Abstract:
A method for detecting an erroneous travel direction or a defective yaw rate sensor  12  is disclosed. Two estimates of the vehicle yaw rate are gathered from separate criteria and are compared with one another as well as the measured vehicle yaw rate. Using these comparisons along with other information regarding vehicle travel, a conclusion of whether a travel direction signal is erroneous is made. This determination can then be used to modify the vehicle&#39;s brake control strategy.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a method of detecting an erroneous signal. More particularly, the present invention relates to a method for detecting an erroneous direction of travel signal as used in a motor vehicle yaw control system. 
     2. Disclosure Information 
     Automotive vehicles with braking systems which respond to vehicle conditions as well as driver input have been produced. For example, when a particular yaw rate that is desired by a driver&#39;s steering wheel operation is not producing an adequate yaw rate, the braking system of the vehicle may compensate by altering a particular wheel&#39;s speed. However, when a vehicle is traveling reverse its normal operation direction, this response may not be appropriate. For this reason, it is necessary to provide the brake system with some indication of vehicle travel direction, e.g. forward or reverse. Further, it would be desirable to provide a system capable of robustly detecting an erroneous travel direction indication. 
     U.S. Pat. No. 5,686,662 (&#39;662) addresses this problem by gathering two estimates of yaw rate, one estimate from the speeds of the left and right undriven wheels, and the other estimate from vehicle speed, steering wheel position, and lateral acceleration. A yaw rate is then measured using a yaw rate sensor. Finally a reverse travel condition is indicated if the signs of the first two estimates are opposite that of the measured yaw rate. However, under various operating conditions, the system described in &#39;662 may have robustness difficulties. For instance, the correction coefficient, ω c , is unknown at vehicle start-up, and thus for an indeterminate period of initial vehicle operation, one of the two estimates of vehicle yaw rate can not be independently determined in a manner which will allow robust detection of a reverse travel direction. Also, the &#39;662 patent assumes that a difference in signs between the estimates and the measured yaw rate indicate a reverse travel direction, whereas such an instance could occur due to a yaw sensor fault. 
     It would be desirable to provide a method for indicating an erroneous travel direction that is robust to faults and disturbances of the wheel speed sensor, steering wheel sensor, and yaw rate sensor, operative from the moment of vehicle start-up. 
     SUMMARY OF THE INVENTION 
     There is disclosed herein a method for detecting an erroneous direction of travel signal for use in actively controlling the yaw of a motor vehicle. In order to detect an erroneous travel direction, the present invention first establishes two yaw rate estimates using different data to calculate each estimate. The present invention then calculates the difference between these two estimates, and halts the analysis if this difference is greater than a predetermined threshold. However, if this difference is less than a predetermined threshold, the present invention continues by calculating the difference between the measured yaw rate of the yaw rate sensor and one of the yaw rate estimates. Finally, if this difference is greater than a predetermined threshold, an erroneous travel direction is indicated. If this difference is less than a predetermined threshold, normal operation is indicated by the present invention, with no erroneous travel direction being indicated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of a control system in accordance with the present invention. 
     FIG. 2 is a logic flow diagram for a yaw control system adapted to use the information provided by the present invention. 
     FIG. 3 is a logic flow block diagram of a method for detecting an erroneous direction of travel signal in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a schematic block diagram of a brake control system is illustrated. The system includes a brake controller  8  that receives data from a steering wheel sensor  10 , a yaw sensor  12 , and wheelspeed sensors  14  for performing its analysis. Brake controller  8  ultimately uses this data to detect whether the travel direction signal indicated by a transmission sensor  16 , such as a gear position indicator, is erroneous. The brake controller  8  uses this data to calculate two estimates of yaw rate, r est.whlspd , r est.swa , which can be used to detect an error in the travel direction signal from the transmission sensor  16 . Based on this information, the Brake Controller determines the appropriate brake control  20  and actuates brakes  24  and possibly a driver display  22 , such as a warning lamp. 
     The first yaw rate estimate, r est.whlspd , is calculated using wheel speed data, according to the following relationship: 
     
       
           r   est.whlspd =( ws   R   −ws   L )/ TW   
       
     
     where ws R  and ws L  are preferably right and left undriven wheel speeds and TW is a vehicle track width at an axle of the undriven wheels. This relationship provides a magnitude of the first yaw rate estimate and relies on the travel direction signal from the transmission, in combination with the sign of r est.whlspd , to determine whether the yaw estimate is clockwise or counter-clockwise. 
     Similarly, the brake controller calculates the second yaw rate estimate, r est.swa , using the steering wheel angle data, SWA according to the following relationship: 
     
       
           r   est.swa   =D*SWA*V   VEH /( L+K*V   VEH   2 ) 
       
     
     where D is a constant determined through vehicle testing; V VEH  is an unsigned longitudinal velocity of the vehicle; K is an understeer constant for the vehicle; and L is the vehicle wheelbase. As with the wheel speed based estimate, this relationship provides only a magnitude of the second yaw rate estimate, requiring the travel direction signal from the transmission, in combination with the sign of r est.swa , to determine whether the estimate is clockwise or counter clockwise. 
     Turning now to FIG. 2, a logic flow diagram of the erroneous travel direction algorithm will now be described. First the brake controller calculates the value of a threshold, T. In the preferred embodiment, this threshold is a variable, dependent on vehicle longitudinal velocity and the steering wheel angle. It has been observed that this allows greater precision in the analysis for detecting an erroneous travel direction signal. Specifically, the threshold is calculated according to the following relationship: 
     
       
         
           T=A+V 
           VEH 
           /B+C*r 
           est.whlspd 
           ,*SWA 
         
       
     
     where V VEH  is the vehicle longitudinal velocity determined from the speed of rotation of the wheels, SWA is the steering wheel angle and r est.whlspd , is a vehicle yaw estimate based on wheel speeds described in detail above. A, B and C are constants selected through testing in a development vehicle. It should be noted that it may be desirable to compare the variable threshold, T, with a predetermined threshold value, T 1 , so that the brake controller returns to starting block  40  when T exceeds T 1 . This would reflect reduced confidence in error detection based on the estimates of yaw rate determined during normal operation of the vehicle at high speeds and large steering wheel angles. The estimates might be biased due to sizable slip angles at the vehicles tires as well as uncertainty about the size and shape of the vehicle tires at high speeds. 
     Proceeding to block  42 , the brake controller calculates a first difference, Δest, which represents the absolute value of the difference between the two independent estimates of the yaw rate, r est.whlspd  and r est.swa . brake controller then compares Δest with the current threshold value, T. If Δest is not less than T, then r est.swa  and r est.whlspd  are not in agreement with one another, indicating that either r est.swa  or r est.whlspd  is an inaccurate estimate of yaw rate. Under this condition, the brake controller halts further error detection analysis, returning through block  58  to block  40  to restart the analysis. The brake controller will continue in this manner until Δest is less than T, indicating that r est.swa  and r est.whlspd  are essentially in agreement with one another, at which point the brake controller continues to block  46 . 
     At block  46 , the brake controller calculates a second difference, Δmeas, which represents the absolute value of the difference between a measured yaw rate of the vehicle, r meas , and one of the two independent yaw rate estimates. In the preferred embodiment, the first yaw rate estimate based on wheel speed data is preferred for determining Δmeas. This preference is due to the likelihood that the yaw rate estimate based on wheelspeed is more precise than that based on steering wheel angle. The brake controller then compares Δmeas with the current threshold value, T. If Δmeas is not greater than T, then r meas  and r est.whlspd , are in agreement with one another, and the brake controller proceeds to block  48  to set the travel direction signal fault false, indicating that that the vehicle travel direction signal is correctly indicated by the transmission. Support for this conclusion rests in the observation that when r est.swa  and r est.whlspd  are in agreement with one another, based on the travel direction signal from the transmission, and r meas  and r est.whlspd , are in agreement with one another, then indicated travel direction is correctly detected. If, however, Δmeas is greater than T, then r meas  and r est.whlspd , are not in agreement and the brake controller proceeds to block  50  to further evaluate the possibility of an erroneous travel direction signal. 
     At block  50 , the brake controller calculates a third difference, Δmeas.rev, which actually represents the absolute value of the sum of the measured yaw rate, r meas , and one of the independent yaw rate estimates. Again, in the preferred embodiment, the first yaw rate estimate, r est.whlspd , is preferred for determining Δmeas.rev. The sum of these two values represents the difference between the measured yaw rate, r meas , and the yaw rate estimate, r est.whlspd , subject to an assumption that the vehicle is actually traveling in the opposite direction than indicated by the transmission. Therefore, if the third difference, Δmeas.rev, is less than T, the brake controller concludes that the travel direction signal indicated by the transmission is in error, as indicated by block  56 . If, however, Δmeas.rev is not less than the T, the brake controller proceeds to block  52  and sets a yaw signal fault flag true. 
     More specifically, the logic of block  50  is supported by the observation that the presence of an erroneous travel direction signal indicated by the transmission causes a properly functioning yaw rate sensor to indicate a yaw rate of the opposite sign and similar magnitude to either of the yaw rate estimates. For example, in this instance, if the yaw rate estimates established by both the steering wheel angle sensor and the wheel speed sensors indicate a clockwise rotation of the vehicle, the properly functioning yaw rate sensor would indicate a counterclockwise rotation. Therefore, when this point in the control logic is reached and Δmeas.rev is not greater than T, the brake controller can reasonably conclude that the travel direction signal indicated by the transmission is erroneous. This being the case, the brake controller proceeds to block  56  where the travel direction signal fault is set to true and the brake controller returns to block  40  through return block  58 . 
     If, on the other hand, Δmeas.rev is not less than the T, the brake controller concludes that measured yaw rate sensor is faulty. This is supported by the deduction that the independent estimates are in agreement and the discrepancy between the estimates and the measured yaw rate is not a reverse direction condition, but rather a disagreement in actual magnitude, which strongly points to a yaw rate sensor fault in the face of two independent estimates being in agreement with respect to magnitude. This being the case, the brake controller proceeds to block  52  where a yaw sensor fault flag is set to true and the brake controller returns to block  40  through block  54  and return block  58 . 
     Referring now to FIG. 3, a logic flow diagram of a brake control system adapted to use the information derived from the present invention is shown. The brake controller  8  executes the logic flow found in FIG. 3, beginning at block  70  and proceeds to block  72 . If an erroneous travel direction has been detected, as indicated by the travel direction signal fault being set to true, the brake controller proceeds to block  74  where any yaw control system the vehicle might possess is disabled. In this instance, the brake controller further proceeds to block  75 , where the driver display is activated appropriately. The brake controller then returns to block  70  through block  80 . 
     If, however, no erroneous travel direction has been detected, as indicated by the travel direction signal fault flag being set to false, the brake controller proceeds to block  76 . At block  76 , if a yaw signal fault has been indicated, the brake controller proceeds to block  78  where the driver of the vehicle is notified as to the faulty condition of the vehicle&#39;s yaw rate sensor, through some type of warning light or sound. Further, the vehicle yaw control system is disabled at block  78 . From either block  78 , or block  76  when no yaw signal fault has been detected, the brake controller returns to block  70  through return block  80 . 
     Various modifications and variations will no doubt occur to those skilled in the arts to which this invention pertains. For example, the particular sensors used in conjunction with the disclosed system may be varied from those herein, as there are numerous possible methods for measuring or estimating the yaw rate of a vehicle. Additionally, the system may be operated with changes to the numerical values of the various thresholds described above while remaining within the calculational and logic flow scheme described herein. These and all other variations which basically rely on the teachings to which this disclosure has advanced the art are properly considered within the scope of this invention as defined by the appended claims.