Electricity: measuring and testing – Particle precession resonance
Reexamination Certificate
1999-05-19
2001-11-06
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Reexamination Certificate
active
06313628
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a device for measuring components of a magnetic field with the aid of a scalar magnetometer. It is used in the measurement of weak magnetic fields (approximately of the order of magnitude of the geomagnetic field, i.e. a few dozen microteslas).
PRIOR ART
For some years research has been carried out on the vectorial measurement of magnetic fields on the basis of intrinsically scalar magnetometers (i.e. sensitive solely to the modulus of the magnetic field and independently of its direction). The underlying idea of such research is to take advantage of the absolute character of scalar measurements (based on the resonance of protons (NMR) or electrons (EPR)) for obviating one of the major shortcomings of vectorial sensors, namely their offsets and associated, low frequency drifts.
Such a construction is e.g. described in FR-A-2 663 751 or the corresponding U.S. Pat. No. 5,221,897. These documents also contain bibliographical references on the subject.
However, these constructions are based on the sequential application of high amplitude polarization fields (generally well above the geomagnetic field). Therefore the magnetometer does not provide a continuous measurement, because it is necessary to take account on the one hand of the polarization field establishment time and on the other of the stabilization time of the measurement on the basis of which the projection of the magnetic field onto the artificially created field application axis is evaluated.
Thus, under these conditions, no anti-aliasing filtering is possible (to prevent the aliasing of the spectrum linked with the sampling) and also such a magnetometer is not adapted to rapidly variable fields, such as is the case during measurements on board satellites. Thus, there are variation rates of the modulus of the field of several dozen nT/s and several hundred or even thousand nT/s for certain components. In addition, for these applications, it is frequently necessary for the vectorial measurements to take place at a speed permitting an analysis band of at least 10 Hz or more in the vicinity of the poles (typically up to 25 Hz).
The object of the invention is to obviate these disadvantages.
DESCRIPTION OF THE INVENTION
To this end, the invention proposes a device for measuring components of a magnetic field using a scalar magnetometer, characterized in that it comprises:
at least two conductor windings placed round said scalar magnetometer, said conductor windings having axes oriented in different directions,
means for supplying each winding with a current having a given frequency individual to said windings,
processing means receiving the output signal supplied by the scalar magnetometer, said means being able to carry out synchronous demodulations at least at the frequencies of the supply currents of the windings, said processing means supplying, for each frequency, a signal corresponding to the component of the magnetic field applied along the axis of the winding supplied at said frequency.
Preferably, the processing means are also able to carry out synchronous demodulations at harmonic frequencies or at the linear combination of the frequencies of the supply currents of the windings.
Preferably, the axes of the conductor windings are mutually orthogonal.
In an advantageous embodiment, the device comprises a first conductor winding with a first axis directed along a first direction and a second conductor winding with a second axis directed along a second direction orthogonal to the first. The processing means supply a first signal corresponding to a first component of the magnetic field in accordance with the first component of the magnetic field along the first direction and a second signal corresponding to a second component of the magnetic field along the second direction. The processing means are also able to calculate, on the basis of the signal supplied by the isotropic scalar magnetometer corresponding to the modulus of the magnetic field applied and on the basis of said first and second signals, a third signal corresponding to a third component of the magnetic field applied along a third direction orthogonal to the first and second directions.
In another embodiment, use is made of three conductor windings having mutually orthogonal axes and the device directly supplies the three components of the field.
In order to comply with the pass band constraints on the vectorial measurements, it is necessary to use a magnetometer, whose use frequency limitation is much higher than the pass bands necessary on vectorial measurements (useful pass band of a minimum of several hundred cycles) and whose scalar measurement resolution is at least two orders of magnitude higher than the precision required on the components of the field (a precision of 100 pT on vectorial measurements implies a scalar sensor resolution of 1 pT or higher).
For these reasons, use is preferably made of a scalar magnetometer with optical pumping using helium, like that described e.g. in FR-A-2 713 347 (or its corresponding U.S. Pat. No. 5,534,776).
REFERENCES:
patent: 4814707 (1989-03-01), Marton
patent: 5245280 (1993-09-01), Beranger et al.
patent: 0 462 001 (1991-12-01), None
patent: 0 616 228 (1994-09-01), None
Commissariat A l'Energie Atomique
Oda Christine
Vargas Dixomara
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