Four wheel drive vehicle with antilock brake device and associated method operation

An antilock brake method and device for a four wheel drive vehicle having front and rear axles, left and right wheels on each axle with brakes associated with each wheel, a power unit drivingly connected to one of the axles, the other axle being coupled for drive together with the driven axle, a hydraulic braking system for controlling the hydraulic pressure applied to the brakes, and an antilock control system coupled to the braking system for controlling the hydraulic braking pressure to reduce the brake pressure when a wheel is about to be locked. The antilock control system includes a front wheel control section for controlling the brakes of the front wheels, a rear wheel control section for controlling the brakes of the rear wheels, and circuit elements included in the front and rear wheel control sections for keeping the hyraulic braking pressure reduced up to the completion of a braking operation, when any of the wheels is about to be locked. In a particular embodiment the rear wheel control section reduces the braking pressure in the rear wheel brakes and maintains the pressure reduced during a braking operation when any of the wheels is about to be locked.

FIELD OF THE INVENTION 
The present invention relates to a four wheel drive vehicle with an 
antilock brake device, in which a power unit is connected to either one of 
the front and rear axles, with the other axle being connected to said one 
axle in intercoupled associated relation such that the drive of the axles 
interferes with each other, and an antilock control unit for controlling 
the braking hydraulic pressure to reduce the pressure when a wheel is 
about to be locked, said antilock control unit being associated with a 
braking hydraulic pressure system for controlling the hydraulic pressure 
of the brakes respectively mounted on each axle. 
The invention further relates to a method of operation of the antilock 
brake device. 
Incidentally, it should be noted here that throughout the specification 
when it is described that the axles interfere with each other, this means 
that the axles are in four-wheel drive state having a substantially rigid 
interconnection therebetween and the application of the brakes on the 
wheels of one axle will have effect on the drive of the wheels of the 
other axle. 
DESCRIPTION OF THE PRIOR ART 
In the conventional four wheel drive vehicle improvements are sought in the 
operating and travel performance on a road surface having a low friction 
coefficient, and attempts have been made to employ an antilock brake 
device on the four wheel drive vehicle. 
However, if an antilock control brake device as employed in the 
conventional two wheel drive vehicle is used in a four wheel drive 
vehicle, a disadvantage is produced in that, in the four wheel drive 
vehicle, the front and rear wheels interfere with each other and 
consequently, a satisfactory antilock effect cannot be obtained. 
SUMMARY OF THE INVENTION 
An object of the present invention is to overcome the above disadvantage of 
providing a four wheel drive vehicle with an antilock brake device in 
which the antilock control of the front and rear wheels is achieved 
efficiently and effectively. 
According to the present invention, an antilock control device is provided 
which includes a front wheel control section for controlling the brakes 
for the front wheels and a rear wheel control section for controlling the 
brakes for the rear wheels, and the front and rear wheel control sections 
are of such a contruction that the braking hydraulic pressure of the rear 
brake is maintained at reduced pressure up to the completion of the 
braking operation, when any of the wheels is about to be locked. 
During antilock control, the hydraulic braking pressure of the rear brake 
is reduced and therefore, the antilock control of the rear wheels can be 
insured by effecting the antilock control of the front wheels, thus 
providing a satisfactory antilock effect.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention will now be described with reference to the 
embodiments illustrated in the accompanying drawing. Referring first to 
FIG. 1 illustrating a first embodiment of the present invention, a pair of 
left and right front wheels Wfl and Wfr and a pair of left and right rear 
wheels Wrl and Wrr are suspended respectively at the front and rear 
portions of a body of a vehicle (not shown). 
A pair of front axles Afl and Afr, which are connected respectively to the 
left and right front wheels Wfl and Wfr, are interconnected through a 
front differential Df, and a pair of rear axles Arl and Arr, which are 
connected respectively to the left and right rear wheels Wrl and Wrr, are 
interconnected a rear differential Dr. The input of the front differential 
Df is connected to the power unit P. The input of the rear differential Dr 
is connected to a rear propeller shaft Pr which is coaxially connected to 
a front prepeller shaft Pf through a viscous clutch 1 serving as a torque 
transmitting mechanism, and the drive force from the power unit P is 
transmitted to the front propeller shaft Pf. 
The viscous clutch 1 includes a closed oil chamber 4 containing inner and 
outer clutch elements 2 and 3 which are rotatable relative to each other. 
A highly viscous oil with a small amount of air to permit the thermal 
expansion of the highly viscous oil is sealed in the closed oil chamber 4. 
The viscous clutch 1 also includes a plurality of outer clutch plates 5 
spline-connected to the outer clutch element 2 and a plurality of inner 
clutch plates 6 spline-connected to the inner clutch element 3, the inner 
and outer clutch plates being interleaved with each other. Each of the 
plates 5 and 6 is provided with an opening or groove (not shown) which 
permits the passage of the oil. The outer clutch element 2 is integral 
with the front propeller shaft Pf, and the inner clutch element 3 is 
integral with the rear propeller shaft Pr. 
In the viscous clutch 1, when relative rotation occurs between the outer 
clutch element 2 and the inner clutch element 3, both the clutch plates 5 
and 6 rotate relative to one another while shearing the highly viscous 
oil, so that the viscous transmission of torque is obtained between the 
clutch plates 5 and 6. When the speed of the relative rotation is further 
increased, a complex temperature gradient is produced in both clutch 
plates 5 and 6 due to the increase in oil temperature, causing a 
synergistic effect of a strain attributable to this temperature gradient 
with an increase in pressure within the closed oil chamber 4 to provide a 
frictionally engaged portion or a portion having an extremely small gap 
between the adjacent clutch plates 5 and 6. As a result, the frictional 
transmission of torque is insured between the outer clutch element 2 and 
the inner clutch element 3. 
With such a viscous clutch 1, the front propeller shaft Pf and the rear 
propeller shaft Pr and thus, the front axles Afl, Afr and the rear axles 
Arl, Arr are always in a substantially rigidly interconnected state, so 
that the front wheels Wfl, Wfr and the rear wheels Wrl, Wrr interfere with 
each other. 
Brakes Bfl, Bfr are respectively mounted on the front wheels Wfl, Wfr, and 
brakes Brl, Brr are respectively mounted on the rear wheels Wfl, Wrr. 
Referring to FIG. 2, a braking hydraulic pressure system 7 for controlling 
the hydraulic pressure of each of the brakes Bfl, Bfr, Brl and Brr 
comprises a tandem type master cylinder 8 having a pair of output ports 8a 
and 8b, modulators Mfl and Mrr for regulating the hydraulic pressure 
supplied from the output port 8a to transmit the pressure to the left 
front wheel brake Bfl and the right rear wheel brake Brr, and modulators 
Mfr and Mrl for regulating the hydraulic pressure supplied from the output 
port Stb to transmit the pressure to the right front wheel brake Bfr and 
the left rear wheel brake Wrl. The braking hydraulic pressure system 7 is 
associated with an antilock control device 9 for controlling the operation 
of the modulators Mfl, Mfr, Mrl and Mrr to prevent the wheels from going 
into a locked state. 
The antilock control device 9 includes a front wheel control section 9a for 
individually controlling the modulators Mfl and Mfr of the front wheels Wfl 
and Wfr, and a rear wheel control section 9b for simultaneously controlling 
the modulators Mrl and Mrr of the rear wheels Wrl and Wrr, so that signals 
from detectors 10l and 10r for detecting the speeds of the front wheels 
Wfl and Wfr are supplied to the front control section 9a, and signals from 
detectors 11l, and 11r for detecting the speeds of the rear wheels Wrl and 
Wrr are supplied to the rear control section 9b. 
Reference is next made to FIG. 3 for the description of the arrangement of 
the front wheel control section 9a, wherein because the portion 
corresponding to one of the modulators Mfl is basically of the same 
construction as the portion corresponding to the other modulator Mfr, the 
parts associated with modulator Mfl are designated by addition of 
reference characters l and they will be explained hereinafter. Meanwhile, 
the parts associated with the modulator Mfr are designated by addition of 
reference characters r and their description will be omitted. 
To determine whether the wheel is ready to go into the locked state, a 
signal representing wheel speed Vw detected in the detector 10l is fed to 
the inverse terminal of a first comparator 13l and to an operator circuit 
12l in which a signal representing wheel acceleration Vw is produced and 
fed to the inverse terminal of a second comparator 14l and the non-inverse 
terminal of a third comparator 15l, respectively. In the first comparator 
13l, the wheel speed Vw is compared with a reference wheel speed Vr stored 
in the non-inverse terminal thereof and when Vr&gt;Vw, a signal .lambda. for 
moderating the hydraulic braking pressure is delivered from the first 
comparator 13l. In the second comparator 14l, the wheel acceleration Vw is 
compared with a reference wheel deceleration -Vw.sub.O stored in the 
non-inverse terminal and when -Vw.sub.O &gt;Vw, a signal .beta. for 
moderating the hydraulic pressure is delivered from the second comparator 
14l. Further, in the third comparator 15l, the wheel acceleration Vw is 
compared with a reference wheel acceleration +Vw.sub.O stored in the 
inverse terminal and when Vw&gt;+Vw.sub.O, a signal .alpha. is delivered from 
the third comparator 15l. The signal .alpha. serves to check whether the 
wheel speed Vw is increasing, so that the period of time for which the 
moderation of the hydraulic braking presure is continued is determined by 
signal .alpha.. 
The output terminal of the first comparator 13l is connected to the input 
terminal of an AND gate 16l and to the input terminal of an OR gate 17l. 
The output terminal of the second comparator 14l is connected to the input 
terminals of AND gate 16l and OR gate 17l. Further, the output terminal of 
the third comparator 15l is connected to the input terminal of OR gate 
17l. 
The output terminal of the AND gate 16l is invertedly connected to the 
input terminals of AND gates 18l and 19l and to an output terminal 20l. 
The output terminal of the OR gate 17l is connected to the input terminal 
of the AND gate 18l whose output terminal is connected to an output 
terminal 22l and invertendly connected to the input terminal of the AND 
gate 19l. Further, the output terminal of the AND gate 19l is connected to 
an output terminal 21l. 
In the front control section 9a, a signal for reducing the braking pressure 
is delivered from the output terminals 20l and 20r, and a signal for 
increasing the braking pressure is delivered from the output terminals 21l 
and 21r, and further, a signal for maintaining the braking pressure 
constant is delivered from the output terminals 22l and 22r. The 
modulators Mfl is operated in response to the signals from the output 
terminals 20l, 21l and 22l, and the modulator Mfr is operated in response 
to the signals from the output terminals 20r, 21r, and 22r, whereby the 
antilock control operations are individually conducted for both the brakes 
Bfl and Bfr. 
Description will next be made of the arrangement of the rear wheel control 
section 9b with reference to FIG. 4. The arrangement of the rear wheel 
control section 9b is similar to that of the front wheel control section 
9a and hence, the parts corresponding to those of the front wheel control 
section 9a are designated by the same reference characters with no letter 
l or r affixed thereto. 
Attention is directed in the rear wheel control section 9b to the 
arrangement in which the signals representing the wheel speeds detected in 
the detectors 11l and 11r are fed to a low speed selector circuit 23 and 
the lower wheel speed selected in the low speed selector circuit 23 is fed 
to the first comparator 13 and the operator circuit 12. More specifically, 
the antilock control is conducted in coordination to that one of the left 
and right rear wheels Wrl and Wrr which is more easily locked, i.e., the 
wheel having the lower speed, and the activation of both modulators Mrl 
and Mrr are simultaneously controlled through control signals derived from 
the output terminals 20, 21 and 22. 
Moreover, in the rear wheel control section 9b, a flip-flop 24 is 
interposed between the AND gate 16 and the AND gates 18 and 19 as well as 
the output terminal 20. More particularly, the output terminal of the AND 
gate 16 is connected to a set input terminal S of the flip-flop 24 whose 
set output terminal Q is connected to the output terminal 20 and 
invertedly connected to the AND gates 18 and 19. In addition, a braking 
operation detector 26, which delivers a high level signal upon the 
detection of the braking operation by a brake pedal 25 (see FIG. 2), is 
invertedly connected to a reset input terminal R of the flip-flop 24. 
With such arrangement of the rear wheel control section 9b, when it is 
detected that one of the rear wheels Wrl and Wrr is about to be locked by 
generation of high level signals .lambda. and .beta. from the first and 
second comparators 13 and 14, so that the output of the AND gate 16 is at 
a high level, the set output of the flip-flop 24 is at a high level until 
the output of the braking operation detector 26 becomes a low level at the 
completion of the braking operation, i.e., until the set input of the 
flip-flop 24 becomes a low level. Consequently, when the rear wheels Wrl 
and Wrr are about to be locked, the output of the output terminal 20 is at 
a high level until the braking operation is completed, and the hydraulic 
braking pressure of the rear wheel brakes Brl and Brr is maintained at a 
decreased level. 
The operation of this embodiment is as follows. When the rear wheels Wrl 
and Wrr are about to be locked upon braking during travel of the vehicle, 
the output of the flip-flop 24 goes into a high level state in accordance 
with the output of the AND gate 16 going into a high level state, so that 
the hydraulic braking pressure of both rear wheel brakes Brl and Brr is 
substantially reduced to the level of atmospheric pressure by the high 
level signal at the output terminai 20. Moreover, such condition is 
continued up to the completion of the braking operation. 
For this duration of time, the rear wheels Wrl and Wrr are responsive to 
the rotation of the front wheels Wfl and Wfr, and the hydraulic braking 
pressure of the front wheel brakes Bfl and Bfr is controlled in the front 
wheel control section 9a, whereby the rotation of the rear wheels Wrl and 
Wrr can be controlled, thus providing a satisfactory antilock effect. 
The above embodiment has been described as being applied to a four wheel 
drive vehicle in which the front axles Afl and Afr are connected with the 
rear axles Arl and Arr through the viscous clutch 1, but the present 
invention is also applicable to a four wheel drive vehicle of a part time 
type having axles Afl, Afr and Arl, Arr interconnected through a clutch 
adapted to be manually shifted between engaged and disengaged states, when 
the clutch is in the engaged state, and to a four wheel drive vehicle 
having axles Afl, Afr and Arl, Arr interconnected through a differential 
having a locking mechanism, when the differential is in a locked state. 
The following is the description of embodiments applied to such four wheel 
drive vehicles. 
FIG. 5 illustrates a rear wheel control section in accordance with a second 
embodiment of the present invention, wherein the parts corresponding to 
those of the first embodiment are denoted by the same reference 
characters. 
The output terminal of AND gate 16 is connected to one of the input 
terminals of OR gate 27 whose output terminal is connected to output 
terminal 20 and invertedly connected to the input terminals of AND gates 
18 and 19. The output terminal of OR gate 17 is connected to the input 
terminal of OR gate 28 whose output terminal is connected to the input 
terminal of the AND gate 18. 
In addition, the output terminal of tne AND gate 16 is also connected to 
the input terminal of an OR gate 29 having other input terminals to which 
are also connected output terminals 20l and 20r in the front wheel controi 
section 9a. The output terminal of the OR gate 29 is connected to one of 
the input terminals of an AND gate 30. A four wheel drive state detector 
33 is connected to the other input terminal of the AND gate 30. The four 
wheel drive state detector 33 detects the engagement of a clutch or the 
locked state of a differential to produce a high level signal. Thus, the 
output of the AND gate 30 goes into a high level state, when the hydraulic 
braking pressure is intended to be reduced upon slipping of either of the 
front wheels Wfl and Wfr or the rear wheels Wrl and Wrr. 
The output terminal of the AND gate 30 is connected to a delay circuit 31. 
When a signal as shown in FIG. 5A (a) is fed to circuit 31 the output of 
delay circuit 31 increases in coordination with the increase of the input 
signal and falls after the lapse of a given time T from the fall of the 
input signal, as shown in FIG. 5A (b). 
The output terminal of the delay circuit 31 is connected to an OR gate 27 
and also to the set input terminal S of a flip-flop 32. A brake operation 
detector 26 is invertedly connected to the reset input terminal R of the 
flip-flop 32 whose set output terminal Q is connected to an OR gate 28. 
In the operation of the second embodiment, when the rear wheels Wrl and Wrr 
are about to be locked during a braking operation, the output of the output 
terminal 20 goes into a high level state, as the output of the AND gate 16 
goes into a high level state. Even after the state of the rear wheels 
being about to be locked is released, the output of the OR gate 27 is at a 
high level state up to the lapse of the given time T and hence, the output 
of the output terminal 20 maintains a high level state during this time T. 
This causes the hydraulic braking pressure of the rear wheel brakes Brl and 
Brr to be significantly reduced. After the lapse of the aforesaid time T, 
the output of the output terminal 22 becomes a high level state as the 
output of the AND gate 18 goes into a high level state. Such state will be 
maintained up to the completion of the braking operation. 
Also when either of the front wheels Wfl and Wfr is about to be locked, the 
output of the OR gate 27 goes into a high level state in the same manner as 
described above, and after the lapse of the given time T, the output of the 
AND gate 18 becomes a high level state. 
Thus, when any of the wheels Wfl, Wfr, Wrl and Wrr is about to be locked, 
the braking hydraulic pressure of the rear wheel brakes Brl and Brr is 
reduced, and such state will be maintained up to the completion of the 
braking operation and consequently, a satisfactory antilock effect is 
provided as in the first embodiment. 
FIG. 6 illustrates a third embodiment of the present invention, wherein the 
elements corresponding to those of the second embodiment are designated by 
the same reference characters as in the second embodiment. 
In this third embodiment, the OR gate 28 and the delay circuit 31 in the 
second embodiment are omitted, and the output terminal of OR gate 17 is 
connected to the input terminal of AND gate 18. The output terminal of AND 
gate 30 is connected to the set input terminal S of flip-flop 32 whose set 
output terminal Q is connected to the input terminal of OR gate 27. 
In this third embodiment, if any of the wheels Wfl, Wfr, Wrl and Wrr is 
about to lock, an output from the OR gate 27, i.e., a signal derived from 
the output terminal 20 is at a high level state up to the completion of 
the braking operation, and the hydraulic braking pressure of the rear 
wheel brakes Brl and Brr continues to be reduced. 
As seen from the above, according to the present invention, the hydraulic 
braking pressure in the rear wheel brakes remains reduced up to the 
completion of the braking operation, when any of the wheels is about to be 
locked. Therefore, travel stability can be maintained and an antilock 
control for the front wheels can be effected without and interference from 
the rear wheels, leading to an effective antilock control for all the 
wheels. 
Although the invention has been described in relation to specific 
embodiments thereof, it will become apparent to those skilled in the art 
that numerous modifications and variations can be made without departing 
from the scope and spirit of the invention as defined in the attached 
claims.