Abstract:
A method of controlling an electric power steering system is provided. The method includes: determining a scale factor based on a handwheel position; applying the scale factor to a hand wheel torque value to scale the hand wheel torque value; and generating a corrected handwheel torque signal based on the scaled hand wheel torque value.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This patent application claims priority to U.S. Provisional Patent Application Ser. No. 61/416,548 filed Nov. 23, 2010 which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject invention relates to methods and systems for controlling harmonic pinion torque correction in steering systems. 
     Steering systems often have unwanted variations in the amount of steering torque that must be applied by a driver. One source is the phasing of the upper and lower cardan joints on an intermediate shaft. Cardan joints phasing imperfections will cause a cyclical variation in the steering torque applied by a driver that repeats every 180 degrees of the steering wheel. A goal in the design of any steering system is to eliminate, or at least minimize, unwanted steering torque variations that are felt by a driver. The best approach to eliminate these torque variations is to eliminate or minimize them at the source, however, due to various limitations and trade-offs, torque variations will always be present to some degree. 
     Accordingly, it is desirable to provide systems and methods for correcting torque variations. 
     SUMMARY OF THE INVENTION 
     In one exemplary embodiment, a method of controlling an electric power steering system is provided. The method includes: determining a scale factor based on a handwheel position; applying the scale factor to a hand wheel torque value to scale the hand wheel torque value; and generating a corrected handwheel torque signal based on the scaled hand wheel torque value. 
     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 
       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 in which: 
         FIG. 1  is a functional block diagram of a steering system that includes a torque correction system in accordance with exemplary embodiments; 
         FIG. 2  is a dataflow diagram illustrating a torque correction system in accordance with exemplary embodiments; and 
         FIG. 3  is flow diagram illustrating a torque correction method in accordance with exemplary embodiments. 
     
    
    
     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  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. 
     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  generate sensor signals based on the observable conditions. In various embodiments, the sensors  31 ,  32  can include, for example, a load sensor, a driver torque sensor, and/or a position sensor. 
     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 torque correction systems and methods of the present disclosure. Generally speaking, the torque correction systems and methods of the present disclosure reduce steering torque variation felt by a driver due to intermediate shaft phasing. For example, the systems and methods receive a handwheel angle signal from a position sensor which measures the handwheel angle and uses this information to modify the torque sensor signal and remove any harmonic variation in the output signal. Once implemented, the systems and methods allow a designer more flexibility in designing steering system geometry because the variation in handwheel torque due to the positioning of the steering system components may be removed analytically. 
     Referring now to  FIG. 2  where a dataflow diagram illustrates exemplary embodiments of the control module  40  of  FIG. 1  used to control the steering system  12  and/or the vehicle  10  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  FIG. 2  can be combined and/or further partitioned to similarly reduce steering torque variation. As can be appreciated, the sub-modules shown in  FIG. 2  can be implemented as a single control module  40  (as shown) or multiple control modules (not shown). 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. In various embodiments, the control module  40  includes a scale factor determination module  50  and a handwheel torque determination module  52 . 
     The scale factor determination module  50  receives as input a relative handwheel position. Based on the relative handwheel position, the scale factor determination module  50  determines a scale factor. In various embodiments, the scale factor determination module  50  determines the scale factor (TorqCorrSclFctr) based on the following equation:
 
TorqCorrSclFctr=1+TorqCorrMag*Cos(2*HwPosRel+TorqCorrPhase).  (1)
 
     Where TorqCorrMag represents a torque correction magnitude. The TorqCorrPhase represents the torque correction phase. The correction magnitude and phase are based on analysis of cardan joint angles and geometry. In various embodiments, a wrapping function may be applied to the handwheel position to accommodate the range of degrees accepted by the cosine function. 
     In various embodiments, the scale factor determination module  50  performs a validity check before computing the scale factor. For example, the scale factor determination module  50  may evaluate one or more diagnostic trouble codes relating to the handwheel position. When a diagnostic trouble code has been set to TRUE, then the scale factor may be set to a predetermined value (e.g., one, or any other value). Otherwise, when the diagnostic trouble code(s) are set to FALSE, then the scale factor is determined as discussed above. 
     In another example, the scale factor determination module  50  may evaluate the status of other algorithms that affect the handwheel position. For example, when the other algorithms have completed, the scale factor can be determined. 
     The handwheel torque determination module  52  receives as input a non-corrected handwheel torque and the correction factor. In various embodiments, the non-corrected handwheel torque can be estimated based on one or more algorithms. Based on the inputs, the handwheel torque determination module  52  generates a handwheel torque signal. For example, the handwheel torque determination module  52  determines a handwheel torque value based on the following equation:
 
HwTrq=HwTrq*TorqCorrSclFctr.  (2)
 
     The handwheel torque determination module  52  then generates the handwheel torque signal based on the handwheel torque value. 
     Referring now to  FIG. 3  and with continued reference to  FIG. 2 , a flow diagram 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. 3 , but may be performed in one or more varying orders as applicable and in accordance with the present disclosure. 
     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 ). 
     In one example, the method may begin at  100 . A validity check is performed at  105 . If the validity check is true at  106 , then the relative handwheel position is received and processed at  110 . For example, any necessary wrapping methods are performed on the handwheel position. The torque correction scale factor is determined, for example, using equation 1 at  120 . The torque correction scale factor is applied to the handwheel torque, for example, using equation 2 at  130 . The handwheel torque signal is generated at  150 . Thereafter, the method may end at  160 . 
     If, however, at  105 , the validity check is false, the torque correction scale factor is set to a predetermined value at  140 . The torque correction scale factor is applied to the handwheel torque, for example, using equation 2 at  130 . The handwheel torque signal is generated at  150 . Thereafter, the method may end at  160 . 
     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.