Magnetic detector

Abstract

A magnetic detector (1, 2) is provided with a magneto-impedance sensor element (10), sample-and-hold circuits (22, 32), amplification circuits (24, 33), and post-stage high-pass filters (25, 35), and wherein the magneto-impedance sensor element (10) is provided with: a magnetosensitive body (11) that generates a magnetization change in accordance with the intensity of a magnetic field when an excitation current (IIN) is supplied; and a detection coil (12) which is wound around the magnetosensitive body (11) and outputs an induced voltage generated by a magnetization change of the magnetosensitive body (11), sample-and-hold circuits (22, 32) which are connected to one end of the detection coil (12) and which hold an output voltage of the detection coil (12) when the sampling switch is turned on, and amplifier circuits (24, 33) which are connected to the output sides of the sample-and-hold circuits (22, 32) and which are connected to the other end of the detection coil (12). The rear-stage high-pass filters (25, 35) are connected to the output sides of the amplifier circuits (24, 33).

Description

Technical Field

[0001] The present invention relates to magnetic detectors. Background Art

[0002] Patent documents 1 and 2 disclose a magnetic detector that uses an amorphous magnetic wire as a magnetically sensitive body and a detection coil to detect the intensity of the magnetic field of the detection object. The amorphous magnetic wire has the property of generating a magneto-impedance effect (MI effect). That is, the amorphous magnetic wire has the property of generating a magnetization change corresponding to the intensity of the magnetic field acting on the amorphous magnetic wire when an excitation current is supplied. In more detail, the magnetic permeability of the amorphous magnetic wire in the circumferential direction changes corresponding to the intensity of the magnetic field acting on it, thereby changing the impedance. A detection coil is wound around the amorphous magnetic wire, and the detection coil outputs an induced voltage generated by the magnetization change of the amorphous magnetic wire.

[0003] The excitation current supplied to the amorphous magnetic wire is, for example, a pulse current or a high-frequency current. If a pulse current or a high-frequency current is supplied to the amorphous magnetic wire, a magnetization change corresponding to the strength of the applied magnetic field occurs in the amorphous magnetic wire when the current increases. At this time, an induced voltage caused by the magnetization change in the amorphous magnetic wire is generated in the detection coil.

[0004] In order to detect the induced voltage generated in the detection coil, a detection circuit is connected to the detection coil. Patent document 1 discloses a structure in which the detection circuit includes a sampling and holding circuit and an amplifier circuit. The sampling and holding circuit includes a sampling switch that performs a conduction operation corresponding to the timing of the supply of the excitation current, and a holding capacitor for holding the output of the sampling switch when the sampling switch is turned on. The amplifier circuit is connected to the output side of the sampling and holding circuit.

[0005] Patent document 2 discloses a structure in which a detection circuit includes a sampling and holding circuit, an amplifier circuit, and a feedback circuit. The feedback circuit is configured by connecting the output terminal of the amplifier circuit to one end of the detection coil and providing magnetic negative feedback. Therefore, when the DC component of the magnetic field acts on the amorphous magnetic wire, a feedback current flows through the detection coil, which can generate a feedback magnetic field that can offset the DC component of the magnetic field.

[0006] Prior Art Literature Patent Literature Patent Document 1: Japanese Patent Application Publication No. 2000-258517 Patent Document 2: Japanese Patent No. 5924503 Summary of the invention

[0007] Technical problem to be solved by the invention There is a demand for further improvement in magnetic detection performance, especially in large DC magnetic fields (e.g. hundreds of meters) such as the Earth's magnetic field and materials with strong magnetism.μ T) to detect tiny AC magnetic fields (e.g., several nT).

[0008] Here, it is found that the output value of the detection circuit disclosed in Patent Documents 1 and 2 varies according to the magnitude of the DC magnetic field. There is a tendency that the larger the magnitude of the DC magnetic field, the larger the output value of the detection circuit. Therefore, it is expected that the output value of the detection circuit is not easily affected by the DC magnetic field.

[0009] The present invention has been made in view of the above background, and an object of the present invention is to provide a magnetic sensor capable of improving magnetic detection performance by making an output value of a detection circuit less susceptible to the influence of a DC magnetic field.

[0010] Technical solutions for solving technical problems One embodiment of the present invention is a magnetic detector comprising: a magneto-impedance sensor element having a magnetically sensitive body and a detection coil, wherein the magnetically sensitive body undergoes a magnetization change corresponding to the intensity of a magnetic field when an excitation current is supplied thereto, and the detection coil is wound around the magnetically sensitive body and outputs an induced voltage generated by the magnetization change of the magnetically sensitive body; A sampling and holding circuit connected to one end of the detection coil to hold the output voltage of the detection coil when the sampling switch is turned on; an amplifier circuit connected to the output side of the sample-and-hold circuit; and A post-stage high-pass filter is connected to the output side of the amplifier circuit.

[0011] Effects of the Invention According to the magnetic detector, even if the offset component affected by the DC magnetic field is included in the output of the amplifier circuit, the offset component can be removed by the post-stage high-pass filter. Therefore, the output value of the detection circuit can not include the offset component affected by the DC magnetic field. As a result, the detection circuit is not easily affected by the DC magnetic field and can detect a tiny AC magnetic field with high accuracy.

[0012] As described above, according to the above aspect, it is possible to provide a magnetic detector capable of improving magnetic detection performance by making the output value of the detection circuit less susceptible to the influence of the DC magnetic field. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a diagram showing the structure of the magnetic sensor in the first embodiment.

[0014] exist Figure 2 middle, Figure 2 (a) is a diagram showing an ideal behavior in a configuration in which the magnetic sensor according to the first embodiment does not include a subsequent high-pass filter. Figure 2(b) is a diagram showing actual behavior in a configuration in which the magnetic sensor according to Embodiment 1 does not include a subsequent high-pass filter. Figure 2 (c) is a diagram showing the behavior of the magnetic sensor in the first embodiment.

[0015] Figure 3 This is a diagram for explaining the operation of the sample-and-hold circuit.

[0016] Figure 4 Involved Figure 2 The output values ​​of the detection circuits in (b) and (c) are graphs showing the relationship with the DC magnetic field.

[0017] Figure 5 This is a diagram showing the structure of a magnetic sensor in the second embodiment.

[0018] exist Figure 6 middle, Figure 6 (a) is a diagram showing an ideal behavior in a configuration in which the magnetic sensor according to the second embodiment does not include a subsequent high-pass filter. Figure 6 (b) is a diagram showing actual behavior in a configuration in which the magnetic sensor according to the second embodiment does not include a subsequent high-pass filter. Figure 6 (c) is a diagram showing the behavior of the magnetic sensor in the second embodiment.

[0019] Figure 7 Involved Figure 6 The output values ​​of the detection circuits in (b) and (c) are graphs showing the relationship with the DC magnetic field.

[0020] Figure 8 This is a diagram showing the structure of a magnetic detector in a first modified form of the first embodiment.

[0021] Fig. 9 This is a diagram showing the structure of a magnetic detector in a second modified form of the first embodiment.

[0022] Fig.10 This is a diagram showing the structure of a magnetic detector in a third modified form of the first embodiment. DETAILED DESCRIPTION

[0023] Magnetic detectors can be used in various applications as long as the purpose is to detect the strength of a magnetic field. For example, magnetic detectors can be used in electronic compasses, foreign object detection sensors, magnetic positioning systems, etc.

[0024] In the magnetic detector, the magnetically sensitive body constituting the magneto-impedance sensor element can use a magnetic wire, in particular, an amorphous magnetic wire. It should be noted that as long as the magnetically sensitive body has the property of producing a magnetization change corresponding to the intensity of the magnetic field when an excitation current is supplied, materials other than amorphous magnetic wire can also be used. In addition, the excitation current supplied to the magnetically sensitive body can be a periodic current such as a pulse current or a high-frequency current.

[0025] As a first aspect, the magnetic detector may further include a front-stage high-pass filter connected to an output side of the sample-and-hold circuit, and the amplifier circuit may be connected to an output side of the front-stage high-pass filter.

[0026] In this case, the magnetic detector is configured to include a magneto-impedance sensor element, a sampling and holding circuit, a front-stage high-pass filter, an amplifier circuit, and a rear-stage high-pass filter. In the past, it was believed that all the influencing components of the DC magnetic field could be removed by having a front-stage high-pass filter. However, it is known that even through the front-stage high-pass filter, all the influencing components of the DC magnetic field cannot be removed by the front-stage high-pass filter, and the influencing components of the DC magnetic field are included in the components that have passed through the front-stage high-pass filter. However, the influencing components of the DC magnetic field included in the components that have passed through the front-stage high-pass filter can be removed by the rear-stage high-pass filter. Therefore, it is possible to achieve a state in which the influencing components of the DC magnetic field are not included in the detection value of the detection circuit.

[0027] In particular, the sample-and-hold circuit can hold the output voltage variation of the detection coil during the conduction action of the sampling switch, the amplifier circuit integrates and amplifies the output of the front-stage high-pass filter, and converts the output voltage variation into an offset component through the integration process, and the rear-stage high-pass filter removes the offset component.

[0028] Due to the relationship between the conduction of the sampling switch and the output of the detection coil, the component passing through the front-stage high-pass filter contains the influence component (offset component) of the DC magnetic field. Even with such a structure, the offset component can be reliably removed by the rear-stage high-pass filter.

[0029] In addition, the sample-and-hold circuit can also maintain the output voltage variation of at least one of the acquisition time and the adjustment time according to the on / off action of the sampling switch, the amplifier circuit integrates and amplifies the output of the front-stage high-pass filter, and converts the output voltage variation into an offset component through the integration process, and the rear-stage high-pass filter removes the offset component.

[0030] Due to the on / off operation of the sampling switch, the component passing through the front-stage high-pass filter contains the influence component (offset component) of the DC magnetic field. Even with such a structure, the offset component can be reliably removed by the rear-stage high-pass filter.

[0031] Furthermore, in the magnetic detector according to the first aspect, the output side of the amplifier circuit is not connected to one end of the detection coil, and no feedback circuit is provided.

[0032] As a second embodiment, the magnetic detector may further include a feedback circuit that connects the output side of the amplifier circuit to one end of the detection coil and is composed of a low-pass filter. In this case, the magnetic detector includes a magneto-impedance sensor element, a sampling and holding circuit, an amplifier circuit, a feedback circuit, and a post-stage high-pass filter.

[0033] According to this structure, for example, when a small AC magnetic field is to be detected under a large DC magnetic field, the magneto-impedance sensor element is not easily affected by the DC magnetic field and can output an induced voltage under the influence of the small AC magnetic field. Therefore, the detection range of the magneto-impedance sensor element can be effectively utilized.

[0034] In particular, in this structure, the feedback circuit uses the feedback magnetic field generated by the feedback current of a given frequency band passing through the low-pass filter to cancel a part of the external magnetic field applied to the magneto-impedance sensor element, and causes the sample-and-hold circuit to hold the sum voltage obtained by adding the induced voltage to the feedback voltage generated by the feedback current. Therefore, the post-stage high-pass filter can remove the component of the feedback voltage generated by the feedback current and included in the output of the amplifier circuit.

[0035] In addition, in the magnetic detector of the first mode and the second mode described above, the post-stage high-pass filter can include any one of the following: a circuit including at least two elements of LCR; a high-pass filter circuit having an operational amplifier; an operation processing unit that performs digital signal processing after digital conversion; a processing unit that subtracts a component of a given frequency band obtained by passing through a low-pass filter relative to the output of the amplifier circuit from the output of the amplifier circuit; and a reference voltage correction unit that provides the DC component in the output of the amplifier circuit as a reference voltage to the amplifier circuit. In either structure, the influence of the DC magnetic field can be suppressed. It should be noted that the circuit composed of at least two elements of LCR includes, for example, a CR circuit, an RL circuit, an LC circuit, an RLC circuit, etc.

[0036] (Implementation Method 1) 1-1. Structure of Magnetic Detector 1 Reference Figure 1 The structure of the magnetic detector 1 in the first embodiment will be described. Figure 1 As shown, the magnetic detector 1 of this embodiment includes a magneto-impedance sensor element 10 (hereinafter referred to as “MI sensor element”) and a detection circuit 20 .

[0037] The MI sensor element 10 is configured to output a magnetic field B corresponding to the magnetic field B acting on the MI sensor element 10. IN The MI sensor element 10 includes a magnetically sensitive body 11 and a detection coil 12.

[0038] The magnetic induction body 11 is supplied with an excitation current I IN When the magnetic field B acting on the magnetic sensitive body 11 IN Specifically, the magnetization changes according to the strength of the magnetosensitive body 11. IN When the pulse current or high frequency current is applied, the magnetic permeability in the circumferential direction is related to the magnetic field B. IN The strength of the magnetic field B changes accordingly, and thus the impedance changes. IN When the excitation current I is supplied to the magnetically sensitive body 11, IN , the magnetization of the magnetic induction body 11 changes. The magnetic induction body 11 can be made of, for example, magnetic wire, especially amorphous magnetic wire.

[0039] The detection coil 12 is wound around the magnetically sensitive body 11. Magnetic field B IN Acting on the magnetic induction body 11, if the excitation current I IN , an induced voltage is generated in the detection coil 12 due to the magnetization change of the magnetic sensitive body 11. Therefore, the detection coil 12 outputs the induced voltage generated by the magnetization change of the magnetic sensitive body 11.

[0040] The detection circuit 20 is electrically connected to the MI sensor element 10 and is configured to detect the magnetic field B acting on the magnetic sensitive body 11. IN The detection circuit 20 includes an excitation circuit 21 and a sampling and holding circuit 22 ( Figure 1 "S / H"), front stage high pass filter 23 ( Figure 1 "HPF1"), preamplifier 24, post-stage high-pass filter 25 ( Figure 1 “HPF2” in the figure), a buffer amplifier 26 and a reference voltage generating amplifier 27.

[0041] The exciting circuit 21 supplies an exciting current I to the magnetically sensitive body 11. IN The excitation circuit 21 and the excitation current I INThe sampling and holding circuit 22 is configured to be connected to one end of the detection coil 12 and to include a sampling switch and a holding capacitor. The sampling switch of the sampling and holding circuit 22 performs an on / off operation based on the signal S1 output from the excitation circuit 21. The sampling and holding circuit 22 holds the output voltage of the detection coil 12 when the sampling switch performs an on operation.

[0042] The front-stage high-pass filter 23 is connected to the output side of the sample-and-hold circuit 22 to extract a signal of a frequency band higher than a given frequency band. For example, the front-stage high-pass filter 23 can be applied to a well-known high-pass filter circuit such as a circuit composed of at least two elements of LCR, a circuit using an operational amplifier such as a differential circuit, etc. In addition to the above-mentioned circuit, the front-stage high-pass filter 23 can also be applied to other structures as long as it can play the intended function.

[0043] The preamplifier 24 is an amplifier circuit connected to the output side of the pre-stage high-pass filter 23. The preamplifier 24 integrates the output of the pre-stage high-pass filter 23 and performs amplification processing.

[0044] The post-stage high-pass filter 25 is connected to the output side of the preamplifier 24 to extract a signal of a frequency band higher than a given frequency band. For example, the post-stage high-pass filter 25 can apply a well-known high-pass filter circuit such as a circuit composed of at least two elements of LCR, a circuit using an operational amplifier such as a differential circuit, etc. In addition to the above-mentioned circuit, the post-stage high-pass filter 25 can also apply other structures as long as it can play the intended function.

[0045] The buffer amplifier 26 is an amplifier circuit connected to the output side of the post-stage high-pass filter 25. The buffer amplifier 26 amplifies the output of the post-stage high-pass filter 25. The output of the buffer amplifier 26 becomes the output of the detection circuit 20. The reference voltage generation amplifier 27 is a circuit that generates a reference voltage for the MI sensor element 10, the sample-and-hold circuit 22, the pre-stage high-pass filter 23, the preamplifier 24, the post-stage high-pass filter 25, and the buffer amplifier 26. For example, when the output voltage of the detection circuit 20 is in the range of 0V to 5V, the reference voltage is set to 2.5V, which is the central value.

[0046] Here, the magnetic detector 1 of this embodiment is configured such that the output side of the preamplifier 24 is not connected to one end of the detection coil 12 and does not include a feedback circuit.

[0047] 1-2. Operation of the Magnetic Detector 1 Next, refer to Figure 2~Figure 4 The operation of the magnetic detector 1 of this embodiment will be described. However, in order to describe the operation of the magnetic detector 1 of this embodiment, for comparison, reference is made to a magnetic detector 1 without Figure 1 Specifically, as a reference, use Figure 2 (a) is used to explain the ideal behavior of the output of each part in a structure without the post-stage high-pass filter 25. Figure 2 (b) is used to explain the actual behavior of the output of each part in a structure without the post-stage high-pass filter 25. Then, using Figure 2 (c) describes the output behavior of each part in this method. Figure 2 In (a), (b), and (c), B IN 、S1~S7 indicates Figure 1 The output of each part is shown.

[0048] 1-2-1. Figure 2 Description of (a) The ideal behavior in the configuration without the post-stage high-pass filter 25 in the magnetic detector 1 will be described. Figure 2 As shown in the first section of (a), the magnetic field B IN , assume that an arbitrary magnetic field is applied from the zero state. Figure 2 In the first section of (a), as the magnetic field B IN , for example, a state in which a given DC magnetic field is applied. Figure 2 As shown in the second section of (a), the excitation circuit 21 periodically outputs a signal S1 for turning on the sampling switch of the sample-hold circuit 22. When the signal S1 is an on signal, the sampling switch is turned on, and when the signal S1 is an off signal, the sampling switch is turned off. The signal S1 of the excitation circuit 21 and the excitation current I supplied to the magnetic inductor 11 are IN synchronous.

[0049] When supplying excitation current I IN timing, due to the excitation current I IN The increase in the magnetic field B acting on the magnetic sensitive body 11 IN The magnetization of the magnetically sensitive body 11 changes in accordance with the strength of the magnetically sensitive body 11. Due to the magnetization change of the magnetically sensitive body 11, an induced voltage is generated in the detection coil 12. The input S2 of the sampling and holding circuit 22 (equivalent to the output of the detection coil 12) becomes Figure 2 The behavior shown in the third paragraph of (a).

[0050] Then, the output S3 of the sample-and-hold circuit 22 becomes Figure 2 The behavior shown in the fourth paragraph of (a) is ideal. That is, when a magnetic field B is applied INAfterwards, when the sampling switch is turned on for the first time, the output S3 of the sampling and holding circuit 22 rises in accordance with the rise of the input S2 of the sampling and holding circuit 22. Afterwards, the output S3 of the sampling and holding circuit 22 rises in the magnetic field B. IN The value is maintained constant during the constant DC magnetic field.

[0051] Then, the output S4 of the preceding high-pass filter 23 extracts the high-frequency component of the output S3 of the sampling and holding circuit 22, and thus becomes Figure 2 That is, when the output S3 of the sample-hold circuit 22 rises, the output S4 of the front-stage high-pass filter 23 also rises. After that, after the output S3 of the sample-hold circuit 22 becomes a constant value, the output S4 of the front-stage high-pass filter 23 gradually removes the low-frequency component and becomes the reference voltage after a period of time.

[0052] The preamplifier 24 integrates the output S4 of the pre-stage high-pass filter 23 and performs amplification. Figure 2 In the case of the behavior shown in the fifth paragraph of (a), the output S5 of the preamplifier 24 is as follows Figure 2 As shown in the sixth paragraph of (a), the output S4 of the pre-stage high-pass filter 23 behaves the same as that of the output S5 of the preamplifier 24. Then, the output S5 of the preamplifier 24 is input to the buffer amplifier 26, so the output S7 of the buffer amplifier 26 is as follows: Figure 2 As shown in the seventh section of (a), the output S5 of the preamplifier 24 is amplified.

[0053] 1-2-2. Figure 2 Description of (b) Next, the actual behavior in the configuration in which the magnetic detector 1 does not include the post-stage high-pass filter 25 will be described. Figure 2 As shown in the first to third paragraphs of (b), the magnetic field B IN , the signal S1 from the excitation circuit 21 and the input S2 of the sampling and holding circuit 22 and Figure 2 The same as (a).

[0054] The output S3 of the sample and hold circuit 22 does not become Figure 2 The conduct indicated in the fourth paragraph of (a) becomes Figure 2 The first reason is that the sampling and holding circuit 22 holds the output voltage variation of the detection coil 12 during the on-action of the sampling switch. The second reason is that the sampling and holding circuit 22 holds the output voltage variation during at least one of the acquisition time and the adjustment time based on the on / off action of the sampling switch.

[0055] For these reasons, see Figure 3 For detailed description. Figure 3 As shown in the upper part of FIG. 1 , the sampling switch is assumed to be turned on / off based on the signal S1 output from the excitation circuit 21. At this time, the output of the detection coil 12, that is, the input S2 of the sampling and holding circuit 22, is displayed. Figure 3 The behavior is shown by the dotted line in the middle section.

[0056] Therefore, in terms of the characteristics of the sample-and-hold circuit 22, as shown in the interval T1, the sample-and-hold circuit 22 has a time from when the sampling switch is just turned on (just after entering the sampling mode) to when the output voltage of the detection coil 12 (the input S2 of the sample-and-hold circuit 22) starts to be tracked, that is, the acquisition time. That is, in the interval T1, the output S3 of the sample-and-hold circuit 22 generates an output voltage variation for approaching the input S2 of the sample-and-hold circuit 22.

[0057] Next, as shown in the interval T2, the sampling and holding circuit 22 tracks the output voltage of the detection coil 12 (the input S2 of the sampling and holding circuit 22) during the remaining period when the sampling switch is turned on. Since the output voltage of the detection coil 12 fluctuates, the output S3 of the sampling and holding circuit 22 fluctuates in accordance with the output voltage fluctuation of the detection coil 12 in the interval T2.

[0058] Next, in the interval T3 , the sample-and-hold circuit 22 has a delay time, namely, an aperture time, from immediately after the sampling switch performs an opening operation to when tracking is completed and holding is started.

[0059] Next, in the interval T4, when the hold mode is entered, there is an adjustment time (setting time) from the time of entering the hold mode to the time of stabilizing within the error range of the hold value. That is, in the interval T4, the output S3 of the sample and hold circuit 22 produces an output voltage fluctuation. After the interval T4, the output S3 of the sample and hold circuit 22 holds the hold value until the next sampling mode.

[0060] In short, if Figure 3 As shown in the middle section of FIG. 1 , the output S3 of the sampling and holding circuit 22 changes in order to maintain the output voltage of the detection coil 12 in the interval T2 during the on-action of the sampling switch. Furthermore, the output S3 of the sampling and holding circuit 22 changes according to at least one of the interval T1 as the acquisition time and the interval T4 as the adjustment time by the on / off action of the sampling switch.

[0061] return Figure 2 Based on the above reasons, if Figure 2 As shown in the fourth paragraph of (b), even if the magnetic field B INThe output S3 of the sampling and holding circuit 22 is not constant but fluctuates. In particular, the magnitude of the fluctuation of the output S3 of the sampling and holding circuit 22 depends on the magnetic field B. IN size.

[0062] The output S3 of the sample-and-hold circuit 22 is input to the pre-stage high-pass filter 23. Figure 2 The fourth paragraph of (b) and Figure 3 As shown in the middle section of FIG. 1 , the output S3 of the sampling and holding circuit 22 changes during the conduction operation of the sampling switch and in the vicinity thereof. Thus, when the output S3 of the sampling and holding circuit 22 changes, the front stage high pass filter 23 extracts the high frequency component of the output S3 of the sampling and holding circuit 22, thereby becoming Figure 2 The fifth paragraph of (b) and Figure 3 Behavior as shown in the following paragraph.

[0063] That is, Figure 2 The fifth paragraph of (b) and Figure 3 As shown in the lower part of FIG, the output S4 of the front stage high pass filter 23 is consistent with the reference voltage (the potential shown by the horizontal thin dotted line) before the sampling switch is switched to the on action, but changes immediately after the sampling switch is switched to the on action, and then returns to the reference voltage. Figure 2 The fifth paragraph of (b) and Figure 3 The lower section of FIG. 1 shows that when the sampling switch is switched to the on state, the output S4 of the preceding high-pass filter 23 changes in the negative direction, but there is also a case where it changes in the positive direction.

[0064] Then, after a given time from the sampling switch switching to the off operation, the output S4 of the front stage high pass filter 23 coincides with the reference voltage again. Thus, the output S4 of the front stage high pass filter 23 becomes a transient waveform that is positively and negatively asymmetrical with respect to the reference voltage during and near the on operation of the sampling switch.

[0065] That is, after the time has passed, regarding the output S4 of the front-stage high-pass filter 23, although the unchanged portion of the output S3 of the sample-and-hold circuit 22 is consistent with the reference voltage, the changed portion of the output S3 of the sample-and-hold circuit 22 still maintains the changed state. In particular, the output S4 of the sample-and-hold circuit 22 becomes a transient waveform that is positively and negatively asymmetrical with respect to the reference voltage at the changed portion.

[0066] The preamplifier 24 integrates the output S4 of the pre-stage high-pass filter 23 and performs amplification. Figure 2 The fifth paragraph of (b) and Figure 3 When the reference voltage shown in the lower section of FIG. 1 becomes asymmetric in positive and negative directions, the output S5 of the preamplifier 24 is as follows: Figure 2 As shown in the sixth section of (b), an offset component with respect to the reference voltage is generated after a period of time has passed.

[0067] That is, the preamplifier 24 converts the output voltage variation of the detection coil 12 during the on-operation of the sampling switch and the output voltage variation during at least one of the acquisition time and the adjustment time based on the on / off operation of the sampling switch into an offset component with respect to the reference voltage through integration processing.

[0068] Furthermore, the output S5 of the preamplifier 24 is input to the buffer amplifier 26, so the output S7 of the buffer amplifier 26 is as follows: Figure 2 As shown in the seventh stage of (b), the output S5 of the preamplifier 24 is amplified. Therefore, the output S7 of the buffer amplifier 26 remains an offset component with respect to the reference voltage after a certain period of time.

[0069] Here, if Figure 2 As shown in (b), the output S7 of the buffer amplifier 26 for the DC magnetic field, that is, the output of the detection circuit 20 becomes Figure 4 That is, the output S7 of the buffer amplifier 26 becomes an offset different from the reference voltage, 2.5 V, depending on the magnitude of the DC magnetic field. Although it is not a linear relationship, it generally has a relationship that the larger the DC magnetic field, the larger the offset component of the output S7 of the buffer amplifier 26.

[0070] 1-2-3. Figure 2 Description of (c) Next, the behavior of each part in the magnetic detector 1 of this embodiment will be described. Figure 2 As shown in the first to sixth paragraphs of (c), the magnetic field B IN , a signal S1 from the excitation circuit 21, an input S2 of the sampling and holding circuit 22, an output S3 of the sampling and holding circuit 22, an output S4 of the front stage high pass filter 23, an output S5 of the preamplifier 24 and Figure 2 In addition, in the magnetic detector 1 of this embodiment, Figure 3 The same applies to the behaviors shown in the upper, middle, and lower sections.

[0071] The magnetic detector 1 of this embodiment includes a post-stage high-pass filter 25 on the output side of the preamplifier 24 and the input side of the buffer amplifier 26. The post-stage high-pass filter 25 inputs the output S5 of the preamplifier 24. The post-stage high-pass filter 25 extracts the high-frequency component of the output S5 of the preamplifier 24, thereby forming Figure 2That is, the post-stage high-pass filter 25 removes the offset component after the time has passed from the output S5 of the preamplifier 24. Therefore, the output S6 of the post-stage high-pass filter 25 matches the reference voltage after the time has passed.

[0072] Furthermore, the output S6 of the post-stage high-pass filter 25 is input to the buffer amplifier 26, so that Figure 2 As shown in the eighth paragraph of (c), the output S7 of the buffer amplifier 26 becomes the behavior of amplifying the output S6 of the subsequent high-pass filter 25. Therefore, the output S7 of the buffer amplifier 26 matches the reference voltage after a period of time. That is, Figure 2 As shown in the seventh section of (a), the output S7 of the buffer amplifier 26 in this embodiment is consistent with the output S7 of the buffer amplifier 26 in the ideal behavior.

[0073] Here, if Figure 2 As shown in (c), the output S7 of the buffer amplifier 26 for the DC magnetic field, that is, the output of the detection circuit 20 becomes Figure 4 That is, the output S7 of the buffer amplifier 26 is always 2.5V, which is the reference voltage, regardless of the magnitude of the DC magnetic field.

[0074] 1-3. Effect According to the magnetic detector 1 of this mode, even if the offset component affected by the DC magnetic field is included in the output S5 of the preamplifier 24, the offset component can be removed by the post-stage high-pass filter 25. Therefore, the output value of the detection circuit 20 can not include the offset component affected by the DC magnetic field. As a result, the detection circuit 20 is not easily affected by the DC magnetic field and can detect a tiny AC magnetic field with high precision. In this way, the magnetic detector 1 of this mode can improve the magnetic detection performance.

[0075] (Implementation Method 2) 2-1. Structure of the Magnetic Detector 2 Reference Figure 5 The structure of the magnetic detector 2 in Embodiment 2 is described. It should be noted that the same symbols used in Embodiment 2 as those used in the previous embodiments represent the same components as those in the previous embodiments unless otherwise specified.

[0076] like Figure 5 As shown in FIG. 1 , the magnetic detector 2 of this embodiment includes an MI sensor element 10 and a detection circuit 30. The detection circuit 30 is electrically connected to the MI sensor element 10 and is configured to detect a magnetic field B acting on the magnetically sensitive body 11. IN The detection circuit 30 includes an excitation circuit 31 and a sampling and holding circuit 32 ( Figure 1"S / H" in the middle), preamplifier 33, feedback circuit 34, post-stage high-pass filter 35 ( Figure 1 “HPF2” in the figure), a buffer amplifier 36 and a reference voltage generating amplifier 37.

[0077] Here, the excitation circuit 31 , the sample-and-hold circuit 32 , and the reference voltage generating amplifier 37 in this embodiment are the same as the excitation circuit 21 , the sample-and-hold circuit 22 , and the reference voltage generating amplifier 27 in the first embodiment.

[0078] The preamplifier 33 is an amplifier circuit connected to the output side of the sample-and-hold circuit 32. The preamplifier 33 integrates the output of the sample-and-hold circuit 32 and performs amplification processing.

[0079] The feedback circuit 34 is connected to the output side of the preamplifier 33 and one end of the detection coil 12, and is composed of a low-pass filter. The feedback circuit 34 extracts a given frequency component through the low-pass filter and passes a feedback current I related to the given frequency component. FB Generate feedback magnetic field B FB In other words, the feedback circuit 34 uses the feedback current I of a given frequency band flowing through the low-pass filter. FB Generate feedback magnetic field B FB Then, the feedback circuit 34 uses the generated feedback magnetic field B FB The external magnetic field B applied to the MI sensor element 10 is canceled IN That is, the feedback circuit 34 frequency-selectively negatively feeds back the magnetic field in the MI sensor element 10. In this case, the MI sensor element 10 cancels the magnetic field signal (low-frequency component mainly composed of DC) of a given frequency band by frequency-selective negative feedback of the magnetic field, and responds only to the AC signal other than the cancellation component.

[0080] The post-stage high-pass filter 35 is connected to the output side of the preamplifier 33 to extract a signal of a frequency band higher than a given frequency band. For example, the post-stage high-pass filter 35 can apply a well-known high-pass filter circuit such as a circuit composed of at least two elements of LCR, a circuit using an operational amplifier such as a differential circuit, etc. In addition to the above-mentioned circuit, the post-stage high-pass filter 35 can also apply other structures as long as it can play the intended function.

[0081] The buffer amplifier 36 is an amplifier circuit connected to the output side of the post-stage high-pass filter 35. The buffer amplifier 36 amplifies the output of the post-stage high-pass filter 35. The output of the buffer amplifier 36 becomes the output of the detection circuit 30.

[0082] 2-2. Operation of the Magnetic Detector 2 Next, refer to Figure 6 as well as Figure 7 The operation of the magnetic detector 1 of this embodiment will be described. However, in order to describe the operation of the magnetic detector 2 of this embodiment, for comparison, reference will be made to a magnetic detector 2 without Figure 5 In detail, as a reference, use Figure 6 (a) is used to explain the ideal behavior of the output of each part in a structure without the post-stage high-pass filter 35. Figure 6 (b) is used to explain the actual behavior of the output of each part in a structure without the post-stage high-pass filter 35. Then, using Figure 6 (c) describes the output behavior of each part in this method. Figure 6 In (a), (b), and (c), B IN 、S11、I FB 、S12~S16 indicates Figure 5 The output of each part is shown.

[0083] 2-2-1. Figure 6 Description of (a) The ideal behavior in the configuration without the post-stage high-pass filter 35 in the magnetic detector 2 will be described. Figure 6 As shown in the first section of (a), the magnetic field B IN , assume that an arbitrary magnetic field is applied from the zero state. Figure 6 In the first section of (a), as the magnetic field B IN , for example, a state in which a given DC magnetic field is applied. Figure 6 As shown in the second section of (a), the excitation circuit 31 periodically outputs a signal S11 for turning on the sampling switch of the sample-hold circuit 32. When the signal S11 is an on signal, the sampling switch is turned on, and when the signal S11 is an off signal, the sampling switch is turned off. The signal S11 of the excitation circuit 31 is related to the excitation current I supplied to the magnetic inductor 11. IN synchronous.

[0084] Here, the magnetic detector 2 of this embodiment has a feedback circuit 34. In the feedback circuit 34, a feedback voltage is generated from the output S14 of the preamplifier 33, and a feedback current I corresponding to the potential difference between the feedback voltage and the reference voltage is generated. FB However, the feedback voltage is from the magnetic field B IN From the moment the change occurs, the response becomes corresponding to the time constant of the low-pass filter. After the time has passed, the feedback voltage becomes able to magnetically cancel the magnetic field B. IN The voltage of the low-frequency component mainly composed of DC contained in the feedback current I FB As the feedback voltage increases, it rises and becomes Figure 6 The behavior shown in the third paragraph of (a).

[0085] The input S12 of the sample-and-hold circuit 32 is preferably Figure 6 The behavior is shown in the fourth paragraph of (a). In the magnetic field B IN Before the change, the input S12 of the sample and hold circuit 32 is approximately the reference voltage. IN Immediately after the change, the input S12 of the sample-and-hold circuit 32 undergoes a large change due to the induced voltage of the detection coil 12. IN Magnetic field B after change IN Without any change, the magnetic field B IN After a period of time has passed since the change in magnetic field B, the input S12 of the sampling and holding circuit 32 becomes approximately equal to the reference voltage. IN The input S12 of the sampling and holding circuit 32 before the change is different from the input S12 of the sampling and holding circuit 32 under the magnetic field B IN Without changing the magnetic field B IN After a period of time has passed since the change in , the input S12 of the sampling and holding circuit 32 becomes substantially consistent.

[0086] Furthermore, it is desirable that the output S13 of the sample-and-hold circuit 32 be Figure 6 The behavior is as shown in the fifth paragraph of (a). That is, in the magnetic field B IN After the change, when the sampling switch is turned on for the first time, the output S13 of the sampling and holding circuit 32 rises in imitation of the rise of the input S12 of the sampling and holding circuit 32. FB Due to the effect of , the voltage held by the sampling and holding circuit 32 gradually decreases, so the output S13 of the sampling and holding circuit 32 becomes consistent with the reference voltage after a period of time.

[0087] The preamplifier 33 integrates the output S13 of the sampling and holding circuit 32 and performs amplification processing. Figure 6 In the case of the behavior shown in the fifth paragraph of (a), the output S14 of the preamplifier 33 is as follows Figure 6 As shown in the sixth paragraph of (a), the output S13 of the sample-hold circuit 32 behaves in the same manner. Furthermore, the output S14 of the preamplifier 33 is input to the buffer amplifier 36, so the output S16 of the buffer amplifier 36 is as follows: Figure 6 As shown in the seventh section of (a), the output S14 of the preamplifier 33 is amplified.

[0088] 2-2-2. Figure 6 Description of (b) Next, the actual behavior in a configuration in which the magnetic detector 2 does not include the post-stage high-pass filter 35 will be described. Figure 6 As shown in the first to third paragraphs of (b), the magnetic field B IN , the signal S11 from the excitation circuit 21 and the feedback current I FB same.

[0089] The input S12 of the sample and hold circuit 32 does not become Figure 6 The conduct is not as indicated in the fourth paragraph of (a), but Figure 6 The reason for this is that the feedback circuit 34 is connected to one end of the detection coil 12. Thus, the feedback circuit 34 holds the summed voltage in the sample-and-hold circuit 32, which is the sum of the induced voltage of the detection coil 12 and the feedback current I FB The voltage is obtained by adding the feedback voltage generated. Therefore, if Figure 6 As shown in the fourth section of (b), the input S12 of the sample-and-hold circuit 32 changes with the feedback current I FB rise with the rise of.

[0090] The output S13 of the sample and hold circuit 32 becomes Figure 6 The operation of the sample-and-hold circuit 32 is similar to that in the first embodiment. Figure 3 The same operation as described above. That is, the output S13 of the sample-and-hold circuit 32 changes at the acquisition time at the beginning of the sampling mode. After that, the output S13 of the sample-and-hold circuit 32 tracks the added voltage obtained by adding the induced voltage of the detection coil 12 and the feedback voltage from the middle of the sampling mode. Then, the output S13 of the sample-and-hold circuit 32 holds the held value after the delay time and the adjustment time after the sampling switch is turned off.

[0091] Therefore, the output S13 of the sample and hold circuit 32 is as follows: Figure 6 As shown in the fifth section of (b), after the passage of time, the state that is deviated from the reference voltage due to the influence of the feedback voltage is added with the fluctuation accompanying the on / off operation of the sampling switch.

[0092] The preamplifier 33 integrates the output S13 of the sampling and holding circuit 32 and performs amplification processing. Figure 6 In the case of the behavior shown in the fifth paragraph of (b), the output S14 of the preamplifier 33 is as follows Figure 6 As shown in the sixth stage of (b), an offset component with respect to the reference voltage is generated after a time has passed. Thus, in the output S14 of the preamplifier 33, a component of the feedback voltage is generated as an offset component after a time has passed.

[0093] Furthermore, the preamplifier 33 converts the output voltage variation of the detection coil 12 when the sampling switch is in the on-action and the output voltage variation in at least one of the acquisition time and the adjustment time due to the on / off action of the sampling switch into an offset component relative to the reference voltage by integration processing. However, in this embodiment, regarding the offset component in the output S14 of the preamplifier 33, the component due to the feedback voltage is relatively large, and the component due to the action of the sampling switch is relatively small.

[0094] Furthermore, the output S14 of the preamplifier 33 is input to the buffer amplifier 36, so the output S16 of the buffer amplifier 36 is as follows: Figure 6 As shown in the seventh stage of (b), the output S14 of the preamplifier 33 is amplified. Therefore, the output S16 of the buffer amplifier 36 remains an offset component with respect to the reference voltage after a certain period of time.

[0095] Here, if Figure 6 As shown in (b), the output S16 of the buffer amplifier 36 for the DC magnetic field, that is, the output of the detection circuit 30 becomes Figure 7 That is, the output S7 of the buffer amplifier 36 becomes an offset different from the reference voltage 2.5 V according to the magnitude of the DC magnetic field. There is a linear relationship that the larger the DC magnetic field, the larger the offset component of the output S7 of the buffer amplifier 26.

[0096] 2-2-3. Figure 6 Description of (c) Next, the behavior of each part in the magnetic detector 2 of this embodiment will be described. Figure 6 As shown in the first to sixth paragraphs of (c), the magnetic field B IN , signal S11 from the excitation circuit 21, feedback current I FB , the input S12 of the sampling and holding circuit 32, the output S13 of the sampling and holding circuit 32, the output S14 of the preamplifier 33 and Figure 6 Same as (b).

[0097] The magnetic detector 2 of this embodiment includes a post-stage high-pass filter 35 on the output side of the preamplifier 33 and the input side of the buffer amplifier 36. The post-stage high-pass filter 35 inputs the output S14 of the preamplifier 33. The post-stage high-pass filter 35 extracts the high-frequency component of the output S14 of the preamplifier 33, thereby forming Figure 6 That is, the post-stage high-pass filter 35 removes the offset component after the time has passed from the output S14 of the preamplifier 33. Therefore, the output S15 of the post-stage high-pass filter 35 matches the reference voltage after the time has passed.

[0098] Then, the output S15 of the post-stage high-pass filter 35 is input to the buffer amplifier 36, so that Figure 6 As shown in the eighth paragraph of (c), the output S16 of the buffer amplifier 36 becomes the behavior of amplifying the output S15 of the subsequent high-pass filter 35. Therefore, the output S16 of the buffer amplifier 36 matches the reference voltage after a period of time. That is, Figure 6 As shown in the seventh section of (a), the output S16 of the buffer amplifier 36 in this embodiment is consistent with the output S16 of the buffer amplifier 36 in the ideal behavior.

[0099] Here, if Figure 6 As shown in (c), the output S16 of the buffer amplifier 36 for the DC magnetic field, that is, the output of the detection circuit 30 becomes Figure 7 That is, the output S16 of the buffer amplifier 36 is always 2.5V, which is the reference voltage, regardless of the magnitude of the DC magnetic field.

[0100] 2-3. Effect According to the magnetic detector 2 of this embodiment, even if the offset component affected by the DC magnetic field is included in the output S14 of the preamplifier 33, the offset component can be removed by the post-stage high-pass filter 35. Therefore, the output value of the detection circuit 30 can not include the offset component affected by the DC magnetic field. As a result, the output value of the detection circuit 30 can be reduced by the feedback current I FB In the value of the canceled DC magnetic field, the detection circuit 30 is less susceptible to the influence of the DC magnetic field and can detect the minute AC magnetic field with high accuracy. In this way, the magnetic detector 2 of this embodiment can improve the magnetic detection performance.

[0101] (First Variation of Embodiment 1) Reference Figure 8 A magnetic detector 3 according to a first variation of Embodiment 1 is described. Only the post-stage high-pass filter 25 of the detection circuit 40 is different. However, the post-stage high-pass filter 25 in this variation has the same function as the post-stage high-pass filter 25 in Embodiment 1, although the structure is different.

[0102] The post-stage high-pass filter 25 in this modified embodiment includes an A / D converter 101, a signal processing unit 102, and a D / A converter 103. The A / D converter 101 converts the output of the preamplifier 24 into a digital signal, the signal processing unit 102 performs correction processing on the digital signal including an offset component, and the D / A converter 103 converts the corrected digital signal into an analog signal.

[0103] That is, the post-stage high-pass filter 25 of this modification is composed of a signal processing unit 102 that performs digital signal processing after digital conversion. The signal processing unit 102 performs Fourier transform, for example, to extract only the desired high-frequency component, and performs inverse Fourier transform on the extracted transform result, thereby achieving a state where the offset component is removed. In this case, all offset components are removed, so when it is necessary to offset the amount of the reference voltage value, the reference voltage value can be added as a correction value.

[0104] Alternatively, the signal processing unit 102 may also obtain the difference between the digital signal voltage and the reference voltage, and perform correction processing by adding or subtracting the difference from the digital signal. In addition, the signal processing unit 102 may perform digital filtering processing represented by an FIR filter, and perform correction processing using the average value of the processing result or the digital value at a specific time as the reference voltage.

[0105] In this variant, the same effect as in Embodiment 1 is achieved. It should be noted that the post-stage high-pass filter 25 in this variant is different from a high-pass filter circuit such as a circuit composed of at least two elements of LCR, but since it functions as a high-pass filter, this description is used as a concept included in the high-pass filter. The same is true in the following variants.

[0106] (Second Variation of Embodiment 1) Reference Fig. 9 A magnetic detector 4 according to a second variation of Embodiment 1 will be described. Only the post-stage high-pass filter 25 of the detection circuit 50 is different. However, the post-stage high-pass filter 25 in this variation has the same function as the post-stage high-pass filter 25 in Embodiment 1, although the structure is different.

[0107] The post-stage high-pass filter 25 in this variant includes a low-pass filter 201 and a differential amplifier 202 (equivalent to a processing unit). The low-pass filter 201 extracts a signal of a frequency band lower than a given frequency band from the output of the preamplifier 24. The differential amplifier 202 subtracts a component obtained by passing through the low-pass filter 201 from the output of the preamplifier 24. In this variant, the same effect as that of the first embodiment is achieved.

[0108] (Third Variation of Embodiment 1) Reference Fig.10 A magnetic detector 5 according to a third variation of Embodiment 1 will be described. Only the post-stage high-pass filter 25 of the detection circuit 60 is different. However, the post-stage high-pass filter 25 in this variation has the same function as the post-stage high-pass filter 25 in Embodiment 1, although the structure is different.

[0109] The post-stage high-pass filter 25 in this modification functions as a reference voltage correction unit 301. The reference voltage correction unit 301 is configured to be connected to the output side of the preamplifier 24 and to give the DC component in the output of the preamplifier 24 as a reference voltage to the preamplifier 24 and the like. Specifically, the reference voltage correction unit 301 extracts the DC component in the output of the preamplifier 24. The reference voltage correction unit 301 corrects the reference voltage of the reference voltage generation amplifier 27 so that the extracted DC component is used as the reference voltage. The reference voltage generation amplifier 27 gives the corrected reference voltage to the MI sensor element 10, the sample-and-hold circuit 22, the post-stage high-pass filter 23, the preamplifier 24, and the buffer amplifier 26.

[0110] In the detection circuit 60 of the present modification, the output of the preamplifier 24 becomes the input of the buffer amplifier 26. In the present modification, the same effects as those of the first embodiment are achieved.

[0111] (other) The first to third modified forms described above can also be applied to the second embodiment in the same manner.

Claims

1. A magnetic detector comprising: A magneto-impedance sensor element including a magnetically sensitive body and a detection coil, wherein the magnetically sensitive body undergoes a magnetization change corresponding to the strength of a magnetic field when an excitation current is supplied thereto, and the detection coil is wound around the magnetically sensitive body and outputs an induced voltage generated by the magnetization change of the magnetically sensitive body; A sampling and holding circuit connected to one end of the detection coil to hold the output voltage of the detection coil when the sampling switch is turned on; an amplifier circuit connected to the output side of the sample-and-hold circuit; and A post-stage high-pass filter is connected to the output side of the amplifier circuit.

2. The magnetic detector according to claim 1, wherein The magnetic detector further includes a front-stage high-pass filter connected to the output side of the sample-and-hold circuit. The amplifier circuit is connected to the output side of the front-stage high-pass filter.

3. The magnetic detector according to claim 2, wherein: The sampling and holding circuit holds the output voltage variation of the detection coil during the conduction operation of the sampling switch. The amplifier circuit integrates and amplifies the output of the preceding high-pass filter, and converts the output voltage variation into an offset component through the integration process. The post-stage high-pass filter removes the offset component.

4. The magnetic detector according to claim 2, wherein: The sample-and-hold circuit holds the output voltage variation of at least one of the acquisition time and the adjustment time according to the on / off operation of the sampling switch. The amplifier circuit integrates and amplifies the output of the preceding high-pass filter, and converts the output voltage variation into an offset component through the integration process. The post-stage high-pass filter removes the offset component.

5. The magnetic detector according to claim 2, wherein: The output side of the amplifier circuit is not connected to one end of the detection coil and has no feedback circuit.

6. The magnetic detector according to claim 1, wherein The magnetic detector further includes a feedback circuit that connects the output side of the amplifier circuit to one end of the detection coil and is composed of a low-pass filter.

7. The magnetic detector according to claim 6, wherein: The feedback circuit uses a feedback magnetic field generated by a feedback current of a given frequency band passing through the low-pass filter to cancel a portion of an external magnetic field applied to the magneto-impedance sensor element, and causes the sample-and-hold circuit to hold a summed voltage obtained by adding the induced voltage to a feedback voltage generated by the feedback current. The post-stage high-pass filter removes a component of the feedback voltage that is included in the output of the amplifier circuit and is generated by the feedback current.

8. The magnetic detector according to any one of claims 1 to 7, wherein: The post-stage high-pass filter includes any one of the following: A circuit comprising at least 2 elements of an LCR; A high-pass filter circuit having an operational amplifier; a signal processing unit that performs digital signal processing after digital conversion; a processing unit that subtracts a component obtained by passing through a low-pass filter with respect to the output of the amplifier circuit from the output of the amplifier circuit; and The reference voltage correction unit is configured to apply a DC component in the output of the amplifier circuit to the amplifier circuit as a reference voltage.

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