Abstract:
Method for operating a steering system for a motor vehicle with at least one steerable wheel, a servo drive and a superimposed gear mechanism, the steering movement, initiated by the driver of the vehicle, and the movement for producing the steering movement of the steerable wheel, initiated by the servo drive for realizing useful functions (VSR, LAFN) being superimposed by the superimposed gear mechanism into a pinion angle, the servo drive for initiating the movement being triggered by a control signal (δ Md ) of a control device. The servo drive is turned back in a controlled manner when a substitute mode of at least one useful function (VSR, LAFN) of the steering system is switched off or on.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for operating a steering system and to a computer program with programming means for carrying out such a method. 
     A generic method is known from DE 197 51 125 A1. For this method, the steering movements, applied by the driver by means of a steering wheel, are superimposed on the motor angle by steering wheel angles, determined by a sensor, by means of a superimposed gear mechanism with the movements of the servo drive. The thus resulting superimposed movement is passed on over the steering gear or the steering linkage to the steerable wheels for adjusting the steering angle. For this purpose, the servo drive is designed as an electric motor. The principle of functioning or the useful applications of such a power steering system consist especially therein that the steering can be carried out indirectly by the gear ratio of the superimposed gear mechanism and, with that, slight steering wheel moments can be attained. As a result, very large steering wheel angles are avoided in that suitable motor angles are superimposed, so that the necessary output angles can be adjusted with steering wheel angles of the usual magnitude. The motor angle or its nominal value, required for steering support, is determined from the steering wheel angle. Moreover, the motor angle may also depend on the signals, which represent the vehicle movements, detected by sensors and/or other vehicle systems, such as an electronic stability program (ESP). This takes place by means of a control device, on which the programs, required for determining the necessary motor angles or for the control of the useful applications can be carried out. 
     SUMMARY OF THE INVENTION 
     Because of the safety requirements, which such a steering system must meet, a safety concept with safety and diagnosis functions is indispensable, especially if for discovering random errors in the sensor system, the control device itself or the actuator system and to react suitably, that is, for example, to switch the useful functions, especially the variable steering gear ratio, suitably and/or to start appropriate substitute modes. The input signals of the control device are tested for plausibility. For example, it would be disadvantageous to accept a distorted speed signal, since the variable steering gear ratio is varied as a function of the speed. 
     The variable steering gear ratio of the steering system calculates the desired motor angle position on the basis of the vehicle speed, pinion angle and steering wheel angle. In the absence of the first two signals, the variable steering gear ratio may be converted into a safe but furthermore active substitute state. In the absence of the steering wheel angle signal, however, there is no information concerning the driver&#39;s steering wishes. For this situation, a meaningful substitute mode is not possible and the variable steering gear ratio must be switched off. However, since a motor angle was superimposed by the superimposed driving mechanism or the servo drive, a so-called inclined state of the steering wheel now results, that is, with the steering wheel in the straight ahead position, the steerable wheels possibly are inclined. 
     It is therefore an object of the present invention to provide a method for operating a steering system and a computer program for implementing the method of the type mentioned above, which, when the useful functions are switched on and off, avoids an inclined position of the steering wheel. 
     The input signals of the control device are monitored constantly by means of suitable safety functions and, because of this monitoring, the useful functions (such as the variable steering gear ratio) are switched on and off appropriately. By means of a controlled turning back of the servo drive, a possibly occurring steering wheel inclined position is removed reliably and almost imperceptibly for the driver when switching into a fall-back plane. By these means, the comfort of the steering system is increased substantially. 
     It is advantageous if steering portions for producing the control signal are superimposed by the useful functions, the steering portions for the controlled turning back of the servo drive being limited appropriately. 
     By these means, the individual steering portions or nominal angle defaults of all useful functions are superimposed on or added to a control signal. This may happen essentially relatively by means of specified priorities. For example, the variable steering gear ratio may have precedents as useful function and flow absolutely into the calculation, whereas the steering portions of the remaining useful functions are taken into consideration only relatively thereto. The steering portions of the useful functions for the controlled turning back of the servo drive when a substitute mode of at least one useful function is switched on or off are limited in a simple manner. This can take place separately for each steering portion, as well as for the already calculated control signal as a whole or also combined. 
     A substitute mode of at least one useful function can be switched off or on because of defective input signals of the control device. 
     A variable steering gear ratio, a lead steering or a driving dynamic stabilization come into consideration as useful functions. Moreover, other useful functions are, of course, also conceivable. 
     In a method development of the invention, provisions can furthermore be made said that the servo drive is turned back slowly, in a controlled manner, by means of a low angular speed and the course of the control signal for the motor angle of the servo drive is steady at the same time. 
     As a result, the driver hardly notices that the servo drive has been turned back. A movement of the servo drive causes a reaction moment at the steering wheel, which should be kept very small. A constant, high driving comfort accordingly is ensured. 
     Advantages with respect to the computer program arise similarly and are evident from the description. 
     An example of the invention is described in principle by means of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  show an outline of the steering system of the state of the art, from which the inventive example starts out, 
         FIG. 3  shows a diagram of a course of a steering portion of a variable steering gear ratio, and 
         FIG. 4  shows a diagram for determining a desired motor angle within an inventive method for operating a steering system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An example of the invention is shown in the following, the starting point being, for example, a superimposed steering, which has already been mentioned. 
     In  FIGS. 1 and 2 , reference numbers  11  and  21  refer to a steering wheel, which can be operated by the driver of the vehicle. By operating the steering wheel  11  or  21 , a steering wheel angle δ S  is supplied over a connection  101  to a superimposed gear mechanism  12  or  22 . At the same time, a motor angle δ M  of a servo drive  13  or  23  is supplied over a connection  104  to the superimposed gear mechanism  12  or  22 , it being possible to construct the servo drive as an electric motor. At the output side of the superimposed gear mechanism  12  or  22 , the superimposed movement or the pinion angle δ G  is supplied over a connection  102 ,  103  to a steering gear mechanism  14  or  24 , which, in turn, acts over a steering linkage  16  in accordance with the superimposed movement or the total angle δ G  on the steerable wheels  15   a  and  15   b . The mechanical gear ratio of the superimposed gear mechanism  12  or  22  for δ M =0, is labeled i ν =δ g /δ S  and the mechanical gear ratio of the steering gear  14  or  24  is labeled i L . 
     A reaction moment M V , which is affected by the street, acts on the wheels  15   a  and  15   b , which are designed to be steered. Furthermore, sensors  26  and  28  can be seen in  FIG. 2 , sensor  28  detecting the steering wheel angle δ S  and supplying a control device  27 , while reference number  26  marks sensors, which detect the movements of the vehicle (such as yaw movements, transverse acceleration, rpm of the wheels, vehicle speed v X , etc.) and supplies corresponding signals to the control device  27 . Depending on the steering wheel angle δ S , which has been determined, and optionally on the vehicle movements, the control device  27  determines a control variable δ Md  for triggering the servo drive  13  or  23  in order to realize useful applications (such as a variable steering gear ratio VSR). The signals of the sensors  26  can also be taken from a CAN bus system of the vehicle. 
     The well-known relationships between the angles and torques shown in  FIGS. 1 and 2 , apply. (i L ( ) is a nonlinear function):
 
 i   L (δ Fm )=(δ S   /i   ü +δ M )  (1)
 
and
 
 M   L   =M   V /( i   L   *i   ü )  (2)
 
     A power steering function is achieved by the steering system shown in  FIGS. 1 and 2  owing to the fact that the steering wheel moment M L  is reduced by a large overall gear ratio (i L *i ü ), that is, by a very indirect steering. A motor angle δ M  is superimposed according to the above equation 1 on the steering wheel angle δ S , so that a desired front wheel angle δ Fm  with a steering angle δ S , which is not too large, can be set. At the same time, by suitably triggering the motor, relationships between the steering wheel angle δ S  and the steering angle δ Fm  can be realized, which may also depend on the driving state, such as the vehicle speed v x  or the steering wheel angle δ S . 
     According to equation 2 above, the steering wheel moment M L  depends only on the reaction moment M V  at the steerable wheels and, accordingly, cannot be affected by the motor intervention. This results in the already mentioned problem that an acceptable steering wheel moment or steering sensitivity cannot be achieved for all driving situations by selecting a constant gear ratio relationship of the steering gear. In particular, the steering wheel moment M L  must not be too large in the stationary state or too small when driving at high speeds. 
     Because of the therefrom resulting safety requirements with the respect to the steering system, a safety concept with safety and diagnosis functions, especially for detecting and appropriately reacting to random errors in sensors  26 ,  28 , the steering device  27  itself or the actuator system, that is, for example, to switch especially the variable steering gear ratio VSR suitably, is indispensable. The input signals of the control device  27 , especially δ S  and the vehicle-specific data of the sensors  26  are checked for plausibility. For example, it would be disadvantageous to accept a wrong speed signal v x  of the vehicle, since the variable steering gear ratio VSR is varied as a function of the speed. 
     The variable steering gear ratio of the steering system calculates a desired motor angle default or a steering portion δ Md   VSR  on the basis of the vehicle speed v X , pinion angle δ G  and steering wheel angle δ S  (refer also to  FIG. 4 ). In the absence of the first two signals, the variable steering gear ratio VSR can be transferred into a safe, but furthermore active alternate state. In the absence of the input signal of the steering wheel angle δ S , there is also no information concerning the driver steering wishes. For this situation, a meaningful alternate mode is not possible and the variable steering gear ratio VSR must be switched off. However, since a motor angle δ M  was previously superimposed by the superimposed gear mechanism  12  or  22  or the servo drive  13  or  23 , a so-called inclined state of the steering wheel now results, that is, when the steering wheel  11  or  21  is in the straight ahead position, the steerable wheels ( 15   a ,  15   b ) possibly are inclined. 
     Pursuant to the invention, when a correction of the inclined state of the steering wheel is necessary, as shown in  FIG. 3  by means of a course of the steering portion δ Md   VSR  of the useful function, the variable steering gear ratio VSR the steering portion δ Md   VSR  is set back in a controlled manner. The steering portion δ Md   VSR  or the superimposed angle portion is plotted on the vertical axis and the horizontal axis represents the time t. Up to time t 1 , the steering portion δ Md   VSR  proceeds normally according to the default of the variable steering gear ratio VSR. At time t 1 , there is no input signal for the steering wheel angle δ S  of the useful function, variable steering gear ratio VSR, after which the steering portion δ Md   VSR  is driven back slowly, that is with a constant angular velocity and continuously, until, at a time t 2 , the steering portion δ Md   VSR  is equal to zero and the inclined state of the steering wheel is eliminated. Moreover, a conceivable, constant course of the steering portion δ Md   VSR  is indicated by a broken line. The latter could be provided in the case of a failure of the input signal for the vehicle speed v K  or of the pinion angle δ G . 
       FIG. 4  shows the determination in principle of a desired motor angle or control signal δ Md  within an inventive method for operating a steering system. In so doing, the variable steering gear ratio VSR and further useful functions LAFN determine steering portions δ Md   VSR  . . . , δ Md   LAFN  continuously at each scanning step, for instance, by means of their input signals. These nominal default values are filtered in a pre-filtering unit  40  and then limited (corresponding to  FIG. 3  between t 1  and t 2 ) in resetting units  41  in the event that an inclined state of the steering wheel, which is to be corrected, is present and finally superimposed or added in a coordinator  42 . This superimposition takes place essentially by means of specified priorities. The variable steering gear ratio VSR, as useful function, has precedence and is entered as absolute angle in the calculation, whereas the steering portions of the remaining useful functions δ Md   LAFN  enter into the calculation only relative to the variable steering gear ratio VSR (as the respective difference between the steering portion δ Md   VSR  and the steering portions δ Md   LAFN ). Finally, there is a further limitation in a unit  43 , which supplies the desired motor angle default or the control signal δ Md  to the servo drive  13  or  23 . 
     The inventive method of operating a steering system is preferably realized as a computer program on the control device  27 . For this purpose, the computer program is stored in a memory element (not shown) of the control device  27 . The inventive method is carried out by processing on a microprocessor of the control device  27 . The computer program may be stored on a computer-readable storage medium (diskette, hard drive, CD-ROM, DVD, SD card, etc.) or an Internet server as computer program product and, from there, be transferred into the memory element of the control device  27 . 
     LIST OF REFERENCE NUMBERS 
     
         
         
           
               11  Steering wheel 
               12  Superimposed gear mechanism 
               13  Servo drive 
               14  Steering gear mechanism 
               15   a  Wheels 
               15   b  Wheels 
               16  Steering linkage 
               21  Steering wheel 
               22  Superimposed gear mechanism 
               23  Sevo drive 
               24  Steering gear mechanism 
               25  - 
               26  Sensors 
               27  Control device 
               28  Sensors 
               40  Pre-filtering unit 
               41  Restoring unit 
               42  Coordinator 
               43  Unit 
               101  Connection 
               102  Connection 
               103  Connection 
               104  Connection 
             δ s  Steering wheel angle 
             δ M  Motor angle 
             δ Md  Nominal motor angle default or control signal 
             δ G  Pinion angle 
             δ Fm  Steering angle 
             v x  Vehicle speed 
             i 0  Mechanical gear ratio of the superimposed gear mechanism 
             i L  Mechanical gear ratio of the steering gear mechanism 
             M V  Reaction moment 
             M L  Steering wheel moment 
             δ Md   VSR  Staring portion of the variable steering gear ratio 
             δ Md   LAFN  Remaining steering portions 
             VSR Variable steering gear ratio 
             LAFN Further useful functions 
             t 1,2  Times