CH641389A5 - Method and apparatus for controlling the advance of a machine for electrical discharge machining - Google Patents

Description

The invention relates to a method and an apparatus for controlling the advance of machining machines using wire-shaped EDM.

We know of machining machines by electro-erosion with elec-45 filiform trode having a pulse generator of the RC type, whose advance control, ensured by a stepping motor, is carried out according to the value average voltage in the working interval. This control system includes a device determining the advance speed at a fixed value for a certain part to be treated. The adjustment of the approximate speed to the expected value is carried out automatically within very narrow limits.

These EDM machining machines have the disadvantage that they cannot maintain a stability of the process during the whole machining, in particular during the machining of a part with variable section. Indeed, the average value of the tension in the working space which must ensure the constant maintenance of the interval varies within limits which are of an order of magnitude from two to three times smaller than the limits within which it the feed speed must be changed when the section of the workpiece changes. This method of controlling the advance as a function of the average value of the tension in the working interval also has the following drawback: it involves a control system which requires continuous interventions to regulate the speed of advance during machining in one piece and this results in a 65 reduction in the productivity of the process and the formation of a variable and uncontrolled interval during machining.

The method of controlling the advance according to the invention as described in the characterizing part of claim 1 eliminates these

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disadvantages in order to maintain the working interval at a constant and controllable value during all the machining, independently of the chosen regime, to achieve a maximum frequency of discharges and to quickly adapt the feed speed to the speed of the process d 'erosion. In order to quickly adapt the feed rate to the speed of the erosion process in the event of an increase in the section of the workpiece without intervening from the outside during machining at the time of appearance of the short circuit, in one embodiment of the invention, the resumption of the advance after the end of the short circuit is carried out at a speed close to the lower limit of the variation thereof. In order to avoid the appearance of short circuits during machining, in another variant of the invention, the slope of variation of the feed speed used is greater in the direction of decrease than in the direction of the increase in it.

The apparatus for implementing the advance control method according to the invention is characterized by the characterizing part of claim 4.

Other variants of the apparatus are described in claims 5 to 9.

An exemplary embodiment of the invention is shown in FIGS. 1, 2, 3, 4 which represent:

fig. 1, the general diagram of the control unit and the generator;

fig. 2, the block diagram for making logical decisions,

and fig. 3 and 4, alternative embodiments of the block for testing the short circuit in the working interval.

Fig. 1 shows the general diagram of the advance control device and the pulse generator. By way of example, it is assumed that the advance is ensured by a system of stepping motors, that is to say the advance speed is proportional to the frequency of the motor control pulses.

The pulse generator comprises a transformer-rectifier group TR, a power transistor T ,, a limiting resistor R, and a capacitor C, which discharges in the working space formed between the wire electrode 1 and the workpiece 2.

The pulsed charging of the capacitor C is ensured by the control of the power transistor T, by means of a multivibrator 3, which is coupled to the base of the transistor T, by means of an electronic switch 4.

The decoupling of the multivibrator 3 from the base of the transistor Tj and the blocking of the latter are controlled in the case of a short circuit in the working interval and in the case of the stopping of the machining process, at the end of the work program or the disappearance of normal conditions of use of the machine. The waveform between the wire electrode 1 and the workpiece 2 is divided by means of a resistive divider R2, R3 and introduced at the input of a logic decision-making system SL which is shown in detail in fig. 2.

The logic system SL monitors the level at which each discharge appears in the workspace, and it is in relation to this level that it takes the decision to increase the speed of advance - at the exit Eh, to reduce the feed speed - at the EL output, or to maintain this at a constant value - at the Es output.

Obtaining variable frequency pulses for controlling stepping motors is ensured by the circuit consisting of a direct counter NDt, a reversible counter NR and a coincidence circuit Cc of the numbers entered in the two counters ND, and NR. At the input of the counter ND, the pulses generated by a tact generator GT are applied via a logic circuit CL, with gate function NO.

If it is assumed that any number is entered in the counter NR, when the number of pulses counted by the counter ND coincides with the number entered in the counter NR, the coincidence circuit Cc controls a bistable rocker CBB, . The bi-stable rocker circuit CBB, controls, by means of a logic circuit CL2 with AND-NO gate function, a monostable rocker CBM ,, which has the role of generating pulses of adequate duration, necessary for the control of stepper motors not. The pulse of the output of the logic circuit CL2 is applied, via a logic circuit CL3 with AND gate function, to one of the inputs of the reversible counter NR according to the decision taken by the logic system SL .

The circuit CL3 is blocked in the case where the logic device SL decides to maintain a constant speed of advance. In this case, the number entered in the reversible counter NR does not change and, at the output of the monostable rocker circuit CBM ,, we will obtain pulses at constant frequency. The bi-stable rocker circuit CBB, controls the erasure of the counter ND, after the determination of each coincidence.

In the case where the logic system SL controls the increase in the speed of advance, therefore the increase in the frequency of control of the stepping motors, a logic signal appears at its output EH which opens a logic circuit CL4 with function AND gate, via a CBB2 bistable rocker circuit. In this way, the pulses from the output of circuit CL2 are transmitted to the counting input in the opposite direction of the counter NR by means of circuits CL3, CL4, CL5 and a frequency divider 5. The circuit CLS has the function of AND-NO gate is blocked in the event that the lowest value imposed by the number entered is reached, which corresponds to the highest frequency with which the stepper motors can be controlled.

The moment when the lowest number entered in the counter NR is reached is determined by means of a circuit for reading the counter Nm.

In the case where the logic system SL controls the reduction of the speed of advance, therefore the reduction of the frequency for the control of the stepping motors, a logic signal appears at its output El which opens a logic circuit CL6 with function of AND gate, via the CBB2 bistable rocker circuit. In this way, the pulses from the output of the circuit CL2 are transmitted to the counting input in the direct direction of the counter NR by means of the circuits CL3, CL6 and of a logic circuit CL, with gate function AND-NO. The circuit CL7 is blocked in the case where the greatest value imposed by the number entered in the counter NR is reached, which corresponds to the lowest frequency with which the stepping motors can be controlled. The moment when the greatest number registered in the counter NR is reached is determined by means of a circuit for reading the states of the counter NM.

The logic system SL produces at its output Es a logic signal which blocks the circuit CL3 in the event that the decision is made to keep the control frequency of the stepping motors constant.

The pulses from the output of the tact generator GT are also applied, via a logic circuit CL8 with gate function NO and a logic circuit CL9 with gate function AND-NO, at the input of an ND2 counter. The pulses produced by the tact generator GT are counted by the counter ND2 only after the appearance of each discharge. When the voltage of the terminal of the capacitor C ,, which charges after having given up its energy in the working interval, reaches a certain level, the logic system SL controls, by the logic signal generated at its output EA, l 'erasing the number entered in the ND2 counter. In the case where, after a discharge, the voltage of the terminals of the capacitor C, no longer reaches the preset level after a longer period, therefore in the case where the erase signal of the counter ND2 does not appear, the number of the pulses registered in the counter ND2 reaches a certain determined value. Then, a logic circuit CL, 0 with AND-NOT gate function controls, on the one hand, the blocking of the circuit CL2 and, on the other hand, the test of the existence of the short-circuit in the space of work simultaneously with blocking of the power transistor T ,, by means of a short-circuit test block BTS, the detailed diagram of which is presented in FIGS. 3 and 4.

In the event that the short circuit test block BTS confirms the existence of a short circuit in the working interval, a signal

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logic appears at the output Ea which controls the withdrawal along the contour of the wire-electrode, while at the input Ef the blocking command of the power transistor T i is maintained.

The logic signal of the output of circuit CL10, which appears in the case of the existence of a short circuit in the working interval, commands the recording in the counter NR of a number of a value close to the maximum value by means of a recording circuit 6. In this way, the resumption of the advance after the disappearance of the short-circuit state in the working space is carried out at a speed close to the limit lower.

When the short-circuit state disappears in the working interval, determined by the short-circuit test block BTS, the multivibrator 3 is coupled to the base of the transistor T, and to the output EA of the logic system SL appears the erase signal of the counter ND2. This unlocks the circuit CL2 and at the output of the monostable rocker circuit CBM, the pulses for controlling the stepping motors at a very low frequency will appear, which allows the resumption of the advance at a speed close to the lower limit.

In fig. 2, the diagram of the logic system SL is presented for making decisions to modify the speed of advance. At input I, of the logic system SL, the electro-erosive discharge pulses are applied between the wire electrode 1 and the workpiece 2, divided by means of a resistive divider R2, R3. These pulses in exponential form are applied to the input of three voltage level comparators A, B, C; the smallest level is analyzed by comparator A, while comparator C determines the largest level.

If it is assumed that the charging of the working capacitor C, has started, at a given moment the predetermined level is exceeded by means of the comparator A; at the output thereof, a logic level 1 is obtained which is inverted by means of a logic circuit CL ,, with gate function NO. In this sequence, comparators B and C have, at the output, logic level 0. In the interval from 0 to 1 for passing the pulse from the output of comparator A, the bistable rocker circuit formed by two logic circuits with door function AND-NOT CLI2 and CL13 goes to the state having logic level 1 at the output of circuit CL12. The control pulse of this bistable rocker circuit is obtained via the circuit formed by the logic circuit with gate function NON CL, 4 and CL, s and by the logic circuit with gate function AND-NON CL16 which has , at an input, an R + C timing circuit;,. The logic signal 0 of the output of the circuit CL ,,, obtained when the level determined by the comparator A is exceeded, is applied to a logic circuit with AND-NON CL gate function, 7, at the output of which logic signal 1 is obtained. If the discharge in the working space appears before reaching the level determined by comparator B, at the output of circuit CL ,, will appear logic signal 0, since comparator A goes to l state 0, while the circuit CL ,, goes to state 1. The logic signal 0 of the output of the circuit CL, 7 determines the passage of the bistable rocker circuit constituted by the logic circuits with gate function AND-NOT CL , 8 and CL, g in the logic signal 0 state at the output of the circuit CL ,,,. Therefore, in the case where the discharge in the working interval has occurred below the level determined by comparator B, at the output EL of the logic system a logic level 0 is obtained, which signifies a decision taken to reduction in feed speed.

If it is assumed that the charging of the working capacitor C ,, after having exceeded the level predetermined by comparator A, also exceeds the level determined by comparator B, at the output of this one will obtain logic level 1, which will be inverted by a logic circuit with a door function NOT CL20. In this sequence, at the output of comparator A the logic level 1 is maintained, while at the output of comparator C the logic level 0 is maintained.

Logic level 0 of the output of circuit CL20 produces a logic signal 1 at the output of circuit CL, 9. In the interval between the comparator B from state 0 to state 1, the bistable rocker circuit made up of two logic circuits with AND-NON gate function CL21 and CL22 will pass to the state having logic level 1 to the output of circuit CL21. The control pulse of this bistable rocker circuit is obtained by means of the circuit formed by the logic circuits with function NON CL23 and CL24 and by the logic circuit with function AND-NON CL25 which has, at an input, a delay circuit RSC3 .

If the discharge in the working space appears before reaching the level determined by the comparator C, the logic signal 0 is obtained at the output of the logic circuit with gate function AND-NOT CL26 which will cause the bistable rocker circuit formed to pass by logic circuits with AND-NO gate function CL27 and CL28 in the state having logic level 0 at the output of circuit CL28. Therefore, in the case where the discharge in the working interval appears at a level between the predetermined levels for the comparators B and C, at the output Es of the logic system SL we obtain the logic signal 0, which corresponds to the advance decision to value before discharge.

If it is assumed that the charging of the working capacitor C ,, after having exceeded the level predetermined by the comparator B, also exceeds the level pre-established by the comparator C, one obtains at the output of the comparator C the logic level 1 which is inverted by means of a logic circuit with door function NOT CL29. At the output EH of the logic system, a logic level 0 is obtained, which corresponds to the decision to increase the speed of advance. The logic circuit with AND function CL30 has the role of passing the output of the circuit CL28 to state 1 and blocking the circuit CL26 by means of the bistable rocker circuit made up of the circuits CL2, and CL22 in the case where the discharge of the capacitor C, occurs from a voltage above the level determined by comparator C or below the level determined by comparator B.

At the output EA of the logic system, the logic signal 0 is obtained after each discharge, that is to say after each reduction of the voltage between the electrodes below the level established by the comparator A. The logic signals of the outputs El Bs, Eh and Ea are then used by the circuit to obtain the frequency of the pulses necessary for controlling the stepping motors by the path described in the presentation of the diagram in FIG. 1.

Fig. 3 shows an alternative embodiment of the block for testing the existence of the short circuit in the BTS working interval in which the electrical resistance of the working space is tested by means of an auxiliary voltage introduced by the relay contacts. The signal of the output of circuit CL10 of fig. 1 is entered at the input Is of the BTS test block. The signal of the input Is of the test block has logic level 0 in the case where, after a discharge, determined by obtaining a logic level 0 at the output Ea of the logic system SL of FIG. 1, the voltage between the electrodes does not once again reach the level preset by the comparator A in FIG. 2, after a predetermined time interval, and thus creates the possibility, for the counter ND2 of FIG. 1, to go to the state which produces the logic signal 0 at the output of the circuit CL, 0.

The signal of the input Is is inverted by the logic circuit CL3, with NO gate function and the monostable switching circuit CBM2 is triggered which will block, for a period, the transistor T2, which makes it possible to close the contacts of the relay d and apply the test voltage U, between the two electrodes. At the same time, the force transistor T is blocked, by means of the electronic switch 4 of FIG. 1, using the logic signal 1 obtained at the output Ef of the test block. If, in the working interval, there is a short circuit, the transistor T3 will block, while being connected in parallel to another transistor T4 which, in turn, is blocked in this sequence by means of the contact of the relay d. Logic level 1 obtained by the collectors of transistors T3 and T4 maintains transistor T2 in the blocked state by saturation of another transistor T5. Logic signal 1 obtained at output E of the test block represents the confirmation of the existence of the short circuit in the working space and controls the withdrawal along the contour of the wire electrode by means of the motor control system step by step SCMpp <, represented by

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a broken line in fig. 1 and which is not the subject of the present invention. When the short circuit disappears, the transistor T3 becomes saturated, the transistor T2 also saturates, the relay opens its contacts, thus removing the voltage Ui between the electrodes and connecting the multivibrator 3 to the base of the tran 5 sistor T, by means of the electronic switch 4 of FIG. 1.

Another alternative embodiment of the test block for the existence of a short circuit in the BTS working interval, in which the electrical resistance of the working space is tested using an auxiliary voltage introduced between the electrodes by means of the transistors, is represented by FIG. 4.

The logic signal 0 of the input Is of the test block, linked by means of the circuit CL3 j, causes the triggering of the monostable switching circuit CBM2, which controls a bistable switching circuit CBB3.

In the case where, after a discharge, the level 15 established by the comparator A of FIG. 2 in a determined time interval, at the output of the bistable rocker circuit CBB3 there is obtained the logic signal 1 which, by means of the logic circuit CL32 with AND gate function, saturates the transistor of an optoelectronic coupler T6, and at the output Ef of the test block there is obtained the logic signal 1 which blocks the force transistor T! of the generator using the electronic switch 4 of fig. 1. Simultaneously with this command, the saturation command of the two transistors T7 and Ta is given by means of a polarity changer of the logic pulses 7. In the case where, in the working interval, there is a short -circuit, a transistor T9 remains blocked and the transistor of the optocoupler T6 remains saturated by means of a polarity changer of the logic pulses 8. At the output Ea of the test block we give the withdrawal signal wire electrode along the contour.

At the time of the disappearance of the short-circuit in the working space, the transistor T9 becomes saturated, and at the output of the logic circuit CL32 we obtain the logic signal 0 which blocks the transistor of the opto-electronic coupler T6 and, of the so apply the processing voltage between the wire electrode and the workpiece. When you reach the level determined by comparator A in fig. 2, the switching circuit CBB3 goes to state 0 at the output thanks to the logic signal 1 obtained at the output EA of the logic system SL and which is inverted by a logic circuit CL33 with gate function NO.

Thus, the short-circuit test block goes into the standby state, with the transistors T7 and Ts with blocking control at the base.

The invention has the following advantages:

- it ensures increased productivity of the machining process;

- it maintains the working interval at a constant and controlled value during machining, while keeping the level of the discharge voltage constant;

- it maintains a very good stability of the process, independently of the modifications of the section of the workpiece and without requiring the intervention of the operator during machining.

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Claims (7)

641,389 2 1. Method for controlling the advance of an EDM machine, to maintain the working interval at a constant value throughout the machining regardless of the speed, to achieve a maximum frequency of discharges and for quickly adapt the feed rate to the erosion rate, characterized in that the feed rate is changed so that the voltage level of each discharge is kept within a predetermined range, the decision to increase the advance speed, to maintain it at a constant value or to reduce it being taken as a function of the appearance of the discharges at a voltage level situated respectively above the upper limit of the range, between the upper limit and the lower range limit, or below the lower range limit, while the speed variation slope is chosen in proportion to the momentary forward speed. 2. Method according to claim 1, characterized in that the resumption of the advance after a short circuit is carried out at a speed close to the lower limit of variation thereof. 3. Method according to claim 1, characterized in that, in order to avoid the appearance of short-circuits during machining, the slope of variation of the feed speed used is greater in the direction of decrease than in the direction of increase. 4. Apparatus for implementing the method according to claim 1, characterized in that it comprises a logic decision device (SL), formed by three voltage comparators (A, B, C) determining the voltage level reached between the electrodes, of a circuit (CLn, CL14, CL15, CL16) for the creation of a pulse at the moment when the voltage exceeds a first preset voltage level, of a circuit (CL12, CL13, CL17, CLiS, CL1S) which takes the decision to reduce the advance speed in the event that a discharge appears at a voltage level located below a second preset level, corresponding to the lower limit of the range, of a circuit (CL20, CL23, CL24, CL2S) for the creation of a pulse when the voltage exceeds this second preset level, of a circuit (CL2j, CL22, CL26, CL27, CL28, CL30) which takes the decision to maintain constant the speed of advance when the discharge appears at a voltage level between this second and a third preset level abli corresponding to the upper limit of the range, and a circuit (CL29) for making the decision to increase the advance speed in the event that the discharge appears at a voltage level located above the third preset level , and a transmitter circuit (CBB2, CL3, CL4, CL6, CL5, CL7, Nm, Nm) transmitting the decisions made by the logic device to a device (GT, ND ,, NR, Cc, CBBj, CL ,, CL2, CBMj) determining the value of the advance speed. 5. Apparatus according to claim 4, characterized in that the device determining the speed of advance comprises a tact generator (GT) which controls a direct counter (NDj) via a logic circuit (CL,), a reversible counter (NR) controlled as a function of the decision taken by the logic device, a circuit (Cc) for coincidence of the numbers entered in said counters, a circuit (CBBj) for erasing the number entered in the direct counter (ND ,) at the time of coincidence, a circuit (ND2) for determining the moment of the appearance of a short circuit, formed by a direct counter which, after each discharge, counts the pulses generated by the generator tact (GT) via logic gates (CL8, CL9), and a gate circuit (CLI0) which, when a preset maximum number of pulses counted by the direct counter is reached, controls, on the one hand, a circuit (6) for recording a number corresponding to a small vi advance in the reversible counter (NR) and, on the other hand, a short circuit test block (BTS). 6. Apparatus according to claim 5, characterized in that, in order to obtain a variation slope of the advance speed proportional to the speed at the moment when the decision to modify it is made, it comprises a gate (CL2) which transmits the pulses obtained at the output of the coincidence circuit (Cc, CBB,), on the one hand, towards the input (CL3) of the transmitter circuit and, on the other hand, towards a monostable rocker circuit (CBMX) for obtaining control pulses of an electromechanical advance system, s 7. Device according to claim 5, characterized in that, in order to avoid the appearance of the short circuit during machining, the transmitter circuit comprises, for the transmission of the decision to increase the advance speed, a divider pulse frequency (5) which reduces the number of pulses written to the reversible counter (NR). 8. Apparatus according to claim 5, characterized in that the test block (BTS) comprises a monostable switching circuit (CBM2) which controls a transistor (T2) which has in its collector a relay (d) whose contacts allow d '' introduce a test voltage 15 between the electrodes and control the blocking of a pulse generator power transistor (TJ of the machine by means of an electronic switch (4), maintaining the relay in state d 'test, in the event of a short circuit, being obtained by an AND logic function circuit (T3, T4, Ts). 9. Apparatus according to claim 5, characterized in that the test block (BTS) comprises a monostable rocker circuit (CBM2) which controls a bistable rocker circuit (CBB3) for memorizing the existence of the short circuit which, in turn, on the one hand, controls a transistor (T7) making it possible to introduce a test voltage 25 between the electrodes, and, on the other hand, via a logic gate (CL32), controls an optoelectronic coupler (T6) intended to control the blocking of a pulse generator power transistor (T,) of the machine by means of an electronic switch (4), the maintenance of the test conditions, in the event short-circuit, being obtained by a transistor (Ts) which transmits the test voltage applied between the electrodes at the base of another transistor (T9) which becomes saturated at the moment when the short-circuit disappears and allows the recoupling of the machining tension which, when it exceeds a preset level, controls the resumption of position i ni-35 tial of the bistable rocker circuit (CBB3).