PURPOSE:To improve the accuracy of detecting the positional deviation of a work having a small shape change by subjecting the AC component of welding current or voltage obtd. by oscillation of a welding torch to Fourier series expansion and determining the ratio of the addition average value of respective groups of the signed numbers determined in such a manner. CONSTITUTION:The positional deviation between the oscillation center of the welding torch and weld line is determined from the change pattern of the welding current or welding voltage obtd. by oscillating the torch in the direction of crossing the weld line on the work. The DC component is removed from the welding current or welding voltage by a means I of this processor. The output signal of the means I is subjected to the Fourier series expansion up to the frequency of frequencies fXN with respect to the integer N larger than 2 and the sign and size of the coefft. of the terms of a sign wave and cosine wave at respective frequencies fXN are determined in a means II. The signed numbers are divided to integer M groups smaller than the predetermined integer N by a means III. The sets of the divided signed numbers are subjected to addition averaging by the multiplication rate predetermined for each of the groups by a means IV. Further, the ratio of the addition averaging results of the specific group predetermined among M pieces of the addition averaging results is determined in a means V.
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