JP2008107119A - Current sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize miniaturization, weight reduction, and cost reduction, by acquiring a constitution having a core and windings of a half of a conventional case, comprising one measuring closed magnetic circuit core and two windings of one excitation winding and one detection winding. <P>SOLUTION: This sensor includes the closed magnetic circuit core 1 wherein a magnetic flux by a current to be measured is generated, the excitation winding 2 and a feedback winding 3 applied to the closed magnetic circuit core 1, a series circuit by an oscillator 4 and a resistance 5 connected between both ends of the excitation winding 2, a positive side peak detection circuit 6 for detecting a positive side peak value of an AC voltage generated in the excitation winding 2 and a negative side peak detection circuit 7 for detecting a negative side peak value of the AC voltage, and an amplifier 8 for magnetic equilibrium for sending a feedback current into the feedback winding 3 so that a difference between the positive side peak value detected by the positive side peak detection circuit 6 and the negative side peak value detected by the negative side peak detection circuit 7 becomes zero. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a “magnetic balance type current sensor” that is used in machine tools, hybrid cars, EV cars, and the like, and that measures current with high accuracy in a non-contact manner, and in particular, magnetically sensitive elements such as Hall elements and MR elements. It is related with the current sensor which can measure not only alternating current but direct current accurately.

  Conventionally, as the non-contact type current sensor, there are (1) a so-called “open loop type” magnetic proportional current sensor that does not apply feedback, and (2) a feedback type current sensor that is magnetically balanced.

(1) An open-loop type magnetic proportional current sensor has a gap in a part of a magnetic core, an element sensitive to DC magnetism such as a Hall element is inserted, and is proportional to the through current passing through the magnetic core. The magnetic flux generated inside the magnetic core is detected and output. Although this magnetic proportional current sensor has a simple configuration, it is of an open loop type, so that the sensitivity change of the Hall element due to temperature directly deteriorates the accuracy of the sensor output, and has a drawback that the temperature dependence is large. It was. However, because of its low price, it is used in applications that do not require much precision.

(2) The feedback type current sensor is a “closed loop type” with respect to the “open loop type” in the above (1). Even with the same configuration as (1) in which a Hall element is inserted in a part of the magnetic core, the feedback current is passed through the feedback winding with the magnetic flux generated by the current to be measured so that the magnetic flux in the gap is always zero. This is offset by the so-called “equal amp turn principle”. Even if the sensitivity characteristic of the Hall element changes due to temperature or the like, there is an advantage that the sensor output is not affected, and a highly accurate current sensor with less temperature dependency is configured. However, there is a drawback that the temperature dependence of the offset of the Hall element is affected.

  The offset of the Hall element is an error component depending on the temperature at which the output does not become zero at low or high temperatures even when the Hall element output is zero at 25 ° C. when the magnetic flux penetrating the Hall element is zero. That is. That is, it means an error due to temperature when the measured current is 0 [A].

  Further, a current sensor that does not use a magnetically sensitive element such as a Hall element has been proposed, and the techniques described in Patent Documents 1 and 2 below are known as known examples.

Japanese Patent Laid-Open No. 2002-22774 JP-A-9-113543

  Patent Document 1 includes two cores, a measurement core and a reference core, in which both coils are wound with excitation coils, excited in parallel from a high-frequency excitation power source, and detection coils are wound on both cores, respectively. Connected. Further, a conducting wire is passed through the measurement core and a reverse excitation coil is wound. Then, when a current to be measured is passed through the conducting wire passing through the measurement core, the difference between the magnetic fields of both cores is detected by the zero magnetic field control circuit, and the reverse excitation circuit is operated so that the zero magnetic field is obtained. A current to be measured flows through the conducting wire by passing a current through the coil and measuring the output at this time.

  As a result, a current sensor with high resolution and high accuracy can be obtained. Further, since the Hall element is not used, there is an advantage that there is no influence of temperature dependency by the Hall element.

  The problem of Patent Document 1 is that two magnetic cores for measurement and reference are used, two excitation coils and two detection coils are provided on both cores, and one reverse excitation coil is provided on the measurement core. This requires a total of five coils, which complicates the configuration. In addition, as the material of the core, an expensive amorphous alloy (or permalloy) having a hysteresis magnetic flux level gradient of nearly 90 degrees is used, and two cores are required. There was a drawback that the shape was large and the weight was heavy.

  Patent Document 2 provides a current detection unit in which two windings are applied to one iron core, an AC voltage is applied to the first winding through a capacitor to induce an induced voltage, and an induced voltage source is detected as a DC voltage. The output voltage is obtained when the detected DC current flows through the DC circuit and is input to the apparatus. A booster using the output voltage as a control signal is provided, and the output current of the booster is passed from one terminal of the second winding to the load resistor connected to the other terminal. The voltage generated at both ends of the load resistor is removed from the alternating current through a smoothing device, and an output voltage proportional to the detected direct current is obtained.

  A conventional current sensor in which a Hall element is inserted by providing a gap in a magnetic core increases the magnetoresistance and decreases the generated magnetic flux, so that the sensitivity is lowered. It can be done.

  The problem of Patent Document 2 is that the difference in electrical angle that occurs between the positive half-wave width and the negative half-wave width of the AC voltage induced in the first winding when a detected DC current flows through the current detector. In addition, since the detection is performed by a DC voltage detection device including waveform shaping, rectification smoothing, and voltage amplification, there is a drawback that the circuit becomes complicated.

  In view of the above points, the present invention is composed of a single closed magnetic circuit core for measurement and a total of two windings, one excitation winding and one detection (feedback) winding. An object of the present invention is to provide a magnetic balance type current sensor that has a half core / winding configuration of the example (Patent Document 1) and can be reduced in size, weight, and cost.

  Further, according to the present invention, since the control circuit that feeds the feedback current to the measurement closed magnetic circuit core operates only by the positive and negative peak values, the positive and negative electrical angles as in the conventional example (Patent Document 2). Another object is to provide a magnetic balance type current sensor capable of reducing the number of parts as a simpler circuit configuration without requiring a complicated circuit for DC conversion and amplifying the difference.

  Other objects and novel features of the present invention will be clarified in embodiments described later.

In order to achieve the above object, a current sensor according to an aspect of the present invention includes a closed magnetic circuit core that generates a magnetic flux due to a current to be measured;
An excitation winding and a feedback winding applied to the closed magnetic circuit core;
A series circuit of an AC voltage source and a resistor connected between both ends of the excitation winding;
A positive-side peak detection circuit for detecting a positive-side peak value of the AC voltage generated in the excitation winding, and a negative-side peak detection circuit for detecting a negative-side peak value of the AC voltage;
Magnetic balance in which a feedback current is passed through the feedback winding so that the difference between the positive peak value detected by the positive peak detection circuit and the negative peak value detected by the negative peak detection circuit is zero. And an amplifier for use.

  In the current sensor, in the magnetic equilibrium state, the positive peak value and the negative peak value are equal, and the minor loop drawn in the BH curve of the closed magnetic circuit core is the first centered on the origin 0. The quadrant and the third quadrant are always symmetrical.

  The current sensor according to the present invention does not use a magnetically sensitive element such as a Hall element, and performs current detection using the characteristic of a BH curve (for example, FIG. 2) of a closed magnetic circuit core that generates a magnetic flux due to a current to be measured. The symmetry of the positive and negative waveforms around the origin 0 of the BH curve is independent of temperature. For this reason, since the offset variation due to the temperature change when using a magnetically sensitive element such as a Hall element is not affected, the stability against the temperature change can be ensured.

  In Patent Document 1, two cores and five windings are required. However, the current sensor of the present invention can be halved to one core and two windings, and can be reduced in size and weight.

  Furthermore, in Patent Document 2, a complex DC voltage detection device including a waveform shaping circuit or the like is used, but the current sensor of the present invention can be controlled by detecting only the peak value, thereby realizing a simpler circuit configuration. it can.

  Hereinafter, as the best mode for carrying out the present invention, an embodiment of a current sensor will be described with reference to the drawings.

  FIG. 1 is a circuit diagram showing the overall configuration of a magnetic balance type current sensor, FIGS. 2 to 4 are explanatory diagrams showing the operation principle, and FIG. 5 is a waveform diagram of each part in the circuit diagram of FIG.

  In FIG. 1, reference numeral 1 denotes a closed magnetic circuit core as a measurement core, on which an excitation winding 2 and a feedback winding 3 are wound. The closed magnetic circuit core 1 is provided with a current path 10 through which a measured current Ip serving as a DC bias current flows (through in the case of illustration).

  A series circuit of an oscillator (oscillator) 4 serving as an AC voltage source and a load resistor (series resistor) 5 is connected between both terminals 2 a and 2 b of the excitation winding 2. The terminal 2b is connected to the ground.

  A terminal 2a of the exciting winding 2 has a positive peak detecting circuit 6 for detecting the positive peak value of the alternating voltage generated in the exciting winding 2, and a negative peak detecting circuit 7 for detecting the negative peak value of the alternating voltage. Is connected to the feedback winding 3 so that the difference between the positive peak value detected by the positive peak detection circuit 6 and the negative peak value detected by the negative peak detection circuit 7 becomes zero. Is provided. A detection resistor 9 for taking out the sensor output voltage is inserted in series with the feedback winding 3. Here, it is assumed that both the positive peak value and the negative peak value have a positive sign.

  Here, prior to the description of the overall operation of the circuit diagram of FIG. 1, the principle of current detection will be described with reference to FIGS.

  FIG. 2 is an example of the BH curve (hysteresis curve) of the closed magnetic circuit core 1 used in the present embodiment. As the core material, the coercive force (Hc) is small and the relative permeability is sufficiently large (for example, 100,000 or more) is desirable, and examples thereof include amorphous alloys and permalloy.

  In the present embodiment, as a specific example of the closed magnetic circuit core 1, PC permalloy (initial permeability 60,000, maximum permeability 180,000, saturation magnetic flux density 0.65 [T], coercive force 1.2 [A / m ] With no gap.

  The DC characteristic of the closed magnetic circuit core 1 is shown by a major loop in FIG. 2. When an AC bias is applied to the excitation winding 2 wound around this core, a small minor loop is drawn in the major loop as described below.

(A) When the DC bias to the closed magnetic circuit core 1 is zero (that is, the current to be measured Ip = 0), when an AC bias is applied to the excitation winding 2, the center is Oa (matches the origin 0) in FIG. As shown in FIG. 3, the operation is performed on a minor loop (a) that is point-symmetric (a figure rotated by 180 degrees overlaps) in the first and third quadrants.

(B) Next, as shown in FIG. 3, when a positive DC bias current (that is, positive measured current + Ip [A]) flows through the closed magnetic circuit core 1, the magnetic field inside the closed magnetic circuit core 1 is reduced. Strength H1 is
H1 = + Ip × 1 [turn] / L [A / m]
However, L [m]: the average magnetic path length of the closed magnetic circuit core, and the measured current Ip flow through the closed magnetic circuit core 1 (one turn).

  As shown in FIG. 2B, the minor loop that operates is asymmetric with respect to Ob.

(C) When a negative DC bias current (that is, negative measured current −Ip [A]) flows through the closed magnetic circuit core 1, an asymmetric minor loop centered on Oc as in (b). Draw.

  4A, 4B, and 4C depict the minor loop (a), the minor loop (b), and the minor loop (c), respectively, with their centers Oa, Ob, and Oc as the centers.

In the case of FIG. 4A, the minor loop is point-symmetric with respect to the center Oa (the figures rotated by 180 degrees overlap), and | B _A1 | = | B _A2 |. For (B), the minor loop is asymmetrical with respect to the center _{Ob, | B B1 | <|} B B2 |, in the case of (C), a minor loop for center Oc is asymmetric, _{| B C1} |> | B _C2 |. Also, the minor loops in FIGS. 4B and 4C have different shapes.

In the current sensor of FIG. 1, when the measured current Ip flows through the current path 10 and is in the state of FIG. 4B or FIG. The measured current Ip is measured from the current value of the feedback current Is. That is, the magnetic field zero in the closed magnetic circuit core 1 can be detected from | B _A1 | = | B _A2 | as shown in FIG. 4A. At that time, according to the “equal ampere-turn principle”,
Ip × Np = Is × Ns (1)
(However, Np: the number of turns of the current path 10 of the current Ip to be measured (one turn in the case of passing through the closed magnetic circuit core 1 once), Ns: the number of turns of the feedback winding 3.
And the measured current Ip is
Ip = Is × Ns / Np (2)
It becomes.

  In the current sensor of FIG. 1, the number of turns of the excitation winding 2 wound around the closed magnetic circuit core 1 as the measurement core is, for example, 50 turns, and the number of turns of the feedback winding 3 is, for example, 10 turns.

  A series circuit of an oscillator 4 and a load resistor 5 as an AC voltage source for applying an AC bias is connected to the excitation winding 2. The reason why the load resistor 5 is inserted is that the oscillator 4 can be regarded as a constant voltage source. Therefore, when the permeability of the closed magnetic circuit core 1, that is, the impedance of the excitation winding 2 changes, both terminals of the excitation winding 2 are changed. This is because the waveform between 2a and 2b changes with the voltage drop of the load resistor 5. The oscillation frequency of the oscillator 4 is several tens kHz to several hundreds kHz, and the oscillation waveform is most preferably a sine wave, and a trapezoidal wave approximated to a triangular wave or a sine wave can also be used.

  One terminal 2 b of the excitation winding 2 is set to the ground level, and the induced voltage appearing on the other terminal 2 a is applied to the positive peak detection circuit 6 and the negative peak detection circuit 7.

  The positive-side peak detection circuit 6 includes an operational amplifier OP1, a diode D1 that rectifies its output voltage, and a capacitor C1 that is charged with the rectified voltage of the diode D1, and a terminal of the capacitor C1 that holds the positive-side peak value. A voltage is input to one input terminal of the magnetic balance amplifier 8.

  The negative-side peak detection circuit 7 includes an inverting amplifier 11 (inverter: composed of an operational amplifier OP2 and resistors 12 and 13) having an amplification factor 1 for inverting the negative waveform to the positive side, an operational amplifier OP3, and its output voltage. A diode D2 for rectification and a capacitor C2 charged with a rectified voltage of the diode D2 are included. The terminal voltage of the capacitor C2 holding the negative peak value (absolute value) is input to the other input terminal of the magnetic balance amplifier 8.

  The magnetic balance amplifier 8 feeds a feedback current to the feedback winding 3 so that the DC voltage value at the output terminal 6c of the positive peak detection circuit 6 is equal to the DC voltage value at the output terminal 7d of the negative peak detection circuit 7. Let Is flow.

Then, in the closed magnetic circuit core 1, the above equation (1) is established according to the “equal ampere-turn principle”. For example, when the number of turns Np of the current path 10 through which the measured current Ip flows 2 [A] is 1 turn and the number of turns Ns of the feedback winding 3 is 10 turns, the feedback current Is = 0.2 [A].
1 [turn] × 2 [A] = 10 [turn] × 0.2 [A] = 2 [AT]

  The measured value of the feedback current Is can be taken out as a sensor output voltage proportional to the measured current Ip from both ends of the detection resistor 9 connected in series to the feedback winding 3.

  The waveform of each part of the current sensor of FIG. 1 is shown in FIG. FIG. 5 (1) shows a state where the measured current Ip = 0 [A] in FIG. 2 (a) is in a magnetic equilibrium state. In this case, the positive peak value m [V] and the negative peak value n [V] are equal, and the feedback current Is = 0.

  FIG. 5 (2) shows a state where the measured current Ip = + 2 [A] is in an unbalanced state in the state of FIG. 2 (b). At this time, the negative peak value q [V] becomes larger than the positive peak value p [V], and the output terminal 6c of the positive peak detection circuit 6 and the output terminal 7d of the negative peak detection circuit 7 in FIG. The voltage values are different (the voltage value of the output terminal 6c <the voltage value of the output terminal 7d), and the feedback current is fed to the feedback winding 3 so that the equation (1) is satisfied and the magnetic equilibrium state of FIG. Will be swept away.

  FIG. 5 (3) shows a state where the measured current Ip = −2 [A] is in an unbalanced state in the state of FIG. 2 (c). At this time, the positive peak value r [V] is larger than the negative peak value s [V], and the voltage values of the output terminal 6c and the output terminal 7d are different (voltage value of the output terminal 6c> output terminal 7d). In this case, the feedback current is passed through the feedback winding 3 so that the magnetic equilibrium state shown in FIG. When the feedback current Is flows and the magnetic equilibrium state is reached, the Ob point and the Oc point return to the origin 0 in the BH curve of FIG. 2, and m = n again.

  Therefore, even if the measured current Ip changes, the feedback current Is flows automatically and accurately so that the magnetic balance amplifier 8 of FIG. 1 is always in a magnetic equilibrium state, and the measured current Ip is monitored with high accuracy. Possible current sensors can be constructed.

  According to this embodiment, the following effects can be obtained.

(1) Current detection is performed using the characteristics of the BH curve (for example, FIG. 2) of the closed magnetic circuit core 1 in which the magnetic flux generated by the current Ip to be measured is generated without using a magnetic sensitive element such as a Hall element The symmetry of the positive and negative waveforms around the origin 0 of the BH curve does not depend on temperature. For this reason, since the offset variation due to the temperature change when using a magnetically sensitive element such as a Hall element is not affected, the stability against the temperature change can be ensured. Therefore, highly accurate current detection that is not affected by the ambient temperature is possible.

(2) In Patent Document 1, two cores and five windings are required. However, the current sensor according to the present embodiment can be halved to one core and two windings, and can be reduced in size and weight.

(3) In Patent Document 2, a complex DC voltage detection device including a waveform shaping circuit is used. However, the current sensor according to the present embodiment can be controlled by detecting only the peak value, so that the circuit configuration is simpler. Can be realized.

  The positive side and negative side peak detection circuits are not limited to the circuit of FIG. 1 using operational amplifiers, and may have other circuit configurations.

  Although the embodiments of the present invention have been described above, it will be obvious to those skilled in the art that the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the claims.

It is a circuit diagram showing an embodiment of a current sensor according to the present invention. It is explanatory drawing which shows the BH curve (hysteresis curve) of the closed magnetic circuit core used by embodiment of this invention, and the minor loop inside it. It is explanatory drawing which shows the direct-current bias current (current to be measured) which flows into a closed magnetic circuit core, the strength of the magnetic field in a core, etc. It is explanatory drawing which shows the relationship between DC bias current and a minor loop. It is a wave form diagram explaining the waveform of each part of the circuit diagram of an embodiment about a magnetic equilibrium state and a non-equilibrium state.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Closed magnetic circuit core 2 Excitation winding 3 Feedback winding 4 Oscillator 5 Load resistance 6 Positive side peak detection circuit 7 Negative side peak detection circuit 8 Magnetic balance amplifier 9 Detection resistance 10 Current path 11 Inversion amplifier C1, C2 Capacitor D1, D2 Diode OP1, OP2, OP3 operational amplifier

Claims (2)

A closed magnetic circuit core that generates magnetic flux due to the current to be measured;
An excitation winding and a feedback winding applied to the closed magnetic circuit core;
A series circuit of an AC voltage source and a resistor connected between both ends of the excitation winding;
A positive-side peak detection circuit for detecting a positive-side peak value of the AC voltage generated in the excitation winding, and a negative-side peak detection circuit for detecting a negative-side peak value of the AC voltage;
Magnetic balance in which a feedback current is passed through the feedback winding so that the difference between the positive peak value detected by the positive peak detection circuit and the negative peak value detected by the negative peak detection circuit is zero. A current sensor.   In the magnetic equilibrium state, the positive peak value and the negative peak value are equal, and the minor loop drawn in the BH curve of the closed magnetic circuit core has a first quadrant and a third quadrant with the origin 0 as the center. 2. The current sensor according to claim 1, which is always symmetrical.

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