Anti-skid control system

An anti-skid control system for use in an automotive vehicle is comprises of a real slip rate detecting device for detecting a real slip rate of the vehicle, a target slip rate setting device for setting a target slip rate of the vehicle, a braking force control device for adjusting a braking force in such a manner that a deviation between the real slip rate and the target slip rate becomes zero, a load measuring device for measuring a load applied to the vehicle, an correcting device for adding a value to the control device which is the product of a coefficient and the deviation between the real slip rate and the target slip rate and a coefficient adjusting device for decreasing the coefficient as the load decreases. If the load decreases during the vehicle's travel, the coefficient to be multiplied to the deviation between the real slip rate and the target slip rate decreases. Thus the quantity to be subtracted from the braking force is decreased and the current braking force is hardly reduced.

BACKGROUND OF THE INVENTION 
The present invention relates to an anti-skid control system for use in an 
automotive vehicle, and in particular to an anti-skid control system in 
which the degree of a locking condition of each road-wheel is restricted. 
An anti-lock braking device has been developed for reducing the braking 
distance by the prevention of the locked condition of each road-wheel. In 
addition, a traction control device also has been developed in order to 
improve the acceleration performance of the vehicle by preventing the slip 
upon initiation of a rapid acceleration. In each device, the prevention of 
the slip between each road-wheel and a road surface is attained by 
adjusting the braking force, which is to be applied to the road-wheel, 
taking into consideration the deceleration and the slip rate of the 
road-wheel, and other factors. Such devices are disclosed, for example, in 
Japanese Patent Laid-open Prints No. Sho 60-154947 and No. Sho 62-31554 
which were published without examination in 1985 and 1987, respectively. 
Sometimes upon a jumping motion or a turning motion of the vehicle, the 
road-wheel may move away from the road surface, which will bring a 
temporary decrease of the load applied to the road-wheel. In general, as 
the load of the road-wheel is decreased, the road-wheel is apt to be in a 
more locked condition. If the foregoing reduction of the load of the 
road-wheel occurs while the braking pressure is being supplied thereto, 
the road-wheel is more or less compelled to be in a more locked condition. 
For the prevention of such situation, immediately upon the load reduction 
the braking force is set to be reduced. Upon recovery of the load of the 
road-wheel, the braking force is also increased up to a value which is the 
same as that when the load reduction occurred. However, the recovery of 
the braking force requires time, which prolongs of the braking distance. 
Similar problems will be raised in the traction control device. 
SUMMARY OF THE INVENTION 
It is therefore a primary object of the present invention to provide an 
anti-skid system for an automotive vehicle which prevents a prolongation 
of a braking distance even though the jumping or turning motion of the 
vehicle occurs. 
In order to attain the foregoing objects, an anti-skid control system for 
use in an automotive vehicle is comprised of real slip rate detecting 
means for detecting a real slip rate of the vehicle, target slip rate 
setting means for setting a target slip rate of the vehicle, a braking 
force control means for adjusting a braking force in such a manner that a 
deviation between the real slip rate and the target slip rate becomes 
zero, load measuring means for measuring a load applied to the vehicle, 
correcting means for adding a value to the control means which is the 
product of coefficient and the deviation between the real slip rate and 
the target slip rate, and coefficient adjusting means for decreasing the 
coefficient as the load decreases.

DETAILED DESCRIPTION OF THE INVENTION 
Embodiments of the present invention will be described hereinafter in 
detail with reference to the accompanying drawings. 
Referring first to FIG. 2, an overall structure of a pressure circuit of an 
anti-skid control system is shown. Conduits 12 and 13 are connected to a 
master cylinder 11. The conduit 12 is connected, via an electromagnetic 
valve 32 and a one-way valve 24, to a conduit 14 which is in fluid 
communication with a wheel cylinder 18 of a front-left road-wheel 44. The 
conduit 14 is also connected via an electromagnetic valve 36 to a drain 
30. The conduit 12 is connected via an electromagnetic valve 35 and a 
one-way valve 27 to a conduit 17 which is in fluid communication with a 
wheel cylinder 21 of a rear-right road-wheel 47. The conduit 17 is 
connected to the drain 30 via an electromagnetic valve 39. 
The conduit 13 is connected, via an electromagnetic valve 33 and a one-way 
valve 25, to a conduit 15 which is in fluid communication with a wheel 
cylinder 19 of a front-right road-wheel 45. The conduit 15 is also 
connected via an electromagnetic valve 34 and a one-way valve 26 to a 
conduit 16 which is in fluid communication with a wheel cylinder 20 of a 
rear-left road-wheel. 46. The conduit 16 is connected to the drain 31 via 
an electromagnetic valve 38. 
Each one-way valve is set to be opened, when an inner pressure of the 
corresponding wheel cylinder is above the pressure in the conduit 112 
(13), for returning the braking pressure thereto, which leads to a 
prevention of an excessive supply of the braking pressure to each 
road-wheel. Each of the electromagnetic valves 32, 33, 34 and 35 is a 
normally open type valve, and is set to be closed when energized or 
actuated. Each of the electromagnetic valves 36, 37, 38 and 39 is a 
normally closed type valve, and is set to be opened when energized or 
actuated. Thus, so long as no electromagnetic valves are being energized 
or actuated, the fluid communication of the conduit 12 with the conduits 
14 and 17 is established as well as the fluid communication of the conduit 
13 with the conduits 15 and 16. Upon depression of a brae pedal 10, the 
master cylinder 11 increases the fluid pressure in each of the conduits 12 
and 13. The resultant pressure increase is transmitted to the wheel 
cylinders 18, 19, 20 and 21 for regulating the rotations of the 
road-wheels 44, 45 46 and 47, respectively. This means that the braking 
force depends on the degree of the depression of the brake pedal 10. 
A pair of pumps 22, 22 are set to be driven by a motor 22a. Outlet ports of 
the pumps 22,22 are connected to the conduits 12 and 13, respectively. As 
previously described, during the de-energized condition of each of the 
electromagnetic valves, the conduit 12 (13) is in fluid communication with 
the wheel cylinders 18 and 21 (19 and 20). Under such condition, if the 
motor 22a is driven or turned on, regardless of the depression of the 
brake pedal 10, the inner pressure of each of the wheel cylinders 18, 19, 
20 and 21 can be increased. Then, both of the electromagnetic valves 32 
and 36 are energized, the electromagnetic valves 32 and 36 are closed and 
opened, respectively, and the fluid in the wheel cylinder 18 is drained 
into the drain 30. Thus activation of the electromagnetic valves 32 and 36 
will decrease the inner pressure of the wheel cylinder 18 of front-left 
road-wheel. Similarly, the inner pressure of the wheel cylinder 21 of the 
rear-right road-wheel 47, the wheel cylinder 21 of the front-right 
road-wheel 45 and the wheel cylinder 20 of the rear-left road-wheel 46 can 
be decreased by the actuation of the electromagnetic valves 33 and 37, and 
the actuation of the electromagnetic valves 34 and 38, respectively. The 
foregoing operation of each of the combination of two electromagnetic 
valves enables the inner pressure of the wheel cylinder to be controlled, 
which results in the adjustment of the control of the respective 
road-wheel. The motor 22a and the electromagnetic valves are under the 
control of an electric control unit 23 which is in the form of a 
microprocessor or CPU. It is to be noted that a one-way valve 28(29) is 
disposed between the conduit 12 (13) and the drain 30 (31) in order that 
when the pressure in the drain 30 (31) exceeds a set value, the pressure 
is returned to the conduit 12 (13). 
Sensor means 40, 41, 42 and 43 are provided to the front-left road wheel 
44, the front-right road wheel 41, the rear-left road-wheel 46, and the 
rear-right road-wheel 47, respectively, in order to feed signals relating 
to the conditions thereof to the electric control unit 23. Each sensor 
means 40, 41, 42, 43 includes a vehicle speed sensor 48 and a lead sensor 
52 (FIG. 3). As shown in FIG. 3, the vehicle speed sensor 48 is set to 
detect the rotational speed of each road wheel and the resulting speed is 
fed as pulse signals SP to the control unit 23. The load sensor 52 is set 
to detect a load F applied to each road-wheel. This load sensor 52 can be 
used as an estimated load sensor which estimate the load on the basis of a 
sprung acceleration, an unsprung acceleration, a vehicle-height and a 
pressure from the suspension. In addition to the foregoing sensors, the 
control unit 23 is connected with a steering angle sensor 49 detecting a 
steering angle .delta. f, a yaw rate sensor 50 detecting a yaw rate 
.gamma., and an acceleration sensor 51 detection accelerations GX and GY 
in the longitudinal and lateral directions, respectively, of the 
vehicle-body. The control unit 23 is also connected with a brake switch 62 
which detects the depression of the brake pedal 10. On the basis of 
signals from the sensors 48 through 52, the control unit 23 is set to 
operate the motor 22a and the electromagnetic valves 42 through 39. 
The control unit 23, as shown in FIG. 1, includes a calculating division 53 
for calculating the vehicle speed, the acceleration, and the estimated 
speed, a road-surface condition recognition division 54, an 
initiation/termination of the control recognition division 55, a motor 
control division 56, a target slip rate calculating division 57, a slip 
rate component calculating division 58, a G-component calculating division 
59, a control mode setting division 60, and a solenoid control division 
61. In the road-surface condition recognition division 54, the condition 
of a road surface is recognized on the basis of the wheel speed and other 
factors. The initiation/termination of the control recognition division 55 
is set to make a decision whether an ABS control should be established or 
not. The motor control division 56 drives the motor 22a for generating the 
fluid pressure depending on the ABS control condition. IN the target slip 
rate calculating division 57, a target slip rate of each rear-wheel is set 
to be calculated. IN the slip rate component calculating division 58, and 
the G-component calculating division 59, a slip rate and an acceleration 
component for setting the control mode are calculated, respectively. In 
the control mode setting division 60, on the basis of the slip rate and 
the acceleration component, the control mode for each road-wheel is 
obtained. In the solenoid control division 61, on the basis of the 
resultant control mode in the control mode setting division 60, the 
electromagnetic valves are controlled corresponding to each road-wheel in 
order to adjust the fluid pressure of the wheel cylinder, thereby 
adjusting the slip condition of each road-wheel. It is to be noted that 
the road-wheels 44, 45, 46 and 47 are set to be controlled independently 
by the foregoing divisions other than the initiation/termination of the 
control recognition division 55 and the motor control division 56. 
Hereinafter each division of the control unit will be detailed. In the 
calculating division 53, a rotational acceleration DVW, a rotational speed 
VW, and an estimated vehicle speed VSO of each road-wheel are calculated 
on the basis of the signal SP from the respective wheel speed sensor 48. 
The estimated speed is defined as the vehicle speed at a portion thereof 
at which each road-wheel is provided. The rotational speed of VW is set to 
be calculated based on the radius of each road-wheel and the width of 
pulse (or a periodic time) of the signal SP. The rotational acceleration 
DVW is obtained by differentiating the rotational speed of VW with respect 
to time t. The estimated vehicle speed VSO of each road-wheel is obtained 
from the respective rotational speed of VW in light of the turning motion 
of the vehicle and other factors. 
The initiation/termination of the control recognition division 55 
establishes an initiation and a termination of the anti-skid control based 
on a brake output BK of the brake switch 62, the estimated vehicle speed 
VSO, the rotational acceleration DVW and the rotational speed VW. If the 
brake switch 62 is in on-condition and the estimated vehicle speed VSO is 
within a predetermined range, the anti-skid control is deemed to be 
initiated. If the termination of the pulse-increase mode is found in each 
road-wheel, then the anti-skid control is deemed to be terminated. 
In the motor control division 56, upon receipt of signals for the 
initiation and termination from the initiation/termination of the control 
recognition division 55, the motor 22a is turned on and turned off, 
respectively. 
The target slip rate calculating division 57 is set to calculate the target 
slip rate on the basis of the longitudinal acceleration GX, the lateral 
acceleration GY, the steering angle .delta. f, the real yaw rate .gamma., 
the load F, the estimated vehicle speed VSO, and the road surface 
condition. The detailed structure of the calculating division 57 is 
illustrated in FIG. 4. The target slip rate SO is calculated at each of 
the calculating units corresponding to the road-wheels, on the basis of 
the maximum deceleration G, the maximum vehicle speed VS1, the yaw rate 
deviation .DELTA..gamma., the road surface, the condition upon initiation 
of the ABS control, each estimated vehicle speed VSO, each rotational 
speed VW, and the load F. 
The maximum deceleration G is obtained at a maximum deceleration 
calculating unit 63 by using the following formula (1). 
EQU G+(GX.sup.2 +GY.sup.2).sup.1/2 (1) 
The maximum vehicle speed V1 is obtained, at a maximum vehicle speed 
calculating unit 64, as the maximum value of the estimated vehicle speed 
VSO of each road-wheel. As for the yaw rate deviation .DELTA..gamma., 
first of all, a target yaw rate .gamma.* is obtained at a target yaw rate 
calculating unit 65 by using the following formula (2). 
EQU .gamma.* =(G.sub.8 .times..delta.f)/(130 
.gamma.S).times.VS1/(1+Kh.times.VS1.sup.2) (2) 
The yaw rate deviation .DELTA..gamma. is calculated at a 
.DELTA..gamma.-calculating unit 66 by using the following formula (3). 
EQU .DELTA..gamma.+.gamma.* -.gamma. (3) 
The acceleration G obtained at the maximum deceleration calculating 
division 63 is fed to a .DELTA. G calculating division 77 to be subtracted 
by the latest value Gt-1 of the acceleration for obtaining an acceleration 
increment .DELTA. G. At the .DELTA. S-calculating portion 78, the 
acceleration increment .DELTA. G is corresponded to a graph shown in FIG. 
6 and a slip ratio increment .DELTA. S is obtained. The present target 
slip ratio S01 is calculated by addition of the resultant slip rate 
increment .DELTA. S and the latest slip rate S01t-1. 
In the foregoing processing, if the deceleration increases with the passing 
of time, .DELTA. G and a .DELTA. become positive, resulting in the 
increase of the target slip rate. Thus the slip quantity of each 
road-wheel is increased which leads to the increase of the vehicle's 
deceleration being restricted. On the other hand, if the deceleration 
decreases with the passing of time, .DELTA. G and .DELTA. S become 
negative, resulting in the decrease of the target slip rate. Thus the slip 
quantity of each road-wheel is decreased which leads to the vehicle's 
deceleration being increased. Thus the continuation of the foregoing 
processing will bring the maximum value of deceleration. In relation to 
the slip rate, the deceleration is proved to be of only one maximum value, 
which results in that this maximum value is the greatest value of the 
deceleration as apparent from real-line graphs A and B in FIG. 5. That is 
to say, in the foregoing processing, except for the slip rate regulating 
portion 80, remaining elements serve for obtaining the target slip rate 
which brings the greatest deceleration. As the graph shows, at a slip rate 
of about 10-20%, the .mu. attains its maximum value on a normal road. The 
.mu. is in proportion to the deceleration. Thus on the normal road, the 
braking operation under which the slip rate ranges from 10 to 20%, will 
bring the minimum braking distance. However, on a gravel road the locked 
condition of each road-wheel brings the minimum braking distance. In such 
case, as the graph B shows, the .mu. reaches its maximum value upon a slip 
rate of 100%. Under such control, on even such a road condition, the 
braking operation or the anti-skid condition is established for obtaining 
the maximum deceleration. Thus, according to the anti-skid condition of 
the present device, the minimum braking distance can be attained 
regardless of the road surface condition. 
In the foregoing control, the acceleration detected by the acceleration 
sensor is set to be maximized. However instead of such acceleration, the 
differential value of the vehicle speed VSO detected at each road-wheel is 
available. In addition, in light of the fact that the .mu. between the 
road surface and the road-wheel can be obtained on the basis of the 
acceleration G and the load F, the same results will be obtained by the 
control wherein the .mu. is maximized after its obtaining from the 
acceleration G and the load F for each road-wheel. The reason is that the 
load should be considered in order to establish more precious control. 
Thus resultant target slip rate S01 is regulated or restricted at the slip 
rate restriction division 80 to which is provided the product of the 
maximum vehicle speed VS1 obtained at the calculating division 67 and the 
absolute value of the yaw rate deviation .DELTA..gamma.. On the basis of 
this product value with reference to a graph shown in FIG. 7, a rate 
.alpha. is obtained and the resultant rate e is multiplied with the target 
slip rate S01 for obtaining the target slip rate S0. As the maximum 
vehicle speed VS1 increase or as the yaw rate deviation .DELTA..gamma., 
the target slip rate SO decreases. Thus so long as the steering operation 
and the actual turning motion of the vehicle are in coincidence, the 
control for chasing the maximum slip rate as will be detailed and if both 
become out of coincidence the target slip rate is set to be decreased for 
effecting the cornering force. 
Referring back to FIG. 1, the obtained target slip rate is set to be fed to 
the component calculating division 58 and on the basis of the following 
formula (4) the slip rate component DINDXS is calculated. 
EQU DINDXS=S0-(VS0-VW-IVW-BVW)/VS0 (4) 
wherein IVW is an integrated value of the rotational speed of the wheel VW, 
and BVW is a constant. 
In this formula, (VS0-VW)/VS0 corresponds to the real slip rate S1. The 
slip rate component DINDXS is then fed via a load invalidation processing 
division 83 to the control mode setting division 60. The load invalidation 
processing division 83 includes a limiter 831 and a coefficient setting 
portion 832. The limiter 831 establishes an invalid zone which serves for 
invalidating the function of the braking operation when the slip rate 
component SOW is less than a value as shown in FIG. 9. The reason is to 
prevent the control in response to a noise included in the slip rate 
component SOW. The foregoing integrated value IVW serves for the 
correction of the slip rate when the slip rate component SOW is generated 
within a minute range for a long time. The constant BVW serves for 
increasing a deviation between the target slip rate and the real slip rate 
when the rotational speed VW is low. As the vehicle speed becomes higher 
VS0 becomes extremely large relative to BVW, BVW becomes neglectably 
small. Thus the slip rate component S0W is a substantial modification of a 
value which is obtained by subtracting the real slip rate from the target 
slip rate SO and shows a slip rate deviation. 
The coefficient setting portion 832 serves for obtaining a coefficient 
.beta. from the load F with reference to a graph shown in FIG. 10. At a 
multiplying portion 833, the modification of a value is obtained which is 
the product of the coefficient .beta. and the slip rate component SOW 
passing through the limiter 831. The foregoing coefficient .beta. becomes 
100% when the load F is equal to or greater than a value of F2, and is 
decreased gradually below F2 which results in that .beta. becomes 0% at a 
value of F1. Thus, so long as the load F is in excess of F2, the 
coefficient .beta. is 100%, and the slip rate component SOW passing 
through the limiter 831 is as it is regarded as the slip rate component 
DINDXS. However as the load F decreases the coefficient .beta. also 
decreases, resulting in that the slip rate component DINDXS becomes less. 
In the G-component calculating division 59, a G-component GW is obtained by 
subtracting a set value GO from the wheel acceleration DVW. The resultant 
G-component GW is passed through a load invalidating processing division 
84 and is fed as the G-component DINDXG to the control mode setting 
division 60. The load invalidating processing division 84, similar to the 
foregoing load invalidating processing division 83, has a coefficient 
setting portion 842 and a multiplying portion 843. The limiter 841, as 
shown in FIG. 9 invalidates the control when the G-component GW is less 
than a set value. The reason is to prevent the control in response to a 
noise included in the G-component GW. At the coefficient setting portion 
842, with reference to a graph shown in FIG. 11, a coefficient .epsilon. 
is obtained on the basis of the load F. At a multiplying portion 843, the 
modification of a value is obtained which is the product of the 
coefficient .epsilon. and the slip rate component GW passing through the 
limiter 841. The foregoing coefficient .epsilon. becomes 100% when the 
load F is equal to or greater than a value of F4 and is decreased 
gradually below F4 which results in that .epsilon. becomes 0% at a value 
of F3. Thus, so long as the load F is in excess of F3, the coefficient 
.epsilon. is 100% and the slip rate component GW passing through the 
limiter 841 is, as it is, regarded as the slip rate component DINDXG. 
However, as the load F decreases, the coefficient .epsilon. also 
decreases, resulting in that the slip rate component DINDXG becomes less. 
The control mode setting division 60 begins to set a control mode upon 
receipt of the slip rate component DINDXS and the G-component. Three 
modes, the pulse-increase mode, the pulse-decrease mode and the 
rapid-decrease mode are available. As described previously, the pressure 
in each of the wheel cylinders is increase thereby to increase the braking 
force when the electromagnetic valves 32-35 are opened and the pressure in 
each of the wheel cylinders is decreased thereby to decrease the braking 
force when the electromagnetic valves 36-39 are opened. In the 
pulse-increase mode, under the closure of each of the electromagnetic 
valves 36-39, each of the electromagnetic valves 32-35 is set to be under 
the duty-control for increasing the braking pressure wherein an opening 
time of each of valves 32-35 is adjusted. In the pulse-decrease mode, 
under the closure of each of the electromagnetic valves 32-35, each of the 
electromagnetic valves 36-39 is set to be under the duty-control for 
decreasing the braking pressure wherein an opening time of each of valves 
36-39 is adjusted. In the rapid-decreasing mode, the pressure in each of 
the wheel cylinder is rapidly decreased by establishing a condition 
wherein valves 32-35 are closed and the valves 36-39 are opened. At the 
mode setting division 60, concurrently with setting the foregoing three 
modes, the valve opening time and the pulse width are set in case of the 
pulse-increasing mode or the pulse decreasing mode. The setting of each 
mode is established based on a chart shown in FIG. 12. In this chart, in 
principle, as the slip rate increases the pulse-increasing mode, the 
pulse-decreasing mode and rapid-decreasing mode are set to be established 
in such order and as the acceleration decreases the pulse-increasing mode, 
the pulse-decreasing mode and the rapid-decreasing mode ar set to be 
established in such order. When the G-component is 0, the pulses increase 
slightly in case the deviation between the target slip rate and the real 
slip rate is 0 and as the slip rate deviation increases, the 
pulse-decreasing mode and the rapid-decreasing mode are established in 
turn. In the chart, under this situation, corrections are set to be made 
toward the pulse-increasing mode and the pulse-decreasing mode when the 
deceleration is increased and decreased, respectively. Thus the braking 
force is so adjusted to establish a coincidence of the target slip rate 
with the real slip rate resulting in that ultimate coincidence is 
attained. Since this adjustment is made previously according to the 
acceleration, the quick control can be established. 
In this chart, a graph A shows a hold condition or another condition 
similar thereto under which the pulse is neither increased or decreased. 
Within a range of the pulse-increasing mode, a time required for 
increasing pressure increases when moving up to the right. In addition, 
within a range of the pulse-decreasing mode, a time required for 
decreasing pressure increases when moving down to the right. Thus in the 
neighborhood of the graph A, the time for increasing or decreasing 
pressure is less, the pressure in the wheel cylinder remains substantially 
unchanged. Since the graph A passes the original point of the chart, when 
either the slip rate component DINDXS or the G-component DINDXG becomes 
zero, the control enters the hold condition. Thus as previously mentioned, 
when the coefficient .beta. and .epsilon. are decreased in response to the 
decrease of the load F, the control approaches the hold condition which 
results in less pressure change in the wheel cylinder. When the load F 
becomes about zero, each coefficient also becomes zero resulting in the 
hold condition completely. Thus the pressure in the wheel cylinder remains 
unchanged. It is to be noted that in FIG. 12, a region within which the 
hold condition is established may be set between the pulse-increasing mode 
and the pulse-decreasing mode areas. 
The solenoid control division 61 serves for controlling the electromagnetic 
valves 32 through 39 based on the set mode at the control mode setting 
division 60, the valve opening time, the valve closing time, and the 
periodic time. In the pulse-increasing mode, the electromagnetic valves 
36-39 are entirely closed and the electromagnetic valves 32-35 are opened 
for a set time. The electromagnetic valves 32-35 are closed for a 
remaining time in a period. This opening and closing operation of each of 
the electromagnetic valves 32-35 is repeated. In the pulse-decreasing 
mode, the electromagnetic valves 32-35 are entirely closed and the 
electromagnetic valves 36-39 are opened for a set time. The 
electromagnetic valves 36-39 are closed for a remaining time in a period. 
This opening and closing operation of each of the electromagnetic valves 
36-39 is repeated. 
As mentioned above, in the present invention, since the pressure in each 
wheel cylinder is adjusted by the increase or the decrease thereof in case 
of the target slip rate, the slip condition of each road-wheel is in 
coincidence with the target value set in the control unit. When the load F 
decreased rapidly during the braking operation, the control is transferred 
to the hold mode or near the hold mode. Thereafter, the load recovers as 
it was and the control is returned to what it was. If no change is 
established relating to the control, the real slip rate is increased 
relative to the target slip rate, resulting in the increase of the slip 
rate component DINDIX in the minimum direction. This movement corresponds 
to the downward one in FIG. 12 which leads to the control into the 
rapid-decreasing mode. Thus the pressure in the wheel cylinder decreases. 
Under the resultant situation if the load F recovers, the pressure in the 
wheel cylinder is set to be increased in order to adjust the real slip 
rate relative to the target slip rate. It takes a long time for the 
pressure recovery which will delay the initiation of the control. This 
leads to the prolongation of the braking distance if such delay occurs 
during the braking operation. In addition, if such delay occurs in the 
course of the acceleration, its performance is not so good. However, 
according to the present invention, the control is brought into an 
insufficient one or a malfunction depending on the load, which ensures the 
prevention of the initiation of the control even if the load decreases 
during a jumping motion or a turning motion of the vehicle. 
As detailed above, the load invalidating processing divisions 83 and 84 
serve for adjusting the sensitivity of the braking operation depending on 
the graphs shown in FIGS. 10 and 11, respectively. Along a straight line 
between a point B and a point C, the coefficient .beta. remains zero which 
brings the malfunction or the invalidity of the newly established braking 
operation and thus the current braking force remains unchanged. Between 
the point C and a point D, the newly established braking operation becomes 
less or insufficient as the load decreases. Thus in the present embodiment 
as the load decreases the coefficient is lowered and the newly established 
braking operation becomes less or insufficient. In addition, when the load 
is less than a set value, the braking force remains unchanged by 
invalidating the newly established braking operation. However, other 
methods can bring the same result. For example, by moving the point C in 
order that the coefficient .beta. becomes zero or a value when the load is 
zero in FIG. 10, a practical result can be attained without realizing the 
complete invalidation of the newly established braking operation. In 
addition, by letting F1=F2, a rapid invalidation of the newly established 
braking operation may be available. In this method, a gradual realization 
of insufficient new establishment of the braking operation is omitted. 
Though in this embodiment the coefficient setting division is provided 
independently, an inclination of the graph shown in FIG. 9 can be changed 
depending on the load which is achieved by changing input-output 
characteristics regarding the load at each limiter 831/841. 
In this embodiment, in order to reduce the braking distance during the 
braking operation, the target slip rate is set to be calculated in order 
that each of the .mu. and the deceleration can be maximized. However, the 
target slip rate can be fixed at a set value previously. 
The invention has thus been shown and described with reference to a 
specific embodiment, however, it should be noted that the invention is in 
no way limited to the details of the illustrated structures but changes 
and modifications may be made without departing from the scope of the 
invention.