Voltage measuring device

Abstract

The present invention comprises: a clamp part 2; an electrode E1 and an electrode E2 that face a cable CBL; a stabilization part 5 for connecting the electrode E1 to the ground via capacitors C1 and C2; an amplifying part A that amplifies voltage Vin obtained from the electrode E1 and that can change the amplification factor; a mode inputting part 81 that receives mode setting; a reference voltage outputting part PS that outputs a periodically changing reference voltage Vs; a measurement part 7 that measures output voltage Vout of the amplifying part A; a switch SW for connecting the electrode E2 to the ground in a measurement mode, and for connecting the electrode E2 to the reference voltage outputting part PS in a calibration mode; and a calibration processing part 82 that, when being in the calibration mode, adjusts an amplification factor G on the basis of a measurement magnification M, a reference voltage Vs, and a measurement value Vm of the measurement part 7 so that the output voltage Vout of the amplifying part A becomes equal to the measurement magnification M-times the amount of a cable voltage Vx.

Description

[Technical Field]

[0001] The present invention relates to a voltage measuring device that clamps a cable to be measured and measures the voltage. [Background technology]

[0002] BACKGROUND ART Conventionally, there has been known a voltage detection device that includes a detection electrode disposed opposite to an object to be detected and detects the voltage of the object to be detected without electrical contact with the object to be detected (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-25918 Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned voltage detection device, if the area of ​​the detection electrode is S, the distance between the detection object and the detection electrode is d, and the dielectric constant between the detection object and the detection electrode is ε, the capacitance C between the detection object and the detection electrode is C=εS / d. When this voltage detection device is used to measure the voltage of a covered cable, the thickness of the cable covering is d and the dielectric constant of the cable covering is ε.

[0005] Therefore, in the above-mentioned voltage detection device, when the cable to be measured is changed, the thickness d of the cable coating and the dielectric constant ε of the cable coating change, and the capacitance C between the detection object and the detection electrode changes. As a result, the voltage detection is affected by the characteristics of the cable to be measured, and the voltage measurement accuracy decreases.

[0006] An object of the present invention is to provide a voltage measurement device that can easily improve the accuracy of voltage measurement. [Means for solving the problem]

[0007] A voltage measuring device according to one embodiment of the present invention comprises a clamping unit that clamps a cable to be measured, a first electrode and a second electrode arranged to face the cable clamped by the clamping unit, a stabilization unit that connects the first electrode to ground via a capacitor, an amplifier unit that is capable of changing its gain and amplifies the voltage obtained from the first electrode, a mode input unit that selectively accepts as a mode setting a measurement mode for measuring the voltage of the cable and a calibration mode for calibrating the output voltage of the amplifier unit, a reference voltage output unit that outputs a predetermined reference voltage that changes periodically, a measurement unit that measures the output voltage of the amplifier unit, a switching unit that connects the second electrode to ground in the measurement mode and connects the second electrode to the reference voltage output unit in the calibration mode, and a calibration processing unit that adjusts the gain in the calibration mode based on the measurement value of the measurement unit, the reference voltage, and the predetermined measurement magnification so that the output voltage of the amplifier unit becomes the measurement magnification times the voltage of the cable. [Effects of the Invention]

[0008] A voltage measuring device having such a configuration can easily improve the voltage measurement accuracy. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a perspective view showing an example of the configuration of a clamp-type voltage measurement device 1 according to an embodiment of the present invention. [Figure 2] 2 is a front view showing the inside of the clamp arm and the front wall of the housing shown in FIG. 1 in a see-through manner. FIG. [Figure 3] FIG. 3 is a front view of the electrodes E1 and E2 shown in FIG. 2, viewed from the Z direction. [Figure 4] 2 is a circuit diagram showing an example of the electrical configuration of the clamp-type voltage measuring device 1 shown in FIG. 1 in a calibration mode. [Figure 5] 2 is a circuit diagram showing an example of the electrical configuration of the clamp-type voltage measuring device 1 shown in FIG. 1 in a measurement mode. FIG. [Figure 6]5 is a flowchart showing an example of the operation of the clamp-type voltage measurement device 1 in the calibration mode. [Figure 7] 4 is a flowchart showing an example of the operation of the clamp-type voltage measuring device 1 in the measurement mode. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In each drawing, the same reference numerals are used to denote the same configuration, and the description thereof will be omitted. In each drawing, XYZ orthogonal coordinate axes are appropriately shown to clarify the directional relationships.

[0011] 1 generally comprises a clamp unit 2 that clamps the cable CBL to be measured, and a roughly box-shaped housing 3 connected to the clamp unit 2. Housing 3 is connected to a measuring device such as an oscilloscope or a data logger via a coaxial cable 4.

[0012] The clamp unit 2 includes a pair of clamp arms 21 and 22. The base end of the clamp arm 21 is pivotally supported by a shaft 27 attached to the housing 3. The clamp arm 21 is able to swing around the shaft 27. Retaining grooves 211 and 221 into which the cable CBL fits are formed on the opposing surfaces of the clamp arms 21 and 22.

[0013] The clamp arm 21 is biased by a torsion spring (not shown) toward the clamp arm 22. The cable CBL is clamped between the clamp arm 21 and the clamp arm 22 by the biasing force of the torsion spring.

[0014] 2, clamp arm 22 is fixedly connected to housing 3. Clamp arm 22 may be swingable like clamp arm 21. Clamp arms 21 and 22 and housing 3 are made of an insulating material, for example, a resin material.

[0015] An accommodation space 28 is provided inside the clamp arm 22. The accommodation space 28 is in communication with the internal space of the housing 3. A substantially plate-shaped electrode E1 (first electrode) and an electrode E2 (second electrode) are disposed in the accommodation space 28.

[0016] Referring to FIG. 3, the electrodes E1 and E2 are arranged to face the cable CBL with a gap therebetween in the direction in which the cable CBL extends.

[0017] The electrodes E1 and E2 may be formed as a conductor pattern on a printed wiring board, for example, or may be metal plates. The electrodes E1 and E2 are disposed facing or in contact with the inner wall surface of the holding groove 221 of the clamp arm 22. This allows the electrodes E1 and E2 to be disposed opposite the cable CBL clamped by the clamp unit 2, via the wall of the holding groove 221 made of an insulating material.

[0018] The housing 3 accommodates a circuit board 31. A terminal T3 and an electrode E1 of the circuit board 31 are connected via a wire W1, and a terminal T4 and an electrode E2 are connected via a wire W2.

[0019] A conductive layer 32, shown by hatching, is formed on the storage space 28 of the clamp arm 22 and the inner wall surface of the housing 3, except for at least the portion located between the cable CBL clamped by the clamp portion 2 and the electrodes E1, E2.

[0020] The conductive layer 32 may be, for example, a metal foil such as aluminum foil, a conductive paint coating, a plated layer, or a metal plate. In the example shown in Fig. 2, the conductive layer 32 is not formed on the wall of the holding groove 221 located between the cable CBL clamped by the clamp unit 2 and the electrode E1, or on the periphery of the electrode E1.

[0021] As a result, the outer wall surface of the clamp-type voltage measuring device 1 is made insulating, and the inner wall surface of the housing 3 and the inner wall surface of the clamp arm 22 are made conductive except for at least the portion located between the cable CBL clamped by the clamp portion 2 and the electrodes E1 and E2.

[0022] Because the outer wall surface of the clamp-type voltage measurement device 1 is insulating, even if the conductor portion of the cable CBL is exposed, there is a reduced risk of current flowing from the cable CBL to the electrodes E1, E2 or the circuit board 31, which could damage the clamp-type voltage measurement device 1. This also improves the safety of the user who operates the clamp-type voltage measurement device 1. Furthermore, the provision of the conductive layer 32 reduces electromagnetic noise from the external environment.

[0023] The electrodes E1 and E2 are not necessarily limited to the example in which they are disposed inside the clamp unit 2 and arranged to face the cable CBL via an insulating material. The electrodes E1 and E2 may be arranged, for example, exposed in the holding groove 221 and arranged to face the cable CBL in a state of contact with it.

[0024] As will be described later, the clamp-on voltage measurement device 1 detects the cable voltage Vx, which is the voltage across the core wire W of the cable CBL, via capacitances Cx1 and Cx2, which are generated when the core wire W of the cable CBL faces the electrodes E1 and E2. The capacitances Cx1 and Cx2 are inversely proportional to the distance d between the core wire W of the cable CBL and the electrodes E1 and E2. Therefore, the shorter the distance d, the greater the capacitances Cx1 and Cx2, making it easier to detect the cable voltage Vx. Furthermore, when the distance d changes, the voltage obtained via the capacitances Cx1 and Cx2 fluctuates.

[0025] On the other hand, the clamping force of the clamp arms 21, 22 may be insufficient to clamp the cable CBL using only the biasing force of the torsion spring (not shown), causing a gap between the cable CBL and the holding groove 221, increasing the facing distance d and decreasing the capacitances Cx1, Cx2. Also, the cable CBL may vibrate, causing the facing distance d to fluctuate, which may result in fluctuations in the capacitances Cx1, Cx2.

[0026] However, the clamp-type voltage measuring device 1 can firmly clamp the cable CBL by fastening the clamp arms 21 and 22 with screws 25, thereby reducing the risk of a decrease in the capacitances Cx1 and Cx2 due to an increase in the opposing distance d, or fluctuations in the capacitances Cx1 and Cx2 due to shaking of the cable CBL.

[0027] 4 and 5 includes electrodes E1, E2, a selector switch SW (switching unit), a reference voltage output unit PS, a stabilizing unit 5, an amplifier A, a measuring unit 7, a mode switch 9, terminals T1, T2, and a control unit 8. The selector switch SW, the reference voltage output unit PS, the stabilizing unit 5, the amplifier A, the measuring unit 7, terminals T1, T2, and the control unit 8 are formed on a circuit board 31 and housed in a housing 3. Note that at least one of the measuring unit 7, the mode switch 9, and the control unit 8 is not necessarily housed in the housing 3, and may be configured outside the housing 3.

[0028] In the cable CBL, a conductor core wire W is covered with an insulating coating J. In Fig. 4 and Fig. 5, the capacitance formed by the core wire W facing the electrode E1 is represented by capacitance Cx1, and the capacitance formed by the core wire W facing the electrode E2 is represented by capacitance Cx2.

[0029] The stabilization unit 5 includes a parallel circuit 51, a resistor R2, and a capacitor C2. The parallel circuit 51 is a parallel circuit of a capacitor C1 and a resistor R1. The electrode E1 is connected to one end P1 of the parallel circuit 51 and the non-inverting input terminal of the amplifier A1 via a terminal T3 and a wiring W1 shown in FIG. 2. The other end P2 of the parallel circuit 51 is connected to the circuit ground GND via the capacitor C2. The other end P2 of the parallel circuit 51 is connected to the circuit ground GND via the resistor R2. The circuit ground GND is connected to the conductive layer 32.

[0030] The stabilization unit 5 stabilizes the potential of the input voltage Vin input from the electrode E1 to the non-inverting input terminal of the amplifier A1, and also functions as a filter, which will be described later. If the clamp-on voltage measurement device 1 did not include the stabilization unit 5, the electrode E1 would simply be connected to the high-impedance non-inverting input terminal of the amplifier A1, resulting in an electrically floating state with no reference potential, and the wiring from the electrode E1 to the non-inverting input terminal acting like an antenna. This would cause the input voltage Vin input to the non-inverting input terminal of the amplifier A1 to become unstable.

[0031] Therefore, by providing the stabilization unit 5 and connecting the electrode E1 to the circuit ground GND via the capacitors C1 and C2, a reference potential for the input voltage Vin is provided, and the input voltage Vin can be stabilized.

[0032] The stabilization unit 5 does not necessarily have to function as a filter, which will be described later, but only needs to be able to stabilize the input voltage Vin. For example, the stabilization unit 5 may have a configuration in which only the capacitor C2 is connected to the electrode E1.

[0033] The capacitances C1 and C2 of the capacitors C1 and C2 are set to be sufficiently large, for example, 100 to 1000 times or more, relative to the expected capacitances Cx1 and Cx2, so that the impedances of the capacitors C1 and C2 are negligibly small relative to the impedances of the capacitances Cx1 and Cx2.

[0034] The amplifier unit A includes an amplifier A1, a feedback resistor Ra, and a variable resistor Rx. The amplifier A1 is a so-called operational amplifier. The non-inverting input terminal of the amplifier A1 is connected to one end P1 of the parallel circuit 51. One end of the feedback resistor Ra is connected to the inverting input terminal of the amplifier A1, and the other end of the feedback resistor Ra is connected to the output terminal of the amplifier A1. The inverting input terminal of the amplifier A1 is connected to the circuit ground GND via the variable resistor Rx. This makes the amplifier unit A a non-inverting amplifier. For example, a digital potentiometer can be used as the variable resistor Rx. However, the variable resistor Rx is not limited to a digital potentiometer. Various components or circuits whose resistance value can be changed can be used as the variable resistor Rx.

[0035] If the resistance value of the feedback resistor Ra is Ra and the resistance value of the variable resistor Rx is Rx, the amplification factor G of the amplifier A is G = 1 + Ra / Rx. Therefore, the amplification factor G can be adjusted by changing the resistance value Rx.

[0036] The feedback resistor Ra may be a variable resistor, and the resistance value Ra of the feedback resistor Ra may be changed to adjust the gain G of the amplifier unit A. However, since the feedback resistor Ra affects the frequency characteristics of the amplifier unit A, it is more preferable to adjust the gain G of the amplifier unit A by changing the variable resistor Rx, which does not affect the frequency characteristics.

[0037] The amplifier A may be an inverting amplifier. However, if the amplifier A is an inverting amplifier, the electrode E1 is connected to the same inverting input terminal as the feedback resistor. In this case, leakage current may flow through the current path from the electrode E1 via the feedback resistor to the output terminal of the amplifier, affecting the input voltage Vin.

[0038] On the other hand, if the amplifier A is a non-inverting amplifier, the electrode E1 is connected to the high-impedance non-inverting input terminal of the amplifier A1, and no leakage current flows through the feedback resistor Ra. As a result, the risk of leakage current affecting the input voltage Vin is reduced. Therefore, it is more preferable to use a non-inverting amplifier for the amplifier A.

[0039] The output terminal of amplifier A1 is connected to the measurement unit 7 and terminal T1. Terminal T2 is connected to circuit ground GND. Terminal T1 is connected to the core wire of coaxial cable 4, and terminal T2 is connected to the shield wire of coaxial cable 4. As a result, the output voltage Vout of amplifier A1 is output to a measurement device such as an oscilloscope or data logger via coaxial cable 4. In other words, the output voltage Vout of amplifier A1 represents the measurement result of the clamp-on voltage measurement device 1.

[0040] The clamp-type voltage measurement device 1 outputs the voltage of the cable voltage Vx multiplied by a measurement magnification factor M as the output voltage Vout. Because the measurement magnification factor M is known, a measurement device connected to the clamp-type voltage measurement device 1 can correctly identify the measured cable voltage Vx from the output voltage Vout. However, if the output voltage Vout is not multiplied by the measurement magnification factor M of the cable voltage Vx, the output voltage Vout will not correctly represent the measured cable voltage Vx. Therefore, using a calibration mode (described later), the amplification factor G is adjusted so that the output voltage Vout becomes the measurement magnification factor M of the cable voltage Vx. The measurement magnification factor M is set to, for example, 1 / 100.

[0041] The cable CBL is not particularly limited, but may be, for example, a power cable for driving the motor of an electric vehicle. In the case of a power cable for driving the motor of an electric vehicle, the cable voltage Vx, which is an AC voltage with a periodic rectangular waveform generated by PWM (Pulse Width Modulation) output from an inverter, is the object of measurement.

[0042] The filter function of the stabilization unit 5 will now be described. The cable voltage Vx of the core wire W is applied to one end P1 of the parallel circuit 51 via the electrostatic capacitance Cx1. For example, in the case of a periodic square wave waveform generated by PWM, high-frequency components are included in the rising and falling edges of the square wave. Therefore, to accurately measure the voltage waveform of the core wire W, it is necessary to detect both the low-frequency components corresponding to the period of the square wave and the high-frequency components corresponding to the rising and falling edges of the square wave.

[0043] Therefore, a series circuit of resistor R1 and capacitor C2 is formed in the clamp-on voltage measuring device 1. The series circuit of resistor R1 and capacitor C2 is a so-called integrating circuit, and functions as a low-pass filter that passes low-frequency components.

[0044] The input terminal of amplifier A1 is applied with the sum of the terminal voltage of capacitor C2 and the terminal voltage of resistor R1. Therefore, the series circuit of resistor R1 and capacitor C2 allows low-frequency components corresponding to the period of the square wave to be input to amplifier A1.

[0045] Furthermore, a series circuit of the capacitor C1 and resistor R2 is formed in the clamp-type voltage measuring device 1. The series circuit of the capacitor C1 and resistor R2 is a so-called differential circuit, and functions as a high-pass filter that passes high-frequency components.

[0046] The input terminal of amplifier A1 is applied with the sum of the terminal voltage of resistor R2 and the terminal voltage of capacitor C1. Therefore, the series circuit of capacitor C1 and resistor R2 allows high-frequency components corresponding to the rising and falling edges of the rectangular wave to be input to amplifier A1.

[0047] That is, the input terminal of amplifier A1 receives a superimposed signal consisting of low-frequency components corresponding to the period of the square wave and high-frequency components corresponding to the rising and falling edges of the square wave. This allows the AC voltage waveform detected from core wire W to be accurately amplified by amplifier A1 and output to the measuring device.

[0048] The reference voltage output unit PS outputs a predetermined reference voltage Vs that changes periodically. The reference voltage Vs may be any voltage that changes periodically, such as a sinusoidal AC voltage or a square-wave pulse, but preferably has a signal waveform similar to the voltage to be detected on the cable CBL. For example, as described above, when the AC voltage to be measured is a square-wave periodic waveform, the reference voltage output unit PS preferably outputs a square-wave voltage as the reference voltage Vs.

[0049] The selector switch SW is a switching unit that connects the electrode E2 to the circuit ground GND in the measurement mode and connects the electrode E2 to the reference voltage output unit PS in the calibration mode. The selector switch SW switches the connection between the measurement mode and the calibration mode in response to a control signal from the control unit 8, for example.

[0050] The mode switch 9 is, for example, a mode setting switch that can be operated by the user. By operating the mode switch 9, it is possible to selectively set the measurement mode or the calibration mode. A signal indicating the mode set by the mode switch 9 is output to the control unit 8.

[0051] The measurement unit 7 measures the output voltage Vout of the amplifier unit A. The measurement unit 7 is configured using, for example, an analog-to-digital converter. The measurement unit 7 outputs a measured value Vm of the output voltage Vout to the control unit 8. Because the cable voltage Vx to be measured is an AC voltage, the output voltage Vout also has an AC waveform. Therefore, it is preferable that the measurement unit 7 measures the peak-to-peak value of the output voltage Vout as the measured value Vm.

[0052] The control unit 8 is configured using, for example, a CPU (Central Processing Unit) that executes predetermined arithmetic processing, a RAM (Random Access Memory) that temporarily stores data, non-volatile storage elements such as flash memory, and peripheral circuits thereof, etc. The control unit 8 functions as a mode input unit 81 and a calibration processing unit 82 by executing programs stored in the storage elements described above.

[0053] Based on the signal output from the mode switch 9, the mode input unit 81 selectively accepts as a mode setting either a measurement mode for measuring a cable voltage Vx, which is the voltage of the core wire W of the cable CBL, or a calibration mode for calibrating the output voltage Vout of the amplifier unit A. When the accepted mode setting is the measurement mode, the mode input unit 81 causes the selector switch SW to connect the electrode E2 to the circuit ground GND, and when the accepted mode setting is the calibration mode, causes the selector switch SW to connect the electrode E2 to the reference voltage output unit PS.

[0054] Note that the present invention is not limited to an example in which the mode input unit 81 switches the selector switch SW. For example, the mode switch 9 and the selector switch SW may be configured as double-pole double-throw switches. One pole of the double-pole double-throw switch may be used as the mode switch 9, and the other pole as the selector switch SW. In this way, the selector switch SW can be switched according to the mode, even if the mode input unit 81 does not switch the selector switch SW.

[0055] Furthermore, the mode input unit 81 only needs to be able to receive a signal indicating a mode setting from the outside, and the clamp-type voltage measuring device 1 does not need to be equipped with the mode switch 9.

[0056] In the calibration mode, the calibration processing unit 82 adjusts the amplification factor G of the amplifier unit A to an amplification factor Gx based on the measurement value Vm of the measurement unit 7, the reference voltage Vs, and a preset measurement magnification M so that the output voltage Vout of the amplifier unit A becomes the measurement magnification M times the cable voltage Vx.

[0057] Specifically, the calibration processing unit 82 calculates, based on the following equation (1), the amplification factor Gx that is the target value after calibration at which the measurement magnification M is obtained, relative to the current amplification factor G, i.e., the amplification factor G when the output voltage Vout is measured by the measurement unit 7 in calibration mode.

[0058] Gx={G×Vs×Cx2 / (Cx1+Cx2)}×M / Vm...(1)

[0059] Here, if the opposing distance between electrodes E1, E2 and core wire W is d, the dielectric constant of coating J is ε, the area of ​​electrode E1 is S1, and the area of ​​electrode E2 is S2, then capacitance Cx1 = εS1 / d and capacitance Cx2 = εS2 / d. Therefore, in the clamp-on voltage measuring device 1, by setting S1 = S2, capacitance Cx1 and capacitance Cx2 can be made approximately equal. When capacitance Cx1 and capacitance Cx2 are equal, equation (1) becomes the following equation (2).

[0060] Gx = (G × Vs / 2) × M / Vm (2)

[0061] Therefore, the calibration processing unit 82 can calculate the amplification factor Gx based on the formula (2). When calculating the amplification factor Gx based on the formula (1), it is necessary to measure the capacitances Cx1 and Cx2, but according to the formula (2), it is not necessary to measure the capacitances Cx1 and Cx2, which makes it easier to calculate the amplification factor Gx.

[0062] That is, by making the capacitance Cx1 and the capacitance Cx2 equal, it becomes easy to calculate the amplification factor Gx. Also, by making the area S1 of the electrode E1 equal to the area S2 of the electrode E2, it becomes easy to make the capacitance Cx1 and the capacitance Cx2 equal.

[0063] That is, to use equation (2), it is sufficient that capacitance Cx1 and capacitance Cx2 are equal, and the example is not necessarily limited to one in which area S1 and area S2 are equal. However, if area S1 and area S2 are equal, capacitance Cx1 and capacitance Cx2 can be made equal by arranging electrodes E1 and E2 so that the opposing distance between electrodes E1, E2 and core wire W is the same opposing distance d. Therefore, making area S1 and area S2 equal is more preferable because it makes it easier to calculate amplification factor Gx using equation (2).

[0064] The calibration processing unit 82 adjusts the resistance value of the variable resistor Rx to adjust the amplification factor of the amplifier unit A to the amplification factor Gx.

[0065] Next, the derivation of equation (1) will be explained. As shown in Figure 4, the explanation will be given assuming that electrode E2 is connected to the reference voltage output unit PS in calibration mode. In calibration mode, no external voltage is applied to cable CBL. In this state, the voltage induced in core wire W based on the reference voltage Vs of the reference voltage output unit PS becomes cable voltage Vx.

[0066] Therefore, the cable voltage Vx is the reference voltage Vs divided by the capacitance Cx1 and the series circuit of the capacitance Cx2 and the stabilization unit 5. As described above, the impedance of the capacitors C1 and C2 of the stabilization unit 5 is negligibly small compared to the impedance of the capacitances Cx1 and Cx2. Therefore, the cable voltage Vx is the reference voltage Vs divided by the capacitances Cx1 and Cx2, and can be approximated by the following equation (3):

[0067] Cable voltage Vx ≒ Vs × Cx2 / (Cx1 + Cx2) (3)

[0068] Here, if the amplification factor of the amplifier A before calibration in the calibration mode is G, and the amplification factor of the amplifier A after calibration that is to be found is Gx, the following equation (4) is established.

[0069] Cable voltage Vx = Gx × Vin / M = Gx × (Vm / G) / M (4) Transforming equation (4) gives

[0070] Amplification factor Gx = G × Vx × M / Vm (5) Substituting equation (3) into the cable voltage Vx in equation (5) gives the above equation (1).

[0071] In order to set the amplification factor of the amplifier A to Gx, the resistance value Rx should be adjusted so as to satisfy the following equation (6).

[0072] Amplification factor Gx=1+Ra / Rx (6) Transforming equation (6) gives

[0073] Resistance value Rx=Ra / (Gx-1) (7)

[0074] From the above, the calibration processing unit 82 calculates the resistance value Rx by substituting the amplification factor Gx obtained from equation (1) or (2) into equation (7), and sets the resistance value of the variable resistor Rx to the resistance value Rx obtained from equation (7), thereby adjusting the amplification unit A to the amplification factor Gx. This allows the clamp-type voltage measuring device 1 to be calibrated so that the output voltage Vout of the amplification unit A satisfies the following equation (8).

[0075] Output voltage Vout=M×Vx (8)

[0076] Next, an example of the operation of the clamp-type voltage measuring device 1 configured as described above will be described. With reference to Fig. 6, when calibrating the clamp-type voltage measuring device 1, the user removes cable CBL from the device, for example, to prevent voltage from being applied to the core wire W. In this state, the user clamps cable CBL with clamp unit 2 and operates mode switch 9 to set the calibration mode. This causes mode switch 9 to output a signal indicating the calibration mode to control unit 8.

[0077] The example shown in Fig. 6 shows the operation when Cx1 = Cx2. When a signal indicating the calibration mode is output from the mode switch 9, the mode input unit 81 accepts the setting of the calibration mode and switches the selector switch SW to the reference voltage output unit PS side (step S1). Then, referring to Fig. 4, the reference voltage Vs output from the reference voltage output unit PS is supplied to the electrode E2, and an input voltage Vin is induced in the electrode E1 from the electrode E2 via the electrostatic capacitance Cx2, the core wire W, and the electrostatic capacitance Cx1.

[0078] The input voltage Vin is amplified by the amplifier A with an amplification factor G and output as the output voltage Vout to the measurement unit 7. As described above, the output voltage Vout has an AC waveform, so the peak-to-peak voltage of the output voltage Vout is measured by the measurement unit 7, and the measured value Vm is output to the control unit 8.

[0079] 6, the calibration processing unit 82 calculates the amplification factor Gx from equation (2) based on the measurement value Vm measured by the measurement unit 7 (step S2). Note that if the capacitances Cx1 and Cx2 are known rather than Cx1=Cx2, the amplification factor Gx can be calculated using equation (1) in step S2.

[0080] Next, the calibration processing unit 82 calculates the resistance value Rx from equation (7) based on the calculated amplification factor Gx (step S3). Next, the calibration processing unit 82 sets the resistance value of the variable resistor Rx to the calculated Rx (step S4). As a result, the amplification factor of the amplifier A is set to Gx.

[0081] This allows the clamp-on voltage measurement device 1 to be calibrated so that the output voltage Vout becomes equal to the measurement magnification M of the cable voltage Vx, that is, so that the formula (8) is satisfied.

[0082] Next, the measurement mode will be described. When measuring the cable voltage Vx while the cable CBL is in use, the user clamps the cable CBL connected to the device with the clamp unit 2 and sets the measurement mode by operating the mode switch 9. This causes the mode switch 9 to output a signal indicating the measurement mode to the control unit 8.

[0083] 7, when a signal indicating the measurement mode is output from the mode switch 9, the mode input unit 81 accepts the setting of the measurement mode and switches the selector switch SW to the circuit ground GND side as shown in Fig. 5 (step S11). Then, the cable voltage Vx of the measurement object supplied to the core wire W induces an input voltage Vin in the electrode E1 via the capacitance Cx1.

[0084] The amplifier A amplifies the input voltage Vin to an output voltage Vout by a calibrated amplification factor Gx. The output voltage Vout is output to a terminal T1 as a signal indicating the measurement result by the clamp-type voltage measuring device 1.

[0085] As described above, the capacitance Cx1 is expressed as εS1 / d, and if the cable CBL to be measured changes and the opposing distance d due to the thickness of the coating J, the dielectric constant ε of the coating J, etc., change, the capacitance Cx1 also changes. Therefore, if the cable CBL changes, the input voltage Vin induced by the cable voltage Vx also changes, making it impossible to obtain the correct output voltage Vout.

[0086] However, with the clamp-type voltage measurement device 1, the cable CBL to be measured is clamped by the clamp unit 2, and calibration is performed in calibration mode, making it easy to obtain a correct output voltage Vout that is suited to the cable CBL to be measured. Therefore, the clamp-type voltage measurement device 1 makes it easy to improve the voltage measurement accuracy.

[0087] That is, a voltage measuring device according to one embodiment of the present invention comprises a clamping unit that clamps a cable to be measured, a first electrode and a second electrode arranged to face the cable clamped by the clamping unit, a stabilizing unit that connects the first electrode to ground via a capacitor, an amplifier unit that is capable of changing the gain and amplifies the voltage obtained from the first electrode, a mode input unit that selectively accepts as a mode setting a measurement mode for measuring the voltage of the cable and a calibration mode for calibrating the output voltage of the amplifier unit, a reference voltage output unit that outputs a predetermined reference voltage that changes periodically, a measurement unit that measures the output voltage of the amplifier unit, a switching unit that connects the second electrode to ground in the measurement mode and connects the second electrode to the reference voltage output unit in the calibration mode, and a calibration processing unit that adjusts the gain in the calibration mode based on the measurement value of the measurement unit, the reference voltage, and the predetermined measurement gain so that the output voltage of the amplifier unit becomes the measurement magnification times the voltage of the cable.

[0088] With this configuration, the first electrode and the second electrode are disposed opposite the cable, generating a capacitance between them. In calibration mode, a preset reference voltage is supplied to the second electrode, and a voltage is generated at the first electrode due to the capacitance between the first and second electrodes and the cable. The voltage generated at the first electrode is amplified by the amplifier, and the amplified output voltage is measured by the measurement unit. Then, the calibration processing unit adjusts the gain of the amplifier based on the measurement value of the measurement unit, the reference voltage, and the measurement magnification so that the output voltage of the amplifier is multiplied by the measurement magnification of the cable voltage. This adjusts the gain of the amplifier according to the cable to be measured, making it easy to improve voltage measurement accuracy.

[0089] The amplifier section is preferably a non-inverting amplifier.

[0090] According to this configuration, a non-inverting amplifier with a high input impedance is used as the amplifier, so that the input impedance of the amplifier is less likely to affect the voltage of the first electrode.

[0091] Furthermore, it is preferable that the amplification unit includes an amplifier having a non-inverting input terminal to which the voltage obtained from the first electrode is input, a feedback resistor connecting the inverting input terminal and output terminal of the amplifier, and a variable resistor connecting the inverting input terminal and ground of the amplifier, and that the amplification factor can be changed by changing the resistance value of the variable resistor.

[0092] With this configuration, the gain can be adjusted by changing the resistance of the variable resistor while keeping the resistance of the feedback resistor fixed. A non-inverting amplifier can also change the gain by changing the resistance of the feedback resistor. However, the feedback resistor also affects the frequency characteristics. Therefore, with this configuration, the gain can be adjusted while keeping the resistance of the feedback resistor fixed, making it possible to change the gain while reducing the impact on the frequency characteristics.

[0093] It is also preferable that the capacitance between the cable and the first electrode is substantially equal to the capacitance between the cable and the second electrode.

[0094] According to this configuration, even if the capacitance between the cable and the first electrode and the capacitance between the cable and the second electrode are unknown, calibration can be performed by the calibration processing unit.

[0095] It is also preferable that the area of ​​the first electrode and the area of ​​the second electrode are substantially equal.

[0096] This configuration makes it easy to make the capacitance between the cable and the first electrode and the capacitance between the cable and the second electrode approximately equal.

[0097] Furthermore, when the capacitance between the cable and the first electrode is Cx1, the capacitance between the cable and the second electrode is Cx2, the reference voltage is Vs, the measurement value of the measurement unit is Vm, the measurement magnification is M, and the amplification factor when the output voltage is measured by the measurement unit in the calibration mode is G, it is preferable that the calibration processing unit adjusts the amplification factor to Gx obtained by the following equation (1):

[0098] Gx={G×Vs×Cx2 / (Cx1+Cx2)}×M / Vm...(1)

[0099] According to this configuration, it becomes easy to calculate an appropriate amplification factor Gx to be adjusted by calibration using equation (1).

[0100] It is also preferable that the capacitance Cx1 and the capacitance Cx2 are substantially equal, and that the formula (1) is approximated by the following formula (2).

[0101] Gx = (G × Vs / 2) × M / Vm (2)

[0102] When the capacitances Cx1 and Cx2 are approximately equal, equation (1) is approximated by equation (2). Therefore, even if the capacitances Cx1 and Cx2 are unknown, the appropriate amplification factor Gx to be adjusted by calibration can be calculated using equation (2). [Explanation of symbols]

[0103] 1. Clamp-type voltage measuring device 2 Clamp section 3. Housing 4 Coaxial Cable 5 Stabilization section 7 Measuring part 8 Control Unit 9 Mode Switch 21,22 Clamp arm 23,24 screw holes 25 screws 26 Nut 27 Shaft 28 Containment Space 31 Circuit Board 32 Conductive layer 51 parallel circuit 81 Mode input section 82 Calibration processing section 211,221 Retaining groove A Amplification section A1 Amplifier C1, C2 capacitors C, C1, C2, Cx1, Cx2 capacitance CBL cable d Opposing distance E1 electrode (first electrode) E2 electrode (second electrode) G,Gx amplification factor GND Circuit Ground J coating M measurement magnification P1 one end P2 other end PS Reference voltage output section R1, R2, resistance Ra feedback resistor Rx variable resistor S1,S2 area SW Changeover switch (changeover section) T1~T4 terminals Vin Input voltage Vm measurements Vout Output voltage Vs Reference voltage Vx Cable voltage W core wire W1, W2 wiring ε dielectric constant

Claims

1. a first electrode and a second electrode disposed to face the object to be measured; a stabilizing unit that connects the first electrode to ground via a capacitor; an amplifier unit that can change the gain and amplifies the voltage obtained from the first electrode; a mode input unit that selectively accepts, as a mode setting, a setting of a measurement mode for measuring the voltage of the measurement object and a calibration mode for calibrating the output voltage of the amplifier unit; a reference voltage output unit that outputs a predetermined reference voltage that changes periodically; a measurement unit that measures an output voltage of the amplifier unit; a switching unit that connects the second electrode to ground in the measurement mode and connects the second electrode to the reference voltage output unit in the calibration mode; a calibration processing unit that, in the calibration mode, adjusts the amplification factor based on the measurement value of the measurement unit, the reference voltage, and a preset measurement magnification so that the output voltage of the amplifier unit becomes the measurement magnification times the voltage of the voltage to be measured.

2. 2. The voltage measuring device according to claim 1, wherein the amplifier section is a non-inverting amplifier.

3. The amplifier unit an amplifier having a non-inverting input terminal to which the voltage obtained from the first electrode is input; a feedback resistor connecting the inverting input terminal and the output terminal of the amplifier; a variable resistor connecting the inverting input terminal of the amplifier to ground; 3. The voltage measuring device according to claim 2, wherein the amplification factor can be changed by changing the resistance value of the variable resistor.

4. 4. The voltage measuring device according to claim 1, wherein the capacitance between the object to be measured and the first electrode is substantially equal to the capacitance between the object to be measured and the second electrode.

5. 5. The voltage measuring device according to claim 4, wherein the area of ​​the first electrode and the area of ​​the second electrode are substantially equal.

6. The voltage measurement device according to any one of claims 1 to 5, wherein the calibration processing unit adjusts the amplification factor to Gx obtained by the following formula (1): where Cx1 is the capacitance between the object to be measured and the first electrode, Cx2 is the capacitance between the object to be measured and the second electrode, Vs is the reference voltage, Vm is the measurement value of the measurement unit, M is the measurement magnification, and G is the amplification factor when the output voltage is measured by the measurement unit in the calibration mode. Gx={G×Vs×Cx2 / (Cx1+Cx2)}×M / Vm...(1)

7. The capacitance Cx1 and the capacitance Cx2 are substantially equal, 7. The voltage measuring device according to claim 6, wherein the formula (1) is approximated by the following formula (2): Gx=(G×Vs / 2)×M / Vm...(2)

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