METHOD AND WELDING DEVICE WITH DETECTION OF ELECTRICAL CONTACTS DURING A WELDING PROCESS

Abstract

In a method in which a welding process is carried out on a workpiece with a welding torch and to a welding device for carrying out the method, a welding current source is supplied in order to provide a welding voltage, and an electrical voltage is applied, at least temporarily, during the welding process to an external element of the welding torch, in particular to an outer wall of a gas nozzle, and a possibly occurring electrical contact between the external element and a further element, in particular the workpiece or a contact tube, is detected by the electrical voltage applied. The welding current source is electrically connected via at least one first resistor to the external element of the welding torch and the external element of the welding torch is connected to the electrical potential of the workpiece via at least one second resistor.

Claims

1: A method in which a welding process is carried out on a workpiece (3) with a welding torch (5), which is preferably arranged on a robot (2), wherein a welding current source (18) is supplied in order to provide a welding voltage (U.sub.SQ), and an electrical voltage (U.sub.D) is applied, at least temporarily, during the welding process to an external element (17) of the welding torch (5), in particular to an outer wall (9′) of a gas nozzle (9), and a possibly occurring electrical contact (13, 14) between the external element (17) and a further element (24′, 24″), in particular the workpiece (3) or a contact tube (16), is detected by means of the electrical voltage (U.sub.D) applied, wherein the welding current source (18) is electrically connected via at least one first resistor (19) to the external element (17) of the welding torch (5) and the external element (17) of the welding torch (5) is connected to the electrical potential of the workpiece (3) via at least one second resistor (21).

2: The method according to claim 1, wherein, in the case of a detected electrical contact (13, 14), in particular touching of the external element (17) with the workpiece (3), a warning signal is emitted, the welding process is interrupted, a welding path is corrected and/or the workpiece (3) is repositioned.

3: The method according to claim 1, wherein a possibly occurring electrical contact (13, 14) of the external element (17) with the further element (24′, 24″) is detected on the basis of a change of a measured voltage (U.sub.mess), which drops across the at least one first (19) and/or the at least one second resistor (21), and/or of a measured current which flows through the at least one first (19) and/or at least one second resistor (21).

4: The method according to claim 3, wherein, on the basis of the change in the measured voltage (U.sub.mess) and/or the measured current, at least two different types of possible electrical contacts (13, 14) are distinguished, namely at least one electrical contact (13) between the external element (17) and a first further element (24′), in particular the workpiece (3), and one electrical contact (14) between the external element (17) and a second further element (24″), in particular a contact tube (16) of the welding torch (5).

5: The method according to claim 1, wherein at least one first capacitor (37) is arranged parallel to the at least one first resistor (19) and at least one second capacitor (38) is arranged parallel to the at least one second resistor (21).

6: The method according to claim 5, wherein a possibly occurring electrical contact (13, 14) of the external element (17) with the further element (24′, 24″) is detected on the basis of a change in a measured current which flows through the at least one first (37) and/or the at least one second capacitor (38).

7: The method according to claim 1, wherein a change in the welding voltage (U.sub.SQ) of the welding current source (18) is used in the detection of a possibly occurring electrical contact (13, 14).

8: A welding device (1), in particular an inert gas welding device, for carrying out a welding process on a workpiece (3) with a welding torch (5), which is preferably arranged on a robot (2), and with a welding current source (18) for providing a welding voltage (U.sub.SQ), wherein an electrical voltage (U.sub.D) is applied to an external element (17) of the welding torch (5), in particular to an outer wall (9′) of a gas nozzle (9), and a detection unit (25) is provided and is configured to detect, during the welding process, a possibly occurring electrical contact (13, 14) between the external element (17) and a further element (24′, 24″), in particular the workpiece (3) or a contact tube (16) of the welding torch (5), wherein, for applying the electrical voltage (U.sub.D), the welding current source (18) is connected via at least one first resistor (19) to the external element (17) of the welding torch (5) and the external element (17) of the welding torch (5) is connected via at least one second resistor (21) to the potential of the workpiece (3).

9: The welding device (1) according to claim 8, wherein at least one first capacitor (37) is arranged parallel to the at least one first resistor (19) and at least one second capacitor (38) is arranged parallel to the at least one second resistor (21).

Description

[0029] The invention will be explained in more detail below with reference to figures, to which, however, it is not intended to be limited.

[0030] The figures show:

[0031] FIG. 1, a schematic representation of a welding device with a welding robot;

[0032] FIG. 2A, a schematic cross-sectional representation of a welding torch according to a first embodiment of the invention;

[0033] FIG. 2B, a schematic cross-sectional representation of a welding torch according to a second embodiment of the invention;

[0034] FIG. 2C, a schematic representation of a welding torch with an auxiliary voltage source;

[0035] FIGS. 3A-3C, respectively, a simplified representation for illustrating voltage drops at a voltage divider;

[0036] FIG. 4, various schematic signal courses of the welding device according to the invention;

[0037] FIGS. 5A and 5B, the course of a welding voltage, the course of currents through capacitors of the welding device, and the course of a detection signal;

[0038] FIG. 6, a simplified block diagram; and

[0039] FIGS. 7A and 7B, the course of a welding voltage, the course of a measuring voltage and the course of a detection signal.

[0040] FIG. 1 shows a welding device 1 with a welding robot 2 which is designed to carry out an inert gas welding process on a workpiece 3. A welding torch 5 is fastened to a flange 4 of the welding robot 2 and is connected via a hose package 6 to a welding current source 18 and a gas supply 7. The gas supply 7 supplies the welding torch 5 with an inert gas, for example helium, argon, CO.sub.2 or mixtures thereof, which emerges at a lower opening 8 of a gas nozzle 9 (see FIG. 2A and FIG. 2B) of the welding torch 5 and protects the arc 12 and the weld seam 10 on the workpiece 3 from the oxygen of the ambient air and chemical reactions. A welding wire 11 (see FIG. 2A and FIG. 2B) is also passed through the gas nozzle 9, for the melting of which an arc 12 is generated between the end of the welding wire 11 and the workpiece 3. The welding robot 2 travels along a predetermined trajectory, while the welding torch 5 carries out the welding process.

[0041] During the welding process, an undesirable contact 13 may occur between the welding torch 5 and the workpiece 3, as shown in FIG. 2A and FIG. 2B. It can also happen that metallic spatter (not shown) of the melting welding wire 11 produces an electrical contact 14 between the gas nozzle 9 and a voltage-carrying contact tube 16 arranged inside the gas nozzle 9. The described contacts 13 and 14 can have disadvantageous effects on the welding process, since damage to the welding torch 5 or to the workpiece 3 can occur due to the touching between the welding torch 5 and the workpiece 3, or the arc 12 can be deflected in the event of an electrical short circuit between the contact tube 16 and the gas nozzle 9.

[0042] The metallic spatter mentioned may also adversely affect the outlet of the inert gas, so that a deterioration of the gas protection for the weld seam 10 and the arc 12 may occur. If a plurality of contacts 13, 14 occur simultaneously, the individual disadvantages of the contacts 13, 14 can also be reinforced. For example, if a contact 13 and a contact 14 occur simultaneously, an arc can be produced between the gas nozzle 9 and the workpiece 3. Contacts 13, 14 should therefore be identified quickly and appropriate countermeasures should be taken. In addition to the two contacts 13 and 14, a contact 15 can also occur between the contact tube 16 and the workpiece 3, which is monitored during the welding process. The contact 15 can take place deliberately during the welding process, in particular a short-circuited welding process, and thus cannot represent a fault.

[0043] According to the invention, to identify the contacts 13 and 14, an electrical voltage U.sub.D is applied to an external element 17 of the welding torch 5, preferably to the gas nozzle 9 of the welding torch 5, in particular to an outer wall 9′ of the gas nozzle 9, and possibly occurring contacts 13 and 14 during the welding process are detected with the aid of this applied voltage U.sub.D or on the basis of the change in the voltage U.sub.D or an electrical magnitude related thereto. This is illustrated in FIG. 2A and FIG. 2B using two exemplary embodiments of the invention.

[0044] FIG. 2A shows a schematic cross-sectional representation of a welding torch 5. The welding torch 5 has a contact tube 16, which acts on a welding wire 11 with a welding voltage U.sub.SQ provided by a welding current source 18. The contact tube 16 is surrounded by the gas nozzle 9. The gas nozzle 9 forms an external element 17 of the welding torch 5 and is electrically connected to the welding current source 18 via at least one first resistor 19, so that a voltage U.sub.D is applied to the gas nozzle 9. The voltage U.sub.D is therefore a part of the voltage U.sub.SQ. It is also possible to provide a plurality of first resistors 19, which in particular can be connected in series or in parallel. For the purpose of a better overview, however, only a first resistor 19 is shown below.

[0045] The gas nozzle 9 is also connected via a second resistor 21 to the potential of the workpiece 3, which is usually connected to the negative pole (ground 34) of the welding device 1. The second resistor 21 is thus connected, on the one hand, to the gas nozzle potential and, on the other hand, to the electrical ground 34. It is also possible to provide a plurality of second resistors 21, which in particular can be connected in series or in parallel. For the sake of clarity, however, only a second resistor 21 is shown below. A voltage divider 22 is formed with the resistors 19, 21, which has a tapping point 23, to which the gas nozzle 9 is electrically connected. The potential of the tapping point 23 is accordingly present at the gas nozzle 9, so that the voltage U.sub.D is present at the gas nozzle 9.

[0046] In order to detect possibly occurring electrical contacts 13, 14 of the gas nozzle 9 with further elements 24′, 24″, for example the workpiece 3 or the contact tube 16, a detection unit 25 is provided. The detection unit 25 is configured to detect a possibly occurring electrical contact 13, 14 by measuring or monitoring a voltage U.sub.mess at the at least one first resistor 19 and/or at least one second resistor 21. Alternatively, the detection unit 25 for detecting contacts 13, 14 could also measure a current through the at least one first resistor 19 or the at least one second resistor 21 instead of the voltage. In FIG. 2A, only the voltage U.sub.mess at the at least one first resistor 19 is measured. To measure the voltage, the detection unit 25 has at least one voltage measuring device 26. The method for detecting and distinguishing the electrical contacts 13, 14 will be described in more detail below. First, a further embodiment of the invention is described.

[0047] FIG. 2B shows an alternative embodiment of the invention. In FIG. 2B, at least one first capacitor 37 is connected in parallel with the at least one first resistor 19. Furthermore, at least one second capacitor 38 is connected in parallel with the at least one second resistor 21. In the embodiment shown, the detection unit 25 has two current measuring devices or current measuring sensors 39, which separately or independently measure the electrical currents through the first capacitor 37 and the second capacitor 38. The branch consisting of the current-measuring sensors 39 and the capacitors 37, 38, one current-measuring sensor 39 and one capacitor 37, 38 each being connected in series, forms a further voltage divider, the tapping point 23′ of which is connected to the tapping point 23 of the voltage divider 22. By monitoring the respective currents flowing through the capacitors 38 or their change, possibly electrical contacts 13, 14 can be detected and distinguished from one another, as will be explained in more detail below.

[0048] In order to be able to detect and distinguish the electrical contacts 13, 14, in the embodiment according to FIG. 2A the voltage U.sub.D or the voltage at one of the resistors 19, 21 is measured or monitored and compared with a target value. Of course, (only) the current through the first 19 and/or second resistor 21 could also be measured or monitored and compared with a target value. In the embodiment according to FIG. 2B, the currents through the first 37 and the second capacitor 38 are detected and compared with a target value. In the fault-free normal state, in which there is no electrical contact 13, 14, the welding voltage U.sub.SQ is divided according to the resistance values of the resistors 19, 21. The voltage U.sub.D, which in the embodiment shown is lower than U.sub.SQ, is applied to the gas nozzle 9. If the resistors 19, 21 have substantially equal resistance values, U.sub.D is substantially 50% of the welding voltage U.sub.SQ. If an electrical contact 13, 14 occurs, voltage shifts occur within the voltage divider 22. These voltage shifts or the associated change in the electrical current can be detected with the aid of the detection unit 25 and an electrical contact 13, 14 can thereby be inferred during the welding process.

[0049] FIG. 2C shows a welding torch with an auxiliary voltage source 20. The auxiliary voltage source 20 can preferably deliver a voltage higher than the welding current source 18. The auxiliary voltage source 20 is preferably isolated, in particular galvanically isolated, from the welding current source 18. The isolation of the welding current source 18 can be made via a decoupling circuit 36. The decoupling circuit 36 can contain at least one electrical switch, in particular a relay, for this purpose. However, an isolation is not mandatory. The auxiliary voltage source 20 is preferably used for detecting the electrical contacts 13, 14, 15 before or after the welding process without the aid of the welding voltage U.sub.SQ. The contact 15 takes place between the contact tube 16 and the workpiece 3. In FIG. 2C, the welding current source 18 and the auxiliary voltage source 20 are electrically connected to one another via the decoupling circuit 36 in a high-impedance manner (values greater than 10 kΩ). The decoupling circuits 36 also serve as current limitation for the auxiliary voltage source 20.

[0050] The effects and the differences between the electrical contacts 13, 14, 15 are explained in more detail below with reference to FIGS. 3A-C. FIGS. 3A-C each show the voltage divider 22 with a voltage measuring device 26 of the detection unit 25. The electrical contacts 13, 14, 15 are each shown with a connection 27.

[0051] A first electrical contact 13 can occur by the touching of the gas nozzle 9 with a first further element 24′, for example the workpiece 3. As a result of the touching of the gas nozzle 9 with the workpiece 3, the second resistor 21 is short-circuited and the entire welding voltage U.sub.SQ drops at the first resistor 19 (FIG. 3A). In relation to the embodiment according to FIG. 2B, this means that no current or only a small current flows through the second capacitor 38 parallel to the second resistor 21 due to the short circuit. By contrast, a current flow can be detected by the first capacitor 37 parallel to the first resistor 19. This case is illustrated in more detail in FIG. 5A. In such a case, the welding path can still be changed during the welding process in order to avoid damage to the workpiece 3 or to the welding torch 5. The welding process does not necessarily have to be interrupted.

[0052] A second electrical contact 14 can occur due to metallic welding spatter of a melting welding wire 11 between the gas nozzle 9 and a second further element 24″, for example the contact tube 16. Through the electrical contact 14 between the contact tube 16 and the gas nozzle 9, the first resistor 19 is short-circuited and the entire welding voltage U.sub.SQ drops at the second resistor 21 (FIG. 3B). In relation to the embodiment according to FIG. 2B, this means that no current or only a small current flows through the first capacitor 37 parallel to the first resistor due to the short circuit. By contrast, a current flow can be detected by the second capacitor 38 parallel to the second resistor 21. This case is illustrated in more detail in FIG. 5B. In the case of a contact of this type, the gas nozzle 9 can be destroyed. In the event of an electrical contact 14, a cleaning recommendation can be issued in order to exchange or clean the welding torch 5, or the welding process can be interrupted.

[0053] A third electrical contact 15, the occurrence of which, however, does not constitute a fault during the welding process, is produced by the touching between the contact tube 16 or the welding wire 11 and the workpiece 3. The electrical contact 14 between the contact tube 16 or the welding wire 11 and the workpiece 3 causes the resistors 19 and 21 to be short-circuited. This case, in particular, before or after a welding process by additionally measuring the welding voltage U.sub.SQ or an auxiliary voltage Un can be detected and distinguished from the other cases, since in such a case the welding voltage U.sub.SQ or the auxiliary voltage U.sub.H also breaks in. In relation to the embodiment according to FIG. 2B, this means that no current flows through the capacitors 37, 38 parallel to the first 19 and to the second resistor 21 due to the short circuit.

[0054] FIG. 4 qualitatively shows a course of a measurement voltage U.sub.mess (in volts) via the first resistor 19 and a course of a welding voltage U.sub.SQ, wherein no welding process is carried out for a better overview, which is why the welding voltage U.sub.SQ (except for the period between t.sub.5 and t.sub.6) remains essentially constant. Different electrical contacts 13, 14, 15 occur over time.

[0055] In the period between the points in time t.sub.1 and t.sub.2, two electrical contacts 14 occur between the gas nozzle 9 and the contact tube 16. It can be seen that the welding voltage U.sub.SQ remains constant, but the voltage U.sub.mess drops to essentially 0 V.

[0056] In the period between the points in time t.sub.2 and t.sub.3 no electrical contacts 13, 14, 15 occur. U.sub.mess corresponds to a target value.

[0057] In the period between the points in time t.sub.3 and t.sub.4, two electrical contacts 13 occur between the gas nozzle 9 and the workpiece 3. It can be seen that the welding voltage U.sub.SQ remains constant, but the voltage U.sub.mess increases because the at least one second resistor 21 is short-circuited.

[0058] In the period between the points in time t.sub.4 and t.sub.5 no electrical contacts 13, 14, 15 occur. U.sub.mess corresponds to a target value.

[0059] In the period between the points in time t.sub.5 and t.sub.6, two electrical contacts 15 occur between the contact tube 16 and the workpiece 3. It can be seen that both the welding voltage U.sub.SQ and the voltage U.sub.mess drop to essentially 0 V. The contact 15 does not constitute a fault during the welding process.

[0060] FIG. 5A shows time courses of a welding voltage U.sub.SQ and currents I.sub.mess-37, I.sub.mess-38 through the first capacitor 37 and the second capacitor 38 (see FIG. 2B) during a welding process. The currents are alternating currents or ripple currents, which are produced by the clocked power section of the welding current source 18. At time point t.sub.1 a contact 13 occurs, so that the second resistor 21 and the second capacitor 38 are short-circuited. Consequently, no current or only a small current can be detected through the second capacitor 38. By contrast, the current through the first capacitor 37 increases. The increase or decrease in the currents through the capacitors 37, 38 allows the contact 13 to be identified when compared with a target value. A detection signal S is output at the time of occurrence.

[0061] FIG. 5B also shows time courses of a welding voltage U.sub.SQ and currents I.sub.mess-37, I.sub.mess-38 through the first 37 and the second capacitor 38 during a welding process. However, at time point t.sub.1 a contact 14 occurs, so that the first resistor 19 and the first capacitor 37 are short-circuited. Consequently, no current or only a small current can be detected through the first capacitor 37. By contrast, the current through the second capacitor 38 increases. The increase or decrease in the currents through the capacitors 37, 38 allows the contact 14 to be identified when compared with a target value. A detection signal S is output at the time of occurrence.

[0062] FIG. 6 schematically shows an exemplary implementation of the welding device 1 in a block diagram. Only the most important components are represented. In this case, on the one hand, the welding voltage U.sub.SQ of the welding current source 18 is measured with a further voltage measuring device 28 and, on the other hand, the voltage U.sub.mess is measured at one of the resistors 19, 21 (not shown). The resistors 19, 21 are implemented together with a voltage measuring device 26 in a circuit 29.

[0063] The welding voltage U.sub.SQ of the welding current source 18 does not have to be constant, but can vary during a welding process. In particular in the individual phases of the welding process, for example in an arc phase, in a basic current phase and in a pulse phase, the welding voltage U.sub.SQ may be different. Consequently, the target value for the measurement voltage U.sub.mess would also vary in the individual phases of the welding process, which would make the evaluation and detection of electrical contacts 13, 14 more difficult. In order to counteract this, a scaling factor P, which results from the wiring, in particular from the voltage divider 22 and the measuring circuit used, can be determined. On the basis of the scaling factor P and a current welding voltage U.sub.SQ, the target value for the measuring voltage U.sub.mess can be determined at any time, so that the electrical contacts 13, 14 can be determined independently of the level of the applied welding voltage U.sub.SQ. The target value corresponds to the voltage which should be present if there is no contact 13, 14. Of course, in the case of the method used, a current measurement can also be carried out instead of a voltage measurement.

[0064] In FIG. 7A and FIG. 7B the course of the welding voltage U.sub.SQ and the course of the measurement voltage U.sub.mess via the first resistor 19 are represented. In FIG. 7A, a contact 13 occurs at time point t.sub.1. In FIG. 7B, a contact 14 occurs at time point t.sub.1. The dashed line represents the course of the target value U.sub.mess-soll determined by means of the scaling factor P for the measuring voltage U.sub.mess. Before a contact 13, 14 occurs, the measured voltage U.sub.mess corresponds essentially to the target value U.sub.mess-soll. After a contact 13, 14 occurs at time point t.sub.1 the measuring voltage U.sub.mess deviates from the course of the target value U.sub.mess-soll. Based on the type of deviation U.sub.diff the measuring voltage U.sub.mess from the target value U.sub.mess-soll the type of contact 13, 14 can be determined. A detection signal S is output at the time of occurrence. The evaluation preferably takes place in the arc phase of a welding process. It can be seen that the course of the target value U.sub.mess-soll is qualitatively equivalent to the course of the welding voltage U.sub.SQ. This is due to the fact that U.sub.mess-soll is continuously determined from the welding voltage U.sub.SQ with the aid of the scaling factor P. Due to the scaling, a detection of contacts 13, 14 is independent of the level of the welding voltage U.sub.SQ is possible.