JP4287735B2 - Measuring instrument for fluxgate magnetometer - Google Patents

Description

  The present invention relates to a measuring device for a fluxgate magnetometer.

  A fluxgate magnetometer is known as a device that measures magnetic flux density by utilizing the symmetry of saturation magnetic flux density of a soft magnetic material core constituting a magnetic sensor. As this magnetic sensor, for example, as shown in FIG. 5, a ring core type has been proposed. An excitation coil 4 for exciting a core 3 made of a soft magnetic material formed in a ring shape, and induced in the core 3. And a detection coil 6 for detecting a change in magnetic flux density. 2 is excited by an alternating magnetic field up to a saturation region by applying an excitation voltage to the excitation coil 4 from the excitation circuit 7 of the measuring device 16 (details will be described in the description of the embodiment) shown in FIG. The change in magnetic flux density induced in the core 3 is detected from the detection coil 6. In FIG. 5, the magnetic sensor 1 has a configuration in which a ring core 3 is housed in a housing 2 made of a nonmagnetic material.

  When no magnetic field other than the excitation magnetic field is applied to the core 3, the induced magnetic flux density is also symmetric, and the detection coil 6 generates only the odd-numbered harmonics of the excitation wave. Here, when a magnetic field is applied to the core 3 from the outside of the sensor, the external magnetic field is added to the exciting magnetic field, so that the magnetic field applied to the core 3 is equivalently biased and the change in magnetic flux density becomes asymmetric. , Even-numbered harmonics are generated in the detection coil 6.

  The fluxgate magnetometer measures the magnetic field by detecting the even harmonics.

That is, a substance having a high magnetic permeability has non-linear magnetic characteristics and has the hysteresis (history curve: BH curve) shown in FIG. When core frequencies of 1 / T shown in FIG. 6 (b) is excited by the magnetic field H _D that fully saturated, the core is saturated at ± H _C, induction flux B is ± B _S.

The core 3 in winding a secondary coil (detection coil), the magnetic flux seen from the detection coil as shown in FIG. 6 (c), a waveform becomes saturated value at ± B _S.

Then, the output signal induced in the sensing coil is a time change of magnetic flux (dB _R / dt), as shown in FIG. 6 (d), opening and closing of the pulse is effected by ± B _S.

  Here, in the state shown in FIG. 6B, if there is no external magnetic field ΔH, the waveform shown in FIG. 6C is symmetric with respect to B = 0, and the positive and negative waveforms in FIG. The pulse output cycle on the side becomes equal.

  On the other hand, when the external magnetic field ΔH exists, the induced magnetic field generated by ΔH + H cos ωt shifts from the zero point excited in FIG. 6B, and therefore, from the positive side to the negative side of the output pulse as shown in FIG. 6C. Is no longer T / 2 (50% duty).

  Permalloy is used as the magnetic detection material that constitutes the core, and the shape of the core is that the magnetic sensor using the ring core shown in FIG. 5 has the characteristics that the excitation power is small and a large output can be obtained. Yes.

  The magnetic sensor using the above-described ring core can measure with high accuracy since the noise is small, but there is a limit to the measurement with higher accuracy.

  The hysteresis curve shown in FIG. 6A is ideal. Actually, as shown in FIG. 7, the magnetic field H reaches the saturation magnetic field, and the rate of change of the induced magnetic flux gradually decreases in the saturation magnetic field region thereafter. Become. Since the point where the magnetic field H reaches the saturation magnetic field can be determined as the origin (H = 0), ΔH can be obtained as shown in FIG.

  However, in order to further increase the accuracy of the magnetic force measurement, a slight noise generated in the history curve greatly affects.

  As a result of examining the generation of such noise, the present inventor has found the following.

  The fluxgate magnetometer measures the magnetic field by applying an alternating magnetic field to the magnetic detection material that constitutes the core, and detecting the component in which the change in magnetic flux density is asymmetric due to the measurement magnetic field. It was speculated that if there was inhomogeneity in the magnetic field, this would appear as noise in the above-mentioned hysteresis curve.

  Then, attention was paid to the magnetic domain, which is a magnet unit in the magnetic detection material, as a factor causing the non-uniformity in the saturation magnetic field. Since the size of the magnetic domains in the magnetic detection material is not uniform, each magnetic domain is saturated by the alternating magnetic field, and the reversal of the magnetic axis in each magnetic domain is non-uniform, resulting in non-uniformity in the saturation method.

  In other words, in the saturation region, the saturation of each magnetic domain varies and unevenness occurs in the change in permeability of the magnetic detection material constituting the core, but the magnetic flux density is proportional to the product of current and permeability. It is considered that the non-uniformity causes fluctuations in the magnetic flux density and causes noise in the magnetometer output.

  For this reason, there has been a limit to increasing the measurement accuracy.

  The object of the present invention was made in view of such conventional problems, and is a flux gate magnetometer measuring device that achieves higher accuracy of magnetic force measurement by improving the excitation circuit that excites the core of the magnetic sensor. It is something to be offered.

  According to a first aspect of the present invention, there is provided an excitation circuit that applies an alternating current to the magnetic detection material of the magnetic sensor by passing an exciting current through the exciting coil of the magnetic sensor. In a measurement apparatus of a fluxgate magnetometer that measures a magnetic field based on a harmonic induced by an asymmetric component of a magnetic flux that reflects a measurement magnetic field detected by a detection coil, the excitation circuit includes the excitation circuit in the magnetic saturation region of the magnetic detection material. It is characterized by having a current fluctuation suppressing circuit for gradually increasing the exciting current.

  According to a second aspect of the present invention, the magnetic sensor includes an excitation circuit that applies an alternating current to the magnetic detection material of the magnetic sensor by passing an exciting current through the exciting coil of the magnetic sensor. In a measuring apparatus of a fluxgate magnetometer that measures a magnetic field based on a harmonic induced by an asymmetric component of a magnetic flux that reflects a measurement magnetic field detected by a detection coil, the excitation circuit causes the excitation current to fall within a predetermined current value. It is characterized by having a current fluctuation suppressing circuit that delays the time until the exciting current that increases and fluctuates in the magnetic saturation region of the magnetic detection material reaches the predetermined current value while maintaining and outputting.

A third invention is as claimed in claim 3, said either of invention, the current variation suppressor is characterized by an inductor.

  According to the first and second aspects of the present invention, the excitation current fluctuates in the magnetic saturation region of the magnetic detection material. However, since the increase fluctuation is not abruptly performed, the magnetic detection material is completely magnetically saturated. Since measurement can be performed in a state, the unevenness (fluctuation) appearing in the magnetic flux density change can be suppressed, the output noise of the magnetometer can be reduced, and the measurement accuracy can be greatly improved.

According to the invention of claim 3 , the measurement accuracy can be drastically improved with a simple configuration in which an inductor is used as the current fluctuation suppressing circuit.

  1 and 2 show an embodiment of the present invention.

  FIG. 2 is a block diagram showing a measuring apparatus and noise measurement system of a fluxgate magnetometer according to the present invention, FIG. 1 is a block diagram of an excitation circuit in the measuring apparatus of the fluxgate magnetometer, and FIG. (B) shows a conventional excitation circuit. As the magnetic sensor, a ring core type shown in FIG. 5 is used.

  FIG. 2 shows a measurement device 16 and a noise level measurement system configured by connecting the vector signal analyzer 11 to the measurement device 16.

  The measuring device 16 receives an excitation circuit 7 for applying an excitation signal, which is an alternating signal, to the excitation coil 4 of the magnetic sensor 1, an amplifier 12 for amplifying the detection signal from the detection coil 6, and an amplified detection signal from the amplifier 12. Second harmonic amplifying circuit 18, phase detector 9, low-pass filter 13, feedback circuit 10, excitation circuit 7 and excitation / phase signal generation circuit 14 for outputting phase signal to phase detector 9, magnetometer EMI filter 15 for supplying the power to each circuit and the like.

When an excitation signal as shown in FIG. 6B is applied from the excitation circuit 7 to the excitation coil 4, a magnetic field is generated in the ring core 3, and the magnetic field gradually increases. When the saturation magnetic field reaches H _D, as shown in FIG. 7, increase the angle of the induced magnetic flux B in the saturation region becomes gentle. As described above, the asymmetric component of the magnetic flux B generated in the core 3 is detected from the detection coil 6 as an electric signal and output to the amplifier 12. The detection signal amplified by the amplifier 12 is sent to the second harmonic amplifying circuit 8. Is output.

  The second harmonic amplifying circuit 8 amplifies the second harmonic induced by the asymmetric component of the magnetic flux reflecting the measurement magnetic field detected by the detection coil 6. The measurement magnetic field amplified by the second harmonic amplifying circuit 8 is extracted as an AC electrical signal.

  The phase detector 9 detects a measurement magnetic field converted into an AC electric signal, which is a second harmonic amplified by the second harmonic amplifying circuit 8, and converts it into a DC electric signal. The measurement magnetic field is a DC voltage. Converted to a signal.

  Then, a signal obtained by converting the measurement magnetic field from the phase detector 9 into a DC electric signal is output to the vector signal analyzer 11 via a low pass filter (LPF) 13. The output signal from the phase detector 9 is output to the feedback circuit 10.

  The feedback circuit 10 converts a DC voltage signal reflecting the measurement magnetic field into a current and supplies 6 to the detection coil. The detection coil 6 provides the core 3 with a feedback magnetic field in a direction to cancel the measurement magnetic field by the current from the feedback circuit 10. That is, the detection coil 6, the second harmonic amplifying circuit 8, the phase detector 9, and the feedback circuit 10 form a feedback loop in which the measurement magnetic field is converted into an electric signal and fed back. Since this feedback loop works to amplify and cancel the difference between the measurement magnetic field and the feedback magnetic field, the feedback magnetic field is equal to the measurement magnetic field, and the current that creates it is proportional to the measurement magnetic field. Since the DC voltage output from the second harmonic amplifying circuit 8 that supplies this current corresponds to the measurement magnetic field, the magnetic field is measured by taking it out as the output of the magnetometer.

  Regarding the configuration of the excitation circuit 7, the configuration of the present embodiment will be described in comparison with the conventional circuit configuration.

  The conventional excitation circuit shown in FIG. 1B amplifies the excitation frequency reference signal by the power amplifier 21 and removes the DC component included in the output component of the power amplifier 21 by the DC component removal capacitor 22. Then, the exciting current is maintained in the output of the power amplifier 21 by the current limiting resistor 23 and is output to the exciting coil 4 as an exciting signal.

  On the other hand, the excitation circuit 7 of the present embodiment shown in FIG. 1A is provided with a current fluctuation control circuit 5 after the current limiting resistor 23 in the conventional excitation circuit shown in FIG. The excitation coil 4 is excited by the output from the fluctuation control circuit 5.

  The current fluctuation control circuit 5 has a function of slowing the increase of the excitation current in the saturation region shown in FIG. 7 and delaying the increase of the current until each magnetic domain has been saturated. is doing.

  FIG. 3 shows a waveform diagram of the excitation current output from the excitation circuit 7. In the waveform diagram of FIG. 3, the horizontal axis represents time, and the vertical axis represents the excitation current flowing through the excitation coil. In the region A in the figure, the magnetic flux density in the core increases as the excitation current increases and enters the saturation region. As the permeability decreases, the inductance decreases and the impedance decreases, so that the excitation current increases.

On the other hand, as described above, due to the non-uniformity of the magnetic domain size of the magnetic detection material constituting the core, the reversal of the magnetic axis is delayed for the large magnetic domain, the reversal of the magnetic axis is fast for the small magnetic domain, and the permeability is saturated in the saturation region. Unevenness occurs in the change of the.
FIG. 3B is a waveform diagram of the excitation current by the excitation circuit of the conventional configuration shown in FIG. 2B, and the curve from reaching the saturation region to reaching a certain value rises with a steep slope (angle θ1). .

  As described above, in the configuration of the conventional excitation circuit, since the excitation current rapidly rises in the saturation region, the magnetic flux density is proportional to the product of the excitation current and the permeability, so that a large unevenness occurs in the change of the magnetic flux density. It is estimated that the fluctuation of the magnetic flux density is made redundant and the output noise is deteriorated.

  On the other hand, FIG. 3A is a waveform diagram of the excitation current by the excitation circuit of the present embodiment shown in FIG. 2A, and the curve from reaching the saturation region to reaching a constant value is gentle (angle θ2). , Θ1> θ2), and is delayed as compared with the excitation in the excitation circuit of the conventional configuration. As a result, the increase in current is delayed until each magnetic domain is saturated (increasing fluctuation in excitation current). Suppression). For this reason, fluctuation (unevenness) in the change in magnetic flux density can be suppressed, the output noise of the fluxgate magnetometer can be reduced, and the measurement limit can be improved.

  4A and 4B are diagrams showing output noise. FIG. 4A shows frequency versus output noise density when the excitation circuit according to this embodiment shown in FIG. 1A is used, and FIG. 4B shows FIG. The relationship between the frequency and the output noise density in the case of using the conventional excitation circuit shown is shown. This output noise density was measured by a vector signal analyzer (Agilent Technology 89410A, manufactured by Agilent Technologies) shown in FIG. The magnetometer sensitivity was set to 500 pTmV, and the frequency response of the magnetometer output was set to 10 Hz.

  When the excitation circuit having the conventional configuration shown in FIG. 4B is used, the output noise density (pTrms / rtHz) is 20 at a frequency of 1 Hz. However, when the excitation circuit of the present embodiment is used. The output noise density (pTrms / rtHz) at a frequency of 1 Hz was dramatically reduced to 0.35.

  As a result, the measurement accuracy has been greatly improved, and the magnetic force measurement in a range that could not be measured conventionally has become possible.

The block diagram of the excitation circuit which comprises the measuring apparatus of a fluxgate magnetometer is shown, (a) shows the circuit structure of embodiment of this invention, (b) shows the conventional circuit structure. The circuit block diagram of the measuring device of a fluxgate magnetometer and a noise measurement system. (A) is a waveform diagram of an excitation current using the excitation circuit of FIG. 1 (a), and (b) is a waveform diagram of an excitation current using the excitation circuit of the conventional configuration of FIG. 1 (b). (A) shows the measurement result of the frequency vs. output noise density by the measuring device using the excitation circuit of FIG. 1 (a), and (b) is by the measuring device using the conventional excitation circuit of FIG. 1 (b). The measurement result of frequency versus output noise density is shown. The exploded perspective view of a magnetic sensor. The excitation principle of a fluxgate magnetometer is shown, (a) is a hysteresis curve, (b) is a waveform diagram of an excitation signal, (c) is a waveform diagram of a detection signal, and (d) is a pulse waveform diagram of an output signal. The figure which shows the actual hysteresis curve of the magnetic sensor in a flux magnetometer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Magnetic sensor 2 Sensor housing 3 Ring core 4 Excitation coil 5 Current fluctuation control circuit 6 Detection coil (pickup coil)
7 Excitation circuit 8 Second harmonic amplifying circuit 9 Phase detection circuit 10 Feedback circuit 11 Vector signal analyzer 12 Amplifier 13 LPF
14 Excitation / phase signal generation circuit 15 EMI filter 21 Power amplifier 22 DC component removal capacitor 23 Current limiting resistor

Claims (3)

An excitation circuit that applies an alternating magnetic field to the magnetic detection material of the magnetic sensor by energizing an excitation current to the excitation coil of the magnetic sensor, and by an asymmetric component of the magnetic flux that reflects the measurement magnetic field detected by the detection coil of the magnetic sensor In a fluxgate magnetometer measuring device that measures the magnetic field based on the induced harmonics,
The flux gate magnetometer measuring apparatus, wherein the excitation circuit includes a current fluctuation suppressing circuit that gently increases and changes the excitation current in a magnetic saturation region of the magnetic detection material. An excitation circuit that applies an alternating magnetic field to the magnetic detection material of the magnetic sensor by energizing an excitation current to the excitation coil of the magnetic sensor, and by an asymmetric component of the magnetic flux that reflects the measurement magnetic field detected by the detection coil of the magnetic sensor In a fluxgate magnetometer measuring device that measures the magnetic field based on the induced harmonics,
The excitation circuit maintains and outputs the excitation current within a predetermined current value, and delays the time until the excitation current that increases and varies in the magnetic saturation region of the magnetic detection material reaches the predetermined current value. An apparatus for measuring a fluxgate magnetometer, characterized by comprising a current fluctuation suppressing circuit for causing the current to flow. The current fluctuation suppression circuit, fluxgate magnetometer measuring apparatus according to claim 1 or 2, characterized in that an inductor.

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