WO1983001693A1

WO1983001693A1 - Fluid flow control system

Info

Publication number: WO1983001693A1
Application number: PCT/GB1982/000318
Authority: WO
Inventors: Henry Christopher Lewis
Original assignee: Henry Christopher Lewis
Priority date: 1981-11-06
Filing date: 1982-11-08
Publication date: 1983-05-11
Also published as: EP0093139A1

Abstract

The flow rate of a fluid to each of several delivery points (for example in a spraying system) is measured by a respective flow meter (17) and is compared by a comparator (21) with a reference signal VR representing a preset, desired flow rate. Any difference between the measured and the desired rates is reproduced as an error signal, and control device (10) controls the flow of the fluid through a passage (11) of variable size in accordance with this error signal. Operation of the device (10) is controlled by a pulsed electrical signal whose mark/space ratio is dependent upon the error signal, and the pulses in the signal cause the device (10) to vibrate the passage (11), thereby reducing the tendency of entrained particles in the fluid blocking the latter.

Description

"FLUID FLOW CONTROL SYSTEM"

This invention relates to a fluid flow control system and to a control device for use in such a system.

In fluid supply systems wherein a fluid is supplied to a plurality of delivery points, it is often required that the fluid flow to each delivery point should be automatically controllable to a respective preset value. Most equipment available for performing this function is, however, relatively complicated and therefore expensive to produce. In addition, little equipment is commercially available for use where low flow rates are involved.

In the case where flow control is achieved by means of a device (such as a valve) which constricts the fluid flow to a greater or lesser degree, problems can arise when the fluid is a liquid having particulate material suspended therein. The particulate material tends to accumulate in the vicinity of the constriction and causes a blockage, thereby reducing the fluid flow. As the constriction is progressively relaxed in order to restore the fluid flow to its former value, the material causing the blockage is often suddenly released producing a surge in the flow rate before the constriction can be narrowed once again. The repeated build-up and release of the particulate material causes the control device to relax and narrow the constriction substantially continuously over a comparatively wide range, which is undesirable both in respect of the excessive wear this imposes on the control device and in respect of the uneven flow rate which results.

It is an object of the present invention to obviate or mitigate the above-described problems.

OMH According to one aspect of the present invention, there is provided a fluid flow control system comprising measuring means operative to measure the flow rate of a fluid to a delivery point, comparison means operative to compare the flow rate measured by the measuring means with a preset, desired flow rate and to produce an error signal dependent upon the difference therebetween, and a fluid flow control device operative to control the flow of the fluid to said delivery point in accordance with said error signal.

According to a second aspect of the present invention, there is provided a fluid flow control device comprising a passage through which a fluid can flow and control means defining a variable constriction in said passage, the control means being operable to vary the size of said constriction and thereby control the fluid flow through the device, the control means being vibrated to cause the size of said constriction to oscillate about a mean value.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of a fluid flow control system according to the present invention, including one embodiment of a fluid flow control device;

Figure 2 is a cross-sectional view of an alternative form of fluid flow control device for use in the system of the invention; Figure 3 is a circuit diagram of a frequency/ voltage converter which forms part of the control system shown in Figure 1; Figure 4 is a circuit diagram of a control circuit which also forms part of the control system shown in Figure 1;

Figure 5 is a circuit diagram of a converter circuit forming part of the control system;

Figure 6 is a graph explaining the operation of the converter circuit;

Figure 7 is a similar diagram to Figure 4, but illustrating a modified form of control circuit; and Figure 8 is a circuit diagram of a further modification which can be made to the control circuit.

Referring first to Figure 1, the fluid flow control system illustrated therein is designed to control the flow rate to separate delivery points (not shown) of a liquid having a suspension of particulate material therein. As such, the system has many and varied applications, for example in agricultural spraying, food processing and the chemical and dyeing industries. The liquid is supplied to each of the delivery points by way of a respective flow control device 10, the control devices for two such delivery points being shown. Each control device 10 comprises a flexible tube 11 (for example made of plastics material) through which the liquid flows, the flow rate of the liquid being controlled by pinching the tube 11 between a fixed abutment 12 and a pivotable lever arm 13. The angular position of the lever arm 13 is controlled by means of a solenoid-operated actuator 14 whose plunger 15 is connected to a free end of the arm. The actuator 14 is powered by a pulsed current supplied along a line 16, the mark/space ratio of the pulsed current being variable in a manner to be described to vary the position in which the plunger 15 and hence the lever arm 13 is held. - k -

The flow rate of the liquid as controlled by each device 10 is measured by a respective flow meter 17, which may take any convenient form. The measured flow rate is then converted into an electrical output signal on a line 18: although the output signal could for example be a simple analogue voltage, in the illustrated arrangement it takes the form of a pulsed signal whose frequency is dependent upon the measured flow rate. The output signal from the flow meter 17 is applied to a frequency-to-voltage converter 19 which produces an output voltage dependent upon the frequency of the signal.

The output voltage of the converter 19 is applied to a control circuit 20 where a comparator 21 compares same with a reference voltage V_ representing the desired flow rate for the particular delivery station. At an output thereof, the control circuit 20 produces a signal which is dependent upon the difference between the two voltages, representing the error of the actual flow rate with respect to the desired value. This error signal is supplied to a converter circuit 22 which produces at an output thereof a pulsed current signal whose mark/space ratio is dependent upon the said error, and the latter signal is amplified by a power amplifier 23 before being applied to the actuator 14 via the line 16. In accordance with the signal received from the circuit 22, the actuator 14 operates the lever arm 13 to increase or decrease the flow rate of the liquid until the latter reaches the desired value.

The output voltage of the converter 19 is also applied by way of an operational amplifier 24 to a meter 25 which is calibrated to indicate the flow rate measured by the flow meter 17. In addition to being connected to the meter 25, the output of the amplifier 24 is also connected to one input of a comparator 26. The other input of the comparator 26 is connected to a reference voltage V_s representing a maximum or minimum permissible flow rate. In the event that the measured flow rate exceeds the permitted maximum or falls below the permitted minimum, as the case may be, a light- emitting diode 27 in the output circuit of the comparator 26 is energised to give a visual fault warning, and preferably also an audible warning to attract the attention of a user to the visual warning.

The above-described meter 25 thus indicates the flow rate of the liquid being supplied to an individual one of the delivery stations. A separate meter 25 can be provided for each delivery station in the manner illustrated, or a single meter can be switched into the various circuits to display the flow rates to the delivery stations in succession. In addition to displaying the various individual flow rates, it is desirable that an indication should also be given of the total flow rate to all of the delivery stations. To this end, the output signals of the converters 19 in the various circuits are supplied via respective resistors 28 to a summation point 29. The summation point 29 is connected to the non-inverting input of an operational amplifier 30, the output of the amplifier 30 being connected to its inverting input so that the amplifier acts as a voltage follower. The output of the amplifier 30 is also connected to a meter 31 which is calibrated to display the sum of the various flow rates measured by the individual flow meters 17, i.e. the total flow rate through the whole system. Instead of using a separate meter 31 for this function, the aforesaid single meter could be employed to display the total flow rate in addition to the individual flow rates.

OMPI_ For certain applications, it may be desirable to indicate not only the total flow rate but also the total volume flow over a given period. This may easily be achieved by providing suitable circuitry (not shown) to integrate the output signal from the amplifier 30.

In the above-described arrangement, the solenoid actuator 14 is powered by a pulsed current signal. The pulsed nature of this signal causes the plunger 15 and hence the lever arm 13 to vibrate, which effect is extremely useful in preventing accumulation of particulate material at the constriction where the tube 11 is pinched. Hence, the control system is able to provide a smooth flow of liquid to each delivery station with no surges due to the sudden clearance of a blockage. Indeed, the control system may be so arranged that the plunger 15 and hence the lever arm 13 are vibrated between two well-defined positions: in this case, the flow rates produced at the extremes of the vibrational stroke of the arm 13 will be averaged. Where -for example the liquid flowing through the system does not contain any entrained matter, vibration of the constriction provided by the tube 11 will not be absolutely necessary and it is possible to power the actuator 14 by an analogue current signal instead of a pulsed one. However, it is preferred that a pulsed signal is still employed in order to overcome "stiction" effects of the liquid within the tube 11.

In the above— escribed control device 10, when the actuator 14 is de-energised the arm 13 does not pinch the flexible tube 11 and consequently the flow rate is a maximum. In situations where zer-o flow is desired when the actuator is de-energised, it is possible in principle to construct a suitable linkage between the actuator and the srm, but this tends to complicate the construction of the control device 10 unnecessarily. Accordingly, it is preferred to employ a valve of the type shown in Figure 2 in place of the flexible tube 11 and the arm 13. In this valve, an axially movable valve member 100 has a configurated head 101 which co-operates with a fixed seat 102 to control the flow rate of the liquid to the respective delivery point. An O-ring 103 is provided between the head 101 and the seat 102 to seal the valve against liquid flow therethrough when the valve member 100 is in a closed position, as illustrated. A weak spring 104 acts on the valve member 100 to bias the latter towards the said closed position.

On the downstream side of the valve, the liquid acts upon a flexible diaphragm 105 against which the valve member 100 is engaged. On the opposite side of the diaphragm 105 to the valve member 100, there is disposed a solenoid-operated actuator 106 whose plunger 107 presses against the valve member 100 through the intermediary of the diaphragm 105. A spring 108 urges the plunger 107 in a direction to close the valve, i.e. to the left as viewed in Figure 2. Arrows 109 indicate the direction of liquid flow through the device.

When it is desired to move the valve out of the illustrated closed condition, the actuator 106 is energised by the aforementioned output signal from the circuit 20 (Figure 1) to move the plunger 107 to the right, as viewed in Figure 2. Such movement is transmitted through the diaphragm 105 to the valve member 100, thereby lifting the head 101 off the seat 102 and permitting the liquid to flow between these parts. Because of constrictions which are imposed on the liquid flow downstream of the control device, for example by the flow meter 17 (Figure 1) , the liquid becomes pressurised behind the diaphragm 105 to a degree which is dependent upon its flow rate. The liquid pressure then acts on the diaphragm 105, thereby urging the latter to the left, i.e. in a direction to oppose the movement of the plunger 107. For a small initial displacement of the plunger 107, the displacement of the valve member 100 will also be small and hence the flow rate of the liquid through the valve will be low. Consequently, the opposing force applied by the diaphragm 105 will be small. At larger displacements of the plunger 107, however, the increased flow rate through the valve will produce an increased force opposing movement of the plunger 107.

If the diaphragm 105 were to be omitted so that the plunger 107 acts directly on the valve member 100, the inherently non-linear force/displacement characteristic of the solenoid of the actuator 106 would result in the force required to open the valve only slightly being rather more than that needed to open the valve fully. Consequently, there would be a tendency for the control device suddenly to move to its fully open condition after it had been opened by only a small amount. This would make it difficult to effect fine adjustment of the liquid flow rate, a problem which would be compounded by the fact that only a small variation in the output signal from the circuit 20 is needed to operate the control device between its closed and fully open conditions. In the control device shown in Figure 2, however, this problem is avoided because of the force which is applied by the diaphragm 105 to the plunger 107, this force being dependent upon the flow rate of the liquid through the valve.

In order to compensate further for the non-linear force/displacement characteristic of the actuator 106, the spring 108 is itself preferably non-linear.

OMPI Wi?θ If a valve of the type shown in Figure 2 is used to replace :^' ach control device 10 in Figure 1, then it will of course be necessary to make suitable modifications to the control circuits 20 to take account of the fact that there is zero rather than a maximum flow rate when the actuator 106 is de-energised.

The above-described valve also has the advantage that its components can be made of material which is not chemically affected by the liquid whose flow is being controlled. In this regard, the flexible tubes 11 employed in the control devices 10 of Figure 1 would normally be made of a rubber or plastics material which is subject to chemical action by certain liquids, which can cause the tubes to become welded in a closed position.

Various elements of the control system will now be described in more detail. Referring first to Figure 3, the frequency-to-voltage circuit 19 makes use of a standard LM2907 integrated circuit (referenced 32) . An input terminal 33 of the circuit 19, to which the signal from the flow meter 17 is supplied, is connected via a capacitor 34 to pin 1 of the integrated circuit 32, the latter pin being connected to earth by way of a resistor 35. Pin 2 of the integrated circuit is connected to earth via a capacitor 36, while pin 3 is connected to earth via a resistor 37 and a capacitor 38 connected in parallel. Pins 4 and 7 of the integrated circuit 32 mS-re commonly connected both to earth via a resistor 39 and to an output terminal 40 of the circuit 19. Pins 5 and 6 of the integrated circuit are commonly connected to a supply voltage V_cc ( for example, 8 volts) , while pin 8 is connected to earth. The circuit 19 operates in a completely conventional manner to convert the frequency of the pulsed signal applied to its input terminal 33 into an analogue voltage which is provided at its output terminal 40, the magnitude of the analogue voltage thus being dependent upon the flow rate measured by the flow .meter 17.

Figure 4 shows the control circuit 20 in detail, which constitutes both an error detector and a three-term controller. An input terminal 41 of the circuit 20 receives the aforementioned signal from the output terminal 40 of the circuit 19, and supplies this by way of a resistor 42 to a summation point 43 which is connected to the inverting input of an operational amplifier 44. The summation point 43 is also connected to the wiper of a variable resistance 45 by way of a resistor 46, and to the wiper of a further variable resistance 47 by way of a further resistor 48. A reference voltage, derived from the connection point between two series-connected resistors 49 and 50, is supplied to the non-inverting input of the-operational amplifier 44, while the output of the latter is connected by way of a resistor 51 to its inverting input. The variable resistance 45 is adjusted so as to supply to the summation point 43 a voltage whose magnitude represents a preset, desired flow rate of fluid to the particular delivery point: arrows L and H denote the directions of movement of the wiper for respectively decreasing and increasing the preset flow rate. The variable resistance 47 is adjusted so that, when the wiper of the variable resistance 45 is moved as far as possible in the direction of arrow L, the flow rate is just zero.

The output voltage of the amplifier 44, which is thus proportional to the difference between the measured and preset flow rates, is supplied on the one hand to a differentiating circuit 52 and on the other hand to an integrating and- summing circuit 53, so that the signal appearing at an output terminal 54 of the control circuit 20 is dependent upon the said difference, its derivative and its integral. The differentiating circuit 52 can be omitted if desired, although this leads to the output signal at terminal 54 having a comparatively slow response to changes in the measured flow rate and a tendency for the system to be less stable.

The differentiating circuit 52 comprises an operational amplifier 55 to the non-inverting input of which is supplied the aforesaid output voltage from the amplifier 44 via a capacitor 56. The non-inverting input of the operational amplifier 55 is connected by way of a resistor 57 to the connection point between two series-connected resistors 58 and 59, while its inverting input is similarly connected by way of a resistor 60 to the connection point between the resistors 58 and 59. The output of the operational amplifier 55 is connected to its inverting input via a resistor 61. The voltage appearing at the output of the amplifier 55 is thus proportional to the time derivative of the above-mentioned difference, the constant of proportionality being dependent upon the value of the resistors 60 and 61 and the value of the ^■ capacitor 56.

The integrating and summing circuit 53 comprises an operational amplifier 62, the inverting terminal of which is connected to a summation point 63. The point 63 is connected on the one hand to the output of the amplifier 55 by way of a high frequency filter comprising a capacitor 64 and a resistor 65 connected in series, and on the other hand to the output of the

OMPI amplifier 44 by way of a resistor 66, the latter having a capacitor 67 connected in parallel therewith. The non-inverting input of the operational amplifier 62 has supplied thereto a reference voltage derived from the connection point between two series-connected resistors 68 and 69, while the output thereof is connected via a capacitor 70 to its inverting input. The voltage appearing at the output of the amplifier 62 thus represents the sum of a first component which is directly proportional to the difference between the measured and preset flow rates, a second component which is proportional to the time derivative thereof, and a third component which is dependent upon the integral with respect to time of the said difference. This voltage is applied directly to the output terminal of the control circuit 20.

Referring now to Figure 5, the converter circuit 22 receives the above-mentioned voltage at an input terminal 71 thereof, and converts same into a pulsed signal whose mark/space ratio is dependent upon the aforesaid difference between the measured and preset flow rates. The circuit 22 employs a conventional LM 556 integrated circuit (referenced 72) which, as is well known, comprises two 555 type timers. One of the timers, (the "A" timer, represented by pins 1 to 6 of the integrated circuit 72) is connected as an astable circuit to ensure that the said pulsed signal is of constant frequency, while the other timer (the "B" timer, represented by pins 8 to 13) is used in the generation of the variable mark/space ratio signal: it is possible for the first of these timers to be omitted where a constant frequency pulsed output is not essential.

O P1 Ay - I3 -

With reference to the "A" timer, pin 1 of the integrated circuit 72 is connected on the one hand to a supply voltage V_cc (for example 8 volts) by way of a resistor 73, and on the other hand to earth by way of a resistor 74 and a capacitor 75 connected in series. Pins 2 and 6 of the integrated circuit are commonly coupled to the connection point between the resistors 74 and the capacitor 75. The values of the resistors 73, 74 and of the capacitor 75 are chosen so that the astable circuit produces an output signal having a frequency of 30 to 50 Hz and a large (fixed) mark/space ratio. Pin 3 of the integrated circuit 72 is connected to earth via a capacitor 76, while pin 4 is connected directly to the supply voltage. Pin 5, together with pin 10, is connected by way of a resistor 77 to the base of a transistor 78, the collector of the latter being connected via a resistor 79 to the supply voltage. Pin 8 of the integrated circuit is connected on the one hand by way of a capacitor 80 to the collector of the transistor 78 and on the other hand via a resistor 81 to the supply voltage. This series of connections serves to invert the signal from the "A" timer in order to reset and trigger the "B" timer.

With reference to the "B" timer, pins 8, and 10 of the integrated circuit 72 are connected in the manner described above. Pin 11 is connected to the input terminal 71 of the circuit 22. Pins 12 and 13 are commonly connected on the one hand to the power supply by way of a resistor 82 and on the other hand to earth by way of a capacitor 83.

Pins 7 and 14 of the integrated circuit 72 are connected directly to earth and to the supply voltage, respectively, in the usual manner. - I k -

The operation of the converter circuit 22 will now be explained with reference to Figure 6. At a time t₀ determined by the above-mentioned astable circuit, a signal appearing at pin 9 goes from a low level to a high level and at the same time a threshold voltage (represented by a solid line in the graph) starts to increase from zero. When the threshold voltage reaches the level of the control voltage applied to the input terminal 71 of the circuit 22, the signal at pin 9 goes from its high level back to its low level. Thus, if the voltage at the terminal 71 is relatively small (as represented by V_) the signal at pin 9 will return to its low level at a time t_, as indicated by waveform (a) , whereas if the voltage at terminal 71 is relatively large (as represented by V2) the signal at pin 9 will return to its low level at a later time t2, as indicated by waveform (b) . Thus, it will be appreciated that the mark/space ratio of the signal appearing at pin 9 will depend upon the voltage at the input terminal 71. After one complete period of the signal produced by the astable circuit, the threshold voltage returns to zero and the process begins once again. Since the period of this signal is fixed, the frequency of the signal appearing at pin 9 will be constant.

In the graph of Figure 6, the solid line indicates the time characteristic of a threshold voltage obtained by means of the charging of a capacitor and, as explained above, the signal at pin 9 of the integrated circuit 72 returns to its low level at a time t^ for a control voltage of Vτ_ and at a time t2 for a control voltage of V2. The times ti and t2 bear a predetermined relationship with one another, determined by the shape of the threshold voltage characteristic. Consider now the case where a linear pulse width

OV.fl generator is employed, as indicated by the chain-dotted line in the graph. For a control voltage of V_]_ the signal at pin 9 now returns to its low level at a time t]_', as indicated by waveform (c) , while for a control - voltage of V2 the signal at pin 9 returns to its low level at a time t2*, as indicated by waveform (d) . The times ti¹ and t2' once again bear a predetermined relationship to one another, but this relationship is different from that between ti and t2 because of the different characteristic of the threshold voltage. It will therefore be appreciated that, whilst the relationship between the mark/space ratio of the signal at pin 9 and the control voltage applied to terminal 71 is predetermined for a given threshold voltage characteristic, this relationship can be altered by changing the said characteristics. This enables the control system as a whole to be adapted in a simple manner to suit the particular requirements of the user.

As an example of a further type of time characteristic which can be employed for the threshold voltage, there is indicated by a broken line in the graph of Figure 6 the characteristic obtained by inverting the control voltage before it reaches the terminal 71 and by inverting the signal after it leaves pin 9 of the integrated circuit 72, assuming that a charging capacitor is employed in the same manner as in obtaining the characteristic indicated by the solid line.

Referring back to Figure 5, pin 9 of the integrated circuit 72 is connected via a resistor 84 to the base of a transistor 85 in the power amplifier 23. The emitter of the transistor 85 is connected directly to earth, while its collector is connected by way of a resistor 86 to the base of a further transistor 87. The emitter of the transistor 87 is connected to a supply voltage V_s (for example 12 volts) , while its collector is connected to an output terminal 88 of the power amplifier 23. A diode 89 is connected between the collector of transistor 87 and earth.

Referring back to Figure 1, when the control system shown therein is brought into operation, the electrical signal supplied to each control device 10 from the respective control circuit 20 is initially at zero and must reach a certain level before proper operation can commence. In an output stage thereof, the circuit 20 has an operational amplifier which is normally set to a relatively low gain: if the amplifier remains at this setting, then the output signal will take too long to reach the required level, and it therefore becomes necessary temporarily to increase the gain of the amplifier. Although the output signal now reaches the general required level much more quickly, it tends to oscillate around the actual required value before finally settling down, so that there is still a significant delay before proper operation can commence.

This particular problem can be overcome by modifying each control circuit 20 in the manner shown in Figure 7, namely by inserting a variable resistance 110 in series between the fixed resistances 49 and 50, and by connecting the wiper of the resistance 110 on the one hand to the non-inverting input of the operational amplifier 44, and on the other hand to the non-inverting input of the operational amplifier 62. The fixed resistances 68 and 69 shown in Figure 4 are now omitted. - I7 -

When the control circuit is brought into operation, a voltage will initially appear at the output terminal 54 which is dependent upon the signal applied to the non-inverting input of the operational amplifier 62. The variable resistance 110 is set so that this voltage will operate the respective control device 10 to give roughly the required liquid flow rate. Thus, when the system commences its operation, the control device 10 will immediately deliver a flow rate which is close to the actual desired value, thereby eliminating the delay previously experienced in initially reaching the desired flow rate.

Where the circuit 20 shown in Figure 7 is used in conjunction with the valve depicted in Figure 2, it is necessary to incorporate an inverter (for example as indicated at 111) into the circuit to take account of the fact that the control device operates in a reverse sense, i.e. when the actuator 106 is de-energised the flow rate is zero.

In the arrangement illustrated in Figure 1, the output signal from each flow meter 17 (representing the flow rate of liquid to the respective delivery point) is supplied via an amplifier 24 to a meter 25 and also to one input of a comparator 26. The meter 25 is calibrated to give a visual indication of the actual flow rate, while the comparator 26 compares the output signal with a reference signal Vs and, for example, energises a light-emitting diode 27 in the event that the actual flow rate exceeds a maximum permissible value. A separate meter 25 may be provided in respect of each delivery point, or a single meter 25 may be connected to switching means such that it can display selectively the flow rates to the various delivery points. This arrangement does, however, require the reference signal Vs to be adjusted by an operator individually for each delivery point, and must be totally re-set in the event that user requirements necessitate the desired flow rate being changed.

In a further modification of the control system, this problem can be overcome by providing comparison means which compares the aforesaid error signal (rather than the output signal from the flow meter) with at least one reference signal representing a maximum permissible error, and which operates an indicator in the event that the error signal exceeds said maximum error. Figure 8 shows one example of a circuit which can put this technique into effect, it being appreciated that one such circuit will be provided for each delivery point. In this circuit, the non- inverting input of an operational amplifier 120 is connected to the output of the operational amplifier 44 in the respective control circuit 20, while its inverting input is connected to the wiper of a variable resistance 121. The output of the amplifier 120 controls the collector/emitter conduction state of a transistor 122, the latter having its collector/emitter current path connected in series with a light-emitting diode 123 (and possibly also an audible warning device, not shown) and a current limiting resistor 124. The resistance 121 is preset so that it applies a reference signal to the amplifier 120 representing the maximum permissible deviation of the measured flow rate below the desired flow rate. When the error signal produced at the output of the amplifier 44 exceeds the maximum permissible deviation, the amplifier produces an output signal which renders the transistor 122 conductive, thereby energising the light-emitting diode 123. A visual and/or an audible warning is thus given to an operator that the actual flow rate has fallen sufficiently to cause a fault situation. In an alternative arrangement, the inputs of the operational amplifier 120 are reversed so that a fault condition is indicated when the error signal exceeds a maximum permissible deviation of the measured flow rate above the desired flow rate, i.e. when the flow rate has become too high. Indeed, separate circuits could be provided to monitor high and low flow rates respectively, each circuit having its input connected in the opposite sense.

Since the circuit detects variations in the error signal, rather than in the signal representing the measured flow rate, it is not necessary for the reference signal from the variable resistance 121 to be adjusted each time the desired flow rate is altered. Thus, there is no need for the resistance 121 to be adjusted unless a different deviation is to be detected.

The circuit shown in Figure 8 is intended to replace the meter 25, the amplifier 26 and their associated parts shown in Figure 1.

The above-described control systems enable the flow rates to individual delivery stations to be controlled automatically and accurately, and involves comparatively little expense in its manufacture. Moreover, the control systems are capable of handling comparatively low flow rates (for example between 10 ml and ll/2 litres per minute) , for which flow control equipment of this type has not previously been widely commercially available. The use of a flexible tube 11 (Figure 1) or a valve (Figure 2) in the manner described above, enables the flow rate to be varied by up to a factor of ten, which is well within the requirements of most fluid supply systems. Furthermore, the systems are quite capable of handling corrosive materials, and are also unaffected by factor which normally have quite significant effects where th fluid supply system is vehicle mounted, for example when used in agricultural spraying, such effects including g-forces and the amount of liquid remaining in a reservoir which supplies the system.

Claims

C L A I S

1. A fluid flow control system comprising measuring means operative to measure the flow rate of a fluid to a delivery point, comparison means operative to compare

5. the flow rate measured by the measuring means with a preset, desired flow rate and to produce an error signal dependent upon the difference therebetween, and a fluid flow control device operative to control the flow of fluid to said delivery point in accordance with 0 said error signal.

2. A control system as claimed in Claim 1, wherein operation of the control device is controlled by a pulsed electrical signal whose mark/space ratio is dependent upon said error signal.

5 3. A control system as claimed in Claim 2, wherein the control device includes a solenoid-operated actuator whose operation is controlled by said pulsed electrical signal, which is a pulsed current signal.

4. A control system as claimed in Claim 1,2 or 3, 0 wherein the control device controls the size of a passage through which the fluid flows, and is vibrated to cause said size to oscillate about a mean value.

5. A control system as claimed in Claim 4 when appended to Claim 2 or 3, wherein the control device is 5 vibrated by the pulses of said pulsed signal.

6. A control system as claimed in any preceding claim, wherein the control device comprises a valve having a valve member which can be moved by means of an actuator to vary the flow rate of the fluid through the valve, and means responsive to the flow rate of the fluid and acting to apply a force to the valve member which opposes movement thereof in a direction to increase the fluid flow rate, which force varies in dependence upon said fluid flow rate.

7. A control system as claimed in Claim 6, wherein said means is responsive to the fluid pressure downstream of the valve.

8. A control system as claimed in Claim 7, wherein said means takes the form of a flexible diaphragm.

9. A control system as claimed in Claim 6,7 or 8, wherein the actuator is solenoid operated, and a spring biasses a plunger of the actuator in a direction to close the valve.

10. A control system as claimed in Claim 9, wherein the spring has a non-linear force/displacement characteristic.

11. A control system as claimed in any preceding claim, wherein operation of the control device is controlled by an output signal of a control circuit of which the comparison means forms part, the output signal being dependent upon said error signal, and the control circuit also includes adjustment means whereby the initial level of the output signal when the system is first brought into operation can be adjusted to a preselected value. - 2 τ

12. A control system as claimed in Claim 11, wherein said error signal is supplied to one input of an operational amplifier, a reference signal is applied to a the other input of said amplifier, said output signal is derived from an output of said amplifier, and adjustment means is operable to adjust the level of said reference signal.

13. A control system as claimed in Claim 12^', wherein the comparison means includes an operational amplifier having an input which is supplied with the same reference signal as said other input of the first-mentioned operational amplifier.

14. A control system as claimed in any preceding claim, wherein further comparison means compares said error signal with a reference signal representing a maximum permissible error, and operates a warning indicator in the event that the error signal exceeds said maximum error.

15. A fluid flow control device comprising a passage through which a fluid can flow and control means defining a variable constriction in said passage, the control means being operable to vary the size .of said constriction and thereby control the fluid flow through the device, the control means being vibrated to cause the size of said constriction to oscillate about a mean value.