WO1989007503A1

WO1989007503A1 - Edm machine with wire electrode, provided with continuous-control device

Info

Publication number: WO1989007503A1
Application number: PCT/CH1989/000026
Authority: WO
Inventors: Michel Jennes
Original assignee: Charmilles Technologies Sa
Priority date: 1988-02-17
Filing date: 1989-02-15
Publication date: 1989-08-24

Abstract

An electroerosion, cutting machine with wire electrode, comprising a numerical control for controlling an electric pulse generator and a cross-motion table as well as other machining parameters, is characterized by a device comprising: a detector for measuring the voltage and current intensity for each pulse in the interelectrode region; other characteristics which determine its profile in function of time; an analyser for continuously recording said characteristics of a train of pulses of predetermined duration and for deducing certain results from said characteristics; a microcomputer for activating and controlling said analyser, equipped with software permitting the calculation, from said results, of the value assumed by an index preventing rupture of the wire electrode, the comparison of said index with a predetermined limiting value 1, and the emission of a signal immediately reaches 1, and an interface for transmitting said signal to the numerical control of the EDM machine.

Description

EDM WIRE CUTTING MACHINE WITH CONTINUOUS MONITORING DEVICE

The present invention relates to a device for continuously monitoring, controlling and optimizing EDM machining with a wire electrode cutting machine.

Up to now, any attempt to increase the performance of EDM machining by wire-electrode has run up against the problems posed by wire breakage. This is to increase the cutting speed as much as possible while avoiding breaking the wire. The main causes of breakage are known: wear of the wire, numerous short-circuits, concentration of discharges in one place, excessive heating of the wire, destabilization of the machining due to cutting in the corners. We have known for a long time how to avoid the first two. It is known to prevent discharge concentrations thanks, for example, to the device described in patent CH 653 585.

But, contrary to the idea widespread in the state of the art, the thermal overload of the wire is not due solely to the energy or to the power dissipated in the wire by certain types of discharges. All the causes of wire overheating must be taken into account, in particular the power dissipated by all the pulses, whether or not these cause an erosive discharge. This is why the object of the present invention is a machine such as that defined in the claims, which takes account of all the pulses emitted.

In fact, the breaking of the wire generally occurs following a sudden deterioration in the machining conditions. To be effective, any prevention must therefore call for control and then adjustment of parameters with short response times. Recall that the dimensions and materials of the electrodes (workpieces, wire electrode) the composition of the machining fluid, certain characteristics of the pulse generator, as well as the precision and the surface condition are given at the start. But other parameters can be modified during machining, provided of course the EDM machine has the equipment and controls necessary to adjust, for example, the unwinding speed and the mechanical tension of the wire, the characteristics of the injection of the machining fluid and its conductivity, the depth of the gap or the speed d 'advance, and above all certain characteristics of the pulses such as their frequency or amplitude, or other quantities conditioning their profile.

The control according to the present invention preferably uses these electrical parameters, that is to say the characteristics of the "profile" of the pulses. The term "pulse profile" means the curves describing the variation as a function of time of the potential difference and the intensity of the current in the interelectrode space (also called "gap").

The parameters used, the pulse profile, as well as the various types of pulses that can be emitted, vary depending on the type of generator used.

The parameters which can condition the profile of an impulse as well as the various types of impulses are shown diagrammatically in Figure 1. Let us recall the meaning of these various parameters:

t: time interval between the beginnings of two pulses of

P successive voltages: t = t + t ≈ t + t + t + t; p o i o d e f t: time to initiate a landfill; d t: duration of discharge; e t: duration of the current rise at the start of the spark; t: duration of the voltage pulse: t = t + t + t; i i d e f t: pause time separating the end of a voltage pulse o from the start of the next pulse;

U no-load voltage; i U discharge voltage; e I intensity of the current crossing the gap at the end of discharge. e

Figure 1 also illustrates the five types of pulses distinguished according to the state of the art (Fig. 1b). Their profile is schematized very roughly, because the real oscillograms of the voltage u and the current i vary from one generator to another. These are the following types:

- open circuit (symbolized by 0): t = t; t = 0 u = constant = U; (so u always> U) die _^ ie

- delayed discharge (symbolized by D): t> limit t; u can reach U and U d dl i e

(we sometimes observe u = U or u = U) i e

- normal discharge (symbolized by N): t <t; u can reach U and U d dl i e

(we sometimes observe u ≈ U or u ≈ U) i e

- arc (symbolized by A): u can reach U but not U e i t = 0; (t ≈ 0); u <U; we sometimes observe u ≈ U d e i e

- short circuit (symbolized by S): t = 0; (t = 0); u cannot reach U and U d e i e

(we always observe u <U) e

By continuously monitoring and analyzing the voltage u and current i signals, we can

- obtain a real-time image of the machining conditions and in particular carry out a real-time control of the latter.

From offline studies we can also

- control and optimize machining,

- calculate certain characteristic quantities allowing an evaluation of the machining conditions,

- experimentally determine the value of certain coefficients, parameters or limit values,

- analyze the role played by certain parameters.

This often requires the classification of the pulses according to the various types which correspond to the generator used, in fact, as mentioned above, the types of pulses observed vary with the various types of generator. Thus, for a static generator emitting pulses of predetermined duration and frequency, the five types defined above will be observed. For a relaxation generator we will not observe "delayed discharges". There are also generators fitted with anti-arc devices. Thus in the case of the generator used in the example of the present invention (EDM machine CHARMILLES-ANDREW EF 330), for which the pulse duration t 'and the pause time t' are pre-set, the profile of the pulses can be shown schematically as in Figure lb.

Note that t = t '+ t' = t + t is a constant, pioio

As shown in FIG. 2, the detector of the device according to the present invention makes it possible to continuously measure, for each pulse, the potential difference u between one of the contacts bringing the current to the wire-electrode and the workpiece, thus as the intensity i of the current with a very high frequency of measurements. This detection of the value of the voltage and of the intensity of the current can be combined with that of the other characteristics of the pulse, such as, for example, its duration t, or the duration t of the discharge. e

These signals are then transmitted to an analyzer activated and controlled by a processor, for example a computer, a microcomputer or a microprocessor or any other suitable processing device. They are arranged so:

- to record these signals and to detect the characteristics of the pulses,

- to classify each pulse in a given type,

- to analyze a train of pulses and to calculate from certain results characteristic quantities allowing either to control the machining in real time, or to carry out diagnostics, to develop strategies or to carry out other studies off-line, as will be developed later.

The processor is connected by an interface to the digital control of the EDM machine.

The detector generally comprises a voltage attenuator, a voltage sensor, for example a differential sensor, a current sensor which can be provided with an amplifier and possibly a device making it possible to display the signals obtained, such as an oscilloscope for example. It performs the measurements at a high frequency (for example 12.5 MHz, that is to say every 0.08 μs). Indeed, it is cutting by wire-electrode; the durations t defined above are short (of the order of a tenth to a few i μs), while i and u are high (U is generally of the order of 20Q V).

The pulses generally follow one another at a few tens of μs and can last approximately from 0.5 to 5 μs, this explains the frequencies at which voltage and current must be measured.

It is possible to provide a device attenuating the voltage signal in order to protect, if necessary, the corresponding sensor. The signals can then be sent to an oscilloscope before being transmitted to the analyzer of the device according to the present invention after having been, preferably, attenuated in order to be compared with reference quantities. The current sensor is arranged to transmit the signal without background "noise". Both signals can be viewed, for example on an appropriate oscilloscope.

Before being transmitted to the analyzer, the signals obtained from current and voltage measurements made by the detector can be stored on a transient digital or analog memory. The latter makes it possible to memorize very fast electrical signals, to store this information and to retransmit it later to equipment for further processing: oscilloscopes, recorders, etc.

The analyzer of the device according to the present invention is arranged so as to extract at high speed different characteristics of the pulses by comparing, for each pulse, these signals with reference values in order to evaluate them. These analytical operations are carried out by electronic circuits with very short response times, such as series of comparators each adjusted to a given voltage or current. For example, a series of three comparators to detect the value of the voltage signal can be set to 5, 30 and 150 V and a series of two comparators to detect the value of the current signal can be set to 5 and 10 A. It is obviously possible to increase the number of these comparators if we want better accuracy. Each of these comparators is connected to a differential receiver indicating whether the quantity to be measured is lower or greater than the setting value of the comparator.

Thanks to electronic circuits constituted in a known manner, the analyzer also makes it possible to determine, if. necessary (i.e. if they are not predetermined by the type of generator used), the durations t, t, idt and t defined above, oe This analyzer is also provided with software making it possible to compare i with I e and u with U and U, to combine these results and to deduce therefrom in real time, ie that is to say in a few tens of μs the type of the pulse thus analyzed, as explained in the example below. At the end of each pulse, the memory must be cleaned, thanks to circuits arranged in order to take account of variations in the duration of the pulses.

The results of these analyzes and classifications are then transmitted to a module comprising a memory and one or more counters which can operate on different time bases. They are stored on these counters. This gives the number of pulses of each type and the total number of pulses analyzed and classified. Several time bases are preferably provided in order to be able to simultaneously conduct a detailed analysis for a few tens of μs, for example, and a long-term analysis (a few hours, for example) without interrupting the latter.

The analyzer and the processor can also be arranged not only in order to classify and to totalize the number of pulses of each type, but also to determine the total duration of the discharges (2 t), the total duration of the e voltage pulses ( _t t), that of the initiation times (Σ t), as well as id the average value of these durations. Thanks to appropriate software, they can above all a quantity characteristic of the risk of wire breakage, and transmit a signal to the digital control as soon as this quantity reaches a given threshold, thanks to the appropriate interface.

The signals generated by the analyzer can also be stored and viewed on suitable devices and their variations can be recorded, for example on a digital recorder.

In the example described below, the generator is that of an EDM CHARMILLES-ANDREW EF 330 machine. The detector includes a voltage alternator, a TEKTRONIX P 6046 voltage sensor, a current sensor with TEKTRONIX P 6302 amplifier and a Trio 2100 oscilloscope.

The analyzer makes it possible to classify each pulse into one of the five types defined above according to criteria such as, for example, those summarized in the following table:

open circuit (0)

delayed discharge (D)

normal discharge (N)

arcs (A) X X

short circuits (S)

It detects, for example:

- if the intensity reaches a limit I (for example I ≈ 10 A) (otherwise it is an open circuit); if it reaches this limit before or after a reference time t 'has elapsed (in the latter case it is a delayed discharge);

- if the voltage reaches a limit U (for example U = 150 V), (the current ii having reached I before t '): it is a normal discharge and otherwise, if ei it reaches a limit U less than U (for example U = 30 V): it ei. it is an arc. If it does not reach this limit it is a short circuit.

These operations can be simplified with other types of generators. Thus, if only pulses of type 0, N and S are emitted, it suffices to detect if:

- the intensity reaches or not the limit I and if e

- the voltage reaches or not the limit U, according to the table

The analyzer also detects the different durations t, t, t, t and i o d e sends the signals corresponding to the processor in order to be counted and stored there. In this example:

- t 'and t are fixed and stored directly; i o

- we only determine and store t if the voltage exceeds U = 150 V; d i

- we only determine and store t if the current exceeds 5 A; e

- the duration t measured corresponds to the time during which the current remains i greater than 5V.

This analysis can be carried out over several hours so as to average the very numerous results obtained. It can also be carried out over a short period taking into account only a few thousand pulses, in order to control only the progress of the machining at a given moment, or to activate, for example, a detailed treatment and the complete memorization of the characteristics of a train of pulses of predetermined duration, as soon as a given number of successive pulses of the same type is detected.

This analyzer is activated and controlled by an adequately programmed microcomputer, such as, for example, a HEWLETT PACKARD HP 85). Other types of microcomputers, microprocessors can also be used.

These durations, as well as the reference value t ', are also stored on counters incorporated in this analyzer.

As we saw in the introduction, the analyzer-processor assembly can be arranged and programmed so as to determine characteristic quantities. So, we can program this organ in order to calculate a quantity instantaneous θ characteristic of the risk of wire electrode breakage and follow the variation. When it reaches a predetermined threshold, this is the signal for the next break in the thread. This signal is transmitted to the digital control of the EDM machine which, in response, adjusts at least one machining parameter, with short response time, in order to avoid wire breakage. When the calculation of θ can be carried out in real time, an adaptive control in real time (and with closed loop) of the machining is thus carried out, at optimum speed and without wire breakage.

According to the present invention, a good rupture indicator θ would be the power P dissipated during the time unit D in the wire electrode at t all the varieties of pulses. As thanks to the device of the present invention, it is possible to follow the variation of P during the machining, it made it possible to carry out a study which showed that

- the rupture of the electrode wire is preceded for a few ms by a sudden increase in the power P dissipated in the gap,

- that an increase in the duration t of the pulse (or of the ratio _ t / Σ i e t) changes the distribution of energy and also increases the heating of the p wire-electrode and, therefore, the risk of rupture. However, remember that if we decrease too much t we are forced to increase the power emitted by the generator in order to maintain a satisfactory cutting speed.

Preferably, a device capable of simultaneously analyzing current and voltage signals is used.

Before being analyzed, the signals obtained from current and voltage measurements made by the detector can therefore be stored on a transient memory. The latter can for example be constituted by a transient digital memory which makes it possible to memorize very fast electrical signals, to store this information and to retransmit it much more slowly towards apparatuses with longer response time: oscillographs, recorders, etc. We can, for example, use a DATALAB DL 922 having a dual 8-bit channel and whose digitization frequency can reach 20 MHz on a single channel or 10 MHz on both. Other installations can be used: a TEKTRONIX 791 2 AD digital recorder, 500 MHz, programmable, with a single channel, which displays ultra-fast electrical signals on the screen of a diode, possibly connected to an analog TV or XYZ monitor or a TEKTRONIX 468 digital oscilloscope with two channels. These various devices are only given as examples. Any other device having the same functions, insensitive to electronic interference, "noise" due to high frequency switching, etc. can be used.

We can therefore prevent the wire electrode from breaking by automatically controlling the instantaneous evolution of its thermal load, this by continuously monitoring the variation of P, the power dissipated in the gap, and keeping it below a given threshold by adjusting at least one machining parameter with short response time. The device of the present invention allows, as we will see below, the experimental determination of this threshold and the adaptive control allowing to machine at optimal speed while preventing wire breakage.

Another quantity can also be used to predict the next break in the wire: the ratio ∑ t / S t. Note, however, that this e p magnitude is less satisfactory than P, unless we keep t constant.

On the other hand, its determination requires much less complex calculations than those for P, and can be carried out in real time. This variant can therefore be very advantageous, because it allows control in real time.

Thanks to appropriate software, the microcomputer activating and controlling the analyzer of the present invention makes it possible to calculate P periodically during machining, this, using the following equation:

P = Σ n E (1)

_ with E Σ a t (x) + b (2) j = J 1

~ i where E is the average of the energies of a given type of pulses j, performed j on n pulses of this type,

_ n the number of pulses of this type per period, a and b of the predetermined coefficients, t the duration of a pulse. x = n i J

Equation (1) can be written

P ≈ Σ (Σ a t x) + b)

J x - 1. ^d - ~

Indeed, as explained above, the analysis of a train of 2 nj pulses, makes it possible to obtain by counting n • (For the indicator P obtained to be significant, a train of pulses must be higher a few tens of ms is analyzed).

Thanks to an off-line study carried out on the basis of the measurements of voltage u (x) and of intensity i (x) carried out for each pulse and of the duration t (x) thereof, it is possible - to calculate l energy of this impulse, E

- to study its variation as a function of t (x), i

This study has shown, surprisingly, that for a given power of the pulse generator, the energy E depends only on one factor, the j duration t of the pulses, and therefore varies linearly with it, but with different slopes depending on the type of pulse: E (x) * a t (x) + b. The slopes a and the constants b are memorized, and it is enough to know the duration and the type of a pulse to calculate its energy.

As soon as the temperature rise indicator P exceeds a threshold, PI, which can also be predetermined by an off-line study carried out using the device of the present invention, the microcomputer transmits digital information which is converted by an interface into appropriate signals. The latter, thanks to the digital control of the EDM machine, can control the pulse generator and the cross-motion table, in order to adjust at least one parameter so as to decrease P, in particular the durations t, the time pause t, the reference voltage io

U, the intensity I. (See the general diagram of the device, figure 2) s p

This interface can also be used to measure and calculate the average cutting speed V.

In case the EDM machine is equipped with a pulse generator with predetermined durations (static generator), it is preferable to work according to the variant already indicated above, that is to say using K =

Σ t / S t, as an overheating indicator, the steps of its calculation being e p less complex than for P.

T is predetermined as a function of the height of the workpiece, thanks to an off-line study carried out using the detector, the analyzer and the microcomputer according to the present invention (for example, t = 0.0125 h + 0.875, h being expressed in mm).

We control t and U, the reference voltage thanks to the same device, o s

The simplicity of the calculation of K, ∑t being the duration of the analysis allows, p to control the variation of K in real time. Indeed, Σ t is the duration of the p pulse train considered and is fixed in advance, therefore known. It suffices to record the durations of the discharges.

One can increase U and decrease t in order to increase the speed s o of machining to its optimum value while monitoring the increase in K which must remain below a limit Kl. This limit can also be set thanks to an off-line study carried out with this device. In the event of an abrupt change or deterioration in the machining conditions, due for example to a breakdown or to a cut in an angle, K increases suddenly.

It suffices to adjust, for example, t and U in order to keep the indicator below the value of Kl. (See diagrams in Figures 4, 5 and 6).

This is very advantageous since it is no longer necessary to stop the machining as recommended in the prior art, by commanding for example the stopping of the cross-motion table or the stopping of the current pulses in the wire as soon as there is a risk of rupture, or of reducing the power of these impulses to an extent which goes beyond what had been necessary to remove the risk of rupture.

In summary, the results obtained by the device of the present invention can be used on-line (in real time) to control the machining or off-line to analyze it. The detector, the analyzer and the microcomputer which activates and controls the analyzer can be placed at a certain distance from the EDM machine proper. The signals obtained by the analyzer can be viewed, or stored before coming to power the microcomputer later.

Let us add that the device according to the present invention not only makes it possible to achieve real-time prevention of wire breakage, which obviously makes it possible to machine at optimum speed (by increasing the cutting speed of a machine by 10 -30% EDM with wire electrode when it is equipped with the device according to the present invention), but also to carry out numerous studies making it possible to optimize certain machining parameters, or else to develop machining strategies in particular for machine in the corners (see diagram in figure 7).

Claims

1. Machine for cutting by electroerosion with a wire electrode comprising a digital control making it possible to control an electric pulse generator and a table with crossed movements as well as other machining parameters, characterized by a device comprising:

- a detector arranged to measure in the inter-electrode area, the voltage and the current intensity for each pulse,

- as well as other characteristics conditioning its profile as a function of time,

- an analyzer arranged to continuously record these characteristics for a train of pulses of predetermined duration and to deduce certain results therefrom,

- a microcomputer activating and controlling this analyzer, equipped with software making it possible to calculate, on the basis of these results, the value taken by an Indicator θ for prevention of wire electrode breakage, to compare it with a limit value θl predetermined, to issue a signal as soon as θ reaches θl, and

- an interface transmitting this signal to the digital control of the EDM machine.

2. Device according to claim 1, in which the generator emits pulses of constant frequency or duration, and the microcomputer is programmed to calculate θ in real time, this calculation operation comprising an arithmetic operation consisting in determining θ as being equal at Σ t / Σ t, ept being the duration of each discharge, and and being the interval separating the start of two successive pulses,

P for a pulse train of predetermined duration.

3. Device according to claim 1, wherein

the analyzer is equipped with software making it possible to deduce the type j from each pulse of a train of pulses of given duration, and to count the number n of the pulses of each type, and

- the microcomputer is programmed to calculate θ, this calculation operation comprising an arithmetic operation consisting in determining, for this train of pulses, θ as being equal to __ n E, i - - a t (x) + bjij with E = - jn

3 where E is the average of the energies of type j, a and b of the coefficients j 3 3 predetermined, and t the duration of a pulse. i