GB2223094A - Pressure transducer - Google Patents

Abstract

A transducer comprises a medium through which a non-conductive signal is transmitted, the medium being such that the transmitted signal varies in relation to distortion of the medium. In a preferred embodiment, polarised light is transmitted through a glass rod 11 and pressure is applied to the glass rod 11 by means of a piston 15 and force transmission member 12. Polarised light is passed through the rod from a light emitting diode 23 to a photo-diode 24. Pressure applied to the glass rod 11 causes the plane of polarisation to be rotated and this produces a detectable change which enables pressure to be measured. Because of the high strength of the glass in compression, and the fact that very little power is required, the pressure transducer is particularly suitable for use in mining conditions, where high overloads may result, and where components must be electrically intrinsically safe to reduce the risk of underground explosions. <IMAGE>

Description

TRANSDUCERS The invention relates to transducers.

Transducers may be used to measure the effect of stress, for example as the result of an applied force.

One known form of transducer comprises a strain gauge in which there is a component having an electrical resistance which varies with stress applied to the component. Thus an electrical signal can be produced which gives a measure of the strain brought about by applied stress.

According to a first aspect of the invention, a transducer comprises a medium through which a nonconductive signal may be transmitted, the medium being such that the signal received through the medium varies in relation to the distortion of the medium.

There may be means for detecting a variation in the transmitted signal and converting the variation into an indication of the distortive force applied to the medium.

The medium may comprise a component through which electromagnetic radiation, e.g. light may be transmitted, the component being such that the manner in which light is transmitted through the component varies in relation to the mechanical stress applied to the component.

The component may be such that the exit plane of polarised light passing through the component varies in relation to the mechanical stress applied to the component.

Preferably the transducer includes first means to direct polarised light into the component and second means to detect polarised light leaving the component.

The transducer may be such that the light travels at right angles to the direction of lines of force applied to the component.

Preferably, the transducer has means for applying fluid pressure to the component, thus enabling the transducer to measure fluid pressure.

Preferably the component is made of glass, although other translucent material may be used, including perspex and silicone rubber. A liquid crystal may also be used.

Preferably the component comprises an elongate glass rod having means to apply pressure to an end of the glass rod.

The means to apply pressure may comprise a piston connectable to the source of fluid pressure to be measured.

A member may be positioned between the piston and the glass rod to distribute load over the end of the glass rod.

The transducer may have a light source in the form of a light emitting diode.

The transducer may have a light detecting device in the form of a photo-diode.

The transducer may have a second photo-diode arranged to stabilise the transducer against external effects.

A transducer according to the invention is particularly effective for measuring pressure in situations where high overload may result, for example, in mining equipment.

Furthermore, the transducer uses very little electrical power and can therefor be made intrinsically safe for mining conditions.

Accordingly, the invention includes mining equipment when fitted with a transducer according to the first aspect of the invention.

The mining equipment may comprise a hydraulic mine roof support.

By way of example, a specific embodiment of the invention will now be described, with reference to the accompanying drawings, in which Figure 1 is a cross-section through an embodiment of transducer according to the invention.

The transducer shown in Figure 1 has been specifically developed for use in measuring pressure, for example, measuring fluid pressure within the hydraulic circuit of a mine roof support.

The transducer comprises a robust body member 10 housing a cylindrical glass rod 11 and a force transmission member 12 which abuts one end of the glass rod 11. The member 12 is retained in position by a robust end cap 13 which has a passage 14 receiving a piston 15. A seal 9 is provided. The passage 14 has an entry port 16. The end cap 13 has an external screw threaded portion 17 so that the transducer can be fitted to a hydraulic circuit so that pressurised fluid enters the port 16 and applies pressure to the end of the piston 15. If the device is used with hydraulic mine roof supports, where very high pressures may be generated, the diameter of the piston 15 can be made relatively small, to reduce the compressive force which is applied to the rod 11 by the piston 15 acting on the member 12.

The glass rod 11 intercepts a passage 18 which extends between a transmitting chamber 19 in the body 10 and a receiving chamber 20 in the body 10.

The chambers 19 and 20 respectively contain crossed polarisers 21 and 22. The chamber 19 contains a light emitting diode 23 and the chamber 20 contains a photo diode 24.

When the device is switched on, light from the light emitting diode 23 passes through the first polariser 21 then through the glass rod 11, then through the second polariser 22, and the resultant liyht is detected by the photo-diode 24 which produces an electrical signal which is dependent upon the intensity of the light received.

When stress is applied to the glass rod 11 by compressing it, it exhibits a photo-elastic effect which rotates the exit plane of the polarised light emerging from the rod. This change in polarisation is experienced by the photo-diode 24 as a change in intensity of the light and so the signal that is produced by the photodiode 24 is directly related to the pressure applied to the piston rod 15 through the port 16.

Thus the device can be used to measure the fluid pressure applied through the port 16.

It is important that any change in the light sensed by the photo-diode 24 results from a change in the plane of polarisation, and not from a change in the intensity of the light emitted by the light emitting diode 23. The transducer therefore includes a secondary photo-diode 25 positioned to observe directly some of the light emitted by the light emitting diode 23. This secondary photo-diode 25 operates through a feedback amplifier to control the current in the light emitting diode to counteract any change in light output, such as might result from temperature change, ageing of the LED or the like.

The two polarisers 21 and 22 preferably each comprise a piece of polaroid material. Their planes of polarisation are arranged at right angles to each other so that light directed through them and through the glass rod is blocked when no pressure is applied to the glass rod. As pressure is applied, the compressive strain in the glass changes the polarisation of the light so that some light passes through, and the increase in transmission will continue with rise in pressure until a maximum is reached.

The rod 11 is a close fit within the body 10 to reduce the risk of the rod buckling under pressure.

This embodiment of pressure transducer is particularly suitable with a hydraulic mine roof support, where substantial pressure overload may arise. This is because the transition from minimum to maximum light transmission corresponds to a very low stress in the glass compared with its ultimate compressive strength.

Thus even if an overload pressure is applied to the transducer which is substantially beyond its working range, this pressure is likely to be resisted by the compressive strength of the glass, without damage.

The body member 10 may conveniently be surrounded by a casing member 26 at one end of which an electrical circuit board 27 may be arranged.

Although glass is a preferred material, other photo-elastic materials may be used. Similarly, a light emitting diode may not necessarily be used as the light source, and an alternative light detector may be used, in preference to the photo-diode.

Preferably the glass rod is annelled so that there is no polarisation resulting from internal strains in the glass.

Other configurations are possible. For example the light may be directed along the glass rod, the compressive load being applied across the width of the rod.

A reflective system may be used, rather than a transmissive system, a transmitter and receiver being placed at one side and a reflective element on the other side.

There may be means to maintain contact between the rod 11 and the member 12, for example a spring.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims (18)

1. A transducer comprising a medium through which a non-conductive signal may be transmitted, the medium being such that the signal received through the medium varies in relation to the distortion of the medium.

2. A transducer as claimed in Claim 1, having means for detecting a variation in the transmitted signal and converting the variation into an indication of the distortive force applied to the medium.

3. A transducer as claimed in Claim 1 or Claim 2, in which the medium comprising a component through which electromagnetic radiation, e.g. light may be transmitted, the component being such that the manner in which light is transmitted through the component varies in relation to the mechanical stress applied to the component.

4. A transducer as claimed in Claim 3, in which the component is such that the exit plane of polarised light passing through the component varies in relation to the mechanical stress applied to the component.

5. A transducer as claimed in Claim 4, including first means to direct polarised light into the component and second means to detect polarised light leaving the component.

6. A transducer as claimed in Claim 4 or Claim 5, which is such that the light travels at right angles to the direction of lines of force applied to the component.

7. A transducer as claimed in any one of Claims 3 to 6, having means for applying fluid pressure to the component, thus enabling the transducer to measure fluid pressure.

8. Atransducer as claimed in any one of Claims 3 to 7, in which the component is of translucent material.

9. A transducer as claimed in Claim 8, in which the component is of glass.

10. A transducer as claimed in Claim 9, in which the component comprises an elongate glass rod having means to apply pressure to an end of the glass rod.

11. A transducer as claimed in Claim 10, in which the means to apply pressure comprises a piston connectable to the source of fluid pressure to be measured.

12. A transducer as claimed in Claim 11, in which a member is positioned between the piston and the glass rod to distribute load over the end of the glass rod.

13. A transducer as claimed in any one of Claims 3 to 12, having a light source in the form of a light emitting diode.

14. A transducer as claimed in any one of Claims 3 to 13, having a light detecting device in the form of a photo-diode.

15. A transducer as claimed in Claim 14, having a second photo-diode arranged to stabilise the transducer against external effects.

16. A transducer constructed and arranged substantially as described herein, with reference to the accompanying drawings.

17. Mining equipment when fitted with a transducer as claimed in any one of the preceding claims.

18. Mining equipment as claimed in Claim 16, comprising a hydraulic mine roof support.