Abstract:
A method of controlling a motor of a power steering system is provided. The method includes: estimating a scale factor based on a vehicle speed and a hand wheel torque; applying the scale factor to a return command; and generating a motor command signal based on the applying the scale factor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of International Application Serial No. PCT/US09/31799 which claims the benefit of U.S. Provisional Application No. 61/023,598 filed Jan. 25, 2008. The disclosure of each of the above applications is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present invention relates to return force in power steering systems. 
     BACKGROUND 
     Power steering systems may use motors or other devices to assist a driver in turning the wheels of a vehicle. When the wheels are in a center position, they are pointing forward such that the vehicle will travel in a straight line. The center position may be referenced as a zero position of a steering wheel or hand wheel of the system. 
     A return assist force may be used to assist the driver in returning the wheels to the center position. The return force may, for example, be a function of vehicle speed and the hand wheel position. In some systems, the use of a return force may result in an undesirable tactile feel for the driver if the driver imparts a torque on the hand wheel in the same direction as that of the return force. An improved system and method that offers better tactile feel for a driver when a return force is used is desired. 
     SUMMARY OF THE INVENTION 
     Accordingly, a method of controlling a motor of a power steering system is provided. The method includes: estimating a scale factor based on a vehicle speed and a hand wheel torque; applying the scale factor to a return command; and generating a motor command signal based on the applying the scale factor. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the embodiments for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 will become apparent from the following detailed description taken in conjunction with the drawings. 
         FIG. 1  illustrates block diagram of an exemplary steering control system. 
         FIG. 2  is a dataflow diagram illustrating an exemplary system for determining return torque. 
         FIG. 3  is a graph illustrating an exemplary embodiment of a table used to determine a return torque. 
         FIG. 4  is a graph illustrating an exemplary embodiment of a table used to determine a scaling for the return torque. 
         FIG. 5  is a flowchart illustrating an exemplary embodiment of a method for determining return torque. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     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 including a power steering system  10  is illustrated. The power steering system  10  includes, for example, wheels  100 , a motor  102 , and a hand wheel  104 . The wheels  100  are mechanically linked to the motor  102 . The hand wheel  104  is mechanically linked to the motor  102 . 
     A torque sensor  106  generates a torque signal  107  based on a torque of the hand wheel  104 . Additional inputs  108  such as, for example, vehicle speed sensors and hand wheel angle sensors sense conditions of the power steering system  10  and/or vehicle and generate signals  109 ,  111  accordingly. The torque sensor  106  and the additional inputs  108  are communicatively linked to a controller  110 . The controller  110  includes, for example, a processor  112 . Based on the torque sensor signal  107  and the additional signals  109 ,  111 , the controller  110  determines a motor command signal  114 . The controller  110  generates the motor command signal  114  and sends the motor command signal  114  to the motor  102  to control the steering system  10 . 
     Referring now to  FIG. 2 , a dataflow diagram illustrates an exemplary embodiment of the controller  110  of  FIG. 1  used to control the steering system  10  of  FIG. 1 . The controller  110  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  FIG. 2  can be combined and/or further partitioned to similarly generate the motor command signal  114 . Inputs to the controller  110  can be generated from the sensors  106 ,  108  ( FIG. 1 ) of the vehicle, can be modeled, and/or can be predefined. In this example, the controller  110  includes a hand wheel return module  208 , a hand wheel torque dependent scaling module  210 , a scaled return command module  212 , and a table datastore  214 . 
     The hand wheel return module  208  receives as input vehicle speed  220  and hand wheel angle  222 . The hand wheel angle  222  may be, for example, an angular position of the hand wheel  104  (of  FIG. 1 ) relative to the center position of the hand wheel  104  (of  FIG. 1 ). The hand wheel return module  208  determines a return command  224  based on the vehicle speed  220  and the hand wheel angle  222 . In various embodiments, the hand wheel return module  208  determines the return command  224  using one or more hand wheel return lookup tables  225 . The lookup tables  225  can be indexed by the vehicle speed  220  and/or the hand wheel angle  222 . The lookup tables  225  can be stored in and accessed from the table datastore  214 . 
     In one example, the hand wheel return module  208  is associated with nine return tables  225 . A unique vehicle speed  220  is defined for each of the nine return tables. A return curve is defined by the data points in the nine return tables. For example, each of the nine return curves is composed of sixteen data points. The data points are defined by the axis, where the axis is defined by hand wheel angle  222 . In one example, the hand wheel angle  222  can range from zero to nine-hundred degrees. In various embodiments, the axis can be selectable. In various embodiments, all return curves share a common axis. As can be appreciated, the data of the return curves can be increasing or decreasing. The speed defined for curve zero can be used as a return speed breakpoint (e.g., return command is ramped to zero below the breakpoint speed). 
       FIG. 3  illustrates an exemplary embodiment of a return table  325 . In this example, the return table  325  includes all nine return curves  302 - 314  (e.g., is a three-dimensional lookup table). As can be appreciated, the return table  325  can be implemented as nine separate return tables, shown collectively in  FIG. 2  as return tables  225 , one for each discrete vehicle speed (e.g., nine two-dimensional lookup tables). 
     In this example, the hand wheel angle  222  is represented on the x-axis  316 . The return command  224  is represented on the y-axis  318 . The curves  302 - 314  represent a range of discrete vehicle speeds  220  as shown in the index  315 . The return command  224  may be calculated by determining the return command value from the input hand wheel angle  222  and the curve representing the input vehicle speed  220 . Interpolation between curves  302 - 314  is used when the vehicle speed  220  does not equal the discrete vehicle speed  220  of one of the given curves  302 - 314 . For example, return command values are looked up from the two nearest return curves  302 - 314  and the return command  224  is determined based on an average between the two return command values. As can be appreciated, other methods of interpolation can be used to determine the return command  224 . 
     Referring back to  FIG. 2 , the hand wheel torque dependent scaling module  210  receives as input hand wheel torque  226  and the vehicle speed  220 . The hand wheel torque dependent scaling module  210  generates a scale factor  228  to tune the return command  224  based on the hand wheel torque  226  and the vehicle speed  220 . In various embodiments, the hand wheel torque dependent scaling module  210  generates the scale factor  228  using one or more scaling lookup tables  229 . The lookup tables  229  can be indexed by the vehicle speed  220  and/or the hand wheel torque  226 . 
     In one example, the hand wheel torque dependent scaling module  210  includes nine scaling tables  229 . As with the return tables  225 , a unique vehicle speed  220  is defined for each of the nine tables  229 . In various embodiments, the unique vehicle speeds  220  are the same as the unique vehicle speeds  220  for the return tables  225 . A scaling curve is defined by the data points in the nine tables. For example, each of the nine scaling curves is composed of four data points. The data points are defined by the axis, where the axis is defined by hand wheel torque  226 . In one example, the hand wheel torque  226  can range from zero to ten Newtonmeters (Nm). In various embodiments, all scaling curves share a common axis. 
       FIG. 4  illustrates an exemplary embodiment of a single scaling table  429 , nine of which are shown collectively in  FIG. 2  as scaling tables  229 . The hand wheel torque  226  is represented on the x-axis  410 . The scale factor  228  is represented on the y-axis  420 . As can be appreciated, only a single curve  422  representing a single vehicle speed  220  is shown as scaling table  429 . In practice, multiple curves (not shown) each representing a different vehicle speed  220 , will comprise scaling tables  229  of  FIG. 2 . 
     In one example, for each scaling curve, the first point  424  on the curve  422  (zero Nm) can be fixed at one hundred percent return scaling. The last point  426  on the curve  422  (10 Nm), for example, can be fixed at zero percent return scaling. The second point  428 , for example, can be defined by the x-coordinate, while the third point  430 , for example, can be defined by both the x- and the y-coordinates. The scale factor  228  is calculated by determining the scale value from the input hand wheel torque  226  and the curve  422  representing the input vehicle speed  220 . Interpolation between curves  422  can be used when the vehicle speed  220  does not equal the discrete vehicle speed  220  of one of the given curves  422 . For example, scaling values are looked up from the two nearest scaling curves and the scale factor  228  is determined based on an average between the two scaling values. As can be appreciated, other methods of interpolation can be used to determine the scale factor  228 . 
     Referring back to  FIG. 2 , the scaled return command module  212  receives as input the return command  224  and the scale factor  228 . The scaled return command module  212  applies the scale factor  228  to the return command  224  and generates the motor command signal  114 . In various embodiments, the scaled return command module  212  multiplies the return command  224  by the scale factor  228  to generate the motor command signal  114 . 
     Referring now to  FIG. 5  and with continued reference to  FIG. 2 , a flowchart illustrates a motor command determination method that can be performed by the controller of  FIG. 2 . 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. 5 , but may be performed in one or more varying orders as applicable and in accordance with the present disclosure. 
     In one example, the method may begin at  500 . The current hand wheel angle  222 , the vehicle speed  220 , and the hand wheel torque  226  are received at  510 . The return command  224  is determined, as discussed above, based on the hand wheel angle  222  and the vehicle speed  220  at  520 . The scale factor  228  is determined based on the hand wheel torque  226  and the vehicle speed  220  at  530 . The scale factor  228  is applied to the return command  224  at  540  and the motor command signal  114  is generated based thereon at  550 . The scaled motor command signal  114  results in an improved tactile feel to the user. Thereafter, the method may end at  560 . 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those of ordinary skill in the pertinent art that various changes may be made and equivalents may be substituted for the elements thereof without departing from the scope of the present disclosure. In addition, numerous modifications may be made to adapt the teachings of the disclosure to a particular object or situation without departing from the essential scope thereof. Therefore, it is intended that the claims not be limited to the particular embodiments disclosed.