JP6823878B2 - Fluxgate magnetic field sensor - Google Patents

Description

The present invention relates to a fluxgate magnetic field sensor, and more particularly to a fluxgate magnetic field sensor having extremely good temperature characteristics by amplifying a pickup voltage with an independent amplification factor, and an adjusting method for the same.

The fundamental wave type orthogonal fluxgate sensor, which is a kind of magnetic field sensor, is a small fluxgate sensor in which the exciting magnetic field of the magnetic core and the magnetic field to be measured are orthogonal to each other. By superimposing a DC bias current on an alternating current as an exciting current that generates an exciting magnetic field, noise reduction, sensitivity improvement, magnetic field detection with a fundamental wave signal, etc. are realized (Patent Document 1). .. In the fundamental wave type orthogonal fluxgate, a large offset may occur in the output due to the magnetic anisotropy of the magnetic core, and a method called bias switching for suppressing this has been proposed (Patent Document 2). In the bias switching, the polarity of the DC bias current in the above-mentioned exciting current (and in some cases, the polarity of the DC bias current and the phase of the alternating current at the same time) is switched to an alternating current at a predetermined cycle, and then each polarity of the exciting current. Offset by subtracting the signal detected below (when only the polarity of the DC bias current is switched) or adding / averaging (when switching the phase of the AC current in addition to the polarity of the DC bias current). And the drift is offset and stabilized.

In a fundamental wave orthogonal fluxgate sensor, if there is a temperature change around the sensor unit, it causes mechanical deformation of the sensor and also changes the anisotropy in the magnetic core. Then, these effects appear in the detection signal, so that the offset drifts depending on the temperature. As described above, the existing bias switching cancels the offset component by adding or subtracting the detection signal obtained by reversing the polarity of the exciting current. In the fundamental wave type orthogonal fluxgate sensor, a sensor unit having a structure in which a pickup coil is wound around an amorphous magnetic wire is typically used as the magnetic core, and a pickup signal in which the amount of the external magnetic field is modulated is acquired. Such a sensor unit is directly affected by the anisotropy of the wire physical properties and has a large offset level that appears at the output, and its temperature stability is extremely poor (about several tens of nT / ° C.), but the polarity of the exciting current is changed. By periodically inverting and averaging the detection waveforms, such fluctuations in the offset level can be canceled, and temperature stability of about 0.1 nT / ° C to 1 nT / ° C can be obtained. However, due to the physical characteristics of the magnetic core and the influence of the circuit that realizes the polarity reversal of the exciting current, the offset component of the pickup signal obtained according to the polarity of the exciting current is not actually truly equivalent. , There is an imbalance. Therefore, there is a signal component derived from the offset that remains without being offset by addition and subtraction, and as a result, in the prior art, there are limits and variations in suppressing the temperature drift of the offset that appears in the output. That is, in the bias switching of the prior art, it is difficult to improve the temperature coefficient, which is the fluctuation ratio of the output with respect to the temperature change of the sensor unit. However, the smaller the output fluctuation due to such temperature, the more desirable it is. In particular, further improvement is required for use in applications such as spacecraft that are exposed to drastic temperature changes. As a solution to this problem, a method has been proposed in which a pickup signal is directly taken into a computer as a digital value and an imbalance is adjusted by post-processing (Non-Patent Document 1). However, as will be described later, this method also has a problem.

Patent No. 4565072 Patent No. 4209114

M. Butta, I. Coroli, "Low offset drift-low-noise orthogonal fluxgate with synchronized polarity flipping", Article. Sequence No. (article number) 4001406, IEEE Transactions on Magnetics, voI.53, no.4, IEEE ( Institute of Electrical and Electronics Engineers), December 9, 2016

As described above, when there is a temperature change around the sensor unit, the offset drifts depending on the temperature. On the other hand, even if the offset components are canceled by adding or subtracting the pickup signal obtained by inverting the polarity of the exciting current by bias switching, the addition or subtraction is due to the imbalance existing between them. The offset component cannot be completely offset, and there is a residual offset-derived signal component. As described above, there is a limit to the suppression of offset temperature drift.

In the conventional bias switching, two types of pickup signals obtained by inverting the polarity of the exciting current are output through a single detection circuit that detects the optimum sensor noise and sensitivity. The temperature coefficient obtained in this state has not been improved for a long time as the physical characteristics of the magnetic core and the limit on the circuit. This is a particular problem in applications such as spacecraft where the temperature changes drastically, and although it hinders the practical use of this sensor, no appropriate improvement measures have been found. In addition, as a technique for this problem, a method of directly capturing the above-mentioned pickup signal as a digital value into a computer has been proposed, but this method not only loses the real-time detectability of the sensor, but also has an open-loop configuration without feedback. Since it can be realized only by itself, the input / output linearity of the sensor deteriorates largely depending on the characteristics of the magnetic material used as the core, and its practicality is extremely poor.

The above problem is solved by the present invention having the following features. That is, the present invention comprises an elongated magnetic material to which a detected magnetic field is applied, in which an exciting current in which a DC bias current whose polarity is periodically switched between positive and negative is superimposed on an AC current is passed. The pickup signal from the pickup coil wound around the magnetic core is amplified by an amplifier, and the detected pickup signal is output by detecting the amplified pickup signal, and the signal that does not correspond to the magnetic field detected from the detected signal is removed. In the flux gate magnetic field sensor that generates the sensor output, the amplifier is characterized in that the pickup signals corresponding to the positive and negative characteristics of the DC bias current are amplified by independent amplification factors. In the present invention, the alternating current may be configured so that the polarity is periodically reversed in the same period as the direct current bias current. In the present invention, the amplifier can be configured such that each amplification factor is independently adjusted so that the temperature coefficient of the sensor output is substantially minimized. In the present invention, when the amplifier changes the temperature around the sensor unit, the absolute values of the offset components contained in the respective detection signals or pickup signals when the DC bias currents are positive and negative remain equal to each other. Each amplification factor can be adjusted independently so as to be. Further, the present invention can also be configured as a closed loop configuration in which the output circuit inputs a footback signal obtained by integrating the magnetic field detection signal to the pickup coil with a polarity that cancels the magnetic field detection signal, and generates a sensor output from the feedback signal.

The present invention provides a mechanism in the bias switching circuit that allows the relative sensitivity relationship between the pickup signals according to each polarity of the DC bias component of the exciting current to be adjusted, thereby providing the DC bias component of the exciting current. It is possible to adjust the ratio of the offset component in the pickup signal corresponding to each polarity on the circuit. This makes it possible to minimize the offset imbalance that has conventionally remained at the time of addition / subtraction of the pickup signal and reduce the temperature coefficient in real time regardless of the open loop configuration or the feedback configuration. Further, when synchronous detection is used for detection, the imbalance can be indirectly changed by adjusting the phase relationship between the reference signal and the pickup signal in the synchronous detection with a phase controller. However, the present invention does not use a method for adjusting such a phase relationship, but directly adjusts the relative sensitivity relationship of the pickup signal according to the polarity of the DC bias component of the exciting current to perform a single phase shift. Even if the instrument is not suitable for adjustment for some reason (such as when the noise characteristics deteriorate significantly when the phase is changed) or when a single phase shifter cannot sufficiently improve the temperature characteristics, the temperature characteristics It has the effect of being able to improve. According to the configuration of the present invention, the temperature coefficient of the fundamental wave type orthogonal fluxgate sensor can be reduced to 1/4 or less as compared with the prior art in real time.

It is a schematic circuit diagram of the conventional fluxgate magnetic field sensor. It is a schematic circuit diagram of the fluxgate magnetic field sensor of this invention. It is a concrete circuit diagram of an example of an amplifier in which an independent amplification factor can be set for each polarity. It is a figure which shows the schematic structure of the fluxgate magnetic field sensor. It is explanatory drawing of the magnetization of the magnetic core of the fluxgate magnetic field sensor in the positive electrode property. It is explanatory drawing of the magnetization of the magnetic core of the fluxgate magnetic field sensor in the negative electrode property. It is a figure explaining the synchronous detection in an ideal state. It is a figure explaining the synchronous detection when the temperature changes in an ideal state. It is a figure explaining the synchronous detection when there is an imbalance in a positive / negative offset. It is a figure explaining the synchronous detection at the time of setting the amplification factor for each polarity so that the positive / negative offset variation becomes the minimum when there is an imbalance in a positive / negative offset. It is a figure explaining the synchronous detection when the temperature changes when the amplification factor for each polarity is set so that the positive / negative offset variation becomes the minimum when there is an imbalance in a positive / negative offset. It is a graph of the measured value of the temperature drift and the temperature coefficient of the fluxgate magnetic field sensor which concerns on the prior art and the present invention. It is a graph of the measured value of the temperature coefficient with respect to the parameter of the amplification ratio according to the polarity of the DC exciting current.

(Structure of conventional fluxgate magnetic field sensor)
In the present invention, as in the case of the conventional magnetic field sensor, the sensitivity and noise of the sensor are adjusted to be good in the detection, while the amplitude of the pickup signal according to the polarity of the DC bias component of the exciting current is independent. An amplifier that amplifies at the increased amplification factor is provided in the circuit. The present invention will be described below with reference to the drawings. First, the structure of the conventional fluxgate magnetic field sensor 100, which is the premise of the present invention, will be described. FIG. 1 is a schematic circuit diagram of a conventional fluxgate magnetic field sensor 100. In this circuit, synchronous detection is used as a typical detection method. In addition, a closed loop configuration is adopted in which the sensor is operated in a region where the characteristics of the magnetic material are good by passing a current that cancels the detected magnetic field through the pickup coil.

The fluxgate magnetic field sensor 100 includes a DC superimposition AC exciter 101, a phase shifter 102, an exciting magnetic pole switching unit 103, a sensor unit 104, a voltage follower 105, an amplifier 106, a synchronous detector 107, a low-pass filter 108, an integrator 109, and feedback. It is composed of a resistor 110 and an output terminal 111. In this example, the phase shifter 102 is arranged immediately after the AC source in the DC superimposition AC excitation unit 101. The alternating current source generates an alternating current having a frequency of fHz, but usually also outputs a reference signal for synchronous detection in a predetermined phase relationship with the alternating current. Since the reference signal is often a square wave, the symbol of the AC source is described in FIG. 1 as representing both a sine wave and a square wave.

The DC superimposed AC excitation unit 101 is a circuit that provides an exciting current in which a DC bias current is superimposed on an AC current. The DC superimposition AC excitation unit 101 and the magnetic field strength switching unit 103 in the subsequent stage form a bias switching type excitation circuit.

The phase shifter 102 is a circuit that adjusts the phase of the AC component of the exciting current of the DC superimposed AC exciting unit 101. The phase shifter 102 adjusts the phase of the signal by shifting the phase by a certain adjustment amount (phase shift amount). In the example of FIG. 1, the phase shifter 102 is provided in the DC superimposing AC exciting unit 101, and adjusts the phase of only the AC component of the exciting current. In the prior art, the phase of the exciting current (and the phase relationship with the exciting current in synchronous detection) is adjusted to maximize the sensitivity of the fluxgate magnetic field sensor 100 (minimum noise in a closed-loop configuration). .. The pickup signal, which is the output from the sensor unit 104, is synchronously detected by the synchronous detector 107 according to the phase relationship with the reference signal referred to by the synchronous detector 107, and the phase shifter 102 adjusts the phase relationship. The phase relationship is adjusted so that the maximum detection sensitivity can be obtained with respect to the output from the sensor unit 104. The phase shifter 102 does not dynamically adjust the phase, and once the optimum phase shift amount is set as the phase shift amount, it holds it and shifts the phase by the phase shift amount. The phase shifter 102 is typically a delay circuit composed of operational amplifiers.

The magnetic field switching unit 103 is a circuit that periodically switches the polarity of the exciting current flowing through the sensor unit 104. As a result, the polarity of the DC bias current component is periodically switched, and the exciting current by the bias switching method is supplied to the magnetic core of the sensor unit 104. The exciting pole switching unit 103 has a clock having a frequency of f _bs Hz that determines the polarity switching cycle, whereby a switch for switching the polarity is driven, and the polarity of the exciting current is periodically switched. A clock having a frequency of f _bs is typically used by _dividing the frequency of the AC component of the exciting current by an integral fraction. As shown in FIG. 1, when the exciting magnetic pole switching unit 103 switches the output from the DC superimposed AC exciting unit 101, the phase of the AC current is also inverted in synchronization with the switching of the polarity of the DC bias current, and the excitation is performed. For each polarity of the current (DC bias current), the offset component due to magnetic anisotropy appears in the opposite polarity, and the sensitivity component of the magnetic field appears in the same polarity.

The exciting current is a DC bias current superimposed on an alternating current. When referring to the DC bias current component, other expressions such as DC bias current, DC component of exciting current, and DC exciting current may be used depending on the context, and when referring to AC current component, Other expressions such as alternating current and alternating current components of the exciting current may be used, depending on the context.

The sensor unit 104 is a sensor element that detects an external magnetic field to be measured, and has a magnetic core made of an elongated magnetic material to which the detected magnetic field is applied, and a pickup coil wound around the magnetic core.

The voltage follower 105 is a component for accurately transmitting the pickup signal from the sensor unit 104 to the subsequent stage by converting the impedance. The amplifier 106 is a component that amplifies the output from the voltage follower 105 to an appropriate level. In FIG. 1, the output from the voltage follower 105 passes through a capacitor to take out only the AC component, and is sent to the amplifier 106 in the subsequent stage. The voltage follower 105, the amplifier 106, and the capacitor between them are additional configurations for accurately transmitting the pickup signal at an appropriate level, and are not necessarily required in the present invention.

The synchronous detector 107 is a component of a circuit that performs synchronous detection on the pickup signal from the sensor unit 104 output from the amplifier 106. The synchronous detector 107 determines the sensor output by synchronously detecting the pickup signal, which is the induced voltage induced in the pickup coil of the sensor unit 104, with reference to the reference signal whose frequency is synchronized with the AC current in the exciting current. It is a detection signal that will be different. The synchronous detector 107 detects the pickup signal from the sensor unit 104 according to the phase relationship with the reference signal having the same frequency as the AC component of the exciting current of the DC superimposed AC excitation unit 101. ..

Here, synchronous detection can be performed by multiplying the detected signal by the sine wave when the signal is conveyed by a sine wave. Since the external magnetic field is modulated by the AC component of the exciting current, synchronous detection can be performed by multiplying the pickup signal by the sine wave, which is the AC component of the exciting current, but normally, synchronous detection is performed with an analog switch or the like. A square wave is used for multiplication instead of a sine wave, which makes the circuit easy to configure. That is, since the square wave has an amplitude of 1, -1, the multiplication result is that the pickup signal is passed through with the same polarity when the amplitude of the square wave is 1, and the pickup signal is passed when the amplitude of the square wave is -1. Synchronous detection can be performed by reversing the polarity and passing through. Here, in order to perform gate control (control of whether or not the polarity of the pick-up signal to be passed is inverted) by the sign of the reference signal, a square wave whose frequency and phase are synchronized with the AC component of the exciting current is used as the reference signal. I'm using it.

A signal having the same frequency as the AC component of the exciting current is input to the synchronous detector 107 as a reference signal, whereby the phase relationship with the pickup signal to be detected is determined. In FIG. 1, the transmission path of the reference signal is shown by a broken line from the AC source having a frequency of fHz in the DC superimposed AC excitation unit 101 toward the synchronous detector 107. As a result, for example, the pickup signal is passed through with the same polarity for half a cycle of the reference signal (for example, when the amplitude is 1), and the polarity of the pickup signal is changed for the other half cycle of the reference signal (for example, when the amplitude is -1). By inverting, detection is performed in synchronization with the AC component generated by the DC superimposition AC excitation unit 101.

The low-pass filter 108 is a component of a circuit that averages the detection signal that is the output from the synchronous detector 107. As a result, the magnetic field detection signal indicating the magnitude of the magnetic field to be detected is extracted from the detection signal. In this example, the phase of the AC current is also inverted in synchronization with the switching of the polarity of the DC bias current, and in the pickup signal, the offset component due to magnetic anisotropy for each polarity of the DC bias current is Since it appears in the opposite polarity and the sensitivity components of the magnetic field appear in the same polarity, a magnetic field detection signal that removes the signal that does not correspond to the magnetic field detected from the detection signal can be obtained by adding (realizing by averaging) the detection signals. ..

The integrator 109 is a component of a circuit that integrates the magnetic field detection signal output from the low-pass filter 108 when the system is operated in a closed loop configuration. The output of the integrator 109 applies negative feedback by a magnetic field to the magnetic core of the sensor unit 104 through the pickup coil of the sensor unit 104 via the feedback resistor 110 described later. As a result, the magnetic field detection signal that is the input of the integrator 109 represents the deviation of this feedback, and the integrator 109 controls the magnetic field detection signal to approach zero.

The feedback resistance 110 is a resistance through which the feedback signal passes when the feedback signal output from the integrator 109 is negatively fed back and input to the pickup coil of the sensor unit 104. Due to the negative feedback, a magnetic field that cancels the external magnetic field to be measured is generated in the pickup coil of the sensor unit 104. The current flowing by the feedback signal corresponds to the external magnetic field to be measured, and the current value detected as a voltage by the feedback resistor 110 is the sensor output indicating the magnitude of the external magnetic field to be measured.

The output terminal 111 is a node that outputs a sensor output obtained by converting the current flowing through the feedback resistor 110 into a voltage representing the magnitude of an external magnetic field.

(Operation of conventional fluxgate magnetic field sensor)
Next, the operation of the conventional fluxgate magnetic field sensor 100, which is the premise of the present invention, will be described. FIG. 4 is a diagram showing a schematic structure of a fluxgate magnetic field sensor. FIG. 4A shows a schematic view of the sensor unit 104. An amorphous magnetic wire extends in the vertical direction, and a pickup coil is wound around it. The circumferential direction of the amorphous magnetic wire is the x direction, and the longitudinal direction is the z direction. The detected external magnetic field flows into the amorphous magnetic wire in the z direction. When the exciting current flows from the bottom to the top in the figure, the magnetic field due to the exciting current is in the x direction and is orthogonal to the direction of the detected external magnetic field. FIG. 4B shows a waveform in which a DC bias current is superimposed on an AC current output from the DC superimposed AC excitation unit 101. Since the excitation pole switching has not been performed yet, it is in a one-biased state. The in amorphous magnetic wire has uniaxial anisotropy K _u, the excitation magnetic field, anisotropy, magnetization J _s at the position where the energy of the external magnetic field is minimized so that the vibrating. The z-axis projection of this vibration induces a voltage in the pickup coil, which can be taken out as a pickup signal. The influence of an external magnetic field is modulated on this voltage, and the output can be taken out by synchronous detection. Even when there is no external magnetic field, an offset component is generated due to the influence of anisotropy, but if the polarity of the exciting pole is reversed by reversing the polarity of the DC bias current, the magnitude of the offset component due to uniaxial symmetry. The polarity of the offset will also be reversed while maintaining the above. By utilizing this property and periodically inverting the polarity and adding (averaging) the output, it is possible to suppress the offset component that appears in the output. In FIG. 4 (c), the exciting current output from the exciting magnetic field switching unit 103 for such inversion, whose polarity is periodically switched, and the DC bias current superimposed on the AC current. The waveform is shown.

Next, the waveform of the signal picked up from the pickup coil by the sensor unit 104 (waveform of the pickup signal) and the vector diagram of the magnetization will be shown to explain the waveform. In the following, the term "positive electrode property" refers to the case where the DC bias current is positive electrode property, and the term "negative electrode property" refers to the case where the DC bias current is negative electrode property. FIG. 5 is an explanatory diagram of the magnetization of the sensor portion of the fluxgate magnetic field sensor in the positive electrode property. FIG. 5A shows the waveform of the pickup signal. When the z-axis projection of the magnetization by the exciting current is J _s · sin (θ (t)), its time derivative is induced in the pickup coil. Waveform of the induced pickup signal includes an offset component caused by uniaxial anisotropy K _u of the amorphous magnetic wire. The (b) of FIG. 5, the external magnetic field H _ex, the amorphous magnetic wire uniaxial anisotropy K _u, and shows the direction of magnetization J _s of the amorphous magnetic wire to the exciting magnetic field H (t) is shown There is. The external magnetic field H _ex can be regarded as zero because of the closed loop control. The x-axis, the excitation magnetic field _{H (t) = H ac ·} sin (2πft) + H dc (H ac AC excitation field, H _dc is the DC bias magnetic field) is applied, also different with even z-axis component Directional K _u exists. The external magnetic field H _ex , the anisotropic K _u , and the exciting magnetic field H (t) generate the magnetization J _s of the amorphous magnetic wire. Assuming that the angle of the magnetization J _{s from} the circumferential direction is the angle θ ₀ , the exciting magnetic field H (t) oscillates due to the AC component, so that the angle θ ₀ oscillates at the same frequency. The exciting magnetic field H (t) is shown in FIG. 5 (c). When the exciting magnetic field H (t) has a positive peak, the angle θ ₀ becomes smaller due to the influence of it, and when the exciting magnetic field H (t) has a negative peak, the influence of it becomes smaller, so that the angle θ ₀ becomes smaller. Becomes larger. That is, when the exciting magnetic field H (t) has a positive peak at (1), the magnetization J _s is most attracted in the direction of the exciting magnetic field H (t), and when the exciting magnetic field H (t) has a negative peak at (2). Sometimes the magnetization J _s is farthest from the direction of the exciting magnetic field H (t). The components of a z-axis component of the magnetization J _s · sin (θ ₀₎ is to generate a pickup coil magnetic flux interlinking to generate a pickup signal to the pickup coil.

Next, the case where the polarity of the DC bias current is reversed will be described. FIG. 6 is an explanatory diagram of the magnetization of the sensor portion of the fluxgate magnetic field sensor in the negative electrode property. In this example, the polarity of the DC bias current is reversed, and the phase of the alternating current on which it is superimposed is also reversed. Figure 6 is similar to FIG. 5, the waveform of the pick-up signal in (a) of FIG. 6, the direction of magnetization J _s of amorphous magnetic wire (b) of FIG. 6, the excitation magnetic field H in FIG. 6 (c) ( t) is shown. In FIG. 6 (c), contrary to FIG. 5 (c), when the exciting magnetic field H (t) has a positive peak, the influence of the peak is reduced, so that the angle θ ₀ becomes larger and the excitation is performed. When the magnetic field H (t) has a negative peak, the angle θ ₀ becomes smaller due to the influence of the peak. That is, when the exciting magnetic field H (t) is (1) and has a negative peak, the magnetization J _s is most attracted in the direction of the exciting magnetic field H (t), and when the exciting magnetic field H (t) is (2) and has a positive peak. Sometimes it is farthest from the direction of the exciting magnetic field H (t).

In this way, when the direction of the DC bias magnetic field H _dc is reversed, the angle θ ₀ is also reversed, and the magnetic flux interlinking with the pickup coil is also reversed. At this time, assuming that anisotropy K _u to ideal imbalance exists, the offset component caused by the anisotropy K _u is manifested in the opposite polarity to the case excitation current of positive polarity, positive polarity Will be present in the same amount as. Therefore, when the polarity of the exciting current is switched, the offset appearing at the output can be set to 0 by finally averaging these offset components on the circuit. Further, even if the anisotropic K _u changes with temperature and the offset caused by this changes, the change always appears symmetrically, so that the average is stable and the offset component appearing at the output is 0. There is up to.

Regarding the sensitivity and the magnitude and polarity of the offset component, a pickup signal is induced in the pickup coil by the time change of the magnetization J _s , and the pickup signal contains information on the external magnetic field H _ex and the offset component. Therefore, as described above, the external magnetic field depends on how the magnetization J _s moves according to the respective factors of the exciting magnetic field H (t) due to the exciting current, the anisotropic K _u , and the external magnetic field H _ex. magnitude and polarity of the offset component due to the magnitude and polarity and anisotropic K _u of sensitivity is determined for.

(Structure of Fluxgate Magnetic Field Sensor of the Present Invention)
Next, the structure of the fluxgate magnetic field sensor 200 according to the present invention will be described. The fluxgate magnetic field sensor 200 amplifies the pickup signals corresponding to the positive electrode property and the negative electrode property of the DC bias current at independent amplification factors. In the conventional fluxgate magnetic field sensor 100, the phase relationship with the reference signal in the synchronous detection is usually adjusted so as to maximize the sensitivity or optimize the noise as the magnetic field sensor. In the fluxgate magnetic field sensor 200 according to the present invention, when bias switching is applied in a general fluxgate magnetic field sensor, the imbalance existing between the pickup signals according to the polarity of the DC bias component of the exciting current is substantially present. The pickup signal is amplified with an independent amplification factor corresponding to the polarity of the DC bias component so as to be canceled by. In this way, the temperature coefficient can be minimized by setting the amplification factor according to the polarity of the DC bias component so as to cancel the imbalance. The fluxgate magnetic field sensor 200 realizes excellent temperature characteristics based on such a principle.

FIG. 2 is a schematic circuit diagram of the fluxgate magnetic field sensor 200. In this circuit, synchronous detection is used as a typical detection method. In addition, a closed loop configuration is adopted in which the sensor is operated in a region where the characteristics of the magnetic material are good by passing a current that cancels the detected magnetic field through the pickup coil by feedback. Alternatively, it is also possible to adopt a closed loop configuration in which a dedicated coil is separately provided without using the pickup coil in order to cancel the detected magnetic field, or an open loop configuration in which feedback is not performed.

The fluxgate magnetic field sensor 200 has the same configuration as the conventional fluxgate magnetic field sensor 100, and has a DC superimposition AC excitation unit 201, a phase shifter 202 (not shown), an exciting magnetic pole switching unit 203, and a sensor unit 204. , Voltage follower 205, amplifier 206, synchronous detector 207, low-pass filter 208, integrator 209, feedback resistor 210, and output terminal 211. The phase shifter 202 adjusts the phase relationship of the exciting current with the AC component in the synchronous detection of the synchronous detector 207 so that the sensitivity is substantially maximized. It can be placed at any position in the path through which the exciting current, pickup signal, reference signal, etc. are transmitted so that it can be adjusted. Therefore, FIG. 2 does not show the phase shifter 202 at a specific position.

The DC superimposed AC excitation unit 201 is a circuit that provides an exciting current in which a DC bias current is superimposed on an AC current. The DC superimposition AC excitation unit 201 is typically a known power supply circuit. By passing an alternating current through the sensor unit 204 to modulate an external magnetic field and superimposing a direct current bias current, a bias switching type excitation circuit is configured together with the exciting magnetic field switching unit 203 in the subsequent stage. In this case, the polarity of the alternating current on which the direct current bias current is superimposed is switched according to the switching of the polarity of the direct current bias current.

In bias switching, it is necessary to periodically switch the polarity of the DC bias current. Here, the AC current on which the DC bias current is superimposed can be configured to switch the polarity according to the change of the polarity of the DC bias current, or to not switch the polarity. In the above example, the polarities of both AC current and DC bias current are switched. In this case, if addition processing (averaging processing) is performed after synchronous detection, signals that do not correspond to the detected magnetic field are removed. The sensor output can be obtained.

Alternatively, in order not to switch the polarity of the AC current together with the polarity of the DC bias current, the DC superimposed AC excitation unit 201 uses an exciting current in which the DC bias current whose polarity is periodically switched is superimposed on the AC current. It can also be configured to provide. In this case, the exciting magnetic field switching unit 203 described later becomes unnecessary. Further, in order to obtain the magnetic field detection signal, it is necessary to take the difference in the detection number at each polarity of the DC bias current after the synchronous detection. For that purpose, for example, it is preferable to perform the addition process (averaging process) after inverting the polarity of the detection signal at the polarity of one of the DC bias currents.

Although not shown, the phase shifter 202 usually has a phase relationship with an AC exciting current from an AC source in the DC superimposed AC excitation unit 201 in synchronous detection, similarly to the conventional fluxgate magnetometer 100. The sensitivity is adjusted to the maximum (the noise is the minimum in the closed loop configuration). It should be noted that the substantially minimum does not mean a mathematically exact minimum, but a minimum within a range of practically adjustable accuracy. The output from the sensor unit 204 is synchronously detected by the synchronous detector 207 with reference to the phase of the reference signal whose AC current and frequency are synchronized, and the phase shifter 202 can adjust the phase relationship with the reference signal. It is possible to arrange it at any position such as.

The magnetic field switching unit 203 is a circuit that periodically switches the polarity of the exciting current flowing through the sensor unit 204. As a result, the polarity of the DC bias current component is periodically switched, and the exciting current by the bias switching method is supplied to the magnetic core of the sensor unit 204. The exciting pole switching unit 203 has a clock having a frequency of f _bs Hz that determines the polarity switching cycle, whereby a switch for switching the polarity is driven, and the polarity of the exciting current is periodically switched. A clock having a frequency of f _bs is typically used by _dividing the frequency of the AC component of the exciting current by an integral fraction. As shown in FIG. 2, when the exciting pole switching unit 203 switches the output from the DC superimposed AC exciting unit 201, the phase of the AC current is also inverted in synchronization with the switching of the polarity of the DC bias current, and the DC For each polarity of the bias current, the offset component due to magnetic anisotropy appears in the opposite polarity, and the sensitivity component of the magnetic field appears in the same polarity. The frequency of the clock is set to be smaller than the frequency of the alternating current generated by the direct current superimposed alternating current excitation unit 201. Further, the clock typically uses a timing signal whose frequency is divided by a fraction of an integer with respect to the AC exciting current generated by the DC superimposing AC exciting unit 201, and the phase is arbitrarily shifted as needed. To. For example, when the frequency of the AC component of the exciting current from the DC superimposed AC excitation unit 201 is 80 kHz, the switching frequency f _bs Hz of the exciting magnetic pole switching unit 203 is set to about 5 kHz, and the DC bias current is about 16 sine waves. It can be configured to repeat polarity switching.

In the bias switching method, it is important to periodically switch the polarity of the DC bias current component, and at that time, both a configuration in which the phases of the AC components are simultaneously inverted and a configuration in which the phases are not inverted are possible. When the exciting magnetic pole switching unit 203 is used, the polarity of the entire exciting current, which is an alternating current on which the direct current bias current is superimposed, is switched, so that the phase of the alternating current component can be inverted in synchronization with the switching of the polarity of the direct current bias current component. It will be. In this case, in the pickup signal, the offset component due to magnetic anisotropy appears with the opposite polarity and the sensitivity component of the magnetic field appears with the same polarity with respect to each polarity of the DC bias current.

Alternatively, if only the polarity of the DC bias current component is periodically switched and the phases of the AC components are not inverted at the same time, the exciting magnetic pole switching unit 203 is unnecessary. In this case, the DC superimposed AC excitation unit 201 is configured to provide an exciting current in which a DC bias current whose polarity is periodically switched is superimposed on the AC current. If the phase of the AC current component is not inverted regardless of the polarity switching of the DC bias current component, the offset component due to magnetic anisotropy has the same polarity with respect to each polarity of the DC bias current in the pickup signal. The sensitivity component of the magnetic field appears in the opposite polarity.

The sensor unit 204 is a sensor element that detects an external magnetic field to be measured, and has a magnetic core made of an elongated magnetic material to which the detected magnetic field is applied, and a pickup coil wound around the elongated magnetic material. The sensor unit 204 typically has a structure in which a pickup coil is wound around an amorphous magnetic wire bent into a U shape. The amorphous magnetic wire has a non-magnetic dwarf composition in which magnetic anisotropy is unlikely to occur. The sensor unit 204 typically has a structure in which a pickup coil is wound around an amorphous magnetic wire bent into a U shape. The shape of the amorphous magnetic wire is not limited to the U-shape, and may be a type II in which the tip is short-circuited with a conducting wire or the like. The magnitude of the external magnetic field is determined by the detection voltage (pickup signal) detected by the pickup coil of the changing magnetic flux generated by the exciting current flowing through the amorphous magnetic wire, including the effect of magnetization by the external magnetic field generated in the amorphous magnetic wire. Measure the magnetic flux. The amorphous magnetic wire operates even in the shape of a single wire I type, but the sensitivity is lower than that of the U type or II type which can obtain high sensitivity with two magnetic materials. Further, in order to increase the sensitivity, a plurality of shapes such as U-type, II-type, and I-type can be used in combination. The sensor unit 204 has a structure of an orthogonal fluxgate in which an exciting current is directly passed through a wire, but by using an alternating current in which a direct current is superimposed as an exciting current, the magnetization in the core is made one direction, and the magnetism accompanying the magnetization reversal. Low noise is achieved by reducing the magnetic noise of the body.

The voltage follower 205 is a component for accurately transmitting the pickup signal from the sensor unit 204 to the subsequent stage by converting the impedance. Since a sufficient current cannot be taken out from the voltage induced in the pickup coil, a voltage drop may occur depending on the impedance of the subsequent stage and the sensitivity may decrease. The voltage follower 205 prevents such a decrease in sensitivity. The voltage follower 205 is typically an amplifier circuit composed of an operational amplifier and having an amplification factor of 1.

The amplifier 206 is a component that amplifies the output from the voltage follower 205 to an appropriate level. The amplifier 206 is configured to be capable of amplifying with independent amplification factors for each of the positive and negative polarities according to the switching of the polarity of the DC bias current having a frequency of f _bs Hz. By doing so, the amplitude of the pickup signal can be adjusted for each polarity. And preferably, the amplifier 206 amplifies at an independent amplification factor according to the polarity of the DC bias component so that the imbalance existing between the pickup signals corresponding to each polarity is substantially canceled. FIG. 3 shows a specific circuit diagram of an example of the amplifier 206 in which an independent amplification factor can be set for each polarity. The resistor that determines the amplification factor is switched between the variable resistor 252A and the variable resistor 252B according to the switching of the polarity of the DC bias current having a frequency of f _bs Hz, and the amplification factor of the amplifier circuit 251 is switched.

In FIG. 2, the output from the voltage follower 205 passes through the capacitor to take out only the AC component, and is sent to the amplifier 206 in the subsequent stage. As a result, unnecessary DC components (such as the voltage due to the feedback signal) included in the pickup voltage are removed. The voltage follower 205 and the capacitor are additional configurations for accurately transmitting the pickup signal at an appropriate level, and are not necessarily required in the present invention.

The synchronous detector 207 is a component of a circuit that performs synchronous detection on the pickup signal from the sensor unit 204 output from the amplifier 206. The synchronous detector 207 synchronously detects the pickup signal, which is the induced voltage induced in the pickup coil of the sensor unit 204, by referring to the phase of the reference signal whose frequency is synchronized with the AC current in the excitation current, thereby outputting the sensor. It is a detection signal that will determine. The synchronous detector 207 detects the pickup signal from the sensor unit 204 according to the phase relationship with the reference signal having the same frequency as the AC component of the exciting current of the DC superimposed AC excitation unit 201. .. A signal having the same frequency as the AC current of the DC superimposed AC excitation unit 201 is input to the synchronous detector 207 as a reference signal, whereby the phase relationship with the pickup signal to be detected is determined. Typically, when the reference signal is a square wave gate control signal synchronized with the AC component of the exciting current in the DC superimposed AC excitation unit 201, this drives an internal switch and the gate control signal becomes negative (LOW). ), The polarity of the pickup signal is inverted and turned back. This makes it possible to obtain a meaningful output consisting of components synchronized with the reference signal. The synchronous detector 207 is typically composed of elements such as an analog switch. In order to obtain a detection signal that determines the sensor output from the pickup signal, other means such as peak value detection can be used without using synchronous detection.

The low-pass filter 208 is a component of a circuit that averages the detection signals that are the outputs of the synchronous detector 207. As a result, the magnetic field detection signal representing the magnitude of the magnetic field detected from the detection signal is extracted. In this example, the phase of the AC current is also inverted in synchronization with the switching of the polarity of the DC bias current, and in the pickup signal, the offset component due to magnetic anisotropy is present for each polarity of the DC bias current. Since it appears in the opposite polarity and the sensitivity components of the magnetic field appear in the same polarity, by adding the detection signals (realized by averaging), a magnetic field detection signal is obtained by removing signals that do not correspond to the magnetic field detected from the detection signal. Can be done. Specifically, the low-pass filter 208 smoothes the waveform of the signal representing the magnitude of the magnetic field that is the pulsating current from the detection signal to make it DC, and removes the offset caused by the DC bias current whose polarity is switched by addition. ..

Alternatively, when only the polarity of the DC bias current component is periodically switched and the phases of the AC components are not inverted at the same time, that is, the DC superimposing AC exciting unit 201 periodically switches the polarity without using the exciting magnetic pole switching unit 203. In the case of providing an exciting current in which the DC bias current is superposed on the AC current, the phase of the AC current component is not inverted regardless of the polarity switching of the DC bias current component. In this case, in the pickup signal, the offset component due to magnetic anisotropy appears with the same polarity and the sensitivity component of the magnetic field appears with the opposite polarity with respect to each polarity of the DC bias current. Therefore, in order to obtain the magnetic field detection signal, it is necessary to obtain the difference between the detection signals for each polarity of the DC bias current. Specifically, after inverting (-1 times) the detection signal at one polarity of the DC bias current, it is preferable to execute an addition process (averaging process) together with the detection signal at the other polarity of the DC bias current. Thereby, the magnetic field detection signal obtained by removing the signal corresponding to the magnetic field detected from the detection signal can be obtained.

The integrator 209 is a component of a circuit that integrates the step waveform output from the low-pass filter 208 when the system is operated in a closed loop configuration. The output of the integrator 209 applies negative feedback by a magnetic field to the magnetic core of the sensor unit 204 through the pickup coil of the sensor unit 204 via the feedback resistor 210 described later. As a result, the magnetic field detection signal that is the input of the integrator 209 represents the deviation of this feedback, and the integrator 209 controls the magnetic field detection signal to approach zero.

The feedback resistor 210 is a resistor through which the feedback signal passes when the feedback signal output from the integrator 209 is negatively fed back and input to the sensor unit 204. For negative feedback, the footback signal integrated with the magnetic field detection signal is input to the pickup coil with a polarity that cancels the magnetic field detection signal. Due to the negative feedback, a magnetic field that cancels the external magnetic field to be measured is generated in the pickup coil of the sensor unit 204. The current flowing by the feedback signal corresponds to the external magnetic field to be measured, and the current value detected as a voltage by the feedback resistor 210 is the sensor output indicating the magnitude of the external magnetic field to be measured.

Since the above example has a closed loop configuration, a feedback signal that generates a magnetic field that cancels the detected external magnetic field that has flowed into the magnetic core is input to the pickup coil, and the magnitude of the feedback signal is used as the sensor output. is there. Therefore, the low-pass filter 208, the integrator 209, and the feedback resistor 210 constitute an output circuit that generates a sensor output based on the magnetic field detection signal obtained by removing the signal that does not correspond to the magnetic field detected from the detection signal.

In the case of the open loop configuration, since the output of the low-pass filter 208 represents the external magnetic field to be measured, the integrator 209 and the feedback resistor 210 are unnecessary. Then, the sensor output can be obtained from the output of the low-pass filter 208.

The output terminal 211 is a node that outputs a sensor output obtained by converting the current flowing through the feedback resistor 210 into a voltage representing the magnitude of an external magnetic field. As a result, the magnitude of the current that generates the magnetic field that cancels the external magnetic field of the measurement target is output as a measurement quantity representing the external magnetic field of the measurement target.

(Synchronous detection in ideal state)
From now on, the operation of synchronous detection in an ideal state, that is, a circuit in which there is no offset imbalance when the DC bias is switched, will be described while showing the signal waveform of each point. For the sake of explanation, it is assumed that the sensor unit 204 is placed in a space where the external magnetic field is sufficiently small, such as in a magnetic shield. FIG. 7 is a diagram illustrating synchronous detection in an ideal state. FIG. 7A shows an exciting current waveform at the time of exciting magnetic field switching immediately after the exciting magnetic field switching unit 103. The waveform is a waveform in which the polarity of the alternating current on which the DC bias component is superimposed is periodically switched. In the figure, the DC bias component is positive in the first half, and the DC bias component is negative in the second half. The voltage when switching from positive electrode to negative electrode is discontinuous. FIG. 7B shows the waveform (after amplification) of the pickup signal at the time of exciting magnetic pole switching immediately after the amplifier 106. The first half is a pickup signal corresponding to the offset appearing at the time of positive electrode property, and the latter half is a pickup signal corresponding to the offset appearing at the time of negative electrode property. The waveform is an AC waveform corresponding to the time derivative of the z-axis projection component of the magnetization excited in the magnetic core by the exciting current. FIG. 7C shows the waveform of the detection signal immediately after the synchronous detector 107. The waveform is generated at the timing when the gate control signal of the synchronous detector 107 generated as a reference signal of a square wave having the same frequency as the AC current from the AC source in the DC superimposed AC excitation unit 101 is negative (LOW). The waveform of the signal is detected by the synchronous detector 107 inverting the polarity and folding back. By detection, the dotted waveform part is inverted in the pickup signal diagram to become the solid waveform part, and the information contained in the waveform is output with the positive and negative of the pickup signal in the positive (HIGH) part of the gate control signal. It becomes. Since the timing of reversing the polarity in the detection is the point where the instantaneous value of the pickup signal becomes zero, the detection output (that is, sensitivity) is maximized. The timing of the gate control signal is fixed at the position where the sensitivity of the fluxgate magnetic field sensor 100 is maximized. Specifically, this fixes the phase shift amount by the phase shifter 102 that determines the phase relationship between the gate control signal and the pickup signal of the synchronous detector 107 at the position where the sensitivity of the fluxgate magnetic field sensor 100 is maximized. Can be done by On the left side of FIG. 7D, the offset appearing at the time of positive electrode property and the offset appearing at the time of negative electrode property in the averaging performed by the low-pass filter 108 are shown. On the right side of FIG. 7D, the final overall average value obtained by the low-pass filter 108 is shown. The overall mean value is always zero because it is in the ideal state and there is no offset imbalance.

(Synchronous detection in ideal state when temperature changes)
Next, the operation of synchronous detection in a circuit in which the temperature of the sensor unit 104 changes and the offset changes in an ideal state where there is no offset imbalance when the DC bias is switched will be described while showing the signal waveform of each point. .. FIG. 8 is a diagram illustrating synchronous detection when the temperature changes in an ideal state. FIG. 8A shows an exciting current waveform at the time of exciting magnetic field switching immediately after the exciting magnetic field switching unit 103. The waveform is a waveform in which the polarity of the alternating current on which the DC bias component is superimposed is periodically switched, as in the case of FIG. 7A. FIG. 8B shows the waveform (after amplification) of the pickup signal at the time of exciting magnetic pole switching immediately after the amplifier 106 when the temperature of the sensor unit 104 changes. The first half is a pickup signal corresponding to the offset that appears at the time of positive electrode property. Compared with the case of (b) in FIG. 7, the amount of the offset component due to anisotropy fluctuates due to the temperature change, and the amplitude is large. It has become. The latter half is a pickup signal corresponding to the offset that appears at the time of negative electrode property. Compared with the case of (b) in FIG. 7, the amount of the offset component due to anisotropy fluctuates due to the temperature change, and the amplitude is large. It has become. However, when comparing the first half and the second half, the amplitudes are larger in both cases, but both waveforms change symmetrically. FIG. 8C shows the waveform of the detection signal immediately after the synchronous detector 107. The waveform has a larger amplitude than the case of FIG. 7 (c), but the ratio of positive and negative offsets does not change. On the left side of FIG. 8D, the offset appearing at the time of positive electrode property and the offset appearing at the time of negative electrode property in the averaging performed by the low-pass filter 108 are shown. On the right side of FIG. 8D, the final overall average value obtained by the low-pass filter 108 is shown. Although the amplitude of the detected waveform is large, if the offset fluctuations of the positive and negative polarities are the same ratio (1: 1), the sum of the positive and negative offsets does not change regardless of the temperature change, so the overall average value is It always remains zero. Therefore, the output does not fluctuate due to temperature changes.

(Synchronous detection when there is an imbalance between positive and negative offsets)
The above description with reference to FIGS. 7 and 8 is based on an ideal state in which there is no imbalance between positive and negative offsets regardless of temperature changes. However, in reality, there is usually an imbalance between positive and negative offsets. Such a case will be described below. FIG. 9 is a diagram illustrating synchronous detection when there is an imbalance between the positive and negative offsets. FIG. 9A shows an exciting current waveform at the time of exciting magnetic field switching immediately after the exciting magnetic field switching unit 103. The waveform is a waveform in which the polarity of the alternating current on which the DC bias component is superimposed is periodically switched, as in the case of FIG. 7A. FIG. 9B shows the waveform (after amplification) of the pickup signal at the time of exciting magnetic pole switching immediately after the amplifier 106 when the temperature of the sensor unit 104 changes from room temperature. Here, as a typical example, a case where distortion exists in the waveform as an imbalance between positive and negative offsets is shown. As an offset imbalance, consider a case where the waveform at the negative electrode property in the latter half is distorted as compared with the positive electrode property in the first half. Here, the amplitude increases due to the offset fluctuation due to the temperature change, and the imbalance between the positive and negative offsets also increases. Specifically, the first half is a pickup signal corresponding to the offset that appears at the time of positive electrode property, which is the same as the case of FIG. 7B. The latter half is a pickup signal corresponding to the offset that appears at the time of negative electrode, but as compared with the case of FIG. 7B, the distortion increases with the amplitude due to the temperature change. FIG. 9C shows the waveform of the detection signal immediately after the synchronous detector 107. The waveform also contains the distortion in the latter half as compared with the case of (c) of FIG. However, in both the first half and the second half, the detection output (that is, sensitivity) is maximized because the timing of reversing the polarity in the detection is the point where the instantaneous value of the pickup signal becomes zero. On the left side of FIG. 9D, the offset appearing at the time of positive electrode property and the offset appearing at the time of negative electrode property in the averaging performed by the low-pass filter 108 are shown. On the right side of FIG. 9D, the final overall average value obtained by the low-pass filter 108 is shown. The overall mean value shown on the right side of FIG. 9D is deviated from zero to the negative side due to the influence of the latter half including distortion. The amount of deviation will change depending on the temperature. As described above, when there is an imbalance between the positive and negative offsets, the overall average value fluctuates due to the imbalance due to the distortion of the waveform due to the temperature change. Therefore, the sensor output does not become zero due to the temperature change.

(Synchronous detection when amplified with an independent amplification factor corresponding to the polarity when there is an imbalance between the positive and negative offsets)
From this, when there is an imbalance in the positive / negative offset fluctuation due to temperature according to the present invention, the final output is not changed due to temperature by amplifying with an independent amplification factor corresponding to the polarity of the DC bias current. The method of making is explained. In FIG. 10, the absolute value of the offset component in the detection signal (or the pickup signal that is the basis thereof) at each polarity of the DC bias current when the temperature is changed when the positive / negative offset fluctuation is unbalanced is shown. Synchronous detection when the amplification ratio is adjusted so that the sum of the positive and negative offsets is minimized by amplifying the pickup signals corresponding to each polarity with independent amplification factors so that they remain substantially equal to each other. It is a figure explaining. The temperature of the sensor unit 204 here is room temperature. FIG. 10A shows an exciting current waveform at the time of exciting magnetic field switching immediately after the exciting magnetic field switching unit 203. The waveform is a waveform in which the polarity of the alternating current on which the DC bias component is superimposed is periodically switched, as in the case of FIG. 7A. FIG. 10B shows the waveform (after amplification) of the pickup signal at the time of exciting magnetic pole switching immediately after the amplifier 206 when the temperature of the magnetic core of the sensor unit 204 is room temperature. Due to the imbalance (not symmetric) of the positive and negative offsets (the offset component when the DC bias component of the exciting current is positive and the offset component when it is negative), the latter half of the pickup waveform when the positive electrode is positive is compared to the latter half. The pickup waveform at the time of negative electrode property is in a state including distortion. FIG. 10C shows the waveform of the detection signal immediately after the synchronous detector 207, and the latter half includes distortion. In both the first half and the second half, the detection output (that is, sensitivity) is maximized because the timing of reversing the polarity in the detection is the point where the instantaneous value of the pickup signal becomes zero.

The amplifier 206 can amplify with independent amplification factors for each of the positive and negative polarities. Here, each amplification factor of the amplifier 206 is independently adjusted so that the temperature coefficient of the sensor output is substantially minimized. Specifically, each amplification factor (amplification ratio) is determined at a constant ratio so that the imbalance of the offset component existing between the pickup signals corresponding to the respective polarities is substantially canceled. That is, the final sensor output values are set to remain substantially equal when the temperature around the sensor unit 204 is changed (without an external magnetic field or kept constant). On the left side of FIG. 10D, the offset appearing at the time of positive electrode property and the offset appearing at the time of negative electrode property in the averaging performed by the low-pass filter 208 are shown. The positive and negative amplification factors of the amplifier 206 are independently adjusted so that the absolute values of these offsets remain equal to each other even when the temperature changes. With this regulation, be varied temperature around the sensor unit 204 is due to the anisotropy K _u in each polarity, the detection signal (or a pickup signal which becomes the base) absolute offset component contained in the Even if the values remain equal to each other and the temperature around the sensor unit 204 changes, the imbalance due to the fluctuation of the offset component can be eliminated. On the right side of FIG. 10D, the final overall average value obtained by the low-pass filter 108 is shown. Since the absolute values of the positive offset appearing at the positive electrode property and the negative offset appearing at the negative electrode property are adjusted so as to remain equal to each other even when the temperature changes, their overall average values are zero. In this way, the pickup signal is amplified by an independent amplification factor for the pickup signals of the positive and negative polarities so that the offset imbalance is canceled, so that the overall average value fluctuates. Instead, it is always zero.

(Synchronous detection when the temperature changes when amplified by an independent amplification factor corresponding to the polarity when there is an imbalance between the positive and negative offsets)
Next, when there is an imbalance between the positive and negative offsets, the case where the temperature when amplified by an independent amplification factor corresponding to the polarity changes from room temperature will be described. FIG. 11 shows a diagram for explaining synchronous detection when the temperature changes from room temperature when there is an imbalance in the positive and negative offsets similar to the case of FIG. FIG. 11A shows an exciting current waveform at the time of exciting magnetic field switching immediately after the exciting magnetic field switching unit 203. The waveform is a waveform in which the polarity of the alternating current on which the DC bias component is superimposed is periodically switched, as in the case of FIG. 7A. FIG. 11B shows the waveform (after amplification) of the pickup signal at the time of exciting magnetic pole switching immediately after the amplifier 206 when the temperature of the sensor unit 204 changes from room temperature. Here, as a typical example, a case where distortion exists in the waveform at the time of negative electrode property is shown as an imbalance of positive and negative offsets similar to the case of FIG. However, as the temperature changes from room temperature, the amplitude increases and the imbalance between positive and negative offsets also increases. Specifically, the first half is a pickup signal corresponding to the offset that appears at the time of positive electrode property, and the amplitude is increased as compared with the case of FIG. 10B. The latter half is a pickup signal corresponding to the offset that appears at the time of the negative electrode property, but the distortion increases with the amplitude as compared with the case of FIG. 10B. FIG. 11 (c) shows the waveform of the detection signal immediately after the synchronous detector 207. The waveform also contains the distortion in the latter half as compared with the case of (c) of FIG. However, in both the first half and the second half, the detection output (that is, sensitivity) remains at the maximum because the timing of reversing the polarity in the detection is the point where the instantaneous value of the pickup signal becomes zero. On the left side of FIG. 11D, the offset appearing at the time of positive electrode property and the offset appearing at the time of negative electrode property in the averaging performed by the low-pass filter 108 are shown. Although the offset is larger than that in the case of FIG. 10, the amplification factor of the amplifier 206 is such that the imbalance of the offset component existing between the pickup signals corresponding to the positive and negative electrodes is substantially canceled. Since it is adjusted at a constant rate, the absolute values of the positive and negative offsets are the same even if the temperature changes. On the right side of FIG. 11D, the final overall average value obtained by the low-pass filter 108 is shown. Since the absolute values of the positive offset that appears when the positive electrode is positive and the negative offset that appears when the negative electrode is negative are adjusted so that they remain equal to each other even if the temperature changes, the temperature changes from normal temperature and the absolute offset is absolute. Even if the values increase, their overall average value will be zero. In this way, the pickup signal is amplified by an independent amplification factor for the positive and negative polarity pickup signals so that the offset imbalance is canceled even if the temperature changes. Even if the temperature changes from normal temperature, the overall average value does not change and is always zero. In this way, the fluctuation of the output due to the temperature change is minimized.

Next, how much the temperature coefficient could be improved by the present invention will be described. The conventional fluxgate magnetometer 100 in which the timing of synchronous detection is adjusted so that the sensitivity and noise characteristics are the best, and the independent amplification factor of the present invention corresponding to the polarity so that the temperature characteristics are the best. The fluxgate magnetic field sensor 200 amplified in 1 was placed in an environment in which the temperature of the sensor unit was changed, and the temperature coefficient at that time was measured. At this time, the sensor unit 204 is installed in a stable magnetic shield box in which the external magnetic field is extremely small so that the measured output change can be regarded as the output offset change itself. FIG. 12 is a graph of measured values of temperature change and temperature coefficient of the fluxgate magnetic field sensor according to the prior art and the present invention. FIG. 12A shows the temperature change given to these sensors. The solid line is the graph of the fluxgate magnetic field sensor 200 of the present invention, and the one-point chain line graph is the graph of the conventional fluxgate magnetic field sensor 100. As for the temperature change, a low temperature side temperature cycle was given in which the temperature was first rapidly cooled to about −30 ° C. and then gradually returned to near room temperature over 3 hours or more. There was an acceptable slight difference in the actual temperature changes of both sensors. FIG. 12B shows the offset when such a temperature change is applied. The temperature coefficient of the offset is 0.026 nT / ° C in the fluxgate magnetic field sensor 200 of the present invention shown by the solid line, but is 0.12 nT / ° C. in the conventional fluxgate magnetic field sensor 100 shown by the one-point chain line. .. As described above, it was confirmed that the fluxgate magnetic field sensor 200 of the present invention has a temperature coefficient reduced by 4 to 5 times as compared with the sensor of the prior art, and exerts a remarkable effect.

(Parameter of amplification ratio according to the polarity of DC exciting current)
Next, how to set the amplification factor (amplification ratio) for each polarity of the DC exciting current so that the positive / negative offset fluctuation is minimized will be described. FIG. 13 shows a graph of an actual measurement example of the temperature coefficient with respect to the amplification ratio according to the polarity of the DC exciting current. The horizontal axis of the figure is a parameter of the normalized amplification ratio, and is a value indicated by (value of variable resistance 252A-value of variable resistance 252B) / value of variable resistance 252A by variable resistors 252A and 252B for setting the amplification factor. Is. The temperature coefficient is shown on the vertical axis. The temperature coefficient at a certain amplification ratio was measured by actually measuring the sensor output while changing the temperature of the environment in a state where the amplification ratio was set by the variable resistors 252A and 252B. As can be seen from FIG. 13, when the amplification ratio parameter is gradually changed from a low value to a high value, the temperature coefficient decreases from a high value, but takes a minimum value at a certain point and then inverts and increases. .. In the example of FIG. 13, the temperature coefficient is the minimum when the amplification ratio parameter is −0.2. Therefore, it is understood that when the variable resistance value in the negative electrode property is made higher than the variable resistance value in the positive electrode property by about 20%, the positive / negative offset fluctuation becomes the minimum and the temperature coefficient becomes the minimum. Since the actual amplification factor takes into account the difference in the ON resistance of the analog switch that switches between the variable resistors 252A and 252B, the case where the amplification ratio parameter is 0 does not directly correspond to the case where the conventional amplifier 106 is used. In this way, the temperature coefficient can be minimized by setting the amplification factor for each polarity of the DC exciting current so that the parameter of the amplification ratio that becomes the minimum value becomes an appropriate value. In the above example, the temperature coefficient is minimized by an amplifier having an amplification factor according to the polarity of the DC exciting current, but the configuration required for that purpose is the level of the pickup signal according to the polarity of the DC exciting current. It is a configuration for adjusting independently. Therefore, it is possible to adjust the temperature coefficient so as to be minimized by adjusting the level of the pickup signal according to the polarity by a method other than amplification. For example, an attenuator having an independent attenuation rate depending on the polarity of the DC exciting current can also minimize the temperature coefficient. In this case, the pickup signals of each polarity can be amplified with the same amplification factor and then attenuated with the attenuation factor corresponding to each polarity. It is also possible to use a configuration such as an amplifier that independently adjusts the level of another signal corresponding to the level of the pickup signal, for example, a detection signal, according to the polarity of the DC exciting current. ..

The present invention can set the temperature coefficient of the fluxgate magnetic field sensor to an extremely good value, and can be suitably applied not only to spacecraft but also to magnetic field sensors used in an environment exposed to temperature changes.

100: Flux gate magnetometer 101: DC superimposition AC exciter 102: Phase shifter 103: Exciting magnetic pole switching unit 104: Sensor unit 105: Voltage follower 106: Amplifier 107: Synchronous detector 108: Low-pass filter 109: Integrator 110: Feedback resistance 111: Output terminal 200: Flux gate magnetic field sensor 200A: Flux gate magnetic field sensor 200B: Flux gate magnetic field sensor 201: DC superimposing AC magnetometer 202: Phase shifter 203: Exciting magnetic pole switching unit 204: Sensor unit 205: Voltage follower 206: Amplifier 207: Synchronous detector 208: Low-pass filter 209: Integrator 210: Feedback resistance 211: Output terminal 251: Amplification circuit 252A: Variable resistance 252B: Variable resistance H: Exciting magnetic field H _dc : DC bias magnetic field H _ex : External Magnetic field J _s : Magnetospheric K _u : Animate f _bs : Switching frequency

Claims (6)

A magnetic core made of an elongated magnetic material to which a detected magnetic field is applied,
The pickup coil wound around the magnetic core and
A DC superimposed AC excitation unit that supplies an exciting current in which a DC bias current whose polarity is periodically switched between positive and negative electrodes is superimposed on an AC current to the magnetic core.
An amplifier that amplifies the pickup signal from the pickup coil and
A detector that outputs a detection signal by detecting the amplified pickup signal, and
In a fluxgate magnetic field sensor including an output circuit that generates a sensor output based on a magnetic field detection signal obtained by removing a signal that does not correspond to the detected magnetic field from the detection signal.
The amplifier is a fluxgate magnetic field sensor that amplifies the pickup signals corresponding to the positive electrode property and the negative electrode property of the DC bias current at independent amplification factors. The fluxgate magnetometer according to claim 1, wherein the detector synchronously detects the amplified pickup signal with reference to a reference signal whose frequency is synchronized with the AC current from the DC superimposed AC excitation unit. .. The fluxgate magnetic field sensor according to claim 1 or 2, wherein the alternating current has a polarity that is periodically reversed in the same cycle as the direct current bias current. The fluxgate magnetic field sensor according to any one of claims 1 to 3, wherein the amplifier is independently adjusted for each amplification factor so that the temperature coefficient of the sensor output is substantially minimized. .. In the amplifier, when the temperature around the sensor is changed, the absolute values of the offset components contained in the detection signals when the DC bias current is positive and negative remain equal to each other. The fluxgate magnetic field sensor according to claim 4, wherein each of the amplification factors is adjusted independently. The output circuit is any one of claims 1 to 5, wherein a footback signal obtained by integrating the magnetic field detection signal is input to the pickup coil with a polarity that cancels the magnetic field detection signal, and the sensor output is generated from the feedback signal. The flux gate magnetic field sensor according to item 1.

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