JP7776199B2 - Voltage measuring device - Google Patents

Description

The present invention relates to a voltage measuring device that clamps the cable to be measured and measures it.

Conventionally, voltage detection devices have been known that have a detection electrode arranged opposite the object to be detected and detect the voltage of the object to be detected without electrical contact with the object to be detected (see, for example, Patent Document 1).

JP 2010-25918 A

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.

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

The object of the present invention is to provide a voltage measurement device that makes it easy to improve voltage measurement accuracy.

A voltage measurement device according to one example 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 has a changeable gain and amplifies the voltage obtained from the first electrode; a mode input unit that selectively accepts a mode setting between 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 periodically changing predetermined reference voltage; 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 is equal to the measurement magnification times the voltage of the cable.

A voltage measurement device with this configuration makes it easy to improve voltage measurement accuracy.

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. 2 is a front view showing the inside of the clamp arm and the front wall of the housing shown in FIG. 1 . FIG. FIG. 3 is a front view of the electrodes E1 and E2 shown in FIG. 2 as viewed from the Z direction. 2 is a circuit diagram showing an example of an electrical configuration of the clamp-type voltage measuring device 1 shown in FIG. 1 in a calibration mode. 2 is a circuit diagram showing an example of an electrical configuration of the clamp-type voltage measuring device 1 shown in FIG. 1 in a measurement mode. 10 is a flowchart showing an example of the operation of the clamp-type voltage measuring device 1 in a calibration mode. 4 is a flowchart showing an example of the operation of the clamp-type voltage measuring device 1 in a measurement mode.

Embodiments of the present invention will be described below with reference to the drawings. Note that components with the same reference numerals in each drawing are identical and will not be described again. X, Y, and Z orthogonal coordinate axes are shown in each drawing as appropriate to clarify directional relationships.

The clamp-type voltage measuring device 1 shown in Figure 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. The housing 3 is connected to a measuring device such as an oscilloscope or data logger via a coaxial cable 4.

The clamp unit 2 has a pair of clamp arms 21, 22. The base end of the clamp arm 21 is 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, 221 into which the cable CBL fits are formed on the opposing surfaces of the clamp arms 21, 22.

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

Referring to Figure 2, the clamp arm 22 is fixedly connected to the housing 3. Note that the clamp arm 22 may be swingable like the clamp arm 21. The clamp arms 21, 22 and the housing 3 are made of an insulating material, for example, a resin material.

An accommodation space 28 is provided inside the clamp arm 22. The accommodation space 28 is connected to the internal space of the housing 3. An electrode E1 (first electrode) and an electrode E2 (second electrode), each having a substantially plate-like shape, are disposed in the accommodation space 28.

Referring to Figure 3, electrodes E1 and E2 are arranged opposite cable CBL with a gap between them in the direction in which cable CBL extends.

Electrodes E1 and E2 may be formed, for example, as a conductor pattern on a printed circuit board, or may be metal plates. Electrodes E1 and E2 are arranged facing or in contact with the inner wall surface of the holding groove 221 of the clamp arm 22. This allows electrodes E1 and E2 to face the cable CBL clamped by the clamp unit 2, via the wall of the holding groove 221, which is made of an insulating material.

The housing 3 contains a circuit board 31. Terminal T3 and electrode E1 of the circuit board 31 are connected via wiring W1, and terminal T4 and electrode E2 are connected via wiring W2.

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 and E2.

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 Figure 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 portion 2 and the electrode E1, or on the periphery of the electrode E1.

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.

Because the outer wall surface of the clamp-type voltage measuring device 1 is insulating, even if the conductor portion of the cable CBL is exposed, the risk of current flowing from the cable CBL to the electrodes E1, E2 or the circuit board 31 and damaging the clamp-type voltage measuring device 1 is reduced. This also improves the safety of users operating the clamp-type voltage measuring device 1. Furthermore, the provision of the conductive layer 32 helps reduce electromagnetic noise from the external environment.

Note that electrodes E1 and E2 are not necessarily arranged inside the clamping portion 2 and are not limited to being arranged so as to face the cable CBL via an insulating material. Electrodes E1 and E2 may be arranged, for example, exposed in the holding groove 221 and arranged so as to face the cable CBL while in contact with it.

As will be described later, the clamp-on voltage measurement device 1 detects the cable voltage Vx, which is the voltage of the core wire W of the cable CBL, through the capacitances _Cx1 and _Cx2 that 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 facing distance d between the core wire W of the cable CBL and the electrodes _E1 and E2. Therefore, the shorter the facing distance d, the greater the capacitances Cx1 and _Cx2 , making it easier to detect the cable voltage Vx. Furthermore, when the facing distance d changes, the voltage obtained via the capacitances Cx1 and Cx2 fluctuates.

On the other hand, the clamping force of the clamp arms 21, 22, when exerted solely by the biasing force of the torsion spring (not shown), may be insufficient to clamp the cable CBL, causing a gap to form between the cable CBL and the holding groove 221, increasing the opposing distance d and decreasing the capacitances _Cx1 and _Cx2 . In addition, the cable CBL may sway, causing the opposing distance d to fluctuate, and the capacitances _Cx1 and _Cx2 to fluctuate.

However, the clamp-type voltage measuring device 1 can firmly clamp the cable CBL by fastening the clamp arms 21 and 22 with the 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 the shaking of the cable CBL.

The clamp-type voltage measuring device 1 shown in Figures 4 and 5 comprises electrodes E1, E2, a selector switch SW (switching unit), a reference voltage output unit PS, a stabilizing unit 5, an amplifier unit 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, an amplifier unit A, a measuring unit 7, terminals T1, T2, and a control unit 8 are formed on a circuit board 31 and housed in the 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.

The cable CBL has a conductor core wire W covered with an insulating coating J. In Figures 4 and 5, the capacitance formed when the core wire W faces the electrode E1 is represented by capacitance _Cx1 , and the capacitance formed when the core wire W faces the electrode E2 is represented by capacitance _Cx2 .

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 the terminal T3 and 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 also connected to the circuit ground GND via the resistor R2. The circuit ground GND is connected to the conductive layer 32.

The stabilization unit 5 stabilizes the potential of the input voltage Vin input from electrode E1 to the non-inverting input terminal of amplifier A1, and also functions as a filter (described below). If the clamp-type voltage measurement device 1 did not include the stabilization unit 5, electrode E1 would simply be connected to the high-impedance non-inverting input terminal of amplifier A1, leaving electrode E1 in an electrically floating state with no reference potential, and the wiring from electrode E1 to the non-inverting input terminal would act like an antenna. This would cause the input voltage Vin input to the non-inverting input terminal of amplifier A1 to become unstable.

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

The stabilization unit 5 does not necessarily have to function as a filter, as described below, but only needs to be able to stabilize the input voltage Vin. For example, the stabilization unit 5 may be configured so that only the capacitor C2 is connected to the electrode E1.

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 .

The amplifier section 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 section A a non-inverting amplifier. The variable resistor Rx can be, for example, a digital potentiometer. Note that 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.

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 section A is G = 1 + Ra/Rx. Therefore, the amplification factor G can be adjusted by changing the resistance value Rx.

The feedback resistor Ra may be a variable resistor, and the gain G of the amplifier unit A may be adjusted by changing the resistance value Ra of the feedback resistor Ra. 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.

Note that amplifier unit A may be an inverting amplifier. However, if amplifier unit A is an inverting amplifier, electrode E1 will be connected to the same inverting input terminal to which the feedback resistor is connected. In this case, leakage current may flow through the current path from electrode E1 via the feedback resistor to the amplifier's output terminal, potentially affecting the input voltage Vin.

On the other hand, if amplifier unit A is a non-inverting amplifier, electrode E1 is connected to the high-impedance non-inverting input terminal of amplifier A1, and no leakage current occurs through 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 amplifier unit A.

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-type voltage measurement device 1.

The clamp-type voltage measurement device 1 outputs the cable voltage Vx multiplied by a measurement magnification factor M as the output voltage Vout. Because the measurement magnification factor M is known, the 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 accurately represent the measured cable voltage Vx. Therefore, using the calibration mode described below, the amplification factor G is adjusted so that the output voltage Vout is multiplied by the measurement magnification factor M of the cable voltage Vx. The measurement magnification factor M is, for example, 1/100.

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 the inverter, is the object of measurement.

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 rectangular wave generated by PWM, high-frequency components are included in the rising and falling edges of the rectangular wave. Therefore, in order 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 rectangular wave and the high-frequency components corresponding to the rising and falling edges of the rectangular wave.

Therefore, the clamp-type voltage measuring device 1 forms a series circuit of resistor R1 and capacitor C2. 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.

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 rectangular wave to be input to amplifier A1.

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

The input terminal of amplifier A1 is applied with the sum of the voltage across resistor R2 and the voltage across 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.

In other words, the input terminal of amplifier A1 receives a superimposed signal consisting of both 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. Therefore, the AC voltage waveform detected from the core wire W can be accurately amplified by amplifier A1 and output to the measuring device.

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 it is preferable that the reference voltage Vs has a signal waveform similar to the voltage to be detected on the cable CBL. For example, as described above, if the measurement target is an AC voltage with a square wave periodic waveform, it is preferable that the reference voltage output unit PS output a square wave voltage as the reference voltage Vs.

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

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.

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 the measured value Vm of the output voltage Vout to the control unit 8. Because the cable voltage Vx to be measured is AC, the output voltage Vout also has an AC waveform. Therefore, it is preferable for the measurement unit 7 to measure the peak-to-peak value of the output voltage Vout as the measured value Vm.

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

Based on the signal output from the mode switch 9, the mode input unit 81 selectively accepts a mode setting between a measurement mode for measuring the cable voltage Vx, which is the voltage of the core wire W of the cable CBL, and 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 connects the electrode E2 to the circuit ground GND using the selector switch SW, and when the accepted mode setting is the calibration mode, it connects the electrode E2 to the reference voltage output unit PS using the selector switch SW.

Note that the example is not necessarily limited to the mode input unit 81 switching 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.

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 a mode switch 9.

When in 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 predetermined measurement magnification M so that the output voltage Vout of the amplifier unit A becomes the measurement magnification M times the cable voltage Vx.

Specifically, the calibration processing unit 82 calculates the amplification factor Gx, which is the target value after calibration that will obtain the measurement magnification M, based on 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, based on the following equation (1):

Gx={G×Vs×Cx ₂ /(Cx ₁ +Cx ₂ )}×M/Vm...(1)

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, capacitance _Cx1 and capacitance _Cx2 can be made approximately equal by setting _S1 = _S2 . When capacitance _Cx1 and capacitance _Cx2 are equal, equation (1) becomes the following equation (2).

Gx=(G×Vs/2)×M/Vm...(2)

Therefore, the calibration processing unit 82 can calculate the amplification factor Gx based on formula (2). When calculating the amplification factor Gx based on formula (1), it is necessary to measure the capacitances _Cx1 and _Cx2 , but according to formula (2), it is not necessary to measure the capacitances _Cx1 and _Cx2 , so that the calculation of the amplification factor Gx becomes easier.

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.

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 distances between electrodes E1, _E2 and core wire W are the same opposing distance d. Therefore, making area _S1 and area _S2 equal is more preferable because it makes it easier to calculate the amplification factor Gx using equation (2).

The calibration processing unit 82 adjusts the amplification factor of the amplifier unit A to the amplification factor Gx by adjusting the resistance value of the variable resistor Rx.

Next, we will explain the derivation of equation (1). As shown in Figure 4, we will explain the case where electrode E2 is connected to the reference voltage output unit PS in calibration mode. In calibration mode, no external voltage is applied to the cable CBL. In this state, the voltage induced in the core wire W based on the reference voltage Vs of the reference voltage output unit PS becomes the cable voltage Vx.

Therefore, the cable voltage Vx is the voltage obtained by dividing the reference voltage Vs between 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 small enough to be ignored compared to the impedance of the capacitances _Cx1 and _Cx2 . Therefore, the cable voltage Vx is the value obtained by dividing the reference voltage Vs between the capacitances _Cx1 and _Cx2 , and can be approximated by the following equation (3).

Cable voltage Vx≈Vs×Cx ₂ /(Cx ₁ +Cx ₂ ) (3)

Here, if the amplification factor of amplifier unit A before calibration during calibration mode is G and the desired amplification factor of amplifier unit A after calibration is Gx, the following equation (4) holds.

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

Amplification factor Gx=G×Vx×M/Vm (5)
By substituting equation (3) for the cable voltage Vx in equation (5), the above equation (1) is obtained.

To set the amplification factor of amplifier section A to Gx, simply adjust the resistance value Rx to satisfy the following equation (6).

Amplification factor Gx=1+Ra/Rx (6)
Transforming equation (6) gives:

Resistance value Rx = Ra / (Gx - 1) ... (7)

From the above, the calibration processing unit 82 calculates the resistance value Rx by substituting the amplification factor Gx obtained from equation (1) or equation (2) into equation (7), and by setting the resistance value of the variable resistor Rx to the resistance value Rx obtained from equation (7), it is possible to adjust the amplification factor Gx of the amplification unit A. 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).

Output voltage Vout = M x Vx (8)

Next, an example of the operation of the clamp-type voltage measuring device 1 configured as described above will be described. Referring to Figure 6, when calibrating the clamp-type voltage measuring device 1, the user removes the cable CBL from the device, for example, to ensure that no voltage is applied to the core wire W. In this state, the user clamps the cable CBL with the clamp unit 2 and operates the mode switch 9 to set the calibration mode. This causes the mode switch 9 to output a signal indicating the calibration mode to the control unit 8.

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 capacitance _Cx2 , the core wire W, and the capacitance _Cx1 .

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 mentioned 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.

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.

Next, the calibration processing unit 82 calculates the resistance value Rx using 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). This sets the amplification factor of the amplifier A to Gx.

This allows the clamp-type voltage measuring device 1 to be calibrated so that the output voltage Vout becomes the measurement magnification M of the cable voltage Vx, i.e., so that equation (8) is satisfied.

Next, we will explain the measurement mode. When measuring the cable voltage Vx while the cable CBL is in use, the user clamps the cable CBL connected to the device using the clamp unit 2 and operates the mode switch 9 to set the measurement mode. This causes the mode switch 9 to output a signal indicating the measurement mode to the control unit 8.

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 .

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

As described above, the capacitance _Cx1 is expressed by _εS1 /d, and if the cable CBL to be measured changes and the opposing distance d due to the thickness of the coating J and the dielectric constant ε of the coating J 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.

However, with the clamp-type voltage measuring 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 the correct output voltage Vout that is appropriate for the cable CBL to be measured. Therefore, the clamp-type voltage measuring device 1 makes it easy to improve voltage measurement accuracy.

That is, a voltage measurement 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 has a changeable gain and amplifies the voltage obtained from the first electrode; a mode input unit that selectively accepts a mode setting between 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 periodically changing predetermined reference voltage; 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 is equal to the measurement magnification times the voltage of the cable.

With this configuration, the first electrode and second electrode are positioned 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, based on the measurement value of the measurement unit, the reference voltage, and the measurement magnification, the calibration processing unit adjusts the amplification factor of the amplifier so that the output voltage of the amplifier is multiplied by the measurement magnification of the cable voltage. This adjusts the amplification of the amplifier according to the cable to be measured, making it easier to improve voltage measurement accuracy.

It is also preferable that the amplifier section is a non-inverting amplifier.

With this configuration, a non-inverting amplifier with high input impedance is used as the amplifier unit, reducing the risk that the input impedance of the amplifier unit will affect the voltage of the first electrode.

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.

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

It is also preferable that the capacitance between the cable and the first electrode is approximately equal to the capacitance between the cable and the second electrode.

With this configuration, calibration can be performed by the calibration processing unit even if the capacitance between the cable and the first electrode and the capacitance between the cable and the second electrode are unknown.

It is also preferable that the area of the first electrode and the area of the second electrode are approximately equal.

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.

Furthermore, when the capacitance between the cable and the first electrode is Cx ₁ , the capacitance between the cable and the second electrode is Cx ₂ , 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).

Gx={G×Vs×Cx ₂ /(Cx ₁ +Cx ₂ )}×M/Vm...(1)

With this configuration, it becomes easy to use equation (1) to calculate the appropriate amplification factor Gx to be adjusted by calibration.

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).

Gx=(G×Vs/2)×M/Vm...(2)

When the capacitances _Cx1 and _Cx2 are approximately equal, the formula (1) is approximated by the formula (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 the formula (2).

1 Clamp-type voltage measuring device 2 Clamp unit 3 Housing 4 Coaxial cable 5 Stabilization unit 7 Measurement unit 8 Control unit 9 Mode switch 21, 22 Clamp arm 23, 24 Screw hole 25 Screw 26 Nut 27 Shaft 28 Housing space 31 Circuit board 32 Conductive layer 51 Parallel circuit 81 Mode input unit 82 Calibration processing unit 211, 221 Holding groove A Amplification unit A1 Amplifier _C1 , C2 Capacitor C, C1, _C2 , _Cx1 , _Cx2 Electrostatic capacitance CBL Cable d Opposite distance E1 Electrode (first electrode)
E2 electrode (second electrode)
G, Gx Amplification factor GND Circuit ground J Covering M Measurement magnification P1 One end P2 Other end PS Reference voltage output section R1, R2, Resistor Ra Feedback resistor Rx Variable resistor _S1 , _S2 Area SW Changeover switch (changeover section)
T1 to T4 Terminal Vin Input voltage Vm Measured value Vout Output voltage Vs Reference voltage Vx Cable voltage W Core wires W1, W2 Wiring ε Dielectric constant

Claims (7)

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 factor so that the output voltage of the amplifier unit becomes equal to the measurement magnification factor of the voltage of the object to be measured. The voltage measurement device of claim 1, wherein the amplifier unit is a non-inverting amplifier. 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. 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. The voltage measurement device described in claim 4, wherein the area of the first electrode and the area of the second electrode are approximately equal. 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) The capacitance Cx1 and the capacitance Cx2 are approximately 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)