GB2260965A - Metering and dispensing system - Google Patents

Abstract

A metering system for liquids which is particularly adapted for accurately controlling small flows of liquid. A pair of metering tubes (3, 10) which are alternately filled and emptied are each provided with level sensors (9, 4; 19, 18) providing input signals to a control circuit. This determines the actual flow rate from the current tube and is also supplied with a set input of a desired flow rate, from which a dispensing pressure signal is determined and applied to an electropneumatic transducer so that corresponding dispensing pressure is applied to the liquid. The control circuit also stores the current value of dispensing pressure which is utilised in the determination of the dispensing pressure at the next dispensing cycle. <IMAGE>

Description

"Liquid Dispensing and Detering System" This invention relates to liquid dispensing systems.

In the processing of certain agricultural products it is necessary to apply accurately controlled amounts of additives, for example as pesticides, insect growth inhibitors or for other purposes such as mould inhibitors.

Usually these additives must be applied so that the concentration of active ingredient in the product falls within specified limits, sometimes defined by law.

Application systems are therefore required which will accurately dispense a known flowrate of liquid additive, and confirm by means of appropriate instrumentation that the correct flow of additive is being delivered. Such a system should also incorporate means to correct error trends, so as to automatically maintain the accuracy of dispensing.

In some instances the flowrate of liquid additive is so small as to be below the range of commonly available flowmeters, or the properties of the liquid may be unsuitable for use with a preferred type of flowmeter (e.g.

the liquid may be electrically non-conductive, which would preclude the use of an electromagentic flowmeter).

The use of positive displacement metering pumps for dispensing small flows is quite common. However, such pumps do not provide confirmation that the appropriate quantity of liquid has been dispensed. Instrumentation on metering pump systems usually comprises some form of sensor which detects the pump motion: from this it is inferred that a corresponding liquid flow has occurred, which may not in fact be true if the pump has developed a defect.

In order to solve these difficulties, the present invention provides a volumetric metering system with automatic control and self-checking features, and the facility to vary the liquid dispensing rate in response to an externally varying set-point signal. This latter feature enables the liquid to be applied for example in the correct ratio to a varying flow of another product measured by a suitable device (e.g. a belt weigher).

Accordingly the present invention provides a volumetric metering system for liquids, comprising a pair of elongate containers of uniform cross-section, each having a pair of spaced apart level sensors arranged along its length, so as to indicate the passage of a known volume of liquid when the level has passed both sensors, whereby liquid can be dispensed continuously by refilling one container whilst liquid is dispensed from the other.

According to another feature of the invention there is provided a volumetric metering system for liquids comprising an elongate container of uniform cross-section having a pair of spaced apart level sensors arranged along its length, means for determining the flow rate in terms of the time taken for the liquid level to pass between the sensors, an electro-pneumatic transducer for supplying dispensing pressure to the liquid in the container, and control circuit means for supplying a signal to the electropneumatic transducer so as to control the flow to a desired rate.

Preferably, the flow rate determined during a cycle of dispensing from one of the containers is used to determine the magnitude of the control signal applied to the EP transducer, and thus the pressure applied to the other container, during the next dispensing cycle. In this way a virtually continuous flow of liquid can be obtained, which is accurately controlled even at very small flow rates. It will be appreciated that when the flow rates are relatively small, the containers will be formed as correspondingly narrow tubes, so that the time taken for the passage of a small amount of liquid can still be accurately determined.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawings.

Figure 1 is a schematic view of metering apparatus in accordance with the invention; and Figure 2 is a control system for the metering apparatus of Figure 1.

Referring to Figure 1, the metering system comprises two vessels of known volume, which as shown are in the form of vertical glass tubes, 3, 10 of uniform crosssection. The ratio of tube bore to length is chosen so that a suitable discharge time is obtained, and end detection errors are minimised. Each vessel is provided with photoelectric sensors 9, 4 and 19, 18 respectively close to the top and bottom, so that the presence or absence of liquid may be detected. If the liquid is transparent, the sensors may be arranged so as to detect the change in effective focal length of the vessel due to the presence or absence of liquid.

The metering vessels are connected by small bore tubing and pilot operated shut-off valves as shown in Figure 1. The preferred sequence of operation is as follows: Liquid flows by gravity from a storage tank 1 to a transfer vessel 2 via valve 5. When metering vessel 3 is empty, as detected by sensor 4, liquid is propelled from the transfer vessel 2 into the metering vessel 3 by closing valve 5 and opening valves 6, 7 and 8. A source 20 of compressed air is connected to the transfer vessel via valve 6 propels the liquid via valve 7, and fills the metering vessel 3. The liquid overflow continues via valve 8 and is returned to the storage tank for a timed period after sensor 9 has detected the presence of liquid. This allows air bubbles to be purged from the metering vessel. Valves 6, 7 and 8 are then closed, and valve 5 is opened, to allow the transfer vessel to re-fill.Metering vessel 10 is filled by a similar process, i.e. by closing valve 5, and opening valves 6, 14 and 16 so as to propel liquid through valve 14 into the vessel.

When it is desired to commence dispensing the liquid, air pressure is applied by electromagnetic converter 24 to the top of one of the metering vessels 3 by opening valve 11. Valve 12 is opened to allow liquid to flow from the metering vessel to the spray via a flow restrictor orifice 13.

The instantaneous liquid flowrate is dependent upon the magnitude of the pressure applied to the top of the metering vessel, and the pressure versus flow characteristic of the restrictor 13.

A control circuit 22 computes the actual liquid flowrate by determining the time which elapses between the absence of liquid being sensed by sensor 9 and subsequently by sensor 4. The flowrate can then be computed by arithmetically dividing the known volume of the metering vessel by the elapsed time.

This measured flowrate can then be compared with the desired flowrate, and if necessary the control signal to the E.P. converter can be modified so as to reduce any error during the next cycle of dispensing.

The completion of a dispensing cycle is signalled by the detection of the absence of liquid at the lower end of the metering vessel. The alternative vessel immediately starts to dispense, and the empty vessels is refilled as described above.

Self checking features can be incorporated into the system to ensure that a malfunction does not go undetected. For example, during the filling sequence, the upper liquid detector 9 should change state from "liquid absent to "liquid present" within a predetermined time from the start of the sequence.

Similarly during the dispense sequence, the upper liquid detector should change state from "liquid present" to "liquid absent" within a preset time from the start of the sequence.

Figure 2 is a schematic representation of the flow control system (22), which would preferably by embodied in a programmable controller or computer. Only those signals relating to one of the metering vessels are shown for reasons of clarity. The desired flowrate to be dispensed is an input (30) to the system, from which the desired dispense time is computed by dividing the flowrate by the known volume of the vessel (32) between the upper and lower sensors 9 and 4.

The actual dispense time is measured by means of a timer, which is initiated when sensor 9 no longer detects liquid, and stops when sensor 4 no longer detects liquid.

This latter event initiates the computation of a correction to the dispense pressure as follows: The ratio of actual dispense time (34) to desired dispense time (36) is computed. An algorithm (38) corresponding to the restrictor (13) flow characteristic is then applied to this ratio, and the resultant correction factor (40) is multiplied by the dispense cycle pressure signal (42), to obtain a new value of dispense pressure (44) to be applied for the duration of the next cycle.

Claims (4)

1. A volumetric metering system for liquids, comprising a pair of elongate containers of uniform crosssection; a pair of sensors spaced apart along the length of each container so as to indicate the passage of a known volume of liquid; means for dispensing liquid alternately from each container; and means for filling each container whilst the other is being emptied, whereby liquid can be dispensed continuously from the system.

2. A volumetric metering system comprising an elongate container of uniform cross-section having a pair of spaced apart sensors arranged along its length, means for determining the flow rate in terms of the time taken for the liquid level to pass between the sensors, an electropneumatic transducer for supplying dispensing pressure to the liquid in the container, and feedback control circuit means for supplying a signal derived from said sensors to the electropneumatic transducer so as to control the flow to a desired rate.

3. A volumetric metering system according to claim 1 or claim 2 further comprising a control circuit having inputs connected to each of said sensors, means for determining a corresponding flow rate for the current dispensing cycle from said sensors; and means for determining a dispensing control signal for the next dispensing cycle in dependence upon said flow rate.

4. A volumetric metering system according to claim 3 in which the control circuit comprises; means for setting a desired dispense flow rate value; means for setting a vessel volume value; means for dividing the said volume by the said flowrate to produce a signal representing a desired dispense time; actual dispense time measuring means comprising a timer, and inputs from each of said sensors which are arranged to start and stop said timeras the liquid passes the sensors; means for calculating the ratio of the desired dispense time and actual dispense time to determine the actual pressure/flow relationship; means for determining a control correction factor from said relationship means for storing the value of dispense pressure applied during a previous cycle; and means for multiplying said previous value by said correction factor to produce a dispensing pressure signal for said electropneumatic transducer.