GB1446652A - Resistance welding - Google Patents

Abstract

1446652 Resistance welding circuits WELDING INSTITUTE 24 Feb 1975 [25 Feb 1974] 8467/74 Heading H2H Method of and apparatus for controlling the duration of a resistance weld comprises monitoring the voltage across the welding electrodes during a weld, detecting and holding the peak voltage to which the monitored voltage rises, comparing the subsequent lower cyclic peaks of the monitored voltage with a voltage which is in the range 2-20% lower than the peak held voltage, and terminating the weld as soon as the subsequent cyclic peaks fall to the lower voltage level. In the apparatus described the voltage present across the electrodes is passed through a steep low-pass filter 1 to eliminate spurious peaks and noise and through a variable gain amplifier 2 to a switch 53 which is set for monitoring either positive or negative peaks. The output from switch 53 is passed to a divider 4 where the monitored voltage is divided by a value representing the peak of the supply voltage to eliminate erroneous operations owing to variations in the supply. The output from divider 4 is passed to a peak detector 5 comprising an operational amplifier 30 with a diode D1 in its feedback path connected to capacitor 15 which holds the peak value of the input to the amplifier. The output is taken via a buffer amplifier 6, which prevents capacitor 15 from discharging, and then is applied to a comparator 9 via a potentiometer which reduces the held peak voltage by an amount preferably the range 10-20%. Incoming voltage values are applied from the output of the divider 4 to the other input of the comparator 9 and a pulse is produced whenever the input exceeds the reduced stored value. A ramp generator 28 is reset by each pulse from the comparator 9 but if one fails to arrive in any one cycle, indicating the peak voltage has fallen the requisite amount, this ramp voltage will rise to activate comparator 31 to stop the weld current via relay 10. A circuit 11-14, responsive to welding voltage cycles may operate a warning lamp 15A via thyristor 15 if greater than a certain number of cycles have been applied to the weld. The circuit 17-24 which is also fed with the welding supply current produces a voltage sample thereof at the peak of the A.C. waveform to be applied to divider 4. A circuit raising to the nth power may be inserted between the devices 4, 5. When a 3-phase supply is used, an RMS waveform is first derived from the monitored electrode waveform and the peak value of this is detected and held. A second peak detector is used and is reset after each cycle, the weld being terminated when the output of the second detector has fallen to the desired proportion of the held value.