Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Reexamination Certificate
2000-08-14
2002-12-17
Le, N. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
C324S244100
Reexamination Certificate
active
06495999
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method and a device for measuring a magnetic field, in particular for measuring an electric current flowing in a current conductor. The device and the method make use of the Faraday effect, also referred to as magnetorotation.
Optical measuring devices for measuring an electric current flowing in a current conductor by utilizing the Faraday effect are known, and are also referred to as magnetooptical current converters. The Faraday effect is understood to be the rotation of the polarization plane of linearly polarized light which is propagated in a medium in the presence of a magnetic field. The angle of the magnetorotation is proportional to the path integral over the magnetic field along the path traced by the light. The proportionality constant is known as the Verdet constant. For its part, the Verdet constant depends on the material in which the light is propagated, on the wavelength of the light and on further interfering variables which influence the properties of the material, for example the temperature and the state of mechanical stress. In order to measure the current, a Faraday element is arranged in the vicinity of the current conductor. The element contains an optically transparent material which exhibits the Faraday effect. Linearly polarized light is coupled into the Faraday element. The magnetic field generated by the electric current has the effect of rotating the plane of polarization of the light propagating in the Faraday element by a polarization rotation angle, which can be evaluated by an evaluation unit as a measure of the strength of the magnetic field and therefore of the intensity of the electric current. It is generally the case that the Faraday element surrounds the current conductor, so that the polarized light runs around the current conductor in a virtually closed path. As a result, the magnitude of the polarization rotation angle is to a good approximation directly proportional to the current intensity.
In one prior art embodiment, disclosed for example in U.S. Pat. No. 4,564,754 (see, also, European Patent 088 419), the Faraday element is designed as a solid glass ring around the current conductor. There, the light runs around the current conductor once.
In another prior art embodiment, disclosed for example in the published PCT Application WO 91/01501, the Faraday element forms a part of an optical monomode fiber, which surrounds the current conductor in the form of a measuring winding. During one passage, the light therefore runs around the current conductor N times, if N is the number of turns of the measuring winding. Two types of such magnetooptical current converters with a measuring winding consisting of an optical fiber are known, namely the transmission type and the reflection type. In the transmission type, the light is coupled into one end of the optical fiber and coupled out at the other end. The light passes through the measuring winding only once. In the reflection type, on the other hand, the other end of the optical fiber is mirrored, so that light coupled in at the first end is then reflected at this other, mirrored end, passes through the measuring winding a second time in the opposite direction and is coupled out at the first end. Due to the nonreciprocity of the Faraday effect, the plane of polarization of the light is rotated once more in the same direction by the same amount during the opposite passage. Given the same measuring winding, the rotation angle is therefore twice as high as in the transmission type. In order to separate the light coupled in and the light coupled out, a beam splitter is provided.
A problem in all of the magnetooptical current converters are disturbing influences which, for example, are brought about by changes in the attenuation constants in the optical transmission paths.
In the above-mentioned magnetooptical current converter disclosed in U.S. Pat. No. 4,564,754 (EP 088 419), the light coupled out of the Faraday element is split, in an analyzer such as a Rochon prism, a Wollaston prism, or a polarization beam splitter, into two linearly polarized light signals A and B with planes of polarization oriented at right angles to each other. These two light signals A and B are transmitted to corresponding light detectors via corresponding optical transmission fibers and converted into electrical signals PA and PB. The two signals PA and PB are used in a computing unit to calculate a Faraday rotation angle as a measurement signal, which corresponds to the quotient (PA−PB/PA+PB) of the difference and the sum of the two signals. By means of this formation of a quotient, a measurement signal is determined which is independent of the attenuation of the light signals A and B in the transmission path.
The above-mentioned U S. Pat. No. 5,764,046 (WO 94/24573) teaches to decompose the electrical signals S
1
and S
2
received by the receivers arranged downstream of a beam-splitting Wollaston prism in each case into a D.C. signal component D
1
and D
2
and an A.C. signal component A
1
and A
2
. For each signal S
1
and S
2
, an intensity-normalized signal P
1
and P
2
is then formed as the quotient P
1
=A
1
/D
1
and P
2
=A
2
/D
2
of its A.C. signal component A
1
and A
2
and D.C. signal component D
1
and D
2
, respectively. As a result of the intensity normalization of the signals S
1
and S
2
, fluctuations in the intensity in the transmission paths provided for the corresponding light signals LS
1
and LS
2
, and differences in sensitivity in these two transmission paths, can be balanced out.
In that prior art method, it is assumed that the changes in attenuation that take place in the transmission path because of environmental influences are virtually static, as referred to the frequency of the alternating current to be measured. However, using that method, any change over time in the attenuation properties of the transmission path with a frequency component in the range of the frequency of the alternating current, for example a vibration of the attenuation at twice the mains frequency, cannot be balanced out. In addition, that prior art method is not suitable for measuring a direct current or a D.C. component. U.S. Pat. No. 4,694,243 (see European Patent 0 247 842) discloses the practice of coupling linearly polarized and unpolarized light whose wavelengths differ one after another into a magnetooptical sensor. Two light sources are provided for that purpose, and the two light sources are activated one after another and emit unpolarized light. A polarizer is arranged in front of the sensor which linearly polarizes the light emitted by one light source and lets the light emitted by the other light source through without polarizing it. Unpolarized and linearly polarized light are therefore coupled into the sensor one after another.
In a first receiver arranged downstream of the sensor, the light signals emerging from the sensor are converted into electrical measurement signals S
1
, S
2
, after passing through an analyzer. The signals are in each case compared with a reference signal S
R
. The light source which belongs to the electrical measurement signal S
2
is driven on the basis of the result of this comparison in such a way that the electrical measurement signal S
2
becomes equal to the reference signal S
R
.
The light intensities emitted by the two light sources are measured with the aid of a second receiver. Using a control unit connected downstream of the receiver, the intensity of the first light source is controlled in such a way that the electrical measurement signal S
1
generated from the linearly polarized light signal in the presence of a magnetic field at the first receiver, and the measurement signal S
2
generated from the unpolarized light signal are equal. In the presence of a magnetic field, the difference between the electrical measurement signal S
1
and the reference signal S
R
is then proportional to the magnetic field and independent of the intensity of the first light s
Beierl Ottmar
Bosselmann Thomas
Greenberg Laurence A.
Kerveros James
Le N.
Locher Ralph E.
Stemer Werner H.
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