Amplifier, and rogowski type current sensor

Abstract

To provide an amplifier that has a long time constant, and to provide a Rogowski type current sensor that has the amplifier.SOLUTION: An amplifier has: an integration circuit composed of a first op-amp 21; and an amplifier circuit composed of a second op-amp 31 connected to a latter part of the integration circuit. The amplifier is configured to: connect an output terminal of the first op-amp 21 to a non-inverted input terminal of the second op-amp 31; provide, in the first op-amp 21, a first feedback capacitor 22, and a second feedback resistor 23; provide, in the second op-amp 31, a second feedback capacitor 32, a second feedback resistor 33, and an input resistor 34 having one end connected to an inverted input terminal and the other end grounded; and satisfy Rf1 Cf1=K Rin2 Cf2, in which Cf1: a capacity of the first feedback capacitor 22, Rf1: a resistance value of the first feedback resistor 23, Cf2: a capacity of the second feedback capacitor 32, Rin2: a resistance value of the input resistor 34, and K: an arbitrary real number equal to or more than 0.9 and equal to or less than 1.1.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an amplifier and a Rogowski type current sensor.

Background Art

[0002] The inventor has been developing a Rogowski type current sensor, creating a Rogowski coil using a printed circuit board or the like (see, for example, Patent Document 1), and connecting an amplifier to this Rogowski coil to form a small current sensor.

[0003] The Rogowski coil is an air-core coil. By crossing the conductor to be measured with the Rogowski coil, an induced voltage is generated in the Rogowski coil by the magnetic field generated by the alternating current flowing through the conductor, and this induced voltage is output as the time derivative value of the current flowing through the conductor, thereby serving as a current sensor.

[0004] That is, current measurement is made possible by inputting the induced voltage generated in the Rogowski coil into an integration circuit. This integration circuit is an integration circuit using an operational amplifier, and the output of the integration circuit is amplified by an amplifier circuit and then used (see, for example, Patent Document 2). The amplifier circuit can also be composed of an operational amplifier, and usually, this integration circuit and amplifier circuit serve as an amplifier for the Rogowski coil.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the operational amplifier that constitutes the integrating circuit, a feedback capacitor is connected between the inverting input terminal and the output terminal. However, when attempting to increase the time constant by increasing the capacitance of this feedback capacitor, there was a problem in that the gain of the operational amplifier would decrease, making it more susceptible to signal noise. Moreover, this signal noise was to be amplified by the amplifier circuit at the subsequent stage of the integrating circuit. Therefore, normally, the capacitance of the feedback capacitor was set in consideration of the measurement accuracy required depending on the measurement target. Consequently, due to the difficulty of increasing the time constant, sufficient sensitivity could not be obtained when the frequency of the alternating current of the measurement target was low.

[0007] Under such circumstances, the inventor conducted research and development to improve the sensitivity when the frequency of the alternating current of the measurement target was low. By utilizing the amplifier circuit at the subsequent stage of the integrating circuit, it became possible to set a long time constant, thereby achieving an improvement in sensitivity when the frequency of the alternating current of the measurement target was low, leading to the present invention.

[0008] That is, the present invention provides an amplifier with a long time constant and a Rogowski-type current sensor using this amplifier.

Means for Solving the Problems

[0009] The amplifier of the present invention is an amplifier having an integrating circuit composed of a first operational amplifier and an amplifier circuit composed of a second operational amplifier connected to the subsequent stage of the integrating circuit. The output terminal of the first operational amplifier is connected to the non-inverting input terminal of the second operational amplifier. The first operational amplifier is provided with a first feedback capacitor and a first feedback resistor, and the second operational amplifier is provided with a second feedback capacitor, a second feedback resistor, and an input resistor having one end connected to the inverting input terminal and the other end grounded, and is made to satisfy the following conditions. Cf1: Capacitance of the first feedback capacitor, Rf1: Resistance value of the first feedback resistor, Cf2: Capacitance of the second feedback capacitor, Rin2: Resistance value of the input resistor Let K be any real number such that 0.9 ≤ K ≤ 1.1, Rf1·Cf1 = K·Rin2·Cf2.

[0010] Further, in the Rogowski type current sensor of the present invention, in the Rogowski type current sensor having a Rogowski coil and an amplifier connected to the output terminal of this Rogowski coil, the amplifier includes an integrating circuit composed of a first operational amplifier and an amplification circuit composed of a second operational amplifier connected to the subsequent stage of the integrating circuit. The output terminal of the first operational amplifier is connected to the non-inverting input terminal of the second operational amplifier. A first feedback capacitor and a first feedback resistor are provided for the first operational amplifier, and a second feedback capacitor, a second feedback resistor, and an input resistor whose one end is connected to the inverting input terminal and the other end is grounded are provided for the second operational amplifier. The amplifier is configured to satisfy the following conditions. Cf1: Capacitance of the first feedback capacitor, Rf1: Resistance value of the first feedback resistor, Cf2: Capacitance of the second feedback capacitor, Rin2: Resistance value of the input resistor Let K be any real number such that 0.9 ≤ K ≤ 1.1, Rf1·Cf1 = K·Rin2·Cf2 (Equation 1).

[0011] Furthermore, the Rogowski type current sensor of the present invention also has the following features. (1) In Equation 1, "0.95 ≤ K ≤ 1.05". (2) In Equation 1, "1 - α < K < 1 + β (0 < β < α)". (3) The displayed value of Cf2 is a power of 10 times the displayed value of Cf1, and the displayed value of Rf1 is the power of 10 times the displayed value of Rin2. (4) The displayed value of Cf2 is a power of 10 times the displayed value of Rf1, and the displayed value of Cf1 is the power of 10 times the displayed value of Rin2.

Advantages of the Invention

[0012] According to the present invention, an amplifier with a long time constant and a Rogowski type current sensor using this amplifier can be provided, and an amplifier with high sensitivity and a Rogowski type current sensor using this amplifier can be provided even when the frequency of the alternating current to be measured is low.

Brief Description of Drawings

[0013]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0014] The amplifier of the present invention is an amplifier used in a Rogowski type current sensor. Hereinafter, embodiments of the Rogowski type current sensor will be described, and at the same time, embodiments of the amplifier will be described.

[0015] The Rogowski type current sensor of this embodiment is composed of a Rogowski coil 1, an amplifier 2, and a control unit 4, as shown in FIG. 1.

[0016] The Rogowski coil 1 is formed as a laminated printed circuit board in which a plurality of printed circuit boards are laminated, and the amplifier 2 and the control unit 4 are mounted on this laminated printed circuit board to form a small Rogowski type current sensor. Note that, as a laminated printed circuit board type Rogowski coil provided with the amplifier 2 without providing the control unit 4, this laminated printed circuit board type Rogowski coil may be arranged in an appropriate measurement area, and appropriate wiring may be provided from the laminated printed circuit board type Rogowski coil to the control unit 4.

[0017] The control unit 4 identifies the current value from the signal output from the amplifier 2 and can display the current value on a display (not shown) provided in the control unit 4 or the like. Further, as will be described later, the control unit 4 is provided with a reset button (not shown), and when this reset button is operated, the reset process of the amplifier 2 is performed, and then current measurement is carried out.

[0018] The amplifier 2, which is the main part of the present invention, has an integrating circuit composed of a first operational amplifier 21 and an amplifying circuit composed of a second operational amplifier 31 connected to the subsequent stage of this integrating circuit. The output terminal of the first operational amplifier 21 is connected to the non-inverting input terminal of the second operational amplifier 31.

[0019] The first operational amplifier 21 is provided with a first feedback capacitor 22 and a first feedback resistor 23. One end of each of the first feedback capacitor 22 and the first feedback resistor 23 is connected to the output terminal of the first operational amplifier 21, and the other end is connected to the inverting input terminal of the first operational amplifier 21.

[0020] Also, the inverting input terminal of the first operational amplifier 21 is connected to the Rogowski coil 1 via a first resistor 24, and integration processing is performed on the output signal of the Rogowski coil 1. The non-inverting input terminal of the first operational amplifier 21 is grounded via a second resistor 25.

[0021] The second operational amplifier 31 is provided with a second feedback capacitor 32 and a second feedback resistor 33, and further, an input resistor 34 whose one end is connected to the inverting input terminal and the other end is grounded is provided. One end of each of the second feedback capacitor 32 and the second feedback resistor 33 is connected to the output terminal of the second operational amplifier 31, and the other end is connected to the inverting input terminal of the second operational amplifier 31.

[0022] A third resistor 26 is provided between the output terminal of the first operational amplifier 21 and the non-inverting input terminal of the second operational amplifier 31. Also, a fourth resistor 35 is provided between the output terminal of the second operational amplifier 31 and the control unit 4.

[0023] Here, Cf1: Capacitance of the first feedback capacitor 22, Rf1: Resistance value of the first feedback resistor 23, Cf2: Capacitance of the second feedback capacitor 32, Rin2: Resistance value of the input resistor 34 Let them be as such, and let K be an arbitrary real number between 0.9 and 1.1 inclusive.

[0024] In the amplifier 2 of this embodiment, Rf1·Cf1 = K·Rin2·Cf2 (Equation 1) is set to be satisfied.

[0025] If using another expression method, 0.9 ≦ Rf1·Cf1 / Rin2·Cf2 ≦ 1.1 is used.

[0026] Preferably, 0.95 ≦ K ≦ 1.05 and more preferably, 0.98 ≦ K ≦ 1.02 is the case.

[0027] Ideally, K = 1, that is, Rf1·Cf1 = Rin2·Cf2 However, when measuring a rectangular current (a waveform that rises stepwise and then becomes a constant current), an increasing waveform may appear in the constant current part, and there is a possibility of misjudging the maximum value of the current. Therefore, in practical use, 0.9 ≦ K ≦ 1.1 within the range of, and 1-α < K < 1+β (0<β<α) it is preferably adjusted to be such. More specifically, for example, when α = 0.5 and β = 0.3, 0.95 ≦ K ≦ 1.03 such an adjustment is preferable.

[0028] Incidentally, when "Rf1·Cf1 = Rin2·Cf2", for example, in the switching of power semiconductors that are frequently used these days, when a current with a rectangular wave-shaped waveform is flowing as the measurement target, the waveform of the output voltage Vint of amplifier 2 can theoretically be the same rectangular wave as the waveform of the current being measured.

[0029] This will be explained in detail. First, let "A" be a constant determined from the cross-sectional area of the coil in Rogowski coil 1, the winding pitch, the resistance value Rin1 of first resistor 24, and the capacitance Cf1 of first feedback capacitor 22. The resistance value Rin1 of first resistor 24 is the resistance value taking into account the internal resistance of Rogowski coil 1.

[0030] First, as shown in the current waveform of the measurement target in Fig. 2(a), it is assumed that the current flows stepwise from 0 A to 1 A. At this time, the voltage Vcoil generated in Rogowski coil 1 will instantaneously generate a voltage, as shown in Fig. 2(b).

[0031] In the integration circuit of amplifier 2, this voltage Vcoil is integrated, and the output voltage Vo1 of the first operational amplifier 21 constituting the integration circuit is given by the following formula.

[0032]

Equation

[0033] That is, the graph of the output voltage Vo1 of the first operational amplifier 21 becomes as shown in Fig. 2(c). First, after reaching the peak magnitude A, the voltage drops with the time constant Cf1·Rf1.

[0034] In the second operational amplifier 31 where the output voltage Vo1 of this first operational amplifier 21 is input to the inverting input terminal, the voltage Vcf2 generated in the second feedback capacitor 32 is given by the following formula.

[0035]

Equation

[0036] That is, the graph of the voltage Vcf2 generated in the second feedback capacitor 32 is as shown in Fig. 2(d). In the second operational amplifier 31 that constitutes the amplifier circuit, when Rf2 is the resistance value of the second feedback resistor 33, the output voltage Vo1 of the first operational amplifier 21 is integrated with a gain of 1 / Cf2·Rf2.

[0037] Therefore, the output voltage Vint of amplifier 2 is "Vint = Vo1 + Vcf2", and it becomes as follows.

[0038]

Equation

[0039]

Equation

[0040] That is, the graph of the output voltage Vint of amplifier 2 is as shown in Fig. 2(e), and it becomes the same waveform as the current waveform to be measured.

[0041] Here, when "Rf1·Cf1 / Rin2·Cf2 = 0.9", the graph of the output voltage Vint of amplifier 2 is as shown in Fig. 2(f), and when "Rf1·Cf1 / Rin2·Cf2 = 1.1", the graph of the output voltage Vint of amplifier 2 is as shown in Fig. 2(g), but it is within the allowable range in practical use.

[0042] It can be made more suitable by setting "0.95 ≦ K ≦ 1.05". Furthermore, by setting "0.98 ≦ K ≦ 1.02", the optimal state in practical use can be achieved.

[0043] In particular, in amplifier 2, since the substantial time constant of the entire amplifier 2 is determined by "Cf2·Rf2", on the integrator side, the gain of the integrator is increased by setting the capacitance Cf1 of the first feedback capacitor 22 to a small value, while on the subsequent amplifier circuit side, the time constant can be increased by setting the capacitance Cf2 of the second feedback capacitor 32 to a large value. As a result, as amplifier 2 having a long time constant, the sensitivity at low frequencies can be improved.

[0044] Also, in amplifier 2, by reducing the gain at high frequencies of the integrator, a signal with high-frequency components attenuated is input to the subsequent amplifier circuit. As a result, the delay generated in amplifier 2 can be limited to only the delay occurring in the integrator, and even in the case of amplifier 2 having a two-stage configuration, a large delay is not generated, and the possibility of the delay caused by amplifier 2 becoming a problem can be eliminated.

[0045] When setting the resistance value Rf1 of the first feedback resistor 23, the capacitance Cf1 of the first feedback capacitor 22, the resistance value Rin2 of the input resistor 34, and the capacitance Cf2 of the second feedback capacitor 32 such that "Rf1·Cf1 = Rin2·Cf2", by using capacitors and resistors where the displayed value of Cf2 is a power of 10 times the displayed value of Cf1 and the displayed value of Rf1 is the same power of 10 times the displayed value of Rin2, it becomes easier to find combinations that satisfy the above conditions. Also, it becomes easier to obtain components that form a highly accurate combination of resistors and capacitors. That is, in commercially available capacitors and resistors, the first two digits of the capacitance value and the resistance value are often discrete skip values such as 10, 12, 15, 18 ···, so it seems difficult to satisfy Rf1·Cf1 = Rin2·Cf2 with high accuracy in other combinations.

[0046] Here, the "displayed value" means paying attention only to the numerical part in the basic unit F (farad) for capacitors and the basic unit Ω (ohm) for resistors. For example, in the above case, the resistance value Rf1 of the first feedback resistor 23 = 30×10 3 (Ω), The capacitance Cf1 of the first feedback capacitor 22 = 1×10 -9 (F), The resistance value Rin2 of the input resistor 34 = 3×10 3 (Ω), The capacitance Cf2 of the second feedback capacitor 32 = 10×10 -9 (F) It can be a combination of capacitors and resistors that are easily available, such as the above.

[0047] Alternatively, the displayed value of Cf2 may be a power of 10 times the displayed value of Rf1, and the displayed value of Cf1 may be the above power of 10 times the value of Rin2. In this case, The capacitance Cf1 of the first feedback capacitor 22 = 1×10 -9 (F), The resistance value Rf1 of the first feedback resistor 23 = 30×10 3 (Ω), The capacitance Cf2 of the second feedback capacitor 32 = 30×10 -9 (F), The resistance value Rin2 of the input resistor 34 = 1×10 3 (Ω) It can be a combination of capacitors and resistors that are easily available, such as the above.

Example

[0048] Finally, a specific example will be described while showing FIG. 3. For the same configuration as the circuit diagram shown in FIG. 1, the same reference numerals are used, and duplicate explanations are omitted.

[0049] In the Rogowski type current sensor of this embodiment, the Rogowski coil 1 is composed of a multilayer printed circuit board, and the amplifier 2 and the control unit 4 are mounted on this multilayer printed circuit board.

[0050] The control unit 4 of this embodiment has reset means, and the first feedback capacitor 22 of the first operational amplifier 21 and the second feedback capacitor 32 of the second operational amplifier 31 described later are reset by the reset signal output from the control unit 4, and current measurement is to be performed.

[0051] The amplifier 2 includes an integrating circuit composed of a first operational amplifier 21 and an amplifying circuit composed of a second operational amplifier 31 connected to the subsequent stage of this integrating circuit. The first operational amplifier 21 and the second operational amplifier 31 are of model number LT1818 respectively.

[0052] The first feedback capacitor 22 connected to the first operational amplifier 21 is 1 nF, and the first feedback resistor 23 connected to the first operational amplifier 21 is 30 kΩ. The first resistor 24 and the second resistor 25 are both 100 Ω.

[0053] The second feedback capacitor 32 connected to the second operational amplifier 31 is 10 nF, and the second resistor 34 is 3 kΩ. The third resistor 26 and the fourth resistor 35 are both 51 Ω.

[0054] A first switch 27 is provided in parallel connection with the first feedback capacitor 22, and a second switch 37 is provided in parallel connection with the second feedback capacitor 32. When a reset signal from the control unit 4 is input, the first switch 27 and the second switch 37 make the output terminal and the inverting input terminal of the first operational amplifier 21 have the same potential, and also make the output terminal and the inverting input terminal of the second operational amplifier 31 have the same potential, thereby resetting the first feedback capacitor 22 and the second feedback capacitor 32. The first switch 27 and the second switch 37 are of model ADG1201.

[0055] A fifth resistor 28 is provided between the control unit 4 and the first switch 27, and a sixth resistor 38 is provided between the control unit 4 and the second switch 37. The fifth resistor 28 and the sixth resistor 38 are both 100 Ω.

[0056] In the Rogowski type current sensor configured as described above, current measurement can be performed with high sensitivity from an alternating current with a low frequency. In this embodiment, no second feedback resistor is provided, but with the above circuit configuration, a desired output can be obtained even without the second feedback resistor.

Explanation of symbols

[0057] 1 Logos coil 2 Amplifier 4 Control unit 21 First operational amplifier 22 First feedback capacitor 23 First feedback resistor 24 First resistor 25 Second resistor 26 Third resistor 27 First switch 28 Fifth resistor 31 Second operational amplifier 32 Second feedback capacitor 33 Second feedback resistor 34 Input resistor 35 Fourth resistor Cf1 Capacitance of the first feedback capacitor Cf2 Capacitance of the second feedback capacitor Rf1 Resistance value of the first feedback resistor Rf2 Resistance value of the second feedback resistor Rin2 Resistance value of the input resistor 37 Second switch 38 Sixth resistor

Claims

1. An amplifier comprising an integrating circuit composed of a first operational amplifier, and an amplifying circuit composed of a second operational amplifier connected to the subsequent stage of the integrating circuit, wherein the output terminal of the first operational amplifier is connected to the non-inverting input terminal of the second operational amplifier, the first operational amplifier is provided with a first feedback capacitor and a first feedback resistor, the second operational amplifier is provided with a second feedback capacitor, a second feedback resistor, and an input resistor having one end connected to the inverting input terminal and the other end grounded, Cf1: capacitance of the first feedback capacitor, Rf1: resistance value of the first feedback resistor, Cf2: capacitance of the second feedback capacitor, Rin2: resistance value of the input resistor and, letting K be an arbitrary real number satisfying 0.9 or more and 1.1 or less, Rf1·Cf1 = K·Rin2·Cf2 An amplifier satisfying the above.

2. In a Rogowski coil type current sensor having a Rogowski coil and an amplifier connected to the output terminal of the Rogowski coil, the amplifier comprises an integrating circuit composed of a first operational amplifier, and an amplifying circuit composed of a second operational amplifier connected to the subsequent stage of the integrating circuit, wherein the output terminal of the first operational amplifier is connected to the non-inverting input terminal of the second operational amplifier, the first operational amplifier is provided with a first feedback capacitor and a first feedback resistor, the second operational amplifier is provided with a second feedback capacitor, a second feedback resistor, and an input resistor having one end connected to the inverting input terminal and the other end grounded, Cf1: capacitance of the first feedback capacitor, Rf1: resistance value of the first feedback resistor, Cf2: capacitance of the second feedback capacitor, Rin2: resistance value of the input resistor and, letting K be an arbitrary real number satisfying 0.9 or more and 1.1 or less, Rf1·Cf1 = K·Rin2·Cf2 (Equation 1) A Rogowski coil type current sensor satisfying the above.

3. In the Equation 1 0.95 ≤ K ≤ 1.05 The Rogowski coil type current sensor according to Claim 2.

4. In the Equation 1 1 - α < K < 1 + β (0 < β < α) The Rogowski coil type current sensor according to Claim 2.

5. The display value of Cf2 is a power of 10 times the display value of Cf1, and the display value of Rf1 is the power of 10 times the display value of Rin2 The Rogowski coil type current sensor according to any one of Claims 2 to 4.

6. The display value of Cf2 is a power of 10 times the display value of Rf1, and the display value of Cf1 is the power of 10 times the display value of Rin2 The Rogowski type current sensor according to any one of claims 2 to 4.

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