GB2246617A

GB2246617A - Valve position detector.

Info

Publication number: GB2246617A
Application number: GB9112599A
Authority: GB
Inventors: David Miller Weir; Nigel John Hall
Original assignee: W H PROMATION Ltd
Current assignee: W H PROMATION Ltd
Priority date: 1990-06-29
Filing date: 1991-06-12
Publication date: 1992-02-05
Anticipated expiration: 2011-06-12
Also published as: GB2246617B; GB9112599D0; GB9014535D0; DE4120485A1

Description

1 - 11; 1 i Valve Position Detector The present invention is a position

detector for monitoring the operation of a valve.

In process industries wherein the successive operation of a number of pneumatic valves is broughtabout automatically, it is essential to ensure that each operation is successfully completed in the appropriate sequence. Thus if a given valve is to be opened or closed, proper control requires provision for checking that the valve has indeed been opened or closed as intended.

With this requirement in mind, various forms of position sensors have been developed which respond in one way or another to the proximity of, say, the shaft of the valve. While in scme situations electrical position is switches fulfil this function adequately, electrical equipment is scarcely appropriate in industries where moisture levels are high. Thus, for example, in the liquid food processing industry, while electrical position switches have been tolerated for this purpose in the absence of an acceptable alternative detection system, expensive equipment such as water-resistant cages and 1 cables have been required in order to ensure their safe installation and operation.

In a smvwhat related setting, namely that of operating underwater control valves in the oil production industry, it has been proposed in TJK Patent Specification No. 2078943 to monitor valve positions by means of a fibre optics system. The sensor system described in that specification employs one or two half opaque discs, which are rotated by the valve shafts into positions in which they interrupt an optical path so as to indicate the position of a valve. While this prior system may be appropriate in the oil production industry, it is scarcely suited to the requirements of food processing. Nonetheless, the liquid food processing industry does require a valve detector system which is less vulnerable to moisture than the electrical detectors currently used. It is an object of the present invention to provide an ixnproved valve position detector which meets that need.

The valve position detector according to the present invention comprises a first magnet mounted for linear reciprocation with the shaft of a valve between positions corresponding respectively to open and closed positions of the valve, aligned fibre optic cables defining a light path,Aey between their adjacent ends, a second magnet mounted for linear movement towards or away from the valve shaft under the influence of the first magnet between a first position in which the light pathway is uninterrupted and 1 i 1 i a second position in which the second magnet interrupts said light pathway, and detector means connected to detect an interruption of said light pathway.

While the valve position detector of the present invention may simply operate to detect whether or not the valve is in one given position, for example the closed position, it then being assLmied that if the valve is not in that position then it must be open, it is much preferred to provide positive detection of each of the limiting positions of the valve. To this end, two of the aforesaid second magnets and two independent light pathways nay be provided, spaced apart along the axis of novement of the valve shaft, one to detect the presence of the valve in its open position and one to detect its presence in the closed position.

The first magnet, which is mounted-for reciprocation with the valve shaft, may be mounted directly upon the shaft or upon an extension to the shaft or even upon a separate menber which abuts the valve shaft and reciprocates with the shaft. Very conveniently it may be annular in shape and surround the shaft or an extension thereto. in a particularly preferred fom, the first magnet comprises two annular components, each macTnetised in an axial direction and mounted with-opposed polarities with an a=UlcErRXnIber of ferramagnetic material betwi--en them.

r i j i 1 The second magnet or magnets is preferably elongateds for example generally cylindrical, and may itself pass between the aligned fibre optic cables or nay be directly or indirectly connected to a body, for example an extension fram said magnet, which interrupts the light pathway.

Preferably means are provided to urge the second magnet or magnets in a linear direction opposed to the direction of the influence thereon of the first magnet, with the object of returning the magnet(s) to a position of rest after the influence of the f irst magnet has been removed. Thus, for example, the second magnet(s) ray be urged towards that position of rest by springs or by means of one or more fixed further magnets.

Udle if desired the fibre optic cables may be of glass, it is much preferred to use cables of synthetic polymeric material. Such polymeric fibre optic cables have the advantage of relative cheapness compared with glass cables but more importantly they are more readily prepared for use and more robust in use. Terminating a glass cable requires special cutting tools and bonding agents and entails fitting a metal sleeve which must be bonded to the glass fibres, before again cutting and polishing the cable end. Using polymeric fibre optic cable, for example a fibre optic cable made of an acrylic polymer, tation of the cable is effected by simple f i I 1 I i 1 i i i d i i i 1 i I I 1 i 1 i i i i cutting. Poly=ic fibre optic cables may be used to Pass light signals for distances in excess of 30 metres.

The fibre optic cable may be led to the position detector in the manner of any other service cable but we have found thatr when the valve being monitored is a pneumatically-operated valve, it is advantageous to associate the fibre optic cable with the pneumatic cable. For example, the fibre optic cable may be fed within the pneumatic cable. However, it is much preferred to dispose the pneumatic cable within a coaxial outer cover and to locate the fibre optic cables between the pneumatic cable and the outer cover. Not only does this arrangement provide ready protection for the fibre optic cables while isolating those cables from the condensation which is =mon in pneumatic lines; it also simplifies the installation of the detector system overall.

As will be apparent from the foregoing, the position of the valve is determined by reference to the interruption of a beam of light. Thus the device includes a detector for recognising an interruption of the light beam. The detector functions by converting the light signal into an electrical signal. In an analogous way, the light beam itself will normally be produced electrically. It therefore becomes possible to optiinise the performance of the device by tuning the light detector to the frequency of the light transmitted, or to modify the frequency of the transmitted 1 light to the optimum frequency for the detector. In this way the capacity of the device to cope with highly-attenuated light signals can be enhanced. In practice, the main factor influencing the choice of the light frequency is likely to be the nature of the fibre optic cable, since any such cable has a particular frequency at which signal attenuation is minimised. For example, in the case of the specific embodiment of the valve position detector described in detail be-low, the acrylic fibre optic cable used had an 10 optimum perforimnce at a light wavelength of 660 rin.

Other optional or preferred features of the invention, and other advantages arising from the invention, will be apparent frcm the following description with reference to the acecrnpanying drawings, which is given by way of example only. In the drawings:-

Fiq. 1 is a horizontal sectional view of one preferred embodiment of the valve position detector according to the present invention; Fig. 2 is a vertical sectional view of the device of Fig. 1, taken along the line II-II in that Figure; Fig. 3 is a vertical sectional view, to a larger scale, of one of the magnetbearing shuttles; and tt i Fig. 4 is a vertical sectional view, also to a larger scale, of the magnet asserribly upon the valve shaft extension.

The illustrated device comprises a housing 10, mounted upon the upper end 11 of a pneumatically operated valve, not shown. The pneumatic cable 12 by which the valve is operated is in turn enclosed in an outer casing 13. Between the cable 12 and the outer casing 13, four acrylic fibre optic cables 14, of which only two are seen in Fig. 1, are run as a spiral around the cable 12. The two illustrated cables 14 are separately led to a housing 15 in which their free ends face each other across a narrow gap 26. The other two cables occupy a similar position in a housing 16 (Fig. 2).

An extension 17 secured to the upper end of the shaft of the valve carries an encircling annular magnet assembly 18 at its upper end. Small cylindrical magnet-bearing shuttles 19, 20 are contained in the housings 15 and 16 respectively for linear movEment towards and awav frcm an axial cavity 23 in the housing 10 within which the shaft extension 17 and annular magnet assembly 18 reciprocate during operation of the valve.

The structure and operation of the magnet-bearing shuttles are shown in Fig. 3, which illustrates the shuttle 19 and housing 15 in vertical section. The shuttle 19 carries a magnet 24 at its end nearer to the shaft 23 and has an axial extension 19a mounted within a short bore 15a in the housing 15. Surrounding the extension 19a and fixed within the housing 15 is an annular magnet 25. The magnets 24 and 25 are axially polarised in opposite directions and therefore tend to repel each other, thereby urging the shuttle 19 towards the cavity 23. hhen the shuttle approaches the cavity, it exposes the gap 26 between the ends of the fibre optic cables. When the shuttle moves to the other end of its path of linear movement, the extension 19a obstructs the gap 26.

When the illustrated device is used, a beam of light of a wavelength of 660 rin is transmitted, in pulses of approximately 1 KHz frequency, down one of the fibre optic cables 14 to the housing 15, where the light beam bridges the gap 26 to the other cable 14 and is conveyed by the latter to a light detector. The light detector issues an electrical signal indicating the arrival of the light.

However, as the valve shaft moves upwardly within the cavity 23, the annular magnet assEnbly 18 repels the magnet 24 in the housing 15 (over-riding the repulsion of the magnet 25) and causes the shuttle 19 to move into a position wherein the extension 19a is interposed between the ends of the cables 14 and thereby interrupts the light beam. The light detector identifies this loss of n i i j i 1 i j i i i light signal and the electrical signal w1dch has been issued by the detector is itself interrupted. Thus this loss of the electrical signal is an indication that the valve shaft is in the position in which.the annular magnet assembly 18 is 5 adjacent to the housing 15.

As the shaft moves downwardly towards the position illustrated in Fig. 2, the magnet 25 repels the nagnet 24 and thereby causes the shuttle 19 to move towards the cavity 23, thus exposing the gap 26 and restoring the signal to the light detector. The magnet carried by the shuttle 20 is now repelled and the shuttle moves to interrupt the light beam within the housing 16.

Thus the presence of the valve shaft in one of its limiting positions as it reciprocates is indicated by the arrival or non-arrival of the relevant light beam at the light detector. Those limiting positions correspond, of course, to the "open" and "closed" conditions of the valve.

One of the features of the illustrated device which assists its reliable operation is the design of the magnet assembly 18.

It is an important consideration that the assembly should present a welldefined magnetic pole at its radially outer edge. However annular magnets magnetised in a radial direction are not readily available. The magnet assembly 18, which is shown to a larger scale in Fig. 4, has been conceived to overcame this problem.

1 The illustrated magnet assembly ccmprises two annular magnet components 27 and 28 and an annular fer-ramagnetic member 29 disposed between them. The assembly is retained upon the;upper end of the valve shaft extension 17 by a retaining ring 30.

The components 27 and 28 are magnetised in an axial direction and are mounted with opposed polarities and thereby cooperate to magnetise-the member 29 in a radial direction. For example, if the component 27 is mounted with its South pole downwards and the component 28 is at the same time mounted with its South pole upwards, then the member 29 beccmes magnetised to present a South pole at its radially outer edge. Thus, in that configuration, the assembly is able to repel the magnet 24 if the latter is orientated with its South pole towards the cavity 23.

A particular advantage of the illustrated construction of the magnet assembly is that the induced circumferential pole is sandwiched in an axial direction between two poles of opposite polarity to the circumferential pole. Thus the circumferential pole is thereby sharply defined and gives enhanced precision.to the operation of the valve position detector according to the invention.

1; j

Claims

1. A valve position detector comprising a first magnet mounted for linear reciprocation with the shaft of a valve between positions corresponding respectively to open and closed positions of the valve, aligned fibre optic cables defining a light pathway between their adjacent ends, a second magnet mounted for linear movement towards or away fran the valve shaft under the influence of the first magnet between a first position in which the light pathway is uninterrupted and a second position in which the second magnet interrupts said light pathway, and detector mans connected to detect an interruption of said light pathway.

2. A valve position detector as claimed in claim 1, having two said second magnets and two independent said light pathways spaced apart along the axis of movement of the valve shaft.

3. A valve position detector as claimed in either of the preceding claims, wherein said first magnet is mounted upon an extension to the valve shaft.

4. A valve position detector as claimed in any of the preceding claims, wherein said first magnet is annular in shape.

4,

5. A valve position detector as claimed in claim 4, wherein said first magnet camprises two annular components, each magnetised in an axial direction and mounted with opposed polarities with an annular nxnter of ferrcmagnetic material 5 between them.

6. A valve position detector as claimed in any of the preceding claims, wherein the second magnet(s) is/are generally cylindrical and is directly or indirectly connected to a body which interrupts the light pathway.

7. A valve position detector as claimed in any of the preceding claims, wherein the second magnet (s) is/are urged by springs in a linear direction opposed to that of influence thereon of the first magnet.

8. A valve position detector as claimed in any of claims 1 to 6, wherein the second magnet (s) is/are urged by one or more fixed further magnets in a linear direction opposed to that of influence of the first magnet.

9. A valve position detector as claimed in any of the preceding claims, wherein the fibre optic cables are of synthetic polymeric material.

10. A valve position detector as claimed in claim 9, wherein the fibre optic cables are of an acrylic polymer.

h

11. A valve position detector as cla in any of the preceding claims, wherein the valve is a pneumatically operated valve and the fibre optic cables are disposed between the pneumatic cable to the valve and a coaxial outer cover.

12. A valve position detector as claimed in any of the preceding claims, wherein the frequency of the light and the optirmn frequency of the detector are selected to correspond to the optimum transmission frequency of the fibre optic cables.

13. A valve position detector substantially as hereinbefore described with reference to, and as illustrated in, the acccnrpanying drawings.

Published 1992 at 7be Patent Office, Concept House. Cardiff Road. Newport. Gwent NP9 I RH. Further copies may be obtained from Sales Branch. Unit 6, Nine Mile Point. Cwmfelinfach, Cross Keys. Newport, NP1 7HZ. Printed by Multiplex techniques ltd. St Mary Cray, Kent.