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.

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.

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.