U.S. patent number 3,847,507 [Application Number 05/361,139] was granted by the patent office on 1974-11-12 for liquid supply system by pump.
This patent grant is currently assigned to Toyo Soda Manufacturing Co., Ltd.. Invention is credited to Tsutomu Hashimoto, Kazutaka Sakiyama.
United States Patent |
3,847,507 |
Sakiyama , et al. |
November 12, 1974 |
LIQUID SUPPLY SYSTEM BY PUMP
Abstract
A system for supplying a liquid at constant pressure for use in
a liquid chromatograph characterized by a pump having a cylinder
and a piston reciprocably disposed therein for drawing liquid into
and discharging liquid from the cylinder, wherein a tacho-generator
is connected to an electric motor which is controlled by an
automatic control circuit for detecting the hydraulic pressure in
the cylinder having a differential amplifier therein, and in which
a signal from the tacho-generator is fed back to the differential
amplifier for controlling the movement of the piston being effected
by the motor through a gearing arrangement, and a screw drive means
connecting the motor and the piston.
Inventors: |
Sakiyama; Kazutaka
(Oaza-Tonda-shin-Nanyo, JA), Hashimoto; Tsutomu
(Oaza-Tonda-shin-Nanyo, JA) |
Assignee: |
Toyo Soda Manufacturing Co.,
Ltd. (Yamaguchi-ken, JA)
|
Family
ID: |
12794994 |
Appl.
No.: |
05/361,139 |
Filed: |
May 17, 1973 |
Foreign Application Priority Data
|
|
|
|
|
May 17, 1972 [JA] |
|
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47-48139 |
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Current U.S.
Class: |
417/22; 417/45;
417/42; 422/921 |
Current CPC
Class: |
F04B
49/06 (20130101); G01N 30/32 (20130101); B01L
3/0206 (20130101); F04B 49/08 (20130101); G01N
2030/326 (20130101) |
Current International
Class: |
B01L
3/02 (20060101); G01N 30/00 (20060101); F04B
49/06 (20060101); G01N 30/32 (20060101); F04B
49/08 (20060101); F04b 049/06 (); F04b
049/00 () |
Field of
Search: |
;417/18,22,44,45,24
;60/423,431 ;73/398AR |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Freeh; William L.
Assistant Examiner: Ward; Arnold F.
Attorney, Agent or Firm: Oblon, Fisher, Spivak, McClelland
& Maier
Claims
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. A system for supplying a liquid at constant pressure
comprising:
a cylinder having an inlet and an outlet,
a piston reciprocably disposed within said cylinder for drawing
liquid into and discharging liquid from said cylinder,
an electric motor,
means connecting said motor to said piston to enable said motor to
reciprocate said piston,
means within said cylinder for detecting a pressure change in said
cylinder,
means responsive to said pressure change detecting means for
actuating said motor,
means for detecting the speed of said motor,
means responsive to said speed detection means for reducing the
speed of said motor.
2. A system in accordance with claim 1, wherein said means for
detecting the speed of said motor comprises a tacho generator.
3. A system in accordance with claim 1, wherein said means for
detecting a pressure change comprises a strain gauge disposed
within said cylinder.
4. A system in accordance with claim 1, wherein said means for
actuating said motor comprises a rotary speed control circuit.
5. A system in accordance with claim 1, wherein said means
responsive to said speed detection means comprises a sychronous
rectifier.
6. A system in accordance with claim 1, wherein said means
responsive to said speed detection means comprises a differential
amplifier.
7. A system in accordance with claim 1, wherein said means for
detecting a pressure change in said cylinder comprises a reference
voltage generator connected to the input of a differential
amplifier.
8. A system for supplying a liquid at constant pressure
comprising:
a cylinder having an inlet and an outlet,
a piston reciprocably disposed within said cylinder for drawing
liquid into and discharging liquid from said cylinder,
a motor,
means connecting said motor to said piston to enable said motor to
reciprocate said piston,
means for detecting a pressure change in said cylinder,
a first differential amplifier,
a second differential amplifier,
a rotary speed control circuit,
a tacho generator,
a reference voltage generator,
means connecting said pressure detecting means to a first input of
said first differential amplifier,
means connecting said reference voltage generator to the second
input of said first differential amplifier,
means connecting the output of said first differential amplifier to
a first input of said second differential amplifier,
means connecting said tacho generator to the second input of said
second differential amplifier,
means connecting said motor to said tacho generator,
means connecting the output of said second differential amplifier
to the input of said rotary speed control circuit,
means connecting the output of said rotary speed control circuit to
the input of said motor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a liquid supply system
utilizing a pump which is especially adapted for use in liquid
chromatography applications because of its capability of
maintaining a constant liquid pressure.
2. Description of the Prior Art
In general, a pump being used for a liquid chromatography system
preferably has no pulsation and is capable of maintaining a
constant pressure therein. Recently, improvements in the separation
columns and detectors of liquid chromatography systems remarkably
increased the accuracy and capabilities of the overall systems and
measurement of quite small amounts of samples are now possible, and
because of such developments, a pump having greater stability has
become an even more important factor in determining the success of
such systems. The conventional cam type pump provides severe
pulsation, because of the principle of its structure, and
accordingly, it has been necessary to set a large volume
accumulator in the path. Thus, even though space efficiency were
disregarded to set the accumulator, a pressure fluctuation still
cannot be prevented, when the repeated operation of a check valve
is not reproducible. As another system, air-hydro-pumps have
heretofore been employed for moving a piston in an air-cylinder
utilizing air-pressure as a driving force. When the air-pressure of
the air-cylinder is kept constant, a constant pressure of the
liquid supply system is given. However, a severe pressure drop is
caused each time the piston is returned. Accordingly, it is
necessary to set valves for preventing backward flow in front and
back of the separation column, and this has proven to be
disadvantageously expensive.
The use of a double screw type pump in such applications has also
been contemplated. In a detailed description, this involves two
pumps for compressing a liquid at a constant speed, by rotating a
screw for moving a liquid transferring piston by a pulse motor or a
servo motor through gears, so that a supply and a suction of the
liquid are alternately provided to prevent intervals therebetween.
However, this system also requires various techniques and labor for
preventing inaccurate timing in the switching process. Thus, even
though accurate conformity of the characteristics of the two pumps
is provided and accurate simultaneous switching is possible, a
pressure fluctuation still results, corresponding to a differential
coefficient of a change in the room temperature, when a solvent
having a high thermal expansion coefficient is used, as a simple
constant speed motor is used.
Accordingly, it is quite difficult to prepare a liquid supply pump
which imparts high and constant pressure. Automatic control of the
rotary speed of an electric motor for rotating the driving screw by
a signal of a pressure detector, when the double screw type pump is
employed, has also been considered. However, it has been difficult
to obtain a desirable pressure control through a feed-back signal
being proportional to a pressure deviation from a predetermined
pressure, because the pressure detector usually detects a pressure
depending upon the bending of the liquid contact position so that a
volume change cannot be prevented. Thus, when the liquid supply
system employs a pressure detector wherein pressure is proportional
to volume change, and a feed-back circuit for accelerating a piston
in proportion to a deviation of the detected pressure from the
predetermined pressure, the following analytical results are
given:
P = a .sup.. S (1)
p = b .sup.. V (2)
wherein P represents the pressure, S represents the flow velocity
of the liquid, V represents the volume of a pressure detector, a
represents a constant depending upon a load in the system, and b
represents a constant depending upon the sensitivity of the
pressure detector.
Then,
v .sup.. A = S + dv/dt , (3)
wherein v represents the driving speed of the piston and A
represents the cross-sectional area of the piston. When the
Equations 1 and 2 are combined with Equation 3, the following
equation is given:
v .sup.. A = P/a + 1/b dP/dt (4)
When the predetermined pressure is Po, the following equation is
given in the condition of the liquid transfer system:
dv/dt = k (Po - P), (5)
wherein k represents a constant depending upon the gain of the
feedback circuit. The following equation is given by cancelling v
from the Equations 4 and 5:
d.sup.2 P/dt.sup.2 + b/a dP/dt + kbA (P - Po) = 0 (6)
Accordingly, the following pressure is given when t = 0 and P =
0.
p = po[1 - e.sup.- .sup.(b/2a) t cos (.sqroot. 4ka.sup.2 bA -
b.sup.2 t )] (7)
As is clear from the above analysis, the pressure P causes a damped
oscillation at a deviation center of the predetermined pressure
Po.
In a practical case, even though the sensitivity of the pressure
detector is possibly increased by employing a resistant wire strain
gauge, it is difficult to obtain a speedy attenuation and to
converge on the predetermined pressure in a practical experiment,
because of a breathing motion of the rest of the system outside the
pressure detector, the response speed of the electric motor, and of
a thermal interaction out of the system.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a system for supplying
a liquid with a pump for compulsorily converging a knocking of a
piston action depending upon a pressure control circuit by
automatically controlling the piston driving speed.
The foregoing and other objects of this invention have been
attained by providing a system for supplying a liquid by a pump
wherein the pressure in a cylinder of the pump structure is
detected and the movement of the piston is controlled by an
automatic control circuit having a differential amplifier through
an electric motor and a feedback signal originating from a
tacho-generator connected to the motor is fed back to the
differential amplifier.
BRIEF DESCRIPTION OF THE DRAWINGS
Various other objects, features and attendant advantages of the
present invention will be more fully appreciated as the same
becomes better understood from the following detailed description,
when considered in connection with the accompanying Drawings,
wherein like reference characters designate like or corresponding
parts throughout the several views and in which:
FIG. 1 is a partially sectional front view of one embodiment of a
novel pump constructed according to this invention;
FIG. 2 is a block diagram of an automatic pressure control circuit
according to the invention;
FIG. 3a is a graph showing the change in pressure when the output
of a tacho-generator is not fed back, wherein the vertical axis
designates a liquid supply pressure P and the horizontal axis
designates a time t;
FIG. 3b is a graph showing the change in the output of the
tacho-generator wherein the vertical axis is for an output level V
and the horizontal axis is for a time t;
FIG. 3c is a graph showing the change of pressure at the time of
feed-back of the output of the tacho-generator wherein the vertical
axis is for a liquid supply pressure P and the horizontal axis is
for a time t; and
FIG. 4 is a schematic view of a liquid supply system according to
this invention, wherein two pumps are connected to one
diaphragm.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Referring now to the Drawing, and more particularly to FIG. 1, a
piston 1 of a pump is reciprocably disposed in a cylinder 2, and a
packing gland 3 for maintaining air-tightness is fitted within the
cylinder 2 being secured therein by a screw nut 4. A diaphragm 5
having a liquid contacting portion 5a, which is elastically
deformable by hydraulic pressure, is air-tightly secured to one end
of the cylinder 2 through a suitable gasket 6. Check valves 7 and 8
are respectively placed in a suction or inlet duct 9 and a
discharge duct 10 so as to control the supply and discharge of
liquid into and out of the cylinder 2.
A d.c. electric motor 11 is directly connected with a reduction
gear 12 having a suitable reduction ratio and is interlocked with
gears 13 and 14 for increasing the strength of the structure. A
screw rod 15 is directly connected with the reduction gear 14 and
one end of the screw rod is inserted into a hollow end of the
piston 1. A driving nut 16 is fixedly mounted on the piston 1 on
the hollow end thereof and is reciprocable along a guide rod 17,
which projects through an arm portion thereof to permit relative
sliding motion of the driving nut therewith while preventing
rotation of the driving nut, such sliding motion being attained
through a threaded engagement of the driving nut 16 and the screw
rod 15. Reciprocation of the piston 1 of course is dependent upon
reciprocating motion of the nut 16 along the guide rod 17.
Micro-switches 18 and 19 are respectively positioned on the guide
rod 17 at both an initial point and an end point of the path of
motion of the driving nut 16 and are connected to control the
rotary direction of the screw rod 15 through the turning of the
motor 11.
Thus, when the driving nut 16 contacts the micro-switch 18, the
motor 11 is driven so as to turn the screw rod 15 to move the
driving nut 16 and the piston 1 fixed thereto in the direction
indicated by arrow A. When the driven nut 16 contacts the
micro-switch 19, the motor is then driven so as to move the driving
nut 16 and the piston 1 affixed thereto in the direction designated
by arrow B.
The reference numeral 20 designates a base frame for the apparatus,
and a plate 21 is provided for fitting the electric motor 11 and
the reduction gear 12, to the base frame 20. A spacer 22 separates
the plate 21 and the frame 20. A resistant wire type strain gauge
23 which converts a pressure change to a signal change is
positioned on the side of the diaphragm 5 opposite the liquid
contacting side 5a thereof. A tacho-generator 24 is directly
connected to the electric motor 11 so as to generate a voltage
proportional to the rotary speed of the motor 11.
A pressure detective circuit 25 for detecting deviation of the
resistant wire type strain gauge 23 fitted to the diaphragm 5 is
shown in FIG. 2 and will transmit a signal of electrical
fluctuation corresponding to the pressure fluctuation, through a
pre-amplifier 26 to an indicator 27. A differential amplifier 28
transmits the output of a reference voltage generator 29 and the
output of the pressure detective circuit 25 being transmitted
through the pre-amplifier 26. A synchronous rectifier 30 receives
the output of the tacho-generator 24 and is connected to a first
input 31a of another differential amplifier 31, which also receives
the output of the differential amplifier 28 at a second input 31b.
The reference numeral 32 designates a rotary speed control circuit
of the motor 11, and a reference numeral 33 designates a load duct
connected to the discharge duct 10, which corresponds to a
separation column in a liquid chromatograph.
Now operation of the embodiment of the invention herein described
will be illustrated.
When the pressure automatic control circuit in FIG. 2 is alerted
from a standstill condition having hydraulic pressure of zero, the
diaphragm 5 does not receive any deformation stress because the
hydraulic pressure is zero. Accordingly, the resistant wire type
strain gauge 23 is not receiving any strain, so that the output
signal of the pressure detective circuit 25 is zero. Accordingly,
the output of the pre-amplifier 26 is zero and the indicator 27
also reflects a zero reading.
When the output of the reference voltage generator 29 is zero, the
output of the differential amplifier 28 is zero and the output of
the differential amplifier 31 also is zero so that the electric
motor 11 is not alerted by the rotary speed control circuit 32.
When the reference voltage generator 29 is actuated to originate a
certain value of output signal, the differential amplifier 28
originates a differential output depending upon the input to cause
operation of the next differential amplifier 31.
Accordingly, the rotary speed control circuit 32 alerts the
electric motor 11 so as to rotate the screw rod 15 through the
reduction gears 12, 13 and 14, move the piston 1 in the direction
of arrow line A through the driving nut 16. At this time, the
rotation of the electric motor 11 results in acceleration which is
substantially in proportion to the output of the differential
amplifier 31. When there is no feed-back signal from the
synchronous rectifier 30, rotation of the electric motor 11, is not
supplied to the differential the movement of the electric motor 11
to cause the piston 1 to move in the direction of the arrow A
causes a fluctuation of pressure as shown in the equation 7 and in
FIG. 3a.
In this embodiment of the invention, the feed-back signal from the
tacho-generator 24 is supplied to the differential amplifier 31,
and accordingly, the electric motor 11 is positively damped based
on the increase of its rotary speed, whereby excess acceleration of
the piston 1 caused by delay in the increase of the hydraulic
pressure resulting from the breathing action of the diaphragm 5 and
the rest of the liquid transfer system, and the oscillation mode,
or hunting, of a constant pressure level can be easily caused to
disappear, as shown in FIG. 3c.
The hunting phenomenon is thus eliminated and a constant pressure
is maintained. The time for reaching the pressure equilibrium is
dependent upon the manner for providing the gains of the two
feed-back circuits. However, a critical damping can be easily
determined, and even though a leakage or a blockage occurs in the
load duct 33, the check valves 7 and 8, the packing 3, and the
like, it is possible to transfer the liquid under a constant
pressure.
When the liquid transferring pump is applied to a liquid
chromatography system, it is necessary to supply liquid for a long
time. Accordingly, two of the same type pumps can be used, if
desired, so that the suction and discharging steps are alternately
repeated by each, as shown in FIG. 4.
In this case, the micro-switches 18 and 19 are used not only for
switching the circuit for positive and reverse rotation of the
electric motor 11, but also for maintaining a constant pressure
without mechanical adjustment of the stroke of the piston. The
switches also serve to operate without fluctuation of the pressure
at the time of switching, the respective pumps when a level
difference is given to the output of the reference voltage
generator 29 of the liquid transferring pump and the output level
is alternately switched.
In the above embodiment, a d.c. electric motor is employed.
However, a corresponding circuit may easily be designed by
replacing it with a synchronous motor or a pulse motor. In order to
form a compact size apparatus, a pressure detector may be placed on
the cylinder and the tacho-generator may be directly connected to
the motor. The fitting place is not limited, but can be changed to
another suitable place.
In accordance with the invention, a liquid can be transferred at a
constant pressure in stable condition and a predetermined pressure
can be automatically maintained without causing a fluctuation of
pressure in or out of the system. When the system is applied to
liquid chromatography which requires a stable constant liquid
transfer for a long period of time, therefore, the results being
obtained can be remarkably improved.
Obviously many modifications and variations of the present
invention are possible in light of the above teachings. It is to be
understood therefore that within the scope of the appended claims
the invention may be practiced otherwise than as specifically
described herein.
* * * * *