JP2015025707A - Current measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current measuring device for accurately measuring a common mode current flowing through a wide conductor by a simple method.SOLUTION: A measurement probe 3 detects, as inductive voltage, a magnetic flux 30 generated by a current 20 flowing through a conductor 2 to be measured using a cylindrical loop antenna. The length of the measurement probe 3 is set substantially equal to the width of the conductor 2 to be measured. A slit 4 is formed in the longitudinal direction at a side surface position of the measurement probe 3, and a feeding point 5 for detecting the inductive voltage from the loop antenna is disposed at a substantially center position of the slit in the longitudinal direction. The inductive voltage detected by the measurement probe 3 is transmitted to signal processing sections 8, 9, and 10 via the feeding point 5, and is converted into a current value.

Description

  The present invention relates to a current measuring device suitable for measuring a current flowing through a cable or the like in an electronic device.

  In recent electronic devices, component parts are modularized, and signals and power are transmitted by connecting them with cables and connectors. In addition, with the increase in speed and functionality of electronic devices, EMC (Electromagnetic Compatibility) measures have become important. In other words, electromagnetic noise emitted from electronic devices is generated by current (common mode component) flowing between the modules, but in order to clarify the noise radiation mechanism, it is necessary to measure the current flowing through cables and connectors.

  In general, a current transformer or a loop antenna is used to measure a current flowing in a conductor, and a magnetic field (magnetic flux) generated by the current is measured and converted into a current value. However, the current transformer has a structure in which a conductor is sandwiched between ring-shaped measuring instruments and is difficult to use for a wide cable or the like. On the other hand, the loop antenna has an advantage that it can be installed and measured on one side without interposing a conductor. For example, Patent Document 1 discloses a device that obtains currents in a common mode and a normal mode by synthesizing outputs of respective loop antennas using four loop antennas.

JP-A-8-68837

  In order to measure the current of a wide conductor such as a flat cable or a multi-pin connector mounted on an electronic device, a loop antenna system that can be installed and measured on one side of the conductor is suitable. However, the measurement apparatus disclosed in Patent Document 1 requires four loop antennas and a circuit for combining outputs, and the configuration is complicated compared to the current transformer method. In addition, since the measurement sensitivity varies depending on the distance between the linear antenna and the conductor in the loop antenna method, the antenna is installed especially when the current flowing through the wide conductor is not uniform (multi-core cable, multi-pin connector). Measurement errors are likely to occur due to position and current distribution.

  An object of the present invention is to provide a current measuring apparatus that measures a common mode current flowing through a wide conductor more accurately by a simple method.

  The present invention relates to a current measuring apparatus for measuring a current flowing through a wide measured conductor, a measuring probe for detecting magnetic flux generated by the current flowing through the measured conductor as an induced voltage by a cylindrical loop antenna, and a measuring probe. A signal processing unit that converts the detected induced voltage into a current value, and the length of the measurement probe is substantially equal to the width of the conductor to be measured.

Here, a slit is formed in the longitudinal direction in the measurement probe, and the slit is provided at a side surface position of the measurement probe as viewed from the conductor to be measured. An induced voltage from the loop antenna is provided at a substantially central position in the longitudinal direction of the slit. A feeding point for detecting the signal and transmitting it to the signal processing unit is provided.
Moreover, it is preferable that the shape of the measurement probe in the longitudinal direction has a curvature of a flow line of magnetic flux generated by a current flowing through the conductor to be measured.

  According to the present invention, a common mode current flowing through a wide conductor can be measured more accurately by a simple method.

The block diagram which shows one Example of the current measuring apparatus by this invention. The figure explaining the shape of the measurement probe 3, and the principle of an electric current measurement. The figure which shows the other shape of the measurement probe 3. FIG. The figure which shows the 1st example (Example 1) of a measurement probe. The table | surface which shows the value of the test electric current sent through each conductor. The figure which shows the measurement result by the measurement probe of FIG. The figure which shows the 2nd example (Example 2) of a measurement probe. The figure which shows the measurement result by the measurement probe of FIG. The figure which shows an example of the manufacturing process of a measurement probe (Example 3).

FIG. 1 is a block diagram showing an embodiment of a current measuring apparatus according to the present invention.
The current measuring apparatus 1 measures a current 20 flowing through a conductor to be measured 2 having a width such as a flat cable (hereinafter simply referred to as “conductor”), and a measurement probe 3 (hereinafter simply referred to as “probe”). And a preamplifier 8, a spectrum analyzer 9, and a personal computer (PC) 10 as signal processing units.

  The probe 3 uses a cylindrical loop antenna made of a conductive material, and detects an induced voltage generated by a magnetic flux generated by the current 20 flowing through the conductor 2. Since the relationship between the induced voltage and the current 20 is uniquely determined by the shape of the probe 3 and the set position, the conversion equation is obtained in advance, and the current 20 can be measured by using the conversion equation. Further, since the magnetic flux penetrating the loop antenna of the probe 3 is determined by the total value of the current flowing through the conductor 2, it is the common mode current (in-phase component) that is measured.

  In the probe 3 of this embodiment, a slit 4 is provided in the longitudinal direction of a cylindrical loop antenna to open one part of the loop, and a feeding point that serves as a detection end of an induced voltage at a substantially central position in the longitudinal direction of the slit 4. 5 is provided. The detection voltage at the feeding point 5 is transmitted to the signal processing unit via the coaxial cable 6 and the connector 7. Since the detection signal of the probe 3 is weak, the preamplifier 8 amplifies the detection signal, and the spectrum analyzer 9 extracts a predetermined frequency component of the detection signal to improve the S / N ratio. The PC 10 stores a conversion formula between the induced voltage and the conductor current for each signal frequency, and calculates the common mode current value flowing through the conductor 2 using this.

2A and 2B are diagrams for explaining the shape of the measurement probe 3 and the principle of current measurement. FIG. 2A is a front view and FIG. 2B is a side view.
The measuring probe 3 detects a magnetic flux generated by the total value of the currents 20 that flow in the wide conductor 2 to be measured. The current 20 flowing in the conductor 2 generates a substantially elliptical magnetic flux 30 around the conductor 2, and a part of this magnetic flux is linked to the cylindrical loop antenna of the probe 3. The current 20 is a high-frequency current, and an induced voltage e determined by the amount of magnetic flux interlinked and the frequency is generated at the feeding point 5 of the loop antenna. The longitudinal direction of the probe 3 is arranged so as to be orthogonal to the direction of the current 20 (longitudinal direction of the conductor 2). By making the length of the probe 3 substantially equal to the width of the current 20 (the width of the conductor 2), the total sum of the currents flowing in the conductor can be accurately detected.

  In the case of a conventional linear loop antenna, the detection sensitivity of the antenna depends on the distance between the antenna and the current, and it is difficult to accurately measure the current flowing in the wide conductor. That is, when the current flows unevenly in the wide conductor, the measured value changes depending on the installation position of the antenna, and it is difficult to accurately measure the common mode current. In contrast, in this embodiment, a cylindrical loop antenna is used instead of a linear loop antenna. Thereby, even if the electric current in a conductor is non-uniform | heterogenous and the induced voltage detected in each position of an antenna differs, it averages within the cylindrical loop antenna of an electrically-conductive material. Therefore, the total current value (common mode current) in the conductor is calculated even when the current in the conductor flows, for example, biased toward the end in the width direction, or when the current flowing through each core wire is different in a multicore cable. It can be detected with high accuracy. In other words, if a current that is reciprocating in the multicore cable and a current with a total value of zero (differential mode current) flows, the induced voltage is canceled in the cylindrical loop antenna, so the differential mode current is Not detected (insensitive). Thus, the cylindrical loop antenna is suitable for measuring the common mode current.

  When the length of the probe 3 is significantly larger than the width of the conductor 2, the magnetic flux interlinked with the antenna is reduced in the portion outside the conductor width, and the induced voltage is reduced. The detection voltage will be lowered. For this reason, the length of the probe 3 is preferably substantially equal to the width of the conductor 2.

  The position of the slit 4 formed in the probe 3 is provided at a position not facing the conductor 2, preferably at a side position of the probe 3 when viewed from the conductor 2. This is to eliminate the inflow / outflow of magnetic flux from the slit 4. That is, if the slit 4 is provided facing the conductor 2, the oblique component magnetic flux flows from the conductor side through the slit 4 into the loop antenna, causing measurement errors. In order to avoid this, the slit 4 is provided at the side surface position of the probe 3 where the magnetic flux of the oblique component from the conductor side is small. Thereby, the oblique component magnetic flux from the conductor side is shielded by the shielding action of the conductive material of the probe 3 and cannot flow. As a result, the magnetic flux 30 generated by the current of the conductor 2 flows in and out from the openings at both ends of the probe 3 and the measurement accuracy is improved. Similarly, providing the slit 4 on the back side of the probe 3 when viewed from the conductor 2 is not preferable because a part of the magnetic flux in the probe flows out of the slit 4.

  In the present embodiment, the cross-sectional shape of the probe 3 is circular. However, the cross-sectional shape is not limited to this, and may be a quadrangle or a rectangle. The cross-sectional area is determined according to measurement conditions such as frequency and current value.

  FIG. 3 is a view (front view) showing another shape of the measurement probe 3. In FIG. 2, the loop antenna of the measurement probe 3 is linear, but in this example, the loop antenna is curved. The curvature is a shape that matches the curvature of the streamline of the magnetic flux 30 created by the current 20 of the conductor 2. Here, the streamline shape of the magnetic flux 30 can be obtained by assuming a current distribution and calculating the magnetic field component at each position generated at that time.

By measuring the shape of the probe 3 along the flow line of the magnetic flux 30, the magnetic flux flows in and out from the openings at both ends of the probe 3, so that the magnetic flux 30 can penetrate the probe 3 most efficiently. Accuracy is improved.
Hereinafter, specific examples of measurement probes and measurement results will be described.

  FIG. 4 is a diagram illustrating a first example (Example 1) of the measurement probe, and corresponds to a case where the loop antenna is linear (FIG. 2). (A) is a front view, (b) is a side view, (c) is a front dimension diagram, and (d) is a side dimension diagram. As the measurement object, the conductor 2 is assumed to be a multi-core cable composed of a plurality of conductors, and a current is passed through each of the conductors 21 to 27. The measurement probe 3 is linear, its length (10 mm) covers the entire conductors 21 to 27, the distance (height) from the conductor 2 is 5 mm, and the loop size (cross-sectional diameter) is 1. 4 mm. Moreover, the slit 4 is provided in the side surface of the probe 3, and the width | variety is 0.2 mm.

  FIG. 5 is a table showing values of test currents flowing through the conductors. As the current mode, a common mode (C0) for evaluating a desired signal level and a differential mode (D1 to D6) for evaluating an unnecessary signal level are provided. The unit of current value is expressed as a relative value. The current value was determined to be a value at which the sum of squares of the currents flowing through the conductors was 1 so that the energy was the same in each mode. In the common mode C0, for the sake of simplicity, the same current is supplied to all conductors in the same direction. By evaluating the common mode C0, it is possible to evaluate the analysis capability for electromagnetic noise. On the other hand, in the differential mode, it is assumed that a current in a reciprocating direction flows through some conductors. For example, in mode D1, the forward current is passed through the conductor 22 and the return current is passed through the conductor 23. In each of the modes D1 to D6, the combination of conductors through which the current is passed is changed. In the differential mode, since the current is in the reciprocating direction, the total current in the same direction is zero in any case, and does not contribute to the generation of electromagnetic noise. Therefore, it is possible to grasp how much unnecessary signals are included by evaluating the differential mode.

FIG. 6 is a diagram showing measurement results obtained by the measurement probe of FIG. 4 and shows each current mode of FIG. The vertical axis represents the induced voltage (relative value), and the horizontal axis represents the frequency.
When the current mode is based on the measurement level in the common mode C0, the differential mode measurement level is 20 dB or more lower than the reference level. The reason why the level of the mode D3 is higher than the others in the differential mode is considered to be because the current distribution is large. Therefore, it can be seen that when measuring the desired signal (common mode current), the level of the unnecessary signal (differential mode current) is sufficiently low, and sufficient accuracy can be obtained without causing an obstacle.

  FIG. 7 is a diagram showing a second example (Example 2) of the measurement probe, and corresponds to a case where the loop antenna is curved (FIG. 3). (A) is a front view, (b) is a side view, (c) is a front dimension diagram, and (d) is a side dimension diagram. The conductor 2 as the measurement target is the same as that of the first embodiment (FIG. 4), and a multicore cable including a plurality of conductors 21 to 27 is assumed.

  The measurement probe 3 of this example is arcuate, and its length covers the entire conductors 21 to 27. The maximum distance (height) from the conductor 2 is 5 mm, and the size of the loop (cross-sectional diameter) is It was set to 1.4 mm. Moreover, the slit 4 is provided in the side surface of the probe 3, and the width | variety is 0.2 mm.

  FIG. 8 is a diagram showing a measurement result by the measurement probe of FIG. 7, and shows each current mode of FIG. The vertical axis represents the induced voltage (relative value), and the horizontal axis represents the frequency. Also in this case, the measurement level in the differential mode is 20 dB or more lower than the measurement level in the common mode C0. In particular, in modes D4 and D6, it is lower than the result of FIG. 1 (FIG. 6), which is considered to be an effect of making the loop antenna curved.

  In Example 3, the manufacturing process of the measurement probe will be described. Here, the case of the probe of Example 2 (FIG. 7) will be described.

FIG. 9 is a diagram illustrating an example of a manufacturing process of the measurement probe.
In S101, measurement conditions such as the width of the conductor to be measured, the distance (height) from the probe, and the signal frequency are input.
In S102, the shape (curvature) of the magnetic flux generated around when a predetermined current is passed through the conductor is calculated.

In S103, the shape of the probe is determined from the shape (curvature) of the magnetic flux at the height position of the probe. The above calculation is executed using, for example, a personal computer, and the probe shape is stored as 3D data.
In S104, the base of the probe is three-dimensionally formed using a resin material or the like. For example, if a 3D printer is used, a 3D object can be created by using 3D data stored in a personal computer as a design drawing and irradiating the liquid resin with ultraviolet rays or the like to gradually cure the resin.

In S105, a metal conductor is formed on the surface of the probe matrix by printing, vapor deposition, or the like. At this time, a non-formed portion of the metal conductor is left as a slit in the longitudinal direction of the probe.
In S106, the connector is attached to the probe and completed.
According to said process, the measurement probe of the shape match | combined with the curvature of magnetic flux can be made easily.

  In each of the embodiments described above, the measurement of the common mode current flowing through the cable or the like as the conductor to be measured has been described, but the measurement target is not limited to this, and the current flowing through the wide bus bar or the housing surface may be measured. it can.

1 ... Current measuring device,
2 ... conductor to be measured,
3 ... Measurement probe,
4 ... Slit,
5 ... Feed point,
6 ... Coaxial cable,
7 ... Connector,
8 ... Preamp,
9 ... Spectrum analyzer,
10 ... Personal computer,
20 ... Current,
30: Magnetic flux.

Claims (5)

In a current measuring device that measures the current flowing through a wide conductor to be measured,
A measurement probe for detecting a magnetic flux generated by a current flowing through the conductor to be measured as an induced voltage by a cylindrical loop antenna;
A signal processing unit that converts the induced voltage detected by the measurement probe into a current value;
The current measuring apparatus is characterized in that the length of the measuring probe is substantially equal to the width of the conductor to be measured. The current measuring device according to claim 1,
The length of the measurement probe is such that the effective detection voltage generated in the width portion of the measured conductor is not reduced by the voltage induced in the portion outside the width of the measured conductor. Current measuring device. The current measuring device according to claim 1,
The measurement probe is formed with a slit in the longitudinal direction, and the slit is provided at a side surface position of the measurement probe when viewed from the conductor to be measured.
A current measuring device characterized in that a feeding point for detecting an induced voltage from the loop antenna and transmitting it to the signal processing unit is provided at a substantially central position in the longitudinal direction of the slit. The current measuring device according to claim 1,
The shape of the measurement probe in the longitudinal direction has a curvature of a streamline of magnetic flux generated by a current flowing through the conductor to be measured. The current measuring device according to claim 1,
When the conductor to be measured is a plurality of conductors, the apparatus measures a common mode current which is a total value of currents flowing through the conductors by the apparatus.

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