Measuring machine

Abstract

To provide a measurement apparatus for allowing a contactless measurement to be made accurately and at low cost.SOLUTION: A measurement apparatus for measuring a DC voltage in a conductor covered by an insulation body includes a buffer circuit. The buffer circuit includes: a guard electrode for continuously inputting a leakage current leaking through the insulation body from the conductor, through a measurement electrode in contact with the insulation body and outputting a voltage which corresponds to the voltage of the leakage current, and approaching the measurement electrode; and an amplification circuit for applying a voltage which corresponds to the output voltage in the buffer circuit to the guard electrode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to measurement instruments. [Background technology]

[0002] Generally, DC voltages flowing through electric wires (conductors) covered with an insulator are measured. For example, such DC voltage measurements are performed without bringing a measuring device into contact with the electric wire to be measured (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2016-080537 Summary of the Invention

[0004] It is desirable that the above-described non-contact measurement of DC voltage be performed with high accuracy and low cost even during operation of an electronic device or the like to which the electric wire is connected. [Means for solving the problem]

[0005] In order to achieve the above-mentioned non-contact measurement, the measuring instrument of the present invention is a measuring instrument that measures a DC voltage in a conductor covered with an insulator, and includes a buffer circuit. The buffer circuit continuously inputs a leakage current leaking from the conductor through the insulator via a measurement electrode that contacts the insulator, outputs a voltage corresponding to the voltage of the leakage current, and includes a guard electrode close to the measurement electrode and an amplifier circuit that applies a voltage corresponding to the output voltage of the buffer circuit to the guard electrode. [Effects of the Invention]

[0006] The measuring device of the present invention makes it possible to perform non-contact measurement with high precision and at low cost. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram of an electric wire 1 and a measuring device 10 according to the first embodiment. [Figure 2] FIG. 2 is a configuration diagram of the electric wire 1 according to the first embodiment. [Figure 3] FIG. 3 is a detailed configuration diagram of the measuring device 10 according to the first embodiment. [Figure 4] FIG. 4 is a detailed configuration diagram of the buffer circuit 6. [Figure 5] FIG. 5 is a schematic diagram of the electric wire 1 and the measuring device 20 according to the second embodiment. [Figure 6] FIG. 6 is a configuration diagram of the electric wire 1 according to the second embodiment. [Figure 7] FIG. 7 is a detailed configuration diagram of the measuring device 20 according to the second embodiment. [Figure 8] FIG. 8 is a diagram illustrating a modified example of the second embodiment. [Figure 9] FIG. 9 is a diagram illustrating a modified example of the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. However, the technical scope of the present invention is not limited to these preferred embodiments.

[0009] [Schematic configuration of electric wire and measuring device in the first embodiment] First, the schematic configuration of the electric wire 1 and the measuring device 10 in the first embodiment will be described. Fig. 1 is a schematic diagram of the electric wire 1 and the measuring device 10 in the first embodiment. Fig. 2 is a diagram of the electric wire 1 in the first embodiment. Specifically, Fig. 2(A) is a diagram of the electric wire 1 when a measuring electrode 3 is attached, and Fig. 2(B) is a cross-sectional view of the electric wire 1 shown in Fig. 2(A) taken along line AA.

[0010] In the example shown in FIG. 1, measuring device 10 is a non-contact voltage measuring device that includes, for example, a buffer circuit 6 and an output circuit 6a configured with a voltmeter (not shown) or the like, and measures a DC voltage Vx (hereinafter simply referred to as voltage Vx) occurring in an electric wire 1 (core wire) covered with an insulator 2. Buffer circuit 6 is, for example, an amplifier circuit with a gain of 1. Measuring device 10 measures voltage Vx in electric wire 1 via an electrode 3 (hereinafter also referred to as measurement electrode 3) attached to insulator 2. Specifically, as shown in FIG. 1, measuring device 10 measures voltage Vx (e.g., a voltage from 1 (V) to 5 (V)) of an instrumentation signal transmitted from an isolator 4, which is an instrumentation device.

[0011] The measurement electrode 3 is, for example, a copper foil. As shown in Fig. 2(A), the measurement electrode 3 is attached to the insulator 2 so as to go around the insulator 2 in the circumferential direction of the insulator 2. That is, when the measurement electrode 3 is attached to the insulator 2, it forms, for example, a cylindrical shape that follows the outer periphery of the insulator 2.

[0012] When the measuring electrode 3 is attached to the insulator 2, a small capacitance Cx (hereinafter also referred to as insulation capacitance Cx) and a large resistance Rx (hereinafter also referred to as insulation resistance Rx) are formed in parallel between the electric wire 1 (core wire) and the measuring electrode 3. Therefore, the measuring device 10 in this embodiment measures the voltage Vx in the electric wire 1 by extracting an instrumentation signal (hereinafter also referred to as leakage current) that passes through the insulation resistance Rx.

[0013] In this regard, a method for measuring the voltage Vx by using a transient current that flows in the insulating capacitance Cx when the measuring electrode 3 is pressed against the electric wire 1 is already known (see, for example, Patent Document 1).

[0014] However, this method uses a transient current that flows through the insulating capacitance Cx, so when measuring the voltage Vx for the second time, it is necessary to release (discharge) the charge on the insulating capacitance Cx after the first measurement, which means that the voltage Vx cannot be measured continuously.

[0015] In contrast, measuring device 10 in this embodiment uses a current that passes through insulation resistance Rx instead of the current that passes through insulation capacitance Cx, thereby eliminating the need to release the charge of insulation capacitance Cx, etc. Therefore, measuring device 10 in this embodiment can continuously measure voltage Vx.

[0016] In the example shown in FIG. 1, a return wire (not shown) may be provided between the isolator 4 and the isolator 5 together with the electric wire 1, for example.

[0017] [Detailed Configuration of Measuring Device in First Embodiment] Next, the detailed configuration of the measuring device 10 in the first embodiment will be described. FIG. 3 is a detailed configuration diagram of the measuring device 10 in the first embodiment. FIG. 4 is a detailed configuration diagram of the buffer circuit 6. Hereinafter, the measurement electrode 3 will be omitted. For convenience, the insulation resistance Rx will be described outside the insulator 2.

[0018] As shown in FIG. 3, the measuring device 10 includes, for example, a buffer circuit 6, a leakage resistor Rz, and an output circuit 6a.

[0019] 3, the current Ix flowing through the insulation resistance Rx and the current Iz flowing through the leakage resistance Rz can be expressed as (1) and (2) below. Hereinafter, the voltage input to the input terminal of the buffer circuit 6 (the voltage at the position between the insulation resistance Rx and the leakage resistance Rz) will be referred to as voltage V2, the voltage output from the output terminal of the buffer circuit 6 will be referred to as voltage Vy, and the voltage Ix flowing through the leakage resistance Rz will be referred to as voltage Vy. Input terminal The bias current input to is called current Ib.

[0020]

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[0021]

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[0022] The relationship between the current Ix, the current Iz, and the current Ib can be expressed as in the following equation (3).

[0023]

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[0024] Then, when the above formulas (1), (2), and (3) are used, the voltage V2 can be expressed as in the following formula (4).

[0025]

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[0026] In this regard, by transforming the above equation (4) into the following equation (5), it becomes possible to express it in a form in which both the voltage term (Vx) and the current term (Ib) are multiplied by a coefficient (Rz (Rz + Rx)).

[0027]

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[0028] Furthermore, the above equation (5) can be transformed into the following equation (6).

[0029]

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[0030] Here, in the above equation (6), for example, when the resistance value of the leakage resistance Rz is at a maximum, Rx / Rz becomes 0. Therefore, in this case, the voltage V2 is expressed as in the following equation (7).

[0031]

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[0032] That is, equation (7) indicates that a voltage drop occurs in voltage V2 according to current Ib and insulation resistance Rx. Therefore, for example, when current Ib is extremely small, it is possible to determine that voltage Vx and voltage V2 are approximately the same value.

[0033] Therefore, for example, the greater the leakage resistance Rz and the smaller the current Ib, the more accurately measuring voltage Vx is possible with measuring device 10 in this embodiment.

[0034] Furthermore, when the above equation (4) is used, the error voltage Verror between the voltages Vx and V2 (hereinafter also referred to as voltage Verror) can be expressed as in the following equation (8).

[0035]

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[0036] In this regard, by transforming the above equation (8) into the following equation (9), it becomes possible to express it in a form in which both the voltage term (Vx) and the current term (Ib) are multiplied by a coefficient (-Rx(Rz+Rx)).

[0037]

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[0038] Furthermore, the above equation (9) can be transformed into the following equation (10).

[0039]

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[0040] In the above equation (10), for example, when the resistance value of the leakage resistance Rz is maximum, Rz / Rx becomes maximum. Therefore, in this case, the voltage Verror can be expressed as the following equation (11).

[0041]

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[0042] As described above, when the resistance value of leakage resistor Rz is maximum (infinite), current Ix and current Ib are equal. Generally, the resistance value of leakage resistor Rz is very large, so current Ix and current Ib are close to each other.

[0043] 4, the buffer circuit 6 is, for example, an amplifier circuit with a gain of 1. The current Ib is input to the positive terminal of the buffer circuit 6, and the signal (voltage) output from the output terminal of the buffer circuit 6 and passed through a resistor Rf is fed back and input to the negative terminal of the buffer circuit 6.

[0044] [Schematic configuration of electric wire and measuring device in the second embodiment] Next, the schematic configuration of the electric wire 1 and the measuring device 20 in the second embodiment will be described. Fig. 5 is a schematic diagram of the electric wire 1 and the measuring device 20 in the second embodiment. Fig. 6 is a structural diagram of the electric wire 1 in the second embodiment. Specifically, Fig. 6(A) is a structural diagram of the electric wire 1 when the measuring electrode 3 is attached, and Fig. 6(B) is a BB cross-sectional view of the electric wire 1 shown in Fig. 6(A). Differences from the first embodiment will be described below. In the second embodiment, unlike the first embodiment, a guard electrode 31 is attached to the insulator 2 together with the measuring electrode 3 to measure the voltage Vx.

[0045] In the example shown in Fig. 5, the measuring device 20 is, for example, a non-contact voltage measuring device that has a buffer circuit 6 and an output circuit 6a and measures the voltage Vx occurring in an electric wire 1 (core wire) covered with an insulator 2. The measuring device 20 measures the voltage Vx in the electric wire 1 by using a measuring electrode 3 covered with a guard electrode 31. Below, a description will be given of a case where the measuring device 20 further includes an amplifier circuit 7, an offset correction circuit 7a, and a gain correction circuit 7b as shown in Fig. 5.

[0046] The guard electrode 31 is made of, for example, aluminum or SUS (stainless steel). The guard electrode 31 is attached to the insulator 2 so as to cover the measurement electrode 3. Specifically, when the measurement electrode 3 is attached to the insulator 2 so as to go around the insulator 2 along the circumferential direction of the insulator 2, the guard electrode 31 is attached to the insulator 2 so as to go around the insulator 2 and the measurement electrode 3 along the circumferential direction of the insulator 2 and the measurement electrode 3, as shown in FIG. 6. That is, when the guard electrode 31 is attached to the insulator 2, it may have a cylindrical or box-like shape. When the guard electrode 31 is cylindrical or box-like, it may have an open structure in which both ends in the longitudinal direction of the electric wire 1 are open (see FIG. 6(B)), or it may have a sealed structure in which both ends are closed.

[0047] The measuring device 20 then applies the voltage measured by the buffer circuit 6 (the measured value of the voltage Vx) to the guard electrode 31. This guarding allows the measuring device 20 to further increase the leakage resistance Rz. It also reduces the voltage difference between the measuring electrode 3 and the guard electrode 31, making it possible to prevent current from flowing from the measuring electrode 3 to the guard electrode 31. This allows the measuring device 20 to suppress current flowing from the measuring electrode 3 to any part other than the buffer circuit 6, making it possible to measure the voltage Vx more accurately.

[0048] The worker may attach the measuring electrode 3 and the guard electrode 31 to the electric wire 1 by using, for example, a clamp (not shown) having the measuring electrode 3 and the guard electrode 31.

[0049] [Detailed Configuration of Measuring Device in Second Embodiment] Next, a detailed configuration of the measuring device 20 according to the second embodiment will be described. Fig. 7 is a detailed configuration diagram of the measuring device 20 according to the second embodiment. Hereinafter, the description of the measuring electrode 3 and the guard electrode 31 will be omitted. For convenience, the insulation resistance Rx will be described on the outside of the insulator 2.

[0050] 7, the measuring device 20 includes, for example, a buffer circuit 6, a leakage resistor Rz, an output circuit 6a, an amplifier circuit 7, an offset correction circuit 7a, and a gain correction circuit 7b. Note that, hereinafter, the resistor formed between the measuring electrode 3 and the guard electrode 31 is also referred to as a leakage resistor Ry.

[0051] 7, the current Ix and the current Iz can be expressed as the above equations (1) and (2), respectively, and the current Iy flowing through the leakage resistance Ry can be expressed as the following equation (12). Note that hereinafter, the voltage output from the amplifier circuit 7 will be referred to as the guard voltage Vg (hereinafter simply referred to as voltage Vg).

[0052]

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[0053] The relationship between the current Ix, the current Iy, the current Iz, and the current Ib can be expressed as in the following equation (13).

[0054]

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[0055] Then, when the above formulas (1), (2), (12), and (13) are used, the voltage V2 can be expressed as in the following formula (14).

[0056]

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[0057] Furthermore, the above equation (14) can be transformed into the following equation (15).

[0058]

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[0059] In this regard, in the above equation (15), for example, when the resistance value of the leakage resistance Ry is at a maximum, the voltage V2 can be expressed in the same way as the above equation (6).

[0060] Furthermore, when the above equation (14) is used, the voltage Verror can be expressed as in the following equation (16).

[0061]

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[0062] Here, when the voltage Verror is set to 0, the voltage Vg can be expressed as in the following equation (17).

[0063]

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[0064] The above equation (17) can be transformed into the following equation (18).

[0065]

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[0066] That is, the measuring device 20 can suppress the voltage Verror to 0 by setting the voltage Vg expressed by the above equation (18) as the voltage at the output terminal of the amplifier circuit 7. Therefore, the offset correction circuit 7a applies a positive offset voltage to the amplifier circuit 7, and the gain correction circuit 7b adjusts the gain of the amplifier circuit 7 to control the amplifier circuit 7 so that the amplifier circuit 7 outputs a guard voltage Vg (see equation (18)) appropriate for the measurement state. Specifically, since (Ry / Rz) is a small value, the guard voltage Vg is a voltage obtained by adding a voltage several percent higher than the voltage Vx to an offset voltage (approximately 1 V or less) proportional to the bias current Ib. The measurement state includes various parameters that affect the guard voltage (e.g., the voltage Vx and resistance Ry in equation (18)).

[0067] This allows the measuring device 20 to extremely reduce the voltage difference between the voltage Vx at the wire 1 and the voltage Vy at the guard electrode 31. Therefore, the measuring device 20 can form a leakage resistance Rz with an extremely large resistance value, thereby extremely reducing the current Iz. Therefore, the measuring device 20 can reduce the error between the currents Ix and Ib, and measure the voltage Vx with greater accuracy. In other words, the measuring device 20 can cancel out measurement error factors due to the voltage drop at the insulation resistance Rx and the influence of the leakage resistance Rz, thereby measuring the voltage Vx with greater accuracy.

[0068] The amplifier circuit 7 is a so-called amplifier, and the output voltage of this amplifier is fed back to the negative terminal of the amplifier, and the output voltage of the buffer circuit 6 and the output voltage (offset voltage) of the offset correction circuit 7a are input to the positive terminal of the amplifier.

[0069] Furthermore, the above formula (18) does not include the insulation resistance Rx of the electric wire 1. Therefore, the measuring device 20 can measure the voltage of electric wires other than the electric wire 1 in the same manner.

[0070] Furthermore, the leakage resistance Ry is determined by the measurement electrode 3 and the guard electrode 31. Therefore, for example, if the measurement electrode 3 and the guard electrode 31 (the clamps used for the measurement) used for the measurement have been determined and the current Ib is stable, the operator can estimate the current term (IbRy) in the above equation (18) in advance. Furthermore, the leakage resistance Rz is determined, for example, by the diameter of the guard electrode 31. Therefore, for example, if the measurement electrode 3 and the guard electrode 31 used for the measurement have been determined, the operator can also estimate the voltage term ((Ry / Rz)+1) in the above equation (18) in advance. Therefore, the operator can estimate the voltage Vg in the above equation (18) in advance.

[0071] [Modification of the second embodiment] Next, a modified example of the second embodiment will be described below. Figures 8 and 9 are diagrams for explaining the modified example of the second embodiment.

[0072] For example, when a guard electrode 31 having a diameter larger than that of the electric wire 1 is attached to the electric wire 1, both ends of the guard electrode 31 in the longitudinal direction of the electric wire 1 are open, as shown in Fig. 6(B). Therefore, in this case, current may leak from both ends of the guard electrode 31, making it impossible to accurately measure the voltage Vx.

[0073] Therefore, as shown in FIG. 8, the electric wire 1 may be provided with side guard electrodes 32 and 33 attached to the front and rear of a guard electrode 31 attached so as to cover the measuring electrode 3.

[0074] The side guard electrodes 32 and 33 are, for example, copper foil. If the axial length of the measurement electrode 3 is, for example, about 25 mm, the axial length of the side guard electrodes 32 and 33 may be, for example, about 15 mm.

[0075] This allows measuring device 20 to prevent signal (current) leakage from both ends of guard electrode 31, enabling accurate measurement of voltage Vx. In particular, measuring device 20 can prevent errors (errors in the measurement value of voltage Vx) caused by changes in temperature and humidity.

[0076] It is to be noted that only one of the side guard electrode 32 and the side guard electrode 33 may be attached to the electric wire 1.

[0077] 9, a resistance (hereinafter also referred to as leakage resistance Rys1) is formed between the measurement electrode 3 (the dotted line portion in FIG. 9) and the side guard electrode 32, and a resistance (hereinafter also referred to as leakage resistance Rys2) is formed between the measurement electrode 3 and the side guard electrode 33. Therefore, in the example shown in FIG. 9, the leakage resistance Ry described in FIG. 7 corresponds to the parallel resistance of the leakage resistance Ry, the leakage resistance Rys1, and the leakage resistance Rys2. [Explanation of symbols]

[0078] 1: Electric wire 2: Insulator 3: Measuring electrode 4: Isolator 5: Isolator 6: Buffer circuit 6a: Output circuit 7: Amplification circuit 7a: Gain correction circuit 7b: Offset correction circuit 10: Measuring equipment 20: Measuring equipment 31: Guard electrode 32: Side guard electrode 33: Side guard electrode Cx: Insulation capacitance Ib: Bias current Ix: Current Iy: Current Iz: Current Vx: Voltage Vy: Voltage V2: Voltage Verror: Error voltage Vg: Guard voltage Rf: Resistance Rx: Insulation resistance Ry: Leakage resistance Rys1: Leakage resistance Rys2: Leakage resistance Rz: Leakage resistance

Claims

1. A measuring instrument for measuring DC voltage across an insulator-coated conductor, A buffer circuit is provided, The buffer circuit a voltage of a leakage current leaking from the conductor through the insulator is continuously input via a measurement electrode in contact with the insulator; outputting a voltage corresponding to the voltage of the leakage current; a guard electrode attached to the insulator so as to cover the measurement electrode; an amplifier circuit that applies a voltage corresponding to the output voltage of the buffer circuit to the guard electrode.

2. 2. The measurement instrument according to claim 1, wherein the amplifier circuit has an adjustable gain and an adjustable offset voltage.

3. The electric power supply further includes a side guard electrode attached to the electric conductor at a position adjacent to the guard electrode, 3. The measuring instrument according to claim 2, wherein the amplifier circuit applies a voltage corresponding to an output voltage of the buffer circuit to the side guard electrode.

4. the side guard electrode includes first and second side guard electrodes attached to the conductor at both ends of the guard electrode, 4. The measuring instrument according to claim 3, wherein the amplifier circuit applies a voltage corresponding to an output voltage of the buffer circuit to each of the first and second side guard electrodes.

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