Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Patent
1997-06-23
1999-08-03
Patidar, Jay M.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
3242441, 324117R, 250225, 356365, 356364, G01R 33032
Patent
active
059330007
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention concerns a process and an arrangement for measuring a magnetic field.
BACKGROUND INFORMATION
Optical measuring arrangements and methods of measuring a magnetic field utilizing the magneto-optic Faraday effect are known. The Faraday effect is defined as the rotation of the plane of polarization of linearly polarized light as a function of a magnetic field. The angle of rotation is proportional to the path integral over the magnetic field along the path traveled by the light with the Verdet constant as a proportionality constant. The Verdet constant depends in general on the material, the temperature, and the wavelength. To measure the magnetic field, a Faraday sensor device made of an optically transparent material such as glass is arranged in the magnetic field. The magnetic field causes the plane of polarization of linearly polarized light passed through the Faraday sensor device to rotate by an angle of rotation that can be analyzed for a measuring signal. Such magneto-optical measuring methods and arrangements are known for use in measuring electric currents. The Faraday sensor device is placed near a current conductor and detects the magnetic field generated by the current in the conductor. The Faraday sensor device generally surrounds the current conductor, so the measuring light travels around the current conductor in a closed path. In this case, the value of the angle of rotation is in good approximation directly proportional to the amplitude of the current to be measured. The Faraday sensor device may be designed as a solid glass ring around the current conductor or it may surround the current conductor in the form of a measuring winding consisting of an optical fiber (fiber coil) with at least one spire.
Advantages of these magneto-optical measuring arrangements and methods, in comparison with traditional inductive current transformers, include electrical isolation and insensitivity to electromagnetic disturbance. In the use of magneto-optic current transformers, however, problems are encountered due to the effects of mechanical vibrations on the sensor device and the optical leads, which can cause changes in intensity that falsify the measurement, as well as the effects of changes in temperature, for example, in the sensor device.
To reduce the effects of vibration on the measurement, it is known that two oppositely directed light signals, i.e., light signals propagating in opposite directions, can be transmitted through a Faraday sensor device. This known measure is based on the idea that the linear birefringences experienced by the two light signals along their common path due to vibrations as a reciprocal effect of the non-reciprocal Faraday effect can can be distinguished using suitable signal processing.
In a first known embodiment, two oppositely directed, linearly polarized light signals are transmitted through an optical fiber coil serving as a Faraday sensor device surrounding a current conductor. A twisted fiber or a spun hi-bi fiber (a high-birefringence fiber twisted during the drawing process) is provided as the optical fiber for the fiber coil. In addition to the Faraday effect, the optical fiber also has a circular birefringence that is high in comparison with the Faraday effect. After passing through the sensor device, each of the two light signals is broken down by a polarizing beam splitter into two components polarized normally to one another. A measuring signal corresponding essentially to the quotient of the Faraday measuring angle and the circular birefringence of the fiber, which is thus independent of the linear birefringence in the optical fiber, is derived by signal processing from a total of four light components. The resulting measuring signal is thus largely free of temperature-induced linear birefringence in the sensor device, but the measuring signal still depends on temperature because of the temperature-dependence of the circular birefringence of the fiber. In this known embodiment, the two oppositely directed light
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Bosselmann Thomas
Menke Peter
Patidar Jay M.
Siemens Aktiengesellschaft
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