HK1210269B - Magnetism detection device - Google Patents

Description

Magnetic detection device

Technical Field

The present invention relates to a magnetic oscillation sensor and an apparatus for detecting magnetism for measuring a leakage magnetic field generated by and existing inside and outside trains and automobiles, and more particularly to an apparatus for detecting magnetism capable of measuring a DC magnetic field and an AC magnetic field having a frequency of about 100kHz at the maximum.

Further, the present invention relates to an apparatus for detecting magnetism to be mounted in a fluxgate type magnetic detector defined in IEC61786 standard or JIS-C1910 standard (i.e., "a detector for detecting a magnetic field by nonlinear magnetic characteristics of a probe or a sensor including a magnetic core made of a ferromagnetic material").

Background

As a magnetic sensor for detecting a leakage magnetic field present inside and/or outside a train or automobile, a search coil type magnetic sensor operating by electromagnetic induction is mainly used. However, a magnetic sensor including a coil is accompanied by a theoretical problem that it cannot detect a DC magnetic field.

A fluxgate-type device that has been practically used to detect magnetism is capable of detecting not only a DC magnetic field but also an AC magnetic field. However, the effective range of frequencies in which the AC magnetic field can be detected is only a few kHz at maximum. There has not been a perfect fluxgate-type magnetic detection apparatus capable of detecting not only a DC magnetic field but also an AC magnetic field having a frequency of about 100kHz at the maximum. This is because it is very difficult to convert a magnetic field in a range from a DC magnetic field to an AC magnetic field having a frequency of about 100kHz and the same magnetic field strength as the DC magnetic field into an electric signal having a constant strength, and furthermore, it is also very difficult to ensure the performance of doing so.

In these years, many trains from which strong leakage magnetic fields are generated have been manufactured, and there is a fear that the leakage magnetic fields exert harmful effects on humans and/or magnetic storage media, and therefore, the japanese industrial standards research council has made "railway vehicle-leakage magnetic field measurement method (JIS E4018)".

The method defines the object to be measured, and the conditions under which the measurement is to be made. The object includes a leakage magnetic field (magnetic flux density) existing inside and outside the train and a device generating the magnetic field. The conditions are defined according to the state of the train. For example, when a train is operating, the leakage magnetic field in the train and near the device generating the magnetic field should be measured at a maximum current through the speed range of the train in which the device is located. Since a DC magnetic field having a magnetic flux density in the range of about 1 to about 2mT is measured when the train starts running, a measuring device including a hall element is mostly used.

Specifically, the X, Y, and Z components of the magnetic field are measured by a measuring device having an accuracy of about ± 5%, the measured components are synthesized according to equation (1), and the magnetic flux density is represented by the synthesized components. In recording the measurement results, the magnetic flux density is recorded in the form of a composite density and a component of each axis.

B＝(B_x ²+B_y ²+B_z ²)^1/2(1)

In the measurement of the magnetic field, the X component, the Y component, and the Z component are measured substantially simultaneously. Since the conventional measuring apparatus is a widely-used apparatus that displays an effective value or a wave height value, and thus cannot ensure performance for performing instantaneous measurement of waveform and broadband frequency characteristics, a resultant value at the time of measurement of an AC magnetic field is calculated according to equation (1) based on effective values, or wave height values, associated with the X-axis, the Y-axis, and the Z-axis. As a result, since the maximum value of the synthesized magnetic field is calculated in the case where the data simultaneity and the phase relationship among the X component, the Y component, and the Z component are neglected, the value thus calculated does not coincide with the true total magnetic force (the strength or the absolute value of the magnetic field vector) to be calculated using the instantaneous value of the X, Y, Z axis.

This is because the displayed significant value or wave height value does not take into account data representing the phase relationship between the X, Y and Z components. For example, the strength of the magnetic field (the strength of the magnetic field vector) calculated based on the displayed wave height value is always larger than the true total magnetic force calculated based on the data obtained when the X, Y and Z components are measured simultaneously except for special cases, and has an error equal to or larger than several 10%, which significantly exceeds the allowed accuracy of ± 5% of the measuring device. As a result, the resultant value of the magnetic field calculated based on the displayed wave height value accompanies the following problem: which differs significantly from the true strength of the magnetic field vector by neglecting errors caused by the phase relation.

From another point of view, a conventional measuring apparatus for measuring a magnetic field based on a displayed effective value or a wave height value is an apparatus that disregards distortion and/or a phase relationship of a waveform of the magnetic field and measures an average value with respect to time as a strength of the magnetic field, and does not guarantee instantaneous response performance to a magnetic field having a high frequency component and accurately reproduce characteristics of the waveform of the measuring magnetic field.

The frequency band of the magnetic field generated from automobiles and trains widely covers the magnetic field in the range of the DC magnetic field to the inversion frequency and the high-frequency noise magnetic field caused by switching. In order to analyze these magnetic fields using FFT (fast fourier transform), it is necessary to use a wide-band type device for measuring magnetic fields including not only low frequency bands such as DC magnetic fields, variable magnetic fields, and magnetic fields for commercial frequencies but also high frequency bands of about 100kHz as a practical device capable of measuring magnetic fields with constant detection sensitivity.

Further, a wide-band type device for measuring a magnetic field needs to have a very wide dynamic range, specifically, a magnetic field in a range capable of measuring a weak magnetic field ranging from a strong magnetic field of several mT to a range of several hundred nT to several tens nT, which is feared to exert a harmful influence on a human body.

The system for detecting magnetism in the magnetic sensor includes a system adapted to measure a DC variable magnetic field from a DC magnetic field to several Hz, a system adapted to measure a DC variable magnetic field from a DC magnetic field to several hundred Hz, a system capable of measuring only an AC magnetic field in a range of several Hz to several tens kHz, a system capable of measuring only a weak magnetic field, a system capable of measuring only a strong magnetic field, and the like according to the theory for measuring magnetism.

For example, the hall element type magnetic sensor has a practically effective accuracy of about several tens μ T, and is therefore suitable for measuring a strong magnetic field because a small magnetic field of about several tens μ T can be ignored as an error when a strong magnetic field in a range of about 1 to about 2mT is measured. However, when a weak leakage magnetic field which is feared to exert a harmful influence on the human body is measured by several μ T or less, an error is larger than a signal, and therefore, a signal indicating the weak magnetic field is mixed with noise and thus cannot be found. Thus, the hall element type magnetic sensor has advantages and disadvantages.

Thus, as one of the solutions to the above-described problems, a technique has been invented in which the low frequency band and the high frequency band including the DC magnetic field are measured by two types of magnetic sensors, respectively.

Specifically, patent document 1 entitled "An apparatus for and a method of measuring a magnetic field in a railway vehicle" discloses a composite type magnetic sensor including a combination of a magnetic oscillation sensor and a search coil type magnetic sensor, both of which complement each other's drawbacks to thereby be able to measure a wide-band magnetic field. The magnetic oscillation sensor is a fluxgate (defined by the IEC61786 standard) that measures a magnetic field by means of a non-linear magnetic characteristic of a probe or a sensing member having a ferromagnetic core.

More specifically, the composite type magnetic sensor includes, as a first triaxial magnetic sensor, a search coil type sensor that is good at detecting an AC magnetic field having a frequency of several tens of Hz or more, and, as a second triaxial magnetic sensor, a magnetic oscillation sensor that is suitable for measuring a DC magnetic field or a variable magnetic field. By combining the strengths of these two types of magnetic detection systems, the composite magnetic sensor has no objects that can not be measured by itself.

Each of the first and second three-axis magnetic sensors is designed to include a magnetic sensing part having a magnetic detection axis (a direction in which the magnetic sensing part senses maximum magnetism). The three magnetic detection axes are arranged perpendicular to each other so that the external magnetic field can be detected by dividing the external magnetic field into an X component, a Y component, and a Z component.

Fig. 7 shows an embodiment of the magnetic sensor disclosed in patent document 1, which has a basic configuration in which each magnetic sensing component is housed in and integrated with a sensor housing.

The first triaxial magnetic sensor 51 includes a magnetic sensor for measuring only an AC magnetic field. The magnetic sensor comprises three search coils perpendicular to each other. The magnetic field signal (induced voltage) detected by the search coil arranged in X, Y and the Z axis is transmitted to the main measurement unit via the sensor cable 53, processed in the signal circuit, and then output.

The second three-axis magnetic oscillation sensor 52 includes a magnetic oscillation sensor for measuring a DC magnetic field and a low frequency magnetic field. The magnetic oscillation sensor includes a magnetic sensor having three core coils each including a magnetic core made of a ferromagnetic material. The magnetic detection axes of the core coils are arranged along the X, Y and Z axes so that they are perpendicular to each other.

Fig. 8 shows a basic circuit of a magnetic oscillation sensor as a device for measuring magnetic fields in three axes. In fig. 8, 100 denotes an X-axis circuit part, 104 denotes a magnetic sensing part for the X-axis, 200 denotes a Y-axis circuit part, 204 denotes a magnetic sensing part for the Y-axis, 300 denotes a Z-axis circuit part, and 304 denotes a magnetic sensing part for the Z-axis. Since the circuits for the three axes have the same structure as each other, the X-axis circuit components are explained below.

The magnetic oscillation sensor has a deformation circuit of a multivibrator. In particular, the deformation circuit of the multivibrator is adapted to be able to oscillate by: the fluctuation of the voltage between the capacitor terminals (i.e., repetition of the voltage fluctuation upon oscillation) is replaced by a phenomenon of a specific fluctuation of the voltage between the terminals of the core coil having a nonlinear characteristic when an AC current flows therethrough.

Since the oscillation in the multivibrator circuit is generated by the nonlinear excitation characteristic of the magnetic material, the oscillation circuit is referred to as a "magnetic oscillation circuit", and the magnetic sensor to which the magnetic oscillation phenomenon is applied is referred to as a "magnetic oscillation sensor" or a "magnetic oscillation type magnetic sensor".

The oscillating current flowing through the magnetic oscillation circuit passes through the core coil 105, and thus alternately excites the core 106 in the positive or negative direction to magnetically saturate the core 106.

Therefore, the oscillating current is also referred to as "excitation current".

The magnetic oscillation sensor in the X-axis circuit includes a magnetic sensor 104, an operational amplifier 108, and resistors 107, 109, and 110 electrically connected to the operational amplifier 108, the magnetic sensor 104 including a core coil 105 including a core 106 as a magnetic core. The core coil 105 includes a terminal P20 electrically connected to the non-inverting input terminal of the operational amplifier 108, and is grounded at the other end. Reference numeral 111 denotes a low-pass filter having a main function of attenuating a magnetic oscillation frequency component included in the magnetic detection signal. Reference numeral 112 denotes an amplification circuit which controls the magnitude of the voltage according to the strength of the external magnetic field detected by the magnetic sensor and outputs the voltage thus controlled via a terminal Q10.

If an excitation magnetic field generated only by an oscillating current is applied to the core 106, the excitation time required for the core 106 to be magnetically saturated in the positive direction is equal to the excitation time required for the core 106 to be magnetically saturated in the negative direction due to the symmetry of the magnetization characteristic (B-H curve) of the magnetic material with respect to the origin.

From another point of view, since the origin at which the core 106 starts its action is the origin of the coordinate axis of the B-H curve, the positive and negative excitation times required for the core 106 to be magnetically saturated in the positive and negative directions are equal to each other, and thus, the time difference is equal to zero. Therefore, the integral of the output voltage having a rectangular waveform in the operational amplifier 108 is equal to zero.

However, if an external magnetic field is applied to the core 106 under the above-described conditions, the external magnetic field overlaps with the excitation magnetic field. As a result, the action point moves from the origin of the coordinate axis of the B-H curve (which is the origin at which the core starts its action) to an extent defined by the strength of the external magnetic field, thus resulting in an interval of times at which the core is saturated with positive or negative magnetism. Specifically, the ratio between the positive half-cycle duration and the negative half-cycle duration in the core (referred to as the "duty cycle") changes due to the external magnetic field, and thus the integral of the output voltage from the operational amplifier 108 also changes accordingly.

In other words, the external magnetic field is detected by the magnetic oscillation sensor as a fluctuation of the integral of the output voltage from the operational amplifier 108.

The oscillation frequency of the magnetic oscillation sensor is initially adjusted (adjusted at the time of shipment) by changing the partial voltage ratio between the resistors 109 and 110 electrically connected to the output terminal of the operational amplifier 108.

However, such a circuit configuration as described above is accompanied by the following problems.

The first problem is: if a difference in oscillation frequency of the plurality of magnetic oscillation sensors is caused, a signal having a beat frequency (a "beat" frequency generated when two waves having frequencies slightly different from each other overlap) is generated.

In other words, a signal that has a beat frequency component and is not present in the external magnetic field overlaps with the detection signal that is noise. It is very difficult to identify the beat frequency component from the magnetic detection signal transmitted from the magnetic oscillation sensor, resulting in the following problems: the magnetic field having the beat frequency component has to be identified as an external magnetic field. Further, if such a phenomenon occurs, the output transmitted from the magnetic oscillation sensor will contain a fluctuation error of the DC level even in the range of about several tens nT to about several thousands nT depending on the intensity of the disturbance magnetic field, resulting in a failure to accurately measure the environmental magnetic field or a failure to measure the magnetic field in a strong field.

Magnetic oscillation sensors have the following tendency in the presence of strong magnetic fields: the magnetic oscillation frequency is lowered while the magnetic field is measured, and therefore, the beat phenomenon easily occurs due to the fluctuation of the frequency, which is a serious drawback that offsets various advantages of the magnetic oscillation sensor with respect to its performance. Thus, it is necessary to quickly solve the problem.

The second problem is: the accuracy with which the disturbing magnetic field is measured is lowered due to electromagnetic noise generated between core coils in the triaxial magnetic oscillation sensor or electromagnetic noise generated in an adjacent search coil type magnetic sensor.

In order to solve this problem, when disposed in the sensor housing, it is necessary to separate the magnetic sensing parts and the circuit parts in the three axes from each other, or to separate the magnetic sensing parts in the magnetic sensor from each other. Specifically, the magnetic oscillation sensor is randomly arranged with a sufficient space therebetween, the core coil or the search coil is randomly arranged with a sufficient space therebetween, and/or the sensor housing that houses the sensor is designed to be large enough to house the magnetic sensor therein.

However, since in the above-described solution, the magnetic field is measured at each position of the sensor, there is newly caused a problem that since the point at which the magnetic field is measured exists randomly, the accuracy of measuring the magnetic field is lowered, and the measurement error is increased.

If the magnetic field is a uniform parallel magnetic field, it is no problem to not consider the random position of the magnetic sensor in the measurement of the magnetic field. However, when measuring a magnetic field that is locally deformed due to a sharp disturbance of the intensity of the magnetic field inside or outside the train or automobile, the magnetic intensities may be very different from each other according to the random position of the magnetic sensor, resulting in the following serious problems: measurement errors inevitably result due to positional gaps of magnetic sensing components of the magnetic sensor, and therefore, the measured magnetic field strength is unreliable.

Reference to the prior art

Patent document

Patent document 1: japanese patent application laid-open No.2005-69829

Disclosure of Invention

Problems to be solved by the invention

Considering a process of supplying an excitation current to a core coil assembled in a magnetic sensing part, a fluxgate type magnetic sensor that measures a magnetic field by a nonlinear magnetic characteristic of a magnetic core made of a ferromagnetic material is grouped into a magnetization system and a self-excitation system.

The foregoing magnetizing system is an external exciting system in which an exciting current is supplied from an external oscillation circuit or an external AC power supply, both of which are independently separated from the core coil. This system was published in 1939 and is now widely used as the basic excitation system in fluxgate-type magnetic sensors. Since the fluxgate-type magnetic sensor including the system can measure not only the DC magnetic field but also the AC magnetic field having a frequency of several kHz, the fluxgate-type magnetic sensor is widely used to measure the weak magnetic field in the low frequency band.

The external excitation system must receive excitation current from an external power source. When an external strong field having an intensity of several thousand μ T or more is to be measured, an excitation current having an intensity several times greater than several thousand μ T needs to be supplied to the magnetic sensor as an AC current having a uniform intensity, and further, the electric power of the excitation current exciting the core of the magnetic sensor needs to be increased.

Conversely, when a weak magnetic field is measured, the excitation current that magnetically saturates the core may be small. However, in order to measure both a weak magnetic field and a strong field having a strength of several thousand μ T, the magnetic sensor needs to be kept overexcited for measuring the strong field as a maximum magnetic field. For this reason, it is necessary to laboriously supply an excitation current having an intensity several times greater than several thousand μ T to the magnetic sensing means.

Thus, there is a need to solve difficult technical problems such as reconfiguration of the magnetic sensing part, countermeasures for abnormal heating coils, and countermeasures for stabilizing the excitation current, all of which are not problems when measuring a weak magnetic field.

There is no commercially available magnetic sensor that meets the performance and the solution to the above problem remains unsolved.

In contrast, a self-excited fluxgate type magnetic sensor described later is referred to as a magnetic oscillation sensor. The simplest circuit for a magnetic oscillation sensor comprises a variable multivibrator, which is reconstructed by: the oscillation generated by the fluctuation of the voltage at the capacitor terminal in the oscillation circuit of the unstable multivibrator including the operational amplifier is replaced with the fluctuation of the voltage which is varied by the nonlinear magnetic characteristic of the core coil.

Since the core coil itself is used as a part of the oscillation circuit in the above-described magnetic sensor, the oscillation current flowing through the oscillation circuit naturally flows through the core coil as the excitation current. In this system, since the oscillating current flowing through the oscillating circuit is used as the exciting current to magnetize the core coil, it is no longer necessary to use an external AC power supply for the excitation. The system can be said to be a self-contained self-exciting system.

In the magnetic oscillation sensor, an AC component, which is an excitation current component for magnetic oscillation, and a component proportional to the intensity of an interference magnetic field flow through a core coil. Since the integral of the excitation current flowing through the core coil of the magnetic oscillation sensor is proportional to the intensity of the external magnetic field, the magnetic oscillation sensor does not waste the excitation current, and can save energy with high efficiency, unlike an excitation system such as a fluxgate-type magnetic sensor in which a magnetization magnetic field having an intensity larger than a measurement limit must be always generated.

The object of the present invention is to improve a magnetic detection device which is capable of accomplishing the above-mentioned performance of the magnetic oscillation sensor to the maximum extent so that a magnetic field including not only a DC magnetic field but also an AC magnetic field can be measured according to the international standard IEC/TS62597 (international standard on measurement of leakage magnetic fields inside and outside a train).

In particular, a first objective to be accomplished by the invention is to minimize the position gap at the point where the magnetic field is measured.

A second object to be accomplished by the present invention is to establish a technique for preventing occurrence of a beat phenomenon generated due to a difference in oscillation frequency between magnetic oscillation sensors.

Solution to the problem

In order to solve the above-described problems, in a first aspect of the present invention, there is provided a magnetic oscillation sensor comprising: a magnetic sensor including a core coil including a core made of a magnetic material and a coil wound around the core; and an operational amplifier circuit that causes an AC excitation current to flow through the coil to generate an output in accordance with the strength of a magnetic field applied to the core, characterized by an air-core coil, wherein the air-core coil is located in the vicinity of the core coil of the magnetic sensor, and by a current being supplied to the air-core coil, wherein a magnetic field of the same strength as that of a leakage magnetic field generated due to the excitation current flowing through the core coil is generated with the current, the magnetic field also having a direction opposite to that of the leakage magnetic field.

It is important to minimize the position gap at the point where the magnetic field is measured to improve the accuracy of measuring the magnetic field. To improve accuracy, it is necessary to place the core coil sensors of the magnetic sensor close to each other to accommodate them in a small-sized sensor housing. For this reason, it is necessary to avoid electromagnetic induction between core coils of the magnetic sensor as much as possible to suppress induction noise as much as possible.

In a first aspect of the present invention, a system is modified in which, in order to reduce the influence exerted on the core coils of adjacent shafts to suppress induced noise as much as possible, an excitation current is supplied not only to the core coils of a magnetic sensor that measures a magnetic field but also to the air-core coils so that the air-core coils generate the following magnetic fields: the magnetic field has the same strength as the leakage magnetic field and also has a direction opposite to the direction of the leakage magnetic field for canceling the leakage magnetic field.

Since the system can minimize the leakage of the leakage magnetic field from the magnetic sensor to the space into which the magnetic sensor enters, the system is useful when the magnetic field is measured only by the magnetic oscillation sensor, ensuring that the induction noise generated in the adjacent electronic devices and circuits can be suppressed.

In a second aspect of the present invention, there is provided a magnetic detection apparatus comprising a plurality of magnetic oscillation sensors each including a magnetic sensor having a core coil and a multivibrator, the core coil includes a core made of a magnetic material and a coil wound around the core, the multivibrator includes the coil of the core coil, a circuit element and an operational amplifier circuit, characterized in that an oscillation synchronizing signal circuit network is reconstructed in which operational amplifier circuits in a magnetic oscillation sensor are electrically connected to each other via an electric connector to unify oscillation frequencies of the magnetic oscillation sensor, each of cores of the magnetic sensor is forcibly excited at the unified magnetic oscillation frequency by a synthetic excitation current, the synthesized excitation current includes an oscillation synchronizing signal flowing through the oscillation synchronizing signal circuit network and an excitation current output from the operational amplifier circuit.

In a second aspect of the present invention, a circuit is configured to share magnetic oscillation frequency components of a plurality of magnetic oscillation sensors. In order to excite each of the cores at a common magnetic oscillation frequency unified by shared data of the circuit, the operational amplifier circuits are electrically connected to each other to construct an oscillation synchronous signal circuit network. The beat phenomenon is avoided by an oscillating synchronization signal flowing through the circuit network.

In a third aspect of the present invention, in the oscillation synchronizing signal circuit network set forth in the second aspect of the present invention, output terminals of the operational amplifier circuit in the magnetic oscillation sensor are connected in a ring via a passive or active element.

In a third aspect of the present invention, output terminals of the operational amplifier circuits are connected via passive or active elements to define a ring type oscillation synchronizing signal circuit network. Each of the cores of the magnetic sensor is forcibly excited at a uniform magnetic oscillation frequency by a synthetic excitation current including an oscillation synchronizing signal flowing through the oscillation synchronizing signal circuit network and an excitation current output from the operational amplifier circuit.

In a fourth aspect of the present invention, in the oscillation synchronizing signal circuit network set forth in the second aspect of the present invention, output terminals of the operational amplifier circuit in the magnetic oscillation sensor are connected in a star shape via a passive or passive element.

In a fourth aspect of the invention, the output terminals of the operational amplifier circuits are connected via passive or active elements to define a star oscillator synchronizing signal circuit network. Each of the cores of the magnetic sensor is forcibly excited at a uniform magnetic oscillation frequency by a synthetic excitation current including an oscillation synchronizing signal flowing through the oscillation synchronizing signal circuit network and an excitation current output from the operational amplifier circuit.

A fifth aspect of the present invention is characterized in that, in the second aspect of the present invention, an external signal transmission circuit that transmits an electric signal having the same frequency as that of the above-described unified magnetic oscillation frequency is electrically connected to the operational amplifier circuit via an electric connector to construct an oscillation synchronous signal circuit network having a fixed unified magnetic oscillation frequency.

In a fifth aspect of the present invention, an external signal transmission circuit that transmits an electric signal having the same frequency as that of the above-described unified magnetic oscillation frequency is electrically connected to the operational amplifier circuit via an electric connector to construct an oscillation synchronizing signal circuit network having the unified magnetic oscillation frequency. Each of the cores of the magnetic sensor is forcibly excited at a uniform magnetic oscillation frequency by a synthetic excitation current including an oscillation synchronizing signal flowing through the oscillation synchronizing signal circuit network and an excitation current output from the operational amplifier circuit.

A sixth aspect of the present invention is characterized in that, in each of the second to fifth aspects of the present invention, each of the magnetic oscillation sensors is designed to include the air-core coil described in the first aspect.

By applying a magnetic oscillation sensor including an air-core coil to each of the second to fifth aspects of the present invention, it is possible to prevent a leakage magnetic field leaking from the core coil of each of the magnetic oscillation sensors from exerting a harmful influence on detection by the other magnetic oscillation sensors, ensuring an improvement in the accuracy of the measurement magnetic field.

Advantages offered by the invention

The present invention relates to improvements imposed on conventional magnetic oscillation sensors under the international standard IEC/TS 62597. The greatest advantage of the present invention is that the applied magnetic field can be canceled by the current flowing through the core coil as long as the normal oscillation condition is maintained, and therefore, a magnetic field having a strength of several thousand μ T or more and a magnetic field in a range from a DC magnetic field to an AC magnetic field having a frequency of about 100kHz can be measured.

The lower limit of the strength of the magnetic field to be measured is lowered to several nT by the reduction of the noise level, and the dynamic range of the strength of the magnetic field to be measured is in the range of a strong field having a strength of several mT to a weak magnetic field having a strength of several nT or less. A wide range of magnetic fields can be measured.

Further, a magnetic field of a wide range of frequencies from a DC magnetic field to an AC magnetic field having a frequency of about several hundred kHz may be measured by a single magnetic detection system (specifically, a magnetic oscillation sensing system) without applying a plurality of magnetic detection systems to the measurement range, respectively. This performance is about 20 times higher than that of the conventional fluxgate type magnetic sensor having the maximum frequency of about 5 kHz.

Since the magnetic oscillation sensor is an energy saving type magnetic sensor in which the excitation current for magnetizing the core is proportional to the intensity of the external magnetic field, it is suitable for simultaneously measuring the intensity distribution of the variable magnetic field at a plurality of measurement points.

As described above, the magnetic detection device according to the present invention provides sufficient performance as a magnetic sensor for measuring a magnetic field in a train or an automobile. In addition, the magnetic detection device according to the present invention is expected to be widely used in learning and research in other technical fields, in industrial fields, and/or in measurement of a magnetic field generated from a power supply line or an environmental magnetic field as one of useful high-performance magnetic sensors.

Drawings

Fig. 1 shows the surroundings of a magnetic sensing component of a magnetic oscillation sensor according to an embodiment of the invention. Fig. 1(a) is a circuit diagram, and fig. 1(b) is for explaining a leakage magnetic field.

Fig. 2 shows the surroundings of the magnetic sensing component of a conventional magnetic oscillation sensor. Fig. 2(a) is a circuit diagram, and fig. 2(b) is for explaining a leakage magnetic field.

Fig. 3 is a circuit diagram of the surroundings of the magnetic sensing component of the magnetic oscillation sensor according to an embodiment of the present invention.

Fig. 4 is a circuit diagram of a magnetic detection device according to a first embodiment of the present invention.

Fig. 5 is a circuit diagram of a magnetic detection device according to a second embodiment of the present invention.

Fig. 6 is a circuit diagram of a magnetic detection device according to a third embodiment of the present invention.

Fig. 7 is a diagram for explaining the magnetic sensor disclosed in patent document 1.

Fig. 8 is a circuit diagram of a three-axis device for measuring magnetic fields.

Detailed Description

Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

Fig. 1 shows an environment around a magnetic sensing component of a magnetic oscillation sensor according to an embodiment of the present invention, and fig. 2 shows an environment around a magnetic sensing component of a conventional magnetic oscillation sensor.

As shown in fig. 2(a) or fig. 8, in the conventional magnetic oscillation sensor, an excitation current is supplied from a terminal P10 to a core coil 105 arranged around a core 106 of a magnetic sensing member via a passive element 107 such as a resistor, and thereafter, a fluctuation of a voltage at the terminal P20 is detected by an operational amplifier 108 to thereby detect the strength of an external magnetic field. In the magnetic oscillation sensor, when an excitation current flows through the core coil 105, a leakage magnetic field is generated around the core coil, as shown in fig. 2 (a). The arrow shown in fig. 2(b) shows the direction of the polarity of the magnetic field generated in the core coil 105.

In the case where only the core coil 105 is used, the excitation magnetic field leaking from the core coil 105 propagates around the coil. Therefore, the electronic devices, the communication devices, and the electronic circuits existing in the space are naturally affected by the inductive noise.

To protect them from induced noise, it is necessary to move them away from the magnetic sensing component or to reduce the size of the space in which the induced noise is present.

The former solution cannot be used in magnetic field measuring devices because the accuracy with which the measurement is made is reduced.

In the latter solution to reduce the size of the noise space, there is a magnetic shielding around the magnetic material. However, if the magnetic shield is located in the vicinity of the magnetic sensing part, the external magnetic field cannot be accurately measured because the external magnetic field is affected by the magnetic shield. Finally, the only solution for shielding the leakage magnetic field is to apply the same magnetic field as the excitation magnetic field in the opposite direction to thereby cancel the leakage magnetic field.

To solve this problem, the magnetic oscillation sensor according to the embodiment of the present invention is designed to use the air-core coil 70 instead of the passive element 107 shown in fig. 2, as shown in fig. 1 (a). The air core coil 70 is possibly placed close to the core coil 5 to generate a magnetic field in the opposite direction. Fig. 1(b) shows the idea of counteracting the leakage magnetic field by means of an air-core coil 70. It can be understood that in fig. 1(b), the size of the noise space in which the electronic device is affected by the induction noise is reduced. The arrows shown in fig. 1 show the directions of the polarities of the magnetic fields generated in the core coil 5 and the air-core coil 70.

Fig. 3 shows an embodiment of a circuit of a single magnetic oscillation sensor capable of preventing fluctuations in oscillation frequency that may be generated when an external oscillation synchronizing signal is applied to an external strong field. The illustrated circuit is a basic circuit of an external synchronous type magnetic oscillation sensor among a plurality of magnetic oscillation sensors.

"a" denotes a circuit for transmitting an external signal. This circuit is newly added externally to the magnetic oscillation sensor circuit. The external signal transmission circuit a is designed to have a frequency at which the magnetic oscillation sensor most stabilizes the oscillation. The component of the frequency is sent as an oscillation synchronization signal from the output terminal PA to the magnetic sensing component via the terminal P1.

The core 6 arranged in the core coil 5 in the magnetic sensing part 4 is excited by a steady excitation current synchronized with the oscillation synchronizing signal by a synthesized excitation current including the oscillation synchronizing signal and the excitation current sent from the operational amplifier circuit.

The connector B comprises electrically passive or active elements.

P13 denotes a passive element including a resistor or a coil, and is designed to have an optimum impedance so that magnetic oscillation continues stably even in the event of a short circuit.

According to the embodiment, by placing the air-core coil 70 in the vicinity of the core coil 5, it is possible to reduce the size of a space in which a leakage magnetic field leaking from the magnetic sensing part 4 exists. Thus, the present embodiment is effective when the magnetic field is measured only by the magnetic oscillation sensor, ensuring that the influence of the induction noise on the adjacent electronic device or circuit can be prevented.

Fig. 4 is a circuit diagram of a triaxial apparatus for measuring a magnetic field according to a first embodiment of the present invention. The illustrated apparatus includes three magnetic oscillation sensors as a typical example of an apparatus including a plurality of magnetic oscillation sensors.

As another example of a circuit including a plurality of magnetic oscillation sensors, simultaneous measurement at a plurality of points where a single magnetic oscillation sensor is located at a plurality of measurement points may be interpreted as a modification of the apparatus described below, and therefore, no description will be given to avoid complexity.

In the case of measuring an external magnetic field by a magnetic sensor having respective sensitive axes, a process of separating its total magnetic force into an X component, a Y component, and a Z component in a vector and measuring each orthogonal component separately is generally selected.

The three-axis apparatus for measuring a magnetic field has a circuit including a combination of an X-axis circuit part 1000, a Y-axis circuit part 2000, and a Z-axis circuit part 3000, each of which is separated from each other.

The magnetic oscillation sensor has a direction. The axial direction in which the magnetic oscillation sensor is most sensitive to magnetic fields is called the magnetic field detection axis. As an example, the magnetic sensing component including the straight linear core and the core coil 5,14, or 23 wound around the core in a direction perpendicular to the longitudinal axis of the core has a magnetic field detection axis extending parallel to the longitudinal axis of the core 6,15, or 24.

In the three-axis device for measuring a magnetic field according to the present embodiment, the magnetic sensing members 4,13, and 22 of the magnetic oscillation sensor are accommodated in the sensor housing such that the magnetic field detection axes thereof are perpendicular to each other by slightly adjusting the angles of the cores 6,15, and 24 in the axial direction thereof.

In order to measure the magnetic field with high accuracy, it is necessary to prevent the occurrence of the beat phenomenon. For this reason, it is necessary to unify the magnetic oscillation frequency of the magnetic oscillation sensor and magnetize the cores 6,15, and 24 of the magnetic sensing part with the excitation current having the thus unified frequency.

The most important technique in the present invention is that operational amplifier circuits in a plurality of magnetic oscillation sensors are electrically connected via an electrical connector to construct an oscillation synchronizing signal circuit network as a member for unifying frequencies to avoid the beat phenomenon, and cores 6,15 and 24 of the magnetic sensing part are excited by an excitation current having a unified frequency by a synthesized excitation current including a combination of an oscillation synchronizing signal flowing through the oscillation synchronizing signal circuit network and an excitation current output from the operational amplifier circuits.

Specifically, the output terminals P3, P6, and P9 of the operational amplification circuits 8,17, and 26 are electrically connected to the output terminals P1, P4, and P7 via passive elements P13, P46, and P79, respectively. The terminals P1 and P9 are electrically connected to each other via the electrical connector 1, the terminals P3 and P4 are electrically connected to each other via the electrical connector 2, and the terminals P6 and P7 are electrically connected to each other via the electrical connector 3 to construct a ring-type oscillation synchronizing signal circuit network so that they can share a signal having a uniform magnetic oscillation frequency. Thus, the cores 6,15, and 24 of the magnetic sensing member can be excited by the synthesized excitation current including a combination of the excitation current output from the operational amplifier circuit and the oscillation synchronizing signal having the uniform magnetic oscillation frequency.

Each of the electrical connectors 1,2 and 3 comprises an electrically passive or active element.

For example, a passive element having the simplest structure is a connector including a single resistor. Alternatively, the passive elements may include resistors, capacitors, and/or coils, etc. The electrical connector may include circuitry to which a power amplification function is added.

In the case where each of the elements denoted by reference numerals 7,16, and 25 and P13, P46, and P79 includes a passive element, a resistor or a coil may be used. Its impedance may be 0 ohm in case of a short circuit, i.e. in case of a circuit constant or configuration according to the oscillating circuit without using passive elements.

Thus, the term "output terminal of the operational amplifier circuit" in the specification includes not only the terminals P3, P6, and P9 but also the output terminals P1, P4, and P7, regardless of the impedances of the passive elements P13, P46, and P79.

Each of the operational amplifier circuits 8,17, and 26 includes an amplification circuit including an operational amplifier as a main component, and may be designed to additionally have a function of amplifying power, if necessary.

The output voltages at the output terminals P3, P6, and P9 of the operational amplifier circuits 8,17, and 26 are divided by the resistors 9 and 10, the resistors 18 and 19, and the resistors 27 and 28, respectively. The terminal voltages of the resistors 10, 19, and 28 are input to the inverting terminals of the operational amplifier circuits. Since the magnetic oscillation frequency is defined by the voltage division ratio between the resistors 9 and 10, the voltage division ratio between the resistors 18 and 19, and the voltage division ratio between the resistors 27 and 28, the voltage division ratio is designed to be able to be slightly changed by a trimmer having a variable resistance.

Each of reference numerals 11,20, and 29 denotes a filter circuit having a function of preventing unnecessary frequency components that are not measured and/or unnecessary magnetic oscillation frequency components from being included in the output voltage via the output terminals Q1, Q2, and Q3.

Each of reference numerals 12,21, and 30 denotes an amplification circuit for controlling amplification for the purpose of calibration. In order to make the measured magnetic field strength reliable, it is necessary to make the measured strength consistent with the strength satisfying the national standard. In the calibration, a standard magnetic field generator having traceability with respect to national standards is used. The magnetic sensing component is positioned in the magnetic field generated by a standard magnetic field generator to control the amplification of the amplification circuits 12,21 and 30.

The first embodiment is characterized in that even if fluctuations in oscillation frequency are generated due to temperature fluctuations of circuit components in the three-axis magnetic sensing device and/or external disturbance magnetic field applied to the magnetic oscillation sensor, the beat phenomenon does not occur because all oscillation frequencies of the magnetic oscillation sensor change together.

Fig. 5 shows a second embodiment according to the invention. Specifically, fig. 5 shows an example of a circuit for a three-axis magnetic sensing device having a countermeasure against a leakage magnetic field. The circuit includes an air core coil to cancel a leakage magnetic field leaking and radiating from the core coil of the magnetic sensing component. The circuit cancels a leakage magnetic field radiated from the core coil to prevent a harmful effect caused by electromagnetic noise from being exerted on a neighboring device. Countermeasures for suppressing electromagnetic noise to a neighboring device are useful even in a single magnetic oscillation sensor.

A circuit is defined by replacing the resistors 7,16 and 25 in the first embodiment shown in fig. 4 with the air-core coils 70,160 and 250 in the second embodiment shown in fig. 5, respectively, to cancel the leakage magnetic field radiated from the core coils 5,14 and 23.

The air-core coils 70,160 and 250 are placed in the vicinity of the core coils 5,14 and 23, respectively, of the magnetic sensing component so that their axes, in which the magnetic fields are measured, are parallel to each other.

The air-core coils 70,160 and 250, and the connection terminals P2, P5 and P8 (the air-core coils are electrically connected to the magnetic sensing means via the connection terminals P2, P5 and P8) are electrically connected to the non-inverting terminals of the operational amplifier circuits 8,17 and 26, respectively, to define a circuit for the magnetic oscillation sensor.

Each of the cores 6,15, and 24 in the core coils 5,14, and 23 of the magnetic sensing parts 4,13, and 22 serves as a magnetic oscillation sensor, respectively, which is excited by an excitation current having a uniform magnetic oscillation frequency by a synthesized excitation current including an oscillation synchronizing current (similar to fig. 4) flowing through the oscillation synchronizing signal circuit network and an excitation current output from the operational amplifier circuit.

Fig. 6 shows a circuit according to a third embodiment, in which a star-shaped oscillation synchronizing signal circuit network is configured for transmitting an oscillation synchronizing signal, and a circuit for transmitting an external signal is added to the circuit network. The magnetic oscillation frequency is fixed by the oscillation synchronizing signal output from the external signal transmitting circuit to completely prevent occurrence of the beat phenomenon caused by the difference between the magnetic oscillation frequencies.

The star-shaped oscillation synchronous signal circuit network for transmitting the oscillation synchronous signal comprises the following circuits: one of the terminals of the passive or active element of the operational amplifier circuit electrically connected to the plurality of magnetic oscillation sensors in the circuit is electrically connected to the common terminal PA via an electrical connector. If the circuit is viewed from the common terminal PA towards the magnetic oscillation sensor, the wires are radially distributed like a star light towards the magnetic oscillation sensor to define a star circuit network. This is thus referred to as a star oscillator synchronizing signal circuit network.

That is, the oscillation signal flowing through the oscillation synchronizing signal circuit network is a signal having a fixed frequency output from the external signal transmitting circuit. Since the excitation current generally has a fixed frequency as an oscillation frequency, the core of the magnetic sensing part is excited by a uniform magnetic oscillation frequency by synthesizing the excitation current including the oscillation synchronous current and the excitation current output from the operational amplifier circuit. The star-shaped oscillation synchronous signal circuit network is one of oscillation synchronous signal circuit networks which are very useful in practice.

However, since the magnetic oscillation sensor is an independent self-excitation system in which the excitation current is generated by the circuit of the sensor itself, it can be influenced by the magnetic field according to the strength of the magnetic field to be measured. Specifically, fluctuations in the oscillation frequency and/or slight fluctuations in the uniform magnetic oscillation frequency are generated.

Since these phenomena prevent the magnetic field from being measured with high accuracy, it is necessary to prevent the occurrence of slight fluctuations in the uniform magnetic oscillation frequency. For this reason, it is necessary to use an oscillation synchronizing signal transmitted from an external oscillation circuit to thereby completely fix the magnetic oscillation frequency, as is done in the embodiment shown in fig. 6.

The oscillation synchronizing signal transmitted from the external oscillation circuit and having the uniform magnetic oscillation frequency is combined with the excitation current transmitted from the operational amplifier circuit through the star oscillation synchronizing signal circuit network configured by the connection of the electrical connectors, and each of the cores of the magnetic sensing member is magnetized as a synthesized excitation current having the uniform magnetic oscillation frequency.

That is, the fixed oscillation synchronizing signal and the excitation current, which are transmitted from the external signal transmission circuit and flow through the oscillation synchronizing signal circuit network, forcibly magnetize each of the cores of the magnetic sensing member at the uniform magnetic oscillation frequency.

In fig. 6, reference symbol a denotes a circuit for transmitting an external signal. The circuit is additionally connected to a common terminal PA outside the star oscillator synchronizing signal circuit network of the magnetic oscillator sensor circuit. The external signal transmission circuit a is designed to have a frequency that allows the magnetic oscillation sensor to oscillate most stably. The oscillation synchronizing signal having the frequency is transmitted to each of the coils via the common terminal PA, the electrical connectors B, C and D, and the terminals P1, P4 and P7.

Each of the cores 6,15, and 24 in the core coils 5,14, and 23 of the magnetic sensing parts 4,13, and 22 is excited by a synthesized excitation current including a fixed oscillation synchronizing signal transmitted from an external signal transmitting circuit and an excitation current transmitted from an operational amplifier circuit, the synthesized excitation current having a uniform magnetic oscillation synchronizing frequency, which is stable and has no frequency fluctuation.

In other words, the embodiment shown in fig. 6 further improves the weakness of the star oscillation synchronizing signal circuit network, and is an embodiment of the magnetic detection device in which the frequency of the oscillation synchronizing signal in the star oscillation synchronizing signal circuit network is synchronized with and fixed to the frequency of the electric signal transmitted from the external signal transmitting circuit by: an external signal transmission circuit that transmits an electric signal having the same frequency as the unified magnetic oscillation synchronization frequency and the operational amplifier circuit are electrically connected to each other via an electric connector.

Each of the connectors B, C and D includes an electrically passive or active element, similar to the connector shown in fig. 4.

In the case where each of P13, P46, and P79 includes a resistor or a coil as a passive element, its impedance is optimally determined so that magnetic oscillation continues stably even in the event of a short circuit.

The technique involving the external synchronization signal suppresses fluctuation of the magnetic oscillation frequency generated when a strong field is measured by a single magnetic oscillation sensor or even a plurality of magnetic oscillation sensors, and can be applied to a magnetic sensor used when an external magnetic field is measured uniaxially with high accuracy or simultaneously at a plurality of points. Thus, this technique is very useful in practice.

INDUSTRIAL APPLICABILITY

The present invention can be preferably used for measuring a leakage magnetic field existing inside or outside the body of a train or an automobile as a technique for improving a magnetic oscillation sensor and a magnetic detection device, thereby allowing the magnetic sensor to achieve its best performance.

Reference symbol representations

1,2,3 connector

4,13,22 magnetic sensing component

5,14,23 core coil

6,15,24 cores

7,16,25 passive components

8,17,26 operational amplifier circuit

9,10,18,19,2728 resistor

11,20,29 filter circuit

12,21,30 amplifying circuit

70,160,250 air-core coil

1000X-axis circuit part

2000Y-axis circuit part

3000Z-axis circuit part

Claims (3)

1. An apparatus for detecting magnetism, comprising a self-exciting fluxgate-type magnetic oscillation sensor in each of three axes perpendicular to each other,

the magnetic oscillation sensor includes: a magnetic sensor including a core coil including a core made of a magnetic material and a coil wound around the core; and an operational amplifier circuit that causes an AC excitation current to flow through the coil to generate an output according to the strength of an external magnetic field applied to the core,

the method is characterized in that:

an air-core coil not wound around the core is arranged in juxtaposition and parallel to the core coil of each magnetic oscillation sensor arranged in each axis,

a node is electrically connected to a non-inverting input terminal of the operational amplifier circuit, wherein a non-ground terminal of the core coil in each axis is electrically connected to one of terminals of the air-core coil via the node,

the other terminal of the air-core coil and the output terminal of the operational amplifier circuit are electrically connected to each other via a passive element,

in order to cancel a leakage magnetic field that leaks outward and is radiated from a core coil of the magnetic sensor when the core coil is excited by an excitation current, the operational amplifier circuit generates a current with which a magnetic field having the same strength as that of the leakage magnetic field and having a direction opposite to that of the leakage magnetic field is generated and supplies the current to the air-core coil,

the output terminal of the operational amplifier circuit included in the magnetic oscillation sensor in each axis and the terminal of the air-core coil in the subsequent axis are electrically connected to each other via an electrical connector to construct a ring-type oscillation synchronizing signal circuit network for unifying the oscillation frequency of the magnetic oscillation sensor in each axis, thereby avoiding a beat phenomenon caused by a difference between the oscillation frequencies of the magnetic oscillation sensors in the respective axes, and

the core of the magnetic sensor is forcibly excited at a uniform magnetic oscillation frequency by an excitation current including a combination of an oscillation synchronizing signal flowing through the oscillation synchronizing signal circuit network and an excitation current output from the operational amplifier circuit.

2. An apparatus for detecting magnetism, comprising a self-exciting fluxgate-type magnetic oscillation sensor in each of three axes perpendicular to each other,

the magnetic oscillation sensor includes: a magnetic sensor including a core coil including a core made of a magnetic material and a coil wound around the core; and an operational amplifier circuit that causes an AC excitation current to flow through the coil to generate an output according to the strength of an external magnetic field applied to the core,

the method is characterized in that:

an air-core coil not wound around the core is arranged in juxtaposition and parallel to the core coil of each magnetic oscillation sensor arranged in each axis,

a node is electrically connected to a non-inverting input terminal of the operational amplifier circuit, wherein a non-ground terminal of the core coil in each axis is electrically connected to one of terminals of the air-core coil via the node,

the other terminal of the air-core coil and the output terminal of the operational amplifier circuit are electrically connected to each other via a passive element,

in order to cancel a leakage magnetic field that leaks outward and is radiated from a core coil of the magnetic sensor when the core coil is excited by an excitation current, the operational amplifier circuit generates a current with which a magnetic field having the same strength as that of the leakage magnetic field and having a direction opposite to that of the leakage magnetic field is generated and supplies the current to the air-core coil,

the terminals of the air-core coil in each axis and the common terminal are electrically connected to each other via an electrical connector to construct a star-shaped oscillation synchronizing signal circuit network for unifying the oscillation frequencies of the magnetic oscillation sensors in each axis, thereby avoiding a beat phenomenon caused by a difference between the oscillation frequencies of the magnetic oscillation sensors in the respective axes, and

the core of the magnetic sensor is forcibly excited at a uniform magnetic oscillation frequency by an excitation current including a combination of an oscillation synchronizing signal flowing through the oscillation synchronizing signal circuit network and an excitation current output from the operational amplifier circuit.

3. The apparatus of claim 2, wherein an external signal transmission circuit for transmitting an electric signal having the same frequency as that of the unified magnetic oscillation frequency, which is fixed, and the common terminal are electrically connected to each other to construct an oscillation synchronization signal circuit network.