Sensor for measuring magnetic field or current

Abstract

This sensor comprises: a magnetic fluid serving as a magnetic core; a container for accommodating the magnetic fluid; and a bridge circuit comprising four coils immersed in the magnetic fluid in the container. The bridge circuit has two terminals pairs, each comprising two terminals facing each other. One of the terminal pairs forms a pair of measurement terminals for detecting a magnetic field to be measured or a current to be measured, and the other terminal pair forms a pair of drive terminals for supplying an excitation current having a predetermined frequency component to the bridge circuit. Of the four coils, two coils of each of the drive terminals, said coils having one end connected to the same drive terminal, are arranged so as to generate excitation magnetic fields in mutually opposite directions, and two coils of each of the measurement terminals, said coils having one end connected to the same measurement terminal, are arranged so as to generate excitation magnetic fields in mutually opposite directions. An excitation current is made to flow from the pair of drive terminals, and the magnetic field to be measured or the current to be measured is measured on the basis of a potential difference generated between the pair of measurement terminals.

Description

Sensor for measuring magnetic field or current

[0001] The present invention relates to a sensor for measuring a magnetic field or current using a magnetic fluid as a magnetic core, and particularly to a magnetic field sensor capable of measuring a static magnetic field or an insulated current sensor capable of measuring a direct current.

[0002] Today, as the use of direct current power expands, highly reliable direct current sensors are in demand. Among these needs, the offset problem is particularly serious. For example, the principle of battery SOC measurement and integrating watt-hour meters for direct current power is integration of current values; if a current sensor has an offset, the error increases in proportion to the passage of time. Furthermore, even for applications that do not require integration, there is market demand for sensor accuracy after a current exceeding several tens of times the rated value has flowed through, and this is a problem that is difficult to solve along the extension of conventional methods.

[0003] In order to solve this problem, the inventor of the present application has made numerous inventions (Patent Documents 1 to 8, Non-Patent Document 1), and by utilizing the non-magnetization characteristic of magnetic fluids, magnetic field sensors and current sensors using a magnetic fluid as a magnetic core have been invented (for example, Patent Documents 1 to 5, 7 to 8).

[0004] The development of a series of magnetic field sensors and current sensors employing a magnetic fluid as a magnetic core has been carried out aiming to maintain high accuracy while overcoming offset caused by magnetization of the magnetic core. Magnetization and magnetic hysteresis are physically the same phenomenon, and among magnetic materials that can currently be used industrially, magnetic fluid is considered the only one that has no magnetic hysteresis. However, the relative permeability of magnetic fluid is as low as less than 10, which is only about one-thousandth that of general soft magnetic materials. Therefore, according to the judgment of ordinary magnetic engineers, magnetic fluid was considered to be a material unsuitable for magnetic application sensors. However, the unique characteristic of not being magnetized is attractive for direct current power applications, for which demand is expected to increase in the future.

[0005] Therefore, a highly sensitive detection method called the magnetic bridge method was invented to compensate for the low permeability, and magnetic field sensors and current sensors using magnetic fluid that fundamentally do not produce offset were invented (Patent Documents 1-2, 5, 7-8). These sensors have performance equivalent to conventional sensors, but are unparalleled in that they absolutely do not produce offset, and are highly valued in the DC power industry. However, the need is always for even higher performance.

[0006] To date, all magnetic field sensors and current sensors using magnetic fluids that have been proposed are magnetically modulated and have a detection coil. To reliably detect weak magnetic flux changes with the detection coil, it is better to increase the number of turns in the detection coil, but increasing the number of turns increases the output impedance of the coil and decreases the signal-to-noise ratio. If the output of the detection coil is small, noise from the electronic circuit that amplifies the signal also becomes a problem. Therefore, one approach is to increase the excitation current in order to strengthen the magnetic flux change detected by the detection coil, but this increases the excitation power, which increases the cost of the drive circuit and increases power consumption, thus reducing its value as a practical product.

[0007] These technologies are useful not only for current sensors but also as magnetic field sensors in applications such as magnetic exploration. Magnetic field sensors using magnetic fluid as a magnetic core eliminate magnetization errors, thus improving accuracy and increasing the reliability of exploration data. Leveraging this, magnetic exploration systems incorporating magnetic fluid-based magnetic field sensors are currently under development. Of course, improvements in the signal-to-noise ratio are also welcome in this field.

[0008] Japanese Patent Publication No. 4515905 Japanese Patent Publication No. 4310373 Japanese Unexamined Patent Publication No. 2013-160549 Japanese Unexamined Patent Publication No. 2016-006823 Japanese Patent Publication No. 6564395 Japanese Unexamined Patent Publication No. 2019-086352 Japanese Unexamined Patent Publication No. 2021-063711 Japanese Unexamined Patent Publication No. 2023-172371

[0009] Takashi Tadatsu, "Development of a DC High-Current Sensor Without Magnetization Offset Using Magnetic Fluid and Magnetic Bridge Method / MAG-22-167," Institute of Electrical Engineers of Japan, IEEJ Research Meeting Materials on Magnetics, Micromachine and Sensor Systems, Biomicrosystems Joint Research Meeting, December 22-23, 2022, pp. 43-48.

[0010] The problem that this invention aims to solve is to improve the signal-to-noise ratio of the detection signal in a magnetic field sensor using a magnetic fluid as a magnetic core and in a current sensor that applies this magnetic field sensor. In particular, the present invention aims to provide a sensor that can measure a magnetic field or current without using a detection coil, using a magnetic fluid as a magnetic core.

[0011] To achieve the above objective, the present invention comprises a magnetic fluid as a magnetic core, a container for containing the magnetic fluid, and a bridge circuit consisting of four coils immersed in the magnetic fluid in the container. The bridge circuit has two pairs of terminals, each consisting of two opposing terminals. One pair of terminals forms a pair of measurement terminals for detecting a magnetic field or current to be measured, and the other pair of terminals forms a pair of drive terminals for supplying an excitation current having a predetermined frequency component to the bridge circuit. Two coils from each of the four coils (coils L1 and L3 for drive terminal D1 and coils L2 and L4 for drive terminal D2 in Figure 1, described later) each have an excitation magnetic field in opposite directions (described later). The device is characterized in that the excitation magnetic fields He1 of coil L1 and He3 of coil L3, and the excitation magnetic fields He2 of coil L2 and He4 of coil L4 are generated in Figure 1, and each of the two coils at each measurement terminal, one end of which is connected to the same measurement terminal, are arranged to generate excitation magnetic fields in opposite directions (He1 of coil L1 and He2 of coil L2, and He3 of coil L3 and He4 of coil L4, as described later in Figure 1), and the excitation current is passed from the pair of drive terminals, and the magnetic field to be measured or the current to be measured is measured based on the potential difference generated between the pair of measurement terminals.

[0012] According to the sensor of the present invention, a magnetic field or current can be measured with a higher signal-to-noise ratio without including a detection coil as a component.

[0013] This figure shows the principle configuration of the sensor of the present invention. This figure shows a bridge circuit having drive terminals and measurement terminals using four coils. This figure shows an example of the arrangement of coils L1-L4 that constitute the bridge circuit. This figure shows an example of the configuration of coil units Ua and Ub. This figure shows the μ-H characteristics and BH characteristics of the magnetic fluid. This figure shows the voltage waveform and current waveform of the sensor. This figure shows an example of the configuration of a magnetic field sensor using the sensor of the present invention. This figure shows the wiring of the coil unit and the direction of magnetic flux in the configuration example of Figure 7. This figure shows another example of the configuration of a magnetic field sensor using the sensor of the present invention. This figure shows an example of the configuration of a current sensor using the sensor of the present invention. This figure shows the wiring of the coil unit and the direction of magnetic flux in the configuration example of Figure 10. This is a block diagram of a measuring device for measuring a magnetic field or current using the sensor of the present invention.

[0014] Embodiments of the present invention will be described below with reference to the drawings. However, these embodiments do not limit the technical scope of the present invention.

[0015] Figure 1 shows the principle configuration of the sensor of the present invention. Unlike conventional detection methods that utilize the electromotive force of a detection coil, the present invention senses the state of the magnetic field to be measured from the behavior of the excitation coil, and consequently measures the magnetic field to be measured and the current to be measured.

[0016] The sensor of the present invention comprises a bridge circuit consisting of four sets of coils immersed in a magnetic fluid, which serves as a magnetic core housed in a container. As shown in Figure 1, the bridge circuit is configured using four coils L1, L2, L3, and L4, and has two pairs of terminals, each consisting of two opposing terminals. One pair of these terminals is designated as measurement terminals M1 and M2, and the other pair as drive terminals D1 and D2. An excitation current having an AC component of a predetermined frequency is passed through this pair of drive terminals D1 and D2, and the magnetic field to be measured or the current to be measured is measured based on the potential difference generated between the pair of measurement terminals M1 and M2. More specifically, the coils L1, L2, L3, and L4 are arranged such that magnetic fields are generated in opposite directions between the coils L1 and L3 connected to the same drive terminal D1, between the coils L2 and L4 connected to the same drive terminal D2, between the coils L1 and L2 connected to the same measurement terminal M1, and between the coils L3 and L4 connected to the same measurement terminal M2. By immersing this bridge circuit in magnetic fluid and further passing an excitation current having an AC component of a predetermined frequency from a pair of drive terminals D1 and D2, the strength and direction of the magnetic field to be measured can be measured based on the magnitude and phase of a frequency component that is twice the fundamental frequency of the excitation current in the potential difference generated between measurement terminals M1 and M2.

[0017] Note that coils L1, L2, L3, and L4 are coils with both ends, such as a solenoid coil, and not coils without ends, such as a toroidal coil. Also, they may be curved, not just straight like a solenoid coil. Furthermore, a single coil is not limited to one formed from a single coil element, but also includes those constructed by connecting multiple coil elements in series or parallel.

[0018] Figure 2 shows the basic configuration of a bridge circuit, which has drive terminals and measurement terminals using four coils. Figure 2 shows the drive terminals D1 and D2, measurement terminals M1 and M2, and coils L1-L4 corresponding to the configuration in Figure 1. The four coils L1-L4 are, for example, all solenoid coils, manufactured to the same specifications, and have the same characteristics if manufacturing tolerances are ignored.

[0019] In this bridge circuit, an alternating current of a predetermined frequency is passed through the drive terminals D1 and D2, and the potential difference between the measurement terminals M1 and M2 is measured. At this time, since the characteristics of the four coils L1-L4 are the same, if these coils are placed in an environment without a magnetic field, the inductance of all four sides of the bridge will be the same, and the bridge circuit will be in a balanced state. Therefore, no potential difference will occur between the measurement terminals M1 and M2.

[0020] Figure 3 shows an example of the arrangement of coils L1-L4 that constitute the bridge circuit of the sensor of the present invention, and is an extract of the arrangement of coils L1-L4 shown in Figure 1. In the present invention, in order to give this bridge circuit a magnetic field detection function, the four coils L1-L4 are arranged as shown in Figure 3. That is, coils L1 and L2 are aligned so that their coil axes are parallel to each other and are arranged as coil unit Ua. Similarly, coils L3 and L4 are aligned so that their coil axes are parallel to each other and are arranged as coil unit Ub. The coils of coil unit Ub are arranged in a vertical column so that their coil axes are aligned on the same magnetic path as coil unit Ua. With this arrangement, coils L1 and L3, one end of which is connected to the same drive terminal D1, generate magnetic fields in opposite directions, coils L2 and L4, one end of which is connected to the same drive terminal D2, generate magnetic fields in opposite directions, coils L1 and L2, one end of which is connected to the same measurement terminal M1, generate magnetic fields in opposite directions, and coils L3 and L4, one end of which is connected to the same measurement terminal M2, generate magnetic fields in opposite directions.

[0021] Figure 4 shows an example configuration of coil units Ua and Ub. Coil unit Ua (or Ub) consists of two air-core coils L1 and L2 (or L3 and L4) arranged in parallel so that their coil axes are parallel to each other. Air-core coils may also be constructed by winding wire onto a bobbin.

[0022] The operating principle of the present invention in a bridge circuit with coils arranged in this manner will now be explained. In Figure 1 or Figure 3, an excitation current of a predetermined frequency is passed from drive terminal D1 and drive terminal D2. Figure 1 or Figure 3 shows the direction of the excitation magnetic fields He1, He2, He3, and He4 (collectively referred to as the excitation magnetic field He if the excitation magnetic fields of each coil are not distinguished) generated in coils L1, L2, L3, and L4 respectively when current flows from drive terminal D1 to drive terminal D2, and the direction of the magnetic field to be measured Hx.

[0023] When the measured magnetic field Hx is zero, the inductances of the four coils L1-L4 are the same, so the measurement terminals M1 and M2 are both at the midpoint potential between the potential of drive terminal D1 and the potential of drive terminal D2, and no potential difference occurs. This potential difference is always zero when the measured magnetic field Hx is zero, regardless of the magnitude or direction of the excitation current. In other words, even when AC excitation current flows from drive terminals D1 and D2, the potential difference between measurement terminals M1 and M2 is always zero.

[0024] Next, when the magnetic field Hx under test is located from measurement terminal M2 to M1 in Figure 1 or Figure 3, the magnetic field Hx is also present in all four coils L1-L4. Therefore, during the time when the AC excitation current flows from drive terminal D1 to drive terminal D2, the magnetic field of coil L1 is the sum of the magnetic field Hx under test and the excitation magnetic field He1, while the magnetic field of coil L2 is the difference between the magnetic field Hx under test and the excitation magnetic field He2. Coil L3 is the same as coil L1, and coil L4 is the same as coil L2. On the other hand, because the excitation magnetic field is AC, the above sum and difference are reversed every half-cycle of the excitation magnetic field. In other words, the excitation magnetic field He is biased by the strength of the magnetic field Hx under test. This "bias" will be described in detail below.

[0025] Fig. 5 is a diagram showing the μ-H characteristics and B-H characteristics of the magnetic fluid. Further, Fig. 6 is a diagram showing the voltage waveform and current waveform of the sensor. In Fig. 5, the magnetic permeability of the magnetic fluid is maximum when the magnetic field is zero, and decreases as the magnetic field increases. Furthermore, since the inductance of a coil is proportional to the magnetic permeability, in a coil using a magnetic fluid as a magnetic core, the stronger the magnetic field is, the smaller the magnetic permeability becomes, and the inductance also decreases. Therefore, for example, the magnitudes of the inductance of the coil L1 and the coil L2 are interchanged in synchronization with the period of the excitation magnetic field, and the potential of the measurement terminal M1 fluctuates in synchronization with the period of the excitation magnetic field.

[0026] Consider the potential Va (see Fig. 1) at the midpoint between the coil L1 and the coil L2, which is the potential of the measurement terminal M1. In Fig. 6, the time during which the drive current Id flows in the direction of the arrow shown in Fig. 1 is expressed as P time with the direction being positive, and the opposite direction is expressed as N time with the direction being negative. Fig. 1 is a diagram for the P time.

[0027] Fig. 6(a) shows waveforms of the drive voltage Vd, the half voltage Vc thereof, and the current Id supplied to the sensor of the present invention. In this description, it is assumed that there is no resistance or stray capacitance of the coils. Furthermore, all the coils have the same characteristics and are Z-wound. Then, in Fig. 1, the measured magnetic field Hx and the excitation magnetic field He1 are in the same direction, and the magnitude of the magnetic field in the coil L1 is Hx+He1 during the P time; on the other hand, the measured magnetic field Hx and the excitation magnetic field He2 are in opposite directions, and the magnitude of the magnetic field in the coil L2 is Hx-He2. Therefore, during the P time, the inductance satisfies L1<L2, and the potential Va is biased toward the point X (corresponding to the position of the drive terminal D1) side in Fig. 1. Note that if the inductance satisfies L1=L2, Va=Vc. Furthermore, during the N time, the current Id is reversed, the inductance satisfies L1>L2, and the potential Va is biased toward the point Y (corresponding to the position of the drive terminal D2) side in Fig. 1. This state is shown in Fig. 6(b).

[0028] This effect also occurs in coils L3 and L4. However, in coils L3 and L4, the direction in which the excitation magnetic field He is generated relative to the measured magnetic field Hx is opposite to that of coils L3 and L4. Therefore, the potential Vb at the midpoint between coils L3 and L4 (see Figure 1) is biased in the opposite direction to the potential Va relative to the midpoint potential Vc. Figure 6(c) shows these potentials Va and Vb, which are biased in opposite directions, and also shows the difference voltage Va-Vb between potential Va and potential Vb. This difference voltage Va-Vb can be generated by a differential amplifier circuit, and since this waveform is twice the drive frequency f, the sensor detection signal can be obtained by synchronous detection with a reference signal of twice the frequency 2f synchronized with the drive signal. The difference voltage between potential Va and potential Vb is proportional to the strength of the measured magnetic field Hx, and the phase inverts if the direction of the measured magnetic field Hx changes.

[0029] The reason why the present invention can improve the S / N ratio, which is a problem, is as follows. As described in the background art above, conventional detection methods use a detection coil to convert changes in magnetic flux into voltage and extract it. This method is not a problem in the case of strong magnetic fields or large currents, but when measuring weak magnetic fields or minute currents, the magnetic field being measured becomes extremely weak. In this case, the excitation magnetic flux of the sensor does not weaken, but the magnetic flux change component acting on the detection coil is only the magnetic flux change component that links with the detection coil due to fluctuations in the magnetic vector, for example, in the case of Patent Document 3 above, and even if the absolute value of the original magnetic vector is large, the magnetic flux acting on the detection coil is weak. Also, in the case of Patent Documents 7 and 8 above, only the difference of excitation magnetic flux generated in opposite directions acts on the detection coil. In this case as well, the original excitation magnetic flux does not weaken, but since only the difference acts on the detection coil, the energy that induces electromotive force in the detection coil is small, the effect of the disturbing magnetic field becomes more noticeable, and the S / N ratio decreases. To obtain a larger electromotive force from a weak change in magnetic flux, it is better to increase the number of turns in the detection coil. However, this increases the output impedance of the coil. Increasing the number of turns in the coil does not reduce noise caused by disturbing magnetic fields. If the number of turns in the coil is small, the measurement signal is small, and the influence of noise in the subsequent amplification circuit becomes larger. Also, if the number of turns is increased, the impedance becomes higher, making it easier for noise from disturbing electric fields to penetrate.

[0030] Therefore, it can be said that it is effective to (1) avoid a detection coil method that performs a two-stage energy conversion by converting measurement information into magnetic flux and then into voltage, (2) directly output the excitation energy as an output signal, and (3) lower the impedance of the output means for measurement information. The present invention extracts the signal from the measurement terminal of the bridge circuit, thereby satisfying the above conditions (1) to (3) and solving the problem.

[0031] The sensor of the present invention functions by immersing the coil constituting the bridge circuit described above in a magnetic fluid. The magnetic fluid is prepared by adsorbing a surfactant onto the surface of magnetic particles of about 10 nm, which are synthesized in a liquid phase, and dispersing them in a solvent solution. For example, the solvent solution may contain isoparaffin (C n H 2n+2 Magnetite (Fe3O4) can be used as the magnetic particles. While commonly used solid magnetic materials all exhibit hysteresis and inevitably generate Barkhausen noise due to pinning of magnetic domain walls, magnetic fluids have no magnetic domain walls and therefore do not produce Barkhausen noise. Consequently, the signal-to-noise ratio is improved, and detection performance is enhanced even at low sensitivity.

[0032] The magnetic fluid is filled into a container, and the shape of the cavity within the container containing the magnetic fluid is the shape of the magnetic core. The optimal cavity shape is selected depending on the intended use of the magnetic field sensor or current sensor.

[0033] In a magnetic field sensor, the cavity of the container housing the magnetic fluid that immerses the bridge circuit is formed into a rod-like shape that is elongated in the direction of the magnetic field being measured, either straight or curved. This causes the magnetic core, which is the magnetic fluid, to exhibit shape magnetic anisotropy, thereby enabling magnetic collection.

[0034] Figure 7 shows an example configuration of a magnetic field sensor using the sensor of the present invention, and shows the coil units Ua and Ub arranged in the container 20. Part of the container 20 is cut open to show the arrangement of the coil units Ua and Ub. Figure 8 shows the wiring and direction of magnetic flux of the coil units Ua and Ub in the configuration example of Figure 7. In the configuration examples of Figures 7 and 8, each coil L1-L4 is configured by connecting four coil elements in series for each coil, and the four coil units Ua (Ua1, Ua2, Ua3, Ua4) and the four coil units Ub (Ub1, Ub2, Ub3, Ub) are arranged in a straight line. The magnetic fluid 30 is injected from the magnetic fluid injection hole 21 and fills the cavity of the container 20, and in Figure 7, the cavity portion of the container 20 is indicated as the magnetic fluid 30.

[0035] Figure 9 shows another configuration example of the magnetic field sensor using the sensor of the present invention, and, similar to Figure 7, shows the coil unit arranged in a container. A part of the container is cut open to show the arrangement of the coil unit. In the configuration example of Figure 9, coils L1-L4, each formed by a single coil element having a relatively long length in the coil axis direction, are arranged inside the container. Although not denoted by reference numerals, the set of coils L1 and L2 constitutes coil unit Ua, and the set of coils L3 and L4 constitutes coil unit Ub. The magnetic fluid 30 is injected through the magnetic fluid injection hole 21 and filled into the cavity of the container 20, and in Figure 9, the cavity portion of the container 20 is indicated as the magnetic fluid 30.

[0036] As shown in the above example of magnetic field sensor configuration, when measuring relatively parallel magnetic fields such as the Earth's magnetic field, a straight magnetic core is usually used. However, for applications such as flaw detection and rebar detection in concrete, a "U" shape can be advantageous. Also, for maintenance purposes, such as measuring corrosion protection currents applied to pipelines and underwater iron structures, using a clamp-type current sensor is not practical due to the large size of the object. Therefore, it is possible to measure the current by wrapping a flexible belt-shaped magnetic field sensor around the object. The present invention makes it possible to create the above-mentioned flexible belt-shaped magnetic field sensor by making a container for magnetic fluid out of a flexible material and sealing it.

[0037] Next, in the case of a typical current sensor, it is preferable to efficiently collect the magnetic field of the current being measured while eliminating the influence of so-called background magnetic fields other than the magnetic field of the current being measured. A torus shape is suitable for this purpose. However, in practical terms, even if it is not a perfect torus shape, various shapes are selected depending on the application, while prioritizing the annular shape of the magnetic path by making the container shape ring-shaped. Preferably, the cavity shape of the container that contains the magnetic fluid is curved or bent, and both ends of the cavity are connected as a magnetic path formed when the cavity is filled with magnetic fluid, forming a closed magnetic path, and the magnetic path forms an annular shape.

[0038] Figure 10 shows an example configuration of a current sensor using the sensor of the present invention, and shows the coil unit arranged in a container. Part of the container is cut open to show the arrangement of the coil unit. Figure 11 shows the wiring of the coil unit and the direction of the magnetic flux in the configuration example of Figure 10. In the configuration examples of Figures 10 and 11, each coil L1-L4 arranged in the annular (the cavity is also annular) container 20 is configured by connecting three coil elements in series, and the three coil units Ua1, Ua2, Ua3 and the three coil units Ub1, Ub2, Ub3 are arranged in an annular pattern. The central hole of the annular container 20 is the wire through hole 22 to be measured. The magnetic fluid 30 fills the cavity of the container 20, and in Figure 10, the cavity portion of the container 20 is indicated as the magnetic fluid 30.

[0039] In the current sensor of the present invention, the shape of the cavity of the container for housing the magnetic fluid that forms the magnetic core is elongated in the direction of the magnetic field to be measured, and then curved or bent. The ends of this cavity are connected as a magnetic path formed when the cavity is filled with magnetic fluid, forming a closed magnetic path, and the problem is solved by making the magnetic path ring-shaped. In the current sensor of the present invention, the shape of the magnetic core, i.e., the shape of the cavity that stores the magnetic fluid, is preferably torus-shaped, but it is practical to adjust the shape according to the application of the current sensor. Incidentally, using magnetic fluid as the magnetic core material is advantageous over using conventional solid magnetic cores because the magnetic core material is liquid, allowing for the creation of complex shapes that balance practicality and magnetic rationality. The sensor of the present invention can further demonstrate this advantage.

[0040] Furthermore, the annular magnetic path does not need to be a perfect annular shape; it may be a closed magnetic path with a magnetic gap in between. The allowance of this magnetic gap is useful when dividing the annular magnetic path to create a so-called clamp-type sensor. In other words, a clamp-type current sensor can be created by dividing the container holding the magnetic fluid at the magnetic gap.

[0041] Figure 12 is a block diagram of a measuring device for measuring a magnetic field or electric current using the sensor of the present invention. The measuring device using the sensor 10 of the present invention comprises an oscillation circuit 11 that generates an excitation current of a predetermined frequency f, a drive voltage output circuit 12 that outputs a drive voltage to the drive terminals D1 and D2 of the sensor 10 of the present invention from the AC signal, a differential amplifier circuit 13 that amplifies the difference voltage Va-Vb between the potentials Va and Vb of the measurement terminals M1 and M2 of the sensor 10 of the present invention, a synchronous detection circuit 14 that performs synchronous detection using a reference signal with a frequency 2f, which is twice the predetermined frequency f, and an LPF circuit 15 for removing the excitation frequency component and its harmonic components caused by excitation.

[0042] Furthermore, as a known technology, there is a measurement method called the magnetic balance method, in which a negative feedback magnetic field opposite in direction to the magnetic field to be measured is generated in the sensor using an output signal of the sensor to bring the magnetic field to be measured inside the sensor close to zero. This method can also be employed in the present invention. In this case, as a means for generating the negative feedback magnetic field, in addition to the method of winding a negative feedback coil, a negative feedback current can be passed through two measurement terminals of a bridge circuit to cancel out the magnetic field to be measured inside the sensor.

[0043] Both of these two negative feedback methods are means for implementing the magnetic balance system. The magnetic balance system is a system that maintains the intensity of the magnetic field to be measured inside the sensor substantially at zero, and it is also a system that maintains the magnetic fluid in a state with the highest magnetic permeability. Therefore, it can also be said to be a means that highlights the performance of the basic configuration of the present invention described above and further improves the S / N ratio. In the case of general soft magnetic materials, the magnetic permeability at a substantially zero magnetic field strength is called initial permeability, which is the lowest below the maximum permeability, and generally, the initial permeability is about one-third to one-half of the maximum permeability.

[0044] The present invention is not limited to the above-described embodiments. It goes without saying that any design changes within a scope that does not depart from the spirit of the present invention, including various variations and modifications that can be conceived by a person having ordinary knowledge in the field of the present invention, are also included in the present invention.

[0045] 10: Sensor, 11: Oscillation circuit, 12: Drive voltage output circuit, 13: Differential voltage amplifier circuit, 14: Synchronous detection circuit, 15: LPF circuit, 20: Container, 21: Magnetic fluid injection hole, 22: Measured wire through hole, 30: Magnetic fluid, L1: Coil, L2: Coil, L3: Coil, L4: Coil, D1: Drive terminal, D2: Drive terminal, M1: Measurement terminal, M2: Measurement terminal, He: Excitation magnetic field, Hx: Magnetic field to be measured

Claims

1. A sensor for measuring a magnetic field to be measured or a current to be measured, comprising: a magnetic fluid as a magnetic core; a container for containing the magnetic fluid; and a bridge circuit consisting of four coils immersed in the magnetic fluid in the container, wherein the bridge circuit has two pairs of terminals, each consisting of two opposing terminals, one of which forms a pair of measurement terminals for detecting a magnetic field to be measured or a current to be measured, and the other pair of terminals forms a pair of drive terminals for supplying an excitation current having a predetermined frequency component to the bridge circuit, wherein of the four coils, two coils at each drive terminal, one end of which is connected to the same drive terminal, are arranged to generate excitation magnetic fields in opposite directions to each other, and two coils at each measurement terminal, one end of which is connected to the same measurement terminal, are arranged to generate excitation magnetic fields in opposite directions to each other, and the excitation current is passed from the pair of drive terminals, and the magnetic field to be measured or a current to be measured is measured based on the potential difference generated between the pair of measurement terminals.

2. The sensor according to claim 1, characterized in that the cavity shape of the container for containing the magnetic fluid is straight or curved like a rod.

3. The sensor according to claim 1, characterized in that the cavity shape of the container for containing the magnetic fluid is curved or bent, and both ends of the cavity are connected to form a closed magnetic path when the cavity is filled with magnetic fluid, and the magnetic path is annular.

4. The sensor according to claim 1, characterized in that each of the four coils is configured by connecting multiple coil elements in series or in parallel.

5. A measuring device comprising: a sensor according to any one of claims 1 to 4; an oscillation circuit that generates an excitation current supplied to the pair of drive terminals of the sensor; and a synchronous detection circuit that detects the potential difference occurring between the pair of measurement terminals of the sensor using a reference signal with a frequency twice that of the excitation current.

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