Electric discharge machine and servo control method

Abstract

To provide an electric discharge machine which enables more accurate and stable servo control.SOLUTION: An electric discharge machine includes: a total sum current area calculation part 631 for calculating a measured value of a total sum current area which is a total sum of areas of a current waveform per prescribed sampling period; a servo command part 612 for comparing the measured value with a preset target value, and outputting a servo control command of bringing the measured value close to the target value; and a servo control device 67 for outputting drive current to servo motors 21, 23 so as to keep the size of a working gap constant, on the basis of the servo control command.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an electric discharge machine and a servo control method for the electric discharge machine. [Background technology]

[0002] In an electric discharge machine, a tool electrode and a workpiece are arranged opposite each other with a machining gap, which is a minute gap. The electric discharge machine applies a predetermined voltage to the machining gap to generate an electric discharge, and moves the tool electrode and the workpiece relatively at a predetermined feed speed to machine the workpiece into a desired shape. In an electric discharge machine, servo control is generally performed to keep the size of the machining gap constant.

[0003] Known parameters used as the basis for servo control include average voltage, machining frequency, and discharge waiting time. The average voltage is the average value of the voltage in the machining gap during a specified sampling period. The machining frequency is the number of discharges during a specified sampling period. The discharge waiting time is the time from when a specified voltage is applied to the machining gap until a discharge occurs. These parameters are compared with a reference value, and the relative distance between the tool electrode and the workpiece is adjusted according to the comparison result, thereby keeping the machining gap constant. Patent Document 1 discloses servo control based on the average voltage. Patent Document 2 discloses servo control based on the machining frequency. Patent Document 3 discloses servo control based on the discharge waiting time. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 2711293 [Patent Document 2] Patent No. 4569973 [Patent Document 3] Special Publication No. 56-000170 Summary of the Invention [Problem to be solved by the invention]

[0005] Servo control based on average voltage is easily affected by disturbances. For example, if the resistivity of the machining fluid decreases, voltage leakage occurs, which affects the average voltage. Servo control based on machining frequency considers the removal amount for each discharge to be the same, and controls only based on the number of discharges. In other words, servo control based on machining frequency does not weight the removal amount according to the removal amount. Servo control based on discharge wait time controls the discharge wait time to be constant. However, even if the discharge wait time is kept constant, the removal amount does not necessarily remain constant. For these reasons, accurate control was difficult to achieve with conventional servo control based on average voltage, machining frequency, or discharge wait time.

[0006] The present invention has been made in consideration of the above circumstances, and has an object to provide an electric discharge machine and a servo control method for an electric discharge machine that performs servo control based on the current area sum, which is the sum of the areas of the current waveforms per specified sampling period. [Means for solving the problem]

[0007] According to the present invention, there is provided an electric discharge machine comprising: a power supply unit which applies a voltage to a machining gap formed by the workpiece and the tool electrode to generate an electric discharge; a relative movement unit which includes a servo motor which moves the workpiece and the tool electrode relatively; a current area sum calculation unit which calculates an actual measurement value of a current area sum which is the sum of the areas of current waveforms per predetermined sampling period; a servo command unit which compares the actual measurement value with a preset target value and outputs a servo control command to bring the actual measurement value closer to the target value; and a servo control unit which outputs a drive current to the servo motor so as to maintain the size of the machining gap constant based on the servo control command.

[0008] Furthermore, according to the present invention, there is provided a servo control method for an electric discharge machine including a power supply unit which applies a voltage to a machining gap formed by the workpiece and the tool electrode to generate an electric discharge, and a relative movement unit including a servo motor which moves the workpiece and the tool electrode relatively, the servo control method calculating an actual measurement value of a current area sum, which is the sum of the areas of current waveforms per prescribed sampling period, comparing the actual measurement value with a preset target value, outputting a servo control command which brings the actual measurement value closer to the target value, and outputting a drive current to the servo motor based on the servo control command so as to maintain the size of the machining gap constant. Effect of the Invention

[0009] In the electric discharge machine of the present invention, servo control is performed based on the current area sum, which is the sum of the areas of the current waveforms per predetermined sampling period. The current area sum has a direct correlation with the energy of the discharge, and therefore with the removal rate. This makes it possible to achieve more accurate and stable servo control. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic configuration diagram of an electric discharge machine according to an embodiment of the present invention; [Diagram 2] FIG. 2 is a block diagram of a power supply device. [Diagram 3] FIG. 2 is a block diagram of a control device. [Figure 4] 4 is a timing chart showing a gap voltage, a gap current, and a gate signal. [Diagram 5] 4 shows an example of an edge locus. [Figure 6] 13 is a graph showing the relationship between the distance and the target value in an edge path during outside corner machining. [Figure 7] 11 is a graph showing the relationship between the distance and the target value in an edge path during corner machining. [Figure 8] An example of a circular arc trajectory is shown. [Figure 9] 13 is a graph showing the relationship between the distance and the target value in a circular arc path during outside corner machining. [Figure 10] 11 is a graph showing the relationship between the distance and the target value in a circular arc path during corner machining. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Various modified examples described below can be implemented in any combination.

[0012] The electric discharge machine 1 of this embodiment is a wire electric discharge machine that uses a wire electrode as the tool electrode T, but the present invention can also be applied to a die-sinking electric discharge machine that uses a formed electrode and a small-hole electric discharge machine that uses a pipe electrode.

[0013] 1, the electric discharge machine 1 of this embodiment includes an upper wire guide unit 11, a lower wire guide unit 13, a machining tank 15, a stand 17, a table 19, and a relative movement device 2. The upper wire guide unit 11 and the lower wire guide unit 13 each have a wire guide that guides a wire electrode serving as a tool electrode T. A workpiece W to be machined by electric discharge machining is fixed on the stand 17 and placed in the machining tank 15. The machining tank 15 is filled with machining fluid L during electric discharge machining. The machining fluid L is a dielectric fluid whose main component is pure water or oil.

[0014] The relative movement device 2 moves the workpiece W and the tool electrode T relative to each other. The relative movement device 2 may be one that moves the workpiece W side, one that moves the tool electrode T side, or one that combines both. The relative movement device 2 of this embodiment includes, as servo motors, an X-axis servo motor 21 that moves the table 19 on which the machining tank 15 is placed in the X-axis direction, which is a predetermined horizontal direction, and a Y-axis servo motor 23 that moves the table 19 in the Y-axis direction, which is a horizontal direction perpendicular to the X-axis direction. In the electric discharge machining, the tool electrode T and the workpiece W are moved relatively to each other so as to approach each other while maintaining non-contact with each other. The gap formed by the workpiece W and the tool electrode is called a machining gap. The size of the machining gap is maintained constant.

[0015] The electric discharge machine 1 includes a power supply 3 that applies a voltage to the machining gap to generate an electric discharge. The workpiece W and the tool electrode T are electrically connected to the power supply 3. As shown in FIG. 2, the power supply 3 includes a first power supply circuit 4 that is connected in series to the machining gap, and a second power supply circuit 5 that is provided in parallel with the second power supply circuit and is directly connected to the machining gap. The first power supply circuit 4 applies a voltage to the machining gap to induce an electric discharge in the machining gap. When the occurrence of an electric discharge is detected, the second power supply circuit 5 applies a voltage to the machining gap and supplies a machining current that is a current larger than the current supplied from the first power supply circuit 4. Hereinafter, the voltage in the machining gap is referred to as the gap voltage, and the current in the machining gap is referred to as the gap current. In addition, as shown in FIG. 3, the electric discharge machine 1 includes a numerical control device 61, an electric discharge control device 63, a pulse generator 65, and a servo control device 67 as a control device 6 that controls each part.

[0016] The first power supply circuit 4 includes a first DC power supply 41, a first main switching element 42, a polarity switching circuit 43, a discharge detection circuit 44, a current limiting resistor 45, a backflow prevention diode 46, and a voltmeter 47.

[0017] The first DC power supply 41 is connected in series to the machining gap and applies a voltage for inducing discharge to the machining gap. The output voltage of the first DC power supply 41 may be large enough to generate a discharge.

[0018] The first main switching element 42 is a switching element provided in series between the first DC power source 41 and the machining gap. The polarity switching circuit 43 is a so-called bridge circuit, and includes a pair of positive polarity switching elements 431 and a pair of reverse polarity switching elements 433. The positive polarity switching element 431 has a switching element provided between the positive electrode side of the first DC power source 41 and the workpiece W, and a switching element provided between the negative electrode side of the first DC power source 41 and the tool electrode T. The reverse polarity switching element 433 has a switching element provided between the negative electrode side of the first DC power source 41 and the workpiece W, and a switching element provided between the positive electrode side of the first DC power source 41 and the tool electrode T.

[0019] The discharge detection circuit 44 is a circuit that detects the occurrence of a discharge, connected in parallel to the first main switching element 42. The discharge detection circuit 44 has a current limiting resistor 441, a detection circuit switching element 443, a shunt resistor 445, and an operational amplifier 447 connected to both ends of the shunt resistor 445. The current limiting resistor 441 has a sufficient resistance value and reduces the current supplied from the first DC power source 41 to the machining gap as much as possible. The detection circuit switching element 443 is a switching element that is turned on / off in synchronization with the first main switching element 42. When a discharge occurs in the machining gap, a current flows through the discharge detection circuit 44, and a potential difference occurs across the shunt resistor 445. This potential difference is amplified by the operational amplifier 447 and output as a discharge detection signal.

[0020] When performing positive polarity machining, that is, when the first power supply circuit 4 applies a voltage such that the workpiece W is positive and the tool electrode T is negative, the pulse generator 65 supplies a gate signal MG to the first main switching element 42 and the detection circuit switching element 443, and a gate signal SGp to the positive polarity switching element 431. Thus, the first main switching element 42, the detection circuit switching element 443, and the positive polarity switching element 431 are turned on. When performing reverse polarity machining, that is, when the first power supply circuit 4 applies a voltage such that the workpiece W is negative and the tool electrode T is positive, the pulse generator 65 supplies a gate signal MG to the first main switching element 42 and the detection circuit switching element 443, and a gate signal SGn to the reverse polarity switching element 433. Thus, the first main switching element 42, the detection circuit switching element 443, and the reverse polarity switching element 433 are turned on.

[0021] The current limiting resistor 45 is provided in series between the first DC power supply 41 and the machining gap. The current limiting resistor 45 has a sufficient resistance value and reduces the current supplied from the first DC power supply 41 to the machining gap as much as possible.

[0022] The reverse current prevention diode 46 is disposed in series between the polarity switching circuit 43 and the machining gap, and protects the first main switching element 42, the detection circuit switching element 443, the positive polarity switching element 431 and the reverse polarity switching element 433 from surge voltages.

[0023] The voltmeter 47 is connected in parallel to the machining gap. The voltmeter 47 measures the machining gap voltage and outputs the measured value to the discharge control device 63.

[0024] The second power supply circuit 5 includes a second DC power supply 51, a second main switching element 52, and a reverse current prevention diode 53.

[0025] The second DC power supply 51 is connected in series to the machining gap and applies a voltage for supplying a machining current to the machining gap. The value of the voltage applied by the second DC power supply 51 is defined as an applied voltage value AV.

[0026] The second main switching element 52 is a switching element provided in series between the second DC power supply 51 and the machining gap. In this embodiment, two second main switching elements 52 are provided on the positive and negative sides of the second DC power supply 51, respectively.

[0027] When it is detected that a discharge has occurred in the machining gap, a gate signal NG is supplied from the pulse generator 65 to the second main switching element 52, which is turned on. Then, a machining current is supplied to the machining gap. After a predetermined time has elapsed, the supply of the gate signals MG and NG is stopped, and the first main switching element 42, the detection circuit switching element 443, and the second main switching element 52 are turned off.

[0028] The reverse current prevention diode 53 is disposed in series between one side of the second main switching element 52 and the machining gap, and protects the second main switching element 52 from surge voltages.

[0029] As shown in FIG. 3, the discharge control device 63 includes a current area sum calculation unit 631, a discharge counter 632, a discharge standby time calculation unit 633, a pulse command unit 634, and a memory 635.

[0030] The current area sum calculation unit 631 calculates the actual value Fm of the current area sum Et based on various data. The current area sum Et refers to the sum of areas E per predetermined sampling period, where E is the area of ​​the current waveform of one discharge.

[0031] The discharge counter 632 stores the number of discharges, which is the number of times that a discharge has occurred, based on the discharge detection signal output by the operational amplifier 447 of the discharge detection circuit 44. In this embodiment, the discharge counter 632 stores the number of discharges when the discharge wait time τw is equal to or greater than a predetermined threshold th, and the number of discharges when the discharge wait time τw is less than the predetermined threshold th, as a first discharge number CL and a second discharge number CS, respectively.

[0032] The discharge waiting time calculation unit 633 calculates the discharge waiting time τw for each discharge. The discharge waiting time τw refers to an unspecified time from when a predetermined voltage is applied to the machining gap until when a discharge occurs. In particular, in this embodiment, the discharge waiting time τw refers to the time from when a predetermined voltage is applied to the machining gap by the first power supply circuit 4 until when a discharge occurs. The discharge waiting time calculation unit 633 uses a timer to measure the time from when the first power supply circuit 4 starts applying a voltage to the machining gap, that is, from when the pulse generator 65 starts outputting the gate signal MG, until when the occurrence of a discharge is detected, and calculates the discharge waiting time τw.

[0033] The pulse command unit 634 sends a gate command to the pulse generator 65 so as to output the gate signals MG, SGp, SGn, and NG at a predetermined timing. In this embodiment, the gate signal NG related to the second main switching element 52 is turned on for a predetermined time set according to the discharge waiting time τw after the occurrence of discharge is detected. In other words, the on time of the gate signal NG is a value that changes according to the discharge waiting time τw. Here, the on time of the gate signal NG is the gate width NGw, the set value of a relatively long gate width among the gate widths NGw is the first gate width NGwL, and the set value of a relatively short gate width is the second gate width NGwS. In this embodiment, the pulse command unit 634 outputs a gate command to the pulse generator 65 so that when the discharge waiting time τw is equal to or greater than a predetermined threshold th, the gate width NGw becomes the first gate width NGwL, and when the discharge waiting time τw is less than the predetermined threshold th, the gate width NGw becomes the second gate width NGwS. In this embodiment, the gate width NGw is divided into two gate widths NGwL and NGwS, but may be divided into three or more gate widths depending on the discharge wait time τw.

[0034] The memory 635 stores data required for calculations and calculation results.

[0035] As shown in Fig. 3, the numerical control device 61 includes a PID calculation unit 611, a servo command unit 612, and a memory 613. The PID calculation unit 611 obtains the operation amount in the PID control, and furthermore, the output speed, based on the actual value Fm of the current area sum Et calculated by the current area sum calculation unit 631 and other data. The servo command unit 612 compares the actual value Fm of the current area sum Et with a preset target value Ft, and outputs a servo control command to the servo control device 67 to bring the actual value Fm closer to the target value Ft. In particular, when servo control is performed by PID control, the servo command unit 612 outputs a servo control command based on the operation amount calculated by the PID calculation unit 611, and furthermore, the output speed, to the servo control device 67. The memory 613 stores data required for the calculation and the calculation results. An operator can input necessary parameters via an input device (not shown) of the numerical control device 61.

[0036] The pulse generator 65 supplies gate signals MG, SGp, SGn, and NG to the first main switching element 42, the detection circuit switching element 443, the positive polarity switching element 431, the reverse polarity switching element 433, and the second main switching element 52, and generates a discharge in the machining gap by switching them on and off. Each switching element is typically a field effect transistor such as a MOSFET.

[0037] The servo control device 67 is a driver, and outputs drive currents to the X-axis servo motor 21 and the Y-axis servo motor 23 of the relative movement device 2 based on a servo control command so as to maintain the size of the machining gap constant.

[0038] The configurations of the power supply device 3 and the control device 6 described above are merely examples, and are not limited thereto. For example, in this embodiment, the occurrence of discharge is detected by the discharge detection circuit 44, but the occurrence of discharge may be detected based on the value of the inter-electrode voltage measured by the voltmeter 47. In other words, it may be considered that a discharge has occurred when the inter-electrode voltage drops by a certain amount or more in positive polarity machining, and when the inter-electrode voltage rises by a certain amount or more in reverse polarity machining. For example, in this embodiment, the calculation related to the current area sum Et is performed by the discharge control device 63, and the calculation related to the PID control is performed by the numerical control device 61, but the calculation may be performed by either the numerical control device 61 or the discharge control device 63. The control device 6 may be configured by combining any software and hardware as long as the desired calculation and control can be performed. Any calculation device such as an FPGA, an ASIC, a DRP, a DSP, or a CPU may be used as each calculation unit and each command unit of the control device 6.

[0039] Here, the servo control of this embodiment will be described in detail. Fig. 4 is a schematic timing chart showing the gap voltage, gap current and gate signals MG, SGp, SGn and NG.

[0040] 4, the waveform of the current between the electrodes is ideally a triangular wave that rises when the gate signal NG turns on after the discharge occurs and has its apex at the time when the gate signal NG turns off. Therefore, the approximate area E of the current waveform of one discharge can be calculated by multiplying the gate width NGw, which is the on time of the gate signal NG, by the peak current value Ip, which is the peak value of the current between the electrodes.

[0041] In this embodiment, servo control is performed based on the current area sum Et, which is the sum of the areas E of the current waveform per predetermined sampling period. Since the current area sum Et has a direct correlation with the energy of the discharge generated in the machining gap, servo control based on the current area sum Et can be performed substantially based on the energy of the discharge. Note that in this embodiment, a value obtained by multiplying the current area sum Et by a predetermined coefficient including 1 is treated as the actual measurement value Fm of the current area sum Et.

[0042] The peak current value Ip, the gate width NGw, a predetermined constant K, and the applied voltage value AV have a proportional relationship as shown in the following formula 1. α is a constant indicating the slope, and can be obtained by experiment.

[0043]

number

[0044] Since "area E = gate width NGw × peak current value Ip", formula 1 can be transformed into the following formula 2.

[0045]

number

[0046] Therefore, the current area sum Et can be obtained by calculating the number of discharges per a predetermined sampling period, for example, 10 ms, and multiplying the area E by the number of discharges. In this embodiment, the gate width NGw is the first gate width NGwL when the discharge waiting time τw is equal to or greater than a predetermined threshold th, and is the second gate width NGwS when the discharge waiting time τw is less than the predetermined threshold th. In addition, the number of discharges is counted as the first discharge number CL when the discharge waiting time τw is equal to or greater than a predetermined threshold th, and is counted as the second discharge number CS when the discharge waiting time τw is less than the predetermined threshold th. That is, the current area sum calculation unit 631 may calculate the current area sum Et based on the following formula 3. Also, as described above, the value obtained by multiplying the current area sum Et by a coefficient may be set as the actual measurement value Fm of the current area sum Et, so Et / α may be calculated instead of Et.

[0047]

number

[0048] The measured value Fm of the total current area Et calculated in this way is a parameter that is not easily affected by disturbances and has a direct correlation with the removal rate. By performing servo control based on the measured value Fm of the total current area Et, more stable and highly accurate machining can be performed.

[0049] In this embodiment, the actual value Fm of the current area sum Et is calculated based on the first discharge number CL, the second discharge number CS, the first gate width NGwL, the second gate width NGwS, and the applied voltage value AV. In this way, the actual value Fm can be calculated using a relatively simple calculation formula, which is preferable. However, if the actual value Fm of the current area sum Et can be calculated, another calculation method may be adopted. For example, an ammeter for measuring the gap current may be provided in the power supply device 3, and the actual value Fm of the current area sum Et may be calculated based on the value of the gap current. Specifically, the actual value Fm may be calculated from the pulse width and peak current value Ip of the gap current, or may be calculated from the integral value of the waveform of the gap current. By performing the calculation directly based on the gap current, the actual value Fm can be calculated more accurately.

[0050] In this embodiment, preferably, servo control is performed by PID control. That is, the PID calculation unit 611 calculates the manipulated variable in the PID control. Then, the servo command unit 612 outputs a servo control command based on the manipulated variable, and in turn, a servo control command including a command related to the output speed calculated based on the manipulated variable, to the servo control device 67. The manipulated variable in the PID control is updated at a predetermined period. Here, an update at a certain point in time is defined as an n-th update, and F n : Operation amount (the subscript n indicates the nth operation amount) E n : Deviation (the subscript n indicates the nth deviation.) E i :integral deviation E d ; Differential deviation K p : Proportional gain K i : Integral gain K d : Differential gain Ft: Target value of total current area Et Fm: Actual measurement value of total current area Et The nth operation amount F n can be calculated using the following formula 4.

[0051]

number

[0052] The update amount ΔF of the operation amount is the nth operation amount F n and the n-1th operation amount F n-1 In addition, there is a proportional relationship between the speed update amount ΔV and the operation amount update amount ΔF with a predetermined slope β as a proportionality constant. Therefore, the nth output speed V n can be calculated using the following formula 5.

[0053]

number

[0054] Conventionally, servo control was performed to simply advance the tool electrode T relatively at a predetermined feed rate when the actual measurement value was equal to or less than the target value, and to relatively retreat the tool electrode T when the actual measurement value exceeded the target value. By performing PID control during servo control, the tool electrode T can be relatively moved at an appropriate feed rate according to the machining shape and plate thickness, compared to the conventional technology. Therefore, high-precision machining can be performed without reducing the feed rate more than necessary. In addition, since PID control is performed based on the current area sum Et, which has a direct correlation with the discharge energy, the feed rate is reduced before the tool electrode T and the workpiece W come into contact with each other and a short circuit occurs. Therefore, the occurrence of a short circuit is prevented.

[0055] The target value Ft of the current area sum Et may be changed according to the machining shape. In particular, in corners, the contour shape is easily distorted, that is, sagging, is easily generated, so by adjusting the target value Ft in the corners, machining with higher accuracy can be performed. Here, the movement path of the tool electrode T is called the machining path. In machining of corners, machining in which the machining path is set inside the desired machining shape is called out-corner machining, and machining in which the machining path is set outside the desired machining shape is called in-corner machining. Corners are roughly divided into edge parts formed by two straight lines with different angles and arc parts formed by a circular arc, and the respective machining paths are called edge paths and arc paths.

[0056] An example of an edge trajectory is shown in Figure 5. Edge trajectories b and c are composed of two linear machining trajectories of a predetermined length that form an acute angle. Here, the linear machining trajectory of a predetermined length that is connected to the starting point of edge trajectory b is defined as approach trajectory a. Also, the linear machining trajectory of a predetermined length that is connected to the end point of edge trajectory c is defined as return trajectory d.

[0057] During out-corner machining, the target values ​​in the edge trajectories b and c are set lower than the target values ​​in the approach trajectory a and the return trajectory d. In particular, as shown in FIG. 6, it is preferable that the target values ​​are set to be gradually lower from the start point of the approach trajectory a to the end point of the approach trajectory a, and to be gradually higher from the start point of the return trajectory d to the end point of the return trajectory d. In this embodiment, the target value LtL1 at the end point of the approach trajectory a and the start point of the return trajectory d is set higher than the target value FtL2 at the edge trajectories b and c. In other words, the target value Ft of the current area sum Et transitions from the normal target value FtN to the target value LtL1 at the approach trajectory a, is changed to the target value FtL2 at the edge trajectories b and c, and then transitions from the target value LtL1 to the normal target value FtN at the return trajectory d. As a result, the feed rate automatically increases at the edge trajectories b and c.

[0058] During corner machining, the target values ​​at the edge trajectories b and c are set higher than the target values ​​at the approach trajectory a and the return trajectory d. In particular, as shown in FIG. 7, it is preferable that the target values ​​are set to be gradually higher from the start point of the approach trajectory a toward the end point of the approach trajectory a, and to be gradually lower from the start point of the return trajectory d toward the end point of the return trajectory d. In this embodiment, the target values ​​LtH1 at the end point of the approach trajectory a and the start point of the return trajectory d are set lower than the target values ​​FtH2 at the edge trajectories b and c. In other words, the target value Ft of the current area sum Et transitions from the normal target value FtN to the target value LtH1 at the approach trajectory a, is changed to the target value FtH2 at the edge trajectories b and c, and then transitions from the target value LtH1 to the normal target value FtN at the return trajectory d. As a result, the feed rate is automatically reduced at the edge trajectories b and c.

[0059] An example of a circular arc trajectory is shown in Figure 8. The circular arc trajectory f is composed of a circular arc-shaped machining trajectory. Here, a linear machining trajectory of a predetermined length connected to the starting point of the circular arc trajectory f is defined as an approach trajectory e. Also, a linear machining trajectory of a predetermined length connected to the end point of the circular arc trajectory f is defined as a return trajectory g.

[0060] During outer corner machining, the target value in the arc trajectory f is set lower than the target values ​​in the approach trajectory e and the return trajectory g. In particular, as shown in FIG. 9, it is preferable that the target value is set to be gradually lower from the start point of the approach trajectory e to the end point of the approach trajectory e, and the target value is set to be gradually higher from the start point of the return trajectory g to the end point of the return trajectory g. In this embodiment, the target value LtL1 at the end point of the approach trajectory e and the start point of the return trajectory g is set higher than the target value FtL2 in the arc trajectory f. In other words, the target value Ft of the current area sum Et transitions from the normal target value FtN to the target value LtL1 in the approach trajectory e, is changed to the target value FtL2 in the arc trajectory f, and then transitions from the target value LtL1 to the normal target value FtN in the return trajectory g. As a result, the feed rate automatically increases in the arc trajectory f.

[0061] During outer corner machining, the target value in the arc trajectory f is set higher than the target values ​​in the approach trajectory e and the return trajectory g. In particular, as shown in FIG. 10, it is preferable that the target value is set to be gradually higher from the start point of the approach trajectory e toward the end point of the approach trajectory e, and that the target value is set to be gradually lower from the start point of the return trajectory g toward the end point of the return trajectory g. In this embodiment, the target value LtH1 at the end point of the approach trajectory e and the start point of the return trajectory g is set higher than the target value FtH2 in the arc trajectory f. In other words, the target value Ft of the current area sum Et transitions from the normal target value FtN to the target value LtH1 in the approach trajectory e, is changed to the target value FtH2 in the arc trajectory f, and then transitions from the target value LtH1 to the normal target value FtN in the return trajectory g. As a result, the feed rate is automatically reduced in the arc trajectory f.

[0062] In this way, by changing the target value Ft at the corner portion, more accurate machining can be achieved and the feed rate can be optimized. In particular, when the target value Ft is controlled to change gradually in the approach trajectories a, e and the return trajectories d, g, a sudden change in the target value Ft is suppressed, so hunting is prevented. In this embodiment, the target value Ft is increased or decreased proportionally in the approach trajectories a, e and the return trajectories d, g, but this is not limited to this. For example, the target value Ft may be increased or decreased in a quadratic function. In this embodiment, a difference is provided between the target values ​​LtL1, FtH1 at the end points of the approach trajectories a, e and the start points of the return trajectories d, g and the target values ​​FtL2, LtH2 at the edge trajectories b, c and the circular arc trajectory f, but both may be the same.

[0063] The control related to the corner portion described above is typically applied to a wire electric discharge machine. However, this control can also be suitably applied to a die-sinking electric discharge machine or a small-hole electric discharge machine, for example, when performing generating machining using a tool electrode T having a simple shape in the die-sinking electric discharge machine or the small-hole electric discharge machine. [Explanation of symbols]

[0064] 1 Electric discharge machine 2 Relative movement device 21 X-axis servo motor 23 Y-axis servo motor 3 Power supply 4 First power supply circuit 41 First DC power source 42 First main switching element 5 Second power supply circuit 51 Second DC power source 52 Second main switching element 611 PID calculation unit 612 Servo Command 631 Current Area Sum Calculation Unit 632 Discharge Counter 634 Pulse Command 67 Servo control device T Tool electrode W Workpiece

Claims

1. a power supply device that applies a voltage to a machining gap formed by the workpiece and the tool electrode to generate an electric discharge; a relative movement device including a servo motor for moving the workpiece and the tool electrode relative to each other; a current area sum calculation unit that calculates an actual measurement value of a current area sum, which is the sum of areas of current waveforms per predetermined sampling period; a servo command unit that compares the actual measurement value with a preset target value and outputs a servo control command that brings the actual measurement value closer to the target value; a servo control device that outputs a drive current to the servo motor so as to maintain the size of the machining gap constant based on the servo control command.

2. The power supply device is a first power supply circuit connected in series to the machining gap and configured to apply a voltage to the machining gap to induce discharge in the machining gap; a second power supply circuit that is provided in parallel with the first power supply circuit and connected in series to the machining gap, and applies a voltage to the machining gap when an occurrence of an electric discharge is detected, thereby supplying a current to the machining gap that is larger than the current supplied from the first power supply circuit, The first power supply circuit includes: a first DC power source connected in series to the machining gap and configured to apply a voltage to the machining gap; a first main switching element provided in series between the first DC power source and the machining gap; The second power supply circuit includes: a second DC power source connected in series to the machining gap and configured to apply a voltage to the machining gap; 2. The electric discharge machine according to claim 1, further comprising: a second main switching element provided in series between said second DC power supply and said machining gap.

3. 3. The electric discharge machine according to claim 2, wherein the current area sum calculation unit calculates the actual value based on a discharge number which is a number of times a discharge occurs per sampling period, a gate width which is an on-time of the second main switching element, and an applied voltage value which is a value of a voltage applied by the second DC power supply.

4. a discharge counter that individually stores a first discharge number, which is the number of discharges when a discharge wait time, which is a time from when a predetermined voltage is applied to the machining gap by the first power supply circuit until a discharge occurs, is equal to or greater than a threshold value, and a second discharge number, which is the number of discharges when the discharge wait time is less than the threshold value; a pulse command unit that outputs a gate command so that the gate width becomes a first gate width when the discharge waiting time is equal to or greater than the threshold value, and so that the gate width becomes a second gate width that is shorter than the first gate width when the discharge waiting time is less than the threshold value; 4. The electric discharge machine according to claim 3, wherein the current area sum calculation unit calculates the actual value based on the first number of discharges, the second number of discharges, the first gate width, the second gate width, and the applied voltage value.

5. A PID calculation unit is further provided for calculating an operation amount in the PID control, The electric discharge machine according to claim 1 , wherein the servo command section outputs the servo control command based on the amount of operation to the servo control device.

6. when a machining locus, which is a moving locus of the tool electrode, is set outside a desired machining shape, the target value in an edge locus constituted by two straight-line machining loci of a predetermined length forming an acute angle is set lower than the target values ​​in an approach locus, which is a straight-line machining locus of a predetermined length connected to a start point of the edge locus, and a return locus, which is a straight-line machining locus of a predetermined length connected to an end point of the edge locus; 2. The electric discharge machine according to claim 1, wherein when the machining trajectory is set inside a desired machining shape, the target value on the edge trajectory is set higher than the target values ​​on the approach trajectory and the return trajectory.

7. When the machining path is set outside the desired machining shape, the target value is set to gradually decrease from the start point of the approach path to the end point of the approach path, The target value is set to gradually increase from the start point of the return trajectory to the end point of the return trajectory, When the machining path is set inside a desired machining shape, The target value is set to gradually increase from the start point of the approach path to the end point of the approach path, 7. The electric discharge machine according to claim 6, wherein the target value is set to gradually decrease from the start point of the return locus toward the end point of the return locus.

8. when a machining locus which is a moving locus of the tool electrode is set outside a desired machining shape, the target value in the arc locus which is a circular arc-shaped machining locus is set lower than the target values ​​in an approach locus which is a linear machining locus of a predetermined length connected to a start point of the arc locus and a return locus which is a linear machining locus of a predetermined length connected to an end point of the arc locus, 2. The electric discharge machine according to claim 1, wherein when the machining path is set inside a desired machining shape, the target value on the arc path is set higher than the target values ​​on the approach path and the return path.

9. When the machining path is set outside the desired machining shape, the target value is set to gradually decrease from the start point of the approach path to the end point of the approach path, The target value is set to gradually increase from the start point of the return trajectory to the end point of the return trajectory, When the machining path is set inside a desired machining shape, The target value is set to gradually increase from the start point of the approach path to the end point of the approach path, 9. The electric discharge machine according to claim 8, wherein the target value is set to gradually decrease from the start point of the return locus toward the end point of the return locus.

10. A servo control method for an electric discharge machine including a power supply device that applies a voltage to a machining gap formed by a workpiece and a tool electrode to generate an electric discharge, and a relative movement device including a servo motor that moves the workpiece and the tool electrode relatively, comprising: Calculating an actual value of a total current area, which is the total area of ​​the current waveform per predetermined sampling period; comparing the actual measurement value with a preset target value, and outputting a servo control command for bringing the actual measurement value closer to the target value; a servo control method for outputting a drive current to the servo motor based on the servo control command so as to maintain the size of the machining gap constant.

11. The power supply device is a first power supply circuit connected in series to the machining gap and configured to apply a voltage to the machining gap to induce discharge in the machining gap; a second power supply circuit that is provided in parallel with the first power supply circuit and connected in series to the machining gap, and applies a voltage to the machining gap when an occurrence of an electric discharge is detected, thereby supplying a current to the machining gap that is larger than the current supplied from the first power supply circuit, The first power supply circuit includes: a first DC power source connected in series to the machining gap and configured to apply a voltage to the machining gap; a first main switching element provided in series between the first DC power source and the machining gap; The second power supply circuit includes: a second DC power source connected in series to the machining gap and configured to apply a voltage to the machining gap; 11. The servo control method according to claim 10, further comprising: a second main switching element provided in series between the second DC power supply and the machining gap.

12. 12. The servo control method according to claim 11, wherein the actual measurement value is calculated based on a discharge number which is a number of times discharge occurs per sampling period, a gate width which is an on-time of the second main switching element, and an applied voltage value which is a value of a voltage applied by the second DC power supply.

13. a first discharge number, which is the number of discharges when a discharge wait time, which is a time from when a predetermined voltage is applied to the machining gap by the first power supply circuit until a discharge occurs, is equal to or greater than a threshold value, and a second discharge number, which is the number of discharges when the discharge wait time is less than the threshold value, are individually stored; a gate command is output so that the gate width becomes a first gate width when the discharge waiting time is equal to or greater than the threshold value, and so that the gate width becomes a second gate width shorter than the first gate width when the discharge waiting time is less than the threshold value; 13. The servo control method according to claim 12, wherein the actual measurement value is calculated based on the first discharge number, the second discharge number, the first gate width, the second gate width, and the applied voltage value.

14. A manipulated variable in the PID control is calculated, The servo control method according to claim 10 , wherein the servo control command is output based on the manipulated variable.

15. when a machining locus, which is a moving locus of the tool electrode, is set outside a desired machining shape, the target value in an edge locus constituted by two straight-line machining loci of a predetermined length forming an acute angle is set lower than the target values ​​in an approach locus, which is a straight-line machining locus of a predetermined length connected to a start point of the edge locus, and a return locus, which is a straight-line machining locus of a predetermined length connected to an end point of the edge locus; The servo control method according to claim 10 , wherein when the machining trajectory is set inside a desired machining shape, the target value on the edge trajectory is set higher than the target values ​​on the approach trajectory and the return trajectory.

16. When the machining path is set outside the desired machining shape, the target value is set to gradually decrease from the start point of the approach path to the end point of the approach path, The target value is set to gradually increase from the start point of the return trajectory to the end point of the return trajectory, When the machining path is set inside a desired machining shape, The target value is set to gradually increase from the start point of the approach path to the end point of the approach path, The servo control method according to claim 15, wherein the target value is set to gradually decrease from the start point of the return locus toward the end point of the return locus.

17. when a machining locus which is a moving locus of the tool electrode is set outside a desired machining shape, the target value in the arc locus which is a circular arc-shaped machining locus is set lower than the target values ​​in an approach locus which is a linear machining locus of a predetermined length connected to a start point of the arc locus and a return locus which is a linear machining locus of a predetermined length connected to an end point of the arc locus, The servo control method according to claim 10 , wherein when the machining trajectory is set inside a desired machining shape, the target value on the arc trajectory is set higher than the target values ​​on the approach trajectory and the return trajectory.

18. When the machining path is set outside the desired machining shape, the target value is set to gradually decrease from the start point of the approach path to the end point of the approach path, The target value is set to gradually increase from the start point of the return trajectory to the end point of the return trajectory, When the machining path is set inside a desired machining shape, The target value is set to gradually increase from the start point of the approach path to the end point of the approach path, The servo control method according to claim 17, wherein the target value is set to gradually decrease from the start point of the return trajectory to the end point of the return trajectory.

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