Abstract:
A method and apparatus for improving the return-to-center motion of a steering wheel ( 16 ) wherein a determination of a first and a second value; secondly, derive a set of two values from the first value; thirdly, compute a third value from the set of two values and the first value according to a predetermined formula; and fourth, calculate a steering command using the third value and the second value for enhancing the return-to-center motion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application Ser. No. 60/183,596 filed Feb. 18, 2000, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to electric power steering, and more particularly, to automatic hysteresis compensation for electric power steering systems. 
     BACKGROUND OF THE INVENTION 
     Typical vehicle steering systems are constructed in such a way that the steering wheel will tend to return to center when let go by the operator of the vehicle. The center is defined as a straight line of travel. This return-to-center function is in response to road forces acting upon the turned front tires of the moving vehicle that tend to force the turned tires to the neutral (or center) position. However, friction in the steering system can interfere with the return-to-center function of the steering. One known method to prevent this is to reduce the overall steering ratio of the steering system, but this has the effect of increasing steering sensitivity, which is not always desirable. 
     In electric power steering systems, a known technique for preventing steering system friction from interfering with the return-to-center function is to generate a command, in relation to vehicle speed, that commands an electric power steering actuator to provide a return-to-center position force. The force exerted by the actuator overcomes the steering friction and allows the return-to-center function. In some systems, electric power steering motors are controlled either in a current mode or voltage mode. Current mode control provides certain advantages because the inherent control damping and speed dependability of voltage mode control are eliminated. However the elimination of the control damping that is present in the voltage mode can prevent the system from obtaining stable closed-loop poles. One solution provided is by creating an apparatus that includes a torque sensor coupled to a vehicle steering system for measuring steering wheel torque provided by a vehicle operator and, a steering angle sensor coupled to the steering system for measuring steering wheel angle. Furthermore, the apparatus may also include a controller, responsive to the measured steering wheel angle and vehicle speed, that provides a sum return-to-center command, as well as a motor responsive to the sum return-to-center command, a detailed description of which is described in Kaufmann et al., US 5,668,722, issued Sep. 16, 1997, for an Electric Power Steering Control. However, as the operating conditions such as temperature, wear, etc. change, a change in hysteresis, also known as friction or lash, occurs. This change in hysteresis adversely affects the performance of the steering system because the system is operating with a different hysteresis value from what the controller was designed to accommodate. 
     SUMMARY OF THE INVENTION 
     Disclosed herein are a method and an apparatus by which the hysteresis of a steering system is measured by the steering system controller and used to compute a value of friction compensation that is optimum for a current system state. 
     In an exemplary preferred embodiment of the invention, a controller continuously monitors the torque and position of the steering system handwheel and records a value of the torque signal each time the position sensor indicates a zero position. These torque values are then separated into two groups, one group denotes a set of torque values for clockwise handwheel movement, and another group denotes a set of torque values for counter-clockwise handwheel movement. A difference between these two groups of data, when suitably filtered for a predetermined period of time forms a measurement of the current system hysteresis. Thus, an enhancement that facilitates consistent performance of the electric power steering feel may be provided. The enhancement equalizes the differences that may exist among different individual electric power steering systems. These differences occur under circumstances such as temperature, wear and tear, and manufacturing variations in different electric power steering systems. From the foregoing it will be seen that this invention is based upon a measurement of a hysteresis of the electric power steering system during its operation. The measurement is possible because a set of sensors senses parameters such as torque or position of the electric power steering system. The relationship of torque and position parameters can be plotted and a symmetrical hysteresis loop curve may be drawn. It is noted that under practical operational circumstances, the loop curve may not be perfectly symmetrical. However, a symmetrical hysteresis curve may be used to best illustrate the concept of the instant invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing an electric power steering control system according to this invention; 
     FIG. 2 illustrates the operation of a method for compensating for an electric power steering system hysteresis; 
     FIG. 3 illustrates the operation of the method in FIG. 2 wherein a detailed illustration is described; and 
     FIG. 4 illustrates an ideal hysteresis loop curve. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, reference numeral  10  generally designates a motor vehicle power steering system suitable for implementation of this invention. The steering mechanism  12  is a rack-and-pinion type system and includes a toothed rack (not shown) and a pinion gear (also not shown) located under gear housing  14 . As the steering wheel  16  is turned, the upper steering shaft  18 , connected to the lower steering shaft  20  through universal joint  22 , turns the pinion gear. Rotation of the pinion gear moves the toothed rack that moves tie rods  24  (only one shown) that in turn move the steering knuckles  26  (only one shown), that turn wheels  28  (only one shown). 
     Electric power steering assist is provided through the unit generally designated by reference numeral  30  and includes a controller  32  and the electric motor  34 . The controller  32  is powered by a vehicle power supply  36  through line  38 . The controller  32  receives a signal representative of the vehicle velocity on line  40 . Steering pinion gear angle is measured through position sensor  42  (which may be an optical encoding type sensor, variable resistance type sensor or any other suitable type of position sensor) and fed to the controller  32  through line  44 . 
     As the steering wheel  16  is turned, a torque sensor  43  senses the torque applied to the steering wheel  16  by the vehicle operator. The torque sensor  43  may include a torsion bar (not shown) and a variable resistive-type sensor (also not shown) that outputs a variable resistance signal to controller  32  through line  46  in relation to the amount of twist on the torsion bar. Although this is the preferable torque sensor, any other suitable torque-sensing device used with known signal processing techniques will suffice. 
     In response to the inputs on lines  40 , 44 , and  46 , the controller  32  sends a current command through line  48  to the electric motor  34 . The motor  34  in turn supplies a torque assist to the steering system through a worm  50  and a worm gear  52 , in such a way as to providing the torque assist to the vehicle steering in addition to a driving force exerted by the vehicle operator. 
     Referring to FIG. 2, reference numeral  60  generally designates a method for implementing the method for automatic compensation for electric power steering system within the controller  32 . A measure of torque, for instance in Newton meters (Nm) is defined. In addition, a measure of position, for instance in degrees (deg) of moving away from the center line either clockwise or counter-clockwise is defined. Attention is drawn to the fact that the measure of torque and the measure of position is based on outputs of sensors. Torque data flows toward block  64  along line  70 . In the block  64 , the measured torque value is recorded and stored in a memory (not shown). It can be appreciated that the memory can be temporary registers in the controller  32  or other form of memories outside the controller  32  such as a random access memory, erasable programmed read only memory, or other suitable forms of memory. The recorded and stored torque values in block  64  then flow along line  72  toward block  66  wherein the recorded and stored torque values are weighted or adjusted according to a friction compensation table or schedule. It is evident that the friction compensation table or schedule is established through the utilization of sensors based, for instance, upon vehicle operating history, certain predetermined formulae, etc. It can be appreciated that a series of compensation settings are determined for a range of possible system hysteresis values. An optimum compensation setting can be determined using various known methods or formulae. After the above adjustment at block  66 , the processed data flows along line  74  to block  68  wherein the incoming adjusted torque values are processed according to a predetermined formula. For example, a difference of the adjusted torque values and the measure of position that is processed in block  62  and transmitted along line  76  is computed and transmitted as output along line  78 . It can be appreciated that data flowing through lines  70 ,  72 ,  74 , and  76  can be processed according to known methods such that undesirable noise, flowing along data streams or lines  70 ,  72 ,  74 , and  76 , can be reduced. For example, defining a moving average as a basis for eliminating undesirable data may be used, etc. 
     Referring to FIG. 3, reference numeral  80  generally designates a detailed method for implementing the method for automatic compensation for electric power steering of FIG. 2. A flow chart is depicted. In block  82 , a set of input value including input torque and position values is generated and flows through line  84 . It is noted that the set of input values may be supplied by a set of sensors that sense and measure the set of requisite values. The input values from block  82  are branched into lines  86  and  88  containing essentially the same information. The input values are checked at diamond block  90  for position information. If the position value is greater than a predetermined limit, the information flows through line  92 , and exits the instant flow-chart. If the steering wheel position is not within a programmable limit, the process stops, thus ensuring that the data used to determine a hysteresis estimate is within an appropriate region of a hysteresis loop curve. If the position value is less than a predetermined limit, the information flows through line  94 , the information flows toward diamond block  96  for determination of a current steering wheel turning direction. The filtered data is separated into two groups depending upon current turning direction of the steering wheel. If the current steering wheel turning direction is clockwise, the information flows along line  98  toward block  100  wherein an accumulated clockwise zero crossing data is determined. The information then flows through line  102  toward block  104  for a calculation of hysteresis. Complementarily, if the current steering wheel turning direction is counter-clockwise, the information flows along line  106  toward block  108  wherein an accumulated counter-clockwise zero crossing data is determined. A known technique in filtering data may be used at this juncture. For example, a filtering technique using exponentially weighted moving average may be used. The information then flows through line  110  toward block  104  for a calculation of hysteresis. A hysteresis value is determined as a difference between a clockwise zero torque cross point and a counter-clockwise zero torque cross point. The calculated hysteresis information then flows along line  112  into block  114  wherein, together with the input values from block  82  that has branched into line  88 , an output is calculated. The output may be used to adjust or create a compensation by increasing a return-to-center command or increasing an amount of assist to offset the hysteresis. The output then flows out along an output line  116 . 
     Referring to FIG. 4, reference numeral  120  generally designates an ideal hysteresis loop. The horizontal co-ordinate denotes steering wheel position measured in degrees as shown. The vertical co-ordinate denotes steering wheel torque measured in Newton Meters as shown. If no hysteresis exists, lines  122  and  124  collapse into a straight line (not shown). Because of the existence of wear and other factors, hysteresis occurs. Assuming that the wear and other factors are symmetrical, the instant loop curve may be drawn. When a clockwise turning occurs, the line  124  moves toward an upper right-hand direction wherein a zero cross point  126  is determined. Symmetrically, when a counter-clockwise turning occurs, the line  122  moves toward a lower left-hand direction wherein a zero cross point  128  is determined. A hysteresis is defined as the difference of the zero cross point  128  and the zero cross point  126 . 
     It will be understood that a person skilled in the art may make modifications to the preferred embodiment shown herein within the scope and intent of the claims. While the present invention has been described as carried out in a specific embodiment thereof, it is not intended to be limited thereby but intended to cover the invention broadly within the scope and spirit of the claims.