GB2259147A

GB2259147A - Pressure sensor

Info

Publication number: GB2259147A
Application number: GB9217443A
Authority: GB
Inventors: Christopher Simon Burrell
Original assignee: BURRENG Ltd; PREMIERFORCE Ltd
Current assignee: BURRENG Ltd; PREMIERFORCE Ltd
Priority date: 1991-08-15
Filing date: 1992-08-17
Publication date: 1993-03-03
Also published as: GB9217443D0

Abstract

A pressure sensor for use in measuring fluid pressure includes a capacitor formed by a metallic diaphragm 50 and a conducting member 60, where the diaphragm is arranged so as to be acted on by the liquid so as to vary the capacitance of the capacitor in dependence on the pressure. The conducting member 60 is formed by a plated area on a circuit board 54 to which the diaphragm is attached via a spacer 52. The board may carry two such sensors and form part of a flow-measuring system e.g. to control a pump for dispensing beer. The board may then carry part or all of the pump control circuitry, and a temperature sensor used for temperature compensation. <IMAGE>

Description

PRESSURE SENSOR The present invention concerns a pressure sensor and its applications, more particularly but not exclusively in measuring fluid flow and for dispensing fluids, in particular pressurized beverages such as beer.

The dispensing of beer must be performed accurately to measured amounts, in the UK typically in pints and half-pints. Normally this is done by eye, using a simple tap, until the beer fills a glass either to the top or to a line on the glass. Since liquids such as beer have a head, and since this head is of variable consistency depending on how the beer is poured, beer is usually poured with some care to ensure that short measure is not given; this often means that, in glasses marked with a line, excess beer is in fact dispensed. This represents considerable lost revenue, and a cheap but accurate method of measuring quantities of dispensed liquid is desirable.

It is possible to use positive-displacement meters to dispense beer, and this is done in some circumstances, such as in clubs. However, there are various disadvantages to the use of such meters. First the meters themselves are expensive. Secondly such meters are relatively complex in design having many mechanical parts which are prone to wear and so maintenance costs can be high.

As a cheaper alternative to positive-displacement meters, flow meters have been tried, in particular the turbine or paddle wheel type. In such arrangements, a measure of volume is obtained by the integration of the flow rate indicated by the paddle wheel. However, these flow meters do have some disadvantages including an inability to distinguish between forward and backward flow; this can lead to an incorrect reading if for some reason a back-pressure develops in the line.

Furthermore, the inertia of the paddle wheel makes it slow to respond to step changes in flow rate causing the accuracy of measurement to be limited.

Flow meters of the differential pressure type have also been used where the beer flows from one side of a membrane via a restricted conduit to the other side of the membrane. The differential pressure across the membrane is indicative of the rate of flow of the beer.

This type of flow meter, however, has a major drawback in the potential formation of pockets (known as "yeast traps") where no significant flow occurs. These pockets are undesirable in the dispensing of beverages for reasons of hygiene.

According to a basic aspect of the present invention there is therefore provided a pressure sensor for measuring fluid pressure, comprising a capacitor, one plate of which is formed by a conducting member and the other plate of which is formed by a conducting diaphragm mounted on the member with a spacing, the diaphragm being arranged to be acted on by the fluid so as to vary the capacitance of the capacitor in dependence on the pressure. Means such as a suitable electric circuit may then be provided for converting the capacitance reading to a suitable pressure reading.

An advantage of using such a sensor is that only a very small movement of the capacitor diaphragm is required, which means that the sensor can be highly linear. Further, the device can be made very simply and in a robust forum, for instance by using a plated area on a printed circuit board as said conducting member of the capacitor. Circuitry for deriving the pressure from the capacitance reading may then be integrated on the circuit board. This simple construction contrasts with the expensive Hall-effect and variable-resistance sensors of the prior art.

Also, when the pressure sensor is used for a larger purpose such as measuring a flow rate or controlling a pump, all or part of the rest of a control circuit can likewise be integrated on the printed circuit board, saving considerable space and providing a very compact arrangement.

The diaphragm may be a thin metal sheet with an insulating layer or foil over it to protect it from the liquid, or it may be made of stainless steel without a protective foil. If the diaphragm is spaced from the board by an insulating spacer then the diaphragm should be solderable, for convenience. However, if the spacer is itself metal then a direct connection to the printed circuit board is made and the diaphragm can be of stainless steel, for instance. This would then not need a protective foil.

A further advantage of the stainless steel diaphragm without a plastics foil is that the liquid can be in direct metallic contact with the printedcircuit board. As a result the board rapidly takes on the temperature of the liquid, allowing measurements of temperature, for instance by a sensor itself on the board. Such measurements can be useful for temperature-compensation calculations.

In order to make the conducting member sufficiently rigid to allow accurate capacitance measurement to be made, a stiffening member may be fixed to the back (i.e. the side opposite the sensor).

This member may be a steel disc or ring screwed to the sensor device, or through the sensor device into the manifold or conduit, thus fastening the assembly. If it is required to measure temperature as mentioned above it is better to use a more thermally insulating disc of, say, plastics, in particular acetal.

One application of the invention is in an apparatus for determining the flow of a fluid along a conduit having a section having a predetermined flow resistance. The apparatus may include a pressure sensor, in particular one as described above, for detecting the pressure of the fluid at one end of the section, and a means for determining the flow of the liquid on the basis of the difference in pressure between this end and the other end of the section.

Advantageously the apparatus includes two sensors integrated with the section of conduit with a given flow resistance, the sensors being arranged at opposite ends of the section, and a circuit for calculating flow rate based on the output of the sensors.

Because the absolute pressure is determined at two spaced points it is not necessary to route the fluid along a circuitous path; this means that, particularly with liquids, a more laminar flow is produced with fewer possibilities for growth of undesirable organisms. This is in turn made possible by the sensor of the present invention which is accurate enough to measure absolute pressure reliably, rather than forcing reliance on a differential sensor. The other side of the diaphragm, in contrast to the differential sensor, is exposed to a known pressure such as atmosphere.

The second point where pressure is determined can most simply be the exit of the conduit to atmosphere, in which case no second sensor is needed: atmospheric pressure can simply be used as the second datum and the single pressure reading is all that is needed.

However, beer dispensers are frequently fitted with "sparklers" for producing a head. These sparklers can vary in specification, which means that the meter cannot be calibrated in the factory, and therefore the two-sensor arrangement is used, the second sensor being downstream of the section of pipe including the flow restriction.

Further circuitry for calculating the flow rate in response to the input or inputs of the pressure sensor(s) can be incorporated on the circuit board. In addition a display controller can be fitted, with a display which shows the rate of flow and the total so far poured, for instance.

The present invention also relates to the use of the pressure sensor in a control device for a pump, in particular one suitable for dispensing pressurised liquids such as beer. Various such systems are known.

Such systems have to address several problems in order to be successful, and many make use of some form of pressure detection, but none is entirely satisfactory.

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 has to be provided.

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 not reached and the timer times out.

Finally, GB 2177523 (Partridge Wilson & Co) shows a further system which 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. However, the sensor of the invention is accurate enough to enable a fluid flow control without reference to any further variables.

A flow controller in accordance with the invention can be constituted as a pump control device for use in dispensing a liquid, including a pressure sensor as aforesaid 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.

For a better understanding of the invention embodiments of it will now be described with the aid of the accompanying drawings, in which: Fig. 1 shows schematically a known differentialpressure sensor for use in a metering apparatus; Fig. 2 shows a schematic view of an arrangement using the present invention for monitoring fluid flow; Figs. 3 and 4 show views of a construction of the pressure-measuring capacitive sensor in one embodiment of the invention; and Fig. 5 shows a diagram of a flow control apparatus using the sensor of the present invention.

Figs. 3 and 4 show an example of a pressure sensor embodying the invention. The sensor is of the capacitive type. The capacitor plates are formed by a metallic diaphragm 50 and a small disc 60 etched on to a printed circuit board. The small disc 60 is connected to the outside world by a thin track 62 so as to minimise the swamping effect of fixed capacitors which are in parallel with the pressure-measurement capacitor.

The conduit section 13, which the liquid passes through, has a recess 57 milled into one side of its interior. The capacitor is applied to this recess via a food-quality gasket 58 which stops the liquid escaping, and consists of the 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 or spacer 52 which has a hole in the middle of it, the gasket being about O.lmm thick, 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 conduit 13 in this order after the metal diaphragm has been soldered to the printed circuit board.Although not shown in the diagram, a stiff disc or ring may be screwed to the back of the board during assembly for added rigidity, which is necessary for accurate measurement.

Brass is advantageous for the material for the diaphragm for two reasons:- (i) it has to be soldered to the main printed circuit board, and (ii) it should be non-ferrous so as not to pick up magnetic interference. The 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, organisms growing on the surface. Once the metal diaphragm, gasket and printed circuit board are fastened together they should never have to be taken apart.

As an alternative to brass stainless steel can be used, which has the further advantage that it does not corrode at all and hence needs no protective foil 58.

Contact to the board 54 can be made simply by making the spacer 52 out of metal, such as copper. In the Fig. 3 59 represents a "dummy" layer of copper formed on the PCB to ensure reproducible spacing of the diaphragm.

As a result of constructing the sensor in this manner 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.

An application of the invention will be described with reference to Figures 1 and 2.

As can be seen in Fig. 1, the known differentialpressure device diverts the liquid round a diaphragm 2 in order for the liquid to be applied to both sides of the diaphragm. This leads to stagnant pockets 5.

In contrast, a flow-measuring apparatus in accordance with the invention as shown in Fig. 2, can have the liquid pass through the apparatus in a straight line, hardly disturbing the laminar flow of the liquid. The apparatus has a housing 11 which the pipeline 1 enters and leaves via suitable connections which seal it to a passage or section 13. This section has a restriction 17 giving rise to a predetermined flow resistance.

Applied to the interior of the section 13 on either side of the restriction 17 are pressure sensors 15, 16 which may be identical. The sensors are integrally mounted on a circuit board 12 which also contains circuitry for calculating the flow rate in dependence on the outputs of the sensors 15, 16.

In an advantageous embodiment, for use in a bar, the pipe 1 comes up from a beer keg in the cellar, the housing 10 stands vertically on the bar and the outlet side of the conduit is adapted for dispensing into a glass. Preferably the two-sensor version is used so that it is not sensitive to sparklers being placed on the outlet side. The housing can incorporate a display controlled by the circuit for indicating the flow rate and the amount so far dispensed.

A second application of the invention is shown in Fig. 5, which shows a block diagram of a pump controller. A typical fluid flow control system in accordance with this embodiment 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. Fig. 5 shows only the control unit and pump.

The measurement capacitor Cp represents the sensor of the present invention. In this embodiment it 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 pressure in 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.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. For further information reference may be made to the co-pending priority application GB 9117626.3. The rest of the circuit is designed to cope with starting and stopping of flow, and again is described in the priority application.

It is clear that while the embodiments described related to the dispensing of beverages such as beer, the device and method are applicable to any situation where pressure or flow rate of a fluid, whether liquid or gas, is to be measured or controlled. It is envisaged that the apparatus be used for dispensing of fluid, but it could also be used as part of a feedback mechanism for controlling flow as part of a larger apparatus.

Claims

Claims: 1. A pressure sensor for measuring fluid pressure, comprising a capacitor, one plate of which is formed by a conducting member and the other plate of which is formed by a conducting diaphragm mounted on the member with a spacing, the diaphragm being arranged to be acted on by the liquid so as to vary the capacitance of the capacitor in dependence on the pressure. 2. A pressure sensor according to claim 1, in which the conducting member comprises a plated conducting area on a printed circuit board. 3. A pressure sensor according to claim 1 or 2, in which the diaphragm is constructed from a corrosionresistant metal. 4. A pressure sensor according to any preceding claim, in which the diaphragm is coated with a corrosion-resistant layer. 5. A pressure sensor according to any preceding claim, in which the diaphragm is spaced from the conducting member by an insulating spacer. 6. A pressure sensor according to any preceding claim, including a temperature compensation means. 7. A pressure sensor according to any preceding claim, in which the conducting member is provided with a stiffening member. 8. A pressure sensor substantially as described herein with reference to the accompanying drawings. Amendments to the claims have been filed as follows

1. A pressure sensor for measuring fluid pressure, comprising a capacitor, one plate of which is formed by a conducting member and the other plate of which is formed by a conducting diaphragm mounted on the member with a spacing, the diaphragm being arranged to be acted on by the fluid so as to vary the capacitance of the capacitor in dependence on the pressure.

2. A pressure sensor according to claim 1, in which the conducting member comprises a plated conducting area on a printed circuit board.

3. A pressure sensor according to claim 1 or 2, in which the diaphragm is spaced from the conducting member by an insulating spacer.

4. A pressure sensor according to claim 2, in which the diaphragm is spaced from the printed circuit board by a conducting spacer which also forms the electrical connection of the diaphragm to the board.

5. A pressure sensor according to any preceding claim, in which the diaphragm is coated with a corrosion-resistant layer.

6. A pressure sensor according to any preceding claim, in which the diaphragm is constructed from a corrosion-resistant metal.

7. A pressure sensor according to any preceding claim, in which the area of the conducting member is small in comparison to that of the conducting diaphragm.

8. A pressure sensor according to any preceding claim, in which the conducting member is provided with a stiffening member on the side opposite the capacitor.

9. A pressure sensor substantially as described herein with reference to the accompanying drawings.

10. A pressure-sensing device incorporating a sensor as claimed in claim 2 or any claim appendant to claim 2, and associated circuitry on the printed circuit board.

11. A pressure-sensing device according to claim 10, in which the associated circuitry is connected by a printed circuit of which the said plated conducting area forms a part.

12. A device for measuring or controlling the flow of fluid using at least one sensor as claimed in any of claims 1 to 9.

13. A method for measuring fluid pressure or flow using a sensor as claimed in any of claims 1 to 9.

14. A method according to claim 13, in which the pressure drop across a predetermined stretch of the said fluid flow is measured using one or more of the claimed sensors.

15. A method according to claim 13 or 14, in which the fluid is a liquid.

16. An assembly for measuring the pressure of a flowing liquid, comprising a conduit for the liquid and a pressure sensor as claimed in any of claims 1 to 9, in which the diaphragm is flat and forms part of the wall of the conduit.

17. An apparatus for determining the flow of a fluid along a conduit including a section having a predetermined flow resistance, the apparatus including a pressure sensor as claimed in any of claims 1 to 9 for detecting the pressure of the fluid at one end of the section, and a means for determining the flow of the liquid on the basis of the difference in pressure between this end and the other end of the section.

18. An apparatus for determining the flow of a fluid along a conduit including a section having a predetermined flow resistance, the apparatus including two pressure sensors arranged at opposite ends of the said section, and a circuit for determining the flow rate based on the output of the sensors.

19. A method for determining the flow of a fluid along a conduit including a section having a predetermined flow resistance, in which the pressure difference across the section is measured using two pressure sensors arranged at opposite ends of the said section, and the flow rate is determined in dependence on the output of the sensors.