Abstract:
Methods and apparatus are provided for automatically determining the steering system calibration of a vehicle in accordance with vehicle loading. The apparatus comprises a combination of a load monitor and a steering controller. The load monitor senses the actual loading of the vehicle and provides a corresponding feedback signal to the steering controller. The steering controller determines a steering calibration appropriate for the load represented by the feedback signal. As a result, the handling performance of the vehicle can be improved over that of a single, fixed steering calibration for a wide range of load conditions.

Description:
TECHNICAL FIELD 
     The present invention generally relates to vehicle steering systems, and more particularly relates to the control of vehicle steering under different vehicle load conditions. 
     BACKGROUND 
     Four wheel steering systems in motor vehicles typically involve a steering control system for the vehicle rear wheels in addition to the conventional steering control of the vehicle front wheels. Rear wheel steering control is typically used, for example, to reduce vehicle yaw (i.e., deviation from the intended course) in lane change maneuvers. Rear wheel steering control can also increase the stability of a loaded vehicle, such as a vehicle towing a trailer, by reducing trailer sway. The widespread use of higher center of gravity vehicles in recent years has led to the further development of steering systems that control the handling of this type of vehicle. 
     Variables such as vehicle speed and axle loading can directly affect the type of steering control that is generally desired for safe and efficient handling of a vehicle. Therefore, four wheel steering (4 ws) systems are typically calibrated for different load and speed conditions. That is, one steering control calibration can be set for a normal load category, such as curb weight, and another steering control calibration can be set for a tow load category, such as a trailer towing condition. 
     However, there are typically significant loading variations within each calibration category. For example, the normal load category may have a weight variation of approximately 2,000 pounds, while the tow load category may include a trailer weight range of approximately 500 to 10,000 pounds. In order to accommodate load variations such as these within a calibration category, 4 ws systems may have their steering calibrations adjusted in order to obtain a performance compromise that can adequately cover the load extremes of that calibration category. For example, the steering calibration for the normal load category may be tuned for reasonably good high speed handling capability at the high weight end of the range, and for acceptable steering sensitivity at the low weight end of the range. Similarly, tuning compromises may also be made in the tow loading category, in order to provide a reasonably satisfactory steering calibration for the wide range of trailer weights, or other towed loads, such as boats and the like. 
     For a driver, however, the selection of an appropriate steering calibration category (normal load or tow load) may not be obvious in certain types of situations. For example, if the driver wants to tow a light trailer, he may find that the normal load calibration category actually provides better steering performance under certain driving conditions than the tow load calibration category. On the other hand, the selection of normal load steering for a towing situation may not provide some of the vehicle handling capabilities that would otherwise be available with the selection of the tow load category. 
     Accordingly, it is desirable to provide an improved steering system that incorporates the actual loading condition of the vehicle in the steering calibration process, in order to achieve a more optimal degree of steering control. In addition, it is desirable to provide the improved steering system capabilities with minimal impact on the system hardware requirements. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY 
     According to various exemplary embodiments, devices and methods are provided for improving the calibration of rear wheel steering systems for vehicles. One exemplary method comprises the steps of:
         a) sensing the loading of the vehicle;   b) generating a feedback signal corresponding to the sensed loading of the vehicle; and   c) determining a steering system calibration for the vehicle based on the feedback signal.       

     The loading of the vehicle is generally sensed with either height or pressure sensors located strategically in the rear of the vehicle, or in the front and rear of the vehicle. The calibration of the rear wheel steering system is typically based on a relationship between the rear wheel angle-to-front wheel angle ratio and the vehicle speed. The calibration may be further refined for improved steering sensitivity through the use of a hand wheel angle multiplier factor. 
     One exemplary device comprises a vehicle rear wheel steering system with multiple calibration curves, based on vehicle loading. A load sensing device is typically configured to generate a load feedback signal corresponding to the actual loading of the vehicle. Concurrently, a speed sensing device is typically configured to generate a speed feedback signal corresponding to the actual speed of the vehicle. A steering controller is typically configured to receive the load feedback signal from the load sensing device and to determine an appropriate rear wheel steering calibration in accordance with the received load feedback signal. This rear wheel calibration generally takes the form of a plot of rear wheel angle-to-front wheel angle ratio versus vehicle speed. In addition, a hand wheel angle multiplier calibration can also be applied to the rear wheel angle calibration to improve steering sensitivity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a block diagram of an exemplary embodiment of a vehicle steering control system; 
         FIG. 2  is a block diagram of an exemplary embodiment of a vehicle leveling and steering control system; 
         FIG. 3  is an illustration of an exemplary vehicle with zero angle front and rear wheel steering; 
         FIG. 4  is an illustration of an exemplary vehicle with positive angle front wheel steering and negative angle rear wheel steering; 
         FIG. 5  is an illustration of an exemplary vehicle with positive angle front wheel steering and positive angle rear wheel steering; 
         FIG. 6  is a graph of exemplary four wheel steering calibration curves for a normal category range and a tow category range; 
         FIG. 7  is a graph of exemplary four wheel steering calibration curves for a lightly loaded normal category and a heavily loaded normal category; 
         FIG. 8  is a composite graph of the exemplary four wheel steering calibration curves of  FIG. 6  and  FIG. 7 ; 
         FIG. 9  is a graph of exemplary calibration curves for a vehicle hand wheel angle multiplier versus the vehicle hand wheel angle. 
         FIG. 10  is a flow diagram of an exemplary method of controlling the rear wheel angle of a vehicle; and 
         FIG. 11  is a flow diagram of an exemplary method of calculating the rear wheel angle of a vehicle. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Various embodiments of the present invention pertain to the area of calibrating the steering system of a vehicle in order to provide a driver with safe and responsive handling characteristics. For example, four wheel steering control systems are typically calibrated for a specific vehicle load category, such as normal load (e.g., curb weight plus passengers and cargo) or tow load (e.g., towing a trailer). However, vehicle loads can vary over a wide weight range within each load category, and especially in the tow category. As such, the steering calibration for a selected load category is typically adjusted to provide a compromise steering control characteristic that can reasonably accommodate the loading extremes within that load category. The exemplary embodiments disclosed herein, however, provide for an improved steering control system with multiple steering calibrations based on the actual loading of the vehicle. The exemplary steering control system typically includes a load monitor that provides actual load information to a steering controller, which then determines an appropriate steering calibration within a selected load category, based on the actual load information. 
     According to a simplified exemplary embodiment of a four wheel steering (4 ws) system  100 , as shown in  FIG. 1 , a vehicle  102  contains a front wheel steering gear  104  connected to front wheels  106 , and a rear wheel steering gear  108  connected to rear wheels  110 . A steering wheel  112  controls the front wheel steering gear  104 , and thereby front wheels  106 , and a steering sensor  114  provides steering wheel angle data (herein designated as hand wheel angle, or HWA) to a rear wheel steering controller  116 . Steering controller  116  is configured to control the action of rear wheel steering gear  108  via any suitable type of drive mechanism  117 , and to thereby control the angle of the rear wheels  110 , designated herein as rear wheel angle (RWA). A rear wheel steering sensor  118  provides RWA feedback data to steering controller  116 . 
     Steering controller  116  also receives actual load data from load sensors  122 , which are associated with load bearing devices  124 . While two load sensors  122  and two load bearing devices  124  are shown in this embodiment of the present invention, it will be appreciated that any appropriate number of load sensors and load bearing devices may be used in different embodiments. Load bearing devices  124  may be in the form of springs or shock mounts, or any type of shock absorbing or springing device. Load sensors  122  may be in the form of height sensors or pressure sensors, or any type of sensor capable of transmitting an output signal proportional to a load quantity. In addition, a vehicle speed sensor  126  provides actual vehicle speed data to steering controller  116 . 
     Steering controller  116  may typically include, but not be limited to, a processor  120 , a digital storage memory  121 , various input/output signal interfaces and the like, and any appropriate number of pre-programmed algorithms and/or calibration curves that may be stored within memory  121 . Processor  120  may be any type of microprocessor, microcontroller or other computing device capable of executing instructions in any computing language. 
     Another exemplary embodiment of a four wheel steering system  200  is shown in  FIG. 2 . In this embodiment, a ride controller  202  typically receives actual load data from sensors  122 , and provides an actual load signal to controller  116 . Ride controller  202  typically uses the actual load data from sensors  122  to provide a type of leveling control for vehicle  102 . The leveling control may take the form of air pressure (pneumatic) or fluid (hydraulic) feedback to load bearing devices  124  via an air compressor or a pump device  204 . Ride controller  202  typically receives vehicle speed data from vehicle speed sensor  126  as well as load data from sensors  122  and HWA data from steering sensor  114 , and is typically configured to generate an appropriate leveling output signal to compressor/pump device  204 , based on the actual loading, speed, and HWA of vehicle  102 . 
     A basic function of four wheel steering for most cars and trucks is the control of rear wheel steering in relationship to the steering of the front wheels. As previously indicated, rear wheel control can enhance the safety and handling performance of a vehicle under conditions such as lane changing and trailering. The relationship between the front wheel angle (FWA) and the rear wheel angle (RWA) is generally designated as the ratio of rear wheel angle-to-front wheel angle, or R/F ratio. This relationship is illustrated in  FIGS. 3-5 , as described below. 
     In  FIG. 3 , a vehicle  302  is equipped with front wheels  304  and rear wheels  306 . In this example, all four wheels are aligned with the longitudinal vehicle axis, such that the difference between the FWA and the RWA is equal to zero degrees. 
     In  FIG. 4 , vehicle  302  is shown in a typical low speed (e.g., below approximately 40 kilometers per hour) configuration. In this example, the FWA is shown as “a” and the RWA is shown as “b”. At low speeds, a four wheel steering controller, such as controller  116  in  FIGS. 1 &amp; 2 , typically controls the RWA so that it is opposite to the FWA direction. That is, angle a can be considered positive in this example, relative to the vehicle longitudinal axis, while angle b is negatively oriented with respect to the vehicle longitudinal axis. Moreover, angle b is typically limited by a four wheel steering controller to a value less than that of angle a. For example, at a speed of approximately 30 kph, a typical R/F ratio (b/a) might be equal to −0.2, where the minus sign indicates the negative orientation of angle b with respect to angle a. If angle a in the  FIG. 3  example is equal to 12 degrees, angle b would be set at (12*−0.2), or −2.4 degrees. 
     In  FIG. 5 , vehicle  302  is illustrated in a typical medium-to-high speed configuration, i.e., at a speed above approximately 40 kph. In this example, both the FWA (a) and the RWA (b) are positively oriented relative to the vehicle longitudinal axis. For an approximate speed of 80 kph, a typical R/F ratio might be equal to 0.3. As such, a FWA of 12 degrees would typically be processed by a four wheel steering controller to a RWA of 3.6 degrees. 
     As previously noted in the Background section, four wheel steering systems typically have one steering calibration for the normal load category, and a separate and different steering calibration for the tow category.  FIG. 6  is a graphical illustration of typical calibration curves for these two categories. For the tow category, calibration curve  602  represents a continuum of pre-programmed relationships between the R/F ratio and the measured speed of vehicle  102 . Point c on curve  602  indicates the crossover point where the RWA changes from negative to positive as the speed increases beyond the approximate value of 37 kph. In similar fashion, point d on normal category calibration curve  604  represents the crossover point for a normal load category R/F ratio at about 47 kph. 
     Calibration curves  602  and  604  in  FIG. 6  represent compromise calibrations that are designed to accommodate anticipated load variations within each load category, as previously described in the background section. As such, the steering control experienced by a driver will generally be adequate with this type of compromise calibration, but will not necessarily be optimal for different loading conditions within that load category. 
     To overcome the potential limitations of this type of compromise steering calibration, the exemplary embodiments described herein take into account the actual loading of a vehicle. That is, the actual load value is introduced into a four wheel steering controller in order to determine a more appropriate calibration curve related to the actual load value, rather than the “one-size-fits-all” compromise calibration curve. For example, in the embodiment illustrated graphically in  FIG. 7 , two calibration curves ( 702  &amp;  704 ) may be pre-programmed into the memory of a steering controller (e.g., into memory  121  in steering controller  116  in  FIGS. 1 &amp; 2 ) for the normal load category. Curve  702  represents an optimally tuned calibration for the heavily loaded portion of the normal category load range, and curve  704  represents an optimally tuned calibration for the lightly loaded portion of the normal category load range. 
     Therefore, with reference to the  FIG. 1  embodiment, when steering controller  116  receives actual load data from load sensors  122 , processor  120  within controller  116  can select an appropriate one of either calibration curve  702  or  704  from memory  121 . Alternately, processor  120  can be configured to generate a load-related calibration curve, such as  702  or  704  or the like, based on one or more pre-programmed algorithms stored in memory  121 . In a similar manner, ride controller  202  ( FIG. 2 ) can receive inputs from load sensors  122 , and can then provide the actual load data to controller  116  to enable the selection or determination of an appropriate load-related calibration curve. 
     For comparison purposes, compromise calibration curves  602  (tow category) and  604  (normal category) are shown together with optimally tuned calibration curves  702  (heavily loaded normal category) and  704  (lightly loaded normal category) in  FIG. 8 . For clarity, optimally tuned calibration curves are not shown for the tow category, but could be developed in the same manner as previously described for the normal category. A series of calibration curves for the tow category might be particularly advantageous due to the typically wide range of trailer weights, as well as other towed objects. 
     A secondary effect of the type of rear wheel steering control described herein is the desensitization of the hand wheel steering response in the normal load category, and especially at low speeds. To compensate for this type of hand wheel responsiveness reduction, a hand wheel angle (HWA) multiplier can be applied to the rear wheel steering control process in such a manner as to enhance the steering wheel control sensitivity. That is, HWA multiplier calibration curves can be used to more closely simulate the handling responsiveness of two wheel steering in the four wheel steering process, as will be explained below. 
     Exemplary HWA multiplier curves are illustrated in  FIG. 9 , where the HWA multiplier is plotted on the y-axis and the steering wheel angle (HWA) is plotted on the x-axis. In this example, the normal category is shown to have HWA multiplier curves  904 ,  906 ,  908 . Also in this example, the tow category is shown to have a multiplier line  902  that is constant at a value equal to one. However, alternate embodiments of the present invention may have multiple tow category curves in the same manner as those shown for the normal category. 
     In the  FIG. 9  example, curve  906  represents a compromise HWA multiplier calibration for the normal category, while curves  904  and  908  represent optimally tuned multiplier calibrations for heavily loaded and lightly loaded portions of the normal category load range, respectively. The manner in which the HWA multiplier is typically applied to the steering control process will be covered in more detail in conjunction with the flow diagrams of  FIGS. 10 and 11 , which depict the methodology of the exemplary embodiments heretofore disclosed. 
     In  FIG. 10 , an exemplary steering control method  1000  starts with an initialization step  1002 . This step represents the pre-programmed status of processor  120  and memory  121  in steering controller  116 . That is, the type of R/F and HWA calibration curves and/or calibration algorithms previously described are typically pre-programmed into processor  120  and memory  121  within controller  116 . 
     In step  1004 , a load category is selected, e.g., normal or tow. The load category can generally be selected by driver activation of an appropriate control button in the passenger cabin of a vehicle. While the load category selection is typically performed by the driver in the exemplary embodiments described herein, it is also possible that alternate embodiments might be configured to have the load category enabled automatically by processor  120 , based on the measured value of the load, and/or on any other relevant parameters. 
     In step  1006 , the vehicle  102  load is measured, either directly by signals from sensors  122  to controller  116  ( FIG. 1 ) or via ride controller  202  ( FIG. 2 ). Controller  116  can optionally output the measured load value to a display (step  1008 ), so that the driver can be made aware of the extent of vehicle loading. 
     In step  1010 , controller  116  can select or generate an appropriate R/F calibration curve for the load category previously selected, based on the measured value of the load (step  1006 ). Controller  116  can also select or generate an appropriate HWA multiplier calibration curve, based on the measured value of the load. For example, referring back to the exemplary embodiments of  FIGS. 7 and 9 , controller  116  would typically select pre-programmed R/F calibration curve  704  for a lightly loaded normal category, and would also typically select pre-programmed HWA multiplier calibration curve  908  for a lightly loaded normal category. 
     In step  1012 , the processor within controller  116  can calculate a desired rear wheel angle (RWA), as will be described in detail in  FIG. 11 , based on the previously selected calibration curves (step  1010 ), as well as on other parameters. 
     In step  1014 , controller  116  can adjust rear wheel steering gear  108  ( FIGS. 1 &amp; 2 ) via drive mechanism  117  to direct rear wheels  110  to turn to the desired RWA. 
       FIG. 11  is an exemplary flow diagram that illustrates the intermediate steps that are typically taken to implement the RWA calculation in step  1012  of  FIG. 10 . Referring again to the embodiments illustrated in  FIGS. 1 &amp; 2 , the steering wheel angle (HWA) is typically measured by steering wheel sensor  114  and entered into processor  120  within controller  116  (step  1102 ). Processor  120  will typically calculate the front wheel angle (FWA) from the HWA value, usually based on a predetermined relationship stored in memory  121  of controller  116  (step  1104 ). In step  1106 , the actual vehicle speed can be determined by processor  120 , based on the vehicle speed signal received by processor  120  from vehicle speed sensor  126 . 
     In step  1108 , processor  120  can “look up” the R/F ratio that corresponds to the vehicle speed determined in step  1106 . For example, at a vehicle speed of 70 kph, the R/F ratio for a lightly loaded normal category (curve  704  in  FIG. 7 ) would be equal to 0.2. Calibration curve  704 , as well as the other calibration curves, may be stored in the form of look up tables in memory  121  of controller  116 , or in any other suitable manner. 
     In step  1110 , processor  120  can calculate a rear wheel angle (RWA 1 ), based on the following relationship:
 
 RWA   1 =( FWA )*( R/F )  (Equation 1)
 
     For example, if the FWA calculated in step  1104  were equal to 12 degrees, the calculated RWA 1  would equal 2.4 degrees, based on the previously determined R/F ratio of 0.2. 
     At this point, controller  116  could implement RWA 1 , as indicated in step  1014  in  FIG. 10 . Alternately, if controller  116  included HWA multiplier calibration curves, as previously described in  FIG. 9 , the exemplary flow diagram of  FIG. 11  can proceed to step  1112 . 
     In step  1112 , processor  120  can look up the HWA multiplier corresponding to the HWA previously determined in step  1102 . For example, if the HWA were equal to 30 degrees, a corresponding HWA multiplier would be approximately 0.3 for a lightly loaded normal category, as indicated by exemplary calibration curve  908  in  FIG. 9 . 
     In step  1114 , processor  120  can use the HWA multiplier obtained in step  1112  to calculate a modified rear wheel angle (RWA 2 ) as follows: 
                           RWA   2     =       ⁢       (   FWA   )     *     (     R   /   F     )     *     (     HWA   ⁢           ⁢   multiplier     )                   =       ⁢       (     RWA   1     )     *     (     HWA   ⁢           ⁢   multiplier     )                     (     Equation   ⁢           ⁢   2     )               
For the previously calculated RWA 1  value of 2.4 degrees, the modified rear wheel angle calculation for RWA 2  would equal (2.4)*(0.3), or 0.72 degrees.
 
     Therefore, the calculated result of either step  1110  (Equation 1) or step  1114  (Equation 2) is represented by step  1012  in  FIG. 10 , and can be implemented accordingly in step  1014 . 
     Accordingly, the shortcomings of the prior art have been overcome by providing an improved rear wheel steering control in a vehicle steering system. The limitations of a single compromise rear wheel steering calibration curve for a given load category are overcome by providing multiple calibration curves to represent different levels of vehicle loading within the load category. The actual vehicle load is measured, and this load value is fed back to the steering controller. The steering controller then selects (or generates) an appropriate calibration curve for the measured load value. As such, the load-related calibration curve can provide better steering and handling responsiveness as compared to that of a single compromise calibration curve for a load category. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.