Abstract:
A method for determining a desired preset value ( Y MSM) for a motor torque of an electric servomotor in a power assisted steering system for a motor vehicle, wherein the desired preset value is determined using a motor torque servo control component (MVSK) which forms a controlled section, and using a motor torque regulator component (MRK) which forms a regulated section, wherein the regulating variable is a torsion bar torque (MDS), the two motor torque components being functionally offset against one another in order to determine therefrom the desired preset value (YMSM) for the motor torque (MSM) of the servomotor (SM).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for operating en electric power assisted steering system. 
     Various methods for generating a motor torque desired preset value for a servomotor are known from the prior art: 
     DE 101 15 018 A1 discloses a vehicle steering system in which a state closed-loop controller generates a manipulated variable for the control device by means of state variables of the vehicle, in which the steering forces to be applied by the driver would be minimal. On the basis of this torque-free steering system it is possible to apply a reference variable. 
     DE 10 2009 002 703 A1 discloses a regulating structure for determining a manipulated variable for actuating a torque actuator in an electrical vehicle steering system as a function of a predetermined desired torsion bar torque, wherein an actual torsion bar torque is detected, a differential torque is formed from the desired torsion bar torque and the actual torsion bar torque, and the manipulated variable is determined by means of an output feedback closed-loop controller as a function of the differential torque. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to propose a method for determining a desired presetting for a motor torque which, compared to the known methods, permits a basic setting which is easy to perform and at the same time permits comprehensive and precise influencing of the steering behavior and the steering sensation communicated to the driver. 
     The method provides that a desired preset value is formed from at least two motor torque components, specifically a motor torque pilot control component which forms an open-loop controlled portion and a motor torque closed-loop controller component which forms a closed-loop controlled portion, wherein the closed-loop control variable is a torsion bar torque. 
     The two motor torque components are functionally combined with one another here in order to determine therefrom the desired preset value for the motor torque of the servomotor. 
     Advantageous developments of the method are represented in the dependent claims which are explained in more detail by means of the description of the figures. 
     Accordingly, there is provision that in order to determine a desired preset value the motor torque pilot control component is determined at least as a function of a torsion bar torque (M DS ), wherein in addition a vehicle speed can be taken into account and the motor-torque closed-loop controller component is determined as a function of a closed-loop control difference formed from the torsion bar torque and an application torsion bar torque based on the toothed rack force. 
     It is advantageous that the motor torque pilot control component is determined using an assistance characteristic curve which determines a pilot control portion at least as a function of the torsion bar torque. In addition, the assistance characteristic curve can be parameterized as a function of the vehicle speed, wherein intermediate values are interpolated. 
     One preferred embodiment provides that the motor torque pilot control component is determined by a closed-loop controller. 
     In this context, a range-limited (saturated) PID controller is preferably used. 
     According to the invention there is provision to limit the motor torque closed-loop controller component to a steering-specific, maximum deliverable limiting value. This preferably takes place as a function of the vehicle speed. 
     Furthermore, there is provision chat the motor torque pilot control component is determined using an assistance characteristic curve to which a vehicle speed is fed as a further input variable. 
     The application torsion bar torque is preferably determined using a determined toothed rack force and/or further steering-specific or vehicle movement dynamics vehicle variables and/or on the basis of different steering function components. 
     The motor torque components are subsequently combined functionally with one another in order to determine therefrom the desired preset value for the motor torque. 
     The subject matter of the invention is also a computer program having program code means in order to carry out a method according to the invention when the program is executed on a microprocessor of a computer, in particular on a control unit of a power assisted steering system or power steering system. 
     The subject matter of the invention is also a corresponding control unit for operating a power assisted steering system or power steering system configured for carrying out a method according to the invention or for executing a corresponding computer program, as well as a power assisted steering system or power steering system equipped with a corresponding control unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a structural diagram on the basis of which a preferred embodiment of the method according to the invention is explained. 
         FIG. 2  shows by way of example different methods for generating the application torsion bar torque. 
         FIG. 3  shows a limitation of the maximum permissible portion of the motor torque closed-loop controller component in the form of an envelope curve. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A pilot control module  100  determines, on the basis of a torsion bar torque M DS  detected at the steering column, a first component, the motor torque pilot control component M VSK . This component represents an open-loop controlled portion of the motor torque. The pilot control module  100  additionally uses the vehicle speed v FZG  for this purpose. The module  100  contains an assistance characteristic curve which determines the motor torque pilot control component M VSK  as a function of the torsion bar torque M DS  and the vehicle speed v FZG . 
     Furthermore, a torsion bar torque application module  200  determines an application torsion bar torque M DS, Appl . This component represents a closed-loop controlled portion of the motor torque. 
     More details are given about the determination of the value of the application torsion bar torque M DS, Appl  in the description of  FIG. 3 . 
     This variable represents, as it were, a reference variable for a subsequent closed-loop control process and is decisive for the desired steering sensation which is to be communicated to the driver. The application torsion bar torque M DS, Appl  is used as a “reference variable”, wherein a closed-loop control difference with respect to the torsion bar torque MDs is determined by means of a closed-loop control difference-determining element  102 . A second component, the motor torque closed-loop control component M RK  is determined by means of a downstream closed-loop controller, the torsion bar-twisting closed-loop controller mode  101 . This component represents a closed-loop controlled portion of the motor torque. The closed-loop controller is preferably embodied as a range-limited (saturated) PID controller. This permits simple and stable application for the respective vehicle. 
     Both components are combined by means of a combination element  103  for forming a desired preset value yM SM  for the motor torque M SM  which is to be output by the servomotor SM. In the simplest case, this is done by summing the two values. 
     The desired preset value yM SM  is fed to an output stage unit which generates corresponding phase currents I M  for the servomotor by means of a subordinate, field-oriented closed-loop control stage (FOR). 
     The motor torque pilot control component M VSK  determined at least as a function of a torsion bar torque M DS  and the motor torque closed-loop controller component M RK  is determined as a function of a closed-loop control difference between the torsion bar torque M DS  and the application torsion bar torque M DS, Appl . The torsion bar torque is measured in the region of the steering column by means of a suitable sensor. 
     In the exemplary embodiment shown, the motor torque pilot control component M VSK  is determined using an assistance characteristic curve to which the vehicle speed v FZG  is fed as a further input variable. 
     The advantages of the method can be seen in particular in the fact that the requirements regarding the dynamics of the system can be met by means of the pilot control portion M VSK , wherein at the same time the closed-loop controlled portion M RK  can be superimposed on a “desired” steering sensation which is to be communicated to the driver. Rapid steering movements of the driver are therefore transmitted in the form of a highly dynamic steering process to the steering system, and at the same time closed-loop controlled influence, which is limited in its effect, can be applied to a driver&#39;s steering torque in accordance with a desired preset value. 
     By means of a pilot control process, initiated by means of the pilot control module  100 , for the motor torque yM SM , a “basic assistance” by the servomotor is impressed into the steering system, in particular in the event of high assistance torques being necessary. This basic assistance is excepted from the closed-loop control and therefore corresponds in functional terms to a “basic assistance open-loop control”. 
     The pilot control module  100  determines a motor torque pilot control component M VSK  on the basis of the vehicle speed V FZG  and the torsion bar torque M DS , corresponding to a “basic assistance”. 
     The behavior of the pilot control module  100  is implemented here in such a way that the value of the motor torque pilot control component M VSM  decreases as the vehicle speed increases and is increased as the torsion bar torque increases. 
     Closed-loop control of the torsion bar torque M DS , initiated by means of the torsion bar twisting closed-loop controller module  101  proves very advantageous, in particular in the event of low or very small assistance torques occurring in the “coupling region”, i.e. in the region relatively close to the straight-ahead position of the steering wheel: in this context, typically relatively low assistance forces of the order of magnitude of approximately 0.5 Nm occur. 
     Here, the torsion bar twisting closed-loop controller module  101  performs the function of a “genuine” closed-loop controller for the torsion bar torque M DS  on the basis of a desired presetting M DS, Appl  of the torsion bar torque application module  200 . 
     By virtue of the transmission ratio of the transmission, these small assistance torques have a correspondingly amplified effect on the steering sensation. For this reason, a closed-loop control process of the torsion bar torque has a particularly positive effect on the steering sensation which is communicated to the driver. 
     it is considerably advantageous here to implement the torsion bar twisting closed-loop controller module  101 , i.e. the actual closed-loop controller which contains the closed-loop control law for performing closed-loop control of the torsion bar torque, as a saturated PID (Proportional-Integral-Differential) controller. This permits the closed-loop controller to be adapted for all conceivable driving situations in a way which involves relatively low expenditure. 
     The saturated region of the closed-loop controller is approximately in a range from +/−0.5 Nm. Within this range, closed-loop controlled, free influencing of the driver steering torque is possible. This has the effect that during straight-ahead travel the closed-loop controlled portion dominates, while in the case of rapid steering movements the open-loop controlled portion is prevalent. 
     For this reason, the motor torque closed-loop controller component M RK  is limited to a steering-specific, maximum deliverable limiting value +/−M RK,lim . The motor torque closed-loop controller component M RK  is limited here as a function of the vehicle speed v FZG . 
       FIG. 2  shows by way of example different methods for generating the application torsion bar torque M DS,Appl . The methods can be applied in isolation or in combination with one another. 
     The torsion bar torque application module  200  includes a toothed rack force-determining module  201  which determines a calculated or estimated toothed rack force (ZSK) value. 
     Furthermore, the module  200  contains a block  242  which contains steering-related or vehicle-specific variables, for example the instantaneous steering angle, the steering angle speed, the current velocity, the rotor rotational speed of the servomotor and further vehicle movement dynamics variables. 
     In the embodiment shown, two ZSK values which are determined in different ways are determined: 
     A dynamic portion  240  (ZSK-D) and a comfort-related portion  241  (ZSK-C). The ZSK-D values are determined here from steering-internal variables which describe a torque or force within the steering device on the basis of mechanical and/or electrical equations. The ZSK-C value is determined using steering-external vehicle variables which describe a movement state on the basis of physical motion relationships. 
     Both ZSK variables can be combined with one another to form a resulting toothed rack force or mixed in any desired ratio with one another, depending on the driving conditions. The resulting toothed rack force is subsequently fed to individual functional units of the basic steering assistance module  202  or the additional steering function module  203 . 
     The functional units of the basic steering assistance module  202  can contain the following components here: A basic steering torque in the coupling region  210 , an active return function  211 , a central sensation functional unit  212 , an active damping function  213  and a friction and hysteresis compensation unit  214 . 
     The functional units of the additional steering function module  203  can contain the following components: 
     An oversteering assistance function  220  and an understeering assistance function  221 . 
     Furthermore, superordinate interventions can be provided by the vehicle manufacturer by means of an interface with OEM functions  230 . These include autonomous steering interventions such as for example, lane keeping and follow-on guidance or the initiation of avoidance manoeuvres. 
     A number of functional units  202 ,  203  require the additional or exclusive use of steering-related variables of the block  242 . For example, the active damping function  213  can determine a damping-specific portion of the application torsion bar torque solely on the basis of the instantaneous rotor speed of the servomotor. 
     In conjunction with the embodiment of the invention described in  FIG. 1  it is accordingly provided that the application torsion bar torque (M DS, Appl ) is determined using a determined toothed rack force (RFM-C, RFM-D) and/or by means of steering-specific or vehicle movement dynamics vehicle variables on the basis of different steering function components  210 ,  211 ,  212 ,  113 ,  214 ,  220 ,  220 ,  221 ,  230 . 
     By means of a summing or weighting point  204  all the determined functional components are combined in order to form an overall application torsion bar torque M DS,Appl . 
     In the simplest case, all the components are added up by means of an addition point  204  in order to determine the overall application torsion bar torque M DS,Appl . 
       FIG. 3  shows a limitation of the maximum permissible portion of the motor torque closed-loop controller component MRK in the form of an envelope curve. 
     The limitation occurs preferably as a function of the vehicle speed v FZG . From the stationary state up to a speed of approximately 10 to 15 km/h (point  1 ), the closed-loop controlled portion is 0 Nm, and starting from this speed the maximum permissible portion is increased in a ramp shape up to a second speed (point  2 ) up to a maximum symmetrical value of +/−M RK, lim . This value is approximately 0.3 to 0.5 Nm. 
     LIST OF REFERENCE SYMBOLS 
     
         
           100  Pilot control module 
           101  Torsion bar twisting closed-loop controller module 
           102  Closed-loop control difference-determining element 
           103  Combination point 
           200  Torsion bar torque application module 
           201  Toothed rack force-determining module 
           202  Basic steering assistance module 
           203  Additional steering function module 
           204  Summing point 
           205  Torsion bar-twisting closed-loop controller module 
           210  Basic steering torque in the coupling region 
           211  Active return 
           212  Central sensation 
           213  Active damping 
           214  Hysteresis 
           220  Oversteering assistant 
           221  Understearing assistant 
           230  External steering functions 
           240  Toothed rack force, dynamic portion (ZSK-D) 
           241  Toothed rack force, comfort portion (ZSK-C) 
           242  Steering-related variables 
         M DS  Torsion bar torque 
         SM Servomotor 
         l M  Servomotor current 
         yM SM  Desired presetting for motor torque 
         M SM  Motor torque 
         M VSK  Motor torque pilot control component 
         M RK  Motor torque closed-loop control component 
         M DS, Appl . Application torsion bar torque 
         v FZG  Vehicle speed