JP2001083223A - Magnetometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high sensitivity magnetometer through a simple arrangement by detecting the phase difference between the oscillation waveform of an oscillator for exciting a resonance circuit and an waveform resonant to the resonance circuit. SOLUTION: The magnetometer has a resonance circuit 1 comprising a coil wound around a ferromagnetic, and a capacitor connected in series therewith. An AC high frequency power supply 3 (oscillator) having a frequency close to a resonance frequency for exciting the resonance circuit 1, a phase difference detection circuit 4 and a bias current source 2 for applying a bias field to the ferromagnetic are also provided. The AC high frequency power supply 3 excites the resonance circuit 1 and the phase difference detection circuit 4 detects the phase difference between the oscillation voltage waveform and the AC current waveform of the high frequency power supply 3 thus detecting variation of external field sensitively as the variation of inductance of the coil. Noise due to amplitude variation of the high frequency power supply 3 does not exist.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetometer for measuring a magnetic field existing in a space or a current flowing through a wire.

[0002]

2. Description of the Related Art A fluxgate magnetometer has been known as a magnetic sensor circuit for detecting a magnetic field. In this magnetometer, a large-amplitude AC frequency current in a region where the magnetic permeability of a ferromagnetic material having no DC component becomes non-linear flows through a coil wound around the ferromagnetic material, thereby exciting the ferromagnetic material. , The magnitude of a double frequency component caused by the interference between the external magnetic field and this frequency is measured.

Another method is to place an operating point in a non-linear region of a magnetic material by flowing an AC frequency current having a relatively small amplitude and a DC current superimposed on the AC frequency current through a coil wound around a ferromagnetic material. There is a method of measuring an AC voltage induced in a coil. This method is based on detecting a change in the magnetic permeability of a ferromagnetic material by an external magnetic field as a change in the inductance of a coil.

In a fluxgate magnetometer, the amplitude of the exciting AC frequency current needs to be large enough to bring the ferromagnetic material close to saturation, so that the loss is large and the noise becomes large. If the AC frequency is higher than about 30 kHz, the degree of non-linearity is reduced, and the sensitivity is lowered. There is also a problem such as heat generation due to loss of the ferromagnetic material. Therefore, the measurable magnetic field frequency is usually limited to about 1 kHz, and the measurement sensitivity cannot be increased due to noise.

In the method of detecting as a change in the inductance of the coil, when the amplitude of the AC frequency current changes, the voltage also changes in accordance with the change. Requires frequency current. It is technically very difficult to realize an AC power supply with high frequency and stable amplitude, and it is also technically difficult to detect an AC voltage with high accuracy, which makes it difficult to increase the measurement sensitivity. . Also, the circuit becomes complicated.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a highly sensitive magnetometer with a simple configuration.

[0007]

According to the present invention, there is provided a magnetometer comprising: a coil wound around a ferromagnetic material; a resonance circuit comprising a capacitor connected in parallel or series to the coil;
An oscillator having a frequency close to the resonance frequency for exciting the resonance circuit, a circuit for detecting a phase difference between an oscillation waveform of the oscillator and a waveform resonating with the resonance circuit, and a device for applying a bias magnetic field to the ferromagnetic material are provided.

[0008]

In the present invention, for example, the resonance frequency determined when the inductance of the coil constituting the resonance circuit 1 is L ₀ and the capacitance of the capacitor is C in FIG.

(Equation 1) When the resonance circuit 1 is excited by the high-frequency AC current source 3 having the following equation, the phase difference δ between the resonance voltage and the high-frequency AC current source 3 is obtained when the Q value of the resonance circuit is Q.

(Equation 2) Becomes Where ΔL is the magnitude of the deviation from the L ₀ of the inductance of the coil. Since Q can easily have a value of several tens or more if the loss of the resonance circuit is reduced, a change in the external magnetic field can be detected sharply as a change in the inductance of the coil by detecting the phase difference.

Further, since the phase difference does not change with the amplitude of the high-frequency AC current source 3, there is no noise due to the amplitude fluctuation. Furthermore, the factors of the phase difference variation not depending on the external magnetic field are the change in the capacitance of the capacitor and the change in the frequency.
It is technically easy to reduce any fluctuations, such as the use of a quartz oscillator, and it is inexpensive, so that a low-cost, high-sensitivity magnetometer can be realized.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Embodiment 1) FIG.
A 0.1 mm diameter enameled wire was wound 200 times around a cobalt-based amorphous magnetic foil having a thickness of × 1 mm and a thickness of 0.025 mm to form a coil.

When the capacitor Cr was set to 200 pF and a DC bias current of 15 mA was applied to the coil, the resonance frequency formed by the coil and the capacitor was 500 kHz.

The oscillation frequency of the rectangular wave oscillator is 500 kHz.
z, a rectangular wave is converted into a triangular wave by an integrating circuit composed of an operational amplifier Q2, a capacitor Ci, and a resistor Ri, and this is given to a high-frequency AC current source having a DC bias composed of Q1 and Rb. The ferromagnetic material was excited and a bias magnetic field was applied. This resonance voltage is applied to the capacitor C
c, amplification by the resistor Rp and the inverter Q3, and conversion into a rectangular wave by a rectangular wave conversion circuit. This output and the output of the square wave oscillator are input to an exclusive OR logic circuit Q4, and this output is integrated by a resistor Rf and a capacitor Cf to form a phase difference detection circuit 4, which is used as an output.

When an external magnetic field was applied to the ferromagnetic material to examine the fluctuation of the output, a change of 1 V was obtained for the external magnetic field change of 1 AT / m. The internal noise at this time was 10 μV. Therefore, a magnetic field sensitivity of 10 μAT / m was obtained.

A coil different from the coil of the resonance circuit was wound around a ferromagnetic material, and a current was applied to the coil to apply a bias magnetic field.
AT / m was obtained.

When a bias magnetic field was applied by disposing a permanent magnet near the magnetic material, the same magnetic field sensitivity of 10 μAT / m as described above was obtained.

(Embodiment 2) As shown in FIG. 3, the same resonance circuit 1, rectangular wave oscillator, and phase difference detection circuit 4 as in Embodiment 1 are used, and the output of the phase difference detection circuit 4 is output by an operational amplifier Q5.
After amplification by an amplifier composed of a resistor Rf and a resistor Ra,
A feedback circuit is configured by adding the DC bias of the high-frequency AC current source by the resistor Rdc.

Thus, by the action of the feedback circuit, the bias magnetic field is controlled in accordance with the external magnetic field so that the output waveform of the rectangular wave oscillator and the resonance voltage waveform have a substantially constant phase difference.

In this case, since the bias magnetic field and the external magnetic field change so as to cancel each other, the external magnetic field could be known by measuring the bias magnetic field. Also, in this method, a constant phase difference could be maintained by changing the bias magnetic field in response to a large change in the external magnetic field. From this, it was found that a wider range of magnetic field measurement was possible.

Note that, even if a coil different from the coil of the resonance circuit is wound around a ferromagnetic material and a current obtained by amplifying the output of the phase difference detection circuit 4 is applied to the coil to apply a bias magnetic field, The same result as was obtained.

(Embodiment 3) As shown in FIG. 4, the same resonance circuit 1 and phase difference detection circuit 4 as in Embodiment 1 are provided, but the output of the phase difference detection circuit 4 is a voltage-controlled rectangular wave oscillator V
CO, the frequency is changed according to this output,
The output waveform of the rectangular wave oscillator VCO and the resonance voltage waveform were controlled so as to have a substantially constant phase difference.

[0002] The external magnetic field is a voltage controlled rectangular wave oscillator VCO.
Output frequency.

(Embodiment 4) As shown in FIG. 5, the ferromagnetic material of Embodiment 2 was formed into a loop shape. Thereby, the current of the current line 5 passing through this loop could be measured.

(Embodiment 5) In the same configuration as that of Embodiment 4, as shown in FIG. 6, a ferromagnetic loop and a conductive film made of a metal foil having a thickness of 0.1 mm outside a coil wound therearound. A sex shield was provided. As a result, the high frequency voltage from the resonance circuit was not induced in the current line, and the influence of the high frequency voltage on the equipment connected to the current line could be eliminated.

(Embodiment 6) As shown in FIG. 7, a conductive shield made of a metal foil having a thickness of 0.1 mm is provided outside a coil wound around a linear ferromagnetic material for measuring an external magnetic field. Provided. This prevents the magnetic field due to the high-frequency current coupled to the resonance circuit from leaking to the outside, so that even if there is a metal object placed close to this, the magnetic field measurement will not be affected by the induced current to this. I could give it.

[00025]

As described above in detail, according to the present invention, a highly sensitive magnetometer which does not require a high-frequency oscillator having a stable amplitude is provided by a simple structure. further,
The ability to excite at high frequencies provides a wide band magnetometer.

[00026]

[Brief description of the drawings]

FIG. 1 is a block diagram for explaining a basic configuration of the present invention.

FIG. 2 is a circuit diagram for explaining a configuration of a magnetometer according to Embodiment 1 of the present invention.

FIG. 3 is a circuit diagram illustrating a configuration of a magnetometer according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram illustrating a configuration of a magnetometer according to a third embodiment of the present invention.

FIG. 5 is a circuit diagram illustrating a configuration of a magnetometer for measuring a current according to a fourth embodiment of the present invention.

FIG. 6 is a diagram for explaining a configuration of a shield outside a coil according to a fifth embodiment of the present invention.

FIG. 7 is a diagram for explaining a configuration of a shield outside a coil according to a sixth embodiment of the present invention.

[Description of Signs] 1 Resonant circuit 2 Bias current source 3 High frequency AC current source 4 Phase difference detection circuit

Claims (6)

[Claims] 1. A resonance circuit comprising a coil wound around a ferromagnetic material, a capacitor connected in parallel or series to the coil, and the resonance outputting a frequency capable of causing resonance in the resonance circuit. An oscillator for exciting a circuit; a circuit for detecting a phase difference between an oscillation waveform of the oscillator and a waveform resonating with the resonance circuit; and a device for applying a bias magnetic field to the ferromagnetic material. Magnetometer. 2. A device for applying a bias magnetic field to a ferromagnetic material according to claim 1, wherein the bias magnetic field can be changed by an external input, and the external input of the device is provided by an output of a circuit for detecting a phase difference. Magnetometer. 3. The magnetometer according to claim 1, wherein the oscillator changes the frequency by an external input, and the external input of the oscillator is provided by an output of a circuit for detecting a phase difference. 4. A magnetometer according to claim 1, wherein the device for applying a bias magnetic field is a device at least partially constituted by a permanent magnet. 5. A magnetometer for detecting a magnetic force generated by a current line passing through the loop, wherein the ferromagnetic material according to claim 1 has a loop shape. 6. A magnetometer according to claim 1, wherein the outer periphery of the coil wound around the ferromagnetic material is covered with a conductive material.