Abstract:
A steering control method is provided. The method includes determining a dynamic load on a steering system based on a dynamic model; and controlling the steering system based on the dynamic load.

Description:
FIELD 
       [0001]    The present invention relates generally to systems and methods of estimating a load associated with a steering system. 
       BACKGROUND 
       [0002]    Steering control systems have traditionally relied on sensors for inputs such as hand wheel angle, driver torque, and vehicle speed. However, some parameters that affect the steering system are neither sensed nor estimated for the purposes of control due to the cost and difficulty of implementation. One example is the load at the steering system rack. 
         [0003]    The load at the rack not only affects vehicle centric behavior, for example, free control and return, but also impacts the overall steering feel. The rack load is a highly nonlinear, highly dynamic, vehicle speed sensitive function of the rack position, thus, making it complicated to estimate. Control methods that treat the vehicle as a simple stiffness (and hence rack load as a gain times rack position) lack capturing the dynamic nature of the steering-chassis linkage. 
       SUMMARY 
       [0004]    In one exemplary embodiment, a steering control method is provided. The method includes determining a dynamic load on a steering system based on a dynamic model; and controlling the steering system based on the dynamic load. 
         [0005]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings. 
           [0007]      FIG. 1  illustrates a functional block diagram of an exemplary vehicle including a controlled steering system in accordance with various embodiments. 
           [0008]      FIGS. 2 through 5  are dataflow diagrams illustrating exemplary steering control systems in accordance with various embodiments. 
           [0009]      FIG. 6  is a flowchart illustrating an exemplary steering control method in accordance with various embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
         [0011]    Referring now to  FIG. 1 , where the invention will be described with reference to specific embodiments without limiting same, an exemplary embodiment of a vehicle  10  including a steering system  12  is illustrated. In various embodiments, the steering system  12  includes a hand wheel  14  coupled to a steering shaft  16 . In one exemplary embodiment, the steering system  12  is an electric power steering (EPS) system that further includes a steering assist unit  18  that couples to the steering shaft  16  of the steering system  12  and to tie rods  20 ,  22  of the vehicle  10 . The steering assist unit  18  includes, for example, a rack and pinion steering mechanism (not shown) that may be coupled through the steering shaft  16  to a steering actuator motor and gearing (hereinafter referred to as the steering actuator). During operation, as the hand wheel  14  is turned by a vehicle operator, the motor of the steering assist unit  18  provides the assistance to move the tie rods  20 ,  22  which in turn moves steering knuckles  24 ,  26 , respectively, coupled to roadway wheels  28 ,  30 , respectively of the vehicle  10 . Although an EPS system is illustrated in  FIG. 1  and described herein, it is appreciated that the steering system  12  of the present disclosure can include various controlled steering systems including, but not limited to, steering systems with hydraulic configurations, and steer by wire configurations. 
         [0012]    As shown in  FIG. 1 , the vehicle  10  further includes various sensors  31 ,  32  that detect and measure observable conditions of the steering system  12  and/or of the vehicle  10 . The sensors  31 ,  32 ,  33  generate sensor signals based on the observable conditions. In various embodiments, the sensors  31 ,  32 ,  33  can include, for example, position sensors, a vehicle speed sensor, or a combination thereof 
         [0013]    In various embodiments, a control module  40  controls the operation of the steering system  12  and/or the vehicle  10  based on one or more of the sensor signals and further based on the steering control systems and methods of the present disclosure. Generally speaking, the steering control systems and methods of the present disclosure estimate a load on the steering system from the vehicle chassis. In particular, the steering control systems and methods estimate a load on the rack of the steering system  12 . In various embodiments, the steering control systems and methods estimate the load based on a dynamic bicycle model of the vehicle  10 . As can be appreciated, various functions can be applied in addition to the bicycle model to improve the overall estimation of the load. 
         [0014]    In various embodiments, the load can be estimated based on the following relationship: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         T 
                         L 
                       
                        
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       
                         x 
                         r 
                       
                        
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         G 
                         ~ 
                       
                       
                         
                           N 
                           str 
                         
                         
                            
                           1 
                         
                       
                     
                     * 
                     
                       
                         
                           
                             F 
                             _ 
                           
                           f 
                         
                          
                         
                           ( 
                           s 
                           ) 
                         
                       
                       
                         
                           δ 
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                             ( 
                             s 
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                            
                           2 
                         
                       
                     
                     * 
                     
                       
                         
                           1 
                           + 
                           
                             ατ 
                              
                             
                                 
                             
                              
                             s 
                           
                         
                         
                           
                             1 
                             + 
                             
                               τ 
                                
                               
                                   
                               
                                
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                             3 
                           
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   1 
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         [0015]    Part 1 of the above equation represents a steady-state gain function. Part 2 of the above equation represents a normalized vehicle transfer function which provides magnitude attenuation with frequency and all the required phase-characteristics. Part 3 of the above equation represents a phase-lead compensator that, by design, has a unit steady-state gain. 
         [0016]    More specifically, the symbol T L  denotes the rack load in column coordinates. The symbol x r  denotes the rack position in column coordinates. The symbol N str  denotes the overall steer ratio. The symbol {tilde over (G)} denotes a steady-state gain function, where: 
         [0000]    
       
         
           
             
               
                 
                   
                     G 
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                   = 
                   
                     
                       G 
                       * 
                       
                         Ff 
                         δ 
                       
                     
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                         s 
                         = 
                         0 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0017]    The symbol G denotes an effective torque gradient. The symbol F f  denotes the lateral force at the front axle. The symbol δ denotes the road wheel angle. The symbols α and τ denote parameters of a first order phase-lead compensator. 
         [0018]    In various embodiments, the values of 
         [0000]    
       
         
           
             
               
                 
                   F 
                   f 
                 
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                 δ 
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             , 
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             , 
           
         
       
     
         [0000]    and α can be a function of vehicle speed. In various embodiments, the steady-state characteristics that are represented by G can be captured as a nonlinear, vehicle speed dependent tabular function. 
         [0019]    Provided the relationship in equation (1), a model is established that evaluates the steady-state characteristics and dynamic characteristics separately to estimate the load. The estimated load can then be used to control the steering system  12  and/or the vehicle  10 . 
         [0020]    As can be appreciated, other variations of the relationship in equation (1) are contemplated to be within the scope of the invention. Such variations may alter the implementation of the model. For example, a model can be implemented based on Part 2 using a bicycle model that is not normalized. In another example, a model can be implemented based on Part 2 and Part 3 using a bicycle that is not normalized. In yet another example, a model can be implemented based on Part 1 and Part 2 using a normalized bicycle model. 
         [0021]    Referring now to  FIGS. 2 through 5 , dataflow diagrams illustrate exemplary embodiments of the control module  40  of  FIG. 1  used to control the steering system  12  of  FIG. 1 . In various embodiments, the control module  40  can include one or more sub-modules and datastores. As used herein the terms module and sub-module refer to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. As can be appreciated, the sub-modules shown in  FIGS. 2 through 5  can be combined and/or further partitioned to similarly estimate load on the steering system  12 . As can be appreciated, the sub-modules shown in  FIGS. 2 through 5  can be implemented as a single control module  40  (as shown) or multiple control modules (not shown). The implementation of the control module  40  will also vary based on the functions used in the model. Inputs to the control module  40  can be generated from the sensors of the vehicle  10  ( FIG. 1 ), can be modeled within the control module  40  (e.g., by other sub-modules (not shown), can be received from other control modules (not shown), and/or can be predefined. 
         [0022]    As shown in the example of  FIG. 2 , the control module  40  includes a steady-state module  50  and a dynamic module  52 . The steady-state module  50  and the dynamic module  52  effectively separate the evaluation of the steady-state characteristics from the dynamic characteristics. The steady-state module  50  receives as input road wheel angle  54 , and vehicle speed  56 . In various embodiments, the vehicle speed  56  can be received from, for example, a sensor or can be communicated from another control module. In various embodiments, the road wheel angle  54  can be directly sensed, can be determined from hand wheel angle and the steer ratio, or can be determined from the motor position. 
         [0023]    Based on the inputs, the steady-state module  50  determines a steady-state load value  58 . In one example, as shown in  FIG. 3 , the steady-state module  50  includes a table sub-module  62 , and an amplifier sub-module  63 . The table sub-module  62  includes a steady-state look-up table. The steady-state look-up table defines a steady-state gain  65  for various road wheel angles and various vehicle speeds. Based on the current road wheel angle  54  and vehicle speed  56 , the table sub-module  62  determines the steady-state gain  65  by interpolating the values in the steady-state look-up table. 
         [0024]    The amplifier sub-module  63  receives as input the steady-state gain  65 , and the road wheel angle  54 . The amplifier sub-module  63  amplifies the road wheel angle  54  based on the steady-state gain  65  to obtain the steady-state output value  58 . The steady-state output is effectively in load units. 
         [0025]    In other embodiments, the steady-state module  50  can include only the table sub-module  62 . The table sub-module  62  includes a steady-state look-up table. The steady-state look-up table defines a steady-state load  58  for various road wheel angles and various vehicle speeds. Based on the current road wheel angle  54  and vehicle speed  56 , the table sub-module  62  determines the steady-state load value  58  by interpolating the values in the steady-state look-up table. 
         [0026]    Referring back to  FIG. 2 , the dynamic module  52  receives as input the steady-state load value  58 , and the vehicle speed  56 . Based on the inputs, the dynamic module  52  adjusts the steady-state load based on a dynamic model of the vehicle  10  ( FIG. 1 ). 
         [0027]    In one example, as shown in  FIG. 4 , the dynamic module  52  includes a bicycle model sub-module and a compensator sub-module. The bicycle model sub-module  64  includes a dynamic bicycle model of the vehicle  10  ( FIG. 1 ). In various embodiments, the bicycle model is a normalized bicycle model. For example, the dynamic bicycle model performs gain attenuation with frequency and relevant phase characteristics to generate an uncompensated load value  68 . The dynamics of the implemented bicycle model is a nonlinear function of vehicle speed  56 . 
         [0028]    The compensator sub-module  66  adjusts any remaining phase characteristics to produce the final output namely, rack load  60 . In one example, as shown in  FIG. 5 , the compensator sub-module  66  includes an alpha table sub-module  70 , a tau table sub-module  72 , and a phase-lead compensator sub-module  74 . The alpha table sub-module  70  includes an alpha table. The alpha table defines the parameter a for various vehicle speeds. Based on the current vehicle speed  56 , the alpha table sub-module determines the parameter a  76  by interpolating the values in the alpha table. 
         [0029]    The tau table sub-module  72  includes a tau table. The tau table defines the parameter v for various vehicle speeds. Based on the current vehicle speed  56 , the tau table sub-module determines the parameter τ  78  by interpolating the values in the tau table. The phase-lead compensator sub-module  74  applies a phase lead compensator to the uncompensated load value  68  based on the parameters τ  78  and α  76 . 
         [0030]    As can be appreciated, the compensator sub-module  66  can be implemented according to various compensation techniques and is not limited to the present example. 
         [0031]    Referring now to  FIG. 6  and with continued reference to  FIGS. 2 through 5 , a flowchart illustrates a steering control method that can be performed by the control module  40  of  FIG. 1 . As can be appreciated in light of the disclosure, the order of operation within the method is not limited to the sequential execution as illustrated in  FIG. 6 , but may be performed in one or more varying orders as applicable and in accordance with the present disclosure. 
         [0032]    As can be appreciated, the steering control method can be scheduled to run based on predetermined events and/or can run at scheduled intervals during operation of the vehicle  10  ( FIG. 1 ). 
         [0033]    In one example, the method may begin at  100 . The road wheel angle  54  is received or determined at  110 . The vehicle speed  56  is received at  120 . The steady-state load value  58  is determined based on the road wheel angle  54  and the vehicle speed  56  using, for example, the steady-state table at  130 . The uncompensated load value  68  is determined based on the steady-state load value  58  and the vehicle speed  56 , for example, as described above at  140 . One or more compensation values  76 ,  78  are determined based on the vehicle speed  56  and applied to the uncompensated load value  68  at  150 . Thereafter, the compensated dynamic load value is output for use as the estimated rack load  60  at  160 . The method may end at  170 . 
         [0034]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.