The present invention relates to an inverter resistance welding electric power supply apparatus for seam welding.
Seam welding is a process in which a pair of relatively thin metal plates are weld-jointed continuously or intermittently forming a seamlike welded line or lines. Seam welding may be applied to seaming metal housings of dry cells and semiconductor devices, fuel tank, etc.
FIG. 7 illustrates, as an example, seam welding of metal plates 100 and 102 along their margins. A pair of seam welding rolls 106 and 108 are moved relative to edges 100a and 102a of the metal plates 100 and 102 along a line 104 to be seamed (see dotted line in FIG. 7). In the meantime, electric current is continuously or intermittently supplied to the roller electrodes 106 and 108 to thereby seam weld the workpieces 100 and 102 along the line 104.
Seam welding may also be applied to sealing or encapsulating a tank 110, such as the one shown in FIG. 8. A pair of seam welding rolls 118 and 120 are moved relative to flanges or brims 112a and 114a of hat-like tank members 112 and 114 along a line 116 to be seamed (see dotted line in FIG. 8). In the meantime welding current is continuously or intermittently supplied to the welding electrodes 118 and 120 to thereby make the seam or joint 116.
Conventionally, the workpieces (100, 102), 110 are successively or constantly moved or translated by a feed gear whereas the seam welding rolls (106, 108, 118, 120) rotate around their axis.
During a seam welding operation, a resistance welding electric power supply apparatus supplies welding current to workpieces via welding electrodes, as mentioned above. The prior art inverter seam resistance welding electric power supply apparatus includes a feedback constant-current control feature which measures welding current on a cycle-by-cycle basis and controls it to be maintained at or near a desired level during a welding operation. The prior art apparatus, however, cannot successfully monitor or evaluate a quantity of welding current which is useful to determine the result of a weld or the quality of the seam welding. The reasons may be explained as follows:
The prior art apparatus employing a constant-current control measures welding current on an inverter switching cycle-by-cycle basis. The measured quantity or magnitude of welding current of a single switching cycle of the inverter contributes very little to weldability of seam welding. Thus, such a single cycle welding current value, as measured, does not carry useful information to reliably determine results of the welding. Going to an opposite extreme, one might propose an inverter seam welding resistance electric power supply apparatus which evaluates an average value of the entire welding current throughout a seam weld time. Such an entire current average value throughout the seam weld time, however, clearly disregards localities of seam welding, thus overlooking local defects of the seam weld since such a seam weld time is generally very long (for instance several minutes); much longer than a single spot weld time.