GB2033686A - Pulse generator - Google Patents

Abstract

A generator generates work pulses for electrically removing metal by penetrating a work gap 5 defined by a pair of electrodes 6,7. The generator comprises a voltage or current source 1, a plurality of main circuits being connected across said voltage or current source 1, each main circuit containing a separate primary winding 1031, 1032, 103N forming part of a single low-induction pulse transformer 100 for the generator. Each main circuit also contains a first switch T11, T21, TN1 serving to control current flow or pulse shape in the main circuit; an energy storing unit 31, 32, 3N; a second switch T12, T22, TN2 for producing an over-voltage in the main circuit in co-operation with the energy storing unit 31, 32, 3N and with the primary winding 1031, 1032, 103N, so that a work pulse penetrates the work gap 5; a third switch T31, T32, T3N for producing a steep trailing edge of the work pulse. <IMAGE>

Description

SPECIFICATION Generator for generating work pulses for treating metal The invention relates to a generator for generating work pulses for the treatment of metals, by removing metal, with at least one main circuit for generating pulse forms which are transmitted to at least one pair of electrodes and, as work pulses, penetrate a work gap defined by the pair of electrodes.

The present invention has the object of modifying the generator described in Swiss Patent Application No. 6878/77.

The invention also has the object of providing a series of pulses to the electrodes defining the work gap, the slope of the leading edges and trailing edges of the pulses being accurately controllable.

According to the invention there is provided a generator for generating work pulses for electrically removing metal comprising a plurality of main circuits for generating pulse forms, which are transmitted to at least one pair of electrodes and, as work pulses, penetrate a work gap defined by the pair of electrodes, the generator comprising a voltage or current source, each main circuit being connected across said voltage or current source, and each main circuit containing a separate primary winding forming part of a single low-induction pulse transformer for the generator, each main circuit containing the following further components:: a first switch which lies in series with the voltage or current source and serves to control current flow or pulse shape in the main circuit; an energy storing unit; a second switch one of whose terminals lies between said energy storing unit and one side of the primary winding and whose other terminal is connected to the voltage source or current source, for producing an over-voltage in the main circuit in co-operation with the energy storing unit and with the primary winding, so that a work pulse penetrates the work gap; a third switch, one of whose terminals is connected to the voltage or current source, and whose other terminal is connected to the other side of the primary winding for producing a steep trailing edge of the work pulse.

An embodiment of the invention is explained in detail below with reference to the accompanying drawings, in which: Fig. 1 shows the whole circuit arrangement of the generator and of the work gap for the electrical removal of metal; and Figs. 2,3 and 4 are details of the circuit arrangement of Fig. 1.

Fig. 1 shows a generator, which consists of a number N of main circuits. Only three circuits are shown so as to facilitate understanding. Each circuit consists of the same circuit components and is con nected, on both its sides, in a specific way, which will be explained in detail below. One end of all the main circuits, of which the number N is provided, is con nected to one terminal of the voltage source or cur rent source 1. Each of the main circuits has its own termination at its other end. This termination is constituted by a primary winding 1031, 1032, 103N. The primary windings of all the main circuits belong to a pulse transformer 100, whose secondary winding 104 is connected, by way of conductors 4, to the tool electrode 6 and workpiece electrode 7.Each main circuit contains a first semi-conductor switch T11, T21, TN1, which are series-connected to the source 1, a measuring sensor 21,22, 2N, an energy storing unit 31,32, 3N, a second semi-conductor switch T12, T22, TN2, a third semi-conductor switch T31, T32, T3N, and also a free-running diode D1, D2, DN, and further diodes D11, D12, D21, D22, DN1, DN2. For the sake of simplicity Fig. 1 only shows single semiconductor switches. In practice it is an easy matterto combine a number of semi-conductor switches to a group of these switches. The manner of operation of the circuit arrangement of Fig. 1 is explained in detail below.It is to be assumed that generator 8 is to have the required shape of work pulse, which is intended to penetrate the work gap 5 between the tool electrode 6 and the workpiece electrode 7. This generator may be a function generator, an adjustable potentiometer, or a specific reference voltage.

In the present instance this generator 8 is intended to contain a step-like leading edge and a vertical trailing edge of the work pulse. A comparator 9 compares the signals, which provide information, which is supplied by the measuring sensors 21,22, 2N acting by way of filters 101,102, 10N, concerning the pattern of current flow in the individual main circuits.

As at the present instant none of the main circuits is carrying current, the comparator 9 passes, by way of conductors 911, 912 and 91 N, control signals to the semi-conductor switches T1 1, T21, TN1,which will be referred to below as first switches. These control signals control the time relationship of the operation of the first semi-conductor switches T1 1, T21 and TN1. In the first main circuit, shown lowermost in Fig. 1, when the first semi-conductor switch T1 1 has become current conductive, the second switch T12 has already been brought into the conductive condition by way of the synchronizing conductor 81, which lies between generator 8 and control circuit 11 and whose presence is merely indicated in Fig. 1.

Due to the presence of source 1, current then flows over the semi-conductor switches T1 1 and T1 2, which have been closed (i.e. rendered current conductive), and in this way charges the energy storage unit 31, which is represented in Fig. 1 as an inductance or choke. A short time after semiconductor switch T11 has become currentconductive, the semi-conductor switch T21 of the next main circuit is rendered current-conductive by the comparator 9. The second switch T22 of the same main circuit has already been rendered current-conductive by way of the synchronization conductor 81. Thus, as soon as the first switch T21 has been brought into its conductive condition, a current flows back to source 1 by way of measuring sensor 22, choke or inductance 32, and second switch T22.The next main circuit is, also after the lapse of a short time offor example 1 to 5,asec, brought into its conductive condition by way of con ductor 91 N. Shortly before the second switch TN2 of this main circuit has also been caused to assume its conductive state. This takes place by way of control switching unit 11. In this circuit also a current flows through measuring sensor 2N, inductance 3N, sec ond switch TN2, and thence backto source 1. It was stated above that the first switches To 1, T21, TN1 of the main circuit are so controlled, by comparator 9 acting by way of conductors 911, 912, 91 N, that they bear a certain timewise relation to one another.

However, it is also readily possible to arrange for all these first switches of the main circuits to be simul- taneously switched on. This would in no way affect the above-described pattern of current flow. In any case, the second switches T12, T22, TN2 can, in both cases, be simultaneously switched on or can be switched on in timed sequence. The current flow present in the individual main circuits is tapped by measuring sensors 21,22, 2N. The tapped signals pass, by way of filters 101,102, 10N, into the comparator 9, which compares the value prescribed by generator 8 with the individual actually-existing shapes or actually-existing values of current flow in the individual main circuits.If prescribed value and actually-existing value coincide, or if the actuallyexisting value is greater than the prescribed value, the first switches T1 1, T21,TN1 are so acted on, by way of their associated conductors 911,912,91 N, as to assume their blocked condition. Accordingly, the actualiy-existing value in the main circuit concerned drops in consequence of the blocked first switch.

However, current flow is maintained by way of the free-running diodes D1 or D2 or DN of the corresponding main circuit. If the actually-existing value of this main circuit is smaller than the prescribed value, the first switch concerned receives a control signal from the comparator 9 and is brought into its conductive state. Thus, comparator 9 ensures that the actually-existing values of the current flow in the individual main circuits at all times matches the value prescribed by the generator 8. For better understanding it is pointed out that the third switches T31, T32, T3N are blocked at this time.

The work pulse is composed of the current flowing in the individual main circuits, the actually-existing values of which currents are accommodated, by means of the first switches To 1, T21, TN1, to the actually-existing value or values. The greater the number of main circuits provided, the smaller are the parts which can compose a work pulse. This means that the smaller the parts the finer will be the resolution of an individual work pulse. The production of a work pulse from the three main circuits shown in Fig. 1 will be described. This pulse production takes place through appropriately actuating the second switches T12, T22, TN2 and the third switches T31, T32, T3N. Let it be assumed that the work pulse is to have a step-like leading edge and a very steep trailing edge.If the second switch T12 is then brought, by way of control circuit 11, into its blocked state, and the third switch T31 of the same main circuit is brought into its conductive state, the current of this main circuit flows, by way of primary winding 1031 and switch T31, back to the source 1.

The current surge is transmitted from the primary winding 1031 to the secondary winding 104Ofthe transformer 100, and reaches the work gap 5 by way of conductors 4; this work gap 5 is defined by the two electrodes 6 and 7. A very short time after this (e.g. nanoseconds or microseconds) control circuit 11 so acts on the second switch T22 and third switch T32 of the next main circuit that the second switch T22 is blocked and the third switch T32 brought into the current conductive state.The currentthen flows, by way of primary winding 1021 and third semiconductor switch T32, back to the source 1. This current flow is transmitted, by pulsetransformer 100, to the secondary winding 104, and'thus passes, by way of conductors 4, to the work gap 5.A short time after (nanoseconds or microseconds) the control circuit causes the second switch TN2 of the next main cir cuitto be brought into its blocked state, and the corresponding third switch T3N to be brought into its conductive state. The current surge, passing by way of the primary winding 103N, is transferred, by the same pulse transformer 100, to the common secondary winding 104 and, by way of conductors 4, to the work gap 5. The energy from the inductances 31,32, 3N and from the primarywindings 1031,1032, 103N produces a higher voltage, i.e. an over-voltage, so that the step-like leading edge of the work pulse, which is applied to the work gap 5, can penetrate this work gap.The trailing edge of the work pulse is produced by a controlled circuit 11 simultaneously bringing the second semi-conductor switches T12, T22, TN2 of all circuits into the conductive state, and the third switches T31, T32, T3N of all the main circuits into the blocked state. In this way current flow is abruptly interrupted in all the primary windings 1031, 1032, 103N, as the current can flow back to the source 1 through the conductive second semiconductor switches T12, T22, TN2. The cycle commences in the same way as has been described for producing the next work pulse.As the times at which the second switches and third switches are brought into their conductive or blocked state must be correlated with very great precision - and as this cannot be realised so accurately and precisely in the one or the other case, particularly when groups or packets of switches are used - undesired over-voltage may occur in each main circuit. These over-voltages may be dissipated by way of diodes D11, D12, D21, D22, DN1, DN2 present in each main circuit. Capacitor C is also provided parallel with the source 1 for this purpose. The generation of work pulses with step-like leading flanks and steep trailing edges has already been described. Naturally, work pulses of any desired shape can be generated. The greaterthe number of main circuits the greater the number of individual parts of the leading edge. The obtainable leading edge of each work pulse can be very precisely and accurately matched to the prescribed leading edge. The prescribed shape of the leading edge may be stored in the generator 8. In this case the control circuit ii, which controls the second and third semi-conductor switches, receives its control commands by way of conductor 81. It is also readily possible for the prescribed shape of the leading edge of a work pulse to be stored in a numerical control unit (not shown). In this case the control circuit 11 will receive its corresponding control command by way of the conductor 82, whose presence is indi cated in the drawing.

In Fig. 1 a voltage source or current source 70, a diode 71 and an adjustable resistance or potentiometer72 are connected to one infeed conductor 4 and to the tool electrode 6. This arrangement sup plies an auxiliary voltage or auxiliary current, which can preheatthetool electrode. This preheating isto be recommended in the case of particularly thin tool electrodes, such as for example wire-like electrodes, the diameter of whose cross-section is 0.1 to 0.8 mm.

The preheating by the d.c. voltage source or this d.c.

source 70 only takes place between the individual work pulses, i.e. in the so-called interval. As this circuit arrangement does not have to be used in all cases, it is connected, in Fig. 1, by a conductor shown in dashed line.

Similarly, in Fig. 1, a voltage source or current source 50 is provided which, in series with a potentiometer or adjustable resistance 51 and with a diode D6, is connected in parallel to the work gap 5. This series connection 50, 51, D6 is shown as being connected to points on a conductor 4 by dashed lines, which are intended to indicatethatthis series circuit is not used in all cases. When the series connection is so connected, the points are not of course directly connected, but only through the series connection.

The source 50, which supplies a d.c. voltage or a direct current, so acts, in the work gap 5, that there is a slight electrolysis between the work pulses. This slight electrolysis is used for cleaning the electroerosively active surfaces of the tool electrode 6 and of the workpiece electrode 7. A voltage lower than 1 Volt is applied to the work gap 5 by means of the adjustable resistance or potentiometer 51. The current is so adjusted that less than 1 Ampere flows through the work gap 5. The diode D6 prevents work pulses from the secondary winding 104 reaching the source 50.

Fig. 2 shows a detail from the circuit arrangement of Fig. 1. In Fig. 2 the source 70 may be used for feeding an electromagnet 110. The electromagnet 110 is used for reinforcing the evacuation of the dirtied dielectric liquid from the very narrow work gap 5, and in this way for improving the cleaning of the spark site. In this case a dielectric liquid is used which has magnetic properties. Such magnetic dielectrics are described in the journal "Elektronik Industrie" 7/8, 1974, in the article "Magnetic Liquids as Versatile Workpieces".

Fig. 3 shows a similar arrangement to Fig. 2. While in Fig. 2 the source 70 is a d.c. voltage or a direct current source, Fig. 3 shows an alternating current (a.c.) source 70. The diode 71 is dispensed with when an a.c. source 70 is used. The electromagnet 110 is fed by the a.c. source 70, so that the evacuation of the contaminated dielectric from the very narrow work gap 5 is reinforced and, hence, the cleaning of the spark site is improved. In this case a magnetic liquid is used as dielectric.

Fig. 4 shows a similar arrangement to Figs. 2 and 3, although with a voltage source 75, whose current or voltage is controlled, by way of conductor 84, by the control circuit 11 or by an optimizing unit. This arrangement can be used with any other generators, or in conjunction with the method and apparatus described in Swiss Patent Specification No. 527,018, for improving the conditions occurring in the work gap during electro-erosive processing.

Claims (12)

1. A generator for generating work pulses for electrically removing metal comprising a plurality of main circuits for generating pulse forms, which are transmitted to at least one pair of electrodes and, as work pulses, penetrate a work gap defined by the pair of electrodes, the generator comprising a voltage or current source, each main circuit being connected across said voltage or current source, and each main circuit containing a separate primary winding forming part of a single low-induction pulse transformer for the generator, each main circuit containing the following further components:: a first switch which lies in series with the voltage or current source and serves to control current flow or pulse shape in the main circuit; an energy storing unit; a second switch one of whose terminals lies between said energy storing unit and one side of the primary winding and whose other terminal is connected to the voltage source or current source, for producing an over-voltage in the main circuit in co-operation with the energy storing unit and with the primary winding, so that a work pulse penetrates the work gap; a third switch, one of whose terminals is connected to the voltage or current source, and whose other terminal is connected to the other side of the primary winding for producing a steep trailing edge of the work pulse.

2. A generator according to claim 1, wherein each main circuit contains a measuring sensor for detecting the current flow controlled by the first switch; and a comparator is responsive to the measuring sensor and serves to compare the current flow value with a reference value prescribed by a reference means, the comparator individually controlling the first switch.

3. A generator according to claim 1 or claim 2, comprising a control circuitforthe second and third switches of all the main circuits and, by controlling the said switches, generates work pulses for the work gap.

4. A generator according to claim 3, wherein the second switch of a main circuit acts in combination, in one of its switching states, with the third switch in the other switching state, to generate part of the leading edge of the work pulse, the whole of the leading edge being formed by all main circuits.

5. A generator according to claim 3, wherein-the third switches of all main circuits combines, in one of their switching states, with the second switches of all the main circuits - these second switches having been acted on so as to assume the other switching state - to generate the trailing edge of the work pulse which penetrates the work gap.

6. A generator according to any of claims 1 to 5, comprising a capacitor for recovering the energy from the storage units arranged in the main circuits.

7. A generator according to any one of claims 1 to 6, comprising a voltage source or a current source arranged in parallel to the work gap and serving to clean the work gap by an adjustable resistance arranged in series with said voltage or current source to alter the value of the voltage or of the current, and a diode arranged so that the work pulses, penetrating the work gap, are isolated from said voltage or current source.

8. A generator according to any one of claims 1 to 7, comprising a series circuit, consisting of an auxiliary source, of a diode, and of an adjustable resistance, for preheating one of said electrodes.

9. A generator according to any one of claims 1 to 7, comprising an auxiliary source, which produces d.c. voltage or direct current, in series with a diode and with an adjustable resistance and feeds a magnetic coil, the magnetic coil so acting on a dielectric, which is provided in the work gap between said electrodes and has magnetic properties, that the evacuation of the dielectric from the work gap is reinforced, and the spark site cleaned.

10. A generator according to any one of claims 1 to 7, comprising an auxiliary source, which supplies a.c. voltage or alternating current, and a variable resistance connected to a magnetic coil, the magnetic coil so acting on a dielectric, which lies in the work gap and has magnetic properties, that the evacuation of the dielectric from the work gap is reinforced, and the spark site cleaned.

11. A generator according to claim 3, or any one of claims 4to 7 when dependent on claim 3, comprising a source for feeding a magnetic coil, whose voltage or current is controlled by said control circuit, the magnetic coil so acting on the dielectric, which lies in the work gap and has magnetic properties, that evacuation of the dielectric from the work gap is reinforced, and the spark site cleaned.

12. A generator substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.