Abstract:
A brake system control method for use in a vehicle in which wheel speed normalization factors are iteratively updated, comprising the steps of: monitoring a plurality of wheel speed signals from a plurality of wheel speed sensors; determining for each wheel a wheel acceleration responsive to the wheel speed signal; determining an acceleration dead band for each wheel, wherein the acceleration dead band is proportional to a measure of vehicle acceleration; comparing the wheel acceleration to the dead band; and if the magnitude of the wheel acceleration is greater than the magnitude of the dead band, inhibiting update of the normalization factors.

Description:
This invention relates to a brake system control method and apparatus. 
     BACKGROUND OF THE INVENTION 
     Many automobiles include anti-lock brake systems as standard or optional features. Some automobiles include traction control systems for preventing wheel slip during positive acceleration of the vehicle. Anti-lock brake systems and many traction control systems utilize wheel speed sensors that provide individual wheel speed information to the brake controller allowing the controller to perform its anti-lock and/or traction control functions. Vehicles have also been provided with a telltale in the instrumentation panel that illuminates when the anti-lock brake system and/or traction control system is activated to indicate to the driver that the vehicle may be on a road surface with low traction or a low coefficient of friction. 
     In operation, typical anti-lock brake systems monitor the wheel speeds of the vehicle wheels and determine a normalization factor for each wheel. This normalization factor is designed to offset differences in wheel rolling radii due to uneven weight distribution of the vehicle on the wheels, uneven tire fill pressures, etc. Typically, the normalization factors are continuously updated, except when the vehicle is in anti-lock brake control mode or traction control mode. 
     SUMMARY OF THE PRESENT INVENTION 
     It is an object of this invention to provide a brake control system according to claim 1. 
     Advantageously, this invention provides a brake system control for scrutinizing wheel speed signals from wheel speed sensors and determining normalized wheel speed signals with increased accuracy. Advantageously, this invention uses the measured wheel speed signals to determine normalization factors for the vehicle wheels and uses the normalization factors to determine the normalized wheel speeds. Advantageously, this invention inhibits updating the normalization factors in a variety of wheel conditions that would provide contaminated wheel speed information, otherwise impairing the accuracy of the normalization factors and, thus, the normalized wheel speed signals. 
     Advantageously, the criteria used to determine whether the normalization factors are to be updated are stricter than previously known and the result is that the normalized wheel speeds determined according to this invention have improved accuracy. 
     Advantageously, this invention provides a brake system control that monitors the vehicle wheel speed information as provided by the vehicle wheel speed sensors and analyzes that information against a variety of criteria to determine whether normalization factors should be updated. If any of the tests indicate that the wheel speed information is not sufficiently free from contamination due to road conditions, the wheel normalization factors are not updated. Additionally, in a preferred example, each time that the system detects that the wheel speed information may be contaminated due to road conditions, a timer is updated and, when the timer reaches a predetermined threshold, a telltale, chime, or other signal for the driver is activated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described by way of example with reference to the following drawings in which: 
     FIG. 1 illustrates a schematic of an example apparatus for implementing this invention; 
     FIG. 2 illustrates a general flow routine for the vehicle brake controller; and 
     FIGS. 3 a-c  illustrate a flow diagram of an example computer routine for implementing this invention in the apparatus shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the vehicle  10  shown includes a controllable brake system with controller  68  for controlling the brakes  20 ,  22 ,  24  and  26  for wheels  12 ,  14 ,  16  and  18 , respectively. Various inputs to the controller  68  include the wheel speed signals on lines  36 ,  38 ,  40  and  42  from wheel speed sensors  28 ,  30 ,  32  and  34 , the brake pedal switch signal on line  84  from brake pedal switch  82 , the brake pedal travel signal on line  83  from pedal travel sensor  85 , the steering wheel angle signal on line  62  from sensor  61  indicating the angle of steering wheel  60 , the vehicle velocity signal on line  78  and the yaw rate signal on line  81  from yaw rate sensor  80 . 
     Each of the sensors  28 ,  30 ,  32 ,  34 ,  61 ,  80 ,  82  and  85  is implemented in a manner known to those skilled in the art. The brake pedal travel sensor  85  is a rotary resistive sensor mounted at the pivot point of pedal  64  providing a resistive output that changes with the amount of arcuate travel of pedal  64 . Alternative sensors for sensor  85  include a pedal travel sensor mounted to the linkage of pedal  64 , a pedal force sensor or a master cylinder pressure sensor. In some implementations, combinations of sensors may be used. 
     Responsive to the various inputs, the controller controls the braking of each wheel in anti-lock brake mode, and, if the vehicle implements active brake control and/or brake by wire braking, the brakes are also controlled in these additional modes. Further, if a traction control system utilizing the friction brakes is implemented, the brake controller controls activation of the brakes during positive vehicle acceleration traction control events to maintain positive tractive force of the drive wheels on the road surface. 
     Brake control is generally achieved as follows. Brake controller  68  receives the various input signals from the various sensors and, responsive to those signals, determines a control commands for the various brake actuators  52 ,  54 ,  56  and  58 . Responsive to the various brake commands, the actuators  52 ,  54 ,  56  and  58  control the hydraulic pressure in hydraulic lines  44 ,  46 ,  48  and  50 , controlling the friction brakes  20 ,  22 ,  24  and  26 . 
     Because example brake by wire, active brake control, traction control and anti-lock brake control systems are known to those skilled in the art and are not germane to this invention, detailed description thereof will not be set forth herein. 
     In one example, the brake actuators  52 - 58  are implemented as reciprocating piston actuators of a type known to those skilled in the art. Such actuators typically include a dc motor positionally controlling a reciprocating piston through a rotary to linear motion converter to increase and/or decrease hydraulic pressure in the wheel brakes. The rear brake actuators  56  and  58  can be implemented in a known manner in which a single motor simultaneously drives two pistons controlling brake fluid to brakes  24  and  26 . 
     In another example, brake actuators  52 - 58  are implemented as solenoid valves for selectively coupling brakes  20 - 26  to a source of pressurized hydraulic fluid to increase brake pressure and for selectively coupling brakes  20 - 26  to a brake fluid reservoir to decrease brake pressure. Implementation of such solenoid valve systems is known to those skilled in the art. 
     In yet another example, the rear brakes and/or the front brakes may be electric motor-driven brakes, in which case the actuator and brake functions are performed by the same unit. An example of a brake system including front hydraulic brakes and rear electric brakes in which all four brakes are drive by wire is set forth in U.S. Pat. No. 5,366,291, assigned to the assignee of this invention. 
     According to the preferred example of this invention, the brake controller  68  performs the anti-lock brake functions and other brake functions that may be implemented in the vehicle. The brake controller  68  also monitors the wheel speed signals and, whether or not the wheel speed signals justify activating the anti-lock brake functions or other brake functions that may be included in the vehicle, provides a signal to the vehicle driver either through telltale  90  or chime  92  or both indicating to the vehicle driver that the vehicle may be on a low coefficient of friction road surface or that one or more wheels may be experiencing low traction. 
     Referring now to FIG. 2, the control routine for controller  68  receives the various inputs from the various sensors at block  200 . At block  202 , the routine determines and then updates wheel normalization factors and uses the normalization factors to determine the normalized wheel speeds. At block  204 , the routine controls a telltale, chime or other signal system in the manner described below in response to the road conditions as determine at block  202 . Blocks  202  and  204  are described in detail below with reference to FIGS. 3 a-c.    
     Next at block  206 , the controller controls the vehicle brake systems, i.e., the anti-lock brake system and traction control system, in a manner known to those skilled in the art utilizing the normalized wheel speed signals. 
     Referring now to FIGS. 3 a-c , the routine for determining the vehicle condition according to this invention starts at block  99  where it receives the various inputs from the various sensors and then moves to block  100  where it calculates the vehicle velocity, the vehicle acceleration and the vehicle steering angle. The vehicle velocity may be determined as the average of the speeds of the undriven wheels or, if all four wheels of the vehicle are driven, as the average of the speeds of all of the wheels. The vehicle acceleration is determined as the derivative of the vehicle velocity. 
     The routine then moves to block  102  where it compares the vehicle velocity v speed  to two velocity thresholds corresponding to, for example, 12 and 80 m.p.h. If the vehicle velocity is not between the two thresholds, the routine is exited. If at block  102  the vehicle velocity is between the two thresholds, the routine continues to block  104  where it keeps a running sum for three control loops of each wheel velocity signal as follows: 
     
       
           S   3   LF   =S   3   LF   +S   LF , 
       
     
     
       
           S   3   RF   =S   3   RF   +S   RF , 
       
     
     
       
           S   3   LR   =S   3   LR   +S   LR , 
       
     
     and 
       S   3   RR   =S   3   RR   +S   RR , 
     where S LF , S RF , S LR  and S RR  are the left front, right front, left rear and right rear filtered wheel velocity signals (the wheel velocity signals are filtered by a low pass filter of a known type to attenuate sensor noise, etc.) and S 3   LF , S 3   RF , S 3   LR  and S 3   RR  are the left front, right front, left rear and right rear wheel velocity signal sums. At block  104 , the routine also increments the timer T. 
     At block  106 , the timer T is compared to 3 and, if T is not equal to 3, the routine is exited. Once T is equal to 3, then the sums S 3   LF , S 3   RF , S 3   RR  and S 3   RR  are completely determined and the routine continues to block  108  where it updates the normalized wheel speed variables. The variable designating the most recent previous normalized wheel speeds are updated as follows: 
     
       
           SNold   LF   =SN   LF , 
       
     
     
       
           SNold   RF   =SN   RF , 
       
     
     
       
           SNold   LR   =SN   LR , 
       
     
     and 
     
       
         SNold RR   =SN   RR , 
       
     
     where SNold LF , SNold RF , SNold LR  and SNold RR  are the most recent previous normalized wheel speeds and SN LF , SN RF , SN LR  and SN RR  are the present values of the normalized wheel speeds. Block  108  also then updates the present values of the normalized wheel speeds as follows: 
     
       
           SN   LF   =S   3   LF   *N   LF , 
       
     
     
       
           SN   RF   =S   3   RF   *N   RF , 
       
     
     
       
           SN   LR   =S   3   LR   *N   LR , 
       
     
     and 
     
       
           SN   RR   =S   3   RR   *N   RR , 
       
     
     where N LF , N RF , N LR  and N RR  are the left front, right front, left rear and right rear normalization factors for the wheel speed signals. Also at block  108 , the flag LT is set to false and the flag IH is set to false. The flag LT controls, in the manner described further below, when the telltale is illuminated and the flag IH, when set, inhibits other control features that depend upon wheel speed signals, i.e., in a brake-by-wire system with dynamic front to rear brake proportioning, the IH flag may inhibit the dynamic front to rear brake proportioning. 
     From block  108 , the routine moves to block  110  where it compares the vehicle speed to the threshold Ksp 3  which is set at, for example, 25 m.p.h.. If the vehicle speed is not greater than the threshold Ksp 3 , the minimum speed at which the telltale will be lit, the routine moves to block  146  described below. The test at block  110  bypasses the normalization factor update (described below) if the vehicle speed is not great enough. 
     If the vehicle speed is greater than the threshold Ksp 3 , the routine continues to block  112  where it determines, by monitoring the output of the brake pedal switch, whether or not the brake pedal has been depressed. If the brake pedal is depressed at block  112 , the routine continues to block  144  described below. The test at block  112  bypasses the normalization factor update and resets the wheel speed sums S 6   LF , S 6   RF , S 6   LR , S 6   RR , used in the normalization factor determination, each time the brake pedal is depressed. If the brake pedal is not depressed at block  112 , the routine continues to block  114  where it determines a dead band value, DB, as follows: 
     
       
         
           DB=|ACCEL|*Kjtracc+V 
           speed 
           *Kjtrspd+Ktrdiff, 
         
       
     
     where ACCEL is the vehicle acceleration, Kjtracc is the acceleration gain term, Kjtrspd is the speed gain term and Ktrdiff is an offset value. The above dead band determination provides an advantage that the dead band is proportional to both vehicle acceleration and vehicle speed. Making the dead band proportional to vehicle speed eliminates unnecessary cycling of the telltale during high speed conditions and making the dead band proportional to vehicle acceleration eliminates unnecessary cycling of the tell tale during high acceleration conditions. The constants Kjtracc, Kjtrspd and Ktrdiff are calibratable as a system designer desires. 
     After the dead bands are computed at block  114 , the routine continues to block  116  where the dead band tests are performed on each wheel. Using the left front wheel as an example, the quantity determined by (SN LF −SNold LF )/SN LF  is compared to the dead band DB. If the quantity is greater than the dead band for any of the vehicle wheels, the routine moves to block  118  where it sets the flags LT and IH to true and then continues to block  146  described below. If at block  116  the determined quantity for each wheel is not greater than the dead band, then the routine continues to block  120  where it compares the steering angle to a predetermined constant Ksg. 
     The steering angle may either be determined from a steering wheel angular position sensor such as represented by reference  61  in FIG. 1 or from the normalized wheel speed signals from either the front or rear vehicle wheels. For example, using the front wheels, the steering angle can be determined as follows: 
     
       
         STEER ANGLE=( SN   LF   −SN   RF ) *K   FRT   
       
     
     where K FRT  is a constant taking into account the track width of the vehicle. Using the rear wheel speeds, the steering angle can be determined as: 
     
       
         STEER ANGLE=( SN   LR   −SN   RR ) *K   RR   
       
     
     where K RR  is a constant taking into account the wheel base and track width of the vehicle. Any of the above ways for determining the steering angle is acceptable. 
     The comparison at block  120  determines whether or not the vehicle is turning at too great of a rate to calculate updated normalization factors for the wheel speed signals. The constant KSG may be scheduled based on vehicle speed to decrease as vehicle speed increases. The test at block  120 , if passed, moves the routine to block  146  described below, bypassing those parts of the routine, block  142  and  144 , that update the normalization factors. This is advantageous because, if the vehicle is turning at too great of a rate, roll of the vehicle body affects the weight distribution between the right and left vehicle tires and the tire rolling radii, thus contaminating the wheel speed information in a manner undesirable for calculating the normalization factors. 
     If at block  120  the test is not passed, the routine continues to block  122 . At block  122  the absolute value of the difference between SN LR  and SN RR  is compared to the quantity (Kst*V speed +Kstacc*ACCEL+Ktrn), where Kst is a speed gain, Kstacc is a vehicle acceleration gain and Ktrn is an offset. The speed and acceleration gain terms increase the quantity with either vehicle speed, acceleration, or both. This comparison of the rear wheel speeds to the quantity determined at block  122  is a threshold test for determining if the vehicle is going straight. If SN LR −SN RR  is greater than the quantity, this indicates that the vehicle is turning and the routine continues to block  124  where it sets the flag IH equal to true (but does not set the LT flag) and then to block  146  described below, bypassing the normalization factor update. This prevents the wheel normalization factors from being updated if vehicle is turning, in which case body roll of the vehicle is assumed to corrupt the un-normalized wheel speed signals. 
     If the test is not passed at block  122 , the routine moves to block  126  where it determines a value Sd equal to the absolute value of ((SN LR −SN RR )*Kd), where Kd is a calibratable gain term. Sd is used below to determine if any relative slip between any two wheels indicates that the normalization should be bypassed and the LT flag set to true. 
     At block  128  the routine determines whether the following quantities all have the same sign: SN LF −SN RF , SN LF −SN RR , SN LR −SN RF , and SN LR −SN RR . These quantities are the left-to-right front and rear and diagonal cross car slips. If all of the quantities at block  128  do not have the same sign, the routine continues to block  132  where it sets the flag LT equal to true and then continues to block  146 , bypassing the normalization factor update. 
     If the differences at block  128  all have the same sign, the routine continues to block  130  where it compares the absolute value of each of the following differences to Sd: SN LF −SN RF ; SN LF −SN LR ; SN RF −SN RR ; SN LF −SN RR  and SN RF −SN LR . If any of the differences determined at block  130  are greater than Sd, this indicates a road condition in which it is not desirable to update the normalization factors and the routine continues to block  132  where it sets the LT flag to true and then continues to block  146 , bypassing the update to the normalization factor routine. If all of the differences at block  130  are not true, then the routine continues to block  134  where it assumes that the vehicle is going in a straight direction and updates the steering wheel position sensor offset if necessary so that the steering wheel output indicates the straight ahead position. 
     From block  134  the routine continues to block  136  where it compares the absolute value of the vehicle acceleration to a constant Kac. If at block  136  the absolute value of the vehicle acceleration is greater than Kac, this indicates that the vehicle is accelerating or decelerating too much to update the normalization factors and, thus, the routine continues to block  146 , bypassing the normalization update. If the test at block  136  indicates that the vehicle acceleration absolute value is not greater than Kac, the routine continues to block  138 . 
     Block  138  begins the update of the normalization factors by keeping track of a sum S 6  for each wheel representing the sum for approximately 6 seconds, or 720 control loops (assuming 120 control loops per second), of the unnormalized wheel speeds as follows: 
     
       
           S   6   LF   =S   6   LF   +S   3   LF , 
       
     
     
       
           S   6   RF   =S   6   RF   +S   3   RF , 
       
     
     
       
           S   6   LR   =S   6   LR   +S   3   LR , 
       
     
     and 
     
       
           S   6   RR   =S   6   RR   +S   3   RR . 
       
     
     Block  136  also increments the timer T 6 . At block  140  the timer T 6  is compared to the value  720 , the selected time out value in this example. If T 6  is not equal to  720 , the normalization factors are not yet updated and the routine continues to block  146 . 
     When, at block  140 , the timer T 6  equals  720 , the routine continues to block  142  where it calculates normalization factor for each wheel as follows: 
       N   i   =S   6   i /( S   6   LF   +S   6   RF   +S   6   LR   +S   6   RR ), 
     where i=LF, RF, LR, RR. In the event that the change in N i  from the previous N i  is greater than +/−Kn, the new N i  is limited so that it is no more than +/−Kn different from the previous N i , where Kn is a predetermined constant. The normalization factor update is repeated for each wheel. 
     From block  142 , the routine continues to block  144  where it resets all of the variables S 6   i  to zero and resets the timer T 6  to zero. From block  144  the routine continues to block  146  where it resets the values S 3   i  all to zero and resets the timer T=0. From block  146  the routine continues to block  148  where it checks the flag in memory indicating whether or not the anti-lock brake system is active. This flag is determined and set in a conventional manner known to those skilled in the art that need not be set forth in detail herein. If ABS is active, the routine is exited as the ABS system typically already includes a feature for illuminating an ABS telltale on the vehicle instrument panel. If ABS is not active at block  148 , the routine continues to block  150  where it checks whether or not the flag LT is set to true. If, at block  150 , the LT flag is true, the routine continues to block  156 . 
     At block  156  the routine determines a threshold value Lc=Kc−V speed *Kls, where Kc represents the default time for the telltale timer and Kls is a gain multiplied by the vehicle speed to reduce, as vehicle speed increases, the time period Lc, that the LT flag must be set before the telltale is illuminated. Thus, the threshold time Lc is variable, inversely proportional to vehicle speed. At block  158  the routine compares the variable LA to Lc. If LA is less than Lc, the routine continues to block  160  where LA is incremented. Blocks  156  and  158  institute a delay in turning on the telltale after the LT flag is set to true. The delay is largest at low vehicle speeds and reduces to virtually zero delay in high vehicle speeds. From block  160 , the subroutine is exited. 
     If, at block  158 , LA is not less than Lc, the routine continues to block  162  where it sends an output signal turning on the telltale or other signal, such as an audible chime for the vehicle driver. If at block  150  the flag LT is not set to true, the routine continues to block  152  where the variable LA is reduced by the value Kla. Then, at block  154 , if LA is less than or equal to zero, the routine continues to block  164  where it outputs a command turning off the telltale lamp, chime or other signal to the vehicle driver and sets the value LA equal to zero. 
     If, at block  154 , LA is not less than or equal to zero, then the routine is exited. In this manner, blocks  152  and  154  institute a delay so that after the LT flag is reset from true to false, the telltale chime or other signal is turned off after a time delay. 
     Further modifications to the above routine may be included. For example, after step  120 , a step may be added to modify the dead band signals, separating out the left and right dead bands based on which way the vehicle is turned by the steering wheel to take into account distribution of vehicle weight that occurs during slow turns that do not pass the test at block  120 , but still have some affect on the right to left weight distribution of the vehicle wheels. Another test can be added during braking to indicate pre-ABS entry conditions, for example, if the sum of the front wheel accelerations as determined from the front wheel speed signals minus the sum of the rear wheel accelerations is greater than a predetermined constant, which may be modified based on steer angle of the vehicle and vehicle speed, for more than a predetermined time period, then the LT flag is set to true. This condition would occur, for example, if the vehicle is braking and moves from a high to low coefficient surface. 
     Additionally, a test can be added to indicate less than optimal wheel to road engagement due to cornering of the vehicle as follows: YAW RATE FRT−YAW RATE RR is compared to a predetermined constant. If the difference in yaw rates is greater than the predetermined constant for more than a predetermined time period, then the LT flag is set to true. The front and rear yaw rates are determined as follows: 
     
       
           YAW  RATE  FRT =STEER ANGLE  FRT*v   speed   
       
     
     
       
           YAW  RATE  RR =STEER ANGLE RR*v   speed , 
       
     
     where STEER ANGLE FRT and STEER ANGLE RR are computed as described above with reference to block  120 . 
     Additionally, deviation of the actual vehicle yaw rate from the commanded vehicle yaw rate can be computed if the vehicle has a steering wheel position sensor that provides an output signal of actual steering angle, in which case, a commanded yaw rate can be determined as: 
     
       
           YAW  RATE COMMAND=ACTUAL STEER ANGLE* v   speed . 
       
     
     This commanded yaw rate can be compared to the yaw rate of the vehicle as measured by the rear wheel signals, wherein if the commanded yaw rate minus that computed using the rear wheel speeds is greater than a Kyaw/v speed , where Kyaw is a predetermined constant, for more than a predetermined time period, the LT flag is set to true. 
     Additionally, if the vehicle is provided with a pedal position sensor  85 , illustrated in FIG. 1, the commanded vehicle deceleration determined by the pedal position sensor and the actual vehicle deceleration can be compared. The commanded deceleration is determined as DECEL=pedal position*Kpdl, where Kpdl is a constant, and the difference between the DECEL and actual vehicle deceleration is compared to a predetermined threshold. If the difference is greater than the predetermined threshold for a predetermined time period, the LT flag is set to true. 
     Alternatively, a SLIP command can be determined as a product of the pedal position and a constant Kpslip and the SLIP command minus actual wheel slip is compared to another predetermined threshold, Kslip. If the difference is greater than the predetermined threshold for a predetermined time period, the LT flag is set to true.