JP2014137359A - Current sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost current sensor excellent in temperature characteristics.SOLUTION: A current sensor 100 comprises: a rectangular wave signal generation section 10a generating a rectangular wave signal oscillating between a positive voltage and a negative voltage that have the same absolute value; a toroidal coil 20 having an annular core 20A through which a conductor 20C for the flow of measurement target current I0 is inserted, and a lead wire 20B which is wound around the core 20A and through which current according to the rectangular wave signal flows; a resistor 30 arranged between an output terminal and a ground terminal of the toroidal coil 20; a pulse signal output part 40 which outputs a pulse signal over a predetermined second period within a first period from inversion of the rectangular wave signal to flux saturation of the core 20A, in a case where the measurement target current I0 does not flow through the conductor 20C and in a case where current based on the rectangular wave signal flows through the lead wire 20B; and a sample hold section 50 sample-holding a voltage between terminals of the resistor 30 on the basis of the pulse signal.

Description

  The present invention relates to a current sensor for measuring a direct current.

  In recent years, the demand for photovoltaic power generation has increased. In such a solar cell panel used for photovoltaic power generation, the output voltage is constant, but the generated current (suppliable output current) varies depending on the power generation situation such as the sunshine rate and the deterioration of the power generation capacity. . For this reason, measuring the output current of the solar cell panel is important from the viewpoint of power generation efficiency and facility maintenance. In recent years, storage batteries have also been widely used. In such a storage battery, charging / discharging is repeatedly performed according to use conditions. In order to appropriately use the storage battery, it is necessary to accurately measure the charging current and the discharging current in such charging and discharging with the same accuracy. As a technique for measuring such an output current of a solar battery panel and a charging current and a discharging current of a storage battery, there is a current sensor using an annular magnetic core having a gap (for example, Patent Document 1).

  A current sensor (DC current sensor) described in Patent Document 1 is made of a substantially annular magnetic material having a gap, and is inserted into a core gap through which a wire through which a current to be measured flows passes radially inward. And a magnetoelectric conversion means for outputting a voltage corresponding to the magnetic flux generated by the current to be measured. A Hall element is used as such a magnetoelectric conversion means. Since such a Hall element is expensive, it causes an increase in the cost of the current sensor. In addition, since the electrical characteristics of the Hall element are highly temperature-dependent, it is not easy to detect the current with high accuracy in a usage environment with temperature fluctuations. Therefore, a current sensor that does not use a Hall element has been studied (for example, Patent Document 2).

  The current sensor described in Patent Document 2 is arranged by penetrating a closed magnetic circuit core through an electric wire that is an object to be measured, and generates a magnetic field generated in the closed magnetic circuit core by the measured current flowing through the electric wire. The current to be measured is measured by causing a feedback current to flow through the winding coil wound around the core so as to cancel it. In particular, this current sensor has a sensing resistor connected in series with the winding coil. A positive peak hold circuit and a negative peak hold to which a voltage signal is applied by applying an AC excitation voltage to a series connection of a winding coil and a detection resistor, converting the AC excitation current generated thereby into a voltage signal by a detection resistor The circuits hold the positive signal and the negative signal, respectively, and acquire the output signals of the positive peak hold circuit and the negative peak hold circuit as a combined sum voltage. This synthesized sum voltage is amplified by an amplifier circuit and fed back as a DC excitation voltage to the winding coil and the detection resistor.

JP 2000-97973 A JP 2007-33222 A

  Since the current sensor described in Patent Document 2 does not have a Hall element, the cost can be reduced compared to the current sensor described in Patent Document 1. Also, since no Hall element is used, the temperature characteristics are improved. However, since the positive peak hold circuit and the negative peak hold circuit are provided, there is room for further cost reduction.

  In view of the above problems, an object of the present invention is to provide a current sensor having low temperature and excellent temperature characteristics.

In order to achieve the above object, the characteristic configuration of the current sensor according to the present invention is as follows:
A rectangular wave signal generating unit that generates a rectangular wave signal that swings between a positive voltage and a negative voltage that are equal in absolute value;
A toroidal coil having an annular core through which a conductor for passing a current to be measured is inserted, and a conductive wire wound around the core and in which a current corresponding to the rectangular wave signal flows;
A resistor disposed between an output terminal of the toroidal coil and a ground terminal;
When no current to be measured flows through the conductor, and when a current based on the rectangular wave signal is passed through the conductor, a first period from when the rectangular wave signal is inverted to when the core is saturated with magnetic flux. A second period shorter than the first period is preset in one period, and a pulse signal output unit that outputs a pulse signal over the second period for each first period;
A sample-and-hold unit that samples and holds a voltage between terminals of the resistor based on the pulse signal;
It is in the point equipped with.

  With such a characteristic configuration, a voltage corresponding to the current to be measured can be generated between the terminals of the resistor, and the voltage can be maintained for a predetermined period. Therefore, since only one sample hold circuit needs to be provided regardless of the sign of the current to be measured, the current sensor can be realized at low cost. Further, since the rectangular wave signal input to the toroidal coil has an amplitude based on GND, even if the core characteristics are not good, the influences can be canceled out with each other positively and negatively. Therefore, since an inexpensive core can be used, it can be realized at low cost. In addition, as described above, since the rectangular wave signal input to the toroidal coil has an amplitude based on GND, the hysteresis generated in the core characteristics can be canceled out with each other. Therefore, the current to be measured can be measured with high accuracy. Furthermore, since a Hall element is not used, a current sensor with excellent temperature characteristics can be realized.

It is a circuit diagram of a current sensor according to the present invention. It is the figure which showed typically the change of the rectangular wave signal and the magnetic flux of a core. It is a figure which shows a measurement principle. It is the figure which showed the waveform of each element from the magnetic characteristic of the coil. It is the figure which showed the waveform of each part of FIG. It is the figure which showed the circuit diagram of the current sensor which concerns on other embodiment.

  The current sensor 100 according to the present invention is configured to be able to measure a direct current flowing through a conductor using a known current transformer. Hereinafter, the current sensor 100 of the present embodiment will be described in detail.

  FIG. 1 is a circuit diagram schematically showing a current sensor 100 according to this embodiment. As shown in FIG. 1, the current sensor 100 includes a rectangular wave signal generation unit 10, a toroidal coil 20, a resistor 30, a pulse signal output unit 40, and a sample hold unit 50. The rectangular wave signal generation unit 10 generates a rectangular wave signal that swings between a positive voltage and a negative voltage having the same absolute value. A positive voltage and a negative voltage having the same absolute value mean that the positive and negative voltages are the same with respect to 0V. Therefore, a rectangular wave signal that is amplified between a positive voltage and a negative voltage having the same absolute value means a rectangular wave signal that is amplified at + V and −V with 0V as a reference.

  A rectangular wave signal generation unit 10 according to this embodiment includes an oscillation circuit unit 1 and a current amplification unit 2. As shown in FIG. 1, the oscillation circuit unit 1 can be configured using, for example, an inverter circuit (particularly preferably configured with a Schmitt trigger). Of course, it may be configured by other known circuits (for example, an oscillation circuit based on OP-AMP). In the present embodiment, the oscillation circuit unit 1 generates a rectangular wave signal having an amplitude of ± V. Such a rectangular wave signal is shown in FIG. The oscillation circuit unit 1 continuously outputs such a rectangular wave signal to the current amplification unit 2.

  In the present embodiment, the current amplifying unit 2 is configured using two bipolar transistors. A pnp transistor 2A is used on the high side, and an npn transistor 2B is used on the low side. The emitter terminal of the pnp transistor 2A is connected to the “+ V” power supply line. The base terminal is connected to the output terminal of the oscillation circuit unit 1 through a resistor. The collector terminal is connected to the collector terminal of the npn transistor 2B. The emitter terminal of the npn transistor 2B is connected to the “−V” power line. The base terminal is connected to the output terminal of the oscillation circuit unit 1 through a resistor. The collector terminal is connected to the collector terminal of the pnp transistor 2A.

  By configuring the rectangular wave signal generation unit 10 in this way, it becomes possible to output a current corresponding to “+ V” from the collector terminal of the pnp transistor 2A when the output of the oscillation circuit unit 1 is Low, and the oscillation circuit When the output of the unit 1 is High, a current corresponding to “−V” can be drawn from the collector terminal of the npn transistor 2B.

  The toroidal coil 20 has a core 20A and a conductive wire 20B. The core 20A is formed of an annular core through which a conductor 20C that allows the current to be measured I0 to flow is inserted. The inner side is a space inside the annular core in the radial direction. The measured current I 0 is a direct current as a measurement target of the current sensor 100. The conductor 20C may be a metal piece or an electric wire. Such a conductor 20C is provided so as to penetrate the space inside the core 20A in the radial direction. The conducting wire 20B is wound around the core 20A and a current corresponding to the driven rectangular wave signal is passed. Conductive wire 20B is coated with an insulator, and is wound by a preset number of turns so that core 20A is the axis.

  The resistor 30 is disposed between the output terminal of the toroidal coil 20 and the ground terminal. The output terminal of the toroidal coil 20 is one end portion of a conducting wire 20B wound around the core 20A. The resistor 30 has a predetermined resistance value R. Therefore, a potential difference is generated between the terminals of the resistor 30 according to the current i flowing through the conductor 20B.

  Here, in the present embodiment, as shown in FIG. 2B, when the measured current I0 does not flow through the conductor 20C and when a current based on the rectangular wave signal flows through the conductor 20B. A second period T1 shorter than the first period T is set in advance within the first period T from when the rectangular wave signal is inverted to when the core 20A is saturated with the magnetic flux. The case where the measured current I0 is not flowing through the conductor 20C means that the measured current I0 is 0A. The case where the current based on the rectangular wave signal is passed through the conducting wire 20B refers to the case where the output current of the rectangular wave signal generating unit 10 is passed through the conducting wire 20B. “After the rectangular wave signal is inverted” means “after the sign of the rectangular wave signal changes”.

  Specifically, it means “after the rectangular wave signal is switched from“ −V ”to“ + V ”” and “after the rectangular wave signal is switched from“ + V ”to“ −V ””. In the present embodiment, when the current to be measured I0 is 0A and the current related to the rectangular wave signal is passed through the conductor 20B, the core 20A is saturated with the magnetic flux until the sign of the rectangular wave signal is reversed. Configured. Therefore, the first period T corresponds to a period from when the sign of the rectangular wave signal is switched until the core 20A is saturated with magnetic flux. The second period T1 is set within a time shorter than the first period T in the first period T.

  The pulse signal output unit 40 outputs a pulse signal for each of the first periods T over the second period T1. Such a pulse signal output unit 40 is preferably composed of an inverter to which the output of the oscillation circuit unit 1 is input and an exclusive OR. More specifically, the pulse signal output from the pulse signal output unit 40 is output with a predetermined time delay T0 set in advance with respect to the rectangular wave signal output from the oscillation circuit unit 1, and the pulse signal is It becomes HIGH only for a preset time T1.

  The sample hold unit 50 samples and holds the voltage between the terminals of the resistor 30 based on the pulse signal. The pulse signal is a signal output from the pulse signal output unit 40 described above. Here, the sample hold unit 50 according to the present embodiment includes a switch SW and a capacitor C. One end of the switch SW is connected to the output terminal of the toroidal coil 20 via the resistor 90. The capacitor C is disposed between the other end of the switch SW and the ground terminal.

  The switch SW is changed in its open / closed state by a pulse signal. In this embodiment, the switch SW is closed when the pulse signal is HIGH, and the switch SW is opened when the pulse signal is LOW. For example, when the pulse signal becomes HIGH and the switch SW is closed, the capacitor C is charged via the resistor 90. Thereafter, when the pulse signal becomes LOW and the switch SW is opened, the charging of the capacitor C is stopped, and the voltage across the terminals of the capacitor C is maintained. This inter-terminal voltage is a voltage based on the current flowing through the conducting wire 20B. In this way, a voltage based on the current flowing through the conductor 20B is acquired. For example, it may be configured to calculate the measured current I0 by dividing this voltage by a predetermined constant, or a map that prescribes the relationship between the voltage between the terminals and the measured current I0 is stored in advance. In addition, it is possible to employ a configuration in which the measured current I0 is calculated based on this map.

  Next, the measurement principle will be described. FIG. 3 is a circuit diagram schematically showing the measurement system of the current sensor 100 according to the present invention. This circuit diagram includes a DC power supply E, a switch SW, a current transformer L, and a resistor R connected in series. In the current transformer L, the number of turns of the primary winding is N1, and the number of turns of the secondary winding is N2. In such a circuit, the voltage E is expressed by the following equation (1).

  This equation (1) is Laplace transformed to obtain equation (2).

ここで、ｉ（０）は初期電流であり、Ｉ（０）×Ｎ１／Ｎ２である。（２）式をＩ（ｓ）について解いて（３）式が得られる。

Here, i (0) is an initial current, which is I (0) × N1 / N2. Equation (3) is obtained by solving equation (2) for I (s).

（３）式を逆ラプラス変換し、（４）式が得られる。

Equation (3) is subjected to inverse Laplace transform to obtain equation (4).

  Here, if the direct current power source E is an excitation power source ± E, the current i (t) after t1 in the positive polarity is the coil current i +, and the current i (t) after t2 in the negative polarity is the coil current i−. These are the following formulas (5) and (6), respectively.

  When the average of the formulas (5) and (6) is calculated, the formula (7) is obtained.

  Here, if t = t1 = t2, equation (8) is obtained.

  Here, the value of the inductance L of the annular coil can be calculated by equation (9).

ここで、Ｎはコイルの巻き数、φは環状鉄心内の磁束、κは磁路の磁気抵抗である。また、磁気抵抗κは、以下の（１０）式で計算できる。

Here, N is the number of turns of the coil, φ is the magnetic flux in the annular core, and κ is the magnetic resistance of the magnetic path. Further, the magnetic resistance κ can be calculated by the following equation (10).

ここで、μは鉄心透磁率、lは鉄心磁路長、Ａは鉄心断面積である。例えば、鉄心材料がパーマロイやナノ結晶の場合を想定して比透磁率５００００、磁路長０．０４７ｍ、断面積を５．５×４ｍｍ²とすると、

となる。これを用いて（９）式によりＬ値を計算すると、

Here, μ is the iron core permeability, l is the iron core magnetic path length, and A is the iron core cross-sectional area. For example, assuming that the iron core material is permalloy or nanocrystal, the relative permeability is 50000, the magnetic path length is 0.047 m, and the cross-sectional area is 5.5 × 4 mm ² .

It becomes. Using this, when calculating the L value according to equation (9),

となる。ただし、コイルの巻き数Ｎは２５００回である。
ここで、例えば抵抗器Ｒの抵抗値が５００Ωである場合の１ｍｓ後の電流ｉは、（８）式より（１３）式のように計算できる。

It becomes. However, the number N of turns of the coil is 2500.
Here, for example, when the resistance value of the resistor R is 500Ω, the current i after 1 ms can be calculated from the equation (8) as the equation (13).

  As shown in the equation (13), there is a 0.26% decrease with respect to i at t = 0. However, this error can be eliminated if detection is performed at a fixed time after the rectangular wave signal is switched. Here, when t = 1 ms, i≈1, and equation (14) is established.

  FIG. 4 is a diagram schematically showing the relationship between the magnetic flux φ generated in the core 20A of the toroidal coil 20 and the coil current i in accordance with the measured current I0. (A) is a current waveform when the measured current I0 is 0 and the excitation voltage E is applied, and (B) is a current waveform when the measured current I0 is I1 and the excitation voltage E is applied. (C) shows a current waveform when the measured current I0 is I2 and the excitation voltage E is applied. Here, 0 <I1 <I2. In such a case, the magnetic flux φ generated in the coil when the excitation voltage E is applied is

である。

It is.

  As shown in the equation (15), the magnetic flux φ is the addition of the time integration of the excitation voltage E and the direct current passing through the coil, that is, the magnetic flux φ0 generated by the current to be measured I0. The magnetic flux φ increases with time. When the saturation magnetic flux φS of the coil is reached, the back electromotive force V generated at both ends of the coil becomes 0, and the current i flowing through the coil becomes a constant value of E / R. Until the saturation magnetic flux φS of the coil is reached, the current I0 expressed by the equation (14) flows. 4A, 4B, and 4C show waveforms of the magnetic flux φ, the back electromotive force V at both ends of the coil, and the coil current i in the cases of (A), (B), and (C), respectively. .

  FIG. 5 shows the waveform of each part of FIG. 5A shows the output of the current amplifying unit 2, and a ± V rectangular wave signal is applied to the toroidal coil 20 and the resistor 30. FIG. This corresponds to the excitation voltage. FIG. 5B is a signal for controlling the opening and closing of the switch SW of the sample hold unit 50 generated by the pulse signal output unit 40 so that the output signal of the oscillation circuit unit 1 is delayed by T0 and becomes active only during the T1 period. is there. (C-1) in FIG. 5 is the coil current i when the measured current I0 is 0A. (C-2) in FIG. 5 is the coil current i when the measured current I0 is, for example, + 25A. (C-3) in FIG. 5 is the coil current i when the measured current I0 is, for example, -25A. FIG. 5D shows a voltage waveform (for example, in the case of + 25A) when the voltage between terminals of the capacitor C is held by the sample hold unit 50 only during the T1 period.

  As shown in FIG. 5D, when the hold voltage holding value when the excitation voltage is + V is A and the hold voltage holding value when the excitation voltage is -V is B, the sample hold unit The voltage waveform is smoothed by 50, and the output voltage E0 is proportional to the measured current I0 (see equation (16)). That is, since the voltage (output voltage E0) held by the sample hold unit 50 corresponds to the measured current I0, the measured current I0 is measured by measuring this voltage (output voltage E0). It becomes possible.

  Here, since the output voltage E0 is proportional to the resistor R and is only inversely proportional to the number of turns N of the coil and is hardly affected by the coil inductance or the iron core permeability, there is no variation among the individual coils. Moreover, since the excitation voltage is de-energized with respect to the influence of the residual magnetic flux, the zero fluctuation does not occur.

  Further, the counter electromotive force induced by the current to be measured I0 can be supplied to the power source of the current sensor 100 through the freewheeling diode 60 to supplement the power consumed by the current sensor 100 and contribute to power saving.

  As described above, according to the current sensor 100, a voltage corresponding to the current to be measured is generated between the terminals of the resistor using a known current transformer, and the voltage is maintained for a predetermined period. be able to. Therefore, since only one sample hold circuit needs to be provided regardless of the sign of the current to be measured, the current sensor can be realized at low cost. Further, since the rectangular wave signal input to the toroidal coil 20 has an amplitude based on GND, even if the characteristics of the core 20A are not good, they can cancel each other positively and negatively, so that no problem occurs. Therefore, an inexpensive core 20A can be used, and can be realized at a low cost. Furthermore, since no Hall element is used, the temperature characteristics are excellent.

[Other Embodiments]
In the above-described embodiment, an example in which the oscillation circuit unit 1, the pulse signal output unit 40, and the sample hold unit 50 of the rectangular wave signal generation unit 10 are independently configured by elements has been described. However, the scope of application of the present invention is not limited to this. For example, as shown in FIG. 6, the oscillation circuit unit 1, the pulse signal output unit 40, and the sample hold unit 50 can naturally be configured by an arithmetic processing unit 70. In such a case, it is preferable to connect the output of the toroidal coil 20 to the AD converter input terminal A / D in order to measure the voltage across the terminals of the resistor R and the DO that outputs the rectangular wave signal. The arithmetic processing unit 70 can naturally be operated by a program.

  In the above embodiment, the current amplifying unit 2 has been described as being configured using two bipolar transistors in the present embodiment. However, the scope of application of the present invention is not limited to this. Of course, a field effect transistor (FET) can also be used.

  In the above-described embodiment, the current amplification unit 2 has been described on the assumption that the pnp transistor 2A is used on the high side and the npn transistor 2B is used on the low side. However, the scope of application of the present invention is not limited to this. Of course, it is possible to use an npn transistor on the high side and a pnp transistor on the low side.

  The present invention can be used for a current sensor for measuring a direct current.

DESCRIPTION OF SYMBOLS 10: Rectangular wave signal generation part 20: Toroidal coil 20A: Core 20B: Conductor 20C: Conductor 30: Resistor 40: Pulse signal output part 50: Sample hold part 100: Current sensor I0: Current to be measured T: First period T1: Second period

Claims (1)

A rectangular wave signal generating unit that generates a rectangular wave signal that swings between a positive voltage and a negative voltage that are equal in absolute value;
A toroidal coil having an annular core through which a conductor for passing a current to be measured is inserted, and a conductive wire wound around the core and in which a current corresponding to the rectangular wave signal flows;
A resistor disposed between an output terminal of the toroidal coil and a ground terminal;
When no current to be measured flows through the conductor, and when a current based on the rectangular wave signal is passed through the conductor, a first period from when the rectangular wave signal is inverted to when the core is saturated with magnetic flux. A second period shorter than the first period is preset in one period, and a pulse signal output unit that outputs a pulse signal over the second period for each first period;
A sample-and-hold unit that samples and holds a voltage between terminals of the resistor based on the pulse signal;
A current sensor comprising:

Images