Abstract
A method for ascertaining the wear of a contact tube during a robot-assisted welding method on a workpiece using a welding torch with a consumable welding wire includes the steps of measuring a welding current, measuring a welding voltage, dividing the welding current so measured by the welding voltage so measured to calculate a conductance value and comparing the conductance value with at least one defined conductance threshold value or dividing the welding voltage so measured by the welding current so measured to calculate a resistance value and comparing the resistance value with at least one defined resistance threshold value, and issuing a warning when at least one of the conductance value equals the at least one defined conductance threshold value and the resistance value equals the at least one defined resistance threshold value.
Claims
1. A method for determining a wear of a contact tube during a robot-assisted welding method on a workpiece using a welding torch with a consumable welding wire, the method comprising the steps of: measuring a welding current, measuring a welding voltage, dividing the welding current so measured by the welding voltage so measured to calculate a conductance value and comparing the conductance value with at least one defined conductance threshold value, or dividing the welding voltage so measured by the welding current so measured to calculate a resistance value and comparing the resistance value with at least one defined resistance threshold value, and issuing a warning when the conductance value equals the at least one defined conductance threshold value or the resistance value equals the at least one defined resistance threshold value.
2. The method according to claim 1, wherein the steps of measuring the welding current and measuring the welding voltage comprise measuring the welding current and the welding voltage at a time at which the welding current has a maximum value, the time corresponding to an end of a pulse phase of a pulsed process or an end of a short-circuit phase during an arcing phase of a short-circuit-based arc welding process.
3. The method according to claim 1, further comprising the step of averaging the conductance value or the resistance value over time.
4. The method according to claim 1, further comprising a step of setting at least one of the at least one defined conductance threshold value and the at least one defined resistance threshold value.
5. The method according to claim 1, wherein the step of issuing the warning comprises issuing the warning acoustically, optically, and/or in tactile form.
6. The method according to claim 1, further comprising a step of reducing a distance from the welding torch or an end of the contact tube to the workpiece, in an event of the warning being issued, thereby compensating for the wear of the contact tube.
7. The method according to claim 1, further comprising a step of determining a distance from the welding torch or an end of the contact tube to the workpiece.
8. The method according to claim 1, further comprising a step of carrying out a weld on a reference metal plate in an event of the warning being issued.
9. The method according to claim 1, wherein the steps of measuring the welding current and measuring the welding voltage comprise measuring the welding current and the welding voltage at a sampling frequency of 10 KHz to 100 KHz.
10. The method according to claim 1, further comprising a step of averaging the welding current and the welding voltage over a time interval of 1 ms to 300 ms.
11. The method according to claim 1, further comprising a step of recording the conductance value or the resistance value as a function of a time of welding.
Description
(1) The present invention will be explained in further detail by reference to the attached drawings. These show:
(2) FIG. 1 a block diagram of a welding device for carrying out a welding process with a consumable welding wire;
(3) FIG. 2A a schematic cross-sectional view through a new contact tube;
(4) FIG. 2B a schematic cross-section through a worn contact tube;
(5) FIG. 3 an example of the schematic temporal waveforms of the welding current, the welding voltage, and the ratio of the welding current to the welding voltage, i.e. the conductance;
(6) FIG. 4 an example of the schematic temporal waveforms of the welding current, the welding voltage, and the ratio of the welding voltage to the welding current, i.e. the resistance; and
(7) FIG. 5 an example of a real ratio of the welding current to the welding voltage as a function of the time or the welding duration.
(8) FIG. 1 shows a block diagram of a welding device 1 for carrying out a welding process with consumable welding wire 6, wherein a welding robot 2 guides a welding torch 4 along a predefined welding path X over at least one workpiece W to be processed. The consumable welding wire 6 is conveyed to the workpiece W via a contact tube 5 in the welding torch 4. In the contact tube 5, the welding current I supplied by the welding current source 3 is passed into the welding wire 6, so that an arc L to the workpiece W burns at the end of the wire that protrudes from the contact tube 5 during the welding process.
(9) FIG. 2A shows a schematic cross-sectional view through a new contact tube 5. The consumable welding wire 6 is conveyed through a corresponding hole 7 in the contact tube 5. In the case of a new contact tube 5, the hole 7 is essentially cylindrical, so that the contact with the welding wire 6 takes place close to the end 8 of the contact tube 5 at which the welding wire 6 exits the contact tube 5. The contact point K is thus located at the opening or the end 8 of the contact tube 5. The length l.sub.id of the current-passing part of the welding wire 6 corresponds here to the free wire length l.sub.s, i.e. the length of the welding wire 6 from the end 8 of the contact tube 5 to the end of the welding wire 6. The resulting length l.sub.id of the current-passing part of the welding wire 6 (from the contact point K to the end of the welding wire 6 at which the arc begins) depends on the material, diameter, feed rate of the welding wire 6, and also on the welding current I flowing through the welding wire 6. The feed rate of the welding wire 6 and the welding current I are usually adjusted in such a way that a desired length l.sub.id of the current-passing welding wire 6 is obtained. The distance ?d from the welding torch 4 or the end 8 of the contact tube 5 to the workpiece W is chosen in such a way that the distance l.sub.L from the end of the welding wire 6 to the workpiece W is suitable for forming the arc L and a stable welding process can take place. To keep the distance l.sub.L constant, the parameters such as the feed rate of the welding wire 6 and/or the welding current I, as well as the distance ?d from the welding torch 4 to the workpiece W, can be varied.
(10) FIG. 2B shows a schematic cross-section through a worn contact tube 5. In the worn contact tube 5, the hole 7 is widened because the permanent current transfer from the contact point K of the contact tube 5 into the welding wire 6 continuously abrades material from the contact tube 5. Thus, the contacting to the welding wire 6 takes place further back. The contact point K here is thus located behind the end 8 of the contact tube 5 at the distance l.sub.K from the end 8 of the contact tube 5. Without changing the feed rate of the welding wire 6 and/or the welding current I, the length l.sub.id of the current-passing welding wire 6 remains the same as in the example according to FIG. 2A and the free wire length l.sub.s is thus shortened compared to FIG. 2A. Thus, the distance l.sub.L from the melted end of the welding wire 6 to the surface of the workpiece W is increased compared to FIG. 2A by the distance l.sub.K from the contact point K to the end 8 of the contact tube 5, and the arc L burning between the end of the welding wire 6 and workpiece W is extended by this distance l.sub.K from the end 8 of the contact tube 5. This degrades the welding characteristics, and the distance l.sub.L must therefore be shortened again to the length according to FIG. 2A.
(11) A length of the arc L or the distance l.sub.L is usually reduced by the following three regulation methods (A, B, C), which can be used individually or in combination to compensate for the wear of the contact tube 5, wherein in method A the length l.sub.id of the current-passing welding wire 6 remains unchanged and in methods B and C it is increased:
(12) A.) By reducing the distance ?d from the welding torch 4 or the end 8 of the contact tube 5 to the workpiece W by means of a corresponding movement of the welding torch 4 using the welding robot 2. In this case the length l.sub.id of the current-passing welding wire 6 and the welding current I and also the feed rate of the welding wire 6 remain unchanged. The advantage of this is that the cross-section of the weld seam and the amount of the base material consumed remain essentially constant.
(13) B.) By increasing the feed rate of the welding wire 6. The length l.sub.id of the current-passing welding wire 6 and the free wire length l.sub.s are increased by the distance l.sub.K from the contact point K to the end 8 of the contact tube 5. The distance ?d from the welding torch 4 to the workpiece W remains unchanged. This method causes a slight increase in the cross-section of the weld seam. The feed rate of the welding wire 6 is controlled or regulated by the current source 3. The advantage of this method is that the welding robot 2 does not need to execute any movement if the contact point K in the contact tube 5 changes and the amount of the base material consumed remains essentially the same, as the welding current I remains unchanged.
(14) C.) By reducing the welding current I. As with method B.), this causes the length l.sub.id of the current-passing welding wire 6 and the free wire length l.sub.s to increase by the distance l.sub.K from the contact point K to the end 8 of the contact tube 5. The distance ?d remains unchanged. The reduction of the welding current I results in a reduction of the amount of the base material consumed (lower penetration depth). The advantage is that the welding robot 2 does not have to perform any movement if the contact point K in the contact tube 5 changes and that the cross-section of the weld seam remains essentially the same, since the feed rate of the welding wire 6 remains unchanged.
(15) If the wear of the contact tube 5 progresses further, the contact point K moves further backwards. In accordance with the limits of the control variables of the methods A.), B.) and C.) described above, replacement of the contact tube 5 will be required. According to the invention, this is possible by specifying the resistance R or conductance G derived from the averaged magnitudes of the welding current I and welding voltage U, irrespective of the combination of the control variables according to methods A.), B.) or C.).
(16) FIG. 3 shows an example of the schematic temporal waveforms of the welding current I(t), the welding voltage U(t) and the ratio of the welding current I(t) to the welding voltage U(t), i.e. the conductance G(t) or the reciprocal resistance 1/R(t), in a welding process for a consumable welding wire. The figures shows the changes in the actual values of the welding current I(t) and the welding voltage U(t) and the magnitude of the conductance value G(t) derived from them, which occur, for example, due to a change in the contact point K in the contact tube 5 by a certain distance, e.g. 6 mm. For the sake of simplification, it was assumed that the wear-related change in the contact point K in the contact tube 5 is linear. In practice, this is usually not the case, but in fact the contact point K of a contact tube 5 normally changes faster at first and then more slowly during the welding process, resulting in a non-linear relationship.
(17) At time t=0 the contact tube 5 is new, the contact point K is very close to the end 8 of the contact tube 5 (see FIG. 2A) and the welding process is carried out with a certain welding voltage U and a certain welding current I. With progressive duration of the arc burn and thus progressive wear of the contact tube 5, a displacement of the contact point K occurs and the welding voltage U increases while the welding current I decreases, when a voltage regulation with a falling characteristic is used (i.e. the welding voltage U is not absolutely constant, but rises with an increase in the resistance R, which means that the welding current I does not drop as much as would be the case if a constant voltage regulation was used). The actual waveforms are dependent on many conditions and factors, such as the particular welding process, the control characteristic, the welding power, the wire material, the contact tube quality, etc. According to the invention, the measured welding current I is divided by the measured welding voltage U and a conductance G is obtained. This conductance G decreases with increasing wear of the contact tube 5, regardless of the form (characteristic) of the regulation of the arc length (constant voltage, constant current, falling, . . . ).
(18) To define threshold values S.sub.si, which indicate the degree of wear of the contact tube 5, reference welds are performed with a new contact tube using a reference weld seam with different distances ?d. The different distances ?d are used to simulate the degree of wear of the contact tube 5 and to record the measured values or mean values of the welding current I and the welding voltage U and their ratios. At the end of each reference weld with the different distances ?d, these are defined as threshold values S.sub.si.
(19) In the example shown according to FIG. 3, two conductance thresholds G.sub.si are defined, with which the currently determined conductance value G(t) is compared. The first conductance threshold value G.sub.S1 corresponds to a medium level of wear of the contact tube 5, in which case the welding process can still be continued. This means that when the conductance reaches or falls below the first conductance threshold value G.sub.S1, a warning is issued at time t.sub.1. The second conductance threshold G.sub.S2 corresponds to a severe level of wear of the contact tube 5, in which case the welding process must no longer be continued. Thus, when the conductance reaches or falls below the second conductance threshold value G.sub.S2, a more urgent warning or even a shutdown of the welding process occurs at time t.sub.2.
(20) FIG. 4 shows the example of the schematic temporal waveforms of the welding current I, the welding voltage U, and the ratio of the welding voltage U to the welding current I, i.e. the resistance R.
(21) At time t=0 the contact tube 5 is new, the contact point K of the contact tube 5 is very close to the end 8 of the contact tube 5 (see FIG. 2A) and the welding process is carried out with a certain welding voltage U and a certain welding current I. As the arc burning duration progresses and hence the contact tube 5 wear increases, a displacement of the contact point K occurs and the welding voltage U increases and the welding current I decreases, if a voltage regulation with a falling characteristic is used. According to the invention, the measured welding voltage U is divided by the measured welding current I and a resistance R is obtained. This resistance R increases with increasing wear of the contact tube 5 regardless of the form of regulation of the arc length (characteristic).
(22) In the example shown according to FIG. 4, two resistance thresholds R.sub.si are defined, with which the currently determined resistance R(t) is compared. The first resistance threshold value R.sub.S1 corresponds to a medium level of wear of the contact tube 5, in which case the welding process can still be continued. This means that when the resistance reaches or exceeds the first resistance threshold value R.sub.S1, a warning is issued at time t.sub.1. The second resistance threshold R.sub.S2 corresponds to a severe level of wear of the contact tube 5, in which case the welding process can no longer be continued. Thus, when the resistance reaches or exceeds the second resistance threshold value R.sub.S2, a more urgent warning is issued, or even a shutdown of the welding process occurs at time t.sub.2.
(23) FIG. 5 shows an example of a real curve of the ratio of the welding current I to the welding voltage U, i.e. the conductance G, as a function of the time t or welding duration, or number of welds. In contrast to the simplified illustrations according to FIGS. 3 and 4, here the fluctuations of the calculated signal due to the fluctuations of the measurement values of the welding current I and the welding voltage U are clearly visible. For example, the welding current I and the welding voltage U are measured at a sampling frequency f.sub.A from 10 kHz to 100 kHz. By averaging the measured values over a certain time period ?t.sub.M of, for example, 1 ms to 300 ms, smoothing of the curve can be achieved. As mentioned above, the measurement of the welding current I and the welding voltage U is preferably carried out at a time when the welding current I has a maximum value I.sub.max, depending on the welding process. This allows the ratio, i.e. the resistance R or the conductance G, to be determined as accurately as possible.
(24) When the conductance reaches or falls below the first threshold value G.sub.S1 for the conductance G at time t.sub.1, an acoustic, optical and/or tactile warning is issued. When the conductance reaches or falls below the second conductance threshold value G.sub.S2 at the time t.sub.2, a warning is output which is different from the warning issued on falling below the first conductance threshold value G.sub.S1, or the welding process is even switched off and the contact tube 5 is replaced.
(25) The present invention describes a reliable method for ascertaining the wear of a contact tube during the welding process.
