GB2082765A - Optical/pressure switch - Google Patents

Abstract

A force or pressure sensitive switch includes a piezo-electric element (18) wherein changes in applied pressure or force produce corresponding changes in electrical potential developed across the element. This potential, being applied to the electrodes of a liquid crystal device (14), varies the light transmissivity thereof in accordance with that potential above a certain threshold level. Light, passed from a light source (10) via optical fibres (11-12-13) through the device (14), is reflected by a mirror (15) back through the device and then travels via optical fibres (13, 12, 16) to a light detector (17). Thus changes in the light reaching the detector depend on changes in the force or pressure being monitored. Alternatively, the fibres coupled to the light source and the detector are on opposite sides of the liquid crystal device. One application of such a switch is as the vortex frequency sensing means in a vortex flow meter. <IMAGE>

Description

SPECIFICATION Optical/Pressure Switch This invention relates to a sensitive pressure or force responsive switch using optical switches and optical fibres.

In an optical fibre transducing system, information about physical changes at one point may be transmitted to another point via an optical fibre. Typically such a system consists of a light source from which light is conducted to a photosensitive detector via one or more optical fibres, with a transducing element or switch interposed at some point in the optical circuit. The transducing element derives from the physical change to be detected a related change in the light transmitted between the light source and the detector.

An object of the present invention is to produce a transducing arrangement for use in such systems which is highly sensitive.

According to the present invention, there is provided an optical pressure or force sensitive switch, which includes a piezo-electric element to which the pressure of force to be monitored is applied so that an electrical potential is generated across the element whose value depends on the pressure or force, a liquid crystal device to which the piezo-electric element is connected so that the potential developed across that element is applied to the liquid crystal device, which potential varies the optical conductivity of the liquid crystal device in accordance with its value above a certain threshold level and an optical fibre arrangement via which light is applied to the liquid crystal element from a light source, which light passes through the liquid crystal element when the latter is in a condition to pass light, so that changes in the light which reaches the light detector depend on the changes in the pressure or force being monitored.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figs. 1 and 2 are simple explanatory diagrams relating to two embodiments of the invention.

Fig. 3 is a somewhat more detailed representation of an arrangement embodying the principles of Fig. 1.

Figs. 4 and 5 illustrate schematically a vortex flowmeter using a switch such as that of Fig, 3 as its pressure/force sensitive switch.

Referring to Fig. 1 we have as the pressure sensitive element a piece 1 of a piezo-electric material which is connected as shown to the two conductive electrode faces of a liquid crystal device 2 such as used in display devices. The device 2 is in the path of light from a light source (not shown) which reaches it via an optical fibre 3. Also connected to the fibre 2 is a light detector (not showntand on the rear face of the liquid crystal element 2 there is a mirror 4. Alternatively, the rear face of the element 2 is silvered so that it is, in effect, its own mirror.

When a mechanical strain-pressure or force is applied to the piezo-electric element 1, which may, for instance, be a flexure element, an electrical potential is developed across the element 1 proportional to the charge generated on the crystal capacitance. This potential is not directly calculable but may be estimated from approximate expressions for charge and capacitance. This potential is applied to the liquid crystal device 2 by virtue of the connection from the element 1 to the device 2. Thus the liquid crystal device changes its opacity to the light transmitted along the fibre, so that the amount of light transmitted via the device 2 and back via the fibre 3 to the detector is dependent on the potential applied to the device 2 from the element 1, and hence dependent on the pressure and force to which that element 1 is subjected.Thus the changes in the pressure or force extended on the piezo-electric element 1 are reflected in the changes in the light which reaches the detector.

The arrangement of Fig. 2 is similar to that of Fig. 1, except that the light from the incident fibre 7, after its passage throughout the liquid crystal element 8, passes via another fibre 9 to the detector (not shown).

Since both the piezo-electric element and the liquid crystal element have finite electrical impedances, the potential generated by the piezoelectric element gradually falls, and the device is only suitable for measuring rapid changes in force or pressure, and not steady values. This feature is common to many piezo-electric devices.

Furthermore, the optical transmissivity of liquid crystal elements is normally only nearly linearly related to the applied voltage over a very small range of voltage and gives no significant change below a certain threshold level. Thus the device is envisaged as being primarily applicable to the indication of sudden changes in force or pressure level and to the indication of the frequency cf a cyclically varying force or pressure.

The frequency response of the device depends on the thickness of the liquid crystal and the operating temperature. With a crystal 10 y thick, the device would be able to switch from its opaque to its transparent condition at a frequency in the order of 100 Hz at normal room temperatures. Liquid crystal devices currently available can operate over the temperature range -400C to +800C.

Since the liquid crystal device can change from an optically opaque condition to an optically transparent condition regardless of the sign of the applied potential, then a cyclic variation of potential applied to the liquid crystal device will cause the device to switch once in every half cycle of electric potential variation. The switch will therefore operate at twice the frequency of the applied potential and hence at twice the frequency of a cyclically varying force applied to the piezo-electric element.

Typical power requirement for current liquid crystal devices is 1 to 10 ,uW/cm2 at 5 V applied, or 100 nA/cm2. Hence a 1 mm2 area needs 1 nA to 10 nA at 10 V.

The above arrangement is thus a transducing system, in which the transducing element needs no external power source and is coupled to active electrical elements only by non-electrically conductive optical fibres. Since the currents generated in the transducing element are very small, and that element is isolated from other electrical compoonents, it may be used where there is risk of fire or explosion.

Referring to Fig. 3, here we have a light emitting diode 10 0 which applies light to an optical fibre 11 from which it passes via a Y-coupler 12 to another fibre 1 3. This fibre ends near a liquid crystal device 1 4 backed with a mirror 1 5 (or a liquid (crystal device with a silvered rear face).

This mirror reflects the light back through the liquid crystal device 14 and back along the fibre 1 3 to the coupler 12. From here some of the liqud reflected back passes up the branch 1 6 to a photodetector 1 7. Thus light which reaches the photodetector 1 7 depends on the condition opaque or transparent-of the device 14, and this in term depends on the pressure or force applied to the piezo-electric element 1 8.

The output of the photo-detector 1 7 is applied, after suitable electrical processing and amplification to a monitoring device, such as the Y plates of an oscilloscope or a measuring instrument. With such an arrangement it was found that gentle pressure applied with a finger to the device 1 8 was sufficient to turn the liquid crystal device 14 from transparent to opaque.

This change in opacity is visible to the naked eye and is accompanied by a change in the output voltage from the photo-detector 17, as indicated on the oscillator tracer other measuring instrument.

We now describe the application of an arrangement such as that of Fig. 3 to a vortex flowmeter, see Figs. 4 and 5 which are highly schematic representations of a flowmeter using the arrangement of Fig. 3.

Vortex flowmeters are based on the fact that when a fluid flows past a bluff body of an appropriate shape, vortices are shed alternately from either side of the bluff body at a frequency which is very nearly proportional to the flow speed. Thus a vortex flowmeter includes such a bluff body, and means for detecting the frequency or number of the vortices shed in a given time.

This shedding of vortices is accompanied by associated changes in the pressure of the fluid on the surfaces of the bluff body, so that the frequency of the vortex shedding can be detected by detecting the frequency of the surface pressure fluctuations.

In the arrangement of Figs. 4 and 5, we have a bluff body 20 formed by a body of a triangular cross-section which extends across the pipe 21 conveying the fluid whose flow is to be measured.

A diaphragm 22 is arranged in the bluff body 20 so that one of its sides is subjected to the pressure on one surface of the body, while the other side is subjected either to a reference pressure or to the pressure on the opposite side of the bluff body, as shown in Fig. 4. The diaphragm is either made of a material with piezo-electric properties such as polyvinylidene fluoride, or has a piece 23 of piezo-electric material bonded to it.

The pressure difference across the diaphragm varies cyclically at the same frequency as the shedding of the vortices. This causes a corresponding cyclic variation in the strain to which the piezo-electric material is subjected, with corresponding changes in the electrical potential generated and applied to the liquid crystal device 24. Since the liquid crystal device can be caused to switch on either a negative or a positive applied potential, the device 24 is switched between its clear and its opaque condition at a frequency which is twice that of the frequency of the shedding of a pair of vortices.

The changes in state of the liquid crystal device 24 are responded to by an optical system similar to that of Fig. 3. This includes a light source 25, a photo-sensitive detector 26 and the optical fibres shown, including the Y coupler 27. As in the arrangement of Fig. 3, the changes in the opacity of the liquid crystal element 24 cause the detector 26 to produce electrical pulses. These are passed via a conventional amplifier/filter arrangement 27 to a pulse counter or frequency measuring circuit 28. The latter is calibrated so as to give an indication of the flow rate in the pipe 21.

In all the arrangements described above we have shown plain fibre ends terminating adjacent to the liquid crystal devices. To improve light transfer it would be possible for a beam expansion arrangement such as a conical end portion to be used at the fibre ends.

Such an arrangement is suited for use in a hazardous environment, and is relatively cheap and inexpensive.

Claims (9)

Claims

1. An optical pressure or force sensitive switch, which includes a piezo-electric element to which the pressure or force to be monitored is applied so that an electrical potential is generated across the element whose value depends on the pressure or force, a liquid crystal device to which the piezoelectric element is connected so that the potential developed across that element is applied to the liquid crystal device, which potential varies the optical conductivity of the liquid crystal device in accordance with its value, above a certain threshold level, and an optical fibre arrnngement via which light is applied to the liquid crystal element from a light source, which light passes through the liquid crystal element when the latter is in a condition to pass light, so that changes in the light which reaches the light detector depends on changes in the pressure or force being monitored.

2. A switch as claimed in claim 1, in which the optical fibre arrangement includes a first optical fibre via which the light is transmitted to one face of the liquid crystal device and a second optical fibre via which light which has passed through the liquid crystal device is conveyed to the light detector after that light has passed through the liquid crystal device.

3. A switch as claimed in claim 1, in which the optical fibre arrangement includes a single optical fibre via which light from the source reaches one face of the liquid crystal device and via which light is conveyed from the device to the light detector, there being a mirror on the other face of the liquid crystal device so that the light makes two passes via that device.

4. A switch as claimed in claim 3, and in which the optical fibre arrangement includes a Y coupler via which light from the source reaches said single fibre and via which light from the single fibre reaches the detector.

5. A switch as claimed in claim 1, 2, 3 or 4, and in which the or each said fibre has at its end adjacent the liquid crystal device a beam expander such as a cone.

6. A vortex fiowmeter which includes a bluff body located in the pipe or the like conveying the fluid whose flow rate is to be monitored, which bluff body produces vortex shedding, and a diaphragm so located as to be subjected to the pressure changes in the fluid caused by the vortex shedding, said diaphragm's movements being monitored by a switch as claimed in claim 1, 2, 3, 4 or 5.

7. A vortex flowmeter as claimed in claim 6, and in which said diaphragm is of a piezo-electric material so that it functions as the piezo-electric element.

8. A pressure or force sensitive switch, substantially as described with reference to Figs.

1, 2 or 3 of the accompanying drawings.

9. A vortex flowmeter substantially as described with reference to Figs. 4 and 5 of the accompanying drawings.