Solenoid-operated hydraulic control device for anti-skid brake system

A hydraulic control device for an anti-skid brake system for a vehicle, including a solenoid-operated directional control valve for communication of a pressure chamber in a hydraulic actuator, selectively with a hydraulic power source and a reservoir, and a solenoid-operated flow control valve disposed between the directional control valve and one of the pressure chamber, hydraulic power source and reservoir, the hydraulic control device being operated to effect rapid and slow rise and fall of a pressure in the hydraulic actuator, in response to a slip ratio of a wheel of the vehicle. The flow control valve comprises a flow restrictor which permits a restricted flow of a fluid through the valve even when its valving member is seated on a valve seat in which a valve hole is open. The flow restrictor is provided in one of various forms except an ordinary orifice with a small diameter. The flow restrictor may be a groove formed in an end face of a valving member opposite to the valve seat, a rough or ragged surface of the valve seat or valving member, or a porous structure formed on the valving member so as to contact the valve seat. Alternatively, the flow restrictor may be formed by a porous structure which fills a hole formed in parallel with the valve hole which is open in the valve seat.

BACKGROUND OF THE INVENTION 
1. Field of the Art 
The present invention relates to a solenoid-operated hydraulic control 
device for an anti-skid brake system for a vehicle, to directly or 
indirectly control a pressure of a fluid in a brake cylinder. 
2. Related Art Statement 
An anti-skid hydraulic brake system for a vehicle is known. For example, 
Japanese Patent Application laid open in 1983 under Publication No. 
58-224839, discloses an anti-skid device of indirect pressure control type 
wherein a pressure of a brake fluid in a brake cylinder is indirectly 
controlled by a pressure regulator which is operated by a fluid delivered 
from a pressure source which is different from a pressure source for the 
brake cylinder. Further, Japanese Patent Application laid open in 1981 
under Publication No. 56-142733 discloses an anti-skid device of direct 
pressure control type wherein a solenoid-operated hydraulic control device 
is disposed in a primary fluid passage which connects a master cylinder 
and a brake cylinder. 
A solenoid-operated hydraulic control device is provided to control a 
pressure of a fluid in a pressure chamber of a hydraulically operated 
actuator, for example, a pressure regulator when the control device is 
used in an anti-skid device of indirect pressure control type, or a brake 
cylinder when the control device is used in an anti-skid device of direct 
pressure control type. 
An example of the solenoid-operated hydraulic control device consists of a 
combination of a solenoid-operated directional control valve and a 
solenoid-operated flow control valve. The directional control valve is 
electromagnetically operated between a first position for communication of 
the pressure chamber in the hydraulic actuator with a hydraulic power 
source such as a master cylinder or a pump, and a second position for 
communication of the pressure chamber in the actuator with a reservoir. 
The fluid from the pressure source may be fed into the hydraulic actuator 
when the directional control valve is placed in the first position. In the 
second position, the fluid in the hydraulic actuator may be discharged 
into the reservoir. The flow control valve is disposed between the 
directional control valve and the hydraulic actuator whose pressure is 
controlled by the hydraulic control device. The flow control valve has a 
restrictor passage for restricting a flow of the fluid through the valve, 
and a non-restrictor passage formed in parallel with the restrictor 
passage. The flow control valve is electromagnetically operated between a 
non-restricting position in which the fluid flows through both of the 
restrictor passage and the non-restrictor passage, and a flow-restricting 
position in which the non-restrictor passage is closed and the fluid is 
forced to flow through the restrictor passage. Thus, the flow control 
device is capable of controlling a rate of flow of the fluid therethrough 
in two steps. 
The abovementioned solenoid-operated hydraulic control device consisting of 
the solenoid-operated directional control and flow control valves is 
capable of effecting a pressure regulating operation in four modes which 
permit a rapid rise, a rapid fall, a slow rise and a slow fall, 
respectively, of the pressure in the hydraulic actuator. Thus, the 
pressure in the hydraulic actuator may be suitably regulated so as to be 
held within an optimum range. 
PROBLEM SOLVED BY THE INVENTION 
In such solenlid-operated hydraulic control device as introduced above, the 
flow restrictor is formed by a conventional orifice having a small 
diameter to provide a resistance to flow of the fluid. However, there is a 
possibility that such an orifice is clogged or plugged with foreign 
substances contained in the working fluid. For this reasons, suitable 
means such as an oil filter should be provided to prevent the orifice from 
being clogged, and considerable cares should be taken in designing such 
preventive means. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a 
solenoid-operated hydraulic control device wherein a flow control valve 
has a flow restrictor which undergoes a minimum variation in its flow 
restriction due to clogging thereof. 
According to the present invention, there is provided a hydraulic control 
device for an anti-skid hydraulic brake system having a hydraulic actuator 
such as a brake cylinder for applying a brake to a wheel of a vehicle or a 
regulator for regulating a pressure in the brake cylinder, the hydraulic 
control device including a solenoid-operated directional control valve for 
communication of a pressure chamber in the hydraulic actuator, selectively 
with a hydraulic power source and a reservoir, and further including a 
solenoid-operated flow control valve disposed between the directional 
control valve and one of the pressure chamber, the hydraulic power source 
and the reservoir, to control a flow of a fluid in two steps, the 
hydraulic control device being operated to effect rapid rise and fall, and 
slow rise and/or fall of a pressure in the pressure chamber, in response 
to a slip ratio of the wheel, the solenoid-operated flow control valve 
comprising: (a) means for defining a valve hole; (b) a valve seat in which 
the valve hole is open; (c) a valving member movable between its first 
position in which the valving member is seated on the valve seat, and its 
second position in which the valving member is spaced apart from the valve 
seat; (d) a solenoid which is energized and deenergized for moving the 
valving member between the first and second positions; and (e) a flow 
restrictor provided on one of the valve seat and the valving member, 
permitting a predetermined rate of restricted flow of the fluid through 
the valve hole while the valving member is placed in the first position, 
the predetermined rate of restricted flow being smaller than a rate of 
flow of the fluid while the valving member is placed in the second 
position. 
As described above, the solenoid-operated flow control valve may be 
disposed between the solenoid-operated directional control valve and the 
hydraulic power source or the reservoir, as well as between the 
directional control valve and the hydraulic actuator. In the case where 
the flow control valve is disposed between the directional control valve 
and the hydraulic actuator, the rate of flow of the fluid into the 
pressure chamber of the hydraulic actuator, and the rate of flow of the 
fluid from the pressure chamber of the hydraulic actuator, may be changed 
in two steps, whereby the pressure in the pressure chamber of the 
hydraulic actuator may be controlled in four different modes, that is, for 
a rapid rise, a rapid fall, a slow rise and a slow fall of the pressure, 
respectively. Where the flow control valve is disposed between the 
directional control valve and the hydraulic power source, only the rate of 
flow of the fluid into the pressure chamber of the hydraulic actuator may 
be changed in two steps, whereby the pressure in the hydraulic actuator 
may be controlled in three different modes for rapid rise and fall and a 
slow rise of the pressure. In the case where the flow control valve is 
disposed between the directional control valve and the reservoir, the 
pressure in the hydraulic actuator may be controlled in three different 
modes corresponding to rapid rise and fall and a slow fall of the 
pressure. 
In the hydraulic control device of the present invention as previously 
described, the flow restrictor permits a restricted flow of the fluid 
through the valve hole at a predetermined limited rate even while the 
valving member is seated on the valve seat. Thus, the rate of fluid flow 
through the flow control valve may be controlled in two steps. The flow 
restrictor replaces an orifice having a small diameter, which is essential 
to a flow control valve of a conventional hydraulic control device. 
Therefore, the flow restrictor of the flow control valve of the present 
hydraulic control device eliminates the need of forming such an orifice, 
and allows an easy manufacture of the control device. In this connection, 
it is noted that the conventional orifice should be formed with a diameter 
of about 0.1 mm, for example, if it is desired to achieve a slow rise of a 
pressure at a rate of 50 kg/cm.sup.2 /sec. where the pressure of the 
hydraulic power source is 100 kg/cm.sup.2. An orifice having such a small 
diameter is considerably difficult to be formed. 
According to one preferred form of the hydraulic control device of the 
invention, the flow restrictor comprises at least one groove formed in a 
contact surface of one of the valving member and the valve seat when the 
valving member is placed in the its first position. 
According to another preferred form of the hydraulic control device, the 
flow restrictor comprises one of contact surfaces of the valving member 
and the valve seat which contact each other while the valving member is in 
its first position. This one of the contact surfaces is formed with minute 
indentations and/or projections which provide gaps between the contact 
surfaces while the valving member is in the first position. In either one 
of the above two cases, a filter or other means for preventing the flow 
restrictor from being clogged may be made relatively simple. If the groove 
in the contact surface of the valving member or the gaps between the valve 
seat and the valving member is/are clogged with foreign substances, these 
substances may be readily washed away with a flow of the fluid after the 
valving member is moved off the valve seat. 
In accordance with a further preferred form of the hydraulic control 
device, the flow restrictor is constituted by a porous structure which 
forms at least end portion of the valving member which contacts the valve 
seat while the valving member is placed in the first position, the porous 
structure having an infinite number of minute continuous pores. In this 
case, a partial or local clogging of the porous structure will not have a 
significant effect on a flow rate of the fluid through the porous 
structure. Further, if a mass of foreign substances of comparatively large 
size sticks to the porous structure of the flow restrictor, the sticking 
mass may be removed with a flow of the fluid after the valving member is 
unseated from the valve seat. 
According to the present invention, there is also provided a hydraulic 
control device for an anti-skid hydraulic brake system having a hydraulic 
actuator such as a brake cylinder for applying a brake to a wheel of a 
vehicle, or a regulator for regulating a pressure in the brake cylinder, 
the hydraulic control device including a solenoid-operated directional 
control valve for communication of a pressure chamber in the hydraulic 
actuator, selectively with a hydraulic power source and a reservoir, and 
further including a solenoid-operated flow control valve disposed between 
the directional control valve and one of the pressure chamber, the 
hydraulic power source and the reservoir, the flow control valve having a 
restrictor passage and a non-restrictor passage formed in parallel with 
each other, and controlling a rate of flow of a fluid therethrough in two 
steps by opening and closing the non-restrictor passage, the hydraulic 
control device being operated to effect rapid rise and fall and slow rise 
and fall of a pressure in the pressure chamber, in response to a slip 
ratio of the wheel, the solenoid-operated directional control valve 
comprising: means for defining a hole in parallel with the non-restrictor 
passage; and a porous structure filling the hole and having an infinite 
number of minute continuous pores, the porous structure cooperating with 
the means for defining a hole, to define the restrictor passage. 
In the above-described hydraulic control device wherein the restrictor 
passage is provided by the hole filled with the porous structure with many 
continuous pores, an effect of flow restriction of the restrictor passage 
as a whole will not be significantly changed even if the porous structure 
is partially or locally clogged with foreign substances. Accordingly, the 
hydraulic control device is effectively protected against abnormal 
functioning due to clogging of the restrictor passage. 
The porous structure of the restrictor passage may preferably be a 
multi-layer laminar structure consisting of a central dense layer having 
fine pores, and a pair of coarse layers which sandwich the central dense 
layer and have coarse pores of sizes larger than the fine pores. The 
laminar structure is positioned in the hole so that the fluid flows in 
directions substantially perpendicular to the layers of the structure. In 
the case, the central dense layer functions primarily for restricting the 
fluid flow, while the outer coarse layers serve primarily as a filter, 
thereby more effectively avoiding a variation in the flow restriction of 
the restrictor passage due to its clogging. The porous structure may be 
formed of a sintered alloy. In particular, a multi-layer porous structure 
of a sintered alloy is preferred. 
As described hitherto, the flow control valve of the hydraulic control 
device constructed according to the present invention comprises a flow 
restrictor or restrictor passage which permits a restricted flow of the 
fluid through the flow control valve even while the flow control valve is 
placed in its positions in which the valve hole or non-restrictor passage 
is closed. The flow restrictor or restrictor passage is formed by various 
means except an ordinary orifice having a small diameter. In this respect, 
the term "orifice" is interpreted in a comparatively narrow sense, namely, 
interpreted to mean a single small aperture or passage which is defined by 
a single member.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the present invention will be described in 
detail, with reference to the accompanying drawing. 
Referring to FIG. 1, reference numeral 10 designates a housing for a 
hydraulic control device. For easy manufacture, this housing 10 is made up 
of a plurality of members which include elements 12, 14, 16, 18, 20, 22, 
24 and 25. These elements are assembled into a unitary housing structure 
which functions as the housing 10. The housing 10 has: a port 26 connected 
to a hydraulic power source such as a master cylinder or a pump of a 
hydraulic system; a port 28 connected to a brake cylinder, regulator, or 
other unit which is controlled by the hydraulic control device; and a port 
30 connected to a reservoir of the hydraulic system. The housing 10 
further incorporates a solenoid-operated directional control valve 32 for 
selective communication of the port 28 with the port 26 or the port 30, 
and a solenoid-operated flow control valve 34 for regulating an amount of 
flow of a hydraulic working fluid through the port 28, in two steps. 
The directional control valve 32 includes a plunger 40 which has two balls 
36, 38 on its opposite ends, respectively. The balls 36, 38 serve as 
valving members which are positioned opposite to corresponding valve seats 
42, 44 formed on the housing 10. The plunger 40 is axially biased by a 
compression coil spring 46 toward the valve seat 42. In this arrangement, 
therefore, the ball 36 is normally held seated on the valve seat 42 so as 
to close a passage 48 leading to the port 30, while the other ball 38 is 
held away from the valve seat 44 so that a passage 50 leading to the port 
26 is held open. The passage 50 communicates with a fluid passage 56 
through a plunger chamber 52 and a communication port 54. 
The above-indicated plunger 40 is supported by a hollow piston 60 made of a 
magnetic material. The hollow piston 60 is axially slidably received 
within a piston chamber 61. Radially outwardly of and coaxially with the 
hollow piston 60, there is disposed a solenoid (first solenoid) 64 with a 
coil wound on a bobbin 62 made of a resin. With the solenoid 64 energized, 
the hollow piston 60 is moved with magnetic attraction in a downward 
direction as viewed in FIG. 1, whereby the plunger 40 is moved against a 
biasing force of the spring 46. As a result, the ball 36 is moved away 
from the corresponding valve seat 42, while the ball 38 is seated on the 
valve seat 44. Namely, the energization of the solenoid 64 will cause the 
directional control valve 32 to be switched from its position for 
communication of the passage 56 with the port 26, to its position for 
communication of the passage 56 with the port 30. 
The previously indicated flow control valve 34 includes a plunger 72, a 
piston 74 for supporting the plunger 72, a compression coil spring 78 for 
biasing the plunger 72 in a direction away from a valve seat 76, a 
solenoid (second solenoid) 82 wound on a bobbin 80, and other elements. 
The flow control valve 34 is almost similar in construction to the 
directional control valve 32, except that the plunger 72 is biased in a 
direction opposite to the biasing direction of the plunger 40, so that the 
plunger 72 is held away from the valve seat 76, and that the plunger 72 
has no ball fixed at its ends. More specifically, the plunger 72 has a 
contact surface 84 on its end opposite to a contact surface 83 of the 
valve seat 76, so that the sealing surface 84 may be seated on the contact 
surface 83. The contact surface 84 has a U-shaped groove 86 formed in a 
diametric direction. The contact surface 84 having this U-shaped groove 86 
cooperates with the opposite contact surface 83 to define a flow 
restrictor which communicates with a valve hole 88 and a piston chamber 
90, even when the plunger 72 is seated on the valve seat 76. The cross 
sectional area of the U-shaped groove 86 is determined so that the flow 
restrictor formed upon seating of the plunger 72 on the valve seat 76 
permits a predetermined small amount (low rate) of flow of the working 
fluid therethrough. 
The solenoid-operated hydraulic control device which has been described is 
applied, for example, to a hydraulic brake system shown in FIG. 2. In this 
example, the solenoid-operated directional control and flow control valves 
32, 34 constituting the hydraulic control device are disposed in a primary 
fluid passage 104 which connects a master cylinder 100 and a brake 
cylinder 102. The primary fluid passage 104 is provided with a check valve 
106. Further, the hydraulic brake system has a by-pass passage 108 which 
by-passes the check valve 106, and the directional and flow control valves 
32, 34. In the by-pass passage 108, there is disposed a check valve 110 
which permits the fluid to flow in a direction opposite to the direction 
of the fluid flow permitted by the check valve 106. 
A reservoir 112 is connected to the directional control valve 32. The 
working fluid in the reservoir 112 is pumped by a pump 114 and is 
delivered to an accumulator 116 for storage therein. As is apparent from 
the foregoing description, the hydraulic system shown in FIG. 2 serves as 
a hydraulic anti-skid brake system of direct pressure control type which 
comprises: a hydraulic power source constituted by the master cylinder 
100, pump 114, accumulator 116, etc.; a hydraulic actuator in the form of 
the brake cylinder 102, whose operating pressure is controlled by the 
solenoid-operated hydraulic control device; and a fluid tank in the form 
of the reservoir 112. 
The solenoid-operated direction control valve 32 and the solenoid-operated 
flow control valve 34 are normally placed in the positions of FIG. 2. In 
these positions, the hydraulic brake fluid pressurized by the master 
cylinder 100 upon application of a brake is delivered through the primary 
fluid passage 104 to the brake cylinder 102 via the check valve 106, and 
the directional and flow control valves 32, 34 which are disposed in the 
passage 104. 
With the valves 32, 34 placed in the above-indicated normal positions of 
FIG. 2, the plunger 72 of the flow control valve 34 is held apart from the 
valve seat 76, and consequently the valve hole 88 is kept open, as 
indicated in FIG. 1. In this condition, the fluid may flow into the brake 
cylinder 102 at a sufficiently high flow rate, whereby there is a minimum 
time lag between the activation of the master cylinder 100 and the actual 
application of the brake by the brake cylinder 102. 
When the braking pressure in the brake cylinder 102 is relatively low in 
relation to a coefficient of friction of a road surface, an anti-skid 
device of the hydraulic brake system is not activated, and the directional 
and flow control valves 32, 34 are held in the positions of FIG. 2. Upon 
releasing the brake, the fluid in the brake cylinder 102 is released 
mainly through the by-pass passage 108, and is returned to the master 
cylinder 100. 
When the braking pressure in the brake cylinder 102 has been raised beyond 
an upper limit in relation to the coefficient of friction of the road 
surface, an amount of slip of a vehicle's drive wheel for which the brake 
cylinder 102 is provided, exceeds a predetermined upper limit, whereby the 
solenoid 82 is energized by a controller (not shown) in response to a 
signal from a sensor (not shown) which has detected an excessive slip 
ratio of the drive wheel. As a result, the flow control valve 32 is 
activated for communication of the brake cylinder 102 with the reservoir 
112. Thus, the brake fluid in the brake cylinder 102 is permitted to be 
discharged therefrom into the reservoir 112, whereby the pressure in the 
pressure chamber in the brake cylinder 102 is lowered. If the controller 
judges at this time that it is necessary to rapidly reduce the brake 
pressure in the brake cylinder 102, the solenoid 82 for the flow control 
valve 34 is held deenergized, so that the valve 34 permits the fluid to 
flow through the valve hole 88 into the reservoir 112 at a sufficiently 
high rate. However, if the controller judges that the pressure in the 
brake cylinder 102 should fall at a slow rate, the solenoid 82 is 
energized to cause the plunger 72 to be seated on the valve seat 76, 
whereby the fluid is forced to flow through the flow restrictor formed by 
the U-shaped groove 86, and thus the rate of flow of the fluid into the 
reservoir 112 is reduced. In this case, therefore, the pressure in the 
brake cylinder 102 falls first at a relatively high rate as indicated by a 
segment AB in FIG. 4, and then at a relatively low rate as indicated by a 
segment BC. 
The brake fluid fed into the reservoir 112 is pumped up by the pump 114 and 
stored in the accumulator 116. Hence, if the slip ratio of the drive wheel 
has been reduced below the upper limit as a result of the fall of the 
pressure in the brake cylinder 102 as described above, and the controller 
operates to deenergize the solenoid 64 for the directional control valve 
32, the highly pressurized fluid stored in the accumulator 116 is allowed 
to be fed into the brake cylinder 102. If if is required to rapidly raise 
the pressure in the brake cylinder 102, the solenoid 82 for the flow 
control valve 34 is deenergized to permit the fluid to flow into the brake 
cylinder 102 at a high rate through the valve hole 88. On the other hand, 
if the braking pressure should be raised at a slow rate, the plunger 72 is 
seated on the valve seat 76 so that the fluid is forced to flow through 
the flow restrictor formed by the U-shaped groove 86, and the fluid is fed 
into the brake cylinder 102 at a relatively low rate. In this case, 
therefore, the pressure in the brake cylinder 102 is raised first at a 
high rate as indicated by a segment CD of FIG. 4, and then at a slow rate 
as indicated by a segment DE. 
Even in the event that the flow restrictor formed by the U-shaped groove 86 
is clogged or plugged with foreign matters during flows of the fluid 
therethrough for relatively slow rise or fall of the pressure in the brake 
cylinder 102, the foreign matters plugging the restrictor passage are 
removed by a flow of the fluid when the plunger 72 is unseated from the 
valve seat 76 and the U-shaped groove 86 is uncovered or opened. Thus, the 
operating reliability of the flow control valve 34 and consequently of the 
solenoid-operated hydraulic control device is improved, and an oil filter 
or other means required for preventing the flow restrictor from being 
clogged, may be made simpler than required in a conventional arrangement. 
Referring next to FIG. 3, there is shown another example of a hydraulic 
system in which the solenoid-operated hydraulic control device is 
incorporated. This hydraulic system provides a hydraulic anti-skid brake 
system of indirect pressure control type, wherein the pressure in the 
brake cylinder 102 is indirectly controlled by means of a pressure of a 
working fluid which is pumped form a reservoir 120 by a pump 122 and 
supplied to another hydraulically operated actuator such as a power 
steering unit 124. The solenoid-operated hydraulic control device 
consisting of the directional and flow control valves 32, 34 is disposed 
between a first regulator 126 and a second regulator 128. The pressure in 
a power pressure chamber 130 in the second regulator 128 is controlled by 
the fluid whose pressure has been regulated by the first regulator 126. 
The first regulator 126 has a pressure regulating piston 134 which is 
slidably and fluid-tightly fitted in a housing 132. On axially opposite 
sides of the piston 134, there are formed a power pressure chamber 136, 
and a brake pressure chamber 138. A pressure-receiving surface on one end 
of the piston 134 exposed to the power pressure chamber 136 is selected to 
be greater than a pressure-receiving surface on the other end exposed to 
the brake pressure chamber 138. The brake pressure chamber 138 receives 
the fluid from the master cylinder 100. In this arrangement, the pressure 
in the power chamber 136 is lower than the pressure in the master cylinder 
100, and is increased in response to an increase in the pressure in the 
master cylinder 100. The pressure in the power pressure chamber 136 is 
hereinafter referred to as "power pressure". This power pressure is 
applied to the power pressure chamber 130 of the second regulator 128 
through the solenoid-operated hydraulic control device 32, 34 provided 
according to the present invention. The power pressure in the power 
pressure chamber 136 is also applied to a power pressure chamber 142 of a 
by-pass valve 140. 
As in the hydraulic system of FIG. 2, the master cylinder 100 is connected 
to the brake cylinder 102 via the primary fluid passage 104. The second 
regulator 128 is disposed in this primary fluid passage 104. The second 
regulator 128 has a pressure regulating piston 146 which is slidably and 
fluid-tightly received in a housing 144. The piston 146 receives at its 
one end the power pressure in the previously indicated power pressure 
chamber 130, and at its other end a pressure in a brake pressure chamber 
148. The piston 146 operated with these pressures is adapted to open and 
close a shut-off valve 156 which comprises a valve seat 150, ball 152 and 
a compression coil spring 154. Further, the piston 146 is adapted to 
change the volume of the brake pressure chamber 148, and thereby regulate 
the power pressure in the brake pressure chamber 148 based on the pressure 
in the power pressure chamber 130. 
The by-pass valve 140 has a piston 158 which holds a ball 160 seated on a 
valve seat 164 against a biasing action of a compression coil spring 162, 
as long as the power pressure is applied to the power pressure chamber 
142, whereby the the pressure regulated by the second regulator 128 is 
applied to the brake cylinder 102. However, if the power pressure is not 
applied to the power pressure chamber 142, the ball 160 is seated on a 
valve seat 166 by the biasing force of the spring 162, and consequently 
the brake fluid from the master cylinder 100 is permitted to be fed into 
the brake cylinder 102 through the by-pass valve 140, while by-passing the 
second regulator 128. 
The solenoid-operated hydraulic control device consisting of the 
directional and flow control valves 32, 34 is connected to: a hydraulic 
power source comprising the pump 122 and the first regulator 126; a 
hydraulic actuator in the form of the second regulator 128; and the 
reservoir 120. The solenoids 64 and 82 are suitably controlled under the 
control of a controller (not shown) which detects an amount of slip (slip 
ratio) of the vehicle's drive wheel according to a signal generated from a 
sensor (not shown), whereby the pressure in the power pressure chamber 130 
of the second regulator is controlled. Based on the pressure in the power 
pressure chamber 130, the second regulator 128 controls the pressure in 
the brake pressure chamber 148, and consequently controls the braking 
pressure in the brake cylinder 102. 
In this anti-skid hydraulic brake system, too, foreign matters which would 
clog or plug the flow restrictor formed by the U-shaped groove 86 during 
flows of the fluid therethrough for slow rise or fall of the pressure in 
the brake cylinder 102, may be removed by a flow of the fluid when the 
plunger 72 is unseated from the valve seat 76 and the U-shaped groove 86 
is uncovered or opened. Thus, the operating reliability of the flow 
control valve 34 and consequently of the solenoid-operated hydraulic 
control device is improved. 
While there have been described the preferred embodiment of the 
solenoid-operated hydraulic control device and the two typical 
applications thereof, the flow restrictor may be provided in other forms. 
For example, the flow restrictor formed by the U-shaped groove 86 in the 
contact surface 84 of the plunger 72 may be replaced by a U-shaped groove 
which is formed in the valve seat 76. In this intance, too, the same 
effect as obtained from the groove 86 is expected. 
A flow restrictor may also be provided by forming the contact surface 84 of 
the plunger 72 or the contact surface 83 of the valve seat 76 with minute 
indentations or projections, as exaggeratedly indicated at 87 in FIG. 11, 
so that there exist small gaps between the contact surfaces 84, 83 when 
the plunger 72 is seated on the valve seat 76. These small gaps serve as a 
flow restrictor which allows a predetermined limited flow of the fluid 
therethrough. The contact surface 84, 83, which may be formed with such 
indentations or projections 87, may be made rough or ragged, by means of 
etching, shot-blast, or other suitable methods. The intended degree of 
flow restriction may be attained by adjusting the surface roughness of the 
contact surface 84, 83. 
Another alternative flow restrictor means is illustrated in FIG. 5, wherein 
the end portion of the plunger 72 opposite to the valve seat 76 is 
constituted by a porous piece 168 which is formed, for example, of a 
porous sintered alloy. The porous structure of the porous piece 168 has an 
infinite number of minute continuous pores (which are too small to be 
visible, and are therefore not indicated in FIG. 5) which allow a 
predetermined restricted rate of flow of the fluid therethrough. 
Referring next to FIG. 6, a modified embodiment of the solenoid-operated 
hydraulic contol device will be described. 
This modified embodiment is different from the preceding embodiment in the 
construction of the flow control valve. That is, the embodiment of FIG. 6 
uses a solenoid-operated flow control valve 170 wherein the plunger 72 has 
a ball 70 fixed at its one end opposite to the valve seat 76, so that the 
ball 70 may be seated on the valve seat 76. The piston chamber 90 
communicates with the fluid passage 56, and at the same time with the port 
28 through a flow restrictor 172 (restrictor passage) and a fluid passage 
174. The flow restrictor 172 is disposed in parallel with the valve hole 
88 which communicates with the fluid passage 174. The valve hole 88 has a 
diameter large enough to permit a sufficiently greater amount of flow of 
the fluid, as compared with the flow restrictor 172. This valve hole 88 is 
open in the previously indicated valve seat 76. 
The ball 70 is normally held apart from the valve seat 76 by a biasing 
force of the spring 78, so as to permit the fluid from the passage 56 to 
flow into the fluid passage 174 through both of the flow restrictor 172 
and the valve hole 88. When the piston 74 is moved in an upward direction 
(in FIG. 6) upon energization of the solenoid 82, the ball 70 is seated on 
the valve seat 76 and closes the valve hole 88. In this condition, 
therefore, the fluid is forced to flow through only the flow restrictor 
172. 
The flow restrictor 172 is formed by a second hole formed in the housing 10 
in parallel with the valve hole 88, and a porous structure in the form of 
a sintered piece 176 which is press-fitted in the second hole. The 
sintered piece 176 has a porous structure which is produced by sintering a 
formed mass of powder of suitable materials such as metal and ceramics. 
Since the porous structure of the sintered piece 176 has a network of an 
infinite number of minute passages, the sintered piece 176 performs 
substantially the same function as a conventionally used orifice. In this 
arrangement, a substantive portion of the fluid which flows into the fluid 
passage 174 flows through the valve hole 88, and a very small amount of 
the fluid flows through the sintered piece 176. Accordingly, there is a 
very low possibility of the sintered piece 176 being plugged by foreign 
substances if contained in the fluid. Further, the flow restricting 
capability of the sintered piece 176 will not be significantly changed 
even if a part of the porous structure is plugged with foreign matters. 
Thus, the sintered piece 176 may serve its intended flow restricting 
function for a long period of time, and assures a reliable operation of 
the flow control valve 170 and consequently of the solenoid-operated 
hydraulic control device as a whole. 
The abovementioned solenoid-operated control device of FIG. 6 may be used 
in anti-skid hydraulic brake systems as illustrated in FIGS. 2 and 3, and 
may be operated in the same manners as previously described. 
While the flow control valve 170 of this modified embodiment uses the 
sintered piece 176 as a flow restrictor, the sintered piece may be 
replaced by a sintered piece 178 of porous structure shown in FIG. 7. This 
modified form of the sintered piece 178 has a multi-layer laminar 
structure which consists of a central dense layer 180 having comparatively 
fine pores, and a pair of coarse layers 182, 182 which sandwich the 
central dense layer 180 and have comparatively large pores. The sintered 
piece 178 is positioned relative to the flow control valve 170, such that 
the line of fluid flows through the sintered pieces 178 is substantially 
perpendicular to the layers 180, 182, 182 of the laminar structure. The 
central dense layer 180 primarily functions to restrict the fluid flows, 
while the outer coarse layers 182, 182 functions mainly as filters for 
protecting the central dense layer 180 from being plugged with foreign 
substances, and further minimizing a variation in the flow restriction by 
the sintered piece 178 due to the plugging. 
The porous structure of the sintered piece used may be made of a suitable 
synthetic resin. While the sintered piece 176, 178 are produced separately 
from the housing 10 of the hydraulic control device, it is possible that 
the housing 10 may be formed with a sintered porous piece such that the 
porous piece fills a hole provided in the housing for flow restriction. At 
any rate, the porous piece should eventually fills such a hole formed in 
the housing. 
Like the flow control valve 34 of the preceding embodiment, the flow 
control valve 170 of the modified embodiment is adapted to be normally 
held open allowing the fluid to flow through the valve hole 88. Therefore, 
the amount of the fluid which flows through the flow restrictor 176, 178 
is effectively reduced for minimizing the possibility of clogging of the 
sintered porous structure of the restrictor. However, the flow control 
valve 34 may be used in a solenoid-operated hydraulic control device in 
which the valve hole 88 is normally closed. In this case, the feature of 
the porous structure that a partial or local clogging thereof will not 
cause a variation in the flow restriction, may be effectively utilized. 
While the embodiments of FIGS. 6 and 7 use the sintered piece 176 or 178 as 
a flow restrictor which permits a restricted flow of the fluid even when 
the ball 70 is seated on the valve seat 76, it is possible to modify the 
flow control valve 170, as illustrated in FIGS. 8, 9 and 10. In this 
modified embodiment, the sintered piece 176, 178 is eliminated, and a 
valve seat member 184 having the valve hole 88 and fixed to the housing 10 
is formed with a valve seat 186 which has a truncated conical shape in 
cross section as shown in FIGS. 9 and 10. The valve seat 186 is provided 
with a V-groove 188 which permits a restricted flow of the fluid even 
while the ball 70 serving as a valving member is seated on the valve seat 
186 as indicated in FIG. 9. Thus, the V-groove 188 acts as a flow 
restrictor. In the event that the V-groove 190 is blocked by a mass of 
foreign substance 190 as depicted in FIG. 9, this foreign substance mass 
190 may be removed by a non-restricted flow of the fluid when the ball 70 
is moved off the valve seat 186, as indicated in FIG. 10. It will be 
understood that the valve seat 186 and the ball 70 of the present modified 
embodiment serve the same function as the valve seat 83 and the end of the 
plunger 72 with the U-shaped groove 86 of the first embodiment of FIG. 1. 
While the present invention has been described in its preferred embodiments 
with a certain degree of particularity, it is to be understood that the 
invention may be embodied with various changes and improvements which may 
occur to those skilled in the art.