GB2173898A

GB2173898A - Apparatus for optically measuring a displacement

Info

Publication number: GB2173898A
Application number: GB08609943A
Authority: GB
Inventors: Dr Evangelos Theocharous
Original assignee: Central Electricity Generating Board
Current assignee: Central Electricity Generating Board
Priority date: 1983-04-26
Filing date: 1986-04-23
Publication date: 1986-10-22
Also published as: GB8609943D0

Abstract

Apparatus for optically measuring a displacement includes an optical displacement transducer comprising a body 80 of optical material having an absorption co-efficient which is graduated in a direction transverse to a light path through the body 80, the body (80) being movable transversely of the light path in dependence upon a displacement to be measured thereby to modulate light passing along the light path, the modulation being a measure of the displacement. The apparatus also includes optical fibre means to feed light from a monitoring position to the transducer and to supply back to the monitoring position an optical signal generated by the transducer by the modulation of the light fed thereto. <IMAGE>

Description

SPECIFICATION Apparatus for optically measuring a displacement The present invention relates to apparatus for optically measuring a displacement.

It is sometimes required to measure external parameters (physical variables) in relatively inaccessible places where the use of electrical transducers for remote monitoring is inappropriate. For example, inside electrical machines such as transformers, generators or motors, no conventional electrical transducer can be used because of the high magnetic fields. It has been proposed to use optical techniques in such cases.

There is therefore a need for an optical transducer sensitive to changes in a parameter to be measured, to modulate light fed to the transucer in dependance upon the changes.

According to this invention there is provided apparatus for optically measuring a displacement, comprising an optical displacement transducer and optical fibre means to feed light from a monitoring position to the transducer and to supply back to the monitoring position an optical signal generated by the transducer by modulation of the light fed thereto, in which the optical displacement transducer comprises a body of optical material arranged in a light path through the transducer, and having an absorption co-efficient which is graduated in a translation direction transverse to said light path, the body being mounted for translational movement relative to said light path in said translation direction in accordance with the displacement to be measured, thereby modulating light passing along said light path through the body.

This invention will now be described by way of example with reference to the drawings, in which: Figure 1 illustrates apparatus according to the invention; Figure 2 is a graphical representation of the absorption band of a neodymium doped glass together with the combined absorption spectrum of a neodymium doped glass in series with a ruby glass plate; and Figure 3 illustrates a method of multiplexing a number of transducers spurred off at various positions along a length of optical fibre.

Referring to the drawing, the apparatus includes a transducer which comprises a body 80 of optical material arranged in a light path between ends 81 and 82 of optical fibres 83 and 84, so that light transmitted along fibre 83 from a source 85 is coupled from the end 81 into the end 82 of fibre 84 through the body 80. The body 80 is formed as two identical wedges 86 and 87 of optical material cemented together to form a uniform thickness body with opposite faces of the body 80 accurately parallel. The materials of the two wedges 86 and 87 have substantially the same refractive index so that there is substantially no deviation of light passing through the body from fibre 83 to fibre 84. The wedge 86 is made of a material having a predetermined co-efficient of absorption per unit of light path length through the body.The wedge 87, on the other hand, is made highly transparent at the wavelength of light from the source 85. It can be seen therefore that the absorption co-efficient for light passing right through the body 80 is dependent on the position of the body relative to the light path between the ends 81 and 82 of the fibres 83 and 84.

The body 80 is mounted for translational movement relative to the fibres 83 and 84 so as to be moved transversely of the light path ie, in the direction of arrows 88, in accordance with the displacement to be measured by the transducer, thereby modulating light passing along the light path through the body 80.

A detector 89 detects light transmitted through the body 80 as conducted by the fibre 84, The energy of light received by the detector 89 provides an indication of the posijtion of the body 80 relative to the light path, and thus of the displacement to be measured.

Several transducers as illustrated in Fig. 1 can be used simultaneously interconnected by lengths of optical fibre forming a string. Then the displacement indicated by any selected one of the transducers can be measured using time-domain reflectometry techniques, for example as described in Patent Application No. 8430717.

Preferably then, the optical material from which the wedge 86 is formed is one having a well defined absorption band but which is highly transparent away from the absorption band. Fig. 2 provides a graphical illustration of an absorption band with a peak at 585nm as provided by glass doped with neodymium (Nd3+). Then, the source 85 may be arranged to emit light at two wavelengths, one in the absorption band, eg, at 585 nm, and the other away from the absorption band, eg, at about 630 nm. Only the light at the wavelength in the absorption band is variably absorbed with displacement of the body 80 of the transducer.If the detector 89 is arranged to detect the absorption co-efficient at the two different wavelengths, these two values can be used to normalise the displacement indication to minimise dependency on other loss co-efficients which may be dependent on various external parameters but have substantially equal effects at both of the two wavelengths.

A single optical fibre system as described in Patent Application No. 8430717 may be used to measure both temperature and displace ment using a single combination transducer. If such a system each sensing device can be formed as a transducer as described above together with a ruby glass plate in series, so that light from one optical fibre passes through both the ruby glass plate and the body of the transducer before entering the next optical fibre. If the displacement transducer includes neodymium doped glass as the absorbing optical material, then the wavelength, 585 nm which is absorbed by the displacement transducer can be different from the wavelength of the absorption edge of a specific form of ruby glass, ie, 550 nm for glass OG550. The combined absorption spectrum of the transducer and the ruby glass plate is illustrated also in Fig. 2.

With this arrangement the light source 85 is arranged to provide three wavelengths of light, at about 550 nm so as to be on the absorption edge of the ruby glass, at about 585 nm to be in the absorption band of the neodymium doped transducer body, and at about 620 nm as a reference wavelength. Temperature variations then have minimal effect on the absorption of light at 585 and 620 nm, whereas displacement variations have minimum effect on the light at wavelengths 550 and 620 nm.

Fig. 3 shows an optical fibre length 90 to which are connected optical fibre spurs 91, 92, 93 and 94. The spurs are connected to the optical fibre length 90 by fibre couplers. A fibre coupler 95 is also used to feed light from a diode source 96 into the fibre length 90 and return modulated light from the fibre length 90 to a photo-sensor 97. Each of the spurs 91 to 94 is terminated at a displacement transducer 98, 99, 100 or 101, each as shown on Fig. 1 except that one face 102 of the body is made reflecting. The end of each fibre spur 91 to 94 is optically linked to the body of the associated displacement transducer so that light emerging from the fibre spur passes through the body, is variably absorbed by the wedge 86 thereof, is reflected at the reflecting surface 102, and passes back through the body to re-enter the end of the fibre spur.

It can be seen that the returning reflected light from a single pulse of light fed into the end of the fibre length 90 from the source 96 is received by the detector 97 as a time spaced series of pulses with the time spacing depending on the length of the fibre elements between one spur connection eg. 103 and the next 104 along the fibre 90. Accordingly, the signals from each of the transducers 98 to 101 can be separately monitored by appropriate timing techniques.

Claims

1. Apparatus for optically measuring a displacement, comprising an optical displacement transducer and optical fibre means to feed light from a monitoring position to the transducer and to supply back to the monitoring position an optical signal generated by the transducer by modulation of the light fed thereto, in which the optical displacement transducer comprises a body of optical material arranged in a light path through the transducer, and having an absorption co-efficient which is graduated in a translation direction transverse to said light path, the body being mounted for translational movement relative to said light path in said translation direction in accordance wihth the displacement to be measured, thereby modulating light passing along said light path through the body.

2. Apparatus as claimed in Claim 1, wherein the optical material has a spectral absorption profile such that the body has said graduated absorption co-efficient at a first wavelength but is uniformally transparent at a second wavelength.

3. Apparatus as claimed in Claim 1 or Claim 2, wherein the body is uniformly refractive in the transmission direction over the range of translational movement, providing no variation with translational movement in any deviation of light passing through the body.

4. Apparatus as claimed in Claim 3, wherein the body has a substantially uniform refractive index and uniform thickness in the transmission direction.

5. Apparatus as claimed in Claim 4, wherein the body is formed of a first wedge of said optical material having a predetermined absorption co-efficient per unit distance in the material and a second wedge of substantially the same angle as the first and made of a material having substantially the same refractive index as the optical material of the first wedge and being highly transparent with substantially no absorption, the two wedges being cemented together to form the uniform thickness body.

6. Apparatus as claimed in Claim 2 or any of Claims 3 to 5 as dependent upon Claim 2, wherein said optical material has an absorption band centred at a predetermined wavelength and said first wavelength is selected to be within said absorption band and said second wavelength is selected to be outside the absorption band.

7. Apparatus as claimed in Claim 6, wherein said optical material is neodymium (Nd3+) doped glass.

8. Apparatus as claimed in Claim 2, or any one of Claims 3 to 7 as dependent upon Claim 2, including light source means at the monitoring position to feed light at each of said wavelengths along the optical fibre means to the transducer, whereby the displacement to be measured modulates the light at said first wavelength but not at said second wavelength, and measuring means comparing the energies of light at the two wavelengths which is supplied back to the monitoring posi tion after transmission through the transducer to provide a value for said absorption co-efficient, and thence the displacement, which is normalised to minimise any dependence on loss co-efficients other than said absorption co-efficient.

9. Apparatus as claimed in Claim 8 as dependent upon Claim 6 or Claim 7, and additionally for measuring temperature, the apparatus further comprising an optical temperature transducer comprising a further body of optical material through which the light passes and having an optical absorption spectrum with an absorption edge which is temperature dependent but extends over a range of wavelengths outside said absorption band, said second wavelength being selected to be away from said edge, said light source means being arranged additionally to supply light at a third wavelength on said absorption edge whereby the displacement modulates the light at said first wavelength but not at said second and third wavelengths and the temperature modulates the light at said third wavelength but not at said first and second wavelengths, said measuring means being arranged additionally to compare the energies of said light at said third and second wavelengths to provide a normalised value for the temperature.

10. Apparatus for optically measuring a displacement, substantially as hereinbefore described with reference to the drawings.