Discharge Detecting Device

Abstract: A method and apparatus for detecting and controlling the operation of an electric discharge machine for machining a workpiece. The machining is conducted by generating pulse discharges at a machining gap formed by a machining electrode and the workpiece opposed to each other. The alternating-current components or high frequency components of at least one of the current, voltage and impedance of the machining gap, the rectified components thereof, or amplitude, frequency or other signal characteristics thereof are detected and processed for control of the machining. The processing may include frequency analysis, pulse counting, threshold detection and the like using analog and digital processing.

Description: The present invention relates to a device for detecting a discharge status at a machining gap according to the preamble of claim 1. A device of this kind is known from FR-A 2 099 228 or from FR-A 2 252 800. Description of the Background Art In electrical discharge machining, a machining status must be detected and controlled to avoid the damage to workpiece surfaces caused by the generation of arc discharges. In an electrical discharge machine known in the art, an average voltage and current in a machining gap formed by a machining electrode and a workpiece opposed to each other were detected and the electrode position was adjusted to maintain the machining status. However, since it is difficult to avoid the generation of arc discharges only by detecting the average machining status of the machining gap, the workpiece is machined under machining conditions biased toward a considerably safe side. To improve the machining performance of the electrical discharge machine, the discharge status must be detected every electrical discharge and the optimum machining status maintained. It is known that high frequency components overlap a voltage waveform, a current waveform and a gap impedance waveform during a normal discharge and they do not overlap the same during a faulty discharge. By utilizing this principle, a device for detecting a discharge status every electrical discharge has been devised. A device for separating these high frequency components with a high-pass filter and judging whether a discharge is normal or faulty is disclosed, for example, in Japanese Laid-Open Patent Publication No. SHO47-13795. Fig. 27 shows an example of such conventional electrical discharge machine, wherein the numeral 1 indicates a machining power supply, 2 denotes an electrode, 3 represents a workpiece, 4 designates low frequency intercepting means, e.g., a high-pass filter, 5 indicates a rectifier, 35, 36 and 37 represent Schmidt circuits which are different in inverse voltage from each other, 38 denotes a switch, and 7 designates a discharge status display device. Operation will now be described. Only while the machining power supply 1 applies a voltage to a machining gap, the switch 38 causes the voltage value of the machining gap to pass through the high-pass filter 4. The output of the high-pass filter 4 is rectified by the rectifier 5, segmented at a plurality of levels by the plurality of Schmidt circuits 35, 36, 37, and digitally displayed as a signal indicating a discharge status. In the conventional electrical discharge machine which operates as described above, since the output of the high-pass filter 4 oscillates finely around approximately 0V, the output of the rectifier 5 oscillates finely between 0V and a maximum amplitude value. Accordingly, even the Schmidt circuits cannot prevent the discharge status output from making frequent changes and, in fact, the discharge status cannot be identified accurately. Also, as the present inventors determined, high frequency components occurred immediately after the start of a discharge, independently of whether the discharge is normal or faulty. Thus, the conventional electrical discharge machine would determine a deteriorated discharge status as normal because the discharge status was detected over a whole discharge period. Further, if a short circuit takes place at the machining gap, the amount of high frequency components generated depends on the degree of the short circuit. The deterioration of the discharge status and the occurrence of the short circuit are essentially different from each other and must be treated differently. However, since the discharge status is only judged according to the magnitude of high frequency components in the conventional electrical discharge machine, the discharge status is judged and machining conditions are controlled independently of whether a short circuit exists or not. It is, accordingly, an object of the present invention to overcome the disadvantages in the conventional art by providing an electrical discharge machining apparatus and method therefor which detect a discharge status precisely at every single discharge and which control electrical discharge machining conditions precisely on the basis of the detected discharge status. SUMMARY OF THE INVENTION According to the present invention this problem is solved as defined in claim 1. The present invention measures the alternating-current components or high frequency components of the current, voltage and impedance of the machining gap not less than 1µsec after the occurrence of a discharge to detect the discharge status, whereby the influence of the high frequency components generated immediately after the start of the discharge can be avoided. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an arrangement diagram of a first apparatus. Fig.2 illustrates waveform diagrams of certain input/output signals in the main area of the first apparatus. Fig. 3 is an arrangement diagram illustrating a second apparatus. Fig. 4 illustrates waveform diagrams of certain input/output signals in the main area of the apparatus of Fig. 3. Fig. 5 is an arrangement diagram illustrating a third apparatus. Fig. 6 is a waveform diagram illustrating input/output signals in the main area of the apparatus in Fig. 5. Fig. 7 is an arrangement diagram illustrating a fourth apparatus. Fig. 8 is a waveform diagram illustrating input/output signals in the main area of the apparatus of Fig. 7. Fig. 9 is an arrangement diagram illustrating a modification of the fourth apparatus. Fig. 10 is an arrangement diagram illustrating a fifth apparatus. Fig. 11 is a waveform diagram illustrating input/output signals in the main area of the apparatus in Fig. 10. Fig. 12 is an arrangement diagram illustrating a sixth apparatus. Fig. 13 is an arrangement diagram illustrating a seventh apparatus. Fig. 14 is a waveform diagram illustrating input/output signals in the main area of the apparatus in Fig. 13. Fig. 15 is an arrangement diagram illustrating an eighth apparatus. Fig. 16 is an arrangement diagram illustrating a ninth apparatus. Fig. 17 is a waveform diagram illustrating input/output signals in the main area of the apparatus in Fig. 16. Fig. 18 is an arrangement diagram illustrating a tenth apparatus. Figs. 19A and 19B illustrate the results of frequency analyses on discharge pulses. Fig. 20 is an arrangement diagram illustrating an eleventh apparatus. Fig. 21 is a waveform diagram illustrating input/output signals in the main area of the apparatus illustrated in Fig. 20. Fig. 22 is an arrangement diagram illustrating a first preferred embodiment of the invention. Fig. 23 is a waveform diagram illustrating input/output signals in the main area of the preferred embodiment of the invention in Fig. 22. Fig. 24 is an arrangement diagram illustrating a second preferred embodiment of the invention. Fig. 25 is a waveform diagram illustrating input/output signals in the main area of the preferred embodiment of the invention in Fig. 24. Figs. 26A-26D illustrate the distribution of high frequency component magnitude of a machining gap voltage in the progress of electrical discharge machining. Fig. 27 is an arrangement diagram of an electrical discharge machine known in the art. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is an arrangement diagram of an apparatus related to the present invention. Parts identical or corresponding to the conventional electrical discharge machine illustrated in Fig. 27 are given identical reference characters and will not be described here. In Fig. 1, the numeral 6 indicates a smoothing device, e.g., a smoother consisting of a resistor and a capacitor. Fig. 2 shows an input/output signal waveform example in the main area of the present apparatus, wherein waveform 2-A shows the voltage waveform of the machining gap, waveform 2-B shows the output waveform of the high-pass filter 4, waveform 2-C shows the output waveform of the rectifier 5, and waveform 2-D shows the output waveform of the smoother 6. A broken line shown in waveform 2-D indicates a judgement threshold value (a given set value). A discharge is normal if high frequency components are located above this threshold value, and is abnormal or is becoming abnormal if the high frequency components are located below the threshold value. Operation will now be described with reference to Figs. 1 and 2. A pulse-shaped voltage is applied to the machining gap by the machining power supply 1 to carry out electrical discharge machining. At this time, the voltage waveform 2-A of the machining gap is, for example, as shown in Fig. 2. From this voltage waveform, only high frequency components are fetched by the high-pass filter 4 and the waveform 2-B changes as shown in Fig. 2. They are then rectified by the rectifier 5 and change into a signal waveform 2-C as shown in Fig. 2, and subsequently, are smoothed by the smoother 6 and change into a signal waveform 2-D, as shown in Fig. 2. According to the level of this output, the display device 7 shows the discharge status. When the high frequency components are rectified and then smoothed as described above, the magnitude of the high frequency components is provided as a voltage level, the variation of the discharge status output is prevented, and the discharge status can be detected accurately. Fig. 3 is an arrangement diagram of a second apparatus. Parts identical or corresponding to the embodiment in Fig. 1 are given identical reference characters and will not be described here. In Fig. 3, the numeral 8 indicates a low-pass filter acting as averaging means for averaging the output of the rectifier 5. Fig. 4 shows an input/output signal waveform example in the main area of the apparatus, wherein the voltage waveform of the machining gap 4-A, the high-pass filter 4 output waveform 4-B, the rectifier 5 output waveform 4-C, and the low-pass filter 8 output waveform 4-E, can be seen. Operation will now be described with reference to Figs. 3 and 4. A pulse-shaped voltage is applied to the machining gap by the machining power supply 1 to carry out electrical discharge machining. At this time, the voltage waveform of the machining gap is, for example, as shown in waveform 4-A. From this voltage waveform, only high frequency components are fetched by the high-pass filter 4 and the waveform changes as shown in waveform 4-B. They are then rectified by the rectifier 5 and change into a signal waveform 4-C, and subsequently, are averaged by the low-pass filter 8 and change into a signal waveform 4-E. According to the level of this output, the display device 7 shows the discharge status. When the high frequency components are rectified and then averaged as described above, the magnitude of the high frequency components is provided as a voltage level, the variation of the discharge status output is prevented, and the discharge status can be detected accurately. Fig. 5 is an arrangement diagram of a third apparatus. Parts identical or corresponding to the apparatus in Fig. 1 are given identical reference characters and will not be described here. In Fig. 5, the numeral 9 indicates an integrator and 10 represents a controller for controlling the integration start and reset of the integrator 9. Fig. 6 shows an input/output signal waveform example in the main area of the present apparatus, wherein the voltage waveform 6-A of the machining gap , the output waveform 6-B of the high-pass filter 4, the output waveform 6-C of the rectifier 5, and the output waveform 6-F of the integrator 9, can be seen. Operation will now be described with reference to Figs. 5 and 6. A pulse-shaped voltage is applied to the machining gap by the machining power supply 1 to carry out electrical discharge machining. At this time, the voltage waveform of the machining gap is, for example, shown as waveform 6-A. From this voltage waveform, only high frequency components are fetched by the high-pass filter 4 and the waveform changes to that illustrated as 6-B. They are then rectified by the rectifier 5 and change into a signal waveform 6-C. In the meantime, when the controller 10 controls the integrator 9 to reset integration at the application of the voltage to the machining gap and to start integration at the occurrence of a discharge, the output waveform of the integrator 9 is as shown as signal 6-F. According to this output value of the integrator 9 detected during stop time, the display device 7 shows the discharge status. When the high frequency components are rectified and then integrated as described above, the magnitude of the high frequency components is provided as a voltage level, the variation of the discharge status output is prevented, and the discharge status can be detected accurately. While the integrator 9 was controlled in synchronization with voltage application or discharge occurrence in this apparatus, it is apparent that the integrator 9 may be controlled at any timing if the output of the rectifier 5 can be integrated per pulse, e.g., integration is started a predetermined period of time after a discharge has occurred, integration is reset and started simultaneously with voltage application, or integration is reset immediately before integration start after discharge occurrence. Also, when this embodiment is designed to hold or check the output of the integrator 9 a predetermined period of time after integration start, the discharge status output can be provided independently of the termination time of the discharge and identical effects are produced. Fig. 7 is an arrangement diagram of a fourth apparatus. Parts identical or corresponding to the apparatus in Fig. 5 are given identical reference characters and will not be described here. In Fig. 7, the numeral 70 indicates a time constant measuring device serving as time constant measuring means for measuring the time constant tH of the high-pass filter 4, 71 represents an output thereof, 72 denotes a logic circuit, 73 designates an output thereof, 74 indicates an output of the integrator 9, and 10 denotes a reset device for the integrator 9. 75 indicates a discharge voltage detector, 76 designates a discharge current detector, 77 denotes a logic circuit, and 78 indicates a comparator. Fig. 8 shows an input/output signal waveform example in the main area of the present apparatus, wherein the voltage waveform of the machining gap 8-A, the output waveform 8-B of the high-pass filter 4, the output waveform 8-G of the logic circuit 77, the output waveform 8-H of the time constant measuring device 70, the output waveform 8-I of the logic circuit 72, and the output waveform 8-F of the integrator are shown. Operation will now be described with reference to Figs. 7 and 8. In Fig. 8, 80 and 81 indicate discharge voltage waveforms of the machining gap, Ton denotes a discharge pulse width, Toff represents stop time, t1 designates discharge detection time of day, t2 indicates time of day after the time constant tH of the high-pass filter 4 starting at discharge detection time of day t1 (t2 = t1 + tH) , 82 represents high frequency components of the discharge voltage, and 83 designates a disturbance waveform attributable to the transient characteristic of the high-pass filter 4, and the time constant measuring device 70 outputs a high level for a period of time tH, starting at the time of day when the output 79 of the logic circuit 77 rises, as shown in waveform 8-H. The logic circuit 72 receives the output 79 of the logic circuit 77 and the output 71 of the time constant measuring device 70 and outputs the signal 73 shown as waveform 8-I. The time of day when the output signal 73 rises is indicated as t2 in waveform 8-I. The reset device 10 resets the integrator 9 while the output 73 of the logic circuit 72 is high. Namely, only while the output 73 of the logic circuit 72 is low, the integrator 9 integrates rectification output Vrec. The comparator 78 compares reference voltage Vref with the integration output 74 shown in waveform 8-F, judges it as a normal discharge pulse if the integration output 74 is larger than reference voltage Vref at the end of discharge pulse width Ton, and judges it as an arc discharge pulse if smaller. Whereas the detection of the high frequency components of the discharge voltage waveform was described in this apparatus, it is apparent that the principles of the apparatus are applicable to the discharge current waveform and the impedance waveform of the machining gap. Also, while the use of the high-pass filter was described in the fourth apparatus, the same effects will be produced by using a band-pass filter as shown in Fig. 9 which has a high-pass characteristic for the detection of high frequency components and further cuts the area of the high frequency. Fig. 10 is an arrangement diagram of a fifth apparatus wherein parts identical or corresponding to the apparatus in Fig. 1 are given identical reference characters and will not be described here. In Fig. 10, the numeral 11 indicates a time measuring device, 12 designates a controller for controlling the reset of the time measuring device 11, 13 denotes a comparator, and 14 represents a comparison reference value generator. Fig. 11 shows an input/output signal waveform example in the main area of the present apparatus, wherein the voltage waveform 11-A of the machining gap, the output waveform 11-J of the comparator 13, and the output waveform 11-K of the time measuring device 12 are shown. Operation will now be described with reference to Figs. 10 and 11. A pulse-shaped voltage is applied to the machining gap by the machining power supply 1 to carry out electrical discharge machining. At this time, the voltage waveform of the machining gap is, for example, as shown in waveform 11-A. This voltage waveform is compared at a certain level by the comparator 13 and changes as shown in waveform 11-J. Meanwhile, the controller 12 resets the time measuring device 11 when the voltage is applied to the machining gap and cancels the reset status when a discharge is detected. Further, since the time measuring device 11 starts time measurement on the leading edge of the comparator 13 output and stops time measurement on the trailing edge thereof, it measures the length of time when the voltage waveform exceeds the comparison level during a single discharge, and the time measurement output is as shown in waveform 11-K. According to the time measurement output during stop time, the display device 7 shows the discharge status. In the above design, the magnitude of high frequency components in a discharge pulse can be measured according to the time when the high frequency components have a predetermined or larger amplitude, whereby the discharge status can be detected accurately. Whereas the time measuring device 11 was controlled in synchronization with voltage application or discharge occurrence in this embodiment, it is apparent that the time measuring device 11 may be controlled at any timing if the length of time when the voltage waveform during a discharge exceeds the comparison level can be measured per pulse, e.g., reset is canceled a predetermined period of time after a discharge has occurred, or the time measuring device 11 is reset immediately before reset cancellation after discharge occurrence. Also, when the present embodiment is designed to hold or check the output of the time measuring device 11 a predetermined period of time after reset cancellation, the discharge status output can be provided independently of the termination time of the discharge and identical effects are produced. Further, when the time measuring device 11 is designed to output as the time measurement output a value obtained by subtracting a time measurement value at a preceding pulse from a time measurement value measured, there is an advantage that the controller 12 for controlling the reset of the time measuring device 11 is not required. Also, while the period of time when the voltage waveform exceeds the comparison level during a single discharge was measured in this embodiment, the length of time when the voltage waveform falls below the comparison level may be measured to provide the same effects, e.g., by starting time measurement on the trailing edge of the comparator 13 output and stopping the time measurement on the leading edge thereof. In this case, it should be noted that as the measurement period of time is made longer, the high frequency components are fewer. Fig. 12 is an arrangement diagram of a sixth apparatus, wherein parts identical or corresponding to the conventional electrical discharge machine in Fig. 49 and the apparatus in Fig. 10 are given identical reference characters and will not be described here. Operation will now be described. The operation is generally identical to that of the above apparatus in Fig. 10; however, since the voltage waveform of the machining gap passes the high-pass filter 4 and is then input to the comparator 13, the influence of direct-current component variations and a low frequency swell can be eliminated and the discharge status detected more accurately. The present apparatus is also identical to that previous apparatus in that the discharge status can be detected by measuring not only the time when the voltage waveform exceeds the comparison level but also the time when it falls below the comparison level. Also, the output of the high-pass filter 4 may be rectified and then input to the comparator 13 to have the same effects. Particularly in the case of full-wave rectification, the amplitude of an area falling below an average level can also be evaluated, whereby the discharge status can be detected much more accurately. Fig. 13 is an arrangement diagram of a seventh apparatus of the invention. Parts identical or corresponding to the apparatus in Fig. 10 are given identical reference characters and will not be described here. In Fig. 13, the numeral 15 indicates a counter and 16 designates a controller for controlling the reset of the counter 15. Fig. 14 shows an input/output signal waveform example in the main area of the present apparatus, wherein waveform 14-A shows the voltage waveform of the machining gap, waveform 14-J shows the output waveform of the comparator 13, and waveform 14-L shows the output waveform of the counter 15. Operation will now be described with reference to Figs. 13 and 14. A pulse-shaped voltage is applied to the machining gap by the machining power supply 1 to carry out electrical discharge machining. At this time, the voltage waveform of the machining gap is, for example, as shown in waveform 14-A. This voltage waveform is compared at a certain level by the comparator 13 and changes as shown in waveform 14-J. Since the counter 15 is arranged to be reset when the voltage is applied to the machining gap and to increment its count value on the leading edge of the comparator 13 output (14-L), it measures the number of times when the voltage waveform exceeds the comparison level during a single discharge. Generally, in the voltage during a discharge in electrical discharge machining, high frequency components overlap an approximately constant bias voltage. Hence, by counting the number of times when the voltage value reaches the comparison level provided slightly above or below the bias voltage, the magnitude of the amplitude of high frequency components can be measured, whereby the discharge status can be detected according to said count value. The display device 7 shows the discharge status according to the value of the counter 15 during stop time. Whereas the counter 15 was controlled in synchronization with voltage application or discharge occurrence in this apparatus, it is apparent that the counter 15 may be controlled at any timing if the number of times when the voltage waveform during a discharge exceeds the comparison level can be measured per pulse, e.g., operation is stopped until predetermine time elapses after discharge occurrence, or reset and start are made simultaneously with discharge occurrence. Also, when the embodiment is designed to hold or check the output of the counter 15 a predetermined period of time after a discharge start or a count start, the discharge status output can be provided independently of the termination time of the discharge and identical effects are produced. Further, when the counter 15 is designed to output as the counter output a value obtained by subtracting a count value at a preceding pulse from a count value measured, there is an advantage that the controller 16 for controlling the reset of the counter 15 is not required. Also, while the leading edge of the comparator 13 out was counted in this apparatus, it is a matter of course that the trailing edge of the comparator 13 output or both the leading edge and the trailing edge thereof may be counted. Fig. 15 is an arrangement diagram of an eighth apparatus of the invention. Parts identical or corresponding to the conventional electrical discharge machine in Fig. 49 and the apparatus in Fig. 13 are given identical reference characters and will not be described here. Operation will now be described. The operation is generally identical to that of the apparatus in Fig. 13; however, since the voltage waveform of the machining gap passes the high-pass filter 4 and is then input to the comparator 13, the influence of direct-current component variations and a low frequency swell can be eliminated and the discharge status detected more accurately. The present apparatus is identical to the above identified embodiment in that the discharge status may be detected by counting not only the number of leading edges of the comparator 13 output but also the number of trailing edges thereof. Also, the output of the high-pass filter 4 may also be rectified and then input to the comparator 13 to provide the same effects. Particularly in the case of full-wave rectification, the amplitude of an area falling below an average level can also be evaluated, whereby the discharge status can be detected further more accurately. Fig. 16 is an arrangement diagram of a ninth apparatus. Parts identical or corresponding to the apparatus in Fig. 1 are given identical reference characters and will not be described here. In Fig. 16, the numeral 17 indicates a peak holder acting as peak holding means and 18 designates a controller acting as control means for controlling the reset of the peak holder 17. Fig. 17 shows an input/output signal waveform example in the main area of the present apparatus, wherein waveform 17-A shows the voltage waveform of the machining gap, waveform 17-B shows the output waveform of the high-pass filter 4, and waveform 17-M shows the output waveform of the peak holder 17. Operation will now be described with reference to Figs. 16 and 17. A pulse-shaped voltage is applied to the machining gap by the machining power supply 1 to carry out electrical discharge machining. At this time, the voltage waveform of the machining gap is, for example, as shown in waveform 17-A. From this voltage waveform, only high frequency components are fetched by the high-pass filter 4 and the waveform changes as shown in waveform 17-B. In the meantime, the controller 18 resets the peak holder 17 when the voltage is applied to the machining gap and cancels the reset status when a discharge is detected. Further, the peak holder 17 provides output as shown in waveform 17-M because it holds the highest voltage value measured during operation. The display device 7 displays the discharge status according to the value of the peak holder 17 during the stop time. Since the above design allows the maximum amplitude value during a discharge to be measured, the discharge status can be detected accurately. Whereas the peak holder 17 was controlled in synchronization with voltage application or discharge occurrence in this apparatus, it is apparent that the peak holder 17 may be controlled at any timing if the maximum amplitude value of all or some voltage values during a discharge can be measured per pulse, e.g., reset is canceled a predetermined period of time after a discharge has occurred, or the peak holder 17 is reset immediately before reset cancellation after discharge occurrence. Also, when this apparatus is designed to hold or check the peak hold output a predetermined period of time after reset cancellation, the discharge status output can be provided independently of the termination time of the discharge and identical effects are produced. The peak holder 17 designed to maintain the highest voltage value in this apparatus may also be arranged to maintain the lowest voltage value to provide the same effects. Also, while this apparatus was designed to hold only one peak, a plurality of peak holder 17 and comparator 13 may be combined or another measure taken to take the add-up value or average value of a plurality of peak values or to take, for example, the third largest peak value, in which case there is an advantage that misoperation is unlikely to take place due to noise. Also, it is a matter of course that peak detection made in response to the output of the high-pass filter 4 in this apparatus may be performed directly with respect to the voltage waveform during a discharge to produce the identical effects. Also, the output of the high-pass filter 4 may be rectified and then input to the peak holder 17 to provide the identical effects. Especially in the case of full-wave rectification, both the minimum value and the maximum value can be evaluated, whereby the discharge status can be detected further more accurately. Fig. 18 is an arrangement diagram of a tenth apparatus. Parts identical or corresponding to the conventional electrical discharge machine in Fig. 49 are given identical reference characters and will not be described here. In Fig. 18, 21 indicates a frequency analyzer and 22 denotes a controller for the frequency analyzer 21. Operation will now be described. As soon as a discharge takes place, the controller 22 starts the frequency analyzer 21. After a predetermined time has elapsed, the controller 22 transmits an output command to the frequency analyzer 21, which then outputs the frequency at the center of gravity of a spectrum waveform as the discharge status and causes the display device 7 to display that status. Subsequently, the controller 22 resets the frequency analyzer 21 to be ready for a next discharge. Figs. 19-A and 19-B show the results of frequency analyses made by the frequency analyzer 21 on pulses 1 and 2 which are different discharge pulses. In these figures, the frequency band shown is up to 5,000,000 Hz. Since the above arrangement operates the frequency analyzer 21 only during a discharge, the influence of repeated frequency changes on the discharge due to the fluctuation of no-load time and the control of stop time can be eliminated and the output can be provided per pulse, whereby the discharge status can be measured and displayed accurately. The frequency analyzer 21 arranged to output the frequency at the center of gravity of a spectrum waveform in this apparatus may also be designed to output any analyzed amount related to the magnitude of high frequency components, such as the area of the spectrum waveform in a specific frequency zone or average strength at one or more specific frequencies. Also, the whole or partial spectrum waveform of the frequency analysis result may be displayed intact as a pattern to provide much information to the operator of the electrical discharge machine. It will be recognized as a matter of course that analysis results over a plurality of pulses may be displayed, e.g., the averages, variances and degrees of change of the analysis results are displayed for a plurality of pulses or displayed in a plurality of display positions in sequence. Also, it is a matter of course that the frequency analyzer 21 started simultaneously with discharge occurrence in this apparatus may be started any time if during a discharge. Also, while the frequency analyzer 21 was provided as hardware in this embodiment, frequency analyses may be made by, for example, the FFT calculation of the waveform during the discharge in order to produce the same effects. Fig. 20 is an arrangement diagram of an eleventh apparatus. Parts identical or corresponding to the apparatus in Fig. 5 are given identical reference characters and will not be described here. In Fig. 20, the numeral 33 indicates a counter. Fig. 21 shows an input/output signal waveform example in the main area of the present embodiment, wherein waveform 21-A shows the voltage waveform of the machining gap, waveform 21-B shows the output waveform of the high-pass filter 4, waveform 21-C shows the output waveform of the rectifier 5, waveform 21-N shows the count-up output waveform of the counter 33, and waveform 21-O shows the output waveform of the integrator 9. Operation will now be described with reference to Figs. 20 and 21. A pulse-shaped voltage is applied to the machining gap by the machining power supply 1 to carry out electrical discharge machining. At this time, the voltage waveform of the machining gap is, for example, shown as waveform 21-A. From this voltage waveform, only high frequency components are fetched by the high-pass filter 4 and the waveform changes as shown in 21-B. They are then rectified by the rectifier 5 and change into a signal waveform 21-C. In the meantime, the counter 33 is designed to be incremented every time a discharge ends, and when incremented up to a preset value, to reset the count value and generate a count-up output until a next discharge occurs. When it is designed to count up at two discharge pulses, the waveform 21-N is output. Also, when the controller 10 instructs the integrator 9 to reset and start integration on the trailing edge of the count-up output, the integration output is as shown in waveform 21-O. According to the output of the integrator 9 on the leading edge of this count-up output, the display device 7 displays the discharge status. Since the above arrangement accumulates integrations over a plurality of discharge pulses, the discharge status can be grasped properly even in a condition under which discharge time is especially short and sufficient integration time cannot be taken. Whereas only pulses in which the discharge current flowed were counted in this apparatus, counting all the generated voltage pulses by, for example, counting the number of times when the application of the voltage pulse to the machining gap has ended allows all voltage pulses including those when the machining gap is open to be counted and produces the same effect. Also, the counter 33 provided to count the discharge pulses in this apparatus may be replaced by a software area provided in an integration controller for counting discharge pulses to have the identical effect. Also, while integration was controlled in synchronization with voltage application or discharge occurrence in this apparatus, the apparatus is identical to the third apparatus in that integration may also be controlled at any timing if the output of the rectifier 5 can be integrated per pulse. Also, count made at the end of a discharge in this apparatus may also be made at any timing if the discharge status output can be checked after a predetermined number of discharge pulses have occurred, e.g., at the application of the voltage to the machining gap. Also, while the technique of the above apparatus employed the rectifier 5 and the integrator 9 to measure high frequency components, it is apparent that the technique described in any of the previous apparatus or any other method that allows the amplitude of high frequency components to be measured may be adopted. While the discharge status detection output provided to the display device 7 in each of the previous apparatus of the invention, clearly it may also be provided to a machining condition controller which is designed to change the machining condition toward a lower risk of arc discharge if there are a few high frequency components and toward a higher risk of arc discharge but higher machining efficiency if there are many high frequency components, in which case the machining condition can be changed faster than by operator control. Also, whereas the analog value was output by the discharge status detector in each of the previous apparatus, a comparator or the like may be provided at the output stage to segment the analog value at one or more levels to be quantized and digitally output. Further, the digital output may be input to the machining condition controller to control the machining condition, whereby there is an advantage that machining condition control can be exercised easily. Fig. 22 is an arrangement diagram of a first embodiment of the invention. Parts identical or corresponding to the apparatus in Fig. 5 are given identical reference characters and will not be described here. In Fig. 23, the numeral 23 indicates a discharge detector, 24 designates a timer, 25 denotes a comparator, 26 represents a comparison reference value generator, and 27 indicates a machining condition controller. Fig. 23 shows an input/output signal waveform example in the main area of the present embodiment, wherein waveform 23-A shows the voltage waveform of the machining gap, waveform 23-B shows the output waveform of the high-pass filter 4, waveform 23-C shows the output waveform of the rectifier 5, waveform 23-P shows the output of the discharge detector 23, waveform 23-Q shows the output of the timer 24, waveform 23-F shows the output of the integrator 9, and waveform 23-R shows the output of the comparator 25. Operation will now be described with reference to Figs. 22 and 23. A pulse-shaped voltage is applied to the machining gap by the machining power supply 1 to carry out electrical discharge machining. At this time, the voltage waveform of the machining gap is, for example, waveform 23-A. From this voltage waveform, only high frequency components are first passed by the high-pass filter 4 and the waveform changes to waveform 23-B. Here, it is desired that a value larger than 50kHz be employed as the cut-off frequency of the high-pass filter 4. Also, to prevent misoperation due to noise, it is further recommended to use a band-pass filter which cuts high frequency components higher than required (for example, components of more than several hundred MHz). Next, the high frequency components obtained are rectified by the rectifier 5, changed into a waveform 23-C, and are then input to the integrator 9. Now, when the voltage applied causes a discharge to occur between the electrode 2 and the workpiece 3, the discharge detector 23 raises a discharge detection output (waveform 23-P). A discharge occurrence detection method considered is, for example, to compare the voltage or current of the machining gap with a reference value. On the leading edge of the discharge detection output, the integrator 9 is reset and the timer 24 is started. The timer 24 used is, for example, a one-shot multivibrator, a binary counter/timer or the like. After a predetermined period of time (1µsec or longer is desired and about half of the discharge time is much better) has elapsed, the timer 24 generates a measurement start output (waveform 23-Q). This output causes the reset status of the integrator 9 to be canceled and the output of the rectifier 5 to be integrated, causing the output waveform of the integrator 9 to be as shown in waveform 23-F. When the discharge ends, the discharge detector 23 drops the discharge detection output. A discharge termination detection method considered is, for example, to compare the voltage or current of the machining gap with a reference value or to detect a signal with which the machining power supply 1 operates the switching device therein. On the trailing edge of the discharge detection output, the comparator 25 compares the output of the integrator 9 with the reference value and outputs the normal/fault judgement of the discharge (waveform 23-R). The machining condition controller 27 checks the output of the comparator 25 during stop time, and extends the stop time if the discharge status is judged to be poor (waveform 23-A). Since the above arrangement causes the measurement of high frequency components to be started soon after discharge occurrence, the influence of the high frequency components generated immediately after a discharge start upon the discharge status detection can be avoided and the discharge status measured precisely. Also, since the high frequency components are integrated over a certain length of time, the present embodiment has an advantage that it is not subject to the influence of noise. While the integrator 9 was designed to be started by the measurement start output in this embodiment, the high-pass filter 4 or the rectifier 5 may also be arranged to be started by the measurement start output, providing the same effects. Also, whereas the comparator 25 was arranged to operate on the trailing edge of the discharge detection output in this embodiment, it is apparent that a second timer may be provided and the comparator 25 designed to operate a predetermined time after the measurement start output, or alternatively, the output of the comparator 25 operating continuously may be held on the trailing edge of the discharge detection output in order to produce the same effects. Furthermore, the integrator 9 arranged to be reset on the leading edge of the discharge detection output in this embodiment may also be reset any time between normal/faulty discharge judgement and integration start, e.g., at a point when the machining power supply 1 has applied the voltage. Also, it is a matter of course that the output of the comparator 25 checked during the stop time may be checked any time after the judgement of a normal/faulty discharge. Also, while the technique of this embodiment of the invention uses the rectifier 5 and the integrator 9 to measure high frequency components, it is apparent that any of the techniques described in the previous apparatuses wherein, for example, the output of the rectifier 5 is smoothed or is caused to pass the low-pass filter, the total time or the number of times when the voltage of the machining gap or the amplitude of the high-pass filter 4 output exceeds a certain level is measured or the maximum value of the amplitude is measured, computation is made on the analog-to-digital conversion output, or the frequency analysis is performed on the voltage of the machining gap, or any other method that allows the amplitude of high frequency components to be measured, may be adopted. Also, whereas the stop time was extended when the discharge status was deteriorated in that embodiment, a further better result will be obtained by reducing the stop time when the discharge status improves. Further, it is apparent that the machining condition to be controlled is not limited to the stop time but may be any machining condition that influences the discharge status, such as machining current, discharge time, electrode pull-up operation (jump), dielectric pressure and servo voltage. Clearly, control may either be exercised toward a lower risk of arc discharge if the discharge status is poor or toward a higher risk of arc discharge but higher machining efficiency if the discharge status is excellent, as disclosed in Japanese Laid-Open Patent Publication No. SHO62-10769, Japanese Laid-Open Patent Publication No. SHO49-92696 or Japanese Laid-Open Patent Publication No. SHO58-50811. Also, the machining condition controlled according to the discharge status output at a single discharge in that embodiment may be controlled according to the output at a plurality of discharges, or only a display may be provided for the operator without the machining condition being controlled. Further, while the machining condition was controlled according to the result obtained by the digital classification of the measurement result of high frequency components by the comparator in the embodiment, it is a matter of course that the measurement result of high frequency components may be displayed intact in the analog value or utilized for machining condition control as described in the previous embodiments of the invention. Fig. 24 is an arrangement diagram of a second embodiment of the invention. Parts that are identical or corresponding to the embodiment in Fig. 22 are given identical reference characters and will not be described here. In Fig. 24, the numeral 28 indicates a short circuit detector serving as short circuit detecting means, 29 designates a first comparator acting as first comparing means, 30 represents a first comparison reference value generator acting as first comparison reference value generating means, 31 denotes a second comparator acting as second comparing means, and 32 indicates a second comparison reference value generator acting as second comparison reference value generating means. Fig. 25 shows an input/output signal waveform example in the main area of the present embodiment, wherein parts identical or corresponding to the embodiment in Fig. 22 are given identical reference characters and will not be described here. In Fig. 25, waveform 25-S shows the output waveform of the first comparator 29 which compares the output of the discharge detector 23 with a first reference value, waveform 25-T shows the output waveform of the second comparator 31 which compares the output of the discharge detector 23 with a second reference value. The waveform 25-U shows the output signal of the short circuit detector 28. Figs. 26A-26D illustrate the distribution of the high frequency component magnitude of the machining gap voltage in the progress of electrical discharge machining. Operation will now be described with reference to Figs. 24, 25 and 26A-26D. The operation up to the integration of high frequency components in the voltage of the machining gap is identical to that in the previous embodiment of the invention and the integration output is input to the first comparator 29 and the second comparator 31. On the trailing edge of the discharge detection output, the first comparator 29 provides the digital output of the comparison result between the integrator 9 output and the first reference value shown as waveform 25-S. At the same time, the second comparator 31 provides the digital output of the comparison result between the integrator 9 output and the second reference value which has been set lower than the first reference value shown as waveform 25-T. Accordingly, discharge pulses are classified by the first and the second comparators 29, 31 into three different types. Namely, as shown in Figs. 26B-26D, as electrical discharge machining progresses from the stable status to the instable status to the arc status, the distribution of the integrator 9 output shows the segmentation of three areas. Hence, more accurate judgement can be made by classifying the discharge pulses into a normal discharge pulse, a quasi-arc discharge pulse and an arc discharge pulse. These pulses are hereinafter referred to as a normal discharge, a quasi-arc discharge and an arc discharge in order of larger high frequency components. In the meantime, the short circuit detector 28 checks the voltage value of the machining gap on the trailing edge of the discharge detection output, compares it with a short circuit reference voltage (15V or less desired), and digitally outputs a short circuit detection signal (waveform 25-U). Here, the short circuit detector 28, which compares the machining gap voltage value during a discharge with the reference value, has been provided not to differentiate the normal discharge from the arc discharge with the reference voltage set to approximately 20V when steel is machined by a copper electrode, as was often seen conventionally, but to judge whether or not the electrode 2 and the workpiece 3 are in a short circuit status. The short circuit status in this case does not only mean the direct contact of the electrode 2 with the workpiece 3 but also includes a short circuit via chips, tar transformed from the dielectric, or a carbon film formed on the electrode surface. Hence, the machining gap voltage value cannot be completely 0V and the voltage of approximately several volts is developed. Since the short circuit reference voltage depends on the electrode material, etc., like the arc discharge voltage, there is not a common value to all cases but it is set to not more than 15V, or more desirably, to not more than 10V, for the electrode 2 of copper and the workpiece 3 of steel. Now, the machining condition controller 27 obtains for each pulse three different judgement outputs, i.e., the normal discharge, the quasi-arc discharge and the arc discharge. These outputs are derived from the first and second comparators 29, 31 and two different judgement outputs, which indicates the presence and absence of the short circuit status, from the short circuit detector 28. As a result, the machining condition controller 27 is given six different judgement outputs. The machining condition controller 27 does not change the machining condition in the cases of a short circuit and a non-short circuit normal discharge, changes the stop time to a longer value in the case of a quasi-arc discharge, and changes the stop time to a further longer value in the case of an arc discharge. In the above arrangement, in addition to the identification of the discharge status by the presence/absence of high frequency components, the present embodiment is designed to judge discharge pulses and control the machining condition in consideration of the presence/absence of the short circuit status, whereby the machining status can be maintained optimally. Whereas relatively simple machining control was adopted in that embodiment, it is a matter of course that separate machining condition changing methods may be adopted for all the six different discharges, e.g., the machining conditions remain unchanged for the short-circuit normal discharge, the stop time is reduced for the non-short-circuit normal discharge, the servo voltage is increased for the short-circuit quasi-arc discharge, the servo voltage is decreased for the non-short-circuit quasi-arc discharge, the electrode pull-up operation (jump) is performed for the short-circuit arc discharge, and the stop time is increased for the non-short-circuit arc discharge, and that the machining condition other than the stop time may also be changed. For such control, reference can be made to the techniques disclosed in the Japanese Laid-Open Patent Publications previously identified herein. While a particular technique was described in the first embodiment of the invention to detect high frequency components, it is a matter of course that all the techniques related to the detection of high frequency components in the voltage, current and impedance of the machining gap described in the prior apparatuses and embodiments of the invention may be adopted and it also is apparent that any other high frequency component detecting technique may be adopted. Also, the discharge pulses classified into three types according to high frequency components in that embodiment may be classified into two, four or more types to provide the same effects. Also, whereas the machining condition was controlled immediately after the discharge pulse was classified, the machining condition may be controlled using the classification results of a plurality of pulses, e.g., the machining condition is changed when the count value of the classified pulse counted reaches a predetermined value or a classification pattern becomes a specific form. Also, only a display may be provided for the operator without the machining condition being controlled. Further, the occurrence of a short circuit detected using the voltage level of the machining gap in that thirteenth embodiment may be detected in any method, e.g., the excess of the discharge current over the reference value or the change in impedance of the machining gap, to produce the same effects. Whereas the voltage waveform was caused to pass the high-pass filter 4 to provide high frequency components in the embodiment, a band-pass filter may be employed in place of the high-pass filter 4 to cut unnecessary high frequency components in order to avoid misoperation due to noise, whereby the discharge status can be detected further more accurately. Also, it is apparent that, for example, alternating-current components may be obtained through a capacitor connected in series, direct-current bias components may be removed by the subtraction of the reference voltage value, desired frequency components may be detected by a resonance circuit provided, or an electromagnetic wave generated from the machining gap may be detected by an antenna provided in the vicinity to detect high frequency components, thereby producing the same effects. Further, in the detection of high frequency components with said resonance circuit or antenna, high frequency components may be detected at a desired single frequency or several frequencies, but when they are detected in a desired frequency zone, e.g., by providing a multiplicity of resonance circuits or detection circuits, the discharge status can be detected further more accurately. Also, while high frequency components in the voltage of the machining gap were detected in the embodiment, it is a matter of course that high frequency components generated in the current or impedance of the machining gap may be detected to provide the same effects. While the detection of high frequency components in a discharge voltage waveform was described in said embodiment, it is apparent that it is also applicable to a discharge current waveform, a machining gap impedance waveform, etc. In the meantime, it is known in electrical discharge machining that electrical discharge characteristics depend on the electrical conditions set to the machining power supply, such as the pulse width, stop time and discharge peak value, and machining conditions from a machining condition setting device not shown (for example, the material and size of the electrode, the dielectric type, the time when the dielectric was changed last, the cycle of electrode up/down, the servo voltage and the dielectric pressure) and that the high frequency components of the discharge voltage waveform detected at the machining gap differ in amplitude or frequency depending on the same. Accordingly, in each of the above described embodiments, the high frequency amplitude measuring parameters, such as the integration time constant, integration time and high-pass filter 4 cut-off frequency, the comparison reference value of the comparator, and/or the analysis reference value of the frequency analyzer may be changed on the basis of the electrical conditions set to the machining power supply, the machining conditions from the machining condition setting device, and the like. In this case, the embodiments can respond to a wide range of machining conditions. It will be apparent that the invention, as detailed above, achieves an electrical discharge machining method and a machine therefor which detect an electrical discharge machining status precisely and improve machining efficiency considerably. The entire disclosure of each and every foreign patent application from which the benefit of foreign priority has been claimed in the present application is incorporated herein by reference, as if fully set forth.

Claims: A device for detecting a discharge status at a machining gap (2, 3) formed by a machining electrode (21) of an electrical discharge machine and workpiece (3) opposed to each other, comprising: discharge detecting means (23) for determining whether or not a discharge has occurred at said machining gap (2, 3); and discharge status detecting means (4, 5, 6) for detecting a discharge status by measuring the alternating-current components or high frequency components of at least one of the current, voltage and impedance of said machining gap (2,3); characterized by said discharge status detecting means being designed to measure said alternating-current components or high frequency components not less than 1 µsec after the occurrence of the discharge. A device according to claim 1, comprising: peak holding means (17) for holding at least either of at least the maximum value and minimum value of one of the current, voltage and impedance of said machining gap (2, 3) or the alternating-current components or high frequency components of the current, voltage and impedance of said machining gap (2, 3) or the rectified outputs of the alternating-current components or high frequency components of the current, voltage and impedance of said machining gap; and control means (18) for controlling the reset of the output of said peak holding means (17).

Inventors: GOTO AKIHIRO, IMAI YOSHIHITO, OZAKI YOSHIO, SATO TATSUSHI, TANEDA ATSUSHI, GOTO, AKIHIRO, IMAI, YOSHIHITO, OZAKI, YOSHIO, SATO, TATSUSHI, TANEDA, ATSUSHI, Goto, Akihiro, c/o Mitsubishi Denki K.K., Imai, Yoshihito, c/o Mitsubishi Denki K.K., OZAKI, YOSHIO, C/O MITSUBISHI DENKI K.K., Sato, Tatsushi, c/o Mitsubishi Denki K.K., Taneda, Atsushi, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: B23H1
IPC codes: B23H7, B23H1