Wire disconnection sensing device and wire disconnection sensing method

Abstract

This disconnection sensing device comprises an acquisition unit and a determination unit. The acquisition unit acquires variation in leakage current flowing in a shield of a power cable for transmitting power supplied from a power supply device to a plasma head for generating plasma. The determination unit determines the presence or absence of disconnection of the power cable on the basis of the variation in the leakage current acquired by the acquisition unit.

Description

Wire break detection device and wire break detection method

[0001] This specification discloses a technique relating to a wire break detection device and a wire break detection method.

[0002] The plasma generator described in Patent Document 1 includes a pair of cables that transmit power from a power supply to a plasma head, a conductive shielding member that shields the pair of cables, a detection device that detects the current flowing through the shielding member, and a control device. The control device executes a determination process to determine whether or not there is a current abnormality based on the result of comparing the current value detected by the detection device with a predetermined value.

[0003] Japanese Patent Application Laid-Open No. 2020-035685

[0004] When a current detector is attached to the shield of a power cable to detect surge noise generated when the power cable is broken, the current detector is susceptible to the influence of external noise other than the break, which can lead to false detection. Furthermore, in order to suppress the influence of external noise other than the break, tuning or the like is required in the environment in which the plasma generator is actually used, which can make the installation work of the plasma generator complicated.

[0005] In view of the above circumstances, this specification discloses a disconnection detection device and a disconnection detection method that can determine whether or not a power cable has a disconnection.

[0006] This specification discloses a wire break detection device including an acquisition unit and a determination unit. The acquisition unit acquires fluctuations in leakage current flowing through the shield of a power cable that transmits power supplied from a power supply device to a plasma head that generates plasma. The determination unit determines whether or not there is a wire break in the power cable based on the fluctuations in leakage current acquired by the acquisition unit.

[0007] This specification also discloses a wire break detection method including an acquisition step and a determination step. The acquisition step acquires fluctuations in leakage current flowing through the shield of a power cable that transmits power supplied from a power supply device to a plasma head that generates plasma. The determination step determines whether or not there is a wire break in the power cable based on the fluctuations in leakage current acquired in the acquisition step.

[0008] In addition, this specification discloses the technical idea of ​​changing "the wire breakage detection device according to claim 1" to "the wire breakage detection device according to any one of claims 1 to 7" in claim 8 of the claims originally attached to the application.

[0009] The above-described disconnection detection device can determine whether or not a disconnection has occurred in a power cable based on fluctuations in the leakage current flowing through the shield of the power cable. What has been described above about the disconnection detection device can also be applied to the disconnection detection method.

[0010] 11 is a schematic diagram showing an example of the configuration of a plasma generation device; FIG. 12 is a schematic diagram showing an example of the electrical configuration of a plasma generation device; FIG. 13 is a partial cross-sectional view showing an example of a first current path and a second current path; FIG. 14 is a waveform diagram showing an example of a change over time in leakage current when there is no disconnection; FIG. 15 is a waveform diagram showing an example of a change over time in leakage current when there is a disconnection; FIG. 16 is a block diagram showing an example of a control block of a disconnection detection device; FIG. 17 is a flowchart showing an example of a control flow by the disconnection detection device; FIG. 18 is a schematic diagram showing an example of the configuration of an apparatus for detecting leakage current flowing through the shield of a power cable; FIG. 19 is a waveform diagram showing an example of a change over time in detected leakage current; FIG. 19 is a waveform diagram showing an example of a waveform obtained by voltage conversion of the waveform of FIG. 9; FIG. 20 is a waveform diagram showing an example of a waveform obtained by DC conversion of the waveform of FIG. 11;

[0011] 1. Embodiment 1-1. Configuration Example of Plasma Generator 1 The wire breakage detection device 90 can be applied to the plasma generator 1. The plasma generator 1 of the embodiment is an atmospheric pressure plasma generator that generates plasma at atmospheric pressure. As shown in FIG. 1, the plasma generator 1 includes a plasma head 10, a power supply device 20, a gas supply device 30, a gas pipe 31, a control device 40, a robot 50, a housing 60, a cover 70, and a power cable 80. For example, the wire breakage detection device 90 is provided in the control device 40.

[0012] The plasma generator 1 supplies power to the plasma head 10 from the power supply device 20 via a power cable 80. The plasma generator 1 also supplies a process gas to be converted into plasma from the gas supply device 30 via a gas pipe 31. This allows the plasma generator 1 to irradiate the plasma gas from the plasma head 10.

[0013] The power supply device 20 supplies power to the plasma head 10, which generates plasma. The power supply device 20 can use a high-frequency power supply capable of generating plasma. As shown in FIG. 2 , the power supply device 20 includes a converter 21, an inverter 22, a smoother 23, a booster 24, and a terminal block 25. The converter 21 converts AC power, such as that from a commercial power source, into DC power. The inverter 22 converts the DC power output from the converter 21 into AC power. The smoother 23 smooths the AC power output from the inverter 22. The booster 24 boosts the AC power smoothed by the smoother 23 to generate power to be supplied to the plasma head 10.

[0014] The power supply device 20 may be any known power device capable of supplying power to the plasma head 10. For example, the inverter 22 may be an inverter including a plurality of (e.g., four) bridge-connected switching elements. The smoother 23 may be a smoother including a coil and a capacitor. The booster 24 may be a transformer. As shown in FIG. 2, the transformer includes a primary winding 24a and a secondary winding 24b.

[0015] The transformer boosts the AC power input to the primary winding 24a (the AC power smoothed by the smoother 23) and outputs the boosted AC power from the secondary winding 24b. For example, the output voltage of the AC power output from the secondary winding 24b is approximately 15 kV at most. Furthermore, since the voltage drops during discharge, the output voltage of the AC power output from the secondary winding 24b is approximately 8 kV to 9 kV. In this way, the power supply device 20 can output AC power of several thousand volts to tens of thousands of volts. Furthermore, the power supply device 20 can output AC power at a frequency of several kilohertz to several tens of kilohertz.

[0016] 2, the terminal block 25 includes an input terminal 25a, an input terminal 25b, an output terminal 25c, and an output terminal 25d. One end of the secondary winding 24b, i.e., an output terminal 24b1, is connected to the input terminal 25a. The other end of the secondary winding 24b, i.e., an output terminal 24b2, is connected to the input terminal 25b.

[0017] One axial end 80a of the power cable 80 is connected to the output terminals 25c and 25d. The other axial end 80b of the power cable 80 is connected to the inner electrode 10a and the outer electrode 10b provided in the plasma head 10. The housing 10h of the plasma head 10, which is the outer electrode 10b, is cylindrical and is disposed so as to surround the inner electrode 10a, which is provided inside the outer electrode 10b.

[0018] The gas supply device 30 pressure-feeds and supplies a gas (e.g., air, dry air, etc.) containing at least one of an inert gas (e.g., nitrogen, etc.) and an active gas (e.g., oxygen, etc.) as a process gas. For this purpose, the gas supply device 30 includes a tank for storing the process gas. The gas supply device 30 may also include a heater for heating the process gas to be supplied to the plasma head 10 as necessary. In this case, the gas supply device 30 can supply the process gas heated by the heater to the plasma head 10, for example. The gas supply device 30 can also mix unheated process gas supplied from the tank with process gas heated by the heater and supply the mixture to the plasma head 10.

[0019] The control device 40 drives and controls the plasma generator 1. As shown in FIG. 1, the power cable 80 and the gas pipe 31 are attached along the robot arm 51 of the robot 50. The plasma head 10 is attached to the tip of the robot arm 51. The robot arm 51 has multiple (two in the figure) arm portions 51a connected in one direction. In other words, the robot 50 is an articulated robot. The robot 50 is driven and controlled by the control device 40. Specifically, the robot 50 drives the robot arm 51 based on commands from the control device 40 to move the plasma head 10, thereby irradiating the workpiece W0 supported on the workpiece table D0 with plasma gas.

[0020] As shown in FIG. 1 , the plasma head 10 has a base end fixed to a housing 60 and a tip end covered by a cover 70. An opening 71 is provided at the tip of the cover 70 for irradiating the plasma gas generated by the plasma head 10. The plasma head 10 generates the plasma gas by converting a process gas into plasma using power supplied from a power supply unit 20. The plasma head 10 irradiates the generated plasma gas onto the surface of the workpiece W0. The plasma gas irradiated onto the surface of the workpiece W0 can be used for various surface treatments of the surface of the workpiece W0, such as a modification treatment for modifying the surface of the workpiece W0 from hydrophobic to hydrophilic.

[0021] 1, the power cable 80 is attached along the robot arm 51 of the robot 50. Therefore, as the robot arm 51 moves, stress such as bending and pulling is applied to the power cable 80, which may cause the wires of the power cable 80 to break. Therefore, it is conceivable that a break in the power cable 80 may be detected by, for example, providing a current detector on the shield of the power cable 80 and detecting surge noise generated when the power cable 80 breaks.

[0022] However, the above-described disconnection detection is susceptible to the influence of external noise other than the disconnection, and there is a possibility of false detection. Furthermore, in order to suppress the influence of external noise other than the disconnection, tuning or the like is required in the environment in which the plasma generator 1 is actually used, which may make the installation work of the plasma generator 1 complicated. Therefore, the plasma generator 1 of the embodiment is provided with a disconnection detection device 90.

[0023] 1-2-1. Principle of Disconnection Detection by Disconnection Detector 90 Figure 3 is a partial cross-sectional view showing an example of a cross section of the plasma head 10 and power cable 80 shown in Figure 1 taken along the axial direction, illustrating an example of a first current path IL1 and a second current path IL2. The power cable 80 may take various forms as long as it can transmit power supplied from the power supply device 20 to the plasma head 10. For example, the power cable 80 shown in Figure 3 includes multiple (two in this figure) electric wires 81, a shield 82, a sheath 83, and an interposer 84.

[0024] The multiple (two) electric wires 81 are members provided on the radial center side of the power cable 80 and transmit power supplied from the power supply device 20 to the plasma head 10. One of the multiple (two) electric wires 81 is referred to as electric wire 81f, and the other of the multiple (two) electric wires 81 is referred to as electric wire 81s. The shield 82 is a conductive member provided radially outward of the power cable 80 relative to the multiple (two) electric wires 81 so as to cover the multiple (two) electric wires 81, and electrically shields the multiple (two) electric wires 81 from the external environment.

[0025] The sheath 83 is an insulating member provided radially outside the shield 82 of the power cable 80 to cover the shield 82 and physically protect the shield 82 from the external environment. The interposer 84 is an insulating member provided radially inside the power cable 80 of the shield 82 and supports the multiple (two) electric wires 81. Note that known members can be used for the power cable 80. The outer electrode 10b, one electric wire 81s of the power cable 80, the shield 82, and one output end 24b2 of the booster 24 are grounded. In the figure, the grounded state is indicated by symbol E0 (the same applies to other figures).

[0026] 2 and 3 , a current path of a main current I1 that flows due to a voltage applied between the outer electrode 10 b and the inner electrode 10 a to generate plasma is designated as a first current path IL1. Specifically, the first current path IL1 passes through the grounded output end 24 b 2 of the booster 24, the grounded electric wire 81 s of the power cable 80, the outer electrode 10 b, the inner electrode 10 a, another electric wire 81 f of the power cable 80, and the other output end 24 b 1 of the booster 24 in this order.

[0027] Furthermore, the current path of leakage current I2 flowing through shield 82 of power cable 80 is referred to as second current path IL2. Second current path IL2 passes through grounded output end 24b2 of booster 24, shield 82 of power cable 80, another electric wire 81f of power cable 80, and another output end 24b1 of booster 24 in this order. As shown in Fig. 3 , leakage current I2 is generated based on parasitic capacitance C0 generated between electric wire 81 and shield 82 of power cable 80.

[0028] When generating plasma, a high-frequency (e.g., several kHz or higher) AC voltage is applied between the inner electrode 10 a and the outer electrode 10 b to prevent arc discharge. At this time, a portion of the current flows to the shield 82 due to the parasitic capacitance C0, generating a leakage current I2. When the discharge between the inner electrode 10 a and the outer electrode 10 b is stable, the leakage current I2 flows stably in accordance with the voltage period T0 of the output voltage of the power supply device 20.

[0029] However, when the discharge between the inner electrode 10a and the outer electrode 10b is unstable (for example, at the start of discharge), the leakage current I2 also becomes unstable, and the leakage current I2 increases compared to when the discharge is stable. When a break occurs in the power cable 80, a discharge occurs at the broken portion because the applied voltage is relatively high. When a discharge occurs at the broken portion, the leakage current I2 increases, similar to when the discharge is unstable (for example, at the start of discharge).

[0030] FIG. 4 shows an example of the change over time in leakage current I2 when there is no disconnection. FIG. 5 shows an example of the change over time in leakage current I2 when there is a disconnection. In these figures, the vertical axis represents current, and the horizontal axis represents time. The change over time in leakage current I2 when there is no disconnection is shown by waveform W1, and the change over time in leakage current I2 when there is a disconnection is shown by waveform W2. As shown in FIG. 4, waveform W1 includes a peak current IN0 due to external noise in a roughly constant leakage current I2. Since peak current IN0 is caused by noise, it occurs irregularly and the peak value also varies.

[0031] In contrast, waveform W2 includes a peak current IP0 that pulses in time with the voltage period T0 of the output voltage of power supply device 20, at a generally constant leakage current I2. As previously mentioned, peak current IP0 is caused by discharge at the disconnected portion, so it occurs regularly with the voltage period T0 and its peak value is generally constant. Furthermore, it is only the portion of peak current IP0 that increases leakage current I2 due to the disconnection, and the overall average value of leakage current I2 is not significantly different from when there is no disconnection.

[0032] The disconnection detection device 90 determines whether or not there is a disconnection in the power cable 80 by taking into consideration the difference between the waveform W1 when no disconnection occurs, as shown in Fig. 4, and the waveform W2 when a disconnection occurs, as shown in Fig. 5. This allows the disconnection detection device 90 to perform disconnection detection that is less susceptible to external noise and does not require tuning to suit the usage environment.

[0033] 6, the disconnection detection device 90 includes an acquisition unit 91 and a determination unit 92. The acquisition unit 91 and the determination unit 92 can be provided in various control devices, management devices, arithmetic devices, etc. For example, at least one of the acquisition unit 91 and the determination unit 92 can be provided in the control device 40.

[0034] At least one of the acquisition unit 91 and the determination unit 92 may be provided in the power supply device 20. At least one of the acquisition unit 91 and the determination unit 92 may be provided in a control device, management device, calculation device, or the like external to the plasma generator 1. At least one of the acquisition unit 91 and the determination unit 92 may be formed on the cloud. As shown in FIG. 6 , the acquisition unit 91 and the determination unit 92 of the embodiment are provided in the control device 40.

[0035] The disconnection detection device 90 executes control in accordance with the flowchart shown in Fig. 7. The acquisition unit 91 performs the processes shown in steps S11 to S15. The determination unit 92 performs the determination and processes shown in steps S16 to S18. The matters described in this specification can be selected and applied as appropriate. The matters described in this specification can also be combined as appropriate.

[0036] 1-2-3. Acquisition Unit 91 The acquisition unit 91 acquires fluctuations in leakage current I2 flowing through the shield 82 of the power cable 80, which transmits power supplied from the power supply device 20 to the plasma head 10, which generates plasma (steps S11 to S15 shown in FIG. 7). The acquisition unit 91 may take various forms as long as it is able to acquire fluctuations in leakage current I2. As described above, the acquisition unit 91 can acquire fluctuations in leakage current I2 that occur based on the parasitic capacitance C0 that occurs between the electric wire 81 and the shield 82 of the power cable 80.

[0037] Specifically, as shown in FIG. 8 , the wire break detection device 90 may include a current detector CT0 that detects leakage current I2 flowing through the shield 82. The current detector CT0 may be any known current detector as long as it can detect leakage current I2. For example, the current detector CT0 may be a current transformer. Also, as shown in FIG. 8 , the wire break detection device 90 may include a conversion board 93 and a processing unit 94.

[0038] A detected current ID0 of the leakage current I2 detected by the current detector CT0 is input to the conversion board 93. The conversion board 93 is equipped with a known signal processing device, etc., and the acquisition unit 91 can acquire a peak current IP0 by signal processing the detected current ID0 in the conversion board 93. The peak current IP0 acquired by the acquisition unit 91 is output from the conversion board 93 to the arithmetic processing device 94. The arithmetic processing device 94 is equipped with a known AD converter 94a, and can convert the input peak current IP0 as an analog signal into a digital signal.

[0039] The acquisition unit 91 may take various forms as long as it can acquire the peak current IP0 by signal processing the detected current ID0 of the leakage current I2 detected by the current detector CT0. For example, the acquisition unit 91 can acquire the peak current IP0 by converting the detected current ID0 to a voltage and then to a direct current, and then performing quasi-peak detection and rectifying the converted current (steps S12 to S15). Figure 9 shows an example of the change over time in the detected current ID0 of the leakage current I2. The vertical axis in the figure represents current, and the horizontal axis represents time. The change over time in the detected current ID0 is indicated by waveform W3.

[0040] The acquisition unit 91 may take various forms as long as it can convert the detected current ID0 of the leakage current I2 detected by the current detector CT0 into a voltage. Fig. 10 shows an example of a waveform obtained by voltage-converting the waveform of Fig. 9. The vertical axis of the figure represents voltage, and the horizontal axis represents time. The voltage-converted waveform is shown as waveform W4. Waveforms W3 and W4 are AC signals of the same shape, but their amplitudes differ due to the conversion from current to voltage.

[0041] The acquisition unit 91 may take various forms as long as it can convert the voltage-converted waveform into a DC signal. Fig. 11 shows an example of a waveform obtained by converting the waveform of Fig. 10 into a DC signal. The vertical axis of the figure indicates voltage, and the horizontal axis indicates time. The voltage-converted waveform is shown as waveform W5. Waveform W5 corresponds to a waveform obtained by folding back the negative value side of waveform W4 to the positive value side. In this way, the acquisition unit 91 can perform DC conversion by full-wave rectification.

[0042] As shown in Figure 5, the leakage current I2 at the time of a disconnection includes a peak current IP0 that is generated in a pulse shape in accordance with the voltage period T0 of the output voltage of the power supply device 20. The leakage current I2 increases only in the peak current IP0 portion due to a disconnection, and the overall average value of the leakage current I2 is not significantly different from when the disconnection is not present. Therefore, if the acquisition unit 91 simply rectifies the DC-converted waveform, it will have difficulty acquiring the peak current IP0. Furthermore, if the acquisition unit 91 performs peak value detection on the DC-converted waveform, there is a possibility that it will erroneously detect a peak wave that momentarily increases due to noise or the like.

[0043] Therefore, it is preferable that the signal processing includes quasi-peak detection. By performing quasi-peak detection on the DC-converted waveform, the acquisition unit 91 can detect the peak current IP0 that occurs in pulses in accordance with the voltage period T0 of the output voltage of the power supply device 20. Furthermore, with quasi-peak detection, even if a peak wave is detected, the detected value will be small if the frequency of occurrence is low. Therefore, by performing quasi-peak detection on the DC-converted waveform, the acquisition unit 91 can reduce false detections when detecting peak waves that momentarily increase due to noise or the like.

[0044] Figure 12 shows an example of a waveform obtained by quasi-peak detection of the waveform in Figure 11. The vertical axis in the figure represents voltage, and the horizontal axis represents time. The waveform obtained by quasi-peak detection is shown as waveform W6. Furthermore, the voltage level corresponding to peak current IP0, which is generated in a pulse shape in accordance with voltage period T0, is shown as voltage VP0.

[0045] The acquisition unit 91 may take various forms as long as it can rectify the quasi-peak detected waveform. Fig. 13 shows an example of a waveform obtained by rectifying the waveform of Fig. 12. The vertical axis of the figure represents voltage, and the horizontal axis represents time. The rectified waveform is shown as waveform W7. Furthermore, the voltage level corresponding to the peak current IP0, which is generated in a pulsed manner in accordance with the voltage period T0, is shown as voltage VP0. The waveform W7 fluctuates more slowly than the waveform W6. In other words, the rectification described above corresponds to a low-pass filter.

[0046] 1-2-4. Determination Unit 92 The determination unit 92 determines whether or not there is a break in the power cable 80 based on the fluctuation in the leakage current I2 acquired by the acquisition unit 91 (steps S16 to S18 shown in FIG. 7). The determination unit 92 may take various forms as long as it can determine whether or not there is a break in the power cable 80 as described above. For example, when the determination unit 92 acquires, by the acquisition unit 91, a peak current IP0 that occurs in pulses in accordance with the voltage period T0 of the output voltage of the power supply device 20 as a fluctuation in the leakage current I2, the determination unit 92 can determine that there is a break in the power cable 80.

[0047] Furthermore, when the peak current IP0 is not acquired by the acquisition unit 91, the determination unit 92 can determine that no break has occurred in the power cable 80. As described above, the peak current IP0 acquired by the acquisition unit 91 is converted from an analog signal (the voltage VP0 shown in FIG. 13 ) into a digital signal by the arithmetic processing unit 94. For example, when the peak current IP0 converted into a digital signal by the arithmetic processing unit 94 is greater than a predetermined threshold (Yes in step S16), the determination unit 92 can determine that a break has occurred in the power cable 80 (step S17). The predetermined threshold is a threshold used to determine whether or not there is a break in the power cable 80, and can be acquired in advance by simulation, verification using an actual device, or the like.

[0048] In this case, the control device 40 of the plasma generator 1 can stop the supply of power from the power supply device 20 to the plasma head 10. The control device 40 can also stop the supply of process gas from the gas supply device 30 to the plasma head 10. Furthermore, the control device 40 can notify the operator, on an operation panel or the like operated by the operator, that a disconnection abnormality has occurred in the power cable 80. This allows the operator to confirm the disconnection abnormality in the power cable 80 and take action.

[0049] If the peak current IP0 converted into a digital signal by the arithmetic processing device 94 is equal to or less than a predetermined threshold (No in step S16), the determination unit 92 can determine that no break has occurred in the power cable 80 (step S18). In this case, the control device 40 of the plasma generator 1 can continue to supply power from the power supply device 20 to the plasma head 10. The control device 40 can also continue to supply process gas from the gas supply device 30 to the plasma head 10. Furthermore, the control device 40 can notify the operator on an operation panel or the like operated by the operator that the power cable 80 is normal (no break has occurred in the power cable 80).

[0050] As already described, the power supply device 20 includes a booster 24 that boosts the smoothed AC power to generate power to be supplied to the plasma head 10. The plasma head 10 also includes a grounded outer electrode 10b and an inner electrode 10a located inside the outer electrode 10b. A first current path IL1 is defined as a current path of a main current I1 that flows due to a voltage applied between the outer electrode 10b and the inner electrode 10a to generate plasma.

[0051] In this case, the first current path IL1 passes through the grounded output end 24b2 of the booster 24, the grounded electric wire 81s of the power cable 80, the outer electrode 10b, the inner electrode 10a, the other electric wire 81f of the power cable 80, and the other output end 24b1 of the booster 24 in this order. The wire breakage detection device 90 can be applied to the leakage current I2 occurring in the above-mentioned first current path IL1.

[0052] The current path of the leakage current I2 is designated as a second current path IL2. In this case, the second current path IL2 passes through the grounded output end 24b2 of the booster 24, the shield 82 of the power cable 80, the other electric wire 81f of the power cable 80, and the other output end 24b1 of the booster 24 in this order. In other words, the leakage current I2 of the second current path IL2 is generated based on the parasitic capacitance C0 generated between the electric wire 81 and the shield 82 of the power cable 80. The disconnection detection device 90 can be applied to the leakage current I2 of the second current path IL2.

[0053] 2. Disconnection Detection Method What has already been described about the disconnection detection device 90 also applies to the disconnection detection method. Specifically, the disconnection detection method includes an acquisition step and a determination step. The acquisition step corresponds to control performed by the acquisition unit 91. The determination step corresponds to control performed by the determination unit 92. Note that duplicated explanations will be omitted in this specification.

[0054] 3. Example of Effect of the Embodiment The disconnection detection device 90 can determine whether or not there is a disconnection in the power cable 80 based on fluctuations in the leakage current I2 flowing through the shield 82 of the power cable 80. What has been described above about the disconnection detection device 90 also applies to the disconnection detection method.

[0055] 10: plasma head, 10a: inner electrode, 10b: outer electrode, 20: power supply device, 24: booster, 24b1: output side end, 24b2: output side end, 80: power cable, 81: electric wire, 81f: electric wire, 81s: electric wire, 82: shield, 90: open circuit detection device, 91: acquisition unit, 92: judgment unit, C0: parasitic capacitance, CT0: current detector, I1: main current, I2: leakage current, ID0: detected current, IP0: peak current, IL1: first current path, IL2: second current path, T0: voltage period.

Claims

1. A disconnection detection device comprising: an acquisition unit that acquires fluctuations in leakage current flowing through the shield of a power cable that transmits power supplied from a power supply device to a plasma head that generates plasma; and a judgment unit that determines whether or not there is a disconnection in the power cable based on the fluctuations in leakage current acquired by the acquisition unit.

2. The disconnection detection device according to claim 1, wherein the acquisition unit acquires fluctuations in the leakage current that occur based on parasitic capacitance that occurs between the electric wires of the power cable and the shield.

3. A disconnection detection device as described in claim 1 or claim 2, wherein the judgment unit determines that a disconnection has occurred in the power cable when the acquisition unit acquires a peak current that occurs in pulses in accordance with the voltage period of the output voltage of the power supply device as a fluctuation in the leakage current.

4. The disconnection detection device according to claim 3, further comprising a current detector for detecting the leakage current flowing through the shield, and the acquisition unit performs signal processing on the detected current of the leakage current detected by the current detector to acquire the peak current.

5. The disconnection detection device according to claim 4, wherein said signal processing includes quasi-peak detection.

6. The disconnection detection device according to claim 5, wherein the acquisition unit converts the detected current into a voltage and then into a direct current, and acquires the peak current by performing the quasi-peak detection and rectifying it.

7. The disconnection detection device according to claim 6, wherein the acquisition unit performs the DC conversion by full-wave rectification.

8. The disconnection detection device of claim 1, wherein the power supply device comprises a booster that boosts smoothed AC power to generate power to be supplied to the plasma head, the plasma head comprising a grounded outer electrode and an inner electrode located more inward than the outer electrode, and a first current path, which is a current path of a main current that flows due to a voltage applied between the outer electrode and the inner electrode to generate the plasma, passes through the grounded output end of the booster, a grounded electric wire of the power cable, the outer electrode, the inner electrode, another electric wire of the power cable, and the other output end of the booster in that order.

9. A wire breakage detection device as described in claim 8, wherein the second current path, which is the current path of the leakage current, passes through the grounded output end of the booster, the shield of the power cable, another electric wire of the power cable, and the other output end of the booster in that order.

10. A disconnection detection method comprising: an acquisition step of acquiring fluctuations in leakage current flowing through the shield of a power cable that transmits power supplied from a power supply device to a plasma head that generates plasma; and a determination step of determining whether or not there is a disconnection in the power cable based on the fluctuations in leakage current acquired by the acquisition step.

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