GB2105126A - Device for controlling the spark starting in an electric or electronic welding machine - Google Patents

Abstract

An arc-welding machine having a potentiometer 19 for manual adjustment of the current delivered by a generator 2 includes a control circuit 25 which controls the current delivered by the generator in dependence on the voltage between the tip 13 of a welding torch 12 and a workpiece. When the tip 13 touches the workpiece 15 with a relay-operated switch 9 open, a comparator 30 turns on an LED 29 so that a phototransistor 36 applies a low resistance across inputs 17, 18 of the generator 2 so that when the switch 9 is closed the generator 2 initially delivers current at its lowest current setting. Subsequently when the tip 13 is lifted from the workpiece 15 to establish an arc, the voltage between the tip 13 and workpiece 15 increases causing the comparator 30 to turn off the LED 29 whereby the current delivered by the generator 2 is that set by the potentiometer 19. <IMAGE>

Description

SPECIFICATION Device for controlling the spark starting in an electric or electronic welding machine This invention relates to a device for controlling the spark starting in an electric or electronic welding machine.

It is knonw that in most of the electronic arcwelding machines, the spark starting is obtained in one of two modes. According to the first working mode, one touches directing with the welding point the joint to be welded and the arc starts automatically when the point is lifted up. According to the second mode it is provided the use of a device generating high-voltage and high-frequency signals, by means of which the spark starting can take place without the point touching the joint to be welded.

Although such systems are widely used, each of them has an inherent disadvantage. In effect, in the first instance, due to the high level of the intensity of the electric currents which establish between point and joint, a very important heating is caused in the point, a portion of which melts and goes directly in the welding bath. As a consequence it is necessary to change frequently the welding point; further, a perfect weld is not obtained, as the welding bath is contaminated with the particles of the point. In the second instance, the said device generating highvoltage and high-frequency signals turns out particularly expensive and difficult to manufacture, as it must comply with very severe specifications which fix top limits for the level ofthe radio-frequency noise that it unavoidably produces.

An object of the present invention is to provide a device for controlling the spark starting in an electric or electronic welding machine, which is free from the drawbacks of the known devices abovementioned.

Said object is attained with the present invention, in that it relates to an electric or electronic arcwelding machine, of the type comprising an electric generator which delivers to its outlet terminal a power signal apt to be fed to a torch in order to make a weld on a workpiece, and means to adjust the level of the current intensity of the said power signal, said welding machine being characterized in that it includes a control circuit operatively connected between the outlet terminals of the said generator and said adjusting means and which generates a first or a second control signal of the said adjusting means according to whether the level of the voltage that, in use, establishes between the said torch and the said workpiece, is respectively lower or higher than a predetermined value.

For a better understanding of the present invention, one preferred embodiment of an electronic welding machine will now be described, as a not limiting example, with reference to the accompanying drawing which is an illustration of a simplified wiring diagram of the same.

With particular reference to the diagram, is shown in general in 1 an electronic arc-welding machine of the type including an electric generator 2 which has inlets 3, 4 respectively connected to terminals 5, 6 suitably connected to the normal electric network, and outlets 7, 8 from which can be drawn a power signal essentially continuous and with negative pole at the outlet 7 and positive pole at the outlet 8. The outlet 7 is connected with a first terminal of an electric contact 9 of the type normally open controlled by a relay 10. A second terminal of the contact 9 is connected with a terminal 11 to which, in use, is connected a torch 12 having at its end a welding point 13. The outlet 8 of the generator 2 is connected with a terminal 14 which is grounded and, in use, is connected with a workpiece 15 on which a weld must be made by means of the point 13 of the torch 12.

The generatore further has inlets 16, 17, 18, respectively connected to a first terminal, the wiper and a second terminal of a potentiometer 19. The potentiometer 19 has the role of adjusting, in a known way, the value of the current intensity I delivered by the generator 2. In particular, reducing the resistive value absorbed between wiper and terminal of the potentiometer 19 respectively connected to the inlets 17 and 18 of the generator 2, it is possible to realize a gradual reduction of the value of the current I till it is attained the lowest value which is that of the short-circuit current of the generator 2 and which preferably corresponds to a current value of some tens of Amperes (for instance 40-50 A).

The relay 10 is fed through a push button 20 associated to the torch 12 and is connected to input terminals 21,22 suitably connected to a supply of electric energy not shown.

According to the present invention, the welding machine 1 includes a control circuit 25 which, in conformity with the voltage measured between the terminals 11 and 14, that is the voltage existing between the point 13 and the workpiece 15, automatically controls, through the potentiometer 19, the current I delivered by the generator 2. In greater details, the control circuit 25 includes a power supply 26 having inlets connected with the terminals 5 and 6 and outlets respectively positive 27 and negative 28 where can be derived a rectified feeding signal suitably equalized (for instance, of the value of 5 V).

The outlet 27 is connected with the anode of a photoemitter diode 29, with a terminal of positive feeding of a comparator 30 and with a first terminal of resistors 31, 32. The cathode of the diode 29 is connected through a resistor 33, with the outlet of the comparator 30, which has a terminal of negative feed connected with the outlet 28 of the circuit 26. A non reversing input of the comparator 30, indicated by the symbol +, is connected with the second terminal of the resistor 32 and with the anode of a diode 34, whose cathode is connected with the terminal 14. A reversing inlet of the comparator 30, indicated by the symbol -, is connected with a second terminal of the resistor 31 and with a first terminal of a resistor 35, a second terminal of which is connected with the outlet 28 of the supply circuit 26; said outlet is also connected with the abovementioned terminal 11.The control circuit 25 finally includes a phototransistor 36 optically coupled with the photoemitter diode 29 and in which the collectoremitter junction is connected in parallel with that portion of the potentiometer 19 which is included between the slider and the terminal of said potentiometer respectively connected with the inlets 17 and 18 of the generator 2.

The operation of the welding machine 1 is as follows.

Let us assume that at first the torch 12 is lifted relative to the workpiece 15 and that the push button 20 associated with it is not pressed. In such situation, the non reversing inlet of the comparator 30 is brought by the resistor 32 to the potential of the outlet 27 of the power supply 26 (in the present example, +5 Volts), while the reversing inlet of the comparator 30 is mantained at a lower potential (for instance, +2,5 Volts) due to the division on the voltage produced by the power supply 26 through the resistors 31 and 35. The outlet of the comparator 30 is therefore at a high level, for instance +5 Volts, and as a consequence the photoemitter diode 29 is switched off.Therefore the collector-emitter junction of the transistor 26 behaves as an open circuit which puts itself in parallel with the inlets 17 and 18 of the generator 2 without changing the resistive value set on the potentiometer 19 by means of the corresponding slider. In this situation, the current that the generator 2 can supply is the current set through the slider the potentiometer 19 and it will be high or low according to whether the slider plugs in between the inlets 17 and 18 of the generator 2 a minimum or maximum value of resistance of the potentiometer 19.

Assuming now that the torch 12 touches, with the point 13, the workpiece 15 and that at the same time the push button 20 is still open, as a consequence also the contact 9 of the relay 10 will be open and the generator 2 cannot make the current circulate. In such a situation, the non reversing inlet of the comparator 30 is brought practically to the same value as the voltage existing at the outlet 28 of the power supply 26 through the diode 34, the work piece 15 and the torch 12. More in particular, the voltage that establishes at the non reversing inlet of the comparator 30 is the direct polarization voltage (anode-cathode) of the diode 34 that, as known, is of the order of some tenths of a Volt.Since the voltage existing at the reversing inlet of the comparator 30 remains at the value of the preceding case, that is about +2,5 Volts, the outlet signal of the comparator 30 shifts to the low level and so produces the switching on of the photoemitter diode 29. In such a situation said diode emits a light which hits the emitter-base junction of the phototransistor 36 and produces in the collector-emitter junction of the said transistor a large reduction of the equivalent resistance. Therefore, is obtained an automatic and substantial reduction of the resistance that occurs at the inlets 17 and 1 8 of the generator 2 with a consequent arrangement of the said generator to the delivery of the lowest current.

If, in such a situation, the push button 20 is pressed, through the relay 10 is obtained the closing of the contact 9 with consequent flow of the current I between the point 13 and the workpiece 15; said current, reaching the before-mentioned lowest value (40-50 A), does not cause any damage to the point 13.

The lifting of the torch 12 from the workpiece 15 causes the spark starting between point 13 and workpiece 15. In such a situation the voltage between point 13 and workpiece 15 increases, and increases correspondingly also the voltage at the non reversing terminal of the comparator 30. When said voltage exceeds the value of threshold voltage (+2,5 V) set through the resistors 31 and 35, it takes place a commutation of the outlet signal produced by the comparator 30 with consequent switching off of the photoemitter diode 29. Therefore is lacking the contribution given by the collector-emitter junction of the phototransistor 36 to the resistance existing at the inlets 17 and 18 of the generator 2 and corresponding now substantially to that set through the potentiometer 19.The result is that the current I delivered by the generator 2 shifts now from the value of lowest current of short-circuit to that set through the potentiometer 19.

The cutoff of the arc will take place or leaving the push button 20 or removing the torch 12 from the workpiece 15 till the arc switches off by itself. In both the above situations, there is a return to the working conditions explained at the beginning ofthedescrip- tion of the operation.

From an examination of the features of the welding machine 1 realized according to the present invention, it is seen that it allows to achieve the objects above stated.

The arc which starts at the beginning of each welding between the point 13 and the workpiece 15 takes place with a minimum value of the current I and therefore are avoided both the damages of the point 13 of the torch 12, and the corresponding polution of the welding bath set on the workpiece 15.

The current values which cause the real welding action establish between point 13 and workpiece 15 when the point 13 itself is already detached from the workpiece 15 and so avoid the disadvantages described above.

It is at least clear that changes and modifications can be made to the welding machine 1 without departing from the scope of the present invention.

For instance, it is not essential prearrange the push button 20 in the way described above; it will suffice prearrange in the generator 2 suitable time constants so that when the point 13 of the torch 12 touches the workpiece 15, the minimum value of current immediately establishes even before that takes place the supply from the generator 2 of the current set by means of the potentiometer 19.

Claims (9)

1. Electric or electronic arc-welding machine, of the type comprising an electric generator (2) which delivers to its outlet terminals (7,8) a power signal suited to be supplied to a torch (12) in order to make a weld on a workpiece (15), and means (25) to adjust the value of the current intensity (I) of the said power signal, the said welding machine (1) being characterized in that it comprises a control circuit (25) which is operatively connected between the outlettermin- als (7,8) of the said generator (2) and the said adjusting means (19) and which produces a first or a second control signal of the said adjusting means (19) according to whether the value of the voltage that, in use, establishes between the said torch (12) and the said workpiece (15) is respectively lower or higher than a predetermined value.

2. Welding machine according to claim 1, characterized in that the said adjusting means (19) cause, following the reception of the said first control signal, a minimum value of the said current intensity (I) deliverable by the said generator (2).

3. Welding machine according to claim 2, characterized in that the said minimum value of the said current (I) corresponds to the value of the shortcircuit current of the said generator (2).

4. Welding machine according to anyone of the preceding claims, characterized in that said predetermined value of voltage is of the order of some Volts.

5. Welding machine according to anyone of the preceding claims, characterized in that the said control circuit (25) comprises a voltage comparator (30) having a first inlet (-) mantained at said predetermined value of tension and a second inlet (+) connected with one of said outlet terminals of the said generator (2).

6. Welding machine according to anyone of the the preceding claims, characterized in that the coupling between the said control circuit (25) and the said adjusting means (19) takes place without any ohmic connection.

7. Welding machine according to anyone of the preceding claims, characterized in that the said adjusting means (19) comprise a potentiometer from which different resistive values can be manually selected, each of which can be modified through the said control signal produced by the said control circuit (25).

8. Welding machine according to claim 7, characterized in that the said control circuit (25) comprises a component (36) whose resistance is variable on the base of the value acquired by the said control signal, and which is connected between the slider and a terminal of the said potentiometer (7).

9. Electric or electronic arc-welding machine, substantially as described with reference to the accompanying drawing.