Abstract:
An electric power steering system includes a steering wheel, an electric assist motor operatively coupled to the steering wheel, and an electronic controller operatively coupled to the assist motor. A steering wheel position sensor provides an output indicative of an angular position of the steering wheel. A motor position sensor provides an output indicative of a relative angular position for the motor. Steering wheel position sensor output is acquired at each of a plurality of angular positions of the motor, thereby establishing a set of points defining a curvilinear relationship between steering wheel sensor output and motor position sensor output. The set of points is sampled to calculate a straight line which approximates the curvilinear relationship, thereby reducing periodic errors in the steering wheel sensor output. The straight line is employed to determine a position offset for the motor position sensor output corresponding to a system zero position. The system zero position is a position for which the vehicle will travel along a linear, substantially straight-ahead path.

Description:
BACKGROUND 
       [0001]    In a vehicle equipped with an electric power steering (EPS) system, a steering assist force is provided by an electric motor operatively coupled to a steering wheel through a steering column or shaft. Typically, such systems include a controller programmed to derive an assist torque signal along with a “return to center” torque signal, thereafter summing these torque signals to generate a motor command signal that is applied to the electric motor. The assist torque signal is derived using an applied torque signal indicative of torque applied to the steering wheel by a vehicle operator. The assist torque signal provides a power steering boost torque, and the “return to center” torque signal provides a bias torque for returning the steering wheel to a system zero position. The system zero position may be defined as a position for which the vehicle will travel along a linear, substantially straight-ahead path. 
         [0002]    During EPS operation, it may be desirable to automatically determine the absolute position of the steering wheel. This position may be used, for example, to cause the steering wheel to return to center following a steering input in the form of torque applied to the steering wheel by a vehicle operator. The return to center effect simulates the self-aligning torque due to positive caster of a conventional hydraulically-operated steering system, and causes the vehicle to be more comfortably controlled by a driver. 
         [0003]    In order to automatically determine the center position of the steering wheel in an EPS system, a motor position sensor may be employed to measure a relative angular position of the motor. However, since the motor position sensor senses relative position, the EPS system utilizes a second sensor to determine an absolute reference for the center position of the steering wheel. This second sensor is provided in the form of a steering wheel sensor mechanically connected to the steering wheel, steering column, or shaft for producing a sensor output signal indicative of the angular position of the steering wheel. Since the steering wheel may be capable of rotating through three or more revolutions, the sensor output signal may exhibit periodic errors. These periodic errors may recur at substantially the same angular position of the steering wheel for each of a plurality of rotational cycles. 
         [0004]    Errors in the steering wheel sensor output signal are partially attributable to the mechanical connection of the steering wheel sensor to the steering wheel, steering column, or shaft. Errors are also attributable to the physical properties of the sensing mechanism used to generate the sensor signal. But regardless of the source of this error, if the steering wheel sensor output signal is used to establish the center position of the steering wheel, either the sensor must provide adequate accuracy, or an appropriate error compensation scheme must be devised. Since improvements to sensor accuracy require increasing the precision of components, materials, and mechanical tolerances, such an approach is costly and impractical. What is needed is an efficient compensation technique for correcting errors in the steering wheel sensor signal. 
       SUMMARY 
       [0005]    The above described and other features are exemplified by the following Figures and Description in which an electric power steering system is disclosed that includes a steering wheel, an electric assist motor operatively coupled to the steering wheel, and an electronic controller operatively coupled to the assist motor. A steering wheel position sensor provides an output indicative of an angular position of the steering wheel. A motor position sensor provides an output indicative of a relative angular position for the motor. Steering wheel position sensor output is acquired at each of a plurality of angular positions of the motor, thereby establishing a set of points defining a curvilinear relationship between steering wheel sensor output and motor position sensor output. The set of points is sampled to calculate a straight line which approximates the curvilinear relationship, thereby reducing periodic errors in the steering wheel sensor output. The straight line is employed to determine a position offset for the motor position sensor output corresponding to a system zero position. The system zero position is a position for which the vehicle will travel along a linear, substantially straight-ahead path. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Referring now to the Figures wherein like elements are numbered alike: 
           [0007]      FIG. 1  is a block diagram of an electric power steering system having a controller; 
           [0008]      FIG. 2  is a graph showing an illustrative relationship between steering wheel position sensor output as a function of motor position reference angle; 
           [0009]      FIG. 3  is a graph showing a line computed as providing the best fit to a relationship setting forth processed steering wheel position sensor output as a function of motor position reference angle. 
           [0010]      FIG. 4  is a flowchart setting forth a method for determining an absolute angle and a system zero position for an electric power steering system; and 
           [0011]      FIG. 5  is a flowchart setting forth a procedure for determining a position offset for the motor position sensor of  FIG. 1  corresponding to a system zero position. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0012]      FIG. 1  is a block diagram of a motor vehicle provided with an exemplary electric power steering (EPS) system  70 . EPS system  70  includes a steering mechanism  72 , illustratively implemented using a conventional rack and pinion steering mechanism that includes a toothed rack (not shown) and a pinion gear (also not shown) under a gear housing  74 . A steering wheel  76  is coupled to an upper steering shaft  78 . As steering wheel  76  is turned, a steered member or upper steering shaft  78 , which is connected to a lower steering shaft  80  through a universal joint  82 , turns the pinion gear. The rotation of the pinion gear moves the toothed rack, which then moves a pair of tie-rods  84  (only one shown) coupled to a pair of steering knuckles  86  (only one shown) to turn a pair of road wheels  88  (only one shown). 
         [0013]    Electric power assist is provided through an assist unit  90  generally designated by reference numeral  90 , which includes a controller  92  and an electric motor  94 . A motor position sensor  95  measures the relative angular rotational position of electric motor  94 . Controller  92  is powered by a vehicle power supply  96  through a supply line  98 . Controller  92  receives a signal indicative of the vehicle velocity on signal line  100 . Initial steering wheel position is measured by a steering wheel position sensor  102  and fed to controller  92  through line  104 . Steering wheel position sensor  102  may be an optical-encoding type of sensor, a variable-resistance type of sensor, or any other suitable type of position sensor for performing the functions of steering wheel position sensor  102 . 
         [0014]    As steering wheel  76  is turned, a torque sensor  103  senses the torque applied to steering wheel  76  by the vehicle operator and provides an input steering torque signal on line  106  to controller  92 . Torque sensor  103  may include a torsion bar (not shown) and a variable-resistance type of sensor (also not shown) that outputs a variable resistance signal to controller  92  through line  106  in relation to the amount of twist on the torsion bar. Other suitable torsion-sensing devices used with known signal processing techniques will suffice in alternate embodiments 
         [0015]    In response to the inputs on lines  100 ,  104  and  106 , controller  92  sends a motor command signal through line  108  to electric motor  94 . Electric motor  94  then supplies a torque assist to upper and lower steering shafts  78 ,  80  through a worm  107  and associated worm gear  109 , in order to provide a steering torque assist to the vehicle steering system in addition to a steering force exerted by the vehicle operator. If torque sensor  103  is of the type that requires upper steering shaft  78  to be separated at the sensor between upper and lower sections (allowing some range of rotational independence), both steering wheel position sensor  102  and worm gear  109  are associated with lower steering shaft  80  below torque sensor  103 , as shown. 
         [0016]      FIG. 2  is a graph showing an illustrative relationship between steering wheel position sensor output  52  as a function of motor position reference angle obtained from motor position sensor  95  ( FIG. 1 ). Illustratively, a raw, unprocessed steering wheel position sensor output  52  ( FIG. 2 ) is obtained from steering wheel position sensor  102  ( FIG. 1 ). Observe that unprocessed steering wheel position sensor output  52  is a periodic signal having a period substantially equal to one revolution of steering wheel  76 . However, it is possible to process steering wheel position sensor output  52  with an appropriate DC offset waveform to produce a continuous waveform shown in  FIG. 2  as processed steering wheel position sensor output  50 . If the output of steering wheel position sensor  102  ( FIG. 1 ) was substantially error-free, processed steering wheel position sensor output  50  ( FIG. 2 ) would appear as a substantially straight line or as a substantially smooth curve. 
         [0017]    Due to variations in mechanical tolerances of steering wheel position sensor  102  ( FIG. 1 ), the physical configurations of devices used to fabricate steering wheel position sensor  102 , hysteresis effects in steering wheel position sensor  102  and motor position sensor  95 , and various other factors, processed steering wheel position sensor output  50  may include periodically recurring error components as those shown in  FIG. 2 . These error components may occur at fundamental frequency and harmonic frequency components corresponding to a single steering wheel  76  revolution. If these error components are not corrected or compensated, EPS system  70  will not be able to properly determine a neutral, straight-ahead position for steering wheel  76 . This neutral, straight-ahead position may also be referred to as a “system zero”. 
         [0018]      FIG. 3  is a graph showing a line  54  that provides the best fit (i.e., the closest match) to a relationship setting forth processed steering wheel position sensor output  50  as a function of motor position reference angle. The motor position reference angle is obtained from motor position sensor  95 . As stated previously, an ideal steering wheel position sensor  102  ( FIG. 1 ) would provide a substantially linear output. However, observe that processed steering wheel position sensor output  50  includes fundamental frequency components and harmonic frequency components related to a single period of revolution of steering wheel  76 , and perhaps additional components unrelated to the revolution of steering wheel  76 . These components represent sources of error that cause position sensor output  50  to deviate from a linear function. 
         [0019]    It would be desirable to compensate for the sources of error in position sensor output  50 . This may be accomplished by defining a linear relationship (i.e., a line), which most closely approximates position sensor output  50 . In this manner, the curves and nonlinearities in position sensor output  50  are effectively ignored or filtered out, and an ideal sensor output function is substituted for the actual position sensor output. In the example of  FIG. 3 , a line  54  is defined by establishing two points that identify the relationship of the motor position reference angle to the steering wheel position sensor output  50 . These points are established by sampling a plurality of points along position sensor output  50 , wherein each of the sampled points has a known reference angle obtained from motor position sensor  95 . Once line  54  is defined, the system zero position is determined from line  54 , and a corresponding position offset is determined for motor position sensor  95 . 
         [0020]    In many situations, it may be desirable to limit sampling of points along position sensor output  50  to a certain range of values, such that only a segment of position sensor output  50  (termed a “sample window”) is utilized to determine line  54 . Moreover, depending upon the harmonic content of position sensor output  50 , it may be desirable to utilize the same segment of position sensor output  50  every time line  54  is to be computed, and possibly when the system zero position is to be computed. Utilizing the same segment of position sensor output  50  may be desired in situations where significant errors result from third-harmonic and other odd harmonic frequency components. Even harmonics tend to cancel in one revolution of steering wheel position sensor  102  ( FIG. 1 ), assuming that a sample window size of one revolution is employed. On the other hand, for a sample window size of one revolution, odd harmonics will not cancel and will introduce a bias effect. If the sample window size is not equal to one mechanical revolution of steering wheel position sensor  102 , then the harmonic content within the sample window should be evaluated to determine if a consistent sample window selection is required. 
         [0021]    Optionally, in some system applications, significant hysteresis may be present in the output of steering wheel position sensor  102  or motor position sensor  95 . In situations where hysteresis is present, the graph of  FIG. 3  can be modified to show the effect of such hysteresis, and this would double the amount of data needed to determine line  54 . For example, data points on the graph of  FIG. 3  would need to be shown for direction-dependent sensor motion where hysteresis occurs, and data points on the graph of  FIG. 3  could also be shown for direction-dependent sensor motion where no hysteresis occurs. 
         [0022]      FIG. 4  is a flowchart setting forth a method for determining an absolute angle and a system zero position for an electric power steering system. The method commences at block  1000  where a steering wheel position sensor output  50  is received. This position sensor output  50  includes periodic error components as were previously discussed in connection with  FIGS. 2 and 3 . Returning to  FIG. 4 , the program proceeds to block  1020  where a “zero offset” procedure is executed to determine a position offset for motor position sensor  95  ( FIG. 1 ) corresponding to a system zero position. Further details regarding this position offset procedure are discussed hereinafter with reference to  FIG. 5 . 
         [0023]    Next, at block  1040  ( FIG. 4 ), the zero offset algorithm returns a position offset for motor position sensor  95  ( FIG. 1 ). At block  1060  ( FIG. 4 ), a signal from motor position sensor  95  is received. This signal may be continuous, but it contains no information as to system zero position. Rather, as indicated at block  1080 , the received signal from motor position sensor  95  includes information specifying a relative reference angle for motor  94  ( FIG. 1 ) position. A summer  1100  ( FIG. 4 ) subtracts the position offset for motor position sensor  95  that was returned at block  1040  from the signal received from motor position sensor (block  1080 ). The output of summer  1100  is a continuous output signal providing an absolute angle reference with a compensated (corrected) system zero position (block  1120 ). 
         [0024]      FIG. 5  is a flowchart setting forth a procedure for determining a position offset for motor position sensor  95  ( FIG. 1 ) corresponding to a system zero position. The procedure commences at block  401  ( FIG. 5 ). At block  403 , a test is performed to determine whether or not a position offset has already been calculated. If so, the procedure exits, but it should be noted that the procedure may then recommence at block  401  as described in greater detail hereinafter. The negative branch from block  403  leads to block  405  where a test is performed to ascertain whether or not this is the first time that the procedure is being executed. If so, the procedure progresses to block  407  where an initial offset is computed. Next (block  409 ), a line segment to be sampled from steering wheel position sensor output  50  is determined (i.e., a sampling window is defined). A data acquisition area is prepared (block  411 ), and a “first time” flag is set to “FALSE” (block  413 ). 
         [0025]    After block  413  is performed, the procedure advances to bock  415 . The procedure also advances to block  415  by following the negative branch from block  405 . At block  415 , a reference angle (and, optionally, a direction) is obtained from motor position sensor  95  ( FIG. 1 ). A test is performed at block  417  ( FIG. 5 ) to determine whether the current data point has been previously sampled and stored. If so, the procedure exits, but it should be noted that the procedure may then recommence at block  401  as described in greater detail hereinafter. The negative branch from block  417  leads to block  419  where the sampled point is stored. The sampled point is defined using an X value representing a reference angle obtained from motor position sensor  95  and a Y value representing a sensor angle obtained from steering wheel position sensor output  50 . 
         [0026]    At block  421 , a test is performed to determine whether all sampling points within the sample window have been taken. If not, the procedure exits, but it should be noted that the procedure may then recommence at block  401  as described in greater detail hereinafter. The positive branch from block  421  leads to block  423  where a best fit line  54  ( FIG. 3 ) is computed from the stored sample points. This line  54  is computed as being the best fit for steering wheel position sensor output  50  ( FIG. 3 ). 
         [0027]    It should be noted that the procedure set forth in the flowchart of  FIG. 5  may be executed periodically and/or repeatedly by controller  92  ( FIG. 1 ) as part of an overall control scheme. In this manner, the procedure of  FIG. 1  will continue to accumulate data points, a maximum of one new point per execution cycle, until all required data points have been collected. After all required data points have been collected, the best fit line is computed (block  423  of  FIG. 5 ). Next, at block  425 , a position offset for motor position sensor  95  ( FIG. 1 ) corresponding to system zero position is computed using line  54 . This position offset is stored ( FIG. 5 , block  427 ), and a “completed” flag is set to “TRUE”. 
         [0028]    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 as the currently preferred best modes contemplated for carrying out the teachings herein, but that the Claims shall cover all embodiments falling within the true scope and spirit of the disclosure.