GB2156445A

GB2156445A - Pump

Info

Publication number: GB2156445A
Application number: GB08507748A
Authority: GB
Inventors: Gary Gordon; Kent Vincent
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1982-01-11
Filing date: 1985-03-26
Publication date: 1985-10-09
Also published as: GB8507748D0; JPH0141220B2; GB2156445B; GB2113315B; JPS58120162A; DE3246067A1; GB2113315A; DE3246067C2

Abstract

A pump has two pistons 30,38 movable relative to cylinders 32,40 in reverse phase, being driven by motor 10 via gears 18,20 and worm drives 34,36 and 42,44. A four-way rotary valve 22 driven by reversible motor 24 connects the piston cylinder pair on its suction stroke with reservoir 28 and the other pair to the load. <IMAGE>

Description

1 GB 2 156 445A 1

SPECIFICATION

An improved high pressure meter pump This invention is concerned with an improved high pressure meter pump and is related to the invention disclosed in our copending UK Patent Appication No. 8230161.

There is a need in liquid chromatography for solvent pump systems which can deliver fluids in accurately metered amounts. It is important in such systems that solvent flow be precisely controlled independently of the load pressure. Liquid chromatography systems and particularly systems utilizing gradient elution require high accuracy at extremely low flow rates. For example, in micro-bore chromatography it is desirable to resolve flow rates as low as one microliter per minute. It has been a problem with most prior art devices that the compressibility of the fluid and the mechanical compliance of the pump combine to cause a severe drop in flow rate as load pressure increases, this phenomenon being commonly referred to as "roll-off". A two pump system overcomes inaccuracies due to roll-off by utilizing a low pressure meter pump which injects controled amounts of solvent into a high pressure slave pump. The meter pump is synchronized with the slave pump to ensure that the meter pump delivers its metered charge into a low pressure portion of the slave pump cycle. Because the meter pump always operates into a low pressure load, roll-off is not a problem affecting meter pump accuracy. Such a pump system is disclosed in U.S. Patent Specification No. 4,003 679 The aforementioned specification describes a system in which the meter pump/slave pump pair are mechanically coupled to cycle at the same fixed rate. Flow rate is controlled by mechanically adjusting meter pump displacement. However, mechanical and fluid compliance in the meter pump introduces some error in each meter pump cycle. This error tends to increase as a function of increasing meter pump cycle rate. As flow rate and hence meier pump displacement is reduced this error and others inherent in scaling become an increasingly greater percentage of the volume of fluid pumped. Any such error is reproduced in each cycle of the meter pump/slave pump pair and reduces system flow rate accuracy, particularly at low flow rates. Hence, the range of flow rates over which a given meter pump can operate is limited by degradation of accuracy. Difficulty in self-priming may also become a problem at low flow rates in pumps that adjust flow rate by controlling displace ment because larger displace ment pu mps tend to selfprime better than small displacement pumps.

The present invention provides a pump for pumping a fluid to a load comprising a first pumping piston having a first fluid port; a second pumping piston having a second fluid port; mechanical driving means coupled to the first and second pumping pistons for moving said first and second pumping pistons each through a push stroke and a pull stroke in a push-pull fashion relative to each other; a reservoir for holding the fluid to be pumped; and fluid switch means interposed between the first fluid port and the reservoir and between the second fluid port and the reservoir for connecting the first pumping piston to the reservoir while the first pumping piston is on the pull stroke and connecting the second pumping piston to the load while the second pumping piston is on the push stroke when the fluid switch means is in a first position, and connecting the first pumping piston to the load while the first pumping piston is on the push stroke and connecting the second pumping piston to the reservoir while the second pumping piston is on the pull stroke when the fluid switch means is in a second position.

In a pump as set forth in the last preceding paragraph, it is preferred that said mechanical driving means further comprises a motor; first and second screws coupled to said first and second pumping pistons respectively; and gears interposed between the motor and the-first and second screws.

In a pump as set forth in the last preceding paragraph, it is preferred that said motor is a first reversible servomotor.

In a pump as set forth in any one of the last three im mediately preceding paragraphs, it is preferred that the reversible servomotor is controlled by a microprocessor.

The reversible servomotors may each be controlled by a microprocessor.

The present invention overcomes problems with inaccuracy associated with roll-off utiliz- ing a meter pump/slave pump pair as in the aforementioned specification. However, further improvements in accuracy are attained by using a servo- driven syringe-type meter pump delivering a charge into a diaphragm slave pump. The volume of the meter pump syringe cylinder is designed to be many times greater than the capacity of the slave pump, so that the slave pump cycles many times for each meter pump cycle. Error in the syringe- type meter pump is very low when expressed as a percentage of the volume of solvent pumped in each meter pump cycle. Because the meter pump is driven by a servomotor at a much lower cycle rate than the slave pump, the small meter pump error is distributed over many slave pump cycles. Consequently, flow rate accuracy is independent of the volume of fluid being pumped. Thus the proposed system is able to resolve very low flow rates such as one microliter per minute. The servomotor may be synchronized to drive the meter pump only during the low pressure input portion of the slave pump cycle. Alternatively, a compliance may be inserted between the meter pump and the slave pump allowing the meter 2 GB2156445A 2 pump to operate independently from the slave pump. Because of the nature of this meter pump/ slave pump pair the system maintains high accuracy over a relatively broad range of flow rates. Additionally, the ability to self- 70 prime is improved.

There now follows a detailed description which is to be read with reference to the accompanying drawings of a pumping system according to the present invention; it is to be clearly understood that this pumping syste m has been selected for description to illustrate the invention by way of example and not by way of limitation.

In the accompanying drawings:

Figure 1 illustrates a pumping system ac cording to a preferred embodiment of the invention; and Figure 2 shows several filling profiles for the system of Figure 1.

In Figure 1, a servomotor 10 drives a pair of syringetype meter pumps 12 and 14 con nected in a push-pull mode. A piston 30 of meter pump 12 is raised and lowered relative to a piston cylinder 32 by engagement of a lead screw 34 with an interior threaded por tion of a piston carrier 36. Similarly, a piston 38 in the pump 14 is raised and lowered in a piston cylinder 40 by the engagement of a lead screw 42 with an interior threaded por tion of a piston carrier 44. The pumps 12 and 14 always move equally in opposite directions in a push-pull mode because a gear 18 at tached to the lead screw 34 of the pump 12 drives a gear 20 attached to the lead screw 42 of the pump 14, and because the gears 18 and 20 and lead screw-piston carrier pairs 34-36 and 42-44 are matched. Hence, the action of the servomotor 10 connected to a gear 16 drives the gears 18 and 20 equally in 105 opposite directions. This in turn moves the pistons 30 and 38 equal distances in opposite directions.

The flow pattern of the push-pull pair of meter pumps 12 and 14 is reversed by a 90 degree rotation of a four-way rotary valve 22 actuated by a servomotor 24, which actuation is synchronized with a reversal of direction of the servomotor 10. This sequence of changes may be controlled by a microprocessor 26, such as an Intel 8085 microprocessor, to ensure that one of the meter pumps 12 or 14 is pumping while the other is refilling from a reservoir 28 of solvent.

At any instant the meter pump which is then pumping, e.g., the pump 14 as shown in Figure 1, delivers its metered charge through a ball valve 46 into a diaphragm pump chamber 48 of a high pressure slave pump 50. A pump motor 52 raises a piston 54, drawing oil from a reservoir 56 through a check valve 58 into a piston cylinder 60. The diaphragm pump chamber 48 is separated from the piston cylinder 60 by a flexible diaphragm 49 such that solvent from the meter pump 12 or 14 is not able to mix with oil from the slave pump 50. The flexible diaphragm 49 transmits pressure between the oil in the piston cylinder 60 and any solvent in the diaphragm pump chamber 48. As the piston 54 is driven down by the motor 52, the check valve 58 closes. A check valve 62 opens-when the pressure in the piston cylinder 60 exceeds load pressure. A spring-loaded ball valve 64, used as a back pressure regulator, together with a compliance 66 and a check valve 68 functions as a high pressure hydraulic override; it is adjusted to open at a pressure above the maximum load pressure.

As the piston 54 continues its downward movement, oil pressure builds up in the piston cylinder 60. The pressure is transmitted across the flexible diaphragm 49 into the solvent in the diaphragm pump chamber 48.

If the ball valve 46 is not already closed, it closes under the increased pressure in the diaphragm pump chamber 48. As pressure in the diaphragm pump chamber 48 reaches load pressure, the check valve 62 opens discharging the solvent at load pressure through a tube 70. After all the solvent has been forced out of the diaphragm pump chamber 48, oil pressure in the piston cylinder 60 again rises, opening the check valve 68 for the remainder of the downward stroke of the piston 54. In this way the slave pump 50 delivers the same volume of fluid (both so]vent and oil) in every cycle. Excess capacity of the slave pump 50 over the amount of solvent delivered by the meter pump 12 and 14 to the diaphragm pump chamber 48 is expended by pumping oil through the check valve 64 into the compliance 66 which is held at a pressure above maximum load pressure by adjustment of the spring loaded ball valve 64. When the pressure in the piston cylinder 60 decreases, the check valve 68 is closed by the pressure retained in the compliance 66. The ball valve 64, however, will remain open as long as its adjusted opening pressure is below the pressure in the compliance 66. In this way the high pressure hydraulic override is able to gradually release any retained excess pressure.

The pressure at which the ball valve 46 is adjusted to open is such that it will not open solely as a result of the suction pressure caused by the upward motion of the piston 54. This is to ensure that only solvent actively delivered by the meter pump 12 or 14 enters the diaphragm pump chamber 48.

In operation, the motor 52 drives the slave pump 50 at a continuous rate while the servomotor 10 incrementally subdivides the volume of the meter pump piston cylinder 32 or 40 into subvolumes, delivering one such subvolume for each slave pump cycle. In this way the meter pump cycles much more slowly than the slave pump. Flow rate is controlled by regulating the size of the subvolumes. In a 3 GB 2 156 445A 3 meter pump/slave pump pair as in the pre- ferred embodiment, for example, assume that the motor 52 is of the constant speed type and drives the slave pump 50 through six hundred complete pump cycles per minute, and the meter pump piston cylinders 32 and 40 have combined volumes of one-fifth of a milliliter, and require eight rotations (four clock wise plus four counterclockwise) (2,880 degrees) of the lead screws 34 and 42 to deliver a volume of one-fifth of a milliliter. Then, at a first flow rate of twelve milliliters per minute, the meter pump pair 12 and 14 must deliver one-fifth of a milliliter per sec- ond. To accomplish this, the servomotor 10 must rotate the gears 18 and 20 through six hundred equal incremental rotations of 4.8 degrees each, per minute. In contrast, a sec ond flow rate of one microliter per minute (or one meter pump cycle in every 200 minutes) 85 requires six hundred incremental rotations of 0004 degrees each, per minute of the gears 18 and 20. In effect the twelve milliliter flow rate subdivides the combined one-fifth millili- ter volume of the meter pump piston cylinders 90 32 and 40 into ten subvolumes averaging 20 microliters each. At the one microliter flow rate each one-fifth milliliter volume is subdivided into 120,000 subvolumes averaging 0.001667 microliters each. Any error in the volume of a particular subvolume is compensated for through the integration of many subvolumes over time.

The microprocessor 26 can be programmed to synchronize the servomoter 10 with the synchronous motor 52 such that the meter pump 12 or 14 only delivers solvent through the ball valve 46 during the portion of the cycle of the slave pump 50 when the piston 54 is moving upwards. This ensures that the 105 meter pump 12 or 14 always experiences minimal roll-off. Alternatively, a compliance 72 would allow the servomotor 10 to operate independently of the synchronous motor 52 by storing solvent during the high pressure portion of the slave pump cycle. Another method of coordinating the action of the slave pump 50 with the amount of solvent delivered by the meter pump 12 or 14 is to combine a "smart" compliance 72 with a variable speed or variable displacement slave pump 50. In this way the slave pump 50 would change the volume of fluid pumped in each cycle in response to the solvent delivered by the meter pump 12 or 14 as communicated by the "smart" compliance 72 to the microprocessor 26. Finally, in liquid chromatography systems utilizing gradient elution the output from multiple meter pump pairs could be mixed and delivered to a single slave 125 pump.

Figure 2 shows a curve 74 representing the harmonic motion of the piston 54 during one cycle of operation of the slave pump 52.

Three curves 76a, 76b and 76c illustrate different possible filling profiles for filling of the slave pump 52 by the meter pumps 12 and 14. Curves 78a, 78b and 78c illustrate the corresponding delivery profiles for the slave pu mp 50 for each of the filling profiles 76a, 76b and ?6c. As discussed above, any excess capacity of the slave pump 50 over the amount of solvent delivered to the slave pump is expended by pumping of oil by the slave pump 50; in Figure 2, this volume is indicated by a refill area 80 and a delivery area 81, both corresponding to solvent profiles 76c, 78c.

Claims

1. A pump for pumping a fluid to a load comprising:

a first pumping piston having a first fluid port; a second pumping piston having a second fluid port; mechanical driving means coupled to the first and second pumping pistons for moving said first and second pumping pistons each through a push stroke and a pull stroke in a push-pull fashion relative to each other; a reservoir for holding the fluid to be pumped; and fluid switch means interposed between the first fluid port and the reservoir and between the second fluid port and the reservoir for connecting the first pumping piston to the reservoir while the first pumping piston is on the pull stroke and connecting the second pumping piston to the load while the second pumping piston is on the push stroke w hen the fluid switch means is in a first position, and connecting the first pumping piston to the load while the first pumping piston is on the push stroke and connecting the second pumping piston to the reservoir while the second pumping piston is on the pull stroke when the fluid switch means is in a second position.

2. A pump according to claim 1 wherein said mechanical driving means further cornprises:

a motor; first and second screws coupled to said first and second pumping piston respectively; and gears interposed between the motor and the first and second screws.

3. A pump according to claim 2 wherein said motor is a first reversible servomotor.

4. A pump according to any one of claims 1 to 3 wherein the fluid switch means cornprises:

a four-way rotary valve; and a reversible servomotor coupled to the fourway rotary valve.

5. A pump according to claim 3 or claim 4 as appended to claim 3 wherein the reversible servomotor is controlled by a microprocessor.

4 Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1985, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

GB 2 156 445A 4