Apparatus for controlling a pump to account for compressibility of liquids in obtaining steady flow

• US 4,797,207 A
• Filed: 09/30/1986
• Issued: 01/10/1989
• Est. Priority Date: 09/30/1986
• Status: Expired due to Term

• Current Assignee: Thermo Instrument Systems, Inc. (Thermo Fisher Scientific Incorporated)
• Original Assignee: Spectra-Physics Incorporated (MKS Instruments Incorporated)
• Inventors: Howe, Robert L., Miller, Leslie A., Honganen, Ronald E.
• Primary Examiner(s): Therkorn, Ernest G.
• Application Number:
  US06/913,356
• Time in Patent Office: 833 Days
• Field of Search: 210/656, 210/659, 210/101, 210/85, 210/198.2, 417/18, 417/53, 417/265
• US Class Current: 210/198.2
• CPC Class Codes: B01D 15/163 Pressure or speed conditioning
• CPC Class Codes: G01N 2030/326 pumps
• CPC Class Codes: G01N 30/32 of pressure or speed G01N30/36 takes precedence
• CPC Class Codes: G01N 30/34 of fluid composition, e.g. gradient G01N30/36 takes precedence
• CPC Class Codes: G01N 30/36 in high pressure liquid systems

Abstract

There is disclosed herein an apparatus and method for controlling a dual piston pump for a liquid chromatography system so as to pump a flow of solvent through the liquid chromatography column at a constant flow rate and with a solvent composition which is substantially equal to the desired solvent composition despite changing conditions of compressibility of the solvent. The control system uses a computer which measures the time it takes the pump shaft to move through an overlap region in the pump cycle when both pistons are simultaneously pumping as normalized to the time taken by the pump to move through a constant velocity portion of the piston travel defined by the user. This time is compared to the time stored in the computer for the particular flow rate used to measure the time defined above for the pump to move through the overlap region for an incompressible solvent at low pressure as normalized to the time taken by the input piston to move through the same user defined segment of the constant velocity portion of the travel of the input piston. The ratio of these two times is then used in an algorithm to derive a correction factor for compressibility. This correction factor is then used to control the flow rate and the makeup of the solvent composition to maintain the correct values over changing conditions of solvent compressibility.

Claims

1. An apparatus for causing controlled flow of a solvent composition through a liquid chromatography column comprising:

means for pumping a solvent composition comprised of one or more solvents through said column at a constant pressure determined by a pressure control signal including a dual piston pump means for pumping said solvent composition at a pressure determined by said pressure control signal where said dual pistons each have their own cylinder and are driven by the same rotating shaft and an associated drive mechanism such that said pistons have an associated overlap interval in the rotation of said rotating shaft during which both pistons are pushing said solvent composition out their respective cylinders, and where each piston has an intake interval in the rotation of said rotating shaft during which said solvent composition is being drawn by each respective piston into the associated cylinder, said intake interval including a linear interval defined by a constant piston velocity, and further including control means including transducer means for detecting data indicating the actual pressure at the output of said means for pumping, said control means for comparing the actual pressure detected by said transducer means to the desired pressure as determined by said pressure control signal and for controlling the speed of rotation of said rotating shaft during intervals including said overlap interval such that the actual pressure at the output of said means for pumping equals the desired pressure; and

computer means for for receiving data defining the desired flow rate and for determining a correction factor determined by the compressibility of the solvent composition by computing the ratio of the duration of a first interval during which said rotating shaft is moving said pistons through said overlap interval for the current solvent composition and the current actual constant pressure to the duration of a second interval during which said rotating shaft is moving said pistons through said overlap interval for incompressible solvent pumped at a constant pressure, each said first and second intervals being normalized to the time it takes said rotating shaft to rotate through a predetermined angle during which the corresponding one of said pistons moves during the corresponding one of said respective linear intervals, where said predetermined angle is equal to the angle through which said rotating shaft moves while said pistons are in said overlap interval, and said computer means for subtracting one from each said normalized first and second interval before computing said ratio and for multiplying said ratio times a volume factor equal to the amount of volume displaced by one of said pistons during said overlap interval to derive a product and for subtracting said product from said volume factor to derive said correction factor, and for updating said pressure control signal by multiplying the value of the old pressure control signal times the quantity equal to the correction factor plus 1.

2. The apparatus of claim 1 further comprising a plurality of valve means each said valve means being coupled to said computer means by a mixture control signal controlling the amount of time said valve means is open, said valve means for coupling a plurality of solvents to the solvent composition input of said means for pumping, said computer means for receiving data defining how much of each said solvent is to be present in the solvent composition at the output of said means for pumping and for generating each said mixture control signal so that each said valve means is opened during a separate, appropriate interval during said linear interval of one of said pistons where said appropriate interval in recalculated and the corresponding one of said mixture control signals is updated on each cycle of said rotating shaft by multiplying the old interval established by the corresponding one of said mixture control signals for each solvent times a quantity equal to one minus the correction factor.

3. The apparatus of claim 2 wherein said computer means recalculates said correction factor on each cycle of said rotating shaft and updates said pressure control signal on each said cycle.

4. An apparatus for causing controlled flow of a solvent composition through a liquid chromatography column at a constant flow rate comprising:

pump means having an input port and having an output port coupled to the solvent input of said column for pumping a solvent composition comprised of one or more solvents from said input port through said column at a flow rate determined by a control signal, said pump means comprising two pistons driven by a rotating shaft which pump said solvent composition out from said output port on different cycles but including an overlap interval during which both said pistons are simultaneously pumping and a linear interval during which at least one of said pistons is traveling at a constant velocity in drawing in said solvent composition, and further comprising pressure sensor means for sensing the actual pressure at said output port and generating an actual pressure signal and control means coupled to receive said actual pressure signal for altering the speed of said pump means to maintain the flow rate defined by said control signal; and

one or more electrically controllable valve means each said valve means coupling a source of each solvent in said solvent composition to said input port of said pump means, each said valve means having a control input for receiving a mixture control signal, each said valve means for controlling the amount of each solvent in said solvent composition in accordance with said mixture control signal;

computer means coupled to said pump means and to said valve means for maintaining the desired flow rate and the desired solvent composition over changing operating pressures and changing solvent compositions and despite changes in other operating conditions by determining the compressibility of the solvent composition by determining a plurality of times and calculating a correction factor for compressibility using the formula,

space="preserve" listing-type="equation">CF=Z-[(Z/((T.sub.df /T.sub.lf)-1))((T.sub.mdf /T.sub.mlf)-1) ]

where,

CF=the correction factor to be used to account for compressibility of the solvent at the solvent composition, operating pressure at the output of said pump means and other operating conditions existing for the current revolution of said rotating shaft,

Z=a constant equal to the percentage of each piston'"'"'s pump stroke volume traversed during said overlap interval,

T_lf =the time for the rotating shaft to rotate through an angle during said linear interval said angle being equal to the angle said rotating shaft rotated through during said overlap interval under the same operating conditions of pressure and flow rate and incompressibility of solvent as were used to determine T_df,

T_mdf =measured time for the rotating shaft to rotate through the angle defining said overlap interval with the current compressible solvent composition at the current constant flow rate, and

T_mlf =measured time for the rotating shaft to rotate through the same angle in said linear interval as used to determine T_lf but with the same flow rate, solvent composition and operating pressure used to measure T_mdf,

and for generating said control signal and for updating said control signal from time to time using said correction factor by multiplying the value of the old control signal times the quantity (CF+1) to establish the desired flow rate and for causing said electrically controllable valve means to be opened for the amount of time necessary for input of each solvent component to cause the final solvent composition to be substantially as desired over all conditions of compressibility by calculating the number of degrees of rotation of said rotating shaft during said linear interval each said valve means is to be opened to obtain the desired amount of intake of each corresponding one of said solvents and for generating one or more of said mixture signals to cause each said valve means to be opened separately for the calculated interval during said linear interval and for, from time to time, updating said mixture signal for each said valve means to take into account the effect of compressibility of said solvent by multiplying the value of each mixture signal by the quantity (1-CF).

5. An apparatus comprising:

means for pumping solvent coposition of one or more solvents through a liquid chromatography column at a constant flow rate including control means for controlling said means for pumping to obtain a flow rate determined by a control signal and for controlling said means for pumping during an overlap interval during which the flow rate will increase unless the means for pumping is slowed down by said control means and for controlling said means for pumping during a linear interval so that said means for pumping is driven at a constant velocity to obtain the desired flow rate; and

means for gating the solvent components into said means for pumping to make up said solvent composition; and

means coupled to said control means for determining the compressibility of the solvent composition being pumped through said column by comparing the time it takes said means for pumping to traverse said overlap interval with the current solvent composition to the time it takes said means for pumping to traverse said overlap interval with an incompressible solvent at the same flow rate and for generating said control signal in such a manner as to obtain the desired constant flow rate over changing conditions of compressibility of the current solvent composition and for controlling the means for gating so that the desired solvent composition is maintained over changing conditions of compressibility and wherein said means for determining the compressibility stores therein a time TDF for travel of a first piston through said overlap region of its cycle with an incompressible solvent at low pressure and constant flow rate and a time T_LF for travel through a portion of the constant velocity portion of its travel equal to the angle of said overlap region under the same conditions under which TDF was determined, said times being stored as constants for each flow rate of solvent through the column that the user might request and wherein the means for compressibility measures the time T_MDF required for the first piston to travel through its overlap region at the desired constant flow rate and the time T_MLF required for the first piston to travel through the same portion of its constant velocity portion of travel as used to determine T_LF at a constant flow rate and under the same conditions under which T_MDF was measured and wherein the means for determining compressibility calculates a correction factor for compressibility by the following expression;

##EQU1## where Z equals the volume displaced as said first piston traverses the overlap region.

6. The apparatus of claim 5 wherein said means for pumping is a dual piston pump wherein each piston is driven by a cam and wherein a first piston pumps solvent from an input through a check valve into the cylinder housing a second piston and wherein said second piston pumps solvent out from its cylinder through an output and wherein said cam contours are such that there is an overlap portion of each cycle wherein both said pistons are pumping simultaneously.

7. The apparatus of claim 5 wherein the means for gating comprises a solenoid operated valve which couples a source of each solvent in the composition to said means for pumping wherein the control signal input of each solenoid operated valve is coupled to said means for determining compressibility.

8. The apparatus of claim 7 wherein the means for determining compressibility prompts the user for data regarding the desired solvent composition and uses the percentage of the minor solvents in the composition times a constant to determine the number of steps to open each solenoid operated valve during the input stroke of said first piston and wherein said constant is multiplied by one minus said correction factor to determine the actual number of steps to open each solenoid valve controlling a minor solvent.

9. The apparatus of claim 8 wherein the first piston has a constant velocity portion of its input stroke and each minor solvent solenoid operated valve is opened separately during a portion of said linear portion of said input stroke and wherein said means for determining compressibility also is coupled to said means for pumping to control its speed by calculating a target pressure control signal based upon a user input regarding the desired flow rate and for correcting said target pressure control signal based upon the compressibility of said solvent composition.