GB2259157A - Liquid dispensing pressure control - Google Patents

Abstract

In a system for dispensing a fluid eg beer, the speed of the pump is adjusted in response to comparison at 120 of the fluid pressure measured by pressure sensor Cp with a desired reference pressure set at 130 to deliver the beer at the desired pressure. To ensure the pump is shut off as the flow is reduced to zero, eg when a tap is slowly shut off, feedback is provided from the output of the control means 120 to reduce the value of the desired reference pressure. Adjustment of the control ratio or gain of the system as the pressure error varies is also disclosed. <IMAGE>

Description

PUMP CONTROLLER The present invention relates to a control device for a pump, in particular one suitable for dispensing pressurised liquids such as beer.

Various systems are known for dispensing beer from one or more taps by means of a pump. Such systems have to address several problems in order to be successful.

One is that the pump must be reliably turned off when the tap is closed, or in the case of a multiple-tap system when all the taps are closed. Another is that the beer should be delivered at a more or less constant rate regardless of the number of-taps opened; this in turn means that the pressure generated by the pump must be kept constant, since the rate of flow affects the taste of the beer. This problem is thus particularly acute in multiple-tap systems supplied by a single pump. A further requirement is that the system be able to cope with the event that the beer runs out.

GB 1 545 447 (Pektron) shows a system designed principally for a single-tap system: the control device comprises a spring-loaded pressure switch which can be set manually to a given limit pressure. This represents that pressure in the line below which the pump is triggered. Since the pressure switch does not discriminate between the high-pressure state existing when the tap is closed and the still fairly highpressure state with the tap open and beer being pumped, a flow sensor in the form of a magnetic float is also provided. When the tap is opened the pump is initially turned full on as a result of the sudden pressure drop; once pressure is re-established by the pump during dispensing of beer, i.e. with the float sensor lifted, the motor control is switched to a lower setting determined by a potentiometer whose setting can be varied by the user.Finally, when the tap is turned off, the float falls and the motor is turned off.

In order to cope with the case that the beer runs out, which means that the dispense pressure is low and the float has fallen, a: timing circuit is included in order to turn the motor off after a certain time, since otherwise the motor runs at full speed in order to try and restore the pressure.

This system is not suitable for multiple taps since there is no automatic flow control for the motor; in other words, the motor can run at only one preset speed other than full speed.

In a later system shown in GB 2193704, also by Pektron, instead of a simple pressure switch a pressure sensor is used which by means of a magnetic moving member and a Hall-effect sensor can detect the pressure in the delivery bore. Circuitry enables a high pressure P to be maintained when the taps are closed and a lower pressure C to be maintained during dispensing by means of a feedback control loop for running the pump. A timer is again used to turn the pump off if pressure is not restored when the taps are closed, i.e. when the beer has run out. A disadvantage of this device is that the Hall effect sensor is nonlinear, and it may further be sensitive to the considerable stray magnetic fields present around the pump.

Both the previous arrangements use a float to detect the presence or absence of flow. A system which does without the cumbersome and inconvenient float switch is shown in GB 2094269. This device is similar to that shown in GB 2193704 in that it uses a Halleffect sensor and two set pressure levels, but no float is present: the pump is turned off either if the upper pressure limit is exceeded or if the lower limit is notreached and the timer times out.

Finally, GB 2177523 (Partridge Wilson & Co) shows a further system which does not use a flow switch and in addition addresses the problem of how to ensure that the pump is reliably turned off even when the tap is turned off slowly. This is done by means of an oscillator which modifies the voltage set by a potentiometer in order to define the set pressure below which the pump is turned on. This modification or modulation ensures that when the tap is turned off, even slowly, the actual pressure will rise above the set pressure and the pump will be turned off. The oscillator is switched off while the pump is turned off.

In addition the circuit of GB-2177523 incorporates a fairly complex arrangement to turn the pump off when the beer runs out. In addition to the timer used also in other arrangements there is a comparator and a delay circuit for detecting whether the pressure is rising or not; to this end the pump is temporarily switched off or slowed, and if the pressure falls it is concluded that flow is still continuing, whereupon the system is reset and the pump continues. If the pressure does not fall then it is concluded that the beer has stopped flowing, whereupon the pump is switched off altogether.

GB 2177523 uses a potentiometer as a pressure sensor, the output voltage from the potentiometer giving the pressure in the bore. Potentiometers are linear but are prone to frictional wear. Other sensors are mentioned as alternatives but there is no suggestion that the arrangement shown could be improved.

As far as is known to the applicant none of the known pump control systems adequately deal with multiple-tap outlets, and an aim of the invention is to provide a simple and reliable device which does so.

According to one aspect of the present invention there is provided a pump control device for use in dispensing a liquid, including a pressure sensor for detecting the pressure of the fluid to be dispensed, a means for setting a desired pressure, and a means for controlling the speed of the pump in order to deliver the liquid at the desired pressure by comparing the reading from the pressure sensor with the desired pressure while liquid is being dispensed and altering the speed of the pump so as to counteract a deviation of the detected pressure from the desired pressure, in which the control means is adapted to operate with an increased control ratio as the magnitude of the said deviation of the detected pressure from the desired pressure increases.By 'control-ratio' is meant the ratio of the change in output to the pump for a given change in the output from the pressure comparison, and could also loosely be termed the 'gain' of the control means.

Such an arrangement can ensure that the dispensing system copes with the situation of very fast pressure drop, as when the first tap is opened, but also provides a stable pressure control for the conditions obtaining when subsequent taps are opened or closed, the consequent variation in pressure being much smaller. The control device need provide the variation in control ratio only for one sign of pressure deviation, namely when the measured pressure drops below the desired pressure, since in the opposite situation the pump is required merely to shut off.

The increase in control ratio need be only a step function, although continuous control ratio variation could be used. For instance, if an operational amplifier is used the control means can be adapted to operate this amplifier at a certain control ratio for normal operation but to switch it to a higher control ratio when the measured pressure drops by more than a predetermined amount below the desired pressure; thus the control means may include a negative feedback loop for controlling the motor speed, and the device may further include a means for disconnecting the said loop when the measured pressure falls below the desired pressure by more than a predetermined amount, so as to drive the pump at full speed.

In a preferred embodiment the feedback loop includes a capacitor for providing AC coupling of the output to the input, and the disconnecting means is adapted to discharge this capacitor while the loop is disconnected. This provides an arrangement with a very fast response time. Further such a system can be used to detect the absence of liquid to-be pumped by the simultaneous occurrence of low pressure and full pump speed, thus rendering the use of a timer unnecessary, although a timer can of course be included if desired.

In a further aspect the invention is directed to a pump control device for use in dispensing a liquid including a pressure sensor for detecting the pressure of the fluid to be dispensed, a means for setting a desired pressure, and a means for controlling the speed of the pump in order to deliver the liquid at the desired pressure by comparing the reading from the pressure sensor with the desired pressure while liquid is being dispensed and altering the speed of the pump so as to counteract a deviation of the detected pressure from the desired pressure, the device further including means for feeding back a proportion of the output of the control means to the pressure-setting means so as to reduce the desired pressure input and thereby to ensure that, in use, the pump is turned off as flow is reduced to zero.

The system of the invention is related in general terms to that described in GB 2177523. However, the circuit disclosed in that document uses several additional circuit sections and is operated in a step fashion in that the modification of the set point is only carried out when the pump is stopping; this needs a separate "starting detector" which turns the set point modifier on and off appropriately. In contrast, in accordance with the analogue nature of the invention a direct feedback construction is used which is extremely simple to incorporate into the control circuit and can consist of a single additional resistor.

From yet another aspect the invention is directed to a pump control device for use in dispensing a liquid, including a pressure sensor for detecting the pressure of the fluid to be dispensed, a means for setting a desired pressure, and a means for controlling the speed of the pump in order to deliver the liquid at the desired pressure by comparing the reading from the pressure sensor with the desired pressure while liquid is being dispensed and altering the speed of the pump so as to counteract a deviation of the detected pressure from the desired pressure, the device further including means for forcing the pressure sensor after a predetermined time to give an output corresponding to a high pressure if a low pressure is present and the pump has continued to run for the said predetermined time, so as to switch off the pump when liquid to be pumped is no longer present.

In another aspect the invention provides a pump control device for use in dispensing a liquid, including a pressure sensor for detecting the pressure of the fluid to be dispensed and a means for controlling the speed of the pump in order to deliver the liquid at a desired pressure, in dependence on the reading from the pressure sensor, in which the pressure sensor includes a capacitor, one plate of which is formed by a diaphragm acted on by the liquid so as to vary the capacitance of the capacitor and hence provide the said pressure reading.

An advantage of using such a sensor is that only a very small movement of the capacitor plate is required, which means that the sensor can be highly linear.

Further, the device can be made very simply and in a robust form, for instance by using a brass diaphragm and a plated area on a printed circuit board as the other plate of the capacitor. This contrasts with the expensive Hall-effect and variable-resistance sensors of the prior art. Also, all or part of the rest of the control circuit can be integrated on the printed circuit board, saving considerable space and providing a very compact arrangement. - For a better understanding of the invention embodiments of it will now be described with the aid of the accompanying drawings, in which: Fig.l is a block diagram of the controller; Fig.2 is a circuit diagram of the controller; and Figs. 3 and 4 show views of a construction of the pressure-measuring capacitor.

A typical application of the system in accordance with the invention comprises: (a) A barrel of liquid under pressure (e.g. beer); (b) An electrically operated pump; (c) A non-return valve; (d) A control unit; and (e) One or more dispense taps, all connected together by lengths of pipe. Figs. 1 and 2 show only the control unit and pump.

The pressure sensor is of the capacitive type.

The capacitor Cp is formed by the diaphragm, which is preferably a brass plate, and a small disc etched onto a printed circuit board. The small disc is connected to the outside world by a thin track so as to minimise the swamping effect of fixed capacitors which are in parallel with the pressure measurement capacitor.

Further description of the physical construction of the sensor follows later in connection with Figs.3 and 4.

The measurement capacitor Cp is charged with a small constant current. This causes the voltage across the plates of the capacitor to rise in a linear fashion (ramp). At regular intervals the capacitor is discharged and allowed to charge again at a frequency of around 20KHz by means of a pulse oscillator 110. As the pressure increases the output voltage decreases due to the increase in capacitance.

The resulting sawtooth waveform is integrated in an integrator 100, producing a steadily varying output voltage indicative of the pressurgin the dispense line - or manifold. This measurement voltage is compared in comparator 120 with a desired voltage corresponding to the set dispensing pressure and produced by set point generator 130 in response to the setting of a potentiometer. An error detection unit 140 ensures among other things that the pump is turned off when no liquid is present. The pump control output is fed to a comparator 150 which produces a pump drive signal for a length of time determined by the point in the cycle of the modulator 160 represented by the said pump control output.

With reference to the circuit diagram shown in Fig.2, the first of four operational amplifiers incorporated into IC2, namely IC2a, is arranged as a voltage follower. This means that the voltage seen at the output is exactly the same as at its non-inverting input but it has a much lower impedance. To achieve the constant current referred to above a boot-strapping idea is used whereby the voltage at the junction of Rll/R4/C5 is changing at the same rate as the output of IC2a but is AC-coupled by C5 and so the voltage at the junction of Rll/R4/C5 is around 15v higher than that of the output of IC2a. This means that there is a constant voltage across R4 and as the voltage is constant and the resistor is fixed then the current must be constant.

Transistor TR3 is used to discharge the sensor capacitor Cp at intervals via the pulse oscillator 110 formed by IC3a/TR5 etc. The way the pulse oscillator works is as follows: IC3a is an analog changeover switch and is arranged so that it works as a non-inverting digital buffer. Capacitor C2 is connected across the buffer so that when the buffer changes state a "snap action" occurs where as the output falls it causes the input to fall which causes the output to fall even further etc.

Altering the voltage at the input to the buffer is done by resistors R22 and VR2. When the output of the buffer is high the transistor TR5 is on and so the resistor R22 is used to discharge C2 until it reaches the switching point of the buffer. The output then goes low and both R22 and VR2 are used to charge the capacitor because TR5 is now off. R12 is connected to supply and is used to pull high the oscillator output when the buffer is inhibited (i.e. all terminals disconnected). This is for when the beer runs out and ensures that the output of the pressure sensor will then go low, which looks like infinite pressure to the rest of the circuit which then switches off the motor in response, as will be described later.

The speed of the oscillator is set such that the output of the pressure sensor is allowed to get to 10v when there is no pressure.

Moving on, R27 and C7 form a filter network which removes the ramp waveform from the output of the pressure sensor. This means that the voltage seen at the junction of R27/C7/R9, which represents the pressure sensor reading, will be half the peak voltage seen at the output of the pressure sensor, in this case 5V at zero pressure The pressure sensor reading is fed to the loop control formed by IC2b/R9/C6 and arranged to be of the type where only proportional and integral action are applied; no derivative action is used. The Op-amp IC2b has a feedback loop formed by C6 and R2 so that it tends always to equalise its two input voltages. The output of IC2b determines the speed of the pump.

Because of the presence of the capacitor C6 in the feedback loop of this stage the output can reach any DC voltage while still maintaining the two inputs the same. Hence the pump motor can be running at any speed required in order to maintain the desired pressure, while the two inputs of IC2b are at the same level.

The set or required pressure is produced by the ladder network R15/VR1/R18 and the control voltage from it is fed into the non-inverting input of IC2b. The output will then swing in a controlled fashion, making the motor run at a speed such as will make the pressure sensor voltage equal to the required pressure voltage.

Of course, if the measured pressure rises above the set pressure and stays there, as will happen when all the taps are turned off, then the pump simply stays off and IC2b cannot equalise its inputs.

To adjust the motor speed a phase angle control method is used whereby, following detection of a zero crossing, the mains is switched on after a delayed period which reduces the total amount of power sent to the pump motor. Such a method is generally known, the system here comprising comparator 150, modulator 160 and drive 170.

To make a phase angle control system the first thing to be done is to detect zero volts across the mains which means that the triac is off. Transformer TX1 provides isolation for the system, which is necessary because one of the plates of the sensor capacitor is close to the liquid. As well as this it provides a simple way of detecting zero volts. After the secondary of the transformer has gone through the bridge rectifier a full wave rectified signal appears at the junction of D5/D4/R25/D2 which goes down to zero volts every time the mains crosses zero. This means that transistor TR4 is switched on nearly all of the time, except when the input is zero. This means that the output of the transistor is allowed to rise by the action of R13, which has the effect of switching on TR2.This discharges C8 and then C8 is allowed to charge again via R19. The voltage at the junction of C8/R19/IC2c is set to reach 10v with a 50Hz mains signal by the time constant of R19 and C8. Regulation of the supply is done by IC4 which has to have a diode feeding it otherwise the charge on C9 would not allow zero volts to be detected.

This zero crossing detector output is then fed into the comparator formed by IC2c whose other input is the loop control output voltage. ' Thus as the voltage at the loop control output rises switching-on of the triac TR1 is delayed and the motor speed is reduced.

The triac is switched on via the opto-coupler IC5 because of the requirement for isolation.

Thus, during normal operation, as the pressure rises the output of the pressure sensor falls and so the output of the loop control rises which slows down the pump motor. This keeps the pressure fixed at the required pressure set by VR1.

The rest of the circuit is designed to cope with starting and stopping of flow.

Firstly, at some stage the liquid is going to stop flowing owing to the dispense tap being closed.

Depending on how fast the dispense tap is closed one of two things will happen: 1. The tap is shut quickly and the pressure in the line rises past the set point because the motor cannot switch off fast enough. In this case the normal operation of the loop control is adequate because its response would be to slow down the motor (in this case it would switch off the motor); 2. The tap is shut very slowly. This gives the loop control time to slow down the pump motor to such an extent that the motor could still be left running even when dispense has stopped. This is an error to which impeller type pumps are prone and is due to the non return'valve being closed and-the pump not running sufficiently fast to generate a greater pressure than that which exists on the dispense side of the non-return valve.

In case 1 everything will work as normal but in case 2 the motor will continue to run indefinitely. To avoid this a small amount of hysteresis is used by feeding part of the loop control output back into the required pressure input via R21. This means that as the motor slows down (i.e. when the loop control output is high) the required pressure is adjusted lower; this causes the motor to slow down more, and so on until the motor switches off. While the motor is running, however, the feedback through R21 is negligible.

When the motor is off, then an error or disparity exists at the inputs 5,6 to the loop control because the high pressure with the taps closed does not match the set (dispensing) pressure. This means that the output of the loop control will be high and the capacitor C6 will have a large charge across it by the action of R2. When the system is asked to dispense again the capacitor will be charged, and would have to discharge again through the loop, which takes time.

The amount of time this would take is too much in view of the time constant which must exist during normal operation (eg for keeping the pressure constant when a further tap is turned on or off).

This is where switch IC3c comes in. When the pressure falls more than a certain amount below the set pressure, say 2 psi (1.4 x 104 Pa) IC3c is switched, C6 is discharged and the loop control is opened which has the effect of switching the pump motor on full speed.

The output of the loop control then falls, eliminating the error of low pressure, and so the capacitor C6 is connected back into the loop. But now the capacitor has zero volts across it, so it can go straight to the required motor speed to obtain the required dispense pressure.

IC3c is switched by error detector circuit 140b as follows. The low-pressure error condition is detected by first amplifying the corresponding voltage error (which is only a few millivolts) and at the same time referencing this output to a fixed voltage, in this case 500mv. R20 and R23 set this reference voltage and R14/R29/R10/R30/IC2d form a differential amplifier.

IClb simply compares the output of IC2d to ground (which is 500mv away from the reference) and when the error is large enough (say 50mV, depending on the ratio of R29 to R30) it causes IC3c to change states thus discharging C6 and opening the loop control. An LED D6 is connected to this error output because when the unit is installed the installation people need to know if when all the dispense taps are opened there is enough pressure in the pump to cope with what is required.

This LED D6 could be marked "can't cope".

The other thing which can happen is that the beer runs out, which is dealt with by error detector circuit 140a. In this case the pressure will be low and the voltage at the output of the pressure sensor will be 10v (as set by VR2). In response to this the output of the loop control will go low, driving the motor at full speed. This condition will be accentuated by the fact that the "can't cope" detector will have opened the loop control. This is the only time that the output of the pressure sensor will be lOv AND the motor will continue to run at full speed. As a result of the high pressure sensor output and low loop output the output of ICla will go low and LED D7, which shows whether the system is primed, will go out. At the same time capacitor C4 is disconnected from ground (released by IC3b) and is thus allowed to charge through R8.

When the voltage at the junction of R8/C4 gets high enough, i.e. "times out", it--causes the-three analog switches IC3 a,b,c to go tri-state (infinite resistance between all pins) which allows the capacitor C4 to go on charging via R8, accentuating the situation. The only way then to get the system to operate again is to discharge C4 via a "PRIME" switch located on the outside of the unit.

When the analog switches go tri-state the oscillator 110 will stop because the pins of IC3a will be disconnected, and TR3 will be switched on continuously by the action of R12. This simulates infinite pressure and so the normal loop control will shut off the motor. This will happen quickly due to the fact that the control loop is open.

The construction of the pressure sensor as used in this embodiment will be described with reference to Figs.3 and 4.

There is a plastic manifold 56 which the liquid passes through, which has a recess 57 milled into one side. The capacitor Cp is applied to this recess via a food-quality gasket 58 which stops the liquid escaping, and consists of a diaphragm 50 which is preferably of brass but may be of any non-corroding mechanically stable material which bends slightly under system pressure, a gasket 52 which has a hole in the middle of it, the gasket being only O.lmm thick (or so), and a printed circuit board 54 which has etched onto it the other plate of the capacitor. This printed circuit may also have all the rest of the circuitry on it for driving the pump. The whole assembly is screwed down to the plastic manifold 56 in this order after the metal diaphragm has been soldered to the printed circuit board.

The material for the metal diaphragm should be either brass for two reasons:- (i) it has to be soldered to the main printed circuit board, and (ii) it has to be non-ferrous so as not two pick up magnetic interference. The metal diaphragm, gasket and printed circuit board are to be sealed together to stop contamination of the inner surfaces. This is important because the value of the capacitor would be altered by either tarnishing or, in the worst case, things growing on the surface. Once the metal diaphragm, gasket and printed circuit board are fastened together they should never have to be taken apart.

The main reason for building the sensor in this manner is that the manufacture of the whole product is very simple, so much so that the whole product is testable as a single printed circuit board, of largest dimensions about 10 cm, without any liquid needing to be present. However, the capacitor could be on a separate assembly.

Claims (8)

Claims:

1. A pump control device for use in dispensing a liquid, including a pressure sensor for detecting the pressure of the fluid to be dispensed, a means for setting a desired pressure, and a means for controlling the speed of the pump in order to deliver the liquid at the desired pressure by comparing the reading from the pressure sensor with the desired pressure while liquid is being dispensed and altering the speed of the pump so as to counteract a deviation of the detected pressure from the desired pressure, the device further including means for feeding back a proportion of the output of the control means to the pressure-setting means so as to reduce the desired pressure input and thereby to ensure that, in use, the pump is turned off as flow is reduced to zero.

2. A pump control device according to claim 1, in which the feedback means consists essentially of a resistor connected between the output of the control means on the one hand and the pressure-setting means on the other hand.

3. A pump control device according to claim 1 or 2, in which the pressure-setting means includes a resistance ladder connected across the supply voltage, one of the resistances being variable so as to set the desired pressure.

4. A pump control device according to any preceding claim, in which the pump control means comprises an operational amplifier, whose non-inverting input comes from the pressure-setting means and whose inverting input comes from the pressure sensor.

5. A pump control device according to any preceding claim, in which the pressure sensor is capacitative.

6. A method of dispensing a liquid using a pump control device including a pressure sensor for detecting the pressure of the fluid to be dispensed, a means for setting a desired pressure, and a means for controlling the speed of the pump in order to deliver the liquid at the desired pressure by comparing the reading from the pressure sensor with the desired pressure while liquid is being dispensed and altering the speed of the pump so as to counteract a deviation of the detected pressure from the desired pressure, in which method a proportion of the output of the control means is fed back to the pressure-setting means so as to reduce the desired pressure input and thereby to ensure that the pump is turned off as flow is reduced to zero.

7. A pump control device for use in dispensing a liquid, including a pressure sensor for detecting the pressure of the fluid to be dispensed, a means for setting a desired pressure, and a means for controlling the speed of the pump in order to deliver the liquid at the desired pressure by comparing the reading from the pressure sensor with the desired pressure while liquid is being dispensed and altering the speed of the pump so as to counteract a deviation of the detected pressure from the desired pressure, in which the control ratio of the control means is increased as the magnitude of the said deviation of the detected pressure from the desired pressure increases.

8. A pump control device substantially as described herein with reference to the accompanying drawings.