Volume controlling device for variable volume pump

The device of this invention has a variable capacity pump, a channel through which the discharged fluid from the pump passes toward an actuator, a variable choke located in the channel and pressure detecting means which detects the pressure difference on opposite sides of the variable choke. The pump varies its volume in relation with the pressure difference detected by the pressure detecting means. The opening area of the channel is varied by the variable choke in relation with the pressure of the fluid being supplied to the actuator, namely the pressure of the fluid flowing downstream of the variable choke. The device of this invention may also have an auxiliary variable choke located in the channel. The auxiliary variable choke can control the amount of fluid being supplied to the actuator in response to the operating condition of the actuator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a volume controlling device for a variable volume 
pump, especially this invention has a special usage for using as an 
automotive power steering device. The amount of fluid discharged from the 
present pump changes as the fluid needs of the power steering device 
change. 
2. Description of the Prior Art 
Many kinds of volume controlling devices have been proposed, especially 
which vary pump volume while maintaining good pump efficiency. For 
example, the device described in Japanese patent KOKAI (laid-open 
publication), 58-110881 maintains the product of the output pressure and 
amount of fluid discharged constant while the device varies the pump 
volume in order to maintain pumping efficiency. 
This conventional device, however, needs a special choke located in a 
special controlling channel. This controlling channel is different from 
the main channel through which the discharged fluid from the pump passes. 
Thus, the special choke needs the special controlling channel. 
Furthermore, the special choke needs working fluid for signaling. Since 
the special working fluid for the special choke should increase in 
relation with the amount of the discharged fluid, the special choke has 
the problem for wasting pump energy. 
SUMMARY OF THE INVENTION 
This invention has the object of controlling the amount of the discharged 
fluid without any special channel for the choke. This device can control 
the amount of the discharged fluid using the actual amount thereof as the 
signal for controlling the choke. 
Another object of this invention is to control the amount of the discharged 
fluid while maintaining good pump efficiency. 
Still another object of this invention is to control the amount of the 
discharged fluid while the product of the discharged pressure and the 
amount of the discharged fluid is kept constant. 
A further object of this invention is to control the amount of fluid which 
is introduced into an actuator by varying the volume of the pump and 
varying the opening area of the choke. 
An additional object of this invention is to control the amount of fluid 
being introduced into the actuator more carefully by using an auxiliary 
choke. 
This invention has another object of controlling the actuator smoothly even 
when the amount of the discharge fluid from the pump becomes minimum. 
According to this invention, these objects are achieved by calculating the 
amount Q of fluid being introduced into the actuator from the opening area 
S of the channel through which the fluid passes and the pressure 
difference .DELTA.P in the channel upstream and downstream of the choke, 
as described by the formula (1). 
EQU Q=S.multidot..DELTA.P.sup.1/2 ( 1) 
Therefore, the device of this invention maintains the pressure difference 
.DELTA.P constant by varying the volume of the pump, and the opening area 
S of the channel is controlled by the choke which varies the opening area 
in relation with the pressure of the fluid being introduced into the 
actuator. 
This invention includes a pump, which can vary its volume, a channel 
through which the discharged fluid from the pump passes towards an 
actuator, a variable choke which is located in the channel and pressure 
detecting means which detects the pressure difference across the variable 
choke. The pump varies its volume in relation with the pressure difference 
.DELTA.P which is detected by the pressure detecting means. The opening 
area of the channel is varied by the variable choke in relation to the 
pressure of the fluid being introduced into the actuator, namely the 
pressure of the fluid flowing downstream of the variable choke. 
The device of this invention may further have an auxiliary variable choke 
located in the channel. The auxiliary variable choke can control the 
amount of the fuel being introduced into the actuator by a signal other 
than the pressure of the fluid. So that the amount Q of the fluid can be 
controlled more carefully. 
The variable choke of this invention may have a bypass passage through 
which a little of the fluid passes. This bypass passage has the following 
special advantage. The amount of the discharged fluid becomes smaller when 
the opening area of the variable choke becomes smaller, because the pump 
varies its volume in accordance with the signal from the pressure 
detecting means. It is well known that the pressure of the discharged 
fluid is varied by the movement of a piston of the pump, and the influence 
of variations in the discharged pressure becomes serious when the pump is 
operated at a small volume. In other words, the varying of the discharged 
pressure influences the actuator, so that the actuator can not operate 
smoothly. The fluid from the pump of this invention can always flow 
through the bypass passage in order to reduce the influence of the varying 
of the discharged pressure.

DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS 
In FIG. 1, a pump 100 discharges working fluid through a channel 200 toward 
a driven actuator 300. Actuator 300 of this embodiment is, for example, a 
power cylinder of an automotive power steering device. Valve 301 switches 
the working fluid being introduced into actuator 300. 
A variable choke 400 is located in channel 200. Variable choke 400 varies 
its opening area in accordance with the pressure of the working fluid 
being introduced into actuator 300, namely the pressure Pi of the fluid 
downstream of variable choke 400. Controlling passage 401 transmits 
pressure Pi of the fluid to variable choke 400. 
A variable capacity means 500 varies the volume of pump 100 and includes 
first and second drive pistons 501 and 502. Variable capacity means 500 is 
controlled by an oil pressure signal from a pressure detecting means 600. 
When high pressure is introduced into first drive piston 501 and low 
pressure is introduced into second drive piston 502, variable capacity 
means 500 varies the volume of pump 100 in order to increase the pump 
volume. When low pressure is introduced into first drive piston 501 and 
high pressure is introduced into second drive piston 502, variable 
capacity means 500 reduces the pump volume. 
Pressures on both the front side and rear side of variable choke 400 are 
introduced into pressure detecting means 600 via first and second signal 
passes 601 and 602. Therefore, pressure detecting means 600 detects the 
pressure difference of the front side and the rear side of variable choke 
400. 
FIG. 2 shows the device shown in FIG. 1 in slightly more detail. Pump 100 
is driven by an automotive engine 700 through a magnetic clutch 701. Pump 
100 takes working fluid from a reservoir 750 through a suction pass 751. 
Variable choke 400 has a control piston 404 reciprocably disposed in a 
cylinder 403. Cylinder 403 is provided in channel 200. Control piston 404 
has a passing area 405 which faces channel 200, so that the area of 
passing area 405 facing to channel 200 is varied in accordance with the 
movement of control piston 404. The opening area of variable choke 400 
becomes larger when a sufficient part of passing area 405 faces channel 
200. The opening area of variable choke 400 becomes smaller when passing 
area 405 does not face channel 200 as entirely. 
Control piston 404 is reciprocated horizontally as illustrated in FIG. 2 by 
the pressure difference between the working fluid in a pressure chamber 
406 defined by cylinder 403 and the rear edge of control piston 404 and 
the predetermined pressure of a spring 407 provided at the outer edge of 
the piston. The pressure Pi of the working fluid downstream of variable 
choke 400 is introduced into pressure chamber 406 via controlling passage 
401. Therefore, control piston 404 moves to the right in FIG. 2 when the 
pressure Pi exceeds the predetermined pressure of spring 407, so that the 
opening area of the channel 200 decreases. 
Pressure detecting means 600 has a pressure chamber 610 provided in a 
housing, a switching valve portion 611 provided in pressure chamber 610, 
an introducing pressure port 612 through which high pressure is introduced 
into pressure chamber 610, drain ports 613 and 614 through which the 
pressure in pressure chamber 610 is released, a first pressure passage 620 
which connects with first drive piston 501 and a second pressure passage 
621 which connects with second drive piston 502. The pressure Pi of the 
working fluid downstream of variable choke 400 is introduced into a first 
pressure chamber 630 which is located on the right side of pressure 
chamber 610 in FIG. 2 via first signal passage 601. The pressure Pd of the 
working fluid upstream of variable choke 400 is introduced into a second 
pressure chamber 631 which is located on the left side of pressure chamber 
610 in FIG. 2 via second signal passage 602. A spring 640 having a 
predetermined pressure is located in first pressure chamber 630. 
Therefore, switching valve portion 611 is reciprocated within pressure 
chamber 610 by the pressure difference .DELTA.P between the pressure in 
first signal passage 601 and that in second signal passage 602 and the 
predetermined pressure of spring 640. A first and a second connecting area 
650 and 651 is provided in switching valve portion 611, so that the 
switching valve portion can switch between introducing pressure port 612 
and first and second pressure passages 620 and 621 and between drain ports 
613 and 614 and first and second pressure passages 621 and 622. 
FIGS. 4 and 5 show pump 100. A shaft 101 is rotated by automotive engine 
700 and is supported by a bearing 102 located on a housing 103. A pintle 
104 is connected with a main housing 900 via an O ring. A cylindrical 
working area 106 is defined by main housing 900, pintle 104 and housing 
103. A rotary ring 107 is provided in working chamber 106 and includes an 
outer race 108 which is supported by main housing 900, an inner race 109 
and a number of balls 110 which are provided between outer race 108 and 
inner race 109. 
Pintle 104 has a convexing portion 111 which convexes toward working area 
106. Convexing portion 111 supports a rotor 113 via a bush 112 in such a 
manner that rotor 113 can rotate freely. Rotor 113 is rotated with shaft 
101. Six cylinders 14 are formed in rotor 113, and are provided with 
pistons 115 that can reciprocate there within. Springs 116 force pistons 
115 outwardly, so that the top edge of pistons 115 always contact inner 
race 109. 
A suction passage 120 and a discharge passage 121 are formed in pintle 104. 
An edge of each of passages 120 and 121 opens at convexing portion 111 and 
faces rotor 113. Suction passage 120 reaches rotor 113 at a suction 
portion thereof via a suction connecting groove 122. Discharge passage 121 
reaches rotor 113 at a discharge portion thereof via a discharge 
connecting groove 123. 
A first holding groove 150 which holds first drive piston 501 and a second 
holding groove 151 which holds second drive piston 502 are formed within 
main housing 900. Springs 152 and 153, which force first and second drive 
pistons 501 and 502 toward outer race 108, are provided within first and 
second holding grooves 150 and 151. 
Both first and second pressure passages 620 and 621 are formed within main 
housing 900, and an edge of each of passages 620 and 621 is connected with 
first and second holding grooves 150 and 151 respectively. Both first and 
second holding grooves 150 and 151 are closed at their ends by a screw 
(not shown), so that closed areas are formed within first and second 
holding grooves 150 and 151. 
Pressure detecting means 600 is provided within main housing 900. Namely, 
pressure chamber 610 is formed within main housing 900, and switching 
valve portion 611 is provided within pressure chamber 610. The opening 
edge of pressure chamber 610 is closed by a screw 170, so that a closing 
area is formed within pressure chamber 610. Main housing 900 has not only 
first and second pressure passages 620 and 621, but also introducing 
pressure port 612 and second signal pass 602 which introduces the signal 
pressure into second pressure chamber 631. Drain ports 613 and 614 are 
also formed within main housing 900, the edges of drain ports 613 and 614 
are connected with area 106. 
A convexing portion 645 is formed at the left edge of switching valve 
portion 611 (as shown in FIG. 6) and is inserted into a groove 646 which 
is formed in main housing 900. The edge of the convexing portion 645 is 
tapered, so that working fluid in groove 646 escapes toward first pressure 
chamber 630 through this tapered portion when convexing portion 645 is 
inserted into groove 646. Therefore, the flow of working fluid from groove 
646 to first pressure chamber 630 is disturbed by a choke formed by the 
tapered portion and the edge of groove 646, so that switching valve 
portion 611 cannot be inserted into groove 646 at a fast speed. 
Though the tapered portion is formed at the edge of convexing portion 645 
(shown in FIG. 6), the taper portion can also be formed at the edge of 
groove 646 (shown in FIG. 7). Furthermore, a ring 648 having a number of 
small ports 647 (shown in FIG. 8) can be used instead of the tapered 
portion. Since working fluid in groove 646 flows toward first pressure 
chamber 630 through small ports 647 (shown in FIG. 9), switching valve 
portion 611 cannot move quickly as a result of ring 648. In the embodiment 
shown in FIG. 9, spring 640 is provided not within first pressure chamber 
630 but groove 646. 
Variable choke 400 is provided in pintle 104. As shown in FIG. 4, discharge 
passage 121 formed in pintle 104 is also channel 200, so that variable 
choke 400 is provided in discharge passage 121. Controlling passage 401 
which introduces pressure Pi is also formed in pintle 104. 
The opening area of variable choke 400 is varied in accordance with the 
pressure Pi, as shown in FIG. 3. While pressure Pi is smaller than the 
predetermined pressure Po of spring 407, piston 404 is not moved, so that 
passing area 405 is kept at a maximum area (shown between A-B in FIG. 3). 
After the pressure becomes larger than the predetermined pressure Po, 
piston 404 is moved rightwardly in FIG. 2, so that the opening area of 
passing area 405 becomes smaller (shown between B-C in FIG. 3). 
As shown in FIG. 10, the amount of working fluid passing through channel 
200 downstream of variable choke 400 is varied in accordance with the 
opening area of variable choke 400. Although the amount of working fluid 
increases, in the area between K-L in FIG. 10, this increase is caused not 
by the variation of the opening area of variable choke 400 but by the 
starting situation of the pump. When the discharged pressure is rather 
small, the pressure forcing first and second drive pistons 501 and 502 is 
smaller than the predetermined pressure of springs 152 and 153. Therefore, 
rotary ring 107 is moved in order to reduce the volume of pump 100 by the 
balance of springs 152 and 153. The predetermined pressure of spring 152 
is larger than that of spring 153. 
The reason why the character shown in FIG. 3 is similar with the character 
shown in FIG. 10 is described as follows: 
As described above, the amount Q of the working fluid passing through 
variable choke 400 is calculated as the product of the opening area S and 
the pressure difference P. 
EQU Q=Cd.multidot.S ((2/.rho.).multidot.(Pd-Pi)).sup.1/2 (2) 
Wherein, Cd represents the flow coefficient and p represents the density of 
the working fluid. Both Cd and .rho. are constant. The pressure difference 
.DELTA.P is also controlled to be constant by controlling the volume of 
pump 100. Therefore, formula (2) shows that the amount Q of the working 
fluid is varied by the opening area S. 
The shape of passing area 405 of variable choke 400 is determined in such a 
manner that the product of the amount Q and the discharged pressure P 
(P.times.Q) is always constant, so that the working efficiency of pump 100 
is maintained. 
FIG. 11 shows the shape of passing area 405, with the ordinate thereof 
indicating the moving distance of piston 404. The opening area of passing 
area 405 is calculated by the following formula if the shape of the 
passing area 405 is described as Y=f(x). 
##EQU1## 
Therefore, the shape of passing area 405 is determined by the condition 
that the product P.times.Q is constant and by formula (3). 
As shown in FIG. 11, the edge of passing area 405 is not the shape obtained 
by formula (3) but a curved shape in order to reduce the opening area of 
passing area 405 immediately when piston 404 moves the maximum amount. The 
area between C-D in FIG. 3 and the area between N-O in FIG. 10 show the 
area related to circular portion 480. 
Even though the shape described in FIG. 11 is the best shape for reducing 
the pump energy, other shapes can be used. FIG. 12 shows another shape 
which is made from straight lines, so that the shape shown in FIG. 12 is 
easy to form. Dotted line b in FIGS. 3 and 10 represents the result when 
passing area 405 is shaped as in FIG. 12. Passing area 405 can be also 
made by one straight line as shown in FIG. 13. 
The edge of passing area 405 may be the shape shown in FIG. 14 or 15 
instead of circular shape 480. Solid line 1 in FIG. 16 shows the result 
from circular shape 480 in FIGS. 11-13, solid line m shows the result from 
the shape of edge 481 shown in FIG. 14 and solid line n shows the result 
from the shape of edge 482 in FIG. 15. 
The operation of the device having the structure described above is 
explained as follows: 
Rotor 113 starts to be rotated with shaft 101 when shaft 101 is driven by 
engine 700. Since the center line of the rotor 113 is eccentric from the 
center line of the rotary ring 107, pistons 115 are reciprocated within 
cylinders 114 when rotor 113 is rotated. The stroke of reciprocation of 
pistons 115 is twice longer than the amount of eccentricity between rotor 
113 and rotary ring 107. 
While pistons 115 are reciprocated, the volumes of the working chambers 190 
defined by pistons 115 and cylinders 114 are varied. While the volumes of 
working chambers 190 are increasing, the working fluid sucked through 
suction passage 120 is introduced into working chambers 190 via suction 
connecting groove 122. When the volumes of working chambers 190 are 
decreasing, the working fluid in working chambers 190 is discharged toward 
discharged passage 121 via discharged connecting groove 123. 
As described above, the reciprocating stroke of piston 115 is varied in 
accordance with the amount of eccentricity between rotary ring 107 and 
rotor 113. Rotary ring 107 is moved horizontally in FIG. 5 in accordance 
with the movement of first and second drive pistons 501 and 502 which are 
located at opposite sides of rotary ring 107. When rotary ring 107 moves 
to the right (shown in FIG. 5) the amount of eccentricity becomes larger, 
so that the variation of the volume of working chamber 190 becomes larger. 
Therefore, the capacity of pump 100 also becomes larger. When the rotary 
ring 107 moves to the left in FIG. 5, the amount of eccentricity becomes 
smaller, so that the capacity of pump 100 becomes smaller. 
The capacity of pump 100 is controlled in such a manner that the pressure 
difference between the front side and the rear side of variable choke 400 
is always constant. The pressure Pi of the working fluid downstream of 
variable choke 400 is introduced into first pressure chamber 630 via first 
signal passage 601, and the pressure Pd of the working fluid upstream of 
variable choke 400 is introduced into second pressure chamber 631 via 
second signal passage 602. Since first pressure chamber 630 is located at 
the opposite side of valve 600 from second pressure chamber 631, the 
pressure difference (Pd-Pi) acts on valve portion 611. Therefore, valve 
portion 611 is moved by the pressure difference and the predetermined 
pressure of spring 640. 
When the pressure difference is smaller than the predetermined pressure of 
the spring 640, the switching valve portion 611 is moved to the right in 
FIG. 5 by spring 640, so that the working fluid discharged from working 
chamber 190 is supplied to first pressure passage 620 via introducing 
pressure port 612 and connecting area 651. Therefore, the high discharge 
pressure is introduced into the back of first drive piston 501. At the 
same time, the working fluid at the back of second drive piston 502 flows 
toward connecting area 650 via second pressure passage 621. Then the 
working fluid returns to working area 106 through drain port 614. 
Therefore, rotary ring 107 is moved by first drive piston 501 to increase 
the amount of eccentricity. Accordingly, the capacity of pump 100 becomes 
larger when the pressure difference is small. 
After the discharged amount from pump 100 becomes larger, the pressure 
difference becomes larger. When the pressure difference becomes larger 
than the predetermined force of spring 640 switching valve portion 611 is 
moved to the left in FIG. 5 against spring 640. In this situation, the 
discharged fluid from working chamber 190 flows toward second pressure 
pass 621 via introducing pressure port 612 and connecting area 650, so 
that high pressure is introduced into the back of second drive piston 502. 
Simultaneously, the fluid at the back of first drive piston 501 escapes 
toward working area 106 through first pressure pass 620, connecting area 
651 and drain port 613. Therefore, rotary ring 107 is moved to reduce the 
amount of eccentricity. Accordingly, when the pressure difference between 
the front side and the rear side of variable choke 400 becomes larger, 
pump 100 reduces its volume. 
Pump 100 can control its volume to keep the pressure difference between the 
front side and the rear side of variable choke 400 constant by repeating 
the operations described above. 
The opening area of variable choke 400 is controlled in accordance with the 
pressure Pi of the fluid downstream of variable choke 400. Pressure Pi is 
the pressure of the fluid supplied to power cylinder 300 of the power 
steering device. When the power steering device needs a large amount of 
working fluid, such as when the steering is operated, the working fluid 
need not be at high pressure. When the steering position is not moved, 
such as when the automobile is driven off-road, the power steering device 
does not need a large amount of working fluid, but the pressure should be 
high. 
In order words, the character of the working fluid supplied to power 
cylinder 300 should be that showing in FIG. 10. A large amount of working 
fluid is needed at normal pressures (the area between L and M in FIG. 10). 
However, a large amount of working fluid is not needed when the required 
pressure becomes maximum. 
Since variable choke 400 controls the opening area of channel 200 in 
accordance with the pressure Pi of fluid supplied to power cylinder 300, 
the device described above can accomplish this type of control. 
Furthermore, since the volume of pump 100 is controlled to maintain the 
product of the pressure P and the amount Q (P.times.Q) constant, the 
working efficiency of pump 100 can be optimized. 
FIGS. 17 and 18 show another embodiment of the present invention. This 
embodiment has an auxiliary variable choke 800 in channel 200 downstream 
of variable choke 400, every other structure of this invention is the same 
as the embodiment described above. Variable choke 400 is controlled by the 
pressure Pi of fluid downstream of auxiliary variable choke 800 via 
controlling passage 401. Pressure detecting means 600 detects the 
difference between the pressure Pi of fluid downstream of auxiliary 
variable choke 800 and pressure Pd of fluid upstream of variable choke 
400. 
Therefore, in the device of this embodiment, the volume of pump 100 is 
controlled to maintain constant the pressure difference between the front 
side of variable choke 400 and the rear side of auxiliary variable choke 
800, and variable choke 400 is controlled in such a manner that the 
product of pressure P and the amount Q of the discharged working fluid is 
maintained constant. 
Since this embodiment employs auxiliary variable choke 800, the amount of 
working fluid supplied to power cylinder 300 is controlled more carefully. 
The opening area auxiliary variable choke 800 is controlled by an electric 
solenoid 801, and electric solenoid 801 is controlled by the electric 
signal from a controller 802. Controller 802 calculates the required 
amount of working fluid to be supplied to power cylinder 300 from the 
signals from a speed sensor 803 and a sensor 804 detecting the degree of 
the steering, and then supplies the electric signal to electromagnetic 
solenoid 801 in order to control the opening amount of auxiliary variable 
choke 800. 
FIGS. 19 and 20 show variable choke 400 of another embodiment of the 
present invention. In this embodiment, variable choke 400 has a bypass 
passage 451 which interconnects a drain chamber 450 and passing area 405. 
The other structure of this embodiment is the same as previous described. 
Since pump 100 controls the amount of working fluid discharged in 
accordance with the requirement of power cylinder 300, the required amount 
of discharged working fluid becomes nearly zero when the required pressure 
of the discharged working fluid becomes maximum. The pressure of the 
discharged working fluid, however, is varied in accordance with the 
operation of working chamber 190. This variation of the discharge pressure 
does not cause serious influence when the amount of discharged working 
fluid is large enough. However, the influence of this variation of the 
discharged pressure becomes serious when the amount of the discharged 
working fluid is not large enough. 
In order to solve the problem described above, the device of this 
embodiment employs bypass passage 401 so that the working fluid in passing 
passage 405 can escape toward drain chamber 450 even when passing passage 
405 closes channel 200. In other words, the device of this embodiment 
ensures a minimum flow of discharged working fluid from pump 100 by 
employing bypass passage 451. The working fluid which escapes to drain 
chamber 450 then flows toward working area 106 within pump 100 or a 
reserve tank 477. 
Bypass passage 451 of this embodiment is provided at an outer surface of 
control piston 404 as shown in FIG. 20. However, bypass passage 451' may 
be formed in pintle 104 of pump 100 as shown in FIGS. 21 and 22. Bypass 
passage 451' shown in FIGS. 21 and 22 is located at a special position of 
pintle 104 so that the working fluid in passing area 405 can flow toward 
drain chamber 450 when passing area 405 closes channel 200. 
Though the devices of these embodiments described above employ a radial 
plunger pump as pump 100, any other type of variable capacity pump, can be 
used with the present invention. Also, an electric motor can be used as 
the power source driving pump 100 instead of engine 700. 
Though pressure detecting means 600 of the embodiments described above 
detects the pressure through first and second signal passages 601 and 602, 
pressure detecting means 600 may use an electric signal from a pressure 
sensor. Furthermore, an electromagnetic solenoid can be used for varying 
the opening area of variable choke 400 instead of the mechanical structure 
such as spring 407. It is needless to say that the device of this 
invention may have many usages other than with a power steering device. 
Since the device of this invention detects the actual working fluid 
supplied to the actuator for controlling the capacity of the pump, special 
signal passages for the working fluid, other than the main channel 
supplying the actuator is not required. Therefore, the channelling for the 
device is not complicated, and the pump of this invention can work very 
efficiently. 
Since the pump of this invention can vary its capacity, its working 
efficiency can be maximized. 
Furthermore, the working fluid supplied to the actuator can be controlled 
more carefully if the device of this invention employs the auxiliary 
variable choke. 
The influence of the varying of the discharged pressure can be reduced even 
when the capacity of the pump becomes smaller if the device of this 
invention employs the bypass passage. Therefore, the actuator can always 
be controlled smoothly. 
Although only a few exemplary embodiments of this invention have been 
described in detail above, those skilled in the art will readily 
appreciate that many modifications are possible in the exemplary 
embodiments without materially departing from the novel teachings and 
advantages of this invention. Accordingly, all such modifications are 
intended to be included within the scope of this invention as defined by 
the following claims.