Anti-skid control apparatus with brake failure detection means

A vehicular anti-skid control apparatus comprises a differential pressure detector for detecting the pressure difference inside brake lines of separate brake channels, a brake failure detector for detecting failure in one of the brake channels based on the anti-skid control state of each brake channel, a brake failure evaluator for evaluating failure of the brake system based on both output signals of the differential pressure detector and brake failure detector wherein the anti-skid control method is changed from normal anti-skid control to a particular anti-skid control when a brake failure occurs.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an anti-skid control apparatus for 
vehicles comprising a brake system separated into a front brake channel 
and a discrete rear brake channel. 
2. Description of the Prior Art 
Anti-skid control apparatuses for vehicles comprising a brake line system 
separated into a front brake channel and a rear brake channel must 
generally give priority to maintaining the directional stability of the 
vehicle over the braking force of the rear wheels when applying anti-skid 
control to the rear wheels. As a result, select-low control, a method 
whereby the right and left wheels are evaluated for a locking tendency, 
and the brake fluid pressure of the right and left wheels is 
simultaneously controlled based on the wheels exhibiting a tendency to 
lock, is normally used. 
However, if the front brake channel has failed and anti-skid control is 
accomplished using only the rear brake channel, vehicle speed will not 
decline sufficiently and the total required stopping distance accordingly 
increases. 
One method, described in Japanese patent laid-open H6-227384 (1994-227384), 
overcoming this problem may switch from the select-low control method 
described above to select-high control, which is similar to select-low 
control except that the brake fluid pressure is controlled based on the 
wheels not exhibiting a tendency to lock; may change, the rear wheel slip 
threshold to permit more slipping than when the front brakes are normal; 
or may set the rear wheel pressure reducing pulse time shorter than when 
the front brakes are normal,, after a predetermined period has passed with 
anti-skid control applied to one of the rear wheels but to neither of the 
front wheels. 
With the method described in Japanese patent laid-open H6-227384 
(1994-227384), however, the brake failure state is detected in software 
alone, permitting brake failure to be detected only after the 
predetermined period has passed, and thus not obtaining sufficient speed 
reduction during the initial braking period. 
Furthermore, if brake failure is erroneously detected and the brake failure 
control mode is entered when the rear wheels lock first due, for example, 
to a road surface with an uneven coefficient of friction, rear wheel 
slipping is effectively over-compensated because the brakes are 
functioning normally, and vehicle stability deteriorates. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide an anti-skid 
control apparatus for vehicles comprising a brake line system separated 
into a front brake channel and a rear brake channel, and relates 
particularly to such an anti-skid control apparatus whereby failure of the 
front brake channel can be quickly and accurately detected, and sufficient 
braking force can therefore be obtained. 
To achieve the aforementioned object, an anti-skid control apparatus 
according to the present invention comprises a differential pressure 
detection means for detecting the pressure difference inside the brake 
lines of the separate brake channels, and outputting a differential 
pressure detection signal when the pressure difference exceeds a 
predetermined level; a brake failure detection means for detecting failure 
in one of the brake channels based on the anti-skid control state of each 
brake channel, and outputting a brake failure detection signal when a 
brake channel failure is detected; and a brake failure evaluating means 
for evaluating failure of the brake system when both the differential 
pressure detection signal and the brake failure detection signal are 
detected, and outputting the brake failure evaluation signal. Thus 
comprised, the anti-skid control apparatus changes the anti-skid control 
method from normal anti-skid control to a particular anti-skid control 
method while the brake failure evaluating means outputs the brake failure 
evaluation signal. 
The brake failure detection means of an anti-skid control apparatus 
according to an embodiment of the invention outputs the brake failure 
detection signal when anti-skid control is applied to one of the rear 
wheels but is not applied to either of the front wheels. 
In the anti-skid control apparatus according to another embodiment of the 
invention, the particular anti-skid control method accomplishes anti-skid 
control by setting the wheel slip threshold value used to evaluate wheel 
slipping to a higher than normal value. 
In the anti-skid control apparatus according to a further embodiment of the 
invention, the particular anti-skid control method accomplishes anti-skid 
control by setting the pressure reducing time of the rear brake fluid 
pressure to a shorter than normal time. 
In the anti-skid control apparatus according to one more embodiment of the 
invention, the particular anti-skid control method accomplishes anti-skid 
control by switching rear wheel braking force control between select-low 
control and select-high control where select-low and select-high control 
evaluate the right and left wheels for a locking tendency, select-low 
control simultaneously controls the brake fluid pressure of the right and 
left wheels based on the wheels exhibiting a tendency to lock, and 
select-high control simultaneously controls the brake fluid pressure of 
the right and left wheels based on the wheels not exhibiting a tendency to 
lock. 
When the pressure difference between the brake channels exceeds a 
predetermined value, and a brake system failure is detected based on the 
anti-skid control status of each wheel, the anti-skid control apparatus 
according to the invention switches from normal anti-skid control to an 
anti-skid control method particularly configured for use during brake 
failure. It is thereby possible to quickly detect brake failure and switch 
to anti-skid control appropriate to brake failure in time to achieve 
sufficient braking force during the initial braking period; to prevent a 
false brake failure determination and a drop in vehicle (control) 
stability when the rear wheels lock first during normal brake operation; 
and to accurately detect brake failure without making a false brake 
failure determination when the differential pressure detection means fails 
and continuously outputs the differential pressure detection signal. 
The brake failure detection means of the anti-skid control apparatus 
according to an embodiment of the invention outputs the brake failure 
detection signal when anti-skid control is applied to one of the rear 
wheels but to neither of the front wheels. It is thereby possible to 
quickly detect brake failure and switch to anti-skid control appropriate 
to brake failure in time to achieve sufficient braking force during the 
initial braking period; to prevent a false brake failure determination and 
a drop in vehicle (control) stability when the rear wheels lock first 
during normal brake operation; and to accurately detect brake failure 
without making a false brake failure determination when the differential 
pressure detection means fails and continuously outputs the differential 
pressure detection signal. 
The particular anti-skid control method selected by the anti-skid control 
apparatus according to another embodiment of the invention accomplishes 
anti-skid control by setting the wheel slip threshold value used to 
evaluate wheel slipping to a higher than normal value. It is thereby 
possible to quickly detect brake failure and switch to anti-skid control 
appropriate to brake failure in time to achieve sufficient braking force 
during the initial braking period; to prevent a false brake failure 
determination and a drop in vehicle (control) stability when the rear 
wheels lock first during normal brake operation; and to accurately detect 
brake failure without making a false brake failure determination when the 
differential pressure detection means fails and continuously outputs the 
differential pressure detection signal. 
The particular anti-skid control method selected by the anti-skid control 
apparatus according to a further embodiment of the invention accomplishes 
anti-skid control by setting the pressure reducing time of the rear brake 
fluid pressure to a shorter than normal time. It is thereby possible to 
quickly detect brake failure and switch to anti-skid control appropriate 
to brake failure in time to achieve sufficient braking force during the 
initial braking period; to prevent a false brake failure determination and 
a drop in vehicle (control) stability when the rear wheels lock first 
during normal brake operation; and to accurately detect brake failure 
without making a false brake failure determination when the differential 
pressure detection means fails and continuously outputs the differential 
pressure detection signal. 
The particular anti-skid control method selected by the anti-skid control 
apparatus according to one more embodiment of the invention accomplishes 
anti-skid control by switching rear wheel braking force control between 
select-low control and select-high control. It is thereby possible to 
quickly detect brake failure and switch to anti-skid control appropriate 
to brake failure in time to achieve sufficient braking force during the 
initial braking period; to prevent a false brake failure determination and 
a drop in vehicle (control) stability when the rear wheels lock first 
during normal brake operation; and to accurately detect brake failure 
without making a false brake failure determination when the differential 
pressure detection means fails and continuously outputs the differential 
pressure detection signal.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The preferred embodiment of an anti-skid control apparatus according to the 
present invention is described hereinbelow with reference to the 
accompanying figures. FIG. 1 is a simplified system diagram of an antilock 
brake control system (ABS) equipped with the anti-skid control apparatus 
of the present invention for a four-wheel motor vehicle in which the brake 
system is separated into discrete front and rear brake channels, each 
commonly connecting the right and left front and rear brakes, 
respectively. FIG. 2 is a system diagram of an antilock brake control 
system showing the ABS fluid pressure control unit 6 shown in FIG. 1. 
As shown in FIG. 1, the brake system is a two-channel (front and rear 
channels) brake system in which the front channel is connected to both 
right and left front brakes, and the rear channel is connected to both 
right and left rear brakes. The master cylinder 2 converts depression of 
the brake pedal 1 to brake fluid pressure supplied to the brake lines 
through the differential pressure switch 3, which as the differential 
pressure detection means detects the pressure difference between the brake 
fluid pressure in the two brake channels, and outputs the detected 
differential pressure as a binary signal. The ABS fluid pressure control 
unit 6 of this system then controls the braking force by increasing, 
decreasing, or maintaining the brake fluid pressure in the wheel cylinders 
5a, 5b, 5c, and 5d of the four wheels 4a, 4b, 4c, and 4d connected to the 
two (front and rear) brake channels. Note that the indices a, b, c, and d 
above and as used below indicate the left front wheel, the right front 
wheel, the left rear wheel, and the right rear wheel, respectively. 
The ABS fluid pressure control unit 6 is discretely connected to the left 
front wheel cylinder 5a and the right front wheel cylinder 5b, and is 
connected to the left and right rear wheel cylinders 5c and 5d in common. 
The speed of each wheel is detected and output as the corresponding wheel 
speed signal by wheel speed sensors 7a, 7b, 7c, and 7d, which are 
connected to the corresponding wheels. Each of the wheel speed sensors 7a, 
7b, 7c, and 7d is also connected to the electronic control unit 8, which 
is described below. 
The electronic control unit 8 is further connected to the differential 
pressure switch 3 and the ABS fluid pressure control unit 6. The 
electronic control unit 8 executes various calculations and evaluations 
based on the wheel speed signals input from the wheel speed sensors 7a-7d 
and the differential pressure detection signal input from the differential 
pressure switch 3, and then outputs the appropriate control signal to the 
ABS fluid pressure control unit 6 to control brake system operation. This 
ABS control operation is described in detail below with reference to the 
ABS fluid pressure control unit 6 shown in FIG. 2. 
As shown in FIG. 2, the ABS fluid pressure control unit 6 comprises inlet 
valves 10a, 10b, and 10r, which are normally-open on/off solenoid valves; 
outlet valves 11a, 11b, and 11r, which are normally-closed on/off solenoid 
valves; reservoir 12f for temporarily storing the brake fluid purged from 
wheel cylinders 5a and 5b when the brake fluid pressure of wheel cylinders 
5a and 5b is reduced; and reservoir 12r for temporarily storing the brake 
fluid purged from wheel cylinders 5c and 5d when the brake fluid pressure 
thereof is reduced. 
The ABS fluid pressure control unit 6 further comprises a front pump 13f 
for pumping the brake fluid stored in the front reservoir 12f to the 
master cylinder 2; a corresponding rear pump 13r for pumping the brake 
fluid stored in the rear reservoir 12r to the master cylinder 2; a common 
motor 14 for driving both front and rear pumps 13f and 13r; and check 
valves 15f, 15r, 16f, and 16r. Note that the indices f and r as used 
herein indicate front and rear wheel components, respectively. 
The master cylinder 2 is connected through the differential pressure switch 
3 to port X of each inlet valve 10a, 10b, and 10r. Ports Y of inlet valves 
10a and 10b are connected to port X of the corresponding outlet valves 11a 
and 11b, and to the corresponding front wheel cylinders 5a and 5b. Ports Y 
of outlet valves 11a and 11b are connected to the front reservoir 12f. 
Port Y of inlet valve 10r is similarly connected to port X of outlet valve 
11r, and to the rear wheel cylinders 5c and 5d, and port Y of the outlet 
valve 11r is connected to the rear reservoir 12r. 
One side of the front pump 13 f is connected through check valve 15 f to 
the connection between ports Y of outlet valves 11 a and 11 b and the 
front reservoir 12f, and the other side of the front pump 13 f is 
connected through another check valve 16f and the differential pressure 
switch 3 to the master cylinder 2. In a similar manner, one side of the 
rear pump 13r is connected through check valve 15r to the connection 
between port Y of outlet valve 11r and the rear reservoir 12r, and the 
other side of the rear pump 13r is connected through check valve 16r and 
differential pressure switch 3 to the master cylinder 2. Note that check 
valves 15f and 15r are disposed to permit the brake fluid to flow in only 
one direction to the corresponding pumps from the connections between 
ports Y of the outlet valves and the corresponding reservoirs, and check 
valves 16f and 16r are disposed to permit the brake fluid to flow in only 
one direction to the master cylinder 2 from the corresponding pumps. 
The electronic control unit 8 is connected to the differential pressure 
switch 3; the wheel speed sensors 7a-7d; the solenoids of the inlet valves 
10a, 10b, and 10r and the outlet valves 11a, 11b, and 11r; and the motor 
14. The electronic control unit 8 performs various calculations and 
evaluations based on the digital signal supplied from the differential 
pressure switch 3, and the wheel speed signals from the wheel speed 
sensors 7a-7d, and then outputs the necessary control signals to the motor 
14, the solenoids of the inlet valves 10a, 10b, and 10r, and the solenoids 
of the outlet valves 11a, 11b, and 11r to appropriately control brake 
operation. 
More specifically, the electronic control unit 8 calculates the speed of 
each wheel based on the signals from the wheel speed sensors 7a-7d, then 
calculates the deceleration of each wheel from the speed of each wheel, 
and then calculates the estimated vehicle speed from the calculated speed 
and deceleration of each wheel. Note that the wheel speed, wheel 
deceleration, and estimated vehicle speed can be calculated using various 
known methods, and further description thereof is thus omitted below. 
When an asynchronous state in which the difference between the calculated 
estimated vehicle speed and any wheel speed signal exceeds a constant 
threshold value is detected, it is determined that at least one of the 
wheels has locked. A "reduce pressure signal" commanding the brake system 
to reduce the brake fluid pressure is then output to the inlet valves 10a, 
10b, and 10r, outlet valves 11a, 11b, and 11r, and the motor 14, and the 
brake system is driven in the pressure reduction mode, i.e., current is 
supplied to excite the solenoids of the inlet valves 10a, 10b, and 10r, 
and the solenoids of the outlet valves 11a, 11b, and 11r, and the motor 14 
is operated to run the pumps 13 f and 13r. 
The pressure reduction mode thus closes ports X and Y of inlet valves 10a, 
10b, and 10r, thereby shutting off the supply of brake fluid from the 
master cylinder 2; opens ports X and Y of outlet valves 11a, 11b, and 11r 
to open the brake fluid outlet channels allowing the brake fluid to flow 
from the wheel cylinders into the reservoirs 12f and 12r; and thus reduces 
the brake fluid pressure in each of the wheel cylinders 5a-5d. When the 
brake fluid pressure in the wheel cylinders 5a-5d begins to drop, pumps 13 
f and 13r operate to return the brake fluid from the reservoirs 12f and 
12r back to the master cylinder 2. 
When, for example, the difference between the estimated vehicle speed and 
the wheel speed drops below the predetermined threshold value and a 
synchronized state is resumed because of the reduction in the brake fluid 
pressure of the wheel cylinders 5a-5d, the electronic control unit 8 
determines that the wheels are no longer locked. To therefore increase the 
brake fluid pressure, the electronic control unit 8 stops the current 
supply to the inlet valves 10a, 10b, and 10r and the outlet valves 11a, 
11b, and 11r. This causes ports X and Y of the inlet valves 10a, 10b, and 
10r to open, opening the brake fluid supply from the master cylinder 2, 
and causes ports X and Y of the outlet valves 11a, 11b, and 11r to close, 
stopping the outflow of brake fluid to the reservoirs and thus causing the 
brake fluid pressure in the wheel cylinders to rise. 
FIG. 3 is a block diagram of the anti-skid control apparatus according to 
the present invention. The preferred embodiment of this anti-skid control 
apparatus applicable to an ABS as described above is described next with 
reference to FIG. 3. Note that like parts in FIGS. 1, 2, and 3 are 
identified by like reference numbers, and further description thereof is 
omitted below. 
As shown in FIG. 3, the electronic control unit 8 comprises a wheel speed 
calculator 30, wheel speed deceleration calculator 31, estimated vehicle 
speed calculator 32, brake failure detector 33, brake failure evaluator 
34, anti-skid control evaluator 35, solenoid drive controller 36, and 
motor driver 37. 
The wheel speed calculator 30 calculates the speed of each wheel based on 
the wheel speed signals input from the wheel speed sensors 7a-7d. 
The wheel speed deceleration calculator 31 then calculates the deceleration 
of each wheel from the wheel speed of each wheel calculated by the wheel 
speed calculator 30. 
The estimated vehicle speed calculator 32 then calculates the estimated 
vehicle speed from each wheel speed and each wheel deceleration. 
The brake failure detector 33 then checks for failure of the brake system 
based on the calculated wheel speeds, wheel deceleration, and estimated 
vehicle speed, and outputs the brake failure detection signal when a 
failure in the brake system is detected. 
The brake failure evaluator 34 then evaluates failure of the brake system 
based on the differential pressure detection signal, which indicates that 
the differential pressure switch 3 has detected a pressure difference 
greater than the predetermined threshold value, and the brake failure 
detection signal. When a brake system failure is determined to have 
occurred, the brake failure evaluator 34 outputs the brake failure 
evaluation signal. 
The anti-skid control evaluator 35 determines whether to apply ABS control 
based on the wheel speed signals, wheel deceleration, and estimated 
vehicle speed, and outputs the ABS control request signal when ABS control 
is determined necessary. If the brake failure evaluation signal is also 
being input from the brake failure evaluator 34, the anti-skid control 
evaluator 35 selects a particular ABS control method. 
In response to the ABS control request signal from the anti-skid control 
evaluator 35, the solenoid drive controller 36 calculates the pressure 
reduction control command values that are used as the control signals for 
the inlet valves 10a, 10b, 10r and the outlet valves 11a, 11b, 11r, and 
thus controls operation of the solenoids for the inlet valves 10a, 10b, 
10r and the outlet valves 11a, 11b, 11r according to the calculated 
pressure reduction control command values. Also in response to the ABS 
control request signal, the motor driver 37 controls the motor 14 to drive 
the pumps 13 f and 13r. 
Note, further, that to simplify the following description, inlet valve 10a 
and outlet valve 11a are shown as actuator 38a, inlet valve 10b and outlet 
valve 11b as actuator 38b, and inlet valve 10r and outlet valve 11r as 
actuator 38r in FIG. 3. Actuator 38a thus adjusts the brake fluid pressure 
of the left front wheel cylinder 5a; actuator 38b adjusts the brake fluid 
pressure of the right front wheel cylinder 5b; and actuator 38r adjusts 
the brake fluid pressure of the rear wheel cylinders 5c and 5d. The 
solenoid drive controller 36 also controls operation of the actuators 38a, 
38b, and 38r by means of the pressure reduction control command values 
calculated as described above. 
Note, further, that the inputs to the wheel speed calculator 30 are 
connected to the wheel speed sensors 7a-7d, and the wheel speed calculator 
30 outputs to the wheel speed deceleration calculator 31, estimated 
vehicle speed calculator 32, brake failure detector 33, and anti-skid 
control evaluator 35. The wheel speed deceleration calculator 31 outputs 
to the estimated vehicle speed calculator 32, brake failure detector 33, 
and anti-skid control evaluator 35. The estimated vehicle speed calculator 
32 outputs to the brake failure detector 33 and anti-skid control 
evaluator 35. The brake failure detector 33 outputs to the brake failure 
evaluator 34. The brake failure evaluator 34 outputs to the anti-skid 
control evaluator 35. The anti-skid control evaluator 35 outputs to the 
solenoid drive controller 36 and the motor driver 37. The solenoid drive 
controller 36 outputs to actuators 38a, 38b, and 38r, and the motor driver 
37 outputs to the motor 14. 
The process whereby the anti-skid control apparatus of the present 
invention as shown in FIG. 3 detects failure of the front brake channel 
and selects the anti-skid control method when brake failure is detected is 
described below with reference to the flow charts shown in FIGS. 4 and 5. 
Starting at step S1 in FIG. 4, the wheel speed calculator 30 first 
calculates each wheel speed based on the signal data supplied from the 
wheel speed sensors 7a-7d; the wheel speed deceleration calculator 31 
calculates the deceleration of each wheel based on the wheel speed values 
calculated by the wheel speed calculator 30; and the estimated vehicle 
speed calculator 32 then calculates the estimated vehicle speed, which is 
an estimate of the actual vehicle speed, from the previously calculated 
wheel speed and wheel deceleration values. 
At step S2, the brake failure evaluator 34 determines whether the 
differential pressure switch 3 is ON, a state occurring when the 
differential pressure switch 3 detects a pressure difference exceeding the 
threshold value. If differential pressure switch 3 is ON, step S2 returns 
YES and control passes to step S3. 
At step S3, the brake failure detector 33 checks the ABS control state of 
each wheel to check for any failure of the front brake channel. The 
failure detection method applied by the brake failure detector 33 at this 
time is described below. 
A synchronization flag SYFLG.sub.i and synchronization timer SYTMR.sub.i 
are set for each wheel. The synchronization flag SYFLG.sub.i indicates 
whether the corresponding wheel speed is synchronized to the estimated 
vehicle speed, and the synchronization timer SYTMR.sub.i is used to count 
the continuous synchronization state time (the time the wheel speed is 
synchronized to the estimated vehicle speed) of the corresponding wheel. 
The brake failure detector 33 thus checks the state of the synchronization 
flag SYFLG.sub.i and the value of the synchronization timer SYTMR.sub.i 
for each wheel. If the synchronization flags SYFLG.sub.0, and SYFLG.sub.1 
for both the left and right front wheels are reset, the values of both 
left and right front wheel synchronization timers SYTMR.sub.0 and 
SYTMR.sub.1 exceed a particular value a, and either left or right rear 
wheel synchronization flag SYFLG.sub.2 or SYFLG.sub.3 is set or either 
left or right rear wheel synchronization timer SYTMR.sub.2 or SYTMR.sub.3 
is less than a particular value a, a failure is detected in the front 
brake channel. 
In other words, the brake failure detector 33 determines a failure in the 
front brake channel if ABS control is applied to neither front wheel (left 
or right) but is applied to either one of the rear wheels (left or right). 
Note that an index i where i=0, 1, 2, or 3 (0 being the right front wheel, 
1 the left front wheel, 2 the right rear wheel, and 3 the left rear wheel) 
is used so that the index value can be counted to determine whether all 
four wheels have been processed. Note, further, that the default 
synchronization flag SYFLG.sub.i setting is RESET, and the default 
synchronization timer SYTMR.sub.i value is 255. 
Returning to FIG. 4, if ABS control is applied to neither front wheel (left 
or right) but is applied to one of the rear wheels (left or right) in step 
S3, a front brake channel failure is detected and step S3 returns YES. At 
step S4, the brake failure evaluator 34 sets a flag FLG indicating that 
the front brake channel was evaluated and determined to have failed, and 
control passes to step S5. 
Note that if step S2 returns NO (differential pressure switch 3 is OFF) or 
step S3 returns NO (front brake failure not detected), this flag FLG is 
cleared (FLG=0) to indicate that the front brake channel was evaluated and 
determined to not have failed. Control then passes to step S5. 
At step S5, the anti-skid control evaluator 35 resets the index counter i 
to 0. The index counter counts the index value set for each wheel as 
described above, and is used later in the control process to determine 
whether all wheels have been processed. 
At step S7, the anti-skid control evaluator 35 then calculates slippage 
SP.sub.i for the wheel corresponding to the current index counter i (e.g., 
SP.sub.0 if the current index counter i=0, indicating the right front 
wheel). This slippage SP.sub.i value indicates the amount of wheel 
slippage relative to the estimated vehicle speed during vehicle braking, 
and is calculated from the wheel speed SPEED.sub.i calculated by the wheel 
speed calculator 30, the wheel deceleration calculated by the wheel speed 
deceleration calculator 31, and the estimated vehicle speed V.sub.REF 
calculated by the estimated vehicle speed calculator 32. Control then 
passes to step S8. 
At step S8, the anti-skid control evaluator 35 determines whether the wheel 
corresponding to the current value of the index counter i is in an 
asynchronous state. If it is (YES is returned), it is determined in step 
S9 whether particular synchronization conditions are satisfied for the 
wheel corresponding to the current value of the index counter i. If these 
conditions are not satisfied (NO is returned), control passes to step S10. 
If these synchronization conditions are satisfied (YES is returned by step 
S9), the synchronization flag SYFLG.sub.i is reset and the synchronization 
timer SYTMR.sub.i is cleared to zero (0) at step S11 for the wheel 
corresponding to the current value of the index counter i, and control 
then passes to step S10. 
If an asynchronous state is not detected at step S8 (=NO), the anti-skid 
control evaluator 35 determines at step S12 the value of the 
synchronization timer SYTMR.sub.i for the wheel corresponding to the 
current value of the index counter i. If SYTMR.sub.i is less than 255, 
step S12 returns NO, the synchronization timer SYTMR.sub.i is incremented 
one in step S13, and control then passes to step S10. If the 
synchronization timer SYTMR.sub.i value is 255 or greater in step S12 (YES 
is returned), control passes directly to step S10. 
The anti-skid control evaluator 35 then determines at step S10 whether the 
flag FLG is set. If it is (=YES), a predetermined value DTH is added to 
the slip threshold value TH.sub.i, the threshold value used to determine 
whether the wheel corresponding to the current value of the index counter 
i is tending to lock, at step S14, and control then passes to step S15 
(FIG. 5). If at step S10 the flag FLG is not set (=NO), control passes 
directly to step S15 (FIG. 5). 
At step S15 (FIG. 5), the anti-skid control evaluator 35 checks for locking 
symptoms in the wheel corresponding to the current value of the index 
counter i. This is accomplished by determining whether the slippage 
SP.sub.i for the wheel corresponding to the current index counter i 
exceeds the slip threshold value TH.sub.i. If locking symptoms are not 
detected (step S15=NO), the anti-skid control evaluator 35 checks the 
value of the synchronization timer SYTMR.sub.i for the wheel corresponding 
to the current value of the index counter i. If the timer value is less 
than the predetermined value a (step S16=NO), the anti-skid control 
evaluator 35 instructs the solenoid drive controller 36 to calculate the 
pressure reduction control command value whereby the actuator for the 
wheel corresponding to the current value of the index counter i is 
operated to gradually increase the brake fluid pressure (step S17), and 
control then passes to step S18. 
If at step S16 the synchronization timer SYTMR.sub.i is greater than or 
equal to predetermined value a (step S16=YES), the anti-skid control 
evaluator 35 instructs the solenoid drive controller 36 at step S19 to 
terminate ABS control of the wheel corresponding to the current value of 
the index counter i, and transfer the fluid pressure in the master 
cylinder 2 directly to the wheel cylinder. Control then passes to step 
S18. 
If locking symptoms are detected at step S15 (=YES), the anti-skid control 
evaluator 35 sets the synchronization flag SYFLG.sub.i for the wheel 
corresponding to the current value of the index counter i at step S20, and 
then determines the state of the flag FLG at step S21. If the flag FLG is 
set (step S21=YES), the brake fluid pressure reducing time for the wheel 
cylinder of the wheel corresponding to the current value of the index 
counter i is set short at step S22 so that the brake fluid pressure is 
reduced only slightly, and control passes to step S18. 
If the flag FLG is not set (step S21=NO), the anti-skid control evaluator 
35 sets the brake fluid pressure reducing time for the wheel cylinder of 
the wheel corresponding to the current value of the index counter i to the 
normal pressure reducing time at step S23, and control passes to step S18. 
At step S18, the anti-skid control evaluator 35 determines whether the 
wheel corresponding to the current value of the index counter i is a rear 
wheel. If it is (step S18=YES), the flag FLG state is checked at step S24. 
If the flag FLG is set (step S24=YES), braking force control of the rear 
wheels is set to select-high control at step S25. As previously described, 
select-high control simultaneously controls the brake fluid pressure of 
both left and right wheels based on the wheels (left or right) not 
indicating a tendency to lock. Control then passes to step S26. 
If the flag FLG is not set (step S24=NO), braking force control of the rear 
wheels is set to select-low control at step S27. As previously described, 
select-low control simultaneously controls the brake fluid pressure of 
both left and right wheels based on the wheels (left or right) indicating 
a tendency to lock. Control then passes to step S26. 
The anti-skid control evaluator 35 increments the index counter i at step 
S26, and then at step S28 checks the value of the index counter i to 
determine whether the process described above has been executed for all 
four wheels (which is the case when i=4 in step S28). If the process has 
been completed for all four wheels (i=4 and step S28 returns YES), the 
solenoid drive controller 36 outputs the brake fluid pressure control 
command values to the respective actuators 38a, 38b, 38c, and 38d as the 
solenoid commands therefor (step S29). Control then returns to step S1 in 
FIG. 4. 
If all four wheels have not been processed (i&lt;4 and step S28 returns NO), 
control loops back to step S7 in FIG. 4. 
FIG. 6 is a timing chart showing the solenoid signals to the actuator 
(SOL.sub.0 and SOL.sub.2 shown in the figure by way of example only), the 
wheel speed, wheel cylinder pressure (W/C), and the synchronization flag 
and synchronization timer states for the right front and right rear wheels 
when the brake system is functioning normally for all wheels and the 
anti-skid control apparatus according to the present invention is used in 
the antilock brake control system. 
FIG. 7 is a similar timing chart showing the solenoid signals to the 
actuator (SOL.sub.0 and SOL.sub.2 shown by way of example only), the wheel 
speed, wheel cylinder pressure (W/C), and the synchronization flag and 
synchronization timer states for the right front and right rear wheels 
when the front brakes are not functioning normally (have failed) and the 
anti-skid control apparatus according to the present invention is used in 
the antilock brake control system. 
As will be known from FIGS. 6 and 7, when the differential pressure switch 
3 (shown as differential pressure SW in the figures) is ON because it has 
detected a pressure difference exceeding a predetermined threshold value 
between the front and rear brake channels, the flag FLG indicating there 
is a failure in the front brake channel is set, and a failure in the front 
brakes is therefore determined, the ABS control method applied to the rear 
brakes is changed to a particular control method whereby the rear wheels 
are permitted to skid more than when all brakes are functioning normally. 
It is to be noted that while the preferred embodiment above has been 
described as detecting a failure in the front brake channel, it is also 
possible to separately check for failure of the left or right front 
brakes, and appropriately change the ABS control method to another control 
method. 
Note, further, that while a differential pressure switch has been described 
as the differential pressure detection means a differential pressure 
sensor outputting an electrical signal that varies linearly to the 
pressure difference may be alternatively used. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.