Electric discharge machining apparatus

• US 4,510,364 A
• Filed: 09/10/1981
• Issued: 04/09/1985
• Est. Priority Date: 01/22/1980
• Status: Expired due to Term

• Current Assignee: Mitsubishi Denki Kabushiki Kaisha (Mitsubishi Electric Corporation)
• Original Assignee: Mitsubishi Denki Kabushiki Kaisha (Mitsubishi Electric Corporation)
• Inventors: Ito, Tetsuro
• Primary Examiner(s): PASCHALL, MARK H
• Application Number:
  US06/302,480
• Time in Patent Office: 1,307 Days
• Field of Search: 219/69 R, 219/69 G, 219/69 C, 219/69 S, 219/69 M
• US Class Current: 219/69.16
• CPC Class Codes: B23H 1/024 Detection of, and response to, abnormal gap conditions, e.g. short circuits preventing short circuits or other abnormal discharges by altering machining parameters using adaptive control B23H7/16
• CPC Class Codes: B23H 7/18 for maintaining or controlling the desired spacing between electrode and workpiece
• CPC Class Codes: B23H 7/30 Moving electrode in the feed direction B23H7/32 takes precedence
• CPC Class Codes: B23H 9/006 Cavity sinking B23H9/14 takes precedence

Abstract

In an electric discharge machining apparatus for machining a workpiece by confronting an electrode and the workpiece through an insulating machining solution, feeding the electrode to the workpiece and causing electric discharge between the confronting electrode and workpiece the difference between the most advanced position of the electrode relative to the workpiece and the present position of the electrode is detected and a signal is outputted by identifying the interelectrode state in accordance with the detected result. Thus, the propriety of the interelectrode machining gap state in the electric discharge machining are accurately observed, and the workpiece is machined under preferable machining conditions.

Claims

1. An electric discharge machining apparatus of the type in which a workpiece is machined by generating an electric discharge across a machining gap between a confronting electrode and said workpiece through an insulating machining solution while feeding said electrode to said workpiece, said apparatus comprising:

electrode position detecting means for detecting, during a present electric discharge machining operation, a difference value between the present position of said electrode relative to said workpiece and a most advanced position of said electrode detected during said present electric discharge of machining operation;

first means for outputting a signal by identifying the interelectrode state on the basis of said difference value detected by said electrode position detecting means; and

means for controlling the operation of said electric discharge machining apparatus in accordance with said signal output from said first means.

2. The apparatus as claimed in claim 1 wherein said first means comprises interelectrode state identifying means for identifying the interelectrode state as abnormal when said difference value is large.

3. The apparatus as claimed in claim 2 wherein said interelectrode state identifying means comprises means for identifying the interelectrode state as abnormal when a detection time, during which said electrode position detecting means detects said large difference value, is longer than a predetermined value.

4. The apparatus as claimed in claim 2 wherein said interelectrode state identifying means comprises means for identifying the interelectrode state as abnormal when the value of the product of said detection time and said difference value becomes higher than a predetermined value.

5. The apparatus as claimed in claim 3 wherein said interelectrode state identifying means comprises means for selectively identifying the interelectrode state as abnormal either when said detection time is longer than a predetermined value or when the value of the product of the detection time and said difference value becomes higher than a predetermined value.

6. The apparatus as claimed in claim 2 wherein said electrode position detecting means comprises a scale for generating a digital signal in response to the advancing or retarding amount of said electrode and a reversible counter for counting the digital signals of said scale.

7. The apparatus as claimed in claim 2 wherein said electrode position detecting means comprises:

a scale for generating a digital signal in response to the advancing or retarding amount of said electrode and a reversible counter for counting the digital signals of said scale, a digital-to-analog converter connected to an output side of said reversible counter for converting the value in said reversible counter into an analog amount, a comparator for comparing the output from said converter with a set value and producing an output signal in response to the compared result, a pulse oscillator for generating pulses at a predetermined interval, a first gate circuit to which the output from said comparator and the output from said pulse oscillator are inputted, and a second gate circuit to which the output from said first gate circuit and the digital signal from said scale are inputted for producing an output signal to said reversible counter.

8. The apparatus as claimed in claim 2 wherein said interelectrode state identifying means comprises a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result.

9. The apparatus as claimed in claim 3 wherein said interelectrode state identifying means comprises:

a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, and a primary delay circuit provided at the output side of said comparator.

10. The apparatus as claimed in claim 3 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element conducting in accordance with the output signal from said comparator, and having a main electrode, a primary delay circuit connected to the main electrode of said switching element and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output signal responsive to the compared result.

11. The apparatus as claimed in claim 4 wherein said interelectrode state identifying means comprises a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result.

12. The apparatus as claimed in claim 4 wherein said interelectrode state identifying means comprises:

a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, and a primary delay circuit provided at the output side of said comparator.

13. The apparatus as claimed in claim 4 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element conducting in accordance with the output signal from said comparator and having a main electrode, a primary delay circuit connected to the main electrode of said switching element, and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output signal responsive to the compared result.

14. The apparatus as claimed in claim 5 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element conducting in accordance with the output signal from said comparator and having a main electrode, a primary delay circuit connected to the main electrode of said switching element, switching means for selectively leading either said difference value or a predetermined constant potential to the main electrode of said switching element, and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output responsive to the compared result.

15. The apparatus as claimed in claim 10 further comprising means for coupling said difference value to said main electrode of said switching element.

16. The apparatus as claimed in claim 13 further comprising means for coupling a predetermined constant potential to said main electrode of said switching element.

17. An electric discharge machining apparatus of the type in which a workpiece is machined by generating an electric discharge across a machining gap between a confronting electrode and said workpiece through an insulating machining solution while feeding said electrode to said workpiece said apparatus comprising:

electrode position detecting means for detecting, during a present electric discharge machining operation, a difference value between the present position of said electrode relative to said workpiece and a most advanced position of said electrode detected during said present electric discharge machining operation;

first means for outputting a state detection signal by identifying the interelectrode state on the basis of said difference value detected by said electrode position detecting means, and power source means for varying electric machining conditions in response to said state detection signal from said interelectrode state identifying means.

18. The apparatus as claimed in claim 17 wherein said power source means comprises means for varying the electric machining conditions to recover a desired interelectrode state in response to the signal from said first means.

19. The apparatus as claimed in claim 17 wherein said first means comprises interelectrode state idenitfying means for identifying the interelectrode state as abnormal when said difference value is large.

20. The apparatus as claimed in any one of claims 17 through 19 wherein said power source means comprises means for varying at least one of a discharge down time and an applying voltage between the electrodes.

21. The apparatus as claimed in claim 19 wherein said interelectrode state identifying means comprises means for identifying the interelectrode state as abnormal when a detection time, during which said difference value is large, is longer than a predetermined value.

22. The apparatus as claimed in claim 19 wherein said interelectrode state identifying means comprises means for identifying the interelectrode state as abnormal when a detection time, during which said difference value is large, is longer than a predetermined value.

23. The apparatus as claimed in claim 21 wherein said interelectrode state identifying means comprises means for selectively identifying the interelectrode state as abnormal either when said detection time is longer than a predetermined value or when the value of the product of the detection time and said difference value becomes higher than a predetermined value.

24. The apparatus as claimed in claim 19 wherein said electrode position detecting means comprises a scale for generating a digital signal in response to the advancing or retarding amount of said electrode and a reversible counter for counting the digital signals of said scale.

25. The apparatus as claimed in claim 19 wherein said electrode position detecting means comprises:

a scale for generating a digital signal in response to the advancing or retarding amount of said electrode and a reversible counter for counting the digital signals of said scale, a digital-to-analog converter connected to an output side of said reversible counter for converting the value in said reversible counter into an analog amount, a comparator for comparing the output from said converter with a set value and producing an output signal in response to the compared result, a pulse oscillator for generating pulses at a predetermined interval, a first gate circuit to which the output from said comparator and the output from said pulse oscillator are inputted, and a second gate circuit to which the output from said first gate circuit and the digital signal from said scale are inputted for producing an output signal to said reversible counter.

26. The apparatus as claimed in claim 19 wherein said interelectrode state identifying means comprises a comparator for receiving said difference value and for comparing the said difference value with a set value and producing an output signal responsive to the compared result.

27. The apparatus as claimed in claim 21 wherein said interelectrode state identifying means comprises:

a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, and a primary delay circuit provided at the output side of said comparator.

28. The apparatus as claimed in claim 21 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element conducting in accordance with the output signal from said comparator and having a main electrode for providing an output, a primary delay circuit connected to the main electrode of said switching element and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output signal responsive to the compared result.

29. The apparatus as claimed in claim 22 wherein said interelectrode state identifying means comprises a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result.

30. The apparatus as claimed in claim 22 wherein said interelectrode state identifying means comprises:

a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, and a primary delay circuit provided at the output side of said comparator.

31. The apparatus as claimed in claim 22 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element controlled to be conducted in accordance with the output signal from said comparator and having a main electrode, a primary delay circuit connected to the main electrode of said switching element, and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output signal responsive to the compared result.

32. The apparatus as claimed in claim 23 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element controlled to be conducted in accordance with the output signal from said comparator and having a main electrode, a primary delay circuit connected to the main electrode of said switching element, switching means for selectively leading either said difference value or a predetermined constant potential to the main electrode of said switching element, and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output responsive to the compared result.

33. The apparatus as claimed in claim 24 wherein said power source means for varying the electric machining conditions comprises:

a down time setting counter for setting an electric discharge down time, a coincidence signal when the counted value of said down time setting counter and the counted value of said reversible counter of said electrode position detecting means become coincident, a gate circuit to which the coincidence signal from said coincidence identifying circuit and the interelectrode state identifying means are inputted, a pulse width setting counter for setting the pulse duration time of an on-off pulse of an applying voltage between said electrodes, a flip-flop circuit to which a signal responsive to the set value of said pulse width setting counter as a set signal and the output from said gate circuit as a reset signal are inputted, said flip-flop having a first output terminal for producing a first output to said down time setting counter and a second output terminal for producing an inverting second output, a switching element conducting in accordance with the second output from said flip-flop circuit and having a main electrode, and a power source connected to the main electrode of said switching element.

34. The apparatus as claimed in claim 28 further comprising means for coupling said difference value to said main electrode of said switching element.

35. The apparatus as claimed in claim 31 further comprising means for coupling a predetermined constant potential to said main electrode of said switching element.

36. The apparatus as claimed in claim 33 wherein said power source means for varying the electric machining conditions comprises a regulator for regulating the output value of said power source connected to the main electrode of said switching element in response to the interelectrode state identifying signal.

37. The apparatus as claimed in claim 36 wherein said regulator comprises means for lowering the applying voltage between said electrode and said workpiece when the machining gap is expanded.

38. An electric discharge machining apparatus of the type in which a workpiece is machined by generating an electric discharge across a machining gap between a confronting electrode and said workpiece through an insulating machining solution while feeding said electrode to said workpiece in response to an electrode feed signal provided to electrode feed means, said apparatus comprising:

electrode position detecting means for detecting, during a present electric discharge machining operation, a difference value between the present position of said electrode relative to said workpiece and a most advanced position of said electrode detected during said present electric discharge machining operation;

first means for outputting a signal by identifying the interelectrode state on the basis of said difference value detected by said electrode position detecting means, and control means for controlling said electrode feed signal to vary the interelectrode state in response to the output signal from said first means.

39. The apparatus as claimed in claim 38 wherein said control means comprises means for controlling said electrode feed signal to recover a desired interelectrode state in response to the output signal from said first means.

40. The apparatus as claimed in claim 38 wherein said control means comprises amplification degree control means for controlling the amplification degree between an interelectrode servo input digital signal based on the difference between a reference voltage and the interelectrode voltage and the electrode feed applied to said electrode feeding means.

41. The apparatus as claimed in any one of claims 38 through 40 wherein said first means comprises interelectrode state identifying means for identifying the interelectrode state as abnormal when said difference value is large.

42. The apparatus as claimed in claim 41 wherein said interelectrode state identifying means comprises means for identifying the interelectrode state as abnormal when a detection time, during which said electrode position detecting means detects said large difference value, is longer than a predetermined value.

43. The apparatus as claimed in claim 41 wherein said interelectrode state identifying means comprises means for identifying the interelectrode state as abnormal when the value of the product of said electrode position detecting means detects said large difference value, becomes higher than a predetermined value.

44. The apparatus as claimed in claim 42 wherein said interelectrode state identifying means comprises means for selectively identifying the interelectrode state as abnormal either when said detection time is longer than a predetermined value or when the value of the product of said detection time and said difference value becomes higher than a predetermined value.

45. The apparatus as claimed in claim 41 wherein said electrode position detecting means comprises a scale for generating a digital signal in response to the advancing or retarding amount of said electrode and a reversible counter for counting the digital signals of said scale.

46. The apparatus as claimed in claim 41 wherein said electrode position detecting means comprises:

a scale for generating a digital signal in response to the advancing or retarding amount of said electrode and a reversible counter for counting the digital signals of said scale, a digital-to-analog converter connected to an output side of said reversible counter for converting the value in said reversible counter into an analog amount, a comparator for comparing the output from said converter with a set value and producing an output signal in response to the compared result, a pulse oscillator for generating pulses at a predetermined interval, a first gate circuit to which the output from said comparator and the output from said pulse oscillator are inputted, and a second gate circuit to which the output from said first gate circuit and the digital signal are inputted for producing an output signal to said reversible counter.

47. The apparatus as claimed in claim 41 wherein said interelectrode state identifying means comprises a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result.

48. The apparatus as claimed in claim 42 wherein said interelectrode state identifying means comprises:

a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, and a primary delay circuit provided at the output side of said comparator.

49. The apparatus as claimed in claim 42 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element conducting in accordance with the output signal from said comparator and a main electrode, a primary delay circuit connected to the main electrode of said switching element and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output signal responsive to the compared result.

50. The apparatus as claimed in claim 43 wherein interelectrode state identifying means comprises a comparator for receiving said difference signal and for comparing said difference signal with a set value and producing an output signal responsive to the compared result.

51. The apparatus as claimed in claim 43 wherein said interelectrode state identifying means comprises:

a comparator for receiving said difference signal and for comparing said difference signal with a set value and producing an output signal responsive to the compared result, and a primary delay circuit provided at the output side of said comparator.

52. The apparatus as claimed in claim 43 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element conducting in accordance with the output signal from said comparator and having a main electrode, a primary delay circuit connected to the main electrode of said switching element, and a second comparator for comparin the output from said primary delay circuit with a predetermined value and producing an output signal responsive to the compared result.

53. The apparatus as claimed in claim 44 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element controlled to be conducted in accordance with the output signal from said comparator and having a main electrode, a primary delay circuit connected to the main electrode of said switching element, switching means for selectively leading either said difference value or a predetermined constant potential to the main electrode of said switching element, and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output responsive to the compared result.

54. The apparatus as claimed in claim 45 wherein said control means comprises:

a machining gap compulsorily expanding time setting counter for setting the time for compulsorily expanding the interelectrode machining gap, a coincidence circuit responsive to the counted value from said machining gap compulsorily expanding time setting counter and to the counted value from said reversible counter of said electrode position detecting means for producing a coincidence signal when both the counted values from said counters become coincident, a machining time setting counter for setting the machining time and reset by the coincidence signal from said coincidence circuit, a logic circuit responsive to the interelectrode abnormality state detection signal representing that the interelectrode state is abnormal and to said coincidence signal for applying a first output signal to said machining time setting counter and a second output signal to said machining gap compulsorily expanding time setting counter, and electrode lifting signal supply means for generating a signal for lifting the feeding electrode preferentially to an ordinary electrode feed control signal to an electrode feeding unit upon receipt of the second output from said logic circuit.

55. The apparatus as claimed in claim 45 wherein said amplification degree control means for controlling the amplification of said control means comprises a digital-to-analog converter responsive to the digital output signal from said reversible counter of said electrode position detecting means and the interelectrode servo input digital signal for producing an analog signal on the basis of said digital input signals.

56. The apparatus as claimed in claim 49 further comprising means for coupling said difference value to said main electrode of said switching element.

57. The apparatus as claimed in claim 52 further comprising means for coupling a predetermined constant potential to said main electrode of said switching element.

58. The apparatus as claimed in claim 55 wherein said logic circuit comprises a flip-flop circuit including a set terminal to which a signal corresponding to the set value of the machining time in the interelectrode abnormality state detection signal is inputted, a reset terminal to which the coincidence signal from said coincidence circuit is inputted, a first output terminal for producing a first output signal to said electrode lifting signal supply means and said coincidence circuit, and a second output terminal for producing a second output signal corresponding to an inverted first output signal to said machining time setting counter.

59. The apparatus as claimed in claim 55 wherein said digital-to-analog converter is a multiplication type digital-to-analog converter.

60. The apparatus as claimed in claim 55 wherein said amplification control means for controlling the amplification degree of said control means comprises:

a digital-to-analog converter responsive to an interelectrode servo input digital signal for converting said input digital signal into the analog amount of the digital output signal from said reversible counter of said electrode position detecting means, an amplifier to which said interelectrode servo input digital signal is inputted, and switching means responsive to the output signal from said interelectrode state identifying means for selectively producing the output signal from said converter or the output signal from said amplifier.

61. The apparatus as claimed in claim 58 wherein said electrode lifting signal supply means comprises:

a switch operating in accordance with the first output from said logic circuit, and a power source connected to said switch.

62. An electric discharge machining apparatus of the type wherein a workpiece is machined by generating an electric discharge across a machining gap between a confronting electrode and said workpiece through an insulating machining solution while feeding said electrode to said workpiece, said apparatus comprising:

electrode position detecting means for detecting, during a present electric discharge machining operation, a difference value between the present position of said electrode relative to said workpiece and a most advanced position of said electrode detected during said present electric discharge of machining operation;

first means for outputting a signal by identifying the interelectrode state on the basis of said difference value detected by said electrode position detecting means, and indicating means for indicating the interelectrode state in accordance with either said difference value or the output signal from said first means.

63. The apparatus as claimed in claim 62 wherein said indicating means is provided at a place isolated from said electrode through a transmission line with said difference value or the output from said first means being provided to said indicating means via said transmission line.

64. The apparatus as claimed in claim 63 wherein said indicating means comprises means for optically indicating the interelectrode state.

65. The apparatus as claimed in claim 63 wherein said indicating means is an instrument for indicating the value of the output signal from said electrode position detecting means.

66. The apparatus as claimed in claim 63 wherein said first means comprises interelectrode state identifying means for identifying the interelectrode state as abnormal when said difference value is large.

67. The apparatus as claimed in claim 66 wherein said first means comprises interelectrode state identifying means for identifying the interelectrode state as abnormal when a detection time, during which said electrode position detecting means detects said large difference value, is longer than a predetermined value.

68. The apparatus as claimed in claim 66 wherein said interelectrode state identifying means comprises means for identifying the interelectrode state as abnormal when the value of the product of said detection time and said difference value becomes higher than a predetermined value.

69. The apparatus as claimed in claim 66 wherein said interelectrode state identifying means comprises means for selectively identifying the interelectrode state is abnormal either when said detection time is longer than a predetermined value or when the value of the product of said detection time and said difference value becomes higher than a predetermined value.

70. The apparatus as claimed in claim 66 wherein said electrode position detecting means comprises a scale for generating a digital signal in response to the advancing or retarding amount of said electrode and a reversible counter for counting the digital signals of said scale.

71. The apparatus as claimed in claim 66 wherein said electrode position detecting means comprises:

a scale for generating a digital signal in response to the advancing or retarding amount of said electrode and a reversible counter for counting the digital signals of said scale, a digital-to-analog converter connected to an output side of said reversible counter for converting the value in said reversible counter into an analog amount, a comparator for comparing the output signal in response to the compared result, a pulse oscillator for generating pulses at a predetermined interval, a first gate circuit to which the output from said comparator and the output from said pulse oscillator are inputted, and a second gate circuit to which the output from said first gate circuit and the digital signal from said scale are inputted for producing an output signal to said reversible counter.

72. The apparatus as claimed in claim 66 wherein said interelectrode state identifying means comprises a comparator for receiving said difference value and said difference value with a set value and producing an output signal responsive to the compared result.

73. The apparatus as claimed in claim 67 wherein said interelectrode state identifying means comprises:

a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, and a primary delay circuit provided at the output side of said comparator.

74. The apparatus as claimed in claim 67 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element conducting in accordance with the output signal from said comparator and having a main electrode, a primary delay circuit connected to the main electrode of said switching element and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output signal responsive to the compared result.

75. The apparatus as claimed in claim 68 wherein said interelectrode state identifying means comprises a comparator for receiving said difference signal and for comparing said difference value with a set value and producing an output signal responsive to the compared result.

76. The apparatus as claimed in claim 68 wherein said interelectrode state identifying means comprises:

a comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, and a primary delay circuit provided at the output side of said comparator.

77. The apparatus as claimed in claim 68 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference value and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element controlled to be conducted in accordance with the output signal from said comparator and having a main electrode, a primary delay circuit connected to the main electrode of said switching element, and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output signal responsive to the compared result.

78. The apparatus as claimed in claim 69 wherein said interelectrode state identifying means comprises:

a first comparator for receiving said difference signal and for comparing said difference value with a set value and producing an output signal responsive to the compared result, a switching element conducting in accordance with the output signal from said comparator and having a main electrode, a primary delay circuit connected to the main electrode of said switching element, switching means for selectively leading either said difference value or a predetermined constant potential to the main electrode of said switching element, and a second comparator for comparing the output from said primary delay circuit with a predetermined value and producing an output response to the compared result.

79. The apparatus as claimed in claim 70 wherein said indicating means comprises a digital-to-analog converter for converting the digital output signal of said reversible counter of said electrode position detecting means transmitted through a transmission line made of twisted pair transmission line group into an analog amount.

80. The apparatus as claimed in claim 74 further comprising means for coupling said difference value to the main electrode of said switching element.

81. The apparatus as claimed in claim 77 further comprising means for coupling a predetermined constant potential to said main electrode of said switching element.