JP7701233B2 - Gradient magnetic field sensor and magnetic object detection device - Google Patents

Description

The present invention relates to a gradient magnetic field sensor and a magnetic object detection device.

Research and development is being conducted on gradient magnetic field sensors that detect the gradient of the magnetic field strength at two different points. Here, the gradient is the difference in the magnetic field strength at the two different points.

A gradient magnetic field sensor has two sensor heads to detect the strength of a magnetic field at each of two different points. In a gradient magnetic field sensor, one of the two sensor heads detects the strength of a magnetic field at one of the two different points. In a gradient magnetic field sensor, the other of the two sensor heads detects the strength of a magnetic field at the other of the two different points. The gradient magnetic field sensor detects the gradient of the strength of the magnetic field at these two different points according to the signals output from the two sensor heads. The gradient of the strength of the magnetic field at these two different points changes depending on the presence or absence of an object (e.g., a magnetic body) that can change the magnetic field. Using this, the gradient magnetic field sensor can detect such an object as a detection target.

However, each of the two sensor heads in the gradient magnetic field sensor has a dead zone, which means that it is sometimes impossible to detect the target object.

In this regard, a gradient magnetic field sensor is known that includes a first sensor head formed by disposing a first detection coil that is magnetically coupled to a part of the linear shape of the magnetic body in a first magnetic core formed of a magnetic body including a linear shape, through which an excitation AC current and a DC current are passed, and a second sensor head formed by disposing a second detection coil that is magnetically coupled to a part of the linear shape of the magnetic body in a rotationally symmetrical position to the first detection coil in a second magnetic core formed of a magnetic body including a linear shape, through which a current common to the excitation AC current and DC current passed through the first magnetic core is passed and which is disposed in parallel to face the first magnetic core, and a sensor circuit that outputs a gradient magnetic field based on the detection voltages output from the first and second sensor heads is known (see Patent Document 1).

JP 2020-165716 A

Here, in the gradient magnetic field sensor described in Patent Document 1, the second detection coil is differentially coupled to the first detection coil in a rotationally symmetrical position, so that the waveform of the detection signal becomes a bimodal waveform having two similar mountain-shaped peaks when the detection object passes near the center of the longitudinal direction of the magnetic core. However, in this gradient magnetic field sensor, the detection signal has a small crest value as a result of the differential coupling between the first detection coil and the second detection coil, because the polarity of the voltage output from the first detection coil and the polarity of the voltage output from the second detection coil are the same in this case. Furthermore, in this gradient magnetic field sensor, when the detection object passes near the tip of the magnetic core, the waveform of the detection signal may become a unimodal waveform having a crest with a different polarity and magnitude when the detection object passes near the tip of the magnetic core. In this way, in this gradient magnetic field sensor, the waveform of the detection signal may differ depending on the passing position of the detection object. This means that the sensitivity of the gradient magnetic field sensor changes depending on the passing position of the detection object, which is not desirable.

The present invention was made in consideration of these circumstances, and aims to provide a gradient magnetic field sensor and a magnetic object detection device that can suppress the occurrence of dead zones while suppressing fluctuations in sensitivity according to the passing position of the object to be detected.

One aspect of the present invention is a gradient magnetic field sensor comprising a first magnetic core having a linear portion, a second magnetic core having a linear portion, N first coils arranged in sequence on the linear portion of the first magnetic core, N second coils arranged in sequence on the linear portion of the second magnetic core, and a main body in which the first magnetic core and the second magnetic core are provided, and the nth first coil from the tip side of the first magnetic core among the N first coils is differentially connected to the nth second coil from the tip side of the second magnetic core among the N second coils, where N is an integer of 2 or more and n is an integer in the range of 1 to N.

The present invention makes it possible to suppress the occurrence of dead zones while suppressing fluctuations in sensitivity according to the passing position of the object to be detected.

1 is a diagram showing an example of the configuration of a magnetic object detection device 1. FIG. FIG. 2 is a diagram showing an example of a circuit configuration of a gradient magnetic field sensor 12. 1 is a diagram showing an example of the configuration of a detection circuit DT1. Note that some or all of the detection circuits DT1 to DT4 may have different configurations from each other as long as the functions of the gradient magnetic field sensor 12 are not impaired. FIG. 2 is a diagram showing an example of a circuit configuration of a signal processing unit PR. 2 is a diagram showing an example of a more detailed configuration of a first sensor head S1 and a second sensor head S2. FIG. 1 is a diagram showing an example of a graph in which the voltage of a differential signal output from a detection coil CL1n differentially connected to a detection coil CL2n is plotted while changing the relative position of a detection object with respect to a gradient magnetic field sensor 12. FIG. 13 is a diagram showing an example of a graph in which the voltage of the detection signal output from the calculation unit O6 is plotted while changing the relative position of the detection object with respect to the gradient magnetic field sensor 12. FIG. 11 is a diagram showing an example of the configuration of a gradient magnetic field sensor 12A according to a first modified example of the embodiment. FIG. FIG. 13 is a diagram showing an example of detection signals output from the detection circuit DT1, the detection circuit DT2, and the differential amplifier DF when a predetermined magnetic moment moving in an orthogonal direction is detected by a gradient magnetic field sensor 12A arranged in a region where a uniform gradient magnetic field is applied. FIG. 11 is a diagram showing another example of detection signals output from the detection circuit DT1, the detection circuit DT2, and the differential amplifier DF when the gradient magnetic field sensor 12A is placed in a region to which a uniform gradient magnetic field is applied. FIG. 11 is a diagram showing yet another example of the detection signals output from the detection circuit DT1, the detection circuit DT2, and the differential amplifier DF when the gradient magnetic field sensor 12A is placed in a region to which a uniform gradient magnetic field is applied. FIG. 2 is a diagram showing an example of a circuit configuration of a gradient magnetic field sensor 12B.

<Embodiment>
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

<Outline of magnetic object detection device>
First, an overview of the magnetic object detection device according to this embodiment will be described.

The magnetic object detection device according to the embodiment includes the gradient magnetic field sensor according to the embodiment. The gradient magnetic field sensor according to the embodiment includes a first magnetic core, a second magnetic core, N first coils, N second coils, and a main body. The first magnetic core has a linear portion. The second magnetic core has a linear portion. The N first coils are arranged in order on the linear portion of the first magnetic core. The N second coils are arranged in order on the linear portion of the second magnetic core. The main body includes a first magnetic core and a second magnetic core. Among the N first coils, the nth first coil from the tip side of the first magnetic core is differentially connected to the nth second coil from the tip side of the second magnetic core among the N second coils. However, N is an integer of 2 or more. And, n is an integer in the range of 1 to N.

As a result, the gradient magnetic field sensor according to the embodiment and the magnetic object detection device according to the embodiment can suppress the occurrence of dead zones while suppressing the variation in sensitivity according to the passing position of the detection target.

The configurations of the magnetic object detection device according to the embodiment and the gradient magnetic field sensor according to the embodiment are described in detail below.

<Configuration of magnetic object detection device>
Hereinafter, the configuration of the magnetic object detection device according to the embodiment will be described with reference to the magnetic object detection device 1 as an example of the magnetic object detection device. Here, in the embodiment, a conductor that transmits an electric signal corresponding to DC power or an electric signal corresponding to AC power will be referred to as a transmission line and described. The transmission line may be, for example, a conductor printed on a substrate, a conductor formed in a linear shape, or another conductor. In addition, in the embodiment, when it is referred to as a voltage, it means a potential difference from a predetermined reference potential, and the illustration and description of the reference potential are omitted. Here, the reference potential may be any potential. In the embodiment, as an example, a case where the reference potential is a ground potential will be described. In addition, in the embodiment, when it is referred to as a ground, it indicates the same ground. In addition, in this specification, the terminal with a dot among the two terminals of the coil in each circuit diagram will be described as the hot side terminal. In addition, in this specification, the terminal without a dot among the two terminals of the coil in each circuit diagram will be described as the cold side terminal.

Figure 1 shows an example of the configuration of a magnetic object detection device 1.

The magnetic object detection device 1 is a device that detects an object to be detected using a magnetic field. The object to be detected is an object that can change a magnetic field. The object to be detected is, for example, a magnetic body.

The magnetic object detection device 1 includes, for example, a frame body CS, a first roller RL1, a second roller RL2, a magnetization device 11, a gradient magnetic field sensor 12, and an information processing device 20. The gradient magnetic field sensor 12 includes two sensor heads, a first sensor head S1 and a second sensor head S2. In FIG. 1, in order to prevent the drawing from becoming complicated, the cable connecting the information processing device 20 in the magnetic object detection device 1 to other members is omitted. The magnetic object detection device 1 may be configured to not include some or all of the frame body CS, the first roller RL1, the second roller RL2, the magnetization device 11, and the information processing device 20. The magnetic object detection device 1 may be configured to include other members, other devices, etc. together with the gradient magnetic field sensor 12 instead of some or all of the frame body CS, the first roller RL1, the second roller RL2, the magnetization device 11, and the information processing device 20. Furthermore, the magnetic object detection device 1 may be configured to include, in addition to the frame body CS, the first roller RL1, the second roller RL2, the magnetization device 11, and the information processing device 20, other members, other devices, etc., along with the gradient magnetic field sensor 12.

The various components of the magnetic object detection device 1 are attached to the frame body CS. The frame body CS is mainly composed of a top plate (not shown), a bottom plate (not shown), and a number of supports connecting the top plate and the bottom plate. In the example shown in FIG. 1, the outer shape of the frame body CS is substantially a rectangular parallelepiped. However, the outer shape of the frame body CS may be any shape.

Inside the frame body CS, a first roller RL1 and a second roller RL2 are provided.

The first roller RL1 is a roller on which the sheet material ST to be inspected for the presence or absence of a detection target is wound in a roll. The second roller RL2 is a roller that winds up the sheet material ST pulled out from the first roller RL1 while transporting it along a predetermined path. The second roller RL2 is rotated by a servo motor (not shown) or the like. This allows the second roller RL2 to wind up the sheet material ST pulled out from the first roller RL1. In other words, the second roller RL2 is a drive roller. The first roller RL1 is a passive roller that rotates while the sheet material ST is wound up in accordance with the rotation of the second roller RL2. The servo motor is controlled, for example, by the information processing device 20 described below.

The sheet material ST is transported through a magnetic field generated by a magnetizing device 11 while being pulled out from the first roller RL1 and wound up by the second roller RL2. As a result, the detection object attached to the sheet material ST is magnetized. After being transported through the magnetic field, the sheet material ST is transported through a predetermined detection area where the gradient magnetic field sensor 12 can detect the presence or absence of the detection object. The sheet material ST is then wound around the second roller RL2.

The magnetizing device 11 is a device that generates a magnetic field of a predetermined strength in a predetermined magnetized area and magnetizes a magnetic body that enters the magnetized area. The magnetizing device 11 is a known device, so further detailed description is omitted.

The gradient magnetic field sensor 12 detects the gradient of the magnetic field strength at two different points in the detection area using sensor heads arranged at the two different points, i.e., the first sensor head S1 and the second sensor head S2. This allows the information processing device 20, which acquires the detection signal output from the gradient magnetic field sensor 12, to determine, based on the detection signal, whether or not a detection target has been attached to the sheet material ST transported through the detection area. For ease of explanation, the gradient of the magnetic field strength at the two points, the point where the first sensor head S1 is arranged and the point where the second sensor head S2 is arranged, will be referred to simply as the gradient below.

The information processing device 20 is communicatively connected to the gradient magnetic field sensor 12. As described above, the information processing device 20 determines whether or not a detection target is attached to the sheet material ST transported through the detection area based on the detection signal output from the gradient magnetic field sensor 12.

For example, when the information processing device 20 determines that a detection target has been attached to the sheet material ST transported through the detection area, it controls the servo motor described above to stop the winding of the sheet material ST by the second roller RL2 and performs a notification process to notify the user that a detection target has been attached to the sheet material ST. The notification process is, for example, a process of displaying information indicating that a detection target has been attached to the sheet material ST on the display of the information processing device 20. This allows the user of the magnetic object detection device 1 to remove the detection target from the sheet material ST or identify the sheet material ST as a defective product. Note that the notification process may be a process of outputting sound, vibration, light, or the like that indicates the information, instead of such a display process.

The information processing device 20 is, for example, a notebook PC (Personal Computer), a tablet PC, a desktop PC, a workstation, a multi-function mobile phone terminal (smartphone), a mobile phone terminal, a PDA (Personal Digital Assistant), etc. Note that the information processing device 20 may be another information processing device such as a microcomputer.

In addition, in the magnetic object detection device 1, some or all of the magnetization device 11, the gradient magnetic field sensor 12, and the information processing device 20 may be configured as an integrated unit.

The object on which the magnetic object detection device 1 inspects for the attachment of a detection target may be, instead of the sheet member ST, another object capable of inspecting for the presence or absence of a detection target. For example, a magnetic object detector equipped with a gradient magnetic field sensor 12 detects the presence or absence of a magnetic object underground. In this case, the magnetic object detector is an example of a magnetic object detection device 1. Also, a magnetic object underground is an example of a detection target in this case.

<Circuit configuration of gradient magnetic field sensor>
The circuit configuration of the gradient magnetic field sensor 12 will be described below with reference to Fig. 2. Fig. 2 is a diagram showing an example of the circuit configuration of the gradient magnetic field sensor 12.

The gradient magnetic field sensor 12 includes an AC power supply connection terminal CT1, a first sensor head S1, a second sensor head S2, a zeroth phase shift circuit PS0, an AC current control unit CC1, a detection circuit DT1, a detection circuit DT2, a detection circuit DT3, a detection circuit DT4, a signal processing unit PR, and a detection signal output terminal CT2.

The first sensor head S1 includes a first magnetic core CR1 having a linear portion and N detection coils CL1, CL11 to CL1N. These N detection coils CL1 are arranged in order on the linear portion of the first magnetic core CR1. In other words, these N detection coils CL1 are wound around the linear portion of the first magnetic core CR1 in order from the tip of the first magnetic core CR1. In this case, each of the N detection coils CL1 is magnetically coupled to the first magnetic core CR1. When a magnetic field is applied to the first sensor head S1, the magnetic fluxes interlinked with each of the N detection coils CL1 are equal to each other (or approximately equal to each other). This is because each of the N detection coils CL1 is wound around the first magnetic core CR1, which is a magnetic body, and the magnetic resistance of the magnetic body is lower than the magnetic resistance of air. The first magnetic core CR1 is made of, for example, an amorphous ribbon, but is not limited to this. In addition, in addition to the N detection coils CL1, one or more other coils may be arranged in the linear portion of the first magnetic core CR1. In this case, in the linear portion of the first magnetic core CR1, the one or more other coils are arranged closer to the tip end side of the first magnetic core CR1 or closer to the base end side of the first magnetic core CR1 than the N detection coils CL1.

The second sensor head S2 has a second magnetic core CR2 having a linear portion and N detection coils CL2, ie, detection coils CL21 to CL2N. That is, the second sensor head S2 has the same number of detection coils CL2 as the number of detection coils CL1 of the first sensor head S1. These N detection coils CL2 are arranged in order on the linear portion of the second magnetic core CR2. In other words, these N detection coils CL2 are wound around the linear portion of the second magnetic core CR2 in order from the tip side of the second magnetic core CR2. In this case, each of the N detection coils CL2 is magnetically coupled to the second magnetic core CR2. When a magnetic field is applied to the second sensor head S2, the magnetic fluxes interlinked with each of the N detection coils CL2 are equal to each other (or approximately equal to each other). This is because each of the N detection coils CL2 is wound around the second magnetic core CR2, which is a magnetic material, and the magnetic resistance of the magnetic material is lower than the magnetic resistance of air. The second magnetic core CR2 is made of, for example, an amorphous ribbon, but is not limited to this. In addition to the N detection coils CL2, one or more other coils may be arranged in the linear portion of the second magnetic core CR2. In this case, in the linear portion of the second magnetic core CR2, the one or more other coils are arranged closer to the tip end side of the second magnetic core CR2 or closer to the base end side of the second magnetic core CR2 than the N detection coils CL2.

Here, N may be any integer equal to or greater than 2. In the following, as an example, a case where N is 4 will be described. In this case, as shown in FIG. 2, the first sensor head S1 has a first magnetic core CR1 and four detection coils, namely, detection coil CL11 to detection coil CL14. In this case, the second sensor head S2 has a second magnetic core CR2 and four detection coils, namely, detection coil CL21 to detection coil CL24.

The 0th phase shift circuit PS0 has two terminals: an input terminal PS01 and an output terminal PS02.

The AC current control unit CC1 has an input terminal CC11, an input terminal CC12, an output terminal CC13, and an output terminal CC14. The AC current control unit CC1 also includes a first phase shift circuit PS1, a first variable resistor VR11, a first capacitor C1, a second phase shift circuit PS2, a second variable resistor VR12, and a second capacitor C2.

Detection circuit DT1 has three terminals: input terminal DT11, input terminal DT12, and output terminal DT13.

The detection circuit DT2 has three terminals: input terminal DT21, input terminal DT22, and output terminal DT23.

Detection circuit DT3 has three terminals: input terminal DT31, input terminal DT32, and output terminal DT33.

Detection circuit DT4 has three terminals: input terminal DT41, input terminal DT42, and output terminal DT43.

The signal processing unit PR has five terminals: input terminal PR1, input terminal PR2, input terminal PR3, input terminal PR4, and output terminal PR5.

The gradient magnetic field sensor 12 is connected to the AC power source P1 via a transmission line. More specifically, the AC power source connection terminal CT1 of the gradient magnetic field sensor 12 is connected to a first power source terminal P11, which is one of the two power source terminals of the AC power source P1, via a transmission line. The other of the two power source terminals of the AC power source P1, the second power source terminal P12, is grounded to the ground via the transmission line. Here, the AC power source P1 may be any AC power source. Note that other circuit elements, other devices, etc. may be connected between the AC power source connection terminal CT1 and the first power source terminal P11, as long as the function of the gradient magnetic field sensor 12 is not impaired. Also, other circuit elements, other devices, etc. may be connected between the second power source terminal P12 and the ground, as long as the function of the gradient magnetic field sensor 12 is not impaired.

The gradient magnetic field sensor 12 is also connected to the information processing device 20 via a transmission line. More specifically, the detection signal output terminal CT2 of the gradient magnetic field sensor 12 is connected to the information processing device 20 via a transmission line. Note that other circuit elements, other devices, etc. may be connected between the detection signal output terminal CT2 and the information processing device 20 as long as the functions of the gradient magnetic field sensor 12 are not impaired.

The AC power supply connection terminal CT1 is connected to the input terminal PS01 of the 0th phase shift circuit PS0, the input terminal DT11 of the detection circuit DT1, the input terminal DT21 of the detection circuit DT2, the input terminal DT31 of the detection circuit DT3, and the input terminal DT41 of the detection circuit DT4 through a transmission line. The output terminal PS02 of the 0th phase shift circuit PS0 is connected to the input terminal CC11 of the AC current control unit CC1 and the input terminal CC12 of the AC current control unit CC1 through a transmission line. Note that other circuit elements, other devices, etc. may be connected between the AC power supply connection terminal CT1 and the input terminal PS01 within a range that does not impair the function of the gradient magnetic field sensor 12. In addition, other circuit elements, other devices, etc. may be connected between the AC power supply connection terminal CT1 and the input terminal DT11, as long as the function of the gradient magnetic field sensor 12 is not impaired. In addition, other circuit elements, other devices, etc. may be connected between the AC power supply connection terminal CT1 and the input terminal DT21, as long as the function of the gradient magnetic field sensor 12 is not impaired. In addition, other circuit elements, other devices, etc. may be connected between the AC power supply connection terminal CT1 and the input terminal DT31, as long as the function of the gradient magnetic field sensor 12 is not impaired. In addition, other circuit elements, other devices, etc. may be connected between the AC power supply connection terminal CT1 and the input terminal DT41, as long as the function of the gradient magnetic field sensor 12 is not impaired. In addition, other circuit elements, other devices, etc. may be connected between the output terminal PS02 and the input terminal CC11, as long as the function of the gradient magnetic field sensor 12 is not impaired. In addition, other circuit elements, other devices, etc. may be connected between the output terminal PS02 and the input terminal CC12 as long as the functionality of the gradient magnetic field sensor 12 is not impaired.

In the AC current control unit CC1, the first phase shift circuit PS1, the first variable resistor VR11, and the first capacitor C1 are connected in series between the input terminal CC11 and the output terminal CC13 via a transmission line. In the AC current control unit CC1, the second phase shift circuit PS2, the second variable resistor VR12, and the second capacitor C2 are connected in series between the input terminal CC12 and the output terminal CC14 via a transmission line. In addition, other circuit elements, other devices, etc. may be connected between the input terminal CC11 and the output terminal CC13, together with the first phase shift circuit PS1, the first variable resistor VR11, and the first capacitor C1, within a range that does not impair the function of the gradient magnetic field sensor 12. In addition, the first phase shift circuit PS1, the first variable resistor VR11, and the first capacitor C1 may be connected in series in any order between the input terminal CC11 and the output terminal CC13. In addition, between the input terminal CC12 and the output terminal CC14, other circuit elements, other devices, etc. may be connected in addition to the second phase shift circuit PS2, the second variable resistor VR12, and the second capacitor C2, as long as the function of the gradient magnetic field sensor 12 is not impaired. In addition, between the input terminal CC12 and the output terminal CC14, the second phase shift circuit PS2, the second variable resistor VR12, and the second capacitor C2 may be connected in series in any order.

The first magnetic core CR1 of the first sensor head S1 is connected between the output terminal CC13 of the AC current control unit CC1 and the ground via a transmission line. The second magnetic core CR2 of the second sensor head S2 is connected between the output terminal CC14 of the AC current control unit CC1 and the ground via a transmission line. In the following, for convenience of explanation, the AC current flowing from the output terminal CC13 to the first magnetic core CR1 is referred to as the first AC excitation current AC1 as shown by the arrow in FIG. 2. In the following, for convenience of explanation, the AC current flowing from the output terminal CC14 to the second magnetic core CR2 is referred to as the second AC excitation current AC2 as shown by the arrow in FIG. In addition, other circuit elements, other devices, etc. may be connected between the output terminal CC13 of the AC current control unit CC1 and the ground within a range that does not impair the function of the gradient magnetic field sensor 12, together with the first magnetic core CR1. In addition, other circuit elements, other devices, etc. may be connected between the output terminal CC14 of the AC current control unit CC1 and ground, as long as the function of the gradient magnetic field sensor 12 is not impaired.

The input terminal DT12 of the detection circuit DT1 is connected to the hot terminal of the two terminals of the detection coil CL11 through a transmission line. The cold terminal of the two terminals of the detection coil CL11 is connected to the cold terminal of the two terminals of the detection coil CL21 through a transmission line. The hot terminal of the two terminals of the detection coil CL21 is grounded to the ground through the transmission line. That is, the detection coil CL11 and the detection coil CL21 are differentially connected. Note that, other circuit elements, other devices, etc. may be connected between the input terminal DT12 and the detection coil CL11 within a range that does not impair the function of the gradient magnetic field sensor 12. Note that, other circuit elements, other devices, etc. may be connected between the detection coil CL11 and the detection coil CL21 within a range that does not impair the function of the gradient magnetic field sensor 12. In addition, other circuit elements, other devices, etc. may be connected between the detection coil CL21 and ground as long as the functionality of the gradient magnetic field sensor 12 is not impaired.

Here, the self-inductance of the detection coil CL11 is the same (or approximately the same) as the self-inductance of the detection coil CL21. This allows the gradient magnetic field sensor 12 to match the sensitivity of the detection coil CL11 and the detection coil CL21 to the uniform magnetic field. In other words, in this case, the voltage output from the detection coil CL11 by applying a magnetic field of a predetermined magnitude X1 to the first sensor head S1 can be a voltage of the opposite polarity and the same magnitude as the voltage output from the detection coil CL21 by applying a magnetic field of a predetermined magnitude X1 to the second sensor head S2. As a result, the gradient magnetic field sensor 12 can suppress the detection of a uniform magnetic field by the combination of the detection coil CL11 and the detection coil CL21. In this specification, the uniform magnetic field refers to a magnetic field whose strength changes uniformly regardless of position in an area where no magnetic field other than the uniform magnetic field is applied, such as geomagnetism, distant disturbance magnetic field, etc. The self-inductance of the detection coil CL11 may be different from the self-inductance of the detection coil CL21. In this case, the gradient magnetic field sensor 12 performs signal processing to match the magnitude of the voltage output from the detection coil CL11 when a magnetic field of a predetermined magnitude X1 is applied to the first sensor head S1 with the magnitude of the voltage output from the detection coil CL21 when a magnetic field of a predetermined magnitude X1 is applied to the second sensor head S2, thereby preventing the detection of a uniform magnetic field.

The input terminal DT22 of the detection circuit DT2 is connected to the hot terminal of the two terminals of the detection coil CL12 through a transmission line. The cold terminal of the two terminals of the detection coil CL12 is connected to the cold terminal of the two terminals of the detection coil CL22 through a transmission line. The hot terminal of the two terminals of the detection coil CL22 is grounded to the ground through the transmission line. That is, the detection coil CL12 and the detection coil CL22 are differentially connected. Note that, other circuit elements, other devices, etc. may be connected between the input terminal DT22 and the detection coil CL12 within a range that does not impair the function of the gradient magnetic field sensor 12. Note that, other circuit elements, other devices, etc. may be connected between the detection coil CL12 and the detection coil CL22 within a range that does not impair the function of the gradient magnetic field sensor 12. In addition, other circuit elements, other devices, etc. may be connected between the detection coil CL22 and ground as long as the functionality of the gradient magnetic field sensor 12 is not impaired.

Here, the self-inductance of the detection coil CL12 is the same (or approximately the same) as the self-inductance of the detection coil CL22. This allows the gradient magnetic field sensor 12 to match the sensitivity of the detection coil CL12 and the detection coil CL22 to the uniform magnetic field. In other words, in this case, the voltage output from the detection coil CL12 by applying a magnetic field of a predetermined magnitude X1 to the first sensor head S1 can be a voltage of the opposite polarity and the same magnitude as the voltage output from the detection coil CL22 by applying a magnetic field of a predetermined magnitude X1 to the second sensor head S2. As a result, the gradient magnetic field sensor 12 can suppress the detection of a uniform magnetic field by the combination of the detection coil CL12 and the detection coil CL22. The self-inductance of the detection coil CL12 may be configured differently from the self-inductance of the detection coil CL22. In this case, the gradient magnetic field sensor 12 performs signal processing to match the magnitude of the voltage output from the detection coil CL12 when a magnetic field of a predetermined magnitude X1 is applied to the first sensor head S1 with the magnitude of the voltage output from the detection coil CL22 when a magnetic field of a predetermined magnitude X1 is applied to the second sensor head S2, thereby preventing the detection of a uniform magnetic field.

The input terminal DT32 of the detection circuit DT3 is connected to the hot terminal of the two terminals of the detection coil CL13 through a transmission line. The cold terminal of the two terminals of the detection coil CL13 is connected to the cold terminal of the two terminals of the detection coil CL23 through a transmission line. The hot terminal of the two terminals of the detection coil CL23 is grounded to the ground through the transmission line. That is, the detection coil CL13 and the detection coil CL23 are differentially connected. Note that, other circuit elements, other devices, etc. may be connected between the input terminal DT32 and the detection coil CL13 within a range that does not impair the functions of the gradient magnetic field sensor 12. Note that, other circuit elements, other devices, etc. may be connected between the detection coil CL13 and the detection coil CL23 within a range that does not impair the functions of the gradient magnetic field sensor 12. In addition, other circuit elements, other devices, etc. may be connected between the detection coil CL23 and ground as long as the functionality of the gradient magnetic field sensor 12 is not impaired.

Here, the self-inductance of the detection coil CL13 is the same (or approximately the same) as the self-inductance of the detection coil CL23. This allows the gradient magnetic field sensor 12 to match the sensitivity of the detection coil CL13 and the detection coil CL23 to the uniform magnetic field. In other words, in this case, the voltage output from the detection coil CL13 by applying a magnetic field of a predetermined magnitude X1 to the first sensor head S1 can be a voltage of the opposite polarity and the same magnitude as the voltage output from the detection coil CL23 by applying a magnetic field of a predetermined magnitude X1 to the second sensor head S2. As a result, the gradient magnetic field sensor 12 can suppress the detection of a uniform magnetic field by the combination of the detection coil CL13 and the detection coil CL23. The self-inductance of the detection coil CL13 may be configured differently from the self-inductance of the detection coil CL23. In this case, the gradient magnetic field sensor 12 performs signal processing to match the magnitude of the voltage output from the detection coil CL13 when a magnetic field of a predetermined magnitude X1 is applied to the first sensor head S1 with the magnitude of the voltage output from the detection coil CL23 when a magnetic field of a predetermined magnitude X1 is applied to the second sensor head S2, thereby preventing the detection of a uniform magnetic field.

The input terminal DT42 of the detection circuit DT4 is connected to the hot terminal of the two terminals of the detection coil CL14 through a transmission line. The cold terminal of the two terminals of the detection coil CL14 is connected to the cold terminal of the two terminals of the detection coil CL24 through a transmission line. The hot terminal of the two terminals of the detection coil CL24 is grounded to the ground through the transmission line. That is, the detection coil CL14 and the detection coil CL24 are differentially connected. Note that, other circuit elements, other devices, etc. may be connected between the input terminal DT42 and the detection coil CL14 within a range that does not impair the functions of the gradient magnetic field sensor 12. Note that, other circuit elements, other devices, etc. may be connected between the detection coil CL14 and the detection coil CL24 within a range that does not impair the functions of the gradient magnetic field sensor 12. In addition, other circuit elements, other devices, etc. may be connected between the detection coil CL24 and ground as long as the functionality of the gradient magnetic field sensor 12 is not impaired.

Here, the self-inductance of the detection coil CL14 is the same (or approximately the same) as the self-inductance of the detection coil CL24. This allows the gradient magnetic field sensor 12 to match the sensitivity of the detection coil CL14 and the detection coil CL24 to the uniform magnetic field. In other words, in this case, the voltage output from the detection coil CL14 by applying a magnetic field of a predetermined magnitude X1 to the first sensor head S1 can be a voltage of the opposite polarity and the same magnitude as the voltage output from the detection coil CL24 by applying a magnetic field of a predetermined magnitude X1 to the second sensor head S2. As a result, the gradient magnetic field sensor 12 can suppress the detection of a uniform magnetic field by the combination of the detection coil CL14 and the detection coil CL24. The self-inductance of the detection coil CL14 may be configured differently from the self-inductance of the detection coil CL24. In this case, the gradient magnetic field sensor 12 performs signal processing to match the magnitude of the voltage output from the detection coil CL14 when a magnetic field of a predetermined magnitude X1 is applied to the first sensor head S1 with the magnitude of the voltage output from the detection coil CL24 when a magnetic field of a predetermined magnitude X1 is applied to the second sensor head S2, thereby preventing the detection of a uniform magnetic field.

In addition, some or all of the self-inductances of the detection coils CL11 and CL21, the self-inductances of the detection coils CL12 and CL22, the self-inductances of the detection coils CL13 and CL23, and the self-inductances of the detection coils CL14 and CL24 may be the same or different from each other. However, if the self-inductances of the detection coils CL11 and CL21, the self-inductances of the detection coils CL12 and CL22, the self-inductances of the detection coils CL13 and CL23, and the self-inductances of the detection coils CL14 and CL24 are all the same, the gradient magnetic field sensor 12 can match the sensitivity of the combination of the detection coils CL11 and CL21, the sensitivity of the combination of the detection coils CL12 and CL22, the sensitivity of the combination of the detection coils CL13 and CL23, and the sensitivity of the combination of the detection coils CL14 and CL24. As a result, the gradient magnetic field sensor 12 can suppress the detection of a uniform magnetic field, and can suppress the detection of a uniform gradient magnetic field generated by a gradient magnetic field coil or the like, as described later. In a region where a uniform gradient magnetic field is applied, the strength of the magnetic field at each position in the region is uniform, and the strength of the magnetic field changes periodically at each position in the region. A cause of the generation of a uniform gradient magnetic field is, for example, vibration caused by a motor driving a production line. For example, the gradient magnetic field sensor 12 installed in the production line may be used while covered by a magnetic body such as a magnetic shield. In this case, the magnetic body also vibrates in response to the vibration of the production line, generating a fluctuating magnetic field. Such a fluctuating magnetic field may be detected as a uniform gradient magnetic field by the gradient magnetic field sensor 12. The uniform gradient magnetic field generated in this way behaves as magnetic noise, and may hinder the detection of the detection target by the gradient magnetic field sensor 12. The gradient magnetic field sensor 12 can suppress the detection of such a uniform gradient magnetic field.

The inductance of the detection coil CL1n and the inductance of the detection coil CL2n can be matched, for example, by matching the number of turns per unit length of the detection coil CL1n and the number of turns per unit length of the detection coil CL2n, and by matching the length of the detection coil CL1n in the central axis direction and the length of the detection coil CL2n in the central axis direction. Note that other methods may be used to match the inductance of the detection coil CL1n and the inductance of the detection coil CL2n.

The input terminal PR1 of the signal processing unit PR is connected to the output terminal DT13 of the detection circuit DT1 via a transmission line. The input terminal PR2 of the signal processing unit PR is connected to the output terminal DT23 of the detection circuit DT2 via a transmission line. The input terminal PR3 of the signal processing unit PR is connected to the output terminal DT33 of the detection circuit DT3 via a transmission line. The input terminal PR4 of the signal processing unit PR is connected to the output terminal DT43 of the detection circuit DT4 via a transmission line. The output terminal PR5 of the signal processing unit PR is connected to the detection signal output terminal CT2 via a transmission line. Note that other circuit elements, other devices, etc. may be connected between the input terminal PR1 and the output terminal DT13 within a range that does not impair the functions of the gradient magnetic field sensor 12. In addition, other circuit elements, other devices, etc. may be connected between the input terminal PR2 and the output terminal DT23, as long as the functions of the gradient magnetic field sensor 12 are not impaired. In addition, other circuit elements, other devices, etc. may be connected between the input terminal PR3 and the output terminal DT33, as long as the functions of the gradient magnetic field sensor 12 are not impaired. In addition, other circuit elements, other devices, etc. may be connected between the input terminal PR4 and the output terminal DT43, as long as the functions of the gradient magnetic field sensor 12 are not impaired. In addition, other circuit elements, other devices, etc. may be connected between the output terminal PR5 and the detection signal output terminal CT2, as long as the functions of the gradient magnetic field sensor 12 are not impaired.

Next, referring to FIG. 3, the circuit configuration of each of the detection circuits DT1 to DT4 will be described. In the following, as an example, a case will be described where each of the detection circuits DT1 to DT4 has the same configuration. For this reason, in the following, the configuration of each of the detection circuits DT1 to DT4 will be described using the configuration of the detection circuit DT1 as an example, and a description of the configuration of each of the detection circuits DT2 to DT4 will be omitted. FIG. 3 is a diagram showing an example of the configuration of the detection circuit DT1. Note that some or all of the detection circuits DT1 to DT4 may have different configurations from each other as long as the functions of the gradient magnetic field sensor 12 are not impaired.

The detection circuit DT1 includes a phase detection circuit PD, a low-pass filter LF, an error amplifier EA, and a resistor R1. The phase detection circuit PD has three terminals: an input terminal PD1, an input terminal PD2, and an output terminal PD3.

In the detection circuit DT1, the input terminal DT11 is connected to the input terminal PD1 of the phase detection circuit PD via a transmission line. The output terminal PD3 of the phase detection circuit PD is connected to the input terminal of the low-pass filter LF via a transmission line. The output terminal of the low-pass filter LF is connected to the inverting input terminal of the error amplifier EA via a transmission line. The non-inverting input terminal of the error amplifier EA is grounded to the ground via a transmission line. The output terminal of the error amplifier EA is connected to the output terminal DT13 of the detection circuit DT1 and one of the two terminals of the resistor R1 via a transmission line. The other of the two terminals of the resistor R1 is connected to the input terminal DT12 of the phase detection circuit PD and one of the two terminals of the third capacitor C3 via a transmission line. The other of the two terminals of the third capacitor C3 is connected to the input terminal PD2 of the phase detection circuit PD through a transmission line. In this way, the detection circuit DT1 is a PSD (Phase Sensitive Detector) circuit. Note that, between the input terminal DT11 and the phase detection circuit PD, other circuit elements, other devices, etc. may be connected within a range that does not impair the function of the detection circuit DT1. Also, between the phase detection circuit PD and the low-pass filter LF, other circuit elements, other devices, etc. may be connected within a range that does not impair the function of the detection circuit DT1. Also, between the low-pass filter LF and the error amplifier EA, other circuit elements, other devices, etc. may be connected within a range that does not impair the function of the detection circuit DT1. Also, between the error amplifier EA and ground, other circuit elements, other devices, etc. may be connected within a range that does not impair the function of the detection circuit DT1. In addition, other circuit elements, other devices, etc. may be connected between the error amplifier EA and the output terminal DT13, as long as the function of the detection circuit DT1 is not impaired. In addition, other circuit elements, other devices, etc. may be connected between the error amplifier EA and the resistor R1, as long as the function of the detection circuit DT1 is not impaired. In addition, other circuit elements, other devices, etc. may be connected between the resistor R1 and the third capacitor C3, as long as the function of the detection circuit DT1 is not impaired. In addition, other circuit elements, other devices, etc. may be connected between the third capacitor C3 and the phase detection circuit PD, as long as the function of the detection circuit DT1 is not impaired. In addition, other circuit elements, other devices, etc. may be connected between the transmission line connecting the resistor R1 and the third capacitor C3 and the input terminal DT12, as long as the function of the detection circuit DT1 is not impaired.

Next, the circuit configuration of the signal processing unit PR will be described with reference to FIG. 4. FIG. 4 is a diagram showing an example of the circuit configuration of the signal processing unit PR.

The signal processing unit PR includes four buffers, buffer B1 through buffer B4, four amplifiers, amplifier A1 through amplifier A4, six calculation units, calculation units O1 through O6, a differential amplifier DA, four high-pass filters, high-pass filter HF1 through high-pass filter HF4, three full-wave rectifiers, full-wave rectifier FR1 through full-wave rectifier FR3, a hysteresis comparator HC, and an inverter IV.

Each of the four buffers, buffer B1 to buffer B4, is, for example, a voltage follower, but is not limited to this.

Each of the four amplifiers A1 to A4 is, for example, an operational amplifier, but is not limited to this.

Each of the six calculation units O1 to O6 is, for example, an adder, but is not limited to this.

In the signal processing unit PR, the input terminal of the buffer B1 is connected to the input terminal PR1 of the signal processing unit PR via a transmission line. The output terminal of the buffer B1 is connected to the input terminal of the amplifier A1 via a transmission line. The output terminal of the amplifier A1 is connected to one of the two input terminals of the calculation unit O1 and one of the two input terminals of the calculation unit O4 via a transmission line. The input terminal of the buffer B2 is connected to the input terminal PR2 of the signal processing unit PR via a transmission line. The output terminal of the buffer B2 is connected to the input terminal of the amplifier A2 via a transmission line. The output terminal of the amplifier A2 is connected to the other of the two input terminals of the calculation unit O1 and one of the two input terminals of the calculation unit O2 via a transmission line. The input terminal of the buffer B3 is connected to the input terminal PR3 of the signal processing unit PR through a transmission line. The output terminal of the buffer B3 is connected to the input terminal of the amplifier A3 through a transmission line. The output terminal of the amplifier A3 is connected to the other of the two input terminals of the arithmetic unit O2 and one of the two input terminals of the arithmetic unit O3 through a transmission line. The output terminal of the buffer B4 is connected to the input terminal of the amplifier A4 through a transmission line. The output terminal of the amplifier A4 is connected to the other of the two input terminals of the arithmetic unit O3 and the other of the two input terminals of the arithmetic unit O4 through a transmission line. Note that other circuit elements, other devices, etc. may be connected between the input terminal PR1 and the buffer B1 within a range that does not impair the function of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the buffer B1 and the amplifier A1, within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the amplifier A1 and the arithmetic unit O1, within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the amplifier A1 and the arithmetic unit O4, within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the input terminal PR2 and the buffer B2, within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the buffer B2 and the amplifier A2, within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the amplifier A2 and the arithmetic unit O1, within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the amplifier A2 and the arithmetic unit O2 within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the input terminal PR3 and the buffer B3 within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the buffer B3 and the amplifier A3 within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the amplifier A3 and the arithmetic unit O2 within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the amplifier A3 and the arithmetic unit O3 within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the input terminal PR4 and the buffer B4 within a range that does not impair the functions of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the buffer B4 and the amplifier A4, as long as the functions of the signal processing unit PR are not impaired. In addition, other circuit elements, other devices, etc. may be connected between the amplifier A4 and the calculation unit O3, as long as the functions of the signal processing unit PR are not impaired. In addition, other circuit elements, other devices, etc. may be connected between the amplifier A4 and the calculation unit O4, as long as the functions of the signal processing unit PR are not impaired.

In addition, in the signal processing unit PR, the output terminal of the calculation unit O1 is connected to one of the two input terminals of the differential amplifier DA via a transmission line. In addition, the output terminal of the calculation unit O3 is connected to the other of the two input terminals of the differential amplifier DA via a transmission line. The output terminal of the differential amplifier DA is connected to the input terminal of the high-pass filter HF1 via a transmission line. In addition, the output terminal of the high-pass filter HF1 is connected to the input terminal of the full-wave rectifier FR1 via a transmission line. In addition, other circuit elements, other devices, etc. may be connected between the calculation unit O1 and the differential amplifier DA within a range that does not impair the function of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the calculation unit O3 and the differential amplifier DA within a range that does not impair the function of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the differential amplifier DA and the high-pass filter HF1, as long as the functions of the signal processing unit PR are not impaired. In addition, other circuit elements, other devices, etc. may be connected between the high-pass filter HF1 and the full-wave rectifier FR1, as long as the functions of the signal processing unit PR are not impaired.

In the signal processing unit PR, the output terminal of the calculation unit O2 is connected to the input terminal of the high-pass filter HF2 via a transmission line. The output terminal of the high-pass filter HF2 is connected to the input terminal of the full-wave rectifier FR2 via a transmission line. Note that other circuit elements, other devices, etc. may be connected between the calculation unit O2 and the high-pass filter HF2, as long as the functions of the signal processing unit PR are not impaired. Note that other circuit elements, other devices, etc. may be connected between the high-pass filter HF2 and the full-wave rectifier FR2, as long as the functions of the signal processing unit PR are not impaired.

In the signal processing unit PR, the output terminal of the full-wave rectifier FR1 is connected to one of the two input terminals of the calculation unit O5 via a transmission line. The output terminal of the full-wave rectifier FR2 is connected to the other of the two input terminals of the calculation unit O5 via a transmission line. Note that other circuit elements, other devices, etc. may be connected between the full-wave rectifier FR1 and the calculation unit O5 as long as the functions of the signal processing unit PR are not impaired. Note that other circuit elements, other devices, etc. may be connected between the full-wave rectifier FR2 and the calculation unit O5 as long as the functions of the signal processing unit PR are not impaired.

In the signal processing unit PR, the output terminal of the calculation unit O4 is connected to the input terminal of the high-pass filter HF3 via a transmission line. The output terminal of the high-pass filter HF3 is connected to the input terminal of the full-wave rectifier FR3 via a transmission line. Note that other circuit elements, other devices, etc. may be connected between the calculation unit O4 and the high-pass filter HF3, as long as the functions of the signal processing unit PR are not impaired. Note that other circuit elements, other devices, etc. may be connected between the high-pass filter HF3 and the full-wave rectifier FR3, as long as the functions of the signal processing unit PR are not impaired.

In the signal processing unit PR, the output terminal of the calculation unit O5 is connected to one of the two input terminals of the calculation unit O6 via a transmission line. The output terminal of the full-wave rectifier FR3 is connected to the other of the two input terminals of the calculation unit O6 via a transmission line. The output terminal of the calculation unit O6 is connected to the input terminal of the high-pass filter HF4 via a transmission line. The output terminal of the high-pass filter HF4 is connected to the input terminal of the hysteresis comparator HC via a transmission line. The output terminal of the hysteresis comparator HC is connected to the input terminal of the inverter IV via a transmission line. The output terminal of the inverter IV is connected to the output terminal PR5 of the signal processing unit PR via a transmission line. In addition, between the calculation unit O5 and the calculation unit O6, other circuit elements, other devices, etc. may be connected within a range that does not impair the function of the signal processing unit PR. In addition, between the full-wave rectifier FR3 and the calculation unit O6, other circuit elements, other devices, etc. may be connected within a range that does not impair the function of the signal processing unit PR. In addition, between the calculation unit O6 and the high-pass filter HF4, other circuit elements, other devices, etc. may be connected within a range that does not impair the function of the signal processing unit PR. In addition, between the high-pass filter HF4 and the hysteresis comparator HC, other circuit elements, other devices, etc. may be connected within a range that does not impair the function of the signal processing unit PR. In addition, between the hysteresis comparator HC and the inverter IV, other circuit elements, other devices, etc. may be connected within a range that does not impair the function of the signal processing unit PR. In addition, other circuit elements, other devices, etc. may be connected between the inverter IV and the output terminal PR5, as long as the functions of the signal processing unit PR are not impaired.

With the above configuration, the gradient magnetic field sensor 12 can detect the presence or absence of a detection target using the first sensor head S1 and the second sensor head S2. More specifically, the gradient magnetic field sensor 12 can detect the presence or absence of a detection target according to the voltages output from each of the four detection coils CL1 of the first sensor head S1 and the voltages output from the four detection coils CL2 of the second sensor head S2.

Now, referring to FIG. 5, a more detailed configuration of the first sensor head S1 and the second sensor head S2 will be described. FIG. 5 is a diagram showing an example of a more detailed configuration of the first sensor head S1 and the second sensor head S2. FIG. 5 is also a cross-sectional view of the gradient magnetic field sensor 12 when the gradient magnetic field sensor 12 is cut in a plane that includes both the central axis AX1 of the first magnetic core CR1 and the central axis AX2 of the second magnetic core CR2.

5, the gradient magnetic field sensor 12 includes a first sensor head S1, a second sensor head S2, and a main body MB. Here, the main body MB is a housing in which the first magnetic core CR1 and the second magnetic core CR2 are provided. The main body MB includes at least some of the AC power supply connection terminal CT1, the 0th phase shift circuit PS0, the AC current control unit CC1, the detection circuit DT1, the detection circuit DT2, the detection circuit DT3, the detection circuit DT4, the signal processing unit PR, and the detection signal output terminal CT2. In the following, as an example, a case will be described in which the main body MB includes all of the AC power supply connection terminal CT1, the 0th phase shift circuit PS0, the AC current control unit CC1, the detection circuit DT1, the detection circuit DT2, the detection circuit DT3, the detection circuit DT4, the signal processing unit PR, and the detection signal output terminal CT2. However, in order to simplify the illustration, the internal structure of the main body part MB is omitted in FIG.

The linear portion of the first magnetic core CR1 is parallel (or approximately parallel) to the linear portion of the second magnetic core CR2. In the example shown in FIG. 5, the first magnetic core CR1 is linear in shape and extends along the central axis AX1. Also, in this example, the second magnetic core CR2 is linear in shape and extends along the central axis AX2. That is, in this example, the linear first magnetic core CR1 is parallel (or approximately parallel) to the linear second magnetic core CR2. The linear portion of the first magnetic core CR1 may be non-parallel to the linear portion of the second magnetic core CR2.

In the example shown in FIG. 5, the first magnetic core CR1 and the second magnetic core CR2 each protrude in a predetermined direction from the main body MB so that their linear portions are approximately parallel to each other. The predetermined direction is the left side of the paper in FIG. 5, but is not limited to this. The linear portion of the first magnetic core CR1 and the linear portion of the second magnetic core CR2 may be configured to be coaxial (or approximately coaxial).

In the example shown in FIG. 5, the linear portion of the first magnetic core CR1 overlaps entirely (or almost entirely) with the linear portion of the second magnetic core CR2 when viewed from an orthogonal direction (i.e., a direction orthogonal to each of the central axis AX1 and the central axis AX2, and along a plane including the central axis AX1 and the central axis AX2) that is perpendicular to each of the linear portions of the first magnetic core CR1 and the second magnetic core CR2. Note that in this case, the linear portion of the first magnetic core CR1 may be configured to overlap partially with the linear portion of the second magnetic core CR2.

5, the n-th detection coil CL1n of the N detection coils CL1 overlaps (or almost overlaps) with the n-th detection coil CL2n of the N detection coils CL2 when viewed from the orthogonal direction. Therefore, in this example, in the gradient magnetic field sensor 12, the shape (structure) of the first sensor head S1 is the same (or almost the same) shape (structure) as the shape (structure) of the second sensor head S2. In other words, in the gradient magnetic field sensor 12, the first magnetic core CR1 around which the four detection coils CL1 are wound and the second magnetic core CR2 around which the four detection coils CL2 are wound are arranged line-symmetrically. This allows the gradient magnetic field sensor 12 to suppress the occurrence of fluctuations in the sensitivity of the gradient magnetic field sensor 12 caused by the difference between the shape of the first sensor head S1 and the shape of the second sensor head S2. In other words, this allows the gradient magnetic field sensor 12 to ensure symmetry between the sensitivity distribution of the first sensor head S1 and the sensitivity distribution of the second sensor head S2.

Here, the winding direction of the n-th detection coil CL1n among the N detection coils CL1 in the predetermined direction is opposite to the winding direction of the n-th detection coil CL2n among the N detection coils CL2 in the predetermined direction. The cold terminal of the two terminals of the detection coil CL1n is connected to the cold terminal of the two terminals of the detection coil CL2n. As a result, the detection coil CL1n is differentially connected to the detection coil CL2n as described above. As a result, the polarity of the voltage output from the detection coil CL1n is opposite to the polarity of the voltage output from the detection coil CL2n. The differential connection between the detection coil CL1n and the detection coil CL2n may be realized by other methods. For example, the detection coil CL1n and the detection coil CL2n may be configured to be differentially connected via an element (e.g., an inverter, etc.) that inverts the polarity of the voltage. In this case, the winding direction of the detection coil CL1n relative to the predetermined direction is the same as the winding direction of the detection coil CL2n relative to the predetermined direction, and the cold side terminal of the detection coil CL1n is connected to the hot side terminal of the detection coil CL2n via an element that reverses the polarity of the voltage. As a result, in this case, the polarity of the voltage output from the detection coil CL1n is the same as the polarity of the voltage output from the detection coil CL2n. In addition, the polarity of the voltage may be reversed, for example, by each of the detection circuits DT1 to DT4, the signal processing unit PR, etc.

In the example shown in FIG. 5, two adjacent detection coils CL1 in the N detection coils CL1 are insulated from each other and are in contact with each other. In the example, two adjacent detection coils CL2 in the N detection coils CL2 are insulated from each other and are in contact with each other. In other words, in the gradient magnetic field sensor 12, two adjacent detection coils CL1 in the N detection coils CL1 are wound around the first magnetic core CR1 so that there is no gap between them. In the gradient magnetic field sensor 12, two adjacent detection coils CL2 in the N detection coils CL2 are wound around the second magnetic core CR2 so that there is no gap between them. As a result, the gradient magnetic field sensor 12 can efficiently capture the magnetic flux generated from the detection object into the first magnetic core CR1 and the second magnetic core CR2, thereby improving the detection accuracy of the detection object. This also allows the gradient magnetic field sensor 12 to match the period of the distribution of sensitivity to the magnetic moment of each of the four detection coils CL1 in the first sensor head S1 with the period of the arrangement interval of these four detection coils CL1. Also, the gradient magnetic field sensor 12 can match the period of the distribution of sensitivity to the magnetic moment of each of the four detection coils CL2 in the second sensor head S2 with the period of the arrangement interval of these four detection coils CL2. As a result, the gradient magnetic field sensor 12 can accurately eliminate the effects of noise.

<Operation of the gradient magnetic field sensor according to the embodiment>
Next, the operation of the gradient magnetic field sensor 12 will be described.

In the gradient magnetic field sensor 12 having the circuit configuration described above, the AC power supply P1 inputs an AC current to the AC power supply connection terminal CT1. The AC current input to the AC power supply connection terminal CT1 is branched into two. One of the two branched AC currents is input to the 0th phase shift circuit PS0 as an AC excitation current. The other of the two branched AC currents is input as a reference signal to the input terminal DT11 of the detection circuit DT1, the input terminal DT21 of the detection circuit DT2, the input terminal DT31 of the detection circuit DT3, and the input terminal DT41 of the detection circuit DT4.

The AC excitation current input to the 0th phase shift circuit PS0 is phase shifted by the 0th phase shift circuit PS0, and then branched into two AC excitation currents, the first AC excitation current AC1 and the second AC excitation current AC2.

The first AC excitation current AC1 flows through the first magnetic core CR1 via the first phase shift circuit PS1, the first variable resistor VR11, and the first capacitor C1. Therefore, the first AC excitation current AC1 is phase-shifted by the first phase shift circuit PS1, and the amplitude is adjusted by the first variable resistor VR11. As a result, a voltage corresponding to the first AC excitation current AC1 flowing through the first magnetic core CR1 and the strength of the magnetic field applied to the detection coil CL1n is induced in the n-th detection coil CL1n among the four detection coils CL1. That is, this voltage is output from the detection coil CL1n. And, this voltage changes according to the strength of the magnetic field applied to the detection coil CL1n from the outside. In the following, for convenience of explanation, the voltage induced in the detection coil CL1n will be referred to as the first induced voltage. Note that the first capacitor C1 is a capacitor for AC (Alternating Current) coupling. The presence of the first capacitor C1 prevents direct current from flowing from the output terminal CC13 to the input terminal CC11 in the gradient magnetic field sensor 12.

The second AC excitation current AC2 flows through the second magnetic core CR2 via the second phase shift circuit PS2, the second variable resistor VR12, and the second capacitor C2. Therefore, the second AC excitation current AC2 is phase-shifted by the second phase shift circuit PS2, and the amplitude is adjusted by the second variable resistor VR12. As a result, a voltage corresponding to the second AC excitation current AC2 flowing through the second magnetic core CR2 and the strength of the magnetic field applied to the detection coil CL2n is induced in the n-th detection coil CL2n among the four detection coils CL2. That is, the detection coil CL2n outputs the voltage. And the voltage changes according to the strength of the magnetic field applied to the detection coil CL2n from the outside. In the following, for convenience of explanation, the voltage induced in the detection coil CL2n is referred to as the second induced voltage. The second capacitor C2 is a capacitor for AC coupling. The presence of the second capacitor C2 prevents direct current from flowing from the output terminal CC14 to the input terminal CC12 in the gradient magnetic field sensor 12.

In the gradient magnetic field sensor 12, a signal corresponding to the first induced voltage induced in the detection coil CL11 and the second induced voltage induced in the detection coil CL21 is input as a differential signal Vout1 to the input terminal DT12 of the detection circuit DT1. In the gradient magnetic field sensor 12, a signal corresponding to the first induced voltage induced in the detection coil CL12 and the second induced voltage induced in the detection coil CL22 is input as a differential signal Vout2 to the input terminal DT22 of the detection circuit DT2. In the gradient magnetic field sensor 12, a signal corresponding to the first induced voltage induced in the detection coil CL13 and the second induced voltage induced in the detection coil CL23 is input as a differential signal Vout3 to the input terminal DT32 of the detection circuit DT3. In addition, in the gradient magnetic field sensor 12, a signal corresponding to the first induced voltage induced in the detection coil CL14 and the second induced voltage induced in the detection coil CL24 is input as a differential signal Vout4 to an input terminal DT42 of the detection circuit DT4.

Here, each of the detection circuits DT1 to DT4 performs the same operation, but the only difference is the differential signal that is input. For this reason, the operation of each of the detection circuits DT1 to DT4 will be explained below using the operation of the detection circuit DT1 as an example. In this embodiment, the explanation of the operation of each of the detection circuits DT2 to DT4 will be omitted.

The phase detection circuit PD of the detection circuit DT1 inputs a signal corresponding to the difference between the input differential signal Vout1 and the reference signal input from the AC power supply P1 to the low-pass filter LF as a detection signal. The low-pass filter LF outputs an output signal obtained by removing components of a predetermined first frequency or higher from the detection signal input from the phase detection circuit PD to the inverting input terminal of the error amplifier EA. The error amplifier EA outputs a signal corresponding to the potential difference between the potential of the output signal input to the inverting input terminal of the error amplifier EA and the ground potential as a detection signal to each of the detection signal output terminal CT2 and the resistor R1. The third capacitor C3 is a capacitor for AC coupling. A buffer circuit (e.g., a voltage follower, etc.) is connected to the terminal of the third capacitor C3 on the opposite side to the terminal connected to the phase detection circuit PD. This buffer circuit is not shown in the figure to simplify the drawing.

The detection signal input to resistor R1 flows as a feedback current from input terminal DT12 to detection coil CL11 without being input to the third capacitor C3 due to the presence of a buffer circuit (not shown).

By adjusting the detection circuit DT1 that performs such operations, when the signal level of the differential signal Vout1 is 0 [V], a detection signal with a signal level of 0 [V] can be output from the detection signal output terminal CT2, and when the signal level of the differential signal Vout1 is not 0 [V], a detection signal with a signal level other than 0 [V] can be output from the detection signal output terminal CT2. Note that since the adjustment of the detection circuit DT1 is known, further detailed explanation is omitted.

In this way, each of the detection circuits DT1 to DT4 outputs a detection signal from the output terminal. That is, the detection circuit DT1 outputs a detection signal corresponding to the differential signal Vout1 to the signal processing unit PR. Furthermore, the detection circuit DT2 outputs a detection signal corresponding to the differential signal Vout2 to the signal processing unit PR. The detection circuit DT3 outputs a detection signal corresponding to the differential signal Vout3 to the signal processing unit PR. Furthermore, the detection circuit DT4 outputs a detection signal corresponding to the differential signal Vout4 to the signal processing unit PR. Here, at this time, each of the detection circuits DT1 to DT4 synchronizes the output timing of the detection signals. The method of synchronizing the output timing may be a known method or a method to be developed in the future.

The signal processing unit PR performs various signal processing operations on the detection signals input from each of the detection circuits DT1 to DT4. Some or all of the functions of the signal processing unit PR may be realized by hardware or by software. However, if some or all of the functions of the signal processing unit PR are realized by software, each detection signal input to the signal processing unit PR is converted into digital data by an A (Analog)/D (Digital) converter. The following describes, as an example, a case where the entire signal processing unit PR is realized by hardware.

Specifically, in the signal processing unit PR, for example, the detection signal input from the detection circuit DT1 is temporarily stored by the buffer B1. In the example shown in FIG. 4, the detection signal is indicated by a symbol of a circle with a 1 inside it. In the following, for convenience of explanation, the symbol is indicated by a circle with a 1. After the detection signal is stored in the buffer B1, the amplifier A1 reads out the detection signal stored in the buffer B1 and amplifies the read detection signal based on a predetermined gain. The detection signal amplified by the amplifier A1 is input to each of the calculation units O1 and O4. Note that the signal processing unit PR may not have a buffer B1. In this case, in the signal processing unit PR, the detection signal input from the detection circuit DT1 is input to the amplifier A1.

In addition, in the signal processing unit PR, for example, the detection signal input from the detection circuit DT2 is temporarily stored by the buffer B2. In the example shown in FIG. 4, the detection signal is indicated by a symbol of a circle with a 2 inside it. In the following, for convenience of explanation, the symbol is indicated by a circle 2. After the detection signal is stored in the buffer B2, the amplifier A2 reads out the detection signal stored in the buffer B2 and amplifies the read detection signal based on a predetermined gain. The detection signal amplified by the amplifier A2 is input to each of the calculation units O1 and O2. Note that the signal processing unit PR may not have a buffer B2. In this case, in the signal processing unit PR, the detection signal input from the detection circuit DT2 is input to the amplifier A2.

In the signal processing unit PR, for example, the detection signal input from the detection circuit DT3 is temporarily stored by the buffer B3. In the example shown in FIG. 4, the detection signal is indicated by a symbol of a circle with a 3 inside it. In the following, for convenience of explanation, the symbol is indicated by a circle 3. After the detection signal is stored in the buffer B3, the amplifier A3 reads out the detection signal stored in the buffer B3 and amplifies the read detection signal based on a predetermined gain. The detection signal amplified by the amplifier A3 is input to each of the calculation units O2 and O3. Note that the signal processing unit PR may not have a buffer B3. In this case, in the signal processing unit PR, the detection signal input from the detection circuit DT3 is input to the amplifier A3.

In addition, in the signal processing unit PR, for example, the detection signal input from the detection circuit DT4 is temporarily stored by the buffer B4. In the example shown in FIG. 4, the detection signal is indicated by a symbol of a circle with a 4 inside it. In the following, for convenience of explanation, the symbol is indicated by a circle 4. After the detection signal is stored in the buffer B4, the amplifier A4 reads out the detection signal stored in the buffer B4 and amplifies the read detection signal based on a predetermined gain. The detection signal amplified by the amplifier A4 is input to each of the calculation units O3 and O4. Note that the signal processing unit PR may not have a buffer B4. In this case, in the signal processing unit PR, the detection signal input from the detection circuit DT4 is input to the amplifier A4.

In the signal processing unit PR, the calculation unit O1 adds, for example, the detection signal amplified by the amplifier A1 and the detection signal amplified by the amplifier A2. The calculation unit O1 inputs the detection signal after addition to the differential amplifier DA. In the example shown in FIG. 4, the detection signal is represented by "○1+○2".

In the signal processing unit PR, the calculation unit O2 adds, for example, the detection signal amplified by the amplifier A2 and the detection signal amplified by the amplifier A3. The calculation unit O2 inputs the detection signal after the addition to the high-pass filter HF2. In the example shown in FIG. 4, the detection signal is represented by "○2+○3".

In the signal processing unit PR, the calculation unit O3 adds, for example, the detection signal amplified by the amplifier A3 and the detection signal amplified by the amplifier A4. The calculation unit O3 inputs the detection signal after addition to the differential amplifier DA. In the example shown in FIG. 4, the detection signal is represented by "○3+○4".

In the signal processing unit PR, the calculation unit O4 adds, for example, the detection signal amplified by the amplifier A1 and the detection signal amplified by the amplifier A4. The calculation unit O4 inputs the detection signal after the addition to the high-pass filter HF3. In the example shown in FIG. 4, the detection signal is represented by "○1+○4".

In the signal processing unit PR, the differential amplifier DA generates, for example, a differential signal between the detection signal input from the calculation unit O1 and the detection signal input from the calculation unit O3 as a new detection signal, and inputs the generated detection signal to the high-pass filter HF1. As a result, the high-pass filter HF1 removes the DC component from the detection signal. In other words, the high-pass filter HF1 inputs the signal obtained by removing components below a predetermined first frequency from the detection signal to the full-wave rectifier FR1 as a new detection signal.

In addition, in the signal processing unit PR, the full-wave rectifier FR1 full-wave rectifies the detection signal input from the high-pass filter HF1, and inputs the full-wave rectified signal to the calculation unit O5 as a new detection signal.

In addition, in the signal processing unit PR, the high-pass filter HF2 removes DC components from the detection signal input from the calculation unit O2. That is, the high-pass filter HF2 removes components below the first frequency from the detection signal, and inputs the resultant signal to the full-wave rectifier FR2 as a new detection signal.

In addition, in the signal processing unit PR, the full-wave rectifier FR2 full-wave rectifies the detection signal input from the high-pass filter HF2, and inputs the full-wave rectified signal to the calculation unit O5 as a new detection signal.

In the signal processing unit PR, the calculation unit O5 adds the detection signal input from the full-wave rectifier FR1 and the detection signal input from the full-wave rectifier FR2. The calculation unit O5 inputs the detection signal after the addition to the calculation unit O6.

In addition, in the signal processing unit PR, the high-pass filter HF3 removes DC components from the detection signal input from the calculation unit O4. That is, the high-pass filter HF3 removes components below the first frequency from the detection signal, and inputs the resultant signal to the full-wave rectifier FR3 as a new detection signal.

In addition, in the signal processing unit PR, the full-wave rectifier FR3 full-wave rectifies the detection signal input from the high-pass filter HF3, and inputs the full-wave rectified signal to the calculation unit O6 as a new detection signal.

In the signal processing unit PR, the calculation unit O6 adds the detection signal input from the calculation unit O5 and the detection signal input from the full-wave rectifier FR3. The calculation unit O6 inputs the detection signal after the addition to the high-pass filter HF4.

In addition, in the signal processing unit PR, the high-pass filter HF4 removes DC components from the detection signal input from the calculation unit O6. That is, the high-pass filter HF4 removes components below the first frequency from the detection signal, and inputs the signal to the hysteresis comparator HC as a new detection signal.

The hysteresis comparator HC also has two thresholds: a first threshold and a second threshold lower than the first threshold. In the signal processing unit PR, the hysteresis comparator HC determines that the detection signal input from the high-pass filter HF4 is a signal when the signal level is greater than the first threshold or greater than the second threshold, and inputs an L-level signal to the inverter IV. On the other hand, the hysteresis comparator HC determines that the detection signal input from the high-pass filter HF4 is noise when the signal level is within the range between the first and second thresholds, and inputs an H-level signal to the inverter IV. This allows the gradient magnetic field sensor 12 to determine whether the detection signal is noise or not, and as a result, it can detect the detection target with high accuracy.

In the signal processing unit PR, when an H-level signal is input from the hysteresis comparator HC, the inverter IV outputs an L-level signal from the output terminal PR5 as a non-detection signal. On the other hand, when an L-level signal is input from the hysteresis comparator HC, the inverter IV outputs an H-level signal from the output terminal PR5 as a detection signal. Therefore, when the gradient magnetic field sensor 12 detects an object to be detected, it outputs an H-level signal as a detection signal. The inverter IV is provided in the gradient magnetic field sensor 12 because it is considered that it is easier for the information processing device 20 to handle if an H-level signal is output as a detection signal to the information processing device 20. Therefore, the gradient magnetic field sensor 12 may be configured not to include the inverter IV. In this case, the gradient magnetic field sensor 12 outputs an L-level signal as a detection signal.

In the signal processing unit PR, the hysteresis comparator HC may be configured to determine that the detection signal input from the high-pass filter HF4 is a signal when its signal level exceeds a first threshold value or exceeds a second threshold value and outputs an H-level signal from the output terminal PR5 as the detection signal. In this case, the hysteresis comparator HC determines that the detection signal input from the high-pass filter HF4 is noise when its signal level is within the range between the first and second threshold values, and outputs an L-level signal from the output terminal PR5 as a non-detection signal. In this case, the signal processing unit PR does not include an inverter IV.

In addition, in the signal processing unit PR, the hysteresis comparator HC may be configured to determine that the detection signal input from the high-pass filter HF4 is a signal when the signal level is greater than the first threshold or greater than the second threshold, and input an H-level signal to the inverter IV. In this case, the hysteresis comparator HC determines that the detection signal input from the high-pass filter HF4 is noise when the signal level is within the range between the first and second thresholds, and inputs an L-level signal to the inverter IV. In this case, when an L-level signal is input from the hysteresis comparator HC, the inverter IV outputs an H-level signal from the output terminal PR5 as a non-detection signal, and when an H-level signal is input from the hysteresis comparator HC, the inverter IV outputs an L-level signal from the output terminal PR5 as a detection signal.

In addition, in the signal processing unit PR, the hysteresis comparator HC may be configured to determine that the detection signal input from the high-pass filter HF4 is a signal when its signal level exceeds a first threshold value or exceeds a second threshold value and outputs an L-level signal from the output terminal PR5 as the detection signal. In this case, the hysteresis comparator HC determines that the detection signal input from the high-pass filter HF4 is noise when its signal level is within the range between the first and second threshold values, and outputs an H-level signal from the output terminal PR5 as a non-detection signal. In this case, the signal processing unit PR does not include an inverter IV.

Here, the relationship between the voltage of the differential signal output from the detection coil CL1n differentially connected to the detection coil CL2n and the dead zone of the gradient magnetic field sensor 12 will be described. FIG. 6 is a diagram showing an example of a graph plotting the voltage of the differential signal output from the detection coil CL1n differentially connected to the detection coil CL2n while changing the relative position of the detection object with respect to the gradient magnetic field sensor 12. The horizontal axis of the graph shown in FIG. 6 indicates the distance in each of the positive and negative directions from the origin of the horizontal axis, which is the boundary between the detection coil CL12 and the detection coil CL13 when the first sensor head S1 and the second sensor head S2 are viewed from the orthogonal direction. In addition, the vertical axis of the graph indicates the voltage of the differential signal output from each of the detection coil CL11 differentially connected to the detection coil CL21, the detection coil CL12 differentially connected to the detection coil CL22, the detection coil CL13 differentially connected to the detection coil CL23, and the detection coil CL14 differentially connected to the detection coil CL24. However, the polarity of the voltage of these differential signals is defined as peak to peak. Specifically, in this example, the polarity of the voltage of these differential signals is defined as positive when the polarity of the induced voltage of the detection coil CL1n is positive. Also, the curve F1 in the graph is a curve that connects in order each point on which the voltage of the differential signal output from the detection coil CL11 differentially connected to the detection coil CL21 is plotted when the detection object is moved from the tip side of the first sensor head S1 and the second sensor head S2 to the base side of the first sensor head S1 and the second sensor head S2 while keeping the distance between each of the first sensor head S1 and the second sensor head S2 and the detection object constant. Also, the curve F2 in the graph is a curve that connects in order each point on which the voltage of the differential signal output from the detection coil CL12 differentially connected to the detection coil CL22 is plotted in the case. Also, the curve F3 in the graph is a curve that connects in order the points on which the voltage of the differential signal output from the detection coil CL13 that is differentially connected to the detection coil CL23 is plotted in the case. Also, the curve F4 in the graph is a curve that connects in order the points on which the voltage of the differential signal output from the detection coil CL14 that is differentially connected to the detection coil CL24 is plotted in the case.

As shown in FIG. 6, the gradient magnetic field sensor 12 does not have a dead zone that existed at the origin of the horizontal axis in the past. For example, at a position of -25 [mm] from the origin of the horizontal axis, as shown by the curve F1, the voltage of the differential signal output from the detection coil CL11 differentially connected to the detection coil CL21 is 0 [mVpp]. However, at that position, the voltage of the differential signal output from each of the detection coil CL12 differentially connected to the detection coil CL22, the detection coil CL13 differentially connected to the detection coil CL23, and the detection coil CL14 differentially connected to the detection coil CL24 is not 0 [mVpp]. In other words, this indicates that the position is not a dead zone in the gradient magnetic field sensor 12. Also, for example, at a position of -10 [mm] from the origin of the horizontal axis, as shown by the curve F2, the voltage of the differential signal output from the detection coil CL12 differentially connected to the detection coil CL22 is 0 [mVpp]. However, at this position, the voltage of the differential signal output from each of the detection coil CL11 differentially connected to the detection coil CL21, the detection coil CL13 differentially connected to the detection coil CL23, and the detection coil CL14 differentially connected to the detection coil CL24 is not 0 [mVpp]. That is, this indicates that this position is not a dead zone in the gradient magnetic field sensor 12. Also, for example, at a position +5 [mm] from the origin of the horizontal axis, as shown by the curve F3, the voltage of the differential signal output from the detection coil CL13 differentially connected to the detection coil CL23 is 0 [mVpp]. However, at this position, the voltage of the differential signal output from each of the detection coil CL11 differentially connected to the detection coil CL21, the detection coil CL12 differentially connected to the detection coil CL22, and the detection coil CL14 differentially connected to the detection coil CL24 is not 0 [mVpp]. That is, this indicates that this position is not a dead zone in the gradient magnetic field sensor 12. Also, for example, at a position +20 [mm] from the origin on the horizontal axis, as shown by curve F4, the voltage of the differential signal output from detection coil CL14 differentially connected to detection coil CL24 is 0 [mVpp]. However, at that position, the voltage of the differential signal output from each of detection coil CL11 differentially connected to detection coil CL21, detection coil CL12 differentially connected to detection coil CL22, and detection coil CL13 differentially connected to detection coil CL23 is not 0 [mVpp]. In other words, this indicates that this position is not a dead zone in the gradient magnetic field sensor 12.

In this way, in the gradient magnetic field sensor 12, even if the voltage of one of the four differential signals becomes 0 [mVpp], the voltages of the remaining three of the four differential signals do not become 0 [mVpp]. For this reason, there is no dead zone in the gradient magnetic field sensor 12. Also, in the gradient magnetic field sensor 12, since the shape of the first sensor head S1 and the shape of the second sensor head S2 are the same, the sensitivity distribution of the first sensor head S1 matches the sensitivity distribution of the second sensor head S2. This is not unique to the case where N is 4 as in this example, but is also the same when N is 2, 3, etc., and is also the same when N is 5 or more.

Figure 7 is a diagram showing an example of a graph in which the voltage of the detection signal output from the calculation unit O6 is plotted while changing the relative position of the detection object with respect to the gradient magnetic field sensor 12. The horizontal axis of the graph shown in Figure 7 indicates the distance in the positive and negative directions from the origin of the horizontal axis, which is the boundary between the detection coil CL12 and the detection coil CL13 when the first sensor head S1 and the second sensor head S2 are viewed from the orthogonal direction. The vertical axis of the graph indicates the voltage of the detection signal output from the calculation unit O6. The curve F5 in the graph is a curve that sequentially connects each point on which the voltage of the detection signal output from the calculation unit O6 is plotted when the detection object is moved from the tip side of the first sensor head S1 and the second sensor head S2 to the base side of the first sensor head S1 and the second sensor head S2 while keeping the distance between each of the first sensor head S1 and the second sensor head S2 and the detection object constant. In other words, the curve F5 indicates the sensitivity distribution of the gradient magnetic field sensor 12. On the other hand, curve F6 plotted on the graph shows the sensitivity distribution of a conventional gradient magnetic field sensor.

As shown in FIG. 7, the voltage indicated by the curve F6 is more than half the maximum value of the voltage indicated by the curve F6 over the range from the tip of the first sensor head S1 and the second sensor head S2 to the base of the first sensor head S1 and the second sensor head S2 (i.e., the range of -30 [mm] to +30 [mm]). This means that, unlike the sensitivity distribution of the conventional gradient magnetic field sensor indicated by the curve F6, there is no dead zone in the sensitivity distribution of the gradient magnetic field sensor 12. In addition, the sensitivity distribution indicated by the curve F6 indicates that there is no dead zone according to the passing position of the detection object relative to the gradient magnetic field sensor 12. In other words, in this sensitivity distribution, the drop in sensitivity according to the passing position of the detection object relative to the gradient magnetic field sensor 12 is suppressed, and the difference in sensitivity due to the difference in the passing position of the detection object relative to the gradient magnetic field sensor 12 is smaller than that of the conventional gradient magnetic field sensor (approaching a uniform sensitivity distribution compared to the sensitivity distribution of the conventional gradient magnetic field sensor). That is, the gradient magnetic field sensor 12 can suppress the occurrence of dead zones while suppressing the variation in sensitivity according to the passing position of the detection target. In addition, because the gradient magnetic field sensor 12 has such characteristics, multiple gradient magnetic field sensors 12 can be arrayed by arranging the sensor heads so that they do not overlap each other in the orthogonal direction.

In this example, the signal processing unit PR of the gradient magnetic field sensor 12 creates four signals, ○1 + ○2, ○2 + ○3, ○3 + ○4, and ○1 + ○4, based on the detection signal ○1 output from the detection circuit DT1, the detection signal ○2 output from the detection circuit DT2, the detection signal ○3 output from the detection circuit DT3, and the detection signal ○4 output from the detection circuit DT4, and then performs signal processing to add three signals, |(○1 + ○2) - (○3 + ○4)|, |○2 + ○3|, and |○1 + ○4|. This signal processing is signal processing obtained by trial and error, and in addition to improving the sensitivity of the gradient magnetic field sensor 12 compared to other signal processing, the sensitivity fluctuation according to the passing position of the detection target relative to the gradient magnetic field sensor 12 is also small. However, the signal processing unit PR may be configured to perform other signal processing. This other signal processing may include subtraction of some or all of the signals indicated by ○1, ○2, ○3, and ○4. In this case, the signal processing unit PR includes a subtractor. For example, in this case, some or all of the calculation units O1 to O6 included in the signal processing unit PR may be subtractors.

Here, by equipping the gradient magnetic field sensor 12 with adders or subtractors such as the calculation units O1 to O6, the gradient magnetic field sensor 12 can artificially reduce the value of N. That is, in this case, the gradient magnetic field sensor 12 can artificially reduce the number of detection coils CL1 and detection coils CL2. As a result, the gradient magnetic field sensor 12 can detect the detection target with a sensitivity distribution of the shape desired by the user.

In addition, by providing the amplifiers A1 to A4 in the gradient magnetic field sensor 12, the gradient magnetic field sensor 12 can perform balancing adjustments on the detection signals output from each of the detection circuits DT1 to DT4.

In addition, by providing the gradient magnetic field sensor 12 with full-wave rectifiers FR1 to FR3, the gradient magnetic field sensor 12 can prevent detection signals from canceling each other out due to differences in polarity.

<First Modification of the Embodiment>
The following describes a first modification of the embodiment. In the first modification of the embodiment, the same components as those in the embodiment are denoted by the same reference numerals and the description thereof is omitted. In the first modification of the embodiment, the gradient magnetic field sensor 12 includes a differential amplifier DF instead of the signal processor PR. In the first modification, the case where N=2 is described as an example. That is, in the first modification, the gradient magnetic field sensor 12 does not include the detector coils CL13 and CL14 among the detector coils CL11 to CL14, does not include the detector coils CL23 and CL24 among the detector coils CL21 to CL24, and does not include the detector circuits DT3 and DT4 among the detector circuits DT1 to DT4. FIG. 8 is a diagram showing an example of the configuration of the gradient magnetic field sensor 12 according to the first modification of the embodiment. In the following, for convenience of description, the gradient magnetic field sensor 12 according to the first modification will be referred to as the gradient magnetic field sensor 12A.

As shown in FIG. 8, in the first modified embodiment, the output terminal DT13 of the detection circuit DT1 is connected to the input terminal DF1 of the differential amplifier DF via a transmission line. The output terminal DT23 of the detection circuit DT2 is connected to the input terminal DF2 of the differential amplifier DF via a transmission line. The output terminal DF3 of the differential amplifier DF is connected to the detection signal output terminal CT2 via a transmission line. Note that other circuit elements, other devices, etc. may be connected between the detection circuit DT1 and the differential amplifier DF as long as the functions of the gradient magnetic field sensor 12A are not impaired. Note that other circuit elements, other devices, etc. may be connected between the detection circuit DT2 and the differential amplifier DF as long as the functions of the gradient magnetic field sensor 12A are not impaired.

In the gradient magnetic field sensor 12A configured in this way, the detection circuit DT1 inputs a detection signal corresponding to the input differential signal Vout1 to the differential amplifier DF.

In addition, in the gradient magnetic field sensor 12A, the detection circuit DT2 inputs a detection signal corresponding to the input differential signal Vout2 to the differential amplifier DF.

The differential amplifier DF outputs the differential signal between the detection signal input from the detection circuit DT1 and the detection signal input from the detection circuit DT2 as a new detection signal to the detection signal output terminal CT2.

By this operation, the gradient magnetic field sensor 12A, like the gradient magnetic field sensor 12 according to the embodiment, can suppress the occurrence of dead zones while suppressing the variation in sensitivity according to the passing position of the detection target. Also, like the gradient magnetic field sensor 12 according to the embodiment, the gradient magnetic field sensor 12A can suppress the detection of a uniform gradient magnetic field such as that generated by a gradient magnetic field coil or the like.

9 is a diagram showing an example of detection signals output from the detection circuit DT1, the detection circuit DT2, and the differential amplifier DF when a predetermined magnetic moment moving in an orthogonal direction is detected by the gradient magnetic field sensor 12 arranged in the region where the uniform gradient magnetic field is applied. However, FIG. 9 shows detection signals obtained by simulation. The horizontal axis of the graph shown in FIG. 9 indicates elapsed time. However, the origin of the horizontal axis of the graph indicates the timing when the magnetic moment passes a position equidistant from both the first sensor head S1 and the second sensor head S2. The vertical axis of the graph indicates the voltage of the detection signal output from each of the detection circuit DT1, the detection circuit DT2, and the differential amplifier DF. In addition, the curve F7 plotted on the graph indicates the change in the voltage of the detection signal output from the detection circuit DT1. In addition, the curve F8 plotted on the graph indicates the change in the voltage of the detection signal output from the detection circuit DT2. In addition, the curve F9 plotted on the graph indicates the change in the detection signal output from the differential amplifier DF. As shown in FIG. 9, the curves F7 and F8 are almost overlapped except around the origin of the horizontal axis of the graph. The curves F7 and F8 are sinusoidal curves except around the origin of the horizontal axis of the graph. This is because the detection coils CL11, CL12, CL21, and CL22 detect the uniform gradient magnetic field. Meanwhile, the curves F7 and F8 form peaks in response to the detection of the magnetic moment around the origin of the horizontal axis of the graph. Here, the differential amplifier DF outputs a differential signal between the detection signal indicated by the curve F7 and the detection signal indicated by the curve F8. Therefore, the curve F9 is the difference between the curves F7 and F8. As a result, the voltage of the curve F9 is 0 [V] except around the origin of the horizontal axis of the graph. This indicates that the gradient magnetic field sensor 12A does not detect the uniform gradient magnetic field, but only detects the magnetic moment.

Figure 10 is a diagram showing another example of detection signals output from each of the detection circuit DT1, the detection circuit DT2, and the differential amplifier DF when the gradient magnetic field sensor 12 is placed in an area where a uniform gradient magnetic field is applied. However, in the example shown in Figure 10, the detection circuit DT1 and the detection circuit DT2 output a sine wave signal estimated to be generated in a uniform gradient magnetic field as a test detection signal. The horizontal axis of the graph shown in Figure 10 indicates the elapsed time. The vertical axis of the graph indicates the voltage of the detection signal output from each of the detection circuit DT1, the detection circuit DT2, and the differential amplifier DF. The curve F10 plotted on the graph indicates the change in the voltage of the detection signal output from the detection circuit DT1. The curve F11 plotted on the graph indicates the change in the voltage of the detection signal output from the detection circuit DT2. The curve F12 plotted on the graph indicates the change in the detection signal output from the differential amplifier DF. In this case, as shown in the graph, curve F12, which is a curve showing the change in the detection signal output from the differential amplifier DF, is the difference between curve F10 and curve F11. As a result, in curve F12, the voltage is 0 [V] over the entire range visible in FIG. 10.

Figure 11 is a diagram showing yet another example of detection signals output from the detection circuit DT1, the detection circuit DT2, and the differential amplifier DF when the gradient magnetic field sensor 12 is arranged in an area where a uniform gradient magnetic field is applied. However, the example shown in Figure 11 shows detection signals output from the detection circuit DT1, the detection circuit DT2, and the differential amplifier DF when a uniform gradient magnetic field is actually detected by the first sensor head S1 and the second sensor head S2. The horizontal axis of the graph shown in Figure 11 indicates elapsed time. The vertical axis of the graph indicates the voltage of the detection signal output from the detection circuit DT1, the detection circuit DT2, and the differential amplifier DF. The curve F13 plotted on the graph indicates the change in the voltage of the detection signal output from the detection circuit DT1. The curve F14 plotted on the graph indicates the change in the voltage of the detection signal output from the detection circuit DT2. The curve F15 plotted on the graph indicates the change in the detection signal output from the differential amplifier DF. In this case, as shown in the graph, curve F15, which is a curve showing the change in the detection signal output from the differential amplifier DF, is the difference between curve F13 and curve F14. As a result, in curve F15, the voltage is 0 [V] over the entire range visible in FIG. 11. This indicates that the actual gradient magnetic field sensor 12A does not detect a uniform gradient magnetic field.

As described above, the gradient magnetic field sensor 12A can suppress the detection of a uniform gradient magnetic field such as that generated by a gradient magnetic field coil, etc.

Here, the principle by which the gradient magnetic field sensor 12A is prevented from detecting a uniform gradient magnetic field will be described. Note that this principle is the same as the principle by which the gradient magnetic field sensor 12 according to the embodiment is prevented from detecting a uniform gradient magnetic field.

<Principle of suppressing detection of uniform gradient magnetic field by gradient magnetic field sensor 12>
In the following, as an example, a case will be considered in which a uniform magnetic field and a uniform gradient magnetic field are applied to the gradient magnetic field sensor 12A, and a predetermined magnetic moment is transported within the detection area of the gradient magnetic field sensor 12A. In addition, in the following, for convenience of explanation, the strength of the magnetic field applied from the magnetic moment to the first sensor head S1 at time t will be represented by B ₁ (t), the strength of the magnetic field applied from the magnetic moment to the second sensor head S2 at time t will be represented by B ₂ (t), the strength of the uniform magnetic field applied to the first sensor head S1 and the second sensor head S2 at time t will be represented by B _u (t), the strength of the uniform gradient magnetic field applied to the first sensor head S1 at time t will be represented by B _g1 (t), and the strength of the uniform gradient magnetic field applied to the second sensor head S2 at time t will be represented by B _g2 (t).

In this case, the first induced voltage _V11 (t) induced in the detection coil CL11 at time t and the first induced voltage _V12 (t) induced in the detection coil CL12 at time t are expressed by the following formula (1). The second induced voltage _V21 (t) induced in the detection coil CL21 at time t and the second induced voltage _V22 (t) induced in the detection coil CL22 at time t are expressed by the following formula (2).

Here, κ in the above formulas (1) and (2) represents a proportional constant for converting the strength of the magnetic field into a voltage. In addition, _Nc in the above formulas (1) and (2) represents the number of turns of each of the detection coils CL11, CL12, CL21, and CL22. In addition, S in the above formulas (1) and (2) represents the cross-sectional area of the coil surface of each of the detection coils CL11, CL12, CL21, and CL22. The coil surface of a certain coil is a virtual plane having a thickness that includes the conductor wound as the coil and the opening of the coil. The cross-sectional area of the coil surface is the area of the upper or lower surface of the plane.

If the detection sensitivity of the magnetic moment by the detection coil CL11 according to the passing position x of the detection object is represented _{by α11} , the detection sensitivity of the magnetic moment by the detection coil CL12 according to the passing position x of the detection object is represented _{by α12} , the detection sensitivity of the magnetic moment by the detection coil CL21 according to the passing position x of the detection object is represented by _α21 , and the detection sensitivity of the magnetic moment by the detection coil CL22 according to the passing position x of the detection object is represented by α22, when the detection sensitivities of the detection coils CL11, CL12, CL21, and CL22 are equal, the following formula (3) is established. Note that each of _α11 , _α12 , _α21 , and _α22 is a function of the passing position x of the detection object.

In this case, a differential signal _V1 (t) between the first induced voltage induced in the detection coil CL11 at time t and the second induced voltage induced in the detection coil CL21 at time t is expressed by the following equation (4): In this case, a differential signal V2(t) between the first induced voltage induced in the detection coil CL12 at time t and the second induced voltage induced in the detection coil _CL22 at time t is expressed by the following equation (5):

As shown in formula (4), the differential signal V ₁ (t) at time t is obtained by the difference between the first induced voltage V ₁₁ (t) induced in the detection coil CL11 at time t and the second induced voltage V ₂₁ (t) induced in the detection coil CL21 at time t. Therefore, in the differential signal V ₁ (t), the component of the uniform magnetic field strength B _u (t) is cancelled out. Also, as shown in formula (5), the differential signal V ₂ (t) at time t is obtained by the difference between the first induced voltage V ₁₂ (t) induced in the detection coil CL12 at time t and the second induced voltage V ₂₂ (t) induced in the detection coil CL22 at time t. Therefore, in the differential signal V ₂ (t), the component of the uniform magnetic field strength B _u (t) is also cancelled out.

Then, the detection signal V ₀ (t) output from the differential amplifier DF at time t is obtained from the difference between the differential signal V ₁ (t) shown in equation (4) and the differential signal V ₂ (t) shown in equation (5), as shown in the following equation (6).

As shown in the above formula (6), in the detection signal V ₀ (t), both the intensity B _g1 (t) of the uniform gradient magnetic field and the intensity B _g2 (t) of the uniform gradient magnetic field are cancelled out. This indicates that the gradient magnetic field sensor 12A suppresses the detection of the uniform magnetic field and also suppresses the detection of the uniform gradient magnetic field. Based on the above principle, the gradient magnetic field sensor 12A can suppress the detection of the uniform magnetic field and also suppress the detection of the uniform gradient magnetic field.

<Modification 2 of the embodiment>
The second modification of the embodiment will be described below. In the second modification of the embodiment, the same components as those in the embodiment will be denoted by the same reference numerals and will not be described. For ease of explanation, the gradient magnetic field sensor 12 according to the second modification of the embodiment will be referred to as the gradient magnetic field sensor 12B. The matters described in the second modification of the embodiment may be applied not only to the embodiment but also to the first modification of the embodiment.

Figure 12 shows an example of the circuit configuration of the gradient magnetic field sensor 12B.

The gradient magnetic field sensor 12B includes an AC power supply connection terminal CT1, a first sensor head S1, a second sensor head S2, a 0th phase shift circuit PS0, an AC current control unit CC1, a detection circuit DT1, a detection circuit DT2, a detection circuit DT3, a detection circuit DT4, a signal processing unit PR, a detection signal output terminal CT2, a DC power supply connection terminal CT3, and a DC current control unit CC2. The DC current control unit CC2 has an input terminal CC21, an output terminal CC22, and an output terminal CC23. The DC current control unit CC2 includes a first variable resistor VR21, a first inductor L1, a second variable resistor VR22, and a second inductor L2.

The gradient magnetic field sensor 12B is connected to the DC power supply P2 in addition to the AC power supply P1 via a transmission line. More specifically, the DC power supply connection terminal CT3 of the gradient magnetic field sensor 12B is connected to the positive power supply terminal of the DC power supply P2 via a transmission line. The negative power supply terminal of the DC power supply P2 is grounded to the ground via the transmission line. Here, the DC power supply P2 may be any DC power supply. Note that other circuit elements, other devices, etc. may be connected between the DC power supply connection terminal CT3 and the positive power supply terminal as long as the function of the gradient magnetic field sensor 12B is not impaired. Note that other circuit elements, other devices, etc. may be connected between the negative power supply terminal and the ground as long as the function of the gradient magnetic field sensor 12B is not impaired.

The DC power supply connection terminal CT3 is connected to the input terminal CC21 of the DC current control unit CC2 via a transmission line. Note that other circuit elements, other devices, etc. may be connected between the DC power supply connection terminal CT3 and the DC current control unit CC2 as long as the functions of the gradient magnetic field sensor 12A are not impaired.

In addition, in the DC current control unit CC2, the first variable resistor VR21 and the first inductor L1 are connected in series between the input terminal CC21 and the output terminal CC22 via a transmission line. In addition, in the DC current control unit CC2, the second variable resistor VR22 and the second inductor L2 are connected in series between the input terminal CC21 and the output terminal CC23 via a transmission line. Note that, between the input terminal CC21 and the output terminal CC22, other circuit elements, other devices, etc. may be connected together with the first variable resistor VR21 and the first inductor L1 within a range that does not impair the function of the gradient magnetic field sensor 12A. In addition, between the input terminal CC21 and the output terminal CC22, the first variable resistor VR21 and the first inductor L1 may be connected in series in any order. In addition, between the input terminal CC21 and the output terminal CC23, other circuit elements, other devices, etc. may be connected in addition to the second variable resistor VR22 and the second inductor L2, as long as the function of the gradient magnetic field sensor 12A is not impaired. In addition, between the input terminal CC21 and the output terminal CC23, the second variable resistor VR22 and the second inductor L2 may be connected in series in any order.

The output terminal CC22 of the DC current control unit CC2 is connected to the transmission line connecting the output terminal CC13 of the AC current control unit CC1 and the first magnetic core CR1 via another transmission line. Note that other circuit elements, other devices, etc. may be connected between the output terminal CC22 and the transmission line connecting the output terminal CC13 of the AC current control unit CC1 and the first magnetic core CR1, as long as the function of the gradient magnetic field sensor 12A is not impaired.

The output terminal CC23 of the DC current control unit CC2 is connected to the transmission line connecting the output terminal CC14 of the AC current control unit CC1 and the second magnetic core CR2 via another transmission line. Note that other circuit elements, other devices, etc. may be connected between the output terminal CC23 and the transmission line connecting the output terminal CC14 of the AC current control unit CC1 and the second magnetic core CR2, as long as the function of the gradient magnetic field sensor 12 is not impaired.

Next, we will explain the operation of the gradient magnetic field sensor 12B.

In the gradient magnetic field sensor 12B having the circuit configuration described above, the DC power supply P2 inputs a DC current to the DC power supply connection terminal CT3. The DC current input to the DC power supply connection terminal CT3 is input to the DC current control unit CC2 as a DC excitation current.

The DC excitation current input to the DC current control unit CC2 is branched into two DC excitation currents: a first DC excitation current DC1 and a second DC excitation current DC2.

In the DC current control unit CC2, the magnitude of the first DC excitation current DC1 is adjusted by the first variable resistor VR21. The first DC excitation current DC1 is input to the first magnetic core CR1 from the output terminal CC22. As a result, a voltage corresponding to the first AC excitation current AC1 flowing through the first magnetic core CR1, the first DC excitation current DC1 flowing through the first magnetic core CR1, and the strength of the magnetic field applied to the detection coil CL1n is induced in the n-th detection coil CL1n of the four detection coils CL1. That is, in the gradient magnetic field sensor 12B, the voltage also changes according to the strength of the magnetic field applied to the detection coil CL1n from the outside. In the following, for convenience of explanation, the voltage is referred to as the third induced voltage. The first inductor L1 is an inductor for DC (Direct Current) coupling. The presence of the first inductor L1 prevents AC current from flowing from the output terminal CC22 to the input terminal CC21 in the gradient magnetic field sensor 12B.

On the other hand, in the DC current control unit CC2, the magnitude of the second DC excitation current DC2 is adjusted by the second variable resistor VR22. The second DC excitation current DC2 is input from the output terminal CC23 to the second magnetic core CR2. As a result, a voltage is induced in the n-th detection coil CL2n of the four detection coils CL2 according to the second AC excitation current AC2 flowing through the second magnetic core CR2, the second DC excitation current DC2 flowing through the second magnetic core CR2, and the strength of the magnetic field applied to the detection coil CL2n. That is, in the gradient magnetic field sensor 12B, the voltage also changes according to the strength of the magnetic field applied to the detection coil CL2n from the outside. In the following, for convenience of explanation, the voltage is referred to as the fourth induced voltage. The second inductor L2 is an inductor for DC coupling. The presence of the second inductor L2 prevents AC current from flowing from the output terminal CC23 to the input terminal CC21 in the gradient magnetic field sensor 12B.

In the gradient magnetic field sensor 12B, a signal corresponding to the third induced voltage induced in the detection coil CL11 and the fourth induced voltage induced in the detection coil CL21 is input to the input terminal DT12 of the detection circuit DT1 as a differential signal Vout1. In the gradient magnetic field sensor 12B, a signal corresponding to the third induced voltage induced in the detection coil CL12 and the fourth induced voltage induced in the detection coil CL22 is input to the input terminal DT22 of the detection circuit DT2 as a differential signal Vout2. In the gradient magnetic field sensor 12B, a signal corresponding to the third induced voltage induced in the detection coil CL13 and the fourth induced voltage induced in the detection coil CL23 is input to the input terminal DT32 of the detection circuit DT3 as a differential signal Vout3. In addition, in the gradient magnetic field sensor 12B, a signal corresponding to the third induced voltage induced in the detection coil CL14 and the fourth induced voltage induced in the detection coil CL24 is input as a differential signal Vout4 to an input terminal DT42 of the detection circuit DT4.

The operations of the detection circuit DT1, the detection circuit DT2, the detection circuit DT3, the detection circuit DT4, and the signal processing unit PR are the same as those described in the embodiment. Therefore, detailed descriptions of these operations will be omitted below.

Even with the above configuration, the gradient magnetic field sensor 12B, like the gradient magnetic field sensor 12 according to the embodiment, can suppress the occurrence of dead zones while suppressing the variation in sensitivity according to the passing position of a magnetized foreign object.

Note that each of the gradient magnetic field sensor 12 according to the embodiment, the gradient magnetic field sensor 12A according to the first modified embodiment, and the gradient magnetic field sensor 12B according to the second modified embodiment may be configured without the zeroth phase shift circuit PS0.

In addition, each of the gradient magnetic field sensor 12 according to the embodiment, the gradient magnetic field sensor 12A according to the first modified example of the embodiment, and the gradient magnetic field sensor 12B according to the second modified example of the embodiment may be configured without including either the first phase shift circuit PS1 or the second phase shift circuit PS2.

In addition, each of the gradient magnetic field sensor 12 according to the embodiment, the gradient magnetic field sensor 12A according to the first modified example of the embodiment, and the gradient magnetic field sensor 12B according to the second modified example of the embodiment may be configured without either the first variable resistor VR11 or the second variable resistor VR12.

In addition, each of the gradient magnetic field sensor 12 according to the embodiment, the gradient magnetic field sensor 12A according to the first modified embodiment, and the gradient magnetic field sensor 12B according to the second modified embodiment may be configured without at least one of the first capacitor C1 and the second capacitor C2. However, as mentioned above, these are capacitors for AC coupling. For this reason, it is preferable that the gradient magnetic field sensor 12 includes both the first capacitor C1 and the second capacitor C2.

In addition, in each of the gradient magnetic field sensor 12 according to the embodiment, the gradient magnetic field sensor 12A according to the first modified embodiment, and the gradient magnetic field sensor 12B according to the second modified embodiment, the AC current control unit CC1 may be configured to include a first amplifier circuit that amplifies the amplitude of the first AC excitation current AC1 instead of the first variable resistor VR11 or in addition to the first variable resistor VR11.

In addition, in each of the gradient magnetic field sensor 12 according to the embodiment, the gradient magnetic field sensor 12A according to the first modified embodiment, and the gradient magnetic field sensor 12B according to the second modified embodiment, the AC current control unit CC1 may be configured to include a second amplifier circuit that amplifies the amplitude of the second AC excitation current AC2 instead of or in addition to the second variable resistor VR12.

In addition, the gradient magnetic field sensor 12 according to the embodiment, the gradient magnetic field sensor 12A according to the modified example 1 of the embodiment, and the gradient magnetic field sensor 12B according to the modified example 2 of the embodiment can adjust the phase shift amount of the first phase shift circuit PS1 of the AC current control unit CC1, the resistance value of the first variable resistor VR11, the phase shift amount of the second phase shift circuit PS2, and the resistance value of the second variable resistor VR12 to bring the sensitivity of the first sensor head S1 and the sensitivity of the second sensor head S2 closer together. In other words, this allows the gradient magnetic field sensor 12 according to the embodiment, the gradient magnetic field sensor 12A according to the modified example 1 of the embodiment, and the gradient magnetic field sensor 12B according to the modified example 2 of the embodiment to balance the sensitivity of the first sensor head S1 and the sensitivity of the second sensor head S2, thereby improving the accuracy of detecting the detection target.

The first magnetic core CR1 described above may also be configured to include two or more magnetic bodies connected in series. In this case, the first magnetic core CR1 may be configured such that some or all of these two or more magnetic bodies are connected in series by a non-magnetic conductor. Here, for example, when the first magnetic core CR1 includes two magnetic bodies, magnetic body CR1A and magnetic body CR1B, in the first magnetic core CR1, the magnetic body CR1A and magnetic body CR1B are connected by a non-magnetic conductor. Then, a terminal connected to the output terminal CC13 of the AC current control unit CC1 is provided at the end of the magnetic body CR1A opposite to the end connected to the conductor. Also, a terminal grounded to the ground is provided at the end of the magnetic body CR1B opposite to the end connected to the conductor CR1C.

The second magnetic core CR2 described above may also be configured to include two or more magnetic bodies connected in series. In this case, the second magnetic core CR2 may be configured such that some or all of these two or more magnetic bodies are connected in series by a non-magnetic conductor. Note that the configuration of the second magnetic core CR2 including multiple magnetic bodies is similar to the configuration of the first magnetic core CR1 including multiple magnetic bodies, so a detailed description will be omitted.

In addition, in the first sensor head S1 described above, the polarity of the voltage output from the detection coil CL1n when a predetermined magnetic field is applied to the first magnetic core CR1 is the same as the polarity of the voltage output from the detection coil CL1n+1 when a predetermined magnetic field is applied to the first magnetic core CR1. Therefore, in the second sensor head S2 described above, the polarity of the voltage output from the detection coil CL2n when a predetermined magnetic field is applied to the second magnetic core CR2 is the same as the polarity of the voltage output from the detection coil CL2n+1 when a predetermined magnetic field is applied to the second magnetic core CR2. This is because the signal processing unit PR can efficiently perform signal processing according to the circuit diagram shown in FIG. 4, which is convenient. For this reason, in the first sensor head S1 described above, when a predetermined magnetic field is applied to the first magnetic core CR1, the polarity of the voltage output from some of the N detection coils CL1 may be different from the polarity of the voltage output from all the remaining of the N detection coils CL1 when a predetermined magnetic field is applied to the first magnetic core CR1, depending on the configuration of the signal processing performed by the signal processing unit PR. In this case, in the second sensor head S2 described above, when a predetermined magnetic field is applied to the second magnetic core CR2, the polarity of the voltage output from some of the N detection coils CL2 also differs from the polarity of the voltage output from all the remaining of the N detection coils CL2 when a predetermined magnetic field is applied to the second magnetic core CR2, depending on the configuration of the signal processing performed by the signal processing unit PR. For example, in the first sensor head S1 described above, the polarity of the voltage output from the detection coil CL12 when a predetermined magnetic field is applied to the first magnetic core CR1 may be different from the polarity of the voltage output from each of the detection coils CL11, CL13, and CL14 when a predetermined magnetic field is applied to the first magnetic core CR1, depending on the configuration of the signal processing performed by the signal processing unit PR. In this case, in the second sensor head S2 described above, the polarity of the voltage output from the detection coil CL22 when a predetermined magnetic field is applied to the second magnetic core CR2 is also different from the polarity of the voltage output from each of the detection coils CL21, CL23, and CL24 when a predetermined magnetic field is applied to the second magnetic core CR2. Also, for example, in the first sensor head S1 described above, the polarity of the voltage output from the detection coil CL1n when a predetermined magnetic field is applied to the first magnetic core CR1 may be configured to be different from the polarity of the voltage output from the detection coil CL1n+1 when a predetermined magnetic field is applied to the first magnetic core CR1, depending on the configuration of the signal processing performed by the signal processing unit PR. In this case, in the second sensor head S2 described above, the polarity of the voltage output from the detection coil CL2n when a predetermined magnetic field is applied to the second magnetic core CR2 is also different from the polarity of the voltage output from the detection coil CL2n+1 when a predetermined magnetic field is applied to the second magnetic core CR2. As a result, the gradient magnetic field sensor 12 according to the embodiment, the gradient magnetic field sensor 12A according to the first modification of the embodiment, and the gradient magnetic field sensor 12B according to the second modification of the embodiment can each improve the efficiency of the signal processing of the signal processing unit PR.

As described above, the gradient magnetic field sensor according to the embodiment (in the examples described above, gradient magnetic field sensor 12, gradient magnetic field sensor 12A, gradient magnetic field sensor 12B) comprises a first magnetic core having a linear portion (in the examples described above, first magnetic core CR1), a second magnetic core having a linear portion (in the examples described above, second magnetic core CR2), N first coils (in the examples described above, four detection coils CL1) arranged in sequence in the linear portion of the first magnetic core, and N detection coils (in the examples described above, four detection coils CL2) arranged in sequence in the linear portion of the second magnetic core. The magnetic field sensor includes N second coils (four detection coils CL2 in the example described above) and a main body (main body MB in the example described above) in which the first magnetic core and the second magnetic core are provided, and the n-th first coil (detection coil CL1n in the example described above) from the tip side of the first magnetic core among the N first coils is differentially connected to the n-th second coil (detection coil CL2n in the example described above) from the tip side of the second magnetic core among the N second coils, where N is an integer of 2 or more and n is an integer in the range of 1 to N. This allows the gradient magnetic field sensor to suppress the occurrence of dead zones while suppressing the fluctuation in sensitivity according to the passing position of a magnetized foreign object.

The gradient magnetic field sensor may also be configured such that the linear portion of the first magnetic core is substantially parallel to the linear portion of the second magnetic core.

The gradient magnetic field sensor may also be configured such that the linear portion of the first magnetic core and the linear portion of the second magnetic core are positioned approximately coaxially.

The gradient magnetic field sensor may also be configured such that the first magnetic core and the second magnetic core each protrude from the main body in a predetermined direction so that their linear portions are substantially parallel to each other.

In addition, the gradient magnetic field sensor may be configured such that the linear portion of the first magnetic core overlaps at least a portion of the linear portion of the second magnetic core when viewed from an orthogonal direction perpendicular to each of the linear portions of the first magnetic core and the second magnetic core.

The gradient magnetic field sensor may also be configured such that the linear portion of the first magnetic core overlaps substantially entirely with the linear portion of the second magnetic core when viewed from an orthogonal direction.

The gradient magnetic field sensor may also be configured such that the nth first coil is substantially completely overlapped with the nth second coil when viewed from an orthogonal direction.

The gradient magnetic field sensor may also be configured such that, among the N first coils, two adjacent first coils are insulated from and in contact with each other, and, among the N second coils, two adjacent second coils are insulated from and in contact with each other.

The gradient magnetic field sensor may also be configured such that the self-inductance of the nth first coil is approximately the same as the self-inductance of the nth second coil.

The gradient magnetic field sensor may also be configured such that the sum of the self-inductances of the N first coils is approximately the same as the sum of the self-inductances of the N second coils.

In addition, the gradient magnetic field sensor may be configured such that a first magnetic core around which N first coils are wound and a second magnetic core around which N second coils are wound are arranged in line symmetry.

In addition, the gradient magnetic field sensor may be configured such that the polarity of the voltage output from the nth first coil when a predetermined magnetic field is applied to the first magnetic core is the same as the polarity of the voltage output from the n+1th first coil from the tip side of the first magnetic core among the N first coils when a predetermined magnetic field is applied to the first magnetic core, and the polarity of the voltage output from the nth second coil when a predetermined magnetic field is applied to the second magnetic core is the same as the polarity of the voltage output from the n+1th second coil from the tip side of the second magnetic core among the N second coils when a predetermined magnetic field is applied to the second magnetic core.

In addition, the gradient magnetic field sensor may be configured such that the polarity of the voltage output from some of the N first coils when a predetermined magnetic field is applied to the first magnetic core is opposite to the polarity of the voltage output from all the remaining of the N first coils when a predetermined magnetic field is applied to the first magnetic core, and the polarity of the voltage output from some of the N second coils when a predetermined magnetic field is applied to the second magnetic core is opposite to the polarity of the voltage output from all the remaining of the N second coils when a predetermined magnetic field is applied to the second magnetic core.

The gradient magnetic field sensor may also be configured to further include a signal processing unit (signal processing unit PR in the example described above) to which a voltage corresponding to the difference between the voltage output from the nth first coil and the voltage output from the nth second coil is input as a voltage signal.

In addition, in the gradient magnetic field sensor, the signal processing unit may be configured to include a calculation unit (in the example described above, each of calculation units O1 to O6) that adds or subtracts voltage signals.

In addition, in the gradient magnetic field sensor, the signal processing unit may be configured to include an amplifier unit (in the example described above, amplifier unit A1 to amplifier unit A4) that amplifies the voltage signal.

In addition, in the gradient magnetic field sensor, the signal processing unit may be configured to include an absolute value output unit that outputs the absolute value of the voltage signal (in the example described above, each of full-wave rectifiers FR1 to FR3).

In addition, in the gradient magnetic field sensor, the signal processing unit may be configured to include a discrimination unit (in the example described above, a hysteresis comparator HC) that discriminates between signals and noise.

The magnetic object detection device according to the embodiment (magnetic object detection device 1 in the example described above) is equipped with the gradient magnetic field sensor described above. This allows the magnetic object detection device to suppress variations in sensitivity according to the passing position of a magnetized foreign object while suppressing the occurrence of dead zones.

The embodiment of the present invention has been described above in detail with reference to the drawings, but the specific configuration is not limited to this embodiment, and may be changed, replaced, deleted, etc., without departing from the spirit of the present invention.

1...magnetic object detection device, 11...magnetization device, 12, 12A, 12B...gradient magnetic field sensor, 20...information processing device, A1, A2, A3, A4...amplification unit, B1, B2, B3, B4...buffer, C1...first capacitor, C2...second capacitor, C3...third capacitor, CC1...AC current control unit, CC2...DC current control unit, CL1, CL1n, CL1N, CL2, C L2n, CL2N, CL11, CL12, CL13, CL14, CL21, CL22, CL23, CL24...detection coil, CR1...first magnetic core, CR2...second magnetic core, CS...frame, CT1...AC power supply connection terminal, CT2...detection signal output terminal, CT3...DC power supply connection terminal, DA, DF...differential amplifier, DT1, DT2, DT3, DT4...detection circuit, EA ...Error amplifier, FR1, FR2, FR3...Full-wave rectifier, HC...Hysteresis comparator, HF1, HF2, HF3, HF4...High-pass filter, IV...Inverter, L1...First inductor, L2...Second inductor, LF...Low-pass filter, MB...Main body, O1, O2, O3, O4, O5, O6...Calculation unit, P1...AC power supply, P2...DC power supply, PD...Phase detection circuit, PR...Signal processing unit, PS0...0th phase shift circuit, PS1...First phase shift circuit, PS2...Second phase shift circuit, R1...Resistance, RL1...First roller, RL2...Second roller, S1...First sensor head, S2...Second sensor head, ST...Sheet member, VR11...First variable resistor, VR12...Second variable resistor, VR21...First variable resistor, VR22...Second variable resistor

Claims (17)

A first magnetic core having a linear portion;
A second magnetic core having a linear portion;
N first coils arranged in sequence in a linear first portion of the first magnetic core;
N second coils arranged in sequence in a linear second portion of the second magnetic core;
a main body portion in which the first magnetic core and the second magnetic core are provided;
Equipped with
Among the N first coils, an n-th first coil from the tip side of the first magnetic core is differentially connected to an n-th second coil from the tip side of the second magnetic core among the N second coils,
N is an integer of 2 or more,
n is an integer ranging from 1 to N,
a polarity of a voltage output from a part of the N first coils when a predetermined magnetic field is applied to the first magnetic core is opposite to a polarity of a voltage output from all the remaining N first coils when the predetermined magnetic field is applied to the first magnetic core;
When the predetermined magnetic field is applied to the second magnetic core, the polarity of the voltage output from some of the N second coils is opposite to the polarity of the voltage output from all the remaining N second coils when the predetermined magnetic field is applied to the second magnetic core.
Gradient magnetic field sensor. The first portion is generally parallel to the second portion.
The gradient magnetic field sensor according to claim 1 . The first portion and the second portion are positioned substantially coaxially.
The gradient magnetic field sensor according to claim 2 . Each of the first magnetic core and the second magnetic core protrudes from the main body in a predetermined direction such that the first portion and the second portion are substantially parallel to each other.
The gradient magnetic field sensor according to claim 2 . The first portion overlaps with at least a part of the second portion when viewed from an orthogonal direction orthogonal to each of the first portion and the second portion.
5. A gradient magnetic field sensor according to claim 4. The first portion substantially entirely overlaps with the second portion when viewed from the orthogonal direction.
The gradient magnetic field sensor according to claim 5 . the n-th first coil substantially entirely overlaps with the n-th second coil when viewed from the orthogonal direction;
7. A gradient magnetic field sensor according to claim 5 or 6. Any two adjacent first coils among the N first coils are insulated from and in contact with each other,
Any two adjacent second coils among the N second coils are insulated from and in contact with each other.
A gradient magnetic field sensor according to any one of claims 1 to 7. The self-inductance of the n-th first coil is substantially the same as the self-inductance of the n-th second coil.
A gradient magnetic field sensor according to any one of claims 1 to 8. A sum of the self-inductances of the N first coils is approximately equal to a sum of the self-inductances of the N second coils.
A gradient magnetic field sensor according to any one of the preceding claims. The first magnetic core around which the N first coils are wound and the second magnetic core around which the N second coils are wound are arranged in line symmetry.
A gradient magnetic field sensor according to any one of the preceding claims. The gradient magnetic field sensor further includes a signal processing unit to which a voltage corresponding to a difference between a voltage output from the n-th first coil and a voltage output from the n-th second coil is input as a voltage signal.
A gradient magnetic field sensor according to any one of the preceding claims. The signal processing unit includes a calculation unit that adds or subtracts a voltage signal.
A gradient magnetic field sensor according to claim 12 . The signal processing unit includes an amplifier unit that amplifies the voltage signal.
A gradient magnetic field sensor according to claim 12 or 13 . The signal processing unit includes an absolute value output unit that outputs an absolute value of the voltage signal.
A gradient magnetic field sensor according to any one of claims 12 to 14 . The signal processing unit includes a discrimination unit that discriminates between a signal and noise.
A gradient magnetic field sensor according to any one of claims 12 to 15 . Equipped with a gradient magnetic field sensor according to any one of claims 1 to 16 ,
Magnetic object detection device.

Images