Patent Publication Number: US-2021179168-A1

Title: Method for operating a power steering system of a vehicle, power steering system and vehicle

Description:
RELATED APPLICATION 
     This application claims priority to German Patent Application No. 102019134568.5 filed Dec. 16, 2019, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for operating a power steering system of a vehicle, a power steering system for performing the method and a vehicle having such a power steering system. 
     Modern steering devices in motor vehicles, for example electrical power steering (EPS), aid the driver in controlling the vehicle by applying power assistance to the steering train of the vehicle. In this way any disruptive steering rack forces occurring can be compensated for combining the driver manual torque applied by the driver with the power assistance generated by the EPS. 
     The steering rack force is substantially influenced by any lateral forces occurring, but also by the condition of the road surface and other factors dependent on the current driving situation. 
     In addition, further contributory factors that affect the handling of the vehicle arise due to the increasing prevalence of driver assistance systems, which can also intervene autonomously in the steering behavior of the vehicle. 
     Balancing these parameters influences the driving “feel” experienced by the driver and presents a complex challenge in designing the vehicle, particularly with regard to the competing steering rack forces applied by the driver and the driver assistance system. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a facility giving the driver a desired and expected driving feel in a variety of driving situations. 
     According to the invention the object is achieved by a method for operating a power steering system of a vehicle, comprising the following steps: 
     a) determination of an initial steering angle of the steering wheel, 
     b) determination of a disturbing force acting on a steering train of the vehicle due to external forces, 
     c) compensation for the disturbing force by means of an applied compensating force in such a way that a steering rack force at the initial steering angle assumes an initial value, in particular the initial value zero, and 
     d) application of a synthesized steering rack force (F 4 ) to produce a desired driving feel. 
     The compensating force serves to correct the steering rack force occurring, which is caused in particular by the external forces acting on the steering train, so that the steering rack force assumes the initial value, at least at the initial steering angle. 
     By then applying an additional synthesized steering rack force, the driving feel to be expected in the neutral position of the steering wheel can be transmitted to any initial steering angle. 
     This allows adjustment of the steering feel at different steering angles without having to undertake any adjustment of the mechanical components of the vehicle. In this way the tuning outlay in order to coordinate the components of the vehicle with one another is considerably reduced. 
     In particular, the magnitude of the applied synthesized steering rack force varies around the initial steering angle and is selected in such a way as to produce a desired steering resistance characteristic. 
     The desired steering resistance characteristic corresponds in particular to the disturbing force characteristic to be expected around a neutral steering angle, the neutral steering angle corresponding to the neutral position of the steering wheel. 
     The method according to the invention may additionally comprise the following steps: first a characteristic curve is prepared and/or calculated, which indicates a power assistance to be applied by the power steering system as a function of a driver manual torque. The driver manual torque applied is then measured. Next, on the basis of the driver manual torque applied, the power assistance is determined by means of the characteristic curve and the power assistance is applied to the steering train of the vehicle. 
     The characteristic curve prepared and/or calculated corresponds in particular to the power assistance of a servomotor, hereinafter also referred to as the servo assistance, to be expected of a normal EPS in the event of steering angle variations around the neutral position of the steering wheel. As a rule, the driver is accustomed to such a characteristic curve and expects a corresponding assistance. 
     The characteristic curve can be prepared and/or calculated on the basis of the current driving maneuver, in particular on the basis of the speed of the vehicle. In this way it is possible to achieve a specific steering rack force as a function of the driving maneuver. The speed of the vehicle here represents one of the greatest factors influencing the steering rack force. 
     The driver manual torque applied may also be zero, for example during straight-line driving of the vehicle. 
     The total force applied by the power steering system is the sum of at least the compensating force, as determined in step c), the synthesized steering rack force, as applied in step d), and optionally the power assistance, which is determined by means of the characteristic curve on the basis of the driver manual torque. 
     In order to compensate fully for the influence of all specific disturbing forces, the disturbing force may be compensated for in such a way that the sum of the disturbing force and the compensating force assumes the initial value for all steering angles. 
     In one variant the power assistance, the compensating force and/or the synthesized steering rack force are exerted on the steering rack by means of an actuator, for example by means of an electric motor. For this purpose, a single actuator may be used for applying both the power assistance, the compensating force and also the synthesized steering rack force. Alternatively, a separate actuator may be available for applying the power assistance, the compensating force and/or the synthesized steering rack force. 
     The power assistance, the compensating force and/or the synthesized steering rack force may also be applied through the application of a torque in the steering column. 
     The disturbing force may result, at least in part, from interference to the lateral control of the vehicle. For example, the disturbing force may be caused by a steep road surface, cross winds and/or uneven weight distributions of the vehicle. 
     In principle it is also possible to only use selected disturbances in determining the compensating force. 
     In a further variant the compensating force is selected so that at least elements of the disturbing force are at least partially and in particular fully compensated for and/or elements of the disturbing force are amplified. 
     The corresponding proportional fractions of the disturbing force may therefore be compensated for and/or especially emphasized depending on the desired driving feel. 
     The disturbing force may be determined by means of a disturbance variable monitor and/or a reference value. The disturbance variable monitor in particular comprises a sensor, which is fitted, for example, directly to the axle of the vehicle. The disturbance variable monitor generally regulates the steering rack moment as the controlled variable. 
     The reference value may be determined on the basis of the vehicle model, on the basis of a characteristics map of the vehicle and/or as a function of the speed. 
     In one variant the vehicle comprises a steering angle control module, which sets the initial steering angle. The steering angle control module may be part of a driver assistance system, for example a lane-keeping assist system or an autonomous or semi-autonomous driving assistance system. 
     The steering angle control module may calculate an assistance force, which is likewise applied to the steering train of the vehicle and which corresponds to a steering lock from the neutral position to the initial steering angle. 
     The steering angle control module therefore does not distort the prepared and/or calculated power assistance characteristic curve. 
     If a steering angle control module is used, the method according to the invention is particularly advantageous, since the initial steering angle will ordinarily not correspond to the neutral position of the steering wheel. Nevertheless, in the event of intervention by the driver the method according to the invention can serve to give the driver the steering feel that he would expect in the neutral position of the steering wheel or around the neutral position. 
     The steering angle control module may exert the assistance force on the steering train by means of an actuator. In particular, the steering angle control module exerts the assistance force by means of the same actuator that is also used to apply the power assistance, the compensating force and/or the synthesized steering rack force. 
     The steering angle control module in particular comprises a closed-loop with no integral component. Accordingly, in the event of a driver intervention, which represents a control interference variable for the steering angle control module, no automatic adjustment of the steering angle will be undertaken. In this way, unwanted fluctuations are avoided in the event of an intervention by the driver. 
     The object of the invention is furthermore achieved by a power steering system having a control module, a steering angle sensor and an actuator for generating a power assistance, a compensating force and/or a synthesized steering rack force, the power steering system being designed to perform the method of the type previously described. 
     The control module may comprise a driver manual torque module for determining the driver manual torque applied and/or a power assistance module for determining the power assistance, the compensating force and/or the synthesized steering rack force. 
     The power steering system furthermore in particular comprises a steering angle control module. 
     Furthermore, according to the invention the object is achieved by a vehicle comprising such a power steering system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and characteristics of the invention emerge from the following description and from the drawings, in which: 
         FIG. 1  shows a motor vehicle according to the invention having a power steering system according to the invention, 
         FIG. 2  shows diagram of the power assistance generated by an EPS as a function of the driver manual torque, 
         FIG. 3  shows a diagram of the steering rack force characteristic as a function of the steering angle, 
         FIG. 4  shows a flow chart of the method according to the invention, 
         FIG. 5  shows a block diagram of the contributors to the force applied by the power steering system in  FIG. 1 , and 
         FIG. 6  shows a block diagram of the control structure of the power steering system in  FIG. 1 . 
     
    
    
     DESCRIPTION 
       FIG. 1  schematically shows a vehicle  10  according to the invention having a power steering system  12  according to the invention. 
     The power steering system  12  comprises a control module  14 , a steering angle sensor  16 , a steering angle control module  17  and an actuator  18  for generating a force F. 
     The vehicle  10  comprises a steering rack  20 , which is connected to a steering train  22 . The steering train  22  can be operated by a driver (not shown) via a steering wheel  24 . 
     The steering angle sensor  16  monitors the position of the steering wheel  24  and is therefore able to determine a steering angle x of the steering wheel  24 . 
     The actuator  18  is, for example, an electric motor and can therefore act on the steering train  22 , in order to steer the vehicle  10 . To put it another way, the actuator  18 , by means of the force F applied, is able to produce a movement, in particular a rotational movement, of the steering train  22 . 
     The control module  14  further comprises a driver manual torque module  26  which can be used to determine the driver manual torque M applied by the driver to the steering wheel  24 . 
     The control module  14  further comprises a power assistance module  28  that serves to determine a power assistance F 1  which in a steering movement is applied to the steering train  22  by means of the actuator  18 . 
     In basic operation of the vehicle  10  the power steering system  12  provides a power assistance F 1 , which varies as a function of the driver manual torque M according to the characteristic curve a represented in  FIG. 2 . The power assistance F 1  is in this case equal to the total force F applied by the power assistance system  12 . 
     Conventional EPS-systems provide the driver with power assistance F 1  that increases symmetrically around a zero point, an ever more rapid increase in the power assistance F 1  being observable with increasing driver manual torque M, as shown in  FIG. 2 . 
     The zero point of the characteristic curve a usually coincides with a neutral position of the steering wheel  24 . The driver of the vehicle  10  therefore receives the same power assistance F 1  for the same driver manual torque M irrespective of the steering direction 
     In  FIG. 3  the steering rack force F 2  of the steering rack  20  is represented as a function of the steering angle x of the steering wheel  24 . It can be seen that in the neutral position of the steering wheel  24 , which corresponds to a neutral steering angle x 0 , the steering rack force F 2  is minimal and increases symmetrically around the steering angle x 0  as a function of the steering angle x, as is illustrated by the characteristic line b. 
     The characteristic line b accordingly describes the characteristic of the steering rack force F 2  around the neutral steering angle x 0 , which is a measure of the steering resistance around the neutral steering angle x 0 . 
     The driving feel experienced by the driver of the vehicle  10  results from the interplay between the driver manual torque M applied, and hence also the power assistance F 1 , and the steering rack force F 2 . 
     The method according to the invention for operating the power steering system  12  of the vehicle  10  serves to transfer this driving feel to any steering angle x, for example to the initial steering angle x 1  represented in  FIG. 3 . 
       FIG. 4  shows a flow chart of the method according to the invention, which is explained in more detail below. 
     In a first mode of the power steering system  12  it is to be operated so that the driving feel experience by the driver is independent of disturbing forces acting on the vehicle  10 . 
     For this purpose, an initial steering angle x 1  of the steering wheel  24  is first determined and an initial value is defined for the steering rack force F 2  which the driver is intended to experience at the initial steering angle x 1  (step S 1 ). The initial steering angle x 1  is, for example, the current steering angle and is determined, for example, by means of the steering angle sensor  16 . 
     A disturbing force which acts on the steering train  22  of the vehicle  10  due to external forces is then determined (step S 2 ). The disturbing force results, at least in part, from interference to the lateral control of the vehicle  10 , for example by steep road surfaces, cross winds and/or uneven weight distributions of the vehicle  10 . The characteristic of the steering rack force F 2  is substantially predetermined by the disturbing force. 
     A disturbance variable monitor  29  (cf.  FIG. 6 ) and/or a reference value may be used for determining the disturbing force. The reference value may likewise be vehicle-based and/or vary as a function of the speed. 
     Now a compensating force F 3  is determined, which is intended to offset the disturbing force to the initial value. To put it another way, the compensating force is selected so that the sum of the disturbing force and the compensating force produces the initial value, in particular for all steering angles. 
     The compensating force is then applied, and the calculated disturbing force is thus at least partially and in particular fully compensated for (step S 3 ). Here the steering rack force F 2 , at least at the initial steering angle x 1 , but in particular for all steering angles, assumes a fixed initial value, which in the embodiment shown is equal to zero. 
     In principle, however, any other desired initial value could also be selected. This is of particular interest if specific contributions to the disturbance variable are even to be amplified, for example in order to provide the driver of the vehicle  10  with information on the road surface condition. 
     In addition, a synthesized steering rack force F 4  is applied to the steering train  22  of the vehicle  10 , for example by the actuator  18  (step S 4 ), so as to produce a desired characteristic of the steering rack force F 2  corresponding to the curve c around the initial steering angle x 1  (cf.  FIG. 3 ). The characteristic of the curve c around the initial steering angle x 1  here corresponds to the expected characteristic of the curve b around the neutral steering angle x 0 , as had resulted in step S 2  before applying the compensating force F 3 —that is to say the expected characteristic of the disturbing force. A desired steering resistance characteristic around the initial steering angle x 1  is therefore generated via the synthesized steering rack force F 4 . 
     It is also feasible for the desired characteristic produced by the synthesized steering rack force F 4  to deviate from the expected characteristic of the disturbing force, in order to tailor the driving feel and/or the steering resistance. 
     Here the desired characteristic may always be based on the characteristic of the disturbing force, since the driver expects a steering resistance or a driving feel that does not deviate too much from the disturbing force. 
     The magnitude of the synthesized steering rack force F 4  may alternatively or additionally be selected on the basis of driving parameters, such as the vehicle speed, the lateral acceleration etc. 
     The magnitude of the compensating force F 3  and/or the synthesized steering rack force F 4  to be applied may likewise be determined by the power assistance module  28 . Alternatively, a separate module could be provided for determining the compensating force F 3  and/or the synthesized steering rack force F 4 . 
     The power assistance F 1  to be applied by the power steering system  12  may optionally be calculated as a function of the driver manual torque M. For this purpose, a characteristic curve, for example the characteristic curve a in  FIG. 2 , is first prepared and/or calculated (step S 5 ). 
     The characteristic curve may be prepared and/or calculated, for example, as a function of the vehicle model, the current driving maneuver and/or the current speed of the vehicle  10 . 
     The use of a characteristic curve as in  FIG. 2 , i.e. a characteristic curve of a conventional EPS, presents itself, since drivers of modern vehicles nowadays expect a power assistance which conforms to this characteristic curve. 
     Then, the driver manual torque M applied is additionally measured, in order to determine the steering intention of the driver (step S 6 ). This is done, for example, by the driver manual torque module  26  of the control module  14 . 
     The driver manual torque M determined is used as a basis for determining, via the characteristic curve a and after compensation for all disturbing variables and application of the synthesized steering rack force, the power assistance F 1  needed in order to give the driver the same driving feel around the initial steering angle x 1  as he would expect around the neutral position of the steering wheel  24  in the previously explained basic operation of the power steering system  12  (step S 7 ). 
     Finally, the power assistance F 1  determined is applied to the steering train  22  of the vehicle  10 , for example via the actuator  18  (step S 8 ). 
     The driver in his steering movement is therefore supported by the power assistance F 1  in precisely the way he would expect, that is to say on the basis of the characteristic curve a in  FIG. 2 . This represents a considerable difference compared to the prior art, in which no compensating force F 3  and no synthesized steering rack force F 4  is applied. In the prior art the driver must himself apply a corresponding steering torque to compensate for the disturbing force, in order to maintain his course. If he now performs a steering movement to change course, the power assistance F 1  would not be calculated from the origin of the characteristic curve a, however, but from the steering torque already applied. This leads to an asymmetrical power assistance F 1 , which the driver is not used to and/or does not expect. 
     The initial steering angle x 1 , particularly in this first mode of the power steering system  12 , may also correspond to the steering angle x 0 . In this case the method according to the invention serves primarily to compensate for the disturbing forces acting on the vehicle  10 , since the contribution of the synthesized steering rack force F 4  in this case may be zero. 
     However, with the aid of the steering angle control module  17  the power steering system  12  according to the invention can also be operated in a second mode. 
     In the second mode the steering angle control module  17  sets the initial steering angle x 1 . For example, the steering angle control module  17  is designed as a driver assistance system, in particular as a lane-keeping assist system and/or an autonomous or semi-autonomous driving assistance system. 
     Accordingly, in this case the driver of the vehicle  10  may happen to intervene in a steering wheel  24  deflected from the neutral position. In this case the driver, immediately on grasping the steering wheel  24 , will obtain the driving feel which he would expect in the neutral position of the steering wheel  24 . 
     For this purpose, the steering angle control module  17  calculates an assistance force F 5 , which corresponds to a steering lock from the neutral position to the initial steering angle x 1 . This force is applied to the steering train  22  of the vehicle even prior to step S 1  or in step S 8  in addition to the power assistance F 1 . 
     The different proportions of the total force F applied to the steering train  22  applied are illustrated schematically in  FIG. 5 , in which the contributions in basic operation, in the first mode and in the second mode are listed from bottom to top. 
       FIG. 6  schematically represents the interaction of the various components of the vehicle  10  and the power steering system  12 . 
     The steering train  22  is connected to the steering rack  20 . 
     Steering rack forces F 2  act on the steering rack  20 . In addition, disturbing forces act on the steering rack  20  and the steering train  22 , in particular disturbing forces acting on the lateral control of the vehicle  10 , that is, for example, in the same direction as the steering rack force F 2  indicated by an arrow in  FIG. 6 . 
     Fitted to the steering train  22  is the steering wheel  24 , the steering angle x of which can be determined by a steering angle sensor  16 . The currently prevailing steering angle x is transmitted as initial steering angle x 1  to the power steering system  12 . 
     In addition, the driver manual torque M applied to the steering wheel  24  is determined by means of a driver manual torque sensor  30  and likewise relayed to the power steering system  12 . 
     In addition, further parameters P to be taken into account can be transmitted to the power steering system  12 , for example the current speed of the vehicle  10 . 
     The power steering system  12 , in accordance with the method described above, calculates a force F which may comprise both the power assistance F 1  and also—depending on the mode in which the power steering system  12  is being operated—contributions by the compensating force F 3 , the synthesized steering rack force F 4  and the assistance force F 5 . 
     The actuator  18  may apply the force F to the steering train  22 . 
     In the embodiment shown in  FIG. 6  an additional second steering angle sensor  32  is provided, which serves to monitor the steering angle x after application of the force F by the actuator  18 .