Vehicle wheel slip control system

Sensors detect rotational speeds of respective vehicle wheels. A slip detection reference value is calculated from the detected rotational speeds of the vehicle wheels. Acceptable slip ranges for the respective vehicle wheels are determined in accordance with the calculated slip detection reference value. The rotational speeds of the vehicle wheels are compared with the corresponding acceptable slip ranges respectively. Slip controls of the respective vehicle wheels are performed when the rotational speeds of the vehicle wheels reside outside the corresponding acceptable slip ranges respectively. The highest of at least two of the rotational speeds of the vehicle wheels is selected. Differences between the respective rotational speeds of the vehicle wheels and the selected highest vehicle wheel rotational speed are calculated. The slip detection reference value is corrected in accordance with the respective calculated differences.

DESCRIPTION OF THE BASIC PREFERRED EMBODIMENT 
With reference to FIG. 1, a vehicle wheel slip control system according to 
a basic embodiment of this invention includes a reference speed calculator 
M2 determining a slip detection reference speed in accordance with 
rotational speeds of vehicle wheels M1. A device M3 compares the 
rotational speeds of the respective vehicle wheels M1 with speed ranges 
dependent on the reference speed. When the comparator M3 detects that the 
rotational speeds of the respective vehicle wheels M1 reside outside the 
speed ranges, a device M4 performs slip controls of the respective vehicle 
wheels M1. 
A device M5 selects the highest of the rotational speeds of predetermined 
at least two of the vehicle wheels M1. A device M6 corrects the reference 
speed in accordance with differences between the selected highest 
rotational speed and the rotational speeds of the respective vehicle 
wheels M1. 
As described previously, the reference speed calculator M2 determines a 
slip detection reference speed in accordance with the rotational speeds of 
the vehicle wheels M1. For example, the reference speed calculator M2 
regards the highest of the vehicle wheel rotational speeds as a vehicle 
speed and calculates the reference speed in accordance with the vehicle 
speed. The reference speed may be varied in accordance with decelerations 
of the vehicle wheels M1. 
The comparator M3 determines whether or not slip controls of the respective 
vehicle wheels M1 are necessary by comparing the rotational speeds of the 
respective vehicle wheels M1 with the reference speed. For example, when 
one of the vehicle wheel rotational speeds is lower than the reference 
speed, the comparator M3 judges the related vehicle wheel to be in an 
unacceptable slip state and supplies the control device M4 with data which 
induces a decrease in the brake force applied to the related vehicle wheel 
M1. 
As described previously, the device M5 selects the highest of the 
rotational speeds of predetermined at least two of the vehicle wheels M1. 
The device M6 corrects the reference speed in accordance with differences 
between the selected highest rotational speed and the rotational speeds of 
the respective vehicle wheels M1. This correction of the reference speed 
is designed in view of a difference in rotational speed between vehicle 
wheels at one side of the vehicle and vehicle wheels at the other side of 
the vehicle which occurs under vehicle cornering conditions or during 
vehicle turns. 
The correction of the reference speed enables the device M4 to perform slip 
controls of the respective vehicle wheels which allow for a difference in 
rotational speed between the vehicle wheels under vehicle cornering 
conditions or during vehicle turns. In addition, the slip controls of the 
respective vehicle wheels are performed on the basis of only the vehicle 
wheel rotational speed data. 
DESCRIPTION OF THE SPECIFIC PREFERRED EMBODIMENT 
FIG. 2 shows a vehicle wheel slip control system according to a specific 
embodiment of this invention. In this specific embodiment, the vehicle 
wheel slip control system is applied to a front wheel steering and rear 
wheel drive automotive vehicle having four wheels. In such an automotive 
vehicle, front vehicle wheels constitute idler wheels. A vehicle wheel 
slip control system of this invention may also be applied to other 
vehicles. 
As shown in FIG. 2, an automotive vehicle has a front-right wheel 1, a 
front-left wheel 3, a rear-right wheel 5, and a rear-left wheel 7. 
Rotational speed sensors 9, 11, 13, and 15 associated with the vehicle 
wheels 1, 3, 5, and 7 output pulse signals representing rotational speeds 
of the vehicle wheels 1, 3, 5, and 7 respectively. For example, the 
vehicle wheel speed sensors 9, 11, 13, and 15 are of the electromagnetic 
pickup type or the photoelectric type. 
Hydraulic brake units 17, 19, 21, and 23 are associated with the vehicle 
wheels 1, 3, 5, and 7 respectively. Hydraulic pressures are applied via 
hydraulic lines 35, 37, 39, and 41 to the brake units 17, 19, 21, and 23 
in accordance with operation of a brake pedal 25 or with operations of 
hydraulic pressure adjustment actuators 27, 29, 31, and 33 of the 
electromagnetic solenoid operated type. The hydraulic pressures applied to 
the brake units 17, 19, 21, and 23 cause corresponding brake forces 
applied to the vehicle wheels 1, 3, 5, and 7. These brake forces are 
adjustable via the actuators 27, 29, 31, and 33, and also via the brake 
pedal 25. 
A stop switch 43 associated with the brake pedal 25 senses the position of 
the brake pedal 25. Specifically, the stop switch 43 generates an ON 
signal when the brake pedal 25 is depressed to brake the vehicle. The stop 
switch 43 generates an OFF signal when the brake pedal 25 is undepressed 
to release the vehicle from the brake. A hydraulic cylinder 45 
mechanically connected to the brake pedal 25 is controllable via the brake 
pedal 25. The hydraulic cylinder 45 is hydraulically connected to the 
brake units 17, 19, 21, and 23. The depression of the brake pedal 25 
causes a hydraulic pressure in the hydraulic cylinder 45 which normally 
allows the vehicle wheels 1, 3, 5, and 7 to be braked. An 
electrically-powered hydraulic pump 47 serves as a source generating a 
hydraulic pressure used for slip controls. An electronic control circuit 
49 electrically connected to the actuators 27, 29, 31, and 33 serves to 
adjust the actuators 27, 29, 31, and 33. The adjustments of the actuators 
27, 29, 31, and 33 allow controls of the hydraulic pressures applied to 
the brake units 17, 19, 21, and 23 from the hydraulic cylinder 45 or from 
the hydraulic pump 47. Accordingly, the brake forces applied to the 
respective vehicle wheels 1, 3, 5, and 7 are independently controllable 
via the adjustments of the actuators 27, 29, 31, and 33. 
A main relay 51 switches the connection between an electric power supply 
and electromagnetic solenoids of the actuators 27, 29, 31, and 33 in 
accordance with an output signal from the electronic control circuit 49. 
When a malfunction occurs in the vehicle wheel slip control system, an 
indicator lamp 53 is activated by an output signal from the electronic 
control circuit 49 to inform the vehicle driver of the occurrence of the 
malfunction. For example, the malfunction consists of a wire breaking of 
the electromagnetic solenoids of the actuators 27, 29, 31, and 33 or a 
wire breaking of the vehicle wheel speed sensors 9, 11, 13, and 15. 
The electronic control circuit 49 is supplied with an electric power when 
an ignition switch 50 is closed or turned on. The electronic control 
circuit 49 receives the signals from the vehicle wheel speed sensors 9, 
11, 13, and 15, and the signal from the stop switch 43. The electronic 
control circuit 49 performs calculation process for slip detection and 
slip control, and other calculation processes in accordance with the 
received signals and outputs signals for controlling the actuators 27, 29, 
31, and 33, the main relay 51, and the indicator lamp 53. 
The actuators 27, 29, 31, and 33 have similar designs. As shown in FIG. 3, 
each of the actuators 27, 29, 31, and 33 includes a regulator 55 and a 
control valve 57. The regulator 55 includes an electromagnetic valve which 
selects one of the hydraulic pressure supplied from the hydraulic cylinder 
45 and the hydraulic pressure supplied from the hydraulic pump 47 in 
accordance with a signal outputted by the electronic control circuit 49. 
The regulator 55 derives a fixed hydraulic pressure applied to the control 
valve 57. The control valve 57 includes an electromagnetic solenoid for 
driving a valve member. The control valve 57 is changeable among three 
positions in accordance with a signal outputted by the electronic control 
circuit 49. When the control valve 57 assumes a first position, the 
hydraulic passage 35, 37, 39, or 41 is exposed to the pressure from the 
regulator 55 so that the hydraulic pressure applied to the brake unit 17, 
19, 21, or 23 usually increases. When the control valve 57 assumes a 
second position, the hydraulic passage 35, 37, 39, or 41 is connected to a 
drain so that the hydraulic pressure applied to the brake unit 17, 19, 21, 
or 23 usually decreases. When the control valve 57 is in a third position, 
the hydraulic passage 35, 37, 39, or 41 is blocked or isolated so that the 
hydraulic pressure applied to the brake unit 17, 19, 21, or 23 remains 
essentially constant. The brake units 17, 19, 21, and 23 include 
respective brake wheel cylinders exposed to the hydraulic pressures 
applied via the actuators 27, 29, 31, and 33. The vehicle wheels 1, 3, 5, 
and 7 are subjected to brake forces dependent on the hydraulic pressures 
applied to the brake wheel cylinders of the brake units 17, 19, 21, and 23 
respectively. The hydraulic pressures applied to the brake units 17, 19, 
21, and 23, and also the brake forces applied to the vehicle wheels 1, 3, 
5, and 7 are controlled in accordance with the signals outputted from the 
electronic control circuit 49 to the actuators 27, 29, 31, and 33 
respectively. 
As shown in FIG. 4, the electronic control circuit 49 includes wave shapers 
or wave shaping amplifiers 60, 62, 64, and 66 converting output signals 
from the vehicle wheel speed sensors 9, 11, 13, and 15 into pulse signals 
fit to be processed by a microcomputer 68. A buffer 70 temporarily holds 
output signal from the stop switch 43. When the ignition switch 50 is 
closed or turned on, a power supply circuit 72 feeds a constant voltage to 
the microcomputer 68 and other devices. The microcomputer 68 includes a 
combination of a central processing unit (CPU) 76, a read-only memory 
(ROM) 78, a random-access memory (RAM) 80, and an input/output (I/0) 
circuit 82. The signals outputted by the devices 60, 62, 64, 66, and 70 
are inputted into the I/0 circuit 82. The microcomputer 68 generates 
control signals in accordance with the input signals. The I/0 circuit 82 
outputs these control signals to drivers or drive circuits 84, 86, 88, 90, 
92, and 94 respectively. 
The devices 84, 86, 88, and 90 drive the electromagnetic solenoids of the 
actuators 27, 29, 31, and 33 in accordance with the input signals supplied 
from the microcomputer 68. The main relay 51 includes a normally open 
switch 96 and a control winding 98. The relay switch 96 is connected 
between the power supply and the electromagnetic solenoids of the 
actuators 27, 29, 31, and 33. The drive circuit 92 energizes and 
de-energizes the relay winding 98 in accordance with the input signal 
supplied from the microcomputer 68. When the relay winding 98 is 
energized, the relay switch 96 is closed so that the electromagnetic 
solenoids of the actuators 27, 29, 31, and 33 can be powered. In this 
case, the actuators 27, 29, 31, and 33 operate in accordance with signals 
supplied from the drive circuits 84, 86, 88, and 90. When the relay 
winding 98 is de-energized, the relay switch 96 is opened so that the 
electromagnetic solenoids of the actuators 27, 29, 31, and 33 is 
deactivated independent of the signals supplied from the drive circuits 
84, 86, 88, and 90. The drive circuit 94 activates and deactivates the 
indicator lamp 53 in accordance with the input signal supplied from the 
microcomputer 68. 
The electronic control circuit 49 operates in accordance with a program 
stored in the ROM 78. This program contains a slip detection and control 
routine. FIG. 5 is a flowchart of the slip detection and control routine 
or program. The slip detection and control routine or program is 
periodically reiterated by a suitable process such as a timer-based 
interruption process. In the following description, the rotational speeds 
of the vehicle wheels 1, 3, 5, and 7 are represented as the corresponding 
speeds at the contacting ground or road surfaces. 
As shown in FIG. 5, a first step 200 of the slip detection and control 
routine derives the current rotational speeds of the vehicle wheels 1, 3, 
5, and 7 from the signals outputted by the vehicle wheel speed sensors 9, 
11, 13, and 15. The step 200 calculates an estimated current vehicle speed 
VSB.sub.n from the current vehicle wheel rotational speeds by referring to 
the following equations. 
EQU V.omega.0=Max(V.omega.FR, V.omega.FL, V.omega.RR, V.omega.RL) . . . (1) 
EQU VSB.sub.n =MED(V.omega.0, VSB.sub.n-1 +.alpha.up.multidot.t, VSB.sub.n-1 
-.alpha.dw.multidot.t) . . . (2) 
In the equation (1), the character V.omega.0 represents the highest 
rotational speed, and the characters V.omega.FR, V.omega.FL, V.omega.RR, 
V.omega.RL represent the rotational speeds of the front-right vehicle 
wheel 1, the front-left vehicle wheel 3, the rear-right vehicle wheel 5, 
and the rear-left vehicle wheel 7 respectively. In addition, the character 
Max represents an operator selecting the highest of the vehicle wheel 
rotational speeds V.omega.FR, V.omega.FL, V.omega.RR, V.omega.RL. 
In the equation (2), the character VSB.sub.n represents an estimated 
current vehicle speed, and the character VSB.sub.n-1 represents the 
estimated vehicle speed determined by the step 200 in the preceding 
execution cycle of the program. The characters .alpha.up and .alpha.dw 
represent preset constants corresponding to given accelerations and used 
in estimating a vehicle speed. For example, the given accelerations 
.alpha.up and .alpha.dw are equal to 0.5G and 1.0G respectively. The 
character t represents a calculation period or interval which essentially 
corresponds to an interval between successive execution cycles of the 
program. The character MED represents an operator selecting the 
intermediate of the speeds equal to V.omega.0, VSB.sub.n-1 
+.alpha.up.multidot.t, VSB.sub.n-1 -.alpha.dw.multidot.t respectively. In 
other words, the second highest or the second lowest of the three speeds 
is selected. 
A step 205 subsequent to the step 200 calculates a slip detection reference 
value or speed VSH by referring to the following equation. 
EQU VSH=KSH.multidot.VSB-KV ... (3) 
where the character KSH represents a preset factor of proportionality, and 
the character KV represents a preset speed constant. For example, the 
values KSH and KV are equal to 0.95 and 5Km/h respectively. In the 
equation (3), the character VSB represents the estimated vehicle speed 
VSB.sub.n determined in the preceding step 200. It is preferable that the 
reference value VSH is chosen to correspond to a slightly small slip ratio 
within a given range in view of response lags of the actuators 27, 29, 31, 
and 33 and in view of calculation lags of the electronic control circuit 
49. After the step 205, the program advances to a step 208. 
As will be made clear hereinafter, during one execution cycle of the slip 
detection and control program, the step 208 is executed four times. The 
step 208 sequentially selects one of the vehicle wheels 1, 3, 5, and 7 
which will be subjected to slip detection and control. For example, the 
front-right wheel 1, the front-left wheel 3, the rear-right wheel 5, and 
the rear-left wheel 7 are sequentially selected in the first, second, 
third, and fourth executions of the step 208 respectively. 
A step 210 following the step 208 determines whether or not the slip 
control is being performed with respect to the selected vehicle wheel. As 
will be made clear hereinafter, the slip control is performed when the 
result of a decision step 260 is positive and thus a step 270 is executed. 
Accordingly, the determination in the step 210 is performed by checking 
whether or not the result of the decision step 260 in the preceding 
execution cycle of the program was positive or by checking whether or not 
the step 270 was executed in the preceding execution cycle of the program. 
When the slip control is being performed, the program jumps to the step 
270. When the slip control is not being performed, the program advances to 
a step 220. 
The step 220 determines whether or not the rotational acceleration 
V.omega.* of the selected vehicle wheel is smaller than a reference value 
KV.omega.. For example, the reference value KV.omega. equals -3G. The step 
220 is to prevent the execution of the following process in cases where 
the coefficient of friction between the road surface and vehicle wheels at 
one side of the vehicle differs from the coefficient of friction between 
the road surface and vehicle wheels at the other side of the vehicle or in 
cases where only the brakes for vehicle wheels at one side of the vehicle 
are effective. When the vehicle wheel rotational acceleration V.omega.* is 
equal to or greater than the reference value KV.omega., the program 
advances to a step 230. When the vehicle wheel rotational acceleration 
V.omega.* is smaller than the reference value KV.omega., the program jumps 
to the step 260. 
The step 230 calculates the difference .DELTA.V.omega. between the 
rotational speed V.omega.* of the selected vehicle wheel and the highest 
of the rotational speeds V.omega.FR and V.omega.FL of the front or idler 
vehicle wheels 1 and 3 by referring to the following equation. 
EQU .DELTA.V.omega.=.vertline.Max(V.omega.FR, V.omega.FL)-V.omega.*.vertline.. 
. . (4) 
After the step 230, the program advances to a step 240. 
The step 240 determines the value .DELTA.V.omega.G by referring to the 
following equation. 
EQU .DELTA.V.omega.G=Min(.DELTA.V.omega., .DELTA.Vmax) . . . (5) 
where the character Min represents an operator selecting the smaller of the 
speed values .DELTA.V.omega. and .DELTA.Vmax, and the character 
.DELTA.VMax represents a preset constant. For example, the constant 
.DELTA.Vmax corresponds to a speed of 7 Km/h. The speed value 
.DELTA.V.omega.G equals the speed value .DELTA.V.omega. when the speed 
value .DELTA.V.omega. is equal to or smaller than the constant 
.DELTA.Vmax. The speed value .DELTA.V.omega.G equals the constant 
.DELTA.Vmax when the speed value .DELTA.V.omega. is greater than the 
constant .DELTA.Vmax. Accordingly, the constant .DELTA.Vmax defines the 
upper limit of the value .DELTA.V.omega.G. As will be made clear 
hereinafter, the value .DELTA.V.omega.G is used in correction of the 
reference speed VSH. The step 240 prevents the speed difference 
.DELTA.V.omega. from causing excessive correction of the reference speed 
VSH. 
A step 250 subsequent to the step 240 corrects the slip detection reference 
value VSH in accordance with the value .DELTA.V.omega.G by referring to 
the following statement. 
EQU VSH=VSH-.DELTA.V.omega.G . . . (6) 
where the right-hand character VSH represents the slip detection reference 
value determined in the previous step 205, and the left-hand character VSH 
represents a new slip detection reference value obtained through the 
correction. After the step 250, the program advances to the step 260. The 
step 260 compares the rotational speed V.omega.* of the selected vehicle 
wheel with the slip detection reference value VSH determined in the 
preceding step 250. The slip detection reference value VSH is chosen so 
that the selected vehicle wheel is excessively slipping when the 
rotational speed V.omega.* of the selected vehicle wheel is smaller than 
the slip detection reference value VSH. When the rotational speed 
V.omega.* of the selected vehicle wheel is actually smaller than the slip 
detection reference value VSH, that is, when the selected vehicle wheel is 
excessively slipping, the program advances to the step 270. When the 
rotational speed V.omega.* of the selected vehicle wheel is equal to or 
greater than the slip detection reference value VSH, that is, when the 
selected vehicle wheel is not excessively slipping, the program advances 
to a step 280. 
The step 270 performs slip control to prevent excessive slip of the 
selected vehicle wheel. The step 270 adjusts the actuator 27, 29, 31, or 
33 associated with the selected vehicle wheel in accordance with a pattern 
which depends on the rotational speed of the selected vehicle wheel and on 
the rotational acceleration or deceleration of the selected vehicle wheel. 
For example, the step 270 controls the actuator 27, 29, 31, or 33 
associated with the selected vehicle wheel and thereby adjusts the 
hydraulic brake pressure applied to the selected vehicle wheel so that the 
actual rotational speed of the selected vehicle wheel can be lower than 
the estimated vehicle speed VSB by 15-20%. In other words, the actual 
vehicle wheel rotational speed can be held equal to 80-85% of the 
estimated vehicle speed VSB. In more detail, the step 270 calculates the 
degree of slip of the selected vehicle wheel from the current vehicle 
wheel speed V.omega.* and the estimated vehicle wheel VSB, or from the 
acceleration or deceleration of the selected vehicle wheel drived through 
differentiation of the vehicle wheel speed V.omega.*. Then, the step 270 
determines whether or not the degree of slip of the selected vehicle wheel 
is acceptable or unacceptable by comparing the degree of slip with a 
reference. When the degree of slip is unacceptable, the actuator 
associated with the selected vehicle wheel is adjusted to decrease the 
hydraulic brake pressure. When the degree of slip is acceptable, the 
actuator is controlled to increase or hold the hydraulic brake pressure. 
After the step 270, the program advances to the step 280. 
It should be noted that the step 270 may control the actuators 27, 29, 31, 
and 33 in view of an induced speed difference between the vehicle wheels 
1, 3, 5, and 7 during a turn of the vehicle. In one example, during slip 
control under vehicle cornering conditions, the rotational speeds of the 
vehicle wheels are held lower than the estimated vehicle speed VSB by 
values which are determined in dependence on the vehicle wheels in view of 
an induced speed difference between the vehicle wheels. In another 
example, during slip control under vehicle cornering conditions, the 
rotational speeds of the vehicle wheels are held within a preset range of 
percents of the estimated vehicle speeds which are corrected in dependence 
on the vehicle wheels in view of an induced speed difference between the 
vehicle wheels. 
The step 280 determines whether or not all of the vehicle wheels have been 
exposed to the slip detection and control process in the present execution 
cycle of the program. Specifically, the step 280 determines whether or not 
all of the vehicle wheels have been selected by the step 208 in the 
present execution cycle of the program. When all of the vehicle wheels 
have been exposed to the slip detection and control process, the present 
execution cycle of the slip detection and control routine ends and the 
program returns to a main routine. When at least one of the vehicle wheels 
is not yet exposed to the slip detection and control process, the program 
returns to the step 208. 
In the next execution cycle of the slip detection and control program, the 
step 210 determines whether or not the slip control is being performed by 
checking whether or not the step 270 was performed in the preceding 
execution cycle of the slip control program. When the slip control program 
is being performed, that is, when the step 270 was performed in the 
preceding execution cycle of the slip detection and control routine, the 
program jumps from the step 210 to the step 270. Accordingly, once the 
step 270 is executed, the steps 220-260 continue to be unexecuted until a 
decision process in the step 270 detects that the slip control ends. 
Specifically, the end of the slip control is detected when the estimated 
vehicle speed VSB drops to or below a preset speed, for example, 3 Km/h, 
or when the stop switch 43 generates the OFF signal. 
In cases where the vehicle is turning to the left or counterclockwise as 
shown in FIG. 6, the radius R1 of turn of the front-left vehicle wheel 3 
about the center of the vehicle turn is smaller than the radius R2 of turn 
of the front-right vehicle wheel 1 about the center of the vehicle turn. 
The rotational speed V.omega.FL of the front-left vehicle wheel 3 differs 
from the rotational speed V.omega.FR of the front-right vehicle wheel 1 in 
accordance with the difference between the turn radiuses R1 and R2 of the 
vehicle wheels 1 and 3. Accordingly, if a common reference value or range 
with respect to the estimated vehicle speed VSB is used in detecting 
excessive slips of the vehicle wheels 1 and 3, the inner vehicle wheel or 
the front-left vehicle wheel 3 would tend to be subjected to excessive 
slip control. As will be made clear hereinafer, the embodiment of this 
invention prevents such excessive slip control. 
As shown in FIG. 7, when the vehicle turns to the left or counterclockwise, 
the rotational speed V.omega.FR of the front-right vehicle wheel differs 
from the rotational speed V.omega.FL of the front-left vehicle wheel. The 
slip detection reference value VSH(V.omega.FR) for the front-right vehicle 
wheel and the slip detection reference value VSH(V.omega.FL) for the 
front-left vehicle wheel are made different in accordance with the 
difference between the rotational speed V.omega.FR of the front-right 
vehicle wheel and the rotational speed V.omega.FL of the front-left 
vehicle wheel. Accordingly, it is possible to reliably detect excessive 
slip of either of the front-light and front-left vehicle wheels. In 
addition, the inner vehicle wheel or the front-left vehicle wheel is 
prevented from undergoing excessive slip control. 
As understood from the previous description, in the embodiment of this 
invention, the vehicle wheels can be respectively subjected to reliable 
slip detections and controls when the vehicle is cornering or turning so 
that the rotational speeds of the right-hand vehicle wheels differ from 
the rotational speeds of the left-hand vehicle wheels. Since these slip 
detections and controls are performed in accordance with only the 
rotational speeds of the vehicle wheels, a special sensor such as a 
vehicle acceleration sensor or a steering sensor is unnecessary. In 
addition, the slip detections and controls respond to the difference 
between the rotational speeds of the vehicle wheels, the slip detections 
and controls are effective also during a straight travel of the vehicle 
under conditions where the rotational speeds of the vehicle wheels differ 
from each other due to a difference between the diameters of the tires or 
a difference between the air pressures within the tires. 
The processes in the steps 200 and 205 correspond to the process in the 
reference speed calculation device M2 of the basic embodiment of FIG. 1. 
The process in the step 260 corresponds to the process in the comparator 
M3 in the basic embodiment of FIG. 1. The process in the step 270 
corresponds to the process in the control device M4 in the basic 
embodiment of FIG. 1. The process in the step 230 corresponds to the 
process in the selection device M5 in the basic embodiment of FIG. 1. The 
process in the step 250 corresponds to the process in the correction 
device M6 in the basic embodiment of FIG. 1. 
Various modifications may be made in the specific embodiment of this 
invention. For example, in the slip detections and controls for the rear 
vehicle wheels, the rotational speed of the output shaft of a transmission 
connected to a differential gear may be used as a mean rotational speed 
V.omega.R of the rear vehicle wheels. In this case, the equation (1) is 
changed as follows. 
EQU V.omega.0=Max(V.omega.FR, V.omega.FL, V.omega.R) 
In addition, the rear wheel actuators 31 and 33 are replaced by a single 
actuator for controlling the hydraulic brake units 21 and 23. 
In a second modification, the four vehicle wheels are separated into two 
groups each having two vehicle wheels. Slip detection reference values are 
determined for the respective groups. In each group, the slip detection 
reference value is corrected in accordance with the higher of the 
rotational speeds of the two vehicle wheels. 
In a third modification, the higher of the rotational speeds of the idler 
vehicle wheels is selected as a parameter for correcting the slip 
detection reference values of the other vehicle wheels. 
In a fourth modification, a step similar to the step 250 is added between 
the steps 210 and 270 in the program of FIG. 5. In other words, the result 
of the decision step 210 is "yes", the program advances from the step 210 
to the step 270 via the added step. As in the step 250, the added step 
corrects the slip detection reference value VSH in accordance with the 
value .DELTA.V.omega.G. The added step uses the value .DELTA.V.omega.G 
which was determined immediately before the slip control step 270 was 
executed for the first time. To this end, the value .DELTA.V.omega.G which 
was determined immediately prior to the onset of the slip control remains 
held by the memory during the slip control. The added step keeps the slip 
detection reference value VSH corrected during the slip control.