GB2188722A - Optical transducer - Google Patents

Abstract

In an optical transducer for providing an optical output representative of a predetermined physical parameter e.g. pressure, temperature or the position of a piston 8, an input light beam is directed along a predetermined path 3 and deflected by an optical deflector 10 to an optical output path 4 is located in a predetermined position relative to the input path. The defector 10 comprises a deformable member such as an elastic hollow ball the shape of which is a function of the parameter. The ball preferably has a reflective outer surface. <IMAGE>

Description

SPECIFICATION Optical transducer The present invention relates to an optical transducer.

A wide variety of optical transducers have been developed to provide optical outputs representative of predetermined physical parameters, for example representative of the pressure within process plant or the position of actuators of control valves in process plant.

Devices providing optical outputs which can operate remote from any electrical supplies are preferred for use in hazardous areas where the risk of sparks for example generated by faulty electrical equipment must be avoided.

Optical transducers capable of signalling the position of a valve member are described in published British Patent Specification 2 159 942. In one of the described arrangement a valve spindle slides in a bore, a portion of the spindle being formed with a reduced diameter so as to leave a cavity between itself and the wall of the bore. Two optic fibres are positioned with their ends facing each other across the bore, the optic fibres being so arranged that when the portion of reduced diameter of the spindle is located adjacent the fibre ends light can be transmitted from one fibre to the other without any obstruction whereas when a portion of full diameter of the spindle is located adjacent the fibre ends only a proportion of the light can be transmitted from one to the other.Thus simply by transmitting a light beam down one fibre and monitoring the intensity of the light emerging from the other fibre it is possible to determine whether or not the axial position of the spindle within the bore corresponds to the position in which the portion of reduced diameter of the spindle is located adjacent the fibre ends. A fault condition, for example breakage of one of the fibres, is indicated by the fact that the monitored light intensity reduces to zero.

In the arrangement described above the intensity of the light in the output fibre changes rapidly in response to a small movement of the spindle as a very small movement of the spindle is sufficient to move the boundary between the portion of reduced diameter of the spindle and the rest of the spindle across the light path between the two fibre ends. This is advantageous in many applications where a rapid switch action is required and where automatic calibration adjustment systems are operating which might erroneously identify a slow change in the light intensity as a calibration drift rather than a true response by the transducer to the monitored condition. Achieving such a rapid switch action does however require mechanisms manufactured to a high tolerance and this makes the known transducers relatively expensive.

It is an object of the present invention to obviate or mitigate the abovementioned problem.

According to the present invention there is provided an optical transducer for providing an optical output representative of a predetermined physical parameter, comprising an optical input means for directing an input light beam along a predetermined path, an optical deflector located in the said path, and an optical output means located in a predetermined position relative to the input means, wherein the optical deflector comprises at least one deformable member the shape of which is a function of the said parameter and which deflects the input beam relative to the output means in a direction which is a function of the said shape, whereby the output means provides an optical output which is representative of the said parameter.

The deflector may be in the form of for example a single a resilient ball having a reflective outer surface. The input and output means may be optic fibres the ends of which project into a chamber housing the ball. The ball is deformed within the chamber, the amount of deformation being a function of the parameter of interest. The proportion of the input beam deflected from the input fibre to the output fibre varies as the shape of the ball changes, and thus the intensity of the light in the output fibre is a function of the forces causing the deformation. The deformation of the ball may be caused in any way, for example by applying mechanical or fluid pressure to the ball, or by changes in the temperature of the ball where the ball is such that thermal deformation results.If pressure is applied mechanically, this may be achieved by for example a piston sliding in the chamber, so that the intensity of the light in the output fibre is a function of the piston position relative to the chamber. If pressure is applied by an alternative means however, for example by a fluid, the ball may be hollow, its shape being responsive to variations in the differential pressure between its interior and its surroundings. The required differential pressures may be produced by controlling the fluid pressure either of its surroundings or its interior.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of an embodiment of the invention in which a deformable ball is positioned within a piston and cylinder arrangement; and Figure 2 illustrates the deformation of the ball of Fig. 1 as a result of movement of the piston within the cylinder.

Referring to the drawings, in the illustrated arrangement an electrically powered instrument panel 1 located in a non-hazardous area is connected to an optical transducer 2 which may be positioned in a hazardous area by an input optic fibre 3 and an output fibre 4.

There is no electrical supply to the transducer 2. The input fibre 3 is connected to a transmitter 5 which supplies a light beam of predetermined intensity to the input fibre 3.

The output fibre 4 is connected to a receiver 6 which provides an electrical output 7 that may be for example a digital output representative of the intensity of the light beam received through the output fibre 4. The instrument panel 1, including the transmitter and receiver, may be entirely conventional and accordingly will now be described further herein.

The transducer 2 comprises a cylinder 7 within which a piston 8 is movable in the direction of arrow 9. The piston 8 may be for example, connected to the spindle of a control valve which is an integral part of a piece of process equipment so that the position of the piston 8 relative to the cylinder 7 is representative of the position of a control member of the valve.

A deformable ball 10 which has a reflective outer surface is positioned within the cylinder.

When the ball is in its free state it is substantially spherical. As the piston 8 moves towards the fibres 3 and 4 however the ball is deformed as shown in Fig. 2, the degree of deformation being a function of the position of the piston 8. When the ball has the shape shown in Fig. 1 a substantial proportion of the light received from fibre 3 is reflected to the output fibre 4. As the ball is deformed however the surface of the ball moves progressively closer to the end face of the fibre 3 and as a result the proportion of the input light beam which is reflected to the output fibre 4 changes. This change is not linear and a rapid change in the intensity of the reflected light beam in fibre 4 results from a relatively small movement of the piston 8, giving a desirable fast switch action.Thus the linear movement of the piston 8 controls the attenuation of the light beam in the output fibre 4 in a very simple manner and by monitoring the intensity of the light in the output fibre 4 the position of the piston in the cylinder can be determined.

Pressure can be applied to the ball mechanically by connecting the piston either directly to a mechanically movable member or linking the position of the piston to another parameter, for example the pressure of a gas or a hydraulic fluid. Fluid pressure could be used to directly influence the shape of the ball however, for example by arranging for a differential pressure to be applied between the interior of a hollow ball and its surroundings. All that is required is a ball which is deformable in a predetermined manner in response to a parameter variations of which are to be detected. When the ball is in its free state it may be spherical or another convenient shape.

Generally the ball will be resilient such that it always returns to the same shape when in its free state although the ball need not necessarily be resilient providing it is exposed to conditions which force it always to assume a predetermined shape when exposed to predetermined conditions.

Claims (6)

1. An optical transducer for providing an optical output representative of a predetermined physical parameter, comprising an optical input means for directing an input light beam along a predetermined path, an optical deflector located in the said path, and an optical output means located in a predetermined position relative to the input means, wherein the optical deflector comprises at least one deformable member the shape of which is a function of the said parameter and which deflects the input beam relative to the output means in a direction which is a function of the said shape, whereby the output means provides an optical output which is representative of the said parameter.

2. An optical transducer according to claim 1, wherein the deflector comprises a resilient ball having a reflective outer surface.

3. An optical transducer according to claim 2, wherein the input and output means comprise optic fibres the ends of which project into a chamber housing the ball.

4. An optical transducer according to claim 3, comprising a piston slidable in the chamber to apply mechanical pressure to the ball, so that the intensity of the light in the output fibre is a function of the piston position relative to the chamber.

5. An optical transducer according to claim 3, wherein the ball is hollow, its shape being responsive to variations in the differential pressure between its interior and its surroundings, and means are provided for adjusting the said differential pressure.

6. An optical transducer substantially as hereinbefore described with reference to the accompanying drawings.