GB2225850A

GB2225850A - Pressure sensing device

Info

Publication number: GB2225850A
Application number: GB8828792A
Authority: GB
Inventors: Alan Frank Cattell
Original assignee: Thorn EMI PLC
Current assignee: Thorn EMI PLC
Priority date: 1988-12-09
Filing date: 1988-12-09
Publication date: 1990-06-13
Anticipated expiration: 2008-12-09
Also published as: GB2225850B; GB8828792D0

Abstract

The device employs no moving parts, and comprises a thin metallic film 2 exposed to a medium which exerts pressure on the device, and means for applying light to the film over a range of angles of incidence to excite resonance in the device. Means are provided to sense where in the range said resonance occurs to generate an indication of the refractive index of the medium for comparison with data indicative of a relationship between refractive index and pressure for the medium. <IMAGE>

Description

PRESSURE SENSING DEVICE This invention relates to a pressure sensing device, and it relates in particular to the provision of such a device employing no moving parts.

According to the invention there is provided a pressure sensing device for sensing the pressure exerted thereupon by a medium, comprising a thin metallic film exposed to said medium means for exciting surface plasmon resonance in said film by the application of light thereto over a range of angles of incidence, and means for sensing at what point in said range said resonance occurs, to generate an indication of the refractive index of said medium for comparison with data indicative of a relationship between refractive index and pressure for said medium.

In order that the invention may be clearly understood and readily carried into effect, one embodiment thereof will now be described by way of example only and with reference to the accompanying drawings of which: Figures l(a) and l(b) are explanatory of the phenomenon of surface plasmon resonance, and Figure 2 shows, in schematic form, a pressure sensing device in accordance with one example of the invention.

Referring now to Figure l(a), there is shown an indication of a basic apparatus exhibiting the phenomenon of surface plasmon resonance (SPR).

One surface of a prism 1 bears a thin film coating 2 of a suitable metal, e.g. silver; the thickness of the coating being approximately 50nm. A collimated polarised beam 3 of light is caused to impinge at an angle a upon one surface of the coating 2. As this angle a is varied, the intensity of the output beam 4 of light from the prism 1 is sharply attenuated, as indicated at 5 in Figure l(b) which plots the normalised intensity of the output beam 4 against angle a, due to the phenomenon of SPR. When SPR occurs, incident light energy is coupled to plasmon modes in the outer surface of the silver coating, and the angle a at which this coupling occurs is sensitive to the refractive index of the medium to which that outer surface of the silver coating 1 is exposed.Thus a change in the refractive index of the medium concerned, occasioned for example, by a change in the pressure to which that medium is subjected, will (other things being equal) result in a shift of the SPR attenuation peak, as shown schematically in dashed lines at 5' in Figure l(b) The intensity of the output light beam 4 may be monitored in a number of ways but if the input beam 3 is scanned, causing it to occupy in sequence various angles a relative to the normal to the surface of the coating 2, then a single large area detector may be used to receive the output beam 4, the attenuation peak being correlated with the angle a by timing relative to the scanning procedure.

Alternatively, an array of detectors may be employed to give angular resolution for the output beam 4 independent of the scan timing. This alternative construction also affords the opportunity of speeding up the analysis by utilising a fanned incident beam of radiation spanning at least a significant number of angles a at one time.

In accordance with the invention, the ability of SPR to detect small changes in refractive index is used to produce a pressure sensor as follows. All materials are compressible to some extent. For example, water has an isothermal compressibility (KT) of around 5x10 N -l M2. Since the pressure, above atmospheric, in water at a depth of 10 m is approximately 10 No , this will result in a fractional volume change, and hence refractive index change, of 1 part in 5x105. This may be detected by a shift in the SPR characteristic. It is therefore clear that SPR may be used directly as a simple pressure sensing device in which there are no moving parts whatsoever; the pressure being read through the change in the property of the material.

In the above, water was given as an example of a compressible medium whose change in refractive index with applied pressure is used to determine the pressure applied.

However, many fluids (gases, vapours or liquids) can be used.

The choice of the fluid determines the range of pressure over which the device is sensitive. For example, ethanol has a compressibility which is approximately three times larger than water. This would therefore increase the sensitivity of the measurement. Furthermore, there is no requirement to use a fluid. Elastomeric solids might be used. The only requirement is that the active element should have a refractive index which varies with applied pressure over the range required. Any suitable material can be encapsulated behind some suitable, flexible, membrane which is impermeable to both the sensing material and the fluid in which the pressure is to be measured.

The mechanical properties of this membrane are not important.

Conveniently, the encapsulated material can consist of or include a liquid crystal material. This is advantageous since changes in pressure can cause changes in ordering of the liquid crystal material, hence giving rise to relatively large changes in the refractive index of the encapsulated material.

Figure 2 shows, schematically, SPR apparatus at 10. The metallic thin film exposed to the medium under examination is shown at 11 and the medium itself at 12. The output 13 of the detector or detectors in the SPR apparatus is applied to a processing unit 14 which also receives data over lines such as 15 indicative of variations in other parameters, such as temperature, which could otherwise confuse the sensor. The processing unit 14 is supplied with a look-up table relating refractive index changes to pressure of the medium concerned, and is arranged after taking due account of input data on lines such as 15, to provide an output signal indicative of the pressure exerted by medium 12 upon the film 11.

As an alternative to the effects of parameters such as temperature being allowed for in a processing unit such as 14, suitable "dummy" pressure sensors could be constructed to act as controls and to permit compensation to be made for the effects of variation in parameters other than pressure.

Some advantages of the invention are as follows. Firstly, the pressure measurement is inherently absolute. For a given temperature, the location of the SPR signal is a function of the applied pressure determined by the fundamental materials properties of the medium under examination. This should be stable over long periods of time assuming that the medium itself is not subject to environmental damage. This could be ensured by suitable choice of the medium and the encapsulation method.

Secondly, the accuracy of the pressure measurement does not depend upon the properties of some displaced membrane. Any outer membrance is only required to encapsulate the sensing medium and provided that its mechanical impedance is substantially less than that of the medium, its physical properties play no part in device performance. Finally, the ability to "read" the pressure change optically, using a simple optical instruction with no moving parts, make the device relatively immune to electrical interference.

Claims

1. A pressure sensing device for sensing the pressure exerted thereupon by a medium the device comprising: a thin metallic film exposed to said medium; means for exciting surface plasmon resonance in said film by the application of light thereto over a range of angles of incidence; and means for sensing where in said range said resonance occurs, to generate an indication of the refractive index of said medium for comparison with data indicative of a relationship between refractive index and pressure for said medium.

2. A pressure sensing device according to Claim 1 wherein said thin metallic film is made substantially of silver.

3. A pressure sensing device according to Claim 1 or 2 wherein said medium is a solid possessing a refractive index which varies with applied pressure over the range required.

4. A pressure sensing device according to Claim 1 to 3 wherein said medium is encapsulated behind some suitable, flexible membrane.

5. A pressure sensing device according to Claims 1 to 4 wherein said means for exciting surface plasmon resonance in said film is a scanning light beam.

6. A pressure sensing device according to Claim 5 wherein said sensing means comprises a single detector.

7. A pressure sensing device according to Claims 1 to 4 wherein said sensing means comprises an array of detectors.

8. A pressure sensing device being substantially as hereinbefore described with reference to, and as illustrated in, Figures l(a) and l(b) of the accompanying drawings.

9. A pressure sensing system comprising: a pressure sensing device according to any of the above claims; a processing unit, including means to relate refractive index changes to pressure of said medium and a means to provide an output signal indicative of the pressure exerted by said medium upon said film.

10. A pressure sensing system according to claim 9 wherein said processing unit comprises means to take account of variations in other parameters other than pressure before providing said output signal indicative of pressure.

11. A pressure sensing system according to claim 9 further comprising control pressure sensors constructed to permit compensation to be made for the effects of variation in parameters other than pressure.

12. A pressure sensing system according to claim 11 wherein said control sensors are encapsulated behind a solid membrane.

13. A pressure sensing system being substantially as hereinbefore described with reference to, and as illustrated in, Figure 2 of the accompanying drawings.