Fluxgate magnetic field sensor, and method for confirming the trend of output changes in a fluxgate magnetic field sensor.

Abstract

To provide a small highly-accurate flux gate magnetic field sensor, and a method for confirming change tendency of output in the flux gate magnetic field sensor.SOLUTION: A flux gate magnetic field sensor comprises: a magnetic core in which an excitation magnetic field and an external magnetic field are generated; a sensor unit having a pickup coil that outputs a pickup signal according to the magnetic field of the magnetized magnetic core; an excitation current supply unit that supplies to the magnetic core an excitation current in which an alternating current is superimposed on a direct current bias current whose polarity is reversed at a predetermined cycle; a fundamental wave component extraction unit that outputs a fundamental wave pickup signal obtained by extracting a fundamental wave component included in the pickup signal; a reference signal generation unit that generates a rectangular wave reference signal having the same frequency as the alternating current; a phase shift unit that adjusts the phase relation between the fundamental wave pickup signal and the reference signal based on a set phase shift amount; a synchronous detection unit that outputs a detection signal obtained by synchronously detecting the fundamental wave pickup signal with reference to the reference signal; and an output unit that outputs an output signal according to the detection signal.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a fluxgate magnetic field sensor and a method for confirming the change tendency of the output in the fluxgate magnetic field sensor.

Background Art

[0002] Conventionally, as a kind of magnetic field sensor for detecting (measuring) the state of a magnetic field, a fundamental waveform orthogonal fluxgate magnetic field sensor is known. The fundamental waveform orthogonal fluxgate magnetic field sensor is a small-sized fluxgate magnetic field sensor in which the excitation magnetic field of a magnetic core and the external magnetic field to be measured are in an orthogonal relationship with each other. In a fundamental waveform orthogonal fluxgate magnetic field sensor (hereinafter simply referred to as "fluxgate magnetic field sensor"), as an excitation current for generating an excitation magnetic field, by superimposing a DC bias current on an AC current, low noise, high sensitivity, magnetic field measurement at the fundamental wave frequency, etc. can be realized.

[0003] Incidentally, in fluxgate magnetic field sensors, large offset components (components that do not correspond to the measured magnetic field) and drift can occur in the output due to the magnetic anisotropy of the magnetic core. A method called bias switching is known to suppress these offset components and drift. In the bias switching method, the magnetic anisotropy of the magnetic core can be considered uniaxial, and the polarity of the DC bias current superimposed on the AC excitation current, and in some cases the phase of the AC current (hereinafter referred to as "excitation current polarity"), are alternately reversed (switched) at a predetermined period. In the bias switching method, after reversing the polarity of the excitation current, subtraction processing (when only the positive and negative of the DC bias current is switched) or addition processing (when the phase of the AC current is also switched) is performed on the signal detected by the pickup coil (pickup signal) for each polarity of the excitation current. These processes are typically achieved by averaging the signals. For example, in the case of subtraction processing, the signal of one polarity is multiplied by -1 before averaging is performed. This allows fluxgate magnetic field sensors using a bias switching method to cancel out potential offset components and their corresponding drift components, thereby stabilizing magnetic field measurements.

[0004] However, when the polarity of the excitation current is reversed, the component of the detected pickup signal that does not correspond to the magnetic field being measured may not appear as an ideally symmetrical state with respect to the reversal of the excitation current's polarity, and may contain a predetermined imbalance. In this case, even if averaging processing (including the subtraction and addition processes described above) is performed in the bias switching method, the predetermined imbalance in the pickup signal cannot be canceled out, and an unavoidable offset component remains in the output of the fluxgate magnetic field sensor. Furthermore, when the temperature around the fluxgate magnetic field sensor (especially the sensor head equipped with the fluxgate magnetic field sensor) changes, the magnetization state of the magnetic core changes in accordance with this temperature change, and a change may be observed in the waveform of the pickup signal. At this time, the contribution of the imbalance to the pickup signal also changes, causing drift in the residual offset component. In other words, in addition to the offset component, drift of the offset component also occurs in the output of the fluxgate magnetic field sensor. This residual offset component and its drift can be an obstacle to high-precision magnetic field measurement with the fluxgate magnetic field sensor.

[0005] In this regard, conventional fluxgate magnetic field sensors have proposed methods to substantially minimize residual offset and its drift, such as those described in Patent Document 1 and Non-Patent Document 1. In the conventional fluxgate magnetic field sensors proposed in Patent Document 1 and Non-Patent Document 1, the phase relationship between the pickup signal and the reference signal is adjusted during the process of synchronous detection of the pickup signal, so that the amount of unbalance detected in the positive and negative excitation sections of the DC bias current is equal. In other words, in conventional fluxgate magnetic field sensors, the detection sensitivity of the positive side excitation section of the DC bias current is balanced with the detection sensitivity of the negative side excitation section. As a result, in conventional fluxgate magnetic field sensors, residual offset and its drift in the pickup signal can be canceled out in the averaging process using the bias switching method. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6823877 [Non-patent literature]

[0007] [Non-Patent Document 1] N. Murata et al., “Practical Method for Drastic Improvement of Output Offset Stability in Bias-Switched Fundamental Mode Orthogonal Fluxgate”, IEEE Sensors Journal, Volume 21, Issue 17, September 2021. [Overview of the project] [Problems that the invention aims to solve]

[0008] However, even when amorphous magnetic wires spun continuously from the same lot are used as the magnetic core of the sensor head of a fluxgate magnetic field sensor, variations in the magnetization state and magnetization response can be expected depending on the position where the amorphous magnetic wire is cut. Furthermore, when multiple sensor heads are manufactured, it is expected that the way imbalance manifests itself will differ for each sensor head depending on conditions such as temperature changes and changes over time. In this case, even if the phase relationship between the pickup signal and the reference signal is adjusted as in conventional fluxgate magnetic field sensors, depending on the magnetization state and magnetization response of the magnetic core, there may be sensors for which the bias switching averaging process cannot sufficiently cancel out the offset and drift remaining in the pickup signal. Consequently, for example, when measuring an external magnetic field in three axes by arranging three sensor heads in directions corresponding to each of the three axes, variations may appear in the measurement results of each fluxgate magnetic field sensor, making it impossible to accurately measure the state of the magnetic field. Furthermore, in applications requiring particularly high-precision magnetic field measurements, such as space science observations and volcanic observations, fluxgate magnetic field sensors can be susceptible to significant impacts on measurement results due to uncancellable offset components and their drift. In other words, the uncancellable offset and drift remaining in the pickup signal of a fluxgate magnetic field sensor can degrade measurement accuracy. Moreover, in space science observations, fluxgate magnetic field sensors may be used not only for measurements on ground-based equipment, such as for creating zero magnetic field spaces, but also on spacecraft exposed to special environments such as outer space (e.g., environments with extreme temperature fluctuations). In such cases, fluxgate magnetic field sensors need to be able to operate over a wide temperature range and withstand long-term measurements.

[0009] This invention is based on the recognition of the above-mentioned problems and aims to provide a compact, high-precision fluxgate magnetic field sensor and a method for confirming the trend of changes in the output of a fluxgate magnetic field sensor. [Means for solving the problem]

[0010] To achieve the above objective, a fluxgate magnetic field sensor according to one aspect of the present invention is a fluxgate magnetic field sensor comprising: a sensor unit having a magnetic core made of an elongated magnetic wire that generates an excitation magnetic field corresponding to a supplied excitation current and an external magnetic field of an object to be measured; a pickup coil wound around the magnetic core and outputting a pickup signal corresponding to the magnetic field of the magnetized magnetic core; an excitation current supply unit that supplies the excitation current to the magnetic core by superimposing an alternating current on a DC bias current whose polarity is reversed at a predetermined period; a fundamental wave component extraction unit that outputs a fundamental wave pickup signal obtained by extracting a fundamental wave component representing the frequency of the alternating current included in the pickup signal; a reference signal generation unit that generates a reference signal of a rectangular wave with the same frequency as the alternating current; a phase shift unit that adjusts the phase relationship between the fundamental wave pickup signal and the reference signal based on a set phase shift amount representing the phase relationship between the fundamental wave pickup signal and the reference signal; a synchronous detection unit that outputs a detection signal obtained by synchronously detecting the fundamental wave pickup signal with reference to the reference signal; and an output unit that outputs an output signal corresponding to the detection signal.

[0011] To achieve the above objective, a method for confirming the trend of change in output of a fluxgate magnetic field sensor according to one aspect of the present invention includes: a sensor unit having a magnetic core made of an elongated magnetic wire that generates an excitation magnetic field corresponding to a supplied excitation current and an external magnetic field of the object to be measured; a pickup coil wound around the magnetic core and outputting a pickup signal corresponding to the magnetic field of the magnetized magnetic core; an excitation current supply unit that supplies the excitation current to the magnetic core by superimposing an alternating current on a DC bias current whose polarity is reversed at a predetermined period; a fundamental wave component extraction unit that outputs a fundamental wave pickup signal obtained by extracting the fundamental wave component representing the frequency of the alternating current included in the pickup signal; and a square-angle component with the same frequency as the alternating current. A method for confirming the trend of change in the output of a fluxgate magnetic field sensor, comprising: a reference signal generation unit that generates a shape wave reference signal; a phase shift unit that adjusts the phase relationship between the fundamental wave pickup signal and the reference signal based on a set phase shift amount representing the phase relationship between the fundamental wave pickup signal and the reference signal; a synchronous detection unit that outputs a detection signal obtained by synchronously detecting the fundamental wave pickup signal with reference to the reference signal; and an output unit that outputs an output signal corresponding to the detection signal, wherein a computer changes the phase shift amount and confirms the trend of change based on the change in the output signal output for each of the changed phase shift amounts. [Effects of the Invention]

[0012] According to one aspect of the present invention, a compact and highly accurate fluxgate magnetic field sensor and a method for confirming the trend of changes in the output of a fluxgate magnetic field sensor can be provided. [Brief explanation of the drawing]

[0013] [Figure 1] This figure shows an example of the configuration of a fluxgate magnetic field sensor according to the embodiment. [Figure 2] This figure shows an example of the configuration of a conventional fluxgate magnetic field sensor. [Figure 3] This figure shows an example of a voltage waveform of a pickup signal. [Figure 4] It is a diagram showing an example of a change in an output signal when an imbalance occurs in a pickup signal. [Figure 5] It is a diagram showing an example of an inspection environment of a fluxgate magnetic field sensor. [Figure 6] It is a flowchart showing an example of a process flow for inspecting a fluxgate magnetic field sensor. [Figure 7] It is a flowchart showing an example of another process flow for inspecting a fluxgate magnetic field sensor. [Figure 8] It is a diagram (part 1) showing an example of an inspection result of a fluxgate magnetic field sensor. [[ID=??]] [Figure 9] It is a diagram (part 2) showing an example of an inspection result of a fluxgate magnetic field sensor. [Figure 10] It is a diagram (part 3) showing an example of an inspection result of a fluxgate magnetic field sensor. [Figure 11] It is a diagram showing an example of noise performance in a fluxgate magnetic field sensor.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, referring to the drawings, embodiments of the fluxgate magnetic field sensor of the present invention and a method for confirming a change tendency of an output in the fluxgate magnetic field sensor will be described. In the following description, it is assumed that the fluxgate magnetic field sensor of the embodiment supplies an excitation current, in which an alternating current whose polarity is inverted in the same period as the period of a direct current bias current that has inverted (switched) the positive and negative polarities at a predetermined period by a bias switching method is superimposed, to a sensor head, and measures an output signal obtained by performing an averaging process on a detection signal obtained by detecting a pickup signal output by the sensor head as a measurement result. Further, it is assumed that the fluxgate magnetic field sensor of the embodiment is configured to be able to adjust a phase relationship between a pickup signal and a reference signal used for reference when detecting the pickup signal.

[0015] Note: There seems to be a typo in the original text where "??" is used instead of a proper ID. It should be corrected in the original for a more accurate translation. [Configuration of Fluxgate Magnetic Field Sensor] FIG. 1 is a diagram showing an example of the configuration of a fluxgate magnetic field sensor according to an embodiment. The fluxgate magnetic field sensor 1 includes, for example, a sensor head unit SH, a DC superposed AC excitation unit 10, an excitation polarity switching unit 20, a voltage follower 30, an amplifier 40, a low pass filter (hereinafter referred to as "LPF") 50, a synchronous detector 60, an LPF 70, an integrator 80, and a feedback resistor 90. The fluxgate magnetic field sensor 1 has a configuration in which an LPF 50 is added to the configuration of a conventional fluxgate magnetic field sensor. Therefore, the basic operation of the fluxgate magnetic field sensor 1 is the same as that of a conventional fluxgate magnetic field sensor. For this reason, a detailed description of the basic operation of the fluxgate magnetic field sensor 1 will be omitted.

[0016] The sensor head unit SH is a sensor element for detecting the state of an external magnetic field to be measured. The sensor head unit SH includes, for example, a magnetic core WC and a pickup coil PC. The magnetic core WC is, for example, an elongated magnetic body wire having magnetic properties of uniaxial magnetic anisotropy, such as an amorphous magnetic wire. In FIG. 1, for the sake of simplicity of the drawing, a configuration in which an excitation current is supplied to both ends of a single amorphous magnetic wire, that is, a magnetic core WC of the I-type configuration is shown, but the configuration of the magnetic core WC is not limited to the I-type configuration. For example, the magnetic core WC may be of a II-type configuration in which one end of each of two amorphous magnetic wires arranged at a narrow interval is connected (shorted) by a copper wire or the like and an excitation current is supplied to the other end of each wire, or may be of a U-type configuration in which an excitation current is supplied to both ends of a single amorphous magnetic wire bent in a U-shape at a narrow interval. Generally, the magnetic core WC has a higher sensitivity for detecting a magnetic field in the II-type configuration or the U-type configuration than in the I-type configuration because the magnetic body wire is doubled. The magnetization of the magnetic core WC vibrates at a position where it is minimized in a state where the excitation magnetic field by the excitation current, the magnetic anisotropy, and the energy of the external magnetic field are balanced.

[0017] The pickup coil PC is an air-core coil composed of wire (e.g., copper wire) wound around the magnetic core WC. The pickup coil PC is electromagnetically induced by the magnetization of the magnetic core WC, which vibrates due to the excitation current and the external magnetic field of the object being measured, and generates (induces) a voltage corresponding to the strength of the magnetic field. The pickup coil PC outputs the generated voltage as a pickup signal.

[0018] The sensor head unit SH is an example of the "sensor unit" in the claims.

[0019] The DC-superimposed AC excitation unit 10 generates an excitation current by superimposing a DC bias current onto an AC current. More specifically, the DC-superimposed AC excitation unit 10 superimposes a DC bias current with a predetermined current value generated by a reference signal source onto a sinusoidal AC current generated by an AC source, thereby generating an excitation current that shifts the AC current, whose current value changes at a predetermined frequency, to either the positive or negative side centered around zero. The DC-superimposed AC excitation unit 10 outputs the generated excitation current to the excitation polarity switching unit 20. Furthermore, the DC-superimposed AC excitation unit 10 generates a square wave signal whose voltage level is inverted between "High" and "Low" levels to match the frequency of the AC source. The DC-superimposed AC excitation unit 10 outputs the generated square wave signal to the synchronous detector 60, which will be described later, as a reference signal for synchronous detection.

[0020] The DC superimposed AC excitation unit 10 includes, for example, a phase shifter 12. The phase shifter 12 adjusts the phase of the AC component in the excitation current output by the DC superimposed AC excitation unit 10. The phase shifter 12 is a delay circuit composed of, for example, an operational amplifier. The phase shifter 12 adjusts the phase of the excitation current generated by the DC superimposed AC excitation unit 10 by shifting the phase of the excitation current by a predetermined adjustment amount (phase shift amount). The phase of the excitation current is also the phase relationship between the pickup signal and the reference signal to be detected in the synchronous detection described later. Therefore, the phase shifter 12 adjusts the phase of the excitation current so that the magnetic field measurement in the fluxgate magnetic field sensor 1 can be performed appropriately. Furthermore, the phase shifter 12 adjusts the phase of the reference signal so that synchronous detection of the pickup signal can be performed appropriately. The phase shifter 12 may adjust the phase relationship between the fundamental wave pickup signal and the reference signal so that the fundamental wave component of the pickup signal extracted by the LPF 50 (hereinafter referred to as the "fundamental wave pickup signal") extracted later is synchronously detected in a balanced ratio between the waveform obtained with positive polarity corresponding to a predetermined period (reversal period) for reversing the polarity of the DC bias current by the bias switching method, and the waveform obtained with negative polarity. The amount of phase adjustment of the excitation current and the reference signal by the phase shifter 12 is set, for example, by an inspection device (not shown) used to confirm the sensitivity of the manufactured fluxgate magnetic field sensor 1. Figure 1 schematically shows the case where the phase shifter 12 is placed between the AC source and the reference signal source, but the phase shifter 12 may be placed after the AC current source (for example, between the AC current source and the resistor above it) when shifting the phase of the AC current side, and in the path between the reference signal source and the synchronous detector 60 when shifting the phase of the reference signal source side.

[0021] The excitation polarity switching unit 20 periodically reverses (switches) the polarity of the excitation current supplied to the sensor head unit SH. The excitation polarity switching unit 20 includes, for example, a switching unit 22. The switching unit 22 switches the switches of the excitation polarity switching unit 20 at a predetermined frequency. The switching unit 22 switches the switches according to, for example, a clock signal of a predetermined frequency. In this case, the clock signal is, for example, obtained by dividing the frequency of the AC current by an integer fraction. Figure 1 shows a case where the switches are switched according to a clock signal with a frequency of fbsHz, obtained by dividing the frequency of the AC current fHz. As a result, in the fluxgate magnetic field sensor 1, an excitation current is supplied to the magnetic core WC of the sensor head unit SH using a bias switching method in which the polarity of the DC bias current component in the excitation current is periodically switched to either the positive or negative pole. Figure 1 shows a configuration in which the phase of the AC current is also switched at the same period as the polarity of the DC bias current.

[0022] The configuration of the DC superimposed AC excitation unit 10 and the excitation polarity switching unit 20 is an example of an "excitation current supply unit" in the claims. The reference signal source is an example of a "reference signal generation unit" in the claims. The phase shifter 12 is an example of a "phase shift unit" in the claims.

[0023] The voltage follower 30 performs impedance conversion on the pickup signal output by the sensor head unit SH (more specifically, the pickup signal output by the pickup coil PC provided in the sensor head unit SH). The voltage follower 30 is a buffer circuit composed of, for example, an operational amplifier. The voltage follower 30 outputs the impedance-converted pickup signal to the amplifier 40. Figure 1 shows a configuration in which the impedance-converted pickup signal output by the voltage follower 30 is transmitted to the amplifier 40 via a capacitor, so that only the AC component contained in the impedance-converted pickup signal is input to the amplifier 40.

[0024] Amplifier 40, which is output by the voltage follower 30, amplifies the AC component pickup signal input via the capacitor to a predetermined level. Amplifier 40 is, for example, a preamplifier circuit composed of an operational amplifier and multiple resistors. Amplifier 40 outputs the amplified AC component pickup signal to LPF 50.

[0025] The voltage follower 30, capacitor, and amplifier 40 are components in the fluxgate magnetic field sensor 1 for amplifying the pickup signal output by the sensor head unit SH to an appropriate level. For example, if the pickup signal output by the sensor head unit SH is at a suitable level in the fluxgate magnetic field sensor 1, the voltage follower 30, capacitor, and amplifier 40 may be omitted.

[0026] The LPF50 extracts the fundamental wave component representing the magnetic field detected by the sensor head SH from the pickup signal of the amplified AC component output by the amplifier 40. More specifically, the LPF50 extracts only the fundamental wave component by passing through signal components in the pickup signal that are below the frequency of the AC current in the excitation current, and attenuating signal components above the frequency of the AC current, i.e., harmonic components. The LPF50 may also be configured to attenuate odd-order harmonic components among the harmonic components in the pickup signal that are considered to have the greatest impact on improving the accuracy of magnetic field measurement. This is because the pickup signal and reference signal that the synchronous detector 60, which will be described later, performs synchronous detection on contain odd-order harmonic components that contribute to the formation of a square wave. Furthermore, the LPF50 may also be configured to attenuate at least the largest third-order harmonic component among the odd-order harmonic components in the pickup signal.

[0027] LPF50 comprises, for example, a passive LPF52 and an active LPF54. The passive LPF52 attenuates the first harmonic component, and the active LPF54 attenuates the second harmonic component. LPF50 is a third-order Sallen-Key low-pass filter combining the first-order passive LPF52 and the second-order active LPF54. The configuration of LPF50 is not limited to the Sallen-Key type. For example, LPF50 may be a state-variable low-pass filter. LPF50 outputs a pickup signal (fundamental pickup signal) with the fundamental wave component extracted to the synchronous detector 60.

[0028] LPF50 is an example of a "fundamental wave component extraction unit" as defined in the patent claims.

[0029] The synchronous detector 60 performs synchronous detection on the fundamental wave pickup signal output by the LPF 50 by referring to the reference signal output by the DC superimposed AC excitation unit 10. In synchronous detection in the synchronous detector 60, the fundamental wave pickup signal is multiplied by a value corresponding to the voltage value of the reference signal. As mentioned above, the reference signal that the synchronous detector 60 refers to when performing synchronous detection is a square wave signal. Therefore, in synchronous detection in the synchronous detector 60, when the voltage value of the reference signal is at a "High" level, the fundamental wave pickup signal is passed through as is with the same polarity, and when the voltage value of the reference signal is at a "Low" level, the polarity of the fundamental wave pickup signal is reversed before passing through. This is equivalent to multiplying the fundamental wave pickup signal by "+1" when the voltage value of the reference signal is at a "High" level, and multiplying the fundamental wave pickup signal by "-1" when the voltage value of the reference signal is at a "Low" level. The synchronous detector 60 outputs a detected signal to the LPF 70, in which the polarity of the fundamental wave pickup signal to be passed through is inverted according to the voltage value of the reference signal, that is, the amplitude of the fundamental wave pickup signal is rectified to the predetermined polarity side.

[0030] The synchronous detector 60 is an example of the "synchronous detection unit" in the claims.

[0031] The LPF70 averages the detection signal output by the synchronous detector 60. This allows the magnitude of the magnetic field represented in the detection signal to be detected. In the fluxgate magnetic field sensor 1, the excitation polarity switching unit 20 periodically reverses (switches) the polarity of the excitation current supplied to the sensor head unit SH. Therefore, in the pickup signal (which is also the fundamental wave pickup signal), the offset component caused by the magnetic anisotropy of the magnetic core WC for each polarity of the excitation current appears with opposite polarity, while the sensitivity component of the magnetic field in the magnetic core WC appears with the same polarity. The LPF70 then averages the detection signal, that is, by adding the detection signals, signals that do not correspond to the external magnetic field being measured are removed from the detection signal. More specifically, the waveform of the signal representing the magnitude of the magnetic field, which is expressed as a pulsating current in the detection signal, is converted into a DC waveform by smoothing by the LPF70, and the offset that appears according to the excitation current whose polarity is switched is removed by adding it. The LPF70 outputs a signal (hereinafter referred to as the "magnetic field detection signal") to the integrator 80, which is obtained by averaging (adding) the detected signals to detect the magnitude of the magnetic field represented in the detected signal (i.e., the fundamental wave pickup signal).

[0032] The integrator 80 integrates the magnetic field detection signal output by the LPF 70. The integrator 80 outputs the voltage value obtained by integrating the magnetic field detection signal to the feedback resistor 90. As a result, the magnetic field detection signal becomes a representation of the feedback deviation due to the negative feedback described later, and the integrator 80 controls the magnetic field detection signal to approach zero.

[0033] The feedback resistor 90 supplies a current corresponding to the voltage output by the integrator 80 to the pickup coil PC of the sensor head unit SH. This creates a negative feedback loop in the fluxgate magnetic field sensor 1 via the feedback resistor 90. This negative feedback causes the fluxgate magnetic field sensor 1 to generate a magnetic field in the pickup coil PC that cancels out the external magnetic field of the object being measured, and this magnetic field is applied to the magnetic core WC of the sensor head unit SH. The current flowing through this pickup coil PC corresponds to the external magnetic field of the object being measured, and the voltage value corresponding to the resistance value of the feedback resistor 90 represents the magnitude of the external magnetic field of the object being measured. This voltage value is output as the output signal of the fluxgate magnetic field sensor 1 to the OUT terminal.

[0034] The configuration of the LPF70, integrator80, and feedback resistor90 is an example of an "output section" as defined in the claims.

[0035] The integrator 80 and the feedback resistor 90 are components for forming a negative feedback loop in the fluxgate magnetic field sensor 1, that is, components for realizing a closed-loop configuration fluxgate magnetic field sensor 1. If the fluxgate magnetic field sensor 1 is in an open-loop configuration, the integrator 80 and the feedback resistor 90 may be omitted. In the open-loop configuration fluxgate magnetic field sensor 1, the magnetic field detection signal output by the LPF 70 becomes the output signal.

[0036] With this configuration, that is, by including the LPF50, the fluxgate magnetic field sensor 1 can suitably measure the magnetic field even if the harmonic components of the pickup signal output by the sensor head SH contain offset components and imbalances due to magnetic anisotropy, by adjusting the phase relationship between the pickup signal and the reference signal with the phase shifter 12.

[0037] [Effects of LPF50] Next, we will explain the effects of the fluxgate magnetic field sensor 1 being equipped with the LPF 50. Here, we will explain the effects of the fluxgate magnetic field sensor 1 being equipped with the LPF 50 by comparing the measurement results of the external magnetic field for the fluxgate magnetic field sensor 1 and a conventional fluxgate magnetic field sensor that is not equipped with the LPF 50.

[0038] First, we will describe the configuration of a conventional fluxgate magnetic field sensor (hereinafter referred to as "fluxgate magnetic field sensor 2") which will be compared with fluxgate magnetic field sensor 1. Figure 2 shows an example of the configuration of a conventional fluxgate magnetic field sensor 2. Fluxgate magnetic field sensor 2 includes, for example, a sensor head SH, a DC superimposed AC excitation unit 10, an excitation polarity switching unit 20, a voltage follower 30, an amplifier 40, a synchronous detector 60, an LPF 70, an integrator 80, and a feedback resistor 90. Fluxgate magnetic field sensor 2 is a configuration in which the LPF 50 is omitted from fluxgate magnetic field sensor 1. In the components of fluxgate magnetic field sensor 2, each component that is given the same reference numeral as the components of fluxgate magnetic field sensor 1 is a similar component, so a detailed explanation will be omitted.

[0039] In both fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2, an excitation current with periodically switched polarity is supplied to the magnetic core WC by a bias switching method, and the synchronous detector 60 synchronously detects the fundamental wave pickup signal by referring to a reference signal, which is a square wave signal. At this time, the pickup signal output by the sensor head unit SH contains not only the fundamental wave component, which is an AC current component (a component with frequency fHz), but also harmonic components that contribute to the formation of a square wave.

[0040] [Example of a pickup signal] Here, we will explain the pickup signal output by the sensor head unit SH. Figure 3 shows an example of the voltage waveform of the pickup signal. The voltage waveform shown in Figure 3 is an example of the pickup signal output by the sensor head unit SH when an excitation current is supplied to either the positive or negative electrode side, while no external magnetic field is applied. In other words, Figure 3 shows an example of the voltage waveform of the pickup signal that includes only components that do not correspond to the external magnetic field being measured, which appear due to the magnetic anisotropy of the magnetic core WC of the sensor head unit SH. The voltage waveform shown in Figure 3 includes the frequency of the fundamental wave, which is modulated with information representing the strength of the magnetic field being measured, and the second and third harmonics of this fundamental wave frequency. The pickup signal output by the sensor head unit SH also includes higher-order harmonics, but these are omitted from the illustration in Figure 3.

[0041] If, in the pickup signal, each frequency component is symmetrical on the positive and negative sides, that is, if the magnetization state in the magnetic core WC is ideally truly symmetrical with respect to the reversal of the polarity of the excitation current, then the waveform WP on the positive side of the pickup signal is expressed by equation (1) below, and the waveform WN on the negative side is expressed by equation (2) below.

[0042]

number

[0043]

number

[0044] In this case, regardless of the phase relationship between the pickup signal and the reference signal, the detected signal synchronously detected by the synchronous detector 60 will have a symmetrical rectification state for both polarities. Furthermore, in both the fluxgate magnetic field sensor 1 and the fluxgate magnetic field sensor 2, the LPF 70 averages the detected signal output by the synchronous detector 60, so that the magnetic field detection signal is always kept at zero. In other words, in both the fluxgate magnetic field sensor 1 and the fluxgate magnetic field sensor 2, a truly ideal bias switching method of processing is performed.

[0045] On the other hand, if an imbalance occurs in the polarity of either component of the pickup signal due to some factor, the magnetization state in the magnetic core WC will no longer be the ideal, truly symmetrical state with respect to the reversal of the excitation current polarity. Here, we consider the case where an imbalance occurs on the negative side of the pickup signal. That is, the waveform WP on the positive side of the pickup signal is given by equation (1) above, but the waveform WN on the negative side is not given by equation (2) above.

[0046] For example, if an imbalance occurs in the negative-side fundamental wave component included in the pickup signal, and an arbitrary amplitude A'1 and phase φ'1 are given to the fundamental wave component, the waveform WN on the negative side becomes a waveform WN1 as shown in equation (3) below.

[0047]

number

[0048] In this case, in both fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2, the phase relationship between the pickup signal and the reference signal causes the rectification state of the detected signal synchronously detected by the synchronous detector 60 to be asymmetrical for each polarity. Furthermore, in both fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2, even if the LPF 70 averages the detected signal output by the synchronous detector 60, the magnetic field detection signal does not become zero due to the phase relationship between the pickup signal and the reference signal, and appears in the output as a residual offset. At this time, if the waveform of the pickup signal changes due to the influence of temperature or other factors, the magnitude of the magnetic field detection signal will also change, and the residual offset will drift. However, it was found that if an imbalance occurs only in the fundamental wave component of the pickup signal, the change in the magnetic field detection signal according to the phase relationship between the pickup signal and the reference signal becomes sinusoidal in both fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2. Furthermore, it was found that even when the waveform of the pickup signal, predominantly the amplitude A'1, changes, the change in the magnetic field detection signal corresponding to the phase relationship with the pickup signal remains sinusoidal. Therefore, in either the fluxgate magnetic field sensor 1 or the fluxgate magnetic field sensor 2, by adjusting the phase of the reference signal with the phase shifter 12 and performing detection at the phase where the magnetic field detection signal becomes zero (the phase that is a node in the change of the sinusoidal magnetic field detection signal), even if the waveform of the pickup signal changes, the offset component and its drift can always be suppressed, and the magnetic field can be measured appropriately.

[0049] Furthermore, for example, if an imbalance occurs in the second and third harmonic components in addition to the fundamental wave component on the negative side of the pickup signal, and an arbitrary amplitude A'2 and phase φ'2 are given to the second harmonic component, and an arbitrary amplitude A'3 and phase φ'3 are given to the third harmonic component, then the waveform WN on the negative side becomes a waveform WN2 as shown in equation (4) below.

[0050]

number

[0051] In this case, it was found that in the fluxgate magnetic field sensor 2, the change in the magnetic field detection signal according to the phase relationship between the pickup signal and the reference signal does not become sinusoidal, but rather the change in the magnetic field detection signal is distorted according to the second harmonic and third harmonic components. Furthermore, when the waveform of the pickup signal, mainly amplitude A'2 and amplitude A'3, changes, the way in which the change in the magnetic field detection signal according to the phase relationship with the pickup signal is distorted will change depending on the situation. For this reason, even if the phase of the reference signal is adjusted by the phase shifter 12 in the fluxgate magnetic field sensor 2, it is not possible to maintain a state in which the magnetic field detection signal becomes zero according to the waveform of the pickup signal so that magnetic field measurement can be performed suitably. On the other hand, in the fluxgate magnetic field sensor 1, only the fundamental wave component is extracted by the LPF 50 (harmonic components are attenuated) and the fundamental wave pickup signal is output to the synchronous detector 60. For this reason, in the fluxgate magnetic field sensor 1, even if the pickup signal contains second harmonic and third harmonic components, the change in the magnetic field detection signal according to the phase relationship between the pickup signal and the reference signal becomes sinusoidal. Therefore, in the fluxgate magnetic field sensor 1, by adjusting the phase of the reference signal with the phase shifter 12, even if the waveform of the pickup signal changes, the offset component and its drift can always be suppressed, and the magnetic field can be measured appropriately.

[0052] Here, we will explain an example of how the magnetic field detection signal, i.e., the output signal, changes when there is an imbalance in the pickup signal. Figure 4 shows an example of how the output signal changes when there is an imbalance in the pickup signal. Figure 4 shows an example of the voltage value [V] of the output signal when the phase relationship between the pickup signal and the reference signal (hereinafter referred to as "detection phase") is changed in increments of 10 [deg]. Figure 4(a) shows the relationship between the detection phase and the output signal when there is an imbalance in the fundamental wave component of the pickup signal, and Figure 4(b) shows the relationship between the detection phase and the output signal when there is an imbalance in the second harmonic component and the third harmonic component of the pickup signal in addition to the fundamental wave component.

[0053] As described above, if an imbalance occurs only in the fundamental wave component of the pickup signal, the change in the output signal when the detection phase is changed will be sinusoidal, as shown in Figure 4(a), for both fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2. Therefore, by adjusting the phase of the excitation current with the phase shifter 12, the magnetic field can be measured appropriately for both fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2. For example, by adjusting the detection phase to one in which the output signal becomes zero, the magnetic field can be measured appropriately for both fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2.

[0054] On the other hand, as mentioned above, if there is an imbalance in the second and third harmonic components in addition to the fundamental wave component of the pickup signal, the output signal of the fluxgate magnetic field sensor 2 will not change sinusoidally when the detection phase is changed, as shown in Figure 4(a), but will be distorted as shown in Figure 4(b). Therefore, even if the fluxgate magnetic field sensor 2 is adjusted to a detection phase where the output signal becomes zero under certain conditions using the phase shifter 12, as the pickup waveform changes due to the influence of temperature and other factors, it will be affected by the imbalance in the second and third harmonic components of the pickup waveform, and the distortion will change moment by moment, as shown in Figure 4(b). In Figure 4(b), the phase at which the output signal crosses zero will be shifted due to the distortion, and it will not be possible to keep the output signal at zero with the adjusted phase. In other words, offset components and their drift caused by the harmonic components will remain. As a result, the measurement results of the fluxgate magnetic field sensor 2 will contain errors. This is because the fluxgate magnetic field sensor 2 performs synchronous detection with the synchronous detector 60 without considering the possibility of an imbalance in the harmonic components of the pickup signal, and without knowing that these effects would alter the distortion of the magnetic field detection signal depending on its phase relationship with the pickup signal. The offset components and their drift caused by the harmonic components remaining in the detection result become more significant as the accuracy of the magnetic field measurement increases.

[0055] In contrast, in the fluxgate magnetic field sensor 1, the LPF 50 attenuates the harmonic components contained in the pickup signal before the pickup signal is synchronously detected by the synchronous detector 60. As a result, when the detection phase is changed, the change in the output signal becomes sinusoidal, as shown in Figure 4(a). In other words, in the fluxgate magnetic field sensor 1, just as in the case where an imbalance occurs only in the fundamental wave component of the pickup signal, the magnetic field can be suitably measured by adjusting the detection phase to one where the output signal becomes zero, for example.

[0056] Incidentally, the distortion of the output signal change with respect to the detection phase, as shown in Figure 4(b), is mainly due to the third harmonic component. This can be confirmed, for example, by performing simulations in which the phase and amplitude arbitrarily assigned to the second harmonic component and the third harmonic component in equation (4) above are individually changed. For this reason, the LPF50 may be configured to attenuate at least the third harmonic component included in the pickup signal, as described above.

[0057] [Configuration for verifying the trend of changes in the output signal of a fluxgate magnetic field sensor] Next, we will explain how to check the trend of change in the output signal as shown in Figure 4 and adjust the detection phase. Figure 5 is a diagram showing an example of an inspection environment for a fluxgate magnetic field sensor (either fluxgate magnetic field sensor 1 or fluxgate magnetic field sensor 2). The inspection environment shown in Figure 5 is an environment in which the influence of an external magnetic field can be ignored (for example, a zero magnetic field space), or an environment in which a constant external magnetic field is applied to the sensor head SH. Figure 5 shows an example in which the inspection device 100 checks the trend of change in the output signal and adjusts the detection phase. Figure 5 also shows the phase shifter 12 and OUT terminal related to checking the trend of change in the output signal and adjusting the detection phase in the fluxgate magnetic field sensor 1 or fluxgate magnetic field sensor 2 under inspection.

[0058] The inspection device 100 is, for example, a computer device such as a personal computer. The inspection device 100 runs an application for inspecting the fluxgate magnetic field sensor. The application also controls an ambient temperature setting device (not shown) to change the temperature of the inspection environment when inspecting the fluxgate magnetic field sensor. The application may also display images representing the inspection status and results on a display device provided by or connected to the inspection device 100.

[0059] The configuration of the inspection device 100 is not limited to computer devices such as personal computers. For example, the inspection device 100 may be composed of dedicated hardware. Dedicated hardware, for example, implements each function by having a hardware processor execute a program (software). Hardware processor means hardware (including circuitry) such as a CPU (Central Processing Unit), GPU (Graphics Processing Unit), Large Scale Integration (LSI), Application Specific Integrated Circuit (ASIC), programmable logic device (e.g., Simple Programmable Logic Device (SPLD) or Complex Programmable Logic Device (CPLD), Field Programmable Gate Array (FPGA)). Some or all of the components of each function implemented by the dedicated hardware may be implemented by dedicated LSIs. The program may be stored (installed) in a dedicated hardware memory device, such as a ROM (Read Only Memory), RAM (Random Access Memory), flash memory, or other semiconductor memory element, or in a storage device such as a hard disk drive (HDD) (a storage device with a non-transient storage medium), or it may be directly incorporated into the hardware processor's circuitry. The program may also be transmitted (downloaded) wirelessly from a computer device or the like and installed in the storage device. The hardware processor realizes its functions by reading and executing the stored program.

[0060] [Process for verifying the trend of changes in the output signal of a fluxgate magnetic field sensor] Next, an example of the overall flow of the process in which the inspection device 100 checks the trend of change in the output signal of the fluxgate magnetic field sensor and adjusts the detection phase will be described. In the following description, it is assumed that the inspection device 100 checks the trend of change in the output signal of the fluxgate magnetic field sensor 1 and adjusts the detection phase.

[0061] Figure 6 is a flowchart showing an example of the process flow for inspecting the fluxgate magnetic field sensor 1. Figure 6 shows an example of an inspection performed within the ambient temperature range in which the fluxgate magnetic field sensor 1 is installed.

[0062] When the inspection device 100 starts processing, it changes the ambient temperature for inspecting the fluxgate magnetic field sensor 1 (step S100). At this time, the inspection device 100 instructs the ambient temperature setting device to set the ambient temperature to one of the ranges from the maximum temperature on the high-temperature side where the fluxgate magnetic field sensor 1 is installed (e.g., +70°C) to the minimum temperature on the low-temperature side (e.g., -60°C). For example, the inspection device 100 instructs the ambient temperature setting device to set the ambient temperature to be used for the initial inspection (e.g., +70°C).

[0063] After the ambient temperature reaches the ambient temperature at which the initial test will be performed, the testing device 100 sets the detection phase for the phase shifter 12 provided by the fluxgate magnetic field sensor 1 (step S102). For example, if the test is performed by changing the detection phase in increments of 10 degrees, the testing device 100 sets the detection phase to be performed first (for example, 0 degrees) in the phase shifter 12.

[0064] The inspection device 100 causes the fluxgate magnetic field sensor 1 to measure the magnetic field at a set detection phase and acquires the output signal (measurement result) output by the fluxgate magnetic field sensor 1 (step S104). The inspection device 100 stores the acquired output signal in a memory device provided by the inspection device 100, for example.

[0065] The inspection device 100 checks whether it has completed acquiring output signals within a predetermined detection phase range (step S106). For example, when testing from 0 to 360 degrees by changing the detection phase in 10-degree increments, it checks whether it has completed acquiring 36 output signals and storing them in the memory. If it is confirmed in step S106 that it has not completed acquiring output signals within the predetermined detection phase range, the inspection device 100 returns to step S102 and sets the next detection phase to be tested (for example, 10 degrees) in the phase shifter 12. The inspection device 100 then repeats the process from step S102 to step S106.

[0066] On the other hand, if step S106 confirms that the acquisition of an output signal within a predetermined detection phase range has been completed, the inspection device 100 checks whether the acquisition of an output signal within a predetermined temperature range has been completed (step S108). If step S108 confirms that the acquisition of an output signal within a predetermined temperature range has not been completed, the inspection device 100 returns to step S100 and instructs the ambient temperature setting device to set the ambient temperature to the next ambient temperature to be inspected (for example, +50[°C]) within the range of ambient temperatures in which the fluxgate magnetic field sensor 1 is installed. The inspection device 100 then repeats the process from step S100 to step S108. For example, the inspection device 100 continues to instruct the ambient temperature setting device to set the ambient temperature to +40[°C], ambient temperature: +25[°C], +10[°C], -10[°C], -50[°C], and -60[°C], and repeats the process from step S100 to step S108 for each ambient temperature.

[0067] On the other hand, in step S108, if it is confirmed that the acquisition of output signals within a predetermined temperature range has been completed, the inspection device 100 determines a detection phase in which the magnetic field measurement at the fluxgate magnetic field sensor 1 is suitably performed, based on each output signal acquired from the fluxgate magnetic field sensor 1 and stored in the memory device (step S110). Typically, the detection phase in which the difference in output signals (temperature coefficient) at each temperature is substantially minimized is determined as the suitable detection phase.

[0068] The inspection device 100 sets the determined detection phase to the phase shifter 12 (step S112).

[0069] Through this process, the inspection device 100 determines the detection phase at which magnetic field measurement is optimally performed within the ambient temperature range in which the fluxgate magnetic field sensor 1 is installed, and sets it in the phase shifter 12.

[0070] Figure 7 is a flowchart illustrating an example of another process flow for inspecting the fluxgate magnetic field sensor 1. Figure 7 shows an example of inspecting the changes over time after operating a stored fluxgate magnetic field sensor 1. Before this inspection, the fluxgate magnetic field sensor 1 is stored in the Earth's magnetic field for a certain period (e.g., several days) in an inactive state. During this storage period, the magnetization state of the magnetic core WC in the sensor head SH of the fluxgate magnetic field sensor 1 changes due to the Earth's magnetic field. As a result, the waveform of the pickup signal output by the sensor head SH of the fluxgate magnetic field sensor 1 differs from that before storage, and a tendency for time-dependent drift in the output signal can be observed depending on the elapsed time since the start of operation.

[0071] When the inspection device 100 starts processing, the flux gate magnetic field sensor 1 starts operating (step S200).

[0072] The inspection device 100 checks whether a predetermined amount of time has elapsed since the start of operation of the fluxgate magnetic field sensor 1 (step S202). For example, if inspections are to be performed at elapsed times of 2 minutes, 30 minutes, 1 hour, 2 hours, 5 hours, and 20 hours after the start of operation of the fluxgate magnetic field sensor 1, the inspection device 100 checks whether the first elapsed time (for example, 2 minutes) has elapsed. If it is confirmed in step S202 that the predetermined elapsed time has not elapsed, the inspection device 100 repeats the process in step S202 and waits for the first elapsed time (2 minutes) to elapse.

[0073] On the other hand, in step S202, if it is confirmed that a predetermined elapsed time has elapsed, the inspection device 100 sets the detection phase for the phase shifter 12 provided by the fluxgate magnetic field sensor 1 (step S204). For example, when performing inspections by changing the detection phase in increments of 10 degrees, the inspection device 100 sets the detection phase to be inspected first (for example, 0 degrees) in the phase shifter 12.

[0074] The inspection device 100 causes the fluxgate magnetic field sensor 1 to measure the magnetic field at a set detection phase and acquires the output signal (measurement result) output by the fluxgate magnetic field sensor 1 (step S206). The inspection device 100 stores the acquired output signal in a memory device provided by the inspection device 100, for example.

[0075] The inspection device 100 checks whether it has completed acquiring output signals within a predetermined detection phase range (step S208). For example, when testing from 0 to 360 degrees by changing the detection phase in 10-degree increments, it checks whether it has completed acquiring 36 output signals and storing them in the memory. If it is confirmed in step S208 that it has not completed acquiring output signals within the predetermined detection phase range, the inspection device 100 returns to step S204 and sets the next detection phase to be tested (for example, 10 degrees) in the phase shifter 12. The inspection device 100 then repeats the process from step S204 to step S208.

[0076] On the other hand, if step S208 confirms that the acquisition of the output signal within a predetermined detection phase range has been completed, the inspection device 100 checks whether the acquisition of the output signal within a predetermined elapsed time has been completed (step S210). If step S210 confirms that the acquisition of the output signal within a predetermined elapsed time has not been completed, the inspection device 100 returns to step S202 and checks whether the next elapsed time (for example, 30 minutes) has elapsed. The inspection device 100 then waits for the next elapsed time (for example, 30 minutes) to elapse. The inspection device 100 then repeats the process from step S202 to step S210 for each elapsed time.

[0077] On the other hand, in step S210, if it is confirmed that the acquisition of the output signal for a predetermined elapsed time has been completed, the inspection device 100 determines a detection phase in which the magnetic field measurement at the fluxgate magnetic field sensor 1 is suitably performed, based on each output signal acquired from the fluxgate magnetic field sensor 1 and stored in the memory device (step S212). Typically, the detection phase in which the difference in the output signal over time (change over time) is substantially minimized is determined as the suitable detection phase.

[0078] The inspection device 100 sets the determined detection phase to the phase shifter 12 (step S214).

[0079] Through this process, the inspection device 100 determines the detection phase at which magnetic field measurement is optimally performed for each elapsed time since the start of operation of the fluxgate magnetic field sensor 1, and sets it in the phase shifter 12.

[0080] In this way, the inspection device 100 can easily determine the phase relationship (detection phase) between the pickup signal and the reference signal such that the magnetic field measurement by the fluxgate magnetic field sensor 1 is performed appropriately. More specifically, the inspection device 100 can easily determine a suitable detection phase by performing a single trial accompanied by temperature changes and time elapsed, without having to go through a long process of checking the input and output of each component of the fluxgate magnetic field sensor 1 and adjusting the phase each time. Instead, it can do this by repeatedly changing the detection phase of the measurement result (output signal) of the fluxgate magnetic field sensor 1 in predetermined increments.

[0081] Next, we will describe an example of the inspection of fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2 by the inspection device 100 under the same conditions for ambient temperature and time-dependent changes. Figures 8 to 10 show examples of inspection results for fluxgate magnetic field sensors. The examples of inspection results shown in Figures 8 to 10 are examples in which the same sensor head unit SH was used for inspection of fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2 in order to clarify the effect of LPF50 in fluxgate magnetic field sensor 1. Figure 8 shows an example of the difference in inspection results between fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2 when the ambient temperature is on the normal to low temperature side, and Figure 9 shows an example of the difference in inspection results between fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2 when the ambient temperature is on the normal to high temperature side. Figure 10 shows an example of the difference in inspection results between fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2 when the inspection is performed over the same period of time. In Figures 8 to 10, (a) shows an example of the test results of fluxgate magnetic field sensor 2, and (b) shows an example of the test results of fluxgate magnetic field sensor 1. In each of Figures 8 to 10, the detection phase is changed in 10 [deg] increments from 0 to 360 [deg].

[0082] First, let's refer to Figure 8 to explain the differences in test results when the ambient temperature is low. Figure 8 summarizes the changes in the voltage value [V] of the output signal with respect to the detection phase when the ambient temperature is set to +25[°C] (room temperature), +10[°C], -10[°C], -50[°C], and -60[°C]. As can be seen in Figure 8(a), in the fluxgate magnetic field sensor 2, the change in the output signal with respect to the detection phase is distorted rather than sinusoidal at each ambient temperature. This is as explained by showing Figure 4(b). In particular, in the example shown in Figure 8(a), the distortion of the change in the output signal becomes larger as the ambient temperature decreases. This is because, in the fluxgate magnetic field sensor 2, when the ambient temperature decreases and the temperature of the sensor head SH decreases, the magnetic properties of the magnetic core WC change, and the pickup signal changes. As a result, the imbalance of harmonic components contained in the pickup signal also changes, and as a result of averaging in the bias switching method processing, the residual offset component changes. As a result, the fluxgate magnetic field sensor 2 exhibits large variations in the output signal at each ambient temperature, making it impossible to identify a point where the phases overlap. The example shown in Figure 8(a) illustrates the drift trend of the offset component of the output signal at each detection phase with respect to ambient temperature in the sensor head SH of the fluxgate magnetic field sensor 2. From the example shown in Figure 8(a), it can be understood that the fluxgate magnetic field sensor 2 is a magnetic field sensor that cannot sufficiently suppress the drift of the offset component of the output signal even when the detection phase is adjusted.

[0083] On the other hand, as can be seen in Figure 8(b), in the fluxgate magnetic field sensor 1, the change in the output signal with respect to the detection phase is sinusoidal at each ambient temperature. In particular, in the example shown in Figure 8(b), the output signal for each ambient temperature is approximately a single point value over a wide range of detection phases from 150 to 170 degrees. From this, as explained with reference to Figure 4, it can be seen that in the fluxgate magnetic field sensor 1, the LPF 50 attenuates the harmonic components contained in the pickup signal, and therefore, even when there is an imbalance in the harmonic components contained in the pickup signal, the tendency of the output signal drift with respect to ambient temperature at the sensor head SH is well reduced.

[0084] Next, with reference to Figure 9, we will explain the differences in test results when the ambient temperature is high. Figure 9 summarizes the changes in the voltage value [V] of the output signal with respect to the detection phase when the ambient temperature is set to +25[°C] (room temperature), +40[°C], +50[°C], and +70[°C]. As can be seen in Figure 9(a), in the fluxgate magnetic field sensor 2, the change in the output signal with respect to the detection phase is distorted rather than sinusoidal at each ambient temperature. Furthermore, the degree of distortion is different from that at low temperatures (see Figure 8(a)). In the example shown in Figure 9(a), the distortion of the change in the output signal increases as the ambient temperature increases. This is because, in the fluxgate magnetic field sensor 2, even when the temperature of the sensor head SH rises in accordance with the ambient temperature, the magnetic properties of the magnetic core WC change, and the unbalance of the harmonic components contained in the pickup signal changes. This change in the unbalance of harmonic components changes the residual offset component as a result of averaging in the bias switching method processing.

[0085] On the other hand, as can be seen in Figure 9(b), in the fluxgate magnetic field sensor 1, the change in the output signal with respect to the detection phase is sinusoidal at each ambient temperature, just as when the ambient temperature is on the low side. And, as in the example shown in Figure 9(b), the output signal for each ambient temperature is approximately a single point value over a wide range of detection phases from 150 to 170 degrees, just as when the ambient temperature is on the low side. From this, it can be seen that in the fluxgate magnetic field sensor 1, the effect of attenuating the harmonic components contained in the pickup signal by the LPF 50 is obtained similarly, regardless of whether the ambient temperature is on the low or high side.

[0086] Next, with reference to Figure 10, the differences in inspection results depending on the time elapsed will be explained. Figure 10 summarizes the changes in the voltage value [V] of the output signal with respect to the detection phase at elapsed times of 2 minutes, 30 minutes, 2 hours, 5 hours, and 20 hours since the start of operation of the fluxgate magnetic field sensor. As can be seen in Figure 10 (a-1), in the fluxgate magnetic field sensor 2, the change in the output signal with respect to the detection phase is distorted rather than sinusoidal at each elapsed time. In particular, in the example shown in Figure 10 (a-1), the distortion of the change in the output signal becomes larger as the elapsed time decreases. From this, it can be seen that in the fluxgate magnetic field sensor 2, the effect of imbalance caused by harmonic components contained in the pickup signal is considerably large immediately after the start of operation. Furthermore, in the example shown in Figure 10 (a-1), as the elapsed time increases, the way in which the change in the output signal with respect to the detection phase is distorted changes, and it can be seen that the imbalance caused by harmonic components changes not only with respect to ambient temperature but also with respect to the change in the magnetization state of the magnetic core WC with respect to elapsed time. As shown in the example in Figure 10 (a-1), the drift trend of the output signal at each detection phase with respect to the elapsed time from the start of operation at the sensor head SH of the fluxgate magnetic field sensor 2 can be easily observed.

[0087] Here, when attempting to determine the phase relationship (detection phase) between the pickup signal and the reference signal that allows for suitable measurement of the magnetic field in the fluxgate magnetic field sensor 2, from the examples shown in Figure 8(a), Figure 9(a), and Figure 10(a-1), a common point is selected in each example. Figure 10(a-2) shows an enlarged view of the change in the voltage value [V] of the output signal in the range of 0 to 60 [deg] in the example shown in Figure 10(a-1). In Figure 10(a-2), auxiliary lines are drawn to represent the connections between measurement points of the output signal at the same elapsed time. In the example shown in Figure 10(a-2), the difference in the measurement results of the output signal with respect to the passage of time is minimized near 30 [deg], that is, the time drift of the offset is minimized, so 30 [deg] is a practical value that can be determined as the detection phase. However, as seen in the example shown in Figure 8(a), the measurement results of the output signal for temperature changes on the low-temperature side vary around 30 degrees, so 30 degrees is not a practical value as a detection phase for temperature changes on the low-temperature side. In the example shown in Figure 8(a), the difference in the measurement results of the output signal for temperature changes on the low-temperature side becomes smaller around 40 degrees. Furthermore, as seen in the example shown in Figure 9(a), the variation in the measurement results of the output signal for temperature changes on the high-temperature side is large at both 30 degrees and 40 degrees, and these are not practical values ​​as detection phases. In the example shown in Figure 9(a), the difference in the measurement results of the output signal for temperature changes on the low-temperature side becomes smaller around 50 degrees. For this reason, it is difficult to determine a single detection phase point that satisfies both the ambient temperature and the time-dependent changes in the fluxgate magnetic field sensor 2.

[0088] On the other hand, as can be seen in Figure 10(b), in the fluxgate magnetic field sensor 1, the change in the output signal with respect to the detection phase is sinusoidal at each elapsed time. Furthermore, in the example shown in Figure 10(b), the output signal at each elapsed time is approximately a single point value over a wide range of detection phases from 120 to 160 degrees, and the offset drift is suppressed to an extremely small extent. From this, it can be seen that in the fluxgate magnetic field sensor 1, the effect of the LPF 50 attenuating the harmonic components contained in the pickup signal is obtained similarly regardless of the ambient temperature (whether it is on the low or high side) or the elapsed time since the start of operation. From this, it can be easily determined in the fluxgate magnetic field sensor 1 as the phase relationship (detection phase) between the pickup signal and the reference signal that satisfies the conditions of ambient temperature and time elapsed, so that the magnetic field measurement can be suitably performed.

[0089] Next, the noise performance of fluxgate magnetic field sensors 1 and 2 will be described. Figure 11 shows an example of the noise performance of fluxgate magnetic field sensors. Figure 11 shows the noise characteristics (noise density) for each frequency measured for fluxgate magnetic field sensors 1 and 2. As can be seen from Figure 11, the noise performance of fluxgate magnetic field sensor 1 and fluxgate magnetic field sensor 2 are similar, but at high frequencies above 1 Hz, fluxgate magnetic field sensor 1 shows less noise than fluxgate magnetic field sensor 2. One possible reason for this is that in fluxgate magnetic field sensor 1, the LPF 50 attenuates the harmonic components contained in the pickup signal, and the fluctuations (so-called jitter) of these harmonic components are also attenuated, reducing the noise in the pickup signal.

[0090] As described above, the fluxgate magnetic field sensor 1 of this embodiment is equipped with an LPF 50 between the amplifier 40 and the synchronous detector 60, which extracts the fundamental wave component (attenuates harmonic components) contained in the pickup signal output by the sensor head unit SH, and outputs the fundamental wave pickup signal to the synchronous detector 60. In the fluxgate magnetic field sensor 1 of this embodiment, the synchronous detector 60 performs synchronous detection of the fundamental wave pickup signal by referring to a square wave reference signal. As a result, the fluxgate magnetic field sensor 1 of this embodiment can measure the external magnetic field of the target while reducing the influence of harmonic components (especially odd-order harmonic components) contained in the pickup signal that cause offset components remaining in the output and their drift in the bias switching method, or more specifically, reducing the influence of the imbalance of harmonic components contained in the pickup signal for each excitation polarity of the bias switching method. As a result, the fluxgate magnetic field sensor 1 of this embodiment can measure the external magnetic field with higher accuracy. Furthermore, since the fluxgate magnetic field sensor 1 of this embodiment is equipped only with an LPF 50, which can be constructed on a relatively small scale, a compact fluxgate magnetic field sensor can be realized.

[0091] Furthermore, in the method for confirming the output change trend of the fluxgate magnetic field sensor 1 of the embodiment, the detection phase can be easily determined by acquiring the measurement result (output signal) of the fluxgate magnetic field sensor 1 in only one trial accompanied by temperature and time-dependent changes. Moreover, the method for confirming the output change trend of the fluxgate magnetic field sensor 1 of the embodiment can be performed by acquiring the measurement result (output signal) rather than checking the signals of internal components. For this reason, the method for confirming the output change trend of the fluxgate magnetic field sensor 1 of the embodiment can confirm the output change trend without requiring any special configuration.

[0092] In the fluxgate magnetic field sensor 1 of the embodiment, as described above, the operation and processing of the configuration when the order of the LPF50 is set to third order in order to appropriately attenuate up to the third harmonic component included in the pickup signal were explained. However, the pickup signal also contains higher order harmonic components such as the fourth and fifth orders, and it is also conceivable that there is a sensor head SH that is more greatly affected by these harmonics. For this reason, in order to configure the fluxgate magnetic field sensor 1 to be less affected by the imbalance of harmonic components included in the pickup signal, the order of the LPF50 should be increased.

[0093] In the fluxgate magnetic field sensor 1 of the embodiment, as described above, the operation and processing of a configuration that includes a low-pass filter as a component for attenuating harmonic components included in the pickup signal have been explained. However, the component for attenuating harmonic components included in the pickup signal is not limited to a low-pass filter. For example, a band-pass filter may be provided instead of a low-pass filter. In this case, the operation and processing of the fluxgate magnetic field sensor should be equivalent to the operation and processing of the fluxgate magnetic field sensor 1 of the embodiment.

[0094] In the embodiment, a fluxgate magnetic field sensor 1 was described in which a synchronous detector 60 performs synchronous detection on a fundamental wave pickup signal. However, detection in a fluxgate magnetic field sensor is not limited to synchronous detection. For example, a configuration may be used that performs peak detection using a diode or the like, or detection that extracts a value at a predetermined timing in the pickup signal using a sample-and-hold circuit. In these cases as well, detection on the pickup signal can be considered to be substantially equivalent to multiplying a rectangular wave-shaped switching pulse signal with an extremely small duty cycle as a reference signal. In this case as well, by performing detection on a fundamental wave pickup signal with attenuated harmonic components, the same effect as the fluxgate magnetic field sensor 1 of the embodiment can be obtained. The operation and processing in this case should also be equivalent to the operation and processing in the fluxgate magnetic field sensor 1 of the embodiment, so a detailed explanation of the operation and processing will be omitted.

[0095] As described above, in the fluxgate magnetic field sensor 1 of the embodiment, in a fundamental wave type orthogonal fluxgate magnetic field sensor, the LPF 50 extracts the fundamental wave component by attenuating the harmonic components contained in the pickup signal output by the sensor head unit SH before synchronous detection of the pickup signal. Then, in the fluxgate magnetic field sensor 1 of the embodiment, the synchronous detector 60 performs synchronous detection on the fundamental wave pickup signal from which the fundamental wave component has been extracted. As a result, the fluxgate magnetic field sensor 1 of the embodiment can eliminate (reduce) the influence of the imbalance of harmonic components that may be contained in the pickup signal for each excitation polarity of the bias switching method on the measurement result of the external magnetic field to be measured, that is, the offset component remaining in the output and its drift, which would reduce the accuracy of the measurement result. As a result, the fluxgate magnetic field sensor 1 of the embodiment can perform highly accurate measurement of magnetic fields.

[0096] Although embodiments for carrying out the present invention have been described above using examples, the present invention is not limited in any way to these embodiments, and various modifications and substitutions can be made without departing from the spirit of the present invention. [Explanation of Symbols]

[0097] 1. Fluxgate magnetic field sensor 10. DC superimposed AC excitation section 12...phase shifter 20. Excitation polarity switching section 22... Switching section 30...Voltage Follower 40... Amplifier 50° LPF 52... Passive LPF 54. Active LPF 60. Synchronous detector 70···LPF 80...integrator 90···Feedback resistor 100... Inspection device SH... Sensor head unit WC... Magnetic core PC pickup coil

Claims

1. A sensor unit comprising: a magnetic core made of an elongated magnetic wire that is magnetized by an excitation magnetic field corresponding to at least a supplied excitation current and an external magnetic field of the object to be measured; and a pickup coil wound around the magnetic core and outputting a pickup signal corresponding to the magnetic field of the magnetized magnetic core; An excitation current supply unit supplies the excitation current to the magnetic core, which is obtained by superimposing an alternating current on a DC bias current whose polarity is reversed at a predetermined period. A fundamental wave component extraction unit has a low-pass filter that outputs a fundamental wave pickup signal in which a fundamental wave component representing the same period as the AC current is extracted by attenuating unbalanced harmonic components included in the pickup signal due to the polarity reversal of the excitation current, A reference signal generation unit that generates a square wave reference signal with the same frequency as the AC current, A phase shift unit adjusts the phase relationship between the fundamental wave pickup signal and the reference signal based on a phase shift amount set to adjust the phase relationship between the fundamental wave pickup signal and the reference signal. A synchronous detection unit outputs a detected signal obtained by synchronously detecting the fundamental wave pickup signal with reference to the aforementioned reference signal, An output unit that outputs an output signal corresponding to the detection signal, A fluxgate magnetic field sensor equipped with the following features.

2. The aforementioned harmonic component is at least a third harmonic component. The fluxgate magnetic field sensor according to claim 1.

3. The aforementioned alternating current is obtained by reversing the polarity of the DC bias current, with the same period as the DC bias current. The fluxgate magnetic field sensor according to claim 1 or claim 2.

4. The phase shift unit adjusts the phase relationship between the fundamental wave pickup signal and the reference signal based on the phase shift amount, which is set so that the waveform obtained with positive polarity corresponding to the reversal period of the DC bias current and the waveform obtained with negative polarity corresponding to the fundamental wave pickup signal are synchronously detected in a balanced ratio. The fluxgate magnetic field sensor according to claim 3.

5. The phase shift unit adjusts the phase relationship between the fundamental wave pickup signal and the reference signal based on the phase shift amount, which is set so that the temperature coefficient of the output signal is substantially minimized. A fluxgate magnetic field sensor according to any one of claims 1 to 4.

6. The phase shift unit adjusts the phase relationship between the fundamental wave pickup signal and the reference signal based on the phase shift amount, which is set so that the change in the output signal over time is substantially minimized. A fluxgate magnetic field sensor according to any one of claims 1 to 5.

7. A sensor unit comprising a magnetic core made of an elongated magnetic wire that is magnetized by an excitation magnetic field corresponding to at least a supplied excitation current and an external magnetic field of the object to be measured, and a pickup coil wound around the magnetic core that outputs a pickup signal corresponding to the magnetic field of the magnetized magnetic core, An excitation current supply unit supplies the excitation current to the magnetic core, which is obtained by superimposing an alternating current on a DC bias current whose polarity is reversed at a predetermined period. A fundamental wave component extraction unit has a low-pass filter that outputs a fundamental wave pickup signal in which a fundamental wave component representing the same period as the AC current is extracted by attenuating unbalanced harmonic components included in the pickup signal due to the polarity reversal of the excitation current, A reference signal generation unit that generates a square wave reference signal with the same frequency as the AC current, A phase shift unit adjusts the phase relationship between the fundamental wave pickup signal and the reference signal based on a phase shift amount set to adjust the phase relationship between the fundamental wave pickup signal and the reference signal. A synchronous detection unit outputs a detected signal obtained by synchronously detecting the fundamental wave pickup signal with reference to the aforementioned reference signal, An output unit that outputs an output signal corresponding to the detection signal, A method for confirming the trend of change in the output of a fluxgate magnetic field sensor equipped with the following: Computers By changing the phase shift amount, Based on the changes in the output signal output for each of the changed phase shift amounts, the trend of change is confirmed. A method for confirming the trend of output changes in a fluxgate magnetic field sensor.

8. The temperature of the environment used to observe the aforementioned trend is changed within a predetermined temperature range. The aforementioned computer, The phase shift amount that minimizes the difference in the change trend of the output signal observed in each of the aforementioned temperature ranges is set in the phase shift unit as the phase shift amount representing the phase relationship of the reference signal synchronously detected by the synchronous detection unit. A method for confirming the trend of change in the output of a fluxgate magnetic field sensor according to claim 7.

9. When checking the aforementioned change trend according to the elapsed time since the start of operation, The aforementioned computer, The phase shift amount that minimizes the difference in the change trend of the output signal observed at each of the aforementioned elapsed times is set in the phase shift unit as the phase shift amount representing the phase relationship of the reference signal synchronously detected by the synchronous detection unit. A method for confirming the trend of change in the output of a fluxgate magnetic field sensor according to claim 7.

Images