Pressure control device wherein shut-off valves are opened and closed in timed relation with suction and delivery strokes of reciprocating pump device

A braking pressure control device having a first shut-off valve in a fluid passage connecting master cylinder and vehicle wheel brake cylinder, a second shut-off valve between the first shut-off valve and the brake cylinder, a reciprocatingly pump device having a variable-volume chamber which is connected to a portion of the fluid passage between the first and second shut-off valves and whose volume is changed by reciprocating movement of a pump piston, and a synchronizer for opening and closing the first and second shut-off valves in timed relationship with suction and delivery strokes of the pump piston for suction and delivery of a brake fluid into the variable-volume chamber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates in general to a device for a hydraulic 
braking pressure, and more particularly to improvements in a hydraulic 
braking pressure control device which is electrically controllable to 
regulate the hydraulic braking pressure. 
2. Discussion of the Related Art 
The hydraulic braking pressure control device of the type indicated above 
is provided in a hydraulically operated braking apparatus for a motor 
vehicle and is used to regulate the hydraulic braking pressure in an 
anti-lock control mode or a traction control mode, as disclosed in 
JP-A-1-190571, for example. 
The hydraulic braking pressure control device as disclosed in the 
above-identified publication employs three solenoid-operated shut-off 
vales for regulating the pressure (hereinafter referred to as "wheel brake 
cylinder pressure") in brake cylinders (hereinafter referred to as "wheel 
brake cylinders") for braking drive wheels of a motor vehicle, and is 
operable in an anti-lock control mode and a traction control mode. The 
three shut-off valves consist of a cut-off valve, a pressure increasing 
valve and a pressure reducing valve. The cut-off valve and the pressure 
increasing valves are provided in series in a fluid passage which connects 
a master cylinder and the wheel brake cylinder. The pressure reducing 
valve is provided in a reservoir passage which is connected at one end 
thereof to a portion of the above-indicated fluid passage between the 
pressure increasing valve and the wheel brake cylinder, and at the other 
end to a reservoir. The cut-off valve and the pressure increasing valve 
are normally-open solenoid-operated shut-off valves, while the pressure 
reducing valve is a normally-closed solenoid-operated shut-off valve. 
In the anti-lock control mode, the cut-off valve is held in an open state 
thereof, and the pressure increasing and reducing valves are suitably 
opened and closed for increasing the wheel brake cylinder pressure by 
permitting a pressurized brake fluid to be from the master cylinder to the 
wheel brake cylinder under control, and reducing the wheel brake cylinder 
pressure by permitting the brake fluid to be discharged from the wheel 
brake cylinder to the reservoir, or for holding the wheel brake cylinder 
pressure by inhibiting supply and discharge flows of the the brake fluid 
into and from the wheel brake cylinder. The brake fluid discharged into 
the reservoir is pumped up by a pump device and returned to a portion of 
the above-indicated fluid passage (connecting the master cylinder and the 
wheel brake cylinder) which portion is between the cut-off and pressure 
increasing valves. 
In the traction control mode, the cut-off valve is held in a closed state 
thereof, and the brake fluid is pumped up from the master cylinder through 
a by-pass passage which by-passes the cut-off valve, and is supplied to 
the wheel brake cylinder for a drive wheel under control, to brake the 
drive wheel without activation of the master cylinder by the vehicle 
operator. In the traction control mode, the wheel brake cylinder pressure 
is suitably increased, reduced or held by opening and closing the pressure 
increasing and reducing valves as needed as in the anti-lock control mode. 
In the conventional hydraulic braking pressure control device, however, the 
rate of increase or decrease in the wheel braking pressure in the 
anti-lock and traction control modes is determined by the actual wheel 
brake cylinder pressure in the wheel brake cylinder under control and the 
actual pressure in the master cylinder. In other words, the rate of 
increase or decrease in the wheel brake cylinder pressure cannot be 
controlled as desired or needed. 
Where the wheel brake cylinder pressure is reduced, for example, the rate 
of decrease in the wheel brake cylinder pressure is relatively high with a 
relatively high rate of discharge flow of the brake fluid from the wheel 
brake cylinder into the reservoir, when the wheel brake cylinder pressure 
upon initiation of the pressure decrease is relatively high. Conversely, 
the pressure decrease rate is relatively low with a relatively low rate of 
discharge flow of the brake fluid from the wheel brake cylinder, when the 
wheel brake cylinder pressure is relatively low. Where the wheel brake 
cylinder pressure is increased in the anti-lock control mode, the rate of 
increase in the wheel brake cylinder pressure is relatively high with a 
relatively high rate of supply flow of the brake fluid from the master 
cylinder into the wheel brake cylinder, when a difference between the 
master cylinder pressure and the wheel brake cylinder pressure is 
relatively large. Conversely, the pressure increase rate is relatively low 
with a relatively low rate of supply flow of the fluid into the wheel 
brake cylinder, when the pressure difference is relatively small. 
In the hydraulic braking pressure control device in which the rates of 
increase and decrease in the wheel brake cylinder pressure are determined 
by the wheel brake cylinder pressure and the master cylinder pressure as 
described above, the accuracy of control of the wheel brake cylinder 
pressure is inevitably and undesirably low. Where the hydraulic braking 
pressure control device is operated in the anti-lock control mode during 
running of the motor vehicle on a road surface having a relatively low 
friction of coefficient, the wheel brake cylinder pressure is relatively 
low, and therefore the wheel brake cylinder pressure cannot be rapidly 
reduced even where it is required to reduce the wheel brake cylinder 
pressure at a comparatively high rate. 
SUMMARY OF THE INVENTION 
It is accordingly an object of this invention to provide a hydraulic 
braking pressure control device which is capable of controlling the rates 
of increase and decrease in the wheel brake cylinder pressure. 
According to the principle of the present invention, there is provided a 
hydraulic braking pressure control device a hydraulic braking pressure 
control device wherein a master cylinder and a wheel brake cylinder for 
braking a vehicle wheel are connected by a fluid passage, the device being 
comprising: 
a first shut-off valve provided in the fluid passage; 
a second shut-off valve provided in a portion of the fluid passage between 
the first shut-off valve and the wheel brake cylinder; 
a reciprocating pump device including a housing, a pump piston which is 
reciprocatingly movable in the housing and which cooperates with the 
housing to define a variable-volume chamber connected to a portion of the 
fluid passage between the first and second shut-off valves, and a pump 
piston drive device for reciprocating the pump piston, the pump drive 
device including an electric motor as a drive source; and 
synchronizing means for opening and closing the first and second shut-off 
valves in timed relationship with suction and delivery strokes of the pump 
piston for suction and delivery of a brake fluid into and from the 
variable-volume chamber. 
In the hydraulic braking pressure control device of the present invention 
constructed as described above, the pressure of the brake fluid in the 
wheel brake cylinder is regulated by suitable combinations of the suction 
and delivery of the brake fluid into and from the variable-volume chamber 
of the reciprocating pump device and the opening and closing of the first 
and second shut-off valves. 
For example, the brake fluid is sucked from the wheel brake cylinder into 
the variable-volume chamber if the first and second shut-off valves are 
closed and opened, respectively, when the volume of the variable-volume 
chamber is increased. In this case, the brake fluid which has been sucked 
into the variable-volume chamber is delivered therefrom to the master 
cylinder, by opening and closing the first and second shut-off valves, 
respectively, when the volume of the variable-volume chamber is then 
reduced. Thus, the pressure in the wheel brake cylinder is reduced. 
If the first and second shut-off valves are opened and closed, 
respectively, when the volume of the variable-volume chamber is increased, 
the brake fluid is sucked from the pressurizing chamber of the master 
cylinder into the variable-volume chamber. In this case, the brake fluid 
which has been sucked into the variable-volume chamber is delivered 
therefrom to the wheel brake cylinder, by closing and opening the first 
and second shut-off valves, respectively, when the volume of the 
variable-volume chamber is then reduced. Thus, the wheel brake cylinder 
pressure is increased. 
Further, the pressure in the wheel brake cylinder can be held at a constant 
level if needed, by closing the first and second shut-off valves or 
closing the second shut-off valve, or by suitably controlling pressure 
holding means different from the first and second shut-off valves, so as 
to hold the brake fluid within the wheel brake cylinder, as described in 
detail with respect to preferred embodiments of the invention. 
It will be understood that the wheel brake cylinder pressure can be 
increased or reduced at a desired rate, by suitably controlling the 
operating speed of the electric motor of the reciprocating pump device and 
thereby adjusting the rate at which the pump device performs the suction 
and delivery strokes. Where the electric motor is operated at a constant 
speed, the rates of increase and decrease of the wheel brake cylinder 
pressure can be held constant, leading to higher accuracy of control of 
the wheel brake cylinder pressure than in the conventional hydraulic 
braking pressure control device. In this respect, it is noted that the 
amount of variation in the operating speed of an electric motor is 
comparatively small. 
According to a first preferred form of the present invention, the first and 
second shut-off valves are solenoid-operated shut-off valves, and the 
synchronizing means includes volume change detecting means for detecting 
directions of change of a volume of the variable-volume chamber, and valve 
control means for opening and closing the first and second shut-off valves 
on the basis of an output of the volume change detecting means. 
In the above preferred form of the invention, the solenoid-operated first 
and second shut-off valves are electrically controlled to be opened and 
closed in timed or synchronized relationship with the directions of change 
of the volume of the variable-volume chamber of the reciprocating pump 
device, so that the wheel brake cylinder pressure increased or decreased, 
or held if necessary. The volume change detecting means determines whether 
the volume of the variable-volume chamber is now increasing or decreasing, 
namely, whether the pump device is now in the sucking stroke or in the 
delivery stroke. For instance, the volume change detecting means uses a 
detector for detecting the position, speed of movement or acceleration of 
the pump piston or other movable member of the pump device. Alternatively, 
the volume change detecting means may detect the directions of change of 
the volume of the variable-volume chamber (determine whether the pump 
device is in the suction or delivery stroke) on the basis of a drive 
current of the electric motor of the pump piston drive device. In this 
instance, the volume change detecting means includes a current detector 
for detecting the electric current of the electric motor, and means for 
determining the changing state of the volume of the variable-volume 
chamber on the basis of an output of the current detector. 
In the above first preferred form of this invention wherein the first and 
second shut-off valves are solenoid-operated shut-off valves, the opening 
and closing of these shut-off valves can be relatively easily controlled 
without timed relationship with the reciprocating movement of the pump 
piston of the pump device, as well as in timed relationship with the 
reciprocating movement. Accordingly, the wheel brake cylinder pressure can 
be relatively easily held at a constant level. Further, the present form 
of the invention facilitates the regulation of the pressures in the wheel 
brake cylinders in two braking systems, even where the same pump piston 
drive device is used for the reciprocating pump devices in the two braking 
systems. 
According to one advantageous arrangement of the above first preferred form 
of the invention, the first and second shut-off valves, the reciprocating 
pump device and the synchronizing means are provided in each of two 
braking systems which have respective wheel brake cylinders for braking 
respective vehicle wheels, and the reciprocating pump devices of the two 
braking system use the same pump piston drive device such that the pump 
pistons of the reciprocating pump device have opposite operating phases. 
In this case, the volume change detecting means comprise shut-off valve 
closing means for closing both of the first and second shut-off valves in 
one of the two braking systems, and change direction determining means for 
determining the directions of change of the volume of the variable-volume 
chamber of each of the two braking systems on the basis of a drive current 
of the electric motor of the pump piston drive device while the first and 
second shut-off valves of the one braking system are both closed by the 
shut-off valve closing means. 
In the above advantageous arrangement, the hydraulic braking pressure 
control device is used for controlling the two braking systems. To open 
and close the first and second solenoid-operated shut-off valves in timed 
relationship with the directions of change of the volume of the 
variable-volume chamber in each of the two braking systems, the directions 
of volume change of the variable-volume chambers of the two reciprocating 
pump devices of the two braking systems should be detected or determined 
independently of each other. In the present arrangement wherein the same 
pump piston drive device is used for the two pump devices, some means is 
required to correctly detect the directions of change of the volumes of 
the two variable-volume chambers of the two pump devices. 
Where the volume change detecting means employs a detector for detecting 
the position of a movable member of each pump device, for example, the 
directions of volume change of the two variable-volume chambers of the two 
pump devices can be comparatively easily detected even where the same pump 
piston drive device is used for the two pump devices. Wherein the 
directions of volume change of the variable-volume chambers are detected 
on the basis of the drive current of the electric motor, as in the present 
arrangement, it is possible to detect that one of the two pump devices is 
in the delivery stroke (or suction stroke), but is not possible to 
determine which one of the two pump devices is in the delivery stroke 
(suction stroke). 
Generally, the current of the electric motor increases during the delivery 
stroke of each pump device. In the present arrangement, the pump pistons 
of the two pump devices and the common pump piston drive device are 
disposed such that the pump pistons have opposite operating phases, so 
that the load acting on the electric motor used by the two pump devices is 
averaged, for example. In this arrangement, the waveform of the drive 
current of the electric motor has peak values at 180.degree. intervals. 
These peak values indicate the delivery stroke of one of the two pump 
devices, but do not indicate which one of the two pump devices is now in 
the delivery stroke. 
In the light of the above, the volume change detecting means in the present 
advantageous arrangement comprises shut-off valve closing means for 
closing both of the first and second shut-off valves in one of the two 
braking systems, while the two pump devices are operating. While the first 
and second shut-off valves are closed, the brake fluid is not sucked into 
the variable-volume chamber in the above-indicated one braking system, or 
delivered through a pressure-relief valve, whereby the drive current of 
the electric motor has a change from that in the normal state in which the 
shut-off valve closing means is not activated. Based on this change of the 
drive current while the first and second shut-off valves are both closed 
by the shut-off valve closing means, the change direction determining 
means determines the directions of change of the volume of the 
variable-volume chamber of each braking system, namely, determines which 
one of the two pump devices is in the delivery stroke (or suction stroke). 
Thus, the above advantageous arrangement permits easy detection of the 
volume change directions of the variable-volume chambers of the pump 
devices of the two braking systems, even where the same pump piston drive 
device is used for the pump devices of the two braking systems. 
Accordingly, the present arrangement permits improved accuracy of control 
of the wheel brake cylinder pressure, while assuring simplified 
construction of the hydraulic braking pressure control device. 
The above advantageous arrangement is desirably adapted such that the 
shut-off valve closing means comprises at least one of anti-lock control 
shut-off valve closing means and traction control shut-off valve closing 
means, and such that the change direction determining means comprises at 
least one of anti-lock control change direction determining means and 
traction control change direction determining means, which corresponds to 
the above-indicated at least one of anti-lock control shut-off valve 
closing means and traction control shut-off valve closing means. The 
anti-lock control shut-off valve closing means is adapted to close the 
first and second shut-off valves of a predetermined one of the two braking 
systems during the suction stroke of the pump piston of the reciprocating 
pump device of one of the two braking systems when the hydraulic braking 
pressure control device is operated in an anti-lock control mode. The 
traction control shut-off valve closing means is adapted to close the 
first and second shut-off valves of a predetermined one of the two braking 
systems during the delivery stroke of the pump piston of the reciprocating 
pump device of one of the two braking systems when the hydraulic braking 
pressure control device is operated in a traction control mode. The 
anti-lock control change direction determining means is adapted to 
determine the direction of change of the volume of the variable-volume 
chamber of each of the two braking systems depending upon whether the 
drive current increases above a predetermined threshold state during the 
delivery stroke of the pump piston immediately after termination of 
closure of the first and second shut-off valves by the anti-lock control 
shut-off valve closing means. The traction control change direction 
determining means is adapted to determine the direction of change of the 
volume of the variable-volume chamber depending upon the drive current 
increases above a predetermined threshold state while the first and second 
shut-off valves are held closed by the traction control shut-off valve 
closing means. 
The desirable arrangement described above provides the volume change 
detecting means which is suitably operated in at least one of the 
anti-lock control operation and the traction control operation of the 
hydraulic braking pressure control device. 
The amount of increase in the drive current of the electric motor when the 
first and second shut-off valves are closed differs depending upon whether 
fluid pressures are present in the master cylinder and the wheel brake 
cylinder, that is, depending upon whether the present hydraulic braking 
pressure control device is in the anti-lock control mode is in the 
traction control mode. The anti-lock control operation is performed when a 
fluid pressure is present in the master and wheel brake cylinders. On the 
other hand, the traction control operation is performed without a fluid 
pressure in the master and wheel brake cylinders. If the brake fluid is 
delivered from the pump device to the master or wheel brake cylinder when 
a fluid pressure is present in the master and wheel brake cylinders, a 
relatively large load acts on the electric motor of the pump piston drive 
device, and the drive current of the motor is relatively large. The load 
acting on the motor is relatively small when a fluid pressure is not 
present in the master and wheel brake cylinders. 
When the first and second shut-off valves of one of the two braking systems 
are closed during the suction stroke of the pump device of the braking 
system in which the anti-lock control operation is performed, the pump 
device of the braking system in which the shut-off valves have been closed 
will initiate its delivery stroke immediately after the expiration of the 
period of closure of the shut-off valves (i.e., after the above-indicated 
suction stroke), if the braking system in which the anti-lock control 
operation is performed is the braking system in which the shut-off valves 
are closed. In this case, the brake fluid is not delivered from the pump 
device during the above-indicated delivery stroke since the brake fluid 
has not been sucked in the variable-volume chamber of the pump device of 
the braking system in which the shut-off valves have been closed. 
Accordingly, the drive current of the electric motor is relatively small. 
If the braking system in which the anti-lock control operation is 
performed is not the braking system in which the shut-off valves are 
closed, the pump device of the braking system in which the shut-off valves 
have not been closed will initiate the delivery stroke immediately after 
the expiration of the closure period of the shut-off valves. In this case, 
the brake fluid is delivered from the pump device during the delivery 
stroke since the brake fluid has been sucked in the variable-volume 
chamber of the pump device of the braking system in which the shut-off 
valves were held open. Accordingly, the drive current of the motor is 
relatively large. Therefore, it is possible to determine whether the pump 
device of each braking system is now in the suction or delivery stroke, or 
whether the volume of the variable-volume chamber of the pump device is 
increasing or decreasing, depending upon whether the drive current of the 
electric motor is relatively large or small during the period during which 
the first and second shut-off valves are closed in one of the two braking 
systems. 
When the first and second shut-off valves of one of the two braking systems 
are closed during the delivery stroke of the pump device of the braking 
system in which the traction control operation is performed, the drive 
current of the electric motor while the shut-off valves are held closed is 
relatively large, if the braking system in which the traction control 
operation is performed is the braking system in which the shut-off valves 
are closed. In the traction control operation in which no fluid pressures 
are present in the master and wheel brake cylinders, the drive current 
during the delivery stroke is smaller than in the anti-lock control 
operation. If the first and second shut-off valves are closed during the 
delivery stroke, the brake fluid which has been sucked in the 
variable-volume chamber in the preceding suction stroke should be 
delivered therefrom through a suitable pressure relief valve, whereby the 
load acting on the motor during the delivery stroke becomes relatively 
large in the braking system in which the shut-off valves are closed. If 
the braking system in which the traction control operation is performed is 
not the braking system in which the shut-off valves are closed, the load 
which acts on the electric motor during the delivery stroke is relatively 
small since no fluid pressures are present in the master and wheel brake 
cylinders. Therefore, it is possible to determine whether the pump device 
of each braking system is now in the suction or delivery stroke, or 
whether the volume of the variable-volume chamber of the pump device is 
increasing or decreasing, depending upon whether the drive current of the 
electric motor is relatively large or small during the period during which 
the first and second shut-off valves are closed in one of the two braking 
systems. 
According to a second advantageous arrangement of the above-indicated first 
preferred form of this invention, the volume change detecting means 
comprises means for detecting the directions of change of the volume of 
the variable-volume chamber on the basis of a drive current of the 
electric motor of the pump piston drive device. 
According to a second preferred form of this invention, the hydraulic 
braking pressure control device further comprises a rapidly pressure 
reducing device including a reservoir passage connected to the portion of 
the fluid passage between the first and second shut-off valves, a 
reservoir connected to the reservoir passage, and a third shut-off valve 
which is provided in the reservoir passage and which is normally closed 
and is opened at least when a pressure in the wheel brake cylinder is 
reduced for the first time. 
In the above second preferred form of the invention, the brake fluid 
discharged from the wheel brake cylinder by the pump device is 
accommodated in the reservoir when the wheel brake cylinder pressure is 
reduced for the first time, so that the wheel brake cylinder is rapidly 
reduced to effectively prevent an increase in the slipping tendency of the 
vehicle wheel, or to rapidly eliminate excessive slipping tendency of the 
wheel in the anti-lock control mode, for example. The third shut-off valve 
is normally closed, so as to prevent an undesirable flow of the brake 
fluid from the pressurizing chamber of the master cylinder into the 
reservoir during an ordinary brake application to the wheel, and thereby 
avoid an unnecessary large amount of operation of a brake operating member 
(e.g., brake pedal) due to such flow of the brake fluid into the 
reservoir. Further, the normally-closed third shut-off valve is also 
effective to avoid the filling of the reservoir with the brake fluid 
before the first reduction of the wheel brake cylinder pressure, which 
prevents the rapid reduction of the wheel brake cylinder. 
If the storage capacity of the reservoir is large enough toe accommodate 
the brake fluid discharged from the wheel brake cylinder even after the 
first reduction of the wheel brake cylinder pressure, the wheel brake 
cylinder pressure can be rapidly reduced upon the second pressure 
reduction of the see wheel brake cylinder, or the pressures in two or more 
wheel brake cylinders if provided in the same braking system can be 
rapidly reduced simultaneously. 
According to one advantageous arrangement of the above second preferred 
form of this invention, the rapidly pressure reducing device includes a 
first pilot passage communicating with the third shut-off valve and the 
variable-volume chamber, and a second pilot passage which communicates 
with the third shut-off valve and which communicates with the first pilot 
passage through flow restricting means, the third shut-off valve being a 
pilot-operated shut-off valve which receives a first and a second pilot 
pressure in the first and second pilot passages, respectively, which first 
and second pilot pressures act on the pilot-operated shut-off valve in 
opposite directions, the pilot-operated shut-off valve being opened when 
the first pilot pressure is lower than the second pilot pressure by more 
than a predetermined value. 
In the above advantageous arrangement, the first pilot pressure in the 
first pilot passage connected to the pump device becomes lower than the 
second pilot pressure in the second pilot passage which communicates with 
the first pilot passage via the flow restricting means, when the pump 
device is in the suction stroke with an increase in the volume of the 
variable-volume chamber. At this time, the pilot-operated shut-off valve 
is opened, and the brake fluid which is discharged from the wheel brake 
cylinder during the suction stroke of the pump device is permitted to flow 
into the reservoir, to that the wheel brake cylinder pressure is rapidly 
reduced this arrangement does not require any electrically controlled 
means for rapidly reducing the wheel brake cylinder with high reliability. 
In the above arrangement, the first shut-off valve may be utilized as the 
flow restricting means. In this case, the first pilot passage is connected 
to a portion of the fluid passage between the master cylinder end the 
first shut-off valve, while the second pilot passage is connected to the 
portion of the fluid passage between the first and second shut-off valves. 
When the wheel brake cylinder pressure is reduced for the first time after 
initiation of the anti-lock control operation, the first and second 
shut-off valves are closed and opened, respectively during the suction 
stroke of the pump device. As a result, there arises a pressure difference 
on the upstream and downstream sides of the first shut-off valve (a 
difference between the pressures in respective portions of the fluid 
passage connected to the master cylinder and wheel brake cylinder), 
whereby the pilot-operated shut-off valve (third shut-off valve) is 
opened, so that the brake fluid in the wheel brake cylinder is discharged 
into the reservoir while at the same time it is sucked into the pump 
device, resulting in a high rate of reduction of the wheel brake cylinder. 
Since the first shut-off valve is closed during the wheel brake cylinder 
pressure, this first shut-off valve functions as the flow restricting 
means for restricting flows of the fluid between the first and second 
pilot passages. 
Alternatively, the flow restricting means may take the form of flow 
resistance applying means provided in a pump passage by which the 
variable-volume chamber is connected to the portion of the fluid passage 
between the first and second shut-off valves. The flow resistance applying 
means is adapted to apply a resistance to a flow of the brake fluid 
through the pump passage in at least a direction toward the 
variable-volume chamber, and the first and second pilot passages are 
connected to respective portions of the pump passage which are on opposite 
sides of the flow resistance applying means and which are respectively 
connected to the variable-volume chamber and the portion of the fluid 
passage between the first and second shut-off valves. When the pump device 
is activated to initiate the anti-lock control operation, there arises a 
pressure difference on the upstream and downstream sides of the flow 
resistance applying means (a difference between the pressures in the 
above-indicated portions of the pump passage), whereby the pilot-operated 
shut-off valve (third shut-off valve) is opened, so that the brake fluid 
in the wheel brake cylinder is discharged into the reservoir while at the 
same time it is sucked into the pump device, resulting in a high rate of 
reduction of the wheel brake cylinder. 
According to a third preferred form of this invention, the synchronizing 
means comprises a motion transmitting device for transmitting a motion of 
a movable member of the pump piston drive device to a movable member of 
each of the first and second shut-off valves. 
In the above third preferred form of the invention, the rotary motion of 
the movable member of the pump piston drive device is transmitted to the 
movable member of each of the first and second shut-off valves, so that 
the first and second shut-off valves are mechanically opened and closed in 
timed or synchronized relationship with the suction and delivery strokes 
of the pump device. This arrangement does not require any electrically 
controlled member for reliable synchronization of the operation of the 
first and second shut-off valves with the operation of the pump device. 
According to one advantageous arrangement of the above third preferred form 
of the invention, the motion transmitting device comprises a motion 
transmitting device of solid member type including solid members which 
contact each other for transmitting the motion of the movable member of 
the pump piston drive device to the movable member of the first and second 
shut-off valves. 
According to a second advantageous arrangement of the third preferred form 
of the invention, the motion transmitting device comprises a motion 
transmitting device of hydraulic pressure type which utilizes a hydraulic 
pressure to transmit the motion of the movable member of the pump piston 
drive device to the movable member of the first and second shut-off 
valves. 
According to a fourth preferred form of this invention, the pump piston 
drive device comprises a cam rotated by the electric motor, and biasing 
means for biasing the pump piston toward a cam surface of the cam. 
In this fourth preferred form of the invention, the pump piston is not held 
in contact with the cam surface of the cam when the brake fluid is not 
sucked into the variable-volume chamber during the suction stroke of the 
pump device, and the pump piston can be stopped at any position between 
the suction and delivery stroke ends. If the first and second shut-off 
valves in one of the two braking systems are closed to detect the 
directions of change of the volume of the variable-volume chamber as 
described above, for example, the cam can be rotated by the electric motor 
while the first and second shut-off valves are held closed, even after the 
pump piston of the pump device has moved to its delivery stroke end in the 
braking system in which the shut-off valves are closed. 
According to a fifth preferred form of this invention, the synchronizing 
means comprises means for opening both of the first and second shut-off 
valves for fluid communication between the master cylinder and the wheel 
brake cylinder, during a portion of a period during which the first and 
second shut-off valves are closed and opened, respectively, for increasing 
a pressure of the brake fluid in the wheel brake cylinder. 
In this preferred form of the invention, the brake fluid is discharged from 
the wheel brake cylinder to the master cylinder, and the wheel brake 
cylinder pressure is reduced, if the master cylinder pressure is lowered 
below the wheel brake cylinder as a result of reduction of the operating 
force acting on the brake operating member during an increase in the wheel 
brake cylinder pressure in the anti-lock control mode, because the master 
cylinder and the wheel brake cylinder are communicated with each other 
through the first and second shut-off valves which are open in the 
predetermined portion of the pressure increase period in the anti-lock 
control operation. 
According to a sixth preferred form of the present invention, the 
synchronizing means comprises pressure change determining means for 
determining whether a pressure of the brake fluid in the wheel brake 
cylinder should be increased or decreased, and means for determining, on 
the basis of an output of the pressure change determining means, whether 
each of the first and second shut-off valves should be opened or closed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIG. 1, there is illustrated a hydraulically operated 
braking apparatus for a motor vehicle, which is equipped with a hydraulic 
braking pressure control device constructed according to a first 
embodiment of this invention. In FIG. 1, reference numeral 1 denotes a 
brake operating member in the form of a brake pedal. Upon depression of 
the brake pedal 10, brake fluid in two mutually independent pressurizing 
chambers of a master cylinder 12 is pressurized to the see pressure level 
corresponding to the amount of depression of the brake pedal 10. Reference 
numeral 14 denotes a reservoir mounted on the master cylinder 12 to supply 
this master cylinder with the brake fluid. 
The fluid pressure generated in one of the two pressurizing chambers of the 
master cylinder 12 is applied through fluid passages 16, 18 to a brake 
cylinder 22 for braking a front left wheel 20 of the vehicle, and through 
fluid passages 16, 24 to a brake cylinder 28 for braking a rear right 
wheel 26 of the vehicle. The fluid pressure generated in the other 
pressurizing chamber of the master cylinder 12 is applied through fluid 
passages 30, 32 to a brake cylinder 36 for braking a front right wheel 34 
of the vehicle, and through fluid passages 30, 38 to a brake cylinder 42 
for braking a rear left wheel 40 of the vehicle. The braking apparatus is 
of a diagonal or X-crossing type. The front left and right wheels 20, 34 
are steerable wheels and drive wheels which are driven by an engine of the 
vehicle. The X-crossing braking apparatus has a first braking system 
including the front left wheel brake cylinder 22 and the rear right wheel 
brake cylinder 28, and a second braking system including the front right 
wheel brake cylinder 36 and the rear left wheel brake cylinder 42. The 
first and second braking systems are provided with respective first and 
second hydraulic pressure control actuators, which are adapted to regulate 
the fluid pressures in the wheel brake cylinders 22, 28, 36, 42 for the 
four wheels 20, 26, 34, 40 in an anti-lock control mode independently of 
each other, and the fluid pressures in the front wheel brake cylinders 22, 
26 of the front drive wheels 20, 26 in the traction control mode. The 
first and second hydraulic pressure actuators for the first and second 
braking systems are identical in construction with each other. In the 
interest of brevity and simplification, only the first hydraulic control 
actuator for the first braking system will be described by way of example. 
It is to be understood that t The same description applies to the second 
hydraulic pressure actuator. 
In the fluid passage 16 indicated above, there is provided a normally-open 
first solenoid-operated shut-off valve 46. In the fluid passages 18, 24, 
there are provided two normally open second solenoid-operated shut-off 
valves 48, 50, respectively. A reservoir 54 is connected through a 
reservoir passage 52 to a portion of the fluid passage 16 which is between 
the first solenoid-operated shut-off valve 46 and the wheel brake cylinder 
22, 28. In the reservoir passage 52, there is provided a check valve 56 
which permits a flow of the brake fluid therethrough in a first direction 
from the reservoir 54 toward the wheel brake cylinders 22, 28, and 
inhibits a flow of the fluid therethrough in a second direction opposite 
to the first direction. The storage capacity of the reservoir 54 is 
determined to permit rapid reduction of the fluid pressures in the wheel 
brake cylinders 22, 28 when the fluid pressures in both of these cylinders 
22, 28 are initially reduced upon simultaneous initiation of anti-lock 
pressure control operations for the front left wheel 20 and the rear right 
wheel 26. 
A by-pass passage 58 is connected to the reservoir passage 52 such that the 
by-pass passage 58 by-passes the check valve 56. A third shut-off valve in 
the form of a pilot-operated shut-off valve 50 is provided in this by-pass 
passage 58. To the pilot-operated shut-off valve 60, there are connected 
two pilot passages 62, 64 so that the pressures on the downstream and 
upstream sides (as viewed in the direction from the master cylinder 12 
toward the wheel brake cylinders 33, 38) of the first solenoid-operated 
shut-off valve 46 act on the pilot-operated shut-off valve 60 in the 
opposite directions. The pilot-operated shut-off valve 60 is opened when 
the upstream side pressure applied thereto through the pilot passage 64 
becomes higher than the downstream side pressure applied thereto through 
the pilot passage 62, by more than a predetermined valve opening pressure 
difference. The reservoir passage 52, reservoir 54 and pilot-operated 
shut-off valve 60 cooperate with the first solenoid-operated shut-off 
valve 46 to constitute a rapid pressure reducing device 66. 
A reciprocating pump device 70 is connected through a pump passage 68 to a 
portion of the above-indicated fluid passage 16 between the first 
solenoid-operated shut-off valve 46 and the fluid passages 18, 24 in which 
the second solenoid-operated shut-off valves 33, 38 are provided. The 
reciprocating pump device 70 has a cylinder housing 72 in which a pump 
piston 74 is fluid-tightly and slidably received. One of opposite end 
faces of the pump piston 74 cooperates with the cylinder housing 72 to 
define a variable-volume chamber 76 to which the pump passage 68 is 
connected. 
The pump piston 74 is biased by a spring 80 disposed in the variable-volume 
chamber 76, so that the other end face of the pump piston 74 is held in 
contact with an outer surface of an eccentric cam 82. This eccentric cam 
82 consists of a rotary shaft 86 and a circular disk fixed to the rotary 
shaft 84 much that the center of the eccentric cam 82 is offset from the 
axis of the shaft 86. The rotary shaft 84 is rotated about its axis by a 
pump drive motor 86, for thereby rotating the eccentric cam 82 about the 
shaft 84 so that the pump piston 74 is reciprocated by the eccentric cam 
82 so as to increase and reduce the volume of the variable-volume chamber 
76, whereby the brake fluid is sucked into and delivered from the chamber 
76. The spring 80, eccentric cam 82 and pump drive motor 86 constitute a 
pump piston drive device 88. 
The pump passage 58 is connected through a pressure relief passage 90 to a 
portion of the fluid passage 16 between the master cylinder 12 and the 
first solenoid-operated shut-off valve 46. In the pressure relief passage 
90, there is provided a pressure relief valve 92 which is opened to permit 
the brake fluid to flow from the pump passage 68 toward the master 
cylinder 12 when the pressure in the pump passage 68 becomes higher than 
the pressure in the master cylinder 12 (hereinafter referred to as "master 
cylinder pressure") by more than a predetermined valve opening pressure 
difference. 
The second hydraulic pressure control actuator for the second braking 
system has the same construction as the first hydraulic pressure control 
actuator which has been described above. In FIG. 1, reference numerals 
used for the components in the first hydraulic pressure control actuator 
are followed by character "A" to identify the corresponding components in 
the second hydraulic pressure control actuator. 
The reciprocating pump device 70A of the second hydraulic pressure control 
actuator for the second braking system uses the same pump piston drive 
device 88 as used by the reciprocating pump device 70 of the first braking 
system. However, reference sign 88A is used in FIG. 1 to denote the pump 
piston drive device of the second hydraulic actuator. The pump piston 74A 
is biased by a spring 80A, so that one of opposite end faces of the pump 
piston 74A is held in contact with the eccentric cam 82, such that the 
points of contact of the two pump pistons 74, 74A are opposed to each 
other diametrically of the eccentric cam 82 or spaced from each other by 
180.degree. in the rotating direction of the eccentric cam 82. 
Accordingly, the suction and delivery of the brake fluid by the pump 
device 70A take place while the delivery and suction by the pump device 70 
take place, respectively. In other words, the pump pistons 70 and 70A have 
opposite operating phases. Namely, the suction and delivery phases of the 
pump device 70A are opposite to those of the pump device 70. 
The rotating speeds of the front left and right wheels 20, 34 and the rear 
left and right wheels 40, 26 are detected by respective wheel speed 
sensors 100, 102, 104, 106, and the output signals of these wheel speed 
sensors are applied to an electronic control unit 110. The electronic 
control unit 110 is constituted principally by a computer incorporating a 
read-only memory (ROM) which stores various control programs including 
speed and acceleration calculating programs, and anti-lock control and 
traction control programs. The speed and acceleration calculating programs 
are used to calculate the rotating speeds and acceleration values of the 
wheels 20, 34, 40, 26 and the running speed of the vehicle, on the basis 
of the output signals of the wheel speed sensors 100-106. The anti-lock 
and traction control programs are used to control the first and second 
solenoid-operated shut-off valves 46, 48, 50, 46A, 48A, 50A and the pump 
drive motor 86 for controlling the fluid pressures in the wheel brake 
cylinders 22, 28, 36, 42 in the anti-lock and traction control modes. To 
execute these control program, the electronic control unit 110 are adapted 
to receive output signals of a brake switch 112 and a current detector 
114. The brake switch 112 detects an operation or depression of the brake 
pedal 10 by the vehicle driver, and the current detector 114 detects a 
drive current applied to the pump drive motor 86. 
There will next be described an operation of the hydraulic braking pressure 
control device according to the present first embodiment. 
When the brake pedal 10 is in a non-operated state, the first 
solenoid-operated shut-off vales 46, 46A and the second solenoid-operated 
shut-off valves 48, 50, 48A, 50A are placed in the states of FIG. 1, and 
the wheel brake cylinders 22, 28, 36, 42 are held in communication with 
the master cylinder 12. When the brake pedal 10 is depressed in this 
condition, the fluid pressure generated in the pressurizing chambers of 
the master cylinder 12 is applied to the wheel brake cylinders 22, 28, 36, 
42 for braking the front left and right wheels 20, 34 and the rear left 
and right wheels 40, 26. 
When the depression force acting on the brake pedal 10 is excessively large 
with respect to the friction coefficient of the road surface on which the 
vehicle is running, the slip of one or more of the wheels exceeds the 
upper limit of an optimum range, and the hydraulic braking pressure 
control device is operated in the anti-lock control mode. In this 
anti-lock control mode, the fluid pressure in the wheel brake cylinder 22, 
28, 36, 42 in question is regulated by a suitable combination of pressure 
decrease, pressure increase and pressure hold operations. The pressure 
decrease operation is effected by a discharge flow of the brake fluid from 
the wheel brake cylinder 22, 28, 36, 42 by a suction stroke of the 
reciprocating pump device 70, 70A. The fluid discharged from the wheel 
brake cylinders is returned to the master cylinder 12 by a delivery stroke 
of the pump device 70, 70A. The pressure increase operation is effected by 
a discharge flow of the fluid from the master cylinder 12 by the suction 
stroke of the pump device 70, 70A and a supply flow of the fluid into the 
wheel brake cylinder 22, 28, 36, 42 by the delivery stroke of the pump 
device 70, 70A. The pressure hold operation is effected by inhibiting both 
of the discharge and supply flows of the fluid from and into the wheel 
brake cylinders 22, 28, 36, 42. 
To increase, decrease and hold the pressure in the wheel brake cylinder in 
question by the suction and delivery of the fluid into and from the 
reciprocating pump device 70, 70A, the suction and delivery strokes of the 
pump device 70, 70A (more precisely, of the pump piston 74, 74A) should be 
suitably timed or synchronized with the opening and closing actions of the 
first and second solenoid-operated shut-off valves 46, 48, 50, 46A, 48A, 
50A so as to effect the pressure increase, decrease and hold operations. 
Described in detail, decreasing the pressure in the wheel brake cylinder 
in question requires: closing of the first solenoid-operated shut-off 
valve 46 and opening of the second solenoid-operated shut-off valve 48, 
50, 48A, 50A during the suction stroke of the pump device 70, 70A; and 
opening of the first solenoid-operated shut-off valve 46 and closing of 
the second solenoid-operated shut-off valve 48, 50, 48A, 50A during the 
delivery stroke of the pump device 70, 70A. On the other hand, increasing 
the wheel brake cylinder pressure requires: opening of the first 
solenoid-operated shut-off valve 46, 46A and closing of the second 
solenoid-operated shut-off valve 48, 50, 48A, 50A during the suction 
stroke of the pump device 70, 70A; and closing of the first shut-off valve 
46, 46A and opening of the second shut-off valve 48, 50, 48A, 50A during 
the delivery stroke of the pump device 70, 70A. To hold the wheel brake 
cylinder pressure requires the first and second shut-off valves 46-50, 
46A-50A to be held closed. As described below, however, the pressure 
increase and hold operations include a period during which the first and 
second shut-off valves 46-50, 46A-50A are both open to permit fluid 
communication between the master cylinder 12 and the wheel brake cylinder 
22, 28, 36, 42 in question. 
If no information is available as to whether the pump device 70, 70A is in 
the suction stroke or delivery stroke upon activation of the pump drive 
motor 86, it is not possible to determine whether each of the first and 
second solenoid-operated shut-off vales 46-50, 46A-50A should be opened or 
closed. In view of this, the electronic control unit 110 is adapted to 
determine whether the pump device 70, 70A is in the suction stroke or 
delivery stroke (suction period or delivery period) before an operation of 
the hydraulic pressure control actuator in the an anti-lock control mode 
is initiated since the same pump drive motor 86 is used for the two pump 
devices 70, 70A of the first and second braking systems, and since the 
suction and delivery phases of the pump device 70 are opposite to those of 
the pump device 70A, the detection of the suction and delivery strokes of 
one of the two pump devices 70, 70A results in automatic detection of the 
suction and delivery strokes of the other pump device. 
The suction and delivery strokes of the pump devices 70, 70A can be 
determined depending upon the direction of change of the volume of the 
variable-volume chambers 76, 76A, namely, depending upon whether the 
volume of the chamber 76, 76A is increasing or decreasing. This 
determination is effected on the basis of a change of the drive current of 
the pump drive motor 86. To this end, the pump drive motor 86 is turned on 
when an anti-lock control operation of the hydraulic pressure control 
actuator is expected to be initiated soon. In this condition where the 
anti-lock operation is about to be initiated, the brake pedal 10 has 
already been depressed, and the brake fluid pressure in the master 
cylinder 12 (wheel brake cylinder 22, 28, 36, 42) acts on the pump piston 
74, 74A. Accordingly, the drive current of the pump drive motor 86 is 
larger in the delivery stroke due to a load acting on the pump piston 74, 
74A, than in the suction stroke. It is also noted that the delivery 
strokes (and the suction strokes) of the two reciprocating pump devices 
70, 70A take place alternately as indicated in the graph of FIG. 2, and 
therefore an increase in the drive current in the delivery stroke of the 
pump device 70 and an increase in the drive current of the pump device 70A 
occur alternately, as also indicated in FIG. 2. Consequently, the change 
in the drive current indicates that one of the two pump devices 70, 70A is 
in the delivery stroke, but does not indicate which one of the two pump 
devices 70, 70A is in the delivery stroke. 
To determine which one of the two pump devices 70, 70A is in the delivery 
stroke, the following determination is effected with the first and second 
solenoid-operated shut-off valves of one of the two braking systems being 
closed. Initially, the electronic control unit 110 calculates the rate of 
change of the drive current of the pump drive motor 86 on the basis of the 
output signal of the current detector 114 during a time period from the 
moment of activation of the pump drive motor 86 and the moment at which 
the fist and second solenoid-operated shut-off valves are closed as 
indicated above. Then, the electronic control unit 110 activates a 
suitable time counter to measure time intervals between adjacent 
zero-crossing points of the waveform of the calculated rate of change of 
the drive current as indicated in the third and sixth rows of FIG. 3. The 
zero-crossing points whose time intervals are to be measured may consist 
of only points negative-to-positive zero-crossing points at which the 
drive current change rate changes from a negative value to a positive 
value, or both the negative-to-positive zero-crossing points and 
positive-to-negative zero-crossing points at which the drive current 
change rate changes from a positive value to a negative value. 
On the basis of the measured time intervals, the electronic control unit 
110 estimates two halves of the operating period (cycle time) of the pump 
devices 70, 70A after the first and second solenoid-operated shut-off 
valves of one of the two braking systems are closed. The operating period 
or cycle time of the pump devices consists of the suction stroke time and 
the delivery stroke time, as indicated in FIG. 2. Then, these shut-off 
valves are closed at the first negative-to-positive zero-crossing point of 
the calculated drive current change rate (indicated in the third and sixth 
rows of FIG. 3) after the activation of the pump drive motor 86. At this 
zero-crossing point, one of the two pump devices 70, 70A initiates the 
suction stroke while the other pump device initiates the delivery stroke. 
The shut-off valves are held closed for a time equal to the estimated 
operating period or cycle time of the pump devices. The electronic control 
unit 110 determines which one of the two pump devices 70, 70A is in the 
delivery stroke (or suction stroke), depending upon whether the drive 
current change rate of the pump motor 86 in the latter half of the 
estimated operating period is higher than a predetermined threshold or 
not. 
If the first and second solenoid-operated shut-off valves 46-50 of the 
first braking system are closed at a point of time corresponding to the 
initiation of the suction stroke of the pump device 70 of the first 
braking system, as indicated in the first row of FIG. 3, the pump piston 
74 cannot actually initiate the suction stroke and remains at the delivery 
stroke end (at which the suction stroke begins). Consequently, only the 
other pump device 70A performs the suction and delivery strokes. Namely, 
the pump device 70 does not perform the delivery stroke in the latter half 
of the period during which the first and second shut-off valves 46-50 are 
held closed. As a result, the drive current in this latter half will not 
considerable increase as indicated in the second row of FIG. 3, and the 
drive current change rate is not higher than the predetermined threshold 
as indicated in the third row of FIG. 3. The latter half corresponds to 
the delivery stroke of the pump device 70. 
If the first and second solenoid-operated shut-off valves 46-50 of the 
first braking system are closed at a point of time corresponding to the 
initiation of the suction stroke of the pump device 70A of the second 
braking system, as indicated in the fourth row of FIG. 3, the pump piston 
74 delivers the fluid to the master cylinder 12 through the pressure 
relief valve 92 in the latter half of the period during which the first 
and second shut-off valves 46-50 are held closed, since the fluid has been 
sucked into the variable-volume chamber 76 before the shut-off valves 
46-50 are closed. Then, the pump piston 74 is stopped at the delivery 
stroke end. In this case, the other pump device 70A also performs the 
suction and delivery strokes. Namely, the pump device 70A delivers the 
fluid in the latter half of the period during which the shut-off valves 
46-50 are held closed, whereby the drive current of the pump motor 86 is 
comparatively larger than in the case described in the preceding 
paragraph, as indicated in the fifth row of FIG. 3, and the drive current 
change rate is higher than the threshold, as indicated in the sixth row of 
FIG. 3. 
Thus, the drive current change rate in the latter half of the period during 
which the first and second solenoid-operated shut-off valves 46-50 are 
held closed differs depending upon whether the shut-off valves 46-50 were 
closed at a point of time corresponding to the initiation of the suction 
stroke of the pump device 70 of the first braking device or at a point of 
time corresponding to the initiation of the suction stroke of the pump 
device 70A of the second braking device. If the drive current change rate 
is not higher than the threshold, it indicates that the shut-off vales 
46-50 were closed at the point of time corresponding to the initiation of 
the suction stroke of the pump device 70 of the first braking system. It 
the drive current change rate is higher than the threshold, it indicates 
that the shut-off valves 46-50 were closed at the point of time 
corresponding to the initiation of the suction stroke of the pump device 
70A of the second braking device. Accordingly, the determination as to 
whether the drive current change rate is higher than the predetermined 
threshold permits the detection of the directions of change of the volumes 
of the variable-volume chambers 76, 76A, which indicate whether each of 
the two pump devices 70, 70A is now in the suction stroke or delivery 
stroke. The thus detected directions of volume change of the 
variable-volume chambers 76, 76A are stored in a pump state memory of the 
computer of the electronic control unit 110. The stored volume change 
directions (indicative of the suction and delivery strokes) are 
subsequently updated at each of the subsequent negative-to-positive 
zero-crossing points of the drive current change rate, by inverting the 
volume change directions (suction and delivery strokes) at each 
negative-to-positive zero-crossing point. Thus, the electronic control 
unit 110 has the information indicative of whether each of the two 
reciprocating pump devices 70, 70A is currently in the suction or delivery 
stroke. 
It is noted that the point of time at which the first and second 
solenoid-operated shut-off valves of one of the two braking systems are 
closed need not correspond to the initiation of the suction stroke of the 
pump device 70, 70A. This point of time may be suitably selected. IN this 
case, however, the determination as to whether the drive current change 
rate is higher than the threshold should be made for successive two 
delivery strokes of the pump device 70, 70A after the closure of the first 
and second shut-off valves 46-50. 
If, for example, the first and second shut-off valves 46-50 are closed 
during the suction stroke of the pump device 70, the pump piston 74 
delivers in the following delivery stroke the fluid which was sucked in by 
the time when the shut-off valves 46-50 were closed. In this case, the 
drive current change rate may exceed the predetermined threshold. If the 
drive current change rate does not exceed the threshold during the 
above-indicated delivery stroke, it is possible to determine that the pump 
device 70 was in the suction stroke. If the change rate exceeds the 
threshold, however, it is not possible to do so, and therefore another 
determination as to whether the drive current change rate exceeds the 
threshold or not should be effected during the next delivery stroke which 
begins one period after the end of the suction stroke during which the 
shut-off valves 46-50 were closed. 
The determination as to whether each of the reciprocating pump device 70, 
70A is in the suction or delivery stroke, that is, the detection of the 
directions of volume change of the variable-volume chambers 76, 76A is 
effected when an anti-lock control operation of at least one of the two 
hydraulic pressure control actuators of the first and second braking 
systems is about to be initiated. The anti-lock control operation is 
initiated for a certain wheel of the vehicle when the rotating speed of 
this wheel becomes lower than the vehicle running speed by more than a 
predetermined first threshold difference value. The above determination is 
initiated when the rotating speed of the wheel in question becomes lower 
than the vehicle speed by more than a predetermined second threshold 
difference value which is smaller than the first threshold difference 
value. To effect the determination, the first and second solenoid-operated 
shut-off valves 46-50, 46A-50A of one of the two braking systems are 
closed. This braking system whose first and second shut-off valves are 
closed is the braking system including the wheel brake cylinder which 
corresponds to the wheel whose amount of slip on the road surface 
increases at the highest rate and for which the anti-lock control 
operation is expected to be initiated first, that is, before the anti-lock 
control operation is initiated for the other wheel brake cylinders. The 
closure of the first and second solenoid-operated shut-off valves results 
in maintaining the pressure in the wheel brake cylinder in question, and 
an increase in the slip amount of the corresponding wheel is prevented or 
restricted. 
If the condition for initiating the anti-lock control operation is 
satisfied, that is, if the difference of the rotating speed of the wheel 
in question from the vehicle speed is larger than the predetermined first 
threshold difference value when the determination of the suction or 
delivery stroke of the corresponding pump device 70, 70A is terminated, 
the anti-lock control operation is initiated for the wheel in question. If 
the front left wheel 20 or the rear right wheel 26 has a rapidly 
increasing slipping tendency and the first and second solenoid-operated 
shut-off valves 46-50 of the first braking system were closed during the 
above determination, the shut-off valves 46-50 are suitably opened and 
closed in timed or synchronized relationship with the suction and delivery 
strokes of the pump device 70 in the anti-lock control operation, so as to 
initially reduce the pressure in the wheel brake cylinder 22, 28 
corresponding to one of the front left and rear right wheels 20, 26 whose 
slipping tendency increases at a higher rate than the other, that is, so 
as to discharge the fluid from that wheel brake cylinder 22, 28. 
If the condition for initiating the anti-lock control operation for the 
wheel brake cylinder 22, 28 is not satisfied at the end of the 
determination of the suction or delivery stroke of the pump device 70, the 
first and second solenoid-operated shut-off valves 46-50 are opened while 
the pump drive motor 86 is kept on. Then, the electronic control unit 110 
continues to monitor whether the condition for initiating the anti-lock 
control operation is satisfied or not. If this condition is satisfied 
within a predetermined time, the anti-lock control operation is initiated. 
If not, the pump drive motor 86 is turned off. 
The wheel cylinder pressure is reduced for the first time after the 
initiation of the anti-lock control operation, the rate of decrease of the 
wheel cylinder pressure is comparatively high. If, for example, the 
slipping tendency of the front left wheel 20 increases at a higher rate 
than that of the rear right wheel 26 and the anti-lock control operation 
is initiated for the front left wheel 20 before initiation of the 
anti-lock control operation for the rear right wheel 26, the first 
solenoid-operated shut-off valve 46 is closed while the second 
solenoid-operated shut-off valve 50 is opened so that the pressurized 
fluid is discharged from the front left wheel brake cylinder 22 into the 
variable-volume chamber 76 of the pump device 70, whereby the pressure 
downstream of the first shut-off valve 46 (which corresponds to the 
pressure in the wheel brake cylinder 22) is made lower than the master 
cylinder pressure. As a result, the pilot-operated shut-off valve 60 is 
opened to permit the pressurized fluid to be discharged from the wheel 
brake cylinder 22 into the reservoir 54. Thus, the pressure in the wheel 
brake cylinder 22 is rapidly reduced. 
While the pressure in the front left wheel brake cylinder 22 is rapidly 
reduced, the second shut-off valve 50 for the rear right wheel 26 is held 
closed, to inhibit a discharge flow of the fluid from the rear right wheel 
brake cylinder 28. If the anti-lock control operation is initiated for the 
rear right wheel 26 during or immediately after the rapid reduction of the 
pressure in the front left wheel brake cylinder 22, the second shut-off 
valve 50 is opened, and the fluid is discharged from the rear right wheel 
brake cylinder 28 into the reservoir 54, whereby the pressure in the wheel 
brake cylinder 28 is rapidly reduced. If the second pressure reduction or 
decrease is initiated for the front left wheel 20 before the anti-lock 
control operation is initiated for the rear right wheel 26, this second 
pressure reduction is also effected at a comparatively high rate. 
When the reservoir 54 is filled with the brake fluid in any operating 
condition of the braking apparatus, the fluid cannot flow into the 
reservoir 54 even with the pilot-operated shut-off valve 60 being open. In 
this state, the rapid pressure reduction of the wheel brake cylinders 20, 
28 is not possible. In this respect, it is noted that the pressure of the 
fluid in the reservoir 54 is only slightly higher than the atmospheric 
pressure and is lower than the pressure in the fluid passages 16, 18, 24, 
the fluid will not be discharged from the reservoir 54 during the 
anti-lock control operation. The pilot-operated shut-off valve 60 is 
closed when the pressure in the wheel brake cylinder 22, 28 becomes higher 
than the master cylinder pressure after the termination of the anti-lock 
control operation, so that the fluid is returned from the reservoir 54 to 
the master cylinder 12 through the check valve 56. 
To effect the pressure decrease, pressure hold and pressure increase 
operations for the wheel brake cylinder 22, 28, 36, 42, the first and 
second solenoid-operated shut-off valves 46-50, 46A-50A are opened and 
closed in synchronization with the suction and delivery strokes of the 
reciprocating pump device 70, 70A, as indicated in FIGS. 5A, 5B and 5C, 
respectively. 
To effect the pressure decrease operation, the first and second 
solenoid-operated shut-off valve are closed and opened, respectively, 
during the suction stroke of the pump device, and are opened and closed, 
respectively, during the delivery stroke of the pump device, as indicated 
in FIG. 5A. 
To effect the pressure hold operation, the first and second shut-off valves 
are closed for a substantive portion of the anti-lock control operation, 
as indicated in FIG. 5B, but are opened for a relatively short time 
corresponding to a portion of the delivery stroke of the pump device, as 
indicated by hatched areas in FIG. 5B, to temporarily establish the fluid 
communication between the wheel brake cylinder and the master cylinder 12. 
To effect the pressure increase operation, the first shut-off valve is 
closed for a substantive portion of the anti-lock control operation while 
the second shut-off valve is held open, as indicated FIG. 5C. The first 
shut-off valve is opened for relatively short times corresponding to the 
initial and terminal portions of the delivery stroke of the pump device, 
as indicated by hatched areas in FIG. 5C, to temporarily the fluid 
communication between the wheel brake cylinder and the master cylinder 12. 
When the fluid communication between the master cylinder 12 and the wheel 
brake cylinder is temporarily established during the pressure hold and 
increase operations, the fluid flows from the master cylinder 12 into the 
wheel brake cylinder so as to increase the wheel brake cylinder pressure 
since the master cylinder pressure is generally higher than the wheel 
brake cylinder pressure. If the master cylinder pressure is lowered below 
the wheel brake cylinder pressure as a result of an operation of the brake 
pedal 10 toward the non-operated position, the fluid is discharged from 
the wheel brake cylinder and the wheel brake cylinder pressure is reduced. 
Consequently, the amount of increase of the wheel brake cylinder pressure 
during the pressure increase operation is reduced so that the wheel brake 
cylinder pressure tends to be reduced by the overall anti-lock control 
operation. If the brake pedal 10 is operated toward the non-operated 
position during the pressure hold operation, the wheel brake cylinder 
pressure is reduced during the pressure hold operation, and the wheel 
brake cylinder pressure tends to be reduced by the overall anti-lock 
control operation. 
The electronic control unit 110 selects one of the pressure increase, 
decrease and hold operations, depending upon the slipping states of the 
vehicle wheels such as the slip amounts, slip ratios, and acceleration 
values of the wheels, and controls the first and second solenoid-operated 
shut-off valves 46-50, 46A-50A to be opened and closed according to the 
selected pressure pressure control operation. The rate of the pressure 
increase or decrease of the wheel in question can be changed as needed by 
controlling the operating speed of the pump drive motor 86. The electronic 
control unit 110 determines the optimum pressure increase and decrease 
rates and controls the drive current to be applied to the pump drive motor 
86 so as to establish the determined optimum pressure increase and 
decrease rates. The pressure increase and decrease rates correspond to the 
operating period or cycle time (suction stroke time plus delivery stroke 
time) of the pump device 70, 70A, and the operating period (opening and 
closing frequency) of the first and second shut-off valves 46-50, 46A-50A 
changes with the operating period of the pump device 70, 70A. 
As described above, the present embodiment is adapted such that the first 
and second solenoid-operated shut-off valves 46-50, 46A-50A are both 
opened for the predetermined times during the pressure increase and hold 
operations, so that the fluid pressurized by the master cylinder 12 is 
usually supplied to the wheel brake cylinder 22, 28, 36, 42 with a result 
of an increase in the wheel brake cylinder pressure. This fact is taken 
into consideration in determining the rates at which the wheel brake 
cylinder pressure is increased or reduced in the pressure increase and 
decrease operations. 
While the first solenoid-operated shut-off valve 46, 46A of each braking 
system is used commonly for the front left and right right wheels 20, 25 
or the rear left and front right wheels 40, 34, the two second 
solenoid-operated shut-off valves 48, 50, 48A, 50A are provided for the 
respective left and right wheels 20, 40 and 40, 34. By opening and closing 
these two second shut-off valves 48, 50, 48A, 50A independently of each 
other, the pressures in the left and right wheel brake cylinders 22, 28 
and 42, 36 can be regulated independently of each other. FIG. 7 indicates 
possible combinations of the pressure control operations (pressure 
increase, decrease and hold) of the left and right wheel brake cylinders 
22 and 28, 42, 36. 
As indicated in FIGS. 5A, 5B and 5C, the opening and closing pattern of the 
first solenoid-operated shut-off valve differs depending upon the selected 
one of the pressure increase, pressure decrease and pressure hold 
operations. Where the pressure hold operation is selected for one of the 
left and right wheel brake cylinders of the braking system in question 
while the pressure increase or decrease operation is selected for the 
other wheel brake cylinder, the opening and closing pattern of the first 
shut-off valve 46, 46A for the left wheel brake cylinder is different from 
that for the right wheel brake cylinder. In this case, the first shut-off 
valve is opened and closed increase or decrease the wheel brake cylinder 
pressure. If the first shut-off valve is opened and closed to hold the 
wheel brake cylinder pressure, the wheel brake cylinder pressure cannot be 
increased or decreased. In the present embodiment, the first shut-off 
valve is opened and closed to increase or decrease the wheel brake 
cylinder pressure, since the wheel brake cylinder pressure can be held by 
the second shut-off valve. 
The determination of the suction and delivery strokes of the pump devices 
70, 70A or the detection of the volume change directions of the 
variable-volume chambers 76, 76A is effected at a predetermined time 
interval even after the initiation of an anti-lock control operation. As 
described above, the volume change directions (suction and delivery 
strokes) stored in the pump stage memory are inverted each time the 
negative-to-positive zero-crossing point of the drive current change rate 
of the pump drive motor 86 is detected. If this negative-to-positive 
zero-crossing point is detected due to noise, therefore, the volume change 
directions are inverted irrespective of the actual operating state of the 
pump device 70, 70A, namely, regardless of whether the pump device is 
actually in the suction stroke or delivery stroke. This inversion due to 
noise leads to inadequate anti-lock control operation of the braking 
system. 
It is noted that an inadequate anti-look control operation of the braking 
system results in an excessively long pressure decrease or increase 
operation. In view of this fact, the inadequate anti-lock control 
operation due to inaccurate information stored in the pump state memory 
can be detected by detecting the excessively long pressure decrease or 
increase operation by providing a time counter for measuring the duration 
of the pressure decrease or increase operation, and detecting means for 
detecting the inadequate anti-lock control operation on the basis of the 
measured pressure decrease or increase operation as compared with a 
threshold time. The detection of the volume change directions of the 
variable-volume chamber 76, 76A may be effected when the inadequate 
anti-lock control operation is detected by the detecting means. 
There will be described the traction control operation of the present 
braking apparatus. 
When the slip ratio of at least one of the left and right front drive 
wheels 20, 34 exceeds a predetermined limit during starting or 
acceleration of the vehicle, the pump drive motor 86 is activated, and the 
first and second solenoid-operated shut-off valves 46-50, 46A-50A are 
suitably opened and closed to supply the pressurized brake fluid from the 
pump device 70, 70A to the wheel brake cylinder or cylinders 22, 36 for 
braking the excessively slipping wheel or wheels 20, 34. 
In the traction control operation, too, the volume change directions of the 
variable-volume chambers 76, 76A of the reciprocating pump device 70, 70A 
are detected on the basis of the rate of change of the drive current of 
the pump drive motor 86, in the manner as described above with respect to 
the anti-lock control operation. To this end, the pump drive motor 86 is 
activated when the condition for initiating the traction control operation 
is about to be satisfied. The traction control operation is initiated when 
the rotating speed of the front drive wheel 20, 34 in question becomes 
higher than the vehicle speed by more than a predetermined first threshold 
difference value, and the pump drive motor 86 is activated when the 
rotating speed of the front drive wheel 20, 34 becomes higher than the 
vehicle speed by more than a predetermined second threshold difference 
value which is smaller than the first threshold difference value. Then, 
the first and second solenoid-operated shut-off valves 46-50, 46A-50A of 
one of the first and second braking systems are closed at a point of time 
corresponding to the initiation of the delivery stroke of the pump device 
70, 70A, namely, at the first negative-to-positive zero-crossing point of 
the drive current change rate of the pump motor 86, as indicated in the 
third and sixth rows of FIG. 4. The first and second shut-off valves are 
held closed during a half of the operating period or cycle time of the 
pump device 70, 70A. 
In the traction control operation, the master cylinder pressure is zero, 
and the load acting on the pump device 70, 70A during the delivery stroke 
of the pump device 70, 70A is relatively small in the braking system in 
which the first and second shut-off valves 46-50, 45A-50A are both open. 
In this braking system, the drive current of the pump drive motor 86 is 
relatively small. If the first and second shut-off valves are closed in 
the braking system in which the delivery stroke of the pump device is 
initiated upon closure of the first and second shut-off valves, the brake 
fluid sucked into the variable-volume chamber 76, 76A of that braking 
system before the initiation of the delivery stroke (before the closure of 
the first and second shut-off valves) is delivered through the pressure 
relief valve 92 during the delivery stroke, and the load acting on the 
pump device 70, 70A is relatively large, resulting in an increase in the 
drive current of the pump drive motor 86, whereby the drive current change 
rate exceeds the predetermined threshold. If the first and second shut-oil 
valves of one of the two braking systems are closed upon initiation of the 
delivery stroke of the pump device of the other braking system, the load 
acting on the pump device 70, 70A is relatively small, and the drive 
current of the pump drive motor 86 is relatively small, so that the drive 
current change rate will not exceed the predetermined threshold. 
Described more specifically referring to FIG. 4, if the first and second 
solenoid-operated shut-off valves 46-50 of the first braking system are 
closed at a point of time corresponding to the initiation of the delivery 
stroke of the pump motor 70 of this first braking system, as indicated in 
the first row of FIG. 4, the brake fluid which was sucked into the 
variable-volume chamber 76 during the preceding suction stroke is 
delivered therefrom through the pressure relief valve 92, so that the 
drive current is relatively large as indicated in the second row in FIG. 
4, whereby the drive current change rate exceeds the predetermined 
threshold as indicated in the third row. 
If the first and second shut-off valves 46-50 of the first braking system 
are closed at a point of time corresponding to the initiation of the 
delivery stroke of the pump motor 70A of the second braking system as 
indicated in the fourth row in FIG. 4, the pump device 70 of the first 
braking system is at the beginning of the suction stroke when the shut-off 
valves 46-50 are closed. Accordingly, the load acting on the pump device 
70 is relatively small. Further, although the pump device 70A of the 
second braking system is at the beginning of the delivery stroke, the load 
acting on this pump device 70A is relatively small since the first and 
second shut-off valves 46A-50A re open. consequently, the drive current is 
relatively small as indicated in the fifth row of FIG. 4, and the drive 
current change rate will not exceed the predetermined threshold as 
indicated in the sixth row of the figure. 
Therefore, it is possible to determine that the pump device of the braking 
system in which the first and second shut-off valves were closed was at 
the beginning of the delivery stroke upon closure of the first and second 
shut-off valves, if the drive current change rate exceeds the 
predetermined threshold during the closure of the first and second 
shut-off valves, and that the pump device of the braking system in which 
the first and second shut-off valve were not closed was at the beginning 
of the delivery stroke upon closure of the first and second shut-off 
vales, if the drive current chance rate does not exceed the threshold. 
The thus detected volume change directions of the variable-volume chambers 
76, 76A (suction and delivery strokes of the pump devices 70, 70A) are 
stored in the pump state memory, and the stored volume change directions 
are inverted upon detection of each negative-to-positive zero-crossing 
point of the detected drive current change rate. 
If the condition for initiating the traction control operation is satisfied 
when the detection of the volume change directions of the variable-volume 
chambers 76, 76A is terminated, the first solenoid-operated shut-off valve 
46, 46A and the second solenoid-operated shut-off valve 48, 48A are 
suitably opened and closed according to the currently stored volume change 
directions, so as to increase, decrease or hold the wheel brake cylinder 
pressure in question, while the second solenoid-operated shut-off valves 
50, 50A corresponding to the non-drive rear left and right wheels 40, 26 
are held closed to inhibit brake application to the rear left and right 
wheels 40, 26. 
If the condition for initiating the traction control operation for the 
front wheel brake cylinder 22, 36 is not satisfied at the end of the 
detection of the volume change directions, the electronic control unit 110 
continues to monitor whether the condition for initiating the traction is 
satisfied or not, while the pump drive motor 86 is kept operated. If this 
condition is satisfied within a predetermined time, the traction control 
operation is initiated. If not, the pump drive motor 86 is turned off. 
In the traction control operation, the pressure increase is effected by 
opening and closing the first and second shut-off valves, as indicated in 
FIG. 6A. To effect the pressure hold, the first and second shut-off valves 
are both closed, as indicated in FIG. 6B. The pressure decrease is 
effected as indicated in FIG. 6C, in the same manner as in the anti-lock 
control operation as indicated in FIG. 5C. As in the anti-lock control 
operation, the rates of the pressure increase and decrease can be changed 
in the traction control operation, by controlling the operating speed of 
the pump drive motor 86. 
The detection of the volume change directions of the variable-volume 
chambers 76, 76A (determination of the suction and delivery stokes) of the 
pump devices 70, 70A is effected at a predetermined time interval even 
after the traction control operation is initiated, or when a predetermined 
condition is satisfied, for example, when an inadequate traction control 
operation is detected. This arrangement prevents erroneous volume change 
directions of the variable-volume chambers 76, 76A stored in the pump 
stage memory, which would occur due to noise generation and which do not 
reflect the actual operating states (suction and delivery strokes) of the 
pump devices 70, 70A. 
If a predetermined condition for terminating the traction control operation 
is satisfied, the first and second shut-off valves 45, 48, 46A, 48A, 50, 
50A are both opened, and the pump drive motor 86 is turned off. The 
predetermined condition indicated above may be an increase of the vehicle 
running speed above a predetermined limit. 
In the present hydraulically operated braking apparatus constructed as 
described above, the wheel cylinder pressures are regulated in the 
anti-lock or traction control mode by the reciprocating pump devices 70, 
70A, which are adapted to supply and discharge the pressurized brake fluid 
to and from the wheel brake cylinders, as needed. Further, the operating 
speed of the pump drive motor 86 can be controlled as desired, so that the 
rates of increase and decrease of the wheel brake cylinders can be 
suitably controlled, irrespective of the wheel cylinder pressures. 
In the pump piston drive device 88, cam followers in the form of the pump 
pistons 74, 74A are forced against the cam surface of the eccentric cam 82 
under the biasing force of biasing means in the form of the springs 80, 
80A, so that the cam followers 74, 74A can remain at the positions 
corresponding to the deliver stroke ends even when the pump devices are in 
the suction strokes. In the first or second braking system in which the 
first and second solenoid-operated shut-off valves 46-50, 46A-50A are 
closed to detect the volume change directions of the variable-volume 
chambers 76, 76A (determine the suction and delivery strokes), the cam 
follower 74, 74A remains at the delivery stroke end in the absence of a 
flow of the brake fluid into the variable-volume chamber 76, 76A due to 
the closure of the shut-off valves. 
If the first and second shut-off valves are closed in the braking system in 
which the pump device is about to initiate the suction stroke, this pump 
device cannot suck in the brake fluid and delivery the brake fluid, 
although the pump drive motor 86 is kept operated. To avoid damages of the 
pump device and the pump drive motor 86, therefore, the pump piston 74, 
74A should not be moved in the suction direction by the pump drive motor 
86. This object is achieved in the present embodiment wherein the pump 
pistons 74, 74A are normally held in contact with the eccentric cam 82 by 
the springs 80, 80A, but are not forcedly moved by the pump drive motor 86 
in the suction direction. 
It will be understood from the foregoing explanation of the present 
embodiment that the electrically operated pump drive motor 86 serves as a 
drive source, while the eccentric cam 82 and the spring 80, 80A constitute 
a motion converting mechanism for converting the rotary motion of the 
drive source into the linear reciprocating motion of the pump piston 74, 
74A. The drive source and the motion converting mechanism cooperate with 
each other to constitute the pump piston drive device 88. It will also be 
understood that the electronic control unit 110 includes shut-off valve 
closing means for closing the first and second solenoid-operated shut-off 
valves 46-50, 46A-50A in the anti-lock control operation, shut-off valve 
closing means for closing these shut-off valves in the traction control 
operation, volume change detecting means for detecting directions of 
change of the volume of the variable-volume chamber 76, 76A in the 
anti-lock control operation, and volume change detecting means for 
detecting the above-indicated volume change directions in the traction 
control operation. 
It will further be understood that the first solenoid-operated shut-off 
valve 46, 46A serves as means for restricting flows of the brake fluid 
therethrough. 
It will further be understood that the spring 80, 80A and eccentric cam 82 
serve as a device for permitting the reciprocating pump device to remain 
at its delivery stroke end without its movement to initiate its suction 
stroke when the first and second solenoid-operated shut-off valves are 
closed for the purpose of detecting the volume change directions of the 
variable-volume chamber 76, 76A in the anti-lock control operation of the 
braking apparatus. Further, the pressure relief valve 92, 92A serve as a 
device for permitting the delivery of the brake fluid under a flow 
resistance not smaller than a predetermined value, from the 
variable-volume chamber 76, 76A of the pump device 70, 70A of the braking 
system in which the first and second shut-off valves are closed to detect 
the volume change directions of the variable-volume chamber 76, 76A. 
While the above embodiment is adapted to detect the volume change 
directions of the variable-volume chambers 76, 76A at a predetermined time 
interval or when needed even after the termination of the anti-lock or 
traction control operation, such subsequent detection of the volume change 
directions is not essential. In this case, the detection is effected only 
once shortly before the initiation of the anti-lock or traction control 
operation. 
In the above embodiment, the detection of the volume change directions of 
the variable-volume chambers 76, 76A is effected by first closing the 
first and second solenoid-operated shut-off valves at the first 
negative-to-positive zero-crossing point of the change rate of the drive 
current of the pump drive motor 86. The detection is possible without 
detecting the negative-to-positive zero-crossing of the drive current 
change rate. In other words, the detection is possible regardless of the 
moment at which the first and second shut-off valves are closed. 
If the first and second solenoid-operated shut-off valves of one of the 
first and second braking systems are closed during the suction stroke of 
the pump device of that braking system to detect the volume change 
directions of the variable-volume chambers, a certain amount of the brake 
fluid which has been sucked into the variable-volume chamber of the 
braking system before the closure of the shut-off valves is delivered in 
the following delivery stroke. In this case, the drive current in the 
delivery stroke is intermediate, and the drive current change rate may 
exceed the predetermined threshold value, resulting in erroneous detection 
of the volume change directions of the variable-volume chambers (erroneous 
determination of the suction or delivery stroke of the pump devices). 
After the delivery of the brake fluid from the variable-volume chamber of 
the braking system in which the first and second shut-off valves were 
closed, the brake fluid is not sucked into that variable-volume chamber in 
the next suction stroke and is not delivered in the following delivery 
stroke, so that the drive current will not increase to such an extent that 
causes the drive current change rate to exceed the threshold value. This 
time, the volume change directions can be correctly detected. This is also 
true in the case where the first and second shut-off valves are closed 
upon termination of the suction stroke of the pump device in question, 
because the drive current change rate will necessarily exceed the 
predetermined threshold in the delivery stroke following the next suction 
stroke. 
In the traction control operation, the volume change directions of the 
variable-volume chambers can be detected in the case where the first and 
second solenoid-operated shut-off valves of one of the two braking systems 
are closed upon initiation of the delivery stroke of the pump device of 
that braking system or within a predetermined time period shortly before 
or after that initiation. If the first and second shut-off valves are 
closed within the predetermined time period before the initiation of the 
delivery stroke, namely, during the latter half of the preceding suction 
stroke, a considerable amount of the brake fluid is sucked into the 
variable-volume chamber 76, 76A of the braking system in question, and the 
fluid remains in the chamber in a considerable amount within a 
predetermined time period before the initiation of the delivery stroke, 
namely, during the former half of the delivery stroke. When the fluid is 
delivery from the chamber while the first and second shut-off valves are 
closed, a considerable load acts on the pump drive motor 86, resulting in 
the drive current change rate exceeding the predetermined threshold value. 
If the shut-off valves are closed at a moment other than the moment of 
initiation of the delivery stroke and the predetermined time period 
shortly before or after the initiation, the load which acts on the pump 
drive motor 86 during the delivery stroke will not increase to such an 
extent that causes the drive current change rate to exceed the threshold 
value, resulting in a failure to correctly detect the volume change 
directions of the variable-volume chambers. 
In the above embodiment, the detection of the volume change directions of 
the variable-volume chambers depends upon whether the drive current change 
rate exceeds the predetermined threshold value in the latter half of the 
operating period of the pump devices during which the first and second 
solenoid-operated shut-off valves are held closed in one of the two 
braking systems. The present braking apparatus may be provided with 
confirming means for confirming the correctness of the detection that the 
drive current change rate does not exceed the threshold value in the 
above-indicated latter half. The conforming means is operated in the next 
half of the operating period of the pump devices, that is, in the delivery 
stroke of the pump device of the other braking system in which the first 
and second shut-off valves were not closed. The confirming means 
determines whether the drive current change rate exceeds the threshold 
value in the delivery stroke of the pump device of the above-indicated 
other braking system. If an affirmative decision is obtained by this 
confirming means, it indicates that the above-indicated other braking 
system is now in the delivery stroke, and that the above-indicated one 
braking system is now in the suction stroke and was in the delivery stroke 
when the drive current change rate not exceeding the threshold value was 
detected in the last operating cycle. If a negative decision is obtained 
by the confirming means, it means that the drive current change rate 
exceeds the threshold value neither in the latter half of the operating 
period of the pump device of one of the two braking systems in which the 
first and second shut-off valves are closed, nor in the following half of 
the operating period of the pump device of the other braking system. In 
this case, there is a high possibility of some abnormality of the braking 
apparatus. 
The braking apparatus according to the above embodiment may be provided 
with confirming means which is operated in the traction control operation 
with the first and second solenoid-operated shut-off valves of one of the 
two braking systems being held closed during the operating period or cycle 
time of the pump devices. If the drive current change rate exceeds the 
threshold in the former half of this period and does not exceed the 
threshold in the latter half, the confirming means determines that the 
pump device of the above-indicated one braking system was in the delivery 
stroke in the former half of the period while the pump device of the other 
braking system was in the delivery stroke in the latter half. If the drive 
current change rate exceeds in none of the former and latter halves, or 
exceeds in both of these two halves, there is a high possibility of some 
abnormality of the braking apparatus. 
Suitable means may be provided for dealing with the above-indicated case of 
high possibility of some abnormality of the braking apparatus. This means 
may be operated in at least one of the anti-lock and traction control 
operations, for repeating the detection of the volume change directions of 
the variable-volume chambers 76, 76A (determination of the suction or 
delivery stroke each pump device 70, 70A), or providing an indication that 
there exists a high possibility of abnormality of the braking system. 
Although the detection of the volume change directions of the 
variable-volume chambers is effected on the basis of the drive current of 
the pump drive motor 86 in the above embodiment, the detection may be 
effected on the basis of the directions of movement of the pump pistons 
74, 74A, which may be detected on the basis of an output signal of a 
position detecting device for detecting the position of the pump pistons 
74, 74A or the position of a suitable member which operates with the pump 
pistons such as the eccentric cam 82 or a component of the pump drive 
motor 86. The position detecting device may be a device for continuously 
detecting the position of the pump pistons 74, 74A, a device for detecting 
at least one of the delivery and suction stroke ends (fully advanced and 
retracted positions) of the pump pistons, or a device for detecting the 
zero or home position of the eccentric cam 82 or pump drive motor 86. 
While the above embodiment is adapted to adjust the rates of increase and 
decrease of the wheel brake cylinder pressure by controlling the operating 
speed of the pump drive motor 86, the pressure increase and decrease rates 
may be adjusted otherwise. For instance, the pressure increase rate may be 
adjusted by controlling the duty ratio of the first and second 
solenoid-operated shut-off valves 46-50, 46A-50A to open and close these 
shut-off valves when the pressurized fluid is sucked from the master 
cylinder 12 into the pump device 70, 70A and/or when the pressurized fluid 
is delivered from the pump device into the wheel brake cylinder 22, 28, 
36, 42. Similarly, the pressure decrease rate may be adjusted by 
controlling the duty ratio of the shut-off valves when the fluid is sucked 
from the wheel brake cylinder into the pump device and/or when the fluid 
is delivered from the pump device into the master cylinder 12. 
Although the pump piston drive device 88 provided in the above embodiment 
includes the eccentric cam 82 and the springs 80, 80A, the pump piston 
drive device may have other arrangements. Fur instance, the pump piston 
drive device may use a cam other than an eccentric cam, such as a positive 
motion cam. Alternatively, the pump piston drive device may include a 
connecting device which includes a crank shaft and a connecting rod and 
which mechanically connects the pump drive motor 86 and the pump pistons 
74, 74A. 
In particular, the pump piston drive device may may use a positive motion 
cam or a connecting device as indicated above, so as to reciprocate the 
pump pistons in a predetermined positional relationship with the drive 
shaft of the pump drive motor, where the hydraulic operated braking 
apparatus is arranged such that the closure of the first and second 
solenoid-operated shut-off valves for the purpose of detecting the volume 
change directions of the variable-volume chambers 76, 76A in the anti-lock 
control operation inhibits the suction of the fluid into the pump device 
of the braking system in which the shut-off valves are closed. 
In the above embodiment wherein the volume change directions of the 
variable-volume chambers 76, 76A are detected shortly before the anti-lock 
or traction control operations, the first and second solenoid-operated 
shut-off valves can be opened and closed in timed relationship with the 
suction and delivery strokes of the reciprocating pump device 70, 70A so 
as to suitably regulate the wheel cylinder pressure in the anti-lock or 
traction control mode without a delay immediately after the predetermined 
condition for initiating the anti-lock or traction control operation has 
been satisfied. However, the detection of the volume change directions of 
the variable-volume chambers may be effected when the predetermined 
condition for initiating the anti-lock or traction control operation is 
satisfied. In this case, the required overall operating time of the pump 
drive motor 86 can be reduced. 
While the above embodiment uses only one first solenoid-operated shut-off 
valve 46 for both of the right and left wheel brake cylinders in each of 
the first and second braking systems, two first solenoid-operated shut-off 
valves may be provided for the respective two wheel brake cylinders in 
each braking system, like the two second solenoid-operated shut-off 
valves. 
Referring next to FIGS. 8-11, a second embodiment of this invention will be 
described. In the present second embodiment, first and second shut-off 
valves of a hydraulic pressure control actuator are mechanically opened 
and closed in timed or synchronized relationship with the suction and 
delivery strokes of the reciprocating pump devices. In the second 
embodiment, the same reference numerals as used in the first embodiment 
will be used to identify the functionally corresponding elements, and 
redundant description of these elements will not be provided in the 
interest of simplification. 
In the hydraulically operated braking apparatus according to the present 
second embodiment, one of the two pressurizing chambers of the master 
cylinder 12 is connected to the brake cylinder 22 of the front left wheel 
20 through fluid passages 120, 122, and to the brake cylinder 28 of the 
rear right wheel 26 through fluid passages 120, 124. In the fluid passage 
120, there are provided a first shut-off valve 128 and a second shut-off 
valve 130. In the fluid passages 122 and 124, there are respectively 
provided two solenoid-operated directional control valves 132, 134. Since 
the first and second shut-off valves 128, 130 have the same construction, 
only the first shut-off valve 128 will be described by way of example by 
reference to FIG. 9. 
The first shut-off valve 128 has a valve housing 140 in which is 
fluid-tightly and slidably received a control piston 142 such that one end 
portion of the control piston 142 projects out of the valve housing 140. 
The other end portion of the control piston 142 cooperates with the valve 
housing 140 to define a valve chamber 144 which communicates with the 
second shut-off valve 130 via a port 146 and with the master cylinder 12 
via a port 148. A valve seat 150 is formed around the inner open end of 
the port 146 and partially defines the valve chamber 144. 
The valve chamber 144 of the second shut-off valve 144 which has the same 
construction as the first shut-off valve 128 communicates with the first 
shut-off valve 128 via the port 148 and with the front left wheel brake 
cylinder 22 via the port 146. 
A valve member in the form of a ball 152 is disposed within the valve 
chamber 144. The ball 152 has an integrally formed engaging projection 
154. A cylindrical holder member 156 is axially movably received in the 
valve chamber 144, and the engaging projection 154 engages this holder 
member 156 such that the engaging projection 154 is movable in the axial 
direction of the holder member 156. The ball 152 is biased by a spring 158 
disposed between the ball 152 and the holder member 156, in a direction 
away from the holder member 156. The spring 158 serves as biasing means in 
the form of an elastic member. The engaging projection 154 has a head 
portion 160 located within a cylindrical portion of the holder member 156, 
so that abutting contact of the head portion 160 with the holder member 
156 limits a maximum axial distance between the ball 152 and the holder 
member 156. A biasing force of the spring 158 is determined so that the 
ball 150 seated on the valve seat 150 is prevented from being moved away 
from the valve seat 150 when the ordinary delivery pressure of a 
reciprocating pump device 170 or the pressure of the wheel brake cylinder 
22, 28 is applied to the port 146. The holder member 156 is biased by a 
spring 162 disposed within the valve chamber 144, and is held in contact 
with the control piston 142 so that the holder member 156 is moved with 
the control piston 142. The spring 162 serves another biasing means in the 
form of an elastic member. 
The solenoid-operated directional control valves 132, 134 are connected to 
the appropriate pressurizing chamber of the master cylinder 12 through 
fluid passages 164, 166, 168. Each of these directional control valves 
132, 134 has a master cylinder position for fluid communication of between 
the wheel brake cylinder 22, 28 with the master cylinder 12, and a pump 
position for fluid communication of the wheel brake cylinder 22, 28 with a 
reciprocating pump device 170 which will be described. When the 
directional control valve 132 is placed in the master cylinder position, 
for example, the fluid delivered from the pump device 170 cannot be 
supplied to the wheel brake cylinder 22. In this case, the ball 152 is 
moved away from the valve seat 150 to permit the fluid delivered from the 
pump device 170 to be returned to the master cylinder 12 through the first 
shut-off valve 128. The biasing force of the spring 158 is determined to 
permit the ball 152 to be moved away from the valve seat 150 when the 
directional control valve 132 is placed in the master cylinder position 
during the delivery stroke of the pump device 170. It will be understood 
that the first shut-off valve 128 also functions as a pressure-relief 
valve. 
The reciprocating pump device 170 is connected through a pump passage 172 
to a portion of the fluid passage 120 between the first and second 
shut-off valves 128, 130. The pump device 170 is identical in construction 
with the pump device 70 used in the first embodiment. The piston 74 is 
held in contact with the eccentric cam 82, and the control pistons 142 of 
the first and second shut-off valves 128, 130 are held in contact with the 
eccentric cam 82 such that the points of contact of the two control 
pistons 142 with the circumferential surface of the eccentric cam 82 are 
spaced apart from the point of contact of the pump piston 74 with the same 
by 90.degree. in the opposite circumferential directions of the eccentric 
cam 82. The biasing force of the spring 162 biasing the holder member 156 
against the end face of each control piston 142 acts on the control piston 
142 so as to hold the control piston in contact with the eccentric cam 82. 
In the pump passage 172, there is provided a flow restrictor 190 for 
restricting flows of the fluid through the pump passage 172 or applying a 
resistance to flows of the fluid through the pump passage 172. A reservoir 
passage 192 is connected at one end thereof to a portion of the pump 
passage 172 between the flow restrictor 190 and the first and second 
shut-off valves 128, 130, and at the other end to a reservoir 194. A third 
shut-off valve in the form of a pilot-operated shut-off valve 196 is 
provided in the reservoir passage 192. The shut-off valve 196 is connected 
through a fluid passage 198 to a portion of the pump passage 172 between 
the flow restrictor 190 and the pump device 170, and connected through a 
fluid passage 200 to the portion of the pump passage 172 between the flow 
restrictor 180 and the first and second shut-off valves 128, 130. Pilot 
pressures applied to the pilot-operated shut-off valve 196 through the 
fluid passages 198, 200 act in the opposite directions, so that the 
shut-off valve 195 is normally closed, and is opened for fluid 
communication of the pump passage 172 with the reservoir 194 through the 
reservoir passage 192 when the pilot pressure applied through the passage 
198 becomes lower than the pilot pressure applied through the passage 200 
by more than a predetermined valve opening pressure difference value. The 
flow restrictor 190, reservoir passage 192, reservoir 194 and fluid 
passages 198, 200 constitute a rapidly pressure reducing device 202. The 
reservoir 194 is connected to the fluid passage 168 through a by-pass 
passage 205 provided with a check valve 204. 
The second braking system is provided with the same hydraulic pressure 
control actuator as described above with respect to the first braking 
system including the pump device 170. The hydraulic pressure control 
actuator of the second braking system includes a reciprocating pump device 
170A which utilizes the same pump drive device 89 as used for the pump 
device 170A. The control pistons 142A of the first and second shut-off 
valves 128A, 130A are held in contact with the eccentric cam 82. The 
eccentric cam 82 has an axial length large enough to permit the contact of 
the control pistons 142 and 142A with the eccentric cam 82 such that the 
points of contact of the control pistons 142A with the eccentric cam 82 
are spaced from those of the control pistons 142 in the axial direction of 
the eccentric cam 82. In the circumferential direction of the eccentric 
cam 82, the second shut-off valve 130A is located at the same position as 
the first shut-off valve 128, while the first shut-off valve 128A is 
located at the same position as the second shut-off valve 130. 
The solenoid-operated directional control valves 132, 134, 132A, 134A and 
the pump drive motor 86 are controlled by an electronic control unit 210, 
so as to regulate the wheel brake cylinder pressures in the anti-lock and 
traction control modes. 
An operation of the first braking system of the braking apparatus of FIG. 8 
will be described by way of example. 
When the brake pedal 10 is in a non-operated state, the first and second 
solenoid-operated directional control valves 132, 134 are placed in the 
states of FIG. 8, and the wheel brake cylinders 22, 28 are held in 
communication with the master cylinder 12. When the brake pedal 10 is 
depressed in this condition, the fluid pressure generated in the 
appropriate pressurizing chamber of the master cylinder 12 is applied to 
the wheel brake cylinders 22, 28 for braking the front left and right 
wheels 20, 34. Since one of the first and second shut-off valves 128, 130 
is open while the other shut-off valve 128, 130 is closed, the fluid is 
inhibited to flow through the fluid passage 120. 
When the depression force acting on the brake pedal 10 is excessively large 
with respect to the friction coefficient of the road surface on which the 
vehicle is running, the slip of one or both of the wheels 20, 26 exceeds 
the upper limit of an optimum range, and the hydraulic braking pressure 
control device is operated in the anti-lock control mode with the pump 
drive motor 86 being operated so as to rotate the eccentric cam 82 in the 
clockwise direction as indicated by solid line arrow in FIG. 8. As a 
result, the pump piston 74 is reciprocated by the eccentric cam 82, so as 
to suck the brake fluid into the variable-volume fluid chamber 75 during 
the suction stroke and delivery the fluid from the chamber 76 during the 
delivery stroke. At the same time, the control pistons 142 of the first 
and second shut-off valves 128, 130 are reciprocated (advanced and 
retracted), whereby these shut-off valves are alternately opened and 
closed. 
As the distance between the rotation axis of the eccentric cam 82 and the 
point of contact of the control piston 142 of the first shut-off valve 128 
with the eccentric cam 82 increases, the control piston 142 is advanced, 
eventually bringing the ball 152 into contact with the valve seat 150. 
After the ball 150 is seated on the valve seat 152, the control piston 142 
is further advanced with the spring 158 being compressed, whereby the 
first shut-off valve 128 is held closed. As the distance between the 
rotation axis of the eccentric cam 82 and the point of contact of the 
control piston 142 with the eccentric cam 82 decreases, the control piston 
142 is retracted, so that the spring 152 is initially expanded, and then 
the ball 152 is moved away from the valve seat 150, whereby the first 
shut-off valve is opened. Since the control pistons 142 of the first and 
second shut-off valves 128, 130 are in contact with the eccentric cam 82 
at the circumferential positions of the latter which are 180.degree. 
spaced apart from each other, the first shut-off valve 128 is opened and 
closed while the second shut-off valve 130 is closed and opened, 
respectively. 
With the eccentric cam 82 rotated in the clockwise direction, the first 
shut-off valve 128 is closed while the second shut-off valve 130 is opened 
during the pump device 170 is in the suction stroke, as indicated in FIG. 
10, whereby the brake fluid is sucked from the wheel brake cylinder 22, 28 
into the pump device 170 through the open second shut-off valve 130. 
During the delivery stroke of the pump device 170, the first shut-off 
valve 128 is opened while the second shut-off valve 130 is closed, as also 
indicated in FIG. 10, whereby the brake fluid is delivered from the pump 
device 170 to the master cylinder 12 through the open first shut-off valve 
128. 
In the anti-lock control operation, the pump device 170 is operated with 
continuous rotation of the eccentric cam 82 in the clockwise direction 
only, so that the first and second shut-off valves 128, 130 are opened and 
closed so as to reduce the pressure in the wheel brake cylinder 22, for 
example. In this condition, the solenoid-operated directional control 
valve 132 is switched between the master cylinder position and the pump 
position, to increase, decrease and hold the pressure in the wheel brake 
cylinder 22. 
If the slip amount of the front left wheel 20 is excessively large and the 
anti-lock control operation is effected for the corresponding wheel brake 
cylinder 22, the solenoid-operated directional control valve 132 is placed 
in the pump position so that the fluid is sucked from the wheel brake 
cylinder 22 into the pump device 170, to decrease the pressure in the 
wheel brake cylinder 22. Since the flow of the brake fluid into the 
variable-volume chamber 76 is restricted by the flow restrictor 190, the 
pressure in the fluid passage 198 becomes lower than the pressure in the 
fluid passage 200 by more than the predetermined difference value, so that 
the pilot-operated shut-off valve 196 is opened permitting the fluid to 
flow from the wheel brake cylinder 22 toward the reservoir 194, whereby 
the pressure in the wheel brake cylinder 22 is rapidly reduced. 
When the slipping tendency of the front left wheel 20 is reduced, the 
solenoid-operated directional control valve 132 is switched to the master 
cylinder position for fluid communication of the wheel brake cylinder 22 
with the master cylinder 12, whereby the pressure in the wheel brake 
cylinder 22 is increased. If the wheel brake cylinder 22 is held in 
communication with the master cylinder 12, the rate of increase of the 
pressure in the cylinder 22 may be excessive. To avoid this, the 
directional control valve 132 is alternately placed in the master cylinder 
position and the pump position, so as to increase the wheel cylinder 
pressure at an optimum rate. 
As long as the pump device 170 is in the delivery stroke, the second 
shut-off valve 130 remains closed preventing the brake fluid to be 
discharged from the wheel brake cylinder 22, even if the wheel brake 
cylinder 22 is in communication with the pump device 170 with the 
directional control valve 132 switched to the pump position. During this 
delivery stroke, therefore, the wheel brake cylinder pressure is not 
reduced. To increase the pressure in the wheel brake cylinder 22 at an 
optimum rate, the duty ratio of the solenoid-operated shut-off valve 132 
is suitably determined to control the ratio of the time during which the 
valve 132 is placed in the master cylinder position, to the time during 
which the valve 132 is placed in the pump position. In this respect, the 
duty ratio should be determined by considering the fact that the brake 
fluid is not discharged from the wheel brake cylinder 22 in a half of the 
time during which the directional control valve 132 is placed in the pump 
position. 
In the anti-lock control operation, the operating speed of the pump drive 
motor 86 is determined so that the rate of decrease of the wheel cylinder 
pressure is controlled to a desired value while the directional control 
valve 132 is held in the pump position. The thus determined operating 
speed of the pump drive motor 86 should also be considered as a 
prerequisite, in determining the duty ratio of the shut-off valve 132 so 
as to obtain the desired rate of increase of the wheel brake cylinder 
pressure. 
To hold the pressure in the wheel brake cylinder 22, too, the the duty 
ratio of the solenoid-operated shut-off valve 132 should be suitably 
determined, by considering the fact that the brake fluid is not discharged 
from the wheel brake cylinder 22 in a half of the time during which the 
directional control valve 132 is placed in the pump position. 
It will be understood that the rate of increase of the wheel brake cylinder 
pressure can be changed as desired, by changing the duty ratio of the 
solenoid-operated directional control valve 132, namely, by changing the 
ratio of the times during which the valve 132 is alternately placed in the 
master cylinder position and the pump position. The wheel brake cylinder 
pressure can be held by zeroing the rate of increase of the wheel brake 
cylinder pressure. 
It will also be understood that the pressures in the wheel brake cylinders 
22, 28 can be controlled (increased, decreased and held) independently of 
each other, by controlling the solenoid-operated directional control 
valves 132, 134 independently of each other depending upon the slip ratios 
of the front left wheel 20 and the rear right wheel 26. 
When the brake pedal 10 is moved toward its non-operated position during 
the pressure increase or hold of the wheel brake cylinder 22, the brake 
fluid is returned from the wheel brake cylinder 22 back to the 
pressurizing chamber of the master cylinder 12 through the fluid passage 
164 when the directional control valve 132 is placed in the master 
cylinder position. Consequently, the wheel brake cylinder pressure is 
reduced. 
When the anti-lock control mode is cancelled, the directional control valve 
132 is switched to the master cylinder position. If the anti-lock control 
mode is cancelled by releasing the brake pedal 10, the brake fluid is 
returned from the wheel brake cylinder 22 to the master cylinder 12 
through the fluid passages 164, 168. The fluid in the reservoir 194 is 
returned to the master cylinder 12 through the check valve 202, pump 
passage 172 and fluid passage 168. 
In the second braking system, the pump piston 75A of the reciprocating pump 
device 170A has an operating phase which is different from that of the 
pump piston 76 of the pump device 170, by an amount corresponding to 
180.degree. rotation of the eccentric cam 82. Similarly, the operating 
phases of the first and second shut-off valves 128A, 130A of the second 
braking system are 180.degree. different from those of the first and 
second shut-off valves 128, 130 of the first braking system. In other 
words, the suction and delivery strokes of the pump device 170A take place 
during the delivery and suction strokes of the pump device 170, and the 
first and second shut-off valves 128A, 130A are opened and closed while 
the first and second shut-off valves 128, 130 are closed and opened, 
respectively. When the eccentric cam 82 is rotated in the clockwise 
direction, the pump device 170A and the first and second shut-off valves 
128A, 130A are operated so as to reduce the wheel cylinder pressure. The 
slip ratios of the front right wheel 34 and the rear left wheel 40 can be 
controlled to within an optimum range by switching the directional control 
valves 132A, 134A between the master cylinder position and the pump 
position, as described above with respect to the first braking system. 
Where the anti-lock control operations are initiated at different times in 
the first and second braking systems, a rotary motion of the eccentric cam 
82 to effect the anti-lock control operation in only one of the two 
braking systems will not cause pressure decrease of the wheel brake 
cylinders in the other braking system, since the two solenoid-operated 
directional control valves in the above-indicated other braking system are 
both placed in the master cylinder position. Thus, the braking apparatus 
provides a sufficiently large overall braking force. 
To effect the traction control operation, the pump drive motor 86 is 
activated to rotate the eccentric cam 82 in the counterclockwise direction 
as indicated by broken line arrow in FIG. 8, namely, in the direction 
opposite to that in the anti-lock control operation. As a result, the 
first and second shut-off valves 128 and 130 are opened and closed, 
respectively, during the delivery stroke of the pump device 170, so that 
the brake fluid is sucked from the master cylinder 12 into the pump device 
170. During the following delivery stroke of the pump device 170, the 
first and second shut-off valves 128, 130 are closed and opened, 
respectively, so that the brake fluid is delivered from the pump device 
170 to the front left wheel brake cylinder 22. 
During the traction control operation, the pump device 170 is operated with 
the eccentric cam 82 kept rotated in the counterclockwise direction only, 
and the first and second shut-off valves 128, 130 are opened and closed so 
as to increase the pressure in the wheel brake cylinder 22. When the 
solenoid-operated directional control valve 132 is switched to the master 
cylinder position, the brake fluid is discharged from the wheel brake 
cylinder 22 to the master cylinder 12, whereby the pressure in the wheel 
brake cylinder 22 is reduced. 
Since the rear right and left wheels 40, 26 are non-drive or idler wheels, 
the pressures in the corresponding wheel brake cylinders 28, 42 are not 
increased with the solenoid-operated directional control valves 134, 134A 
being held in the master cylinder position. 
Described in detail, if the traction control operation is effected when the 
slip amount of the front left drive wheel 20 is excessive, the directional 
control valve 132 is switched to the pump position so that the brake fluid 
sucked from the master cylinder 12 into the pump device 170 is delivered 
to the wheel brake cylinder 22, for braking the front left wheel 20. When 
the excessive slipping tendency of the front left wheel 20 is eliminated, 
the directional control valve 132 is switched to the master cylinder 
position. Since the master cylinder pressure is zero during the traction 
control operation with the brake pedal 12 placed at its non-operated 
position, the brake fluid is discharged from the wheel brake cylinder 22 
to the corresponding pressurizing chamber of the master cylinder 12 
through the directional control valve 132, whereby the pressure in the 
wheel brake cylinder 22 is reduced. 
The pressure in the wheel brake cylinder 22 may be maintained by 
alternately placing the directional control valve 132 in the master 
cylinder position and the pump position, so that the brake fluid supplied 
from the pump device 170 to the wheel brake cylinder 22 when the valve 132 
is in the pump position is returned to the master cylinder 12 when the 
valve 132 is placed in the master cylinder position. If the duty ratio of 
the solenoid-operated directional control valve 132 is controlled so as to 
place the valve 132 in the master cylinder position for a time longer than 
that for which the valve 132 is placed in the pump position, the pressure 
in the wheel brake cylinder 22 is reduced. In this respect, it is noted 
that the brake fluid is not supplied to the wheel brake cylinder 22 in a 
half of the period during which the valve 132 is placed in the pump 
position, namely, during the suction stroke of the pump device 170. This 
fact should be taken into account in determining the duty ratio of the 
directional control valve 132. 
In the traction control operation, the operating speed of the pump drive 
motor 86 is determined so as to increase the wheel brake cylinder pressure 
at a desired rate with the directional control valve 132 placed in the 
pump position. The thus determined operating speed of the pump drive motor 
86 is a prerequisite in determining the duty ratio (energization current) 
of the solenoid-operated directional control valve 132 so as to obtain a 
desired rate of decrease of the wheel brake cylinder pressure or to hold 
the wheel brake cylinder pressure. 
It will be understood from the above explanation of this second embodiment 
that the eccentric cam 82 functions as a movable member of the pump piston 
drive device 88, while the control pistons 142, 142A function as movable 
members of the first and second shut-off valves 128, 128A, 130, 130A, and 
that the eccentric cam 82 and the control pistons 142, 142A which contact 
each other and which are both solid members constitute a motion 
transmitting device of solid member type which functions as synchronizing 
means for synchronizing the opening and closing actions of the first and 
second shut-off valves 128, 128A, 130, 130A with the suction and delivery 
strokes of the reciprocating pump devices 170, 170A. 
Referring next to FIGS. 12-18, there will be described a hydraulically 
operated braking apparatus constructed according to a third embodiment of 
the present invention. In this braking apparatus, too, the first braking 
system including the front left and rear right wheel brake cylinders and 
the second braking system including the front right and rear left wheel 
brake cylinders are identical in construction with each other. Hence, only 
the first braking system is illustrated and will be described by way of 
example. 
As shown in FIG. 12, one of the two pressurizing chambers of the master 
cylinder 12 is connected to the brake cylinder 22 of the front left wheel 
20 through fluid passages 220, 222, and to the brake cylinder 28 of the 
rear right wheel 26 through fluid passages 220, 224. A first shut-off 
valve 226 is provided in the fluid passage 220, and two second shut-off 
valves 228, 230 are provided in the fluid passages 222, 224, respectively. 
These first and second shut-off valves 226, 228, 230 have the same 
construction, and the first shut-off valve 226 will be explained by way of 
example by reference to FIG. 13. 
As shown in FIG. 13, the first shut-off valve 226 has a valve housing 234 
in which is fluid-tightly and slidably received a control piston 236. The 
control piston 236 cooperates with the valve housing 234 to define an 
operating pressure chamber 238 on one side of the control piston 236, and 
a valve chamber 240 on the other side of the control piston 236. The valve 
chamber 240 communicates with the master cylinder 12 through a port 242, 
and with a port 242 of each of the second shut-off valves 228, 230 through 
a port 244. Each second shut-off valve 228, 230 has a port 244 
communicating with the corresponding wheel brake cylinder 22, 28. 
The control piston 236 has a valve member 246 integrally formed on one of 
its opposite end faces which partially defines the valve chamber 240. The 
control piston 236 is biased by a spring 248 disposed in the valve chamber 
240, in a direction away from a valve seat 250 which is formed around an 
inner open end of the port 244. Under the biasing force of the spring 248, 
the control piston 236 is normally held at its fully retracted position 
with the other end face in abutting contact with a stop surface 252 
provided on the valve housing 234. Thus, the first shut-off valve 226 is 
normally open with the valve member 246 held apart from the valve seat 
250. 
The wheel brake cylinders 22, 28 are connected to the master cylinder 12 by 
respective two by-pass passages 254 which by-pass the respective first and 
second shut-off valves 226, 228, 230. In each of these by-pass passages 
254, there is provided a pressure relief valve 256 whose relief pressure 
is set at about 7.8 MPa (80 kg.multidot.f/cm.sup.2). This pressure relief 
valve 256 permits a flow of the brake fluid in a first direction from the 
wheel brake cylinder 22, 28 toward the master cylinder 12 when the 
pressure difference across the valve 256 exceeds the relief pressure, and 
inhibits a flow of the brake fluid in a second direction opposite to the 
first direction. 
A reciprocating pump device 262 is connected through a pump passage 260 to 
a portion of the fluid passage 220 between the first shut-off valve 226 
and the second shut-off valves 22, 230. The pump device 262, which has the 
same construction as the pump device 70 of the first embodiment, includes 
a disk-like eccentric cam 266 similar to the eccentric cam 82, a rotary 
shaft 267 for rotating the eccentric cam 266, and a pump piston 74 held in 
contact with a circumferential cam surface of the eccentric cam 266. The 
rotary shaft 267 supports the eccentric cam 266 such that the axis of the 
rotary shaft 267 is offset from the center of the eccentric cam 266. As in 
the second embodiment of FIG. 8, the rapidly pressure reducing device 202 
is provided in the pump passage 260. 
The eccentric cam 266 has two cam grooves 268, 270 formed in the respective 
opposite end faces, as shown in FIGS. 15 and 16. First and second 
operating pistons 272 engage these two cam grooves 268, 270, respectively. 
The first and second operating pistons 272, 274 are fluid-tightly and 
slidably supported by respective cylinder housings 276, 278, such that one 
end portion of each piston 272, 274 protrudes out of the corresponding 
cylinder housing 276, 278, for engagement with the corresponding cam 
groove 268, 270 through an engaging projection 280, 282. 
As shown in FIG. 15, the cam groove 268 has two circular arc portions 268a, 
268b which have an arc center on the axis of the rotary shaft 267 and 
different radii. The cam groove 268 also has two curved portions 268c, 
268d which connect the two circular arc portions 268a, 268b. The curved 
portions 268c, 268d are formed such that a distance of the curved portions 
from the axis of the rotary shaft 267 gradually changes. As the engaging 
projection 280 is moved through the curved portions 268c, 268d, the first 
operating piston 272 is advanced and retracted go that the volume of a 
variable-volume chamber 286 defined between the piston 272 and the 
cylinder housing 276 is increased and decreased. 
The variable-volume chamber 286 is connected through a fluid passage 288 to 
the operating pressure chamber 230 of the first shut-off valve 226. The 
fluid passage 288 is connected through a fluid passage 290 to the 
pressurizing chamber of the master cylinder 12. In the fluid passage 290, 
there are provided a controllable check valve 292 and a check valve 294 in 
parallel connection with each other. These two check valves 292, 294 
permit flows of the brake fluid therethrough in opposite directions. 
Namely, the check valve 292 permits a flow of the fluid in a first 
direction from the fluid passage 288 toward the master cylinder 12, while 
the check valve 294 permits a flow of the fluid in a second direction 
opposite to the first direction. The controllable check valve 292 has an 
operating state in which the check valve 292 is enabled to perform a 
function of inhibiting a flow of the fluid in the second direction, and a 
non-operating state in which the check valve 292 is disabled to perform 
the fluid flow inhibiting function. The check valve 294 is opened to 
permit the fluid flow in the second direction when the pressure difference 
across the check valve 294 exceeds a value which is set to be considerably 
large. 
The controllable check shown in is constructed as shown in FIG. 14. This 
check valve 292 has a housing 300 in which is fluid-tightly and slidably 
received a stepped control piston 302. The housing 300 and the control 
piston 302 cooperate to define an operating pressure chamber 304, an 
atmospheric pressure chamber 306, and a valve chamber 308. The control 
piston 302 is biased in a direction toward its fully retracted position of 
FIG. 14, under a biasing force of a spring 311 which acts on the control 
piston 302 via a cylindrical holder member 310. The operating pressure 
chamber 304 has a cross sectional area which is sufficiently larger than 
that of the valve chamber 308, so that a comparatively low pressure 
applied to the operating pressure chamber 304 advances the control piston 
302 to its fully advanced position, irrespective of the pressure in the 
valve chamber 308. The cylindrical holder member 310 supports a valve 
member 312 such that an engaging projection 314 of the valve member 312 is 
movable relative to the holder member 310 in the axial direction of the 
holder member 310. The valve member 312 is biased by a spring 316 in a 
direction away from the control piston 302. A distance of the valve member 
312 and the control piston 302 in the axial direction of the control 
piston 302 is limited by abutting contact of the engaging projection 314 
and the holder member 310. 
With an advancing movement of the control piston 302, the valve member 312 
is seated on a valve seat 318 before the control piston 302 reaches its 
fully advanced position. As the control piston 302 is further advanced to 
its fully advanced position, the spring 316 is compressed. This spring 315 
is pre-loaded so that an amount of change of the biasing force of the 
spring 316 due to its compression is relatively small. 
The operating pressure chamber 304 communicates with a small pump 322 
through a port 320 and a fluid passage 321. The small pump 322 is a 
reciprocating pump device of small delivery capacity, which is also driven 
by the eccentric cam 266. When the small pump 322 is operated by either 
clockwise or counterclockwise rotation of the eccentric cam 266, the brake 
fluid is sucked from the reservoir 14 through a suction valve 323, and 
delivered to the fluid passage 321 through a delivery valve 324. A 
downstream side of the delivery valve 324 and an upstream side of the 
suction valve 323 are connected to each other through a fluid passage 
provided with a flow restrictor 325. 
The brake fluid is delivered to the fluid passage 321 during the delivery 
stroke of the small pump 322. A relatively small portion of the delivered 
fluid is returned to the reservoir 14 through the flow restrictor 325, and 
the relatively large remaining portion is supplied to the operating 
pressure chamber 304 of the controllable check valve 292, whereby the 
control piston 302 is advanced. During the suction stroke of the small 
pump 322, the brake fluid is sucked into the small pump 322 from the 
reservoir 14 through the suction valve 323, and from the operating 
pressure chamber 304 through the flow restrictor 325 and the suction valve 
323. However, the amount of the fluid discharged from the operating 
pressure chamber 304 during the suction stroke of the small pump 322 is 
smaller than the amount of the fluid delivered to the same chamber 304 
during the delivery stroke of the small pump 322, so that the control 
piston 302 is moved to its fully advanced position to force the valve 
member 312 on the valve seat 318 after several reciprocations of the small 
pump 322. 
The valve seat 318 is formed around an inner open end of a port 328 through 
which the valve chamber 308 communicates with a portion of the fluid 
passage 290 on the side of the fluid passage 288. When the valve member 
312 is seated on the valve seat 318, the port 328 is disconnected from a 
port 328 through which the valve chamber 308 communicates with a portion 
of the fluid passage 290 on the side of the master cylinder 12. The valve 
member 312, valve seat 318 and spring 316 constitute a check valve adapted 
to permit a flow of the brake fluid in the above-indicated first direction 
from the fluid passage 288 toward the master cylinder 12 and inhibit a 
flow of the fluid in the opposite, second direction, while the control 
piston 302 is located at or near its fully advanced position. As indicated 
above, the spring 316 is pre-loaded, so that the pressure in the fluid 
passage 288 that causes the controllable check valve 292 to be opened is 
sufficient for advancing the control piston 236 of the first shut-off 
valve 226 against the biasing force of the spring 248. 
When the engaging projection 280 of the first operating piston 272 is moved 
from the smaller-radius circular arc portion 268a to the larger-radius 
circular arc portion 268b through the curved portion 268c or 268d, during 
rotation of the eccentric cam 266, the first operating piston 272 is 
advanced with the engaging projection 280 being pushed by the curved 
portion 258c, 268d, and the brake fluid is delivered from the 
variable-volume chamber 286 to the operating pressure chamber 238 of the 
first shut-off valve 226, so that the control piston 236 is advanced to 
force the valve member 245 onto the valve seat 250, whereby the first 
shut-off valve 226 is closed. At this time, the control piston 236 
receives the master cylinder pressure applied to the valve chamber 240, 
but the pressure applied to the operating pressure chamber 238 due to the 
advancing movement of the first operating piston 272 is higher than the 
master cylinder pressure by an amount equal to the opening pressure of the 
controllable check valve 292, so that the control piston 236 is advanced 
against the biasing force of the spring 248. The curved portion 268c is 
formed so as to advance the first operating piston 272 by a small distance 
after the closure of the first shut-off valve 226, to assure stable 
seating of the valve member 246 on the valve seat 250. This advancing 
movement is permitted by a discharge flow of the brake fluid through the 
controllable check valve 292 toward the master cylinder 12. 
While the engaging projection 280 of the first operating piston 272 is 
moved through the larger-radius circular arc portion 268b, the first 
shut-off valve 226 is held closed. As the engaging projection 280 is moved 
to the smaller-radius circular arc portion 280a through the curved portion 
268c or 268d, the first operating piston 272 is retracted. As a result, 
the brake fluid is discharged from the operating pressure chamber 238 into 
the variable-volume chamber 286, so that the control piston 236 is 
retracted, with the valve member 246 being moved away from the valve seat 
250, whereby the first shut-off valve 226 is opened. The valve opening 
pressure of the check valve 294 is determined to be high enough to prevent 
a flow of the brake fluid in a direction from the master cylinder 12 
toward the fluid passage 288 before the control piston 236 is brought into 
abutting contact with the stop surface 252. The curved portions 268c, 268d 
are formed so as to retract the first operating piston 272 by a small 
distance after the abutting contact of the control piston 236 with the 
stop surface 242. This retracting movement is permitted by a flow of the 
brake fluid through the check valve 294 toward the chamber 286. 
As shown in FIG. 15, the cam groove 268 is formed in the corresponding end 
face of the eccentric cam 266 such that the larger-radius circular arc 
portion 268b is located at a circumferential portion of the eccentric cam 
226 at which the distance from the axis of the rotary shaft 267 is the 
smallest. The first operating piston 272 engages the cam groove 268 at a 
circumferential position of the eccentric cam 266 which is spaced by 
90.degree. from the circumferential position at which the control piston 
74 of the pump device 262 is held in contact with the eccentric cam 266. 
Accordingly, the first shut-off valve 226 is opened and closed in timed 
relationship with the suction and delivery strokes of the reciprocating 
pump 262, as indicated in FIGS. 17 and 18. Basically, the first shut-off 
valve 226 is closed and opened during the suction and delivery strokes of 
the pump device 262, respectively, when the eccentric cam 266 is rotated 
in one of the clockwise and counterclockwise directions, and during the 
delivery and suction strokes, respectively, when the eccentric cam 266 is 
rotated in the other direction. In FIGS. 17 and 18, solid lines indicate a 
movement of the operating piston 272, while broken lines indicate a 
movement of the valve member 246 of the control piston 236. 
However, the smaller-radius circular arc portion 268a of the cam groove 268 
is formed such that the first shut-off valve 226 is opened for short times 
at the beginning and the end of each delivery stroke of the pump device 
262, as well as during the suction stroke, as indicated in FIG. 18, when 
the wheel brake cylinder pressure is increased. Thus, the total opening 
time of the first shut-off valve 226 is longer than the total closing 
time. 
Like the cam groove 268, the cam groove 270 with which the engaging 
projection 282 of the second operating piston 274 engages has two circular 
radius portions 270a, 270b which have an arc center on the axis of the 
rotary shaft 267 and different radii. The cam groove 270 also has two 
curved portions 270c, 270d which connect the two circular arc portions 
270a, 270b. The cam groove 270 is formed in the corresponding end face of 
the eccentric cam 266 at the see circumferential portion as the cam groove 
268. The second piston 274 engages the cam groove 270 at a circumferential 
position of the eccentric cam 266 which is 180.degree. spaced apart from 
the circumferential position at which the first piston 272 engages the cam 
groove 268. 
The cam groove 270 is formed so that the total opening time and the total 
closing time of each of the second shut-off valves 228, 230 are equal to 
each other. However, a movement of the control piston 236 of each second 
shut-off valve 228, 230 is initiated upon initiation of a movement of the 
second operating piston 274, and is terminated a short time before the 
termination of the movement of the second operating piston 274. 
Accordingly, the opening and closing actions of the second shut-off valves 
228, 230 are slightly out of synchronization of the suction and delivery 
stokes of the reciprocating pump device 262. 
The second piston 274 cooperates with a cylinder housing 278 to define a 
variable-volume chamber 330 which communicates with the operating pressure 
chambers 238 of the second shut-off valves 228, 230 through fluid passages 
331, 332, respectively. These fluid passages 331, 332 are connected to the 
fluid passage 220 through respective fluid passages 333, 334. Each of the 
fluid passages 333, 334 is provided with a controllable check valve 335 
and a check valve 336 which are identical with the controllable check 
valve 292 and the check valve 294. 
With the eccentric cam 226 being rotated, the engaging projection 282 of 
the second operating piston 274 is moved through the cam groove 270, 
whereby the second operating piston 274 is advanced and retracted to 
thereby open and close the second shut-off valves 228, 230. To this end, 
the cross sectional area of the second operating piston 274 is two times 
that of the control piston 236 of each second shut-off valve 228, 230, 
while the cross sectional area of the first operating piston 272 is the 
same as that of the control piston 236 of the first shut-off valve 226. 
Although the circumferential position of the cam groove 270 on the 
eccentric cam 266 is the same as that of the cam groove 268 as shown in 
FIGS. 15 and 16, the circumferential positions of the eccentric cam 266 at 
which the first and second operating pistons 272 and 274 engage the 
respective cam grooves 268, 270 are offset by 180.degree. from each other 
in the circumferential direction of the cam 266, as indicated above. 
Consequently, the second shut-off valves 228, 230 are opened and closed 
while the first shut-off valve 226 is closed and opened, respectively. 
Further, the relationship between the opening and closing actions of the 
second shut-off valves 228, 230 with the suction and delivery strokes of 
the pump device 262 when the eccentric cam 266 is rotated in the clockwise 
direction is reversed with respect to the relationship when the eccentric 
cam 266 is rotated in the counterclockwise direction, as indicated in 
FIGS. 17 and 18. Since the cam groove 268 is formed so that the total 
opening time of the first shut-off valve 226 is longer than the closing 
time, the opening time of the first shut-off valve 226 includes a portion 
during which the second shut-off valves 228, 230 are also open. 
In the fluid passages 331, 332, there are provided respective 
solenoid-operated directional control valves 338, 340. Each of these 
directional control valves 338, 340 has a first state for bidirectional 
fluid communication of the operating pressure chamber 238 of the second 
shut-off valve 227, 230 with the variable-volume chamber 330 of the second 
operating piston 274, and a second state for unidirectional fluid 
communication between the operating pressure chamber 238 and the 
variable-volume chamber 330 through a check valve 343, 344, which permits 
a flow of the fluid in a direction front the variable-volume chamber 330 
toward the operating pressure chamber 238 and inhibits a flow of the fluid 
in the opposite direction. 
The solenoid-operated directional control valves 338, 348 and the pump 
drive motor 86 are controlled by an electronic control unit 346 which is 
principally constituted by a computer. Like the electronic control unit 
110 provided in the first embodiment, the electronic control unit 346 
obtains slipping tendencies of the individual wheel brake cylinders by 
calculation on the basis of output signals of the wheel speed sensors 100, 
106, etc., and controls the solenoid-operated directional control valves 
338, 340 and pump drive motor 86 so as to control the wheel brake cylinder 
pressures in the anti-lock or traction control mode. 
An operation of the braking apparatus of the present third embodiment of 
the invention will be described. 
When the brake pedal 10 is not depressed, the control piston 246 of each of 
the shut-off valves 226, 228, 230 is held in abutting contact with the 
stop surface 252 under the biasing action of the spring 248, and the 
shut-off valves 226, 228, 230 are held open. When the brake pedal 10 is 
depressed in this condition to brake the front left and rear right wheels 
20, 26, the fluid pressurized by the master cylinder 12 is supplied to the 
wheel brake cylinders 22, 28, whereby the wheels 20, 26 are braked. 
The electronic control unit 346 initiates an anti-lock control operation 
when the slip amount of the wheel 20, 26 exceeds an upper limit of an 
optimum range, due to an excessively large depression force acting on the 
brake pedal 10, with respect to the friction coefficient of the road 
surface on which the vehicle is running. The anti-lock control operation 
is initiated with forward rotation of the pump drive motor 86 as indicated 
by solid line arrow in FIG. 12. As a result, the brake fluid is delivered 
from the small pump 322 to the controllable check valves 292, 335, so that 
these check valves are placed in their operating state for performing 
their fluid flow inhibiting function. Then, the first shut-off valve 226 
is closed and the second shut-off valve 228, 230 is opened while the pump 
device 262 is in the suction stroke, as indicated in FIG. 17. 
Consequently, the fluid is sucked from the wheel brake cylinder 22, 28 
into the pump device 262. During the delivery stroke of the pump device 
262, the first shut-off valve 226 is opened while the second shut-off 
valve 228, 230 is closed, as also indicated in FIG. 17, so that the fluid 
is delivered from the pump device 262 to the master cylinder 12. Thus, the 
wheel brake cylinder pressure is reduced. As in the preceding embodiments, 
the wheel brake cylinder pressure is rapidly reduced upon initiation of 
the anti-lock control operation. 
When the slipping tendency of the wheel 20, 26 is eliminated, the wheel 
brake cylinder pressure is increased. In this case, the pump drive motor 
86 is rotated in the reverse direction as indicated by broken line arrow 
in FIG. 12. As a result, the first shut-off valve 226 is opened and the 
second shut-off valve 228, 230 is opened while the pump device 262 is in 
the suction stroke, as indicated in FIG. 18, whereby the fluid is sucked 
from the master cylinder 12 into the pump device 262. During the delivery 
stroke of the pump device 262, the first shut-off valve 226 is closed and 
the second shut-off valve 228, 230 is opened, as also indicated in FIG. 
18, whereby the fluid is delivered to the wheel brake cylinder 22, 28. 
The cam groove 288 is formed so that the first shut-off valve 226 is opened 
for short times at the initial and terminal portions of the opening period 
of the second shut-off valve 228, 230, as indicated in FIG. 18, when the 
wheel brake cylinder pressure is increased. In this arrangement, the brake 
fluid is discharged from the wheel brake cylinder 22, 28 to the master 
cylinder 12 and the wheel cylinder pressure is reduced, when the master 
cylinder pressure is lowered below the wheel brake cylinder pressure as a 
result of a movement of the brake pedal 10 toward its non-operated 
position while the wheel brake cylinder pressure is being increased. Even 
when the wheel brake cylinder 22 is not in communication with the master 
cylinder 12, the wheel brake cylinder pressure is reduced with a discharge 
flow of the fluid from the wheel brake cylinder 22, 28 to the master 
cylinder 12 when the wheel brake cylinder pressure becomes higher than the 
master cylinder pressure by more than the predetermined relief pressure of 
the pressure relief valve 256. 
To hold the wheel brake cylinder pressure, the solenoid-operated 
directional control valves 338, 340 are switched to their second state for 
unidirectional fluid communication between the operating pressure chamber 
238 and the variable-volume chamber 330 through the check valve 343, 344, 
irrespective of the rotating direction of the pump drive motor 86. In this 
state, an advancing movement of the second operating piston 274 causes the 
brake fluid to be supplied from the variable-volume chamber 330 to the 
operating pressure chamber 238 of the second shut-off valve 228, 230, so 
that the second shut-off valve 228, 230 is closed. Since the brake fluid 
is not discharged from the operating pressure chamber 238, the second 
shut-off valve 228, 230 is held closed. In the meantime, the fluid is 
supplied to the variable-volume chamber 330 through the check valve 330, 
and the fluid is discharged from the variable-volume chamber 330 through 
the controllable check valve 335, so that the second operating piston 274 
is permitted to be reciprocated. 
If the brake pedal 10 is moved toward the non-operated position while the 
wheel brake cylinder pressure is held, the brake fluid is discharged from 
the wheel brake cylinder 22, 28 to the master cylinder 12 and the wheel 
brake cylinder pressure is reduced, when the wheel brake cylinder pressure 
becomes higher than the master cylinder pressure by more than the relief 
pressure of the pressure relief valve 256. 
The pressures in the front and rear wheel brake cylinders in the same 
braking system are controlled by suitable combinations of the pressure 
increase, decrease and hold, as indicated in FIG. 7, as in the first 
embodiment of FIGS. 1-7. 
When the brake pedal 10 is released during the anti-lock control operation, 
the pump drive motor 86 is turned off, and the eccentric cam 266 is 
stopped at a given angular position unknown. In this condition, one of the 
first and second shut-off valves 226, and 228, 230 is closed while the 
other shut-off valve is open. Since the delivery of the fluid from the 
small pump 322 is stopped when the drive pump motor 86 is turned off, the 
brake fluid is discharged from the operating pressure chambers 304 of the 
controllable check valves 292, 335 into the reservoir 14 through the flow 
restrictor 325, so that the control piston 302 is retracted, and the 
controllable check valves 292, 335 are placed in their non-operating state 
in which the check valves 292, 334 are disabled to perform their fluid 
flow inhibiting function. Consequently, the brake fluid is permitted to be 
discharged from the operating pressure chambers 238 of the first and 
second shut-off valves 226, 228, 230, and the control piston 236 is moved 
to the fully retracted position, whereby the first and second shut-off 
valves 226, 228, 230 are all returned to the initial open state in which 
the pressure if generated by the master cylinder 12 is applied to the 
wheel brake cylinders 22, 28. 
If the anti-lock control operation is initiated by operation of the brake 
pedal 10 when the first shut-off valve 226 is open while the first 
operating piston 272 is placed at its fully advanced position (for holding 
the first shut-off valve 226 in the closed state), for example, the 
rotation of the eccentric cam 266 by the pump drive motor 86 will cause 
the first operating piston 272 to be retracted, resulting in a flow of the 
fluid into the fluid passage 288 through the check valve 294, whereby the 
first operating piston 272 is permitted to be retracted. When the first 
operating piston 272 is then advanced, the control piston 236 is also 
advanced, and the first shut-off valve 228 is closed. 
When the traction control operation is performed, the pump drive motor 86 
is initially turned on. To prevent braking of the rear right wheel 26 
which is a non-drive or idler wheel, the solenoid-operated directional 
control valve 340 is placed in the second state for unidirectional fluid 
communication between the operating pressure chamber 238 of the second 
shut-off valve 230 and the variable-volume chamber 330 through the check 
valve 344, so that the second shut-off valve 230 is closed. 
The controllable check valves 292, 335 are placed in their operating state 
during several turns of the eccentric cam 266. In the traction control 
operation, the eccentric cam 266 is first rotated in the direction 
indicated by the broken line arrow in FIG. 12, to supply the pressurized 
fluid to the brake cylinder 22 of the front wheel 20. As a result, the 
brake fluid sucked from the master cylinder 12 by the pump device 262 is 
delivered from the pump device 252 to the wheel brake cylinder 22. The 
pressure in the wheel brake cylinder 22 to be controlled in the traction 
control operation is determined by the relief pressure of the pressure 
relief valve 256. This wheel brake cylinder pressure is sufficient to 
eliminate excessive slipping of the front drive wheel 20. When it becomes 
necessary to reduce the wheel brake cylinder pressure as a result of 
elimination of the excessive slipping tendency of the front drive wheel 
20, the eccentric cam 266 is rotated in the direction indicated by the 
solid line arrow in FIG. 12. When it becomes necessary to hold the wheel 
brake cylinder pressure, the solenoid-operated directional control valve 
338 is switched to the second state for unidirectional fluid communication 
between the operating pressure chamber 238 of the second shut-off valve 
228 and the variable-volume chamber 330 through the check valve 342. 
It will be understood from the foregoing explanation of the third 
embodiment that the first and second operating pistons 272, 274, fluid 
passages 288, 331, 332, variable-volume chambers 286, 330 and operating 
pressure chambers 238 constitute a motion transmitting device of hydraulic 
pressure type for transmitting a motion of a movable member in the form of 
the eccentric cam 266 of the pump piston drive device 88 to a movable 
member of the form of the control pistons 236 of the first and second 
shut-off valves 226, 228, 230. 
Referring next to FIGS. 19 and 20, there will be described a fourth 
embodiment of the present invention wherein first and second shut-off 
valves are spool valves. In this embodiment, too, the first and second 
braking systems of the braking apparatus have the same construction. 
Therefore, only the first braking system including the front left wheel 
brake cylinder 22 and the rear right wheel brake cylinder 28 is shown in 
FIG. 19 and will be described by way of example. 
As shown in FIG. 19, one of the two pressurizing chambers of the master 
cylinder 12 is connected to the brake cylinder 22 of the front left wheel 
20 through fluid passages 352, 353, and to the brake cylinder 28 of the 
rear right wheel 26 through fluid passages 352, 354. A first shut-off 
valve 355 is provided in the fluid passage 352, and two second shut-off 
valves 357, 356 are provided in the fluid passages 353, 354, respectively. 
Since all of these three shut-off valves 255, 356, 357 have the same 
construction, only the first shut-off valve 355 will be described by 
reference to FIG. 20 by way of example. 
The first shut-off valve 355 has a valve housing 360 in which a spool 362 
is fluid-tightly and slidably received. The spool 362 has an annular 
groove 362 formed in an axially intermediate portion thereof, and a first 
land 366 and a second land 368 which are formed on the opposite sides of 
the annular groove 364. The spool 362 is fitted in the valve housing 360 
such that the first and second lands 366, 368 slidably contact the inner 
circumferential surface of the valve housing 360. 
One of the opposite end faces of the spool 362 cooperates with the valve 
housing 360 to define an operating pressure chamber 370, while the other 
end face of the spool 362 cooperates with the valve housing 360 to define 
a master cylinder pressure chamber 372 which communicates with the 
corresponding pressurizing chamber of the master cylinder 12 through a 
fluid passage 374. In the master cylinder pressure chamber 372, there is 
disposed an elastic member in the form of a spring 376, which serves as 
biasing means for biasing the spool 362 in a direction for abutting 
contact of the first land 366 with the bottom surface of the operating 
pressure chamber 370. 
The valve housing 360 has a port 378 communicating with the master cylinder 
12, and a port 380 communicating with the ports 378 of the second shut-off 
valves 356, 357. When the spool 362 biased by the spring 376 is placed in 
a first position of FIG. 20 in which the first land 366 is in abutting 
contact with the bottom surface of the operating pressure chamber 370, the 
two ports 378, 380 communicate with each other through the annular groove 
364. 
As shown in FIG. 20, the wheel brake cylinders 22, 28 are connected to the 
master cylinder 12 through respective by-pass passages 382, 382 which 
by-pass the first and second shut-off valves 355, 356, 357. In each of 
these by-pass passages 382, there is provided a pressure relief valve 384 
whose relief pressure is about 7.8 MPa (80 kg.multidot.f/cm.sup.2). 
A reciprocating pump device 388 is connected to a portion of the fluid 
passage 352 between the first shut-off valve 355 and the second shut-off 
valves 356, 357. The pump device 388 has the same construction as the pump 
device 170. First and second operating pistons 390, 392 are held in 
contact with the outer circumferential cam surface of the eccentric cam 82 
such that the points of contact of the pistons 390, 392 with the eccentric 
cam 82 are spaced 90.degree. from the point of contact of the pump piston 
74 with the eccentric cam 82, in the opposite circumferential directions 
of the eccentric cam 82. These first and second operating pistons 390, 392 
are fluid-tightly and slidably supported by cylinder housings 394, 396, 
respectively, such that the pistons 390, 392 cooperate with the cylinder 
housings 394, 396 to define variable-volume chambers 398, 400. The 
variable-volume chamber 398 is connected to the operating pressure chamber 
370 of the first shut-off valve 355 through a fluid passage 402, while the 
variable-volume chamber 400 is connected to the operating pressure 
chambers 370 of the second shut-off valves 357, 356 through respective 
fluid passages 404, 406. 
The fluid passages 402, 404, 406 are connected to the master cylinder 12 by 
respective fluid passages 410, 412, 414 in which are flow restrictors 416, 
418, 420 are provided, respectively. When the operating pistons 390, 392 
are slowly advanced, a relatively large portion (about 4/5) of the brake 
fluid delivered from the variable-volume chambers 398, 400 is supplied to 
the operating pressure chambers 370 of the shut-off valves 355, 356, 357, 
and a relatively small portion (about 1/5) of the delivered brake fluid is 
returned to the master cylinder 12 through the flow restrictors 416, 418, 
420. 
The solenoid-operated directional control valves 422, 424 and the pump 
drive motor 86 are controlled by an electronic control unit 430, to 
perform the anti-lock and traction control operations. 
An operation of the first braking system of the present braking apparatus 
will be described. 
When the brake pedal 10 is placed in its non-operated position, the spool 
362 of each shut-off valve 355, 356, 257 is held at its fully retracted 
position under the biasing action of the spring 376, with the first land 
366 in abutting contact with the bottom surface of the operating pressure 
chamber 370. In this position, the ports 378, 380 communicate with each 
other through the annular groove 364, so that the master cylinder 12 
communicates with the wheel brake cylinders 22, 28. 
When the brake pedal 10 is depressed to brake the front left and rear right 
wheels 20, 26, the pressure generated in the corresponding pressurizing 
chamber of the master cylinder 12 is applied to the wheel brake cylinders 
22, 28 through the first and second shut-off valves 355-357, whereby the 
wheels 20, 26 are braked. At this time, the master cylinder pressure is 
applied to the master cylinder pressure chamber 372 and operating pressure 
chamber 370 of each shut-off valve 355-357. However, since the pressures 
in these two chambers 372, 370 are equal to each other, the spool 362 is 
held in its fully retracted position of FIG. 20 by the spring 376, and the 
shut-off valves 355-357 are held open. 
If the depression force acting on the brake pedal 10 is excessive with 
respect to the friction coefficient of the road surface and the slip of 
the wheel 20, 26 exceeds an upper limit of an optimum range, the anti-lock 
control operation is initiated with the pump drive motor 86 rotated in the 
direction indicated by the solid line arrow in FIG. 19. As a result, the 
reciprocating pump device 388 performs alternate suction and delivery 
strokes of the brake fluid while the first and second operating pistons 
390, 392 are alternately advanced and retracted to alternately open and 
close the first and second shut-off valves 355-357. 
When the first operating piston 390 is advanced, the brake fluid is 
supplied to the operating pressure chamber 370 of the first shut-off valve 
355, so that the spool 363 is advanced whereby the ports 378, 380 are 
disconnected from each other by the first land 366. The spool 362 is 
further advanced after the disconnection of the ports 378, 380. When the 
first operating piston 390 is retracted, the brake fluid is discharged 
from the operating pressure chamber 370 into the variable-volume chamber 
398, so that the spool 362 is retracted, and the ports 378, 380 are 
brought into communication with each other, whereby the first shut-off 
valve 355 is opened. 
As the eccentric cam 82 is rotated in the direction of the solid line arrow 
of FIG. 19, the first and second shut-off valves 355-357 are opened and 
closed in timed or synchronized relationship with the suction and delivery 
of the pump device 388, as indicated in FIG. 10, so that the pressure in 
the wheel brake cylinder 22, 28 is reduced. When the excessive slip of the 
wheel 20, 26 is eliminated, the wheel cylinder pressure is increased by 
reversing the direction of rotation of the eccentric cam 82. In this case, 
the first and second shut-off valves 355-357 are opened and closed in 
timed relationship with the suction and delivery of the pump device 388, 
as indicated in FIG. 11, so that the pressurized fluid is supplied to the 
wheel brake cylinder 22, 28 to increase the wheel brake cylinder pressure. 
When it becomes necessary to hold the wheel brake cylinder pressure, the 
solenoid-operated directional control valve 422, 424 is switched to the 
second state for unidirectional fluid communication between the operating 
pressure chamber 370 and the variable-volume chamber 400 through the check 
valve 426, 428. In this state, the brake fluid supplied to the operating 
pressure chamber 370 of the second shut-off valve 356, 357 is not 
discharged, and the second shut-off valve 356, 357 is held closed. 
If the brake pedal 10 is moved toward its non-operated position while the 
wheel brake cylinder pressure is being increased or held, the pressure 
relief valve 384 is opened when the wheel brake cylinder pressure becomes 
higher than the master cylinder pressure by more than the predetermined 
relief pressure of the valve 384, whereby the brake fluid is returned from 
the wheel brake cylinder 22, 28 to the master cylinder 12, and the wheel 
brake cylinder pressure is reduced. 
When the anti-lock control operation is terminated, the rotation of the 
eccentric cam 82 is stopped, and the solenoid-operated directional control 
valves 422, 424 are restored to their original position of FIG. 19. 
Although the angular position of the eccentric cam 82 at which the 
eccentric cam 82 is stopped is not fixed and unknown, the brake fluid is 
returned from the operating pressure chambers 370 to the master cylinder 
12 through the fluid passages 410, 412, 414 and the flow restrictors 416, 
418, 420, since the brake fluid is no longer delivered from the 
variable-volume chambers 398, 400. Consequently, the first and second 
shut-off valves 355-357 are opened. 
If the anti-lock control operation is again performed with the brake pedal 
10 depressed, the pump drive motor 86 is turned on irrespective of the 
positions of the first and second operating pistons 390, 932 at which 
these pistons were stopped at the end of the previous anti-lock control 
operation. The spools 362 of the shut-off valves 355, 356, 357 are 
eventually moved following the movements of the first and second operating 
pistons 390, 392, so that the shut-off valves 355-357 are opened and 
closed in timed or synchronized relationship with the suction and delivery 
strokes of the pump device 388. 
The traction control operation is also initiated with the activation of the 
pump drive motor 86 so as to rotate the eccentric cam 82 in the direction 
indicated by the broken line arrow in FIG. 19, so that the brake fluid is 
supplied to the front left drive wheel 20. Since the rear right wheel is 
not a drive wheel, a supply flow of the brake fluid to the rear right 
wheel brake cylinder 28 is prevented by switching the solenoid-operated 
directional control valve 424 to its position for inhibiting the discharge 
flow of the brake fluid from the operating pressure chamber 370 of the 
second shut-off valve 356. As a result, the second shut-off valve 356 is 
closed when the second operating piston 392 is moved to its fully advanced 
position. Thus, the supply flow of the brake fluid to the rear right wheel 
brake cylinder 28 to brake the rear right wheel 26 is prevented. 
when it becomes necessary to reduce the front left wheel brake cylinder 22 
as a result of elimination of the excessive slip of the front left wheel 
20, the eccentric cam 82 is rotated in the direction indicated by the 
solid-line arrow in FIG. 19. If it becomes necessary to hold the pressure 
in the brake cylinder 22, the directional control valve 422 is switched to 
the second state for unidirectional fluid communication between the 
operating pressure chamber 370 and the variable-volume chamber 400 through 
the check valve 426. 
It will be understood from the above explanation of the fourth embodiment 
of this invention that the first and second pistons 390, 393, operating 
pressure chamber 370, fluid passages 402-406, and variable-volume chambers 
398, 400 constitute a motion transmitting device of hydraulic pressure 
type for transmitting a motion of a movable member in the form of the 
eccentric cam 82 of the pump device 88 to a movable member in the form of 
the spool 362 of the first and second shut-off valves 355-357. 
While the several presently preferred embodiments of this invention have 
been described in detail by reference to the accompanying drawings, it is 
to be understood that the invention is not limited to the details of the 
illustrated embodiments, but may be otherwise embodied. 
For instance, the rapidly pressure reducing device 202 used in the 
embodiments of FIGS. 8-20 may be used in the first embodiment of FIGS. 
1-7, in place of the rapidly pressure reducing device 66. Conversely, the 
rapidly pressure reducing device 66 may be used in the embodiments of 
FIGS. 8-20, in place of the rapidly pressure reducing device 202. 
However, the rapidly pressure reducing device is not essential, and may be 
eliminated, particularly where the reciprocating pump device has a 
relatively large capacity and is capable of sucking the brake fluid into 
the variable-volume chamber at a rate high enough to permit rapid 
reduction of the wheel brake cylinder pressure. 
In the third embodiment of FIGS. 12-18 and the fourth embodiment of FIGS. 
19-20, the see drive device as used for driving the pump pistons 74 in the 
first braking system may be used in the second braking system. Similarly, 
the same drive device as used for driving the first and second operating 
pistons 272, 274, 390, 392 in the first braking system may be used in the 
second braking system. 
Although the illustrated embodiments take the form a hydraulic braking 
pressure control device of a hydraulically operated braking apparatus of 
diagonal or X-crossing type wherein each of the first and second braking 
systems includes a front wheel brake cylinder and a rear wheel brake 
cylinder, the principle of the present invention is equally applicable to 
a hydraulic braking pressure control device of a hydraulically operated 
braking apparatus having a front braking system including front right and 
left wheel brake cylinders and a rear braking system including rear right 
and left wheel brake cylinders. 
It is to be also understood that the present invention may be embodied with 
suitable combinations of elements used in the different embodiments which 
have been described for illustrative purpose only. 
It is to be further understood that the present invention may be embodied 
with various other changes, modifications and improvements, which may 
occur to those skilled in the art, without departing from the spirit and 
scope of the invention defined in the following claims.