JPS63318222A - Detecting device for short circuit of wire electric discharge machine - Google Patents

Abstract

PURPOSE:To eliminate useless control by holding peak voltage values within each defined period determined by first and second peak holding means, comparing them by a voltage difference comparing means and judging whether a short circuit is one which must be truly detected for an electric discharge machining. CONSTITUTION:As signals are inputted into a terminal 2 during a period from the application of discharge pulses to an end, an FET 6 is turned on and a discharge pulse voltage inputted from a terminal 1 is charged into a capacitor 10 and a peak voltage value is held by a peak hold circuit 14 formed with an operating amplifier 12. Also, as signals are inputted into a terminal 3 during a period from when a time in which normally discharge is supposed to be under way after the application of discharge pulses elapsed to the end, an FET 7 is turned on and, equally, a peak voltage value is held by a peak hold circuit 15. And, a difference voltage between both is outputted from an operating amplifier 16 and compared with a reference voltage V5 which is larger than the difference voltage of a complete short circuit period by an operating amplifier 17, and only a complete short circuit period is distinguished and detected. Thereby, useless control can be eliminated preventing the lowering of machining efficiency.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a device for detecting a short circuit between a wire and a workpiece in a wire electric discharge machine.

[Conventional technology]

In a wire electric discharge machine using a wire as an electrode, a suitable gap is maintained between the wire and the workpiece, and a voltage is applied to this gap to generate electric discharge. If there is no gap between the wire and the workpiece, in other words, if they are in contact and electrically short-circuited, no electrical discharge will occur, and electrical discharge machining cannot be performed.
Such short circuits should be detected promptly, and measures such as reducing the energy supplied by the power supply or stopping the feed of the workpiece (moving the workpiece so that the part to be processed approaches the wire) may be taken. It is necessary to take action. A first technique for detecting a short circuit that has been conventionally used is to detect a short circuit by monitoring the voltage between a brush for feeding power to a wire and a workpiece. If a short circuit occurs, the voltage will drop, so you can know that there is a short circuit. The second conventional technique is to detect a short circuit by monitoring the average voltage between the brush and the workpiece. If a short circuit occurs, the average voltage also decreases, so the short circuit can be detected. I can do it.

[Problems to be Solved by the Invention] (Problems) The first prior art has problems such as the difficulty in determining a comparison reference voltage to distinguish between short circuits and non-short circuits, and There has been a problem in that it is not possible to distinguish between short circuits that do not cause problems and short circuits that cause problems. The second problem with the conventional technology is that in order to calculate the average voltage, factors such as no-load voltage and pulse pause time must be taken into account, making calculations complicated, and short circuit detection is not accurate. was there. (Explanation of the problem) Fig. 3 shows a voltage waveform diagram of a discharge pulse. Differential voltage between the voltage and...
Actual discharge voltage R1...Wire resistance R5...Brush contact resistance I2...Current peak value Tw...No load time Tos...Pulse width TOFF...Pulse rest time. When measuring the voltage during the discharging period (T (1% period) in Figure 3, that is, the discharge voltage V), it is actually the voltage between the brush that supplies power to the wire and the workpiece. Strictly speaking, it is the voltage beyond the brush when viewed from the power supply side.
The discharge voltage V, is Vs −I F X (Re + R
h ) " V
(above)) is used. Since the current peak value is high, the voltage drop due to the contact resistance between the brush and the wire (IF xR,) or the voltage drop at the wire portion (I, x
R1) becomes larger. For example, when the voltage beyond the brush is 70V, 50V of it is accounted for by these voltage drops, and the remaining 20V
It accounts for a fairly large proportion of the voltage to be measured, such that V is the real discharge voltage (V, ). In this case, even if a short circuit were to occur, it would only drop 20V out of 70V. It is necessary to detect this drop by setting the comparative reference voltage.However, since the discharge voltage itself (voltage equivalent to 70V in the previous example) changes depending on the following factors, it is necessary to take this into account. However, it was quite difficult to set the comparison reference voltage to an appropriate value. Factors that affect the discharge voltage include the current peak value (which changes as the machining conditions change) and the wire used. There are factors such as material, diameter, distance from the machined part of the workpiece to the brush, degree of wear on the brush, etc.When the diameter of the wire changes, the cross-sectional area changes by the square of it, which affects the wire resistance value. In addition, if the current is large, the wear on the brush will be severe, and the contact resistance with the wire will change even during machining. , and there may be no problem in actual machining even without special control. However, short circuit detection using the first prior art
Since this is carried out for every single shot, even in such a case it will be detected as a short circuit, and therefore control will be carried out in case of a short circuit even though there is no need to do so. In other words, if you over-control, the machining speed will decrease. It is possible to provide an additional circuit to determine whether a short circuit requires control in the event of a short circuit or a short circuit that does not, but doing so when using the average voltage makes it difficult to operate in practical terms. It made things complicated. The average voltage used in the second conventional technique is calculated by the following formula: Vp x"l', + (V, + (Re + Rh
) X Ep ) XTosT u + T ON
+T (IyyIn addition to the factors that affect the discharge voltage mentioned earlier, there are also no-load voltage (VP) and pulse rest time (TO).
FF) etc. are also involved in the equation, making it even more difficult to set the reference voltage for comparison to an appropriate value, and accurate detection cannot be expected. The present invention aims to solve the above problems.

[Means to solve the problem]

In order to solve the above-mentioned problems, the present invention takes the following measures to detect only short circuits that should really be detected in electrical discharge machining. That is, the short circuit detection device for a wire electric discharge machine according to the present invention includes a first peak hold means that holds a peak voltage value after application of a discharge pulse between the wire and the workpiece. , a second peak hold means for holding a peak voltage value after a point in time when normal discharge would occur; A difference voltage comparison means is provided for comparing the difference in peak voltage value of the second peak hold means with a reference voltage.

[For production]

Said 1st. The second peak hold means functions to hold the peak voltage value in each predetermined period. The differential voltage comparison means compares the held peak voltage values to determine whether a short circuit that must be truly detected in electrical discharge machining has occurred.

【Example】

(Regarding the Detection Principle) First, the principle of short circuit detection of the present invention will be explained. FIG. 2 shows periods of various states when a discharge pulse is applied, ie, a normal period and a short-circuit period. The voltage waveform of a discharge pulse during a complete short-circuit period is shown. The normal period is a period during which discharge is performed normally. As shown in Figure 2 (A), the voltage waveform shows a high no-load voltage followed by a low voltage indicating that the discharge is in progress (the peak voltage value before the start of discharge at this time and the peak voltage after the start of discharge) The voltage difference Vl11 from the voltage value (for example, 70V) is large.The short-term short-circuit period means that the wire and the workpiece are short-circuited in a point-to-point contact state or a light (semi-contact state at the level of a touch). This is the period when the current is in the range of (from 100A to several thousand A or more), the current capacity of the contact part cannot cover it and flows to the other party by discharge.The time from voltage application to the start of discharge is short;
The waveform will be as shown in Figure 2 (b). The peak voltage value before the start of discharge at this time and the peak voltage value after the start of discharge (e.g. 7
0v) is smaller than the differential voltage Vll+. During the short-circuit period, the point contact portion or semi-contact portion is instantaneously blown away by heat caused by the discharge current, so that the discharge is approximately the same as the discharge during the normal period. Therefore, electrical discharge machining may be continued during this period. The complete short-circuit period is a period when the wire and the workpiece are short-circuited in a secure contact state where they are strongly pressed over a large area. The voltage waveform initially rises slightly, and then quickly drops to a value (eg, 50V) that is the sum of voltage drops due to wire resistance and brush contact resistance. The differential voltage vl13 in this case is small. During the complete short-circuit period, the electrical discharge required for electrical discharge machining does not occur, so the workpiece is fed (moved) toward the wire.
It is necessary to perform controls such as stopping the energy supply or stepping down the energy supply. Based on the above considerations, the present invention focuses on the fact that the differential voltage VD3 is smaller during a complete short-circuit period than the differential voltages in other periods, and uses this as a clue to perform short-circuit detection. It is. That is, a reference voltage is set that is smaller than the differential voltage during the short-circuit period and larger than the differential voltage during the complete short-circuit period, and a short circuit is detected when the measured differential voltage is smaller than this reference voltage. This is the principle. (Example) Hereinafter, an example of the present invention will be described in detail based on the drawings. FIG. 1 shows a short-circuit detection device for a wire electrical discharge machine according to an embodiment of the present invention. In FIG. 1, 1 to 5 are terminals, and 6 to 9 are FETs (field effect transistors).
, 10.11 is a capacitor, 12.13 is an operational amplifier,
14.15 is a peak hold circuit, 16.17 is an operational amplifier, 18 is a digital to analog converter, 19
is a logic element. FETs 6 to 9 play the role of switches. The voltage applied between the brush that feeds the wire and the workpiece is input to the terminal 1. Therefore, during the period when the discharge pulse is applied, a voltage corresponding to the discharge pulse is inputted, and during the rest period, a zero voltage is inputted. Terminal 2 is connected to the point at which the discharge pulse is applied (t in Fig. 3).
,) to the end of the discharge pulse (t! in Figure 3).
The signal is input until the point in time). This signal is FE
When entering the gate of T6, FET6 is turned on. The voltage input to terminal 1 charges capacitor 10 while FET 6 is on. Therefore, the voltage of the capacitor 10 has a peak voltage value during the period from time t0 to t2. The operational amplifier 12 has its output fed back to the inverting input terminal as it is, and serves as a buffer circuit with an amplification degree of 1, and together with the capacitor 10, a well-known peak hold circuit is formed. Therefore, the peak voltage value (VP in FIG. 3) after the time t0 when the discharge pulse is applied will eventually appear at the output terminal of the peak hold circuit 14. After the application of the discharge pulse to terminal 3, there is a signal at the time when the time that would normally be considered to be discharging has elapsed (time t1 in Fig. 3).
The signal is applied from t0 to the end of the discharge pulse. When this signal enters the gate of FET7, FET7
turns on and leads the voltage input to terminal 1 to capacitor 11. Then, in the same manner as the peak hold circuit 14, the peak voltage value (indicated by ■ in FIG. 3) at the time t and thereafter appears at the output terminal of the peak hold circuit 15. The operational amplifier 16 outputs the difference voltage between the peak voltage values. Assuming that the differential voltage is V, ① is compared with the reference voltage vt in the operational amplifier 17, and when it is smaller than the reference voltage V, the output of the operational amplifier 17 becomes high. The reference voltage v1 is the digital-to-analog converter 1
8, and its value is set to be larger than the differential voltage during the complete short-circuit period (see FIG. 2) and smaller than the differential voltage during the short-circuit period. This makes it possible to distinguish and detect only the complete short-circuit period. Logic element 19 is provided to ensure that the output of the short-circuit detection device of the wire electrical discharge machine is output only during the application of the discharge pulse. To effect this operation, a signal is applied to one of the input terminals of the logic element 19 via the terminal 5, which goes high during the application of the discharge pulse.

【Effect of the invention】

As described above (according to the present invention), among the short circuits that occur, it is possible to detect only short circuits (complete short circuits) that are a hindrance to continuing electrical discharge machining. Based on the signal from such short circuit detection, If control is performed such as stopping the feed of the workpiece or stepping down the input discharge energy, there will be no waste in the control, and machining efficiency will not be unnecessarily reduced.

[Brief explanation of drawings]

Fig. 1: Short-circuit detection device for a wire electrical discharge machine according to an embodiment of the present invention Fig. 2: Voltage waveforms of discharge pulses during normal period, short-circuit period, and complete short-circuit period Fig. 3: Discharge In the pulse voltage waveform diagram, 1 to 5 are terminals, and 6 to 9 are FETs (
field effect transistor), 10.11 is a capacitor, 1
2.13 is an operational amplifier, 14.15 is a peak hold circuit, 16.17 is an operational amplifier, 18 is a digital-to-analog converter, and 19 is a logic element.

Claims (1)

[Claims] A first peak hold means for holding the peak voltage value after the application of the discharge pulse applied between the wire and the workpiece, and a peak voltage after the point at which the discharge would normally occur. Wire electrical discharge machining characterized by comprising: second peak hold means for holding a value; and differential voltage comparison means for comparing the difference between the peak voltage values of the first and second peak hold means with a reference voltage. Machine short circuit detection device.