator, the steering angle, specified by the device for manipulating the steering being converted with the help of the steering linkage onto the steerable wheel of the vehicle and the steering actuator producing a supporting moment. Alternatively or additionally to a supporting moment, it is also possible, in the case of an embodiment as active steering system, to specify a superimposing steering angle, which is superimposed on the steering angle desired by the driver. Basically, the method is suitable for use in an EPS steering system (electrical power steering) with an electric motor as steering actuator. However, other steering systems, such as an electrohydraulic steering system, also come into consideration. 
     Further advantages and appropriate embodiments are given in the description of the Figures and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a diagrammatic representation of a steering system in a vehicle with a steering gear, which is preceded by a superimposing gear; 
         FIG. 2  shows a diagrammatic view of a vehicle combination, consisting of a towing vehicle and a trailer, a steering system of  FIG. 1  being realized in the towing vehicle; 
         FIG. 3  shows a block circuit diagram for implementing a method of stabilizing the vehicle combination of towing vehicle and trailer; and 
         FIG. 4  shows a diagram with a limiting function, which can be used to limit the controller input and/or the controller output. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A steering system  1  in a motor vehicle, shown in diagrammatic representation in  FIG. 1 , comprises a device for manipulating the steering, which is constructed as a steering wheel  2 , a steering shaft  3 , which is connected with the steering wheel  2 , a steering gear  6  with a steering actuator  9  and a steering linkage  7 , which is connected with the steerable front wheels  8 . The steering angle δ S , specified by the driver over the steering wheel  2 , is transferred over the steering shaft  3  and the steering gear  6  into a gear rack travel of the steering linkage  7 , as a result of which a wheel steering angle δ F  is set in the steerable front wheels  8 . Depending on the situation, a motor correcting moment can be fed over the steering gear  6  into the steering system to support the steering over the steering actuator  9 , which preferably is constructed as an electric motor. Instead of being constructed as an electric motor, a steering actuator  9  can also be constructed as an electrohydraulic control element. 
     Furthermore, the steering system  1  has a superimposing steering gear  4  with a servo motor  5 , the superimposing gear  4  being interposed in the steering shaft  3 . When the servo motor  5  is actuated, a superimposing steering angle δ M  is produced, which is superimposed on the steering angle δ S , produced by the driver, to form the resulting steering angle δ′ S . No superimposing steering angle δ M  is produced when the servo motor is not in operation. In this case, the steering angle δ S  produced by the driver is supplied directly to the steering gear  6  as an input quantity. 
     It is also possible to do without the superimposing gear  4  including the servo motor  5 . 
     The steering actuator  9  is adjusted by means of a correcting variable S, which is produced in a regulating or controlling device, through which the method for stabilizing a vehicle combination passes. The correcting variable S adjusts the steering actuator  9  and, with that, brings about the desired setting of the front wheel angle δ F . 
       FIG. 2  shows a vehicle combination  10 , which consists of a towing vehicle  11  and a trailer  12 , which is coupled pivotably over a fixed tow bar  13  to the towing vehicle  11 . The front wheels  8  of the towing vehicle  11  are constructed to be steerable. The angles Ψ 1 , Ψ 2  and γ, of which Ψ 1  and Ψ 2  represent the yaw angle of the towing vehicle  11  or of the trailer  12  and γ represents the trailer angle, which represents the angular deviation between the longitudinal axis of the trailer  12  and the longitudinal axis of the towing vehicle  11 , are entered in  FIG. 2 . 
       FIG. 3  shows a block circuit diagram for carrying out the method. The first block  20  represents the kinematics of the system. Depending on the different state variables and parameters, especially the longitudinal speed of the vehicle v x , the maximum friction μ max  between the wheels and the road, the attitude angle β and the current wheel steering angle δ F , the nominal value Ψ d  of the yaw rate is determined on the basis of the kinematic relationships in block  20 . In the subsequent block  21 , the dynamic vehicle behavior is modeled by means of filters with a frequency-dependent phase shift; the thereby obtained nominal value Ψ d  of the yaw rate reflects the dynamic vehicle behavior. 
     For determining the control error, the assigned actual value Ψ m  of the yaw rate is subtracted from the nominal value Ψ d  of the yaw rate in the subsequent block  22 . The difference ΔΨ between the yaw rates is supplied to a subsequent block  23 , as input value. In block  23 , a limiter is realized, which has the task of limiting the difference ΔΨ between the yaw rates with the help of a dead-time function in such a manner that, when the difference is below a threshold value, steering interventions are not carried out. Additionally or alternatively, the difference can also be capped at a maximum. The difference function is described in  FIG. 4  and will be explained in detail there. 
     In addition, the longitudinal speed of the vehicle v x , the maximum friction μ max , the attitude angle β, the wheel steering angle δ F , as well as the difference between the nominal value γ d  and the actual value γ m  of the trailer can flow into the limiting function in block  23  as input quantities. These state variables or parameters can be generated in a preceding block  24 , in which an observer model is realized. The quantities sought are calculated on the basis of a mathematical model in the observer as a function of measured state variables or parameters, especially of the rpm of the wheels ω U , the steering angle δ S  specified by the driver, as well as the actual value γ m  of the trailer angle. 
     The difference ΔΨ of the yaw rate, limited in block  23 , is supplied subsequently to block  25 , which represents a controller. A process variable, which is generally referred to as a state variable x and is at the output of the controller, is generated in the controller. In particular, this state variable is a quantity, which characterizes the transverse dynamics of the vehicle combination, such as the yaw acceleration. 
     In the further course, the process variable or state variable x is supplied as input quantity to the block  26 , in which a so-called inverse mathematical vehicle model is realized. Together with the controller in block  25 , a nonlinear control formulation can be carried out, in which the controller is constructed as a compensation comptroller. If the inverse vehicle model in block  26  is of sufficient quality, the real vehicle, shown in  FIG. 3  in block  29 , is compensated and a linear control relationship results. State variables and parameters, generated by the observer model from block  24  are additional input quantities in block  26 . In addition, the wheel steering angle δ F  is supplied as input quantity in a returning loop to the inverse vehicle model. 
     The inverse vehicle model supplies the superimposing steering angle δ M  as output quantity from block  26 . In a subsequent block  27 , which represents the superimposing steering gear of the superimposing steering system and is constructed, for example, as a planetary gear, this superimposing steering angle δ M  is added to the steering angle δ S , which is specified by the driver and converted in a block  28 , which represents the steering gear. If superimposing steering is not provided in the steering system, block  27  can also represent a moment superimposition of an electric power steering (EPS). 
     The steering angle δ 1  of the driver and the superimposition steering angle δ M  lead to a wheel steering angle δ F  or to a corresponding correcting variable (labeled S in  FIG. 1 ), which is supplied as input quantity to an actuator in the real vehicle, which is represented in block  29 . Thereupon, a desired actual value Ψ of the yaw rate sets in. If superimposing steering is provided, the correcting variable S is the nominal value of the superimposing steering angle δ M . 
     The yaw angle Ψ, considered in  FIG. 3 , is, in particular, the yaw angle of the towing vehicle. Optionally, however, the yaw angle of the trailer or some other state variable also comes into consideration, especially a state variable representing the transverse dynamics of the system. 
       FIG. 4  shows a limiting function, which can be used for limiting the controller input in block  23  in  FIG. 3  or the controller output in block  26  in  FIG. 3 . Input and output of the limiter in  FIG. 4  is a state variable, which is generally labeled x and modulated corresponding to the limiting function. On the one hand, the state variable x can be subjected to a dead time, in that, below a threshold value b, the output of the state variable is set at 0 or at least a reduced value. The concept of “dead time” is not time-dependent here. Instead, it is to be understood generally as a reaction, which sets in with delay. 
     On the other, the state variable can be limited to a maximum value c. The increase to the maximum value c takes place after the expiration of the dead time at point b linearly between b and a further ordinate value a. 
     The values of a, b and c may be specified as fixed values in the limiter or calculated from vehicle state variables and/or parameters of the vehicle. 
     LIST OF REFERENCE SYMBOLS 
     
         
           1  Steering system 
           2  Steering wheel 
           3  Steering shaft 
           4  Superimposing gear 
           5  Servo motor 
           6  Steering gear 
           7  Steering linkage 
           8  Front wheel 
           9  Steering actuator 
           10  Vehicle combination 
           11  Towing vehicle 
           12  Trailer 
           13  Tow bar 
         Ψ m  Actual value of yaw rate 
         Ψ d  Nominal value of yaw rate 
         ΔΨ Yaw rate difference 
         Ψ 1  Yaw angle of trailer 
         Ψ 2  Yaw angle of towing vehicle 
         γ Trailer angle 
         γ m  Actual value of trailer angle 
         γ d  Nominal value of trailer angle 
         v x  Longitudinal speed of the vehicle 
         β Attitude angle 
         μ Friction 
         μ max  Maximum friction 
         δ F  Wheel steering angle 
         δ S  Steering angle 
         δ S ′ Resultant steering angle 
         δ M  Superimposition steering angle 
         ω y  Rpm of wheels 
         S Control variable