Control valve for controlling fluid passage

A piston in a first cylinder is driven as a piezoelectric element group, which includes a plurality of laminar piezoelectric elements stacked in layers, extends or contracts. A second cylinder is formed so as to connect with a first fluid pressure chamber which is defined in the first cylinder by the piston. A piston-shaped valve member is disposed in the second cylinder. The valve member is urged toward the first fluid pressure chamber by a spring. The first fluid pressure chamber is supplied with a fluid whose pressure is set by means of a selector valve. When the selector valve is closed, the fluid pressure chamber is defined as an independent compartment. In this state, high voltage is applied to the piezoelectric element group to elongate it, thereby driving the piston. Thus, the valve member is actuated by the fluid pressure inside the first fluid pressure chamber, and a fluid passage is opened and closed by the valve member.

BACKGROUND OF THE INVENTION 
The present invention relates to a control valve for controlling a fluid 
passage, and more specifically to a control valve effectively used as an 
fluid pressure passage control valve, adapted to open and close a passage 
for fluid pressure supplied from a master cylinder to, for example, wheel 
cylinders of a braking system of an automobile provided individually for 
wheels of the vehicle, and capable of executing braking force control, 
such as antiskid control, with high response characteristics. 
In a conventional braking system of an automobile, a master cylinder 
produces an fluid pressure in response to a force with which a brake pedal 
is worked, and the pressure is transmitted to wheel cylinders arranged 
individually for wheels of the vehicle. In each wheel cylinder, a brake 
piston is driven by the supplied fluid pressure so that a braking force 
responsive to the working force on the pedal acts on each corresponding 
wheel. 
In performing a braking operation on the braking system constructed in this 
manner, if a strong braking force is applied to a wheel, the wheel may 
sometimes lock and slip on a road surface. In such a case, it is necessary 
to execute antiskid control such that the slip between wheel and road 
surface is quickly removed to increase the contact resistance between 
them, thereby ensuring stable driving of the vehicle. The antiskid control 
may be effected by manually adjusting the working force on the brake 
pedal. It is desirable, however, that the braking force on the slipped 
wheel should automatically be reduced when a locked state of the wheel is 
detected. 
A system for such automatic antiskid control has conventionally been 
proposed. In this system, a valve mechanism is provided in each of 
hydraulic circuits arranged individually between a master cylinder and 
wheel cylinders, and the braking force is reduced by discharging braking 
fluid from that wheel cylinder which corresponds to the slipped wheel. 
Usually, a solenoid-operated valve is used as a fluid control valve for 
operating the hydraulic circuit. In this case, the fluid pressure passage 
is opened and closed electrically. In the solenoid valve, however, a 
solenoid coil for supplying exciting current has an inductor, so that the 
valve plug is actuated with a delay after the solenoid coil is energized. 
Thus, it is hard to obtain high response characteristics suited to 
antiskid control. 
In consideration of these circumstances, piezoelectric elements, as a 
high-response drive source, may be used in place of the solenoid 
mechanism. The coefficient of thermal expansion of these elements is as 
low as 1.times.10.sup.-6 /.degree. C., while those of a mechanism for 
supporting the elements and other members constituting the control valve 
range from 12.times.10.sup.-6 /.degree. C. to 23.times.10.sup.-6 /.degree. 
C. Thus, there is a considerable difference between these values. 
When using the control valve including the piezoelectric elements, 
therefore, if the temperature of the control valve section or that of a 
fluid as an object of control greatly changes, a difference in thermal 
displacement related to thermal expansion or contraction is caused between 
the piezoelectric elements and other members. Thus, even with the same 
control instruction, the opening of the valve plug of the control valve 
varies with temperature conditions. It is therefore difficult to 
constantly maintain satisfactory conditions for high-accuracy control. 
More specifically, when using the control valve with the piezoelectric 
elements as its drive source in a motor vehicle, the working temperature 
of the valve widely ranges from -30.degree. to +120.degree. C. 
Accordingly, the control valve cannot easily maintain its normal 
functions. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a control valve for 
opening and closing a fluid passage, enjoying satisfactory response 
characteristics in operation and adapted for effective control of, e.g., 
hydraulic circuits for antiskid control in a braking system. 
Another object of the invention is to provide a control valve specifically 
using piezoelectric elements as its drive source for higher response 
characteristics, and capable of maintaining conditions for high-accuracy 
control over a wide temperature range. 
Still another object of the invention is to provide a control valve 
specially designed for miniaturization and capable of opening and closing 
a fluid passage with a satisfactory response characteristic and high 
accuracy, thus ensuring high-speed, high-accuracy fluid control of 
hydraulic circuits of a braking system for passenger vehicles and various 
industrial machines. 
In a control valve for operating a fluid passage according to the present 
invention, a piston in a cylinder is driven by a piezoelectric element 
group which includes a plurality of laminar piezoelectric elements stacked 
in layers, and a compressed control fluid is supplied to a fluid pressure 
chamber which is defined in the cylinder by the piston. A voltage is 
applied to the piezoelectric elements so that the element group extends or 
contracts in accordance with the applied voltage to control the piston in 
movement. The capacity of the fluid pressure chamber is changed in 
response to the movement of the piston. A piston-shaped valve member is 
inserted in another cylinder which connects with the fluid pressure 
chamber. As the chamber changes in capacity, the valve member is driven to 
control the fluid passage. 
According to the control valve constructed in this manner, if high voltage 
is applied to the piezoelectric elements constituting the piezoelectric 
element group, the element group is extended in accordance with the 
applied voltage, so that the piston is driven in a direction to reduce the 
capacity of the fluid pressure chamber. As a result, the valve member is 
moved away from the chamber, and the fluid passage is controlled by a 
valve plug which is formed integrally on the valve member. 
Since the fluid pressure chamber is pressurized with fluid to a 
predetermined pressure, the fluid pressure inside the chamber can be kept 
at a predetermined level, despite the difference in the coefficient of 
thermal expansion between the piezoelectric element group and members 
constituting the piston section, or change of the chamber capacity caused 
by temperature change, if any. In response to the moved distance of the 
piston in the cylinder, the valve member is driven for the control of the 
fluid passage without being influenced by temperature change.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a fundamental embodiment of a control valve for controlling a 
fluid passage. In FIG. 1, first cylinder 11 is formed inside case 12 made 
of, e.g., metal. First piston 13 is movably fitted in cylinder 11. Piston 
13 is supported by piezoelectric element group 14. Spring mechanism 15 is 
used to urge piston 13 toward element group 14. Thus, piston 13 is located 
in a position depending on the length of group 14, defining fluid pressure 
chamber 16 in cylinder 12. 
Piezoelectric element group 14 is a laminate structure formed of a 
plurality of laminar piezoelectric elements 141, 142, . . . . Electrode 
plates (not shown) are sandwiched between elements 141, 142, . . . , so 
that the elements are connected in parallel with one another by means of 
the plates. Voltage signals are applied to the parallel-connected elements 
141, 142, .... If supplied with high voltage, the elements are extended in 
thickness to elongate element group 14. 
More specifically, if high voltage is applied to piezoelectric element 
group 14, the group is extended to move piston 13 against the urging force 
of spring mechanism 15 in a direction such that fluid pressure chamber 16 
is reduced in capacity. If the voltage applied to group 14 is lowered, the 
element group is shortened, so that piston 13 is moved by the urging force 
of mechanism 15 in a direction to increase the capacity of chamber 16. 
Second cylinder 17 smaller in diameter than piston 13 is formed coaxial 
with first cylinder 11, opening into fluid pressure chamber 16. Cylinder 
17 is penetrated by valve member 18 which constitutes a second piston 
whose active area is smaller than that of the first piston. Member 18 is 
urged toward chamber 16 by spring 19. 
If high voltage is applied to piezoelectric element group 14 so that fluid 
pressure chamber 16 is reduced in capacity by piston 13, valve member 18 
is actuated against the urging force of spring 19. If element group 14 is 
contracted without the supply of any voltage, member 18 is urged toward 
chamber 16 by spring 19. 
As valve member 18 moves corresponding to the contraction or extension of 
piezoelectric element group 14, valve plug 22 opens or closes the fluid 
passage between first and second ports 20 and 21. More specifically, if 
high voltage is applied to element group 14, member 18 is moved from the 
position shown in FIG. 1 against the urging force of spring 19, so that 
plug 22 closes the fluid passage. If no voltage is applied to group 14, 
member 18 is located in the position of FIG. 1, so that the passage 
between first and second ports 20 and 21 is opened. 
A suitable high-pressure fluid is supplied to fluid pressure chamber 16 
through selector valve 23. In this embodiment, the fluid is fed from 
second port 21 so that the fluid pressure inside chamber 16 is adjusted to 
a specified level. Valve 23 is switched in accordance with a pilot 
pressure. The pilot pressure is produced when high voltage is applied to 
piezoelectric element group 14 so that piston 13 is moved in the direction 
to reduce the capacity of fluid pressure chamber 16. Thus, the pilot 
pressure causes chamber 16 to become a closed container. 
According to a control valve constructed in this manner, when it is in a 
normal state such that high voltage is not applied to piezoelectric 
element group 14, the element group is contracted, and the fluid passage 
between first and second ports 20 and 21 is open, as shown in FIG. 1. 
Thereupon, the fluid from second port 21 is fed into fluid pressure 
chamber 16 through selector valve 23, and the fluid pressure inside 
chamber 16 is adjusted to a value such that valve member 18 is not 
actuated by the urging force of spring 19. 
In this state, if an instruction for a closure of the control valve is 
given, the pilot pressure is produced correspondingly, so that selector 
valve 23 is closed. As a result, high voltage is applied to piezoelectric 
elements 141, 142, ... which constitute piezoelectric element group 14. 
Thus, element group 14 is extended, and piston 13 is driven in the 
direction to reduce the capacity of fluid pressure chamber 16. In this 
case, since valve 23 is closed, the fluid pressure inside chamber 16 rises 
as piston 13 moves. Accordingly, valve member 18 is driven against the 
urging force of spring 19, so that the fluid passage connecting first and 
second ports 20 and 21 is closed by valve plug 22. 
In this control valve, if the ambient temperature greatly increases, the 
space between piston 13 and valve member 18 and the capacity of fluid 
pressure chamber 16 change due to the substantial difference in the rate 
of thermal expansion between the materials of element group 14 and case 
12. However, since the fluid under a predetermined pressure is continually 
fed into chamber 16 through selector valve 23, the fluid pressure inside 
chamber 16 can be kept constant without being influenced by the 
temperature change. Thus, if valve 23 is closed and high voltage is 
applied to element group 14 to actuate piston 13, the action of piston 13 
is precisely reciprocated in that of valve member 18, permitting accurate 
switching control of the fluid passage. 
FIG. 2 shows a second embodiment of the control valve of the invention. 
This valve is used, for example, to switch hydraulic circuits of a braking 
system of a vehicle. In FIG. 2, seat member 25 and guide member 26 are 
fixedly arranged side by side in first cylihder 11. Fluid pressure chamber 
16 is defined between member 25 and piston 13 which is moved by 
piezoelectric element group 14. Piston 13 is urged toward element group 14 
by spring mechanism 15. 
Arranged in series as aforesaid, seat member 25 and guide member 26 are 
formed with coaxial second cylinder 17. Valve member 18 is movably 
inserted in cylinder 17. Member 18 is formed with fluid passage 27 along 
its central axis. Check valve member 28 is formed on member 18 on the side 
of fluid pressure chamber 16, and piston member 29 on the opposite side. 
Member 28, which includes steel ball 281 and spring 282, allows the fluid 
to flow only from passage 27 to chamber 16. A narrow gap 283 is defined 
between the outer peripheral surface of member 28 and second cylinder 17. 
The fluid produces a small flow between passage 27 and chamber 16 through 
gap 283. 
Piston member 29 includes tapered valve plug 291. Outlet passage 30 is 
formed between seat member 25 and guide member 26, corresponding in 
position to plug 291. Valve member 18 is urged toward fluid pressure 
chamber 16 by spring 19 so that a slanting surface of plug 291 abuts 
against an edge portion of member 25. A selector passage is defined 
between outlet passage 30 and branch passage 271 around valve member 18. 
Valve plug 291 selectively blocks the selector passage as plug 291 abuts 
against member 25. Thus, passage 30 is disconnected from branch passage 
271 which diverges from fluid passage 27 in member 18. 
Second fluid pressure or pilot chamber 31 is defined beside piston member 
29 of valve member 18 so that the fluid in chamber 31 is fed into fluid 
passage 27 of member 18. In this case, restriction 272 for restricting the 
fluid flow is formed in that portion of passage 27 which corresponds to 
piston member 29. Fluid inlet hole 32 is formed in chamber 31, and outlet 
hole 33 in outlet passage 30. 
According to the control valve constructed in this manner, if no voltage is 
applied to piezoelectric element group 14 so that group 14 is contracted, 
valve member 18 is urged toward first fluid pressure chamber 16 by spring 
19, cutting off fluid passage 27 from outlet passage 30, as shown in FIG. 
2. 
Here let it be supposed that inlet passage 32 of the control valve is 
supplied with high-pressure braking fluid from, for example, a master 
cylinder of a braking system for a vehicle. Thereupon, the fluid pressure 
acts on steel ball 281 of check valve member 28 through restriction 272. 
As a result, valve 28 is opened against the urging force of spring 282 to 
allow the braking fluid to be fed into first fluid pressure chamber 16. In 
this case, the fluid pressure produces, at valve plug 291, a force 
opposite in direction to the force to close passage 32 and acting on valve 
member 18. Since this force can be made substantially equivalent to an 
fluid pressure acting in second fluid pressure chamber 31, member 18 can 
be held in the position shown in FIG. 2. 
If a high voltage of, e.g., 300 to 500 volts is applied to piezoelectric 
element group 14 in this state, group 14 extends. As a result, piston 13 
is moved in the direction to reduce the capacity of first fluid pressure 
chamber 16, against the urging force of spring mechanism 15. Valve member 
18 is moved at the same time. Accordingly, valve plug 291 separates from 
the edge portion of seat member 25 to form a passage between fluid passage 
27 and outlet passage 30, thereby connecting inlet and outlet holes 32 and 
33. 
When the fluid passage is thus formed to permit a fluid flow from inlet 
hole 32 to outlet hole 33, the pressure on the lower-course side of 
restriction 272 in the passage becomes lower than that on the upper-course 
side. Accordingly, the fluid in first fluid pressure chamber 16, whose 
internal pressure is equal to the pressure on the upper-course side of 
restriction 272, starts to flow through gap 283 into fluid passage 27. 
Valve member 18 is then driven by the urging force of spring 19 to cut off 
passage 27 from outlet passage 30. 
In this state, if the high voltage applied to piezoelectric element group 
14 is removed, the element group, which has so far been in the extended 
state, is contracted, allowing piston 13 to move in the direction to 
increase the capacity of first fluid pressure chamber 16. In this case, 
the fluid is fed from fluid passage 27 into first fluid pressure chamber 
16 through check valve member 28. 
Here it is to be supposed that a specified voltage is pulsatively applied 
to piezoelectric element group 14, and that the pulse duration is t.sub.1 
which is shorter than time interval T required for the extension of 
element group 14 to its maximum length L. Then, every time the pulsative 
voltage rises, piston 13 is driven in the direction to reduce the capacity 
of first fluid pressure chamber 16. When piston 13 is moved by distance 
l.sub.1 (&lt;L), the voltage supplied to element group 14 falls, so that the 
group contracts to move piston 13 in the direction to increase the 
capacity of chamber 16. In this case, the time interval for the impression 
of the voltage on element group 14 is short, so that there is no time for 
the fluid to flow from chamber 16 into fluid passage 27 through gap 283. 
When the applied voltage falls, therefore, the fluid in passage 27 flows 
through check valve member 28 into chamber 16. Then, valve member 18 is 
moved toward first fluid pressure chamber 16 with delay. 
Thus, if the pulsative voltage is applied to piezoelectric element group 14 
at, e.g., 100 to 500 Hz, the fluid pressure inside first fluid pressure 
chamber 16 is kept at a level corresponding to the extended state caused 
when the voltage with pulse duration t.sub.1 is supplied. Also, valve 
member 18 is kept in the moved position at distance l.sub.1. As a result, 
the fluid passage area between inlet and outlet holes 32 and 33 is set to 
correspond to duration t.sub.1, so that the quantity of fluid flowing 
through the fluid passage corresponds to the pulse duration. Thus, if the 
pulse duration is set to t.sub.2 (.ltoreq.T) longer than t.sub.1, valve 
member 18 is moved distance l.sub.2, and the quantity of the fluid 
delivered to outlet hole 33 increases. 
Thus, the fluid passage between inlet and outlet holes 32 and 33 can be 
opened and closed by controlling the impression of the pulsative voltage 
on piezoelectric element group 14 by means of control circuit 34. Also, 
the opening of the passage can be varied by controlling the pulse duration 
and pulse frequency. In this case, the time interval between the voltage 
supply to element group 14 and the extension of the group to a 
predetermined degree, i.e., response speed, is approximately 0.0005 
second. Thus, there may be provided a fluid control valve with a very high 
response characteristic. 
FIG. 3 shows a specific configuration of an antiskid system of a vehicle 
braking device using control valves 100a and 100b as described above. In 
normal braking control, fluid pressure is produced in master cylinder 300 
by working brake pedal 200, and is supplied to wheel cylinder 500 through 
solenoid valve 400. As a result, a braking force corresponding to the 
force applied to pedal 200 is produced in wheel 900 which corresponds to 
cylinder 500. At this time, valves 100a and 100b are closed. When pedal 
200 is released, the fluid pressure inside cylinder 500 is returned to 
cylinder 300 through check valve 800, so that the braking force is removed 
from wheel 900. 
If wheel 900, with brake pedal 200 down, locks and begins to cause 
skidding, this situation is detected by, for example, a wheel speed sensor 
(not shown). Thus, solenoid valve 400 is closed, and hydraulic pump 700 is 
actuated. 
A detection signal indicative of the skidding gives an instruction to the 
control circuit of control valve 100b such that the valve is opened to 
feed braking fluid from wheel cylinder 500 into reservoir 600. Thus, the 
fluid pressure inside cylinder 500 is lowered, so that wheel 900 unlocks. 
When the rotating speed of wheel 900 increases to a predetermined value or 
more, control valves 100b and 100a are closed and opened, respectively. 
Thereupon, fluid pressure produced by hydraulic pump 700 is fed into wheel 
cylinder 500 via valve 100a, and the braking force acting on wheel 900 
increases again. Thus, wheel 900 is adjusted to a proper skid for optimum 
braking force control. 
As described above, the fluid pressure inside wheel cylinder 500 can be 
regulated by selectively operating control valves 100a and 100b. The 
changing pressure may be controlled on the basis of the opening of valves 
100a and 100b, that is, the frequency and pulse width of control pulse 
voltage applied to the valves. To attain this, a voltage of 300 volts is 
applied to the piezoelectric element groups of valves 100a and 100b at 200 
or 400 Hz. Pulse voltages at 200 and 100 Hz may alternatively be applied 
to valve 100a with valve 100b closed. Quick and slow compression modes for 
cylinder 500 are established by applying the pulse voltages at 200 and 100 
Hz, respectively, to valve 100a. 
Moreover, a quick or slow decompression mode for wheel cylinder 500 is 
established alternatively by applying the pulse voltage of 200 or 100 Hz 
to control valve 100b with valve 100a closed. 
FIGS. 4A and 4B show control voltages supplied to the piezoelectric element 
groups of control valves 100a and 100b, respectively. The fluid pressure 
in wheel cylinder 500 is changed as shown in FIG. 4C by operating valves 
100a and 100b in accordance with voltage signals indicative of the control 
voltages. Thus, the braking force acting on wheel 900 is controlled in 
accordance with the fluid pressure. 
FIG. 5 shows a third embodiment of the present invention. In FIG. 5, like 
reference numerals are used to designate like components as shown in FIG. 
2. In this embodiment, check valve member 28 and the fluid passage are 
formed independently of valve member 18. The fluid from inlet hole 32 is 
supplied to member 28 through first restriction 35, and then to first 
fluid pressure chamber 16. Chamber 16 communicates with outlet passage 30 
by means of second restriction 36, which serves in the same manner as gap 
283 of the control valve of FIG. 2. In the control valve of the third 
embodiment, as in the one shown in FIG. 2, the fluid passage between inlet 
and outlet holes 32 and 33 is opened or closed by controlling the voltage 
applied to piezoelectric element group 14. 
FIG. 6 shows a fourth embodiment used in a braking system capable of 
antiskid control as shown in FIG. 3. In this embodiment, two control 
valves 100a and 100b shown in FIG. 2 are combined together. Inlet and 
outlet holes 322 and 332 of valve 100b are connected to wheel cylinder 500 
and reservoir 600, respectively. Inlet hole 321 of valve 100a is connected 
to master cylinder 300, while outlet hole 331 communicates with second 
fluid pressure chamber 162 of valve 100b by means of fluid passage 37. 
FIG. 7 shows another example of a vehicle braking system using the control 
valve. In this case, the skidding control of rear wheels Rr and Rl as 
driving wheels and front wheels Fr and Fl as driven wheels is performed by 
means of three normally-open control valves 41a, 41b and 41c and three 
normally-closed control valves 42a, 42b and 42c. 
Fluid pressure produced in master cylinder 43 in response to the operation 
of brake pedal 44 is supplied through control valves 41a to 41c to wheel 
cylinders WC which constitute braking systems of the individual wheels. 
Thus, a braking force responsive to the operation of pedal 44 acts on each 
wheel independently. 
Fluid pressures of wheel cylinders WC set for the individual wheels are 
released by means of pumps 45a and 45b when control valves 42a to 42c are 
open. Valves 42a to 42c serve to reduce the braking force. 
If right-hand rear wheel Rr, for example, locks and causes a slip hetween 
itself and a road surface when brake pedal 44 is worked, the locking of 
the wheel is detected by a wheel rotation sensor (not shown) or the like, 
and an antiskid control instruction for wheel Rr is given. The control 
instruction closes control valve 41c which is set in a hydraulic circuit 
for feeding fluid pressure to the wheel cylinder of wheel Rr. Thus, 
cylinder WC of wheel Rr is isolated from master cylinder 43 by valve 41c. 
As a result, the fluid pressure inside cylinder WC rises to a high level 
corresponding to the force on brake pedal 44, which opens valve 42c. 
Thereupon, the fluid pressure inside cylinder WC of wheel Rr decreases and 
unlocks the wheel. 
In this embodiment, the two rear wheels are controlled in common for the 
braking force by control valves 41c and 42c, in consideration of the 
straight-advancing characteristics the vehicle, in particular. 
Control valves 41a to 41c and 42a to 42c are controlled by instructions 
from an electronic control unit (not shown in detail) which is formed of a 
microcomputer or the like. The control unit is supplied with signals from 
rotating speed sensors of the individual wheels, and monitors the wheels 
for slip check. If a slip or locking of a wheel is detected, the control 
valve corresponding to the slipped wheel is controlled in the aforesaid 
manner, thereby unlocking the wheel. When the wheel unlocks, it is 
supplied again with fluid pressure from master cylinder 43 to increase the 
braking force to a proper skidding condition. Thus, braking efficiency is 
improved. 
FIG. 8 shows a fifth embodiment of the control valve adapted to be 
effectively used in the aforementioned braking system. In FIG. 8, like 
reference numerals are used to designate like components as included in 
the embodiment shown in FIG. 2. 
The control valve of this embodiment is a normally-open valve which may 
constitute any of control valves 41a to 41c. In this valve, valve member 
18 includes valve plug 47 for connecting and disconnecting first and 
second ports 45 and 46 which communicate with inlet and outlet holes 32 
and 33, respectively, as member 18 moves axially. Inlet hole 32 is 
connected to the master cylinder, and outlet hole 33 to the wheel 
cylinder. Thus, the control valve switches hydraulic circuits between the 
master and wheel cylinders. 
In this control valve, moreover, if high voltage is applied to 
piezoelectric element group 14 so that the element group is extended to 
reduce the capacity of first fluid pressure chamber 16, valve member 18 is 
moved against the urging force of spring 19 in a manner such that valve 
plug 47 cuts off fluid passage between first and second ports 45 and 46. 
Also, the control valve is formed with pilot fluid pressure chamber 48. 
Chamber 48 is supplied with a compressed fluid from pilot control hole 49, 
which is fed with fluid pressure from, e.g., the master cylinder. Thus, a 
high fluid pressure is established in fluid pressure chamber 48 especially 
when the brake pedal is worked strongly. 
Pilot fluid pressure chamber 48 communicates with first fluid pressure 
chamber 16 by means of fluid leak passage 50 which constitutes a 
restriction mechanism for limiting the flow quantity. Passage 50 contains 
pilot piston 51 therein. When the fluid inside chamber 48 rises, piston 51 
is driven toward chamber 16 against the urging force of spring 52, so that 
passage 50 is closed by valve plug 511 of piston 51. 
Pilot piston 51 is formed with narrow passage 512 as a restriction 
extending along its axis. When passage 50 is opened by valve plug 511 of 
piston 51, the fluid in pilot fluid pressure chamber 48 is fed into first 
fluid pressure chamber 16 at a limited flow rate. 
Thus, according to the control valve of this embodiment, when fluid 
pressure is produced in the master cylinder by operating, e.g., the brake 
pedal, it is transmitted through first and second ports 45 and 46, and fed 
into the wheel cylinder for a braking operation. 
During this normal braking operation, piezoelectric element group 14 is 
contracted as shown in FIG. 8, without being supplied with any high 
voltage. Therefore, the pressure inside first fluid pressure chamber 16 is 
equal to that inside pilot fluid pressure chamber 48. 
In this state, if the brake pedal is strongly worked to greatly increase 
the fluid pressure of the master cylinder, the fluid pressure of pilot 
fluid pressure chamber 48 also greatly increases to drive pilot piston 51 
against the urging force of spring 52. Thus, fluid leak passage 50 is 
closed, so that first fluid pressure chamber 16 is defined as a 
compartment. If any of the wheels is locked by the powerful braking 
action, high voltage is applied to piezoelectric element group 14 to 
elongate the same. As the element group extends in this manner, the fluid 
pressure of chamber 16 rises to drive valve member 18 against the urging 
force of spring 19. As a result, the fluid passage between first and 
second ports 45 and 46 is closed. 
Thus, if a braking operation with a certain intensity or more is executed 
to cause a slip of a wheel or wheels, fluid leak passage 50 is closed by 
pilot piston 51, so that first fluid pressure chamber 16 is defined as an 
independent compartment. When a slip is detected, high voltage is applied 
to piezoelectric element group 14 to effectively actuate valve member 18. 
In the braking system shown in FIG. 7, there are used control valves 42a to 
42c which normally are closed and are adapted to be opened when given an 
instruction, as well as normally-open control valves 41a to 41c as 
described in connection with the foregoing embodiment. FIG. 9 shows a 
configuration of one such normally-closed control valve. Basically, the 
valve of FIG. 9 has the same construction as the one shown in FIG. 8. The 
difference between the two valves lies only in the structure of valve 
member 18. In FIG. 9, valve plug 471 of valve member 18 closes the fluid 
passage between first and second ports 45 and 46 when it is urged toward 
first fluid pressure chamber 16 by spring 19. When fluid leak passage 50 
is closed and if high voltage is applied to piezoelectric element group 
14, therefore, valve member 18 is driven against the urging force of 
spring 19. Thus, a fluid passage is formed between first and second ports 
45 and 46. 
In the fifth and sixth embodiments described above, pilot fluid pressure 
chamber 48 and first fluid pressure chamber 16 are connected by fluid leak 
passage 50 which contains pilot piston 51 therein. As in a seventh 
embodiment shown in FIG. 10, however, passage 50 may be only disposed 
between chambers 48 and 16. In this control valve, the fluid pressure 
inside first fluid pressure chamber 16 varies following that inside pilot 
fluid pressure chamber 48 with delay. Thus, if high voltage is applied to 
piezoelectric element member 14 to drive piston 13, valve member 18 is 
actuated in response to the action of piston 13. 
FIG. 11 shows an eighth embodiment of the invention, in which pilot fluid 
pressure chamber 48 and first fluid pressure chamber 16 are connected by 
fluid leak passage 50, and check valve 53 is disposed parallel to passage 
50. In this case, valve 53 allows the fluid to flow only from chamber 48 
to chamber 16. If the pressure of chamber 48 rises, that of chamber 16 
follows it without delay. If high voltage is applied to element group 14 
to move piston 13 in the direction to reduce the capacity of chamber 16, 
valve member 18 is driven with a satisfactory response characteristic. 
In the embodiments described above, the valve member is switched with the 
piezoelectric element group extended by the impression of high voltage. 
Alternatively, however, it may be switched when the element group is 
contracted. FIG. 12 shows a further embodiment based on this concept. 
Basically, this embodiment is the same as the one shown in FIG. 1. In FIG. 
12, valve member 18 is pushed out of first fluid pressure chamber 16 by 
spring 55 so as to close the fluid passage between first and second ports 
20 and 21. 
Normally, in this case, high voltage is applied to piezoelectric element 
group 14 to keep it elongated. In switching the control valve, selector 
valve 23 is closed, and the voltage applied to element group 14 is then 
cut off to contract the group. As a result, piston 13 is moved by spring 
mechanism 15 in the direction to increase the capacity of first fluid 
pressure chamber 16. Thus, valve member 18 is driven against the urging 
force of spring 55, defining the fluid passage between first and second 
ports 20 and 21. 
According to this embodiment, first fluid pressure chamber 16 normally 
cannot be supplied with any fluid pressure from second port 21, so that it 
is fed with a valve 23. Spring mechanism 15 for urging piston 13 in 
chamber 16 toward piezoelectric element group 14 is strong enough to 
actuate valve member 18 against the urging force of spring 55.