GB2233084A

GB2233084A - Position indicator

Info

Publication number: GB2233084A
Application number: GB8911807A
Authority: GB
Inventors: Neil Emerton; Dexter Robert Plummer
Original assignee: PA Consulting Services Ltd
Current assignee: PA Consulting Services Ltd
Priority date: 1989-05-23
Filing date: 1989-05-23
Publication date: 1991-01-02
Anticipated expiration: 2009-05-23
Also published as: GB8911807D0; GB2233084B

Abstract

A position indicator, particularly for a beer meter, in which the position of an object is detected by an optical sensor which reacts to light reflected from a reflector 31 associated with the object, the sensor incorporating two detectors 37, 38 which compare light reflectance from the leading and trailing edges 33, 34 of the reflector. The object may be a piston freely reciprocable within a transparent cylinder 35. The light may be modulated infra-red. The leading and trailing edges may be of the same reflector or of spaced-apart reflectors. Alternatively the reflector is stationary and the object is a trolley carrying the two detectors. <IMAGE>

Description

Position Indicator This invention relates to position indicators and has particular, but not exclusive, reference to position indicators for use in beverage dispense apparatus.

Beverage dispense apparatus for the dispensing of beer in measured quantities is well known. A particular form of such apparatus comprises a cylinder containing a freely movable piston which can reciprocate within the cylinder from one end to the other. Means are provided to supply beer to both ends of the cylinder and to remove beer from either end to a central dispense point. Valves are provided so that beer in one half of the cylinder can be displaced under pressure of beer entering the other end of the cylinder to a suitable dispense head. It will be appreciated that for each stroke of the piston within the cylinder from one end to the other, a constant volume of beer is dispensed.

As is clearly described in UK Patent Application 2 192 860 A in the name of Bass Public Limited Company - the contents of which specification are incorporated herein by reference - such devices do have problems when dispensing beer containing dissolved nitrogen. For the reasons set out in that specification, the nitrogen in solution can be released from solution when the dispense piston hits the end stop at the end of its cylinder. The shock of hitting the end stop in conjunction with the moving column of beer can result in a layer of pressure locally within the beer resulting in nitrogen coming out of solution. This nitrogen does not redissolve as does carbon dioxide which comes out of solution, and the resultant gas can bubble upward through the system and be released from the dispense nozzle in an explosive burst when the apparatus is next operated.

The invention described in the UK application referred to above, provides for a mechanism whereby the piston is brought to a stop without hitting an end stop. There is therefore no sudden drop of pressure and the nitrogen is not released from solution.

The invention described in the application includes an optical sensor which detects reflected light - normally infra red light - reflecting off a shiny surface on the reciprocating piston. A pair of optical sensors are provided at each end, one of the pair operating a mechanism whereby beer is dispensed in an agitated or creamed state, and the other sensor cutting off the supply of beer completely.

It has been found however that such optical sensors are in practice not completely predictable in their operation. The optical sensors tend to be influenced by extraneous lights and also by variations in illumination level. Typically, the energy which is emitted by an infra red light emitting diode will fall to one half its peak level during the normal life of the apparatus.

The invention therefore is concerned with providing improved optical sensing devices capable of detecting the position of a movable member relative to a fixed member and in particular is concerned with beverage dispensing apparatus incorporating such improved positioning device.

By the present invention there is provided a position detector for detecting the relative position of a movable member to a fixed member, one of the movable member and the fixed member having an associated reflecting surface in fixed positional relationship thereto, there being provided means to illuminate the reflecting surface, the other of the movable member and the fixed member having a first means to detect reflected electromagnetic wave energy from the reflecting surface, and a second means to detect reflected electromagnetic wave energy from the reflecting surface, the two means being spaced apart along the direction of travel of the movable member, so that the reflected energy from a trailing edge of the reflecting surface is falling and producing a falling electrical output signal from the first means as the reflected energy from a leading edge of the reflecting surface is rising and producing a rising electrical output signal from the second means, means to compare the electrical output signal of the first means with the electrical output signal of the second means and to identify when the signals are in a defined relationship to locate the position of the movable member relation to the fixed member.

Preferably the defined relationship is an equal relationship.

The reflecting surface may be located on the movable member. The leading edge and trailing edge may be of the same reflecting surface.

The reflecting surface may be on a piston movable within a cylinder. The cylinder may be formed of glass or other transparent material, with the reflecting surface inside the cylinder on the piston.

The radiation may be infra-red light, and may be modulated.

The two means may be focussed onto the leading and trailing edges of the ring. The cylinder may have a position detector at both ends, the piston and cylinder may form part of a beverage dispense unit.

The present invention also provides a beverage dispensing apparatus of the type incorporating a cylinder having a freely movable piston reciprocal therein in which the piston carries a reflecting surface, there being a position detector of the type set out above on the exterior of the cylinder, which cylinder is formed of transparent material.

The reflecting surface preferably comprises a reflecting ring. There may be provided a position detector at each end of the cylinder to detect the presence of the piston within the cylinder.

The reflecting surface may be directly connected to the movable member or alternatively may be electromagnetically connected so as to move with the movable member.

The electromagnetic radiation may be provided by light emitting diodes.

By way of example embodiments of the present invention will now be described with reference to the accompanying drawings of which FIGURE 1 is a schematic view of a beverage dispensing apparatus incorporating the invention, FIGURE 2 is an enlarged view of a cylinder of Figure 1, FIGURES 3 and 4 illustrate detectors for use in the embodiment of Figure 2, FIGURES 5 to 9 are graphs of signal against distance, FIGURE 10 illustrates an alternative form of detector, FIGURE 11 is a graph of absorbtion against wavelength, FIGURE 12 is a graph of signal against distance, and FIGURE 13 is a schematic view of an alternative form of position indicator.

The invention will be described with particular reference to a beverage dispense apparatus which has particular application in dispensing carbonated beverages such as beer. Certain beer systems have been developed for dispensing beer under the top pressure of a mixed nitrogen/carbon dioxide gas product and certain of the beers are brewed to incorporate nitrogen in solution. In many cases, beer is dispensed via a meter which is intended to dispense exactly half a pint of beer in each operating stroke.

These meters essentially comprise a cylinder containing a freely movable piston which can reciprocate from one end to the other.

As is shown in Figure 1, beer is held in a barrel 1 and is provided with a top pressure of carbon dioxide or carbon dioxide and nitrogen from a cylinder 2. The beer is forced under the pressure of the carbon dioxide into a pump 3 via a line 4. The pump 3 supplies beer into two lines 5, 6 which are connected to opposite ends 7, 8 of a cylinder 9. Lines 10, 11 lead from the ends 7, 8 respectively to a suitable dispense point 12. A non-return valve 13 controls the flow of beer from pump 3 into lines 5 and 6 and valves 14, 15 control the flow of beer into ends 7 and 8 of the cylinder 9.

Valves 16 and 17 control the flow of beer from the cylinder 9 through lines 10 and 11 respectively and a further valve 18 controls the flow of beer through nozzle 12.

Located within the bore of the cylinder is a piston generally indicated by 19.

It will be appreciated that if valves 15, 16 and 18 are opened and pump 3 operates, beer will be forced into the chamber 20 and will displace piston 19 to the left, thus displacing beer in chamber 21 through the nozzle 12. Closure of the valves 16, 18 and 15 will bring the piston 19 to a halt and cause a cessation of beer flow through nozzle 12.

Subsequently, opening valves 14, 17 and 18 and operating pump 3 will cause piston 19 to move to the right, refilling chamber 21 and forcing beer from chamber 20 out through nozzle 12. To operate the system a suitable electrical circuitry is provided whereby actuation of the circuit opens the set of valves such as 15, 16 and 18 and energises pump 3 to cause the piston 19 to move to the left and to dispense a half pint of beer. When the piston reaches the left hand side of the cyinder 9, an optical sensor 22 detects the presence of a shiny ring 23 in a manner described in more detail below and closes valve 18, followed shortly by valves 15, 16 and switches off pump 3 to terminate the flow of beer.Subsequently, a further initiation of the switch opens valves 14, 17 and 18 and causes movement of the piston 19 to the right, until reflecting ring 24 is detected by sensor 25 which again shuts down valve 18 followed shortly by valves 14 and 17 and switches off pump 3.

This means that the piston 19 does not contact the ends of the cylinder 9 and there is thus no sudden shock wave set up in the beer resulting in a localised pressure drop and nitrogen coming out of solution.

The present invention is particularly concerned with the accurate detection of the movable member 19. In theory, it might be possible to illuminate the movable member with some form of light and simply to detect reflections of the light from the reflecting rings 23, 24 and detect and determine the position of the movable member. Thus, for example, the movable member 26 shown in Figure 2 has a pair of reflecting surfaces 27, 28 which are detected by detectors 29, 30. However, for the reasons set out below, it is not possible accurately to position the movable member merely by simply reflecting light from the rings and determining their position. The invention provides a method of accurately determining the position of the movable member and its associated reflecting ring as is shown in Figures 3 and 4.

In Figure 3 the reflecting portion 31 of the piston, which is moving to the right in the direction of arrow 32 has a leading edge 33 and a trailing edge 34. Infra-red light illuminates the region of the cylinder 35 from a suitable LED 36. A first infra-red detector 37 detects the presence of the leading edge 33 and a second infra-red detector 38 detects the presence of the trailing edge 34. Suitable lenses 39, 40 focus light from the leading and trailing edges onto the detectors as is shown in the drawing.

Although the detectors 37 and 38 are shown detecting the presence of leading and trailing edges 33, 34 on the same reflecting ring, it may be possible to use two separate reflecting rings 41, 42 and use a detector 43 to detect the leading edge of ring 41 and detector 44 to detect the trailing edge of 42 when the piston 45 is moving in the direction of arrow 46. The spacing of the two rings 41 and 42 would have to be chosen to comply with the optical requirements of the detector, for the reasons set out below.

Referring to Figure 5, this shows an idealised graph of distance along axis 47 against voltage emitted by the detectors such as 37, 38 along axis 48. In an ideal position, the detector detects the presence of the reflected light from the light emitting diode and the output moves from level 49 to a high level 50 instantly as is shown in the drawing.

However, it has been found in practice that the electrical output of the photo-diode used in the detectors is not as shown in Figure 5.

The output of a diode is more as shown by graph 51 in Figure 6. It can be seen that there is a gradual increase in output voltage along portion 53 of the output curve together with a constant region 54 when the diode is looking at a continuously reflecting portion of ring 31 followed by a fall off along line 55 as the trailing edge passes the field of vision of the detector. In practice, the detectors detect over a finite region and the passage of the leading edge into the region results in a gradual build up in reflection and hence in voltage. Thus, a single detector has no discrete point at which the output voltage sharply moves from one state to another.

Furthermore, the light emitted by a light emitting diode reduces with time and ageing of the diode and hence it is not possible to measure the height of line 54 and use that as an accurate determinant of the position of the piston.

However, by cleverly taking advantage of the shape of curve 51 it is possible accurately to determine the position of the reflecting portion 31.

Thus, if curve 51 shows the output of the diode first encountered by the reflecting ring, curve 56 shows the output of the diode which is reached secondly by the ring passing along the cylinder. Again, it can be seen that there is a rising portion 57, a plateau 58 and a falling portion 59. By superimposing the two output curves as is shown in the bottom portion of Figure 6, it can be seen that there is a crossover point 60 where the outputs of 51 and 56 are the same.

This output corresponds to a fixed position of the reflecting ring 31. As can be seen in Figure 7 the height of the curves 51 and 56 does not affect the crossover points and the three curves 51 a, b and c cross at the same longitudinal point with curves 56 a, b anc c, namely at points 60 a, b and c where it can be seen that they are in the same longitudinal position.

This invention cleverly takes advantage of the variation in output of the receptors to permit an accurate positioning of the reflecting ring and hence of the piston within the cylinder.

It will be appreciated that it is important for the rising and falling power outputs of the two sensors neither to coincide as is shown at 61 in Figure 8, nor to be totally separated as shown at 62, 63 in Figure 9. In the case of 61, there is no crossover point and hence no accurate determinant position and in the case of 62, 63 there is no meeting and hence no accurate determination.

As is shown in Figure 10, a single lens 64 may be used to collimate reflected light 65, 66 onto two photo-diode receptors 67, 68. However, two separate lenses as are shown in Figures 3 and 4 are preferred.

In some embodiments it may be preferable to detect the point at which the outputs of the two sensors are in a specific relationship other than equality, eg in a specific ratio which may compensate for a reflection from some part of the apparatus.

It has fortunately been found that beers such as stouts are transparent to infra-red radiation in the 900 nm range as is shown in Figure 11 which shows the absorption characteristics of stout in dB/cm over the wavelengths 500-1000 nm.

In practice, the voltage output of a sensor such as sensor 38 is shown in Figure 12 and it can be seen that mV output increases clearly over a short distance range from 8-8.2 mm irrespective of whether the product being dispensed is stout, line 70, water, line 71 or a mixture, line 72.

It will be appreciated that the position detection system can be used to detect the position of a movable member such as a trolley 74 relative to a reflecting mirror 75, Figure 13.

It will also be appreciated that the mirror could therefore be stationary and the sensors could move.

An improved sensitivity can be obtained by modulating the light output from the light emitting diodes at a high frequency, ie at a frequency greater than 50 cycles per second so that the device would become insensitive to normal ambient or domestic lighting, typically, a frequency other than 100 cycles or 120 cycles to avoid domestic pulsing lights.

Suitable electronic circuitry for example a phase sensitive demodulator would be used to filter out the extraneous radiation and make the detectors responsive only to modulated light.

In practise to provide measuring information fast enough for good control, a modulating frequency of at least 10 kHz is likely to be preferred.

It will also be appreciated that the reflecting surface need not directly be provided on the movable member, it could be associated therewith, for example, it could be magnetically coupled.

In a preferred embodiment of the invention, only a single mirror is provided in the form of a reflecting ring on the piston and both optical sensors would detect the presence of the same ring.

In its preferred form, the device is capable of detecting the position of the movable piston to an accuracy of about 50 mem.

Claims

CLAIMS:

1. A position detector for detecting the relative position of a movable member to a fixed member, one of the movable member and fixed member having an associated reflecting surface in a fixed position relationship thereto, there being provided means to illuminate the reflecting surface, the other of the movable member and the fixed member having a first means to detect reflected electromagnetic wave energy from the reflecting surface, and a second means to detect reflected electromagnetic wave energy from the reflecting surface, the two means being spaced apart along tile direction of travel of the movable member, so that the reflected energy from a control edge of the reflecting surface is falling and producing a falling electrical output signal from the first means as the reflected energy from a leading edge of the reflecting surface is rising and producing a rising electrical output signal from the second means, means to compare the electrical output signal of the first means with the electrical output signal of the second means and to identify when the signals are in a defined relationship to locate the position of the movable member relative to the fixed member.

2. A detector as claimed in Claim 1 in which the relationship is equal.

3. A detector as claimed in Claim 1 or 2 in which the reflecting surface is located on the movable member.

4. A detector as claimed in Claim 1,2 or 3 in which the leading and trailing edge are of the same reflecting surface.

5. A detector as claimed in any one of Claims 1 to 4 in which the reflecting surface is on a piston movable within a cylinder.

6. A detector as claimed in any one of Claims 1 to 5 in which the cylinder is formed of glass or other transparent material, with the reflecting surface inside the cylinder on the piston.

7. A detector as claimed in any one of Claims 1 to 6 in which the radiation is infra-red light.

8. A detector as claimed in Claim 7 in which the infra-red light is modulated.

9. A detector as claimed in any one of Claims 1 to 8 in'which the two means are focused onto the leading and trailing edges of the ring.

10. A detector as claimed in any one of Claims 5 or 6 in which the cylinder has a position detector at both ends.

11. A detector as claimed in Claim 10 in which the piston and cylinder form part of a beverage dispense unit.

12. Beverage dispensing apparatus incorporating a cylinder having a freely movable piston reciprocal therein in which the piston carries a reflecting surface, there being a position detector of the type claimed in any one of Claims 1 to 11 on the exterior of the cylinder, the cylinder being formed of transparent material.

13. Beverage dispensing apparatus as claimed in Claim 12 in which the reflecting surface comprises a reflecting ring.

14. Beverage dispensing apparatus as claimed in Claim 12 or 13 in which there is a position detector at each end of the cylinder to detect the presence of the piston within the cylinder.

15. Beverage dispensing apparatus as claimed in any one of Claims 12 to 14 in which the reflecting surface is directly connected to the piston.

16. Beverage dispensing apparatus as claimed in any one of Claims 12 to 14 in which the reflecting surface is electromagnetically connected to the piston so as to be movable therewith.

17. Beverage dispensing apparatus as claimed in any one of Claims 12 to 16 in which the electromagnetic radiation is provided by light emitting diodes.

18. Beverage dispensing apparatus substantially as herein described with reference to and as illustrated by the accompanying drawings.