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 (PAxe2x88x92PB/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 S1 and S2 received by the receivers arranged downstream of a beam-splitting Wollaston prism in each case into a D.C. signal component D1 and D2 and an A.C. signal component A1 and A2. For each signal S1 and S2, an intensity-normalized signal P1 and P2 is then formed as the quotient P1=A1/D1 and P2=A2/D2 of its A.C. signal component A1 and A2 and D.C. signal component D1 and D2, respectively. As a result of the intensity normalization of the signals S1 and S2, fluctuations in the intensity in the transmission paths provided for the corresponding light signals LS1 and LS2, 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 S1, S2, after passing through an analyzer. The signals are in each case compared with a reference signal SR. The light source which belongs to the electrical measurement signal S2 is driven on the basis of the result of this comparison in such a way that the electrical measurement signal S2 becomes equal to the reference signal SR.
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 S1 generated from the linearly polarized light signal in the presence of a magnetic field at the first receiver, and the measurement signal S2 generated from the unpolarized light signal are equal. In the presence of a magnetic field, the difference between the electrical measurement signal S1 and the reference signal SR is then proportional to the magnetic field and independent of the intensity of the first light source.
In this way, changes in the attenuation properties of the transmission path over time can also be compensated for. In addition, a direct current or a D.C. component can be measured with that method. However, with regard to the electronic processing of the measurement signals and the control of the light sources, the prior art measuring device is complicated and susceptible to interference because of the large number of electronic components needed to control the light sources.
The object of the invention is to provide a method and device for measuring a magnetic field with the aid of the Faraday effect which overcome the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which permit simple compensation of the attenuation present in the transmission path.
With the above and other objects in view there is provided, in accordance with the invention, a method of measuring a magnetic field with the Faraday effect, which comprises:
coupling light having a first component with a first wavelength and a second component with a second wavelength different from the first wavelength into a transmission path leading to a Faraday element subjected to a magnetic field;
transmitting the first and second components together through a polarizing optical fiber to the Faraday element, such that the light coupled from the optical fiber into the Faraday element has a linearly polarized first light component with the first wavelength and a substantially unpolarized second component with the second wavelength, whereby the first light component is linearly polarized in the polarizing optical fiber;
optically splitting a light signal coupled out of the Faraday element into a first light signal component with the first wavelength and a second light signal component with the second wavelength;
deriving a first measurement signal from the first light signal component and a second measurement signal from the second light signal component, and generating from the first and second measurement signals a corrected measurement signal.
In other words, light is coupled into the Faraday element that is subjected to a magnetic field, the light having a linearly polarized first component and an unpolarized second component. The two components have a mutually different wavelength. The output light signal issuing from the Faraday element is split optically into the two wavelength components. A first measurement signal is thereby derived from the first light signal component and a second measurement signal is derived from the second light signal component. The two components are used to form a corrected measurement signal.
In this way, attenuation influences in the transmission paths can largely be compensated for, even when measuring a magnetic field which is constant over time or a magnetic field with a component which is constant over time. The method according to the invention thus permits the measurement of a direct current or of a current with a D.C. component which is accurate and largely independent of the attenuation properties of the transmission path.
The fact that the linear polarization of the first light component is generated in the polarizing optical fiber leading to the Faraday element, whereby the two light components are transmitted together makes for a particularly simple and loss-free assembly, since the polarizing optical fiber at the same time contributes to transmitting the light over part of the total transmission path.
In accordance with an added feature of the invention, the corrected measurement signal is formed by simple division of the first measurement signal by the second measurement signal.
With the above and other objects in view there is also provided, in accordance with the invention, a device for measuring a magnetic field with the aid of the Faraday effect.
The device has the following elements:
a light-source assembly for generating light having a first component with a first wavelength and a second component with a second wavelength;
a Faraday element;
a light transmission path connected between the light-source assembly and the Faraday element, the light transmission path including a polarizing optical fiber for linearly polarizing the first component and for jointly transmitting the first and second components to the Faraday element such that a light coupled into the Faraday element has a linearly polarized first light component with the first wavelength and a substantially unpolarized second component with the second wavelength different from the first wavelength; and
a device, connected o receive an output light signal coupled out from the Faraday element, for analyzing the output light signal, the device including a wavelength-selective beam splitter for splitting the output light signal into a first light signal component with the first wavelength and a second light signal component with the second wavelength, a receiving unit connected to the beam splitter for forming a first measurement signal derived from the first light signal component and a second measurement signal derived from the second light signal component, and an evaluation device connected to the receiving unit for forming a corrected measurement signal from the first and second measurement signals.
In accordance with a concomitant feature of the invention, the evaluation device is configured to divide the first measurement signal by the second measurement signal so as to form the corrected measurement signal.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for measuring a magnetic field with the aid of the faraday effect, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.