Patent ID: 12253544

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a current measurement device according to embodiments of the present invention will be described in detail with reference to the drawings. In the following description, an outline of the embodiments of the present invention will be described first, and then details of each embodiment of the present invention will be described.

[Outline]

Embodiments of the present invention make it possible to measure a current flowing through a measurement target conductor using a compact device in a non-contact manner with high accuracy. Specifically, it can be installed in a very small space during current measurement, and direct current and alternating current up to several tens of [MHz] can be measured in a non-contact manner with high accuracy.

In recent years, in a development process of hybrid cars (hybrid vehicles; HVs) and electric vehicles (EVs), there has been a demand to measure large currents flowing between a battery and a power unit, and between a converter and an inverter. Moreover, in recent years, since acceleration in a short period of time has been pursued and an upper limit of current to be used has increased, the wiring tends to be thicker. On the other hand, space-saving, integration, and miniaturization of devices are being promoted to expand passenger space and reduce weight. For this reason, devices and connection wirings are densely packed, and it makes it difficult to secure a space for disposing a current sensor. Under such circumstances, there is a demand for a current measurement device capable of measuring direct current and alternating current flowing in wirings with limited peripheral space in a non-contact manner with high accuracy.

However, in the current measurement device of the zero-flux method disclosed in Japanese Unexamined Patent Application Publication No. 2005-55300 described above, since a magnetic core having a certain size (for example, about 20 [cm] in diameter) needs to be provided around the measurement target conductor, installation in a narrow place is difficult. In addition, since the current measurement device of the Rogowski method described above detects the voltage induced in the Rogowski coil, it cannot measure direct current in principle. In addition, an output signal is weak and the phase is shifted in a low-frequency region, resulting in poor measurement accuracy. Moreover, the current measurement devices disclosed in Japanese Unexamined Patent Application Publication No. 2005-55300 and Japanese Unexamined Patent Application Publication No. 2011-164019 described above are affected by a magnetic field (external magnetic field) other than the magnetic field generated by the current flowing through the measurement target conductor, and therefore have poor measurement accuracy.

In an embodiment of the present invention, a first sensor that detects a direct current magnetic field and a low-frequency alternating current magnetic field generated by a current flowing through a measurement target conductor, a hollow magnetic shielding member that has a cutout portion into which the measurement target conductor is inserted and in which the first sensor is accommodated, a fixing mechanism that fixes the measurement target conductor such that a distance between the measurement target conductor inserted into the cutout portion of the magnetic shielding member and the first sensor is a predetermined reference distance, and a first calculator that calculates a current flowing through the measurement target conductor based on the detection result of the first sensor are provided. As a result, it is possible to measure the current flowing through the measurement target conductor using a compact device in a non-contact manner with high accuracy.

First Embodiment

<Configuration of Current Measurement Device>

FIG.1is an external perspective view of a current measurement device according to a first embodiment of the present invention. As shown inFIG.1, a current measurement device1of the present embodiment includes a sensor head10and a circuit section20connected by a cable CB, and measures a current I flowing through a measurement target conductor MC in a non-contact manner. The measurement target conductor MC is an arbitrary conductor such as a power semiconductor pin or a busbar. To simplify the description below, the measurement target conductor MC is assumed to be a cylindrical conductor.

In the following description, the positional relationships of the members will be described with reference to an XYZ orthogonal coordinate system set in the drawings as necessary. In the XYZ orthogonal coordinate system shown inFIG.1, an X axis is set in a longitudinal direction of the measurement target conductor MC (a direction of the current I), a Y axis is set in a horizontal direction, and a Z axis is set in a vertical direction. However, for convenience of description, an origin of the XYZ orthogonal coordinate system shown in each drawing is not fixed, but its position is appropriately changed for each drawing.

The sensor head10is a member for measuring the current I flowing through the measurement target conductor MC in a non-contact manner. This sensor head10is used as, so to speak, a probe for measuring the current I flowing through the measurement target conductor MC in a non-contact manner. It is desirable that the sensor head10be decreased in size as much as possible so that the current I can be measured even when space around the measurement target conductor MC is limited.

As shown inFIG.1, the sensor head10is fixedly disposed with respect to the measurement target conductor MC when the current I flowing through the measurement target conductor MC is measured. That is, the sensor head10is brought into physical contact with the measurement target conductor MC. However, the sensor head10is electrically insulated from the measurement target conductor MC, and the current I flowing through the measurement target conductor MC does not flow into the sensor head10. For this reason, the sensor head10can be used to measure the current I flowing through the measurement target conductor MC in a non-contact manner.

FIG.2is a perspective view of the sensor head provided in the current measurement device according to the first embodiment of the present invention. As shown inFIG.2, the sensor head10includes a housing11, a magnetic shield12(magnetic shielding member), and a fixing mechanism13. The housing11is a rectangular parallelepiped hollow member formed by combining a first housing11aand a second housing11band having a cutout portion CP1formed on a −Z side. The housing11(the first housing11aand the second housing11b) is formed of a material (for example, a non-magnetic material such as resin) that does not affect a magnetic field generated by the current I. The housing11accommodates the magnetic shield12and the fixing mechanism13therein.

The cutout portion CP1is formed on the −Z side of the housing11to extend from an end of a +X side to an end of a −X side. This cutout portion CP1is for making it possible to dispose a part of the measurement target conductor MC inside the housing11. Note that the cutout portion CP1exemplified inFIG.2has a substantially inverted U shape when viewed in an X-axis direction. Since the cutout portion CP1is formed to extend from the end of the +X side of the housing11to the end of the −X side of the housing11, a part of the measurement target conductor MC can be easily disposed inside the housing11. That is, the sensor head10is disposed with the cutout portion CP1facing the measurement target conductor MC so that the cutout portion CP1and the measurement target conductor MC are parallel to each other, and a part of the measurement target conductor MC can be easily disposed inside the housing11simply by moving the sensor head10toward the measurement target conductor MC.

FIGS.3A to3Care views which show a magnetic shield provided by the current measurement device according to the first embodiment of the present invention.FIG.3Ais an external perspective view of the magnetic shield12, andFIG.3Bis a cross-sectional arrow view along a line A-A inFIG.3A. As shown inFIG.3A, the magnetic shield12is a rectangular parallelepiped hollow member formed by combining a first shield member12aand a second shield member12band having a cutout portion CP2on the −Z side. The magnetic shield12(the first shield member12aand the second shield member12b) is formed of a soft magnetic material (for example, permalloy) having a small holding force and a high magnetic permeability. This magnetic shield12is provided to shield magnetic fields (external magnetic fields) other than a magnetic field (a signal magnetic field) generated by the current I flowing through the measurement target conductor MC, and to improve the measurement accuracy of the current1.

The cutout portion CP2is formed on the −Z side of the magnetic shield12to extend from the end of the +X side to the end of the −X side. This cutout portion CP2is provided so that a portion of the measurement target conductor MC is disposed inside the magnetic shield12when the current I flowing through the measurement target conductor MC is measured. That is, the cutout portion CP2is provided to dispose a portion of the measurement target conductor MC inside the magnetic shield12from which an external magnetic field is shielded. The cutout portion CP2is set to have the same (or substantially the same) shape and size as the cutout portion CP1formed in the housing11when viewed in the X-axis direction. The magnetic shield12is accommodated in the housing11such that the cutout portion CP2overlaps the cutout portion CP1of the housing11when viewed in the X-axis direction.

A thickness of the magnetic shield12is set so that magnetic saturation does not occur even if a maximum current that can be measured by the current measurement device1(an upper limit current: for example, 1000 [A] in effective value) flows through the measurement target conductor MC. For example, the magnetic shield12may be created using a soft magnetic material having a thickness that does not cause the magnetic saturation described above, and may also be created using a laminate having a thickness that does not cause the magnetic saturation described above by stacking a plurality of thin soft magnetic materials. Alternatively, the magnetic shield12may be formed by casting, cutting, or the like to have a thickness that does not cause magnetic saturation described above.

The magnetic shield12accommodates a magnetic sensor SE1(a first sensor) therein, as shown inFIG.3B. A reason for installing the magnetic sensor SE1in the magnetic shield12is to improve the measurement accuracy of the current I by detecting a signal magnetic field in a state where an influence of an external magnetic field is eliminated. The magnetic sensor SE1detects a direct current magnetic field and an alternating current magnetic field of a low frequency (for example, up to several [kHz]) generated by the current I flowing through the measurement target conductor MC. For example, when the current I flowing through the measurement target conductor MC is a direct current, a magnetic field H shown inFIG.3Bis detected.

The magnetic sensor SE1is required to have responsiveness because it is used to measure the current I flowing through the measurement target conductor MC. The magnetic sensor SE1preferably has a delay time of, for example, less than 1 [msec]. The magnetic sensor SE1may be an analog sensor or a digital sensor as long as the response speed is sufficiently fast. Sensing axes of the magnetic sensor SE1are desirably three or more, but may also be one or two. When the sensing axes of the magnetic sensor SE1are two axes or less, the magnetic sensor SE1is disposed in the magnetic shield12such that the sensing axes (a magnetic sensing direction) are a direction of a magnetic field generated by the current I (a tangential direction of the measurement target conductor MC) when the sensor head10is fixed to the measurement target conductor MC by the fixing mechanism13. By disposing the magnetic sensor SE1in this manner, the influence of the external magnetic field flowing into the magnetic shield12from the cutout portion CP2on the magnetic sensor SE1can be reduced.

The magnetic sensor SE1with low noise is used to ensure the measurement accuracy of the current measurement device1. For example, the magnetic sensor SE1which has a magnitude of a noise component included in a detection result of the magnetic sensor SE1being about 0.1% of a measurement range of the current measurement device1is used. As the magnetic sensor SE1, for example, a Hall element, a magneto-resistive effect element, or the like can be used.

The magnetic sensor SE1is installed at a position where a strength ratio (an SN ratio (a signal-to-noise ratio)) between the signal magnetic field and the external magnetic field is about 100 to 1 or more in the magnetic shield12.FIG.4is a view which shows simulation results of the SN ratio in the first embodiment of the present invention. In a graph shown inFIG.4, the horizontal axis represents a distance in a Z direction from a center of the measurement target conductor MC, and the vertical axis represents the SN ratio (logarithm).

In addition, the simulation results shown inFIG.4show that there is another conductor through which a current equivalent in magnitude to the current I flowing in the measurement target conductor MC flows adjacent to the measurement target conductor MC, like a three-phase alternating current. InFIG.4, a graph indicated by a solid line is a simulation result when the magnetic shield12is provided, and a graph indicated by a dashed line is a simulation result when the magnetic shield12is omitted. In addition, as shown inFIG.4, in the simulation, a radius of the measurement target conductor MC is set to 5 [mm].

Referring toFIG.4, it can be known that the SN ratio is much higher when the magnetic shield12is provided than when the magnetic shield12is omitted. In addition, it can be known that the SN ratio gradually decreases as a distance from the measurement target conductor MC (a distance from the center of the measurement target conductor MC) increases. According to the simulation results shown inFIG.4, the SN ratio is 100 to 1 or more if the distance from the center of the measurement target conductor MC is within 5 to 8 [mm] The magnetic sensor SE1is installed, for example, at a position where the distance in the Z direction from the center of the measurement target conductor MC is 8 [mm] in the magnetic shield12.

The fixing mechanism13is a mechanism for fixing the sensor head10to the measurement target conductor MC, and is disposed on the +X side of the magnetic shield12as shown inFIG.2. This fixing mechanism13fixes the measurement target conductor MC such that a distance between the center of the measurement target conductor MC and the magnetic sensor SE1becomes a predetermined reference distance r regardless of a diameter of the measurement target conductor MC. For example, the fixing mechanism13fixes the measurement target conductor MC such that the distance between the center of the measurement target conductor MC and the magnetic sensor SE1is 8 [mm].

A reason for setting, by the fixing mechanism13, the distance of the magnetic sensor SE1to the measurement target conductor MC to the reference distance r is to calculate the current I by using only the detection result of the magnetic sensor SE1. That is, the current I flowing through the measurement target conductor MC is calculated by multiplying the detection result of the magnetic sensor SE1by a constant uniquely determined based on the reference distance r. If the distance of the magnetic sensor SE1to the measurement target conductor MC is set to the reference distance r by the fixing mechanism13, the current I flowing through the measurement target conductor MC can be calculated using only the measurement result of the magnetic sensor SE1.

The fixing mechanism13, as shown inFIG.2, is a guided screw mechanism including a pair of guide members13a, a pair of fixing arms13b, left and right screws13c, and a knob member13d. Members (the pair of guide members13a, the pair of fixing arms13b, the left and right screws13c, and the knob member13d) constituting the fixing mechanism13are formed of a non-magnetic material such as resin.

The guide member13ais a cylindrical member for guiding the fixing arm13bin a Y direction. The pair of guide members13aare disposed with a certain interval in a Z direction such that the longitudinal direction thereof is along the Y direction. The fixing arm13bis a quadrangular prism-shaped member having a V-shaped gripping portion GR, a guide hole into which the guide member13ais inserted, and a screw hole into which the left and right screws13care inserted. The pair of fixing arms13bare disposed so that gripping portions GR face each other, the guide members13aare inserted into the guide holes, and the left and right screws13care screwed into the screw holes.

The left and right screws13care, for example, cylindrical screws with a right screw provided on the right side (+Y side) from the center and a left screw provided on the left side (−Y side) from the center. The knob member13dis a cylindrical member attached to one end (an end on the +Y side) of the left and right screws13c, and is used to rotate the left and right screws13caround its axis according to an operation of a user. When the left and right screws13crotate in one direction around the axis by operating the knob member13d, the pair of fixing arms13bmove in the Y direction to be away from each other. On the other hand, when the left and right screws13crotate in the other direction around the axis by operating the knob member13d, the pair of fixing arms13bmove in the Y direction so as to approach each other.

Here, when the measurement target conductor MC is fixed to the sensor head10, the measurement target conductor MC is sandwiched between the V-shaped gripping portions GR formed on the pair of fixing arms13b. When the measurement target conductor MC is sandwiched between the gripping portions GR, a position in the Y direction and a position in the Z direction are regulated, and the position in the Y direction and the position in the Z direction of the center of the measurement target conductor MC are the same position regardless of the diameter of the measurement target conductor MC. In this manner, regardless of the diameter of the measurement target conductor MC, the measurement target conductor MC is fixed to the sensor head10such that the distance between the center of the measurement target conductor MC and the magnetic sensor SE1is the predetermined reference distance r.

The circuit section20shown inFIG.1measures the current I flowing through the measurement target conductor MC on the basis of the detection result output from the sensor head10(the detection result of the magnetic sensor SE1). The circuit section20outputs or displays a measurement result of the current I to the outside. As the cable CB that connects the sensor head10and the circuit section20, any cable can be used, but it is desirable to have flexibility, to be easy to handle, and to be hard to disconnect.

FIG.5is a block diagram which shows a configuration of a main part of the current measurement device according to the first embodiment of the present invention. InFIG.5, blocks corresponding to the constituents shown inFIG.1are denoted by the same reference numerals. In the following description, details of an internal configuration of the circuit section20will be mainly described with reference toFIG.5. As shown inFIG.2, the circuit section20has a calculator21(first calculator) and an output section22.

The calculator21calculates the current I flowing through the measurement target conductor MC based on the detection result of the magnetic sensor SE1. Here, since the magnetic sensor SE1detects the direct current magnetic field and the low-frequency alternating current magnetic field as described above, the current I calculated by the calculator21is direct current and low frequency components. Distance information indicating the reference distance r described above is stored in calculator21in advance. The calculator21calculates the current I flowing through the measurement target conductor MC by calculating a product of the detection result (magnetic field H) of the magnetic sensor SE1and a constant uniquely determined based on the reference distance r.

The output section22outputs the measurement result of the current I calculated by the calculator21to the outside. The output section22may include, for example, an output terminal for outputting a signal indicating the measurement result of the current I and a display device (for example, a liquid crystal display device) for displaying the measurement result of the current I to the outside.

Here, as shown inFIG.1, the circuit section20is separated from the sensor head10and is connected to the sensor head10via the cable CB. With such a configuration, a magnetic field detection function (the magnetic sensor SE1) and a calculation function (the calculator21and the output section22) can be separated. As a result, handling of the sensor head10becomes easy, and, for example, the sensor head10can be easily installed in a narrow space. In addition, various problems (such as temperature characteristics and insulation resistance) and the like that occur when the calculation function is provided in the sensor head10can be avoided, and thereby applications of the current measurement device1can be expanded.

<Operation of Current Measurement Device>

Next, an operation at the time of measuring the current I flowing through the measurement target conductor MC using the current measurement device1will be described. First, a user of the current measurement device1performs an operation of fixing the sensor head10to the measurement target conductor MC as shown inFIG.1to measure the current I flowing through the measurement target conductor MC.

Specifically, the user of the current measurement device1disposes the sensor head10with the cutout portion CP1facing the measurement target conductor MC such that the cutout portion CP1and the measurement target conductor MC are parallel to each other, and moves the sensor head10toward the measurement target conductor MC. As a result, a portion of the measurement target conductor MC is disposed inside the housing11of the sensor head10. The user of the current measurement device1operates the knob member13dof the fixing mechanism13to move the pair of fixing arms13btoward each other, and performs work to make the measurement target conductor MC sandwiched by the gripping portions GR of the pair of fixing arms13b.

When the work is done, the measurement target conductor MC is fixed to the sensor head10by the fixing mechanism13, and the measurement target conductor MC is inserted into the cutout portion CP2of the magnetic shield12provided on the sensor head10. Note that, in this state, the distance of the magnetic sensor SE1to the measurement target conductor MC inserted into the cutout portion CP2of the magnetic shield12is set to the reference distance r.

When the work described above is completed, processing of measuring the current I flowing through the measurement target conductor MC is performed. Specifically, first, processing of detecting, by the magnetic sensor SE1, a magnetic field formed by the current I flowing through the measurement target conductor MC is performed. Next, the calculator21performs processing of calculating the current I flowing through the measurement target conductor MC based on the detection result of the magnetic sensor SE1. Specifically, the calculator21calculates a product of the constant stored therein (a constant uniquely determined based on the reference distance r) and the detection result (the magnetic field H) of the magnetic sensor SE1, thereby performing processing of calculating the current I (direct current and low frequency components) flowing through the measurement target conductor MC. When the processing described above is completed, the output section22performs processing of outputting information indicating the current I calculated by the calculator21. The processing described above is performed continuously or repeatedly at regular intervals (for example, 1 second).

As described above, in the present embodiment, the hollow magnetic shield12that has the cutout portion CP2into which the measurement target conductor MC is inserted, and in which the magnetic sensor SE1is accommodated, the fixing mechanism13that fixes the measurement target conductor MC such that the distance between the center of the measurement target conductor MC and the magnetic shield12is the predetermined reference distance r, and the calculator21for calculating a current flowing through the measurement target conductor MC based on the detection result of the magnetic sensor SE1are provided. For this reason, in the present embodiment, the current I (direct current and low-frequency components) flowing through the measurement target conductor MC can be measured using a compact device in a non-contact manner with high accuracy.

Second Embodiment

<Configuration of Current Measurement Device>

FIG.6is an external view of a current measurement device according to a second embodiment of the present invention. As shown inFIG.6, the current measurement device2of the present embodiment includes a sensor head10A and a circuit section20A connected by a cable CB, and measures the current I flowing through the measurement target conductor MC in a non-contact manner. The measurement target conductor MC is, for example, an arbitrary conductor such as pins or busbars of a power semiconductor, but for the sake of simplicity of description in the present embodiment, it is assumed that the measurement target conductor MC is a cylindrical conductor.

The sensor head10A has a configuration in which a Rogowski sensor SE2(second sensor) is added to the sensor head10of the first embodiment. The current measurement device1of the first embodiment described above measures the direct current and low frequency components of the current I flowing through the measurement target conductor MC. On the other hand, the current measurement device2of the present embodiment is capable of measuring not only direct current and low-frequency components of the current I flowing through the measurement target conductor MC but also low to high frequency components.

The Rogowski sensor SE2detects an alternating current magnetic field of a low frequency (for example, several [kHz]) to a high frequency (for example, several tens of [MHz]) generated by the current I flowing through the measurement target conductor MC. The Rogowski sensor SE2is a sensor using a Rogowski coil (air core coil), and is disposed to surround the measurement target conductor MC. The Rogowski sensor SE2is configured to have one end thereof being detachable from the sensor head10A to facilitate its disposition around the measurement target conductor MC.

The circuit section20A measures the current I flowing through the measurement target conductor MC on the basis of the detection results output from the sensor head10A (the detection results of the magnetic sensor SE1and the Rogowski sensor SE2). The circuit section20A outputs or displays the measurement result of the current I to the outside. As in the first embodiment, it is desirable that the cable CB have flexibility, be easy to handle, and be hard to disconnect.

FIG.7is a block diagram which shows a configuration of a main part of the current measurement device according to the second embodiment of the present invention. InFIG.7, blocks corresponding to constituents shown inFIG.5are denoted by the same reference numerals. In the following description, details of an internal configuration of the circuit section20A will be mainly described with reference toFIG.7. As shown inFIG.7, the circuit section20A has a calculator23(a second calculator) and a synthezer24in addition to the calculator21and the output section22.

The calculator23calculates the current I flowing through the measurement target conductor MC based on the detection result of the Rogowski sensor SE2. Here, since the Rogowski sensor SE2detects an alternating current magnetic field from a low frequency to a high frequency as described above, the current I calculated by the calculator23is low-frequency to high-frequency components. The detection result of the Rogowski sensor SE2indicates a voltage induced in the Rogowski coil by an alternating current magnetic field generated around the current I (alternating current) flowing through the measurement target conductor MC. The calculator23calculates the current I flowing through the measurement target conductor MC by converting the detection result (the voltage) of the Rogowski sensor SE2into a current value.

The synthesizer24synthesizes a calculation result of the calculator21and a calculation result of the calculator23. Specifically, the synthesizer24includes a low-pass filter24a, a high-pass filter24b, a signal level adjuster24c, and an adder24d. The low-pass filter24aremoves high-frequency components from the calculation result of the calculator21to cause low-frequency components to pass therethrough, and calculates a signal with desired frequency characteristics suitable for synthesis processing, which will be described below. The high-pass filter24bremoves the low-frequency components from the calculation result of the calculator23and allows the high-frequency components to pass through, thereby obtaining a signal with desired frequency characteristics suitable for synthesis processing, which will be described below.

The signal level adjuster24cadjusts a level of a signal output from the low-pass filter24a. For example, the signal level adjuster24cadjusts a signal level of a signal output from the low-pass filter24ato be the same as a signal level of a signal output from the high-pass filter24bwhen a direct current and an alternating current with the same effective value are flowing in the measurement target conductor MC. For example, a variable resistor can be used as the signal level adjuster24c.

Although the signal level adjuster24cfor adjusting only the level of the signal output from the low-pass filter24ais included in the present embodiment, the present invention is not limited thereto. For example, a signal level adjuster for adjusting the level of the signal output from the high-pass filter24bmay be provided instead of the signal level adjuster24c, or may be provided in addition to the signal level adjuster24c. Alternatively, a signal level adjuster capable of adjusting each of a level of a first signal and a level of a second signal may be provided.

The adder24dadds a signal whose signal level is adjusted by the signal level adjuster24cand a signal output from the high-pass filter24b. The signal whose signal level has been adjusted by the signal level adjuster24cis a signal representing the direct current and low frequency components of the current I. The signal output from the high-pass filter24bis a signal indicating the high-frequency component of the current I. For this reason, by adding these, it is possible to calculate a signal indicating a direct current component and an alternating current component to a high frequency.

FIGS.8A to8Care views for describing processing performed by a synthesizer of a circuit section in the second embodiment of the present invention.FIG.8Ais a view which shows an example of filter characteristics of the low-pass filter24a. FIG.8B is a view which shows an example of filter characteristics of the high-pass filter24b.FIG.8Cis a view which shows an example of frequency characteristics of the synthesizer.

As shown inFIGS.8A and8B, cutoff frequencies of the low-pass filter24aand the high-pass filter24bare both fc. That is, the low-pass filter24agenerally has characteristics of removing components of a frequency higher than the cutoff frequency fc and causing components of a frequency lower than the cutoff frequency fc to pass through. In addition, the high-pass filter24bgenerally has characteristics of removing the components of a frequency lower than the cutoff frequency fc and causing the components of a frequency higher than the cutoff frequency fc to pass through.

As shown inFIG.8C, the frequency characteristics of the synthesizer24are characteristics obtained by synthesizing the characteristics shown inFIG.8Aand the characteristics shown inFIG.8Bsuch that they are flattened at and around the cutoff frequency fc. That is, the frequency characteristics of the synthesizer24are characteristics in which an effective value of an alternating current from a low frequency to a high frequency is flattened, and an effective value (current value) of direct current is a value approximately the same as the effective value of alternating current current, which is flattened in this frequency characteristics.

Here, in order for a signal synthesized by the synthesizer24(the measurement result of the current I) to become a reproduction of the current I flowing through the measurement target conductor MC, a relationship between a delay time Way of the magnetic sensor SE1and the cutoff frequency fc needs to satisfy a relationship of tdelay<(1/fc)×(1/100). The delay time tdelaydescribed above is time required from when a current flowing through the measurement target conductor MC changes (that is, after the magnetic field applied to the magnetic sensor SE1changes) to when the magnetic sensor SE1outputs the detection result.

The output section22outputs the signal synthesized by the synthesizer24(the measurement result of the current I) to the outside. Note that the output section22may include, for example, an output terminal for outputting a signal indicating the measurement result of the current I to the outside, as in the first embodiment, and may also include a display device (for example, a liquid crystal display device) that displays the measurement result of the current I to the outside.

Here, as shown inFIG.7, the circuit section20A is separated from the sensor head10A and connected to the sensor head10A via the cable CB. With such a configuration, the magnetic field detection function (the magnetic sensor SE1, the Rogowski sensor SE2) and the calculation function (the calculators21and23, the synthesizer24, and the output section22) can be separated. As a result, handling of the sensor head10A becomes easy, and, for example, the sensor head10A can be easily installed in a narrow place. In addition, various problems (such as temperature characteristics and insulation resistance) and the like that occur when the calculation function is provided in the sensor head10A can be avoided, and thereby applications of the current measurement device2can be expanded.

<Operation of Current Measurement Device>

Next, an operation at the time of measuring the current I flowing through the measurement target conductor MC using the current measurement device2will be described. First, the user of the current measurement device2performs work to fix the sensor head10A to the measurement target conductor MC using the fixing mechanism13as in the first embodiment to measure the current I flowing through the measurement target conductor MC. When the work described above is performed, the measurement target conductor MC is fixed to the sensor head10A by the fixing mechanism13, and the distance of the magnetic sensor SE1to the measurement target conductor MC is set to reference distance r as in the first embodiment.

In addition, as shown inFIG.6, the user of the current measurement device2performs work to dispose the Rogowski sensor SE2to surround the measurement target conductor MC. At this time, the user of the current measurement device2removes one end E1of the Rogowski sensor SE2from the sensor head10A, if necessary, and performs work to fix the sensor head10A to the measurement target conductor MC. After the work is completed, the Rogowski sensor SE2may be disposed in the state described above, and then one end E1of the Rogowski sensor SE2may be attached to the sensor head10A.

When the work above is completed, processing of measuring the current I flowing through the measurement target conductor MC is performed. Specifically, first, processing of detecting, by the magnetic sensor SE1and the Rogowski sensor SE2, a magnetic field formed by the current I flowing through the measurement target conductor MC is performed. Next, processing of calculating the current I flowing through the measurement target conductor MC based on the detection results of the magnetic sensor SE1and Rogowski sensor SE2is performed by the calculators21and23. Specifically, the calculator21calculates a product of a constant stored therein (a constant uniquely determined based on the reference distance r) and the detection result (the magnetic field H) of the magnetic sensor SE1, thereby the processing of calculating the current flowing through the measurement target conductor MC (direct current and low frequency components) is performed. In addition, processing of calculating the current I (low-frequency to high-frequency components) flowing through the measurement target conductor MC is performed by the calculator23converting the detection result (voltage) of the Rogowski sensor SE2into a current value.

Subsequently, the synthesizer24performs processing of synthesizing currents calculated by the calculators21and23. Specifically, first, a calculation result of the calculator21is input to the low-pass filter24ato remove the high frequency component therefrom, and a calculation result of the calculator23is input to the high-pass filter24bto remove the low frequency component therefrom. Next, processing of adjusting levels of a signal output from the low-pass filter24a(a low-frequency component having passed through the low-pass filter24a) and a signal output from the high-pass filter24b(a high-frequency component having passed through the high-pass filter24b) is performed by the signal level adjuster24c.

Then, processing of adding the signal whose signal level has been adjusted by the signal level adjuster24cand the signal output from the high-pass filter24bis performed by the adder24d. In this manner, the currents calculated by the calculators21and23are synthesized. When the above processing is completed, the current synthesized by the synthesizer24is output from the output section22. The above processing is performed continuously or repeatedly at regular intervals (for example, 1 second).

As described above, in the present embodiment, the magnetic sensor SE1, which is accommodated in the magnetic shield12and detects a direct current magnetic field and a low-frequency alternating current magnetic field generated by the current flowing through the measurement target conductor MC, and the Rogowski sensor SE2, which detects low-frequency to high-frequency alternating current magnetic fields generated by the current flowing through the measurement target conductor MC are provided. Then, the current (direct current and low-frequency alternating current) flowing through the measurement target conductor MC is calculated based on the detection result of the Rogowski sensor SE1, the current I flowing through the measurement target conductor MC (from a low frequency to a high frequency) is calculated from the Rogowski sensor SE2, and each of the calculation results are synthesized. For this reason, in this embodiment, the current I (direct current and low-frequency components and low-frequency to high-frequency components) flowing through the measurement target conductor MC can be measured using a compact device in a non-contact manner with high accuracy.

Third Embodiment

<Configuration of Current Measurement Device>

FIG.9is an external view of a current measurement device according to a third embodiment of the invention. As shown inFIG.9, a current measurement device3of the present embodiment has a configuration in which the sensor head10A included in the current measurement device2shown inFIG.6is replaced with a sensor head10B. Like the current measurement device2of the second embodiment, the current measurement device3of the present embodiment can measure low to high frequency components in addition to the direct current and low frequency components of the current I flowing through the measurement target conductor MC.

FIG.10is a cross-sectional arrow view of a magnetic shield of a sensor head provided in the current measurement device according to the third embodiment of the present invention.FIG.10corresponds to the cross-sectional arrow view shown inFIG.3B. The sensor head10B has a configuration in which the Rogowski sensor SE2of the sensor head10A of the second embodiment is omitted, and a coil SE3(a second sensor) is provided instead.

Like the Rogowski sensor SE2, the coil SE3detects an alternating current magnetic field from a low frequency (for example, several [kHz]) to a high frequency (for example, several tens of [MHz]) generated by the current I flowing in measurement target conductor MC. The coil SE3is designed according to the maximum current (an upper limit current) which can be measured by the current measurement device3and the maximum frequency which can be measured by the current measurement device3, and is accommodated in the magnetic shield12as shown inFIG.10.

Specifically, the coil SE3is disposed in the magnetic shield12such that a sensing axis (a magnetic sensing direction) is in a direction of a magnetic field generated by the current I (a tangential direction of the measurement target conductor MC) when the sensor head10B is fixed to the measurement target conductor MC by the fixing mechanism13. By disposing the coil SE3in this manner, the influence of an external magnetic field flowing into the magnetic shield12from the cutout portion CP2on the coil SE3can be reduced. In an example shown inFIG.10, the coil SE3is disposed at a position in which a distance from the center of the measurement target conductor MC inside the magnetic shield12becomes longer than a distance between the magnetic sensor SE1and the center of the measurement target conductor MC. The position of the coil SE3inside the sensor head10B is not limited to the position exemplified inFIG.10, and may be provided at a position different from the position exemplified inFIG.10.

The circuit section20A has the same configuration as the circuit section20A provided in the current measurement device2according to the second embodiment. In a circuit configuration of the current measurement device3of the present embodiment, the Rogowski sensor SE2shown inFIG.7can be read as the coil SE3, and the sensor head10A can be read as the sensor head10B. As in the first and second embodiments, the cable CB desirably has flexibility, is desirably easy to handle, and is desirably hard to disconnect.

<Operation of Current Measurement Device>

The operation of the current measurement device3of the present embodiment is basically the same as the operation of the current measurement device2of the second embodiment, so detailed description will be omitted. In the present embodiment, since the Rogowski sensor SE2provided in the current measurement device2of the second embodiment is omitted, work to dispose the Rogowski sensor SE2(work to dispose the Rogowski sensor SE2to surround the measurement target conductor MC) performed in the second embodiment will be omitted.

As described above, in the present embodiment, the magnetic sensor SE1for detecting the direct current magnetic field and the low-frequency alternating current magnetic field generated by the current flowing through the measurement target conductor MC and the coil SE3for detecting the low frequency to high frequency alternating current magnetic fields generated by the current flowing through the measurement target conductor MC are accommodated in the magnetic shield12. Then, the current (direct current and low-frequency alternating current) flowing through the measurement target conductor MC is calculated based on the detection result of the magnetic sensor SE1, and the current (a low frequency to a high frequency) flowing through the measurement target conductor MC is calculated based on the detection result of the coil SE3to synthesize each calculation result. For this reason, in the present embodiment, the current I (direct current and low-frequency components, and low-frequency to high-frequency components) flowing through the measurement target conductor MC can be measured using a compact device in a non-contact manner with high accuracy.

Moreover, in the present embodiment, instead of the Rogowski sensor SE2provided in the current measurement device2of the second embodiment, the coil SE3accommodated in the magnetic shield12is provided. For this reason, it is possible to omit work to dispose the Rogowski sensor SE2(work to dispose the Rogowski sensor SE2to surround the measurement target conductor MC), which is necessary in the second embodiment, and to further decrease the sensor head in size.

Fourth Embodiment

A current measurement device according to a fourth embodiment of the present invention includes a beam member BM inside the magnetic shield12of the sensor heads10,10A, and10B provided in the current measurement devices1to3according to the first to third embodiments described above. Such a current measurement device improves an SN ratio within the magnetic shield12to improve the measurement accuracy. In the followings, an example in which the beam member BM is provided inside the magnetic shield12of the sensor head10of the current measurement device1according to the first embodiment will be described.

FIGS.11A to11Dare views which show a configuration of a magnetic shield provided in the sensor head of the current measurement device according to the fourth embodiment of the present invention.FIG.11Ais an external perspective view of the magnetic shield12,FIG.11Bis a cross-sectional arrow view of the magnetic shield12, andFIGS.11C and11Dare perspective views which exemplify the beam member BM provided inside the magnetic shield12. Note thatFIG.11Bcorresponds to the cross-sectional arrow view shown inFIG.3B.

The beam member BM is, for example, formed of the same material as the magnetic shield12(for example, permalloy, or the like). A thickness of the beam member BM is set to be equal to or greater than a thickness of the magnetic shield12such that magnetic saturation does not occur even if the maximum current (upper limit current) that can be measured by the current measurement device1flows through the measurement target conductor MC. Incidentally, the beam member BM may be produced by the same manufacturing method as the magnetic shield12.

In addition, the beam member BM may be formed integrally with the magnetic shield12or may be formed separately from the magnetic shield12. When the beam member BM is formed separately from the magnetic shield12, for example, the beam member BM shown inFIG.11C or11Dcan be used. The beam member BM shown inFIG.11Cis a member composed of a quadrangular prism-shaped beam bin and a pair of columnar support portions sp provided at both ends of the beam bm. The beam member BM shown inFIG.11Dis a member composed of the quadrangular prism-shaped beam bm, the pair of columnar support portions sp provided at both ends of the beam bm, and a connecting portion cn for connecting the other ends of the support portions sp.

The beam member BM is disposed in the magnetic shield12such that, for example, the quadrangular prism-shaped beam bm extends in the X direction on the −Z side of the magnetic sensor SE1(between the magnetic sensor SE1and the cutout portion CP2). At this time, in the beam member BM, both ends (the pair of columnar support portions sp) of the beam bm are in contact with inner walls of the first shield member12aand the second shield member12b, respectively. Note that the beam member BM does not necessarily have to be disposed so that the beam bm extends in the X direction. For example, the beam member BM may also be disposed such that the beam bin extends in the Y direction.

FIGS.12A and12Bare views which show simulation results of magnetic flux density distribution in the fourth embodiment of the present invention. Specifically, the simulation results shown inFIG.12is calculated by simulating a magnetic flux density in the magnetic shield12when an external magnetic field EM in the Z axis direction is present.FIG.12Ashows an example in which beam member BM is not provided, andFIG.12Bshows an example in which the beam member BM is provided.

Referring toFIG.12A, the magnetic flux density distribution in the magnetic shield12when no beam member BM is provided approximately has elliptical distribution in which the measurement target conductor MC is set as the center, and magnetic flux density gradually decreases as a distance from the measurement target conductor MC increases. On the other hand, referring toFIG.12B, the magnetic flux density distribution within the magnetic shield12when the beam member BM is provided has a region R1where the magnetic flux density decreases on the +Z side of the beam member BM.

FIG.13is a view which shows simulation results of the SN ratio in the fourth embodiment of the present invention. In a graph shown inFIG.13, as in the graph shown inFIG.4, the horizontal axis represents the distance from the center of the measurement target conductor MC in the Z direction, and the vertical axis represents the SN ratio (logarithm). As shown inFIG.13, in the simulation, a radius of the measurement target conductor MC is set to 5 [mm], and the beam member BM with a thickness of 2 [mm] is set at a position6[mm] away from the center of the measurement target conductor MC.

Referring toFIG.13, when the beam member BM is omitted, it is known that the SN ratio gradually decreases as the distance from the measurement target conductor MC (the distance from the center of the measurement target conductor MC) increases. On the other hand, when the beam member BM is provided, it is known that there is a point where the SN rises sharply and becomes 100 to 1 or more at a position where the distance from the center of the measurement target conductor MC is around 11.5 [mm]. Therefore, by disposing the magnetic sensor SE1at this position, it is possible to improve the measurement accuracy.

Although the current measurement device according to the embodiment of the present invention has been described above, the present invention is not limited to the embodiments described above and can be freely modified within the scope of the present invention. For example, the current measurement device in the embodiments described above has the sensor head and the circuit section connected by the cable CB, but a function of the circuit section may be provided in the sensor head, and the sensor head and the circuit section may also be integrated.

In the embodiment described above, although a case in which the fixing mechanism13of the sensor head is a screw mechanism with a guide has been described as an example, the fixing mechanism13is not limited to the screw mechanism with a guide. The fixing mechanism13can adopt any structure as long as it can fix the measurement target conductor MC so that the distance between the center of the measurement target conductor MC and the magnetic sensor SE1becomes the predetermined reference distance r regardless of the diameter of the measurement target conductor MC. For example, a leaf spring or the like made of resin configured to hold a side surface of the measurement target conductor MC can be employed.

Also, in the embodiments described above, an example in which the fixing mechanism13of the sensor head is disposed on the +X side of the magnetic shield12has been described. However, the fixing mechanism13may be disposed on the −X side of the magnetic shield12or may be disposed in the magnetic shield12.

Directional terms such as “front, back, up, down, right, left, vertical, horizontal, longitudinal, lateral, rows and columns” herein in the present specification refer to these directions in the device of the present invention. Accordingly, these terms in the specification of the present invention should be interpreted relatively for the device of the present invention.

The word “configured” is used to indicate a configuration, an element, or a portion of a device that is configured or configured to perform the functions of the present invention.

Furthermore, language expressed as “means plus function” in a claim should include any structure that can be utilized to perform the functions included in the present invention.

The term “unit” is used to indicate a component, unit, piece of hardware or software programmed to perform a desired function. Hardware is typically, but not limited to, devices and circuits.

Although preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments. Configuration additions, omissions, substitutions, and other changes can be made within a range not departing from the scope of the present invention. The present invention is not limited by the description described above, but is limited only by the scope of the appended claims.

REFERENCE SIGNS LIST

1to3Current measurement device10,10A,10B Sensor head12Magnetic shield13Fixing mechanism20,20A Circuit section21,23Calculator24SynthesizerBM Beam memberCP2Cutout portionI CurrentMC Measurement target conductorr Reference distanceSE1Magnetic sensorSE2Rogowski sensorSE3Coil