Abstract:
The duration and average deceleration of a vehicle wheel are measured during a wheel speed departure and compared to duration and deceleration thresholds to determined if a wheel sneakdown condition exists for the wheel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Nos. 60/070,045, filed Dec. 30, 1997, and is a continuation of PCT Patent Application No. PCT/US98/27,813 filed on Dec. 30, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates in general to algorithms for anti-lock brake systems and in particular to a control algorithm for detecting wheel speed sneakdown on a low mu surface. 
     Braking a vehicle in a controlled manner under adverse weather conditions, such as rain, snow or ice, generally requires precise application of the vehicle wheel brakes by the vehicle operator. Under these conditions, or in panic stop situations, a driver will often apply excessive brake pressure which causes the vehicle wheels to lock-up such that excessive slippage between the wheels and the road surface takes place. Wheel lock-up conditions can lead to loss of directional stability and, possibly, uncontrolled vehicle spinout. Accordingly, an Anti-lock Brake System (ABS) is often included as standard or optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of the vehicle wheel brakes to prevent lock-up of the associated vehicle wheels. One type of ABS controls only the rear vehicle wheel brakes. Such a system is referred to as a RWAL in the following description. 
     A typical prior art RWAL system  10  is illustrated in FIG.  1 . The RWAL system  10  includes a normally open solenoid valve  22  connected between the vehicle master cylinder  14  and the controlled rear wheel brakes  20   a  and  20   b . When actuated, the normally open solenoid valve  22  closes to isolate the rear wheel brakes  20   a  and  20   b  from the master cylinder  14 . Accordingly, the normally open solenoid valve  22  is referred to below as an isolation valve. The isolation valve  22  also can be selectively opened to increase the pressure at the rear wheel brakes  20   a  and  20   b . The RWAL system  10  also includes a normally closed solenoid valve  26 , which is referred to below as a dump valve. The dump valve  26  is selectively opened to reduce the pressure at the rear wheel brakes by bleeding brake fluid from the rear wheel brakes  20   a  and  20   b  to an accumulator  28 . The isolation and dump valves  22  and  26  are mounted within a control valve  21 . 
     The vehicle brake system master cylinder  14  provides a source of pressurized hydraulic brake fluid to the RWAL system  10 . Thus, a separate hydraulic source, such as a motor driven pump, which is usually included in a four wheel ABS, is not needed. This reduces the complexity and cost of manufacturing the RWAL system  10 , which is typically referred to as a passive system. The RWAL system  10  further includes an electronic control module  30  which is electrically connected to a wheel speed sensor  40  and to the isolation and dump valves  22  and  26 . The control module  30  can be mounted directly upon the control valve  21  or located remotely therefrom. 
     The control module  30  includes a microprocessor (not shown) which is programmed to control the RWAL system in accordance with a control algorithm and parameters permanently stored in a Read Only Memory (ROM). Typically, the control algorithm is trimmed for the particular vehicle in which the ABS is installed. The microprocessor also can access a Random Access Memory (RAM) for temporary storage and retrieval of data. A detailed description of the RWAL system  10  illustrated in FIG. 1 is included in U.S. Pat. Nos. 4,790,607 and 4,886,322. 
     During vehicle operation, the microprocessor in the ABS electronic control module  30  continuously receives speed signals from the wheel speed sensor  40 . During a vehicle braking cycle, the ABS microprocessor monitors the rear wheel speed and deceleration. The microprocessor calculates a theoretical speed ramp, which represents the speed the vehicle would travel if decelerated at a predetermined maximum rate, such as, for example, 1.0 g. The microprocessor compares the actual rear wheel speed to the theoretical ramp. If the rear wheel deceleration reaches a predetermined value, such as, for example, 1.3 g, the microprocessor determines that the rear wheel brakes  20   a  and  20   b  may be approaching a rear wheel lock-up condition. Accordingly, the ABS microprocessor closes the isolation valve  22  to isolate the rear wheel brakes  20   a  and  20   b  from the master cylinder  14 . If the rear wheel speed departs form the theoretical ramp in addition to, or in place of, the deceleration condition, the ABS microprocessor determines that the rear wheel brakes  20   a  and  20   b  are certainly approaching a lock-up condition and the microprocessor maintains the isolation valve  22  in the closed position. The ABS microprocessor then selectively opens the dump valve  26  to reduce the pressure applied to the rear wheel brakes  20   a  and  20   b  to correct the rear wheel speed departure. Once the wheel speed departure has been corrected and the controlled wheel has spun up to the vehicle speed, the microprocessor opens the isolation valve to initiate a second wheel speed departure to adjust the rear wheel brake pressure upward. 
     The operation of the RWAL system is illustrated by the graphs shown in FIG.  2 . The upper curve shows the rear wheel speed as a function of time while the lower curve shows the rear wheel brake pressure as a function of time. The middle curves illustrate the operation of the isolation and dump valves  22  and  26  as a function of time. The solid curve labeled  60  represents the velocity of the rear wheels while the dashed curve labeled  64  represents the vehicle velocity. The first and second wheel speed departures are labeled  60   a  and  60   b , respectively. Following correction of the second wheel speed departure, which occurs at time t 7 , the rear wheel brake pressure is maintained a constant level P e , as shown in the lower curve. 
     If the vehicle transitions from a low mu to a high mu road surface, a key feature included in the algorithm utilized by the RWAL system  10  is that the braking effort exerted by the rear wheel brakes  20   a  and  20   b  can be increased to utilize the increased mu. An example of such a transition is shown at t 8  in FIG.  2 . The transition can be detected by monitoring the deceleration of rear wheels which can increase due to the greater braking effect of the uncontrolled front wheel brakes  19   a  and  19   b  upon the higher mu road surface. If the rear wheel deceleration increases by a predetermined amount, such as 1.0 g, the microprocessor assumes that the change is due to the road surface transition and reopens the isolation valve  22  to generate an unlimited series of reapply pulses  62   b . The resulting increased pressure to the rear wheel brakes  20   a  and  20   b  initiates a third wheel speed departure, which is labeled  60   c  in FIG.  2 . At time t 10 , a dump pulse is generated to open the dump valve  26  to reduce the rear wheel brake pressure to a level P g  to correct the third rear wheel departure. Thereafter, the rear wheel brake pressure is held at the level P g , which is greater than the previously held level P e . 
     SUMMARY 
     This invention relates to an improved control algorithm for an anti-lock brake system which detects wheel speed sneakdown on a low mu surface. 
     During an anti-lock brake cycle, it is possible for the rear wheel speed to follow an overall trajectory approaching 1.0 g even though the friction coefficient of the road surface may be in the neighborhood of only 0.1. This condition is often referred to as wheel speed sneakdown. Wheel speed sneakdown can occur gradually or following a wheel speed excursion. An example of wheel speed sneakdown occurring following a wheel speed excursion is shown in FIG. 3 where the solid line represents the rear wheel speed and the dashed line represents the vehicle speed. Similar to FIG. 2, at t 6  a second wheel speed excursion is initiated. At t 14 , the wheel speed departure and recovery cycle appears to the ABS microprocessor to have been completed, causing the microprocessor to decide that the rear wheel speed has returned to the vehicle speed and that the wheel speed excursion has ended. Actually, the rear wheel is following a wheel speed curve approximating 1.0 g. Accordingly, when microprocessor samples the rear wheel speed, the microprocessor will determine that a low-to-high road surface transition has occurred. The microprocessor algorithm will then initiate a third wheel speed departure with an unlimited series of reapply pulses even though the vehicle is still on a low mu road surface. The resulting wheel speed departure cycle is wasteful of the limited amount of pressurized brake fluid available in the master cylinder  14 . 
     While the above example is for a second wheel speed excursion, it will be appreciated that wheel speed sneakdown also may occur during an anti-lock braking cycle following the first or any subsequent wheel speed excursion. Wheel speed sneakdown can occur in any ABS not having a G-sensor, but is most severe for RWAL systems, which have only one speed sensor  40 . Thus, it would be desirable to detect the presence of wheel speed sneakdown to avoid initiating an unneeded wheel speed departure. 
     The present invention contemplates a system for controlling at least one vehicle wheel brake which includes a valving device connected to the controlled vehicle wheel brake. The valving device being operable to control application of pressurized fluid to the controlled wheel brake. The system also includes a wheel speed sensor for monitoring the speed of a vehicle wheel associated with the controlled wheel brake. The system further includes a microprocessor coupled to the valving device and the so wheel speed sensor. 
     The microprocessor is operative to selectively actuate the valving device to control the wheel brake. The microprocessor also is operative to measure a duration of a wheel speed departure of the wheel associated with the controlled wheel brake and to calculate an average deceleration of the wheel during the wheel speed departure. The microprocessor is further operative to set a wheel speed sneakdown flag if the wheel speed departure duration is greater than or equal to a predetermined wheel speed departure duration threshold and the average deceleration greater than or equal to a deceleration threshold. 
     After setting said sneakdown flag, the microprocessor is further operative to cause the valving device to decrease the pressure applied to the controlled wheel brake. Following the decrease of pressure applied to the controlled wheel brake, the microprocessor can cause the valving device to increase the pressure applied to the controlled wheel brake. Alternately, if the wheel speed departure duration is less than said predetermined wheel speed departure duration threshold, the microprocessor can cause the valving device to increase the pressure applied to the controlled wheel brake. Furthermore, if the average deceleration is less than the deceleration threshold, the microprocessor can cause the valving device to maintain the pressure applied to the controlled wheel brake. 
     In the preferred embodiment, the deceleration threshold is non-linear function of the wheel speed departure. However, the deceleration threshold also can be predetermined constant or a linear function of the wheel speed departure duration. Also, in the preferred embodiment, the controlled wheel brake is a rear wheel brake and the system is included in a rear wheel anti-lock brake system. 
     The present invention also contemplates a method for controlling at least one vehicle brake which includes providing a valving device for controlling the application of a pressurized fluid to the controlled wheel brake and a wheel speed sensor for monitoring the speed of a vehicle wheel associated with the controlled wheel brake. Both the valving device and wheel speed sensor are coupled to a microprocessor. The microprocessor measures the duration of a wheel speed departure of the wheel associated with the controlled wheel brake and computes an average deceleration for the wheel during the wheel speed departure duration. The microprocessor then compares the wheel speed departure duration to a predetermined wheel speed departure duration threshold and the average deceleration to a deceleration threshold. The microprocessor sets a wheel speed sneakdown flag if the wheel speed departure duration is greater than or equal to the wheel speed departure duration threshold and the average deceleration is greater than or equal to the deceleration threshold. 
     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a prior art passive rear wheel anti-lock brake system. 
     FIG. 2 illustrates the operation of the rear wheel anti-lock brake system shown in FIG.  1 . 
     FIG. 3 is a graph which illustrates wheel speed sneakdown. 
     FIG. 4 is a chart showing criteria utilized by the present invention to determine if wheel speed sneak down is present. 
     FIG. 5 is a flow chart of an algorithm for applying the criteria shown in FIG.  4 . 
     FIG. 6 is a chart showing an alternate criteria utilized by the present invention to determine if wheel speed sneak down is present. 
     FIG. 7 is a flow chart of an algorithm for applying the criteria shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention contemplates comparing the duration of a wheel speed departure cycle and the average rear wheel deceleration to predetermined values to determine if wheel speed sneakdown is occurring. The wheel speed departure cycle duration is the time period between the first pressure release pulse and the time at which reapply pulses might begin and is shown as t d  in FIG.  3 . It is known that on a relatively high mu road surface, such as wet or dry ordinary asphalt or concrete, the wheel speed departure cycle duration never exceeds a relatively small value. A threshold, t cycle , for the maximum departure cycle duration on a high mu road surface is determined for a particular vehicle. In the preferred embodiment, t cycle  is equal to 300 milliseconds; however, other values can be used for t cycle . If t cycle  has been exceeded, then it is known that the road surface is not high mu. If, concurrently, the average rear wheel deceleration during the wheel speed departure cycle is relatively high, then wheel speed sneakdown must be occurring. In the preferred embodiment, a threshold, T DEC , for the deceleration of rear wheels of 0.5 g is used; however, other values also can be used. 
     The relationship between the wheel speed departure cycle duration and rear wheel speed deceleration is illustrated by the graph shown in FIG. 4 with the horizontal axis representing the slope of the rear wheel speed curve and the vertical axis representing the wheel speed departure cycle duration. The solid curve labeled  70  in FIG. 4 represents the locus of points which correspond to wheel speed departure cycle duration as a function of the rear wheel speed slope. Thus, a rear wheel speed slope of 1.0 g corresponds to a wheel speed departure cycle duration of 100 milliseconds. The points to the left of and below the solid curve  70  represent operation where wheel speed sneakdown is not occurring. The dashed line, which is labeled  71 , represents a shifted locus of points to define a guard band, the purpose for which will be explained below. The shaded region, which is bounded by the threshold values of t cycle =300 milliseconds and T DEC =0.5 g, represents part of the region for which wheel sneakdown is occurring. 
     Upon determining that wheel sneakdown is occurring, the present invention contemplates generating one or two small pressure release, or dump, pulses to let the wheel speed recover completely to the actual vehicle speed. The algorithm then may generate a small pressure reapply pulse to establish a level of brake torque consistent with achieving good stopping distance on the low mu road surface. The guard band shown in FIG. 4 between the solid curve  70  and the dashed curve  71  is provided to avoid unnecessary dump pulses. 
     A flow chart for an algorithm for implementing the wheel speed control described above is shown in FIG.  5 . In functional block  80 , a counter is indexed to measure the length of the wheel speed departure duration, T d . In functional block  81 , the rear wheel speed is sampled and the average rear wheel speed deceleration during the wheel speed departure, AVG DEC, is computed. In decision block  82 , the algorithm checks to determine if the wheel speed departure cycle has ended. If the cycle has not ended, the algorithm transfers back to functional block  80  to continue timing the cycle duration t d  and computing the average rear wheel deceleration AVG DEC. If the cycle has ended, the algorithm transfers to decision block  83  where the duration t d  of the wheel speed departure is compared to the departure duration threshold t cycle . If the cycle duration t d  is less than the threshold t cycle , a low-to-high surface mu transition has occurred during the cycle and the algorithm transfers to functional block  84  to set a corresponding flag. The algorithm then transfers to functional block  85  to begin an unlimited series of apply pulses to initiate another wheel speed departure. If the cycle duration is greater than or equal to the departure duration threshold t cycle , the algorithm transfers to decision block  86 . 
     In decision block  86 , the average deceleration AVG DEC is compared to the deceleration threshold T DEC . If the average deceleration is less than the threshold T DEC , sneakdown is not present and the algorithm transfers to functional block  87  to return to the main program. If the average deceleration is greater than or equal to the threshold T DEC , sneakdown is present and the algorithm transfers to functional block  88  to set a corresponding flag. The algorithm then transfers to function block  89  is initiate a small pressure release pulse as described above. 
     Instead of a fixed deceleration threshold T DEC , as described above, the invention also contemplates utilization of a deceleration threshold which is a linear or nonlinear function of cycle duration. A certain minimum cycle duration would still have to be exceeded, but the deceleration threshold would be determined from a functional relationship, such as a point along the line  71  in FIG.  4 . An example of such a non-linear relationship is illustrated in FIG. 6 where the wheel sneakdown region has been extended to the line  71  values of t cycle  which are greater than 300 millisec. Accordingly, the value for the deceleration threshold T DEC  is given by the following relationships: 
     
       
           T   DEC   =−K   1   *t   d   +K   2 , for t d  greater than 300 millisec, 
       
     
     and 
     
       
           T   DEC =0.5*g for t d  less than or equal to 300 millisec, 
       
     
     with K 1  equal to the slope of the line  71  in FIG. 6 and K 2  being the y-intercept of the line in FIG.  6 . 
     Thus, the deceleration threshold, T DEC , for detecting sneakdown would decrease as total cycle duration t d  increases beyond the duration threshold t cycle . 
     A flow chart for an algorithm which utilizes the relationship illustrated in FIG. 6 is shown in FIG.  7 . Blocks in FIG. 7 which are the same as blocks shown in FIG. 5 have the same numerical designators. As shown in FIG. 7, the difference from the flow chart shown in FIG. 5 is the addition of a functional block  90  in which the deceleration threshold T DEC  is calculated as a function of the cycle duration t d . The calculated value for T DEC  from functional block  90  is then used in decision block  86 . Otherwise, the operation of the flow chart is the same as described above for FIG.  5 . 
     As indicated above, the invention further contemplates that the deceleration threshold T DEC  can also be a linear function of the cycle time t d  (not shown). 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope. For example, the flow charts shown and described above are intended to be illustrative and it will be appreciated that the invention also can be practiced with algorithms based upon flow charts which differ from the ones shown and described above.