GB2101302A

GB2101302A - Optical fibre sensor

Info

Publication number: GB2101302A
Application number: GB08214032A
Authority: GB
Inventors: Raymond Eric Robert Berry
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1981-06-22
Filing date: 1982-05-14
Publication date: 1983-01-12
Also published as: GB2101302B

Abstract

Optical fibre sensor for enabling a physical parameter to be assessed and comprising an emitter 23 and a detector 25 which are relatively displaceable, the size of any relative displacement being dependent on and correlatable with the value of said parameter, and wherein the emitter comprises one end of an optical fibre 26 of which the other end is illuminatable by a light source, and the detector comprises the ends of a plurality of optical fibres 27 positioned to be sequentially illuminatable by the light from the emitter as displacement occurs, the other ends being connectable with means for assessing any displacement in terms of the instantly illuminated optical fibre(s). The physical parameter is particularly pressure, where the relative displacement is effected by the movement of the tip of a Bourdon tube, or the juxtaposed oppositely movable tips 22, 24 of two such tubes 20, 21 connected in series. <IMAGE>

Description

SPECIFICATION Optical fibre sensor The present invention relates to an optical fibre sensor.

Numerous types of sensors for assessing various physical parameters, such as pressure, temperature and material level are well known. It is now very common for sensors to be designed such that their output is in the form of an electric signal as this is a convenient input for electronic devices, such as computers or microprocessors, which are becoming increasingly important adjuncts of many types of operation. Thus e.g.

many industrial plant operations, particularly chemical plant processes, are designed to be monitored and/or controlled by such electronic devices, and the electric signal-generating sensors employed in these plant operations are therefore in some sort of electrical connection with the electronic devices. The existence of these electric signals and connections creates a safety hazard in many types of operation, particularly in those where the sensor is employed in or near a region containing inflammable or explosive material where the possibility of an electrically-caused spark has to be taken into account. There are of course ways and means to decrease the safety hazard of electric signal-generating sensors but these are invariably costly.

I have now invented a sensor which does not generate an electric signal output but yet is able to provide a convenient input for the type of electronic device discussed above.

According to the present invention there is provided an optical fibre sensor for enabling a physical parameter to be assessed which sensor comprises an emitter and a detector which are relatively displaceable, the size of any instant relative displacement being dependent on and correlatable with the value of said parameter at that instant, and wherein the emitter comprises one end of an optical fibre of which the other end is illuminatable by a light source, and the detector comprises the ends of a plurality of optical fibres positioned to be sequentially illuminatable by the light from the emitter as displacement occurs, the other ends of the detector optical fibres being connectable with means for assessing any displacement in terms of the instantly illuminated detector optical fibre(s).

Thus the sensor of the present invention generates as an output a light signal along an optical fibre (as defined above), and this can be used as the input for the means to assess any instant relative displacement whereby said means is able to assess the value of the physical parameter at that instant (or provide information which can be correlated with this value -- which comes to the same thing). Accordingly, the sensor of the invention is intrinsically safe (due to the absence of the electrical signals and connections) and yet is simple and cheap to produce and operate.

The means for assessing the relative displacement in terms of the instantly illuminated detector fibre(s) is, as indicated above, generally an electronic device such as a computer or a microprocessor. Such electronic devices are usually employed for control as well as monitoring purposes, the value(s) of the physical parameter under assessment, possibly inter alia, being used for the purpose of controlling an operation, e.g. a chemical plant process, by means of such a device. Conveniently the light source, which may e.g. be a laser diode, can also be connected with the electronic device (although this is not essential) so that the emitter of the sensor (as well as the detector) can be monitored; this is useful e.g. as a check against spurious values being obtained due to failure of the light source.

The sensor of the present invention comprises an emitter and a detector which are relatively displaceable, the size of any instant relative displacement being dependent on and correlatable with the value at that instant of the physical parameter being assessed. In general, any change in the relative displacement of the emitter and detector is caused directly or indirectly by a change in the value of the physical parameter being assessed.

Frequently the emitter includes a support for the end of the emitter optical fibre which support is fixed, or is attached to or part of a movable part of the sensor, and the detector includes a support for the ends of the detector optical fibres which support is fixed, or is attached to or part of a movable part of the sensor, so that the detector and emitter are relatively displaceable. Thus, both the emitter and detector may be capable of being moved, or the emitter may be fixed and the detector capable of being moved, or the detector may be fixed and the emitter capable of being moved.

The arrangement of optical fibres in the detector should be such that the fibre ends are sequentially illuminatable by light from the emitter as the relative displacement occurs. Usually the detector fibre ends will be positioned to lie on a straight or curved locus such that each detector fibre end is substantially equidistant from the emitter fibre end when in a position to be illuminated by the emitter fibre end and at its nearest approach. The locus will of course depend on the nature of the relative displacement.

The minimum number of detector optical fibres is of course 2 - although such a detector would only allow a very crude assessment of a physical parameter (which, however, in some cases may be all that is required). Obviously the greater the number of illuminatable detector optical fibres and the closer their spacing, the greater will be the resolution and range of the sensor. There will of course be a limit to the number of detector optical fibres which may be used for any particular sensor determined by, inter alia, sensor shape, size and location and optical fibre diameter. Generally speaking, however, I envisage 4 to 25 as a reasonably convenient number of detector optical fibres.

Examples of physical parameters which may be assessed by the use of a sensor according to the invention are pressure, temperature and material level (e.g. fluid or powder level), particularly pressure. Thus, the change in force due to the change in pressure of a system may be channelled or otherwise used or adapted to cause a movement in at least one part of the sensor so that the above-defined relative displacement occurs, the size of any instant relative displacement being dependent on and correlatable with the pressure at that instant.

In a preferred embodiment of the invention, the sensor is an optical fibre pressure sensor and the part(s) of the sensor caused to undergo movement by changes in pressure is the movable tip of a Bourdon tube, or the juxtaposed oppositely movable tips of two Bourdon tubes connected in series. (A Bourdon tube, which is well known in the art for use in pressure gauges, is a C-shaped tube of oval section having a sealed free end, the other end being connectable with the system under pressure. Any pressure in the tube in excess of external or atmospheric pressure causes the outer material of the tube to expand and the inner material to contract as the tube attempts to change its section to a more circular one: the resulting stresses in the tube tends to cause it to uncoil so that the sealed free end moves upwards.

The amount of the movement is dependent on and correlatabie with the pressure increase; consequently the tip displacement can be employed to assess the pressure at that instant.

The reverse effect occurs under vacuum, when the pressure in the tube is less than the external or atmospheric pressure. The movement of the tube at the sealed free end is called tip travel.) Thus, in the case of using one Bourdon tube, either the emitter or the detector fibre support is attached to, or part of, the movable tip of the Bourdon tube, the support not attached to or part of the movable tip being fixed.In the case of using two Bourdon tubes connected in series and having juxtaposed oppositely movable tips, the emitter fibre support is attached to or part of one of the movable tips and the detector fibre support is attached to or part of the other movable tip. (It is to be understood that the term "movable tip" in relation to a Bourdon tube includes not only the free sealed end but also any surround or attachment thereto which moves with it.) Accordingly, in a preferred embodiment of the invention there is provided an optical fibre pressure sensor which comprises:: a) an emitter and a detector which are relatively displaceable, the emitter comprising the supported end of an optical fibre of which the other end is illuminatable by a light source and the detector comprising the supported ends of a plurality of optical fibres positioned to be sequentially illuminatable by the light from the emitter as displacement occurs; and b) either (i) one Bourdon tube whose movable tip provides or is attached to either the support for the end of the emitter optical fibre or the support for the ends of the detector optical fibres, the support which is not attached to or part of the movable tip being fixed, or (ii) two Bourdon tubes connected in series and having juxtaposed oppositely movable tips, the support for the emitter fibre end being attached to or part of one of the movable tips and the support for the detector fibre ends being attached to or part of the other movable tip;; wherein the relative displacement of the emitter and detector is effected by the tip movement of the Bourdon tube or tubes which movement is caused by a pressure change, the size of any displacement being dependent on and correlatable with the pressure change causing it, and wherein the ends of the detector optical fibres remote from the emitter are connectable with means for assessing any displacement in terms of the instantly illuminated detector optical fibre(s).

Since any pressure change in a system may be directly linked to the actual pressure of that system after the change by suitable calibration, any relative displacement of the emitter and detector is dependent on and correlatable with the pressure. As mentioned above, the means for assessing the displacement in terms of the instantly illuminated detectorfibre(s) is usually an electronic devide such as a computer or a microprocessor. The light source is preferably also linked with the electronic device, and e.g. the light source may be a computer-derived light source, be it a conventional white light emitter or a lightemitting diode.

It is to be understood that by the term "light" is meant any electromagnetic radiation capable of transmission along an optical fibre, and in particular radiation in the visible part of the electromagnetic spectrum and infra-red radiation.

Another preferred physical parameter which may be assessed by the use of the invention is temperature, e.g. by employing the moving tip of a bimetallic strip to effect the displacement of the emitter and detector.

The present invention is further illustrated by the following description with reference to the accompanying drawings in which: Figure 1 is a schematic sketch of an emitter and detector as used in the present invention showing a typical optical fibre layout, Figure 2 is a schematic diagram of an elementary circuit incorporating a sensor according to the invention, Figure 3 is a schematic representation of a vertical view of an optical fibre sensor according to the invention, and Figure 4 is a schematic representation of a side view of the sensor of Figure 4.

In Figure 1, an emitter 1 comprises an optical fibre 2 with a fibre end 3, the fibre end having a support 4. A detector 5 is juxtaposed opposite the emitter and comprises a plurality of fibres 6 with equidistantly spaced fibre ends 7, the fibre ends having a support 8. The emitter and detector are relatively displaceable in opposite directions as indicated by the arrows 9, the locus of the detector fibre ends 7 being such that each end is substantially equidistant from the emitter fibre end 3 when in a position to be illuminated therewith and at its nearest approach.Light from the fibre 2 is thus able to sequentially illuminate the fibres 6 as displacement occurs; accordingly, it is possible to assess the displacement in terms of the instantly illuminated detector fibres, thereby allowing the value of the parameter under investigation to be assessed (or information to be provided which can be correlated with this value).

The resolution of the system will depend, inter alia, on the available movement and the fibre diameters.

It is preferable for the light from the emitter to be capable of only illuminating one, or at most two, detector fibres at any single time.

In the circuit sketched in Figure 2, an optical fibre sensor according to the invention 10 is associated with some feature 11 of an operation (e.g. a pressurised vessel), the association being such that the sensor enables a particular physical parameter of the operation (e.g. pressure) to be assessed. The detector optical fibres extend from the sensor in the form of a multicore cable 12; similarly the emitter optical fibre extends from the sensor as a cable 13. (N.B. it is conceivable that the sensor according to the invention comprises more than one such emitter/detector pair.) The emitter optical fibre and detector optical fibres extend to a zone (or zones) remote from the operational feature 11 and sensor 10, the remoteness being limited only by the practicable length of the optical fibres.Thus, for example, the remote zone(s) could be up to 500 m, or even up to 2.5 km and beyond, from the operational feature and sensor. The detector fibres are able to carry a light input for and are connectable to a means 1 4 to assess any relative displacement of the emitter and detector in terms of the instantly illuminated detector optical fibres(s), thereby allowing the physical parameter to be assessed or allowing information to be provided which is correlatable with the physical parameter.

Typically, the means 14 is an electronic device such as a microprocessor, or a computer (which computer may, e.g. have a devoted microprocessor linked with it which receives the detector signal). The device may provide a direct read-out of the physical parameter, or it may merely store or note the value (or information which correlates therewith) e.g. for the purpose of controlling the operation, or both. The emitter fibre is illuminatable by a light source 15, e.g. a laser diode; the source is optionally linked with the assessment means 14.

It can be seen that the zones in and/or around (or near) the operational feature and sensor, which may be hazardous, can be safely isolated by virtue of any electrical signals or connections being present in a remote non-hazardous zone.

The optical fibre sensor of Figures 3 and 4 is intended for the assessment of pressure. It comprises two facing Bourdon tubes 20 and 21 connected in series. The tube 20 has a sealed movable tip 22 having attached thereto an emitter 23 while the tube 21 has a sealed movable tip 24 having attached thereto a detector 25, the tips being in a juxtaposed oppositely movable relationship; the optical fibre layout of the emitter and detector is substantially as shown in Figure 1, the fibres exiting through cables 26 and 27 respectively. The tube 20 has a mounting thread 28 by which the sensor is attached to a vessel whose pressure is being assessed, and is connected in series to the tube 21 by means of a balance pressure tube 29. The tubes 20 and 2 1 are mounted on a rigid backbone support 30.

It is seen that an increase in pressure causes the tip 22 and its attached emitter 23 to move in an upward direction while the tip 24 and its attached detector 25 is simultaneously caused to move in a downward direction. Accordingly, a relative displacement of the emitter and detector takes place which may be assessed in terms of the instantly illuminated detector optical fibre(s), the displacement being dependent on and correlatable with the magnitude of the pressure increase and hence dependent on and correlatable with the magnitude of the pressure itself at the end of the increase; similar reasoning applies for a decrease in pressure..Therefore the pressure in the vessel may also be assessed (or inJormation provided which correlates therewith, which comes to the same thing).

It is apparent that the emitter 23 could be attached to the tip 24 and the detector 25 to the tip 22 (although the system would need to be recalibrated). It is also apparent that an optical fibre sensor employing only one Bourdon tube could be constructed, with the emitter being fixed and the detector attached to the movable tube tip, or vice versa. Such a sensor would, however, be less sensitive than the one described above.

Claims

1. An optical fibre sensor for enabling a physical parameter to be assessed which sensor comprises an emitter and a detector which are relatively displaceable, the size of any instant relative displacement being dependent on and correlatable with the value of said parameter at that instant, and wherein the emitter comprises one end of an optical fibre of which the other end is illuminatable by a light source, and the detector comprises the ends of a plurality of optical fibres positioned to be sequentially illuminatable by the light from the emitter as displacement occurs, the other ends of the detector optical fibres being connectable with means for assessing any displacement in terms of the instantly illuminated optical fibre(s).

2. A sensor according to claim 1 wherein the emitter includes a support for the end of the emitter optical fibre which support is fixed, or is attached to or part of a movable part of the sensor, and the detector includes a support for the ends of the detector optical fibres which support is fixed.

or is attached to or part of a movable part of the sensor, at least one of said supports being attached to or part of a movable part of the sensor, whereby said relative displacement is effectable.

3. A sensor according to claim 2 which comprises an optical fibre pressure sensor.

whereby the change in force due to a change in pressure is able to effect said relative displacement by causing the movable part(s) of the sensor to move.

4. A sensor according to claim 3 wherein the movable part(s) of the sensor is the movable tip of a Bourdon tube, or the juxtaposed oppositely movable tips of two Bourdon tubes connected in series.

5. An optical fibre pressure sensor which comprises a) an emitter and a detector which are relatively displaceable, the emitter comprising the supported end of an optical fibre of which the other end is illuminatable by a light source and the detector comprising the supported ends of a plurality of optical fibres positioned to be sequentially iliuminatable by the light from the emitter as displacement occurs; and b) either (i) one Bourdon tube whose movable tip provides or is attached to either the support for the end of the emitter optical fibre or the support for the ends of the detector optical fibres, the support which is not attached to or part of the movable tip being fixed, or (ii) two Bourdon tubes connected in series and having juxtaposed oppositely movable tips, the support for the emitter fibre end being attached to or part of one of the movable tips and the support for the detector fibre ends being attached to or part of the other movable tip;; wherein the relative displacement of the emitter and detector is effected by the tip movement of the Bourdon tube or tubes which movement is caused by a pressure change, the size of any displacement being dependent on and correlatable with the pressure change causing it, and wherein the ends of the detector optical fibres remote from the emitter are connectable with means for assessing any displacement in terms of the instantly illuminated detector optical fibre(s).

6. A sensor according to any one of the preceding claims wherein the detector fibre ends are positioned to lie on a straight or curved locus such that each detector fibre end is substantially equidistant from the emitter end when in a position to be illuminated by the emitter fibre end and at its nearest approach.

7. A sensor according to any one of the preceding claims wherein the changes in relative displacement correspond linearly with the changes in value of the parameter.

8. A sensor according to any one of the preceding claims when connected with the means for assessing the displacement in terms of the instantly illuminated detector fibre(s).

9. A sensor according to claim 8 wherein the means is an electronic device.

1 0. A sensor according to claim 9 wherein the electronic device is at least one of a computer and a microprocessor.

11. A sensor according to any one of claims 8 to 10 wherein the means is linked with the light source.

12. A sensor according to any one of the preceding claims wherein the optical fibres are illuminatable by visible or infra-red irradiation.