Hydraulic brake system for controlling fluid flow to a wheel cylinder

A hydraulic brake system is provided which does not need a mechanism to feed back the wheel cylinder pressure. The hydraulic brake system generates a brake force by supplying the brake fluid to a wheel cylinder under a high pressure generated by a high pressure source. The brake fluid is supplied to the high pressure source from a reservoir tank storing the brake fluid under atmospheric pressure. A fluid passage connects the high pressure source to both the wheel cylinder and the reservoir tank. An amount of the brake fluid flowing from the high pressure source to the wheel cylinder is controlled by a constant flow valve.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a hydraulic brake system and, 
more particularly, to an automobile hydraulic brake system which gene 
rates a brake force by introducing fluid pressure generated by a high 
pressure source into a wheel cylinder. 
2. Description of the Related Art 
A conventional hydraulic brake system is described in Japanese Laid-Open 
Patent Application No.6-344894 which generates a desired brake force by 
introducing a hydraulic pressure generated by a hydraulic pump into a 
wheel cylinder when a brake pedal is operated. The hydraulic pressure 
introduced into the wheel cylinder is responsive to a force applied to the 
brake pedal. 
In a hydraulic brake system of this type, since the hydraulic pressure 
introduced into the wheel cylinder is generated by the hydraulic pump, a 
large brake force can be obtained by applying only a small force to the 
brake pedal. Accordingly, a brake system having such a hydraulic brake 
provides a good brake feeling. 
The hydraulic brake system described in the above-mentioned patent 
application has a spool valve which selectively connects one of the 
hydraulic pump and the reservoir tank to the wheel cylinder. One end of 
the spool valve is provided with a first pressure receiving surface onto 
which a master cylinder pressure generated by the master cylinder is 
applied. Thus, a force equal to a multiplication of the master cylinder 
pressure and the pressure reviving area of the first pressure receiving 
surface is applied to the spool valve. This force is hereinafter referred 
to as a spool valve driving force. The other end of the spool valve is 
provided with a second pressure receiving surface onto which a wheel 
cylinder pressure generated by the wheel cylinder is applied. Thus, a 
force equal to a multiplication of the wheel cylinder pressure and the 
pressure receiving area of the second pressure receiving surface is 
applied to the spool valve. This force is hereinafter referred to as a 
feedback driving force. When the spool valve driving force is greater than 
the feedback driving force, the spool valve is moved in a direction where 
the hydraulic pump is connected to the wheel cylinder. Or the other hand, 
when the spool valve driving force is less than the feedback driving 
force, the spool valve is moved in a direction where the reservoir tank is 
connected to the wheel cylinder. 
According to the above-mentioned structure, when the spool valve driving 
force is greater than the feedback driving force, the wheel cylinder is 
connected to the hydraulic pump until the feedback driving force becomes 
equal to the spool valve driving force. On the other hand, when the 
feedback driving force is greater than the spool valve driving force, the 
wheel cylinder is connected to the reservoir tank until the feedback 
driving force becomes equal to the spool valve driving force. In this 
case, the feedback driving force accurately balances with the spool valve 
driving force. Thus, according to the above-mentioned hydraulic brake 
system, a precisely controlled pressure, which is responsive to a force 
applied by an operator and supplied to the wheel cylinder, can be 
generated by using the hydraulic pump as a high pressure source. 
However, since one of the hydraulic cylinder and the reservoir tank is 
selectively connected to the wheel cylinder, the wheel cylinder pressure 
is continuously increased while the connection between the hydraulic pump 
and the wheel cylinder is selected. Thus, it is required to interrupt the 
connection between the wheel cylinder and the hydraulic pump, when the 
wheel cylinder pressure reaches a predetermined pressure relative to the 
master cylinder pressure, by applying to the spool valve the feedback 
driving force which is against the spool valve driving force generated by 
the master cylinder pressure. That is, a mechanism is needed to apply the 
wheel cylinder pressure to the spool valve as the feedback driving force 
which is against the spool valve driving force. In this respect, the 
above-mentioned conventional hydraulic brake system has room for 
improvement in that a complex mechanism is needed to control the wheel 
cylinder pressure. 
SUMMARY OF THE INVENTION 
It is a general object of the present invention to provide an improved and 
useful hydraulic brake system. 
A more specific object of the present invention is to provide a hydraulic 
brake system which does not need a mechanism to feed back the wheel 
cylinder pressure due to controlling a flow of brake fluid from a high 
pressure source to the wheel cylinder when a high pressure source is 
connected to both of the wheel cylinder and the reservoir tank. 
In order to achieve the above-mentioned objects, there is provided 
according to the present invention, as illustrated in FIG. 1, a hydraulic 
brake system for generating a brake force by supplying brake fluid to a 
wheel cylinder (P12) under a high pressure generated by a high pressure 
source (P10), the hydraulic brake system comprising: 
a reservoir tank (P14) storing the brake fluid under atmospheric pressure, 
a fluid passage (P16) connecting the high pressure source (P10) to both of 
the wheel cylinder (P12) and the reservoir tank (P14); and 
flow control means (P18) for controlling an amount of the brake fluid 
flowing from the high pressure source (P10) to the wheel cylinder (P12). 
In the above-mentioned invention, the high pressure source (P10) is 
connected to the reservoir tank (P14) as well as the wheel cylinder (P12). 
Thus, the brake fluid discharged from the high pressure source (P10) and 
flows to the wheel cylinder (P12) and the reservoir tank (P14) via the 
fluid passage (P16). The amount of the brake fluid flowing from the high 
pressure source (P10) to the wheel cylinder (P12) is controlled by the 
flow control means (P18). Flow control means (P18) comprises a variable 
volume pump as shown in FIG. 13, or may comprise a variable orifice as 
shown in FIG. 15. The pressure in the wheel cylinder (12) increases as the 
amount of the brake fluid flowing into the wheel cylinder (P12) is 
increased. On the other hand, the amount of the brake fluid flowing from 
the high pressure source (P10) to the reservoir tank (P14) increases as 
the pressure in the wheel cylinder (P12) is increased. After the pressure 
in the wheel cylinder (P12) has reached a pressure controlled by the fluid 
control means (P18), the whole amount of the brake fluid discharged by the 
high pressure source (P10) flows to the reservoir tank (P14). At the same 
time a brake force corresponding to the pressure in the wheel cylinder 
(P12) is generated. Thus, the brake force generated by the wheel cylinder 
(P12) is controlled by the flow control means (P18). Accordingly, the 
hydraulic brake system of the present invention has a simple construction 
without a feedback mechanism for the pressure in the wheel cylinder, and a 
precisely controlled brake force corresponding to an operational force 
applied to a brake can be generated. 
The hydraulic brake system of the present invention may further comprise, 
as illustrated in FIG. 2: 
brake means (P20) for generating a predetermined force corresponding to an 
operational force applied thereto; 
a first opening/closing valve (P24) provided in a part (P22) of the fluid 
passage (P16) between the high pressure source (P10) and the wheel 
cylinder (P12) to close the part (P22) of the fluid passage (P16) when the 
operational force applied to the brake means (P20) is less than a 
predetermined value; 
a pressure release passage (P26) connecting the wheel cylinder (P12) to the 
reservoir tank (P14); and 
a second opening/closing valve (P28) provided in the pressure release 
passage (P26) to open he pressure release passage (P26) when the 
operational force applied to the brake means (P20) is less than a 
predetermined value. 
According to this invention, when the operational force applied to the 
brake means (P20) is less than the predetermined value, the first 
opening/closing valve (P24) closes the part (P22) of the fluid passage 
(P16) and second opening/closing valve (P28) opens the pressure release 
passage (P26). Thus, the flow of the brake fluid from the high pressure 
source (P10) to the wheel cylinder (P12) is stopped, and the flow from the 
high pressure source (P10) to the reservoir tank (P14) is permitted. In 
this condition, occurrence of brake drag is prevented since the pressure 
in the wheel cylinder is maintained at atmospheric pressure. The brake 
means P20 includes a master cylinder (not shown) which provides fluid flow 
(not shown) to the wheel cylinder P12 when opening/closing valve P24 is 
closed. On the other hand, when the operational force applied to the brake 
means (P20) exceeds the predetermined value, a par (P22) of the fluid 
passage (P16) is opened by the first opening/closing valve (P24) and the 
pressure release passage (P26) is closed by the second opening/closing 
valve (P28). In this condition, the wheel cylinder (P12) is connected to 
the high pressure source (P10) so as to generate an appropriate brake 
force controlled by the flow control means (P18). 
In the hydraulic brake system of the present invention, as illustrated in 
FIG. 3, the high pressure source (P10) may comprise a variable volume pump 
(P30), and the flow control means (P18) comprises variable volume control 
means for controlling an output volume from the variable volume pump 
(P30). 
In this invention, the amount of brake fluid flowing to the wheel cylinder 
(P12) is increased when the amount of brake fluid discharged by the 
variable volume pump (P30) is increased. On the other hand, the amount of 
brake fluid flowing to the wheel cylinder (P12) is decreased when the 
amount of brake fluid discharged by the variable volume pump (P30) is 
decreased. Thus, an appropriate amount of the brake fluid can be supplied 
to the wheel cylinder (P12) by controlling the output volume of the 
variable volume pump (P30), resulting in a desired brake force being 
generated. Energy input to the variable volume pump (P30) is decreased as 
the output of the variable volume pump (P30) is decreased. Thus, if the 
output volume of the variable volume pump (P30) is decreased when a 
required brake force is small, energy is not wasted especially when the 
brake is not actuated. Accordingly, the hydraulic brake system of this 
invention can reduce energy consumption. 
Additionally, in the hydraulic brake system according to the present 
invention, as illustrated in FIG. 4, the fluid passage (P16) may comprise, 
high pressure source passage (P32) connected to the high pressure source 
(P10), a wheel cylinder passage (P34) connecting the high pressure source 
passage (P32) to the wheel cylinder (P12) and a reservoir passage (P36) 
connecting the high pressure source passage (P32) to the reservoir tank 
(P14), and wherein the flow control means (P18) comprises a variable 
throttle (P38) provided in the reservoir passage (P26). 
In this invention, the brake fluid discharged from the high pressure source 
(P10) flows through the high pressure source passage (P32) and flows into 
both of the wheel cylinder (P12) and reservoir tank (P14) via the wheel 
cylinder passage (P34) and the reservoir passage (P36), respectively. The 
amount of brake fluid flowing to the wheel cylinder (P12) is increased as 
flow resistance of the reservoir passage (P36) is increased, and is 
decreased as the flow resistance of the reservoir passage (P36) is 
decreased. In this invention, the flow resistance of the reservoir passage 
(P36) is determined by the effective opening area of the variable throttle 
(P38). Accordingly, a desired brake force can be generated by controlling 
the effective opening area of the throttle (P38) to supply an appropriate 
amount of brake fluid to the wheel cylinder (P12). The control of the 
variable throttle can be achieved by a known technique in the art. Thus, 
the hydraulic brake system according to this invention can easily and 
precisely control the brake force generated by the wheel cylinder (P12). 
Additionally, the hydraulic brake system according to the present invention 
may further comprise, as illustrated in FIG. 5: 
a master cylinder (P40) generating a fluid pressure corresponding to an 
operational force applied to a brake; 
a master cylinder passage (P42) connecting the master cylinder (P40) to the 
wheel cylinder (P12); and 
an opening/closing valve (P44) provided in the master cylinder passage 
(P42) to open the mater cylinder passage (P42) when the high pressure 
generated by the high pressure source (P10) is less than a predetermined 
pressure. 
In this invention, the master cylinder (P40) generates a pressure (a master 
cylinder pressure) corresponding to the operational force applied to the 
brake. The master cylinder pressure is introduced into the master cylinder 
passage (P42) which is connected to the wheel cylinder (P12). The master 
cylinder passage (P42) is closed by the third opening/closing valve (P44) 
when the high pressure generated by the high pressure source (P10) exceeds 
the predetermined value. In this condition, the master cylinder pressure 
is not supplied to the wheel cylinder (P12). On the other hand , when the 
high pressure source (P10) generates a pressure less than the 
predetermined value, the third opening/closing valve (P44) opens the 
master cylinder passage (P42). In this condition, the master cylinder 
pressure is supplied to the wheel cylinder (P12). Accordingly, when the 
high pressure source (P10) does not generate a normal high pressure, the 
master cylinder pressure is supplied to the wheel cylinder (P12). 
Accordingly, the master cylinder pressure is supplied to the wheel 
cylinder (P12) so as to generate a brake force even when a malfunction 
occurs in the high pressure source (P10). 
Additionally, the hydraulic brake system according to the present invention 
may further comprise, as illustrated in FIG. 6; 
a master cylinder (40) generating a fluid pressure corresponding to an 
operational force applied to a brake; 
a master cylinder passage (P42) connecting the master cylinder (P40) to the 
wheel cylinder (P12); and 
a check valve (P46) provided in the master cylinder passage to permit a 
flow of the brake fluid only in a direction from the master cylinder (P40) 
to the wheel cylinder (P12). 
In this invention, the master cylinder pressure is supplied to the check 
valve (P46) in a normal direction, and the wheel cylinder pressure is 
applied to the check valve (P46) in a reverse direction. When the high 
pressure source (P10) is generating a normal high pressure, the high 
pressure is supplied to the wheel cylinder (P12) when the brake is 
actuated. If the wheel cylinder pressure is higher than the master 
cylinder pressure, the check valve (P46) does not open. However, if the 
high pressure source (P10) does not generate a normal high pressure, that 
is, if the master cylinder pressure is higher than the wheel cylinder 
pressure, the check valve (P46) is opened and the master cylinder pressure 
is supplied to the wheel cylinder (P12). Accordingly, in the hydraulic 
brake system of the present invention, the master cylinder pressure is 
supplied to the wheel cylinder (P12) so as to generate a brake force even 
when a malfunction occurs in the high pressure source (P10). 
Additionally, the hydraulic brake system shown in FIG. 4 may further 
comprise, as illustrate in FIG. 7, a master cylinder (P40) generating a 
fluid pressure corresponding to an operational force applied to a brake, 
wherein the variable throttle comprises a spool valve (P38) for decreasing 
an amount of the brake fluid flowing in the reservoir passage (P36) as the 
fluid pressure generated by said master cylinder is increased. 
In this invention, the variable throttle comprises the spool valve (P38) 
which decreases the flow of the brake fluid flowing in the reservoir 
passage (P36) as the master cylinder pressure is increased. In this case, 
a brake force corresponding to the operational force applied to the brake 
can be realized by a mechanical construction in which the spool valve 
(P38) and the master cylinder (P40) are connected to each other. 
Accordingly, in the present invention, a hydraulic brake system generating 
a precisely controlled brake force can be realized with a simple 
construction. 
Additionally, in the hydraulic brake system shown in FIG. 4, as illustrated 
in FIG. 8, the variable throttle (P38) comprises an effective opening area 
varied by an external input and adjust means (P50) for adjusting the 
effective opening area based on a predetermined brake force control. 
In this invention, the adjust means (P50) adjusts the effective opening 
area of the variable throttle (P38). The brake force generated by the 
wheel cylinder (P12) varies in response to the effective opening area of 
the variable throttle (P38). Accordingly the wheel cylinder (P12) can 
generate a brake force controlled by a predetermined brake force control 
logic known in the art such as an antilock brake system or a traction 
control system. 
Additionally, the hydraulic brake system shown in FIG. 4 may further 
comprise, as illustrate in FIG. 9, constant flow means (P52) for 
maintaining a constant flow rate of the brake fluid flowing in the high 
pressure source passage (P32). 
In this invention, the constant flow means (P52) maintains the flow rate of 
the brake fluid flowing in the high pressure source passage (P32). The 
wheel cylinder pressure is determined by the effective opening area of the 
variable throttle (P38) and the amount of brake fluid flowing in the high 
pressure source passage (P32). Thus, when the flow rate in the high 
pressure source passage (P32) is maintained to be constant, a pressure 
precisely corresponding to the effective opening area of the variable 
throttle (P38) is generated. Accordingly, the hydraulic brake system 
according to this invention can precisely controls a brake force by 
controlling the effective opening area of the variable throttle (P38). 
Additionally, the hydraulic brake systems shown in FIGS. 5 and 6 may 
further comprise, as illustrated in FIG. 10, an opening/closing valve 
(P56) provided in the reservoir passage (P36) to close the reservoir 
passage (P36) when the high pressure generated by the high pressure source 
(P10) is less than a predetermined pressure. 
In this invention, when the high pressure source (P10) is generating a 
normal high pressure, the fourth opening/closing valve (P56) opens the 
reservoir passage (P36). In this state, since the wheel cylinder (P12) 
connects to the reservoir tank (P14), no residual pressure remains in the 
wheel cylinder (P12). On the other hand, the fourth opening/closing valve 
(P56) closes the reservoir passage (P36) when the pressure generated by 
the high pressure source (P10) is less than a predetermined value. In this 
condition, a pressure can be supplied to the wheel cylinder (P12) from the 
master cylinder (P40). Accordingly, if a malfunction occurs in the high 
pressure source (P10), a pressure can be supplied from the master cylinder 
(P40) to the wheel cylinder (P12) to positively and effectively pressurize 
the wheel cylinder (P12). 
Other objects, features and advantages of the present invention will become 
more apparent from the following detailed description when read in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A description will now be given, with reference to FIGS. 11 to 16, of a 
first embodiment of a present invention. FIG. 11 is a perspective view of 
a vehicle 10 which is equipped with a hydraulic brake system according to 
the first embodiment of the present invention. The vehicle 10 has left and 
right front wheels FL and FR and left and right rear wheels RL and RR. The 
wheels FL, FR, RL and RR are provided with disk brakes 12F, 12FR, 12RL and 
12RR, respectively. 
The disk brakes 12FL and 12FR provided in the left and right front wheels 
FL and FR have disk rotors 14FL and 14FR and wheel cylinders 16FL and 
16FR, respectively. The disk rotors 14FL and 14FR rotate with the 
respective wheels. The wheel cylinders 16FL and 16FR applies a supplied 
brake torque corresponding to a brake fluid pressure to the respective 
disk rotors 16FL and 16FR. 
The disk brakes 12RL and 12RR provided in the left and right rear wheels RL 
and RR have disk rotors 14RL and 14RR and wheel cylinders 16RL and 16RR, 
respectively. The disk rotors 14RL and 14RR rotate with the respective 
wheels. The wheel cylinders 16RL and 16RR applies a brake torque 
corresponding to a supplied brake fluid pressure to the respective disk 
rotors 16FL and 16FR. Additionally, the disk rotors 14RL and 14RR are 
provided with parking brakes 18RL and 18RR, respectively. The parking 
brakes 18RL and 18RR generate a brake force when a parking brake lever 20 
is pulled or actuated. 
The above-mentioned wheel cylinders 16F, 16FR, 16RL and 16RR are connected 
to a hydraulic booster actuator 24 via respective brake conduits 22FL, 
22RL, 22RL and 22RR. The hydraulic booster actuator 24 supplies a 
hydraulic pressure responsive to an operational force applied to a brake 
pedal 26. The hydraulic brake system according to the present embodiment 
generally comprises the hydraulic booster actuator 24 and the wheel 
cylinders 16FL, 16FR, 16RL and 16RR, and particularly relates to a 
structure of the hydraulic booster actuator 24. 
FIG. 12 is a hydraulic circuit diagram of a hydraulic circuit formed by the 
hydraulic booster actuator 24 and the wheel cylinder 16FL. Although 
hydraulic circuits corresponding to other wheel cylinders 16FR, 16RL and 
16RR are formed in the hydraulic booster actuator 24, a description will 
be given of the hydraulic circuit correspond to the wheel cylinder 16FR 
for the sake of convenience because the construction of other hydraulic 
circuits are similar to each other. 
As shown in FIG. 12, the brake pedal 26 is connected to an input shaft 28a 
of a master cylinder 28. The master cylinder 28 converts a thrust force 
applied to the input shaft 28a into a hydraulic pressure. Thus, when an 
operational force is applied to the brake pedal 26, the master cylinder 28 
generates a brake fluid pressure corresponding to the operational force. 
The master cylinder 28 is connected to the wheel cylinder 16FL via a master 
cylinder passage 30. A first opening/closing valve 30 is provided in the 
middle of the master cylinder passage 30. The first opening/closing valve 
32 has a valve member 32b having a first end to which a pressing force is 
applied by a spring 32a and a second end to which a discharge pressure of 
a variable volume pump 34 is applied. When the valve member 32b is 
displaced in a direction where the pressing force by the spring 32a is 
applied (rightward in FIG. 12) the master cylinder passage 30 is open. The 
master cylinder passage 30 is open when the operational force applied to 
the brake means is less than the predetermined value. When the operational 
force is above a predetermined value, the hydraulic fluid flows to the 
wheel cylinder directly from the variable volume pump 34. The operational 
force is provided to the variable volume pump as illustrated by the dotted 
line as shown in FIG. 12. When the valve member 32b is displaced in a 
direction where the discharge pressure of the variable volume pump 34 is 
applied (leftward in FIG. 12), the master cylinder passage 30 is shut. 
In the present embodiment, the spring 2 is designed so as to satisfy the 
conditions where the first opening/closing valve 32 is closed when the 
variable volume pump 34 is generating an appropriate discharge pressure, 
and the first opening/closing valve 32 is open when the variable volume 
pump 34 is not generating an appropriate discharge pressure. Accordingly, 
the master cylinder passage 30 is open when the variable volume pump 34 is 
generating a normal discharge pressure, and is closed when the variable 
volume pump 34 is not generating a normal discharge pressure. 
The variable volume pump 34 is a pump which pressurizes and delivers a 
fluid by using a motor as a I driving source. As shown in FIG. 13, the 
variable volume pump 34 comprises a swash plate 34a which rotates while 
maintaining a predetermined angle to a rotational shaft 36a of the motor 
36. The swash plate 34a is connected with pistons 34b and 34c via ball 
joints 34d and 34e, respectively. The pistons 34b and 34c are slidably and 
sealingly inserted into respective cylinders 34g and 34h formed in a 
cylinder block 34f. 
In the above mentioned structure of the variable volume pump 34, the 
pistons 34b and 34c reciprocate in the respective cylinders 34g and 34h as 
the rotational shaft 36a of the motor 36 is rotated. The stroke of each of 
the pistons 34b and 34c is determined by an angle of the swash plate 34a. 
The cylinder block 34f is provided with through holes 34l and 34m which 
connect the cylinder 34g and 34h to an inlet port 34j or an outlet port 
34k of a valve plate 34i. The valve plate 34i is not rotated by the 
rotational shaft 36a. The inlet port 34j an the outlet port 34k are 
elongated openings having a predetermined radius of curvature. The inlet 
port 34j connects to the through holes 34l and 34m when the pistons 34b 
and 34c are moved in the direction where the pistons 34b and 34c are 
pulled out from the respective cylinders 34g and 34h. The outlet port 34k 
connects to the through holes 34l and 34m when the pistons 34b and 34c are 
moved in the direction where the pistons 34b and 34c are pushed into the 
respective cylinders 34g and 34h. 
In the above-mentioned structure of the variable volume pump 34, when the 
pistons 34b and 34c reciprocate in the respective cylinders 34g and 34h 
with a predetermined travel, a predetermined amount of fluid, which 
corresponds to the travel of the pistons 34g and 34h, is suctioned through 
the inlet port 34j and discharged through the outlet port 34k. Since the 
travel of the pistons 34b and 34c is determined by the angle of the swash 
plate 34a, the flow of the fluid discharged from the outlet port 34k per 
revolution of the rotational shaft 36a is determined by the angle of the 
swash plate 34a. 
The variable volume pump 34 has a mechanism to change the angle of the 
swash plate 34a. In the present embodiment, a two-step changing mechanism 
is used in which the angle of the swash plate 34a is changed stepwise in 
increments of two degrees. The change of the angle of the swash plate 34a 
is not limited to the stepwise change, and the angle may be continuously 
or linearly changed. Additionally, as shown in FIG. 12, the variable 
volume pump 34 is connected with a brake switch 38 which detects an 
operational state of the brake pedal 26. That is, when the motion of the 
brake pedal 26 is detected by the brake switch 38, the angle of the swash 
plate 34a is increased. 
Accordingly, in the hydraulic brake system according to the present 
embodiment, the variable volume pump 34 discharges a small amount of flow 
when the brake pedal 26 is not depressed, whereas a large amount of flow 
is discharged when the brake pedal 26 is depressed. 
The variable volume pump 34 acts as a high pressure source for supplying a 
brake fluid pressure to the wheel cylinder 16FL. Thus, a large amount of 
flow is not required for the variable volume pump 34 when a braking 
operation is not performed. In this respect, the variable volume pump 34 
having the above-mentioned structure in which the amount of flow is 
changeable allows a sufficient amount of flow to be maintained when a 
brake operation is performed while maintaining driving energy at a minimum 
when a brake operation is not performed. 
As shown in FIG. 12, the discharge side of the variable volume pump 34 is 
connected to a high pressure source passage 39. The high pressure source 
passage 39 is connected with an accumulator 40. Additionally, a check 
valve 42 is provided on the downstream side of the accumulator 40. The 
check valve 42, permits a flow in the direction from the variable volume 
pump 34 to the wheel cylinder 16FL. The check valve 42 comprises a spring 
42b which presses a ball valve 42a in a closing direction. Thus, the check 
valve has an opening pressure which corresponds to a pressing force 
generated by the spring 42b. In this structure, a residual pressure is 
accumulated in a part of the high pressure source passage 39 from a 
variable volume pump 34 to the check valve 42 as well as in the 
accumulator 40. 
On the downstream side of the check vale 42 in the high pressure source 
passage 39, a constant flow valve 44 is provided. The constant flow valve 
44 comprises a spool valve 44b which is slidable in a housing 44a in a 
direction of the longitudinal axis as shown in FIG. 14. The spool valve 
44b has an annular groove 44c along an outer surface thereof, and a fluid 
pressure introducing passage 44d inside thereof. The fluid pressure 
introducing passage 44d opens at one end of the spool valve 44b (the right 
side in FIG. 14) and opens on a side surface of the spool valve 44b near 
the opposite end. 
A space 44g is formed interiorly of the housing 44a for accommodating a 
piston 44e and a spring 44f which urges the piston 44e in addition to a 
space or accommodating the spool valve 44b. The piston 44 contacts the end 
of the spool valve 44b, and slides in the space 44g. The opening of the 
fluid pressure introducing passage 44d on the side surface opens in the 
space 44g. 
A fluid inflow passage 44h and a fluid outflow passage 44c are also formed 
in the housing 44a. The fluid inflow passage 44h connects to the high 
pressure source passage 39. The fluid outflow passage 44i connects to a 
wheel cylinder passage 46. The fluid inflow passage 44h overlaps with the 
annular groove 44c by a predetermined length when the spool valve 44b is 
positioned at the leftmost position in FIG. 14. On the other hand, the 
fluid inflow passage 44n overlaps with the annular groove 44c irrespective 
of the position of the spool valve 44b. The inflow passage 44n has a 
throttle 44j. The fluid inflow passage 44n connects to the right side of 
the piston 44e on the upstream side of the throttle 44j, an connects to 
the left side of the piston 44e on the downstream side of the piston 44e. 
In the above-mentioned structure, when fluid enters into the fluid inflow 
passage 44h, the fluid flows into the fluid outflow passage 44i via the 
angular groove 44c of the spool valve 44b. This flow of the fluid causes a 
pressure difference in the fluid inflow passage between the upstream side 
and the downstream side with respect to the throttle 44j due to a 
throttling action of the throttle 44j. A thrust force from right to left 
in FIG. 14 is exerted on the spool valve 44b and the piton 44e due to a 
pressure P1, where P1 is a pressure on he upstream side of the throttle 
44j. On the other hand a thrust force from left to right in FIG. 14 is 
exerted on the piston 44e due to a pressure P2 (P1&gt;P2), where P2 is a 
pressure on the downstream side of the throttle 44j. Accordingly, the 
spool valve 44b moves to a position where the thrust forces due to the 
pressures P1 an P2 and a force of the spring 44f are balanced. 
If the difference between the pressures P1 and P2 is small, the spool valve 
44b moves to a right side position in FIG. 14. In this state, a large 
opening is formed between the annular groove 44i and the fluid inflow 
passage 44h, and thereby a large amount of fluid is supplied to the fluid 
inflow passage 44n. When a large amount of fluid flows through the fluid 
inflow passage 44n, the pressure P1 on the upstream side of the throttle 
44j is increased. Thus, the difference between the pressures P1 and P2 is 
increased. 
When the difference between the pressures P1 and P2 is large, the spool 
valve 44b is positioned on the left side in FIG. 14. In this state, a 
small opening is formed between the annular groove 44c and the fluid 
inflow passage 44h. Thus, a small amount of fluid flows through the fluid 
inflow passage 44i, and the pressure P1 on the upstream side of the 
throttle 44j is decreased. Thus, the difference between the pressures P1 
and P2 is decreased. 
As mentioned above, in the constant flow valve 44, the position of the 
spool valve 44b is controlled so that a predetermined pressure difference 
is always generated between the pressures P1 and P2. Thus, the constant 
amount of fluid flowing out from the high pressure source passage 39 can 
be precisely maintained even if pulsation is generated in the pressure on 
the upstream side of the constant flow valve 44 provided in the high 
pressure source passage 39. 
The high pressure source passage 39 is branched to a wheel cylinder passage 
46 connected to the wheel cylinder 16FL and a reservoir passage 50 
connected to the reservoir tank 48. Accordingly, the fluid flowing out 
from the high pressure source passage 39 flows into the wheel cylinder 
16FL via the wheel cylinder passage 46 or flows into the reservoir 48 via 
the reservoir passage 50. It should be noted that the reservoir tank 48 is 
adapted to maintain the pressure therein at atmospheric pressure when the 
high pressure fluid flows into the reservoir tank 48 from the high 
pressure source passage 39. 
The reservoir tank 48 is connected to the inlet port 34j of the variable 
volume pump 34 via a check valve 51. The high pressure source passage 39 
is connected to the outlet port 34k of the variable volume pimp 34 via the 
check valve 42. The check valve 51 permits only a flow from the reservoir 
tank 48 to the variable volume pump 34. Accordingly, the fluid flows into 
the reservoir tank 48 via the reservoir tank passage 50 is then suctioned 
into the variable volume pump 34, and discharged from the variable volume 
pump 34 to the high pressure source passage 39. 
A second opening/closing valve 52 and a variable throttle 54 are provided 
in the middle of the reservoir passage 50. The second opening/closing 
valve 52 comprises a valve member 52b having one end on which a pressing 
force of a spring 52a and the master cylinder pressure are exerted and the 
other end on which the discharge pressure of the variable volume pump 34 
is exerted. If the valve member 52b is moved in a direction where the 
valve member 52 is pressed by the spring 52a (rightward in FIG. 12), the 
reservoir passage 50 is closed. On the other hand, if the valve member 52b 
is moved in a direction where the valve member 52 is pressed by the 
discharge pressure of the variable volume pump 34 (leftward in FIG. 12), 
the reservoir passage 50 is open. 
In the present embodiment, the spring 52a is designed so as to satisfy the 
conditions where the second opening/closing valve 52 is open when the 
variable volume pump 34 is generating an appropriate discharge pressure, 
and the second opening/closing valve 52 is closed when the variable volume 
pump 34 is not generating an appropriate discharge pressure. Accordingly, 
the reservoir passage 50 is open when the variable volume pump 34 is 
generating a normal discharge pressure, and closed when the variable 
volume pump 34 is not generating a normal discharge pressure. 
The variable throttle 54 comprises, as shown in FIG. 15, a spool 54b having 
one end to which spring 54a is contacted and the other end on which a 
master cylinder pressure is exerted. The spool 54b has an angular groove 
54c. The annual groove 54c increasingly opens to the reservoir passage 50 
which is connected to the reservoir tank 48 as the spool moves in the 
direction pressed by the spring 54a (leftward in FIG. 15). Therefore, the 
flow resistance between the annular groove 54b an the reservoir passage 50 
is increased as the master cylinder pressure is increased. 
When the variable volume pump 34 is generating a normal pressure, the 
opening/closing valve 52 is open. Thus, the fluid flowing out from the 
high pressure source passage is returned to the reservoir tank 48 via the 
reservoir passage 50. As discussed above, when the fluid flows to the 
reservoir passage 50, a pressure higher than the pressure in the reservoir 
tank 48 side of the variable throttle 54 is generated in the high pressure 
source passage 39 side of the variable throttle 54. 
When the high pressure is generated in the high pressure source passage 39 
side of the variable throttle 54, the high pressure is introduced into the 
wheel cylinder 16FL via the wheel cylinder passage 46. When the variable 
volume pump 34 is generating a normal pressure, the opening/closing valve 
32 is closed. Thus, when the pressure is introduced into the wheel 
cylinder 16FL as mentioned above, the wheel cylinder pressure Pwc, is 
increased, and a brake force is generated by the wheel cylinder 16FL. 
It is known that the following equation (1) is established where: Pwc is 
the wheel cylinder pressure which corresponds to the pressure on the high 
pressure source passage 39 side of the variable throttle 54; Pres is the 
reservoir pressure which corresponds to the reservoir tank 48 side of the 
variable throttle 54; Q is an amount of the fluid flowing out from the 
high pressure source passage 39; A is the effective opening area of the 
variable throttle 54; and, .tau. is a density of the fluid. 
EQU Q=C.multidot.A|{2(Pwc-Pres)/.tau.} (1) 
In the above-mentioned equation (1), the reservoir pressure Pres can be 
regarded as the atmospheric pressure (0.1 MPa). Accordingly, the wheel 
cylinder pressure Pwc is expressed as follows. 
EQU Pwc=(.tau./2) Q.sup.2 /(C.sup.2 .multidot.A.sup.2)+0.1 (2) 
The pressure value "0.1 MPa" in the equation (2) is negligibly small as 
compared to the pressure Pwc. 
In the present embodiment, the fluid amount Q is maintained to be constant 
by the constant flow valve 44. Additionally, the equation (2) indicates 
that the wheel cylinder pressure Pwc is reversely proportional to a square 
of the effective opening area A of the variable throttle 54. Thus, 
according to the hydraulic brake system of the present embodiment, the 
wheel cylinder pressure Pwc can be precisely controlled by controlling the 
effective opening area A. That is, the wheel cylinder pressure Pwc can be 
increased by decreasing the effective opening area A, and the pressure Pwc 
can be decreased by increasing the effective opening area A. 
Additionally, the hydraulic brake system of the present embodiment is 
adapted to generate the wheel cylinder pressure Pwc corresponding to the 
operational force of the brake when a braking operation is performed. 
Thus, an appropriate brake force corresponding to the operational force of 
the brake can be generated when the brake pedal 26 is depressed. 
In the hydraulic brake system of the present embodiment, if the variable 
volume pump 34 malfunctions, the opening/closing valve 32 is turned from 
the open state to the closed state. Then, the opening/closing valve 52 is 
turned from the open state to the closed state. Thereafter, the brake 
fluid pressure can be supplied from the master cylinder 28 to the wheel 
cylinder 116FL while preventing the release of the brake fluid pressure 
from the wheel cylinder 16FL to the reservoir tank 48. 
Especially, each component part in the resent embodiment is designed, so 
that when a situation occurs where the variable volume pump 34 cannot 
generate a normal pressure, the opening/closing valve 32 is open first, 
and the opening/closing valve 52 is subsequently closed. This structure 
prevents an occurrence of a state in which the wheel cylinder 16FL is 
disconnected from both the master cylinder 28 and the reservoir tank 48 
while the opening/closing valves 32 and 52 are switched. 
According to this structure, if a high pressure remains in the accumulator 
40 when the variable volume pump 54 malfunctions, the wheel cylinder 
pressure will not rapidly increase due to the pressure in the accumulator 
40 while the opening/closing valves 32 and 52 are switched. Additionally, 
if a high pressure remains in the wheel cylinder when the variable volume 
pump 34 malfunctions, the pressure in the wheel cylinder will not be 
unnecessarily maintained. 
As discussed above, in the present embodiment, a brake force can be 
precisely controlled by controlling the effective opening area A of the 
variable throttle 54 in response to the master cylinder pressure. 
Additionally, when the variable volume pump 34 malfunctions, a brake force 
is positively maintained by introducing the master cylinder pressure into 
the wheel cylinder 16FL. 
In the present embodiment, the amount Q of the fluid flowing out from the 
high pressure source passage 39 is stabilized by providing the constant 
flow valve 44 in the high pressure source passage 39. The amount Q of the 
fluid is one of parameters which determines the wheel cylinder pressure 
Pwc as indicated by the equation (2). Accordingly, it is difficult to 
precisely control the wheel cylinder pressure Pwc no matter how the 
effective opening area A of the variable throttle 54 is precisely 
controlled under the condition in which the amount Q fluctuates in a wide 
range. In this respect the hydraulic brake system of the present 
embodiment can provide a more precise control as compared to a system 
which does not have the constant flow valve 4. 
FIG. 16 is a hydraulic circuit diagram of an entire circuit including the 
hydraulic actuator 24 and the wheel cylinders 16FL, 16FR, 16RL and 16RR in 
the first embodiment of the present invention. In FIG. 16, parts that are 
the same as the parts shown in FIG. 12 are given the same reference 
numerals, and descriptions thereof will be omitted. 
The vehicle 10 has the four wheel cylinders 16FL, 16FR, 16RL and 16RR 
provided to the respective wheels. In an automobile of a front-engine 
rear-drive type, generally two separate hydraulic fluid circuits are 
provided, one for the left and right front wheels FL and FR and the other 
for the left and right rear wheels RL and RR. Such a hydraulic brake 
system is provided by the circuit shown in FIG. 16. 
In FIG. 16, the master cylinder 28 is a tandem brake master cylinder having 
two hydraulic pressure generating chambers therein. The master cylinder 
passages 30 and 56 are connected to the respective chambers of the master 
cylinder 28. The hydraulic circuit for the wheel cylinders 16FL and 16FR 
is realized by connecting the wheel cylinders 16FL and 16FR in parallel to 
the master cylinder passage 30 to which, in FIG. 2, only the wheel 
cylinder 16FL is connected. In this structure, a brake force corresponding 
to the operational force applied to the brake pedal 26 can be generated in 
both the wheel cylinders 16FL and 16FR. 
The hydraulic circuit for the wheel cylinders 16RL and 16RR is provided by 
connecting the wheel cylinders 16FL and 16FR in parallel to the master 
cylinder passage 56 and providing a proportioning valve (P valve) 58 in 
the master cylinder passage 56. The F valve 58 is a valve to decrease the 
master cylinder pressure with a predetermined ratio. 
Weight of the vehicle 10 is shifted toward the front wheels since the 
engine is situated in the front part of the vehicle. Additionally, the 
weight of the vehicle 10 is sifted to the front end during a braking 
operation. Thus, if the same master cylinder pressure Pwc is supplied to 
the rear wheel cylinders 16FL and 16RR and the front wheel cylinders 16FL 
and 16FR, the rear wheels RL and RR corresponding to the rear wheel 
cylinders 16RL and 16RR tend to be locked. 
In order to eliminate such a problem, the P valve 58 is provided in the 
master cylinder passage 56 so as to supply a pilot pressure to the 
variable throttle 54 on the downstream side of the P valve 58. According 
to this structure, the master cylinder pressure is directly supplied as a 
pilot pressure to the variable throttle 54 provided in the hydraulic 
circuit for the front wheels FL and FR, whereas the pilot pressure which 
is decreased with a predetermined ratio is supplied to the variable 
throttle 54 provided in the hydraulic circuit for the rear wheels RL and 
RR. As a result, a brake force generated in the rear wheels RL and RR is 
smaller than a brake force generated in the front wheels FL and FR. 
Accordingly, in the hydraulic brake system of the present embodiment, 
brake forces generated in the front wheels a in the rear wheels can be 
determined by considering the load ratio of the rear wheels to the front 
wheels. 
A description will now be given, with reference to FIG. 17, of a second 
embodiment of the present invention. FIG. 17 is a hydraulic circuit 
diagram of a part of a hydraulic brake system according to the second 
embodiment of the present invention. In FIG. 17, parts that are the same 
as the parts shown in FIG. 16 are given the same reference numerals, and 
description thereof will be omitted. 
In an automobile of a front-engine front drive type, generally two separate 
hydraulic fluid circuits are provided, one for the left front and right 
rear wheels FL and RR and the other for the right front and left rear 
wheels FR and RL. Such a hydraulic brake system is provided by the circuit 
shown in FIG. 17. 
In FIG. 17, the master cylinder 28 is a tandem brake master cylinder having 
two hydraulic pressure generating chambers therein. The master cylinder 
passages 30 are connected to the respective chambers of the master 
cylinder 28, one for the hydraulic circuit for the left front and right 
rear wheels FL and RR and the other (not shown in the figure) for the 
hydraulic circuit for the right front and left rear wheels FR and RL. 
Since the two hydraulic circuits are identical to each other, a 
description will be given only for the hydraulic circuit for the left 
front and right rear wheels FL and RR shown in FIG. 17. 
The wheel cylinder 16FL corresponding to the left front wheel FL is 
connected to the master cylinder passage 30 via the opening/closing valve 
32. Another master cylinder passage 60 is also connected to the master 
cylinder passage 30 to connect the right rear wheel cylinder 16RR to the 
master cylinder 28. The P valve 56 is provided in the middle of the master 
cylinder passage 60. A pilot pressure is supplied from the downstream side 
of the P valve 56 to the variable throttle 5 for the right rear wheel 
cylinder 16RR of the right ear wheel RR. 
According to the above-mentioned structure, the master cylinder pressure is 
directly supplied as a pilot pressure to the variable throttle 54 provided 
for the left front wheel, whereas the pilot pressure which is decreased 
with a predetermined ratio is supplied to the variable throttle 54 
provided for the right rear wheel RR. As a result, a brake force generated 
in the right rear wheel RR is smaller than a brake force generated in the 
left front wheel FL. Accordingly, in the hydraulic brake system of the 
present embodiment, brake forces generated in the front wheels and the 
rear wheels can be determined by considering the load ratio of the rear 
wheels to the front wheels while using the hydraulic circuit for the left 
front and right rear wheels FL and RR and the hydraulic circuit for the 
right front and left rear wheels FR and RL. 
A description will now be given, with reference to FIG. 18, of a third 
embodiment of the present invention. FIG. 18 is a hydraulic circuit 
diagram of a part of a hydraulic brake system according to the third 
embodiment of the present invention. The hydraulic brake system according 
to the third embodiment has two separate hydraulic circuit systems 
similarly to the second embodiment, that is, one for the left front and 
right rear wheels FL and RR and the other for the right front and left 
rear wheels FR and RL. In FIG. 18, parts that are the same as the parts 
shown in FIG. 16 are given the same reference numerals, and descriptions 
thereof will be omitted. 
In FIG. 18, the master cylinder 28 is a tandem brake master cylinder having 
two hydraulic pressure generating chambers therein. The master cylinder 
passages 30 and 56 are connected to the respective chambers of the master 
cylinder 28. The master cylinder passage 30 connects the left front and 
right rear wheels FL and RR to the master cylinder 28. The master cylinder 
passage 56 connects the right front and left rear wheels FR and RL. The 
wheel cylinder 16RR corresponding to the right rear wheel RR is connected 
to the master cylinder passage 30 via a proportional valve (P valve) 59. 
The wheel cylinder 16RL corresponding to the left rear wheel RL is 
connected to the master cylinder passage 58 via another P valve 59. 
According to the above-mentioned structure, the master cylinder pressure is 
directly supplied to the left and right wheel cylinders 16FL and 16FR 
corresponding to the left and right front wheels FL and FR via the 
respective master cylinder passages 30 and 60, whereas a pressure reduced 
by the respective P valve is supplied to the left and right wheel 
cylinders 16LR and 16RR corresponding to the left and right wheels LE and 
RR. Accordingly, in the hydraulic brake system of the present embodiment, 
brake forces generated in the front wheels and the rear wheels can be 
determined by considering the load ratio of the rear wheels to the front 
wheels. 
A description will now be given, with reference to FIGS. 19, 20 and 21, of 
a fourth embodiment of the present invention. FIG. 19 is a hydraulic 
circuit diagram of a part of a hydraulic brake system according to the 
fourth embodiment of the present invention. In FIG. 19, parts that are the 
same as the parts shown in FIG. 12 are given the same reference numerals, 
and descriptions thereof will be omitted. 
A structure of the hydraulic brake system according to the fourth 
embodiment is the sale as that of the hydraulic system according to the 
first embodiment except for the variable throttle 54 being replaced by an 
electrically controlled variable throttle 62 
The variable throttle 62 comprises, as shown in FIG. 20, an electromagnetic 
coil 62a and a spool 62b which together constitute a linear solenoid 
actuator. When an electric current flows in the coil 62a, the plunger 62b 
is moved in a direction to the right in FIG. 20. An end (a right end in 
FIG. 20) of the plunger 62b contacts a spool 62c. The spool 62c is 
slidable within the variable throttle 62. A spring 62d contacts an 
opposite end of the spool 62c so that the spool 62c is pressed by the 
spring 62d. The spool 62c has an annular groove 62e. The annual groove 62e 
increasingly opens to the reservoir passage 50 which is connected to the 
reservoir tank 48 as the spool 62c moves in the direction pressed by the 
spring 62d (leftward in FIG. 20). 
In the above-mentioned structure, the flow resistance between the annular 
groove 62e an the reservoir passage 50 is decreased as the spool 62c is 
moved to the left by the spring 62d. Accordingly, the flow resistance of 
the reservoir passage 50 is minimized when no current flows in the 
electromagnetic coil 62a. The flow resistance is increased as a current 
flowing in the electromagnetic coil 62a is increased. Thus, in the present 
invention, a brake force generated by the wheel cylinder 16FL can be 
controlled by controlling the current flowing in the electromagnetic coil 
62a. 
In the present embodiment, pressure sensors 64 and 66 are provided on the 
upstream and downstream side of the opening/closing valve 32 in the master 
cylinder passage 301 respectively. The pressure sensor 64 senses the 
master cylinder pressure, and the pressure sensors 66 senses the wheel 
cylinder pressure. 
Output signals from the pressure sensor are supplied to an electronic 
control unit (ECU) 68. The ECU 68 controls the variable throttle 62 based 
on the output signals of the pressure sensors 64 and 66. The ECU 68 stores 
data of an amplifying ratio for the brake pressure. The ECU 68 controls a 
current flowing in the electromagnetic coil 62a of the variable throttle 
62 so that the wheel cylinder pressure becomes equal to a multiple of the 
master cylinder pressure and the amplifying ratio. 
When the ECU 68 performs such a control, a wheel cylinder pressure 
corresponding to the master cylinder pressure, that is, a wheel pressure 
corresponding to an operational force applied to the brake pedal 26 is 
generated in the wheel cylinder 16FL. Accordingly, in the hydraulic brake 
system of the present embodiment, an appropriate brake force corresponding 
to the operational force applied to the brake pedal 26 can be generated by 
the wheel cylinder 16FL. 
In the field of automobile technology, an antilock brake system (ABS) and a 
traction control system (TRC) are known. The ABS is for preventing locking 
of a wheel due to an excessive brake force. The TRC is for preventing 
spinning of a wheel due to an excessive driving force. 
A function of the ABS is achieved by controlling the wheel cylinder 
pressure. That is, the wheel cylinder pressure is forcibly decreased or 
maintained when it is determined that one of the wheels tends to fall into 
a locked condition during a braking operation, and an increase of the 
wheel cylinder pressure is permitted after the possibility of locking is 
eliminated. 
A function of the TRC is also achieved by controlling the wheel cylinder 
pressure. That is, the wheel cylinder pressure is forcibly increased when 
it is determined that one of the wheels tends to fall into a spin 
condition during an acceleration, and is decreased when the possibility of 
spinning is eliminated. 
Accordingly, in the hydraulic brake system shown in FIG. 19, if the ECU 68 
controls the variable throttle 62 based on the above-mentioned scheme, the 
functions of the ABS and TRC are provided in the hydraulic brake system 
according to the present embodiment. 
FIG. 21 a block diagram of a control mechanism which provides the 
above-mentioned functions in the hydraulic brake system according to the 
present embodiment. In FIG. 21, parts the same as the parts shown in FIG. 
19 are give the same reference numerals, and descriptions thereof will be 
omitted. 
In FIG. 21, wheel sensors 70FL, 70FR, 70RL and 70RR generate pulse signals 
according to rotational speeds of the respective wheels FL, FR, RL and RR. 
The ECU 68 determines the rotational speed of each of the wheels by 
detecting the cycle period of the pulse signals. A throttle sensor 72 
senses a degree of opening of a throttle valve in the automobile having 
the hydraulic brake system according to the present embodiment. The ECU 68 
can determine whether or not the vehicle is in an acceleration state by 
detecting the degree of opening of the throttle sensor 72. 
The ECU 68 determines, when a brake switch is on, that a braking operation 
is being performed so as to execute a control (hereinafter referred to as 
an ABS control) for performing an ABS function. Additionally, the ECU 68 
determines, when a predetermined acceleration state is detected by the 
throttle sensor 72, that an acceleration is performed so as to execute 
control (hereinafter referred to as a TRC control) for performing a TRC 
function. 
In the ABS control, the ECU 68 calculates an assumed vehicle speed based on 
the output signals of the wheel speed sensors 70FL, 70FR, 70RL and 70RR. 
In the present embodiment, the fastest wheel speed among the wheel speeds 
detected by the wheel speed sensors 70FL, 70FR, 70RL and 70RR is used as 
the vehicle speed. Thereafter, the ECU 68 compares the calculated assumed 
vehicle speed with the wheel speed of each of the wheels FL, FR, RL and 
RR. If one of the wheel speed is extremely slower than the assumed vehicle 
speed, it is determined that the wheel corresponding to the particular 
wheel speed is possibly in a locked state. 
If it is determined from the results of the comparison that there is no 
possibility that the wheel FL is in a locked state, the ECU 68 controls 
the variable throttle 62 so that the wheel cylinder pressure corresponding 
to the master cylinder pressure is generated in the wheel cylinder 16FL. 
Hereinafter this control mode is referred to as a pressure increase mode. 
Additionally if it is determined that there if a possibility that the 
wheel FL is in a locked state, the ECU 68 controls the variable throttle 
62 so as to decrease or maintain the wheel cylinder pressure in the wheel 
cylinder 16FL. 
The decrease in the wheel cylinder pressure can be achieved by increasing 
the effective opening area of the variable throttle 62. Hereinafter this 
control mode is referred to as a pressure decrease mode. Additionally, the 
pressure can be maintained at substantially the same level by alternately 
repeating the pressure increase mode and the pressure decrease mode. 
Hereinafter this control mode is referred to as a pressure maintain made. 
It should be noted that the wheel cylinder pressure can be maintained by 
maintaining the effective opening area of the variable throttle 62. 
As discussed above, in the hydraulic brake system according to the present 
embodiment, the wheel FL is positively prevented from falling into a 
locked state during a braking operation due to an excessive brake force. 
Although the above-mentioned control is related only to the left front 
wheel FL, the same control is applied to other wheel FR, RL and RR so as 
to provide the ABS function. 
In the TRC control, the ECU 68 calculate assumed vehicle speed based on the 
output signals from each of the wheel speed sensors 70FL, 70FR, 70RL and 
70RR. In the present embodiment, an average of the wheel speeds detected 
by the wheel speed sensors 70RL and 70RR of the idler wheels (in this 
case, the rear wheels RL and RR) is used as the vehicle speed. Thereafter, 
the ECU 68 compares the calculated assumed vehicle speed with each of the 
wheel speeds of the driving wheels FL and FR. If the wheel speeds of the 
driving wheels FL and FR are extremely faster than the assumed vehicle 
speed, it is determined that a spin condition has occurred in the driving 
wheels. 
In the above-mentioned comparison, if it is determined that no spin 
condition exists in the wheel FL, no special control is performed. Thus, 
the ECU 68 controls the variable throttle 62 so that the wheel cylinder 
pressure corresponding to the master cylinder pressure is generated. On 
the other hand, if it is determined that a spin is occurring in the 
driving wheel FL, the ECU 68 controls the variable throttle 62 so as to 
forcibly increase the wheel cylinder pressure of the wheel cylinder 16FL. 
When the wheel cylinder pressure is forcibly increased, a brake force is 
generated in the wheel cylinder 16FL and the spin of the drive wheel is 
suppressed. It should be noted that the forcible increase in the wheel 
cylinder pressure can be realized by decreasing the effective opening area 
of the variable throttle 62. 
As discussed above, in the hydraulic brake system according to the present 
embodiment, the wheel FL is positively prevented from falling into a spin 
condition during an acceleration due to excessive driving force. Although 
the above-mentioned control is related only to the left front wheel FL, 
the same control is applied to other wheel FR, RL and RR so as to realize 
the TRC function. 
A description will now be given, with reference to FIGS. 22, of a fifth 
embodiment of the present invention. FIG. 22 is a hydraulic circuit 
diagram of a part of a hydraulic brake system according to the fifth 
embodiment of the present invention. In FIG. 22, parts that are the same 
as the parts shown in FIG. 9 are given the same reference numerals, and 
descriptions thereof will be omitted. 
The hydraulic brake system according to the present embodiment uses a flow 
sensor 74 provided in the reservoir passage 52 between the variable 
throttle 62 and the reservoir tank 48 instead of the pressure sensor 66 of 
the fifth embodiment. 
If the amount Q of fluid flowing out from the high pressure source passage 
39 is constant, the wheel cylinder pressure Pwc is determined by the 
effective opening area A of the variable throttle 62. On the other hand, 
if the amount Q is constant, the amount q of the fluid flowing from the 
variable throttle 62 to the reservoir tank 48 is determined by the 
effective opening area A of the variable throttle 62 and the difference 
Pwc-Pres (nearly equal to Pwc) between pressures across the variable 
throttle 62. Accordingly, if the an amount q of the fluid flowing from the 
variable throttle 62 to the reservoir tank 48 is known, a level of the 
pressure difference Pwc-Pres can be assumed, that is, a level of the wheel 
cylinder pressure Pwc can be assumed. 
In the present embodiment, an ECU 76 stores therein two-dimensional map 
data of the wheel cylinder pressure Pwc in which the amount q of the fluid 
flowing from the variable throttle 62 to the reservoir tank 48 is used as 
a parameter. Additionally, the ECU 76 has a function to assume the 
effective opening area A of the variable throttle 62 based on the current 
supplied to the electromagnetic coil of the variable throttle 62. 
The ECU 76 calculates, when controlling the variable throttle 62, the 
amount q of the fluid based on an output of the flow sensor 74 and assumes 
the effective opening area A based on the current value supplied to the 
electromagnetic coil 62a. Thereafter, the two-dimensional map is searched 
based on q and A to determine the wheel cylinder pressure Pwc. If the 
pressure Pwc is less than a value obtained by a multiple of the master 
cylinder pressure and a predetermined amplification ratio, the current 
supplied to the electromagnetic coil 62a is increased. If the pressure Pwc 
is greater than a value obtained by a multiple of the master cylinder 
pressure and a predetermined amplification ratio, the current supplied to 
the electromagnetic coil 62a is decreased 
When the ECU 76 performs such a control the wheel cylinder pressure Pwc in 
the wheel cylinder 16FL is precisely controlled to the pressure obtained 
by the multiple of the master cylinder pressure and the predetermined 
amplification ratio. Accordingly, in the hydraulic brake system according 
to the present embodiment, an appropriate brake force corresponding to an 
operational force to the brake pedal 26 can be generated by the wheel 
cylinder 16FL. 
It should be noted that if the control mechanism described with reference 
to FIG. 21 is adopted in the present embodiment by using the ECU 76, The 
ABS and TRC functions can also be achieved in the present embodiment. 
A description will now be given, with reference to FIGS. 23, of a sixth 
embodiment of the present invention. FIG. 23 is a hydraulic circuit 
diagram of a part of a hydraulic brake system according to the sixth 
embodiment of the present invention. In FIG. 23, parts that are the same 
as the parts shown in FIG. 12 are given the same reference numerals, and 
descriptions thereof will be omitted. 
The hydraulic brake system according to the present embodiment has an 
opening/closing valve 78 in the middle of the wheel cylinder passage 46 
between the high pressure source passage 39 and the wheel cylinder 16FL. 
Additionally, in the present embodiment, the wheel cylinder 16FL is 
connected to the reservoir tank 48 via a fluid pressure releasing passage 
82 having an opening/closing valve 80. 
The opening/closing valve 78 is a normally closed type solenoid valve which 
opens when a drive signal is supplied thereto. The opening/closing valve 
78 is connected to a brake switch 38 which supplies the drive signal to 
the opening/closing valve 78 when the brake pedal 26 is depressed. Thus, 
the wheel cylinder passage 46 is closed when the brake pedal 26 is not 
pressed, and is open when the brake pedal 26 is pressed. 
The opening/closing valve 80 is a normally open type solenoid valve which 
closes when a drive signal is supplied thereto. The opening/closing valve 
80 is connected with the brake switch 38 which supplies the drive signal 
to the opening/closing valve 80 when the brake pedal 26 is pressed down. 
Thus, the of fluid pressure releasing passage 82 is open when the brake 
pedal 26 is not pressed, and is closed when the brake pedal 26 is pressed. 
In the above-mentioned structure of the present embodiment, the wheel 
cylinder passage 46 is open and the fluid pressure releasing passage 82 is 
closed when the brake pedal 26 is pressed. Thus, the present embodiment 
has a structure substantially the same as the structure of the hydraulic 
brake system shown in FIG. 12. Accordingly, similar to the hydraulic brake 
system shown in FIG. 12, the present embodiment can generate an 
appropriate brake force corresponding to the master cylinder pressure by 
the wheel cylinder 16FL. 
On the other hand, when the brake pedal 26 is not pressed in the hydraulic 
brake system according to the present embodiment, the wheel cylinder 
passage 46 is closed and the fluid pressure releasing passage 82 is open. 
Thus, all of the fluid flowing out of the high pressure source passage 39 
is returned to the reservoir tank 48 via the reservoir passage 50 when the 
brake pedal 26 is not pressed. Additionally, the brake fluid pressure 
which remains in the wheel cylinder 16FL when the brake pedal 26 is not 
pressed is released to the reservoir tank 48 via the fluid pressure 
releasing passage 82. Accordingly, in the hydraulic brake system according 
to the present embodiment, an occurrence of a phenomenon in which a brake 
force is generated when a braking operation is not performed, that is, 
so-called brake drug can be positively prevented. 
Although the opening/closing valves 78 and 80 are comprised of solenoid 
valves in the present embodiment, the present invention is not limited to 
this and the opening/closing valves 78 and 80 may be valve mechanisms 
which are operated by the master cylinder pressure as a pilot pressure. 
A description will now be given, with reference to FIGS. 24, of a seventh 
embodiment of the present invention. FIG. 24 is a hydraulic circuit 
diagram of a part of a hydraulic brake system according to the seventh 
embodiment of the present invention. In FIG. 24, parts that are the same 
as the parts shown in FIG. 23 are given the same reference numerals, and 
descriptions thereof will be omitted. 
The hydraulic brake system according to the present embodiment has another 
master cylinder passage 86 parallel to the master cylinder passage 30. The 
master cylinder passage 86 is provided between the master cylinder 28 and 
the wheel cylinder 16FL, and has a check valve 84. The check valve 84 is a 
one-way valve which permits a flow in a direction from the master cylinder 
28 to the wheel cylinder 16FL. When the master cylinder pressure which is 
higher than the wheel cylinder pressure is generated in the master 
cylinder 28, the pressure in the master cylinder is rapidly introduced 
into the wheel cylinder 16FL. 
In the hydraulic brake system according to the present embodiment, in view 
of the prevention of brake drag, a structure is used in which the 
opening/closing valve 78 is provided to prevent an introduction of the 
brake fluid pressure from the high pressure source passage 39 into the 
wheel cylinder 16FL when a braking operation is not performed. This 
structure is effective in terms of preventing brake drag, however, there 
is a problem in that a delay in a response to a braking operation is 
possibly generated by the response time of the opening/closing valve 78. 
However, in the present embodiment, since the master cylinder pressure is 
introduced into the wheel cylinder 16FL by providing the master cylinder 
passage 86, a pressure increasing operation for the wheel cylinder 
pressure can be started before the opening/closing valve 78 is open. Thus, 
a brake force can be rapidly increased after a braking operation is 
started. 
Additionally, the hydraulic brake system according to the present 
embodiment uses a structure in which a brake fluid pressure is introduced 
from the master cylinder 28 into the wheel cylinder 16FL by opening the 
opening/closing valve 32 when a malfunction occurs in the variable volume 
pump 34. In this structure, a delay in a response of the opening/closing 
valve 32 may be generated from a time when a malfunction occurs in the 
variable volume pump 34 until a time when the fail-safe function is 
effected. 
However, in the hydraulic brake system according to the present embodiment, 
the master cylinder pressure is introduced into the wheel cylinder 16FL 
immediately after the master cylinder pressure having a higher level that 
the wheel cylinder pressure is generated even if the opening/closing valve 
32 is still closed. Accordingly, the hydraulic brake system according to 
the present embodiment can achieve a fail-safe mechanism having a quick 
response without being influenced by the response of the opening/closing 
valve 32. 
The present invention is not limited to the specifically disclosed 
embodiments, and variations and modifications may be made without 
departing from the scope of the present invention.