GB2255635A

GB2255635A - Measurement method using an optical transducer arrangement

Info

Publication number: GB2255635A
Application number: GB9209511A
Authority: GB
Inventors: James David Hobby; Steven John Butcher
Original assignee: Focas Ltd
Current assignee: Focas Ltd
Priority date: 1991-05-03
Filing date: 1992-05-01
Publication date: 1992-11-11
Also published as: GB9209511D0; GB9109715D0

Abstract

A method of measurement of a physical parameter comprises the steps of sensing a value of the parameter by means of a transducer (2, 6) comprising an actuator (6) which is movable in a manner representative of variation in the parameter value, applying the actuator to a portion of an optical fibre (8) such that the actuator movement produces a variable strain in the fibre portion, inputting polarised light to an input end (10) of the fibre portion such that light transmitted through the fibre portion undergoes a change of state of polarisation which is strain dependent and connecting an output end (12) of the fibre portion to a remotely located receiver (13) by a fibre optic means, and sensing the state of polarisation by means of the receiver to obtain an output representative of the parameter. The method has application in measurement of current in power transmission lines. The transducer actuator (6) may be an electromechanical device constructed from electro-strictive or non-electro-strictive materials (such as piezo-electric or magneto-strictive materials). <IMAGE>

Description

REMOTE SENSING USING FIBRE OPTICS This invention relates to remote sensing using fibre optics and in particular but not exclusively to the remote sensing of electric current.

It is known to use fibre optic devices for the remote sensing of physical parameters. For example it is known to sense electrical current by directly sensing fields associated with a current carrying conductor using the Kerr or Faraday effect.

Typically light passed through a fibre optic exposed to such a field is input to a receiver which compares by means of an interferometer the received light with.

light received from a reference fibre.

A disadvantage of such methods is that the field effects are of small magnitude thereby requiring sophisticated detection apparatus and requiring screening from the effects of interference.

According to the present invention there is disclosed a method of measurement of a physical parameter comprising the steps of sensing a value of the parameter by means of a transducer comprising an actuator which is movable in a manner representative of variation in the parameter value, applying the actuator to a portion of an optical fibre such that the actuator movement produces a variable strain in the fibre portion in a direction transverse to the longitudinal extent of the fibre portion, inputting polarised light to an input end of the fibre portion such that light transmitted through the fibre portion undergoes a change of status polarisation which is strain dependent and connecting an output end of the fibre portion to a remotely located receiver by a fibre optic means which preserves the state of polarisation of light emerging from the output end, and sensing the state of polarisation by means of the receiver to obtain an output representative of the parameter.

Preferably the fibre optic means comprises a polarisation maintaining single mode optical fibre which preserves the state of polarisation of light emerging from the output end.

Preferably a single optical fibre extends from a source of polarised light to the receiver and unitarily incorporates the portion of fibre upon which the actuator acts.

An advantage of such a method is that the value of the physical parameter is optically encoded in a manner which relies upon a single optical fibre extending from the actuator to the receiver.

Preferably the fibre optic means comprises polarisation splitting means located adjacent to the actuator, the method including the step of splitting the light emergent from the strained fibre portion into components having mutually orthogonal planes of polarisation and transmitting the respective components to the receiver.

The respective components may then be transmitted via respective optical fibres to the receiver.

Preferably the method includes the step of applying a clamping force to the actuator so as to continuously apply a stress to produce a level of strain in the fibre portion.

Both positive and negative movements of the actuator will thereby be encoded in the transmitted light as a variation in the state of polarisation.

Preferably the clamping force is selected to provide a level of strain on the fibre portion which rotates the plane of polarisation of light transmitted through the fibre portion by substantially 450.

The transducer may comprise a magnetic element which is movable in response to variation in a magnetic field associated with the parameter to be sensed, the actuator being operatively connected to the magnetic element.

Where for example the parameter being sensed is an electric current the magnetic element can be arranged to be movable in response to the strength of magnetic field associated with the current.

The transducer may alternatively be arranged to produce an electrical signal representative of the sensed parameter value and the actuator may then comprise an electromechanical device driven by the electrical signal.

The electromechanical device may comprise a piezoelectric actuator.

The electromechanical device may alternatively comprise an electrostrictive actuator operable to produce a variable strain in the fibre portion at double the frequency of the electrical signal.

It is a known property of such electrostrictive actuators that the mechanical actuation produced in response to a signal is doubled in frequency. This provides a particular advantage when used in the method of the present invention in that by doubling the frequency the signal is encoded in a manner which is inherently resistant to interference effects associated with the parameter being sensed since any such interference effects acting on the fibre will have a characteristic frequency which is not subject to the frequency doubling effect of the electrostrictive actuator.

The method may include the further step of applying a second actuator driven by the electrical signal, the second actuator being other than an electrostrictive actuator so as to produce a component of strain to the fibre portion at the same frequency as the variation in parameter value. Suitable signal processing may then be used in the receiver to reduce the effects of frequency dependent noise.

The two actuators may alternatively be driven from different electrical signals representative of different parameter values to be sensed. This is useful for example where two alternating current amplitudes are to be measured, each current having the same frequency but different amplitude, the encoded parameter values thereby being distinguishable at the receiver by virtue of the frequency doubling effect of the electrostrictive actuator on the first electrical signal.

Specific embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings of which: Figure 1 is a schematic drawing of a system for measuring electric current; Figure 2 is a schematic drawing of an alternative apparatus for measuring electric current; and Figure 3 is a sectional view of a bow tie fibre.

In Figure 1 a measurement system 1 is arranged to measure the alternating electric current passing through a conductor 2. A Rogowski pick-up coil 3 is wound around the conductor 2 such that an induced e.m.f. appears across output wires 4 and 5 which are connected to the input terminals of an actuator 6.

An optical fibre 7 of several metres length includes a clamped portion 8 which is clamped between the actuator 6 and a fixed block 9. The actuator 6 is arranged such that when energised by an input voltage across wires 4 and 5 the extent to which the clamped portion 8 is compressed varies according to the magnitude of the voltage.

The optical fibre 7 has an input end 10 coupled to a helium neon laser 11 and the fibre has an output end 12 connected to a receiver 13.

In the system 1 the fibre 7 is a particular type of single mode optical fibre referred to as a bow tie fibre having a refractive index profile illustrated schematically in Figure 3. The fibre 7 has a core 14 surrounded by a cladding layer 15 which in profile resembles a bow tie. The refractive index of the cladding layer 15 is less than that of the core 14 and the layer 15 is in turn surrounded by an outer layer 16 having a refractive index comparable with that of the core.

The characteristics of bow tie fibre are that there is relatively low cross-coupling between components of light having E vectors in the X and Y directions respectively. If for example plane polarised light having an E vector lying in a plane defined by the X direction is input to one end of a fibre then substantially all of the light emerging at the other end of the fibre will retain this polarisation with negligible transfer of energy to light with an E vector in the plane of the Y direction.

The fibre 7 is also selected to exhibit a high degree of stress induced bi-refringence. The fibre 7 is coupled to the laser 10 such that light propagated through the fibre has a strong component of polarisation in the X direction. The clamped portion 8 is oriented relative to the actuator 6 such that the clamping force between the actuator 6 and the fixed block 9 creates a strain in the fibre in a direction which rotates the plane of polarisation through an angle A. The clamping force is determined such that angle A is about 450.

In use when measuring an alternating current in the conductor 2 an induced e.m.f. (typically of the order of 100 volts) from the pickup coil 3 drives the actuator 6 at the frequency f of the alternating current. It is a property of such electrostrictive actuators that the resulting mechanical motion is driven at twice the frequency of the input voltage so that the fibre 7 is stressed with a modulation frequency 2f.

Since the angle A is proportional to the applied stress, the light intensities in the X and Y polarisations exiting from the clamped portion 8 are proportional to cos A and sin A respectively. It follows that the X and Y components of polarisation delivered to the receiver 13 are modulated at frequency 2f.

The receiver 13 contains a polarisation splitting element (such as a polarising fibre splitter) to separate the X and Y components which are sensed using separate photodetectors whose outputs are fed to a comparator. The comparator output is converted from analogue to digital form and the resulting value is converted using a look-up table to provide an alphanumeric display of the value of measured current.

The system 1 thereby enables the current in the conductor 2 to be remotely sensed at the receiver 13 by means of a fibre optic link constituted by the optical fibre 7.

There are no active electrical components in the vicinity of the conductor 2 so that the system is inherently free from errors induced by electromagnetic interference.

An alternative measurement system 20 is shown in Figure 2 in which corresponding reference numerals to those of Figure 1 are used where appropriate for corresponding elements.

The alternative system 20 comprises an electrostrictive actuator 6 which is connected to a pickup coil 3 for sensing current in conductor 2. A first optical fibre 21 has an input end 22 coupled to a helium neon laser 11 and extends from the laser to an output end 23 which is located adjacent the actuator 6.

The first optical fibre 21 is a polarisation preserving fibre of the bow tie type referred to with reference to the optical fibre 7 of Figure 1. Plane polarised light entering the input end 22 is arranged to have a strong electric field component in the X direction as referred to above with reference to Figure 3 so that the light reaching the output end 23 is constituted predominantly of an X component with minimal Y component due to the low cross-coupling properties of the selected fibre.

The output end 23 is coupled by means of a connector 24 to a second optical fibre 25 which is a conventional single mode fibre and includes a clamped portion 26 which is clamped between the actuator 6 and a fixed block 9. The effect of the clamping force applied between the actuator 6 and the fixed block 9 is arranged so as to produce a rotation through angle A of the plane of polarisation which is approximately 450.

The second optical fibre 25 has an input end 27 connected to the connector 24 in close proximity with the actuator 26 and an output end 28 which is connected to a polarising fibre splitter 29 (alternatively referred to as a 1 x 2 polarising coupler). Third and fourth optical fibres 30 and 31 are connected to the output of the polarising fibre splitter 29 so as to carry X and Y components of the transmitted light and these fibres extend to a remotely located receiver 32 where the third and fourth fibres are connected to respective detectors 33 and 34. Light intensity levels measured by the detectors 33 and 34 are processed by an electronics unit 35 comprising a de-multiplexing solid state circuit, analogue to digital convertor, look-up table and output display of the type referred to above with reference to receiver 13.

In use the actuator 6 receives an input signal at frequency f corresponding to the frequency of current within the conductor 2 and the strain applied to the clamped portion 26 of second optical fibre 25 is modulated in proportion to the current amplitude but at a frequency of 2f.

The angle A through which the light polarisation is rotated is likewise modulated at frequency 2f and the resulting X and Y components transmitted through the third and fourth optical fibres 30 and 31 receive corresponding modulations in anti-phase at frequency 2f.

In the alternative measurement system 20 the effects of extraneous interference on the second optical fibre 25 are minimised by locating both the connector 24 and the polarising fibre splitter 29 as close as possible to the actuator 6 such that the length of the second optical fibre 25 is minimised.

The third and fourth optical fibres 30 and 31 are multi-mode fibres, there being no requirement for these fibres to preserve the state of polarisation of the X and Y light components since the angle of rotation A is represented by the relative intensities of the light beams carried by these fibres. The system 20 may be modified by replacing the third and fourth optical fibres 30 and 31 by a single bow tie fibre of a type selected such that both X and Y components exhibit low attenuation characteristics, the receiver 35 being provided with suitable means of separating the X and Y components before sensing their respective light intensities.

The apparatus of the above examples may be modified by the addition of an additional actuator acting on the optical fibre adjacent to the actuator 6 and acting in the same direction. The actuator may be of a different type selected such that the result of driving the actuator with a voltage at frequency f produces a mechanical output at the same frequency f.

A piezo-electric transducer for example would achieve this result. If both actuators are then driven from the same source then the output at the receiver will contain components at both f and 2f. Suitable signal processing may then be used to reduce the effects of frequency dependent noise on the output signal.

The two actuators may alternatively be driven from different sources in order to simultaneously measure two different current values.

The above examples refer to the sensing of electrical current. It will be apparent that other physical parameters may be sensed using the measurement systems of the present invention provided that the parameter is initially sensed as an alternating voltage.

In the preferred embodiment a helium neon laser is used as a light source. Other types of laser such as solid state lasers may be used. It may also be possible to use light sources other than lasers if they provide polarised light of narrow bandwidth.

The actuators of Figures 1 and 2 may be replaced by non-electrostrictive actuators such as piezo-electric or magnetostrictive actuators.

The present invention has particular application to the measurement of current in power transmission lines where characteristically high currents or voltages are encountered.

Throughout the above disclosure the term "stress" is used in accordance with its usual meaning to denote a deforming force per unit area and the term "strain" to denote the resulting fractional change in dimension. Stress and strain are generally linearly related for a given material, the ratio of stress to strain being characterised by Young's modulus.

Claims

CLAIMS:

1. A method of measurement of a physical parameter comprising the steps of sensing a value of the parameter by means of a transducer comprising an actuator which is movable in a manner representative of variation in the parameter value, applying the actuator to a portion of an optical fibre such that the actuator movement produces a variable strain in the fibre portion in a direction transverse to the longitudinal extent of the fibre portion, inputting polarised light to an input end of the fibre portion such that light transmitted through the fibre portion undergoes a change of state of polarisation which is strain dependent and connecting an output end of the fibre portion to a remotely located receiver by a fibre optic means, and sensing the state of polarisation by means of the receiver to obtain an output representative of the parameter.

2. A method as claimed in claim 1 wherein the fibre optic means comprises a polarisation maintaining single mode optical fibre which preserves the state of polarisation of light emerging from the output end.

3. A method as claimed in claim 2 wherein a single optical fibre extending from a source of polarised light to the receiver unitarily incorporates the portion of fibre upon which the actuator acts.

4. A method as claimed in claim 1 wherein the fibre optic means comprises polarisation splitting means located adjacent to the actuator, the method including the step of splitting the light emergent from the strained fibre portion into components having mutually orthogonal planes of polarisation and transmitting the respective components to the receiver.

5. A method as claimed in claim 4 wherein the respective components are transmitted via respective optical fibres.

6. A method as claimed in any preceding claim including the step of applying a clamping force to the actuator so as to continuously produce a strain to the fibre portion.

7. A method as claimed in claim 6 wherein the clamping force is selected to provide a level of strain in the fibre portion which rotates the plane of polarisation of light transmitted through the fibre portion by substantially 450.

8. A method as claimed in any preceding claim wherein the transducer comprises a magnetic element which is movable in response to variation in a magnetic field associated with the parameter to be sensed and wherein the actuator is operatively connected to the magnetic element.

9. A method as claimed in any of claims 1 to 9 wherein the transducer produces an electrical signal representative of the sensed parameter value and the actuator comprises an electromechanical device driven by the electrical signal.

10. A method as claimed in claim 9 wherein the electromechanical device comprises a piezoelectric actuator.

11. A method as claimed in claim 9 wherein the electromechanical device comprises an electrostrictive actuator operable to apply a variable strain to the fibre portion at double the frequency of the electrical signal.

12. A method as claimed in claim 11 including the further step of applying a second actuator driven by the electrical signal to the fibre portion, the second actuator being other than an electrostrictive actuator so as to produce a component of strain to the fibre portion at the same frequency as the variation in parameter value.

13. A method as claimed in claim 11 including the further step of applying a second actuator driven by a second electrical signal to the fibre portion, the second actuator being other than an electrostrictive actuator.

14. A method as claimed in any preceding claim wherein the sensed parameter is an alternating electric current.

15. A method of measurement of a physical parameter substantially as hereinbefore described with reference to and as shown in any of the accompanying drawings.