Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Patent
1998-03-16
2000-03-07
Snow, Walter E.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
3242441, 359280, G01R 33232, G01R 1527, G01R 1900, G01J 902
Patent
active
060345225
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a transducer which incorporates a coil of optical fibre and, in one of its aspects, to a transducer that provides for referencing of optical signals that pass through the coil. The invention has been developed in the context of a fibre optic transducer that is employed for sensing the magnitude of electric current flow through a conductor and the invention is hereinafter described in relation to such a transducer. However, it will be understood that the invention does have broader application, to any transducer that incorporates an optical fibre having a length sufficient to provide a discernible light transmission time.
BACKGROUND OF THE INVENTION
Fibre optic current sensors typically are employed in high voltage electricity generating/distribution systems, and they make use of the Faraday effect to provide a measure of current flow through a conductor. Counter-propagating light signals are directed into opposite ends of a coil of the optical fibre that surrounds the current conductor and signal detecting/processing circuitry is provided to detect a phase shift between the polarisation modes of the counter-propagated light beams. This is achieved alternatively by using a polarising beam splitter and detecting directly for the phase shift or by inferring the phase shift from the interference of the counter-propagating light beams in a coupling device.
The current sensors rely upon the accurate measurement of the optical power levels of the light signals that have counter-propagated through the coil. However, the power levels are influenced not only by the electric current but, also, by changes in power level of the light source and by attenuation in the optical fibre leading to and from the coil. Also, electronic processing and measurement of the optical signals may give rise to errors due to changes in receiver sensitivity, amplifier gain and offset. Therefore, to stabilise absolutely the measurement of optical power levels would be an impossible task, even if consideration was to be given only to one or two influencing factors such as temperature induced variations and aging effects.
SUMMARY OF THE INVENTION
The present invention seeks to avoid these problems by providing a referencing system which is arranged to compensate for extraneous influencing factors and which makes use of the difference in transmission times of light through light paths that have significantly different path lengths.
Thus, the invention may be defined broadly as providing a transducer which comprises a first light path which is defined by a light transmitting element and a second light path which is defined by an optical fibre, the optical fibre defining the second light path having a length which is significantly greater than that of the light transmitting element which defines the first light path. A light source is provided for directing light simultaneously into both the first and the second light paths, and means are provided for cyclically modulating the light from the source between upper and lower levels. Also, means are provided for detecting for light exiting from the two light paths during successive time periods that include a first time interval in which light exits from the first light path only and a second time interval in which light exits from the second light path only, and further means are provided for referencing the level of light that exits from one of the paths with respect to the level of the light that exits from the other path.
Referencing may be made with respect to light that exits from either the first or the second light path, depending upon the construction and application of the transducer. That is, if the optical fibre which defines the second light path is wound into a coil and the coil is employed, for example, as a sensing coil in the transducer, the relatively short light transmitting element may be employed as the reference path. However, where a relatively short length of optical fibre is employed as or in conjunction with a sensing
REFERENCES:
patent: 4370612 (1983-01-01), Puech et al.
patent: 4686371 (1987-08-01), Birch et al.
Snow Walter E.
The University of Sydney
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