The object of the invention is a pulse generator intended for sinking electrical discharge machining that allows the adjustment and control of increase of the pulse current ie(t) during a discharge time, between an electrode and a workpiece in a dielectric liquid. The invention is classified to Class B 23H 7/26 of the International Patent Classification.

Technical problems, solved by a circuit (d) (Figure 1 ), include adjusting the speed of the increase of an average pulse current ie(t), which is selected for individual combinations of electrode and workpiece materials and machining modes. In a power module of a generator, a difference between a source voltage for the power module of the generator uj and the discharge voltages ue is important for a current increase: duej = uj - ue.

The constant value of a selected voltage dueji is important for the stability of a technology of a selected mode. The voltage of the power module of the generator uj can be changed without process interruption continuously.

In an ignition module of the generator, a difference between a source voltage for the ignition module of the generator uv and the discharge voltage ue is important for a current increase: duev = uv - ue. For each machining mode a supply voltage of the power module of the generator ujt is preselected and is not changed in some cases of machining while the discharge voltages ue are adjusted. That is the case when a higher supply voltage of the power module of the generator uj is not desired, due to a higher discharge voltage ue, e. g. in first roughing stages of the machining procedure.

A further problem, solved by a circuit (e), is the interruption of bad pulses, which are identified by a too rapid increase of the current. If the time of current increase trn for a selected difference in the current iejn is smaller than selected, the pulse is interrupted.

If pulse interruption occurs too frequently, the circuit (e) generates a signal Sod that uses CNC to move the toole electrode away from the workpiece and provides for a supply of a fresh dielectric into a working gap-

A problem that occurs in generators with a resistor Rj is that the value of the resistor changes as a function of the current selected by the iejmi technology. This influences the speed of the current increase and results in thermal losses. The resistor Rj starts performing its function only when the selected value of the current iejmi is reached. The-problem is solved by a transistor TRRj connected parallel with the resistor Rj. When the transistor TRRj is turned on, the resistance in the electrical circuit of the power module of the generator is reduced to the resistance of the transistor TRRj. When the current of the power module of the generator iej reaches the value iejmi of the circuit (e), a signal is emitted that turns off the transistor TRRj and the entire current flows through the resistor Rj. In many instances, the discharge current iej fails to reach the value of the current iejmi. In these instances the transistor TRRj is turned on the whole time.

In generators without the resistor Rj the current iejm regulation is realized in a way that the transistor TRj turns off once the current iej reaches the selected value iejmi. When the current drops for a selected value diejmi, the transistor TRj turns on again, until the value of the current iejmi is reached (Figure 4). This is repeated until the end of a pulse. To have an adequate frequency of turned-on and turned-off pulses, the circuit of the power module of the generator must have adequate inductance. If the sum of inductances of a cable Lk and a generator Lj is not sufficient, the inductance needs to be increased by a series-connected additional inductance L. This inductance starts performing its function only when the current of the power module of the generator iej reaches the selected value iejmi. If the inductance L is turned on during the entire duration of a pulse, the speed of the current increase can be reduced. This problem is solved by a transistor TRL connected parallel to the inductance L. When the transistor is turned on, the impact of the inductance L is practically cancelled.

Hence, when the pulse circuit iej reaches the value iejmi, the circuit (e) sends a signal to a circuit (j) that turns off the transistor TRL and the entire current uninterruptedly flows through the inductance L.

In electrical discharge machining side discharges having a higher discharge voltage ue occur. This phenomenon is described in patent EP 2 848 349 A1. If the discharge voltage ue is higher than the supply voltage of the power module uj, the current of the power module iej does not flow and the discharge is only carried out by an ignition current iev. Once the discharge voltage ue drops below the value of the voltage uj, the current flows from the power module of the generator iej as well (Figure 5). Lower energy with smaller removal of material is delivered in these pulses. Moreover, these discharges occur at a lateral surface of the electrode and increase a side gap. This is not desirable in the roughing stage of machining and such pulses should be interrupted and a new pulse should be started after a break to that is selected in the technology.

A majority of modern devices for electrical discharge machining provide a change in voltage of the power module of the generator in several stages but do not provide for a change in these voltages continuously and as a function of the course of the electrical discharge process. A generator manufactured according to patent SI 20254 A enables gradual changes in voltages of the power module but fails to enable adaptation of these voltages to the course of the electrical discharge process during machining. A generator according to patent SI 22476 A enables changes in voltages of the power module during machining but with the purpose of changing the speed of increase and size of the discharge current due to stabilization of the electrical discharge process and not stabilization of the technology of the selected mode.

The pulse generator of the invention will be explained on the basis of an embodiment and the enclosed drawings, in which:

Figure 1 : shows a diagram of a generator for electrical discharge machining;

Figure 2: shows the discharge parameters in electrical discharge machining;

Figure 3: shows the influence of the impedance of a gap on the discharge voltage ue and the increase of the discharge current iej;

Figure 4: shows the current increase in a generator without a resistor Rj

and added inductance L;

Figure 5: shows an interrupted pulse due to excessive delay in turning- on of the power module of the generator dtrO.

The pulse generator of the invention consists of (Figure 1 ) a power module of the generator with a power source (b), a transistor TRj, a resistor Rj, a diode D, connections in the generator with inductance Lj and added inductance L. The power module of the generator has its own capacitance Cj. A voltage sensor (c) measures the discharge voltage ue and a current sensor (h) measures the discharge current iej.

A signal of the discharge voltage ue is delivered into a circuit (d) in the last third of normal discharges, when the discharge voltage is lowest. To be able to control the voltage of the power module of the generator uj, an average discharge voltage uep needs to be calculated: it is calculated in a way that the voltage supplied to the circuit (d) by the sensor (c) is measured in the last third of the discharge ue and the circuit (d) calculates the average voltage uep for the selected number of pulses n from a unit

(f) . To this value the selected value of a voltage difference between the voltage of the power source and the discharge voltage dueji is added and the sum uj = uep + dueji from the circuit (d) controls the supply voltage of the power source (b), of the generator.

The discharge voltage ue is influenced by the size of the gap and its conductivity (Figure 3). The setting of the servo-system influences the size of the gap. In more demanding conditions, the machining is carried out with a larger gap due to better flushing. This results in a higher discharge voltage ue and consequently in a smaller difference of the voltage duej. The average discharge current iej is smaller. An identical value of duej is reached by increasing the supply voltage of the power module of the generator uj. Of course, the voltage of uj can be changed with a speed provided for by the power supply of the power module of the generator. These times range from several ms to several 10 to 100 ms.

An ignition module of the generator consists of a power source (a), a transistor TRv, a resistor Rv, cables with inductance Lv and its own capacitance Cv. The ignition voltage of the source uv is selected in a unit

(g) and is not changed during machining with a selected mode.

Both parts of the generator (ignition and power module) are connected with the tool electrode and the workpiece by cables with inductance Lk and capacitance Ck. A majority of generators have additional capacitors with different capacitance C connected in parallel to the gap, said capacitors being turned on or off by a switch Sc. The capacitance in the circuit of the ignition generator and the power generator has the greatest impact in fine machining. In case of large-surface electrodes the capacitance Ceo of the gap between the electrode and the workpiece has a great influence as well. The size of this capacitance may prevent reaching a low level of roughness since capacitive discharges occur with an energy that depends on the size of the capacitance Ceo.

A signal of discharge current is conducted from a sensor (h) iej to a circuit (e). From a unit (f) parameters can be entered for selected values of the discharge current iejni, which are needed to measure the time of increase in the current trn, individual levels of the current dieji, a selected technological value of the supply voltage of the power module of the generator ujt, a selected value of the minimum time of increase to a selected value of the pulse current trni, and a selected value of the minimum time of increase for a selected value of the level of the pulse current dtrni. Circuit (d) delivers an average value of the discharges uep and the voltage sensor (c) delivers a signal of the beginning of the discharge of the power module of the generator trO. The circuit (e) has a setting which indicates when discharging is interrupted due to a too rapid current increase. The signal is delivered to an oscillator (i) that controls the transistors TRj and TRv.

The discharge voltage in point tj ue(tj) (Figure 2) depends on a combination of the electrode and the workpiece materials and on the resistance of the working gap Re(tj). The resistance of the working gap is influenced by local pollution of the working gap and the size of the working gap. The local pollution of the working gap is influenced by the shape of the electrode and by the removal of erosion products from the working gap. The size of the working gap is influenced by the direction of the electrode movement with respect to a machining surface and setting of the servo-system. A gap in direction of the movement of the servo-system - the front gap - is always smaller than the side gap. The servo-system is used to regulate the size of the front gap.

The discharge current in point tj ie(tj) is a sum of the currents of the ignition module of the generator iev(tj) and the power module of the generator iej(tj) (Figure 2):

ie(tj) = iev(tj) + iej(tj).

The resistance of the working gap during a pulse Re(tj) cannot be measured, however the resistance of the circuit can be calculated from the voltage and the pulse current in the selected point tj.

Both the impedance of the circuit connected to this module of the generator Ztj(tj) and the voltage of the power module of the generator uj are important for the speed of current increase of the power module of the generator:

Ztj(tj) = uj/iej(tj).

At known constant resistances in the circuit of the power module of the generator Rgj, the resistance of the circuit is a sum of the constant resistor Rj, of the resistor of the power supply cables of the power module of the generator Rkj, of the resistor of a diode RD and the resistor of a transistor in point tj Rtrj(tj) in the power module of the generator:

Rgj(tj) = Rj + Rkj + Rtrj(tj) +RD

A transistor TRRj is connected in parallel to the resistor Rj. When the transistor TRRj is turned on, the resistance in the circuit of the power module of the generator is reduced practically to the resistance of a transistor TRRj Rtrrj which is much smaller than the resistance Rj (Rj » Rtrrj). When the current of the power module of the generator iej reaches the value iejmi, a signal is generated by the circuit (e) that turns off the transistor TRRj and the entire current flows through the resistor Rj. In many instances the discharge current iej does not reach the value of the current iejmi. In these instances the transistor TRRj is turned on the whole time. When the transistor TRRj is turned on, the resistance in the power module of the generator is:

Rgj(tj) = Rtrrj + Rkj + Rtrj(tj) +RD.

The resistance of the gap connected to the power module of the generator Rej(tj) is the difference between Ztj(tj) and Rgj(tj)

Rej(tj) = Ztj(tj) - Rgj(tj).

The current which flows through the circuit of the power module of the generator in point tj iej(tj) depends on the source voltage of uj and the impedance of the circuit for the power module of the generator Ztj(tj):

iej(tj) = uj / Ztj(tj).

The resistance of the gap and consequently the value of the impedance of the circuit Ztj(i) get reduced by the discharge time ie and the discharge current iej(t) increases accordingly. The size of the current iej(t) at identical impedance value Ztj(t) depends on the size of the voltage of the power module of the generator uj.

The speed of the current increase in the ignition module of the generator is considerably dependent on the part of the impedance of the circuit connected to this module of the generator Ztv(tj) and the supply voltage of the ignition module of the generator uv

Ztv(tj) = uv / iev(tj).

The resistance of the gap connected to the ignition module of the generator Rev(tj) is the difference between the impedance Ztv(tj) and the resistance of the ignition module of the generator Rgv(tj):

Rev(tj) = Ztv(tj) - Rgv(tj).

At known constant resistances in the circuit of the ignition module of the generator, the resistance of the ignition module of the generator Rgv is the sum of the constant resistor Rv, of the resistor of cables for energy supply Rkv and of the resistor of the transistor in point tj Rtvj(tj) in the ignition module of the generator:

Rgv(tj) = Rv + Rkv + Rtvj(tj).

The current flowing through the circuit of the ignition module of the generator in point tj iev(tj) depends on the source voltage uv and the impedance of the circuit of the ignition module of the generator Ztv(tj). iev(tj) - uv / Ztv(tj).

The resistance of the gap and consequently the value of the impedance of the circuit Ztv(t) get reduced by the discharge time and the discharge current iev(t) increases accordingly. Since the supply voltage of the ignition module uv is high compared to the supply voltage of the power module uj, the speed of current increase from the ignition module of the generator is high as well (Figure 2).

The impedance and thus the speed of the current increase from the power module of the generator diej/dt is influenced by the inductance of the circuit of the power module of the generator Lj with the inductance of the joint cable Lk and added inductance L and a difference between the voltage of the power module of the generator uj and the discharge voltage ue duej:

diej/dt = duej / (Lj + Lk + L).

The impedance and thus the speed of the current increase from the ignition module of the generator diev/dt is influenced by the inductance of the circuit of the ignition module of the generator Lv with the inductance of the joint cable Lk and a difference between the voltage of the ignition module of the generator uv and the discharge voltage ue duev:

diev/dt = duev / (Lv + Lk).

At normal discharges, the current increase from both the power and the ignition modules of the generator is smaller than allowed by the inductance of the power circuit of the generator (Lj + LK) and the inductance (Lv + LK) of the ignition circuit of the generator. If the inductance L is turned on during the whole pulse time, it can reduce the speed of current increase. Some manufacturers of electrical discharge devices thus get a trapezoidal shape of the current.

To eliminate the influence of the inductance L, the transistor TRL is turned on parallel to the inductance L. When said transistor is turned on, the impact of the inductance L is practically cancelled. Therefore, the circuit (e) sends a signal to the circuit (j) in case when the pulse current iej reaches the value iejmi. Said signal turns off the transistor TRL and the whole current flows through the inductance L In many cases iej fails to reach iejmi. The transistor TRL remains turned on in these cases and the influence of the inductance L is eliminated (Figure 4).

The occurrence of side discharges is detected by an increased discharge voltage ue. The beginning of discharge tdO is detected by the circuit (d) by a drop in the voltage in the gap below the selected value utd from the unit (f). A delay in the switch-on of the power module of the generator dtrO is measured in the circuit (e). The power current iej flows when the discharge voltage drops below the supply voltage of the power module of the generator uj and is detected by a sensor (h). In the circuit (e) the dtrO is compared with the selected value from the unit (f). If the dtrO exceeds the selected value dtrOi (Figure 5), the pulse is interrupted and a next pulse continues after a selected break W.

A successful use of electrical discharge machining requires a generator that provides for a constant nature of the optimally selected technology and prevents the occurrence of harmful pulses in the electrical discharge process. The generator must also be energetically efficient without unnecessary energy losses. Further, the influence of parameters having constant values (e. g. Rj and L) on the current increase must be reduced to a minimum. Only in this case will the course of the current increase in the electrical discharge process be considerably influenced by the course of the process by changing the impedance of the working gap. An important parameter that provides for a constant nature of the selected technology is the speed of the current increase especially through the power module of the generator diej/dt. The speed depends on the difference between the selected voltage of the power module of the generator uj and the discharge voltage ue: duej = uj - ue. Since the discharge voltage ue depends on the course of the electrical discharge process (Figure 3), the supply voltage of the power module uj needs to be adapted in order to get a constant difference duej.

The pulse generator of the invention allows that the circuit (d) calculates the average value uep for the selected number of normal discharges n on the basis of the discharge voltage measured in the last third of the normal discharges ue by the sensor (c). Therefore a signal from the oscillator (i) is needed as well. To the value uep the selected value due/7 (d) is added and consequently the voltage for the control of the power module uj is calculated:

uj = uep + duej.

The speed of the current increase of the power module of the generator diej/dt depends on the resistance of the working gap. This changes on a pulse to pulse basis (Figure 3). By reducing the resistance, the discharge current iej increases. If the resistance is reduced too rapidly, the current increases more rapidly and the density of the supplied energy is reduced. Reduced density of the supplied energy results in poorer overheating of the material in a future cavity at the discharge spot and consequently in a poorer removal of the material. When the energy density is too low, no material is removed and the supplied energy is high enough to cause additional pollution of the working gap due to a breakdown of the dielectric. The energy supply must be interrupted at such pulses since it is harmful for the normal course of the electrical discharge machining. Therefore such pulses are interrupted by a signal from the circuit (e) (Figure 1 ). First, a beginning of discharge trO needs to be detected by a current from the power module of the generator. It is determined in a way that the voltage sensor (c) detects a drop in voltage on the power module of the energy source. The current sensor (h) is used to measure the pulse current iej. The circuit (e) measures the time of increase in the current tr between the beginning of the discharge trO and the selected value of the pulse current iejn. If the current increase time trn is smaller than the selected current increase time trn < trni, the pulse is interrupted by a signal from the circuit (e).

Another option is that the current pulse is divided into several stages with an identical difference in the current diej. The measurement of the time of current increase starts at trO and the measured increase time up to iejl equals diej:

iej1 = diej.

A next current ie 2 is a sum of the currents iejl and diej:

iejl = iejl + diej.

The currents are added until the maximum current pulse iejm is reached (Figure 2):

iejm = iej(n-1) + diej. For each stage of current increase for diej the time of increase in current dtr is measured. If the time of increase in the current dtr is smaller than the selected value dtr < dtri, the pulse is interrupted.

In resistance generators, the energy yield of the power module of the generator is influenced by the constant resistance of the generator Rj. Its value changes as a function of the selected current iejmi. This resistance also influences the speed of the current increase. Both influences are cancelled in the phase of the increase in the discharge current iej when the resistor Rj is turned off. This is why the circuit (j) only turns on the resistor Rj when the discharge current reaches the value iejmi. In the event that the discharge current of the power module of the generator iej fails to reach the value iejmi, no additional resistor Rj is turned on (Figure 3)·

In generators without a resistor in the power circuit of the generator, certain inductance is needed which allows the current regulation during a pulse. In the circuit of the generator there are cable inductance Lk, the inductance of the power module of the generator Lj and additional inductance L. The latter is included in the circuit of the power module of the generator if the sum of the cable inductance Lk and the power module of the generator Lj is too low. If the additional inductance L is constantly turned on, it can limit the speed of the current increase diej/dt to a smaller value than allowed by the course of the electrical discharge process due to a change in the impedance of the working gap. If the circuit (j) turns on the inductance L only when it reaches the value iejmi, the influence of the additional inductance L is cancelled. Should the discharge current of the power module of the generator iej not reach the value iejmi until the end of the pulse duration, the additional inductance L does not turn on (Figure 4).

The efficiency of the generator can also be increased by interrupting side discharges having lower energy. An interruption following a selected break between the pulses tO is followed by a next pulse (Figure 5). The occurrence of side discharges is detected by an increased discharge voltage ue. Until this voltage drops below the voltage of the source of the power module uj, the current from the power module of the generator iej does not flow. If the time from the beginning of the discharge until the moment when the discharge voltage ue drops below the voltage of the source of the power module uj dtrO exceeds the selected value dtrOi, the pulse is interrupted and a next pulse starts after the selected break tO.