GB2069726A

GB2069726A - Fluid flow control apparatus and method

Info

Publication number: GB2069726A
Application number: GB8104544A
Authority: GB
Original assignee: Lovelock J E
Current assignee: Lovelock J E
Priority date: 1980-02-14
Filing date: 1981-02-13
Publication date: 1981-08-26

Abstract

A method and apparatus for maintaining a constant rate of flow of fluid in a flow line (2) in which an electrically operable valve (1) in the flow line is cycled repeatedly such that it is open during one part of the cycle and closed during the remainder of the cycle. The fluid flow is controlled by varying the opening and closing periods during each cycle and this is achieved either by varying the switching frequency at a constant on/off ratio or by varying the on/off ratio at a constant frequency, or both, in response to electrical signals generated from a fluid flow sensor (5) located in the flow line. The flow sensor may be of the type which measures temperature changes resulting from flow and is said to emit pulses. <IMAGE>

Description

SPECIFICATION Fluid flow control apparatus and method The present invention is concerned with a method of and apparatus for the control of fluid flow and has particular application to the control of fluid flow in analytical instruments wherein very accurate control of the rate of flow is essential so that a constant flow of fluid can be maintained for small quantities of fluid, for example, as low as 0.1 cc per minute.

The control of regulation of fluid flows at these low levels as encountered in analytical instruments, such as gas chromatographs and automatic analysers, can be difficult when using manually operable control means such as needle-type valves.

The present invention provides a method of controlling the flow of fluid in a flow line, the method comprising allowing fluid to flow through the flow line which includes an electrically-operable valve, sensing the flow of fluid by flow sensor means located in the flow line downstream of the valve and providing an electrical feedback signal from said flow sensor means through electrical circuit means to the valve to open and close the valve successively and repeatedly, thereby to maintain a constant rate of flow of fluid in the flow line of at least O.lcc/min.

As stated above, the invention finds particular application in connection with gas chromatography and analysis of small quantities of gases where accurate control of gas flow is fundamental to accuracy of measurement. More specifically, a method of controlling flow of fluid according to the invention should be capable of providing a constant rate, in a flow line of internal diameter such as is usual in the field of chromatography, of from approximately 1 cc/min to approximately 100ccimin, but not more than 1 litre/min.

The rate of flow of fluid is controlled by the periods for which the valve is open or closed in a given time span. This control can be achieved either by varying the frequency with which the valve is opened and closed, assuming the open and close times to be the same, or by varying the open period/close period ratio of the valve, or by combining both methods together.

The apparatus according to the invention comprises an electrically operable valve capable of allowing a fluid flow rate of at least 0.1ccfmin, the valve being mounted in a fluid flow conduit including a fluid flow sensor capable of detecting the quantity of fluid flowing therepast and of providing a feedback signal to the valve via electrical circuit means to control the open/close periods of the valve to thus maintain the flow of fluid in the flow line at a constant rate.

There now follows a description which is to be read with reference to the accompanying drawings, of a method and apparatus which have been selected for description to illustrate the invention by way of example and not by way of limitation.

In the accompanying drawings: Figure lisa circuit diagram of an apparatus according to the invention; and Figure 2 is a diagrammatic illustration of an embodiment of a flow sensor of an apparatus according to the invention.

The apparatus illustrated in Figure 1 comprises an electrically operated valve 1, which for example can be a Clippard Model EVO3 12V D.C. valve manufactured by Clippard Instrument Laboratory Inc. of Cinncinnati, Ohio, U.S.A., and is arranged in a flow line 2 for a fluid; this flow line will typically be provided by small bore piping of internal diameter between imm and 4mm such as is used for flow of gases in gas chromatography where flow rates between 0.1cc/min and 1 litre/min are usual. The valve 1 is capable of being cycled continuously between open and closed states and has a minimum response time of 5ms. so that it can cycle with a frequency in the range of 1 to 100Hz. Fluid flow from the valve is smoothed by a low pass network comprising a flow resistor 3 and a dead space 4.The smoothed flow is sensed by a mass flow sensor 5, (see Figure 2). The mass flow sensor comprises a heating coil 10 which extends around a portion of the flow line. Within the flow line are the two ends 12,14 of a hot wire thermometer, a first, upstream, end 10 of which wire is maintained at a first temperature and a second, downstream end 12 of which is maintained at a second temperature higher than the first temperature. The hot wire thermo meter is then calibrated for different, constant, flow rates, so that the mass flow sensor can provide a feedback signal which is related to the difference between the temperatures of the two ends of the thermometer for each given rate of flow and thus to the rate of flow.The feedback signal supplied by the mass flow sensor is a pulsed signal which may be supplied to an indicator 6 to provide, if required, visual indication of the flow rate, and to a first input of a comparator 16. A second input of the comparator 16 is connected to a controller 18 which provides a digital output such that at the comparator 16 it can be compared with that from the sensor 5. When identity between the two signals is achieved, the comparator provides an output signal to a switching oscillator 7 which in turn controls the mark space ratio of the valve driving frequency. i.e., the proportion of the time that the valve 1 is on and off.

The result is that the flow at the output of the sensor 5 remains substantially constant and is independent of changes in input and output flow resistances and pressure over a useful range. The outlet flow can be regulated to a required desired value by changing the mark space ratio of the switching oscillation by means of a variable set flow.

If it is desired to change the rate of flow of fluid in the line 2, this can be achieved by resetting the controller 18 so that it emits a different sequence of pulses corresponding to the adjusted flow rate. The pulse sequence is then compared in the comparator 16 with the sequence of pulses from the sensor 5.

When identity is achieved between the two sequ ences, a signal is emitted from the comparator to control the oscillator 7 and thus the frequency of operation of the valve 1. The controller 18 may be operated under the control of a computer.

Instead of using a hot wire thermometer as the fluid flow sensor, a hot wire anemometer, a thermis tor or a thermocouple may be employed.

Claims

1. A method of controlling the fíow of fíuid in a flow line, the method comprising allowing fluid to flow through the flow line which includes an electrically-operable valve, sensing the flow of fluid by flow sensor means located in the flow line downstream of the valve, and providing an electrical feedback signal from the flow sensor means through electrical circuit means to the valve to open and close the valve successively and reapeatedly, thereby to maintain a constant rate of flow of fluid in the flow line of at least 0.1cc/min.

2. A method according to claim 1 wherein fluid flow is controlled by varying the frequency with which the valve is opened.

3. A method according to either one of claims 1 and 2 wherein fluid flow is controlled by varying the open time of the valve.

4. A method according to any one of the preced ing claims wherein the mark space ratio of the valve is automatically adjusted by a feedback signal derived from a sensed value of the flow downstream of the valve whereby to maintain the flow at a substantially constant desired level.

5. A method of controlling the flow of fluid in a flow line substantially as hereinbefore described with reference to the accompanying drawings.

6. Apparatus for controlling flow of fluid in a fluid flow line, the apparatus comprising the fluid flow line, an electrically-operable valve positioned in the flow line, a fluid flow sensor mounted in the flow line downstream from the valve, and electrical circuit means connecting the fluid flow sensor to the valve whereby to control the opening and closing of the valve to maintain a constant rate of fluid flow in the flow line of at least 0.1cc/min.

7. Apparatus according to claim 6 wherein the fluid flow line comprises a fluid flow resistor portion and a dead space portion between the valve and the sensor.

8. Apparatus according to either one of claims 6 and 7 wherein the circuit means comprises comparator means having an output, a first input connected to the sensor means for receiving said feedback signal from the sensor, and a second input, connected to a fluid flow rate control means, for receiving a control signal from the control means for comparison with said feedback signal; means connected to the valve for operating the valve and connected to the output of the comparator means for receiving an output signal therefrom to operate the valve when said feedback signal and said control signal are identical.

9. Apparatus according to claim 8 wherein said means connected to the valve comprises an oscillator for continuously cycling the valve, and the output signal from the comparator means controls the mark-space ratio -of the oscillator.

10. Apparatus for controlling flow of fluid in a fluid flow line substantially as hereinbefore described with reference to the accompanying drawings.